Multiple Roles of Oxygenated Adsorbates, Nature of Active Sites, and

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Electrocatalytic O Reduction on Pt: Multiple Roles of Oxygenated Adsorbates, Nature of Active Sites and Origin of Overpotential Junxiang Chen, Liwen Fang, Siwei Luo, Yuwen Liu, and Shengli Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01082 • Publication Date (Web): 01 Mar 2017 Downloaded from http://pubs.acs.org on March 12, 2017

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The Journal of Physical Chemistry

Electrocatalytic O2 Reduction on Pt: Multiple Roles of Oxygenated Adsorbates, Nature of Active Sites and Origin of Overpotential Junxiang Chen, Liwen Fang, Siwei Luo, Yuwen Liu and Shengli Chen* Hubei Key Laboratory of Electrochemical Power Sources, Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), Department of Chemistry, Wuhan University, Wuhan 430072, China.

ABSTRACT: This work aims to reveal the origin of the large onset overpotential and sluggish kinetics of the oxygen reduction reaction (ORR) on Pt, a standing problem that hinders the progress of fuel cell technology. By investigating various possible reaction steps and pathways through detailed DFT calculations on Pt(111) surface covered by rationalized phase structures of oxygenated adsorbates, we show that the ORR overpotential and Tafel kinetics are originated from the potential-dependent formation of a siteblocking spectator phase, 33-structured oxygen ad-atoms (O*), which co-exists with a relatively weak blocking phase, (33)R30-patterned adsorption network of hydroxyl group (OH*) and water molecule (H2O*) at ORR relevant potentials. The OH*/H2O* phase provides sites for ORR to proceed through a dissociative pathway consisting of four proton/electron transfer (PET) steps. The first step, PET-coupled O2 adsorption, is identified as the activity determining step. Different from the usual believes, we found the O2 and O* don’t directly accept proton during the reduction steps; rather, OH* and H2O* act as PET mediators to facilitate the O2 adsorption and dissociation and the O* reduction. These findings unveil the distinctly multiple roles of various oxygenated adsorbates as intermediates, spectators and PET mediators in ORR. The implications of these findings on designing Pt-based catalysts are discussed. It is concluded that the binding strength of O* impacts the ORR activity of Pt-based surface predominantly by modulating the number of the available active sites, rather than the activation barriers for the rds.

KEYWORDS: Oxygen reduction, active sites, overpotentials, Tafel kinetics, DFT calculations.

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INTRODUCTION

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The oxygen reduction reaction (ORR) on Pt is among the current research focuses in energy science, due to the seemingly indispensable role of Pt in catalyzing the ORR in fuel cells and the considerable activity and durability gap of Pt remaining to the requirements for fuel cells to be a cost-effective electricity generation technology.1-9 As the best metal catalyst, Pt-based materials still exhibit ORR onset potentials nearly 200 mV negative to the thermodynamic equilibrium potential (~1.23V vs RHE). The origin of this large overpotential remains largely elusive, which is not only a fundamental embarrassment in electrochemistry, but also hampers the electrocatalyst progress. To understand the ORR kinetics, knowledge on the nature of the rate determining step(s) (rds) and its thermodynamic/kinetic features are essential. Electrochemical measurements mostly show reaction orders of ca. 1 with respect of O2 and Tafel slopes of 100120 mV/dec at relatively large overpotentials,2-19 which have led to the proposition of the electron-transfer coupled O2 adsorption as rds, although there was a dispute on whether the proton-transfer is important or not7-12,18,19. But recent microkinetic analysis of steadystate polarization curves by Wang et al.13 has suggested the ORR is kinetically limited by the reduction of either the adsorbed oxygen atom (O*) or hydroxyl group (OH*), depending on electrode potentials that dictate their coverages. Due to the limited accessibility of electrochemical interfaces to modern surface-science techniques, density functional theory (DFT) calculations have been mainly used to gain atom-level insights into ORR mechanism in recent years.20-40 Despite the significant progress in treating electrode potential, solvent and electric-double-layer,20,21,25-33 theoretical calculations also have failed to provide unambiguous views on the rds in ORR. The calculations on pristine Pt surface mostly favoured an rds of O* or OH* reduction;20,25,26,3437

while those on surfaces covered by oxygenated adsorption phases, e.g., OH*/H2O* network,21-23,32 O*,37 or O*/OH*/H2O* net-

