MXene as a Charge Storage Host - ACS Publications - American

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Article Cite This: Acc. Chem. Res. 2018, 51, 591−599

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MXene as a Charge Storage Host Published as part of the Accounts of Chemical Research special issue “Energy Storage: Complexities Among Materials and Interfaces at Multiple Length Scales”. Masashi Okubo,†,‡ Akira Sugahara,† Satoshi Kajiyama,† and Atsuo Yamada*,†,‡ †

Department of Chemical System Engineering, School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan ‡ Elemental Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan S Supporting Information *

CONSPECTUS: The development of efficient electrochemical energy storage (EES) devices is an important sustainability issue to realize green electrical grids. Charge storage mechanisms in present EES devices, such as ion (de)intercalation in lithium-ion batteries and electric double layer formation in capacitors, provide insufficient efficiency and performance for grid use. Intercalation pseudocapacitance (or redox capacitance) has emerged as an alternative chemistry for advanced EES devices. Intercalation pseudocapacitance occurs through bulk redox reactions with ultrafast ion diffusion. In particular, the metal carbide/nitride nanosheets termed MXene discovered in 2011 are a promising class of intercalation pseudocapacitor electrode materials because of their compositional versatility for materials exploration (e.g., Ti2CTx, Ti3C2Tx, V2CTx, and Nb2CTx, where T is a surface termination group such as F, Cl, O, or OH), high electrical conductivity for high current charge, and a layered structure of stacked nanosheets for ultrafast ion intercalation. Various MXene electrodes have been reported to exhibit complementary battery performance, such as large specific capacity at high charge/discharge rates. However, general design strategies of MXenes for EES applications have not been established because of the limited understanding of the electrochemical mechanisms of MXenes. This Account describes current knowledge of the fundamental electrochemical properties of MXenes and attempts to clarify where intercalation capacitance ends and intercalation pseudocapacitance begins. MXene electrodes in aqueous electrolytes exhibit intercalation of hydrated cations. The hydrated cations form an electric double layer in the interlayer space to give a conventional capacitance within the narrow potential window of aqueous electrolytes. When nonaqueous electrolytes are used, although solvated cations are intercalated into the interlayer space during the initial stage of charging, the confined solvation shell should gradually collapse because of the large inner potential difference in the interlayer space. Upon further charging, desolvated ions solely intercalate, and the atomic orbitals of the desolvated cations overlap with the orbitals of MXene to form a donor band. The formation of the donor band induces the reduction of MXene, giving rise to an intercalation pseudocapacitance through charge transfer from the ions to MXene sheets. Differences in the electrochemical reaction mechanisms lead to variation of the electrochemical responses of MXenes (e.g., cyclic voltammetry curves, specific capacitance), highlighting the importance of establishing a comprehensive grasp of the electrochemical reactions of MXenes at an atomic level. Because of their better charge storage kinetics compared with those of typical materials used in present EES devices, aqueous/nonaqueous asymmetric capacitors using titanium carbide MXene electrodes are capable of efficient operation at high charge/discharge rates. Therefore, the further development of novel MXene electrodes for advanced EES applications is warranted.

1. INTRODUCTION

from renewable energy. The low cost and elemental abundance of each component in EES devices are minimal attributes for wide deployment and market share. However, none of the currently used EES devices can meet all of these requirements. Lithium ion batteries, which store charge through lithium ion (de)intercalation, have limited power density because lithium ion diffusion is slow. Although electric double layer (EDL) formation is fast, EDL capacitors cannot store a large amount of

Building clean, sustainable electrical grids is an urgent global demand in light of recent serious concerns over climate change. One technology that could potentially address this challenge is electrochemical energy storage (EES), as the efficient operation of EES devices distributed in a grid will enable seamless integration of carbon-free renewable energy.1,2 However, grid-scale EES devices must meet various requirements. For example, high energy density is of primary importance to power electric vehicles over long distances. High power density is required to load-level intermittent power © 2018 American Chemical Society

