Electrochemical Supercapacitors for Energy Storage and Conversion

Transcription

1 Electrochemical Supercapacitors for Energy Storage and Conversion Brian Kihun Kim 1, Serubbable Sy 1, Aiping Yu 1, and Jinjun Zhang 2 1 University of Waterloo, Waterloo, Canada 2 National Research Council Canada, Vancouver, Canada 1 INTRODUCTION With the increase in energy demand, developing clean, sustainable, and efficient energy storage and conversion technologies has become one of the necessary approaches for the world communities of science and technology. Among different energy storage and conversion technologies, electrochemical ones such as batteries, fuel cells, and electrochemical supercapacitors (ESs) have been recognized as important. Particularly, the ES, also known as supercapacitor, ultracapacitor, or electrochemical double-layer capacitor, can store relatively higher energy density than that of conventional capacitor. With several advantages, such as fast charging, long charge discharge cycles, and broad operating temperature ranges, ESs have found wide applications in hybrid or electric vehicles, electronics, aircrafts, and smart grids. There are nevertheless still some challenges in ES systems such as relatively low energy density and high manufacturing cost. 1.1 Conventional capacitors and electrochemical supercapacitors A conventional capacitor, also known as a condenser or an electrostatic capacitor, is an energy storing device consisting of two electrically conductive plates (sometimes called electrodes), which are separated by a dielectric layer. The dielectric materials are insulators such as ceramic, glass, paper, This article was published in the Handbook of Clean Energy Systems in 2015 by John Wiley & Sons, Ltd. plastic, and aluminum oxide (Al 2 O 3 ). The charging process of capacitors is simple. When the two conductive plates are connected to an external power source, which induces a potential difference between the two plates, positive charges accumulate on one plate and negative charges build up on the other plate. The charges remain on their corresponding plates even after the removal of the external power source; this is the charged state of a capacitor. During discharge, the capacitor releases the positive/negative charges to a connected resistive load to deliver its stored energy. However, the applications for these conventional capacitors are limited by their low energy capacity. As a result, the search for a new material led to a new type of capacitor called supercapacitors or ECs. Unlike conventional capacitors, ES electrodes are normally composed of high surface porous materials such as carbon particle materials and the separator is either solid or liquid, thus electrode/electrolyte interfaces are generated. These interfaces, called electric double layer, have higher surface area than dielectric capacitors and thus can store more charges. It is recognized that the improved structure of an ES allows better energy storage than conventional capacitors. Regarding the detailed discussion about the fundamentals of ES, a section is presented to take care of that. Before diving into the ES principles, it would be beneficial to briefly learn about the history of this energy storage device. 1.2 History In antiquity, the phenomenon of rubbing amber with a cloth was found to attract small particles. In 1745, this unexplained phenomenon was used to develop the very first known capacitor: the Leyden jar. At that time, the

2 2 Electrochemical Storage Leyden jar comprised glass where its interior and exterior surfaces were laminated with metal foils. The jar itself served as a dielectric material, while the laminated surfaces acted as conductive plates (Yu, Chabot, and Zhang, 2013). Since then, capacitor technology rapidly evolved through steady research. In 1957, the very first ES was patented by Howard Becker of General Electric (GE) (Becker, 1957). He pioneered a capacitor with a porous carbon electrode and sulfuric acid as its electrolyte, forming an electric doublelayer system. However, GE did not pursue this development. Meanwhile, in 1970, Standard Oil of Ohio (SOHIO) patented an electrolytic capacitor based on carbon (Boos, 1970). Nippon Electric Company (NEC) licensed the patent from SOHIO and marketed aqueous electrolyte capacitors as ESs (Endo et al., 2001). Subsequently, NEC acquired the first commercial success by promoting supercapacitors as backup memory for electronics. In the late twentieth century, numerous companies initiated the production of supercapacitors to compete in the market. Pinnacle Research Institute (PRI) designed supercapacitors with low internal resistances for high powered portable energy storage (Pandolfo and Hollenkamp, 2006). In 1992, Maxwell Technologies took over PRI s development and manufactured their own supercapacitors named Boost Caps. This continuous research has led to today s high performing commercially available supercapacitors. 1.3 Importance of electrochemical supercapacitors ESs possess advantages that complement the many deficiencies of other commercial energy storage devices, which in return have aroused great interest academically and commercially. ESs are capable of generating higher power densities than fuel cells and batteries and greater energy densities than conventional capacitors. Figure 1 shows a Ragone plot of specific power with respect to the specific energy of significant energy storage and conversion devices. From the plot in Figure 1, it can be seen that supercapacitor technology can evidently bridge the gap between batteries and capacitors in terms of both power and energy densities. Furthermore, supercapacitors have longer cycle life than batteries because the chemical phase changes in the electrodes of a supercapacitor are much less than that in a battery during continuous charging/discharging (Yu, Davies, and Chen, 2012). These key attributes make supercapacitors more attractive and versatile as high powered energy storages. The US Department of Energy (DOE) has spotlighted batteries and supercapacitors as major future energy storage technologies (Goodenough, 2007). Power density (W kg 1 ) Capacitors Supercapacitors Batteries ,000 Energy density (Wh kg 1 ) Fuel cells Figure 1. Ragone plot for significant energy storage and conversion devices. 1.4 Application market and economy of electrochemical supercapacitors The earliest application of ESs was a backup power supply for electronics. On one hand, supercapacitors, capable of discharging large amounts of power in a matter of seconds, are ideal for supplying instant and uninterruptable backup power in response to energy surges or a shutdown. Batteries, on the other hand, are less-than-ideal for this type of application because they are more expensive and may induce uncontrollable temperature escalations (Yu, Chabot, and Zhang, 2013; Yu, Davies, and Chen, 2012). Supercapacitors with its long cycle life and high power delivery are applicable to both consumer and military devices. Coleman s portable cordless screwdrivers, which are powered by supercapacitors, are currently listed on the market for home usage. This power tool is fully charged within 90 s for immediate use (Miller and Burke, 2008). In military applications, supercapacitors are generally implemented and used as alternative power for electronics in armed vehicles, black boxes on helicopters, and so on (Yu, Chabot, and Zhang, 2013). Energy recovery in public transportations and hybrid electric vehicle (HEV) has further reenergized interest in supercapacitors. This is because the primary challenge for public transportation was harnessing the regenerative energy when braking for the frequent stops and reusing the stored energy when accelerating to depart. Supercapacitors are capable of storing instantaneous brake energy and discharging upon demand, which improves fuel efficiency. Moreover, current HEVs encounter temperature stability challenges where the charging/discharging mechanisms

3 Electrochemical Supercapacitors for Energy Storage and Conversion 3 in HEVs generate undesirable heat from resistances and exothermic reactions. Possessing the ability to dissipate heat effectively, supercapacitors can be implemented into HEVs to manage the thermal issues, thus enabling them easier entry into the HEV market (Miller and Burke, 2008). 1.5 Current research and development status In spite of the rapid progression, supercapacitor technology still faces the low energy density challenge. In order to overcome this challenge, governments, industries, and academic organizations around the world are making great efforts to develop novel and high performing electrode and electrolyte materials for supercapacitors. In addition, some efforts are also putting on the system design and performance optimization for some niche applications. For example, many research groups have developed special carbon and polymer materials to fabricate flexible supercapacitors for applications in electronic devices, as shown in Figure 2 (Yuan et al., 2012; El-Kady et al., 2012; Davies et al., 2011; Meng et al., 2010). In addition, some effort has also been given to the hybridization of the electrode to increase the electrode s (a) (c) 1 cm Figure 2. (a) Carbon nanoparticles/mno 2 nanorods composed all solid-state supercapacitors. Source: Reproduced with permission from Yuan et al., American Chemical Society. (b) Flexible graphene/polyaniline nanofiber composite film. Source: Reproduced with permission from Wu et al., American Chemical Society. (c) Graphene-based flexible supercapacitors with electropolymerized polypyrrole. Source: Reproduced with permission from Davies et al., American Chemical Society. (d) Flexible all-solid-state paper-like polymer supercapacitors. Source: Reproduced with permission from Meng et al., American Chemical Society. (b) (d) capacitance. In this approach, battery and supercapacitor electrode materials are integrated together to yield higher energy density hybrid supercapacitors. The results showed that an improvement in energy density without compromising its high power density, which is inherent in supercapacitors, could be achieved with this kind of hybrid supercapacitors (Wang and Xia, 2013; Lang et al., 2011). However, these advantageous properties must always be balanced against the decrease in cycle stability and loss in power performance. Extensive research will continue to overcome these challenges and evolve the supercapacitor technology to be superior and eventually dominate the energy storage market. 2 TYPES OF SUPERCAPACITORS As discussed in the previous section, in the mid-1900s, researchers pioneered electrostatic double-layer capacitors, which are capable of accumulating exceptionally high amounts of charge. These kinds of capacitors were later called ESs because the charges stored inside the device reside within the electric double layer at the electrode/electrolyte interfaces. With rapid growths of mobile electronics and hybrid vehicles, research in ES technology developed other two additional variations: pseudocapacitors and asymmetric supercapacitors. The following sections explain the energy storage mechanisms behind conventional capacitors and the three categories of ESs, such as electrostatic doublelayer supercapacitors, pseudocapacitors, and asymmetric supercapacitors. 2.1 Conventional capacitors Conventional capacitors, formerly referred to as condensers, store energy electrostatically. Manifold forms of capacitors exist commercially, but their fundamental design is similar. Capacitors are generally composed of dielectric materials such as glass, plastic, ceramic, and paper, which separates two electrically conductive plates (electrodes). Inducing a potential difference across the plates can generate an electric field in the dielectric layer that accumulates equal magnitudes of positive and negative charges, as shown in Figure 3. Capacitance is the measure by which capacitors store energy through this arrangement of charges. Capacitance (C) is measured in farad (F) and can be calculated by C = A ε d, (1) where A is the surface area of the plates, ε is the permittivity of the dielectric material, which measures the resistance

