Advances in Electrochemical Energy Devices Constructed with Tungsten Oxide-Based Nanomaterials

Tungsten oxide-based materials have drawn huge attention for their versatile uses to construct various energy storage devices. Particularly, their electrochromic devices and optically-changing devices are intensively studied in terms of energy-saving. Furthermore, based on close connections in the forms of device structure and working mechanisms between these two main applications, bifunctional devices of tungsten oxide-based materials with energy storage and optical change came into our view, and when solar cells are integrated, multifunctional devices are accessible. In this article, we have reviewed the latest developments of tungsten oxide-based nanostructured materials in various kinds of applications, and our focus falls on their energy-related uses, especially supercapacitors, lithium ion batteries, electrochromic devices, and their bifunctional and multifunctional devices. Additionally, other applications such as photochromic devices, sensors, and photocatalysts of tungsten oxide-based materials have also been mentioned. We hope this article can shed light on the related applications of tungsten oxide-based materials and inspire new possibilities for further uses.


Introduction
Energy exhaustion and environment deterioration has caused more and more scientific and public concern. To slow down the speed of resources running out and to ameliorate our living condition, turning to other inexhaustible energies including solar, wind, and tidal energy and employing high-efficiency devices to save energy have naturally become very important. Under the uncontrollable weather conditions, it is obviously challenging to get reliable and stable energy supply merely from inexhaustible energies. Therefore, those energy converting systems have to be used in conjunction with high-efficiency energy storage devices to store the converted energy [1,2]. As is known to us, supercapacitors [3,4] and lithium ion batteries [5] are two types of widely used efficient energy storage devices (ESDs). Moreover, electrochromic devices (ECDs) [6][7][8] are a well-known high-efficient application through controlling sunlight intensity and the amount of heat crossing it by changing transmittance.
Supercapacitors (SC) are one promising energy storage device for its unique advantages like high power density, ultra-long cycling life (over 10 5 times), fast charging speed (within tens of seconds), and outstanding performance under low temperature [9]. There are two main types of SCs, electrical double-layer capacitors and pseudo-capacitors [3]. The former works basing on the centralization and decentralization of charge at the interface between electrode and electrolyte, while the latter operates mainly relying on Faradic reactions with their double layer capacitance making a relatively small contribution to

The Crystal Structure of Tungsten Oxides
In light of the fact that tungsten trioxide (WO3) is the most widely used of the tungsten oxides, we will mainly concentrate on WO3-based materials. The perfect WO3 is a kind of ReO3 type cubic structured material in which octahedral WO6 links each other by corner-sharing [25]. In a WO6 octahedron, the W atom lies at the center and the remaining six O atoms form the octahedral framework. As temperature and pressure change, the WO6 octahedron will tilt and rotate at certain angles, leading to the formation of several different phases: tetragonal phase, orthorhombic phase, monoclinic phase, triclinic phase, and cubic phase [35][36][37]. Figure 2a shows the phase transformation of WO3 as temperature changes. Within WO3, there are sites and tunnels between octahedrons so that atoms with small diameters such as H + , Li + , and K + can transfer into WO3 and be stored. There is also another phase of WO3, hexagonal phase, that can be obtained from the hydrate-losing process of hydrated tungsten oxides [38]. As shown in Figure 2b,c, in the a-b plane of this phase, there are trigonal cavities and hexagonal windows. After stacking of the WO6 octahedron, trigonal and hexagonal tunnels along the c axel are formed [39,40]. These tunnels are conducive to the fast transfer of ions and electrons so the electrochemical activity of hexagonal tungsten oxides is better than the WO3 of other phases. The most common phases of tungsten trioxide are monoclinic phase (m-WO3), hexagon phase (h-WO3), and cubic phase (c-WO3).

The Crystal Structure of Tungsten Oxides
In light of the fact that tungsten trioxide (WO 3 ) is the most widely used of the tungsten oxides, we will mainly concentrate on WO 3 -based materials. The perfect WO 3 is a kind of ReO 3 type cubic structured material in which octahedral WO 6 links each other by cornersharing [25]. In a WO 6 octahedron, the W atom lies at the center and the remaining six O atoms form the octahedral framework. As temperature and pressure change, the WO 6 octahedron will tilt and rotate at certain angles, leading to the formation of several different phases: tetragonal phase, orthorhombic phase, monoclinic phase, triclinic phase, and cubic phase [35][36][37]. Figure 2a shows the phase transformation of WO 3 as temperature changes. Within WO 3 , there are sites and tunnels between octahedrons so that atoms with small diameters such as H + , Li + , and K + can transfer into WO 3 and be stored. There is also another phase of WO 3 , hexagonal phase, that can be obtained from the hydrate-losing process of hydrated tungsten oxides [38]. As shown in Figure 2b,c, in the a-b plane of this phase, there are trigonal cavities and hexagonal windows. After stacking of the WO 6 octahedron, trigonal and hexagonal tunnels along the c axel are formed [39,40]. These tunnels are conducive to the fast transfer of ions and electrons so the electrochemical activity of hexagonal tungsten oxides is better than the WO 3 of other phases. The most common phases of tungsten trioxide are monoclinic phase (m-WO 3 ), hexagon phase (h-WO 3 ), and cubic phase (c-WO 3 ).
Oxygen deficiencies are very common in the naturally existing WO 3 , causing the existence of substoichiometric tungsten oxides, WO 3−x (0 < x <1), in which the valence of W might be +3, +4, or +5 [42]. Among them, W 18 O 49 , W 20 O 58 , and W 24 O 68 are the most common and the oxygen deficiencies within them can promote their conductivity. This characteristic renders WO 3 an n-type semiconductor whose electric conductivity can be adjusted by controlling the amount of O vacancy in it [43][44][45].
Tungsten oxides made from liquid-related methods are often hydrated tungsten oxides before heat treatment, WO 3 ·xH 2 O, mainly including WO 3 ·0.33H 2 O, WO 3 ·0.5H 2 O, WO 3 ·H 2 O, and WO 3 ·2H 2 O. The structure of WO 3 ·xH 2 O is largely decided by the value of x. For example, the structure of WO 3 ·H 2 O is that H 2 O lies in the gap between the layers of WO 6 octahedrons [46,47], and the structure of WO 3 ·2H 2 O is that in addition to the same kind of H 2 O molecule in WO 3 ·H 2 O, another type of H 2 O molecule directly links to the tungsten atom at the bottom or top of the octahedron [48]. This structure is good for easy transport of ions and electrons, especially protons by means of the hydrogen bond network within them. Usually, hydrated tungsten oxides have much better conductivity than pure tungsten oxide, translating into enhanced electrochemical performance [47].

