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

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.