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OPERANDO FUEL CELL SPECTROSCOPY

A dissertation presented

by

Ian Michael Kendrick

to

The Department of Chemistry and Chemical Biology

In partial fulfillment of the requirements for the degree of

Doctor of Philosophy

in the field of

Chemistry

Northeastern University

Boston, Massachusetts

April 24, 2013

2

OPERANDO FUEL CELL SPECTROSCOPY

by

Ian Michael Kendrick

ABSTRACT OF DISSERTATION

Submitted in partial fulfillment of the requirements

for the degree of Doctor of Philosophy in Chemistry

in the College of Science of

Northeastern University

April 24, 2013

3

Abstract

The active state of a catalyst only exists during catalysis(1)

provided the motivation for

developing operando spectroscopic techniques. A polymer electrolyte membrane fuel cell

(PEMFC) was designed to interface with commercially available instruments for acquisition of

infrared spectra of the catalytic surface of the membrane electrode assembly (MEA) during

normal operation. This technique has provided insight of the complex processes occurring at the

electrode surface. Nafion, the solid electrolyte used in most modern-day polymer electrolyte

membrane fuel cells (PEMFC), serves many purposes in fuel cell operation. However, there is

little known of the interface between Nafion and the electrode surface. Previous studies of

complex Stark tuning curves of carbon monoxide on the surface of a platinum electrode were

attributed the co-adsorption of bisulfite ions originating from the 0.5M H2SO4 electrolyte used in

the study(2)

. Similar tuning curves obtained on a fuel cell MEA despite the absence of

supplemental electrolytes suggest the adsorption of Nafion onto platinum(3)

. The correlation of

spectra obtained using attenuated total reflectance spectroscopy (ATR) and polarization

modulated IR reflection-absorption spectroscopy (PM-IRRAS) to a theoretical spectrum

generated using density functional theory (DFT) lead to development of a model of Nafion and

platinum interaction which identified participation of the SO3- and CF3 groups in Nafion

adsorption.

The use of ethanol as a fuel stream in proton exchange membrane fuel cells provides a

promising alternative to methanol. Relative to methanol, ethanol has a greater energy density,

lower toxicity and can be made from the fermentation of biomass(4)

. Operando IR spectroscopy

was used to study the oxidation pathway of ethanol and Stark tuning behavior of carbon

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monoxide on Pt, Ru, and PtRu electrodes. Potential dependent products such as acetaldehyde,

acetic acid and carbon monoxide are identified as well as previously unobserved peaks

corresponding to adsorbed ethanol.

A modification to the operando fuel cell design allowed for acquisition of Raman spectra.

A confocal Raman microscope enabled characterization of the MEA through depth profiling.

The potential dependent peaks of an Fe-Nx/C catalyst were identified and compared to the

theoretical spectra of the proposed active sites. It was determined that oxygen adsorbed onto

iron/iron oxide carbon nanostructures were responsible for the experimentally obtained peaks.

This finding was supported by additional Raman studies carried out on a catalyst with these

active sites removed through peroxide treatments.

1. Topsoe, H., Developments in operando studies and in situ characterization of heterogeneous catalysts. Journal of Catalysis, 2003. 216(1-2): p. 155-164.

2. Stamenkovic, V., et al., Vibrational properties of CO at the Pt(111)-solution interface: the anomalous stark-tuning slope. Journal of Physical Chemistry B, 2005. 109(2): p. 678-680.

3. Kendrick, I., et al., Elucidating the Ionomer-Electrified Metal Interface. J. Am. Chem. Soc., 2010. 132(49): p. 17611-17616.

4. Lamy, C. and Leger, J.M., FUEL-CELLS - APPLICATION TO ELECTRIC VEHICLES. Journal De Physique Iv, 1994. 4(C1): p. 253-281.

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Acknowledgements

I am extremely grateful for the opportunity to have been invited to pursue my graduate degree

at Northeastern University. I could not have gone this far alone and wish to acknowledge the

help and support I have received over the years.

My advisor Dr. Eugene Smotkin for his constant support, advice and for pushing me further

than I ever thought possible.

Graham Jones for all of the opportunities have gave me during the first half of my graduate

career.

My thesis committee: Drs. Graham Jones, Sanjeev Mukerjee and Max Deim.

Lab members past and present especially: Sara Evarts, Adam Yakaboski, Jonathan Doan,

Mike Bates and Mike Finch. Thank you for all of the support and friendship.

I am fortunate to have made many wonderful friends that I have made here at Northeastern,

especially Meghan Johnston. Thank you for reminding me that work and research isn’t

everything.

Maiann Good for her constant support and understanding.

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Table of contents

Abstract ........................................................................................................................................... 2

Acknowledgements ......................................................................................................................... 5

Table of contents ............................................................................................................................. 6

List of Figures ................................................................................................................................. 7

List of Tables ................................................................................................................................ 10

List of Schemes ............................................................................................................................. 11

List of Abbreviations .................................................................................................................... 12

Chapter 1: Introduction ................................................................................................................. 14

Chapter 2: Complex Stark Tuning of CO ..................................................................................... 26

Chapter 3: Elucidating the ionomer-metal interface ..................................................................... 35

Chapter 4: Minimal Vibrational Mode Analysis of Nafion Infrared Spectroscopy ..................... 51

Chapter 5: Operando Infrared Spectroscopy of Ethanol Oxidation in Polymer Electrolyte Fuel

Cells .............................................................................................................................................. 68

Chapter 6: Operando Raman Spectroscopy of a non-Pt Cathode Nafion Membrane Electrode .. 87

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List of Figures Figure 1.1 PEM fuel cell schematic .............................................................................................. 14

Figure 1.2. Schematic of operando IR cell developed by Fan et al .............................................. 17

Figure 1.3. Operando XAS spectroscopy fuel cell and schematic (from ref. 28) ......................... 17

Figure 1.4. Operando IR-XAS fuel cell schematic. ...................................................................... 18

Figure 1.5. Cell installed in a diffuse reflectance accessory ......................................................... 19

Figure 1.6. IR-XAS cyclic voltammogram. ................................................................................. 20

Figure 1.7. IR-XAS fuel cell polarization curve. ......................................................................... 20

Figure 2.1. Potential-dependent spectra of CO on Pt. .................................................................. 29

Figure 2.2. Stark tuning plots of linearly adsorbed CO adsorbed on Pt at 50 °C. ....................... 29

Figure 2.3. CO electro-oxidation onset potentials versus adsorption potentials .......................... 30

Figure 2.4. Operando CO/Pt Stark tuning ..................................................................................... 31

Figure 2.5. The potential at which CO begins to oxidize as a function of temperature. ............. 32

Figure 3.1. DFT calculated normal modes and Nafion ATR spectrum ....................................... 40

Figure 3.2. Theoretical and experimental spectra of Nafion ....................................................... 41

Figure 3.3. Normal mode coordinate animation snapshots of the Nafion side-chain anion and

backbone fragment ........................................................................................................................ 44

Figure 3.4. Gaussian 03 Viewer Nafion-Pt interface model ........................................................ 48

Figure 4.1. Flow chart for generation of Nafion repeat group MVM spectra. ............................. 55

Figure 4.2. Definition of the internal coordinates for a) C3v, b) C2v, and c) the Nafion Backbone.

....................................................................................................................................................... 58

Figure 4.3. Top: DFT calculated normal modes decomposed into MVM spectra. Composite

DFT lines show contributions of minimal vibrational modes to each normal mode. ................... 60

Figure 4.4: The ATR-IR spectrum of Teflon superimposed over Nafion backbone MVMs. ...... 61

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Figure 4.5: ATR spectrum of PFMHSA (left) and PFEBSA (right) superimposed over SO3- νas,

SO3- νs, CF3 νas, CF3 νs MVMs. ..................................................................................................... 61

Figure 5.1. Stark tuning curve of CO adsorbed onto Pt and potential dependent spectra of a Pt

black electrode in the presence of ethanol vapor. ......................................................................... 72

Figure 5.2. Stark tuning curve of CO adsorbed onto PtRu potential dependent spectra of an

unsupported PtRu electrode in the presence of ethanol vapor. ..................................................... 73

Figure 5.3. A general mechanism for the oxidation of ethanol on Pt. ......................................... 75

Figure 5.4. Potential dependent spectra of a Pt black electrode in the presence of ethanol vapor.

....................................................................................................................................................... 76

Figure 5.5. Potential dependent spectra of a Ru black electrode in the presence of ethanol vapor.

....................................................................................................................................................... 79

Figure 5.6. Potential dependent spectra of a PtRu black electrode in the presence of ethanol

vapor ............................................................................................................................................. 81

Figure 6.1. Exploded view of operando Raman cell components. ............................................... 90

Figure 6.2. Operando Raman cell beneath a confocal Raman microscope. ................................ 90

Figure 6.3: Total free energy, in Hartrees, for the hypothetical Fe-Nx/C active site at various spin

states. ............................................................................................................................................. 92

Figure 6.4. Schematic outlining the concept of depth profiling a membrane electrode assembly

using a confocal Raman microscope. Depth dependent spectra of a membrane electrode

assembly consisting of an Fe/N/C catalyst and Nafion.. .............................................................. 94

Figure 6.5. The potential dependent Raman spectra of an iron based non-PGM cathode catalyst

obtained under O2. ........................................................................................................................ 96

Figure 6.6. Raman fuel cell polarization curve obtained under oxygen with a MEA consisting of

a Pt anode and an Fe-Nx/C cathode............................................................................................... 97

Figure 6.7. Potential dependent control experiments of a non-PGM cathode catalyst. (Left):

Spectra obtained under O2 with a catalyst prepared without iron. (Right): Spectra obtained under

N2 catalyst prepared with iron....................................................................................................... 98

Figure 6.8. DFT generated theoretical spectra of an iron based non-PGM catalyst with a N4

pyridinic active site. . ................................................................................................................. 100

Figure 6.9. DFT generated theoretical spectra of an iron based non-PGM catalyst with a N4

pyrrolic active site. ...................................................................................................................... 100

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Figure 6.10. DFT generated theoretical spectra of an iron nanoparticle with and without adsorbed

oxygen ......................................................................................................................................... 103

Figure 6.11. Potential dependent Raman spectra of non-PGM catalyst treated with H2O2 to

remove nanoparticles. Spectra were obtained under presence of oxygen. ................................ 104

Figure 6.12. Fuel cell polarization curve obtained under oxygen with a MEA consisting of a Pt

anode and an Fe-Nx/C cathode subjected to the structural distortion treatment. ........................ 105

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List of Tables

Table 3.1. PM-IRRAS and DFT IR adsorption peaks and assignments. ………………………45

Table 3.2. Average partial charges of selected Nafion segments. ……………………………...47

Table 4.1. Internal coordinate system MVMs of the most prominent IR modes for Nafion. …...57

Table 4.2 Pure mode assignments from visualization of normal mode animations in comparison

to MVM assignments of this work. ……………………………………………………………..63

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List of Schemes Scheme 3.1. Segment and atom labeling for Nafion. …………………………………………42

Scheme 4.1: Nafion chemical repeat unit. ……………………………….……………………53

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List of Abbreviations

ATR attenuated total reflection

CCM catalyst coated membrane

CO carbon monoxide

CV cyclic voltammogram

DEMS differential electrochemical mass spectroscopy

DFT density functional theory

DMFC direct methanol fuel cell

FTIR Fourier transform infrared spectroscopy

IR infrared

GDL gas diffusion layer

MEA membrane electrode assembly

MVM minimal vibrational mode

NBO natural bond order

NHE normal hydrogen electrode

Non-PGM non-platinum group metal

ORR oxygen reduction reaction

PEMFC proton exchange membrane fuel cell

PFEBSA perfluoro(2-ethoxybutane) sulfonic acid

PFMHSA perfluoro(3-methyl-2,4-dioxahexane) sulfonic acid

PM-IRRAS polarization modulated infrared reflection-absorption spectroscopy

Psig pounds per square inch (gauge)

RHE reversible hydrogen electrode

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Sccm standard cubic centimeter per minute

TR transmission spectrum

XAS X-ray absorption spectroscopy

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Chapter 1: Introduction

That the active state of a catalyst exists only during catalysis(1) is succinct rationale for

operando methods of catalyst characterization. The primary challenge to operando

spectroscopy is conversion of a practical device into a spectroscopic cell with minimal

perturbation of device functionality. Figure 1.1 schematizes the terminal end-cell of a fuel cell

stack.(2)

Figure 1.1 PEM fuel cell schematic (From ref. 2)

Graphite flow-field plates distribute fuel and oxidant to the 5-layer membrane electrode

assembly (MEA). An ionomer membrane (e.g., Nafion) supports electrocatalytic layers that

contact gas diffusion layers (i.e., porous carbon paper or cloth) that are optimized for reactant

transport and electronic conductivity. MEA fabrication methods have been reviewed.(3)

Catalyst particles (carbon supported or metal blacks) (4) are dispersed in alcoholic solutions of

solubilized ionomer. These “inks” are deposited onto the gas diffusion layers and hot pressed to

the membrane. Alternatively, a catalyst-coated-membrane (CCM) can be prepared by

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immobilizing the membrane on a heated vacuum table (NuVant Systems Inc., Crown Point, IN)

for direct ink deposition onto the membrane. Catalytic layers are a complex blend of ionomer,

catalyst particles and, sometimes at the cathode, Teflon dispersion. The relative amounts depend

on the catalyst composition and device application. The final step of MEA preparation occurs in

the operating fuel cell. The hot pressing of the gas diffusion electrode and/or ink drying on the

heated vacuum table causes delamination of the ionomer from the catalyst particles. The

conditioning of the MEA in the operating fuel cell re-wets the catalyst with ionomer and

removes the deep oxides that are typical of as-prepared catalysts.(5) There are no intended

“triple phases”. The catalyst active area must be coated with a sub-micron gas permeable, ion

conducting layer.(6) A Nafion layer on Pt has been shown to enhance electrocatalysis.(6, 7)

Ionomer electrolytes have no mobile ions other than protons or hydroxide ions. Supplemental

electrolytes such as aqueous H2SO4 or HClO4 contribute mobile ions that competitively adsorb

onto the surface.(8-10) Supplemental aqueous electrolytes preclude fuel cell operation at the high

end of relevant temperatures (e.g. 70-120 oC).

Operando spectroscopy requires control of the temperature, flow rate and the humidity of

the anode and cathode reactant streams while potential dependent spectra are acquired. Cell

component materials must not fluoresce at energies similar to the X-ray edge energies of the

catalysts. The uniformity of the polymer electrolyte resistance, governed by the ionomer

membrane thickness (e.g., 7 mil for Nafion 117)(11) depends on careful water management and

proper flow field design. Whether studying anode or cathode catalysts, the counter electrode can

serve as the auxiliary and the reference electrode (counter-reference electrode).(12) Gurau

delivered hydrogen to the counter-reference electrode while acquiring liquid feed direct

methanol fuel cell anode polarization curves.(13) Pure water can be delivered to the counter-

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reference electrode: Hydrogen evolution at the counter electrode and the hydrogen ion activity,

set by the ionomer equivalent weight and state of hydration, poises(14) the counter-reference

electrode. Although the counter-reference is polarized at high currents, the alternative of

developing a 3rd

electrode as a reference is more complex than correcting for reference electrode

polarization. Fortunately, the exchange current density for the hydrogen electrode is many orders

of magnitude larger than that for the oxygen reduction reaction.(15) On a practical level, the

use of the fuel cell counter-reference affords greater reproducibility between laboratories.

Operando infrared spectroscopy of the catalyst-electrolyte interface is ideal for adsorbate

characterization. Adsorbates effect dynamic changes on the catalyst surface such as surface

restructuring where surface atoms can be entirely displaced in order to make stronger surface-

adsorbate bonds.(16) These changes can be observed, in operando and in situ spectroscopy, as

changes in the vibrational modes of either the surface or the adsorbate atoms. Fan introduced

operando fuel cell spectroscopy in 1996.(17, 18) He incorporated a CaF2 window on a fuel cell

flow field, cut a slot into the GDL to expose the catalytic layer, and installed the modified fuel

cell into a 1990’s model Harrick Praying Mantis (Pleasantville, NY) diffuse reflection IR

accessory. Figure 1.2 schematizes the cell used to identify methanol oxidation products,

including formic acid, in spectra acquired over a range of temperatures from 25 - 90oC with

steady state reactant flow to both electrodes. Bo used the same cell to measure Stark tuning

rates(19) of CO/Pt at 50oC .(20)

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Figure 1.2. Schematic of operando IR cell developed by Fan et al

Viswananthan(21) and Stoupin(22) introduced hydrogen and liquid feed direct methanol fuel

cell (DMFC) operando X-ray absorption spectroscopy respectively using the cell of Fig. 1.5.

