Binder-free 2D MXene/activated carbon for High-performance Supercapacitors and Methylene Blue Adsorption
Yue Lia,Lanshu Xua, Pascal Kamdemb, Xiao-Juan Jina11Corresponding 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.