Thin film TiO2/TiN bifunctional interface enables integration of Ni5P4 electro-catalyst with GaInP2/GaAs III-V tandem photoabsorber for stable solar-driven water splitting

Abstract Hydrogen production by photoelectrochemical (PEC) water splitting has remained challenging for commercialization mainly due to the limited durability of integrated photoelectrodes owing to failure of the interfacial connection between the photoabsorber(s) and catalyst(s). Although multijunction III-V semiconductors have been popular choices as photoabsorbers in PEC devices and have yielded a high solar-to-hydrogen (STH) efficiency > 19% using platinum group metal (PGM) catalysts, they rapidly corrode in aqueous electrolytes. Here, a bifunctional interface comprised of a titanium dioxide (TiO 2 ) corrosion barrier and titanium nitride diffusion barrier (TiN, 1nm thickness) was created to protect the interface between the GaInP 2 /GaAs III-V tandem photocathode and the PGM-free catalyst. This strategy enables use of elevated temperatures to crystallize and fuse together the Ni 5 P 4 catalyst particles and the TiO 2 without damaging the sensitive III-V photoasbsorber. The combination of ultrathin catalyst and protective bilayers (Ni 5 P 4 /TiO 2 /TiN) produces negligible optical loss. The resulting III-V tandem photocathode exhibits an initial STH efficiency in a range of 11.4 - 13.2% in sodium phosphate electrolyte at neutral pH 7. It operated continuously for over 200 h at a STH efficiency > 10%, exceeding the previous benchmarks. Fused TiO 2 /TiN bilayers expand the synthesis temperatures for surface modification of and catalyst integration on III-V multijunction semiconductors without compromising the device photocurrent densities. The Ni 5 P 4 catalyst replaces costly PGM catalysts with comparable HER activity and operates at neutral pH in less corrosive electrolytes than either acid or alkaline electrolytes.