MolybdenumâTungsten Mixed Oxide Deposited into Titanium Dioxide

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Molybdenum-Tungsten Mixed Oxide Deposited into Titanium Dioxide Nanotube Arrays for Ultrahigh Rate Supercapacitors He Zhou, Xiaopeng Zou, Kaikai Zhang, Peng Sun, Md. Suzaul Islam, Jianyu Gong, Yanrong Zhang, and Jiakuan Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

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ACS Applied Materials & Interfaces

Molybdenum-Tungsten Mixed Oxide Deposited into Titanium Dioxide Nanotube Arrays for Ultrahigh Rate Supercapacitors He Zhou,† Xiaopeng Zou,‡ Kaikai Zhang,† Peng Sun,†,§ Md. Suzaul Islam,† Jianyu Gong,† Yanrong Zhang,*,† Jiakuan Yang*,† †

School of Environmental Science and Engineering, Huazhong University of Science and Technology, Luoyu Road 1037#, 430074, Wuhan, China. ‡

Materials Science and Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.

§

Institute of Energy and Environment, Inner Mongolia University of Science and Technology, Arding Street 7#, 014010, Baotou, China. KEYWORDS: molybdenum oxide; tungsten oxide; mixed oxide; titania nanotube arrays; ultrahigh rate supercapacitor; energy storage ABSTRACT: A novel method involving the fabrication of Mo-W mixed oxide (MoxW1-xO3) is proposed to modify the modest reaction kinetics and poor cycling stability of MoO3 material. By a simple co-electrodeposition method, a series of MoxW1-xO3 oxides is deposited on a TiO2 nanotube array substrate. Due to the differences between Mo6+ and W6+ in nature, there is significant distortion existing in the mixed oxides, leading to their decreased crystallite size and enlarged lattice space, which facilitates ion diffusion in the solid. As results, the mixed oxides show much better balance between specific capacitance and cycling stability than the bare MoO3 or WO3 sample, which suffers from either poor cycling stability or low electrochemical activity. Impressively, the optimal Mo-W mixed oxide exhibits a high specific capacitance of 517.4 F g−1 at 1 A g−1, and moreover, it retains 89.3% of the capacitance even at a high current density of 10 A g−1, demonstrating ultrahigh rate capability. These findings reveal the potential of the Mo-W mixed oxide for constructing advanced ultrahigh power supercapacitors.

1. Introduction Supercapacitors are of emerging importance and research interest nowadays because of their high power density, long cycle life, and bridging function for the energy and power gap between traditional dielectric capacitors and batteries.1, 2 Up to now, there are three main categories of active materials used in supercapacitors, namely carbonaceous materials, transition metal oxides, and conducting polymers.2, 3 Generally, carbonaceous materials adopt electrostatic charge diffusion and accumulation at the electrode/electrolyte interface as charge storage mechanism, behaving as electrical double-layer capacitors, and thus deliver high power density and long cycling stability but low energy density.4 On the other hand, transition metal oxides and conducting polymers employ fast and reversible surface or near-surface redox reactions for charge storage, exhibit higher specific capacitances compared to carbonaceous materials and therefore have the potential to meet the higher requirements of future energy storage systems.5, 6 Nevertheless, there are also some intrinsic drawbacks facing these pseudocapacitive materials, such as poor electrical conductivity,7 low cycling sta-

bility,8 and limited rate capability.9, 10 To address these issues, various strategies have been proposed and proved effectively, such as dimension tuning at nanoscale,11, 12 incorporating nanosized active material into electrically conductive scaffold,13, 14 hybridizing different type active materials,15 and combining two metal oxides to form a ternary oxide (AxByOz).16, 17 As a typical transition metal oxide, orthorhombic αMoO3 is an electroactive two-dimensional (2D) material consisting of alternately stacked layers held together by covalent forces in the a- and c-axis directions and by weak van der Waals forces in the b-axis direction. Its unique crystal structure allows the insertion/extraction of small cations into/from the interlayer and intralayer. Impressively, it can accommodate up to 1.5 Li/Mo18 and thus delivers a remarkable theoretical capacity of 1117 mAh g−1.19 However, the modest reaction kinetics and poor cycling behavior of α-MoO3 extremely hinder its practical application for batteries.10 These problems become more severe when exploring its application for supercapacitors because cycling stability and power are the most important properties of supercapacitors.20 To realize the

