Electrochemical performance of zirconia/graphene oxide nanocomposites cathode designed for high power density supercapacitor
© Mudila et al. 2016
Received: 20 September 2015
Accepted: 12 January 2016
Published: 22 January 2016
Carbon-based metal oxide nanocomposites are always been the prime material for study in the field of energy storage due to their rich abundance, low toxicity, high surface area, electrical conductivity and diverse oxidation states.
In this direction, novel zirconia/graphene oxide (ZrO2/GO) nanocomposites are fabricated on the surface of 316 stainless steel for studying their specific capacitance and power performance. ZrO2 and GO in varying mass ratio (1:1, 1:2, and 2:1) were used to fabricate the electroactive material. The physical interaction between the two was determined by Fourier transform-infrared, X-ray diffraction and scanning electron microscopy. TG-DTA-DG informs about the exhibited thermal property by the variants. The cyclic voltammetry was done to study the specific capacitance of the electroactive materials with reference to Ag/AgCl at scan rate (V/s) ranging 0.15–0.001 in 1.0 M KOH. The specific capacitance of ZrO2 was found to be 17.13 Fg−1 at 0.001 Vs−1. The representative (nanocomposite) NC-II shows the maximum specific capacitance of 299.26 Fg−1 at similar rate of scan with power density of 59.40 W/kg.
The nanocomposites show comparable level of charge-discharge behavior with long-term cycleability, suggesting that fabricated ZrO2/GO nanocomposite electrodes are promising candidate for the high-performance energy storage devices.
KeywordsZirconium oxide Graphene oxide Specific capacitance Cyclic voltammetry
Metal oxides depending upon their structural, geometries, and electronic structure play a very significant role in areas of chemistry, physics, biology and material sciences (Marcos and Rodriguez 2007; Graves 2014). Technical applications of these metal oxides involve the fabrication of electronic circuits, sensors, optical devices, piezoelectric devices, fuel cells, corrosion less surfaces coatings and catalysis, etc. (Sahu and Rao 2000; Wang et al. 2010; Ye et al. 2013; Lee et al. 2010; Mugniery et al. 1999). The proliferation of personal electronics and commercialization of electric and hybrid electric vehicles have popularized the needs for rechargeable and portable power sources and thus had increase the exploration of metal oxide in this direction. Supercapacitors in 21st era had attracted intense research interest as an auxiliary and clean source of power and energy. Due to their low molecular weight and their favorable electrochemical and solid‐state properties, first and second row transition metal oxides seem to be especially attractive as cathode materials in electrochemical energy storage systems. In this perspective, many transition metal oxides alone and with nanostructured carboneous filler had got special attention due to their low price, rich abundance, low toxicity, and diverse oxidation states (Wu et al. 2012; Lee et al. 2011; Zhao et al. 2013; Mai et al. 2014; Chen et al. 2012). In this exploration, a large number of metal oxide-graphene oxide combinations are synthesized to study the supercapacitive behavior (Wu et al. 2012; Lee et al. 2011; Wu et al. 2012). The electronic properties of the metal oxide are effected by particle size of material, the quantum size or confinement effects are produced in nanostructured materials which essentially arise from the presence of discrete, atom-like electronic states (Marcos and Rodriguez 2007), and also nanosized material is observed to have small band gap in its lattice. Thus, metal oxide nanoparticles can exhibit unique physical and chemical properties due to their limited size and a high density of corner or edge surface sites. Apart from this, metal oxide materials exhibit ionic or mixed ionic/electronic conductivity and are influenced by the nanostructure of the material (Tuller 2000).
Nano-phase zirconia (ZrO2) is a widely used heterogeneous catalyst and is an n-type semiconductor with band-gap energy of 5.0 eV (Pouretedal and Hosseini 2010). Zirconium oxide (ZrO2) is a smart material being studied for various applications such as oxygen sensor, solid state electrolytes for fuel cell, and gradient refractive index lenses due to its excellent mechanical, thermal, optical, and electrical characteristics (Pouretedal and Hosseini 2010; Liu et al. 2013). Studies on depositing ZrO2 onto carbon nanotubes (CNTs) (Lu et al. 2008; Shan and Gao 2005; Song et al. 2009; Guo et al. 2009), graphene (Wu et al. 2012, Liu et al. 2013), etc. for fuel cells and transistors as advanced gate dielectrics (Javey et al. 2002) had placed it in the row of promising candidate for high-power energy storage material.
