Recent Progress in MXene‐Based Electrochemical Actuators and Capacitors

A new family of two‐dimensional (2D) transition metal carbides, carbonitrides, and nitrides (MXenes) has attracted increasing attention owing to their electrical, chemical, and physical properties. Together with various attractive properties of the MXenes, they also exhibit electrochemical induced deformation (i.e., expansion/contraction) via ion intercalation/de‐intercalation in/from MXene layers. In this respect, MXenes offer the possibility of application as the electrode of electrochemical actuators (ECAs). While the MXene‐based ECAs are still in their infancy stage, researchers have made great effort to achieve high‐performance MXene‐based electrochemical capacitors (ECs). As the name suggests, both ECAs and ECs shared common traits in which their operations are based on electrochemical processes. This review provides the recent progress in the MXene‐based ECAs in parallel with that in the MXene‐based ECs to gain insights from the relatively mature developments in EC applications. Finally, based on the findings from previous studies on both electrochemical applications, perspectives on future MXene‐based ECAs in terms of electrode, electrolyte, and cell configuration are provided.

etc. Therefore, it is of great opportunity for MXene-based ECAs to take a large step in advancement based on the insights from MXene-based ECs.
In this review, we provide recent progress in MXene-based ECAs and ECs.Unlike plenty of review papers regarding MXene-based energy storage devices such as supercapacitors and/or batteries, [14][15][16] only a few review papers briefly touched on MXene-based ECAs as a minor part. [17,18]Considering that many of the MXene-based ECAs have been recently developed, it is timely to deliver a review focusing on MXene-ECAs.Starting from the brief introduction of various types and synthetic methods of MXenes, this review covers recent research on MXene-based ECAs and ECs to gain better understanding of the electrochemical properties of MXene.Lastly, we highlight future perspectives on MXene-based ECAs in terms of electrode, electrolyte, and cell configuration.

Types and Preparation Methods of MXene
Typically, MXenes are 2D materials with a large surface area and a few atom-thickness.2D MXenes are composed of M nþ1 X n structure, where M and X are transient metals, such as titanium (Ti), vanadium (V), molybdenum (Mo), niobium (Nb), hafnium (Hf ), and so on, and carbon (C)/nitrogen (N), respectively. [19]epending on the combination of elements, the 2D MXene with unique electrical, electrochemical, and thermal properties can be obtained.For example, titanium carbide-based MXene, such as Ti 3 C 2 , has a high electrical conductivity and good mechanical properties, the titanium carbide MXene showed potential as a promising electrode in the electrochemical energy storage applications. [20]Meanwhile, molybdenum or niobium-based MXene, such as Mo 2 C or Nb 2 C, respectively, showed potential in thermoelectric energy harvesting applications. [13]o apply MXene in the practical applications, the preparation of the 2D MXene should be the primary focus.Generally, selective etching of A element in M nþ1 AX n structure, where A is an atom of 13 or 14 groups, is employed widely.For instance, 2D Ti 3 C 2 MXene, the most representative 2D MXene, is prepared by the selective etching of aluminum (Al) in Ti 3 AlC 2 structure with hydrofluoric acid (HF). [7]Owing to the stronger bonding between Ti and C than the bonding between Ti and Al, the chemically stable Ti 3 C 2 structure can be obtained.In this process, several functional groups, such as F, O, or OH, are terminated, resulting in Ti 3 C 2 T x (T = F, O, and OH).Selective etching of A element with fluorine-free solution is practical with hydrothermal method or electrochemical reaction.Elaborating further, Al in Ti 3 AlC 2 can react in sodium hydroxide solution, resulting in Ti 3 C 2 T x formation. [21,22]In this process, the terminated functional groups can be engineered with oxygen-contained functional groups, such as O or OH.In addition, selective etching process with Lewis acidic molten salt is another way to prepare 2D MXenes.Huang et al. introduced selective Al etching with molten salts, such as ZnCl 2 . [23]In this case, the Cl À from the molten salt reacted thermodynamically with Al atoms, and the excessive Cl À can be terminated on the 2D MXene.Apart from the selective etching process, bottom-up process, such as chemical vapor deposition (CVD), can be the alternative for preparing 2D MXenes.Similar to CVD-grown graphene, the 2D MXene can be obtained by the CVD process. [24]Although high quality 2D MXene can be obtained by the CVD method, there are several issues, including the limited gas source (mostly Mo-based source) or large-scale production issues.As the name suggests, both ECAs and ECs have common ground in involving electrochemical process for their operations.In this regard, the basic cell structures of MXene-based ECAs and ECs are identical to each other, consisting of two MXene electrodes and electrolyte (and/or separator) between them (Figure 1).When the cell voltage is applied into two MXene electrodes, the positive/negative charged ions of electrolyte are electrostatically attracted to the negative/positive MXene electrodes.For the ECAs, an ion intercalation/de-intercalation in MXene layers exhibit an expansion/contraction behavior of the electrodes, leading to a mechanical deformation from the difference in volume between two MXene electrodes (Figure 1a).Therefore, the size and/or number of intercalated/de-intercalated ions greatly affect the interlayer spacing of MXene electrodes and therefore the bending displacement of ECAs.
Meanwhile, the MXene-based ECs store the charge from the electrical double layer capacitance and hybrid pseudocapacitance (surface and intercalation pseudocapacitances) (Figure 1b).In general, there are two types of charge storage mechanisms associated with supercapacitors.For the electrical double layer capacitance, charges are accumulated at the electrode/electrolyte interface, forming an electrical double layer. [25,26]This process does not involve electron transfer or any redox reaction.Instead, energy is stored in the materials through a non-Faradic process.The charge accumulation occurs without the transfer of ions across the electrode.In contrast, in the case of pseudocapacitors, electron transfer or faradic redox reactions take place on the surface of the electrode materials.There are two main mechanisms of charge storage for pseudocapacitors.First, the redox pseudocapacitive materials, such as RuO 2 , [27,28] TiO 2 , [29] MnO 2 , [30] metal organic frameworks (MOFs), [31,32] and conducting polymers, [33][34][35] can store and release energy by utilizing the redox reactions with Faradaic charge transfer at or near the surface of the materials.[38] However, most of the pseudocapacitive materials have poor electronic conductivity, resulted in a relatively lower power density than electrical double-layer capacitors (EDLCs).Unlike the MXene-based ECAs, the MXene-based ECs do not show an electrochemical induced bending behavior due to the usage of rigid current collector and/or cell components (e.g., coin cell).Despite the relatively low specific surface area of MXenes (20-100 m 2 g À1 ) compared to the carbon-based materials (500-3500 m 2 g À1 ), their flakes possess remarkable energy density due to their hybrid pseudocapacitance. [39,40]heir ordered layered structures provide the adsorption sites for intercalation pseudocapacitance, while the functional groups (─T) at the terminal position offer the redox reactions for redox pseudocapacitance.These exceptional features make MXenes extremely desirable for supercapacitor applications.

