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Importance of oxygen spillover for fuel oxidation on Ni/YSZ anodes in solid oxide fuel cells† Zhaoming Fu,a Mingyang Wang,a Pengju Zuo,a Zongxian Yang*a and Ruqian Wu*b Using first principles simulations and the Monte Carlo method, the optimal structure of the triple-phase boundaries (TPB) of the Ni/Yttria-Stabilized Zirconia (YSZ) anode in solid oxide fuel cells (SOFCs) is determined. Based on the new TPB microstructures we reveal different reaction pathways for H2 and CO
Received 2nd December 2013, Accepted 6th February 2014
oxidation. In contrast to what was believed in previous theoretical studies, we find that the O spillover
DOI: 10.1039/c3cp55076a
rapidly, by means of both the H and O spillovers, whereas the CO oxidation can only proceed through the
from YSZ to Ni plays a vital role in electrochemical reactions. The H2 oxidation reaction can proceed very O spillover pathway. Further understanding of the roles of defects and dopants allows us to explain puzzling experimental observations and to predict ways to improve the catalytic performance of SOFCs.
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Introduction Solid-oxide fuel cells (SOFCs) are very promising for direct conversion of chemical energy to electricity due to their high efficiency and unique scalability.1 A typical anode for a SOFC consists of nickel and yttria-stabilized zirconia (YSZ), which has excellent catalytic activity for fuel oxidation, conductivity for current collection, good heat stability, and compatibility with YSZ electrolyte for easy cell fabrication.2–6 The main hurdles for the development of SOFC technologies are carbon deposition and sulfur poisoning over the Ni/YSZ anodes by contaminants and fuels.7–13 In particular, sulfur impurities can degrade significantly the catalytic performance of Ni/YSZ anodes even at the level of a few parts per million (ppm).10 To solve these problems, it is crucial to understand the fuel oxidation mechanism at the triple-phase boundary (TPB) of Ni, YSZ and gas, starting from studies of reaction pathways of fuel combustion at TPB.14 Although extensive experimental data are available, it is still a major challenge in this area to understand clearly the electrochemical reaction mechanism, and further technological advances await fundamental breakthroughs.1 As an example, there is no consensus regarding the actual reaction pathways and elementary steps of hydrogen combustion in SOFCs. As the simplest fuel for SOFCs, it is expected that H2 oxidizes near the Ni–YSZ–Gas TPBs as H2 + O2 - H2O + 2e + VO2+ a
(1)
College of Physics and Electronic Engineering, Henan Normal University, Xinxiang, Henan 453007, People’s Republic of China. E-mail: [email protected] b Department of Physics and Astronomy, University of California, Irvine, California 92697-4575, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cp55076a
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Here, VO2+ is a doubly charged oxygen vacancy in YSZ. However, several mechanisms have been proposed: (a) two consecutive spillovers of hydrogen atoms from Ni to YSZ;15,16 (b) oxygen spillover from the YSZ surface to the Ni surface;17–20 (c) hydroxyl spillover from the YSZ surface to the Ni surface.21 Similar controversies also exist for the understanding of the oxidation of CO, another important fuel for SOFCs.1 Ab initio simulations based on the density functional theory (DFT) were conducted to distinguish reaction paths and to provide comprehensive insights into the reaction mechanisms.14,22,23 So far, these considerations are nonetheless rather inadequate since only one Ni–YSZ–Gas TPB model has been widely used for studies of H2 combustion. It is conceivable that TPB structures in actual anodes are diverse and complex, because of the presence of different nucleation, doping and defect configurations. In experiments, it was found that the anode performance depends strongly on the microstructure of the samples,24 reflecting the close relation between the electrochemical reaction and the morphology of Ni/YSZ anodes. Therefore, it is critical first to determine and optimize stable TPB configurations for a better understanding of actual electrochemical reactions in SOFCs. In this work, based on the Monte Carlo method and systematic ab initio simulations, we search for the stable Ni46/YSZ adhesion configurations and investigate the oxidation mechanism of the primary fuels (H2 and CO) in SOFCs, using various TPB structures beyond previous studies.14,22,23 We identify close correlations between fuel oxidation pathways and TPB microstructures, and the importance of the O spillover process in hydrogen and CO oxidation. Based on the understanding of microscopic reaction processes, we explain a puzzling experimental observation: the low electrochemical oxidation rate of CO fuel in SOFCs. We believe that the new mechanisms
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and insights are extremely useful for the development of efficient SOFCs.
