Intrinsic Properties Affecting Catalytic Activity of 3d-Transition-Metal

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Intrinsic Properties Affecting Catalytic Activity of 3d-Transition-Metal Carbides in Li-O Battery 2

Yingying Yang, Yuan Qin, Xiaowan Xue, Xudong Wang, Man Yao, and Hao Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04285 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 21, 2018

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The Journal of Physical Chemistry

Intrinsic Properties Affecting Catalytic Activity of 3d-Transition-Metal Carbides in Li-O2 Battery Yingying Yang, Yuan Qin, Xiaowan Xue, Xudong Wang, Man Yao*, Hao Huang* School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China Corresponding author: Tel: 86-411 84707347; Fax: 86-411 84709248. Email: [email protected] (M. Yao), [email protected] (H. Huang).

ABSTRACT All 3d-transition-metal carbides (3d-TMCs) in the NaCl structure have been constructed to compare the catalytic activity of Li-O2 battery by first-principle calculations. The interfacial catalytic models of LixO2 (x=4, 2 and 1) molecules adsorbed on 3d-TMCs surfaces were used to simulate discharging (ORR, oxygen reduction reaction) and charging (OER, oxygen evolution reaction) processes. The calculated results indicate that TiC surface has smaller ORR and OER overpotentials, which may be the maximum catalytic activity of 3d-TMCs. Taking overpotentials as measurement of catalytic activity, some intrinsic properties related to catalytic activity are determined, including adsorption energies of Li and LiO2, surface energy and binding energy of O. The catalytic activities of 3d-TMCs for ORR and OER are inversely proportional to the adsorption energies of Li and LiO2. The ORR overpotentials are proportional to the surface energies of 3d-TMCs surfaces, but the relationship between OER overpotentials with the surface energies is not clear. TiC has a moderate binding energy of O atom. Additionally, when bonding state tends to be saturated, namely Fermi level happens to be in pseudogap, the catalytic activity reaches its maximum. 1

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By calculating ORR and OER overpotentials and establishing the correlation between catalytic activity and relevantly intrinsic properties of 3d-TMCs, our investigation is helpful for screening and designing highly active catalysts to enhance ORR and OER in Li-O2 battery. 1. INTRODUCTION In recent years, rechargeable lithium-oxygen (Li-O2) batteries have attracted great attention owing to their high theoretical energy density (11680Wh/kg) and even the practically available energy density which is nearly comparable to that of gasoline (1700Wh/kg)

1-4

. Hence, they have potential applications in electric vehicles and other

high-energy storage devices. The chemistry of nonaqueous aprotic Li-O2 batteries is simple, involving reversible formation and dissociation of Li2O2, which mainly occurs in three phase interface

5

(cathode/Li2O2/electrolyte)

.

However,

many

problems

such

as

low

discharge/charge rate, low round-trip efficiency and poor cycling life limit their practical application 6-8. These issues all boil down to large overpotentials which is caused by sluggish ORR/OER kinetics during discharge/charge processes. Electrocatalysts play an important role in accelerating the sluggish electrochemical reactions, thus effectively improving the performance of Li-O2 battery. Noble metals bimetallic alloys

11-12

, metal oxides

and carbides (TMCs)

16-17

13-14

, perovskites

15

9-10

,

and transition metal nitrides (TMNs)

as the electrochemical catalysts for Li-O2 battery have been

extensively investigated in experimental and theoretical studies. Especially, some TMCs materials greatly reduce side reactions and exhibit better reversible discharge/charge performance. In experiment, the TiC-based cathode can reduce side reactions causing by 2


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electrode and electrolyte decomposition, and even more than 98% capacity retention after 100 cycles in a aprotic Li-O2 battery

17

. The ordered mesoporous TiC-C material as

bifunctional catalyst can obviously reduces dicharge/charge overpotentials and enhances specific capacity of a non-aqueous Li-O2 battery

18

. The Fe/Fe3C carbon nanofibers as

bifunctional catalyst for rechargeable Li-O2 batteries exhibit superior electrochemical properties including high specific capacity, high rate capability and good cycle stability, in which the high electrocatalytic activity is provided by Fe/Fe3C composites 19. Meanwhile, TMCs show good electrochemical performance in fuel cells, Li-ion batteries, Na-ion batteries, supercapacitors, and electrocatalytic reactions which include ORR, OER and hydrogen evolution reaction (HER)

20

. The reasons are summarized as follows: 1) The

hybridization between carbon s-, p-orbitals and metal d-orbital results in a broadening of transition-metal d-band. This redistribution of DOS in turn lead to catalytic performance similar to that of noble metals 21; 2) TMCs possess three different interactions among atoms: covalent bond, ionic bond and metallic bond, which simultaneously combine good ionic conductivity and high electronic conductivity in a system; 3) TMCs have good chemical stability, which are not easily eroded by electrolyte 22. The crystal structures of group IV–VI metal carbides as potential electrocatalysts for HER have been systematically examined

23

.

