Electrocatalytic Nitrate Reduction to Ammonia Using Co ₃ O ₄ Nanowires Supported on TiO ₂ /Ti

The electrochemical nitrate reduction reaction (NO3RR) offers a sustainable alternative for ammonia synthesis under mild conditions, addressing both nitrate pollution and the energy-intensive nature of the Haber–Bosch process. In this work, Co3O4 nanowires supported on TiO2/Ti were synthesized and evaluated for NO3RR in an H-type electrochemical cell using Ar-saturated 1.0 mol L–1 NaOH containing 20 mmol L–1 NaNO3. After confirming NO3RR activity via cyclic voltammetry, 1 h chronoamperometry was performed at potentials ranging from 0 to −0.4 V vs RHE (in 100 mV steps). The catalyst exhibited ammonia production at 0.0 V vs RHE, achieving a maximum faradaic efficiency (FE) of 80.0 ± 5.8% at −0.3 V vs RHE and a maximum yield rate (YR) of 93.3 ± 5.7 μmol h–1 cm–2 at −0.4 V vs RHE. This combination of high FE and low onset potential is rarely achieved in NO3RR. The catalyst maintained stable performance over 12 consecutive 1 h chronoamperometric tests. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analyses before and after electrolysis revealed a significant increase in nanowire diameter (from 97 to 562 nm), no changes in the crystalline structure, and partial reduction of Co3+ to Co2+ at the electrocatalyst surface, while preserving the Co3O4 phase. In situ X-ray absorption near edge structure (XANES) further confirmed the stability of the Co3O4/TiO2/Ti under operating conditions. These results provide a basis for future investigations aiming to enhance ammonia yield rate at even lower overpotentials.