Optimization, and analysis of carbon supported VS2 nanocomposites as potential electrodes in supercapacitors

Journal of Energy Storage 27 (2020) 101074

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Optimization, and analysis of carbon supported VS2 nanocomposites as potential electrodes in supercapacitors Edson Meyera, Asanda Bedea,b, Dorcas Mutukwaa,b, Raymond Taziwac, Nyengerai Zingwea,b,

T ⁎

a

Fort Hare Institute of Technology (FHIT), Private Bag X1314, Alice 5700, South Africa Department of Chemistry, University of Fort Hare, Alice 5700, South Africa c Walter Sisulu University, Department of Applied Science, Old King Williams Town Road, Fort Jackson, East London 5200 South Africa b

A R T I C LE I N FO

A B S T R A C T

Keywords: Vanadium disulphide Supercapacitors Nanocomposite electrodes Capacitance

The hydrothermal synthesis and optimization of carbon supported vanadium disulphide nanocomposites which could potentially be utilized as electrodes in supercapacitors is hereby reported. The superior electrical conductivity of multi walled carbon nanotubes coupled with the modest electrocatalytic capability of vanadium disulphide creates a synergy that enhances effective redox charge transfer leading to greater capacitance. Minimum charge transfer resistance and peak to peak potential difference for the optimum electrode, measured at 0.32 Ω and 0.15 mV respectively signified relatively higher charge transfer which was attributed to the greater electrical conductivity shored up by the increased carbon content. Thus, the optimum electrode was determined to have a specific capacitance of 33F∙g−1at 1 mA current density. The results obtained show that the carbon supported vanadium disulphide electrodes could potentially be used in supercapacitors.

1. Introduction Increased energy demand as well as global warming due to use of fossil fuels has led to research focusing on alternative energy generation from clean and renewable energy sources such as solar, wind and hydro. However, energy generation from these renewable energy is spasmodic and development of sustainable energy storage is very crucial to ensure reliable energy supply [1,2]. Supercapacitors or electrochemical capacitors are one of the most promising energy-storage devices due to their excellent properties such as high power density, relative low cost, longer life cycle than secondary batteries and faster charger/discharge [3–5]. There are three types of charge storage mechanisms in electrochemical capacitors which are based on non-Faradic reaction or Faradic reaction and can be divided into electric double-layer capacitors (EDLC), pseudocapacitors and hybrid capacitors respectively. In EDLC, energy storage occurs as result of charge separation at the surfaces of the electrode and electrolyte which give rise to the double-layer capacitance. While in pseudocapacitors, energy storage occur as a result of fast Faradic processes which are reversible occurring at the electrode and electrolyte surfaces and give rise to pseudocapacitance [6]. Hybrid capacitors are composed of both mechanisms with EDLC and pseudocapacitive electrodes functioning as either positive and negative

electrodes thereby forming an asymmetrical cell. Hybrid supercapacitors can also consist of composite electrodes made from materials normally used as electrodes in EDLC and pseudocapacitors. Carbonaceous materials have been studied extensively as electrodes in supercapacitors and typically exhibit EDLC charge storage mechanism. The carbonaceous materials such as activated carbon, reduced graphene oxide (R-GO) and multiwalled carbon nanotubes (MWCNTs) have high surface area and have exhibited high electrical conductivity, good mechanical stability as well as good cycle stability [7,8]. Metal sulphides, metal oxides, metal hydroxide and transition metal dichalcogenides which typically exhibit Faradic behaviour have been studied extensively in pseudo and hybrid capacitors. Amongst the advantages associated with pseudocapacitors are high power and energy densities whilst being also limited by poor cycle stability and low electric conductivity. On the other hand, EDLCs suffer from low energy density [9]. This has led to focus on combining metal oxides/transition metal chalcogenides/ metal sulphides with carbonaceous materials in order to overcome their limitations. Metal sulphides such as CuS [10], MoS2 [11], NiS [12] and WS2 [13] have been explored as potential electrodes due to their good performance in supercapacitors. VS2 nanosheets possess high electric conductivity as well as high surface area and hence have been applied as electrodes in supercapacitors [14]. Feng et al. [14] reported an

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Corresponding author. E-mail addresses: [email protected] (E. Meyer), [email protected] (A. Bede), [email protected] (D. Mutukwa), [email protected] (R. Taziwa), [email protected] (N. Zingwe). https://doi.org/10.1016/j.est.2019.101074 Received 18 July 2019; Received in revised form 23 October 2019; Accepted 12 November 2019 2352-152X/ © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).

