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High performance supercapacitor based on carbon coated V2O5 nanorods
Journal of Electroanalytical Chemistry 758 (2015) 111–116
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jeac
Short communication
High performance supercapacitor based on carbon coated V2O5 nanorods Balakrishnan Saravanakumar a, Kamatchi Kamaraj Purushothaman b, Gopalan Muralidharan c,⁎ a b c
Faculty of Physics, Mahalingam College of Engineering and Technology, Pollachi, Tamilnadu, India Faculty of Physics, TRP Engineering College (SRM Group), Irungalur, Trichy, Tamilnadu, India Department of Physics, Gandhigram Rural University, Gandhigram, Tamilnadu, India
a r t i c l e
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Article history: Received 22 May 2015 Received in revised form 14 September 2015 Accepted 26 October 2015 Available online 28 October 2015 Keywords: Nanostructures Oxides Precipitation Electrochemical properties Supercapacitor
a b s t r a c t Conductive material coating on redox efficient transition-metal oxides is one of the approaches to enhance the conductivity of the metal oxides. This approach could greatly boost the electrochemical performance of supercapacitors. In this article an attractive low cost method to coat conductive carbon on V2O5 nanostructures is reported. When utilized as supercapacitor electrodes, this smart combination of C@V2O5 nanorods exhibits attractive capacitive features, including higher capacitance (417 Fg−1 at 0.5 Ag−1), superior rate capacity (341 Fg−1 at 10 Ag−1), very low charge transfer resistance (1.1 Ω) and better cyclic stability. An asymmetric supercapacitor device based on C@V2O5 nanorod electrode delivers an energy density of 9.4 Wh kg−1 at a power density of 170 W kg−1. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The supercapacitors (SCs) are rechargeable devices that store huge energy. The storage capacity is thousands of Farads higher when compared to conventional capacitors. The SCs possess fascinating characteristics such as ultrafast charge/discharge rates, high power uptake, exceptional cycle life and safety. [1] These elegant features of SCs favour their potential use in hybrid electrical vehicles, portable electronic devices, memory backup systems etc. The overall electrochemical performance of SCs is much influenced by innovative electrode materials. Over a decade, large variety of components have been examined and exploited as electrode materials for supercapacitors. [2–6] In addition, a few of them have found their way in the manufacture of commercial supercapacitors. Among various compounds tested so far, vanadium pentoxide (V2O5) is highly attractive owing to its structural richness. The crystal structure of this material is composed of stacking of V2O5 layers perpendicular to the c-axis through Vander Waals interaction. [7] The layered structure aids the intercalation of electrolyte ions through the electrode. V2O5 has been rigorously studied as an electrode material for battery and supercapacitor applications after the first report by Whittingham [8]. Unfortunately, practical applications of V2O5 based electrode materials for supercapacitors are hampered by the poor electrical conductivity and cyclic stability due to the large specific volume expansion during electrolyte ion insertion/deinsertion processes [9]. It leads to lesser contact with the current collector and significantly increases the electrical ⁎ Corresponding author. E-mail address: [email protected] (G. Muralidharan).
http://dx.doi.org/10.1016/j.jelechem.2015.10.031 1572-6657/© 2015 Elsevier B.V. All rights reserved.
resistance. Furthermore, most of the V2O5 based electrodes suffer from poor rate capacity. The most important reasons for poor rate performance are higher value of electrode resistance and large interfacial charge transfer resistance. For example, recently Zhu et al. [10] published one of the most significant research works on V2O5 based supercapacitor electrode materials. Even though higher specific capacitance is achieved (451 Fg−1 at 0.5 Ag−1) for 3D-V2O5 electrodes, it retains only about 50% of the specific capacitance (~ 230 Fg− 1) at 10 Ag−1. Further this material was reported to possess a higher charge transfer resistance (Rct) of 10 Ω. Lee et al. [11] reported graphene decorated V2O5 nanobelts and achieved a maximum specific capacitance of 134 Fg−1 at 0.25 Ag− 1 in 1 M Na2SO4 aqueous electrolyte. These graphene/V2O5 nanobelts could lose 23% of specific capacitance (~ 104 Fg− 1) when current density rose from 0.25 to 1 Ag − 1. This material possesses Rct value of 5.7 Ω. V2O5/graphene hybrid aerogels fabricated by Wu et al. [12] showed a specific capacitance of 486 Fg− 1. This composite material retained only 37% of specific capacitance (~ 180 Fg− 1) at 10 Ag− 1 and its Rct was 5 Ω. V2O5 nanowires/CNT composites were reported by Chen and et al. [13] could exhibit a specific capacitance of 440 Fg− 1 and 200 Fg− 1 at 0.25 and 10 Ag− 1 respectively. This composite material was capable of retaining nearly 40% of specific capacitance at 10 Ag− 1. Even, in the authors' very recent publication on MnO2 grafted V2O5 nanostructures, nearly 50% specific capacitance (251 Fg− 1) at 5 Ag− 1is reported. [14] These studies have demonstrated the inability of V2O5 based electrode materials, which retain a higher specific capacitance at higher current rates. It is also indicated that the lower values of charge transfer resistance influence the rate capacity, even if they could yield higher capacitance at lower current rate.
