Phosphorus‐Rich Colloidal Cobalt Diphosphide (CoP 2 ) Nanocrystals for Electrochemical and Photoelectrochemical Hydrogen Evolution

: Developing earth-abundant and efficient electrocatalysts for photoelectrochemical water splitting is critical to realizing a high-performance solar-to-hydrogen energy conversion process. Herein, phosphorus-rich colloidal cobalt diphosphide nanocrystals (CoP2 NCs) are synthesized via hot injection. The CoP2 NCs show a Pt-like hydrogen evolution reaction (HER) electrocatalytic activity in acidic solution with a small overpotential of 39 mV to achieve -10 mA cm-2 and a very low Tafel slope of 32 mV dec-1 . Density functional theory (DFT) calculations reveal that the high P content both physically separates Co atoms to prevent H from over binding to multiple Co atoms, while simultaneously stabilizing H adsorbed to single Co atoms. The catalytic performance of the CoP2 NCs is further demonstrated in a metal-insulator-semiconductor photoelectrochemical device consisting of bottom p-Si light absorber, atomic layer deposition Al-ZnO passivation layers, and the CoP2 cocatalyst. The p-Si/AZO/TiO2 /CoP2 photocathode shows a photocurrent density of -16.7 mA cm-2 at 0 V versus reversible hydrogen electrode (RHE) and an output photovoltage of 0.54 V. The high performance and stability are attributed to the junction between p-Si and AZO, the corrosion-resistance of the pinhole-free TiO2 protective layer, and the fast HER kinetics of the CoP2 NCs.