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energy storage capability of α-MoO3, some of the aforementioned strategies were adopted to overcome these limitations. Brezesinski etc. reported an ordered mesoporous α-MoO3 with enhanced pseudocapacitance, which contributed 70% to the total stored charge.10 This promotion was owing to the mesoporous morphology and unique properties of crystallographically oriented films that enabled high-rate Li+ insertion into the quasi-2D van der Waals gaps of α-MoO3 at time scale comparable to traditional redox pseudocapacitance. Hanlon etc. reported the mixing of exfoliated MoO3 with carbon nanotubes could extremely enhance its capacitance because of the much improved conductivity.21 Riley and coworkers reported conformal surface coating of MoO3 electrode with a thin film of Al2O3 increased the cycling stability of the electrode, due to the coating created some adhesion between MoO3 with the conductive additive that mitigated the mechanical failure caused by the extreme volume expansion/contraction during cycling.22 Tang etc. demonstrated the coating of MoO3 nanoplates with polypyrrole resulted in not only enhanced cycling performance but also excellent rate capability of the composite, which was attributed to the polypyrrole coating inhibited the dissolution of Mo ions, buffered the possible volume changes during cycling process, and decreased the charge transfer resistance.23 In addition, it was reported that excellent cycling performance and high specific capacitance could also be achieved by polyaniline or reduced graphene oxide wrapped MoO3.24, 25 Besides, no such a specific method involving the fabrication of Mo-based ternary oxide was exploited to overcome the drawback of MoO3 for energy storage application. In the present work, the strategy of constructing a ternary oxide is used to modify the electrochemical performance of MoO3 for supercapacitor application. We hypothesize that synergistic effects of pure oxides could also be achieved by the Mo-based ternary oxides, such as superior conductivity, more active sites and improved stability, analogous to the cases of other ternary oxides previously reported.16, 17 Specially, WO3 was chosen as the partner oxide because of its quite similar physical and chemical properties with MoO3, which facilitated their homogeneously blending at micro scale during the fabrication process and provided the possibility in the formation of a ternary oxide. Moreover, a TiO2 nanotube array material was employed as substrate for the deposited mixed oxide, because its ideal porous structure and satisfied conductivity at the applied potential window make it an excellent current collector. These designs indeed resulted in the successful fabrication of ternary oxide with high specific capacitance, much improved cycling performance as well as high-rate capability. To our knowledge, this is the first time for a Mo-based ternary oxide applied in energy storage device reported, though there are several studies focused on the structural and optical properties of Mo-W mixed oxide.26-31 Interestingly, the results of this study show both some similarities and differences with that of previous work.

2. Experimental section

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2.1 Preparation of MoO3, WO3, and Mo-W mixed oxides All chemicals were analytical grade and used without further purification. Firstly, a TiO2 nanotube array substrate was fabricated by an anodization process. In brief, well cleaned Ti foil was anodized in a two-electrode system at a constant potential of 60 V for 8 h with a Pt mesh as cathode, using ethylene glycol containing 0.25 wt % NH4F and 12.5 wt % H2O as electrolyte. The distance gap between the electrodes was fixed as 2.5 cm, and the electrolyte temperature was constantly maintained at 25 °C using a thermostat. The as-prepared TiO2 nanotube arrays were calcined at 450 °C in air for 2 h. Then an electrodeposition method 32 was employed to prepare the MoO3, WO3, and molybdenum-tungsten mixed oxide samples. The electrodeposition procedure was conducted in a three-electrode system at 25 °C on an electrochemical workstation (CS310, CorrTest, China), using the TiO2 nanotube array sample, a Pt mesh, and an SCE (saturated calomel electrode) as working, counter, and reference electrodes, respectively. The electrolyte used was an aqueous mixture solution of 0.1*x M sodium molybdate dihydrate (Na2MoO4•H2O), 0.1*(1−x) M sodium tungstate dihydrate (Na2WO4•H2O), 0.1 M ethylenediamine tetraacetic acid disodium (Na2EDTA), and 0.1 M ammonium acetate (CH3COONH4). For the preparation of each sample, galvanostatic plating at −1 mA cm−2 was carried out for 400 s. With different x value adopted, seven samples were fabricated and denoted as MoO3, 0.9MoW, 0.7MoW, 0.5MoW, 0.3MoW, 0.1MoW, and WO3 respectively, according to the following principle: the sample prepared at x = 1 was denoted as MoO3, at x = 0 was denoted as WO3, at 0 < x < 1 was denoted as xMoW. Additionally, for the analysis of the mixed oxide formation mechanism, a sequential deposition of WO3 and MoO3 was also conducted. After a pre-deposition of WO3 onto the TiO2 nantube arrays in an aqueous solution of 0.1 M Na2WO4•H2O + 0.1 M Na2EDTA + 0.1 M CH3COONH4 at −1 mA cm−2 for 400 s, a secondary deposition of MoO3 was carried out in an aqueous solution of 0.1 M Na2MoO4•H2O + 0.1 M Na2EDTA + 0.1 M CH3COONH4 at −1 mA cm−2 for y seconds. The samples prepared as per this procedure with y values of 100, 150, and 200, etc., were denoted as W400+Mo100, W400+Mo150, and W400+Mo200, etc., respectively. Finally, all the electrodeposited samples were subjected to a thermal treatment at 450 °C in air for 2 h. 2.2. Material characterization and electrochemical measurements The morphology of the samples was characterized by field emission scanning electron microscopy (FE-SEM; NANOSEM 450, FEI) and field emission transmission electron microscopy (FE-TEM; Tecnai G2 F30, FEI). The phase and elemental composition of the samples were investigated using energy dispersive X-ray spectrometer (EDXS, equipped on FE-SEM and FE-TEM), X-ray diffraction technique (XRD; Empyrean, PANalytical B.V.) with Cu Kα radiation (λ = 1.54056 Å) scanned at a rate of 2° min−1, Raman spectroscopy (HORIBA Jobin Yvon


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Figure 1. SEM images of the samples: (a) TiO2 nanotube array substrate, (b) MoO3, (c) 0.9MoW, (d) 0.7MoW, (e) 0.5MoW, (f) 0.3MoW, (g) 0.1MoW, and (h) WO3.

