To compute the Gibbs free energy of electrochemical reactions involving proton-electron transfer, the computational hydrogen electrode (CHE) model was used. In this model, the reference potential is set to be the standard hydrogen electrode (SHE). This makes the free energy of (H + + e -) pair correspond to that of 1⁄2 H2(g) under standard conditions (1 bar, 298 K and pH = 0).