PdCo Alloys for Oxygen Reduction

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Effects of Co Content in Pd-Skin/PdCo Alloys for Oxygen Reduction Reaction: Density Functional Theory Predictions Do Ngoc Son, Le Kim Ong, Viorel Chihaia, and Kaito Takahashi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06439 • Publication Date (Web): 01 Oct 2015 Downloaded from http://pubs.acs.org on October 6, 2015

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The Journal of Physical Chemistry

Effects of Co Content in Pd-skin/PdCo Alloys for Oxygen Reduction Reaction: Density Functional Theory Predictions

Do Ngoc Son1,*, Ong Kim Le1, Viorel Chihaia2, Kaito Takahashi3 1

Ho Chi Minh City University of Technology, 268 Ly Thuong Kiet Street, District 10, Ho

Chi Minh City, Vietnam. 2

Institute of Physical Chemistry “Ilie Murgulescu” of the Romanian Academy, Splaiul

Independentei 202, Sector 6, 060021 Bucharest, Romania. 3

Institute of Atomic and Molecular Sciences, Academia Sinica, No. 1, Roosevelt Road,

Section 4, P.O. Box 23-166, Taipei, 10617, Taiwan, ROC. *

Email: [email protected]

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ABSTRACT: Improving the slow kinetics of oxygen reduction reaction (ORR) on the cathode of the proton exchange membrane fuel cells to achieve the performance at a practical level is an important task. PdCo alloys appeared as a promising electrocatalyst. Much attention has been devoted to the study of the effects of the Co content on the ORR activity of PdCo films and PdCo/C nanoparticles where the Co atoms can be at the topmost surface layer. While Pdskin/PdCo alloys with the topmost layer formed only by Pd have been proved to provide a very high ORR activity and high durability, no researches are available in the literature for the effects of the Co content on the ORR activity of Pd-skin/PdCo alloys. Hence, the effects of the Co content on the ORR activity of Pd-skin/PdCo alloys are clarified in this work by using the density functional theory calculations and Norskov’s thermodynamic model. Our results predicted that the ORR activity increases monotonically with the increase of the Co content. This behavior is particularly different compared to the Volcano behavior previously obtained in the literature for PdCo films and PdCo/C nanoparticles.


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I. INTRODUCTION

The efficiency of proton exchange membrane fuel cells (PEMFCs) is limited mainly to the cathode side due to the slow kinetics of the oxygen reduction reaction (ORR), O2 + 4 (H+ + e-) → 2 H2O.

(1)

Platinum is a well-known but expensive cathode electrocatalyst.1 In addition, the Pt electrocatalyst is unstable under the operating conditions of the PEMFCs due to Pt dissolution caused by the surface oxide formation.2-5 Many types of electrocatalysts were proposed for the ORR,6-21 where alloys of inexpensive metals were shown to improve the ORR activity and reduce the cost compared with the Pt electrocatalyst. Investigations have indicated an improved ORR activity of the Pt-based alloys in comparison to the pure Pt.1,6-9 Recently, the Pt-free alloys have attracted much attention especially binary alloying of Pd with Co, Fe, Cr, Ni, Cu, Au.10-12 Of these alloys, Pd with Co has emerged as a good candidate that satisfies not only high activity but also high durability. Several methods have been employed to prepare the PdCo alloys for the ORR such as sputtering, electro-deposition, impregnation, micro-emulsion, electrochemical dealloying, and ultrasonic spray reaction.10,22,23 The PdCo alloys have been synthesized in two forms as films and carbon-supported nanoparticles. Generally, the ORR activity of PdCo alloys depends on the preparation methods, preparation conditions, particle sizes, morphologies, surface compositions, structures of the alloy, degree of alloying, heat treatment, and the Co content, where the Co content is one of the most important factors that directly affects the ORR activity.22-24 Many works have found that the ORR reactivity of non-treated PdCo films and carbon-supported nanoparticles is a parabola of the Co content and the maximum ORR activity was found at around 30% Co.25-32 From the theoretical point of view, Norskov et al.33 developed a description of the free-energy landscape of the ORR as a function of applied bias in combination with the density functional theory calculations and the thermodynamic data; they suggested that 3 ACS Paragon Plus Environment


