Defect-induced triple synergistic modulation in copper for superior electrochemical ammonia production across broad nitrate concentrations

Excessive nitrate in wastewater and groundwater, stemming from fertilizer overuse and industrial contamination, poses environmental and health risks. Electrochemical nitrate reduction to ammonia offers an environmentally friendly alternative to traditional methods, providing a valuable resource while treating pollutants. However, achieving both high Faradaic efficiency (FE) and production rate across the broad range of nitrate concentrations (1–100 mM) found in real wastewater remains a significant hurdle. Existing studies often focus on high nitrate concentrations, neglecting the challenges posed by low concentrations where hydrogen evolution reaction (HER) competes strongly. This study aims to address this gap by developing a catalyst that effectively reduces nitrate across a wide concentration range, thereby offering a practical solution for wastewater treatment. The authors highlight the importance of low-concentration nitrate reduction, citing the work of Kang et al. (NiFe LDH/Cu foam) and Junqueira et al. (CuCoSP catalyst) which achieved commendable results but still had limitations in either FE, current density, or broader concentration range applicability. The need to address both strong HER at low concentrations and insufficient active “H” supply at typical concentrations to efficiently treat real waste streams with varying nitrate levels is emphasized.

The existing literature demonstrates significant progress in electrochemical nitrate reduction to ammonia. Several studies have explored various catalytic materials and strategies to enhance the efficiency of this process. However, many studies have focused on specific concentration ranges or idealized conditions, limiting their applicability to real-world wastewater treatment scenarios. For example, while some research has shown high Faradaic efficiencies at high nitrate concentrations, the performance often deteriorates at lower concentrations due to the competing hydrogen evolution reaction. The review highlights the importance of developing catalysts that can maintain high efficiency across a wide range of nitrate concentrations. This necessitates a deeper understanding of the reaction mechanisms and the ability to control competing reactions. The scarcity of research on nitrate reduction in authentic wastewater further emphasizes the need for a more practical and efficient solution.

The study involved the synthesis of a defect-rich copper nanowire array electrode (V-Cu NAE) through an in-situ electrochemical reduction of Cu3N nanowires. The synthesis process began with the preparation of copper hydroxide nanowires (Cu(OH)2 NWs) by oxidizing copper foam. These were then converted to copper nitride nanowires (Cu3N NWs) through sintering under ammonia gas. Finally, the Cu3N NWs underwent in-situ electrochemical reduction in a potassium sulfate solution, resulting in the formation of the V-Cu NAE. This process introduced defects, specifically copper vacancies, into the copper nanowire structure. The materials were characterized using various techniques, including powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), synchrotron radiation X-ray absorption near edge structure (XANES), and X-ray photoelectron spectroscopy (XPS). The electrochemical performance of the V-Cu NAE was evaluated using a three-electrode H-type cell with a 0.5 M K2SO4 electrolyte. The effects of varying nitrate concentrations (1–1000 mM) were investigated. Operando synchrotron radiation Fourier transform infrared (SR-FTIR) spectroscopy was employed to study the reaction mechanism in-situ. Density functional theory (DFT) calculations were performed to understand the catalytic behavior of both defect-free and defect-rich copper surfaces, focusing on the adsorption of nitrate, water dissociation, and hydrogen evolution. Finally, a two-electrode flow electrolytic cell was used to test the practical applicability of the V-Cu NAE in treating actual industrial wastewater, coupled with a glycerol oxidation reaction (GOR) at the anode for enhanced energy efficiency.

The defect-rich V-Cu NAE exhibited significantly enhanced electrochemical performance compared to defect-free copper nanowires and other control samples. It achieved high current densities (50–1100 mA cm⁻²) across a wide range of nitrate concentrations (1–100 mM), maintaining a Faradaic efficiency above 90%. Operando SR-FTIR spectroscopy revealed the formation of ammonia and the presence of nitrite intermediates, confirming the efficient nitrate reduction pathway. DFT calculations showed that the copper vacancies in the V-Cu NAE led to a triple synergistic modulation: enhanced nitrate adsorption, promoted water dissociation, and suppressed hydrogen evolution. This explained the superior catalytic activity observed experimentally. The two-electrode system, integrating NOxRR with GOR, demonstrated excellent performance in treating actual industrial wastewater, achieving a current density of 550 mA cm⁻² at -1.4 V with 99.9% ammonia selectivity, 99.9% nitrate conversion, and outstanding stability for over 100 hours. The high-purity ammonia products (NH4Cl and NH3·H2O) were successfully collected using air stripping and subsequent acid treatment, showcasing the practical feasibility of the developed technology. Electrochemical double-layer capacitance measurements confirmed a significantly higher electrochemically active surface area (ECSA) for V-Cu NAE compared to defect-free Cu NWs.

The findings demonstrate the remarkable effectiveness of defect engineering in enhancing the performance of copper-based catalysts for electrochemical nitrate reduction to ammonia. The triple synergistic modulation mechanism, elucidated through experimental and computational studies, provides valuable insights into the key factors governing the catalytic activity. The high current densities and Faradaic efficiencies achieved across a broad concentration range address a critical limitation of existing technologies. The successful application of the V-Cu NAE in a two-electrode system for treating real industrial wastewater highlights its significant potential for practical implementation. The high selectivity and stability demonstrated by the system suggest its suitability for large-scale applications. This technology has significant implications for sustainable wastewater treatment and the distributed production of green ammonia, contributing to a more environmentally friendly and resource-efficient approach.

This study successfully synthesized a high-performance defect-rich Cu NAE catalyst for electrochemical nitrate reduction to ammonia. Operando characterization and DFT calculations revealed a unique triple synergistic modulation mechanism that explains the observed high activity and selectivity. The successful demonstration of this catalyst in a two-electrode system for treating real industrial wastewater, coupled with efficient ammonia product recovery, indicates significant potential for practical application in sustainable wastewater treatment and green ammonia production. Future research could focus on exploring other defect types and catalyst compositions to further enhance performance and explore large-scale implementation.

While the study demonstrates exceptional performance, several limitations should be noted. The long-term stability tests were conducted under specific conditions, and further investigation may be needed to assess performance under a wider range of operating parameters. The scaling up of the two-electrode system for industrial applications requires further optimization and engineering design. Furthermore, a comprehensive life cycle assessment of the technology, including energy consumption and material costs, would provide a more complete evaluation of its sustainability.