Screening of Oxygen-Reduction-Reaction-Efficient Electrocatalysts

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Screening of oxygen reduction reaction (ORR) efficient electrocatalysts based on Ag-M (M = 3d, 4d and 5d transition metals) nanoalloys: A Density functional theory (DFT) study Mahesh Datt Bhatt, Geunsik Lee, and Jae Sung Lee Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02991 • Publication Date (Web): 27 Dec 2016 Downloaded from http://pubs.acs.org on January 4, 2017

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Energy & Fuels

Screening

of

oxygen

reduction

reaction

(ORR)

efficient

electrocatalysts based on Ag-M (M = 3d, 4d and 5d transition metals) nanoalloys: A Density functional theory (DFT) study

Bhatt, Mahesh Datt1; Lee, Geunsik2; Lee, Jae Sung1* 1

School of Energy and Chemical Engineering, Ulsan National Institute of Science and

Technology, Ulsan, 689-798, Republic of Korea 2

Department of Chemistry, Ulsan National Institute of Science and Technology, Ulsan,

689-798, Republic of Korea

Abstract Due to the high cost and scarcity of Pt and Pt-based materials as electrocatalysts with high oxygen reduction reaction (ORR) performance at the carbon supported oxygen cathode of polymer electrolyte membrane fuel cells (PEMFCs), we perform a screening of ORR efficient electrocatalysts based on Ag-M nanoalloys, where M is 3d or 4d or 5d transition metal using density functional theory (DFT) methods. We consider atomic oxygen adsorption energy Eads (O) as a descriptor to explore the cheap and ORR efficient Ag-M (111) (M = 3d, 4d and 5d transition metals) surfaces in various sub-alloying configurations compared to Pt (111) surface. Our calculated results reveal that the

Ag-shelled catalysts by subsurface alloying with all 3d, 4d and 5d transition metals are more stable than pure Ag(111) by analyzing the surface energy and surface segregation energy of Ag-M alloys and consistent with Pt-M alloys sub-alloying with 3d transition metals. Moreover, the d-band center of the same

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Ag-M alloy with different sub-alloying configurations is found to be in the order of Ag-M-skin < Ag-M-subsurafce < Ag3M-mixing < pure Pt < Ag-M-overlayer in Ag-shelled catalysts sub-alloying with all 3d, 4d and 5d transition metals. We finally propose Mn, Fe and Co (3d), Zr, Mo, and Ru (4d), and Ta and W (5d) are suitable catalysts for ORR on Ag3M-mixing surfaces; and Mn, Fe and Co (3d) and Ta, W (5d) for ORR on Ag-M-overlayer surfaces based on the fact that any catalysts with the strength of atomic oxygen reduction higher (but not very high) than that of pure Pt would be a suitable catalyst for enhanced ORR, which should be confirmed by further investigating ORR mechanisms on these catalyst surfaces in alkaline media both experimentally and theoretically. Moreover, the trends of oxygen reduction activity plotted against O binding energy, relative adsorption energies of ORR intermediates and scaling relations between ORR intermediates (O, OH and OOH) also stress our proposition illustrated above. Such type of DFT investigations may open the room for the researchers working in this direction.

Key Words: Metal alloys, stability, electrocatalysts, ORR, DFT, surface-segregation *E-mail: [email protected]; [email protected]