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the trends in the rate of the ORR for different transition and noble metals are related to the atomic oxygen and hydroxyl adsorption energies. Wang and Balbuena34 proposed a thermodynamic guideline for the design of binary alloy catalysts for the ORR. To enhance the ORR activity, they suggested that the bimetallic catalysts must be formed from two different types of metals; one that favors the formation of OOH and the other one favors the reduction of the adsorbed O on the surface of the catalysts. Using the method of Norskov and co-workers,33 several works have been performed for studying the ORR mechanism and activity on PdCo alloys.35,36 We previously studied the Pd-skin type of PdCo alloy with 30% Co based on the stability of enthalpy of mixing.35 Furthermore, we pointed out that maximizing the number of Co atoms in the second layer of substrates significantly improves the ORR activity. Lamas and Balbuena36 discussed about possible ORR mechanisms on Pt, Pd, Pd0.75Co0.25, Pt0.75Co0.25 catalysts. Based on the Gibbs free energy profiles and the magnitude of the energy barriers, they showed that both the direct and series O2 reduction mechanisms might be operating in parallel and the highest thermodynamic barriers occur in the first hydrogenation steps for both mechanisms. Using the Hammer-Norskov d-band model that correlates the electronic structure of the surface metal to its catalytic activity,37 many investigations have successfully explained the ORR activity and the electrochemical behavior of strained surfaces and of metal overlayers; and simultaneously predicted several good alloying candidates for enhancing the ORR activity.26,38-40 Shao and co-workers found that the downshifting of the d-band center of the Pd skin is a major factor ensuring a high ORR activity of Pd2Co/C electrocatalyst. Stamenkovic et al.40 established a new approach for screening new alloying catalysts for the ORR. They showed that for Pt skins, one should select metal surfaces that bind the atomic oxygen a bit weaker than Pt. This was shown to be achieved by looking for surfaces with a down shift of the Pt d states relative to the Fermi level. Fernández and co-workers41 introduced a strategy that combines the density functional theory calculations and the scanning electrochemical microscopy for rapidly screening new electrocatalysts for the ORR and also illustrated it for the case of Pd-Co 4 ACS Paragon Plus Environment

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catalysts. The strategy goes through seven steps in which carrying out theoretical studies is an important step to support experiments for new material selection. Suo et al.42 attempted to gain insight into the Pd-alloy catalyzed ORR by combining experimental studies and DFT calculations. They reported the volcano relationship between the ORR activity and the degree of alloying, and elucidated the contrary influences of the latticestrain and surface-ligand effects. At a low surface concentration of Co, the lattice-strain effect is predominant, which weakens the metal-oxygen bonding and increases the ORR activity. At a high surface concentration of Co, the surface-ligand effect becomes significant and leads to a reduction of the ORR activity. Using DFT calculations, Li et al.43 calculated the atomic oxygen binding energy, as an ORR descriptor, on Pd-Co and Pd-Ni alloys. They found that for the bulkterminated alloys the oxygen binding energy becomes stronger with more alloying element atoms in the top surface layer, but for the Pd skin alloys the oxygen binding energy becomes weaker with more alloying element atoms in the subsurface layers. Based on the electronic structure analysis, Zuluaga and Stolbov44 found that the hybridization of dPd and dCo electronic states is the main factor controlling the electrocatalytic properties of Pd/Pd0.75Co0.25. The dPd–dCo hybridization causes low energy shift of the surface Pd d-band with respect to that for Pd(111). This shift weakens the chemical bonds between the ORR intermediates and the Pd/Pd0.75Co0.25 surface, which is favorable for the ORR reaction. Manogaran and Hwang45 studied the role of the surface–subsurface interlayer interaction in enhancing the oxygen hydrogenation towards water in Pd3Co alloy catalysts. Their work clarified that the subsurface Co atoms facilitate the ORR by lowering the activation barriers for O/OH hydrogenation; however, the Co atoms lying below the subsurface far from the surface layer have no significant involvement in the modification of the surface reactivity towards O hydrogenation. Experimental and theoretical works confirmed that the Pd-skin alloy catalysts are the key systems for improving the ORR activity.26,35,36,39-49 Many works have been performed to clarify the ORR activity versus the Co content for PdCo films and carbonsupport PdCo nanoparticles where Co atoms can be at the topmost surface layer.25-32 5 ACS Paragon Plus Environment


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Despite the fact that the Pd-skin/PdCo electrocatalysts are very stable and active for the ORR,26,35,36,39-49 no similar works are available in the literature for Pd-skin/PdCo alloy catalysts. Understanding the effects of the Co content on the ORR activity of Pdskin/PdCo electrocatalysts is of great use for rational designs of better electrocatalytic cathodes for proton exchange membrane fuel cells. Therefore, this is the topic for the present work. The density functional theory calculations within the framework of Norskov and co-workers’ model33 will be utilized to achieve our target. The remaining of this paper is organized as follows: details of computational method used in this study are given in section II. Results and discussion are presented in section III in which the searching for intermediates of the ORR on the most stable substrate of Pd-skin/PdCo electrocatalysts and on the substrate with the maximum number of Co atoms in the second layer at each Co percentage, the proposing of the ORR reaction pathways, and the constructing of free energy diagrams are reported. Finally, conclusions are provided in section IV. II. COMPUTATIONAL METHODS We use the supercell approach with a 5-layer 2×2 slab model having a vacuum space of at least 13 Å, where the first three atomic layers are allowed to fully relax during simulation. Density functional theory calculations within a plane wave basis set, the Perdew-Burke-Ernzerhof generalized gradient approximation pseudopotentials for the exchange correlation energy,50,51 and the projector-augmented-wave method for the electron-ion interactions52,53 are used for optimizing structures and calculating total energies. The plane-wave basis cutoff energy is set at 400 eV. The surface Brillouin zone integration is done by using the special point sample technique of Monkhorst and Pack54 with k-point mesh sample 7×7×1 for relaxation of atomic positions and then 13×13×1 for the total energy. Dipole corrections55,56 are also included in the simulation for periodic supercells. Methfessel−Paxton smearing57 of order 1 with the sigma value of 0.2 is used to aid the convergence of the position relaxation, but the linear tetrahedron method with


