Stable zigzag edges of transition-metal dichalcogenides with high catalytic activity for oxygen reduction

Journal Pre-proof Stable zigzag edges of transition-metal dichalcogenides with high catalytic activity for oxygen reduction Yu Hao, Li-Chun Xu, Jibin Pu, Liping Wang, Liang-Feng Huang PII:

S0013-4686(20)30257-7

DOI:

https://doi.org/10.1016/j.electacta.2020.135865

Reference:

EA 135865

To appear in:

Electrochimica Acta

Received Date: 1 December 2019 Revised Date:

21 January 2020

Accepted Date: 5 February 2020

Please cite this article as: Y. Hao, L.-C. Xu, J. Pu, L. Wang, L.-F. Huang, Stable zigzag edges of transition-metal dichalcogenides with high catalytic activity for oxygen reduction, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.135865. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Yu Hao: Conceptualization, Formal Analysis, Investigation, Writing. Li-Chun Xu: Review & Editing, Supervision. Jibin Pu: Review & Editing. Liping Wang: Review & Editing. Liang-Feng Huang: Conceptualization, Data Curation, Formal Analysis, Investigation, Supervision, Writing.

Stable Zigzag Edges of Transition-Metal Dichalcogenides with High Catalytic Activity for Oxygen Reduction Yu Haoa,b , Li-Chun Xua,c,∗, Jibin Pub , Liping Wangb , Liang-Feng Huangb,d,∗ a

College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China. Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China. c Key Lab of Advanced Transducers and Intelligent Control System, Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan, 030024, China. d Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, USA. b

Abstract Developing non-precious and efficient oxygen reduction reaction (ORR) catalysts to replace Pt-based materials is of vital importance for hydrogen fuel cells. Transition-metal dichalcogenides (TMD) are a typical group of catalysts that have been previously applied for hydrogen-evolution and hydrodesulfurization reactions. Using density-functional-theory calculations, we explore the prospect of both traditional and Janus TMDs as electrochemical ORR catalysts by studying their planar surfaces and different kinds of edges. Among the tens of surfaces, armchair edges, and zigzag edges of Mo- and W-based systems screened here, we find that both excellent stability and high catalytic activity can be simultaneously achieved for the zigzag edges of WSe2 and WSSe. The overpotentials of different zigzag edges of WSe2 and WSSe vary between 0.43 and 0.64 V, as low as that of the prototypical Pt electrode (∼ 0.45 V). The comprehensive ORR activities of TMD-based surfaces and edges studied here, as well as the high ORR activity discovered for specific zigzag edges, can not only be readily validated by experiments but also facilitate their future related applications by providing a complete structure–property relationship. Those low overpotentials are benefited from the moderate OH–edge bonding strength, and the revealed microscopic electornic-structure mechanisms here will also be useful for further optimizing the ORR activities of TMD edges through, e.g., chemical, mechanical, and potentiostatic approaches. Keywords: Transition-Metal Dichalcogenide, Oxygen Reduction Reaction, Janus Structure, Density-Functional Theory 1. Introduction ∗

Corresponding author Email addresses: [email protected] (Li-Chun

Xu), [email protected] (Liang-Feng Huang) Preprint submitted to Electrochimica Acta

Oxygen reduction reaction (ORR) is vitally important in various energy conversion and storage appliFebruary 6, 2020

cations, i.e., fuel cells and metal-air batteries [1, 2].

it is even possible to maximize the exposed edge sites

However, the sluggish kinetics of the ORR greatly

by preparing nanoscale structures, e.g., nanoparti-

limit the development of these devices, and efficient

cles, defect-rich films, and nanoflakes [6, 7, 17, 18].

catalysts are urgently needed to promote the reac-

The efficiencies of hydrogen production and uti-

tion [3]. Currently, the Pt-based catalysts always

lization can be promoted by using hydrogen evolu-

exhibit superior catalytic activity among the avail-

tion reaction (HER) and ORR catalysts, respectively

able catalysts. However, Pt has a very low reserve

[19, 20]. Regardless of the well studied HER catal-

(only 5 µg/kg in earth’s crust [4]) and a high price

ysis of TMD edges, the knowledge on their ORR

(774∼1281 USD/oz in recent five years, according

performance still require a comprehensive establish-

to the data from London Platinum and Palladium

ment, especially when the enhanced ORR has been

Market). This reality definitely precludes its further

pointed out on some selected edges in few recent

prevailing applications, and has motivated a huge

reports [21, 22]. Considering the controllable frac-

amount of effort to pursue non-precious electrocat-

tion of edge structures in synthesized samples, it is

alysts with high ORR efficiency [5].

highly necessary to locate the most superior candidates (e.g., among various surfaces and edges) for the

Two dimensional (2D) materials have many ad-

ORR catalysis, which can promote the development

vantages for catalysis, such as excellent mechan-

of energy technology using TMD-based materials.

ical strength, ultrahigh specific surface area, and high exposure of surface atoms [8–10]. Transition-

Furthermore, compared with traditional TMD ma-

metal dichalcogenide (TMD), as a typical 2D rep-

terials, a family of so-called Janus TMDs have also

resentative, has promising electrocatalytic activities

drawn widespread interest due to their particular

in hydrogen evolution reaction (HER) [11] and hy-

sandwiched asymmetric structures with many attrac-

drodesulfurization reaction [12], which has been as-

tive properties such as tunable dipole, carrier mobil-

cribed (in both experiment and theory) to the catalyt-

ity, magnetism, and band gap [23–25]. The Janus

ically active edge sites with the special hydrogenase-

MSSe (M = Mo, W) structures have been prepared

like structure [11, 13]. Apart from the high abun-

by replacing the S atoms on one side of the mono-

dance and low cost, TMD not only has a high sta-

layer MS2 with Se atoms, as well as sulfurizing one

bility in various solvents and oxygenated environ-

side of monolayer MSe2 [26, 27]. There exist intrin-

ments, but also possesses preferred tolerance to ex-

sic strain and electric field in Janus TMDs, which

treme thermal and baric conditions [14–16]. A large

have been found responsible for the higher HER ac-

amount of edge sites usually inevitably appear in the

tivity of Janus TMDs than the conventional ones

experimentally synthesized TMD samples [11], and

[27, 28]. In addition, the Janus structures with sym2

Figure 1: (a, b) The scanning electron microscopy and the transmission electron microscopy images of MoS2 samples synthesized in experiment with a large number of exposed edges [6, 7] (Copyright 2013, Wiley Online Library and Copyright 2013, American Chemical Society). (c) The lattice structure of Janus MSSe with the surface anion site denoted as X-side. (d-f) The zigzag edges (denoted as M-edge and X-edge) and armchair edge of MSSe, which will turn into the counterpart edges of traditional MX2 if all of the anion atoms are identical.

