Mass Transport Modifies the Interfacial Electrolyte to Influence Electrochemical Nitrate Reduction

The electrochemical nitrate reduction reaction (NO3RR) can facilitate remediation of nitrate-polluted wastewater and sustainable production of ammonia. As an important component of the reaction microenvironment, the interfacial electrolyte substantially influences NO3RR but remains underexplored. Mass transport modifies the interfacial electrolyte properties (e.g., pH, solute concentrations) and thus regulates NO3RR activity and selectivity. In a representative flow-cell configuration with a titanium NO3RR electrode, we systematically controlled mass transport conditions and demonstrated their impacts on NO3RR performance. With continuum model simulation and in situ infrared absorption spectroscopy, we characterized the interfacial electrolyte environment under varied mass transport conditions. Furthermore, we strategically tuned the interfacial electrolyte properties and experimentally deconvoluted their impacts on NO3RR activity and selectivity. We found that diffusion layer thickness and background electrolyte concentration govern NO3RR activity, while interfacial pH steers NO3RR selectivity. Inspired by these findings, we applied pulsed potential to periodically refresh the interfacial electrolyte environment and lower the local pH, successfully tripling the relative ammonia-to-nitrite selectivity. Distinct from NO3RR studies that focus on reaction kinetics, this study was conducted under commonly observed mass transport limitations to advance mechanistic understanding behind mass transport effects and to help identify engineering opportunities that optimize ammonia production.