(Invited) Flexible Mxene/Graphene Films for Ultrafast Supercapacitors

Due to their unique properties and a great diversity, various two-dimensional nanomaterials have attracted interest from scientific community for energy storage applications in recent years. Herein, we present a facile strategy to prepare MXene/graphene films with excellent flexibility and electrical conductivity for supercapacitors by using layer-by-layer electrostatic self-assembly from solution between positively charged graphene and negatively charged MXene nanosheets. Graphene nanosheets are intercalated in-between MXene layers after electrostatic assembly. As a result, the face-to-face self-restacking of MXene nanosheets is effectively prevented, leading to the considerably increased interlayer spacing of MXene layers and more active sites accessible to electrolyte ions, which ensures high diffusion rate and transport of electrolyte ions during the charge/discharge process. The as-prepared freestanding MXene/graphene (5wt.%) film displays outstanding volumetric capacitance of 1040 F cm -3 at a scan rate of 2 mV s -1 and impressive rate capability with 61% capacitance retention at 1000 mV s -1 . Meanwhile, the film electrode exhibits a long cycle life with almost no capacitance decay after 20,000 cycles. More importantly, our fabricated symmetric supercapacitor shows a maximum volumetric energy density of 34.6 Wh L -1 and an ultrahigh volumetric power density up to 74.4 kW L -1 , which is among the highest values reported for carbon and MXene based materials in aqueous electrolytes so far. This work not only provides fundamental insights to the effect of interlayer spacing on the electrochemical performance of emerging MXene materials but also sheds light on future design of next-generation flexible, portable and highly integrated supercapacitors with high volumetric and rate performances. Figure 1