metry breaking also have promising and wide ap-

possess excellent stability and high ORR catalytic

plications in the fields, e.g., photocatalysis [29] and

performance. Electronic structure analysis is also

electronic equipment [30], and it is quite attractive to

conducted to reveal the underlying mechanism for

explore the ORR performances of their surfaces and

such superior ORR performance.

edges. 2. Methodology In this work, both the co-generic molybdenum 2.1. Computational Parameters

and tungsten dichalcogenides (MX2 , M = Mo, W; X = S, Se) and their corresponding Janus structures

The considered structures, as well as their en-

(MSSe) are considered to explore their ORR perfor-

ergies and electronic structures, are calculated us-

mance. The ORR activities of tens of structures (sur-

ing density-functional theory (DFT) implemented in

faces, armchair edges, and zigzag edges) are com-

the Vienna Ab initio Simulation Package (VASP)

prehensively screened using density-functional the-

[31, 32], where the electronic wave functions and

ory calculations, based on which we have found that

potentials are pseudized using the projector aug-

the zigzag edges of WSSe and WSe2 simultaneously

mented wave method [33]. The electronic exchange 3

Figure 2: The schematic presentation of the possible OOH association and O2 dissociation reaction mechanisms for an ORR process, where ∗ represents the adsorption state, the gray rectangle represents catalyst.

and correlation are described by the spin-polarized

model implemented in the VASPsol code package

PBE functional [34, 35] in the generalized gradi-

[39, 40], where the dielectric constant of water sol-

ent approximation. To simulate the van der Waals

vent is set to be 78.

forces existing in the systems under study, the zero2.2. ORR Model

damping DFT-D3 functional [36] is used to describe

In acid solution, a complete ORR process has

such dispersive electronic potential. A cutoff energy

two possible pathways as shown in Figure 2: the

of 450 eV is used for the plane-wave expansions

adsorbed O2 molecule is hydrogenated into OOH

of the electronic wave functions and electron den-

species by accepting a proton coupled with an elec-

sities. The convergence thresholds for atomic force

tron transfer (i.e., the OOH association mechanism),

and electronic energy are 0.01 eV/Å and 10−5 eV, re-

or directly dissociated into two individual O∗ adsor-

spectively.

bates (i.e., the O2 dissociation mechanism). The inThe ORR reactions are considered on various

volved elementary steps are indicated by numbers

structure sites as shown in Figure 1. To effectively

(1)–(6) in Figure 2. Both the two mechanisms must

exclude the interactions between the neighboring pe-

go through the same last two steps (5-6), where the

riodic slabs, a vacuum spacing of at least 15 Å is

adsorbate O∗ is successively hydrogenated into OH∗

used. The Brillouin zone is sampled by gamma cen-

and H2 O. The equations of the reaction steps can be

tered reciprocal meshes and the allowed reciprocal-

found in Section (A) of Supporting Information (SI).

−1

point spacing is 0.04 (2π · Å ). The frozen-phonon

The change in Gibbs free energy of each elemen-

method [37] is used to obtain the vibrational fre-

tary ORR step (∆G) can be calculated using

quencies, from which the vibrational free energy of

∆G = ∆ε0 + ∆εzpe − T ∆s + ∆gpH + ∆gU

a material can be directly derived using the thermo-

(1)

dynamic functions of phonons [38]. The solution en-

where ∆ε0 is the change in electronic-energy; ∆εzpe

vironment is simulated using the implicit solvation

is the change in vibrational zero-point energy; T is 4

Figure 3: (a) The formation energies (Ef s) of various TMD edge structures, where S-term, Se-term and M-term represent the Medge terminated with S, Se and M atoms, respectively, and arm represents the armchair edge. (b) The relationships between the formation energies of the S- and Se-term M-edges of MoSSe (and WSSe) and the chemical potential of S (and Se, upper axis from right to left), respectively. The blue line varies with the upper axis coordinate, while the red line with the lower axis coordinate.

temperature (= 298.15 K here); and ∆s is the change

been widely proved reliable for many ORR catalysts

in entropy. The entropy of the adsorption structure is

such as transition-metal, alloy, group VB TMDs, and

calculated from vibrational frequency [38]. The last

doped MoS2 [44–47].

two energetic terms only need to be considered for the protonation steps, which are the contributions of

3. Results and Discussion

solution acidity (∆gpH = kB T ln 10 × pH) and elec-

3.1. Structures and Stability

trode potential (∆gU = −qU, q = |e| here). The ideal

Traditional monolayer TMD is typically denoted

open-circuit potential generated by an ORR process

as MX2 , where the transition metal M (= Mo or

increases with increasing the environmental acidic-

W) layer is sandwiched between two identical layers

ity [41], thus, we consider the standard acidic condi-

with chalcogen atom X (= S or Se). The asymmetric

tion at pH 0 and the equilibrium electrode potential at

Janus MSSe can be obtained by changing the type

1.23 V (with respect to the standard hydrogen elec-

of chalcogen atoms on one side to a different one, as

trode) [41, 42]. Gas-phase H2 O and H2 are used as

shown in Figure 1 (c), where the anion site on the pla-

reference states and the entropy of H2 O is calculated

nar surface is denoted as X-side. This substitutional

at 0.035 bar, i.e., the equilibrium pressure at 298.15

method in constructing the Janus structure is consis-

K. The free energy of O2 is obtained from the reac-

tent with the microscopic processes happening in the

tion O2 + 2H2 ↔ 2H2 O, and turns out to be 4.92 eV

experimental synthesis methods [27]. Furthermore,

[43]. The involving electrochemical methods have

it is unavoidable and controllable to produce edge 5

Figure 4: The adsorption structures and stability of ORR intermediates on WSSe. (a) The O2 -adsorbed structures for Se-edge, W-edge, S-side and arm-W (the W site of armchair edge). (b) Adsorption free energies (∆Gads ) of O∗ , OH∗ , O∗2 , and OOH∗ on various sites, where the horizontal dashed line resides at the thermodynamically neutral state (i.e., 0 eV).

