Combined Effects of Concentration, pH, and Polycrystalline Copper Surfaces on Electrocatalytic Nitrate-to-Ammonia Activity and Selectivity

Wastewater nitrates (NO3–) represent an untapped source for nutrient recovery. This study explores the effects of NO3– concentration ranging from 0.1 to 1 M and pH conditions of 8, 10, and 14 on the electrochemical reduction to ammonia (NH3) with polycrystalline Cu electrodes. Cyclic voltammograms prove pH- and concentration-dependent reaction kinetics. Chronoamperometry tests probed the reaction selectivity to NH3 production for a fixed potential across different pH conditions. The maximum NH3 Faradaic efficiency achieved was 46% ± 11% for 1 M NaNO3 at pH 14 at −0.55 V vs the reversible hydrogen electrode (RHE), while the minimum was 25% ± 6% for 1 M NaNO3 at pH 8. Distinctly, at pH 8 and 10, 0.1 M NaNO3 results in higher NH3 Faradaic efficiencies compared to the 1 M solution. Product quantification reveals that as the pH decreases, more charge is utilized for the formation of NO2– as compared to NH3 as a product. Large trial-to-trial uncertainties motivated the application of in situ electrochemical impedance spectroscopy to provide insights into the causal factors. Fitted parameters from impedance measurements correlate with measured contributions of net charge utilized for NH3 and NO2– production. Trial-to-trial variations map with changes in both the charge-transfer resistance and the effective double-layer capacitance. Changes in surface roughness and consequently the electrochemically active surface area are more dominant for 0.1 M NaNO3 solutions, while other variations play a significant role for 1 M NaNO3 tests. Overall, these results indicate that catalytic performance of NO3– reduction on Cu is highly sensitive to pH, concentration, secondary ions, and surface composition.