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 N 2 is a feasible and environmentally friendly route to close the anthropogenic nitrogen-based cycle. However, it is currently hindered by sluggish kinetics and low N 2 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 N 2 in the chlorine-free system at 100 mg-N·L –1 NO 3 – 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/Cu 2 O 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 NO 3 – to N 2 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 N 2 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.

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