Electrochemical performance and material characterization of reduced grapheneoxide (rGO) / titanium dioxide (TiO2) composite as electrodes for supercapacitors

Electrochemical performance and material characterization of reduced grapheneoxide (rGO) / titanium dioxide (TiO2) composite as electrodes for supercapacitors M. D. R. De Costa 1, , R. C. L. De Silva, L. D. C. Nayanajith, H. C. D. P. Colombage, M. D. Y. Milani, S. R. D. Rosaand I. R. M. Kottegoda 1 Materials Technology Section, Industrial Technology Institute, Colombo 07, Sri Lanka. 2 Department of Physics, University of Colombo, Colombo 03, Sri Lanka. ____________________________________________________________________________


INTRODUCTION
Titanium Dioxide (TiO2) has been extensively explored for different applications including catalysis, energy storage and energy conversion devices such as supercapacitors, rechargeable batteries and solar cells owing to its good stability, nontoxicity and low cost [1,2]. It possesses low charge transfer resistance (Rct = 0.34 Ω) and high specific capacitance (Csp =280.3 Fg -1 ) at current density of 1 Ag -1 (3). Carbon doped TiO2 composite is a promising electrode for supercapacitors when compared to sulfur/TiO2 or R/TiO2 (R= Ag, Pt, Fe) composites due to good electrochemical performances [4,5].
In order to raise a composite with TiO2, graphene is a promising material. Graphene is identified as a single layer carbon atom which was first exfoliated from graphite by Novoselov et al. by using a mechanical tape striping method in 2004 [6]. Graphene possesses excellent thermal and electrical conductivity as well as earns a high surface area. Because of the unique structure of graphene, it results in relatively higher specific capacitance compared to other carbon materials [7]. Therefore, graphene/TiO2 composite would results unique properties.
Chemical and thermal reduction of graphene oxide has been used at mass production of rGO [8][9][10].
In the present study, a novel method one-step hydrothermal reaction was followed for synthesis of rGO/TiO2 composite. The material characterization was done using X-ray diffractrometer (XRD), a Fourier Transformation Infrared Spectrometer (FTIR) and a Scanning Electron Microscope (SEM). The electrochemical properties of rGO/TiO2 based supercapacitor were investigated.

Synthesis of rGO/ TiO2 composite
Graphite oxide (GO) was synthesized by modified Hummers method [11] using natural graphite (99.5%) from Kahatagaha graphite mine in Sri Lanka. After the oxidation reaction, GO was washed with 5% HCl until sulphate ions could not be detected with BaCl2.
Subsequently, GO was washed with distilled water several times to remove ions. Thereafter the residue was dried at 60 °C in vacuum oven over night. GO (1g) was dissolved in distilled water (1600ml) and kept for 30 min under the ultrasonication at room temperature. 2g of TiO2 (bulk of TiO2 powder available in the lab) was added into the above solution under the vigorous stirring for 1h. The suspension was transferred into an autoclave, and it was heated at 120°C for 2hrs. The final solution was washed with ethanol and distilled water several times and dried at 80°C for 2hrs. Finally, the black colour composite of rGO/TiO2 was obtained. The schematic of the experimental procedure of preparation of rGO/TiO2 composite is shown in Figure. 1. Figure. 1: The schematic of the experimental procedure of formation of rGO/TiO2 composite

Electrode constructions and assembling of supercapacitors
Supercapacitor includes three main parts: working electrode (anode), counter electrode (anode) and appropriate electrolyte. To prepare the electrodes, 80% of active material rGO/TiO2 composite was mixed with 15% of active carbon and 5% of polytetraflurethyleno (PTFE) as a binding agent. Known amount of active material was applied on the surface of two identical stainless-steel electrodes. Two electrodes consisted of a mixture of active material (80%), carbon black (15%) and polyvinylidene fluoride (PVDF) as binder agent (5%). A piece of filter paper (50 um) was used as a separator and 1M H2SO4 used as an electrolyte.

Characterization
The phase composition of rGO and rGO/TiO2 materials were studied by a Regaku-ultima VI, X-ray diffractometer with Cu Kα (λ= 1542 Å) radiation with range varying from 5° to 100°.
Crystallographic information of the materials was obtained with the aid of the ICSD data base.
The functional groups of rGO and rGO/TiO2 materials were analyzed using Bruker Tensor 27 Fourier transformed infrared spectrometer. The morphology of TiO2 and rGO/TiO2 materials were studied by a LEOm1420vp scanning electron microscopy. Charge/Discharge performance, cyclic voltammograms, 1 st discharge curves and impedance spectra of supercapacitors were obtained using Biologic sp-150 potentiostat / impedance analyzer. When the GO was reduced to rGO, the diffraction peak at 2θ = 10.5° was disappeared and a new low intensity broader peak appeared at 2θ = 26° corresponding to the rGO (0 0 2) plane as shown in the Fig. 2(b). For the rGO/TiO2 composite, as in the Fig. 2(c), a series of characteristic peaks are observed at 2θ = 25.5°, 37.8°, 48.1°, 54.1°, 55.5°, 63.9°, 70.3° and 74.1° relevant to TiO2, where peak corresponding to rGO is not observed probably due to overlapping with high intense TiO2 diffraction peaks. All the peaks of X-ray diffraction of anatase phase of TiO2 could be identified using ICSD collection code 154604.

SEM Analysis
The SEM image of rGO/TiO2 composite is shown in figure. 4 at 12 K magnification. Though the presence of TiO2 particles is not visible in the image, crumpled flexible nature of rGO has been shown. Higher magnification through FE-SEM or SEM coupling to EDX would obviously enable observing TiO2 particles. Further studies are required for such analysis.
Nevertheless, the presence of rGO in the composite was clearly confirmed by FTIR spectra.   Table 1.   where Cm is the specific capacitance (F g -1 ) based on the mass of electroactive materials, m is the weight of active material (g), ʋ is the scan rate (V s -1 ), ΔV is the potential window (V) and 'I' is the response current density (A). Specific capacitance of supercapacitor with rGO/ TiO2 composite shows a significant value compared to that with the individual materials.  Scan rate 1mVs -1

First Discharge Test
The

Impedance Analysis
The Nyquist impedance plot of rGO/TiO2 supercapacitor is shown in figure. 8(a). The possible fitted line for the original curve represented in solid line. The equivalent circuit of the rGO/ TiO2 composite is shown in fig. 8 is observed due to lower conductivity of the current collector which is to be improved.

CONCLUSIONS
The rGO/TiO2 composite was synthesized by one step hydrothermal reaction without using high temperature calcinations. The phase composition, functional groups and morphology structure of rGO/TiO2 composite were analyzed by using XRD, FTIR and SEM respectively.
The SEM image was showed flexible corrugated nature of rGO sheet while XRD and FTIR verified the presence of individual material in the composite. The specific capacitance of rGO/

ACKNOWLEDGEMENT
The financial support is provided by NRC Grant (No. 16-138).