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.