Revisiting CaMnO₃ as a Proton Conductor─A Theoretical Perspective

High-temperature water electrolysis is presently discussed as a technique for conversion of electrical energy into chemical energy, which can be employed for renewable energy storage. The proton ceramic electrolysis cell (PCEC) is a promising technique for this, which is hampered by long-term stability and efficiency issues of available electrode materials. CaMnO3 is a promising electrode material due to its stability and the ability of reversibly forming oxygen vacancies. Its stability with respect to decomposition can be further improved by Mn/Fe substitution and Ca/Sr substitution. However, in a previous theoretical study [Phys. Rev. B 2010, 82, 014103], a rather high proton migration barrier of 1.8 eV was obtained for cubic CaMnO3. This would exclude CaMnO3 as an efficient electrode material. In this work, much lower proton migration barriers of ≈0.2 eV were computed for cubic CaMnO3. For the experimental room-temperature orthorhombic phase, the barriers are only slightly larger (0.2 to 0.5 eV). Mn/Fe and Ca–Sr substitution is essential for negative hydration energies and does not significantly change the barriers. Based on the present theoretical results, we therefore suggest to reconsider substituted CaMnO3 as an PCEC electrode material.