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1. Introduction Polymer electrolyte membrane fuel cells (PEMFCs) are considered as highly efficient, high power density and environmentally friendly energy sources with various energy applications [1-4]. However, due to sluggish oxygen reduction reaction (ORR) kinetics at the carbon supported oxygen cathode of PEMFC, an electrocatalyst is required to speed-up the ORR process. Platinum (Pt) and Pt-based alloys with other transition metals [5-18] have been utilized as potential electrocatalysts for efficient, stable and durable ORR. There is hurdle in commercialization of PEMFCs due to the high cost and scarcity of Pt. Therefore, alternative cost-effective Pt-free metals and their alloys as highly ORR efficient electrocatalysts have been explored by researchers experimentally and theoretically [19-32]. Nowadays, the researchers have been interested to explore the Ag-based bimetallic alloys with other d metals as highly ORR-efficient due to the fact that Ag is 50 times cheaper than Pt [33] and has a superior electrochemical stability in alkaline media. However, Ag atoms are approximately 10 times less active than Pt atoms [34-38]. Recent studies reported that alloying of Ag with 3d transition metals such as Co, Fe, Ni, and Cu [39] and 4d transition metals such as Rh and Pd [40, 41] for efficient ORR to our best knowledge. For example, Holewinski et al. [39] explored Ag-3d transition metal alloys as robust and active ORR catalysts in alkaline media compared to Pt both experimentally and theoretically. Moreover, Roy and co-workers [40] reported the enhanced catalytic activity of Ag/Rh bimetallic nanoparticles both experimentally and theoretically. In addition, Slanac et al. [41] investigated an ensemble effect in Ag-rich Ag/Pd bimetallic nanoalloy catalysts for oxygen reduction in alkaline media experimentally. In this regard, we have strong motivation to explore the Ag-M (M =3d, 4d, 5d

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transition metals) nanoalloys as cost-effective electrocatalysts for active, stable and efficient oxygen reduction in alkaline media. We screen first the Ag-M (M= 3d, 4d, 5d transition metals) nanoalloys as ORR efficient catalysts based on atomic oxygen adsorption energy with various alloying configurations and then compare Ag-3d transition metal alloys with Pt-3d transition metal alloys to confirm for high ORR efficient Ag-M (M= 3d, 4d, 5d transition metals) alloys as electrocatalysts. In our work, the atomic oxygen adsorption energy Eads (O) is used as the descriptor of the ORR activity [16], where the weaker oxygen adsorption energy would increase the ORR activity [42]. Moreover, the stability of the Ag-M (M= 3d, 4d, 5d transition metals) alloys as electrocatalysts can be estimated in terms of surface energy and surface segregation energy. The d-band center, i.e. the average energy of the d-band [43] of the top-layer atoms of these alloy surfaces is also calculated to measure the chemical activity of metallic catalysts.

2. Computational Methodology We perform density functional theory (DFT) calculations for the slab model using Vienna Ab-initio Simulation Package (VASP) [44-47]. The spin-polarized generalized gradient approximation (GGA) is used to describe the exchange and correlation potential. The cut-off energy for the plane-wave basis set is set to 400 eV in all calculations. The Monkhorst-Pack mesh k-points (13 ╳ 13 ╳ 13) and (3 ╳ 3 ╳ 1) are used for the bulk and slab calculations respectively. The convergence criteria for the electronic self-consistent iteration and ionic relaxation are set to 0.0001 eV and 0.01eV/Å respectively. The slab alloys are modeled with a 5 layer slab with 2 ╳ 2

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supercells using BIOVIA Materials Studio Visualization Software [48]. A vacuum gap of 15 Å in the z-direction is introduced to separate two subsequent slabs. The atoms in the top 2 layers are allowed to relax, while the atoms on the remaining 3 layers are fixed at their ideal bulk positions. The adsorbate (say, O atom) is placed on the top layer of the slab at the four adsorption sites namely top, bridge, fcc and hcp. After analyzing the O atom adsorption energy at four sites, it was found that the fcc site is the most stable adsorption site. The calculated fcc lattice constant of Ag (4.10 Å) is in good agreement with the measured value 4.08 Å [49]. The calculated bond length of a gas-phase oxygen molecule (1.236 Å) in 15 Å ╳ 15 Å ╳ 15 Å box also agrees well with the experimental value 1.210 Å [50].

(a)

(b)

(c)

(d)

(e)

Fig. 1 Configurations of the slab models: (a) Ag(111), (b) Ag3M(111), (c) Ag-M (111)-overlayer, (d) Ag(111)-skin- AgM (in short, Ag-M(111)-skin), and (e) Ag(111) with a subsurface monolayer of other 3d or 4d or 5d metals (in short, Ag-M(111)-subsuraface). Here, grey and blue colored spheres represent 3d, 4d, 5d metals and Ag respectively.

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In this work, we perform DFT calculations to investigate the ORR activity and stability of Ag-shelled catalysts by subsurface alloying with 3d, 4d and 5d transition metals. We systematically and comparatively study the ORR activity of the Ag-M subsurface alloy with the various configurations Ag3M-mixing (top 2 layers), Ag-M-overlayer, Ag-M-skin (or Ag-M-mixing with Ag on top layer), Ag-M subsurface and pure Ag and then compare with pure Pt. These geometries are shown as in Fig. 1 (a-e).

Results and Discussion In order to estimate the stability of the Ag-subsurface alloy, the surface energy of the Ag-subsurface alloy relative to that of Ag(111) can be calculated as follows and compared to that of Pt(111) [39]. ∆Esurf = EAg-subsurface – EAg(111)

(1)

Where EAg-subsurface and EAg(111) represent the surface energy of the Ag subsurface alloy and Ag(111), respectively. EAg-subsurface is defined as [51] EAg-subsurface = 1/A (Eslab –NM x Ebulk M - NAg x EbulkAg) Where Eslab is the total energy of a surface of Ag-M subsurface slab, EbulkAg

(2) and EbulkM

refer to the energy per atom of the bulk Ag and bulk M, NAg and NM denote the number of atoms of Ag and M in the Ag-subsurface slab, A is the cross-sectional area of the Ag-M surface slab unit cell. In our case, we freeze the atomic coordinates of bottom 3 layers and relax only upper 2 layers of Ag-M subsurface slab. So, we consider only the upper surface area as it is asymmetric.

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In order to further illustrate the stability of the Ag-subsurface alloy, we also analyze the surface segregation energy, which can be calculated as [52] ∆Eseg= EAg-overlayer – EAg-subsurface

(2)

Here, EAg-overlayer represents the surface energy of the Ag-overlayer with a monolayer M on the top layer of Ag(111). The relative surface energy and surface segregation energy of the Ag-M-subsurface alloy (M = 3d, 4d, 5d) are shown in Table I and also in Fig. 2 (a-c) respectively.

From Table I (S1 in supporting information) and Fig. 3, it is interesting to note that a more negative value of ∆Esurf indicates that the subsurface structure is more stable than the pure Ag. As shown in Fig. 3 (a, b, c), all ∆Esurf are negative 3d, 4d, and 5d transition metals with respect to pure Ag(111) surface. In this regard, Ag-shelled catalysts by subsurface alloying with all 3d, 4d and 5d transition metals are more stable than pure Ag(111). Surface energy Segregation energy

4

0

-4

Energy (eV)

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-8

-12

-16

-20 Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

3d transition metals

(a)

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Surface energy Segregation energy

4

0

Energy (eV)

-4

-8

-12

-16

-20 Y

Zr

Nb

Mo

Tc

Ru

Rh

Pd

Cd


(b)

Surface energy Segregation energy

4

0

-4

Energy (eV)

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

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-8

-12

-16

-20 Lu

Hf

Ta

W

Re

Os

Ir

Au

Hg


(c) Fig. 2 The relative surface energy and segregation energy of the Ag-M-subsurface alloy catalysts: (a) 3d, (b) 4d, and (c) 5d transition metals.

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Moreover, the values of ∆Eseg all are positive except Zn in 3d, Cd in 4d, Ir, Au and Hg in 5d transition metals, signifying that subsurface structure is stable, which is in consistence of the previous work for Pt-shelled catalysts sub-alloying with 3d transition metals [53]. In order to measure the chemical activity of metallic catalysts, we calculate the d-band center, i.e. the average energy of the d-band of top layer atoms of Ag-M (M = 3d, 4d and 5d transition metals) alloy surface.

5

Ag_5 Ag_10 Ag_15 Ag_20

5

Pt_5 Pt_10 Pt_15 Pt_20

4

3

2

1

d-DOS of Ag-surface atoms (states/eV)

d-DOS of Pt surface atoms (states/eV)

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4

3

2

1

0

0 -10

-8

-6

-4

-2

0

2

4

6

-10

-8

-6

-4

Energy (eV)

-2

0

2

4

6

Energy (eV)

(a) (b) Fig. 3 (a) The d-dos of pure Pt surface atoms(Pt5, Pt10, Pt15, Pt20) 5-layer Pt slab and (b) d-dos of pure Ag surface atoms (Ag5, Ag10, Ag15, Ag20) in 5-layer Ag slab as an example to estimate the d-band center.