They end-milled a rectangular core out of both flow field blocks and endplates. Palladium was

used at the cathode to mitigate interference with the absorption edge of the Pt based anode

catalysts. The “cored” cell design has since been used by a number of workers.(23-27)

Figure 1.3. Operando XAS spectroscopy fuel cell and schematic (from ref. 28)

A number of operando XAS(28-35) studies followed that of Viswananthan with many

focused on the oxidation state of the metal components. All found that at relevant fuel cell anode

operating potentials, platinum is metallic. The oxidation of CO was studied both on Pt(32), and

CO, D2O

inlet

D2, D2O

heating pad

Nafion

membrane

Exhaust

CaF2

Incident IR Integrating sphere

Exhaust

PtRu

Pt

CO, D2O

inlet

D2, D2O

heating pad

Nafion

membrane

Exhaust

CaF2


Exhaust

PtRu

Pt

CO, D2O

inlet

D2, D2O

heating pad

Nafion

membrane

Exhaust

CaF2


Exhaust

PtRu

Pt

CO, D2O

inlet

CO, D2O

inlet

D2, D2O

heating pad

Nafion

membrane

Exhaust

CaF2


Exhaust

PtRu

Pt

CO, D2O

inlet

D2, D2O

heating pad

Nafion

membrane

Exhaust

CaF2


Exhaust

PtRu

Pt

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PtRu,(28, 31, 36, 37) confirming the electronic benefits of Ru as a co-catalyst for CO oxidation.

Studies of adsorbed oxygen reduction reaction intermediates on fuel cell catalysts show that

adsorption is both potential-dependent and site-specific.(28, 38-41)

The Viswananthan cell was designed for transmission X-ray absorption measurements.

Fluorescence measurements provide better signal-to-noise because interference due to cell

components is minimized. The most recent cell design (Fig. 1.8) combines features of the

Viswananthan cell(21) with the specular reflectance infrared cell reported by Fan et al.(18) The

IR-XAS cell enables operando XAS (in transmission and fluorescence) and specular reflectance

FTIR spectroscopy. The top flow field accommodates a CaF2 window for IR reflectance

studies.(20, 42) A pin-style upper flow field optimizes flow distribution around the CaF2

window inset. The CaF2 window can be removed when the working electrode of interest is an

air-breathing electrode.

Figure 1.4. Left: IR-XAS cell with DE9 connector for electrodes, heater cartridge and thermister.

Right: 1) X-ray transmission, 2) Reflectance IR or fluorescence X-ray, 3) CaF2 window housing,

4) Teflon gasket, 5) gas outlet insert, 6) slider assembly, 7) thermocouple/heater cartridge port,

8) lower flow field, 9) MEA, 10) upper flow field, 11) top plate.

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The IR-XAS cell slides into a Pike Technologies (Watertown, WI) diffuse reflectance

accessory (with a modified front plate) that is accommodated by most commercial FTIR

instruments (Fig. 1.9).

Figure 1.5. Cell installed in Pike Technologies (Watertown, WI) diffuse reflectance accessory in

a Bruker Optics (Billerica, MA) Vertex 70 FTIR Spectrometer.

A slot under the slider assembly ensures precise positioning of the cell under the Diffuse-IR

accessory integrating mirrors. Figure 1.10 shows background CV (solid) and the CO stripping

wave (dotted) obtained at 50 oC. The CO stripping wave (10 mV/sec) extends from 600 - 900

mV. Figure 1.11 is a performance curve of a Pt/Pt electrode

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Figure 1.6. Cyclic voltammetry (10mV/s) of fuel cell working electrode (5-cm2 geometric)

under humidified nitrogen at 50°C (solid). Humidified H2 (50 sccm) at the counter-reference.

CO stripping wave (dashed).

Figure 1.7. IR-XAS fuel cell polarization curve.

0.05 0.00 -0.05 -0.10 -0.15 -0.20 -0.25 -0.30 -0.35

0.2

0.4

0.6

0.8

1.0

Po

ten

tial (m

V)

Current (A)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

-0.02

-0.01

0.00

0.01

0.02

0.03

0.04

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

-0.02

-0.01

0.00

0.01

0.02

0.03

0.04

Curr

ent

(mA

)

Potential (mV)

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Here the first fully operando FTIR and X-ray absorption studies of fuel cells is described.

The features of these legacy cells are then combined into a single cell that can accommodate both

FTIR and X-ray absorption spectroscopy of operating fuel cells. Operando FTIR spectroscopy,

complemented with density functional theory (DFT) and polarization modulated infrared

reflection absorption spectroscopy (PM-IRRAS) of ionomer-platinum interfaces, suggests a

model for self-assembly of Nafion onto a Pt surface. The assignment of bands responsible

Nafion’s self-assembly onto Pt motivated the development of an automated method to identify

functional group contributions to theoretical vibrational modes. Separating these contributions

to into their own spectra enables the prediction of changes in experimentally obtained spectra of

Nafion caused by derivitization, state-of-hydration and ion exchange.

The utility of operando FTIR spectroscopy is exemplified with the study of electro-

oxidation of ethanol. The oxidation pathways of ethanol on Pt, Ru and PtRu electrodes are

elucidated. The bipolar nature of peaks associated with adsorbed species enables differentiation

from desorbed species without the use of a polarized light source allows for a comprehensive

assessment of oxidation processes with a single technique. Previous in-situ FTIR studies of

ethanol oxidation use HClO4 as an electrolyte yielding an intense peak at 1033 cm-1

. Because

operando spectra are acquired without mobile anions, a peak associated with O-adsorbed ethanol

is observed for the first time. The oxidation mechanisms elucidated through operando

spectroscopy is consistent with the established mechanisms and has been used to characterize

ethanol oxidation on experimental catalysts.

A simple modification to the design of the slider assembly enabled the collection of

Raman spectra. The use of a confocal Raman microscope allowed for a complete

characterization of the MEA. This new technique was used to identify potential dependent

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features of the Raman spectra of the surface of iron/nitrogen based supported oxygen reduction

catalysts. The exact nature of the active site and mechanism of these catalysts is still disputed.

The latest theory is that in acidic media, the ORR proceeds via a two-step mechanism: a Fe-N4/C

moiety reduces oxygen to hydrogen peroxide and peroxide is reduced to water on a second active

site. Recent studies suggest that the active site consists of a metal/metal oxide cluster.(44) DFT

was used to generate theoretical spectra of two proposed Fe-N4/C active sites and one Fe

nanocluster. Theoretical spectra were generated for these models both in the presence and

absence of adsorbed O2. The position of the potential dependent features of the Raman spectra

matches the position of Fe-O stretching on the Fe nanocluster theoretical spectrum. The

potential dependent peaks are missing in the Raman spectra Fe-Nx/C catalysts treated to remove

the Fe/FeOx moieties.

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35. Croze, V., et al., The use of in situ X-ray absorption spectroscopy in applied fuel cell research. Journal of Applied Electrochemistry, 2010. 40(5): p. 877-883.

36. Russell, A.E., et al., In situ X-ray absorption spectroscopy and X-ray diffraction of fuel cell electrocatalysts. Journal of Power Sources, 2001. 96(1): p. 226-232.

37. Scott, F.J., Mukerjee, S., and Ramaker, D.E., CO Coverage/Oxidation Correlated with PtRu Electrocatalyst Particle Morphology in 0.3 M Methanol by In Situ XAS. Journal of The Electrochemical Society, 2007. 154(5): p. A396-A406.

38. Teliska, M., O'Grady, W.E., and Ramaker, D.E., Determination of H Adsorption Sites on Pt/C Electrodes in HClO4 from Pt L23 X-ray Absorption Spectroscopy. J. Phys. Chem. B, 2004. 108(7): p. 2333-2344.

39. Teliska, M., et al., Site-Specific vs Specific Adsorption of Anions on Pt and Pt-Based Alloys. J. Phys. Chem. C, 2007. 111(26): p. 9267-9274.

40. Teliska, M., et al., In situ determination of O(H) adsorption sites on Pt based alloy electrodes using X-ray absorption spectroscopy. Proceedings - Electrochemical Society, 2005. 2003-30(Fundamental Understanding of Electrode Processes): p. 212-216.

25

41. Roth, C. and Ramaker, D.E., 3 XAS Investigations of PEM Fuel Cells, in Interfacial Phenomena in Electrocatalysis, C.G. Vayenas, Editor. 2011, Springer New York. p. 159-201.

42. Gasteiger, H.A., et al., Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs. Applied Catalysis B: Environmental, 2005. 56(1-2): p. 9-35.

43. FTIR studies require a slot cut into the GDL to enable beam access to the catalytic surface.

44. Olson, T.S., et al., Bifunctional Oxygen Reduction Reaction Mechanism on Non-Platinum Catalysts Derived from Pyrolyzed Porphyrins. Journal of the Electrochemical Society, 2010. 157(1): p. B54-B63.

26

Chapter 2: Complex Stark Tuning of CO

2.1 Introduction The chemisorption of CO on a metal surface results from electron transfer from the 5σ

orbital of CO to the metal. Additionally, electrons from dz metal orbital back donate in the CO

2π* antibonding orbital of CO.(1) The degree of back donation can be modulated when the

metal surface is used as an electrode. The adsorption of CO onto a metal surface can be directly

studied through in-situ infrared spectroscopy.(2-4) At more positive potentials, the bond order is

increased due to 2π* antibonding orbital being less occupied. This results in a shift in the peak

position known as Stark tuning. The peak position of adsorbed CO provides information on

adlayer structures.(5-7) In this study, the Start tuning of CO peaks adsorbed onto a Pt electrode

will be examined in a working fuel cell. The complex behavior elucidates co-adsorption of the

polymer electrolyte and enthalpies of adsorption will be correlated to adsorption potentials.

2.2 Experimental

Membrane electrode assembly preparation: Nafion-117 (E. I. DuPont) was immersed in

boiling ~8 M nitric acid for 20 min, rinsed with Nanopure™ water, and finally immersed in

boiling water for one hr. Catalyst inks were prepared as previously described.(8, 9) Briefly, Pt

black (Johnson Matthey) was dispersed in 5 wt% Nafion ionomer solution (Sigma Aldrich,

Milwaukee, WI) diluted with Nanopure™ water and isopropanol. Inks were applied directly to a

5 cm2 area of Nafion immobilized on a temperature controlled vacuum table (NuVant Systems

Inc., Crown Point, IN) at 70° C. The catalyst loadings were 4mg/cm2 of Pt black. The carbon

paper gas diffusion layers (Toray Industries, Tokyo, Japan) were blocked with Vulcan XC-72

(Cabot Corporation, Billerica, MA).

27

Operando IR spectroscopy: Spectra from 800-4000 cm-1

, at 4 cm-1

resolution, were acquired

with a Vertex 70 Spectrometer (Bruker Optics, Billerica, MA) and analyzed with Opus 6.5™

software. The potential dependent IR specular reflectance spectra of adsorbed CO (COads) was

obtained for CO dosing potentials (Eads) of 100 to 400 mV vs. RHE in 100 mV increments. Prior

to acquisition of the background IR spectrum, the cell was brought to 50 °C with H2 (50 sccm) at

the counter-reference electrode and N2 at the working electrode (200 sccm) for 15 min. The

working electrode feed was switched to CO (40 sccm) at the selected adsorption potential for 15

min prior to purging the working electrode with N2 (200 sccm). The potential was set to 100

mV prior to acquisition of four signal-averaged (250 scans) spectra, at 50mV increments, until

the CO vibrational bands were no longer observable.

2.3 Results and discussion

Potential dependent spectra of CO adsorbed at 100 mV vs. NHE are shown in Figure 2.1. The

corresponding Stark tuning plots for CO adsorbed at 100, 200, 300 and 400 mV (Fig. 2.2) show

the effects of co-adsorption on the CO stretching frequencies. These results are similar to those

of Stamenkovic et al. studying CO on Pt(111) in 0.5 M H2SO4.(5) Stamenkovic correlated the

complex Stark tuning of COads stretching frequencies (νCO) to the compression/dissipation of

COads islands: A linear Stark tuning region with a subtle νCO blue shift from the extrapolated

linear region was followed by a precipitous drop in νCO, followed by an upturn. The potential

where νCO precipitously drops (Eonset) correlates with COads oxidation. Stamenkovic identified the

co-adsorbate as HSO3- originating from the 0.5 M H2SO4 electrolyte used in their study.

Stamenkovic attributed the blue shift from the linear region to the oxidation of a small amount of

adsorbed CO caused by the presence of activated water adsorbed onto defect sites. Kendrick,

using no supplemental electrolyte (i.e., operando conditions), identified co-adsorbates as the

28

Nafion sulfonate exchange group and the side chain CF3 group. COads oxidation, induced by OH-

adsorption, diminishes dipole-dipole coupling and thus precipitously decreases νCO. The upturn

is ascribed to increased adsorption of sulfonate species relative to OH-, which reestablishes

repulsive dipole interactions that compress COads islands and increases νCO.

The Stark tuning rates of this work, 6.8 0.6 cm-1

/mV, are within the range of 2.5 cm-1

/V to

18.3 cm-1

/V of previous operando Stark tuning studies of direct methanol fuel cells at potentials

negative of 0.5 V vs. NHE.42

The wavelength precision of an FTIR is determined by the stability

of the reference HeNe laser. Virtually all FTIR spectrometers manufactured today are capable of

0.1 cm-1

or better precision on the wavelength axis. The Bruker Vertex 80V is a research

spectrometer that is routinely precise to better than 0.07 cm-1. This does not mean that spectral

features are resolved to this extent, but it does mean that band positions can reliably be reported

to this level of precision.

2200 2175 2150 2125 2100 2075 2050 2025 2000

Wavenumber (cm-1)

100 mV

150 mV

200 mV

250 mV

300 mV

350 mV

400 mV

450 mV

500 mV

29

Figure 2.1. Potential-dependent spectra of CO on Pt; Adsorption potential of 100 mV operating

at 50 °C.

Figure 2.2. Stark tuning plots of linearly adsorbed CO adsorbed on Pt at 50 °C.

Figure 2.3 shows how Eonset dependends on Eads. At 400 mV, Eonset and Eads coincide because at

higher potentials COads favors adsorption sites with higher enthalpies of adsorption (e.g., steps

and kinks) (10, 11) and there is no thermodynamic force driving migration to sites of lower

ΔHads. At an Eads of 300 mV, the Eonset exceeds Eads by 60 mV because the distribution of

adsorption sites are more heterogeneous and there is a driving force for COads on terraces to

migrate to steps, kinks and ad atoms. Thus the difference between Eonset on Eads would be

100 200 300 400 500 6002074

2076

2078

2080

2082

2084

2086

2088

2090

2092

Wavenum

ber

(cm

-1)

Potential (mV)

100 200 300 400 500 6002074

2076

2078

2080

2082

2084

2086

2088

2090

2092

Wavenum

ber

(cm

-1)

Potential (mV)

100 200 300 400 500 6002074

2076

2078

2080

2082

2084

2086

2088

2090

2092

Wavenum

ber

(cm

-1)

Potential (mV)

100 200 300 400 500 6002074

2076

2078

2080

2082

2084

2086

2088

2090

2092W

avenum

ber

(cm

-1)

Potential (mV)

400 mV 300 mV

100 mV 200 mV

30

expected to increase as Eads is decreased. At Eads of 200 mV and lower, the Eonset is leveled to 320

mV

Figure 2.3. CO electro-oxidation onset potentials versus adsorption potentials

31

Stark tuning plots of CO/Pt obtained at 30° C, 50° C and 70° C (Fig. 2.4) are similar to those of

figure 2.2.

Figure 2.4. Operando CO/Pt Stark tuning. (12)

100 200 300 400 500 600

2072

2074

2076

2078

2080

2082

2084

2086

2088

Waven

um

ber

(cm

-1)

Potential (mV)

0 100 200 300 400 500 600

2072

2074

2076

2078

2080

2082

2084

2086

2088

Waven

um

ber

(cm

-1)

Potential (mV)

0 100 200 300 400 500 600

2072

2074

2076

2078

2080

2082

2084

2086

2088

Waven

um

ber

(cm

-1)

Potential (mV)

30° C

50° C

70° C

32

Figure 15 shows that Eonset decreases linearly (slope = -2.88 cm-1

/K) with temperature. The

relationship of this linear variation to the kinetics of the inner sphere processes(13) will be

addressed in future work.

Figure 2.5. The potential at which CO begins to oxidize as a function of temperature.

Figures 2.2 and 2.4 show an adsorption process despite the lack of mobile ions typical of

aqueous sulfuric acid. The operando spectroscopy suggests a need for elucidation of Nafion

functional groups responsible for the complex Stark tuning plots.

2.4 Conclusion

Stark tuning of CO adsorbed at 100, 200, 300 and 400 mV vs. the fuel cell counter-

reference electrode (i.e., NHE) is interpreted in terms of the difference between the oxidation

30 40 50 60 70

280

300

320

340

360

380

400

420

Po

ten

tia

l (m

V)

Temperature (°C)

Slope: -2.88

R2: 0.9911

33

onset potential and the potential at which the CO was adsorbed. At 400 mV, the oxidation onset

potential (Eonset) is coincident with the adsorption potential (Eads) because CO preferentially

adsorbs only on sites with high Hads and there is no driving force for migration. As Eads, Eonset

decreases relative to Eads as CO migrates to sites of higher enthalpies of adsorption. At Eads at or

below 200 mV the Eonset is leveled to 325 mV.(14)

References:

1. Blyholder, G., Molecular orbital view of chemisorbed carbon monoxide. Journal of Physical Chemistry, 1964. 68(10): p. 2772-8.

2. Beden, B., et al., Infrared Study of Adsorbed Species on Electrodes - Adsorption of Carbon-Monoxide on Pt, Rh, and Au. Journal of Electroanalytical Chemistry, 1982. 142(1-2): p. 345-356.

3. Russell, J.W., et al., Infrared-Spectrum of CO on a Platinum-Electrode in Acidic Solution. Journal of Physical Chemistry, 1982. 86(16): p. 3066-3068.

