Mechanisms of Enhanced Electrocatalytic Activity for Oxygen

Aug 11, 2015 - ... The Hong Kong University of Science & Technology, Clear Water Bay,. Kowloon ... energies along the step compared to the terrace pla...
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Mechanisms of Enhanced Electrocatalytic Activity for Oxygen Reduction Reaction on High-Index Platinum n(111)−(111) Surfaces Jeffrey Yue,† Zheng Du,‡ and Minhua Shao*,† †

Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong ‡ National Supercomputing Center in Shenzhen, Shenzhen, Guangdong 518055, P.R. China ABSTRACT: Oxygen reduction reactions (ORRs) on high-index planes of Pt n(111)−(111) were studied by density functional theory (DFT). The stepped surfaces, where n = 2, 3, and 4, showed that O2, O, and OH exhibited higher binding energies along the step compared to the terrace plane. The Pt atoms along the step can become distorted through the binding of the O and OH, where the shift in position of the Pt atoms is the largest along the stepped sites, hence forming stronger bonds with O atoms. One of the two O atoms produced from the bond dissociation of O2 will push the other one down a step with lower binding energies, consequently reducing the energy required for the protonation reaction (O + H+ → OH, and OH + H+ → H2O). The quicker recovery back to the clean Pt surface would therefore improve the catalytic properties of Pt nanoparticles, especially those with exposure to high-indexed facets.

E

density, despite the higher oxygen and hydroxyl adsorption energies. Recently, researchers have introduced various experimental and theoretical techniques to study nanoparticles to enhance their efficiencies as electrocatalysts.20,21 Density functional theory (DFT) in particular have been a useful tool to predict efficiencies of the ORR by calculating the binding energies and free energy of the intermediate reaction steps.22,23 For instance, it has been successfully applied to explain the ORR activity enhancement in Pt alloys,12,21,24,25 Pt skins,26−29 and core− shell structures,30−32 as well as to predict possible reaction pathways for the ORR reaction.5,23,33−35 Although this technique allowed researchers to predict and explain the enhanced ORR process, they have been limited to the simple facets, such as (100) and (111). A deeper understanding of the high-indexed planes is necessary in order to explain the enhanced ORR activity and explore possible nanoparticle morphologies for their applications. In this study, DFT simulations of the ORR were performed on the high-index surfaces of Pt n(111)−(111). It was found that the binding energy of O2, O, and OH were the highest along the edge of the step. However, the mechanism and binding sites have shown to differ from the nonstepped surface due to the limited positions for certain atoms/molecules to adsorb. The stronger binding of one of the O atoms will force the other O atom to shift to a weaker binding sites during its protonation into OH and, hence, reducing the energy required for ORR on the stepped surfaces.

ver since the major reform in energy consumption of fuels, proton exchange membrane (PEM) fuel cells have been the major focus for their application in automotives.1,2 They are strong candidates to replace the combustion engine because they operate with advantages such as zero emission, high efficiency, lightweight, low temperature operations, and fast start-up capabilities.3,4 However, one of the major problems that needs to overcome is not the fuel itself but the sluggish cathodic oxygen reduction reaction (ORR) even on the best metallic platinum catalysts.5−8 Recently, shape-controlled nanocatalysts with particular active sites exposed for catalytic reactions have been explored.9−11 For instance, octahedral Pt nanoparticles are bound by (111) facets whereas cubic ones are bound by (100) facets. The Pt(111) surface is found to be more active than Pt(100) in ORR.12 Nanoparticles with high-index planes were also explored. The tetrahexahedron {hk0}, trapexezohedron {hkk}, and trisoctahedron {hhk} with at least one Miller index being larger than the unit have demonstrated that the open surface structures exhibit much higher activity than {111} or {100}.13 This activity enhancement has been assigned to the high density of low-coordinated atoms situated on steps, edges and kinks.14,15 Some experimental work based on high-index planes of bulk Pt crystals, such as n(111)−(111), n(111)− (100), n(100)−(111), and n(100)−(110) have been performed by Hoshi et al.16 and Feliu et al.17−19 using a rotating disk electrode technique. They proposed that the surfaces with (111) terrace had higher ORR activities than on the (100) terrace, and the most active sites were located between the edge and the terrace atomic row. Also, Feliu et al.14 found that for stepped Pt n(111)−(111) with large terraces (n ≥ 20), the inclusion of a small amount of surface steps enhanced the ORR activity that increased linearly with increasing surface step © XXXX American Chemical Society

