(437f) Harnessing Mass Transport to Optimize Ammonia Production during Electrochemical Nitrate Reduction

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 a widely used commodity chemical and fertilizer, thus balancing the nitrogen cycle while reducing energy consumption from the traditional Haber-Bosch process. Efforts to date have primarily focused on improving NO₃RR at the electrode surface. In parallel, mass transport throughout the system dictates the supply of reactants and supporting ions while defining the local reaction environment (i.e., electric potential, pH, ion concentration). Therefore, mass transport plays an important, and sometimes determining, role in NO₃RR activity and selectivity.

We used a membrane-separated flow cell and titanium foil electrode to study the influence of mass transport (Fig. 1a). Considering the two serial interfaces in the working electrode chamber (electrode/electrolyte and electrolyte/membrane), different strategies are employed to enhance mass transport across them. Nitrate removal, ammonia N-selectivity and Faradaic efficiency are chosen as key performance metrics.

Cross-membrane ion transport resistance can significantly limit the total current density of electrochemical systems. We have found that pairing membrane type (proton vs. cation exchange membrane) with the primary charge carrier species in bulk electrolyte can improve total current density and nitrate removal (Fig. 1b). Meanwhile, the structure of the diffusion layer at the electrode/electrolyte interface is strongly impacted by bulk electrolyte flow rate, which can be utilized to tune nitrate removal and ammonia production. Interestingly, the dependence of nitrate selectivity on flow rate shows opposite trends in different co-existing cation concentrations (Fig. 1c). Continuum models will be applied to solve interfacial potential and concentration profiles to illustrate the underlying mechanisms.

As an underexplored topic, mass transport processes in electrochemical NO₃RR are being systematically studied and will be engineered to optimize ammonia production from polluted waters.