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Effect of residual ions of hydrothermal precursors on the thickness and capacitive properties of WO3 nanoplates
Journal of Alloys and Compounds 823 (2020) 153715
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Effect of residual ions of hydrothermal precursors on the thickness and capacitive properties of WO3 nanoplates Jinzhi Jia a, b, Xu Dong Liu a, Xiaojia Li a, Linhong Cao b, Meng Zhang a, b, Botao Wu a, b, Xiuwen Zhou a, * a b
Research Centre of Laser Fusion, China Academy of Engineering Physics, Mianyang, 621900, China School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang, 621900, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 July 2019 Received in revised form 25 December 2019 Accepted 6 January 2020 Available online 8 January 2020
The morphology and size of nanostructures have been considered to be directly related to capacitive performance. Additionally, hydrothermal synthesis is a common method to control nanostructures. In practice, researchers paid more attention to the direct effect of “useful ions” on morphology or size of nanostructures, e.g. the combination and decomposition processes of Hþ and WO2 4 to form WO3. Unfortunately, the effects of other incidental residual ions on the nanostructures have been often neglected. Herein, we add an extra procedure (cleaning the precursor) in the as-optimized hydrothermal synthesis routine to remove residual ions in the solution around tungstic acid colloids. A comparative study shows such an extra procedure would result in a further considerable reduction of WO3 nanoplates in the thickness by 50% and in a prominent enhancement of the specific capacitance from 203.5 F g1 to 334.0 F g1 at a scan rate of 2 mV s1. The thin WO3 nanoplates also show outstanding cycle stability for 5000 cycles. Interestingly, when we refilled the precursors solution with the Naþ and Cl ions again, the thickness of WO3 nanoplates gradually thickens as the increase of NaCl content. In this way, the effect of residual ions on the thickness of nanoplates has been experimentally demonstrated. Otherwise, this facile method can be helpful to establish a bridge between the physical deposition method, which is suitable for the complete fabrications of single thin layer materials with less active sites and the common hydrothermal synthesis method, which tends to produce thicker fragmental nanoplates but more abundant reaction sites. Also, this simple and efficient method can be extended to the preparation of thin nanoplates of other transition metal oxides and better electrochemical performance can be expected. © 2020 Published by Elsevier B.V.
Keywords: WO3 Nanoplates Supercapacitor Hydrothermal Thickness
1. Introduction In the face of the increasing energy demands, exhausting fossil energy resources and the environmental impacts associated with greenhouse gas emissions, developing renewable and sustainable energy has become extraordinary urgent [1e3]. In the past few decades, wind, tidal and solar energy have undergone rapid growth with huge cost reductions, which is becoming increasingly competitive with fossil fuels [4,5]. However, due to the intermittency of these renewable sources, energy storage devices, such as Lithium-ion batteries (LIBs), traditional capacitor, lithium-sulfur batteries (LSBs), fuel cells and supercapacitors (SCs), play a vital role in storing these energies [6,7]. Among the various energy-
* Corresponding author. E-mail address: [email protected] (X. Zhou). https://doi.org/10.1016/j.jallcom.2020.153715 0925-8388/© 2020 Published by Elsevier B.V.
storage devices, supercapacitors, also known as electrochemical capacitors (ECs), are widely regarded as one perspective candidate for the energy storage devices, because it can provide fast chargingdischarging (within seconds), great power density (ten times more than LIBs), superior electrochemical reversibility with extremely long service lifetimes (100 000 cycles) [6,8e10]. At present, SCs have been extensively used in the fields of electrical vehicles, pacemakers, power systems, and back-up power [2,10,11]. As well known, supercapacitors are mainly classified into electric doublelayer capacitors (EDLCs) and pseudocapacitors according to the different charge storage mechanisms [3,12]. The energy storage mechanism of EDLCs is that the electrolyte ions are electrostatically adsorbed/separated at the interface between the electrode active material and the electrolyte, and simultaneously forming an electric double layer [5,13]. The latter is a surface faradaic reaction by electrolyte ion adsorption-desorption to the surface of the
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electrode material and a highly reversible insertion or deintercalation of the electrolyte ions inside the crystal lattice of the material [5e7]. Compared to EDLCs, pseudocapacitors have a faster Faraday response and a larger specific capacitance [3,14]. As for the pseudocapacitive electrode materials, the transition metal oxides (MnO2, Co3O4, V2O5, RuO2, MoO3, NiO and WO3) have performed excellent properties, such as superior thermal stability, multiple oxidation states, rich redox reactions and high specific capacitance [5,7,10,15e19]. Among pseudocapacitive electrode materials, tungsten oxide (WO3) is a very well-known functional material that has been widely studied for multiple applications, such as electrochromic windows, dye-sensitized solar cells, photoelectrocatalysis, photocatalysis, optical data storage, light therapy, gas sensing, field emitting, lithium-ion batteries and supercapacitors [7,20e23]. Among them, the application of WO3 in supercapacitors has received extensive attention due to its extraordinary physical and chemical properties, such as reversible electrochemical redox reaction, excellent electronic conductivity, high intrinsic density, low toxicity, reversible electrochemical redox reaction, inexpensiveness, various crystalline phases and oxidation states [7,23e27]. Nanosizing of WO3 electrode materials has attracted more investigative interest, and the improvement of electrochemical performance can be achieved because of the shorter diffusion path of electrolyte ions [28,29]. Hydrothermal synthesis is a common method to realize nanosizing. In practice, researchers paid more attention to the direct effect of some “kind of residual ions and acid” on morphology or size of nanostructures, e.g. the combination and decomposition processes of Hþ and WO2 4 to form WO3 [30,31]. Unfortunately, whether positive or negative effects of other incidental residual ions on the nanostructures have been completely neglected. To highlight the influence of residual ions on nanosizing and further on capacitive performance, the selection of research objects seems important. It has been widely known that the thickness of nanoplates has a greater impact on the capacitive performance than their width due to the primary charge storage in the basal plane [28,29]. Thus, nanoplates are the suitable research objects because the influencing factors of nanosizing, associated with the capacitive performance, are limited to a single one. Otherwise, the non-bending nature of nanoplates and relatively low specific surface area can exclude the effect of the possible construction of 3D architecture on the improvement of capacitive properties. For nanoplates, physical deposition and hydrothermal synthesis are both common methods for achieving thickness control, however, each method has its advantages and disadvantages. Zhuiykov et al. synthesized