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Model and computation method Spin-polarized density functional theory (DFT) calculations are performed by the Vienna Ab Initio Simulation Package (VASP).25 The exchange and correlation interaction among electrons are described at the level of the generalized gradient approximation (GGA), using the Perdew–Burke–Ernzerhof (PBE) formula.26 The electron–ion interactions are treated using the projector augmented wave (PAW) method.27,28 The Kohn–Sham orbitals are expanded using plane waves with a well-converged cutoff energy of 400 eV. A vacuum layer of 12 Å along the z direction is placed between the slabs to avoid periodic interactions. The Monkhorst–Pack k-point mesh of 1 1 1 is used for Brillouin zone (BZ) sampling. The atoms in the bottom multilayer are kept fixed for all calculations. Structural optimization of all systems is performed until the atomic forces drop below 0.02 eV Å1. The climbing image nudged elastic band method (CI-NEB),29 with a spring constant between adjacent images of 5.0 eV Å1, is adopted to identify the reaction paths and transition states. The classical Monte Carlo method is used to search for stable adhesion configurations through the conformational parameter space of the Ni/YSZ adsorption system, spanned by rotating and moving the Ni cluster on the YSZ surface. The distances between Ni atoms and surface O atoms are used to judge whether O–Ni bonds are formed. The interfacial O–Ni bond lengths between Ni(111) and YSZ(111) are set to 2.0 Å,30,31 and the maximum allowable deviation is 0.2 Å in our bonding criteria. In the previous image simulations of high-resolution transmission electron microscopy (HRTEM),32 the interlayer distances (H) between the Ni and YSZ were set to be 1.95 Å for the O-terminated YSZ(111) model. Here the H values are sampled in the range from 1.85 to 2.05. The interfacial matching is estimated according to the number of O–Ni bonds. By random sampling of the adsorption configurations, the maximum bonding numbers can be calculated according the mentioned criteria. In this way, we can get a small number of possible stable configurations by omitting the equivalent and similar configurations. Then we perform DFT optimization to obtain the accurate adsorption energies of these systems and get the most stable Ni46/YSZ configuration from them. In this paper, the adsorption energy (Ead) is defined by Ead = Eadsorbate/substrate Eadsorbate Esubstrate where the Eadsorbate/substrate is the total energy of the adsorbed assembly, and Esubstrate and Eadsorbate are those of the Ni/YSZ(111) and the adsorbate, respectively. Supporting details of the methodology can be obtained in the ESI.†
Results and discussions In this study, the Ni/YSZ interface is modeled with a large supercell, shown in Fig. 1. A Ni cluster with 46 atoms is placed
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Fig. 1 The Ni/YSZ adsorption models and the corresponding interfaces between Ni(111) and YSZ(111). Blue (dark gray), red (medium gray), gray, and green (medium–light gray) spheres represent Ni, O, Zr and Y atoms, respectively. (a) The side view of the Ni/YSZ(111) supercell model. (b) The top view of three stable adsorption configurations. The yellow arrows mark various h110i orientations on the YSZ(111) surface.
on the YSZ slab that consists of 144 O, 63 Zr, 12 Y atoms, and six constitutional vacancies. Different interfaces can be formed between the YSZ(111) surface and the (111) facet of Ni clusters, depending on the atomic alignment at the base. The main motivation for the present computational simulations is to identify which configuration is more stable and more catalytically effective. The microstructures along TPBs are determined by optimizing the adsorption configurations of Ni46/YSZ. The classical Monte Carlo method is employed for the search of possible stable adsorption structures in the vast phase space, and the best configurations are further examined through density functional theory calculations. Interestingly, we found that Ni atoms at the cluster edges tend to arrange along the h110i orientations on the substrate, as shown in Fig. 1(b). This suggests that h110i TPBs are the most important regions for studies of reactions near Ni/YSZ(111) nanocatalysts. As a matter of fact, these TPBs have been adopted by several authors, even though their stability and matching conditions were not considered.14,22,23 The TPB length has been widely viewed as a key structural parameter,33 since electrochemical reactions are assumed to occur only along the TPB. To explore further the effect of local structural changes along different TPBs, we study three adsorption configurations, as displayed in Fig. 2(a)–(c). They belong to two types [denoted as type-I and type-II in Fig. 2(d)], with the TPB in Fig. 2(a) corresponding to type-I and those in Fig. 2(b) and (c) corresponding to type-II. Each oxygen atom at TPB (denoted as TPB-O) has three Zr neighbors, one (two) of them is (are) covered under the Ni cluster in the type-I (type-II) TPB. Since the chemical bonds between TPB-O and Zr atoms that are covered under Ni are strongly affected, the activity and mobility of TPB-O atoms should be very different in the two types of TPBs. Previous studies used the type I adsorption model shown in Fig. 2(a), in which TPB-O atoms are tightly bound to their Zr and Ni neighbor. Without swappable TPB-O ions, H2 oxidation was proposed to occur through the hydrogen spillover pathway, with the active O atoms being extracted from the interior region for the reaction with H atoms on the Ni facets.14,22,23 Using this structural model, we also confirm that the H spillover pathway must be assisted by O-migration. The calculated activation energy is about 1.3 eV for the formation of hydroxyl, in agreement with Heyden’ result (about 1.3 eV)14 and close to the range that were advised by impedance spectroscopy measurements
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Fig. 2 The TPB microstructures. The insets (a) (b) and (c) display the TPB configurations in the three stable adsorption systems, respectively. The sketch (d) is the front view of TPB structure, illustrating two types of representative TPBs (type I and II) determined by the different adsorption structures, where the TPB in the inset (a) belongs to type I, and ones in (b) and (c) belong to type II. The black characters represent the Zr atoms under the Ni cluster; the pink represents the Zr atoms not covered by the Ni cluster. In type II the oxygen atom bonding to two Ni has high activity, and the dashed lines represent the broken O–Zr bond with TPBs formation.
