Inverted Pyramid Textured p-Silicon Covered with Co₂P as an Efficient and Stable Solar Hydrogen Evolution Photocathode

Silicon (Si) has been investigated as a promising photoelectrode material for use in photoelectrochemical water splitting. However, development of Si photocathodes that can operate at a high photocurrent density for solar-driven hydrogen production with long-term stability remains challenging. Herein, we report the fabrication of inverted pyramid textured p-Si photocathodes covered conformally and continuously with a thickness-gradient cobalt phosphide (Co₂P) layer, which not only effectively isolates p-Si from aqueous electrolyte to avoid corrosion but also efficiently catalyzes the solar-driven hydrogen evolution reaction (HER). Thanks to the unique inverted pyramid structure, the drop-cast Co₂P can distribute all over the p-Si photocathode and form a macroscopically continuous but locally nonuniform layer on the sidewalls of each inverted pyramid. The local nonuniform distribution enables light absorption to be partially separated from catalytic activity. Consequently, the as-fabricated Co₂P-coated p-Si photocathode exhibits a high photocurrent density of 35.2 mA cm^-2 at 0 V versus the reversible hydrogen electrode under AM 1.5G illumination and can photoelectrochemically catalyze the HER above 30 mA cm^-2 at least 150 h without notable degradation.