Design,Synthesis And Application Of MnO₂-based Materials For Supercapacitor Electrodes

Supercapacitors have a greater power density and longer cycle life compared to batteries,and a higher energy storage capacity compared with conventional capacitors,so there is a wide range of market value and application prospects.However,the lower energy density is the main factor restricting the development of supercapacitor,and the research of electrode materials is the most likely solution to this problem.The electrode materials for supercapacitors are mainly divided into three categories:carbon-based materials,conductive polymers and their derivatives,transition metal oxides and their hydroxides.Among them,manganese oxides are ones of the most important supercapacitor materials as they have the advantages of high theoretical capacity,environmental friendliness,low cost and so on.However,their poor conductivity and poor cycle stability restrict their energy storage capacity,fast charge and discharge property,cycle life and so on.So,the key point to solve the problem is to improve the conductivity and stability of manganese oxides.In order to improve the conductivity and stability of MnO2,we achieved through the following three programs:(1)composite with the conductive materials;(2)design of the electrode structure;(3)chemical modification.The main work and conclusions are as follows:(1)Composite with the conductive materials:In order to improve the charge transfer rate of MnO2,different methods were used to incorporate conductive PANI with MnO2,and three kinds of novel MnO2-PANI composites were prepared.(ⅰ)A core-shell structure with MnO2 nanorods as the core,and conductive PANI as the shell was synthesized by using template method.The MnO2 nanorods,as oxidant and template,leaded the growth of conductive PAN I on its surface,and thus the morphology of the composites can be controlled.(ⅱ)We synthesized a new morphology composite like the sea urchin with MnO2 nanorods insert the conductive PANI sphere by using water bath method,(ⅲ)Hydrothermal method was innovatively applied to the polymerization of PANI,and synthesized a new morphology of nanobundles structure with MnO2 nanorods as main part while conductive PANI as adhesive.Electrochemical tests were carried out on the three different composites.The improvement of the properties for the MnO2-PANI composites were mainly due to(ⅰ)the good conductivity of PANI,(ⅱ)the good flexibility of PANI,(ⅲ)the strong chemical bonds between the components of the composite.(2)Design of the electrode structure:In order to improve the utilization of bulk materials and solve the difficulty for the charge carrier transfer in the bulk phase,a layer of MnO2 on nickel foam was prepared by electrochemical deposition method,with a subsequent in situ polymerization of aniline to apply PANI layer as a coating.Following the PANI deposition,another layer of MnO2 was electrochemical deposited on the top to create a sandwich-structured MnO2-PANI-MnO2 electrode.We discussed the mechanism of deposition process and the importance of the electrochemical depositioin method.The impact of the polymerization time to the PANI layer and the key role of the conductive PANI to the sandwich-structured electrode were also studied.The electrochemical measurement showed that the sandwich-structured MnO2-PANI-MnO2 electrode exhibited better electrochemical property than pure MnO2 electrode.Its specific capacitance achieved 204 F/g at a current density of 1 A/g and the capacitance still remained 85%after 3000 cycling test.SEM,TEM,Raman,XPS and EIS results showed that the good capacitive properties were mainly due to the conductive PANI layer,which influenced the structure of active materials and improved the conductivity of electrode.The porous PANI made it easier for electrolyte to permeate into the deeper layer and made good use of materials in bulk phase.Besides,the good michenical properties of PANI enhanced the cycling stability of the sandwich-structured electrode.(3)Chemical modification:There are fewer reports about composite materials based on MnO2 and CuO as supercapacitor electrode materials.In our work,CuO-MnO2 composite with novel morphology was successfully synthesized.Nanostructured CuO materials were deposited on Cu net in a short time by using etching solution oxidation method.We explored the coordinate complex diffusion-deposition mechanism.CuO nanomaterials with different morphologies(nanowall,nanosheet,nanowire)were prepared on the Cu net surface by controlling the concentration of coordination complexes.Electrochemical tests showed that the nanowire structured CuO had the best electrochemical performance.The active MnO2 nanosheets were coated on the CuO surface by hydrothermal method subsequently.Due to their synergistic effect,the CuO-MnO2 composite electrode exhibited a wide voltage window(～1.3 V)and a good cycle stability with no capacitance decrease after 2000 cycles test.