LabRAM) with Ar+ laser of 514.5 nm excitation, and X-ray photoelectron spectroscopy (XPS; AXIS-ULTRA DLD600W, Kratos). In Raman study, the cosmic ray lines were removed from the spectra and Lorentzian function was used to resolve the overlapped bands. In XPS study, the binding energy was calibrated keeping the C 1s photoelectron peak at 284.6 eV as a reference. Cyclic voltammetry (CV), galvanostatic chargedischarge test, and electrochemical impedance spectroscopy (EIS) were performed on an electrochemical workstation (CS310, CorrTest, China) to investigate the electrochemical properties of the prepared samples. The measurements were carried out in a conventional threeelectrode cell using the tested sample, SCE, and Pt mesh as working, reference, and counter electrodes, respectively, and 1 M H2SO4 aqueous solution was employed as electrolyte. EIS measurements were performed between 100 kHz and 0.o1 Hz with 10 mV AC amplitude at −0.05 V DC potential on each sample with geometrical area of 2 cm2. The cycling stability of the samples was assessed via CV test performed up to 5000 cycles at 20 mV s−1. All the electrochemical tests were carried out at ~25 °C.

3. Results and discussion 3.1. Morphology study Figure 1 and Figure S1 show the microstructure of the fabricated samples. The TiO2 nanotube array substrate consists of highly ordered nanotubes with inner diameter of 150 ~ 200 nm, wall thickness of 10 ~ 20 nm, and tube length of ca. 7.2 μm. The MoO3 and 0.9MoW samples show morphology of nanowires stacked on the top of TiO2 nanotubes, and an increase in the nanowire diameter could be evidently identified for the latter. The 0.7MoW sample comprises sparsely distributed blocks in larger size with one side anchored in the nanotubes. While for the 0.5MoW sample, most of the species were deposited into the nanotubes with the mouths of some neighboring tubes filled, and a more homogenous distribution of the deposits in the entire nanotube layer was

observed for the 0.3MoW, 0.1MoW, and WO3 samples. Actually, after a careful examination, it could be found that some of the species were also deposited into the nanotubes in the cases of the MoO3, 0.9MoW, and 0.7MoW samples, reflected as an increase in the thickness of the nanotube walls (Figure 1b−d) compared to the bare TiO2 nanotube arrays (Figure 1a). This finding was further corroborated by the side-view FE-SEM images, from which a gradually smoothing of the bamboo-like ribbons on the outside walls by the deposits was observed for these samples (Figure S1a−d). Thus, after a comprehensive comparison on the microstructure of all the samples, it could be concluded that a promotion of W/Mo ratio in the electrolyte during electrodeposition will yield a more homogenously distribution of the deposits in the TiO2 nanotube layer. 3.2. Phase and chemical composition studies Figure 2a shows the XRD patterns for the samples. All the spectra contain the peaks of anatase TiO2 (JCPDS no. 21-1272) and Ti metal (JCPDS no. 44-1294) without distinguishable character, which belong to the particular substrate (TiO2 nanotube arrays grown on the surface of Ti foil).33 Besides these peaks, the pattern of MoO3 sample shows additional peaks at ca. 12.8°, 25.7°, and 27.3°, etc., which could all be assigned to the thermodynamically stable orthorhombic α-MoO3 (JCPDS no. 05-0508),10, 24 indicating the phase composition of this sample is mainly α-MoO3 (the substrate is excluded from the discussion here and hereafter). Similarly, the WO3 sample comprises only thermodynamically stable monoclinic m-WO3 phase (JCPDS no. 72-1465).34 Interestingly, for the other samples deposited from electrolytes containing both Mo and W precursors, none of their XRD spectra could be simply indexed as a mixture of α-MoO3 and m-WO3. As highlighted in Figure 2b, strong peaks centered at 22.8° to 23.2° emerge, which are apparently in different shapes with that of m-WO3 and shift slight to negative positions, suggesting the formation of new phases. Based on the



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Figure 2. (a, b) XRD and (c, d) Raman spectra of the samples. In panel a, the black asterisk, red asterisk, black arrow, and red arrow indicate the peaks of anatase TiO2, Ti metal, α-MoO3, and m-WO3, respectively. Panel b is an enlarged view of the region from 20° to 30° in panel a, with the vertical lines in the bottom signify the standard XRD patterns for β-MoO3 and m-WO3. Panel d is an enlarged showing of the Raman spectra for the MoO3, 0.9MoW and 0.7MoW, with the insets elucidating the deconvolution of the overlapped peaks.

following concerns, it is believed that these new phases are a series of Mo-W mixed oxides (in the form of MoxW1xO3, identification of the elemental states will be shown below). (1) First, the main elemental composition of these samples consists of only Mo, W, O, and Ti, detected by EDXS and XPS. (2) These new peaks cannot be indexed as any MoO3 or WO3 phases, and a variation of the peak character is observed with a change of Mo/W ratio in the preparation procedure. (3) Due to the quite similar properties of Mo6+ and W6+, there is a possibility for them to form a complex oxide, as demonstrated in the literature.26-31 Thus, it could be concluded that the 0.9MoW and 0.7MoW consist of α-MoO3 and some MoxW1-xO3 oxides, the 0.5MoW and 0.3MoW consist of some MoxW1-xO3 oxides, and the 0.1MoW consists of an m-WO3 like MoxW1-xO3 oxide (a mixture of m-WO3 and an m-WO3 like MoxW1-xO3 is not probable because of the MoxW1-xO3 formation mechanism discussed later). However, the detailed structural composition of these mixed oxides could hardly be understood merely from the XRD study.