Received: June 24, 2015 Accepted: August 11, 2015

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DOI: 10.1021/acs.jpclett.5b01345 J. Phys. Chem. Lett. 2015, 6, 3346−3351

Letter

The Journal of Physical Chemistry Letters Spin-polarized DFT calculations were performed using the commercial software CASTEP. 36 All calculations were performed using ultrasoft pseudopotentials, with kinetic energy cutoff of 340 eV. Electronic exchange and correlation effects were described within the generalized gradient approximation (GGA) using PW91 functionals. Ionic positions were optimized until Hellman−Feynman force on each ion becomes less than 0.02 eV/Å−1. Bulk Pt has a face-centered cubic lattice structure, where the lattice parameter was determined to be 4.00 Å in this work. The Pt(111) surface was modeled using a slab supercell containing five layers of Pt atoms with the top two layers relaxed and a 15 Å vacuum region. The stepped surfaces of Pt(111) were created according to their number of steps, n = 2, 3, and 4 corresponding to surfaces (110), (331), and (221), respectively.16,37 The Monkhorst−Pack fixed at 0.035 Å−1 was used to determine the k-points, corresponding to 6 × 6 × 1 for Pt(111), 5 × 4 × 1 for Pt(110), 5 × 5 × 1 for Pt(331), and 6 × 4 × 1 for Pt(221) surfaces. For the (111) and (110) surfaces, the binding energies (BE = Etotal − Eslab − Egas‑phase adsorbate) were calculated on a 2 × 2 unit cell with the adsorption of the one adsorbate (O2, O, and OH) corresponding to 1/4 ML coverage. Because the (331) and (221) surfaces are relatively larger, we believe there is sufficient space to avoid the interaction with adjacent cells. They were calculated on a 2 × 1 unit cell, such that the (331) surface consists of four fcc sites and the (221) surface consists of six fcc sites on the terrace, corresponding to 1/4 and 1/6 ML coverage, respectively. The adsorption sites being considered in this study includes the fcc (F), bridge (B), and edge step (E). The transition states of reaction were located using the Halgren−Lipscomb method, in which the linear synchronous transit (LST) maximization was performed followed by a single-line search minimization reaction pathway that was reduced to be less than 0.05 eV/Å−1. The heat of reaction (ΔE) refers to the energy difference between the final state (products) and the initial state (reactants) of the chemical reaction. The activation energy barrier (Ea) is defined to be Ea = ETS − EIS, where IS and TS refer to the reactant state and corresponding transition state, respectively. All binding energy and activation energy barrier values include zero-point energy (ZPE) corrections, which were calculated as ZPE = Σi 1/2hvi, where i’s correspond to the different modes of vibration for the respective species. Three possible mechanisms have been proposed to describe the oxygen reduction reaction: (1) oxygen dissociation mechanism, (2) peroxyl dissociation mechanism, and (3) hydrogen peroxide dissociation.20,38 These theories involve the dissociation at the oxygen−oxygen bond, that is, oxygen (O− O), peroxyl (O−OH), and hydrogen peroxide (HO−OH), followed by intermediate reactions involving atomic hydrogen (H), atomic oxygen (O), and hydroxyl (OH) to form water (H2O) on the metallic catalyst, as illustrated in eqs 1−4.38 Because that the O−O bond scission process is the most facile, and the reactions of O and OH are the significant intermediate steps, we will focus on mechanism 1 as suggested by previous studies.39 The O2 dissociation reaction can also be viewed as representative O−O bond breaking, and the hydrogenation of O can be viewed as the O−H bond formation in OOH. Both these reactions are elementary reaction steps in ORR regardless of the exact reaction intermediates.27 The analysis of the two O atoms would allow the reaction mechanism to be explained from their primitive state

O2 → O2 *

(1)

O2 * → 2O*

(2)

O* + H+ + e− → OH* +

−

OH* + (H + e ) → H 2O

(3) (4)

In order to understand the reactivity of the stepped surface of n(111)−(111), we first compare the geometry and the arrangement of the Pt atoms of the surfaces. As seen in Figure 1a, higher indexed surfaces are merely made of (111) and (110)