sites in TMD samples by recently developed experi-

bilities of the M-edge covered by X atoms and the

mental techniques [6, 7], and a large number of edge

armchair edge, the formation energy Ef is calculated

sites in MoS2 samples have been obtained, as shown

using

in Figure 1 (a) and (b). To comprehensively consider

Ef =

different types of reaction sites on these TMD ma-

1 (εedge − nM µunit − ∆nX µ0X ), 2L

(2)

where L is the number of MX2 unit cells (= 4)

terials, we consider the X atom on surface (X-side),

along the edge in the used supercell; εedge is the elec-

armchair edge and zigzag edge (edge terminated with

tronic energy of the supercell; nM is the number of

X atoms (X-edge) and terminated with M atom (M-

M atoms; ∆nX is the number of terminated X atoms

edge)) for all of MoS2 , MoSe2 , WS2 , WSe2 , MoSSe,

on M-edge, µunit is the chemical potential per one for-

and WSSe, as shown in Figure 1 (c-f). In addition,

mula unit and µ0X is the chemical potential of elemen-

due to the tendency of metal atoms at edges to further

tal X (= S or Se) with a X8 -ring molecular structure

bond with anion atoms (e.g., S and Se) [48], we con-

(ground state) [49, 50]. The width of the supercells

sider the M-edges with both bare metal atoms and

for the edge structures is carefully tested (as shown

adsorbed with anion X atoms (Figure 1 (d)). The

in Section (B) of SI), which reveals that the width of

zigzag edges of these six TMD structures exhibit to-

6 rows can be large enough to accurately describe the

tal magnetization of 2µB , while the armchair edges

chemical reactions on the edges.

have no magnetization. To explore the relative sta-

The calculated edge formation energies are shown 6

in Figure 3 (a), where S-term, Se-term and M-term

tials of S2 and Se2 can be found in Section (C) of

represent the S, Se and M atoms terminated M-edge

SI. Regarding the relative stabilities between S- and

respectively. It can be seen that the exposed M atoms

Se-term edges, the corresponding formation-energy

on MX2 edges are highly reactive to bond with X

differences are −0.04 and −0.09 eV for MoSSe and

atoms, and in MSSe, the exposed M atoms are eas-

WSSe at 0 K, respectively (by Equation 2, see Fig-

ier to cover with S atoms than Se atoms due to the

ure 3 (a)), while these differences are enlarged to be

lower formation energy (by 0.04 eV for MoSSe and

−0.07 and −0.12 eV at 800 ◦ C (see Figure 3 (b)).

0.09 eV for WSSe). Comparing to zigzag edges, 3.2. Stabilities of Adsorbates

the Ef s of the armchair edges in these TMD materials are higher by 0.94 ∼ 1.26 eV, indicating that the

An ORR process starts when an adsorbate is cap-

much higher stability (i.e., existence probability) of

tured by the catalysts immersed in a solution, and

zigzag edges in realistic samples. Moreover, among

then the whole progress proceeds with many inter-

the zigzag edges, the S- and Se-terminated W-edges

mediate adsorbates, e.g., O∗ , OH∗ , O∗2 , and OOH∗

in WSSe and the Se-terminated W-edge in WSe2 are

[43]. It is indispensable to clearly understand/predict

the three edges with the lowest formation energies.

the structures and stabilities of these adsorbates, due to their the key role in determining the occurrence of

In order to determine the most possible chalco-

related ORR processes. The most stable adsorption

gen atoms terminating the Janus zigzag M-edges, we

sites for the adsorbates are located by examining the

calculate the relationships between their edge for-

adsorption free energy ∆Gads , which is derived based

mation energies and the chemical potential of S (or

on the electrochemical reactions of [53]

Se). The chemical potential of the additional edgeterminating X atoms varies with the environmental

H2 O+∗ → O∗ + H2

(3)

condition, thus the term µ0X in Equation 2 should be

H2 O+∗ → OH∗ + 1/2H2

(4)

replaced with the variable µX (Figure 3 (b)). In the

O2 +∗ → O∗2

(5)

experimental preparation condition, the samples are

2H2 O+∗ → OOH∗ + 3/2H2

(6)

kept at 800 ◦ C and the atmospheric pressure [27]. In such condition, the diatomic S2 and Se2 gases are

where the free energy of reaction for each equation

the mostly favored states of S and Se, respectively

(i.e., ∆Gads ) can indicate the stability of the produced

[51, 52], and their corresponding ∆µX s (= µX − µ0X )

adsorbate on the right side, e.g., O∗ , OH∗ , O∗2 , or

are -0.77 and -0.90 eV, respectively, as indicated by

OOH∗ . A negative (positive) ∆Gads corresponds to an

the vertical lines in Figure 3 (b). The calculation de-

exothermic (endothermic) adsorption process, and a

tails and the temperature-dependent chemical poten-

lower ∆Gads to a stronger adsorption. 7

Figure 5: Gibbs free energy diagrams at 1.23 V for the ORR steps happening on WSe2 and WSSe along the OOH association (left panels) and O2 dissociation (right panels) mechanisms. The reaction steps highlighted using thicker lines are the rate-determining step with their corresponding overpotentials (in V) labeled alongside. The keys are arranged in the same order as the free-energy heights at OH∗ .

Take WSSe as a representative, the calculated

have similar ORR properties. The adsorption modes

∆Gads for different sites (e.g., zigzag edges, side sur-

of O∗2 on these three zigzag edges are end type (Fig-

faces, and armchair edges) are shown in Figure 4,

ure 4 a), indicating that the OOH∗ is easy to form and

together with the structural details for the O2 ad-

the ORR process will tend to adopt the OOH associ-

sorbed surfaces/edges. As the first step of an ORR

ation mechanism. For the side surface sites (S-side

process, the initial adsorption configuration and sta-

and Se-side), the ORR species generally have higher

bility of O∗2 is highly important in the subsequent re-

∆Gads s and longer O-X bonds, due to the too stable

action pathway. Other TMD materials are similar

basal plane X atoms than those at exposed edges.

to WSSe and are listed in Section (D) of SI. From

There is no stable OOH adsorption at the armchair

Figure 4, ∆Gads of all the four adsorbates between

edge, no matter on the S, Se, or W site, indicating

S-edge, Se-edge, and W-edge of WSSe have little

that the ORR process on armchair edge will adopt

changes (around 0.5 eV), implying that they may

the O2 dissociation mechanism. On the W site, the 8

adsorption structure of O∗2 is side type (Figure 4 a),

catalysis reactions, which has been confirmed by

where both O atoms are closely bonded to the active

many experimental measurements [43, 54, 55]. For

W, reflecting by the very negative ∆Gads (∼ −3.76

armchair edge, the ORR processes follow the O2 dis-

eV). Accordingly, due to the existence of adjacent W

sociation mechanism, where the RDS of the M site is

atoms at armchair edge, the S and Se sites are diffi-

at the step 6 (see Fig. 2), i.e. the desorption of OH∗ .

cult to have stable O∗2 adsorption.

