Electrocatalysis in Alkaline Electrolytes - Research … HClO4 0.1M NaOH E 1/2 = 0.775V E 1/2 =...
Transcript of Electrocatalysis in Alkaline Electrolytes - Research … HClO4 0.1M NaOH E 1/2 = 0.775V E 1/2 =...
Sanjeev Mukerjee
Nagappan Ramaswamy, Qinggang He, Daniel Abbott,
and Michael Bates
Department of Chemistry and Chemical Biology
Northeastern University, Boston, MA 02115
Electrocatalysis in Alkaline Electrolytes - Research Overview
AMFC Workshop Seminar – May 8, 2011
Acidic pH Alkaline pH
1e- + 1H+C
HH3C
Pt
OHOH2
CH3CH2OH
Pt
1e- + 1H+
Pt
C
OH3C
Pt
COH3C
H
OH2
Pt
O
H
Pt
O
C
OH3C
+ H
H
C
OH3C
O
Pt
1e- + 1H+
-H2 at low
coord Pt
Pt
CHx
Pt
C
O
CO2
Pt
2OH
111
sites
O
HxC
Pt
C
Acetyl
AA
O
HxC
Pt
C
di-
OVERVIEW
General Overview – Advantages of High pH
ORR Mechanistic Understanding
Question of Kinetic Facility in Alkaline Medium?
Inner- vs. Outer-sphere Electron Transfer
Non-noble Metal Macrocycle Electrocatalyst
Origin of ORR Activity
Redox Potential Tuning
Influence of Graphitic Defects
Smart Catalysts for Anodic Oxidation
Homogeneous Mediation using
Metal-organic Complexes
Alkaline Membrane Fuel Cell Studies
Interfacial Challenges and Future Prospects
Acidic Medium
Alkaline Medium
Direct: O2 + 4H+ + 4e- 2H
2O E
o = 1.230V
Series: O2 + 2H+ + 2e- H
2O
2 E
o = 0.695V
H2O
2 + 2H+ + 2e- 2H
2O E
o = 1.763V
Direct: O2 + 2H
2O + 4e- 4OH
- E
o = 0.401V
Series: O2 + H
2O + 2e- HO
2
- + OH
- E
o = -0.065V
HO2
- + H
2O + 2e- 3OH
- E
o = 0.867V
O2,aq
O2,ads
H2O
2,adsH
2O
H2O
2
k1 (4e-)
k2(2e-) k
3(2e-)
k4 k
5
O2,aq
O2,ads
HO-
2,adsOH
-
k1 (4e-)
k2(2e-) k
3(2e-)
k4 k
5
HO-
2
In alkaline medium: pH > 12 ˜ HO2
-
H+/H
2
O2/H
2O
O2/OH
-
0.000V
1.230V
0.401V
H2O/H
2-0.828V
SHE
ScaleO/R
O2/O
2
--0.301V
η=1.53V
η =0.70V
ORR - Acid vs. Alkaline Medium: Thermodynamic Advantages
1) Nernstian Potential Shift
2) Affects Adsorption Strengths of spectators,
and intermediate species
Tafel Plots
log ik [A/cm2geo]
1e-4 1e-3 1e-2P
ote
nti
al
[V V
s R
HE
]
0.84
0.86
0.88
0.90
0.92
0.94
0.96
0.1M HClO4
0.1M NaOH 25 mV
O2 Satd.
20 mV/s900 rpmRoom Temp.
How Facile is ORR in Alkaline Medium?
