DFT Calculation Of The Effect Of Doping Configurations On The Activity Of Carbon-based Catalysts For Oxygen Reduction Reaction

Oxygen reduction reaction(ORR)electrodes are the cathode of fuel cell,metal-air battery and other electrochemical device.As the ORR kinetics is 6-10 orders of magnitude slower than the kinetics of anode,i.e.hydrogen electrode or metal electrode,the ORR electrdes have greatly affected the efficiency of the above electrochemical energy conversion devices.At present,platinum-based catalysts are the most effective ORR catalysts,but problems such as low earth abundance and high costs have greatly restricted their large-scale application.Therefore,to improve the kinetics of the ORR,it is necessary to deeply understand the mechanism of the ORR and develop low-cost and high-performance ORR catalysts.Owing to relative high ORR activity and low cost,carbon-based catalysts such as nitrogen-doped(NC)and iron-nitrogen co-doped carbon-based catalysts(FeNC)have become potential alternatives to platinum-based ORR catalysts.However,they still exhibit limited performance due to their relative low intrinsic activity and activity density.To further improve its catalytic ORR performance,it is necessary to understand the contribution of each doping type to the activity,the interaction between doped active units,and its influence on the ORR reaction mechanism,intrinsic activity and active site density from the atomic level.Based on this backgroud,this thesis mainly focuses on NC and FeNC catalysts and contains the following two research works:(1)For NC catalysts,the influence of edge structure(armrest type A and zigzag type Z),N doping type(pyridine nitrogen N_p and graphitic nitrogen N_g)of graphene and their interactions on the mechanism and catalytic activity of ORR were studied.Results indicate that single N doping can destroy the spin density of zigzag graphene(G_Z-A),while dinitrogen doping at a close distance has less damage to the overall spin density.The single N doping in the armchair graphene(G_A-A)can introduce the spin density,and the dinitrogen doping at the appropriate position can further increase the overall spin density of G_A-A.Further,the adsorption free energy of OH(?G_^*OH)can be used as a descriptor of ORR mechanism,and when?G_^*OH in the range of 0eV-0.91eV,0.91eV-0.98eV and large than 0.98eV,the ORR mainly follows the 4e^-,a mixed zone of2e^-and 4e^-,the 2e^-reaction mechanism respectively.The overpotential on each active site was calculated to evaluat the ORR activity of each doped configuration.And the overpotential in the range of 0.3V-0.6V,0.6V-0.9V,0.9V-1.2V corresponding to active sites of type?,?,and ?,respectively.For the G_Z-A,it has high intrinsic active site density of type?(?_?),and except for the N_p doping at the edge of the armrest,the single N doping can decreases its?_? and the switching frequency(TOF).And dinitrogen doping can effectively compensate for the damage of single N doping to the spin density of the configuration,and increase the values of?_? and TOF,which are equivalent to the G_Z-A configuration.For G_A-A,it not has active sites of type?,?,and?,while single N doping increases the?_? and TOF.When dinitrogen doping position is moderate,the?_?and TOF can be further increased.Therefore,increase the N doping content at the edge of the armres and increase the N doping density at the appropriate position can improve the ORR catalytic activity of G_Z-A and G_A-A respectively.(2)For FeNC catalysts,the influence of the doping distance and interaction of FeN₄elements on the thermodynamic stability,ORR mechanism and catalytic activity of the catalyst were studied.The thermodynamic stability calculations found that it is easy to form two FeN₄ units which set far apart(the number of C atoms?8)or very close(the number of C atoms=0)to form a double Fedoped configuration,it is easy to prepare and stable.Further,the electronic structure results show that when the number of C atoms between FeN₄ doped elements is 3-8,the overall charge transfer of the doped configuratio becomes larger.When the FeN₄units are far or close,the charge transfer between the doped elements and the carbon atoms is smaller.Besides,the double FeN₄units systems hane larger spin density than that of single FeN₄ unit systems.Specially,the spin density of the two Fesites in the double Fedoping configuration becomes one positive and one negative,and the other doping configurations are all positive.Then,the ORR active sites of the doped configuration were counted,and the overpotentials in the range of 0.3V-0.6V,0.6V-0.9V,and 0.9V-1.2V are the ORR active sites of type?,?,and ?,respectively.When the two FeN₄ units set far apart,the type ? active sites are twice that of the single FeN₄ configuration,which can be approximated as the sum of the activities of the two single doped elements.When the number of C atoms between the two FeN₄ units is 3-8,the type ? active sites ncrease with C atoms;the double Fedoping configuration is formed,type? active sites are produced,and the activity is improved.As for ORR mechanism,the determing step on Fesites is^*OH desorption in other FeN₄ units doping configurations due to the OH adsorption is too strong.While the determing step becomes the adsorption and protonation of O₂in the double Fedoped configuration due to the change of the spin density of Fesites weaken the adsorption of OH.In all doping configurations,the ORR activity of Fesites that occupied by^*OH is enhanced.And the optimal overpotential value of FeN₄ unit doped-graphene configuration is better than that of nitrogen-doped graphene as an oxygen reduction catalyst.