where µ Θ and g h represents the standard chemical potential and homogeneous free energy density of the mixture, respectively. The reservoir chemical potential (µ res ), or cell voltage (V ), acts as the nonequilibrium chemical potential of the system. The difference between the internally controlled potential (µ h ) and the externally controlled potential (µ res ) is given by the reaction affinity, A = µ res -µ h , which controls the rate of a reduction reaction. This can also be expressed as the activation overpotential: 4,7,8