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According to Stoller and Ruoff [15] an electrode material should be able to store larger amount of energy at higher current rates. Only such a material is favourable for supercapacitor device applications. Here it is valuable to mention that a good supercapacitor electrode material should possess high conductivity with large electrolyte access area and lower value Rct to achieve ultrafast charge/discharging at higher current rates. In this context, many inspiring works confirmed adding carbonaceous materials to V2O5 nanostructures to be an effective strategy to mitigate these issues and improve the electrochemical performance. [16–19] But these works involve complex experimental procedures and sophisticated instruments. Further, most importantly, this kind of approach is mainly adopted to improve the rate capacity in Li ion battery applications. Considering all these important concerns, the present study proposes a simple and cost-effective strategy to coat carbon onto V2O5 nanostructures. The presence of carbon on the surface of the V2O5 nanostructure forms an electrical network and improves the electrical conductivity and reduces volume changes due to ion intercalation/ deintercalation in the V2O5 and ensures the better cyclic stability. [20] In this communication, findings on the fabrication of carbon-coated V2O5 nanorods (C@V2O5) via simple and economically viable combined co-precipitation and hydrothermal process using dextrose as carbon source are presented. With this smart combination of C@V2O5, higher specific capacitance, superior rate capacity and better cyclic stability could be achieved when it is used as a supercapacitor electrode material. 2. Experimental methods 2.1. Material synthesis Vanadium pentoxide, hydrogen peroxide (30%) and potassium sulphate were obtained from SD fine Chemicals Ltd., India. Disodium citrate dextrose was purchased from Sigma Aldrich. All chemicals were of analytical grade and used without further purification. The V2O5 nanostructures were prepared following a previously reported protocol [21]. Thus prepared V2O5 nanostructure was mixed with dextrose (0.1773 g) which is used as a cost-effective carbon source. The mixture was transferred to a 100 mL Teflon lined stainless steel autoclave and kept at 180 °C for 12 h. The resulting material was properly washed using de ionized water (DI). The washed material was dried at 80 °C for 12 h. The final product was mentioned as C@V2O5 nanostructure. The performances of commercial V2O5 powder and V2O5 nanostructures without carbon prepared by the same route were also investigated and mentioned as pristine V2O5 and V2O5 networks respectively. 2.2. Material characterization The C@V2O5 nanostructure was characterized by X-ray diffraction (XRD) using a PANalyticalX'pert-PRO diffractometer with Cu Kα sealed tube (λ = 1.5406 Å). The FTIR analysis was performed using a PerkinElmer Spectrum BX-II spectrophotometer utilizing the KBr pellet method. The SEM investigations were carried out on a TESCAN VEGA 3 LMU scanning electron microscope. TEM characterizations were performed on a TEM-Philips JEOL CM12 microscope. 2.3. Electrochemical measurements Electrochemical investigations on C@V2O5 nanorod were performed using a three-electrode cell set-up, which consists of C@V2O5 as the working electrode, Ag/AgCl as the reference electrode and platinum wire as the counter electrode. The working electrode was prepared by mixing 85 wt.% sample, 10 wt.% activated carbon (Sigma-Aldrich), 5 wt.% polytetrafluoroethylene (Sigma-Aldrich) and a few drops of ethanol. This mixture was coated on to ultrasonically cleaned nickel foam (1 cm2). The electrodes were dried at 80 °C for 6 h to get the working
electrodes. The electrochemical tests were carried out at room temperature using 0.5 M K2SO4 aqueous electrolyte. The mass of the active material in the electrode was 1 mg. Electrochemical measurements were performed using CHI 660D electrochemical workstation (CH Instruments). An asymmetric supercapacitor was designed using C@V2O5 nanostructure and activated carbon (Sigma-Aldrich, 1100 m2 g−1) as the electrode materials. The polypropylene films (Celgard, 2400) soaked in 0.5 M K2SO4 was used as a separator and electrolyte. The basic equations adopted to calculate specific capacitance, energy and power densities are reported elsewhere. [21].
