TiO2/TiN Interface Enables Integration of Ni5P4 Electrocatalyst with a III–V Tandem Photoabsorber for Stable Unassisted Solar-Driven Water Splitting

H2 production by direct photoelectrochemical (PEC) water splitting has remained unachievable commercially, mainly due to rapid failure at the interface between the photoabsorber(s) and catalyst(s). PEC devices made from multijunction III–V semiconductors with platinum group metal (PGM) catalysts have yielded impressive initial solar-to-H2 (STH) efficiency >19%, which rapidly corrodes in aqueous electrolytes. Here, TiO2/TiN layers were fused to create a bifunctional interface between a GaInP2/GaAs III–V tandem photoabsorber and a polycrystalline Ni5P4 HER catalyst. The TiO2 serves as a conducting corrosion barrier, while a thin layer of much denser TiN (1 nm) blocks interlayer diffusion during fabrication. This strategy allows the elevated temperatures needed to crystallize the Ni5P4 nanoparticles and fuse to the TiO2/TiN junction to achieve minimal optical loss without damaging the sensitive photoasbsorber. The resulting photocathode exhibits an initial STH efficiency of 11.4%–13.2% in sodium phosphate electrolyte at neutral pH 7. It operated continuously for over 200 h without failure above 10% STH efficiency, exceeding all previous benchmarks. The earth-abundant Ni5P4 catalyst replaces costly PGM catalysts at comparable HER activity in neutral, acidic, or basic pH electrolytes.