To further illustrate the crystal structure of the ternary oxides, Raman spectroscopic study was conducted. Figure 2c shows the Raman spectra for the samples. The intense bands located at 395, 517, and 636 cm−1 in all the patterns are characteristic bands of the TiO2 substrate.35 All the additional bands in the spectrum of the MoO3 sample (at 666, 816, and 995 cm−1, etc.) can be assigned to orthorhombic α-MoO336, 37 and those in WO3 (at 274, 326, 714, and 805 cm−1) can be assigned to m-WO3.34, 38 The spectra of 0.9MoW and 0.7MoW also exhibit characteristic bands of α-MoO3, indicating the presence of α-MoO3 phase in them, which is also consistent with the XRD results. After scrutinizing the spectra, it was noticed the bottoms of the peak at 816 cm−1 are quite similar in the spectra of 0.9MoW and 0.7MoW, with some broadening compared to that of the MoO3 sample, as shown in Figure 2d. A deconvolution process was carried out to exclude the interference of the peak belongs to α-MoO3 phase at 816 cm−1, and the results clearly showed the emerging of a broad band at ~820 cm−1 in the spectra of 0.9MoW and 0.7MoW (insets of Figure 2d). Combined with their similar XRD patterns, it could be understood that they have the same


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phase composition, i.e., α-MoO3 and a MoxW1-xO3 phase, which exhibits XRD peak at 22.8° and Raman bands at ~820 and 952 cm−1. At this stage, it is valid to make a comprehensive comparison on the Raman bands for all the Mo-W ternary oxides in the samples. The MoxW1-xO3 oxides in 0.1MoW and 0.3MoW show four bands at positions close to that of m-WO3 with gradual positive shift and significant broadening, suggesting m-WO3 type phases with some modification. Actually, the m-WO3 type phase in the 0.1MoW sample is apparently evidenced by its XRD spectrum, and some hints are also present in that of 0.3MoW, such as the triplet peaks at 33.3° ~ 34.2° (Figure 2a). Another obvious alteration of the Raman spectrum caused by introducing Mo into the m-WO3 matrix is the emerging of a broad tail at 965 and 957 cm−1 in the case of 0.1MoW and 0.3MoW, respectively. This tail is also observable in the spectrum of the WO3 sample, located at 975 cm−1 but with much weaker intensity, as enlarged shown in Figure S2. Typically, the Raman band at 950 ~ 975 cm−1 is originated from the stretching mode of W═O terminal bond existing in all types of tungsten trioxide hydrates38, 39 as well as on the surface of m-WO3.40 Importantly, it showed a positive shift of its position accompanied with a decrease in the intensity when WO3 was calcined at elevated temperature,39, 40 which was also demonstrated in this study (Figure S2), and therefore it could be used as an indicator to gauge the particle size and crystallinity of WO3.40 By applying this principle to the 0.1MoW and 0.3MoW, it was known that these samples are also in monoclinic phases but with lower crystallinity and smaller crystallite size than the WO3 sample. Interestingly, the MoxW1-xO3 oxides in 0.5MoW, 0.7MoW, and 0.9MoW also show Raman bands at ~952 cm−1 but no those bands with typical m-WO3 character. Taking into account the band at 805 cm−1 gradual shifts to ~820 cm−1 in the order of WO3, 0.1MoW, 0.3MoW, 0.5MoW, 0.7MoW, and 0.9MoW, it could be understood that the MoxW1-xO3 oxides in 0.5MoW, 0.7MoW, and 0.9MoW still maintain monoclinic phases containing W═O terminal bonds but exhibit properties far from m-WO3. This deduction was considered to be rational because the introduction of too much Mo into the m-WO3 matrix will certainly lead to the material behaves more like a monoclinic MoO3. In fact, with a promotion of introduced Mo, the Raman and XRD spectra of the MoxW1-xO3 oxides indeed show a gradually approaching to those of monoclinic MoO3 phase, i.e., β-MoO3 (JCPDS no. 47-1081). Besides the strongest Raman band of the monoclinic phase gradually shifts to approach that of β-MoO3 at 850 cm−1, two weak bands at ~318 and ~753 cm−1 (indicated by the dashed lines in Figure 2c) are observed in the spectra of 0.3MoW and 0.5MoW, which are also close to the strong peaks of β-MoO3 at 353 and 776 cm−1.36, 37 Similarly, the strongest XRD peak of the MoxW1-xO3 oxides is close to that of mWO3 and β-MoO3, as indicated in Figure 2b. Moreover, the emerging of a weak peak located at 24.8° in the spectra of 0.5MoW (circled in Figure 2b) makes the entire pattern quite analogous to that of β-MoO3. After prolonging the electrodeposition time during the preparation procedure, this peak unambiguously showed up even in

