thermodynamics and electrochemistry of low temperature ... · friedrich wilhelm ostwald...
TRANSCRIPT
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Thermodynamics and Electrochemistry
Of Low Temperature Fuel Cells
Frano BarbirFrano Barbir
Professor, FESB, Split, Croatia
Joint European Summer School for Fuel Cell and Hydrogen Technology
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energy partners
Prof. Dr. Frano Barbir
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heat
What is fuel cell?What is fuel cell?
Fuel cell is an electrochemical energy converter.
It converts chemical energy of fuel (H2) directly into electricity.
Fuel cell is like a battery but with constant fuel and oxidant supply.
hydrogen
DC electricity
_
+
BATTERYoxygen
water
FUEL CELL
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Invention of fuel cell
Fuel cell historyFuel cell historyChristian Schönbein
(1799-1868):
Prof. of Phys. and Chem.
in Basel, first
observation of fuel cell
effect
(Dec. 1838, Phil. Mag.)
William Robert Grove
1839
(1811 –1896):
Welsh lawyer turned
scientist, demonstration
of fuel cell
(Jan. 1839, Phil. Mag.)
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Invention of fuel cell
Scientific curiosity
Friedrich Wilhelm Ostwald
(1853-1932, Nobel prize 1909):
founder of “Physical Chemistry”,
provided much of the theoretical
Fuel cell historyFuel cell history
1839
provided much of the theoretical
understanding of how fuel cells operate
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Friedrich Wilhelm Ostwald – visionary ideas:
“I do not know whether all of us realise fully what an
imperfect thing is the most essential source of power
which we are using in our highly developed engineering
– the steam engine”
Energy conversion in combustion engines
limited by Carnot efficiency limited by Carnot efficiency
unacceptable levels of atmospheric pollution
vs.
Energy conversion in fuel cells
direct generation of electricity
highly efficient, silent, no pollution
Transition would be technical revolution
Prediction: practical realization could take a long time!
Source: Z. Elektrochemie, Vol. 1, p. 122-125, 1894.
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Invention of fuel cell
Reinvention
Scientific curiosity
Fuel cell historyFuel cell history
1839 1939
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Industrial curiosity
Invention of fuel cell
Reinvention
Scientific curiosity
Fuel cell historyFuel cell history
1960’s1839 1939
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Space program
Industrial curiosity
Invention of fuel cell
Reinvention
Scientific curiosity
Fuel cell historyFuel cell history
Space program
1960’s1839 1939
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PEM Fuel Cells used in Gemini program
Apollo program used alkaline fuel cells
Space Shuttle uses alkaline fuel cellsSpace Shuttle uses alkaline fuel cells
Renewed interest in PEM fuel cells
Courtesy of UTC Fuel Cells
First PEM Fuel Cell
Grubb and Niedrach (GE)Gemini Space Program
1960s
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Space program
Industrial curiosity
Invention of fuel cell
Reinvention
Scientific curiosity
Fuel cell historyFuel cell history
Space program
1990’s
Entrepreneural phase
1960’s1839 1939
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Space program
Industrial curiosity
Invention of fuel cell
Reinvention
Scientific curiosity
Fuel cell historyFuel cell history
Space program
1990’s
Entrepreneural phase
1960’s1839 1939
Birth of new industry
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Automobiles
Buses
Scooters
Bicycles
Golf-carts
Distributed power generation
Cogeneration
Back-up power
Fuel Cell Applications Fuel Cell Applications -- DemonstrationsDemonstrations
Back-up power
Portable power
Space
Airplanes
Locomotives
Boats
Underwater vehicles
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Why fuel cells
Promise of high efficiency
Promise of low or zero emissions
Run on hydrogen/fuel may be produced from
indigenous sources/issue of national security
Simple/promise of low cost
No moving parts/promise of long life
Modular
Quiet
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electrodehydrogen
What is fuel cell?What is fuel cell?
Fuel cell is an electrochemical energy converter.
It converts chemical energy of fuel (H2) directly into electricity.
Fuel cell is like a battery but with constant fuel and oxidant supply.
electrolyte
electrode
hydrogen
oxygen
water
FUEL CELL
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electrodehydrogen
What is fuel cell?What is fuel cell?
Fuel cell is an electrochemical energy converter.
It converts chemical energy of fuel (H2) directly into electricity.
electrolyte
electrode
hydrogen
oxygen
water
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Types of fuel cellsTypes of fuel cells(based on electrolyte)
Alkaline
Acid polymer membrane
or Polymer Electrolyte Membrane
or Proton Exchange Membrane
PEM
Phosphoric acid
Molten carbonate
Solid oxide
- Zinc/Air fuel cell is not a fuel cell
- Direct methanol fuel cell is PEM fuel cell
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Reactions in fuel cells
H2 O2H+
H2 O2
H2OOH-
H2O
depleted oxidant and
product gases out
depleted fuel and
product gases out
load
e-
AFC
PEMFC
65-220 °C
60-80 °CH2 O2
H2OH+
H2O2
H2O
CO3=
H2 O2
H2OO=
CO2
CO2
oxidant infuel in
anode cathodeelectrolyte
PEMFC
PAFC
MCFC
SOFC
(CO)
(CO)
(CH4)
60-80 °C205 °C
650 °C
600-1000 °C
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Types of fuel cellsTypes of fuel cells(based on electrolyte)
Alkaline
Acid polymer membrane
or Polymer Electrolyte Membrane
or Proton Exchange Membrane
PEM
Phosphoric acid
Molten carbonate
Solid oxide
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Why PEM Fuel Cells?Why PEM Fuel Cells?
