Efficient Phosphorus Doping into the Surface Oxide Layers on TiN to Enhance Oxygen Reduction Reaction Activity in Acidic Media

Oxide catalysts have been developed in polymer electrolyte fuel cell cathodes to replace platinum-group-metal catalysts while retaining moderate oxygen reduction reaction (ORR) activity. Recently, carbon support-free titanium oxide (TiO2) catalysts formed on the outer surface of titanium nitride (TiN) have been shown to significantly increase ORR activity. In this study, ORR activity was further enhanced by increasing the surface phosphorus content on the disordered TiO2 layer by using hypophosphorous acid as a phosphorus source. Both the half-wave potential and limiting current density in acidic media were maximized to 0.66 V and 5.18 mA cm–2, respectively. Besides, the durability against high potential cycles between 1.0 and 1.5 V versus the reversible hydrogen electrode, which is critical to be used in vehicles, was enhanced. The decrease in half-wave potential during the 5000 cycles was suppressed to 0.08 V, which is 0.03 V lower than that in a previous study in which phosphoric acid was used. Furthermore, nitrogen doping on the catalyst under an NH3 flow improved the 4-electron ORR selectivity and reduced the hydrogen peroxide yield to less than half of the value with undoped catalysts. Although some phosphorus and nitrogen atoms were removed from the catalyst under high potential cycles, the surface oxides protected inner TiN for 5000 cycles. Codoping the TiO2 surface with large amounts of both phosphorus and nitrogen atoms is necessary to use this catalyst without relying on a high-cost system in which the potential is kept below 1.0 V.