work27, have suggested an rds of O2 adsorption or dissociation, with or without proton transfer. 27 These calculations, however, have indicated the vital importance of oxygenated adsorbates in ORR mechanism and kinetics. The main problem is lacking a clear view on their phase structures in ORR condition. Besides, their roles in ORR are also in dispute. As reaction intermediates, the properties of oxygenated adsorbates impact the energetics of ORR steps, which has been the basis for theoretical screening of catalysts,20-24 and for explaining the Tafel slope decrease above ca. 0.85 V in terms of adsorption mode transition.7-12 In some studies, oxygenated adsorbates such as OH* and O* were proposed to affect ORR kinetics mainly through blocking surface sites.7-9,13-16,41,42 Some researchers even suggested the site-blocking OH* and/or O* produced from water dissociation are different in nature from the oxygenated intermediates in ORR. 41,42 In this work, we attempt to solve the mechanistic and kinetic puzzles in ORR electrocatalysis by investigating various possible reaction steps on Pt(111) surface bearing rationalized spectator phases of oxygenated adsorbates, the (33)R30-structured OH*/H2O* network and 33-structured O*. The two phases, which will be denoted as OH*/H2O* phase and O* phase respectively, have been suggested to form from water oxidation on Pt(111), 20-23,32,33,43-46 and will be shown in present study also to form in ORR condition. Our calculations reveal distinctly multiple roles of H 2O*, OH* and O* in ORR. Especially, we show that OH* and H2O* in OH*/H2O* network act as proton/electron transfer (PET) mediators to facilitate the O 2 adsorption and dissociation, and the O*

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reduction. Based on these findings, the ORR overpotential and kinetics can be well explained.

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RESULTS AND DISCUSSION Different Blocking Effects of the OH*/H2O* Network Phase and O* Phase on O2 adsorption. The previous studies has shown that water oxidation on Pt(111) surface in O 2-free media would yield an OH*/H2O* phase, with the OH* coverage reaching 1/3 monolayer (ML) at potential of ca. 0.85 V; the thus formed half-dissociated OH*/H2O* phase, OH*N/3H2O*N/3 (N is the total number of surface Pt atoms) is very stable, so that the OH* dissociation initially requires a potential as high as ca. 0.95 V.21,43 Once an O* forms, however, the OH*s nearby would be destabilized, which allows O* phase to grow at potentials as low as 0.85 V, forming an 33-structured O* cluster.43 This nucleation-and-growth (N&G)-like formation of O* phase on Pt(111) surface has also been demonstrated experimentally.45,46 In this case, an O* atom acts as the nucleus. As will be shown later on, O* atom produced through O2 dissociation also induces O* cluster formation. Moreover, the presence of O2 can shift the nucleation to potential as early as ca 0.85 V, thus making the OH*/H2O* networks and O* clusters coexist on Pt(111) surface at potentials between ca. 0.85 V and 1.07 V in ORR condition. This means that locally ordered OH*/H2O* networks and O* clusters would act as spectator phases in ORR. To investigate their blocking effects, we calculated the reaction free energies of various possible pathways for O2 adsorption, the starting step of the ORR, on Pt(111) surface covered by them. Molecular O2 may undergo direct or PET-coupled adsorption, with different configurations and on different surface sites (Figure 1a). We have investigated various possible O2 adsorption pathways on Pt(111) surface covered by an OH*/H2O* phase or an O* phase. The optimized adsorption structures for the energetically most favorite pathways on the two types of surface are shown in Figure 1b-d; and those for other pathways are given in Figure S1 in Supporting Information (SI). The number in each of these figures indicates the calculated standard reaction free energies (G0) for the corresponding adsorption pathway. Unless stated, G0 for PETcoupled adsorption was calculated at 0.9 V, the activity at which is used as a benchmark for assessing Pt-based ORR electrocatalysts. The calculation details are given in the section of COMPUTATIONAL DETAILS.

(a)

(b)

0.03

(c)

(d)

0.21

Figure 1. (a) End-on and top-bridge-top (t-b-t) configurations of O2* and three types of surface sites on OH*/H2O* phase-covered surface: underneath OH*s (A) and H2O*s (B) and inside OH*/H2O* hexagon (C); (b) optimized structure for the energetically most favoured O2 adsorption pathway on Pt(111) bearing an OH*/H2O* phase; (c) configuration of 1/3 ML O* phase (the arrow illustrates shifting of an O* aside to make room for O2 adsorption); (d) optimized structure for the energetically most favoured O 2 adsorption pathway on O* phase-covered surface. The numbers in (b) and (d) indicate the corresponding G0 values (in eV). Direct adsorption of O2 on the OH*/H2O* phase-covered surface may yield a tilted end-on O2 atop site C (Figure S1a), t-b-t O2



