Transition-metal-based Catalysts for Electrochemical Synthesis of Ammonia by Nitrogen Reduction Reaction: Advancing the Green Ammonia Economy

Ammonia (NH₃), a cornerstone in the chemical industry, has historically been pivotal for producing various valuable products, notably fertilizers. Its significance is further underscored in the modern energy landscape, where NH₃ is seen as a promising medium for hydrogen storage and transportation. However, the conventional Haber–Bosch process, which accounts for approximately 170 million ton of NH₃ produced globally each year, is energy-intensive and environmentally damaging. The electrochemical nitrogen reduction reaction (NRR) emerges as a sustainable alternative that operates in ambient conditions and uses renewable energy sources. Despite its potential, the NRR faces challenges, including the inherent stability of nitrogen and its competition with the hydrogen evolution reaction. Transition metals, especially ruthenium (Ru) and molybdenum (Mo), have demonstrated promise as catalysts, enhancing the efficiency of the NRR. Ru excels in catalytic activity, while Mo offers robustness. Strategies like heteroatom doping are being pursued to mitigate NRR challenges, especially the competing hydrogen evolution reaction. This review delves into the advancements of Ru and Mo-based catalysts for electrochemical ammonia synthesis, elucidating the NRR mechanisms, and championing the transition towards a greener ammonia economy. It also seeks to elucidate the core principles underpinning the NRR mechanism. This shift aims not only to address challenges inherent to traditional production methods but also to align with the overarching goals of global sustainability.