Figure-of-Merits for ECAs and ECs
Figure-of-Merits for ECAs can be classified into the actuation performance and the mechanical strength of MXene-based electrode and/or ECA.More specifically, the actuation performance can be evaluated by: 1) how much it can be deformed with respect to electrochemical response (e.g., change in c-lattice parameter (Å), relative deformation (volumetric %), bending strain (%), amplitude of strain (volumetric %), and curvature change (mm À1 )), 2) how much force can be achieved when completely blocked (e.g., blocking force (mN)), 3) how fast is the electrochemical bending behavior (e.g., response time (s) and frequency response (Hz)), and 4) how stable it can maintain the pristine bending performance over the cycles/duration time of repetitive potential variation (e.g., bending retention (%)).In terms of mechanical strength of MXene-electrode/ECA, Young's modulus (Pa) was used to express how rigid/soft it is.
For the ECs, there are plenty of Figure-of-Merits used for evaluating their performance.To facilitate a more comprehensive comparison, we aim to present several valuable Figure-of-Merits.Capacitance (F) is a fundamental concept in the ECs, describing the ability of a device to store electrical charge.It shows how much electric charge can be accumulated on a device for a given electric voltage range.The cell voltage (V) is also one of the crucial factors in terms of total amount of stored energy, proportional to the square value of cell voltage.The energy and power densities are used to determine the actual energy that can be stored in the devices and to represent its ability to quantify the speed at which energy can be delivered or extracted from energy storage devices, respectively.They are widely plotted as the Ragone plot, converted into watt-hours (Wh) and watt (W), respectively, normalized to mass and/or area of active materials for comparative analysis of different energy storage devices.Based on the intrinsic connection between two electrochemical devices, the bending/curvature displacement and response speed of the ECAs would be highly linked with the energy and power density of ECs, respectively.

Pristine MXene-Based ECAs
MXenes mainly attracted significant attention in the fields of batteries and electrochemical capacitors owing to their good electrical conductivity, hydrophilic surfaces, and 2D layered structures.Despite the low specific surface area of multi-layer exfoliated MXene (e.g., 23 m 2 g À1 ), their intercalation capacitances are comparable to those from high-specific-surface-area carbon materials (e.g., 1000-3000 m 2 g À1 ). [16,41]Therefore, researchers endeavored to investigate the charge storage mechanism of MXene and found the phenomenon that the intercalation of cations changes the interlayer spacing of MXene (expansion/ contraction) in aqueous and non-aqueous electrolyte systems.

Aqueous Electrolytes
In the early stage, Lukatskaya et al. observed that the cations (e.g., þ , and Al 3þ ) can be intercalated into MXene layers in both spontaneous (immersed MXene in electrolyte) and electrochemical ways (potential applied to MXene) by using in situ X-ray diffraction (XRD) analysis. [41]Interestingly, the spontaneous intercalation was entirely attributed to the cations, which was supported by the comparable c-axis expansions in three sodium salts with different radii of anions and the absence of element of anion (e.g., sulfur) in energy-dispersible X-ray spectroscopy (XPS) analysis after intercalation.In terms of electrochemical ion intercalation, the c-lattice parameters of multi-layer MXene showed contraction/expansion when decreasing/increasing potential value with various aqueous electrolytes (e.g., Li 2 SO 4 , Mg 2 SO 4 ) (Figure 2a,b). [42]These contraction/expansion behaviors can be attributed to the increased/decreased electrostatic interaction between positively charged cations and MXene layers when applying negative/ positive potential.The electrochemical induced deformation of MXene was also confirmed by using in situ atomic force microscopy (AFM) measurement (Figure 2c-f ). [43,44]During the AFM operation, the tip was kept contacting at the surface of MXene electrode to investigate its deformation and/or elastic changes resulted from electrochemical intercalation of cations.Interestingly, Gao et al. reported that pre-intercalation of K þ adversely affected the subsequent intercalation of Mg 2þ into MXene. [44]After the pre-intercalation process with K þ , the relative displacement of MXene decreased from ≈23 to ≈16% (contraction) due to the remaining trapped K þ in MXene.This electrochemical induced deformation of MXene was also observed by the elastic changes of MXene with respect to the applied potential via in situ AFM measurement.
[47] Pang et al. demonstrated the electrochemical actuation of MXene with both 1 M H 2 SO 4 liquid and H 2 SO 4 /PVA gel electrolytes in three-and two-electrode cell configurations, respectively. [45]The changes in curvature and strain of MXene were clearly shown as a function of scan rates and frequency at maximum voltage in both MXenebased cells with 1 M H 2 SO 4 (0.083 mm À1 and 0.29%) or H 2 SO 4 / PVA electrolytes (0.038 mm À1 and 0.26%).This actuation would be attributed to proton intercalation into MXene layers and changes in surface functional group from ═O to ─OH, which corresponded well with the variation of c-lattice parameter in in situ XRD data. [45,46]Especially, the solid-state MXene-based actuator with H 2 SO 4 -PVA gel electrolyte gave promise of future applications such as robotic arms.Moreover, Zhao et al. achieved the largest change in interlayer spacing of MXene in aqueous electrolytes (2.58 Å) by using environmentally friendly methanesulfonic acid (CH 3 SO 3 H) electrolyte (Figure 2g-i). [47]However, this large displacement in MXene cannot be explained by simple cationic (de-) intercalation because of its small cationic size (H þ or H 3 O þ ).Therefore, they suggested the possibility of the intercalation of cations with large-sized solvation shell or multi-layered cations into MXene layers, or the concurrent intercalation of cations and anions (CH 3 SO 3 À ), or their combination.Despite additional ex situ XPS analysis after electrochemical test and electrochemical characterization with electrolytes consisting of same cation (H 2 SO 4 ) and anion (CH 3 SO 3 Na) as methanesulfonic acid (CH 3 SO 3 H), it remained unclear and still required further investigation to clarify the roles of the respective ions.

Non-Aqueous Electrolytes
In addition to various aqueous electrolytes, the change in interlayer spacing of MXene was also observed from the intercalation of ionic liquid electrolyte using in situ XRD and/or electrochemical dilatometry measurement. [48,49]Firstly, Lin et al. exhibited the increase/decrease in the c-axis of MXene with 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide (EMImTFSI) electrolyte under negative/positive polarizations via in situ XRD analysis. [48]The steric effect of intercalated EMIm þ cation into negatively charged MXene (expansion) and the de-intercalated EMIm þ cation and/or the electrostatic attraction of intercalated TFSI À anion at positively charged MXene (contraction) led to increased/decreased interlayer spacing of MXene, respectively.In Jackel's study, the volumetric change of MXene electrode with respect to applied potential was further investigated with two different ionic liquids (EMImTFSI and 1-butyl-3-methylimidazolium tetrafluoroborate, BMImBF 4 ) via dilatometry as well as in situ XRD measurements. [49]Both ionic liquids showed a similar trend in electrochemical deformation of MXene, but contrary to expectations, the BMImBF 4 exhibited lower strain than the EMImTFSI (4% versus 9%) at negative potential region despite the larger cationic size of BMIm þ than EMIm þ (0.90 versus 0.76 nm) (Figure 2j,k).This lower expansion of BMImBF 4 was explained by the formation of more complete monolayer of BMIm þ inside MXene layers, confirmed by in situ XRD analysis with the variation of (002) peak under negative polarization (saturated peak shift, increased peak intensity, and decreased full width at half maximum of peak).In addition, the BMImBF 4 has a higher viscosity than the EMImTFSI, which results in slower ionic mobility of BMIm þ , and therefore lower total electrochemical deformation of MXene at the applied potential.

