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Enhanced charge storage by optimization of pore structure in nanocomposite between ordered mesoporous carbon and nanosized WO3−x
Journal of Power Sources 244 (2013) 777e782
Contents lists available at SciVerse ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Enhanced charge storage by optimization of pore structure in nanocomposite between ordered mesoporous carbon and nanosized WO3x Yuanyuan Zhou a, Seunghyun Ko a, Chul Wee Lee a, Sung Gyu Pyo b, Soo-Kil Kim b, Songhun Yoon b, * a Green Chemical Technology Division, Korea Research Institute of Chemical Technology (KRICT), University of Science and Technology (UST), Daejeon 305-600, Republic of Korea b School of Integrative Engineering, Chung-Ang University, 221, Heukseok-Dong, Dongjak-Gu, Seoul 156-756, Republic of Korea
h i g h l i g h t s Novel ordered mesoporous carbon/tungsten oxide nanocomposite was prepared. Small pores within wall were homogeneously filled by tungsten oxide nanocrystals. Ordered pores were optimized for electrolyte transport and high charge accessibility. Volumetric capacity was as high as 125 F cm3 and rate capability was excellent. Capacitance increase was observed after long-term cycles (113%).
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 December 2012 Received in revised form 9 April 2013 Accepted 11 April 2013 Available online 23 April 2013
Herein, we report preparation of a novel ordered mesoporous carbon/reduced tungsten oxide nanocomposite by post-addition into the ordered mesoporous carbon prepared by triconstituent co-assembly method. The as-prepared composite material is characterized using various analysis methods and nitrogen sorption isotherms, which reveal that small mesopores located within pore wall are selectively filled by tungsten oxide and also reduced tungsten oxide crystals with size of 6 nm are uniformly dispersed in the carbon matrix. Hence, our prepared nanocomposite possesses an ordered pore structure optimized for fast electrolyte transport and highly accessible charge storage sites. When applied as supercapacitor electrode, it exhibits a high volumetric capacity (125 F cm3), an excellent rate capability (79%) and capacitance increase after long-term cycles (113%). Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Ordered mesoporous carbon Reduced tungsten Supercapacitor
1. Introduction Nowadays, high-performance supercapacitors (SCs) are in high demand in the application to portable electric devices and electrical vehicles [1e3]. However, power capability and energy density remain unsatisfactory in the case of commercial carbonaceous electrode materials and therefore, many researchers have focused on searching for novel electrode materials with high performance. Amongst these, ordered mesoporous carbon (OMC) materials have been intensely investigated as supercapacitor electrodes due to superior electrolyte transport through their highly ordered structures [4e8]. In spite of the high rate capability, the specific capacitance of OMC has been reported to have low specific * Corresponding author. Tel.: þ82 2 820 5769; fax: þ82 2 814 2615. E-mail address: [email protected] (S. Yoon). 0378-7753/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2013.04.054
capacitance (<200 F g1) and volumetric one, which are ascribed to limited charge storage on the electric double layer and low material density (about 0.5 g cm3) [4e8]. In contrast, pseudocapacitive materials based on transient metal oxides (MnO2, CoOx, MoOx, WO3x, etc.) show high capacitance but low rate capability is inevitable due to intrinsically slow Faradaic charging [9e12]. Hence, one can expect that incorporation of transient metal oxides into OMC is desirable for combination of large capacitance and high rate capability. In our previous work, MoO2 was successfully incorporated into the mesopores of a bimodal ordered mesoporous carbon (tOMC) and a large increase of capacitance was observed due to high capacitive contribution from MoO2 [13]. Because of the dissolution of MoO2 in the high acid environment, unfortunately, steep capacitance decay happened, which retarded its practical application [13]. However, WO3x, which is highly resistant to the strong acid electrolyte, could be a good alternative to MoO2. Form
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our previous work, especially, nanostructured WO3x with high electrical conductivity exhibited a fairly high volumetric capacitance and good rate capability [12]. Thus, an incorporation of conductive WO3x instead of MoO2 into tOMC is expected to be beneficial for high volumetric capacitance, stable cyclability and excellent rate capability. Herein, we propose a novel preparation of OMC/WO3x nanocomposite by impregnating a tungsten-rich precursor (phosphate tungsten acid) into OMC material and following calcination in a reducing atmosphere. The electrochemical properties as supercapacitor electrode are investigated in detail. 