Origin of Anomalous Activities for Electrocatalysts in Alkaline

Sep 19, 2012 - What Can We Learn in Electrocatalysis, from Nanoparticulated Precious and/or Non-Precious Catalytic Centers Interacting with Their Supp...
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Origin of Anomalous Activities for Electrocatalysts in Alkaline Electrolytes Ram Subbaraman,† N. Danilovic,† P. P. Lopes,†,‡ D. Tripkovic,† D. Strmcnik,† V. R. Stamenkovic,† and N. M. Markovic*,† †

Materials Science Division, Argonne National Laboratory, Lemont, Illinois 60439 United States Instituto de Química de São Carlos/USP, C.P.780, CEP 13560-970, São Carlos, SP, Brazil

‡

S Supporting Information *

ABSTRACT: Pt extended surfaces and nanoparticle electrodes are used to understand the origin of anomalous activities for electrocatalytic reactions in alkaline electrolytes as a function of cycling/time. Scanning tunneling microscopy (STM) of the surfaces before and after cycling in alkaline electrolytes was used to understand the morphology of the impurities and their impact on the catalytic sites. The nature of the contaminant species is identified as 3d-transition metal cations, and the formation of hydr(oxy)oxides of these elements is established as the main reason for the observed behavior. We find that, while for the oxygen reduction reaction (ORR) and the hydrogen oxidation reaction (HOR) the blocking of the sites by the undesired 3d-transition metal hydr(oxy)oxide species leads to deactivation of the reaction activities, the CO oxidation reaction and the hydrogen evolution reaction (HER) can have beneficial effects from the same impurities, the latter being dependent on the exact nature of the adsorbing species. These results show the significance of impurities present in real electrolytes and their impact on electrocatalysis.

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(ORR),11 the oxidation of small organic molecules,11,12 the hydrogen oxidation reaction (HOR),13,11 and the HER.13,14 The influence of noncovalent reactions on kinetic rates can be either catalytic (as in a case of the HER and CO oxidation reaction) or inhibiting (e.g., ORR, HOR, and the oxidation of methanol), a fact that significantly contributes to the richness of methods that can be used to control the activity of electrochemical interfaces in alkaline environments. A central complication involved with experimentally exploring the role of covalent and noncovalent interactions in alkaline solutions, however, is that the measured rates are influenced substantially (or even predominantly) by the presence of varying levels of impurities. The puzzling role of impurities in alkaline electrochemical measurements has been a topic of interest for over 25 years. Conway15−17 addressed some of the issues employing recrystallization protocols to purify electrolytes to remove some transition elements, while Spendelow et al.18 demonstrated the use of sacrificial adsorbers, in particular for Fe. However, the impact of the impurity on the electrocatalytic activities, particularly for O2 and H2 reactions, was seldom studied. Recently, effort has been focused on identifying the role of impurities from the reaction of the glass components of the electrochemical cell and how they affect the

INTRODUCTION Alkaline energy conversion devices (alkaline fuel cells, AFCs) and fuel generation devices (alkaline electrolyzers, AEs) have garnered significant attention lately due to the applicability of low-cost materials as catalysts in comparison to their acid counterparts. For example, while in PEM fuel cells Pt-based catalysts are required for efficient transformation of chemical energy of hydrogen and oxygen into electricity,1,2 in alkaline electrolytes efficient energy conversion in AFCs can be achieved with much cheaper nonprecious materials, including Ag and abundantly available transition metal elements.2−4 A major limiting factor thus has been the development of robust alkaline anion-exchange membranes, for which some significant progress is being made.5,6 The same is true for the fuel production reactions, where the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) can efficiently be carried out on metal surfaces modified with inexpensive 3d transition metal (3d-TM) oxides, especially for the HER or on 3d-TM compounds, such as oxides and sulfides.7−9 In addition to such “material factors” (e.g., strong covalent metal−adsorbate interactions), the reactivity of interfaces in alkaline environments is also governed via rather weak noncovalent interactions between hydrated cations (located in the double layer at ca. 3.5 Å10) and covalently bound oxygenated species. This issue has been recently discussed especially for understanding the role of the noncovalent interactions in the oxygen reduction reaction © 2012 American Chemical Society

