Fuel Cells in Energy Technology
Tutorial 8 / SS 2013 - solutions
Prof. W. Schindler, Jassen Brumbarov /
Celine Rüdiger
26.06.2013
1. List the most common fuel cell types. Write down the reactions taking place on the anode,
cathode and the overall reactions. In which temperature and power ranges do these types of
fuel cells work?
AFC: there are two possibilities to write the reaction equations:
+
+
(i)
Anode: H2 -> 2e + 2H , 2 H + 2 OH -> 2 H2O
Cathode: ½ O2 + 2e + H2O -> 2OH
(ii)
-
-
Anode: H2 + 2OH -> 2e + 2H2O
Cathode: ½ O2 + 2e + H2O -> 2OH ;
Overall: H2 + ½ O2 -> H2O
temperature range: <100°C, power range: 5-150 kW
PEMFC: Hydrogen and Alcohols possible fuels at ~ 80°C
H2:
+
Anode: H2 -> 2e + 2H
+
Cathode: ½ O2 + 2H + 2e  H2O
Overall: H2 + ½ O2 -> H2O
temperature range: ~80°C, power range: 5-250 kW
CH3OH:
+
Anode: CH3OH + H2O  CO2 + 6H + 6e
+
Cathode: 3/2 O2 + 6H + 6e 3H2O
Overall: CH3OH + 3/2 O2  CO2 + 2H2O
temperature range: ~80°C, power range: 5 kW
PAFC:
+
Anode: H2 -> 2e + 2H
+
Cathode: ½ O2 + 2H + 2e  H2O
Overall: H2 + ½ O2 -> H2O
temperature range: 160-220°C, power range: 50-11000 kW
MCFC: Hydrogen and internally reformed Hydrocarbons are possible fuels
H2:
2Anode: H2 + CO3 -> CO2 + 2e + H2O
2Cathode: ½ O2 + CO2 + 2e -> CO3
Overall: H2 + ½ O2 -> H2O
CH4:
Reforming: CH4 + H2O  CO + 3H2
Gas shift: CO + H2O  CO2 + H2
Overall Reforming: CH4 + 2H2O  CO2 + 4H2
2Anode: 4H2 + 4CO3 -> 4CO2 + 8e + 4H2O
2Cathode: 2O2 + 4CO2 + 8e -> 4CO3
temperature range: 600-800°C, power range: 100-250 kW
combined heat and power possible; combination with gas turbine for higher efficiency
SOFC: Hydrogen and internally reformed Hydrocarbons are possible fuels
H2:
2Anode: H2 + O -> 2e + H2O
2Cathode: ½ O2 + 2e -> O
Overall: H2 + ½ O2 -> H2O
CH4:
Reforming: CH4 + H2O  CO + 3H2
Gas shift: CO + H2O  CO2 + H2
Overall Reforming: CH4 + 2H2O  CO2 + 4H2
2Anode: 4H2 + 4O -> 8e + 4H2O
2Cathode: 2O2 + 8e -> 4O
temperature range: 800 – 1000°C, power range: 0.1 - 2000 kW
combined heat and power possible; combination with gas turbine for higher efficiency
2. Calculate for a DMFC:
a. The maximum cell voltage and the thermodynamic efficiency for standard conditions.
b. The theoretical maximum cell voltage at T=100°C. Assume that enthalpy and entropy
do not depend on temperature.
c. The voltage (electric) efficiency for a measured cell voltage of 0.85V.
(CH3OH + 1.5 O2 -> CO2 + 2 H2O; n=6 number of transferred electrons; H =-726.6 kJ/mol;
0
G =-702.5 kJ/mol; F=96485 C/mol)
0
G 0
G 0
 1.213V ,  th 
 0.967
nF
H 0
H 0  G 0
J
0
0
0
0
b. G  H  TS  S 
, with T=298K
 80.87
T
mol  K
 H 0  TS 0
U 100C 
 1.203V , with T=373K
nF
U measured
 0.701
c. V 
U max
a.
U max  
3. Does the strategy of bottoming cycles make sense? Write down the general equations for the
maximum thermodynamic efficiency of a fuel cell operating at ambient temperature T A, the
thermodynamic efficiency of the same fuel cell combined with an ideal heat engine operating
at temperature TF (TF>TA) and the maximum thermodynamic efficiency of a heat engine with
Tcold=TA. Sketch the temperature dependent efficiency curves for the above mentioned
systems (fuel cell: H2/O2 fuel cell).