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on sites B and C (Figure S1b), or t-b-t O2 on sites A and C (Figure S1c). In the optimized structure in Figure S1b the H2O above B was slightly lifted up and pushed aside; but the hydrogen bonding network was maintained. When performing structural optimization for the t-b-t O2 in Figure S1c, the OH above A spontaneously seized a hydrogen atom from a proximate H2O. This inspired us to consider the PET-coupled O2 adsorption reaction that forms the structure in Figure 1b, namely, O2+OH*N/3H2O*N/3+H++e-OH*N/3-1H2O*N/3+1O2* (R1) in which O2 adsorption is synergistically accompanied by the reduction of OH* above site A, with the proton coming from solution. The resulting structure, OH*N/3-1H2O*N/3+1O2*, is shown in Figure 1b. This PET-coupled adsorption pathway is different from the earlier proposition that the O2 adsorption is coupled by an PET to O2 itself to form an intermediate of O2H*.10-12,23 We also considered the PET-coupled O2 adsorption pathways that forms end-on O2H atop site C (Figure S1d) or B (Figure S1e). For the later, the H2O* originally atop B would be ejected into solution. Figure S1f shows a structure with an O2H atop A, which was proposed to form in the first step of ORR in a recent study. 21-23 The formation of this intermediate structure might not be an elementary step since it actually

involves

two

PET

processes,

+ 1H2O*N/3O2*+H +eOH*N/3-1H2O*N/3O2H*.

that

are,

O2+OH*N/3H2O*N/3+H++eOH*N/3-1H2O*N/3O2*+H2Ol

and

OH*N/3-

The OH* atop A has to be protonated first, forming a water molecule entering into

solution; and then a second PET process to O2* occurs to form O2H*. According to the calculated reaction free energy values, we may conclude that various direct O2 adsorption pathways and PETcoupled adsorption pathways to form O2H structures are unfavorable on Pt (111) surface covered by an OH/H2O network, as compared with the PET-coupled adsorption pathway of R1. In this energetically most favored adsorption pathway, the t-b-t adsorption of O2 is synergistically coupled by the PET to an OH* in the OH*/H2O* network. In this way, the OH* not only acts as spectator, but also participate in reaction as PET mediator. It is noticed that R1 consists of chemically two events, namely, proton transfer and O2 adsorption, which may be expressed as OH*N/3H2O*N/3+H++e-OH*N/3-1H2O*N/3+1 (R1a) and O2+OH*N/3-1H2O*N/3+1OH*N/31H2O*N/3+1O2*

(R1b). The calculated reaction free energies for the two reactions are 0.1 eV and -0.07 eV respectively. From the

thermodynamic point of view, the proton transfer (R1a) seems to be the rate-determining step (rds). However, we think R1b should require higher kinetic barrier. One can imagine that the formation of the t-b-t O2* (Figure 1b) should involve the end-on O2* as the intermediate/transition state. The latter has a formation reaction free energy of ca. 0.47 eV (Figure S1a in SI). Furthermore, the transition from end-on configuration in Figure S1a to the t-b-t configuration in Figure 1b should have to overcome additional activation barrier. Therefore, the main kinetic barrier for R1 should be contributed by the O2 adsorption. We speculate that R1a and R1b might not be separate processes; rather, they are synergistically concerted processes. The formation of the end-on O2* inside an OH*/H2O* hexagon would destabilize the nearby OH*, as has been demonstrated in a previous study43. Therefore, the proton transfer to OH* would be facilitated. As stated above, we find that the OH* above A spontaneously seizes a hydrogen atom from a proximate H2O* when performing structural optimization for the t-b-t O2* on sites A and C (Figure S1c), which also implies the synergy between O2 adsorption and proton transfer to OH*. On the other hand, the protonation of OH* to H 2O* would cause weakened Pt-H2O* interaction and increased Pt-H2O*, thus facilitating the formation of t-b-t O2*.


