SWCNT/BiVO₄ composites as anode materials for supercapacitor application

Abstract

In this report, we have systematically investigated the effect of SWCNTs on the electrochemical performance of BiVO₄: a material not studied for energy storage. Loading of 20 wt% SWCNTs with BiVO₄ exhibited the highest specific capacitance (395 F g⁻¹) and up to 88% coulombic efficiency was obtained over 200 cycles of charge–discharge for this composite.

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Supercapacitors, commonly known as electrochemical capacitors (ECs), are unique energy storage devices due to their high power density compared to secondary batteries and they also have a high energy density when compared to conventional electrostatic capacitors.^1,2 ECs, depending on charge storage mechanism, can be classified into two main categories, namely, electrical double layer capacitors (EDLC) and pseudocapacitors. The latter derive their energy from fast and reversible faradic reactions that occur at the electrode–electrolyte interface. Since such fast redox reactions usually take place in materials where metal ions have multiple oxidation states, therefore, transition metal oxides are of great importance for pseudocapacitors.³ RuO₂, MnO₂, NiO, Co₃O₄, V₂O₅ and Ni(OH)₂ materials have been investigated for pseudocapacitor based applications as cathode materials in order to enhance the power and energy densities.^4–7 Moreover, predominantly in these cases, activated carbon (AC) has been used as the anode material which normally does not contribute substantially to the overall energy density.⁸ It has been purported that energy density of the supercapacitors can be improved by replacing the traditional AC with oxides materials which can work as the anode. So far there have been few reports on formation of new anode materials.^8–11

Presented here is the use of bismuth vanadate (BiVO₄), a novel anode material for supercapacitor based applications. This is an attractive transition metal oxide because of its unique properties and thus far it has predominantly been evaluated for gas sensing, ferroelasticity and most recently for photocatalytic activity. It is known, however, to suffer from low electronic conductivity.¹² A valuable strategy for improving electronic conductivity has been to form a composite with a conductive agent. It should be noted that thus far, predominantly various porous and conductive carbon materials having high a surface area such as activated carbons, graphene oxide, graphene and carbon nanotubes (CNTs) have been widely studied to form a composite with the metal oxide for energy storage based devices.^13,14 SWCNTs (single walled carbon nanotubes) are found to be an attractive material which can improve the electronic conductivity predominantly in combination with electroactive materials because of their excellent conductivity, low density and good mechanical and chemical stability.¹⁵ Therefore, herein, we investigated the effect of SWCNTs on the electrochemical performance of BiVO₄. The electrochemical properties of pure and SWCNT/BiVO₄ composite were examined by using cyclic voltammetry (CV) and galvanostatic charge–discharge (CD) technique in presence of exclusively Na⁺ ion containing aqueous electrolyte.

BiVO₄ was synthesised by solvothermal route and loading of SWCNT was done by simple ultrasonication method (see ESI† for details). We have systematically studied the effect of 5, 10, 20, 25, 50 and 75 wt% loading of SWCNTs on the electrochemical performance of BiVO₄ however, data will only be presented for 10, 20 and 25 wt% loadings (see ESI† for data on the rest of the samples). 10, 20 and 25 wt% SWCNT/BiVO₄ composite have been termed as S1, S2 and S3. For electrochemical characterization, Pt as counter electrode and Hg/HgO as reference electrode was used with 2 M NaOH electrolyte in all experiments.

The purity and crystallinity of the solvothermally synthesized BiVO₄ was analysed by powder X-ray diffraction, Fig. 1. It is clear that the product crystallized well and the diffractogram fitted perfectly to a monoclinic scheelite structure and the peaks could be indexed according to reported data (JCPDS no. 75-2480). From FE-SEM images it can be observed that BiVO₄ consisted of 15–20 nm sized spherical particles that agglomerated and formed larger porous oval shaped structures with sizes in the range of 200–300 nm (inset Fig. 1). The energy-dispersive X-ray (EDX) pattern for BiVO₄ confirmed the presence of Bi, V and O elements (Fig. S1a and b, ESI†). The quantitative analysis revealed the atomic ratio of Bi³⁺ : V⁵⁺ as 1 : 1, thereby confirming the stoichiometry. BET surface area of BiVO₄ was measured by N₂ adsorption–desorption isotherm (Fig. S2, ESI†). The N₂ sorption measurement displayed a typical type III isotherm and BET surface area was found to be 15 m² g⁻¹. This value is 10 times higher than the reported value for BiVO₄ thus far.¹⁶

Fig. 1 Powder X-ray diffraction pattern and inset shows FE-SEM image of solvothermally synthesised BiVO₄.