2. Experimental section 2.1. Preparation of C/WO3x nanocomposite 2.1.1. Preparation of the bimodal ordered mesoporous carbon (tOMC) The tOMC material was prepared according to the literature. In a typical procedure, 1.6 g of F127 and 1 g of 0.1 M HCl was added into 8 g of ethanol. The mixture was stirred at room temperature until a clear solution was acquired. Then 2.08 g of TEOS and 5 g of 20 wt% resol ethanolic solution (prepared according to the literature [14]) were dropped. Upon stirring at room temperature for 2 h, the clear solution was poured into a big dish and kept in ambient environment for fully ethanol-evaporation until a glue-like film was formed. Then the dish was transferred into an oven at 100 C for 24 h until polymerized film with darker colour obtained. After grinding, as-obtained powders were calcined in a tubular furnace at 450 C for 2 h and further calcined at 900 C for 2 h under N2 flow. After cooling down to the room temperature, the powders were taken out and immersed into a 10 wt% HF aqueous solution for etching process. After filtration, washing and drying, the final product was collected and indexed as tOMC. 2.1.2. Preparation of OMC/WO3x nanocomposite (tOMC-W) The preparation of OMC/WO3x nanocomposite is similar to that of our previous OMC/MoO2 nanocomposite. In a typical procedure, tOMC powders were immersed in 20 wt% HNO3 (Aldrich) solution at 70 C for 24 h for the purpose of surface acidification. After drying, 0.95 g of phosphate tungsten acid (Aldrich) was dissolved in 14 g of ethanol, 0.34 g of the acidified tOMC powders were mixed into the solution. After one-day stirring, the powders were filtered and washed for three times using ethanol and gathered after drying in the oven. The powders were calcinated in a tubular furnace at
600 C for 2 h in a reductive atmosphere (5 vol% H2/N2)(the temperature rate is 5 C min1). Finally, the powders were removed from the furnace and indexed as tOMC-W. 2.2. Material characterization and electrochemical performance testing The pore size distribution (PSD) was analysed using an N2 adsorption measurement (Micromeritics ASAP 2010). The external morphology of the carbon was examined using a scanning electron microscope (SEM, JEOL JSM-840A), whereas the pore image was scanned by a transmission electron microscope (TEM, JEOL JEM2010). The energy dispersive X-ray (EDX) analysis was conducted using the EDX analyzer equipped with the TEM in point mode. The small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) patterns were obtained with a Rigaku D/Max-3C diffractometer equipped with a rotating anode and Cu Ka radiation (l ¼ 0.15418 nm). X-ray photoelectron spectroscopy (XPS) experiments were carried out in an ultrahigh vacuum using the Scienta ESCA-300 high-resolution X-ray photoelectron spectrometer (HRXPS). Thermogravimetric analysis (TGA) was conducted using the TGA-HP50 in an air atmosphere with a temperature rate of 2 C min1. The supercapacitor electrode fabrication procedure and electrochemical cell structure were identical with the reported literature. Here, Pt flag counter and SCE reference electrodes were used for three electrode cell system using 1.0 M H2SO4 electrolyte. Ivium potentiostat was utilized to conduct cyclic voltammetry (CV). Scan rate was varied from 2 to 20 mV s1 in the CV experiment. Galvanostatic chargeedischarge experiment was carried out using the WBCS-3000 battery cycler (Xeno Co.). The capacitance C from the CV experiment is calculated using Eq. (1):
C ¼ i=v
(1)
Here, i and v are the observed current and the scan rate, respectively. 3. Results and discussion 3.1. Characteristics of the prepared OMC/WO3x nanocomposite Fig. 1(a) shows the small-angle X-ray scattering (SAXS) patterns of the tOMC and tOMC-W material. As seen, the SAXS patterns of the tOMC sample showed two peaks at 2q ¼ 1 and 1.5 ,
Fig. 1. (a) Small-angle X-ray scattering patterns of the tOMC-W and tOMC material. (b) Wide-angle X-ray diffraction patterns of the tOMC-W and tOMC material. Insets are peak indexes of WO2.83.
Y. Zhou et al. / Journal of Power Sources 244 (2013) 777e782
Fig. 2. N2 sorption isotherms for the tOMC-W and tOMC material and the inset shows the pore size distributions from the BJH calculation.
respectively, indexed as (100) and (110) plane of a typical 2D hexagonal structure [15,16]. After impregnation of tungsten precursor and calcination, one obvious peak at 2q ¼ 1 was still observed in the tOMC-W sample, indicating that the ordered mesoporous structure was generally maintained. Fig. 1(b) shows the wide-angle X-ray diffraction patterns of the tOMC and tOMC-W material. For tOMC sample, two typical board peaks are associated with the amorphous nature of the tOMC material. The peaks at 2q ¼ 22 and 42 correspond to the (002) and (100) plane of graphite, respectively. For tOMC-W, characteristic peaks of WO2.83 (JSPD PDF #00036-0103) appeared in the amorphous carbon background, indicative of the formation of tungsten oxide crystals. Using the Scherer’s equation, the average crystal size was calculated as 6 nm, reflecting that serious crystal growth hardly happened [17]. And from the TGA result, the weight fraction of WO3x was about 33 wt %. Fig. 2 shows the nitrogen isotherms of the tOMC and tOMC-W samples. Both tOMC and tOMC-W showed typical isotherms of Type IV corresponding to mesoporous structures [13,15,16,18,19]. Similar hysteresis loops between P/P0 ¼ 0.4 and P/P0 ¼ 0.7 for both samples indicated narrow pore size distribution of the mesopores. Compared with the tOMC material, the adsorbed volume of tOMCW decreased largely as expected due to the incorporation of WO3x. The calculated surface areas by the BET method for tOMC and tOMC-W were 1639 m2 g1 and 741 m2 g1, respectively. And from the pore volume of both samples (0.96 cm3 g1 for tOMC, 1.58 cm3 g1 for tOMC-W), the densities of tOMC and tOMC-W
Fig. 3. XPS spectra for the tOMC-W sample for W 4f.
