The reversible intercalation of sodium ion into tungsten oxide WO 3 appears as an interesting alternative to hydrogen or lithium ion reduction in order to get the characteristic transition from clear transparent to bluish coloration in electrochromic devices, but it has been comparatively less considered. In order to address further viable all-ceramic devices based on sodium ion intercalation and overcome the issue of WO 3 degradation in aqueous media, three configurations of WO 3 thin film-based electrochromic half-cells were tested, namely in (i) aqueous acidified Na 2 SO 4 electrolyte, (ii) room temperature ionic liquid BEPipTFSI electrolyte and (iii) aqueous acidified Na 2 SO 4 electrolyte associated with an amorphous NASICON-cap onto WO 3 film. We compared their electro-optical characteristics during 100 voltammetry cycles, including the Na + diffusion coefficient calculated through electrochemical method. It is found that sputter-deposited amorphous WO 3 thin films on transparent conductive substrates is promising for electrochromic all-ceramic devices based on Na ion insertion. Electrochemical characterization in aqueous medium is not relevant to extract relevant data when WO 3 is in direct contact with the electrolyte as the electrochromic film is progressively dissolved. In contrast, WO 3 capped with oxide amorphous Na-ion conductor readily operates over 100 cycles, the capping layer preventing degradation by the aqueous medium. Alternatively, ionic liquid does not degrade the WO 3 film and can be employed to efficiently characterize the electro-optical performances.
A series of compounds with the general formula (Cu 65 Ni 20 Fe 15 ) 100-x O x , with x = 0.3, 1.4, 3.3 and 7.2 were prepared by high energy ball milling and evaluated as oxygen-evolving anodes for aluminum electrolysis. In a first step, elemental Cu, Ni and Fe powders were milled together to form a face-centered-cubic (fcc) phase (γ-phase). Then, the milling operation was resumed in presence of the desired amount of oxygen. Upon heat-treatment at 1000 • C during the subsequent powder consolidation, the added oxygen reacted with Fe to form Fe 2 O 3 . Aluminum electrolyses conducted for 20 h in low-temperature (700 • C) KF-AlF 3 electrolyte at an anode current density of 0.5 A cm −2 showed that the electrode stability and aluminum purity are strongly dependent on the amount of oxygen added. The best results were obtained for x = 1.4. In that case, the cell voltage was stable at ca. 4.0 V and the Cu, Fe and Ni contaminations of the produced Al and electrolyte were minimal, resulting in an anode erosion rate of 0.8 cm year −1 . In this case, the size, dispersion and concentration of Fe 2 O 3 precipitates in the consolidated powder were optimal to give rise to the formation of a protective NiFe 2 O 4 -rich layer.
The coloration mechanism of tungsten trioxide (WO3) upon insertion of alkali ions is still under debate after several decades of research. This Letter provides new insights into the reversible insertion and coloration mechanisms of Na + ions in WO3 thin films sputterdeposited on ITO/glass substrates. A unique model based on a constrained spline approach was developed and applied to draw out ε1+iε2 from spectroscopic ellipsometry data from 0.6 to 4.8 eV whatever the state of the electrochromic active layer, i.e. as-deposited, colored or bleached. It is shown that electrochemically intercalated sodium-tungsten trioxide, NaxWO3 (x=0.1, 0.2, 0.35), exhibits an absorption band centered at ca. 1.14 eV in 2 governing the coloration mechanism.
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