nanomaterials Review The Recent Advances in the Mechanical Properties of Self-Standing Two-Dimensional MXene-Based Nanostructures: Deep Insights into the Supercapacitor 1, 2, 3 3, Yassmin Ibrahim y, Ahmed Mohamed y , Ahmed M. Abdelgawad , Kamel Eid * , Aboubakr M. Abdullah 1,* and Ahmed Elzatahry 4,* 1 Center for Advanced Materials, Qatar University, Doha 2713, Qatar; [email protected] 2 Department of Mechanical and Industrial Engineering, College of Engineering, Qatar University, Doha 2713, Qatar; [email protected] 3 Gas Processing Center, College of Engineering, Qatar University, Doha 2713, Qatar; [email protected] 4 Materials Science and Technology Program, College of Arts and Sciences, Qatar University, Doha 2713, Qatar * Correspondence: [email protected] (K.E.); [email protected] (A.M.A.); [email protected] (A.E.) These authors contributed equally to this work. y Received: 11 August 2020; Accepted: 2 September 2020; Published: 25 September 2020 Abstract: MXenes have emerged as promising materials for various mechanical applications due to their outstanding physicochemical merits, multilayered structures, excellent strength, flexibility, and electrical conductivity. Despite the substantial progress achieved in the rational design of MXenes nanostructures, the tutorial reviews on the mechanical properties of self-standing MXenes were not yet reported to our knowledge. Thus, it is essential to provide timely updates of the mechanical properties of MXenes, due to the explosion of publications in this filed. In pursuit of this aim, this review is dedicated to highlighting the recent advances in the rational design of self-standing MXene with unique mechanical properties for various applications. This includes elastic properties, ideal strengths, bending rigidity, adhesion, and sliding resistance theoretically as well as experimentally supported with various representative paradigms. Meanwhile, the mechanical properties of self-standing MXenes were compared with hybrid MXenes and various 2D materials. Then, the utilization of MXenes as supercapacitors for energy storage is also discussed. This review can provide a roadmap for the scientists to tailor the mechanical properties of MXene-based materials for the new generations of energy and sensor devices. Keywords: MXene; mechanical properties; 2D materials; metal carbide; young modules; supercapacitors 1. Introduction Carbon-based nanostructures (C-Ns) such as graphene, carbon nanotubes, and carbon nitride are of great interest due to their unique physiochemical merits such as high surface area, thermal stability, and outstanding mechanical properties [1–4]. These properties promoted the utilization of C-Ns in structural composites, protective coatings, fibers, energy storage, catalysis, and durable wearable sensors; however, their complicated fabrication process remains a major challenge [5–7]. Y. Gogotsi and M.W. Barsoum groups discovered a novel family of 2D transition metal carbides or nitrides called MXene (pronounced “maxenes”) [8]. The general formula of MXene is Mn+1XnTx (n = 1–4), where M represents transition metals, A is an A-group element of group 13 to 15 in the periodic table, X is carbon or nitrogen, and Tx is surface functional groups (OH, O, Cl, F) (Scheme1)[ 8]. There are around three main structures of MXenes, including M2XTx,M3X2Tx, and M4X3Tx, derived from the selective etching Nanomaterials 2020, 10, 1916; doi:10.3390/nano10101916 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, x FOR PEER REVIEW 2 of 29 1–4),Nanomaterials where2020 M represents, 10, 1916 transition metals, A is an A-group element of group 13 to 15 in the periodic2 of 27 table, X is carbon or nitrogen, and Tx is surface functional groups (OH, O, Cl, F) (Scheme 1) [8]. There are around three main structures of MXenes, including M2XTx, M3X2Tx, and M4X3Tx, derived from the of MAX phases (M, A, and X elements are in Scheme1) including M AX, M AX , and M AX . To this selective etching of MAX phases (M, A, and X elements are in Scheme2 1) 3including2 M42AX,3 M3AX2, end, more than 30 MXenes compositions have prepared, such as Ti CT , Nb CT ,V CT , Ti C T , and M4AX3. To this end, more than 30 MXenes compositions have prepared,2 x such2 asx Ti22CTxx, Nb3 2CT2 x, Mo2TiC2Tx, Mo2Ti2C3, TiyNb2 yCTx, and NbyV2 yCTx, along with additional dozens were explored V2CTx, Ti3C2Tx, Mo2TiC2Tx, Mo2Ti− 2C3, TiyNb2−yCTx, −and NbyV2−yCTx, along with additional dozens were exploredby computational by computational methods [methods9–11]. [9–11]. MXenes possess unique physical and chemical chemical merits merits such such as as great great miscibility, miscibility, high surface area to volume volume ratio, ratio, accessible accessible active active sites, sites, surface surface charge charge