Received: July 31, 2012 Revised: September 5, 2012 Published: September 19, 2012 22231

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electrochemical behavior of polycrystalline Pt electrodes.19 While the use of non-glass-based cells have addressed some of the inconsistencies, it has been found that depending on the prehistory of electrolyte preparation, temperature of electrolyte, duration of experiment, scan rates, and applied potential windows, adsorption properties and the reaction activities may vary substantially even if experiments were performed in fluoroethylenepolymer-based electrochemical cells. Consequently, unraveling both the true nature of these impurities as well as how they may affect the key electrochemical processes in alkaline solutions should be of paramount importance for the development of technologies that can efficiently transform and store energy at electrochemical interfaces at high pH values. Here, in order to probe the impurities effect on the surface electrochemistry in alkaline electrolytes, we employ the use of well-defined metal single-crystal surfaces in a glass-free cell (made with fluoropolymer). The use of single crystal surfaces for “detecting” impurities in electrochemical environments is a well-established tactic, as has been demonstrated where they are used for monitoring the effects of trace levels of halides, nitrates, and other anions on adsorption and catalytic properties of metal−electrolyte interfaces in acidic media.1 Combining scanning tunneling microscopy (STM) with traditional electrochemical methods, applied to the Pt(111) surface, we identify the signature for the presence of the impurities and establish the role of these impurities present in the alkaline electrolyte on both the adsorption and reaction properties of the electrode for the various reactions. Furthermore, intentionally adding compounds of the 3d elements (Ni, Co, Cr, Fe), we further establish the nature of the common impurities present in the alkaline electrolyte as that of the 3d-TMs. Lastly, it is also shown that the formation of 3d-TM hydr(oxy)oxides is mainly responsible for the anomalous behavior observed in alkaline electrocatalytic measurements, and depending on the nature of these elements, we can observe beneficial or poisoning effects for the various reactions.

electrode very uniformly. Once dry, these electrodes were washed with water to ensure/verify the good adhesion of particles to the glassy carbon substrate, after which they were introduced into the alkaline cell. During the alkaline ORR, HOR measurements, in order to ensure that particles had not been dislodged during the experiments, the underpotentially deposited hydrogen based charge was measured both before and after in 0.1 M HClO4. Given that the 3d-TM hydr(oxy)oxides are very soluble in acid solutions, we were able to obtain accurate measures of the surface area. A standard rotating disk electrode (RDE) setup with Ag/ AgCl reference (−0.96 V vs RHE), was used for electrochemical measurements. All the results reported in the manuscript are versus the RHE. A Pt counter electrode was used for all the experiments. The sweep rates used in the cyclic voltammetry (CV) experiments were 50 mV s−1, while the rotation rate was either 1600 or 2500 rpm. Typical experiments were conducted at this sweep rate. We also tried slower sweep rates, which showed similar behavior at much smaller number of cycles and are not reported here. Most experiments were performed at room temperature (RT) except for those mentioned in the Results and Discussion section conducted at 60 °C. For HOR and HER experiments, the potential was swept in the cathodic direction from the hold potential; the data presented is taken from first sweep curves. First scans were used for deriving baselines for clean conditions. The first scans are always obtained within 1−2 min of introduction of the electrode into the electrolyte under potential control. This was helpful to protect the electrode from electrolyte impurities. Ohmic resistaances20 were corrected for all the data reported here using the Autolab PGSTAT 302N potentiostat. Electrolyte resistance was also measured with AC impedance spectroscopy. The gases used were research grade (5N) Ar and H2. CO cylinders were purchased from Airgas at research plus grade (aluminum container, to avoid Fe contamination due to stainless steel materials).

