BaCe1âxYxO3âÎ´ Anodes for Solid Oxide Fuel Cells

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Coke-Tolerant Ni/BaCe YO Anodes for Solid Oxide Fuel Cells: DFT+U Study Maxim Shishkin, and Tom Ziegler J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp312485q • Publication Date (Web): 19 Mar 2013 Downloaded from http://pubs.acs.org on March 20, 2013

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The Journal of Physical Chemistry

Coke-Tolerant Ni/Bace1-Xyxo3-δδ Anodes For Solid Oxide Fuel Cells: DFT+U Study. M. Shishkin* and T. Ziegler. Department of Chemistry, University of Calgary, University Drive 2500, Calgary, AB, T2N 1N4, Canada.

Abstract. Carbon removal from the anode triple phase boundary (TPB) of solid oxide fuel cells (SOFCs) by adsorbed water molecules has been studied by density functional theory (DFT). We evaluated the energy pathways of water adsorption with subsequent oxidation of interfacial carbon for the case of Ni/BaCe1-xYxO3-δ and conventional Ni/YSZ anodes. It has been found that oxidation of interfacial carbon, which occurs via a reaction with hydroxyls, released by adsorbed water molecules, is significantly more favorable on Ni/BaCe1-xYxO3-δ anodes as compared to Ni/YSZ. We argue that favorable carbon oxidation is governed by the ability of the oxide to adsorb and partially split water molecules. We also analyzed the underlying reasons for favorable water adsorption on the oxide surface and found that more favorable water adsorption occurs on the surfaces of the oxides with the lower value for the electronic work function and higher Fermi basicity. To generalize the latter principle we studied water adsorption on CaO and SrO oxides, which have the same crystal structure as BaO and found that the more favorable water adsorption takes place on a surface of BaO, which also has the lowest electronic work function. In line with previously published works, our findings indicate that a Ni/oxide anode, where the oxide surface has BaO termination, has a high resistance towards blocking of the TPB with carbon atoms in a water-containing atmosphere.

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*Corresponding author: Department of Chemistry, University of Calgary, University Drive 2500, Calgary, AB, T2N 1N4, Canada, email: [email protected]; tel 1-4032109779 Key words: perovskites, anodes, ab initio, fuel cells.

1. Introduction.

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The potential ability to use a wide range of gaseous fuels is a key advantage of solid oxide fuel cells (SOFCs) as compared to other types of fuel cell technologies.1 This fuel flexibility is achieved by reducing the oxygen molecules in the cathode compartment and transporting the oxygen ions for fuel oxidation through the solid-state electrolyte, which requires high operating temperatures (600-10000C). Unfortunately, the high operating temperatures of SOFC lead to a number of complications that can be detrimental for the cell performance. These include low anode stability to redox cycling,2,3 adverse reactions between adjacent cell components4,5 and temperature gradients in the cell parts occurring during the start-up and shut-down phase.6,7 Moreover, if hydrocarbon fuel is used, the metallic (i.e. Ni) part of the SOFC anode is also rapidly covered by a carbon film (coke formation) at elevated temperatures (most severely at T>8000C), which results in chemical deactivation of the anode.8 Numerous attempts have been made by researchers worldwide to either diminish or fully eliminate coke formation on the anode surface.9,10 For instance, Gorte et al proposed an anode with a chemically inactive metal (i.e. Cu), that would only function as a current collector.11-13 On the other hand, the metal oxide (i.e. ceria or doped ceria) would both conduct the oxygen anions and oxidize the fuel.11-13 The problem associated with such anodes is a low melting temperature of Cu, which causes a structural and functional instability of the anode at the cell operating conditions (T>7000 C).14,15 This difficulty may be partially circumvented by Cu alloying with Ni or adding Co to Cu, as these metals will stabilize the cermet structure. However they will at the same time reintroduce susceptibility towards coke formation.16-19

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Replacing the metals with perovskite materials is another approach towards the development of coke-tolerant anodes.20 LaMnO3-based anodes with various substitutional dopants at both A and B sites have been used in conjunction with the YSZ electrolyte.21-24 The general drawback of such anodes is a poor catalytic activity for hydrocarbon oxidation, accompanied by the additional problem of a substantial loss of electronic conductivity of the perovskite, incorporated into YSZ, as compared to the bulk perovskite structure.10 Other anode cermet compositions with high resistance to coke formation have been proposed recently.25-27 However, coke-resistance in the respective anodes is usually achieved at the expense of losing electrochemical activity, which emphasizes the need for the development of a coke-tolerant anode that does not suffer from loss of power generation. In this regard, the Ni/BaZr0.1Ce0.7Y0.2-xYbxO3-δ anodes, studied by Yang et al, fall into the category of anodes, characterized by a long operational time (>100 hours), when using propane as a fuel, and high output power densities.28 The resistance to coke formation of these anodes heavily relies on supply of water (~3 vol % steam), indicating that adsorbed water molecules affect the removal of carbon from the anode triple phase boundary (TPB). Indeed, a fully dry hydrocarbon fuel causes coke formation on the Ni surface of these anodes, leading to a rapid drop of a cell voltage with time.28 It has been hypothesized that water, adsorbed on a perovskite surface, oxidizes impurity atoms (i.e. C or S), leading to more stable operation of the cell, unhindered by the coke formation or sulfur poisoning. We note that the suppression of coke by adding water on a perovskite surface has been reported previously. Coors have shown that carbon deposition is less critical if water molecules are aggregated on a BaCe0.9Y0.1O3-δ surface, as this layer of

