Hybrid mesostructured electrodes for fast-switching proton-based solid state electrochromic devices

Abstract

Tungsten oxide, the most commonly used electrochromic material, reversibly changes its color from transparent to blue under applied bias. The rate of color modulation, a parameter critical for device applications, is determined by the rate of cation injection into WO₃ and its diffusion inside the solid. Here we show that fast switching could be obtained by processing mesostructured hybrid electrochromic electrodes with high organic–inorganic interfacial area and intimate contact between an organic electrolyte and WO₃. The hybrid electrode is prepared by infiltrating a polymer electrolyte, Nafion, into a highly porous sol–gel processed WO₃ matrix. Energy filtered-transmission electron microscopy is used to map the distribution of Nafion in the micro- and nano-sized pores. The images corroborate the formation of a continuous Nafion network through the inorganic scaffold and high WO₃–Nafion interfacial area, for proton conductivity and proton transfer, respectively. The use of a polymer electrolyte, in contrast to commonly used liquid electrolytes, allows integration of the hybrid electrode into fully solid state devices. The device shows dramatically reduced response times compared to the corresponding bi-layer WO₃–Nafion electrodes, associated with the mesostructured morphology of the solid hybrid electrode.