where the electron chemical potential µ i e = µ 0,i e + k B T ln[a i e ] is an intrinsic material property and depends on the standard state chemical potential µ 0,i e and activity a i e , which is, for example, particularly important for semiconductor electrodes. ϕ i is inner potential i.e. the electrostatic potential of a phase i, z the electron charge, and F the Faraday constant.