(c) Polarization Curves
Potential [V Vs RHE]
0.0 0.2 0.4 0.6 0.8 1.0
Cu
rren
t D
ensi
ty [
A/c
m2
geo
]
-5e-3
-4e-3
-3e-3
-2e-3
-1e-3
00.1M HClO4
0.1M NaOH
(e) Ring Current
Potential [V Vs RHE]
0.0 0.2 0.4 0.6 0.8 1.0
Rin
g C
urr
ent
[A]
0
1e-5
2e-5
3e-5
4e-5
0.1M HClO4
0.1M NaOH
900 rpm
20 mV/s
Potential [V Vs RHE]
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Cu
rren
t D
ensi
ty [
A/c
m2
geo]
-4e-4
-2e-4
0
2e-4
4e-4
0.1M HClO4
0.1M NaOH
E1/2 = 0.775V
E1/2 = 0.810V
Specific Adsorption of Hydroxide Anion
in 0.1M NaOH
Water activation
in 0.1M HClO4
30% Pt/C - 20 mV/s
Outer sphere electron transfer process
responsible for peroxide anion formation
in alkaline medium
ORR on Pt better in non-adsorbing acidic
electrolyte like 0.1M HClO4
Most claims of increased kinetic facility in alkaline
fuel cell are based on comparisons with phosphoric
acid fuel cell
Polariztion Curves
Disk Potential [V Vs RHE]
0.0 0.2 0.4 0.6 0.8 1.0
Cu
rren
t D
ensi
ty [
A/c
m2g
eo]
-5e-3
-4e-3
-3e-3
-2e-3
-1e-3
0
1e-3
0.1 M NaOH
1.0 M NaOH
ERing
= 1.1 V
Disk Potential [V Vs RHE]
0.0 0.2 0.4 0.6 0.8 1.0
Rin
g C
urr
ent
[A]
0.0
5.0e-6
1.0e-5
1.5e-5
2.0e-5
2.5e-5
3.0e-5
3.5e-5
0.1 M NaOH
1.0 M NaOH
HO2ˉ related to specifically adsorbed oxides on Pt
Pt-O(H) + O2 + H2O + 2e‾ ------> Pt-O(H) + HO2‾ + OHˉ
Effect of Specific Adsorption of Hydroxide Anions in Alkaline Medium
Outer sphere electron transfer process
responsible for peroxide anion formation
in alkaline medium
Gold Ring Electrode:
Au + HO2ˉ + OHˉ → Au + O2 + H2O + 2eˉ
Inner-sphere and Outer-sphere Electron Transfer Mechanisms
Desired 4eˉ transfer on oxide free Pt site Acidic pH
Pt + O2 + e-
+ H2O + OH-
+ e-
H2O
+ 2OH-
+ e-Pt + OH-
+ e-
Pt
O
O
Pt
O
O
Pt
O
O
H
Pt
O
O
H
Pt
O
O
H
Pt
O
O
H
Pt
O
H
Pt
O
H
Alkaline pH
Outer-Sphere Electron Transfer
[O2.(H2O)n]aq + eˉ → [O2•ˉ.(H2O)n]aq
[O2•ˉ.(H2O)n]aq → (O2
•ˉ)ads + nH2O
(O2•ˉ)ads + H2O → (HO2
•)ads + OHˉ
(HO2•)ads + eˉ → (HO2ˉ)ads
(HO2ˉ)ads → (HO2ˉ)aq
Specific adsorption of hydroxide anions
promote outer-sphere electron transfer
in alkaline medium
Inner-sphere and outer-sphere electron transfer compete in alkaline medium
Literature Survey on non-PGM Cathode Catalysts
Metal Porphyrin
Mukerjee et al
Cofacial Porphyrin
Anson et al
Pentlandite Co9Se8
Anderson et al
Metal Phthalocyanine
D. Chu et al - ARL
The starting complex is immaterial. Its important to know how the self-assembly
and distribution of the active site takes place!!!
Bottom-up approach needed in catalyst synthesis rather than a random mixing of
precursor materials and heat treatment.
