Journal of Colloid and Interface Science xxx (2013) xxx–xxx
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Facile synthesis of hierarchical porous VOx@carbon composites for supercapacitors Chunxia Zhao a, Jinqiao Cao a, Yunxia Yang b, Wen Chen a,⇑, Junshen Li a a State Key Laboratory of Advanced Technology for Materials Synthesis and Progressing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, PR China b Earth Science and Resource Engineering, CSIRO, VIC 3168, Australia
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Article history: Available online xxxx Keywords: Hierarchical porous structure VOx@carbon composite Micro–nano structure One-step synthesis Supercapacitor
a b s t r a c t Hierarchical or micro–nano structured porous VOx@carbon composites were synthesized by a one-step method using phenolic resin as the carbon precursor and ammonium metavanadate as the source of vanadium oxides. The effects of the vanadium source loading on the microstructure and electrochemical properties of the composites were investigated. X-ray diffraction results showed that as the vanadium oxides source loading increased, vanadium oxides in the composites changed oxidation states from V2O3 to mixed states of V2O3 and VO2. Electrochemical test results indicated that the micro–nano porous structure of the composites could facilitate the ion diffusion in the rich porous structure and then promote the electrochemical reaction. More importantly, we found that vanadium oxides greatly enhanced the electrochemical performance of the materials, due to the faradic capacitance generated from vanadium oxide nanoparticles. A maximum specific capacitance of 171 F/g was obtained from VOx@carbon composite with vanadium loading of 44 wt%. Further increasing the VOx loading over this fraction was not beneficial. Our results suggested that hierarchical porous VOx@carbon composites were promising candidates for supercapacitor applications. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Energy storage devices made from nanostructured electrodes are reported to be lightweight and possess promising performance [1]. Interest is growing in new energy storage devices that are low cost, lightweight and deliver high energy and power densities. Supercapacitors are important energy storage devices that can deliver high power during short periods of time, making them attractive for electric vehicle applications. Vanadium oxides are used in energy-related devices, such as lithium-ion batteries [2–4] and supercapacitors [5–7], due to their multi-oxidation states and low cost. However, bulk vanadium oxide electrodes in supercapacitors have been limited by their low electronic conductivity and slow electrochemical kinetics [7–9]. A vanadium oxide/carbon system that explored the effect of mixing carbon black with the working electrode showed that the carbon black increased electronic conductivity, leading to improved electrochemical kinetics [9]. Meanwhile, it also revealed that generating porosity in the electrode material would improve electrolyte diffusion, resulting in better electrochemical properties. Since then, an intriguing question has been raised as to whether
⇑ Corresponding author. Fax: +86 27 87760129. E-mail address: [email protected] (W. Chen).
carbon black is the best medium for establishing an electronically conducting network for vanadium oxides. In recent years, progress in nanoscience and nanotechnology has provided an impetus for the development of new hybrid porous nanostructures, especially metal oxide nanoparticles (NPs) confined in porous carbons. These carbon-based metal oxides are highly regarded as potential supercapacitor electrodes. A common route to synthesize hybrid porous nanocomposites is via the post-synthesis method, in which porous carbon supports are used as the hosts and metal oxides procurers as the guest particles. The utilization of nanoporous carbon networks leads to a facile design of hybrid materials containing vanadium oxides. For example, Sakamoto’s group integrated vanadium oxides with carbon nanotubes (CNTs) via sol–gel methods, and showed that CNT conductivity helped to improve the specific capacities of the materials [2]. Perera et al. [1] used a pulsed laser deposition method to coat carbon fiber fabrics with V2O5, followed by hydrothermal treatment to create vanadium oxide nanotube spherical clusters on the carbon fiber for use as electrode materials for supercapacitors. The resulting electrodes, with high-surface-area carbon fibers serving as the conducting material and freestanding substrate, exhibited promising electrochemical properties with long-term stability [1]. Another study showed that grafting layers of vanadium oxides onto the surface of functionalized CNTs can obtain cumulative high-capacity [6].
0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.11.086
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Among various carbon support candidates, ordered mesoporous carbons (OMCs) have attracted tremendous attention. Their fascinating properties include high surface area, uniform pore size, chemical inertness and good electrical conductivity [10]. Vanadium oxide has been incorporated into OMC CMK-3 by a combination of the impregnation and sonochemical methods, with improvement in capacitance reported [11]. Conventionally, OMCs are fabricated by the nanocasting method (CMK-3), which is fussy, high-cost, and thus industrially unfeasible. Recently, several groups have developed soft templating approaches to prepare OMCs through organic–organic self-assembly of amphiphilic block copolymers and phenolic resins. Numerous attempts have been made to obtain hybrid materials with dispersed NPs in these OMCs, using microwave, impregnation and sonochemical methods [12–14]. In some cases, presurface modifications were made to the carbon surface to increase its ability to anchor metal oxide precursors [15,16]. The above methods were done through post-synthesis routes. Disadvantages of these time-consuming, multi-step routes include low guest loading and guest particle aggregation. For commercial, mass-production applications, a low cost, facile and one-step synthesis procedure is preferable. Incorporating the metal guest nanoparticles into the carbonizable carbon precursor during carbon synthesis can provide an alternative in situ route to obtain hybrid porous nanostructures [17,18]. In this work, VOx@carbon composites with a hierarchically porous structure were synthesized via a facile one-step route through self-assembly of amphiphilic block copolymers, phenolic resins and ammonium metavanadate, and then annealed at 700 °C. We also studied the effect of the amount of ammonium metavanadate on the morphology and structural properties of the composites, and investigated the electrochemical potential of the materials in a nontoxic aqueous electrolyte for supercapacitor applications.
