Probing the Oxygen Reduction Reaction Intermediates and Dynamic Active Site Structures of Molecular and Pyrolyzed Fe–N–C Electrocatalysts by In Situ Raman Spectroscopy,ACS Catalysis

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Probing the Oxygen Reduction Reaction Intermediates and Dynamic Active Site Structures of Molecular and Pyrolyzed Fe–N–C Electrocatalysts by In Situ Raman Spectroscopy
ACS Catalysis ( IF 11.3 ) Pub Date : 2022-06-16 , DOI: 10.1021/acscatal.2c00771
Jie Wei ₁ , Dongsheng Xia ₁ , Yinping Wei ₁ , Xuya Zhu ₁ , Jia Li ₁ , Lin Gan ₁

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While FeN₄ species are widely suggested as the active sites of noble-metal-free Fe–N–C oxygen reduction reaction (ORR) electrocatalysts, the ORR mechanism, particularly the rate-determining steps (RDSs) at the Fe centers, and the likely contribution of co-existed C–N active site remain disputed. Moreover, the dynamic structures of the FeN₄ active sites during ORR electrocatalysis also remain elusive. By in situ (isotope-labeled) Raman spectroscopy of molecular Fe phthalocyanine (FePc) model catalysts and pyrolyzed Fe–N–C catalysts, we achieve direct, simultaneous spectral identification of the ORR intermediates/RDSs at different active sites under different pH conditions, from which their intrinsic activities and ORR mechanisms can be quantitatively decoupled. Besides the single-atomic Fe–N_x site, two kinds of C–N sites were pinpointed and clarified as separate active sites in pyrolyzed Fe–N–C catalysts, showing different ORR intermediates (*O₂^– and *OOH) and RDSs. Furthermore, from the FePc model catalyst, we reveal a pH-dependent structural switching of the FeN₄ between planar and non-planar structures during ORR electrocatalysis, which provides important insights into their pH-dependent ORR activity (RDS) and stability.

中文翻译：

用原位拉曼光谱研究分子和热解 Fe-N-C 电催化剂的氧还原反应中间体和动态活性位点结构

虽然 FeN ₄物种被广泛认为是无贵金属 Fe-N-C 氧还原反应 (ORR) 电催化剂的活性位点，但 ORR 机制，特别是 Fe 中心的速率决定步骤 (RDS) 和共存的 C-N 活性位点的可能贡献仍然存在争议。此外，FeN _{4的动态结构}ORR 电催化过程中的活性位点仍然难以捉摸。通过分子 Fe 酞菁 (FePc) 模型催化剂和热解 Fe-N-C 催化剂的原位（同位素标记）拉曼光谱，我们实现了在不同 pH 条件下不同活性位点的 ORR 中间体/RDS 的直接、同时光谱鉴定，它们的内在活动和 ORR 机制可以从数量上解耦。除了单原子 Fe-N _x位点外，两种 C-N 位点被精确定位并阐明为热解 Fe-N-C 催化剂中的独立活性位点，显示出不同的 ORR 中间体（*O ₂^-和 *OOH）和 RDSs . _{此外，从 FePc 模型催化剂，我们揭示了 FeN 4}的 pH 依赖性结构转换ORR 电催化过程中平面和非平面结构之间的关系，这为了解其 pH 依赖性 ORR 活性 (RDS) 和稳定性提供了重要的见解。

更新日期：2022-06-16

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