Electrochromism is the reversible modulation of a material's optical properties in response to an applied electrical voltage in the presence of an electrolyte. Electrochromic materials find broad utility in technologies such as energy-saving smart windows, anti-glare rearview mirrors, adaptive displays, and responsive wearable devices. [1][2][3] The growing demand for tunable optical components has driven extensive research into both the discovery of new materials and a deeper mechanistic understanding of established materials to enable precise control over electrochromic behavior. Among the electrochromic materials explored to date, tungsten oxide (WO3) stands out due to its pronounced color change-from transparent to deep blue-upon cathodic polarization. 4 This coloration is associated with the reversible formation of tungsten bronzes, AxWO3, where A represents an intercalated cation such as H⁺, Li⁺, or Na⁺. 5 Cation-insertion coupled electron transfer into the WO3 framework to form AxWO3 modifies its electronic structure and optical absorption characteristics. Fundamentally, this behavior stems from the electronic band structure of WO3: the valence band is dominated by fully occupied O 2p states, while the conduction band comprises empty W 5d orbitals, yielding a semiconductor with a band gap of approximately 3.0-3.3 eV in its bleached (transparent) state. 6 Electron transfer leads to partial reduction of W 6+ and population of the conduction band or localized states in the band gap, contributing to light absorption in the "colored" state. 7 At the atomic level, electrochromism in WO3 is determined by the local structure of tungsten metal centers. Subtle structural distortions, such as octahedral tilts, bond length asymmetries, and ligand identity changes can induce shifts in d-orbital splitting energies and thus electrochromic behavior. This connection opens opportunities to tailor electrochromic performance through precise structural modifications.
One such structural modification involves the incorporation of structural water into the tungsten oxide lattice, yielding crystalline and amorphous hydrates, WO 3 •nH 2 O. [8][9][10] These hydrated oxides adopt distinct crystal structures and possess different ion transport properties relative to anhydrous WO3. Early investigations into hydrous tungsten oxides noted enhanced optical switching speeds, attributed to faster solid state ion transport in the hydrous polymorphs. 8 Building on these findings, our previous work 10 explored how structural water impacts not just the inserted ion mobility but the nature of optical modulation. We compared the electrochromic behavior of crystalline γ-WO3 and orthorhombic WO3•H2O in a non-aqueous Li⁺ electrolyte. Our studies revealed that the hydrate exhibited dual-band electrochromism, where initial polarization led to decreased optical transmission in the near-infrared (NIR) spectrum followed by visible light attenuation at higher states of charge. In contrast, γ-WO 3 displayed simultaneous changes in both spectral regimes across all states of charge. We hypothesized that the open framework of WO3•H2O supports an extended solid-solution insertion region, in contrast to the more constrained γ-phase. This further led us to hypothesize that solid solution insertion would facilitate electrochromic performance at low temperatures. However, the influence of structural water on the electronic structure remained unresolved, as did the influence of the temperature.
The present study aims to bridge that gap by investigating the effects of structural water and temperature on the electrochromic response of crystalline tungsten oxides, this time focusing on protons as the charge-compensating electrolyte species. We conducted operando UV/Vis/NIR spectroelectrochemical experiments on thin films of γ-WO3 and monoclinic WO3•2H2O in aqueous inorganic acidic electrolytes between -20°C to 20°C. We performed complementary density functional theory (DFT) calculations to elucidate the electronic density of states (DOS) and provide atomic-level insight into structure-optical property relationships. Our results reveal that the distinct electrochromic behaviors of γ-WO 3 and monoclinic WO 3 •2H 2 O stem from differences in the local coordination environment of tungsten induced by the presence of structural water. We further find that in strong acid electrolytes, the electrochromic response improves at low temperatures due to decreased influence of the parasitic hydrogen evolution reaction (HER).
These findings deepen the mechanistic understanding of electrochromism in tungsten oxides and highlight strategies to fine-tune the electrochromic properties of transition metal oxides through rational control of the crystal structure and transition metal coordination.
