Electrochemistry Basics - Lehrstuhl für Technische ... AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt p. 65 Principle of Fuel Cells Direct electrochemical conversion

2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt p. 64

Electrochemistry Basics

- electrochemical cells & ion transport

- electrochemical potential

- half-cell reactions

Lithium Ion Batteries (LiBs)

- battery materials

- application of batteries

- “post-LiBs”

Fuel Cell Basics & Applications

- fuel cell types and materials

- basic electrocatalysis

- H2 reduction & O2 reduction kinetics

- transport resistances


Principle of Fuel Cells

Direct electrochemical conversion of chemical energy into electrical energy:

H2 2 H+ + 2e-

“spatial separation” of oxidation and reduction on electrocatalysts, e.g.:

0.5 O2 + 2 H+ + 2e

- H2O

anode (oxidation):

cathode (reduction):

overall reaction: H2 + 0.5 O2 H2O

simplest configuration:

Pt

H2 O2

Pt

2H+ 2e-

H2 ½ O2

2e-

H2SO4

H2O

load e

- e

-

catalysts to enable half-cell reactions

(electronically conducting)

ion-conducting electrolyte

(electronically non-conducting)

effective three-phase interface

(reactant, ion-conductor, catalyst)


Fuel Cell Types generally distinguished by type of electrolyte and conducting ion:

high-temperature fuel cells (SOFC, MCFC):

temperature required to obtain sufficient electrolyte conductivity

ability to oxidize CO and to use CH4 reactant via internal reforming

low/medium-temperature fuel cells:

temperature maximum dictated by H2O-loss (no conductivity without H2O)

require clean H2: CO-tolerance of 1% for PAFC and <<100ppm for others;

CO2-free O2/air for AFCs (carbonate formation: pH change, precipitation)

fuel cell type temp. anode reaction conducting ion cathode reaction

SOFC(Solid Oxide FC)

1000°C H2 + O-2

H2O + 2e- O

-2

(Y-stabilized ZrO2)½ O2 + 2e

- O

-2

MCFC( Molten Carbonate FC)

650°C H2 + CO3-2

H2O + CO2 + 2e- CO3

-2

(alkali carbonates)½ O2 + CO2 + 2e

- CO3

-2

PAFC( Phosphoric Acid FC)

200°C H2 2H+ + 2e

- H+

(H3PO4)½ O2 + 2H

+ + 2e

- H2O

PEMFC

( H+ Exchange Membrane FC)

80°C H2 2H+ + 2e

- H+

(solid polymer)½ O2 + 2H

+ + 2e

- H2O

DMFC( Direct Methanol FC)

80°C CH3OH + H2O CO2 + 6H+ + 6e

- H+

(solid polymer)1.5 O2 + 6H

+ + 6e

- 3H2O

AFC( Alkaline FC)

80°C H2 + 2OH- 2H2O + 2e

- OH-

(KOH)½ O2 + H2O + 2e

- 2OH

-


H2 for Fuel Cells H2 supply for fuel cell systems:

- stationary systems: - reforming of CH4 (natural gas), petroleum, gasoline

- automotive systems: - on-board reforming of methanol (CH3OH),

gasoline (avg. molar composition CH1.5), or

Diesel fuel (avg. molar composition CH2)

- stored H2 (liquid, high-pressure, chemical hydride)

- portable systems: - direct electrooxidation of methanol (DMFC)

energy (storage) density of various fuels:

kWh/kg kWh/l

*) based on the density of liquefied gas

(from: P. Piela and P. Zelenay, Fuel Cell Review 1 (2004) 17)

(from: A. Bouza et al., DOE Annual

Hydrogen Program Review (2004))

practical H2 storage densities liquid hydrocarbons

made on-board hydrocarbon reforming in cars attractive

H2 tank systems


catalyst cost & supply (100kW car):

currently: 0.5 gPt/kW 50gPt/car at $50/gPt-as-catalyst $25/kW ($50/kWFC-system

target)

long-term: <0.1gPt/kW <10gPt/car with current automotive Pt use: >15 million cars/year

catalyst durability:

1500 hours*) vs. 6000 hour target carbon-support corrosion & Pt-dissolution

advanced catalysts & controls

H2-Fuel Cell Electric Vehicles (FCEVs)

GM H2-FC (2008): 500 km (70 MPa H2)

meets vehicle range target

refueling in < 5 minutes

*) DOE test fleet data: K. Wipke, S. Sprik, J. Kurtz, J. Garbak, in: Handbook of Fuel Cells