4 4 Electrochemical Storage (d) Dielectric (ε) Plates (A) Figure 3. Arrangement of charges in a dielectric material during the charged state of conventional capacitors. Electrodes Separator : Electrolyte Ions : Interface Figure 4. A schematic of charged EDLCs. of the material during the formation electric fields, and d is the distance between the plates. Commercial capacitors can store energy in the range between pico- and microfarads. 2.2 Electrostatic double-layer capacitor (EDLC) EDLCs share a similar mechanism with conventional capacitors. However, instead of storing charges in the dielectric layer, EDLCs utilize the interfaces between the electrode and the electrolyte for their energy storage. As shown in Figure 4, a potential difference across the EDLCs induces electrodes with different polarity, leading to the migration of the electrolyte ions to the micropores of the electrodes. Unlike conventional capacitors, the capacitance is determined by the thickness of the separator (d in Equation 1). EDLC s capacitance is determined by the thickness of the double layer at the electrode/electrolyte interface (Figure 4). The thickness of the double layer is much smaller than that of the separator, indicating that EDLC must have a much higher capacitance than that of conventional capacitor according to Equation (1) (Zhang and Zhao, 2009). In addition to shorter separation distance, significantly larger area of electrode/electrolyte interface than that in conventional capacitor can further give increased capacitance of EDLC. In addition, excellent electrical conductivity of the electrolyte in EDLC can reduce the internal resistance of electrodes, while good wettability of the electrolyte will facilitate increased mobility of ions into the pores of electrodes to further enhance the capacitive performance of EDLCs (Zhang and Zhao, 2009; Yu, Davies, and Chen, 2012). Normally, a specific type of abundant and inexpensive carbon known as activated carbon (AC) is widely used in commercial EDLCs. ACs can exhibit specific capacitance values of F/g in organic electrolyte (Simon and Gogotsi, 2008). To further improve the capacitance of AC material, further advances have engineered sophisticated carbon structures, such as ordered mesoporous carbon, carbon nanotubes (CNTs), and graphene. These materials are generally more expensive than ACs; however, their capacitive capabilities surpass that of ACs. For example, Zhu et al. (2011) synthesized KOH-activated graphene that demonstrates a superior specific capacitance of 166 F/g in organic electrolyte. 2.3 Pseudocapacitor In order to increase the capacitance of a supercapacitor, some electrochemical active redox material can be composited with the carbon material such as AC to make electrode materials for supercapacitor. In this way, the electron storage at the electrode/electrolyte interface of EDLC is not simply a physical process, some fast reversible oxidation/reduction reaction(s) will occur to give times more capacitance than that of pure carbon-based EDLC (Conway, 1991). This kind of ES is called pseudocapacitors. One of the more thoroughly explored materials for pseudocapacitors is ruthenium oxide. This precious metal has multiple redox phases [i.e., Ru(IV)/(III) and Ru(III)/Ru(II)]

5 Electrochemical Supercapacitors for Energy Storage and Conversion 5 Load Graphene/MnO 2 e e e e e e e e ACN 50 nm Cation MnO 2 1 μm Ni foam Separator Figure 5. Schematic of an asymmetric MnO 2 -graphene/ac supercapacitor. Source: Reproduced with permission from Fan et al., John Wiley & Sons, Ltd. in proton-rich environments [e.g., sulfuric acid (H 2 SO 4 )], supplying more electron transfers for greater capacitance (Zhang and Zhao, 2009; Conway, 1999). For example, Hu et al. (2006) synthesized RuO 2 nanotubes that gave around 1300 F/g in H 2 SO 4 electrolyte. Unfortunately, although this RuO 2 electrode material can give outstanding performance, its toxicity, high cost, and scarcity limit its practical application in supercapacitors. In this regard, research and development have shifted their resources to less toxic and inexpensive alternatives: nonprecious transitional metal such as cobalt oxide, nickel oxide, and conductive polymers. For example, cobalt oxide nanowires displayed capacitance of F/g (Gao et al., 2010) and nickel oxide nanoflowers presented a capacitance range of F/g, respectively (Yuan et al., 2009; Kim, Chabot, and Yu, 2013). A conductive polymer, polypyrrole (PPY), produced by Zhang et al. (2010) demonstrated capacitance values of F/g. Although these alternatives could exhibit promising performances, their low conductivity (relative to carbonaceous materials) and lack of long cycle stability seemed to inhibit their application in pseudocapacitors. To address these drawbacks, carbonaceous additives were explored to integrate into the electrode for improved performance (Kim, Chabot, and Yu, 2013). 2.4 Asymmetric supercapacitor As the name implies, an asymmetric supercapacitor is configured with dissimilar electrodes: a battery-like Faradaic electrode and a capacitive carbonaceous electrode. This unique design can adjust the operating voltage window and increase the energy density due to the electrochemical redox process at the Faradaic electrode (Malak et al., 2010). However, optimization of both electrodes through careful design is mandatory. The prototype of asymmetric supercapacitors was invented by Amatucci et al. (2001) and it was further researched by many groups. For example, Wu et al. (2010) assembled a high energy density asymmetric supercapacitor from MnO 2 nanowires and graphene with an energy density of 30.4 Wh/kg. In comparison with symmetrically configured supercapacitors composed of graphene/graphene (2.8 Wh/kg) and MnO 2 /MnO 2 (5.2 Wh/kg), the asymmetric MnO 2 /graphene cell reported by Wu et al. displayed a significant superiority in energy density. Fan et al. (2011) further improved the energy density by incorporating MnO 2 into graphene as an electrode and complementing it to an AC nanofiber-based electrode. Figure 5 illustrates the design of the MnO 2 graphene/ac supercapacitor that possesses an outstanding energy density of 51.1 Wh/kg. Furthermore, Fuji Heavy Industry designed a commercial lithium-ion-doped AC/AC asymmetric supercapacitor that could supply an energy density up to 25 Wh/kg (Naoi and Simon, 2008). It should be noted that although the energy densities of asymmetric supercapacitors could be increased when compared to those of symmetric supercapacitors, implementing the Faradaic materials into electrodes to fabricate asymmetric supercapacitors may lead to the reduced cycle stability. Nevertheless, asymmetric supercapacitors have great potential for future energy storage devices in terms of energy density improvement. 3 SUPERCAPACITOR COMPONENTS AND MATERIALS Optimizing supercapacitor design will typically enhance performance. The optimization includes careful selection of electrodes, electrolyte, conductive current collectors, and

