ARTICLE pubs.acs.org/JPCC
Effect of Solvation on the Oxygen Reduction Reaction on Pt Catalyst Stephen P. Walch Department of Mechanical Engineering, Stanford University, Stanford, California 94305, United States ABSTRACT: Previous models of PEM fuel cells have not explicitly considered the effect of a bulk water layer on the mechanism of the oxygen reduction reaction. The dominant mechanism is considered to be PtO2 þ H þ þ e- f PtO2 H, PtO2 H f PtO þ PtOH, PtO þ H þ þ e- f PtOH, and PtOH þ H þ þ e- f PtOH2 . Using the B3LYP functional with the LANL2DZ basis set and effective core potential on Pt and the CC-pVDZ basis set on O and H with two clusters, the first a planar Pt10 cluster and the second a three-layer Pt20 cluster, we have considered the effect of a solvation model consisting of two stacked hexagonal water structures with another water in the center above a hexagon of surface Pt atoms. We find that dissociation of O2H is much less favorable on the water covered surface as compared to the bare surface unless at least one water molecule is removed to create two adjacent surface Pt atoms. We find an H2O 3 O2 complex, where one H2O has been turned so as to form a hydrogen bond with the O2, and this complex can be viewed as the branch point in the mechanism. O2 can be reduced to HO2, which can dissociate to OH plus O if at least one water is removed from the surface. Alternatively, we find a new low energy pathway: H 2 O þ O2 f OH þ HO2 , HO2 þ H 2 O f H 2 O2 þ OH, and H 2 O2 f OH þ OH. This reaction sequence is found to be all downhill or thermoneutral. We believe both of these pathways are involved in the oxygen reduction reaction in PEM fuel cells.
I. INTRODUCTION In a previous paper,1 we discussed calculations to predict barriers and rate constants for elementary reaction steps in the oxygen reduction reaction (ORR) on Pt catalyst for a PEM fuel cell. In that paper, we reviewed the existing mechanisms for the ORR but we did not take into consideration the role of the water. The mechanism, which is usually postulated, includes PtO2 þ Hþ þ e- f PtO2 H
ð1Þ
PtO2 H f PtO þ PtOH
ð2Þ
PtO þ Hþ þ e- f PtOH
ð3Þ
PtOH þ Hþ þ e- f PtOH2
ð4Þ
Reaction 1 produces the HO2 species and is usually preferred over direct dissociation of O2, though recent experiments have shown that the rate of O2 dissociation2 is larger than had been previously predicted. Both calculations by Sidak and Anderson3 as well as those in ref 1 showed a very small barrier to dissociation of HO2 (reaction 2). Rai, Areyanpour, and Pitsch4 studied the rates for the electrochemical steps (1), (3), and (4). A difficulty in this work is that it is difficult to accurately calculate the reversible potential for these redox processes. Experiment5 showed that water forms a hexagonal overlayer structure on the Pt(111) surface, as we later discuss in section 3. Also, Norskov et al.6 predicted that at cathode potentials greater than ∼0.9 V water is oxidized to OH, leading to an alternating r 2011 American Chemical Society
water-OH hexagonal structure which has also been observed experimentally.7 Rai8 proposed a site blocking model based on the water overlayer structures discussed above. In this model, the rate of the elementary reactions depended on the species present at adjacent sites. They postulated that dissociation of HO2 required at least two adjacent free surface Pt atoms. In this way, they developed a kinetic Monte Carlo model for the ORR reaction at the cathode of a PEM fuel cell. The present studies began by looking at the bonding of O2 and HO2 to the Pt(111) surface with a water monolayer. Here, we used the hexagonal water overlayer structures for cathode potentials less than 0.9 V and the mixed water OH overlayer for cathode potentials greater than 0.9 V. We also looked at the barrier for dissociation of HO2 as a function of the number of water molecules present on the surface in order to test the assumptions in the model of Rai.8 Jinnouchi and Anderson9a have developed a method for determining the effect of solvation on redox potentials using an electrostatic continuum solvent model developed by Fattenbert and Gygi.9b In the present work, we have adopted the alternative method of explicitly including water molecules up to two layers of water above the Pt surface. The role of H2O2 in the ORR reaction has not been well established. Rai8 proposes a parallel pathway to reaction 2 where a different O2 binding site, possibly with the O2 perpendicular to the surface, leads to H2O2. In this work, we consider an alterReceived: July 13, 2010 Revised: January 21, 2011 Published: March 28, 2011 7377
dx.doi.org/10.1021/jp106497h | J. Phys. Chem. C 2011, 115, 7377–7391
The Journal of Physical Chemistry C
ARTICLE
Table 1a. Energy for Pt10 as a Function of Spin Multiplicity
Table 1b. Energy for Pt20 as a Function of Spin Multiplicity
multiplicity
energy, EH
multiplicity
energy, EH
11
-1191.61098
21
-2383.56431
9
-1191.61877
19
-2383.59895
7
-1191.61537
17
-2383.62659
5
-1191.60314
15
-2383.63078
3
-1191.59498
13
-2383.62243
1
-1191.56302
native pathway to H2O2 starting with the direct reaction between a surface water and a surface O2 (direct H2O-O2 reaction).