Modified MXene-Based ECAs
Pristine MXene gave promise of being a suitable electrode for ECA based on the evidence of electrochemical deformation in previous studies.However, the relatively densely packed structure of MXene provided a narrow interlayer spacing, a relatively low mechanical strength, and an inefficient ion transportation, hindering further development of MXene-based ECAs.Therefore, researchers have made efforts to enhance the electrochemical actuation performance of MXene electrode by forming composite materials with polymers or introducing additional pore structures.
Moreover, a polypyrrole (PPy) conducting polymer was adopted to achieve high electrochemical actuation performance. [52]The MXene/PPy bilayer composite film was strategically designed to take advantage of high electrical conductivity of MXene and large electrochemical deformability of PPy.Unlike the MXene/PEDOT:PSS composite, the MXene mainly acted as a current collector in MXene/PPy composite to enhance the redox reaction rate of PPy and therefore its electrochemical actuation performance.The MXene/PPy bilayer film was fabricated by electrodeposition of PPy on MXene surface, showing improved Young's modulus and strain (0.323% versus 0.301%) and curvature changes (0.083 mm À1 versus 0.071 mm À1 ) .Reproduced with permission. [51]Copyright 2022, American Chemical Society.c-f ) Schematics and morphologies of pristine MXene (c,e) and MXene/polystyrene-MXene films (d,f ).Reproduced with permission. [53]Copyright 2021, Springer Nature.g) Schematic of hierarchical porous MXene/metal organic framework nanoarchitecture.h,i) Demonstrations of eyelid-blinking (h) and eyeball-movement in a doll (i) with MXene/ metal organic framework-based ECA.Reproduced with permission. [55]Copyright 2023, Wiley-VCH.
compared to neat PPy.However, weaker cycle stability was observed in MXene/PPy bilayer-based ECA (69.2% over 10 000 cycles), which can be explained by the separation of PPy layer from MXene layer due to the shear forces arising from repetitive large expansion/contraction of PPy.
In addition to the conducting polymers, non-conducting polymers were also studied as flexible spacer and/or 3D framework of MXene electrode for ECAs.In Wang's study, a spherical polystyrene (PS) polymer (1 μm) was introduced as a framework of 3D MXene/PS-MXene structure via vacuum filtration method. [53]he addition of PS into MXene provided facile migration pathways for electrolyte ions, leading to a fast ion intercalation process into MXene composite structure (Figure 3c-f ).Therefore, the fabricated ECA with MXene/PS-MXene composite films exhibited larger bending deformation (peak-to-peak strain and displacement = 1.18% and 35 mm versus 0.68% and 18 mm) and broader frequency response (up to 10 versus 5 Hz) compared to MXene-based counterpart, excellent stable durability (90% bending retention after 10 000 cycles), and fairly high Young's modulus (246 MPa).Moreover, Lee's group adopted a methylcellulose polymer as a flexible spacer of MXene, fabricating a molecular-level MXene/methylcellulose (MC) composite films with enhanced layer spacing of MXene (1.2 !1.4 nm). [54]nterestingly, the hybrid MXene/MC showed in-plane sliding of MXene sheets as well as conventional out-of-plane electrochemical deformation of MXene.Since the MXene sheets surrounded by MC polymer chains formed a nanogap container, the increase in internal pressure of container under negative potential allowed MXene sheets to slide away.Owing to the additional in-plane electrochemical deformation and the enlarged layer distance of hybrid film, the MC/MXene-based ECAs showed high electrochemical actuation performance with large peak-to-peak strain (0.541%) and displacement differences (8.17 mm) with decent blocking force (0.826 mN).

MXene with Pore Architecture
The stacked structure of MXene suffers from not only a narrow interlayer spacing but also a long ion transport pathway through MXene layers.In this respect, the well-designed pore structure embedded in MXene layers could provide efficient ion transportation and intercalation process, leading to fast actuating response of MXene-based ECAs.Recently, Oh's group opened the possibility of porous MXene materials by introducing hierarchical manganese-based 1,3,5-benzenetricarboxylate metal organic framework (MnBTC) nanoarchitectures inside MXene layers. [55]The hierarchical porous manganese-based 1,3,5-benzenetricarboxylate MOF (MnBTC) was directly synthesized on MXene sheets (Figure 3g).Therefore, the MXene/MnBTC exhibited an enhanced interlayer spacing (2θ = 6.5°versus 7.4°) compared to neat MXene and a hierarchical porous structure with a majority of mesopore (d ≥ 2 nm), enabling facile ion movement through MXene layers.The ECA performance of the MXene/ MnBTC was evaluated after hybridizing with PEDOT:PSS as a conductive matrix.As a result, the MXene/MnBTC/PEDOT: PSS-based ECA achieved outstanding electrochemical actuating performance including high bending displacement (12.5 mm) and ultrafast response speed (0.77 s), broad frequency response (0.1-10 Hz), and excellent cyclability and durability (96% bending retention after 604 800 cycles or 7 days).Notably, its exceptional cycle stability performance was achieved when operating in an open-air environment, which can meet the requirement of real-world robotic application.Considering that the pristine MXene-based ECAs with H 2 SO 4 /PVA electrolyte exhibited a decent cycle stability of 80.4% over 10 000 cycles, [45] the stability of MXene-based ECAs could be enhanced by adopting composite materials and pore architectures properly.Moreover, the fabricated ECA successfully demonstrated the eyelid blinking and eyeball movement in a doll based on the working principle of artificial human robotic eye (Figure 3h,i).The important properties of MXene-based electrode/ECAs including strain change, blocking force, frequency response range, stability, and Young's modulus are summarized in Table 1 for comparison.

In-Depth Studies for Revealing ECA Mechanism of MXene
To make a huge leap in performance of MXene-based ECAs, it is necessary to look more closely at the relationship between ion intercalation/de-intercalation process and electrochemical actuation phenomena.Several in-depth studies were performed to elucidate the ECA mechanism of MXene via electrochemical quartz-crystal admittance (EQCA), in situ XRD with density functional theory, and time-resolved operando X-ray reflectivity.