Received: October 1, 2017 Published: February 22, 2018 591

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Accounts of Chemical Research charge, leading to poor energy density.3 Therefore, advanced EES devices need an alternative chemical system that is capable of more efficient charge storage than those presently available. Pseudocapacitance (or redox capacitance) is a potential alternative to current technology because a large amount of charge can be stored through fast surface redox reactions.4,5 EES devices using pseudocapacitor electrodes (e.g., heteroatom-doped carbons, 6,7 conducting polymers, 8 RuO 2 · nH2O,9−11 and MnO212) have achieved high gravimetric energy and power densities simultaneously. However, from a practical point of view, the large surface area of the nano/mesostructured materials generally causes irreversible surface reactions arising from electrolyte decomposition. Furthermore, their low electrode density leads to low volumetric energy density. Despite the past few decades of efforts to develop pseudocapacitor electrodes, these drawbacks still severely limit their application to practical EES devices. In 2013, Dunn and co-workers13 proposed the concept of “intercalation pseudocapacitance” as a more practical pseudocapacitor mechanism. As demonstrated for T-Nb2O5, the intercalation pseudocapacitance occurs through bulk redox reactions with ultrafast ion intercalation.14,15 Importantly, the limited surface exposure to the electrolyte suppresses unfavorable irreversible reactions without sacrificing the amount of charge storage. Therefore, the concept of intercalation pseudocapacitance has been extended to other compounds such as VOPO4,16 MoS2,17 and VS218 with reasonable compatibility of power and energy densities. Further materials exploration to best exploit this novel concept is urgently needed. To develop intercalation pseudocapacitor electrodes, host electrode materials should possess both high electronic conductivity and fast ion diffusivity. In these regards, twodimensional transition metal carbides/nitrides called MXenes, which were discovered by the research group led by Barsoum and Gogotsi in 2011,19 are a promising platform for intercalation pseudocapacitance.20−22 As shown in Figure 1a, MXenes have a layered structure of stacked M n+1 X nT x nanosheets (M = Ti, V, Cr, Zr, Nb, Mo, etc.; X = C, N; T = F, OH, Cl, O, etc.).23 We expect that while the stacked nanosheet morphology minimizes active surface exposure to electrolytes, the open but minimal interlayer space contributes to fast ion intercalation as well as high-density charge storage, satisfying the criteria to realize intercalation pseudocapacitance. Indeed, the use of MXenes as electrode materials for EES devices has been experimentally examined, theoretically predicted, and systematically reviewed by many research groups.24−32 However, the reaction mechanisms of MXene electrodes are not fully understood, and their optimal exploitation in EES devices has not been established. This Account describes current understanding of the fundamental electrochemical properties of MXenes. We attempt to overview how the electrode reactions of MXenes occur by careful comparison of aqueous and nonaqueous systems.

Figure 1. (a) Transformation of MAX phase Mn+1AXn to MXene Mn+1XnTx, where M is an early transition metal such as Ti, V, Nb, or Mo; A is Al; X is C and/or N; and T is a surface termination group such as −OH, −F, Cl, or −O. (b) SEM and TEM images of MXene Ti3C2Tx. A delaminated structure for electric double layer formation and a layered structure of stacked nanosheets for ion intercalation coexist. Adapted from ref 41. Copyright 2016 American Chemical Society.

giving Mn+1XnTx nanosheets. The surface termination groups are −F, −OH, and −O after HF treatment33−35 and −F, −Cl, −OH, and −O after LiF/HCl treatment.36 MXenes with various compositions such as Ti2CTx,23 Ti3C2Tx,19 V2CTx,37 Nb2CTx,38 Mo2CTx,39 (Mo2Ti)C2Tx,40 Mo1.33CTx,30 and Ti3(CN)Tx23 have been synthesized. Transmission electron microscopy (TEM) revealed that MXenes possess a layered structure of stacked nanosheets (Figure 1b); the open interlayer space is capable of ion intercalation.41 Scanning electron microscopy (SEM) confirmed that even without any delamination process such as sonication, MXenes have partially delaminated structures (Figure 1b). Presumably, conventional EDLs are formed on the delaminated surface of MXenes.42 Therefore, MXenes are expected to exhibit charge storage by ion intercalation as well as by surface EDL formation. From the viewpoint of electronic structure, hard X-ray photoelectron spectroscopy (HXPES) of Ti 2p indicated that Ti is oxidized upon the transformation of Ti3AlC2 to Ti3C2Tx (Figure 2a), whereas the chemical state of C is not influenced (Figure 2b).43 This result suggests that the frontier orbitals in Ti3AlC2 and Ti3C2Tx mainly consist of Ti 3d states. The density of states calculated by density functional theory (DFT) calculations for Ti3C2F2 (Figure 3a) support that Ti t2g states are dominant near the Fermi level. 41,44−46 The DFT calculations also indicate that C 2p states hybridize mainly with Ti eg states. Importantly, MXenes are metallic compounds. For example, the magnetic susceptibilities of Ti2CTx and Ti3C2Tx are small and independent of temperature (Figure 2c),36,43 which is typical behavior for metallic compounds (Pauli paramagnetism). Presumably, the orbital hybridization between M eg and C 2p orbitals is relatively weak because of the large separation of

