Variable-Tint Electrochromic Supercapacitor with a Benzyl Hexenyl Viologen-Prussian Blue Architecture and Ultralong Cycling Life

A highly efficient, separator-free, low-cost, dual-function electrochromic supercapacitor (ESC) with a benzyl hexenyl viologen (BHV) based gel film and a Prussian blue (PB) film as the anode and cathode, respectively, has been fabricated. The BHV//PB ESC operates over a wide potential window of 2 V and offers a specific capacitance of 67 F g–1 with energy and power density maxima of 37.2 Wh kg–1 and 6.7 kW kg–1, respectively. Simultaneously, during the charging process, the ESC undergoes a reversible and uniform color transition from pale blue to a deep rich purple hue, corresponding to a transmission modulation of ∼68% at 550 nm and an integrated visible light modulation of ∼50%, superior to those of most ESCs in the literature. The ESC offers a visually perceptible level of charging through the variable tints it acquires during charging or discharging. The ESC also sustains high external temperatures with ease, as even at 60 °C, the contrast is ∼80% at 550 nm, making it suitable for hot weather conditions. The ESC delivers an ultralong cycling life, for it does not undergo any major optical modulation loss even after 5000+ color–bleach cycles and retains 85% of its initial capacitance after 5000 charge–discharge cycles. The evolution of the charge–discharge mechanism from being predominantly ionic diffusion controlled to largely surface reaction controlled with scan rate is also demonstrated. The remarkable chemical and electrochemical cycling stability, the very high optical contrast, the reasonably fast switching kinetics, and the color purity of the bleached and colored states achieved over a geometric area of 4 × 3 cm2 are clear indicators of the potential the BHV//PB ESC has for practical deployment as a window that not only provides visual/thermal comfort but also has stored-up energy to power any external electronic device.