Colloidal WO_2.72 Nanocrystals with Hexagonal Tunnels for Sodium-ion Supercapacitors

Although sodium-ion supercapacitors are promising candidates for next-generation electrochemical systems, their performance is often hindered by the relatively large ionic radius of Na⁺, which limits facile ion insertion and thus suppresses capacity enhancement. The high surface area and large hexagonal tunnels make colloidal monoclinic WO_2.72 nanocrystals attractive as electrode materials, but their use in Na⁺-based supercapacitors remains unexplored. In this study, we introduce colloidally synthesized monoclinic WO_2.72 nanocrystals capable of effectively accommodating Na⁺ ions within their hexagonal sites for Na⁺-based supercapacitors. By removing insulating organic ligands, the nanocrystals are uniformly deposited onto hydrophilic carbon cloth (CC) through a controllable, additive-free dip-coating process. Furthermore, applying a conformal 1 nm Al₂O₃ overlayer via atomic layer deposition (ALD) enhances mechanical robustness and prevents nanocrystal delamination, enabling highly reversible charge-storage behavior. A symmetric supercapacitor fabricated with ALD-treated WO_2.72/CC and a PVA/Na₂SO₄ gel electrolyte demonstrates stable performance without degradation under bending, achieving an areal energy density of 10.08 µWh/cm² at 0.3724 mW/cm². These results highlight the potential of colloidal nanocrystal-based electrodes for high-performance sodium-ion supercapacitors suitable for practical, wearable electronic applications.