2. STRUCTURE, MORPHOLOGY, AND ELECTRONIC STATE OF MXENE MXenes are synthesized by the removal of aluminum (or gallium) from a metal carbide/nitride Mn+1AlXn (called the MAX phase) using HF or LiF/HCl solution (Figure 1a).19,21 After the removal of aluminum (or gallium), surface termination groups T attach on the remaining Mn+1Xn layers, 592

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Figure 2. Electronic structure of MXene. (a, b) Ti 2p and C 1s hard X-ray photoelectron spectroscopy (HXPES) spectra for Ti3AlC2 and Ti3C2Tx. (c) Temperature dependence of the magnetic susceptibility χ for Ti2CTx and Ti3C2Tx. Adapted with permission from ref 36. Copyright 2017 WileyVCH. (d) Schematic illustration of the density of states for MXene Mn+1XnTx. Weak orbital hybridization between M d and C 2p orbitals gives a small ligand-field splitting 10Dq relative to the d bandwidth W, leading to a metallic band structure.

the orbital energy levels. Therefore, the ligand-field splitting (10Dq) is small relative to the bandwidth (W) of the d orbitals, leading to a metallic band structure (Figure 2d). The metallic conduction of MXenes is advantageous for electrode applications, especially at high charge/discharge rates.

3. ELECTROCHEMISTRY OF MXENE As described above, MXenes have a layered structure that is partially delaminated, allowing both ion intercalation and EDL formation to contribute to charge storage. Delamination of the nanosheets by sonication, which is frequently used to fabricate free-standing MXene films,47−51 alters the relative fraction of these two contributions to charge storage. For simplicity, the electrochemical properties of the MXene electrodes fabricated without a delamination process are summarized in this Account. Figure 4 shows cyclic voltammetry (CV) curves for Ti2CTx and Ti3C2Tx electrodes in aqueous and nonaqueous electrolytes at a sweep rate of 1 mV/s.26,36,41,43 In aqueous electrolytes, the MXene electrodes exhibit rectangular CV curves (Figure 4a,c), which are typical of EDL capacitor electrodes. The ex situ X-ray diffraction (XRD) patterns for Ti2CTx during charge/discharge (Figure 5a) show the reversible change in the interlayer distance d (12.7 and 13.2 Å, respectively), indicating ion intercalation into the interlayer space. Because the d values during charge/discharge are much larger than that of the pristine anhydrous MXene obtained by drying at 200 °C under vacuum (d = 8.7 Å) (Figure 5b), water molecules and hydrated

Figure 3. Total and partial densities of states (DOS) of (a) Ti3C2F2 and (b) Na-intercalated Ti3C2F2. Adapted from ref 41. Copyright 2016 American Chemical Society.

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reversible Li+ (de)intercalation (Figure 5a,b). Meanwhile, the ex situ X-ray absorption spectroscopy of the Ti K-edge of Ti2CTx indicated reversible reduction/oxidation of Ti upon charge/discharge (Figure 5c).36 Therefore, the MXene electrodes in nonaqueous electrolytes exhibit intercalation pseudocapacitance (redox capacitance). Because the d values during charge/discharge are close to that of pristine MXene (d = 8.7 Å), desolvated ions are solely intercalated into the interlayer space.

4. REACTION MECHANISMS OF MXENE ELECTRODES To establish a firm design strategy to produce better capacitors, an atomic-scale reaction scheme describing the charge/ discharge reactions should be revealed. In this regard, we still do not clearly understand (i) whether an EDL is formed in the interlayer space within the narrow potential window of aqueous electrolytes and (ii) why the CV curves and specific capacitances are different in aqueous and nonaqueous electrolytes. Theoretically, the specific capacitance (C) of an electrode is expressed as C = ΔQ , where Δ(φcc − φb) is the change in

Figure 4. Cyclic voltammetry curves of MXene Ti2CTx and Ti3C2Tx electrodes at a sweep rate of 1 mV/s: (a) 0.5 M Li2SO4/H2O, (b)1.0 M LiPF6/ethylene carbonate (EC)-dimethyl carbonate (DMC), (c) 0.5 M Na2SO4/H2O, and (d) 1.0 M NaPF6/EC-diethyl carbonate (DEC). Adapted with permission from refs 26, 36, and 41. Copyright 2015 Springer Nature, 2017 Wiley-VCH, and 2016 American Chemical Society, respectively.

