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

Abstract

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.

Elucidating the Intrinsic Activity and Selectivity of Cu for Nitrate Electroreduction

“…Particularly, we focus on the impact of often overlooked factors on the NO 3 RR performance, including Cu facet exposure, nitrate concentration, and electrode area. 44 For Cu foils cleaned by two commonly used pretreatment methods (wet etching and electropolishing), we find that the electropolished Cu foil exhibits a higher activity and selectivity for NH 3 production as compared to the wet-etched Cu foil, which originates from the pretreatment induced different exposures of Cu facets. Moreover, the NH 3 selectivity increases monotonically with Cu electrode surface area, and such a dependence is attributed to a promoted NO 2 − conversion to NH 3 on a larger electrode, as validated by a 15 N isotope labeling experiment.…”

“…In this work, we use high-purity polycrystalline Cu foils as a model catalyst and benchmark electrode to investigate the intrinsic activity and selectivity of Cu for the NO 3 RR. Particularly, we focus on the impact of often overlooked factors on the NO 3 RR performance, including Cu facet exposure, nitrate concentration, and electrode area . For Cu foils cleaned by two commonly used pretreatment methods (wet etching and electropolishing), we find that the electropolished Cu foil exhibits a higher activity and selectivity for NH 3 production as compared to the wet-etched Cu foil, which originates from the pretreatment induced different exposures of Cu facets.…”

“…Therefore, understanding the intrinsic activity and selectivity of Cu for the NO 3 RR is essential to achieve a benchmarking and a consistent picture of the NO 3 RR catalytic performance for rational catalyst design. 44 In this work, we use high-purity polycrystalline Cu foils as a model catalyst and benchmark electrode to investigate the intrinsic activity and selectivity of Cu for the NO 3 RR. Particularly, we focus on the impact of often overlooked factors on the NO 3 RR performance, including Cu facet exposure, nitrate concentration, and electrode area.…”

“…Based on the above results, the electropolished Cu foil with a higher activity was selected as the “standard” Cu foil sample for further investigations below. As the NO 3 – concentration was reported to influence the NO 3 RR selectivity, ,, such as a higher concentration leading to an increased N 2 production or a suppressed HER, we evaluated the NO 3 RR on the electropolished Cu foil in 1 M KOH electrolytes containing various concentrations of NO 3 – . As shown in Figure a, the current density of the bulk electrolysis on the Cu foil at −0.2 V vs RHE increased linearly from 4.7 to 47.3 mA cm –2 as the NO 3 – concentration increased from 5 to 50 mM.…”

“…Based on the above results, the electropolished Cu foil with a higher activity was selected as the "standard" Cu foil sample for further investigations below. As the NO 3 − concentration was reported to influence the NO 3 RR selectivity, 33,43,44 such as a higher concentration leading to an increased N 2 production We further examined the role of Cu electrode area in the NO 3 RR catalysis, as nanostructured electrodes with high surface areas were often used in previous studies, while the surface area was not considered to correlate with the NO 3 RR selectivity. Here we varied the Cu foil surface area from 0.25 to 2 cm 2 , and the electrolysis current on the Cu foil increased almost linearly from 1.5 to 9.3 mA under typical test conditions (5 mM NO 3 − , −0.2 V vs RHE), as shown in Figure 3c.…”