Computational Discovery and Design of MXenes for Energy Applications

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Computational Discovery and Design of MXenes for Energy Applications: Status, Successes, and Opportunities Cheng Zhan,†,∥ Weiwei Sun,‡ Yu Xie,‡ De-en Jiang,† and Paul R. C. Kent*,‡,§ †

Department of Chemistry, University of California, Riverside, California 92521, United States Center for Nanophase Materials Sciences and §Computational Sciences and Engineering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ∥ Quantum Simulation Group, Lawrence Livermore National Laboratory, Livermore, California 94551, United States

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‡

ABSTRACT: MXenes (Mn+1Xn, e.g., Ti3C2) are the largest 2D material family developed in recent years. They exhibit significant potential in the energy sciences, particularly for energy storage. In this review, we summarize the progress of the computational work regarding the theoretical design of new MXene structures and predictions for energy applications including their fundamental, energy storage, and catalytic properties. We also outline how high-throughput computation, big data, and machine-learning techniques can help broaden the MXene family. Finally, we present some of the major remaining challenges and future research directions needed to mature this novel materials family.

KEYWORDS: MXene, energy storage, electrocatalysis, simulation, high-throughput computation, machine learning

1. INTRODUCTION Two-dimensional (2D) materials are becoming increasingly significant in scientific research because of their unique physical and chemical properties and promise for practical applications. In earlier years, the search for novel 2D materials has been mainly focused on those with only one or two constituent elements, such as graphene,1 silicene,2−4 phosphorene,5,6 borophene,7−9 germanene,10,11 transition-metal dichalcogenites,12 and oxides.13 Beyond graphene and known 2D semiconducting materials, transition-metal carbides, nitrides, and carbonitrides, known as MXenes, stand out as a new class of 2D materials with characteristic metallic behavior. The first MXene Ti3C2Tx was successfully synthesized in 2011 by Naguib, Gogotsi, and Barsoum et al.14 Following this pioneering work, tens of MXene family members have been synthesized, including vanadium-15 and niobium-based carbides.16 Today, the family of MXenes is the largest class of 2D materials and is still growing rapidly, expanding both in size and scope in the range of potential applications.17,18 In this review article, we will begin with an introduction to the parent MAX phases and current methods for MXene synthesis. We will then consider (Section 3) the role of surface terminations, which is one area where MXenes differ significantly from other 2D material families. In Section 4, we review the range of measured and predicted intrinsic © XXXX American Chemical Society

properties in MXenes and several compositional variations that greatly expand the size of the MXene family. In Section 5, we review the status of MXenes for energy storage in metal-ion and supercapacitor applications. In Section 6, we consider the current experimental and theoretical situation for electrocatalysis using MXenes. Because of the large size of the MXene family, use of high-throughput computation and machinelearning techniques may potentially accelerate the search for idealized MXenes for specific applications. We review efforts in this area in Section 7. Finally, in Section 8, we give our outlook for the key challenges to predictive theories of MXene properties that would accelerate materials discovery.

2. MXENE SYNTHESIS Laboratory scale synthesis of MXenes is now well-developed. The standard method of MXene synthesis is to chemically and selectively etch out the A-elements, usually group IIIA and IVA elements, such as Al3+and Si4+, from the parent bulk MAX phase precursor,14 where the M stands for the early transition metals usually from group IIIB to VIIB, and X is either C or N. Special Issue: Materials Discovery and Design Received: January 8, 2019 Accepted: April 30, 2019

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DOI: 10.1021/acsami.9b00439 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) Experimental steps of synthesizing MXenes by chemical etching the MAX phases. (b) Surface terminal groups on MXenes with fcc, hcp, and top sites.

In the early stages, fluoride-containing acidic solution such as hydrofluoric acid solution or LiF−HCl mixture solution was the most widely used for the preparation of MXenes.19,20 Some MXenes such as Mo2CTx and Zr3C2Tx were obtained by etching the A-elements from the non-MAX phases Mo2Ga2C and Zr3Al3C5 that are also crystallizing in the space group of P63/mmc.21,22 A recent development in the etching method is the use of molten salts that enable the A-element to be removed at high temperature. The obtained MXenes are subsequently found to be purer. For instance, the reported synthesis of Ti4N3 is performed by etching the Ti4AlN3 with molten fluoride salt.23 MXenes obtained by chemical etching are usually terminated by O, F, and OH groups on the surface.24Alternatively, other than wet etching, the chemical vapor deposition (CVD) is also a possible means for MXene synthesis,25 and one of the advantages of this approach is to leave the 2D MXenes unterminated. Inherited from the MAX phases, the chemical formula of MXene is Mn+1XnTx, where the T stands for the terminal groups, such as −F, −O, and −OH. So far, there have been about 30 MXenes that were experimentally synthesized by etching away the A-element from MAX phase precursor, as listed in Figure 1.17 The building block of MXene, Mn+1Xn terminated by various functional groups, is determined by the structure of MAX phase precursor and the etching method. With the expansion of MAX phases by alloying new metals, the synthesized MXenes are actually not limited to the regular MXenes like Ti3C214 and Mo2C21 but also can be extended to more complicated MXenes, such as double-metal MXene ((Mo 2 Ti)C 2 , (Cr 2 Ti)C 2 , etc. 26 ), carbonitride MXene (Ti3(C,N)2),27 and nonstoichiometric MXene (Nb4/3C,28 Mo4/3C,29 and W4/3C30). Structurally, in the multiple-layered MXenes, M and X atoms are assembled in an A−B−C pattern along the z-axis, as shown in Figure 1b. From a computational perspective, Ashton et al. comprehensively studied the thermodynamic stability of MAX phase with different compositions using density functional theory (DFT) and indicated the stable M2AX phases, including solid solutions.31

3. SURFACE TERMINATIONS When MXenes are functionalized by terminal groups, in principle, three choices of placing terminations can be generated according to the symmetry, the fcc, hcp, and top sites. For MXene monolayers, more possible configurations can be theoretically generated by having different configurations on each surface. Admittedly, experimental determination of surface terminal configurations at the atomic scale is very difficult due to the complexity of structures and the composition (mixtures), but to date, there is no unique map relating MXene termination with synthesis approach. Here, first-principles-based simulation, such as those based on DFT, is a powerful tool to find the preferred surface structure of MXene. For example, what are the controlling factors on the configurational stability of MXene, and what is the preferred termination (T)? Ashton et al. applied DFT to study the dependence of the thermodynamics stability of MXenes with different terminal groups on their chemical composition and hydrogen chemical potential, which indicates that a majority of the MXene candidates are theoretically synthesizable.32 Khazaei et al. studied the relative stability of M2X type with −F, −O, and −OH terminal groups and indicated that most of the M2XT2 are more stable with the termination configurations in either fcc or hcp positions, although the top site is not preferable in the considered cases.33 However, exceptional MXenes, such as Sc2CO2 and Zr2C(OH)2, energetically favor the fcc−hcp mixing mode.33 Seh’s work also reported the relative stability of −O and −OH groups on MXene and found that Cr-, Mo-, and W-based MXenes prefer the hcp site, whereas Sc-, Ti-, V-, Zr-, and Hf-based MXenes prefer the fcc site.34 Moreover, nitride MXene tends to have relatively more stable terminations in the hcp site.34 A more comprehensive study on the stability of −O and −OH-terminated MXene was reported by Zhan et al.35 and Jiang et al.36 More recently, Ashton et al. investigated the Pourbaix diagram and thermodynamic stability of MXene under electrochemical conditions, which revealed the synthesizability of new MXene materials by etching.37 Although a number of theoretical B


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Figure 2. (a) Band structure of semiconducting MXene systems. The Fermi energy is at zero. (b) Calculated total magnetic moments (in μB per formula unit) of monolayer M2X under biaxial strain. For both Ti2C and Ti2N, we also observe different physical properties under different strains. Both V2C and V2N are metallic under all the strains studied. (c) Work functions of MXenes with various terminations. The representations of different surfaces are the bare surface by the black square; O-terminated surface by the red circle; OH-terminated surface by the blue up-triangle; and the F-terminated surface by the cyan down-triangle. For comparison, work functions of Sc and Pt metal are indicated by dashed lines. (d) Work function variation induced by surface termination, as a function of surface dipole moment density. Figure 2a is reproduced with permission from ref 33. Copyright 2012 John Wiley & Sons, Inc. Figure 2b is reproduced with permission from ref 46. Copyright 2016 The Royal Society of Chemistry. Figure 2c,d is reproduced with permission from ref 48. Copyright 2016 American Chemical Society.

