Preparation Of Manganese-Cerium Composite Oxide And Its Photo-Assisted Electrochemical Energy Storage Performance

To solve the energy crisis and environmental problems facing by human beings,it is critical to develop energy storage devices that can efficiently use energy,in particular renewable energy.Supercapacitors have become one of the most promising energy storage devices due to their high-power density,fast charge-discharge capability,ultra-long cycle life,and green safety features.As the core of these devices,electrode materials and their assembly into rational configurations are the main factors governing the charge storage properties of supercapacitors.Due to their abundance,low cost,environmental friendliness,and high theoretical specific capacitance,manganese oxides have become a research hotspot for supercapacitors.However,manganese oxides have inherent problems such as poor electrical conductivity and slow ion transport rate,which seriously hinder their further applications.To address these problems,it is crucial to develop green and sustainable methods to improve the charge storage efficiency of energy storage devices.Hence,manganese-cerium composite oxide electrodes with photo-assisted capacitance enhancement effect were constructed in this thesis through the rational material design for electrodes.Their electrochemical properties and photoelectrochemical properties were investigated,and their photo-assisted energy storage mechanism was explored.Detailed research results were as follows:（1）A two-step in-situ growth process was developed to fabricate carbon fiber paper-supported CeO₂/Mn O₂ composite（CeO₂/Mn O₂-CFP）as a binder-free photo-electrode for the photo-assisted electrochemical charge storage.Due to the formation of CeO₂/Mn O₂ type II heterojunction,the separation efficiency of photogenerated charge carriers were largely enhanced,resulting in a substantially enhanced photo-assisted capacitance enhancement capability to enhance its specific capacitance for～20%in the traditional pseudocapacitive Mn O₂-based material systems from the pseudo-capacitance enhancement.Furthermore,it possessed an even more interesting capability to retain a large part of its photo-assisted enhanced specific capacitance（～56%）in dark after the illumination was switched off for an extended period of time of 12 h,which could be attributed to the interesting photocatalytic memory effect.Its slow release of store photogenerated electrons originated from its specific band structure to avoid their release in dark from the reaction with O₂.This study proposed the design principles for supercapacitors with both the photo-assisted capacitance enhancement capability and its long-lasting retainment in dark,which is beneficial for promoting solar energy utilization and the development of new electrode materials and related technologies.（2）Based on the first part of work,Mn₃O₄@CeO₂ heterojunctions were synthesized using OMS-2 nanofiber templates to extend this photo-assisted capacitance enhancement capability from rigid electrodes to flexible free-standing electrodes.These flexible,self-supporting film electrodes were prepared by stacking OMS-2（lower part）and Mn₃O₄@CeO₂（upper part）layers by a simple vacuum filtration operation.Compared with the OMS-2 film electrode,as-prepared Mn₃O₄@CeO₂/OMS-2 flexible film electrodes exhibited more significant photo-assisted charging capability in the three-electrode system.For example,the areal capacitance of Mn₃O₄@CeO₂/OMS-2 electrode increased～11.2%to 21.6%at a series of current densities(0.2～1.0 m A cm^-2)under illumination,while the areal capacitance of OMS-2 thin film electrode only increased～1.7%to 8.6%accordingly.This obvious difference could be attributed to its better light absorption properties and photogenerated charge carrier separation capability,which could generate more charge carriers to enhance the conductivity of the electrode and promote the charge carrier transfer reaction.The photo-assisted capacitance enhancement effect was found to be dominated by a pseudo-capacitance enhancement.Furthermore,the Mn₃O₄@CeO₂/OMS-2 electrode retains its photo-assisted increased specific capacitance for a longer time in dark after the illumination was switched off from the photocatalytic memory effect.This study provided a new approach for developing flexible energy storage devices with the photo-assisted capacitance enhancement effect and its long-lasting retainment in dark.