(eds.: W. Vielstich, H.A. Gasteiger, H. Yokokawa), Wiley (2009): vol. 6, pp. 893.

advanced catalysts required: ultra-high activity Pt catalysts or non-Pt catalysts


Thermodynamic Fuel Cell Efficiency

fuel energy content released by combustion:

DHR = DHf

H2O – [DHfH2 + 0.5DHf

O2 ]

(enthalpies of formation from thermodynamic tables)

Wheat produced depends on the state of water:

DHRH2O(liquid) = -285.8 kJ/mol Higher Heating Value (HHV)

DHR(101kPa vapor) = -241.8 kJ/mol Lower Heating Value (LHV)

Wheat = DHR

for H2 + 0.5 O2 H2O :

at 25°C, pH2 =pO2 = 101.3 kPaabs:

DHRH2O(liquid) = -285.8 kJ/mol – [0 kJ/mol + 0 kJ/mol) ] = -285.8 kJ/mol

DHRH2O(101.3kPa vapor) = -241.8 kJ/mol – [0 kJ/mol + 0 kJ/mol) ] = -241.8 kJ/mol

thermodynamic fuel cell efficiency (usually based on HHV):

th = DGRH2O(liquid/vapor)/DHR

H2O(liquid) = -DGRH2O(liquid/vapor)/285.8 kJ/mol

th = 237.1 kJ/mol / 285.8 kJ/mol = 83.05%

for a H2/O2 fuel cell at 25°C and pH2 = pO2 = 101 kPa producing H2O(liquid) :


Dependence of th on Different Fuels

Gibbs-Helmholtz relation:

if DSR < 0 (decrease of number of moles):

th = 1 - TDSR(T)/DHR

(T) <100%

Wheat,rev = -TDSR(T) is released

(applies for most fuel cell reactions)

DGR(T) = DHR

(T) - TDSR(T)

th for most fuel cell reactions ranges from 80 to 100%

(W. Vielstich, in: Handbook of Fuel Cells (eds.: W. Vielstich, A. Lamm, H.A. Gasteiger), Wiley (2003): vol. 1, chapter 4, p. 26)


Overall Efficiency of Fuel Cells

in actual fuel cells: Ecell << Erev due to resistive, kinetic, and mass-transport losses

Eth(HHV) DHR(H2O liquid)/(2F) = 1.48 V

to evaluate Wheat formation, it is convenient

to define a thermal-equivalent voltage, Eth :

voltage = Ecell/Erev = (i)

Eth(LHV) DHR(H2O vapor)/(2F) = 1.25 V

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 0.5 1 1.5 i [A/cm2 ]

Ecell [

V]

Eth(HHV)

H2/air (s=2/2) at 80C, 100%RH,

150kPaabs

Eth(LHV)

Ecell

Erev

Wheat(H2O liquid) Wheat(H2O vapor)

Welectrical

FC = th voltage = Ecell/Eth(HHV) = Ecell/1.48V

heat formation in actual fuel cells:

if water is produced in liquid form:

Wheat = i (Eth(HHV) – Ecell )

if water is produced in vapor form:

Wheat = i (Eth(LHV) – Ecell )

the ratio of Wheat/Welectric as i

heat rejection requirement

increases with power density


Proton Exchange Membrane Fuel Cell Materials


H2/Air PEMFC Performance Model

Ecell = Erev – iRW(RH) – hHOR – | hORR | – iRH+,an&ca (RH) – htx,O2(dry) – htx,O2(wet)

Erev : thermodynamic voltage

RW : RW-membrane(RH) + RW-el purely Ohmic resistances

hHOR , hORR : H2 oxidation and O2 reduction kinetic losses

RH+,an&ca(RH) : electrode H+-conduction resistance

h tx,O2 : O2 diffusion through H2O-free DM at <100% local relative humidty (RH)

htx,O2(wet) : additional O2 diffusion loss in H2O-filled pores (>100%RH)

determine performance gaps via quantification of each term


Membrane Electrode Assembly

Diffusion Media (DM)

Diffusion Media (DM)

Bipolar Plate (BP)

Bipolar Plate (BP)

PEMFC Stack Single-Cell Repeating Units

e--conducting plates

H2 & air distribution

via flow-fields

gas diffusion layer

channel-to-land

distribution (gas, e-)

MEA: electrodes on

H+-conducting PEM

full-size PEMFC stacks: - 100’s of single cells

- MEA active areas of 200 to 800 cm2

20-40 cm

1-2 mm


Single Cell Assembly & Diffusion Medium Structure

M.F. Mathias et al., in: Handbook of Fuel Cells; Wiley, v.3 (2003)


Proton (H+) Exchange Membrane

from: K.D. Kreuer, in: Handbook of Fuel Cells: Fundamentals,

Technology & Applications (eds: W. Vielstich, A. Lamm, H.A.