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electric capacitance of 4 760 µF∙cm−2 and observed no significant reduction in capacitance after 1000 discharge/charge cycles. VS2 which also belong to the transition metal dichalcogenides (TMDs) family has a structure which can be described as vanadium metal sandwiched between two layers of sulphur layers forming S-V-S tri-layer stacked together via weak Van der Waals interactions [15]. The structure makes it possible to insert guest ions without distorting its form thus allows efficient and fast Faradic processes [16]. Wang et al. [17] investigated the capacitance of VS2 graphene supported nanoparticles in lithium-ion battery yielding a discharge capacity of 114.2 mA∙h∙g−1 at a high current density of 20 C compared to 90 mA∙h∙g−1 for pristine VS2. Addition of carbonaceous materials to VS2 help improve its energy storage properties. Masikhwa et al. [18] developed composite electrode composed of molybdenum disulphide and graphene foam with activated carbon which exhibited high maximum specific capacitance of 59F∙g−1, with power and energy densities of 758 W∙kg−1 and 16 Wh∙kg−1 respectively. After 2000 charge discharge cycles the MoS2 electrode exhibited 95% capacitance retention. Obtained results are indicative of the efficient synergy created by the good electrocatalytic activity of MoS2 and excellent conductivity of graphene. MWCNTs have also been employed in supercapacitors and have shown high chemical, high electric conductivity, high surface area and mechanical stability [19]. About three times increase in capacitance was observed for MoS2 supported MWCNTs (452.7 F∙g−1) as compared to pristine MoS2 (149.6 F∙g−1). The MoS2/MWCNT composite also exhibited 95.8% retention after 1000 discharge/charge cycles [20]. In TMD/carbonaceous composites, carbonaceous materials are known to present a conductive channel and enhance interface contact between electrolyte and electrode. Overall the electrochemical performance is increased due to short ion diffusion pathways as well as short electron transport facilitated by TMDs [9]. Therefore, in here we report the structure, morphological properties and the potential use as electrode in supercapacitors of VS2 supported MWCNTs prepared by a simple hydrothermal process.

Table 1 Ratio of reactants utilized in the synthesis and optimization of VS2-MWCNTx nanocomposites. Sample

VS2 (mg)

MWCNT (mg)

PVF (mg)

Mole Ratio

VS2-MWCNT(8:1:1) VS2-MWCNT(6:3:1) VS2-MWCNT(3:6:1) VS2-MWCNT(1:8:1)