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The Journal of Physical Chemistry On O* phase-covered surface, O* atoms occupy fcc hollow sites with 1/3 ML coverage (Figure 1c). The available surface sites

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for O2 adsorption all have closely neighbouring fcc O* atoms, which would result in strong repulsion interaction. Therefore, an fcc O* atom had to move aside (arrow in Figure 1c) in various adsorption pathways (see SI for details). The adsorption structures formed in these pathways (Figure 1d and Figure S1g-i) all have a more positive G0 value than that for R1. To this end, we may conclude that the O* phase inhibits O2 adsorption more strongly than the OH*/H2O* phase. In the other words, the region covered by the H2O/OH* network phase is mainly effective region for ORR. Therefore, we will only investigate the ORR mechanism on H2O/OH* network-coved surface. ORR Mechanism on OH*/H2O* Phase-covered Surface. We now investigate the possible reactions that the t-b-t O2* formed through R1 may undergo, namely, the direct dissociation into two fcc O*s or the PET-coupled dissociation as described by R2. The latter yields an fcc O*, an OH* in place of the H2O* formed in R1 (yellow-colored in Figure 2b), and a solution water molecule (H2Ol). The calculated G0 for R2 was ca. -0.74 eV, which was 0.43 eV more negative than that for the direct dissociation, suggesting R2 is preferred. The first two steps of the ORR thus produce an fcc O* inside the OH*/H2O* network (Figure 2c) and a H2Ol, with an OH* and the formed H2O* acting as PET mediators for O2 adsorption and dissociation. OH*N/3-1H2O*N/3+1O2*+H++e-OH*N/3H2O*N/3O*+H2Ol (R2)

(c*) 2 1

IS (b)

(a) H++e

2 1

O2 OH N/3H2O N/3

(c) H++e

2

-H2O

OH N/3-1H2O N/3+1O2*

1 OH N/3H2O N/3O*

(e)

(d) H++e

H++e

2

2

-H2O

1

1 OH N/3-1H2O N/3+1O*

OH N/3H2O N/3

Figure 2. Structural illustration of the optimized ORR pathway on Pt(111) covered by a half-dissociated OH*/H2O* phase. (a)(b) represents R1. The oxygen atoms in the adsorbed O2* molecule are marked as 1 and 2. The OH*s and H2O*s marked yellow are those involved in an ORR pathway. The proposed intermediate state (IS) between (b) and (c) is also given in (c*). There might be intermediate step between Figure 2b and Figure 2c. We think that the O* numbered as 1 in Figure 2b should obtain a hydrogen from the water molecule marked in yellow in Figure 2b, forming an atop OH*; and a proton from solution simultaneously transfers to the split water, forming a water molecule entering into the solution. This should yield an intermediate state



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indicated in Figure 2c*. The O* numbered as 2 then shifts to the nearby fcc site which is more favorite for O*. The calculated reaction free energies are ca. -0.29 eV for transition from Figure 2b to 2c* and -0.48 eV for that from Figure 2c* to 2c. The intermediate in Figure 2c may be reduced through protonation of the O* or an OH* (R3 and R3). The latter should be preferred because it exhibits a G0 of ca. -0.45 eV, which is 0.23 eV more negative than that of R3. The more positive G0 of R3 should be due to the saturated OH* number in the OH*/H2O* network in this intermediate; the formed OH* thus has to sit atop site C, causing repulsion between OH*s. OH*N/3H2O*N/3O*+H++e- OH*N/3+1H2O*N/3

(R3)

OH*N/3H2O*N/3O*+H++e- OH*N/3-1H2O*N/3+1O*

(R3)

OH*N/3-1H2O*N/3+1O*+H++e-OH*N/3H2O*N/3+H2Ol

(R4)

The intermediate formed in R3 (Figure 2d) would be reduced through R4, with which a 4-electron O2 reduction completes and the original OH*/H2O* network recovers (Figure 2e). Again, an OH* and the formed H2O* act as PET mediators in the final two steps for O* reduction. To this end, neither O2* nor O* directly accepts proton in the entire ORR pathway. This is fairly different from the usual belief.

6

Reaction Free Energy (eV)

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0.85 V 1.05 V 0.0 V

4

2

0 O2+OH* +

+4H +4e

O2*(t-b-t)2+H2O +

+3H +3e

O*+H2O(l)+OH* +

+2H +2e

O*+H2O(l)+H2O* +

+H +e

2H2O(l)+OH*

Reaction Coordination

Figure 3. Free energy variation for the optimized reaction pathway of the ORR at potentials of 0 V, 0.85 V and 1.05 V respectively. Figure 3 illustrates the free energy profile for the optimized ORR steps at potentials of 0 V, 0.85 V and 1.05 V respectively. The first step, i.e., the PET-coupled O2 adsorption (R1), is least favorable in energetics; while steps following it are much more exoergic. It can be imagined that R1 may involve the intrusion and configuration rearrangement of O 2 in OH*/H2O* network, while the following steps involve only proton transfer between surface oxygenated species (O*, OH*, or H2O*) or between surface oxygenated species and the hydronium ions in the double layer. Therefore, we believe R1 is the rds in ORR. It is worth mentioning that the ORR mechanism on OH/H2O phase-covered Pt(111) surface have also been investigated by Tripkovic et al21 In a recent study. These authors have created an OH vacancy in the OH/H2O network for O2 adsorption and reduction; whereas in present study the whole OH/H2O network has been maintained during the ORR. In Ref. 21, the first step of ORR also produces an adsorbed intermediate of t-b-t O2*, but the OH/H2O network breaks off at the site of O2* (Figure 5b in Ref.


