Electrochromic coordination nanosheets: Achievements and future perspective

Electrochromic (EC) materials and electrochromic devices (ECDs) have drawn immense attention in the last few decades for various colour-tuning and energy-saving applications. However, EC materials that show high optical contrast and long-term durability, are of particular interest for fabrication of high durable ECDs. The coordination nanosheets (CONASHs) are emerging as a new class of EC materials in recent years with attractive EC properties. Among various types of EC materials reported till date, CONASHs are unique EC materials by their virtue of interfacial synthesis which offers a simple and in situ preparation of a free-standing thin film that can be transferred on any desired transparent conducting substrate for EC applications. Moreover, the thin film of CONASHs is porous in nature and insoluble in any solvent, which make them superior EC materials for future displays with large optical contrast, fast EC switching, high coloration efficiency, and high durability. In this review article, the developments of electrochromic CONASHs achieved so far are highlighted systematically in detail, which we hope will aid further progress of this exciting class of EC materials. Among the reported electrochromic CONASHs, it has been observed that terpyridine ligand based CONASHs are attractive for making of multi-color and NIR active EC materials, whereas a phenanthroline-Fe2+ based CONASHs has been found to display best performance for high durable ECDs fabrication with more than 15,000 EC switching cycle in solid state device. Finally, based on the reported research results, future challenges and opportunities associated with electrochromic CONASHs are presented.