Simple
Quick start-up
Fast response
High efficiency
High power density (kW/kg and kW/l)
Zero emissions
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PEM Fuel Cell Basic ComponentsPEM Fuel Cell Basic Components
proton
exchange
membrane
porous
electrode
porous
electrode
catalyst
layer
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Porous electrode structure
Gas diffusion layer surface
(carbon fiber paper)
80 µµµµm
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proton
exchange
membrane
porous
electrode
PEM Fuel Cell Basic ComponentsPEM Fuel Cell Basic Components
porous
electrode
catalyst
layer
MEA – Membrane Electrode Assembly
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cell frame / bipolar plate
Pt-catalyst
current collector /
gas diffusion layer
MEA Cross-Section
current collector /
gas diffusion layer
cell frame / bipolar plate
Pt-catalyst
polymer membrane
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PEM Fuel Cell: How does it work?PEM Fuel Cell: How does it work?
membraneelectrode electrode
MembranePorous
electrode
structure
Carbonsupport
Platinum
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2H+
collector
plate
collector
plate
2e-
O2
oxygen feed
How does it work?How does it work?
membraneelectrode electrode
external load
2H+
1/2O2 + 2H+ + 2e- → H2O
H2OH2
hydrogen feed
H2 → 2H+ + 2e-
MembranePorous
electrode
structure
Carbonsupport
Platinum
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electrode membrane electrode
oxygen feed
collector
plate
O2
external load
2e-
electrode membrane electrode
oxygen feed
O2
PEM Fuel Cell - Basic Principle
2H+
H2 → 2H+ + 2e–
H2
2e-
H2O
hydrogen feed
bi-polar
collector
plate
2H+
H2 H2O
hydrogen feed
2e-
½O2 + 2H+ + 2e– → H2O
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end plate
oxidant in
hydrogen out
bus plate
bi-polar collector plates
anode
coolant in
FUEL CELL STACKFUEL CELL STACK
oxidant + water out
hydrogen in
tie rod
membrane
anode
cathode
coolant out
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Actual fuel cell stacksActual fuel cell stacks
BallardTeledyne
UTC Fuel Cells
Honda
Hydrogenics
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exhaust
pressure
regulator
compressor
load
DC/
DC
hydrogen tankgorivni članak
Fuel cell needs supporting equipment/balanceFuel cell needs supporting equipment/balance--ofof--plantplant
pressure
regulator
compressor
heat
exchanger
water
pump
MM
M
fan
heat exchanger/
humidifier
battery
©2002 by Frano Barbir.
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compressor
battery
heat
exchangers
hydrogen tank
pressure
regulator DC/DC inverter
Actual fuel cell systemActual fuel cell system
fuel cell stack
battery
humidifierwater tank
water pump
el. motor
©2002 by Frano Barbir.
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Understand the difference between:Understand the difference between:
Single cell
StackStack
Systemexhaust
pressure
regulator
pressure
regulator
compressor
heat
exchanger
water
pump
load
MM
M
DC/
DC
hydrogen tank
fan
gorivni članak
heat exchanger/
humidifier
battery
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Fuel Cell Theory Fuel Cell Theory -- ThermodynamicsThermodynamics
Hydrogen side: anode – hydrogen oxidation
Oxygen side: cathode – oxygen reduction
Mnemonic: reduction of oxygen on cathode: red-cat
Basic reactions
Anode reaction: H2 →→→→ 2e– + 2H+ oxidation
Cathode reaction: ½O2 + 2e– + 2H+ →→→→ H2O reduction
Overall reaction: H2 + ½O2 →→→→ H2O
Same as combustion of hydrogen
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Combustion of hydrogen is an exothermic reaction – heat is generated
How much heat?
H2 + ½O2 →→→→ H2O + heat
From the tables:
Heat of formation: ∆∆∆∆H = (h ) – (h ) – ½(h ) = – 286 – 0 – 0 = – 286 kJ/mol
Heat of Formation
∆∆∆∆ means difference between products and reactants
Heat of formation: ∆∆∆∆H = (hf)H2O – (hf)H2 – ½(hf)O2 = – 286 – 0 – 0 = – 286 kJ/mol
mol is a measure of quantity
1 mol of hydrogen contains 2 grams of hydrogen (2.0159 to be more precise)
1 mol of oxygen contains 32 grams of oxygen
1 mol contains 6.022 x 1023 molecules (Avogadro’s number)
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Chemistry can be confusing!!!
Don’t forget – fuel cell is an electrochemical device
Enthalpy = heat of formation
Negative heat means heat is released →→→→ exothermic reaction
Forget about plus and minus →→→→ use common sense
Of course that combustion of hydrogen and oxygen produces heat.
How much heat? →→→→ 286 kJ/mol
This enthalpy of hydrogen combustion reaction is also called:
Hydrogen Heating Value (or to be more precise Higher Heating Value)
obviously – how much heat we can get from hydrogen
Important note: all chemical tables refer to 25ºC.
That means that both reactants and products are at 25 ºC.
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Heat of combustion – heating value
1 mol of H2 + ½ mol of O2
2 g of H2 + 16 g of O2
T = 25 °C (298.15 K)
PV = GRT or PV = NRT
1 mol of H2O (l)
18 g of H2O (l)
T = 25 °C (298.15 K)
Heat = 286 kJ
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Heat of combustion – heating value
1 mol H2 + x mol O2
2 g of H2 + 32x g of O2
T = 25 °C (298.15 K)
PV = GRT or PV = NRT
1 mol of H2O (g) + (x- ½) mol O2
18 g of H2O (g)
T = 25 °C (298.15 K)
Heat = 241 kJ
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Heat of combustion – heating value
Saturation pressure at 25°C:
Psat = 3.169 kPa
Maximum water content in air(oxygen)
- at saturation:
1 mol of H2O (g) + (x- ½) mol O2
18 g of H2O (g)
T = 25 °C (298.15 K)
Heat = 241 kJ
mol H2O/mol O2 = PH2O/PO2
= 3.169/(101.3 – 3.169)
= 0,0323
x – ½ >>>>= 1/0.0323 = 31
X >>>> = 31.5
1 mol H2 + 32 mol O2
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Liquid water produced: 286 kJ/mol
Water vapor produced: 241 kJ/mol
Higher heating value
Lower heating value
The difference is: 45 kJ/mol Heat of evaporation
45 kJ/mol
18 g/mol= 2.5 kJ/g = 2500 kJ/kg
Higher and lower heating value
18 g/mol= 2.5 kJ/g = 2500 kJ/kg
Why do we care about heating value?
There is no combustion in fuel cell!
We use heating value as a measure of energy going into the fuel cell:
mol/s x kJ/mol = kJ/s = kW Beauty of SI system!