6 6 Electrochemical Storage sealants. Matured engineered designs should not be contingent on performance but should also consider cost and safety. 3.1 Supercapacitor components and materials As briefly mentioned in Section 2, depending on electrode materials, supercapacitors can utilize two energy storage mechanisms. EDLCs, which are primarily composed of carbonaceous materials, statically deposit charges within the porous structures of electrodes. Pseudocapacitive supercapacitors, which consist of transitional metal oxides/nitrides and conducting polymers such as electrodes, accumulate energy through fast, reversible electrochemical redox reactions on the active surfaces of the electrodes. Both mechanisms share common qualifications for selecting appropriate materials in electrodes, which include the following: 1. Large surface area and porosity 2. Good surface wettability 3. High electrical conductivity 4. Long cycle stability (>10 5 cycles) 5. Facile manipulation of morphology (e.g., pore sizes, pore distributions, particle dimensions/distributions) 6. Thermodynamic stability for a wide operational potential range EDLC electrode materials The widely accepted materials for EDLC electrodes are carbon. Its low manufacturing cost, abundance, large surface area, controllable morphology, and high electrical conductivity are ideal characteristics of EDLC electrodes. Furthermore, carbon materials can be post-treated to alter their structures and chemical/mechanical properties for various applications. Common classifications of modified carbon include AC, CNT, and graphene Activated carbon (AC). In general, current commercially available supercapacitors utilize AC as their electrode material. AC is processed either via thermal or chemical activation of carbonaceous precursors such as petroleum pitches, coals, woods, and hard-shells. Potassium hydroxide (KOH) activation is one of the most prevalent techniques used to produce AC due to its low processing cost. During the activation, sufficient energy is applied to small hexagonal carbon rings, which are also known as graphene sheets. This energy breaks the linkage between these sheets, deforming the structure and creating pores (Qu, 2002). The creation of pores increases the surface area, which contributes to the high capacitive performance of AC. AC can store specific capacitance between 100 and 200 F/g in aqueous electrolytes and 50 and 150 F/g in organic electrolytes (Frackowiak, 2007). Wang et al. (Wang, Wang, and Liang, 2003) and Wen et al. (2009) achieved capacitances of 160 and 225 F/g, respectively, with KOH-treated AC in aqueous electrolytes Carbon nanotubes (CNTs). CNTs are engineered carbon with superior properties compared to AC. CNTs are composed of graphitic walls assembled in nearone-dimensional (1-D) cylinders (Figure 6). The thickness of the tubes divides CNTs into single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). The long 1-D structure of CNTs offers excellent mechanical properties and prevents the scattering of electrons, exceeding the electrical conductivity of AC (Baughman, Zakhidov, and De Heer, 2002). Highly ordered CNT arrays have significant surface area, offering large porous area for electron storage (Hu et al., 2009). Depending (a) (b) 85 nm 1 μm Figure 6. (a) Transmission electron microscope (TEM) image of catalytically grown carbon nanotubes. Source: Reproduced with permission from Niu et al., AIP Publishing LLC. (b) Scanning electron microscope (SEM) image of a carbon nanotube forest. Source: Reproduced with permission from Lau et al., American Chemical Society.

7 Electrochemical Supercapacitors for Energy Storage and Conversion 7 on morphology and purity, the specific capacitance of CNTs ranges between 15 and 200 F/g (Liu et al., 2010b). Unfortunately, a major deterrent from the potential usage of CNTs in commercial supercapacitors is its high production cost. Carbon arc discharge and chemical vapor deposition are currently implemented techniques in producing lab-scale amounts of high purity CNTs (Ding et al., 2001). However, these techniques require further research before scaling up production and purification Graphene. Possessing high thermal/electrical conductivity, mechanical strength, chemical stability, and large surface area, graphene distinguishes itself from other competitive carbon materials. Recently, it became the rising advanced material for supercapacitor applications (Wang et al., 2009b). Graphene is a flat 2-D honeycomb-shaped monolayer sheet of carbon atoms that is the basic building blocks of other carbon materials as illustrated in Figure 7 (Geim and Novoselov, 2007). Graphene may possess a theoretical surface area of up to 2675 m 2 /g and translates to 550 F/g if all of the theoretical area is fully utilized (Liu et al., 2010a). However, as with all nanostructures, especially graphene sheets, agglomeration and restacking limit the specific capacitance of graphene between 100 and 200 F/g. To reduce the restacking, Wang et al. (2011) incorporated CNTs as a spacer between 2-D graphene sheet graphene sheets (Figure 8a). This drastically improved the capacitance to 318 F/g. Other techniques such as KOH-activated graphene by Zhu et al. (2011) exceeded the theoretical surface area of graphene, achieving 3100 m 2 /g through exposing hidden sheets and creating extra pores. Hassan et al. (2013) doped the graphene planes with nitrogen atoms (Figure 8b) to increase the electrical conductivities and promote graphene ion interaction in electrolyte solutions. The nitrogen-doped graphene demonstrated a capacitance value of 194 F/g Pseudocapacitor electrode materials Transition metal oxides and electrically conductive polymer are commonly selected as the electrode materials for pseudocapacitors. Pseudocapacitor electrodes utilize redox reactions on the surfaces of electroactive materials. The redox reactions are electrode potential dependent and change with charging and discharging. This mechanism provides superior capacitance and energy density compared to purely carbon-based EDLCs. However, these advantageous properties are normally counterbalanced with poor life cycles. For example, EDLCs can achieve up to half a million cycles, whereas pseudocapacitors are compromised with cycling issues. This is because the multiple cycles of chemical reactions can damage the pseudocapacitive material and induce undesirable changes to the morphology, leading to reduction in performance as cycling progresses. In response, carbon supports are often added to resolve the detriment. Particle 1-D CNT 3-D graphite Figure 7. Illustration of 2-D graphene sheet as the building blocks for other carbon materials such as particle, 1-D CNT, and 3-D graphite. Source: Reproduced with permission from Geim et al., Nature Publishing Group Transition metal oxides. Common transition metal oxides that are currently researched for pseudocapacitive electrodes are RuO 2 (Zheng and Jow, 1995), Co 3 O 4 (Lin, Ritter, and Popov, 1998), MnO 2 (Pang, Anderson, and Chapman, 2000), NiO (Liu and Anderson, 1996), and V 2 O 5 (Reddy and Reddy, 2006). These metal oxides undergo multiple oxidation states at specific potentials, leading to increased capacitance. RuO 2 is one of the most popular pseudocapacitive materials because of its good reversibility, three oxidation states within a potential range of 1.2 V, and an acceptable life cycle (Simon and Gogotsi, 2008). For example, Hu, Chen, and Chang (2004) maximized the performance of hydrous RuO 2 by annealing it at high temperature achieving a capacitance of 1340 F/g. Unfortunately, ruthenium metal is toxic and expensive due to scarce availability. This shifted the attention of researchers toward nonprecious metals that are more commercially available, including cobalt, manganese, nickel, and vanadium oxides. Table 1 lists the specific capacitances and the morphologies of the previously mentioned metal oxides and Figure 9 illustrates each of their morphologies.

8 8 Electrochemical Storage N O N O N N N N N N (a) N (b) N O N N Figure 8. (a) Model of graphene sheets separated by CNTs. Source: Reproduced with permission from Wang et al., American Chemical Society. (b) Nitrogen-doped graphene structure. Source: Reproduced with permission from Hassan et al., Royal Society of Chemistry. Table 1. Morphology and performance of currently researched metal oxides for pseudocapacitor applications. (a) (b) Compound Morphology Specific Capacitance (F/g) Reference Co 3 O 4 Brush-like nanowires 1525 Rakhi et al. (2012) MnO 2 Nanowires 800 Chin, Pang, and Anderson (2010) NiO Nanopetals 710 Lee et al. (2011) V 2 O 5 Nanoporous network 316 Saravanakumar, Purushothaman, and Muralidharan (2012) (c) 50 μm (d) 100 nm Nonetheless, metal oxides generally suffer performance loss for possessing lower electrical conductivity than carbon materials. Additions of carbon scaffolds can normally resolve this issue. For example, Li and Zhitomirsky (2009) incorporated MnO 2 into MWCNTs to enhance the capacitive performance. Kim, Chabot, and Yu (2013) investigated the effect of adding different carbon supports on nickel oxides. For example, addition of SWCNT could significantly increase the capacitive performances from 474 to 810 F/g at rapid charge/discharge rates Conducting polymer. Besides transition metal oxides, electrically conductive polymers [e.g., PPY, polyaniline (PANI)] are also explored due to their inexpensive and facile synthesis. Electrochemical deposition and chemical oxidation are the main techniques in synthesizing these materials. Tailoring the synthesis conditions could result in different morphologies. During the charging and discharging periods, conducting polymers switch between two doping states (p-doping/n-doping) where electrolyte 2 μm 1 μm SEI Figure 9. SEM images illustrating the detailed morphologies of (a) brush-like Co 3 O 4 nanowire. Source: Reproduced with permission from Rakhi et al., American Chemical Society. (b) MnO 2 nanowire. Source: Reproduced with permission from Chin, Pang, and Anderson, Elsevier. (c) NiO nanopetals. Source: Reproduced with permission from Lee et al., Elsevier. (d) V 2 O 5 nanoporous network. Source: Reproduced with permission from Saravanakumar et al., American Chemical Society. ions are inserted/extracted from the polymers backbones (Figure 10) (Rudge et al., 1994). Unlike metal oxides, entire polymer structures are exposed to the doping of ions, which grants high capacitance. For example, PANI synthesized by Zhou et al. (2005) exhibited a capacitance of 609 F/g. However, the fast charge/discharge nature of supercapacitors can damage the polymer structure