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Blöchl corrections58 is employed for the calculations of the total energy. More information about the slab model with the PdCo configurations for different Co concentrations can be found in Ref. 35. Adsorption Energy. To understand the binding strength of the reaction intermediates on different adsorption sites, the adsorption energy is calculated by using the formula: E = E[Sub+Ad] − (ESub + EAd).

(2)

Here, E[Sub+Ad] is the total energy of a substrate−adsorbate system. The total energy of the isolated substrate and that of the isolated adsorbate is denoted by ESub and EAd, respectively. Gibbs Free Energy. To understand the thermodynamic stability of the reaction intermediates, we construct free energy diagrams following the method of Norskov et al.33 In this research, we are concerned with the free energy diagrams at the equilibrium potential of 1.23 V, the standard atmospheric pressure of 1 bar, the room temperature of 300 K, and pH = 0, without corrections for double layer electrical field and water media. The free energy calculations take into account reaction energies ( ∆E ), changes of zero point energies ( ∆ZPE ), and changes of entropies ( ∆S ) by the formula: ∆G = ∆E + ∆ZPE − T∆S .

(3)

Here, ∆E and ∆ZPE are estimated from the total energies and vibrational energies that were calculated by using the Vienna Ab initio Simulation Package (VASP); and ∆S is taken from the standard table for molecules in gas phase in Ref. 33. The effects of the electrode potentials are taken into account by ∆GU = − eU , where U is the electrode potential relative to the standard hydrogen electrode. At a pH equal to 0, the Gibbs free energy with electrode potential corrections will be:

∆G (U ) = ∆G − ∆GU .

(4)



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III.

RESULTS AND DISCUSSION

Pd-skin/PdCo Substrates. The most stable structure for each Co percentage of 10, 20, 30, 40, and 60 % was found in our previous work35 and correspondingly shown in Figures 1(a)-(e). They will be used as the substrates for the ORR. Furthermore, the structures with a maximum number of Co atoms below the surface were predicted to give a very high ORR activity. Thus we selected structures presented in Figures 1(f)-(i) for 10, 20, 30, and 40 % Co, respectively. The PdCo substrates are fully optimized before studying the adsorption of the ORR intermediates. The total energy of the substrates in Figures 1 (a)-(i) correspondingly are E = -103.207, -107.121, -111.073, -114.743, 122.010, -103.193, -107.117, -110.899, and -114.435 eV. The lattice constant of the substrates are 3.88, 3.85, 3.82, 3.80, 3.72 Å for 10, 20, 30, 40, 60 % Co, respectively.

Figure 1. From a) to e) are the most stable substrates that represent the most stable structure of the Pd-skin/PdCo electrocatalysts for Co contents of 10, 20, 30, 40, and 60 %, respectively. From f) to i) correspondingly are the substrates with the maximum number of Co atoms in the underneath layer of the surface for 10, 20, 30, and 40% Co.


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Similarly to our previous work,35 the ORR is supposed to proceed through two scenarios that begin with dissociative and associative adsorptions of O2. Figure S1 (see the Supporting Information) shows possible adsorption positions of the ORR intermediates on each substrate including four top (T) sites, five bridge (B) sites, one hcp hollow (HCP) site, and one fcc hollow (FCC) site. Dissociative Adsorption of O2. In this scenario, the ORR begins with an atomic oxygen adsorption. To simulate, an oxygen atom, then a hydrogen atom will be loaded subsequently into the simulation cell. Optimized geometrical structures of ORR intermediates in this scenario are obtained by relaxing their initial structures. The initial structures are constructed with an initial position of the oxygen atom of about 2 Ǻ over the PdCo substrates’ surface at the preferential sites shown in Figure S1. The initial position of the hydrogen atom is 1 Ǻ right above the previously optimized oxygen atom (O*) and HO*. The asterisk denotes that the atom/molecule is adsorbed on the substrate surface. The corresponding total energies are obtained for the optimized structures of all possible ORR intermediates on all possible adsorption sites of the PdCo substrates for 10, 20, 30, 40, and 60 % Co. By using the eq 2, we calculate the adsorption energies of the ORR intermediates at the adsorption sites. Atomic Oxygen Adsorption. The adsorption energy of O* on each substrate is listed in Table S1 (see the Supporting Information), and is presented in Figure 2, where the total energy of an isolated oxygen atom EO = EO2 / 2 = − 4.928 eV was used to calculate the adsorption energy of O* following eq 2. From Figure 2 we find that the favorable order of atomic oxygen adsorption sites for all the most stable substrates is FCC ≥ HCP > B > T, except for the most stable substrate of 10% Co where HCP > FCC > B > T. When comparing the dashed lines with each other, we also find the favorable order of O adsorption sites as FCC ≥ HCP > B > T. This result is in agreement with previous publications,35,36 In addition, the atomic oxygen adsorption energy tends to decrease (less negative) monotonically with the increase of the Co content for the most stable substrates shown by the solid lines, this result is in good agreement in comparison with that of Ref. 9 ACS Paragon Plus Environment


