Subscriber access provided by CMU Libraries - http://library.cmich.edu Article 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 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication They are posted online prior to technical editing, formatting for publication and author proofing The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version 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original U.S Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties Page of 34 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 The 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 Ho Chi Minh City University of Technology, 268 Ly Thuong Kiet Street, District 10, Ho Chi Minh City, Vietnam Institute of Physical Chemistry “Ilie Murgulescu” of the Romanian Academy, Splaiul Independentei 202, Sector 6, 060021 Bucharest, Romania Institute of Atomic and Molecular Sciences, Academia Sinica, No 1, Roosevelt Road, Section 4, P.O Box 23-166, Taipei, 10617, Taiwan, ROC * Email: dnson@hcmut.edu.vn ACS Paragon Plus Environment The Journal of Physical Chemistry 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 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 ACS Paragon Plus Environment Page of 34 Page of 34 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 The Journal of Physical Chemistry 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 + (H+ + e-) → 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 ACS Paragon Plus Environment The Journal of Physical Chemistry 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 the 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 ACS Paragon Plus Environment Page of 34 Page of 34 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 The Journal of Physical Chemistry 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 ACS Paragon Plus Environment The Journal of Physical Chemistry 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 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 with the sigma value of 0.2 is used to aid the convergence of the position relaxation, but the linear tetrahedron method with ACS Paragon Plus Environment Page of 34 Page of 34 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 The Journal of Physical Chemistry 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 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) ACS Paragon Plus Environment The Journal of Physical Chemistry 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 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 (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 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 ACS Paragon Plus Environment Page of 34 Page of 34 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 The Journal of Physical Chemistry 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 Ǻ over the PdCo substrates’ surface at the preferential sites shown in Figure S1 The initial position of the hydrogen atom is 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 / = − 4.928 eV was used to calculate the adsorption energy of O* following eq From Figure 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 ACS Paragon Plus Environment The Journal of Physical Chemistry 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 34 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* + H (U ) = ∆G0 (0) + eU , (18) ∆G1 (U ) = GO* + H (U ) − GHOO* +( / ) H (U ) = ∆G1 (0) + eU , (19) ∆G2 (U ) = G2 H 2O (U ) − GHOO* +( / ) H (U ) = ∆G2 (0) + eU , (20) ∆G3 (U ) = G2 H 2O (U ) − GHO* +O* +( / ) 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 where ∆Gi(0) (i = 0, 1, 2, 3, 4, 5) are obtained by using eq Table 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 20 ACS Paragon Plus Environment Page 21 of 34 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 The Journal of Physical Chemistry 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 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 bar Table 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 On the forwarding direction 21 ACS Paragon Plus Environment The Journal of Physical Chemistry 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 22 of 34 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) > 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 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- 22 ACS Paragon Plus Environment Page 23 of 34 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 The Journal of Physical Chemistry 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 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 23 ACS Paragon Plus Environment The Journal of Physical Chemistry 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 Figure The energy barrier of the rate limiting step in the dissociative and associative mechanisms for each Co percentage Figure Relationship between the occupied d-band center with Co content and with the energy barrier of the ORR 24 ACS Paragon Plus Environment Page 24 of 34 Page 25 of 34 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 The Journal of Physical Chemistry 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 The Journal of Physical Chemistry 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 26 of 34 REFERENCES (1) Markovic, N M.; Schmidt, T J.; Stamenkovic, V.; Ross, P N Oxygen Reduction Reaction on Pt and Pt Bimetallic Surfaces: A Selective Review Fuel Cells 2001, 1, 105-116 (2) Topalov, A A.; Katsounaros, I.; Auinger, M.; Cherevko, S.; Meier, J C.; Klemm, S O.; Mayrhofer, K J J Dissolution of Platinum: Limits for the Deployment of Electrochemical Energy Conversion? 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Medlin, J W Effects of Electronic Structure Modifications on the Adsorption of Oxygen Reduction Reaction Intermediates on Model Pt(111)-Alloy Surfaces J Phys Chem C 2007, 111, 17052-17060 33 ACS Paragon Plus Environment The Journal of Physical Chemistry 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 TOC Image 34 ACS Paragon Plus Environment Page 34 of 34 ... 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. .. 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... 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,