Pulsed Electrolysis in Membrane Electrode Assembly Architecture for Enhanced Electrochemical Nitrate Reduction Reaction to Ammonia

Abstract

Electrochemical nitrate reduction reaction (NO₃^–RR) to ammonia offers a promising solution to environmental and energy challenges, converting a ubiquitous pollutant in aquatic environments into a carbon-free energy carrier and essential chemical feedstock. While considerable research has focused on electrocatalyst development, relatively less attention has been given to device engineering and electroanalytical techniques that play crucial roles in enhancing the performance of the electrocatalytic NO₃^–RR, especially at such low concentrations. Here, Cu_xRu_y alloy catalysts were synthesized, and their electrocatalytic performance was investigated by using various electroanalytical techniques in H-type and membrane-electrode-assembly (MEA) configurations. The results revealed the poor performance of the electrocatalytic NO₃^–RR at low NO₃^– concentrations (0.01 M) in H cells due to the mass transfer loss, promoting the competing hydrogen evolution reaction. Pulsed electrolysis was leveraged as an effective strategy to enhance ammonia yield rate (3-fold) and Faradaic efficiency (FE) (2-fold) compared to the potentiostatic (i.e., constant voltage) condition at low nitrate concentrations, primarily by impacting the local microenvironment. Additionally, an MEA cell was constructed with anionic and bipolar membranes, and a comparative study was conducted by examining cell voltage, selectivity, and energy efficiency. The findings exhibited that membrane type significantly influences cell voltage and system efficiency. Notably, the CuRu alloy catalyst in an MEA system with an anion exchange membrane achieved a FE exceeding 90% at 200 mA cm^–2 with the highest NH₃ yield rate of 5.74 ± 0.27 mmol h^–1 cm^–2 and stability over 100 h assessed at 600 mA cm^–2. The insights gained from this work could inform the rational design of the electrochemical NO₃^–RR to ammonia with enhanced catalytic performance at low nitrate concentrations.