Effect of nickel catalyst on physicochemical properties of carbon xerogels as electrode materials for supercapacitor

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Abstract

Carbon xerogels and Ni-doped carbon xerogels prepared by the sol-gel polymerization were examined to reveal the effect of metallic nickel incorporated in carbon matrix on the physicochemical properties of carbon xerogels and their electrochemical performance for supercapacitor electrode in aqueous 6 M KOH solution. As shown by XRD and XPS measurements, the decomposition of nickel precursor in carbon matrix led to the creation of well-crystalline particles of metallic nickel which gave rise to the changes in the morphology, chemical and porous structure of carbon xerogels. Due to the modification of porous structure the surface area increased from 595 m2/g via 632 m2/g up to 660 m2/g for carbon xerogel, 7 wt% and 10 wt% Ni-doped composites, respectively. The enhancement of the surface area occurred along with diminishing the BJH average pore diameter. The value for nickel free xerogel amounted to 11.35 nm, whereas the value of 5.71 nm was measured for 10 wt% Ni xerogel. The changes in the porous and chemical structure created during the formation of carbon-nickel composites as well as the pseudo-capacitive effects arising from the redox reaction of nickel particles present in carbon matrix brought about a significant improvement of electrode capacitance. Electrochemical measurements showed that in comparison with capacitances measured for nickel free electrode (82.1 F/g calculated using the cyclic voltammetry and 88.8 F/g calculated using the galvanostatic charge/discharge method), the respective capacitances for 10 wt% Ni-doped carbon xerogel increased up to 103.0 F/g and 103.4 F/g. These values correspond to 25 and 16% improvement, respectively.

Highlights

► Electrochemical supercapacitors; carbon xerogels-nickel composite electrodes. ► Improved specific capacitance as effect of incorporation of Ni/NiO dopants to carbon xerogel matrix. ► New preparation method and properties.

Introduction

Carbon gels are materials which were prepared for the first time by Pekala [1] through polycondensation of resorcinol (R) with formaldehyde (F), and sodium carbonate added as a catalyst. The obtained resorcinol-formaldehyde (RF) gels are dried, most often by the exchange of a solvent [1], drying with supercritical CO2 [1], [2], drying at ambient temperature [3], [4], [5], [6] or by freeze drying [7]. Supercritically dried gels are called aerogels, whereas drying in an inert atmosphere gives xerogel. When liquid solvent is removed by a freeze-drying method, the gel obtained is named cryogel. The products of drying are then subjected to thermal treatment to obtain carbon gels. Organic gels can also be obtained from other substrates, e.g., polyvinyl chloride [8], cresol [9] and in solvents different from water [10]. However, not all organic gels can be used as reagents for obtaining carbon aerogels by carbonization. Aerogel type resorcinol-formaldehyde (RF) has the most widespread usage because of the possibility of its conversion during pyrolysis into carbon aerogel.

Among different carbons with a highly developed surface, carbon gels and metal-doped carbon gels are considered as promising electrochemical materials [2], [11], [12], [13], [14], [15]. Carbon gels can be obtained in the form of monoliths, beads, powders or thin layers. Their unique properties, associated with well-developed and controlled micro- [16] and mesopores [17], [18], create a chance for the application of these materials in the adsorption [16] and catalysis processes [19], whereas their good electrical conductivity makes them attractive materials for electrochemical applications [4], [12], [20], [21], [22], [23].

It is worth mentioning that various metals can be easily incorporated into carbon xerogels to create carbon-metal composites. Such materials can be produced by the addition of the soluble metal salt in the process of a sol-gel polymerization reaction. During this process the precipitation or crystallization of metal salts within the pore network occurs. The presence of metal salts can also play the role of catalyst modifying the degree of polymerization or gelation, and can have an influence on the morphology and structure of pores of the organic gels.