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surface water would act as a source of hydroxyls, involved in oxidation of the surface carbon species.29 It should be mentioned that in the work of Coors,29 the water molecules are delivered to the anode surface via migration through the electrolyte, rather than direct adsorption from the gas phase as is done in the experiments of Yang et al.28 In their more recent study, Yang et al have proposed to cover the Ni surface of Ni/YSZ anode with BaO nano-islands, achieving a similar effect of carbon removal from the metal surface (hydrocarbon fuel was used in this case as well) when a small amount of water (~3 vol % steam) is added to the fuel feed.30 The BaO nano-islands were found stable on a Ni surface under SOFC operating conditions and did not cause a drop in generated power. To elucidate the mechanism of carbon removal from the Ni surface and the role of water in this process, Yang et al performed DFT modeling on the oxidation of a carbon atom, located on a Ni surface, close to BaO/Ni interface. The energetics of adsorption of a water molecule on a BaO surface with subsequent formation of COH and surface hydrogen has been evaluated using a proposed model of BaO islands on a surface of a Ni slab. The interfacial COH was found to favorably dissociate into CO and H species at the metal/oxide interface. The overall reaction of water adsorption with production of interfacial CO and hydrogen species on the Ni surface, led to an enthalpy decrease of 3.16 eV, indicating that water-assisted carbon removal is indeed a very favorable reaction. The BaO surface was found to be an effective catalyst for water adsorption (as compared to Ni), whereas hydroxyl groups from adsorbed water favorably interact with carbon, leading to formation of CO species. In this work we perform a similar study of water-assisted carbon removal from a more complex system, a Ni/BaCe1-xYxO3-δ interface, which is used as a prototype for the anode

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proposed in the work of Yang et al.28 The paper is organized as follows. The computational details are provided in Section 2. The study of methane interaction with the surfaces of BaCeO3 is provided in Section 3. In Section 4 we introduce our model for the Ni/BaCe1-xYxO3-δ interface and demonstrate that water adsorbs very favorable on a BaO termination of the BaCe1-xYxO3-δ perovskite with subsequent formation of COH and then CO species at the interface. We also study a similar mechanism of interfacial carbon removal from a conventional Ni/YSZ system and compare the obtained energy pathway with that obtained in the study on Ni/BaCe1-xYxO3-δ. Moreover, using microkinetic modeling, we show that the rate of carbon oxidation is significantly higher in the case of Ni/BaCe1-xYxO3-δ and argue that favorable carbon oxidation requires a strong adsorption of a water molecule on the oxide surface. In Section 5 we analyze the factors, contributing to water adsorption on an oxide surface using a combined charge and energy decomposition scheme. These findings allow for the formulation of a more general correlation between the energy of water adsorption and the work function of the respective materials relative to their Fermi basicity. The summary of the results is finally provided in Section 6.

2. Computational framework. For the calculation of the atomic structures and reaction energies we have used the VASP ab initio package.31-33 For all calculations a local PBE functional has been employed.34 The Hubbard corrections for f-electrons of cerium atoms have been applied within the DFT+U methodology.35 We used a value of Ueff = 5eV, as has been determined in our previous work on BaCeO3.36 The Kohn-Sham orbitals have been expanded in plane

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waves with an energy cut off of 400 eV. Spin-polarized calculations have been applied throughout.37 We used a 2×2×1 Monkhorst-Pack k-point mesh for the smaller slabs, employed in the study of methane oxidation on a CeO2(100) and BaO(100) termination of BaCeO3. A more moderate 1×2×1 k-point mesh was employed for the Ni/BaCe1xYxO3-δ

slabs, in view of the longer cell dimension in one of the horizontal directions (see

Section 4 for details).38 It should be mentioned that these settings for cutoff energy and the k-point sampling have been shown to be sufficient for convergence (e.g. of vacancy formation energies) as demonstrated in our previous work.36 Projector augmented plane wave (PAW) potentials have been used in this work for the treatment of the electron-ion interactions.39,40 For optimization of the atomic structure, the Hellmann-Feynman forces have been minimized to the maximum value of 0.03 eV/Å on the atoms of the slabs. The barriers of molecular adsorption and surface reactions have been evaluated using the nudged elastic band (NEB) method, implemented in VASP.