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43. Moreover, the adsorption energy at T sites on all the substrates is positive. This implies that O* is unstable at T and O* may diffuse to more stable sites such as B, HCP, or FCC.

Figure 2. Adsorption energy of O* on the PdCo substrates with different Co percentages. The solid lines (or solid marks) are for the most stable substrates, while the dashed lines (or open marks) are for the substrates with the maximum number of Co atoms in the second layer of the surface. Diamonds, triangles, circles, and squares corresponds to the adsorption on T, B, HCP, and FCC.

HO* Adsorption. The optimized structures of HO* on the substrates at different adsorption sites are obtained, see Figure S2 in the Supporting Information. While HO* orients along the surface normal at FCC and HCP, it forms an angle with the surface normal at the T and B sites on the PdCo substrates.


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The adsorption energy of HO* is calculated by using eq 2, listed in Table S2 (see the Supporting Information), and presented in Figure 3, where the total energy of an isolated HO, EHO = -7.674 eV, was used to calculate the adsorption energy of HO*. Figure 3 shows that the adsorption energy of HO* decreases as the Co content increases, and that B is the most favorable site for HO* adsorption for all the most stable substrates, which is in agreement with the result of previous publications.35,36 This confirms that the change of the Co content affects the adsorption strength of reaction intermediates and subsequently has an effect on the ORR activity. Besides, the adsorption energies of HO* are nearly identical at B, HCP, and FCC on the substrates with the maximum number of Co atoms in the second layer, except that at 30 % Co the adsorption energy of HO* is significantly different at HCP in comparison to that at B and FCC, where the dashed-open triangle, circle, and square curves indicate the B, HCP, and FCC, respectively.

Figure 3. Adsorption energy of HO* as a function of the Co content. The solid lines (or solid marks) are for the most stable substrates, while the dashed lines (or open marks) are for the 11 ACS Paragon Plus Environment


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substrates with the maximum number of Co atoms in the second layer of surface. Diamonds, triangles, circles, and squares correspondingly indicate the T, B, HCP, and FCC.

Associative Adsorption of O2. This scenario starts with a molecular adsorption of O2 before proton transfers. On each substrate of the 2x2 unit cell, possible adsorption sites are 5 top-top (T-T), 3 top-bridge over the hcp hollow denoted by T-B (HCP), 3 topbridge over the fcc hollow denoted by T-B (FCC), 6 bridge-bridge (B-B), and 1 hcp hollow-fcc hollow (HCP-FCC). Structural optimization for O2* is obtained by relaxing the geometry starting with O2 placed 2 Ǻ above the substrate surface. We observe that no stable adsorption of O2 is found at B-B and HCP-FCC while it is found at T-T, T-B (HCP), and T-B (FCC) sites, see Figure S3 and Table S3 in the Supporting Information. The adsorption energy and stable sites of O2* are listed in Table S3, where the total energy of an isolated O2, EO2 = -9.856 eV, was used for calculating the adsorption energy of O2* on the substrates. Table S3 shows that T-T is the most favorable adsorption site of O2* for all the substrates, similar to that found in the previous work for 30% Co.35 We note that for substrate g) it is slightly different where T-B is the most stable site. Proton Transfer to O2*. This is the first proton transfer process in the associative adsorption scenario. To obtain the ORR intermediates, we load a hydrogen atom into the simulation cell consisting of the substrate and the previously optimized O2*, and then relax the system. The initial position of H is about 1 Å on top of either oxygen atom of O2*. The obtained structures are either HOO* or HO* + O*, see Figure S4 and Table S4 in the Supporting Information. The adsorption energy and the corresponding stable sites of these intermediates are listed in Table S4, where the total energy of an isolated HOO, EHOO = -13.296 eV, was employed to calculate the adsorption energy of HOO* and HO* +

