Phase Evolution of MOF-Derived CuO Nanoparticles Supported on Carbon Nanotubes during Electrochemical Nitrate Reduction in Acidic, Neutral, and Alkaline Media

Abstract Herein, we report the successful synthesis of MOF-derived CuO nanoparticles supported on carbon nanotubes (CNTs) for electrochemical nitrate reduction reaction (eNO3RR). X-ray photoelectron spectroscopy (XPS) analysis reveals that MOF-derived CuO contains an enhanced Cu(I) oxidation state. Morphological characterization using field emission scanning electron microscopy, transmission electron microscopy, and scanning transmission electron microscopy demonstrated deposition of CuO nanoparticles along the CNT network. The MOF-derived CuO/CNT catalyst was utilized for eNO3RR performances in acidic, neutral, and alkaline media. Postelectrolysis powder X-ray diffraction analysis revealed significant phase evolution of CuO during the reaction, whereas postelectrolysis XPS confirmed that the Cu–C bond remained intact irrespective of the electrolyte pH. The Cu–C interaction suppressed Cu dissolution during eNO3RR in different electrolytes, and the catalyst remained stable for over five consecutive cycles (3 h per cycle). The optimized catalyst showed an average NH3 production rate of 16.63 mg h–1cm–2 [Faradaic efficiency (FE) 91.7%], 9.36 mg h–1cm–2 (FE 80.42%), and 17.45 mg h–1cm–2 (FE 70.98%) in 0.5 M H2SO4, 0.5 M Na2SO4, and 0.5 M NaOH, respectively, at –0.5 V vs RHE. Thus, the coexistence of Cu(0) and Cu(I) generated during pH-dependent reconstruction may provide complementary active sites that contribute synergistically to the observed eNO3RR performance.