2.3. Electrochemical capacitors
2.3.1. Classification of supercapacitors
The complete assembled device, i.e. the electrode materials and electrolytes, influences the EC capacitive properties. Charges accumulate within the electrolyte at solid electrodes and the double layer thickness depends on the ion size and concentration, i.e. the former two determine the quantity of ions adsorbed in a given area [2.14,2.72]. The nature of the electrode material determines the capacitive behaviour of ECs [2.43,2.46,2.69]. A wide range of materials based on carbon nanostructured materials, conducting polymers and transition metal oxides, such as Ti, Mo, Mn, Ni, Ir, Co, Fe and V, have been used to fabricate various types of ECs [2.28].
They can be categorised according to a wider range of criteria such as electrode material, cell design and electrolyte [2.14]. Aqueous, non-aqueous or solid state are the three classes arising on the basis of the electrolyte system [2.73]. ECs are commonly classified into two categories, i.e. as pseudocapacitors and electrochemical double layer capacitors (EDLCs), subject to both the charge storage mechanism and material technology exploited in the fabrication of electrodes [2.10,2.46,2.74] (Figure 2.4) [2.8,2.43,2.66,2.75,2.76].
43 Figure 2.4. Types of ultra-capacitors.
2.3.1.1. Electrochemical double layer capacitors
The double layer concept was developed by von Helmholtz in the 1900s and General Electric pioneered the EDLCs in 1957 (Figure 2.5) [2.1,2.15,2.30]. Interest in EDLCs is particularly growing because of the drive towards ‘greener’ environments and sustainable energy systems.
An EDLC based on nano-structured carbons can be fabricated by using two carbon electrodes with high surface areas that are immersed in an appropriate electrolyte [2.34]. The separator must be permeable to ions; charge separation occurs over a small inter-phase space between the electrolyte and electrode. Unlike batteries, EDLCs are independent of chemical phase changes and heterogeneous charge transfers [2.77].
In EDLCs, charging of the double layer at the electrolyte-electrode interface forms the basis of the technique (Figure 2.5) [2.30,2.37]. They operate through a non-Faradaic process [2.37,2.75,2.78] which involves no chemical or compositional changes due to the absence of electron transfer activities [2.30]. Additionally, the thickness of the dielectric in a solid state
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capacitor is often higher than that in the Helmholtz layer in carbon nanostructured material- based capacitors, i.e. 0.5 – 1 nm [2.14,2.79]. Hence, EDLCs normally have rapid response and reversible interfaces with respect to changes in electrode potentials. Energy is stored electrostatically as opposite electric charges, i.e. through a charge separation mechanism (Figure 2.5) [2.37,2.75,2.77,2.80]. Electrolyte ions are reversibly adsorbed into the micropores of the electrode material [2.29,2.69]. The basic requirements for quick formation of a double layer in such capacitors are porosity and accessible surface area [2.29]. The ion movement, associated with the double layer formation in the pores, from the electrode via the electrolyte to the counter electrode, is facilitated by a diffusion process [2.1,2.29]. Some authors view EDLCs as substitutes for dielectric capacitors [2.12]. ECs have a higher energy density by virtue of their quick response, high ion mobility in electrolytes and inverse proportionality relationship with double layer thickness, but they usually have a low power density [2.78,2.81, 2.82].
Figure 2.5. Representation of an EDLC illustration of Helmholtz layer.
Carbon nanostructured material-based EDLCs are highly conductive, porous, associated with low costs and ease of processability, and are electrochemically stable [2.75,2.83]. Related energy storage capability is restricted by the electrostatic nature of the finite electrical charge separation at the interface, but they store more energy per unit volume/mass [2.76]. Use of
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carbon nanostructures in this regard is affected by oxidation during recycling and ageing, but can exceed 500 000 cycles [2.37,2.76,2.77]. This aspect will be reviewed in later sections.
2.3.1.2. Pseudocapacitors
Contrary to EDLCs, pseudo/redox capacitors utilize both double layer charge and Faradaic reactions [2.43]. Redox capacitors are battery-like in behaviour but are non-electrostatic in nature and transfer charge electrochemically [2.1,2.37,2.44,2.80]. Charge is transported near the exterior or in bulk region close to the electrode surface [2.29]. The proposed mechanism involves space charge density increase and attraction of protons [2.84]. They are based on electrode charge storage mechanisms and require highly reversible surface redox processes between electrode-electrolyte interfaces or through intercalation of ions [2.37,2.75,2.80,2.85].