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were estimated (0.42 g cm3 for tOMC, 0.74 g cm3 for tOMC-W). Note that the surface area and density of both materials are essential factors to influence their electrochemical properties. Also, in the inset of Fig. 3, the BJH pore size distributions of both samples were plotted. Clearly, it was observed that the intrinsic bimodal peak distribution of pore diameter of tOMC changed into a monodispersed peak distribution. The change implied that small mesopores located within pore walls were completely filled by the WO3x while large mesopores remained unoccupied. With disappearance of the smaller mesopores, the pore distribution became less distributed, and thus a more intense peak at around 4 nm was observed. Importantly, such optimized structure can be beneficial to improved electrolyte transport and higher accessibility of charge storage sites in SC electrode. In Fig. 3, the XPS W4f spectra for tOMC-W are shown. After careful fitting, it could be easily seen that the W4f spectra was composed of two doublets: one doublet with two peaks at 34.9 eV and 37.0 eV for the W6þ and the other one with peaks at 33.8 eV and 35.8 eV for the W5þ [20]. From the integrated calculation of peak area, the molar ratio of W5þ in the W element was around 10% and thus the stoichiometric composition of tungsten oxide in tOMC-W could be simply represented as WO2.95, which differentiated the stoichiometric composition of WO2.83 detected from the XRD patterns. This difference is possibly related with the less-crystalized and less-reduced WO3x incorporated in the small mesopores in the carbon walls. For those WO3x precursors adsorbed into the small mesopores, they were probably difficult to get crystallized and reduced because of limited exposal to the external atmosphere and nanosized confinement of the rigid carbon phase. This WO3x fraction possibly contributed to a higher fraction of W6þ. The presence of such WO3x within the smaller mesopores was confirmed by the EDX point analysis, which will be discussed below. Fig. 4 shows the morphology of the tOMC material. In Fig. 4(a), the overview of a typical tOMC-W particle and magnified SEM image of the selected area on the particle were presented. As can been seen, the surface of the composite particle was generally smooth, indicative of negligible aggregation of WO3x crystals in the external surface of the particles. More importantly, the mesoporous structure with pore size of several nanometres was clearly seen from Fig. 4(b), reflecting that the main large mesopore channels remained unfilled by the incorporation of WO3x. This is in good agreement with the nitrogen sorption results. Fig. 4(c) and (d) shows the TEM and high-resolution TEM (HR-TEM) images of tOMC-W material. In Fig. 4(a), ordered array of tubular-like mesopores were observed, which coincided with the SAXS results. The pore size of around 4 nm was roughly measured, close to the pore size distribution. Also, the high-contrast nanocrystals were dispersed uniformly in the ordered mesoporous carbon matrix. The interstitial distance of the as-appeared crystal fringes was measured as 0.378 nm, corresponding to the (010) plane of WO2.83 and the crystal size was within several nanometres, which accorded with the XRD result. As discussed above, the as-formed tungsten oxides were composed of two different morphologies. First, WO2.83 crystals uniformly distributed within carbon phase have been detected from XRD and TEM. On the other hand, tungsten oxides probably with amorphous nature were located in the small mesopores of tOMC. In order to elucidate the material structure, EDX was conducted at two representative points of one typical tOMC-W particle; one in the high-contrast area and the other one in the lowcontrast area. As shown in Fig. 5, peaks assigned as tungsten element were detected at both points, indicative of the presence of WO3x in both areas. The high-contrast area has already been identified as WO3x crystals from the TEM images. In contrast, the presence of tungsten element in the low-contrast area should be
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Fig. 4. Morphology of the tOMC-W material: (a and b) SEM images of the tOMC-W material; (c and d) TEM and high-resolution TEM image of the tOMC-W material.
Fig. 5. STEM images and the corresponding EDX point analysis of the tOMC-W material.
Y. Zhou et al. / Journal of Power Sources 244 (2013) 777e782
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Fig. 6. (a) Cyclic voltammogram of tOMC-W at 2 mV s1; and (b) cyclic voltammograms from 2 mV s1e20 mV s1 for tOMC-W.
explained by these tungsten oxides accommodated in the smaller mesopores on the carbon walls. Due to their amorphous nature, these tungsten oxides were hard to be identified from the TEM or STEM or XRD. Herein, the material morphology and structure have been fully demonstrated and confirmed. 3.2. Electrochemical performance Fig. 6(a) shows the cyclic voltammogram of tOMC-W at 2 mV s1 (the y axis has been converted to capacitance using Eq. (1)). It was clearly shown that the capacitance was composed of two parts, the shadowed part and unshadowed one, which are, respectively, contributed by the electric double-layer capacitance (EDLC) occurring on the carbon phase and the pseudocapacitance from the redox reactions (2) on the surface of WO3x. As seen from the figure, the capacitance contribution from carbon was about 75 F g1. In the OMC materials, EDLC was generally proportional to the surface area and therefore, the decrease of EDLC when compared with the tOMC material (about 150 F g1, see Table 1) was mainly on account of the decrease of surface area from 1639 m2 g1 to 741 m2 g1. As for the unshadowed area, a board peak located at 0.2 V vs. SCE in the anodic scan should be associated with pseudocapacitive reactions (see Eq. (2)) on the incorporated nano-WO3x, which was similar to a pure nanostructured WO3x case [12]. Simply estimated from the peak capacitance of around 50 F g1 and the WO3x mass fraction of 33 wt %, the peak capacitance from WO3x itself was about 150 F g1. This value was close to the ordered mesoporous WO3x material case, indicative of an approximately optimized utilization of WO3x in our tOMC-W material [12]. In the cathodic scan, however, an interesting plateau appeared between 0.2 V and 0.4 vs. SCE, which required more elucidation.