WO3 films via the atomic layer deposition (ALD) technique, and also investigated the effect of nano-thickness of WO3 plates in the thickness below 6 nm on their electrochemical performance [32]. Those studies indicated that the dependence of capacitance on nanoplates thickness below 6 nm. However, the ALD deposition method can only form a uniform layer with less active sites. The electrochemical behavior may change as the thickness increases further. Usually, hydrothermal synthesis methods can be used to obtain a large number of fragmented plates, but the thickness of the individual plates (>50 nm) is difficult to further reduce. There is currently a lack of researching on the capacitive performance of WO3 nanoplates with the thickness between 10 nm and 50 nm. In this work, we used WO3 nanoplates as research objects and added an extra procedure (cleaning the precursor) in the asoptimized hydrothermal synthesis routine to remove residual ions in the solution around tungstic acid colloids. A comparative study shows such an extra procedure would result in a further considerable reduction of WO3 nanoplates in the thickness by 50%
and in a prominent enhancement of the specific capacitance from 203.5 F g1 to 334.0 F g1 at a scan rate of 2 mV s1. Simultaneously, the corresponding thinning mechanism of the nanoplates is also discussed in this paper. Also, besides the thickness of the nanoplates can be controlled between 6 and 50 nm by this method. The manuscript also systematically compares the differences of capacitive performance between our nanoplates and that of below 6 nm. More importantly, such a method has the potential to further reduce the thickness of nanoplates when hydrothermal synthesis meets its bottlenecks. 2. Materials and methods 2.1. Preparation of WO3 nanoplates WO3 nanoplates were synthesized based on the previous reports with an extra cleaning step [31]. In a typical process of synthesis, 5 mmol Na2WO4$2H2O was added to 25 mL of deionized water (DW, 18.2 M U) and magnetically stirred for 10 min to form a clear white solution. Subsequently, an aqueous solution of 5 mL HCl (37%) was dropped into the above mixture solution under stirring for 3 h to form yellow H2WO4 precipitate. Then, the as-obtained yellow products (H2WO4) were rinsed with DW for 3 times and redispersed in 30 mL DW. Then, 0.1 g oxalic acid (H2C2O4) was added into the above solution following by transferring the mixed solution into a 50 mL autoclave, then sealed and kept at a temperature of 70 C for 24 h. After the reaction was completed, the asobtained products were centrifuged and rinsed alternately with DW and ethanol for 3 times to remove impurities. The as-prepared products were dried and finally annealed at 350 C for 24 h. The resultant WO3 powders were named as r-WO3. The preparation conditions of other WO3 products (donated as m-WO3) are likely to that of the r-WO3 except without the additional cleaning step (named as CH step) of intermediate products (H2WO4). To further study the effect of residual ions on the growth of nanoplates, NaCl with different amounts (10 mmol and 20 mmol) were refilled into the as-cleaned prescursors solution. 2.2. Electrode preparation For working electrodes fabrication, the 80 wt% as-obtained WO3 as electroactive material, 10 wt % carbon black as a conducting material and 10 wt% polyvinylidene fluoride (PVDF) as a binder were mixed in N-methyl-2-pyrrolidone (NMP) to form the homogeneous slurry, then coated onto glassy carbon substrates as a current collector with a working area of 2.0 cm2, and finally dried at 60 C for 24 h under a vacuum. The total active materials mass per electrode was set at about 6 mg. 2.3. Characterization The crystalline structure and phase formation of the assynthesized samples was calculated by X-ray diffraction (XRD) using Cu Ka radiation (l ¼ 1.5418). The X-ray photo-electron spectroscopy (XPS) was used for investigating element contents and oxidation states of as-obtained samples. The field emission scanning electron microscope (FE-SEM) equipped with an Energy Dispersive X-ray Spectroscopy (EDS) and transmission electron microscopic studies (TEM) were used to observing the detailed morphologies and element contents of the as-prepared samples. The thickness of r-WO3 and m-WO3 measured by Atomic force microscopy (AFM). Ion chromatography (IC) was conducted to determine the content of Na element in the different precursors solution.
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2.4. Electrochemical measurements All electrochemical measurements of as-obtained WO3 nanoplates were systematically investigated in a conventional threeelectrode system with a CHI 660e electrochemical workstation (Chenhua, Shanghai) in 0.5 M H2SO4 as the electrolyte. The asprepared active material served as a working electrode, a standard Ag/AgCl electrode used as a reference electrode and a Pt foil served as a counter electrode. The electrochemical properties of the as-obtained materials were examined via cyclic voltammetry (CV, potential range: 0.3 - 0.2 V) and electrochemical impedance spectroscopy spectra (EIS, AC voltage: 5 mV amplitude, frequency range: 0.1e105 Hz) and long cycle life test (5000 cycles at 20 mV s1). 3. Results and discussion In order to explore the applicability of CH step method to reduce the thickness of nanoplates, we firstly optimized the thickness of as-synthesized WO3 nanoplates by hydrothermal method. The detailed optimization process and experimental details are shown in the supporting documents. Finally, we can find that the nanoplates, prepared by adding oxalic acid as a structure-directing agent, are thinnest (See Fig. S1). It is worth noting that the use of oxalic acid as a capping agent would not introduce Naþ and Cl, which originally existed in the precursor solution. We further used a hydrothermal synthesis routine containing the CH step to remove residual ions in the solution around tungstic acid colloids. EDS results show that owing to the extra CH step, the Na and Cl contents of the tungstic acid colloids are significantly reduced as shown in Table S1. Then m-WO3 and r-WO3 materials were prepared using the same hydrothermal process. Fig. 1a and b show the corresponding FE-SEM images and the corresponding histograms of the thickness of m-WO3 and r-WO3 measured from FE-SEM images are shown in Fig. 1c and d. Fig. 1a and b display that the as-prepared nanoplates have a smooth, dense surface with no aggregation. And the pores formed via stacking between nanoplates are obviously visible, which facilitates the penetration of the electrolyte and the transfer of ions, therefore improving the capacitance characteristics. The mean thickness measured from FE-SEM images. displays Gaussian-like distribution, and can be estimated to be 65 and 32 nm for m-WO3 and r-WO3, respectively. Meanwhile, the narrow thickness distribution of nanoplates centralizes at about 49 and 35 nm for m-WO3 and r-WO3, respectively, showing that the distribution of thickness for r-WO3 is more concentrated. The AFM results further prove a difference in the thickness of nanoplates as shown in Fig. S2. The r-WO3 is thinner than that of m-WO3. This result is consistent with SEM tests. As expected, these thinner nanoplates would enhance the resultant electroactive sites for electrochemical reactions to improve the electrochemical performance. After the CH step, the reason for the decrease in the thickness of the nanoplates has aroused our strong interests. As previously reported, the cations and anions were assumed to accelerate the growth of WO3 in a particular direction [33]. By adding a CH step, it is possible to remove additional Naþ and Cl , which may promote the growth of WO3 in the direction of thickness, resulting in a decrease in the thickness. In order to verify the assumption, we have designed and performed the following experiments. On the one hand, EDS tests of the tungstic acid colloids were conducted. The EDS results show that the contents of Na and Cl elements decrease after conducting the CH step (see Table S1). The results of ion chromatography indicate that the content of Na element in the precursors solution remarkably decreases from 10389 to 488.9 mg L1. This can be responsible for the phenomenon that the
