Polyoxometalate/MXene hybrid film with a 3D porous structure for high-performance electrochromic supercapacitors

MXenes (2D transition metal carbides, nitrides, and carbonitrides) are attractive pseudocapacitive electrode materials for electrochemical energy conversion and storage applications. The layered structure of MXenes provides intercalation pseudocapacitance properties and numerous reaction sites; however, nanosheet agglomeration and stacking reduce the reaction site availability and limit the electrochemical reaction dynamics of the interlayer ions. In this study, a unique electrochromic-energy storage hybrid film (NW/P2W17Ni/Ti3C2Tx), based on a Ni mono-substituted Dawson-type polyoxometalate (P2W17Ni) and a 2D transition metal carbide (Ti3C2Tx), was produced by the combination of a hydrothermal process and layer-by-layer self-assembly method. With positively charged polyelectrolyte polyetherimide (PEI) as a linker, negatively charged Ti3C2Tx layered sheets and P2W17Ni polyanions were deposited on a TiO2 nanowire (NW) array to form a 3D porous structure which prevented the stacking of nanoparticles and sheets, provided more ion transmission paths, and enhanced the ion transfer kinetics. The NW/P2W17Ni/Ti3C2Tx hybrid film demonstrated elevenfold higher areal capacitance (16.1 mF cm(-2), 0.6 mA cm(-2)) than a pure Ti3C2Tx film (1.4 mF cm(-2), 0.6 mA cm(-2)), and the retention rate reached 100% after 1000 cycles. The asymmetric electrochromic supercapacitor (ECSC) assembled with the NW/P2W17Ni/Ti3C2Tx film as the cathode exhibited a high capacity with significant dual-band regulation in the visible and near-infrared regions. The ECSC illuminated a light-emitting diode and achieved high-speed color switching between olive green and dark azure blue during the charging and discharging processes, demonstrating the energy storage status. These results could lead to considerable advances in the development of MXene-based ECSCs. (C) 2023 Elsevier Ltd. All rights reserved.