Yttria-Stabilized Zirconia Interface: A

ACS2GO © 2019. ← → → ←. loading. To add this web app to the home screen open the browser option menu and tap on Add to homescreen...
1 downloads 0 Views 2MB Size
Download PDF
J. Phys. Chem. C 2010, 114, 11209–11214

11209

Hydrogen Oxidation at the Ni/Yttria-Stabilized Zirconia Interface: A Study Based on Density Functional Theory M. Shishkin* and T. Ziegler Department of Chemistry, UniVersity of Calgary, UniVersity DriVe 2500, Calgary, Alberta T2N 1N4, Canada ReceiVed: April 5, 2010; ReVised Manuscript ReceiVed: May 14, 2010

A new mechanism for hydrogen oxidation on the nickel/yttria-stabilized zirconia (Ni/YSZ) interface is proposed based on density functional theory. The new mechanism involves oxidation of hydrogen by the oxygen atoms that are bound to both nickel and zirconium (or yttrium) at the interface. The free energy change (∆G) for this pathway is compared to ∆G for reaction steps of previously proposed oxidation mechanisms involving spillover of oxygen from YSZ to nickel or hydrogen spillover from nickel to YSZ. For all mechanisms, we consider both a stoichiometric (YSZ) surface as well as an oxygen enriched YSZ surface (YSZ+O) where in the latter case a vacant site is filled by an oxygen atom transferred from the YSZ bulk. The release of water as the final product in hydrogen oxidation is facilitated at high temperatures by entropy. The difference between the current and previous mechanisms is that for the hydrogen oxidation now we only consider the involvement of oxygen atoms that are bound to both nickel and zirconium (or yttrium). In previous studies we only considered oxygen atoms that initially were bound to zirconium (or yttrium) only. 1. Introduction Electrochemical oxidation of fuel molecules is a key process in the solid oxide fuel cell (SOFC) operation, which is known to occur in a small area close to the interface between the metal (Ni) and the oxide (yttria-stabilized zirconia) called the anode triple phase boundary (TPB).1 Recent studies using kinetic modeling2 and ab initio calculations3 indicate that fuel molecules, in particular hydrogen, generally adsorb on the Ni, rather than the YSZ surface. Surface hydrogens subsequently form a water molecule with an oxygen atom supplied by YSZ. The overall reaction can be presented as: x 2+ 2HNi + OYSZ f v H2O + VYSZ + 2eNi

(1)

where Ox and V2+ are respectively an oxygen atom and a vacancy of the YSZ oxide. Further, the charge of two electrons is formally released from the YSZ to Ni. An understanding of the mechanisms of oxidation of the surface hydrogen atoms is essential for modeling and enhancing the SOFC performance.4 Several possible scenarios for hydrogen oxidation at the nickel/yttria-stabilized zirconia (Ni/YSZ) interface have been proposed.5-11 Recent kinetic modeling studies2,12 considered three possible processes of hydrogen oxidation via the so-called spillover reactions, i.e., reactions which include migration of surface species (*O, *H, and *OH) from YSZ to the Ni surface and vice versa. These include (a) oxygen spillover from YSZ to Ni with subsequent water formation on the Ni surface,2,12 (b) spillover of two consecutive hydrogens from Ni to YSZ with water formation on the YSZ surface,12 and (c) hydroxyl formation on the YSZ surface (via hydrogen spillover from Ni to YSZ) followed by hydroxyl spillover to Ni and water formation on the Ni surface.2 Vogler et al.12 concluded that consecutive spillover of two hydrogens followed by water formation on the YSZ surface is the mechanism which correctly predicts the observed relation between polarization resistance and the partial pressures of hydrogen and water.13 By contrast