state, state, electron-rich electron-rich density, density, and and absorption absorption of electromagneticof electromagnetic waves waves [12]. [12 This]. Thisis besides is besides the impr theessive impressive properties properties of 2D ofcarbide 2D carbide transition transition metal carbides/nitrides,metal carbides/nitrides, such suchas multil as multilayeredayered structures structures with with excellent excellent mechanical mechanical properties, properties, strength, strength, flexibility,flexibility, and high electrical conductivity [[12].12]. Additionally, Additionally, the the fabrication process process of MXene is scalable, productive,productive, controllable, controllable, facile, facile, and and feasible feasib forle for large-scale large-scale applications applications [12]. [12]. MXenes MXenes with with high highnegative negative zeta potential zeta potential are miscible are miscible in various in various solvents, solvents, polymeric polymeric materials, materials, and other and C-Ns other materials C-Ns materialsresulting inresulting the formation in the offormation unlimited of compositesunlimited composites with various with properties various [13properties]. The impressive [13]. The impressivemechanical mechanical properties ofproperties MXenes areof MXenes one of theareunique one of featuresthe unique for MXenefeatures [2for,14 MXene–16]. Despite [2,14–16]. the significant progress in the synthesis of MXene nanostructures, Ti C T compound is the most widely Despite the significant progress in the synthesis of MXene nanostructures,3 2 x Ti3C2Tx compound is the moststudied widely material, studied for material, various applications, for various applications, due to its impressive due to its electricalimpressive conductivity, electrical conductivity, mechanical mechanicalproperties, andproperties, electrochemical and electrochemical properties electromagnetic properties electromagneti shielding [2c ,shielding14–16]. [2,14–16]. There are are numerous numerous published published reviews reviews in inthe the fields fields of MXenes of MXenes for energy, for energy, catalysis, catalysis, and environmentaland environmental remediation remediation [12,17–2 [123].,17 However,–23]. However, the reviews the reviews on the onmechanical the mechanical properties properties of self- standingof self-standing MXenes MXenes are not areyet reported not yet reported [24]. Many [24 ].studies Many have studies shown have that shown MXenes that exhibits MXenes excellent exhibits mechanicalexcellent mechanical ion adsorption ion adsorption properties, properties,which in turn which will inset turn the stage will setfor theexploring stage forthe exploringpossibility the of theirpossibility use in of sensors their useand in flexible sensors devices and flexible [6,24–26]. devices For instance, [6,24–26]. the For strain-tunable instance, the electrochemical strain-tunable propertieselectrochemical of MXenes properties enable of MXenes them to enable be a thempropit toious be a solution propitious for solution flexible for and flexible stretchable and stretchable devices [6,24–26].devices [6 Regarding,24–26]. Regarding the electrochemical the electrochemical properties properties of MXenes, of their MXenes, large specific their large surface specific area surface makes themareamakes a promising them acandidate promising for candidate various applicatio for variousns applicationssuch as supercapacitor, such as supercapacitor, Li-ion and Sodium-ion Li-ion and batteries,Sodium-ion hydrogen batteries, storage, hydrogen adsorption, storage, and adsorption, catalysts and[6,24–26]. catalysts Due [6 to,24 the–26 ].abundant Due to theresearch abundant and ceaselessresearch andpublications ceaseless on publications the mechanical on the properties mechanical of MXene properties (more ofMXene than 146 (more articles, than according 146 articles, to SciFinder),according to it SciFinder),is crucial to it provide is crucial a timely to provide update a timely of research update efforts of research in this e ffarea.orts in this area. Inspired by by this, this, the the presented presented review review summarizes summarizes the the recent recent progress progress of research of research work work on the on mechanicalthe mechanical properties properties of self-standing of self-standing MXenes, MXenes, from fromboth boththeoretical theoretical and experimental and experimental views. views. This includes:This includes: (1) elastic (1) elastic properties properties and and superior superior strengths, strengths, (2)(2) bending bending rigidity, rigidity, (3) (3) adhesion, adhesion, and and sliding sliding resistance with their fundamental mechanism mechanism suppo supportedrted with with numerous numerous representative representative