J. Phys. Chem. C 2010, 114, 21411–21416
21411
The Electronic Structure and Chemical Properties of a Ni/CeO2 Anode in a Solid Oxide Fuel Cell: A DFT + U Study M. Shishkin* and T. Ziegler Department of Chemistry, UniVersity of Calgary, UniVersity DriVe 2500, Calgary, Alberta T2N 1N4, Canada ReceiVed: June 9, 2010; ReVised Manuscript ReceiVed: October 20, 2010
Using a model for the Ni/ceria anode of a solid oxide fuel cell (SOFC), we have performed a DFT + U study of H2 and CH4 activation on the ceria surface and elucidated the mechanisms of charge transfer as a result of ceria reduction. Bader’s Atoms in Molecules (AIM) analysis scheme establishes that deposition of nickel on stoichiometric ceria to form the Ni/ceria anode results in the transfer of electron density from Ni to CeO2. The subsequent removal of an oxygen atom from the ceria surface of the Ni/ceria anode in the process of fuel oxidation results in a minor transfer of electron density from ceria to Ni. The modest amount of transferred electron charge following vacancy formation can be explained by the fact that the rather shallow vacancyinduced occupied band in ceria has a lower energy than the top of the Ni valence band. We also show that the removal of an oxygen atom not bonded to Ni is more favorable (by 28 kcal/mol) than extraction of an oxygen, originally bonded to both Ni and Ce at the Ni/CeO2 interface. This finding would indicate that fuel oxidation should occur predominantly at the ceria surface of the cermet rather than at the Ni/CeO2 interface. Moreover, our study demonstrates that the energies of activation of the fuel molecules (H2 and CH4) on the ceria surface of the Ni/CeO2 cermet are close to the respective values in the case of fuel adsorption on a pure ceria surface. We also show that the reduction of a ceria film on top of the electrolyte (YSZ, yttria-stabilized zirconia), with subsequent migration of an oxygen from the YSZ bulk to ceria, results in accumulation of charge on Ni, accounting for generation of a current in a SOFC. 1. Introduction As a material with outstanding redox properties,1 ceria is widely used in a large variety of fields, including automotive three-way catalysis,2 hydrogen production,3 and the water-gas shift reaction,4 in addition to serving as a support for metal catalysts. Cerium oxide can also be used for oxidation of hydrocarbons, which makes it a suitable candidate as anode material in solid oxide fuel cells (SOFCs).5,6 In this respect, ceria/metal composites (cermets) are particularly attractive as possible replacements for Ni/YSZ (yttria-stabilized zirconia), which is often used as a SOFC anode.6–9 The anode material Ni/YSZ has several important advantages, such as low cost, the ability of the Ni catalyst to activate hydrocarbons, and thermal stability at high operating temperatures, leading to the generation of high power densities. However, Ni also catalyzes adverse side reactions, such as surface carbon film formation (coking). The resulting coverage of the Ni surface by carbon fibers makes the anode chemically inactive.6 To combat the effect of carbon coking on the metal surface, Gorte and co-workers pioneered the use of a Cu/CeO2 cermet as an alternative to Ni/YSZ.10–12 Cu has been proposed as a metal, since it does not catalyze hydrocarbon activation and coking. Thus, its sole role is to serve as a conductor of current generated in response to the electrochemical oxidation of the fuel. On the other hand, ceria is used to facilitate both activation and oxidation of hydrocarbon molecules. However, the low melting temperature of Cu (as compared to Ni) makes copperbased anodes unstable at elevated temperatures (>1073 K), which results in the loss of Cu conductivity due to sintering of the metal into unconnected networks.13,14 As a possible remedy, * Corresponding author. E-mail:
[email protected].
