adsorbed species i per unit cell surface area, A. In the CHE scheme, it is assumed that the electrode and the electrolyte are thermodynamic reservoirs for electrons and ions such as protons, respectively, whereas the reference system typically corresponds to the standard hydrogen electrode (SHE) . At standard conditions (T=298 K, and P= 1 atm, and pH = 0), molecular hydrogen in the gas phase is in equilibrium with the solvated proton and the electron ( 1 2 H 2 (g) H + (aq) + e -). Therefore, in thermodynamic equilibrium the corresponding chemical potential of hydrogen in the gas phase is equal to that of a protonelectron pair. Consequently, referring the potential to the SHE, we can avoid the calculation of the proton solvation energy in water and instead use the gas-phase energy of H 2 which can easily be calculated based on DFT: