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Article Cite This: Chem. Mater. 2018, 30, 2687−2693

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Universal Descriptor for Large-Scale Screening of High-Performance MXene-Based Materials for Energy Storage and Conversion Wei Jiang,† Xiaolong Zou,*,‡ Hongda Du,† Lin Gan,† Chengjun Xu,† Feiyu Kang,† Wenhui Duan,§ and Jia Li*,† †

Guangdong Provincial Key Laboratory of Thermal Management Engineering and Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, People’s Republic of China ‡ Tsinghua-Berkeley Shenzhen Institute (TBSI), Tsinghua University, Shenzhen 518055, People’s Republic of China § Department of Physics and State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *

ABSTRACT: Density functional theory calculations are employed to systematically investigate the trend of hydrogen evolution reaction (HER) performance of oxygen-terminated MXenes. By studying 30 transition-metal carbides and 30 transition-metal nitrides, Mn+1CnO2 and Mn+1NnO2 (M = Sc, Cr, Hf, Mo, Nb, Ta, Ti, V, W, Zr; n = 1, 2, 3), the tendency of oxygen desorption after hydrogen adsorption is elucidated to play a key role in HER performance of oxygen-terminated MXenes. On the basis of these observations, we propose a suitable HER descriptor, oxygen vacancy formation energy (Ef), which scales linearly with the adsorption free energy of hydrogen, ΔGH. In addition, this new descriptor is linearly correlated with the lithium binding strength on oxygen-terminated MXenes. Therefore, Ef is a universal descriptor for identifying the trend of adsorption processes where adsorbed species donate electrons to oxygen-terminated MXenes. This work provides a general guideline for large-scale screening of promising MXene-based materials for energy storage and conversion.

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INTRODUCTION Hydrogen is a promising alternative to fossil fuels such as petroleum, coal, and natural gas in the future. To take full advantage, it should be able to produce hydrogen sustainably.1,2 Hydrogen evolution from water splitting is an attractive but challenging approach for mass production of hydrogen.3,4 One critical issue of this approach is the lack of efficient and economic catalysts for hydrogen evolution reaction (HER). Platinum (Pt) and other noble metals such as palladium (Pd) and iridium (Ir) are well-known as the most active catalysts for HER.5,6 However, the scarcity of noble metals limit its largescale application. Therefore, the search for noble metal-free catalysts with high activates for HER is attracting a lot of attention.7−17 With novel electronic and excellent mechanical properties, two-dimensional (2D) materials have shown vast potential applications as functional and structural materials. In terms of HER catalysts, 2D materials hold various advantages comparing with traditional materials. Especially, the large specific surface area of 2D materials could afford a great © 2018 American Chemical Society

number of active sites for HER, and various 2D materials with versatile chemical characteristics provide a good platform to tune their catalytic properties.18,19 All these make it possible to design promising HER catalysts based on 2D materials. Hinnemann20 et al. carried out density functional theory (DFT) calculations of molybdenum disulfide (MoS2) and suggested MoS2 could be an efficient HER catalyst because of optimal adsorption free energy (ΔGH). However, only its edges show high activities for HER.21 To compete with noble metals, effective routes should be applied to enhance HER performance of MoS2. These methods include not only the delicate structural design to maximize the exposure of edges,22−25 but also the search for other surface-active 2D materials. Very recently, a new group of 2D materials named as MXenes has been emerging as promising candidates for energy Received: January 12, 2018 Revised: March 28, 2018 Published: March 28, 2018 2687