the 0.7MoW sample, and the emerging of another peak at 26.2° further drew the spectrum of 0.5MoW closer to that of β-MoO3 (Figure S3). All these findings corroborate the deduction that every MoxW1-xO3 oxide is in monoclinic phase and with the increase of Mo content, its crystal structure gradually changes from m-WO3 like to β-MoO3 like. Similar results were also obtained in previously reported researches.26, 29, 31 Specially, differing from the literatures, no such a MoxW1-xO3 species with extremely high Mo content was obtained in the present study; adopting a very high Mo/W ratio in the preparation process will only attain mixed phases of α-MoO3 and MoxW1-xO3. That is to say, the x value in MoxW1-xO3 cannot get close to 1 and the crystal structure of the obtained MoxW1-xO3 oxides still shows significant differences with that of β-MoO3. There are several proofs for these differences, for instance, one important reflection of these differences is the strongest Raman band only shifted to ~820 cm−1. Second, the Raman spectra in the region of 750 ~ 850 cm−1 did not obviously split into two bands as in the case of β-MoO3. The third, the MoxW1-xO3 in 0.9MoW (or 0.7MoW) shows a broad XRD peak shifted to a more negative position than that of β-MoO3, implying a significantly distorted monoclinic matrix with larger lattice space than that of m-WO3 and β-MoO3. The phenomenon that a MoxW1-xO3 behaving similarly to β-MoO3 could not be found is a natural result of the thermodynamic equilibrium, which forbids the existence of β-MoO3 or a MoxW1-xO3 similar to βMoO3 because of the thermodynamically stable phase of α-MoO3. Finally, the formulas of the MoxW1-xO3 oxides were identified by EDXS and XPS technique (Figure S1, Table S1, and Figure S4), as summarized in Table S2. 3.3. Formation mechanism of mixed oxide Besides MoxW1-xO3, the successful synthesis of other Mo-based mixed oxides has also been reported, such as MoxRe1−xO341 and MoxSe1−xOy.42 These ternary oxides are a group of substitutional solid solution formed by replacing some solvent atoms with solute atoms. This type of solid solution is easier to be attained if the solute and solvent obey the Hume-Rothery rules. In the present study, MoW mixed oxides were successfully obtained because of the striking similarities between W6+ and Mo6+ in valence, electronegativity and ionic radius (W6+: 74 pm; Mo6+: 73 pm). And importantly, they are all in monoclinic phase plausibly due to the Mo and W oxides only share a ReO3type crystal structure belonging to monoclinic system. These findings completely followed the Hume-Rothery rules and thus were well explained in reverse. However, notwithstanding their striking similarity, the substitution of W6+ with Mo6+ still distorted the crystal lattice and disrupted the long-term order of the host matrix, resulting in decreased crystallinity, smaller crystallite size, and a slightly broadening of the lattice space as illustrated above. In this case of too much Mo6+ was introduced into the matrix, the structure was more proper to be perceived as some of the Mo6+ in β-MoO3 matrix was replaced by W6+. The structure of all the MoxW1-xO3 oxides obtained in this study could be systematically described as: there is a ReO3-type monoclinic matrix with all the positions of Re6+



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Figure 3. (a) XRD and (b) Raman spectra of the sequentially deposited samples. (c) and (d) are EDXS spectra of the W400+Mo200 sample before and after calcined, respectively. The insets in panel c and d show the FE-TEM images of the corresponding sample respectively, in which the lines indicate where the EDXS data were recorded.

empty, and these vacancies are to be filled by either Mo6+ or W6+; if more positions are fixed with W6+, the whole structure resembles m-WO3, otherwise it is more close to β-MoO3; owing to the quite similar properties of Mo6+ and W6+, the matrix maintains its monoclinic structure with some distortion but cannot hold long-term order. From the perspective of achieving a minimal energy, the Mo6+ and W6+ are likely to be homogeneously distributed in the matrix. Thus it was hypothesized that there should be a diffusion, blending, and redistribution of Mo6+ and W6+ during the calcination process since the aselectrodeposited samples were amorphous in nature (Figure S5). A strategy involving sequential deposition of WO3 and MoO3 was designed to have an insight into the proposed MoxW1-xO3 formation mechanism. That is, WO3 was predeposited onto the TiO2 nanotube arrays in an electrolyte containing W precursor, and then a post-deposition of MoO3 was conducted in a Mo precursor electrolyte (see Experimental section). A series of samples was prepared and named as W400+Mo100, W400+Mo150, and W400+Mo200, etc., according to the deposition time applied. Figure 3a and Figure 3b show the XRD and Raman spectra for these samples respectively. Impressively, with an increase of the deposited MoO3 amount, quite similar phenomena with that shown in Figure 2 were observed. Importantly, these results reveal that the same Mo-W mixed oxide could be synthesized via both the sequential deposition method and the co-deposition method merely

on condition the same ratio of Mo/W was deposited. Considering the character of the sequential deposition method, that is, MoO3 was deposited on the predeposited WO3, it could be deduced that there was indeed a diffusion, blending, and redistribution of Mo6+ and W6+ during the calcination process. We further proved this deduction by using FE-TEM and EDXS analyses. Figure 3c shows the EDXS results of the W400+Mo200 sample without calcination scanned linearly across a nanotube as indicated in the inset, which clearly demonstrate a layer of MoO3 about 2 ~ 5 nm is coated on the surface of WO3. While the signals of Mo and W are always synchronous in the EDXS spectra after calcination of the sample (Figure 3d), revealing their homogeneous distribution. This synchronization was also observed in the EDXS spectra of the calcined W400+Mo300 and 0.7MoW samples (Figure S6), which declared this phenomenon was not a coincidence. After scrutinizing the Raman spectra of the sequentially deposited samples, it was found that once αMoO3 existed in the sample, there was no m-WO3 or any m-WO3 like MoxW1-xO3 present in the sample otherwise the band at 714 cm−1 would not completely disappear. This finding revealed that the entire WO3 layer had been penetrated by Mo during the thermal treatment. Though a precise evaluation on how far Mo6+ and W6+ can diffuse during the calcination process is not valid, it could be estimated that this value exceeds 10 nm from the thickness of the WO3 layer in Figure 3c. Actually, this is the reason why we do not suggest the presence of mixed