(1.04–1.68 eV).34–36 The detailed calculations of transition state for the formation of hydroxyl are given by Fig. S1 in the ESI.† It is important to point out that the configuration in Fig. 2(a) is not the most stable structure according to our total energy calculations. Its energy is as much as 0.7 eV higher than that of the model shown in Fig. 2(c). Such a large energy difference indicates that the population of the type-I TPBs is much less than that of type-II TPBs in real samples. Therefore, it is more relevant to discuss reaction pathways and vital intermediate states using models in Fig. 2(b) and (c). Interestingly, some TPB-O atoms in Fig. 2(c) relax to higher positions and bind to two Ni atoms. These TPB-O atoms are more detachable than those in Fig. 2(a) and they may participate directly in chemical reactions. As a result, the O spillover from YSZ to Ni becomes rather easy in the type-II TPBs. The overall H2 oxidation process therefore can be accomplished through the new O spillover pathway, as displayed in Fig. 3. Now let us study the importance of this new O-spillover mechanism for the performance of YSZ based nanocatalysts in SOFCs. Our calculations using the nudged-elastic-band (NEB) approach indicate that the migration of a TPB-O atom from its stable position in type-II to the adjacent site on the Ni facet, the first step for O spillover, has an energy barrier of 1.36 eV. The initial and final states of this process are depicted as configurations 1 and 2 in Fig. 3. This O atom may easily diffuse on the Ni cluster afterwards, as shown in configuration 3, with an energy barrier of only 0.73 eV. Obviously, H oxidation may occur on the Ni surfaces without the need for O-migration from the bulk YSZ. To study further the formation of hydroxyl or H2O through this pathway, we add one H atom into the system and calculate its co-adsorption with the spillover O atom. Configuration 30 shows an initial state for the formation of hydroxyl, with the H atom taking the hollow site near the spillover O atom. The H atom can migrate to the O atom and form the hydroxyl as depicted in configuration 4 in Fig. 3 with a barrier of 0.9 eV. Similarly, we add one more H atom to another neighboring hollow site on Ni with the hydroxyl as configuration 4 0 in Fig. 3. From this initial state, the H atom can react with the hydroxyl and form H2O in the final state (configuration 5 in Fig. 3) with an
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Fig. 3 The energy profile along the O spillover pathway for H oxidation. The energy of the intermediate state 3 is set to zero, and the values on the vertical coordinate are relative energies with respect to state 3. The O spillover and diffusion pathways in the pure Ni/YSZ, doped Ni/YSZ with a Ni dopant, and defective Ni/YSZ with an interface O vacancy are represented by the blue thick, thin and dashed curves; the green curve represents H2 oxidation by the spillover O. All energy barriers are given in the pictures. All configurations of intermediate states along the reaction coordinate are displayed. The states 3 0 and 4 0 are the results with the state 3 and 4 with the addition of an adsorbed H, respectively.
activation energy of 0.95 eV. Overall, the O spillover mechanism based on the most stable Ni/YSZ TPB model has an energy barrier of 1.36 eV for the rate-determining step: O detachment. This barrier is comparable with that of the H spillover mechanism, 1.3 eV14 or 1.04–1.68 eV.34–36 Moreover, we also study the energetics of the H-spillover pathway (details of NEB simulations are given in ESI,† Fig. S1) near the type-II TPBs. The activation energy of the first H spillover step, formation of hydroxyl, is 1.37 eV. This is only slightly larger than the value of the O spillover pathway, 1.36 eV. Therefore, we perceive that the two mechanisms coexist for hydrogen oxidation near the stable type-II TPBs displayed in Fig. 2(b) and (c). The reaction rates of both channels should be comparable to that of the hydrogen spillover pathway on the type-I TPB. Evidently, the Ni/YSZ anodes in SOFCs are more efficient than what was thought in previous theoretical studies,14,22,23 because of the additional unexplored oxygen spillover pathway on more stable TPBs. Since the availability of TPB-O atoms is crucial for the oxygen spillover reaction dynamics, it is instructive to use the number of them (denoted as NTPB-O) as a microscopic parameter to characterize the anode catalytic performance. For TPBs along the h110i orientation on YSZ(111), we can estimate NTPB-O from the TPB length by using the formula. NTPB-O = TPB length/(2dO–O),
(2)
where dO–O is the distance of two neighboring TPB-O atoms along the h110i orientation on the YSZ(111) surface. As sketched in Fig. 2(d) dO–O is 3.6 Å in our model for the type-II structure. When TPB-O atoms become reactants, one crucial issue for catalytic reactions is their mutual influence and supply. Multiple
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Fig. 4 The comparison of the two supposed CO oxidation pathways, displayed by the colored solid and dash curves, respectively. The blue curve represents the O spillover and diffusion pathway; and the red curve represents CO oxidation by the spillover O (Osp). All energy barriers are given in the pictures. The left inset shows CO spillover from the Ni to YSZ surface, where the energy difference between state 1 and 2 in the inset is 1.9 eV. The detailed geometry structure of each state can be found in the ESI,† Fig. S4. The right inset shows two hollow sites on the Ni cluster with different distances to the active interface O.
