Theoretical Design On The High Perfomance Cathode Materials Based On Graphene And Tungsten Carbide For Proton Exchange Membrane Fuel Cell

The proton exchange membrane fuel cell has received more and more attention due to its properties of clean,high-efficiency and environmental-friendly.However,the traditional cathode catalysts,i.e.the platinum nanoparticles supported on the carbon black,suffer from many serious problems including high cost and easy to be poisoned by CO of the precious platinum.The carbon black substrate is also sensitive to the corrosion.To this end,the first principles methods and molecular dynamics method are carried out for the purpose of designing the graphene-based catalysts with heteroatom doping and the supported catalysts with different metal monolayers on tungsten carbide.First,the effects of the different dopants on the geometric and electronic characteristics of graphene are studied,and then the structural stability,catalytic activity and reaction mechanisms of the oxygen reduction reaction（ORR）and CO oxidation are investigated and compared with the tranditional Pt-based catalysts.It is found that doping is an effective method for tuning the reactivity of graphene.The heteroatom doped graphene could function as the direct catalyst or as the substrate to stabilize the supported catalyst.We futher focused on the tungsten carbide supported platinum,parradium and gold monolayers systems,and also found that the tungsten carbide acting as substrate could enhance the stability,ORR activity and anti-CO poisoning of the supported metal monolayers.These results will provide theoretical guidances to design the highly-efficent,stable and cost-efficetive graphene-based catalysts and the supported catalysts with high-performances.The systems studied and the results arrived are as following:1.Tuning the activity of graphene towards ORR by doping（1）The ORR mechanisms on the metal-free phosphorus doped graphene（P-Gra）are investigated.It is found that all the ORR species can be strongly adsorbed on the P-Gra except for the H₂O molecule.Our calculation results show that all of the possible ORR elementary reactions could take place within a small region around the dopant and the ORR could proceed firstly by a 2e^-process to form an OOH intermediate,which is followed by the 4e^-process to break the O-O bond of OOH.Along this reaction path,the reduction of OH to H₂O is the rate-determining step（RDS）with the largest barrier of 0.88 eV.To the best of our knowledge,this is the first theoretical investigation on the mechanisms for the entire process of the ORR on the P-Gra.（2）The kinetic activation barriers and the thermodynamic free energy changes for the probable elementary reaction steps of ORR on CoN4 embedded graphene（Co N4-gra）are calculated,and the reaction mechanisms are revealed.It is found that Co N4-gra can promote the ORR to proceed along a 4e^-pathway and to finally generate two H₂O molecules by successive hydrogenation reactions.The reduction of OOH into O and H₂O with the largest barrier of 0.69 eV is suggested to be the kinetic rate-determining step.The thermodynamics results show that the elementary steps of ORR along the 4e^-pathway are downhill（exthermic）at the electrode potential lower than 0.58 V.When the electrode potential is higher than 0.58 V,the reduction of OH into H₂O functions as the thermodynamic RDS of the 4e^-pathway.2.CO oxidation on the catalysts based on nitrogen doped graphene（1）The stability and catalytic activity of single Pt atoms supported on nitrogen doped graphene（Pt/NG）are revealed.Single-atom catalysts,especially the single Pt atom catalyst,have attracted more and more attention due to their high catalytic activity for CO oxidation.It is found that the stability of a Pt atom on the NG can be promoted by adopting an appropriate doping configuration.The exceptionally stable Pt/NG catalyst exhibits excellent catalytic activity for CO oxidation via a new tri-molecular Eley-Rideal mechanism（2CO+O₂→OCO-OCO→2CO₂）with an energy barrier of 0.16 eV for the RDS of OCO-OCO dissociation,which is more preferable than the other two normal Langmuir-Hinshelwood and Eley-Rideal mechanisms.（2）CO oxidation on the catalyst with a single cobalt atom incorporated with pyridinic nitrogen graphene（Co-N3-gra）.The low operating temperature,high efficiency and noble-metal-free are the desired properties of the ideal catalysts.The catalytic activity for CO oxidation via various mechanisms on the Co-N3-gra catalyst is investigated.It is found that the CO oxidation occurs preferably following the Langmuir-Hinshelwood mechanism（CO+O₂→OOCO→CO₂+O）with an energy barrier of 0.86 eV for the RDS of OOCO formation.Our findings prove that the activity of noble-metal-free Co-N3-gra catalyst is comparable to or even higher than some other single noble-metal-graphene based catalysts3.Bifunctional CoNx embedded graphene electrocatalysts for OER and ORRThe bifunctional catalysts for both oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）are of importance to the development of electrochemical energy systems such as reversible fuel cells,metal-air batteries,and water electrolyzers.Here,the Co and N codoped graphene systems（CoNx-gra）with high ORR activity were further suggested as efficient OER catalysts.It was found that the activity on Co site of CoNx-gra towards ORR and OER would be affected by both the N-dopant concentration and configuration.The extrapolated overpotential of 0.37 V for either ORR or OER on CoNx-gra systems is comparable to those of noble metal benchmark catalysts.The origin of the activity stems from moderate hybridization between Co 3d orbital and p-orbital from O species,governed by the neighboring N coordination environment.Our results highlight the potential application of transition metal and non-metal codoped graphene as efficient non-precious bifunctional catalysts.4.Oxygen reduction and CO oxidation on Pt,Pd,Au monolayers on WC（0001）The geometric and electronic structures of the metal monolayers supported on WC（0001）surfaces(M_ML/WC（0001）（M=Pt,Pd,and Au）)and their catalytic activity toward the oxygen reduction reaction（ORR）were comparatively studied.Both the kinetics and the density of states results demonstrated that the direct dissociation of O₂ on all three M_ML/WC（0001）surfaces are almost impossible.Yet the barriers of the formation and dissociation of OOH on Au_ML/WC（0001）are much smaller than those on the Pt_ML/WC（0001）and the Pd_ML/WC（0001）surfaces,implying that the Au_ML/WC（0001）exhibits the highest catalytic activity for ORR via a combination of 2e^-hydrogenation of O₂ and 4e^-dissociation of OOH.The rate-limiting step barrier of 0.83 eV for the hydrogenation of OH forming H₂O is also comparable to that on the traditional Pt-based catalysts.Catalysts with weak adsorption yet high reactivity towards CO are urgently required to solve the serious problem of CO poisoning that occurs in many important reactions,e.g.,in fuel cells.We further found a promising electrocatalyst for this purpose:Au monolayer on WC（0001）(Au_ML/WC),which has both high oxygen reduction activity and high tolerance to CO poisoning.The advantages of using Au_ML/WC as electrocatalyst in fuel cells are demonstrated through analyses on energetics of different reaction steps as well as interaction properties of reactants and products.We anticipate that the present results are useful to advance the development of efficient catalysts with high tolerance to CO poisoning.