Goodwin et al.2 proposed that hydroxyl spillover with water formation on the Ni surface was able to provide an adequate fit to the experimentally observed Tafel plots.14,15 Recently we presented an ab initio study of hydrogen oxidation at the Ni/YSZ interface based on DFT calculations and a stoichiometic YSZ surface.3 It was shown that hydrogen oxidation involving oxygen atoms initially bound to zirconium (or yttrium) have a high thermodynamic barrier (>45 kcal/mol) for oxygen spillover reactions. We suggested as a result that the stoichiometric YSZ surface initially is “activated” by the migration of oxygen anions from the bulk to form an oxygen enriched YSZ surface (YSZ+O) where extrinsic vacancy sites are filled. Indeed our preliminary study has shown that such a migration is feasible with a moderate thermodynamic barrier of 20 kcal/mol.3 We found that for hydrogen oxidation involving oxygen atoms initially bound to zirconium (or yttrium), Ni/ (YSZ+O) is more active than Ni/YSZ cermet. All three types of spillover reactions (*O, *H, and *OH) have been found to be possible in the case of the Ni/(YSZ+O) system due to moderate barriers of oxidation (e26 kcal/mol). We have also shown that removal of oxygen from either the YSZ+O or YSZ surface results in accumulation of extra charge on the Ni part of the cermet, indicating a charge transfer as a result of the electrochemical reaction in eq 1.3 The previous study was limited to a single layer of YSZ to which an additional oxygen was added to produce YSZ+O. It was assumed that this oxygen came from the bulk, but the bulk as such was not included in the model, see YSZ top of Figure 1. Further, the oxygen atoms involved in the hydrogen oxidation via hydrogen spillover to the oxide were all initially bound to zirconium (or yttrium) and oxygens bound to both zirconium (or yttrium) and nickel were not considered. In this work we revisit the proposed mechanisms of hydrogen oxidation on a Ni/YSZ cermet by explicitly introducing an additional bulk layer of the oxide, which permits the study of oxygen-vacancy pair formation with oxygen migrating to the Ni/YSZ interface (see YSZ bottom of Figure 1). Moreover we shall in the present study consider hydrogen oxidation by oxygen atoms bound

10.1021/jp1030575  2010 American Chemical Society Published on Web 06/09/2010

11210

J. Phys. Chem. C, Vol. 114, No. 25, 2010

Shishkin and Ziegler tions. We rely on the approximation that entropy does not change in the absence of desorption/adsorption and therefore for these reactions ∆G ∼ ∆H. For the reaction steps that include desorption into the gas phase, the increase in entropy (∆S) has been evaluated as the sum of translational and rotational components of a desorbed molecule as it is assumed that the vibrational component, which is usually smaller than the other two, is canceled out by the vibrational component of the surface species. For evaluation of free energy, we adopt a characteristic temperature of 1000 K. 3. Model of Ni/YSZ Cermet

Figure 1. Model of Ni/YSZ:Ni cluster (blue balls) on top of the YSZ support. The Ni cluster is periodic in the direction perpendicular to the plane of the figure. The YSZ is comprised of four units with 9 mol% concentration of yttria: two bottom units (YSZ bottom) and two top units (YSZ top). Vacancies are indicated as open circles whereas yttrium atoms are denoted as green balls.

initially to zirconium (or yttrium) only in the vicinity to the TPB as well as oxygen atoms bound originally to both nickel and zirconium (or yttrium) at the TPB. We propose finally based on free energies of reaction the most prominent mechanism of hydrogen oxidation at the Ni/YSZ interface. 2. Computational Details All calculations have been performed using a periodic DFT method as implemented in the VASP code.16-18 The PBE functional has been employed for treatment of exchangecorrelation effects19 within the spin-polarized approximation.20 The PAW methodology has been applied for the description of electron-ion interactions.21,22 The Kohn-Sham orbitals have been expanded in a plane wave basis set, using cut off energies of 300 eV. We used a slab with horizontal dimensions of 12.56 × 7.25 Å and a k-point mesh of 1 × 2 × 1. The vacuum layer of 9 Å has been employed. The cut off energies, the number of k-points, and the magnitude of the vacuum layer have been found to yield well converged energies, as is described in our previous work.3 For the calculation of migrational barriers and transition state geometries, the nudged elastic band (NEB) method has been used for initial “determination” of the image trajectory,23 whereas climbing image NEB calculations have been applied for more precise calculations.24 Calculations have been considered converged if the Hellmann-Feynmann forces on the atoms drop below 0.03 eV/Å in the case of structural optimization. For evaluation of Bader charges,25 we used the program developed by Henkelman and co-workers.26-28 The relative free energy (∆G) of the surface and gas species has been evaluated as