2. Experimental 2.1. Materials synthesis Phenolic resin was prepared using phenol and formaldehyde in a base-catalyzed process [19]. The typical procedure was: 10.0 g of phenol was melted at 40–42 °C, then 2.1 g of 20 wt% NaOH solution was added slowly while stirring. Ten minutes later, 17.7 g of 37 wt% formalin was added dropwise, and the mixed solution was stirred at 70–75 °C for 1 h. After cooling to the room temperature, the pH value of the mixture was adjusted to 7.0 using 1.0 mol/L HCl solution. Water was then removed at 50 °C in a vacuum oven. The final product was diluted in ethanol to prepare the 20 wt% ethanolic resol solution before use. Hierarchical, porous VOx@carbon composites were prepared through a one-step co-assembly procedure, typically as follows: 1.6 g Pluronic F127, 0.1 g ammonium metavanadate (NH4VO3) and 1.0 g of 0.2 M HCl were added to 8.0 g ethanol, followed by mixing with 5.0 g of 20 wt% ethanolic resol solution. After stirring for 2 h at 40 °C, the solution was cast onto Petri dishes and dried at room temperature to evaporate ethanol, and then the dishes were put in an oven for thermopolymerization at 100 °C for 24 h. The dark-colored film was torn off the dishes and pyrolysed at 700 °C for 3 h in a tube furnace with a ramping rate of 1 °C/min under a nitrogen atmosphere. The final obtained VOx@carbon composites were denoted as C–V-n, where n refers to the input weight of NH4VO3 (n = 0, 0.2, 0.5, 1.0, 1.5 and 2.0 g). The blank was made using 2.0 g of NH4VO3, while keeping all other conditions the same as before, but without addition of resol. The resultant sample was named ‘blank VOx’.
2.2. Structural and morphology characterization X-ray diffraction (XRD) experiments were performed on an X’pert powder diffractometer (PANalytical, The Netherlands) with Cu Ka radiation (k = 1.5418 Å). The thermal behaviors of the obtained samples were investigated on a Netzsch STA 449C differential thermal analyzer under nitrogen atmosphere. Nitrogen sorption analysis was measured on an ASAP2020 adsorption analyzer (Micromeritics) at 77 K. The Brunauer–Emment–Teller (BET) method was employed to calculate the specific surface areas. The pore size distributions were derived from the adsorption branches of isotherms by using the Barrett–Joyner–Halenda (BJH) model. Field emission scanning electron microscope (SEM) images and energy-dispersive X-ray spectroscopy (EDX) were obtained on a Hitachi S-4800 microscope operated at 20 kV. Transmission electron microscope (TEM) images of the samples were recorded on a JEOL JEM-2100F microscope operated at 200 kV. 2.3. Electrochemical characterization The electrochemical experiments were performed on an Autolab PGSTAT30 (Eco Echemie B.V. company) with a three-electrode electrochemical cell system that consisted of a working electrode, a platinum counter electrode and an Hg/HgO reference electrode. The working electrode was prepared firstly by mixing the powder composites and polytetrafluoroethylene binder with a weight percent ratio of 9:1 in isopropyl alcohol. The mixture was then rolled into a thin sheet of uniform thickness. The thin sheet was coldpressed onto a nickel foam which was used as a current collector and vacuum dried at 80 °C for 12 h to obtain a sample electrode. The geometric surface area of the working electrode was 1.0 cm2. Cyclic voltammetry was carried out in the potential range from 0.20 to 0.30 V in 1 mol/L KNO3 neutral electrolyte at room temperature. Constant current charge–discharge at a current density of 100 mA/g was conducted on Autolab PGSTAT30. The specific capacitance sC (F/g) is calculated from the gravimetric discharge process according to the following equation:
sC ¼
i Dt d m DV
where i is the constant current density used for charge/discharge (mA/g), m is the weight of the activated substance (g), Dtd is the time elapsed for the discharging cycle (s) and DV is the voltage interval of the discharge (V). 3. Results and discussion Fig. 1 presents the XRD patterns of C–V-n. The prepared pure carbon (C–V-0) shows only two broad diffraction peaks at 2h = 23° and 43°, suggesting that it has an amorphous nature. With addition of NH4VO3 to the precursor, very weak peaks emerge at 33.00°, 36.25°and 53.95° in the XRD patterns of samples C–V-0.1 and C–V-0.2, which can be indexed to (1 0 4), (1 1 0) and (1 1 6) diffractions of V2O3 (85–1411). This suggests that at low vanadium precursor loading (n = 0.1 and 0.2), vanadium oxides exist in the V2O3 phase, which is due to the reduction of V(V)–V(III) in the presence of carbon at 700 °C under a flow of inert gas. As greater amounts of the vanadium precursor are added, peaks appear at 24.32°, 41.24°, 49.83°, 63.16° and 65.22° in samples C–V-n (n = 0.5, 1.0 and 1.5), which can be indexed to (0 1 2), (1 1 3), (0 2 4), (2 1 4) and (3 0 0) diffractions of V2O3. This suggests that V2O3 started forming and became more crystalline. Moreover, two more weak peaks are found at 27.83° and 55.48° in these three samples, which are indexed to (1 1 0) and (1 2 1) diffractions of VO2 (74–1642), suggesting reduction of V2O3 to VO2. Vanadium oxides
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Fig. 1. XRD patterns of blank carbon, C–V-n and blank VOx samples.
