Theoretical Design And Electrocatalytic Mechanism Of Graphene-based Cathode Materials For Fuel Cells

Owing to increasing demand of energy resource and worsening environment,it is strongly expected to develop renewable clean energy technology.Therefore,the clean energy technologies represented by fuel cells,Li-ions batteries,photocatalytic hydrogen production,and solar cells have attracted much attention.As a green and efficient energy conversion device,fuel cells become an important development direction of energy-resource field in recent years.Developing highly efficient and low-cost electrocatalysts is crucial for the realization of large-scale applications of fuel cells.Heteroatom-doped graphene was regarded as one of the most important candidate materials for the large-scale applications of fuel cells due to their low cost,environment-friendly,and excellent electrocatalytic activity.Therefore,the study of the microstructure and oxygen reduction mechanism of graphene-based catalysts is very significant for the experimental preparation and practical applications.In this thesis,geometries,electrocatalytic oxygen-reduction reaction?ORR?activity and mechanism of nonmetal-doped graphene and metal-nonmetal codoped graphene have been studied by using first-principles calculations within the density functional theory.The main research contents and results are as follows:1.The electrocatalytic activity and design principles of nonmetal-doped graphene catalysts with different doping configurations have been studied by using first principles calculations.Our results have demonstrated that binding energies of ORR intermediates?i.e.,*OH?on catalysts can sever as a good descriptor for the ORR activity,attaining the optimal value at the vicinity of².6 eV.The analysis of electronic structures indicates that the ORR activity of doped graphene catalysts depends on the abundance of electronic states at the Fermi level?D_F?,which dominates the charge transfer and binding ability between ORR intermediates and the catalysts.Using the binding energy as a descriptor,we predict the realization of highly active graphene-based electrocatalysts by the dual-doping scheme,which is supported by recent experimental reports.Moreover,we find that the catalytic activity of graphene basal planes can be activated by the B-Sb and B-N codoping approaches.2.The electrocatalytic activity and oxygen-reduction mechanism of Co-N codoping at graphene basal plane are investigated by combining first-principles calculations with ab initio molecular dynamics simulations.The calculated results indicated that the activity of the graphene-based electrocatalysts depends on the binding energies of oxygenated species on the catalysts.The N-doped graphene with a week surface activity is difficult to activate O₂ moleclues,thus facilitating a series two-electron reduction pathway and the formation of peroxide.The Co-doped graphene with highly active surface can strongly bind with the oxygenated species but difficultly release the production.Hence,the oxygen-reduction reaction on Co-doped graphene surface prefers to the four-electron reduction pathway but has a large ORR overpotential?1.93 V?.The Co-N codoped graphene basal plane has a moderate surface activity,which is responsible for the high ORR activity and the selectivity of four-electron reduction pathway.The result mainly originates from that the coupling between Co-3d and N-2p states medicate the activity of Co atom,which contributes to the activation of O₂ dissociation and the H₂O desorption.Hence,the Co-N codoped graphene surface has a smaller ORR overpential?¹.00 V?.3.The electrocatalytic ORR mechanism and activity-evaluation rule of Fe-N codoped graphene are investigated by the density-functional calculations and ab initio molecular dynamics simulations.The calculated results suggest that most of N-doped graphene structures have very weak ORR activity,except for few N-doped structures located at graphene edges.These N-doped structures with weak surface activity are difficult to activate O₂ molecules,resulting in that the protonation of O₂occurs at the electrolyte.In contrast,Fe-doped and Fe-N codoped graphene structures have higher surface activity,facilitating to the adsorption of O₂ and the ORR on the surface of catalysts,which is the adsorption-induced ORR mechanism.The results make a perfect supplement for the tradition ORR mechanism.Finally,we find that the ORR activity of Fe-N codoped graphene can be significantly improved by reducing the activity of Fe atoms through the optimization of the Fe-N composition.