NiCu15–17 or CoCu18,19 alloys can be utilized because higher melting temperatures of both Ni and Co have a stabilizing effect on the metal conductor. Moreover, both Ni and Co increase the chemical activity of the cermet, albeit at the risk of reintroducing coking. Alternatively, cermets of gadolinia-doped ceria and pure ceria with a low concentration of Ni are also being considered as candidates that might achieve a higher tolerance toward carbon coke formation20 as well as diluting the effect of intense volume changes responsible for damaging the cermet or the electrolyte structure.21 As part of the development of a coke-tolerant anode, the mechanisms of fuel oxidation on the SOFC anode need to be understood. It is known experimentally that oxidation of fuel at the anode of SOFC (i.e., Ni/YSZ) is an electrochemical reaction that occurs close to the metal/oxide interface.22 Recently we presented a detailed description of electrochemical oxidation of hydrogen and methane molecules at the Ni/YSZ interface and described the mechanisms of charge accumulation on Ni in response to fuel oxidation using DFT calculations.23 Oxidation of potential fuel molecules (hydrogen,24 methane25 and CO26) on a ceria (111) surface has been studied previously by ab initio methods. With respect to a Ni/CeO2 system, adsorption of Ni on a CeO2 surface has been investigated by the DFT method,27 whereas recently, ab initio investigation of Ni doping in ceria bulk has been reported.28 These studies, however, do not address electrochemical oxidation of fuel molecules at the metal(Ni)/ ceria interface as well as spillover reactions, relevant to SOFC operation. The objective of this work is to perform such a study involving the interaction of fuel molecules with the Ni/ceria interface and elucidate the mechanisms of charge transfer from ceria to Ni in response to ceria reduction. We study in this
10.1021/jp105316p 2010 American Chemical Society Published on Web 11/11/2010
21412
J. Phys. Chem. C, Vol. 114, No. 49, 2010
respect the influence of Ni on the adsorption of fuel molecules on the ceria surface in close vicinity to Ni in order to establish whether the presence of the Ni cluster has any influence on this process. We also investigate the effect of vacancy formation on the ceria surface (as a result of fuel oxidation) and compare the induced changes of the electronic structure with those of the Ni/YSZ cermet in the case of YSZ reduction. Then we evaluate thermodynamic barriers of oxygen migration from the ceria surface to Ni and transfer of hydrogen atoms (produced as a result of hydrogen adsorption on the Ni surface) to ceria: the so-called spillover reactions that are considered to play an important role in electrochemical oxidation of fuel molecules on the Ni/YSZ interface.29,30 Finally, we model the Ni/CeO2/ YSZ system (a Ni/CeO2 anode on top of the YSZ electrolyte of SOFC) and study the impact of ceria reduction with subsequent oxygen migration from YSZ bulk to the ceria surface on the accumulation of negative charge on the Ni part of the anode. 2. Treatment of f-Electrons In this work, we employ the DFT + U framework in Dudarev’s approxiation31 for a corrected description of on-site Coulomb repulsion of f-electrons on the cerium atoms because it is well established that inclusion of the Hubbard U term in the DFT framework leads to realistic localization of the f-electrons on cerium atoms, as opposed to an unphysical delocalized charge distribution, as predicted by regular generalized gradient corrected (GGA) functionals, such as PBE.32 The characteristic f-state is predicted in the gap of reduced ceria within DFT + U, in agreement with experimental observations,32 whereas regular GGA calculations do not yield a gap state in the case of vacancy formation. Moreover, the DFT + U method can be considered as a reasonable compromise between inaccurate regular DFT and computationally demanding hybrid functionals. The latter have been recently applied in the description of the electronic structure and energetics of ceria oxide by several groups.33–37 Although hybrid functionals certainly improve upon regular DFT calculations (i.e., they also remove an unphysical charge delocalization),35 there is no conclusive indication of a more superior performance as compared with DFT + U. For instance, Branda et al. favored the DFT + U over HSE functionals,36 whereas the work of Kullgren et al. demonstrated a better performance of LDA + U and PBE + U over PBE0 and B3LYP functionals in the description of the electronic (and in some cases structural) properties of CeO2 and Ce2O3.37 In this work, we restrict the calculations to the DFT + U framework due to the large size of the cells employed in our study. 3. Computational Details In all calculations presented herein, we use the ab initio code VASP.38–40 Exchange correlation effects are treated employing the PBE functional.41 The projector augmented plane wave (PAW) method has been used for the description of electronion interactions.42,43 The Hubbard parameter, U ) 5 eV, is employed in our work because it has been demonstrated previously that this value yields a qualitatively correct distribution of f-electrons, localized on the ceria atoms, whereas larger values of U result only in a marginal change of the electronic structure.32 The plane wave basis set with a characteristic cutoff energy of 400 eV has been enforced. We employed the k-point mesh of 1 × 3 × 1 to model the slabs with horizontal cell dimensions of 13.44 × 7.76 Å. Spin-polarized calculations have been applied throughout.44 A Hellmann-Feynman force of 0.03
Shishkin and Ziegler
Figure 1. Models of (a) the Ni/CeO2 cermet and (b) the Ni/CeO2/ YSZ structure. The atoms fixed in the process of structural optimization are sandwiched between the broken lines.
