DFT Study Of Catalytic Oxygen Electrode Reaction For Transition Metal Atoms Supported On Graphyne

The continuous depletion of fossil fuels and the gradual deterioration of global environment are attracted widespread research interest for the clean and renewable energy storage and conversion devices,such as,metal-air batteries,fuel cells,and water splitting.The overall efficiency of these energy storage and conversion devices is determined by the oxygen evolution reaction（OER）and oxygen reduction reaction（ORR）.Nevertheless,both OER and ORR need high-efficient catalysts to overcome high overpotential and slow kinetics.At present,ruthenium and iridium oxides（Ru O₂/Ir O₂）are regarded as the most active OER catalysts,while Pt is identified as the best ORR catalysts.However,their large-scale application is greatly impeded by high cost,low reserves,and poor tolerance.Moreover,none of these commercial noble metal electrocatalysts have satisfactory bifunctional OER and ORR catalytic activities.Therefore,it is necessary to develop high-efficient and low-priced bifunctional electrocatalysts for OER and ORR.Recently,single-atom or single-cluster catalysts are rapidly developing due to the high atom utilization,high catalytic activities and low cost,and considered as potential alternatives for noble metal catalysts.The appropriate substrate material is very important for single-atom or single-cluster catalysts.The novelly synthesized two-dimensionalγ-graphyne（γ-GY）and graphitic dysene（GDY）have a natural acetylene ring macroporous structure,which is a kind of excellent single-atom or single-cluster catalysts substrate material.Based on this,we designed a series of single-atom or single-cluster catalysts by supporting transition metal（TM）atoms onγ-GY and GDY substrates in this thesis,and systematically investigated the stability,catalytic performance,activity origin,reaction mechanism and activity trend of designed single-atom or single-cluster catalysts to screen stable and efficient bifunctional OER and ORR catalysts by using density functional theory（DFT）from the thermodynamic and kinetic perspectives.The research results are summaraized as follows:（1）Although single-atom catalysts based onγ-GY are used in various electrocatalytic reactions,except for the two post TM atoms anchored on theγ-GY substrate are applied to nitrogen reduction recaction,otherγ-GY based catalysts are the TM atoms of the first period,and the OER and ORR performance of theseγ-GY based catalysts are still far behind the precious metals Ru O₂/Ir O₂ and Pt.Therefore,a series ofγ-GY@TM single-atom catalyst were designed by supporting six kinds of post transition metal atoms（Ru,Rh,Pd,Os,Ir,and Pt）onγ-GY substrates in this work,and the stability,OER and ORR catalytic activities as well as the origin of activity were investgated in detail.The DFT calculated results indicate that the six transition metal atoms can be stably anchored on the acetylene ring ofγ-GY.Among the sixγ-GY@TM candidates,the OER overpotential is 0.42 V forγ-GY@Rh,which is comparable to precious metal Ru O₂（0.43 V）,suggestingγ-GY@Rh is a potential OER catalyst.Compared with the other fiveγ-GY@TM catalysts,the high OER activity ofγ-GY@Rh can be attributed to the middling adsorption strength between Rh active site and intermediates.（2）The research results of work（1）show that althoughγ-GY@Rh has efficient OER activity,its ORR performance is not good.The method of heteroatom doping to improve the catalytic activity has been confirmed experimentally and theoretically,but the complete picture of how heteroatom doping regulates the electrocatalytic activity of single-atom catalysts is still elusive.In this work,therefore,a series of gra N-γ-GY@TM catalysts were designed by doping graphitic N atom intoγ-GY@TM（TM=Ru,Rh,Pd,Os,Ir,Pt）catalysts.And the stability,OER and ORR catalytic activities,the effect of graphitic N atom on the catalytic activities of OER and ORR as well as its essential reasons were systematacially investigated from the perspective of thermodynamics and kinetics.The calculation results demonstate that the gra N-γ-GY@TM catalysts have good thermodynamic and kinetic stability.Due to the doping of graphitic N regulating the electronic structure of the catalysts,gra N-γ-GY@Rh exhibits excellent bifunctional OER and ORR catalytic activity with overpotential of 0.27 V for OER and 0.33 V for ORR,which are remarkably superior to the precious metals Ru O₂（0.43 V）and Pt（0.45V）.In addition,the doping of graphitic N significantly decreases the activation energy barriers of rate-limiting step of OER and ORR,thereby accelerating the kinetic process of OER and ORR.（3）Single-atom catalysts are not necessarily more stable than single-cluster catalysts for GDY substrate materials that can support single or multiple TM atoms in a vacancy.As we know,the stability is a prerequisite for excellent catalysts.Therefore,three new single-cluster catalysts GDY@TM₃（TM=Co,Ni,Cu）were designed in this work,and the stability,OER and ORR catalytic activities as well as the origin of activity were investgated in detail.The calculation results prove that GDY@TM₃ is more stable than GDY@TM single-atom catalysts.Nevertheless,GDY@TM₃ dosen’t exhibit satisfactory OER or ORR performance due to the adsorption strength between active site and intermediate too strong.However,when the self-structured adsorbed O^*and OH^*（GDY@TM₃-O and GDY@TM₃-OH）in the catalytic process are used as the catalytic model,GDY@Cu₃-O and GDY@Ni₃-OH exhibit excellent ORR performance with overpotential of 0.43 V and 0.40 V,which is slightly better than the noble metal Pt（0.45V）.Importantly,GDY@Ni₃-O exhibits efficient bifunctional OER and ORR catalytic preformance,its OER overpotential is 0.31 V,which is remarkably superior to 0.43 V of Ru O₂;and its ORR overpotential is 0.46 V,which is comparable to that of Pt.This method of self-construction in the catalytic process provides us with new insight and guidance for improving the catalytic performance.