Phase Transformation in Tungsten Oxides toward Energy Storage
As we mentioned above, SCs and LIBs are two typical ESDs, and ECDs can be seen as ESDs too. Here, we give a rough introduction and comparison between their working mechanisms and requirements of tungsten oxide electrodes. Figure 3 depicts their similarities and differences in terms of structures and mechanisms. SCs consist of five main parts, two electrodes, electrolyte, and current collectors at both electrodes (Figure 3a) [4]. As for pseudo-capacitors, they mainly rely on fast Faradic reactions on the Nanomaterials 2021, 11, 692 5 of 39 interface between electrode and the electrolyte. Take WO 3 as an example. When used as electrode material for pseudo-capacitors, WO 3 works according to Equation (1) [49][50][51]: 3 (1) where M can be H, Li, Na, K, and other atoms or groups with small volumes. Different from tungsten oxides with other phases, h-WO 3 , except for this Faradic reaction, has another way of energy-storage by placing atoms in tunnels and cavities within its inner structure [52].
Nanomaterials 2021, 11, 692 5 of 41 WO3 + xM + + xe − = MxWO3 (1) where M can be H, Li, Na, K, and other atoms or groups with small volumes. Different from tungsten oxides with other phases, h-WO3, except for this Faradic reaction, has another way of energy-storage by placing atoms in tunnels and cavities within its inner structure [52]. LIB also shares the five-layer sandwich structure (Figure 3b). It works depending on Li + ions' movement between cathode and anode. Compared with that of pseudocapacitors, the time needed for the charging and discharging process of LIBs is usually much longer because the redox reactions happen not only at the surface of the electrode but also in its deep bulk. Crystalline WO3 follows the conversion mechanism when as anode material in LIB, as presented in Equations (2) and (3) [15]. From the equations, we also get the theoretical capacity of WO3, 695mA h g −1 , when every W atom accommodates 6 Li + . Nevertheless, it is a double-edged sword because tungsten oxides suffer from large volume change at the same time, causing structural collapses and fast capacity decreases during cycling. Additionally, their low conductivity results in poor rate performance. As shown in Figure 4, the morphology and phase changes in the WO3 made by magnetron sputtering after initially fully discharged (at 0.01 V) and charged (at 4.0 V) have also been explored by scanning electron microscopy (SEM) and transition electron microscopy (TEM), revealing a large volume change of phase variation [15].
. LIB also shares the five-layer sandwich structure (Figure 3b). It works depending on Li + ions' movement between cathode and anode. Compared with that of pseudo-capacitors, the time needed for the charging and discharging process of LIBs is usually much longer because the redox reactions happen not only at the surface of the electrode but also in its deep bulk. Crystalline WO 3 follows the conversion mechanism when as anode material in LIB, as presented in Equations (2) and (3) [15]. From the equations, we also get the theoretical capacity of WO 3 , 695 mA h g −1 , when every W atom accommodates 6 Li + . Nevertheless, it is a double-edged sword because tungsten oxides suffer from large volume change at the same time, causing structural collapses and fast capacity decreases during cycling. Additionally, their low conductivity results in poor rate performance. As shown in Figure 4, the morphology and phase changes in the WO 3 made by magnetron sputtering after initially fully discharged (at 0.01 V) and charged (at 4.0 V) have also been explored by scanning electron microscopy (SEM) and transition electron microscopy (TEM), revealing a large volume change of phase variation [15].
Usually, as shown in Figure 3c, to assemble a classic ECD, five components are needed, namely electrochromic layer, ion storage (IS) layer, counter electrode (CE) layer, and two transparent conducting (TC) layers. When the EC layer is WO 3 , the CE layer is usually anode EC materials such as nickel oxide [53,54], manganese dioxide [55], vanadium pentoxide [56], prussian blue [57,58], and some organic material like polyaniline (PANI) [59], etc., for they also present specific color change when the applied voltage changes. Their colors can have a synergy effect to strengthen the color of the ECD, or color superposition effect with the color of WO 3 to enable the ECD to have multiple colors. For instance, when nickel oxide works as the CE layer, it turns brown when WO 3 is colored, hence the color of ECD is deepened. When PANI performs the CE layer, the EC device can have four different colors (light green, green, light blue, and dark blue) as voltage regulates [59]. Sometimes, pure indium tin oxide (ITO) film [54,60] or fluorine doped tin oxide (FTO) film [61] can serve as CE layer as well, because they also have a large capacity of ions, leaving the ECD composed of four functional layers. However, it has been reported that an ECD with only single EC layer of tungsten oxide has relatively poorer EC performances than those of a complementary one [62,63].
but also in its deep bulk. Crystalline WO3 follows the conversion mechanism when as anode material in LIB, as presented in Equations (2) and (3) [15]. From the equations, we also get the theoretical capacity of WO3, 695mA h g −1 , when every W atom accommodates 6 Li + . Nevertheless, it is a double-edged sword because tungsten oxides suffer from large volume change at the same time, causing structural collapses and fast capacity decreases during cycling. Additionally, their low conductivity results in poor rate performance. As shown in Figure 4, the morphology and phase changes in the WO3 made by magnetron sputtering after initially fully discharged (at 0.01 V) and charged (at 4.0 V) have also been explored by scanning electron microscopy (SEM) and transition electron microscopy (TEM), revealing a large volume change of phase variation [15].
. WO 3 is a cathodic EC material. Its color change from colorless to blue appears as a result of the insertion of ions and electrons when a negative voltage is applied. This process is reversible as the applied voltage turns positive. Thus, we can see ECDs as transparent ESDs. It has also been reported that these color changes happen owing to the change of band gap induced by the insertion and extraction of ions [64,65]. Figure 3d displays the optical image of the color changing process of WO 3 . This process can be implied as following [66]: where M + can be H + , Li + , K + , et al. As introduced above, it is found that the electrochemical reactions of WO 3 in ECDs are similar to those in SCs and LIBs, which is very helpful for the development of integrated devices from WO 3 -based ESD and ECD. These three kinds of devices share the same sandwich structure. However, their different working mechanisms call for different requirements for tungsten oxide-based electrodes. Nevertheless, for all three kinds of devices, to get fast Faradic reactions, large specific surface area and good electrochemical conductivity of the tungsten oxides electrode are necessary.

WO 3 Electrode Materials of Supercapacitors
WO 3 , with a theoretical capacitance of 1112 F g −1 , is promising as an electrode material for pseudo-capacitors, but it also has drawbacks like unpleasant conductivity, poor rate performance, and less-satisfying cycling stability. The main improving methods can be divided into two parts, getting nanostructured single-phased tungsten oxide and getting multi-phased structures consisting of tungsten oxides with other materials such as carbon material, transitional oxides, and organic materials. Table 1 lists tungsten oxides as anode in SCs and their synthesizing methods and electrochemical performances, indicating that most of the WO 3 nanostructures for the SCs were prepared by hydrothermal processes. It is well known that nanostructured materials have larger specific surface area by fining the size of materials, which makes them fully exposed to the electrolyte. The inner active materials can be very accessible to ions and electrons so that redox reactions can be accelerated. WO 3 -based nanostructures, including quantum dots [95] (Figure 5a) and nano particles, nanofibers [67], nanorods [68,72,73], nanotubes [70], nanochannels [81] and nanowires [74,75], nanoflakes [51], nanoplates and nanosheets [69] (Figure 5c), nanospheres [76], and nanoflowers have all been researched. Cong et al. [95] demonstrated that the WO 3 quantum dots have better reversibility according to the more symmetric charge-discharge curve ( Figure 5b) and more excellent rate performance. In particular, the WO 3 nanosheets made by Yin et al. [69] can retain a capacity retention almost 100% after 10,000 cycles ( Figure 5d). Huang et al. [96] got WO 3 samples with different morphology by hydrothermal method: nanorods, nanoplates, and microspheres assembled of numerous nanorods. Among them, the ball cactus-like WO 3 microspheres had larger specific surface area and more tunnels across these nanorods, translating into lower equivalent series resistance (R s ) and excellent cycling stability, showing the best capacitive performance. It is well known that nanostructured materials have larger specific surface area by fining the size of materials, which makes them fully exposed to the electrolyte. The inner active materials can be very accessible to ions and electrons so that redox reactions can be accelerated. WO3-based nanostructures, including quantum dots [95] (Figure 5a) and nano particles, nanofibers [67], nanorods [68,72,73], nanotubes [70], nanochannels [81] and nanowires [74,75], nanoflakes [51], nanoplates and nanosheets [69] (Figure 5c), nanospheres [76], and nanoflowers have all been researched. Cong et al. [95] demonstrated that the WO3 quantum dots have better reversibility according to the more symmetric charge-discharge curve ( Figure 5b) and more excellent rate performance. In particular, the WO3 nanosheets made by Yin et al. [69] can retain a capacity retention almost 100% after 10,000 cycles ( Figure 5d). Huang et al. [96] got WO3 samples with different morphology by hydrothermal method: nanorods, nanoplates, and microspheres assembled of numerous nanorods. Among them, the ball cactus-like WO3 microspheres had larger specific surface area and more tunnels across these nanorods, translating into lower equivalent series resistance (Rs) and excellent cycling stability, showing the best capacitive performance. Apart from the aforementioned nanostructures, there are also other more complex and interesting morphologies assembled by smaller nano-units. For example, Shao et al. [77] prepared frisbee-like WO3·nH2O microstructure assembled with numerous nanorods (Figure 6a,b). Thanks to this special micro/nano structure, it had high specific capacity of 391 F g −1 at 0.5 A g −1 and good rate capacity of 298 F g −1 under 10 A g −1 . After 2000 chargedischarge cycles, its capacitance retention is around 100% (Figure 6c). By doping Pd, Gupta et al. [78] changed the morphology from nanosheets-assembled cabbage pure WO3 into nanobricks-assembled cauliflower Pd-doped WO3, achieving larger surface area. He Apart from the aforementioned nanostructures, there are also other more complex and interesting morphologies assembled by smaller nano-units. For example, Shao et al. [77] prepared frisbee-like WO 3 ·nH 2 O microstructure assembled with numerous nanorods (Figure 6a,b). Thanks to this special micro/nano structure, it had high specific capacity of 391 F g −1 at 0.5 A g −1 and good rate capacity of 298 F g −1 under 10 A g −1 . After 2000 charge-discharge cycles, its capacitance retention is around 100% (Figure 6c). By doping Pd, Gupta et al. [78] changed the morphology from nanosheets-assembled cabbage pure WO 3 into nanobricks-assembled cauliflower Pd-doped WO 3 , achieving larger surface area. He et al. [50] fabricated a special furball-like microsphere, of which the core was assembled by a large number of nanorods and the shell was many other fluffy nanorods connecting each core, resulting in a porous 3D structure network. Notably, when used as the electrode in SC, even after 10,000 charge-discharge cycles, 93.4% of its initial capacitance was maintained. In addition, WO 3 3D nanorods array [71], cactus-like microspheres hierarchy 3D structure assembled by numerous nanorods [79,80], mesoporous pancake-like h-WO 3 [52], and WO 3 ·H 2 O flower-like hierarchical architecture composed of nanosheets [82] have also been reported, showing much enhanced performance in comparison to most WO 3 .
Nanomaterials 2021, 11, 692 11 of 41 et al. [50] fabricated a special furball-like microsphere, of which the core was assembled by a large number of nanorods and the shell was many other fluffy nanorods connecting each core, resulting in a porous 3D structure network. Notably, when used as the electrode in SC, even after 10,000 charge-discharge cycles, 93.4% of its initial capacitance was maintained. In addition, WO3 3D nanorods array [71], cactus-like microspheres hierarchy 3D structure assembled by numerous nanorods [79,80], mesoporous pancake-like h-WO3 [52], and WO3·H2O flower-like hierarchical architecture composed of nanosheets [82] have also been reported, showing much enhanced performance in comparison to most WO3.

