2D titanium and vanadium carbide MXene heterostructures for electrochemical energy storage

Two-dimensional (2D) heterostructured electrodes built from vertical stacking of different 2D materials are among the most promising electrode architectures for electrochemical energy storage devices. These materials offer interesting opportunities for energy storage applications such as versatility in the structural design of electrode, and the possibility to integrate individual 2D building blocks with different properties into heterostructures. These features can potentially enable new materials with improved or new electrochemical features. Here, we report on large-scale liquid phase self-assembly of 2D heterostructures built from two different 2D transition metal carbides (MXenes), Ti3C2Tx and V2CTx. A cation-driven self-assembly process was used to assemble the negatively-charged flakes of the two MXenes into heterolayered flakes. The freestanding and binder-free MXene heterostructure films could deliver a high volumetric capacitance of ~1473 F cm−3 and showed no capacitance loss after 50,000 charge-discharge cycles in 3 M H2SO4 electrolyte. Due to coupling of redox reactions of Ti3C2Tx and V2CTx, the heterostructure electrodes showed a nearly constant current over their entire potential window, which is reminiscent of traditional pseudocapacitive materials. This electrochemical behavior differs from individual MXene electrodes or most other emerging pseudocapacitive materials whose maximum performance is usually achieved in a narrow potential range.