Levi et al. investigated the electrochemical deformation of MXene with neutral electrolytes including various cations
Ca 2þ , Ba 2þ , and three tetraalkylammonium cations) by using EQCA measurement. [56]As shown in Figure 4a-c, the geometric parameters of MXene electrodes including effective thickness, h, and permeability length, ξ, were determined by fitting the experimental data into the admittance model.The EQCA results exhibited the different contraction/ expansion behaviors of MXene tested with the chloride solutions having different cations.These data indicated that the ionic size is the main factor for electrochemical deformation of MXene when using larger radii of cations 4a,c).In the case of Na þ cations, there was trade-off between the expansion from intercalation and the contraction from strong attraction between MXene sheets and increased Na þ cations (Figure 4a).The alkali-earth cations (Mg 2þ , Ca 2þ , and Ba 2þ ) generally showed contraction during negative polarization regardless of their large ionic radii, indicating that the cationic charge is more critical for the deformation of MXene than its ionic size (Figure 4b).Lastly, the smallsized Li þ ions followed the trend of alkali-earth cations despite single charged ions, resulting in contraction at negative potential region (Figure 4a).As a result, it was concluded that the higher/ lower ratio of the cationic charge to its size leads to the contraction/expansion of MXene, respectively.
In Mu's study, the proton intercalation into MXene with acid electrolyte (e.g., H 2 SO 4 ) was intensively studied in three divided potential ranges via in situ XRD measurement and density functional theory (DFT) calculation. [46]In Figure 4d, the electrochemical in situ XRD results displayed mixed expansion/shrinkage states of MXene with H 2 SO 4 in an irregular manner with applied potential values.With the consideration of cyclic voltammetry data and DFT calculation, they suggested a reliable hypothesis categorized by three main potential regions.Firstly, within the potential range from -0.25 to 0 V versus.Ag, only electrochemical double layer was formed at MXene without proton intercalation (almost no c-axis variation).Below -0.25 V versus Ag, the occurrence of proton intercalation led to electrostatic contraction of MXene.Together with proton intercalation, the redox reaction involving conversion of some O-terminations (Ti 3 C 2 O 2 ) to OH-terminations of MXene (Ti 3 C 2 (OH) 2 ) was just beginning.Reproduced with permission. [56]Copyright 2015, Wiley-VCH.d-f ) The variation in c-lattice parameter of MXene with H 2 SO 4 electrolyte in response to applied potential (d) and simulation results of O-terminated (e) and OH-terminated MXenes (f ) according to intercalation of H 2 O and H þ ions, respectively.Reproduced with permission. [46]Copyright 2019, Wiley-VCH.g-i) Dynamical structural response of MXene with the indications of two different contraction regimes (slow and fast) (g) and illustrations presenting two different charge storage mechanisms corresponding to slow and fast contractions, respectively.Reproduced with permission. [57]Copyright 2022, American Chemical Society.In addition, Sobyra et al. reported the first operando measurements of dynamical structure change of MXenes according to electrochemical Li þ ion intercalation via X-ray reflectivity measurement. [57]They revealed that there are two types of different dynamical responses: slow and fast lattice contractions associated with the capacitive and redox features in CV curve (Figure 4g).Each contraction rates (Δðd Ti 3 C 2 Þ=Δt, Å s À1 ) obtained in slow and fast contraction regions showed the linear trend of square root of sweep rate (v 1/2 ) and sweep rate (v), respectively.These different linear trends in two regions indicated that the slow contraction is limited by ion diffusion through MXene layers (∝ v 1/2 ) but the fast contraction is intrinsically rapid without mass transport limitation (∝ v) (Figure 4g-i).These findings provided insight into the correlations between charging storage mechanism and electrochemical actuation performance of MXene, and therefore the development of fast-response MXene-based ECAs in the future.
Recently, Lee's group investigated the structural changes of MXenes in real time with various in situ/ex situ techniques. [58]he neat and tetrabutylammonium-functionalized MXenes (TBA-MXene) have been studied to understand the large improvement in actuation performance of TBA-MXene.Notably, the in situ Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM) analyses were newly adopted to substantiate the desertion/insertion of cation from/into MXene electrodes and the in-plane strain of MXene layers at high/low potentials, respectively.Based on the multi-perspective characterizations including these two in situ techniques, the large actuating behavior of TBA-MXene was clearly explained by its more efficient in-plane actuation mechanism compared to that of neat MXene.More specifically, the sheet sliding mechanism is induced by the co-(de)insertion of TBA and Li ions in TBA-MXene, leading to the large improvement in its actuation.In addition, this work successfully demonstrated the enhancement in force output of TBA-MXene-based ECAs by reducing the length or increasing the width of fabricated actuator, respectively.This research shows the great importance of real-time in situ techniques to gain a deeper understanding of electrochemical actuation mechanism of neat/modified MXenes.

MXene-Based ECs
MXenes have attracted significant attention for their superb electronic conductivity and high surface area, making them highly promising for supercapacitor applications.The inner interlayer spacing within MXene flakes creates an efficient pathway for ion movement, making it suitable as materials for an EDLC.Additionally, the outermost layers of MXenes offer abundant redox sites, enabling the pseudocapacitive energy storage.Owing to their distinctive electronic structures, as well as their remarkable physical and chemical properties, MXenes have garnered significant attention and are now recognized as cutting-edge functional materials for supercapacitors in the next generation.

Pristine MXene-Based ECs
MXenes flakes tend to self-restack due to the strong interlayer van der Waals interactions and hydrogen bonds even after successful exfoliation.This phenomenon, which is easily observed in other 2D materials, leads to the limited active sites and hindered ion-transport channels, resulting in a substantial decrease in the electrocapacitive properties of MXene electrodes.Hence, manipulating the interlayer structure and controlling their surface chemistry of MXenes can significantly enhance the electrochemical performance of MXene-based supercapacitors.

Manipulating the Interlayer Structure on MXene
Enhancing the performance of MXenes can be achieved by manipulating the interlayer structure, which improves ion transport and accessibility to active sites.Therefore, the design of a 3D or porous electrode structure offers even greater potential for improving the high-rate capacitance performance.Such a structure provides a large ion-accessible active surface area and interconnected pores, creating efficient channels for ion transport.
Li et al. introduced a novel 3D MXene aerogel through a simple ethylenediamine (EDA)-assisted self-assembly process. [59]his unique aerogel structure (Figure 5a,b) effectively prevents restacking and allows the interconnected nanosheets to form numerous channels and pores which was also confirmed by the XRD pattern (Figure 5c).As a result, the aerogel exhibits an impressive specific surface area of up to 176.3 m 2 g À1 , along with a significantly increased pore volume, which is twelve times greater than that of the stacked MXene films.The porous aerogel demonstrates remarkable performance, achieving a high areal capacitance of 1012.5 mF cm À2 in a 1 M KOH electrolyte (Figure 5d).Additionally, it exhibits excellent rate capability and cycling stability (Figure 5e).
Likewise, the freeze-drying method offers a convenient approach to achieving the preparation of 3D structures.During the freeze-drying process of the MXene film, the water molecules within the MXene interlayer undergo a transition, converting into small ice grains.These ice grains then serve as self-sacrificial templates, enabling the opening of the structure and the construction of a porous domain within the MXene film.Yang et al. adjusted the concentration and the size of the MXene flakes to facilitate to 3D inkjet printing. [60]The printable Ti 3 C 2 T x was directly printed on top of the glass to fabricate the symmetric micro-supercapacitors (MSCs) with interdigitated electrode.Following the freeze-drying process, the Ti 3 C 2 T x nanosheets were structured into an internal network, featuring a diverse range of pore sizes ranging from 3 to 35 μm.With large specific surface area of 177 m 2 g À1 .The fabricated symmetric supercapacitors with solid H 2 SO 4 /PVA electrolyte, to deliver the areal capacitance of 2.1 F cm À2 at 1.7 mA cm À2 .Shi et al. successfully prepared porous Ti 3 C 2 T x -foam by subjecting the thermal treatment with the assistance of hydrazine vapor. [61]The resulting Ti 3 C 2 T x -foam exhibited an interconnected porous structure, which effectively prevented restacking of the Ti 3 C 2 T x nanosheets.The thickness of the Ti 3 C 2 T x film was around 6 μm.However, upon thermal treatment with 80 μL hydrazine monohydrate, interconnected porous structure was formed while the thickness increasing to around 50 μm with enlarged c-lattice parameter.This porous structure reduced the diffusion path of ions and facilitated faster intercalation/de-intercalation of ions.As a result, the Ti 3 C 2 T x -foam demonstrated a high areal capacitance of 271.2 mF cm À2 at a scan rate of 5 mV s À1 in 1 M KOH electrolyte.The rational design and construction of porous MXenes have been shown to suppress the restacking of MXene flakes, enabling high utilization of active sites and fast ion transfer channels.This design approach enhances the specific capacitance of MXenes.

