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J. Phys. Chem. C 2009, 113, 5014–5024
Electrocatalytic Activity of Gold-Platinum Clusters for Low Temperature Fuel Cell Applications Wei Tang,†,‡,| Shrisudersan Jayaraman,†,‡,⊥ Thomas F. Jaramillo,‡,# Galen D. Stucky,‡,| and Eric W. McFarland*,§,| Chemistry and Biochemistry Department, and Department of Chemical Engineering, UniVersity of California Santa Barbara, California 93106, and Mitsubishi Chemical Center for AdVanced Materials, Santa Barbara, California 93106 ReceiVed: October 8, 2008; ReVised Manuscript ReceiVed: January 23, 2009
The electrocatalytic activity of gold-platinum (Au-Pt) clusters was investigated in acidic and alkaline electrolytes. The clusters were synthesized by electrodeposition on fluorinated tin oxide (FTO) substrates and carbon disks with dimensions from 50 to 200 nm. Methanol electrooxidation (fuel cell anode) and oxygen electroreduction (fuel cell cathode) activities were measured using cyclic voltammetry and chronoamperometry. The results for methanol electrooxidation showed that platinum could be partially substituted by gold to achieve higher resistance to poisoning without affecting the activity in acid electrolyte, while in alkaline environment, a significant overall improvement in performance was observed. On the other hand, for the oxygen electroreduction, kinetic results obtained using a rotating disk electrode (RDE) indicated that Au alloying with Pt does not significantly alter the catalytic activity in acidic electrolyte, while a significant improvement in activity was observed in an alkaline electrolyte, which is attributed to a synergistic effect between the Au and Pt. Introduction One of the major challenges facing the world today is the development of sustainable and cost-effective systems for energy interconversion. Fossil fuels are finite in supply and may have unacceptable shorter term environmental consequences. Fuel cells can convert chemical fuels, including some renewables, directly into electricity; however, their less-than-ideal efficiency and prohibitively high cost have limited the widespread commercialization of fuel cell technology. Currently, one of the major cost factors in polymer electrolyte membrane (PEM) fuel cell is the expensive platinum-based electrocatalyst, which has unique activity and selectivity for oxygen reduction and oxidation of several potential renewable chemical fuels. Because of the limited supply of Pt and its susceptibility to poisoning by oxidation products, many research programs have been focused on reducing or eliminating the Pt in fuel cell catalysts and increasing the resistance to poisoning. One approach is based on alloying decreasing quantities of Pt with other affordable metal(s).1 Pt/Ru2-8 is a popular combination with a better performance for methanol electrooxidation. According to Davies et al., the addition of Ru into Pt reduces the overpotential to dissociate H2O to produce the surface oxidizing species to achieve higher performance.9 Besides ruthenium, Mo,4,10 Os,11 Rh,12 Sn,13,14 and other metals have also been shown to improve the catalytic activity of Pt. Jayaraman et al. also have reported that tungsten oxide (WO3) * Corresponding author. Tel.: (805) 893-4343. Fax: (805) 893-4731. E-mail:
[email protected]. † These authors contributed equally to this work. ‡ Chemistry and Biochemistry Department, University of California Santa Barbara. § Department of Chemical Engineering, University of California - Santa Barbara. | Mitsubishi Chemical Center for Advanced Materials. ⊥ Current address: Corning Inc., Corning, NY 14831. # Current address: Department of Chemical Engineering, Stanford University, Stanford, CA 94305.