The d-band center of Pt is estimated by us is -1.84 eV, which is consistent with -1.90 eV in previous work [53] and the d-band center of Ag is estimated to be -2.91 eV. In this way, we estimate the d-band center for Ag-shelled catalysts subsurface alloying with 3d, 4d and 5d transition metals for various configurations as arranged in Table II. The d-dos of Pt surface atoms (Pt5, Pt10, Pt15, Pt20 ) in a 5-layer Pt slab and of Ag surface atoms (Ag5, Ag10, Ag15, Ag20) in a 5-layer Ag slab is shown in Fig. 3 (a, b) as an example to

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estimate the d-band center. The d-band center curves of top-layer of atoms of the Ag-shelled catalysts sub-alloying with 3d, 4d and 5d transition metals compared to the

d-band center (eV)

pure Pt(111) surface in the slab model are shown in Fig. 4 (a-c).

2

Ag3M-mixing

1

Ag-M-overlayer Ag-M-skin Ag-M-subsurface

0

-1

-2

-3

-4 Ag

Pt

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn


(a)

Ag3M-mixing Ag-M-overlayer Ag-M-skin Ag-M-subsurface

2

1

d-band center (eV)

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

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0

-1

-2

-3

-4 Ag

Pt

Y

Zr

Nb

Mo

Tc

Ru

Rh

Pd

Cd


(b)

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2


1

d-band center (eV)

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

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0

-1

-2

-3

-4 Ag

Pt

Hf

Ta

W

Re

Os

Ir

Hg

Au

5d-transition metals

(c) Fig. 4 The d-band center of the Ag-M-subsurface alloy catalysts: (a) 3d, (b) 4d, and (c) 5d transition metals.

From Table II (S2 in supporting information) and Fig. 4, it is clear that the d-band center of the same Ag-M alloy with different sub-alloying configurations is found to be in the order of Ag-M-skin < Ag-M-subsurafce < Ag3M-mixing < pure Pt < Ag-M-overlayer in Ag-shelled catalysts sub-alloying with 3d, 4d and 5d transition metals. It means that the more negative the d-band center, the weaker is Eads(O), which is in good agreement with the previous work [54]. The strength of O atom adsorption depends on the effect of electronic configuration of d-electrons (or effect of d-band center). It is suggested that the d-band center is mainly affected by whether the d shell is fully occupied or not. The fully filled d shell or closed shell is energetically stable, and it is likely to lower the d-band center. For example, when the surface layer is covered by Ag atoms such as the

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skin and the subsurface, its d-band center is close to that of pure Ag surface and rather independent of alloying element with the lowest d-band center among heterostructures. On the other hand, when an element with partially filled d shell is mixed at the surface layer, it makes the d-band center increase because some partially filled d shells are present. For the overlayer case, all the d shells at the surface layer are partially filled, thus it shows the highest d band center, where a significant decrease can be seen with increasing atomic number towards the completely filled d shell. In order to compare our calculations with the previous work on Pt [53], we also calculate the adsorption energy of O atom on pure Pt(111) and Ag(111) surface as shown in Fig. 5 (a-c). We calculate for Eads (O) for all possible sites and found that the most stable site of O adsorption is fcc.

Fig. 5 (a) The optimized structure of 5-layer slab of pure Pt(111), (b) O adsorption at pure Pt(111) surface, and (c) O adsorption at pure Ag(111) surface.

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Table III. The Eads (O) (in eV) of atomic adsorption on the pure M(111) (M=Pt, Ag) at fcc site and O-M distance (M=Pt, Ag) (in Å) at fcc site at the oxygen coverage of 0.5ML M(111)

Site

Eads (O) our work

Pt(111) Ag(111)

fcc fcc

O-M distance

previous work

-4.84 -3.99

our work

-4.69 [53] -4.00 [55]

previous work

2.06 2.09

2.05 [53] 2.17 [55]