For arm-S and arm-Se, the RDSs are located at the step 5 (see Figure 2), the reduction of O∗ is relatively

3.3. ORR Performance

difficult, which is consistent with the trend of adsorp-

The ORR performances of the basal-plane sur-

tion energy calculated above. In contrast, on zigzag

faces and various edges can be described by the

edge and basal-plane surface, the adsorption of O2 is

corresponding Gibbs free energy diagrams (FEDs)

relatively weak without dissociation, and there will

for the involved reaction steps, which can directly

be stable OOH∗ adsorbate to form, thus, all of the

present the change of energy barrier as the reaction

ORR processes therein will follow the OOH associ-

proceeds. The overpotential η is defined as the min-

ation mechanism (Figure 5 and Section (F) of SI).

imum additional potential required for a successful

Furthermore, we also examine the possibility of the

ORR process, thus can also reflect the possibility for

generation of H2 O2 after the formation of OOH∗ on

an ORR to occur [42], i.e., the reaction is easier to

the W-edge of WSSe that possesses the lowest over-

complete with a lower overpotential. After calculat-

potential, and find that it has a much higher energy

ing the FEDs of all of the considered surfaces and

barrier (1.22 eV) than that for the OOH∗ –O∗ tran-

edges, we find the lowest overpotentials (0.43 ∼ 0.64

sition (−1.13 eV), making H2 O2 very difficult to be

V) on zigzag edges of WSSe and WSe2 , thus, FEDs

produced.

for WSSe and WSe2 at the equilibrium potential 1.23 V are shown in Figure 5, while the other TMD struc-

From FEDs, it can be found that the RDSs for

tures are shown in Figure S7 – S10 of SI. The cor-

these TMD structures mainly reside at the last two

responding adsorption structures and the calculated

steps (see Figure 2), i.e., the ORR is limited by the

zero-point energies and entropies of the critical struc-

protonation of O∗ or OH∗ . Therefore, it is possible to

tures W-edge of WSSe and WSe2 are also provided

better understand the relationship between the over-

in Section (E) of SI.

potential of each structure and the adsorption energy

The one reaction step within the whole ORR

of ORR species by constructing the ∆GO –∆GOH –

process that has the maximum free-energy rising

overpotential map, as shown in Figure 6. In Figure

(i.e., maximum overpotential) actually is the rate-

6, the exposed transition-metal M atoms on armchair

determining step (RDS) in many electrochemical

edge that bind O and OH very strongly reside at the 9

Figure 6: The ∆GO –∆GOH –overpotential map for all the studied TMD structures, where the red (blue) regions represent the low (high) overpotential.

left bottom corner of the map with very large overpo-

and WSe2 have the preferred high ORR activities as

tentials. The X atoms on the basal-plane surface and

shown in Figure 5, where the highest overpotential of

armchair edge locate at the upper half of the volcano

them is only 0.64 V, indicating that their zigzag edges

plot due to the weak OH binding.

are more active than MoS2 and other TMD materials studied here. This comparative trends can be readily

Importantly, except for the Mo-edge of MoS2 , the

validated in experiment. In addition, HER and ORR

overpotentials of all of the zigzag edges are less than

are two important component processes responsible

or close to 0.7 V, and the lowest overpotential (0.43

for the hydrogen production and utilization, respec-

V) appears at the W-edge of WSSe. Such a low over-

tively, and catalyzing it with the same materials will

potential of WSSe is even lower than those of many

be highly attractive and can greatly facilitate the de-

superior catalysts, such as 0.45 V for benchmark Pt

velopment of fuel cells. It is interesting to find that

electrode [55], 0.47 V for defective graphene [56],

the HER performance of WSSe and WSe2 has been

and 0.67 V for N-doped graphene [57]. For MoS2 ,

identified [28, 58, 59], and together with their active

it has been proved possessing ORR activity in ex-

ORR on stable zigzag edges (controllable in synthe-

periment [21] by reducing particle size, where the

sis, see Figure 1) discovered here, the prospect of

S-edge therein has a low ORR overpotential of 0.66

these materials in energy applications can be natu-

V, while the overpotential of Mo-edge is as high as

rally derived.

1.29 V, indicating the inert ORR behavior. Comparing with MoS2 , all of the zigzag edges of WSSe 10

Figure 7: Differential electron densities (∆ρ) for the OH adsorption on the (a) W-edge of WSSe, (b) Mo-edge of MoS2 , and (c) W-edge of WSe2 . The red (blue) regions represent electron accumulation (depletion).

To further understand the electronic-structure

3.4. Electronic-Structure Mechanisms For an ORR process, the essence of it is the charge

mechanisms in a different perspective, we calculate

redistribution between the catalysts and the adsor-

the projected densities of states (PDOS) of S atoms in

bates. The electronic-structure analysis can reveal

bulk WSSe (Figure 8(a)) and at the W-edge of WSSe

the underlying mechanism deeply for the variation

(Figure 8(b) before OH adsorption; Figure 8(b) af-

of ORR performance on different structures and ad-

ter OH adsorption). The S rows leaving away from

sorption sites. From the overpotential shown in Fig-

the edge are labeled as 1L (front), 2L , 3L, and 4L,

ure 6, it is interesting that the Mo-edge of MoS2

successively (see Figure 8), and some localized elec-

has an ORR performance different from those of the

tronic edge states appearing as peaks in the 1L/2L

zigzag edges of other TMD materials. Therefore, we

PDOSs within the bulk band gap are labeled as α, β,

calculate the electron-density difference (∆ρ) of the

and γ, respectively (see Figure 8(b) and (c)). With

Mo-edge of MoS2 , after and before the OH adsorp-

the S–edge distance increasing from 1L to 4L, the

tion, and also compare it with the W-edges of WSSe

edge-state peaks become more and more negligible

and WSe2 that both have lower ORR overpotentials.