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In a fuel cell both heat and electricity are generated.
But how much electricity?
Can all heat be converted into useful work (electricity)?
Remember 2nd law of thermodynamics?
Remember entropy?
∆∆For mechanical processes: W = Q - T∆∆∆∆S
For chemical reactions: ∆∆∆∆G = ∆∆∆∆H - T ∆∆∆∆S
∆∆∆∆G = Gibbs free energy
∆∆∆∆ means difference between products and reactants
∆∆∆∆G = 237.2 kJ/mol (at 25ºC)
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Electrical work = charge x voltage
Mechanical work = distance x force
Charge =
Electrical power = current x voltage
Mechanical power = flow rate x pressure
electrons/molecule of H2 molecules/mol Coulombs/electron
2 6.022x1023 1.602x10-19X X
Faraday’s constant
Theoretical cell voltage
∆∆∆∆G = – 2FE
E = voltage
Faraday’s constant
96,485 C/electron mol
Electrical work =
= – ∆∆∆∆G
2F=
237,200
2x96.485= 1.23
J/mol Ws/mol
C/mol As/mol
Coulomb = Amper.second
= = V
V
Beauty of SI system!
(at 25ºC)
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∆∆∆∆G = ∆∆∆∆H - T ∆∆∆∆S
∆∆∆∆ means difference between products and reactants
hf (J/mol) s (J/mol,K)
H2 0 130.59
O2 0 205.14
H2O (l) –285,838 70.05
H2O (g) –241,827 188.83
More thermodynamics – effect of temperature
∆∆∆∆h = f(T)
∆∆∆∆s = f(T)
∆∆∆∆H = (hf)H2O – (hf)H2 – ½(hf)O2 = – 286 – 0 – 0 = – 286 kJ/mol
∆∆∆∆S = (s)H2O – (s)H2 – ½(s)O2 = 70.05 – 130.59 – ½x205.14 = – 163.11 J/mol,K
Cp = f(T)
∫∫∫∫++++====T
15298
p15298T dTchh.
. ∫∫∫∫++++====T
15298
p15298T dTcT
1ss
.
.
(at 25ºC)
∆∆∆∆G = f(T)
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31.00
33.00
35.00
37.00
39.00
41.00
43.00S
pe
cif
ic h
ea
t (J
/mo
l,K
)
Cp = f(T)
H2O(g)
O2
H2
25.00
27.00
29.00
273 373 473 573 673 773 873 973 1073
Temperature (K)
Sp
ec
ific
he
at
(J/m
ol,
K)
H2
Dashed lines: equations from Larminie&Dicks pp.392-393
Solid lines: equations from Fuel Cell Handbook
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∆∆∆∆H and ∆∆∆∆G as a function of temperature
200
250
300
350d
H o
r d
G (
kJ/m
ol)
G (
kJ/m
ol)
0
50
100
150
0 200 400 600 800 1000 1200
temperature (C)
dH
or
dG
(kJ/m
ol)
H (l)
G (l)
H (g)
G (g)
∆∆ ∆∆H
or ∆∆ ∆∆
G (
kJ/m
ol)
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Theoretical cell voltage – function of temperature
1.05
1.1
1.15
1.2
1.25
1.3C
ell
Po
ten
tial
(V)
liquid water
water vapor
0.8
0.85
0.9
0.95
1
0 200 400 600 800 1000 1200
temperature (C)
Cell
Po
ten
tial
(V)
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Efficiency of any process
Fuel cell (ideal) efficiency
Theoretical Fuel Cell Efficiency
Ein Eout
Edeg
Eout < Einηηηη =
Eout
Ein
∆∆∆∆H ∆∆∆∆G∆∆∆∆G 237.2
∆∆∆∆H ∆∆∆∆G
Q = T ∆∆∆∆S∆∆∆∆G < ∆∆∆∆H
ηηηηfc = ∆∆∆∆G
∆∆∆∆H
237.2
286.0= = 0.83
Remember:∆∆∆∆G
2F= 1.23 V
∆∆∆∆H
2F= 1.48 VBy analogy: thermoneutral voltage
Fuel cell (ideal) efficiency ηηηηfc = = 0.831.23 V
1.48 V
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Sometimes the lower heating value is used to express the efficiency
In internal combustion engines the products leave at a temperature
above 100ºC – product water is steam indeed.
Therefore, the heat of condensation cannot be utilized, and because
of that the lower heating value is used to calculate the engine efficiency.
Hydrogen higher heating value: 286 kJ/mol
Hydrogen lower heating value: 241 kJ/mol
2237G .============
∆ηEfficiency based on higher heating value: 830
0286
2237
H
G.
.
.============
∆∆
η
Efficiency based on lower heating value: 9800241
2237
H
G.
.
.============
∆∆
η
Remember:
∆∆∆∆G/2F = 1.23 V
∆∆∆∆HHHV /2F = 1.482 V
∆∆∆∆HLHV /2F = 1.254 V
Efficiency is also:
(case of condensing boilers)
4821
Vcell
.====η
2541
Vcell
.====η
HHV
LHV
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Maximum efficiency for mechanical systems
Qin
Wout
Qdeg
Wout < Qin
TH
TC
Carnot efficiency
or
Efficiency of Carnot engine
ηηηη = Wout
Qin
= 1 –TC
THQin TH
Carnot efficiency is the maximum efficiency any process/engine
taking place between the two temperatures can have.
However, from any practical point of view the Carnot efficiency
is meaningless!
An engine operating with Carnot efficiency would produce zero power!
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0.5
0.6
0.7
0.8
0.9
eff
icie
ncy
Max efficiency = 1 – Tc/Th
Efficiency at
max. power = 1 – Tc/Th
Carnot efficiency
0
0.1
0.2
0.3
0.4
0.5
0 20 40 60 80 100 120
power
eff
icie
ncy
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An engine operating with Carnot efficiency would produce zero power!
A fuel cell operating with ideal efficiency would produce zero power!
E = – ∆∆∆∆G
2Fis maximum cell voltage
Similarity between Carnot efficiency and fuel cell ideal efficiency
E = 2F
is maximum cell voltage
Maximum voltage is achieved when there is no current.
