Fig.6. (a) Effect of temperature on adsorption capacity (b) Effect of pH on adsorption capacity (c) Plot of the MB removal rate and time for different adsorbents (d) Adsorption isotherms of MB on pure MXene and MXene/AAC hybrids
The MXene/AAC could not only possess merits of 3D sandwich structure, but also presents hydrophilia, great specific surface area, chemical stabilities and noteworthy electrical conductivity, which endows MXene/AAC hybrids promising candidates for wastewater treatment. To further explore and understand possible application, the adsorption property of pure MXene and MXene/AAC hybrids is explored to research the adsorptive removal of methylene blue from wastewater.
The effects of initial solution temperature and pH on MB adsorption are researched within the temperature range from 15 to 40 ℃ and pH range between 1 to 12 and the corresponding imagines shown in Fig.6. (a-b). The adsorption capacities of MB show an obvious increase, such as the adsorption capacitance of MXene/AAC 2:1 adsorbent increased from 55.6 mg/g to 335 mg/g with the increase of pH in the range of 1-7. Similarly, the pure MXene and MXene/AAC hybrids exhibit the highest adsorption capacities when the temperature is 25℃. Adsortion isotherms of MB on pure MXene and MXene/AAC are shown in Fig.6. (d). MXene/AAC 2:1 exhibits the highest MB adsorption capacity, achieving ca. 343 mg/g. This could be explained that the compact and restacked MXene flakes with inadequate AAC, resulting in poor ion transport and electrolyte infiltration when the mass proportion of MXene and AAC over 2:1. Fig.6. (c) exhibits variation of methylene blue removal rate vs . time for pure MXene and MXene/AAC hybrids. The adsorption capacity of pure MXene and MXene/AAC hybrids is evaluated with increasing the adsorption time from 10 to 180 min. The MXene/AAC 2:1 exhibits the highest adsorption capacities (311.5 mg/g) at 180 min among all MXene/AAC and MXene adsorbents (189.4 mg/g for MXene, 243.8 mg/g for MXene/AAC 1:1, 305.1 mg/g for MXene/AAC 3:1, 233.8 mg/g for MXene/AAC 5:1). Actually, pure MXene and MXene/AAC hybrids have achieved a high adsorption capacity nearly as same as the equilibrium adsorption capacities. The high adsorption capacities maybe mainly ascribe to the great specific surface area and oxygen-containing functional groups of AAC which provided some ion-change sites by cation substitution. The oxygen-containing functional groups are prone to adsorb cationic dyes as followed equations:
\(\left[Ti-O\right]^{-}H^{+}+\text{MB}^{+}\rightarrow\left[Ti-O\right]^{-}\text{MB}^{+}+H^{+}\)(7)
\(\left[Ti-O\right]^{-}M^{+}+\text{MB}^{+}\rightarrow\left[Ti-O\right]^{-}\text{MB}^{+}+M^{+}\)(8)
The adsorption kinetics of pure MXene and MXene/AAC are examined using the pseudo-first-order and pseudo-second-order kinetics models as followed formulas:
\(\ln\left(q_{e}-q_{t}\right)=lnq_{e}-k_{1}t\) (9)
\(\frac{t}{q_{t}}=\frac{1}{k_{2}q_{e}^{2}}+\frac{t}{q_{e}}\) (10)
Where q e (mg/g) represents the equilibrium adsorption capacity, q t (mg/g) represents the adsorption capacity at any time, k 1(min-1) and k 2 (g mg-1min-1) are the pseudo-first-order and pseudo-second-order rate constant, respectively. The kinetic parameters k 1,k 2, qe and correlation coefficients (R2) are determined by linear regression exhibited in Table S2. It is obviously observed that the experimentally measured data to the pseudo-first-order are not consistent with low correlation coefficients (R2) (ranging from 0.526 to 0.831). On the contrary, the R2 is much higher than 0.831 demonstrating that data are well in agreement with pseudo-second-order which is calculated according to Fig.S6 and Fig.S7. The better agreement of the pseudo-second-order model than the pseudo-first-order model indicates that the MB adsorption over the pure MXene and MXene/AAC adsorbents mainly depends on the adsorbent concentration in the MB solution.
4. Conclutions :
In conclusion, we have demonstrated a simple method to encapsulate AAC particles into MXene flakes, shaping a 3D sandwich architecture which are expected to avoid the serious restacking problem of 2D MXene flakes. Besides, the AAC particles with great specific surface area are encapsulated between the MXene flakes and enlarge the interlayer space of MXene. When employed as a binder-free electrode for supercapacitors, the MXene/AAC hybrids exhibits noteworthy electrochemical performance, expecially the MXene/AAC 2:1 electrode. The MXene/AAC 2:1 electrode delivers the specific capacitance of 378 F g-1 at 0.5 A g-1 and a capacitance retention of 88.9% at 30 A g-1. Besides, asymmetrical supercapacitors have been fabricated using MXene/AAC hybrids as positive electrode and exhibits exceptional electrochemical properties. The MXene/AAC 2:1//AAC achieves a high energy density of 63.95 Wh kg-1 at the power density of 0.23 kW kg-1 and a 97.4% retention after 10000 circles of GCD measurements at 5 A g-1. In addition, great specific surface exhibits noteworthy adsorption capacity (311.5 mg/g) to remove Methylene blue (MB).
5. Acknowledgements
The authors gratefully acknowledge the financial support of National Natural Science Foundation of China (31470605). And thanks for the support of Chinese Scholarship Council (CSC)