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Three Phase Interfaces at Electrified Metal-Solid Electrolyte Systems 1. Study of the Pt(hkl)-Nafion Interface Ram Subbaraman,† Dusan Strmcnik,‡ Vojislav Stamenkovic,‡ and Nenad M Markovic*,‡ Nuclear Engineering Department and Material Sciences DiVision, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: January 27, 2010; ReVised Manuscript ReceiVed: March 22, 2010
A voltammetric fingerprinting approach has been used to probe the nature of Pt-Nafion three phase interfaces for Pt(hkl) and polycrystalline platinum surfaces. Nature of adsorbing species is identified as the sulfonate anions via CO charge displacement technique. The affinity for the sulfonate anions to adsorb on the electrode surface is investigated. Adsorption strength of the sulfonate anions with the electrode surface is compared with other strongly adsorbing anions such as (bi) sulfates and chlorides. Various factors that influence the adsorption properties of the sulfonate anions are studied. Nature and strength of the anion interaction with various surface geometries is also discussed. A physical model is presented to describe the observed phenomena. Introduction The growing demand for renewable energy, along with the restrictions on green house gas emissions, has made polymer electrolyte membrane fuel cells (PEMFCs) a major part of the overall energy landscape. While this technology has been in existence for over 100 years, significant advancements have happened in the last few decades. At the core of the PEMFC, the electrode, reaction sites are composed of an ion conducting solid polymer electrolyte (ionomer) typically Nafion, next to the electrocatalytic surface such as platinum or its alloy nanoparticles embedded in an interpenetrating network of electronic and ionic conductors. The reactants, typically hydrogen at the anode and oxygen at the cathode react at these interfaces generating energy and producing water as the only byproduct. Such a local reaction site is often referred to as the three phase interface where the ion conducting polymer electrolyte phase (which also affects the transport of species in and out of the interface), is in contact with an electronic phase, an electrocatalyst surface, and the reactant phase components, which are usually comprised of adsorbing anions (supporting electrolyte) and/or reactants and reaction intermediates, from the “solution” environment. Despite many years of studies, very little is known regarding the true nature of such interfaces; a longstanding difficulty has been the lack of experimental methods for preparing and characterizing well-defined threephase interfaces for electrochemical studies. Not surprisingly, these circumstances have encouraged the use of mathematical models applicable to relevant three-phase interfaces. The primary component of the interface, the ionomer, behaves very differently from that of aqueous electrolytes. Various models have been proposed in the literature for describing the structure of the ionomer component.1–10 For example, Yeager and Steck8,9 proposed that the ionomer is a three phase material comprised of a polymeric matrix, an ionic cluster, and an interfacial zone. Hsu and Gierke2,3,10 further improved on this idea where, under well hydrated conditions, sulfonic acid groups are treated as aggregates of ionic clusters * Corresponding author. E-mail:
[email protected]. † Nuclear Engineering Department. ‡ Material Sciences Division.
