The electrochemical performance of carbon xerogels with the addition of graphite intercalation compound

Highlights

• The morphology and structure of composites was changed by NiCl₂-FeCl₃-GIC addition.

• Carbon xerogel and carbon-GIC composites were active in 6 M KOH aqueous solution.

• The higher specific capacitances were obtained for the xerogels with the addition of GIC.

Abstract

The sol-gel polycondensation method was used to prepare a resorcinol-formaldehyde (RF) hydrogels. The RF gels and RF gels with the addition of graphite intercalation compound (GIC) were carbonised at 1000 °C in N₂ atmosphere to obtain carbon xerogels (CX) and CX-GIC composites, respectively. The purpose of this paper was to describe the impact of admixture of GIC on the electrochemical performance of CX-GIC composites. Studies of morphological, crystalline and porous properties of obtained xerogels showed that the addition of GIC resulted in changes of the carbon matrix. It has been proved that xerogel CX and composites CX-GIC are electrochemically active in aqueous alkaline solution. The addition of GIC caused an enhanced of electrochemical activity. The specific capacitances for CX-GIC electrodes were also higher, from 21 to 65%, than for the carbon xerogel electrode. The increase of capacitance depended on the amount of GIC added.

Introduction

Carbon gels are porous carbon materials that can be obtained following Pekala's method [1]. The synthesis leading to the formation of carbon gels consists of the receiving of organic gels, which are subjected to drying and pyrolysis in an inert atmosphere to form carbon gels [[2], [3], [4], [5]]. Depending on the type of drying method used in the synthesis process, i.e. supercritical drying [1,6], drying at ambient condition [[7], [8], [9], [10]] or freeze drying [11,12], they are classified as aerogels, xerogels or cryogels, respectively.

These materials can be prepared in monolithic shape, as well as in the form of globules, ashes or slight films. Their specific features, such as well-developed and monitored micro- and mesoporosity [[13], [14], [15]], give options for the production of materials to be used in adsorption [[16], [17], [18], [19], [20], [21]], catalysis [22], and as materials for gas sensors [23]. Carbon gels, among different coals with more developed surface, can be very promising electrode materials [[24], [25], [26]]. This is due to their good electrical conductivity and highly accessible surface. Carbon gels are also promising electrode materials for supercapacitors [[27], [28], [29], [30]].

Graphite intercalation compounds (GICs) can be obtained by introducing of certain atoms, molecules or ions (intercalates) into the interlayer space of graphite lattice. Intercalates are bound to a graphite matrix as a consequence of the formation of chemical bonds [31]. GICs can be transformed into expanded graphite (EG) due to exfoliation. The process of exfoliation can be realized by three methods: thermal (carried out at high temperatures) [[32], [33], [34], [35], [36], [37]], chemical (performed at ambient temperature in a solution containing a chemical agent) [38,39], electrochemical (GIC is decomposed under a given potential or current) [40,41].

During thermal treatment intercalates can be partially or completely removed from the graphite matrix, in consequence the existing a swollen product composed of highly wrinkled platelets with very torn edges is formed. The above mentioned treatment commonly results in an enormous distention of graphite flakes along the c-axis and an enormous development of specific surface area [32,33,38]. GICs containing intercalates of catalytic activity in the graphite matrix are of a particular interest, because due to exfoliation they can be transformed into an exfoliated graphite/catalyst system.

In our earlier work [42] the effect of the addition of EG to carbon gel on the electrochemical properties of the composites in the process of capacitance was checked. Due to the presence of EG in the carbon matrix composites a significant increase in the capacity of a double layer was shown.

The purpose of this study was to obtain carbon xerogels with the addition of graphite intercalation compound (CX-GIC) by the admixture of NiCl₂-FeCl₃-GIC to carbon matrix. We anticipated that this addition can affect the increase of the BET surface area of the carbon gels as an effect of exfoliation taking place during process of thermal treatment, carried out at 700 °C. Moreover, GIC incorporated in the carbon matrix was expected to enhance the specific capacitance of material, therefore electrodes made of carbon gels and carbon-GIC composites were examined for the use as electrical double-layer capacitors.