氧还原催化材料与催化机理及活性位点的研究进展

Research Progress of Oxygen Reduction Catalytic Materials and Catalytic Active Sites

燃料电池被认为是解决能源枯竭、环境污染的新能源方案之一。氧还原催化剂是其最关键的材料之一。在氧还原催化剂中,Pt/C是目前商业化最好的催化剂,但其资源有限、价格昂贵等缺点已经阻碍了燃料电池的发展与大规模生产。在非铂燃料电池催化材料中,氮掺杂纳米管阵列、氮掺杂石墨烯等非金属催化材料展示出了氧还原催化的“四电子”过程及优异的抗一氧化碳“毒性”,同时具有较高的催化活性及抗甲醇稳定性。原子掺杂非金属催化剂已被证明是非常有前途的潜在催化材料。但关于原子掺杂碳基催化材料的研究中,对原子催化物质与活性位点的认识存在争议,虽然原子掺杂改变了碳基能带,修饰了电子特性并操纵表面元素的变化,但对其活性位点协同效应、详细机理的研究还在探讨之中。掺杂原子与碳原子间不同的电负性所引起的电荷转移能有效提高催化材料的氧还原催化性能。由于掺杂原子与碳原子电负性不同,增加了碳的n-型导电性与正电荷,改变了碳六环原有完整结构,形成了具有特定杂化的共轭体系。同时由于掺杂原子与碳原子原子半径、键能不同,在碳六环结构中会出现缺陷、电荷不匀,使得电中性被破坏,一方面利于氧气的吸附,另一方面利于氧气的活化解离,促进了氧还原活性的提高。但目前很难确定活性位点确切的位置结构、电荷转移机理和化学性质。纳米碳材料的催化活性往往与其表面化学性能/缺陷、表面官能团的类型、密度相关,当碳表面出现空位、间隙、边界等缺陷时更容易与外界极性原子或非饱和官能团结合,进而具备一定的氧化还原催化活性。碳缺陷对于材料结构与性能起着重要的作用,碳材料中各类型的缺陷不但改变纳米材料的结构和物化性质,而且改变了碳材料本身的形态。虽然目前对缺陷类型的确定与定性分析还没有形成统一认识,但其电催化性能的贡献已被认可。本文主要从原子掺杂、分子内电荷转移、原子类型及转化、表面缺陷角度出发,归纳了氧还原催化剂材料的研究进展,介绍了催化材料与电催化活性位点间的作用机理;分析了氧还原催化材料面临的问题并展望了新型氧还原催化材料的研究方向,以期为制备高性能、稳定和廉价的新型非金属催化材料提供参考。

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.