Fe/N/C
Dodelet et al M-Polymer Composite
Zelenay et al M-Organic Framwork
D. Liu - ANL
Open Framework
Atanassov et al
ORR Activity of FeTPP/BPC Pyrolyzed at 800˚C
Pyrolyzed FeTPP exhibits
remarkably low H2O2 yield
due to its exotic structure as
explained in the next slides FeTPP on Black
Pearl Carbon
pyrolyzed at 800°C
ED [V Vs RHE]
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
i [A
/cm
2
geo
]
-5e-3
-4e-3
-3e-3
-2e-3
-1e-3
0
FeTPP
Pt
[c] Tafel Plots
log ik [A/cm2
geo]
10-5 10-4 10-3 10-2
E V
s R
HE
0.7
0.8
0.9
1.0
[a] Polarization Curves
[b] Ring Current
ED [V Vs RHE]
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
Rin
g C
urr
ent
[A]
2e-6
4e-6
6e-6
8e-6
1e-5
FeTPP
Pt
Square Wave Voltammetry - Evolution of Fe2+/Fe3+ Redox Potential
Potential [V Vs RHE]
1.0 1.2 1.4 1.6 1.8
Cu
rren
t [A
]
-1e-4
0
1e-4
2e-4
3e-4
4e-4
5e-4
Potential [V Vs RHE]
0.0 0.2 0.4 0.6 0.8 1.0
Cu
rren
t [A
]
-1e-4
0
1e-4
2e-4
3e-4
4e-4
5e-4
0300 C
0500 C
0600 C
0800 C
0900 C
300o
C 300o
C
500o
C
500o
C600o
C
800o
C
900o
C
600o
C800
oC
900o
C
Square Wave Voltammetry in 0.1M NaOH at 10Hz
Anodic shift in the Fe2+/Fe3+ redox potential
upon pyrolysis is observed to 1.25V
Similar mechanism observed in acidic medium
also
Pyrolysis Temperature [oC]
0 200 400 600 800 1000
EP
eak [
V]
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Two Fe2+/Fe3+
redox couples
Scan 5
Potential [V Vs RHE]
0.3 0.6 0.9 1.2 1.5 1.8
Cu
rren
t D
ensi
ty [
A/c
m2
geo
]
0
1e-3
2e-3
3e-3
4e-3Cyclic Voltammetry
E Vs RHE
-0.2 0.0 0.2 0.4 0.6 0.8 1.0
0.1
mA
/cm
2
geo
0.314V Vs RHE
Fe(II)/Fe(III)
Ligand Oxidation
1.508V Vs RHE
Square Wave Voltammetry
FeTPPCl/BPCNon-pyrolyzed
Sqaure wave voltammetry removes all capacitive current contribution
Synchrotron Principles
Principles of X-ray Absorption Spectroscopy
XANES (< 50 eV)
•Absorber site symmetry
(e.g. Td, Oh, etc)
• Electronic configuration
• Geometric Binding Site
EXAFS (> 50 eV)
• Geometric information
• Bond length
• Coordination number
•BULK short range order
Synchrotron
Monochromator
Io
Spectro-electrochemicalCell
Fluorescence detector
ItIr
Referencefoil
300oC
R [Å]
0 1 2 3 4 5 6
| (R
)| [
Å-3
]
0.1V
Fit
800oC
R [Å]
0 1 2 3 4 5
| (R
)| [
Å-3
]
0.1V
Fit
Fe-N4
Fe-O
Fe-N4
Fe-O
Metallic Fe
Insitu Extended X-ray Absorption – Fe-Nx Short Range Structure
Fe-N
Coordination
Number
Fe-N
Bond
Length [Å]
300°C 4.00 1.996
800°C 4.00 1.976
In situ 0.1M NaOH electrolyte @ 0.1 V vs. RHE
Delta Mu – Identification of Active Site and Oxygen Adsorption Mode
Increasing defect sites is key to increasing active site density
Defective sites are regions of higher chemical potential and hence anodic shift in redox potential
ORR on FeTPP/C: Acid vs. Alkaline
(c) Peroxide Reduction
Potential [V Vs RHE]
0.2 0.4 0.6 0.8 1.0
i [A
/cm
2
geo
]
-2e-3
-1e-3
0
1e-3
2e-3
ORR
HRR
(b) Ring Current
Potential [V Vs RHE]
0.0 0.2 0.4 0.6 0.8 1.0
I R [
A]
0
1e-6
2e-6
3e-6
4e-6
5e-6
0.1M NaOH
0.1M HClO4
(a) ORR Polarization Curves
Potential [V Vs RHE]
0.0 0.2 0.4 0.6 0.8 1.0
i D [
A/c
m2
geo
]
-5e-3
-4e-3
-3e-3
-2e-3
-1e-3
0
0.1M NaOH
0.1M HClO4
0.1M HClO4
0.1M NaOH
