Activated carbons capacitance limits

The results presented in this section confirm that an adequate pore size is more important than a high surface area for an optimization of the capacitance values. For the production of compact systems, an important objective is to limit as much as possible the useless porosity in order to enhance the volumetric capacity. Moderately activated carbons, with pores at the boarder of the ultramicropore region, e.g., 0.7-0.9 nm, are the most profitable for ions electrosorption. 

One can see in the table that the ratio between the pore wall thickness and the field screening length decreases with increasing capacitance. However, for surfaces areas around 1200 m g, the average pore wall thickness becomes close to the screening length of the field. Hence, the observed capacitance limitation in hi ly activated carbons can be also ascribed to a space constriction for charge accommodation inside some pore walls. 

The energy capacity of ECs arises from either double-layer capacitance for electric doublelayer capacitors (EDLCs) or pseudocapacitance for redox capacitors [2, 3]. The energy storage mechanism of EDLCs is based on non-faradic phenomena in electric double layer formed at an electrode/electrolyte interface. In regard to electrode active materials for EDLCs, carbon materials such as activated carbons have been most widely used [4] because of their reasonable cost, good electrical conductivity, and high specific surface area. However, there is a limitation in their specific capacitance the gravimetric capacitance of most carbon materials does not linearly increase with an increase in the specific surface area above 1,200 m g [5]. 

Barbieri O, Hahn M, Herzog A, Kotz R (2005) Capacitance limits of high surface area activated carlxms for double layer capacitors. Carbon 43 1303—1310... 

Despite the widespread applicability and high capacitance values of activated carbon materials, other electrolyte and ion transport issues may arise and limit the performances of electrodes fabricated from these materials. Fabricating ordered electrode structures seems promising for overcoming this limitation. For example, CNTs have been extensively investigated due to their one-dimensional structures that result in porous electrode networks. 

Prior to the use of graphene, a carbon-based material such as activated carbon was extensively used as an electrode material in the construction of supercapacitors due to its large surface area and low cost. However, in such stmctures, there are a lot of carbon atoms that cannot be accessed by the electrolyte ions, as illustrated in Fig. 4.11, and are effectively wasted consequently this is a major factor that limits the specific capacitance (which is F/g) of activated carbon electrodes. Additionally, it is reported that the low electrical conductivity of... 

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