Chemicals: All chemicals were used as received. Sodium tungstate dihydrate (Na2WO4ᐧ2H2O; 99+%), platinum wire (0.5 mm diameter, 99.99% trace metals basis), and FTO-coated glass (2.2 mm thickness, 7 Ω/sq) were purchased from Millipore Sigma. Concentrated sulfuric acid (H2SO4; 95.0 -98.0%, ACS Grade), potassium hydroxide (KOH, Certified ACS), and ammonium hydroxide (NH4OH, Certified ACS Plus) were purchased from Fisher Scientific. Argon gas (industrial grade) was purchased from Arc3 Gases. Hydrogen peroxide (30% by weight in H2O, ACS) was purchased from Macron Fine Chemicals.
Electrodeposition: WO3•2H2O thin films were prepared by electrochemical deposition onto cleaned fluorinated tin oxide (FTO)-coated glass via a previously reported method. 11 Briefly, the deposition solution was prepared by heating 25 mL of a 12 mM aqueous solution of Na2WO4ᐧ2H2O in deionized (DI) water to 75°C. 687 μL of concentrated H2SO4 was added to the heated Na2WO4ᐧ2H2O solution while stirring to obtain a solution with a concentration of 0.5 M H2SO4. The electrodeposition solution was placed in a 50 mL glass three-neck-flask (Kontes). The working electrode was 0.5 x 3 cm FTO-coated glass, the counter electrode was platinum wire, and the reference electrode was Ag/AgCl in saturated KCl (Pine Instruments). Films were electrodeposited onto a 1 cm 2 geometric area of the working electrode via cyclic voltammetry between -0.2 and 1 V vs. Ag/AgCl at 100 mV/s for 3 hours (450 cycles). After electrodeposition, the working electrode was immersed in 0.5 M H2SO4 overnight to promote film crystallinity. Anhydrous WO3 films were prepared by heating the as-electrodeposited WO3•2H2O films at 350°C in air for 12 hours. FTO-coated glass was cleaned by sonicating for 10 minutes in soapy water, acetone, ethanol, 1 M KOH, deionized (DI) water, and a basic piranha solution consisting of NH4OH, 30% hydrogen peroxide, and DI water, respectively. Physical Characterization: Room-temperature Raman spectra were collected using a Witec Alpha300r confocal microscope. Spectra were acquired using a 532 nm Nd:YAG laser at 100x magnification (Zeiss), averaged over 7 scans of 8 seconds each. Cryo-electron microscopy (cryo-EM) was performed on an FEI/Thermo Fisher Titan Themis cryoS/TEM microscope operated at 300 kV with a Gatan 626 cryo-transfer holder. Measurements were acquired at liquid nitrogen temperatures, and the sample temperature was estimated as the sample holder temperature, measured by a Gatan model 1905 temperature controller. In situ heating to 25 °C was performed with the same temperature controller and images were acquired after the temperature stabilized.
Temperature-dependent UV/VIS spectroelectrochemistry was obtained using an Ocean HDX spectrometer (OceanOptics) equipped with a tungsten halogen light source (OceanOptics HL-2000) and a Qpod 3e temperature-controlled cuvette holder (Quantum Northwest). Spectra were collected using 0.1 second acquisition time, averaged over 10 spectra. Temperatures down to -20°C were achieved by cooling the temperature-controlled cuvette holder with a constant flow of ice-water and Ar gas (to prevent condensation). The electrochemical cell consisted of a quartz cuvette (Perkin Elmer 10 mm path length) containing WO3•2H2O or WO3 on FTO-coated glass as the working electrode, Pt wire as the counter electrode, and a leakless Ag/AgCl in saturated KCl reference electrode. A custom-made cuvette cap was 3D-printed using a polylactic acid filament to hold the electrodes in place within the cuvette. The electrolyte was 3 mL of 5 M H 2 SO 4 . Cyclic voltammetry (CV) was performed with a BioLogic MPG-2 potentiostat at scan rates between 1 and 100 mV/s.