9 Electrochemical Supercapacitors for Energy Storage and Conversion 9 Polymer backbone e (a) Current collector plate Undoped conducting polymer film Solution Cations Anions P-doping P-dedoping Current collector plate P-doped conducting polymer film Solution e (b) Current collector plate Undoped conducting polymer film Solution Cations Anions N-doping N-dedoping Current collector plate N-doped conducting polymer film Solution Figure 10. Illustration of conducting polymer s charging/discharging mechanism with two doping states: (a) p-doping and (b) n-doping. Source: Reproduced with permission from Rudge et al., Elsevier. through swelling and cracking, which shortens the polymers overall life cycle. To improve the life cycle, conductive polymers and carbon supports are combined. Wang et al. (2009a) demonstrated the improved capacitance by comparing PANI only (216 F/g) to graphene oxide (GO)- doped PANI (531 F/g). Figure 11 displays PANI arrays on GO sheet and PPY on graphene by Xu et al. (2010) and Davies et al. (2011). 3.2 Electrolyte Apart from electrodes, electrolytes also play a key role in supercapacitor performance. Electrolytes directly affect the cell s operational voltage window and its resistance. Energy density is proportional to the square of the voltage window, while the ionic resistivity is inversely proportional to the cell s power capability (Burke, 2007). Therefore, careful selection of the most fitting electrolyte is crucial in constructing high performing supercapacitors. Three types of electrolytes are currently available for supercapacitors: aqueous, organic, and ionic liquids (ILs). Table 2 lists the density, ionic resistivity, and voltage window for various electrolytes and Figure 12 visually illustrates the effects of the different electrolytes on energy density and specific capacitance. The following sections discuss each electrolyte type in detail Aqueous electrolyte Aqueous electrolytes are primarily adopted in research stages for their low cost and abundance. H 2 SO 4, KOH, and potassium chloride (KCl) are the common aqueous electrolytes bearing the merits of easy handling in an open environment and low ionic resistivity. The diverse variety between acid, base, and neutral electrolytes offer researchers many solutions for enhancing performance. For example, switching from H 2 SO 4 to KOH for nickel oxide electrodes could increase the specific capacitance from 16 to 155 F/g because nickel oxide will undergo a Faradaic reaction in OH ionrich environment (Srinivasan and Weidner, 2000). Unfortunately, aqueous electrolytes commercial success was stinted due to their narrow voltage window (only 1.2 V). Increasing the voltage window evokes water decomposition, that is, oxygen/hydrogen evolution, and builds pressure within the system, which would ultimately result in rupturing the cell. Moreover, strong ph acid base can induce corrosion on metals (e.g., nickel and aluminum). Chloride ions from KCl can also tamper with these metals.

10 10 Electrochemical Storage (a) (b) 300 nm 1 μm Figure 11. SEM images of (a) PANI arrays on GO sheet. Source: Reproduced with permission from Xu et al., American Chemical Society. (b) PPY deposited on graphene. Source: Reproduced with permission from Davies et al., American Chemical Society. Table 2. Density, ionic resistivity, and voltage window for various electrolytes. Electrolyte Density (g/cm 3 ) Resistivity (Ω cm) Cell Voltage (V) KOH Sulfuric acid Propylene carbonate Acetonitrile Ionic liquids (25 C) 28 (100 C) Source: Reproduced with permission from Burke, Elsevier Organic electrolyte Commercially available supercapacitors employ organic electrolytes such as acetonitrile and propylene carbonate (PC) because of their wide operating voltage windows (0 to V). The broad voltage range definitely raises the energy density to the standard of commercial demand. Acetonitrile is more favorable than PC because it only bears one third of PC s ionic resistivity. However, acetonitrile s toxicity and flammability are an issue for safety. Overall, aging (i.e., degradation) of carbon electrodes are observed when exposed to organic electrolytes. The decomposition of organic electrolyte blocks the pores of the electrodes, reducing the capacitive performance and cyclic stability (Azais et al., 2007) Ionic liquids (ILs) ILs are composed of solvent-free molten salts in liquid form at room temperature, which is due to their low melting temperatures. This type of electrolyte is nontoxic and nonflammable and provides the widest voltage window (0 to 3 5 V), without thermal or chemical instability (Galinski, Lewandowski, and Stepniak, 2006). ILs can also be heated up to 300 C without any vaporization (Burke, 2007). E* (Wh/kg) V Ionic liquids Organic electrolytes 3 V Graphene-based electrodes 2 V Other carbonaceous materials MnO 2 hybrids Carbonaceous materials Aqueous electrolytes RuO 2 1 V C G (F/g) Figure 12. Energy density of a two-electrode cell as a function of voltage window and specific capacitance of various electrolytes. Source: Reproduced with permission from Pope et al., ECS Publications. However, the major drawback of ILs is the inadequate ionic conductivity compared to the other two types of electrolytes. The conductivity can be improved by heating ILs to 125 C, but this raises other design challenges. El-Kady et al. (2012) prepared laser-scribed graphene (LSG) supercapacitors and characterized the products with aqueous, organic, and IL electrolytes. Figure 13 summarizes the results, where IL-based LSG supercapacitors surpass both organic- and aqueous-based devices with respect to both energy density and power density. Although the performance of ILs is promising, several challenges, including high costs, lie ahead for commercial applications.

11 Electrochemical Supercapacitors for Energy Storage and Conversion 11 Energy density (Wh/cm 3 ) V/500 μah Li thin-film battery 2.75 V/44 mf commercial AC-EC LSG-EC Gelled electrolyte Power density (W/cm 3 ) LSG-EC, lonic liquid LSG-EC, TEA-BF 4 /CH 3 CN Organic electrolyte LSG-EC Aqueous electrolyte 3 V/300 μf Al electrolytic capacitor Figure 13. Illustration of energy density of two electrode cell as a function of voltage window and specific capacitance of various electrolytes. Source: Reproduced with permission from El-Kady et al., AAAS Publications. 3.3 Separators While many advances have been established in improving performance of supercapacitor electrodes, little research has been initiated in developing well-engineered separators. Separators can negatively influence the performance of supercapacitors because poorly designed separators can induce additional resistances in the cell. This can, in the worst case, short circuit the cell. Considerations in selecting adequate separators for supercapacitors include the following: 1. Nonconductive (prevent electron transport between electrodes) 2. Electrolyte ion permeable with minimum ionic resistance 3. Chemical resistance to electrolytes and electrode materials 4. Mechanical resistance to pressure and volume changes such as swelling 5. Easily wetted by electrolytes Natural materials, such as glass, cellulose paper, and ceramics, were originally used as separators in the infantile stages of supercapacitor development. Nevertheless, evolution of polymer-based separators dominated the separator markets because of their low cost, amazing flexibility, and high porosity. Polymer separators can be classified into two categories: fibrous structure and monolithic network with defined pores. Figure 14 illustrates the two separator structures. It is worth pointing out that Shulga et al. (2014) successfully utilized GO films as separators for supercapacitor applications. 3.4 Current collectors The majority of energy storage devices require current collectors that complement performance because of the active materials inadequate conductivity. Normally found within the cell, a current collectors role is to transport current from electrodes to external loads. Therefore, they must be electronically conductive and resilient in the cell environment withstanding chemical abuse from electrolytes. With these caveats, aluminum, steel, and iron are popular current collectors. Moreover, coating active materials directly onto current collectors can provide firm molecular contact, which amplifies the performance by minimizing the interfacial resistance between active layers and current collectors (Wu et al., 2009). Unfortunately, owing to the extensive cycling of the supercapacitors and the physical disorientation of the active materials, a determination between the active material and the collector may occur, leading to high resistance. To address this, polymeric binding agents (i.e., Nafion and polytetrafluoroethylene) are supplemented to inhibit the dislocation of active materials from the current collectors. In addition, it is also important for current collectors to dissipate heat generated within the cell. As such, aluminum is commonly used. 3.5 Sealants Proper sealing in cell assembly is vital to avoid performance loss of supercapacitors. A sealants function is to prevent foreign contaminants (i.e., water and air) from entering the cell. The impurities can provoke electrolyte degradation and surface oxidation on electrodes, resulting in loss of life cycles. For commercial applications, multiple supercapacitors are linked in series to supply a high voltage window, but this connection requires sophisticated sealing. Improper sealing of cells in series can cause shunt resistances between neighboring cells. Shunt resistances can reduce the overall efficiency of the device by providing alternative current paths (Kotz and Carlen, 2000). For sealants, polymer materials are generally selected for their dimensional flexibility, mechanical stability, moisture resistance, and electrical insulation. 4 SUPERCAPACITOR PERFORMANCE, TESTING, AND DIAGNOSIS Performance evaluation of supercapacitor components and the full cell/stack is vital in optimizing the technology. The major parameters used to quantitatively evaluate the supercapacitor s performance are capacitance, energy and power