Gasteiger), Wiley (2003): vol. 3.

ionomeric membranes for PEMFCs (25 mm) and DMFCs (100 mm)

- H+-donating sulfonic acid groups –SO3-

& organic/aqueous phase-segregation

hydrophobic backbone

C.K. Mittelsteadt & H. Liu, in:

Handbook of Fuel Cells: Fund.,

Techn. & Appl. (eds: W. Vielstich,

H.A. Gasteiger, H. Yokokawa),

Wiley (2009): vol. 5.

RH+(membrane) & RH+(electrodes) are strong functions of RH


Sulfonic Acid Ionomers – l vs. RH

Dowex 50: ion-exchange resin made of 4% cross-linked polystyrene divinyl benzene.

BPSH 40: 2 mil 40% randomly sulfonated biphenol sulfone provided by James McGrath, Virginia Tech.

700 EW PFSA: 1 mil membrane with similar structure to Nafion.

Nafion 112 : 2 mil extruded membrane.

PAEK triblock: 1 mil triblock polyaryl ether ketone with a sulfonated middle block from PolyMaterials, Germany.

up to 80%RH:

same l vs. RH for

very different ionomers

water-uptake of various

sulfonic acid ionomers at 80°C:

at >80%RH (high l):

l-value now strongly

depends on ionomer

0

1

2

3

4

5

6

7

8

9

10

0 20 40 60 80 100

Relative Humidity %

Sulfuric Acid

Crosslinked 4% DVB Dowex 50 (Pushpa)

BPSH 40% (McGrath) GES Measurement

700 EW PFSA

Nafion 112

sulfonated-nonsulfonated multi-block

l n

H2O

/n-S

O3H

from: C. Mittelsteadt & H. Liu, in: Handbook

of Fuel Cells (eds.: W. Vielstich, H.A. Gasteiger,

H. Yokokawa), Wiley (2009): vol. 5, p. 345.


Catalyst Carbon-Supports Typical Pt/C Catalyst:

pri

mary

ag

glo

mera

te

primary C-particles (20-40nm)

high “structure” of primary carbon agglomerates

leads to highly porous packing

primary agglomerates cannot be “broken”

by typical shears/pressures

agglomerates breakage occures during

carbon corrosion

SEM picture: Jim Mitchell, Ted Gacek, and Mike Budinski (GM Fuel Cell Activities)

two primary functions:

- high surface area to support catalyst

nanoparticles (e.g., Pt)

- high-structure material,

creating highly porous electrodes


PEFC Electrode Composition & Structure

C / Pt / ionomer 1 / 1 / 1 mass-ratio morphology via high-structure carbon-blacks

40 nm

46% Pt/carbon

40 nm

46% Pt/carbon

60% void volume & dpore 50-100nm

from: Z.Y. Liu, B.K. Brady, R.N. Carter, B. Litteer,

M. Budinski, J. Electrochem. Soc. 155 (2008) B979.

membrane

H+

e-

O2

O2 + 4H+ + 4e- 2H2OPt

Diffusion

Medium

membrane

H+

e-

O2

O2 + 4H+ + 4e- 2H2OPt

O2 + 4H+ + 4e- 2H2OPt

O2 + 4H+ + 4e- 2H2OPt

Diffusion

Medium

ionomer-film model consistent

with SEM & AC-impedance data


Pt Dispersion: m2/gPt, roughness factor, ... face-centered cubic structure (fcc) of Pt: 1 Pt atom at each cube corner & face center

(100) surface (“number 5”): 1.28·1015 atoms/cm2 = 2.13·10-9 mol/cm2 = 205mC/cm2

(110) surface (“number 6”): 0.92·1015 atoms/cm2 = 1.53·10-9 mol/cm2 = 147mC/cm2

(111) surface (“hexagonal”): 1.5·1015 atoms/cm2 = 2.49·10-9 mol/cm2 = 240mC/cm2

Coulombs from cyclic voltammetry divided by 210mC/cm2 : cm2Pt surface

specific surface area: cm2Pt/gPt m2/gPt - from 30-120m2/gPt for Pt/C (catalyst property)

roughness factor: 0.4mgPt/cm2MEA · 60m2

Pt/gPt = 240cm2Pt/cm2

MEA

(note: 210mC/cm2Pt = 2.04 nmolPt/cm2

Pt 235m2Pt/gPt using MPt =195.7 gPt/molPt)

