Performance of electric double-layer capacitors using carbons prepared from phenol–formaldehyde resins by KOH etching

doi:10.1016/S0008-6223(01)00027-6

Carbon

Volume 39, Issue 13, November 2001, Pages 1981-1987

https://doi.org/10.1016/S0008-6223(01)00027-6 Get rights and content

Abstract

Phenol–formaldehyde resins were used to produce carbons with different porosities by KOH etching. Through the introduction of a binder, carbon-binder composites were prepared to serve as electrodes in electric double-layer capacitors. Nitrogen adsorption was used to characterize the porous structure of the carbons and electrodes. The performance of the capacitors in 1 M H₂SO₄ was investigated with charge–discharge cycling experiments. The faradaic leakage current and the specific capacitance of the electrodes were found to increase with the specific surface area, while the resistance determined from the IR drop showed an opposite trend. The increased electrode resistance, which would probably result from the increased diffusion path in the pores upon etching, has caused a decrease in the capacitance per unit carbon area. A specific capacitance larger than 100 F/g was achieved with an electrode consisting of 80% carbon particles with a specific surface area of 1900 m²/g and 20% polyvinylidenefluoride as the binder.

Introduction

High porosity carbons, which can be collectively called activated carbon, have attracted strong interest in applications using these carbons as electrode materials for electric double-layer capacitor (EDLC) manufacture [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. The intention of using activated carbon in EDLC manufacture is attributed to the recognized fact that the larger surface area that an EDLC can provide for adsorption of electrolytes on electrodes, the more energy can be stored in the EDLC [9], [10], [11], [12], [13]. However, how the carbon preparation process and the resulting surface characteristics can affect the performance of the EDLC has rarely been discussed at length in the literature.

Most commercial grade activated carbons are derived from naturally occurring carbonaceous materials such as coal and coconut shell [17]. However, the large amount of mineral species contained in these naturally occurring carbon precursors generally affects the properties and thus the performance of the resulting carbon in various applications. Phenol–formaldehyde resins have structural features similar to those in coal, but contain much fewer mineral impurities, which can be controlled to very low levels during their synthesis. Because of the low content of the retained ash, which has a negligible porosity, the specific porosity of the carbon prepared from the resins is expected to be high.

The formation of pores in carbon is generally achieved through carbonization of the precursors followed by etching of the resulting char with oxidizing agents. The etching can remove carbon atoms to open up closed pores and enlarge existing micropores. The type of precursor determines the principal structure of the resulting carbon. Apart from the influence of the precursor structure, the method of etching may also affect the final features of the porosity [18], [19]. In the present work, the creation of porosity in the carbon derived from phenol–formaldehyde resins was conducted by treating the resins with potassium hydroxide. The key mechanisms in porosity development by KOH etching are presumed to be associated with carbon gasification by oxygen contained in the alkali [20], [21]. In addition to alkalis, the etching process can also be conducted by so-called physical activation methods, such as gasification of a char in oxidizing gases like CO₂ or steam [22]. Carbon etching with KOH has been reported to be more effective in porosity development than etching approaches involving physical methods [19], [21].

The porous structure of carbon is usually characterized by gas adsorption. Changes in porous structure during etching are often monitored and controlled to produce various carbons for different applications. However, it has been shown that the adsorption capacity of microporous carbons for phenol in aqueous solutions does not show a linear increase with the surface area determined from gas adsorption [23]. The accessibility of the surface in pores may have been affected by the diffusion of the adsorbate. In the present work an aqueous electrolyte solution is employed to form double layers on the interface between the carbon surface and the solution. The information on how the double-layer capacitance is affected by the variation of surface area may shed light on the mechanism of double layer formation in carbon micropores.

Within the above scope this study is devoted to the applicability of carbons derived from phenol–formaldehyde resins as electrodes of EDLCs. Carbons with different porous structures are prepared by KOH etching to different extents and the effects of the porous structure on the electrochemical behavior and capacitance of the resulting EDLCs are investigated.

Section snippets

Carbon preparation

The phenol–formaldehyde resins were synthesized under an environment of nitrogen at 95°C for 36 h, using an initial formaldehyde-to-phenol molar ratio of 1.33. Reagent grade phenol (Nacalai Tesque Co., Tokyo, Japan) and formaldehyde (Nihon Shiyaku Industrial LTD., Tokyo, Japan) were used. A base-catalyzed method was used in the synthesis, with ammonium hydroxide (reagent grade, Nihon Shiyaku Industrial LTD., Tokyo, Japan) as the base. After synthesis, the resins were cured by heat treatment in

Characteristics of the porous carbons used in electrodes

Carbons with different porosities were prepared by varying the chemical KOH/resin weight ratio. The porous structures of the carbons are summarized in Table 1. Each sample has been designated by using the nomenclature of its precursor (PF, phenol–formaldehyde resin), followed by the chemical ratio employed in the preparation. The untreated resin has a negligible porosity with BET surface area less than 1 m²/g. It can be seen that both the surface area and pore volume increase with the

Conclusions

Carbon particles with different surface areas were prepared from phenol–formaldehyde resins by KOH etching. These carbons are mainly microporous and have a similar distribution of pore sizes. The surface area was found to increase with the extent of etching and reached a maximum value of 1902 m²/g for the highest KOH/resin ratio employed (i.e., 3). Electrodes made of these carbons with PVdF as the binder show stable electrochemical performance in 1 M H₂SO₄ at potentials within −0.3 to 1 V

Acknowledgements

Financial support from the National Science Council of Taiwan is gratefully acknowledged. The project number is NSC 89-2214-E-006-009.

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Performance of electric double-layer capacitors using carbons prepared from phenol–formaldehyde resins by KOH etching

Abstract

Introduction

Section snippets

Carbon preparation

Characteristics of the porous carbons used in electrodes

Conclusions

Acknowledgements

Carbon

Electrochim Acta

J Power Sources

J Power Sources

J Power Sources

Electrochimica Acta

J Non-Cryst Solids

J Power Sources

Electrochim Acta

Carbon

Carbon

Carbon

Carbon