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Scheme 1. Schematically illustrating the formation mechanism and crystal structure of the samples: (a) α-MoO3, (b) m-WO3, and (c) Mo-W mixed oxides. In panel c, to demonstrate the variation in the structure with an increase of the Mo/W ratio, the mixed oxides similar or close to m-WO3 were constructed with the matrix of m-WO3, while those close to β-MoO3 were constructed with the matrix of β-MoO3. The distortion in the host matrix caused by the introduction of exotic atoms was not de43-45 picted in the drawing. The data needed to construct the matrix of α-MoO3, m-WO3, and β-MoO3 were taken from references , respectively.

phases of m-WO3 and MoxW1-xO3 in the 0.1MoW sample because of its similar dimension with the WO3 sample (Figure 1g and 1h). In conclusion, the MoxW1-xO3 formation mechanism could be depicted as a diffusion, blending, and redistribution of Mo6+ and W6+ at nanoscale to homogeneously occupy the sites where Re6+ stands in a ReO3-type monoclinic matrix during calcination, as schematically illustrated in Scheme 1. 3.4. Electrochemical property of the samples TiO2 is known as a typical n-type semiconductor, but two cases should be strictly distinguished when examining its electrical conductivity. When subjected to anodic polarization, TiO2 materials are indeed suffering from poor conductivity, and only those significantly modified ones could be used as anode materials.35, 46, 47 On the flip side, when bearing a cathodic bias, TiO2 is of good conductivity behaving as semimetal or even metal because of the formation of accumulation layer, and thus it could be directly used as cathode material.48, 49 Analogous to the latter case, good conductivity of TiO2 nanotubes was achieved in this study, because the applied potential window is negative to the flat band potential of TiO2 nanotubes, as illustrated in Figure S8. Relying on this benefit and their ideal morphology, the TiO2 nanotube arrays was selected as current collectors for the deposited samples

(Figure S9a and S9b). Figure 4a compares the CV curves of the samples, which could be distinguished into three categories based on their character: (i) the curves of the MoO3, 0.9MoW, and 0.7MoW samples, with two pairs of redox peaks (as an example, those are indicated by A1/C1 and A2/C2 for the MoO3 sample in Figure 4a); (ii) those of the 0.5MoW and 0.3MoW samples with a pair of very broad redox peaks centered at ca. −0.5 V, as enlarged shown in Figure S9c; and (iii) the curves of the 0.1MoW and WO3 without apparent redox peaks. Those redox peaks in category (i) were described as charge insertion into the interlayer (A1/C1) and intralayer (A2/C2) of αMoO3, respectively, according to previous report,10 which is quite consistent with the phase composition of the samples. In contrast, the CV profiles of category (ii) are characteristic of a surface confined charge transfer process, which suggests that the stored charge is mainly capacitive in nature.10 While in the case of category (iii), it seems an intense reaction did not happen, revealing low electrochemical activity of the 0.1MoW and WO3 samples at applied potential window. To further illustrate the charge storage capability and mechanism of the samples, their specific capacitances were quantitatively compared and the contributions from intercalation process and capacitive process were divided



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−1

Figure 4. Electrochemical characterization of the samples: (a) CV curves recorded at a scan rate of 20 mV s in 1 M H2SO4 aqueous solution; (b) Specific capacitance versus the reciprocal square root of scan rate; (c) Decomposed capacitances into contributions from intercalation charge and capacitive charge, with a number beyond each column gives the ratio of capacitive charge to total charge; (d) Coulombic efficiency as a function of scan rate; (e) Cycling performance of the samples.

using a Trasatti method.50 If semi-infinite linear diffusion of proton in the solid state is assumed, capacity is expected to be linearly related to the reciprocal square root of scan rate (v−1/2) and the extrapolation of v−1/2 to 0 gives the capacitive charge because capacitive contribution is independent of scan rate; similarly, a linear relationship between the reciprocal of capacity and v1/2 is expected and the extrapolation of v1/2 to 0 gives the total capacity because every active site is accessible and contributes at scan rate of 0 (for details, see references50, 51). Figure 4b plots the specific capacitances (C) (see Calculations in Supporting Information) versus v−1/2, in which two distinct regions were observed. In the region of scan rate 12 mV s−1, the plots deviate from linearity. This deviation can arise from numerous sources such as ohmic resistance and irreversible redox processes.52-54 Therefore, only the plots at low scan rates (examples are shown in Figure S9d) were fitted to obtain the capacitance contributed from capacitive charge.52, 53, 55 A similar manipulation gave the value of total capacitance (Figure S9e) and the portion from intercalation charge was obtained by subtracting that from capacitive charge. Figure 4c shows the results derived from the Trasatti method. Interestingly, the MoO3 exhibits the highest total capacitance but a modest portion of 67.2% contributed from capacitive charge, which reveals a modest reaction kinetic limited by diffusion controlled proton intercalation. Actually, this value is comparable to


