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J. Phys. Chem. C 2008, 112, 18566–18571
Theoretical Study of Early Steps in Corrosion of Pt and Pt/Co Alloy Electrodes Feng Tian and Alfred B. Anderson* Department of Chemistry, Case Western ReserVe UniVersity, 10900 Euclid AVenue, CleVeland, Ohio 44106-7078 ReceiVed: August 8, 2008; ReVised Manuscript ReceiVed: September 23, 2008
Theoretical studies of Pt and PtCo alloy electrode dissolutions in PEMFCs were made using adsorption energies from VASP slab band calculations on the metal surfaces in a linear Gibbs energy relationship. The reversible potential for Co-OH formation from oxidation of H2O on the Pt3Co(111) surface was calculated to be 0.50 V, which is significantly lower than the calculated 0.70 V reversible potential for Pt-OH formation on the Pt(111) surface. Co(OH)2 formation from oxidation of H2O on the Pt3Co(111) surface was calculated to take place at a lower potential than Pt(OH)2 formation on the Pt(111) surface, with the respective reversible potentials 0.58 and 1.92 V. These results for what may be the initial steps of anodic dissolution indicate that Co will dissolve at lower potential than Pt in the alloy surface and leave behind a Pt surface skin when the potential is >∼0.50 V. In addition, the reversible potential for OH formation on a step of edge atom on Pt(111) surface from oxidation of H2O was calculated to be 0.60 V, which is 0.10 V lower than reversible potential on the terrace sites, which implies that edge atoms are removed first. The final step of dissolution is believed to be the hydrolysis of the Pt2+ hydroxide, which considering the small equilibrium constant reported by Pourbaix, should proceed slowly. Introduction Proton exchange membrane fuel cells, PEMFCs, are pollution-free energy sources that continue to be a subject of research since being invented in the early 1960s. Research in the past decades has focused on seeking more efficient but lower cost catalysts for the oxygen reduction reaction, ORR, on fuel cell cathodes since the overpotential in PEMFCs is mainly caused by slow kinetics of the cathode reaction.1-3 To improve catalytic activity and lower the cost, platinum alloys have been substituted for pure platinum cathodes. Many platinum alloy catalysts such as PtCuCo,4 PtCo,5-17 PtNi, PtFe, PtCr, PtV, PtTi, PtW, PtAl, and PtAg5 have been investigated and most of them show higher activities than pure platinum. Very recently a PtCuCo alloy catalyst was reported by Strasser et al. to have the highest reported activity for ORR.4 Besides catalytic efficiency and cost, catalyst durability, a topic disregarded for a long time, is also critical to the development of a highly efficient PEMFCs with acceptable lifetimes for use in the portable and stationary applications. Cathodes are susceptible to oxidation and dissolution because of the high potential and high O2 activity, relative to anodes. Low durability results in unacceptable performance loss in PEMFCs after too short an operating time. Since 2003, more and more efforts have been directed at improving the durability of catalysts in PEMFC’s to meet the application requirements.18-21 The low durability of catalysts has been attributed to electrode metal dissolution leading to active surface area loss of cathode catalysts. Active area loss of cathode catalysts under fuel cell operating condition was reported by Guilminot et al. and platinum dissolution was evidenced by detection of Ptz+ in the polymer electrolyte membrane.22 By accelerating durability testing, ADT, and potential holding * To whom correspondence should be addressed. E-mail: aba@ po.cwru.edu. Phone: 216-368-5044. Fax: 216-368-3006.
experiments, Yasuda et al. established strong evidence for platinum dissolution taking place during degradation of cathode catalyst.23,24 Darling and Meyer proposed a kinetic model for platinum oxidation and dissolution in PEMFCs.25,26 The model includes three electrochemical reactions: platinum dissolution, electrochemical formation of a passivating surface oxide, and chemical dissolution of the oxide. The parametrized model for platinum oxidation fits reasonably well to cyclic voltammograms, CVs, for Pt/C electrodes. Molecular orbital theory was used 25 years ago in this laboratory to characterize the anodic dissolution of iron.27 The effects of changing the potential were modeled by shifting the valence band, and Fermi level, of the cluster model by changing atomic parameters in the semiempirical MO theory. More recently, the procedure was used to explore dissolution of a Cr atom from a Pt surface.28 In the past two years the Balbuena group has performed self-consistent cluster29 and slab band30,31 studies of structures and stabilities of models for alloy systems going from metallic to various oxidized states. Despite measurements and modeling, the mechanism of metal electrode anodic dissolution remains not fully understood. PtCo alloy catalysts have long been considered to be among the most promising Pt-based bimetallic O2 reduction catalysts because they have high activity and stability in acid. Although most previous experimental studies on PtCo alloy catalysts focused on catalytic activity, some studies were devoted to their durability. Durability studies of PtCo alloy catalysts in the literature have reported that the effects vary from increased durability to negligible changes in durability to decreased durability. Stability and catalytic activity of Pt/C and PtCo/C electrocatalysts in the form of Pt or PtCo alloy particles supported on carbon were studied experimentally by Yu et al.8 and Popov et al.6 It was concluded from Yu’s work that alloying with cobalt improved specific activity and also retarded platinum dissolution. Yu’s work showed cobalt dissolution was a cause of performance loss. Popov et al. performed ADTs on Pt/C and
10.1021/jp807094m CCC: $40.75 2008 American Chemical Society Published on Web 11/05/2008
Corrosion of Pt and Pt/Co Alloy Electrodes
J. Phys. Chem. C, Vol. 112, No. 47, 2008 18567 Pt3Co(111) surface was used for studying adsorption. The model for the Pt(111) surface is the same as that for Pt3Co(111) except Co atoms are replaced by Pt atoms. A Pt15Co slab model with a single Pt atom in the surface layer replaced by Co, representing one-quarter of the surface Pt atoms substituted by Co, was also used. Structures were optimized by the VASP program using the quasi-Newton algorithm by minimizing the HellmannFeynman forces acting on atoms. The initial Pt and Pt3Co slab structures were taken from ref 39 and in the adsorption calculations only the first layer was allowed to relax. Theoretical Methodology
Figure 1. (A) Four-layer slab model for the Pt3Co(111) surface with a × b ) 2 × 2. (B) 2 × 2 cell in the surface of slab model A. Aqua spheres are Pt atoms, and blue ones are Co atoms.
several Pt-based bimetallic alloys supported by carbon. With the potential held at 0.80 and 0.90 V in 0.3 M sulfuric acid for up to 1600 h, the non-noble metals on the cathodes dissolved rapidly in the first hundred hours and slowly thereafter. By a combination of hydrogen adsorption and transmission electron microscopy, TEM, studies it was found that sintering was relatively slight for alloys, compared with Pt/C. According to Yu’s work, the ORR catalytic activities of all the studied catalysts decreased with time, but the Pt-alloy catalysts maintained higher activity than pure Pt. More recently Popov has prepared a PtCo/C catalyst that has smaller and more uniformly distributed particles. The activity and long-term stability of the catalyst are better than for catalysts prepared by other methods.32 Interesting questions concerning identities of surface species causing the overpotential, the underlying mechanism for surface oxide formations and the reason for the rapid catalyst dissolution remain unanswered. Here we present results of an elementary theoretical study of the early steps in catalyst passivation and dissolution. By determining approximate reversible potentials for their occurrence on surfaces of Pt/C and PtCo/C, we conclude that the more electropositive Co oxidizes before Pt in alloy (111) surfaces and that Pt at edge sites of