Since its discovery, graphene oxide and graphene have become the key star in research associated with energy production and energy storage because of their sp2 carbon nanoform arrangement in 2D network with remarkable electronic, mechanical properties, and high morphological anisotropy (Stoller et al. 2008; Kim et al. 2009; Yoo et al. 2011; Tian et al. 2012, Yee et al. 2011). Graphene oxide (GO) provided large accessible surface area for effective transportation of ions onto the material surface, thus accomplishing high electric-double-layer capacitance in aqueous electrolytes (Yang et al. 2011). Nanostructured metal oxides can effectually prevent the Van der Waals-induced agglomeration of GO, resulting large accessible electrochemical active surface area for energy storage. Size and morphology affect the performance of metal oxides used as electrode materials. Designing nanostructured semiconducting metal oxide materials with graphene family has been the key to success in developing large-specific capacitive energy storage devices.
Studies based on I/V characteristics reveal very low CS and stability for ZrO2 and other metal oxides (Ye et al. 2013; He et al. 2013). While GO is a well-known material for its implication in the field of energy production and storage, this accounts of higher CS of GO over ZrO2 which prompted to investigate the effect of GO concentration on the CS of ZrO2 in the respective nanocomposites (NCs). In the present study, three variant of ZrO2/GO NCs were synthesized with molar concentrations of ZrO2 and GO (1:1, 1:2, and 2:1). GO dispersed in methanol was sonicated into a solution comprising of tetraethyl ammonium bromide (TEAB). To this, requisite amounts of ZrO2 were supplied and the contents were further sonicated. The resulting suspension was then washed repeatedly with methanol and distilled water (20:80) to obtain nanosized ZrO2/GO composites which shows high level of specific capacitance, energy, and power density along with good capacitive retention of 98.6 %. Along with this, workers had also found out that specific capacitance of ZrO2/GO is comparable to ZrO2/graphene nanocomposite which was found to be 10 F/g at 0.05 V/s by Liu et al. (2013).
ZrO2 (MW 123.22, 99.5 %, APS 45 nm) was purchased from Sisco Research Laboratories Pvt. Ltd., India. Polysulphone (PSO, Mn, 26,000, ρ = 1.24 g/cm3) and chlorosulfonic acid (bp., 152 ± 1 °C/755 mmHg, ρ = 1.753 g/cm3 at 25 °C) were procured from Aldrich Chemical Comp. Inc., USA. Tetraethyl ammonium bromide (TEAB, MW 210.17, 99 %) was procured from Molychem, and graphite (98.0 %, surface area 500 μm) was purchased from Otto Chemicka-Biochemica reagents, India. All other chemicals and solvents were purchased from s.d. fine Chemical, India. All the chemicals and solvents were used without further purifications. The commercially available 316-stainless steel (SS) was used as substrate for the preparation of electrode.
Preparation of ZrO2/GO NCs
Synthetic molar ratio of constituents
Particle size (nm)
Fabrication of working electrode
Prior to use, well-finished 1 cm2 316-SS was supplied as substrate and then de-greased with acetone and subjected to surface oxidation at 50 ± 1 °C over 1 hr under vacuum. The working electrode materials were prepared by mixing the electroactive material, graphite, and sulphonated polysulphone (SPS) in a mass ratio of 65:10:25. After sonicating the mixtures for 1 h in N-methylpyrrolidone (NMP), the resulting slurry was pressed on a stainless steel substrate which acts as a current collector. The treated substrate was dried at room temperature for 4 hr, followed by 100 °C/400 mmHg for 48 hr. The mass of the material deposited on the substrate was determined from the weight difference between the electrode before and after deposition by using a high precision microbalance. This has afforded cathodes with mass thickness of electroactive materials by 1.0 ± 0.1 mg/cm2 over SS substrate. The electrodes were tested after 24 hr of fabrications.
Result and discussion
Morphology of graphene oxide, ZrO2, and ZrO2/GO NCs
The morphological change in the as-grown ZrO2 on GO (NC-II) into SPS matrix indicates the formation of multiphase system created due to component used for preparation of electrode. Such multiphase system has been well distinguished in Fig. 5g–i. All the nanocomposites show porous stacking structures of graphene oxide sheets. There is no remarkable dependency of crystal morphology on the mixing ratio of zirconium oxide and graphene oxide, confirming homogeneous distribution of the two. The observed porous morphology of the present nanocomposite is quite different from the nonporous morphology of the pristine ZrO2 material which may be due to the presence of GO in the former.
