Universal Descriptor for Large-Scale Screening of High-Performance

Subscriber access provided by UNIVERSITY OF TOLEDO LIBRARIES

A 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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00156 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

A Universal Descriptor for Large-Scale Screening of High Performance MXene-based Materials for Energy Storage and Conversion Wei Jiang,a Xiaolong Zou,*,b Hongda Du,a Lin Gan,a Chengjun Xu,a Feiyu Kang,a Wenhui Duan,c and Jia Li*,a a

Guangdong Provincial Key Laboratory of Thermal Management Engineering and Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, People’s Republic of China b

Tsinghua-Berkeley Shenzhen Institute (TBSI), Tsinghua University, Shenzhen 518055, People’s Republic of China

c

Department of Physics and State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084, People’s Republic of China

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 oxygenterminated MXenes. Based on 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 oxygenterminated MXenes. This work provides a general guideline for large-scale screening of promising MXene-based materials for energy storage and conversion.

INTRODUCTION

Hydrogen, is a promising alternative to fossil fuels such as petroleum, coal and natural gas in future. To take its 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 large-scale 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

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,2225 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 storage and conversion. MXenes, twodimensional transition-metal carbides or nitrides, were firstly synthesized by selectively etching certain atomic layers from precursors and have been widely

ACS Paragon Plus Environment

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

Figure 1. Geometrical 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.

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

Page 2 of 10

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 electron 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. 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) 40 parameterization. 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 Γ-centered 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 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 2


Page 3 of 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 While the number of electron 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

pristine Mn+1Xn and two different kinds of Mn+1XnO2. 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

RESULTS AND DISCUSSION

MXenes were firstly 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 oxygen-terminated 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 fcc-type and hcp-type, resulting in two different structures of oxygen-terminated MXenes, respectively.29 Figure 1 shows the atomic structure of

Figure 2. The linear relationship between free energy of hydrogen adsorption (ΔGH) and (a) the lowest unoccupied state (εLUS) of Mn+1CnO2, (b) the number of electron 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.

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 2(a) shows 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 determination R2 only 0.25. In the case 3



of MX2, the “states-filling” 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 oxygen-terminated MXenes, and induces nonnegligible changes to intrinsic electronic structure of the substrates. Therefore, the linear relation of εLUS versus ΔGH on oxygen terminated MXenes is not maintained.

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.

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 electron 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 anti-bonding 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

Page 4 of 10

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 2(b). 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 3(a)), the projected density of states of the oxygen atoms with/without H adsorption are demonstrated in Figure 3(b). 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, while Ef was evaluated through removing a single oxygen atom from a fully covered 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 4


Page 5 of 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


rearrangement in the calculation of Eb. The poor linear relationships between Eb and Ef further corroborates fore-mentioned 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 4(a). In comparison with other parameters, such as εLUS and Ne, the superiority of the Ef in predicting HER performance of oxygen-terminated 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 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 oxygen-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’s why ΔGH

shows strong linear relationships against Ef (proportional to Edist). Since 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

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. 5



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 eV 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 transitionmetal 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 4(b). 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 4(c), 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

Page 6 of 10

S8, which confirms Ef is a universal and robust descriptor to scale the trends of HER performance of MXene-based 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 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

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).

between ΔELi and Ef is observed, as shown in Figure 5. Different from H, Li interacts 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 anti-bonding 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 oxygen-terminated MXenes, i.e., orbitals coupling for H and charge transfer for Li, they both tend to weaken the oxygen 6


Page 7 of 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 oxygen-terminated 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 oxygen-terminated 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.

The linear fitting of Ef against Eb, Eb against Ne, Ef against Ne and ΔGH against Eb, in the case of Mn+1CnO2; Ef and ΔGH of Crn+1NnO2, Mon+1NnO2 and Wn+1NnO2 (n=1, 2, 3); serious structure distortion of Cr4N3O2 after oxygen vacancy formation and hydrogen adsorption; O-H bond length of hydrogen adsorption on oxygenterminated 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.

CONCLUSION

*(X.Z.) [email protected]

In this letter, a systematic study focusing on HER performance of oxygen-terminated MXenes has be carried out with first-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 oxygen-terminated 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 electron 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, Mg-ion and Li-S batteries. Our results provide a general guideline for designing promising MXenesbased HER catalysts and cathode materials by finetuning the oxygen vacancy formation energy via varying chemical composition or atomic layers.