4. Golden, W.G., Dunn, D.S., and Overend, J., A Method for Measuring Infrared Reflection-Absorption Spectra of Molecules Adsorbed on Low-Area Surfaces at Monolayer and Submonolayer Concentrations. Journal of Catalysis, 1981. 71(2): p. 395-404.


6. Villegas, I. and Weaver, M.J., Carbon-Monoxide Adlayer Structures on Platinum(111) Electrodes - A Synergy Between in-situ Scanning-Tunneling-Microsopy and Infrared-Spectroscopy. Journal of Chemical Physics, 1994. 101(2): p. 1648-1660.

7. Yoshimi, K., Song, M.B., and Ito, M., Carbon monoxide oxidation on a Pt(111) electrode studied by in-situ IRAS and STM: Coadsorption of CO with water on Pt(111). Surface Science, 1996. 368: p. 389-395.

8. Gurau, B. and Smotkin, E., Methanol crossover in direct methanol fuel cells: a link between power and energy density. J. Power Sources, 2002. 112: p. 339-352.

9. Stoupin, S., et al., Pt and Ru X-ray Absorption Spectroscopy of PtRu Anode Catalysts in Operating Direct Methanol Fuel Cells. Journal of Physical Chemistry B, 2006. 110(20): p. 9932-9938.

10. Kim, C.S. and Korzeniewski, C., Vibrational Coupling as a Probe of Adsorption at Different Structural Sites on a Stepped Single-Crystal Electrode. Analytical Chemistry, 1997. 69(13): p. 2349-2353.

11. Yoshinobu, J., et al., Lateral displacement by transient mobility in chemisorption of CO on Pt(997). Phys Rev Lett, 2003. 90(24): p. 248301.


34

13. Bard, A.J., Inner-Sphere Heterogeneous Electrode Reactions. Electrocatalysis and Photocatalysis: The Challenge. Journal of the American Chemical Society, 2010. 132(22): p. 7559-7567.

14. Iwasita, T. and Pastor, E., A DEMS and FTIR Spectroscopic Investigation of Adosbed Ethanol on Polycrystaline Platinum. Electrochimica Acta, 1994. 39(4): p. 531-537.

35

Chapter 3: Elucidating the ionomer-metal interface

3.1 Introduction

A solubilized version of Nafion(1) (ionomer solution) is often used to prepare catalyst “inks”

that are directly painted or decal transferred to the membrane.(2) The ionomer-metal interface

formed after evaporation of the ink solvent is central to PEMFC electrocatalysis. For example, a

spin coated Nafion layer on polycrystalline Pt enhances electrocatalysis.(3, 4) Little is known

about ionomer-metal interfaces. Markovic and co-workers probed Pt-electrolyte interfaces by

measurements of CO oxidation currents, in sulfuric, perchloric and KOH solutions, synchronized

with IR absorption-reflection spectroscopy of linear (νCOl) and bridge bound (νCO

b) COads on

Pt(111).(5) More recently their electrochemical studies on Pt(hkl)-Nafion interfaces suggest that

the Nafion sulfonate group adsorbs onto the Pt surface.(6) A fuel cell membrane electrode

assembly uniquely enables study of the ionomer metal interface without interferences due to

mobile anions characteristic of aqueous acidic electrolytes. Operando fuel cell infrared (IR)

spectroscopy was introduced by Fan et al.(7) In this report the aggregate of operando

spectroscopy of fuel cell membrane electrode surfaces, attenuated total reflectance spectroscopy

of Nafion 117, polarization modulated IR reflection absorption spectroscopy of Nafion spin-

coated onto Pt, and density functional theory calculated Nafion spectra suggest a model for the

Pt-Nafion interface the includes the Nafion CF3 group as an important co-adsorbate at the

ionomer-Pt interface.

3.2 Experimental:

Attenuated total reflectance (ATR) spectroscopy: A surface pressure of 815 psi was

maintained over the 1.8 mm diameter ATR crystal. Spectra were obtained using a Bruker™

36

Vertex 70 and Vertex 80V vacuum FTIR spectrometer (Bruker, Billerica, MA). A MIRacle™

ATR accessory (Pike Technologies Spectroscopic Creativity, Madison, WI) with a ZnSe ATR

crystal was used. The spectra were signal averaged from 100 scans at 4 cm-1

resolution with a

dry-air purge at ambient temperature. Atmospheric compensation (to eliminate H2O and CO2

interference in the beam path) was used in all measurements. Data processing for all infrared

data was done with the Bruker™ OPUS 6.5™ software.

Preparation of arc-melt Pt: The preparative method for arc melted electrodes has been

described.(8) Briefly, the arc-melter (Materials Research Furnaces, Sun Cook, NH) was charged

with 3 mm Pt shot (99.9+%, Sigma-Aldrich, St. Louis, MO) The chamber was evacuated to -29

psig and purged with argon three times. The Pt was arc-melted at 75 amps under an Ar bleed.

The chamber was vented to flip and arc-melt the sample three times. The Pt slug was epoxied

(Devcon HP250, Danvers, MA) to a modified glass syringe barrel, cut flat using a diamond cut-

off saw (Buehler IsoMet 1000, Lake Bluff, IL), and finally polished to a mirror finish using 0.05

µm aluminum oxide (Magner Scientific, Dexter, MI). The electrode was sonicated in

Nanopure™ water (Milli-Q, Billerica, MA) for 10 min. Nafion ionomer solution (20µL) was

pipetted onto the Pt electrode assembly mounted on an inverted electrode rotator (Pine

Instrument Company, Grove City, Pa) and then rotated (1000 rpm, one min).

Polarization modulated infrared reflection absorption spectra (PM-IRRAS): The Vertex

80V spectrometer was equipped with a Hinds II/ZS50 photoelastic modulator (Hinds

Instruments, Hillsboro, OR), SR830 lock-in amplifier (Stanford Research Systems, Sunnyvale,

CA) and a D3131\6 MCT detector (Infrared Associates, Stuart, FL). The angle of incidence was

60° and the photoelastic modulator frequency was 50.14 kHz. The PM-IRRAS cell design have

37

been reported.(9, 10) Spectra were averaged (710 scans; 4 cm-1

resolution). Li-exchanged

Nafion was prepared by soaking Nafion samples in 0.1M salt solutions.

Computational method: Unrestricted DFT(11, 12) with the X3LYP(13) functional was used

for geometry optimization and calculations of the normal mode frequencies and corresponding

IR spectra of the deprotonated and protonated Nafion side-chain and backbone segment. The

backbone-segment terminal ends were substituted with CH3 groups to eliminate computational

interference with the Nafion CF3 group. Jaguar 6.5 (Schrodinger Inc., Portland, OR) was used

with the all-electron 6-311G**++ Pople triple- basis set (“**” and “++” denote polarization

(14) and diffuse (15) basis set functions, respectively). Output files were converted to vibrational

mode animations using Maestro (Schrodinger Inc). Calculations were carried out on a 55 node

(dual core Xeon processors with 4GB RAM) High Performance Computing Cluster at the

University of Texas, Pan American.

Operando spectroscopy: Temperature dependant COads Stark tuning data were acquired by

operando specular reflectance IR spectroscopy using a cell (Fig. 1.8) based on the design of Fan

et al.(7) The cell, controlled by an EZstat potentiostat (NuVant Systems Inc), interfaces to a

diffuse reflectance accessory (Pike Technologies, Madison, WI) installed on the Vertex 70

Spectrometer. The IR beam accesses the working electrode surface through a CaF2 window

inserted into the upper flow field and a small slot in the carbon gas diffusion layer. The lower

flow field-electrode serves as both a hydrogen reference and counter electrode when charged

with hydrogen. The small CO oxidation currents do not measurably polarize the hydrogen

counter electrode. The working electrode was cycled (50 times) from 0 to1.2 V vs. the hydrogen

counter electrode. Spectra were obtained by averaging 250 scans at 4 cm-1

resolution. The cell

was brought to the desired temperature and potential (300 mV) for acquisition of reference

38

spectra. Carbon monoxide was passed over the working electrode for 15 min. The cell was

purged with N2 (15 min) prior to setting the potential to 100 mV. Replicates of four spectra were

acquired at 50 mV increments until the CO vibrational bands were no longer observable.

3.3 Results and Discussion:

Operando Spectroscopy: Stark tuning plots of COads on Pt in a fuel cell operated at 30° C,

50° C and 70° C (Fig. 2.4) show a remarkable similarity to plots obtained by Stamenkovic et al.

in sulfuric acid.(5) They correlated complex potential dependences (Stark tuning), of COads

vibrational frequencies (i.e., dνCOl/dE and dνCO

b/dE) in 0.5 M H2SO4, to the

compression/dissipation of COads islands: After a linear region from 0.1 to 0.3 V (dνCOl/dE = 31

cm-1

/V), a subtle νCOl blue shift from the extrapolated linear region was followed by a precipitous

drop, initiating at 0.5 volts, that finally upturns at 0.65V. They attribute this behavior to COads

island compression due to repulsive dipole interactions with co-adsorbed bisulfate ion initially

observable at 0.35V. COads oxidation initiating at 0.5 volts, induced by OH- absorption,

diminishes dipole-dipole coupling(16) and thus decreases dνCOl/dE. The upturn is attributed to

increased absorption of HSO3- relative to OH

-, which reestablishes repulsive dipole interactions

that compress COads islands and increase νCOl. The plots in Figure 2.4 demonstrate an adsorption

phenomenon onto Pt despite the lack of mobile ions typical of dilute sulfuric acid solutions. The

fuel cell operando spectroscopy suggests a need for elucidation of Nafion functional groups

responsible for modulation of CO/Pt interactions (i.e., Stark tuning).

Analysis of IR spectra: The DFT calculated spectrum of a 55-atom Nafion side-chain and

backbone segment provides 159 normal mode frequencies and intensities. Figure 3.1 shows the

theoretically derived peak positions and intensities (black lines) superimposed upon the ATR

spectrum (red line) of hydrated Nafion. ATR is a spectroscopic technique that requires that the

39

sample come into with what’s known as an internal reflection element (IRE). When light with

an angle of incidence θA passing through a transparent material passes through another material

with a different index of refraction, the resulting light will be refracted at angle θB. If θA exceeds

a critical angle θc, the light will be reflected internally. The value of θc is defined as:

equation 1

Where nA is the index of refraction of the first material and nB is the index of refraction of the

second. In the case of ATR, material A is the IRE and material B is the sample. As shown in

equation 1, the IRE must have an index of refraction greater than the sample for light to be

reflected internally at the interface. Even though the light is reflected internally within the IRE,

energy is lost via an evanescent wave emanating from the IRE perpendicular to the surface,

penetrating the sample. The evanescent wave’s depth of penetration is calculated by:

√

equation 2

Where λ is the wavelength. Given that the equation 2 is linear with only variable during an

acquiring a spectrum is λ, the loss of peak intensity at higher wavenumbers can easily be

compensated for.

40

Figure 3.1. DFT calculated normal modes (black lines) and Nafion ATR spectrum (red).

PM-IRRAS enhances (relative to the ATR) vibrational modes of functional groups ordered by

the Pt surface. PM-IRRAS is technique that enhances peaks associated with molecules ordered

by a surface. A polarizer generates light perpendicular the plane of incidence (p-polarized). An

angle of incidence higher than ~50° is required. At angles below 50°, the electric vector of p-

polarized light undergoes a phase change of 180° resulting in destructive interference between

the incident and reflected rays. The optimum angle of incidence of IRRAS techniques is usually

around 70°. A dipolar molecule at the metal surface results in an image dipole inside the

metal.(17) When the molecule is oriented parallel to the metal, the negative charge induces a

positive charge in the image dipole. Thus, upon absorption, there is no net change in dipole

moment. When the molecule’s dipole moment is perpendicular to the surface, is in the case of

41

adsorbed molecules, the image dipole is such that the net change in dipole moments creates an

additive effect (18)

Figure 3.2 shows the ATR spectrum (red), the PM-IRRAS spectra of Nafion-H/Pt interface

(grey line), Nafion-Li/Pt interface of Li+ exchanged Nafion (blue line) and 6 selected (from the

159 calculated) DFT calculated frequencies and intensities.

Figure 3.2. Theoretical and experimental spectra. ATR of hydrated Nafion (red); PM-IRRAS of

Nafion-H on Pt (grey); PM-IRRAS of Nafion-Li on Pt (blue); Selected DFT peaks (black lines 1-

6).

Scheme 3.1 is the Nafion structure with functional groups labeled for ease of discussion. The

low-frequency ATR band (Fig. 3.2) at 971 cm-1

(corresponding to theoretical 984 cm-1

; line-1)

and the 1056 cm-1

band (corresponding to theoretical 1059 cm-1

; line-3) have recently been

thoroughly assigned by Webber et al.(19) They obtained high resolution transmission spectra of

hydrated and thoroughly dehydrated Nafion and analyzed them in the context of the

spectroscopy of the short chain ionomer (formerly DOW membrane), the Nafion sulfonyl

fluoride(20) and the Nafion sulfonyl imide.(21) Animations of the DFT calculated internal

coordinates reveal that the observed 1056 cm-1

and 971 cm-1

peaks both have internal coordinates

42

resulting from the mechanical coupling of the adjacent the sulfonate and COC (A) ether link.(19)

Thus these peaks shift concertedly with changes in the sulfonate environment. Consider the

ATR and PM-IRRAS spectra of the protonic form of Nafion (Fig. 3.2, grey line). The 1056 cm-1

and 971 cm-1

peaks concertedly shift to higher frequencies in the PM-IRRAS because of the

interaction of the sulfonate functional group with the Pt surface. A similar effect is observed

with Li+ exchange of the adsorbed Nafion (blue line).

Scheme 3.1. Segment and atom labeling for Nafion

A convention for correlating PM-IRRAS enhanced peaks to the calculated DFT peaks would

enable identification of functional groups ordered by the Pt surface: The association of observed

PM-IRRAS peaks with DFT peaks, assigned by visualization of mechanically coupled internal

coordinates,(19, 22) provides the basis for such a convention. Normal mode coordinate

animations (generated by Maestro from DFT output files) explicitly show how neighbor

functional groups (called out in scheme 1) are mechanically coupled. The calculated internal

coordinates are viewed in the context of calculated normal modes of relevant small molecules

43

(e.g., triflic acid, CF3OCF3, 10-carbon CF2 backbone, etc.) hereafter are referred to as “pure

modes,” which serve as the basis-elements for assigning DFT calculated normal modes

associated with observed peaks. Figure 3.3 shows the assignments of the 6 selected DFT peaks

and snapshots of the corresponding Maestro animations. The atoms contributing to the

dominating motion (black circles) and the next most significant atom motions (dotted circles)

comprise pure modes that form the basis for the assignments. An alternate strategy for

determining the dominant mode is to consider the contribution to the potential energy surface on

an atom by atom basis.(23) While this may change the selection of the dominant mode, it does

not alter what pure modes contribute to the assignments. The correlation of the DFT to PM-

IRRAS peaks (Fig. 3.2) and the resulting assignments in terms of the mechanically coupled

modes are tabulated in Table 3.1.

44

Figure 3.3. Normal mode coordinate animation snapshots of the Nafion side-chain anion and

backbone fragment (see scheme 1). Left and right views are extrema positions of the vibrational

mode. Functional groups associated with the dominant internal coordinates and next most

significant motions are designated by solid and dotted boundary lines respectively.

2. 992 cm-1 CF2

(BBdef) + COC(B) δs

6. 1322 cm-1 CF2 δs (BBdef)

1. 984 cm-1 SO3

- s + COC (A) as + COC (B) ρr

3. 1059 cm-1 COC (A) as + SO3

- s

4. 1168 cm

-1 CF2 δs (BBdef) + CF2 ρr (SCdef) + COC(A)

5. 1254 cm-1 CF3

as + COC (A) δs + COC (B) as

45

Symmetric stretch, s; Asymmetric stretch, as

Wagging, ; Scissoring, δs; Twisting, ; Rocking, ρr

Backbone deformation, BBdef; Side-chain deformation, SCdef; Backbone Stretching, BBstre

Table 3.1. PM-IRRAS and DFT IR adsorption peaks and assignments.

The pure mode peak assignments (Table 1) elucidate functional groups ordered by the Pt

surface. The rational for the key functional group assignments (Table 1) is supported by the

overlap of the DFT calculated peak positions with the PM-IRRAS peaks. Consider the DFT and

PM-IRRAS peaks in the contexts of the bulk-Nafion ATR and the report by Cable et al.(20) that

the 1056 cm-1

and 971 cm-1

peaks shift with alterations of the sulfonate group environment. The

bulk ATR peak at 1056 cm-1

(red), the PM-IRRAS peak of protonated Nafion adsorbed on Pt

(grey line) at 1061 cm-1

and the PM-IRRAS peak of lithiated Nafion adsorbed on Pt (blue line) at

1077 cm-1

(Fig. 3.2) confirm that Pt surface atoms induce frequency shifts, as do extent-of-

hydration(19) and ion exchange of Nafion.(20) Thus the PM-IRRAS enhances bulk-Nafion-

modes that are shifted due to functional group interactions with Pt. Less explicit than the 1056

cm-1

peak, are PM-IRRAS peaks derived from bulk-Nafion-modes that are convoluted within the

Nafion ATR broad envelop region (1100 - 1300 cm-1

), in particular the 1164 and 1260 cm-1

PM-

IRRAS peaks. Di Noto et al.(24) extensively deconvoluted the broad envelop region. Their

Wavenumber (cm-1) Pure Mode Components

PM-IRRAS DFT

1 971 984 SO3- s + COC(A) as + COC(B) ρr

2 984 992 CF2 (BBdef) + COC(B) δs

3 1061 1059 COC(A) as + SO3- s

4 1164 1168 CF2 δs (BBstre) + CF2 (BBdef) ρr + COC(A)

5 1260 1254 CF3 as + COC(A) δs + COC(B) δs

6 1322 1322 CF2 δs (BBdef)

46

resulting peak library includes 1148, 1245 cm-1

, which could be reconciled with an association of

the DFT peaks (Fig. 3.2, line-4 and line-5 ) with the shifted PM-IRRAS peaks at 1164 and 1260

cm-1

.

