Structurally tailored carbon xerogels produced through a sol–gel process in a water–methanol–inorganic salt solution

Wojciech Kicin ski 0 Mateusz Szala 0 Marcin Nita 0 0 W. Kicinski (&) M. Szala M. Nita Military University of Technology , Kaliskiego 2, 00-908 Warsaw, Poland The impact of solvent composition as well as inorganic salt content and type on carbon xerogel structure was investigated. Carbon xerogels were derived from the sol-gel polycondensation of resorcinol with furfural in a water-methanol-inorganic salt solution. As inorganic salts, NaCl, NH4ClO4 and FeCl3 were used. In order to conduct an accurate examination of the carbon xerogel structures and textures, inorganic salts were removed prior to carbonization. The xerogel structures can be tailored according to the water/methanol ratio and, to a lesser extent, according to the inorganic salt content and type in the starting solution. As a result, a significant amount of salt can be introduced to the gel network of the desired structure. The morphology and physical properties of the organic xerogels, carbon xerogels and their composites were characterized by means of SEM, N2 sorption and XRD. It was found that samples derived from mixtures with FeCl3 manifest well developed mesoporosity and depleated microporosity in comparison to samples prepared from mixtures with NaCl and NH4ClO4. Iron ions chemically bond to the xerogel matrix and cause its partial graphitization during the carbonization process, resulting in enhanced mesoporosity. - Owing to their high porosity, open pore network, easily tailored and replicable continuous structure, high surface area and attractive electrochemical properties, carbon gels (aero-, xero-, and cryo-gels) are drawing a lot of attention lately [129]. Their interesting properties can be additionally modified and enhanced through activation, graphitization, composition with conductive nanostructures (i.e. carbon nanotubes, carbon nanofibers) and doping with inorganic materials (metal, metal oxides, carbides, or phosphides) [317, 23]. The modified carbon gels have a range of potential applications, in particular, in adsorption, catalysis, fuel cells, supercapacitors and batteries [1831]. Complicated and time-consuming supercritical extraction and freeze-drying make aero- and cryogels less attractive for economical reasons, thus favoring carbon xerogels (CXs), which are obtained through simple evaporative drying and carbonization of organic gels [32]. From a short literature review one can conclude that many methods to prepare xerogels with enhanced porosity have been proposed [3344]. Job and co-workers showed in detailed research that for the set resorcinol/formaldehyde molar ratio and dilution, monolithic, porous xerogels with tailored structure and high values of pore volume and specific surface area (*2 cm3/g and *700 m2/g, respectively) can be obtained by choosing an appropriate pH of the precursor solution [18, 4043]. This procedure can even be shortened if microwaves are employed to dry organic gels [44]. Moreover, they pointed out that for a given pH, the catalyst (substance used to adjust pH) chemical formula itself, i.e. the particular cations from the catalyst and their concentration, can affect the gel structure [45, 46]. Additionally, research pursuing the doping of resorcinol formaldehyde gels through salt solubilization revealed that the presence of inorganic salt in the solution of gel precursors significantly affects the gel structure [58, 11, 17, 4749]. This finding was attributed not only to the pH modification (salts usually possess acido-basic properties) but also to the cation nature. The solubilization of inorganic salts in the solution of precursors, followed by evaporative drying, yields salt-doped organic xerogels. High homogeneity of those hybrid organic polymer/inorganic salt composites creates an opportunity to obtain new functional materials. For example, organic xerogels doped with perchlorates or nitrates can serve as explosives [5052], while carbonization of an organic xerogel/metal salt composite leads to metal doped carbon xerogels [6, 8, 47, 53], and in the case of transitions metals, porous graphitic carbon can be obtained after metal removal [15, 27, 30, 54, 55]. Metaldoped carbon xerogels and highly graphitic porous carbons have been widely investigated as candidates for carbon supported metal catalysts [14, 27, 30, 5457]. All of that suggests that doping of organic gels with inorganic salts via salt solubilization can be an easy and safe way to obtain many interesting functional nanostructured materials. Since evaporative drying is economically reasonable and doping by salt solubilization is convenient whilst enabling control of the gel structure, it is justified to continue research in this direction. This paper proves that the resorcinol-furfural (R-F) xerogel structure is controllable when the solgel process is carried out in watermethanol mixture and the salt is added by solubilization into the precursor solution. It was shown that for a fixed water/methanol ratio, a significant amount of salt can be incorporated into the resorcinol-furfural (R-F) gel network of the desired structure. Since the aim of this research was to precisely analyze the structure of carbon xerogels depending on the starting solution composition (water/methanol ratio and salt amount and type), the salts were removed from the organic xerogel structure, which results in a pure resorcinol-furfural xerogel and, after carbonization, in a carbon xerogel. Salts with three different cations (Na?, NH4?, Fe3?) were chosen in order to check if this observation can be generalized and extended to other resorcinol-aldehyde/inorganic salt systems. The obtained organic and carbon materials were characterized by means of scanning electron microscopy, surface area analysis, and X-ray diffraction. 2 Experimental 2.1 Preparation Organic xerogels (OXs) were obtained from condensation of resorcinol with furfural (R-F gel) carried out in a solution of water, methanol (MeOH) and sodium chloride, ammonium perchlorate or anhydrous Fe(III) chloride. The molar ratio of furfural to resorcinol was held at a constant value of 2.3, while water/methanol ratio and salts contents were varied gradually. The experimental data of the synthesis of organic xerogel materials are shown in Tables 1, 2 and 3. Gelation in water/salt solution (without methanol) did not occur; in this case flocky precipitates were obtained. Water acts as a solvent for inorganic salts, while methanol acts as a co-solvent to increase the solubility of organic components, especially furfural which does not mix with water. Additionally methanol is very easy to remove during conventional drying. With the exception of mixtures with FeCl3, which easily hydrolyzes to HCl, sol gel polymerization was initiated using concentrated hydrochloric acid [58]. In a typical synthesis, resorcinol, furfural and inorganic salt were dissolved in a water/ methanol mixture. After that, HCl (37 wt%, 0.8% of the starting solution in each case) was added to (...truncated)