Selectivity modulation during electrochemical reduction of nitrate by electrolyte engineering

This article explores how electrolyte engineering can control product selectivity and kinetics of electrochemical reduction of nitrate (ERN). This is an alternative approach to the conventional catalyst engineering methodology for controlling the electrode/electrolyte interface and impacting on ERN activity and selectivity. Electrolytic treatment was conducted in a membrane-less plug flow reactor (PFR) under batch recirculation using a tin cathode. Operational parameters related to solution flow rate, mass transport regime, initial pH, and dissolved oxygen demonstrated to have negligible impact on nitrate (NO₃^–) removal under the operation conditions studied. In stark contrast, the presence of different alkali cations in solution (Li⁺, Na⁺, K⁺ and Cs⁺) sharply impacted on NO₃^– removal rate and steered product selectivity in ERN, as well as they did it for the case of nitrite (NO₂^–) reduction reaction. An evident increase in ammonia (NH₃) production is achieved in both NO₃^– and NO₂^– removal by following the order Li⁺ < Na⁺ ≈ K⁺ < Cs⁺. These close tendencies observed for NO₃^– and NO₂^– reduction reactions point to the electrostatic effect stabilizing negatively charged species at the electrode interface as the main responsible of selectivity modulation through electrolyte engineering. Thus, we present the first evidence of a significant shift in products selectivity in ERN from N_gas towards NH₃ production on tin electrodes by tuning the electrode–electrolyte interface with suitable cations. Furthermore, an approximately 2-fold decrease in electrical energy per order is achieved by solutions containing Cs⁺ instead of Li⁺ for both NO₃^– and NO₂^– reduction reactions. These results open the pathway towards understanding interfacial impacts associated to different ionic species present in solution that can enhance electrochemical pollutants removal, and resource recovery, as well as lowering the process cost.