calculations by Kazaei et al.33,40 for metal carbides M2C (M = Sc, Ti, V, Cr, Zr, Nb, Ta) and metal nitrides M2N (M = Ti, Cr, Zr) with O, OH, or F termination found that, with some exceptions, the MXenes are generally metallic and nonmagnetic. However, Sc2CT2 (T = O, OH, F), Ti2CO2, Zr2CO2, and Hf2CO2 are semiconducting (Figure 2a), whereas Cr2CT2 (T = OH, F) and Cr2NT2 (T= O, OH, F) are magnetic. Thicker MXenes tend to display greater metallicity. Computationally, local density functionals were found to show similar results to more costly hybrid DFT.41 Provided van der Waals interactions are accounted for, and calculated structural and electronic properties are in fair agreement with experimental measurements. For the bare M2C MXene, DFT study has shown that only Ti2C and Zr2C have spontaneous magnetism, whereas other bare MXenes are nonmagnetic.42 In addition to carbides, some nitrides, such as Mn2NTx (T = F, O, OH), were predicted to have a desirable ferromagnetic ground state, which leads to promising potential for spintronics.43 For the semiconducting MXenes, their band gaps are tunable by mechanical strain. Lee et al. reported that Sc2CO2 exhibited an indirect to direct band gap transition by a small

studies have aimed to determine the configuration of terminations on various MXene surfaces by omitting the full complexity of the surface by only considering one or two types of T-group in the symmetric manner, they may be far away from the true physical picture determined by the etching methods and conditions employed.24 To reach an accurate match and even assuming that the structures are in thermodynamic equilibrium, this might require even more sophisticated simulation techniques that can take a large number of compositions and configurations space of MXenes in order to search the energetic landscape and locate the energy minima.

4. INTRINSIC PROPERTIES OF MXENES 4.1. Electronic Properties of Conventional MXenes. The electronic structures of the first MXenes were rapidly established. Starting with Ti3C2,14,19 initial DFT calculations found the bare MXenes to be metallic and the band structures of terminated MXenes to be dependent on the surface termination. The experimentally measured electrical conductance of Ti3C2Tx has attained 3250 S/m,38 which is higher than that of graphene (∼2500 S/m).39 More extensive DFT C


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ACS Applied Materials & Interfaces critical strain of 2%.44 Similar band modulation phenomenon was also observed by Yu et al. in Ti2CO2, Zr2CO2, and Hf2CO2.45 The indirect to direct band gap transition in Ti2CO2 was caused by biaxial strain of ∼4% and uniaxial strain of ∼6%, whereas Zr2CO2 and Hf2CO2 need 10 and 14% biaxial strains for the band gap modulation. Similarly, as shown in Figure 2b, the magnetic property of MXene also can be affected by external strain or introducing defects, which induce the nonmagnetic to magnetic transition.42,46,47 The work function is also an important aspect of MXenes for their application as a part of 2D metal−semiconductor junctions. Liu et al. studied the work functions of bare and functionalized MXenes by DFT simulation in Figure 2c.48 DFT results showed that the work functions of bare MXenes are mostly located in the range of 3.5−5.5 eV. F-termination was found to have slight impact on the work function, whereas O-termination could greatly increase the work function, and OH-termination greatly decreases the work function. The calculated work functions of terminated MXenes were further found to correlate with the surface dipole moment, as shown in Figure 2d.48 Khazaei et al. found a strong linear correlation between the work function and surface dipole moment in the F- and O-terminated MXenes, which originated from the charge transfer of terminal groups to substrate. The ultralow work function of OH-terminated MXene was explained by the intrinsic dipole moment of the −OH group.49 Tahini et al. further found that the 2p band center of terminal group T has a “volcano” relation with the work function, which enables the work function modulation by surface engineering for specific applications.50 The large tunability of MXene’s work function enables broader applications, such as semiconductor/metal heterojunctions.51 4.2. Multiple-Transition-Metal MXenes. The range of possible compositions and intrinsic properties of MXenes was greatly expanded by the discovery of multiple-metal-atom MXenes. Liu et al.52 first discovered an ordered Cr2TiAlC2 MAX phase structure, where the Ti atom layer is sandwiched between outer Cr layers. Anasori et al.26 later applied DFT calculations to predict the existence of a large family of ordered carbides M′2M″C2 and M′2M″2C3, where M′ and M″ are early transition metals. Mo2TiC2Tx, Mo2Ti2C3Tx, and Cr2TiC2Tx were synthesized and shown to have distinct electrochemical behavior from that of the conventional single-metal Ti−Cbased MXenes. The range of potential properties in multiple-metal-atom MXenes is increased due to the lower symmetry and the ability to choose combinations of metal atoms to tailor the electronic properties (e.g., by varying the electron filling or inducing magnetic ordering). As with conventional MXenes, attractiveness for energy storage applications is reduced in thicker materials because of the difficulty of reducing the central metal-atom layers, which could lower achievable volumetric and gravimetric capacities. Many new properties have been predicted in this materials family that were not found in the single-metal-atom case. For example, Dong et al.53 predicted potential spintronic materials. They found robust ferromagnetism in Ti2MnC2Tx independent of surface termination as well as in oxidized Hf2MnC2O2 and Hf2VC2O2. Semimetal−semiconductor and ferromagnetic−antiferromagnetic transitions were also predicted to occur at small strains. More recently, Sun et al.54 predicted a range of surface-metal- and termination-dependent metal− insulator transitions in TiCr2N2 and TiMn2N2 MXenes with Ti

as the middle layer. TiCr2C2Tx was predicted as semiconducting independent of the termination using both hybrid DFT and DFT + U techniques. Khazaei et al. found that the double-transition-metal M′2M″C2O2 MXenes (M′ = Mo, W; M″ = Ti, Zr, Hf) were potential topological insulators due to the strong spin−orbital coupling (SOC) between M′ and M″.55 Anasori et al. found the metal to semiconductor transition by replacing the Ti outer layer by Mo in the metallic Ti3C2(OH)2 and Ti4C3(OH)2despite both Ti and Mo being metal atomswhich provides a new avenue for tuning the electronic property of MXene.56 The experimental discovery of double-transition-metal MXenes opened a new door to the more extensive exploration of new functional MXene materials. Although the number of multiple-metal MXenes that has been synthesized to date is small, theory predicts a very large number of possible multiplemetal MXenes to be thermodynamically stable, particularly when solid solutions are also considered.26,57 Because of the improved tunability of the electronic and magnetic structures, these may display properties not yet realized in the conventional MXenes. 4.3. Nonstoichiometric MXenes (i-MXenes). A new class of MAX phase and MXene was outlined by Tao et al.58 in 2017. They first designed and realized a new “i-MAX” phase (Mo2/3Sc1/2)2AlC, where the Mo and Sc atoms exhibited inplane chemical ordering. Selective etching was able to remove both the Al and Sc atoms leaving 2D MXene Mo4/3C sheets, “iMXene”. This new class of MXene displays an ordered array of divacancies in the metal-atom layer. The initial measured capacitances of 1153 F·cm−3 and 339 F·g−1 exceeds those of Mo2C papers by 65 and 28%, respectively. These new phases therefore open up new performance regimes. Several additional i-MAX phases including (V2/3Zr1/3)2AlC, (Mo2/3Y1/3)2AlC, and (Cr2/3Sc1/3)2AlC have since been predicted and synthesized.29,59,60 In the general family (W2/3M1/3)2AlC, Meshkian and co-workers30 predicted only (W2/3Sc1/3)2AlC and (W2/3Y1/3)2AlC to be stable on the basis of DFT calculations. These MAX phases and the first 2D MXene W4/3C with ordered vacancies were successfully synthesized. The MXene Nb4/3C was synthesized, and by controlling the etching, films with random vacancies produced. 28 The lower density and presumed higher permeabilities and reactivities may be attractive for energy storage and catalytic applications. To date, computational and predictive work on the i-MAX phases and i-MXenes has primarily focused on their stability. Crucially, quaternary i-MAX phases may be thermodynamically stable where the parent ternary MAX phases are not. When these quaternaries are etched, this opens new compositional space of MXenes. Besides enhanced volumetric or gravimetric capacities for energy storage, the additional structural complexity of the M4/3C MXenes may enable unique functionalities not found in conventional MXenes. For example, Khazaei et al. predicted that certain combinations might become semiconducting and, owing to the lack of centrosymmetry, become piezoelectric.61 By combining the ideas of i-MAX phases with the multiple-metal-atom MAX phases, the compositional and properties space may potentially be expanded even further. This could be particularly beneficial for properties that are highly surface geometry and electronicstructure-dependent. D