240 180 90 30

30 90 180 240

30 30 30 30

8:1:1 6:3:1 3:6:1 1:8:1

2.2.1. Characterization techniques The morphology of the prepared VS2 nanosheets and carbon-supported VS2 nanocomposites was studied using a high-resolution Zeiss Ultra plus 55 scanning electron microscope (SEM) (Carl Zeiss, Oberkochen, Germany), coupled with EDS, Smart SEM software for elemental analysis. Transmission electron microscopy (TEM) microimages were obtained with a JEOL JEM-2100F microscope operated at 200 KV (JEOL, Massachusetts, USA). To analyse the crystallographic phase and associated parameters of the unsupported VS2 and carbonsupported VS2 samples x-ray diffraction (XRD) spectra were obtained using a Bruker D8 Advance X-Ray diffractometer (Bruker, Wisconsin, USA) with a Cu anode, generating Kά radiation of wavelength 1.544 Å and operating at 40 kV and 40 mA. Electrochemical analysis was conducted using a Biologic VMP-300 potentiostat (Knoxville, Tennessee USA). A standard three electrode system with scanning rates from 5 to 100 mVs−1 was used to obtain the cyclic voltammetry (CV) profiles. The as-synthesized palladium alloys, Ag/AgCl electrode, and carbon black served as the working, reference and counter electrode respectively. A 6 M KOH electrolyte solution was used for electrochemical measurements. Electrochemical impedance spectroscopy (EIS) was conducted in a frequency range from 0,01–100 kHz at 0 V bias with an amplitude of 10 mV, (Fig. 1). 3. Results Powder X-Ray diffraction was conducted so as to determine the crystalline structure and composition of the carbon supported VS2 nanosheets. Vanadium disulphide has two known crystal phases i.e. hexagonal and trigonal. The hexagonal VS2 phase has been observed to show peaks at 2θ angles of 18.3, 30.2, 34.3, 36, 43.4, 47.6, 56.8, 57.2 and 67.1 corresponding to the (001),(002), (100), (011), (102), (003), (110), (103), (108) lattice planes respectively [21]. Whereas the trigonal crystal phase has diffraction peaks at 2θ angles of 15.6, 36.1, 45.3, 57.2 and 69.5° which correspond to the (001), (100), (102), (110) and (201) lattice planes [22]. The XRD diffractograms for the as-synthesized unsupported and carbon supported VS2 nanosheets are depicted in Fig. 2. Peaks were revealed at 2θ of 15.4, 28.2, 34.2, 36.2, 43.3, 48.3, 54.4, 57.7 and 66.2° corresponding to the (001), (002), (100), (011), (102), (003), (110), (103) and (201) lattice planes of hexagonal VS2 as per JCPDS card 36–1139. The high intensity at 28.2° indicates that crystal growth was oriented in the (002) lattice plane. Furthermore, two peaks were observed for the carbon content from the multi walled carbon nanotubes situated at 26.6° and 41.3° which correspond to the (002) and (001) lattice planes respectively. Using the (002) VS2 lattice plane as a reference, it is evident that upon incorporation of the MWCNT, a shift towards lower 2θ angles is observed, with the highest shift and intensity being observed for the 3:6:1 ratio. The morphological and elemental composition of the fabricated VS2 nanoparticles were determined by the scanning electron microscopy (SEM). Fig. 3 depicts the SEM diagrams for the prepared unsupported VS2 samples as well as pure multi walled carbon nanotubes which were taken at different magnifications. It can be observed from Fig. 3a and b that the VS2 samples were composed of aggregated nanosheets with different directional orientation. Corresponding EDX patterns for the assynthesized VS2 nanosheets and multi walled carbon nanotubes are shown in Fig. 3(b and d) respectively. The EDX patterns confirm that

2. Materials and methods 2.1. Synthesis of unsupported VS2 nanoparticles The fabrication procedure for the vanadium sulphide nanoparticles VS2 involved dissolving 3 mmol sodium orthovanadate (Na3VO4∙2H2O) and 15 mmol thioacetamide in 40 ml deionized water. The as prepared solution was then vigorously stirred until homogeneity was attained, and subsequently transferred into an autoclave for hydrothermal synthesis at 180 °C for 12 h. The reaction conditions were utilized since they. The as -prepared black metal sulphide precipitate retrieved after the reaction time had lapsed, was subsequently washed with ethanol and deionized water so as to remove any impurities, followed by drying in an oven. The prepared metal sulphide was subsequently stored for characterization and further analysis.

2.2. Synthesis of carbon supported VS2 nanoparticles The preparation of the carbon supported VS2 nanocomposites was conducted through mixing appropriate ratios of the as synthesized VS2 precursor with different amounts of multi walled carbon nanotubes, and polyvinyl fluoride. Table 1 shows the mixing ratios of the reagents utilized for synthesis and optimization of the carbon supported VS2 nanocomposites. Each prepare mixture was then dissolved in 40 ml deionized water and stirred until homogeneity. The as-prepared precursor solutions were separately transferred into a Teflon lined autoclave for hydrothermal synthesis at 180 °C for 12 h. The rest of the procedure for the fabrication of the carbon supported VS2 nanocomposites mirrors the procedure outlined in 2a). 2

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Fig. 1. Hydrothermal synthesis procedure for carbon supported vanadium disulphide (VS2) nanocomposites.

