Studies On The Electrochemical Reaction Kinetics And Performance Optimization Of MnO₂ Supercapacitor Electrodes

MnO₂ has been used for a long time in the research history as electrode materials for supercapacitors,mainly due to the advantages like high performance,low cost,safety,and being suitable for the neutral aqueous environment.However,at present,the research on the energy storage mechanism of MnO₂ in water environment is not thorough,mainly lack systematic research from the perspective of electrochemical kinetics.Because of the lack of kinetic studies,the experimental design is blind and the feasibility is low.And the cycle stability of energy storage devices based on MnO₂needs to be improved.In addition,prolonging the atomic economy of MnO₂?that is,the utilization rate of MnO₂?has not been well played.Therefore,it is very important to make clear the MnO₂ energy storage mechanism,understand the electrochemical kinetic process of MnO₂ during energy storage,explore the influence of ion participation on the structure and performance of MnO₂,design the structure reasonably,and configure the appropriate electrolyte for improving the utilization rate of MnO₂ and the performance of supercapacitor.For some important issues mentioned above,we first systematically designed experiments to study the energy storage mechanism of MnO₂ electrode materials.Based on the guidance of electrochemical kinetics,the changes of structure,morphology,composition and electrochemical properties of MnO₂ electrode materials in neutral aqueous electrolyte were investigated in detail.The structural reconstruction model was constructed to explain the microscopic changes of MnO₂ in different electrochemical stages by studying the changes of Na⁺ distribution and Mn chemical valence states.MnO₂ morphological transformation is the result of Mn²⁺dissolution and redeposition,which is mainly affected by cations located the interface between the electrode and the electrolyte.Na⁺ intercalation and deintercalation process in the MnO₂ is reflected by the change of the chemical valence state of manganese element.Based on the exploration and optimization of the electrochemical reaction kinetics of MnO₂ electrode materials,this paper carried out the following four parts of research:?1?Based on the exploration and understanding of the electrochemical kinetics of the MnO₂-based supercapacitor electrode,the concept of electrochemical activation was proposed.In the experiment,the discharge current density during the electrochemical activation process was changed to optimize the MnO₂ morphology and the pre-embedded content of Na⁺.The study found that the electrochemically activated MnO₂ electrode can achieve a specific capacitance up to 404 F/g,and the intercalation pseudocapacitance accounts for 18%of the total specific capacitance.Systematic research reveals the superior electrochemical performance is mainly due to the well-distributed pre-embedded Na⁺ions and MnO₂ nanosheets generation after activation,which improve the surface-controlled and diffusion-controlled capacitance during the electrochemical cycle.?2?By optimizing the charge distribution on the electrode surface,the problem of resistance to ion insertion in the MnO₂ electrochemical process is solved.We introduced a bipolar electrolyte,4-hydroxy-2,2,6,6-tetramethylpiperidine oxygen radical?HTEMPO?,which becomes anion after getting an electron and combines with a Mn²⁺.After forming a neutral group,which can reduce the repelling effect of Mn²⁺on the subsequent Na⁺,promotes the insertion of Na⁺,and finally achieves the purpose of increasing the proportion of intercalation pseudocapacitance?49.6%?in the total capacitance contribution?773 F/g?.?3?Through the exploration of the energy storage system of multi-electron reaction,the energy transfer efficiency of multi-electron electrolyte ions in the MnO₂ electrode material during the electrochemical reaction has been greatly improved.Because Mg²⁺ has two positive charges,the MnO₂ electrode material can transfer two charges at a time during the charge storage process,which has a higher charge transfer efficiency than Na⁺.We systematically studied the changes of the electrode structure and electronic state information of MnO₂ under the action of different charge and discharge potentials,and concluded that when the voltage window is 0-1.2 V,the MnO₂ electrode material has the best electrochemical performance and cycling stability.The highest specific capacitance can reach 967 F/g,and the intercalation pseudocapacitance accounts for 48.4%.?4?Based on the correlation between the kinetic mechanism of the electrochemical reaction of MnO₂-based supercapacitors and the structural properties of MnO₂ itself,we propose that through reasonable design and optimization of MnO₂ structure,MnO₂ electrodes can be obtained for practical applications in experiments.In the experiment,a self-supporting electrode composed of MnO₂ tiny nanosheets with high mass loading?about 9.14 mg/cm²?was obtained through an improved electrodeposition method.This electrode structure can greatly shorten the diffusion path of electrolyte ions and enhance the insertion and de-insertion of Na⁺,and then get excellent electrochemical performance.The highest areal capacitance increased to 1570 mF/cm²,and the proportion of intercalation pseudocapacitance has reached 80.8%.