The Eads (O) (in eV) of atomic adsorption on the pure M(111) (M=Pt, Ag) at fcc site and O-M distance (M=Pt, Ag) (in Å) at fcc site is shown in Table III. From Table III, it is clear that the calculated values of Eads (O) and O-M distance (M = Pt, Ag) are consistent with previous work. The atomic oxygen adsorption energy Eads (O) on Ag-M shelled catalysts sub-alloying with 3d, 4d and 5d transition metals for different configurations as shown in Fig. 1 (b-e) is given in Table IV (S3 in supporting information) and Fig. 6 (a-c). Ag3M-mixing Ag-M-overlayer Ag-M-skin Ag-M-subsurface

0

-2

Eads(O)(eV)

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

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-4

-6

-8

-10 Ag

Pt

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

3d transition metals including pure Ag and Pt

(a)

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0

Eads(O)(eV)

-2

-4

-6

-8

-10 Ag

Pt

Y

Zr

Nb

Mo

Tc

Ru

Rh

Pd

Cd

4d transition metals with pure Ag and Pt

(b) Ag3M-mixing Ag-M-overlayer Ag-M-skin Ag-M-subsurface

0

-2

Eads(O)(eV)

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

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-4

-6

-8

-10 Ag

Pt

Lu

Hf

Ta

W

Re

Os

Ir

Au

Hg

5d transition metals with pure Ag and Pt

(c) Fig. 6 The oxygen atom adsorption energies (in eV) for Ag-shelled catalyst subsurafces alloying with (a) 3d, (b) 4d and (c) 5d transition metals.

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From Table IV (S3 in supporting information) and Fig. 6, it is worthy to note that the strength of atomic oxygen adsorption is in inverse proportion to d-band center for all Ag-shelled catalysts sub-alloying with 3d, 4d and 5d transition metals for various alloying configurations. The strength of O atom adsorption depends on the effect of electronic configuration of d-electrons (or effect of d-band center). It is suggested that the d-band center is mainly affected by whether the d shell is fully occupied or not. The fully filled d shell or closed shell is energetically stable, and it is likely to lower the d-band center. Based on this, pure Ag(111) surface is energetically stable as it has fully filled d shell. For Ag-M (111) based catalysts, the metals with partially filled d shell may have strong O atom adsorption energy, when alloying with Ag. The number of partial d-electrons increases as we go from earlier transition metals from late transition metals and the O atom adsorption energy decreases accordingly.

According to previous studies on Pt-shelled catalysts alloying with 3d transition metals [39], any catalyst with Eads(O) higher (not very high) could not be suitable catalyst for enhanced ORR. Here, Eads(O) higher means stronger oxygen adsorption energy or more negative Eads(O). According to such theory, we can propose that Mn, Fe and Co (3d); Zr, Mo, Nb and Ru (4d); and Ta and W (5d) for Ag3M-mixing surfaces; and Mn, Fe (3d) and Ta, W (5d) for Ag-M-overlayer surfaces are suitable catalysts for efficient ORR in alkaline media in applications of fuel cells. However, ORR on all these Ag-shelled catalyst

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surfaces in alkaline media with Ag-M mixing and Ag-M-overlayer configurations should be confirmed by investigating ORR using both by experimental and DFT techniques for Ag alloying with 4d and 5d transition metals with Eads(O) larger than pure Pt(111) surface, except for Ag alloying with 3d transition metals already reported [39]. Moreover, we calculate Eads(OH) and Eads(OOH) for Ag3M-mixing configurations (M = 3d, 4d and 5d transition metals) for example (say). We first make volcano activity diagram for Eads(OH) versus Eads(O) for Ag3M-mixing configurations (M = 3d, 4d and 5d transition metals), for example. We secondly plot the linear scaling relationship for trends of adsorption energies of ORR intermediates (O, OH and OOH) of Ag-M(111) surfaces with respect to Ag(111) surface as a function of valence electrons, where M represents 3d, 4d and 5d transition metals. We finally plot Eads(O), Eads(OH) and Eads(OOH) linearly for 3d, 4d and 5d transition metals in Ag3M-mixing configurations. The aim of this investigation is to confirm our proposition of metals, which are alloying with Ag for high ORR activity. These are shown in Fig. 7 (a-g). Regarding Figs. 7 (a-c), it is interesting to note that the trends of oxygen activity plotted against O atom binding energy of pure Pt and Ag seems to be consistence as in case of activity diagram in Norskov et al. [56]. Although the trends of oxygen reduction activity of Ag-M nanoalloys plotted as a function of O atom binding energy, where M stands for 3d, 4d and 5d transition metals, our proposition metals alloying with Ag having high ORR activity in Ag3M-mixing configurations, which is in consistence with 3d transition metals [39]; however, metals for 4d and 5d should be investigated in future.

