DFT Study On Oxygen Reduction Reaction Catalysts

Proton exchange membrane fuel cells?PEMFC?have been generally regarded as one of the most promising energy conversion technology owing to their excellent energy conversion efficiency and environmental friendliness.However,the commercialization of PEMFC still meets with many challenges.On one hand,the high loading and low utilization rate of Pt will largely increase the cost of fuel cell;on the other hand,the insufficient duribility under harsh operating condition,especially caused by oxidation of Pt and errosion of carbon support,will shorten the life time of catalysts remarkably.Therefore,the requirements including improvement of catalyst stability,reduction of Pt consumption,and cutting down the cost are urgly to be satisfied for fuel cell.On these grounds,the catalysts of ORR can be promoted by following methods: first,we can improve the activity and the stability of Pt,including adopting oxide instead of carbon as support to benefit both the activity and stability of Pt,and developing PtM alloy to obtain high activity under reduced Pt usage;second,we can invent non-Pt and even non-precious metal catalysts,such as Pd-based materials and nitrogen doped carbon materials,to lower the cost of catalyst significantly.Although those novel catalysts show plenty of merits compared with Pt/C,numerous foundamental issues still accompanying with them are necessary to be resolved,such as the impact of oxide support on activity and stability of Pt catalyst,the influence of nature of doped atoms in PtM alloy on ORR activity and mechanism on Pt,the reason for complete reductin of O₂ on Pd supported on reduced-polyoxometalate and the contribution of different N-doped partten to activity of N-doped graphene.Theoretical computation combined with experiment,and especially the density functional theory?DFT?were used to research on characteristic of materials,reaction mechanism,and relationship about nature of materials vs activity and stability,and quatum chemistry factors and microscopic mechanism behind aforementioned issues,thus providing theoretical basis for designing other catalysts with higher activity and duribility.Firstly,the research on hydrogenated Pt/CaMnO₃ shows that the Pt particle supported on oxide has better ORR performance than Pt/C catalyst in aspects of activity and stability.To uncover the mechanism under this phenonmenon,the DFT method was used to investigate the effect of O-vacancies on electrical conductivity of CaMnO₃ support,and the influence of SMSI between Pt and oxide support on activity and stability of the Pt catalyst.Band structure results show that CaMnO₃ will go through the process from semiconductor to metal to half-metal as increasing the concentration of O-vacancies,which indicates a moderate amount of O-vacancies can obviously improve the electrical conductivity of CaMnO₃,inducing a lower electron transfer resistance.When the Pt5 cluster is loaded on CaMnO₃,a stronger interaction between the Pt5 cluster and CaMnO₃ than that between Pt5 cluster and graphene will form,owing to more charge transfer from support to CaMnO₃,and larger overlap between Op states of CaMnO₃ and Ptd states of Pt5 cluster.In addition,for O₂ and O adsorption on Pt5/CaMnO₃,it can be found that more charge transfer from catalyst to O₂ promotes dissociation of O₂,and lower d-band center of Pt5 on oxide facilitates desorption of intermediates when compared with Pt5/graphene.These two aspects collectively contribute to the promoted activity of Pt/CaMnO₃.Secondly,with regard to Pt-based alloy,although Au and Os are in the two extremes in chemical properties,namely Au is a chemically inert metal while Os is quite active to react with oxygen,unexpectedly,both of them can enhance the oxygen reduction reaction?ORR?activity of Pt based alloys.In this work,a systematical DFT calculation was used to elucidate the mechanisms of enhanced activity in PtM/Pd with doped atoms changing from chemically inert Au to active Os.The computional results indicate that the PtAu/Pd and the PtOs/Pd have heterogeneous electronic structure,such as uneven surface charge and unequal d-band center,which results in different roles of surface Pt atoms in the process of ORR.The calculations of the adsorption of ORR intermediates and the ORR mechanism demonstrate that the PtOs/Pd-S adopts the dissociative pathway as the most favorable pathway for ORR,whereas the Pt,Pt/Pd,PtAu/Pd,and the PtOs/Pd-W approve the associative pathway.More importantly,all of the ORR steps on the sites far away from the doped atoms in the PtAu/Pd and PtOs/Pd,such as the PtAu/Pd-S and the PtOs/Pd-W site,display similar activation energy corresponding to better catalytic activity than the other sites.Combined with electronic structure,it can be concluded that the catalytic activity is mainly affected by the ligand effect,and a proper distance between the doped atoms and the Pt atoms should induce the catalysts to possess the highest catalytic activity.These results also uncover why the different content of doped atom can lead to the different activity of catalysts.Thirdly,experimental results show that the Pd/rPOM largely reduces the production of H2O₂ leading to a 4e-ORR process on it.For the sake of uncovering impact of rPOM support on H2O₂ reduction activity of Pd catalyst,the reaction mechanism of H2O₂ reduction on the rPOM support and the supported Pd cluster were considered to reveal the reason behind the fact that Pd/rPOM can catalyze the complete reduction of O₂ more easily than Pd/C.Calculation results show that although the Mo-Od bond can be elongated significantly as the POM is reduce to rPOM,the Mo site in rPOM is inadequate to supply the main source of activity because of the large overpotential on isolate rPOM,insteadly,the supported Pd becomes dominated active sites where H2O₂ is reduced.As long as the Pd is supported on rPOM,the overpotential of H2O₂ reduction on Pd reaches to-0.978 V,which is far lower than that on isolated Pd cluster?-1.057V?.The overpotantial difference?79mV?between them is nearly equal to the difference?55mV?in experimental results.The analysis of electronic structure suggests that the higher activiy of Pd/rPOM is ascribed to the lower d-band center of Pd supported on rPOM leading to weaker adsorption of OH.Finally,N-doped graphene has aroused general concern owing to its high activity and stability in oxygen reduction reaction?ORR?catalysis.However,the contribution of different types of N-doped graphene to ORR activity is still in dispute.Based on this issue,this paper conducts a comparative study about the ORR on graphitic N-doped graphene?GNG?and pyridinic N-doped graphene?PNG?.Calculations of band structure show that as the nitrogen content increases,the conductivity of GNG decreases monotonously;while that of PNG increases first,after reaching the highest at 4.2 at.% of nitrogen content,and then decreases.The conductivity of PNG is always higher than GNG after the content of doped nitrogen is greater than 2.8 at.%.Additionally,the free energy diagram of ORR shows that protonation of O₂ is potential-determining step among the whole ORR process,and the free energy change of this step on GNG is higher than on PNG,which suggests GNG has higher ORR activity than PNG if the electron transport ability keeps the same.When N-doped content is lower than 2.8 at.%,the ORR activity of N-doped graphene is controlled by capacity of O₂ protonation owing to small conductivity difference between GNG and PNG,thus GNG has higher ORR activity than PNG;when N-doped content is over 2.8 at.%,in this case,conductivity instead of free energy change will be in domination,therefore,ORR on PNG will conduct faster than GNG due to its higher conductivity.