Black Silicon for Solar Fuels.

Photoelectrochemical (PEC) solar energy conversion offers a promising route for transforming sunlight into chemical fuels and value-added products through light-driven redox reactions at semiconductor-electrolyte interfaces. Among semiconductor photoabsorbers, black silicon (b-Si) is particularly attractive because its micro-/nanostructured surface enables ultralow reflectance, broadband light absorption, a large electrode-electrolyte interfacial area, and versatile integration of catalysts and protective layers. These characteristics allow b-Si to combine effective photon management with interfacial reaction engineering, making it relevant to a broad range of PEC processes. This review first introduces the fundamental characteristics of b-Si and summarizes representative fabrication methods, with emphasis on the relationships among processing, morphology, and physicochemical properties. It then examines how these properties affect PEC water splitting, nitrate reduction, nitrogen reduction, and carbon dioxide reduction. Particular attention is given to the roles of surface morphology, junction design, passivation, catalyst integration, charge separation and transport, and the local reaction environment in governing activity, selectivity, and stability. Finally, the major limitations of b-Si-based PEC systems-including charge-carrier recombination, photocorrosion, parasitic reactions, and mass-transport constraints-are critically assessed. On this basis, future research directions are proposed for developing reaction-adaptive b-Si photoelectrodes capable of efficient, selective, durable, and practically relevant solar-fuel and chemical production.