getting started with electrochemistry in polymer electrolyte membrane fuel cells (pemfc):

Electrochemistry in membrane fuel cells1

Getting started with electrochemistry in polymer electrolyte membrane fuel cells (PEMFC):

Francois LapicqueLaboratoire des Sciences du Génie Chimique, CNRS –ENSIC, Nancy

• Background of electrochemical phenomena in FC• Features of electrochemical reactions• Transport and transfer

• Available electrochemical methods for their investigation

Presented by: Dr Bradley [email protected]


½ O2 + 2e + 4 H+ H2O

H2 H+ + 2e Chargeeg. Engine

O2/air

H2

Electron flux in the external circuit

3/2 O2 + 6e + 6 H+ 3 H2O

CH3OH + H2O CO2 + 6H+ + 6e Chargeeg. Engine

O2/air

Methanol

Electron flux in the external circuit

PEMFC

H2 + ½ O2

H2O + H

DMFC

CH3OH + 3/2 O2

CO2 + 2 H2O + H

AnodeMembrane

Cathode

AnodeMembrane

Cathode

Operation principle of membrane fuel cells


Particularités de la réaction électrochimique

Adsorption

Desorption

Charge transfer

Transfer to the electrode

Transport

Current :Electrons Current: Ions

Heterogeneous process involving the exchange of charges

(Chemical Processes)

Anode: A B + e

Cathode:C + e D

Specific features of electrochemical reactions


ss gradi

Specific features of electrochemical reactions (C’td)

Faraday’s law

Ae FNnI

A + ne e - → B

AeAe Frn

A

FNn

A

Ii

e

Ae Frni

Existence of several reactions

Current yield

Ohm’s lawConsequences

Ohmic drop : linked to Joule effect

• Reduce the electrode gap• Improve the electrical conductivity of the medium

To be minimised


H2

O2

H2

O2

Outlet

Feed

Externalplate

Bipolarplate

Backing

Membrane-electrodeassembly

PEMFC:Electrolyte = Conducting polymer• Reduce the membrane thickness• Improve the electrical connections

Split view of a polymer electrolyte membrane fuel cell


Active layer

Activelayer

BackingBacking

Cathode Anode

Membrane =

HydratedConductin

g gel Carbon materials- conducting- hydrophobic

Carbone 30 nmPlatinum 2 nm 50-150 m 20 m 300 m

Electrodes and membrane

Pt-Ru catalyst deposited on XC-72X. Xue et al. Electrochem. Comm. 8 (2006) 1280


H2 2H+ + 2e

½ O2

+ 2 H++ 2e H2O

O2 transport by convectionand diffusion

Cathode AnodeMembrane

WaterFeed ?

Liquid water Formation?

Migration H+

Electroosmosis (H2O)

Diffusion of H2O

Diffusionto Pt

Diffusionto Pt

ElectronsElectrons

Water management(excessive) Drying Flooding

Heat Heat

H2 transport by convectionand diffusion


- DIFFUSEUR : (180 µm)matrice poreuse engraphite (Toray paper)

+ agent hydrophobe (PTFE)

- ELECTRODE : (50 µm)mince couche en matériaucarboné (Vulcan XC-72R)

+ particules de platine

- MEMBRANE : (125 µm)échangeuse de protons(Nafion)

Thin layer of carbonMaterials (Vulcan XC-72R+ platinum particles

(Proton exchanging)e.g. Nafion

DIFFUSION LAYER (backing)Graphite porous structure (e.g. Toray paper)+ hydrophobic agent (PTFE)


Ohmic behaviour of PEMFC’s

* Membrane resistance

Importance of hydration

* Other resistance sources :• Electrodes• Backings• Bipolar plates• Electrical connectors• Current leads

Nafion 117 (Wöhr, 2000)

0

10

20

30

0 10 20 30

Nombre : n(H2O)/site sulfonate

Con

duct

ivit

é (S

m-1)

R < 0.3 cm2


ss gradi

Calculation of the membrane resistance

Area S

Thickness eI

I

Ohm’s law:

1-D modelDemonstrate :

S

e

IR

1

Calculate R for Nafion 112, 115 et 117 with S=100 cm2 and =0.1 S cm-1

Calculate the ohmic drop for current density at 0.1, 0.3 et 1 A cm-2


C

R

Time constant of a capacitor and a resistor in series

Calculation of the equivalent complex impedance

2221

1.