Multi-Phased Tungsten Oxide Nanocomposites
Combining WO3 with other materials into composite is another direct way to achieve better performances in terms of good conductivity, high capacitance, and excellent stability. These composites obtained may possess the strengths of a single component and it is much easier to get a special structure that will further optimize its performances.
Carbon materials have been frequently chosen for their attractive conductivity and low cost. Additionally, they are also used as an electrode in double-layer capacitors. The combination of double-layer capacitor material with pseudo-capacity material can have strengthened stability, capacitance, and rate performance. Di et al. [83] fabricated a feather duster-like carbon nanotube (CNT)@WO3 composite, in which WO3 nanosheet grows uniformly on the surface of CNT. After 8000 cycles of repeating cyclic voltammetry (CV) test at 100 mV s −1 , this composite still retained 96.3% of its initial capacitance. Through a two-step hydrothermal method, Shinde et al. [49] made a composite in which multiwalled carbon nanotubes (MWCNTs) uniformly grew on the carbon cloth substrate and with WO3 nanorods growing on the MWCNTs. The prepared 3D structure had a large surface area and good structural stability. Chu et al. [84] synthesized WO3 nanoflower, well-coated with reduced graphene oxide (rGO) nanosheets. The capacity of WO3 and WO3-rGO composite were 127 F g −1 and 495 F g −1 respectively at current density of 1 A g −1 , and when the current density was 5 A g −1 , capacity of the composite was as high as 401 F g −1 . These improvements were down to the shorter ion diffusion paths and 3D nanostructure of the composite. Liu et al. [85] embedded WO3 hybrid nanowires and nanoparticles in carbon aerogel and the electrode also showed a high capacity of 609 F g −1 .

Multi-Phased Tungsten Oxide Nanocomposites
Combining WO 3 with other materials into composite is another direct way to achieve better performances in terms of good conductivity, high capacitance, and excellent stability. These composites obtained may possess the strengths of a single component and it is much easier to get a special structure that will further optimize its performances.
Carbon materials have been frequently chosen for their attractive conductivity and low cost. Additionally, they are also used as an electrode in double-layer capacitors. The combination of double-layer capacitor material with pseudo-capacity material can have strengthened stability, capacitance, and rate performance. Di et al. [83] fabricated a feather duster-like carbon nanotube (CNT)@WO 3 composite, in which WO 3 nanosheet grows uniformly on the surface of CNT. After 8000 cycles of repeating cyclic voltammetry (CV) test at 100 mV s −1 , this composite still retained 96.3% of its initial capacitance. Through a two-step hydrothermal method, Shinde et al. [49] made a composite in which multi-walled carbon nanotubes (MWCNTs) uniformly grew on the carbon cloth substrate and with WO 3 nanorods growing on the MWCNTs. The prepared 3D structure had a large surface area and good structural stability. Chu et al. [84] synthesized WO 3 nanoflower, well-coated with reduced graphene oxide (rGO) nanosheets. The capacity of WO 3 and WO 3 -rGO composite were 127 F g −1 and 495 F g −1 respectively at current density of 1 A g −1 , and when the current density was 5 A g −1 , capacity of the composite was as high as 401 F g −1 . These improvements were down to the shorter ion diffusion paths and 3D nanostructure of the composite. Liu et al. [85] embedded WO 3 hybrid nanowires and nanoparticles in carbon aerogel and the electrode also showed a high capacity of 609 F g −1 . Later, they used the same method dispersing size-selected WO 3 nanoparticles in carbon aerogel and the results were also very pleasing [86].
Transition oxide materials such as V 2 O 5 , MnO 2 , CuO, and TiO 2 are other typical electrode materials for pseudo-capacitors owing to their high capacitance and stability, and they are often used to form a composite with WO 3 . Shinde et al. [87] got a nanostructured composite of WO 3 and MnO 2 , which had high capacity of 540 F g −1 at 2 mA cm −2 and good stability with 89% retention of initial capacitance after 2000 CV tests. Yuan's group [88] also prepared nano-WO 3 *H 2 O/MnO 2 composite with high capacity of 363 F g −1 at 0.5 A g −1 .
Periasamy et al. [89] reported a rod-shaped WO 3 -V 2 O 5 composite prepared by microwaveassisted wet-chemical method. When in KOH electrolyte, its capacity was higher than pure WO 3 by some distance. Moreover, it was noteworthy that after 5000 long cycles, the composite showed excellent capacity retention of 126% and had Coulombic efficiency of 100% up to 5000 cycles. In addition, WO 3 /TiO 2 composites [90,91] and WO 3 @CuO composites [92] have been researched.
As well as carbon materials and transition oxide materials, organic materials, especially conductive polymers, such as PANI, poly-3,4-ethylenedioxithiophene (PEDOT), and poly-pyrrole (PPy), are also preferred to combine with WO 3 for their high conductivity, low cost, and easy fabrication. Zhuzhelskii's group [93] dispersed WO 3 in PEDOT. The porous PEDOT matrix ensured fast ion and electron transfer, thus promoting the electrochemical performance. Similarly, Das et al. [94] fabricated a WO 3 @PPy composite, in which WO 3 nanostick is the core and PPy capsulated WO 3 . Owing to the high conductivity of PPy and the specific structure, shorter diffusion path length and greater stability were realized.

Tungsten Oxide-Based Materials as Anodes in Lithium Ion Battery
As mentioned before, when used as anode material in LIB, tungsten oxides suffer from structural collapses and fast capacity decreases during the charge-discharge cycling owing to the large volume change. Additionally, their low conductivity results in poor rate performance. So far, as listed in Table 2, some effective methods have been offered to improve the electrochemical performances of tungsten oxides. When O vacancies are introduced into tungsten oxides, its conductivity may be largely improved, and changing vacancy concentration to get increased conductivity has been tried. Nanostructures are not only adopted in SCs but also in LIBs. Moreover, adding carbon materials such as graphite, reduced graphite, and carbon nanotubes into tungsten oxide to get complex is also often adopted due to their high conductivity and structural stability.  Cauliflower-like WO 3 decorated with carbon [110] Hydrothermal + firing 67% 0-3.0 V, 50 mA g −1 , 750 mA h g −1 (1st) and 500 mA h g −1 (2nd) 50 mA/g, 50 cycles, 650 mA h g −1 (~Li 5.5 WO 3 ) Carbon-coated 3D WO 3 [111] Template assisted process 60. Ultrathin WO 3−x /C nanosheets [113] Acid-assisted one-pot process 39.4% 0-3.0 V, 200 mA g −1 , 1866 mA h g −1 (1st), 893 mA h g −1 (2nd)

Non-Stoichiometric Tungsten Oxides
As introduced above, substoichiometric tungsten oxides are common in the natural world. O vacancies within them have a positive effect on the transport of electrons. In addition, WO 3 is an n-type semiconductor, whose conductivity mainly depends on the concentration of free electrons in their conduction bands or, in other words, the concentration of donor within it [114]. By adjusting the ratio of W and O within tungsten oxides, the concentration of vacancies is changed accordingly, and its conductivity can drastically increase [115]. Thus, substoichiometric tungsten oxides such as W 18 (Figure 7a), transforming it into ultrafine disordered clusters (Figure 7b). The introduction of N offered many redox sites and facilitated the electrochemical kinetics, thus getting superior high-rate performance (Figure 7c). Aside from introducing O vacancies into tungsten oxides, excess O in tungsten oxides is also helpful because excess O can result in distortion of tunnels within tungsten oxides. Inamdar et al. [99] obtained tungsten oxide with excess O by adjusting the ratio of Ar to O 2 in radiofrequency (RF) magnetron sputtering process. Results showed that the charge transfer resistance of WO x under the gas ratio of 7:3 was tested to be 215 Ω, much lower compared with 370.8 Ω when the gas is pure Ar. They attributed this to the increased donor concentration induced by excess O in tungsten oxide. It is worth noting that the performance of tungsten oxide with excess O under high current density was also impressive.

