Epitaxial Thin-Film Spinel Oxides as Oxygen Reduction Electrocatalysts in Alkaline Media

Designing nonprecious electrocatalysts for the challenging oxygen reduction reaction (ORR) is of paramount importance to advance hydrogen fuel cell technologies. Elucidating the ORR mechanism requires well-controlled surface structures and compositions at the atomic scale, which are often lacking in nanoparticle studies. Here, we report the growth and ORR activity in alkaline media of (100)-oriented, crystalline spinel oxide thin films prepared with atomic layer precision by molecular beam epitaxy. Co active sites were incorporated into a more conductive Fe3O4 host to mitigate the insulating nature of Co3O4. Co-doped Fe3O4, with Co-doping levels from 0 to 25%, were grown epitaxially on single-crystal MgO (100) substrates, prepared as thin-film electrodes with a front contact, and tested in a customized rotating disk electrode (RDE) assembly. The reliability of this RDE assembly for quantitative ORR activity measurements was validated with both polycrystalline Pd thin films and conductive RuO2 (110) on an insulating substrate. Co-doped Fe3O4, with a 15% Co-doping level, achieved the optimal ORR activity in alkaline media. This was ascribed to the maximum number of Co active sites that could be incorporated without affecting the conductivity of the Fe3O4 matrix. This study enables investigating the ORR processes in a new realm—on epitaxial thin-film spinel oxides customized with a precision previously reserved/achieved for the semiconductor industry—and provides new strategies to overcome the generally low conductivity of metal oxide films as electrocatalysts in alkaline fuel cells.