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H2WO4 precursor of r-WO3 adsorbs less Na to some extent. On the other hand, we re-dispersed the tungstic acid colloids in the NaCl solution and kept it for 30 min to ensure adsorption. Figs. S3a and b display the SEM results of the as-prepared WO3 nanoplates with different amounts of NaCl and Figs. S3c and d show the corresponding histograms of WO3 measured from FE-SEM images. The thickness of tungsten oxide nanoplates increased to more than 50 nm again by immersing the H2WO4 colloids in the NaCl solution. From these experiments, it can be verified that Naþ and Cl ions in the precursors solution around H2WO4 colloids can promote the growth of tungsten oxide nanosheets along the direction of thickness. In other words, our experimental results validate previous assumptions. Based on the above experimental results, the formation mechanism (See Fig. 2) of the tungsten oxide nanoplates can be explained as follows: Na2WO42H2O / 2Naþ þ WO2 4 þ 2H2O
(1)
þ WO2 4 þ 2H / H2WO4
(2)
H2WO4 / WO3 þ H2O
(3)
First, Na2WO42H2O dispersed into distilled water and decomposed into Naþ and WO2 4 ions. And then, when HCl was added dropwise into the above-mentioned solution, the WO2 ions 4 reacted immediately with the Hþ ions to form a great number of H2WO4 colloids. According to the results of DFT calculations in the as-reported literature, Naþ ions are easier to be adsorbed on the (200) crystal plane and can promote the growth of WO3 crystals [29]. The concentration of Naþ decreased after conducting the CH step (see Table S1), and the growth rate of WO3 along the direction of thickness became evidently slower than that of the samples prepared without the CH step. Finally, m-WO3 and r-WO3 show great thickness differences. The experiment of immersion in NaCl solution further verified the rationality of the growth mechanism. The XRD, TEM, and XPS techniques were used for investigating the crystal structure, phase purity and the surface/near-surface element contents/oxidation states of the as-synthesized materials. Fig. 3a demonstrates XRD patterns of the as-obtained WO3 nanoplates. And the diffraction peaks of both WO3 at the 2q of 23.1, 23.6 , 24.4 , 26.6 , 28.9 , 34.2 , 41.9 , 48.2 , and 49.9 , corresponding to the (002), (020), (200), (120), (112), (202), (222), (040) and (140) planes, are well indexed to the monoclinic phase of WO3 (JCPDS no. 43e1035). No characteristic peaks from other impurity phases were discovered, showing the superior purity of the two WO3 samples. It is worth mentioning that the intensity of (020), (002) and (200) peak in the standard pattern are close, while the intensity of (002) peak for r-WO3 have obvious less than that of (020) and (200) peak owing to the inhibition of growth of crystallographic plane. The XPS survey spectra (Fig. 3b) exhibit that the surface/near-surface elements of both as-prepared samples are composed of W, O and trace amount of C, which derived from the sample holder. Fig. 3c and d shows the W4f spectra of r-WO3 and m-WO3. As revealed, m-WO3 and r-WO3 have similarly two obvious peaks at about 35.5 and 37.6 eV, corresponding to W 4f7/2 and W 4f5/2, respectively, which belong to the typical binding energies of W6þ oxidation states. The binding-energy gap between W 4f7/2 and W 4f5/2 for W6þ state is 2.1 eV, which is entirely consistent with the earlier report [34]. HRTEM images of m-WO3 and r-WO3 are shown in Fig. 3e and f respectively and indicate that the interplanar gaps of 0.375 nm correspond well to the (020) planes of the monoclinic phase of WO3. Based on the above analyses, the crystal structure, exposed crystal surface and tungsten valence state of the two samples are almost in the consistance. Therefore, it could be
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Fig. 1. The FE-SEM images of (a) m-WO3, (b) r-WO3 and corresponding the histogram of the thickness of nanoplates measured from FE-SEM images: (c) m-WO3, (d) r-WO3.
considered that the difference in the capacitive performance of the as-prepared materials is mainly due to the discrepancy in the thickness of r-WO3 and m-WO3. The electrochemical properties of both WO3 electrodes were studied by cyclic voltammetry (CV) in a potential window of 0.3e0.2 V (vs. Ag/AgCl) with 0.5 mol L1 H2SO4 electrolyte and electrochemical impedance spectroscopy (EIS) in the frequency region of 0.1e100 kHz. The comparative CV curves of m-WO3 and rWO3 electrodes at a scan rate of 2 mV s1 are exhibited in Fig. 4a. The two prominent oxidation and reduction peaks in CV curves can be attributed to the insertion/de-insertion of Hþ ions into/out of the WO3 host with the injection/de-injection of electrons. Charge storage mechanism of WO3 electrode may be interpreted via following redox reaction: W6þO3 þ xHþ þ xe /HxW5þO3-x (0 x 1). The integral area and peak current responses of rWO3 electrodes are much larger than those of m-WO3 electrodes, demonstrating that r-WO3 electrodes possess higher specific capacitance than m-WO3 electrodes as expected. This also stresses the importance of the thickness of nanoplates in outstandingly improving the capacitance of the WO3 electrode. The calculated specific capacitances of r-WO3 and m-WO3 electrode are 334.0 and 203.5 F g1 at 2 mV s1, respectively. The discrepancy in the thickness of the nanoplates is responsible for the difference in the
capacitive performance of the as-prepared materials as expected. The Nyquist plots of m-WO3 and r-WO3 are shown in Fig. 4b. The inherent resistance (Rs) of r-WO3 and m-WO3 electrode is 0.75 U and 1.36 U, respectively. It evidently illustrates that r-WO3 in the thinner thickness possesses minor intrinsic resistance, which accelerates the transmission of electrons more accessibly between rWO3 active materials and the current collector. This may also account for the excellent performance of r-WO3. At the same time, the slope of the straight line of the r-WO3 electrode is also higher than that of m-WO3, demonstrating that the ion diffusion resistance in this electrode is lower than the other one. The CV curves of r-WO3 and m-WO3 at different scan rates are revealed in Fig. 4c and d. The CV curves with obvious redox peaks exhibit Faradaic pseudo-capacitors’ characteristics. With the increase of scan rates, the CV curves of r-WO3 and m-WO3 are gradually inclined, which may be responsible for the three-dimension deformation. The specific capacitance is calculated from CV curves as follows [35,36]:
ð IdV Cs ¼
2mvDV
(4)
where Cs is the specific capacitance (F g1), m is the mass of active
Fig. 2. Schematic diagram of WO3 fabrication.
J. Jia et al. / Journal of Alloys and Compounds 823 (2020) 153715
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Fig. 3. (a) The XRD patterns and (b) the XPS survey spectra of m-WO3 and r-WO3. W 4f spectra of (c) r-WO3 and (d) m-WO3. HRTEM images of (e) m-WO3 and (f) r-WO3.
