Achieving 12.0% Solar-to-Hydrogen Efficiency with a Trimetallic-Layer-Protected and Catalyzed Silicon Photoanode Coupled with an Inexpensive Silicon Solar Cell

n-Type silicon (n-Si), with surface easily oxidized and passivated in an aqueous electrolyte, has suffered from sluggish oxygen evolution reaction (OER) kinetics for photoelectrochemical (PEC) water splitting. Herein, a trimetallic Ni_0.9Fe_0.05Co_0.05 protective layer is successfully electrodeposited on a p⁺n-Si substrate by underpotential deposition. The prepared Ni_0.9Fe_0.05Co_0.05/p⁺n-Si photoanode exhibits excellent stability and activity for PEC water oxidation, with a low onset potential of 0.938 V versus a reversible hydrogen electrode (RHE) and a remarkable photocurrent density of about 33.1 mA∙cm⁻² at 1.23 V versus RHE, which significantly outperforms the Ni/p⁺n-Si photoanode as a reference. It is revealed that the incorporation of Fe into the Ni layer creates a large band bending at the Ni_0.9Fe_0.05Co_0.05/p⁺n-Si interface, promoting interfacial charge separation. Moreover, the incorporation of Co produces abundant Ni³⁺ and oxygen vacancies (O_v) that act as active sites to accelerate the OER kinetics, synergistically contributing to a major enhancement of PEC water oxidation activity. Encouragingly, by connecting the Ni_0.9Fe_0.05Co_0.05/p⁺n-Si photoanode to an inexpensive Si solar cell, an integrated photovoltaic/PEC (PV/PEC) device achieved a solar-to-hydrogen conversion efficiency of as high as 12.0% without bias. This work provides a facile approach to design efficient and stable n-Si-based photoanodes with a deep understanding of the structure–activity relationship, which exhibits great potential for the integration of low-cost PV/PEC devices for unassisted solar-driven water splitting.