Tailored interface stabilization of FTO transparent conducting electrodes boosting electron and Li ion transport for electrochromic energy-storage devices

Transparent conducting electrodes (TCEs) play an important role in transporting electrons to an active layer, which directly affects electrochemical reactions in electrochromic (EC) energy-storage devices. However, homogeneous and fast electron supply to electrochemically active layer is mainly limited by interfacial properties of the TCE. Especially, a rough interfacial structure leads to redundant voids for electron scattering, and an oxygen vacancy acts as an intrinsic electron-trapping site in TCE. Thus, we propose a highly smooth morphology and oxygen vacancy passivated TCE to boost electron and Li ion transport without an active material (WO3) adjustment. These nanostructures are synthesized with simultaneous effects of W co-doping and H2O2 during spray pyrolysis deposition (W-FTO/H2O2) for application in EC energy-storage devices. The highly dense and smooth surface of W-FTO/H2O2 provides a homogeneous electron supply to WO3, which induces uniform Li ion transport into WO3. And the oxygen vacancy passivated structure encourages electron mobility, which leads to in-depth Li ion transport. Consequently, the EC energy-storage electrodes fabricated with W-FTO/H2O2 as a TCE exhibited ultra-fast switching speeds (2.3 s for coloration and 0.6 s for bleaching) and a high rate capability because of the high electron mobility. An all-solid-state cell fabricated with W-FTO/H2O2 as a TCE exhibited remarkable cyclic stability (transmittance retention of 92% and specific capacitance retention of 95.8% after 2,000 continuous cycles) because of the homogeneous electron transfer at the interface. Therefore, we demonstrate that tailoring interface structure of TCE is a promising strategy to improve the performance of EC energy-storage devices.