84.0% energy-efficient nitrate conversion by a defective (Fe,Cu,Ni)2O3 electrocatalyst

Abstract

Decentralized ammonia (NH₃) production as a way of environmental remediation for nitrate (NO₃⁻) removal is a current issue due to the massive impact of nitrate on human well-being and the environment. Converting these contaminating NO₃⁻ species to valuable commodities such as NH₃ in a green and sustainable way with a high NO₃⁻ conversion rate at low energy consumption efficiency can help to protect the environment and improve the livelihood of human beings. We synthesized an in situ grown and nanoflower-type (Fe,Cu,Ni)₂O_3−x electrocatalytic active material via a facile hydrothermal method. This catalyst exhibited a high NO₃⁻ conversion of 92.6% into a dominant ammonia with a selectivity of 98.3%. The NH₃ was produced at a high yield rate of 9.2 mg h⁻¹ cm⁻² with faradaic efficiency (FE) of 94.8% at −0.3 V vs. the reversible hydrogen electrode (RHE) in an H-cell configuration. For a viable commercialization test, the scaled-up model of a single-stack flow cell electrolyzer was investigated and it accelerated the NH₃ yield rate to 49.3 mg h⁻¹ cm⁻² and 96.0% FE while holding a high energy efficiency of 84.0% and a low energy consumption of 21.0 kW h kg⁻¹. The NH₃ yield exceeds that of the current state-of-the-art due to the reactants' continuous and high mass transfer rate on the trimetallic oxide active sites. The elucidated electrokinetic reaction mechanism reveals the direct reduction pathway enhanced by the abundant oxygen-vacancy (OV) sites in a synergetic trimetallic oxide system. These OV sites play a crucial role in the reaction mechanism by facilitating the trapping of oxygen atoms in NO₃⁻, which are then surrounded by Fe^2+/3+, Cu⁺, and Ni^2+/3+ as sites of proton adsorption for NO₃⁻ hydrogenation. The trapping interaction enhancing the bond weakening of NO₃⁻ makes the electrochemical NO₃⁻ reduction reaction (eNO₃⁻RR) mechanism meaningful.