Cobalt-Induced Magnéli Phase Transformation Towards Oxygen Evolution and Oxygen Reduction Reactions.

Hydrothermal deposition of cobalt on mixed-phase titanium suboxides, Ti5O9 and Ti8O15 (TSO), was performed under mild conditions. High-temperature treatment under a reducing atmosphere led to a phase reversal, contrary to the expected Co-doping into the TSO. Surface cobalt species led to "unshearing" of crystallite planes, reverting to Co-decorated rutile. SEM-EDX measurements confirmed homogeneous dispersion of cobalt on the surface amounting to 4.15 wt.%. Raman and FTIR measurements confirmed the presence of Co at the surface, albeit in oxidized form, converting the Co to Co3O4 under atmospheric conditions. Modification of TSO by Co significantly improved OER and, to a certain degree, ORR. The charge transfer resistance for OER diminished from 17.5 kΩ to 47.4 Ω, indicating excellent activity towards oxygen evolution, with the onset potential for OER shifting to 1.68 V vs. RHE. RRDE measurement revealed that upon Co-surface deposition, the mechanism of ORR changes only slightly towards the 4e- branch and is still dominated by the 2e- pathway, characteristic of TSO, due to low cobalt content, with an onset potential for ORR of 0.72 V vs. RHE.