in the composites were converted from V(V) to mixed valences of V(IV) and V(III) at high vanadium loadings. When the amount of NH4VO3 was increased to 2.0 g (sample C–V-2.0), the diffraction intensity of peaks associated with V2O3 increased, suggesting better crystallinity of V2O3. Apart from the peaks identified in sample C–V-n (n = 0.5, 1.0 and 1.5) at 27.83° and 55.48° that are associated with VO2, two peaks at 45.06° and 55.29° are identified in sample C–V-2.0 (indexed to (5 1 1), (6 0 3) diffractions of VO2), suggesting the development of a much more crystalline VO2. The main
peaks on the XRD patterns of both C–V-1.0 and C–V-1.5 have similar intensities, indicating that the vanadium oxides have similar crystallinity and crystal size. The diffraction peaks assigned to V2O3 and VO2 are also found in the blank VOx, which results from oxygen lose and reduction from the carbon formed from the amphiphilic block copolymers in inert gas atmosphere calcinations. The vanadium element weight ratios in the composites are listed in Table 1. Fig. 2 shows representative SEM images of the synthesized samples. Without addition of the vanadium source, the obtained blank carbon sample C–V-0 shows block morphology as depicted in Fig. 2a. At relatively low vanadium precursor loadings, the composites C–V-n (n = 0.1, 0.2) display similar morphology to the blank carbon sample (Fig. 2b and c). After increasing vanadium precursor loading to n = 0.5, spherical ‘cells’ start to scatter on the surface of sample C–V-0.5 (Fig. 2d). The open diameter of the large cell is 1.0– 1.7 lm, and that of the small cell is 0.4–0.6 lm. When vanadium precursor loading is further increased, more cells are observed on the surface. The cross-section sides of sample C–V-1.0, which has a vanadium precursor loading of 1.0 g (Fig. 2e), reveal that the bulk sample is also porous. Spherical particles are found inside the open cells. The open diameter of the larger cells is 1.2–3.0 lm, and the smaller cells are around 0.2–0.3 lm in diameter. The C–V-1.5 sample with a vanadium precursor loading of 1.5 g possesses many more, well-dispersed cells than the C–V-1.0 sample. The cell size in C–V-1.5 samples is about 0.5–1.5 lm (Fig. 3a). The majority of cells are around 1 lm in diameter, which is much more uniform than the samples with vanadium precursor loadings of less than 1.0 g. Sample C–V-2.0 (Fig. 3b) has a similar micro-structure to sample C–V-1.5, with spherical cells dispersed
Table 1 Textural parameters and specific capacitance of C–V-n and blank VOx.
a
Sample
V (wt%)a
Pore size (nm)
BET surface area (m2/g)
Pore volume (cm3/g)
Specific capacitance (F/g)
C–V-0 C–V-0.1 C–V-0.2 C–V-0.5 C–V-1.0 C–V-1.5 C–V-2.0 Blank VOx
0 7.82 18.45 28.82 38.61 43.86 60.24 93.46
3.41 3.81 3.81 3.81 3.83 3.94 3.93 3.75
519 714 307 414 327 204 198 62
0.35 0.60 0.31 0.43 0.32 0.16 0.16 0.12
26 57 91 116 125 171 147 88
Vanadium element weight ratio in the composites calculated by MV/(MC + MV) 100% based on thermogravimetric analysis.
Fig. 2. SEM images of blank carbon C–V-0 (a), C–V-n with n = 0.1 (b), 0.2 (c), 0.5 (d) and 1.0 (e).
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Fig. 3. SEM images of C–V-1.5 (a), C–V-2.0 (b) and blank VOx (c); energy-dispersive X-ray spectroscopy results of the C–V-1.5 sample at the selected area (d and e).
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Fig. 4. TEM images of blank carbon C–V-0 (a), C–V-0.5 (b–c), C–V-1.5 (d–f) and C–V-2.0 (g–h).
throughout the bulk structure. The majority of cells have a diameter of around 0.4 lm, which is much smaller and even more uniform than the cells in sample C–V-1.5. Both EDX spectrums (Fig. 3d and e) obtained from the encapsulated particles inside the cells and the bulk area for sample C–V-1.5 confirm the existence of vanadium oxide particles. These spherical particles are formed by the aggregation of excess vanadium precursor during the carbonization step. When the vanadium precursor loading is low, vanadium precursors assemble with phenolic resin and F127. Because the loading is low, the amount of vanadium oxides produced from the precursor is insufficient to aggregate and form larger particles, and thus show the same morphology as blank carbon C–V-0. But, at a higher vanadium loading (n > 0.5), the excess vanadium precursors migrate from adjacent areas and aggregate into particles ranging in size from 400 nm to 2 lm. Fig. 3c shows the SEM image of blank VOx. It exhibits a similar micro-morphology to C–V-n composites (n = 1.5 and 2.0), with cells and particles. The size of cells/particles is between 0.4 and 1.0 lm. Many spherical particles are embedded into a sponge-like framework, which has a clearly porous structure with a mean size of 20 nm. The porosity parameters of the samples obtained from the nitrogen sorption analysis are listed in Table 1. Sample C–V-0 has a specific surface area of 519 m2/g, a pore volume of 0.35 cm3/g and pore size is 3.4 nm. With addition of vanadium precursor, sample C–V-0.1 shows a higher specific surface area of 714 m2/g. Its pore volume of 0.60 cm3/g and pore size of 3.81 nm are also both larger than C–V-0’s, suggesting that a very
small addition of vanadium precursor has activated the carbon structure and increased the porosity. When the vanadium precursor is increased to 0.2 g (C–V-0.2), the specific surface area decreases significantly to 307 m2/g, and the pore volume is reduced to 0.31 cm3/g. This suggests that with further addition of vanadium precursor, the blocking effects of precursor overwhelm its activation effects, reducing both surface area and pore volume. When precursor loading increases to 0.5 g, instead of further decreasing the surface area, sample C–V-0.5 has a higher specific surface area (414 m2/g) and larger pore volume (0.43 cm3/g) than sample C–V-0.2. We believe that the surface area increase is due to the development of many cells in the sample, as shown in Fig. 2d. With further additions of vanadium precursor, the specific surface areas and the pore volumes of C–V-n (n = 1.0, 1.5 and 2.0) start decreasing gradually because of the blocking effects from the vanadium oxides produced in the composites. The blank VOx shows a low specific surface area of 62 m2/g and a low pore volume of 0.12 cm3/g. To further examine the structural information of the samples, we characterized a selection of them using TEM. Fig. 4a shows that the blank carbon C–V-0 has a well-ordered mesoporous structure. The average pore size of C–V-0 is around 3 nm, which is in accordance with the nitrogen sorption analysis results. The TEM image of sample C–V-0.5 reveals that the regular structure of the blank carbon has disappeared (Fig. 4b). Small particles around 4 nm in diameter are observed (Fig. 4c). According to the XRD results in Fig. 1, these crystallites are vanadium oxides, which are dispersed throughout the carbon substrate.
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Fig. 5. Cyclic voltammograms (A) and discharge curves (B) of C–V-n and blank VOx electrodes.