(100)-face (110)-face (111)-face

average: 197mC/cm2 commonly used for polycrystalline Pt: 210mC/cm2


Pt and Pt-Alloys on Carbon-Supports

supported Pt crystallites: cubo-octahedra:

PtMo/C

m2/gPt vs. dPt for spherical geometry approximation: m2/gPt 6/(dPt rPt)

figures courtesy Lawrence Berkeley Lab. (P. Ross, N. Marković)


-40

-30

-20

-10

0

10

20

0.0 0.2 0.4 0.6 0.8 1.0 1.2 E/V [RHE]

i [ A

/g P

t ]

thin-film GC in 0.1M HClO4

50cm2 MEA w. 63 sccm N2

In-situ cathode CV's at 20mV/s and 25°C:MEA: H2(500sccm) / N2 (62sccm); both overhumidified

RDE: N2 (1000sccm) in 0.1M HClO4

Catalyst Characterization: In-Situ Surface Area

• H-adsorption/desorption on RDE (13mgPt/cm2) or in MEA (0.4mgPt/cm2): “H-titration” of Pt

• State-of-the-art 47%wt Pt/HSC (TKK): 92/79m2/gPt (RDE/MEA) vs. 235 m2/gPt theoretical limit

• H-adsorption/desorption is independent of H2 partial pressure, H2-evolution is not

Nafionfilm

Glassy-Carbon(RDE)

Catalysts

~6mm

~1µm

RDE

MEA

N2 , W

ork

ing

Ele

ctro

de

H2 , C

ounte

r/

Refe

rence

Ele

ctro

des

Pt-Had Pt + H+ + e-

Pt + H+ + e- Pt-Had

2H+ + 2e- H2

Pt + H2O PtOHad + H+ + e-

PtOHad + H+ + e- Pt + H2O

H2 2H+ + 2e-







- battery materials


- “post-LiBs”







Kinetics vs. Thermodynamics

deviation from thermodynamics when drawing a current

kinetic and ohmic limits for PEMFCs:

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 i [A/cm2

MEA]

E [

V]

Erev 1.18V for H2/O2 at 101kPa & 80C

Erev

Ecathode

Eanode

DEohmic

EPEMFC = Ecathode - DEohmic - Eanode

hcathode

assumptions:

- pure H2/O2 at 101 kPaabs & 80C

- 0.4 mgPt/cm2 on cathode (60 m2/gPt)

- 0.1 mgPt/cm2 on anode (60 m2/gPt)

- 25mm membrane with 0.1 S/cm

- contact resistance of 0.03 Wcm2

- no mass-transport resistances

significant kinetic losses for the ORR

major scientific challenge!


Electrocatalysis heterogeneously catalyzed electrochemical reactions are a complex sequence of possibly

many steps: adsorption of reactants, desorption of products, solvation, etc.

in this sequence of processes, the rate-determining steps (rds) may be different on

different electrocatalyts

noO + ne- nRR ; Erev(O/R)


Electrokinetics

h

h

TR

F

TR

F

ccT

ca

ROeerfii ),,(0

the Butler-Volmer equation is generally used to describe the overpotential, temperature,

and reactant concentration dependence of the current of an electrode reaction *) :

*) for derivations/details, e.g.: [1] A.J. Bard & L.R. Faulkner, Electrochemical Methods, John Wiley & Sons (1980); [2] J. O’M. Bockris

& A.K.N. Reddy, Modern Electrochemistry, Plenum Press: (1970); [3] J.S. Newman, Electrochemical Systems, Prentice Hall (1991).

definition according to [2, 3]:

(used throughout these slides)

ianodic ( > 0 ) icathodic ( < 0 )

noO + ne- nRR ; Erev(O/R)

with:

i0(T,cO,cR) [A/cm2real]: exchange current density, a kinetic reaction rate constant, which

depends on the specific electrocatalyst (note: i = i0 at h = 0)

rf [cm2real/cm2

electrode]: electrode roughness factor, relating the real surface area of the

catalyst (e.g., BET, etc.) to the electrode’s geometric area

a, c [dimensionless]: anodic/cathodic transfer coefficients, describe the energy barrier

symmetry and the numberelectrons in the rds (a,c = 0.5 2.5 [2])

other constants: F is the Faraday constant (96485 As/mol), T is temperature [K],

and R is the gas constant (8.314 J/mol/K)