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that reported for an ordered mesoporous α-MoO3 crystalline film (70%),10 suggesting the nanowire architecture presented in this study also enables the fast intercalation of proton into the interlayer and intralayer of α-MoO3 at time scale comparable to redox pseudocapacitance. In contrast, the samples containing Mo-W mixed oxide deliver decreased total capacitance but promoted portion from capacitive charge, revealing improved rate capability. Remarkably, the capacitive charge contributes as much as 95.3% to the total capacitance in the 0.5MoW sample, which makes it behave almost as an electrical double-layer capacitor. This high rate capability was further proved by galvanostatic charge-discharge test. As shown in Figure S10, the 0.5MoW sample delivers a specific capacitance of 517.4 F g−1 (209.5 mF cm−2) at a chargedischarge current density of 1 A g−1; and it keeps capacitance retention as high as 89.3% with an increase of the charge-discharge current density to 10 A g−1, far exceeding the values reported for other MoO3- or WO3-based materials, such as rGO/MoO3 composite (~60.6%),25 MoO3/PANI coaxial nanobelts (~60.0%),24 and h-WO3 nanopillars (~73.7%, from 0.5 to 5 A g−1).56 To our knowledge, the high-rate capability of the 0.5MoW sample is the best ever achieved by MoO3- or WO3-based materials. Relying on the ultrahigh rate charge/discharge kinetics, the 0.5MoW sample delivers a specific energy of 16.0 Wh kg−1 even at a high specific power of 2.5 kW kg−1 (Figure S10b), which makes it attractive for the construction of advanced ultrahigh power supercapacitors.

structure in two aspects. (i) First, the long-term order of the matrix was disrupted in the mixed oxides, resulting in a decrease in crystallite size, which provides greater surface area for electrolyte to access and shorter ion diffusion path length in the domain. (ii) Second, the significant disrupting in the monoclinic matrix yields a broader lattice space in the mixed oxide than both the m-WO3 and β-MoO3, which diminishes the ion diffusion resistance in the solid and thus also promotes the rate response of the sample. To further confirm these deductions, EIS studies were conducted. As shown in Figure S11, Figure S12, Table S3, and Table S4, the results unambiguously demonstrate that the WO3 possesses much larger charge-transfer and ion diffusion resistances, accounting for the limited rate capability as illustrated in the CV study. While for the other samples, these resistances are small, implying excellent rate capability. And the quite small charge-transfer and ion diffusion resistances are responsible for the ultrahigh rate capability of the 0.5MoW sample, originated from the modified crystal structure with decreased crystallite size and broader lattice space, as illustrated in Scheme 2. As one of the most important characteristics of supercapacitor, the long-term cycling stability of the samples

As to the WO3 sample, low specific capacitance and quite poor rate capability are exhibited, which could be attributed to the non-sufficient negative potential window applied and the good crystallization of this material as discussed above, respectively. Typically, intense proton insertion into WO3 takes place at potential extent more negative than −0.2 V (vs SCE) in acidic electrolyte.56-58 However, this process is always accompanied by a risk of hydrogen evolution, which should be avoided for energy storage application. In the present study, strong hydrogen evolution occurred with bubbles could be observed by naked eyes when the WO3 sample was subjected to a potential of ca. −0.4 V (vs SCE). To quantitatively evaluate the reversibility of electrochemical processes, the coulombic efficiency, termed as anodic charge/cathodic charge in this situation, was compared at different scan rates for the samples. As shown in Figure 4d, all the samples show higher coulombic efficiency at elevated scan rates, and the co-deposited samples show higher values than the bare MoO3 or WO3 sample. Remarkably, the coulombic efficiency of the 0.5MoW is always as high as 95 ~ 100%, demonstrating quite excellent reversibility of the electrochemical processes. These findings hint better rate capability and reversibility of the sample at the same time, both originated from the facilitated ion insertion/extraction. After a comprehensive comparison on the morphology, structure, and electrochemical property of the samples, it could be understood the superiority of the 0.5MoW sample in rate capability is barely related to its morphology, but mainly benefited from the modification on crystal

Scheme 2. Schematic of ion transportation into the (a) αMoO3, (b) m-WO3 and (c) Mo-W mixed oxide.