material on the electrode (mg), DV is the potential window (V), v is the scan rate (mV s1) and I is the current response at certain potentials (mA), respectively. The calculated specific capacitances of rWO3 at 2, 5, 10, 20, 50 mV s1 are 334., 217.1 152.8, 110.0 and 67.0 F g1, respectively, while those of m-WO3 are 203.5, 120.3, 79.1, 51.6, 32.6 F g1, respectively, as shown in Fig. 4e. The changing trend of Cs is opposite to that of scan rates. At a lower scan rate, Hþ ions have enough time to diffuse from the surface to the deep inside of the electrode, therefore resulting in more specific capacitance. Fig. S4 displays the areal capacitance of m-WO3 and r-WO3 electrodes calculated from CV curves according to Eq. (S1). Areal capacitances of r-WO3 electrode at the scan rates of 2, 5, 10, 20 and 50 mV s1 are calculated to be 935.1, 608.0, 428.0, 308.0 and 187.5 mF cm2, while those of m-WO3 are 610.3, 360.8, 237.2, 155.0, 97.9 mF cm2, respectively. It can be seen that the specific capacitance of r-WO3 is higher than that of m-WO3 at each scanning rate. The long cycle life of the electrode material is another crucial and desired aspect for practical application in SCs. So as to confirm the long-term cycling stability of r-WO3 and m-WO3 electrode, the CV curves are tested at 20 mV s1 for 5000 cycles as revealed in Fig. 3f. And the capacity retention of r-WO3 has hardly changed after 5000 cycles, which is much better than m-WO3 at 20 mV s1. The detailed comparison of the electrochemical performance for WO3 in thicknesses is shown in Table S2. According to the report of [32], when the thickness of the WO3 thin films (prepared by ALD method)
increases from 0.7 to 6 nm, the specific capacitance decreases gradually from 650.3 to 225.4 F g-1 and the capacitance retention after long-cycle tests increases from 65.8% to 91.7%. The specific capacitance of r-WO3 is less than that of 0.7 nm-thickness film but larger than that of 6 nm-thickness film. This can be attributed to the fact that the fragmental nanoplates provide more active sites. Meanwhile, the cyclic stability is further improved with the increase of the thickness of WO3 nanoplates. This is consistent with the trend reported in the literature [32]. It is thus suggested that rWO3 could be a perspective electrode material candidate for SCs, owing to its high capacitances and relatively outstanding cycling stability. An approach reported elsewhere was used, in order to separate the contribution of the Hþ insertion capacitance and the electric double-layer capacitor (EDLCs) to the total capacitance using. The mechanisms of energy storage can be calculated by analyzing the CV data at various sweep rates according to the following equation [26,]:
Q ðvÞ ¼ Qc þ k*v1=2 where Qc represents the capacitance contribution from EDLCs, k is a constant and relates to the Hþ insertion capacitance. The Q value can be obtained based on the integrated area of the measured CV curve. Qc can be calculated by extrapolating the plots of Q versus
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Fig. 4. (a) The CV curves of m-WO3 and r-WO3 electrodes at a scan rate of 2 mV s1. (b) Nyquist plots of m-WO3 and r-WO3. CV curves of (c) r-WO3 and (d) m-WO3 at different scan rates. (e) Specific capacitance versus the scan rates of r-WO3 and m-WO3. (f) Cycling performance of m-WO3 and r-WO3 electrodes collected at a scan rate of 20 mV s1 for 5000 cycles.
v1/2 and the calculated values of Qc are about 30 and 7 F g1 (Fig. 5 and Fig. S5), indicating that the values of EDLCs are inthe positive correlation with the specific surface area. As can be seen from Fig. 5a, the approximately linear relationship, which the Q value increases with the decrease of the reciprocal of the square root of the scan rates, represents a limited EDLCs process and the Q is sensitive to the change of the scan rates. According to the estimated Qc, the Hþ insertion capacitance and the EDLCs contribution are
vividly shown in Fig. 5b. The Hþ insertion capacitive charge occupies 88.4% at 2 mV s1 while the EDLCs controlled part contributes a little. However, with the scan rate increases, the Hþ insertion capacitive section gently decreases. When the scan rate increases to 50 mV s1, the Hþ insertion capacitive section only accounts for 55.2%. Based on these analyses, the energy storage mechanism of monoclinic tungsten oxide can mainly be attributed to the combination of Hþ insertion capacitance and the electric double-layer
Fig. 5. (a) Dependence of Q on v1/2 for the r-WO3 electrode. (b) Separation of contributions from Hþ insertion capacitance and EDLCs at different scan rates.
J. Jia et al. / Journal of Alloys and Compounds 823 (2020) 153715
capacitor. And the Hþ insertion mechanism is dominated. 4. Conclusions In summary, an extra cleaning process before hydrothermal synthesis was conducted to remove the residual ions in the precursors solution around H2WO4 colloids and thinner WO3 nanoplates were achieved. By such a cleaning process, the average thickness of the nanoplates can be further reduced by half (from 65 nm to 32 nm). With the decrease of the average thickness, the specific capacitance increases from 203.5 F g1 to 334.0 F g1 at a scan rate of 2 mV s1, and simultaneously exhibits outstanding electrochemical stability and good cycle stability. Subsequently, the H2WO4 colloids were immersed into NaCl solution with different contents, and the thickness of WO3 nanoplates re-increases. As a result, the fact has been verified that residual ions can promote the growth of WO3 nanosheets along the direction of thickness. More importantly, we believe that such a scalable method possesses the potential to further reduce the thickness of nanoplates when hydrothermal synthesis meets its bottlenecks. In addition, this work displays a facile method to reduce the thickness of WO3 nanoplates and can be extended to other metal oxides or other functional applications, e.g. gas-sensitive sensors, various ion batteries, and catalysts, etc. Author contributions J. J experimented and prepared the manuscript. X. Z and L. C developed the concept. D. L designed and supervised the experiments. M. Z, X. L, and B. W helped with analysis and discussion. All of the authors have discussed the results and commented on the manuscript. Declaration of competing interest There are no conflicts of interest to declare. CRediT authorship contribution statement Jinzhi Jia: Writing - original draft, Formal analysis, Investigation, Writing - review & editing, Data curation. Xu Dong Liu: Formal analysis, Writing - review & editing, Supervision, Methodology. Xiaojia Li: Data curation, Project administration. Linhong Cao: Conceptualization, Methodology, Writing - review & editing, Supervision. Meng Zhang: Data curation, Investigation, Project administration. Botao Wu: Investigation, Project administration, Data curation. Xiuwen Zhou: Conceptualization, Methodology, Writing - review & editing, Supervision. Acknowledgments This work was funded by the National Natural Science Foundation of China (NSFC 11804317) and Science and Technology on Plasma Physics Laboratory (ZY2018-05). The authors gratefully acknowledge Q. Yang, Y. Zeng, X. Liu, H. Ju, J. Li (Research Centre of Laser Fusion, China Academy of Engineering Physics, Mianyang, China), Y. Liu and H. Hu (Analysis and Testing Centre, Southwest University of Science and Technology. Mianyang, China) for the TEM, SEM, XRD, XPS, IC and AFM measurements, respectively. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.153715.