Nano-Structured Tungsten Oxides
Through the sol-gel method, hydrothermal method, and template method, nanostructured tungsten oxides can be easily obtained. Wu et al. [70] made WO3 nanotube bundles by one-step hydrothermal and post-annealing process. Its initial specific discharge capacity and initial Coulombic efficiency were 871.9 mA h g −1 and 77.8%, respectively. Lim et al. [100] prepared WO3 nanocrystals and nanowires. Both samples showed high initial capacity of 867 and 954 mA h g −1 at 0.1C. For instance, after 100 cycles, the specific discharge capacity of WO3 nanowires retained 552 mA h g −1 , and its average Coulombic efficiency was 97.2% during 2-100 cycles. Yang et al. [101] synthesized hierarchical flower-like WO3 using HCOOH as structure-directing agent in the hydrothermal method (Figure 8a). Every flower petal consisted of numerous nanorods (Figure 8b). At a current density of 100 mA g −1 , its reversible capacity was 766 mA h g −1 after 50 cycles and still remained at 720 mA h g −1 even after 100 cycles. Additionally, under current density of 500 mA h g −1 , its capacity was as high as 576.8 mA h g −1 . All the results demonstrated good cycling and rate performance for the hierarchical flower-like WO3. Sasidharan et al. [102] used poly(styrene-b-[3-(methacryloylamino) propyl] trimethylammonium chloride-b-ethylene oxide) micelles (PS-PMAPTAC-PEO) as the template to produce WO3 hollow nanospheres. The whole triblock copolymer is composed of PS as its core, PMAPTAC as its shell and PEO as its corona. PMAPTAC can effectively bind with WO4 2+ cations. After the following calcinations, the polymeric template is completely removed and the WO3 hollow nanosphere can be produced.

Nano-Structured Tungsten Oxides
Through the sol-gel method, hydrothermal method, and template method, nanostructured tungsten oxides can be easily obtained. Wu et al. [70] made WO 3 nanotube bundles by one-step hydrothermal and post-annealing process. Its initial specific discharge capacity and initial Coulombic efficiency were 871.9 mA h g −1 and 77.8%, respectively. Lim et al. [100] prepared WO 3 nanocrystals and nanowires. Both samples showed high initial capacity of 867 and 954 mA h g −1 at 0.1C. For instance, after 100 cycles, the specific discharge capacity of WO 3 nanowires retained 552 mA h g −1 , and its average Coulombic efficiency was 97.2% during 2-100 cycles. Yang et al. [101] synthesized hierarchical flower-like WO 3 using HCOOH as structure-directing agent in the hydrothermal method (Figure 8a). Every flower petal consisted of numerous nanorods (Figure 8b). At a current density of 100 mA g −1 , its reversible capacity was 766 mA h g −1 after 50 cycles and still remained at 720 mA h g −1 even after 100 cycles. Additionally, under current density of 500 mA h g −1 , its capacity was as high as 576.8 mA h g −1 . All the results demonstrated good cycling and rate performance for the hierarchical flower-like WO 3 . Sasidharan et al. [102] used poly(styrene-b-[3-(methacryloylamino) propyl] trimethylammonium chloride-b-ethylene oxide) micelles (PS-PMAPTAC-PEO) as the template to produce WO 3 hollow nanospheres. The whole triblock copolymer is composed of PS as its core, PMAPTAC as its shell and PEO as its corona. PMAPTAC can effectively bind with WO 4 2+ cations. After the following calcinations, the polymeric template is completely removed and the WO 3 hollow nanosphere can be produced.

Tungsten Oxide-Carbon Composites
Introducing carbon materials into tungsten oxide to get composite can have several advantages. One is that the composite can integrate the advantages of both tungsten oxide and carbon material. The other is that it is more possible to form facile structures with high structure stability. Graphene is a flat monolayer based on single carbon atoms layer in a honeycomb lattice. This specific 2D structure gives it a super high theoretical specific surface area (2675 m 2 g −1 ) [117] and offers high thermal and electronic conductivity. Zeng et al. [106] synthesized hierarchical sandwich composite consisting of WO3 nanoplatelets and graphene (Figure 8c). They added WO3*H2O nanoplatelets into the well-dispersed graphene oxide solution and then stirred the solution to form homogeneous suspension.

Tungsten Oxide-Carbon Composites
Introducing carbon materials into tungsten oxide to get composite can have several advantages. One is that the composite can integrate the advantages of both tungsten oxide and carbon material. The other is that it is more possible to form facile structures with high structure stability. Graphene is a flat monolayer based on single carbon atoms layer in a honeycomb lattice. This specific 2D structure gives it a super high theoretical specific surface area (2675 m 2 g −1 ) [117] and offers high thermal and electronic conductivity. Zeng et al. [106] synthesized hierarchical sandwich composite consisting of WO 3 nanoplatelets and graphene (Figure 8c). They added WO 3 *H 2 O nanoplatelets into the well-dispersed graphene oxide solution and then stirred the solution to form homogeneous suspension. Following that was the vacuum filtration process. After WO 3 *H 2 O nanoplatelets-GO was peeled from the membrane, it then underwent heat treatment and finally the WO 3 nanoplatelets and graphene were obtained (Figure 8d). WO 3 was embedded uniformly in the interlayer of graphene so that the electrode had stable cycling performance since the volume expansion of WO 3 can be effectively relieved during cycling. At current density of 1080 mA g −1 , its reversible capacity was kept at around 615 mA h g −1 after 1000 cycles (Figure 8e). Kim et al. [104] reported a 2D nanocomposite consisting of graphene nanosheets with WO 3 nanoplates well scattered on it. After 50 cycles at 80 mA g −1 , its capacity was 688.8 mA h g −1 compared with 555.2 mA h g −1 of pure WO 3 . Gu et al. [105] produced bamboo-like WO 3 nanorods anchored on the N-doped 3D graphene frameworks. This composite can effectively bear the volume change and it provided higher conductivity for superior high-rate capability Reduced graphene oxide (rGO), usually obtained by reducing graphene oxide [118], is widely used to achieve better electrochemical performances of tungsten oxides. Dang et al. [106] successfully embedded WO 3 nanoplates in a rGO matrix with a hydrothermal method followed by a heating treatment. Surprisingly, after 150 cycles under 100 mA g −1 , its discharge capacity remained at 1005.7 mA h g −1 , nearly twice that (565 mA h g −1 ) of pure WO 3 . The main reasons for this improvement can be ascribed to the fact that rGO can not only offer easier access for ions and electrons but also largely buffer the damage to its structure during cycling. Huang et al. [107] produced h-WO 3 nanorods embedded in the rGO matrix doped with N and S. At a current density of 100 mA g −1 , the composite possessed a specific discharge capacity of 1030.3 mA h g −1 at the first cycle and was down slightly to 816 mA h g −1 in the second cycle. Moreover, at a high current density of 1500 mA g −1 , its specific discharge capacity was averaged at 196.1 mA h g −1 over 200 cycles. Park et al. [108] dispersed WO 3 particles on 3D macroporous rGO frameworks. This special structure and rGO's good conductivity jointly improved its rate capability and cycling stability.
Mesoporous carbon material is another kind of carbon material with high electrical and thermal conductivity, highly porous structure, and large specific surface area [119]. Wang et al. [109] dispersed ultrasmall WO 3 nanocrystals into mesoporous carbon matrix. During the preparing process, the W species were limited by the carbon matrix, making the particle size of WO 3 around 3 nm and high surface area of 157 m 2 g −1 for the composite. After 100 cycles at current density of 100 mA g −1 , its specific discharge capacity was 440 mA h g −1 . Kim et al. [120] also achieved a nanocomposite in which WO x nanoparticles were uniformly embedded in the mesoporous carbon matrix. Its main improvement was the lower polarization during the delithiation process owing to the high conductivity of mesoporous carbon matrix and shorter lithium on diffusion pathway.
Aside from the above-mentioned carbon materials, amorphous carbon materials are also often used to achieve better electrochemical performance. For example, Yoon et al. [110] coated cauliflower-like WO 3 with a thin layer of carbon (Figure 9a-c), which strengthened the electrochemical correlation between active WO 3 and current collector and buffered the volume change as well. It showed much better cycling stability and rate performance than the pure WO 3 (Figure 9d). Herdt et al. [111] made WO 3 nanorod arrays encapsulated in a thin layer of carbon. After 200 cycles of charge-discharge at C/20, the vertical arrangement of nanorods were maintained, indicating the outstanding structural stability of this composite. In addition, Liu et al. [112] obtained a WO 3 *0.33H 2 O@C composite in which amorphous carbon was coated around WO 3 *0.33H 2 O. Interestingly, in his study, an appropriate amount of carbon coating can have positive effects while it can be self-defeating when the amount of carbon is in excess because it decreased the crystallinity of WO 3 ·0.33H 2 O and sacrificed the capacity of the composite as well. Furthermore, Bao et al. [113] reported that the ultrathin WO 3-x nanoplate doped with carbon also showed excellent electrochemical performance.