Peroxide Intermediate is more stable on
FeTPP in alkaline medium
O2
H2O
e-
+ OH -
H2O
e-
e-e-
-OH -
FeII
Active Site
O
H
FeIII
Ferric-
Hydroperoxyl
O
OH
FeIII
Ferric
Hydroxyl
O
H
+ 2OH -
FeIII
Ferric
Superoxo
O
O
FeII
Adsorbed O2
O
O
FeII
Ferrous-
Hydroperoxyl
O
OH
Oxygen Reduction Mechanism on Fe-N4 Sites
In Alkaline Medium
Electrostatic Stabilization of peroxide intermediate
in alkaline medium is key to efficient 4eˉ Reduction Fe
II
Ferrous-
Hydroperoxyl
HO
OH
In Acidic Medium
Alkaline hydrogen oxidation – AEMFC Anodes
HOR/HER i0 much lower in alkaline vs. acid media
Need high loading non-PGM catalyst on AEMFC anode
Pmax of 50mW/cm2 obtained with: Cr-decorated Ni nanoparticles on anode & Ag nanoparticles on cathode
Gasteiger et al. JECS 2010 Lu et al. PNAS 2008
Literature Survey on Methanol/Ethanol Oxidation Anode Catalysts
EtOH binds to surface through Cα
Acetaldehyde/acetic acid pathways are
preferred
Selectivity is needed at electrode surface to
promote CO2 pathway
Adzic, R. R., Electrochim. Acta 2010, 55, 4331-4338
Markovic. N.M., Electrochim. Acta 2002, 47, 3707
1e- + 1H+C
HH3C
Pt
OHOH2
CH3CH2OH
Pt
1e- + 1H+
Pt
C
OH3C
Pt
COH3C
H
OH2
Pt
O
H
Pt
O
C
OH3C
+ H
H
C
OH3C
O
Pt
1e- + 1H+
-H2 at low
coord Pt
Pt
CHx
Pt
C
O
CO2
Pt
2OH
111
sites
O
HxC
Pt
C
Acetyl
AA
O
HxC
Pt
C
di-
Methanol Oxidation: Effect of pH, and Carbonates
Effect
of pH
Effect of
Carbonates
H+ OHˉ CO32-
Mobility
(cm2 V-1 s-1)
36.25 20.5 3.5
Infinite Diffusion
Coefficient (10-5
cm2/s)
9.3 5.3 0.9
Increasing pH or NaOH concentration increases MOR activity
Effect of carbonate is two-fold
Decreasing conductivity and increasing viscosity
Decreasing MOR activity by competitively
adsorbing with hydroxide anions
K. Scott et al, 2004, Electrochem. Acta, 49, 2443
K. Scott et al, 2003, J. Electroanal. Chem, 547, 17
Enhanced Electrocatalytic Processes: Polyvalent smart catalyst (TM) effect on C-C cleavage
Labeled Ethanol: C13 label study
0 2000 4000 6000
0.0
0.1
0.2
0.3
0.4
0.5
0 2000 4000 6000
time [s]
with co-catalyst
without cocatalyst
CO
2 f
ract
ion
0.67 V 0.77 V
time [s]
Clean Pt Pt-Ox
Grey = bulk
∆μ= μ(Pt-Ox) – μ(Pt-clean)
Atomic Level Picture of Electrocatalytic Pathways via
Determination of Specific Adsorption Sites
Proof of Pi-bonded ethylene pathway using synchrotron based in situ XANES on
Pt for EtOH oxidation in KOH solution with and without co-catalyst in solution
Presence of co-catalyst favors formation of pi-bonded Ethylene
type intermediate and hence enhances C-C Bond Cleavage
Experimental Signatures • Short dash (0.1 V) • Long dash (0.3 V) • Solid line (0.5V)
Comparison of theoretical and experimental signatures for oxidation of ethanol with and without co-catalyst for identifying species on the surface as a function of potential
pi-bonded ethylene is the mechanistically favored route to CO2
Acetyl species is mechanistically favored toward acetic acid route
Literature Survey on Alkaline Membrane Fuel Cell Performance
Pt-catalysts, H2/Atmospheric Air
Acta, 2010
Use of alcohol feed in the absence of KOH on the
anode leads to power densities of only ~10 mW/cm2.