in Pt-WO3 mixed catalyst takes part in the removal of CO in addition to abstracting protons achieving improved poison tolerance.15 For the oxygen reduction reaction (ORR), several alloys including Pt/Fe,16 Pt/Co,17,18 Pt/Cr,19 and Pt/Zn20 have been reported to have higher activities than pure Pt.21,22 The alloys have also been used to modify the strength of the surface adsorption of atomic oxygen and OH for the ORR,23-25 which is thought to account for the activity changes. Norskov, et al. have predicted using Density Functional Theory (DFT) that Pt alloying with specific metals could lead to a higher rate of oxygen reduction.22,23 There are also efforts to find nonplatinum electrocatalysts,forexample,Pd/Co,17,24,26 Pd/Co/Mo,24 Pd/Co/Au,27,28 and Au/Co,29 which have been shown to have high exchange current densities. As an important metal throughout human history, gold has attracted an enormous interest recently due to the possibility of using certain preparations as an alternative to platinum. Au has a filled d-orbital and a relatively low (energetically) d-band center as compared to the other transition metals, which provides Au with stability in both acidic and alkaline solution and reduces binding by potential poisons. This stability is why Au is relatively unreactive in bulk. In the form of small particles, Au has been shown to have a size and support dependence for gasphase oxidation of carbon monoxide.30,31 The limited bulk reactivity of Au has excluded it from serious consideration as an anode or cathode electrocatalyst in PEM fuel cells.26 Oxygen reduction on Au is thought to be limited to a two-electron pathway with stoichiometric generation of H2O2 (associative mechanism), which can degrade the membrane material and produces only one-half the usable current as the four-electron pathway. Au alone cannot yet be used as a substitute for platinum. A Pt-Au alloy catalyst may overcome the individual limitations of the pure phase materials for electrooxidation and electroreduction in fuel cell applications. In methanol electrooxidation, Mott has reported a synergistic effect in the Pt-Au
10.1021/jp8089209 CCC: $40.75 2009 American Chemical Society Published on Web 02/26/2009
Electrocatalytic Activity of Gold-Platinum Clusters alloy nanoparticles through a two-phase synthesis.32-35 The size of the nanocluster is considered a key factor in the high activity.34 As a cathode electrocatalyst, Zhang et al. have reported the stabilization effect on the activity of Pt by Au clusters deposited on the surface.36 For oxygen electroreduction, the stability of the Pt surface suffers from the formation of Pt-OH and Pt-O with further dissolution into electrolyte (surface corrosion). The stabilization effect of Au was explained by the apparent decrease in Pt oxidation on the surface. Zhang’s group postulated that without the formation of Pt-OH and Pt-O, the Pt surface is preserved and retains a constant activity and surface area during repetitive oxidation/reduction cycles. In their study, the change of the surface Pt d-bands after Au cluster deposition was related primarily to the surface stability, whereas the high ORR activity was attributed to the close proximity of Pt and Au sites where a spillover of H2O2 from Au to Pt occurs at nanoscale.36,37 A similar explanation was provided by Luo et al. in their work on Pt-Au bimetallic nanoclusters.37-39 Another strategy has been to electrodeposit Pt nanoclusters onto gold electrodes.40 Observed improvements in this case were thought to be due to a support effect, rather than alloying. In our work, electrodeposition is utilized to create Au-Pt alloy clusters using a synthesis route that is scalable to industrial production. We examined Au clusters that are larger than those previously reported to exhibit high oxidation activity.30,41 Our hypothesis is that the synergistic interactions of Pt and Au provide the increased electrocatalytic activity observed in methanol electrooxidation and oxygen electroreduction. The synergistic effects were investigated by examining the oxidation states of Pt in the alloy system. This Article addresses the following questions: (1) Can electrodeposition be used for the controlled synthesis of Au-Pt mixed metal clusters with high electrocatalytic activity for methanol oxidation and oxygen reduction? (2) What is the relationship of the Au:Pt ratio to the electrocatalytic activity for methanol oxidation in acidic and basic electrolytes? (3) What is the relationship of the Au:Pt ratio to the electrocatalytic activity of oxygen reduction reactivity in acidic and basic electrolytes? Experimental Methods Catalyst Synthesis. Hydrogen tetrachloroaurate (HAuCl4, 99%) and