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Activity of Oxygen Reduction (Eads(OH)) (eV)

0.0 -0.5 -1.0 -1.5 -2.0 -2.5

Pt

-3.0 -3.5

AgZn

AgCo

Ag

AgNi

-4.0

AgCu

-4.5

AgFe

AgV AgCr

-5.0

AgMn

AgTi

-5.5 -6.0

AgSc

-6.5 -10

-8

-6

-4

Eads(O) (eV)

(a) 0.0

Activity of Oxygen Reduction (Eads(OH)) (eV)

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

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-0.5 -1.0 -1.5 -2.0 -2.5

Pt

-3.0

AgPd

AgRu

-3.5

AgRh

Ag

-4.0

AgMo

AgNb

-4.5 -5.0 -5.5

AgY

-6.0

AgZr

AgTc

-6.5 -10

-8

-6

-4

Eads(O) (eV)

(b)

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AgIr

0.0 -0.5

AgHg

-1.0 -1.5 -2.0 -2.5

AgAu

Pt

-3.0

AgOs

Ag

-3.5

AgW

-4.0 -4.5

AgRe

AgTa

-5.0

AgHf

-5.5 -6.0 -6.5 -10

-8

-6

-4

-2

0

2

4

Eads(O) (eV)

(c Eads(O) Adsorption energies of ORR intermediates (eV)

Activity of Oxygen Reduction (Eads(OH) (eV))

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Eads(OH) 2

Eads(OOH)

0

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-8

-10 Ag

Pt

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

Ag-M (M= 3d transition metals) alloys including Ag and Pt metals

(d)

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Adsorption energies of ORR intermediates (eV)

Eads(O) Eads(OH) 2

Eads(OOH)

0

-2

-4

-6

-8

-10 Ag

Pt

Cd

Mo

Nb

Pd

Rh

Ru

Tc

Y

Zr


(e) Adsorption energies of ORR intermediates (eV)

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

Energy & Fuels

Eads(O)

2

Eads(OH) Eads(OOH)

0

-2

-4

-6

-8

-10 Ag

Pt

Au

Hf

Hg

Ir

Os

Re

Ta

W


(f) Fig. 7 The trends of oxygen activity diagram plotted with respect to O atom 19


Energy & Fuels

binding energy for Ag-M nanoalloys under Ag3M-mixing configurations (a) M =3d, (b) M =4d, (c) M =5d transition metals and relative ORR intermediates adsorption energies for Ag-M nanoalloys under Ag3M-mixing configurations (d) M =3d, (e) M =4d, (f) M =5d transition metals.

Also, Fig. 7 (d-f) shows a proportional increase in the adsorption energies of ORR intermediates O, OH and OOH, which also stresses our propositions about metals (3d, 4d and 5d transition metals) alloying with Ag to have high ORR activity. The scaling relationships for trends of adsorption energies of ORR intermediates Eads(OH) vs Eads(O) and Eads(OH) vs Eads(O) is plotted in Figs. 8 (i-iii) respectively. Eads(OH) vs. Eads(O)

Eads(OOH) vs. Eads(O) Linear fit of Eads(OOH) vs. Eads(O)

0

0

-2

-2

Eads(OOH) (eV)

Eads(OH) (eV)

Linear fit of Eads(OH) vs. Eads(O)

Eads(OH) = 0.5 Eads (O) – 1.16 -4

Eads(OOH) = 0.59 Eads (O)

-4

-6

-6

-9

-8

-7

-6

-9

-5

-8

-7

(a)

-5

(b) (i)

Eads(OH) vs. Eads(O) Linear fit of Eads(OH) vs. Eads(O)

0

-6

Eads(O) (eV)

Eads(O) (eV)

Eads(OOH) vs. Eads(O) Linear fit of Eads(OOH) vs. Eads(O)

0

-2

Eads(OOH) (eV)

-2

Eads(OH) (eV)

