 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
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 64
Principle of Fuel Cells
Direct electrochemical conversion of chemical energy into electrical energy:
 “spatial separation” of oxidation and reduction on electrocatalysts, e.g.:
H2  2 H+ + 2eanode (oxidation):
cathode (reduction): 0.5 O2 + 2 H+ + 2e-  H2O
overall reaction:
H2 + 0.5 O2  H2O
e-
e-
load
 simplest configuration:
 catalysts to enable half-cell reactions
(electronically conducting)
 ion-conducting electrolyte
(electronically non-conducting)
2e
Pt
 effective three-phase interface
(reactant, ion-conductor, catalyst)
H2
2H+
½ O2
2e-
Pt
H2O
H2
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
H2SO4
-
O2
p. 65
Fuel Cell Types
generally distinguished by type of electrolyte and conducting ion:
fuel cell type
temp.
anode reaction
SOFC
1000°C
H2 + O  H2O + 2e
650°C
(Solid Oxide FC)
MCFC
( Molten Carbonate FC)
PAFC
( Phosphoric Acid FC)
PEMFC
+
( H Exchange Membrane FC)
DMFC
( Direct Methanol FC)
AFC
( Alkaline FC)
conducting ion
cathode reaction
 O-2
(Y-stabilized ZrO2)
½ O2 + 2e-  O-2
H2 + CO3  H2O + CO2 + 2e
 CO3-2
(alkali carbonates)
½ O2 + CO2 + 2e  CO3
200°C
H2  2H+ + 2e-
H 
(H3PO4)
½ O2 + 2H+ + 2e-  H2O
80°C
H2  2H+ + 2e-
H+ 
(solid polymer)
½ O2 + 2H+ + 2e-  H2O
80°C
CH3OH + H2O  CO2 + 6H+ + 6e-
H 
(solid polymer)
1.5 O2 + 6H+ + 6e-  3H2O
80°C
H2 + 2OH  2H2O + 2e
 OH(KOH)
½ O2 + H2O + 2e  2OH
-2
-
-2
-
-
+
-
+
-
-
 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)
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 66
-2
-
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
H2 tank systems
*) based
on the density of liquefied gas
(from: P. Piela and P. Zelenay, Fuel Cell Review 1 (2004) 17)
 practical H2 storage densities  liquid hydrocarbons
 made on-board hydrocarbon reforming in cars attractive
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
(from: A. Bouza et al., DOE Annual
Hydrogen Program Review (2004))
p. 67
H2-Fuel Cell Electric Vehicles (FCEVs)
GM H2-FC (2008): 500 km (70 MPa H2)
 meets vehicle range target
 refueling in < 5 minutes
 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
 advanced catalysts required:
*) DOE
ultra-high activity Pt catalysts or non-Pt catalysts
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.
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 68
Thermodynamic Fuel Cell Efficiency
fuel energy content released by combustion:
Wheat = DHR
for H2 + 0.5 O2  H2O :
 DHR = DHfH2O – [DHfH2 + 0.5DHfO2 ]
(enthalpies of formation from thermodynamic tables)
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
 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)
thermodynamic fuel cell efficiency (usually based on HHV):
th = DGRH2O(liquid/vapor)/DHRH2O(liquid) = -DGRH2O(liquid/vapor)/285.8 kJ/mol
 for a H2/O2 fuel cell at 25°C and pH2 = pO2 = 101 kPa producing H2O(liquid) :
 th = 237.1 kJ/mol / 285.8 kJ/mol = 83.05%
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 69
Dependence of th on Different Fuels
Gibbs-Helmholtz relation: DGR(T) = DHR(T) - TDSR(T)
 if DSR < 0 (decrease of number of moles):
 th = 1 - TDSR(T)/DHR(T) <100%