WO3x ðH2 OÞy 4WOð3xþdÞ H2 OðydÞ þ 2Hþ þ 2de
(2)
Fig. 6(b) shows the cyclic voltammograms from 2 to 20 mV s1 for tOMC-W. Although highly pseudocapacitive contribution from anodic peaks was difficult to be practically utilized, it was of great help in rate performance analysis of the tOMC-W electrode. As scan rate increased, the capacity retention by electric double layer charging remained invariant while the pseudocapacitance decayed relatively fast. This was associated with the intrinsic difference of charge storage mechanisms; fast charge storage on electric double layer but slow surface Faradaic reaction by pseudocapacitive charging [2]. Our preliminary experiment showed that the tOMC electrode exhibited almost invariable rectangle CV curves with an increase of scan rate. Nevertheless, the overall shape of the CV curves for the tOMC-W electrode was basically maintained even at 20 mV s1, shown in Fig. 6(b), implying a still good rate capability. In order to directly compare the rate capability of both electrodes, capacity retention vs. current density are plotted as shown in Fig. 7(a) tOMC-W showed similar rate capability with the tOMC material. Compared with the previous report on OMC/MoO2, better rate performance of our tOMC electrode is ascribed to higher conductivity of WO3x and more optimized nanostructure [12,13,21]. Fig. 7(b) shows cycle performance of the tOMC-W electrode. Surprisingly, a gradual capacitance increase was observed, probably due to more WO3x exposal to the electrolyte and/or amorphization of the WO3x phases. In contrast, the capacitance of tOMC material remained invariant after long-term cycling. In order to investigate the increment of the capacitance, CV curves between before and after cycling were compared as shown in Fig. 7(c). Here, capacitance was increased in anodic scan but it remained unchanged in cathodic direction. Interestingly, three pairs of broad peaks at around 0.1, 0.1 and 0.3 V vs. SCE, respectively, appeared after 1100 cycles in the anodic scan, which should be explained by gradual evolution of pseudocapacitive sites according to the
Table 1 Physiciochemical and electrochemical properities of tOMC-W and tOMC materials.
tOMC-W tOMC a b c d e f g h i
WO3x (wt%)
Ca (wt%)
Poreb (nm)
ABETc (m2 g1)
Vpored (cm3 g1)
De (g cm3)
Cf (F g1)
Cvg (F cm3)
Rateh (%)
Cyclei (%)
33 0
67 100
4.3 2.7 and 4.3
741 1639
0.96 1.58
0.74 0.42
175 150
129 63
77 81
133 100
The weight fraction of WO3x and carbon from TGA analysis. Average pore diameter. BET surface area. Pore volume from N2 soprtion. Theoretical density. Specific capacitance. Volumetric capacitance. Rate performance at 2 A cm2 current rate. Capacity retention at 1100 cycle.
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Fig. 7. Galvanostatic charge/discharge profiles of the tOMC-W (a) and tOMC (b) electrode at different current densities; rate (c) and cycle (d) performance comparison between tOMC-W and tOMC.
increase of cycles. Meanwhile, the capacitance during cathodic direction kept almost constant, indicative of an increase in Coulomb efficiency during chargeedischarge. In order to confirm this tendency, the galvanostatic charge/discharge profiles at 2 mA cm2 of the fresh and cycled tOMC-W electrode were shown in Fig. 7(d). The calculated efficiency for the fresh and cycled electrode was 75% and 85%, respectively. Although the reason is not clear, it is assumed that long-term cycling preferentially activated pseudocapactive sites for anodic reaction in tOMC-W electrode, which required further investigation. In addition, note that the volumetric capacitance of the tOMC materials was highly increased. From Fig. 7(c), a remarkable volumetric capacitance of 125 F cm3 was achieved after 1100 cycles, which was about twice as high as that of tOMC. As regards to the cycle performance, tOMC-W electrode exhibited a superior cycle performance to the previous OMC/MoO2 electrode, which are ascribed to its high resistivity to the strong acid electrolyte [13,21]. 4. Conclusions In this paper, we successfully incorporated nanosized WO3x into a bimodal mesoporous carbon material (tOMC) using a simple postaddition method. Small mesopores of the tOMC were selectively filled by tungsten precursor while large mesopores remained unoccupied, which is considered as an optimized nanostructure for higher electrolyte transport and better accessibility of charge storage sites. Also, WO2.83 crystals with size of several nanometres were uniformly dispersed within the tOMC matrix. Due to the optimized structure properties and intrinsic advantages of WO3x material, the asprepared OMC/WO3x composite material showed a high volumetric capacitance, an excellent cyclability and a good rate performance in the application to SCs. Also, considering the physicochemical activity of WO3x and the highly ordered mesoporous structure, we expect our tOMC-W could be employed in other electrochemical energy devices such as fuel cells and lithium ion batteries.
Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2012004731). References [1] L.L. Zhang, X.S. Zhao, Chem. Soc. Rev. 38 (2009) 2520e2531. [2] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum Publisher, New York, 1999. [3] L.-Q. Mai, F. Yang, Y.-L. Zhao, X. Xu, L. Xu, Y.-Z. Luo, Nat. Commun. 2 (2011) 381. [4] H.-Q. Li, R.-L. Liu, D.-Y. Zhao, Y.-Y. Xia, Carbon 45 (2007) 2628e2635. [5] H.-J. Liu, X.-M. Wang, W.-J. Cui, Y.-Q. Dou, D.-Y. Zhao, Y.-Y. Xia, J. Mater. Chem. 20 (2010) 4223e4230. [6] H.-J. Liu, W.-J. Cui, L.-H. Jin, C.-X. Wang, Y.-Y. Xia, J. Mater. Chem. 19 (2009) 3661e3667. [7] S. Yoon, J. Lee, T. Hyeon, S.M. Oh, J. Electrochem. Soc. 147 (2000) 2507e2512. [8] D.-D. Zhou, H.-J. Liu, Y.-G. Wang, C.-X. Wang, Y.-Y. Xia, J. Mater. Chem. 22 (2012) 1937e1943. [9] M. Ghaemia, F. Ataherian, A. Zolfaghari, S.M. Jafari, Electrochim. Acta 53 (2008) 4607e4614. [10] J. Xu, L. Gao, J. Cao, W. Wang, Z. Chen, Electrochim. Acta 56 (2010) 732e736. [11] J. Rajeswari, P.S. Kishore, B. Viswanathan, T.K. Varadarajan, Electrochem. Commun. 11 (2009) 572e575. [12] S. Yoon, E. Kang, J.K. Kim, C.W. Lee, J. Lee, Chem. Commun. 47 (2011) 1021e1023. [13] Y. Zhou, C.W. Lee, S. Yoon, Electrochem. Solid-State Lett. 14 (2011) A157e A160. [14] Y. Meng, D. Gu, F.Q. Zhang, Y.F. Shi, H.F. Yang, Z. Li, C.Z. Yu, B. Tu, D.Y. Zhao, Angew. Chem. Int. Ed. 44 (2005) 7053e7059. [15] R. Liu, Y. Shi, Y. Wan, Y. Meng, F. Zhang, D. Gu, Z. Chen, B. Tu, D. Zhao, J. Am. Chem. Soc. 128 (2006) 11652e11662. [16] Y. Zhou, Y. Kim, C. Jo, J. Lee, C.W. Lee, S. Yoon, Chem. Commun. 47 (2011) 4944e4946. [17] P. Scherrer, Göttinger Nachrichten Math. Phys. 2 (1918) 98e100. [18] Y. Zhou, C. Jo, J. Lee, C.W. Lee, G. Qao, S. Yoon, Micropor. Mesopor. Mater. 151 (2012) 172e179. [19] Y. Zhou, C.W. Lee, S.-K. Kim, S. Yoon, ECS Electrochem. Lett. 1 (2012) A17eA20. [20] A.P. Shpak, A.M. Korduban, M.M. Medvedskij, V.O. Kandyba, J. Electron Spectrosc. Relat. Phenomena 156e158 (2007) 172e175. [21] E. Kang, S. An, S. Yoon, J.K. Kim, J. Lee, J. Mater. Chem. 20 (2010) 7416e7421.
Contents lists available at SciVerse ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Enhanced charge storage by optimization of pore structure in nanocomposite between ordered mesoporous carbon and nanosized WO3x Yuanyuan Zhou a, Seunghyun Ko a, Chul Wee Lee a, Sung Gyu Pyo b, Soo-Kil Kim b, Songhun Yoon b, * a Green Chemical Technology Division, Korea Research Institute of Chemical Technology (KRICT), University of Science and Technology (UST), Daejeon 305-600, Republic of Korea b School of Integrative Engineering, Chung-Ang University, 221, Heukseok-Dong, Dongjak-Gu, Seoul 156-756, Republic of Korea
h i g h l i g h t s Novel ordered mesoporous carbon/tungsten oxide nanocomposite was prepared. Small pores within wall were homogeneously filled by tungsten oxide nanocrystals. Ordered pores were optimized for electrolyte transport and high charge accessibility. Volumetric capacity was as high as 125 F cm3 and rate capability was excellent. Capacitance increase was observed after long-term cycles (113%).
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 December 2012 Received in revised form 9 April 2013 Accepted 11 April 2013 Available online 23 April 2013
Herein, we report preparation of a novel ordered mesoporous carbon/reduced tungsten oxide nanocomposite by post-addition into the ordered mesoporous carbon prepared by triconstituent co-assembly method. The as-prepared composite material is characterized using various analysis methods and nitrogen sorption isotherms, which reveal that small mesopores located within pore wall are selectively filled by tungsten oxide and also reduced tungsten oxide crystals with size of 6 nm are uniformly dispersed in the carbon matrix. Hence, our prepared nanocomposite possesses an ordered pore structure optimized for fast electrolyte transport and highly accessible charge storage sites. When applied as supercapacitor electrode, it exhibits a high volumetric capacity (125 F cm3), an excellent rate capability (79%) and capacitance increase after long-term cycles (113%). Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Ordered mesoporous carbon Reduced tungsten Supercapacitor
1. Introduction Nowadays, high-performance supercapacitors (SCs) are in high demand in the application to portable electric devices and electrical vehicles [1e3]. However, power capability and energy density remain unsatisfactory in the case of commercial carbonaceous electrode materials and therefore, many researchers have focused on searching for novel electrode materials with high performance. Amongst these, ordered mesoporous carbon (OMC) materials have been intensely investigated as supercapacitor electrodes due to superior electrolyte transport through their highly ordered structures [4e8]. In spite of the high rate capability, the specific capacitance of OMC has been reported to have low specific * Corresponding author. Tel.: þ82 2 820 5769; fax: þ82 2 814 2615. E-mail address: [email protected] (S. Yoon). 0378-7753/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2013.04.054
capacitance (<200 F g1) and volumetric one, which are ascribed to limited charge storage on the electric double layer and low material density (about 0.5 g cm3) [4e8]. In contrast, pseudocapacitive materials based on transient metal oxides (MnO2, CoOx, MoOx, WO3x, etc.) show high capacitance but low rate capability is inevitable due to intrinsically slow Faradaic charging [9e12]. Hence, one can expect that incorporation of transient metal oxides into OMC is desirable for combination of large capacitance and high rate capability. In our previous work, MoO2 was successfully incorporated into the mesopores of a bimodal ordered mesoporous carbon (tOMC) and a large increase of capacitance was observed due to high capacitive contribution from MoO2 [13]. Because of the dissolution of MoO2 in the high acid environment, unfortunately, steep capacitance decay happened, which retarded its practical application [13]. However, WO3x, which is highly resistant to the strong acid electrolyte, could be a good alternative to MoO2. Form