*(J.L.) [email protected]; [email protected]

ASSOCIATED CONTENT Supporting Information

AUTHOR INFORMATION

Corresponding Author

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the National Key Research and Development Program of China (Grant Nos. 2017YFB0701600 and 2014CB932400), the National Natural Science Foundation of China (Grant No. 51232005), Shenzhen Projects for Basic Research (Grant Nos. 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. REFERANCES (1) Lewis, N. S.; Nocera, D. G. Powering the planet: Chemical challenges in solar energy utilization. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729-15735. (2) Chu, S.; Majumdar, A. Opportunities and challenges for a sustainable energy future. Nature 2012, 488, 294-303. (3) Turner, J. A. Sustainable hydrogen production. Science 2004, 305, 972-974. (4) Hosseini, S. E.; Wahid, M. A. Hydrogen production from renewable and sustainable energy resources: Promising green energy carrier for clean development. Renewable Sustainable Energy Rev. 2016, 57, 850-866. (5) Greeley, J.; Jaramillo, T. F.; Bonde, J.; Chorkendorff, I.; Norskov, J. K. Computational high-throughput screening of electrocatalytic materials for hydrogen evolution. Nat. Mater. 2006, 5, 909-913. (6) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F. Combining theory and experiment 7



in electrocatalysis: Insights into materials design. Science 2017, 355, eaad4998. (7) Cao, B.; Veith, G. M.; Neuefeind, J. C.; Adzic, R. R.; Khalifah, P. G. Mixed close-packed cobalt molybdenum nitrides as nonnoble metal electrocatalysts for the hydrogen evolution reaction. J. Am. Chem. Soc. 2014, 45, 19186-19192. (8) Chen, W.-F.; Sasaki, K.; Ma, C.; Frenkel, A. I.; Marinkovic, N.; Muckerman, J. T.; Zhu, Y.; Adzic, R. R. Hydrogen-evolution catalysts based on non-noble metal nickel-molybdenum nitride nanosheets. Angew. Chem., Int. Ed. 2012, 51, 6131-6135. (9) Kong, D.; Cha, J. J.; Wang, H.; Lee, H. R.; Cui, Y. First-row transition metal dichalcogenide catalysts for hydrogen evolution reaction. Energy Environ. Sci. 2013, 6, 3553-3558. (10) Kong, D.; Wang, H.; Lu, Z.; Cui, Y. CoSe2 nanoparticles grown on carbon fiber paper: an efficient and stable electrocatalyst for hydrogen evolution reaction. J. Am. Chem. Soc. 2014, 136, 4897-4900. (11) Liu, Q.; Tian, J.; Cui, W.; Jiang, P.; Cheng, N.; Asiri, A. M.; Sun, X. Carbon nanotubes decorated with CoP nanocrystals: A highly active non-noble-metal nanohybrid electrocatalyst for hydrogen evolution. Angew. Chem., Int. Ed. 2014, 126, 6828-6832. (12) Lu, Q.; Hutchings, G. S.; Yu, W. T.; Zhou, Y.; Forest, R. V.; Tao, R.; Rosen, J.; Yonemoto, B. T.; Cao, Z. Y.; Zheng, H. M.; Xiao, J. Q.; Jiao, F.; Chen, J. G. Highly porous non-precious bimetallic electrocatalysts for efficient hydrogen evolution. Nat. Commun. 2015, 6, 6567. (13) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured nickel phosphide as an electrocatalyst for the hydrogen evolution reaction. J. Am. Chem. Soc. 2013, 135, 9267-9270. (14) Yang, Y.; Lun, Z.; Xia, G.; Zheng, F.; He, M.; Chen, Q. Nonprecious alloy encapsulated in nitrogen-doped graphene layers derived from MOFs as an active and durable hydrogen evolution reaction catalyst. Energy Environ. Sci. 2015, 8, 3563-3571. (15) Zheng, Y.; Jiao, Y.; Zhu, Y.; Li, L. H.; Han, Y.; Chen, Y.; Du, A.; Jaroniec, M.; Qiao, S. Z. Hydrogen evolution by a metal-free electrocatalyst. Nat. Commun. 2014, 5, 3783. (16) Zou, X.; Huang, X.; Goswami, A.; Silva, R.; Sathe, B. R.; Mikmekova, E.; Asefa, T. Cobalt-embedded nitrogen-rich carbon nanotubes efficiently catalyze hydrogen evolution reaction at all pH values. Angew. Chem. 2014, 126, 4461-4465. (17) Zou, X.; Zhang, Y. Noble metal-free hydrogen evolution catalysts for water splitting. Chem. Soc. Rev. 2015, 44, 5148-5180. (18) Deng, J.; Ren, P.; Deng, D.; Bao, X. Enhanced electron penetration through an ultrathin graphene layer for highly efficient catalysis of the hydrogen evolution reaction. Angew. Chem., Int. Ed. 2015, 54, 2100-2104. (19) Ito, Y.; Cong, W.; Fujita, T.; Tang, Z.; Chen, M. High catalytic activity of nitrogen and sulfur Co-doped nanoporous graphene in the hydrogen evolution reaction. Angew. Chem., Int. Ed. 2015, 54, 2131-2136. (20) Hinnemann, B.; Moses, P. G.; Bonde, J.; Joergensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendorff, I.; Noerskov, J. K. Biomimetic hydrogen evolution: MoS2 nanoparticles as catalyst for hydrogen evolution. J. Am. Chem. Soc. 2005, 127, 5308-5309. (21) Jaramillo, T. F.; Jørgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Identification of active edge sites for electrochemical H2 evolution from MoS2 nanocatalysts. Science 2007, 317, 100-102. (22) Kibsgaard, J.; Chen, Z.; Reinecke, B. N.; Jaramillo, T. F. Engineering the surface structure of MoS2 to preferentially expose active edge sites for electrocatalysis. Nat. Mater. 2012, 11, 963-969. (23) Karunadasa, H. I.; Chang, C. J. A molecular MoS2 edge site mimic for catalytic hydrogen generation. Science 2012, 335, 698702.

