High-Performance Complementary Electrochromic Device Based on Iridium Oxide as a Counter Electrode

In complementary electrochromic devices (ECDs), nickel oxide (NiO) is generally used as a counter electrode material for enhancing the coloration efficiency. However, an NiO film as a counter electrode in ECDs is susceptible to degradation upon prolonged electrochemical cycling, which leads to an insufficient device lifetime. In this study, a type of counter electrode iridium oxide (IrO₂) layer was fabricated using vacuum cathodic arc plasma (CAP). We focused on the comparison of IrO₂ and NiO deposited on a 5 × 5 cm² indium tin oxide (ITO) glass substrate with various Ar/O₂ gas-flow ratios (1/2, 1/2.5, and 1/3) in series. The optical performance of IrO₂-ECD (glass/ITO/WO₃/liquid electrolyte/IrO₂/ITO/glass) was determined by optical transmittance modulation; ∆T = 50% (from T_bleaching (75%) to T_coloring (25%)) at 633 nm was higher than that of NiO-ECD (ITO/NiO/liquid electrolyte/WO₃/ITO) (∆T = 32%). Apart from this, the ECD device demonstrated a fast coloring time of 4.8 s, a bleaching time of 1.5 s, and good cycling durability, which remained at 50% transmittance modulation even after 1000 cycles. The fast time was associated with the IrO₂ electrode and provided higher diffusion coefficients and a filamentary shape as an interface that facilitated the transfer of the Li ions into/out of the interface electrodes and the electrolyte. In our result of IrO₂-ECD analyses, the higher optical transmittance modulation was useful for promoting electrochromic application to a cycle durability test as an alternative to NiO-ECD.