Batteries and Supercapacitors

Electrochemical devices have come in recent years to the forefront in many applications. An example is the provision of electrical energy for electrical vehicles, where high energy storage density is provided by (rechargeable) batteries. Another example is pulsed lasers, where high power, as delivered by supercapacitors, is needed. 

The choice of the electrolyte devolves around its adequate solubility on the one hand and avoidance of ion pairing in the resultant solutions on the other in order to provide adequate conductivity of the solution. Electrolytes, such as H SO, KOH, NaClO, LiClO, LiAsE, or quaternary phosphonium salts may be dissolved in water as the solvent to produce useful EDLCs. However, the use of aqueous solutions limit the voltage of the supercapacitor to 2.3 V. Organic solvents provide a somewhat larger applicable voltage, 2.7 V, and a wider temperature range than obtainable with aqueous solutions. The criteria listed above for the selection of the solvent apply to supercapacitors as weU. 

Acetonitrile and propylene carbonate (PC) are currently widely used in commercial devices. Acetonitrile-based electrolytes have high electrochemical stability and high conductivities (due to low viscosity) even at low temperatures (down to -40°C). On the other hand, this solvent has a low flashpoint (6°C) and is toxic and hence 

TABLE 8.1 Comparison of Features of Widely Used Batteries 

Acetonitrile and PC were tested with a quaternary ammonium tetrafluoroborate as the electrolyte in devices described by Beguin et al. [6]. The ions of the electrolyte need to be able to enter the pores of the electrodes and hence should not be too large, or else must be (partly) desolvated in order to fit the pores of the activated carbon electrodes. As for batteries, asymmetric hnear sulfones and dinitriles may permit operation at higher voltages than acetonitrile and PC permit. 

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M. Fujiinoto, K. Ueno, T. Nouina, M. Takaha-shi, K. Nishio, T. Saito, Proc. Symp. on New Sealed Rechargeable Batteries and Supercapacitors, 1993, p. 280. 

JT. Takamura, M. Kikuchi, J, Ebana, M. Naga-shima, Y. lkezawa, in New Sealed Rechargeable Batteries and Supercapacitors (Eds. B. M, Barnett, E. Dowgiallo, G, Halpert, Y. Matsuda, Z. Takehara), The Electrochemical So-... 

Conducting polymers have found applications in a wide variety of areas,44 45 and many more have been proposed. From an electrochemical perspective, the most important applications46 appear to be in batteries and supercapacitors 47,48 electroanalysis and sensors49-51 electrocatalysis,12,1, 52 display and electrochromic devices,46 and electromechanical actuators.53... 

X.Y. Song, Xi Chu and K. Kinoshita, in New Sealed Rechargeable Batteries and Supercapacitors, MRS Symposium Proceedings Volume 393, Materials Research Society, Warrendale, 1995, p321. 

Graphite finds wide range of applications in the electrodes for certain types of rechargeable batteries and supercapacitors, in electro-sorption/desorption electrodes, as anodes in a number of processes of... 

Graphite materials produced at 600-1100°C may find applications in lithium batteries and supercapacitors. Currently, similar flakes are produced in a complex process including graphitization at above 2500°C,16 followed by intercalation and exfoliation of graphite15. Here we demonstrate that synthesis of graphite from iron carbide can be done in one step at moderate temperatures. 

If the supporting electrolyte and the electrode material are chosen appropriately, the potential window in such protophobic aprotic solvents as AN, NM, PC and TMS easily exceeds 6 V (Table 8.1, see also 15) in Chapter 8). In aqueous solutions, the potential window never exceeds 4.5 V, even when a mercury electrode is used on the negative side and a diamond electrode on the positive side. This difference is important not only for electrochemical measurements but also for electrochemical technologies of, for example, rechargeable batteries and supercapacitors. For more information on the potential windows in non-aqueous solutions, see Ref. [10]. 

Detailed information about batteries and supercapacitors can be found in specialized textbooks [1-5]. Some useful concepts are reviewed below. 

Carbon science and electrochemistry are interconnected since the early days of both disciplines [1]. Electrochemistry provides significant inputs for characterization and, eventually, practical applications of carbon materials, e.g. in Li-ion batteries and supercapacitors. The discovery of fullerenes and nanotubes promoted further electrochemical research on carbons in general... 

Porous carbons and nanotubes have attracted considerable attention in relation to such practical issues as hydrogen storage, lithium batteries, and supercapacitors. In general, the electrochemical behavior of porous carbons and CNTs solely consists of double-layer charging processes with small or zero contribution of faradaic pseudocapacitance of surface oxide functionalities. This is in sharp contrast with the rich electrochemistry of fullerenes. 

In a nonexhaustive way, this chapter shows that Li-ion batteries and supercapacitors are very important electrical energy storage systems, where the carbon material plays a central role in the performance. Lately, many types of carbons have been investigated more or less empirically in these cells. However, the works performed recently pay a special attention to find correlations with specific parameters of nanostructured carbons, which is rather difficult because of the highly disordered state of these materials. 

For instance, if there are high amp draws from motor start ups, etc., put a supercapacitor in parallel connection with a 12 volt rechargeable battery, and use this to supply those intermittent load needs adequately. To use a rechargeable battery alone, as mentioned, simply connect the output from the fuel cells to the battery and draw power from the battery. To use a supercapacitor and rechargeable battery, connect the battery and supercapacitor in parallel, connect the fuel cell output to these, and draw your power from the supercapacitor and battery connected leads. With these system additions you will need a diode so that reverse flow does not occur to the fuel cell stack, and fuse the circuit on both sides in case of shorts. A switch, either remote or direct, should be used to connect the power supply with any lines or equipment being powered. If you have AC power requirements you will need an inverter to convert DC to AC electricity. 

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