Variants of the Butler-Volmer Equation

ca

RO

ca

RO

bb

ccT

TR

F

TR

F

ccT rfirfii

hhh

h

10101010 ),,(0

303.2303.2

),,(0

F

TRb

ca

a,c

,

303.2

often the Butler-Volmer equation is written in log10 form :

where: is referred to as the anodic/cathodic Tafel slope, representing the

overpotential increase required for a 10x increase in current

at 25°C, b commonly ranges from 120 mV/decade (a,c = 0.5)

to 30 mV/decade (a,c = 2)

alternative definition acc. to [1]:

h

h

TR

Fn

TR

Fn

ccT eerfiiRO

)1(

),,(0

here, is also referred to as transfer coefficient, even though its meaning is

different from the definitions in [2] and [3] (actually, in [1], the symmetry factor is

meant! ...detailed explanation of differences: E. Gileadi, Electrode Kinetics...VCH)

care must be taken to not mix up these two different definitions

the more general definition with a,c as transfer coefficients will be used here

[1] A.J. Bard & L.R. Faulkner, Electrochemical Methods, John Wiley & Sons (1980); [2] J. O’M. Bockris



Dependence of i0 on the Electrocatalyst exchange current densities vary by many order of magnitudes for different electrocatalysts

e.g., for the H2 evolution reaction (2H+ + 2e- H2 ) in acid electrolytes:

according to Sabatier’s Principle, high reaction rates (i0’s) require bonding of

the reaction intermediate (M-H) which is not too weak and not too strong

Au

NiCo

Fe

Cu

PtRe

Rh

Ir

W

Mo

NbTi

Ta

Sn BiAgZn

GaPb

CdTl

from: S. Trasatti, J. Electroanal.

Chem. 39 (1972) 163

Au

NiCo

Fe

Cu

PtRe

Rh

Ir

W

Mo

NbTi

Ta

Sn BiAgZn

GaPb

CdTl

Au

NiCo

Fe

Cu

PtRe

Rh

Ir

W

Mo

NbTi

Ta

Sn BiAgZn

GaPb

CdTl

Au

NiCo

Fe

Cu

PtRe

Rh

Ir

W

Mo

NbTi

Ta

Sn BiAgZn

GaPb

CdTl

from: S. Trasatti, J. Electroanal.

Chem. 39 (1972) 163


Temperature and Concentration Dependence of i0

),,(0 ***RO ccT

i

as any rate constant of a chemical reaction, the exchange current density depends

on temperature and reactant/products concentrations

it is frequently based on the definition used in [3]:

[1] A.J. Bard & L.R. Faulkner, Electrochemical Methods, John Wiley & Sons (1980); [2] J. O’M. Bockris


with:

[A/cm2real]: exchange current density at defined reference temperature and

reference reactant/product (c*R , c

*O ) concentrations

g, d [dimensionless]: reaction orders, describing the concentration dependence of i0

Eact [J/mol]: activation energy of the exchange current density

TR

E

O

O

R

R

ccTccT

act

ROROe

c

c

c

cii

dg

**),,(0),,(0 ***


Characteristics of the Butler-Volmer Reaction

ca

RO

bb

ccTnet rfii

hh

1010),,(0

the Butler-Volmer equation is the summation of anodic and cathodic currents; this is shown

for the example of the HOR/HER kinetics on low-index Pt single crystals:

Pt face (110) (100) (111)

i0 [mA/cm2] 1.35 0.76 0.83

b [mV/decade] 33 44 66

(Markovic et al., J. Phys. Chem. B 101 (1997) 5405))

HOR/HER on Pt(hkl) (0.05M H2SO4 at 60°C)

where rf = 1 for flame-annealed Pt(hkl) single crystals

ianodic icathodic

at h =0: ianodic = icathodic = i0(T,H2, H+)

dynamic equilibrium

at h +ba/2: ianodic 10 icathodic

reverse reaction negligible

-8

-6

-4

-2

0

2

4

6

8

-40 -30 -20 -10 0 10 20 30 40

overpotential h [mV]

cu

rre

nt

de

ns

ity

[ m

A/c

m 2

Pt ]

icathodic : 2H+ + 2e

- H2

ianodic : H2 2H+ + 2e

-

inet

(i 0 rf )


-25

-20

-15

-10

-5

0

5

10

15

20

25

-100 -80 -60 -40 -20 0 20 40 60 80 100


cu

rre

nt

de

ns

ity

[ m

A/c

m 2

Pt ]