∆G ) ∆H - T∆S

(2)

where ∆H and ∆S are the relative enthalpy and entropy of the species and T is the absolute temperature. The enthalpy and entropies are calculated relative to the reactants of eq 1 represented by two hydrogen atoms on the Ni surface of Ni/ YSZ. The enthalpy change is determined using DFT calcula-

To study the process of hydrogen oxidation on Ni/YSZ and to introduce a possible mechanism of oxygen migration from the YSZ bulk to the surface, a model of Ni/YSZ has been constructed. The YSZ part is formed from four YSZ units with 9 mol% of yttria (Figure 1), thereby consisting of four extrinsic oxygen vacancies (indicated by the open circles) and four pairs of yttrium atoms. The Ni cluster on top of YSZ is periodic in the direction normal to the plane of Figure 1 and covers only a part of the YSZ surface. Both Ni and YSZ are exposed to the gas phase by low energy (111) surfaces. The atoms of the very bottom multilayer (sandwiched by the broken lines) have been fixed, whereas positions of all other atoms have been fully optimized. Essentially our current model of the Ni/YSZ cermet differs from the one used in our previous study3 by the addition of two more units of YSZ, increasing the depth of the YSZ slab. This larger model allows us to study migration of an oxygen atom from the deeper bulk layers of YSZ (YSZ bottom, Figure 1) to the extrinsic vacancy sites of the YSZ surface (YSZ top, Figure 1). Thus, the model accounts for the formation of oxygen enriched YSZ regions in proximity to the interface with Ni. We shall indicate an oxygen enriched YSZ region by YSZ+O where the extra oxygen comes from the layer below (this layer is in turn named YSZ-O). The interface near an oxygen enriched YSZ region will be termed Ni/(YSZ+O) as opposed to an interface near a stoichiometric surface which we denote Ni/YSZ. 4. Vacancy Formation on Both the Stoichiometric and Oxygen Enriched YSZ Surfaces of Ni/YSZ In this section we shall discuss vacancy formation on Ni/ YSZ (Figure 2a). By that we will understand the removal of an oxygen atom in excess of the extrinsic vacancies of stoichiometric YSZ. We shall further differentiate between the formation of an interface vacancy (Figure 2b) and a surface vacancy (Figure 2c). Here the interface vacancy comes from the removal of an oxygen atom that is bound both to nickel and zirconium (or yttrium), whereas the formation of a surface vacancy is due to the removal of an oxygen atom bound only to zirconium (or yttrium). We start with the simple case of a stoichimetric YSZ surface before we deal with YSZ+O. We find in line with our previous study3 that the formation of an interface vacancy with ∆H ) 186 kcal/mol is more favorable than that of a surface vacancy with ∆H ) 211 kcal/ mol. These values are very close to those obtained previously with a smaller slab model where the bottom layer in Figure 1 was missing. In that study we obtained 185 and 210 kcal/mol,3 respectively. The close agreement indicates a rapid convergence of the vacancy formation energy with respect to the number of YSZ multilayers. To analyze the charge transfer in response to formation of various vacancies, we monitored the total charge on Ni as well as on the top and bottom parts of YSZ as it is shown on Figure

Hydrogen Oxidation at the Ni/YSZ Interface

Figure 2. Vacancy formation on the YSZ surface and oxygen spillover to Ni. The extrinsic vacancy of YSZ is denoted as an open circle whereas the additional vacancy is indicated by an open square: (a) Ni/ YSZ interface, (b) formation of an interface vacancy, (c) formation of a surface vacancy, and (d) oxygen spillover to Ni, resulting in formation of an interface vacancy.

