context: In summary, active catholyte has been applied to assemble a high-performance hybrid supercapacitor, which relies on the reversible conversion between soluble Mn2+ and solid MnO2 at the cathode and the redox of Ti–O with the bonding/de-bonding of H3O+ at the 400-KOH-Ti3C2 anode. The well-separated potential window between the cathode and anode leads to a hybrid supercapacitor with a wide voltage window of 1.7 V. This hybrid supercapacitor achieves a high energy density of 43.4 W h kg−1, without using any ion-selective membrane, and super-long cycle life (75% retention after 20000 cycles). Furthermore, the hybrid supercapacitor can operate well even with the frozen electrolyte at −70 °C. These inspiring results provide a new strategy to design high-performance hybrid supercapacitors for low temperature applications.
answer: MnO2 