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Introduction to SOFC Technologies
Fundamentals of Electro-Chemistry, Electrochemical Kinetics &
Solid State Chemistry
Robert Mücke
Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research(IEK-1: Materials Synthesis and Processing)
Joint European Summer School for Fuel Cell and Hydrogen TechnologyViterbo, ItalyAugust 22nd 2011
2 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
1. Introduction• electrochemical cells • thermodynamic potentials
2. The Driving Force• 1.+2. law of thermodynamics• chemical potentials • Nernst‘s potential • open cell voltage
3. Drawing a Current• i-V characteristics • ohmic losses • polarisation• electrochemical impedance spectroscopy
4. Operation of SOFCs• operation field • degradation
Contents
3 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
1. IntroductionElectrochemical Cells
Electrochemical Cells
Galvanic CellsElectrolytic Cells
Substance producers
not carried out spontaneously (free
energy change ΔΔΔΔG>0)
Energy producers
carried out spontaneously (free
energy change ΔΔΔΔG<0)
Batteries Fuel Cells
Electrodes contain fuel(being dissociated)
Fuel separately added
4 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Electrochemical Cells - Principle
anode electrolyte cathode
e–
I–
I+
or
electrical work(input or output)
IX → e– + I+ + X
I– + Y → e– + IYor
electronation/oxidation
ionconduction
de-electronation/reduction
e– + I+ + Y → IY
e– + IX → I– + Xor
IX + Y → IY+ X
(simplified)overall cell reaction
SOFC:X= –
Y = fuelI = ½ O2I–= O2–
Li-Ion (e.g.)X= C
Y = Li1-xCoO2I = LiI+= Li+
5 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Electrolytic cells
Electrical work is used for splitting a HI solution (e.g. I=O2)
positive negative
6 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Galvanic Celle.g. SOFC
positivenegative
7 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Chemical vs. Electrochemical Reactions
Chemical Reaction Electro-chemical reaction
Activation thermal electric
Intermediates formed
collision of reactants separated electron / ion transfer
Electrons directly released and taken during reaction
continuously transferred from anode to cathode in external circuit
Products anywhere in reacting system
at the 2 electrode/electrolyte interfaces
Additional degree of freedom: Electrical field(supplied voltage / current load)
8 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Repetition of Basic Principle
electrolyte
cathode
anode
oxidation
reduction
L.G.J. de Haart, IEF-3, FZJ
fuel
air / O2
user
9 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Umolecular energy
F = U –TSmolecular energy minus environmentenergy
H = U + pVmolecular energy plus energy of constrained room
G = U +pV–TS
molecular energy plus energy of constrained room minus environment energy
+ p Vp V = work needed to get a certain pressure + volume
Thermodynamic PotentialsInternal Energy, Entropy, Enthalpy, Free Energy
10 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Umolecular energy
F = U –TSmolecular energy minus environmentenergy
H = U + pVmolecular energy plus energy of constrained room
G = U +pV–TS
molecular energy plus energy of constrained room minus environment energy
+ p V
– T S
p V = work needed to get a certain pressure + volume
–T S = energy provided from the environment limits the system work
300K
QH
QCW
600K
0 K
water wheel analogy
Thermodynamic PotentialsInternal Energy, Entropy, Enthalpy, Free Energy
11 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
+
-
Anode side
= 0.21 bar
~ 10–21 bar
Ucell
Cathode side
2. The Driving ForceThe Chemical Potential
1 O2
4 e–
2 O2–
Assuming homogenous temperature
Chemical potential
Current flow
Power (if no losses)
Open cell voltage
~1 V
12 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
The Ideal SOFCFirst Law of Thermodynamics
Fuel cellT, p