DOI: 10.1021/acs.chemmater.8b00156 Chem. Mater. 2018, 30, 2687−2693

Article

Chemistry of Materials

investigate the adsorption of hydrogen or lithium, and the formation energy of oxygen vacancy. A Γ-centered 4 × 4 × 1 k-point samplings were adopted for the relaxation of these structures. In these settings, the coverage of hydrogen and lithium is 1/9 ML, and the concentration of oxygen vacancy is 1/9. To avoid the interaction between periodic images, a vacuum layer lager than 10 Å was selected in all calculations. The binding energy between oxygen and Mn+1Xn substrates is defined as Eb = {E(MnCn+1O2) − E(MnCn+1) − E(O2)}/2, where E(MnCn+1), E(MnCn+1O2) and E(O2) are the total energies of pristine and oxygen terminated MXenes, and an oxygen molecule, respectively. The formation energy of oxygen vacancy is obtained as Ef = 1/2E(O2) + E(M9n+9X9nO17) − E(M9n+9X9nO18), where E(O2), E(M9n+9X9nO18), and E(M9n+9X9nO17) are the energies of an oxygen molecule, 3 × 3 × 1 Mn+1XnO2 substrates, and 3 × 3 × 1 Mn+1XnO2 substrates with an oxygen vacancy, respectively. The Gibbs free energy is calculated by ΔGH = ΔEH + ΔZPE − TΔS, where ΔEH is the adsorption energy, ΔZPE is the zero-point energy change, T is the absolute temperature, and ΔS is the entropy change of hydrogen adsorption.20,41,42 The Li binding energy is defined as ΔELi = E(Li-X) − E(X) − E(Li), where E(Li-X), E(X) and E(Li) are the energies of the Li-adsorbed substrates, pristine substrates and Li in bulk structure, respectively. Regarding to two previously proposed descriptors, the lowest unoccupied state (εLUS) indicates the Fermi level of a metal, or the conduction-band minimum of a semiconductor,35 and the number of electrons gained by oxygen atom (Ne) is evaluated by subtracting 6 (number of valence electrons for one free O atom) from the number of valence electrons of O in Mn+1XnO2.37

storage and conversion. MXenes, two-dimensional transitionmetal carbides or nitrides, were first synthesized by selectively etching certain atomic layers from precursors and have been widely studied both experimentally and theoretically since then.26−29 MXenes can be denoted with a unit formula as Mn+1XnTx (n = 1,2,3), where M indicates an early ,transition metals, such as Sc, Ti, V, Cr, Mo, Nb, Ta, Hf, Zr, W and so on, X represents carbon and/or nitrogen and Tx is surface terminations, for instance, hydroxyl, oxygen or fluorine. The M layers can be composed of just one type of metal (such as Ti2C and V2C), a random solid solution of two or more early transition metals (such as TixV3−xC2) or ordered double early transition metals. In the last case, one type of metal occupies the surface layers and the other fills the central M layers (for example, Mo2TiC2 and Mo2Ti2C3, in which Mo occupies the outer M layers and Ti fills the central M layers).30,31 Similar to M layers, X layers may present as C, N, or the solid solution of them. Furthermore, the termination functional groups (hydroxyl, oxygen or fluorine) provide another degree of freedom to adjust the structures of MXenes. Given rich varieties of structures in MXenes family, their electronic structure, chemical and physical properties can be tuned in a wide range. Until now, more than 20 different MXenes have been synthesized successfully, and a greater number of MXenes have been proved feasible theoretically.29,31 It is of great interest to search suitable MXenes candidates for certain applications, such as HER and energy storage.29,32−34 To achieve this goal, efficient screening techniques are critical for exploring configurational space of MXenes. In this work, taking HER performance of oxygen terminated MXenes as an example, we systematically studied 30 2D transition-metal carbides (TMCs) and 30 2D transition-metal nitrides (TMNs), namely Mn+1Cn and Mn+1Nn (M = Sc, Cr, Hf, Mo, Nb, Ta, Ti, V, W, Zr; n = 1,2,3) to search for a suitable descriptor for fast screening of efficient catalysts. To achieve this goal, the descriptor should describe the underlying mechanisms of HER process on Mn+1XnO2 and accurately correlate with the trend of ΔGH on Mn+1XnO2. We first tested two recently proposed indicators, the lowest unoccupied state (εLUS)35,36 and the number of electrons gained by oxygen atom (Ne),37 which are verified as competent descriptors for predicting HER performances of transition metal dichalcogenides (MX2) and M2CO2, respectively. However, when it comes to Mn+1XnO2 (X = C, N; n = 1,2,3), these two descriptors were not very satisfactory. Basing on the observation that H tends to pull O out of substrates after adsorbing on Mn+1XnO2, we propose a new descriptor, oxygen vacancy formation energy (Ef), which scales well with ΔGH of Mn+1XnO2. Besides, Ef correlates linearly with lithium binding energy (ΔELi) of Mn+1XnO2, which suggests it can also be used to search for Mn+1XnO2-based materials as electrodes in energy storage devices.