The electrochemical behaviour of pure BiVO₄ was evaluated by CV under various scan rates ranging from 5 to 100 mV s⁻¹ (Fig. 2a). The redox peaks observed were assigned to the reversible faradic processes of Bi [Bi(III) ↔ Bi] via electrolytic ion diffusion.¹⁷ A single peak with high current was detected for reduction (−0.8 V) while for oxidation, two peaks were observed (−0.57 V and −0.42 V). The single reduction peak was attributed to the reduction of Bi(III) to Bi metal whereas the two oxidation peaks were due to the oxidation of Bi metal to Bi(III). The oxidation peak A1 has been purported to the oxidation of metallic Bi to Bi⁺ due to the formation of Bi metal layer at Bi-metal–solution interface.¹⁸ The redox peaks were found to be reproducible at higher scan rates, however, the reduction and oxidation peaks were shifted upon increasing scan rate assigned to the internal resistance drop.¹⁹ An increase in the current response with higher scan rate clearly indicated that the kinetics of interfacial faradic redox reactions and the rates of electronic and ionic transport were rapid enough as they were easily observed at high scan rates. This suggests that BiVO₄ could be a good anode material for aqueous supercapacitors.

Fig. 2 (a) CV and (b) charge–discharge profile of BiVO₄ at various scan rates.

The electrochemical capacitance of BiVO₄ was evaluated by galvanostatic CD. The CD curves (Fig. 2b), at different current densities ranging from 1 to 3 A g⁻¹ between voltage window −1.0 and 0.0 V appeared to be non-symmetric with an associated IR drop. This behaviour is different from typical EDLC which insinuates pseudocapacitive nature of BiVO₄. Despite steep voltage drop (upon the first few seconds of discharge) which is ascribed to internal resistance, we observed strikingly prolonged plateau of voltage output due to the involvement of faradaic process in BiVO₄. The calculated specific capacitances were 168, 115, 97, and 84 F g⁻¹ at current densities of 1.0, 1.5, 2.0 and 3 A g⁻¹, respectively. It was also detected that with increasing current densities, the specific capacitance decreased gradually, which can be attributed to the diffusion of electrolytic ions into the electroactive materials at low current densities. At higher current densities, diffusion effect limits the migration of the electrolytic ions, thereby this caused some of the active surface of the material to become inaccessible for charge storage.²⁰ Good cycling stability is another significant feature for high-performance supercapacitors. The cyclic performance of BiVO₄ is displayed in Fig. S3, ESI† at 1 A g⁻¹ current density for 200 cycles. It was observed that the discharge capacity retained only 42% up to 200 cycles. Unexpectedly, BiVO₄ displayed enormous (38%) drop in specific capacitance within first 10 cycles. This huge drop in capacitance may be due to the low electronic conductivity of BiVO₄. Therefore, in order to improve its electrochemical activity, SWCNTs (at varying ratios) were combined with BiVO₄ via ultrasonication.

It was determined through powder X-ray diffraction that the prepared S1, S2 and S3 composites clearly showed well-resolved peaks due to presence of BiVO₄ and there were no deleterious effects observed due to the ultrasonication procedure (Fig. S4, ESI†). This was valid whether SWCNTs were incorporated or not. In addition, SEM images of the composite clearly showed the separate two phases of BiVO₄ and the SWCNT (Fig. S5, ESI†). N₂ adsorption desorption isotherm was performed to evaluate surface area of S1, S2 and S3 samples and it was observed that all adsorption desorption isotherms followed type III isotherm (Fig. S6, ESI†). The BET surface areas for S1, S2 and S3 increased gradually with increased loading level of SWCNTs and values of 35 m² g⁻¹, 53 m² g⁻¹ and 56 m² g⁻¹ were obtained for the three samples respectively.