O spillover processes may happen simultaneously around TPBs. These processes may affect each other because of the absence of adjacent TPB-O atom(s). To see if this issue is important, we calculate the O-spillover transition state at an O-deficient TPB (the structural details are displayed in ESI,† Fig. S2). From the energy barriers shown by the dashed curve in Fig. 3, the activation energy is hardly changed (1.39 eV). Therefore, the mutual influence between O spillover steps even from the same TPB is rather weak. In fact, when the vacancy concentration become large at the anode, the O vacancies can be refilled by O from YSZ bulk under the SOFC work conditions. Therefore we only discuss the effects of one O defect on TPB-O spillover in this paper. To accelerate the O spillover, one may consider using dopants near the TPBs so as to reduce the energy barrier and energy cost for the detachment of TPB-O atoms. To this end, Ni diffusion into YSZ is beneficial since the intermixing of two materials significantly change the chemistry in the TPB region. This phenomenon was noticed in experiments,37–39 but the underlying mechanism has never been addressed. Here, we also study the effect of Ni dopants near the typeII TPBs on the O spillover process. Dramatically, the activation energy for the TPB-O detachment decreases to only 0.88 eV, as shown by the thin blue curve in Fig. 3. Details for the O spillover reactions near the Ni-doped TPBs can be found in the ESI,† Fig. S3. Clearly, one can modulate conveniently the activity of Ni/YSZ anodes by changing the interfacial structures and chemical components. Further studies along this line should be very valuable. Now we investigate the effect of O spillover on the electrochemical reaction CO oxidation,1 described as CO + O2 - CO2 + 2e
(3)
Fig. 4 and its left-inset show the energy profiles along O and CO spillover pathways, respectively. The possible initial CO
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adsorption sites on Ni are shown in the right inset (additional structural information can be seen in the ESI,† Fig. S4 and S5). Unlike the small H atom that takes the hollow site A next to the active TPB-O, the CO molecule can only take site B or other hollow sites farther apart due to its larger van der Waals size. As a result, CO spillover requires large activation energies (41.9 eV) to extract the TBP-O atom from YSZ. Therefore, the CO spillover is quite difficult and this pathway should be ruled out. In contrast, the activation energy for CO oxidation with an O adatom on the Ni facet is only 0.83 eV. Therefore, the new oxygen spillover mechanism is applicable for both H2 and CO oxidation in SOFCs. Now we see that H2 oxidation may proceed with both H spillover and O spillover pathways whereas CO oxidation can only adopt the O spillover pathway. These results provide clues to understand the low efficiency of Ni/YSZ catalysts toward the oxidation of dry CO as observed in experiments.40,41 Without the straightforward insights discussed above, this observation was attributed vaguely to the difference in the mass-transfer resistance of the active species. It was also observed that the electrochemical reaction rate of humidified CO fuel gas is high, even to the level of H2. This can also be interpreted with our new results. Humidified CO first undergoes the water gas shift reaction at temperatures above 250 1C on Ni catalysts,40,42,43 as described in the following equation: CO + H2O - CO2 + H2
(4)
Therefore, the actual fuel that is involved in the electrochemical process becomes H2 instead of CO. Since H2 oxidation may proceed with both the H spillover and O spillover pathways, the CO - H2 conversion leads undoubtedly to significant enhancement of the electrochemical reaction rate of wet CO.
Conclusions Ab initio simulations suggest that the local TPB structures have a strong influence on the reaction mechanism for the H2 and CO oxidation on Ni/YSZ anodes in SOFCs. Based on the newly optimized stable TPB structures, we find that O spillover plays a crucial role in the oxidation reaction of both H2 and CO. With the coexistence of O spillover and H spillover pathways, we believe that the H2 oxidation rate in SOFCs is high. Importantly, we find that the catalytic performance of Ni/YSZ nanocatalysts can be enhanced further by doping, or even by Ni–YSZ interdiffusion. For CO oxidation, the CO spillover pathway is essentially blocked and the O spillover pathway is dominant. This result can be applied for explaining the observation of sharply different reaction rates for the combustion of dry and wet CO. Our studies provide useful new insights for the understanding of oxidation processes of different fuels near TPBs and, furthermore, for the optimization of the performance of SOFCs. Finally, we emphasize that, in many occasions (not only in SOFCs) the chemical reactions occurring at the triple-phase boundaries are a multi-path process due to the multiple local structures of TPBs, and any single mechanism cannot give an
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unified explanation for the corresponding experimental results and chemical phenomena.
Acknowledgements
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This work was supported by the National Natural Science Foundation of China (Grant No. 11247012 and 11174070). Work at UCI was supported by the National Science Foundation under CHE-0802913 and computing time at XSEDE.
