Electrochemical nitrate reduction on boron-doped diamond cathodes in a single-compartment cell for selectivity control and nitrite suppression

Nitrate contamination in water sources poses significant environmental and public health concerns, motivating the development of selective electrochemical remediation strategies. Herein, a boron-doped diamond (BDD) cathode was evaluated for nitrate electroreduction in a single-compartment electrochemical cell and compared with a conventional Cu cathode under galvanostatic operation. Nitrate conversion, nitrogen product distribution, and specific energy consumption were assessed as a function of applied current density (6–30 mA cm-2). For both cathodes, nitrate conversion increased with current density; however, BDD consistently outperformed Cu at current densities ≥ 12 mA cm-2, reaching 50 ± 2% nitrate conversion at 30 mA cm-2, compared with 34.4 ± 5.0% for Cu under the same conditions. Notably, under the tested single-compartment conditions (NaCl as supporting electrolyte and a DSA® anode), BDD provided improved control of nitrogen speciation, with nitrite remaining below the detection limit (< 0.05 mg L-1 as NO₂⁻) across all operating conditions. At low current densities, BDD favored ammonium formation, reaching a maximum of 3.00 mg-N L-1 (as NH₄⁺-N) at 12 mA cm⁻², which was twofold higher than the maximum value obtained with Cu (1.50 mg-N L-1). In contrast, Cu promoted nitrite formation at high current densities, with detectable NO2--N concentrations appearing at ≥ 24 mA cm-2 and increasing up to 1.45 mg-N L-1 (as NO2--N) at 30 mA cm-2. Energy consumption increased markedly with current density for both electrodes, ranging from 195 to 4519 kWh kg-1 N–NO3- for BDD and 198 to 3546 kWh kg-1 N–NO3- for Cu. Although BDD does not compete with state-of-the-art catalysts in terms of ammonium productivity, its chemical stability and the observed low nitrite accumulation under single-compartment conditions make it a promising material for applications prioritizing selectivity control and operational robustness.