Porous MXenes enable high performance potassium ion capacitors

High power K+ ion capacitors have great potential in various large-scale applications because of the cost advantages and the low redox potential of K/K+. However, the large ionic radius of potassium brings huge challenges for the development of suitable electrode materials. Here we demonstrate a general strategy for preparing porous MXene electrodes that can significantly enhance K+ storage performance. Using V2C MXene as a model system, we show that the K+ ion storage capacity can be greatly boosted by a simple sequential acid/alkali treatment. The resulting product, K-V2C, not only delivers a capacity of 195 mAh g(-1) (in contrast to 98 mAh g(-1) of pristine V2C) at 50 mA g(-1), but also good rate performance. The charge storage mechanism was carefully studied and is shown to involve a solvent co-intercalation process. In addition, full cells were fabricated by coupling the K-V2C anode and Prussian blue analogous (KxMnFe(CN)(6)) cathode, which can work at a high average operating voltage of similar to 3.3 V within a wide range (0.01 V-4.6 V). Moreover, the devices can achieve a high energy density of 145 Wh kg(-1) at a power density of 112.6 W kg(-1), suggesting that K-V2C, and other porous MXenes prepared by our approach, are promising electrodes in mobile ion capacitors.