12 12 Electrochemical Storage (a) (b) S kv 10.2 mm x 500 ESED 20 Pa 9/19/ um Figure 14. SEM images of (a) Millipore JVWP separator with fibrous structure and (b) GE Osmotics K50CP01300 separator with monolithic/defined pores. Source: Reproduced with permission from Cameron, ECS Publications. density, internal resistance, and cycle stability. All methods used to obtain these parameters are electrochemical ones, as described. 4.1 Electrochemical cell design for performance testing Working electrode (recycled carbon fibre sheet) Potentiostat V I Reference electrode (Ag/AgCl) Counter electrode (platinum rod) For characterization of supercapacitors, testing apparatus are classified into two configurations: three-electrode and two-electrode systems. The former configuration focuses on the evaluation of electrode materials by screening electrode materials with minimal amounts of the active material. The latter system, resembling the structure of fully assembled supercapacitors, evaluates the performance of a cell under less-than-ideal conditions Three-electrode system The three-electrode system consists of a working electrode, reference electrode, and counter electrode, which are all connected to a potentiostat. This potentiostat is used to control the electrode potential while recording the change in electrode current with potential, or controlling the current passing through the electrode and then recoding the change in electrode potential with current. Figure 15 illustrates the three operating components: (a) working electrode, (b) reference electrode, and (c) counter electrode. In Figure 15, the working electrode is normally prepared by coating the active material onto the surface of a stable electrode, that is, glassy carbon or platinum metal. The active material is dispersed in a selected solvent (e.g., water, ethanol, or isopropanol) until an ink with a uniform dispersion is acquired. Carbon supplements are integrated into (a) (b) (c) Electrolyte Figure 15. Schematic drawing of a three-electrode system: (a) working electrode, (b) reference electrode, and (c) counter electrode. Source: Reproduced with permission from Pang et al., Elsevier. this ink if the active material is conductively deficient. A desired amount of ink is then pipetted onto the surface of prepolished electrodes. Often, a small amount of polymeric binder (e.g., Nafion) is incorporated following the ink deposition to prevent the ink from diffusing away into the electrolyte. A reference electrode establishes a base potential in the three-electrode system by acquiring a fixed potential. There are many different types of reference electrodes with discrete fixed potentials. Normal hydrogen electrode (NHE), silver chloride electrode (Ag/AgCl), and saturated calomel electrode (SCE) are a few common reference electrodes. Lastly, a counter electrode, also known as an auxillary electrode, balances the reaction that is occurring in the working electrode by adjusting its potential. For this

13 Electrochemical Supercapacitors for Energy Storage and Conversion 13 purpose, highly conductive yet inert materials such as platinum meshes or graphitic rods are employed as the reference electrodes. techniques, cyclic voltammetry (CV) and galvanostatic charge discharge (GCD), are used to evaluate the capacitance of materials Two-electrode system Figure 16 illustrates a two-electrode system designed by Tsay, Zhang, and Zhang (2012). In a two-electrode system of Figure 16, two active electrodes are included: a cathode and an anode. A separator is placed between these electrodes to avoid short circuits. Metal plates as current collectors adjoin the two electrodes and their separator. Lastly, a metal casing with three screws applying pressure evenly encloses the cell. Before testing, the cell is submerged in a vessel filled with electrolyte, and then, it is dried in a vacuum oven to remove any trapped air. The anode and cathode are synthesized by creating inks/pastes from the active material(s). The inks are subsequently pasted or sprayed onto the stable electrode sheets (e.g., carbon papers or carbon fibers) and trimmed to specified dimensions. To measure the potential of individual electrodes, a reference electrode can be installed in the cell; however, this may give extra design challenges Cyclic voltammetry CV assesses quantitative and qualitative data relating to the electrochemical phenomena occurring in the active materials of the working electrode. This technique applies a potential to the working electrode, with respect to the reference electrode s fixed potential, which linearly sweeps back and forth between the two predefined potentials. The potential range is limited by the electrolyte s operating stability. Scanning the potential range yields a time-dependent current and plotting this current (I) against the scanned potential (E) graphs a cyclic voltammogram (CV) curve for capacitance diagnosis (Figure 17). (b) (c) 4.2 Capacitance (C) Capacitance (C), which was briefly introduced in Section 2, is one of the key performance parameters evaluated during the analysis of supercapacitors. It is measured in (SI unit) farads (F). Specific capacitance (C s ), which is the capacitance normalized by the mass of the active material, is calculated to compare the performance between electrodes with different masses. Two electrochemical I (ma) E (V) Figure 17. Cyclic voltammogram curves of (a) ideal capacitor, (b) EDLC, and (c) pseudocapacitive materials. (a) Reference electrode End plate (metal) Current collectors Separator Cell plate (Teflon) Pressure coin Cell plate (Teflon) Electrodes End plate (metal) Figure 16. Illustration of a two-electrode system. Source: Reproduced with permission from Tsay, Elsevier.

14 14 Electrochemical Storage Theoretically, a rectangular CV, shown in Figure 17a resembles an ideal capacitor; however, EDLC materials do not behave ideally, resulting in a deformed rectangular shape as displayed in Figure 17b. Faradaic reactions from pseudocapacitors output redox peaks as seen in Figure 17c. From these curves, capacitance can be appraised. Capacitance can be calculated as (Kim, Chabot, and Yu, 2013) C = idv 2V s ΔV, (2) where idv is the integrated area under the CV curve, V s the potential scan rate, and ΔV the potential range. Dividing the capacitance by the mass of the active materials gives the specific capacitance (C s )as C s = C m, (3) where C is the calculated capacitance from Equation (2) and m the mass of the active material. Moreover, potential scan rates have significant effects on measured capacitances. At lower rates (e.g., mv/s), CV curves exhibit near ideal capacitive behavior with a rectangular shaped curve, as shown in Figure 18. However, increasing scan rates distort the ideal rectangular CV curve. At extreme scan rates, the electrochemical kinetics cannot contend with the rapid change in potential. Considering the sluggish transportation of ions and the underutilized micropores in active materials, a decline in performance is to be expected. This consequence is commonly observed with pseudocapacitive materials due to their slow Faradic reactions in the mid of fast scan rates Galvanostatic charge discharge (GCD) In addition to CV, GCD is an alternative method to measure the capacitance of the material. The GCD technique applies a constant current density (e.g., A/g) and measures the responsive potential with respect to time. Generally, the working electrode is charged to a preset potential and the discharge process is then monitored to assess the capacitance. Figure 19 illustrates GCD plots for EDLC and pseudocapacitive materials. From Figure 19, it can be seen that similar to CV plots, the two materials demonstrate dissimilar responses. EDLC materials charge and discharge linearly, while the nonlinearity of pseudocapacitive materials is due to the redox reactions. Because of this discrepancy, each type of material has its unique equation to calculate its capacitance. For EDLC, the slope of the discharging section is utilized, which gives (Stoller and Ruoff, 2010) C = I dv dt, (4) where C is the capacitance of the material, I the applied current, and dv/dt the slope of the discharging GCD curve. For pseudocapacitive material, an altered form of the equation without the slope is employed (Kim, Chabot, and Yu, 2013), which gives Specific current (A/g) C = (Δt)(I) ΔV, (5) where Δt is the total discharge time, I the applied current, and ΔV the potential difference at the discharging phase. (a) Charging Discharging mv/s 20 mv/s 5 mv/s Potential vs (H H 2 ) (V) Figure 18. Cyclic voltammograms (CVs) of ordered graphitic mesoporous carbon at different scan rates. Source: Reproduced with permission from Zhang et al., Elsevier. E (V) t (s) Figure 19. Galvanostatic charge discharge plots of (a) EDLC and (b) pseudocapacitive material. (b)

