Fe–N–C catalysts are emerging as potential alternatives to platinum in the oxygen reduction reaction (ORR) for fuel cell cathodes. The challenge in optimizing these catalysts lies in their structural complexity and the multiplicity of reaction pathways. Here, we employ a series of model catalysts with varying amounts of Fe–N_x and Fe nanoparticles (NPs) and estimate their turnover frequency (TOF) for apparent H₂O and H₂O₂ production at different catalyst loadings. This approach highlights the importance of the surface site density (SD) of Fe–N_x moieties in determining the overall ORR activity, selectivity, and even stability. We uncover that increasing the SD of Fe–N_x moieties fosters the indirect 4e^– ORR pathway and consequently promotes their TOF toward preferential H₂O production. In contrast, Fe NPs, often formed at high Fe contents, behave as anticatalysts (or spectators) in this context. Indeed, an online inductively coupled plasma-mass spectrometry (ICP-MS) study reveals that a higher SD can lead to the faster leaching of Fe–N_x moieties during operation, resulting in accelerated activity decline. Taken together, the comprehensive understanding of the intricate dependence of catalytic activity and stability on the nature and amount of Fe species provides a basis for design principles of next-generation Fe–N–C catalysts.

中文翻译：

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Fe-N-C 催化剂上电化学氧还原复杂性的单点水平解读

Fe-N-C 催化剂正在成为燃料电池阴极氧还原反应 (ORR) 中铂的潜在替代品。优化这些催化剂的挑战在于其结构复杂性和反应途径的多样性。在这里，我们采用了一系列具有不同数量的 Fe-N _x和 Fe 纳米颗粒 (NP) 的模型催化剂，并估算了不同催化剂负载量下表观 H ₂ O 和 H ₂ O ₂产量的周转频率 (TOF)。这种方法强调了 Fe-N _x部分的表面位点密度 (SD) 在确定整体 ORR 活性、选择性甚至稳定性方面的重要性。我们发现增加 Fe-N _x部分的 SD会促进间接 4e ^- ORR 途径，从而促进其 TOF 优先产生 H ₂ O。相比之下，通常在高铁含量下形成的铁纳米颗粒在这种情况下充当抗催化剂（或旁观者）。事实上，一项在线电感耦合等离子体质谱 (ICP-MS) 研究表明，较高的 SD 会导致操作过程中 Fe-N _x部分的浸出速度更快，从而导致活性加速下降。总而言之，对催化活性和稳定性对 Fe 物种的性质和数量的复杂依赖性的全面理解为下一代 Fe-N-C 催化剂的设计原理提供了基础。