Elsevier

Journal of Power Sources

Volume 236, 15 August 2013, Pages 215-223
Journal of Power Sources

Copper nanocrystal modified activated carbon for supercapacitors with enhanced volumetric energy and power density

https://doi.org/10.1016/j.jpowsour.2013.02.036Get rights and content

Abstract

Copper nanocrystals homogeneously dispersed in activated carbon (AC) were synthesized by an in situ solution-based absorption–reduction method, and the resulting composites were characterized and studied for supercapacitor application. The effects of copper nanocrystals and their content on the electrochemical properties of the obtained AC composites were investigated using multiple techniques, including X-ray photoelectron spectroscopy, nitrogen sorption analysis, cyclic voltammetry, galvanic cycling, and electrochemical impedance spectroscopy. Results showed that the addition of copper nanocrystals in the AC composites has little effect on the surface area and porosity of AC, and is helpful to reduce its equivalent series resistance, which is favorable to achieve higher charge/discharge rate and improve its power density when used as electrodes for supercapacitors. Copper nanocrystals and the partially oxidized phase CuO act as pseudo-active materials and improve the capacity of the AC composite electrode. The volumetric capacity, energy density, and power density of copper nanocrystal modified activated carbon are much higher than those of AC and AC mechanically mixed with copper particles. Copper nanocrystal modified AC presents a high volumetric capacitance of 62 F cm−3, more than twice of that of unmodified AC.

Highlights

► Cu modified activated carbon for supercapacitor was synthesized by in situ solution-based absorption–reduction method. ► The incorporation of copper nanocrystals in AC has little effect on the surface area and porosity of activated carbon. ► The incorporation of copper nanocrystals improves the electrical conductivity of the carbon network. ► The addition of Cu can greatly increase the volumetric capacitance and power density of AC. ► The effects of copper content on the electrochemical properties of AC were investigated.

Introduction

Currently, electric double layer capacitors (EDLCs) have evoked wide interest in recent years due to their ability to supply high power in short-term pulses, which makes them very good energy storage devices for applications such as hybrid power sources for electric vehicles and portable electronic devices [1], [2], [3], [4]. The working mechanism of EDLCs is based on the quick formation of a double layer of surface charges and counter-ions at the electrode/electrolyte interface. For the attainment of high capacity and fast kinetics, electrode materials for EDLCs should have large surface area to accumulate a large amount of charges, and a size-controlled porous channel system for easy access of the electrolyte [1], [3]. A lot of efforts have been devoted to develop highly porous electrodes with very high specific surface area and low density. These electrode materials offer high gravimetric energy and power densities; however, the volumetric energy and power densities are typically very low.

So far, activated carbons (AC) are the mostly widely used EDLC electrode materials due to their large surface area, relatively good electrical properties, moderate cost, and somewhat controllable pore size [1], [2], [3], [4], [5], [6], [7]. However, despite the high specific surface area, application of porous carbon materials has been limited due to their low conductivity and incomplete use of all pores for charge accumulation. For AC, typically only about 10–20% of the “theoretical” capacitance was observed due to the presence of micropores that are inaccessible to the electrolyte, wetting deficiencies of electrolytes on electrode surface, and/or the inability of a double layer to form successfully in the pores [8]. Several methods have been used to control the microstructure of porous carbon, but these techniques are onerous and expensive [2], [7], [8], [9].

Introducing pseudocapacitance to traditional EDLCs can significantly increase their capacitance. This involves voltage dependent faradaic reactions between the electrode and the electrolyte, either in the form of surface adsorption/desorption of ions, redox reactions with the electrolyte, or doping/undoping of the electrode materials [1], [2], [3], [4]. The most commonly studied materials include those with oxygen- and nitrogen-containing surface functional groups, electrically conducting polymers, such as polyanilines [1], [2], [3], [4], [10] and polypyrrole [1], [2], [3], [4], [11], [12], transition metal oxides e.g., RuO2, MnOx, SnO2, NiO, CuO, etc. [4], [13], [14], [15], [16], [17], [18], [19], [20], or the combination of conductive polymers and transition metal oxides [4], [21]. Incorporating metal onto the surface of porous carbon can work to improve its conductivity due to electronic interactions between the two components [20]. One metal that is of particular interest is copper, which is an excellent conductor, low cost, abundant, and nontoxic. Previous studies [22], [23] have shown that introducing copper to mesoporous activated carbons can significantly enhance the capacitance of an electrochemical capacitor. Hwang and Teng reported that electrodeposition of copper on activated carbon fabric can provide a simple and economical means to substantially enhance the capacitance of carbonaceous electrodes [22]. Men et al. achieved high capacitance using a doping method to fabricate Cu-doped activated carbon composites from phenolic resins [23]. In this study, we attempt to introduce copper nanocrystals both on the surface and in the pores of activated carbon; modification can increase the accessible surface area, improve the wettability of the electrolyte, and reduce the electrical resistance. In addition, copper nanocrystals act as a pseudo-active material and add faradaic capacitance to the EDLCs, similar to noble metals such as Pt and Pd [4], but much more economic. All these factors will result in a greatly enhanced active material in a given volume and, thus, significantly improve its volumetric capacity, energy density, and power density.

Microsized copper particles directly admixed with AC are difficult to incorporate uniformly, and can cause a decrease the surface area and porosity of AC. Therefore, in this paper, copper nanocrystals deposited directly on the surface and inside pores of AC were synthesized by an in situ solution-based absorption–reduction method. Fig. 1 illustrates the rationale and intended structure of the AC–Cu nanocomposites. Using the in situ solution-based absorption–reduction technique, copper nanocrystals with small size and high dispersibility were deposited directly onto the surface and inside the pores of AC. The microstructures and electrochemical properties of the resultant AC–Cu nanocomposites were systematically studied with particular focus devoted to the effects of the copper content on the electrochemical properties of the AC composites.

Section snippets

Materials synthesis

AC–Cu composites were synthesized by an in situ absorption–reduction method. In order to improve the reactive activity, especially the absorbing ability of activated carbon (AC Calgon), the AC was first acid treated with HNO3. 5 g of AC was refluxed in 50 mL 6 M HNO3 solution while stirring for 1 h at 80 °C to introduce negative groups onto its surface. The obtained product was labeled as AC–H. Then 0.34 g AC–H and a given amount of CuCl2·2H2O were ultrasonically dispersed in 12 mL distilled

Composition and structure

In order to understand the structure and the difference between the samples obtained by the in situ technique and mechanical mixing method, AC, AC–Cu (12%) and AC–Cu-mix (12%) were chosen as models to undergo the following characterization.

To determine the existing state of copper in the AC–Cu (12%) composite, XRD patterns of AC–Cu-mix and AC are presented in Fig. 2. It can be seen that AC is noncrystalline. After Cu modification (Fig. 2a), the peaks corresponding to Cu (111) and (200) were

Conclusion

Modification of activated carbon with copper nanocrystals, both on the surface and in the internal pore surface of the carbon, has proved to increase the capacitive behavior for supercapacitor applications, especially for the improvement of volumetric capacity. The Cu nanocrystals tuned the porous structure of carbon to match the size of the electrolyte ions and induced faradaic reactions, which contribute to the capacitance that is already present from electric double-layer formation.

Acknowledgments

This work is supported in part by National Science Foundation (DMR-0605159 and CMMI-1030048) and Intel Corporation. Lili Zhang would like to acknowledge the Program for Jiangsu Higher Institutions Key Basic Research Projects of Natural Science (10KJA430005), Technological Research Foundation of Huai'an City (HAG2011008) and Qing Lan Project for financial support.

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