Preparation, Modification And Electrochemical Capacitance Of MnO₂and Graphene-based Aerogels Electrode Materials

Supercapacitor is considered to be a new kind of high-efficiency energy storagedevice thanks to its relatively high power density, long cycle life, wide potentialwindow and environmental friendliness. In recent years, supercapacitors are usuallycoupled with primary high-energy batteries or fuel cells to serve as a temporaryenergy storage device with a high-power capability to store energies when braking,which also can be widely used in our daily life, aerospace, military, industrialproduction and other fields.On the basis of summarizing the process in research on supercapacitor electrodematerials, our efforts mainly devoted to the preparation of manganese dioxide andgraphene-based aerogels. We also have made various attempts to improve theirelectrochemical performance. The main contents of our research paper are describedas follows.(1) Three different kinds of bivalent manganese salts have been adopted to reactwith permanganate to prepare manganese dioxide. Depending on some physical andelectrochemical characterization, it can be found that bivalent manganese salt has aneffect on the morphology, crystal type, specific surface area, pore size distribution andelectrochemical performance of the resultant manganese dioxide. Amorphousmanganese dioxide prepared from manganese sulfate or manganese chloride exhibitslarge specific surface area and ideal capacitive behavior. While the sample preparedfrom manganese acetate has a long cycle life due to its relatively complete crystalphase. According to the comprehensive evaluation of the electrochemical properties,electroactive manganese dioxide prepared from manganese sulfate is considered to bethe most promising electrode material among these products.(2) Based on the charge and discharge mechanism of manganese dioxide,tourmaline with adsorption properties has been adopted to incorporate with optimizedmanganese dioxide for the purpose of improving the capacitance performance and cycling stability of the electrode through mechanical mixing method. The highestspecific capacitance value of347F/g has been achieved as the mass ratio ofmanganese dioxide and adding tourmaline is4:5. The cycle stability of electrode alsohas been enhanced by tourmaline, after400cycles, the capacitance retention ofelectrode still remained above90%.(3) Graphene oxide (GO) is composed of hydroxyl and ether groups on bothsides and carboxyl groups on the edge, so it greatly separates the layers. Whenreducing the hydroxyl, ether, and carboxyl groups, the graphene layers usuallyundergoes irreversible agglomeration owing to the strong van der Waals interactionbetween the adjacent layers, leading to the distance of adjacent sheets similar to thatin graphite. The large accessible surface area of graphene sheets will be sacrificed inelectrochemical reactions. For the purpose of improving these problems, orderedmesoporous carbon (OMC) served as a spacer to prevent the restacking of adjacentgraphene layers. At the same time, the graphene oxide could be partially reduced tographene and further spontaneously assembled into a hydrogel through a simplehydrothermal procedure. On the basis of the physical characterization andelectrochemical tests on different proportions of the GO/OMC hybrid aerogels, theresults indicate that a certain amount of OMCs are able to tailor the morphology andelectrochemical properties of products. OMC can effectively avoid restacking andagglomeration of graphene as the mass ratio of GO/OMC is1:1. This electrode showgood rate capability and cycle stability, but not very high specific capacitance of144F/g. When the mass ratio of GO/OMC increased to2:1, the product exhibit betterelectrical conductivity and capacitive performance (191F/g). The results also indicatethat carbon materials with a3D ordered porous texture, which facilitates ion transportby providing a smaller resistance and shorter diffusion pathways, are promisingcandidates for electrode materials. (4) Among various electrode materials, carbon materials can meet the demands ofEDLCs thanks to their high surface area, chemical stability, relatively low cost andenvironmental friendliness, but they inevitably undergo limited energy density due totheir single electrostatic surface charging mechanism. As one of the most promisingcandidates among conductive polymers for electroactive materials, PANI has attractedconsiderable attention due to its fast and stable transition between doped and reducedstates, easy synthesis and low-cost alternative source. Unfortunately, conductivepolymers usually suffer from a limited long-term stability during cycling due to thedegradation caused by swelling and shrinking. In our work, we prepared a kind ofnew graphene/PANI hybrid aerogel using PANI-NWs as spacers and orient agents.During the hydrothermal self-assembling process, PANI nanowires could help keepthe neighboring GO sheets separate and tailor the hierarchically porous network ofobtained hybrid. The combination of electric double-layer capacitance and faradaicpseudocapacitance makes a contribution to the capacitance of the hybrid aerogel. Inaddition, the hybrid with porous and interconnected network enables a fast electrolyteions transport and provide continuous charge transfer pathways. After systematicelectrochemical tests, the highest specific capacitance value of520F/g has beenachieved from the graphene/PANI hybrid aerogel, which is much higher than purePANI electrode (343F/g), implying a high improvement. The hybrid aerogelelectrode also exhibits a better rate capability and cycling stability.