Figure 3. Total charge on Ni and the YSZ-top and YSZ-bottom parts of the Ni/YSZ slab (the charges are indicated by upper indices): (a) both top and bottom of YSZ are stoichiometric, (b) interface vacancy formed on stoichiometric YSZ, (c) surface vacancy formed on stoichiometric YSZ, (d) oxygen migration from the bottom to the extrinsic vacancy of the top YSZ: formation of local YSZ+O (top) and YSZ-O (bottom) layers, (e) formation of interface vacancy on YSZ+O top unit, and (f) formation of surface vacancy on YSZ+O top unit.

3. The charges on all three parts have been determined as a sum of the positive charges of respectively the nuclei (pseudoatoms) plus the negative charges of the (valence) electrons localized on those atoms according to the Bader method.25-28 In Figure 3a are shown the stoichiometric Ni/YSZ system prior to the migration or removal of an oxygen atom. The net charges on the Ni cluster as well as the top and bottom parts of YSZ are rather small. This would indicate that there only is a small net flow of electron charge between the three regions, notably from Ni to the YSZ. Oxygen removal from the interface causes a flow of charge from the YSZ top to Ni (∼1e-). On the other hand, the YSZ bottom remains close to neutral (Figure 3b). This observation is in line with results from calculations based on a smaller slab3 where a similar charge transfer was observed. From the

J. Phys. Chem. C, Vol. 114, No. 25, 2010 11211 perspective of SOFC operation, this result indicates that fuel oxidation which leads to the removal of an oxygen atom from YSZ indeed causes a flow of charge to Ni, thus accounting for the electric current generation. We have already demonstrated in our previous work3 that a more favorable vacancy formation is associated with a higher degree of charge transfer from YSZ to Ni. Indeed for the less favorable surface vacancy formation we find smaller transfer of electron charge compared to the more feasible interface vacancy formation (Figure 3, panels b and c). Moreover a characteristic peak right below the Fermi level of the combined PDOS of d electrons on the three atoms neighboring the surface vacancy (Zr and Y) is introduced in connection with surface vacancy formations. This points to the fact that a substantial amount of charge is localized in the vacancy region, rather than moving to nickel, in case of surface vacancy formation. By contrast interface vacancy does not yield a defect state below the Fermi level in PDOS of respective Zr and Y, indicating a higher amount of charge transferred to Ni.3 We shall next turn to the study of vacancy formation on a locally oxygen enriched YSZ surface Ni/(YSZ+O). The motivation for such an investigation is the notion that facile vacancy formation and hydrogen oxidation might require the prior migration of an oxygen atom to the surface. This notion was in part born out of our previous investigations.3,29 We find that oxygen migration from the bottom unit of Ni/ YSZ to the extrinsic vacancy site of the surface results in substantial changes in the charges in the different layers of the model. The charge at the bottom YSZ layers becomes substantially positive (+1.53e-), indicating a transfer to the upper part for which the negative charge increases by about the same amount (-1.55e-) (Figure 3d). The Ni cluster stays almost neutral (+0.01 e-) and thus its charge is not significantly affected by the oxygen migration to the surface. Migration of oxygen from the bottom unit in Ni/YSZ to the top layer to form Ni/(YSZ+O) leads to an enthalpy increase of 29 kcal/mol. This is in reasonable agreement with our previous study where a value of 20 kcal/mol has been obtained.3 The deviation can be attributed to that use was made of a different slab elongated in the vertical direction. Also the type of Ni cluster employed was different.3 The removal of an oxygen atom from the Ni/(YSZ+O) interface causes an increase of negative charge on the Ni to 0.66e- (Figure 3e). Remarkably the top YSZ part (locally stoichiometric) is negatively charged (-0.24e-), whereas the bottom part (with an extra vacancy) is positively charged (+0.90e-). This result indicates that oxygen transfer to the YSZ surface and subsequent removal as a result of fuel oxidation also leads to accumulation of charge on the Ni cluster. The formation energy of an interface vacancy for Ni/(YSZ+O) is 169 kcal/mol. This value however is still smaller than the interface vacancy formation for Ni/YSZ of 186 kcal/mol. On the other hand, the removal of an oxygen atom from the surface of Ni(YSZ+O) in the vicinity to the interface with Ni causes smaller increase of negative charge on Ni, (-0.43e-) (Figure 3f) similar to respective reaction on Ni/YSZ. Smaller amount of charge transfer between YSZ and Ni in response to surface vacancy formation results in a higher vacancy formation energy (+190 kcal/mol), indicating that oxygen removal is more favorable on the interface of Ni/YSZ in the case of both oxygen enriched and stoichiometric YSZ. 5. Hydrogen Oxidation on the Ni/YSZ Interface We shall first discuss hydrogen oxidation on Ni/YSZ (Figure 2a) without the presence of an oxygen enriched YSZ surface