hydrogen hexachloroplatinate(IV) (H2PtCl6 · xH2O, 99.995%) were purchased from Aldrich and used as received. Deionized water (18 MΩ) was used to prepare all of the solutions. For the methanol oxidation studies, fluorine-doped tin oxide (FTO) coated on glass (Tec 8, 2.3 mm thick glass, Hartford Glass Co., Hartford City, IN) was used as substrate. Prior to catalyst deposition, the substrates were prepared as follows: 0.5” × 1” sections of FTO were first sonicated in a mixture of acetone:ethanol:DI water with 1:1:1 volume ratio for 15 min followed by rinsing and further sonication in DI water for 15 min. The substrates were dried under a nitrogen stream subsequently, at which point they were ready for electrochemical experimentation. The reactivity of the catalysts for oxygen electroreduction was measured using a rotating disk electrode (RDE). Five millimeter diameter carbon disks of 4 mm thickness were used as substrates (geometric area ) 0.196 cm2). The carbon disks were machined from carbon rods (Grade 1, Ted Pella Inc.), polished, cleaned, and dried prior to catalyst deposition. The electrodeposition was carried out from aqueous solutions containing Pt4+ and Au3+ ions. Because gold is reduced at a more anodic potential than Pt (Figure S1.a), the codeposition
J. Phys. Chem. C, Vol. 113, No. 12, 2009 5015 is challenging because of preferential deposition of the Au. To minimize this problem, the codeposition was carried out in the diffusion regime where the growth of particles is limited by delivery of the precursor ions from solution to the surface without kinetic limitations. To ensure this condition, the concentrations of precursor metal ions were kept low (the total metal ion concentration being equal to 1 mM), and the reduction was carried out at a potential more cathodic than the standard reduction potential (in the mass-transport limited region). Pulse electrochemical reduction was employed to obtain uniform particle sizes. A square wave pulse with a positive potential limit of +1.0 V and a negative potential limit of -1.0 V was used, with Ag/AgCl as reference. Several different pulse times were tested for the pure Au and pure Pt cases, and it was found that a pulse time of 50 µs resulted in similar particle sizes and reasonably narrow particle size distributions for Pt and Au (Figure S1.b-g). In other words, the nucleation and growth rates of Au and Pt are comparable at a pulse time of 50 µs, providing the best electrochemical condition for depositing mixed Au-Pt catalysts. Based on the concerns above, the series of Au-Pt catalyst samples for further investigation were electrochemically deposited on fluorine-doped tin-oxide (FTO) and carbon disk substrates from electrolytes containing varying amounts of chloroplatinic acid (H2PtCl6) and hydrogen tetrachloroaurate (HAuCl4) with a total metal ion concentration of 1 mM using a pulsed potential program. The pulsing limits were -1.0 and 1.0 V, the pulse time was 50 µs, and the total deposition time was 1 min for all of the samples. Physical Characterization. The surface morphology of the catalyst samples was characterized using scanning electron microscopy (SEM, FEI XL40 Sirion FEG digital scanning microscope). X-ray photoelectron spectroscopy (XPS) was employed for two purposes: first, to determine surface ratios of Pt and Au for each sample, and, second, to observe the oxidation states of Pt and Au in the samples. The XPS system (Kratos Axis Ultra) used a monochromated Al KR source for incident radiation and an eight-channel detector for measurement of photoelectrons. Charge compensation was utilized during measurement, and base pressure was maintained at 0.87 V followed by water oxidation beyond 1.13 V. A cathodic peak corresponding to the reduction of the surface oxides is observed at ∼0.98 V. The CV data obtained from the sample with 100% Pt, S11, exhibit the well-known signature of Pt with the H-UPD peaks in the range -0.2 to +0.15 V, formation of surface oxides (Pt-OxHy) at >0.6 V, and a subsequent peak at 0.52 V during the cathodic scan corresponding to the reduction of surface oxides. The qualitative appearance of the CVs obtained from samples S1-S11 follows a clear trend with the Au-rich samples exhibiting “Au-like” response, which diminishes as the Pt concentration increases and increasing “Pt-like” behavior is observed. The oxide-reduction peak corresponding to platinum is shifted to more negative values in the Au-containing samples and moves gradually toward that of pure Pt with increasing Pt content. This shift in the oxide-reduction peaks corroborates our earlier observation that Au and Pt are atomically mixed in these bimetallic catalyst samples.
Tang et al.