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Wang, Boosting the cycling stability of transition metal compounds-based supercapacitors, Energy Storage Mater. 16 (2019) 545e573. [12] J. Jung, D. Kim, W18O49 nanowires assembled on carbon felt for application to supercapacitors, J. Power Sources 433 (2017) 750e755. [13] S. Park, H. Shim, C. Lee, H. Song, J. Kim, D. Kim, High-power and long-life supercapacitive performance of hierarchical 3-D urchin-like W18O49 nanostructure electrodes, Nano Res. 9 (2016) 633e643. [14] Y. Gao, K. Huang, NiCo2S4 materials for supercapacitor applications, Chem. Asian J. 12 (2017) 1969e1984. [15] R. Wang, J. Mitchall, Q. Cao, W. Tsai, S. Boyd, B. Pharr, P. Balke, B. Augustyn, Operando atomic force microscopy reveals mechanics of structural water driver battery to pseudocapacitor transition, ACS Nano 12 (2018) 6032e6039. [16] L. Kang, C. Huang, J. Zhang, M. Zhang, N. Zhang, Y. He, C. Luo, C. Wang, X. Zhou, X. 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Li, Macroporous defective tungsten oxide nanostructure electrodes deliver ultrhigh capacitance and remarkable long cyclic durability in LiCl electrolyte, Ceram. Int. 44 (2018) 22718e22724. [25] F. Zheng, H. Gong, Z. Li, W. Yang, P. Hu, Y. Li, Y. Gong, Q. Zhen, Tertiary structure of cactus-like WO3 spheres self-assembled on Cu foil for supercapacitive electrode materials, J. Alloy. Comp. 712 (2017) 345e354. [26] M. Qiu, P. Sun, L. Shen, K. Wang, X. Yu, S. Tan, C. Zhao, W. Mai, WO3 nanoflowers with excellent pseudo-capacitive performance and the capacitance contribution analysis, J. Mater. Chem. 4 (2016) 7266e7273. [27] G. Yang, X. Liu, Electrochemical fabrication of interconnected tungsten bronze nanosheets for high performance supercapacitor, J. Power Sources 383 (2018) 17e23. [28] Z. Gholamvand, D. McAteer, A. Harvey, C. Backes, J. Coleman, Electrochemical
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of residual ions of hydrothermal precursors on the thickness and capacitive properties of WO3 nanoplates Jinzhi Jia a, b, Xu Dong Liu a, Xiaojia Li a, Linhong Cao b, Meng Zhang a, b, Botao Wu a, b, Xiuwen Zhou a, * a b
Research Centre of Laser Fusion, China Academy of Engineering Physics, Mianyang, 621900, China School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang, 621900, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 July 2019 Received in revised form 25 December 2019 Accepted 6 January 2020 Available online 8 January 2020
The morphology and size of nanostructures have been considered to be directly related to capacitive performance. Additionally, hydrothermal synthesis is a common method to control nanostructures. In practice, researchers paid more attention to the direct effect of “useful ions” on morphology or size of nanostructures, e.g. the combination and decomposition processes of Hþ and WO2 4 to form WO3. Unfortunately, the effects of other incidental residual ions on the nanostructures have been often neglected. Herein, we add an extra procedure (cleaning the precursor) in the as-optimized hydrothermal synthesis routine to remove residual ions in the solution around tungstic acid colloids. A comparative study shows such an extra procedure would result in a further considerable reduction of WO3 nanoplates in the thickness by 50% and in a prominent enhancement of the specific capacitance from 203.5 F g1 to 334.0 F g1 at a scan rate of 2 mV s1. The thin WO3 nanoplates also show outstanding cycle stability for 5000 cycles. Interestingly, when we refilled the precursors solution with the Naþ and Cl ions again, the thickness of WO3 nanoplates gradually thickens as the increase of NaCl content. In this way, the effect of residual ions on the thickness of nanoplates has been experimentally demonstrated. Otherwise, this facile method can be helpful to establish a bridge between the physical deposition method, which is suitable for the complete fabrications of single thin layer materials with less active sites and the common hydrothermal synthesis method, which tends to produce thicker fragmental nanoplates but more abundant reaction sites. Also, this simple and efficient method can be extended to the preparation of thin nanoplates of other transition metal oxides and better electrochemical performance can be expected. © 2020 Published by Elsevier B.V.
Keywords: WO3 Nanoplates Supercapacitor Hydrothermal Thickness
1. Introduction In the face of the increasing energy demands, exhausting fossil energy resources and the environmental impacts associated with greenhouse gas emissions, developing renewable and sustainable energy has become extraordinary urgent [1e3]. In the past few decades, wind, tidal and solar energy have undergone rapid growth with huge cost reductions, which is becoming increasingly competitive with fossil fuels [4,5]. However, due to the intermittency of these renewable sources, energy storage devices, such as Lithium-ion batteries (LIBs), traditional capacitor, lithium-sulfur batteries (LSBs), fuel cells and supercapacitors (SCs), play a vital role in storing these energies [6,7]. Among the various energy-
* Corresponding author. E-mail address: [email protected] (X. Zhou). https://doi.org/10.1016/j.jallcom.2020.153715 0925-8388/© 2020 Published by Elsevier B.V.
storage devices, supercapacitors, also known as electrochemical capacitors (ECs), are widely regarded as one perspective candidate for the energy storage devices, because it can provide fast chargingdischarging (within seconds), great power density (ten times more than LIBs), superior electrochemical reversibility with extremely long service lifetimes (100 000 cycles) [6,8e10]. At present, SCs have been extensively used in the fields of electrical vehicles, pacemakers, power systems, and back-up power [2,10,11]. As well known, supercapacitors are mainly classified into electric doublelayer capacitors (EDLCs) and pseudocapacitors according to the different charge storage mechanisms [3,12]. The energy storage mechanism of EDLCs is that the electrolyte ions are electrostatically adsorbed/separated at the interface between the electrode active material and the electrolyte, and simultaneously forming an electric double layer [5,13]. The latter is a surface faradaic reaction by electrolyte ion adsorption-desorption to the surface of the
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electrode material and a highly reversible insertion or deintercalation of the electrolyte ions inside the crystal lattice of the material [5e7]. Compared to EDLCs, pseudocapacitors have a faster Faraday response and a larger specific capacitance [3,14]. As for the pseudocapacitive electrode materials, the transition metal oxides (MnO2, Co3O4, V2O5, RuO2, MoO3, NiO and WO3) have performed excellent properties, such as superior thermal stability, multiple oxidation states, rich redox reactions and high specific capacitance [5,7,10,15e19]. Among pseudocapacitive electrode materials, tungsten oxide (WO3) is a very well-known functional material that has been widely studied for multiple applications, such as electrochromic windows, dye-sensitized solar cells, photoelectrocatalysis, photocatalysis, optical data storage, light therapy, gas sensing, field emitting, lithium-ion batteries and supercapacitors [7,20e23]. Among them, the application of WO3 in supercapacitors has received extensive attention due to its extraordinary physical and chemical properties, such as reversible electrochemical redox reaction, excellent electronic conductivity, high intrinsic density, low toxicity, reversible electrochemical redox reaction, inexpensiveness, various crystalline phases and oxidation states [7,23e27]. Nanosizing of WO3 electrode materials has attracted more investigative interest, and the improvement of electrochemical performance can be achieved because of the shorter diffusion path of electrolyte ions [28,29]. Hydrothermal synthesis is a common method to realize nanosizing. In practice, researchers paid more attention to the direct effect of some “kind of residual ions and acid” on morphology or size of nanostructures, e.g. the combination and decomposition processes of Hþ and WO2 4 to form WO3 [30,31]. Unfortunately, whether positive or negative effects of other incidental residual ions on the nanostructures have been completely neglected. To highlight the influence of residual ions on nanosizing and further on capacitive performance, the selection of research objects seems important. It has been widely known that the thickness of nanoplates has a greater impact on the capacitive performance than their width due to the primary charge storage in the basal plane [28,29]. Thus, nanoplates are the suitable research objects because the influencing factors of nanosizing, associated with the capacitive performance, are limited to a single one. Otherwise, the non-bending nature of nanoplates and relatively low specific surface area can exclude the effect of the possible construction of 3D architecture on the improvement of capacitive properties. For nanoplates, physical deposition and hydrothermal synthesis are both common methods for achieving thickness control, however, each method has its advantages and disadvantages. Zhuiykov et al. synthesized WO3 films via the atomic layer deposition (ALD) technique, and also investigated the effect of nano-thickness of WO3 plates in the thickness below 6 nm on their electrochemical performance [32]. Those studies indicated that the dependence of capacitance on nanoplates thickness below 6 nm. However, the ALD deposition method can only form a uniform layer with less active sites. The electrochemical behavior may change as the thickness increases further. Usually, hydrothermal synthesis methods can be used to obtain a large number of fragmented plates, but the thickness of the individual plates (>50 nm) is difficult to further reduce. There is currently a lack of researching on the capacitive performance of WO3 nanoplates with the thickness between 10 nm and 50 nm. In this work, we used WO3 nanoplates as research objects and added an extra procedure (cleaning the precursor) in the asoptimized hydrothermal synthesis routine to remove residual ions in the solution around tungstic acid colloids. A comparative study shows such an extra procedure would result in a further considerable reduction of WO3 nanoplates in the thickness by 50%
and in a prominent enhancement of the specific capacitance from 203.5 F g1 to 334.0 F g1 at a scan rate of 2 mV s1. Simultaneously, the corresponding thinning mechanism of the nanoplates is also discussed in this paper. Also, besides the thickness of the nanoplates can be controlled between 6 and 50 nm by this method. The manuscript also systematically compares the differences of capacitive performance between our nanoplates and that of below 6 nm. More importantly, such a method has the potential to further reduce the thickness of nanoplates when hydrothermal synthesis meets its bottlenecks. 2. Materials and methods 2.1. Preparation of WO3 nanoplates WO3 nanoplates were synthesized based on the previous reports with an extra cleaning step [31]. In a typical process of synthesis, 5 mmol Na2WO4$2H2O was added to 25 mL of deionized water (DW, 18.2 M U) and magnetically stirred for 10 min to form a clear white solution. Subsequently, an aqueous solution of 5 mL HCl (37%) was dropped into the above mixture solution under stirring for 3 h to form yellow H2WO4 precipitate. Then, the as-obtained yellow products (H2WO4) were rinsed with DW for 3 times and redispersed in 30 mL DW. Then, 0.1 g oxalic acid (H2C2O4) was added into the above solution following by transferring the mixed solution into a 50 mL autoclave, then sealed and kept at a temperature of 70 C for 24 h. After the reaction was completed, the asobtained products were centrifuged and rinsed alternately with DW and ethanol for 3 times to remove impurities. The as-prepared products were dried and finally annealed at 350 C for 24 h. The resultant WO3 powders were named as r-WO3. The preparation conditions of other WO3 products (donated as m-WO3) are likely to that of the r-WO3 except without the additional cleaning step (named as CH step) of intermediate products (H2WO4). To further study the effect of residual ions on the growth of nanoplates, NaCl with different amounts (10 mmol and 20 mmol) were refilled into the as-cleaned prescursors solution. 2.2. Electrode preparation For working electrodes fabrication, the 80 wt% as-obtained WO3 as electroactive material, 10 wt % carbon black as a conducting material and 10 wt% polyvinylidene fluoride (PVDF) as a binder were mixed in N-methyl-2-pyrrolidone (NMP) to form the homogeneous slurry, then coated onto glassy carbon substrates as a current collector with a working area of 2.0 cm2, and finally dried at 60 C for 24 h under a vacuum. The total active materials mass per electrode was set at about 6 mg. 2.3. Characterization The crystalline structure and phase formation of the assynthesized samples was calculated by X-ray diffraction (XRD) using Cu Ka radiation (l ¼ 1.5418). The X-ray photo-electron spectroscopy (XPS) was used for investigating element contents and oxidation states of as-obtained samples. The field emission scanning electron microscope (FE-SEM) equipped with an Energy Dispersive X-ray Spectroscopy (EDS) and transmission electron microscopic studies (TEM) were used to observing the detailed morphologies and element contents of the as-prepared samples. The thickness of r-WO3 and m-WO3 measured by Atomic force microscopy (AFM). Ion chromatography (IC) was conducted to determine the content of Na element in the different precursors solution.
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2.4. Electrochemical measurements All electrochemical measurements of as-obtained WO3 nanoplates were systematically investigated in a conventional threeelectrode system with a CHI 660e electrochemical workstation (Chenhua, Shanghai) in 0.5 M H2SO4 as the electrolyte. The asprepared active material served as a working electrode, a standard Ag/AgCl electrode used as a reference electrode and a Pt foil served as a counter electrode. The electrochemical properties of the as-obtained materials were examined via cyclic voltammetry (CV, potential range: 0.3 - 0.2 V) and electrochemical impedance spectroscopy spectra (EIS, AC voltage: 5 mV amplitude, frequency range: 0.1e105 Hz) and long cycle life test (5000 cycles at 20 mV s1). 3. Results and discussion In order to explore the applicability of CH step method to reduce the thickness of nanoplates, we firstly optimized the thickness of as-synthesized WO3 nanoplates by hydrothermal method. The detailed optimization process and experimental details are shown in the supporting documents. Finally, we can find that the nanoplates, prepared by adding oxalic acid as a structure-directing agent, are thinnest (See Fig. S1). It is worth noting that the use of oxalic acid as a capping agent would not introduce Naþ and Cl, which originally existed in the precursor solution. We further used a hydrothermal synthesis routine containing the CH step to remove residual ions in the solution around tungstic acid colloids. EDS results show that owing to the extra CH step, the Na and Cl contents of the tungstic acid colloids are significantly reduced as shown in Table S1. Then m-WO3 and r-WO3 materials were prepared using the same hydrothermal process. Fig. 1a and b show the corresponding FE-SEM images and the corresponding histograms of the thickness of m-WO3 and r-WO3 measured from FE-SEM images are shown in Fig. 1c and d. Fig. 1a and b display that the as-prepared nanoplates have a smooth, dense surface with no aggregation. And the pores formed via stacking between nanoplates are obviously visible, which facilitates the penetration of the electrolyte and the transfer of ions, therefore improving the capacitance characteristics. The mean thickness measured from FE-SEM images. displays Gaussian-like distribution, and can be estimated to be 65 and 32 nm for m-WO3 