The density of states (DOS) was calculated with spin-polarized density functional theory (DFT) calculations, which were performed using the CP2K software package. 12 Unit cells were optimized using the Perdew-Burke-Ernzerhof functional with Grimme's 3 rd order dispersion corrections (PBE-D3). PBE-D3 is known to accurately predict lattice parameters. 13,14 Subsequently, the Heyd-Scuseria-Ernzerhof (HSE06) [15][16][17] range-separated hybrid exchangecorrelation functional was used to calculate the DOS on the PBE-D3-optimized cells. Hybrid functionals are known to improve the accuracy of the calculated DOS when compared with commonly used generalized gradient approximation (GGA) functionals. 18,19 Goedecker, Teter, and Hutter pseudopotentials were used to approximate the effect of core electrons.
20,21 Double-ζ valence plus polarization (DZVP) basis sets were used for W, and triple-ζ (TZVP) for H, O atoms.
A 600 Ry kinetic energy cutoff was applied for plane-wave expansion as implemented in the mixed Gaussian and plane-waves method in CP2K. 22 The HSE06 calculated DOS was constructed using the auxiliary density matrix method. 23 The cFIT10 auxiliary basis set was used for W, while cFIT3 was used for H, O. DOS were shifted by the Fermi level, EF, that corresponds to the energy of the highest occupied molecular orbital (HOMO) in our calculations. The degree of protonation,
x, is equal to 0.5 for the protonated systems in the DFT calculations.
Additional information on the computational procedures (Section 1-3 and Figures S1- S7), physical and electrochemical characterization of the materials (Section 4, Figures S8-S11)
and electrochemical kinetic analysis (Section 5, Figure S12, Table S4) can be found in the Supplementary Information, SI. [24][25][26][27][28][29][30][31]
We investigated the temperature-dependent UV-VIS spectroelectrochemistry of tungsten oxides using electrodeposited thin films of WO3ᐧ2H2O and WO3. We used Raman spectroscopy (Figure S8), SEM (Figure S9) 25 , and cryo-TEM (Figure 1) to determine the crystal structure and microstructure of the deposited films. Raman spectroscopy shows that electrodeposition led to monoclinic WO3ᐧ2H2O thin films while SEM images show that they consist of platelet-like nanoparticles. Annealing the films at 350°C in air led to the formation of γ-WO3 with a similar microstructure. We performed cryo-TEM to study the dehydration mechanism of a single WO3ᐧ2H2O platelet (synthesized via a precipitation reaction 32 ) and determine whether it influenced the microstructure. This is important because the electrochromic response is sensitive to the particle size and shape. 33,34 Dehydration took place via in situ heating from -195°C to 25°C under vacuum (10 -8 Torr). Figure 1 shows that the dehydration of a platelet of monoclinic WO3ᐧ2H2O to γ-WO3 occurs with no change in the platelet shape, although the overall thickness decreased from 208 Å to 169 Å. This is expected given that the interplanar spacing of the WO3ᐧ2H2O (010) is 6.9
Å and removal of the interlayer water should lead to a higher density crystal structure. As a result, in line with the SEM images in Figure S9 25 , dehydration of WO3ᐧ2H2O leads to minimal microstructural changes and we can associate differences in the electrochromic response with differences in their crystal structure. heating from -195°C to 25°C under vacuum. At -195°C (a) the platelet is predominantly monoclinic WO3ᐧ2H2O as shown by the presence of crystal planes with an interlayer spacing of ~6.9 Å. Partial dehydration of the platelet under vacuum was observed as indicated by some collapsed layers with a decreased interlayer spacing of ~5.6 Å. After heating to 25°C (b), the structure of the entire platelet is consistent with γ-WO3 (lattice spacing ~5.5 Å), indicating successful complete dehydration.
