Combined Effects of Concentration, pH, and Polycrystalline Copper Surfaces on Electrocatalytic Nitrate-to-Ammonia Activity and Selectivity
Luisa Barrera, Rachel Silcox, Katherine Giammalvo, Erika Brower, Emily Isip, Rohini Bala Chandran
Abstract
Wastewater nitrates (NO 3 – ) represent an untapped source for nutrient recovery. This study explores the effects of NO 3 – concentration ranging from 0.1 to 1 M and pH conditions of 8, 10, and 14 on the electrochemical reduction to ammonia (NH 3 ) with polycrystalline Cu electrodes. Cyclic voltammograms prove pH- and concentration-dependent reaction kinetics. Chronoamperometry tests probed the reaction selectivity to NH 3 production for a fixed potential across different pH conditions. The maximum NH 3 Faradaic efficiency achieved was 46% ± 11% for 1 M NaNO 3 at pH 14 at −0.55 V vs the reversible hydrogen electrode (RHE), while the minimum was 25% ± 6% for 1 M NaNO 3 at pH 8. Distinctly, at pH 8 and 10, 0.1 M NaNO 3 results in higher NH 3 Faradaic efficiencies compared to the 1 M solution. Product quantification reveals that as the pH decreases, more charge is utilized for the formation of NO 2 – as compared to NH 3 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 NH 3 and NO 2 – 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 NaNO 3 solutions, while other variations play a significant role for 1 M NaNO 3 tests. Overall, these results indicate that catalytic performance of NO 3 – reduction on Cu is highly sensitive to pH, concentration, secondary ions, and surface composition.