Nitrates Reduction using NiO Nanoparticles as Electrocatalyst

The introduction of the Haber-Bosch (HB) process in the early twentieth century revolutionized the NH3 chemical production, contributing in a particularly significant way in the agricultural sector, given the widespread use of this product as a synthetic fertilizer. Its significance further extends to recent explorations in renewable energy, with NH3 emerging as a potential storage medium for hydrogen. Despite its extensive adoption, the Haber-Bosch process poses environmental challenges. Reliance on fossil fuels, coupled with elevated temperatures and pressures required to catalyse the synthesis reaction, leads to the production of a quantity of CO2 equal to approximately 1.6% of annual global production. 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.