Transparent Metal–Organic Framework-Based Gel Electrolytes for Generalized Assembly of Quasi-Solid-State Electrochromic Devices

Metal–organic framework (MOF)-based electrolytes under gel/solid states have been widely used for electrochemical devices recently due to their designable metal centers/ligands and diffusion channels in the porous structures. Therefore, it is always desired to apply the MOF-based electrolytes in electrochromic (EC) fields. Yet, challenges exist in realizing their high optical transparency to satisfy the unique optical requirements of EC devices. Herein, a transparent MOF-based gel electrolyte (MGE) is demonstrated through the incorporation of 2-methylimidazole among MOF nanocrystals to prevent the strong light scattering of MOF nanocrystals. As a result, the gel electrolyte showed an improved average transmittance of ca. 82.2% compared with the MOF electrolytes without 2-methylimidazole (ca. 59.2%). In addition, because of the designed large channels in the porous MOF structure, the gel electrolyte exhibited a high ionic conductivity of 2.66 × 10–3 S cm–1. At last, we used the transparent MGEs to assemble two types (rigid and flexible) of quasi-solid-state EC devices based on inorganic WO3 and organic poly(3,4-ethylenedioxythiophene) (PEDOT), respectively. Both devices showed great EC performances, and the flexible devices exhibited high mechanical stability under the bending state or even after being cut and punched, advancing the general applications of our transparent MGEs in EC fields.