Study Of High-performance Electrode Materials And The Novel Asymmetric Supercapacitors

As the depletion of fossil fuels and environmental pollution leads to the accelerateddevelopment of renewable and clean energy conversion/storage systems, two types ofelectrochemical devices such as batteries and electrochemical capacitors (ECs) havebeen paid more attention. Batteries with high energy densities suffer from slow powerdelivery, whereas ECs exhibit high power but low energy densities. Hence both highenergy and high power without sacrificing cycle life will be demanded for futureelectrochemical energy storage applications. This demand can be met by the ECs iftheir energy densities can be enhanced efficiently. High power density, long cycle lifeand environment-friendly, compared to traditional batteries, make ECs promisingenergy storage devices in a wide range of applications, such as hybrid electricvehicles, mobile electronic devices, large industrial equipments and space or militarydevices, wherein relatively light weight and small size are desirable. However, thedisadvantage of ECs including low energy densities and high production costs hasbeen identified as a major challenge for the capacitive storage science.Advanced supercapacitors must be developed with higher energy withoutsacrificing the power delivery and cycle life to meet the demands of practicalapplications in the future. The improvements of energy density (E) can be achieved bymaximizing the specific capacitance (C) and/or the cell voltage (V) according to the energy density equation (E12CV2). Considering above research background andapproach, the main contents of this thesis are summarized as follows:Firstly, vertically aligned graphene nanosheets (GNS) have been synthesizedusing radio frequency plasma enhanced chemical vapor deposition (PECVD) on Nifoam and that have been used as substrate for electrodeposition of Co(OH)2nanosheets. The hybrid material exhibits a high specific capacitance and ratecapability, i.e. a maximum specific capacitance of693.8Fg-1at a current density of2Ag-1and506.2Fg-1at32Ag-1in1M KOH aqueous solution. The capacitance canretain as high as91.9%after3000charge-discharge cycles at40Ag-1. These resultsindicate that GNS play an important role in high rate capability and long-term cyclingstability. This binder-free nano-electrode possesses a high specific capacitance,excellent power performance and long-term cyclic stability, which offer great promisefor applications in hybrid capacitors.Different carbon nanomaterials have been synthesized by PECVD on nickel-foamcurrent collectors and that have been used as substrates for cathodic electrodepositionof cobalt hydroxide nanosheets. The effect of carbon nanomaterials includingcrystallinity and morphology on the specific capacitance and cycling stability for theCo(OH)2are explored. The carbonaceous supports can not only act as a template todirect the growth of the as-deposited hydroxide but also affect the conductivity of theelectrodes and finally the electrochemical properties of the whole system, which mayprovide a technical reference for improvement in the controllable preparation andcombination of composites in the future.A lot of researches show that the energy density cannot be improved significantlyvia only relying on the active electrode materials. Therefore a simple and effectivemethod has been implemented to enhance the electrochemical performance forCo(OH)2/GNS electrode through introducing K3Fe(CN)6into the conventional KOHelectrolyte, realizing that both solid electrode and liquid electrolyte can contribute tothe pseudocapacitance simultaneously. The novel complex electrode system exhibitsnot only an ultrahigh specific capacitance and coulombic efficiency (a maximum specific capacitance of7514.2Fg-1and coulombic efficiency of541.4%at a highcurrent density of16Ag-1in the mixed1M KOH and0.08M K3Fe(CN)6solution),but also an excellent cycling stability (the capacitance retention is91.1%after2000continuous charge-discharge cycles and75.0%after20000continuous cycles at ahigh current density of80Ag-1in mixed1M KOH and0.04M K3Fe(CN)6solution).Upon increasing the K3Fe(CN)6concentration, both specific capacitance andcoulombic efficiency increase, but the rate property is worsened. The energy densityand power density for the system can be compromised via well controlling theK3Fe(CN)6concentration according to the practical application, and the highcoulombic efficiency (more than100%) can be recovered by charging the electrolyte.This encouraging investigation shows great potential in developing a battery-typesupercapacitor with a high energy density, power density, coulombic efficiency andlong-term cycling stability.We have prepared novel AC/carbon paper-KOH/PPD negative electrode system.The AC/carbon paper electrode in1M KOH exhibits capacitances of150.9and84.4Fg-1at current density of4and32Ag-1, respectively. And the AC/carbon paperelectrode in1M KOH and0.025M PPD mixed electrolyte exhibits high capacitancesof463.3and285.1Fg-1at current density of4and32Ag-1, respectively. That indicatethe introduction of PPD into conventional KOH electrolyte can contribute extrapseudocapacitance, improving capacitive properties effectively. This study provides anegative complex system with high-performance for novel asymmetricsupercapacitor.Finally, in order to maximize the specific capacitance and the cell voltage (V)simultaneously, we prepared a novel asymmetric supercapacitor, usingCo(OH)2/GNS-KOH/K3Fe(CN)6as positive electrode and AC/carbonpaper-KOH/PPD as negative electrode. The aqueous asymmetric supercapacitor hasdemonstrated a high voltage of2V, high energy density of124.4Wh kg-1at powerdensity of2000W kg-1, and outstanding cycleability (during20000continuouscharge-discharge cycles the capacitance almost remains the same). The highelectrochemical performance results from synergies between positive electrode and negative electrode.