15 Electrochemical Supercapacitors for Energy Storage and Conversion 15 To compute the specific capacitance, Equation (3) can again be used. Similar to the dependence on scan rates, current density also affects capacitance. A high current density leads to a rapid discharge where the utilization of the materials properties is hindered and the electrochemical kinetics is limited. On the other hand, an extremely low current density induces leakage current and self-discharge that is discussed in detail in Section Energy and power densities Assessing the energy and power densities of a supercapacitor is imperative in diagnosing its deliverable performance for real-life applications. Both CV and GCD techniques can determine both energy and power densities Energy density Specific energy density (watt hour per kilogram) is expressed by ED = 1 2 C s(δv) 2 = 1 C 2 m (ΔV)2, (6) where C s is the specific capacitance from CV or GCD techniques and ΔV the operating potential range. The latter part of the equation correlates specific energy density to the capacitance and mass of active materials. The squared potential range indicates that the operating voltage window significantly dictates the energy density. As discussed in Section 3.2, the electrolyte determines the operating potential range. Research is continuing to direct its resources toward organic electrolytes and ILs to widen the potential window and ultimately resolve the supercapacitors lack of a high energy density. However, moving toward electrolytes that possess a wider voltage window will give a net reduction in power density Power density Specific power density (Watt per kilogram) defines how quickly a device can deliver energy under a constant current density to external loads. Maximum specific power density is calculated as PD max = (ΔV)2 4mR ESR, (7) where ΔV is the potential range, m the mass of the active materials, and R ESR the equivalent series resistance (ESR) within the cell. The ESR includes ohmic resistance of the electrolytes, resistances from the cell design, and other resistances. This parameter is explained in a later section. Using organic or ILs as electrolytes can widen the voltage window; however, their inferior ionic conductivities contribute to an increased R ESR, which can reduce the overall power performance. The maximum power can be averaged by applying a lucid relationship between specific energy density and average specific power density, which gives PD average = ED Δt, (8) where Δt is the rate of discharge of the cell Ragone plot Plotting the specific power density against its specific energy density generates a Ragone plot (as shown in Figure 1), which provides an overview of the performance in terms of energy and power. Actually, Figure 1 illustrates Ragone plots of several well-known electrochemical energy storage devices, including supercapacitors. A trend of diminishing power density with increasing energy density is evident with all of the devices. Similarly, supercapacitors also conform to this trend and encounter a challenge where increase in the maximum energy density lowers its maximum power density. Figure 20 shows that for supercapacitors, maximum energy density can be improved using organic electrolytes with a wider operating voltage; nonetheless, organic electrolyte-based supercapacitors will yield lower maximum power densities than aqueous electrolyte-based supercapacitors. This inverse relationship between the two densities emphasizes that optimization of cell design, including the electrolyte, is necessary in improving both power and energy of storage devices. Maximum power density (W/I) Aqueous σ = 0.8 S/cm 1 μm Organic σ = S/cm 10 μm 100 μm 1000 μm Maximum energy density (Wh/I) Figure 20. Calculated maximum achievable power density and energy density of two capacitors in aqueous and organic electrolytes. Source: Reproduced with permission from Kotz and Carlen, Elsevier.

16 16 Electrochemical Storage 4.4 Internal resistances Most supercapacitors do not exhibit ideal capacitive behavior due to internal resistances. Comprehending the internal resistances of supercapacitors offers in-depth knowledge about their power performance. The principal resistance is the ESR, which involves resistances of cell components, for example, electrolyte resistance and contact resistance between current collectors and electrodes (Zhao et al., 2010; Conway, 1999). ESR directly affects the power delivery of a cell, as shown in Equation (7). It restricts the rate at which supercapacitors can be charged and discharged, leading to power reduction and energy dissipation. Two methods, which can determine ESR, include GCD and electrochemical impedance spectroscopy (EIS) Galvanostatic charge discharge (GCD) As mentioned in a previous section, the GCD technique monitors the response in potential with respect to time during charging and discharging. At the initial discharge, a sudden drop in potential is observed that is referred as the IR drop, as shown in Figure 21. Calculating the ESR requires dividing the IR drop (e.g., the change in potential at initial discharge) by twice the current applied. This method can be employed for three-electrode systems as well as for two-electrode systems, including fully assembled supercapacitors Electrochemical impedance spectroscopy (EIS) EIS can also be used to measure and compute the ESR. Moreover, EIS also provides further insight about other electrical characteristics. In this method, an alternating current (AC) with small magnitude is supplied to supercapacitors over a frequency range 0.01 Hz to 1 MHz. The response is charted on a Nyquist plot, which plots imaginary resistance (Z ) against real resistance (Z). From the Nyquist plot, the intersection of the impedance curve at the x-axis corresponds to the ESR. Figure 22 shows that both (a) ideal capacitors and (b) supercapacitors possess the same ESR. However, a supercapacitor s impedance response deviates from the ideal capacitor at low frequencies. This deviation is caused by the equivalent distributed resistance (EDR), which accounts for the ionic resistance of electrolyte within the pores of the electrodes (Kotz and Carlen, 2000; Conway, 1999). Therefore, increasing the number of pores in an active electrode will induce an elevated EDR. 4.5 Cycle stability Another important property of supercapacitors is their cycle stability. Commercial supercapacitors are well recognized for possessing a merit of long life cycles in which they would operate at full capacity, even after half a million cycles. At lab-scale testing, 1000 to 10,000 cycles are generally Frequency High Low 1.4 (a) (b) Charging-discharging voltage (V) Charging V V IR ESR 2IR ESR Discharging Z imaginary resistance δ Charging-discharging time (s) ESR φ EDR Figure 21. Charge discharge curves recorded using a symmetric supercapacitor cell with a geometric area of 4.0 cm 2 for each electrode in 0.5 M Na 2 SO 4 aqueous solution at ambient conditions. The IR drop is highlighted in blue box. Source: Reproduced with permission from Ban et al., Elsevier. Z real resistance Figure 22. Schematic representation of the Nyquist plot of (a) an ideal capacitor and (b) a supercapacitor. Source: Reproduced with permission from Kotz and Carlen, Elsevier.

17 Electrochemical Supercapacitors for Energy Storage and Conversion Capacity retention (%) Capacity retention (%) 80 CNT-sponge at 10 V/s min MnO 2 -CNT-sponge at A/g Voltage across capacitor Charged to 2.4 V Charged to 2 V Charged to 1.55 V Cycle number Cycle number Figure 23. Performance retention versus cycle number for CNTsponge and MnO 2 -CNT-sponge. Source: Reproduced with permission from Chen et al., ACS Publications. conducted for investigating the cell s cycle durability. One cycle would equal to one charge/discharge cycle at a constant current density. As charging and discharging are required for the test, GCD method is employed. Extensive cycling degrades the electrodes and induces corrosion in the cell s components, resulting in capacitance reduction and rise in ESR. Therefore, comparing the initial and the final performance from the cycle testing provides a foresight on how the material will perform in real applications where it will be utilized to extreme amounts of cycles. In addition, the type of material impacts the cycle durability. EDLC materials with static storing mechanism tend to have finer stability than pseudocapacitive materials. For example, when compared to a CNT-sponge s cycle stability, Chen et al. (2011) illustrated higher rates of degradation of pseudocapacitive MnO 2 - deposited CNT-sponge as cycling advanced. Figure 23 shows that a 2% drop in performance retention is visible from the CNT-sponge over the course of 100,000 cycles, whereas a 4% reduction in capacitance is evident from MnO 2 -CNT-sponge from a mere 10,000 cycles. If the MnO 2 composite sponge was subjected to 100,000 cycles, severe reduction in performance would be apparent. 4.6 Self-discharge All capacitors, including supercapacitors, suffer from capacitance loss because of self-discharge. This phenomenon involves the gradual decaying of voltage in a fully charged cell due to a constant (leakage) current in an open-circuit cell. The self-discharge reduces the deliverable power and energy from supercapacitors; thus, it is considered a major Minutes Figure 24. Self-discharge of carbon-based supercapacitors in organic electrolyte at different charged voltages. Source: Reproduced with permission from Ricketts and Ton-That, Elsevier. issue for commercial applications. One reason behind selfdischarge is the thermodynamic instability in the charged state, implying that the charged potential may surpass the thermodynamic limitation of the electrolyte, resulting in a Faradaic decomposition of the solution. Another reason for the self-discharge is that the ions redistribute themselves within the pores of the electrodes. Alternatively, fast charging behavior of the supercapacitor does not allow sufficient time for the entirety of pores to be thoroughly saturated with ions. Once charging is completed, the excess ions in the pores will be allocated to adjacent vacant pores, uniformly distributed across the electrode. This redistribution results in voltage decay. In order to investigate self-discharge, supercapacitors are first fully charged, then idled to endure self-discharge. The voltage drop is monitored and calculated. The measurement duration can take between a few minutes to a couple of weeks. Figure 24 is an example of self-discharge in a carbon-based supercapacitor. Using organic electrolytes and assembling cells in moisture-free environment can eliminate the possibility of self-discharge originating from Faradaic reactions (Zhang et al., 2011). 5 ASSEMBLY AND MANUFACTURING OF SUPERCAPACITORS Electrodes and electrolytes are the largest contributors to the performance of supercapacitors. The fabrication of each

