Direct Chemical Synthesis of MnO₂ Nanowhiskers on Transition-Metal Carbide Surfaces for Supercapacitor Applications

Transition-metal carbides (MXenes) are an emerging class of two-dimensional materials with promising electrochemical energy storage performance. Herein, for the first time, by direct chemical synthesis, nanocrystalline ϵ-MnO₂ whiskers were formed on MXene nanosheet surfaces (ϵ-MnO₂/Ti₂CT_x and ϵ-MnO₂/Ti₃C₂T_x) to make nanocomposite electrodes for aqueous pseudocapacitors. The ϵ-MnO₂ nanowhiskers increase the surface area of the composite electrode and enhance the specific capacitance by nearly 3 orders of magnitude compared to that of pure MXene-based symmetric supercapacitors. Combined with enhanced pseudocapacitance, the fabricated ϵ-MnO₂/MXene supercapacitors exhibited excellent cycling stability with ∼88% of the initial specific capacitance retained after 10000 cycles which is much higher than pure ϵ-MnO₂-based supercapacitors (∼74%). The proposed electrode structure capitalizes on the high specific capacitance of MnO₂ and the ability of MXenes to improve conductivity and cycling stability.