Mechanism Study And Rational Design Of Oxygen Reduction And Hydrogen Evolution Reaction Of Atomically Dispersed Electrocatalysts

With the increasing demand for energy and the aggravation of environmental problems,it is imperative to develop green and sustainable energy conversion technologies.Among various promising technologies,the interconversion of renewable electricity and pollution-free hydrogen energy has become one of the most attractive methods.For example,fuel cells,supercapacitors and water electrolysis systems are considered to be the most practical options for energy storage and conversion.However,there are two major reasons that limit the large-scale application of related devices.First,the kinetics of oxygen reduction reaction（ORR）and hydrogen evolution reaction（HER）as key reactions are slow.Second,the most widely used ORR and HER catalysts are Pt/C catalysts with small reserves and high prices.Therefore,the research and development of ORR and HER electrocatalysts with low cost,high activity and high durability have important theoretical significance and application value for realizing efficient energy conversion.Atomically dispersed metal catalysts that reduce the size of the supported metal to the atomic level have two characteristics.First,they have a high utilization rate of metal atoms,which can effectively reduce the cost of using metal.Second,they have an the active center of unsaturated field,which can effectively improve the catalytic activity of metal center.Therefore,based on the application background of ORR and HER,this work systematically studies the reaction mechanism of atomically dispersed metal catalysts for ORR and HER based on the theoretical calculation method of first principles.Clarifying the reaction mechanism of ORR and HER applied to atomically dispersed catalysts is helpful for rational design of ORR and HER electrocatalysts.The main contents are as follows:（1）Theoretical design of Fe-N-C single-atom catalysts doped with heteroatoms for ORR.The structure models of 100 kinds of Fe N_xD（D=S,P,B）single atom catalysts（SACs）were constructed by doping S,P,B heteroatoms in different coordination shells of Fe-N-C,and the regulation of ORR activity by the introduction of heteroatoms was studied.It is found that the doping distance between heteroatom and Fe center can effectively regulate ORR activity.Doping S or B atoms as environmental atoms into Fe-N-C active groups,and P as coordination atoms into Fe-N-C active groups can effectively improve ORR activity.This doping rule successfully rationalizes the results of the existing experimental literature.In addition,a structure descriptor that can be easily measured by experiment is found to predict ORR activity,which lays a theoretical foundation for rational design of high performance ORR catalysts.（2）Switching of ORR reaction mechanism from single-atom to double-atom and triple atom catalysts.A total of 15 Fe Co double-atom catalysts（DACs）and 20 Fe Co triple atom catalysts（TACs）were established on graphene to evaluate the ORR process.It has been found that there are two electron acceptance-feedback channels between the adjacent metal sites and O-O bonds of DACs/TACs,which promotes the further active dissociation of O-O bonds,which is the key to switch from the unit point association pathway to the two-site dissociation pathway.Compared with the traditional association pathway,the two-site dissociation pathway is dominant in the ORR total reaction network,and following the dissociation path makes the theoretical ORR activity of DACs/TACs more consistent with the existing experimental results,which explains the superior kinetic behavior of DACs/TACs compared with SACs.（3）Oxygen dissociation mechanism and theoretical design of double-atom catalysts for ORR.The feasibility of ORR dissociation mechanisms of 180 DACs in Mn M,Fe M,Co M,Ni M,and Cu M（doped metal M is a 3d,4d,5d transition metal from IVB to IB）was further explored by high-throughput theoretical calculations.It is found that the new structure-activity relationship under the dissociation path can avoid the original adsorption-property scaling relationship,and a new active volcano map is established.The modified volcanic curve makes the initial potential closer to the ideal value.In addition,the pH and potential dependence of DACs were revealed by the potentiostatic calculation method.Co Rh@NC and Co Ir@NC DACs have the best ORR activity under alkaline conditions following the ORR dissociation mechanism.In future studies,the dissociation mechanism should be regarded as an important way for DACs to catalyze ORR.These theoretical insights will also help design high-performance catalysts for other electrochemical reactions.（4）The pH effect on HER over Pt SAC loaded on different carriers.In this work,the reaction mechanism of HER in acidic and alkaline media was studied using Pt@C₃,Pt@WS₂ and Pt@Mo S₂ catalysts as models.The surface preadsorption conditions of the three systems are different at different pH,which can effectively regulate the electronic structure of the active Pt site,resulting in changes in the HER mechanism of the corresponding systems in acidic and alkaline.Moreover,the thermodynamic and kinetic activities of HER of each system were analyzed based on the implicit solvent model,which is consistent with the results of previous experimental reports.Based on the analysis of surface preadsorption and its electronic structure,the preset HER mechanism of each system in acid and alkaline can be used to explain the pH effect of its kinetics.