CR

jRCRZ

Time constant: RC

C: double layer capacitance (see above). 30 µF cm-2

Calculation of the time constant in two cases: Flat electrode plane, S=100 cm2

Electrode of PEMFC, S=100 cm2, =200


Thermodynamics and theoretical yields of PEMFC’s

Uth, thermoneutral voltage

Uth = - H / nF

Urev, reversible voltage

Urev = - G / nF

Theoretical yield th

th = G / H

PEMFC DMFC

n 2 6

H (kJ/mol) -285.83 -726.51

G (kJ/mol) -237.13 -703.35

Uth (V) 1.481 1.229

Urev (V) 1.229 1.215

th 83.0% 96.8%


S

FnT

U

eP

.10

0

00 .

1S

FnT

E

eP

V

FnP

U

eT

.10

0

00 .

1V

FnP

E

eT

Variations with temperature Variations with pressure

TP

Atm

dTSF

dPVF

AtmUPTU15.2981

00 .2

1.

2

1)1,15.298(),(

Present case: Water formation from O2 and H2

OH

OH

P

PP

F

RTTU

2

22

2/1

0 ln2

15.298.00085.0229.1


FC cell voltage at zero current: the real case

E0, Zero current voltage << Voltage predicted by the thermodynamics. Why ?

1- Oxygen reduction: slow processH2O2 is an inetermediate, with E(H2O2 /H2O)=0.68 V

2- Presence of Pt oxides, shift of the equilibrium potential

3- Existence of an internal current caused by hydrogen diffusion through the membrane

followed by combustion at the cathode H2 + ½ O2 H2O

Internal current density (cross over), in = proport. Flux of H2 diffusionPotential variation proport. to Ln(in)

Usually, E0 = 0.9 - 1.04 V


Kinetics of electrochemical processes

Butler-Volmer’s model

A + e B Model assumptions:

• Reversible reaction• One electron exchanged• Overall process controlled by charge transfer rate

Development of the model: theory of the activated complex between A et B

Expression for the current density i versus the overpotential = E - E0

RT

F

RT

Fii

)1(expexp0

Exchange currentdensity

Charge transfer coefficient


-200

-150

-100

-50

0

50

100

150

200

250

300

350

400

-0.3 -0.2 -0.1 0 0.1 0.2 0.3

Overpotential (V)

Cur

rent

den

sity

(A/m

2)

1 A/ m2

10 A/ m2

Example = 0.5, i0 variable

Linear part

Exponential part(irreversible) : Tafel

Tafel’s law for large enough

=a+blog(i)

Kinetics of electrochemical processes (C’td)


Electrode reactions: Hydrogen oxidation

Platinum : Excellent catalyst

« Easy reaction »

Volmer-Tafel’s model :

2 Pt + H2 2 PtH)ads Slow process

2H2O + 2PtH)ads 2Pt + 2H3O+ + 2e Fast process

1

RTF2exp

C

Cii

0H

H0

2

2

Currentdensity

Overpotential

+ 30 mV

i 10 i


Platinum : One of the less worse catalysts Overall slow reaction Kinetics and mechanism : Pt or PtO2 ?

* Potential < 0.8 V (High cd)Pt + O2 PtO2)ads Fast process

PtO2)ads + H+ + e PtO2H)ads Slow process

PtO2H)ads + 3 H+ + 3e 2H2O + Pt Fast

+ 120 mV i 10 i

* Potential > 0.8 V (Low cd) PtO2

+ 60 mV i 10 i

Electrode reactions: Oxygen reduction


Charge transfer resistance , Ract

iSiSI

Ract

1.

1 bi

RT

Fii expexp 00

ibSRact ..

1

T=60°CS=100 cm2i=0.5 A/cm2

b=17.4 V-1 (56 mV/decade) and Ract = 1.13 m

C

Ract

Calculation of the time constant Ract.C


Case of high current densities: mass transfer can become rate-controlling

CAS

CAb

i Electrolyticmedium

Electrode

CAS

CAb

i

CAS

CAb

i Electrolyticmedium

Electrode

As

Abd C

C

F

RTln

)1(

Existence of an additionaloverpotential

The overpotential is the sum of the charge transfer overpotential (Butler Volmer)and the concentration overpotential d

More complex relationship between i and



-100

-80

-60

-40

-20

0

20

40

60

80

100

-0.3 -0.2 -0.1 0 0.1 0.2 0.3

Overpotential (V)

Cur

rent

den

sity

(A/m

2)

1 A/ m2

10 A/ m2

100 A/ m2

d: depends on mass transfer rate (diffusion and convection)

Whentends to infinite, CAs = 0 and i tends to iL, limiting current density

iL=96 A/m2Example : = 0.5



Control by mass transfer phenomena in FC’s

The involved phenomena

Gas Convection (bipolar plates, backing)Diffusion (backing, active layers)Knudsen diffusion (active layers)

