Interfacial electron spillover modulated Cu+–Ti4−δ active sites facilitating nitrate hydrogenation for photoelectrochemical ammonia synthesis

Abstract

The photoelectrochemical nitrate reduction reaction (PEC-NO₃RR) has emerged as a sustainable route for environmental remediation and value-added ammonia synthesis, offering a promising pathway to close the global nitrogen cycle. However, the efficient PEC-NO₃RR is critically limited by the kinetic mismatch between active hydrogen (*H) supply and nitrate hydrogenation. To address this challenge, a Cu/TiO₂-M photoelectrode featuring well-defined Cu⁺–Ti^4−δ interfacial active sites was constructed via an interfacial electron spillover effect (IESE). This photoelectrode achieved a notable NH₃ faradaic efficiency of 85.2% and a production rate of 0.38 mg h⁻¹ cm⁻² at −0.5 V vs. RHE. Systematic experiments indicated a volcano-type dependence of the Cu⁺–Ti^4−δ interfacial site population on Cu loading, optimum at the intermediate Cu/TiO₂-M. Mechanistic insight revealed that IESE delocalized electrons at the Cu⁺–Ti^4−δ interfacial sites, stabilizing nitrate in a bridged bidentate adsorption configuration to enhance its hydrogenation capacity. This enhanced hydrogenation capacity was matched by moderate *H supply capability at the intermediate copper loading, steering the reaction selectively toward ammonia. This work establishes a design paradigm in which targeted interfacial electron engineering programs the coupling between *H supply and nitrate hydrogenation, offering a transferable strategy for high-performance catalysts in sustainable nitrogen-cycle transformations and related multi-step proton-coupled electron-transfer reactions.