3. Results and discussion Herein, the formation of carbon coated V2O5 nanorods from network structured V2O5 as shown in Scheme 1 is proposed. Initially the network structured V2O5 was prepared by the same protocol which was adopted in the authors' previous work [21]. The V2O5 sol was prepared by blending of V2O5 powder, H2O2 and deionized water. The presence of H2O2 initiates gelation process within a few minutes [22]. To avoid gelation, disodium citrate was added as the controlling agent to the sol under stirring condition. After 3 days V2O5 nanostructures were collected by centrifugation and dried at 100 °C for 10 h. Secondly, as synthesized V2O5 nanostructures were mixed with dextrose and transferred to auto clave and kept at 180 °C for 2 h. It is well reported that the presence of carbon source under hydrothermal conditions leads the change of morphology of metal oxide nanostructures. [11] The mixing of carbon precursor (dextrose) with V2O5 networks under hydrothermal conditions leads the formation of rod like structures. The phase, purity and structure of C@V2O5 nanorods were investigated by X-ray diffraction (XRD) analysis. The XRD pattern of pristine V2O5, V2O5 network and C@V2O5 nanorods is presented in Fig. 1. The diffraction peaks (200), (001), (101), (110), (400), (011), (310), (002), (411), (600), (020) and (710) in the spectrum are consistent with the orthorhombic structured layered V2O5 (JCPDS no 41-1426 space group: Pmmn(59)) and confirms its crystalline nature. The lattice parameters are a = 1.151 nm, b = 3.565 nm, and c = 4.372 nm. The absence of any other peaks in the XRD confirms the purity of the material. Further the FTIR spectrum was recorded for the C@V2O5 nanorods and presented in Fig. S1a (Supplementary data). The spectrum shows a strong absorption band at 1006 cm−1 attributed to the stretching vibration of V_O. The magnified FTIR spectrum of C@V2O5 between 400–700cm−1 (Fig. S1b) shows bands at 485 and 576 cm−1 which are assigned to the symmetric and the asymmetric stretching modes of V– O–V vibrations respectively [23]. The absorption at 525 cm−1 represents the V–O–V stretching vibration [24]. The absorption peak at 1615 cm−1 is due to the bending vibrations of the water molecules [25]. The FT-Raman spectra of the C@V2O5 nanostructures are shown in Fig. S2. The spectrum consists of two high frequency bands at 1362 and 1578 cm−1 which correspond to D band and G band that confirms the presence of carbon. All other peaks 302, 407, 697, 854, and 994 cm−1 marked as “⁎” are analogous to Raman bands of V2O5. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies were carried out to gain further insight into the morphology of C@V2O5 nanostructures. The SEM images of C@V2O5 nanostructures are shown in Fig. 2a, b. These figures evidenced the presence of well-defined rod like morphology with lengths ranging from 1 to 3 μm and diameter ranging from 300–600 nm. The energy dispersive X-ray spectrum (EDS) of C@V2O5 is presented in Fig. S3. It confirms the presence of vanadium (44.7%), oxygen (46.7%) and carbon (8.5%) in the prepared material. Further, the EDS elemental mapping images presented in Fig. S3 confirms the presence of these three elements. In addition the EDS line scanning spectrum of the C@V2O5 is presented in Fig. S4. The signals of V, O and C across the line trace confirm the presence of carbon in the sample. The high resolution TEM images of C@V2O5 (Fig. 2c, d, e) clearly indicate the presence of a thin carbon layer that
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Scheme 1. Schematic illustration of the fabrication of C@V2O5 nanorods.
covers the surface of V2O5. The presence of lattice fringe of 0.44 nm indicates crystallinity of V2O5 [26]. To examine the supercapacitor behaviour and to estimate the capacity of the C@V2O5 nanorods as electrode material, cyclic voltammetry (CV) measurements were performed. The CV traces of pristine V2O5, V2O5 network and C@V2O5 nanorod electrodes between 0-1 V at scan rate of 2 mV s−1 are displayed in Fig. 3a. The CV curve of C@V2O5 electrode reveals a quasi-rectangular structure with oxidation and reduction peaks indicating the supercapacitor behaviour of the electrode. The C@V2O5 nanorod electrode exhibits higher specific capacitance as 426 Fg−1, which is 82% higher than pristine V2O5 and 35% higher than the values obtained for V2O5 network at a scan rate of 2 mV s−1 [21]. The CV curves of C@V2O5 nanorod at various scan rates from 2 to 50 mV s−1are shown in Fig. 3b. These curves indicate better reversibility of C@V2O5 electrode. In addition to this, CV curves of pristine V2O5 and V2O5 networks at different scan rates were presented in Fig. S5 (a, b). Fig. 3c shows the variation of specific capacitance with scan rate for
Fig. 1. XRD pattern of pristine V2O5, V2O5 network and C@V2O5 nanorods.
pristine V2O5, V2O5 network and C@V2O5 nanorod electrodes. The C@ V2O5 electrode is found to retain 85% of the specific capacitance even at a larger scan rate of 50 mV s−1 (321 and 272 Fg−1 respectively at scan rates of 10 and 50 mV s−1) compared to pristine V2O5 and V2O5 networks. These values are much higher than the values reported recently by Lee et al. [11] (graphene decorated V2O5 nanobelt, 288 Fg−1 at 10 mV s− 1), Fu et al. [27] (graphene/V2O5 nanotube, 225 Fg− 1 at 10 mV s−1) and Bonso et al. [28] (exfoliated graphite nanoplatelets/ V2O5 nanotubes 35 Fg−1 at 10 mV s−1). This higher withstanding capacity of C@V2O5 electrodes at higher scan rates is quite attractive, when it is employed in device applications. Further, the charge/discharge characteristics of C@V2O5 electrode under galvanostatic conditions (GCD) were studied to evaluate the electrochemical performance at different rates of charge/discharge. The charge/discharge profiles of pristine V2O5, V2O5 network and C@V2O5 nanorod electrode at a current density of 0.5 Ag− 1 are presented in Fig. 3d. The nonlinear GCD curve of C@V2O5 electrode confirms the pseudocapacitive nature of the electrode. The specific capacitance calculated from GCD curve for C@V2O5 electrode is found to be 417 Fg−1, which is about 78% higher than pristine V2O5 and 32% higher than those obtained at similar current density for the pristine V2O5 and V2O5 networks [21]. The retention of capacitance at ultrafast charge/discharge rate is an important concern to achieve high performance supercapacitor devices. Fig. 3e shows GCD curves of C@V2O5 electrode at different current densities. Furthermore, the GCD curves of pristine V2O5 and V2O5 networks at different current densities are presented in Fig. S6(a, b). Fig. 3f displays a plot on the variation of specific capacitance with current density for pristine V2O5, V2O5 network and C@V2O5 nanorod electrode. Besides the elevated specific capacitance, the C@V2O5 electrode holds the specific capacitance of 341 Fg−1 at higher current density of 10 Ag−1 compared to pristine V2O5 and V2O5 networks. The material prepared in the present work exhibits excellent retention of capacitance (82% retention of capacitance when the current was increased by a factor of 20). This higher rate capacity of C@V2O5 electrode is compared with the V2O5/carbon based supercapcitor electrodes reported by different workers presented in Table S1. The retention of capacitance at higher rates of discharge is a unique feature of C@V2O5 electrode which will be much useful for performing charge/discharge at higher rates.