Surface Chemistry on Pristine MXene
The surface terminal functional groups play a crucial role in the electrochemical performance of MXene materials, as these functional groups provide the redox active sites.The surface chemistry of MXene materials is commonly influenced by the synthesis methods and conditions employed during their fabrication.Most of the Ti 3 C 2 T x MXenes for supercapacitors were prepared by removing the "A" elements by hydrofluoric acid (HF) or fluoride-based salt with etching process, generating various negatively charged functional groups, e. g., ─OH, ─F, and ─O.
At the early stage, Ghidiu et al. reported etching in mixed lithium fluoride (LiF) and concentrated hydrochloric acid (HCl) solution for the MXene flakes to have the (002) peak much lower angle than that of the MXene flakes by HF etching, indicating the broaden layer spacing to provide the ease ion transport in MXene flakes. [62]Subsequently, NMR characterization revealed that this method also affects the surface termination. [63]They demonstrated that LiF/HCl etched sample has more ═O termination groups and less ─F and ─OH groups than HF etched sample.Also, ─OH termination groups are less than ─F or ═O groups because vicinal ─OH groups tend to be ═O termination groups through the condensation reaction, showing that there is no neighbouring ─OH termination.This study has presented a comprehensive depiction of realistic map of the surface terminations on Ti 3 C 2 sheets.Through the DFT simulation, Mu et al. found that the ═O terminal groups are contracting when the H 3 O þ ion is incorporated while ─OH terminal groups have a higher c-lattice parameter at the same situation. [30]Based on additional in situ XRD pattern analysis, it is believed that Ti 3 C 2 undergoes the following chemical equation in an H 2 SO 4 electrolyte: [46,64]  .Reproduced with permission. [66]Copyright 2019, American Chemical Society.
In order to replace the inert ─F terminal group to ═O groups, alkali treatment with KOH and calcination method could successfully enhance the capacitance over 500 F g À1 (at 1 mV s À1 ). [65]fter the treatment by KOH, the intensity of the Ti-F peaks decreases quickly while the contents of ─OH significantly increased confirmed by the XPS analysis increasing the interlayer distance (9.6 Å to 12.5 Å) indicating K þ ion intercalation.
Subsequent annealing process at 400 °C reduces the ─OH termination.However, high temperature annealing process induced the oxidation of Ti atom to generate small amount of anatase TiO 2 .To suppress the oxidation of the Ti atom at the surface and enhance ═O termination group on the MXene flakes, Chen et al. suggested the n-BuLi treatment following the water washing process resulted in much more ═O while limiting the ─F terminal group (Figure 5f,g). [66]This surface functional group substitution method provides a high capacitance of 523 F g À1 at 2 mV s À1 with 96% capacitance retention even after 10 000 cycles.
To avoid the ─F terminal group, Li et al. pave the way to the fluorine-free synthesis of MXene flakes using NaOH aqueous solution in high concentration of NaOH (27.5 M) and high temperature (270 °C). [22]Although slight aluminum was detected on the surface of the MXene flakes, they are successfully synthesized the MXene flakes without using fluorinated solution showing further improvement could be achievable with fine-tuning of the concentration and the temperature.However, their fluorine-free etching process also affected to the terminal Ti atom to oxidize resulted in the appearance of the partial TiO 2 , and low conductivity and capacitance retention.
Heteroatom substitution of the terminal functional groups can be realized by Lewis acid molten salt method.In 2019, Li et al. first presented the method to prepare Cl-terminated MXene flakes.They replaced Ti 3 AlC 2 to Ti 3 ZnC 2 using ZnCl 2 , which have relatively low meting points and strong Lewis acidity. [23]Consequently, the removal of Zn was easily achieved through HCl treatment, resulted in the disappearance of XRD peaks associated with Zn, while the peaks corresponding to Ti 3 C 2 Cl 2 remained unchanged.Furthermore, through the combination of the AlBr 3 /NaBr/KBr eutectic molten salts, F-terminated MXene could be substituted by the Br-terminated MXene with increased d-spacing from 11.2 to 14.7 Å (Figure 6a-c).In an 1 M LiPF 6 /EC/DMC electrolyte, this particular MXene showed an outstanding coulombic efficiency of 96% and achieved a maximum capacity of 229 mAh g À1 at a specific current of 0.1 A g À1 , which was twice as high as the pristine MXene. [67]Owing to the relatively weaker bonding energies of Ti─Cl and Ti─Br compared to the Ti─F and Ti─OH bonds.Cl─ and Br-terminated MXenes can provide a versatile platform for the synthesis of diverse surface terminal groups (e.g., O, S, Se, Te, NH, vacancy). [68]For instance, post-thermal treatment of Reproduced with permission. [67]Copyright 2022, Springer Nature.d) Schematic illustration of the fabrication process of N-containing Ti 3 C 2 T x .Atomic resolution high-angle annular dark-field (HAADF) images of e,f ) Cl-containing and j,k) N-containing Ti 3 C 2 T x samples at different scale.g) c-lattice parameter changes in the electrochemical process.h) CV graph of the N-containing (blue), Cl-containing (black) films and the film with conventional etching process (orange).i) The rate performance of the N-containing MXene film.Reproduced with permission. [69]Copyright 2023, Wiley-VCH.
Cl-terminated Ti 3 C 2 in Li 3 N successfully substituted ─Cl to ─N, resulted in extremely high-rate performance over 300 F g À1 at 2 V s À1 (Figure 6d-k). [69]This method makes it feasible to produce new types of functionalized MXenes that are difficult or even impossible to prepare using conventional synthesis routes such as HF etching.However, due to the requirement of high temperatures (>300 °C) and the need for an Ar-filled glove box with extremely low oxygen and moisture levels (below 1 ppm), these methods have not been widely adopted and implemented.Therefore, further research should focus on reducing the reaction temperature and conducting the synthesis under ambient conditions.Additionally, more investigations are needed to explore the electrochemical performance of MXenes with precisely controlled surface termination groups.