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

Page 20 of 25

-4

-6

-4

Eads(OOH) = 1.21 Eads (O) + 4.77

-6

Eads(OH) = 0.61 Eads (O) – 0.57

-8

-8

-10 -9

-8

-7

-6

-5

-10

-4

-9

-8

Eads(O) (eV)

-7

-6

-5

-4

Eads(O) (eV)

(a)

(b) (ii)

20


Page 21 of 25

Eads(OH) vs. Eads(O)

1

Eads(OOH) vs. Eads(O)

1

Linear fit of Eads(OOH) vs. Eads(O)

Linear fit of Eads(OH) vs. Eads(O) 0

0

-1

Eads(OOH) (eV)

-1

Eads(OH) (eV)

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

Energy & Fuels

-2

-3

-4

Eads(OH) = 0.63 Eads (O) – 0.62

-5

-2

-3

-4

Eads(OOH) = 0.83 Eads (O) + 2.44

-5

-6

-6 -8

-6

-4

-2

-8

0

-6

-4

-2

0

Eads(O) (eV)

Eads(O) (eV)

(a)

(b) (iii)

Fig. 8 The scaling relations for trends of adsorption energies of ORR intermediates (a) Eads(OH) vs Eads(O) and (b) Eads(OH) vs Eads(O) on Ag-M(111) surfaces for (i) 3d, (ii) 4d and (iii) 5d transition metals.

From Figs. 8 (a-c), it is interesting to note that there are simple linear relations between the adsorption energies for OH vs. O and OOH vs. O for 3d, 4d and 5d transition metals alloying with Ag. The different slopes for the adsorption energies for OH vs. O and OOH vs. O for 3d, 4d and 5d transition metals alloying with Ag signifies that these metals alloying with Ag have different oxygen adsorption energy with respect to that of intermediates. Such type of scaling relations between the adsorption energies of ORR intermediates also favor our proposition about 3d, 4d and 5d transition metals for enhanced ORR activity.

Conclusion We use density functional theory (DFT) methods to explore the suitable cost-effective Ag-based alloy catalysts for oxygen reduction reaction (ORR). Our calculated results reveal the Ag-shelled catalysts by subsurface alloying with all 3d, 4d and 5d

21


Energy & Fuels

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Page 22 of 25

transition metals are more stable than pure Ag(111) by analyzing the surface energy and surface segregation energy of Ag-M alloys. In order to measure the chemical activity of metallic catalysts, we calculate the d-band center, i.e. the average energy of the d-band of top layer atoms of Ag-M (M = 3d, 4d and 5d transition metals) alloy surfaces and found that the d-band center of the same Ag-M alloy with different sub-alloying configurations is found to be in the order of Ag-M-skin < Ag-M-subsurafce < Ag3M-mixing < pure Pt < Ag-M-overlayer in Ag-shelled catalysts sub-alloying 3d, 4d and 5d transition metals. Finally, we calculate

and analyze atomic oxygen adsorption energy on Ag-M alloy catalyst surfaces. Our calculated results reveal that Mn, Fe and Co (3d), Zr, Mo, and Ru (4d), and Ta and W (5d) are suitable catalysts for ORR on Ag3M-mixing surfaces; and Mn, Fe (3d) and Ta, W (5d) for ORR on Ag-M-overlayer surfaces. Moreover, the trends of oxygen activity plotted against O binding energy and relative adsorption energies of ORR intermediates also stress our proposition illustrated above. Such type of investigations should be confirmed by further investigation for ORR on above proposed Ag-M alloy catalysts by our group and other research group else.

Acknowledgement This work was supported by the Korea Center for Artificial Photosynthesis (KCAP, 2009-0093880, 2009-0093886), Basic Science Research Program (No. 2012-017247), BK Plus Program and A3 Foresight Program, all funded by the Ministry of Science, ICT and Future Planning through the National Research Foundation (NRF) Korea. 22


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Energy & Fuels

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Abstract Figure

O2+2H2O+4e-

4OH-

Ag-M-shelledcatalysts suballoyin suballoying alloying with 3d, 4d and 5d metals

25


Screening of Oxygen-Reduction-Reaction-Efficient Electrocatalysts

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