Electrochemical Activation of Expanded Graphite Electrode for Electrochemical Capacitor

Abstract

An expanded graphite ͑e-MCMB, mesocarbon microbeads͒ having a wider interlayer spacing ͑d 002 = 0.404 nm͒ than that of common graphites is prepared by heat-treatment of an oxidized MCMB. When the e-MCMB electrode, which gives a negligible capacitance due to a small surface area, is polarized over a certain onset potential ͓4.6-4.8 V ͑vs Li/Li + ͒ for positive and 1.3-1.0 V for negative direction͔, it is electrochemically activated to be a high-capacitance positive and negative electrode for electrochemical capacitor. The activation process involves an ion intercalation into the interlayer space to generate ion-accessible sites. The intercalation is evidenced by the presence of a voltage plateau in the charge-discharge profiles, and by the widening of the interlayer distance ͑by in situ X-ray diffraction study͒ and concomitant electrode swelling ͑by electrochemical dilatometry͒ that occur at the same potential region. The electrochemically activated e-MCMB particles carry slitlike pores of ca. 0.45 nm in the mean interlayer distance, into which ions very likely enter either bare or with partial solvent shells with a mixed adsorption/ intercalation charge storage behavior. A full cell fabricated with two e-MCMB electrodes delivers a volume specific capacitance of 30-24 F mL Until now, electric double-layer capacitors ͑EDLCs͒, which deliver a higher rate capability and longer cycle life as compared to the modern secondary batteries, have been used as the energy storage device for memory back-up systems. [1][2][3][4][5][6] Recently, the markets for EDLCs have been extended to the higher power and higher energy systems such as hybrid electric vehicles. Energy density of the present EDLC, however, does not meet the market's need. Hence, to exploit such new applications, it is necessary to develop electrode materials having a higher energy density than the conventional ones.The energy density of EDLC is given by E = 1/2 CV 2 , where C stands for the capacitance per volume or weight, and V the working voltage. An enlargement in either C or V can thus be the way to achieve a high energy density in EDLCs. 3,5,7 One way to increase the cell voltage is the use of nonaqueous electrolytes. Normally, the cell voltage of EDLCs employing aqueous electrolytes is below 1.2 V, which can, however, be enlarged up to 3.0 V by using nonaqueous electrolytes. The other approach to increase the energy density is the employment of high-capacitance electrode materials, which are normally high-surface-area conductive materials as the electric double layer is formed at the electrode/electrolyte interface.

Opening Two‐Dimensional Materials for Energy Conversion and Storage: A Concept

“…This review mainly focuses on the opening of 2D materials and their subsequent applications in energy conversion and storage fields, expecting to promote the development of such a new class of materials, namely expanded 2D materials. [17] In addition, Seung et al prepared an expanded graphite with a larger interlayer spacing (d 002 = 0.404 nm), [18] and it was concluded that the prerequisite for electrochemical activation was the expansion of interlayer distance in graphitic carbons. Furthermore, prospects and further developments in these exciting fields of the expanded 2D materials are also commented.…”

“…For example, experimental results from Zhang et al showed that the electrical transport properties of MoS 2 nanosheets could be tuned by intercalating phosphorus atoms (P). [17] In addition, Seung et al prepared an expanded graphite with a larger interlayer spacing (d 002 = 0.404 nm), [18] and it was concluded that the prerequisite for electrochemical activation was the expansion of interlayer distance in graphitic carbons.In this review, we mainly focus on how to open 2D materials and their subsequent applications in energy conversion and storage fields, hoping to promote the development of expanded 2D material (Scheme 1). To begin with, a variety of methods for opening 2D materials are described.…”

Hydrogen ion conducting starch-chitosan blend based electrolyte for application in electrochemical devices

“…In the development of electrochemical double layer capacitor (EDLC), many materials have been used as electrode active material like carbon aerogel [20,21], manganese oxide (MnO 2 ) [22], graphite [5,23] and activated carbon [24][25][26]. However, activated carbon is preferred due to its outstanding properties such as high surface area, low cost, high conductivity and good chemical stability [26].…”

Facile synthesis of high-performance Ni(OH)2/expanded graphite electrodes for asymmetric supercapacitors

“…EG has been investigated owing to its unique properties in terms of excellent electrical conductivity, good chemical stability, cheap price, relatively wider interlayer spacing and larger specific surface area as compared to graphite [31]. Guo et al [32] prepared AC/EG composite by a novel supersonic wave shocking method, the largest specific capacitance (Cs) can reach to 305 F/g at 1 mA charging current.…”