separated by ∼4.0-5.0 nm throughout the ionomer. The ionomer components within each cluster are expected to be in equilibrium with various interaction forces such as the electrostatic interaction with the solvated proton, the dipole interaction with neighboring clusters, a van der Waals type interaction with the other polymer segments, a covalent interaction with the matrix, and the solvation energy arising from the waters of hydration. Other approaches which treat the ionomer as a dielectric continuum also have been reported in the literature. In spite of the simplicity of the ionomer models, application of such structures to describe the three phase interface is seldom trivial. For an accurate depiction of the three phase interface, the impact of the presence of the electrocatalyst surface, in terms of specific vs nonspecific interaction with the electrolyte in the presence of electrical potential, along with the reactants, and reaction intermediates will need to be considered. Inclusion of these interaction components makes such models tedious even with modern computers. Some of these complications are overcome by treating the ionomer to be a nonadsorbing electrolyte. Recently, Malek et al.11,12 treated the interaction between the hydrophilic groups of the ionomer (sulfonic acid clusters) and the carbon particles using a Lennard-Jones type potential with variations in the strength of interaction for the various components in the system. Other models that make use of energies developed based on London forces or lattice-gas models incorporating the impact of electrostatic interaction between the ionomer and the electrode surfaces also have been reported.13–16 Results from such models provide a first order approximation of the three phase interface. It is important to note that these models predominantly consider the electrolyte’s interaction with the catalyst particle to be described by a “bulklike” force. As a result, impact of reactions such as specific adsorption of anions on the catalyst is poorly described. Although all of these approaches provide some viable information about the possible structure of the three-phase interface, a great deal still needs to be learned in order to obtain a molecular-level structural description of the three-phase interface. This situation contrasts markedly that for conventional (Nafion-free) electrode-electrolyte interfaces (denoted hereafter as the two-phase interface), for which a variety of characterization methods have been well established since the 1990s. These
10.1021/jp100814x 2010 American Chemical Society Published on Web 04/06/2010
The Pt(hkl)-Nafion Interface have enabled the extensive use of metal single crystal surfaces to study two-phase interfaces.17–24 These studies have helped establish the structure of metal electrodes, the nature and structure of the adsorbing species, as well as relationships between interfacial properties and the rate of charge transfer through interfaces. Such results also have helped pave the way for physical models representative of the conventionally accepted picture of an electrochemical double layer, as described later. This interim disparity in characterization, however, tends to mask the inherently close ties between metal surface phenomena at the two-phase and three-phase interfaces. In this article, we present an approach based on using welldefined surfaces to establish the true nature of ionomer interactions with the catalyst surface. These conclusions can then be applied toward understanding the behavior of complex interfaces such as those that exist in a fuel cell electrode. We also present a physical model for the three phase interface based on the experimental findings. Experimental Section 1. Nafion-Free Pt Electrode Preparation. Pt(111), Pt(110), Pt(100), and polycrystalline Pt (Pt-poly) electrodes were prepared by inductive heating for 15 min at ∼1100 K in an argon hydrogen flow (3% hydrogen).25,26 Polycrystalline electrode behaviors are known to be significantly influenced by the preparation procedure and when prepared by inductive heating leads to extensive faceting which can be obtained by high temperature treatments such as high temperature annealing. The annealed specimens were cooled slowly to room temperature under an inert atmosphere and immediately covered with a droplet of DI water. Electrodes were then assembled into a hanging meniscus electrode (HME) ensemble. Voltammograms were recorded in Argon saturated electrolytes. The Ag/AgCl reference electrode was used, but all potentials in the paper are shown versus the reversible hydrogen electrode (RHE). 2. Chemicals. All metal perchlorates, chloride salts used in our experiments were obtained in the highest purity from Sigma Aldrich. Electrolytes used for our experiments 0.1 M perchloric acid, 0.05 M sulfuric acid were obtained from JT baker. All gases (Argon, oxygen, Hydrogen) were of 5N5 quality purchased from Airgas Inc. A typical three electrode glass cell was used. Experiments were controlled using an Autolab PGSTAT 302N potentiostat. 3. Nafion-Covered Pt Electrode Preparation. Solution. 