in the S PDOSs, and the PDOS of 4L S atom has be-

From the ∆ρs in Figure 7, it can be seen that the OH

come quite close to the bulk one, regardless of the

adsorption and the consequent OH–X bonding on the

edge status (adsorbed with OH or not). Comparing

W-edges of WSSe and WSe2 have influences within

the PDOSs before and after the OH adsorption, it can

larger ranges than those on the Mo-edge of MoS2 , in-

be seen that the 1L and 2L PDOSs are changed sig-

dicating the stronger OH–X bonding strengths on the

nificantly by the OH bonding, while the change in the

former two W-edges. This explains the higher ORR

3L and 4L ones are negligible. This clearly indicates

activities of the two W-edges of WSSe and WSe2

the covalent OH–edge bonding, and the depth pro-

than the Mo-edge of MoS2 .

file of the PDOS change reflects the influence range 11

Figure 8: The projected densities of states (PDOS) of (a) the S atom in bulk WSSe and the S atoms at the W-edges of WSSe (b) before and (c) after the OH adsorption. The magnetism mechanism is not our focus here, thus the spin-up and spin-down parts are summed together in (b) and (c) for simplicity. The reference electronic energy (i.e., 0 eV) in PDOS is the Fermi level. The distributions of the electron densities of the localized α edge states also are shown alongside.

of such bond, which is consistent with the distri-

of WSSe and WSe2 with zigzag edges (Figure S12

bution of the electron density for the α state. The

in SI), their metallic characters have been readily ob-

electron densities for the other two peaks for edges

served, which will also be beneficial to the electro-

with/without OH can be found in Figrue S11 of SI,

chemical ORR processes by providing itinerant car-

which have the same depth profile as the α state. The

riers.

OH–edge bonding also stabilizes and downshifts the

The Sabatier principle [60] has pointed out that

α state, making it partially occupied. The covalent

both strong and weak adsorptions of ORR interme-

character of the OH–edge bond can also be seen from

diates on substrates are adverse to the proceeding of

the significantly broadened PDOS of the O atom in

ORR: (1) the strong binding strength between ex-

OH (Figure 8(c)). In addition, from the total DOS

posed M and O makes the catalyst poisoned; (2) the 12

weak adsorption of ORR species results in a negli-

obtained on the W-edge of WSSe can be down to

gible intermediate products participating in the fol-

0.43 V, and even the highest overpotential on the Se-

lowing reaction. To clearly probe the OH adsorption

edge of WSSe still is as low as 0.64 V, which are very

strength on different structures, the Crystal Orbital

close to the that of 0.45 V on prototypical Pt elec-

Hamilton Population (COHP) [61–64] is employed

trode. The origin of such superior ORR performance

to analyze the characters of the O–substrate bonds

has been attributed to the favored moderate O–S

(Figure S14 of SI), and the bonding and antibonding

bonding strength, and the covalent character of such

electronic states can be readily derived from the cal-

bond has been analyzed from different aspects, e.g.,

culated −COHP spectra. To quantitatively determine

projected density of states, distribution of electron

the overall bond strength, the integral of −COHP

density, and COHP spectra. The structure–ORR re-

(−ICOHP) is calculated up to the Fermi level. For

lationship established in this comprehensive compu-

WSSe, the obtained values follow the magnitude or-

tational work, as well as the promising ORR activity

der of 5.81 (arm-W) >3.53 (W-edge) >1.63 (S-side),

found on the stable zigzag edges of WSSe and WSe2 ,

with a lower −ICOHP value indicating a weaker

can be easily validated by future experiments. The

bond strength. Therefore, it has consistently demon-

established understanding on the OH–edge bonding

strated that the moderate strength of the covalent S–

character here can also shed light on further opti-

O bonding at the W-edge of WSSe is responsible for

mizing the ORR activities of TMD edges through,

the favored ORR activity.

e.g., chemical, mechanical, and potentiostatic approaches, all of which tend to alter the electronicstructure nature of the edges states.

4. Conclusion In summary, using density-functional theory cal-

Conflicts of interest

culations, we have comprehensively evaluated the There are no conflicts to declare.

electrochemical adsorptions and ORR diagrams of the planar surfaces, armchair edges, and zigzag

Acknowledgements edges of MoS2 , MoSe2 , WS2 , WSe2 , MoSSe, and WSSe. The underlying microscopic thermodynamic

Y.H., J.P., L.W., and L.F.H. are supported by

and electronic-structure mechanisms are also re-

the National Science Fund for Distinguished Young

vealed in depth. It is a profound finding that the

Scholars of China (Grant No. 51825505), the Na-

stable zigzag edges of WSSe and WSe2 exhibit ex-

tional Natural Science Foundation of China (Grant

cellent ORR activities, regardless of their chemical

No.

composition. In particular, the lowest overpotential

of Frontier Science, Chinese Academy of Sciences 13

U1737214), and the Key Research Projects

(Grant No. QYZDY-SSW-JSC009). L.C.X. is spon-

hanced electrocatalytic hydrogen evolution, Adv. Mater.

sored by the National Natural Science Foundation of

25 (2013) 5807–5813. doi:10.1002/adma.201302685. [7] D. Kong, H. Wang, J. J. Cha, M. Pasta, K. J. Koski, J. Yao,

China (Nos. 11604235) and the Scientific and Tech-

Y. Cui, Synthesis of mos2 and mose2 films with vertically

nological Innovation Programs of Higher Education

aligned layers, Nano Lett. 13 (2013) 1341–1347. doi:

Institutions in Shanxi (2019L0309). The Supercom-

10.1038/NMAT3439.

puting Center at Ningbo Institute of Materials Tech-

[8] L. Yang, Z. Cai, L. Hao, L. Ran, X. Xu, Y. Dai, S. Pan,

nology are acknowledged for providing the comput-

B. Jing, J. Zou, Increase of structural defects by n doping in mos2 cross-linked with n-doped cnts/carbon for

ing resources.

enhancing charge transfer in oxygen reduction, Electrochim. Acta 283 (2018) 448–458. doi:10.1016/j. electacta.2018.06.152.

