Mechanism of electrocatalytic nitric oxide reduction reaction for the synthesis of ammonia and recent progress

Ammonia (NH3) serves as a crucial industrial feedstock, playing an indispensable role in human societal advancement and economic development. However, its synthesis and utilization pose significant environmental challenges. Currently, industrial-scale NH3 production primarily relies on the Haber−Bosch process, which consumes substantial fossil fuels and releases large quantities of greenhouse gases. Therefore, it is essential to develop more efficient and sustainable techniques for synthesizing NH3. In recent years, the electrochemical synthesis of NH3 has garnered significant attention due to its potential to reduce energy consumption, cut down the carbon footprint, streamline the reactor design, and offer a more eco-friendly and cost-effective approach to ammonia generation. Among them, electrocatalytic N2 reduction reaction (NRR) yields NH3 in ambient conditions and thus is beneficial for environmental sustainability, which has been attracting a great deal of attention. Nevertheless, the inert N≡N (cleavage energy 941 kJ mol−1) places serious restrictions on the NRR activity and NH3 selectivity. Therefore, NH3 production is urgently needed to explore more green and economizing alternative technologies for the NH3 demand. By contrast, electrocatalytic NO reduction reaction (NORR) represents a promising strategy for mitigating air pollution and reducing energy consumption in ammonia synthesis, with significant potential to supplant the resource- and energy-intensive selective catalytic reduction (SCR) and Haber−Bosch processes. The practical implementation of this technology largely hinges on the development of efficient electrocatalysts, which can catalyze NORR for NH3 synthesis at low overpotentials, exhibiting high reaction rates and selectivity. Consequently, the design strategies for catalysts with outstanding synthesis performance have garnered significant attention from researchers. This paper reviews the mechanism of electrocatalytic NORR to NH3, focusing on the key aspects of electrocatalysis such as single-atom coordination, crystal plane orientation and defect engineering. We aim to summarize recent advancements in catalytic NORR systems while also exploring future directions and potential breakthroughs that may significantly advance the conversion of nitrogen oxides to ammonia. Over the past few years, there has been a notable increase in research and reports focused on NORR, yet despite these efforts, the materials and catalysts developed to date still fall short of achieving the energy efficiency and ammonia yield that is comparable to the traditional Haber−Bosch process, which remains the industry standard for ammonia synthesis. The electrochemical catalysis process still faces several bottlenecks including high costs, complex preparation methods, low Faradaic efficiency (FE), elevated overpotential, intense competition from hydrogen evolution reactions (HER), and poor catalytic stability. Although numerous obstacles remain to be addressed, advancements in theoretical calculations, experimental optimizations and novel characterization techniques have contributed significantly to enhancing the synthesis of NORR catalysts. Addressing the challenges surrounding electrocatalytic NORR is essential for realizing a cleaner and more sustainable future. A breakthrough in the development of highly efficient and stable NORR electrocatalysts could not only contribute to environmental protection by reducing harmful NO emissions but also open the door to new, cleaner energy carriers. Furthermore, it could help establish a sustainable green ammonia synthesis industry that could revolutionize the way ammonia is produced, moving away from the energy-intensive Haber−Bosch process and toward a more environmentally friendly alternative. With continued innovation and research in this area, electrocatalytic NORR holds the potential to play a crucial role in advancing both environmental sustainability and clean energy technologies.