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Fig. 2. (a, b) SEM images of C@V2O5 nanorods, (c, d, e) TEM images of C@V2O5 nanorods.
Cyclic stability test of C@V2O5 electrode is also performed at 10 Ag−1. A higher rate is chosen as it would represent the real world applications. As shown in Fig. 4a, C@V2O5 electrode retains 76% of its initial capacitance up to 1000 cycles which is much better to those exhibited by the pristine V2O5 and V2O5 networks [21]. V2O5 is a known material to suffer more and exhibit poor cyclic performance at nanolevel due to the stress produced during intercalation/deintercalation of charges. For instance, Perera et al. [23] reported the 100% capacity retention up to 50 cycles for vanadium oxide nanotube spherical clusters prepared on carbon fabric. Zhang et al. [29] reported 70% stability up to 1000 cycles for RGO/V2O5 hydrogel. Perera et al. [30] reported 70% stability up to 70 cycles for vanadium oxide nanowire/graphene composites. Compared to these reports C@V2O5 electrode shows much better cyclic stability. It is noteworthy that the carbon coating on V2O5 acts as a physical support and reduces the stress during the ion intercalation/ deintercalation. The reduced stress during ion intercalation seems to improve the cyclic stability of C@V2O5 electrode.
To get a complete insight into the electrochemical performance of C@V2O5 electrode electrochemical impedance spectroscopy (EIS) measurements were carried out from 0.01 to 100 kHz. The Fig. 4b presents the Nyquist plot of pristine V2O5, V2O5 network and C@V2O5 nanorod electrode. The inset of Fig. 4b presents the enlarged view of Nyquist plot of C@V2O5 nanorod electrode. The impedance data was fitted to an equivalent circuit model (Fig. S7) involving the bulk solution resistance (Rs), charge transfer resistance (Rct), mass capacitance (CL), double layer capacitance CDL and Warburg resistance (W). The impedance spectrum is composed of a linear part at low frequency and semicircle at high frequency region. To a large extent the performance of the supercapacitor depends on the charge transfer resistance of the electrode. A smaller value of Rct confirms the presence of large number of highly conductive pathways that are present in the active material. The observed Rct value of C@V2O5 is 1.1 Ω, which is quite low when compared to the values for pristine V2O5 (11 Ω) and V2O5 network (2 Ω) and many other reported V2O5 based materials. For example,
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Fig. 3. (a) CV curves (vs Ag/AgCl) at a scan rate of 2 mV s−1 for pristine V2O5, V2O5 network and C@V2O5 nanorod electrode. (b) CV curves of C@V2O5 at different scan rates. (c) Variation of specific capacitance with a scan rate. (d) Charge/discharge profiles of pristine V2O5, V2O5 network and C@V2O5 electrode at the current density of 0.5 Ag−1(vs Ag/AgCl).(e) Charge/discharge profiles of C@V2O5 electrode at different current densities. (f) The variation of specific capacitance as a function of current density.
Perera et al. [30] reported 7.6 Ω for vanadium oxide nanowire/graphene paper electrodes. Xu et al. [31] reported 2.33 Ω for graphene/V2O5 xerogel nanocomposites. This result also demonstrates that C@V2O5 electrode is one of the most favourable high conductivity electrodes for supercapacitor applications. An asymmetric supercapacitor improves energy and power density simultaneously due to the use of metal oxide based positive electrode and carbon based negative electrode. An asymmetric type supercapacitor was fabricated using C@V2O5nanorods as positive electrode, activated carbon as the negative electrode and a polypropylene film as a separator. It was employed to assess the performance of the carbon coated vanadium pentoxide in a complete device. The CV traces of C@V2O5 nanostructure based asymmetric supercapacitor acquired at different scan rates are presented in Fig. S8a. The absence of redox peaks in the CV curve confirms the capacitive behaviour of the device. The GCD profiles of C@V2O5//AC asymmetric supercapacitor with current densities are presented in Fig. S8b. The better
reversibility of the device is confirmed by the symmetrical GCD curves. The specific capacitance values of 68, 53 and 32 Fg−1 at the current densities of 0.5, 1 and 2.5 Ag−1 were obtained from GCD curves. The C@ V2O5//AC asymmetric supercapacitor shows the energy density of 9.4 W h kg−1 with the power density of 170 W kg−1.
4. Conclusions A high performance carbon coated V2O5 nanorods were prepared via simple and scalable solution based hydrothermal process. The carbon coating on V2O5 significantly enhances the electronic conductivity and ion diffusion at electrode/electrolyte interface leading to higher specific capacitance (417 Fg−1), excellent rate capacity (341 Fg−1at 10 Ag−1), low charge transfer resistance (1.1 Ω) and better cyclic stability (76% after 1000 cycles).
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Fig. 4. (a) Charge/discharge cycling test of pristine V2O5, V2O5 network and C@V2O5 electrode at a current density of 10 Ag−1. (b) Nyquist plot of pristine V2O5, V2O5 network and C@V2O5 electrode.
