Highly dispersed single‐wall carbon nanotube thin film on WO 3 film for ultrafast multi‐functional electrochemical devices

A highly dispersed single-wall carbon nanotube (HD-SWCNT) thin film is introduced on a WO3 film (HD-SWCNT/WO3) by ultrasonic spray coating method to accelerate electron and Li-ion transport for realizing ultrafast multi-functional electrochromic (EC) energy-storage electrodes. Uniform grafting of polyvinylpyrrolidone onto the SWCNTs induces their amicable debundling without any surface defects. The highly debundled and continuous morphology of the HD-SWCNT thin film enables accelerated electron transport along the sp2 carbons, which leads to excellent electrical properties (electrical conductivity of ~1361 S/cm and sheet resistance of ~7.3 Ω/□). Functional groups such as amides and carbonyls on the HD-SWCNTs enhance Li-ion wettability, which accelerates Li-ion diffusion kinetics. In addition, the uniform structure of the HD-SWCNT thin film with its porosity effectively shortens the Li-ion diffusion pathways and increases the contact area between the functional groups and the electrolyte, improving the electrochemical activity of the electrode. Such behaviors to promote electron and Li-ion transport at the interface between the electrolyte and the WO3 film enhance the EC energy-storage performances compared to those of aggregated SWCNT film on WO3 and a bare WO3 electrode. The corresponding performances of HD-SWCNT/WO3 include the transmittance modulation (58.7% at 633 nm), switching speeds (3.1 s for coloration and 4.5 s for bleaching), coloration efficiency (51.9 cm2/C), and specific capacitance (87.9 F/g at 2 A/g). In particular, owing to the synergistic effect of the accelerated electrical conductivity and the Li-ion diffusivity of the HD-SWCNT thin film for ultrafast electrochemical kinetics, HD-SWCNT/WO3 exhibits a remarkable high-rate capability (82.9%, specific capacitance retention at 20 A/g compared to 2 A/g), which demonstrates ultrafast charge/discharge characteristics. In this regard, the introduction of an HD-SWCNT thin film as a functional layer to improve the ultrafast charge transport at the interface between a WO3 and an electrolyte could be a promising strategy for ultrafast multi-functional electrochemical devices.