totalreactants’enthalpy
> 0
totalproducts’enthalpy
< 0
Qreleased heatQ < 0
W
delivered workW < 0
energies that are transferred into the system are positive,ones that are released are negative
Conservation of energy:
Energy of fuel isdecreased
13 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Reaction Enthalpies for SOFC Fuels(Direct Oxidation)
H2 + ½O2 → H2O
CO + ½O2 → CO2
CH4 + 2O2 → 2 H2O+CO2
reaction enthalpies from Singhal & Kendall, High Temperature Solid Oxide Fuel Cells, Elsevier, Oxford 2003, p.61
–242 kJ/mol
–283 kJ/mol
–802 kJ/mol
at standard conditions(25°C, 1bar)
binding energy(enthalpy)
14 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
The Ideal, Reversible SOFCSecond Law of Thermodynamics
energies that are transferred into the system are positive,ones that are released are negative
Fuel cellT, p
totalreactants’entropy
> 0
released heat(reversible)
Q < 0
reversible work
totalproducts’entropy
Ideal reversible system:
Entropy of reactants isdecreased
15 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
The Ideal, Reversible SOFCUsable Work
Putting first & second law of thermodynamics together:
16 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Entropy (S)
Measure of disorder, non-usable energy
Standard state conditionsPartial pressures of gases involved: 0.1 MPaConcentrations of aqueous solutions: 1 MTemperature of 25 C (298 K)
Entropy increases (S ), if:• degrees of freedom (f ) • solid liquid gas• T• number of particles (n) • number of soluted phases
During chemical reaction:changes dS measured from heat transfer δQ at certain temperature T
17 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Recation Entropies for SOFC Fuels(Direct Oxidation)
H2 + ½O2 → H2O
CO + ½O2 → CO2
CH4 + 2O2 → 2 H2O+CO2
reaction entropies from Singhal & Kendall, High Temperature Solid Oxide Fuel Cells, Elsevier, Oxford 2003, p.61
–44.3
–86.4
–5.13
1 mol + ½ mol → 1 mol
1 mol + ½ mol → 1 mol
1 mol + 2 mol → 2 mol + 1 mol
at standard conditions(25°C, 1bar)
(800°C, 1bar)
usablebinding energy
–242
–283
–802
binding energy
[J/(mol K)] [kJ/mol] [kJ/mol]
–195
–192
–797
18 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
The Ideal, Reversible SOFCThermal Efficiency
from first & second law of thermodynamics:
theoretical efficiency decreaseswith temperature (in contrast to thermal engines)
efficieny:
19 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
0%
20%
40%
60%
80%
100%
0 200 400 600 800 1000
Operating temperature [°C]
Eff
icie
ncy
Carnot cycle (against 25°C)
endoreversible Carnot cycle
reversible oxidation H2
reversible oxidation CO
reversible oxidation CH4
Comparison with Thermal Engines
[kJ/mol]
H2 CO CH4
ΔrH
ΔrS
reaction enthalpies / entropies from Singhal & Kendall 2003, p.61, (reversible oxidation of fuels)
CH4
H2
CO
H2 + ½O2 → H2O
CO + ½O2 → CO2
CH4 + 2O2 →2 H2O+CO2
20 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Reversible Cell Voltage(Gibbs Potential)
2H+ + 2e–
½O2H2OO2–
2 e–
anode electrolyte cathode
2 e–
O2–H2
work
fuel H2 CO CH4
nel 2 2 8
Current
electrons per fuel molecule
Power (if no losses occur)
Reversible voltageuse of tabulated values only valid for standard conditions and negligible fuel utilisation (mixing of fuel+products is irreversible)
fuel H2 CO CH4
nel 2 2 8
U
also termed (standard) Gibbs potentialdoes not include any ohmic etc. losses
open circuit voltage
21 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Pressure Dependence (Nernst Equation)
Pressure depending entropy (ideal gas)
Cell voltage without losses (Nernst potential):
Reaction entropy difference after integration yields
p0with equilibrium constant K
pj
vj
Terming: Nernst potential is the electromotoric force (EMF)UNernst(i = 0 A/cm²) = Open Cell Voltage = Open Circuit Voltage (OCV) = UOCV
22 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Cell Voltage vs. Temperature and Pressure