eV/Å was chosen as a convergence criteria for optimization of the atomic structure. To analyze the net charges on ceria and Ni parts of the cell, the Bader’s atoms in molecules (AIM)45 method has been used in the implementation of Henkelmann and co-workers.46–48 The charge on the atoms has been calculated as the sum of the positive nuclei of the pseudoatoms plus the negative charge of the valence electrons as determined by the AIM method. 4. The Model of Ni/CeO2 Cermet As a support for the Ni cluster in this work, we used an orthogonal slab of ceria with three O-Ce-O trilayers (Figure 1a). Our model of Ni/CeO2 cermet consists of three O-C-O trilayers, which should be sufficient for obtaining converged results, at least for the reactions occurring on the ceria surface. Indeed, our recent study has demonstrated that in case of a Ni/ YSZ system (which is less computationally demanding to treat), addition of three extra trilayers has a negligibly small effect on electronic properties and energetics in the case of surface reduction reactions.23b A theoretical lattice constant of ceria (5.49 Å) has been employed.25 The low energy (111) plane is used as a termination surface. The Ni cluster is mounted on ceria support, covering only a part of ceria to permit interactions of fuel molecules with the ceria surface. The Ni cluster is periodic in the direction perpendicular to the plane of Figure 1 and faces the gas phase by a (111) surface. The bottom trilayer of ceria (sandwiched by the broken lines) has been kept fixed during the structural optimization, whereas the positions of the two outward trilayers and atoms of the Ni cluster as well as the adsorbed atoms (if present) have been fully optimized. The model of Ni/CeO2/YSZ has been constructed using a unit of 9 mol % of yttria in YSZ with a (111) termination surface, as described in our previous work (Figure 1b).49 The YSZ surface is covered by a single trilayer of ceria, which also has a (111) termination surfaces. The cluster of four Ni atoms is set on top of the ceria trilayer. The horizontal cell dimensions of the Ni/CeO2/YSZ slab are identical to those used for modeling of oxidation reactions on the YSZ surface.49 5. Electronic Structure of Ni/CeO2 Cermet with Stoichiometric and Reduced Oxide Deposition of the Ni cluster on the ceria surface induces transfer of electronic charge from the Ni cluster to the stoichiometric ceria. Thus, Bader AIM analysis shows that the electron density of the Ni cluster is depleted by 1.82e- as compared with the isolated and neutral Ni cluster (Figure 2a). The transfer of charge from the metal, in particular,Pd and Au, to ceria has been observed on the basis of DFT + U studies previously.50,51 DFT + U calculations revealed that Au adatoms
Ni/CeO2 Anode in a Solid Oxide Fuel Cell
Figure 2. Schematics of a Ni/CeO2 cermet (Figure 1a) with the total charge on Ni in the case of (a) stoichiometric ceria, (b)ceria reduction (surface vacancy), and (c)ceria reduction (interface vacancy).
adsorbed on the most stable sites of the ceria surface adopt a positive total charge as evaluated by the AIM method.50 Moreover, concomitant reduction of the valence of a surface cerium atom (Ce4+ f Ce3+) has been observed.50 It has also been found that a Pd adatom, too, causes reduction of the formal valence of a surface cerium atom, which is evidenced by the introduction of an occupied f-state in the PDOS of this cerium atom.51 Similar to these works, we also observe reduction of a formal valence of three cerium atoms on the ceria surface of the Ni/CeO2 cermet, as is revealed by the induced occupied f-states in the PDOS of the respective cerium atoms, which accounts for the charge transferred to the oxide mentioned above. It should be pointed out that in the case of Ni/YSZ cermet as studied by us previously, the magnitude of the charge transferred from Ni to a stoichiometric YSZ is substantially smaller (