between the electrode and the electrolyte the higher is the rate of the catalytic process since more enhanced charge transfer will occur. As the carbon content increase from the 8:1:1 ratio to 6:3:1 ratio depicted in Fig. 4a significant increase in nanosheet agglomeration was observed with the 1:8:1 ratio experiencing the most severe agglomeration. The increased agglomeration as the carbon composition increases can be attributed to the multiwalled carbon nanotubes causing nucleation of vanadium atoms in sulphur atoms which results in the extension of the nanosheets. Fig. 4c depicting the SEM images of the 3:6:1 ratio shows that the carbon supported VS2 nanosheets are densely attached to the multiwalled carbon nanotubes which could limit channels for electron transfer. The further increase in the carbon content to the 1:8:1 ratio produces SEM images depicted in Fig. 4d which possess a porous and irregular surface that could enhanced electrolyte adsorption thus enables more efficient charge transfer. EDX patterns depicted in Fig. 5(a–d) confirm that the synthesized carbon supported VS2 nanocomposites are composed of carbon, vanadium and sulphur. Fig. 5 shows the High Resolution Transmission Electron Microscopy (HRTEM) images for the carbon supported VS2 nanocomposites and unsupported VS2 nanosheets. Fig. 5a depicts layered VS2 nanosheets with an interlayer spacing of 0.571 nm belonging to the (001) lattice plane of H-VS2 structure. Edge to edge length of the nanosheets was observed to be in the range 0.294–1.248 nm. Additionally, numerous disentangled and non cleaved lattice structure was observed in the VS2 indicating that there is no formation of defects during the hydrothermal synthesis of VS2 nanosheet. Fig. 5b depicts the HRTEM images of the MWCNT with a length and width of 7–15 nm and 0.5–10 µm respectively. HRTEM analysis revealed lattice spacing of 0.342 nm for MWCNTs corresponding to the (002) lattice plane of MWCNT. The intertwined and unorderly nature of the MWCNT depicted in Fig. 5b could potentially help in binding of VS2 and increase the electrode surface area. Figures c-f depicts the HRTEM images for the carbon supported VS2 nanocomposites. All the carbon supported VS2 samples

Fig. 2. X-ray diffraction (XRD) images of carbon supported vanadium disulphide (VS2) nanocomposites and VS2 nanosheets.

the as-synthesized VS2 nanosheets and the MWCNT are composed of vanadium, sulphur and carbon. Fig. 4 depicts the SEM images of the carbon supported VS2 nanosheets. The SEM images for the carbon supported VS2 nanosheets in Fig. 4a and b depict samples consisting of small, intertwined micropores that could potentially provide a larger surface area for greater interaction between the electrode and electrolyte as compared to the unsupported VS2 nanosheets. The greater the surface area of contact

3

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Fig. 3. Scanning Electron Microscopy (SEM) images of (a) and (b) pure VS2 nanosheets, (c) and (d) multi-walled carbon nanotubes (MWCNTs). Additionally, insert in (b) and (d) shows the elemental composition of elements in the VS2 NSs and MWCNTs samples respectively.

Fig. 4. SEM images of carbon supported VS2 nanocomposite samples obtained at 20 µm magnification. (a) VS2-MWCNT1, (b) VS2-MWCNT2, (c) VS2-MWCNT3, (d) VS2-MWCNT4. 4

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Fig. 5. Figure. High resolution transmission electron microscopy (HRTEM) images for (a) VS2 nanosheets, (b) multi walled carbon nanotubes, (c) VS2MWCNT(1:8:1), (d) VS2-MWCNT(6:3:1), (e) VS-MWCNT(3:6:1), (f) VS2-MWCNT(8:1:1).

developed VS2 nanosheets. CV analysis provides two significant values i.e. current density and peak to peak potential difference (ΔEpp) which are utilized as indicators of the catalytic process occurring on the electrode surface. The higher the current density the greater is the redox catalytic activity at the electrode surface. Peak to peak potential difference in a redox system is inversely proportional to the rate of progression of the redox reaction. From Fig. 6 two broad peaks can be observed for each of the VS2 nanosheets indicative of their faradaic redox capability. As the carbon content increases from the 8:1:1 ratio to1:8:1, there is also an increase in the values of the redox peaks. Nevertheless, peak current densities for all the carbon supported VS2 nanosheets are lower than those of unsupported VS2 nanosheets. Peak to peak potential difference calculations for the developed samples show that the unsupported VS2 nanosheets partake in the redox reaction with more vigour than the