Sample C–V-1.5 contains nanosized pores of around 20–100 nm in diameter (Fig. 4d and e), suggesting that this sample has both macro-sized spherical cells and mesopores. In addition, some small crystallites of vanadium oxides are observed in the homogeneous bulk area seen in the SEM images. The crystallites are 4 nm (Fig. 4f). Nanoparticles of vanadium oxides with clear lattice fringes are also observed in sample C–V-2.0 (Fig. 4g). Interestingly, the large particles observed in the SEM images are also vanadium oxides, but are multi-crystallites with individual sizes of 4 nm (Fig. 4h). Compared with sample C–V-n (n 6 1.5), many vanadium oxide NPs are found in the C–V-2.0 sample. In all samples, the vanadium oxide NPs have a mean size of 4 nm. This suggests that although increasing the vanadium loading will increase the number of the vanadium oxide NPs, the particles do not grow further once they reach a certain size (4 nm in this case). Cyclic voltammogram (CV) tests for the electrodes prepared using VOx@carbon composites are presented in Fig. 5(A). The CV curves of the C–V-0 electrode exhibit a rectangular shape at a voltage sweep rate of 1 mV/s, as shown in Fig. 5(A), indicating the typical electric double-layer capacitive (EDLC) behavior. As is known, galvanostatic charge/discharge can be used to evaluate charge storage capacity. Fig. 5(B) shows the discharge curves of blank carbon, VOx@carbon composites and blank VOx at a current density of 100 mA/g; their calculated specific capacitances (sC)
are listed in Table 1. The sC of C–V-0 is 26 F/g. With a small loading of vanadium oxides, electrode C–V-0.1’ CV curves maintain the rectangular shape, but with a clearly enlarged cover area. Since the areas of CV curves are proportional to the capacitance, the result suggests that the C–V-0.1 electrode possesses basic EDLC behavior, but with increased capacitance sC of 57 F/g which is more than doubled than that of C–V-0. No clear redox peak is observed, suggesting little pseudo capacitance. In this case, we believe the increased capacitance is mainly due to the increased specific surface area of the sample. Moreover, the well-developed micro–nano porous structure of the sample can also facilitate electrolyte filtration and increase ion diffusion, which promotes the electrochemical reaction that improves capacitance. Electrode C–V-0.2 shows a distorted-rectangular CV shape. Small redox peaks around 0.0 V and 0.14 V can barely be seen, suggesting the presence of faradic reaction and pseudo capacitance contributed by the vanadium oxides. The surface area of sample C–V-0.2 is much less than sample C–V-0.1, while its capacitance sC of 91 F/g is much higher instead. Therefore, we believe the increased capacitance is contributed by the pseudo capacitance of the vanadium oxides. With continued addition of vanadium precursor to 0.5 g, electrode C–V-0.5 has a similar cover area as electrode C–V-0.2. But the peak of C–V-0.5 around 0.14 V is clearer than that of C–V-0.2. The calculated sC of C–V-0.5 is 116 F/g, which is much higher than that of sample C–V-0.1 even though it has lower specific surface area. Electrode C–V-1.0 exhibits significantly enlarged cover area. The redox peaks remain the same as in the previous samples, suggesting that the vanadium oxides in these samples have similar faradic properties. C–V-1.0 electrode obtains an sC of 125 F/g. Since the surface area of sample C–V-1.0 is similar to that of sample C–V-0.2, the increase in capacitance is clearly not affected by the surface area but contributed by the loading amount of vanadium oxides. When the loading amount of vanadium precursor is further increased to 1.5 g, electrode C–V-1.5 particularly shows an outstandingly high sC of 171 F/g. It further demonstrates that increased amount of vanadium oxides in the sample contribute to increased capacitance because of the pseudo capacitance generated by the vanadium oxides. However, we have observed that further increasing the vanadium precursor loading amount to 2.0 g as in sample C–V-2.0, sample’s sC falls to 147 F/g. Considering both C–V-2.0 and C–V-1.5 have similar specific surface areas (204 and 198 m2/g, respectively) and similar crystallinity (XRD patterns and TEM images), the reduced capacitance therefore must result from the overloading of vanadium oxides (60 wt%), which reduce the conductivity of the composites and thereby influence the overall capacitance. The CV curve of the blank VOx sample shows a dramatically reduced cover area with an sC of 88 F/g, revealing poor electrochemical properties of the sample and further suggesting higher content of VOx in the sample (93 wt%) lead to reduced overall capacitance. Overall, C–V-n composites (n P 0.2) show much improved capacitance compared with pure carbon sample C–V-0 and blank VOx. When the vanadium precursor loading is less than 44 wt% (sample C–V-1.5), with the increase in vanadium precursor loadings, pseudo capacitance makes a significant contribution to the overall capacitance. However, as the vanadium loadings increase, the carbon contents correspondingly decrease (e.g. C–V-2.0, which has a high vanadium ratio of 60 wt%). This leads to poor electronic conductivity and decreased capacitance.
4. Conclusion Hierarchical, porous VOx@carbon composite was successfully synthesized using a one-step method, with ammonium metavanadate as the vanadium source. For the composite with low vanadium loading (n = 0.1, 0.2), V2O3 was produced after calcination
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at 700 °C and the composites showed bulk film morphology. However, when the vanadium loading was increased to 0.5 6 n P 2.0, both VO2 and V2O3 phases were found in the composites, and micrometer-size cells and spheres were observed. Moreover, a large amount of cells with uniform size distribution developed in the composites when n value was 1.5 and 2.0, leading to hierarchically macro- and meso-porous structures. Electrochemical analysis showed that VOx@carbon composites possessed improved capacitance, which was due to their high surface area at lower VOx loadings (n 6 0.1) and their pseudo capacitance at higher VOx loadings (0.1 < n 6 1.5). The micro–nano porous structure also contributed to the increased capacitance, because it facilitated electrolyte filtration and ion diffusion and then promoted the electrochemical reaction. However, our finding also suggested that there was a maximum loading amount of the vanadium precursor to optimize the pseudo capacitance of the electrode material when it was synthesized. It is 44 wt% in our case. When the loading amount is over 44 wt%, the trade off property from the vanadium oxides started to appear as they reduced the conductivity of the material and resulted in decreased pseudo capacitance.