Water Transport through the membrane

Sharper problemsFor dilute reacting gases(air, reforming hydrogen)

Problems raised by liquid water:Flow hindrance in the various parts: lower transfer rates

i(lim) = 0.5 – 2 A cm-2


Cell voltage

Anode aCathode c

Ohmicdrop Separator

Ec

Ea

E0,c

E0,a

a+da

c+dc

Anode aCathode c

Ohmicdrop Separator

Ec

Ea

E0,c

E0,a

a+da

c+dc

Usual reactors

Fuel cells

IREEU edcdacacac 00

IREEU edcdacacac 00

For usualelectrochemical reactors


Available voltage in PEMFC’s

Ohmic drop

Diffusion control

Revrsible voltage Urev = -G/2F

Zone 2Zone 3

Hydrogen cross-over, PtO2, H2O2 etc.

Electrochemicalactivation

Cell voltage (V)

Current density (A/cm2)

Zone 1

Urev

Zero current voltage


0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 2 4 6 8 10Current / A

Cell

volt

age /

VExp. 2

Exp. 3

Exp. 5

Exp. 6

Exp. 7

Experimental data

Dry air

Humid H2

Humid

air

Example of i-E curves


Dynamics of diffusion processes

Transient Fick’s law, 1-D

2

2

x

CD

t

C

Characteristic timeD

td

2

, characteristic dimensionThickness of the Nernst’s filmThickness of the electrode?

10 µm

D, diffusion coefficient 10-10 m2/s (in liquids or in the gel)

std 1


Technology of electrochemical cells

I

I

I

I

4 x I

Uc

Uc

I

I

I

I

4 x I

Uc

Uc

I5 x Uc

Uc

+ - + - + - + - + -

I5 x Uc

Uc

+ - + - + - + - + -

Electrical connection with monopolar electrodes

Series

Parallel

Selection of the connection: * Significance of energy losses in the E-converter* Avoid too large currents and low voltages!!


Electrochemical methods for FC investigations

Fuel cellCurrent

Voltage

Voltage

Current

Steady-state techniques Fixed currentLow-rate scanning (of potential or current)

Transient methodsHigh-rate scanningImpedance spectroscopyCurrent step

Frequency range: 50 kHz – 10 mHz

Interpretation

Estimation of the ohmic drop

In most cases:No reference electrodes


Impedance spectroscopy• Principle

Tension

Courant

u+u

i+i

tcosuu)t(u

t cos ii)t(i

) t ( cos î

t)( cos ûZ

Complex variable

– Varying the frequency (10kHz to 10mHz)– Plotting data: Nyquist (-Z’’ vs. Z’), or

– Bode (|Z] and vs.

– Modelling using equivalent circuits or various balances

Current

Voltage


0

5

10

15

20

25

0 5 10 15 20 25

Z' (ohm .cm ²)

- Z

" ( o

hm

.cm

²)

5 kHz10 mHz

Tension

Q

Equivalent electrical circuit

Ract

Rm

Response of the electrodes

<1 Hz100 Hz


Equivalent electrical circuit: a simple case

2221

1.

CR

CjRRRZ

t

ttm

Tension

C

Rt

Rm

infinite Z = Rm

= 0 Z = Rp = Rm + Rt

-Z ’’

Z’

Rm Rt

Rp

=1/(RtC)


-0.01

0

0.01

0.02

0 0.02 0.04 0.06 0.08Z' (ohm)

Z"

(ohm

)

FCO39

Rs (ohmic) Rp (polarisation)

Diff usion

Electrochemical impedance: equivalent circuit

Rt (charge transfer)

In most cases,only one loopcan be observed.

Tilted loop in most cases:CPE

ZCPE,c

Rct,cWdiff,c

RΩ

Rct,a

ZCPE,a


Some fuel cell references

• Larminie, J. and Dicks, A. (2000) Fuel Cell Systems Explained, Wiley, England.

• Vielstich W (2003) Handbook of Fuel Cells (4 volumes), Wiley, England.

• Grove, W. (1839) On voltaic series and the combination of gases by platinum, Philosophical Magazine Series 3 14:127 – 130.

• Fuel Cell Today www.fuelcelltoday.com [funded by Johnson Matthey, worlds largest producer of Platinum, including that used by Mr Grove, producer of catalyst and MEAs]

http://www.fuelcelltoday.com/

getting started with electrochemistry in polymer electrolyte membrane fuel cells (pemfc):

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