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21). We think this should be also a PET coupled O2 adsorption which may be expressed as O2+OH*N/3H2O*N/3+H++e-OH*N/31H2O*N/3O2*+

H2Ol. It suggests that the O2 adsorption causes reduction of an OH* to water entering into solution; while in present

study we proposed that the produced water remains staying in the OH/H2O network (R1). We find that the Tripkovic et al’s adsorption pathway has a reaction free energy which is ca. 0.1 eV more positive than that of R1. So, we believe that the OH/H2O network should maintian during ORR. Furthermore, one can find from the present study that the existence of the OH/H2O network can greatly facilitate the ORR steps following R1 (Figure 3), owing to that the OH* (or H2O) serves as the PET mediator. That’s why R1 is identified as the rds in present study; while Tripkovic et al have concluded that the step of OH*+H++eH2O is the rds. Surface Phase Structures of Oxygenated Adsorbates in ORR condition. In above sections, we have considered various possible reduction reaction pathways in the presence of O2. Since the ORR takes place at relatively high potentials, there should also have some oxidation reactions occurring on the surface. The balance between the oxidation and reduction processes determines the surface phase structures during the ORR. In the previous study, we have found that the formation of an O* through OH* dissociation in an OH*/H2O* network could destabilize the nearby OH*s, thus facilitating the further OH* dissociation to form O* clusters.43 In the absence of O2, the initial formation of O* in the half-dissociated OH*/H2O* network requires a potential as high as ca. 0.95 V. 43 As have shown in above sections, O* also forms through R2 in the presence of O2. The thus formed O* should also destabilize the nearby OH*s. The largely negative G0 for R3, i.e., the protonation of an OH* in the intermediate produced through R2 (Figure 2c) should be due to this destabilization effect. It is also possible for the destabilized OH* to dissociate, i.e., to be oxidized. Therefore, we investigated the OH* dissociation reaction for the intermediate in Figure 2c, namely, H2ON/3OHN/3O*+H2O(l)→H2ON/3+1OHN/3-1O*+H++e (R5) The calculated G0 for R5 was ca. 0.15 eV, which is fairly more positive than that for R3. Since R5 and R3 both cause an OH*H2O* conversion in the OH*/H2O* network, the corresponding change of configuration entropy (ΔSconf) should be very similar, approximately equal to kln((1/3-θOH*)/θOH*)) under the mean-field approximation. Therefore, the reaction free energies (G=G0TΔSconf) of R5 and R3 should differ mainly by their G0 values, which indicate that R3 is much more favorable. This suggests that the intermediate in Figure 2c mainly undergo R3, forming the intermediate in Figure 2d. For the intermediate in Figure 2d, the OH* nearby O* also has the possibility to undergo dissociation, that is H2ON/3+1OHN/3-1O*+H2O(l)→H2ON/3+2OHN/3-2O*2+H++e (R6) One can find that R6 causes an OH*H2O* conversion in the OH*/H2O* network, while the reduction of this intermediate (R4) does oppositely. Therefore, the ΔSconf for R6 and R4 would be plus and minus kln((1/3-O*)/O*) respectively under the mean-field approximation (O* is O* coverage). The calculated G0 for R6 and R4 are 0.9-eU and -1.04+eU respectively, where U and e refer to electrode potential and electron charge. On this basis, R6 should have more negative G than R4 at relatively low O* coverages and high potentials.



