A silicon-based hybrid photocathode modified with an N5-chelated nickel catalyst in a noble-metal-free biomimetic photoelectrochemical cell for solar-driven unbiased overall water splitting

Abstract

Modification of the surface of semiconductor-based photoelectrodes with molecular redox catalysts provides a way to realize atom-efficient catalysis for photoelectrochemical (PEC) H₂ and O₂ evolution. However, the diversity of immobilized molecular catalysts for the reported hybrid photocathodes is very limited to date, and most of the semiconductor/molecular catalyst hybrid photocathodes still suffer from low efficiency and poor stability. This work demonstrates that the efficiency and stability of a silicon-based photocathode for PEC H₂ production were substantially improved by surface modification with N₅-chelated nickel and cobalt catalysts (denoted as M(N₅)^HA, M = Ni, Co) through a hydroxamate anchor (HA). Compared to previously reported molecular catalyst-modified planar p-Si photocathodes, p-Si/TiO₂/Ni(N₅)^HA displayed higher photocurrent density (up to −1.54 mA cm⁻²) and superior stability for PEC H₂ production at 0 V vs. RHE under simulated 1 sun illumination. Furthermore, a noble-metal-free biomimetic Z-scheme PEC cell was assembled by coupling the newly-built p-Si/TiO₂/Ni(N₅)^HA photocathode with a Co₄O₄ cubane-modified BiVO₄ photoanode. This fully molecular PEC cell produced H₂ and O₂ in a molar ratio of 2 : 1 from water under illumination at zero bias voltage, achieving a solar-to-hydrogen efficiency of 0.04%, which is the highest efficiency reported so far for molecular catalyst-modified water splitting PEC cells. This is the first efficient noble-metal-free PEC cell assembled with two molecular catalyst-modified hybrid photoelectrodes, either with or without dye sensitization, for unbiased overall water splitting under illumination.