reaction entropies from Singhal & Kendall, High Temperature Solid Oxide Fuel Cells, Elsevier, Oxford 2003, p.61
p = overallpressure
CH4: reactantsand products:almost same volume
23 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Example of Nernst Potential
H2 + ½O2 → H2O
1 mol + ½ mol → 1 mol
vj
Slightly rewritten form of Nernst potential:
24 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
0.8
0.9
1.0
1.1
1.2
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Wasserstoffpartialdruck / bar
Off
ene
Zel
lsp
ann
un
g (O
CV
) vs
Lu
ft /
Vo
lt
900 °C
850 °C800 °C
750 °C700 °C
650 °C600 °C
Wasserdampf-partialdruck
0,03 bar
A,OH
A,H½
K,O0OCV
2
22
p
ppln
F2RT
UU +=OHO½H 222 →+
open
cell
volta
ge(O
CV
) vs
air,
V
hydrogen partial pressure, bar
water vaporpressure0.03 bar
Example of Nernst PotentialpH2, Different Temperatures
25 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Example of Nernst Potentialvs. Fuel Utilisation (Uf)
(excess of air)
Singhal & Kendall, High Temperature Solid Oxide Fuel Cells, Elsevier, Oxford 2003, p.65
Higher fuel utilisation leads to mixing of gases (additional ΔS>0) and changesof gas concentration (partial pressures)
26 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
pO2 Form from Nernst Potential
H2 + ½O2 → H2O H2 + ½O2
Keq→←
Equilibrium coefficient of reaction (anode side)
Relationship to Gibbs standard energy:
Use for overall fuel cell reaction in Nernst equation:
superscript A: anode sidesuperscript C: cathode side
27 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
3. Drawing a Currentno current, i = 0 with current load, i > 0
- -
28 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Experimental Setup
29 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
OCV
Vol
tage
, U
[V]
0
Nernst potential (EMF)
gas consumption
ohmic losses
polarisation losses
: overpotentials
actual cell voltage
Current density, i [A/cm²]
Drawing a Current – Overpotentials
30 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Drawing a Current – Overpotentials
OCV
Current density, i [A/cm²]
Vol
tage
, U
[V]
0
gas consumption
ohmic losses
polarisation losses
: overpotentials
activationoverpotential
ohmic range
concentrationoverpotential
Nernst potential (EMF)
31 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Ohmic Losses(also Ohmic Polarisation)
Ω
indexACE
ΩΩ
σ
Current density, i [A/cm²]
Vol
tage
, U [
V]
0
Nernst potential
ohmic range
if Nernst potential is constant over Δi (low or constant Uf only!)
Nernst potential (emf)
Otherwise
32 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
anode supported cell (ASC)(800°C)
electrolyte supported cell (ESC)(850°C)
thickness ASR[mΩ cm²]
% total ASR
thickness[μm]
ASR[mΩ cm²]
% total ASR
Anode ~50 ~25% 310 32%
Electrolyte 50 25% 500 53%
Cathode ~150 ~50% 140 15%
Ohmic LossesAn Practical Example
electrolyte support
cathode
anode
anodesupport
33 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Comparison of Measured i-V curvesASC vs. ESC
ASC = anode supported cellESC = electrolyte supported cellspecifications: see last slide
ASR~0.2 Ω cm²
ASR~0.95 Ω cm²
Better ion-conducting materials for ESCsgive now upto i = 0.8 A/cm² at 0.7 V
0.7
34 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Potential of Thinner ASC ElectrolyteASRs
even more obvious in linear scale
35 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Potential of Thinner ASC ElectrolyteCurrent Densities Results (I)
(La0.58Sr0.4Fe0.2Co0.8O3- )
36 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Potential of Thinner ASC ElectrolyteCurrent Densities Results (II)
Han, PhD Thesis, 2010
37 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Real LifeContact Layers / Contacting
Cr alloyinterconnect steel
Cr O2 3
(Cr,Mn) O3 4
MnOx
cathode contact layer
cathode current collector
~10 m cm²Ω}
~120 μm
~50 μm
~10 μm
~0,02 S/cm
~10 S/cm
~100 S/cm
thicknessmaterialsconduct.
