WO₃@Nd-Co₃O₄ bilayer composites for functionally coupled electrochromic energy storage materials

Precise and reconfigurable control of structural coloration represents a pivotal advancement toward multifunctional device capabilities and the expansion of application frontiers. Electrochromic energy storage systems, which unite optical modulation with energy storage, are emerging as promising candidates for intelligent building technologies. In this study, we report the design and implementation of a high-performance bilayer electrochromic energy storage device based on a WO₃@Nd-doped Co₃O₄ (W@Nd-C) composite thin film, engineered for simultaneous optical modulation and energy retention. The thin Co₃O₄ overlayer, strategically doped with neodymium ions, introduces abundant redox-active sites, defect-induced ion diffusion pathways, and lattice distortions that synergistically enhance electrochemical kinetics without impairing the intrinsic optical response of WO₃. Comprehensive structural, morphological, and spectroscopic analyses confirm the formation of a heterointerface, facilitating accelerated charge transport and high structural stability under prolonged cycling. The W@Nd-C electrode delivers an outstanding areal capacitance of 39.23 mF/cm², a transmittance modulation of 79.3 % at 600 nm, and a superior coloration efficiency of 107.31 cm²/C, coupled with electrochemical reversibility exceeding 99 % and cycling durability over 12,000 GCD cycles. When integrated into a sandwich-structured device architecture, the device exhibits extended voltage operation, rapid switching kinetics, and the ability to visually indicate charge status in real time via distinct blue coloration transitions. Moreover, the device demonstrates sufficient energy output to drive multicolor LEDs, underscoring its practical viability. This work establishes a new paradigm in smart material systems by uniting rare-earth dopant engineering, hierarchical nanostructure design, and dual-functionality into a single platform, offering a scalable and intelligent solution for energy-adaptive optical technologies.