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Figure 3. (a) Top views of the considered migration paths for Li diffusion on Ti3C2, I−Ti3C2F2, and I−Ti3C2(OH)2 monolayers, where the black, red, and blue lines represent pathways I, II, and III, respectively. Below are the corresponding diffusion barrier profiles of Li on Ti3C2, I−Ti3C2F2, and I−Ti3C2(OH)2 monolayers through the predesigned pathways, and the lowest part shows the local atomic configuration at the barrier state that is associated with the optimal diffusion path with the lowest energy barrier. (b) Lithiated Ti3C2O2 with one (left) and two (right) layers of Li on each side. (c) Considered ion migration pathways and corresponding energy barriers of Li and Na on the Mo2C monolayer. (d) Side and top views of ion adsorption on Ti2CO2 nanosheet, (e) ion adsorption energies, and (f) theoretical capacities on O-terminated MXene nanosheets. (g) Side and top views of ion adsorption on a metalized Ti2CO2 nanosheet; (h) second-metal-layer adsorption energies for Mn+1CnO2A2; (i) Mg capacity variation as a function of adsorbed number of Mg layers. Figure 3a is reproduced with permission from ref 77. Copyright 2012 American Chemical Society; Figure 3b is reproduced with permission from ref 79. Copyright 2014 American Chemical Society; Figure 3c is reproduced with permission from ref 80. Copyright 2016 American Chemical Society; Figure 3d−i is reproduced with permission from ref 73. Copyright 2014 American Chemical Society. capacity of 5.9 mAh/cm2 in the constant-like behavior.66 Sun et al. suggested that introducing a larger amount of fluorine into the MXene surface enhances the capacity of MXene.67 Hybridizing MXene with other materials (e.g., carbon nanotubes) and using filtration to synthesize the MXene composites could also lead to higher capacities compared with the performances of pure MXenes.16 For example, Nb2CTx (Nb2CTx/CNT) showed more than 400 mAh/g capacity at 0.5 C as a lithium-ion battery as well as a promising high volumetric capacitance of 325 F/cm3 as a lithium-ion capacitor.16 On the other hand, by introducing sizable pores on MXene flakes, the lithium capacity can be further increased up to 1250 mAh/g at 0.1 C, which opened a new door of enhancing the lithium capacity by the usage of porous MXene.68 For non-lithium-ion battery applications, MXene has also shown rather promising performance in terms of both capacity and stability after hundreds of cycles. Kajiyama et al. studied the Na+ intercalation in Ti3C2Tx and found that Ti3C2Tx could reach 270 mAh/g at the first

5. MXENE MATERIALS FOR ENERGY STORAGE 5.1. Metal-Ion Batteries. 5.1.1. Current Experimental Progress. MXene has been considered to show great potential for anode materials in both Li- and non-Li-ion batteries18,62,63 because of its high charge capacity originating from the large surface area and the surface activity induced by transition-metal surface terminated by T (for example, a mixture of −O and −OH groups). Intuitively, the series of the M2C MXenes is expected to exhibit higher capacity than other series, such as M3C2 and M4C3, in the perspective of gravimetric capacity. Naguib et al. studied the feasibility of Ti2C applied to the lithium-ion battery (LIB), and it was concluded that the lithiation and delithiation peaks are situated at 1.6 and 2.0 V vs Li+/Li with a stable capacity of 225 mAh/g at a C/25 rate, which indicated that Ti2C fulfils the requirements of being an anode material.64 Later, other MXenes such as Nb2C and V2C were found to exhibit 170 and 260 mAh/g at 1C and 110 and 125 mAh/g at 10C.65 Kim et al. reported the high mass loading and binder-free features in Ti3C2 and the high E


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Figure 4. (a) Bonding energies of “M2AlB2” type MAB and MAX phases. The insets are the atomic configurations of M2AlB2 (MAB phases) and M2AlC (MAX phases). (b) Lowest-energy atomic configuration during the step-by-step interaction of HF via the Mo2AlB2 edge and extraction of AlF3 and H2, where the red, green, black, cyan, and pink spheres represent Mo, B, Al, F, and H atoms, respectively. (c) The possible Li adsorption sites and the considered diffusion pathways on MBenes, where the red and green spheres represent M and B atoms. (d) Calculated adsorption energy for Li atoms on various adsorption sites. The diffusion energy curves for Li atoms on 2D (e) Mo2B2 and (f) Fe2B2. (g) Relative energetic stabilities of TiC3Nan (n = 1−4) with respect to an elemental solid Na and TiC3 monolayer at 0 K. The optimized monolayers corresponding to the data points located on the convex hull are thermodynamically stable. (h) The most stable structures with different Na concentrations in TiC3Nan. For clarity, the Na atoms of the first and second adsorbed layers are colored in yellow and green, respectively. Figure 4a−f is reproduced with permission from ref 85. Copyright 2017 The Royal Society of Chemistry. Figure 4g,h is reproduced with permission from ref 86. Copyright 2018 American Chemical Society. new battery materials (e.g., ion−electrode interaction,73 ion partition,74 phase transition,75 and ion diffusion).76 These properties and performances are controlled by the ion−electrode interactions at the solid−liquid interface. The ion−electrode interaction together with the open circuit voltage (OCV) can almost determine the capacity of a metal ion and the stability. Tang et al. applied DFT to study lithium adsorption and its diffusion on the Ti3C2 surface in Figure 3a, leading to the result that adsorption energies of Li on Ti3C2 surface are −0.504, −0.951, and −0.201 eV/Li for bare, −F, and −OH-terminated Ti3C2 respectively, simultaneously accompanied by the transfer of 0.22, 0.41, and 0.33 electrons from Li to MXene, respectively.77 The theoretically predicted capacities were about 320, 130, and 67 mAh/g for Ti3C2, Ti3C2F2, and Ti3C2(OH)2 with OCVs of 0.62, 0.56, and 0.14 V. Further study showed that the Li-ion diffusion on Ti3C2 has a barrier as low as 0.05 eV, which suggests the bare Ti3C2 can be an anode material for the lithium-ion battery. Beyond the DFT simulation of monolayer MXenes, Ashton et al. indicated that modeling the LIB by bilayer MXene with the dispersion interactions included in DFT could give results in better agreement with experiment.78

sodiation process (2Na/Ti3C2Tx), whereas the lithium capacity fell to 100 mAh/g over 100 cycles.69 Guo et al. reported an excellent performance for sodium-ion storage in Sb3O2/Ti3C2Tx composites with the charge capacity reaching 295 mAh/g at 2 A/g current and 472 mAh/g at 0.1 A/g after 100 cycles,70 which is primarily resulting from the fast ion transport in the structure of 3D networks.70 Xie et al. studied a porous self-assembled Ti3C2Tx/CNT composite (Ti3C2Tx/ CNT-SA) and measured a high volumetric sodium capacity of 421 mAh/cm3 at 20 mA/g.71 Lian et al. synthesized the alkalized Ti3C2 MXene nanoribbon, which was observed to possess a high reversible sodium and potassium capacity of 168 mAh/g and 136 mAh/g at 20 mA/g and notably with an excellent cyclability,72 suggesting the alkalization method is a plausible mean of improving the energy storage performance in the MXene-based materials as the metal-ion battery/capacitor. Overall, even this small selection of experimental results shows the potential for promising anode performance. Ideally, theory and computation would guide the MXene selection and optimization. 5.1.2. Computational Understanding of the Metal−Ion Interactions. Multiscale modeling has been widely and extensively applied to help understand the working principles and to predict and design F