Table 1c. Energetics for Dissociation of HO2 on Pt10 Cluster
structure Pt10 HO2
HO2 þ H2 O f H2 O2 þ OH In this paper, we have looked at how the presence of water on the surface affects the dissociation of HO2 and the direct H2O-O2 reaction. To the best of our knowledge, these calculations are the first systematic study of the effect of explicit inclusion of water on these pathways for the ORR. The details of the calculations are presented in section II. The results are discussed in section III, and finally, in section IV, we present the conclusions.
Pt10 3 O2H 3 outer Pt10 3 O2H 3 sp Pt10 3 O2H 3 inner
energy
ΔE, eV
-1191.61098 -150.91488
energy
ΔE, eV
-1191.65333 -150.91488
-1342.52586
0.0
-1342.56821
0.0
-1342.54985
-0.65
-1342.59010
-0.60
-1342.53492
-0.25
-1342.573
-0.13
-1342.60363
-2.10
-1342.64263
-2.03
Table 1d. Energetics for Dissociation of HO2 on Pt20 Cluster LANL2DZ þ f
LANL2DZ
II. COMPUTATIONAL DETAILS The method used here has been described in detail in ref 1. The Pt atom is in the third transition row and has the atomic configuration
structure
½Kr 4d10 4f 14 5s2 5p6 6s1 5d9 5d10 6s2 5d8 where 6s15d9 is the ground state configuration and the 5d10 and 6s25d8 states are 0.5 and 0.6 eV higher in energy. The 6s and 5d levels are considered to be valence, the 5s and 5p levels are semicore, and the 4d and 4f levels are core. Thus, normally 60 electrons are taken as core. As a third transition row element, an accurate description of the Pt atom requires inclusion of relativistic effects. In this work, we included these effects using the relativistic effective core potentials developed by Wadt and Hay.10 Here, we used the LANL2DZ potential, which replaces the 60-electron core. Wadt and Hay also developed double-ζ level basis sets for use with their ECPs, which were used here without modification. We used a planar Pt10 cluster and a three-layer Pt20 cluster, which were described in ref 1. The calculations used the DFT method with the B3LYP11 hybrid12 functional and were carried out with Gaussian 03.13 Calculations for the singlet and triplet states of Pt2 indicate that the Pt-Pt bond is mainly Pt 6s-6p like and the 5d levels are only weakly coupled. A CASSCF/MRCI calculation for Pt2 at a bond length of 4.75 au found the singlet state, which has a Pt-Pt 5dσ bond, to be only 0.15 eV below the triplet state, which has the 5d electrons in 3dδ orbitals. With respect to the choice of spin state, it is important to understand that DFT is an approximation to Hartree-Fock, or UHF in the case of the formulation in Gaussian. Thus, DFT has some of the same problems as Hartree-Fock. One well documented problem is the inability to correctly dissociate H2. In the case of Pt clusters, the 3d levels are only weakly overlapping and the singlet coupling would be poorly described, as for H2 at large internuclear separation. Thus, we have adopted a cluster model in which the Pt atoms are assumed to be in a 6s15d9 atomic configuration and we assume the 5d levels are only weakly
LANL2DZ þ f
LANL2DZ
H2 O þ O2 f OH þ HO2
energy
Pt20
-2383.56431
HO2
-150.91488
Pt20 3 O2H 3 outer Pt20 3 O2H 3 sp Pt20 3 O2H 3 inner Pt20 3 O2H 3 outer2 Pt20 3 O2H 3 sp2 Pt20 3 O2H 3 inner2
ΔE, eV
energy
ΔE, eV
-2383.65931 -150.91488
-2534.47919
0.0
-2534.57419
0.0
-2534.54198
-1.71
-2534.6387
-1.76
-2534.52255
-1.18
2534.6176
-1.18
-2534.550
-1.93
-2534.6471
-1.98
-2534.54568
-1.81
-2534.6414
-1.83
-2534.5288 -2534.5650
-1.35 -2.33
-2534.6258 -2534.6699
-1.40 -2.60
interacting and are high-spin coupled. Similar conclusions have been reached by Kua and Goddard14 who also note that the metal-metal bonding mainly involves the metal 6s level and the metal to O bonding is mainly through the metal 5d levels. Table 1a shows the energy of the planar Pt10 cluster as a function of spin multiplicity. Note the preference for high-spin states over low-spin states. The ground state has a spin multiplicity of 9 but is only slightly lower in energy than the spin multiplicity 11 state, which we assume to be the ground state. Table 1b shows similar results for Pt20, where again high spin states are favored over low spin states. The waters are closed shell singlets and thus do not change the spin multiplicity. However, OH or HO2 tie up one 3d orbital in a chemical bond and thus reduce the spin multiplicity by 1. O2 in a peroxy state removes one 3d orbital but adds an O2p radical orbital so it does not change the spin multiplicity. While we noticed in our previous work1 that significant cluster size effects are observed for the relatively small clusters we have used, we have found that most of this effect derived from the approximation of treating the central two Pt atoms at a higher level than the surrounding nearest neighbor Pt atoms. In the present work, all the Pt atoms were treated equivalently and we see from Tables 1c and 1d that the best estimates for the barriers for dissociation of HO2 are 0.47 and 0.43 eV for the Pt10 and Pt20 7378
dx.doi.org/10.1021/jp106497h |J. Phys. Chem. C 2011, 115, 7377–7391
The Journal of Physical Chemistry C
Figure 1. Structure of the cyclic water hexamer. Red spheres are O atoms, and white spheres are H atoms.