Their performances are governed by ion kinetics and electrode electron transport as well as the electrode-electrolyte interface [2.86]. The electrolyte ions participate in three main processes at the working electrode of a pseudocapacitor, i.e. electrosorption, redox reactions and intercalation [2.29,2.30]. Their intercalation is a bulk process largely dependent on electrode porosity whilst the other two are surface activities with a high dependency on surface area.
Pseudocapacitors have a high energy storage potential due to the associated specific capacitance (Cs), high power density, low self-discharge, safe operation, high cycling stability and fast charge/discharge capability [2.28,2.75,2.76]. Hence, their applications include aeroplanes, subway trains, power electronics and smart grids. Conversely, pseudocapacitors have low porosity, low mechanical strength and conduct electricity poorly.
The active electrodes of pseudocapacitors are mostly either transition metal oxides or based on conducting polymers [2.30,2.52]. The capability of carbon nanostructured materials in supercapacitors can be enhanced by the use of transition metal-based oxides and conducting polymers such as polyaniline and polythiophene [2.46]. Pseudocapacitors fabricated from transition metal oxides have high energy density and large charge transfer reaction characteristics; this is due to variable oxidation-state structures. For example, noble metal oxide-based capacitors of approximately 700 F g-1 have been reported [2.28]. Conducting RuO2 and IrO2 were the most preferred in ECs and have found use in space charge and military applications [2.14]. However, cost and economic reasons, such as 90% of the cost being linked
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to the electrode material, point to the necessity of exploring other alternatives. Also, other options, that can offer decent electrical conductivity with sufficient surface area and high capability to participate in Faradaic EC reactions, are required for energy storage in distinctive devices [2.28].
Carbon nanostructured materials such as carbon nanotubes (CNTs) and graphene (G), enhance the charge transfer efficiency of pseudocapacitors [2.76]. If functionalised, the acidic oxide surfaces serve as adsorption sites for polar molecules and hence improve wettability and hydrophilicity. Consequently, the acid-base nature of carbon nanomaterials enhances the pseudocapacitive behaviour. Thus, modifying the surface chemistry and porous nanostructure of carbon materials, particularly man-made carbon nanostructured materials, is necessary especially for generating better functionality at high scan speeds and durability for continuous short and long pulse power intervals. The characteristics of the earlier two mentioned classes of capacitors can be combined into one device (Figure 2.4) to make a new class, hybrid electrochemical capacitors.
2.3.1.3. Hybrid electrochemical capacitors
Composites of carbon nanostructured materials and other nanomaterials combine EDLC and pseudocapacitance features in hybrid electrochemical devices [2.10,2.30,2.85]. Each ESS method and EC, in particular, has their own strengths, limitations and operational characteristics [2.6]. Andrew Burke’s [2.29] view is that separating energy and power requirements via peak power supply with the use of a capacitor charged periodically using an energy storage unit could be a reasonable approach to sustainable energy. Hence, combinations of ECs with either fuel cells or batteries produce advanced hybrid power systems [2.7,2.87].
Various designs of hybrid EC devices bridging the gap between batteries, fuel cells and EDLCs have been reported. The design of their device is an example of an internal series hybrid capacitor since the two electrodes function with different mechanisms [2.88]. Likewise, the components of hybrid capacitors can have mechanisms of both pseudocapacitors and EDLCs [2.29].
Hybrid electrochemical devices rely on Faradaic charge transfer processes of surface atoms and non-Faradaic processes [2.28,2.30]. Examples of such capacitors, according to their
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electrode configurations, include nano-hybrids, battery-like and asymmetrical ECs (AECs) [2.30] (Figure 2.4). An AEC is a combination of an EDLC and a battery in order to improve cycleability and energy density [2.66,2.70]. They have two different active materials combined to make one device. The transition metal electrode relies on reversible redox Faradaic processes whereas the counter carbon-based electrode is influenced by reversible surface adsorption/desorption activities occurring via non-Faradaic mechanisms. Advantages of AECs over EDLCs include their slower self-discharge rate and this arises from the lithium revocable redox reactions. Hence, amongst other reasons, commercial automobile companies such as Honda, Nissan, Chrysler and General Motors have become increasingly interested in typical hybrid systems [2.64].