Additionally, the investigations of a series of high surface area TMCs prepared and applied for HER and ORR have shown that some TMCs are not only good ORR catalysts but also good catalyst supports

24

. Inspired by these findings, we found that there is no relevant

systematic researches on TMCs for Li-O2 battery. The electrochemical reactions of ORR in

3



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fuel cells are similar to the ORR and OER in Li-O2 battery, whether the catalysis is still available requiring systematic investigation. Extensive exploration of TMCs catalyst for formation/decomposition of Li2O2 in Li-O2 batteries not only determines the optimal TMC but also establishes a basic correlation between material properties and catalytic activity. The downshift of d-band center relative to Fermi level of TMCs in group IV–VI reduces its HBE (hydrogen binding energy) that is a useful parameter for identifying HER catalysis in fuel cells

23

. In Li-ion batteries, the

relationship between surface structure and Li-ion storage capacities for some functionalized two-dimensional TMC involving Sc2C, Ti2C, Ti3C2, V2C, Cr2C, and Nb2C has also been established

25

. The Pt3M bimetallic alloys (M=3d, 4d, and 5d transition metals) as

electrocatalysts in Li-O2 batteries have been systematic investigated, then the correlations of overpotentials with adsorption energies of reactive intermediates and d-band center for ORR and OER respectively are build 26. In addition, surface acidity is identified as a descriptor of catalytic activity for OER in Li-O2 battery 14. However, intrinsic properties of TMCs and their relationships with catalytic activity have not yet been established in Li-O2 battery, or the mentioned material properties are applicable, which are fundamental for screening and designing more active catalyst to improve ORR and OER catalysis. In this paper, a detailed and systematic first-principles study of formation and dissociation of Li2O2 on 3d-TMCs with the NaCl structure in Li-O2 batteris is presented. The focus is to find the maximum catalytic activity of 3d-TMC and to correlate catalytic activity with material properties. The calculated ORR and OER overpotentials were used to evaluate catalytic activity, and some intrinsic properties related to catalytic activity were determined. 4


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Then correlations between ORR and OER overpotentials with adsorption energies of Li and LiO2, surface energy, binding energy of O atom and electronic structure were established. These studies provide a fundamental base for reliable screening and designing highly active catalysts that can be applied in Li-O2 battery. 2. COMPUTATIONAL DETAILS All first-principle calculations in this work were based upon the spin-polarized periodic density functional theory (DFT) using Vienna Ab-initio Simulation Package (VASP) 27. The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) was used to formulate electron-electron exchange-correlation functional

28

, and the valence electron-ion

interaction was modeled by projector augmented wave (PAW) potential. No dipole and van der Waals (vdW) corrections were applied due to the dipole and vdW effects on the total energies were verified to be negligible. A plane-wave cutoff energy of 500 eV and 8×8×1 k-point Monkhorst-Pack grid were chosen by testing corresponding parameters. The stopping-criterion of force and energy for geometry optimization were less than 0.02 eV/Å and10-4 eV, respectively. The 3d-transition-metal carbides considered in present study including ScC, TiC, VC, CrC, MnC, FeC, CoC and NiC crystallize in the NaCl structure, in which except for ScC (at normal pressures), TiC and VC are stable, and others are metastable compounds. The choice of 3d-TMCs with the rocksalt structure is necessary in our calculations considering the number of varying parameters should be kept small when performing a trend study

29

. The

3d-TMCs (100) surfaces with both carbon and metal reaction sites were chosen to examine their catalytic activity. A 2×2 supercell with a 15 Å vacuum added in Z direction was built in 5