Figure 1. (a) Cross section of the atomic arrangement of n(111)− (111) surface. (b) Side and top view of the Pt(221) plane adsorption sites considered in the current study, which consist of oxygen (red), hydrogen (white), and platinum (aqua on the top layer; dark blue on the second layer; and black on the bottom layer).

surfaces, where the {331} is a combination of a row with (110) and (111), and {221} is a row of (110) and 2 × (111), and the {110} is a symmetrical image of a tilted {111} surface. It is believed that the kink created along these surfaces will produce unique reaction mechanisms.16 By physical observation, the Pt atoms on the {110} are relatively close together between the steps. It is possible that atoms inside the stepped region is unstable from the tight area enclosed which will be discussed later. The analysis of the first reaction step in Table 1 shows that the oxygen molecule has a significantly stronger interaction with stepped surfaces. In a typical reaction on Pt(111), the oxygen molecule will become adsorbed on the bridge site of the surface with BE of −0.80 eV, which then dissociate and bind onto the fcc sites.40,41 The fcc orientation is the most preferred binding site for atomic O because of its low BE of −4.46 eV. Moreover, the activation energy required for bond dissociation of O2 is 0.61 eV. These results are in close agreement with previous studies.40,42 However, the attraction of the O2 molecule on the stepped surfaces is much stronger with BEs of −1.67, − 1.61, and −1.64 eV for n = 2, 3, and 4, respectively. 3347

DOI: 10.1021/acs.jpclett.5b01345 J. Phys. Chem. Lett. 2015, 6, 3346−3351

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The Journal of Physical Chemistry Letters Table 1. Calculated Activation Energies, Ea, and Heat of Reaction, in Electronvolts, for the Oxygen Bond Dissociation Reaction, O2* → 2O*, on Various n(111)− (111) Surfaces surface {111}, {110}, {331}, {221},

n n n n

= = = =

∞ 2 3 4

Ea

ΔE

BE (O2)

0.61 0.09 0.19 0.16

−0.99 −0.35 −0.05 −0.15

−0.80 −1.67 −1.61 −1.64

on Pt surfaces will result in a slower ORR reaction rate.21,43 It is possible that other mechanisms play a role for the higher ORR activity on the stepped surfaces. An alternate explanation for this phenomena is that the two O atoms dissociated will occupy the F1 (fcc site down a step) and F2 (fcc site next to edge) sites, as illustrated in Figure 2. During the protonation reaction,

The binding configurations of the dissociated O atoms are also unique for the stepped surfaces (which will be explained later) where the two closest stable sites are on F1 and F2, as shown in Figure 1b. The activation energies required to break the O−O bond are significantly reduced to 0.09, 0.19, and 0.16 eV for n = 2, 3, and 4, respectively. These results suggests that the step sites tend to attract O2 more easily, as well as significantly lowering the energy barrier required to break the O−O bonds and improve the reaction rate required for ORR. The second most important factor for improving ORR is to reduce the surface coverage of O and OH on the metal surface, which inhibits the catalyst surface for further reactions. In order to recover the metal back to the clean surface, the binding energies of O and OH must be lowered such that the protonation reactions eqs 3 and 4 becomes more efficient. Tables 2 and 3 shows the calculated binding energies of O and

Figure 2. Schematic diagram of the protonation reaction (O + H+ → OH) describing the relocation of the (a) O atom on F1 toward B1 site, and (b) O atom on F2 toward E site on the Pt(221) surface, consisting of oxygen (red), hydrogen (white), and platinum (aqua on top layer; dark blue on second layer; and black on bottom layer).

