How much of the energy in the electrochromic energy storage window can be reused?

Electrochromic energy storage technology that can store energy electrochemically while controlling the optical transmittance, could be mainly used in the development of next-generation smart window systems for net-zero energy buildings. The resultant apparatus can first implement indoor optical and thermal modulation, then their involved residual electric energy would be reused to power small electronic products. However, there has been confusion as to how much of the electric energy can be used for secondary purposes. Herein, a hyperbranched polyamide with dual redox centra of triphenylamine and pentaaniline was synthesized with good processability and multi-band absorption. Coupled with a zinc frame electrode, an electrochromic energy storage window (EESW) was manufactured, which simultaneously exhibited approving electrochromic performance (high optical contrast of >48 % and good thermal insulation), as well as ideal zinc ions energy storage properties (wide voltage window of 2.4 V and large capacitance of 71.06 mF cm−2). Furthermore, the spectrochronoamperometry was first coupled with rate performance to investigate its energy reused properties. As implementing electrochromism upon 2.4 V for 50 s, the engaged EESW possessed an energy density of 20.21 μWh cm−2, due to the nonoptimal constant-voltage charging mode. After 1-hour transmittance maintenance without any applied voltage, an 80 % energy density of 16.54 μWh cm−2 was determined, which should be considered available electric energy that can be reused.