Strengthening P–S bonding in TiO2 for enhanced fuel cell startup/shutdown durability with an N, P, S–TiO2/S–TiN catalyst†

Abstract

Durability is crucial in polymer electrolyte fuel cells (PEFCs). The carbon supports currently employed in cathodes are oxidized during startup/shutdown, by increasing the cathode potential up to 1.5 V, causing the supported platinum–cobalt (PtCo/C) catalysts to lose activity. Therefore, system-level measures are currently used to control the potential below 1.0 V, which increases the cost of PEFCs. A recently reported S-doped TiN-supported N, P, S-tridoped TiO₂ catalyst is a promising candidate to replace currently available PtCo/C catalysts because, unlike other platinum group metal (PGM)-free catalysts, it is free from carbon supports. During the startup/shutdown cycles, the doped N³⁻ and S²⁻ anions substituted for O²⁻ in the TiO₂ lattice were stable, but some of the P⁵⁺ cations substituted for Ti⁴⁺ were removed from the TiO₂ surface, causing activity loss. Herein, P⁵⁺ dopants are stabilized by increasing the S²⁻ doping level, resulting in excellent startup/shutdown durability and enhanced intrinsic activity. The resulting reduction of half-wave potential after 5000 cycles between 1.0 and 1.5 V is the lowest of any reported PGM-free catalysts, at only 0.02 V. The P–S bonds formed in the TiO₂ lattice were found to be responsible for the durability of P⁵⁺, which provides a new strategy to accelerate the development of low-cost PGM-free catalysts with excellent durability.