A semi-solid hydrogel state electrochromic device based on asymmetric disubstituted viologen containing a quaternary ammonium group with enhanced electrochromic stability

This work aims to investigate the electrochromic behavior of an asymmetric disubstituted viologen containing a quaternary ammonium group in semi-solid hydrogel state electrochromic devices. An asymmetric disubstituted viologen BTMAPP3+3〖Br〗^- was synthesized as the electrochromic material, and two symmetric disubstituted viologens (DTMAPP4+4〖Br〗^- and DBP2+2〖Br〗^-) were synthesized as the control electrochromic materials. Polyacrylamide hydrogel state ECDs based on three electrochromic materials were assembled by in-situ photo-initiated polymerization of acrylamide under UV light, and their electrochemical and electrochromic performance were investigated. They showed similar electrochemical performance. They also required a same working voltage and achieved a similar color change from colorless bleached state to blue-purple colored state. It’s noteworthy that the ECD based on BTMAPP3+3〖Br〗^- showed a better cycling stability (retaining of the initial optical contrast after 16,000 s monitored: 95.6% versus 70.7% and 78.5%) and a higher coloration efficiency (115 cm2/C versus 96 cm2/C and 103 cm2/C) compared to the PAM ECDs based on two symmetric disubstituted viologen. The results indicate that an asymmetric disubstituted viologen containing a quaternary ammonium group can enhance the cycling stability of the gel-state ECD through suppressing the formation of radical cation dimers. The all-in-one semi-solid hydrogel state ECDs based on BTMAPP3+3〖Br〗^- with advantages of easy-to-make process, good cycling stability, low operating voltage, high coloration efficiency and low power consumption offer a new choice for electronic labels, solar cell powered displays, smart windows, etc.