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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 8536
Importance of oxygen spillover for fuel oxidation on Ni/YSZ anodes in solid oxide fuel cells† Zhaoming Fu,a Mingyang Wang,a Pengju Zuo,a Zongxian Yang*a and Ruqian Wu*b Using first principles simulations and the Monte Carlo method, the optimal structure of the triple-phase boundaries (TPB) of the Ni/Yttria-Stabilized Zirconia (YSZ) anode in solid oxide fuel cells (SOFCs) is determined. Based on the new TPB microstructures we reveal different reaction pathways for H2 and CO
Received 2nd December 2013, Accepted 6th February 2014
oxidation. In contrast to what was believed in previous theoretical studies, we find that the O spillover
DOI: 10.1039/c3cp55076a
rapidly, by means of both the H and O spillovers, whereas the CO oxidation can only proceed through the
from YSZ to Ni plays a vital role in electrochemical reactions. The H2 oxidation reaction can proceed very O spillover pathway. Further understanding of the roles of defects and dopants allows us to explain puzzling experimental observations and to predict ways to improve the catalytic performance of SOFCs.
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Introduction Solid-oxide fuel cells (SOFCs) are very promising for direct conversion of chemical energy to electricity due to their high efficiency and unique scalability.1 A typical anode for a SOFC consists of nickel and yttria-stabilized zirconia (YSZ), which has excellent catalytic activity for fuel oxidation, conductivity for current collection, good heat stability, and compatibility with YSZ electrolyte for easy cell fabrication.2–6 The main hurdles for the development of SOFC technologies are carbon deposition and sulfur poisoning over the Ni/YSZ anodes by contaminants and fuels.7–13 In particular, sulfur impurities can degrade significantly the catalytic performance of Ni/YSZ anodes even at the level of a few parts per million (ppm).10 To solve these problems, it is crucial to understand the fuel oxidation mechanism at the triple-phase boundary (TPB) of Ni, YSZ and gas, starting from studies of reaction pathways of fuel combustion at TPB.14 Although extensive experimental data are available, it is still a major challenge in this area to understand clearly the electrochemical reaction mechanism, and further technological advances await fundamental breakthroughs.1 As an example, there is no consensus regarding the actual reaction pathways and elementary steps of hydrogen combustion in SOFCs. As the simplest fuel for SOFCs, it is expected that H2 oxidizes near the Ni–YSZ–Gas TPBs as H2 + O2 - H2O + 2e + VO2+ a
(1)
College of Physics and Electronic Engineering, Henan Normal University, Xinxiang, Henan 453007, People’s Republic of China. E-mail: [email protected] b Department of Physics and Astronomy, University of California, Irvine, California 92697-4575, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cp55076a
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Here, VO2+ is a doubly charged oxygen vacancy in YSZ. However, several mechanisms have been proposed: (a) two consecutive spillovers of hydrogen atoms from Ni to YSZ;15,16 (b) oxygen spillover from the YSZ surface to the Ni surface;17–20 (c) hydroxyl spillover from the YSZ surface to the Ni surface.21 Similar controversies also exist for the understanding of the oxidation of CO, another important fuel for SOFCs.1 Ab initio simulations based on the density functional theory (DFT) were conducted to distinguish reaction paths and to provide comprehensive insights into the reaction mechanisms.14,22,23 So far, these considerations are nonetheless rather inadequate since only one Ni–YSZ–Gas TPB model has been widely used for studies of H2 combustion. It is conceivable that TPB structures in actual anodes are diverse and complex, because of the presence of different nucleation, doping and defect configurations. In experiments, it was found that the anode performance depends strongly on the microstructure of the samples,24 reflecting the close relation between the electrochemical reaction and the morphology of Ni/YSZ anodes. Therefore, it is critical first to determine and optimize stable TPB configurations for a better understanding of actual electrochemical reactions in SOFCs. In this work, based on the Monte Carlo method and systematic ab initio simulations, we search for the stable Ni46/YSZ adhesion configurations and investigate the oxidation mechanism of the primary fuels (H2 and CO) in SOFCs, using various TPB structures beyond previous studies.14,22,23 We identify close correlations between fuel oxidation pathways and TPB microstructures, and the importance of the O spillover process in hydrogen and CO oxidation. Based on the understanding of microscopic reaction processes, we explain a puzzling experimental observation: the low electrochemical oxidation rate of CO fuel in SOFCs. We believe that the new mechanisms
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and insights are extremely useful for the development of efficient SOFCs.
