Reversible Zn2+/Al3+ intercalation in niobium-substituted polyoxometalates and demonstration of energy storage smart windows

Abstract

Electrochromic energy storage devices with diversified functions that can realize intelligent visualization of the energy status with the naked eye are highly desirable for intelligent electronic technology. Polyoxometalates (POMs) as a promising electrochromic energy storage material are bottlenecked by limited choices in the electrolyte to achieve high-performance electrochromic smart windows. Herein, we report a novel tri-niobium substituted Dawson-type POM α-K₉[P₂W₁₅Nb₃O₆₂]·18H₂O (P₂W₁₅Nb₃) coated TiO₂ as an electrochromic electrode for non-aqueous electrochromic energy storage smart windows, utilizing a dual-ion electrolyte with Zn²⁺ and Al³⁺. Our findings suggest that the synergistic effects of the dual-ion Zn²⁺/Al³⁺-ion electrolyte on P₂W₁₅Nb₃ films greatly promote the electrochromic and electrochemical properties: high optical contrast (90% at 650 nm) in the 350–1500 nm range, large areal capacity (52 mF cm⁻² at 0.66 mA cm⁻²), fast switching time and good cycling stability. Remarkably, a bi-functional electrochromic energy storage device (EESD) was also successfully assembled using a P₂W₁₅Nb₃ composite film and a hydrothermally prepared NiO film. This dual-ion Zn²⁺/Al³⁺ electrolyte opens a new direction for the development of POM based smart windows with high contrast, long life-time stability and excellent energy storage.