Page 8 of 10

(24) Xie, J.; Zhang, H.; Li, S.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.; Lou, X. W.; Xie, Y. Defect-rich MoS2 ultrathin nanosheets with additional active edge sites for enhanced electrocatalytic hydrogen evolution. Adv. Mater. 2013, 25, 5807-5813. (25) Huang, Z.; Luo, W.; Ma, L.; Yu, M.; Ren, X.; He, M.; Polen, S.; Click, K.; Garrett, B.; Lu, J. Dimeric [Mo2S12]2- cluster: A molecular analogue of MoS2 edges for superior hydrogen-evolution electrocatalysis. Angew. Chem., Int. Ed. 2015, 127, 15181-15185. (26) Naguib, M.; Kurtoglu, M.; Presser, V.; Lu, J.; Niu, J.; Heon, M.; Hultman, L.; Gogotsi, Y.; Barsoum, M. W. Two-dimensional nanocrystals produced by exfoliation of Ti3AlC2. Adv. Mater. 2011, 23, 4248-4253. (27) Naguib, M.; Mashtalir, O.; Carle, J.; Presser, V.; Lu, J.; Hultman, L.; Gogotsi, Y.; Barsoum, M. W. Two-dimensional transition metal carbides. ACS Nano 2012, 6, 1322−1331. (28) Naguib, M.; Mochalin, V. N.; Barsoum, M. W.; Gogotsi, Y. 25th anniversary article: MXenes: a new family of twodimensional materials. Adv. Mater. 2014, 26, 992-1005. (29) Anasori, B.; Lukatskaya, M. R.; Gogotsi, Y. 2D metal carbides and nitrides (MXenes) for energy storage. Nat. Rev. Mater. 2017, 2, 16098. (30) Anasori, B.; Xie, Y.; Beidaghi, M.; Lu, J.; Hosler, B. C.; Hultman, L.; Kent, P. R. C.; Gogotsi, Y.; Barsoum, M. W. Twodimensional, ordered, double transition metals carbides (MXenes). ACS Nano 2015, 9, 9507-9516. (31) Tan, T. L.; Jin, H. M.; Sullivan, M. B.; Anasori, B.; Gogotsi, Y. High-throughput survey of ordering configurations in MXene alloys across compositions and temperatures. ACS Nano 2017, 11, 4407-4418. (32) Gao, G.; O’Mullane, A. P.; Du, A. 2D MXenes: A new family of promising catalysts for the hydrogen evolution reaction. ACS Catal. 2016, 7, 494-500. (33) Seh, Z. W.; Fredrickson, K. D.; Anasori, B.; Kibsgaard, J.; Strickler, A. L.; Lukatskaya, M. R.; Gogotsi, Y.; Jaramillo, T. F.; Vojvodic, A. Two-dimensional molybdenum carbide (MXene) as an efficient electrocatalyst for hydrogen evolution. ACS Energy Lett. 2016, 1, 589-594. (34) Ran, J.; Gao, G.; Li, F. T.; Ma, T. Y.; Du, A.; Qiao, S. Z. Ti3C2 MXene co-catalyst on metal sulfide photo-absorbers for enhanced visible-light photocatalytic hydrogen production. Nat. Commun. 2017, 8, 13907. (35) Liu, Y.; Wang, Y. M.; Yakobson, B. I.; Wood, B. C. Assessing carbon-based anodes for lithium-ion batteries: a universal description of charge-transfer binding. Phys. Rev. Lett. 2014, 113, 028304. (36) Liu, Y.; Wu, J.; Hackenberg, K. P.; Zhang, J.; Wang, Y. M.; Yang, Y.; Keyshar, K.; Gu, J.; Ogitsu, T.; Vajtai, R.; Lou, J.; Ajayan, P. M.; Wood, Brandon C.; Yakobson, B. I. Self-optimizing, highly surface-active layered metal dichalcogenide catalysts for hydrogen evolution. Nat. Energy 2017, 2, 17127. (37) Ling, C.; Shi, L.; Ouyang, Y.; Wang, J. Searching for highly active catalysts for hydrogen evolution reaction based on Oterminated MXenes through a simple descriptor. Chem. Mater. 2016. 28, 9026-9032. (38) Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, 11169-11186. (39) Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comp. Mater. Sci. 1996, 6, 15-50. (40) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865. (41) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. Origin of the overpotential 8


