Tailoring electrochromic and energy storage performance through side chain functionalization of electrosynthesized poly(3,4-propylenedioxythiophene)s in rigid/flexible devices

Electrochromic energy storage devices (EcESDs) still meet many challenges, demanding continuous materials development and device engineering for practical applications. Herein, we introduce two conjugated 3,4-propylenedioxythiophene (ProDOT)-based polymers directly deposited by electropolimerization on rigid/flexible substrates (ITO-coated glass/PET) as electrodes for EcESDs. The starting monomers were functionalized with two different polar groups, short hydroxymethyl and long oligooxyethylene-benzoester fragment, to boost the ionic conductivity and enhance the coating adhesion to the working electrode surface for improved charge transfer. Among electropolymerized polymers, the one substituted with a long polar chain exhibited outstanding capacitive and electrochromic performance in a three-electrode configuration, e.g. capacitance of 9.96 mF/cm 2 at 0.01 mA/cm 2 , high optical contrast of 83%, color switching in sub-second time frames, and remarkable coloration efficiency, of 1036 cm 2 /C, outperforming many conjugated polymers reported to date. The EcESDs assembled on both rigid and flexible electrodes without the use of any additional storage layer also confirmed a better electrochromic response from this polymer, affording optical contrast of 54.86% and coloration efficiency of 824 cm 2 /C. Unexpectedly, both electropolymerized polymers displayed similar energy storage performance (1.04 mF/cm 2 at 0.01 mA), as a consequence of a more ideal device state attained by the hydroxymethyl-functionalized polymer in terms of electrical contacts, internal series resistance, balanced charge between the electrodes, and others. Notably, the flexible devices also showed capacitive performances comparable to those of the rigid counterparts, even in a bent state, placing the investigated polymers among promising candidates for the next-generation bendable and smart energy storage devices.