We performed operando UV/Vis/NIR spectroelectrochemistry during cyclic voltammetry in a strong acid electrolyte, 5 M H2SO4, at varied scan rates and at room temperature (20°C). We selected this acid concentration because aqueous solutions of 30 -50 wt% H2SO4 have a freezing point below -20°C, thus allowing us to perform spectroelectrochemistry over a broad temperature range. 35 Upon cathodic polarization in acidic electrolytes, tungsten oxides (WO3ᐧnH2O where n = 0 or 2) undergo proton-insertion coupled electron transfer (PICET) which leads to an electrochromic response as the initially d 0 metal cation undergoes a 1-electron transfer. This is described by the following reversible electrochemical reaction in a strongly acidic aqueous electrolyte, where the proton donor is H3O + and x is < 1:
For an insertion process like PICET, the cyclic voltammogram (CV) response is sensitive to the crystal structure of the insertion host. 36 Correspondingly, the room temperature CV of WO3 (Figure 2a) exhibits two pairs of redox peaks at 5 mV/s, labeled 1/1' and 2/2'. We attribute the large current magnitude of peak 2 to the overlap of PICET with the onset of the HER, a competitive proton-coupled electron transfer (PCET) reaction at low potentials. Transmission spectra collected at these redox peaks (Figure 2b) indicate broad-spectrum decreased transmission (increased absorption) accompanying peak 1, which is completely reversed at peak 1'. The 1'. c) Color-mapped operando electrochemical UV/Vis/NIR transmission spectra from 400 -1100 spectra at selected pointed during cyclic voltammetry: initial (open circuit, point 0), peak 3, peak 4, peak 4', and peak 3'. c) Operando electrochemical UV/Vis/NIR transmission spectra from 400 -1100 nm. The corresponding CV data is shown in the left panel.
We further examined the electrochromic behavior by differentiating the transmission response of the tungsten oxides with respect to time at specific wavelengths, ΔT(%)/Δt(sec). By plotting the resulting rate-of-change data against the applied potential, we constructed a spectral analog of a CV. This juxtaposition of voltammetry data with the correlated differential UV/Vis data was utilized previously to characterize electrochromic mechanisms and charge storage processes. 10,37 We selected two wavelengths for this analysis, 550 nm and 1050 nm, to compare the response in the visible and near-IR regions, respectively. At 20°C, we observe peaks in the WO3 spectral data which correlate with the first redox couple (1/1') in the CV (Figure 4a). The 550 nm peaks occur at lower potentials than the 1050 nm peaks by 50 mV. We also observe that there is little-to-no change in transmission at either wavelength for the low-potential redox couple
(2/2'). This may be due to the formation of hydrogen bubbles on the electrode surface from the HER, which could obfuscate the electrochromic response of the underlying film in this transmission-based measurement. The differential transmission of WO3•2H2O shows coloration at 1050 nm over a broad potential range, with the greatest changes coinciding with redox peaks 3 and 4 (Figure 4b). However, at 550 nm, the electrochromic response is decoupled from the CV response, exhibiting a gradual increase in differential transmission as a function of potential. This analysis further emphasizes that while both WO3 and WO3•2H2O undergo PICET, their CV and electrochromic response differ substantially. into WO3ᐧ2H2O for two cycles under the same conditions as for WO3 in a).
To understand this key difference in the room temperature electrochromic behavior between WO3 and WO3•2H2O, we considered the differences in their solid-state structures and more specifically the local coordination environment of tungsten (Figure 5). In monoclinic WO 3 , W 6+ is coordinated by six oxygens but they do not form an ideal octahedron: tungsten is displaced from the octahedral center producing three shorter and three longer bonds (Figure 5). In the extended structure, these distorted octahedra connect via corners with non-linear W-O-W bond angles, termed octahedral tilting, which has a direct influence on the electronic properties of perovskite-like oxides like WO3. 38 In monoclinic WO3•2H2O, W 6+ also resides off the polyhedral center and is coordinated by five oxygens with one water molecule occupying a terminal ligand position (Figure 5). The W-OH2 bond is longer (2.3 Å) than the W-O bonds, which range from 1.7 to 1.9 Å. 39 This lengthening of one apical W-OH2 bond leads to further distortion than in the case of WO3, such that the coordination environment of W 6+ in WO3•2H2O approaches that of a square pyramid.
Crystal field theory (CFT) is a simple model that allows us to make predictions about the arrangement of W d-orbitals in different coordination environments. Here we consider increasing octahedral distortion along the continuum between ideal octahedral and ideal square pyramidal coordination (Figure 5). Relative to the octahedral case, W d-orbitals with z-components (dxz, dyz, and dz 2 ) decrease in energy, while orbitals which overlap the x-y plane (dxy and dx 2 -y 2 ) increase in energy, resulting in the square pyramidal arrangement of d-orbitals. 40 We regard the coordination of W in WO3 to be closer to, but not exactly like, the ideal octahedral coordination, while W approaches square pyramidal coordination in WO3•2H2O. This coordination environment, which we considered from the standpoint of a single octahedron, has a direct influence on the electronic structure of tungsten oxides, since the conduction band is primarily composed of the W d-states, and thus the electrochromic response.
