Mass loading and self-discharge challenges for MXene-based aqueous supercapacitors

MXene-based aqueous supercapacitors (SCs) have rapidly developed during the last decade because of their excellent cycling stability, fast charging capabilities, and environmental benignity. However, despite the practical importance of mass loadings (MLs) and self-discharge (SD) rates, these two issues have, for the most part, been neglected by the MXene community. MXene-based devices with MLs > 10 mg cm−2 are vital for the development of the next generation of SC devices. However, poor electrolyte accessibility to active materials and high electrical resistances at high MLs can reduce the specific capacitances significantly, leading to low energy/power density devices. Most MXene SC papers do not report the SD, despite its great importance in terms of applications. SCs with high SD rates will have many fewer applications. In this review, we are focusing on the ML and SD challenges in MXene-based aqueous SCs. The strategies for constructing high-performance MXene-based aqueous SCs with high MLs and/or slow SD rates are summarized with key challenges and perspectives outlined. Moreover, this review also attempts to raise awareness in the MXene SC community of the importance of ML and SD for a large host of applications.