NiO Nanoparticles as Electrocatalyst for Nitrates Reduction

The introduction of the Haber-Bosch (HB) process in the early twentieth century enabled the large-scale production of NH3, swiftly becoming one of the most crucial chemical products worldwide due to its extensive application in agriculture as a fertilizer. Moreover, NH3 has recently garnered significant interest as a potential renewable energy storage system, given its capacity to serve as a source of hydrogen. However, the Haber-Bosch process, reliant on atmospheric nitrogen and fossil fuel, requires H2 for NH3 production, as well as high process temperature and pressure, contributing to approximately 1.6% of the annual global CO2 emissions. A promising alternative lies in the electrochemical nitrogen reduction reaction (E-NRR) to synthesize NH3 under ambient conditions. However, to date, this process has a limited yield production and a low selectivity due to the high stability of the N2 molecule and the presence of parasitic reactions, primarily leading to water (solvent) conversion into hydrogen. A more recent focus has emerged towards the reduction of NO3‒, as it can be more easily converted into NH3 with a significant Faradaic efficiency (FE) and high yield. Moreover, owing to the prevalent use of nitrogen-based fertilizers, this process possesses a significant real-case application towards wastewater treatment were high NO3‒ levels have often been detected. This study presents the utilization of a nanostructured NiO electrocatalyst, prepared by precipitation in aqueous medium and calcinated at 600 °C, for the reduction of NO3‒ into NH3, achieving an average FE of 36% and a production rate ranging from 28 to 107 μg h cm‒2, depending on the initial NO3‒ concentration. The experiments were conducted in a H-type cell, utilizing three different concentrations of KNO3 (NO3‒ source), i.e. 0.1, 0.05, and 0.008 M. A second investigated experimental parameter was the concentrations of the supporting electrolyte (i.e., K2SO4), that was used at 0.4, 0.45, and 0.492 M. The tests were conducted under an applied potential (E) of ‒1.4 V vs. Ag/AgCl for a duration of 2 h. In this contribution, we will show the main outcomes derived from this newly explored electrocatalysts, highlighting the main structure-performance correlations.