First-Principles Study Of Graphene-Based Dual-Atom Catalysts For Oxygen Reduction Reaction

Despite the high catalytic efficiency of noble metal electrocatalysts for oxygen reduction（ORR）,their high cost and scarcity still hinder their large-scale application.Dual-atom catalyst（DAC）is a novel ORR electrocatalyst with promising applications.However,the design of highly effective DACs is still limited by the understanding of ORR mechanisms on DACs due to their complex coordination environment and the electronic interplay of transition metal sites.To understand the ORR mechanism occurring on DACs,we applied density functional theory（DFT）calculations to systematically investigate a series of nitrogen-doped graphene-based DACs for ORR,and the main results are as follows:（1）ORR Activity of graphene-based DACs:Firstly,42 nitrogen-doped graphene-based DACs with two different coordination patterns,M₁M₂NC-3 and M₁M₂NC-4（3and 4 stand for the number of coordinated nitrogen atoms of each transition metal atom M₁ or M₂）,were constructed by using Materials Studio software,and stable structures were determined based on the results of formation energies and binding energies.Then,the adsorption behavior and ORR catalytic activity of these DACs were calculated for oxygen-containing species based on three different reaction pathways,and it was found that the M₁M₂NC-4 DACs exhibit weaker and more appropriate adsorption capacities and superior ORR performances compared to M₁M₂NC-3.Among them,the Fe Fe NC-4 catalyst with the unconventional pathway（O₂→*OH+*O→*OH+*OH→*OH）is demonstrated to be the most efficient catalyst with the lowest overpotential of0.15 V.Finally,by constructing the volcano plots between the adsorption free energy of ORR intermediates and the activity,it was found that pathway C was the thermodynamically most suitable ORR reaction mechanism.（2）Mechanism of coordination environment regulating ORR activity of DACs:In order to further elucidate the synergistic effect between the dual-atom sites of DACs,the electronic properties of bimetallic sites with different nitrogen-coordinated configurations were analyzed in terms of the d orbital interactions and electronic structure calculations such as density of states,magnetic moments,d-band center and Bader charge.The results show the coordination environment of the M₁M₂NC-4structure could induce the downshift of the d orbital(especially for the d_z2 orbital)of central metal atoms,resulting in the weaker spin polarization and more suitable interaction strength for the adsorption and desorption process of the oxygen-containing intermediates.In addition,a key descriptor was proposed by establishing the linear relationship between adsorption energy and the p-band center of the adsorbed OH,which can help estimate the catalytic activities of the DACs system towards ORR.This work constructs a relationship between ORR activity and ligand environment,which can provide further guidance for the rational design of DACs.