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In fact, the intermediate in Figure 2d is identical to that produced through OH* dissociation in the half dissociated OH*/H2O*

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network in O2-free conditions (RS1 in SI);43 and R6 is the reaction for O* phase growth that have been investigated in a previous study (RS2 in SI)43. Figure S3 shows the calculated isotherm for this nucleation-and-growth (N&G)-like process. One can see from Figure S3 that R6 could have a more negative free energy than R4 when the electrode potential is above ca. 0.85 V, as far as the O* coverage is below a certain value that depends on the electrode potential (see the section S2 in SI for details). At potentials below 0.85 V, R6 would always have a more positive reaction free energy than R4. Therefore, we may conclude that O* cluster would form at potentials above ca. 0.85 V in ORR condition; while a pure OH*/H2O* network phase should cover the surface at potentials below 0.85 V. This is different from that in O2-free condition, in which a much higher potential (ca. 0.95 V) is required to for O* formation. In the presence of O2, the initial O* formation (nucleation) occurs through the dissociation of O 2 (R2). At a given potential above 0.85 V, the growth of an O* cluster would become disfavored as the O* coverage reaches certain value, making the surface covered by both O* clusters and OH*/H2O* networks. Based on the isotherm in Figure S3, a pure O* phase should form at ca.1.07 V, above which bulk oxidation may occur through the so-called place-exchanging processes.32,33,47 Origin of ORR Overpotential and Tafel Kinetics. As know from the previous section, the ORR takes place mainly in the OH*/H2O* network regions on Pt(111) surface. Since the strongly blocking O* phase takes over the surface at potentials above ca. 1.07 V, the ORR would hardly occur. This should be the reason why ORR exhibits an onset potential close to 1.1 V on Pt(111) electrode. For the same reason, the kinetics of ORR at potentials above 0.85 V should be crucially affected by the O* coverage. As will be shown in the following, the decreased Tafel slope of ORR observed in experiments can be reasonably explained by the O* isotherm. According to the results presented above, the mass-transport free current density (jk) due to ORR at a given potential U can be expressed as

jk  j *

1/ 3   exp[  (G 0 R1  G 0 R1* ) / RT ] * 1/ 3  

(1)

in which  and G0R1 refers to the O* coverage and the standard reaction free energy of R1 at U, while * and G0R1* are those at a reference potential, U*, at which jk=j*;  is the transfer coefficient (Brønsted-Evans-Polanyi constant48) describing how the change in the reaction free energy alters the activation free energy. For convenient, U* is set to 0.85 V here, at which *≈0. Because the ORR takes place at very large overpotentials, the backward reaction has been ignored here. G0R1 can be divided into chemical and electric contributions, G0c and eU, where G0c represents the free energy of the following chemical adsorption process on surface covered by the OH*/H2O* networks. O2+OH*N /3H2O*N /3+1/2H2OH*N /3-1H2O* N /3+1O2* (R5) At potentials between 0.85V and 1.07 V, the value of G0c should change little with U since the OH*/H2O* networks keep a halfdissociated structure. In this case, G0R1-G0R1*≈e(U-U*). By assuming =0.5 and T=298 K, we then have

lg jk  lg( j */(1/ 3   * ))  lg(1/ 3   )  (U  0.85) / 0.118 (2) 8 Environment ACS Paragon Plus

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2

Tafel slope ( mV / dec )

100

Current (mA/cm )

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10

1

-30

-60

-90

-120 0.85

0.90

0.95

1.00

Electrode potential (V)

0.1

0.01

0.85

0.90

0.95

1.00

Electrode Potential (V)

Figure 4. Tafel relation between current density and electrode potential for the ORR at potentials above 0.85 V. The magenta points are the experimental data from Ref. 4b, and the solid line is the fit with Eq. 2 with j*= 2.7810-2 mA/cm2 and =1/3 at 1.07 V. As an approximation, we may estimate the ~U dependence above 0.85 V by integrating the O* reduction peak in the CV obtained in non-adsorbing electrolyte at a slow potential scanning rate usually used for ORR polarization measurements (Figure S4). 49 With the integrated charge, the experimental Tafel data at potentials above 0.85 V can be reasonably fitted with Eq. 2 (Figure 45 and insert). This suggests that the significantly decreased Tafel slopes at potentials above 0.85 V are due to the decrease in active site numbers (increase in coverage of the strong blocking spectator, O*), rather than the transition from Langmuir to Termkin adsorption of oxygenated intermediates as proposed in earlier literatures.7-12 In a potential region below ca. 0.85 V, the Pt(111) surface should be covered by a global OH*/H2O* network. In this case, θ approximately equals 0 regardless of U. Eq. 2 thus predicts a potential-independent Tafel slope, 118 mV/dec. Electrochemical kinetics studies indeed have shown approximately invariant Tafel slopes for ORR in low potential region, 7-17 but slightly smaller in values than 118 mV/dec in non-adsorbing electrolyte solution.14 It is noted that the composition of OH*/H2O* network (OH* coverage) varies with potential below 0.85V,43 which could cause variation in the value of G0c. Our preliminary calculations showed a decrease of ca. 0.06 eV when moving from the half-dissociated network to that with OH* coverage of ca. 0.27 ML, which corresponds to a potential change from ca. 0.85 V to 0.8 V. Taking this G0c decrease into account, the Tafel slope should be ca. 97 mV/dec, which is fairly close to the experiment results (Figure 4).14 Thus, by forming hydrogen-bonding network, OH* and H2O* act as spectator in a single phase, which makes the numbers of the active sites keeps nearly constant in a potential region below 0.85 V despite of a continuous increase of OH* coverage up to ca. 1/3 ML. If the OH* is considered separately as a spectator species, one should expect a continuous change of Tafel slope, according to either the energetic 7-12 or site-blocking effect42 of spectator adsorbates. Implications to ORR Catalyst Design. With various oxygenated adsorbates as the intermediates, the ORR rate depends on the oxophilicity of the catalyst surface. Because the binding energies of various oxygenated adsorbates scale with each other, the ORR rate may be described with the binding energy of a single adsorbate, e.g, O*.21-23 Recent results from experiments and theory have generally suggested that a weakening of the O* binding make Pt catalyst better ORR catalyst. If one merely looks the reaction energies shown in Figure 3, a conclusion could be drawn that increasing the oxophilicity would improve the ORR performance of Pt(111),