 Wheat,rev = -TDSR(T) is released
(applies for most fuel cell reactions)
(W. Vielstich, in: Handbook of Fuel Cells (eds.: W. Vielstich, A. Lamm, H.A. Gasteiger), Wiley (2003): vol. 1, chapter 4, p. 26)
 th for most fuel cell reactions ranges from 80 to 100%
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 70
Overall Efficiency of Fuel Cells
in actual fuel cells:
Ecell << Erev
due to resistive, kinetic, and mass-transport losses
H2/air (s=2/2) at 80C, 100%RH,
E150kPa
th(HHV) abs
 voltage = Ecell/Erev = (i)
1.4
Eth(HHV)  DHR(H2O liquid)/(2F) = 1.48 V
1.2
Ecell [V]
 to evaluate Wheat formation, it is convenient
to define a thermal-equivalent voltage, Eth :
Eth(LHV)
Eth(LHV)  DHR(H2O vapor)/(2F) = 1.25 V
Erev
1.0
Wheat(H2O liquid)
0.8
Wheat(H2O vapor)
Ecell
0.6
0.4
Welectrical
0.2
 FC = th  voltage = Ecell/Eth(HHV) = Ecell/1.48V
0.0
0
0.5
1
1.5
2
i [A/cm ]
 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 )
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
 the ratio of Wheat/Welectric  as i 
 heat rejection requirement
increases with power density
p. 71
Proton Exchange Membrane Fuel Cell Materials
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 72
H2/Air PEMFC Performance Model
Ecell = Erev – iRW(RH) – hHOR – | hORR | – iRH+,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
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 73
PEMFC Stack Single-Cell Repeating Units
Bipolar Plate (BP)
e--conducting plates
 H2 & air distribution
via flow-fields
Diffusion Media (DM)
gas diffusion layer
 channel-to-land
distribution (gas, e-)
1-2 mm
Membrane Electrode Assembly
MEA: electrodes on
H+-conducting PEM
Diffusion Media (DM)
Bipolar Plate (BP)
 20-40 cm
full-size PEMFC stacks: - 100’s of single cells
- MEA active areas of 200 to 800 cm2
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 74
Single Cell Assembly & Diffusion Medium Structure
M.F. Mathias et al., in: Handbook of Fuel Cells; Wiley, v.3 (2003)
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 75
Proton (H+) Exchange Membrane
 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.
from: K.D. Kreuer, in: Handbook of Fuel Cells: Fundamentals,
Technology & Applications (eds: W. Vielstich, A. Lamm, H.A.
Gasteiger), Wiley (2003): vol. 3.
 RH+(membrane) & RH+(electrodes) are strong functions of RH
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 76
Sulfonic Acid Ionomers – l vs. RH
10
Sulfuric Acid
 water-uptake of various
sulfonic acid ionomers at 80°C:
Crosslinked 4% DVB Dowex 50 (Pushpa)
9
BPSH 40% (McGrath) GES Measurement
700 EW PFSA
8
 up to 80%RH:
same l vs. RH for
very different ionomers
 at >80%RH (high l):
l-value now strongly
depends on ionomer 
l nH2O/n-SO3H
Nafion 112
sulfonated-nonsulfonated multi-block
7
6
5
4
3
2
from: C. Mittelsteadt & H. Liu, in: Handbook
of Fuel Cells (eds.: W. Vielstich, H.A. Gasteiger,
H. Yokokawa), Wiley (2009): vol. 5, p. 345.
1
0
0
20
40
60
80
100
Relative Humidity %
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.
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 77
Catalyst Carbon-Supports
primary agglomerate
Typical Pt/C Catalyst:
 two primary functions:
- high surface area to support catalyst