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Model and computation method Spin-polarized density functional theory (DFT) calculations are performed by the Vienna Ab Initio Simulation Package (VASP).25 The exchange and correlation interaction among electrons are described at the level of the generalized gradient approximation (GGA), using the Perdew–Burke–Ernzerhof (PBE) formula.26 The electron–ion interactions are treated using the projector augmented wave (PAW) method.27,28 The Kohn–Sham orbitals are expanded using plane waves with a well-converged cutoff energy of 400 eV. A vacuum layer of 12 Å along the z direction is placed between the slabs to avoid periodic interactions. The Monkhorst–Pack k-point mesh of 1 1 1 is used for Brillouin zone (BZ) sampling. The atoms in the bottom multilayer are kept fixed for all calculations. Structural optimization of all systems is performed until the atomic forces drop below 0.02 eV Å1. The climbing image nudged elastic band method (CI-NEB),29 with a spring constant between adjacent images of 5.0 eV Å1, is adopted to identify the reaction paths and transition states. The classical Monte Carlo method is used to search for stable adhesion configurations through the conformational parameter space of the Ni/YSZ adsorption system, spanned by rotating and moving the Ni cluster on the YSZ surface. The distances between Ni atoms and surface O atoms are used to judge whether O–Ni bonds are formed. The interfacial O–Ni bond lengths between Ni(111) and YSZ(111) are set to 2.0 Å,30,31 and the maximum allowable deviation is 0.2 Å in our bonding criteria. In the previous image simulations of high-resolution transmission electron microscopy (HRTEM),32 the interlayer distances (H) between the Ni and YSZ were set to be 1.95 Å for the O-terminated YSZ(111) model. Here the H values are sampled in the range from 1.85 to 2.05. The interfacial matching is estimated according to the number of O–Ni bonds. By random sampling of the adsorption configurations, the maximum bonding numbers can be calculated according the mentioned criteria. In this way, we can get a small number of possible stable configurations by omitting the equivalent and similar configurations. Then we perform DFT optimization to obtain the accurate adsorption energies of these systems and get the most stable Ni46/YSZ configuration from them. In this paper, the adsorption energy (Ead) is defined by Ead = Eadsorbate/substrate Eadsorbate Esubstrate where the Eadsorbate/substrate is the total energy of the adsorbed assembly, and Esubstrate and Eadsorbate are those of the Ni/YSZ(111) and the adsorbate, respectively. Supporting details of the methodology can be obtained in the ESI.†
Results and discussions In this study, the Ni/YSZ interface is modeled with a large supercell, shown in Fig. 1. A Ni cluster with 46 atoms is placed
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Fig. 1 The Ni/YSZ adsorption models and the corresponding interfaces between Ni(111) and YSZ(111). Blue (dark gray), red (medium gray), gray, and green (medium–light gray) spheres represent Ni, O, Zr and Y atoms, respectively. (a) The side view of the Ni/YSZ(111) supercell model. (b) The top view of three stable adsorption configurations. The yellow arrows mark various h110i orientations on the YSZ(111) surface.
on the YSZ slab that consists of 144 O, 63 Zr, 12 Y atoms, and six constitutional vacancies. Different interfaces can be formed between the YSZ(111) surface and the (111) facet of Ni clusters, depending on the atomic alignment at the base. The main motivation for the present computational simulations is to identify which configuration is more stable and more catalytically effective. The microstructures along TPBs are determined by optimizing the adsorption configurations of Ni46/YSZ. The classical Monte Carlo method is employed for the search of possible stable adsorption structures in the vast phase space, and the best configurations are further examined through density functional theory calculations. Interestingly, we found that Ni atoms at the cluster edges tend to arrange along the h110i orientations on the substrate, as shown in Fig. 1(b). This suggests that h110i TPBs are the most important regions for studies of reactions near Ni/YSZ(111) nanocatalysts. As a matter of fact, these TPBs have been adopted by several authors, even though their stability and matching conditions were not considered.14,22,23 The TPB length has been widely viewed as a key structural parameter,33 since electrochemical reactions are assumed to occur only along the TPB. To explore further the effect of local structural changes along different TPBs, we study three adsorption configurations, as displayed in Fig. 2(a)–(c). They belong to two types [denoted as type-I and type-II in Fig. 2(d)], with the TPB in Fig. 2(a) corresponding to type-I and those in Fig. 2(b) and (c) corresponding to type-II. Each oxygen atom at TPB (denoted as TPB-O) has three Zr neighbors, one (two) of them is (are) covered under the Ni cluster in the type-I (type-II) TPB. Since the chemical bonds between TPB-O and Zr atoms that are covered under Ni are strongly affected, the activity and mobility of TPB-O atoms should be very different in the two types of TPBs. Previous studies used the type I adsorption model shown in Fig. 2(a), in which TPB-O atoms are tightly bound to their Zr and Ni neighbor. Without swappable TPB-O ions, H2 oxidation was proposed to occur through the hydrogen spillover pathway, with the active O atoms being extracted from the interior region for the reaction with H atoms on the Ni facets.14,22,23 Using this structural model, we also confirm that the H spillover pathway must be assisted by O-migration. The calculated activation energy is about 1.3 eV for the formation of hydroxyl, in agreement with Heyden’ result (about 1.3 eV)14 and close to the range that were advised by impedance spectroscopy measurements
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Fig. 2 The TPB microstructures. The insets (a) (b) and (c) display the TPB configurations in the three stable adsorption systems, respectively. The sketch (d) is the front view of TPB structure, illustrating two types of representative TPBs (type I and II) determined by the different adsorption structures, where the TPB in the inset (a) belongs to type I, and ones in (b) and (c) belong to type II. The black characters represent the Zr atoms under the Ni cluster; the pink represents the Zr atoms not covered by the Ni cluster. In type II the oxygen atom bonding to two Ni has high activity, and the dashed lines represent the broken O–Zr bond with TPBs formation.
