This way, the adsorption free energy is approximated by the total binding energy obtained using periodic DFT calculations, while the electrode potential U and pH enter this equilibrium expression to account for deviations from the standard conditions. The CHE is not only applicable to coupled proton-electron transfer processes but it can also be applied to any solvated ionic species . For alkali metal cations A + , this can be realized by the half-cell reaction, A + (aq) + e - A (s) at U = U 0 vs. SHE, where U 0 corresponds to the standard redox potential, as listed in Tab. 2.