Supercapacitor surface

A classic definition of electrochemical ultracapacitors or supercapacitors summarizes them as devices, which store electrical energy via charge in the electrical double layer, mainly by electrostatic forces, without phase transformation in the electrode materials. Most commercially available capacitors consist of two high surface area carbon electrodes with graphitic or soot-like material as electrical conductivity enhancement additives. Chapter 1 of this volume contains seven papers with overview presentations, and development reports, as related to new carbon materials for this emerging segment of the energy market. 

In the sixth paper of this chapter, Kierzek et al., mainly focus on modeling of pore formation vs surface area growth phenomena upon activation of coal and pitch-derived carbon precursors. These authors briefly touch on other precursor carbons as well. The properties of newly synthesized materials are being looked at from the point of view of their application as active materials in the supercapacitor electrodes. Editors thought this work by the Institute of Chemistiy and Technology of Petroleum and Coal in Poland, could be of genuine interest to the practical developers of carbon materials for the supercapacitor industry. 

In supercapacitors, apart from the electrostatic attraction of ions in the electrode/electrolyte interface, which is strongly affected by the electrochemically available surface area, pseudocapacitance effects connected with faradaic reactions take place. Pseudocapacitance may be realized through carbon modification by conducting polymers [4-7], transition metal oxides [8-10] and special doping via the presence of heteroatoms, e.g. oxygen and/or nitrogen [11, 12]. 

Under severe conditions (above 700°C), a potassium vapor is formed. It plays a special role in the activation of carbonaceous materials, easily penetrating in the graphitic domains that form cage-like micropores. The efficient development of micropores, which often gives a few-fold increase of the total specific surface area, is very useful for the application of these materials in supercapacitors [13-14]. 

The activation with KOH of selected parent materials under appropriate process conditions (temperature, time, reagent ratio) can provide highly porous carbons of controlled pore size distribution and surface chemistry, also suitable for use as electrode materials in supercapacitors. 

Fig. 12.3 Fabrication of the nanocomposite paper units for battery, (a) Schematic of the battery assembled by using nanocomposite film units. The nanocomposite unit comprises LiPF6 electrolyte and multiwalled carbon nanotube (MWNT) embedded inside cellulose paper. A thin extra layer of cellulose covers the top of the MWNT array. Ti/Au thin film deposited on the exposed MWNT acts as a current collector. In the battery, a thin Li electrode film is added onto the nanocomposite, (b) Cross-sectional SEM image of the nanocomposite paper showing MWNT protruding from the cel-lulose-RTIL ([bmlm] [Cl]) thin films (scale bar, 2pm). The schematic displays the partial exposure of MWNT. A supercapacitor is prepared by putting two sheets of nanocomposite paper together at the cellulose exposed side and using the MWNTs on the external surfaces as electrodes, (c) Photographs of the nanocomposite units demonstrating mechanical flexibility. Flat sheet (top), partially rolled (middle), and completely rolled up inside a capillary (bottom) are shown (See Color Plates)...

G. Ning, Z. Fan, G. Wang, J. Gao, W. Qian, F. Wei, Gram-scale synthesis of nanomesh graphene with high surface area and its application in supercapacitor electrodes, Chemical Communications, 47 (2011) 5976. 

Electrostatic interactions have recently been exploited for the synthesis of gra-phene-CNT hybrids. For example, poly(ethyleneimine) (PEI) coated graphene has been mixed with acid treated CNTs in a layer-by-layer method to form high surface area electrodes for supercapacitors [90]. Furthermore, Lu et al. prepared a supercapacitor electrode by mixing PDDA coated CNT-Mn02 hybrid with RGO [91]. 

After the description of chemical structure and control of meso-architecture and surface area, selected applications of such carbon materials as battery electrodes, supercapacitors, and in the design of controlled hybrid heterojunctions were presented. In the Li battery, coating or hybridization with hydrothermal carbon brought excellent capacities at simultaneous excellent stabilities and rate performances. This was exemplified by hybridization with Si, Sn02 (both anode materials) as well as LiFeP04 (a cathode material). In the design of supercapacitors, porous HTC carbons could easily reach the benchmark of optimized activated traditional carbons, with better stability and rate performance. 

Energy Storage—CNTs have a very high surface area (about 10 m /g), good electrical conductivity and can be made very linear (straight). They have been used to make lithium batteries with the highest reversible capacity of any carbon material and employed to make supercapacitor electrodes. CNTs are used in a variety of fuel cell applications where durability is important. 

Fig. 9.27 shows the variation of capacitance as a function of the surface area from which it may be concluded that in principle it is possible to obtain double-layer supercapacitors having specific capacitance of the order of 100 F/g so that values of practical capacitance density as high as 30 F/g can be predicted. 

Fig. 9.27 Specific capacitance of a double-layer supercapacitor as a function of surface area. (By permission of Chim Ind . E. Spila et al., 1995, 77, 333.)...

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