surface steps on pure Pt(111) oxidizes at lower potential than Pt in the terraces. Computational Details All calculations were performed using spin-polarized version of the Vienna Ab initio Simulation Package, VASP.33-35 The Kohn-Sham equations are solved with plane wave basis sets and periodic boundary conditions. PW91 was employed for electron exchange and correlation and the generalized gradient approximation, GGA, was used to take into account the nonlocality part in the exchange-correlation functional. The k-points meshes were 5 × 5 × 1 and k-point sampling was performed with the Monkhorst-Pack scheme.37 An energy cutoff of 400 eV was used for all calculations. Sufficient convergence of energy with respect to the number of k-points and energy cutoff was reached. Criteria for structure relaxations were set to be 0.01 eV/Å for the force acting on each atom. A fourlayered slab model was used in previous work in this laboratory because it was found to be a suitable for calculating adsorption of molecules on the close-packed face-centered cubic (fcc) (111) surface.38-41 The surface models for this study are four-layer slabs. The one for the Pt3Co(111) surface is shown in Figure 1A. The repeating slabs are separated by five interlayer distances of vacuum. The 2 × 2 surface unit cell shown in Figure 1B for
To obtain reversible potential of surface electron transfer reaction, a linear Gibbs energy relationship is used. It is a model for calculating reversible potentials for forming intermediates that use known potentials for the corresponding electron transfer reaction in aqueous solution and the adsorption energies of the reactant and product molecules.39-41 The method has given good results in previous studies of the Pt skin effect such as in refs 39-41. It is introduced next. The relationship between the standard reversible potential, U°, of an electron transfer reaction in aqueous solution and the change in Gibbs free energy, ∆G°, for the reaction can be written as
U° ) -∆G ° ⁄ nF
(1)
where F is the Faraday constant and n is the number of electrons transferred in the reaction. Early work discovered that to a good approximation ∆G° can be divided into two parts for reduction reaction of HnOm species in solution,42 and eq 1 changes to
U° ) -(Er ⁄ nF) - 4.6 V + c
(2)
where Er is change of internal energy for the reaction defined by E(products) - E(reactants). The constant c represents the difference between ∆G° and Er and includes P∆V and T∆S components, and 4.6 V is the thermodynamic workfunction of the standard hydrogen electrode (SHE).43 Assuming the same relation for the adsorbed reactant and product, the following is written
U°cs ) -(Er(cs) ⁄ nF) - 4.6 V + ccs
(3)
Here the subscript ‘cs’ indicates physical properties for the electrode catalyst surface reaction. Assuming the difference between ccs and c is negligible, subtracting eq 2 from eq 3 yields
U°cs ) U° - [(Er(cs)-Er) ⁄ nF]
(4)
where U° is the standard experimental value for the solution phase reaction and (Er(cs) - Er) is a function of the adsorption energies, Eads, of the products and reactants on electrode catalyst surface:
Er(cs) - Er)Eads(reactants) - Eads(products)
(5)
By using eq 4, one is able to obtain reversible potential (U°cs) of surface electron transfer reaction from the standard reversible potential (U°) for corresponding aqueous reaction and adsorption energies of products and reactants on the electrode catalyst surface. Adsorption energies from measurement or theoretical predictions are applicable. The applicability of eq 4 has been demonstrated in refs 39-41 and subsequent papers. Results and Discussion Adsorptions of OH and H2O at 0.25 ML Coverage. First, adsorptions of OH and H2O on the Pt(111), Pt15Co(111), and
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TABLE 1: Calculated Adsorption Energies and Critical Geometric Parameters for OH on Pt(111), Pt15Co(111), and Pt3Co(111) Surfaces at Coverage of 0.25 MLa surface Pt(111) Pt15Co(111) Pt3Co(111)
TABLE 2: Calculated Adsorption Energies and Critical Geometric Parameters of H2O on Pt(111), Pt15Co(111), and Pt3Co(111) Surfaces at Coverage of 0.25 MLa
adsorption site Eads d(M-O) d(O-H) R(M-O-H) Pt Pt Co Pt Co
2.35 2.45 2.74 2.38 2.78
2.00 2.01 1.82 2.03 1.82
0.98 0.98 0.98 0.98 0.98
surface
106.5 107.0 114.5 106.4 113.7
adsorption R1(M- R2(Msite Eads d(M-O) d(O-H) O-H) O-H)
Pt(111) Pt15Co(111)
Pt Pt Co Pt Co
Pt3Co(111)
a
d(M-O) and d(O-H) are internuclear distances in Å, and R(M-O-H) is the angle between H, O, and M in deg.