Small Molecule Intercalation
As mentioned above, restacking phenomenon is a major culprit to degrade the energy storage performance of the MXene, making the less ion transport to the active sites.Enlarging the interlayer spacing with pillaring the small molecules can be an effective strategy to overcome the restacking problems.
Luo et al. compared the effects of three different cationic surfactants.Immersing in 40 °C of cationic surfactant solution, it was found that cetyltrimethylammonium bromide (CTAB) achieved the largest interlayer spacing of MXene flakes at 2.230 nm at 40 °C (Figure 7a). [70]Additional ion-exchange of Sn 4þ intercalation, Sn 4þ formed a Sn (IV) nanocomplex effectively and uniformly anchored within the MXene flakes.The asymmetric Li-ion capacitors fabricated by assembling the MXene/Sn(IV) complex and activated carbon (AC) delivered 45.31 Wh kg À1 of energy density at 10.8 kW kg À1 of power density.Another approach to expanding the interlayer spacing of MXenes involved the use of amorphous carbon derived from organic sources as intercalation agents.Shen et al. demonstrated a simple strategy for in situ carbonization of long-chain fatty amines between MXene flakes, resulting in a carbon-intercalated structure. [71]The incorporation of amorphous carbon expanded the interlayer spacing from 0.99 to 2.83 nm, facilitating rapid diffusion of electrolyte ions into MXene layers and improving the gravimetric capacitance performance compared to pristine MXene flakes.
In another study, Xia et al. introduced hexaethylene glycol monododecyl ether (C 12 E 6 ) as a pillar and lamellar structuring agent (Figure 7b). [72]They achieved vertical alignment by mechanically shearing with a discotic lamellar liquid-crystal Figure 7. a) Schematic illustration of preparation of CTAB-Sn(IV)@Ti 3 C 2 by HF etching.Reproduced with permission. [70]Copyright 2017, American Chemical Society.b) Schematic illustration of ion transport in MXene lamellar liquid crystal (MXLLC) films.Reproduced with permission. [72]Copyright 2018, Springer Nature.c) Schematic illustration with the optical photographs of the synthesis process of the 3D porous MXene-rGO film.Cross-section SEM images of d) 3D porous MXene-rGO-20 film and e) dense MXene film.f ) XRD patterns of the MXene-rGO films and dense MXene film.g) Gravimetric capacitances of the MXene-rGO films and dense MXene film at various scan rates.(c-g) Reproduced with permission. [73]Copyright 2021, Elsevier.
phase of the pillared MXene.They clearly confirmed the lamellar structure of the MXene lamellar liquid crystal (MXLLC) with small angle X-ray scattering and SEM image.The vertically aligned MXene films, with a thickness of 200 μm, demonstrated outstanding rate capacitance performance, reaching 600 mF cm À2 at a high scan rate of 2000 mV s À1 .Furthermore, these films exhibited an impressive capacitance retention of nearly 100% over the course of 20 000 cycles.

Composite Structure
The consideration of an interlayer structure modification, specific surface chemistry, and the creation of electrochemically active sites provide the MXene for suitable energy storage applications.However, the challenges associated with MXene, such as restacking and lower mechanical properties, impede further exploration of its potential.To overcome these obstacles and enhance supercapacitor performance, a promising approach is the composite formation with other electroactive materials.By incorporating MXene into the composite structures, the drawbacks can be mitigated, thanks to the MXene's exceptional elastic modulus, ability to adjust surface terminal groups, and high hydrophilicity, making it an ideal filler for composite materials.
Carbon-based materials possess exceptional physical and chemical properties, including high surface area and excellent electronic conductivity, making them highly desirable for energy applications.To enhance the electronic conductivity of MXene, prevent restacking, and enhance electrochemical storage performance, numerous carbon-based materials have been incorporated into MXene-based composites.Notably, graphene, carbon nanotubes (CNTs), and mesoporous carbon are among the carbon materials that have received significant attention in this field.
Miao et al. presented 3D MXene-reduced graphene oxide (rGO) composite film using self-propagation method (Figure 7c-g). [73]A MXene/GO film was prepared by vacuum filtration of a mixture of MXene and GO solution, followed by contact with a 300 °C hot stage under an argon atmosphere to initiate a self-propagating reaction.This reaction rapidly propagated throughout the entire film within a short time of 1.25 s, leading to the reduction of GO and the instantaneous release of a significant amount of gas.As a result, the macropores of rGO were formed between the MXene multilayers, creating a threedimensional (3D) porous structure.After the self-propagating process, the 3D MXene/rGO film exhibited increased thickness (5.66-80.8μm for 20 wt% MXene-rGO composite) while retaining the electrical conductivity comparable to the compact MXene film.In terms of electrochemical performance, the 3D MXene/rGO film with 20% rGO content exhibited a high capacitance of 329.2 F g À1 at 5 mV s À1 .The composite film, benefiting from its porous structure, displayed remarkable rate performance by achieving a capacitance of 260.1 F g À1 at a challenging scan rate of 1000 mV s À1 .This enhanced performance can be attributed to the improved connectivity between the MXene and rGO components within the film.Furthermore, this composite film demonstrated remarkable capacitance retention, with over 90% retention even after 40 000 cycles at a high current density of 100 A g À1 .Gao et al. reported 3D MXene/CNT knotted composite by mixing Ti 3 C 2 suspension with knotted CNT solution via selfassembly process followed by vacuum filtration. [74]The knotted CNT were prepared by carbonization of Ni-Mn-Al-O catalyst, through two-step temperature-assisted programmed at 750 and 950 °C under H 2 /C 2 H 2 atmosphere.The CNT impeded the restacking of the MXene, thus the rate performance was significantly enhanced.By employing the EMIM-TFSI/ LiTFSI/ACN electrolyte, a capacitance of 130 F g À1 at a scan rate of 10 mV s À1 was attained.Notably, the composite film exhibited excellent capacitance retention, maintaining 73 F g À1 , which corresponds to 56% of the initial capacitance, at 10 mV s À1 , while the pristine MXene only retained 20% of its initial capacitance.
On the other hand, the metal oxides (MOs) are one promising class of materials for energy storage application due to their impressive specific capacitance from their unique redox reaction, ease processability, and abundant availability.However, the limited electrical conductivity of MOs hinders their performance in the development of practical application.To address this constraint, the integration of MXene in the formation of nanocomposite materials offers a solution by enhancing conductivity, enabling fast electron transport.Additionally, the nanoparticle structure of MOs effectively impedes the restacking of MXene, leading to improved electrochemical properties and high-rate performance for supercapacitors.So far, RuO 2 , [28] MnO 2 , [75] MoO 3 , [76] NiO, [77] WO 3 , [78] and TiO 2 [79] are the most commonly used MOs with MXene composite.Commonly, after mixing the metal ion with delaminated MXene flakes, vacuum filtration and successive reduction process to fabricate the composite film.Ma et al. synthesized Fe 2 O 3 @MXene composite film using 450 °C annealing process. [80]This composite film showed ultrahigh volumetric capacitance of ≈2607 F cm À3 (584 F g À1 ) at 10 mV s À1 , and excellent capacitance retention after 13 000 cycles.They fabricated symmetric supercapacitors using 3 M PVA-H 2 SO 4 gel electrolyte, resulted in volumetric energy density of 29.7 Wh L À1 at a power density of 213.8 W L À1 and a volumetric power density of 1718.8W L À1 at an energy density of 9.6 Wh L À1 .
There is another class of studies that focus on depositing conducting polymers (such as PANI, PPy and PEDOT) on the surface of MXene sheets to promote the electrochemical properties of independent MXene electrodes.Wu et al. synthesized MXene/ PANI nanotubes composites with a hierarchical structure using a one-pot in situ polymerization method. [81]The incorporation of MXene nanosheets ensures mechanical stability and high conductivity in the composite material, while PANI nanotubes act as one-dimensional high-speed ion channels, offering additional active sites.The introduction of PANI effectively prevents the self-stacking of MXene nanosheets, leading to increased spacing between MXene layers and improved accessible surface area for ions.Under 1 M H 2 SO 4 electrolyte, the composite electrode exhibited a specific capacitance of 596.6 F g À1 at 0.1 A g À1 , with a specific capacitance retention rate of 94.7% after 5000 cycles.A symmetric supercapacitor was fabricated, achieving an energy density of 25.6 Wh kg À1 at a power density of 153.2 W kg À1 .The capacitive properties of MXene-based materials are summarized in Table 2 for comparison.