H2 2H+ + 2e

-

2H+ + 2e

- H2

Tafel Slope Impact – HOR/HER Example

1066.0

303.2,

FV

TRHERHOR

for the HOR/HER on low-index Pt single crystal faces, nearly identical i0’s were

reported, while the Tafel slopes varied from 33 66 mV/dec:

Pt face (110) (100) (111)

i0 [mA/cm2] 1.35 0.76 0.83




2033.0

303.2,

FV

TRHERHOR

from the definition of b:

at 60ºC (333 K) and bHOR/HER from table

the overpotential effect on current density

is the stronger, the lower the Tafel slope

near h = 0, i h


Butler-Volmer: Linear Approximation

h

h h

h

TR

Fe

TR

Fe cTR

F

aTR

F ca

1 and1

F

TR

F

TR

ba

h

h and1and1

h

h

TR

F

TR

F ca

in the region of small overpotentials, the Butler-Volmer equation can be linearized:

for: i.e.,

h

h

TR

F

TR

F

ccT

ca

ROeerfii ),,(0

therefore:

hh

caccT

caccT bb

rfiTR

Frfii

RORO

11303.2)(

),,(0),,(0

special case as defined by Bard and Faulkner: nn ca and)1(

h

TR

nFrfii

RO ccT ),,(0


Butler-Volmer: Linear Approximation

txech

caccT

RF

TR

i

F

TRrfi

RO

arg

),,(0

1

)(h

for the previous example of HOR/HER on Pt(110):

obtain i0 from the slope in the

linear reagion, if a,c & rf are known

rf from cyclic voltammetry, XRD,

or TEM

a,c from Tafel plots

since:

-6

-4

-2

0

2

4

6

-40 -30 -20 -10 0 10 20 30 40


cu

rren

t d

en

sit

y [

mA

/cm

2P

t ]

< 10% error at -h < b /3

< 10% error at h < b /3

Pt face (110) (100) (111)

i0 [mA/cm2] 1.35 0.76 0.83





Butler-Volmer: Tafel Approximation

ca bandb hh1and1 ca bb

hh

a

RO

ca

RO

b

ccT

bb

ccTanodic rfirfii

hhh

101010 ),,(0),,(0

at large overpotentials, one of the Butler-Volmer equation terms becomes negligible:

for: i.e.,

for anodic processes (h ba,c):

or, more commonly:

F

TRb

ca

a,c

,

303.2

(where )

10 0.1

ha

ccTanodicb

rfiiRO

1log)log( ),,(0

for cathodic processes (h ba,c): hc

ccTcathodicb

rfiiRO

1log)log( ),,(0

the log(i) vs. h relationship at high h is commonly referred to as Tafel equation


0.1

1

10

100

-100 -80 -60 -40 -20 0 20 40 60 80 100


log

( |i

| ) [

mA

/cm

2P

t ]

H2 2H+ + 2e

-2H

+ + 2e

- H2

(i 0 rf )

(b a )-1(b c )

-

Butler-Volmer: Tafel Approximation

Pt face (110) (100) (111)

i0 [mA/cm2] 1.35 0.76 0.83




for the previous example of HOR/HER on Pt(110) and Pt(111):

the accuracy of the Tafel equation

at |h| ½ba,c is better 10%

from Tafel plots, both (i0 rf) and

ba,c can be determined

for slow kinetics, determination of

(i0 rf) requires extrapolation over

many orders of magnitude

large errors for ORR in PEMFCs


HOR/HER Kinetic Models

possible HOR reaction mech.: Tafel: H2 + 2Pt 2 Pt-H

Pt-H Pt + H+ + e- Volmer: (K. Krischer & E.R. Savinova; in: Handbook

of Het. Catalysis; Wiley (2007): ch. 8.1.1.)

H2 + Pt Pt-H + H+ + e- Heyrovski:







- battery materials


- “post-LiBs”







H2 Oxidation Reaction (HOR) Kinetics



HOR/HER Kinetics: 80°C, 100kPa H2

PtPt

caoALF

TRii

h

caoi 0.47 A/cm2Pt

K.C. Neyerlin et al., J. Electrochem. Soc. 154 (2007) B631.

data fit with symmetric Butler-Volmer equation (a = c = ):

from linear region:

h

h

RT

F

RT

F

PtPtelectPtPto eemgcmcmmgcmAii ]/[1000/003.0]/[22

.

