Arya Gopal S, Edathiparambil Poulose Anuroop, Sudakar Chandran, Muthukrishnan Azhagumuthu
School of Chemistry, Indian Institute of Science Education and Research, Thiruvananthapuram, Maruthamala (P.O.), Vithura 695551, Kerala, India.
Multifunctional Materials Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India.
ACS Appl Mater Interfaces. 2021 Sep 22;13(37):44195-44206. doi: 10.1021/acsami.1c10114. Epub 2021 Sep 13.
Since the inception of cobalt phthalocyanine for oxygen reduction reaction (ORR), non-platinum group metals have been the central focus in the area of fuel-cell electrocatalysts. Besides Fe-N active sites, a large variety of species are formed during the pyrolysis, but studies related to their ORR activity have been given less importance in the literature. FeO is one among them, and this study describes the role of FeO in the ORR. The FeO is carefully synthesized on various carbon supports and characterized using X-ray photoelectron spectroscopy (XPS) spectra, high-resolution transmission electron microscopy (HRTEM) images, and surface area analysis. The characterization techniques reveal that the FeO nanoparticles are present in the pores of the carbon supports, having a particle size ranging from 4 to 15 nm. The current density of the ORR on FeO/C catalysts is increased compared with bare carbon supports, as discerned from the rotating ring-disk electrode (RRDE) voltammetry experiments, demonstrating the role of size-confined FeO nanoparticles. The overall number of electrons in the ORR is increased by the introduction of FeO on the carbon support. Based on the kinetic analysis, the ORR on FeO/C follows a pseudo-4-electron or 2+2-electron ORR, where the first 2-electron ORR to HO and second 2-electron HO reduction reaction (HPRR) to HO are assigned to the graphitic carbon (carbon defects) and FeO active sites, respectively. Theoretical studies indicate that the role of FeO is to decrease the free energy of O adsorption and reduce the energy barrier for the reduction of *OOH to OH. The onset potential estimated from the free energy diagram is 0.42 V, matching with the HPRR activity demonstrated using the potential-dependent rate constants plot. FeO/C shows higher stability by retaining 95% of the initial activity even after 20 000 cycles.
自从酞菁钴用于氧还原反应（ORR）以来，非铂族金属一直是燃料电池电催化剂领域的核心关注点。除了Fe-N活性位点外，热解过程中还会形成各种各样的物种，但文献中对它们的ORR活性相关研究的重视程度较低。FeO就是其中之一，本研究描述了FeO在ORR中的作用。FeO在各种碳载体上精心合成，并通过X射线光电子能谱（XPS）光谱、高分辨率透射电子显微镜（HRTEM）图像和表面积分析进行表征。表征技术表明，FeO纳米颗粒存在于碳载体的孔隙中，粒径范围为4至15 nm。从旋转环盘电极（RRDE）伏安法实验可以看出，与裸碳载体相比，FeO/C催化剂上ORR的电流密度有所增加，这证明了尺寸受限的FeO纳米颗粒的作用。通过在碳载体上引入FeO，ORR中的电子总数增加。基于动力学分析，FeO/C上的ORR遵循伪4电子或2 + 2电子ORR，其中第一步将2电子ORR还原为HO以及第二步将HO还原反应（HPRR）还原为HO分别归因于石墨碳（碳缺陷）和FeO活性位点。理论研究表明，FeO的作用是降低O吸附的自由能并降低将*OOH还原为OH的能垒。从自由能图估计的起始电位为0.42 V，与使用电位依赖速率常数图所展示的HPRR活性相匹配。即使在20000次循环后，FeO/C仍能保留95%的初始活性，显示出更高的稳定性。
