Specific capacitance

According to the method developed by Izumi [593], magnesium chloride is added to the reactor as a diluent, along with K2TaF7 and NaCl, prior to the reduction process. The powder obtained by the above method is washed and treated thermally at 1200°C in vacuum. The final product has a specific capacitance of 12,000 p.C/g, contains 1800 ppm of oxygen, and its Mg content is as low as 20 ppm. 

Sample Galvanostatic discharge C/Fg1 Cyclic voltammetry C/Fg1 Impedance spectroscopy C/Fg1 Specific capacitance uF cm 2... 

The KOH-activated carbons give excellent capacitance values reaching ca. 300 F g 1 despite a rather moderate specific capacitance per surface area, being in the range of 7-11 pF cm 2. Apparently, this can be... 

Table 1 contains the values of specific capacitance obtained when different amounts of carbon nanotubes are added to a-Mn02-nH20 for the preparation of the composite electrodes. 

Table 1. Specific capacitance in 1 mol.L 1 Na2SO i medium for a-Mn02nH20 loaded with...

Another important aspect for a material to be used as electrode for supercapacitors is its electrochemical stability. In Figure 2, presenting the specific discharge capacitance versus the cycle number for the optimized a-Mn02-nl FO/CNTs composite in 2 molL 1 KNO3 (pH=6.5), it can be observed that the specific capacitance loss after 200 cycles is about 20%. 

It is interesting to note in Figure 2 that the value of the initial specific capacitance is more important at high voltage, but it rapidly decreases with cycling. 

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.

The cycling data is summarized in Table 1. It becomes clear from the data in Table 1 that the initial specific capacitive parameters of some types of NGZ are higher than NGF by a factor of 1.5-2.0. Nevertheless, the values of N°max for NGF are considerably higher than those for NGZ. Thus, NGZ are less resistant to electrochemical oxidation than NGF. 

A variety of methods have been developed to study exocytosis. Neurotransmitter and hormone release can be measured by the electrical effects of released neurotransmitter or hormone on postsynaptic membrane receptors, such as the neuromuscular junction (NMJ see below), and directly by biochemical assay. Another direct measure of exocytosis is the increase in membrane area due to the incorporation of the secretory granule or vesicle membrane into the plasma membrane. This can be measured by increases in membrane capacitance (Cm). Cm is directly proportional to membrane area and is defined as Cm = QAJV, where Cm is the membrane capacitance in farads (F), Q is the charge across the membrane in coulombs (C), V is voltage (V) and Am is the area of the plasma membrane (cm2). The specific capacitance, Q/V, is the amount of charge that must be deposited across 1 cm2 of membrane to change the potential by IV. The specific capacitance, mainly determined by the thickness and dielectric constant of the phospholipid bilayer membrane, is approximately 1 pF/cm2 for intracellular organelles and the plasma membrane. Therefore, the increase in plasma membrane area due to exocytosis is proportional to the increase in Cm. 

The material properties of PS offer new ways of making electronic devices. For the manufacture of cold cathodes, for example, oxidized microporous polysilicon has been found to be a promising material. The application of basic semiconductor processing steps such as doping, oxidation and CVD to a macroporous material enable us to fabricate silicon-based capacitors of high specific capacitance. Both devices will be discussed below. 

The electrical characteristics of the SIKO are summarized below. The specific capacitance of the device shown in Fig. 10.20 is close to 4 pFV mnT3, and with smaller designs values of up to 20 pFV mnT3 are feasible. The range of manufacturable capacitance is not only a question of chip size but also of defect density of the ONO. Surprisingly, a defect density of porous structure, which is much smaller than the defect density of planar ONO layers, which is in the order of 0.1 cnT2. This low defect density can be understood if the defect density of planar films is assumed to be due to particles that do not penetrate into the pores. 

Fig. 7.11 Electrochemical performance of different carbons using a three-electrode cell in 1 mol L 1 H2S04 (a) cyclic voltammograms at a scan rate of 1 mV s 1, (b) galvanostatic charge/discharge curves at a current density of 0.2 Ag 1, (c) relationship of the specific capacitance with respect to the charge/discharge specific currents, and (d) Ragone plots.

Fig. 11.8 (a) Galvanostatic charge-discharge curve of the supercapacitor using G/CNTs (10 1) as the active material at constant current densities of 100, 250, 500, and 1000 mAg-1 using 30wt% KOH electrolyte, (b) Specific capacitance of 3D hybrid G/CNTs (10 1) measured at different current densities. Reprinted with permission from [88]. 

ZnO [79] have been used in ECs. These hybrids all show enhanced electrochemical performance in terms of the high reversible capacity of LIBs or specific capacitance of ECs, rate capability, and cycling performance. 

---