Gradient‐Structuring Manipulation in Ni₃S₂ Layer Boosts Solar Hydrogen Production of Si Photocathode in Alkaline Media

Abstract Using silicon as a photocathode has long been considered as an ideal pathway toward cost‐effective photoelectrochemical (PEC) solar hydrogen production. However, the trade‐off between charge transfer efficiency and stability severely restricts the practical application of Si‐based PEC devices in alkaline media. Herein, a facile thermo‐electrodeposition process to integrate a gradient‐structuring Ni₃S₂ (G‐Ni₃S_xO_2−x) layer to simultaneously protect and act as a catalyst in Si photocathodes in alkaline solutions is reported. The G‐Ni₃S_xO_2−x layer not only provides abundant active sites for the hydrogen evolution reaction but also promotes the charge separation and transport and mass transfer. Consequently, the as‐fabricated Si photocathodes exhibit an excellent PEC activity under simulated AM1.5G illumination with a high onset potential of 0.39 V versus reversible hydrogen electrode (RHE) and a photocurrent density of −33.8 mA cm⁻² at 0 V versus RHE, outperforming the state‐of‐the‐art p‐Si based photocathodes. Moreover, the G‐Ni₃S_xO_2−x layer possesses a good interfacial contact with the Si substrate with negligible stress at the G‐Ni₃S_xO_2−x/Si interface, affording a good durability of over 120 h at >30 mA cm⁻² in alkaline media. This gradient‐structuring strategy paves new way for engineering highly efficient and durable PEC devices.