Figure 2. (a) Adsorption of OH at top sites of Pt(111) surface with a coverage of 0.25 ML. (b) Adsorption of OH at top sites of Pt3Co(111) surface with a coverage of 0.25 ML. Red spheres are oxygen atoms and white spheres are hydrogen atoms.
Pt3Co(111) surfaces are investigated with the method and the slab models described previously. The adsorption energies for OH and H2O were calculated using total energies of the unit cell
Eads(R) ) E(R) + E(slab)-E(R ⁄ slab)
(6)
where R stands for OH or H2O. A surface coverage of 0.25 ML was chosen to calculate the adsorption energies of OH and H2O. The top site adsorptions were previously calculated to bond OH and H2O most strongly on the Pt(111) surface.40 Therefore, in the present calculations, only top sites on the Pt3Co(111), Pt(111) and Pt15Co(111) surfaces were investigated. Table 1 gives adsorption energies for OH on the different surfaces and geometric parameters for the optimized structures. With a bonding energy of 2.35 eV, the 1-fold atop site is most stable for the Pt(111) surface. Calculated adsorption energies of OH on top of Pt in Pt15Co(111) and Pt3Co(111) surfaces are 2.45 and 2.38 eV, respectively. There are increases of 0.1 eV for Pt15Co(111) and 0.03 eV for Pt3Co(111) compared to adsorption energy on Pt(111), showing adjacent substitutional Co atoms make OH adsorb Pt more strongly. The stronger adsorptions could be attributed to the electropositive nature of Co compared to Pt. The Pt atom becomes negatively charged by drawing electron density from Co and adsorption of OH, forming, OHδ-, will be stronger on the more negatively charged Pt atom. The adsorption bond energies of OH on top of Co atoms in Pt15Co(111) and on Pt3Co(111) surfaces are calculated to be even stronger 2.74 and 2.78 eV, respectively. The structures of OH(ads) are similar for the three (111) surfaces, examples are in Figure 2. Calculated adsorption energies and geometric parameters for H2O on different surfaces are in Table 2. Water molecules bond to the surfaces with their planes nearly parallel to the surfaces as shown in Figure 3. Adsorption energies for atop Pt sites in the three surfaces are very close, 0.24, 0.24, and 0.20 eV. As previously discussed, Pt atoms in the Pt3Co(111) surface are negatively charged. The negative charge makes the donation of electrons from H2O to Pt in the Pt3Co(111) surface decrease, leading to a slightly lower adsorption energy on Pt3Co(111) relative to the pure Pt(111) surface. The adsorption energies of
0.24 0.24 0.55 0.20 0.47
2.51 2.52 2.16 2.63 2.19
0.98 0.98 0.98 0.98 0.98
94.0 91.6 100.0 89.7 98.9
93.5 96.1 100.0 94.2 99.3
a d(M-O) and d(O-H) are internuclear distances in Å, and R1(M-O-H) and R2(M-O-H) are two angles for the two H atoms in H2O with O and M in deg.
Figure 3. (a) Adsorption of H2O at top sites of Pt(111) surface with a coverage of 0.25 ML. (b) Adsorption of H2O at top sites of Pt3Co(111) surface with a coverage of 0.25 ML.
