Coordination Nanosheet-Based Electrochromic Supercapacitor with High Energy Storage, Switching Durability, and Long Optical Memory Properties

Electrochromic (EC) supercapacitors have attracted considerable attention as energy storage systems integrated with optical functions. EC supercapacitors with high-performance and long-term optical memory properties were successfully fabricated by a combination of coordination nanosheets (CONASH), composed of Fe(II) ions and a tristerpyridine ligand having a nonconjugated linker, and nickel hexacyanoferrate (NiHCF) as a redox-complementary counter material. The EC supercapacitor exhibited EC changes between purple and pale yellow with large optical contrast (57.4% at 556 nm), short switching times (1.28/1.69 s), exceptionally high coloration efficiency (619 cm² C^-1), significantly small energy consumption (3.6 mJ/cm²), and excellent EC switching stability of more than 50,000 cycles. The EC supercapacitor also demonstrated high volumetric capacitance (248.1 F/cm³), energy density (29.37 mW h/cm³), and power density (7.5 W/cm³), maintaining stable performance over 40,000 galvanostatic charge-discharge cycles. Most notably, the device showed a drastically reduced self-discharge property as only 33% optical contrast was returned after 36 h under open-circuit conditions, paving the way for an efficient energy storage solution by exploiting the long optical memory of the device. Combining superior EC functionality with robust supercapacitive performance, this study offers a foundation for sustainable energy technology.