3. Methane oxidation on CeO2(100) and BaO(100) terminations of BaCeO3. Adsorption and oxidation of hydrocarbon fuel (i.e. methane) can take place on a metal as well as on an oxide surface of the anode cermet. Methane oxidation on the Ni surface has been studied previously by several groups.41 A similar study of methane oxidation on the BaCeO3 surfaces is however still absent. Of particular interest here would be the investigation of interactions of fuel molecules with the most favorable surface terminations of BaCeO3 under SOFC operating conditions. In our recent work we have determined that (100) terminations (CeO2 and BaO) have the lowest energy of formation via crystal cleavage, indicating that formation of these terminations is the most feasible.36

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Moreover we have performed an evaluation of the surface free energy of several terminations of BaCeO3 as a function of temperature and partial pressure of oxygen.36 It has been determined that only BaO(100) termination is stable under SOFC operating conditions, whereas CeO2(100) termination is unstable with respect to precipitation of cerium oxide. Additionally, we have also shown that vacancy formation on a CeO2(100) termination is more favorable than on a BaO(100) termination, indicating that CeO2(100) has a higher chemical activity. Although CeO2(100) termination is not thermodynamically stable, we evaluated the energy pathway of methane oxidation and monitored the electronic properties of the structures at each stage of methane dehydrogenation for comparison with similar reaction steps on BaO(100). Such a comparison is useful for understanding the impact of the surface reduction ability (vacancy formation energy) on the thermodynamic pathway of methane adsorption as well as the influence of dehydrogenation of methane on the reduction of the cerium oxidation state. In our study we use a BaCeO3 slab which consists of 8 atomic layers (i.e. CeO2 and BaO layers), with CeO2(100) and BaO(100) terminations, as depicted in Fig. 1. The number of atomic layers should be sufficient, as converged values of vacancy formation energies have been obtained if more atomic layers are used. We used a vacuum spacing layer of 12 Å, which provides converged values of surface vacancy formation energy (within 2 kcal/mol). In this work each aforementioned slab consists of 80 atoms of the oxide with 8.953Å ×8.954Å×27.838 Å cell dimensions. Large horizontal dimensions (8.953Å ×8.954Å) are found sufficient to prevent non-physical interactions between periodic images, as the water adsorption energy changes by less than 2 kcal/mol upon increasing of one horizontal cell dimension.

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Positions the atoms of the bottom layer (i.e. BaO and CeO2 layers for the slabs with CeO2(100) and BaO(100) terminations respectively) have been frozen during the optimization procedure, whereas positions of all other atoms have been fully optimized. In this study we used asymmetric slabs for evaluation of the energies of adsorption and surface reaction. Although the dipole moment is induced in such slabs, particularly upon vacancy formation and/or addition of the Ni cluster on the slab surface (see subsequent Section), the employed slabs provide reliable energies of adsorption and surface reaction. As a proof of the principle we have evaluated the energy of a vacancy formation on the surface of the asymmetric slab and found it to be very close (within 4 kcal/mol) to the respective value for the symmetric slab when a vacancy is introduced at the top and the bottom of a slab.36 The energy pathway of methane adsorption and dehydrogenation on CeO2(100) termination is presented in Fig. 2 (top). For each dehydrogenation step, we studied the energy of hydrogen release at the nearby surface oxygen site and a subsequent configuration which corresponds to dehydrogenated *CHx species and surface hydrogens, separated infinitely far on the CeO2(100) surface (we use an asterisk symbol as a notation for the surface species, i.e. *CH3). These hydrogens, “removed” infinitely far on a surface, are denoted as *H ∞ and their relative energy is evaluated as

∆E(∗H ∞ ) = E(∗H) − E(∗)

(1)

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where E(∗H) is the energy of a slab with a single hydrogen on a surface, whereas E(∗) is the energy of a slab with a clean surface. The relative enthalpies of *CHx species are calculated as:

∆E(*CH x ) = E(*CH x ) + (4 − x) * ∆E(*H ∞ ) − E(↑CH 4 )

(2)

where E(∗CH x ) and E(↑ CH 4 ) are the energies of a slab with a *CHx species and a slab with clean surface and a methane molecule, introduced into the vacuum region. For the configurations, which include an abstraction of hydrogen at the nearby site, the relative enthalpies have been calculated as:

∆E(*CH x + *H ) = E(*CH x + *H ) + (3 − x)* ∆E(*H ∞ ) − E(↑ CH 4 )

(3)

Methane adsorption is associated with a kinetic barrier of 63 kcal/mol, whereas the relative enthalpy of the adsorbed species is almost the same as for a reference state (methane molecule in the gas phase (Fig.2, top)). An adsorbed methane molecule splits into two parts: one surface oxygen forms a bond with *CH3, whereas the other hydrogen forms the *H surface species. This results in breaking of two oxygen-cerium bonds of the oxide, leading to the oxidation state reduction of two cerium atoms. Our calculations show that subsequent dehydrogenation steps, up until *CH formation, result in an enthalpy increase (Fig. 2, the respective number of *H ∞ is assumed to be present on a surface, but we do not include these species for brevity of notations). Additionally, for each *CHx and *CHx +*H on the surface (1

BaCe1âxYxO3âÎ´ Anodes for Solid Oxide Fuel Cells

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BaCe1âxYxO3âÎ´ Anodes for Solid Oxide Fuel Cells