Supercapacitors electrode pore size

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. 

Recent reports describe the use of various porous carbon materials for protein adsorption. For example, Hyeon and coworkers summarized the recent development of porous carbon materials in their review [163], where the successful use of mesoporous carbons as adsorbents for bulky pollutants, as electrodes for supercapacitors and fuel cells, and as hosts for protein immobilization are described. Gogotsi and coworkers synthesized novel mesoporous carbon materials using ternary MAX-phase carbides that can be optimized for efficient adsorption of large inflammatory proteins [164]. The synthesized carbons possess tunable pore size with a large volume of slit-shaped mesopores. They demonstrated that not only micropores (0.4—2 nm) but also mesopores (2-50 nm) can be tuned in a controlled way by extraction of metals from carbides, providing a mechanism for the optimization of adsorption systems for selective adsorption of a large variety of biomolecules. Furthermore, Vinu and coworkers have successfully developed the synthesis of... 

Sun, G., W. Song, X. Liu, D. Long, W. Qiao, and L. Ling. 2011. Capacitive matching of pore size and ion size in the negative and positive electrodes for supercapacitors. Electrochimica Acta 56 9248-9256. 

To enhance the energy density of supercapacitors, it is desirable to use electrodes with very small pores. Experimentally, it has been shown that when pores in carbon electrodes are narrower than 1.0 run, the area-normalized capacitance of these nanopores increases as pore size reduces. This breakthrough was discovered for supercapacitors using organic electrolytes and later for supercapacitors using a RTIL [EMIM] [Tf2N] as electrolyte. 

The performance of supercapacitors (power density, specific capacitance and lifetime) is governed by parameters such as porosity, pore size distribution, stability, surface area, conductivity, etc., of the electrode materials. High surface area, stability and conductivity of CNTs make them promising materials for supercapacitor electrodes. Owing to the high conductivity of CNT electrodes, the supercapacitors can be operated in a wide frequency range. 

In the industrial applications of electrochemistiy, the use of smooth surfaces is impractical and the electrodes must possess a large real surface area in order to increase the total current per unit of geometric surface area. For that reason porous electrodes are usually used, for example, in industrial electrolysis, fuel cells, batteries, and supercapacitors [400]. Porous siufaces are different from rough surfaces in the depth, /, and diameter, r, of pores for porous electrodes the ratio Hr is very important. Characterization of porous electrodes can supply information about their real surface area and electrochemical utilization. These factors are important in their design, and it makes no sense to design pores that are too long and that are impenetrable by a current. Impedance studies provide simple tools to characterize such materials. Initially, an electrode model was developed by several authors for dc response of porous electrodes [401-406]. Such solutions must be known first to be able to develop the ac response. In what follows, porous electrode response for ideally polarizable electrodes will be presented, followed by a response in the presence of redox processes. Finally, more elaborate models involving pore size distribution and continuous porous models will be presented. 

The activated carbon (AC) reveals EDL capacitor. The capacitance of AC-based supercapacitors is dependent on specific conductivity, surface area, and porosity of the electrode. The porosity of the electrode material is estimated by Bmnauer— Emmet-Teller (BET) [34]. The porosity in the material has been understood by measuring the diameters of the surface pores of the material. The diameter of microparticles has been categorized as less than 2 nm whereas meso lies between 2 and 50 nm and macropores are greater than 50 nm. The capacitance varies depending on the size of the ions interacting with the surfaces as shown in the Fig. 5. 

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