Nafion/Pt adsorption model: The animation of the theoretical peak at 1254 cm-1

(Fig. 3.2,

line-5), associated with PM-IRRAS peaks at 1260 cm-1

(blue and grey lines), suggests that the

CF3 internal coordinates dominate the normal mode. The insensitivity of the 1201 cm-1

peak, to

ion exchange, suggests that the internal coordinates are not substantially coupled to the sulfonate

group. The 1260 cm-1

band-intensity is over an order-of-magnitude greater than that the cluster

of peaks (i.e., associated with theoretical lines 1 and 2) that are mechanically coupled to the

sulfonate pure mode: The CF3 functional group is a co-adsorbate of comparable importance to

the sulfonate exchange group in formation of the Nafion/Pt interface. Further support for this

model is provided by Mulliken population(25) analysis. Atomic charges of the 55 Nafion

fragment atoms were calculated. Table 3.2 shows the average charges of the backbone, side

chain and CF3 group fluorine atoms and the average charges on the sulfonate oxygen atoms. The

charges for chemically equivalent atoms (e.g., CF3 fluorine and sulfonate oxygen atoms) differ

because the calculations are done for the lowest energy Newman projections where the atomic

environments are different for chemically equivalent atoms because of the absence of symmetry

in the full molecule. The chemically equivalent atoms have smaller charge standard deviations

as would be expected. The average charge of the CF3 fluorine atoms are the highest amongst the

three classes of fluorine atoms (Table 2) and are about 18% that of the sulfonate oxygens.

47

Segment Backbone (F) Side-chain (F) CF3 (F) Sulfonate (O)

13 atoms 8 atoms 3 atoms 3 atoms

Avg. Partial Charge -0.0665 -0.0816 -0.0876 -0.4879

Standard Deviation 0.027 0.055 0.013 0.012

Table 3.2. Average partial charges of selected Nafion segments

A Gaussian 03 Viewer (Gaussian, Wallingford, CT) used to construct a 2-equivalent (1100

g/equiv) model of Nafion 117, enables rotation of dihedral angles while maintaining the native

bond angles associated with each and every functional group. The CF3 and SO3- groups, oriented

with the two planes defined by the CF3 fluorine and sulfonate oxygen atoms parallel to a Pt

surface, effect ordering of the CF2 backbone segments with respect to the Pt surface. Ordering of

the CF2 groups would be expected to yield PM-IRRAS peaks. The PM-IRRAS peak at 1164 cm-

1 is associated with the theoretical peak (line-4) at 1168 cm

-1. The line-4 animation shows that

CF2 backbone internal coordinates dominate the 1168 cm-1

mode, supporting the suggestion of

ordered CF2 groups. Figure 3.4 is the Gaussian View model resulting from orienting the CF3 and

SO3- groups for adsorption to the Pt surface. The numbers (yellow) associate DFT calculated IR

peaks (line-1 – 6, Fig. 3.2) and associated PM-IRRAS peaks with regions of order induced by the

CF3 and SO3- functional group adsorbates.

48

Figure 3.4. Gaussian 03 Viewer Nafion-Pt interface model. Oxygen (red), Sulfur (yellow),

Fluorine (light blue), Carbon (grey), Pt (dark blue).

The ordering of the backbone CF2 groups in the Gaussain model is a natural consequence

of adjusting the dihedral angles of the anchoring groups for adsorption, while maintaining

functional group native bond angles. Thus the aggregate of the Stark tuning data of Figure 2.3,

the PM-IRRAS and DFT calculations support Figure 3.4 as a model for Nafion functional group

adsorption to Pt. The details of exactly how adsorbed CF3 functional groups influence the

operando Stark tuning curves is not yet established. The low density of functional group

adsorption sites, relative to the number of backbone CF2 groups suggests an explanation as to

why Nafion is observed to enhance electrode processes.(3, 26) The methodology of assigning IR

bands in the context of mechanically coupled internal coordinates of neighboring functional

groups, and correlating those assignments to functional groups interactions with metal surfaces,

has broad applications towards characterization of ionomeric interfaces.

2.4 Conclusion:

Operando IR spectroscopy, PM-IRRAS of Nafion-Pt interfaces, and ATR spectroscopy of

Nafion, correlated with DFT calculated normal mode frequencies confirm that Nafion side-chain

sulfonate and CF3 co-adsorbates are structural components of the Nafion-Pt interface. These

5 1

2,4,6

3

49

“anchoring” functional groups reduce degrees of freedom available for backbone and side-chain

CF2 dynamics. The partial ordering of Nafion CF2 groups is supported by observed PM-IRRAS

and DFT calculated peaks possessing vibrational internal coordinates dominated by, and

mechanically coupled to side-chain CF2 group motions.

References

1. Martin, C.R., Rhoades, T.A., and Ferguson, J.A., Dissolution of perfluorinated ion-containing polymers. Analytical Chemistry, 1982. 54(9): p. 1639-1641.

2. Wilson, M.S. and Gottesfeld, S., Thin-Film Catalyst Layers For Polymer Electrolyte Fuel-Cell Electrodes. Journal of Applied Electrochemistry, 1992. 22(1): p. 1-7.

3. Liu, L., et al., Methanol oxidation on nafion spin-coated polycrystalline platinum and platinum alloys. Electrochemical and Solid State Letters, 1998. 1(3): p. 123-125.

4. Ploense, L., et al., Spectroscopic study of NEMCA promoted alkene isomerizations at PEM fuel cell Pd-Nafion cathodes. Solid State Ionics, 2000. 136-137: p. 713-720.


6. Subbaraman, R., et al., Three Phase Interfaces at Electrified Metal-Solid Electrolyte Systems 1. Study of the Pt(hkl)-Nafion Interface. Journal of Physical Chemistry C, 2010. 114(18): p. 8414-8422.

7. Fan, Q., et al., In situ FTIR-diffuse reflection spectroscopy of the anode surface in a direct methanol/oxygen fuel cell. Journal of the Electrochemical Society, 1996. 143(2): p. L21-L23.

8. Ley, K.L., et al., Methanol oxidation on single-phase Pt-Ru-Os ternary alloys. Journal of the Electrochemical Society, 1997. 144(5): p. 1543-1548.

9. Kunimatsu, K., et al., Carbon-Monoxide Adsorption on a Plationum-Electrode Studied by Polarization Modulated FT-IRRAS.1. CO Adsorbed in the Double-Layer Potential Region and Its Oxidation in Acids. Langmuir, 1985. 1(2): p. 245-250.

10. Kunimatsu, K., Infrared Spectroscopic Study of Menthanol and Formic-Acid Adsorbates on a Platinum-Electrode .1. Comparison of the Infrared-Adsorption Intensities of Linear CO(A) Derived From CO, CH3OH AND HCOOH. Journal of Electroanalytical Chemistry, 1986. 213(1): p. 149-157.

11. Hohenberg, P. and Kohn, W., Inhomogeneous Electron Gas. Physical Review B, 1964. 136(3B): p. B864-&.

12. Kohn, W. and Sham, L.J., Self-Concsistent Equations Including Exchange And Correlation Effects. Physical Review, 1965. 140(4A): p. 1133-&.

13. Xu, X., et al., An extended hybrid density functional (X3LYP) with improved descriptions of nonbond interactions and thermodynamic properties of molecular systems. Journal of Chemical Physics, 2005. 122(1): p. 14.

50

14. Frisch, M.J., Pople, J.A., and Binkley, J.S., Self-Consistent Molecular-Orbital Methods 25. Supplementary Functions For Gaussian-Basis Sets. Journal of Chemical Physics, 1984. 80(7): p. 3265-3269.

15. Clark, T., et al., Efficient Diffuse Function-Augmented Basis-Sets For Anion Calculations 3. The 3-21+G Basis Set For 1ST-Row Elements, LI-F. Journal of Computational Chemistry, 1983. 4(3): p. 294-301.

16. Persson, B.N.J. and Ryberg, R., Vibrational Interaction Between Molecules Adsorbed On A Metal-Surface - The Dipole-Dipole Interaction. Physical Review B, 1981. 24(12): p. 6954-6970.

17. King, F.W., Duyne, R.P.V., and Schatz, G.C., Theory of Raman scattering by molecules adsorbed on electrode surfaces. The Journal of Chemical Physics, 1978. 69(10): p. 4472-4481.

18. Griffiths, P.R. and de Haseth, J.A., Specular Reflection, in Fourier Transform Infrared Spectrometry. 2006, John Wiley & Sons, Inc. p. 277-301.

19. Webber, M., et al., Mechanically Coupled Internal Coordinates of Ionomer Vibrational Modes. Macromolecules, 2010. 43(13): p. 5500-5502.

20. Cable, K.M., Mauritz, K.A., and Moore, R.B., Effects of hydrophilic and hydrophobic counterions on the Coulombic interactions in perfluorosulfonate ionomers. Journal of Polymer Science, Part B: Polymer Physics, 1995. 33(7): p. 1065-72.

21. Byun, C.K., et al., Infrared Spectroscopy of Bis (perfluoroalkyl)sulfonyl Imide Ionomer Membrane Materials. Journal of Physical Chemistry B, 2009. 113(18): p. 6299-6304.

22. Warren, D.S. and McQuillan, A.J., Infrared spectroscopic and DFT vibrational mode study of perfluoro(2-ethoxyethane) sulfonic acid (PES), a model Nafion side-chain molecule. Journal of Physical Chemistry B, 2008. 112(34): p. 10535-10543.

23. Johansson, P. 2010: Gothenburg, Sweden. 24. Di Noto, V., et al., Structure, properties and proton conductivity of Nafion/ (TiO2)center

dot(WO3)(0.148) (psi TiO2) nanocomposite membranes. Electrochimica Acta, 2010. 55(4): p. 1431-1444.

25. Mulliken, R.S., Electronic population analysis on LCAO-MO [linear combination of atomic orbital-molecular orbital] molecular wave functions. I. Journal of Chemical Physics, 1955. 23: p. 1833-40.

26. Ploense, L., et al., Proton spillover promoted isomerization of n-butylenes on Pd-black cathodes/Nafion 117. Journal of the American Chemical Society, 1997. 119(47): p. 11550-11551.

51

Chapter 4: Minimal Vibrational Mode Analysis of Nafion Infrared

Spectroscopy

4.1 Introduction:

Ionomer membranes have a rapidly growing range of applications, including separators,

reaction media, and electrolytes that motivate the investigation of structural variations to

accommodate new functionalities.(1-10) Ionomers typically feature a backbone structure with

side-chains terminated by an ion exchange group. Infrared spectroscopy (IR) is an important tool

for correlating the response of the ionomer exchange group and backbone to ion exchange and

absorption of solutions of molecular species. The infrared spectra of polymers, in general are

deceptively simple. Bower and Maddams explain this by considering a polymer as a series of

chemical repeat units,(11) noting that the wavelength of the IR radiation absorbed by the

polymer is orders of magnitude larger than the repeat unit dimensions. Thus, each wavelength

will reflect the interaction between the IR radiation and an ensemble of repeat units with the

number of normal modes reduced to about 3n where n is the number of atoms in the repeat unit

(vs. 3N-6 where N is the number of atoms in the molecule): The chemical repeat unit of Nafion

(Scheme 1) would give rise to well over 100 normal modes (with intensity) within the fingerprint

region (900-1400 cm-1

). Yet there are only six bands and associated shoulders within the finger

print region. It is difficult to assign individual bonds in this region than at higher frequencies due

to the different bending vibrations within the molecule. In the past, researchers have used the

distinct –OH stretching bands at higher frequencies to study hydration.(12, 13) The discussion

of the fingerprint region is generally limited to the well defined peaks at 1060, 969 and 980 cm-1

and their relationship with the SO3- and ether groups. Analysis of the broad envelope region

52

from 1070-1350 cm-1

is incomplete or simply assigned as backbone vibrations. Despite over

7000 publications on Nafion since 1975,(14) no consensus has been reached regarding the

assignment of the fingerprint region.

Using density functional theory (DFT), the theoretical spectrum of a 55-atom Nafion proton

dissociated repeat unit (scheme 1, right) was generated.(15) The 159 normal mode spectrum was

accompanied by an eigenvector animation for each normal mode. These animations support the

notion introduced by Cable et al.(16) and Warren and McQuillan(17) that normal modes should

be assigned as group modes rather than the prevalent single-functional-group assignments. .

The contributions of different functionalities and their vibrations (e.g. stretching, scissoring, etc)

were assigned through visualization of each normal animation. (18) This method is lacking

given that the process is time consuming, precludes the assignment of the relative contribution of

each functionality, and is ultimately subjective. The thorough assignment of polymer vibrational

spectra demands a quantitative method of assigning each theoretical normal mode. This work

describes a method which assigns the fingerprint region of the Nafion theoretical spectrum.

Visualization of normal mode animations are used only to identify functional groups

participating in a group mode. For each normal mode, the selected functional groups are then

represented as subsets of the generalized coordinates. These subsets provide a set of eigenvectors

now introduced as minimal vibrational modes (MVMs). The MVMs are coded as color bars,

where bar-lengths scale with the MVM contribution to the normal mode. The addition of a

categorical MVM axis clarifies the complex effects of the mechanical coupling in another

dimension as well. Along an MVM axis, the distribution of selected functional group MVM

amongst all normal modes is easily visualized (Fig. 3 top) vida infra.

53

Scheme 4.1: Nafion chemical repeat unit.

4.2 Experimental:

IR Spectroscopy: Attenuated total reflectance (ATR) spectra of Nafion 117 were obtained

using a Bruker™ Vertex 70 and Vertex 80V vacuum FTIR spectrometer (Bruker, Billerica, MA)

equipped with a MIRacle™ ATR accessory (Pike Technologies Spectroscopic Creativity,

Madison, WI) using a ZnSe ATR crystal with 45° beveled edges. A surface pressure of 815 psi

was maintained over the 1.8 mm diameter ATR crystal. Atmospheric compensation (to

eliminate H2O and CO2 interference in the beam path) was used in all measurements and the

spectra were corrected to take into account for the depth of penetration of the IR beam. Spectra

were signal averaged (100 scans at 4 cm-1

resolution) using a DLaTGS detector. Data processing

was done with the Bruker™ OPUS 6.5™ software.

Density Functional Theory: Unrestricted DFT(19, 20) with the X3LYP(21) functional was

used for geometry optimization and normal mode calculations of the Nafion side-chain and

backbone-segment. The Nafion backbone-segment is artificially terminated to yield the

computational chemical repeat unit used in this study. CH3 groups were used to terminate the

backbone segment rather than CF3 groups in order to avoid computational interference with the

Nafion side-chain CF3 group (i.e. the only type of perfluorinated methyl group in the molecule).

54

Jaguar 6.5 (Schrodinger Inc., Portland, OR) was used with the all-electron 6-311G**++ Pople

triple- basis set (“**” and “++” denote polarization (22) and diffuse (23) basis set functions,

respectively). Output files were converted to vibrational mode animations using Maestro

(Schrodinger Inc.). Calculations were carried out on a 55 node (dual core Xeon processors with

4GB RAM) High Performance Computing Cluster at the University of Texas, Pan American.

Automated Normal Mode Assignments: The normal mode displacements were calculated

using our normal mode output file.(15) A Mathematica (Wolfram Res., Champaign, IL) program

calculates normal mode displacements as:

and Eq. (1)

where xi are the atomic coordinates of the geometry optimized repeat-unit and N is the number

atoms of the repeat unit. The generalized coordinates (q) are defined in terms of and xi. The

MVMs are described in terms of generalized coordinate subsets (đ) obtained from q. The MVMs

(e.g., bending, symmetric, asymmetric, wagging, twisting, rocking, and scissoring), are

represented by a đ, are assigned a color. The đ2 are calculated and normalized to the largest đ

2 on

a per-color basis. The program color-codes DFT line spectra by re-normalizing the đ2 on a per

mode basis. The methodology is summarized (Fig. 1). To demonstrate the reproducibility of this

technique, the theoretical vibrational spectrum of benzyltrimethylammonium hydroxide was

assigned. Benzyltrimethylammonium hydroxide was selected because it has been suggested that

it is the ion exchange head group in commercially available anionic exchange membranes

(AEMs).

55

Figure 4.1. Flow chart for generation of Nafion repeat group MVM spectra.

Consider the normal modes dominated by the Nafion side-chain groups (e.g., SO3- , CF2,

CF, CF3 and COC(A,B)). In the case of C3v type functional groups (SO3– and CF3) the

symmetric stretch is the average of the S-O or C-F bonds as given by Eq. 2 and shown on Fig.

