Carbon properties and their role in supercapacitors

doi:10.1016/j.jpowsour.2006.02.065

Journal of Power Sources

Volume 157, Issue 1, 19 June 2006, Pages 11-27

https://doi.org/10.1016/j.jpowsour.2006.02.065 Get rights and content

Abstract

Supercapacitors (also known as ‘ultracapacitors’) offer a promising alternative approach to meeting the increasing power demands of energy-storage systems in general, and of portable (digital) electronic devices in particular. Supercapacitors are able to store and deliver energy at relatively high rates (beyond those accessible with batteries) because the mechanism of energy storage is simple charge-separation (as in conventional capacitors). The vast increases in capacitance achieved by supercapacitors are due to the combination of: (i) an extremely small distance that separates the opposite charges, as defined by the electric double-layer; (ii) highly porous electrodes that embody very high surface-area. A variety of porous forms of carbon are currently preferred as the electrode materials because they have exceptionally high surface areas, relatively high electronic conductivity, and acceptable cost. The power and energy-storage capabilities of these devices are closely linked to the physical and chemical characteristics of the carbon electrodes. For example, increases in specific surface-area, obtained through activation of the carbon, generally lead to increased capacitance. Since only the electrolyte-wetted surface-area contributes to capacitance, the carbon processing is required to generate predominantly ‘open’ pores that are connected to the bulk pore network. While the supercapacitors available today perform well, it is generally agreed that there is considerable scope for improvement (e.g., improved performance at higher frequencies). Thus it is likely that carbon will continue to play a principal role in supercapacitor technology, mainly through further optimization of porosity, surface treatments to promote wettability, and reduced inter-particle contact resistance.

Introduction

Supercapacitors, ultracapacitors and electrochemical double-layer capacitors (EDLCs) are commonly used names for a class of electrochemical energy-storage devices that are ideally suited to the rapid storage and release of energy. The term ‘supercapacitor’ is adopted in this paper. Compared with conventional capacitors, the specific energy of supercapacitors is several orders of magnitude higher (hence the ‘super’ or ‘ultra’ prefix). Supercapacitors also have a higher specific power than most batteries, but their specific energy is somewhat lower. Through appropriate cell design, both the specific energy and specific power ranges for supercapacitors can cover several orders of magnitude and this makes them extremely versatile as a stand-alone energy supply, or in combination with batteries as a hybrid system. This unique combination of high power capability and good specific energy, allows supercapacitors to occupy a functional position between batteries and conventional capacitors (Fig. 1 and Table 1).

Supercapacitors are particularly useful because their parameters complement the deficiencies of other power sources such as batteries and fuel cells. Due to their highly reversible charge-storage process, supercapacitors have longer cycle-lives and can be both rapidly charged and discharged at power densities exceeding 1 kW kg⁻¹ [1]. These features have generated great interest in the application of supercapacitors for a wide, and growing, range of applications that include: consumer electronics, hybrid electric vehicles, and industrial power management [2], [3], [4].

Research into supercapacitors is presently divided into two main areas that are based primarily on their mode of energy storage, namely: (i) the redox supercapacitor and (ii) the electrochemical double layer capacitor.

In redox supercapacitors (also referred to as pseudocapacitors), a reversible Faradaic-type charge transfer occurs and the resulting capacitance, while often large, is not electrostatic in origin (hence the ‘pseudo’ prefix to provide differentiation from electrostatic capacitance). Rather, capacitance is associated with an electrochemical charge-transfer process that takes place to an extent limited by a finite amount of active material or available surface [5]. The most commonly investigated classes of pseudocapacitive materials are transition metal oxides (notably, ruthenium oxide) and conducting polymers such as polyaniline, polypyrrole or derivatives of polythiophene [6], [7], [8], [9]. Given that charge storage is based on a redox process, this type of supercapacitor is somewhat battery-like in its behaviour.

By comparison, the EDLC stores energy in much the same way as a traditional capacitor, by means of charge separation. Supercapacitors can, however, store substantially more energy (per unit mass or volume) than a conventional capacitor (by several orders of magnitude) because: (i) charge separation takes place across a very small distance in the electrical double-layer that constitutes the interphase between an electrode and the adjacent electrolyte [5]; (ii) an increased amount of charge can be stored on the highly extended electrode surface-area (created by a large number of pores within a high surface-area electrode material). The mechanism of energy storage is inherently rapid because it simply involves movement of ions to and from electrode surfaces. In batteries, additional steps, such as heterogeneous charge-transfer and chemical phase changes, introduce relatively slow steps into the process of energy storage and delivery. For similar reasons, EDLCs exhibit a very high degree of reversibility in repetitive charge–discharge cycling—demonstrated cycle lives in excess of 500,000 cycles have been achieved [10].