both OH molecules will try to occupy the E (edge) site since it is the most stable. However, the weaker binding of the O atom at F1 (denoted as OF1) must move to the B1 to form OH (denoted as OHB1) because the site on F2 (denoted as OF2) is already occupied by the O atom and the O atom can only repel because of the close distance. Now that the two O atoms are further away, the E site is available and the OF2 can move toward the E site (denoted as OHE) with higher stability. Similar reaction mechanisms have been discussed by Bard and co-workers in their study of ORR on Pd0.89Co0.11 alloy surface.44 In their case, one O atom was strongly adsorbed on the Co fcc site, whereas the other was forced to move toward a weaker Pd fcc site. The weaker binding energy on the Pd fcc site therefore allowed the ORR to occur quickly. In other words, once the O2 molecule is dissociated only one O atom can occupy the most stable site, whereas the other is forced to occupy the second most stable site that is much weaker in comparison. This is only exceptional for the Pt(110) surface. As mentioned previously, the geometry within the step is relatively tight for O2, O, or OH to become stable. The calculations also show that all sites within the (110) including the bridges or top sites are unstable. The stepped geometry of the surface forces the atoms/molecules to repel from all these sites. The only position that is relatively stable for OH is the site between the step (B1) with BE = −2.10 eV. This site is the only stable site within the (110) geometry besides the E site, and anywhere within the hole is unstable. The similar structure within the Pt(331) and Pt(221) surfaces would also explain how the O and OH can be weaker within this area. The weaker binding on the longer bridge (B1) is possibly due to the geometry of the larger distance between Pt atoms, which creates less lattice distortion when the OH molecule is adsorbed on the Pt surface. On the other hand, the E site of {110} is an openly exposed set of Pt atoms. Such geometry may distort the Pt atoms, hence forming a stable Pt−O or Pt−OH bond.45 This is a reasonable explanation because the critical criteria for enhanced ORR activity is the weaker binding energy of the O and OH atom on the Pt surface. For example, in the core− shell systems, this has been often done by changing the core material of the nanoparticle while still using 1−2 layers of Pt skin.39,46,47 The binding energy can be significantly reduced but

Table 2. Binding Energies, in Electronvolts, of O on Various Sites on the n(111)−(111) Surfaces, n = ∞, 2, 3, and 4 surface {111} {110} {331} {221}

F1 −4.23 −4.22 −4.34

E

F2

−4.46 −4.41 −4.42

−4.46 −4.23 −4.55 −4.58

F3

−3.79 −4.27

F4

avg. coadsorbed (OF1+OF2)

−3.88

−4.26 −4.03 −4.20 −4.26

Table 3. Binding Energies, in Electronvolts, of OH on Various Sites on the n(111)−(111) Surfaces, n = ∞, 2, 3, and 4 surface

B1

E

{111} {110} {331} {221}

−2.10 −2.05 −2.03

−2.47 −2.44 −2.47

B2

B3

−2.06 −1.94 −1.87

−1.86

OH on Pt{111},{110},{331}, and {221}. The comparison of the results reveals that the most stable site is along the row of fcc next to the edge (or F2) sites, with exception to {110} where the edge (E) site is the most stable. The binding energies of O on {111} is similar to that on {110} but is slightly weaker than those on {331} (−4.55 eV) and {221} (−4.58 eV). However, we have also realized that the binding energy of the coadsorbed O atoms is lower than the single O atom. This indicates that the protonation reaction occurs more quickly straight after O−O dissociation than single O atoms. For the binding of OH, the most stable site is on the edge (E), and clearly OH binds much more strongly on all high-index facets (≥ −2.44 eV) than on {111} (−2.06 eV). However, it has been concluded in the previous studies that a stronger binding of reaction intermediates (O, OH, OOH, etc.) 3348

DOI: 10.1021/acs.jpclett.5b01345 J. Phys. Chem. Lett. 2015, 6, 3346−3351

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The Journal of Physical Chemistry Letters

site) with a high binding energy. However, one O atom will locate at the F2, whereas the other will locate at the (100) bridge after bond dissociation. DFT studies have shown that the BE of the O atom at the center of the (100) bridge is stronger than the fcc site.50 According to previous DFT calculations,33,51 the strongest adsorptions on Pt(111) and Pt(100) are at the hollow fcc and bridge site, respectively, with the former weaker than that of the later. For this reason, both O atoms dissociated near the step sites will prolong the protonation reaction into OH and also into H2O and inhibit the overall ORR reaction. It is possible that all n(100)−(111) planes has this property, which therefore reduces the electrocatalytic activity of these surfaces. Moreover, the current discussion is based on the reactions in acidic media. Under alkaline conditions, Pt(111) has been found to be more active than Pt(331).18 The OH binding energy on a certain Pt surface is pH independent. But the roles of OH species in ORR are different in alkaline and acidic solutions. In the former, OH is not only a surface poisoning species, but also a reaction intermediate during ORR. In the alkaline solution, it is simply a spectator. The adsorption of OH on steps is much stronger than that on the terrace, which causes Pt atoms on the steps inactive for ORR in alkaline media because they are easily blocked by OH. In conclusion, the DFT calculations have been performed on various sites of high-index surfaces of n(111)−(111) for the bindings of O2, O, and OH. The strongest binding sites are located on the fcc site closest to the edge for O, and at the edge for OH. However, the enhanced ORR is more efficient for stepped surfaces because one of the O atoms is forced to locate at a weaker site, which also forces the OH molecule to occupy another weaker binding site. Although one O atom has strong binding on the edge of the Pt surface, the overall reaction benefits from the lower activation energy at the edge for O2 bond dissociation, as well as weaker binding for O and OH sites as one of the O atoms moves down the step. These results are useful for the explanation of enhanced ORR activity in stepped surfaces and provide additional insights for catalytic reactions occurring near edges and kinks of nanocatalysts. Further studies including the effects from the electrolytes, electrode potentials will be carried out to have a more complete picture of ORR on high-index Pt surfaces.