18 18 Electrochemical Storage component is diverse due to the large variety of materials made available. Different supercapacitor designs allow more options for manufacturing and assembling of the devices. 5.1 Supercapacitor designs Coin cells, cylindrical cells, and pouch cells are the three prevalent supercapacitor designs used commercially. Coin cell design as illustrated in Figure 25a is suitable for supplying energy to small devices. In the assembly of coin cell, two electrodes and electrolyte-saturated separators are assembled in a conductive metal casing. An insulating polymer, such as Teflon, is added between the casings to prevent electrolyte from leaking and the cell from shorting. When the assembly is completed, the metal casing can be crimped under high pressure to seal the cell. However, excessive pressure may induce a short circuit within the cell. The coin cell design requires thin electrodes with low active material mass; hence, alternative designs are preferable for bulkier supercapacitors. The cylindrical cell design is widely employed in commercial supercapacitors. As shown in Figure 25b, the design consists of rolled layers of electrode sheets with separator sheets. Then, these layered sheets are fitted into a cylindrical metal casing to reinforce its mechanical durability. Subsequently, current collector tabs are soldered onto the rolled sheets. Finally, electrolyte is injected in the cell followed by sealing of the metal casing. As a safety precaution, a one-way safety vent must be installed in each device to release any pressure accumulation within the cell. Figure 25c shows an alternative design that also utilizes layer-by-layer sheets of electrodes and separators, called a pouch cell. The pouch cell design is effective in stacking multiple electrodes provided that each soldered current collector tab is correctly connected to the proper corresponding terminals in the cell. These components are assembled within polymer bags, which are flexible in accommodating undesired volume expansions. Moreover, this simple design minimizes the number of components required in the cell, resulting in thin devices with low ESR and excellent performance. The flexibility and slim design Stainless steel cap MWNT buckypaper Positive electrode Separator Negative electrode Teflon ring Nomex separators with electrolyte Polymer-based cell case MWNT buckypaper Stainless steel cap (a) (c) Separator (b) Positive electrode Separator Negative electrode Figure 25. Schematic of (a) coin cell design, (b) cylindrical cell design, and (c) pouch cell design. Source: (a) Reproduced with permission from Hu et al., ACS Publications.

19 Electrochemical Supercapacitors for Energy Storage and Conversion 19 grant pouch-cell supercapacitors more spatial freedom than coin cells and cylindrical cells. 5.2 Component fabrication The fabrication techniques of commercial supercapacitors are normally trade secrets that manufacturers base their sole success on. On the other hand, laboratory techniques for fabricating electrodes, electrolytes, membranes, and current collectors are well established and made public. These processes may resemble some of the commercial techniques. Electrode fabrication requires preparing a paste from mixing active materials (e.g., AC) and binding agents (e.g., polyvinylidene fluoride) in solvents (e.g., isopropanol). Additional sonication homogenizes the paste. For the fabrication of electrodes, the prepared mixture can then be processed via one of two methods. The paste can be spread onto a plate, hot pressed, and then dried into thin sheets or the paste can be diluted into a slurry with more solvent and sprayed onto current collectors. Compared to the electrode fabrication, electrolyte preparation is fairly simple. However, special conditions need to be met depending on the electrolyte type. With safe handling of strong acids/bases and neutral salts, aqueous electrolytes (e.g., H 2 SO 4 and KOH) can be simply prepared at ambient conditions. Conversely, organic electrolyte (e.g., acetonitrile and polycarbonate) and ILs [e.g., 1-ethyl-3- methyl imidazolium tetrafluoroborate (EMIMBF 4 )] must be formulated in a moisture-free environment (e.g., glove box) to avoid water-based contamination of the electrolyte. Presence of water molecules in organic and ILs stimulates gas evolutions at high operating voltages, which leads to an increase in cell pressure and, consequently, safety concerns. In terms of separators, a wide range of materials from filtration papers to polymer membranes are used. Electrolyte ions must be permeated through separators, yet the separators should be electrically insulating. Ideally, separators should be saturated with electrolyte before the assembly of the cell. For better contact, polymer separators are sandwiched between the electrodes, hot-pressed, cooled down, and then soaked with electrolyte. Nonprecious metal foils (e.g., aluminum and stainless steel) that are chemically resistant and mechanically durable are extensively used in commercial current collectors. These foils are etched by acids to eliminate any impurities and to increase the surface area. As mentioned earlier, electrode pastes can be directly applied to current collectors. However, poorly adhered active materials on current collectors will suffer from high resistances and, as a result, performance losses. 5.3 Stacking of supercapacitors and stack performance Stacking of supercapacitors allows multiple cells to operate collectively, and thus exceed the limits of a single cell. When individual cells are connected in series, the pack can supply operating voltage up to 60 V. Supercapacitor packs are often assembled in metal casings (i.e., modules) that provide sturdiness at the cost of additional weight and volume. To quantify its performance, capacitance in series is expressed as 1 C stack = 1 C 1 1 C 2 1 C 3 = n i 1 C i, (9) If identical cells are assumed, then Equation (9) can be simplified to C Stack = C i n, (10) where n is the number of cells. Voltage in a stacked series can be expressed similarly as V stack = V 1 V 2 V 3 = with identical cells assumption: n V i, (11) V Stack = nv i, (12) Lastly, the energy density of a pack can be expressed as E Stack = 1 2 C Stack(V Stack ) 2 = 1 2 C Stackn(V i ) 2, (13) This equation states that the energy density of a stacked cell is equivalent to the energy density of a single cell multiplied by the number of cells. Stacking the cell increases the performance, and it also creates complications that are not present in a single cell. Although cooling systems are built within modules, nonuniform heat distribution in the stack may impair physical properties and reduce cycle durability. Moreover, despite the fixed stack voltage of modules, the potential difference across each cell can differ. The discrepancy between each capacitor can actually overcharge/discharge individual cells, leading to electrode degradation, electrolyte decomposition, and gas evolution within the packs. Charging the cells in parallel and discharging in series can be implemented as a potential solution; however, tailoring such configuration would be a challenge Bipolar electrode configuration An alternative stacking design involves a bipolar electrode configuration, as shown in Figure 26. The bipolar electrode i

20 20 Electrochemical Storage Casing E 1 E 2 A B A B : Bipolar electrodes : Electrolyte Figure 26. A schematic of bipolar electrode configuration. arrangement effectively minimizes the volume of the stack and circumvents additional connections between cells, lowering the internal resistance. Electrodes are coated on both sides of each plate, serving as bipolar electrodes. The plates are spaced out in the assembly, whereas the gap between each electrode is filled with electrolyte. The bipolar plates must prohibit the transfer of electrolyte ions; otherwise, the lack of electrolyte insulation will trigger short circuits. Sealants are often applied to ensure the isolation of electrolytes. The primary drawback of the bipolar arrangement is inadequate heat distribution within the cells. Therefore, implementation of a cooling system is necessary. 6 APPLICATIONS For decades, rechargeable lithium ion batteries have dominated the energy storage market. However, with the increasing demand of improved energy storage for manifold applications from portable electronics to HEVs, supercapacitors are recognized for their high power density, rapid charge/discharge capability, and long life cycle. New technologies are being developed to optimize the performance and simultaneously reduce the cost of production. The steady advance in supercapacitor research has opened opportunities for real-world applications. This section discusses the existing markets for supercapacitors and prospective applications. 6.1 Consumer application One of the first consumer products was the cordless screw driver solely powered by supercapacitors. This tool from Coleman Company Inc. cannot supply the same capacity as lithium-ion battery-based screw drivers; nonetheless, it Figure 27. An image of a supercapacitor-powered speaker with charging time of 5 min and playback time of 6 h. Source: Reproduced with permission from Beck, Blueshift. can be operational within minutes of charging. Furthermore, users can disconnect the screwdriver at any time while it is charging without damaging its components (Conner, 2007). In addition to household tools, Maxwell Technologies, a supercapacitor manufacturer, collaborated with Celadon to develop supercapacitor-powered remote controllers. The controller originally operated with two AAA batteries but has recently been replaced with smaller supercapacitors. The controller can now be charged instantly. Additionally, the life span of the supercapacitors can outlast the remote controllers (Wald, 2013). Recently, Sam Beck from Blueshift constructed portable speakers equipped with supercapacitors. The speaker is fully charged within 5 min and operates up to 6 h of playback at full volume (Denison, 2013). Figure 27 illustrates the speaker, which is currently available for purchase. 6.2 Industrial application For industrial applications, supercapacitors are generally employed as an emergency backup power source because of their instant discharging capability. In the event of a power failure in computer components, hospitals, and factories, supercapacitors can assure a supply of uninterruptable energy to prevent catastrophic failures until the power is restored. For example, implementing supercapacitors in solid-state drives can prevent the deletion of data due to power disruption (Cellergy, 2012). Also, supercapacitors are implemented as an emergency power source in airplanes. An array of supercapacitors is installed in the Airbus A380 to immediately power the emergency doors in any circumstance (Simon and Gogotsi, 2008). Unlike batteries, inert

