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 agent33-36.
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.