11212

J. Phys. Chem. C, Vol. 114, No. 25, 2010

Figure 4. Free energy pathway for the two possible scenarios of hydrogen oxidation on the Ni/YSZ interface: (a) oxygen and hydroxyl spillover from YSZ to the Ni surface with subsequent water formation and (b) first hydrogen spillover to the Ni/YSZ interface with water formation on the interface. Red line is the free energy profile at T ) 1000 K, blue line is the enthalpic energy profile.

where oxygen has migrated from the YSZ bottom to the YSZ top. For this system the spillover of an interface oxygen (Figure 2a) to the nickel surface (Figure 2d) has a thermodynamic barrier of 46 kcal/mol. An identical barrier has been found previously for the same process using a smaller Ni/YSZ model.3 As anticipated based on the discussion in the previous section, an even higher barrier (85 kcal/mol) is associated with a spillover of an oxygen atom from the YSZ surface. The calculated thermodynamic barriers for the two oxygen spillover processes render hydrogen oxidation on the nickel surface unlikely. We depict the free energy profile for hydrogen oxidation involving the more facile process of interface oxygen spillover in Figure 4a. After the actual oxygen transfer to nickel (ONi + 2HNi) with an energy of 46 kcal/mol, one hydrogen is transferred to oxygen under formation of a hydroxyl group (OHNi + HNi) resulting in an energy increase to 52 kcal/mol. The last step is the associative desorption of OHNi + HNi under the formation of vH2O. This step is exergonic with a free energy decrease of 43 kcal/mol due to the large entropy of H2O desorption from nickel at 1000 K given by 55 cal/(mol*K). At the same time, the last step is endothermic by 12 kcal/mol and thus only possible at elevated temperatures. We are not able to trace the free energy profile of the last water desorping step in any details. However, we do not expect any significant barrier on the free energy surface. The point OHNi + HNi on Figure 4a is also part of the free energy profile for the alternative process involving spillover of a hydrogen atom from nickel to the YSZ surface followed by back spillover of hydroxyl to the nickel surface. Thus this process too will have a thermodynamic barrier of at least 52 kcal/mol. These results indicate that oxygen and hydroxyl spillover are unlikely reactions, at least from the stoichiometric YSZ surface. Finally, we have previously used our smaller model to show that the successive spillover of two hydrogen atoms from nickel to the YSZ surface with subsequent formation of water has a thermodynamic barrier of 70 kcal/mol.3 We do not expect this barrier to change with the current model and conclude that this process is unlikely as well as an integral part of the SOFC

Shishkin and Ziegler

Figure 5. Formation of a water molecule on the Ni/YSZ interface: (a) a pair of hydrogens on the Ni surface, (b) spillover of the first hydrogen to the interface oxygen, (c) migration of the second hydrogen toward the interface oxygen, and (d) the water molecule in the gas phase with accompanied formation of the interface vacancy.