Figure 4. (a) Chronoamperometry of Au-Pt samples in 0.5 M methanol/0.5 M H2SO4 at 0.5 V vs Ag/AgCl. Samples S1-S6 are not shown as they exhibited negligible activity toward methanol oxidation at this potential. (b) Methanol oxidation currents at various times from chronoamperometry as a function of Pt composition (derived from (a)).
The cyclic voltammograms of the Au-Pt samples in a solution containing 0.5 M methanol/0.5 M H2SO4 are also shown in Figure 3 (thick lines). Sample 1 (100% Au) shows minimal activity for methanol electrooxidation. On sample 11 (pure Pt), methanol oxidation currents can be observed at potentials greater than 0.15 V during the anodic scan. The surface Pt atoms are covered by methanol absorbate (a combination of one or more of the following species: -CH3O, -CH2O, -CHO, -CO) at the start of the sweep (-0.2 V). As the potential is scanned to more positive values, successive dehydrogenation of methanol occurs while the Pt surface remains partially covered with adsorbates.8 The onset of methanol oxidation current occurs at ∼0.15 V. The current increases until it peaks at ∼0.68 V, at which point the surface oxides on Pt block the sites for further oxidation. During the cathodic scan, the surface oxides are subsequently reduced, exposing Pt sites that become available for methanol adsorption and oxidation.15 The adsorbed methanol is oxidized by the residual surface oxides on Pt (Pt-OxHy). This results in the observed oxidation current during the cathodic scan, which peaks at ∼0.45 V. CO and other methoxy groups (-CHxO) poison the surface at lower potentials, resulting in a drop in the current. Methanol oxidation currents can be observed for all of the Pt-containing samples (S2-S11). It can clearly be observed that the peak currents increase with increasing Ptcontent. The monotonic increase in the peak current densities during methanol oxidation indicates the number of Pt sites is determining the peak current. Although the addition of Au does not increase the peak current, the onset potential is shifted. Pure Pt (S11) has an onset potential of approximately 0.4 V. Alloying
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Figure 5. Cyclic voltammograms of Au-Pt samples in 0.5 M KOH (thin line, primary axis, as indicated by the left arrow in S11) and in 0.5 M methanol/0.5 M KOH (thick line, secondary axis, as indicated by the right arrow in S11) at a scan rate of 0.1 V s-1. The composition of each sample (as % Pt) is shown in parentheses.
TABLE 2: Methanol Oxidation Peak Potential of Various Au-Pt Catalyst Samplesa
a
Figure 6. Performance graph of the Au-Pt system toward methanol oxidation in alkaline environment. The methanol oxidation current density is plotted as a function of Pt composition at several different potentials. The outer dark line connects the maximum current densities at various potentials and represents the Au-Pt composition that has highest activity at those potentials. This graph provides a potentialcomposition-activity map for the Au-Pt bimetallic system.
with Au (S6-S10) shifts the onset potential to 0.2 V. The shift in the onset potential indicates that the addition of Au to Pt facilitates methanol oxidation at a lower overpotential (0.2-0.4 V). Subsequently, the catalytic performance relative to deactivation-resistance on Au-Pt alloy samples was also examined.
sample no.
Pt composition (%)
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11
0 2 5 7 12 25 35 49 58 78 100
methanol oxidation peak potential (V)
-0.07 0.02 0.12 0.23 0.33 0.16 -0.02 -0.08 -0.23
Deduced from Figure 5.
Figure 4a shows the first 10 min of chronoamperometric response of Au-Pt catalysts in 0.5 M methanol/0.5 M H2SO4 at a constant potential of 0.5 V. Sample S11 (pure Pt) shows the highest initial methanol oxidation current density; however, the current decays faster than samples S10, S9, and S8, indicating a higher rate of catalyst deactivation in the pure Pt sample. The rate of deactivation decreases with increasing Au amounts. The methanol oxidation current densities of the 11 samples are compared at the beginning (t ) 0 s), at 300 s, and at 600 s in Figure 4b. Although possessing a lower initial current density, sample S9 (58% Pt, 42% Au) shows stable and higher activity at 5 and 10 min. The deactivation is consistent with poisoning by CO.48 In the Blyholder Model49 for poisoning, the CO molecule adsorbs on a transition metal surface atom in a linear
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Figure 7. CO-stripping voltammetry of the Au-Pt samples in a nitrogen-purged solution of 0.5 M KOH at a scan rate of 0.1 V s-1. The dotted line depicts the potential at which CO-stripping peak occurs on pure Pt.
