Nitrate is a prevalent waterborne pollutant that threatens the health of humans and aquatic systems. By selectively producing ammonia, electrochemical nitrate reduction reaction (NO₃RR) can directly transform nitrate pollutants into widely used commodity chemicals and fertilizers, thus balancing the nitrogen cycle while reducing energy consumption from the traditional Haber-Bosch process.

Although research efforts to date have primarily focused on the development of efficient nitrate reduction catalysts, the electrolyte environment has been found to substantially influence NO₃RR activity and selectivity. Specifically, the composition of the first ten nanometers of electrolyte away from the electrode surface dictates the immediate environment within which reactions take place. As has been demonstrated in the electrochemical CO₂ reduction reaction, this interfacial reaction microenvironment comprised of near-surface electrolyte and electrocatalyst influences reaction activity and selectivity. In electrochemical NO₃RR, where an anionic species reacts at a negatively charged electrode, the reaction rate can often be limited by the mass transport of the reactant nitrate. Additionally, mass transport phenomena define the electrolyte side of the reaction microenvironment by influencing other interfacial properties (i.e., electric potential, pH and ion concentrations). Therefore, we investigated the influence of mass transport phenomena on NO₃RR activity and selectivity.

In this study, we used a membrane-separated flow cell as a representative and translatable reactor and titanium foil as a common ammonia-selective NO₃RR electrode. By varying the electrolyte flow rate, we controlled the mass transport phenomena in the flow cell and empirically determined the diffusion layer thickness. We found that NO₃RR activity generally increased with increasing electrolyte flow rate, as predicted from the decreasing diffusion layer thickness, whereas NH₃ selectivity slightly decreased, showing an activity-selectivity trade-off.

To unravel the origins of the experimentally observed mass transport effects, simulations using the 1D generalized modified Poisson-Nernst-Planck (GMPNP) model were conducted to spatially resolve interfacial properties. Interfacial cation concentration and pH were the two properties that changed most significantly with electrolyte flow rate. Informed by simulation results, we controlled the bulk electrolyte composition to modify the interfacial cation concentration and pH and examine their impacts on NO₃RR activity and selectivity. Nitrate removal rate decreased by half when the bulk cation concentration was lowered to 1/100 of the original bulk concentration but the product distribution was similar. Meanwhile, in phosphate buffered electrolytes with different bulk pH values, NH₃ selectivity decreased as bulk pH increased, with NO₃RR activity remaining largely unchanged. Therefore, we attributed the influence of mass transport on NO₃RR selectivity to changes in the interfacial pH during reaction, likely as the result of the varying NO₃RR activity under different flow rates. By comparing the nitrogen species product distribution in different electrolytes, we inferred that the interfacial pH increased rapidly to basic in non-buffered systems regardless of their bulk pH.

In summary, we systematically investigated the underexplored effects of mass transport on NO₃RR and found that it influenced the reaction activity and selectivity by influencing interfacial properties. This study underscores the importance of scrutinizing the interfacial electrolyte environment in electrocatalyst development and provides insight to optimize NH₃ production by engineering the bulk electrolyte.