nanoparticles (e.g., Pt)
- high-structure material,
creating highly porous electrodes
 high “structure” of primary carbon agglomerates
leads to highly porous packing
 primary agglomerates cannot be “broken”
by typical shears/pressures
primary C-particles (20-40nm)
 agglomerates breakage occures during
carbon corrosion
SEM picture: Jim Mitchell, Ted Gacek, and Mike Budinski (GM Fuel Cell Activities)
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 78
PEFC Electrode Composition & Structure
 C / Pt / ionomer  1 / 1 / 1 mass-ratio  morphology via high-structure carbon-blacks
from: Z.Y. Liu, B.K. Brady, R.N. Carter, B. Litteer,
M. Budinski, J. Electrochem. Soc. 155 (2008) B979.
46% Pt/carbon
40 nm
 60% void volume & dpore 50-100nm
membrane
Diffusion
Medium
H+
O2
eO2 + 4H+ + 4e-
Pt
2H2O
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
 ionomer-film model consistent
with SEM & AC-impedance data
p. 79
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
 average: 197mC/cm2  commonly used for polycrystalline Pt: 210mC/cm2
(note: 210mC/cm2Pt = 2.04 nmolPt/cm2Pt  235m2Pt/gPt using MPt =195.7 gPt/molPt)
(100)-face
(110)-face
(111)-face
 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 · 60m2Pt/gPt = 240cm2Pt/cm2MEA
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 80
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)
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 81
Catalyst Characterization: In-Situ Surface Area
H2  2H+ + 2e-
RDE
Pt-Had  Pt + H+ + e-
Catalysts
Nafionfilm
In-situ cathode CV's at 20mV/s and 25°C:
20
~1µm
MEA: H2(500sccm) / N2 (62sccm); both overhumidified
RDE: N2 (1000sccm) in 0.1M HClO4
Glassy-Carbon
(RDE)
Pt + H2O  PtOHad + H+ + e-
~6mm
0
MEA
-20
thin-film GC in 0.1M HClO4
50cm2 MEA w. 63 sccm N2
-30
H2, Counter/
Reference
Electrodes
PtOHad + H+ + e-  Pt + H2O
-10
N2, Working
Electrode
i [ A/g Pt ]
10
Pt + H+ + e-  Pt-Had
-40
0.0
0.2
0.4
0.6
0.8
1.0
1.2
E/V [RHE]
2H+ + 2e-  H2
•
•
•
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
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 82
 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
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 83
Kinetics vs. Thermodynamics
 deviation from thermodynamics when drawing a current
 kinetic and ohmic limits for PEMFCs: Erev  1.18V for H2/O2 at 101kPa & 80C
EPEMFC = Ecathode - DEohmic - Eanode
Erev
1.2
1.1
hcathode
1.0
0.9
Ecathode
E [V]
0.8
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
DEohmic
0.7
0.6
0.5
0.4
 significant kinetic losses for the ORR
 major scientific challenge!
0.3
0.2
0.1
Eanode
0.0
0
0.2 0.4 0.6 0.8
1
1.2 1.4 1.6 1.8 i [A/cm2MEA]
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 84
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
 nRR ; Erev(O/R)
noO + ne- 
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 85
Electrokinetics
 the Butler-Volmer equation is generally used to describe the overpotential, temperature,
and reactant concentration dependence of the current of an electrode reaction *) :
 nRR ; Erev(O/R)
noO + ne- 
definition according to [2, 3]:
(used throughout these slides)
with:
 i0(T,cO,cR) [A/cm2real]:
 c  F
 Ra TF h
h
R