5% solution of Nafion (1100 eq wt) was obtained from Sigma Aldrich. This solution was diluted 100 times using a mixture of 10-20% Dimethylformamide (DMF) and rest DI water. The DMF solution used for all experiments was a 99.9% purity Chromasolv solution from Sigma Aldrich. The above solution was ultrasonically mixed for 30 min. Multiple batches of Nafion solution from Sigma Aldrich were tested to ensure the quality of dispersion used for our experiments were similar for different batches of solution. Nafion solutions without the DMF solvent were prepared by diluting the 5% Nafion solution 100 times with DI water. Coating. Disc electrodes of platinum crystals were washed thoroughly with DI water after the initial voltammograms were recorded in Argon saturated electrolyte. The surface of the electrode was then protected with a droplet of DI water. A known volume of Nafion solution was then added to the droplet following which the electrode was transferred to a hot plate at 115 °C to be dried under an inert atmosphere. Plastic micropipet tips were used to add the solution in order to remove possible contamination from glass pipettes. The drying times were varied
J. Phys. Chem. C, Vol. 114, No. 18, 2010 8415 depending on the volume of the Nafion solution added. Typical drying times ranges from 15-30 min. Experiments were also performed with Nafion solutions in the absence of DMF. In this case the electrodes were dried at 60 °C in order to dry the solvent at a moderate rate. The electrodes after drying were washed with DI water thoroughly before reintroduction into the electrochemical cell. In this work, Nafion coating thickness is denoted as n× where n ) 1, 2, 3, etc. This measure is based on the volume of Nafion solution added to the surface of the electrode; For example, 2× refers to the case of 10 µL of solution (0.005 wt %) applied to the electrode. Thickness can then be calculated by using a dry density of 1.5 g/cm3. 2× thick Nafion film was used for all experiments presented in this paper. Other thicknesses were found to behave qualitatively similar to 2×. DMF is expected to help in “crystallizing” the polymer by helping the polymer flow at temperatures close to the glass transition temperature (120 °C). Physico-mechanical stability of films was found to improve significantly with the addition of DMF, especially for the transfer of the electrode onto the RDE setup. All data presented in this work are for films prepared with DMF. No differences in voltammograms were observed for the cases of with and without DMF in the solution. 4. Cation Exchange Experiments. The Nafion coated Pt(111) electrode was removed from the electrochemical cell after recording the steady state voltammogram for Pt(111) with Nafion and was washed thoroughly. This electrode was then equilibrated in 1 M solutions of perchlorates and chlorides of various cations for 1 h. The equilibration time was fixed at one hour. This was expected to provide >80% exchange of protons with various cations. No measurements were made to estimate the level of cation exchange during these experiments. After equilibration, these electrodes were washed repeatedly with water followed by soaking in lukewarm DI water for 30 min to completely remove any residual salts. Care must be taken while washing the electrodes to not disturb the integrity of the Nafion film. Also, electrodes must be washed thoroughly to prevent any possible contamination from chlorides and other contaminants present in the salt. On reintroduction of the electrode into the electrolyte, voltammograms were recorded within 2 min of introduction. This short time was necessary in some cases to prevent an extensive reverse exchange of cations with the protons in the electrolyte. For this work, no measures were taken to control or measure the extent of exchange and the composition at the exact moment of measurement. Direct addition of cation salts to the acid electrolyte to prepare known activities of cations in solution was also tried. However, the contamination from these salts was a significant problem. So for the results presented here, the electrodes were cation exchanged ex situ. Results and Discussions 1. Well-DefinedPlatinumSingleCrystalSurfaces. a. NafionFree Pt(111), Pt(100), and Pt(110) Surfaces. We begin with a brief description of cyclic voltammograms (CVs) for Pt(hkl) single crystal surfaces in 0.05 M H2SO4 and 0.1 M HClO4 solutions, which are summarized in Figure 1. These particular systems have been chosen as “typical electrochemically relevant” two phase interfaces, that can shed some light on the corresponding three phase interfaces. As suggested previously,17,18,20,21,29–36 comparison of the CVs in the so-called nonadsorbing electrolytes such as perchloric acid solution with environments containing strongly adsorbing ions, in our case (bi)sulfate anions, may be used as an “electrochemical fingerprint” to delineate specific adsorption of anions from other adsorbates present in supporting electrolytes. For example, for Pt(111) in 0.1 M HClO4 three potential regions are clearly visible
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Figure 1. Characteristic voltammograms for Pt(hkl) surfaces in nonadsorbing (0.1 M HClO4) and strongly adsorbing (0.05 M H2SO4) supporting electrolytes. Also shown are the voltammograms for Nafion covered Pt(hkl) surfaces in 0.1 M HClO4. Notice the features arising from coadsorption of (bi) sulfate anions and sulfonate anions with Hupd for Pt(110) and Pt(100) surfaces (a and b). Occurrence of reversible butterfly and irreversible “mini-butterfly” features for Pt(111) in the double layer region (0.4-0.645 V) for (bi) sulfate and sulfonate anions respectively. OHads is also influenced by the presence of strongly adsorbing anions. (scan rate 50 mV/s).