A number of reviews have discussed the science and technology of supercapacitors for various configurations and electrode materials [5], [11], [12], [13]. The EDLC version of the supercapacitor is the most developed form of electrochemical capacitor. Carbon, in its various forms, is currently the most extensively examined and widely utilised electrode material in EDLCs with development focusing on achieving high surface-area with low matrix resistivity. A number of carbon manufacturers are now targeting supercapacitors as a market for their products [14], [15].

Carbon materials have long been incorporated into the electrodes of energy-storage devices as: electro-conductive additives, supports for active materials, electron transfer catalysts, intercalation hosts, substrates for current leads, and as agents for the control of heat transfer, porosity, surface-area and capacitance [16]. For these reasons, carbons are of course also well suited as electrode materials for EDLCs.

It is clear that the ultimate performance of carbon-based supercapacitors will be closely linked to the physical and chemical characteristics of the carbon electrodes. Due to the enormous range of carbon materials that are available, an understanding of carbon materials and their properties is desirable for matching carbon characteristics with supercapacitor applications. In this study, we describe the role of carbon in EDLCs and discuss the implications of carbon structure on the physical and chemical properties of these devices.

Section snippets

Energy storage in electrochemical double-layer capacitors

The concept of the double layer has been studied by chemists since the 19th century when von Helmholtz first developed and modelled the double layer concept in investigations on colloidal suspensions [17]. This work was subsequently extended to the surface of metal electrodes in the late 19th and early-mid-20th centuries [18], [19], [20], [21], [22]. In 1957, the practical use of a double-layer capacitor, for the storage of electrical charge, was demonstrated and patented by General Electric

EDLC construction

Supercapacitors are constructed much like a battery in that there are two electrodes immersed in an electrolyte, with an ion permeable separator located between the electrodes (Fig. 2). In such a device, each electrode–electrolyte interface represents a capacitor so that the complete cell can be considered as two capacitors in series. For a symmetrical capacitor (similar electrodes), the cell capacitance (C_cell), will therefore be $\frac{1}{C_{Cell}} = \frac{1}{C_{1}} + \frac{1}{C_{2}}$ where C₁ and C₂ represent the capacitance of the

Electrode material characteristics

The attraction of carbon as a supercapacitor electrode material arises from a unique combination of chemical and physical properties, namely:

•
high conductivity,
•
high surface-area range (∼1 to >2000 m² g⁻¹),
•
good corrosion resistance,
•
high temperature stability,
•
controlled pore structure,
•
processability and compatibility in composite materials,
•
relatively low cost.

In general terms, the first two of these properties are critical to the construction of supercapacitor electrodes. As will be seen, the

Summary

Carbon, in its various forms, is the most extensively examined and widely utilised electrode material in supercapacitors. Continuing efforts are aimed at achieving higher surface-area with lower matrix resistivity at an acceptable cost. Carbons with BET surface areas of up to 3000 m² g⁻¹ are available in various forms, viz., powders, fibres, woven cloths, felts, aerogels, and nanotubes. Even though surface-area is a key determinant of capacitance, other factors such as carbon structure, pore

Acknowledgements

The authors wish to acknowledge financial support from the CSIRO Energy Transformed National Research Flagship initiative.

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^☆: This review is one of a series dealing with the role of carbon in electrochemical energy storage. The review covering the role of carbon in valve-regulated lead-acid battery technology is also published in this issue, J. Power Sources, volume 157, issue 1, pages 3–10. The reviews covering the role of carbon in fuel cells and the role of carbon in graphite and carbon powders were published in J. Power Sources, volume 156, issue 2, pages 128–150.

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ReviewCarbon properties and their role in supercapacitors☆

Abstract

Introduction

Section snippets

Energy storage in electrochemical double-layer capacitors

EDLC construction

Electrode material characteristics

Summary

Acknowledgements

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Chem. Rev.

Proc. R. Soc.

Carbon Sci.

Review
Carbon properties and their role in supercapacitors☆