O*. The structural parameters of the isolated HOO are O-O = 1.35 Å, O-H = 0.99 Å, and the angle of H-O-O = 104.93 o. Table S4 shows that the most stable adsorption site of HOO* depends on the Co %. It is T-T for substrates f), b), h) and d); T-B (HCP) for substrates a), g), i), and e); but 12 ACS Paragon Plus Environment

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T-B (FCC) for the substrate c). The intermediate HO* + O* is also obtained with the more negative adsorption energy compared to that of HOO* on each substrate. It suggests that HOO* is likely to dissociate and form HO* + O*. The most favorable adsorption site of HO* + O* is B-H for the substrate c); and T-H for all other substrates, where HO* adsorbs on bridge (B) or top (T) sites, while O* adsorbs always on a hollow (H) site such as FCC or HCP. Proton Transfer to HOO* and HO* + O*. This is the second proton transfer process in the associative adsorption scenario. The ORR intermediates are obtained by loading a hydrogen atom onto top of the available oxygen atom of the previously optimized HOO* and HO* + O*, and then by relaxing the system, we get the intermediates HO* + HO* (or 2HO*) and O* + H2O, see Figure S5; together with its adsorption energy listed in Table S5 (see the Supporting Information), where the total energy of an isolated HOOH, EHOOH = -18.135 eV, was used to calculate the adsorption energy of 2HO* and O*+H2O. The structural parameters of the isolated HOOH are O-O = 1.48 Å, O-H = 0.98 Å, and angles of H-O-O = 104.6o. As seen from Table S5 the most favorable adsorption site of 2HO* is B-B for the substrate e); T-B for the substrates a), b), and c); and T-T for the other substrates. The most stable site for O* + H2O is similar for all the substrates. O* is on the hollow site such as HCP or FCC, while the oxygen atom of H2O adsorbs on a top site or is desorbed away from the substrates’ surface. Clearly, if another hydrogen atom is loaded into the simulation cell of O* + H2O, O* will be hydrogenated to form HO*, and the reaction will continue to progress in the same way with the dissociative adsorption scenario but in the presence of H2O. Generally, the results showed that the Co content significantly affects the optimized structure and the adsorption energy of the ORR intermediates. Table 1 is the summary on the most favorable adsorption site and the adsorption energy of all the ORR intermediates in the associative adsorption scenario, and Figure 4 presents the corresponding adsorption energy as a function of the Co content. As seen from Figure 4,



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there are no simple trends, but the absorption energy generally decreases with the increase of the Co content.

Table 1. The most favorable adsorption site and the adsorption energy (in eV) of all the ORR intermediates in the associative adsorption scenario: Top (T-T), Top-Hollow (T-H), Top-Bridge (T-B), Bridge-Hollow (B-H), Bridge-Bridge (B-B).

Intermediates and its most favorable adsorption sites Co content

Substrate

O2*

HOO*

HO* + O*

2HO*

O*+H2O O* at hollow

10%

20%

30%

40%

60%

a)

-0.607(T-T)

-0.953 (T-B)

-2.102 (T-H) -1.665 (T-B)

-2.145 (FCC)

f)

-0.557(T-T)

-0.952(T-T)

-1.963 (T-H) -1.823 (T-T)

-2.058 (FCC)

b)

-0.389(T-T)

-0.871 (T-T)

-1.727 (T-H) -1.767 (T-B)

-2.222 (FCC)

g)

-0.220 (T-B)

-0.830 (T-B)

-1.704 (T-H) -1.634 (T-T)

-1.689 (FCC)

c)

-0.392(T-T)

-0.937 (T-B)

-1.956 (B-H) -1.644 (T-B)

-1.763 (HCP)

h)

-0.316(T-T)

-0.818 (T-T)

-1.586 (T-H) -1.592 (T-T)

-1.789 (HCP)

d)

-0.277(T-T)

-0.810 (T-T)

-1.410 (T-H) -1.566 (T-T)

------

i)

-0.277(T-T)

-0.760 (T-B)

-1.613 (T-H) -1.599 (T-T)

-1.735 (FCC)

e)

-0.211(T-T)

-0.807 (T-B)

-1.591 (T-H) -1.599 (B-B)

-1.975 (FCC)


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Figure 4. Adsorption energy of the ORR intermediates in the associative adsorption scenario at its most stable site. The solid lines (or solid marks) are for the most stable substrates, while the dashed lines (or open marks) are for the substrates with the maximum number of Co atoms in the second layer of surface. Triangles, diamons, and circles indicate HO*+O*, 2HO*, and O*+H2O, respectively.

Reaction mechanisms. Based on the obtained results for the possible intermediates and its adsorption energy, we propose two reaction mechanisms that are dissociative and associative mechanisms as follows. The dissociative mechanism will proceed through 3 intermediate steps: ½ O2 + * → O*,

(5)

O* + (H+ + e-) → HO*,

(6)

HO* + (H+ + e-) → H2O + *.