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jeac
Short communication
High performance supercapacitor based on carbon coated V2O5 nanorods Balakrishnan Saravanakumar a, Kamatchi Kamaraj Purushothaman b, Gopalan Muralidharan c,⁎ a b c
Faculty of Physics, Mahalingam College of Engineering and Technology, Pollachi, Tamilnadu, India Faculty of Physics, TRP Engineering College (SRM Group), Irungalur, Trichy, Tamilnadu, India Department of Physics, Gandhigram Rural University, Gandhigram, Tamilnadu, India
a r t i c l e
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Article history: Received 22 May 2015 Received in revised form 14 September 2015 Accepted 26 October 2015 Available online 28 October 2015 Keywords: Nanostructures Oxides Precipitation Electrochemical properties Supercapacitor
a b s t r a c t Conductive material coating on redox efficient transition-metal oxides is one of the approaches to enhance the conductivity of the metal oxides. This approach could greatly boost the electrochemical performance of supercapacitors. In this article an attractive low cost method to coat conductive carbon on V2O5 nanostructures is reported. When utilized as supercapacitor electrodes, this smart combination of C@V2O5 nanorods exhibits attractive capacitive features, including higher capacitance (417 Fg−1 at 0.5 Ag−1), superior rate capacity (341 Fg−1 at 10 Ag−1), very low charge transfer resistance (1.1 Ω) and better cyclic stability. An asymmetric supercapacitor device based on C@V2O5 nanorod electrode delivers an energy density of 9.4 Wh kg−1 at a power density of 170 W kg−1. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The supercapacitors (SCs) are rechargeable devices that store huge energy. The storage capacity is thousands of Farads higher when compared to conventional capacitors. The SCs possess fascinating characteristics such as ultrafast charge/discharge rates, high power uptake, exceptional cycle life and safety. [1] These elegant features of SCs favour their potential use in hybrid electrical vehicles, portable electronic devices, memory backup systems etc. The overall electrochemical performance of SCs is much influenced by innovative electrode materials. Over a decade, large variety of components have been examined and exploited as electrode materials for supercapacitors. [2–6] In addition, a few of them have found their way in the manufacture of commercial supercapacitors. Among various compounds tested so far, vanadium pentoxide (V2O5) is highly attractive owing to its structural richness. The crystal structure of this material is composed of stacking of V2O5 layers perpendicular to the c-axis through Vander Waals interaction. [7] The layered structure aids the intercalation of electrolyte ions through the electrode. V2O5 has been rigorously studied as an electrode material for battery and supercapacitor applications after the first report by Whittingham [8]. Unfortunately, practical applications of V2O5 based electrode materials for supercapacitors are hampered by the poor electrical conductivity and cyclic stability due to the large specific volume expansion during electrolyte ion insertion/deinsertion processes [9]. It leads to lesser contact with the current collector and significantly increases the electrical ⁎ Corresponding author. E-mail address: [email protected] (G. Muralidharan).
http://dx.doi.org/10.1016/j.jelechem.2015.10.031 1572-6657/© 2015 Elsevier B.V. All rights reserved.
resistance. Furthermore, most of the V2O5 based electrodes suffer from poor rate capacity. The most important reasons for poor rate performance are higher value of electrode resistance and large interfacial charge transfer resistance. For example, recently Zhu et al. [10] published one of the most significant research works on V2O5 based supercapacitor electrode materials. Even though higher specific capacitance is achieved (451 Fg−1 at 0.5 Ag−1) for 3D-V2O5 electrodes, it retains only about 50% of the specific capacitance (~ 230 Fg− 1) at 10 Ag−1. Further this material was reported to possess a higher charge transfer resistance (Rct) of 10 Ω. Lee et al. [11] reported graphene decorated V2O5 nanobelts and achieved a maximum specific capacitance of 134 Fg−1 at 0.25 Ag− 1 in 1 M Na2SO4 aqueous electrolyte. These graphene/V2O5 nanobelts could lose 23% of specific capacitance (~ 104 Fg− 1) when current density rose from 0.25 to 1 Ag − 1. This material possesses Rct value of 5.7 Ω. V2O5/graphene hybrid aerogels fabricated by Wu et al. [12] showed a specific capacitance of 486 Fg− 1. This composite material retained only 37% of specific capacitance (~ 180 Fg− 1) at 10 Ag− 1 and its Rct was 5 Ω. V2O5 nanowires/CNT composites were reported by Chen and et al. [13] could exhibit a specific capacitance of 440 Fg− 1 and 200 Fg− 1 at 0.25 and 10 Ag− 1 respectively. This composite material was capable of retaining nearly 40% of specific capacitance at 10 Ag− 1. Even, in the authors' very recent publication on MnO2 grafted V2O5 nanostructures, nearly 50% specific capacitance (251 Fg− 1) at 5 Ag− 1is reported. [14] These studies have demonstrated the inability of V2O5 based electrode materials, which retain a higher specific capacitance at higher current rates. It is also indicated that the lower values of charge transfer resistance influence the rate capacity, even if they could yield higher capacitance at lower current rate.
