S-doped TiN supported N, P, S-tridoped TiO₂ with hetero-phase junctions for fuel cell startup/shutdown durability

TiO₂ layers codoped with N and P formed on S-doped TiN are recently developed platinum-group metal (PGM)-free catalysts (N, P-TiO₂/S-TiN) for the oxygen reduction reaction (ORR) in acidic media. Conventional Pt-based catalysts lack durability as the necessary carbon supports are oxidized at high potentials (>1.0 V) during startup and shutdown. N, P-TiO₂/S-TiN catalysts do not require carbon supports and are expected to tolerate oxidation at high potentials. However, N, P-TiO₂/S-TiN loses ORR activity during startup or shutdown at potentials >1.0 V due to removal of N- and P-atoms. We therefore report a new pathway to enhance durability and ORR activity of N, P-TiO₂/S-TiN. By annealing N, P-TiO₂/S-TiN with NH₄F under N₂, ORR active anatase/rutile TiO₂ hetero-phase junctions are produced in bulk with remaining TiN supports, and the surface O-atoms in the TiO₂ lattice are substituted by S²⁻. S-doped TiN-supported N, P, S-tridoped TiO₂ exhibits the highest ORR activity among reported oxide/oxynitride catalysts, although the ORR activity of the N, P, S-TiO₂/S-TiN catalyst remains lower than that of state-of-the-art carbon-based catalysts. The N, P, S-TiO₂/S-TiN catalyst exhibits superior durability among PGM-free catalysts as the anionic dopants are not removed after 5000 potential cycles (1.0-1.5 V), leading to a 0.02 V reduction in half-wave potential.