Photoelectrochemical nitrate reduction to ammonia on CuFeO₂ photocathodes modified with metallic cocatalysts

Photoelectrochemical nitrate reduction (p-NO₃RR) is a sustainable route to green ammonia that simultaneously removes excess nitrate from wastewater thereby preventing ecosystem damage. In this work, CuFeO₂, a p-type delafossite semiconductor, is investigated as a photocathode for p-NO₃RR. Unmodified CuFeO₂ is demonstrated to be unable to facilitate liquid p-NO₃RR product formation, however, cocatalyst screening (Pt, Au, and Co) revealed that for all cocatalyst-modified CuFeO₂ photoelectrodes, ammonia formation is observed, with Pt being the superior cocatalytic material for selective nitrate reduction. Optimisation of the Pt hydrothermal immobilization procedure resulted in the preparation of CuFeO₂/Pt photocathodes, achieving a faradaic efficiency (FE) of 92% toward ammonia at a low potential of 0.63 V vs. RHE, with improved short-term stability. Surface characterisation of the photocathodes indicated that the enhanced stability of the metal-modified CuFeO₂ photocathodes during p-NO₃RR compared to unmodified CuFeO₂ is attributable to the formation of a Cu²⁺-containing surface layer obtained during hydrothermal processing under strongly alkaline conditions. This layer may impede reduction of Cu⁺ to metallic copper, a frequently reported degradation mechanism for delafossite photoelectrodes. While further improvements in long-term stability are still required, these findings clearly demonstrate the viability of CuFeO₂-based photocathodes for selective p-NO₃RR, i.e. for sustainable ammonia synthesis from nitrate waste streams, and highlight the material's potential for solar-to-chemical energy conversion.