T
i  i0(T ,cO ,c R )  rf   e
 e


ianodic ( > 0 )




icathodic ( < 0 )
exchange current density, a kinetic reaction rate constant, which
depends on the specific electrocatalyst (note: i = i0 at h = 0)
 rf [cm2real/cm2electrode]: 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)
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).
*) for
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 86
Variants of the Butler-Volmer Equation
 often the Butler-Volmer equation is written in log10 form :
a  F
 c  F

h
h
2.303 R T
2.303 R T

i  i0(T ,cO ,c R )  rf  10
 10


where: ba,c 
2.303  R  T
 a ,c  F

i
 rf
 0 ( T , cO , c R )

h

  10 ba


 10
h
bc




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]:
  n  F
h 
 (1 R)T n F h
R

T

i  i0(T ,cO ,c R )  rf   e
e




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
& A.K.N. Reddy, Modern Electrochemistry, Plenum Press: (1970); [3] J.S. Newman, Electrochemical Systems, Prentice Hall (1991).
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 87
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:
Pt
Rh
Au
Cu
Re
Ir
Ni
Co
Fe
W
Sn
Zn
Pb
Tl
Bi
Ag
Ga
Cd
Mo
Nb
Ti
Ta
from: S. Trasatti, J. Electroanal.
Chem. 39 (1972) 163
 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
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 88
Temperature and Concentration Dependence of i0
 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]:
i0(T ,cO ,cR )  i0(T * ,c* ,c*
O
with:
 i
0(T
*
,cO* ,c *R )
R
 cR 
 
)  * 
 cR 
g
d
E
act
 cO 
  *   e R T
 cO 
[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
[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).
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 89
Characteristics of the Butler-Volmer Reaction
 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:
(0.05M H2SO4 at 60°C)
(100)
0.76
44
(111)
0.83
66
(Markovic et al., J. Phys. Chem. B 101 (1997) 5405))

inet  i0(T ,cO ,c R )  rf   10 ba


h
ianodic
 at h =0:
 10
h
bc




icathodic
ianodic = icathodic = i0(T,H2, H+)
 dynamic equilibrium
 at h  +ba/2:
ianodic  10  icathodic
 reverse reaction negligible
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
where rf = 1 for flame-annealed Pt(hkl) single crystals
8
current density [ mA/cm 2Pt ]
HOR/HER on Pt(hkl)
Pt face
(110)
2
i0 [mA/cm ]
1.35
b [mV/decade]
33
6
4
ianodic : H2  2H+ + 2e-
2
inet
0
(i 0  rf )
-2
icathodic : 2H+ + 2e-  H2
-4
-6
-8
-40
-30
-20
-10
0
10
20
30
overpotential h [mV]
p. 90
40
Tafel Slope Impact – HOR/HER Example
 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:
HOR/HER on Pt(hkl)
Pt face
(110)
2
i0 [mA/cm ]
1.35
b [mV/decade]
33
(0.05M H2SO4 at 60°C)
(100)
0.76
44
(111)
0.83
66
from the definition of b:
at 60ºC (333 K) and bHOR/HER from table
(Markovic et al., J. Phys. Chem. B 101 (1997) 5405))
25
  HOR ,HER 
current density [ mA/cm 2Pt ]
20
2.303 R  T
2
0.033V  F
  HOR ,HER  2.303 R  T 1
0.066V  F
15
10
5
0
H2  2H+ + 2e2H+ + 2e-  H2
 the overpotential effect on current density
is the stronger, the lower the Tafel slope
-5
-10
-15
 near h = 0, i  h
-20
-25
-100 -80
-60
-40
-20
0
20
40
60
80
100
overpotential h [mV]
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 91
Butler-Volmer: Linear Approximation
 in the region of small overpotentials, the Butler-Volmer equation can be linearized:
for:  a  F h   1 and  c  F h   1
R T
 e
i.e.,
R T
a  F
R T
h
1
a  F
R T
h
and
e
 c  F
h
R T
1
h 
c  F
R T
R T
a  F
and h  
R T
b  F
h
 c  F
 Ra TF h
h
R T

therefore: i  i
 e
0 ( T ,cO ,c R )  rf  e






 i  i0(T ,c ,c )  rf  ( a   c )  F h  i0(T ,c ,c )  rf  2.303  1
O
R
O
R
R T
special case as defined by Bard and Faulkner:
 ba
 a  (1   )  n
 i  i0 ( T ,c
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
and
1
 h
bc 
c    n
 rf 
O ,c R )
F n
h
R T
p. 92
Butler-Volmer: Linear Approximation
 for the previous example of HOR/HER on Pt(110):
current density [ mA/cm 2Pt ]
6
4
HOR/HER on Pt(hkl)
Pt face
(110)
i0 [mA/cm 2]
1.35
b [mV/decade]
33
since:
(0.05M H2SO4 at 60°C)
(100)
0.76
44
(111)
0.83
66
i0 (T ,cO ,cR )  rf  ( a   c ) 
(Markovic et al., J. Phys. Chem. B 101 (1997) 5405))
2
< 10% error at h < b /3
0
< 10% error at -h < b /3
-4
-6
-30
-20
-10
0
10
20
R T
1