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RESULTS AND DISCUSSION MXenes were first synthesized through the etching of MAX (MAX is a family of layered hexagonal ternary metal carbides and nitrides, where “M” is an early transition metal, “A” is mostly an element of group 13 and 14, and “X” is carbon and/ or nitrogen) precursors using hydrofluoric acid.26,27 Because the basal planes of pristine MXenes are terminated by transition metals with high chemical activity, the as-produced MXenes are naturally passivated by functional groups (such as *F, *OH, and *O).28 Recent theoretical study indicates that pristine MXenes thermodynamically prefer to be functionalized by atomic oxygen.43 Therefore, we only focus on oxygenterminated MXenes hereafter. In terms of oxygen-terminated MXenes, oxygen atoms usually locate at the hollow sites of surface transition metal layers to maximize the coordinate number of surface transition metals. There are two different hollows sites on the surfaces of pristine MXenes, named as fcctype and hcp-type, resulting in two different structures of oxygen-terminated MXenes, respectively.29 Figure 1 shows the atomic structure of pristine Mn+1Xn and two different kinds of M n+1 X nO 2 . In this work, we choose the most stable configurations of Mn+1XnO2 with the lowest energies (Table S2). Based on the rigid-band approximation, Liu35,36 et al. suggested that the εLUS could serve as an intrinsic descriptor for the screening of basal-plane-active transition-metal dichalcogenides (MX2) catalysts with excellent HER performance. They successfully identified that H-TaS2 and H-NbS2 were efficient HER catalysts, which have been verified by experiments. What’s more, under the framework of rigid-band model, Fan44 et al. found that the energy gained for electrons to transfer from Li/Na to MX2 could serve as a descriptor for characterizing voltages of MX2. Considering MX2 and pristine MXenes share some similar structural characteristics, it is expected the εLUS would also be a good descriptor for HER performance of oxygen-terminated MXenes. Figure 2a shows

METHODS

DFT calculations were carried out by using the Vienna Ab initio Simulation Package (VASP),38,39 adopting generalized gradient approximation (GGA) with exchange-correlation functional according to Perdew, Burke, and Ernzerhof (PBE) parametrization.40 An energy cutoff of 500 eV and force convergence criterion of 0.02 eV/Å were set throughout all calculations. For primary cell structures of pristine Mn+1Xn and its oxygen terminated counterparts Mn+1XnO2, the Γcentered 10 × 10 × 1 and 20 × 20 × 1 k-point samplings were chosen to optimize the structures and obtain their electronic structures, respectively. A 3 × 3 × 1 supercell of Mn+1XnO2 was employed to 2688


