Comparative study of various metal oxides in different oxidation states for the Electrochemical Reduction of Nitrate to Ammonia (NO3-RR)

Ammonia, the basic building block of fertilizers and an essential industrial reagent is synthesized globally at a scale exceeding 150 million tons annually, making it the world’s second most commonly produced chemical after sulfuric acid (H2SO4). However, its production is typically based on the Haber Bosch process, an energy-intensive process (>600 KJ mol-1 ammonia), which entails demanding conditions of high pressures and high temperatures (150-350 atm, 350-550 °C). Moreover, in this process, H2 is entirely produced through steam-reforming natural gas, which consumes 3-5% of the global natural gas supply and is responsible for 450 million metric tons of CO2 emission annually. In addition to the challenges associated with conventional NH3 production, nitrate pollution is engendering severe threats to the whole ecosystem. On the other hand, the electrochemical reduction of nitrate to ammonia (NO3 - RR) is emerging as an efficacious method to synthesize NH3 under ambient conditions and remove nitrates from the environment while avoiding the limitations and problems involved in the conventional synthesis pathways. Besides, if electricity is provided through renewable energy, this process can be considered totally green and could contribute to electrifying, at least partially, an industrial sector such as NH3 production, which is crucial to reaching the complete decarbonization set by the EU in 2050. [1] However, the sluggish kinetics of NO3 - RR and competitive hydrogen evolution reaction (HER) are the main bottlenecks of the given technology and demand urgent scientific endeavors. Therefore, the development of efficient electrocatalysts with specific structures capable of suppressing competitive reactions i.e. HER is the need of the hour. To this end, the aim of this work was to compare the electrochemical performance of various metal oxides in different oxidation states (e.g. ZnO, CuO, Cu2O, Co3O4, Fe3O4, Fe2O3) to identify which electrocatalyst exhibit higher efficiency for the selective reduction of nitrate to ammonia between them. Each material was characterized using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) to evaluate its morphology and crystalline structure. The electrochemical performance were then tested in nitrogen-saturated (oxygen-free) phosphate-buffered solution (PBS) containing potassium nitrate through Linear Sweep Voltammetry (LSV) using a rotating disk electrode (RDE), and by Chronoamperometry (CA) in a H-type cell. Following the chronoamperometric tests, the concentrations of NH₄⁺ formed during the electroreduction were quantified by ion chromatography (IC). Among the various synthesized materials, Co₃O₄ nanoparticles obtained via an optimized co-precipitation synthesis route exhibited the best performance, achieving a Faradaic Efficiency (FE) of above 96% and an ammonia production rate of roughly 110 μmol h⁻¹ cm⁻² at -0.8 V vs Reference hydrogen electrode (RHE), providing a simple synthesis approach that can lead to highperforming, Pt-free electrocatalyst for the NO3 - RR.