TABLE 3: Reversible Potentials for OH Formation on Sites of the Pt(111), Pt15Co(111), and Pt3Co(111) Surfaces Obtained Using the Linear Gibbs Energy Relationship surface
formation site
U°cs (V)
Pt(111) Pt15Co(111)
Pt Pt Co Pt Co
0.70 0.60 0.62 0.63 0.50
Pt3Co(111)
H2O on top of Co are larger, 0.47 eV for Pt3Co(111) and 0.55 eV for Pt15Co(111). These higher adsorption energies on Co compared to adsorption energies on Pt are due to the more electropositive nature of Co in the alloying surfaces, which leads to stronger lone-pair σ-donation bonding. Since at coverage of 0.25 ML, the strength of hydrogen bonding adsorbed molecules is negligible due to their separation, the adsorption energies will not change much with different orientations. However, as the coverage increases, hydrogen bonding should lead to a more rigid structure. Reversible Potential for OH(ads) Formation at 0.25 ML Coverage. Adsorbed OH on the electrode surface is believed to be one of the causes of the overpotential in fuel cells. Adsorbed OH forms in acid solution by the water oxidation reaction
H2O(ads) h OH(ads) + H+(aq) + e-
(7)
OH(ads) is also a reduction intermediate during the four-electron reduction of oxygen to water. Reversible potentials, Urev, calculated using the Linear Gibbs Energy relationship for OH formation on the Pt(111), Pt15Co(111), and Pt3Co(111) surfaces, are in Table 3. The lowest reversible potential for OH(ads) formation on the different sites of the three surfaces is 0.50 V for OH(ads) formation on Co in Pt3Co(111). On the Pt sites the potential is 0.63 V, meaning Co is oxidized at lower potential than Pt in this alloy. For substituted Co in the surface, the potentials for forming OH(ads) on Co and Pt sites are about the same, 0.62 and 0.60 V, respectively. The reversible potential for OH(ads) formation from oxidation of H2O on Pt(111) is
Corrosion of Pt and Pt/Co Alloy Electrodes
J. Phys. Chem. C, Vol. 112, No. 47, 2008 18569
TABLE 4: Adsorption Energies of OH and H2O on Surfaces Covered with 0.25 ML H2O or OHa surface Pt(111) Pt15Co(111) Pt3Co(111)
Eaads(OH) Ebads(OH) Ebads(H2O) Ecads(OH) Ecads(H2O) 1.14 2.47 2.47
2.87 2.76
0.74 0.60
2.92
0.50
3.33
0.62
a The Eaads(OH) column entries are for the second 0.25 ML of OH as in Figure 4. The Ebads(OH) and Ebads(H2O) column entries are for 0.25 ML adsorption of OH or H2O on Pt adjacent to OH adsorbed on Co as in Figure 5. The Ecads(OH) and Ecads(H2O) column entries are for 0.25 ML adsorption of OH or H2O adjacent to H2O adsorbed on Pt as in Figure 6.
Figure 4. (a) Top and side views of two OH(ads) per translational cell on Pt(111). (b) Top and side views of two OH(ads) per translational cell on Pt15Co(111). (c) Top and side views of two OH(ads) per translational cell on Pt3Co(111).
0.70 V. The first step in anodic dissolution is OH(ads) formation and these results are consistent with the possibility that as the potential of an alloy electrode is increased Co will be removed from the surface, leaving behind a platinum layer or layers. Previous theoretical work concerning oxygen reduction reaction on a monolayer Pt skin on platinum electrodes alloyed with chromium or cobalt has shown higher reversible potentials of OH formation from water molecules compared with pure Pt.37-39 With the higher reversible potentials, the blocking of O2 adsorption is shifted to higher potentials and the overpotential is decreased. Adsorption of OH and H2O and Reversible Potentials for OH(ads) Formation on Partially Covered Surfaces. As mentioned in last section, hydrogen bonding is absent for OH and H2O adsorption at 0.25 ML. Further investigations of OH and H2O adsorption at higher coverage were performed. Adsorption energies of a second OH on the different surfaces are summarized as Eaads(OH) in Table 4 and optimized structures are shown as 1a, 1b, and 1c in Figure 4. Adsorption energies for the second OH on Co in the Pt15Co(111) and Pt3Co(111) surfaces are significantly larger than on Pt in the Pt(111) surface. Compared to adsorption energies of the first OH, adsorption energies of the second OH are smaller. The optimized structures
Figure 5. (a) Top and side views of Pt15Co(111) with 0.50 ML OH(ads). (b) Top and side views of Pt3Co(111) with 0.50 ML OH(ads). (c) Top and side views of Pt15Co(111) with 0.25 ML OH(ads) and 0.25 ML H2O(ads). (d) Top and side views of Pt3Co(111) with 0.25 ML OH(ads) and 0.25 ML H2O(ads).