Summary and Perspectives
In the last decade or so, increasing efforts have been made to explore and expand the understanding on electrochemical properties of MXenes and derivatives.With establishing better understanding in ECs, there are successive breakthroughs in electrochemical actuation phenomena of 2D MXene.Firstly, the electrochemical induced actuation or deformation of pristine MXene has been studied with various electrolytes mainly with respect to applied potentials and types of cations.To address the goal of higher electrochemical actuation, enhancing the mechanical strength while tailoring interlayer spacings is the primarily focus.Substantial progress has been made in modifying the MXene via fabricating composite materials with polymers or adopting additional pore architecture.In-depth studies have been carried out to promote a better understanding of ECA mechanism of MXene using various in situ/operando techniques with theoretical calculation.Moreover, the blocking force of MXenebased ECAs has been improved by controlling the length/width of actuators in a recent study.Interestingly, the pristine MXene-based ECAs provided a higher maximum strain change (0.29 versus 0.12%) compared to traditional 2D graphene-based ECAs. [45,82]Furthermore, the MXene-based ECAs showed competitive actuation performances compared to other ECAs based on metals, carbons, and polymers: [1,83,84] a similar strain change to metal-and carbon-based devices (≈1%), a high cycle stability (≈90%) comparable to carbon-based (≈95%) or more than polymer-based ones (≈65% over 10 000 cycles), and a relatively high blocking force (≈5 mN) compared to carbon-based ECAs (0.3 mN for CNT; 1.92 mN for graphene; 0.93 mN for g-C 3 N 4 ; 3.37 mN for graphdiyne).[87] However, they showed much lower force output properties (≈5 mN) than those of typical ionic-polymer metal composite (IPMC) actuators (1-100 mN). [88]n addition to ECAs, the MXene-based ECs showed notable accomplishments with remarkable gravimetric energy and power density.Due to their distinctive advantages over other conventional 2D materials, such as reduced graphene oxide, MXenes stand out with their exceptional metallic conductivity and substantial redox activity derived from abundant surface termination groups and surfaces that can be functionalized.These characteristics make MXenes highly promising materials for electrochemical supercapacitors.Despite these promising advantages, MXenes still face limitations that hinder their practical applications.Further research and development efforts are necessary to overcome these challenges to ensure the successful integration of MXenes into industrial applications.A significant challenge associated with MXenes is the utilization of hazardous chemicals such as HF.While numerous synthesis methods have been developed, there remains an urgent need to identify environmentally friendly and safe etchants for large-scale industrial use of MXenes.These green etchants also aimed at providing reduced costs with short processing times, ensuring the outstanding electrochemical performance of MXenes.In addition to this, another significant challenge for practical applications is the poor ambient stability of MXenes in outdoor environments.[91] Therefore, the need to achieve MXenes with improved environmental stability or develop non-aqueous MXene devices has led to essential research to mitigate the oxidation and degradation of the MXenes.Mathis et al. suggested that modifying the synthesis of Ti 3 AlC 2 with excess aluminum (Al-Ti 3 AlC 2 ) could result in an enhanced structural stability.Both the aqueous suspension and freestanding film of Al-Ti 3 C 2 demonstrated an excellent storage life of over 6 months with minimal oxidation, maintaining a conductivity of 6000 S cm À1 . [92]Since oxidation reactions typically initiate from the ─OH functional group along the flake edge sites, [93] thermal annealing has been found to retard the oxidation process of MXenes by passivating the outermost layer.This passivation layer through the chemical changes enables MXene films to maintain their conductivity in water for over 10 months. [94]oreover, Lee et al. observed that MXene films annealed at 900 °C in a H 2 atmosphere can effectively prevent oxidation, even in harsh conditions (100% relative humidity, 70 °C).
97][98] To achieve the better electrochemical properties of ECs, diverse approaches have been utilized, including 3D architecturing, surface chemistry modulation, interlayer spacing control using small molecules, and composite structures incorporating other effective capacitive materials.These approaches utilized for supercapacitor application can be readily employed to realize the effective ECAs.For example, generating MXene 3D structures via freeze-drying techniques to enhance active surface area, thereby attracting more ions and facilitating the expansion of electrodes, which is advantageous for electrochemical actuators.Moreover, the improved rate performance of the porous structure in electrochemical supercapacitor also offers a viable method for constructing efficient electrochemical actuators, improving the response time and frequency response through accelerated ion intercalation and extraction.Indeed, the collaborative use of the polymers with 3D structuring methods have proven to be effective in reducing the Young's modulus of the MXene electrodes, one of the significant factors for enhancing the bending deformation of the electrochemical actuators. [99,100]or instance, the introduction of the bacterial cellulose along with MXene, coupled with the fabrication of a 3D structure using the freeze-drying technique, demonstrated a significant improvement in capacitance along with a notable decrease in Young's modulus. [101]oreover, the employment of a molten salt etching method, which allows precise control over the MXene surface, enables the introduction of diverse functionalities.Kamysbayev et al demonstrated that when atoms with large ionic radius such as Br and Te are present on the surface, the resulting extended Ti-Ti distance due to the surface effect can induce internal strain within the in-plane structures. [68]These results imply the electrochemical actuation could occur not only for a z-directional actuation by the ion intercalation, but also for a x-directional actuation through the crystal lattice structure with modulation of Ti-Ti atom distance.Additionally, further investigation is needed to explore electrode structures utilizing small molecules for pre-intercalation and employing diverse composites, including carbon-based materials or conducting polymers, to finely tune the interlayer distance, aiming to improve electrochemical actuation performance.
With regards to electrolytes selection, the electrochemical actuation behavior of MXene has been explored using various aqueous/non-aqueous electrolytes with different radii and/or charge of cations.However, most of MXene-based ECAs have been studied in liquid-based electrolytes; only a few types of gel electrolytes (e.g., H 2 SO 4 /PVA, EMImBF 4 (1-ethyl-3-methylimidazolium tetrafluoroborate)/Nafion or PVDF) were adopted in MXene-based ECAs so far. [45,50,51,55]In terms of practical aspects, it is more appropriate for ECAs to operate under atmospheric condition.Therefore, more effort should be devoted to researching on various gel-based electrolytes for real-life application of MXene-ECAs.
Together with the developments of electrodes and electrolytes, the ECA devices must be judiciously designed to maximize their electrochemical actuating or bending performance.All the MXene-based ECAs adopted symmetric cell structure consisting of identical MXene-based materials for the two electrodes configuration.Since the extent of electrochemical expansion/contraction of MXene-based electrodes showed differences with respect to applied potential range and types of cations in electrolyte, the asymmetric cell design leads to further increase in the bending performance of ECA cells.The asymmetric can be established by using MXene-based electrodes with different capacitances (e.g., different mass, thickness, and pore structures) and/or anion exchange membrane and gel electrolytes with two different types of cations.In addition, the low blocking force of MXene-based ECAs (≈5 mN) could be improved by optimizing their cell designs.The force output is one of the important properties for actuators.However, it is surprising that researchers have not paid much attention to this issue yet in the MXenebased ECA's works.One recent paper suggested the way to improve the force output of MXene-based ECA by controlling the length or the width of actuator.Firstly, the blocking force was enhanced from 1.01 to 1.81 mN by reducing the free length of MXene-based ECAs (17 !6 mm).After then, the further improvement was made from 1.81 to 5.02 mN by increasing the width of actuators (3 !18 mm).Based on the recent approach in MXene-based ECA and strategies from IPMCs, the well-controlled dimension of actuators (thickness, width, length, etc.) and strong interface adhesion among cell components would be crucial factors to be considered for enhancing the blocking force. [58,102,103]he great advantages of ECAs can lead to realize potentially a wide range of future applications such as bionic robots, haptic systems, light weight low voltage braille displays, artificial muscles, low-voltage human-machine interfaces, autofocus camera modules, space and medical applications, etc.For example, owing to their flexibility, low driving voltage, moderate bending displacement, and operation in liquid or corrosive environments, various bionic robots have been successfully demonstrated based on the ECAs (e.g., imitation of fins, limbs, joints, and trunks, flowers etc.). [83,104,105]However, like other actuators, the current ECAs still cannot meet the requirements of several future applications (e.g., artificial muscles and haptic/tactile feedbacks), for example, a large strain change (≈50%), a fast response speed with wide frequency bandwidth (0-1000 Hz), and a high force range (0-10 N), etc. [106,107] Moreover, the high-performance ECAs consisting of nanoporous noble metals, carbons, MXenes, and others require a high manufacturing cost and/or a complicated preparation process, restricting them from production on a large scale. [83]The ECAs are still in the experimental stage and necessitate further investigation with various aspects toward practical usage for potential future applications.
Pooi See Lee is a Professor in the School of Materials Science and Engineering and the President's Chair Professor at Nanyang Technological University, Singapore.Her research focuses on nanomaterials and composites for energy and electronics applications, flexible and stretchable devices, electrochemical inspired devices, and human-machine interface.She is named the National Academy of Inventors Fellow 2020, Fellow of the Materials Research Society 2022, and Fellow of the Royal Society of Chemistry 2022.

