A polyoxotungstate/W ₁₈ O ₄₉ nanocomposite with a bird-nest-like network supports high-performance electrochromic supercapacitors

Electrochromic supercapacitors (ECSCs) offer real-time visual monitoring of the energy storage status, highlighting their applicability in cutting-edge electronic devices. Herein, we employed a layer-by-layer self-assembly approach to fabricate a W₁₈O₄₉/[PEI/P₂W₁₈]₂₀ nanocomposite via depositing the Dawson-type polyoxometalate (POM) K₆[P₂W₁₈O₆₂]·14H₂O (P₂W₁₈) onto W₁₈O₄₉ nanofibers. The distinctive bird-nest-like network structure of the nanocomposite, featuring a 3D conductive pathway and unobstructed ion diffusion channels, significantly enhanced the kinetics of ion/electron transfer and facilitated its excellent electrochemical performance. Compared to the pure W₁₈O₄₉ film, this W₁₈O₄₉/[PEI/P₂W₁₈]₂₀ film exhibited superior electrochromic (EC) and energy storage capabilities, with a significantly higher areal capacitance of 30.45 mF cm^-2 at a current density of 0.05 mA cm^-2 and an excellent coloration efficiency of 224.15 cm² C^-1. These enhancements can be attributed to the unique bird-nest-like microstructure and the synergistic effect resulting from the combination of two cathode EC components in this hybrid material. An asymmetric ECSC was successfully fabricated using W₁₈O₄₉/[PEI/P₂W₁₈]₂₀ and NiO as the cathode and anode, respectively. The ECSC exhibited a high retention rate of 84.76% after undergoing 1000 continuous charge/discharge cycles and an excellent areal capacitance of 4.38 mF cm^-2 at a current density of 0.35 mA cm^-2. Moreover, this ECSC demonstrated the ability to seamlessly transition between the transparent and dark blue states throughout the charge/discharge process. Upon a charging period of 2 min, the resulting ECSC exhibited the capability to power an LED load for 3 min, while its color variations served as a convenient visual indicator of the energy storage level.