2

h’s too low to obtain

a c 0.5-1

-60

-40

-20

0

20

40

60

-3 -2 -1 0 1 2 3

i [A/cm2]

hH

ER

/h

HO

R [

mV

]

4

2

1

ca

ca

ca


HOR/HER Kinetics (80°C, 100 kPa H2)

at 1.5 A/cm2 & 0.05 mgPt/cm2 : 30 A/mgPt

with pure H2 hHOR 2 mV

MEA (aa + ac) TS [mV/dec.] i0 [mA/cm2

Pt]

#1 1 2 140 70 470 240

#2 1 2 140 70 600 300

600 240-60

-40

-20

0

20

40

60

-1000 -500 0 500 1000im [A/mgPt]

hH

ER

/h

HO

R [

mV

]

MEA 1

MEA 2

i0’s on the order of 100’s of mA/cm2Pt

most literature i0’s 10-100x too low (2-3x for 2580°C for Eact 10-20 kJ/mol in lit.)

reaction catalyst electrolyte T [°C] i0 [mA/cm2

Pt] reference

HOR/HER 5% Pt/C PEMFC 80 240 - 600 this study

HOR 5% Pt/C 96% H3PO4 80 50 W. Vogel et al. , Electrochim. Acta 20 (1975) 79

HOR/HER Pt(hkl) 0.05M H2SO4 60 0.8 - 1.4 N.M. Markovic et al. , J. Phys. Chem. B 101 (1997) 5405

HOR Ptnano/C 0.1M H2SO4 25 20 S. Chen et al. , J. Phys. Chem. B 108 (2004) 13984

HOR 10% Pt/C 0.5M H2SO4 25 1 J.X. Wang et al. , J. Electrochem. Soc. 150 (2003) A1108

HOR Ptpc 0.01M HClO4 25 2.5 R. Notoya et al. , J. Phys. Chem. 93 (1989) 5521

HER Pt(hkl) 0.5M H2SO4 25 1 H. Kita et al. , J. Electroanal. Chem. 334 (1992) 351

HER Pt(hkl), Ptpc 0.1M HClO4 25 1.7 - 3.0 K. Seto et al. , J. Electroanal. Chem. 226 (1987) 351

HER Ptpc 0.5M H2SO4 25 3 S. Trasatti, J. Electroanal. Chem. 39 (1972) 163


O2 Reduction Reaction (ORR) Kinetics



ORR Kinetics

0

Ecell = Erev – DEW(RH) – hHOR – |hORR| – hH+,an&ca(RH) – htx,O2

measured

-blog i[A/cm2]

L[mgPt/cm2] APt[cm2/mgPt] EiR-free = Ecell + DEW(RH)

0 at 100%RH 0 with H2/O2

hORR follows Tafel-relation


ORR on Carbon-Supported Pt (Pt/C)

2.3RT/(cF)log i[A/cm2]

LPt [mgPt/cm2] APt [cm2/mgPt] i0 (pO2,T) [A/cm2]

EiR-free - hORR = ?

with: i0 (pO2,T) = i0 (100kPa, 353K) * pO2

100kPa

g Ea

R

exp

* 1

T

1

353 K

-

(J.S. Newman, Electrochem. Systems, Prentice Hall (1991))

fitted data range:

0.03 0.5 A/cm2

35 95°C

40 400 kPaa

fitted paramters with c 1*) :

i0 (100kPa, 353K) = 2.110-8 A/cm2Pt literature?

Ea = 67 kJ/mol literature ?

g 0.5

*

hORR follows Tafel-relation in PEMFC range

*) K.C. Neyerlin et al., J. Electrochem. Soc. 153 (2006) A1955.

0.77

0.79

0.81

0.83

0.85

0.87

0.89

0.91

0.77 0.79 0.81 0.83 0.85 0.87 0.89 0.91

measured EiR-free [V]

fitt

ed

E i

R-f

ree [V

]


Direct Methanol Fuel Cells (DMFCs)


DMFC Performance with Air-Feed literature data with Nafion 117® :

[1] M.P. Hogarth et al., Plat. Met. Rev. 46 (2002) 146.

[3] R. Dillon et al., J. Power S. 127 (2004) 112.

[4] M. Baldauf and W. Preidel, J. Power S. 84 (1999) 161.

[2] S.C. Thomas et al., Electrochim. Acta 47 (2002) 3741.

power densities of 0.05-0.1 W/cm2 at 0.5 V and 0.1-0.2 W/cm2 at 0.4 V

5-10x lower W/cm2 and 100x higher mgPt/W than PEMFC (0.5 mgPt/W)

Tcell cmeth. Pair sair anode catalyst cath. catalyst loadinganode loadingcath. 0.5V

performance

0.4V

performance

Ref.