materials ASR
~1-4 μm
~0,1 m cm²Ω
~0,1 m cm²Ω
~50 S/cm ~0,1 m cm²Ω
+ contact potentials + contacting area + longitudinal resistance
Interconnect
Interconnect
Cell
~1-3 mm
Currently dominating issue!upto 50% performance loss forhigh performance ASC due to contacting
38 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
O2- O2- O2- O2-
O2 N2
Concentration Polarisation(or Concentration Overpotential)
Definition:is the resistance in mass transfer of the electroactive speciesto or from the electrodes (diffusion/permeation limit, cell starves)
O2- O2- O2- O2-
H2 H2O
binary diffusivity
anode cathode
39 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Activation PolarisationAnode Side
surface diffusion (?)
or ?
Nickel
Electrolyte
H2 (internal reforming)
fuel adsorption
TPB
anodic reaction (model)
O2–
2e–
VO••
40 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Activation PolarisationAnode Side
surface diffusion (?)
or (after all, these are just models)
Nickel
Electrolyte
H2 (internal reforming)
fuel adsorption
TPB
anodic reaction (model)
O2–
2e–
VO••
(Tafel equation)
41 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Activation PolarisationCathode Side
catalyst without ionic conductivity(only electronic)
catalyst with high ionic conductivitymixed ionic electronic conductor (MIEC)
42 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Mixed Ionic Electronic ConductorCathodes
direct tp
b
reaction
O2(g)
O x(a
d)
O2-
VÖ
Ox(ad)
Ogb
Ox(ad)
O2(g)
VÖ VÖ
O2(g)
Ox(ad)
gasdiffusion
surf
ace
diff
usio
n
volu
me
diff
uion
grai
n bo
unda
ry d
iffus
ion
YSZ
(Ln,A)MTO3+δδδδ
VÖh+
(h+)
e-
VÖ
1 μm
micropores
e-/h+-(surface)
conductivity of electrolyte
h+h+
h+
(h+)
h+
(h+)
h+
43 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
ηα−−ηα= AcAas
OHrH,0 RT
nFexp
RTnF
expppjj222H
AK , ηη
Butler-Volmer equation
ηα−−ηα= Acs
OHrH,0Aa
sOH
rH,0 RT
nFexpppj
RTnF
expppjj222H222H
−+ += jjj
process of charge transfer (no diffusion limit)
−− +↔+ e2OHOH 22
2
Butler-Volmer Equation
44 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
cathodicanodictotal
η
+j
j
−+ += jjj
−j
ηα−−ηα= AcAas
OHrH,0 RT
nFexp
RTnF
expppjj222H
Butler-Volmer Equation
Tafelregion
−ηα= Aas
OHrH,0 RT
nFexpppjj
222H
45 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Contribution of Different Polarisations
0 3.02.52.01.51.00.5
cathodeactivation
anodeactivation
ohmic
Current density, i [A/cm²]
Model calculation for ASC 800°C
S.H. Chan et al., Journal of Power Sources, 93 (2001), 130-140
46 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Electrochemical Impedance Spectroscopy (EIS)
I. Finke, IEF-3, FZJ
47 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Phase Angles
I. Finke, IEF-3, FZJ
48 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Z´´
Z´
10 mHz
1 Hz1 kHz
100 kHz
impedance spectrum (Nyquist-Plot)
≈ 1 μm
cathode
electrolyte
+ + + + + + +- - - - - - -
Equivalent Circuits and Impedance Spectra
O2–
h+h+ ½ O2(g)
A. Weber, IWE, Karlsruhe
anodecathode electrolyte
RΩ,AnRΩ,catRΩ,El
Rpol,anCpol,an
Rpol,catCpol,cat
simplified equivalent circuit
R0 = RΩ,Cat + RΩ,An + RΩ,El
Rpol,catCpol,cat
Rpol,anCpol,an
ohmiclosses
cathode + anodepolarisations
simplified reaction on tri phase boundary
Rpol,catCpol,cat
49 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Equivalent Circuit Elements
Frequently used:
• Charge transfer R
• Double layer capacitance CZ'
–Z''
parallel
• Diffusion processes Warburg (W)
Z'
–Z''
Z'
–Z''
• real processes:constant phase element Qinstead of C
Q parallel with R
• Coupled reactions anddiffusion
Gerischer (G)
Z'
–Z''
50 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Multiple Equivalent Circuitsfor Simulating Results
Z'
–Z''
All three equivalent circuits are possible!
