Modeling of Electrochemical Cells:Proton Exchange Membrane Fuel Cells

HYD7007 - 01HYD7007 01

Dept. of Chemical & Biomolecular EngineeringYonsei Universityy

Spring, 2011

Prof David KefferProf. David Kefferdkeffer@utk.edu

Course Information

Class Meeting Location and Times● GS Caltex building, 1st floor seminar room ● Wednesday 6:00 PM – 9:00 PM

Course Website● http://utkstair org/clausius/docs/fuelcells/index html● http://utkstair.org/clausius/docs/fuelcells/index.html

Instructor Information● Office YERC 174B● Office YERC 174B ● Office telephone: 2123-5748● email: dkeffer@utk.edu

Course Objective

Objective

The objective of this portion of the course is to understand the molecular-level structure and transport processes of Proton Exchange Membranes (PEM) fuel cells.

Organization and Scheduling

The course is organized into four parts:● Structure (May 11, 2011)● Water & Charge Transport (May 18 2011)● Water & Charge Transport (May 18, 2011)● Membrane Composition (May 25, 2011)● Polymer Dynamics (June 8, 2011)

Course Grades

Overall Course Grades● The total course grade is an average of the three grades for each

instructor.

Course Grade for this Portion ● Attendance: 20%● Homework Assignments: 30%● Homework Assignments: 30%● Final Exam: 50%

Homework Assignmentsi t 1 A i d M 11 2011 D M 18 2011 b i i f l● assignment 1. Assigned: May 11, 2011. Due: May 18, 2011, beginning of class.

● assignment 2. Assigned: May 18, 2011. Due: May 25, 2011, beginning of class. ● assignment 3. Assigned: May 25, 2011. Due: June 8, 2011, beginning of class.

Final Examination● Covers only this final third of the class● date, time and location: To be determined.

h i l i l l l l d t i l d l

Instructor: Prof. Keffer

Slide on experiment

chemical engineer, molecular-level process and materials modeler

Slide on experimentMinneapolis, MNUniversity of MNPh.D. 1996

Washington, DCNaval Res. Lab

SeoulYonsei Univ.Visiting Prof. 2010 2011

Kansas City, MO

Naval Res. LabPostdoc 1996-7

2010-2011

Knoxville, TNUniversity of TNAsst. Prof. 1998Assoc Prof 2004

Gainesville, FLUniversity of FLB S 1992

Assoc. Prof. 2004Prof. 2009

B.S. 1992

Modeling of Electrochemical Cells:Proton Exchange Membrane Fuel CellsProton Exchange Membrane Fuel Cells

HYD7007 – 01

Lecture 01. Overview of PEM Fuel Cell Structure

Dept. of Chemical & Biomolecular EngineeringYonsei University

Spring, 2011

Prof. David Kefferdkeffer@utk.edu

Lecture Outline

● Motivation● Macroscopic Structure of Fuel Cells● Structure and Properties of Nafion● Examination of Macroscopic Models

Sustainable Energy Cycles

inputsunlight

outputelectricity

H2OH2O

hydrogen fuel cycle

H2biologicalproduction hydrogen

H2hydrogen t t i lproduction

of hydrogen fuel cellsstorage materials

O2 O2

Sustainable Energy Cycles

H2 production H2 storage H2 conversion

H O Ph t l i i Al Materials discovery of novel nanoporous adsorbents with high capacity and fast

Understanding structure-property relationships in proton exchange membrane fuel cells to

H2O Photolysis in Algae and Cyanobacteria

charging aid design of next generation devices

● metal-porphyrin frameworks● decorated carbonfullerenes

Each task has significant challenges

Why Hydrogen Fuel Cells?

Big (perhaps premature) emphasis on hydrogen under the Bush administration (2000-2008)administration (2000 2008).

Originally Obama administration zeroed out the hydrogen budget. It was restored and some say that Secretary of DoE Steven Chu now

d it th i t k P d F l C lladmits the mistake. Program renamed as Fuel Cells.

There is no single silver bullet to the energy issue (except perhaps fusion). Many technologies must be explored.) y g p

Battery electric vehicle technology is closer to widespread implementation but pure BE vehicles have limited range. (30-40 miles)

Fuel cell vehicles would have much longer range (> 200 miles) but other issues (takes a while to “warm up”).

A fuel cell/battery electric hybrid may be eventual solution.

Understanding Structure/Property Relationships in Fuel Cells

Interdisciplinary Research

Understanding Structure/Property Relationships in Fuel Cells

synthesis characterization modeling

• neutron tomography• SEM/TEM• small angle x-

• quantum mechanical calculations

• classical molecular dynamics simulation

• novel fluorinated, sulfonated block co-polymers

small angle xray scattering• dielectric spectroscopy• proton

dynamics simulation • mesoscale modeling• mean field modeling

proton conductivity

0.10

0.12

0.14

DOW 858DOW 1084

Nafion

• precise control

0 5 10 15 20 25 30

/ S

cm-1

0.00

0.02

0.04

0.06

0.08

T = 303K

• precise control over architectural elements• two scales of morphology

Theory guides functionalization of next iteration of materials.