Power = current x voltage
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0.3
0.4
0.5
0.6
0.7
0.8
0.9
eff
icie
ncy l
imit
Difference between Carnot efficiency and fuel cell ideal efficiency
fuel cell
Carnot
0
0.1
0.2
0.3
0 200 400 600 800 1000 1200
temperature (C)
eff
icie
ncy l
imit
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Ideal cell Voltage at different temperature and pressure
∆∆∆∆G = ∆∆∆∆H - T ∆∆∆∆S
nF
ST
nF
H
nF
G ∆∆∆−−−−==== At 25ºC V231
nF
G.====
∆V4821
nF
H.====
∆V2520
nF
ST.====
∆
ET = 1.482 – 0.000845T Note: ∆∆∆∆H and ∆∆∆∆S are f(T)
For T<373K ET ≈≈≈≈ 1.482 – 0.000845TFor T<373K ET ≈≈≈≈ 1.482 – 0.000845T
All of the above considerations were at ambient pressure
P = 1 atm, P = 101.3 kPa; P = 1.013 bar, P = 14.698 psi; P = 0 psig
Does the ideal voltage change with pressure?
Unfortunately – yes!
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A little bit of basic thermodynamics
– dG = VmdP
For an ideal gas:
PVm = RT
– dG = RTdP
P
– G = – G0 + RT ln P
P0
Fuel cell reaction: H2 + ½ O2 →→→→ H2O
⋅⋅⋅⋅++++====
OH
50OH
0P
2
22
P
PP
nF
RTEE
.
ln
Nernst Equation
⋅+
∆−=
∆−
OH
50
2O2H0
2P
PP
nF
RT
nF
G
nF
G .
ln
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Summary
E0 = E1atm,298.15K = 1.23 V
Ideal fuel cell voltage
⋅⋅⋅⋅++++−−−−====
OH
50OH
TP
2
22
P
PP
F2
RTT00084504821E
.
, ln..
⋅⋅⋅⋅
++++−−−−====50
OH 22PP
T00004310T00084504821E
.
ln...
This is voltage at 0 current
So called open circuit voltage (OCV)
⋅⋅⋅⋅++++−−−−====
OH
OH
TP
2
22
P
PPT00004310T00084504821E , ln...
Caution: valid only for low temperatures
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Ideal Cell voltage: 1.23 V
Function of temperature, pressure and concentration of reactants:
At 80ºC: 1.186 V
Air instead of O2: 1.174 V
Actual voltage is always lower!
0.97
(H2/O2 at 25ºC, 1 atm)
0.97
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Ideal Cell voltage: 1.23 V
Function of temperature, pressure and concentration of reactants:
At 80ºC: 1.186 V
Air instead of O2: 1.174 V
Actual voltage is always lower!
(H2/O2 at 25ºC, 1 atm)
0.97 1.1740.950.900.800.700.600.500.97
current
vo
ltag
e
0.97
1.174
0
0.950.900.800.700.600.50
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Relationship between current and voltage
Butler-Volmer equation
( ) ( ) ( )
−α−−
−α−=
RT
EEF1
RT
EEFii rr0 expexp
i0 = exchange current density
αααα = charge transfer coefficientαααα = charge transfer coefficient
F = Faraday’s constant
R = gas constant
T = temperature
E = potential
Er = reversible potential
αααα ≅≅≅≅ 1
Note: in some literature nαααα is used instead of ααααn = 2 for the anode and n = 4 for the cathode
Larminie&Dicks suggest ααααa = 0.5 and ααααc = 0.1 to 0.5
Newman specifies αααα in a range between 0.2 and 2.0
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Factors affecting the exchange current densityCatalyst
Roughness/actual surface area
Temperature
Reactants concentration
Pressure
−−
=
γ
Crefr
ccref
T
T
RT
E
P
PLaii 1
00 exp
ref
refr
ccTRTP00
i0ref = exchange current density at reference conditions (T,P) in A/cm2 Pt surface
ac = catalyst specific area (for Pt theoretically: 2,400 cm2/mg
practically: 600-1,000 cm2/mg
in electrode: 420-700 cm2/mg
Lc = catalyst loading (today 0.3 to 0.5 mg/cm2)
Ec = activation energy (66 kJ/mol)
R = universal gas constant (8.314 J/molK)
γγγγ = pressure coefficient (0.5 to 1.0)
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( ) ( ) ( )
−α−−
−α−=
RT
EEF1
RT
EEFii rr0 expexp
( ) ( ) ( )
−α−−
−α−=
RT
EEF1
RT
EEFii
araaaraaa0a
,,, expexp
(E – Er) = overpotential
On the fuel cell anode:
Er,a = 0 definition of zero potential – hydrogen electrode
(Ea – Er,a) > 0
( ) ( )
−α−−=
RT
EEF1ii
araaa0a
,, exp
negative sign means current is leaving the electrode
– oxidation reaction
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( ) ( ) ( )
−α−−
−α−=
RT
EEF1
RT
EEFii
crcccrccc0c
,,, expexp
On the fuel cell cathode:
Er,c = 1.23 V
(E – E ) < 0(Ec – Er,c) < 0
( )
−α−=
RT
EEFii
crccc0c
,, exp
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Types of losses in fuel cell:
Activation losses (activation polarization)
Resistive (Ohmic) losses
Mass transfer losses (concentration polarization)
Internal current and fuel crossover
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Activation polarization losses
Tafel equation: ∆∆∆∆Vact = a + b . log(i) (empirical)
α=∆
0i
i
F
RTV ln
∆α=
RT
VFii 0 exp
i = current density (mA/cm2)
i0 = exchange current density (mA/cm2)
R = universal gas constant = 8.314 J/mol
T = temperature of the process (K)T = temperature of the process (K)
α = charge transfer coefficient
n = number of electrons involved (2)
F = Faraday’s constant = 96,485 C
( ) ( )iF
RTi
F
RTV 0 lnln
α+
α−=∆
( ) ( )00 iF
RT32i
F
RTa log.ln
α−=
α−=
F
RT32bα
