Photovoltage-regulated interfacial carrier delivery enables deep nitrate reduction and bias-free paired conversion

Photoelectrochemical nitrate reduction offers a route to couple solar-energy conversion with nitrogen recovery, yet the role of a photovoltage-mediated interfacial driving force in governing the depth of multielectron NO_x reduction remains poorly understood. Here, we develop a scalable Cu₃BiS₃/InCdS/TiO₂/CoCu multilayer photocathode and use a matched CdS-buffered counterpart to isolate the influence of absorber/buffer interfacial regulation on nitrate-to-ammonium conversion. Replacing CdS with InCdS enhances the photovoltage response, accelerates carrier extraction, prolongs carrier lifetime and lowers interfacial resistance while retaining the same TiO₂/CoCu reaction interface. These changes coincide with suppressed NO₂^- accumulation and enhanced NH₄⁺ formation, providing an experimentally accessible relationship between interfacial carrier energetics, carrier-delivery kinetics and NO_x reduction depth. The optimized photocathode reaches an NH₄⁺ faradaic efficiency of 92.67% ± 2.33% at 0 V versus RHE and a yield rate of 14.93 ± 0.48 µmol h^-1 cm^-2 at -0.3 V versus RHE. Coupling this Cu₃BiS₃-based photocathode with a Cu-based 5-hydroxymethylfurfural oxidation anode further enables bias-free paired conversion, sustaining cathodic NH₄⁺ production with faradaic efficiencies above 83% and selective anodic HMFCA formation above 98%. This work identifies interfacial photovoltage and carrier delivery as experimentally resolvable variables associated with the progression of the PEC NO₃RR and extends these insights to integrated solar-driven nitrate valorization and biomass upgrading.