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our calculation. The thickness of 7 atom layers effectively display bulk characteristic of the slab in which the top four layers allow to relax freely and the bottom three layers are fixed during geometric optimization. Two different reaction pathways involving either in solution or on electrode surface have been proposed for Li2O2 formation/decomposition 30-31. L. Johnson et al. 32 found out that O2 reduction in high-DN electrolytes and at high potentials follows the solution pathway, while in high-DN electrolytes and at low potentials or in low-DN electrolytes at all potentials follows the surface pathway. This is due to the high-DN electrolyte normally has a strong solubility of LiO2. Taken together, the surface pathway is selected in our work for considering the effect of electrolyte in Li-O2 battery, and the surface pathway is also widely used to predict the electrochemical ability of catalyst in relevant theoretical researches 11-14, 26, 33. The LixO2 (x=4, 2 and 1) molecules adsorbed on 3d-TMCs surfaces were calculated to represent the intermediates during ORR and OER processes. The reaction free energy of each step in Li-O2 battery can be formulated by

33

, ∆G = E − E0 + ∆nLi ( µ Li − eU ) + ∆nO2 µO2 , where E

and E0 are the total energies of slab at a certain step and initial step. ∆nLi and ∆nO2 are the numbers of Li and O2 adsorbed/desorbed. µ Li is the chemical potential of Li, which is referenced to the DFT energy of one Li atom in Li bulk 11-12. µO2 is the chemical potential of O2 molecule. For the well-known overbinding issue of O2 molecule using DFT

34

, in this

work, the calculated O atomic energy and experimental value of O2 binding energy (5.12 eV) 35

were used to calculate the energy of O2. By the same method, the total energy of O2 was

calculated in previous publications

13-14, 36-37

. The term −eU added on µ Li means the

influence of U on energy of electron because of external potential can easily change the 6


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chemical potential of electron. To ensure energy conservation, this way is commonly used in electrochemical reaction

33, 38-39

. It is noted that the ORR/OER occur at low temperature and

pressure, so the impacts of entropy (-TS) and volume (PV) on E and E0 are ignored. The ORR and OER overpotentials (ηORR and ηOER ) were calculated by ηORR =U 0 − U Dc and ηOER =U C − U 0 , where U 0 , U Dc and U C are the equilibrium, discharging and charging potentials, respectively. Specifically, U 0 in theory is the thermodynamic equilibrium potential corresponding to the ORR/OER occur spontaneously ( ∆G ≤ 0 ). U Dc is the highest discharging potential in which all steps are energetically downhill, while U C is the lowest charging potential in which all steps are energetically downhill. This theoretical calculation procedure of overpotentials has been widely used in previous published works 11-12, 26,39-40. The adsorption energies ( Eb ) were expressed as Eb = Esub − ads − Esub − Eads , where Esub − ads , Esub , Eads stand for the total energies of adsorption system, substrate slab and adsorbed molecule or atom, respectively. The surface energies (γ) in stoichiometric slab were calculated using the following equation γ =1 2 A( Eslab − NEbulk ) , where Eslab and Ebulk are the total energies of slab and bulk. N= N slab N bulk , where Nslab and N bulk are the number of formula units in slab and bulk, respectively. The fraction of 1 2 accounts for two surfaces of slab, and A stands for the surface area.

3. RESULTS AND DISCUSSION We investigated the ORR and OER overpotentials of 3d-TMCs in the NaCl structure. It is well known that some 3d-TMCs play an important role in improving the catalysis of Li-O2 battery as well as in fuel cell. Here, all catalytic reactions including ORR and OER were 7



Page 8 of 41

examined on 3d-TMCs (100) surface which was considered as the catalyst surface in this study. By calculating reaction free energy of each step, the discharging, charging and equilibrium potentials are determined, then overpotenticals are obtained to evaluate the catalytic activity of 3d-TMCs.

3.1 Evaluation of Catalytic Activity Figure 1 shows the ORR and OER energy diagrams of 3d-TMCs surfaces. Three elementary

ORR/OER

reaction

steps

are

listed

as:

1)

O2+(Li++e-)→LiO2*,

2)

LiO2*+(Li++e-)→Li2O2* and 3) Li2O2*+2(Li++e-)→(Li2O)2*, where the * indicates the molecules adsorbed on surface. The LixO2 (x=4, 2 and 1) molecules represent intermediate products of surface catalytic reactions. In fact, all these adsorbed species have been confirmed in previous experimental researches

41-43

, and the electrochemical performance of

catalyst are often predicted by analyzing the interaction between reactive intermediates and catalyst surface in relevant theoretical studies

