Stability-enhanced indium hexacyanoferrate electrodes: Morphological characterization, in situ EQCM analysis in nonaqueous electrolytes and application to a WO₃ electrochromic device

This paper presents a promising transparent counterelectrode system for a WO₃ electrochromic device (ECD) on the basis of a stability-enhanced indium hexacyanoferrate (InHCF) electrode and a NaClO₄/propylene carbonate (PC) electrolyte. Through SEM characterization it was found that clusters of granular InHCF nanoparticles (ca. 80-140 nm) were deposited on ITO substrates in HCl and KCl-stabilized plating solutions, and uniform micrometer thick films with high charge capacity could be obtained. From in situ electrochemical quartz crystal microbalance study, it was discovered that Na⁺ would enter or move out from the InHCF film in the 'desolvated' form during the redox process in a PC electrolyte. Besides, NaClO₄/PC resulted in higher electrochemical activity and reversibility than LiClO₄/PC. With these discoveries, a durable WO₃-InHCF ECD featuring blue-to-colorless electrochromism was fabricated successfully. The device remained 73.6 and 88.7% of its initial ΔT values at 600 and 800 nm after 40,000 rapid and successive coloring/bleaching cycles, respectively. Moreover, the cycling-induced loss of electrochromic performance almost completely restored after 1-month rest and kept unchanged for another month. Thus, the applicability of this nonaqueous InHCF counterelectrode system to ECDs was verified