revealed uniform distribution of the two constituent materials which could be potentially. advantageous for redox catalytic activity since it exposed more active catalytic sites. The VS2 NSs appear to be stacked layer by layer, surrounding the fish-bone shaped carbon nanotubes which is vital for surface support that is conductive. As the carbon content increases as depicted in Fig. 5c and d the VS2 NSs become more interconnected and vertically distributed on the surface of MWCNTs. Cyclic voltammetry was utilized to determine the electrochemical properties of the developed VS2 nanosheets. Analysis was conducted in a three electrode system using a 6 M potassium hydroxide KOH as the electrolyte. Measurements were taken for VS2 nanosheets at a scanning rate of 100 mV∙s−1 in a potential range from 0 to 0.4 V. The synthesized nanosheets were utilized as the working electrodes, whilst glass carbon acted as the counter electrode with Ag/AgCl system being `the reference electrode. Fig. 6 shows the cyclic voltammetry curves for all the 5

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faradaic behaviour of the electrode materials. The initial increase in the carbon content does not affect the discharge capability of the materials as indicated by the same area for the unsupported VS2 nanosheets compared to the nanocomposites with ratios (8:1:1) and (6:3:1). When the carbon content reaches the (1:8:1) ratio a greater discharge capacity is experienced which is indicative of higher capacitance as the carbon content increases. Further comparison of the electrochemical properties of the developed VS2 nanosheets samples was conducted through calculation of their specific capacitance. Fig. 7b shows the variation in the specific capacitance for each of the developed VS2 nanocomposites against the prevailing current density. For all the developed samples specific capacitance decreases as the current density increases with the (1:8:1) ratio exhibiting the highest specific capacitance at 33F∙g−1 at 1 mA current density. The higher capacitance at low current densities can be attributed to the greater and effective interaction between the electrode and the electrolyte thus relatively more efficient charge transfer occurs whereas at higher current densities only minimum interaction occurs leading to diminished charge transfer and low capacitance. As clearly shown higher capacitance and charge transfer is observed as the carbon content increases which can be attributed to its excellent electrical conductivity as well providing a greater surface area for electrolyte adsorption. Nevertheless, the relationship between the carbon content in the supported VS2 nanocomposites and the capacitance does not follow and ideal pattern as indicated by a lower than usual specific capacitance for the (3:6:1) ratio as compared to unsupported VS2 and the (6:3:1) ratio. Additional analysis of the electrochemical capability of the developed nanocomposites was done using electrochemical impedance spectroscopy (EIS). Electrochemical impedance spectroscopy gives a measure of the charge transfer activity occurring between the electrodes and the electrolyte. Thus, the technique is utilized to quantify the electrical conductivity the electrode material possesses. The greater the electrical conductivity, the less is the impedance to electron flow hence the lower the charge transfer resistance obtained in the analysis. In this work impedance measurements were taken in the frequency range from 5 MHz to 100 kHz before and after cycling. Fig. 8a shows the Nyquist plots for the VS2 nanocomposites. The intersection of the curves with the real Z' axis gives the total resistance value exhibited by the electrode. The total resistance consists of the ionic resistance of the electrolyte, interfacial contact resistance between the electrode and current collector as well as the intrinsic resistance of the active materials on the electrode. Table 3 shows the resistances obtained for all the developed samples. The least resistance was observed for the (1:8:1) ratio with the unsupported VS2 nanosheets exhibiting the highest impedance at 0.44 Ω. The low charge transfer resistance for the (1:8:1) nanocomposite is due to the greater electrical conductivity provided by the multiwalled carbon nanotubes. A greater active surface area for electrolyte interaction as well as high carrier mobilities results in greater charge transfer hence more capacitance is exhibited. The results obtained in the EIS analysis are in mutual agreement with the cyclic voltammetry and charge discharge results confirming that, the 1:8:1 with the highest carbon composition is the optimum ratio for effective functionality of the electrode. Consequently, this material could potentially be used as an electrode in hybrid supercapacitors since it has higher capacitance attributed to high redox activity, high electrical conductivity and very low charge transfer resistance.