Upon electrochemical PICET, WO3 undergoes two structural transitions associated with the two redox features observed in the CV. The 1/1' redox couple is associated with the transition between monoclinic WO3 and a tetragonal hydrogen bronze (t-HxWO3) and the 2/2' redox couple is associated with the transition between t-HxWO3 and a cubic bronze (c-HxWO3). 18 These structural transitions correspond with changes in the pattern of octahedral tilting as well as changes in the local coordination environment of the W-centers, where the W-atom shifts to the centroid of the polyhedral unit and the W-O bond lengths become more similar in length.
Extending this CFT analysis to these hydrogen bronze phases, we see that as the degree of PICET increases in WO3, the polyhedral units become more similar to a perfect octahedral coordination environment (Figure 5). For WO3•2H2O, our previous results indicate that no discrete structural transitions occur during PICET, instead we propose that there may be changes to the degree of octahedral tilting and W-distortion 18 , albeit the presence of the axial water molecule inherently distorts the coordination geometry. Though CFT is a simple tool, the prediction of the relative energy of W d-orbitals is corroborated by the DFT-calculated DOS for WO3 and WO3•2H2O. Tungsten coordination in WO3 and the HxWO3 structures is more like the ideal octahedral case while in WO3•2H2O it approaches the square pyramidal case. 41 We calculated the DOS of pristine and proton intercalated tungsten oxides (x = 0.5) to validate the orbital splitting hypothesis developed with CFT. As expected, DOS results show that in the pristine oxides (i.e., before PICET), most oxygen states occupy the valence band, while most low-lying conduction band states are composed of W d orbitals (Figure 6 and Figures S1 -S2) 25 . In WO3•2H2O, we found a narrow energy band centered at approximately 2.9 eV above EF. This narrow band arises from the dz 2 orbitals in the model, consistent with the CFT prediction that the dz² orbital decreases in energy due to changes in axial bond lengths. However, our CFT hypothesis could not adequately predict that the dz 2 orbital would shift to lower energies than the dxy, dxz, and dyz orbitals , as suggested by DFT. This inadequacy results from the differences in the axial bond lengths as well as the different identities and field strengths of the ligands (O 2-vs.
H2O) in these axial positions. No such narrow band is observed for WO3, where a continuum of low-lying states instead originates from these molecular orbitals (dxy, dxz, and dyz). The DOS calculations on the pristine oxides are therefore consistent with the prediction from CFT and confirm the presence of a narrow conduction band in WO3•2H2O which we hypothesize is critical to explain its unique electrochromic response. 0.5): Total (a -pristine, c -protonated), and projected on W atoms (b -pristine, d -protonated). The inset figures on panels (b) and (d) focus on areas of interest and show the DOS projected on the W dz 2 orbital. DOS profiles are normalized by their respective maximum values. States that considerably contribute to W -d orbitals are shown with colored arrows based on corresponding systems. Right panel: visualization of the LUMO (around W atoms). Blue and green solid surfaces represent opposite phases of the wave function. Orientation vectors in Cartesian space: x -red, y -green, z -blue. Atom color code: W -ochre, O -red, H -white. Upon PICET to form H0.5WO3•2H2O, dz 2 remains the dominant contributor to the LUMO in Figure S3 and Table S3. 25 Profiles of DOS projected on the specific orbital types are provided in Figures S4 -S7. 25 In the case of PICET forming H0.5WO3, while dxy is the major contributor to the LUMO in WO3, there is a comparable contribution from dxz and dyz orbitals to the LUMO of H0.5WO3 (Figure S3) 25 consistent with the CFT predictions for the more symmetric coordination environments of cubic H0.5WO3. Consequently, distinct dxz and dyz states at low energies in the conduction band of WO3 overlapped after PICET contributing to the same DOS peak, i.e., the peak corresponding to the LUMO (at ~ 0.5 eV). Furthermore, mixed contributions from dxy, dxz, dyz orbitals are observed in the rest of the H0.5WO3 low-energy DOS peaks in the conduction band.
