DSpace at VNU: Effects of Co Content in Pd-Skin PdCo Alloys for Oxygen Reduction Reaction: Density Functional Theory Predictions

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

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

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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: dnson@hcmut.edu.vn

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

Pd-skin/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

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),

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

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

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 de-alloying, 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

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

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

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

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

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

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.42attempted 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 lattice-strain 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 terminated 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

bulk-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,

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 carbon-support PdCo nanoparticles where Co atoms can be at the topmost surface layer.25-32

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

Pd-skin/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

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

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

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

to aid the convergence of the position relaxation, but the linear tetrahedron method with

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

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:

S T ZPE E

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 ∆G U =−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:

U G G U

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

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 O 2 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

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

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

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

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

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 O 2 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 (T-T), 3 bridge over the hcp hollow denoted by T-B (HCP), 3

top-bridge over the fcc hollow denoted by T-B (FCC), 6 top-bridge-top-bridge (B-B), and 1 hcp

hollow-fcc hollow (HCP-FCC) Structural optimization for O2* is obtained by relaxing

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, E O2 = -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 O 2

*

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,

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

T-B (FCC) for the substrate c) The intermediate HO* + O* is also obtained with the more

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

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

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

= -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

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

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,

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)

Co content Substrate

Intermediates and its most favorable adsorption sites

O2* HOO* HO* + O* 2HO* O*+H2O

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:

The associative mechanism is more complicated in comparison to the dissociative one In

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 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

+ 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

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

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