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determination R2 only 0.25. In the case of MX2, the “statesfilling” mechanism35 is based on the observation that adsorbed hydrogen leaves the intrinsic electronic structure of MX2 largely intact, and the electrons of the adsorbed hydrogen directly transfer to the unoccupied states of MX2. However, the situation is different in the case of oxygen-terminated MXenes. The adsorbed hydrogen tends to couple with oxygenterminated MXenes, and induces non-negligible changes to intrinsic electronic structure of the substrates. Therefore, the linear relation of εLUS versus ΔGH on oxygen terminated MXenes is not maintained. As discussed above, a good descriptor for HER performance should be able to describe the orbital coupling interaction between the adsorbed hydrogen and substrates. Ling37 et al. suggested that the number of electrons gained by oxygen atom (Ne) can be adopted as an efficient descriptor for HER performance of oxygen terminated-MXenes, M2CO2. The coupling between H 1s and O 2pz orbitals leads to two split states, a fully occupied bonding state (σ) and a partially occupied antibonding state (σ*). The more the number of electrons occupying σ*, the weaker the adsorption of H on M2CO2. As a result, Ne correlates linearly with ΔGH on M2CO2. When the same descriptor is applied to general Mn+1CnO2 (n = 1, 2, 3) considered here, it is found that as “n” increases from 1 to 3, the linear relation between Ne and ΔGH becomes weaker, as shown in Figure 2b. These results suggest that Ne is not suitable for all kinds of Mn+1CnO2, and the chemical properties as well as HER performance of MXenes could be tuned by the number of its atomic layers. The deficiency of current descriptors for H adsorption on oxygen terminated MXenes manifests the importance of understanding the details of the H adsorption process, which is helpful for establishing a universal and robust descriptor for HER performance. From the optimized structures after H adsorption on Mn+1XnO2, it is found that the oxygen atom with adsorbed H protrudes from the basal plane, and tends to be pulled out from the pristine MXenes by the adsorbed H. Taking Hf2CO2 as an example (Figure 3a), the projected density of states of the oxygen atoms with/without H adsorption are demonstrated in Figure 3b. The strong coupling between H 1s orbital couples and O 2pz orbitals leads to the formation of a bonding state σ with low energy. Accordingly, the orbital distribution of the O atom with H adsorption changes significantly. When H 1s and O 2pz form a σ bond, the electrons of O 2pz orbitals will transfer from O-M bonds to the O−H σ bond. As a result, the binding strength between O and the transition-metal atoms will be weakened, and then the oxygen atom with hydrogen adsorption tends to protrude from the substrate. To account for such interaction between H and O, one can employ two indicators, oxygen binding energy (Eb) and oxygen vacancy formation energy (Ef), to sever as descriptors for ΔGH of oxygen-terminated MXenes. Because the process of oxygen absorption onto the pristine MXenes can be regarded as the inverse process of oxygen desorption from the substrates forming oxygen vacancies, vice versa, Eb and Ef can be seemingly treated as equivalent parameter describing the oxygen binding strength without marked difference. However, it should be pointed out that there exists subtle difference between these two parameters. Eb was derived from the average binding energy of all oxygen atoms which fully cover the surfaces of a pristine MXenes, whereas Ef was evaluated through removing a single oxygen atom from a fully covered

Figure 1. Geometric structures of two-dimensional pristine MXenes and its oxygen-terminated counterparts. Pristine MXenes (a) M2X, (d) M3X2, (g) M4X3; fcc-type oxygen-terminated MXenes (b) M2XO2, (e) M3X2O2, (h) M4X3O2; hcp-type oxygen-terminated MXenes (c) M2XO2, (f) M3X2O2, (i) M4X3O2. Dashed line indicates the unit cell. Gray, black, and red spheres represent transition metals, carbon or nitrogen, and oxygen atoms, respectively.

Figure 2. Linear relationship between free energy of hydrogen adsorption (ΔGH) and (a) the lowest unoccupied state (εLUS) of Mn+1CnO2, (b) the number of electrons gained by oxygen atom (Ne) of Mn+1CnO2. Black, red and blue symbols represent M2CO2, M3C2O2, and M4C3O2, respectively. Gray dashed line represents linear fitting of the data.

the relationship between εLUS and ΔGH of Mn+1CnO2. Compared to MX2, the linear relation of εLUS versus ΔGH on oxygen terminated MXenes is weaker with coefficient of 2689


