Comparative study of cobalt, copper and zinc oxides for the electrochemical reduction of nitrate to ammonia

Electrochemical nitrate reduction (NO₃RR) is gaining attention as a sustainable route for ammonia synthesis, offering a greener alternative to the energy-intensive Haber–Bosch process. In this work, we present a systematic and direct comparison of three representative metal oxide electrocatalysts (ECs), named CuO, ZnO, and Co₃O₄. These materials were synthesized following a simple co-precipitation route in an alkaline environment. This synthesis approach ensures consistent preparation conditions, allowing a more reliable assessment of their intrinsic electrocatalytic behavior. Structural and morphological characterizations, including XRD, SEM, HR-TEM, STEM, EDX, XRF and XPS, confirmed the formation of phase-pure oxides with distinct nanostructures: fibrous, needle-like CuO, plate-like Co₃O₄, and cauliflower-like ZnO. Moreover, binary mixtures of these oxides were also investigated to explore potential synergistic effects. Co₃O₄ emerged as the most promising EC to form NH4+, achieving a Faradaic efficiency (FE) of 94.3 % and a yield rate of 149.5 μmol h⁻¹ cm⁻² at −0.8 V vs RHE. In contrast, ZnO exhibited the lowest activity for NO₃RR, with FE below 16 % and yield rates below 7 μmol h⁻¹ cm⁻² at −0.8 V vs RHE. Instead, CuO showed high selectivity towards the partial reduction of NO3− to NO₂⁻ (FE up to 79.6 % and yield rate of 586.9 μmol h⁻¹ cm⁻²), with limited NH₄⁺ formation (FE 20.8 %, 40.8 μmol h⁻¹ cm⁻²). These findings highlight the potential of Co₃O₄ as an efficient platinum group metal-free (PGM-free) EC for selective and efficient ammonia production.