Pulsed Electrolysis\nin Membrane Electrode Assembly\nArchitecture for Enhanced Electrochemical Nitrate Reduction Reaction\nto Ammonia

Electrochemical nitrate reduction reaction (NO<sub>3</sub><sup>–</sup>RR) to ammonia offers a promising solution to\nenvironmental\nand energy challenges, converting a ubiquitous pollutant in aquatic\nenvironments into a carbon-free energy carrier and essential chemical\nfeedstock. While considerable research has focused on electrocatalyst\ndevelopment, relatively less attention has been given to device engineering\nand electroanalytical techniques that play crucial roles in enhancing\nthe performance of the electrocatalytic NO<sub>3</sub><sup>–</sup>RR, especially at such low concentrations. Here, Cu<sub><i>x</i></sub>Ru<sub><i>y</i></sub> alloy catalysts were synthesized,\nand their electrocatalytic performance was investigated by using various\nelectroanalytical techniques in H-type and membrane-electrode-assembly\n(MEA) configurations. The results revealed the poor performance of\nthe electrocatalytic NO<sub>3</sub><sup>–</sup>RR at low NO<sub>3</sub><sup>–</sup> concentrations (0.01 M) in H cells due\nto the mass transfer loss, promoting the competing hydrogen evolution\nreaction. Pulsed electrolysis was leveraged as an effective strategy\nto enhance ammonia yield rate (3-fold) and Faradaic efficiency (FE)\n(2-fold) compared to the potentiostatic (i.e., constant voltage) condition\nat low nitrate concentrations, primarily by impacting the local microenvironment.\nAdditionally, an MEA cell was constructed with anionic and bipolar\nmembranes, and a comparative study was conducted by examining cell\nvoltage, selectivity, and energy efficiency. The findings exhibited\nthat membrane type significantly influences cell voltage and system\nefficiency. Notably, the CuRu alloy catalyst in an MEA system with\nan anion exchange membrane achieved a FE exceeding 90% at 200 mA cm<sup>–2</sup> with the highest NH<sub>3</sub> yield rate of 5.74\n± 0.27 mmol h<sup>–1</sup> cm<sup>–2</sup> and\nstability over 100 h assessed at 600 mA cm<sup>–2</sup>. The\ninsights gained from this work could inform the rational design of\nthe electrochemical NO<sub>3</sub><sup>–</sup>RR to ammonia\nwith enhanced catalytic performance at low nitrate concentrations.