Acknowledgments This work was financially supported by the National Basic Research Program of China (2013CB934103), the National Natural Science Foundation of China (50772085), the Fundamental Research Funds for the Central Universities (2012-IV-004) and
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Facile synthesis of hierarchical porous VOx@carbon composites for supercapacitors Chunxia Zhao a, Jinqiao Cao a, Yunxia Yang b, Wen Chen a,⇑, Junshen Li a a State Key Laboratory of Advanced Technology for Materials Synthesis and Progressing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, PR China b Earth Science and Resource Engineering, CSIRO, VIC 3168, Australia
a r t i c l e
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Article history: Available online xxxx Keywords: Hierarchical porous structure VOx@carbon composite Micro–nano structure One-step synthesis Supercapacitor
a b s t r a c t Hierarchical or micro–nano structured porous VOx@carbon composites were synthesized by a one-step method using phenolic resin as the carbon precursor and ammonium metavanadate as the source of vanadium oxides. The effects of the vanadium source loading on the microstructure and electrochemical properties of the composites were investigated. X-ray diffraction results showed that as the vanadium oxides source loading increased, vanadium oxides in the composites changed oxidation states from V2O3 to mixed states of V2O3 and VO2. Electrochemical test results indicated that the micro–nano porous structure of the composites could facilitate the ion diffusion in the rich porous structure and then promote the electrochemical reaction. More importantly, we found that vanadium oxides greatly enhanced the electrochemical performance of the materials, due to the faradic capacitance generated from vanadium oxide nanoparticles. A maximum specific capacitance of 171 F/g was obtained from VOx@carbon composite with vanadium loading of 44 wt%. Further increasing the VOx loading over this fraction was not beneficial. Our results suggested that hierarchical porous VOx@carbon composites were promising candidates for supercapacitor applications. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Energy storage devices made from nanostructured electrodes are reported to be lightweight and possess promising performance [1]. Interest is growing in new energy storage devices that are low cost, lightweight and deliver high energy and power densities. Supercapacitors are important energy storage devices that can deliver high power during short periods of time, making them attractive for electric vehicle applications. Vanadium oxides are used in energy-related devices, such as lithium-ion batteries [2–4] and supercapacitors [5–7], due to their multi-oxidation states and low cost. However, bulk vanadium oxide electrodes in supercapacitors have been limited by their low electronic conductivity and slow electrochemical kinetics [7–9]. A vanadium oxide/carbon system that explored the effect of mixing carbon black with the working electrode showed that the carbon black increased electronic conductivity, leading to improved electrochemical kinetics [9]. Meanwhile, it also revealed that generating porosity in the electrode material would improve electrolyte diffusion, resulting in better electrochemical properties. Since then, an intriguing question has been raised as to whether
⇑ Corresponding author. Fax: +86 27 87760129. E-mail address: [email protected] (W. Chen).
carbon black is the best medium for establishing an electronically conducting network for vanadium oxides. In recent years, progress in nanoscience and nanotechnology has provided an impetus for the development of new hybrid porous nanostructures, especially metal oxide nanoparticles (NPs) confined in porous carbons. These carbon-based metal oxides are highly regarded as potential supercapacitor electrodes. A common route to synthesize hybrid porous nanocomposites is via the post-synthesis method, in which porous carbon supports are used as the hosts and metal oxides procurers as the guest particles. The utilization of nanoporous carbon networks leads to a facile design of hybrid materials containing vanadium oxides. For example, Sakamoto’s group integrated vanadium oxides with carbon nanotubes (CNTs) via sol–gel methods, and showed that CNT conductivity helped to improve the specific capacities of the materials [2]. Perera et al. [1] used a pulsed laser deposition method to coat carbon fiber fabrics with V2O5, followed by hydrothermal treatment to create vanadium oxide nanotube spherical clusters on the carbon fiber for use as electrode materials for supercapacitors. The resulting electrodes, with high-surface-area carbon fibers serving as the conducting material and freestanding substrate, exhibited promising electrochemical properties with long-term stability [1]. Another study showed that grafting layers of vanadium oxides onto the surface of functionalized CNTs can obtain cumulative high-capacity [6].
0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.11.086
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Among various carbon support candidates, ordered mesoporous carbons (OMCs) have attracted tremendous attention. Their fascinating properties include high surface area, uniform pore size, chemical inertness and good electrical conductivity [10]. Vanadium oxide has been incorporated into OMC CMK-3 by a combination of the impregnation and sonochemical methods, with improvement in capacitance reported [11]. Conventionally, OMCs are fabricated by the nanocasting method (CMK-3), which is fussy, high-cost, and thus industrially unfeasible. Recently, several groups have developed soft templating approaches to prepare OMCs through organic–organic self-assembly of amphiphilic block copolymers and phenolic resins. Numerous attempts have been made to obtain hybrid materials with dispersed NPs in these OMCs, using microwave, impregnation and sonochemical methods [12–14]. In some cases, presurface modifications were made to the carbon surface to increase its ability to anchor metal oxide precursors [15,16]. The above methods were done through post-synthesis routes. Disadvantages of these time-consuming, multi-step routes include low guest loading and guest particle aggregation. For commercial, mass-production applications, a low cost, facile and one-step synthesis procedure is preferable. Incorporating the metal guest nanoparticles into the carbonizable carbon precursor during carbon synthesis can provide an alternative in situ route to obtain hybrid porous nanostructures [17,18]. In this work, VOx@carbon composites with a hierarchically porous structure were synthesized via a facile one-step route through self-assembly of amphiphilic block copolymers, phenolic resins and ammonium metavanadate, and then annealed at 700 °C. We also studied the effect of the amount of ammonium metavanadate on the morphology and structural properties of the composites, and investigated the electrochemical potential of the materials in a nontoxic aqueous electrolyte for supercapacitor applications.