F Rch arg e  tx
 obtain i0 from the slope in the
linear reagion, if a,c & rf are known
-2
-40

R T i

F h
30
40
 rf from cyclic voltammetry, XRD,
or TEM
 a,c from Tafel plots
overpotential h [mV]
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 93
Butler-Volmer: Tafel Approximation
 at large overpotentials, one of the Butler-Volmer equation terms becomes negligible:
for:
h
ba
 1 and
h
bc
1
i.e., h  ba and h  bc
(where ba,c 
h
 bh
a
 for anodic processes (h  ba,c): i

 10 bc
anodic  i0 ( T , cO , c R )  rf  10


 10



h

  i0(T ,c ,c )  rf  10 ba
O R


 0.1
or, more commonly: log(ianodic)  log i0(T ,c ,c )  rf 
O R
 for cathodic processes (h  ba,c):
2.303 R  T
)
 a ,c  F
1
h
ba

log( icathodic )  log i0(T ,cO ,cR )  rf 
1
h
bc
 the log(i) vs. h relationship at high h is commonly referred to as Tafel equation
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 94
Butler-Volmer: Tafel Approximation
 for the previous example of HOR/HER on Pt(110) and Pt(111):
HOR/HER on Pt(hkl) (0.05M H2SO4 at 60°C)
Pt face
(110)
(100)
(111)
i0 [mA/cm2]
b [mV/decade]
1.35
33
0.76
44
0.83
66
(Markovic et al., J. Phys. Chem. B 101 (1997) 5405))
100
log( |i| ) [ mA/cm2Pt ]
 the accuracy of the Tafel equation
(b c )-
at |h|  ½ba,c is better 10%
(b a )-1
10
 from Tafel plots, both (i0  rf) and
ba,c can be determined
2H + 2e  H2
+
H2  2H + 2e
-
+
-
 for slow kinetics, determination of
1
(i0  rf) requires extrapolation over
(i 0  rf )
0.1
-100
-80
-60
-40
-20
0
20
40
many orders of magnitude
 large errors for ORR in PEMFCs
60
80
100
overpotential h [mV]
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 95
HOR/HER Kinetic Models
 possible HOR reaction mech.:
(K. Krischer & E.R. Savinova; in: Handbook
of Het. Catalysis; Wiley (2007): ch. 8.1.1.)
Tafel: H2 + 2Pt  2 Pt-H
Volmer:
Heyrovski:
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
Pt-H  Pt + H+ + eH2 + Pt  Pt-H + H+ + e-
p. 96
 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
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 97
H2 Oxidation Reaction (HOR) Kinetics
Ecell = Erev – iRW(RH) – hHOR – | hORR | – iRH+,an&ca (RH) – htx,O2(dry) – htx,O2(wet)
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 98
HOR/HER Kinetics: 80°C, 100kPa H2
 data fit with symmetric Butler-Volmer equation (a = c = ):
 F
  F
h
h 

2
2
2
RT
RT

i  io [ A / cm Pt ]  0.003 mg Pt / cm elect .  1000 [cm Pt / mg Pt ]   e
e



h HER /h HOR [mV]
60
 h’s too low to obtain 
a c  1
a c  2
a c  4
40
20
 a  c  0.5-1
from linear region:
0
io   a   c  
-20
R T
h F  LPt  APt
i