°C mol/l1 kPaabs -- -- -- mgPt/cm

2 mgPt/cm

2 W/cm

2 mgPt/W W/cm

2 mgPt/W

90 0.75 300 5 60%wt Pt1Ru1/C Pt-black 1.0 4.0 0.11 45 0.18 28 [1]

90 0.75 300 2 60%wt Pt1Ru1/C Pt-black 1.0 4.0 0.17 29 0.18 28 [1]

80 0.5 300 ? 1)

Pt1Ru1-black Pt-black anode/cath.=2.6 0.06 43 0.11 24 [2]

100 0.5 300 ? 1)

Pt1Ru1-black Pt-black anode/cath.=2.6 0.10 26 0.15 17 [2]

110 1.0 300 ? 2)

85%wt Pt1Ru1/C 85%wt Pt/C anode/cath.=2.0 0.04 50 0.09 22 [3]

90 0.5 150 >5 PtRu 3)

Pt-black 0.7 4.0 0.05 94 0.09 52 [4]

1) the air stoichiometry was only referred to as “high” and no specific value was given

2) air stoichiometry was not specified

3) the used PtRu catalyst was unspecified wrt. composition (assumed 1:1 atomic ratio in above calculation) and support (black or C-supported)


best-possible performance

with current catalysts

improvements only via

novel catalysts

Ideal Performance of Air-Fed DMFC assumptions :

membrane with “zero” CH3OH permability use 25mm thin membrane

no transport resistances in electrodes/DM (Rsheet 0, htx,O2 0 , htx,CH3OH 0)

conditions: 1mgPtRu/cm2 (80m2/gPtRu), 0.4mgPt/cm2 (80m2/gPt), 80oC

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

0.0 0.2 0.4 0.6 0.8 1.0 i [A/cm2]

E [

V]

iR-loss

hORR

hanode

EDMFC(ideal)

Erev(80C, 21kPa O2, 1M CH3OH) = 1.165 V

DMFCideal

From: H.A. Gasteiger and J. Garche,

in: Handbook of Heterogeneous Catalysis,

2nd edition, Wiley (2007), in press.







- battery materials


- “post-LiBs”








detailed model in: W. Gu, D.R. Baker, Y. Liu, H.A. Gasteiger, in: Handbook of Fuel Cells

(eds.: W. Vielstich, H.A. Gasteiger, H. Yokokawa), Wiley (2009): vol. 6, pp. 631.

only measured kinetic & transport properties (DH2O , sionomer(RH) , DO2,eff , kthermal ,...)

no fitting parameters !

K.C. Neyerlin, W. Gu, J. Jorne, H.A. Gasteiger, J. Electrochem. Soc. 153 (2006) A1955.

K.C. Neyerlin, W. Gu, J. Jorne, H.A. Gasteiger, J. Electrochem. Soc. 154 (2007) B631.

E.L. Thompson, J. Jorne, H.A. Gasteiger, J. Electrochem. Soc. 154 (2007) B783.

Y. Liu, M.W. Murphy, D.R. Baker, W. Gu, C. Ji, J. Jorne, H.A. Gasteiger,

J. Electrochem. Soc. 156 (2009) B970

hHOR :

hORR :

RH+,(RH) :


MEA Performance Analysis

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.0 0.3 0.6 0.9 1.2 1.5 [A/cm2]

Vo

ltag

e (

V)

Ecell

hHFR=90 mV (hmem=30 mV)

ST19-S0559 (Nano-x coating) RC FCPM op-line

MEA: Gore 5720 (18 mm, 0.2/0.3 mgPt/cm2, I/C=1.2)

DM/MPL: Pre-compressed SGL 25BC

htx,O2(dry)=26 mV

hORR=410 mV

htx,H+ =18 mV

htx,O2(wet)=18 mV

kW

g

cm

W

cm

mg

Pt

MEA

MEA

Pt

5.0

9.0

45.0

2

2

at 1.5 A/cm2:

H2/air (s=1.5/2), 150kPaabs, <50% RHinlet

25mm membrane and 0.05/0.4mgPt/cm2MEA

( 60 mV Rcontact)

undefined losses, htx,O2(wet), of only 20mV

improvements require new materials

need 10x better ORR catalysts to reach 0.05/0.04 mgPt/cm2MEA 0.1 gPt/kW

from: W. Gu, D.R. Baker, Y. Liu, H.A. Gasteiger, in:

Handbook of Fuel Cells, Wiley (2009): vol. 6, pp. 631.

hHOR < 5 mV*) )

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