2. Experimental 2.1. Materials synthesis Phenolic resin was prepared using phenol and formaldehyde in a base-catalyzed process [19]. The typical procedure was: 10.0 g of phenol was melted at 40–42 °C, then 2.1 g of 20 wt% NaOH solution was added slowly while stirring. Ten minutes later, 17.7 g of 37 wt% formalin was added dropwise, and the mixed solution was stirred at 70–75 °C for 1 h. After cooling to the room temperature, the pH value of the mixture was adjusted to 7.0 using 1.0 mol/L HCl solution. Water was then removed at 50 °C in a vacuum oven. The final product was diluted in ethanol to prepare the 20 wt% ethanolic resol solution before use. Hierarchical, porous VOx@carbon composites were prepared through a one-step co-assembly procedure, typically as follows: 1.6 g Pluronic F127, 0.1 g ammonium metavanadate (NH4VO3) and 1.0 g of 0.2 M HCl were added to 8.0 g ethanol, followed by mixing with 5.0 g of 20 wt% ethanolic resol solution. After stirring for 2 h at 40 °C, the solution was cast onto Petri dishes and dried at room temperature to evaporate ethanol, and then the dishes were put in an oven for thermopolymerization at 100 °C for 24 h. The dark-colored film was torn off the dishes and pyrolysed at 700 °C for 3 h in a tube furnace with a ramping rate of 1 °C/min under a nitrogen atmosphere. The final obtained VOx@carbon composites were denoted as C–V-n, where n refers to the input weight of NH4VO3 (n = 0, 0.2, 0.5, 1.0, 1.5 and 2.0 g). The blank was made using 2.0 g of NH4VO3, while keeping all other conditions the same as before, but without addition of resol. The resultant sample was named ‘blank VOx’.
2.2. Structural and morphology characterization X-ray diffraction (XRD) experiments were performed on an X’pert powder diffractometer (PANalytical, The Netherlands) with Cu Ka radiation (k = 1.5418 Å). The thermal behaviors of the obtained samples were investigated on a Netzsch STA 449C differential thermal analyzer under nitrogen atmosphere. Nitrogen sorption analysis was measured on an ASAP2020 adsorption analyzer (Micromeritics) at 77 K. The Brunauer–Emment–Teller (BET) method was employed to calculate the specific surface areas. The pore size distributions were derived from the adsorption branches of isotherms by using the Barrett–Joyner–Halenda (BJH) model. Field emission scanning electron microscope (SEM) images and energy-dispersive X-ray spectroscopy (EDX) were obtained on a Hitachi S-4800 microscope operated at 20 kV. Transmission electron microscope (TEM) images of the samples were recorded on a JEOL JEM-2100F microscope operated at 200 kV. 2.3. Electrochemical characterization The electrochemical experiments were performed on an Autolab PGSTAT30 (Eco Echemie B.V. company) with a three-electrode electrochemical cell system that consisted of a working electrode, a platinum counter electrode and an Hg/HgO reference electrode. The working electrode was prepared firstly by mixing the powder composites and polytetrafluoroethylene binder with a weight percent ratio of 9:1 in isopropyl alcohol. The mixture was then rolled into a thin sheet of uniform thickness. The thin sheet was coldpressed onto a nickel foam which was used as a current collector and vacuum dried at 80 °C for 12 h to obtain a sample electrode. The geometric surface area of the working electrode was 1.0 cm2. Cyclic voltammetry was carried out in the potential range from 0.20 to 0.30 V in 1 mol/L KNO3 neutral electrolyte at room temperature. Constant current charge–discharge at a current density of 100 mA/g was conducted on Autolab PGSTAT30. The specific capacitance sC (F/g) is calculated from the gravimetric discharge process according to the following equation:
sC ¼
i Dt d m DV
where i is the constant current density used for charge/discharge (mA/g), m is the weight of the activated substance (g), Dtd is the time elapsed for the discharging cycle (s) and DV is the voltage interval of the discharge (V). 3. Results and discussion Fig. 1 presents the XRD patterns of C–V-n. The prepared pure carbon (C–V-0) shows only two broad diffraction peaks at 2h = 23° and 43°, suggesting that it has an amorphous nature. With addition of NH4VO3 to the precursor, very weak peaks emerge at 33.00°, 36.25°and 53.95° in the XRD patterns of samples C–V-0.1 and C–V-0.2, which can be indexed to (1 0 4), (1 1 0) and (1 1 6) diffractions of V2O3 (85–1411). This suggests that at low vanadium precursor loading (n = 0.1 and 0.2), vanadium oxides exist in the V2O3 phase, which is due to the reduction of V(V)–V(III) in the presence of carbon at 700 °C under a flow of inert gas. As greater amounts of the vanadium precursor are added, peaks appear at 24.32°, 41.24°, 49.83°, 63.16° and 65.22° in samples C–V-n (n = 0.5, 1.0 and 1.5), which can be indexed to (0 1 2), (1 1 3), (0 2 4), (2 1 4) and (3 0 0) diffractions of V2O3. This suggests that V2O3 started forming and became more crystalline. Moreover, two more weak peaks are found at 27.83° and 55.48° in these three samples, which are indexed to (1 1 0) and (1 2 1) diffractions of VO2 (74–1642), suggesting reduction of V2O3 to VO2. Vanadium oxides
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Fig. 1. XRD patterns of blank carbon, C–V-n and blank VOx samples.
in the composites were converted from V(V) to mixed valences of V(IV) and V(III) at high vanadium loadings. When the amount of NH4VO3 was increased to 2.0 g (sample C–V-2.0), the diffraction intensity of peaks associated with V2O3 increased, suggesting better crystallinity of V2O3. Apart from the peaks identified in sample C–V-n (n = 0.5, 1.0 and 1.5) at 27.83° and 55.48° that are associated with VO2, two peaks at 45.06° and 55.29° are identified in sample C–V-2.0 (indexed to (5 1 1), (6 0 3) diffractions of VO2), suggesting the development of a much more crystalline VO2. The main
peaks on the XRD patterns of both C–V-1.0 and C–V-1.5 have similar intensities, indicating that the vanadium oxides have similar crystallinity and crystal size. The diffraction peaks assigned to V2O3 and VO2 are also found in the blank VOx, which results from oxygen lose and reduction from the carbon formed from the amphiphilic block copolymers in inert gas atmosphere calcinations. The vanadium element weight ratios in the composites are listed in Table 1. Fig. 2 shows representative SEM images of the synthesized samples. Without addition of the vanadium source, the obtained blank carbon sample C–V-0 shows block morphology as depicted in Fig. 2a. At relatively low vanadium precursor loadings, the composites C–V-n (n = 0.1, 0.2) display similar morphology to the blank carbon sample (Fig. 2b and c). After increasing vanadium precursor loading to n = 0.5, spherical ‘cells’ start to scatter on the surface of sample C–V-0.5 (Fig. 2d). The open diameter of the large cell is 1.0– 1.7 lm, and that of the small cell is 0.4–0.6 lm. When vanadium precursor loading is further increased, more cells are observed on the surface. The cross-section sides of sample C–V-1.0, which has a vanadium precursor loading of 1.0 g (Fig. 2e), reveal that the bulk sample is also porous. Spherical particles are found inside the open cells. The open diameter of the larger cells is 1.2–3.0 lm, and the smaller cells are around 0.2–0.3 lm in diameter. The C–V-1.5 sample with a vanadium precursor loading of 1.5 g possesses many more, well-dispersed cells than the C–V-1.0 sample. The cell size in C–V-1.5 samples is about 0.5–1.5 lm (Fig. 3a). The majority of cells are around 1 lm in diameter, which is much more uniform than the samples with vanadium precursor loadings of less than 1.0 g. Sample C–V-2.0 (Fig. 3b) has a similar micro-structure to sample C–V-1.5, with spherical cells dispersed
Table 1 Textural parameters and specific capacitance of C–V-n and blank VOx.