References

[9] Y. Wang, Y.-J. Tang, K. Zhou, Self-adjusting activity induced by intrinsic reaction intermediate in fe-n-c single-

References

atom catalysts, J. Am. Chem. Soc. 141 (2019) 14115– 14119. doi:10.1021/jacs.9b07712.

[1] I. Katsounaros, S. Cherevko, A. R. Zeradjanin, K. J. J. Mayrhofer, Oxygen electrochemistry as a cornerstone for

[10] B. Liu, K. Zhou, Recent progress on graphene-analogous

sustainable energy conversion, Angew. Chem. Int. Ed. 53

2d nanomaterials: Properties, modeling and applications,

(2014) 102–121. doi:10.1002/anie.201306588.

Prog. Mater. Sci. 100 (2019) 99–169. doi:10.1016/j. pmatsci.2018.09.004.

[2] X. Zhao, P. Pachfule, S. Li, T. Langenhahn, M. Ye, G. Tian, J. Schmidt, A. Thomas, Silica-templated cova-

[11] T. F. Jaramillo, K. P. Jørgensen, J. Bonde, J. H. N. abd

lent organic framework-derived fe-n-doped mesoporous

Sebastian Horch, I. Chorkendorff, Identification of active

carbon as oxygen reduction electrocatalyst, Chem. Mater.

edge sites for electrochemical h2 evolution from mos2

31 (2019) 3274–3280. doi:10.1021/acs.chemmater.

nanocatalysts, Science 317 (2007) 100–102. doi:10.

9b00204.

1126/science.1141483.

[3] M. K. Debe, Electrocatalyst approaches and challenges

[12] P. Raybaud, J. Hafner, G. Kresse, S. Kasztelan, H. Toul-

for automotive fuel cells, Nature 486 (2012) 43–51. doi:

hoat, Ab initio study of the h2 -h2 s/mos2 gas-solid inter-

10.1038/nature11115.

face: The nature of the catalytically active sites, J. Catal.

[4] D. R. Lide (Ed.), CRC Handbook of Chemistry and

189 (2000) 129–146. doi:10.1006/jcat.1999.2698.

Physics, 90th Edition, 2010, Ch. Abundance of Elements

[13] B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jørgensen, J. H. Nielsen, S. Horch, I. Chorkendorff, J. K. Nørskov,

in the Earth’s Crust and in the Sea, pp. 14–18. [5] Y. Jiao, Y. Zheng, M. Jaroniec, S. Z. Qiao, Design of elec-

Biomimetic hydrogen evolution: Mos2 nanoparticles as

trocatalysts for oxygen- and hydrogen-involving energy

catalyst for hydrogen evolution, J. Am. Chem. Soc. 127

conversion reactions, Chem. Soc. Rev. 44 (2015) 2060–

(2005) 5308–5309. doi:10.1021/ja0504690. [14] S. Z. Butler, S. M. Hollen, L. Cao, Y. Cui, J. A. Gupta,

2086. doi:10.1039/c4cs00470a. [6] J. Xie, H. Zhang, S. Li, R. Wang, X. Sun, M. Zhou,

H. R. Guti´errez, T. F. Heinz, S. S. Hong, J. Huang, A. F.

J. Zhou, X. W. D. Lou, Y. Xie, Defect-rich mos2 ultra-

Ismach, E. Johnston-Halperin, M. Kuno, V. V. Plashnitsa,

thin nanosheets with additional active edge sites for en-

R. D. Robinson, R. S. Ruoff, S. Salahuddin, J. Shan,

14

L. Shi, M. G. Spencer, M. Terrones, W. Windl, J. E. Gold-

[22] S. Jayabal, G. Sarany, J. Wu, Y. Liu, D. Geng, X. Meng,

berger, Progress, challenges, and opportunities in two-

Understanding the high-electrocatalytic performance of

dimensional materials beyond graphene, ACS Nano 7 (4)

two-dimensional mos2 nanosheets and their composite

(2013) 2898–2926. doi:10.1021/nn400280c.

materials, J. Mater. Chem. A 5 (2017) 24540.

[15] R. J. Britto, J. D. Benck, J. L. Young, C. Hahn, T. G.

doi:

10.1039/c7ta08327k.

Deutsch, T. F. Jaramillo, Molybdenum disulfide as a pro-

[23] W. J. Yin, B. Wen, G. Z. Nie, X. L. Wei, L. M. Liu,

tection layer and catalyst for gallium indium phosphide

Tunable dipole and carrier mobility for a few layer janus

solar water splitting photocathodes, J. Phys. Chem. Lett.

mosse structure, J. Mater. Chem. C 6 (7) (2018) 1693–

7 (2016) 2044–2049.

1700. doi:10.1039/c7tc05225a.

doi:10.1021/acs.jpclett.

[24] M. Wang, Y. Pang, D. Y. Liu, S. H. Zheng, Q. L. Song,

6b00563. [16] J. Choi, H. Zhang, H. Du, J. H. Choi, Understanding sol-

Tuning magnetism by strain and external electric field in

vent effects on the properties of two-dimensional transi-

zigzag janus mosse nanoribbons, Comp Mater Sci 146

tion metal dichalcogenides, ACS Appl. Mater. Interfaces

(2018) 240–247. doi:10.1016/j.commatsci.2018.

8 (2016) 8864–8869. doi:10.1021/acsami.6b01491.

01.044.

[17] J. Kibsgaard, Z. Chen, B. N. Reinecke, T. F. Jaramillo,

[25] H. H. Wu, Q. Meng, H. Huang, C. T. Liu, X. L.

Engineering the surface structure of mos2 to preferentially

Wang, Tuning the indirect-direct band gap transition in

expose active edge sites for electrocatalysis, Nat. Mater.

the mos2−x se x armchair nanotube by diameter modula-

11 (2012) 963–969. doi:10.1038/NMAT3439.

tion, Phys. Chem. Chem. Phys. 20 (2018) 3608. doi:

[18] J. D. Benck, Z. Chen, L. Y. Kuritzky, A. J. Forman, T. F.

10.1039/c7cp08034d.

