Preparation And Properties Of Cobalt-and Manganese-based Oxide Supercapacitor Electrode Materials By Microwave-assisted Hydrothermal Process

As an energy storage device with outstanding advantages such as fast charge and discharge rate,high power density and long cycle life,supercapacitors have been well used in vehicles,ships and other transportation tools.As the main component of supercapacitors,electrode materials play a key role in their performance.Cobalt and manganese-based oxides（Co O,Co₃O₄,Mn₃O₄）as non-precious metal oxides have widely studied as supercapacitor electrode materials due to their high theoretical specific capacity,abundant redox pairs,and easy synthesis.However,the poor electrical conductivity,narrow potential window and volume expansion of the above materials limit their application and development in the field of supercapacitors.Therefore,it is of great significance to develop suitable paths to prepare cobalt and manganese-based oxide electrode materials with excellent electrochemical performance to promote the construction of high energy density supercapacitors and accelerate their application in carrier vehicles.The microwave-assisted hydrothermal synthesis process relies on the dipole polarization and ion conduction heating mechanism under microwave radiation,which can effectively control the microstructural characteristics such as product purity,crystal structure,micromorphology and size,and surface interface properties.Based on this,a series of cobalt-and manganese-based oxide electrode materials with high specific capacity,wide potential window and long cycle life were prepared by microwave-assisted hydrothermal combined with heat treatment process,using microwave-assisted effect to regulate the microstructure of the product,and combining the strategies of doping,substitution and recombination.The main conclusions are as follows:（1）Porous Co O and Co Co₂O₄ nanowire array electrode materials were prepared on Ni foam substrate by microwave-assisted hydrothermal treatment combined with annealing heat treatment process.Among them,the obtained Co Co₂O₄ nanowire electrode can achieve a specific capacity of 743.8 C g^-1(206.6 m A h g^-1)at 1 A g^-1,the potential window of 0.85 V（-0.3～0.55 V vs.SCE）and high cycling stability(the initial capacity retention was 89.7%after 5000 cycles at 15 A g^-1).The assembled Co Co₂O₄//m-CNT asymmetric supercapacitor（ASC）device achieved an energy density of 26.8 Wh kg^-1 at a power density of 775.4 W kg^-1.It was found that the introduction of microwave assistance in the process of hydrothermal synthesis effectively induced significant changes in the crystal structure and morphology,improved the electronic conductivity,broadened the voltage window,and enhanced the redox reaction,thereby effectively improving the electrochemical performance.（2）Using the microwave-assisted hydrothermal method,heterogeneous Mn ions were introduced in the process of synthesizing porous Co Co₂O₄ nanowires,and the multivalent Mn ion isomorphically doped Co Co₂O₄ porous nanowire array electrodes（Mnx Co Co₂O₄（x=1,2,3））were obtained.Among them,the obtained Mn-doped Co Co₂O₄（Mn2Co Co₂O₄）nanowire array electrode achieved a specific capacity of 1528.0 C g^-1/424.4 m A h g^-1 at 1 A g^-1 with an operating voltage window of 1.1 V（-0.3～0.8 V vs.SCE）and excellent cycling stability(91.5%initial capacity retention after 5000 cycles at 15 A g^-1).The assembled Mn2Co Co₂O₄//N-C ASC device showed a high stable operating voltage of 1.9 V,a high energy density of 58.6Wh kg^-1(power density of 950 W kg^-1)and an excellent cycling stability(83.7%initial capacity retention after 8000 cycles at 10 A g^-1).The reversible redox reactions occurring in Mn ions doped in Co Co₂O₄ can not only increase the specific capacity,but also widen the potential window,thereby significantly improving the electrochemical performance.（3）Carambola-like Co WO₄ microsphere with preferred crystal planes and oxygen vacancies were synthesized by introducing heterogeneous W ions with the microwave-assisted hydrothermal combined with heat treatment.Density functional theory analysis reveals that tuning the preferred crystal plane and introduction of oxygen vacancy into Co WO₄ can not only improve the conductivity,but also promote the adsorption process of OH^-and achieve more reaction charge transfer,thereby enhancing its electrochemical performance.As a result,the as-obtained carambola-like Co WO₄ microsphere with preferred crystal planes and oxygen vacancies delivered high specific capacity(493.7 C g^-1/137.1 m A h g^-1 at 1 A g^-1)with superior rate capability(148.6 C g^-1/41.3 m A h g^-1 at 15 A g^-1).Moreover,the cycling-induced morphology evolution lead to unconventional capacity increasing during cycling(123.8%initial capacity retention after 10000 cycles at 15 A g^-1).The preferred orientation and oxygen vacancies of carambola-like Co WO₄ prepared by microwave-assisted hydrothermal process effectively improved its conductivity,charge storage capacity and cycle stability.（4）Based on the Co WO₄ electrode material,a CNT-Mn₃O₄/Co WO₄ composite electrode material containing conductive carbon nanotubes and double metal oxides was synthesized by microwave-assisted hydrothermal processes.It was found that granular Mn₃O₄ and spheroid-like Co WO₄ with preferred orientation and oxygen vacancies were arranged on CNTs skeletons to construct a rational designed hybrid nanocomposite with abundant heterointerfaces and interfacial chemical bonds.The CNT-Mn₃O₄/Co WO₄ hybrid electrode delivered an ultra-high specific capacity(1907.5 C g^-1/529.8 m A h g^-1 at 1 A g^-1),a wide operating voltage window（1.15 V,-0.2～0.95 V vs.SCE）,and excellent cycling stability(117.2%initial capacity retention after 13000 cycles at 15 A g^-1).In addition,the assembled CNT-Mn₃O₄/Co WO₄//HN-C ASC device delivered a stable working voltage of 2.05 V and superior energy density of 67.5 Wh kg^-1 at power density of 1025 W kg^-1 as well as excellent stability(92.2%specific capacity retained at 5 A g^-1 for 12600 cycles).The prepared CNT-Mn₃O₄/Co WO₄ nanocomposites can be used as one of the candidate electrode materials for high energy density supercapacitors.（5）In order to explore the applicability of microwave effect to regulate the microstructure of metal oxides and improve electrochemical performance,manganese-based electrode materials were synthesized by microwave-assisted hydrothermal method.Based on microwave-assisted hydrothermal process,the microwave effect was used to induce Ni substitute Mn in Mn₃O₄ for the synthesis of Ni Mn₂O₄ microsphere and CNT@Ni Mn₂O₄core-shell electrode materials.It was found that the microwave effect introduced in the synthesis process also achieved the improvement of the electrochemical performance of both.The above work can provide reference and guidance for the design and preparation of supercapacitor electrode materials with excellent comprehensive performance by microwave-assisted process.