= [H2O] / [SO3H]morphology

Fuel Cell-powered vehicles

understanding starts at the

t l lquantum level

leads to high-fidelity

H2-powered autosbecome a reality

coarse-grained models

impacts fuelcell performance

improved nanoscale design of membrane/electrodeassembly

how fuel cells work: conceptual level

inputsH2 O2

proton exchange membrane

Pt alloycatalyst Pt catalyst

H+ H+

cathodeanode

e- e-

H2Oelectrical workoutputs

Overview of Structure

http://www.fueleconomy.gov/feg/fcv_pem.shtml

Overview of Structure

10 mm membrane/vaporcatalyst nanoparticle (gold)10 mm membrane/vaporcatalyst nanoparticle (gold)

A membrane electrode assembly from the macroscale to the molecular scale

10 mm

10 mminterface

ionomer film (blue)

catalyst nanoparticle (gold)carbon particles (gray)

10 mm

10 mminterface

ionomer film (blue)

catalyst nanoparticle (gold)carbon particles (gray)

~800 m

50 m

membrane/vapor/Ptinterface

~30 nm~800 m

50 m

~800 m

50 m

membrane/vapor/Ptinterface

~30 nm

polymer backbone

aqueous phasevapor phase

50 m10 m

anod

e

cath

ode

membrane/vapor/C

accessible wet catalystaccessible dry catalystisolated catalystburied catalyst

polymer backbone

aqueous phasevapor phase

50 m10 m

anod

e

cath

ode

50 m10 m

anod

e

cath

ode

membrane/vapor/C

accessible wet catalystaccessible dry catalystisolated catalystburied catalyst

accessible wet catalystaccessible dry catalystisolated catalystburied catalyst

carbon particlecarbon particle

a c

polymer electrolyte membranecatalyst layer (+ recast ionomer)

membrane/vapor/Csupport interface

carbon particlecarbon particle

a c

polymer electrolyte membranecatalyst layer (+ recast ionomer)

a c

polymer electrolyte membranecatalyst layer (+ recast ionomer)

membrane/vapor/Csupport interface

catalyst layer (+ recast ionomer)carbon fiber + carbon layer

catalyst layer (+ recast ionomer)carbon fiber + carbon layer

catalyst layer (+ recast ionomer)carbon fiber + carbon layer

Fuel cell issues

Pt is expensive.Gemini space program 28 mg/cm2

today <0.2 mg/cm2

today $200/kWtoday $200/kW

Goal $35/kW (electrode is 14% of cost)(from Partnership for a New Generation of Vehicles)of Vehicles)

Pt can be poisoned by COneed very pure H2 fuel

Part of the solution:Run at higher temperaturePt (and other metal catalysts) are

more active and less susceptible to B O t l J P S 2002more active and less susceptible to poisoning

BUT other parts of the fuel cell don’t work at high temperatures

Bar-On et al. J. Power Sources, 2002.

work at high temperatures.

proton exchange membranes are polymer electrolytes

Industry Standard Membraneproton exchange membranes are polymer electrolytes

industry standard: Nafion (DuPont)

sulfonic acid at end of side chainNafion (DuPont)

perfluorosulfonic acidend of side chainprovides protons

monomer backbone contains CF2.

side chain

CF2 = gray, O = red, S = orange, cation not shown.

Industry Standard Membrane

Membrane must be Hydrated

Kreuer et al., Chem. Rev. 2004.

Diffusion data

Kreuer et al., Chem. Rev. 2004.

Conductivity data

Kreuer et al., Chem. Rev. 2004.

High Temperature Fuel Cells

Running a fuel cell at higher temperatures has advantages but poses technological challenges.

Advantages:● requires less catalyst

catalyst more inherently more active at higher temperature because ● lessens the requirement on fuel purity because● lessens the requirement on fuel purity because

catalyst is less susceptible to CO poisoning○ at lower temperature, CO adsorbs irreversibly, occupies sites on the Pt catalyst, preventing those sites from participating in useful electrochemical reactions ○ at higher temperature the CO adsorption isotherm is shifted to the gas phase freeing○ at higher temperature, the CO adsorption isotherm is shifted to the gas phase, freeing up more of the catalyst surface to participate in useful electrochemical reactions

Technological Challenges:● requires advanced membrane technology that maintains a high conductivity at higher● requires advanced membrane technology that maintains a high conductivity at higher

temperatures, by either ○ retaining more moisture at higher temperature, or ○ having a higher conductivity at reduced moisture contents○ moving to a water free membrane○ moving to a water-free membrane

Macroscopic Modeling

Wang, Chem. Rev. 2004.

Macroscopic Modeling

M i d l i d b l diMacroscopic models use macroscopic mass, energy and momentum balances to predict fluxes.

Effective mass transfer coefficients and conductivities are required as inputs.

Wang, Chem. Rev. 2004.

Macroscopic Modeling

Example outputs from macroscopic models: flow field in the bipolar plate channels.

Wang, Chem. Rev. 2004.

Macroscopic Modeling

Wang, Chem. Rev. 2004.

Macroscopic Modeling

Modeling of Catalyst Layers (Electrode/Electrolyte Interface)

Wang, Chem. Rev. 2004.

Modeling Reviews

Weber & Newman, Chem. Rev. 2004.

Modeling Reviews

Weber & Newman, Chem. Rev. 2004.

Modeling Reviews

M d li f C t l t L (El t d /El t l t I t f )Modeling of Catalyst Layers (Electrode/Electrolyte Interface)

Weber & Newman, Chem. Rev. 2004.

Conclusions

In the development of next generation fuel cells, there are several points that become apparent:

● Optimization of membranes with high performance at higher temperatures requires an understanding of the molecular-level mechanism for water and charge transport through the membrane.● This understanding can guide synthetic chemists to develop new membrane● This understanding can guide synthetic chemists to develop new membrane materials with superior performance.● Lecture 02 in this course provides a picture of the current understanding of molecular-level structure of proton exchange membranes.

● The interface between the electrode and electrolyte (the catalyst layer) is the area of greatest ignorance.area of greatest ignorance.● Molecular level understanding of the structure of and transport through these interfaces would be very useful to design nanostructured interfaces with enhanced performance.

L t 03 i thi id i t f th t d t di f● Lecture 03 in this course provides a picture of the current understanding of molecular-level structure of the electrode/electrolyte interface in PEM fuel cells.