No abstract

It is shown that charged defect generation, through argon ion-based plasma processing, in few layer graphene, could substantially enhance the electrical capacitance for electrochemical energy storage. Detailed consideration of the constituent space charge and quantum capacitances were used to delineate a new length scale, correlated to electrically active defects contributing to the capacitance, and was found to be smaller than a structural correlation length determined through Raman spectroscopy. The study offers insights into an industrially viable method (i.e., plasma processing) for modifying and enhancing the energy density of graphene-based electrochemical capacitors.

The theories to describe the rate at which electrochemical reactions proceed, to date, do not consider explicitly the dimensionality or the discreteness and occupancy of the energy levels of the electrodes. We show experimentally that such quantum mechanical aspects are important for dimensionally confined nanostructured materials and yield unusual variation of the kinetic rate constants with applied voltage in single-layer graphene. The observed divergence from conventional electrokinetics was ascribed to the linear energy dispersion as well as a nonzero density of states at the Dirac point in the graphene. The obtained results justify the use of density of states-based rate constants and considerably add to Marcus-Hush-Chidsey kinetics.

The capacitive characteristics of a model macroporous electrode, comprised of vertically aligned carbon nanotubes, with a relatively large volume /unit mass (of ∼40 cm 3 /g) are discussed. The faradaic characteristics of the capacity were evidenced through a voltage plateau in the galvanostatic discharge experiments. It is shown that the confinement of electrolyte within the spacing between the nanostructures with respect to an appropriate choice of the voltage scan rate yields a regime where both high capacity and rate capability may be realized. Additionally, the absence of diffusional processes allows for large capacitance retention even at discharge current densities of the order of 200 mA/cm 2 . For 1M K 3 Fe(CN) 6 , the maximum observed capacity was found to be ∼26 mAh/cm 3 (normalized to the pore volume) or 0.28 mAh/cm 2 (normalized to the projected area of the electrode) and it was noted that the capacity could be cycled at very high rates for 5000 cycles.Electrochemical capacitors (ECs) typically store charge either (a) electrostatically (in the double layer at the electrode/electrolyte interface, as an electrochemical double-layer capacitor: EDLC) or (b) pseudocapacitively 1 (at the electrode surface through redox reaction mediated mechanisms). The latter mode of charge storage in ECs relies on faradaic charge transfer, may occur over a relatively smaller voltage range compared to EDLCs, and varies as a function of the surface coverage area. Such pseudocapacitive characteristics have been observed in transition metal oxides (such as RuO 2 , MnO 2 , etc.) as well as conductive polymers (such as polyaniline) 2-4 and seem to be related to specific functional groups.At the very outset, a major benefit of capacitive methods of storing charge is the high power density that may be obtained due to the charge being predominantly on the surface. However, this very same attribute results in a lower energy density and consequently, there have been significant efforts to raise the energy density to the level of batteries through using interstitial spaces within the material. For instance, an intercalation pseudocapacitance 5,6 has been recently proposed as due to nanostructuring, in oxides such as TiO 2 and Nb 2 O 5 , and closely mimics battery-like behavior, while avoiding diffusional limitations. Such charge storage may also be realized through the use of high surface area nanostructured materials, such as activated carbon (AC) and carbon nanotubes (CNTs) 7-9 in conjunction with redox electrolytes. Optimized pore diameter or structure spacing mitigates diffusional constraints and could yield high charge/discharge rates without a detrimental effect on the cycling stability, which is an issue when using surface functional groups 10 for pseudocapacitance based storage. Concomitantly, a combination of redox electrolytes, 11,12 e.g., KI (for the positive electrode) and vanadyl sulfate (VOSO 4 , for the negative electrode), or through the use of an electrochemically active compound such as hydroquinone (HQ) added to a s...

The use of hierarchical assemblies constituted from macroporous structures (e.g., reticulated vitreous carbon, RVC) where the internal pore area is covered with closely spaced nanostructures (e.g., carbon nanotubes, CNT) is proposed for substantially enhancing the energy density of electrochemical capacitors, while maintaining large charge/discharge rates. While the macroscale pores enable storage of substantial electrolyte volumes that would contribute through redox reactions to the energy density, the closely spaced nanostructures provide a large geometric area and capacitance in addition to enabling rate independent Faradaic charge storage via thin layer electrochemistry (TLE). A fifty fold increase in the double layer capacitance, in addition to increased Faradaic charge densitywith potential for orders of magnitude improvement, was observed for the RVC-CNT electrodes, in comparison to the bare RVC foam electrode. It was seen that the hierarchical assembly enables the contribution from ~ 94% of the net volume of the wetted RVC-CNT electrode for active Faradaic charge storage.