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.