Theoretical Study On The Catalytic Performance Of M-N-C Type Carbon-Based Single-Atom Catalysts For Acidic Oxygen Reduction Reactions

The energy crisis caused by the massive consumption of fossil energy and the environmental problems caused by the emission of greenhouse gases are becoming more and more serious.Proton exchange membrane fuel cells（PEMFCs）,as a clean and green energy device,can reduce the dependence on fossil energy and make better use of sustainable energy.However,efficient catalysts are needed to catalyse the slow kinetic cathodic oxygen reduction reaction（ORR）,among which the N-atom coordination transition metal monatomic catalyst supported by carbon material（M-N-C）is considered to be the most promising catalytic materials.However,M-N-C catalyst has some problems such as poor stability and unsatisfactory catalytic activity under acidic conditions.Based on this,in order to explore the degradation mechanism of M-N-C carbon-based monatomic catalyst and screen the catalyst structure with high performance,this work carried out a systematic theoretical study on M-N-C carbon-based monatomic catalyst by using the density functional theory（DFT）calculation software VASP.Specific research contents include:（1）The ORR acid stability and degradation mechanism of three M-N-C monatomic catalysts were calculated theoretically.The thermal stability region was constructed by U-p H diagram to illustrate the solubility resistance of the three catalysts.It was found that marginal MN₄C₈catalyst had the largest thermal stability region,followed by pyridine N coordination MN4C10,that is,the solubility resistance of MN₄C₈configuration was better than that of MN₄C₁₀configuration than that of MN₄C₁₂configuration.The energy required for protonation of N atoms around the active site increases with the voltage,and a higher thermodynamic barrier must be crossed at 0.8 V.In addition,the formation energy of·OH radical was calculated.The results show that the exothermic activity of H₂O₂to form OH-in Co,Ni and Zn catalysts is stronger than that of forming·OH active radical,and the exothermic activity of Fenton reaction to form·OH active radical in Fe N₄C₁₀structure is the strongest.Therefore,for carbon-based monatomic catalyst,its poor acid stability is mainly due to the removal of metal active sites caused by electrochemical corrosion in acidic environment.It is worth special note that the structure of Fe N₄C₁₀is also affected by·OH active free radicals.（2）Doping of B,N,O,P,S nonmetallic elements into the first and second coordination spheres of M-N-C monatomic catalysts with pyridine N coordination to explore the influence of coordination environment changes around the active site on catalytic performance.Five catalysts,Fe N₄S_c,Co N₄N_c,Co N₃S_a,Ni N₃O_aand Ni N₄S_d,have high ORR activity.In particular,the overpotential of Fe N₄S_cstructures is lower than 0.4 V in vacuum and implicit solvent environment,and the thermodynamic stable region of the above structures is calculated.Fe N₄S_ccatalyst with high activity and stability in acid ORR was screened.The adsorption energyΔG_*OHof ORR intermediate*OH has a volcanic relationship with the overelectric potential(η^ORR),and the catalyst has good ORR activity whenΔG_*OHis in the range of 1.0±0.1 e V.The calculation results of Bader charge revealed the charge transfer between the active site of the catalyst metal and the intermediate,and the calculation results of state density showed that the electron distribution of the metal dz²orbital and the Pz orbital of O atom affected the adsorption strength of the catalyst to the intermediate.（3）The effect of doping S element in marginal M-N-C monatomic catalyst on the catalytic performance of ORR was investigated.The ORR activity of Co N4Sb and Ni N3Sa structures is higher.The overpotential of Co N4Sb structure in solvent environment is 0.31 V,and this structure has high stability in the condition of PEMFCs.Marginal M-N-C also follows the volcanic relationship between adsorption energyΔG_*OHand ORR overpotential(η^ORR).The 2-dimensional charge density distribution diagram revealed that the doping of S mainly affected the redistribution of charge around the active site.The Hamiltonian layout analysis of the crystal orbit confirmed that the electron filling of the bonding orbital increased in the M-O bond formed by the metal active site and O atom,which improved the adsorption strength of O₂.In summary,this work first explores the stability of different configurations of carbon-based monatomic catalysts under acidic conditions and the calculation and characterization methods that affect the stability.Secondly,after doping modification of pyridine type and marginal carbon-based monatomic catalysts,the stability verification is carried out to screen high-performance oxygen reduction reaction catalysts.It is of great significance to improve the oxygen reduction reaction rate and reduce the operation cost of proton exchange membrane fuel cells.