Hydrogen-Bonded Organic Framework-Polyoxometalate-Based System for Electrochromic Devices

A porous hydrogen-bonded organic framework (HOF) structure was explored for the first time in the design of high-performance electrochromic devices (ECDs) using polyoxometalate (POM)-based charge-balancing layers as counter electrodes (CEs). The novelty of this work lies in the facile construction of films using small molecule-based EC materials to form a porous HOF structure. A full-cell model of an ECD was constructed by utilizing a POM-based CE to optimize the voltage distribution on the HOF-coated working electrode (WE). The addition of PW₁₂O₄₀^3- (PW₁₂) on CE significantly enhanced the voltage distribution on EC electrodes and decreased the overvoltage on the WE, further preventing the formation of non-EC species and resulting in a 3.3-fold increase in the lifetime of the ECD. The optical contrast was enhanced from 47% (TiO₂ only) to 68%, and the coloration efficiency was enhanced from 185 (TiO₂ only) to 373 cm² C^-1. The optimized voltage distribution on the WE, leading to the fast response time and high optical EC contrast, could be explained by the charge-balancing effect. Overall, this new finding provides a robust framework for designing high-performance ECDs, taking advantage of the porous morphology and potential matching of the HOF and PW₁₂.