still allow O2 dissociation at the same time. The (110) surface with a unique hole geometry can significantly strengthen the BE of O2 along the edge but at the same time weaken the BE of the O atom, as well as force OH onto weaker sites.48 When the B1 sites are compared for n = 2, 3, and 4, we can observe that the binding energy of OH gradually decreases with higher indexed planes (Table 2). Closer observations of the adsorption of OH with the two Pt bridge atoms reveal that this energy is related to their interactive distance. On the B1 site of Pt(110), the distances of the Pt−O bonds are measured to be 2.23 and 2.25 Å. The OH molecule is centered almost symmetrically between the two Pt atoms, where both Pt atoms appear to shift their positions slightly to adjust to form the Pt− OH−Pt interaction putting more stress along the edge of the Pt atoms.49 However, the measured distances between neighboring Pt atoms are 2.22 and 2.44 Å for Pt(331), and 2.21 and 2.52 Å for Pt(221), respectively. The larger variation in the distances with increasing steps on the surface indicates that OH will be favored to one side of the Pt atoms and become less symmetrical. Moreover, the distortion of Pt atoms along the step is less significant compared with that of Pt(110). This shows that higher order surfaces will have more stable atoms along the step, which are less willing to adjust to the adsorbing species. Therefore, the unstable geometry reduces the binding energy of the OH molecule and benefits the ORR, because the energy required for protonation can be reduced when OH is bonded toward only one Pt atoms.39 The current explanation for the ORR occurring on the stepped edge can only explain the weakened mechanism for the first O atom; however, the second O atom will occupy the E site during the protonation reaction to become OH (BE = −2.47 eV). This is possibly one of the reason why the Pt(110) is only slightly more active than the Pt(111). As seen in Table 4, the lower ΔE for the protonation reaction of OHE to H2O Table 4. Heat of Reaction ΔE of the Protonation Reaction for the Two Coadsorbed O Atoms on the n(111)−(111) Surfaces, n = ∞, 2, 3, and 4 surface {111}, n=∞ {110}, n=2 {331}, n=3 {221}, n=4

OF1* + H+→ OHB1

OHB1* + H+ → H2O

OF2* + H+ → OHE

OHE* + H+ → H2O

−0.89

−0.83

−1.39

−0.64

−0.97

−0.42

−1.02

−0.81

−0.64

−0.50

■

−1.06

−0.88

−0.69

−0.46

*E-mail: [email protected].

AUTHOR INFORMATION

Corresponding Author Notes

The authors declare no competing financial interest.

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compared to OHB1 indicates a less efficient reaction. As explained previously, the coadsorbed O atoms can lower its binding energy possibly due to repulsive forces. After the first O atom is reduced and shifts away from its original position the lower repulsive forces will cause the second O atoms to bind strongly to the surface. Although, the activation energy for bond dissociation of O2 is lower and the reduction of one of the O atoms is relatively faster as discussed above, the second O atom is still ineffective in becoming reduced to H2O. This can be one of the reasons why n(111)−(111) is more efficient than the n(100)−(111).16 For example, the (331) is quite similar to (311) except the step is a (100) on the (F1) location for (311), whereas (111) for the (331) surface. As shown previously, the O2 molecule can adsorb on the step (E

ACKNOWLEDGMENTS The authors acknowledge the financial support from the Research Grant Council of the Hong Kong Special Administrative Region (IGN13EG05) and start up fund from the Hong Kong University of Science and Technology. The authors thank Manos Mavrikakis, Luke Roling, and Lang Xu for valuable discussions and comments.

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DOI: 10.1021/acs.jpclett.5b01345 J. Phys. Chem. Lett. 2015, 6, 3346−3351