2 (a).

Table 1 shows the MVM internal coordinate system of the most prominent IR modes for

Nafion. Figure 2 shows the definition of the functional group internal coordinates used in this

work. An internal coordinate that involves a distance between two atoms is defined as,

| | | |, Eq. (2)

56

where is the vector defined by the atomic coordinates of the geometry optimized repeat-

unit, and is the vector defined by

of Eq. 1. Similarly, an internal coordinate that

involves an in-plane angle between three atoms is defined as,

| |-| |, Eq.

(3)

where is the angle formed by and , and is the angle formed by and

. Rocking γ is represented by the summation of the angles formed by

and and by

and . An out-of-plane angle is formed between

and , whereas the angles

δi’ measuring twisting and scissoring modes are formed by and for i=k, l.

57

Table 4.1. Internal coordinate system MVMs of the most prominent IR modes for Nafion.*

*In plane angles as follows: Out-of-plane angles as follows: is the angle formed by

and . For BB is the average between angles formed by

and and

by and . See Fig.2 for definitions of distances and bond angles.

Symmetry

type

Functional

Group

Minimal Vibrational Modes

C3V

SO3– and CF3

Symmetric stretch = average among d1, d2, and d3.

Asymmetric stretch = all sums of d1, d2, and d3, where one of the

factors is multiplied by -1.

Symmetric bend = average among α1, α2, and α3.

C2V

CF2 and COC Symmetric stretch = average between d1 and d2.

Asymmetric stretch = -d1+d2.

Wagging= average between δ1’ and δ2’.

Twisting= δ1’- δ2’.

Scissoring= δ.

Rocking= γ.

C1 Backbone BB Stretches = sum of e4,16, e16,28, e4,13, e13,19, e16, 22, e22,7, e13,25,

e19,10, and e7, 10, sum of e4,16, -e16,28, e4,13, -e13,19, e16, 22, -e22,7, -e13,25, -e19,10, and e7,

10,

sum of e4,16, -e16,28, -e4,13, e13,19, e16, 22, -e22,7, -e13,25, e19,10, and e7, 10,

sum of e4,16, e16,28, e4,13, e13,19, -e16, 22, -e22,7, -e13,25, -e19,10, and -e7,

10,

sum of e4,16, -e16,28, -e4,13, e13,19, -e16, 22, e22,7, e13,25, -e19,10, and e7, 10.

BB out-of-plane deform. = sum of 7,4,10 , 22,16,7

, 19,13,10 ,

52,28,22 ,and 48,25,19

,

sum of 7,4,10 , , 22,16,7

, 19,13,10 , 52,28,22

, and 48,25,19 ,

sum of 7,4,10 , 22,16,7

, 19,13,10 , 52,28,22

,and 48,25,19 ,

sum of 7,4,10 , 22,16,7

, 19,13,10 , 52,28,22

,and 48,25,19 ,

sum of 7,4,10 , , 22,16,7

, 19,13,10 , 52,28,22

,and 48,25,19 .

58

Figure 4.2. Definition of the internal coordinates for a) C3v, b) C2v, and c) the Nafion Backbone.

4.3 Results and discussion:

Figure 3 (top) shows the đ2 contributions of each normal mode distributed along a MVM

categorical axis. For example, the 982*, 984* and 1059

* cm

-11 normal modes all have SO3

-

MVMs. The utility of the MVM categorical axis can be seen with the MVMs corresponding

with the Nafion backbone. The perfluorinated backbone, when considered separately from the

Nafion side-chain, shares similar morphology with Teflon®. Figure 4 (left) shows MVMs

corresponding to backbone stretching and out-of-plane bending overlaid upon the ATR-IR

spectrum of Teflon®. Clusters of backbone MVMs are found at wavenumbers coincident with

experimentally obtained IR bands and have similar relative intensities. The ATR-IR spectra of

perfluoro(3-methyl-2,4-dioxahexane) sulfonic acid (PFMHSA) and perfluoro(2-ethoxybutane)

1 ATR wavenumbers are denoted with xATR, high-resolution transmission: xTR, DFT calculated: x*. Unsuperscripted values are generic values for

discussion.

59

sulfonic acid (PFEBSA) obtained by Danilczuk et al.(24) allows for a similar analysis of the Nafion

side-chain. Figure 5 shows the ATR-IR spectra of PFMHSA and PFEBSA, small molecule analog

of the Naifon side-chain, superimposed over the CF3 and SO3 peaks MVM peaks. Upon initially

viewing the PFMHSA spectrum, it would be tempting to correlate the 1228ATR

cm-1

to the SO3-

νas MVM2 found at 1223* cm-1. However, the diminished intensity of the corresponding PFEBSA

band at 1225ATR

cm-1

suggests that this band should be associated with a CF3 MVM rather than a

SO3-. This is supported by the proximity of these bands to 1201* cm

-1 which is the DFT peak

with the largest CF3 νas MVM contribution. Similarly, the 1196/1197ATR cm-1 bands should be

associated with the SO3- νas MVM of 1224* cm-1 because their intensity is not diminished through

derivitization. The transposition of these two MVM’s relative to their occurrence in the

experimentally obtained spectra is not surprising: Scaling factors inherent in DFT calculations can

shift calculated peaks (vida infra). It should be noted that comparison of these specific MVM’s to

experimentally obtained spectra is enabled by their abundance in a narrow region of the

theoretically obtained vibrational spectrum. The broad occurrence of MVM’s corresponding to

other functional groups (e.g. CF2(1),(2) and COC(A),(B), preclude similar analysis.

2 νs: symmetric stretching; νas asymmetric stretching; ω: wagging; δs: scissoring; τ: twisting; ρr: rocking.

60

Figure 4.3. Top: DFT calculated normal modes decomposed into MVM spectra (i.e., s(CF3)

light orange; CF3 as, dark orange; SO3- s, light brown; etc.) along the categorical axis. Middle:

Nafion ATR spectra (black). Composite DFT lines show contributions of minimal vibrational

modes to each normal mode. Bottom: Color code legend for minimum vibrational modes.

Str: unspecified stretching; Str, OOP: Stretching out-of-plane.

61

Figure 4.4: The ATR-IR spectrum of Teflon superimposed over Nafion backbone MVMs.

Figure 4.5: ATR spectrum of PFMHSA (left) and PFEBSA (right) superimposed over SO3- νas,

SO3- νs, CF3 νas, CF3 νs MVMs. ATR spectra are reproduced from reference (24).

The use of MVM spectra as a tool for the interpretation of Nafion spectra is exemplified

by analysis of the multiplet between 940-1000 cm-1

and the 1060 band. Aqueous solvation of

the COC(A) group has been invoked to explain the concerted shift of the 1060 cm-1

and 970 cm-

1 bands with changes in the sulfonate environment.(16) This is reconciled by the fact that all

models of hydrated Nafion have the sulfonate group immersed within the confines of an aqueous

phase.(16, 25-29) However, Hamrock suggests that solvation of COC(A) is unlikely because of

the electron withdrawing effects of the fluorine atoms.(30) This motivated our Natural Bond

Orbital (NBO) analysis(31) of dimethyl ether versus the perfluorinated analog. The charge

62

density of oxygen in the perfluorinated analog is 48 % that of oxygen in dimethyl ether.3 Thus

solvation of the COC(A) group in water, or complexation to cations is unlikely. Consideration

of the Nafion spectrum as a series of mechanically coupled group modes allows for proper

analysis of the shift in the 970 cm-1

and 1060 cm-1

bands. Specifically, any normal mode with

SO3- MVM’s are expected to concurrently vary with changes in the exchange group environment

or changes due to synthetic modification of the ionomer. The composite spectrum (Fig. 3 middle)

provides an overview of where these regions of the spectrum are. This is why the 970 cm-1

and

the 1060 cm-1

bands shift concertedly with changes in the sulfonate environment (e.g. they both

vanish with rigorous dehydration(15)). The 983ATR

cm-1

peak is another example where

composite spectra can be used to predict which peaks are sensitive to derivitization. Figure 3

(middle) shows that COC(A) νas and CF3 νs MVM’s contribute to both 970* cm-1

and 973* cm-1

.

It is therefore reasonable to assume that if these functionalities were not present, there would be

significant change in the experimentally obtained spectrum in this region. This is observed in the

spectrum of PFEBSA where the 985 cm-1

peak found in the PFMHSA spectrum missing. The

presence of the 962ATR

cm-1

peak in the PFEBSA also supports the correlation of the Nafion

970ATR

cm-1

to a SO3 νs MVM.

Previously, we assigned the 984* cm-1

mode to primarily SO3- s with a substantially

smaller contribution from COC(A) as through visualization of normal mode animations.(15)

The MVM spectra show that CF2 motions are greater contributor to 984* cm-1

than the COC(A)

as. Additionally, the s(CF3) contribution to the multiplet of bands from 940 – 1010 cm-1

was

elucidated by the MVM spectra. The animations showed the dominant 1059* cm-1

pure mode as

as(COC(A)) with a lesser contribution from SO3-. The quantitative MVMs show the SO3

- νs

3 NBO 5.0 is incorporated within the Jaguar code that is commercially available from Schrodinger.

63

đ2contributing 2% to the normal mode. The importance of this 2% is scaled by the fact that the

S-O bonds have the largest force constants. Table 2 shows the advantage of MVM assignments

over previous assignments based on visualization of eigenvector animations.

DFT tends to overestimate vibrational frequencies by a few percent: Scaling factors are

applied to calculated peak frequencies.(33) However the applications of scaling factors to

calculated spectra of chemical repeat units has caveats. Bower and Maddams(11) explain that

polymer chemical repeat units are not physical repeat units: Hydrated ion exchange membranes

have geometrical irregularities that include interfacial regions separating complex aqueous

confines from hydrophobic regions.(28, 34, 35) A chemical repeat unit can be approximated as a

separate molecule in its own local environment. This is applicable to the motion of side-groups,

with little motion of the backbone. Broad infrared absorption peaks are expected because groups

of the same atoms in separate repeat units have frequencies that vary as a consequence of the

different physical environments. Atoms in close proximity to the ion exchange site would exhibit

more substantial frequency shifts with changes in the environment.(36, 37) This can scramble the

order that normal modes appear in DFT line spectra in comparison to the order experimentally

observed. This severely limits the reliability of using DFT intensities and frequencies for

deconvolution and assignment of resulting bands.

Table 4.2 Pure mode assignments from visualization of normal mode animations in comparison to

MVM assignments of this work.

ν (cm-1

) (15) Legacy assignments Pure mode components MVM assignments 984* COC(A) νas SO3

- νs, COC(A) νas, COC(B) ρr (15, 18) SO3- νs, CF2(2) ω, CF2 (1) νs, BBStr,OOP, COC(A) νas,

1059* SO3- νs COC(A) νas, SO3

- νs (15, 18) COC(A) νas, CF2(2) νs, COC(B) νs, SO3- νs, CF3 νas,

1168* - BBStr, CF2 ρr, COC(A) ω(32) CF2(2) νas, COC(A) ω, COC(B) τ, BBStr,OOP, SO3- νas

1254* - CF3 νas, COC(A) δS, COC(B) δS25

CF3 νas, COC(B) ω, BBStr,OOP,COC(A) ω, C1-C2Str

1299* - C1-C2Str, BBStr(32) C1-C2Str, BBStr, COC(A) δS, C3-C4Str, COC(B) δS

1302* - BBStr, C1-C2Str(32) BBStr, C1-C2Str, COC(B) δS, COC(A) δS, C3-C4Str

1322* - BBStr,OOP(32) BBStr, COC(B) ω, CF3 νas, COC(A) νas, CF2(2) νas

1357* - BBStr,OOP, COC(B) ρr(32) BBStr, COC(B) δS, CF3 νs bend, COC(A) τ, C1-C2Str

64

Experimental data are essential for correlation of DFT calculated peaks to experimental

bands. Consider the 940 – 1010 cm-1

multiplet, and the1060 cm-1

band. If the criterion of

nearness of a theoretical peak to an experimental band is used, the 984* cm-1

mode would be

correlated with the green band centered at 983TR

cm-1

. However, the band at 983TR

cm-1

is

entirely insensitive to changes to the SO3- environment. The MVM spectra show that the 984*

cm-1

mode is predominantly an SO3- s mode. Thus we associate the 984* cm

-1 with the

experimental 969TR

cm-1

peak because of its high sensitivity to ion exchange and changes in the

state-of-hydration.(15) The correlation of the sulfonate MVM segments at 973*, 982*, and 984*

cm-1

(light brown) with the 969TR

cm-1

and/or the 971ATR

cm-1

bands of our previous studies(15,

18) are further supported by the extensive SO3

- MVMs within the multiplet of bands between 940

cm-1

and 1010 cm-1

.

Fortunately the ion exchange group, sensitive to the state of hydration and the nature of

the counter-ions, facilitates the correlation of theoretical lines to experimental bands.(15, 18) A

direct correlation of theoretical lines to experimental bands without additional data (e.g., ion

exchange, dehydration or synthetic derivatization) would require knowledge of the physical

repeat unit under the conditions in which the spectra are obtained. Molecular dynamics may be a

source of more realistic physical repeat units.(38, 39)

4.4 Conclusion:

Single-functional-group normal mode assignments have been a persistent source of confusion

to the ionomer electrolyte community. The ionomer side-chain functional groups are distributed

between the restricted confines of aqueous and hydrophobic regions. It has previously been

impossible to explain how variations in the exchange group environment cause concerted shifts

of peaks in disparate parts of the IR spectra within the framework of individual functional group

65

assignments. The mechanical coupling of the internal coordinates of side-chain functional

groups that can reside in different phases (e.g., aqueous or hydrophobic) explains the complex

behavior of IR peaks that accompanies ion exchange, variations in the state-of-hydration and

chemical derivatization. The representation of functional group contributions to normal modes,

as subsets of the generalized coordinates, provides a set of eigenvectors now introduced as

minimal vibrational modes. The addition of a categorical axis representing minimal vibrational

modes coded as colors adds a dimension (i.e., wavenumbers, intensities, colors) to IR

spectroscopy that clarifies the complex effects of the mechanical coupling of internal coordinates

of functional groups in ionomer electrolytes.

References:

1. Hickner, M.A., Ion-containing polymers: new energy & clean water. Materials Today,

2010. 13(5): p. 34-41. 2. Hamrock, S.J. and Yandrasits, M.A., Proton exchange membranes for fuel cell

applications. Polymer Reviews, 2006. 46(3): p. 219-244. 3. Li, S.K., Zhu, H., and Higuchi, W.I., Enhanced transscleral iontophoretic transport with

ion-exchange membrane. Pharmaceutical Research, 2006. 23(8): p. 1857-1867. 4. Allegrezza, A.E., Jr., et al., Chlorine resistant polysulfone reverse osmosis modules.

Desalination, 1987. 64: p. 285-304. 5. Mauritz, K.A. and Moore, R.B., State of understanding of Nafion. Chemical Reviews,

2004. 104(10): p. 4535-4585. 6. Elabd, Y.A., et al., Transport properties of sulfonated poly (styrene-b-isobutylene-b-

styrene) triblock copolymers at high ion-exchange capacities. Macromolecules, 2006. 39(1): p. 399-407.

7. Elabd, Y.A. and Hickner, M.A., Block Copolymers for Fuel Cells. Macromolecules, 2011. 44(1): p. 1-11.

8. Ploense, L., et al., Proton spillover promoted isomerization of n-butylenes on Pd-black cathodes/Nafion 117. Journal of the American Chemical Society, 1997. 119(47): p. 11550-11551.

9. Liu, Z.J., et al., Reductive dehalogenation of gas-phase chlorinated solvents using a modified fuel cell. Environmental Science & Technology, 2001. 35(21): p. 4320-4326.

10. Hernandez-Pagan, E.A., et al., Resistance and polarization losses in aqueous buffer-membrane electrolytes for water-splitting photoelectrochemical cells. Energy & Environmental Science, 2012. 5(6): p. 7582-7589.

66

11. Bower, D.I. and Maddams, W.F., The Vibrational Spectroscopy of Polymers. 1989, Cambridge: Cambridge University Press.

12. Korzeniewski, C., Snow, D.E., and Basnayake, R., Transmission infrared spectroscopy as a probe of Nafion film structure: Analysis of spectral regions fundamental to understanding hydration effects. Applied Spectroscopy, 2006. 60(6): p. 599-604.

13. Basnayake, R., Wever, W., and Korzeniewski, C., Hydration of freestanding nation membrane in proton and sodium ion exchanged forms probed by infrared spectroscopy. Electrochimica Acta, 2007. 53(3): p. 1259-1264.

14. ISI Web of Knowledge Home Page,, 2009. 15. Webber, M., et al., Mechanically Coupled Internal Coordinates of Ionomer Vibrational

Modes. Macromolecules, 2010. 43(13): p. 5500-5502. 16. Cable, K.M., Mauritz, K.A., and Moore, R.B., Effects of hydrophilic and hydrophobic

counterions on the Coulombic interactions in perfluorosulfonate ionomers. Journal of Polymer Science, Part B: Polymer Physics, 1995. 33(7): p. 1065-72.

17. Warren, D.S. and McQuillan, A.J., Infrared spectroscopic and DFT vibrational mode study of perfluoro(2-ethoxyethane) sulfonic acid (PES), a model Nafion side-chain molecule. Journal of Physical Chemistry B, 2008. 112(34): p. 10535-10543.