Δ(φcc − φ b)

the inner potential difference between a current collector (φcc) and a bulk electrolyte (φb) and ΔQ is the stored charge per the weight of the electrode (Figure S1). Using the equilibrium conditions of the electron and ion electrochemical potentials,

ions should be cointercalated into the interlayer space, considering the small ionic radius of Li+ (0.9 Å). In contrast, MXene electrodes in nonaqueous electrolytes exhibit distorted capacitive CV curves (Figure 4b,d). For example, a Ti2CTx electrode in a nonaqueous Na+ electrolyte displayed a cathodic peak at 1.8 V and anodic peak at 2.3 V in addition to a rectangular-shaped capacitive current of approximately 250 F/g between 0.1 and 2.0 V vs Na/Na+. The ex situ XRD patterns of Ti2CTx upon charge/discharge in a nonaqueous Li+ electrolyte showed a reversible d change (9.4 and 9.8 Å, respectively), which was presumably caused by

Δ(φcc − φb) is equal to Δ(φEe − φEi ) −

Δ(μeE + μiE ) F

, in which μEe

and μEi are the chemical potentials of an electron and an ion in the electrode, respectively, and φEe and φEi are the inner potentials of the electron and ion in the electrode, respectively. The first term gives an EDL capacitance, whereas the second term gives a non-EDL capacitance (pseudocapacitance, quantum capacitance, or chemical capacitance). To address the above-mentioned issues on the basis of this principle, in this

Figure 5. (a, b) Structural changes of MXene Ti2CTx upon charge/discharge: (a) ex situ X-ray diffraction patterns and (b) interlayer distance changes in aqueous and nonaqueous electrolytes. (c) Ti K-edge X-ray absorption spectra for Ti2CTx in 1 M LiPF6/EC-DMC. Adapted with permission from ref 36. Copyright 2017 Wiley-VCH. 594

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Figure 6. Schematic illustration of structural changes and the electronic structure changes of MXene in (a) aqueous and (b) nonaqueous Li+ electrolytes. φ is the inner potential, and ηe is the electron electrochemical potential.

interactions are so small that both μEe and μEi are almost constant during the entire charge/discharge process. In this regard, the MXene electrodes with aqueous electrolytes behave as EDL capacitors to deliver a constant capacitance (Figure 4). In other words, MXene has a negligible pseudocapacitance relative to the EDL capacitance within the potential window of aqueous electrolytes. It is also important to note that when acidic electrolytes (e.g., H2SO4) are used, distorted rectangular CV curves are observed because of the contribution of the Faradaic reaction Mn+−O + H+ + e− → M(n−1)+−OH.54 The reduction of M was confirmed by in situ X-ray absorption spectroscopy.55 Presumably as a result of this Faradaic reaction, acidic electrolytes provide the highest capacitance among aqueous electrolytes.

section we will closely trace the elemental steps of the electrochemical reactions of an MXene electrode. 4.1. Aqueous System

In aqueous electrolytes, hydrated cations (e.g., Li+) are intercalated without dehydration, as demonstrated in Figure 4. Because isolated atomic orbitals of the cation in a hydration shell cannot hybridize with the orbitals of the MXene, separated positive and negative charges generate an inner potential difference Δφ = φEe − φEi in the interlayer space, forming an EDL within the interlayer space (Figure 6a). Because the amounts of stored electrons and ions are small (vide infra), the chemical potentials of the electron and ion should be almost constant during the charge/discharge processes. Therefore, Δφ dominates the electrode potential during charge/discharge, as shown in Figure 6a. Within this region, a conventional EDL model can be applied, where accumulated electrons occupy states above the original Fermi states. The specific capacitance is expressed as C = εrε0A , where εr is the dielectric constant b−a between the MXene wall and the ion, ε0 is the vacuum permittivity, A is the specific surface area of the MXene walls, 2b is the separation of the MXene walls, and a is the ionic radius.52 The EDL capacitance is constant, giving rectangular CV curves like those in Figure 4, and is not affected by the electronic structure (e.g., density of states) of the MXene. Because εrε0 and A are inherent to the electrode and electrolyte, decreasing b − a through the careful selection of the electrolyte or modification of the surface termination groups T is necessary to enhance C.36,53 Concerning the electronic structure of MXene upon charge/ discharge with aqueous electrolytes, stored electrons occupy the originally unoccupied state of MXene, and reduction of MXene occurs. Indeed, the Ti K-edge X-ray absorption spectrum of Ti2CTx in a 1 M Li2SO4 aqueous electrolyte showed a slight shift of the peak top position of the main edge to lower energy after charging from −0.05 to −0.45 V vs Ag/AgCl (Figure S2), suggesting the reduction of Ti. However, within the potential window of aqueous electrolytes, the amounts of stored electrons and ions are small (ca. 20 mA h/g for Ti2CTx in a 1 M Li2SO4 aqueous electrolyte, i.e., 0.05 e− and 0.05 Li+ per Ti atom). Therefore, the band-filling effect and intercation