clusters, respectively. This can be compared to a barrier of 0.37 eV obtained with a periodic boundary condition by Tripkovic et al.6b Note the barrier height for the Pt20 cluster is derived from the Pt20 3 O2H 3 outer2 and Pt20 3 O2H 3 sp2 energies. See section III.c for further discussion of the results in Tables 1c and 1d. Thus, we conclude that cluster size effects are actually quite small for this system. We also observed that the limitations of the DFT methods used in codes such as VASP introduce errors that are larger than the cluster size effects. For example, we find if we change the functional from B3LYP to PBE for a Pt10 3 2H2O 3 4OH cluster we find the binding energies of O2 and O2H increase by 0.85 and 0.38 eV, respectively. We find very similar results with the PW91 functional. Thus, the cluster size effects are smaller than the variations from choice of functional. The RPBE functional has been modified to reproduce binding energies to surfaces, and thus, Tripkovic et al.6b find results very similar to ours for the dissociation of HO2 using this functional. In general, we have found that DFT methods tend to overestimate binding energies on metal surfaces. Normally, ab initio methods tend to underestimate binding energies, since they are variational methods; i.e., the energy is evaluated with an approximate wave function for the correct electrostatic Hamiltonian, and improvement in the wave function improves the energy. However, in DFT, the Hamiltonian is modified to give better agreement with experiment and hence the method is not variational. Therefore, it is possible to get binding energies larger than experiment with DFT. In ref 1, comparison of DFT to ab initio calculations showed that the DFT surface was too attractive. However, in general, the whole surface was shifted to lower energy, so the energy differences, which are critical in chemistry, are more accurate than the absolute energies. In the calculations reported here, we also find some overestimation of binding energies. For example, in ref 1, we computed a binding energy for an O atom on Pt(111) of 3.79 eV compared to experimental results of 3.43-3.71 eV for low coverage as quoted by Jacob, Muller, and Goddard.14b Thus, the overestimation of the binding energy is not large for the B3LYP functional. The method used here has several distinct advantages including the following: (1) we are able to find actual saddle points, (2) we are able to accurately describe spin states, (3) we are able to examine systems with broken symmetry, and (4) we are able to do a vibrational analysis from which we can derive thermodynamic properties.
ARTICLE
Figure 2. Structure of Pt10 with a water overlayer consisting of a water hexamer and one additional water. Red spheres are oxygen atoms, white spheres are hydrogen atoms, and small blue spheres are platinum atoms. The same conventions apply to the other figures.
Figure 3. Structure of the water overlayer with an electric field.
III. DISCUSSION III.a. Water Overlayer Structures. We first looked at the structure of (H2O)6, since the water overlayer has a hexagonal structure similar to the known cyclic structure of the water hexamer.15 Figure 1 shows the structure of the water hexamer. Here, one sees that one OH bond of each water is directed toward the O atom of an adjacent water, leading to a cyclic hydrogen bonding structure. The remaining OH bond of each water is alternately up and down, which reduces the repulsion between the dipole moments of the polar OH bonds. Figure 2 shows the structure of Pt10 with a water overlayer consisting of a water hexamer and one additional water. As is evident from the figure, the water hexamer has a good registry with the surface Pt atoms. The water hexamer distorts in the following way: (1) The waters with up H become parallel to the surface and move closer to the surface. (2) The waters with down hydrogen remain with hydrogen down and move further above the surface. This leads to the experimental structure found by Ogasawara et al.5 At the cathode of a fuel cell, the metal surface is positively charged, which repels the H end of the OH bonds pointed toward the surface. This electrostatic effect switches the orientation of these bonds to pointing up, as seen in Figure 3. In the calculations, the up orientation of the OH bonds was obtained by including an electric field perpendicular to the surface. (The electric field is x-60 in Gaussian, which corresponds to an electric field of 0.31 V/ Å. We estimate an electric field at the cathode of