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Figure 5. (a,b) In situ Raman spectra of Ti3C2Tx MXene recorded on positive electrode in H2SO4 electrolyte and negative electrode in H2SO4 electrolyte. (c) Selected spectra and Lorentzian fits of bands at 590, 630, 672, 708, 721, and 726 cm−1 of the negative electrode in H2SO4 electrolyte, and the change in Raman vibration modes during the charging and discharging. Note that there is a reversible electrochemical transformation between M=O and M−OH in the presence of hydronium. The atomic indexes are green spheres for Ti atoms, black spheres for C atoms, red spheres for O atoms, and white spheres for H atoms. (d) Ex situ XRD patterns and TEM images for Ti3C2Tx upon sodiation and desodiation. The scale bar in the TEM images indicates 5 nm. (e) Optimized structures for pristine Ti3C2Tx and Na+-intercalated Ti3C2Tx with the various termination groups: −OH/−F, −O/−F, and −F/−F. Figure 5a−c is reproduced with permission from ref 89. Copyright 2016 American Chemical Society. Figure 5d,e is reproduced with permission from ref 69. Copyright 2016 American Chemical Society. Xie et al. studied the Na+, K+, Mg2+, Ca2+, and Al3+ adsorption on the MXene surface and found strong binding ability for the O-terminated MXenes with the metal ions. More interestingly, adsorbed Mg and Al ions prefer to form a stable multilayer on the MXene surface, leading to an excellent charge capacity over 800 mAh/g, as shown in Figure 3d−i.73 Eames et al.83 performed an extensive global screening of M2C MXenes for both lithium and non-lithium applications, concluding that the most promising compounds are those containing light transition metals with either no termination (bare) or Otermination. In summary, the configuration of terminations on MXene surfaces is crucial to the Li- and non-Li-ion absorption as well as to the storage capacity. 5.1.3. Designing Novel MXene and MXene Analogues for MetalIon Batteries. It is known that the great potential of the MXene as a high-capacity metal-ion battery is closely related to its low dimensionality, layered structure, composition, as well as low weight. On the basis of the structures of MXenes, novel MXenes and MXene analogues have been proposed by computational simulations, which have pushed the boundary of this series of materials and stimulated experimental synthesis. Chen et al. studied the lithium storage on Ti3CN via first-principles simulation and found the Li adsorption

Xie et al. combining the theory with experiment comprehensively studied the Li adsorption on Sc2C, Ti2C, Ti3C2,V2C, Cr2C, and Nb2C (Figure 3b) and proposed the essentiality of terminations (e.g., −O and −OH).79 It was suggested that the O-terminated MXene exhibits the strongest binding with lithium and the highest charge capacity, and the lithiated oxygen on MXene can attract additional lithium atoms to form a lithium layer on the MXene surface, providing a fundamental reason for the high lithium storage performance.79 In addition, Cr2C was predicted to be a promising electrode material because of its high lithium capacity and low diffusion barrier.79 Sun et al. employed DFT to study the lithium adsorption and diffusion on Mo2C and reported excellent theoretical lithium capacity on Mo2C as high as 526 mAh/g with a low Li diffusion barrier of 0.14 eV in Figure 3c.80 Zhao et al. indicated that the lithiation process on the Ti2C layer should proceed one side at a time instead of occurring on both sides simultaneously, and the adsorption energy of Li is surprisingly independent of Li concentration.81 The lithium capacity of V2CO2 was studied to reach an extremely high capacity of 735 mAh/g with a good structural stability.82 Outside of lithium applications, MXene also exhibits a strong capability for interacting with other metal ions. G


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Figure 6. (a) Simulated specific capacitance of Ti2CO2 and Ti2CF2 nanosheets, and the calculated method is illustrated as the inset. (b) OCV (red) and lattice parameters of the c-axis (blue) as a function of Na concentration in NaxTi2CT2. (c) Faradaic charge (blue, to balance proton transfer), EDL charge (black, because of surface net charge), and total charge (red, net electron transfer number) stored at different electrode potentials. (d) Simulated differential capacitance of Ti3C2Tx in 1 M H2SO4. The voltage windows from two experiments are also shown: Expt-1 is from ref 90, and Expt-2 is from ref 89. (e) Proportion of each storage mechanism (counterion intercalation, co-ion deintercalation, and co-ion/counterion exchange) at the negative and positive electrodes when the system is fully charged. (f) Evolution of [EMIM]+, [TFSI]− density profiles and charge-density profiles inside the negative electrode pore. (g) Schematic illustration of pyrrole polymerization using MXene. The terminating groups on the latter contribute to the polymerization process. Figure 6a,b is reproduced with permission from ref 94. Copyright 2016 PCCP Owner Societies. Figure 6c,d is reproduced with permission from ref 96. Copyright 2018 American Chemical Society. Figure 6e,f is reproduced with permission from ref 97. Copyright 2018 John Wiley & Sons, Inc. neutral aqueous electrolyte, Ti3C2Tx shows a pronounced capacitive behavior as the cations intercalate into the MXene layers, associated with the volume shrinking/expansion during the discharging/charging process.87 For a pure MXene (no hybridizing with other materials and without additives), the highest volumetric capacitance of ∼400 F/cm3 was observed in the KOH electrolyte for Ti3C2.87 In addition to the cation intercalation capacitance, Ti3C2Tx also exhibits pseudocapacitive behavior that would further enhance the capacitance, which was observed and explained to be due to the surface redox reaction on −O and −OH groups in the H2SO4 electrolyte.20 The experimentally reported volumetric capacitance of Ti3C2Tx can maximally reach 900 F/cm3,20 which is 2 times higher than that measured in the KOH electrolyte.87 It was found that the volumetric capacitance of Ti3C2Tx in H2SO4 electrolyte could go beyond 1400 F/cm3 by fabricating the Ti3C2Tx into hydrogel electrode.88 Beyond the prototype MXene (i.e., Ti3C2Tx), the capacitive performances of many other MXenes have been investigated. For example, the capacitance of Mo2CTx was reported as 700 F/cm3 and 196 F/g, respectively, in 1 M H2SO4 electrolyte;21 V2CTx is able to reach a capacitance as high as 487 F/g in 1 M H2SO4 electrolyte; the nonstoichiometric MXene Mo4/3C exhibits a surprisingly high capacitance of 1310 F/cm3 in 1 M H2SO4.29 All the above are strong indicators that MXenes have an ultrahigh capacitive storage due to their large capability of storing intercalated cations and an additive

prefers the nitrogen site on the bare surface and the carbon site on the functionalized Ti3CN MXene surface (T = O, OH, F), initiating the investigation of the nitrogen-enriched MXene for battery applications.84 Recently, Guo et al. designed a new class of transition-metal borides with the M2B2 formula (MBenes) showing similarity to MXene in terms of a good mechanical stability, which expanded the metallic 2D materials to a wide range.85 In MBenes, Mo2B2 and Fe2B2 are found to possess excellent electrical conductivity, lithium capacity (444 and 665 mAh/g, respectively), and low diffusion barrier (0.27 and 0.24 eV, respectively), which fulfill the basic requirements of the anode material for a lithium-ion battery, as shown in Figure 4a−f.85 Furthermore, Yu et al. designed a new titanium carbide monolayer TiC3 by using configurational search.86 The new MXene analogue not only exhibits good static thermodynamic and dynamical stability but also an extremely high sodium capacity (1278 mAh/g), a low diffusion barrier (0.18 eV), and a high OCV (0.18 V) as shown in Figure 4g,h.86 This excellent example strongly supports the capability of configuration search to identify new materials for improving energy storage. 5.2. Supercapacitors. 5.2.1. Experimental Performance and Capacitive Mechanism of MXene-Based Supercapacitors. On the other hand, MXenes also exhibit outstanding performance in capacitive energy storage, owing to their excellent conductivity, stability, redox activity, and large-area surface. In the MXene family, Ti3C2Tx has been so far the most widely studied material. In the H


