Efficient and Selective Electrochemical Nitrate Reduction to N₂ Using a Flow-Through Zero-Gap Electrochemical Reactor with a Reconstructed Cu(OH)₂ Cathode: Insights into the Importance of Inter-Electrode Distance

Abstract

Electrochemically converting nitrate, a widely distributed nitrogen contaminant, into harmless N₂ is a feasible and environmentally friendly route to close the anthropogenic nitrogen-based cycle. However, it is currently hindered by sluggish kinetics and low N₂ selectivity, as well as scarce attention to reactor configuration. Here, we report a flow-through zero-gap electrochemical reactor that shows a high performance of nitrate reduction with 100% conversion and 80.36% selectivity of desired N₂ in the chlorine-free system at 100 mg-N·L^-1 NO₃^- while maintaining a rapid reduction kinetics of 0.07676 min^-1. More importantly, the mass transport and current utilization efficiency are significantly improved by shortening the inter-electrode distance, especially in the zero-gap electrocatalytic system where the current efficiency reached 50.15% at 5 mA·cm^-2. Detailed characterizations demonstrated that during the electroreduction process, partial Cu(OH)₂ on the cathode surface was reconstructed into stable Cu/Cu₂O as the active phase for efficient nitrate reduction. In situ characterizations revealed that the highly selective *NO to *N conversion and the N-N coupling step played crucial roles during the selective reduction of NO₃^- to N₂ in the zero-gap electrochemical system. In addition, theoretical calculations demonstrated that improving the key intermediate *N coverage could effectively facilitate the N-N coupling step, thereby promoting N₂ selectivity. Moreover, the environmental and economic benefits and long-term stability shown by the treatment of real nitrate-containing wastewater make our proposed electrocatalytic system more attractive for practical applications.