operation. We wish also to point out that this finding is similar to conclusions of Anderson and Vayner who found a consecutive spillover of two hydrogens from Ni to YSZ to be a very unfavorable endothermic reaction (by ∼5 eV).30 Much larger value of the thermodynamic barrier as compared to the one found by us can be attributed to the different (cluster) model of Ni/YSZ cermet, utilized by Anderson and Vayner.30 In addition to these mechanisms of hydrogen oxidation, which have been studied in our previous work, we also analyzed an alternative scenario of water formation. It includes two subsequent hydrogen spillovers to the same oxygen on the Ni/YSZ interface followed by water formation and desorption, Figure 5. The enthalpy and free energy profiles of this path are shown in Figure 4b. The first step in this new mechanism of water formation is hydrogen spillover to oxygen at the Ni/YSZ interface with a thermodynamic barrier of 27 kcal/mol (Figure 5, panels a and b). Moreover only a small additional kinetic barrier (∼3 kcal/mol) is associated with this process according to calculations based on a smaller model.3 The next step includes the second hydrogen migration to the hydroxyl with water formation and desorption (Figure 5, panels c and d). Using climbing image NEB method it has been determined that the kinetic barrier for this reaction is equal to thermodynamic. The final water desorbing step is again favored by entropy and exergonic. We do not expect it to add any significant barrier on the free energy profile. Of the different scenarios for hydrogen oxidation examined here the path involving two subsequent hydrogen spillovers to the same oxygen on the Ni/YSZ interface followed by water formation and desorption, Figure 4b, seems the most facile. 6. Hydrogen Oxidation on Ni/(YSZ+O) Interface Oxidation of hydrogen on an oxygen-enriched surface YSZ+O will be discussed next. The thermodynamic barrier for spillover of an interface oxygen atom to nickel is 29 kcal/mol relative to two hydrogen atoms on Ni/(YSZ + O), Figure 6a. This barrier is slightly higher than the one found previous in our study using a smaller cell (20 kcal/mol).3 The barrier is still lower than in the case of interface oxygen spillover from the stoichiometric YSZ (46 kcal/mol), indicating that oxygen enrichment makes the oxide surface more reactive in agreement with our previous conclusions.3 The overall barrier of OH

Hydrogen Oxidation at the Ni/YSZ Interface

J. Phys. Chem. C, Vol. 114, No. 25, 2010 11213

Figure 7. Free energy pathway of hydrogen oxidation via spillover of two hydrogens from Ni to YSZ + O surface. Red line is the free energy profile at T ) 1000 K, blue line the enthalpic profile. The initial reactant (2HNi) has an increased energy due to migration of the bulk oxygen to the YSZ surface.

Figure 6. Free energy pathway of two possible scenarios of hydrogen oxidation on the Ni/(YSZ + O) interface: (a) oxygen and hydroxyl spillover from YSZ + O to Ni surface with subsequent water formation; (b) first hydrogen spillover to Ni/(YSZ + O) interface with water formation on the interface. Red line is the free energy pathway at T ) 1000 K, blue line shows the enthalpic energy profile. The initial reactant (2HNi) has an increased energy due to migration of the bulk oxygen to the YSZ surface.

spillover, which includes hydrogen migration to the interface with hydroxyl spillover to Ni, is 33 kcal/mol relative to two hydrogen atoms on Ni/(YSZ + O). This is again higher than in the case of smaller cells (20 kcal/mol) but lower than OH spillover from stoichiometric YSZ (52 kcal/mol). The barriers cited so far are with respect to two hydrogen atoms on Ni/(YSZ + O). However in order to compare with the pathways involving Ni/YSZ in Figure 4, we must add the energy (29 kcal/mol) required to migrate an oxygen atom to the surface. As a result the total energy needed to form 2HNi + ONi and HNi + OHNi on Ni/(YSZ + O), Figure 6a, is larger than the energy necessary to form 2HNi + ONi and HNi + OHNi on Ni/YSZ, Figure 4a, by ∼10 kcal/mol. As a result, water formation via an oxygen and hydroxide spillover pathways involving surface oxygen on YSZ is even less likely to proceed via Ni/(YSZ+O) generation. The direct water formation on the interface discussed in the previous section for Ni/YSZ, Figure 4b, has also been studied for the case of the oxygen-enriched interface Ni/(YSZ + O), Figure 6b. We find that the first hydrogen transfer to an oxygen atom on the interface is associated with a barrier of 15 kcal/ mol relative to two hydrogen atoms on Ni/(YSZ + O). The next step includes the second hydrogen migration to the hydroxyl with water formation and desorption. The final water desorbing step is again favored by entropy and exergonic. We do not expect it to add any significant barrier on the free energy profile. The corresponding pathway involving Ni/YSZ in Figure 4b had a barrier of 27 kcal/mol relative to two hydrogen atoms on Ni/YSZ. To make a direct comparison between Figures 4b and 6b, we must add the energy (29 kcal/mol) required to migrate an oxygen atom to the surface to the profile in Figure 6a. As a result the total energy needed to form HNi + OHNi is now 44 kcal/mol on Ni/(YSZ + O), Figure 6b, compared to 27 kcal/ mol on Ni/YSZ. Thus, water formation involving transfer of two hydrogen atoms does not seem to be facilitated by the prior formation of Ni/(YSZ + O). We shall finally report on the study of two hydrogens transferred from Ni to the surface of YSZ + O in Ni/(YSZ +

