Modulating single-atom M-N-C electrocatalysts for the oxygen reduction: the insights beyond the first coordination shell

The oxygen reduction reaction (ORR) is a pivotal process in electrochemical energy systems such as fuel cells and metal-air batteries. Recent advancements have highlighted the single-atom metal-nitrogen-carbon (M-N-C) catalysts for their exceptional ORR electrocatalytic performance. However, further exploration is needed for the optimization methods of single atomic active sites. Significantly, the modulation of coordination environment emerges as a pivotal technique for the enhancement of M-N-C catalysts, while extending this modulation beyond the first coordination shell has ignited substantial investigation. This review delves into the frontier of M-N-C optimization by transcending the first coordination shell, presenting a comprehensive analysis of innovative strategies that modulate the electronic structure and reactivity of MN4 sites. The primary focus lies in three regulation approaches: regulating atomic entities, introducing metallic species and tailoring non-metallic modulators. These strategies are scrutinized for their ability to fine-tune the ORR activity and stability at the atomic level. By providing a clearer trajectory for future research, this review should be able to inspire novel designs of high-performance M-N-C ORR catalysts.