Jaramillo, Amorphous molybdenum sulfide catalysts for

[26] A. Y. Lu, H. Y. Zhu, J. Xiao, C. P. Chuu, Y. M. Han, M. H.

electrochemical hydrogen production: Insights into the

Chiu, C. C. Cheng, C. W. Yang, K. H. Wei, Y. M. Yang,

origin of their catalytic activity, ACS Catal. 2 (2012)

Y. Wang, D. Sokaras, D. Nordlund, P. D. Yang, D. A.

19161923. doi:10.1021/cs300451q.

Muller, M. Y. Chou, X. Zhang, L. J. Li, Janus monolayers of transition metal dichalcogenides, Nat. Nanotech. 12 (8)

[19] X. Lv, W. Wei, H. Wang, B. Huang, Y. Dai, Multifunctional electrocatalyst ptm with low pt loading and high

(2017) 744–749. doi:10.1038/Nnano.2017.100.

activity towards hydrogen and oxygen electrode reac-

[27] J. Zhang, S. Jia, I. Kholmanov, L. Dong, D. Er, W. Chen,

tions: A computational study, Appl. Catal. B: Environ.

H. Guo, Z. Jin, V. B. Shenoy, L. Shi, J. Lou, Janus mono-

255 (2019) 117743. doi:10.1016/j.apcatb.

layer transition-metal dichalcogenides, ACS Nano 11 (8) (2017) 8192–8198. doi:10.1021/acsnano.7b03186.

[20] Z. Lu, J. Wang, S. Huang, Y. Hou, Y. Li, Y. Zhao, S. Mu, J. Zhang, Y. Zhao, N,b-codoped defect-rich graphitic car-

[28] D. Er, H. Ye, N. C. Frey, H. Kumar, J. Lou, V. B. Shenoy,

bon nanocages as high performance multifunctional elec-

Prediction of enhanced catalytic activity for hydrogen

trocatalysts, Nano Energy 42 (2017) 334–340.

evolution reaction in janus transition metal dichalco-

doi:

genides, Nano Lett. 18 (2018) 3943–3949.

10.1016/j.nanoen.2017.11.004. [21] T. Wang, D. Gao, J. Zhuo, Z. Zhu, P. Papakonstantinou,

doi:10.

1021/acs.nanolett.8b01335.

Y. Li, M. Li, Size-dependent enhancement of electrocat-

[29] X. C. Ma, X. Wu, H. D. Wang, Y. C. Wang, A janus

alytic oxygen-reduction and hydrogen-evolution perfor-

mosse monolayer: a potential wide solar-spectrum water-

mance of mos2 particles, Chem. Eur. J. 19 (36) (2013)

splitting photocatalyst with a low carrier recombination

11939–11948. doi:10.1002/chem.201301406.

rate, J. Mater. Chem. A 6 (5) (2018) 2295–2301. doi:

15

dynamics of solids, Comput. Mater. Sci. 120 (2016) 84–

10.1039/c7ta10015a.

93. doi:10.1016/j.commatsci.2016.04.012.

[30] C. Zhang, Y. Nie, S. Sanvito, A. Du, First-principles prediction of a room-temperature ferromagnetic janus vsse

[39] K. Mathew, V. S. C. Kolluru, S. Mula, S. N. Steinmann,

monolayer with piezoelectricity, ferroelasticity, and large

R. G. Hennig, Implicit self-consistent electrolyte model

valley polarization, Nano Lett. 19 (2019) 1366–1370.

in plane-wave density-functional theory (2016). arXiv:

doi:10.1021/acs.nanolett.8b05050.

1601.03346.

[31] G. Kresse, J. Furthm¨uller, Efficient iterative schemes for

[40] K. Mathew, R. Sundararaman, K. Letchworth-Weaver,

ab initio total-energy calculations using a plane-wave ba-

T. A. Arias, R. G. Hennig, Implicit solvation model for

sis set, Phys. Rev. B 54 (16) (1996) 11169–11186. doi:

density-functional study of nanocrystal surfaces and re-

10.1103/PhysRevB.54.11169.

action pathways, J. Chem. Phys. 140 (8) (2014) 084106.

[32] G. Kresse, D. Joubert, From ultrasoft pseudopotentials

doi:10.1063/1.4865107.

to the projector augmented-wave method, Phys. Rev. B

[41] L. F. Huang, J. R. Scully, J. M. Rondinelli, Modeling

59 (3) (1999) 1758–1775. doi:10.1103/PhysRevB.

corrosion with first-principles electrochemical phase di-

59.1758.

agrams, Annu. Rev. Mater. Res. 49 (2019) 53–77. doi:

[33] P. E. Bl¨ochl, Projector augmented-wave method, Phys.

10.1146/annurev-matsci-070218-010105. [42] J. K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist,

Rev. B 50 (24) (1994) 17953–17979. doi:10.1103/

J. R. Kitchin, T. Bligaard, H. J´onsson, Origin of the over-

PhysRevB.50.17953. [34] J. P. Perdew, K. Burke, M. Ernzerhof, Generalized gradi-

potential for oxygen reduction at a fuel-cell cathode, J.

ent approximation made simple, Phys. Rev. Lett. 77 (18)

Phys. Chem. B 108 (2004) 17886–17892. doi:10.1021/

(1996) 3865–3868. doi:10.1103/PhysRevLett.77.

jp047349j. [43] N. Yang, L. Li, J. Li, W. Ding, Z. Wei, Modulating

3865. [35] J. P. Perdew, J. A. Chevary, S. H. Vosko, K. A. Jack-

the oxygen reduction activity of heteroatom-doped car-

son, M. R. Pederson, D. J. Singh, C. Fiolhais, Atoms,

bon catalysts via the triple effect: Charge, spin density

molecules, solids, and surfaces: Applications of the gen-

and ligand effect, Chem. Sci. 9 (26) (2018) 5795–5804.

eralized gradient approximation for exchange and corre-

doi:10.1039/c8sc01801d. [44] J. Greeley, J. K. Nørskov, Combinatorial density func-

lation, Phys. Rev. B 46 (11) (1992) 6671–6687. doi:

tional theory-based screening of surface alloys for the

10.1103/PhysRevB.46.6671.

oxygen reduction reaction, J. Phys. Chem. C 113 (2009)

[36] S. Grimme, J. Antony, S. Ehrlich, H. Krieg, A consistent

4932–4939. doi:10.1021/jp808945y.

and accurate ab initio parametrization of density func-

[45] H. Zhang, Y. Tian, J. Zhao, Q. Cai, Z. Chen, Small

tional dispersion correction (dft-d) for the 94 elements h-pu, J. Chem. Phys. 132 (15) (2010) 154104.

dopants make big differences: Enhanced electrocatalytic

doi:

performance of mos2 monolayer for oxygen reduction re-

10.1063/1.3382344. [37] G. Kresse, J. Furthm¨uller, J. Hafner, Ab initio force con-

action (orr) by n- and p-doping, Electrochim. Acta 225

stant approach to phonon dispersion relations of diamond

(2017) 543–550. doi:10.1016/j.electacta.2016.

and graphite, Europhys. Lett. 32 (9) (1995) 729–734.

