Copper cobalt oxide nanowire arrays for selective electrochemical nitrate reduction to ammonia

ABSTRACT Electrochemical reduction of nitrate to ammonia (NO 3 RR) presents a sustainable pathway for simultaneous remediation of nitrogen-polluted wastewater and production of a carbon-neutral chemical feedstock. A copper‑cobalt bimetallic oxide nanowire catalyst, designated CuCo 2 O x /CF, underwent preparation through hydrothermal growth on copper foam substrate followed by electrochemical activation. The resulting self-supported electrode displays a hierarchical nanowire architecture with individual wire diameters of 120 nm and lengths reaching 5.8 μm, yielding a Brunauer–Emmett–Teller (BET) specific surface area of 124.6 m 2 g −1 . X-ray diffraction confirmed the coexistence of CuO and Co 3 O 4 phases, whilst X-ray photoelectron spectroscopy identified Cu 2+ /Cu + mixed valency at 933.8 eV and 932.4 eV and Co 3+ /Co 2+ states at 780.3 eV and 782.1 eV. Electrochemical impedance spectroscopy demonstrated a charge transfer resistance of 2.3 Ω, substantially lower than monometallic CuO x (8.7 Ω) and CoO x (12.4 Ω) counterparts. At an applied potential of −0.4 V relative to the reversible hydrogen electrode, CuCo 2 O x /CF achieved a Faradaic efficiency (FE) of 95.4% towards ammonia with a yield rate of 847.3 μmol h −1 cm −2 . Nitrite FE remained at 1.8% and hydrazine remained undetected under any condition tested. Density functional theory calculations on the CuCo 2 O x (110) surface revealed that the rate-determining *NO→*N hydrogenation step carries an energy barrier of +0.34 eV, compared with +0.67 eV on CuO(111), rationalising the superior selectivity and activity of the bimetallic composition. The catalyst retained more than 91.2% of its initial Faradaic efficiency after 50 h of continuous electrolysis, with post-test X-ray diffraction confirming structural integrity.