(1.04–1.68 eV).34–36 The detailed calculations of transition state for the formation of hydroxyl are given by Fig. S1 in the ESI.† It is important to point out that the configuration in Fig. 2(a) is not the most stable structure according to our total energy calculations. Its energy is as much as 0.7 eV higher than that of the model shown in Fig. 2(c). Such a large energy difference indicates that the population of the type-I TPBs is much less than that of type-II TPBs in real samples. Therefore, it is more relevant to discuss reaction pathways and vital intermediate states using models in Fig. 2(b) and (c). Interestingly, some TPB-O atoms in Fig. 2(c) relax to higher positions and bind to two Ni atoms. These TPB-O atoms are more detachable than those in Fig. 2(a) and they may participate directly in chemical reactions. As a result, the O spillover from YSZ to Ni becomes rather easy in the type-II TPBs. The overall H2 oxidation process therefore can be accomplished through the new O spillover pathway, as displayed in Fig. 3. Now let us study the importance of this new O-spillover mechanism for the performance of YSZ based nanocatalysts in SOFCs. Our calculations using the nudged-elastic-band (NEB) approach indicate that the migration of a TPB-O atom from its stable position in type-II to the adjacent site on the Ni facet, the first step for O spillover, has an energy barrier of 1.36 eV. The initial and final states of this process are depicted as configurations 1 and 2 in Fig. 3. This O atom may easily diffuse on the Ni cluster afterwards, as shown in configuration 3, with an energy barrier of only 0.73 eV. Obviously, H oxidation may occur on the Ni surfaces without the need for O-migration from the bulk YSZ. To study further the formation of hydroxyl or H2O through this pathway, we add one H atom into the system and calculate its co-adsorption with the spillover O atom. Configuration 30 shows an initial state for the formation of hydroxyl, with the H atom taking the hollow site near the spillover O atom. The H atom can migrate to the O atom and form the hydroxyl as depicted in configuration 4 in Fig. 3 with a barrier of 0.9 eV. Similarly, we add one more H atom to another neighboring hollow site on Ni with the hydroxyl as configuration 4 0 in Fig. 3. From this initial state, the H atom can react with the hydroxyl and form H2O in the final state (configuration 5 in Fig. 3) with an
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Fig. 3 The energy profile along the O spillover pathway for H oxidation. The energy of the intermediate state 3 is set to zero, and the values on the vertical coordinate are relative energies with respect to state 3. The O spillover and diffusion pathways in the pure Ni/YSZ, doped Ni/YSZ with a Ni dopant, and defective Ni/YSZ with an interface O vacancy are represented by the blue thick, thin and dashed curves; the green curve represents H2 oxidation by the spillover O. All energy barriers are given in the pictures. All configurations of intermediate states along the reaction coordinate are displayed. The states 3 0 and 4 0 are the results with the state 3 and 4 with the addition of an adsorbed H, respectively.
activation energy of 0.95 eV. Overall, the O spillover mechanism based on the most stable Ni/YSZ TPB model has an energy barrier of 1.36 eV for the rate-determining step: O detachment. This barrier is comparable with that of the H spillover mechanism, 1.3 eV14 or 1.04–1.68 eV.34–36 Moreover, we also study the energetics of the H-spillover pathway (details of NEB simulations are given in ESI,† Fig. S1) near the type-II TPBs. The activation energy of the first H spillover step, formation of hydroxyl, is 1.37 eV. This is only slightly larger than the value of the O spillover pathway, 1.36 eV. Therefore, we perceive that the two mechanisms coexist for hydrogen oxidation near the stable type-II TPBs displayed in Fig. 2(b) and (c). The reaction rates of both channels should be comparable to that of the hydrogen spillover pathway on the type-I TPB. Evidently, the Ni/YSZ anodes in SOFCs are more efficient than what was thought in previous theoretical studies,14,22,23 because of the additional unexplored oxygen spillover pathway on more stable TPBs. Since the availability of TPB-O atoms is crucial for the oxygen spillover reaction dynamics, it is instructive to use the number of them (denoted as NTPB-O) as a microscopic parameter to characterize the anode catalytic performance. For TPBs along the h110i orientation on YSZ(111), we can estimate NTPB-O from the TPB length by using the formula. NTPB-O = TPB length/(2dO–O),
(2)
where dO–O is the distance of two neighboring TPB-O atoms along the h110i orientation on the YSZ(111) surface. As sketched in Fig. 2(d) dO–O is 3.6 Å in our model for the type-II structure. When TPB-O atoms become reactants, one crucial issue for catalytic reactions is their mutual influence and supply. Multiple
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Fig. 4 The comparison of the two supposed CO oxidation pathways, displayed by the colored solid and dash curves, respectively. The blue curve represents the O spillover and diffusion pathway; and the red curve represents CO oxidation by the spillover O (Osp). All energy barriers are given in the pictures. The left inset shows CO spillover from the Ni to YSZ surface, where the energy difference between state 1 and 2 in the inset is 1.9 eV. The detailed geometry structure of each state can be found in the ESI,† Fig. S4. The right inset shows two hollow sites on the Ni cluster with different distances to the active interface O.
