The electrochemical oxygen reduction reaction (ORR) is a critical reaction in many energy conversion and storage systems, including fuel cells and metal–air batteries. Nonprecious-metal-based catalysts have captured attention for realizing viable and sustainable devices. However, in Fe-based catalysts, the efficient utilization of active sites, Fe–N_x and Fe coated on a carbon layer (Fe@C), is challenging owing to difficulties in controlling these active sites during synthesis. In this study, FeNC electrocatalysts with varying Fe/C ratios show different Fe@C/Fe–N_x ratios and ORR activities. Increasing the carbon content increases the Fe–N_x site density while decreasing the size of the Fe nanoparticles and the thickness of the carbon coating layer, thus enhancing the ORR activity. As cathode materials for anion exchange membrane fuel cells and Zn–air batteries, the FeNC electrocatalysts exhibit excellent performance when compared to Pt catalysts and previously reported transition-metal-based catalysts. Based on the structural changes observed by in situ X-ray absorption fine structure analysis during electrochemical operation, these catalysts were found to contain electrocatalytically efficient Fe–N₄ sites. This work provides efficient strategies for designing high-performance catalysts for the ORR.

电化学氧还原反应（ORR）是许多能量转换和存储系统（包括燃料电池和金属空气电池）中的关键反应。非贵金属基催化剂已吸引人们关注，以实现可行且可持续的装置。然而，在铁基催化剂中，由于在合成过程中难以控制这些活性位，因此有效利用活性位，涂覆在碳层上的Fe–N _x和Fe（Fe @ C）颇具挑战性。在这项研究中，具有不同Fe / C比的FeNC电催化剂显示出不同的Fe @ C / Fe– _Nx比和ORR活性。增加碳含量会增加Fe–N _x降低Fe纳米粒子的尺寸和碳涂层的厚度，从而提高ORR活性。作为阴离子交换膜燃料电池和Zn-空气电池的阴极材料，与Pt催化剂和先前报道的过渡金属基催化剂相比，FeNC电催化剂具有出色的性能。根据电化学操作过程中原位X射线吸收精细结构分析观察到的结构变化，发现这些催化剂包含电催化有效的Fe–N ₄位。这项工作为设计ORR高性能催化剂提供了有效的策略。