a
Sample
V (wt%)a
Pore size (nm)
BET surface area (m2/g)
Pore volume (cm3/g)
Specific capacitance (F/g)
C–V-0 C–V-0.1 C–V-0.2 C–V-0.5 C–V-1.0 C–V-1.5 C–V-2.0 Blank VOx
0 7.82 18.45 28.82 38.61 43.86 60.24 93.46
3.41 3.81 3.81 3.81 3.83 3.94 3.93 3.75
519 714 307 414 327 204 198 62
0.35 0.60 0.31 0.43 0.32 0.16 0.16 0.12
26 57 91 116 125 171 147 88
Vanadium element weight ratio in the composites calculated by MV/(MC + MV) 100% based on thermogravimetric analysis.
Fig. 2. SEM images of blank carbon C–V-0 (a), C–V-n with n = 0.1 (b), 0.2 (c), 0.5 (d) and 1.0 (e).
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Fig. 3. SEM images of C–V-1.5 (a), C–V-2.0 (b) and blank VOx (c); energy-dispersive X-ray spectroscopy results of the C–V-1.5 sample at the selected area (d and e).
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Fig. 4. TEM images of blank carbon C–V-0 (a), C–V-0.5 (b–c), C–V-1.5 (d–f) and C–V-2.0 (g–h).
throughout the bulk structure. The majority of cells have a diameter of around 0.4 lm, which is much smaller and even more uniform than the cells in sample C–V-1.5. Both EDX spectrums (Fig. 3d and e) obtained from the encapsulated particles inside the cells and the bulk area for sample C–V-1.5 confirm the existence of vanadium oxide particles. These spherical particles are formed by the aggregation of excess vanadium precursor during the carbonization step. When the vanadium precursor loading is low, vanadium precursors assemble with phenolic resin and F127. Because the loading is low, the amount of vanadium oxides produced from the precursor is insufficient to aggregate and form larger particles, and thus show the same morphology as blank carbon C–V-0. But, at a higher vanadium loading (n > 0.5), the excess vanadium precursors migrate from adjacent areas and aggregate into particles ranging in size from 400 nm to 2 lm. Fig. 3c shows the SEM image of blank VOx. It exhibits a similar micro-morphology to C–V-n composites (n = 1.5 and 2.0), with cells and particles. The size of cells/particles is between 0.4 and 1.0 lm. Many spherical particles are embedded into a sponge-like framework, which has a clearly porous structure with a mean size of 20 nm. The porosity parameters of the samples obtained from the nitrogen sorption analysis are listed in Table 1. Sample C–V-0 has a specific surface area of 519 m2/g, a pore volume of 0.35 cm3/g and pore size is 3.4 nm. With addition of vanadium precursor, sample C–V-0.1 shows a higher specific surface area of 714 m2/g. Its pore volume of 0.60 cm3/g and pore size of 3.81 nm are also both larger than C–V-0’s, suggesting that a very
small addition of vanadium precursor has activated the carbon structure and increased the porosity. When the vanadium precursor is increased to 0.2 g (C–V-0.2), the specific surface area decreases significantly to 307 m2/g, and the pore volume is reduced to 0.31 cm3/g. This suggests that with further addition of vanadium precursor, the blocking effects of precursor overwhelm its activation effects, reducing both surface area and pore volume. When precursor loading increases to 0.5 g, instead of further decreasing the surface area, sample C–V-0.5 has a higher specific surface area (414 m2/g) and larger pore volume (0.43 cm3/g) than sample C–V-0.2. We believe that the surface area increase is due to the development of many cells in the sample, as shown in Fig. 2d. With further additions of vanadium precursor, the specific surface areas and the pore volumes of C–V-n (n = 1.0, 1.5 and 2.0) start decreasing gradually because of the blocking effects from the vanadium oxides produced in the composites. The blank VOx shows a low specific surface area of 62 m2/g and a low pore volume of 0.12 cm3/g. To further examine the structural information of the samples, we characterized a selection of them using TEM. Fig. 4a shows that the blank carbon C–V-0 has a well-ordered mesoporous structure. The average pore size of C–V-0 is around 3 nm, which is in accordance with the nitrogen sorption analysis results. The TEM image of sample C–V-0.5 reveals that the regular structure of the blank carbon has disappeared (Fig. 4b). Small particles around 4 nm in diameter are observed (Fig. 4c). According to the XRD results in Fig. 1, these crystallites are vanadium oxides, which are dispersed throughout the carbon substrate.
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Fig. 5. Cyclic voltammograms (A) and discharge curves (B) of C–V-n and blank VOx electrodes.
