Co-based tandem catalysts for electrocatalytic nitrate reduction: mechanisms and challenges

Electrochemical nitrate reduction (NO₃^-RR) offers an environmentally benign route for nitrate removal and sustainable ammonia synthesis. However, achieving high selectivity and efficiency in this process remains a major challenge. Cobalt, as a low-cost transition metal with flexible oxidation states, has garnered significant interest in NO₃^-RR electrocatalysis. Yet, single-site cobalt catalysts often struggle to simultaneously regulate the adsorption and conversion of nitrate and its intermediates, as well as promote water dissociation and optimal hydrogen adsorption, resulting in limited faradaic efficiency and NH₃ selectivity. Tandem catalysts, which integrate multiple functionally complementary sites, have emerged as a promising solution to decouple and accelerate the complex multi-step reaction involved in NO₃^- reduction. This review analyzes recent advances in Co-based tandem electrocatalysts for nitrate-to-ammonia conversion, with a focus on active-site design, synergistic mechanisms, and reaction pathway regulation. We also discuss key challenges and opportunities related to the practical application of tandem catalysts in electrochemical NO₃^-RR systems. Finally, we propose future research directions for sustainable nitrate conversion.