Activated carbon electrodes

10 Capacitance ( ) and resistance ( ) variation with thickness for all solid gel electrolyte comprising PEO/PC/LiCI04 along with isotropic high density graphite (HDG) electrode as EDLC.  

72 XRD spectra of HDG electrodes (a) before charging, (b) after charging (anode), (c) after charging (cathode).  

The above reports indicate that there are not many studies with EDLC using a non-aqueous solid electrolyte. This might be because the macropores of carbon give easy access to the solid electrolyte. Relatively more studies were concentrated on aqueous-based solid electrolytes. Matsuda 

As silica gel is known as a thermally stable material and can provide excellent thermal conductivity with the incorporation of salts, more studies were concentrated on silica compounds. Tatsumisago et aL reported on silica-gel films containing perchloric acid HCIO4, dodecatungstophosphoric acid and tetra-n-butylammonium perchlorate prepared by the sol-gel method using tetraethoxysilane. The films exhibited maximum conductivities of S/cm at room temperature (RT) with 19 wt% of HCIO4. 

There was some interest in making a gel using acrylic acid and its salts. Iwakura et al. reported on an EDLC capacitor using a cross-linked potassium poly(acrylate) hydrogel. ° The capacitance with a polymer electrolyte was 150 F/g, which was a little higher than with the aqueous KOH electrolyte (144 F/g).The effect was attributed to pseudocapacitance owing to the use of the hydrogel in the capacitor cell. Moreover, the capacitance retention after 20000 cycles was found to be 82% of the initial capacitance for the polymer-based electrolyte while it was only 72% for the aqueous KOH electrolyte. 

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Fig. 7. A schematic diagram of an electric double-layer capacitor using active carbon electrodes.

In Figure 17, the rate capability Ycap was determined to be 0.60 for the as-activated carbon electrode specimen. This value is lower than that for the as-reactivated carbon electrode specimen 0.76. From these results, it is confirmed that the SAFG reduces the ion penetration depth, and hence impedes the ion penetration into the pores during double layer charging of the carbon electrodes. 

Endo, M., Maeda, T., Takeda, T., Kim, Y.J., Koshiba, K., Hara, H., and Dresselhaus, M.S. Capacitance and pore-size distribution in aqueous and nonaqueous electrolytes using various activated carbon electrodes. J. Electrochem. Soc. 148, 2001 A910-A914. 

FIGURE 8.31 Voltage profiles of various hybrid systems in nonaqueous electrolytes (NAH) including Li-intercalation electrodes compared with a nonaqueous (NA) EDLC based on activated carbon electrodes cycled to 3 V. All profiles are normalized with respect to each other. (From Plitz, I., et al., Appl. Phys. A, 82, 615, 2006. With permission.)... 

Therefore, a hybrid cell has been designed in lmol L 1 LiPF6 in 1 1 ethylene carbonate/ diethyl carbonate electrolyte by combining graphite and activated carbon as negative and positive electrodes, respectively [113], The activated carbon electrode is stable in the potential window between 1.0 and 5.0 V vs. Li, whereas the graphite electrode can be polarized down to low potential values. The mass of the electrodes should be balanced to fully take profit of the performance... 

Increase of the electronic state density in the carbon pore walls with the voltage. Hahn et al. [52] have measured double-layer capacitance and electronic conductance of an activated carbon electrode in an aprotic electrolyte solution, 1 mol/dm3 (C2H5)4NBF4 in acetonitrile. Both quantities show a similar dependency on the electrode potential with distinct minima near the potential of zero charge. This correlation suggests that the capacitance, like the conductance, is governed substantially by the electronic properties of the solid, rather than by the ionic properties of the solution in the interface of the double layer. 

Aqueous electrolyte Carbon, activated carbon electrode H2S04 aqueous... 

Table 9 Experimental Tafel Slopes, Charge Transfer Coefficients and Surface Coverages for Halogen-Active Carbon Electrodes at Various Temperatures and Coverages... 

For commercial double-layer capacitors using activated carbon electrodes, a nonaqueous solution such as 0.5 to 1 mol dm (=M) Et4NBp4 in propylene carbonate (PC) or an aqueous solution such as 3.7 to 4.5 M (30 35 wt%) H2SO4 is used. The advantages of the nonaqueous liquid electrolytes are as follows ... 

Based on these properties, the double-layer capacitor comprising from a pair of the activated carbon electrodes and the nonaqueous liquid electrolyte is the most favorable one from the viewpoint of energy density. Ionic liquids are a kind of nonaqueous liquid electrolytes. 

Gadkaree KP, Mach JF, and Stempin J. Ion-removal from water using activated carbon electrodes. US Patent no. 6,214,204. April 2001. 

A powdered active carbon electrode consists of a continuous matrix of electrically conducting solid that is interspersed with interconnecting voids or pores whose characteristic dimensions are small compared to the size of the electrode. The electrochemical reactions in such electrodes occur predominantly in the pores, which represent the major fraction of the total surface area. The external surface area is relatively small with respect to the area of the pore walls. It is the high interfacial surface area available for electrochemical reaction that provides the major advantage of porous electrodes over smooth electrodes (e.g., glasslike car-... 

FIG. 1 Schematic representation of the powder active carbon electrode (a) and the porous structure of a carbon granule (b) L, sedimentation bed thickness. (Inspired by Refs. 4 and 29.)... 

Considerable attention has also been paid to the potentiometric response of powdered active carbon electrodes, which in considerable part depends on the type and concentration of functional groups on the surface [7,70,160,161]. The response of a carbon electrode to ionic species in aqueous solution arises from the adsorption behavior of surface functional groups. In addition, physically and/ or chemically adsorbed gases (mainly CO and oxygen) affect this process significantly. 

IV. VOLTAMMETRY OF POWDERED ACTIVE CARBON ELECTRODES (PACE)... 

TABLE 6 Potentials of Hydrogen and Oxygen Evolution for Active Carbon Electrodes in Different Electrolytes (V vs. SCE)... 

The electrochemical behavior of the powdered active carbon electrode depends on the surface chemistry, and cyclic voltammetry can be used as a simple method of characterizing active carbon materials. A new heterogeneous copper catalyst was developed using highly porous active carbon as the catalyst support [282]. The advantages of a porous-medium supported catalyst are that the active phase could be kept in a dispersed but stable state, and that, as an example, the oxidized organic pollutant is adsorbed onto carbon, thereby enhancing its surface concen-... 

The properties of active carbon render it a difficult material to use as an electrode. Electrochemical processes occur more often in the inner cavities (pore structure) of active carbon particles than on their outer, planar surface. For this reason, three-dimensional electrochemical activity is observed rather than the planar responses characteristic of solid carbon electrodes. It is generally a.s.sumed that the total area of the internal structure of the porous carbon electrode is completely wetted by electrolyte, although this may not be the case with high-surface-area carbons containing micropores inaccessible to electrolyte. The main difficulty is estimating the electrochemically active part of the total surface area of the active carbon electrode material. 

Figure 2. Plots of cell potential against operation temperature measured on the electrodialysis cell at the applied current of 2A. The apparent area of activated carbon electrode was 25 cm ...

FIG U RE 7.1 Conductivity of activated carbon electrodes prepared from naphthalene-derived mesophase chemically activated by KOH and thermally treated at 600 and 1000°C, mixed with polyvinylidene fluoride, and subsequently thermally activated at the above temperatures. (Adapted from Ruiz et al., 2008. 7. Electroanal. Chem. 618, 17-23, with permission.)... 

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