The efficiency of a fuel cell is defined as the ratio of useful work obtained from the cell to the total energy released by the combustion of the fuel. The maximum useful electrical work that can be obtained is equal to the change in Gibbs free energy ($\Delta G$), and the total thermal energy is the enthalpy change of the reaction ($\Delta H$). Therefore, the thermodynamic efficiency is given by $\eta = \frac{\Delta G}{\Delta H}$.