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attributed to the wider interlayer spacing and more stable layered framework.93 5.2.2. First-Principles Understanding of the MXene Pseudocapacitance. Modeling the pseudocapacitance of MXene under electrochemical conditions is difficult and remains challenging to perform accurately because of the complex interfacial problems, such as electrode/electrolyte interactions, electron/charge transfer in redox reactions, and the varying of the bias electrode potential. Ji et al. applied first-principles DFT to study Ti2CT2 as a sodium-ion intercalation pseudocapacitor.94 On the basis of the electronic density of states (DOS) and the intrinsic capacitance (also known as quantum capacitance (CQ)), Ti2CO2 was found to be the most suitable cathode material in the MXene family. By computing the change of charge (Na number) with respect to the electrode potential, they deduced the differential capacitance of Ti2CF2 and Ti2CO2 shown in Figure 6a.94 The calculated integral capacitance of Ti2CO2 can reach 291 F/g with a broad voltage window and a low Na+ diffusion barrier,94 which provides an important insight into the capacitive mechanism of ion intercalation capacitance in MXene. Different from the ion intercalation derived capacitance, the redox pseudocapacitance of MXene in acidic aqueous electrolyte, such as H2SO4, depends largely on the effects of solvation and bias potential as well as double-layer charging on the surface redox, which requires a far more careful treatment. Because MXene has a similar surface structure to that of the transition-metal oxides, such as RuO2, a partially oxidized/reduced state could leave a net charge on surface.95 Analogously, Zhan et al. proposed a generic model of MXene pseudocapacitance in H2SO4 based on the combination of DFT with the implicit solvation model.96 By explicitly deducing the voltagedependent Gibbs free energy of partially oxidized Ti3C2Tx (T is a mixture of O and OH groups), one can then deduce the statistical average surface H coverage, surface net charge, and net electron transfer at the specified voltage window, as shown in Figure 6c.96 The calculated differential capacitance shown in Figure 6d is consistent with experimental measurement in both capacitance and the surface change.89,90 Moreover, the calculated variance of the oxidation state of Ti during the capacitive charging process is comparable to that of the in situ XANES measurement.90 The above studies support the feasibility of applying this model to other MXenes to predict and explain the expected capacitance and for high-throughput computational screening. 5.2.3. Theoretical Understanding of Electric Double-Layer Charging and Ion−Solvent Confinement in MXene Nanopores. Because most MXenes are metallic and highly electrically conductive,17,19,98 they should also be suitable for electric double-layer capacitor (EDLC) electrodes, the model of which is often described by the intrinsic capacitance of an electrode, the quantum capacitance (CQ). Xin et al. studied the CQ of niobium-based MXenes and found they have extremely high CQ beyond 1000 F/g.99 A recent review focusing on the electronic property of MXenes also indicated their metallic features are in the majority,17 which suggests the sufficiency of considering the EDL on charged MXene surfaces to capture the EDLC’s charging behavior. Xu et al. employed classical molecular dynamics (CMD) to study the charging behavior of Ti3C2Tx in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([EMIM]+[TFSI]−) electrolyte,97 and the calculated interlayer distance was in a good agreement with experiment and varied due to ion intercalation/exclusion and solvent rearrangement during the charging/discharging process.97 According to the population analysis of ions shown in Figure 6f, the charging mechanisms at positive and negative electrodes are different. On the negative electrode, the electrode charge is compensated mainly by counterion intercalation. On its counterpart, the electrode charge is compensated by both ion deintercalation and ion exchange.97 However, the contribution from each mechanism depends on the surface terminal groups and is facilitated by −OH for a faster ion exchange as shown in Figure 6e.97 Another effective way of enhancing the ion dynamics in the subnanometer MXene can be realized by spontaneous surface wetting by ionic liquid observed in CMD calculations.100 Similar phenomena of spontaneous intercalation of ion and solvent into MXenes was also

contribution from the pseudocapacitance governed via surface redox reactions. The capacitive mechanism of MXene is highly dependent on the choice of electrolyte: MXene always exhibits pseudocapacitance in acidic electrolytes and electric double-layer or intercalation capacitance in neutral electrolyte. The pseudocapacitive mechanism of Ti3C2Tx in H2SO4 has been quantitively characterized. Hu et al. applied in situ electrochemical Raman spectroscopy to study the vibrational signature change during the charging/discharging,89 and the Raman shift signal at 726 cm−1 was responsible for the vibration of the C atom along the out-of-plane in Ti3C2O2, whereas the signal in 708 cm−1 is for the vibration of C in Ti3C2O(OH), as shown in Figure 5a−c.89 The Raman shift with the electrode potential exactly reflects the surface redox reaction between −O and −OH groups, which contributes to the high capacitance of Ti3C2Tx observed in H2SO4. To capture a more direct measurement, the change of oxidation states is considered and reported in many studies by using in situ X-ray absorption spectroscopy (XAS). Lukatskaya et al. applied in situ XAS to study the change of oxidation states of Ti in electrode materials,90 and based on the measured K-edge XANES spectrum of Ti compared with the references of bulk TiO and TiO2, the redox state of Ti varies in the range between +2.34 and +2.43 in the voltage window from −0.35 to +0.35 vs Ag/AgCl.90 The sodium-ion intercalation mechanism of MXene was investigated by Yamada’s team through using ex situ X-ray powder diffraction (XRD), nuclear magnetic resonance (NMR), and transmission electron microscopy (TEM), as shown in Figure 5d.69 During the first sodium intercalation cycle, the interlayer distance was expanded from 9.7 to 12.5 Å, and with the Na concentration increasing, the distance remains constant, which indicates the Naconcentration-independent feature in the long term cycling. The NMR characterization revealed the three Na types in the system: Na+ in bulk electrolyte, MXene edge, and MXene interlayer region.69 After the first sodiation cycle, the intercalated Na+ and solvent molecules both preferred to stay around the interlayer region and could not be fully displaced during the discharging process.69 These Na+ and solvent can serve as a pillar to support the MXene layers and keep the interlayer distance unchanged.69 DFT calculations in Figure 5e showed that the measured stable interlayer distance of 12.5 Å is determined by the repulsion between the −OH group and mobile Na+ within the interlayer region.69 In the aqueous electrolyte, X-ray spectroscopy has revealed that large anions, such as SO42−, are not accessible to the MXene interlayer region during the charging/ discharging process.87 More recently, Shpigel et al. studied the cation intercalation with the confined water in MXene by the surface acoustic technique and found that the intercalation of the mixed Al3+, Mg2+, and Li+ ions is driven by the cointercalation of water molecules, whereas Cs+ and TEA+ have the poor solubility with the water molecules during the intercalating process.91 All of these studies provide an improved understanding on the chemical and physical processes behind MXene’s capacitive mechanism and also encourage further computational work to verify, explain, and propose new insights into the complicated but important process. MXenes can also be used as a substrate to enhance total capacitive energy storage. It is a rational strategy to combine MXene with other redox-active compounds to form hybrid electrodes because of its high electrical conductivity and stability. One successful example is the composite of a polypyrrole (PPy)/Ti3C2Tx hybrid electrode experimentally conducted in H2SO4 electrolyte, which exhibited much higher capacitance and better performance at a high scan rate than either Ti3C2Tx or PPy.92 The high capacitance was resulting from the synergic effect of combining PPy and MXene, because they are all pseudocapacitive, PPy has a higher specific capacitance than MXene per weight, and the hybrid electrode overcomes the degradation and exhibits promising cyclability due to the PPy chain stabilized by the terminal groups on MXene.92 Moreover, the glycinefunctionalized Ti3C2Tx was reported by Chen et al.,93 who revealed that the glycine molecule is in a more stable status via the strong N− Ti bonding supported by Ti3C2O2. Then, the improved capacitive performance at the low and high scan rates were both observed and I


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Figure 7. (a) HER volcano plot with the theoretical overpotentials for the investigated MXenes. Red (blue) circles represent carbides (nitrides) MXenes, where Mo2C is marked by star. (b) Surface Pourbaix diagrams of Ti2C, V2C, Nb2C, Ti3C2, and Nb4C3. The most thermodynamically stable states of the surface under relevant USHE conditions and pH values are labeled by the terminations. (c) Free energy diagrams for hydrogen evolution at zero potential and pH = 0 on the edges of various MXene nanoribbons with a width of N = 12. The black dashed line indicates the ideal DGH* for HER catalysis. (d) Correlation between the d band center (ed) and DGH* for the MXene nanoribbons. The red, blue, and green symbols represent M2C, M3C2, and M4C3 MXene nanoribbons, respectively. The depth of each color (from dark to light) indicates increasing H* binding strength. (e) Calculated reaction free energies of hydrogen absorption ΔGH of V2CO2 with 12.5, 16.7, and 25%ML TM coverage as a function of promoter and active site. The blue and red lines present the variations of ΔGH with the changes of promoter type and active site, respectively. The yellow and transparent planes present the cases of 12.5 and 16.7%ML TM covered V2CO2, respectively, which can describe the effect of the coverage of TM on the ΔGH. Figure 7a is reproduced with permission from ref 34. Copyright 2016 American Chemical Society. Figure 7b is reproduced with permission from ref 124. Copyright 2017 American Chemical Society. Figure 7c,d is reproduced with permission from ref 125. Copyright 2018 PCCP Owner Societies. Figure 7e is reproduced with permission from ref 126. Copyright 2016 John Wiley & Sons, Inc. observed in aqueous electrolyte by Osti et al.101 In addition, a joint experiment and theory work employing X-ray, neutron scattering, and CMD revealed that K+ enables a stabilization of the water in the MXene nanopore and decreases the self-diffusion coefficient of H2O.101