 io   a   c   0.47 A/cm2Pt
-40
-60
-3
-2
-1
0
1
2
3
i [A/cm2]
K.C. Neyerlin et al., J. Electrochem. Soc. 154 (2007) B631.
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 99
HOR/HER Kinetics (80°C, 100 kPa H2)
h HER /h HOR [mV]
60
 at 1.5 A/cm2 & 0.05 mgPt/cm2 : 30 A/mgPt
40
 with pure H2 hHOR  2 mV
20
 i0’s on the order of 100’s of mA/cm2Pt 
0
-20
MEA 1
MEA 2
-40
-60
-1000
-500
0
500
im1000
[A/mgPt]
MEA
(aa + ac)
TS [mV/dec.]
i0 [mA/cm2Pt]
#1
12
140  70
470  240
#2
12
140  70
600  300
600  240
 most literature i0’s 10-100x too low (2-3x for 2580°C for Eact  10-20 kJ/mol in lit.)
T [°C] i0 [mA/cm2Pt]
80
240 - 600
80
50
reaction
HOR/HER
HOR
catalyst
5% Pt/C
5% Pt/C
electrolyte
PEMFC
96% H3PO4
HOR/HER
Pt(hkl)
Ptnano/C
0.05M H2SO4
60
0.8 - 1.4
N.M. Markovic et al. , J. Phys. Chem. B 101 (1997) 5405
0.1M H2SO4
25
20
S. Chen et al. , J. Phys. Chem. B 108 (2004) 13984
10% Pt/C
Ptpc
0.5M H2SO4
25
J.X. Wang et al. , J. Electrochem. Soc. 150 (2003) A1108
0.01M HClO4
25
1
2.5
0.5M H2SO4
25
0.1M HClO4
25
1
1.7 - 3.0
H. Kita et al. , J. Electroanal. Chem. 334 (1992) 351
HER
Pt(hkl)
Pt(hkl), Ptpc
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
HOR
HOR
HOR
HER
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
reference
this study
W. Vogel et al. , Electrochim. Acta 20 (1975) 79
R. Notoya et al. , J. Phys. Chem. 93 (1989) 5521
p. 100
O2 Reduction Reaction (ORR) Kinetics
Ecell = Erev – iRW(RH) – hHOR – | hORR | – iRH+,an&ca (RH) – htx,O2(dry) – htx,O2(wet)
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 101
ORR Kinetics
measured
0
0 at 100%RH 0 with H2/O2
Ecell = Erev – DEW(RH) – hHOR – |hORR| – hH+,an&ca(RH) – htx,O2
i[A/cm2]
 EiR-free = Ecell + DEW(RH)  -blog L[mg /cm2]  A [cm2/mg ]
Pt
Pt
Pt
 hORR follows Tafel-relation
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 102
ORR on Carbon-Supported Pt (Pt/C)
i[A/cm2]
EiR-free  - hORR = 2.3RT/(cF)log
LPt [mgPt/cm2]  APt [cm2/mgPt]  i0 (p
?
[A/cm2]
,T)
O2
g
E*a 1
 exp
 - 1
with: i0 (pO2,T) = i0 (100kPa, 353K) 
R
T 353 K
100kPa
pO2
*
(J.S. Newman, Electrochem. Systems, Prentice Hall (1991))
fitted data range:
 0.03  0.5 A/cm2
 35  95°C
 40  400 kPaa
0.91
fitted paramters with c  1*) :
 i0*(100kPa, 353K) = 2.110-8 A/cm2Pt  literature?
 Ea = 67 kJ/mol  literature ?
 g  0.5
fitted E iR-free [V]
0.89
0.87
0.85
0.83
0.81
0.79
 hORR follows Tafel-relation in PEMFC range
*) K.C.
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
0.77
0.77 0.79 0.81 0.83 0.85 0.87 0.89 0.91
measured EiR-free [V]
Neyerlin et al., J. Electrochem. Soc. 153 (2006) A1955.
p. 103
Direct Methanol Fuel Cells (DMFCs)
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 104
DMFC Performance with Air-Feed
 literature data with Nafion 117® :
Tcell cmeth.
Pair
sair
anode catalyst
°C mol/l1 kPaabs
--
--
--
mgPt/cm2
mgPt/cm2
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