51 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Getting OCV and CurrentCharacteristics Simultaneously
reference electrode:• to determine the potential drop / overpotentialat the working electrode• as close as possible to the working electrode as possible to avoid Ohmic contributionsdraw
current
reference electrode
52 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
3 Electrode Setup
I. Finke, IEF-3, FZJ
Potentiostat
53 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
EIS Example withDistribution of Relaxation Times
Leonide et. al, Journal of The Electrochemical Society, 155, B36-B41 (2008)
Nyquist-Plot
Equivalent Circuit
Distribution of Relaxation Times
ASC/800°C
54 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen TechnologyLeonide et. al, Journal of The Electrochemical Society, 155, B36-B41 (2008)
Cathode p(O2) Anode p(H2O)
EIS Example withDistribution of Relaxation Times
55 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
OCVU
00
Stromdichte / A/cm²
Zel
lspa
nnun
g /
V
Leis
tung
sdic
hte
/ W
/cm
²
Pmax
cell
volta
ge, V
current density, A/cm²
pow
er d
ensi
ty, W
/cm
²
4. OperationOperating Field of SOFCs
p = U · i
0.6V
56 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
OCVU
00
Stromdichte / A/cm²
Zel
lspa
nnun
g /
V
Leis
tung
sdic
hte
/ W
/cm
²
0.6V
Pmax
cell
volta
ge, V
current density, A/cm²
pow
er d
ensi
ty, W
/cm
²
fuel cells are not operated at pmax!
Operating Field of SOFCs
57 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
OCVU
Stromdichte / A/cm²
Zel
lspa
nnun
g /
V
Nut
zung
sgra
d /
%
konstante Brenngas- und Luftdurchflüßen
Brenngasnutzung
Sauerstoffnutzung
Fu
oxu
100
50
80
cell
volta
ge, V
current density, A/cm²
effic
ienc
y, %
constant fuel and air flow
fuel utilization
air utilization
Operating Field of SOFCsEfficiency
00
58 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
OCVU
00
Stromdichte / A/cm²
Zel
lspa
nnun
g /
V
constant fuel and air flow
fuel utilisation%80uF =
constant fuel and air utilisation
cell
volta
ge, V
current density, A/cm²
Constant Fuel/Air Flow vs. Utilisation
59 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
00
Stromdichte / A/cm²
Zel
lspa
nnun
g /
V
cell or electrode areaoxidant: air or oxygen
fuel compositionair and fuel flow rates
air and fuel utilisationtemperature
cellR linear region
maxP V7.0P
cell
volta
ge, V
current density, A/cm²
Summary of Factors that InfluenceElectrochemical Performance
60 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
00
Stromdichte / A/cm²
Sta
cksp
annu
ng/
V
cell or electrode areaoxidant: air or oxygen
fuel compositionair and fuel flow rates
air and fuel utilisationtemperature distribution
maxP V7.0P
amount of cells in stack
air / fuel inair/ fuel outmax / min / medium
temperaturecellR linear region
stac
kvo
ltage
, V
current density, A/cm²
Summary of Factors that InfluenceElectrochemical Performance in a Stack
61 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
cell
volta
ge /
V
current density / A/cm2
single cell M0611 / 16 cm²
temperature: 800 °Coxidant: 1000 ml/min air
fuel: H2 H2Oml/min ml/min %1000 31 31000 333 33750 250 33
1000 1000 50500 500 50
1000 4000 80500 2000 80300 1200 80
Single Cell Performance for SelectedH2/H2O ratios at 800°C
