Flexible electrochromic reflectance device based on tungsten oxide for infrared emissivity control

Instead of the usual sputtered anhydrous tungsten oxide thin films, a powder of monohydrated tungsten oxide ${\mathrm{(WO}}_{\mathrm{3}}{\mathrm{.H}}_{\mathrm{2}}\mathrm{O})$ was used for the making of an electrochromic infrared emissivity modulator. The ${\mathrm{WO}}_{\mathrm{3}}{\mathrm{.H}}_{\mathrm{2}}\mathrm{O}$ powder was embedded in a porous plastic matrix before being laminated with other appropriate layers of the battery-like device, leading then to a complete flexible emissivity modulator. The widely open structure of the hydrated tungsten oxide makes lithium intercalation easier, which is particularly suitable for the realization of plasticized devices. Compared to a classical battery assembly, a porous plastic graphite layer laminated with a conductive grid was sandwiched between the ${\mathrm{WO}}_{\mathrm{3}}{\mathrm{.H}}_{\mathrm{2}}\mathrm{O}$ and the electrolyte layers. Such an original device allowed both a perfect uniformity in current collection and a sufficient porosity for the liquid electrolyte displacement. The complete device demonstrated a satisfying electrochemical behavior under 1 mV/s potential sweeps, allowing the insertion of a large amount of lithium ions into the ${\mathrm{WO}}_{\mathrm{3}}{\mathrm{.H}}_{\mathrm{2}}\mathrm{O}$ structure. Hemispherical reflectance measurements were carried out both over the VIS/NIR (0.4–2.5 μm) and the mid-infrared (2.5–25 μm) spectral ranges. Reflectance over the 2.5–25 μm spectral range was found to switch from 2% to 32% upon intercalation of 0.65 Li per tungsten. This value is comparable to previous literature results obtained for rigid devices.