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our previous work, especially, nanostructured WO3x with high electrical conductivity exhibited a fairly high volumetric capacitance and good rate capability [12]. Thus, an incorporation of conductive WO3x instead of MoO2 into tOMC is expected to be beneficial for high volumetric capacitance, stable cyclability and excellent rate capability. Herein, we propose a novel preparation of OMC/WO3x nanocomposite by impregnating a tungsten-rich precursor (phosphate tungsten acid) into OMC material and following calcination in a reducing atmosphere. The electrochemical properties as supercapacitor electrode are investigated in detail. 2. Experimental section 2.1. Preparation of C/WO3x nanocomposite 2.1.1. Preparation of the bimodal ordered mesoporous carbon (tOMC) The tOMC material was prepared according to the literature. In a typical procedure, 1.6 g of F127 and 1 g of 0.1 M HCl was added into 8 g of ethanol. The mixture was stirred at room temperature until a clear solution was acquired. Then 2.08 g of TEOS and 5 g of 20 wt% resol ethanolic solution (prepared according to the literature [14]) were dropped. Upon stirring at room temperature for 2 h, the clear solution was poured into a big dish and kept in ambient environment for fully ethanol-evaporation until a glue-like film was formed. Then the dish was transferred into an oven at 100 C for 24 h until polymerized film with darker colour obtained. After grinding, as-obtained powders were calcined in a tubular furnace at 450 C for 2 h and further calcined at 900 C for 2 h under N2 flow. After cooling down to the room temperature, the powders were taken out and immersed into a 10 wt% HF aqueous solution for etching process. After filtration, washing and drying, the final product was collected and indexed as tOMC. 2.1.2. Preparation of OMC/WO3x nanocomposite (tOMC-W) The preparation of OMC/WO3x nanocomposite is similar to that of our previous OMC/MoO2 nanocomposite. In a typical procedure, tOMC powders were immersed in 20 wt% HNO3 (Aldrich) solution at 70 C for 24 h for the purpose of surface acidification. After drying, 0.95 g of phosphate tungsten acid (Aldrich) was dissolved in 14 g of ethanol, 0.34 g of the acidified tOMC powders were mixed into the solution. After one-day stirring, the powders were filtered and washed for three times using ethanol and gathered after drying in the oven. The powders were calcinated in a tubular furnace at
600 C for 2 h in a reductive atmosphere (5 vol% H2/N2)(the temperature rate is 5 C min1). Finally, the powders were removed from the furnace and indexed as tOMC-W. 2.2. Material characterization and electrochemical performance testing The pore size distribution (PSD) was analysed using an N2 adsorption measurement (Micromeritics ASAP 2010). The external morphology of the carbon was examined using a scanning electron microscope (SEM, JEOL JSM-840A), whereas the pore image was scanned by a transmission electron microscope (TEM, JEOL JEM2010). The energy dispersive X-ray (EDX) analysis was conducted using the EDX analyzer equipped with the TEM in point mode. The small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) patterns were obtained with a Rigaku D/Max-3C diffractometer equipped with a rotating anode and Cu Ka radiation (l ¼ 0.15418 nm). X-ray photoelectron spectroscopy (XPS) experiments were carried out in an ultrahigh vacuum using the Scienta ESCA-300 high-resolution X-ray photoelectron spectrometer (HRXPS). Thermogravimetric analysis (TGA) was conducted using the TGA-HP50 in an air atmosphere with a temperature rate of 2 C min1. The supercapacitor electrode fabrication procedure and electrochemical cell structure were identical with the reported literature. Here, Pt flag counter and SCE reference electrodes were used for three electrode cell system using 1.0 M H2SO4 electrolyte. Ivium potentiostat was utilized to conduct cyclic voltammetry (CV). Scan rate was varied from 2 to 20 mV s1 in the CV experiment. Galvanostatic chargeedischarge experiment was carried out using the WBCS-3000 battery cycler (Xeno Co.). The capacitance C from the CV experiment is calculated using Eq. (1):
C ¼ i=v
(1)
Here, i and v are the observed current and the scan rate, respectively. 3. Results and discussion 3.1. Characteristics of the prepared OMC/WO3x nanocomposite Fig. 1(a) shows the small-angle X-ray scattering (SAXS) patterns of the tOMC and tOMC-W material. As seen, the SAXS patterns of the tOMC sample showed two peaks at 2q ¼ 1 and 1.5 ,
Fig. 1. (a) Small-angle X-ray scattering patterns of the tOMC-W and tOMC material. (b) Wide-angle X-ray diffraction patterns of the tOMC-W and tOMC material. Insets are peak indexes of WO2.83.
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Fig. 2. N2 sorption isotherms for the tOMC-W and tOMC material and the inset shows the pore size distributions from the BJH calculation.
respectively, indexed as (100) and (110) plane of a typical 2D hexagonal structure [15,16]. After impregnation of tungsten precursor and calcination, one obvious peak at 2q ¼ 1 was still observed in the tOMC-W sample, indicating that the ordered mesoporous structure was generally maintained. Fig. 1(b) shows the wide-angle X-ray diffraction patterns of the tOMC and tOMC-W material. For tOMC sample, two typical board peaks are associated with the amorphous nature of the tOMC material. The peaks at 2q ¼ 22 and 42 correspond to the (002) and (100) plane of graphite, respectively. For tOMC-W, characteristic peaks of WO2.83 (JSPD PDF #00036-0103) appeared in the amorphous carbon background, indicative of the formation of tungsten oxide crystals. Using the Scherer’s equation, the average crystal size was calculated as 6 nm, reflecting that serious crystal growth hardly happened [17]. And from the TGA result, the weight fraction of WO3x was about 33 wt %. Fig. 2 shows the nitrogen isotherms of the tOMC and tOMC-W samples. Both tOMC and tOMC-W showed typical isotherms of Type IV corresponding to mesoporous structures [13,15,16,18,19]. Similar hysteresis loops between P/P0 ¼ 0.4 and P/P0 ¼ 0.7 for both samples indicated narrow pore size distribution of the mesopores. Compared with the tOMC material, the adsorbed volume of tOMCW decreased largely as expected due to the incorporation of WO3x. The calculated surface areas by the BET method for tOMC and tOMC-W were 1639 m2 g1 and 741 m2 g1, respectively. And from the pore volume of both samples (0.96 cm3 g1 for tOMC, 1.58 cm3 g1 for tOMC-W), the densities of tOMC and tOMC-W
Fig. 3. XPS spectra for the tOMC-W sample for W 4f.