(7)



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The associative mechanism is more complicated in comparison to the dissociative one. In this mechanism, O2 will not dissociate before the hydrogenation steps. O2 + * → O2*,

(8)

O2* + (H+ + e-) → HOO*,

(9)

HOO* → HO* + O*,

(10)

(HO* + O*) + (H+ + e-) → HO* + HO*,

(11)

(HO* + O*) + (H+ + e-) → O* + H2O,

(12)

O* + (H+ + e-) → HO*,

(13)

HO* + (H+ + e-) → H2O + *.

(14)

The ORR will begin with eq 8, by a molecular adsorption of O2 to form O2* adsorbed on the substrates’ surface, and followed by eq 9, that is the first proton transfer to form HOO*. However, HOO* is less stable than HO* + O* as shown in Figure 4, and hence HOO* may decompose to HO* + O* following eq 10. The second proton transfer to HO* + O* can proceed through two possibilities: (i) a proton transfers to O* to form 2HO* as pointed by eq 11, and (ii) a proton transfers to HO* to form O* + H2O as shown by eq 12. The adsorbed oxygen atom O* on the right side of eq 12 will receive another proton to form HO*. The intermediate HO* obtained in eqs 11 and 13 gains another proton to form H2O as shown by eq 14. Steps 13 and 14 are definitely the same as steps 6 and 7 in the dissociative mechanism. Based on the results of the stable adsorption site and the adsorption energy of the ORR intermediates in the associative adsorption scenario shown in Table S5 (see the Supporting Information), we find that all intermediate steps of the associative mechanism are stable except for step 12 for substrate d) of 40 % Co. Gibbs Free Energy. Based on the method of Norskov et al.,33 the Gibbs free energy is calculated for the ORR mechanisms at the equilibrium potential of 1.23 V, the room 16 ACS Paragon Plus Environment

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temperature of 300 K, the atmosphere pressure of 1 bar, and pH = 0. The purpose of calculating the Gibbs free energy is to identify the rate limiting steps of the ORR on each substrate, then evaluate the efficiency of the Co content toward the ORR. To calculate the Gibbs free energy by using eq 4, we must calculate the reaction energy of each intermediate step via the total energies of the intermediates, and the zero point energy difference (∆ZPE) via the zero point energy (ZPE) of the ORR intermediates, that is the vibrational energy, (1/2)ħω. The vibrational energy of the ORR intermediates is listed in Table S6 in the Supporting Information.

Gibbs free energy diagram for the dissociative mechanism. The Gibbs free energy for each intermediate step in this mechanism is calculated as follows:

∆G1 (U ) = GHO* +(1/ 2 ) H (U ) − GO* + H (U ) = ∆G1 (0) + eU ,

(15)

∆G2 (U ) = GH 2O (U ) − GHO* +(1 / 2 ) H (U ) = ∆G2 (0) + eU ,

(16)

∆G0 (U ) = GH 2O (U ) − GO* + H (U ) = ∆G0 (0) + eU ,

(17)

2

2

2

2

where ∆Gi (0) (i = 0,1, 2) are obtained using eq 3. The values of the Gibbs free energy are listed in Table 2.

Table 2. Gibbs free energy of the intermediate steps in the dissociative mechanism.

Gibbs free energy in eV at equilibrium potential 1.23 V Co content

10%

Substrate ∆G1(U)

∆G2(U)

∆G0(U)

a)

0.660

0.047

0.707

f)

0.789

0.055

0.843



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20%

30%

40%

60%

Page 18 of 34

b)

0.699

-0.042

0.657

g)

0.590

-0.180

0.410

c)

0.565

-0.043

0.522

h)

0.561

-0.081

0.480

d)

0.539

-0.106

0.433

i)

0.687

-0.252

0.435

e)

0.519

-0.133

0.386


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Figure 5. The Gibbs free energy diagram for the dissociative mechanism at the equilibrium potential 1.23 V, the temperature 300 K, and the pressure 1 bar. The Gibbs free energy corresponding to the substrates a), g), h), d), and e) for 10, 20, 30, 40, and 60 % Co, respectively, are presented.

Previous studies12,35,36 found that the first hydrogenation step is the rate limiting step of the ORR shown by ∆G1 . From Table 2, we select the substrate (the most stable one or the one with the maximum number of Co atoms in second layer) having the smaller value of

