Antimony species was chemically anchored on graphene oxide using antimony (III) chloride precursor and then converted to the reduced graphene oxide-antimony species composite by a well-established polyol method. The resultant composite was successfully used as supercapacitor electrodes in a two-electrode symmetric system with aqueous electrolyte. The specific capacitance calculated from the galvanostatic charge/discharge curves obtained for this composite was 289 F/g. The enhanced capacitance results were confirmed by the electrochemical impedance spectroscopy and cyclic voltammetry. The high capacitance of the reduced graphene oxide-antimony species composite arises from the combination of double-layer charging and pseudocapacitance caused by the Faradaic reactions of the intercalated antimony species and residual surface-bonded functional groups.
Antimony is widely used in semiconductors, antifriction alloys, small arms and tracer bullets, and cable sheathing and in large quantities as a flame retarding additive [
In the lithium-ion batteries, antimony is thought to play two roles. First, it acts as a spacer to prevent large volume changing of electrode during charging/discharging cycles; second, it can accommodate about 3 lithium atoms and give an additional capacitance. Problem of anode pulverization during electrode working was examined mainly by Besenhard [
These researches indicate that antimony chemistry may play a crucial role in the new energy storage systems. As far as we know, application of graphene oxide-antimony species composites as a supercapacitor electrode was never reported before, though we have found some preliminary trials to synthesize peroxoantimonates on graphene oxide [
It has high theoretical surface area up to 2630 m2/g, Young’s modulus as high as 1 TPa, and thermal conductivity of 5000 W/(mK) [
Graphite oxide (GO) was synthesized by a modified Staudenmaier method [
Materials were characterized with powder X-ray diffraction (XRD, X’Pert Pro, Philips) with a step size 0.02° and Cu K
All electrochemical experiments were performed with Autolab PGSTAT 30 workstation using the two-electrode symmetric system. The working electrode and counter/reference electrode, composed of 15 mg of active material and 2.5 mg polytetrafluoroethylene (PTFE) binder (Sigma Aldrich, 35
As mentioned previously, the polyol process using mixture of ethylene glycol and water was used to prepare reduced graphene oxide-antimony species composite. In comparison to the traditional reduction method using hydrazine or metal hydrides, the polyol process demonstrates an enhanced control of uniform metal dispersion and deposition as well as homogenous in situ generation of reducing species [
The exact formula of the antimony species after hydrolysis at high pH is precisely not known.
It was not possible to univocally identify the obtained XRD pattern. Except for typical and characteristic broad graphitic signal located near 25 degrees satisfying GO reduction, there were sharp signals belonging to antimony species recognized as the antimony oxide hydroxide of the form Sb3O6OH and antimony oxide hydrated Sb2O5*4H2O. Figure
XRD patterns of graphite (gray), GO (red), and reduced graphene oxide-antimony species composite (blue).
RGOSb composite was prepared in ethylene glycol that satisfied complete and uniform reduction of graphene oxide. Consequently, the broad signal in the graphitic region with the interlayer distance similar to that of parent graphite was observed. The XRD pattern of the RGOSb composite revealed also well-developed reflections of antimony species. This finding implies that the antimony salt truly cannot simply convert into antimony oxide but the oxide hydroxide or hydrated forms are preferred. Presence of antimony (III) and (V) in the composite may suggest partial oxidation of antimony by GO.
SEM micrographs show the morphology of graphite, graphite oxide, and reduced graphene oxide-antimony species. Graphite intercalated by the oxygen-containing groups after oxidation has a rose-like, corrugated structure that can be observed in a Figure
SEM images of graphite (a), GO (b), and RGOSb (c).
Raman spectroscopy is a useful method to investigate hybridization of carbon atoms and defects and crystal disorder of graphene composites. Figure
Raman spectra of graphite, GO, and RGOSb.
