(1) Developing the nano-structural and composite electrode materials for increasing the capacitance. Designing and controlling the composition and structure to improve the electrical and ionic conductivities.
(2) Employing the organic liquid, gel solid polymer and ionic liquid electrolytes to enlarge the working potential window.
(3) Micro-scale supercapacitors as an important power source with comparable size in variety of miniaturized electronics seem ideal for capturing and storing energy from renewable resources, for on-chip operation, and for excellent nano-/micro-scale peak power.
We analyzes the high-voltage charge-storage behavior of electric double-layer capacitors in which two ionic-liquid electrolytes are used, 1-ethyl-3-methylimidazolium and 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imides (EMIm- and MPPy-TFSIs), and are operated at 3.5 and 4.1 V, respectively. Symmetric two-electrode capacitor cells assembled using micropore-rich activated mesophase pitch (aMP) and activated carbon fiber (aCF) carbons show a standard capacitive behavior in cyclic voltammetry analysis, whereas cells featuring templated mesoporous carbon (tMC) show ion-intercalating peaks in high-voltage scans. Impedance analysis performed at high voltages reveals that the aMP and aCF cells show lower charge-storage resistance than the tMC, although tMC facilitates ion transport more efficiently than aMP and aCF. The experimental results indicate that micropore-rich aMP and aCF accommodate single ions at high voltages, whereas the carbon structure is destroyed in micropore-deficient tMC because of graphitic-layer intercalation. The aMP carbon, which contains hierarchically connected micropores and mesopores, is effective in storing charge at a high rate at high voltages. Because of the compact feature of aMP, incorporating ionic liquids with aMP represents a very promising strategy for assembling capacitors of ultrahigh volumetric energy densities. (J. Mater. Chem. A In Press)
This study examines the linear triblock copolymer design of poly(acrylonitrile)-b-poly(ethylene glycol)-b-poly(acrylonitrile) (PAN-b-PEG-b-PAN) for a gel polymer electrolyte (GPE) swollen with dimethylformamide dissolving LiClO4. The study demonstrates the synergistic effect of the nitrile and ether functionalities in facilitating ion transport in the carbon films of electric double-layer capacitors. A GPE with a tuned AN/EG ratio exhibits ionic conductivity at approximately 10-2 Scm-1. The linear configuration incorporates the GPE border into the carbon electrodes. The PAN chain promotes ion solvation and transport into the carbon interior, and the PEG chain coordinates the solvent molecules to form ion motion channels. The synergistic effect of the PAN and PEG blocks enables a GPE EDLC delivering more energy and power than EDLCs with a liquid phase electrolyte. The GPE EDLC delivers 20 Whkg-1 (approximately 10 WhL-1) at a high power of 10 kWkg-1 (approximately 5 kWL-1) when using a high-porosity carbon electrode derived from mesophase pitch activation. (J. Phys. Chem. C 2013, 117, 16751)
The synthesis of a gelled polymer electrolyte (GPE) using poly(ethyleneglycol) blending poly(acrylonitrile) (i.e., PAN- b -PEG- b -PAN) as a host, dimethyl formamide (DMF) as a plasticizer and LiClO4 as an electrolytic salt for EDLCs is reported. The PAN-b-PEG-b–PAN copolymer in the GPE has a linear configuration for high ionic conductivity and excellent compatibility with carbon electrodes. When assembling the GPE in a carbon-based symmetric EDLC, the copolymer network facilitates ion motion by reducing the equivalent series resistance and Warburg resistance of the capacitor. This symmetric cell has a capacitance value of 101 Fg-1 at 0.125 Ag-1 and can deliver an energy level of 11.5 Whkg-1 at a high power of 10,000 Wkg-1 over a voltage window of 2.1 V. This cell shows superior stability, with little decay of specific capacitance after 30,000 galvanostatic charge-discharge cycles. The distinctive merit of the GPE film is its adjustable mechanical integrity, which makes the roll-to-roll assembly of GPE-based EDLCs readily scalable to industrial levels. (Adv. Funct. Mater. 2012, 22, 4677)
A novel composite of KOH activated mesophase pitch (aMP) and carbon nanotubes (CNTs) shows outstanding performance as an electrode for electric double-layer formation in 2 M H2SO4. The aMP powder is highly porous and the KOH activation may produce pores that are populated with graphitic edges. The resulting aMP electrode has a capacitance value of 295 Fg-1 at 0.125 Ag-1 discharge and decreases to 180 Fg-1 at 100 Ag-1. With particle milling, the pore diffusion resistance of the aMP electrode decreases significantly because of the elimination of a hindered diffusion mode from the particle interior. CNT addition provides inter-particle spacing and bridging media for the milled aMP and reduces the Warburg diffusion and electrical resistance. The composite of milled aMP and CNTs has capacitance values of 305 Fg-1 at 0.125 Ag-1 and 214 Fg-1 at 100 Ag-1. With a small potential window of 1 V, the resulting symmetric cells can deliver an energy level of 8.2 Whkg-1 at a high power of 10,000 Wkg-1. These cells show superior stability, with no decay of specific capacitance after 10,000 cycles of galvanostatic charge and discharge. (J. Mater. Chem. 2012, 22, 7314)
Graphene sheets are an ideal carbon material with the highest area available for electrolyte interaction and can be obtained by reducing graphite oxide (GO). This study presents the photocatalytic reduction of GO in water with mercury-lamp irradiation. The specific capacitance of the reduced GO in an H2SO4 aqueous solution reached levels as high as 220 Fg-1. This is because of the double layer formation and the reversible pseudocapacitive processes caused by oxygen functionalities at the sheet periphery. The rate capability for charge storage increases with irradiation time due to the continued reduction of oxygenated sites on the graphene basal plane. Alternating current impedance analysis shows that prolonged light irradiation promotes electronic percolation in the electrode, significantly reducing the capacitive relaxation time. With a potential widow of 1 V, the resulting symmetric cells can deliver an energy level of 5 Whkg-1 at a high power of 1000 Wkg-1. These cells show superior stability, with 92% retention of specific capacitance after 20,000 cycles of galvanostatic charge-discharge. (J. Phys. Chem. C 2011, 115, 20689)
Carbon nanofibers were grafted onto mesoporous carbon spheres to produce ‘‘sea urchinlike’’ mesoporous carbon with a nanofiber content of 25 wt.%. Because of its combined features of high electronic conductivity and efficient electrolyte transport, the sea urchin-like mesoporous carbon assembled in electric double layer capacitors shows outstanding high-rate performance with a voltammetric scan rate as high as 3000 mVs-1. Ac impedance analysis shows that this method of carbon nanofiber grafting promotes electronic percolation and ionic transportation in the carbon electrode, reducing the capacitive relaxation time to less than one fourth of its original value. Electrochemical oxidation in sea urchin-like mesoporous carbon produces a capacitance increase of ca. 200% while retaining high electronic and ionic conductivities in the electrode. (Carbon 2011, 49, 895)