and r-WO3, respectively. Meanwhile, the narrow thickness distribution of nanoplates centralizes at about 49 and 35 nm for m-WO3 and r-WO3, respectively, showing that the distribution of thickness for r-WO3 is more concentrated. The AFM results further prove a difference in the thickness of nanoplates as shown in Fig. S2. The r-WO3 is thinner than that of m-WO3. This result is consistent with SEM tests. As expected, these thinner nanoplates would enhance the resultant electroactive sites for electrochemical reactions to improve the electrochemical performance. After the CH step, the reason for the decrease in the thickness of the nanoplates has aroused our strong interests. As previously reported, the cations and anions were assumed to accelerate the growth of WO3 in a particular direction [33]. By adding a CH step, it is possible to remove additional Naþ and Cl , which may promote the growth of WO3 in the direction of thickness, resulting in a decrease in the thickness. In order to verify the assumption, we have designed and performed the following experiments. On the one hand, EDS tests of the tungstic acid colloids were conducted. The EDS results show that the contents of Na and Cl elements decrease after conducting the CH step (see Table S1). The results of ion chromatography indicate that the content of Na element in the precursors solution remarkably decreases from 10389 to 488.9 mg L1. This can be responsible for the phenomenon that the
3
H2WO4 precursor of r-WO3 adsorbs less Na to some extent. On the other hand, we re-dispersed the tungstic acid colloids in the NaCl solution and kept it for 30 min to ensure adsorption. Figs. S3a and b display the SEM results of the as-prepared WO3 nanoplates with different amounts of NaCl and Figs. S3c and d show the corresponding histograms of WO3 measured from FE-SEM images. The thickness of tungsten oxide nanoplates increased to more than 50 nm again by immersing the H2WO4 colloids in the NaCl solution. From these experiments, it can be verified that Naþ and Cl ions in the precursors solution around H2WO4 colloids can promote the growth of tungsten oxide nanosheets along the direction of thickness. In other words, our experimental results validate previous assumptions. Based on the above experimental results, the formation mechanism (See Fig. 2) of the tungsten oxide nanoplates can be explained as follows: Na2WO42H2O / 2Naþ þ WO2 4 þ 2H2O
(1)
þ WO2 4 þ 2H / H2WO4
(2)
H2WO4 / WO3 þ H2O
(3)
First, Na2WO42H2O dispersed into distilled water and decomposed into Naþ and WO2 4 ions. And then, when HCl was added dropwise into the above-mentioned solution, the WO2 ions 4 reacted immediately with the Hþ ions to form a great number of H2WO4 colloids. According to the results of DFT calculations in the as-reported literature, Naþ ions are easier to be adsorbed on the (200) crystal plane and can promote the growth of WO3 crystals [29]. The concentration of Naþ decreased after conducting the CH step (see Table S1), and the growth rate of WO3 along the direction of thickness became evidently slower than that of the samples prepared without the CH step. Finally, m-WO3 and r-WO3 show great thickness differences. The experiment of immersion in NaCl solution further verified the rationality of the growth mechanism. The XRD, TEM, and XPS techniques were used for investigating the crystal structure, phase purity and the surface/near-surface element contents/oxidation states of the as-synthesized materials. Fig. 3a demonstrates XRD patterns of the as-obtained WO3 nanoplates. And the diffraction peaks of both WO3 at the 2q of 23.1, 23.6 , 24.4 , 26.6 , 28.9 , 34.2 , 41.9 , 48.2 , and 49.9 , corresponding to the (002), (020), (200), (120), (112), (202), (222), (040) and (140) planes, are well indexed to the monoclinic phase of WO3 (JCPDS no. 43e1035). No characteristic peaks from other impurity phases were discovered, showing the superior purity of the two WO3 samples. It is worth mentioning that the intensity of (020), (002) and (200) peak in the standard pattern are close, while the intensity of (002) peak for r-WO3 have obvious less than that of (020) and (200) peak owing to the inhibition of growth of crystallographic plane. The XPS survey spectra (Fig. 3b) exhibit that the surface/near-surface elements of both as-prepared samples are composed of W, O and trace amount of C, which derived from the sample holder. Fig. 3c and d shows the W4f spectra of r-WO3 and m-WO3. As revealed, m-WO3 and r-WO3 have similarly two obvious peaks at about 35.5 and 37.6 eV, corresponding to W 4f7/2 and W 4f5/2, respectively, which belong to the typical binding energies of W6þ oxidation states. The binding-energy gap between W 4f7/2 and W 4f5/2 for W6þ state is 2.1 eV, which is entirely consistent with the earlier report [34]. HRTEM images of m-WO3 and r-WO3 are shown in Fig. 3e and f respectively and indicate that the interplanar gaps of 0.375 nm correspond well to the (020) planes of the monoclinic phase of WO3. Based on the above analyses, the crystal structure, exposed crystal surface and tungsten valence state of the two samples are almost in the consistance. Therefore, it could be
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Fig. 1. The FE-SEM images of (a) m-WO3, (b) r-WO3 and corresponding the histogram of the thickness of nanoplates measured from FE-SEM images: (c) m-WO3, (d) r-WO3.
considered that the difference in the capacitive performance of the as-prepared materials is mainly due to the discrepancy in the thickness of r-WO3 and m-WO3. The electrochemical properties of both WO3 electrodes were studied by cyclic voltammetry (CV) in a potential window of 0.3e0.2 V (vs. Ag/AgCl) with 0.5 mol L1 H2SO4 electrolyte and electrochemical impedance spectroscopy (EIS) in the frequency region of 0.1e100 kHz. The comparative CV curves of m-WO3 and rWO3 electrodes at a scan rate of 2 mV s1 are exhibited in Fig. 4a. The two prominent oxidation and reduction peaks in CV curves can be attributed to the insertion/de-insertion of Hþ ions into/out of the WO3 host with the injection/de-injection of electrons. Charge storage mechanism of WO3 electrode may be interpreted via following redox reaction: W6þO3 þ xHþ þ xe /HxW5þO3-x (0 x 1). The integral area and peak current responses of rWO3 electrodes are much larger than those of m-WO3 electrodes, demonstrating that r-WO3 electrodes possess higher specific capacitance than m-WO3 electrodes as expected. This also stresses the importance of the thickness of nanoplates in outstandingly improving the capacitance of the WO3 electrode. The calculated specific capacitances of r-WO3 and m-WO3 electrode are 334.0 and 203.5 F g1 at 2 mV s1, respectively. The discrepancy in the thickness of the nanoplates is responsible for the difference in the
capacitive performance of the as-prepared materials as expected. The Nyquist plots of m-WO3 and r-WO3 are shown in Fig. 4b. The inherent resistance (Rs) of r-WO3 and m-WO3 electrode is 0.75 U and 1.36 U, respectively. It evidently illustrates that r-WO3 in the thinner thickness possesses minor intrinsic resistance, which accelerates the transmission of electrons more accessibly between rWO3 active materials and the current collector. This may also account for the excellent performance of r-WO3. At the same time, the slope of the straight line of the r-WO3 electrode is also higher than that of m-WO3, demonstrating that the ion diffusion resistance in this electrode is lower than the other one. The CV curves of r-WO3 and m-WO3 at different scan rates are revealed in Fig. 4c and d. The CV curves with obvious redox peaks exhibit Faradaic pseudo-capacitors’ characteristics. With the increase of scan rates, the CV curves of r-WO3 and m-WO3 are gradually inclined, which may be responsible for the three-dimension deformation. The specific capacitance is calculated from CV curves as follows [35,36]:
ð IdV Cs ¼
2mvDV
(4)
where Cs is the specific capacitance (F g1), m is the mass of active
Fig. 2. Schematic diagram of WO3 fabrication.