(Figure 1c): adsorption of hydrogen (denoted as underpotentially deposited hydrogen, H+ + e- T Hupd) between 0.05 and 0.4 V is followed first by a double layer region (0.4 to 0.645 V) and then by adsorption of OH (hereafter denoted as OHad) above 0.6 V. In H2SO4 the hydrogen Hupd potential region is the same as in HClO4, confirming that on Pt(111), (bi)sulfate anions are not coadsorbed with Hupd. The adsorption of (bi)sulfate anions is, however, observed between 0.35 and 0.5 V as a reversible, butterfly like, pseudocapacitive feature (charge under the peak is ∼75 µC/cm2), presumably due to the long-range ordering of the (bi)sulfate anion on the (111) surface geometry.29,32,34,35,37 Due to strong adsorption of (bi)sulfate anions, OH adsorption
Subbaraman et al. is almost completely attenuated on Pt(111) and is represented, at least in part,38 by a small pseudocapacitance between 0.7 and 0.8 V. Interaction of (bi)sulfate anions with Pt(100) and Pt(110) is substantially different.34,39 For example, the initial anion adsorption of bisulfate on Pt(100) is observed in the Hupd potential region (Figure 1b), causing the appearance of sharp peaks centered at 0.3 and 0.45 V. Furthermore, while in perchloric acid solutions the Hupd region is immediately followed by OH adsorption (in Figure 1b between 0.6 < E < 0.8 V), in the presence of (bi)sulfate anions, OH adsorption, which appears as a small peak at 0.8 V, is separated from the Hupd potential region by a narrow double layer feature. In the case of Pt(110), in solutions containing (bi)sulfate anions the Hupd region appears as a single sharp peak that is centered at 0.15 V (Figure 1a); in perchloric acid solutions, however, the Hupd is extended up to 0.3 V with two clearly visible Hupd peaks (Figure 1a). b. Nafion-CoWered Pt(111), Pt(100), and Pt(110) Surfaces. CVs recorded on Nafion-free and Nafion-covered Pt(hkl) single crystal surfaces in 0.1 M HClO4 are also shown in Figure 1. Careful inspection of the CVs reveals several significant features on the Nafion-covered surfaces. First, the appearance of sharp peaks in the Hupd potential regions are observed on Pt(110) and Pt(100), indicative of anion specific adsorption on these two surfaces (Figure 1, panels a and b). Second, suppression of the charge under the OHad peaks on Pt(100) and Pt(110) surfaces (Figure 1, panel a and b) is also seen signaling that OH adsorption is in a strong competition with some coadsorbing species. The peak for OHad on Pt(100) is found to lay between that observed in 0.1 M HClO4 and 0.05 M H2SO4 electrolytes. Third, an irreversible, “mini-butterfly”, pseudocapacitive feature is observed on Pt(111) in the same potential region where the reversible (bi)sulfate adsorption is observed in sulfuric acid solutions (Figure 1c). Interestingly, the observed changes in CVs signal that, in fact, Nafion is not a nonadsorbing electrolyte, i.e., in contrast to the existing viewpoints, it may contain some ions that can significantly alter the two-phase interfaces of Pt(hkl) surfaces. Having established the effect of Nafion on the CVs of Pt(hkl), a key question concerning the nature of adsorbing species needs to be addressed. In an attempt to resolve this issue, voltammetric behaviors of the Nafion-covered Pt(111) surfaces will be investigated in more detail. The choice of Pt(111) was prompted by several factors, including, well-defined separation between the Hupd and mini-butterfly potential regions and distinguished attenuation of OH adsorption. The area under the anodic peak is ∼21-24 µC/cm2, corresponding to surface coverage of ∼0.1 ML. In general, this charge can be produced either by adsorption of some anionic species, or desorption of some cationic species or even an anion adsorption process which is accompanied by a cation desorption process. i. Nature of the Adsorbing Species from Nafion Coatings. To resolve this issue, we utilize the so-called CO displacement experiment,40 a method which relies on monitoring the quantity as well as the polarity of the charge assessed via chronoamperometric measurements. For our purposes here, three electrode potentials were chosen in the CO displacement experiments on Pt(111) (Figure 2), 0.09, 0.45, and 0.56 V. These three potentials correspond to the Hupd potential region, and just before and just after the observed mini-butterfly peak, respectively. At 0.09 V, a positive charge of ∼160 µC/cm2 is assessed in the CO displacement experiment, indicative of Hupd being completely displaced from the Pt(111) surface. In contrast, a negative charge of ∼10 µC/cm2 at 0.45 V and ∼32 µC/cm2 at 0.56 V is assessed
The Pt(hkl)-Nafion Interface
Figure 2. CO displacement experiments for Nafion covered Pt(111) surfaces. Three potentials correspond to Hupd region (0.09 V), before mini-butterfly (0.45 V), and after mini-butterfly (0.56 V) regions. A slight positive charge at 0.56 V corresponds to the onset of CO bulk oxidation on Pt(111).
