Controlling the Dimensions of 2D MXenes for Ultrahigh-Rate Pseudocapacitive Energy Storage

The capacitive properties of two-dimensional (2D) transition metal carbides/nitrides (MXenes) have been the focus of much research in recent years. MXenes store charge by the pseudocapacitance mechanism (fast surface redox reactions) but can deliver their stored charge at much higher rates compared to other pseudocapacitive materials. Herein, the dependence of the electrochemical properties of MXenes on their lateral dimensions is reported. We show that synthesizing MXenes with controlled dimensions enables the design and fabrication of electrodes with high electronic and ionic conductivities. At low scan rates, electrodes fabricated using a mixture of small and large flakes could deliver very high specific gravimetric and volumetric capacitances of about 435 F g^-1 and 1513 F cm^-3, respectively. At a very high scan rate of 10 V s^-1, the performance of the electrodes remained capacitive, demonstrating their ultrahigh-rate energy storage capability. This work outlines an effective method for the design and fabrication of MXene electrodes with high energy and power densities.