= . Tafel slope
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Activation polarization losses
α=∆
0i
i
F
RTV ln
∆∆∆∆Vact = a + b . log(i)
0
0.2
0.4
0.6
0.8
1
1.2
po
ten
tia
l lo
ss
(V
)
i0a
slope = b
log scale
For αααα = 1, T = 333 K ⇒⇒⇒⇒ b = 0.066 V/dec
0
0.2
0.4
0.6
0.8
1
1.2
0 500 1000 1500 2000
current density (mA/cm²)
po
ten
tial
loss (
V)
0
0.0001 0.001 0.01 0.1 1 10 100 1000 10000
current density (mA/cm²)
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Resistive losses
Ohmic law: ∆∆∆∆V = I.R = i.Ri
I = current (Amps)
R = resistance (Ohms)
i = current density (A/cm2)
Ri = aerial resistance (Ohm-cm2)
A
dR ρ==== ρ = resistivity (Ohm-cm)
d = distance, length (cm)
A = cross sectional area (cm2)
Ri = R.A
0
0.2
0.4
0.6
0.8
1
1.2
0 500 1000 1500 2000
current density (mA/cm²)
po
ten
tial
loss (
V)
Resistance through:
Membrane (ionic)
Electrodes
Bi-polar plates
Interfacial contacts
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electrode membrane electrode
oxygen feed
collector
plate
O2
external load
electrode membrane electrode
O2
2e-
V
bus
plate
Resistive losses
2H+
H2 H2O
hydrogen feed
bi-polar
collector
plate
2H+
H2 H2O
hydrogen feed
2e-
½O2 + 2H+ + 2e– → H2O
2e-
H2 → 2H+ + 2e–
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Concentration polarization or mass transport losses
Po
iLi
P ii
PPP
L
00 −−−−====
L0 i
i1
P
P−−−−====
−−−−====
====ii
i
F2
RT
P
P
F2
RTV
L
L0 lnln∆
−−−−−−−−====
Li
i1
F2
RTV ln∆or
0
0.2
0.4
0.6
0.8
1
1.2
0 500 1000 1500 2000
current density (mA/cm²)
po
ten
tia
l lo
ss (
V) theoretical
more realistic
⋅=∆d
icVconc exp
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Fick’s First Law of Diffusion:(((( ))))
ACCD
N SB
δ−−−−⋅⋅⋅⋅
====
N = flow rate, mol/s
D = diffusion coefficient, cm2/s
CB = bulk concentration, mol/cm3
CS = concentration at
catalyst surface, mol/cm3
δ= diffusion path, thickness
of diffusion layer, cm
A = active area, cm2
nF
IN ====
====
S
B
C
C
nF
RTV ln∆Voltage gain/loss
(Nernst equation)
Alternative way to express concentration polarization
A = active area, cm2
(((( ))))δ
SB CCDnFi
−−−−⋅⋅⋅⋅⋅⋅⋅⋅====
δB
L
CDnFi
⋅⋅⋅⋅⋅⋅⋅⋅====
LB
S
i
i1
C
C−−−−====
−=∆
L
L
ii
i
nF
RTV ln
Voltage loss due to concentration polarization:
Limiting current CS = 0
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0.6
0.8
1
1.2p
ote
nti
al
loss (
V)
Voltage losses compared
0
0.2
0.4
0.6
0 500 1000 1500 2000
current density (mA/cm²)
po
ten
tial
loss (
V)
activation
resistive
concentration
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Actual fuel cell voltage
concresactPTcell VVVEV ∆∆∆ −−−−−−−−−−−−==== ,
(((( )))) (((( )))) (((( )))) (((( ))))caconcanconcrescaactanactPTcell VVVVVEV ∆∆∆∆∆ −−−−−−−−−−−−−−−−−−−−==== ,
Losses occur on both anode and cathode:
−+⋅−
α−=
Li
0PTcell
i
i1
nF
RTRi
i
i
F
RTEV lnln,
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0.8
0.2
0.4
0.6
Cathode solid phase potential
Anode overpotential
Ecell
E
DL DLCL CL
membrane1 A/cm2
po
ten
tial (V
)
0
0.2
-0.4
-0.2
Anode solid phase potential
Anode overpotential
Cathode overpotential
electrolyte phase potential
Eloss
Eeq
po
ten
tial (V
)
0 50 150100nominal cell thickness (µµµµm)
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0.6
0.8
1
1.2cell
po
ten
tial
(V)
Fuel cell polarization curve
activation losses
resistance losses
theoretical (ideal) voltage
0
0.2
0.4
0.6
0 500 1000 1500 2000
current density (mA/cm²)
cell
po
ten
tial
(V)
mass transport
lossesresulting V vs. i curve
©2002 by Frano Barbir.
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0.6
0.7
0.8
0.9
1p
ow
er
den
sit
y (
W/c
m²)
Power density vs. current density
Power density = V.i (W/cm2 or mW/cm2)
0
0.1
0.2
0.3
0.4
0.5
0.6
0 500 1000 1500 2000
current density (mA/cm²)
po
wer
den
sit
y (
W/c
m²)
©2002 by Frano Barbir.
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0.5
0.6
0.7
0.8
0.9
1
cell
po
ten
tial
(V)
Cell potential vs. power density
0
0.1
0.2
0.3
0.4
0.5
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
power density (W/cm²)
cell
po
ten
tial
(V)
©2002 by Frano Barbir.
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2e-
O2
e- loss
Internal currents and fuel crossover
2H+
1/2O2 + 2H+ + 2e- → H2O
H2OH2
H2 → 2H+ + 2e-
H2 loss
©2002 by Frano Barbir.
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Hydrogen consumption in fuel cell
Faraday’s law:
I = nFN
(Amps = Amp-second/mol x mol/s) nF
IN ====
Hydrogen consumption in fuel cell: NH2 = I/(2F)
mol/s
Oxygen consumption in fuel cell: NO2 = ½ NH2 = I/(4F)
Water generation in fuel cell: NH2O = I/(2F)
©2002 by Frano Barbir.
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Both internal currents and hydrogen crossover mean loss of current:
Iloss = 2FNloss
This means that even at 0 external current there is hydrogen consumption.