orientation with the 5σ orbital on the carbon end polarized toward the transition metal accompanied by “back donation” by the metal d-orbital into the unoccupied CO 2π* orbital. Pt forms a particularly strong bond with CO, which prevents further reaction and poisons the surface. It was expected that the presence of gold would provide less d orbital back-donation and shift the polarization of CO 5σ orbital, therefore weakening the Pt-CO bond.23,50,51 Our results suggest that the optimal surface composition of a Pt and Au alloy was approximately represented by sample S9 (58% Pt, 42% Au). Such a catalyst would allow for an approximately 40% reduction in Pt loading for a fuel cell catalyst while achieving resistance to poisoning without a significant decrease in activity. Figure 5 (thin lines) shows the cyclic voltammograms of the Au-Pt samples in a 0.5 M KOH electrolyte, which would be suitable for an alkaline fuel cell.37,41,52,53 The signature CVs of Au and Pt were exhibited in samples S1 and S11, respectively. Qualitatively, as the Pt content is increased from sample S1 to S11, the CVs for samples with the surface-oxide reduction peak on Pt shift more positive to -0.25 V. This is due to the weakening of the Pt-OxHy bonds, which is consistent with the XPS results as well as the results obtained in the acidic electrolyte (Figures 1 and 3). As the Au concentration is increased, additional currents appear in the anodic and cathodic scans corresponding to the surface reactions on Au. Figure 5 also shows the CVs of the Au-Pt catalysts in 0.5 methanol/0.5 M KOH (thick lines). The methanol oxidation current densities increase with increasing Pt composition but subsequently drop off at higher Pt amounts. The peak methanol
oxidation current densities observed from samples S5, S6, and S7 (corresponding to Pt contents of 12%, 25%, and 35%) are as high as 14 times that on pure Pt (S11). Samples with highest Au compositions (S1 and S2) exhibit no activity toward methanol oxidation; however, with the addition of as little as 5% Pt to Au, significant methanol oxidation activity is observed. The peak current density of the sample containing 5% Pt is 6 times that of 100% Pt. This high activity at very low Pt concentrations is precisely the target behavior desired for lower cost fuel cell catalysts. Figure 6 depicts the performance for the Au-Pt system for methanol oxidation in an alkaline environment and is derived from the data in Figure 5 with the current densities at several potentials plotted as a function of Pt composition. As observed above, the methanol oxidation currents for Au-Pt mixtures are higher as compared to that of pure Pt for all of the potentials. This plot provides the potential-composition-activity map for the Au-Pt bimetallic system, which might be used to select the composition of a fuel cell catalyst. It can be noted that pure Pt exhibits a symmetric peak in methanol oxidation current around a potential of approximately -0.23 V (Figure 5, S11). As mentioned earlier, this peak in the oxidation current is due to the blocking of Pt sites by -OxHy species at more anodic potentials (greater than -0.23 V). With the addition of Au to Pt, the peak potential shifts to more positive values until, at a composition of 35% Pt (samples S11-S7), the peak potential shifts back in the negative direction with decreasing Pt composition (samples S6-S3). The peak potentials are summarized in Table 2. In addition to shifts in the peak
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Figure 8. (a) Linear sweep voltammograms of representative Au-Pt samples on carbon disk electrodes at several different rotation speeds (200, 300, 500, and 1000 rpm) in an oxygen-saturated solution of 0.5 M H2SO4 at a scan rate 0.01 V s-1. The direction of the arrows indicates increasing rotation speeds. (b) Levich plots of Au-Pt samples at a potential of 0.3 V vs Ag/AgCl. Samples with 32%, the currents at potentials negative to 0.4 V increase in magnitude with increasing rotation speed, indicating the reaction is diffusion limited and the intrinsic rates of oxygen reduction on the catalyst are relatively high. On samples with Pt compositions