17, 38, 44

. The optimized LixO2 (x=4, 2 and 1)

adsorption structures as top view along reaction coordinate are displayed in Figure 1a-h. The stable adsorption structures of LixO2 (x=4, 2 and 1) molecules on 3d-TMCs surfaces are different, which are structures with the lowest total energy of molecules adsorbed on various active sites. We tested about 20 sites, including vertical and horizontal adsorption for per adsorbed intermediates, then optimized to obtain the most stable adsorption structures. It is noted that the absolute values of overpotentials for a certain catalytic surface depends on the selected elementary reactions. Meanwhile, our work focuses on intrinsic electrochemical reaction of Li-O2 battery in ideal electrochemical environment to compare the catalytic performance of substrates. As mentioned above, U Dc is the electrode potential ensure all 8


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ORR proceeds energetically downhill from left to right side, while U C is the electrode potential ensure all OER proceeds energetically uphill from left to right side in Figure 1. The specific values of discharging ( U Dc ), equilibrium ( U 0 ) and charging ( U C ) potentials are listed in Table 1. Here, our theoretical charging potential result of TiC surface is 3.7 V, which is in agreement with the experimental date of 3.5 V (in DMSO at a current density of 1 mA/cm2) and 3.6 V (in TEGDME at a current density of 0.5 mA/cm2) for TiC cathode in aprotic Li-O2 battery 17. The calculated equilibrium potential 2.51 V is close to other theoretical prediction result of thermodynamic equilibrium potential for TiC catalyst in Li-O2 battery 14. VC has the smallest U Dc , while CoC has the largest U Dc . The smallest value of U 0 corresponds to VC, while CoC corresponds to the largest U 0 . The maximum and minimum values of U Dc or

U 0 correspond to the same TMC surface. TiC has the smallest U C , while NiC has the largest U C . However, potentials are not the common parameters for evaluating performance of Li-O2 battery. In experimental and theoretical researches, the catalytic activity of Li-O2 battery is often evaluated by overpotentials.

9



Figure 1 The calculated energy diagrams of ORR and OER process with optimized geometries of intermediates on (a) ScC, (b) TiC, (c) VC, (d) CrC, (e) MnC, (f) FeC, (g) CoC, 10


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(h) NiC. Green, red, brown and other spheres indicate Li, O, C and 3d-transition-metal atoms, respectively.

Table 1 The discharging ( U Dc ), equilibrium ( U 0 ), charging ( U C ) potentials, ORR (η OER (η

OER

) overpotentials, their sum (η

) and binding energy of O Eb (O ) for 3d-TMCs.

TiC

VC

CrC

MnC

FeC

CoC

NiC

UDc (V)

1.60

1.82

1.32

1.60

1.75

1.84

2.73

1.76

U0 (V)

2.35

2.51

2.25

2.57

3.04

2.85

4.02

3.62

UC (V)

4.07

3.70

4.01

4.03

4.63

4.00

5.73

6.10

(V)

0.75

0.69

0.93

0.97

1.29

1.01

1.29

1.86

(V)

1.72

1.19

1.76

1.46

1.59

1.15

1.71

2.48

(V)

2.47

1.88

2.69

2.43

2.88

2.16

3.00

4.34

-4.62

-2.90

-2.56

-3.32

-4.91

-4.72

-5.47

-5.84

ORR

η

OER

η

TOT

Eb(O) (eV)

),

TOT

ScC

η

ORR

We calculated the OER and ORR overpotentials according to equations of

ηOER =U C − U 0 and ηORR =U 0 − U Dc , respectively. The calculated ηORR , ηOER and their sum ηToT = ηORR + ηOER are shown in Table 1. The values of ηOER are all higher than that of ηORR for 3d-TMCs surfaces, reflecting more sluggish kinetics during OER process. The lowest values of ηORR , ηOER and ηTOT correspond to TiC , FeC and TiC, respectively. This means that TiC can effectively catalyze the ORR ,while FeC can effectively catalyze the OER. Taken together, TiC has the best catalytic activity of 3d-TMCs. We have compared the catalytic activities of 3d-TMCs surfaces, next some intrinsic material properties of TMCs and their relationships with catalytic activity will be discussed.

3.2. Intrinsic Properties Affecting Catalytic Activity 11



3.2.1 Adsorption Energies of Li and LiO2. We attempt to extract available material properties affecting catalytic activities from the energies of reactive intermediates in three elementary reactions. In Figure 1, we can clearly see that the energies before and after the reverse reaction 2) LiO2*+(Li++e-)→Li2O2* are the same in ORR process corresponding to the discharge potential U Dc , while the energies before and after the reverse reaction 1) O2+(Li++e-)→LiO2* are the same in OER process for most 3d-TMCs (except ScC, CoC and NiC) corresponding to the charge potential U C . These findings indicate that the ORR/OER overpotentials can predominantly described by the adsorption energies of Li and LiO2 species.