O). The free energy is found to increase to 60 kcal/mol relative to two hydrogen atoms on Ni/YSZ before the water desorption can begin, Figure 7. It is thus not likely that two subsequent migrations of hydrogen from nickel to the surface of YSZ + O (or YSZ) play an important role in the operation of SOFC’s based on Ni/YSZ. 7. Conclusion In this work we have carried out a DFT study of hydrogen oxidation on the TPB of the Ni/YSZ anode considering both previously proposed oxygen and hydroxyl spillover mechanisms as well as a new mechanism of direct water formation on the interface. In the case of the stoichiometric YSZ surface we find that oxygen and hydroxyl spillover reactions are associated with high energy barriers (46 and 52 kcal/mol respectively). On the other hand, interface water formation is considered more feasible with an estimated barrier of 27 kcal/mol, Figure 4b. Finally, water formation by a hydrogen spillover mechanism to YSZ is also ruled out based on previous calculations3 since this process was found to have a barrier of 70 kcal/mol. For all the mechanisms the free energy of the overall reaction 1 increases by 9 kcal/mol. In the case of an oxygen enriched YSZ surface, Ni/(YSZ + O), oxygen and hydroxyl spillover barriers relative to two hydrogen’s on Ni/(YSZ + O) are found to be lower than in the respective reactions on the Ni/YSZ interface relative to two hydrogen’s on Ni/YSZ. However when the energy of 29 kcal/ mol for the formation of an oxygen enriched YSZ surface from Ni/YSZ is taken into account these processes become even less likely than on Ni/YSZ, and this is also the case for hydrogen spillover leading to water formation on the YSZ + O surface. The favored hydrogen oxidation reaction on Ni/YSZ is water formation on an oxygen atom at the interface and this is also the most facile hydrogen process on Ni/(YSZ + O). However the prior formation of an oxygen enriched surface in Ni/(YSZ + O) does not make this process more facile. Thus there seems not to be any reason to suggest that prior migration of oxygen is a necessary trigger mechanism for hydrogen oxidation as it was hinted in our previous studies.3,29 There has been considerable discussion2,12,31,32 as to whether hydrogen oxidation involves initial hydrogen spillover to YSZ or oxygen spillover to nickel. Our study seems to indicate that neither of these mechanisms is at work. Instead the oxidation takes place on the oxygen atoms in the narrow interface between nickel and YSZ. Acknowledgment. This research was supported through funding to the NSERC Solid Oxide Fuel Cell Canada Strategic Research Network from the Natural Science and Engineering