There are the three main methods for incorporation of metal into the carbon matrix. The first one consists in the introduction of dissolved precursor of metal to the initial resorcinol-formaldehyde mixture [24], [25]. The second method uses parent derivatives of resorcinol containing ion–exchange moieties that could be polymerized using sol-gel techniques [26], [27], [28], [29]. As a result, each repeated unit of organic polymer becomes a binding site for metal ions, ensuring an uniform dispersion of the admixture. The third method involves the deposition of metallic precursor on the organic or carbon gel via impregnation with metal salt followed by pyrolysis [14].

Various metal-doped carbon gels have already been prepared and characterized from the point of view of physicochemical properties, including Fe-, Co-, Ni-, Cu-, Mn-, Ag-, Ti-, Pd-, Ru- or Pt-doped carbon aerogels [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]. Some of these (containing Fe, Co, Cu, Mn, Zn and Pt dopants) have been studied electrochemically [11], [14], [15], [35]. The influence of Ni dopant on the capacitance of carbon aerogels has been reported recently [36]. In this paper, carbon aerogels, prepared by polycondensation of resorcinol with formaldehyde using sodium carbonate as a catalyst, were then impregnated with nickel(II) chloride hexahydrate dissolved in ethanol, precipitated in alkaline solution, filtered and finally calcined at 250 °C. Thus prepared nano-sized Ni-doped carbon aerogels with Ni content ranging from 21 to 82 wt% were examined as electrode materials. The highest capacitance of 92 F/g and 120 F/g were attained for 35 wt% Ni-doped carbon aerogel using the cyclic voltammetry and galvanostatic charge/discharge method, respectively.

The aim of this work was to prepare carbon-nickel composites by the dispersion of metallic nickel in the carbon gel matrix. In contrast to the preparation procedure used in the paper [36], we prepared nickel-doped carbon gels by polycondensation of resorcinol with formaldehyde using nickel acetate playing a role of a catalyst instead of sodium carbonate and taking a part in the development of porous structure of carbon gels as effect of thermal decomposition to nickel metal accompanied by the evolution of CO2 as an activating agent. Moreover, nickel particles incorporated in carbon matrix were expected to enhance the specific capacitance of material, similar to beneficial effect attained using different method of preparation [36].

Both carbon xerogel and carbon-nickel composites were examined by the cyclic voltammetry (CV) and galvanostatic method in alkaline solution in order to determine their electrochemical activity. Special attention has been paid to the influence of nickel present in the carbon gel matrix on the physicochemical properties of composites and the resulting changes in the specific capacitances of electrodes examined for the use as electrical double-layer capacitors.

Section snippets

Preparation of carbon gel and carbon-nickel composites

RF hydrogels were synthesized by the polycondensation of resorcinol (R) with formaldehyde (F), and sodium carbonate (K) as a catalyst. Molar ratios R/F and R/K were 0.5 and 1000, respectively. Resorcinol was mixed with the catalyst and deionized water. After forming a homogeneous solution, formaldehyde was added and the solution was stirred for 1 h. The weight percentage of the reactants in solution was set at RF = 40%. After mixing was completed the initial pH of RF solution was equal to 6.0.

Influence of nickel doping on morphological properties of xerogels

The SEM images of carbon gels shown in Fig. 1, Fig. 2, Fig. 3 allow observing the changes in the morphology of carbon gels caused by adding nickel salt to the reaction mixture. As mentioned above, because carbon gel obtained by heat treatment of aerogel free of nickel salt appeared to be much harder than those prepared due to pyrolysis of xerogels admixed with nickel salt, hence more intense conditions of milling had to be used. As effect of such a treatment the carbon material composed of

Conclusions

It was shown that electrochemical performance of carbon gels examined as electrode materials for supercapacitor undergoes a strong modification due to nickel metal particles incorporated in carbon matrix. Based on the SEM and XRD data it has been demonstrated that carbon-nickel composites are composed of sphere-like granules incrusted with well-crystalline nickel particles. On increasing the nickel content these granules becomes more spherical and tend to join each other to form linearly

Acknowledgements

The financial support from the grant No. DS 31-199/2010 is gratefully acknowledged.

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