Cationic Ionomer Coatings Promote Anion Electroreduction: Nitrate Reduction as a Model Reaction

Abstract Electrolysis technologies represent a promising pathway to valorize abundant feedstocks using renewable electricity. While ionomers serve critical roles as ion conductors and catalyst binders in electrolyzers, their impact on electrochemical transformations of ionic species remains poorly understood compared to extensively studied neutral-molecule conversions like water splitting and CO2 reduction. Herein, we use the electrochemical nitrate reduction reaction as a model transformation to investigate the influence of ionomer coatings in anion electroreduction reactions, which face fundamental challenges from electrostatic anion depletion near negatively polarized cathodes and premature desorption of anionic intermediates. We demonstrate that cationic ionomer coatings increase local nitrate concentration up to 10-fold compared to bulk electrolyte due to the interfacial Donnan potential. While prior studies have shown that ionomers can influence the local reaction microenvironment, this work provides, to the best of our knowledge, the first quantitative measurement of nitrate concentration within ionomer coatings as a function of charge density and its impact on catalysis. A 5-fold enhancement in ammonium partial current density was achieved with the ionomer bearing the highest charge density. Using in situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), we quantified enhanced local nitrate concentration (up to 1.1 M versus 0.1 M in bulk electrolyte), conversion, and supply with increasing charge density. This study establishes guiding principles for tuning ionomer charge identity and density to control interfacial ion concentrations in electrochemical transformations of dilute charged substrates.