19. Hohenberg, P. and Kohn, W., Inhomogeneous Electron Gas. Physical Review B, 1964. 136(3B): p. B864-&.

20. Kohn, W. and Sham, L.J., Self-Concsistent Equations Including Exchange And Correlation Effects. Physical Review, 1965. 140(4A): p. 1133-&.

21. Xu, X., et al., An extended hybrid density functional (X3LYP) with improved descriptions of nonbond interactions and thermodynamic properties of molecular systems. Journal of Chemical Physics, 2005. 122(1): p. 14.



24. Danilczuk, M., et al., Understanding the fingerprint region in the infra-red spectra of perfluorinated ionomer membranes and corresponding model compounds: Experiments and theoretical calculations. Journal of Power Sources, 2011. 196(20): p. 8216-8224.

25. Lehmani, A., Durand-Vidal, S., and Turq, P., Surface morphology of Nafion 117 membrane by tapping mode atomic force microscope. Journal of Applied Polymer Science, 1998. 68(3): p. 503-508.

26. Moore, R.B., III and Martin, C.R., Morphology and chemical properties of the Dow perfluorosulfonate ionomers. Macromolecules, 1989. 22(9): p. 3594-9.

27. Gierke, T.D. and G. E. Munn, F.C.W., The morphology in Nafion perfluorinated membrane products, as determined by wide- and small-angle x-ray studies. Journal of Polymer Science: Polymer Physics Edition, 1981. 19(11): p. 1687-1704.

67

28. Hsu, W.Y. and Gierke, T.D., Ion transport and clustering in nafion perfluorinated membranes. Journal of Membrane Science, 1983. 13(3): p. 307-326.

29. Yeager, H.L. and Steck, A., Cation and Water Diffusion in Nafion Ion Exchange Membranes: Influence of Polymer Structure. Journal Of The Electrochemical Society, 1981. 128(9): p. 1880-1884.

30. Hamrock, S. 2010: St. Paul, MN. 31. Weinhold, F. and Landis, C., Natural bond order and extensions of localized bonding

concepts. Chem. Educ. Res. Pract. Eur, 2001. 2(2): p. 91-104. 32. Byun, C.K., et al., Thermal Processing as a Means to Prepare Durable, Submicron

Thickness Ionomer Films for Study by Transmission Infrared Spectroscopy. Analytical Chemistry, 2012. 84(19): p. 8127-8132.

33. Dimakis, N., et al., A band dispersion mechanism for Pt alloy compositional tuning of linear bound CO stretching frequencies. Journal of Physical Chemistry B, 2005. 109(5): p. 1839-1848.

34. Elliott, J.A. and Paddison, S.J., Modelling of morphology and proton transport in PFSA membranes. Physical Chemistry Chemical Physics, 2007. 9(21): p. 2602-2618.

35. Gebel, G. and Lambard, J., Small-angle scattering study of water-swollen perfluorinated ionomer membranes. Macromolecules, 1997. 30(25): p. 7914-7920.

36. Moukheiber, E., et al., Investigation of ionomer structure through its dependence on ion exchange capacity (IEC). J. Membr. Sci., 2012. 389: p. 294-304.

37. Kumari, D., Vibrational spectroscopy of ion exchange membranes. 2011, Northeastern University.

38. Brandell, D., et al., Molecular dynamics studies of the Nafion (R), Dow (R) and Aciplex (R) fuel-cell polymer membrane systems. Journal of Molecular Modeling, 2007. 13(10): p. 1039-1046.

39. Brandell, D., Karo, J., and Thomas, J.O., Modelling the Nafion (R) diffraction profile by molecular dynamics simulation. Journal of Power Sources, 2010. 195(18): p. 5962-5965.

68

Chapter 5: Operando Infrared Spectroscopy of Ethanol Oxidation

in Polymer Electrolyte Fuel Cells

5.1 Introduction:

Ethanol and methanol are an attractive alternative to hydrogen gas in proton exchange

membrane (PEM) fuel cells. When compared to hydrogen gas, the production, storage and

delivery of these fuels can be achieved with less modification to existing infrastructure. The

theoretical mass energy density for methanol and ethanol is 6.1 and 8.0 kWh/kg, respectively,

close to that of gasoline. (1) Ethanol is significantly less toxic to methanol and can be obtained

from biomass. The rate of ethanol crossover across a proton exchange membrane is lower than

that of methanol.(2) The crossover of ethanol or methanol reduces the number of sites for

oxygen reduction and creates a mixed potential at the cathode reducing cell voltage.(3) While

platinum is generally considered the best catalyst for the oxidation of ethanol and methanol,

cleaving the C-C bond presents a significant challenge with oxidizing ethanol. The cleavage of

the C-C bond leads to the complete oxidation of ethanol yielding CO2 and generating 12

electrons. Acetic acid is the result of an incomplete oxidation pathway yielding four electrons.

Several studies have investigated the oxidation pathway of ethanol on a Pt electrode using both

in-situ Fourier transform infrared spectroscopy (FTIR) (4-13) and differential electrochemical

mass spectroscopy (DEMS)(4, 9, 12, 14, 15). Each of these techniques has their drawbacks. It is

difficult to study the evolution of acetic acid in ethanol oxidation using DEMS due to its low

volatility. In-situ IR techniques using liquid electrolytes often have uneven current distribution

in the thin layer between the working electrode and the IR window resulting in observing

oxidation products associated with lower potentials despite the cell being set to a higher

69

potential.(16) Peaks in the IR spectrum resulting from the electrolytes used in these in-situ

methods (e.g. sulfuric and perchloric acids) often overlap portions of the IR spectrum useful in

identifying products of ethanol oxidation. It is better to study the products of ethanol oxidation

under conditions associated with an operating fuel cell. Operando IR spectroscopy allows for the

acquisition of IR spectra under actual operating conditions without interference from

supplemental electrolytes. In spectra acquired using operando spectroscopy adsorbed species

are observed as bipolar peaks,(17) often eliminating using polarization techniques in

differentiating between adsorbed and desorbed compounds. The oxidation of ethanol on Pt, Ru

and PtRu catalysts are studied using operando spectroscopy. All of the principle products of

ethanol oxidation are detected using a single technique under real world operation conditions.

This demonstrates operando spectroscopy’s viability in the study of new ethanol oxidation

catalysts.

5.2 Experimental

Membrane electrode assembly preparation: Nafion 117 (Dupont, was immersed in

boiling 8 M nitric acid for 20 minutes, rinsed with Nanopure™ water and immersed in boiling

water for one hour. Catalyst inks are comprised of the catalyst nanoparticles (4 mg/cm2) and 5

wt% Nafion ionomer solution (Sigma Aldrich, Milwaukee, WI) dispersed in Nanopure™ water.

Inks were applied to a 5 cm2 portion of Nafion immobilized on a heated vacuum table at 70 °C.

Carbon paper gas diffusion layers (Toray Industries, Tokyo, Japan) are placed between each flow

field and electrode during fuel cell assembly. When charged with hydrogen, a Pt electrode

served at both the counter and reference electrode. The working electrodes used for this study

were: Pt (Johnson Matthey), Ru (Sigma-Aldrich) and PtRu (1:1, Johnson-Matthey). Membrane

electrode assemblies (MEA’s) were initially conditioned in a fuel cell operating at 50 °C by

70

cycling the potential from 800 mV to 600 mV at a rate of 40 mV/min five times. Anode and

cathode reactant streams were humidified H2 (50 sccm) and air (200 sccm) respectively.

Operando Spectroscopy: Operando specular reflectance IR spectra were obtained using

a fuel cell described by Lewis et al.(18) Briefly, a CaF2 window in the upper flow field and an

aperture in the gas diffusion layer, exposes the working electrode to the IR beam. The cell

interfaces with a commercially available diffuse reflection stage (Pike Technologies, Madison,

WI) connected to a Vertex 70 spectrometer (Bruker). Spectra were obtained by averaging 100

scans at 4 cm-1

resolution using a liquid nitrogen cooled MCT detector. The cell, operating at 50

°C, was fed humidified H2 (50 sccm) and N2 (200 sccm) to the counter/reference and working

electrodes, respectively. Immediately prior to acquiring spectra, the working electrode was

conditioned by cycling the potential from 0 mV to 1200 mV 50 times. Ethanol vapor was fed to

the cell using a modified GOW- MAC 350 gas chromatograph (Bethlehem, PA) being fed 10 M

ethanol at a rate of 2.5 µL/min with a N2 carrier gas (60 sccm). Once the ethanol feed began, the

cell potential was held at 0 mV and equilibrated for 30 minutes and a reference spectrum was

obtained. Potential dependent spectra were obtained at 100 mV increments between 100 mV and

900 mV. Before increasing the potential, reference spectra were taken 0 mV to maintain a

consistent baseline and to reduce contamination from atmospheric CO2 and water.

5.3 Results and discussion:

At low potentials adsorbed carbon monoxide is produced via the breaking of the C-C

bond of the adsorbed acetaldehyde species.(19, 20) Adsorbed species are observed in operando

FTIR spectra as bipolar peaks.(17) Carbon monoxide adsorption to the catalytic surface inhibits

catalysis by blocking sites for the adsorption of ethanol. At higher potentials, adsorption of

hydroxide allows for the oxidation of CO to CO2 freeing up sites for additional

71

ethanol/acetaldehyde adsorption.(11) The potential dependent shift in the CO peak, referred to

as Stark tuning, is the result of back donation of dπ electrons from the electrode into the 2π*

orbital of CO.(21) Figure 1 (left) shows the Stark tuning curve of CO adsorbed onto a Pt

electrode. The complex Stark tuning behavior of CO on Pt in an acidic medium has been

previously discussed.(22, 23) Briefly, the peak position of adsorbed CO increases linearly until

the potential is high enough to allow for adsorption of –OH and subsequent oxidation of CO

causing a drop in peak position caused by decreased dipole-dipole interactions. At higher

potentials, the adsorption of the Nafion sulfonate and CF3 groups onto Pt re-establishes dipole-

dipole interaction between the Nafion adsorbed species and the remaining clusters of adsorbed

CO thereby increasing the CO peak position.(23) The Stark tuning curve and potential dependent

spectra of figure 1 agree with previous studies reporting the onset of CO oxidation at 500 mV

with a sharp increase in oxidation at 650 mV.(24, 25) The Operando Stark tuning curves

demonstrate the sensitivity of adsorbed peak position to dipole-dipole interactions. The amount

of CO oxidized is low enough to relax the dipole-dipole interaction (i.e. lower peak position), yet

the amount of CO2 observed in the FTIR spectrum were barely noticeable at potentials lower

than 700 mV.

Alloying Ru to Pt yields a catalyst showing both a high activity for ethanol oxidation

while allowing for CO oxidation at much lower potentials. Ruthenium on its own, shows poor

kinetics for breaking the C-C bond of ethanol but adsorbs –OH at much lower potentials (300

mV) than Pt (26, 27). In the literature, there are two generally accepted theories to explain this

phenomenon. The alloying of Ru to Pt contributes to the electron density of Pt thereby

weakening Pt-CO bond.(28-32) This theory is supported by DFT studies showing either no Pt-C

bond contacting or bond elongation expected with electron back donation.(33) Alternatively, the

72

bifunctional theory states that Ru activates water at lower potentials yielding a tightly bound Ru-

OH species. The adsorbed CO on Pt sites to interact with Ru-OH sites allowing for its oxidation

to CO2.(27, 34-39) The spectra of figure 2 show the potential dependent spectra of ethanol on a

PtRu electrode (1:1). The spectra agree with Lima and Gonzales(40), whose electrochemical

studies of ethanol studies on PtRu show an onset of CO oxidation at 400 mV.

Figure 5.1. Stark tuning curve of CO adsorbed onto Pt (left). Potential dependent spectra of a

Pt black electrode in the presence of ethanol vapor (right). This region of the spectrum focuses

on peaks associated with adsorbed CO (~2065 cm-1

) and CO2 (2350 cm-1

).

73

Figure 5.2. Stark tuning curve of CO adsorbed onto PtRu (right). Potential dependent spectra of

an unsupported PtRu electrode in the presence of ethanol vapor (left). This region of the

spectrum focuses on peaks associated with adsorbed CO (~2060 cm-1

) and CO2 (2350 cm-1

).

Oxidation Mechanism

The oxidation mechanism of ethanol on Pt electrodes has been extensively studied.(2, 11,

12, 41-48) The potential dependent products of ethanol oxidation are acetaldehyde, acetic acid

and CO2. In a generally accepted mechanism, the initial step of ethanol oxidation is its

adsorption at the carbon alpha to the oxygen. The transfer of two electrons from the adsorbed

species yield acetaldehyde which dissociates from the surface. The acetaldehyde then re-adsorbs

at low potentials at which point one of two things can happen: 1) The C-C bond is broken

yielding CO or 2) The acetaldehyde is oxidized to acetic acid. In the former scenario, the

ethanol is completely oxidized yielding twelve electrons whereas in the latter, only 4 electrons

are produced. This is summarized in figure 3 below. Desorbed acetaldehyde is generally

74

observed around 400 mV with acetic acid in lower amounts. Bands corresponding to both

products intensify at 600 mV coinciding with and increased availability in adsorption sites due to

the oxidation of adsorbed CO.(13) Equally important are adsorbed species. As previously

discussed, ethanol is adsorbed at low potentials at the alpha carbon and two Pt atoms.(11, 12)

The band corresponding to adsorbed C-adsorbed ethanol is observed 1256 cm-1

.(12)

Acetaldehyde can be adsorbed at the carbon, oxygen or both.(49-53) All three modes of

adsorption yield bands from 1645-1665 cm-1

. The two oxygen atoms of acetic acid adsorb to a

Pt surface via two proposed mechanisms: Each oxygen is bound to its own unique Pt atom or

both oxygens are bound to a single Pt atom.(54) Surface bound acetic acid is referred to as

adsorbed acetate. In the case of Pt single-crystal electrodes, the bands corresponding to adsorbed

acetate form generally form between 300-350 mV but in the case of Pt(110), adsorbed acetate

can be found at 100 mV(54). Adsorbed acetate bands are found at 1395-1420 cm-1

. (7, 54-56)

75

Figure 5.3. A general mechanism for the oxidation of ethanol on Pt. For brevity, adsorbed

species are depicted only once despite the existence of several bonding configurations.

Reproduced from (11, 57)

In order fully elucidate the oxidation mechanism of ethanol on Pt, or any other catalyst, it

is not enough to detect the presence of desorbed species but the potential dependent evolution of

adsorbed species must also be identified. An advantage of studying ethanol oxidation using

operando techniques is that both of these types of products can be observed. Figure 4 is a series

of spectra taken of a Pt electrode in the presence of ethanol vapor under normal operating

conditions.

76

Figure 5.4. Potential dependent spectra of a Pt black electrode in the presence of ethanol vapor.

Vapor phase acetaldehyde and acetic acid were obtained ex-situ.

1900 1800 1700 1600 1500 1400 1300 1200 1100 1000

1394

1627

1258

1066

1369

1763

Wavenumber (cm-1)

Acetic Acid

Acetaldehyde

900 mV

800 mV

700 mV

600 mV

500 mV

400 mV

300 mV

200 mV

0 mV

100 mV

77

The spectra in figure 5.4 illustrate and follow the accepted mechanism for ethanol oxidation on

Pt. In order to distinguish between adsorbed and desorbed species, recall that bands

corresponding to adsorbed species are observed as bipolar bands.(17) At 100 mV bands

corresponding to adsorbed CO are observed. Also at 100 mV, bipolar peaks at 1258 cm-1

and

1066 cm-1

appear. These to peaks correspond to C and O adsorbed ethanol respectively.(12)

While the observation of the 1258 cm-1

has been reported, this is the first time that a peak

corresponding to O adsorbed ethanol has been observed.(12) Iwasita and Pastor reported the

possibility of O adsorbed ethanol theorizing a corresponding band from 1000 cm-1

and 1050 cm-

1, noting that a dipole moment of a C-O bond perpendicular to the surface would be small and

difficult to observe. Previous in-situ ethanol FTIR studies on Pt used HClO4 as an electrolyte.

ClO4- peaks are observed at 1033 cm

-1 (58) and dominate this region of the spectrum making the

observation of O adsorbed ethanol under non-operando conditions unlikely. Bipolar peaks

appearing at 1627 cm-1

are associated with adsorbed acetaldehyde(50, 51) evolve at the same

potentials as peaks associated with adsorbed ethanol. Adsorbed acetic acid is observed starting

at 200 mV around 1394 cm-1

. This peak position is slightly lower than that reported by Shin et

al, however, the potential at which these peaks evolve is consistent with previous studies and

subsequent experiments in this report (vida infra).(7, 54) At 400 mV, small amounts of adsorbed

CO begin to oxidize to CO2 decreasing CO coverage and allowing for other adsorbates. This is

correlated in the spectra of figure 4 by a marked increase in the intensities of peaks

corresponding to all the other adsrobed species. Acetaldehyde is the intermediate produced

between adsorption of ethanol and the cleavage of the C-C bond. Therefore it is not unexpected

that desorbed acetaldehyde is detected in potentials above 400 mV with and marked increase in

production above 600 mV coinciding with CO oxidation. Since Pt is shows activity for cleaving

78

the C-C bond of ethanol, CO2 is observing in signficant ammounts. It is difficult to ascertain the

amount of desorbed acetic acid produced using this technique. Any carbonyl peaks arising from

acetic acid (1730 cm-1

) would be overwhelmed by carbonyl peaks related to acetaldehyde.