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Figure 3. (a) Optimized structure of hydrogen adsorption on Hf2CO2. Δ indicates the distance O protrudes from the basal plane after H absorption. Black, orange, red, and white spheres represent carbon, hafnium, oxygen, and hydrogen atoms, respectively. (b) Projected density of states (PDOS) of oxygen of Hf2CO2 before and after H adsorption. And PDOS of H after H adsorption is also shown. The Fermi level is set to 0 eV.

oxygen-terminated MXenes. For the latter case, atoms around an oxygen vacancy must relax to reach a new ground state, which means local structure distortion is inevitable. This is varying from the average effects without local structural rearrangement in the calculation of Eb. The poor linear relationships between Eb and Ef further corroborates forementioned subtle difference between them (Figure S1). The Eb and Ef of Mn+1CnO2 both indicate that the variation of O binding strength of Mn+1CnO2 relative to transition metals is in accordance with the periodic law. For transition metals in the same periodic row, metals with smaller atomic number show stronger O binding strength, while in the same periodic column, metals with smaller atomic number show weaker O binding strength. It should be mentioned that the Ne shows a prominent linear relationship with Eb of MnCn+1O2 (Figure S2), with larger Ne corresponding to stronger O binding. On the contrary, Ef versus Ne is more scattering (Figure S3). To figure out which descriptor, Eb or Ef, is more efficient to identify the HER performance of MnCn+1O2, we plotted ΔGH against Eb and Ef separately. A large deviation from linear relationship (R2= 0.61) between ΔGH and Eb is observed (Figure S4), in contrast to strong correlation (R2 = 0.88) between ΔGH and Ef, which indicates Ef is a good descriptor for screening efficient HER catalysts among MnCn+1O2, as shown in Figure 4a. In comparison with other parameters, such as εLUS and Ne, the superiority of the Ef in predicting HER performance of oxygenterminated MXenes is mainly due to its ability to describe subtle changes of both the atomic and electronic structures after H adsorption. As the adsorption energy of hydrogen (ΔEH) consists of two parts: one is the distortion energy (Edist) deriving from the atomic relaxation of the substrates after

Figure 4. Linear relationship between oxygen vacancy formation energy (Ef) and free energy of hydrogen adsorption (ΔGH) of (a) Mn+1CnO2 and (b) Mn+1NnO2. Black, red, and blue symbols represent M2XO2, M3X2O2 and M4X3O2, respectively. (c) Linear relationship between Ef and ΔGH of both Mn+1CnO2 and Mn+1NnO2. Gray dashed line represents linear fitting of the data.

hydrogen adsorption; the other is the binding strength between hydrogen and the oxygen of the substrates (Eb(O−H)). When hydrogen atom interacts with the oxygen-terminated MXenes, the substrates distortion and charge redistribution take place simultaneously, so it is impossible to obtain the exact value of Edist and Eb(O−H) separately by conventional approaches. However, the values of Eb(O−H) can be qualitatively evaluated by the bond length of O−H. Table S1 shows the O−H bond length of hydrogen adsorption on different oxygen-terminated MXenes. It can be seen that the O−H bond length of hydrogen adsorption on various Mn+1CnO2 investigated in this work have almost same value, ranging from 0.974 to 0.981 Å. The similar bond length of O−H indicates Eb(O−H) for different oxygen2690


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between active adsorbates (e.g., Li+ for Li-ion batteries and polysulfide species for Li−S batteries) and electrode materials is critical for their performances, including capacity, voltage, and energy density.46,47 For efficient search for potential MXenes based electrodes, a good descriptor correlating with the adsorption energies of active adsorbates is important. In the following, we take Li as an example to explore the relationship between its adsorption strength and Ef of oxygen-terminated MXenes, specifically Mn+1CnO2. Except for Scn+1CnO2 (n = 1, 2, 3) that notable structure distortion occurs after Li adsorption (Figure S9), a good linear relationship between ΔELi and Ef is observed, as shown in Figure 5. Different from H, Li interacts