Challenge lies at anode interface (vide infra)
Specific Adsorption of Quaternary Ammonium
Cations is the killer
i [A/cm2
geo]
0.0 0.2 0.4 0.6 0.8 1.0 1.2
EC
ell [
V]
0.0
0.2
0.4
0.6
0.8
1.0
1.2
PD
[m
W/c
m2
geo
]
0
50
100
150
200
250
Pt/C
FeTPP/C
H2/O
2 - 50oC
Thicker Electrodes with
Non-PGM Cathodes
Better Ionomer Solutions
Needed
Anode: PtRu on Toray Paper (4mgPtRu/cm2 + 1mgAS4/cm2)
Cathode: BASF 30% Pt/C on GDL (1mgPt/cm2, 28.5% AS4 + 1mgAS4/cm2)
Membrane: Tokuyama A201
Steep cell voltage loss right in the activation overpotential region
Direct Ethanol AMFC – PtRu Anode
Polarization Curves
50oC - 100% RH
Current Density [mA/cm2
geo]
0 20 40 60 80 100
EC
ell [
V]
0.0
0.2
0.4
0.6
0.8
1.0
0.25M KOH + 1M EtOH
1M EtOH
PtRu Anode/A201/Pt CathodePossible Reasons
1) Methanol Crossover
2) Poor Ionic Conductivity
3) Carbonate Formation
4) Specific Adsorption of NR4+
1) Loss in Surface Area
2) Electrostatic Effects
Ethanol Oxidation on Pt Anode at 50oC
Potential [V Vs RHE]
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Curr
ent
Den
sity
[m
A/c
m2
geo
]
-20
0
20
40
60
80
100
0.25M KOH + 1M EtOH
1M EtOH
50oC
20 mV/s
Cathode: H2
Significant overpotential for EtOH oxidation in the absence of KOH
Understanding the origin of this overpotential is the key towards improving performance
Cell Temp: 50˚C, H2 Feed on Cathode used as reference electrode, Anode Feed: 8ml/min
Ethanol Oxidation on Pt and PtRu Anodes
Ionic
Resistance
[Ohm]
Charge
Transfer
Resistance
[Ohm]
0.25M KOH +
1M EtOH/O2
0.109 1.31
1M EtOH/O2 0.591 16.85
Z' [Ohm]
0 2 4 6 8 10 12 14 16 18 20
Z''
[O
hm
]
0
1
2
3
4
5
6
7
0.25M KOH + 1M EtOH
1M EtOH
Presence of KOH
Decreases the charge transfer resistance
Increases the ionic conductivity
Concentration of Contaminant, mM
0 20 40 60 80 100 120
% L
oss
in
Cu
rren
t D
ensi
ty
-100
-80
-60
-40
-20
0
(CH3)4N+OH-
(CH3CH2)4N+OH-
(CH3CH2CH2)4N+OH-
(C6H5CH2)(CH3)3N+OH-
30% Pt/C
0.1 M KOH + 0.5 M MeOH
0.6 V vs. RHE
Percent loss in MeOH oxidation current
in 0.1M KOH electrolyte in the presence
of various ammonium group based
contaminants
Specific Adsorption of Quaternary Ammonium Ions
Indicates strong specific adsorption of NR4+
and its effect on interfacial electron
transfer
1. Loss of surface area (blocking effect)
2. Electrostatic effects (may hinder OH- transport across IHP)
E [V vs. RHE]
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4C
urr
ent
[A]
-6e-3
-4e-3
-2e-3
0
2e-3
4e-3
6e-3
(w/ ionomer at anode) - No KOH
(w/ ionomer at anode) - w/ 0.1M KOH
(No ionomer at anode) - No KOH
(No ionomer at anode) - w/ 0.1M KOH
0.750 V
0.985 V
0.633 V
approx. -235 mV shift in
presence of ionomer
Experiments with charged redox couples
(φ2)
φ2(E1)
φ2(E2)
Specific Adsorption of Quaternary Ammonium Ions
Current Hypothesis
Potential drop across the
electrochemical double layer
Unfavorable Electrostatics
in the Inner-Helhmholtz Plane
Future work needed in this
direction
ORR - General Mechanistic Considerations
Inner- and Outer-sphere electron transfer steps compete in alkaline medium
Higher stability of peroxide intermediate is the primary kinetic advantage in
alkaline media
ORR - Pyrolyzed Non-noble Catalysts
Redox potential shift of Fe2+/Fe3+ metal center upon pyrolysis
Defect sites on graphitic plane hosts active sites
Ethanol Oxidation - Smart catalyst (TM) redox couple
Exhibits the highest reported enhancement of ethanol oxidation.
Selectivity to produce CO2
Delta-Mu indicates pi-bonded ethylene intermediate
Future Prospects - Interfacial Challenges
Results with H2/O2 are promising
Understanding specific adsorption of quaternary ammonium ions at the
anode interface is key
Conclusions
Important Considerations for Future Electrocatalysis Research
• Need to unravel Anode Electrode Issues: Electrostatics, Reaction Centers for Hydride formation and Oxidation
• Need for Increase in site density for Non PGM reaction centers
• Role of liquid electrolytes from an interfacial perspective
• Interaction of ionomer with active site: Specific adsorption etc.
• Durability issues need to be determined in conjunction with the operating conditions and the choice of ionomer.
Acknowledgements
Funding Agencies