An indium hexacyanoferrate–tungsten oxide electrochromic battery with a hybrid K+/H+-conducting polymer electrolyte

Electrochromic tungsten oxide (WO3) and indium hexacyanoferrate (InHCF) thin-film electrodes, in combination with a polymer electrolyte that accommodates conduction of both K+ and H+, were assembled into a thin-film electrochromic battery (ECB). A typical InHCF–WO3 ECB (designated as IWECB) can be charged and discharged reversibly between 0.5 and 1.5 V with a theoretical voltage of around 1.24 V. A hybrid K+/H+-conducting solid polymer electrolyte (SPE) was prepared through doping different amounts of KCl into poly-2-acrylamido-2-methylpropane sulfonic acid (PAMPS). The resultant SPE fulfilled the dual requirements: H+ for the WO3 and K+ for the InHCF insertion/extraction. The KCl-doping level, evaluated from the molar ratio of (KCl)/(AMPS), plays a crucial role in determining the SPE properties, including its ionic conductivity and water content, and thus strongly affects the charge–discharge characteristics of the IWECB. Furthermore, both properties of the SPE exhibited a very similar, concave-up dependence on the doping level, and a minimum existed at (KCl)/(AMPS)=0.44. It was found that the SPE with a higher KCl-doping level could achieve a larger discharge capacity and a higher cell voltage, but would result in a poorer cycle life. Although the charge capacity of the IWECB was limited, it is enough to drive many low-watt electronic devices for several hours. Finally, the capability of the IWECB was also demonstrated by storing solar energy and by visualizing the state-of-charge (SOC), in which a highly contrasting blue-to-colorless electrochromism in response to discharging was visualized.