terminated MXenes are comparable. Therefore, Eb(O−H) can be approximately considered as a constant in the case of hydrogen adsorption on oxygen-terminated MXenes. Consequently, ΔEH positively correlate with Edist, and that is why ΔGH shows strong linear relationships against Ef (proportional to Edist). Because H adsorption usually has more configurations than oxygen vacancy formation, moreover, free energy calculation is more complicated and time-consuming than Ef calculation, the new descriptor provides more efficient ability of screening. More importantly, the proposed descriptor could potentially be used for other more complex reaction processes.45 For HER, we can set a screening criteria for various candidates as 3.41 eV < Ef < 4.48 eV, which corresponds to −0.3 eV < ΔGH < + 0.3 eV, and this window includes following promising candidates: Mo2CO2, Zr4C3O2, Nbn+1CnO2, Tin+1CnO2 and Wn+1CnO2 (n = 1, 2, 3). Recently, Seh33 et al. verified HER activities of Ti2CTx and Mo2CTx experimentally, and Mo2CTx exhibited higher HER activity than Ti2CTx. According to our computational results, the ΔGH of Mo2CO2, Ti2CO2 are −0.36 and 0.26 eV respectively, seemingly disagreeing with experiments measurements. Considering Mo2CO2 is metallic and Ti2CO2 is a semiconductor, the relatively poorer electrical conductivity of Ti2CO2 could be detrimental for the HER process. Moreover, there are many candidates with more suitable ΔGH than Mo2CO2, including Nb2CO2, W2CO2, Nb3C2O2, Ti3C2O2, W3C2O2, Nb4C3O2, Ti4C3O2, and Zr4C3O2, and they all are good electronic conductors. Experimental studies should shed more light on these promising HER candidates. To check out whether Ef could be used to identify HER performance of oxygen terminated 2D transition-metal nitrides as well, we apply this new descriptor to a training set of 30 MnNn+1O2 (M = Cr, Hf, Mo, Nb, Ta, Ti, V, W, Zr; n = 1, 2, 3). Surprisingly, except group-VI B MnNn+1O2 (CrnNn+1O2, MonNn+1O2 and WnNn+1O2, as show in Figure S5), a linear relationship (R2 = 0.94) is observed, as shown in Figure 4b. The deviation from the fitting line for group-VI B MnNn+1O2 (M = Cr, Mo, W; n = 1, 2, 3) may result from the serious structure distortion of MnNn+1O2 (M = Cr, Mo, W; n = 1, 2, 3) after H absorption and formation of oxygen vacancy (Figure S6). Even for the combination of Mn+1CnO2 and Mn+1NnO2, a good linear relationship (R2 = 0.86) between Ef and ΔGH can be obtained, as shown in Figure 4c, which suggests that the new descriptor is robust and universal for screening efficient HER catalysts from oxygen-terminated MXenes, including both transition-metal carbides and transition-metal nitrides. Among various Mn+1NnO2 investigated in this work, Nb2NO2, Ta3N2O2, and TinNn+1O2 (n = 1, 2, 3) show moderate ΔGH and good electrical conductivity, which indicates their potential high HER performance. Moreover, in most experimental studies, the mixture of terminations (O, F, and OH) exists on the pristine MXenes’ surface. We take M2CO2 with one O atom substituted by F/ OH as an example to check out whether this descriptor is also suitable in the case of MXenes with the mixture of terminations, as shown in Figure S7. F and OH have little influence on the linear relationship between Ef and ΔGH, as shown in Figure S8, which confirms Ef is a universal and robust descriptor to scale the trends of HER performance of MXenebased materials. Besides HER catalysts, MXenes have been proposed for the application in energy storage devices, such as Li-ion, Na-ion and Li−S batteries. For all energy storage devices, the binding

Figure 5. Linear relationship between oxygen vacancy formation energy (Ef) and Li binding energy (ΔELi) of Mn+1CnO2. Black, red, and blue symbols represent M2CO2, M3C2O2, and M4C3O2, respectively. Gray dashed line represents linear fitting of the data, except for Scn+1CnO2 (n = 1, 2, 3).

