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Sustainable hydrogen production is one of the most challenging topics in modern energy economics. Electrochemical and thermal splitting of water are promising techniques, but are highly energy demanding. Efficient hydrogen evolution reaction catalysts can play a key role to lower the electrolysis potential and to make water splitting more feasible. Among other perovskites, CaMnO 3  has been identified as efficient electrode material due to its relatively high oxygen vacancy concentrations at elevated temperatures. But this compound needs to be further improved for technical use. In this study, the effect of Mn substitution in CaMnO 3  by 3d metals M = Sc, Ti, V, Cr, Fe, Co, Ni, Cu and Zn on the oxygen vacancy formation energy is investigated theoretically at DFT level. Vacancy formation energies, enthalpies and free enthalpies are calculated with a combination of hybrid and GGA density functionals. Configuration entropy is taken into account by calculating all possible configurations of M and oxygen vacancy site in supercell models. The calculated oxygen vacancy formation energies are strongly affected by Mn/M substitution, the most promising heteroelement being Cu. CaMn 0.875 Cu 0.125 O 2.875  and CaMn 0.875 Cu 0.125 O 2.75  are in equilibrium at 537 K, compared to 1231 K for CaMnO 2.875 /CaMnO 2.75  and 1350 K for CaMnO 3 /CaMnO 2.875 .

Partial substitution of the Mn atoms in CaMnO 3 by first row transition metal atoms: effect on oxygen vacancy formation