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were estimated (0.42 g cm3 for tOMC, 0.74 g cm3 for tOMC-W). Note that the surface area and density of both materials are essential factors to influence their electrochemical properties. Also, in the inset of Fig. 3, the BJH pore size distributions of both samples were plotted. Clearly, it was observed that the intrinsic bimodal peak distribution of pore diameter of tOMC changed into a monodispersed peak distribution. The change implied that small mesopores located within pore walls were completely filled by the WO3x while large mesopores remained unoccupied. With disappearance of the smaller mesopores, the pore distribution became less distributed, and thus a more intense peak at around 4 nm was observed. Importantly, such optimized structure can be beneficial to improved electrolyte transport and higher accessibility of charge storage sites in SC electrode. In Fig. 3, the XPS W4f spectra for tOMC-W are shown. After careful fitting, it could be easily seen that the W4f spectra was composed of two doublets: one doublet with two peaks at 34.9 eV and 37.0 eV for the W6þ and the other one with peaks at 33.8 eV and 35.8 eV for the W5þ [20]. From the integrated calculation of peak area, the molar ratio of W5þ in the W element was around 10% and thus the stoichiometric composition of tungsten oxide in tOMC-W could be simply represented as WO2.95, which differentiated the stoichiometric composition of WO2.83 detected from the XRD patterns. This difference is possibly related with the less-crystalized and less-reduced WO3x incorporated in the small mesopores in the carbon walls. For those WO3x precursors adsorbed into the small mesopores, they were probably difficult to get crystallized and reduced because of limited exposal to the external atmosphere and nanosized confinement of the rigid carbon phase. This WO3x fraction possibly contributed to a higher fraction of W6þ. The presence of such WO3x within the smaller mesopores was confirmed by the EDX point analysis, which will be discussed below. Fig. 4 shows the morphology of the tOMC material. In Fig. 4(a), the overview of a typical tOMC-W particle and magnified SEM image of the selected area on the particle were presented. As can been seen, the surface of the composite particle was generally smooth, indicative of negligible aggregation of WO3x crystals in the external surface of the particles. More importantly, the mesoporous structure with pore size of several nanometres was clearly seen from Fig. 4(b), reflecting that the main large mesopore channels remained unfilled by the incorporation of WO3x. This is in good agreement with the nitrogen sorption results. Fig. 4(c) and (d) shows the TEM and high-resolution TEM (HR-TEM) images of tOMC-W material. In Fig. 4(a), ordered array of tubular-like mesopores were observed, which coincided with the SAXS results. The pore size of around 4 nm was roughly measured, close to the pore size distribution. Also, the high-contrast nanocrystals were dispersed uniformly in the ordered mesoporous carbon matrix. The interstitial distance of the as-appeared crystal fringes was measured as 0.378 nm, corresponding to the (010) plane of WO2.83 and the crystal size was within several nanometres, which accorded with the XRD result. As discussed above, the as-formed tungsten oxides were composed of two different morphologies. First, WO2.83 crystals uniformly distributed within carbon phase have been detected from XRD and TEM. On the other hand, tungsten oxides probably with amorphous nature were located in the small mesopores of tOMC. In order to elucidate the material structure, EDX was conducted at two representative points of one typical tOMC-W particle; one in the high-contrast area and the other one in the lowcontrast area. As shown in Fig. 5, peaks assigned as tungsten element were detected at both points, indicative of the presence of WO3x in both areas. The high-contrast area has already been identified as WO3x crystals from the TEM images. In contrast, the presence of tungsten element in the low-contrast area should be
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Fig. 4. Morphology of the tOMC-W material: (a and b) SEM images of the tOMC-W material; (c and d) TEM and high-resolution TEM image of the tOMC-W material.
Fig. 5. STEM images and the corresponding EDX point analysis of the tOMC-W material.
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Fig. 6. (a) Cyclic voltammogram of tOMC-W at 2 mV s1; and (b) cyclic voltammograms from 2 mV s1e20 mV s1 for tOMC-W.
explained by these tungsten oxides accommodated in the smaller mesopores on the carbon walls. Due to their amorphous nature, these tungsten oxides were hard to be identified from the TEM or STEM or XRD. Herein, the material morphology and structure have been fully demonstrated and confirmed. 3.2. Electrochemical performance Fig. 6(a) shows the cyclic voltammogram of tOMC-W at 2 mV s1 (the y axis has been converted to capacitance using Eq. (1)). It was clearly shown that the capacitance was composed of two parts, the shadowed part and unshadowed one, which are, respectively, contributed by the electric double-layer capacitance (EDLC) occurring on the carbon phase and the pseudocapacitance from the redox reactions (2) on the surface of WO3x. As seen from the figure, the capacitance contribution from carbon was about 75 F g1. In the OMC materials, EDLC was generally proportional to the surface area and therefore, the decrease of EDLC when compared with the tOMC material (about 150 F g1, see Table 1) was mainly on account of the decrease of surface area from 1639 m2 g1 to 741 m2 g1. As for the unshadowed area, a board peak located at 0.2 V vs. SCE in the anodic scan should be associated with pseudocapacitive reactions (see Eq. (2)) on the incorporated nano-WO3x, which was similar to a pure nanostructured WO3x case [12]. Simply estimated from the peak capacitance of around 50 F g1 and the WO3x mass fraction of 33 wt %, the peak capacitance from WO3x itself was about 150 F g1. This value was close to the ordered mesoporous WO3x material case, indicative of an approximately optimized utilization of WO3x in our tOMC-W material [12]. In the cathodic scan, however, an interesting plateau appeared between 0.2 V and 0.4 vs. SCE, which required more elucidation.