Tungsten Oxides as EC Electrode in Visible Light Area
Previous works about WO3 mostly concentrated on improving its EC performances in visible light spectrum area, with EC performances in the near infrared (NIR) and infrared (IR) spectrum area neglected. Their objects focused on getting WO3 EC films with wider optical modulation amplitudes, shorter response times, and higher coloration efficiency in the visible light area. To achieve these, efforts involving getting nanostructured tungsten oxides, porous structured tungsten oxides, and doped tungsten oxides were widely made.

Tungsten Oxides as EC Electrode in Visible Light Area
Previous works about WO 3 mostly concentrated on improving its EC performances in visible light spectrum area, with EC performances in the near infrared (NIR) and infrared (IR) spectrum area neglected. Their objects focused on getting WO 3 EC films with wider optical modulation amplitudes, shorter response times, and higher coloration efficiency in the visible light area. To achieve these, efforts involving getting nanostructured tungsten oxides, porous structured tungsten oxides, and doped tungsten oxides were widely made.

Tungsten Oxides as EC Electrode in NIR Area
Research shows that WO3 thin films have good control of the transmittance of not only visible light but also NIR and IR light so the temperature can be dynamically modulated, since IR light is the main resource of heat from the sun [36,48,62,149]. Jian et al. [150] reported that the WO3/PEDOT: PSS (poly (3,4-ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS).) smart window can effectively reduce the heat across it. They detected the temperature of the back side of a small chamber that was assembled with an EC window as its front side (Figure 11a-c). When a halogen lamp worked as a radiation resource, it turned out that the temperature of the back side of the chamber was 3.3 °C lower as the EC window was darkened compared to the value as the EC window was bleached, demonstrating the EC film can effectively block heat (Figure 11d). Li et al. [151] made 1D W18O49 nanomaterials for NIR shielding. These films all had high transmittance in the visible light area. However, they did not explore the transmittance change during the color-changing process. Liu et al. [62] made a flexible ECD with transmission modulation of 63% between 760 and 1600 nm while they did not dig into the relationship between the transmittance of visible light and NIR light.

Tungsten Oxides as EC Electrode in NIR Area
Research shows that WO 3 thin films have good control of the transmittance of not only visible light but also NIR and IR light so the temperature can be dynamically modulated, since IR light is the main resource of heat from the sun [36,48,62,149]. Jian et al. [150] reported that the WO 3 /PEDOT: PSS (poly (3,4-ethylenedioxythiophene):poly (styrene sulfonate) (PEDOT:PSS).) smart window can effectively reduce the heat across it. They detected the temperature of the back side of a small chamber that was assembled with an EC window as its front side (Figure 11a-c). When a halogen lamp worked as a radiation resource, it turned out that the temperature of the back side of the chamber was 3.3 • C lower as the EC window was darkened compared to the value as the EC window was bleached, demonstrating the EC film can effectively block heat (Figure 11d). Li et al. [151] made 1D W 18 O 49 nanomaterials for NIR shielding. These films all had high transmittance in the visible light area. However, they did not explore the transmittance change during the color-changing process. Liu et al. [62] made a flexible ECD with transmission modulation of 63% between 760 and 1600 nm while they did not dig into the relationship between the transmittance of visible light and NIR light. Adapted with permission from [150]. Copyright Elsevier, 2018.

Inverse Opal-Structured Tungsten Oxides
Inverse opal (IO) structure is a kind of 3D layered porous structure. It is favorable for its large specific surface area and artificially-ordered periodic layered configuration. This structure is often achieved by the template-assisted method, in which the template used is opal structure. After the material is deposited on the template, the template is removed, and the IO structure is obtained. Its large specific surface area was a result of the porosity obtained after the removal of template material so that electrolyte penetration can be bettered and the transmission of both electrons and ions are accelerated [146,147]. Owing to the periodicity and uniformity of the template, the final product also has a periodic and uniform structure, thus light reflection and refraction can be enhanced and it is beneficial for effective reduction of visible and NIR light transmittance [146,148,149].
Yang et al. [152] produced an ordered microporous tungsten oxide IO film using PS with different diameters as the template, followed by the process of removing PS through immersing the sample into tetrahydrofuran (THF), following which the porous tungsten oxide film was obtained (Figure 12a,b). Compared with the dense tungsten oxide film, this porous film showed high optical density and coloration efficiency in the NIR area ( Figure  12c,d). They also found that smaller diameter of the porous and higher integration of ordered porous structure can translate into better EC performances. Later, a uniform SnO2-WO3 core-shell IO structure was reported by Nguyen et al. [153], aiming at good control of the transmission of NIR radiation without reduction in the transmission of visible light. They firstly got the SnO2 IO structure on the ITO coated base after removing PS. Following that was the electrodeposition of WO3. Finally, the specific core-shell SnO2-WO3 IO structure was successfully obtained. This EC film displayed high visible transparency of 70.3%, 67.1% at the wavelength of 400 nm at colored state, and blocked 62% of the NIR radiation at the same time. Later, adopting a similar method, Ling et al. [154] also made a TiO2-WO3 core-shell IO structure, which displayed well improved electrochromic performance in the NIR region as well.

Inverse Opal-Structured Tungsten Oxides
Inverse opal (IO) structure is a kind of 3D layered porous structure. It is favorable for its large specific surface area and artificially-ordered periodic layered configuration. This structure is often achieved by the template-assisted method, in which the template used is opal structure. After the material is deposited on the template, the template is removed, and the IO structure is obtained. Its large specific surface area was a result of the porosity obtained after the removal of template material so that electrolyte penetration can be bettered and the transmission of both electrons and ions are accelerated [146,147]. Owing to the periodicity and uniformity of the template, the final product also has a periodic and uniform structure, thus light reflection and refraction can be enhanced and it is beneficial for effective reduction of visible and NIR light transmittance [146,148,149].
Yang et al. [152] produced an ordered microporous tungsten oxide IO film using PS with different diameters as the template, followed by the process of removing PS through immersing the sample into tetrahydrofuran (THF), following which the porous tungsten oxide film was obtained (Figure 12a,b). Compared with the dense tungsten oxide film, this porous film showed high optical density and coloration efficiency in the NIR area (Figure 12c,d). They also found that smaller diameter of the porous and higher integration of ordered porous structure can translate into better EC performances. Later, a uniform SnO 2 -WO 3 core-shell IO structure was reported by Nguyen et al. [153], aiming at good control of the transmission of NIR radiation without reduction in the transmission of visible light. They firstly got the SnO 2 IO structure on the ITO coated base after removing PS. Following that was the electrodeposition of WO 3 . Finally, the specific core-shell SnO 2 -WO 3 IO structure was successfully obtained. This EC film displayed high visible transparency of 70.3%, 67.1% at the wavelength of 400 nm at colored state, and blocked 62% of the NIR radiation at the same time. Later, adopting a similar method, Ling et al. [154] also made a TiO 2 -WO 3 core-shell IO structure, which displayed well improved electrochromic performance in the NIR region as well. Adapted with permission from [152]. Copyright Elsevier, 2012.

