Fuel Cell Efficiency

The fuel cell thermodynamic efficiency is given by the ratio of the Gibbs function change to the Enthalpy change in the overall cell reaction. The Gibbs function change measures the electrical work and the enthalpy change is a measure of the heating value of the fuel.
Efficiency = (dG/dH)

For the hydrogen -oxygen reaction: dH = - 68,317 cal/g mole of H₂, and dG = - 56,690 cal/g mole of H₂ . The efficiency of the Ideal Fuel Cell is therefore:

Efficiency = (56,690/68,317) = 83%

Another measure of the fuel cell efficiency is known as the "Voltage Efficiency" and is the ratio of the actual voltage under operating conditions to the theoretical cell voltage.

Voltage Efficiency = (Actual Voltage)/(Theoretical Voltage) = (V_A / 1.23)