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because it will make the reaction free energy of R1 less positive. This seems to be against the previous observations. However, as

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indicated by Eq. 1, the overall rate of the ORR on Pt(111) is affected simultaneously by the reaction free energy of the R1 (rds) and the number of the reactive sites on the surface, both of which are determined by the oxophilicity of the surface. Increasing the oxophilicity of the surface may facilitate the ORR through the reaction energetics (rate constant) of R1, but could inhibit the ORR by increasing the coverage of the strong blocking spectator of O*. In last section, we have shown that the number of O* coverage plays a major role in ORR rate at potentials above ca. 0.85 V. According to the scaling relation between binding energies of O* and OH*, the oxophilicity of surface change the binding energy of O* more severely than OH*.50 Therefore, a weakening of the O* binding is expected to improve the ORR activity. This indeed has been a general strategy in designing Pt-based ORR catalysts in recent years. The present calculations show that the binding strength of O* impacts the ORR activity of Pt-based surface predominantly by modulating the number of the available sites, rather than the activation barriers for the rds as suggested in recent theoretical studies20-23. Although OH*/H2O* network provides a platform allowing ORR to proceed, the OH*s and H2O*s still act as spectators by occupying nearly 2/3 of surface sites. Therefore, considerable space should remain to promote the ORR activity on Pt(111) surface, provided the ordered OH*/H2O* network can be broken or modified. Recent studies have reported significant enhancement of ORR activity on stepped single-crystal surfaces such as Pt(332), Pt(331), P(544), etc.,51-53 and Pt(111) surface with vacancy defects54, which may be due to the local broken of OH*/H2O* network at the step and defect sites, although alternative explanations have also been proposed.51-54 Further Discussions. It should be pointed out that, besides the stable OH*/H2O* network and ordered O* phase, there might form some meta-stable phases on catalyst surface during ORR, for examples, isolated OH*s, O* and/or their co-adsorption structures with water molecules. These meta-stable surface phase may lead to altered pathways and kinetics of the ORR. However, considering the high stability of the OH*/H2O* network and ordered O* phases, the surface should be overwhelmingly occupied by these major phases. The overall ORR pathways and kinetics would be negligibly affected by these meta-stable phases. Finally, it is worth mentioning that the DFT energy calculations in present study have been performed at a 0 K; while in practice the ORR takes place at room temperature. To calculate the free energies of various reaction steps in ORR, we have included the temperature effect on the enthalpies and entropies of gaseous molecules. For adsorbed intermediates, the temperature effect on enthalpies and entropies should be negligible at room temperature. At nonzero temperature, the energetics of electrocatalytic reaction steps might be also impacted by the dynamic change of double layer structure at interface. However, this may be also negligible for ORR. At ORR relevant potentials, the major surface adsorbates are the relatively stable spectator phases, the OH*/H 2O* networks and/or ordered O* clusters; while the reaction intermediates are only minor surface adsorbates. In this way, the change in the structure of the interfacial region beyond the surface phases (e.g., the solvation layers of surface adsorbates) may have rather minor contribution to the energetics of each step.