Figure 2 .
Figure 2. Electrochemical deformation of MXene in aqueous and non-aqueous electrolytes.a,b) Cyclic voltammetry curves at 2 mV s À1 (b) and relative deformations of MXene via in situ atomic force microscopy (AFM) with various aqueous neutral electrolytes.Reproduced with permission.[42]Copyright 2015, Elsevier.c,d) Maximum difference in elastic modulus of MXene between charged and discharged states with aqueous 1 M Li 2 SO 4 (c) and 0.5 M K 2 SO 4 electrolytes (d).Reproduced with permission.[43]Copyright 2016, Wiley-VCH.e,f ) CV curves at 5 mV s À1 (e) and relative displacements (f ) of MXene in aqueous electrolytes of 1 M MgSO 4 , 0.5 M K 2 SO 4 , and 1 M MgSO 4 after pre-cycling with 0.5 M K 2 SO 4 electrolyte.Reproduced with permission.[44]Copyright 2017, Royal Society of Chemistry.g-i) Electrochemical deformation performance of MXene with methanesulfonic acid (MSA) electrolyte: Electrochemical in situ XRD data (g), the plot of corresponding c-lattice parameter versus applied potential (h), and schematic of expansion/contraction of MXene with MSA electrolyte (i).Reproduced with permission.[47]Copyright 2019, Wiley-VCH.j,k) Strain change of MXene with respect to applied potential tested in EMImTFSI (j) and BMImBF 4 ionic liquid electrolytes (k).Reproduced with permission.[49]Copyright 2016, American Chemical Society.

Figure 4 .
Figure 4. In-depth studies for understanding electrochemical actuation behavior of MXene.a-c) The changes in effective electrode layer thicknesses, h, and permeability lengths, ξ, of MXene with respect to applied potential, tested in 0.05 M chloride-based electrolytes including various cations.Reproduced with permission.[56]Copyright 2015, Wiley-VCH.d-f ) The variation in c-lattice parameter of MXene with H 2 SO 4 electrolyte in response to applied potential (d) and simulation results of O-terminated (e) and OH-terminated MXenes (f ) according to intercalation of H 2 O and H þ ions, respectively.Reproduced with permission.[46]Copyright 2019, Wiley-VCH.g-i) Dynamical structural response of MXene with the indications of two different contraction regimes (slow and fast) (g) and illustrations presenting two different charge storage mechanisms corresponding to slow and fast contractions, respectively.Reproduced with permission.[57]Copyright 2022, American Chemical Society.

Figure 5 .
Figure 5. a) Schematic Image of the process for the Ti 3 C 2 aerogel using EDA as a mediator.b) Cross-sectional SEM image and c) XRD pattern of Ti 3 C 2 aerogel.d) Areal capacitance as a function of scan rates with different mass and e) cycle stability of the Ti 3 C 2 aerogel electrode.Reproduced with permission.[59]Copyright 2017, Elsevier.f ) Schematic illustration of the n-BuLi-modified and alkali-modified MXene synthesis.g) The atomic ratio of the resultant films measured by the EDS (M-Ti 3 C 2 T x : HF etched sample, F-Ti 3 C 2 T x : LiF/HCl etched sample, L-: LiOH treated sample, n-: n-BuLi treated sample).Reproduced with permission.[66]Copyright 2019, American Chemical Society.

Figure 6 .
Figure 6.a) Schematic of the preparation of Lewis-basic halides treated Ti 3 C 2 T x film (LB-Ti 3 C 2 T x ).SEM image of b) the HF etched film and c) Lewis-basic bromide treated MXene film (scale bar: 1 μm).Reproduced with permission.[67]Copyright 2022, Springer Nature.d) Schematic illustration of the fabrication process of N-containing Ti 3 C 2 T x .Atomic resolution high-angle annular dark-field (HAADF) images of e,f ) Cl-containing and j,k) N-containing Ti 3 C 2 T x samples at different scale.g) c-lattice parameter changes in the electrochemical process.h) CV graph of the N-containing (blue), Cl-containing (black) films and the film with conventional etching process (orange).i) The rate performance of the N-containing MXene film.Reproduced with permission.[69]Copyright 2023, Wiley-VCH.