Durable bias-free solar Water-Splitting cell composed of n+p- Si/Nb2O5/NiPt photocathode and W:BiVO4/NiCo(O-OH)2 photoanode

Development of photoactive materials and design of tandem configurations for photoelectrochemical (PEC) water-splitting is a rapidly emerging solar technology. However, the recorded solar-to-hydrogen efficiency (STH, %) still falls below expectations owing to slow reaction kinetics and active material loss, which limit large-scale hydrogen production. To suppress the high corrosion resistance required for optimum PEC activity, combination strategies such as addition of surface protection and active co-catalysts are necessary for photoelectrode materials. Moreover, a proper combination based on well-developed photoelectrode materials is crucial for improving solar energy-capturing capabilities. In this study, a vertically oriented n+-doped p-Si photocathode (denoted as n+p-Si) was developed using alkali etching and phosphorization to improve the bulk carrier separation. The n+p-Si surface was passivated with a thin crystalline niobium pentoxide (Nb2O5) nanolayer (∼40 nm) using a simple spin-coating method, providing a maximum stability of ∼ 70 h at 0 VRHE in the semiconductor/liquid junction between n+p-Si and the electrolyte. Furthermore, the n+p-Si/Nb2O5 surface was electrochemically decorated with a nickel–platinum (NiPt) alloy composite as an efficient electrocatalyst to reduce the charge-transfer overpotential. The n+p-Si/Nb2O5/NiPt exhibited a more positive onset potential of ∼ 0.62 V, compared with the reversible hydrogen electrode (RHE, abbreviated as VRHE) with a photocurrent density (J) of ∼ 32 mA·cm−2 at 0 VRHE in 0.5 M H2SO4. As a counterpart, the bismuth vanadate (BiVO4) photoanodes progressed significantly to improve bulk charge separation and injection with 2 wt% W doping and electrodeposited NiCo(O-OH)2 electrocatalysts, which delivered excellent PEC performance with a maximum Jph of 5.6 mA·cm−2 at 1.23 VRHE. A solar water-splitting system composed of a n+p-Si/Nb2O5/NiPt photocathode and a W:BiVO4/NiCo(O-OH)2 photoanode was fabricated in a PEC–PEC tandem configuration, and a stable STH conversion efficiency of 3.9% was achieved with an illumination of 1 sun, with stable H2/O2 production for 50 h maintaining a Jph of ∼ 3.1 mA·cm−2 in the bias-free condition.