A Study On The Electrode Materials Of Supercapacitors With High Energy Density

As an energy storage device, supercapacitor has been studied and applied widely. Itcan store much more energy than conventional capacitors and offer much higher powerdensity than batteries. The performance of the electrode materials is the one of keyfactors which may improve its energy density. In the present paper, the effect of theactivated carbons' microstructure on the specific capacitance and different chargestorage mechanisms of manganese dioxide were investigated deeply, andgraphene/MnO₂composite materials with good performance was prepared based on theelectrostatic interactions.The activated carbons (ACs) with hierarchical pore distribution were preparedfrom mesophase pitch by chemical activation process. The pre-carbonization processbefore activation changed the microstructure of ACs and was in favor of thecontribution of micropore surface area on the capacitance. By changingpre-carbonization temperature from560℃to750℃, the arrangement ofmicrocrystallines was controllable, and correspondingly the pore size distributionbetween1.5nm and2.4nm could be adjusted. The electrochemical performance of ACswas investigated in1M Et₄NBF₄/PC. It was found that the pore larger than or equal to1.85nm was in favor of moving of ions, which tended to obtain a high capacitance withhigh current density; on the contrary, the ions can not move freely in the pore smallerthan or equal to1.75nm. And the pore range of1.75¹.85nm may be the critical one in1M Et₄NBF₄/PC.For improving the conductivity of porous carbons, a new pathway to prepareporous graphitic carbons was proposed by combining chemical activation with catalyticgraphitization. The activating agent ZnCl2increased the surface area of porous carbon,and the catalyst (Fe, Ni) accelerated the graphitization. By changing heat treatmenttemperature and the ratio of the reactants, the surface area and the graphitization degreemight be adjusted.The co-existence of two mechanisms, the surface chemisorption mechanism (SCM)and the tunnel storage mechanism (TSM), for the charge storage of MnO₂wasinvestigated. Alpha-MnO₂whose tunnels (2×2) were almost vacant was synthesized by chemical co-precipitation technique. To verify whether the charge storage ofas-prepared α-MnO₂involves the charges based on the SCM besides the TSM, Ba²⁺andK+were stabilized into the tunnels of α-MnO₂by ion-exchange-type reaction andsamples Ba-MnO₂and K-MnO₂were obtained, respectively. Based on electrochemicalanalyses, it was found that the charge storage of Ba-MnO₂and K-MnO₂basicallydepends on the SCM, as Ba²⁺and K⁺stabilized in the tunnels in advance blocked theintercalation of cations from the electrolyte; that for α-MnO₂two mechanismscontributed to the charge storage. The contributions of α-MnO₂'s mechanisms wereestimated. The charges based on the TSM predominantly accounted for81.2⁸9.9%ofthe total charges. The charges on the SCM may contributed to7.9¹6.8%and were notnegligible. The charges from double-layer capacitance may be negligible, since it wasonly1.1⁴.1%.The graphene/MnO₂composite materials were synthesized based on theelectrostatic interactions. Graphite oxide (GO) was synthesized by a modified Hummersmethod, and converted to graphene nanosheets (GNs) by chemical reduction. GNsdispersed in water were negatively charged. Hexadecyl trimethyl ammonium bromide(CTAB) chosen as the surfactant, nano MnO₂powder was prepared by a microemulsionroute. Also CTAB made the particle of MnO₂positively charged. The specificcapacitance of as-prepared graphene/MnO₂composite in0.1M Na₂SO₄was206Fg^-1,40%higher than that of MnO₂.