Fig.5. (a) GCD curves of the
MXene/AAC 2:1//AAC asymmetrical capacitances at 0.1 A
g-1 at different voltage (b) GCD curves of the
MXene/AAC 2:1//AAC asymmetrical capacitances at 0.1 mV
s-1 at different voltage (c) The GCD curves of
MXene/AAC hybrids asymmetrical capacitances with various mass ratios at
the current density of 0.5 A g-1 (d) CV curves of
MXene/AAC hybrids asymmetrical capacitances with various mass ratios at
the scan rate of 0.1 V s-1 (e) specific capacitance of
MXene/AAC hybrids asymmetrical capacitances with various mass ratios at
different current densities (f) Ragone plot for energy density and power
density.
In order to further investigate the capacitive efficiency of pure MXene
and MXene/AAC electrodes for practical application, we have successfully
fabricated asymmetrical capacitances using MXene/AAC hybrids as positive
electrode and AAC as the negative electrode in 7.0 M KOH solution. In
order to determine the working voltage window of MXene/AAC, GCD and CV
images of MXene/AAC 2:1//AAC asymmetrical capacitances are measured. As
shown in Fig.5. (a), the GCD images shows ideal capacitive property in
the voltage window from -0.8-(-0.3) V to -0.8-1.2 V. Furthermore, GCD
curves of MXene/AAC 2:1//AAC asymmetrical capacitances in the voltage
range of -0.8-1.2 V at different current densities shown in Fig.S4 (c)
exhibits symmetric shapes demonstrate exceptional reversibility and
outstanding coulombic efficiency. After a series of experimental
measurements and study, an operating voltage window from -0.8 V to 1.2 V
was selected to research the electrochemical properties of the
MXene/AAC//AAC devices. Fig.5. (b) exhibits the CV images of the
synthesized MXene/AAC2:1//AAC at different working voltage windows from
-0.8-(-0.3) V to -0.8-1.2 V at the scan rate of 100 mV
s-1. Obviously, the stable voltage could range from
-0.8 V to 1.2 V with no noticeable sharp current increase due to obvious
oxygen evolution, which might originate from the electrolyte
decomposition/water splitting. The GCD images of pure MXene//AAC and
MXene/AAC//AAC devices at the current density of 100 mA
g-1 shown in Fig.5. (c) have triangular shape without
apparent voltage drop, suggesting the ideal electric double layer
properties. The specific capacitances of pure MXene//AAC and
MXene/AAC//AAC devices are calculated at various current densities and
the corresponding images are shown in Fig.5. (e). The specific
capacitance of MXene//AAC asymmetrical capacitor (114 F
g-1) is lower than those of MXene/AAC//AAC (157 F
g-1 of MXene/AAC 1:1//AAC, 177 F g-1of MXene/AAC 2:1//AAC, 167 F g-1 of MXene/AAC
3:1//AAC, 163 F g-1 of MXene/AAC 1:1//AAC). Besides,
when the current density increases from 0.5 A g-1 to
15 A g-1, the MXene//AAC exhibits the minimum rate
retention of 49.58%, which is lower than those of MXene/AAC1:1//AAC
(66.88%), MXene/AAC2:1//AAC (76.84%), MXene/AAC3:1//AAC (67.66%) and
MXene/AAC5:1//AAC (51.55%). In addition, the repetitive
charge-discharge cycling of MXene//AAC and MXene/AAC2:1//AAC exhibit
noteworthy cycling stability shown in Fig.S5, about 88.6% and 97.4% of
the initial capacity are retained after 10000 circles of GCD
measurements at the current density of 5 A g-1,
respectively.
Fig.5. (d) exhibits the CV images of pure MXene//AAC and MXene/AAC//AAC
devices. The quasi-rectangular CV shapes, reflecting an excellent
symmetry and capacitance characteristics.
The Ragone curve of MXene//AAC and MXene/AAC//AAC for energy density (E,
Wh kg-1) and power density (P, kW
kg-1) in a voltage window of -0.8-1.2 V at different
current densities is illustrated in Fig.5. (f). The expansion of voltage
window beyond 2 V is one of the main factor for improving the
electrochemical performance as energy and power density increase with
the square of the potential window. The specific energy density of
MXene//AAC and MXene/AAC2:1//AAC decreased from 23.89 Wh
kg-1 to 13.89 Wh kg-1 and from 63.95
Wh kg-1 to 19.8 Wh kg-1 based on the
total mass of active material. Nevertheless, the power density has
raised from 0.112 kW kg-1 to 5.89 kW
kg-1 and from 0.23 kW kg-1 to 11.55
kW kg-1 at the current density ranging from 1 to 15 A
g-1. The excellent electrochemical properties for
energy storage could be comparable to reported similar systems, as shown
in Table 1.
Table 1. Comparison of obtained results with reported similar systems