catalysts are based in part on semiconductors, with relatively low electrical conductivity, which limits their performance as solar water-splitting devices.118 Thus, there is still a huge gap for noble metals or semiconducting materials to show higher efficiency as electrochemical catalysts. 6.1.1. Current Experimental Progress of MXene as HER Catalyst. The 2D conductive transition-metal carbides have displayed promising catalytic performance for water splitting. Seh et al. first theoretically studied the application of M2XTx MXene on HER catalysis, shown in Figure 7a, which was later proven in experiments.34 The measured HER overpotential of Mo2CTx in acidic conditions reaches 283 mV at 10 mA/cm2, which is superior to those of many other 2D catalysts, like gC3N4.119 Xu et al. found that phosphor doping can further improve the HER performance of Mo2CTx.120 The phosphorized Mo2CTx (P−Mo2CTx) showed an even lower overpotential of 114 mV at 10 mA/cm2 compared with the 100 mV measured in the undoped Mo2CTx. Subsequently, DFT simulations revealed that the introduction of P atoms in the Mo2CTx lowers the hydrogen adsorption energy (more close to thermoneutral) and also improves the electrical conductivity, which both contribute to the HER activity.120 A more recent study also indicated that nitrogen doping on the

6. ELECTROCATALYTIC PROPERTIES OF MXENES 6.1. Electrochemical Water Splitting and Fuel Cells: HER, OER, and ORR Catalysis. The realization of electrochemical water splitting at low cost would be an important breakthrough. In the process of electrochemical conversion from H2O to H2, one of the key questions is how to lower the overpotentials of the cathode reaction (known as the hydrogen evolution reaction, HER) and the anode reaction (known as oxygen evolution reaction, OER) in water splitting, by relatively cheap methods and with high efficiency.102−104 Today, a range of inorganic materials are emerging with performance comparable or superior to that of noble metals, including transition-metal carbides,105−108 borides,105,109 nitrides,110 phosphides,111,112 and sulfides.113,114 Design of a noble-metal-free catalyst is emphasized to replace conventional noble metal catalysts, such as Pt,115 IrO2, and RuO2, among others,116 because of their high cost.117 Many of the new J


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ACS Applied Materials & Interfaces Ti2CTx surface can greatly improve the HER activity.121 Intrinsically, the HER activity of Ti2CTx was found to be improved by maintaining a larger amount of F-groups on the surface.122 The resultant HER overpotential of F-enriched Ti2CTx is 75 mV at a current of 0.41 mA/cm2.122 The origin of enhancement due to the F-group is strengthening of the surface’s proton binding ability and lowering of the chargetransfer resistance.122 The synthesis of the Ti3C2 fiber was achieved by the new etching method applied to Ti3AlC2 in the alkaline condition, and the as-synthesized material was introduced with more active sites, eventually benefiting the reaction kinetics.123 The measured low overpotential of 169 mV at the current density of 10 mA/cm2 and the suppressed Tafel slope of 97 mV/dec suggest the excellent performance of the Ti3C2 nanofiber as the HER catalyst.123 6.1.2. Theoretical Understanding on the HER Activity of MXene. With the continued expansion of the MXene family, experimentally screening the catalytic performance for each case and material will be cumbersome and expensive. Thus, an effective prior theoretical screening is highly desired. Gao et al. comprehensively studied the electrical conductivity, thermal stability, and HER activity of titanium, vanadium, and niobium carbides MXenes by DFT.124 Ti2C is the most exceptional, because it will show a bandgap around 0.78 eV when functionalized by −O and −OH groups. Meanwhile, the preferable surface terminations under different electrode potentials and varying pH values were also predicted. The Pourbaix diagram (Figure 7b) showed that at standard conditions (0 V vs SHE and 0 pH), Ti2C, V2C, and Ti3C2 tend to have a mixture of terminal groups of −O and −OH, whereas Nb2C and Nb4C3 are energetically preferred to be fully covered with −O groups.124 The HER free energy diagram demonstrated that Ti2CO2 exhibits a very low overpotential under the hydrogen coverage, regarded as a promising HER catalytic material.124 Yang et al. studied the HER activity (Figure 7c) of the MXene nanoribbon and located HER active sites in the edge area, and the hydrogen binding ability is related to the d-band center of metal in MXene in Figure 7d. For the considered MXene ribbons, Ti3C2 and (Ti, Nb)C showed almost zero hydrogen adsorption free energy with Tafel barriers of 0.42 and 0.17 eV, respectively.125 In addition, the HER activity of MXene can also be enhanced by decorating MXene surfaces with transition metals: Ling et al. designed the transition-metal-promoted V2CO2 (TM−V2CO2) in DFT calculations and concluded that the introduction of transition metals to the V2CO2 surface or strain can significantly weaken the binding between the hydrogen and oxygen group and eventually leads the hydrogen adsorption free energy to approach 0 eV, as shown in Figure 7e.126 Jiang et al. recently studied the HER activity of the MXene family and found that the oxygen vacancy formation energy can be a good descriptor to assess the HER activity.36 6.1.3. Potential of MXene in OER and ORR Catalysis. MXene can also serve as a matrix or substrate hybridizing with other materials to form hybrid OER catalysts with improved performance. Ma et al. reported that the OER activity of titanium−carbon-g-C3N4 (TCCN) nanosheets showed an onset potential of 1.44 V, comparable to the known excellent catalyst IrO2/C.127 Note that an even lower overpotential of 250 mV was observed in the cobalt−borate−MXene (Co−Bi− Ti3C2Tx) hybrid catalyst at 10 mA/cm2 current.128 Moreover, Li et al. reported the synergic enhancement of OER activity of the Ti3C2Tx supported FeOOH quantum dot.129 Although lots

of accumulated knowledge from the many studies on the MXene-based hybrid catalyst for OER have been established, how exactly the OER activity is improved by the MXene matrix/substrate is not clearly understood, which calls for further computational effort. Returning to traditional fuel cell applications, MXene is also of importance in catalyzing the oxygen reduction reaction (ORR). Xue et al. applied the Mn3O4/MXene nanocomposite to catalyze the ORR reaction, which ended up with a promising onset potential of 0.89 V.130 The enhancement of ORR activity is primarily due to the good electrical conductivity of MXene and the enhanced stability because of the strong binding ability of the MXene surface with metal oxide particles.130 The synergic improvement of ORR activity was also observed in the combination of FeN4 and MXene131 and Ag/MXene composite.132 Cheng et al. investigated the ORR activity of a metal monolayer (Cu, Pd, Pt, Ag, Au) on Mo2C by means of DFT,133 by which the metal−Mo2C interactions were explained: these transition-metal atoms energetically prefer to form a monolayer on Mo2C instead of metal clusters. The calculation of the adsorption energy of ORR intermediates showed that Au-monolayer/Mo2C has the best ORR performance as well as stability because of the strong interaction between the Au monolayer and Mo2C and the stronger adsorption to oxygen-containing species.133 A similar study by Wei et al. also concluded a better ORR activity of a Pt nanoparticle supported by Ti3C2Tx (Pt-NP/Ti3C2Tx composite) and a higher durability than that of Pt/C catalyst.134 Recently, Zhou et al. designed a heterostructure of N-doped graphene and MXene as a bifunctional ORR−HER catalyst,135 showing the high activity in both ORR (0.36 eV as of the overpotential with the kinetic barrier of 0.2 eV) and HER. The improved ORR activity is attributed to the strong coupling between the N−graphene layer and MXene,135 which paves a plausible way of designing MXene hybrid materials as highperformance catalysts. 6.2. Chemical Conversion: CO2 Reduction Reaction and N2 Fixation. 6.2.1. Experimental Performance of MXene as the CO2RR Catalyst. Not limited to only electrochemical water-splitting and fuel cell applications, MXene also exhibits potential for electrochemical catalysis for energy conversion, such as CO2 reduction and N2 fixation. Several experimental studies have shown a considerable enhancement in the catalytic activity, when MXene was used as a cocatalyst in the CO2 reduction. For instance, Cao et al. reported the excellent photocatalytic CO2 reduction in the newly developed 2D heterostructure of Ti3C2/Bi2WO6, showing the enhanced interfacial charge-transfer ability and the strengthened CO2 adsorption on Ti3C2.136 Ye et al. also measured the enhanced photocatalytic activity of CO 2 reduction of commercial titania on the surface-alkalized Ti3C2, originating from the improved binding ability between catalyst and the absorption of CO2 on the −OH-terminated MXene.137 Zeng et al. recently reported the improved photocatalytic activity of Cu2O nanowires decorated with Ti3C2 quantum dots, which enhance the overall charge transfer, carrier density, light absorption, and stability.138 So far, no direct experimental examination of single MXene in CO2 reduction reaction (CO2RR) catalysis has been reported, although the result has been indicating strong binding or adsorption capability with CO2 molecules. The capability of binding or adsorption is a prerequisite to activating the reactions and also the key step in the electrochemical K