To demonstrate its efficacy on catalysts outside of Pt, the operando methodolgy was used

to study ethanol oxidation on Ru and PtRu. Figure 5 is a series of a Ru catalyst in the presence

of ethanol at increasing potentials. Recall that compared to Pt, Ru displays lower activity

towards cleaving the C-C bond of ethanol. Therefore it reasonable to expect a majority of the

peaks in the spectra of figure 5 to correspond to acetic acid and its adsorbed intermediates.

Peaks corresponding to ethanol adsorption on Ru (1258 cm-1

and 1065 cm-1

) do not arise until

possibly 300 mV with O adsorption favored compared to Pt. The two bands used to identify

acetic acid in figure occur at 1730 cm-1

and 1172 cm-1

with adsorbed acetate appearing at 1417

cm-1

. The presense of adsorbed water is needed for the oxidation of acetaldehyde to acetic acid.

Because Ru adsorbs water at lower potentials than Pt, the oxidation of acetaldehyde will also

occur at lower potentials. This explains why peaks corresponding to desorbed acetaldehyde are

observed at significantly lower intensities compared to Pt.

79

Figure 5.5. Potential dependent spectra of a Ru black electrode in the presence of ethanol vapor.

Vapor phase acetaldehyde and acetic acid were obtained ex-situ.

The potential dependent spectra of a PtRu (1:1) catalyst in the presense of ethanol vapor

is shown in figure 6. The bifunctional mechanism ethanol oxidation will take place on the Pt

sites while Ru sites adsorb activated water species(59). The result is a lower potential at which

1900 1800 1700 1600 1500 1400 1300 1200 1100 1000

1065

1172

1276

1417

Acetic Acid

Acetaldehyde

900 mV

800 mV

700 mV

600 mV

500 mV

400 mV

300 mV

200 mV

100 mV

Wavenumber (cm-1)

0 mV

80

CO oxidizes, lowering coverage and freeing up sites for further ethanol oxidation. As shown

earlier in figure 2, CO2 evolves at 200 mV on PtRu coinciding with the plateau of the Stark

tuning curve. The shift in the CO band position signifies a relaxed dipole-dipole interaction from

lower CO coverage.(22) However, despite a lower CO coverage, there are less Pt sites making

complete oxidation of ethanol not as favorable than on pure Pt. This is exemplified by the

substantially lower signal for bipolar peaks indicative of ethanol adsorption. Consequently, there

is stronger signal of acetaldehyde relative to that of Pt (Fig 4) as there are fewer sites for

acetaldehyde to readsorb for complete oxidation.

81

Figure 5.6. Potential dependent spectra of a PtRu black electrode in the presence of ethanol

vapor. Vapor phase acetaldehyde and acetic acid were obtained ex-situ.

1900 1800 1700 1600 1500 1400 1300 1200 1100 1000

Wavenumber (cm-1)

Acetic Acid

13731056

1763

Acetaldehyde

900 mV

800 mV

700 mV

600 mV

500 mV

400 mV

300 mV

200 mV

100 mV

0 mV

1256

82

5.4 Conclusion

Operando fuel cell spectroscopy allows for elucidation of the ethanol oxidation pathway

on a variety of cataysts. Both adsorbed and desorbed species can be observed and differentiated

without employing a polarized light source. By studying this process with an electrolyte system

without any mobile anions, previously unreported peaks associated with O-adsorbed ethanol

have been observed. Under these conditions it is clear that the complete oxidation of ethanol is

favored over partial pathway. The appearance of strong absorptions consitant with the presence

CO, CO2 and acetaldehyde support this claim. This methodology operates under conditions that

reflect real world conditions providing data with more relevancy than in-situ techniques and will

be a useful tool in characterizing new catalysts.

References:

1. Lamy, C. and Leger, J.M., FUEL-CELLS - APPLICATION TO ELECTRIC VEHICLES. Journal De Physique Iv, 1994. 4(C1): p. 253-281.

2. Wang, J., Wasmus, S., and Savinell, R.F., Evaluation of Ethanol, 1-Propanol, and 2-Propanol in a Direct Oxidation Polymer-Electrolyte Fuel Cell. J. Electrochem. Soc., 1995. 142(12): p. 4218.

3. Rivera, H., et al., Effect of Sorbed Methanol, Current, and Temperature on Multicomponent Transport in Nafion-Based Direct Methanol Fuel Cells. The Journal of Physical Chemistry B, 2008. 112(29): p. 8542-8548.

4. Bittins Cattaneo, B., et al., Intermediates and Products of Ethanol Oxidationon Platinum in Acid-Solution. Berichte Der Bunsen-Gesellschaft-Physical Chemistry Chemical Physics, 1988. 92(11): p. 1210-1218.

5. Leung, L.W.H., Chang, S.C., and Weaver, M.J., Real-Time FTIR Spectroscopy as an Electrochemical Mechanistic Probe - Electrooxidation of Ethanol and Related Species on Well-Defined Pt(111) Surfaces. Journal of Electroanalytical Chemistry, 1989. 266(2): p. 317-336.

6. Holze, R., On the Adsorption and Oxidation of Ethanol on Platinum as Studied with in-situ IR Spectroscopy. Journal of Electroanalytical Chemistry, 1988. 246(2): p. 449-455.

7. Shin, J., et al., Elementary steps in the oxidation and dissociative chemisorption of ethanol on smooth and stepped surface planes of platinum electrodes. Surface Science, 1996. 364(2): p. 122-130.

83

8. Vigier, F., et al., Development of anode catalysts for a direct ethanol fuel cell. Journal of Applied Electrochemistry, 2004. 34(4): p. 439-446.

9. Iwasita, T., Fuel cells: Spectroscopic studies in the electrocatalysis of alcohol oxidation. Journal of the Brazilian Chemical Society, 2002. 13(4): p. 401-409.

10. Iwasita, T., et al., A SNIFTIRS Study of Ethanol Oxidation on Platinum. Electrochimica Acta, 1989. 34(8): p. 1073-1079.

11. Hitmi, H., et al., A kinetic analysis of the electro-oxidation of ethanol at a platinum electrode in acid medium. Electrochimica Acta, 1994. 39(3): p. 407-415.

12. Iwasita, T. and Pastor, E., A DEMS and FTIR Spectroscopic Investigation of Adosbed Ethanol on Polycrystaline Platinum. Electrochimica Acta, 1994. 39(4): p. 531-537.

13. Xia, X.H., Liess, H.D., and Iwasita, T., Early stages in the oxidation of ethanol at low index single crystal platinum electrodes. Journal of Electroanalytical Chemistry, 1997. 437(1-2): p. 233-240.

14. de Souza, J.P.I., et al., Electro-oxidation of ethanol on Pt, Rh, and PtRh electrodes. A study using DEMS and in-situ FTIR techniques. Journal of Physical Chemistry B, 2002. 106(38): p. 9825-9830.

15. Willsau, J. and Heitbaum, J., Elementary Steps of Ethanol Oxidation on Pt in Sulfuric-Acid as Evidenced by Isotope Labeling. Journal of Electroanalytical Chemistry, 1985. 194(1): p. 27-35.

16. Iwasita, T. and Vielstich, W., The electrochemical oxidation of ethanol on platinum: a SNIFTIRS study. Journal of electroanalytical chemistry and interfacial electrochemistry, 1988. 257(1-2): p. 319-324.

17. Burgi, T., ATR-IR spectroscopy at the metal-liquid interface: influence of film properties on anomalous band-shape. Physical Chemistry Chemical Physics, 2001. 3(11): p. 2124-2130.

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19. Shao, M.H., et al., In situ ATR-SEIRAS study of electro oxidation of dimethyl ether on a Pt electrode in acid solutions. Electrochemistry Communications, 2005. 7(5): p. 459-465.

20. Souza-Garcia, J., Herrero, E., and Feliu, J.M., Breaking the Oxidation Reaction on Platinum Electrodes: Effect of Steps and Ruthenium Adatoms. ChemPhysChem, 2010. 11(7): p. 1391-1394.

21. Blyholder, G., Molecular orbital view of chemisorbed carbon monoxide. Journal of Physical Chemistry, 1964. 68(10): p. 2772-8.



24. Salgado, J.R.C., et al., Carbon monoxide and methanol oxidation at platinum catalysts supported on ordered mesoporous carbon: the influence of functionalization of the support. Physical Chemistry Chemical Physics, 2008. 10(45): p. 6796-6806.

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25. Bellows, R.J., Marucchi-Soos, E.P., and Buckley, D.T., Analysis of Reaction Kinetics for Carbon Monoxide and Carbon Dioxide on Polycrystalline Platinum Relative to Fuel Cell Operation. Industrial & Engineering Chemistry Research, 1996. 35(4): p. 1235-1242.

26. Tanaka, S., et al., Preparation and evaluation of a multi-component catalyst by using a co-sputtering system for anodic oxidation of ethanol. Journal of Power Sources, 2005. 152(1): p. 34-39.

27. Lu, C., et al., UHV, Electrochemical NMR, and Electrochemical Studies of Platinum/Ruthenium Fuel Cell Catalysts. Journal of Physical Chemistry B, 2002. 106(37): p. 9581-9589.

28. Goodenough, J.B., et al., Methanol Oxidation on Unspported and Carbon Supported Pt + Ru Anodes. Journal of Electroanalytical Chemistry, 1988. 240(1-2): p. 133-145.

29. Iwasita, T., Nart, F.C., and Vielstich, W., An FTIR Study of the Catalytic Activity of a 85-15 Pt-Ru Alloy for Methanol Oxidation. Berichte Der Bunsen-Gesellschaft-Physical Chemistry Chemical Physics, 1990. 94(9): p. 1030-1034.

30. Krausa, M. and Vielstich, W., Study of the Electrocatalytic Influence of Pt/Ru and Ru on the Oxidation of Residues of Small Organic-Molecules. Journal of Electroanalytical Chemistry, 1994. 379(1-2): p. 307-314.

31. Frelink, T., Visscher, W., and vanVeen, J.A.R., Measurement of the Ru surface content of electrocodeposited PtRu electrodes with the electrochemical quartz crystal microbalance: Implications for methanol and CO electrooxidation. Langmuir, 1996. 12(15): p. 3702-3708.

32. Frelink, T., Visscher, W., and Vanveen, J.A.R., On the Role of Ru and Sn as Promoters of Methanol Electrooxidation Over Pt. Surface Science, 1995. 335(1-3): p. 353-360.


34. Wang, J.X., et al., In situ X-ray reflectivity and voltammetry study of Ru(0001) surface oxidation in electrolyte solutions. Journal of Physical Chemistry B, 2001. 105(14): p. 2809-2814.

35. Watanabe, M. and Motoo, S., ELECTROCATALYSIS BY AD-ATOMS .1. ENHANCEMENT OF OXIDATION OF METHANOL ON PLATINUM AND PALLADIUM BY GOLD AD-ATOMS. Journal of Electroanalytical Chemistry, 1975. 60(3): p. 259-266.

36. Watanabe, M. and Motoo, S., ELECTROCATALYSIS BY AD-ATOMS .2. ENHANCEMENT OF OXIDATION OF METHANOL ON PLATINUM BY RUTHENIUM AD-ATOMS. Journal of Electroanalytical Chemistry, 1975. 60(3): p. 267-273.

37. Watanabe, M. and Motoo, S., ELECTROCATALYSIS BY AD-ATOMS .3. ENHANCEMENT OF OXIDATION OF CARBON-MONOXIDE ON PLATINUM BY RUTHENIUM AD-ATOMS. Journal of Electroanalytical Chemistry, 1975. 60(3): p. 275-283.

38. Tremiliosi, G., et al., Reactivity and activation parameters in methanol oxidation on platinum single crystal electrodes 'decorated' by ruthenium adlayers. Journal of Electroanalytical Chemistry, 1999. 467(1-2): p. 143-156.

39. Davies, J.C., Hayden, B.E., and Pegg, D.J., The modification of Pt(110) by ruthenium: CO adsorption and electro-oxidation. Surface Science, 2000. 467(1-3): p. 118-130.

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40. Lima, F.H.B. and Gonzalez, E.R., Ethanol electro-oxidation on carbon-supported Pt–Ru, Pt–Rh and Pt–Ru–Rh nanoparticles. Electrochimica Acta, 2008. 53(6): p. 2963-2971.

41. Rightmire, R.A., et al., Ethyl Alcohol Oxidation at Platinum Electrodes. Journal of the Electrochemical Society, 1964. 111(2): p. 242-247.

42. Pastor, E. and Iwasita, T., D/H Exchange of Ethanol at Platinum-Electrodes. Electrochimica Acta, 1994. 39(4): p. 547-551.

43. Lamy, C., et al., Recent advances in the development of direct alcohol fuel cells (DAFC). J. Power Sources, 2002. 105: p. 283-296.

44. Souza, J.P.I., et al., Performance of a co-electrodeposited Pt-Ru electrode for the electro-oxidation of ethanol studied by in situ FTIR spectroscopy. Journal of Electroanalytical Chemistry, 1997. 420(1-2): p. 17-20.

45. Delime, F., Leger, J.M., and Lamy, C., Enhancement of the electrooxidation of ethanol on a Pt-PEM electrode modified by tin. Part I: Half cell study. Journal of Applied Electrochemistry, 1999. 29(11): p. 1249-1254.

46. Lamy, C., Belgsir, E.M., and Leger, J.M., Electrocatalytic oxidation of aliphatic alcohols: Application to the direct alcohol fuel cell (DAFC). Journal of Applied Electrochemistry, 2001. 31(7): p. 799-809.

47. Bonarowska, M., Malinowski, A., and Karpinski, Z., Hydrogenolysis of C-C and C-Cl bonds by Pd-Re/Al2O3 catalysts. Applied Catalysis a-General, 1999. 188(1-2): p. 145-154.

48. Aboulgheit, A.K., Menoufy, M.F., and Elmorsi, A.K., Hydroconversion of N-Heptane on Catalysis Containing Platinum, Rhenium and Platinum Rhenium on Sodium Mordenite. Applied Catalysis, 1990. 61(2): p. 283-292.

49. McCabe, R.W., DiMaggio, C.L., and Madix, R.J., Adsorption and reactions of acetaldehyde on platinum(S)-[6(111) .times. (100)]. The Journal of Physical Chemistry, 1985. 89(5): p. 854-861.

50. Zhao, H.B., Kim, J., and Koel, B.E., Adsorption and reaction of acetaldehyde on Pt(111) and Sn/Pt(111) surface alloys. Surface Science, 2003. 538(3): p. 147-159.

51. Rodriguez, J.L., et al., Reaction intermediates of acetaldehyde oxidation on Pt(111) and Pt(100). An in situ FTIR study. Langmuir, 2000. 16(12): p. 5479-5486.

52. Perez, J.M., et al., Adsorption of Acetaldehyde on Pt(100) and (111) Faces - A Semiempirical Quantum Mechanical Study. Surface Science, 1990. 235(2-3): p. 307-316.

53. Shekhar, R., et al., Adsorption and reaction of aldehydes on Pd surfaces. Journal of Physical Chemistry B, 1997. 101(40): p. 7939-7951.

54. Rodes, A., Pastor, E., and Iwasita, T., An FTIR study on the adsorption of acetate at the basal planes of platinum single-crystal electrodes. Journal of Electroanalytical Chemistry, 1994. 376(1–2): p. 109-118.

55. Shao, M.H. and Adzic, R.R., Electrooxidation of ethanol on a Pt electrode in acid solutions: in situ ATR-SEIRAS study. Electrochimica Acta, 2005. 50(12): p. 2415-2422.

56. Corrigan, D.S., et al., Adsorption of acetic acid at platinum and gold electrodes: a combined infrared spectroscopic and radiotracer study. The Journal of Physical Chemistry, 1988. 92(6): p. 1596-1601.

57. Vigier, F., et al., Electrocatalysis for the direct alcohol fuel cell. Topics in Catalysis, 2006. 40(1-4): p. 111-121.

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58. Karelin, A.I., Grigorovich, Z.I., and Rosolovskii, V.Y., Vibrational spectra of perchloric acid-I. Gaseous and liquid HClO4 and DClO4. Spectrochimica Acta Part A: Molecular Spectroscopy, 1975. 31(5–6): p. 765-775.

59. Scott, F.J., Mukerjee, S., and Ramaker, D.E., CO coverage/oxidation correlated with PtRu electrocatalyst particle morphology in 0.3 M methanol by in situ XAS. Journal of the Electrochemical Society, 2007. 154(5): p. A396-A406.

87

Chapter 6: Operando Raman Spectroscopy of a non-Pt Cathode

Nafion Membrane Electrode

6.1 Introduction:

The increasing cost of Pt has motivated the development of non-platinum group metal

(non-PGM) compounds to catalyze the reduction of molecular oxygen to water in proton

exchange membrane fuel cells (PEMFC). The ability of Fe and Co N4 macrocycles to catalyze

the oxygen reduction reaction (ORR) has been known since the 1960s.(1, 2) The activity of

these catalysts was subsequently improved through pyrolysis in an inert atmosphere. (3-10)

Despite their increased activity, the cost of the macrocycle precursors made their use impractical.

