development of educational tools for pem fuel cellsjmfent/pemfc tutorial_final.pdfdevelopment of...

Development of Educational Tools Development of Educational Tools for PEM Fuel Cellsfor PEM Fuel Cells

Vijay Ramani, Ruichun Jiang, Harold R. Kunz, Vijay Ramani, Ruichun Jiang, Harold R. Kunz, James M. FentonJames M. Fenton

Department of Chemical EngineeringDepartment of Chemical EngineeringUniversity of ConnecticutUniversity of ConnecticutStorrs, CT Storrs, CT –– 06269, USA06269, USA

OutlineOutline

• Motivation• Experimental Techniques, equipment • Data Analysis• Undergraduate experiments• Summary

MotivationMotivation

• CT – Hub of fuel cell industry• Need for trained fuel cell engineers • Stimulate interest in graduate fuel cell research among

undergraduate students• Dual graduate – undergraduate course in PEM fuel cells:

- Developed at UConn – Chem. Engg. Dept.- Offered in Spring 2003- To be offered again at UTC Fuel Cells –

Spring 2004• Highlights from the course presented in this tutorial• Focus on experimental techniques and systematic

data analysis

ObjectivesObjectives

• Special emphasis placed on proton exchange membrane (PEM) fuel cells.

• Present fundamental principles of:- thermodynamics- electrode kinetics- mass transfer

as they apply to fuel cell engineering and data analysis

• Provide hands on fuel cell operating experience

Experimental TechniquesExperimental Techniques

Linear Sweep Voltammetry (LSV)Linear Sweep Voltammetry (LSV)

• Hydrogen gas passed through counter / reference electrode (anode) nitrogen passed through cathode

• Working electrode: fuel cell cathode subjected to a potential sweep from open circuit to 500 mV

• Sweep done using a potentiostat• Fixed sweep rate – 4 mV/s• Faradaic current monitored

Potentiostat and Frequency Response Analyzer – LSV and CV measurements

Mass transport limitMass transport limit• Above a certain E, reaction becomes mass transport

limitedFaraday’s law

J (H2) [mols/cm2-s] = il /n F (2.5 mA/cm2)

Crossover Current Density of NTPA MEAs (4 mv/s)

Potential (V)0.1 0.2 0.3 0.4 0.5

Cur

rent

Den

sity

(mA

/cm

2 )

0

1

2

3

4

5

normal

short1/R=1/Rm+1/Rs

Cyclic voltammetryCyclic voltammetry• Potential sweep experiment (similar to LSV)• Additional reverse sweep incorporated• Input function:

- start from E = E1- sweep up to E = E2 - sweep down, back to E = E1

• Performed using a potentiostat• Fixed sweep rate - ~ 30mV/s• Faradaic current monitored

Typical CV at a fuel cell cathodeTypical CV at a fuel cell cathode

Working Electrode Potential (V)

0.0 0.2 0.4 0.6 0.8 1.0

Cur

rent

(A)

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

t

0 V

2H+ + 2 e H2

H2 2H+ + 2 e; Oxidation as E becomes more positive

2H+ + 2 e H2; Reduction as E becomes more negative

Overall Reaction:

Input Function

Output (Response)

0 s

(Area under peak)

(scan rate)*(210µC/cm2-Pt)*(Pt loading)ECA =

E0.8 V

Load Box

Single Cell

Flow Loop

Polarization experimentsPolarization experiments

Re=.04 mOhms/cm2 Correction by Using Io/Ia=~4.76 Current vs Voltage Data

300.00

400.00

500.00

600.00

700.00

800.00

900.00

0.00 500.00 1000.00 1500.00 2000.00 2500.00Current Density (mA/cm2)

Volta

ge (m

V)

Air Actual

4%Oxygen Actual

Oxygen Actual

Data AnalysisData Analysis

Polarization in PEM fuel cellsPolarization in PEM fuel cells

Current Density (mA/cm2)0 5 10 15 20 25 30 35

Cel

l Vol

tage

(V)

0.0

0.5

1.0

0.0

0.5

1.0

Pola

rizat

ion

(V)

Ideal Potential

Region of Activation Polarization(Reaction Rate Loss)

Total Loss

Region of Ohmic Polarization(Resistance Loss)

Region of Concentration Polarization

(Gas Transport Loss)

Cathode Loss

Membrane Internal Resistance Loss

Anode Loss

]/)ln[()(ln llemOCV iiicRRiibaEE −−+−−−=

VEOHeHOCathode

VEeHHAnode

red

red

23.1;44:

0;22:

022

20

=→++

=+→

+

+

(1.23 V)

Reactions:

Membrane resistanceMembrane resistance• Determined by using techniques such as current

interrupt and AC impedance spectroscopy. • Data then corrected as follows:

• Recall that the voltage drop across a resistor is given by V = IR :

mFreeiR iREEm

+=

Result of correctionResult of correction

E (V)

10000 i (mA/cm2) [log scale]

0

1

1

Original curve

Corrected curve

Various Stages of Correction for Current vs Voltage Data

R2 = 0.995

R2 = 0.999

R2 = 0.9993

0.00

100.00

200.00

300.00

400.00

500.00

600.00

700.00

800.00

900.00

1000.00

1.00 10.00 100.00 1000.00 10000.00 100000.00

Current Density (mA/cm2)

