Capacitors are inactive electrical constituents that comprise of two conducting plates disconnected with a dielectric material (Figure 2.2a). They form part of our everyday lives and their common applications include television satellite receivers, television channel setting, car audio systems, taxi meters, process controllers, cameras and mobile phones, among others [2.14]. Hence, companies, such as Panasonic, Maxwell, ELIT, US Army and Siemens Matsushita, are investing in capacitors (Figure 2.2b) [2.14,2.28-2.30]. Capacitors store electrostatic charges, a form of electrical energy, in the Helmholtz layer with distribution of equal and opposite charges on each plate [2.1,2.14,2.29]. The plates are the terminals of the ECs and their performance is influenced by electro-active surface area and the separation between electrode charge and electrolyte charge [2.1]. The energy storage mechanism in ECs is a physical process devoid of chemical changes. Therefore, stored energy is released upon connecting a capacitor to an external circuit and the process is reversible on charging.
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Supercapacitors are associated with some self-discharge caused by various transfer mechanisms or some device imperfections. One disadvantage associated with EC designs, among others, is the need to critically consider mass balance to avoid masking the performance of one of the components, i.e. charge balance [2.31,2.32]. Also, performance is influenced by design parameters such as the thickness of the ultimate electrode.
Figure 2.2. (a) A schematic diagram showing a typical capacitor [2.28] and (b) images of commercially available capacitors [2.14].
In reality there exists an inverse relationship between energy and power [2.33], i.e. a higher rate of releasing energy implies a higher power, and this is associated with smaller energy capacity [2.28]. There is need for balance between these two phenomena of ECs and, therefore, capacitors are an interesting area of research.
The dielectric medium, an electrolyte in ECs, determines the permittivity and therefore influences or is affected by the electric field [2.14]. In simple terms, it is the quantity of electric flux generated per unit charge and high amount implies low electrical flux. Hence, the capacity of a material to polarise in response to the inside field influences its permittivity. The maximum charge storage in this context is determined by the breakdown characteristics of the dielectric medium [2.29]. Furthermore, in case of carbon nanostructured material-based ECs, the voltage window is subject to the electrolyte solvent [2.34]. This is signal emphasises the importance of dielectric materials, i.e. electrolytes in capacitors. The energy generation
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processes occurring at the electrolyte/electrode interphase of supercapacitors are similar, though the mechanisms are different, to those of fuel cells and batteries [2.35,2.36].
Batteries have numerous applications and this is due to a variety of reasons, such as lower cost per unit charge [2.37], more established markets with commercial availability [2.7], and the ability to store enormous energy quantities in comparatively lesser weight and volume units [2.1]. Anticipated energy and power outputs are obtainable by connecting batteries in series or parallel [2.11]. However, batteries have several disadvantages relative to supercapacitors and these include the use of toxic heavy metals, limited charge/discharge abilities and small power densities [2.1,2.38,2.39]. Supercapacitors are preferred to batteries where high power is needed because they release energy at faster rates [2.1]. Also, short shelf and cycle life are common problems associated with battery use [2.5,2.39]. These shortcomings have become globally acceptable due to lack of competitive alternatives. However, the fast growing and competitive markets, compels scientists and engineers in industry, as well as in academia, to search for alternatives that can enhance battery capacity amongst other aspects [2.40].
The average life span of conventional capacitors is about five years and they have a 60-70%
cycle efficiency [2.41]. Supercapacitors have several pluses over other energy storage systems, especially fuel cells and batteries, such as high safety levels, absence of rotating parts and cooling requirements, large modularity in terms of voltage and capacitance, less chances of discharging on their own, high recyclability, low production cost and no service requirements [2.14]. Supercapacitors undergo reversible charge storage processes, have better power density, longer life cycles, lower internal resistance and a broader range of working temperatures compared to batteries (106 W dm-3) [2.14,2.42-2.46]. Supercapacitors undergo self-discharge over a period of time and this lowers their voltage in a similar manner to batteries but, unlike batteries, without degrading, and thus they retain their maximum capacitance [2.29].
The uppermost power density of supercapacitors can be 100-fold more than the power output of batteries [2.1]. Compared with fuel cells, supercapacitors have a higher efficiency of recovery of 85-98% than the 25-85% of fuel cells [2.5]. Disadvantages of fuel cells over supercapacitors include lower power density per volume, lower durability, and shortened cell- life due to pulse strains and impurities associated with the gas stream [2.35]. Fuel cells have a reduced dynamic response due to the involvement of air compression at the cathode [2.47] and, hence, often require supercapacitors to achieve maximum power outputs [2.39]. They are still behind both batteries and supercapacitors in terms of market development [2.35]. Unlike fuel
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cells, supercapacitors do not involve expensive catalysts, hence they have lower capital costs [2.5,2.35]. However, although fuel cells utilise more toxic and corrosive electrolytes than supercapacitors, they exist in several forms and are therefore suitable for more applications.
The major forms of electrical capacitors are electrochemical, electrolytic and electrostatic [2.33], and these are discussed in the following sections. A pair of conductors disjointed by a dielectric material [2.14] constitute electrostatic capacitors and energy is stored in the form of separated electrical charges [2.1]. Common dielectric media of electrostatic capacitors include air and mica. Electrostatic capacitors have a charge time of approximately 10-9 s and store the lowest energy relative to the other two. Hence, more ideas and research inputs are still required in this regard.
Electrolytic capacitors are made -up of a narrower and greater dielectric constant material built on a robust metal [2.14,2.48]. Their response time is approximately 10-4 s and have ten-fold better energy storage than electrostatic capacitors. This makes them commercially preferred to electrostatic capacitors and they can be applied in small electronic devices [2.49]. However, the interest of this review is towards ECs often referred to as supercapacitors in literature [2.50].