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

H-2 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-H-2 (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.