from chronoamperometric measurements summarized in Figure 2, signaling that the displaced species are anionic in nature and the anodic peak observed is mainly due to anion adsorption onto the surface. Given that the supporting electrolyte is 0.1 M HClO4, it is reasonable to suggest that the displaced anion is an anionic component of Nafion itself: the sulfonate anion. As with (bi)sulfate anions, sulfonate anions from the Nafion ionomer exhibit a structure with a C3V symmetry that is compatible with the (111) surface geometry. The appearance of a mini-butterfly peak may, therefore, be a consequence of the adsorption of sulfonate groups onto Pt(111). Although both (bi)sulfate and sulfonate anions have similar symmetries, there are some important differences in adsorption properties between these two anions; the surface coverage of the sulfonate anions of Nafion is lower than that observed for (bi) sulfate anions (0.33-0.4 ML). Maximum surface coverage of the sulfonate anions from Nafion is limited by the finite spacing between the anions, arising from the structure of the ionomer, which prevents a “compact” arrangement on the surface. Also, the mini-butterfly peak is found to be irreversible unlike that for the (bi)sulfate adsorption. On close examination, the irreversibility which is markedly visible for Pt(111) is also present for Pt(110) and Pt(100) and as shown later for Pt-Poly surfaces. Such potential shifts between the adsorption and desorption peaks are expected to be dependent on the strength of adsorption of the anions on these surfaces. ii. Role of the Countercation on the Sulfonate Anion Adsorption. To get further insight into the irreversibility of the minibutterfly peaks, we decided to probe the local environment of the sulfonate anion. This was done by modifying the countercation within the ionomer. Before discussing the results, however, it is appropriate to note that the exchange of protons in Nafion with cations has been shown to significantly alter the physical, mechanical and to some extent chemical properties of the polymer.1,27,28,41,42 It has also been found that the strength of interaction of Li+, Na+, K+, Mg2+, etc. is stronger/weaker than that of the protons due to increase in size and charge density of the cation, as well as a decrease in number of waters of hydration (in some cases) of these cations compared to the protons. The number of water molecules is known to play a role in shielding the ions within the ion-pairs.43 In our experiments, we chose to first exchange the protons in Nafion
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Figure 3. Cation exchange experiment for Nafion covered Pt(111). 1 M perchlorate salts of potassium, and magnesium were used for cation exchange. Voltammograms were recorded 2 min after immersion into the acid electrolyte. Adsorption peak and desorption peaks are found to shift more positively with exchange of protons with larger cations. Observe decrease in anion adsorption charge with increased anion-cation interaction. This is possibly due to increased steric interaction between ionic clusters. (scan rate 50 mV/s). Inset’s voltages are also vs RHE.
