Oxygen Reduction Catalysis Of Single-atom Transition Metal Carbon-nitrogen Composites

Proton Exchange Membrane Fuel Cell(PEMFC)has been rapidly developed in recent years due to its advantages of high energy conversion rate,abundant raw materials,and environmental protection,and is widely used in vehicles,portable electronic equipment and other fields.Proton exchange membrane fuel cells react very fast on the anode,but on the cathode,even on the best Pt-based catalysts at present,the oxygen reduction reaction kinetics are still slow,and Pt is a rare and expensive metal.Therefore,the development of cheap and efficient non-noble metal catalysts to replace Pt is the key to the development of proton exchange membrane batteries.This paper combines the theoretical calculation of quantum mechanics and experimental research,using density functional theory to study the relationship between the structural properties of single-atom transition metal carbon-nitrogen composite materials and electrocatalytic activity,explore the microscopic reaction properties in the relationship,and through theoretical calculations Guided experimental research,studied its catalytic activity in oxygen reduction reaction through different characterization methods,and provided theoretical and experimental basis for the development of efficient and cheap electrochemical catalysts.The main research contents and results are as follows:(1)M/g-C₃N₄(M=Fe,Co,Ni)single-atom catalyst model formed by metal single atom M supported on nitrogen carbide(g-C₃N₄)was built,and the structural stability and electrons of the catalyst were studied.Structure and electrocatalytic activity.By comparing the electronic structure of the three catalysts with the electronic results of g-C₃N₄,it was found that the introduction of metal atoms can significantly improve the conductivity of the overall material and enhance the electron transport capacity of the catalyst,among which Co/g-C₃N₄has the strongest conductivity.According to the analysis of d-band center theory and front-line orbit theory,the electrocatalytic activities of the three catalysts are Co/g-C₃N₄,Ni/g-C₃N₄,and Fe/g-C₃N₄in order from strong to weak,because there is a stronger interaction between Co and g-C₃N₄,there is a greater effective overlap between the Co d orbit and the N p orbit.The analysis of the adsorption results of the oxygen reduction reaction reactants,intermediates and products in M/g-C₃N₄,through the adsorption energy of the intermediate species,the reaction path of the four-electron process of the oxygen reduction reaction of M/g-C₃N₄is analyzed.According to the calculated Gibbs free energy change diagram,the oxygen reduction reaction of M/g-C₃N₄can be known.The rate-determining steps are all O*hydrogenation process,and the calculated overpotential of Co/g-C₃N₄is 0.56 V,which is smaller than that of Ni/g-C₃N₄,and Fe/g-C₃N₄,showing better oxygen reduction active.(2)Based on the theoretical calculation of quantum mechanics,Co/g-C₃N₄,Ni/g-C₃N₄,and Fe/g-C₃N₄single-atom catalysts were synthesized by high-temperature roasting method.In order to confirm that the synthesized material is consistent with the calculated structure,the structural characterization confirmed that the metal single atoms were supported on the carrier g-C3N4,and single atom-dispersed Co/g-C₃N₄,Ni/g-C₃N₄,Fe/g-C₃N₄and cyclic voltammetry scan and linear scan for M/g-C₃N₄oxygen reduction reaction activity test.The results showed that the three catalysts showed good electrocatalytic activity.Among the three catalysts,Co/g-C₃N₄-900 showed a corrected starting point(E_onset=0.92 V)and half-wave potential(E_1/2=0.89 V),Its excellent electrocatalytic activity comes from the electronic structure of Co/g-C₃N₄structure which is more affinity to electrocatalytic reaction.By calculating the number of transferred electrons of the three catalysts,we know that the reaction paths of the three catalysts are all four-electron paths,which is also consistent with the premise of theoretical calculation.