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was strictly interrogated by CV test performed up to 5000 cycles. Figure 4e shows the capacitance retention during the cycling for the samples. It is not surprising that the MoO3 sample exhibits quite poor cycling stability with capacitance retention of only 13.3% of its initial value after 5000 cycles. In fact, analogous to the behavior previously reported for α-MoO3,22, 23 most of its capacitance loses in the initial 100 cycles (~71%), which is the principal drawback of α-MoO3 that hinders its practical application for energy storage. The 0.9MoW and 0.7MoW samples also show fast degradation of capacitance in the initial 100 cycles, due to the loss of contribution from α-MoO3 phase in them, evidenced by the gradually vanishing of the characteristic redox peaks of α-MoO3 in the CV curves (Figure S13) and the morphology study of the samples after cycling (Figure S14). Actually, α-MoO3 is a high volume expansion material with at least 100% volume change during ion insertion/extraction.22 The repeated expansion/contraction of the nanowires or blocks in the MoO3, 0.9MoW, and 0.7MoW samples led to the detachment of them into the electrolyte rapidly. On the contrary, the WO3 sample shows gradually enhancement in capacitance during the 5000 cycles. This activation in capacitance could be explained as follows. The ion diffusion in WO3 was sluggish as analyzed above, which determined the protons only penetrated into the surface of WO3 with a certain depth during the charge-discharge process at relevant time scale. With the repeating of proton insertion/extraction, the lattice was expanded, which facilitated the proton diffusion in return. And thus, proton penetrated deeper into WO3 at given time, leading to an increase in capacitance. As to the samples consisting of only Mo-W mixed oxide, the 0.1MoW delivered excellent cycling stability with capacitance retention of 103.0% after 5000 cycles; while the 0.3MoW and 0.5MoW showed gradually decrease in capacitance during the cycling, and retained 69.4% and 60.2% of their initial capacitances respectively. All these results are supported by the EIS analysis on the samples after cycling (Figure S11a−g and Table S5), such as the capacitance degradation of the MoO3, 0.9MoW, 0.7MoW, 0.5MoW, and 0.3MoW samples, the capacitance elevation of the 0.1MoW and WO3 samples, and the decreased ion diffusion resistance in the WO3. A comprehensive comparison on specific capacitance and cycling performance among all the samples gives that the one who is active suffers from poor stability and vice versa. To determine the sample with optimal balance between specific capacitance and cycling stability, we use a term — average capacitance, that is, the quotient of the added up capacitance in every cycle divided by 5000. As shown in Figure S15, the 0.7MoW and 0.5MoW samples delivered similar average capacitance of ~340 F g−1, higher than the other samples. Taking into account the 0.5MoW possesses the ultrahigh rate capability, it was considered as the one with the best electrochemical performance. In addition, this sample always maintained excellent coulombic efficiency of 95 ~ 100% in its lifespan (Figure S13f), demonstrating the potential for practical application.

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At last, to further demonstrate the capability of the 0.5MoW sample for practical application, a preliminary supercapacitor device was assembled by combining the 0.5MoW (negative electrode) with a polyaniline/TiO2 electrode (see Supporting Information for details). This device delivered an energy density of 8.3 Wh kg−1 (at 400 W kg−1) and a remarkable power density as high as 10 kW kg−1 (at 4.0 Wh kg−1). The power density achieved is not only much higher than other MoO3- or WO3- based supercapacitor devices25, 57, 59-61 but also those made of other materials reported in the literatures.62-64 This supercapacitor device also showed good cycling stability with energy retention of 61.7% after 5000 cycles. Given that the polyaniline/TiO2 electrode did not match the 0.5MoW so well, the performance of the supercapacitor device could be further boosted in the future by replacing the polyaniline/TiO2 with a more proper positive electrode.

4. Conclusions In summary, a strategy involving the fabrication of MoW mixed oxide was proposed to modify the rate capability and cycling stability of MoO3. By a simple co-deposition method, a series of Mo-W mixed oxide in the form of MoxW1-xO3 was successfully coated on a TiO2 nanotube array substrate. As results, the co-deposited samples showed lots of differences with the bare MoO3 or WO3 sample in morphology, phase structure, and electrochemical properties. It was found all the MoxW1-xO3 oxides are in monoclinic phase, and the Mo-rich ones are close to βMoO3 in structure, while those W-rich ones can even behave similar to m-WO3. This is because a MoxW1-xO3 similar to β-MoO3 is thermodynamically forbidden, and will decompose into a mixture of α-MoO3 and a MoxW1-xO3 close to β-MoO3. It was also found that the monoclinic MoxW1-xO3 and WO3 were more easily to be deposited into the TiO2 nanotube arrays, while the α-MoO3 species preferred to be stacked on the top of the TiO2 nanotube layer. By adopting a sequential deposition method, Mo-W mixed oxides similar with those prepared by codeposition were also obtained, which proved the proposed formation mechanism of Mo-W mixed oxides. That is a diffusion, blending, and redistribution of Mo6+ and W6+ at nanoscale to homogeneously occupy the sites where Re6+ stands in a ReO3-type monoclinic matrix during calcination. Importantly, the distortion existing in the mixed oxides leads to decreased crystallite size and enlarged lattice space of them, which have a decisive impact on their high rate capability by facilitating ion diffusion in the solid. Remarkably, as the one with the optimal balance between electrochemical activity and cycling stability, the 0.5MoW sample exhibits a high specific capacitance of 517.4 F g−1 at 1 A g−1, and retains 89.3% of the capacitance even at a current density of 10 A g−1, demonstrating ultrahigh rate capability. Furthermore, the potential of this sample for the construction of ultrahigh power supercapacitors was justified by assembling a PANI//0.5MoW supercapacitor device, which could stably operate at a high power density of 10 kW kg−1. Using the strategy and material presented in this study for con-


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structing ultrahigh rate supercapacitors as well as other energy storage devices is exciting.

ASSOCIATED CONTENT Supporting Information. SEM, Raman, XPS analysis, calculation of the capacitance, CV and charge-discharge curves, EIS spectra, fabrication and performance of supercapacitor device. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

ACKNOWLEDGMENT This work was supported by International Science & Technology Cooperation Program of China (No. 2013DFG50150 and 2016YFE0126300) and the Innovative and Interdisciplinary Team at HUST (2015ZDTD027). The authors thank the Analytical and Testing Center of HUST for the use of SEM, XRD, and XPS equipment.

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