Page 9 of 10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


for oxygen reduction at a fuel-cell cathode. J. Phys. Chem. B 2004, 108, 17886-17892. (42) Skúlason, E.; Tripkovic, V.; Björketun, M. E.; Gudmundsdóttir, S. d.; Karlberg, G.; Rossmeisl, J.; Bligaard, T.; Jónsson, H.; Nørskov, J. K., Modeling the electrochemical hydrogen oxidation and evolution reactions on the basis of density functional theory calculations. J. Phys. Chem. C 2010, 114, 1818218197. (43) Pandey, M.; Thygesen, K. S. Two-dimensional MXenes as catalysts for electrochemical hydrogen evolution: A computational screening study. J. Phys. Chem. C 2017, 121, 13593-13598. (44) Fan, S.; Zou, X.; Du, H.; Gan, L.; Xu, C.; Lv, W.; He, Y.-B.; Yang, Q.; Kang, F.; Li, J. Theoretical investigation of the intercalation chemistry of Li/Na ions in transition metal dichalcogenides. J. Phys. Chem. C 2017. 121, 13599-13605. (45) Lee, Y.-L.; Kleis, J.; Rossmeisl, J.; Shao-Horn, Y.; Morgan, D. Prediction of solid oxide fuel cell cathode activity with firstprinciples descriptors. Energy Environ. Sci. 2011, 4, 3966-3970. (46) Eames, C.; Islam, M. S. Ion intercalation into two-dimensional transition-metal carbides: global screening for new high-capacity battery materials. J. Am. Chem. Soc. 2014, 136, 16270-16276. (47) Xie, Y.; Naguib, M.; Mochalin, V. N.; Barsoum, M. W.; Gogotsi, Y.; Yu, X.; Nam, K. W.; Yang, X. Q.; Kolesnikov, A. I.; Kent, P. R. Role of surface structure on Li-ion energy storage capacity of two-dimensional transition-metal carbides. J. Am. Chem. Soc. 2014, 136, 6385-6394.

9



Page 10 of 10

TOC

10


Universal Descriptor for Large-Scale Screening of High-Performance

Recommend Documents