+
α−=
0
lossPTcell
i
ii
F
RTEV ln,
Internal currents and fuel crossover
0.900
0.920
0.940
0.960
0.980
1.000
1.020
0 2 4 6 8 10 12
current density (mA/cm2)
ce
ll p
ote
nti
al
(V)
iloss = 1 mA/cm2
iloss = 2 mA/cm2
iloss = 4 mA/cm2
iloss = 8 mA/cm2
©2002 by Frano Barbir.
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Fuel cell efficiency4821
Vcell
.====η HHV
Efficiency = Electricity out
Hydrogen in=
I.V
N.HHHV
=I.V
F2
IN ====
I.HHHV/(2F)
Vcell
1.482=
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Cell potential vs. power densityefficiency
0.5
0.6
0.7
0.8
0.9
1
cell
po
ten
tial
(V)
cell
eff
icie
ncy (
HH
V)
0.7
0.6
0.5
0.4
0
0.1
0.2
0.3
0.4
0.5
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
power density (W/cm²)
cell
po
ten
tial
(V)
cell
eff
icie
ncy (
HH
V)
0.3
0.2
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Fuel cell efficiency4821
Vcell
.====η HHV
Efficiency = Electricity out
Hydrogen in=
I.V
N.HHHV
=I.V
F2
IN ====
I.HHHV/(2F)
Vcell
1.482=
Hydrogen in = hydrogen converted into current + hydrogen loss
II++++ iVIV
F2
IIN loss++++====
loss
cell
loss
cell
ii
i
4821
V
II
I
4821
V
++++====
++++====
..η⇒⇒⇒⇒
electrolyte
electrode
electrode
hydrogen in
oxygen in
hydrogen out
oxygen outelectrolyte
electrode
electrode
hydrogen in
oxygen in
hydrogen out
oxygen out
In some cases excess hydrogen is fed into the fuel cell – some H2 is wasted
fucellV
η=η4821.
hydrogen consumed
hydrogen suppliedηηηηfu =
![Page 80: Thermodynamics and Electrochemistry Of Low Temperature ... · Friedrich Wilhelm Ostwald –visionary ideas: “I do not know whether all of us realise fully what an imperfect thing](https://reader035.vdocuments.mx/reader035/viewer/2022062914/5e6d0d8639262460801dde58/html5/thumbnails/80.jpg)
Fuel cell efficiency vs. power density
0.5
0.6
0.7
0.8
0.9
1eff
icie
ncy (
HH
V)
0
0.1
0.2
0.3
0.4
0.5
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
power density (W/cm²)
eff
icie
ncy (
HH
V)
![Page 81: Thermodynamics and Electrochemistry Of Low Temperature ... · Friedrich Wilhelm Ostwald –visionary ideas: “I do not know whether all of us realise fully what an imperfect thing](https://reader035.vdocuments.mx/reader035/viewer/2022062914/5e6d0d8639262460801dde58/html5/thumbnails/81.jpg)
Additional slides
![Page 82: Thermodynamics and Electrochemistry Of Low Temperature ... · Friedrich Wilhelm Ostwald –visionary ideas: “I do not know whether all of us realise fully what an imperfect thing](https://reader035.vdocuments.mx/reader035/viewer/2022062914/5e6d0d8639262460801dde58/html5/thumbnails/82.jpg)
0.6
0.8
1
1.2c
ell
po
ten
tia
l (V
)Tafel = 77 mV/dec
Tafel = 66 mV/dec
Tafel = 55 mV/dec
−+⋅−
+
α−=
Li
0
lossPTcell
i
i1
nF
RTRi
i
ii
F
RTEV lnln,
0
0.2
0.4
0.6
0 500 1000 1500 2000
current density (mA/cm²)
ce
ll p
ote
nti
al
(V)
![Page 83: Thermodynamics and Electrochemistry Of Low Temperature ... · Friedrich Wilhelm Ostwald –visionary ideas: “I do not know whether all of us realise fully what an imperfect thing](https://reader035.vdocuments.mx/reader035/viewer/2022062914/5e6d0d8639262460801dde58/html5/thumbnails/83.jpg)
0.8
1
1.2c
ell
po
ten
tia
l (V
)
io = 0.0003 mA/cm²
io = 0.003 mA/cm²
io = 0.03 mA/cm²
−+⋅−
+
α−=
Li
0
lossPTcell
i
i1
nF
RTRi
i
ii
F
RTEV lnln,
0
0.2
0.4
0.6
0 500 1000 1500 2000
current density (mA/cm²)
ce
ll p
ote
nti
al
(V)
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0.2
0.4
0.6
0.8
1
1.2
ce
ll p
ote
nti
al
(V)
ix = 0 mA/cm²
ix = 2 mA/cm²
ix = 20 mA/cm²
1.2ix = 0 mA/cm²
−+⋅−
+
α−=
Li
0
lossPTcell
i
i1
nF
RTRi
i
ii
F
RTEV lnln,
0
0.2
0 500 1000 1500 2000
current density (mA/cm²)
0.6
0.7
0.8
0.9
1
1.1
0 20 40 60 80 100
current density (mA/cm²)
ce
ll p
ote
nti
al
(V)
ix = 2 mA/cm²
ix = 20 mA/cm²
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0.8
1
1.2c
ell
po
ten
tia
l (V
)
R = 0.1 Ohm-cm²
R = 0.15 Ohm-cm²
R = 0.2 Ohm-cm²
−+⋅−
+
α−=
Li
0
lossPTcell
i
i1
nF
RTRi
i
ii
F
RTEV lnln,
0
0.2
0.4
0.6
0 500 1000 1500 2000
current density (mA/cm²)
ce
ll p
ote
nti
al
(V)
