High-Performance Flexible Electrochromic Supercapacitor with a Capability of Quantitative Visualization of Its Energy Storage Status through Electrochromic Contrast

Flexible electrochromic supercapacitors (ECSCs) are currently under considerable investigation as potential smart energy storage components in wearable intelligent electronics. However, the lack of a suitable strategy for precisely judging its real-time energy storage status has hindered its development toward practical application. Herein, an optical-energy feedback strategy based on electrochromic contrast as a quantitative indicator of the state of charge (SOC) in flexible ECSCs has been developed using a polypyrrole (PPy)-based electrochromic electrode: carbon nanotubes (CNTs)/Au/PPy/poly(ethylene terephthalate) (PET). A linear dependence of real-time electrochromic contrast on the SOC is established, enabling a quantitative, accurate, and on-site visualization of the energy level from digitized information. Benefiting from the inherent high color-to-color contrast in PPy during electrochromism, the strategy shows a high detection sensitivity and resolution during charging/discharging. Moreover, the good flexibility, high capacitance of the electrode, and asymmetric device design can synergistically benefit the energy storage performance of the flexible ECSC, resulting in a high energy density of 4.03 μWh cm–2 and high electrochemical stability under deformations. This work largely explores the potential and advantages of PPy for fabricating a high-performance flexible smart supercapacitor and opens up an alternative methodology for realizing a convenient, low-cost, and nondestructive SOC self-monitoring ability in flexible energy storage devices.