Pseudocapacitors carbons

The total capacity of a ruthenium oxide electrode [the usual double-layer capacity plus the pseudocapacity of reaction (21.4)] is rather high (i.e., several hundred F/g), even more than at the electrodes of carbon double-layer capacitors. The maximum working voltage of ruthenium oxide pseudocapacitors is about 1.4 V. 

ECs are another promising electrical energy storage device with a higher energy density than electrical capacitors, and a better rate capability and cycling stability than LIBs [32]. Carbon-based electric double layer capacitors and metal oxide- or polymer-based pseudocapacitors are two main types of ECs. The charge-... 

Recently supercapacitors are attracting much attention as new power sources complementary to secondary batteries. The term supercapacitors is used for both electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. The EDLCs are based on the double-layer capacitance at carbon electrodes of high specific areas, while the pseudocapacitors are based on the pseudocapacitance of the films of redox oxides (Ru02, Ir02, etc.) or redox polymers (polypyrrole, polythiophene, etc.). 

Capacity also influences the energy value. Actually, most of the research effort is dedicated to improving the capacity of electrode materials. Since ACs are taken as reference, the cost of the materials is an important criterion at the industrial level. Most of the materials developed for EDLC are of little interest for applications, because of their cost and only slightly improved performance. By contrast, it will be shown that advanced carbon materials can be developed for pseudocapacitors. 

Miller JR. Performance of mixed metal oxide pseudocapacitors Comparison with carbon double layer capacitors, Proceedings of the 2nd International Seminar on Double Layer Capacitors and Similar Energy Storage Devices, Deerfield Beach, FL, 1992. 

Both EDLCs and pseudocapacitors benefit from tailored, high surface area architectures because they each store charge on the surface by electrostatic or faradaic reactions, respectively. There are numerous examples in the hterature which show that materials possessing such features as nanodimensional crystallite size and mesoscale porosity exhibit significantly higher specific capacitance as compared to nonpotous materials or materials composed of micron-sized powders. The assembly of nanoscale materials is also important. One structure envisioned to be of interest is an array of vertically aligned carbon nanotubes where the spacing between the tubes is matched to the diameters of the solvated electrolyte ions (3). 

Most commercial applications for EDLCs and pseudocapacitors are for backup and pulse power sources for electronic devices. More recently, they have been explored as a power source for hybrid vehicles when used in combination with fuel cells or batteries. Their role is in load levehng, start-up, and acceleration (28). The traditional electrodes used in EDLCs are high surface area carbon (1000 m /g) which have specific capacitances around 100 F/g for a single electrode (5). Low-voltage devices (2.3 V) with capacitance values of 470, 900, and 1500 F are available commercially (29). By comparison, the specific capacitance of Ru02 electrode pseudocapacitors is in the range of 720 to 900 F/g (30-33). For the most part, ECs based on this material have yet to reach the market. 

Electrodes of various types are used in hybrid (asymmetric) supercapacitors (HSCs). For example, one of the electrodes is highly dispersed carbon, that is, a double-layer electrode, and the other electrode is a battery one or one of the electrodes is carbon and the other one is a pseudocapacitor, for example, based on electron-conducting polymer (ECP). The main advantage of HSCs as compared EDLCs is an increase in energy density because of the wider potential window. The main fault of HSCs, meanwhile, as compared to electric double-layer capacitors (EDLCs), is a decrease in cyclability following the limitations posed by the nondouble-layer electrode. 

As follows from these tables, symmetrical double-layer supercapacitors based on C/C-type carbon electrodes and, in limited quantities, some hybrid C/NiOOH-type ECSCs are produced at present. In the case of pseudocapacitors based on electron-conducting polymers, there are as yet only laboratory prototypes. 

Furthermore, some pseudocapacitors and hybrid ESs also used mixture electrolytes containing imidazolium-based ILs and organic solvents [678,694,695]. Zhang et al. [678] found the specific capacitance of asymmetric AC//Mn02 ES was increased and the internal resistance was decreased after the addition of DMF into the [BMIM][PF6] IL electrolyte, due to the improved ion mobility and penetration of the electrolyte compared with the pure IL. The optimal volume ratio of [BMIM][PF6] and DMF was 1 1, and further increasing the DMF concentration would decrease the performance. For intercalation-type carbon materials (e.g., partially reduced graphite oxide), the dilution of [EMIM][BF4] with ACN was found to increase the specific capacitance from below 100 F g up to approximately 180 F g. ... 

At the same time, a fundamental understanding of supercapacitor design, operation, performance, and component optimization led to improvements of supercapacitor performance, particularly increasing their energy density. To further increase energy density, more advanced supercapacitors called pseudocapacitors, in which the electroactive materials are composited with carbon particles to form composite electrode materials, were developed. The electrochemical reaction of the electroactive material in a pseudocapacitor takes place at the interface between the electrode and electrolyte via adsorption, intercalation, or reduction-oxidation (redox) mechanisms. In this way, the capacitance of the electrode and the energy density can be increased significantly. 

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