Fig. 6. Cyclic voltammetry (CV) curves for carbon supported VS2 nanocomposites and unsupported VS2 nanosheets obtained using a scan rate of 100 mV∙s. Table 2 Current densities and peak to peak (ΔEPP) potential differences of the synthesized CEs. CE

Current density (Ag−1)

Ep1 (V)

Ep2(V)

ΔEpp (V)

VS2 8:1:1 6:3:1 3:6:1 1:8:1

47 40 44 38 53

0.22 0.13 0.13 0.16 0.17

0.39 0.34 0.28 0.34 0.32

0.17 0.21 0.15 0.18 0.15

carbon supported nanosheets since they possess the lowest values at 0.14 V. Table 2 shows the peak to peak potential differences as well as the reduction and oxidation peaks for all the VS2 samples. Table 2 also shows that upon addition of multi walled carbon nanotubes to the VS2 nanosheets diminished redox capability is experienced leading to higher peak to peak potential difference as compared to the unsupported VS2 nanosheets. This is indicative of a reduction in the rate of redox reaction as carbon content is introduced. Nevertheless, the catalytic activity seemingly increases as the carbon content increases to levels closer to those exhibited by unsupported VS2 nanosheets. The reduction in the redox catalytic activity at higher carbon concentrations can be attributed to the aforementioned densely packed nature of the nanocomposite particles which could have potentially reduced electron transportation channels and limited electrolyte interaction with the active sites on the electrode. Since the redox catalytic process occurs through charge transfer thus inference can be made that the incorporation of the multi walled carbon nanotubes diminishes electron transportation in the VS2 nanocomposites. The lower redox capacity of carbon nanotubes and other carbon based materials was well documented by can Charge discharge analysis was conducted so as to determine the capacitance of the VS2 nanocomposites. Charge discharge is a technique utilized for calculation of the specific capacitance of any material under current. The greater the discharge durability any material exhibits the higher is its specific capacity, the material possesses. Fig. 7a shows the charge discharge curves for the VS2 nanocomposites. The charge -discharge (CD)analysis was conducted at 1 A∙g−1 current density in the potential range from 0 to 0.4 V. Fig. 7a shows that the CD curves for the VS2 nanocomposites are non linear indicative of the redox reaction taking place during the discharge process. Each discharge curve shows two clear voltage steps with a quick potential drop from 0.36 to0.225 V as well as a voltage plateau from 0.22to 0.16 V indicative of the

4. Conclusions Carbon supported VS2 nanocomposites as well as unsupported VS2 were successfully synthesized via a hydrothermal route. The unsupported VS2 samples were observed to exhibit a layered nanosheet structure whilst the carbon supported VS2-MWCNTx nanocomposites 6

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Fig. 7. Comparison of (a) charge-discharge curves (b) specific capacitance curves for carbon supported VS2 nanocomposites to unsupported VS2 nanosheets.

the Govan Mbeki Research and Development Centre at the University of Fort Hare for their support. CRediT authorship contribution statement Edson Meyer: Funding acquisition. Asanda Bede: Conceptualization. Dorcas Mutukwa: Conceptualization, Writing - review & editing. Raymond Taziwa: Visualization. Nyengerai Zingwe: Conceptualization, Writing - review & editing. Data for reference Research data will be made available upon request. Declaration of Competing Interest “The authors declare no conflict of interest.” The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 8. A comparison of the electrochemical impedance spectroscopy curves for carbon supported VS2 nanocomposites to the unsupported VS2 nanosheets. Measurements were obtained at 100 mV∙s1.

Acknowledgments Table 3 Total resistances exhibited by the VS2 samples.

We are grateful for the support from the Fort Hare Institute of Technology and the University of Fort Hare Chemistry department

CEs

VS2

VS2-CNT1

VS2-CNT2

VS2-CNT3

VS2-CNT4

Resistance/Ω

0.44

0.41

0.38

0.36

0.32

Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.est.2019.101074.

exhibited densely packed and interconnected micropore structure which was potentially vital for fast ion transfer. Through cyclic voltammetry analysis the carbon supported VS2-MWCNT(1:8:1) electrode was determined to possess the highest reduction current density and least peak to peak potential difference of 53A∙g−1 and 0.15 V respectively. Electrochemical impedance spectroscopy showed that the VS2MWCNT(1:8:1) electrode provided the least impedance to electron transfer with 0.32 Ω thus possessed the highest possible specific capacitance of 33F∙g−1 at a current density of 1 mA, which was ascribed to the superior electrical conductivity shored up by the increased higher carbon content.

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Funding This research was funded by South African National Research Foundation (NRF), Eskom (TESP and the South African Department of Science and Technology (DST). We also extend our sincere gratitude to 7

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