show that Co or Pt is drawn above the surface when one or two OH are bonded to them, which has the appearance of the probable precursor to oxidative dissolution. Using adsorption energies of the second OH and adsorption energies of H2O at 0.25 ML coverage, respective reversible potentials of formations of the second OH on Pt in Pt(111), Co in Pt15Co(111), Co in Pt3Co(111) were estimated to be 1.92, 0.59, and 0.58 V. These estimates show that forming the second OH on a Pt atom in Pt(111) requires a high potential but for Co in the PtCo alloys the potentials are much lower. Second, we investigated the adsorptions of OH and H2O on Pt15Co(111) and Pt3Co(111) following the OH adsorption on Co in the two surfaces. Adsorption energies in Table 4 and Figure 5 shows the optimized structures. Third, we turned our attention to the adsorptions of OH and H2O with an adjacent H2O adsorbed on the surface. Adsorption energies are shown in Table 4 and the optimized structures are presented in Figure 6. The results show that adsorption energies of OH and H2O increase, resulting from the hydrogen bonds from adsorbed H2O on the surfaces. For Pt(111), the adjacent H2O lowers the reversible potential for OH formation from 0.70 to 0.39 V. The decrease of the reversible potential indicates that hydrogen bonding will favor the OH formation on Pt(111) by stabilizing it preferentially over adsorbed H2O. For Pt3Co(111), the reversible potential decrease from 0.50 to 0.10 V by an adjacent H2O molecule.
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Figure 7. Pt(111) step with (a) adsorbed H2O and (b) adsorbed OH.
terrace it is 0.70 V. According to the calculations, to form a second OH on a Pt atom in the Pt(111) surface requires a very high potential of 1.92V or greater. However, the reversible potential forming a second OH on Co on the Pt3Co surface is calculated to be 0.58 V, suggesting that two OH’s are able to form on the same Co atom on this alloy surface. These results for what may be the initial steps of anodic dissolution indicate that Co will dissolve prior to Pt in the alloy surface leaving behind a Pt surface skin at potential in the 0.5-1.0 V region. The final step of dissolution is believed to be the hydrolysis of the Pt2+ hydroxide, which proceeds according to the small equilibrium constant reported by Pourbaix.44 To apply a new more complete physical theory to these reactions and study the thermodynamics of the hydrolysis is our future work.
Figure 6. (a) Top and side views of OH adsorbed on Pt(111) adjacent to adsorbed H2O.(b) Top and side views of H2O adsorbed on Pt(111) adjacent to adsorbed H2O. (c) Top and side views of OH adsorbed on Co in Pt3Co(111) adjacent to adsorbed H2O. (d) Top and side views of H2O on Co in Pt3Co(111) adjacent to adsorbed H2O.
Adsorptions of H2O at a Step Site on Pt(111). Adsorption of H2O at a step site was investigated to find if the potential for OH(ads) formtion was less, which would be consistent with dissoultion at lower potential than for flat surfaces. The step models with adsorbed molecules may be seen in From the calcualtions, adsorption energies for H2O and OH at the step site of Pt(111) are 0.50 and 2.71 eV, respectively. The reversible potential is calculated to be 0.60 V, 0.10 V lower than on the Pt(111) surface. Conclusions In this paper, adsorptions of OH and H2O on different surface models including Pt(111), Pt15Co(111), and Pt3Co(111) at 0.25 ML coverage were systematically investigated and reversible potentials for a series of reactions occurring in the initial stage of electrode dissolutions were obtained by employing the linear Gibbs energy relationship. On the Pt3Co alloy, OH(ads) forms from H2O(ads) oxidation on a Co site at the lowest reversible potential of 0.50 V compared to 0.70 V for Pt(111). On the PtCo alloy surface, as the potential is increased from low values, the first OH forms on the top of surface Co atom rather than on surface Pt atom, with a reversible potential of 0.58 V on the Co compared to 0.65 V on a Pt atom. On a stepped Pt(111) surface, the first OH formation is more likely to take place on the Pt atom at the step site rather than on a terrace Pt atom, since the reversible potential at the step is 0.60 V and on the
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