Fig.4. Electrochemical property of pure MXene and MXene/AAC hybrids electrodes: (a) The GCD curves of MXene/AAC hybrids electrodes with various mass ratios at the current density of 0.5 A g^-1 (b) CV curves of MXene/AAC hybrids electrodes with various mass ratios at the scan rate of 0.1 V s^-1 (c) specific capacitance of MXene/AAC hybrids electrodes with various mass ratios at different current densities (d) Nyquist plots of MXene/AAC hybrids electrodes with various mass ratios in 7 M KOH in the frequency range from 10^-2 to 10⁵ Hz

MXene/AAC composites present exceptional mechanical strength, enhanced interlayer spacing, high porosity and conductivity which could be directly employed as the flexible supercapacitance electrodes without any conducting additive and binding agent^33-36. Therefore, they are expected to possess enhanced electrolyte accessibility, noteworthy rate capability and high volumetric capacitance. The electrochemical properties of the as-obtained MXene/AAC hybrids are examined using a three-electrode system in 6 M KOH aqueous electrolyte. Fig.4. (a) exhibits the galvanostatic charge-discharge (GCD) curves of pure MXene and MXene/AAC hybrids electrodes at 0.5 A g^-1. The GCD images have triangular profiles without obvious plateaus and distinct voltage drop in all charge-discharge images, demonstrating the ideal electric double layer capacitors of pure MXene and MXene/AAC. Obviously, the MXene/AAC 2:1 shows the longest discharge time among all the MXene/AAC electrodes, indicating the highest specific capacitance. Fig.S4 (a) reveals GCD curves of MXene/AAC 2:1 at different current densities ranging from 0.5 to 15 A g^-1. The nonlinear charge/discharge curves with negligible voltage drop even at 15 A g^-1 further state clearly the excellent performance of electric double layer and small internal resistance of MXene/AAC 2:1 electrode. The specific capacitances of pure MXene and MXene/AAC are calculated and the corresponding numerical value are exhibited in Fig.4. (c). The specific capacitance of pure MXene (253 F g^-1) is much lower than MXene/AAC electrodes (357 F g^-1 of MXene/AAC 1:1, 378 F g^-1 of MXene/AAC 2:1, 369 F g^-1 of MXene/AAC 3:1, 363 F g^-1 of MXene/AAC 5:1) because of the lower of AAC content which results in the restacked MXene layer. The restacked MXene flakes with inadequate AAC powers adverse to electrolyte infiltration and ion transport, resulting in poor rate capabilities. According to our precious, the specific capacitance of AAC electrode is 264 F g^-1 due to its high specific surface area⁹. The specific capacitances of MXene/AAC electrodes based on the total mass of MXene/AAC hybrids increase with the increase of AAC mass ratio due to the enhanced BET and pore volume of MXene/AAC hybrids and the enlarged distance between the MXene layers opened by AAC powers. However, when the mass proportion of AAC and MXene increase from 1:2 to 1:1, the material density decreases from 0.309 to 0.216 g cm^-1, the MXene/AAC hybrids exhibit loose structures even could not shape a membrane. MXene/AAC hybrids exhibits increased rate capability even at 15 A g^-1 better than pure MXene electrode. When the current density increases from 0.5 A g^-1 to 15 A g^-1, the MXene/AAC 2:1 exhibits the best rate capability with the capacitance retention of 88.9%, which is higher than those of MXene/AAC 1:1(87.1%), MXene/AAC 3:1(84.3%), MXene/AAC 5:1(79.3%) and pure MXene (77.8%). The enhanced rate properties of the MXene/AAC electrodes attribute to the 3D sandwich structure constructed by MXene and AAC. The cyclic voltammograms (CVs) of the pure MXene and MXene/AAC hybrids electrodes are presented in Fig.4. (b). It is obviously noted that the shape of the CV profiles at 100 mV s^-1 for all MXene and MXene/AAC electrodes are nearly rectangular and the CV curve of MXene/AAC 2:1 electrode exhibits the utmost scanning area, indicating the low contact resistance and ideal charge propagation. Fig.S4 (b) exhibits CV images of MXene/AAC 2:1 electrode at different scan rates. The CV curves remain rectangle even at 5 V s^-1 which is the excellent characterization for the porous carbon-based electrode.

The electrochemical impedance spectroscopy (EIS) measurement is employed in 7.0 M KOH over the frequency range from 0.01 Hz to 100 KHz and presented in Fig.4.(d). It is state clearly that all the Nyquist plots include a similar semicircle in the high-frequency range and an observed straight line in the low frequency region responding to the diffusion-controlled Warburg impedance. Obviously, the semicircles in the high-frequency range for MXene/AAC 2:1 electrode are relatively smaller than other electrodes prepared under same preparation which reveal MXene/AAC 2:1 electrode has a smaller resistance and fast/easy transfer of electrons/ions to the inlayer of the material. This is possibly because Further confirm MXene/AAC 2:1 electrode is favorable for electrolyte to increase the kinematic velocity of ions through the surface of the active materials, thus increasing the contactable area between the electrolyte and electrode³⁷.