Dynamic Control of Visible and NIR Light of Tungsten Oxide ECDs
Different from the aforementioned studies that realized mere transmittance control of visible or NIR light, there were reports about dynamic transmittance control of both at the same time. In other words, when these materials are adopted in EC windows, three modes can be achieved, namely bright mode when both the transmittance of visible and NIR light are relatively high; cool mode when the transmittance of visible light is high while that of NIR light is relatively low; and dark mode when transmittance of both of them is low [155]. Zhang et al. [155] synthesized oxygen-deficient tungsten oxide nanowires that was able to control the transmittance of NIR and visible light independently. The film showed bright mode when the potential applied on active film was 4.0 V, cool mode when the potential was between 2.8 and 2.6 V, and dark mode when the potential was 2.0 V (Figure 13a-d). Reports have shown that amorphous and polycrystalline WO3 have different responses of light; that is, the light absorption peak of amorphous WO3 is more shifted into blue than that of crystalline WO3. Lia et al. [151] prepared WO3 films with hybrid phases to adjust the transmittance of visible and NIR light. It is reported that the WO3 flexible ECD they made had three different modes for the absorption of light in response to the applied voltage because an amorphous and a hexagonal phase of WO3 were both observed in the film. When the applied potential was lower than 1.1 V, the response of the NIR area was more active since the hexagonal portion of WO3 was at play, while under higher voltages, the response of visible area active for the amorphous portion was reduced (Figure 13e,f).

Dynamic Control of Visible and NIR Light of Tungsten Oxide ECDs
Different from the aforementioned studies that realized mere transmittance control of visible or NIR light, there were reports about dynamic transmittance control of both at the same time. In other words, when these materials are adopted in EC windows, three modes can be achieved, namely bright mode when both the transmittance of visible and NIR light are relatively high; cool mode when the transmittance of visible light is high while that of NIR light is relatively low; and dark mode when transmittance of both of them is low [155]. Zhang et al. [155] synthesized oxygen-deficient tungsten oxide nanowires that was able to control the transmittance of NIR and visible light independently. The film showed bright mode when the potential applied on active film was 4.0 V, cool mode when the potential was between 2.8 and 2.6 V, and dark mode when the potential was 2.0 V (Figure 13a-d).
Reports have shown that amorphous and polycrystalline WO 3 have different responses of light; that is, the light absorption peak of amorphous WO 3 is more shifted into blue than that of crystalline WO 3 . Lia et al. [151] prepared WO 3 films with hybrid phases to adjust the transmittance of visible and NIR light. It is reported that the WO 3 flexible ECD they made had three different modes for the absorption of light in response to the applied voltage because an amorphous and a hexagonal phase of WO 3 were both observed in the film. When the applied potential was lower than 1.1 V, the response of the NIR area was more active since the hexagonal portion of WO 3 was at play, while under higher voltages, the response of visible area active for the amorphous portion was reduced (Figure 13e,f). Adapted with permission from [155]. Copyright Royal Society of Chemistry, 2014. (e) Visible-NIR spectra showing the change in absorbance when a voltage is applied on the device, between the on (i.e., negative voltage, reduced tungsten oxide) and the off (i.e., positive voltage, oxidized tungsten oxide) states at 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, 1.7, and 1.9 V; (f) zoom of the spectra obtained with lower voltages. Adapted with permission from [156]. Copyright American Chemical Society, 2012.

Electrochromic Energy Storage Devices (ECESDs)
As mentioned above, tungsten oxide is not only one of the candidates of electrode material in ESDs, including LIBs and SCs, but also an excellent material for ECDs. One device integrating these two functions has come into reality [157,158]. The idea of this integration rests chiefly on the following arguments. Firstly, ESDs share almost the same structure with ECDs, the sandwich structure [7,159]. Secondly, the working mechanisms of these two types of devices are also very similar. They both run relying on the redox reactions of ions in the electrolyte and active electrodes [160]. Thirdly, in the integrated device, tungsten oxide materials can be the electrode of the energy storage part and EC part at the same time [161,162]. Of course, aside from tungsten oxides, many other materials, especially some transitional metal oxides and conductive polymers, can be used as the active material in an integrated device as well. For example, bifunctional devices based on nickel oxide [163], vanadium pentoxide [164], and PANI [165,166] have all been reported before. These bifunctional devices can show us dynamic color signals, of which we can make good use to monitor how the device is running and to judge whether the device needs charging in case of energy cut-off. For another use, the energy stored in ECDs can be further used. Another relation is that the close charging and discharging time of SCs and switching time of EC devices also links them together. Consequently, integration of ECDs and SCs is more common compared with the integration of ECDs and batteries. However, in some situations where switching time does not matter that much, like smart windows and smart sunglasses, integrated ECBs can still have their place. Furthermore, as researchers are making dedicated efforts towards fast charging techniques of batteries, this gap is being filled in. In the following subsection, we will discuss tungsten oxides ECESDs from two main angles: research on single tungsten oxide electrode and exploration on complete ECSCs containing tungsten oxides as an electrode.  [155]. Copyright Royal Society of Chemistry, 2014. (e) Visible-NIR spectra showing the change in absorbance when a voltage is applied on the device, between the on (i.e., negative voltage, reduced tungsten oxide) and the off (i.e., positive voltage, oxidized tungsten oxide) states at 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, 1.7, and 1.9 V; (f) zoom of the spectra obtained with lower voltages. Adapted with permission from [156]. Copyright American Chemical Society, 2012.

Electrochromic Energy Storage Devices (ECESDs)
As mentioned above, tungsten oxide is not only one of the candidates of electrode material in ESDs, including LIBs and SCs, but also an excellent material for ECDs. One device integrating these two functions has come into reality [157,158]. The idea of this integration rests chiefly on the following arguments. Firstly, ESDs share almost the same structure with ECDs, the sandwich structure [7,159]. Secondly, the working mechanisms of these two types of devices are also very similar. They both run relying on the redox reactions of ions in the electrolyte and active electrodes [160]. Thirdly, in the integrated device, tungsten oxide materials can be the electrode of the energy storage part and EC part at the same time [161,162]. Of course, aside from tungsten oxides, many other materials, especially some transitional metal oxides and conductive polymers, can be used as the active material in an integrated device as well. For example, bifunctional devices based on nickel oxide [163], vanadium pentoxide [164], and PANI [165,166] have all been reported before. These bifunctional devices can show us dynamic color signals, of which we can make good use to monitor how the device is running and to judge whether the device needs charging in case of energy cut-off. For another use, the energy stored in ECDs can be further used. Another relation is that the close charging and discharging time of SCs and switching time of EC devices also links them together. Consequently, integration of ECDs and SCs is more common compared with the integration of ECDs and batteries. However, in some situations where switching time does not matter that much, like smart windows and smart sunglasses, integrated ECBs can still have their place. Furthermore, as researchers are making dedicated efforts towards fast charging techniques of batteries, this gap is being filled in. In the following subsection, we will discuss tungsten oxides ECESDs from two main angles: research on single tungsten oxide electrode and exploration on complete ECSCs containing tungsten oxides as an electrode.