21 Electrochemical Supercapacitors for Energy Storage and Conversion 21 carbon-based supercapacitors are stable in extreme conditions. This advantageous property created an opportunity for FastCAP systems to develop supercapacitor-powered drills for oil, petroleum, and geothermal exploration. The drill was fully functional at 150 C without accompanying overcharge (FastCAP, 2013). Furthermore, Ioxus developed a technology that conjoins supercapacitor and battery together to harness the advantages from each energy storage technology. The hybrid system can supply more energy than supercapacitors and more power than batteries. Lastly, a Japanese company, JSR Micro, constructed these hybrid devices to serve as a backup power source for medical imaging equipment (Patel, 2010). 6.3 Public sector application As a clean and alternative energy source, the wind energy market is constantly expanding, presenting more opportunities for supercapacitors. Pitch control in wind turbines adjusts the blades to current conditions in order to maximize generation. Batteries have been the dominant energy source to power the pitch control; however, setbacks, such as insufficient power output and low life cycle, have redirected wind turbine operators to more reliable supercapacitors. Supercapacitors are capable of responding instantly to unpredictable weather conditions, supplying short bursts of power to operate the turbines (Andrews, 2011). Additionally, the low maintenance cost and long cycle life of supercapacitors are suitable to be installed in unmanned wind turbines. Another application of supercapacitors can be found with the Emirates Airline cable cars. Each cable car is equipped with its own supercapacitor-based energy system that supports accommodations (e.g., lights and air conditioning) for tourists. The energy system is rapidly charged at each station to power the 5-min cable car ride (Hubley, 2012). As another example, three Japanese companies have collaborated to build stand-alone, environment-friendly LED street lamps. The LED lamps consist of solar panels to harness the solar energy and supercapacitors to store the energy during the day. At night, the energy is discharged to power the LED lamps (Nippon-Chemi-Con, 2010). The combination of long-life LED lamps and supercapacitors provide maintenance-free street lamps for years at a time. 6.4 Public transportation applications The prospective utilization of supercapacitors began with public transportation. Heavy vehicles such as buses and trucks frequently stop during their operations, wasting enormous amounts of heat energy. Supercapacitors would be capable of harnessing and storing this instantly released momentum when it is transformed into electrical energy. Then, the charged supercapacitors can discharge the energy to assist acceleration, reducing overall fuel consumption and CO 2 emissions. Regenerative breaking involving supercapacitors has already been applied to many public transportation vehicles. In Switzerland, 1 ton of supercapacitors was installed onto a tram to capture its brake energy. The stored energy was successfully deployed to power the vehicle for a short distance without an external source (Railway-Gazette, 2012). Similarly, supercapacitors are also implemented on trams in Paris. The trams, holding a bank of 48 supercapacitors on each vehicle, were able to travel between stations via regenerative braking and short charging during the idling time at each station ( 20 s) (Hondius, 2009). On the road side, Sinautec developed 41-seat municipal buses powered entirely by supercapacitors. The buses can traverse a few miles to the adjacent stop where the buses can be recharged again. On the basis of a vehicle s expected 12-year life, a supercapacitor bus can save up to $200,000 in fuel than a diesel bus and it is 40% cheaper than a lithium ion battery bus when considering capital costs (Hamilton, 2009). 6.5 Future applications Scientists and entrepreneurs are exerting much effort into uncovering innovative applications for supercapacitors. Volvo developed lightweight structural energy storage components composed of carbon fibers and polymer resins. The components, behaving like supercapacitors, are considerably lighter than the conventional batteries already used in electronic vehicles. In addition, the energy storage components can be easily molded into desired shapes (e.g., car chassis). Volvo s S80 prototype replaced its trunk lid and plenum cover for these supercapacitive materials to store the brake energy (Ingram, 2013). Another development in electric vehicles arouse when South Korean scientists synthesized extremely porous graphene supercapacitors that can exhibit similar energy densities to that of lithium ion batteries. A charge time of only 16 s was required for this supercapacitor. One gram of this extremely porous graphene has a surface area of a basketball court, which translates to an extremely high energy density. Incorporating this graphene supercapacitor to electric vehicles will allow effective storage of the braking energy, but scaling-up the production of this specialized graphene for commercial application remains a challenge (Estes, 2013). Alternatively, the concept of wearable electronics as shown in Figure 28 is becoming more realistic as scientists are integrating supercapacitor technology into clothing. Li from the University of South Carolina created a T-shirt that functioned like a supercapacitor. Li purchased a T-shirt from a local store

22 22 Electrochemical Storage Power communication devices that control your phone/music player Harvest energy from body movements to keep your textilesupercapacitor charged Energy storing material made as a textile with seamless knitting Figure 28. Concept of integrated energy storage in wearable electronics. Source: Reproduced with permission from Jost et al., RCS Publications. and soaked it in fluoride solution, then baked it at a high temperature in the absence of oxygen. The surfaces of the clothing fibers transformed into AC, exhibiting supercapacitive behavior. Additionally, Li deposited manganese dioxide on the activated-carbon T-shirt to further enhance its energy performance (Powell, 2012). 7 CHALLENGES AND PERSPECTIVES As the world explores new options for clean and safe energy sources, the supercapacitors contribution is becoming more evident. Supercapacitors can instantly charge/discharge with high power and retain its performance even after half a million cycles. Despite these benefits, commercialization of supercapacitors is currently limited by several challenges. Insufficient energy density of supercapacitors is a pitfall for this type of energy system, which restricts its potential application. Comparatively, lithium ion batteries stores up to 20 times more energy than supercapacitors at given size or mass (Harrop, 2013). High power density and long life cycle properties of supercapacitors are not appealing enough for industries and consumers to totally replace batteries with supercapacitors. Another hindrance to the ubiquity of supercapacitors is its cost. Over 200 lithium ion battery manufacturers are established internationally, massively producing their devices, whereas only 80 supercapacitor companies exist to supply the demands (Harrop, 2013). As of this moment, supercapacitors cannot surpass lithium ion batteries in terms of price per performance. Another challenge to nationally implement supercapacitors is the deficit of supporting infrastructure. For example, electric vehicle markets would greatly benefit from the numerous prospective opportunities from supercapacitors. However, commercialization of these vehicles will require installation of new charging stations throughout the nation, which may take several years for implementation (De Angelis, 2013). Nevertheless, the future of supercapacitors is expected to grow as significant resources have been invested in technological research and in the initiation of entrepreneurial applications. Supercapacitor companies such as Ioxus raised $15 million equity for research and development in 2013 (Harrop, 2013). Scientists at the University of Illinois have experienced great success in transforming forest waste to supercapacitors in an environment-friendly way (Yates, 2013). Therefore, otherwise unusable forest waste can now be recycled into energy storage devices. Overall, industries foresee that the supercapacitor market will increase at a compound annual growth rate of 19.85% through 2016 (De Angelis, 2013). In addition, numerous supercapacitor manufacturers will expand in the distant future, which will in turn increase the supply, as well as lower the price of supercapacitors. The preference for supercapacitors in commercial applications is continuously growing. For example, Maxwell Technologies is selling $100 million of supercapacitors every year for use in windmills, transit buses, and automobiles (Batzdorf, 2013). Similarly, the new 2014 Mazda 3 vehicles incorporate a regenerative braking system utilizing supercapacitors called i-eloop. Brake energy from the car is stored in the supercapacitors and then discharged to supply power to the interior electrical systems (Gastelu, 2013). Currently, many potential developments are underway in the world to make supercapacitors more viable and affordable. Within a few years, supercapacitors will be a major energy source, alongside batteries, to offer the world more options for clean and efficient energy storage. RELATED ARTICLES Control of Electricity Loads in Future Electric Energy Systems Electric Vehicles and the Electric Grid Introduction: Energy Storage Technologies Modern Energy Storage Applications Barriers to the Development of Energy Storage Systems Rechargeable Battery Energy Storage System Design Flow Battery Technology Sodium Sulfur Batteries Lithium-Ion Batteries: Thermomechanics, Performance, and Design Optimization Recycling of Lithium-Ion Batteries Electric Vehicles as a Mobile Storage Device