films attached to the Pt(111) surface with K+ and Mg2+ ions (see experimental section for protocol) and then to record the corresponding CVs in 0.1 M HClO4. As shown in Figure 3, the anodic peak position is strongly dependent on the nature of the cation; it shifts in the positive direction from 0.51 V in the case of H+ to 0.53 V for K+, and to 0.54 V for Mg2+, i.e, the peak position increases in the same order as the corresponding cationsulfonate bond strength: H+ · · · SO3- < K+ · · · SO3- < Mg+ · · · SO3-. Figure 3 shows that under the negative sweep direction desorption of the sulfonate groups, again exemplified by the peak position for the cathodic scan, follows a trend related to the strength of the ion-pairs. The Hupd region of the voltammograms is shown in the inset of Figure 3. No significant changes were observed in the Hupd region of the Pt(111) voltammograms with cation exchanged ionomer films. This suggests that junction potential (Donnan equilibrium), arising from the different proton activities, for cation exchanged ionomer on introduction into the acid electrolyte, is negligible in our experiments. The potential shifts observed in the anodic and cathodic peaks are in fact related to the change in adsorption properties of the sulfonate anion in the presence of different cations. We would like to emphasize that, in order for such a trend to be absolute, one must ensure the composition of the ionomer is same for all the samples. Composition variation of the cation within the ionomer is also expected to modify the peak position as can be observed from the reverse exchange voltammograms shown in Figure 4. Time resolved voltammograms for cation exchanged membranes show that, as the cation, in this case Mg2+, is reverse exchanged by the protons in the acid electrolyte, the adsorption/desorption peaks shift back close to their original position. Also, the charge under the adsorption/desorption peaks increases with increasing proton concentration. The final voltammograms are seldom same as the proton form because of some residual cations present in the ionomer as well as the possible morphology change induced by cation exchange into the ionomer. Therefore, the differences in the reverse exchange rates of various cations make it difficult to control the composition at the exact recording time and thus the exact position of the peaks is admittedly questionable. In spite of the subtle
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Figure 4. Relaxation of cation exchanged ionomer film where the cations are reverse exchanged by the protons in the supporting electrolyte. As the concentration of cations decrease due to exchange with protons, the peaks shift back to original position corresponding to the proton form (scan rate 50 mV/s).
differences in the compositions, we can still qualitatively conclude, at least for K+ and Mg2+, that as the protons are exchanged with larger cations the interaction forces play a major role in the observed voltammograms. Also, the potentials of adsorption and desorption are a function of the Mn+ · · · SO3interaction. Recently, a similar type of interaction at two-phase interfaces has been characterized as the noncovalent interaction.44,45 For our purposes here, we will adopt this nomenclature for characterizing cation-sulfonate interactions as well. iii. Effect of Strongly Adsorbing Anions from the Supporting Electrolyte-(Bi)Sulfate and Chloride. Having established the role of cations, it is important to understand the strength of platinum-sulfonate energetics. Adsorption energetics can be probed, as discussed widely in the literature30,46–48 for other anions, at least qualitatively, by monitoring how different anions may affect the CVs. For our purposes here, we summarize the CVs obtained on the Nafion-covered Pt(111) in 0.05 M H2SO4 (Figure 5a) and in 0.1 M HClO4 + 5 × 10-6 M Cl- (Figure 5b). Figure 5a shows that apart from some small differences (compare Figures 1c and 5a), such as the lack of sharp peaks in the case of butterfly feature, the CVs of Pt(111) and Pt(111)Nafion are similar in 0.05 M H2SO4, suggesting that the (bi)sulfate anions are more strongly adsorbed on Pt than the sulfonate anions. Such behavior, of course, is expected since, while the (bi)sulfate adsorption is controlled only by the applied electric field, sulfonate adsorption should, in addition to the applied electric field, overcome some additional energetic barriers linked to sulfonate-cation and sulfonate-backbone interactions (see the last section). Similarly, Figure 5b shows that in the presence of chloride anions (strongly adsorbing) the CV of Pt(111)-Nafion is the same as corresponding Nafionfree Pt(111), implying that the anions from supporting electrolytes can easily “penetrate” through thin Nafion films. The thickness of the film (submicrometer) along with higher activity of the anion overcomes the anion transport retardation effect of the cation exchange membrane (Donnan forbidden energy). Also, the interaction of strongly adsorbing anions with Pt can significantly alter the nature of the three phase interface between Pt and Nafion layer. These results show that in the presence of strongly adsorbing (bi)sulfate and chloride anions one can easily overlook the adsorption properties of the ionomer onto the catalyst surface. 2. Polycrystalline Platinum. From the various studies reported in the literarture49–57 for systems involving polycrys-
Figure 5. (a) Comparison of voltammograms for ionomer covered Pt(111) in perchloric and sulfuric acid electrolytes. No significant difference in the Hupd region. For ionomer covered surfaces, only difference from bare Pt(111) in sulfuric acid appears to be the lack of sharp features at 0.55 V. These peaks are related to the (bi) sulfate ordering, presence of ionomer affects the long-range ordering on the surface (scan rate 50 mV/s). (b) Impact of strongly adsorbing anions such as chloride on the overall voltammogram of Nafion covered Pt(111). Sulfonate anions are easily displaced by anions with high affinity to Pt surface such as chloride (scan rate 50 mV/s).