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According to Stoller and Ruoff [15] an electrode material should be able to store larger amount of energy at higher current rates. Only such a material is favourable for supercapacitor device applications. Here it is valuable to mention that a good supercapacitor electrode material should possess high conductivity with large electrolyte access area and lower value Rct to achieve ultrafast charge/discharging at higher current rates. In this context, many inspiring works confirmed adding carbonaceous materials to V2O5 nanostructures to be an effective strategy to mitigate these issues and improve the electrochemical performance. [16–19] But these works involve complex experimental procedures and sophisticated instruments. Further, most importantly, this kind of approach is mainly adopted to improve the rate capacity in Li ion battery applications. Considering all these important concerns, the present study proposes a simple and cost-effective strategy to coat carbon onto V2O5 nanostructures. The presence of carbon on the surface of the V2O5 nanostructure forms an electrical network and improves the electrical conductivity and reduces volume changes due to ion intercalation/ deintercalation in the V2O5 and ensures the better cyclic stability. [20] In this communication, findings on the fabrication of carbon-coated V2O5 nanorods (C@V2O5) via simple and economically viable combined co-precipitation and hydrothermal process using dextrose as carbon source are presented. With this smart combination of C@V2O5, higher specific capacitance, superior rate capacity and better cyclic stability could be achieved when it is used as a supercapacitor electrode material. 2. Experimental methods 2.1. Material synthesis Vanadium pentoxide, hydrogen peroxide (30%) and potassium sulphate were obtained from SD fine Chemicals Ltd., India. Disodium citrate dextrose was purchased from Sigma Aldrich. All chemicals were of analytical grade and used without further purification. The V2O5 nanostructures were prepared following a previously reported protocol [21]. Thus prepared V2O5 nanostructure was mixed with dextrose (0.1773 g) which is used as a cost-effective carbon source. The mixture was transferred to a 100 mL Teflon lined stainless steel autoclave and kept at 180 °C for 12 h. The resulting material was properly washed using de ionized water (DI). The washed material was dried at 80 °C for 12 h. The final product was mentioned as C@V2O5 nanostructure. The performances of commercial V2O5 powder and V2O5 nanostructures without carbon prepared by the same route were also investigated and mentioned as pristine V2O5 and V2O5 networks respectively. 2.2. Material characterization The C@V2O5 nanostructure was characterized by X-ray diffraction (XRD) using a PANalyticalX'pert-PRO diffractometer with Cu Kα sealed tube (λ = 1.5406 Å). The FTIR analysis was performed using a PerkinElmer Spectrum BX-II spectrophotometer utilizing the KBr pellet method. The SEM investigations were carried out on a TESCAN VEGA 3 LMU scanning electron microscope. TEM characterizations were performed on a TEM-Philips JEOL CM12 microscope. 2.3. Electrochemical measurements Electrochemical investigations on C@V2O5 nanorod were performed using a three-electrode cell set-up, which consists of C@V2O5 as the working electrode, Ag/AgCl as the reference electrode and platinum wire as the counter electrode. The working electrode was prepared by mixing 85 wt.% sample, 10 wt.% activated carbon (Sigma-Aldrich), 5 wt.% polytetrafluoroethylene (Sigma-Aldrich) and a few drops of ethanol. This mixture was coated on to ultrasonically cleaned nickel foam (1 cm2). The electrodes were dried at 80 °C for 6 h to get the working
electrodes. The electrochemical tests were carried out at room temperature using 0.5 M K2SO4 aqueous electrolyte. The mass of the active material in the electrode was 1 mg. Electrochemical measurements were performed using CHI 660D electrochemical workstation (CH Instruments). An asymmetric supercapacitor was designed using C@V2O5 nanostructure and activated carbon (Sigma-Aldrich, 1100 m2 g−1) as the electrode materials. The polypropylene films (Celgard, 2400) soaked in 0.5 M K2SO4 was used as a separator and electrolyte. The basic equations adopted to calculate specific capacitance, energy and power densities are reported elsewhere. [21].
3. Results and discussion Herein, the formation of carbon coated V2O5 nanorods from network structured V2O5 as shown in Scheme 1 is proposed. Initially the network structured V2O5 was prepared by the same protocol which was adopted in the authors' previous work [21]. The V2O5 sol was prepared by blending of V2O5 powder, H2O2 and deionized water. The presence of H2O2 initiates gelation process within a few minutes [22]. To avoid gelation, disodium citrate was added as the controlling agent to the sol under stirring condition. After 3 days V2O5 nanostructures were collected by centrifugation and dried at 100 °C for 10 h. Secondly, as synthesized V2O5 nanostructures were mixed with dextrose and transferred to auto clave and kept at 180 °C for 2 h. It is well reported that the presence of carbon source under hydrothermal conditions leads the change of morphology of metal oxide nanostructures. [11] The mixing of carbon precursor (dextrose) with V2O5 networks under hydrothermal conditions leads the formation of rod like structures. The phase, purity and structure of C@V2O5 nanorods were investigated by X-ray diffraction (XRD) analysis. The XRD pattern of pristine V2O5, V2O5 network and C@V2O5 nanorods is presented in Fig. 1. The diffraction peaks (200), (001), (101), (110), (400), (011), (310), (002), (411), (600), (020) and (710) in the spectrum are consistent with the orthorhombic structured layered V2O5 (JCPDS no 41-1426 space group: Pmmn(59)) and confirms its crystalline nature. The lattice