Achieving low-driving voltage electrochromic devices with N-methylphenothiazine derived ionic liquid

Abstract

• N-methylphenothiazine derived redox ionic liquid (NMP-IL) was synthesized. • NMP-IL shows superior ionic conductivity with high thermal stability. • An extremely low-driving voltage (0.6 V) electrochromic device (ECD) was reported. • The proposed ECD gives a large transmittance change and coloration efficiency. • The ECD exhibits good long-term cycling stabilities at elevated temperatures. A novel N-methylphenothiazine based ionic liquid (NMP-IL) was designed for achieving low-driving voltage electrochromic device (ECD) with thermal durability. NMP-IL was synthesized via a five-step reaction, including thionation, methylation, substitution, ionization, and anion exchange. By chemically bonding with the ionic liquid, NMP-IL exhibited a large optical contrast and high thermal stability simply to take the advantages of both NMP and ionic liquid features. Compared to NMP, NMP-IL would provide the electrolyte with a higher ionic conductivity, thus reducing the driving voltage of the ECDs. Most importantly, an extremely low-driving voltage (0.6 V) ECD based on NMP-IL and nickel hexacyanoferrate (NiHCF) thin film, known as the ion storage layer, was further demonstrated. The proposed NMP-IL/NiHCF ECD accomplished a large transmittance change of 56.6% at 575 nm, a desirable coloration efficiency of 166 cm 2 /C, and attenuated 120.9 W/m 2 solar irradiance between bleached state and colored state with a minimal consumed power per unit area of 1.08 W/m 2 . Besides, impressive long-term stabilities of the proposed ECDs, under both room temperature (92.3% retention of its original ΔT after 10,000 cycles) and high temperature (80.2% retention of its original ΔT after 3,000 cycles at 65 °C), were achieved. These merits reveal that NMP-IL has the potential to become the new anodically coloring material for application in energy-saving ECDs.