4D printing of MXene hydrogels for high-efficiency pseudocapacitive energy storage

2D material hydrogels have recently sparked tremendous interest owing to their potential in diverse applications. However, research on the emerging 2D MXene hydrogels is still in its infancy. Herein, we show a universal 4D printing technology for manufacturing MXene hydrogels with customizable geometries, which suits a family of MXenes such as Nb₂CT_x, Ti₃C₂T_x, and Mo₂Ti₂C₃T_x. The obtained MXene hydrogels offer 3D porous architectures, large specific surface areas, high electrical conductivities, and satisfying mechanical properties. Consequently, ultrahigh capacitance (3.32 F cm^-2 (10 mV s^-1) and 233 F g^-1 (10 V s^-1)) and mass loading/thickness-independent rate capabilities are achieved. The further 4D-printed Ti₃C₂T_x hydrogel micro-supercapacitors showcase great low-temperature tolerance (down to -20 °C) and deliver high energy and power densities up to 93 μWh cm^-2 and 7 mW cm^-2, respectively, surpassing most state-of-the-art devices. This work brings new insights into MXene hydrogel manufacturing and expands the range of their potential applications.