11214

J. Phys. Chem. C, Vol. 114, No. 25, 2010

Research Council (NSERC) and other sponsors listed at www.sofccanada.com. T.Z. thanks the Canadian government for a Canada Research Chair in theoretical inorganic chemistry. References and Notes (1) Sun, C.; Stimming, U. J. Power Sources 2007, 171, 247. (2) Goodwin, D. G.; Zhu, H.; Colclasure, A. M.; Kee, R. J. J. Electrochem. Soc. 2009, 156, B1004. (3) Shishkin, M.; Ziegler, T. J. Phys. Chem. 2009, 113, 21667. (4) Atkinson, A.; Barnett, S.; Gorte, R. J.; Irvine, J. T. S.; McEvoy, A. J.; Mogensen, M.; Singhal, S. C.; Vohs, J. Nat. Mater. 2004, 3, 17. (5) Bieberle, A.; Gaukler, L. J. Solid State Ionics 2002, 146, 23. (6) Bessler, W. G. Solid State Ionics 2005, 176, 997. (7) Bessler, W. G.; Warnatz, J.; Goodwin, D. G. Solid State Ionics 2007, 177, 3371. (8) Zhu, H.; Kee, R. J.; Janardhana, V. M.; Deutschmann, O.; Goodwin, D. G. J. Electrochem. Soc. 2005, 152, A2427. (9) Kek, D.; Mogensen, M.; Pejovnik, S. J. Electrochem. Soc. 2001, 148, A878. (10) Chebotin, V. N.; Glumov, M. V.; Neuimin, A. D.; Palguev, S. F. SoV. Electrochem. 1971, 7, 55. (11) Holtappels, P.; Vinke, L. C.; deHaart, L. G. J.; Stimming, U. J. Electrochem. Soc. 1999, 146, 2976. (12) Vogler, M.; Bieberle-Hu¨tter, A.; Gauckler, L.; Warnatz, J.; Bessler, W. G. J. Electrochem. Soc. 2009, 156, B663. (13) Bieberle, A.; Meier, L. P.; Gauckler, L. J. J. Electrochem. Soc. 2001, 148, A646. (14) Mizusaki, J.; Tagawa, H.; Saito, T.; Kamitani, K.; Yamamura, T.; Hirano, K.; Ehara, S.; Takagi, T.; Hikita, T.; Ippommatsu, M.; Nakagawa, S.; Hashimoto, K. J. Electrochem. Soc. 1994, 141, 2199.

Shishkin and Ziegler (15) Mizusaki, J.; Tagawa, H.; Saito, T.; Kamitani, K.; Yamamura, T.; Kamitani, K.; Hirano, K.; Hikita, T.; Ippommatsu, M.; Nakagawa, S.; Hashimoto, K. Solid State Ionics 1994, 70, 52. (16) http://cms.mpi.univie.ac.at/vasp/. (17) Kresse, G.; Furthmu¨ller, J. Phys. ReV. B 1996, 54, 11169. (18) Kresse, G.; Furthmu¨ller, J. Comput. Mater. Sci. 1996, 6, 15. (19) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (20) vonBarth, U.; Hedin, L. J. Phys. C 1972, 5, 1629. (21) Blo¨chl, P. Phys. ReV. B 1994, 50, 17953. (22) Kresse, G.; Joubert, D. Phys. ReV. B 1999, 59, 1758. (23) Jo´nsson, H.; Mills, G.; Jacobsen, K. W. Nudged elastic band method for finding minimum energy paths of transitions. In Classical and Quantum Dynamics in Condensed Matter Phase Simulations; Berne, B. J., Ciccotti, G., Coker, D. F., Eds.; World Scientific: Singapore, 1998; pp 385-404. (24) Henkelman, G.; Uberuaga, B. P.; Jonsson, H. J. Chem. Phys. 2000, 113, 9901. (25) Bader, R. F. W. Atoms in Molecules: a Quantum Theory; New York: Oxford University Press, 1990. (26) Henkelman, G.; Arnaldsson, A.; Jo´nsson, H. Comput. Mater. Sci. 2006, 36, 254–360. (27) Sanville, E.; Kenny, S. D.; Smith, R.; Henkelman, G. J. Comput. Chem. 2007, 28, 899–908. (28) Tang, W.; Sanville, E.; Henkelman, G. J. Phys.: Condens. Matter 2009, 21, 084204. (29) Shishkin, M.; Ziegler, T. J. Phys. Chem. C 2008, 112, 19662. (30) Anderson, A. B.; Vayner, E. Solid State Ionics 2006, 177, 1355. (31) Kleis, J.; Jones, G.; Abild-Pedersen, F.; Tripkovic, V.; Bligaard, T.; Rossmeisl, J. J. Electrochem. Soc. 2009, 156, B1447. (32) Rossmeisl, J.; Bessler, W. G. Solid State Ionics 2008, 178, 1694.

JP1030575