62 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2. T H2 H2O Ar CH4 Luft
°C ml/min ml/min ml/min ml/min ml/min802 1000 30.0 -- -- 1000797 500 500.0 -- -- 1000800 125 560.0 35 280 1000804 235 470.0 60 235 1000
cell
volta
ge /
V
current density / A/cm2
Single Cell Performance for SelectedH2/H2O/CH4 ratios at 800°C
63 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
electrolyte
fuel
fuel electrode(anode)
−− +→+ e2OHOH 22
2−− +→+ e2COOCO 2
2
reforming
O2- O2- O2- O2-
224 H3COOHCH +→+
shift reaction222 HCOOHCO +→+
Internal Reforming
64 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Gradual Reforming
M. Mogensen, RisoeChanges in gas composition(additional to fuel consumption)
65 Robert Mücke, LargeSOFC Summer School, Ancona, 2009
0 50 100 150 200 250 300 350
Temperatur / °C
Brenngasnutzung 70 %Laufzeit 320 hEbene 8
Ofentemperatur 800°CStrom 179,3 A (-dichte 0,5 A/cm²)Spannung 7,36 V Leistung 1,32 kW
CH4
4,35 l/minH
20,5 l/min
H2O 8,71 l/min
Luft 200 l/min
Bre
nnga
s ei
n
Luft
ein
Position / mm
650 675 700 725 750 775 800 825 850
Temperature Distribution in200x200mm Cells
fuel: methane; fuel utilization: 70%
air
in
fuel
in
66 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Overall Efficiency
thermodynamic efficiency(laws of thermodynamics)
electrochemical efficiency(all polarisations)
fuel utilisation
total electrical efficiency
(up to 60% achieved)
67 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Electrical Net Efficiency, Based on Natural Gas Fuel
Basis: Siemens 100 kW system
Gas Input
cellvoltage650 mV
100%
62.5%
52.5% 50.4%46.9%
fuelutil.84%
BoPeffic.96%
invertereffic.93%
celleffic.62,5%
improvement of cell properties
0%
10%
20%
30%
40%
50%
60%
70%
80%
600 650 700 750 800 850 900
cell voltage [mV]
el. n
etef
fici
ency
El. net efficiency as a functionof cell voltage
61.3%
improvement of cell performance
all efficiencies optimised 67.8%
optimised: 81,7% 90% 96% 96%
Net efficiency ηηηηel = ηηηηz × uF × ηηηηp × ηηηηInv = ηηηηstack × ηηηηp × ηηηηInv
(cell efficiency × fuel utilisation × BoP efficiency × inverter efficiency)
L. Blum, IEK-3
68 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Sources of Degradation
Cr poisoning / cathode change cathode activation + ohmic
coking (occupation of TPBs) anode activation
contact loss ohmic
cathode / electrolyte reaction ohmic + cathode activation
coarsening of anode microstructure anode activation
Sorted by approx. relevance
69 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Measurement of Degradation
A. Mai, Hexis
better to measure ASR instead of ΔV, however, requires i-V cures taken at intervals
70 Robert Mücke, Joint European Summer School for Fuel Cell and Hydrogen Technology
Acknowledgements
Special thanks for graphics and results are due to
Prof. E. Ivers-Tiffée • Dr. A. Weber • A. Leonide (IWE/KIT)Dr. A. Mai (Hexis)
Prof. M. Mogensen (Risoe National Lab.)
Prof. Ludger Blum • Dr. Martin Bram • Dr. Izaak C. Finke •Dr. V.A.C. Haanappel • Dr. L.G.J. de Haart • Dr. Natividat Jordan-Escalona •
Dr. Norbert H. Menzler • Dr. Sven Uhlenbruck (all FZJ)
The fuel cell project team under Dr. Robert Steinberger-Wilckens.
The SOFC group of IEK-1 in Jülich under Dr. H.-P. Buchkremer(and fomerly Prof. Dr. D. Stöver).