4.2. Nonaqueous System

For nonaqueous electrolytes, the solvation energy is much weaker than that for hydration. Partial desolvation thus occurs more easily at the MXene−electrolyte interface, and after intercalation, the atomic orbitals of cations hybridize with the orbitals of the surface termination groups of the MXene. The existence of a donor band of cations was supported by DFT calculations (Figure 3b).41,45,46 This orbital hybridization forms a donor band to reduce MXene (Figure 6b). Importantly, charge transfer between MXene and intercalated cations occurs through the orbital hybridization, which should significantly screen the electric field between the electron and cation to reduce Δ(φEe − φEi ). In addition, the large potential window of organic electrolytes allows the storage of large amounts of electrons and cations, leading to considerable changes in μEe and μEi due to the band-filling effect and intercation interactions. Therefore, the capacitance of MXene electrodes in nonaqueous electrolytes arises mainly from pseudocapacitance dominated by the chemical potentials, which is larger than that in aqueous electrolytes (Figure 4). As the pseudocapacitance is influenced by the density of states above the original Fermi level (the band-filling effect), control of the electronic structure of MXenes is important to increase their capacitance. Furthermore, because the DFT calculations predicted that the orbital hybridization is strongly influenced by the cations and surface termination groups,41,45,46 appropriate selection of the intercalant or modification of the 595

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Figure 7. (a) First charge curve at 20 mA/g for Ti2CTx in a 1 M LiPF6/EC−DMC electrolyte and corresponding ex situ X-ray diffraction patterns. (b) Schematic illustration of the MXene electrode reaction in nonaqueous electrolytes during the initial stage of charge.

5. CONCLUSION AND OUTLOOK This Account illustrates that MXenes are attractive electrode materials for various EES devices. MXene electrodes in aqueous electrolytes exhibit charge storage through EDL formation with hydrated cations in the interlayer space. In nonaqueous electrolytes, charge is stored through redox reactions of MXene associated with charge transfer between cations and surface termination groups (donor band formation of desolvated cations). Despite the fact that much effort has been devoted to the EES applications of MXenes, their charge storage mechanism is yet to be fully understood, mainly because the morphology of MXenes is strongly influenced by the electrode fabrication procedure. A completely delaminated MXene should show different electrochemical behavior from that observed in stacked MXene nanosheets. It is necessary to quantify the effects of delamination on the capacitance/capacity of MXenes through precise control of the degree of delamination. It is also important to clarify the effects of the chemical compositions of MXenes on their electrode performance. As discussed in this Account, the pseudocapacitance of MXene in organic electrolytes is influenced by the band-filling effect. Because a low density of states above the Fermi level decreases the pseudocapacitance, it is necessary to design MXenes that possess a high density of states above the Fermi level. Materials screening using computational informatics should be an efficient method for the exploration of better MXene electrodes. For example, a large pseudocapacitance (quantum capacitance) is theoretically predicted for Nb2CTx.56

surface termination groups is essential to maximize the capacitance. For example, when Na+ is intercalated into MXene, the larger 2s orbital of Na+ (relative to the 1s orbital of Li+) interacts significantly with the surface termination groups to induce a large charge transfer δ (Mm+−Tn− + Na+ + e− → M(m−1+η)+−T(n+δ+η)−−Na(1−δ)+). The large δ enhances the electron donation η from M to T, which suppresses the valence change of M. Indeed, the valence change of Ti in Ti3C2Tx during Na+ (de)intercalation is small relative to that observed for Li+ (de)intercalation (Figures 5c and S3).36,41 Nonetheless, it should be emphasized that the change in the electron/ion chemical potentials, i.e., the redox reaction of MXene, plays a dominant role in determining the pseudocapacitance. Interestingly, during the initial stage of charging in a 1 M LiPF6/ethylene carbonate (EC)−dimethyl carbonate (DMC) electrolyte, the ex situ XRD patterns (Figure 7a) indicated a considerable increase in d from 8.7 to 10.5 Å (A → C). Then this phase with large d disappears, and one with a small d of 8.8 Å appears upon further charging (C → F). We presume that (partially) solvated Li+ is intercalated into the interlayer space in the initial stage. Because further charging induces strong Coulombic attraction between Li+ and the MXene walls, the solvation shell of intercalated Li+ collapses to give a small d, causing charge transfer to the MXene sheets (Figure 7b). Therefore, a transition from intercalation capacitance to intercalation pseudocapacitance occurs in this potential region (ca. 2.0 V vs Li/Li+), and it is this transition that explains the distorted CV curves in Figure 4b. 596