talline platinum coated with recast Nafion, very little is clear regarding the nature of the interface. Yeager and co-workers54 used CVs to demonstrate the impact of Nafion on deactivation of the irreversible-oxide’s formation on polycrystalline gold and platinum electrodes and attributed it to be due to possible anion adsorption. Recently, Kanamura et.al55 and Malewich et al.56,57 used surface enhanced infrared spectroscopy to probe the interface between the polycrystalline platinum and the ionomer. Preliminary observations by these groups suggest the existence of a potential dependent interaction between Nafion and the platinum electrode. Results in Figure 6 reveal that anion adsorption indeed takes place on Pt-poly especially in the presence of nonadsorbing supporting electrolyte. As for Pt(110) and Pt(100), the presence of sharp peaks in the Hupd potential region is consistent with adsorption of (bi)sulfate33/sulfonate in the Hupd potential region. The sharpness of the peak is indicative of the presence of relatively large domains of “well-ordered” facets, which are usually created by the annealing method used in our experiments. Obviously the domains of facets with the (111) geometry is rather small, given that in the double layer region even a trace of mini-butterfly feature is not observed. Although, it is not as clearly seen as was in the case of Pt(111) and Pt(100) surfaces, the reversible oxide region of Pt-poly is also found to be slightly deactivated. In line with the results
The Pt(hkl)-Nafion Interface
Figure 6. Voltammograms for Pt-Poly surface in nonadsorbing (0.1 M HClO4) and strongly adsorbing (0.05 M H2SO4) supporting electrolyte for bare and film covered surfaces. Sharp features in Hupd correspond to coadsorption on anions. Slight suppression of OHad in perchloric acid is observed. No change in sulfuric acid voltammogram (scan rate 50 mV/s).
for Pt(hkl), the CVs of Pt-poly with and without Nafion are the same in sulfuric acid solution (Figure 6b), consistent with the previous suggestion that the specific adsorption of sulfonate groups can be easily suppressed by the strongly adsorbing (bi)sulfate anions. We notice that, the results for the Nafioncovered Pt-poly surfaces would be very difficult to interpret
J. Phys. Chem. C, Vol. 114, No. 18, 2010 8419 without having the corresponding results on well-characterized single crystal surfaces. We see further below that the same models developed for Pt(hkl) surfaces can be used to explain the three phase interfaces of Pt extended surfaces. 3. Nature of Interfaces. a. Two Phase Interfaces. The development of methods for preparing and characterizing welldefined metal single crystal surfaces for electrochemical use have had a substantial impact on understanding physicochemical properties of the electrified two-phase interfaces. Figure 7 is a schematic of the conventionally accepted picture of such interfaces. In general, the interface between a metal and an electrolyte solution is characterized by the presence of an electric double layer, formed by an electrical charge on the metal surface and an opposing charge in the adjacent solution. Depending on the nature of adsorbates, solvated ions and the charge of the metal electrode, at electrochemical interfaces three types of metal-adsorbate interactions are possible:58–60 (i) strong covalent bonding (bond energy >200 kJ/mol) that includes orbital overlap of adsorbates with the surface (so-called “specifically” adsorbed ions) at distances of shorter than 2 Å; (ii) weak electrostatic forces ( Mg2+ · · · SO3-. A larger ionomer spring force (energy of interaction) facilitates desorption of the sulfonate anions when the sulfonate-metal interaction becomes weak (electrostatic), as the anion is no longer specifically adsorbed. These two effects are exemplified in Figures 3 and 4 as anodic and cathodic peak position shifts, respectively. As schematically depicted in Figure 8, at potentials below the specific adsorption of sulfonate anions, the Mn+ · · · SO3- ion-pairs behave as spectator species and have a negligible effect on the adsorption of ions/reactant from supporting electrolyte. In particular, the hydrogen adsorption on Pt(111) is not affected by the Nafion film (Figure 1c). Consequently, then, the initial model of the Pt(111)-Nafion interface may be one in which the key component of the three phase interface (the H+ · · · SO3- ion pairs) are part of the traditionally accepted double layer structure and the