0.05
94
0.09
52
[4]
PtRu 3)
cath. catalyst loadinganode loadingcath.
Pt-black
0.7
4.0
0.5V
performance
0.4V
performance
W/cm2 mgPt/W W/cm2 mgPt/W
1)
the air stoichiometry was only referred to as “high” and no specific value was given
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)
2)
 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)
[1] M.P. Hogarth et al., Plat. Met. Rev. 46 (2002) 146.
[2] S.C. Thomas et al., Electrochim. Acta 47 (2002) 3741.
[3] R. Dillon et al., J. Power S. 127 (2004) 112.
[4] M. Baldauf and W. Preidel, J. Power S. 84 (1999) 161.
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
Ref.
p. 105
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
Erev(80C, 21kPa O2, 1M CH3OH) = 1.165 V
1.1
hORR
1.0
From: H.A. Gasteiger and J. Garche,
in: Handbook of Heterogeneous Catalysis,
2nd edition, Wiley (2007), in press.
0.9
0.8
iR-loss
E [V]
0.7
EDMFC(ideal)
0.6
0.5
0.4
 best-possible performance
0.3
hanode
0.2
DMFCideal
0.1
0.0
0.0
0.2
0.4
0.6
0.8
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
with current catalysts
 improvements only via
novel catalysts
1.0 i [A/cm2]
p. 106
 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
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 107
Ecell = Erev – iRW(RH) – hHOR – | hORR | – iRH+,an&ca (RH) – htx,O2(dry) – htx,O2(wet)
hHOR :
hORR :
K.C. Neyerlin, W. Gu, J. Jorne, H.A. Gasteiger, J. Electrochem. Soc. 154 (2007) B631.
K.C. Neyerlin, W. Gu, J. Jorne, H.A. Gasteiger, J. Electrochem. Soc. 153 (2006) A1955.
E.L. Thompson, J. Jorne, H.A. Gasteiger, J. Electrochem. Soc. 154 (2007) B783.
RH+,(RH) : Y. Liu, M.W. Murphy, D.R. Baker, W. Gu, C. Ji, J. Jorne, H.A. Gasteiger,
J. Electrochem. Soc. 156 (2009) B970
 only measured kinetic & transport properties (DH2O , sionomer(RH) , DO2,eff , kthermal ,...)
 no fitting parameters !
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.
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 108
MEA Performance Analysis
ST19-S0559 (Nano-x coating) RC FCPM op-line
0.90
H2/air (s=1.5/2), 150kPaabs, <50% RHinlet
2 2
MEA: Gore 5720
(18 0.05/0.4mg
mm, 0.2/0.3 mgPtPt
/cm
, I/C=1.2)
25mm membrane
and
/cm
MEA
DM/MPL: Pre-compressed SGL 25BC
Voltage (V)
0.85
hORR=410 mV  hHOR < 5 mV*) )
0.80
0.75
( 60
mV
hHFR=90 mV (h
=30Rcontact
mV) )
mem
0.70
htx,H+ =18 mV
htx,O2(dry)=26 mV
0.65
from: W. Gu, D.R. Baker, Y. Liu, H.A. Gasteiger, in:
Handbook of Fuel Cells, Wiley (2009): vol. 6, pp. 631.
0.60
0.0
0.3
0.6
0.9
1.2
 undefined losses, htx,O2(wet), of only 20mV
 improvements require new materials
htx,O2(wet)=18 mV
Ecell
1.5 [A/cm2]
 at 1.5 A/cm2:
mg Pt
2
g
cm MEA
 0.5 Pt
W
kW
0.9 2
cm MEA
0.45
 need 10x better ORR catalysts to reach 0.05/0.04 mgPt/cm2MEA  0.1 gPt/kW
2012-06-05 AMS Battery & FC Lectures - Fuel Cell (Michele P. for Hubert G.).ppt
p. 109