Voltammetric Studies of Nitrate Reduction at Boron‐Doped Diamond Thin‐Film Electrodes of Different Microstructure

The electrochemical reduction of nitrate at polycrystalline boron‐doped diamond (BDD) electrodes possessing three different microstructures was studied. The BDD thin films were grown on conducting Si substrates using 2 and 1% CH 4 /H 2 and 1% CH 4 /Ar source gas mixtures. All films were highly boron‐doped using 0.1% B 2 H 6 /H 2 in the source gas to produce a doping level of ca. 10 21 cm −3 or ~5000 ppm. BDD films with a hydrogen surface termination were investigated. All BDD films microstructures exhibited a high degree of electrochemical activity for a suite of redox systems: (IrCl 6 −2/−3 , Fe(CN) 6 −3/−4 , Ru(NH 3 ) +3/ + 2 , and methyl viologen +2/+/0 ). The reduction of nitrate proceeded similarly on the BDD electrodes in terms of onset and peak potentials in 0.1 M Na 2 SO 4 . In other words, the BDD electrode microstructure, at least for these three film types, had no differential influence on the nitrate reduction polarization curves. However, the largest peak current densities were observed for the 1% CH 4 /H 2 nanocrystalline BDD film. The reaction proceeds through a two‐step process involving the initial reduction of nitrate to nitrite at less negative potentials (ca. −1.2 V vs. Ag/AgCl), followed by further reduction to ammonia or other products at more negative potentials (ca. −1.6 V). Initial batch electrolysis experiments confirmed ammonia as the primary electrolysis product at the nanocrystalline BDD thin films in 1 M NaOH (−1.8 V for 1 h), achieving a production rate of up to 42 µmol/cm 2 h at a Faradaic efficiency of 85%. The results align with earlier reports in the literature and contribute meaningfully to the advancement of nitrate reduction research using BDD electrodes.