Sample C–V-1.5 contains nanosized pores of around 20–100 nm in diameter (Fig. 4d and e), suggesting that this sample has both macro-sized spherical cells and mesopores. In addition, some small crystallites of vanadium oxides are observed in the homogeneous bulk area seen in the SEM images. The crystallites are 4 nm (Fig. 4f). Nanoparticles of vanadium oxides with clear lattice fringes are also observed in sample C–V-2.0 (Fig. 4g). Interestingly, the large particles observed in the SEM images are also vanadium oxides, but are multi-crystallites with individual sizes of 4 nm (Fig. 4h). Compared with sample C–V-n (n 6 1.5), many vanadium oxide NPs are found in the C–V-2.0 sample. In all samples, the vanadium oxide NPs have a mean size of 4 nm. This suggests that although increasing the vanadium loading will increase the number of the vanadium oxide NPs, the particles do not grow further once they reach a certain size (4 nm in this case). Cyclic voltammogram (CV) tests for the electrodes prepared using VOx@carbon composites are presented in Fig. 5(A). The CV curves of the C–V-0 electrode exhibit a rectangular shape at a voltage sweep rate of 1 mV/s, as shown in Fig. 5(A), indicating the typical electric double-layer capacitive (EDLC) behavior. As is known, galvanostatic charge/discharge can be used to evaluate charge storage capacity. Fig. 5(B) shows the discharge curves of blank carbon, VOx@carbon composites and blank VOx at a current density of 100 mA/g; their calculated specific capacitances (sC)
are listed in Table 1. The sC of C–V-0 is 26 F/g. With a small loading of vanadium oxides, electrode C–V-0.1’ CV curves maintain the rectangular shape, but with a clearly enlarged cover area. Since the areas of CV curves are proportional to the capacitance, the result suggests that the C–V-0.1 electrode possesses basic EDLC behavior, but with increased capacitance sC of 57 F/g which is more than doubled than that of C–V-0. No clear redox peak is observed, suggesting little pseudo capacitance. In this case, we believe the increased capacitance is mainly due to the increased specific surface area of the sample. Moreover, the well-developed micro–nano porous structure of the sample can also facilitate electrolyte filtration and increase ion diffusion, which promotes the electrochemical reaction that improves capacitance. Electrode C–V-0.2 shows a distorted-rectangular CV shape. Small redox peaks around 0.0 V and 0.14 V can barely be seen, suggesting the presence of faradic reaction and pseudo capacitance contributed by the vanadium oxides. The surface area of sample C–V-0.2 is much less than sample C–V-0.1, while its capacitance sC of 91 F/g is much higher instead. Therefore, we believe the increased capacitance is contributed by the pseudo capacitance of the vanadium oxides. With continued addition of vanadium precursor to 0.5 g, electrode C–V-0.5 has a similar cover area as electrode C–V-0.2. But the peak of C–V-0.5 around 0.14 V is clearer than that of C–V-0.2. The calculated sC of C–V-0.5 is 116 F/g, which is much higher than that of sample C–V-0.1 even though it has lower specific surface area. Electrode C–V-1.0 exhibits significantly enlarged cover area. The redox peaks remain the same as in the previous samples, suggesting that the vanadium oxides in these samples have similar faradic properties. C–V-1.0 electrode obtains an sC of 125 F/g. Since the surface area of sample C–V-1.0 is similar to that of sample C–V-0.2, the increase in capacitance is clearly not affected by the surface area but contributed by the loading amount of vanadium oxides. When the loading amount of vanadium precursor is further increased to 1.5 g, electrode C–V-1.5 particularly shows an outstandingly high sC of 171 F/g. It further demonstrates that increased amount of vanadium oxides in the sample contribute to increased capacitance because of the pseudo capacitance generated by the vanadium oxides. However, we have observed that further increasing the vanadium precursor loading amount to 2.0 g as in sample C–V-2.0, sample’s sC falls to 147 F/g. Considering both C–V-2.0 and C–V-1.5 have similar specific surface areas (204 and 198 m2/g, respectively) and similar crystallinity (XRD patterns and TEM images), the reduced capacitance therefore must result from the overloading of vanadium oxides (60 wt%), which reduce the conductivity of the composites and thereby influence the overall capacitance. The CV curve of the blank VOx sample shows a dramatically reduced cover area with an sC of 88 F/g, revealing poor electrochemical properties of the sample and further suggesting higher content of VOx in the sample (93 wt%) lead to reduced overall capacitance. Overall, C–V-n composites (n P 0.2) show much improved capacitance compared with pure carbon sample C–V-0 and blank VOx. When the vanadium precursor loading is less than 44 wt% (sample C–V-1.5), with the increase in vanadium precursor loadings, pseudo capacitance makes a significant contribution to the overall capacitance. However, as the vanadium loadings increase, the carbon contents correspondingly decrease (e.g. C–V-2.0, which has a high vanadium ratio of 60 wt%). This leads to poor electronic conductivity and decreased capacitance.
4. Conclusion Hierarchical, porous VOx@carbon composite was successfully synthesized using a one-step method, with ammonium metavanadate as the vanadium source. For the composite with low vanadium loading (n = 0.1, 0.2), V2O3 was produced after calcination
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at 700 °C and the composites showed bulk film morphology. However, when the vanadium loading was increased to 0.5 6 n P 2.0, both VO2 and V2O3 phases were found in the composites, and micrometer-size cells and spheres were observed. Moreover, a large amount of cells with uniform size distribution developed in the composites when n value was 1.5 and 2.0, leading to hierarchically macro- and meso-porous structures. Electrochemical analysis showed that VOx@carbon composites possessed improved capacitance, which was due to their high surface area at lower VOx loadings (n 6 0.1) and their pseudo capacitance at higher VOx loadings (0.1 < n 6 1.5). The micro–nano porous structure also contributed to the increased capacitance, because it facilitated electrolyte filtration and ion diffusion and then promoted the electrochemical reaction. However, our finding also suggested that there was a maximum loading amount of the vanadium precursor to optimize the pseudo capacitance of the electrode material when it was synthesized. It is 44 wt% in our case. When the loading amount is over 44 wt%, the trade off property from the vanadium oxides started to appear as they reduced the conductivity of the material and resulted in decreased pseudo capacitance.
Acknowledgments This work was financially supported by the National Basic Research Program of China (2013CB934103), the National Natural Science Foundation of China (50772085), the Fundamental Research Funds for the Central Universities (2012-IV-004) and
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