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Figure 8. (a,b) Minimum energy pathway for the CO2 conversion into CH4 and H2O over (a) Mo3C2(OH)2 and (b) Mo3C2O2. Gibbs free energies along the pathway, vs SHE, are shown in eV (top). (c) 3D volcano plot of UL as a function of the binding energies ECOOHb and EHCOOHb for each elementary reaction step along the minimum energy pathway. The numbers next to each UL plane denote the reaction number that the plane represents. (d) Contour plot of the most negative UL(CO2) values obtained by taking the lowest UL(CO2) plane in (c). (e) Plot of UL(CO2)−UL(H2) as a function of UL(CO2) showing the selectivity of CO2RR relative to HER for MXenes. Vertical line in (e) marks the equilibrium potential for the reduction of CO2 to CH4 (0.17 V vs RHE). Figure 8a,b is reproduced with permission from ref 140. Copyright 2017 American Chemical Society. Figure 8c−e is reproduced with permission from ref 142. Copyright 2018 The Royal Society of Chemistry.

CO2RR.139 Thus, a systematic computational exploration of MXene’s CO2RR activity in the atomic level could effectively accelerate the explorations of MXene-based electrocatalysts and also gain a deeper understanding and new insight into CO2RR catalysis. 6.2.2. Theoretical Investigation on the CO2RR Activity of MXene. DFT has been widely used to study the electrocatalytic process. Li et al. systematically studied the CO2RR activity of M3C2 MXenes containing transition metals from groups IV, V, and VI by first-principles calculations.140 The theoretical calculations of CO2 adsorption on MXenes showed that the considered MXenes all have strong binding ability with CO2 molecules via chemical absorption. The obtained free energies of the intermediate species indicate that Cr3C2 and Mo3C2 have the best selective conversion capability to CH4, and for the CO2 conversion to CH4, the theoretical input was estimated to be 1.05 and 1.31 eV on the bare Cr3C2 and Mo3C2, respectively, which can be further lowered to 0.35 and 0.54 eV with the −O and −OH-terminations, as shown in Figure 8a,b.140 Zhang et al. investigated the electrocatalytic CO2RR activity of O-terminated monolayer MXenes including V 2 CO 2 , Ti 2 CO 2 , Ti 3 C 2 O 2 , and Ti 2 CO 2 with oxygen

vacancy.141 The preferred reaction pathway was predicted to be CO2 → HCOO → HCOOH with an energy barrier of 0.53 eV on an O-vacancy site.141 Handoko et al. studied CO2RR activity of MXenes, gained a scaling relation between thermodynamic binding energy and kinetic barriers for a series of M2XO2 by DFT, and identified the preferred reaction pathway from CO2 to CH4.142 W2CO2 and Ti2CO2 were found to be the most promising CO2RR catalysts, showing overpotentials of 0.52 and 0.69 V and excellent selectivity in theory. According to the scaling relation, the binding energy of each intermediate shows that the *HCOOH and *COOH binding has the strongest relation to the limiting potential (UL), which can be further used as two descriptors to estimate the reaction barrier in Figure 8c,d. From UL (CO2) and UL (H2), the catalytic activity and conversion selectivity can be easily obtained in all MXene candidates, and for instance, the scaling relation proposed in Hondoko’s work also suggested that nitride MXenes, such as Cr2NO2, Nb2CO2, Ti2, and NO2, among others, are more favorable to be CO2RR catalyst (Figure 8e) than carbides.142 In parallel, the promising performance of nitride MXene on CO2 activation was indicated by other DFT calculations.143 L


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Figure 9. (a) Proposed routes for the N2 conversion mechanism. Minimum energy path for the N2 conversion into NH3 catalyzed by (b) V3C2 and (c) Nb3C2 MXenes, calculated in the DFT + D3 functional. Structures and energies of the intermediates and transition states (TS) indicated. Gibbs free reaction (black text) and activation (red text) energies, vs SHE [pH = 0, f(H2) = 101 325 Pa, and U = 0 V], at T = 298.15 K, are shown in eV. Selected M−N distances are shown in the unit of Å. Reactive hydrogen atom during the N2 electroreduction is colored in pink in each step. Shading indicates spontaneous (blue) versus nonspontaneous (red) steps. Figure 9a−c is reproduced with permission from ref 145. Copyright 2016 The Royal Society of Chemistry.

pointed out that MXenes with d3 and d4 metals display higher adsorption free energies because of the stronger binding with N2 than CO2 and H2O. The preferable reaction pathway (Figure 9a) for N2 hydrogenation is unveiled as the side-byside mechanism undergoing the transition from NNH to NNH2 and then N + NH3. The rate-limiting step is predicted to be the binding of hydrogen, from N−N to N−NH. In the view of the reaction pathways, V3C2 and Nb3C2 were predicted to be the most promising NRR catalysts with reaction energies of 0.32 and 0.39 eV. The maximum overpotentials of the above two MXenes were predicted to 0.64 and 0.94 eV, corresponding to the first and second hydrogenations on V3C2 and Nb3C2, shown in Figure 9b,c.145

6.2.3. Theoretical Exploration of MXene as NRR Catalyst. In addition to CO2RR, MXene was also investigated as a possible nitrogen reduction reaction (NRR) catalyst. Distinct from CO2RR and HER, the activation of N atoms and generation of N2 gas is considerably more challenging and difficult electrochemically because of the high stability of the N2 molecule’s triple bonds. Zhao et al. reported the performance of Ti3C2Tx as the NRR catalyst in 0.1 M HCl,144 showing that at −0.4 V vs reversible hydrogen electrode, the NH3 yield was measured to 20.4 mg·h−1·mgcat−1, and the Faradaic efficiency was about 9.3%. The rate-limiting step of *NH2 → NH3 with excellent activity is predicted to be due to the weakened NN bond on the Ti3C2Tx surface.144 A systematic theoretical investigation on the NRR activity of M3C2 with different M elements (i.e., varied number of valence electrons (d2, d3, d4)) was conducted by Azofra et al.,145 who M


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Figure 10. (a) Formation of MXene alloys with varying degrees of ordering, where M1 and M2 can be Ti, V, Nb, Ta, and Mo. Left: M3C2 MXenes consist of two different transition metals occupying the outer layers and the middle layer (blue and orange spheres), respectively. (b) Atomic occupancies of representative (M11−xM2x)3C2 alloys at x = 2/3 for T = 300, 1100, and 1900 K. Larger spheres represent metallic atoms, and their colors vary from red (100% occupation by M1) to blue (100% occupation by M2) depending on the average occupancies of the lattice site during MC simulation. Intermediate compositions are colored according to the color chart. Small gray spheres are C atoms. (Ti1/3Mo2/3)3C2 is a representative of (M11/3Mo2/3)3C2 alloys with strong interlayer ordering, where Mo occupies the outer layer. (Ta1/3Ti2/3)3C2 represents alloy MXenes with intermediate interlayer ordering with Ti occupying the outer layer. (Ti1/3V2/3)3C2 represents a disordered solid solution. Figure 10a,b is reproduced with permission from ref 57. Copyright 2017 American Chemical Society.

