First-Principles Design of New Electrodes for Proton-Conducting Solid-Oxide Electrochemical Cells: A‑Site Doped Sr₂Fe_1.5Mo_0.5O_6−δ Perovskite

Electrolyzer and fuel cells based on proton-conducting solid-oxide ceramics (PC-SOEC/FC) are gaining wide interest as promising green technologies for H₂ production and conversion. Despite major advances in PC electrolytes, large-scale deployment of PC-SOEC/FC has been hindered by severe limitations at electrodes, which must ensure catalytic activity, electronic conduction, and high proton diffusion rates. Designing electrodes with mixed proton and electron conduction capability represents a great challenge. Several attempts have been based on composite materials made of common electrocatalysts and PC electrolytes, but the resulting electrodes have often suffered stability and conductivity problems. Inspired by the good performance in PC regime of some perovskite oxides, here we propose an alternative approach by designing a new single-phase triple-conducting oxide (TCO) from the recently proposed and well-tested mixed ion-electron conductive electrocatalyst Sr₂Fe_1.5Mo_0.5O_6−δ (SFMO) double perovskite. We investigated with first-principles methods (DFT+U) the key processes that promote proton transport, i.e., oxygen vacancy formation, water dissociative incorporation into the defective lattice, and proton transfer along the oxide sublattice. We focused on SFMO and A-substituted derivatives with Ba or K cations. Both dopants lower the proton migration barrier of SFMO, thus improving proton transport effectiveness. In particular, we found K-doped SFMO to be the best candidate thanks to its peculiar and very favorable structural and electronic properties. Moreover, from our ab initio analysis, we identified a general design principle to enhance proton transport in perovskite oxides at the nanoscale. Our computational results can be easily implemented to develop and test new low-cost TCO-based electrodes for PC-SOEC/FC