Research Progress of Oxygen Reduction Catalytic Materials and Catalytic Active Sites
PAN Yun1, WU Chengren1, CHEN Shaowei1, WU Xiaobo2,3
1 GAC Automotive Research & Development Center, Guangzhou 511434 2 State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083 3 College of Metallurgy and Materials Engineering, Hunan University of Technology, Zhuzhou 412007
Abstract: Fuel cells is considered as one of the new energy solutions to energy depletion and environmental pollution. Oxygen reduction catalyst is one of the key materials for fuel cells. Among the oxygen reduction catalysts, Pt/C is the best commercial catalyst at present, but limited resources and expensive prices have hindered the development and mass production of fuel cells. In the non-platinum fuel cell catalytic materials, non-metal catalytic materials such as nitrogen-doped nanotube array and nitrogen-doped graphene show the "four-electron" process of oxygen reduction catalysis and excellent anti-carbon monoxide "toxicity", and have high catalytic activity and anti-methanol stability. Atomic doped nonmetallic catalysts have been proved to be very promising potential catalysts. However, in the case of atom doped carbon based catalysis mate-rials, the recognition of the atomic catalytic material and the active site is controversial. Although atom doping changes the carbon based energy band, modifies the electron characteristics and manipulates the changes of surface elements, the research on the synergistic effect of the active site and detailed mechanism is still under discussion. The charge transfer induced by different electronegative properties between doped atoms and carbon atoms can effectively improve the catalytic performance of oxygen reduction. Due to the difference in electronegativity between doped atoms and carbon source, the incorporation of atoms increases the n-type conductivity and more positive charge of carbon, changes the original complete structure of carbon six ring, and forms a conjugate system with specific hybridization. At the same time, due to the differences in atomic radius and bond energy between doped atoms and carbon atoms, defects and uneven charges can occur in the carbon hexyclic structure, resulting in the destruction of electrical neutrality. On the one hand, it is conducive to the adsorption of oxygen; on the other hand, it is conducive to the living dissolution of oxygen, which promotes the improvement of oxygen reduction activity. However, it is difficult to determine the exact site structure, charge transfer mechanism and chemical properties of the active site. The catalytic activity of nano-carbon materials is often related to their surface chemical properties/defects, types and densities of surface functional groups. When defects such as vacancy, gap and boundary occur on carbon surface, it is easier to combine with polar atoms or non-saturated functional groups in the outside world, so as to have certain catalytic activity of REDOX. Carbon defects for materials play an important role, structure and performance of various types of defects in carbon materials not only the change of the structure and physicochemical properties of nanomaterials, and change the configuration of the carbon material itself, although it is not sure about the type of defect and qualitative analysis to form a unified understanding, but for the contribution of its electric catalytic performance has been recognized. In this paper, from the perspective of atomic doping, intra-molecular charge transfer, atomic type and transformation, surface defects, the research progress of oxygen reduction catalyst materials is summarized, and the interaction mechanism between catalytic materials and electrocatalytic active sites is introduced. The problems faced by oxygen reduction catalytic materials are analyzed and the research direction of new oxygen reduction catalytic materials is prospected. It is expected to provide reference for the preparation of new non-metallic catalytic materials with high performance, stability and low cost.
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2019, Vol. 33 