![Page 86: Thermodynamics and Electrochemistry Of Low Temperature ... · Friedrich Wilhelm Ostwald –visionary ideas: “I do not know whether all of us realise fully what an imperfect thing](https://reader035.vdocuments.mx/reader035/viewer/2022062914/5e6d0d8639262460801dde58/html5/thumbnails/86.jpg)
0.8
1
1.2c
ell
po
ten
tia
l (V
)
iL = 1200 mA/cm²
iL = 1600 mA/cm²
iL = 2000 mA/cm²
−−−−++++⋅⋅⋅⋅−−−−
++++−−−−====
Li
0
lossPTcell
i
i1
nF
RTRi
i
ii
nF
RTEV lnln, α
0
0.2
0.4
0.6
0 500 1000 1500 2000
current density (mA/cm²)
ce
ll p
ote
nti
al
(V)
![Page 87: Thermodynamics and Electrochemistry Of Low Temperature ... · Friedrich Wilhelm Ostwald –visionary ideas: “I do not know whether all of us realise fully what an imperfect thing](https://reader035.vdocuments.mx/reader035/viewer/2022062914/5e6d0d8639262460801dde58/html5/thumbnails/87.jpg)
0.8
1
1.2c
ell
po
ten
tia
l (V
)
P = 300 kPa
P = 200 kPa
P = 101.3 kPa
−+⋅−
+
α−=
Li
0
lossPTcell
i
i1
nF
RTRi
i
ii
F
RTEV lnln,
Operating pressure
0
0.2
0.4
0.6
0 500 1000 1500 2000
current density (mA/cm²)
ce
ll p
ote
nti
al
(V)
![Page 88: Thermodynamics and Electrochemistry Of Low Temperature ... · Friedrich Wilhelm Ostwald –visionary ideas: “I do not know whether all of us realise fully what an imperfect thing](https://reader035.vdocuments.mx/reader035/viewer/2022062914/5e6d0d8639262460801dde58/html5/thumbnails/88.jpg)
++++−−−−====
============
3P0
loss3PT3Pcell
i
ii
nF
RTEV
,
,, lnα
(((( )))) (((( )))) (((( ))))loss3P03T3Pcell iiF250
RTi
F250
RTP
F4
RTEV ++++
⋅⋅⋅⋅−−−−
⋅⋅⋅⋅++++++++==== ======== ln
.ln
.ln ,,
(((( )))) (((( )))) (((( ))))loss1P01T1Pcell iiRT
iRT
PRT
EV ++++−−−−++++++++==== ======== lnlnln ,, (((( )))) (((( )))) (((( ))))loss1P01T1Pcell iiF250
iF250
PF4
EV ++++⋅⋅⋅⋅
−−−−⋅⋅⋅⋅
++++++++==== ======== ln.
ln.
ln ,,
++++
====−−−− ========1
3
F
RT
1
3
F4
RTVV 1Pcell3Pcell lnln,,
Vcell,P=3 – Vcell,P=1 = 0.039 V (at 60º)
![Page 89: Thermodynamics and Electrochemistry Of Low Temperature ... · Friedrich Wilhelm Ostwald –visionary ideas: “I do not know whether all of us realise fully what an imperfect thing](https://reader035.vdocuments.mx/reader035/viewer/2022062914/5e6d0d8639262460801dde58/html5/thumbnails/89.jpg)
0.6
0.8
1
1.2c
ell
po
ten
tia
l (V
)air
oxygen
−+⋅−
+
α−=
Li
0
lossPTcell
i
i1
nF
RTRi
i
ii
F
RTEV lnln,
Air vs. oxygen
0
0.2
0.4
0.6
0 500 1000 1500 2000
current density (mA/cm²)
ce
ll p
ote
nti
al
(V)
V0560210
1
F
RT
210
1
F4
RTVV 1Pcell3Pcell .
.ln
.ln,, ====
++++
====−−−− ========
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Evidence of Mass Transport Losses
Source: T.P. Ralph and M.P. Hogarth, Platinum Metals Review, Vol. 46, No. 1, pp. 3-14, 2002
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Constant current
Constant voltage
Constant resistance
Constant power
Variable power
0.6
0.7
0.8
0.9
1
ce
ll p
ote
nti
al
(Vo
lts
)V OP1
OP2
R=2ΩΩΩΩcm2 1 0.5
w=1W/cm2
0
0.1
0.2
0.3
0.4
0.5
0.6
0 200 400 600 800 1000 1200 1400 1600 1800 2000
current density (mA/cm2)
ce
ll p
ote
nti
al
(Vo
lts
)V OP1
0.2
w=1W/cm
0.8
0.6
0.4
0.2
![Page 94: Thermodynamics and Electrochemistry Of Low Temperature ... · Friedrich Wilhelm Ostwald –visionary ideas: “I do not know whether all of us realise fully what an imperfect thing](https://reader035.vdocuments.mx/reader035/viewer/2022062914/5e6d0d8639262460801dde58/html5/thumbnails/94.jpg)
Operational conditions.
Operating pressure
Flow rates of reactants
Operating temperature
Humidity of reactants
What else can affect the polarization curve?
Heat transfer
Fluid mechanics
Fluid mechanics/chemistry
PsychrometryHumidity of reactants Psychrometry
Thermodynamics of moist gases
We must know some basics!
We must know fuel cell inner working!
![Page 95: Thermodynamics and Electrochemistry Of Low Temperature ... · Friedrich Wilhelm Ostwald –visionary ideas: “I do not know whether all of us realise fully what an imperfect thing](https://reader035.vdocuments.mx/reader035/viewer/2022062914/5e6d0d8639262460801dde58/html5/thumbnails/95.jpg)
What else can affect the polarization curve?