parameters are a = 1.151 nm, b = 3.565 nm, and c = 4.372 nm. The absence of any other peaks in the XRD confirms the purity of the material. Further the FTIR spectrum was recorded for the C@V2O5 nanorods and presented in Fig. S1a (Supplementary data). The spectrum shows a strong absorption band at 1006 cm−1 attributed to the stretching vibration of V_O. The magnified FTIR spectrum of C@V2O5 between 400–700cm−1 (Fig. S1b) shows bands at 485 and 576 cm−1 which are assigned to the symmetric and the asymmetric stretching modes of V– O–V vibrations respectively [23]. The absorption at 525 cm−1 represents the V–O–V stretching vibration [24]. The absorption peak at 1615 cm−1 is due to the bending vibrations of the water molecules [25]. The FT-Raman spectra of the C@V2O5 nanostructures are shown in Fig. S2. The spectrum consists of two high frequency bands at 1362 and 1578 cm−1 which correspond to D band and G band that confirms the presence of carbon. All other peaks 302, 407, 697, 854, and 994 cm−1 marked as “⁎” are analogous to Raman bands of V2O5. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies were carried out to gain further insight into the morphology of C@V2O5 nanostructures. The SEM images of C@V2O5 nanostructures are shown in Fig. 2a, b. These figures evidenced the presence of well-defined rod like morphology with lengths ranging from 1 to 3 μm and diameter ranging from 300–600 nm. The energy dispersive X-ray spectrum (EDS) of C@V2O5 is presented in Fig. S3. It confirms the presence of vanadium (44.7%), oxygen (46.7%) and carbon (8.5%) in the prepared material. Further, the EDS elemental mapping images presented in Fig. S3 confirms the presence of these three elements. In addition the EDS line scanning spectrum of the C@V2O5 is presented in Fig. S4. The signals of V, O and C across the line trace confirm the presence of carbon in the sample. The high resolution TEM images of C@V2O5 (Fig. 2c, d, e) clearly indicate the presence of a thin carbon layer that
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Scheme 1. Schematic illustration of the fabrication of C@V2O5 nanorods.
covers the surface of V2O5. The presence of lattice fringe of 0.44 nm indicates crystallinity of V2O5 [26]. To examine the supercapacitor behaviour and to estimate the capacity of the C@V2O5 nanorods as electrode material, cyclic voltammetry (CV) measurements were performed. The CV traces of pristine V2O5, V2O5 network and C@V2O5 nanorod electrodes between 0-1 V at scan rate of 2 mV s−1 are displayed in Fig. 3a. The CV curve of C@V2O5 electrode reveals a quasi-rectangular structure with oxidation and reduction peaks indicating the supercapacitor behaviour of the electrode. The C@V2O5 nanorod electrode exhibits higher specific capacitance as 426 Fg−1, which is 82% higher than pristine V2O5 and 35% higher than the values obtained for V2O5 network at a scan rate of 2 mV s−1 [21]. The CV curves of C@V2O5 nanorod at various scan rates from 2 to 50 mV s−1are shown in Fig. 3b. These curves indicate better reversibility of C@V2O5 electrode. In addition to this, CV curves of pristine V2O5 and V2O5 networks at different scan rates were presented in Fig. S5 (a, b). Fig. 3c shows the variation of specific capacitance with scan rate for
Fig. 1. XRD pattern of pristine V2O5, V2O5 network and C@V2O5 nanorods.
pristine V2O5, V2O5 network and C@V2O5 nanorod electrodes. The C@ V2O5 electrode is found to retain 85% of the specific capacitance even at a larger scan rate of 50 mV s−1 (321 and 272 Fg−1 respectively at scan rates of 10 and 50 mV s−1) compared to pristine V2O5 and V2O5 networks. These values are much higher than the values reported recently by Lee et al. [11] (graphene decorated V2O5 nanobelt, 288 Fg−1 at 10 mV s− 1), Fu et al. [27] (graphene/V2O5 nanotube, 225 Fg− 1 at 10 mV s−1) and Bonso et al. [28] (exfoliated graphite nanoplatelets/ V2O5 nanotubes 35 Fg−1 at 10 mV s−1). This higher withstanding capacity of C@V2O5 electrodes at higher scan rates is quite attractive, when it is employed in device applications. Further, the charge/discharge characteristics of C@V2O5 electrode under galvanostatic conditions (GCD) were studied to evaluate the electrochemical performance at different rates of charge/discharge. The charge/discharge profiles of pristine V2O5, V2O5 network and C@V2O5 nanorod electrode at a current density of 0.5 Ag− 1 are presented in Fig. 3d. The nonlinear GCD curve of C@V2O5 electrode confirms the pseudocapacitive nature of the electrode. The specific capacitance calculated from GCD curve for C@V2O5 electrode is found to be 417 Fg−1, which is about 78% higher than pristine V2O5 and 32% higher than those obtained at similar current density for the pristine V2O5 and V2O5 networks [21]. The retention of capacitance at ultrafast charge/discharge rate is an important concern to achieve high performance supercapacitor devices. Fig. 3e shows GCD curves of C@V2O5 electrode at different current densities. Furthermore, the GCD curves of pristine V2O5 and V2O5 networks at different current densities are presented in Fig. S6(a, b). Fig. 3f displays a plot on the variation of specific capacitance with current density for pristine V2O5, V2O5 network and C@V2O5 nanorod electrode. Besides the elevated specific capacitance, the C@V2O5 electrode holds the specific capacitance of 341 Fg−1 at higher current density of 10 Ag−1 compared to pristine V2O5 and V2O5 networks. The material prepared in the present work exhibits excellent retention of capacitance (82% retention of capacitance when the current was increased by a factor of 20). This higher rate capacity of C@V2O5 electrode is compared with the V2O5/carbon based supercapcitor electrodes reported by different workers presented in Table S1. The retention of capacitance at higher rates of discharge is a unique feature of C@V2O5 electrode which will be much useful for performing charge/discharge at higher rates.