Tungsten Oxides Based ECESDs
Approaches to enhance bifunctional performances of tungsten oxides electrode are very similar to those that improve electrochromic performance and energy storage performances. They are merely getting porous nanostructure, doping, and integrating tungsten oxide with other materials, especially organic materials (see Tables 1-3). Figure 14 sketches the main modification methods adopted by researchers. Most often, these methods are not adopted alone but two or three methods are adopted at the same time. Table 3 presents the electrochemical and EC performances of tungsten-based bifunctional electrodes.  performances. They are merely getting porous nanostructure, doping, and integrating tungsten oxide with other materials, especially organic materials (see Tables 1-3). Figure  14 sketches the main modification methods adopted by researchers. Most often, these methods are not adopted alone but two or three methods are adopted at the same time. Table 3 presents the electrochemical and EC performances of tungsten-based bifunctional electrodes.  Nanostructures can significantly facilitate the electrochemical activities of the electrode. For example, by hydrothermal method, He et al. [167] made different tungsten oxide nanostructures including nanospindles, nanopetals, nanosheets, and nanobricks. Among them, nanosheets had better EC performance of wider optical contrast, faster switching speed, higher coloration efficiency and capacitive performance of greater areal capacitance owing to its significantly increased active sites, and facilitated Li + ions diffusion by the large surface area and porous structure. Soon after, their group [168] achieved WO 3 ·H 2 O nanosheets by a one-step citric acid-assisted hydrothermal method with no need for a seed layer under a relatively low temperature of 90 • C. The film possessed large optical modulation of 79.0% at 633 nm and high areal capacitance of 43.30 mF cm −2 . When the film was fully charged, its color turned blue. Wang et al. [161] fabricated a mesoporous WO 3 film by a template assisted sol-gel method. It had faster switching time and higher storage capacity compared with microporous WO 3 film owing to the reduced diffusion length and more exposed active sites in the mesoporous WO 3 film. Later, their group [170] also made Nb-doped WO 3 mesoporous cathode film by a sol-gel method. The results showed that slight doping of Nb enables the film to have wider optical modulation range, shorter switching speed, and higher capacitance because the introduction of the Nb element was accompanied by the introduction of O vacancy which increased the conductivity of the electrode. Other studies [162,171] reported that Mo-doped WO 3 film had amorphous and porous structure, and was able to offer more channels for fast ion transfer and active sites for redox reaction.
Many organic materials have large energy storage capacity and outstanding conductivity and are also good candidates for EC materials and have colorful color changes when voltage changes. When inorganic and organic materials are integrated together, some synergistic effects will occur. As an inorganic material, tungsten oxide has high electrochemical stability and long cycling life span but poor conductivity. When the former organic materials integrate with tungsten oxides, the final performance of the composite can be improved and the weaknesses of the single component can be attenuated. For example, the single-color display problem of tungsten oxide can be solved. One popular integrating idea is adding a material that can form donor-acceptor systems with tungsten oxide based on tungsten trioxide being an n-type semiconductor. It has been reported that this kind of donor-acceptor pair can have a strong combining force and it follows that the stability of the composite is highly strong so that the composite may have long life span [176]. On the other hand, the transport of ions and electrons will be facilitated. In addition, owing to the introduction of the organic material, the voltage window of the composite can also be widened. Many researchers have designed novel donor-acceptor pairs. Wei et al. [172] electropolymerized PANI onto the surface of tungsten oxide to form nanocomposite. They observed an obvious reduction of oxide peak currents in the CV curves of the nanocomposite compared to pure PANI owing to the donor-acceptor system. Results showed that not only did the nanocomposite have higher color efficiency (98.4 cm 2 C −1 ) than pure tungsten oxide film (36.3 cm 2 C −1 ) and pure PANI film (50.0 cm 2 C −1 ), but also that its cycling performance was much more stable. Additionally, the potential window of the nanocomposite is wider than that of the pure PANI. Similarly, Nwanya et al. [173] also mentioned the slight reduction in CV curves of the WO 3 /PANI composite, which led to a more reversible redox reaction of the composite. Zhang et al. [174] used urchin-like WO 3 @PANI composite and showed that the composite electrode had better cycling stability than pure WO 3 and maintained a reversible capacity of 516 mA h g −1 after 1200 charge-discharge cycles (Figure 15a,c). Furthermore, the colors of the composite were enriched, showing purple, green, yellow, gray, to blue as voltage changed (Figure 15b). Other organic materials have also been added to form this similar system. Guo's group prepared the composite of WO 3 and poly (indole-5-carboxylic acid) (P5ICA), where P5ICA is a p-doped material [158], and nanocomposites of WO 3 and poly(5-formylindole) (P5FIn) where P5FIn acted as the p-doped semiconductor [175]. Both composite electrodes showed high capacitance and excellent cycling stability.

Quantitative Judgement of Energy Level Function of Tungsten Oxide Based ECESDs
Full tungsten oxide-based bifunctional devices provide a wider voltage window, possess larger capacity and have richer colors if an appropriate counter electrode is selected. Similar to tungsten oxide-based ECDs, the counter electrode of the device needs to be transparent. In order to get higher capacitance, a counter electrode with large capacitance is preferred. There have been reports [179,180] about the basic introduction of ECESDs, therefore, here, we just focus on an interesting characteristic of these devices.
There is a relation between energy storage level (ESL) of the tungsten oxide-based bifunctional device and its color, which is why we can get a direct qualitative message of the device's state. Yet, in order to get precise judgement about the energy storage state, Figure 15. (a) The galvanostatic charge-discharge profiles of the urchin-like WO 3 @PANI electrode at current density of 0.2 A g −1 ; the photographs of (b) WO 3 and WO 3 @PANI under different voltages; (c) the durability test of the urchin-like WO 3 @PANI composite film for 1200 cycles at a wavelength. Adapted with permission from [174]. Copyright Springer Nature, 2010. (d) Optical density variation with respect to the charge density at 633 nm. Adapted with permission from [177]. Copyright John Wiley and Sons, 2015. (e) Basic structure and mechanism of the in situ monitoring system composed of PANI//WO 3 ECSCs and CsPbBr 3 perovskite photodetector. Adapted with permission from [178]. Copyright John Wiley and Sons, 2019.

Quantitative Judgement of Energy Level Function of Tungsten Oxide Based ECESDs
Full tungsten oxide-based bifunctional devices provide a wider voltage window, possess larger capacity and have richer colors if an appropriate counter electrode is selected. Similar to tungsten oxide-based ECDs, the counter electrode of the device needs to be transparent. In order to get higher capacitance, a counter electrode with large capacitance is preferred. There have been reports [179,180] about the basic introduction of ECESDs, therefore, here, we just focus on an interesting characteristic of these devices.
There is a relation between energy storage level (ESL) of the tungsten oxide-based bifunctional device and its color, which is why we can get a direct qualitative message of the device's state. Yet, in order to get precise judgement about the energy storage state, exploration of the accurate relationship between optical parameters and electrochemical parameters or getting help from other detectors is needed. We know that the more charges inserted into the EC film, the deeper the color displayed on the tungsten oxide-based bifunctional device and the higher ESL it has. It follows that we can get the qualitative monitoring by the relationship of transmittance and ESL. Coloration efficiency (CE) is the bridge between optical parameters and electrochemical parameters. Their relationship is expressed by the following equation [181,182]: where Q is the inserted charge density, T b and T c are transmittances of bleached state (fully discharged state) and colored state (fully charged state), respectively. Because the CE of a device is almost fixed, we can get rough qualitative information of ESL through the relation of Q and log (T b /T ESL ), as in the reports of Cai's group [177] (Figure 15d), Bi's group [183], and He's group [167]. They painted the curve of optical density and charge density, though the curve is often used to calculate CE. At the same time, it reflected the rough correspondence between optical parameters and electrochemical parameters. Guo et al. [158] further changed the form of the formula into a more direct expression: where Q t is the total charge that can be stored in the device, T 0 is the transmittance of the device fully discharged, and T ESL is the real-time transmittance of the device. They achieved quantitative monitoring of device by getting the linear link of ESL and the optical density at the wavelength of 600 nm. Moreover, Sun et al. [178] successfully achieved the in-situ monitoring of the ESL of bifunctional device constantly and accurately with the help of an inorganic CsPbBr 3 perovskite photodetector, based on the PANI//WO 3 bifunctional ECESD they reported earlier (Figure 15e). This photodetector detects according to the change of response current to a green laser with the wavelength of 520 nm penetrated through the bifunctional device. It was so sensitive that even a voltage difference as small as 47.2 mV (charge variation of 0.33 mC) could be detected.