Volta

ge (m

V)

Air Corrected for RmAir ActualAir Corrected for Rm+ReOxygen Corrected for RmOxygen ActualOxygen Corrected for Rm+Re4% Oxygen Corrected for Rm4% Oxygen Actual4% Oxygen Corrected for Rm+ReO2 Corrected for ILAir Corrected for IL4% Corrected for ILLog. (O2 Corrected for IL)Log. (Air Corrected for IL)Log. (4% Corrected for IL)

Electrode (cathode) ionic resistance (RElectrode (cathode) ionic resistance (Ree))

• Contribution of Re is crucial in optimizing the structure of the electrode

• Re lowered by:- increasing the electrolyte content of the

electrode, - designing better gas diffusion layers that

permit water retentivity in the electrode• Not possible to directly separate out the contribution of

Re

The current ratioThe current ratio

• The ratio of the currents at a given (membrane resistance corrected) voltage when oxygen and air are used at the cathode

• Can be shown to equal the oxygen concentration ratio (~ 4.8) for first order kinetics

• The current ratio (C.R.) can be used to extract Re - rearrange data until iO2 / iAir = 4.8 at a given voltage.

• Iterative procedure, best done using a spreadsheet

Effect of REffect of Re e correctioncorrection


R2 = 0.995

R2 = 0.999

R2 = 0.9993

0.00

100.00

200.00

300.00

400.00

500.00

600.00

700.00

800.00

900.00

1000.00

1.00 10.00 100.00 1000.00 10000.00 100000.00


Volta

ge (m

V)


E (V)

i (mA/cm2) [log scale]

1 10000 0

1Re corrected data – note that the

mass transport effects are not eliminated

Substrate DiffusionSubstrate Diffusion• Need to determine limiting current • May be done by guessing value for iL, and plotting

data as:

V vs.

• The value of iL that yields a linear data fit should be the true limiting current (since other losses are accounted for, Tafel behavior should be seen)

)/1/( liii −

Effect of substrate diffusion correctionEffect of substrate diffusion correction


R2 = 0.995

R2 = 0.999

R2 = 0.9993

0.00

100.00

200.00

300.00

400.00

500.00

600.00

700.00

800.00

900.00

1000.00

1.00 10.00 100.00 1000.00 10000.00 100000.00


Volta

ge (m

V)



1 10000 0

1Linearization obtained with

iterative il estimation

Reacting DiffusionReacting Diffusion

• If oxygen reduction reaction entirely activation controlled:

• Data should have a theoretical Tafel slope (70mV/decade at 80°C) – if 1st order B-V kinetics are followed

• Any deviation– due to uncorrected diffusion losses (e.g. in electrodes)

• These losses may be determined by estimating difference between theoretical Tafel line and corrected data

Effect of correctionEffect of correction

E (V)“Corrected data”


1 10000 0

1

Theoretical Tafel slope assuming 1st order B-V

kinetics


R2 = 0.995

R2 = 0.999

R2 = 0.9993

0.00

100.00

200.00

300.00

400.00

500.00

600.00

700.00

800.00

900.00

1000.00

1.00 10.00 100.00 1000.00 10000.00 100000.00


Volta

ge (m

V)


Correction was not performed for this data set

RecapRecap

• Original data from test stand• Correct for membrane resistance• Correct for electrode ionic resistance – current

ratios• Estimate limiting current – air or 4% O2 data• Correct for substrate diffusion – iterative –

determine value of iL that yields linear fit• Deviation between corrected curve and

theoretical Tafel behavior: losses due to diffusion in electrode

Typical resultsTypical resultsRe=.04 mOhms/cm2 Correction by Using Io/Ia=~4.76 Current vs Voltage Data

300.00

400.00

500.00

600.00

700.00

800.00

900.00

0.00 500.00 1000.00 1500.00 2000.00 2500.00


Volta

ge (m

V)

Air Actual

4%Oxygen Actual

Oxygen Actual


R2 = 0.995

R2 = 0.999

R2 = 0.9993

0.00

100.00

200.00

300.00

400.00

500.00

600.00

700.00

800.00

900.00

1000.00

1.00 10.00 100.00 1000.00 10000.00 100000.00


Volta

ge (m

V)


Undergraduate ExperimentsUndergraduate Experiments

• Solar panel / Water electrolyzer / Fuel cell system

• Demonstration of Renewable Hydrogen Economy

• Students – obtain electrolyzer characteristics, fuel cell polarization using manual potentiostat

• Efficiencies calculated at each stage {assuming incident light is free (solar)}

Solar panel / Water electrolyzer / Fuel cell system – potentiostat not shown

Cost: $239.00Manufacturer http://www.h-tec.com/Retailer http://www.fuelcellstore.com

SummarySummary• Systematic analysis of data – yields useful information

about overpotentials distribution• Very useful in developing better materials and structures

– helps highlight target areas• All parameters extracted have physical significance –

unlike multi parameter data fitting.• Solar panel / Water electrolyzer / Fuel cell system – used

to demonstrate hydrogen economy to undergraduate students

• Hands on experience provided• Excellent educational tool for undergraduate and

graduate fuel cell education

Other fuel cell toysOther fuel cell toys

Fuel Cell Car & Experiment KitRetail Price: $150.00

http://www.thamesandkosmos.com/

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