The G band that corresponds to sp2 carbons is broadened after oxidation and its intensity with respect to the D band diminishes due to change of the carbon atoms hybridization from sp2 to sp3. This effect is more marked in the RGOSb, where most of the residual sp2 carbon atoms are converted to sp3 and bonded to the intercalating molecules.
The intensity ratio
Qualitative analysis of composite and its precursor was performed using infrared spectroscopy. Figure
FTIR spectra of the GO and RGOSb.
After reduction of the composite in ethylene glycol, most of the signals disappeared, only one signal of the residual OH groups (higher magnification) located around 1,350 cm−1 was observed. This may indicate very efficient graphene oxide reduction in ethylene glycol; however, difficulties in removal of the OH groups from graphene oxide by chemical reduction were confirmed [
Electrodes were characterized in the two-electrode cell by the cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy. The CV curves show the reversible charge/discharge characteristics of the composite electrode. Experiments were carried out at scan rate 5 mV/s, 20 mV/s, and 500 mV/s in two potential windows from 0 to 1 V and from −0.5 to +0.5 V. The potential window 0-1 V is more often used in aqueous electrolyte supercapacitors experiments [
CV curves of graphite (gray), GO (red), and RGOSb composite (blue) registered in the two potential windows 0-1 V (a) and −0.5+ to 0.5 V (b) at the scan rate 20 mV/s.
The well-developed supercapacitor has to satisfy two conditions, namely, operate in a possibly high current and have a box-like rectangular shape. Based on a Figure
CV curves of graphite (gray), GO (red), and RGOSb (blue) at scan rate 500 mV/s and 1000 cycles (a) and comparison of CV curves recorded for RGOSb at 5 mV/s and 100 cycles in two potential ranges (b).
After 1000 cycles the specific capacitance of graphite dropped to 50% of the initial with a charging current of 0.0015 A, while for GO difference between the first and the last step was quite small with the average charging current of 0.0004 A. In RGOSb, once again the highest average charging current of 0.009 A and more rectangular shape were recorded; however, the specific capacitance dropped to 80% of the initial value. Additionally, Figure
These results show univocally that antimony species in oxide hydroxide or hydrated form is able to enhance the specific capacitance of graphite oxide many times. This is mainly attributed to the additional Faradaic reactions of the active antimony species with the electrolyte. Although it has been demonstrated in literature that the oxygen-containing functional groups of graphite oxide can enhance the total capacitance through Faradaic reactions and may improve the wettability of porous carbon with electrolytes [
The galvanostatic charge/discharge is the most credible method to calculate the specific capacitance. It is used to show real capacitance value and charge/discharge characteristics of energy storing materials. Figure
Galvanostatic charge/discharge curves of RGOSb at 0.004 A with discharge time 700 s (light blue) and 1000 s (dark blue).
RGOSb was galvanostatically charged at 0.004 A for 100 s (light blue) and 200 s (dark blue) and discharged for 700 s and 1000 s, respectively. The specific capacitance obtained from these measurements was 244 F/g at shorter discharge time and 289 F/g after 1000 s. In both cases, quite unsymmetrical galvanostatic curves with quick charging and very long discharge time were obtained preceded by iR drop. The specific capacitance for RGOSb was much higher than that obtained for graphite and graphite oxide, 12 F/g and 4 F/g, respectively.
Figure
Nyquist plots of graphite (gray), GO (red), and RGOSb composite (blue) over the frequency range from 100 kHz to 100 mHz.
Generally, in the RGOSb composite, two regions can be distinguished including the semicircle corresponding to the Faradaic charge transfer resistance [
In this work, the new reduced graphene oxide-antimony species composite has been prepared and characterized for supercapacitor electrodes. The polyol process was used to prepare dispersion of antimony active species in reduced graphene oxide. This method did not require thermal annealing of the active species into antimony oxides. It showed the enhanced specific capacitance, as high as 289 F/g, resulting from the pseudocapacitance and very good cyclability with respect to the parent graphite and graphite oxide.
The authors declare that there is no conflict of interests regarding the publication of this paper.