WO3x ðH2 OÞy 4WOð3xþdÞ H2 OðydÞ þ 2Hþ þ 2de
(2)
Fig. 6(b) shows the cyclic voltammograms from 2 to 20 mV s1 for tOMC-W. Although highly pseudocapacitive contribution from anodic peaks was difficult to be practically utilized, it was of great help in rate performance analysis of the tOMC-W electrode. As scan rate increased, the capacity retention by electric double layer charging remained invariant while the pseudocapacitance decayed relatively fast. This was associated with the intrinsic difference of charge storage mechanisms; fast charge storage on electric double layer but slow surface Faradaic reaction by pseudocapacitive charging [2]. Our preliminary experiment showed that the tOMC electrode exhibited almost invariable rectangle CV curves with an increase of scan rate. Nevertheless, the overall shape of the CV curves for the tOMC-W electrode was basically maintained even at 20 mV s1, shown in Fig. 6(b), implying a still good rate capability. In order to directly compare the rate capability of both electrodes, capacity retention vs. current density are plotted as shown in Fig. 7(a) tOMC-W showed similar rate capability with the tOMC material. Compared with the previous report on OMC/MoO2, better rate performance of our tOMC electrode is ascribed to higher conductivity of WO3x and more optimized nanostructure [12,13,21]. Fig. 7(b) shows cycle performance of the tOMC-W electrode. Surprisingly, a gradual capacitance increase was observed, probably due to more WO3x exposal to the electrolyte and/or amorphization of the WO3x phases. In contrast, the capacitance of tOMC material remained invariant after long-term cycling. In order to investigate the increment of the capacitance, CV curves between before and after cycling were compared as shown in Fig. 7(c). Here, capacitance was increased in anodic scan but it remained unchanged in cathodic direction. Interestingly, three pairs of broad peaks at around 0.1, 0.1 and 0.3 V vs. SCE, respectively, appeared after 1100 cycles in the anodic scan, which should be explained by gradual evolution of pseudocapacitive sites according to the
Table 1 Physiciochemical and electrochemical properities of tOMC-W and tOMC materials.
tOMC-W tOMC a b c d e f g h i
WO3x (wt%)
Ca (wt%)
Poreb (nm)
ABETc (m2 g1)
Vpored (cm3 g1)
De (g cm3)
Cf (F g1)
Cvg (F cm3)
Rateh (%)
Cyclei (%)
33 0
67 100
4.3 2.7 and 4.3
741 1639
0.96 1.58
0.74 0.42
175 150
129 63
77 81
133 100
The weight fraction of WO3x and carbon from TGA analysis. Average pore diameter. BET surface area. Pore volume from N2 soprtion. Theoretical density. Specific capacitance. Volumetric capacitance. Rate performance at 2 A cm2 current rate. Capacity retention at 1100 cycle.
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Fig. 7. Galvanostatic charge/discharge profiles of the tOMC-W (a) and tOMC (b) electrode at different current densities; rate (c) and cycle (d) performance comparison between tOMC-W and tOMC.
increase of cycles. Meanwhile, the capacitance during cathodic direction kept almost constant, indicative of an increase in Coulomb efficiency during chargeedischarge. In order to confirm this tendency, the galvanostatic charge/discharge profiles at 2 mA cm2 of the fresh and cycled tOMC-W electrode were shown in Fig. 7(d). The calculated efficiency for the fresh and cycled electrode was 75% and 85%, respectively. Although the reason is not clear, it is assumed that long-term cycling preferentially activated pseudocapactive sites for anodic reaction in tOMC-W electrode, which required further investigation. In addition, note that the volumetric capacitance of the tOMC materials was highly increased. From Fig. 7(c), a remarkable volumetric capacitance of 125 F cm3 was achieved after 1100 cycles, which was about twice as high as that of tOMC. As regards to the cycle performance, tOMC-W electrode exhibited a superior cycle performance to the previous OMC/MoO2 electrode, which are ascribed to its high resistivity to the strong acid electrolyte [13,21]. 4. Conclusions In this paper, we successfully incorporated nanosized WO3x into a bimodal mesoporous carbon material (tOMC) using a simple postaddition method. Small mesopores of the tOMC were selectively filled by tungsten precursor while large mesopores remained unoccupied, which is considered as an optimized nanostructure for higher electrolyte transport and better accessibility of charge storage sites. Also, WO2.83 crystals with size of several nanometres were uniformly dispersed within the tOMC matrix. Due to the optimized structure properties and intrinsic advantages of WO3x material, the asprepared OMC/WO3x composite material showed a high volumetric capacitance, an excellent cyclability and a good rate performance in the application to SCs. Also, considering the physicochemical activity of WO3x and the highly ordered mesoporous structure, we expect our tOMC-W could be employed in other electrochemical energy devices such as fuel cells and lithium ion batteries.
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