An air-stable electrochromic conjugated microporous polymer as an emerging electrode material for hybrid energy storage systems

Abstract

The oxidation states of polymers and their stabilities are of great importance for their application in energy storage systems. In this paper, we report an air-stable triphenylamine–triazine-based conjugated microporous polymer (pTTPATA) with the smart function of changing color by simply changing the applied voltage. Uniquely, the yellow colored neutral polymer switches to a red color upon oxidation (p-doping) and the red color remains stable even for ten hours after removal of the applied potential, which allows an in-depth analysis of the pTTPATA in the oxidized state. X-ray photoelectron spectroscopy (XPS) confirms that large amounts of radical cations from the triazine groups as well as a small proportion of those from oxidized triphenylamine are present in the stable red-colored oxidized state of the pTTPATA film. Electrochemistry and density functional theory (DFT) calculations demonstrate that only the triphenylamine group is oxidized in the pTTPATA polymer under the applied voltages. Thus, we conclude that most of the oxidation of the pTTPATA polymer occurs from the structural resonance from the oxidation of the triphenylamine group to the relatively more stable radical cation of the triazine group, which results in a stable red colored oxidation state. More importantly, the structural resonance in the special oxidation state induces the charge storage of triazine except the charge storage of triphenylamine, which results in a high specific capacity of ∼81 mA h g⁻¹, among the best reported values for conducting polymers-based energy storage systems. The combination of these fascinating properties results in an intelligent energy storage device that changes its color based on its charged state so that the state of charge can be monitored by simple visual inspection.