Figure 2 The ORR overpotentials as a function of the adsorption energies of (a) Li and (b) LiO2, the OER overpotentials as a function of the adsorption energies of (c) Li and (d) LiO2 for 3d-TMCs. 12


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We found that the adsorption energies of Li and LiO2 are available factors affecting ORR/OER overpotentials for 3d-TMCs surfaces. Figure 2(a) exhibits that the ORR overpotentials are inversely proportional to the adsorption energies of Li with a high correlation coefficient of R2=0.96. This relationship suggests that weaker Li adsorption leads to higher ORR catalytic activity of 3d-TMCs. Figure 2(b) presents that the ORR overpotentials are also inversely proportional to the adsorption energies of LiO2 with a high correlation coefficient of R2=0.93. This relationship means that weaker LiO2 adsorption induces higher ORR catalytic activity of 3d-TMCs. Figure 2(c) exhibits that the OER overpotentials have a poor correlation with the Li adsorption energies, as verified by a low correlation coefficient of R2=0.61. Figure 2(d) shows that the OER overpotentials are inversely proportional to the adsorption energies of LiO2 with a high correlation coefficient of R2=0.72. This relationship means that weaker LiO2 adsorption induces higher OER catalytic activity of 3d-TMCs. The remarkable catalytic activity of TiC is closely associated with the smallest adsorption energies of Li and LiO2 during charging process. As a result, the catalytic activities of 3d-TMCs for OER cannot be fully explained by the Li adsorption energies. Consequently, the catalytic activities of 3d-TMCs in Li-O2 battery are associated with the LiO2 adsorption energies. It is worth mentioning that the linear correlation between catalytic activities with these adsorption energies is different from the volcano-shape dependence of O2 desorption energy and equilibrium potential in Li-O2 battery for transition-metal compounds 14.

3.2.2 Surface Energy. Surface energy is another important factor to correlate catalytic activity of 3d-TMCs surfaces. A surface with high surface energy is usually highly reactive. 13



Meanwhile, high surface areas and large reaction sites as well as sufficient contact area between reactants/electrolyte and active materials can accelerate reaction rate. Figure 3 shows that the ORR overpotentials are proportional to the surface energies with a high correlation coefficient of R2=0.88. This relationship means that higher surface energy leads to higher catalytic activity of 3d-TMCs for ORR. The remarkable catalytic activity of TiC is closely associated with the largest surface energy during discharging process. However, the relationship between OER overpotentials and catalytic activity is not clear, which is not shown in Figure 3.

Figure 3 The ORR overpotentials and surface energies for 3d-TMCs. 3.2.3 Binding Energy of O. The intermediate states of elementary reaction involving Li4O2*, Li2O2* and LiO2* correspond to oxygen ions at different chemical valence. The binding strength of all intermediates can be described by the binding energy of O* ( Eb (O ) ) which is a widely accepted descriptor of ORR and OER for a broad range of catalytic materials 45. A good catalyst should have a moderate Eb (O) , including both good adsorption and desorption abilities, to improve the bi-functional catalytic activity. The values of Eb (O ) 14


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for 3d-TMCs surfaces are listed in the last line of Table 1. Among them, TiC has a moderate binding energy of O*, which corresponds to smaller ORR and OER overpotentials. Additionally, to further characterize the correlation of electronic properties and Eb (O) , we compared the projected density of states (PDOS) of O adsorbed on 3d-TMCs. And only the PDOS of ScC, TiC, VC and NiC are shown in Figure 4. As O atom adsorbed on ScC, some hybridized electronic states of Ti-3d, C-2p and O-2p occur around the Fermi level. In TiC, the hybridization between Ti-3d and O-2p is weakened, and the Ti-3d state is dominated for VC, CrC, MnC, FeC and CoC. Anomaly occurs in NiC, which is mainly caused by the Ni-3d. Therefore, the maximum catalytic activity of 3d-TMCs is TiC for Li-O2 battery.

Figure 4 The projected density of states (PDOS) of O atom adsorbed on ScC, TiC, VC and NiC. The vertical dashed line at E=0 eV represents the Fermi energy.

15



As a result, the highly active catalyst should have weaker adsorption energies of Li and LiO2, higher surface energy and moderate binding energy of O atom. These intrinsic properties may be the potential descriptor of catalytic activity in Li-O2 batteries. Next, We qualitatively analyze the density of states of 3d-TMCs, which will further understand from underlying electronic structure of 3d-TMCs for catalysis.

