Supercapacitors asymmetric

In the third paper by French and Ukrainian scientists (Khomenko et al.), the authors focus on high performance a-MnCVcarbon nanotube composites as pseudo-capacitor materials. Somewhat surprisingly, this paper teaches to use carbon nanotubes for the role of conductive additives, thus suggesting an alternative to the carbon blacks and graphite materials - low cost, widely accepted conductive diluents, which are typically used in todays supercapacitors. The electrochemical devices used in the report are full symmetric and optimized asymmetric systems, and are discussed here... 

Electronically conducting polymers (ECPs) such as polyaniline (PANI), polypyrrole (PPy) and po 1 y(3.4-cthy 1 cncdi oxyth iophcnc) (PEDOT) have been applied in supercapacitors, due to their excellent electrochemical properties and lower cost than other ECPs. We demonstrated that multi-walled carbon nanotubes (CNTs) prepared by catalytic decomposition of acetylene in a solid solution are very effective conductivity additives in composite materials based on ECPs. In this paper, we show that a successful application of ECPs in supercapacitor technologies could be possible only in an asymmetric configuration, i.e. with electrodes of different nature. 

Table 1. Electrochemical characteristics of some asymmetric supercapacitors. U is the maximum available cell voltage. Cs is the specific capacitance of a pellet electrode calculated from Cs = Ct 4/M, where Q is the capacitance of the asymmetric supercapacitor, M is the total mass of both electrodes.

As it was already described in Section 8.2.1, the total capacitance of a supercapacitor is given by Equation 8.1. In a symmetric capacitor with equal values of capacitance for the positive (Cj) and negative (f2) electrodes, the total capacitance of the system is half the capacitance of one electrode. In the asymmetric device, as the capacitance of the battery electrode is much higher than the capacitive one, the capacitance of the system approaches that of the electrode with the smallest value. In other words, the capacitance of the asymmetric configuration combining a battery-like electrode with a capacitive one will be close to the value of the capacitive electrode, i.e., twice larger than that for a symmetric configuration with two capacitive electrodes. 

Table 8.5 summarizes the electrochemical performance of different types of symmetric and asymmetric supercapacitors in aqueous medium, including the maximum cell voltage (Vmax), the... 

In conclusion, while being environment-friendly, the asymmetric construction in aqueous electrolyte offers electrochemical characteristics comparable with EDLCs in organic electrolytes. Combining materials with pseudocapacitance properties in an asymmetric cell is a very promising issue for developing a new generation of high-performance supercapacitors. 

Khomenko V, Raymundo-Pinero E, Beguin F, Frackowiak E. High-voltage asymmetric supercapacitors operating in aqueous electrolyte. Applied Physics 2006 A82 567-573. 

Taking into account the underestimated advantages to operate in aqueous electrolyte, it seems also important to look for other applications of carbon materials where the unique combination of electrical conductivity, surface functionality and porous texture may be useful. Such applications as electrochemical hydrogen storage [116, 117], asymmetric supercapacitors [118] open future perspectives where aU the information previously collected on other systems will be useful. 

Fan, Z. et al.. Asymmetric supercapacitors based on graphene/MnOj and activated carbon nanofiber electrodes with high power and energy density. Adv. Funct. Mater. 2011,27(72 , 2366-2375. 

Guo, CX.,Yilmaz, G., Chen, S., Chen, S., Lu, X., 2015. Hierarchical nanocomposite composed of layered V205/PED0T/Mn02 nanosheets for high-performance asymmetric supercapacitors. Nano Energy... 

Yu, D., Goh, K., Zhang, Q., Wei, L., Wang, H., Jiang, W, Chen, Y, 2014. Controlled functionalization of carbonaceous fibers for asymmetric solid-state micro-supercapacitors with high volumetric energy density. Adv. Mater. 26, 6790-6797. 

Zhou, C., Zhang, Y, Li, Y, Liu, J., 2013. Construction of high-capacitance 3D CoO polypyrrole nanowire array electrode for aqueous asymmetric supercapacitor. Nano Lett. 13, 2078-2085. 

According to chemical analysis carried out using a number of methods, especially X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy, the chemical composition of carbons for different electrodes changes variously. All changes depend on the electrode polarity negative electrodes (cathodes) age much less than positive electrodes (anodes). This must be taken into account in new supercapacitor schemes, for example, anodes must be made thicker than cathodes in an asymmetric scheme to compensate losses of capacitance or, in the case of presence of temperature gradients, to position an anode from the colder side. The most important measurements showed formation as a result of the process of the aging of C-H bonds on the cathode and bonds C-N and C-F on the anode and also polymerization of acetonitrile on the cathode and decomposition of tetrafluoroborate. The practically unavoidable... 

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. 

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