Electrochemistry-based processes to produce ammonia have recently gained widespread interest by the scientific community because they provide a way to reduce the high carbon emissions currently associated with ammonia production. Global NH₃ demand is expected to rise since it is included in various energy scenarios as a possible green transportation fuel and storage medium of renewable electricity involving fuel cells.¹ Several electrochemical and hybrid electrochemical processes with plasma integration are currently being investigated to produce NH₃ via different nitrogen sources, such as N₂, NO₃^-/NO₂^- or NO. The utilization of NO₃^- from wastewater sources can be valuable for the environmental and health issues associated with NO₃^- contamination. Additionally, NO₃^- electroreduction in aqueous media is an attractive approach to investigate the catalysis due to its high solubility and the low dissociation energy needed to break the N=O bond. Cu and Cu-based materials have extensively been studied for NH₃ production through NO₃^- electroreduction. Earlier studies on Cu single crystals² in alkaline media showed that Cu (100) performs better than Cu (111) in reducing the NO₂^- intermediate to hydroxylamine, which is believed to be the precursor to NH₃. On the contrary, recent density functional theory calculations³ showed that a Cu−Cu couple with 2.57 Å distance serves as a catalytic center, leading to the conclusion that Cu (111) is the most active surface termination. In this work, preferentially oriented Cu₂O crystals with (111), (100), and (110) terminations were obtained via electrodeposition and evaluated for their performance in NO₃^- electroreduction to NH₃ (Figure 1). To understand which oxidation state of Cu represents the active phase, the samples were investigated by in-situ Raman spectroscopy. According to a recent study, the active phase was a combination of Cu and Cu⁺.⁴ Our results point to the immediate reduction of remaining Cu⁺ species during application of -0.3V vs. Ag/AgCl and formation of Cu(OH)₂ at more negative potentials. The presence of Cu(OH)₂ was also apparent from ex-situ XPS and UV-Vis analysis. Moreover, post-reaction XRD analysis showed a retainment of the crystal orientation on the reduced Cu surfaces. All the nanostructured catalysts were active for NO₃^- reduction and produced NH₃ with faradaic efficiencies above 40%. Cu₂O (111) was the most efficient for NH₃ synthesis reaching a faradaic efficiency of up to 99%.

1 D. Anastasiadou, Electrochem. Sci. Adv.,submitted.

2 E. Pérez-Gallent, Electrochim. Acta, 2017, 227, 77–84.

3 T. Hu, ACS Catal., 2021, 11, 14417–14427.

4 Y. Wang, Angew. Chemie Int. Ed., 2020, 59, 5350–5354.