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