Understanding MXene-Based "symmetric" Supercapacitors and Redox Electrolyte Energy Storage

With the development of renewable energy sources, supercapacitors (SCs) have drawn considerable attention due to their high rate performance and power density. Ti₃C₂T_x (MXene) shows an excellent physical and chemical nature, making it a promising electrode material for SCs. Here, Ti₃C₂T_x-based symmetric SCs are examined by employing in situ Raman and X-ray diffraction, revealing that the redox of Ti-O with the bonding/debonding of hydronium ions occurs in the negative electrode, while the positive electrode mainly involves the intercalation pseudocapacitance of hydronium ions. Meanwhile, the results reveal the asymmetric split of voltage between the two electrodes in the H₂SO₄ electrolyte. Furthermore, the shift of the open circuit potential is observed because of the partial oxidation of the positive electrode. As a result, the negative electrode exhibits a much larger capacitance than that of the positive electrode. Thus, a strategy is proposed by introducing redox-active species into the traditional H₂SO₄ electrolyte. The results show that the positive electrode is converted to battery-type capacitance and the working potential range for the negative electrode is expanded, leading to an obvious improvement in capacitance for SCs. Moreover, the Ti₃C₂T_x-based SCs show an energy density of 33.2 Wh L^-1 in the mixed electrolyte, which is much higher than that in the H₂SO₄ electrolyte (14.3 Wh L^-1). Therefore, this method can be used as a highly effective strategy for improving the energy storage capacity of MXene-based SCs.