with oxygen-terminated MXenes mainly through charge transfer of its out-shell electrons to substrates. Similar to “states-filling” mechanism, these excessing electrons would fill the unoccupied antibonding states produced by the interaction between oxygen and the transition metal of pristine MXenes. Therefore, the binding between oxygen and pristine MXenes would be weakened after Li adsorption. Despite different interaction mechanisms between H and Li on oxygenterminated MXenes, i.e., orbitals coupling for H and charge transfer for Li, they both tend to weaken the oxygen binding strength of the substrates. Therefore, Ef is suitable to scale the trends of both H and Li adsorption on oxygen-terminated MXenes. It is also expected that Ef is a universal descriptor for the binding strength of adsorbates which interact with oxygenterminated MXenes substrates through charge transfer. Hence, the binding between Na, K, Mg and oxygen-terminated MXenes should share similar trend as that of Li on oxygenterminated MXenes. Moreover, the trend of lithium polysulfide species interacting with the oxygen-terminated MXenes may also be described by Ef, because Li in polysulfide species usually transfers electrons to the substrates. The detailed relationship between Ef and the binding strength of Na, K, Mg and polysulfide species on oxygen-terminated MXenes deserves further comprehensive studies.

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CONCLUSION In this letter, a systematic study focusing on HER performance of oxygen-terminated MXenes has be carried out with first2691


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(Grants KQCX20140521161756227 and JCYJ20170307154206288), and the National Program for Thousand Young Talents of China. Tianjin Supercomputing Center is also acknowledged for allowing the use of computational resources including TIANHE-1.

principle calculations. We point out that the trend of oxygen desorption after adsorbing hydrogen is the predominant mechanism in HER process of oxygen-terminated MXenes. Based on this mechanism, we propose a competent descriptor, oxygen vacancy formation energy, to scale ΔGH of oxygenterminated MXenes. The subtle difference between oxygen binding energy and oxygen vacancy formation energy of oxygen-terminated MXenes has also been discussed. In addition, comparing with oxygen vacancy formation energy, the deficiency of the lowest unoccupied states and the number of electrons gained by oxygen atom in indicating ΔGH of oxygen-terminated MXenes mainly derive their inability in revealing local structure distortion caused by hydrogen adsorption. Furthermore, the descriptor of oxygen vacancy formation energy is appropriate to scale lithium binding strength of oxygen-terminated MXenes, verifying that this new descriptor is suitable to indicate the trend of charge transfer process taking place in oxygen-terminated MXenes. This is critical for efficient screening of viable cathode materials of energy storage devices, such as Li-ion, Na-ion, K-ion, Mgion, and Li−S batteries. Our results provide a general guideline for designing promising MXenes-based HER catalysts and cathode materials by fine-tuning the oxygen vacancy formation energy via varying chemical composition or atomic layers.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b00156. Linear relationships between Eb and Ef, Ne and Eb, Ne and Ef, Eb and ΔGH of Mn+1CnO2; relationship between Ef and ΔGH of Mn+1NnO2; serious structure distortion of Cr4N3O2 after oxygen vacancy formation and hydrogen adsorption; O−H bond length of hydrogen adsorption on oxygen-terminated MXenes investigated in this work; total energies of fcc- and hcp-type oxygen-terminated MXenes; models of MXenes with the mixture of surface terminations; linear relationships between Ef and ΔGH of MXenes with the mixture of surface terminations; structure distortion of Sc2CO2 after Li adsorption (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.Z.). *E-mail: [email protected] or [email protected] (J.L.). ORCID

Xiaolong Zou: 0000-0002-3987-6865 Lin Gan: 0000-0003-3486-6016 Wenhui Duan: 0000-0001-9685-2547 Jia Li: 0000-0001-9361-2281 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (Grants 2017YFB0701600 and 2014CB932400), the National Natural Science Foundation of China (Grant 51232005), Shenzhen Projects for Basic Research 2692


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