Highly selective and efficient electrochemical nitrate removal via Ni-doped Cu(OH)2 nanowires promoting H* reaction

Abstract

Electrochemical reduction of nitrate (NO₃⁻) offers a promising approach for nitrate removing from water. Development of non-noble metal electrocatalysts with high reactivity and selectivity is crucial for the electrochemical reduction of NO₃⁻. Herein, Ni-doped Cu(OH)₂ nanowires (Ni-Cu(OH)₂ NW/CF) were synthesised on copper foam (CF) and utilised as a cathode, enabling efficient and selective nitrate removal with further conversion to harmless N₂. The Ni-Cu(OH)₂ NW/CF electrode achieved a 99.8% NO₃⁻-N conversion rate within 40 min, with a reaction rate 25 times higher than CF and 1.51 times than that of Cu(OH)₂. It demonstrated a rapid nitrate reduction apparent rate constant (0.02072 cm⁻² min⁻¹) without nitrite accumulation. With the assistance of active chlorine at the anode, the electrocatalytic system effectively converted NO₃⁻ into harmless N₂, achieving a final N₂ selectivity of 99.9%. The Cu(OH)₂ nanowires provided abundant active sites to enhance NO₃⁻ adsorption and conversion, while Ni doping mainly accelerated the reduction of NO₂⁻ intermediate by facilitating the atomic hydrogen (H*)-mediated reaction pathway on the electrode surface. This relay catalytic effect enhanced the electrode's adsorption and reduction capabilities for NO₃⁻. Notably, the Ni-Cu(OH)₂ NW/CF electrode maintained consistent performance over 10 experimental cycles. This study provides novel strategy for designing non-noble metal electrocatalysts with high selectivity and efficiency for NO₃⁻ removal.