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)