Rationally Designed Bilayer Heterojunction Electrode to Realize Multivalent Ion Intercalation in Bifunctional Devices : Efficient Aqueous Aluminum Electrochromic Supercapacitor with Solution-Processed Transparent Nanostructured Tio2/Moo3

Bifunctional electrochromic energy storage devices are of great potential in energy-efficient technologies. Present electrochromic capacitors utilize monovalent ion intercalation, and switching to a multivalent ion such as Al 3+ is sought to sustainably overcome the prevailing performance limitations. However, the realization of such a bifunctional device has remained a challenge, owing to the strong electrostatic interaction of multivalent ions with the electrode material and the requirement of enhanced charge transfer. To cross this hurdle, we present here a bi-layer electrode having a host with wide lattice spacing and forming a type II heterojunction at the interface. Specifically, a nanostructured transparent anatase TiO 2 /α-MoO 3 bi-layer electrode is designed and demonstrated to exhibit the efficient intercalation/de-intercalation of aqueous Al 3+ ions in an electrochromic supercapacitor. The favourable features offered by MoO 3 and the built-in potential at the heterojunction interface resulted in an excellent and stable performance in terms of coloration efficiency (128 cm 2 /C), transmittance change (54%), switching time (~1s) and areal capacitance (218.8 mF/cm 2 ). Thus, a new approach of electrode design for achieving multivalent ion intercalation in bifunctional devices is exemplified with an aqueous aluminum electrochromic supercapacitor, and the best-in-class bifunctional performance of the TiO 2 /MoO 3 bilayer heterojunction electrode proves its potential to be used in energy-saving technologies of the future