Atomic force microscopy
Thermogram of ZrO2 reveals that it is free from any contamination. ZrO2 shows rapid decomposition up to 700 °C leaving 98.81 % and was associated with moisture of 0.15 % (Fig. 7). No observed degradation peaks occur in DTA and DTG analysis of ZrO2 proving that the material is quite resistible towards thermal degradation (Figs. 8 and 9, inset).
The NC-III derived from ZrO2 shows two-step decomposition (Fig. 7). The first-step decomposition of NC-III appeared at 152 °C leaving 98.2 % residue. This was supported with a DTG at 197 °C with the rate of degradation 84.2 μJ/min. Before this temperature, a weight loss of 1.27 % at 99.8 °C may be assigned content of NC. Such removal of moisture of NC was progressed at the rate of 32 μJ/min at 48 °C. Decomposition of NC was in the range of 152 to 200 °C leaving weight residue 96.04 %. In comparison to GO, such range of decomposition was shifted to lower temperature. This may be due to the formation of multiphase system that has reduced the thermal stability of NC over GO. During this temperature change, the NC was decomposed at 84 μJ/min at 197 °C. Increase in temperature from 200 °C has induced a steep weight loss up to 400 °C leaving the weight residue 90.97 % at 400 °C; further increase in temperature to 500 °C leaving weight residue 83.74 % was associated with a DTA signal of 34.9 μV at 461 °C with heat of decomposition 1.71 J/mg. This was supported with a DTG at 458 °C with rate of decomposition 125 μg/min; beyond 500 °C, no remarkable weight was appeared in the NC. At 817 °C, the decomposition of NC was terminated leaving weight residue 83.07 %.
The electrochemical activity of the Zirconium oxide-graphene oxide nanocomposites was studied by measuring cyclic voltammograms (CV) and galvanostatic charge-discharge cycles of these nanomaterials. Full range CV of all the samples were scanned on Iviumstat electrochemical work station (Ivium technologies, Netherlands) at current compliance 10 mA and ranges of voltage compliance 0.0 to −1.5 V, at scan rate 0.05–0.15 V/s using a conventional three-electrode cell assembly with reference to Ag/AgCl electrode; Pt foil with 1 cm2 area was used as counter electrode and commercially available SS electrode as a working electrode. 1.0 M KOH solution was used as electrolyte in whole experiment.
Comparative oxidative and reductive potential of materials at varying scan rates
Scan rate (V/s)
With various concentrations of ZrO2 and GO, under identical I/V compliance, NC-I corresponds to the CS ranging 4.40 to 175.37 F/g at proposed scan rates, NC-III shows CS ranges from 4.58 to 250.41 F/g, while for NC-II, it was reported to be ranging from 10.58 to 299.26 F/g. All such NCs metal oxide and GO display stable I/V compliance till 1000 cycles (Fig. 18). At a high scan rate, diffusion of electrolyte ions was limited due to the time constraint and only the outer active surface was utilized for charge storage; thus, specific capacitance decreased with increase in scan rate from 0.001 to 0.15 V/s. (Fig. 16). The present results of electrochemical measurement clearly demonstrate that the increase in concentration of graphene oxide leads to the increase in the specific capacitance to a limit beyond which the decrease in specific capacitance of NCs was encountered. This strongly suggests that the high content of graphene oxide is not advantageous for improving the electrode performance of the NCs.
In summary, the fabricated ZrO2/GO nanocomposites present efficient high-power material for supercapacitive studies. These materials were observed to have good thermal stability, specific capacitance, and cycleability as confirm by TG-DTA-DTG and CV studies, respectively. The electrochemical outcome clearly demonstrates that the increase of graphene oxide after a certain limit (here ZrO2: GO, 1:2) leads to the decline of the specific capacitance of nanocomposite. This strongly advocates that the high content of graphene oxide is not advantageous for enhancing the electrode performance of the nanocomposite. The remarkable performance of ZrO2/GO nanocomposite could be credited to the nanosize and high surface area of ZrO2 particles along with the good electronic conductivity of GO. Both factors can contribute to the enhanced charge-transfer-reaction pseudocapacitance of the NCs, which is based on rapid and reversible redox reactions at the surface of electrode. FT-IR SEM studies were performed to confirm the intermixing of reactant material, while XRD confirms the nanosized materials with layered matrix.
tetraethyl ammonium bromide
- ZrO2 :
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