∆G1 at each Co percentage and display their Gibbs free energy in Figure 5. These substrates are a), g), h), d), and e) for 10, 20, 30, 40, and 60 % Co, respectively. From left to right, the arrows show the intermediate steps that are the atomic oxygen adsorption O*, the first hydrogenation of O* to form HO*, and then the second hydrogenation of HO* to form H2O. When ∆G > 0 , chemical reactions are difficult to proceed. Considering the forward direction of the Gibbs free energy diagram in Figure 5, the first hydrogenation is the most difficult step because ∆G of this step is positive and larger compared to those of other steps, and hence, the first hydrogenation step is the rate limiting step of the ORR in the dissociative mechanism on all the PdCo substrates.12,35,36 Considering the backward direction, we find that H2O on the PdCo 10% substrate can reversely form HO* and then O* easily. This event may decrease the ORR efficiency, and hence, decrease the performance of PEMFCs. Furthermore, for 30 % Co, ∆G1 of the substrate h) is almost identical to that of the substrate c), while in our previous study35 ∆G1 of the substrate h) is lower than that of the substrate h) by approximately 0.1 eV. This difference is attributed to the accuracy level of the calculations. In this study, three upper layers of the substrate are allowed to fully relax, while in the previous work the substrate surface was fixed. However, the prediction in our previous work35 that the maximum number of Co atoms in the second layer provides a very high ORR activity is still valid for 20 and 30 % Co, and it is not valid for 10 and 40 % Co. The reason may be that at 20 and 30 % Co the PdCo alloy has the best enthalpy of mixing, see Figure 2 of Ref. 35.



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Gibbs free energy diagram for the associative mechanism. The Gibbs free energies ∆Gi(U) (i = 0, 1, 2, 3, 4, 5) for the ORR intermediate steps, which are also shown in Table 3, are calculated by using the following formulas:

∆G0 (U ) = G2 H 2O (U ) − GO* + 2 H (U ) = ∆G0 (0) + eU ,

(18)

∆G1 (U ) = GO* + 2 H (U ) − GHOO* +( 3 / 2 ) H (U ) = ∆G1 (0) + eU ,

(19)

∆G2 (U ) = G2 H 2O (U ) − GHOO* +( 3 / 2 ) H (U ) = ∆G2 (0) + eU ,

(20)

∆G3 (U ) = G2 H 2O (U ) − GHO* +O* +( 3 / 2 ) H (U ) = ∆G3 (0) + eU ,

(21)

∆G4 (U ) = G2 H 2O (U ) − GO* + H O + H (U ) = ∆G4 (0) + eU ,

(22)

∆G5 (U ) = G2 H 2O (U ) − GHO* + HO* + H (U ) = ∆G5 (0) + eU ,

(23)

2

2

2

2

2

2

2

2

2

2

where ∆Gi(0) (i = 0, 1, 2, 3, 4, 5) are obtained by using eq 3. Table 3. Gibbs free energy of the ORR intermediate steps in the associative mechanism.

Gibbs free energy in eV at equilibrium potential 1.23 V Co Content

10%

20%

30%

Substrate ∆G1(U)

∆G0(U)

∆G2(U)

∆G3(U)

∆G4(U)

∆G5(U)

a)

1.171

0.372

-0.799

0.375

0.303

-0.175

f)

1.345

0.324

-1.021

0.243

-0.127

-0.396

b)

1.046

0.159

-0.888

-0.033

0.335

-0.088

g)

1.078

-0.057

-1.136

-0.216

-0.480

-0.583

c)

0.973

0.156

-0.818

0.196

-0.070

-0.208

h)

1.166

0.120

-1.045

-0.254

-0.342

-0.528


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40%

60%

d)

0.978

0.047

-0.931

-0.336

---

-0.299

i)

1.162

-0.015

-1.177

-0.315

-0.460

-0.626

e)

0.926

-0.011

-0.937

-0.172

0.085

-0.285

Figure 6. The Gibbs free energy diagram for the associative mechanism on the substrates a), b), c), d), and e) correspondingly for 10, 20, 30, 40, and 60 % Co at the equilibrium potential 1.23 V, the room temperature 300 K, and the pressure 1 bar.

Table 3 shows that the energy barrier ∆G1 for the first hydrogenation step of the most stable substrate is smaller than that of the substrate with the maximum number of Co atoms in the second layer at each Co percentage. Therefore we present the results for the most stable substrate at each Co percentage in Figure 6. On the forwarding direction 21 ACS Paragon Plus Environment


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the rate limiting step is also at the first hydrogenation.12,35,36 On the other hand, at the proton transfer steps to O* + H2O to form 2H2O, the Pd-Co 30% substrate gives ∆G4(U) < 0, while other Co percentages give ∆G4(U) > 0. This implies these proton transfer steps are favorable for 30% Co, while difficult for other Co percentages. Considering the backward direction, we find that H2O on the Pd-Co 10%, Pd-Co 20%, and Pd-Co 60% substrates can reversely form again O*. This may hinder the performance of the substrates for the ORR.

