To confirm the improved energy density, a hybrid supercapacitor was fabricated based on a solid-state 400-KOH-Ti3C2 anode and an active catholyte containing Mn2+ in 2 M H2SO4 electrolyte. Before assembling the hybrid supercapacitor, the CF as a current collector was pretreated by electrochemical predeposition with 3 mA h cm−2 MnO2 on the surface. The electrochemical performance of the as-assembled hybrid device was systematically investigated to demonstrate its unique advantages. As shown in Fig. 4a, in situ potential detection was performed, in which the Ag/AgCl reference electrode was introduced into the two-electrode system to explore in situ the potential variation of the respective electrode. This hybrid supercapacitor achieves a wide voltage window up to 1.7 V (red line), benefiting from the potential difference between the solid 400-KOH-Ti3C2 anode (dark cyan line) and active catholyte containing Mn2+ (black line). It is worth mentioning that such a wide operating voltage window is superior to that of most recently reported hybrid supercapacitors (see Table S2†) and even comparable to that of the “water in salt” electrolyte-based hybrid capacitors.Fig. 4b displays the CV curves of the hybrid supercapacitor at various scan rates. All the CV curves exhibit an approximate rectangular shape, which suggests that the operation mechanism in this hybrid device is bonding/debonding-induced pseudocapacitance in nature. Additionally, it is found that the addition of Mn2+ does not significantly affect the reaction kinetics of the 400-KOH-Ti3C2 anode (see Fig. S16† and the corresponding discussions). On the other hand, the near linear symmetric triangular charge/discharge curves at various current densities also confirm the pseudocapacitance characteristics of the hybrid device (Fig. 4c), which agrees well with the CV results. The corresponding specific capacitance was calculated based on the charge–discharge curves (see detailed calculations in the ESI†). Impressively, the hybrid device delivers an appreciable specific capacitance of 312.8 F g−1 at 1 A g−1 and 131.2 F g−1 at 80 A g−1, showing good rate performance (Fig. S17†). The energy and power densities of the present hybrid supercapacitor were also calculated on the basis of the total mass of 400-KOH-Ti3C2 and consumed MnO2, which is given as a Ragone plot profile (Fig. 4d). As displayed in Fig. 4d, the hybrid supercapacitor exhibits a maximum energy density of 43.4 W h kg−1 at a power density of 488.7 W kg−1. Even at a high power density of 40 kW kg−1, the energy density still remains 18.2 W h kg−1. It is worth noting that the energy density achieved here is markedly superior to those of most recently reported hybrid supercapacitors, as listed in Table S3.† These facts indicate that benefiting from both the high operating voltage and specific capacity of the active catholyte containing Mn2+, the energy density of the hybrid supercapacitor is significantly enhanced. In addition, this hybrid supercapacitor exhibits prominent cycling stability with a high-capacitance retention of 75% over 20000 cycles at 4 A g−1 (Fig. 4e). The charge/discharge curves at different cycles are shown in Fig. S18† to illustrate the high coulombic efficiency of the hybrid supercapacitor.

Question: What's the electrolyte?
Answer: Mn2+ in 2 M H2SO4