First-principles study of O2 reduction on BaZr1-xCoxO3 cathodes in protonic-solid oxide fuel cells

Bulk proton mobility and catalytic activity to surface oxygen reduction are the two factors for determining the effectiveness of cathode materials for protonic-solid oxide fuel cells (p-SOFCs). In this work, a mixed protonic/electronic conductor (MPEC) of BaZr0 75Co0.25O3(BZCO) was selected as a potential cathode for p-SOFCs, and its bulk proton transporting and oxygen reduction behaviours at the microscopic level were investigated using the first-principles approach. Two plausible proton migration pathways in BZCO were examined, and the highest proton migration barrier was calculated to be 0.63 eV, which agrees remarkably well with the experimental findings. Compared with the weak adsorption of oxygen on BaZrO3(100) surface, the BZCO(100) surface provides a relatively large adsorption energy of -0.64 eV, indicating that Co doping enhances the oxygen adsorption on the surface. Furthermore, an oxygen reduction reaction over the MPEC cathode surface was explored using a hydrogenated BZCO(100) surface model, where four protons are located to react with one O-2 molecule to generate two water molecules. For the formation and desorption of the first water molecule on the BZCO surface, four possible reaction pathways have been mapped. The potential energy profiles indicate that the reaction with two protons simultaneously migrating to the adsorbed oxygen molecule to break the O-O bond (path-4) is the most feasible process for the formation of the first water molecule. Our study presents an atomistic level understanding of oxygen reduction and proton migration over or inside the MPEC cathode for the first time.