Solar Cell and Tungsten Oxide-Based EC Integrated Multifunctional Devices
The perfect use of tungsten oxide-based bifunctional ECESDs is the use of smart windows, as it can control the brightness and temperature in the room under a comfortable state by changing its transmittance of sunlight [184]. In addition, they can serve as a power to drive small electrical appliance like bulbs. Based on the position of this device, the introduction of a photovoltaic solar cell is natural and feasible. This idea is backed by the following reasons. Firstly, the open circuit voltage of the solar cells matches the relevantly low driving voltage of the tungsten oxide-based bifunctional device well [30,140]. Secondly, their positions in a building are not exclusive so that they can operate free of the influence of each other, thus achieving highly efficient energy use [185].
Generally, the introduction of a solar self-powering part may have two main different forms. One is that the ECESD and the sunlight conversion device are separate and they are connected through wires outside the device. For example, Zhong's group [59], Bi's group [31] (Figure 16a,b), and Xie's group [55] connected the tungsten oxide-based ECESD with silicon solar cell, Xia's group [166] with perovskite solar cell, and Xie's group [186] with dye-sensitized solar cells to achieve a self-powering system. Results showed that the bifunctional device can be fully charged without external power. Then, they can output energy in many forms including lighting up an LED bulb and the whole process can be continuously monitored by the color on the bifunctional device. Xu et al. [184] designed a new structure of the integrated system in which a tungsten oxide-based bifunctional device played as the glass and the fiber-shaped dye-sensitized solar cells served as the frame of the window so that space can be effectively used (Figure 17a). The shining point of this new system is that the numbers of the solar cells can be changed and when converting, they are free of the incidence angle since the frame was designed in a cylindrical configuration.  In addition to merely powering the bifunctional device, the whole self-powering smart window system can modulate its transmittance automatically in response to the changing weather condition. That is to say, when it is sunny, the solar cell can generate enough power to turn the window into dark state so that most of the sunlight is blocked to keep the room with a comfortable atmosphere and when it is cloudy, the generated  In addition to merely powering the bifunctional device, the whole self-powering smart window system can modulate its transmittance automatically in response to the changing weather condition. That is to say, when it is sunny, the solar cell can generate enough power to turn the window into dark state so that most of the sunlight is blocked to keep the room with a comfortable atmosphere and when it is cloudy, the generated In addition to merely powering the bifunctional device, the whole self-powering smart window system can modulate its transmittance automatically in response to the changing weather condition. That is to say, when it is sunny, the solar cell can generate enough power to turn the window into dark state so that most of the sunlight is blocked to keep the room with a comfortable atmosphere and when it is cloudy, the generated power is not enough for the window to turn dark and light can partly penetrate through so that the room is still bright. This process was achieved in a report by Yun et al. [188] who took the solar cell as an automatic smart controller to modulate the transmittance of the tungsten-based bifunctional device according to the weather.
The other type is that one device has three functions at the same time, namely solarelectricity conversion, EC and energy storage and powering, with no need for external wires. The EC part and PV part share the same electrolyte and typically, this kind of integrated method has to sacrifice a certain area of the window. For example, Zhang et al. [30] designed a trifunctional device, photoelectrochromic device (PECD), in which WO 3 and TiO 2 were both used as the anode and Pt was used as the counter electrode (Figure 18a,b). When the device is open circuit and exposed to sunlight, dye molecules in the electrolyte are excited and electron-hole pairs are generated. Then, the electrons are injected into the conduction band of TiO 2 and later diffused to the ITO-PET substrate and then WO 3 , so WO 3 film turns blue and the sunlight is blocked by the device as a result. The colored device can discharge when connected with electric applications. Based on tungsten oxide and PANI as the cathode and Al as the anode, Chang et al. [187] developed a multifunctional device that can be powered through two methods, by sunlight and by being exposed to the air (Figure 17b). Compared to the charging method without sunlight radiation, the time needed for the device to be fully charged is six times shorter. power is not enough for the window to turn dark and light can partly penetrate through so that the room is still bright. This process was achieved in a report by Yun et al. [188] who took the solar cell as an automatic smart controller to modulate the transmittance of the tungsten-based bifunctional device according to the weather. The other type is that one device has three functions at the same time, namely solarelectricity conversion, EC and energy storage and powering, with no need for external wires. The EC part and PV part share the same electrolyte and typically, this kind of integrated method has to sacrifice a certain area of the window. For example, Zhang et al. [30] designed a trifunctional device, photoelectrochromic device (PECD), in which WO3 and TiO2 were both used as the anode and Pt was used as the counter electrode ( Figure  18a,b). When the device is open circuit and exposed to sunlight, dye molecules in the electrolyte are excited and electron-hole pairs are generated. Then, the electrons are injected into the conduction band of TiO2 and later diffused to the ITO-PET substrate and then WO3, so WO3 film turns blue and the sunlight is blocked by the device as a result. The colored device can discharge when connected with electric applications. Based on tungsten oxide and PANI as the cathode and Al as the anode, Chang et al. [187] developed a multifunctional device that can be powered through two methods, by sunlight and by being exposed to the air (Figure 17b). Compared to the charging method without sunlight radiation, the time needed for the device to be fully charged is six times shorter.

Other Applications of Tungsten Oxides-Based Materials
Tungsten oxide is also an outstanding material in the photochromism field. When light is shed on tungsten oxide, electron-hole pairs are generated, leading to the change of optical absorption of them. What reflects directly to us is their color changes, and they can be applied as displays, smart windows, and optical signal processing. Wei et al. [189] indicated intrinsic WO3 with color change between yellow and blue when exposed to UV illumination. Zhang et al. [190] also synthesized tungsten oxide@poly(N-isopropylacrylamide) composite spheres that had excellent response speed and impressed coloration efficiency. Finally, Wang et al. [191] have a comprehensive review about tungsten oxidesbased photochromic materials.
The bandgap of WO3 in bulk is around 2.6 eV [192], so that it can absorb light with wavelengths up to 475 nm, indicating that it is a superior photocatalyst in the visible light area compared with TiO2 of which the band gap is 3.2 eV [33]. Currently, tungsten oxidebased materials as photocatalysts, are widely applied in CO2 photoreduction [193], air purification [194], and other fields. Specifically, in the photoelectrochemical cell (PEC)

Other Applications of Tungsten Oxides-Based Materials
Tungsten oxide is also an outstanding material in the photochromism field. When light is shed on tungsten oxide, electron-hole pairs are generated, leading to the change of optical absorption of them. What reflects directly to us is their color changes, and they can be applied as displays, smart windows, and optical signal processing. Wei et al. [189] indicated intrinsic WO 3 with color change between yellow and blue when exposed to UV illumination. Zhang et al. [190] also synthesized tungsten oxide@poly(N-isopropylacrylamide) composite spheres that had excellent response speed and impressed coloration efficiency. Finally, Wang et al. [191] have a comprehensive review about tungsten oxidesbased photochromic materials.
The bandgap of WO 3 in bulk is around 2.6 eV [192], so that it can absorb light with wavelengths up to 475 nm, indicating that it is a superior photocatalyst in the visible light area compared with TiO 2 of which the band gap is 3.2 eV [33]. Currently, tungsten oxide-based materials as photocatalysts, are widely applied in CO 2 photoreduction [193], air purification [194], and other fields. Specifically, in the photoelectrochemical cell (PEC) water-splitting field, tungsten oxides are also widely used. For example, J. Fu et al. [195] produced 2D/2D WO 3 /g-C 3 N 4 and it showed good H 2 -production activity. Moreover, owing to its relative lower band gap, tungsten oxides are often applied to composite with TiO 2 to expand the photoresponse of the composite into the visible light range [196,197]. For example, the composite of WO 3 /TiO 2 made by Castro et al. [197] has a bandgap energy of 3.23 eV, which is between the values of pristine TiO 2 and WO 3 , resulting in better light absorption at the visible range and higher efficiency of water splitting.
Another useful application of tungsten oxides is as gas sensors. As mentioned above, WO 3 is a kind of n-type semiconductor of which the conductivity largely depends on the electrons within it. When it touches reductive gas, its conductivity will increase, while when it is exposed to oxidative gas, it decreases. By having the data of the conductivity changes, we can realize the detection of specific gases, like H 2 , H 2 S [198], CO, NH 3 [199], NO x [200,201], and some organic gases [202,203]. For instance, Liang et al. [203] produced ultra-thin WO 3 nanosheets with dominant {002} crystal facets that have been proven to have improved xylene-sensing performance. Ji et al. [199] reported that tightly arranged nanosheet-assembled flower-like WO 3 nanostructures showed better NH 3 -sensing performances compared with tightly arranged WO 3 nanostructures.

Summary and Outlook
In summary, tungsten oxides are attractive for their numerous possibilities in various fields, particularly in energy storage like LIBs, SCs, and electrochromisms. Tungsten oxide-based multifunctional devices have also been widely explored based on the links between ESDs and ECDs. Furthermore, integration of solar converting system is a very effective way to realize a green application. Despite that the fact that much effort has been made in the research of tungsten oxides, there are many challenges to deal with. When tungsten oxides are used as the electrode in ESDs, low specific capacity, poor electric conductivity, and unsatisfied cycling stability still need to be improved. Furthermore, research about the tungsten oxide-based full ESDs are still rare. When they are applied in ECDs, their performance in the NIR area needs more attention. The bifunctional applications mentioned in this article also have weaknesses like single color, narrow voltage window, and low capacity, restricting their application to real uses. Apart from the aforementioned integration with solar cells, tungsten oxide-based devices call for new functions. For novel applications, such as flexible devices and self-powering methods by air or electrochemical reactions that have been reported before, improvements are still needed.