Hybrid Supercapacitor Based On MnO₂ Nanomaterials

Manganese dioxide(MnO2)has become one of the hot spots in the field of alternative energy because of its excellent theoretical capacitance,abundant natural reserves,low cost,environmental protection characteristics and highly compatible with other energy storage materials.In the future design and application of supercapacitor,metal oxide nanomaterials represented by MnO2 will play an important role.However,the relatively poor conductivity of MnO2 materials,as well as the problems of caking,dissolution and volume expansion during the electrochemical cycle,all seriously restrict its practical application.During electrode preparation,the combination of the active material with the substrate with excellent conductivity and large specific surface area,and the improvement of charge transfer efficiency in the electrolyte are all key factors affecting the performance of the MnO2 based electrode.Therefore,in order to make full use of the advantages of MnO2 in supercapacitors,electrode preparation methods with high conductivity,structural stability and electron transfer efficiency must be combined,which will promote the development and application of MnO2 containing supercapacitors.In this paper,α-MnO2 and Fe2O3 nanomaterials were prepared on carbon cloth(CC)modified with carbon nanotubes(CNTs),core-shell positive(α-MnO2@CNTs/CC)and negative(Fe2O3@CNTs/CC)were obtained.The electrochemical performance of single electrode was evaluated in 1 M Na2SO4.Meanwhile,in order to evaluate the potential of the electrodes,a supercapacitor consisting of α-MnO2@CNTs/CC and Fe2O3@CNTs/CC electrodes was prepared,one using 1 M Na2SO4 electrolyte corresponding to a 2 V voltage window,and the other using EMImBF4 ionic liquid electrolyte corresponding to a 4 V voltage window.Through the voltammetry cycle test,it is found that the CV curve of the supercapacitor with 2 V voltage window does not show obvious deformation even at the scanning rate of up to 5000 mV/s,which proves that the supercapacitor has stable electrochemical performance and rapid charge-discharge ability.In addition,the energy density of the supercapacitor reaches 57.29 W h/kg when the power density reaches 833.35 W/kg.After 20000 constant-current charge-discharge(GCD)cycles,the capacity retention rate reached 87.06%.The supercapacitor with 4 V voltage window shows 1260 s discharge time and 124.8 F/g specific capacitance under the current density of 0.5 mA/cm2.After 5000 GCD cycles,the capacity retention rate reaches 87.77%.The flexible supercapacitor based on EMImBF4 electrolyte can withstand large angle physical bending and can continuously power LED arrays,demonstrating its good mechanical stability and application potential.Meanwhile,the supercapacitor reaches a maximum energy density of 57.29 W h/kg at a power density of 833.35 W/kg,maintaining 87.06%of its capacity after 20000 constant-current GCD cycles.The supercapacitor with 4 V voltage window shows 1260 s discharge time and 124.8 F/g specific capacitance at the current density of 0.5 mA/cm2,and 87.77%capacity retention rate after 5000 GCD cycles.Flexible supercapacitors based on EMImBF4 electrolytes can withstand physical bending at large angles and provide continuous power for LED arrays,proving their excellent mechanical stability and application potential.By using the synergistic effect between nanomaterials,the surface composite of MnO2 and carbon-based materials can not only give full play to the advantages of MnO2,but also effectively reduce its disadvantages.In order to take advantage of the conductivity,stability and high specific surface area of carbon materials,metal-organic frames(MOFs)were first prepared on carbon cloth(CC)surfaces.After high temperature carbonization and ion etching treatment,a conductive porous carbon nanosheet(CPCN)was prepared,and carbon cloth formed a flexible conductive substrate CPCN@CC.Then,a layered δ-MnO2 nanomaterial was composite on the surface of CPCN@CC to form a core-shell structure δ-MnO2/CPCN@CC flexible composite electrode,which significantly increased the specific surface area of MnO2.Finally,AuNPs were uniformly deposited on the surface of the prepared MnO2/CPCN@CC electrode using the excellent electrical conductivity of gold nanoparticles(AuNPs),and the modified Au-MnO2/CPCN@CC flexible electrode with enhanced activity was obtained.The Au-MnO2/CPCN@CC electrode exhibited a specific capacitance of 503.7 F/g at a current density of 0.125 mA/cm2 in a single electrode test in a 1 M Na2SO4 electrolyte.After 10000 cycles,the capacitance retention rate is 87.68%.However,when the MnO2/CPCN@CC electrode is tested under the same conditions,its capacitance retention rate is only 61.07%.The ratios of pseudocapacitance of AuMnO2/CPCN@CC,Au/CPCN@CC and MnO2/CPCN@CC at 1 mV/s are 83.80%,78.43%and 40.88%,respectively.The CV curve at the same scanning rate can reflect the energy storage mechanism of the electrode,and the large pseudocapacitance ratio can reflect the proportion of fast redox reaction on the surface of the electrode in the process of energy storage.In this study,AuMnO2/CPCN@CC was used as the positive electrode,carbon cloth coated with activated carbon(AC)was used as the negative electrode,and 1 M NaPF6 was used as the electrolyte to construct the supercapacitor.The experimental results show that the energy density of the capacitor can reach 71.48 W h/kg and the power density can reach 79.96 W/kg.After 10000 cycles,the capacitor retention rate is 81.93%.MOFs-derived CPCN can effectively improve the conductivity,load and mobility of the electrode active material,play a good supporting role for MnO2,and effectively alleviate the material agglomeration.At the same time,AuNPs greatly promote the surface electron transmission of MnO2,and the advantages of various materials complement each other,cooperate and promote the good performance of the supercapacitor.