O spillover processes may happen simultaneously around TPBs. These processes may affect each other because of the absence of adjacent TPB-O atom(s). To see if this issue is important, we calculate the O-spillover transition state at an O-deficient TPB (the structural details are displayed in ESI,† Fig. S2). From the energy barriers shown by the dashed curve in Fig. 3, the activation energy is hardly changed (1.39 eV). Therefore, the mutual influence between O spillover steps even from the same TPB is rather weak. In fact, when the vacancy concentration become large at the anode, the O vacancies can be refilled by O from YSZ bulk under the SOFC work conditions. Therefore we only discuss the effects of one O defect on TPB-O spillover in this paper. To accelerate the O spillover, one may consider using dopants near the TPBs so as to reduce the energy barrier and energy cost for the detachment of TPB-O atoms. To this end, Ni diffusion into YSZ is beneficial since the intermixing of two materials significantly change the chemistry in the TPB region. This phenomenon was noticed in experiments,37–39 but the underlying mechanism has never been addressed. Here, we also study the effect of Ni dopants near the typeII TPBs on the O spillover process. Dramatically, the activation energy for the TPB-O detachment decreases to only 0.88 eV, as shown by the thin blue curve in Fig. 3. Details for the O spillover reactions near the Ni-doped TPBs can be found in the ESI,† Fig. S3. Clearly, one can modulate conveniently the activity of Ni/YSZ anodes by changing the interfacial structures and chemical components. Further studies along this line should be very valuable. Now we investigate the effect of O spillover on the electrochemical reaction CO oxidation,1 described as CO + O2 - CO2 + 2e
(3)
Fig. 4 and its left-inset show the energy profiles along O and CO spillover pathways, respectively. The possible initial CO
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adsorption sites on Ni are shown in the right inset (additional structural information can be seen in the ESI,† Fig. S4 and S5). Unlike the small H atom that takes the hollow site A next to the active TPB-O, the CO molecule can only take site B or other hollow sites farther apart due to its larger van der Waals size. As a result, CO spillover requires large activation energies (41.9 eV) to extract the TBP-O atom from YSZ. Therefore, the CO spillover is quite difficult and this pathway should be ruled out. In contrast, the activation energy for CO oxidation with an O adatom on the Ni facet is only 0.83 eV. Therefore, the new oxygen spillover mechanism is applicable for both H2 and CO oxidation in SOFCs. Now we see that H2 oxidation may proceed with both H spillover and O spillover pathways whereas CO oxidation can only adopt the O spillover pathway. These results provide clues to understand the low efficiency of Ni/YSZ catalysts toward the oxidation of dry CO as observed in experiments.40,41 Without the straightforward insights discussed above, this observation was attributed vaguely to the difference in the mass-transfer resistance of the active species. It was also observed that the electrochemical reaction rate of humidified CO fuel gas is high, even to the level of H2. This can also be interpreted with our new results. Humidified CO first undergoes the water gas shift reaction at temperatures above 250 1C on Ni catalysts,40,42,43 as described in the following equation: CO + H2O - CO2 + H2
(4)
Therefore, the actual fuel that is involved in the electrochemical process becomes H2 instead of CO. Since H2 oxidation may proceed with both the H spillover and O spillover pathways, the CO - H2 conversion leads undoubtedly to significant enhancement of the electrochemical reaction rate of wet CO.
Conclusions Ab initio simulations suggest that the local TPB structures have a strong influence on the reaction mechanism for the H2 and CO oxidation on Ni/YSZ anodes in SOFCs. Based on the newly optimized stable TPB structures, we find that O spillover plays a crucial role in the oxidation reaction of both H2 and CO. With the coexistence of O spillover and H spillover pathways, we believe that the H2 oxidation rate in SOFCs is high. Importantly, we find that the catalytic performance of Ni/YSZ nanocatalysts can be enhanced further by doping, or even by Ni–YSZ interdiffusion. For CO oxidation, the CO spillover pathway is essentially blocked and the O spillover pathway is dominant. This result can be applied for explaining the observation of sharply different reaction rates for the combustion of dry and wet CO. Our studies provide useful new insights for the understanding of oxidation processes of different fuels near TPBs and, furthermore, for the optimization of the performance of SOFCs. Finally, we emphasize that, in many occasions (not only in SOFCs) the chemical reactions occurring at the triple-phase boundaries are a multi-path process due to the multiple local structures of TPBs, and any single mechanism cannot give an
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unified explanation for the corresponding experimental results and chemical phenomena.
Acknowledgements
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This work was supported by the National Natural Science Foundation of China (Grant No. 11247012 and 11174070). Work at UCI was supported by the National Science Foundation under CHE-0802913 and computing time at XSEDE.
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