Rationally designed bilayer heterojunction electrode to realize multivalent ion intercalation in bifunctional devices: Efficient aqueous aluminum electrochromic supercapacitor with 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 Al3+ 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. To cross this hurdle, we present here a nanostructured transparent anatase TiO2/ alpha-MoO3 bi-layer electrode having a host with wide lattice spacing and forming a type II heterojunction at the interface. The favourable features offered by MoO3 and the built-in potential at the heterojunction interface resulted in a superior and stable performance in terms of coloration efficiency (128 cm2/C), transmittance change (54%), switching time (~1s) and areal capacitance (218.8 mF/cm2). 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 TiO2/MoO3 bilayer heterojunction electrode proves its potential to be used in energy-saving technologies of the future.