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Accounts of Chemical Research Control of the intercation interactions is another important issue for the design of better MXene electrodes, as it affects the pseudocapacitance through the ion electrochemical potential. For example, electron donation from a surface termination group to a cation suppresses the intercation interactions by screening the Coulombic repulsion. Therefore, surface termination groups with lower electronegativity (e.g., Cl < O, F) are expected to enhance the pseudocapacitance. Indeed, Ti2CTx with chloride termination delivers a larger capacitance than that without chloride termination.36 In this regard, it is of primary importance to establish the procedure for the control of the termination groups. It is also essential to experimentally and theoretically clarify the interaction between the surface termination groups and the intercalated cations. To realize practical EES devices with MXene negative electrodes, it is necessary to develop a large-capacity positive electrode that is able to operate at high rates. One potential strategy is to use an MXene as a positive electrode that exhibits anion (de)intercalation. However, the best-studied titanium carbide MXene electrodes easily decompose under anodic conditions. Therefore, it is necessary to develop a novel MXene that is robust against oxidation. For example, doping electrons into the M d bands by M substitution (e.g., Ti2CTx−V2CTx, Nb2CTx−Mo2CTx) could be an effective approach to realize an MXene positive electrode. Although MXenes display various advantages over conventional electrodes (composition versatility, high electronic conductivity, and high electrochemical activity), the electrode performance of MXene electrodes at present is not satisfactory for practical EES applications. During this early stage of development, systematic evaluation of the fundamental electrochemical properties of MXenes is of absolute necessity to determine the essential factors that dominate their electrode performance. In view of the tunability and versatility of MXenes, establishing full knowledge of them will enable rational MXene design for practical EES applications.

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Tokyo. His research focuses on materials exploration for electrochemical energy storage. Akira Sugahara is a postdoctoral researcher in the Department of Chemical System Engineering, School of Engineering, The University of Tokyo, under the supervision of Prof. Atsuo Yamada. He received his Ph.D. in 2012 from The University of Tokyo. His research includes the investigation of the electrochemical properties of MXene. Satoshi Kajiyama was a postdoctoral researcher in the Department of Chemical System Engineering, School of Engineering, The University of Tokyo, under the supervision of Prof. Atsuo Yamada. He is currently an assistant professor in the Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo. He received his Ph.D. in 2013 from The University of Tokyo. His research focuses on the fabrication and characterization of nano/mesoscale structured materials. Atsuo Yamada is a professor in the Department of Chemical System Engineering, School of Engineering, The University of Tokyo. He received his Ph.D. in 1996 from the University of Tsukuba. He is directing multidisciplinary research on materials for energy storage and conversion.

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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, a Grant-in-Aid for Specially Promoted Research (15H05701), and JSPS KAKENHI grants (JP15H03873 and JP16H00901). HXPES spectra were measured at SPring-8 BL46XU (2015A1936). X-ray absorption spectroscopy was conducted under the approval of the Photon Factory Program Advisory Committee (Proposal 2016G031). We are grateful to L. Szabova and Y. Tateyama for providing DFT calculation results and fruitful discussions.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.accounts.7b00481. Methods, inner potential difference in an electrode, and X-ray absorption spectra for Ti2CTx in an aqueous electrolyte and Ti3C2Tx in a nonaqueous Na+ electrolyte (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Masashi Okubo: 0000-0002-7741-5234 Atsuo Yamada: 0000-0002-7880-5701 Notes

The authors declare no competing financial interest. Biographies Masashi Okubo is an associate professor in the Department of Chemical System Engineering, School of Engineering, The University of Tokyo. He received his Ph.D. in 2005 from The University of 597

DOI: 10.1021/acs.accounts.7b00481 Acc. Chem. Res. 2018, 51, 591−599

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DOI: 10.1021/acs.accounts.7b00481 Acc. Chem. Res. 2018, 51, 591−599