Theoretical Study On Acidic Oxygen Reduction Reaction Mechanisms Catalyzed By Transition Metal-Nitrogen-Carbon

Oxygen reduction reaction(ORR)is an important electrochemical reaction at the cathode of proton exchange membrane fuel cell(PEMFC),which is generally catalyzed by platinum.However,the sluggish ORR kinetics requires platinum with high loading,which limits the PEMFC's large-scale commercialization.Transition metal-nitrogen carbon(TM-N-C)materials,especially Fe-N-C system,are expected to be alternative catalysts because their activity is approaching the commercial platinum-based catalysts.Nevertheless,the durability of Fe-N-C in acid electrolyte is poor,which constitutes the bottleneck of Fe-N-C further development and makes it crucial to study the degradation mechanisms of the catalysts in order to design the high efficiency catalysts rationally.Due to the complex composition of catalyst and the difficulty of in-situ characterization,it is difficult to provide direct evidence for the reaction mechanism by experiments,while theoretical calculation can study the reaction mechanism of specific active sites and becomes an important tool.However,the widely used computational hydrogen electrode(CHE)method does not explicitly consider the electrode potential and can only study thermodynamics.In this work,the electrode potential and solvent molecules were considered to overcome the deficiency of CHE.The reaction kinetics of the degradation mechanism on TM-N-C electrocatalyst at molecular and atomic levels was studied,which provided important insight for the development of more stable TM-N-C catalyst.After explicitly considering the potential-dependent kinetics,we screened the DM-N-C models and provided important theoretical guidance for the development of TM-N-C catalyst with superior performance.The main results of this dissertation are summarized as follows.1.Theoretical study on acidic oxygen reduction reaction mechanisms catalyzed by iron-nitrogen-carbonTraditional CHE method only considers the thermodynamics.We further considered the electrode potential and explicit solvent water molecules to study the reaction kinetics,and explored the effect of OH axial coordination on the stability of Fe-N-C catalyst.It is found that even though OH ligand can increase the theoretical limiting potential to some extent,it can also substantially increase the H₂O₂ selectivity,pushing ORR diverted to the 2e^-+2e^-pathway.In the latter 2e^-process(H₂O₂ reduction),harmful hydroxyl radicals could be produced upon H₂O₂ dissociation.Therefore,from the perspective of catalysts'stability,OH ligand coordination on the metal center is not a good way to develop stable ORR catalysts.Alternatively,we suggest the development of DM-N-C due to their ability to dissociate the O-O bond so that the hydroxyl radical formation can be greatly suppressed to improve the stability.This work provides important insight for the development of more stable TM-N-C catalyst for acid oxygen reduction.2.Screening of dual metal-nitrogen-carbon models for acid oxygen reduction.Although some theoretical works have screened DM-N-C materials,these works are basically based on CHE method,which ignores the kinetic effect.Therefore,the results are still controversial.To meet this challenge,three kinds of active centers reported in the experiments are considered,Aside from thermodynamics,we focused on the potential dependent kinetic effects.The results not only confirm the experimental reported Fe Fe N₆,Fe Co N₆,and Co Zn N₆,but also predict Mn Co N₆.Meanwhile,we proposed a descriptor called“antibonding center”to simplify the assement of kinetics of ORR.This work not only provides direct guidance for the development of DM-N-C materials for acid ORR,but also provides important insights for the study of electrochemical reaction kinetics.