12.144. [46] H. Kobayashi, M. Teranishi, R. Negishi, S. ichi Naya,

doi:10.1209/0295-5075/32/9/005. [38] L.-F. Huang, X.-Z. Lu, E. Tennessen, J. M. Rondinelli, An

H. Tada, Reaction mechanism of the multiple-electron

efficient ab-initio quasiharmonic approach for the thermo-

oxygen reduction reaction on the surfaces of gold and

16

platinum nanoparticles loaded on titanium(iv) oxide, J.

tion reaction, Chem. Rev. 118 (2018) 2302–2312. doi:

Phys. Chem. Lett. 7 (2016) 5002–5007. doi:10.1021/

10.1021/acs.chemrev.7b00488. [56] Y. Jia, L. Zhang, A. Du, G. Gao, J. Chen, X. Yan, C. L.

acs.jpclett.6b02026. [47] S. Zhao, K. Wang, X. Zou, L. Gan, H. Du, C. Xu,

Brown, X. Yao, Defect graphene as a trifunctional cata-

F. Kang, W. Duan, J. Li, Group vb transition metal

lyst for electrochemical reactions, Adv. Mater. 28 (2016)

dichalcogenides for oxygen reduction reaction and strain-

9532–9538. doi:10.1002/adma.201602912. [57] U. V. W. S. Sinthika, R. Thapa, Structural and electronic

enhanced activity governed by p-orbital electrons of chalcogen, Nano Res. 12 (2019) 925–930.

descriptors of catalytic activity of graphene-based materi-

doi:10.

als: First-principles theoretical analysis, Small 14 (2018)

1007/s12274-019-2326-7.

1703609. doi:10.1002/smll.201703609.

[48] M. V. Bollinger, K. W. Jacobsen, J. K. Nørskov, Atomic

[58] D. A. Henckel, O. M. Lenz, K. M. Krishnan, B. M.

and electronic structure of mos2 nanoparticles, Phys. Rev. B 67 (2003) 085410.

Cossairt, Improved her catalysis through facile, aqueous

doi:10.1103/PhysRevB.67.

electrochemical activation of nanoscale wse2 , Nano Lett.

085410.

18 (2018) 2329–2335. doi:10.1021/acs.nanolett.

[49] R. D. Burbank, The crystal structure of alpha-monoclinic selenium, Acta Crystallographica 4 (2) (1951) 140–148.

7b05213. [59] V. Maz´anek, C. C. Mayorga-Martinez, D. Bouˇsa,

doi:10.1107/S0365110X5100043X.

Z. Sofer, M. Pumera, Wse2 nanoparticles with en-

[50] M. W. J. Chase, NIST-JANAF Thermochemical Tables,

hanced hydrogen evolution reaction prepared by bipo-

4th ed, American Institute of Physics, New York, 1998.

lar electrochemistry: application in competitive magneto-

[51] H.-P. Komsa, A. V. Krasheninnikov, Native defects in bulk and monolayer mos2 from first principles, Phys. Rev.

immunoassay, Nanoscale 10 (2018) 23149.

B 91 (12) (2015) 125304. doi:10.1103/PhysRevB.91.

1039/c8nr04670k.

doi:10.

[60] A. J. Medford, A. Vojvodic, J. S. Hummelshøj, J. Voss,

125304. [52] K. Hayashi, M. Nakahira, Stability and the equilib-

F. AbildPedersen, F. Studt, T. Bligaard, A. Nilsson, J. K.

rium selenium vapor pressure of the vse2 phase, J.

Nørskov, From the sabatier principle to a predictive theory

Solid State Chem. 24 (1978) 153–161. doi:10.1016/

of transition-metal heterogeneous catalysis, J. Catal. 328

0022-4596(78)90004-X.

(2015) 36–42. doi:10.1016/j.jcat.2014.12.033.

[53] Z. Wang, J. Zhao, Q. Caia, F. Li, Computational screening

[61] R. Dronskowski, P. E. Bl¨oechl, Crystal orbital hamil-

for high-activity mos2 monolayer-based catalysts for the

ton populations (cohp): Energy-resolved visualization of

oxygen reduction reaction via substitutional doping with

chemical bonding in solids based on density-functional

transition metal, J. Mater. Chem. A 5 (20) (2017) 9842–

calculations, J. Phys. Chem. 97 (1993) 8617–8624. doi:

9851. doi:10.1039/c7ta00577f.

10.1021/j100135a014. [62] V. L. Deringer, A. L. Tchougr´eeff, R. Dronskowski, Crys-

[54] M. T. M. Koper, Analysis of electrocatalytic reacDistinction between rate-determining

tal orbital hamilton population (cohp) analysis as pro-

and potential-determining steps, J. Solid State Elec-

jected from plane-wave basis sets, J. Phys. Chem. A 115

trochem. 17 (2) (2013) 339–344.

(2011) 21. doi:10.1021/jp202489s.

tion schemes:

doi:10.1007/

[63] S. Maintz, V. L. Deringer, A. L. Tchougr´eeff, R. Dron-

s10008-012-1918-x. [55] A. Kulkarni, S. Siahrostami, A. Patel, J. K. Nørskov, Un-

skowski, Analytic projection from plane-wave and paw

derstanding catalytic activity trends in the oxygen reduc-

wavefunctions and application to chemical-bonding anal-

17

ysis in solids, J. Comput. Chem. 34 (2013) 2557–2567. doi:10.1002/jcc.23424. [64] S. Maintz, V. L. Deringer, A. L. Tchougr´eeff, R. Dronskowski, Lobster: A tool to extract chemical bonding from plane-wave based dft, J. Comput. Chem. 37 (2016) 1030–1035. doi:10.1002/jcc.23424.

18

Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Declarations of interest: none