Question

A typical H2–O2 PEMFC might operate at a voltage of 0.75 V and λ = 1.10. At STP, what is the efficiency of such a fuel cell (use HHV and assume pure oxygen at the cathode)?

Answer 1

Given: Operating Voltage, V=0.75 V and =1.10

And assuming pure oxygen at cathode.

The efficiency of H₂-O₂ cell is given by

where is total efficiency of the cell

is the reversible thermodynamic efficiency of fuel cell

is the voltage efficiency of the fuel cell

is fuel utilization efficiency

For pure H₂-O₂ at STP, =0.83

(At standard conditions of reaction, the chemical energy in the hydrogen/oxygen reaction is 285 kJ/mole and free energy available for useful work is 237kJ/mole. The thermal efficiency of an ideal fuel cell operating reversibly on hydrogen and pure oxygen at standard conditions will be =237/285=0.83)

where V is the operating voltage

E is thermodynamically reversible voltage

E=1.23V for hydrogen/oxygen system

(E is calculated using the relation ,

where is free energy change, =-229kJ/mol

n is number of electrons, n=2

F is Faraday's constant, F=96500 C/g mole electron)

We have =0.83, =0.6097 and =0.909

Thus the efficiency of the cell is

%