We evaluated the effect of water on the DOS of WO3 at various degrees of hydration and states of charge. In this context, DOS is recalculated in the absence of water, i.e., by removing water molecules while keeping the tungstate structure frozen. The two different types of water molecules in WO3•2H2O are gradually removed. First, H-bonded water molecules are removed, while W-coordinated water molecules remain in the structure, resembling the structure of the monohydrate analogue (WO3•H2O). Then, the coordinated water molecules were removed, leaving a vacant ligand site. The resulting DOS profiles are provided in Figure 7. We observe the same low-lying dz² energy band in the pristine oxide. This finding suggests that the origin of this effect is geometric in nature. This supports our assumption from the CFT model, where the extension of axial bonds due to bound water approximates a square pyramidal coordination environment. We observe a similar trend upon PICET of WO3 . 2H2O, i.e., in H0.5WO3•2H2O. Based on this theoretical and computational analysis, we consider the impact of the different electronic structures on the electrochromic response during PICET, as electrons populate the conduction band and possibly undergo transitions in energy upon the absorption of optical photons. In WO3 and WO3•2H2O, tungsten is in the 6 + oxidation state with a d 0 electron configuration and thus at room temperature there is minimal occupancy of the outermost W dorbitals which compose the conduction band. Therefore, both materials are semiconductors with experimentally determined bandgaps of 3.0 and 2.6 eV for WO3 and WO3•2H2O, respectively. 42 PICET leads to the formation of HxWO3•nH2O (Equation 2) and transfer of electrons to the oxide; the state of charge (x) increases as the cathodic potential decreases. We expect electrons to fill the lowest energy conduction band states first (i.e., dxy in WO3), with the next-lowest energy level available for excitation being the dxz or dyz states. In short, we expect PICET to result in shallow electron doping at the bottom of the conduction band. In WO3, electrochromism is associated with optical absorption due to direct transitions of electrons in the conduction band. 43 In both oxides, transmittance at 1050 nm decreases even at low states of charge, and its derivative tracks with the CV for all potentials (Figure 3). In other words, low energy photons can be absorbed by both oxides at all states of charge (x > 0). Transmittance in the visible at 550 nm behaves differently than the NIR response. In WO3, it tracks with the CV but at slightly higher states of charge than at 1050 nm. This shift in onset potential for photons of different energies, known as the Moss-Burstein effect, indicates that absorption of higher-energy photons occurs only at higher states of charge or greater electron filling in WO3. On the other hand, in WO3•2H2O, the transmittance at 550 nm does not track with the CV except at the highest states of charge. We propose that the larger energy gap between the top and bottom energy levels of the d-orbitals in the conduction band of WO3•2H2O, as predicted by CFT and DFT, is responsible for this wavelengthdependence.
We next consider the influence of temperature between -20 and 20°C on the electrochromic response of tungsten oxides since we expect this to have an impact on the kinetics of PICET as well as competitive PCET reactions like HER. The temperature-dependent CVs for WO3 at 5 and 100 mV/s are shown in Figure S10. 25 The overall shape of the CVs is maintained with temperature although the current slightly decreases leading to a reduction in the capacity with decreasing temperature at both 5 and 100 mV/s. On the other hand, the coulombic efficiency increases as temperature decreases, with a larger improvement at 5 mV/s than 100 mV/s. The largest temperature differences arise in the low potential regions at both scan rates: Peak 2 contains contributions from both PICET and HER, but peak 2' only contains contributions from PICET. We hypothesize that temperature-dependent changes in the low potential region are largely due to suppression of the HER, which explains the trend in coulombic efficiency. The HER (2H + + 2e -→ H2) requires the transfer of two protons and two electrons, whereas PICET only requires one. As a result, we expect the interfacial kinetics associated with the HER to require higher activation energies than PICET and thus be more sensitive to changes in temperature.