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CONCLUSION In summary, the present calculations have revealed the multiple roles of oxygenated adsorbates (OH*, H2O* and O*) as intermediates, spectators and PET mediators in ORR, which allows atom-level understanding on some long standing questions in ORR electrocatalysis. The main findings are detailed as follows, The oxygenated adsorbates are present in ordered phases rather than randomly distribute on Pt(111) surface in ORR conditions. The OH* and H2O* form single phase of co-adsorption networks; while the O* forms clusters. A pure OH*/H2O* co-adsorption phase should cover the surface at potentials below ca. 0.85 V; while above this potential O* clusters form through O2 dissociation with an N&G-like mechanism. The OH*/H2O* phase provides the major active sites for ORR to proceed through a dissociation mechanism consisting of four PET steps, while the initial and final two steps can be illustrated as O2+2H++2e-O*+H2Ol, and O*+2H++2e-H2Ol, respectively. The large onset overpotential of ORR is due to the full occupation of surface by the strong blocking spectator, i.e., O* phase, at ca. 1.07 V, which inhibits the O2 adsorption. And the Tafel kinetics of ORR are also explainable from the potential-dependent occupation of surface by the O* phase. Neither O2 nor O* acts as direct PET acceptors in ORR steps. Instead, OH*s and H2O* in OH*/H2O* network serve as PET mediators for O2 adsorption and dissociation, and for O* reduction. The propositions that OH*s produced from water dissociation act solely as spectators and that they are different in nature from OH*s involved in ORR seem not correct, neither is the general belief that the first and rate-determining step is the PET-coupled O2 adsorption which leads to O2H* intermediate. These findings provide insightful implications on designing Pt-based catalysts for the ORR.

COMPUTATIONAL DETAILS The calculations were performed using periodic super-cells under the framework of spin-polarized DFT with the generalized gradient approximation, Perdew-Burke-Ernzerhof functional and ultrasoft pseudopotentials. The Kohn-Sham orbitals were expanded in a plane-wave basis set with a kinetic energy cutoff of 30 Ry and the charge-density cutoff of 300 Ry. The smearing technique of Methfessel and Paxton was used to deal with the Fermi-surface effect, using a smearing parameter of 0.02 Ry. The Pt(111) electrode was modelled with a periodically repeated four-layer slab with face centered cube (fcc) structure and 3√3×3√3 size. The relatively large slab size can eliminate the effect of coverage change in the reaction energy calculations. We have shown in previous study that the calculated reaction energies for H2O*OH* and OH*O* inter-conversion converge as the slab size is larger than that with 21 surface atoms.43 In calculations, the bottom two layers of the slab were fixed to DFT-optimized equilibrium lattice constant of 4.00 Ǻ; while the top two layers were allowed to relax with the adsorbates till the Cartesian force components acting on each atom were below 10-3 Ry/Bohr and the total energy converged to within 10-5 Ry. The Brillouin-zones were sampled with a 4×4×1 k-point mesh. The PWSCF program contained in the Quantum ESPRESSO distribution55 were used to implement all the calculations. Considering that the high-spin ground state of O2 molecule is poorly described in current DFT scheme, the standard free energy of gaseous O2 was calculated from the equilibrium of 4H++4e+O2(g)2H2O (l) at potential of 1.229 V. When calculating the free energies of



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reactions involving H+ and electron, the chemical potential of H+ in solution (μH+) is substituted by the standard chemical potential of

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gaseous H2 molecule (μH2) since they are the same in value as the potential (U) is referenced to the reversible hydrogen electrode (RHE); and the free energy of electron is set to -eU. The entropies and zero-point-energies (ZPE) of H2O and H2 molecules, and the ZPE of O* were included in calculating the free energy of various surface reactions. The data are listed in Table S1.

ASSOCIATED CONTENT Supporting Information O2 adsorption pathways and adsorption structures; OH* dissociation reactions for ORR intermediates; estimation of O* isotherm from CV charge; supporting figures and tables. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * Shengli Chen: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Ministry of Science and Technology of China under the National Basic Research Program (Grant No. 2012CB932800), the National Natural Science Foundation of China (Grant No. 21633008 & 21073137).

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TOC graphic

H++e

OH N/3H2O N/3

H++e

H++e -H2O

2 1

O2

OH N/3-1H2O N/3+1O2*

H++e

2 1

-H2O

OH N/3-1H2O N/3+1O*

2 1 OH N/3H2O N/3O*

2 1 OH N/3H2O N/3


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Multiple Roles of Oxygenated Adsorbates, Nature of Active Sites, and

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