Performance of carbon xerogel-graphene hybrids as electrodes in aqueous supercapacitors

Abstract

A pristine carbon xerogel (AX) and two hybrid samples (AX-3% and AX-9%), with different graphene percentages (3 and 9 wt%), were synthesized using a fast and economical process. It was observed that graphene produces less shrinkage of the xerogel structure during synthesis. Moreover, the electrical conductivity of the materials increases linearly with the percentage of graphene added. Thus, AX-9% presents an electrical conductivity 135 and 321% greater than that of AX-3% and AX, respectively.As a result of the good pore size distribution and high electrical conductivity of AX-9%, when this material is used as electrode in supercapacitors, the resistance of the cell is reduced; therefore, better power densities are obtained. However, its capacitance values are the same as those of AX-3%. The performances of these materials as electrodes in supercapacitors were evaluated taking into account the influence of their porosity and electrical conductivity. Moreover, AX and AX-9% were subjected to mild oxidation with air to study the effect of air on the porosity, electrical conductivity, and performance of these treated samples as electrodes in supercapacitors.

Graphitized Carbon Xerogels for Lithium-Ion Batteries

“…Table 1 shows that the ordered part of both of the graphitized carbon xerogels (GX-300 and GX-100) present values of d 002 that are close to that of graphite (i.e., 0.335 nm at 2θ = 26.6 • ). Moreover, their crystalline dimensions along the c-axis (Lc) are larger than those reported for carbon xerogels in the literature [33] although much smaller than that of the reference graphite. In any case, there is a relevant difference between these two GXs, i.e., that the graphitic contribution of sample GX-300 (22.6%) is three times greater than that of GX-100 (7.5%), indicating that the pore size of the carbon xerogels exercises a marked influence on the graphitization of these samples.…”

“…Instead, two broad peaks are located at ca. 25 • and 45 • 2θ [33,46]. In general, the graphitization process leads to the narrowing and the displacement of the maximum of the peak at around 2θ = 25 • attributed to the (002) diffraction peak of the graphitic framework.…”

“…As observed in Figure 3a, where the isotherms of CX-100 and GX-100 are compared, the graphitization process produces a decrease in the volume of micropores, which in turn leads to a decrease of 88% in the S BET of the graphitized sample (Table 1), while the macroporosity remains unchanged (Figure 3b). It would appear, therefore, that graphitization reorganizes the basal planes inside the nodules of the polymeric structure of the xerogels producing a collapse of the micropores [33], but the heat treatment does not modify the size of the nodules and the space between them (i.e., the size of the macropores).…”

“…To perform the electrochemical characterization, electrodes were prepared by mixing 92 wt% of carbon xerogel, 8 wt% of polyvinylidene fluoride (PVDF) as binder and N-methyl-2-pyrrolidone (NMP). The mixture was kept under stirring to obtain a homogeneous slurry and then deposited by dripping onto pressed copper disks, with a diameter of 10 mm and a thickness between 105-110 µm, subsequently, they were left to dry overnight at 60 • C [33]. After this process, electrodes with an active material thickness of 35-40 µm and a mass loading in the range of 1.3-3.5 mg cm −2 were obtained.…”

“…This high volume of pores makes xerogels to be carbons that have a lot of defects and it is this that causes the electrons to move more slowly than in other more ordered materials resulting in a lower electrical conductivity which is one of the essential requirements of electrochemical devices. To overcome this drawback, some authors have introduced a highly conductive additive into the xerogel structure to facilitate a better electron mobility [25][26][27][28][29][30][31][32][33][34][35] and to obtain a better performance as electrode in supercapacitors, while others have used graphitization processes at high temperatures (2000-3000 • C) to obtain highly ordered carbon xerogels with enhanced conductivities [33,[36][37][38]. Each different approach gives rise to final carbons that combine ordered with turbostratic parts in a single material, resulting in diverse chemical properties, all of which contribute in different ways to the energy storage mechanism.…”