The electrochemical performances of S1, S2 and S3 were also evaluated by CV and CD technique at various scan rates. It was observed that the CV profiles for these three composites were similar to the BiVO₄ profile indicating that SWCNTs did not affect BiVO₄ chemically and the observed redox peaks were due to faradic processes (Fig. S7, ESI†) of BiVO₄.

The charge–discharge curves of S1, S2 and S3 (Fig. 3) at different current densities in voltage window −1.0 and 0.0 V display nonlinearities, different from typical EDLC behaviour and similar to CD profile of BiVO₄ (Fig. 2b) which indicated the pseudocapacitive nature of materials for charge storage application. Similar CD profile of S1, S2 and S3 w.r.t. BiVO₄ confirms that SWCNTs were not playing any role in the faradic process. The specific capacitance was found to be 350 F g⁻¹ for S1 at 2.5 A g⁻¹ which improved with increased loading level of SWCNTs (for S2 395 F g⁻¹ at 2.5 A g⁻¹). Note that mass loadings of the electrode have been provided in Table S1, ESI.† The high conductive nature of SWCNT leads to good electron transport, thereby improving the specific capacitance of composites. However, further enhancement in loading level of SWCNTs caused a drop in specific capacitance (for S3 224 F g⁻¹ at 2.5 A g⁻¹). Thus, it was observed that S2 gave the highest specific capacitance at 2.5 A g⁻¹ current density compared to BiVO₄, S1 and S3. This could be attributed to improved surface area and enhanced electronic conductivity due to the presence of SWCNTs. However, the decrease in specific capacitance for S3 may be ascribed to the inaccessibility of redox sites of BiVO₄ due to high concentration of SWCNTs. Hence, the optimized the loading level of SWCNTs with BiVO₄ was found at 20 wt%. Upon increasing the current density, the specific capacitance for S2 decreased gradually from 395 F g⁻¹ to 272 F g⁻¹. This can be ascribed to the diffusion effect of electrolytic ions. It was analogously observed that the For complete data set for all samples synthesized, refer to Table S2, ESI.†

Fig. 3 (a–c) Charge–discharge profile and table shows specific capacitance at of S1, S2 and S3 at different current densities.

Fig. 4 displays the cycle performance of BiVO₄, S1 and S2 measured at current density 1.0 A g⁻¹, 2.5 A g⁻¹ and 2.5 A g⁻¹, respectively for 200 cycles. From Fig. 4, noticeably, 42% and 58% capacitance was retained for BiVO₄ and S1. However, it was observed that the stability of material was improved dramatically by increasing the loading amount of SWCNT up to 20 wt% and 88% of capacitance was retained up to 200 cycles at current density of 2.5 A g⁻¹ (compare to BiVO₄) which showed the high rate capability and excellent long-term stability of S2. As discussed in galvanostatic CD section for BiVO₄, we noted 38% of specific capacitance dropped within first 10 cycles, however, for S2 no such large drop in specific capacitance was detected. This unique behaviour can be ascribed to improved electronic conductivity by the addition of SWCNTs. Subsequently, the coulombic efficiency was also found to be 84% retained at high current density 2.5 A g⁻¹, indicating excellent long-term stability of S2 sample (Fig. S8, ESI†).

Fig. 4 Cyclic performance in terms of specific capacitance (%) of BiVO₄, S1 and S2 at current density of 1.0 A g⁻¹, 2.5 A g⁻¹ and 2.5 A g⁻¹, respectively.

In conclusion, BiVO₄ and SWCNT/BiVO₄ composites have been synthesized and their electrochemical performance was evaluated by CV and galvanostatic charge–discharge measurements in 2 M NaOH electrolytic solution. It was observed that 20 wt% SWCNT/BiVO₄ composite was the optimized anode material when compared to the others in terms of specific capacitance, cyclic stability and coulombic efficiency. It is believed, that this study provides a new direction to explore SWCNT/BiVO₄ based anode materials for aqueous supercapacitors with high capacitance.

Acknowledgements

We sincerely acknowledge Mr R. D. Bapat and Ms B. Chalke's assistance with FE-SEM measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and characterization data. See DOI: 10.1039/c4ra01273a

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