Co-sputtering construction of Gd-doped WO3 nano-stalagmites film for bi-funcional electrochromic and energy storage applications

Gd-doped WO3 nano-stalagmites (GW-NSs) film has been successfully prepared on indium-doped tin oxide (ITO)-coated glass substrates by co-sputtering deposition. Gd dopants can facilitate the vertically-aligned growth of WO3 nano-stalagmites on the WO3-nanoparticle seed layer. The microstructure analysis shows that the GW-NSs film with optimized aspect ratio and array density is fabricated with Gd/W = 0.6 % in atomic ratio. Such GW-NSs electrode (active area: 1 × 1.5 cm2) exhibits large transmittance modulation (ΔT = 52.7 % at 633 nm), high coloration efficiency (87.3 cm2 C−1 at 633 nm), fast switching speed (5.9/13.1 s for coloration/bleaching), large diffusion coefficient (9.52 × 10−10 cm2 s−1) and long-term cycling stability (>2000) under an applied voltage of ± 1.0 V, which can be attributed to a shorten Li+ diffusion distance and a large-area contact with the electrolyte. Importantly, the GW-NSs film also shows superior capacitive performance (calculated areal capacitance of 8.44 mF cm−2 at 0.4 mA cm−2) and long-term cyclic stability (capacitance retention rate is 87.4 % after 2000 cycles). Furthermore, we fabricated the bifunctional smart window device combined with NiO counterpart electrode, which has the ability to visually monitor the level of stored energy from rapid and reversible coloration, indicating the potential applications of GW-NSs in both electrochromic and energy-storage fields.