Polarity Modulation Enhances Electrocatalytic Reduction of Nitrate by Iron Nanocatalysts

Electrocatalytic reduction to convert nitrate (NO3-) to N-2 or NH3 is of great interest for water and wastewater treatment, as well as N cycle management. However, the inherent electrostatic repulsion between the negatively charged nitrate ion and the cathode hinders nitrate adsorption on the catalyst and decreases reaction kinetics. In this study, we demonstrate that a simple polarity modulation strategy greatly enhances NO3- reduction catalyzed by an iron nanocatalyst immobilized on a carbon black electrode (Fe@CB). By switching between a positive and a negative potential, the system cycled through a short electrosorption step and a longer electrocatalytic reduction step. This increases the pseudo-first-order reaction rate constant of nitrate reduction from 2.46 to 3.09 h(-1), a 25.6% increase compared to the constant potential operation. The improved nitrate reduction kinetics was attributed to the enhanced nitrate adsorption during the electrosorption step, which improved the subsequent electrocatalytic reduction of NO3- upon reversal of the applied voltage. A short 30 s adsorption step at +0.1 V was found to enhance NO3- adsorption while avoiding reoxidation of reduced species formed in the previous reduction step. This operational strategy can be easily applied to other electrodes and catalysts, offering a simple, chemical-free, and cost-effective method for the removal of NO3- and valorization.