Ammonia Synthesis from Electrochemical Reduction of Nitrate Using Boron-Doped Diamond Electrodes

In this study, we investigated ammonia synthesis from electrochemical nitrate reduction using a boron-doped diamond (BDD) electrode. Several parameters were optimized, including the boron doping level in BDD, reduction potential, cell type, and electrolyte, to reach an ammonia production rate of 67 ± 12 μmol cm–2 h–1 with a Faradaic efficiency of 98 ± 6%. The ammonia production rate could be enhanced up to 184 μmol cm–2 h–1 by adjusting the boron doping level. From kinetic measurements during ammonia synthesis, the time dependence of NO3–, NO2–, and NH3 concentrations confirmed that the reaction of electrochemical nitrate reduction can be described by two sequential reactions: NO3– reduction to NO2– and the susbequent reduction of NO2– to NH3. Among the conditions studied, the addition of NaOH, which decreased the charge transfer resistance of electron transfer and increased the ionic conductivity, was found to enhance the rate constant of both reactions, with a significantly larger effect on NO2– to NH3. In addition, comparison with other electrode materials, namely Cu, Ti, and glassy carbon confirmed the superior quality of the BDD electrode in terms of production rate, Faradaic efficiency, and durability.