Database (COD)149 are now complemented by multiple computational material databases. These include the Materials Project (MP) database,150 the Open Quantum Materials Database (OQMD),151 and the AFLOW database.152 Once the databases are assembled, material screening for particular problems is performed. For instance, Zhang et al. performed a HTC screening of 2D heterojunctions in the MP database for photocatalytic water splitting by using the stability and positions of the band edge as criteria.153 Ong et al. conducted computational screening to explore novel inorganic phosphors for light emission that was later confirmed experimentally.154 A more recent use of computational materials databases has been in the application of neural-network-based machine learning (ML). Artificial neural networks (ANN) have been used to generate interatomic potentials and to predict a variety

7. APPLICATION OF HIGH-THROUGHPUT COMPUTATION AND MACHINE LEARNING TO MXENES 7.1. High-Throughput Computation and Machine Learning for Material Design. With the development of computing hardware and software, high-throughput computation (HTC) has emerged as a powerful way to explore the chemical space of materials by building up large databases,146,147 which can later be screened for a particular desired properties. For targeted applications where the desired property is well-known, these two processes can be combined in a guided optimization. The traditionally experimentally focused materials databases such as the Inorganic Crystal Structure Database (ISCD)148 and the Crystallography Open N


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Figure 11. (a) Work flow filtering semiconductors from the MXene database and feeding its subset as input to machine-learning algorithms. (b) Density plot for the standardized data set with zero mean and unit variance in the preprocessing for machine learning. (c) Statistical heat map showing correlation of individual primary features with themselves. Figure 11a−c is reproduced with permission from ref 162. Copyright 2018 American Chemical Society.

of energy related quantities.155−157 The faster evaluation of the ANN compared to performing a full quantum-mechanical calculation enables more complex structures or a wider range of compositions to be studied. The methods of decision trees and shrinkage are also widely used in the predictive purpose of materials having certain features or properties.158 To avoid the expensive computational cost to access high-dimensional descriptors, feature regularization and dimensional reduction via the least absolute shrinkage and selection operator (LASSO)158 or its improvement are performed.159 For example, Ghiringhelli et al. reported a successful example of applying an ML-based descriptor to structure-energy prediction of rocksalts.159 Jonayat et al. studied the stability of stoichiometric and nonstoichiometric monolayer metal oxides (MMO) by ML technique.160 7.2. Examples of Using High-Throughput Computation and Machine Learning in MXenes. The well-defined structures of MXenes, combined with the large configurational and composition space, makes them ideal targets for HTC and ML techniques, particularly when solid solutions and nonstoichiometric MXenes are also considered. Although quantitative and accurate predictions are most desired, the identification of reliable materials’ property trends would help drive synthesis efforts. Tan et al. carried out HTC of the structure stability of binary MXene alloys (Figure 10a) by combining first-principles DFT with the cluster expansion

method. 5 7 They found that in Mo-rich MXenes (M1−xMox)3C2, Mo often tends to occupy the surface layers, and the predicted ordering is robust even at high temperature (Figure 10b, middle row). In the Ti-containing MXenes, the preferred Ti location is dependent on the alloying metals: when alloying with Ta, Ti prefers to occupy the surface layer (Figure 10b, top); when alloying with V, Ti tends to form a solid solution (Figure 10b, top row). HTC has also been applied to discovering MXene with promising catalytic properties. Ling et al. performed the HTC screening on the HER activity of M2CO2 single- and double-metal MXenes. A new descriptor, electron number, gained by the surface O atom, was found to efficiently estimate the HER activity, and TiVCO2 was then predicted to be a promising HER catalyst.161 Rajan et al. proposed an ML algorithm to accurately predict bandgaps of functionalized MXenes. Among 23 870 MXene candidates in the database, 76 semiconducting MXenes were selected to establish the predictive model via the supervised ML (see the workflow in Figure 11a). The LASSO method eventually generated 15 main features (Figure 11b), which surprisingly did not include the DFT bandgap or thermodynamic inputs. The Pearson’s correlation coefficients (Figure 11c) indicated that these features are not strongly correlated. Recently, four methods are becoming widely used to train the ML models: kernel ridge (KRR), support vector (SVR), Gaussian process (GPR), and bootstrap aggregating (bagging). O


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Figure 12. Band gap predictions of MXene. Scatter plots showing band gap predictions versus true (i.e., GW) gaps of important primary (top panel) and compound (bottom panel) feature combinations. Best model predictions are shown with accuracy metrics such as R2 and rmse (train/ test) of (a,e) KRR, (b,f) SVR, (c,g) GPR, and (d,h) bagging corresponding to 90% training (colored red) and 10% testing (green) data. Figures are reproduced with permission from ref 162. Copyright 2018 American Chemical Society.

modeling approach by taking into account the reaction kinetics and the large configurational space of mixed surface terminal groups.24 Alternatively, the recent study by Sang et al. suggested that the atomic layer growth by thermal exposure and electron-beam irradiation might be a way to obtain pure MXene without surface termination165 in a bottom-up manner. A high-throughput study of different possible constituent elements could identify MXenes that are most suited to this process and thereby focus experimental studies. For electrocatalysis and energy storage, understanding the interface of the MXene with the electrolyte is key. As a result of the complexity in MXene surface chemistry, advanced computations are needed to include the effect of mixed terminal groups, confined electrolyte, transition-metal ordering (for double- and multiple-transition-metal MXene), and stoichiometry (for i-MXene), all while under electrochemical conditions. Today, the study of electrocatalytic and energy storage properties under bias is little developed. Our understanding cannot be considered complete until comprehensive studies of the effect of bias on surface terminations, coverage, and electrolyte structure on capacity are performed. This is uniquely important in MXenes because of their greater complexity than materials such as graphene, where the surface structure is effectively fixed. A combination of multiscale simulations, such as CMD, mesoscale continuum modeling, as well as HTC and ML, may be necessary. This will allow one to accurately answer the question of which combination of MXene, electrolyte, and ion gives highest capacities for electrodes in batteries and pseudocapacitors. In addition, studies of the ion transport behavior are required to address the rate performance of these systems. This is an intrinsically multiscale process involving transport in the confined MXene layers up to transport in and around MXene particles (or however the MXene is processed). A key technical hurdle to overcome is the lack of classical potentials or sufficiently fast quantum-mechanics-based methods suitable to describe the wide range of possible ions, electrolytes, and MXenes, with sufficient accuracy to address rate capability. It is possible that the promisingly high capacities and rate capabilities observed experimentally can be further enhanced with improved materials and electrolyte selection and further optimization. Finally, although we have focused on computational studies, a

KRR and SVR were found to be more promising and feasible due to their relatively low root-mean-squared error (rmse) than other methods in the prediction of band gaps of MXenes (Figure 12). For learning models, the volume/atom and mean boiling point were found to be more reliable than others and have strong positive correlation with GW band gaps.162 Although it is easy to predict the continued expansion of HTC and ML in the domain of MXenes, their use is likely to be most important for properties such as capacitive performance and for electrocatalysis, where the complex processes and range of important length scales precludes performing many direct calculations. For example, optimizing ionic diffusivity is required for high rate performance, but diffusivity is challenging to compute by direct application of high-level methods.

8. SUMMARY AND OUTLOOK Aiming to accelerate the process of materials discovery, many significant computational studies on MXenes have been performed in the past several years. To restate only two successful examples: first, in catalysis, the superior HER activity of Mo2CTx first predicted by DFT calculation was later confirmed experimentally.34 Second, in energy storage, computational screening of MXene anodes inspired follow-up experimental studies.73 High-throughput computational screening and machine learning of the MXene family have also been successfully applied.83,162−164 However, despite these and other successes, there are still significant gaps between what has been achieved through theory and modeling and what is necessary to design materials with optimized properties. Fundamentally, although there has been success in describing the structure and thermodynamic stability of MXenes, there is still a gap between the computational predictionsusing necessarily simple, idealized structures and the current complex experimental realizations. To close this gap requires developing a more thorough theory of synthesis that incorporates details of the processes employed. For example, what is the full extent of the range of salts, temperatures, pH values, and parent MAX phases that can be combined via etching to produce high-purity MXenes? Besides considering the formation or reaction energy with acidic solutions or molten salts, one can further pursue a multiscale P


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key component to this work must involve experimental validation with well-characterized materials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Weiwei Sun: 0000-0002-5535-0660 Yu Xie: 0000-0002-7782-5428 De-en Jiang: 0000-0001-5167-0731 Paul R. C. Kent: 0000-0001-5539-4017 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is sponsored by the Fluid Interface Reactions, Structures, and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences. Part of this work (C.Z.) was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. C.Z. is partially supported by the PLS-Postdoctoral Grant of the Lawrence Livermore National Laboratory.

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Computational Discovery and Design of MXenes for Energy Applications

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