Porous polyoxotungstate/MXene hybrid films allowing for visualization of the energy storage status in high-performance electrochromic supercapacitors

Electrochromic supercapacitors (ECSCs) have recently received growing attention for potential smart energy storage components in intelligent electronics. However, in the development of ECSCs, the design and assembly of high-performance electrode materials remain ongoing challenges. In this study, Ti3C2Tx MXene and polyoxotungstate (P2W18) were deposited on TiO2 nanowires to construct a unique three-dimensional (3D) porous hybrid film, NW@MXene/P2W18, via a convenient layer-by-layer self-assembly approach. The 3D porous structure of the nanocomposite reduced the aggregation and stacking of Ti3C2Tx MXene nanosheets during self-assembly, leading to the formation of unobstructed ion diffusion channels and interfacial charge transfer between adjacent layers, resulting in a good electrochemical performance. Compared to the tightly packed structure, the porous hybrid film demonstrated an enhanced electrochromic energy storage performance with a higher areal capacitance (i.e., 19.0 mF cm-2 at a current density of 0.6 mA cm-2), in addition to a high cycling stability (i.e., 90.7% retention rate after 2000 cycles), and an excellent color rendering efficiency. Subsequently, an asymmetric ECSC was fabricated using an NW@MXene/P2W18 film as the cathode and a TiO2 nanowire film as the anode. This ECSC exhibited a high areal capacitance of 4.0 mF cm-2 at a current density of 0.1 mA cm-2 with a wide operating window of 4.5 V, whilst also achieving high-speed color switching between olive green and dark blue during the charge/discharge processes, ultimately offering new avenues for the development of electrochromic energy storage electrode materials and the design of novel devices.