J. Phys. Chem. C, Vol. 114, No. 18, 2010 8421 adsorption properties at the two-phase interfaces are not disturbed by the presence of the third component until the necessary activation energy for adsorption is overcome by the electrode potential. Finally, we would like to point out that, while the proposed “spring model” does not include details regarding the water structure, polymer orientation, kinetics of rearrangement of ionomer, cluster-network rearrangement/breaking, as well as electrode properties such as work function, defects on the surface, etc., it is aimed at providing an uncomplicated approach to modeling three phase interfaces. In spite of its simplistic nature, by virtue of accounting for the true adsorbing nature of solid electrolyte, this type of model will provide a more realistic approach to model fuel cell electrodes and the reaction at these interfaces. Conclusions Voltammetric analysis was used to probe the nature of PtNafion interfaces for Pt(hkl) surfaces as well as for more complex surfaces. Adsorption-desorption features were observed as a result of application of Nafion coating to the electrode surface. The nature of the adsorbing species was identified as anionic in nature and using voltammetric fingerprinting, the adsorbing species was identified as the sulfonate anions from the ionomer. Sulfonate anions are found to adsorb on the platinum surface in the absence of other strongly adsorbing anions. While the overall behavior exhibited by the sulfonate anions is similar to that of (bi) sulfate anions, the nature and strength of adsorption of sulfonate anions are significantly influenced by the presence of the native interactions of these anions with its host polymer matrix. Impact of cation substitution in the ionomer leading to modification of local environment was also discussed. Finally, a physical model for the three phase interface structure is proposed accounting for the various structural and energetic parameters. Acknowledgment. The authors would like to acknowledge the Department of Energy for project funding under Contract No. DE-AC03-76SF00098. R.S. would like to acknowledge Argonne National Laboratory postdoctoral fellowship for his funding. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Science Division under Contract No. DE-AC02-06CH11357. References and Notes (1) Yeager, H. L.; Eisenberg, A. ACS Symp. Series 1982, 180, 1–6. (2) Hsu, W. Y.; Gierke, T. D. J. Membr. Sci. 1983, 13, 307–326. (3) Hsu, W. Y.; Gierke, T. D. J. Electrochem. Soc. 1982, 129, C121– C121. (4) Eikerling, M.; Kornyshev, A. A.; Spohr, E. Fuel Cells I 2008, 215, 15–54. (5) Kreuer, K. D. J. Membr. Sci. 2001, 185, 29–39. (6) Pratt, L. R.; Eikerling, M.; Paddison, S. J.; Zawodzinski, T. A. Abstr. Papers Am. Chem. Soc. 2002, 224, U480–U480. (7) Paddison, S. J.; Reagor, D. W.; Zawodzinski, T. A. J. Electroanal. Chem. 1998, 459, 91–97. (8) Yeager, H. L.; Kipling, B.; Steck, A. J. Electrochem. Soc. 1980, 127, C404–C405. (9) Yeager, H. L. Acs Symp. Series 1982, 180, 41–63. (10) Hsu, W. Y.; Gierke, T. D. ACS Symp. Series 1982, 180, 283–307. (11) Malek, K.; Eikerling, M.; Wang, Q. P.; Liu, Z. S.; Otsuka, S.; Akizuki, K.; Abe, M. J. Chem. Phys. 2008, 129. (12) Malek, K.; Eikerling, M.; Wang, Q. P.; Navessin, T. C.; Liu, Z. S. J. Phys. Chem. C 2007, 111, 13627–13634. (13) Subbaraman, R. PhD Thesis, 2008. (14) Zhdanov, V. P. Phys. ReV. E 2003, 67. (15) Zhdanov, V. P.; Kasemo, B. Electrochem. Commun. 2006, 8, 561– 564. (16) Zhdanov, V. P.; Kasemo, B. Surf. Sci. 2004, 554, 103–108.
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