Synthesis And Properties Research Of MnO₂-based Composite Materials For Supercapacitors

Fossil energy（coal、petroleum、natural gas）is exploited excessively by humans at unprecedented speed since the industrial revolution at late 18th century,rapid economics development and enhanced production capacity have greatly promoted living quality of human beings.However,environmental issues are increasingly prominent.Global warming gives rise to more frequent extreme climates,ocean acidification poses harsh ordeal for species diversity.Zero release or negative emission of carbon dioxide is urgent requirement of contemporary world.New energy,possessing great possibility to replace traditional energy,has received increasing attention.Supercapacitors,owning double peculiarities of larger specific capacitance than tranditional capacitors and greater power density than rechargeable batteries,as electronic energy storage devices comparable to rechargeable batteries,has received extensive and comprehensive research.But it is noteworthy that energy density of supercapacitors is smaller comparable to secondary cell,this badly astricts the development potential and commercial application of supercapacitors.According to computational formula of energy density:E=1/2CV²,the value of energy density is proportional to the specific capacitance and the spuare of the voltage window.Hence,exploring the active materials with the characteristics of high specific capacitance and large voltage window is pivotal for high energy density supercapacitors.The known electrode materials of water-based supercapacitors are mainly divided into three categories:transition metal oxides and hydroxides、conductive polymers and various kinds of deuterogenic materials、carbon-based materials（graphene and carbon nanotube et al）.MnO₂,as one kind of representative electrode materials of transition metal oxides,has always been a hot materials for water-based supercapacitors.MnO₂,possessing the advantages of high theoretical specific capacitance,abundant reserves,non-toxic,pollution-free and diversified preparation methods,hold incalculable potential in the field of energy storage materials.However,the current shortcoming of MnO₂-based supercapacitors are still narrow window voltage and small specific capacitance.Exploring the various energy storage mechanisms of MnO₂,optimizing the preparation process of MnO₂ nanomaterials,increasing the specific surface area and surface contact sites of MnO₂,and pre-intercalating alkali metal ions show important effects on improving the electrochemical performance of MnO₂-based supercapacitors.In this paper,we proposed three new preparation schemes through systematic research on MnO₂-based supercapacitors in-depth.The first scheme uses Co-MOFs as precursors to prepare hollow Co₃O₄ nano-particles with catalytic effect.Birnessite-type MnO₂ with rich K⁺is synthesized on the surface of Co₃O₄ by pyrolysis of KMnO₄.Co₃O₄@MnO₂ composite electrode materials holds extra-high output potential（1.4 V VS Ag/Ag Cl）under three-electrode system.In the second scheme,considering the inherent shortcoming of the powder electrode,on the basis of the first scheme,one new method of directly growing leaf-shaped Co₃O₄@MnO₂ nanosheets on carbon cloth is developed.Co₃O₄@MnO₂@CC can be directly used as the electrode materials for supercapacitors,avoiding the use of conductive agents and adhesives,with a high output voltage of 1.2 V and a high specific capacitance of 616.7 F/g.In addition,the close contact between the Co₃O₄@MnO₂ nanosheets and the carbon cloth gives the electrode materials excellent structural stability.The third scheme simplifies the experimental steps,directly growing MnO₂ nanosheets on the foam Ni substrate.The electrode MnO₂@Ni shows a high output potential of 1.2 V and a high specific capacitance of 335.8 F/g.Our studies have great reference value for the development of high-voltage MnO₂-based supercapacitors,and also promote the commercialization of supercapacitors.