3.2.4 Electronic Structure Analysis. To further characterize the correlation of electronic properties and catalytic activity, we compared the total density of states (TDOS) of 3d-TMCs in Figure 5. As can be seen from Figure 5, the outline of TDOS for 3d-TMCs surfaces in the identical rocksalt structure are almost the same, but the relative position of Fermi level changes with the increasing number of 3d electrons. This phenomenon is related to the occupation of electrons on bonding or antibonding orbitals. The valence electrons occupy the bonding state with lower energy firstly and then occupy the antibonding state with higher energy. Fermi level lies to the left of pseudogap in ScC, indicating that the bonding state is partially filled and additional electrons are required for achieving the maximum stability of 3d-TMCs. In TiC, Fermi level happens to be in pseudogap, which indicates that the stability of the system is significant improved by increasing covalent interaction. When the bonding state tends to be saturated, the catalytic activity reaches its maximum. Fermi level locates on the right of the pseudogap from VC to NiC, indicating that all the bonding states are fully filled and extra electrons are partially filled into the antibonding state, which means that the surfaces become unstable. The occupation of antibonding state weakens the bonding between surface atoms, resulting in a decrease of catalytic activity. Here, the stability of system is based on the quantum size effect, which is different from the aforementioned 16


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stability of surface effect. For the same structure, the pseudogap position of 3d-TMCs moves from right to left as the increasing number of electrons. The saturation of bonding states reaches at TiC which corresponds to smaller ORR and OER overpotentials.

Figure 5 The total density of states (TDOS) of 3d-TMCs. The vertical dashed line at E=0 eV represents the Fermi energy.

4. CONCLUSION In this study, we systematically investigated the catalytic activity of 3d-transition-metal carbides in the rocksalt structure to improve sluggish ORR and OER kinetics in Li-O2 battery using first-principle calculations. Our calculations show that TiC has the best catalytic activity with the lowest overpotentials of ORR and OER are 1.19 and 1.88 V, respectively. 17



Then some intrinsic properties are determined and their correlation with catalytic activity are established. The ORR overpotentials have strong linear correlations with the adsorption energies of Li and LiO2, while OER overpotentials correlate well with the adsorption energies of LiO2. The ORR overpotentials are proportional to the surface energies of 3d-TMCs surfaces, but the relationship between OER overpotentials with catalytic activity is not clear. TiC surface has a moderate Eb (O ) , which is reasonable to improve the overall catalytic activity. Consequently, the highly active catalyst should have weaker adsorption energies of Li and LiO2, higher surface energy and moderate binding energy of O atom. These intrinsic properties may be the potential descriptor of catalytic activity in Li-O2 batteries. In addition, the catalytic activity reaches its maximum when Fermi level happens to be in the pseudogap. Here, only 3d-transition-metal carbides are considered, whether the calculated results are applicable to 4d-TMCs and 5d-TMCs, and further systematic studies are required. Our calculations and analysis yield a deep insight into understanding the relationship of catalytic activity and material property, which layer a reliable foundation for the future catalyst screening and design in Li-O2 battery.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 21233010), Fundamental Research Funds for the Central Universities (Grant No. DUT16ZD102), the Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province) and Supercomputing Center of Dalian University of Technology.

18


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Figure 1 The calculated energy diagrams of ORR and OER process with optimized geometries of intermediates on (a) ScC, (b) TiC, (c) VC, (d) CrC, (e) MnC, (f) FeC, (g) CoC, (h) NiC. Green, red, brown and other spheres indicate Li, O, C and 3d-transition-metal atoms, respectively. 254x190mm (300 x 300 DPI)


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Figure 2 The ORR overpotentials as a function of the adsorption energies of (a) Li and (b) LiO2, the OER overpotentials as a function of the adsorption energies of (c) Li and (d) LiO2 for 3d-TMCs. 289x203mm (300 x 300 DPI)


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Figure 3 The ORR overpotentials and surface energies for 3d-TMCs. 289x203mm (300 x 300 DPI)



Figure 4 The projected density of states (PDOS) of O atom adsorbed on ScC, TiC, VC and NiC. The vertical dashed line at E=0 eV represents the Fermi energy. 127x127mm (300 x 300 DPI)


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Figure 5 The total density of states (TDOS) of 3d-TMCs. The vertical dashed line at E=0 eV represents the Fermi energy. 127x203mm (300 x 300 DPI)



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Intrinsic Properties Affecting Catalytic Activity of 3d-Transition-Metal

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