Unsteady conditions (flooding/drying)
Contaminants
Crossover leaks
Aging (loss of catalyst surface, crossover leaks,
change in membrane properties)
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Summary
Voltage losses:
Activation polarization
Ohmic (resistive losses)
Concentration polarization or mass transport losses
Internal currents and reactants crossover
++++ iRTiiRT
Resulting polarization curve:
4821
Vcell
.====ηFuel cell efficiency:
−−−−++++⋅⋅⋅⋅−−−−
++++−−−−====
L
i
0
lossPTcell
i
i1
nF
RTRi
i
ii
nF
RTEV lnln, α
Effect of operating pressure
Effect of oxygen vs. air
![Page 97: Thermodynamics and Electrochemistry Of Low Temperature ... · Friedrich Wilhelm Ostwald –visionary ideas: “I do not know whether all of us realise fully what an imperfect thing](https://reader035.vdocuments.mx/reader035/viewer/2022062914/5e6d0d8639262460801dde58/html5/thumbnails/97.jpg)
PEM Specific Issues – Water transport, gas diffusion
![Page 98: Thermodynamics and Electrochemistry Of Low Temperature ... · Friedrich Wilhelm Ostwald –visionary ideas: “I do not know whether all of us realise fully what an imperfect thing](https://reader035.vdocuments.mx/reader035/viewer/2022062914/5e6d0d8639262460801dde58/html5/thumbnails/98.jpg)
PEM Fuel Cell:
Membrane/Electrode Processes
e-e-
e-
e-
e-e-
Hydrogen
Oxygen
e-
H2 → 2H+ + 2 e- 2H+ + 1/2O2 + 2 e- → H2O
e-
+
+
+ +
+
+
e-e-
e-
Platinum
Carbon
Functional Group
Polymer Electrolyte
e- Electron
Materials Science and Technology Division
![Page 99: Thermodynamics and Electrochemistry Of Low Temperature ... · Friedrich Wilhelm Ostwald –visionary ideas: “I do not know whether all of us realise fully what an imperfect thing](https://reader035.vdocuments.mx/reader035/viewer/2022062914/5e6d0d8639262460801dde58/html5/thumbnails/99.jpg)
Water content and conductivity
Source: T.A. Zawodzinski, et al., J. Electrochem. Soc., Vol. 140, No. 7, pp. A1981-A1985, 1993
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λλλλ = water molecules per sulfonate groupλλλλ = water molecules per sulfonate group
![Page 101: Thermodynamics and Electrochemistry Of Low Temperature ... · Friedrich Wilhelm Ostwald –visionary ideas: “I do not know whether all of us realise fully what an imperfect thing](https://reader035.vdocuments.mx/reader035/viewer/2022062914/5e6d0d8639262460801dde58/html5/thumbnails/101.jpg)
Water content and conductivity
Source: T.A. Zawodzinski, et al., J. Electrochem. Soc., Vol. 140, No. 7, pp. A1981-A1985, 1993
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Nafion conductivity: ~0.1 S/cm
1/conductivity = resistivity = 10 Ohm-cm
Nafion 115 thickness: 5 mils = 0.0127 cm
Nafion conductivity
Resistance = 10 Ohm-cm x 0.0127 cm = 0.127 Ohm-cm2
At 1 A/cm2 it would cause 0.127 V loss
Nafion conductivity is a function of water content and temperature!
![Page 103: Thermodynamics and Electrochemistry Of Low Temperature ... · Friedrich Wilhelm Ostwald –visionary ideas: “I do not know whether all of us realise fully what an imperfect thing](https://reader035.vdocuments.mx/reader035/viewer/2022062914/5e6d0d8639262460801dde58/html5/thumbnails/103.jpg)
Nafion resistivity – function of membrane thickness?
• unisotropic structure
• resistance from the electrodes
• thickness of the membrane in the cell different from measured before assembly
Source: F.N. Buchi and G.G. Scherer, J. Electrochem. Soc., Vol. 148, No. 3, pp. A181-A188, 2001
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• Electro-osmotic
drag removes
water from the
anode side.
With thicker 0.6
0.8
1.0
HF R
ES
ISTA
NC
E (ohm
cm
2)
or C
ELL V
OLTA
GE
(V
)
Cell Resistance and Performance:
PEM Thickness Effects
Increasing
Membrane
Thickness
With thicker
membranes,
back diffusion
of water is
difficult - the
anode side loses
water content.
0.0
0.2
0.4
0.0 0.5 1.0 1.5 2.0 2.5 3.0
HF R
ES
ISTA
NC
E (ohm
cm
or C
ELL V
OLTA
GE
(V
)
CURRENT DENSITY (A/cm2)
Fg 3, TF69,109,74,117 Ox's
(Neat
Oxygen
Cathodes)
Materials Science and Technology Division
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Membrane resistance – function of current density
Source: F.N. Buchi and G.G. Scherer, J. Electrochem. Soc., Vol. 148, No. 3, pp. A181-A188, 2001
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Permeation of gases through the membrane
Permeability is a product of diffusivity and solubility:
Pm = D x S
cm2/s x mol/cm3atm =mol
cm.s.atm
For ideal gas: Pvm = RT; at standard conditions vm = 0.024 m3/mol
mol.cm
cm2.s.atmor
For ideal gas: Pvm = RT; at standard conditions vm = 0.024 m3/mol
vm = 24,060 cm3/mol
cm3.cm
cm2.s.atm
cm3.cm
cm2.s.cmHg1 atm = 75 cmHg = 1010 barrers
Example:
DH2 = 6.65x10-7 cm2/s
SH2 = 2.2x10-5mol/cm3atm
Pm = 1.5x10-11mol.cm-1s-1atm-1
1.5x10-11mol.cm-1s-1atm-1 x 24,060 cm3/mol =
= 3.6x10-7 cm3.cm.cm-2.s-1atm-1=
= 3.6x10-7 cm3.cm.cm-2.s-1atm-1/75 cmHg/atm =
= 4.8x10-9 cm3.cm.cm-2.s-1cm-1Hg = 48 Barrers
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Permeation rate, N (mol/s)
N = Pm.A.∆P/d
N = I/nF
i = I/A = nF.Pm.∆P/d
Example:
Pm = 1.5x10-11mol.cm-1s-1atm-1
A = 100 cm2
d = 51 µm = 0.0051 cm
N = 2.94x10-7mol/s
I = nFN = 0.057 A
NV = N.24060 (cm3/mol) = 0.125×I cm3/s
NV = 7.5 cm3/min/Amp
d = 51 µm = 0.0051 cm∆P = 1 atm (partial pressure)
I = nFN = 0.057 A
i = I/A = 0.00057 A/cm2 = 0.57 mA/cm2
Additional variable (complication)
DH2 = f(T) = 0.0041e-2602/T
0.00E+00
1.00E-06
2.00E-06
3.00E-06
4.00E-06
5.00E-06
0 20 40 60 80 100 120
temperature (°C)
dif
fusiv
ity (
cm
²/s)
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Permeation of gases
through Nafion
Source: T. Sakai, et al., J. Electrochem. Soc., Vol. 133, No. 1, pp. 88-92, 1986