Theoretical Study on Proton Migration on the CaMnO3 Surface

Recently, it was shown theoretically that Ca1-xSrxMn1-yFeyO3 has low proton migration barriers ranging from 0.2 to 0.5 eV [Grimm, B.; Bredow, T. J. Phys. Chem. C 2024, 128, 5429. DOI: 10.1021/acs.jpcc.3c07594]. In this article, the proton migration mechanism from the CaMnO3 surface into the bulk is investigated theoretically. The process is one of the key steps in proton conducting electrolysis cells (PCEC). In the theoretical models, the (100) and (010) surfaces are considered, which together account for about 50% of the total surface area of CaMnO3 single crystals. It turns out that the proton migration mechanism on the (010) surface is feasible and is further improved by Mn/Fe substitution. The (010) surface of CaMn0.75Fe0.25O3 has strongly negative hydration energies of -2.40 eV in the first layer and -2.10 eV in the inner layers. The largest barrier for proton migration is calculated for the hopping process from the first to the second layer. This is due to the relatively large hydration energy difference of about 1 eV between the two layers, which is also found for the known proton conductor BaFeO3.