DFT Study On Electrode Catalytic Reaction Of Direct Methanol Fuel Cell

Direct methanol fuel cells（DMFC）can replace energy conversion devices due to their many advantages such as environmental friendliness and high energy conversion efficiency.The most central material for fuel cells is the electrocatalyst,which directly affects the cost,performance and lifetime of the cell.Currently,Pt-based electrocatalysts are the most widely studied catalysts,but traditional Pt-based catalysts suffer from potential oscillations,high prices,limited resources,CO poisoning,low atomic utilisation and poor reaction kinetics,which greatly limit their large-scale use in electrochemical energy conversion devices.To address the above critical issues of Pt-based catalysts,the methanol fuel cell anode is mainly based on Pt,and low-Pt catalysts are carried out;the cathode is mainly based on Fe N₄catalysts,and the catalytic performance after axial ligand modification is carried out,and the specific works are carried out as follows:（1）For the anodic methanol oxidation reaction,the performance of Pt_mPd_n catalysts with different Pt Pd fractions and morphologies was investigated,starting from reducing the amount of Pt.Firstly,the stability of three low index crystal surfaces（100）,（110）and（111）was investigated,and the calculation results showed that（111）had the smallest surface energy and the most stable structure.Then four different dehydrogenation paths of methanol on the surface of Pt（111）were investigated,and the adsorption capacity of methanol and intermediate species CH₂OH*,CHOH*,COH*,and CO*were moderate,and the results showed that the best dehydrogenation path for methanol oxidation was:CH₃OH→CH₂OH→CHOH→COH→CO.Then the effect of atomic ratio on the catalyst activity of Pt_mPd_n was investigated.The results showed that the 1:1 disordered doped Pt Pd^dsurface has moderate adsorption energy of intermediate species for methanol oxidation,and Pt Pd^d possesses a suitable d-band center,while the weakest adsorption of CO.The catalytic performance of Pt Pd^d is the best among the studied Pt Pd alloys.（2）The CO activation mechanism of the non-precious metal single-atom catalyst TMN₄（TM=Fe,Co,Ni）was investigated for the catalytic oxidation of CO,the key substance clock of the methanol oxidation reaction.Firstly,the stability of TMN₄ was studied.According to the binding energy of TM atoms and nitrogen-doped graphene and the cohesion energy of the three metals,the binding energy of Co with nitrogen-doped graphene is the largest among the three,while the cohesion energy of Co atoms is smaller than its binding energy with the surrounding substrate,and the structure of Co N₄ is the most stable.Secondly,based on the ER mechanism,the oxidation process of CO on the surface of TMN₄ was investigated.The results showed that on the surface of Co N₄,the reaction energy barrier of CO oxidation was the lowest,and the order of its reaction energy barrier was Ni N₄>Fe N₄>Co N₄,and the catalytic activity of Co N₄ was the best among the three catalysts studied.（3）To further investigate the catalytic activity of Co N₄ catalysts,the effect of axial ligand L（L=F,Cl,Br,I）modification of halogenated elements on Co N₄ catalysts was further calculated.Firstly,the stability of Co N₄L was investigated,and according to the binding energy of ligands to Co atoms,the stability of Co N₄L decreases in order of increasing atomic number,and Co N₄F has the most stable structure.Then,based on the ER mechanism,the oxidation process of CO on the surface of Co N₄L was investigated,and the results showed that on the surface of Co N₄Cl,the reaction energy base of CO oxidation was the lowest and its catalytic activity was the best.On the Co N₄I surface,the reaction energy barrier is slightly larger than that of the Co N₄Cl surface,and its catalytic activity is second only to that of Co N₄Cl.The order of catalytic activity of Co N₄L is:Co N₄Cl>Co N₄I>Co N₄F>Co N₄Br.（4）The effect of S,P,and Cl as axial ligand modifications on the catalytic activity of Fe N₄ was investigated computationally for the cathodic oxygen reduction reaction.Firstly,the stability of Fe N₄X was investigated and the adsorption energies of the ligands on the Fe N₄ surface were calculated with values of-1.79 e V,-3.30 e V,and-2.97 e V,respectively.The results indicated that the Fe N₄X catalyst was stable and the subsequent ORR could proceed smoothly.The adsorption of ORR intermediates*OOH,*O,and*OH on Fe N₄X was then investigated computationally.The results show moderate adsorption of the intermediates on the Fe N₄Cl surface.According to the calculated Gibbs free energy step diagram,Fe N₄Cl has the lowest overpotential with a value of 0.51 e V and the best catalytic activity for ORR.