Energy barrier of the rate limiting step. From the analyses of the obtained Gibbs free energies, we found that the highest energy barrier are achieved at the first hydrogenation steps, which is in agreement with the previous works.12,35,36 Moreover, a simple trend of the energy barrier of the first hydrogenation step was found, that is the monotonically decrease of the energy barrier as an increase of Co content, for both mechanisms as presented on Figure 7. The energy barrier of both mechanisms is in the order: Pd-Co 10% > Pd-Co 20% > Pd-Co 30% > Pd-Co 40% > Pd-Co 60%. The smaller the energy barrier the higher the ORR activity is, and hence, the ORR activity is also expected to increase monotonically as an increase of Co content. This behavior of the ORR activity for Pdskin/ PdCo electrocatalysts is rather different compared to the Volcano behavior of PdCo films and PdCo/C nanoparticles that were obtained in the literature.12,22-24 Li et al.43 used the oxygen binding energy as an ORR descriptor to predict the ORR activity. They found that the oxygen binding energy of the Pd-skin type of PdCo alloys decreases when the number of Co atoms increases, i.e. the Co content increases, in the subsurface layers. Weakening the oxygen binding energy is one of the key factors to improve the ORR activity, and hence, their result implied that the increase of the Co content can improve the ORR activity. In such sense, our result is in agreement with their findings. The Hammer-Norskov d-band model was used for elucidating the ORR activity and establishing the correlation between the electronic properties of the substrates and the ORR activity in many publications. In the d-band model, the electronic properties are characterized by the average energy of the valance d-band density of states, i.e., the d-


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band center ε d . Nevertheless, the values of ε d obtained in the past are inconsistent for several transition metals, for examples, for Pt(111) ranging from -2.25 eV to -2.7 eV59,60 while for Pd(111) being -1.83 eV61 and -1.98 eV.62 If the lower and upper bounds of the energy range for the integration for the calculation of ε d are not well-defined, it may lead to an arbitrary in the values of ε d . When working with the substrates of the same alloying elements, the values of the d-band center of the substrates with different alloying percentages are very close. This comes from the arbitrary nature of the energy bounds, and they must be improved to increase the distinguishability of the ε d values. Hyman et al.63 calculated the center of the occupied portion of the d-band, the occupied d-band center, up to the Fermi level instead of the whole energy range. The occupied d-band center relative to the Fermi level was calculated for the substrates including the pure Pd with lattice constant of 3.90 Å, a), g), h), d), e) and listed in Table S7 (see Supporting Information). The adsorption energy of the atomic oxygen and the energy barrier of the dissociative pathway are also listed. The Co content and the energy barrier in Table S7 is also presented as a function of the occupied d-band center on Figure 8. It shows a monotonic downshift of the occupied d-band center as increasing of the Co content and leading to a monotonic reduction of the energy barrier. From Table S7, we found that the adsorption energy of atomic oxygen decreases as a downshift of the occupied d-band center. This result agrees with the Hammer-Norskov d-band model.37,60,61 The weaker adsorption of atomic oxygen on the PdCo alloy implied a faster electroreduction of the ORR intermediates and hence a faster ORR kinetics, which explains the predicted behavior of the ORR activity in the present work. We also studied the occupied d-band center for different configurations of 30 % Co. However, we did not find any simple trend of the ORR activity versus the occupied d-band center due to the strong competition of the Co arrangements.



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Figure 7. The energy barrier of the rate limiting step in the dissociative and associative mechanisms for each Co percentage.

Figure 8. Relationship between the occupied d-band center with Co content and with the energy barrier of the ORR. 24 ACS Paragon Plus Environment

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IV. CONCLUSION

In this work, the effects of Co content on the ORR activity of Pd-skin/PdCo alloys were studied for the first time. Based on the obtained results from analyses of the Gibbs free energy diagrams and the energy barrier of the rate limiting steps for each Co percentage, we found that the energy barrier monotonically decreases or the ORR activity monotonically increases with the increase of the Co content. This behavior is totally different compared to that obtained in the literature for the PdCo films and the PdCo/C nanoparticles, whereas the ORR activity behaves as a parabola of the Co content. Probably, the Pd-skin/PdCo electrocatalysts are the only candidate in the PdCo-based alloys that was found to provide this simple behavior of the ORR efficiency so far.

ACKNOWLEDGMENT This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.01-2013.74. We acknowledge the usage of the computer time and software granted by the Institute of Physical Chemistry of Romanian Academy, Bucharest (HPC infrastructure developed under the projects Capacities 84 Cp/I of 15.09.2007 and INFRANANOCHEM 19/01.03.2009). KT thanks Academia Sinica, and National Center for High Performance Computing of Taiwan for the usage of supercomputer system. The authors would like to thank Dr Jen-Chang Chen at Institute of Atomic and Molecular Science - Taiwan for technical supports.

ASSOCIATED CONTENT Supporting Information. Schematic denotations of possible adsorption sites on a substrate surface; and detailed information on adsorption energy, favorable adsorption site of the ORR intermediates, and zero point energy of the ORR intermediates at the most favorable adsorption site. This material is available free of charge via the Internet at http://pubs.acs.org. 25 ACS Paragon Plus Environment


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PdCo Alloys for Oxygen Reduction

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