Binder-free 2D
MXene/activated carbon for High-performance Supercapacitors and
Methylene Blue Adsorption
Yue Lia,Lanshu Xua, Pascal
Kamdemb, Xiao-Juan Jina11∗Corresponding
author. Tel.: +8613718160441, Fax: None.
E-mail
address:jxj0322@163.com(XiaojuanJin).
aMOE Engineering Research Center of Forestry Biomass Materials and
Bioenergy, Beijing Forestry University, 35 Qinghua East Road, Haidian,
100083, Beijing, China.
b College of Agriculture and Natural Resources, School
of Packaging Michigan State University, East Lansing, MI 48864
Abstract: In this work,
we
have assembled the 2D MXene flakes and acid activated carbon (AAC) into
a 3D sandwich architecture which are expected to avoid the serious
restacking problem of 2D MXene flakes and develop MXene-based functional
materials. When used for energy storage, 2D MXene, as a flexible,
conductive and electrochemically active binder, furnishes a new
perspective and possibility to assemble flexible and conductive
electrodes without any current collector, binders or conductive
additives. In the 3D sandwich
architecture, the AAC particles are encapsulated between the MXene
flakes and enlarge the interlayer space of MXene, significantly
perfecting the electrochemical
performance.
The flexible 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. Impressively,
asymmetrical supercapacitors have been assembled with MXene/AAC hybrids
as positive electrode and delivers exceptional electrochemical
properties. The MXene/AAC 2:1//AAC achieves 177 F g-1at 0.5 A g-1, 97.4% retention after 10000 circles of
GCD measurements at 5 A g-1. Moreover,
the
3D sandwich architecture of MXene/AAC hybrids with great specific
surface exhibits noteworthy adsorption capacity (311.5 mg/g) to remove
Methylene blue (MB).
Keywords: MXene, acidified activated carbon, asymmetric
supercapacitor, adsorption, Methylene blue
Introduction:
Recently, fast advances of wearable and portable electrochemical
capacitors require miniaturized, lightweight, self-powered systems.
Currently, flexible supercapacitors, as a new type of energy storage
devices, have gained substantial interest in related fields due to their
fast charge-discharge rate, high power density and wide temperature
operating range and low maintenance cost1.
Manufacturing electrodes with remarkable mechanical flexibility is the
main parament for fabricating flexible
supercapacitors2. Many researches have focused
primarily on using the carbon-based electrode materials for
supercapacitors such as carbon nanotubes, carbon aerogels and activated
carbon fibers and graphene3, 4. Especially activated
carbons are the most commonly used owing to their well-developed
microstructure, high specific surface area, low cost and relatively high
packing density5-8. However, activated carbon, as
electrode materials requires polymer binders which usually occupy about
10% of the electrode mass without any capacitance contribution and
reduce the energy density of supercapacitors9, 10.
Furthermore, the polymer binders are insulator to electricity, which
usually increase the resistance, lowering the power density of devices.
Therefore, using activated carbon and conductive additives to fabricate
electrodes could increase the electrical
conductivity11. Flexibility is also a main parament to
consider in the as-obtained carbon electrodes. Because the carbon
electrodes are rather stiff and could not be employed in flexible
devices12, 13. To offset this defect, flexible
substrates are often employed to add flexibility. The traditional
flexible substrates are main carbon nanotubes, graphene and textiles
electrodes with thin conductive layers. These additives exhibit
relatively low flexibility with lightweight and occupying a large part
of the electrode volume14.
2D nanomaterials, for instance graphene, transition metal
dichalcogenides, transition metal oxides/hydroxides and MXene have
opened new possibilities in terms of energy storage applications because
they could significantly improve the electrochemical properties as
binder-free electrodes materials15, 16.
MXenes, as an emerging family of
2D transition metal carbides and nitrides has opened new possibilities
in terms of energy harvesting and storage because they exhibit
impressive mechanical performance, high electronic conductivity and
exceptional rate capability15-17. 2D transition metal
carbides MXene are generally produced by selective etching of the
A-group (generally, group Ⅲ A or Ⅳ A) element layers from MAX phase
precursors with a general formula of
Mn+1XnTx, where M is an
early transition metal, X is carbon and/or nitrogen, Txdenotes surface functional groups such as -OH, -F and/or -O and n
is1-318-20. MXene are employing as a promising,
versatile platform for energy storage equipment with the combination of
hydrophilic surface, noteworthy mechanical stability and metallic
conductivity (6000 -8000 S cm-1) which distinguish
them from other 2D materials and make them promising candidates for
energy storage, transparent conductors, sensor, catalysis and
electromagnetic-interference applications21-23.
However, similar to other 2D
nanomaterials, the electrochemical properties of MXenes are impeded by
their self-restacking and aggregation during drying and electrode
fabrication processes owing to the strong van der Waals interaction
between adjacent nanosheets24, 25, which severely
reduced their functions in their corresponding field. To compete the
shortcoming, many researches concentrate on introducing interlayer
spacers, integrating MXene sheets into 3D structures and creating porous
structures, which are proposed to tackle these
issues26, 27. Typically, Gogotsi’s groups have
proposed a fabrication of flexible, sandwich-like Mxene/CNT composite
paper electrodes through alternating filtration of CNT and MXene
dispersion28. Gogotsi’s groups have prepared
MXene/graphene electrode films for outstanding volumetric capacitance by
using electrostatic self-assembly between negatively charged MXene and
positively charged rGO with rGO inserting in between MXene
layers29.
Herein, we report a novel method to fabricate and assemble flexible
electrodes for supercapacitors by exploring
Ti3C2Tx MXene as the
binder,a flexible backbone, a conductive additive and an additional
active material. In the as-prepared electrode materials, activated
carbon particles are enclosed by MXene layers and could expand the layer
distance of the MXene which ensure easy charge and electrolyte
infiltration. The MXene layers could enhance flexibility of the hybrids
and fabricate a 3D constructive network to speed up the electronic
transmission. When fabricated as the supercapacitors electrode, the
as-fabricated flexible MXene/activated carbon hybrids show a superb
gravimetric capacitance and outstanding rate properties owing to the
rapid ion transport and electron transfer to redox-active sites in the
unique 3D conductive network. Besides, some researches have proved that
MXene could be applied for wastewater treatment and ion separation as a
promising candidate. Mahmoud’s groups have fabricated lamellar
Ti3C2Tx for selective
reduction and remediation of BrO3-from water system. Atieh’s groups have prepared the adsorption of barium
from natural and produced/co-produced water using
Ti3C2 nanoparticles as a sorbent
material. However, the organic dyestuff such as methylene blue
adsorption behavior and mechanism of MXene has not been clear yet. In
this research, we have explored the adsorption properties of MXene and
revealed its adsorption mechanism. The MXene/activated carbon has an
excellent hardness and Young’s modulus which revealed their potential
use in environmental cleaning, mechanics and energy storage.