The decrease in the HER contribution also leads to an increased current response for peak 2' as temperature decreases, which suggests that some inserted protons could also be participating in the HER, as we proposed in a previous study. 18 The temperature-dependent CVs for WO3•2H2O at 5 and 100 mV/s also show an overall reduction in current, with a slight decrease in capacity, but overall improvement in the coulombic efficiency at 5 mV/s as temperature decreases (Figure S11). 25 We also note an observed broadening of the redox peaks as the temperature decreases with peak 3 appearing to nearly disappear completely, though the effect is most notable at 100 mV/s. Unlike for WO3, the increase in coulombic efficiency with decreasing temperature is only observed at 5 mV/s. We attribute this difference to the reduced contribution of HER in WO3•2H2O. We previously found that H x WO 3 •2H 2 O was a worse HER catalyst than H x WO 3 , which agrees with these results. 18 A more detailed kinetic analysis of the temperature dependence of the PICET process is discussed in the SI (Section 5 and Figure S12). 25 Overall, in both materials we found that PICET is more favored at low temperatures that suppress the parasitic HER.
We performed operando electrochemical UV/Vis/NIR as a function of temperature from -20 to 20°C at 5 and 100 mV/s (Figure 8 for WO3 and 9 for WO3•2H2O). We found a gradual decrease in the background transmission at low temperatures over the timescale of the experiment. We attribute this to the condensation of water on the quartz cuvette from the atmosphere, which was not totally excluded by the Ar gas flowing through the stage. For WO3 (Figure 8), we observed an increase in coloration for all wavelengths when measured at -10°C and -20°C. As the temperature decreases, transmittance decreases (absorbance increases) while cathodic capacity decreases, leading to an overall improvement in coloration efficiency (the change in optical density per unit charge). This again emphasizes that PICET is more favorable at lower temperatures. For each respective temperature, cycling the WO3 thin film at 5 mV/s generally yields greater absorbance than cycling at 100 mV/s, though not by a large margin. There is no cycling condition we found for which a sample exhibiting an electrochromic response at 5 mV/s did not also exhibit a similar response at 100 mV/s, which is indicative of both the similar degrees of PICET achieved at both scan rates and the speed of the electrochromic response. In WO3•2H2O, at both 5 mV/s and 100 mV/s the film achieves similar degrees of reduction in transmission (Figure 9) and demonstrates the same general triangular shape in the operando UV/Vis/NIR response. Unlike WO3, WO3•2H2O does not exhibit an increase in coloration at decreased temperatures, perhaps due to its smaller overall degree of PICET. Rather, the operando spectra at -10 and -20°C become partially obfuscated by the aforementioned changes in background spectra due to water condensation within the sample stage.
In this study, we analyzed the electrochromic response of WO3 and WO3•2H2O thin films in 5 M H2SO4 electrolyte. Differentiating the operando UV/Vis/NIR response shows that the visible wavelength response in WO3•2H2O is strongly potential dependent, where the near-IR response is more strongly correlated with the degree of PICET. The correlation between the current response and the degree of coloration is also observed for both the visible and near-IR in WO3.
We present our hypothesis based on the d-orbital splitting predicted by CFT, with the increased degree of splitting for WO3•2H2O resulting in a larger intra-band transition from the lowest energy dz 2 orbital to the higher energy dxy and dxz orbitals. This hypothesis is corroborated by DOS calculations for HxWO3•nH2O (n = 0 or 2, x = 0.0 or 0.5). These calculations demonstrate that the formation of a ~ 2.9 eV dz 2
We acknowledge the contributions of Prof. Lena Kourkoutis towards useful discussions on low temperature structural transitions and cryo-TEM of tungsten oxides, to whom we dedicate The tool was used in a manner that does not conflict with APS ethical policies and the authors take full responsibility for the content.
(1) Gu, C.; Jia, A. B.; Zhang, Y. M.; Zhang, S. X. A. Emerging Electrochromic Materials and Devices for Future Displays. Chem. Rev. 2022, 122 (18), 14679-14721. https://doi.org/10.1021/acs.chemrev.1c01055. 11684-11696. https://doi.org/10.1021/acs.chemmater.4c02814. (43) Wang, W.; Peelaers, H.; Shen, J. X.; Van De Walle, C. G. Carrier-Induced Absorption as a Mechanism for Electrochromism in Tungsten Trioxide. MRS Commun. 2018, 8 (3), 926-931. https://doi.org/10.1557/mrc.2018.115.