J. Jia et al. / Journal of Alloys and Compounds 823 (2020) 153715
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Fig. 3. (a) The XRD patterns and (b) the XPS survey spectra of m-WO3 and r-WO3. W 4f spectra of (c) r-WO3 and (d) m-WO3. HRTEM images of (e) m-WO3 and (f) r-WO3.
material on the electrode (mg), DV is the potential window (V), v is the scan rate (mV s1) and I is the current response at certain potentials (mA), respectively. The calculated specific capacitances of rWO3 at 2, 5, 10, 20, 50 mV s1 are 334., 217.1 152.8, 110.0 and 67.0 F g1, respectively, while those of m-WO3 are 203.5, 120.3, 79.1, 51.6, 32.6 F g1, respectively, as shown in Fig. 4e. The changing trend of Cs is opposite to that of scan rates. At a lower scan rate, Hþ ions have enough time to diffuse from the surface to the deep inside of the electrode, therefore resulting in more specific capacitance. Fig. S4 displays the areal capacitance of m-WO3 and r-WO3 electrodes calculated from CV curves according to Eq. (S1). Areal capacitances of r-WO3 electrode at the scan rates of 2, 5, 10, 20 and 50 mV s1 are calculated to be 935.1, 608.0, 428.0, 308.0 and 187.5 mF cm2, while those of m-WO3 are 610.3, 360.8, 237.2, 155.0, 97.9 mF cm2, respectively. It can be seen that the specific capacitance of r-WO3 is higher than that of m-WO3 at each scanning rate. The long cycle life of the electrode material is another crucial and desired aspect for practical application in SCs. So as to confirm the long-term cycling stability of r-WO3 and m-WO3 electrode, the CV curves are tested at 20 mV s1 for 5000 cycles as revealed in Fig. 3f. And the capacity retention of r-WO3 has hardly changed after 5000 cycles, which is much better than m-WO3 at 20 mV s1. The detailed comparison of the electrochemical performance for WO3 in thicknesses is shown in Table S2. According to the report of [32], when the thickness of the WO3 thin films (prepared by ALD method)
increases from 0.7 to 6 nm, the specific capacitance decreases gradually from 650.3 to 225.4 F g-1 and the capacitance retention after long-cycle tests increases from 65.8% to 91.7%. The specific capacitance of r-WO3 is less than that of 0.7 nm-thickness film but larger than that of 6 nm-thickness film. This can be attributed to the fact that the fragmental nanoplates provide more active sites. Meanwhile, the cyclic stability is further improved with the increase of the thickness of WO3 nanoplates. This is consistent with the trend reported in the literature [32]. It is thus suggested that rWO3 could be a perspective electrode material candidate for SCs, owing to its high capacitances and relatively outstanding cycling stability. An approach reported elsewhere was used, in order to separate the contribution of the Hþ insertion capacitance and the electric double-layer capacitor (EDLCs) to the total capacitance using. The mechanisms of energy storage can be calculated by analyzing the CV data at various sweep rates according to the following equation [26,]:
Q ðvÞ ¼ Qc þ k*v1=2 where Qc represents the capacitance contribution from EDLCs, k is a constant and relates to the Hþ insertion capacitance. The Q value can be obtained based on the integrated area of the measured CV curve. Qc can be calculated by extrapolating the plots of Q versus
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J. Jia et al. / Journal of Alloys and Compounds 823 (2020) 153715
Fig. 4. (a) The CV curves of m-WO3 and r-WO3 electrodes at a scan rate of 2 mV s1. (b) Nyquist plots of m-WO3 and r-WO3. CV curves of (c) r-WO3 and (d) m-WO3 at different scan rates. (e) Specific capacitance versus the scan rates of r-WO3 and m-WO3. (f) Cycling performance of m-WO3 and r-WO3 electrodes collected at a scan rate of 20 mV s1 for 5000 cycles.
v1/2 and the calculated values of Qc are about 30 and 7 F g1 (Fig. 5 and Fig. S5), indicating that the values of EDLCs are inthe positive correlation with the specific surface area. As can be seen from Fig. 5a, the approximately linear relationship, which the Q value increases with the decrease of the reciprocal of the square root of the scan rates, represents a limited EDLCs process and the Q is sensitive to the change of the scan rates. According to the estimated Qc, the Hþ insertion capacitance and the EDLCs contribution are
vividly shown in Fig. 5b. The Hþ insertion capacitive charge occupies 88.4% at 2 mV s1 while the EDLCs controlled part contributes a little. However, with the scan rate increases, the Hþ insertion capacitive section gently decreases. When the scan rate increases to 50 mV s1, the Hþ insertion capacitive section only accounts for 55.2%. Based on these analyses, the energy storage mechanism of monoclinic tungsten oxide can mainly be attributed to the combination of Hþ insertion capacitance and the electric double-layer
Fig. 5. (a) Dependence of Q on v1/2 for the r-WO3 electrode. (b) Separation of contributions from Hþ insertion capacitance and EDLCs at different scan rates.
J. Jia et al. / Journal of Alloys and Compounds 823 (2020) 153715
capacitor. And the Hþ insertion mechanism is dominated. 4. Conclusions In summary, an extra cleaning process before hydrothermal synthesis was conducted to remove the residual ions in the precursors solution around H2WO4 colloids and thinner WO3 nanoplates were achieved. By such a cleaning process, the average thickness of the nanoplates can be further reduced by half (from 65 nm to 32 nm). With the decrease of the average thickness, the specific capacitance increases from 203.5 F g1 to 334.0 F g1 at a scan rate of 2 mV s1, and simultaneously exhibits outstanding electrochemical stability and good cycle stability. Subsequently, the H2WO4 colloids were immersed into NaCl solution with different contents, and the thickness of WO3 nanoplates re-increases. As a result, the fact has been verified that residual ions can promote the growth of WO3 nanosheets along the direction of thickness. More importantly, we believe that such a scalable method possesses the potential to further reduce the thickness of nanoplates when hydrothermal synthesis meets its bottlenecks. In addition, this work displays a facile method to reduce the thickness of WO3 nanoplates and can be extended to other metal oxides or other functional applications, e.g. gas-sensitive sensors, various ion batteries, and catalysts, etc. Author contributions J. J experimented and prepared the manuscript. X. Z and L. C developed the concept. D. L designed and supervised the experiments. M. Z, X. L, and B. W helped with analysis and discussion. All of the authors have discussed the results and commented on the manuscript. Declaration of competing interest There are no conflicts of interest to declare. CRediT authorship contribution statement Jinzhi Jia: Writing - original draft, Formal analysis, Investigation, Writing - review & editing, Data curation. Xu Dong Liu: Formal analysis, Writing - review & editing, Supervision, Methodology. Xiaojia Li: Data curation, Project administration. Linhong Cao: Conceptualization, Methodology, Writing - review & editing, Supervision. Meng Zhang: Data curation, Investigation, Project administration. Botao Wu: Investigation, Project administration, Data curation. Xiuwen Zhou: Conceptualization, Methodology, Writing - review & editing, Supervision. Acknowledgments This work was funded by the National Natural Science Foundation of China (NSFC 11804317) and Science and Technology on Plasma Physics Laboratory (ZY2018-05). The authors gratefully acknowledge Q. Yang, Y. Zeng, X. Liu, H. Ju, J. Li (Research Centre of Laser Fusion, China Academy of Engineering Physics, Mianyang, China), Y. Liu and H. Hu (Analysis and Testing Centre, Southwest University of Science and Technology. Mianyang, China) for the TEM, SEM, XRD, XPS, IC and AFM measurements, respectively. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.153715.
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