More recently, it has been demonstrated that compounds capable of catalyzing the ORR can be

made through pyrolysis of cost effective transition metal, nitrogen and carbon precursors.(11-14)

Pyrolysis of these materials results in a catalyst M-Nx active site embedded in a graphite plane

(M-Nx/C).(15-20)

There is no generally consensus regarding the nature of the active complex of M-Nx/C

catalysts. The proposed structure of this active site could exist in two possible configurations M-

N2/C and M-N4/C.(21, 22) In the case of the former, the iron would be bound to two pyridinic

nitrogens at the edge of a graphene crystallite(21, 23) while he latter describes the active site as

an iron atom bound to four nitrogens at the center of porphyrin moiety.(22) Given that previous

studies show iron coordinated by four nitrogens(24-30) in a phenanthrolinic configuration,22,24

Dodelet further proposed a Fe-N2+2/C active site where the iron is bound to four nitrogens from

two separate graphene crystallites.(31) Additionally, a second active site has been proposed

consisting of metal nanoparticles and/or oxides (M/MOx) encapsulated in a carbon

88

nanostructure.(32, 33) These M/MOx structures have been shown stable even subjected to

prolonged periods of time in an acidic environment.(32)

The proposed mechanism in acidic media for the ORR on M-Nx/C catalysts is a two-step

process where oxygen is first reduced to hydrogen peroxide followed a reduction to water.

Ramaswamy et al demonstrated using a Fe-N4 porphryin catalyst that in, acidic media, the Fe-N4

active site is incapable of reducing hydrogen peroxide to water.(34) This suggests the existence

of second active site which catalyzes the second step of the ORR. While Jaouen et al(19) noted

the catalytic activity from the presence of M/MOx complexes, it is not until recently that their

role as a second active site for the reduction of peroxides has been considered.(35, 36)

In an effort to gain insight into the reduction mechanism active sites of these new

catalysts, a fuel cell was developed for the purpose of obtaining Raman spectra of a Fe-Nx/C

catalyst under normal operating conditions. The goal was to identify and characterize the

potential-dependent peaks arising during catalysis. The confocal Raman microscope allowed for

depth profiling of the membrane electrode assembly (MEA) allowing for comprehensive

characterization of the MEA. Density functional theory was used to calculate the theoretical

vibrational spectrum of the hypothetical Fe-N2+2 and Fe-N4 active site along with a cluster of Fe

atoms to represent a nanopartricle. For each of these models, spectra were generated both in the

presence and absence of adsorbed O2. A comparison between these two theoretical spectra

elucidates which peaks corresponds to O2 adsorption.

6.2 Experimental:

Membrane electrode assembly preparation: Nafion 117 (Wilmington, DE) was

pretreated by immersing sheets in boiling 8 M nitric acid followed by two hours in boiling

Nanopure™ water. Non-PGM Catalysts were prepared using the method described by

89

Barton.(37) Briefly, Ketjen black 600JD, iron acetate (0.75 wt % Fe) and melamine (6.3 wt %

N) was heat treated at 800 °C. Catalyst inks were prepared by diluting the catalyst in 5 wt %

Nafion ionomer solution (Sigma Aldrich, Milwaukee, WI), water and isopropyl alcohol. 4

mg/cm2 Pt black (Johnson Matthey) and 1.6 mg non-PGM catalyst was used as the anode and

cathode catalysts, respectively. Catalyst inks were applied to a 5 cm2 area of a sheet of Nafion

117 immobilized on a heated vacuum table (NuVant Systems, Inc., Crown Point, IN). MEA’s

were initially conditioned by cycling the potential from 200-800 mV in the operando Raman cell

operating at 50 °C with humidified H2 (50 sccm) and O2 (200 sccm) flowing over the anode and

cathode, respectively.

Cell design: The operando Raman fuel cell design is based on the operando IR-XAS cell

described by Lewis et al.(38) Briefly, the upper flow field connects to the working electrode and

contains an aperture that accommodates a G.E. 124 fused quartz window. The lower flow field

connects to the counter/reference electrode. An aluminum housing slider (Fig 1.) delivers the

reactant feeds to the graphite flow fields and contains the heating and temperature control

elements. A DB9 connector seated in the housing slider connects the cell to the potentiostat.

The slider is dimensioned to position the catalytic surface at the working distance of the

microscope objective (Fig 2).

90

Figure 6.1. Exploded view of operando Raman cell components. (1) Top plate, (2) upper flow

field, (3) membrane electrode assembly, (4) lower flow field, (5) Assembly gasket, and (6)

assembly stage.

Figure 6.2. Operando Raman cell beneath a confocal Raman microscope.

91

Operando Raman spectroscopy: All operando Raman spectra were acquired using a

WITec Inc. (Ulm, Germany) Confocal Raman Microscope (CRM 200). A 488 nm (23 mW)

solid state laser (WITec Inc.) was used as the excitation source, which was coupled into a Zeiss

(Thornwood, NY) microscope via a 50 m wavelength-specific single-mode optical fiber. The

incident laser beam was focused onto the sample using a Nikon (Tokyo, Japan) Fluor (10x/0.25,

WD: 7.00 mm) objective. The Raman backscattered radiation was focused through a

holographic notch filter, onto a 50 m multimode optical fiber, and into a 300 mm focal length

monochromator (600/mm grating, blazed at 500 nm). The Raman spectrum was detected via a

back-illuminated, deep-depletion CCD camera (1024 x 128 pixels) operating at -82C. Single

Raman spectra were aquired for 15 s.

Prior to obtaining spectra, the cell potential was cycled from 0 to 1200 mV at 50 °C with

humidified H2 (50 sccm) and N2 (200 sccm) fed to the counter/reference and working electrodes,

respectively. The working electrode reactant feed was then switched to humidified O2 (200

sccm). Raman spectra were obtained between 1100 mV and 0 mV and collected at decreasing

100 mV increments.

Density Functional Theory: The hypothetical active sites of a Fe-Nx/C catalyst were

used as the input structure to be modeled in the presence and absence of O2. Calculations were

performed using Jaguar 7.2 (Schrodinger Inc., Portland, OR) at the X3LYP/LACV3P++** (“**”

and “++” denote polarization (39) and diffuse (40) basis set functions, respectively) level of

theory. Output files were converted to vibrational mode animations using Maestro (Schrodinger

Inc.). Calculations were carried out on a 55 node (dual core Xeon processors with 4GB RAM)

High Performance Computing Cluster at the University of Texas, Pan American. By varying the

spin state of catalyst and calculating the total energy values of each, DFT shows that the ground

92

state of [catalyst] as having a spin multiplicity of 3. The calculated values are depicted in Figure

3. All three cases have the lowest energy spin state as a triplet. For the lowest energy state,

normal mode calculations were performed

Figure 6.3: Total free energy, in Hartrees, for the hypothetical Fe-Nx/C active site at various spin

states.

93

Structural distortion treatment procedure of Fe-Nx/C: The Melamine-Fe catalyst was

incorporated on MEA specifically designed for in-situ operando cell. The ready MEA electrode

was assembled into a flow-through half-cell which was equipped with carbon cloth as counter

electrode and RHE reference. While the treatment, acid-based electrolyte saturated with oxygen,

0.5 M H2SO4 was circulating through the cell. The half-cell was connected to an Autolab

(Ecochemie Inc. model-PGSTAT 30) and subjected to CV-cycling at potentials range of 0.05-1.1

V (vs. RHE). The structural alteration of Melamine-Fe-C catalyst was achieved by addition of a

controlled amount of 35% hydrogen peroxide to the electrolyte (pH=1) reservoir. After the

treatment, the half-cell was reassembled and the electrode was rinsed with Milipore DI water and

dried prior further studies.

6.3 Results

There are several advantages to studying the ionomer-metal interface of a fuel cell using

Raman spectroscopy. A hydrated membrane is essential for the normal operation of a proton

exchange membrane fuel cell. In previous FTIR operando studies the reactant stream

humidification, flow rate and cell temperature needed to be rigorously controlled or potentially

useful regions of the IR spectrum would be obliterated from peaks resulting from an excess or

deficit of water. Peaks in Raman spectra related to water do not appear in significant intensity in

the region of interest for this study making the acquisition and analysis of spectra comparatively

simple. The confocal microscope allows for depth profiling of the membrane electrode assembly

providing a comprehensive representation of the interface. Figure 4 outlines the utility of this

technique. In order to obtain a reference point, the microscope was positioned along the z-axis

such that the spectrum of bulk Nafion was the strongest and will be referred to as +0 µm (all

subsequent positions on the z-axis will refer to points above +0 µm). The signal for bulk Nafion

94

remains prevalent at positions from up to +100 µm. At this point peaks related to the carbon

support begin to appear. The two prominent features of the graphite are the E2g vibration mode

1360 cm-1

disorder peak (D peak) at 1600 cm-1

.(16) The D peak arises from a A1g breathing

mode is observed at the edges of graphene planes on clusters smaller than 200 Å.(16) Residual

peaks from Nafion are due the Nafion dispersion used to prepare catalyst inks. Peaks from

molecular oxygen also evolve beginning at +100 µm and continue to intensify as the distance

from the reference point increases.

Figure 6.4. (Left) Schematic outlining the concept of depth profiling a membrane electrode

assembly using a confocal Raman microscope. (Right) Depth dependent spectra of a membrane

electrode assembly consisting of an Fe/N/C catalyst and Nafion. The focal position where the

spectrum of bulk Nafion is most intense serves as the reference point.

Potential dependent changes to the surface of a Fe-Nx/C cathode during ORR were

assessed using operando Raman spectroscopy. Figure 5 is a series of spectra acquired at

95

decreasing potentials in the presence of oxygen. Spectra were obtained at 350 µm above the

reference point. Potential dependent peaks at 631 cm-1

, 600 cm-1

, and 564 cm-1

appear at 700

mV and are present in subsequent spectra taken at lower potentials. The polarization curve

shown in figure 6 indicates that current is produced at potentials of 700 mV and lower. It is

therefore reasonable to correlate these peaks to catalytic activity. This experiment was

duplicated on the same catalyst in the presence of nitrogen and using a catalyst prepared without

the iron precursor. In the resulting spectra shown, in figure 7, the 631 cm-1

, 600 cm-1

, and 564

cm-1

peaks to do not appear in either of these control experiments.

96

Figure 6.5. The potential dependent Raman spectra of an iron based non-PGM cathode catalyst

obtained under O2.

1300 1200 1100 1000 900 800 700 600 500 400

1300120011001000900800700600500400

564 cm-1

600 cm-1

0mV

100mV

200mV

300mV

400mV

500mV

600mV

700mV

800mV

900mV

1000mV

1100mV

629 cm-1

97

Figure 6.6. Raman fuel cell polarization curve obtained under oxygen with a MEA consisting of

a Pt anode and an Fe-Nx/C cathode.

98

Figure 6.7. Potential dependent control experiments of a non-PGM cathode catalyst. (Left):

Spectra obtained under O2 with a catalyst prepared without iron. (Right): Spectra obtained under

N2 catalyst prepared with iron.

Theoretical spectra generated using DFT are useful for analyzing experimentally acquired

vibrational spectra. These spectra provide the approximate location of each normal mode along

with an animation which describes the vibration. Scaling factors inherent in DFT calculations

generally preclude an exact agreement between theoretically and experimentally obtained data:

peak assignments must therefore be supported with additional experimental data.(41)

Generating a theoretical spectrum of a hypothetical active site a Fe-Nx/C catalyst will provide a

basis for locating regions of the Raman spectra where peaks corresponding to oxygen adsorption

may be found. The selection of the hypothetical active site provides a challenge in this analysis.

1200 1000 800 600 400

Wavenumber (cm-1)

0mV

100mV

200mV

300mV

400mV

500mV

600mV

700mV

800mV

900mV

1100mV

1000mV

629 cm-1

600 cm-1 564 cm

-1

1200 1000 800 600 400

600 cm-1

564 cm-1

629 cm-1

1100mV

Wavenumber (cm-1)

0mV

100mV

200mV

300mV

400mV

500mV

600mV

700mV

800mV

900mV

1000mV

1100mV

99

In order to ensure that both forms of nitrogen are considered (pyridinic and pyrrolic), Fe-Nx/C

proposed active sites were modeled. Theoretical spectra both of these models were generated,

each with O2 adsorbed onto the iron and without. Figure 8 shows the series of theoretical spectra

representing the Fe-N2+2/C active site with pyridinic nitrogens and figure 9 shows the N4 pyrrolic

active site.

100

Figure 6.8. (Top) DFT generated theoretical spectra of an iron based non-PGM catalyst with a

N4 pyridinic active site. (Bottom) Molecular representations of the theoretical catalyst with and

without adsorbed oxygen.

Figure 6.9. (Top) DFT generated theoretical spectra of an iron based non-PGM catalyst with a

N4 pyrrolic active site. (Bottom) Molecular representations of the theoretical catalyst with and

without adsorbed oxygen.

101

The theoretical spectra of the hypothetical Fe-N/C catalyst active sites in the presence

and absence of adsorbed oxygen were used to elucidate which normal modes corresponded to

oxygen adsorption. Specifically, differences between the two spectra could be used to identify

normal modes corresponding with Fe-O and O-O stretching. Observing peaks associated with

O-O stretching is unlikely given that they are found in regions of the spectra dominated by E2g

and D graphene peaks. The spectra of figure 8, corresponding to a Fe-N2+2/C pyridinic active

site, show only a handful of normal modes present in the oxygenated spectra that are missing the

deoxygenated spectra. Normal mode animations show that Fe-N vibrations are responsible for

the 843 cm-1

and 930 cm-1

modes while the 1216 cm-1

peak is from O-O stretching. A complete

inspection of all of the DFT normal modes of the oxygenated sample show Fe-O stretching at

372 cm-1

. Similarly the differences between the Fe-N4/C pyrrolic active site spectra (mainly the

regions near 608 cm-1

, 725 cm-1

, 788 cm-1

, 866 cm-1

, 1009 cm-1

, and 1105 cm-1

) also correspond

with Fe-N vibrations. The peaks associated with O-O are found at 1592 cm-1

and Fe-O stretching

are found at 148 cm-1

, 192 cm-1

, and 223 cm-1

. In both models Fe-O peaks are at frequencies

which are too low to be associated with the experimentally obtained potential dependent peaks.

An explanation for position of the experimentally obtained potential dependent peaks is

that they result oxygen adsorbed onto Fe/FeOx nanostructures. This can be observed in figure

10, as the Fe-O stretch from the theoretical spectrum of an iron nanoparticle cluster with

adsorbed O2 appears at 537 cm-1

. As previously discussed, additional experimental data is

required for associating real peaks with theoretical normal modes. To that end, a Fe-Nx/C

electrode was subjected to the structural distortion treatment described in the experimental

section. This treatment, as described by Tylus et al., contributes to the dissolution of Fe/FeOx

nanostructures.(42) The potential dependent Raman spectra of the Fe-Nx/C catalyst after the

102

peroxide/CV treatment is shown in figure 11. The fact that none of the potential dependent

features of figure 5 are present in figure 11, suggests that they result from oxygen species

adsorbed onto Fe nanoparticles and not the Fe-Nx moiety. A polarization curve of the

peroxide/CV treated catalyst (Fig. 12) shows that despite a higher overpotential, there is only a

small decrease in current density at lower potentials when compared to the intact catalyst. A

study by Olson et al proposes a bifunctional mechanism where a M-Nx moiety reduces O2 to

H2O2 and the reduction of H2O2 to water is catalyzed by M/MOx clusters.(35) The idea that

H2O2 reduced by M/MOx clusters is supported by the observations of Guillet et al who reported a

92% selectivity in the production of H2O2 on a Co-based porphyrin catalysts prepared under

conditions that limited formation Co/CoOx nanostructures.(36)

103

Figure 6.10. (Top) DFT generated theoretical spectra of an iron nanoparticle with and without

adsorbed oxygen. (Bottom) Molecular representations of iron nanoparticle with and without

adsorbed oxygen.

104

Figure 6.11. Potential dependent Raman spectra of non-PGM catalyst treated with H2O2 to

remove nanoparticles. Spectra were obtained under presence of oxygen.

1200 1000 800 600 400

0mV

100mV

200mV

300mV

400mV

500mV

600mV

700mV

800mV

900mV

1000mV

1100mV

Wavenumber (cm-1)

105

Figure 6.12. Fuel cell polarization curve obtained under oxygen with a MEA consisting of a Pt

anode and an Fe-Nx/C cathode subjected to the structural distortion treatment.

6.4 Conclusions:

Operando Raman spectroscopy to observe potential-dependent peaks on a Fe-Nx/C

catalyst in the presence of oxygen. It is reasonable to associate these peaks with catalysis as their

appearance coincides with current production. A proposed mechanism for the ORR on these

catalysts suggests two active sites: Fe-Nx moieties where O2 is reduced to H2O2 and Fe/FeOx

nanostructures that catalyze the reduction of H2O2 to water. Positions of Fe-O stretching modes

observed in theoretical spectra of oxygen adsorbed onto hypothetical models of both active sites

suggests the experimentally obtained peaks are associated with the latter. Additional Raman

studies on an Fe-Nx/C catalyst with these Fe/FeOx nanostructures removed, do not show the

potential dependent peaks observed on the intact catalyst.

References:

5 0 -5 -10 -15

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Po

ten

tia

l (m

V)

Current Density (mA/cm2)

106

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