Mechanism And Activity Of Oxygen Reduction Rection On Novel Catalysts:a First-principles Study

Due to the climate change,environmental pollution,and depletion of fossil fuels,the research and development of new clean energy sources is of great concern.Proton exchange membrane fuel cells（PEMFC）as a kind of sustainable green energy that directly convert the chemical energy of fuels and oxidants into electrical energy have received considerable attention.However,the slow kinetic property of the oxygen reduction reaction（ORR）at the cathode of fuel cells severely limits the performance of the fuel cell.At present,the serious impediment to the large-scale commercial application of PEMFC is that cathode requires a large amount of expensive platinum-based catalysts for oxygen reduction reaction.Therefore,over the past few decades,tremendous efforts have been performed to develop alternative catalysts with low-cost,high catalytic activity and long durability.Metal-N4 macrocycles and nitrogen doped carbon materials are evaluated as competitive candidates to replace platinum due to their high catalytic activity.In this thesis,the first-principles calculations are used to investigate the electrocatalytic mechanism and activity of the ORR on cobalt porphyrin compounds and nitrogen-doped carbon materials.In this work,three topics are studied as follow:Firstly,The effect of the functional groups on the ORR catalytic activity of metal porphyrin compounds is investigated based on three metal porphyrin compounds:cobalt porphyrin（CoP）,cobalt tetramethoxyphenyl porphyrin（CoTMPP）,and cobalt protoporphyrin（CoPP）.Our results show that the electronic structure of central metal atom and ORR catalytic activity depend on on the structures of metal porphyrin compounds.Among the three models,cobalt tetramethoxyphenyl porphyrin and cobalt porphyrin exhibit higher catalytic activity than cobalt protoporphyrin.Secondly,in order to understand the mechanism and active sites of ORR on nitrogen-doped carbon materials,the atomic mechanisms of ORR on nitrogen-doped graphene nanoribbons have been studied.Adsorption of O₂ was found to depend on the structure of nitrogen-doped graphene nanoribbons.Compared to nitrogen-doped armchair graphene nanoribbons,graphitic-type nitrogen-doped zigzag graphene nanoribbons could stably capture O₂ and efficiently catalyze ORR.ORR on graphitic-type nitrogen-doped zigzag graphene nanoribbons exhibited a four-electron reduction process with the formation of second H₂O molecule as the rate-determining step.Finally,the size effect of the ORR catalyzed by nitrogen-doped graphene quantum dots（GQDs）is studied.In this work,the mechanism of ORR on nitrogen-doped GQDs with different sizes has been investigated based on first-principles calculations.Our results show that the size of the nitrogen-doped GQDs affects the ORR activity.The adsorption strength of ORR intermediates,the reaction free energy of the rate-determining step,and the overpotential increase with the increase of the size of nitrogen-doped GQDs.The nitrogen-doped GQDs with the smallest size possess the smallest overpotential,indicating the highest ORR catalytic activity.