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Fig. 2. (a, b) SEM images of C@V2O5 nanorods, (c, d, e) TEM images of C@V2O5 nanorods.
Cyclic stability test of C@V2O5 electrode is also performed at 10 Ag−1. A higher rate is chosen as it would represent the real world applications. As shown in Fig. 4a, C@V2O5 electrode retains 76% of its initial capacitance up to 1000 cycles which is much better to those exhibited by the pristine V2O5 and V2O5 networks [21]. V2O5 is a known material to suffer more and exhibit poor cyclic performance at nanolevel due to the stress produced during intercalation/deintercalation of charges. For instance, Perera et al. [23] reported the 100% capacity retention up to 50 cycles for vanadium oxide nanotube spherical clusters prepared on carbon fabric. Zhang et al. [29] reported 70% stability up to 1000 cycles for RGO/V2O5 hydrogel. Perera et al. [30] reported 70% stability up to 70 cycles for vanadium oxide nanowire/graphene composites. Compared to these reports C@V2O5 electrode shows much better cyclic stability. It is noteworthy that the carbon coating on V2O5 acts as a physical support and reduces the stress during the ion intercalation/ deintercalation. The reduced stress during ion intercalation seems to improve the cyclic stability of C@V2O5 electrode.
To get a complete insight into the electrochemical performance of C@V2O5 electrode electrochemical impedance spectroscopy (EIS) measurements were carried out from 0.01 to 100 kHz. The Fig. 4b presents the Nyquist plot of pristine V2O5, V2O5 network and C@V2O5 nanorod electrode. The inset of Fig. 4b presents the enlarged view of Nyquist plot of C@V2O5 nanorod electrode. The impedance data was fitted to an equivalent circuit model (Fig. S7) involving the bulk solution resistance (Rs), charge transfer resistance (Rct), mass capacitance (CL), double layer capacitance CDL and Warburg resistance (W). The impedance spectrum is composed of a linear part at low frequency and semicircle at high frequency region. To a large extent the performance of the supercapacitor depends on the charge transfer resistance of the electrode. A smaller value of Rct confirms the presence of large number of highly conductive pathways that are present in the active material. The observed Rct value of C@V2O5 is 1.1 Ω, which is quite low when compared to the values for pristine V2O5 (11 Ω) and V2O5 network (2 Ω) and many other reported V2O5 based materials. For example,
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Fig. 3. (a) CV curves (vs Ag/AgCl) at a scan rate of 2 mV s−1 for pristine V2O5, V2O5 network and C@V2O5 nanorod electrode. (b) CV curves of C@V2O5 at different scan rates. (c) Variation of specific capacitance with a scan rate. (d) Charge/discharge profiles of pristine V2O5, V2O5 network and C@V2O5 electrode at the current density of 0.5 Ag−1(vs Ag/AgCl).(e) Charge/discharge profiles of C@V2O5 electrode at different current densities. (f) The variation of specific capacitance as a function of current density.
Perera et al. [30] reported 7.6 Ω for vanadium oxide nanowire/graphene paper electrodes. Xu et al. [31] reported 2.33 Ω for graphene/V2O5 xerogel nanocomposites. This result also demonstrates that C@V2O5 electrode is one of the most favourable high conductivity electrodes for supercapacitor applications. An asymmetric supercapacitor improves energy and power density simultaneously due to the use of metal oxide based positive electrode and carbon based negative electrode. An asymmetric type supercapacitor was fabricated using C@V2O5nanorods as positive electrode, activated carbon as the negative electrode and a polypropylene film as a separator. It was employed to assess the performance of the carbon coated vanadium pentoxide in a complete device. The CV traces of C@V2O5 nanostructure based asymmetric supercapacitor acquired at different scan rates are presented in Fig. S8a. The absence of redox peaks in the CV curve confirms the capacitive behaviour of the device. The GCD profiles of C@V2O5//AC asymmetric supercapacitor with current densities are presented in Fig. S8b. The better
reversibility of the device is confirmed by the symmetrical GCD curves. The specific capacitance values of 68, 53 and 32 Fg−1 at the current densities of 0.5, 1 and 2.5 Ag−1 were obtained from GCD curves. The C@ V2O5//AC asymmetric supercapacitor shows the energy density of 9.4 W h kg−1 with the power density of 170 W kg−1.
4. Conclusions A high performance carbon coated V2O5 nanorods were prepared via simple and scalable solution based hydrothermal process. The carbon coating on V2O5 significantly enhances the electronic conductivity and ion diffusion at electrode/electrolyte interface leading to higher specific capacitance (417 Fg−1), excellent rate capacity (341 Fg−1at 10 Ag−1), low charge transfer resistance (1.1 Ω) and better cyclic stability (76% after 1000 cycles).
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Fig. 4. (a) Charge/discharge cycling test of pristine V2O5, V2O5 network and C@V2O5 electrode at a current density of 10 Ag−1. (b) Nyquist plot of pristine V2O5, V2O5 network and C@V2O5 electrode.
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