MXene-based Zn-ion hybrid supercapacitors: Effects of anion carriers and MXene surface coatings on the capacities and life span

Energy storage devices such as rechargeable batteries and supercapacitors are of great importance for establishing clean energy sources. Accordingly, the production of these devices needs to rely on sustainable and environmentally friendly materials. This report provides an insight on the use of two-dimensional transition metal carbides (MXene) based electrodes, here shown for Mo1.33CTz-Ti3C2Tz mixed MXene, in Zn-ion hybrid supercapacitors (ZHSC) using aqueous and nonaqueous (acetonitrile-based) electrolytes. The effect of anion carriers on the accessible capacity, rate capability, and life span of the MXene//Zn hybrid supercapacitor is explored in-depth. Halide carriers such as chloride (Cl−) and iodide (I−) feature a superior performance, however, a fast passivation is observed in Cl− based electrolytes and a narrow potential window is achieved in I− based electrolytes. Importantly, a few micron layer of Ti3C2Tz MXene coated on the surface of the Zn anode is found to inhibit the side reactions and passivation observed in ZnCl2 solutions, which enables the use of such low-cost Zn salt in MXene//Ti3C2Tz-coated-Zn cells. The cells can be reversibly cycled over 10,000 cycles, delivering a capacity up to 200 mAh g−1 at low rate (0.5 mV s−1) and a capacity retention of about 36% at high rate (100 mV s−1). Furthermore, the Ti3C2Tz surface coating layer enhanced the coulombic efficiency in Zn(CF3SO3)2 electrolyte without affecting the accessible capacity or the rate capability. This work sheds light on the use of MXenes in sustainable low-cost ZHSC with high energy density and power density as a positive electrode material as well as a surface coating material for the Zn negative electrode.