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RESULTS AND DISCUSSION We begin by presenting the CVs obtained for Pt(111) under three different experimental conditions (Figure 1a): the first and 25th sweep in “clean” 0.1 M KOH at RT, the 15th sweep in 1 M KOH, and the 10th sweep for 1 M KOH at 60 °C. The CV recorded on Pt(111) (see corresponding STM image) during the first sweep shows all the expected characteristics of a wellordered surface in “clean” KOH environments: reversible adsorption of hydrogen (Hupd: 0.05 < E < 0.4), formation of the double layer between 0.4 and 0.6 V, and adsorption of hydroxyl species (OHad: 0.6 < E < 0.95 V). However, after 25 cycles within the same potential region, three distinctive CV features are observed: (i) the Hupd potential region is suppressed; (ii) OHad formation starts at more negative potential; and (iii) initial adsorption of the OHad becomes highly irreversible. Similar distortions are observed for the other two cases considered here as well. In the past, such variation in the OH region has often been associated with an irreversible oxide formed on the Pt surface with unknown stoichiometries.21,22 Importantly, Figure 1b reveals that the degree of Hupd and OHad distortion is strongly dependent on both the concentration as well as temperature of the electrolyte. For example, in 1 M KOH, only 15 cycles were sufficient to produce a similar degree of changes as previously observed after 25 potential cycles in dilute electrolyte solutions. The temperature of the electrolyte plays an even bigger role, i.e., at 60 °C, merely 10

EXPERIMENTAL SECTION A standard three-electrode cell made from fluoroethylenepolymer (FEP) was used for the experiments. The chemical solutions were prepared from KOH obtained from different sources (GFS, Sigma Aldrich, Fluka, and Alfa Aesar) and MilliQ deionized (DI) water. The perchlorate salts of Ni, Co, Fe, Mn, and Cr were all purchased from Sigma Aldrich at the highest purity levels available and were made into 0.01 M solution with the DI water. Small volumes of this solution were added to the electrochemical cell to study the effect of the concentrations. Six millimeter disk electrodes were used for all experiments. The electrodes were prepared by radio frequency (RF) annealing at ∼1100 °C in a 3% H2−Ar gas mixture for 7 min. The samples were transferred into the electrochemical cell with the surface protected with a drop of DI water and immersed under potential control at 0.05 V vs reversible hydrogen electrode (RHE). The Pt/C catalyst obtained from Tanaka (TKK) was mixed with water in the concentration of 1 mg/mL. This dispersion was then ultrasonically mixed for 1 h, following which a stable suspension was obtained. A glassy carbon disk (6 mm diameter) was then mechanically polished. Known volumes of the suspensions were then added using a micro pipet onto the glassy carbon disk electrode. The electrode was dried at 60 °C in an inert atmosphere. The suspension was applied so that it coats the surface of the 22232

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Figure 1. continued change in the surface coverage, indicative of the absence of a “simple CO displacement” mechanism. Enhanced CO oxidation activities are observed in the presence of 3d-TM hydr(oxy)oxides on the surface.

potential cycles were sufficient to completely transform the perfect to rather anomalous pseudocapacitive features of Hupd and OHad. Interestingly, with the exception of the CV, which is recorded during the very first potential sweep, the anomalous shapes display a close resemblance to current−potential curves recorded on Pt(111) modified with 3d-TM hydr(oxy)oxide clusters.23 The chemical nature of such species as a function of potential was also established previously for different 3dmetals.23 In turn, this was the first indication that the observed changes might be due to the adsorption of 3d-TM elements, which are known to be present in alkaline electrolytes at ppm− ppb levels even in the most clean alkaline salts. Here, the existence of trace levels (0.5−20 μg/L depending on the supplier and batch of the electrolyte salt; averages for all the cations and their levels are shown in Table S1, Supporting Information) of 3d-TM elements in dilute alkaline electrolytes was confirmed by using inductively coupled plasma mass spectrometry (ICPMS). To further confirm that the 3d-TM impurities are the main contributors to the observed anomalous behavior of Pt(111) in Figure 1a, we intentionally “spiked” the electrolyte with small concentrations (25 ppm) of 3d-TM cations. As in ref 23, CV/STM results are used to establish the correlation between the coverage of 3d-TM hydr(oxy)oxide clusters and their effects on Hupd and OHad formation. Inspection of Figure 1b indicates several features of interest in this regard. First, the STM results for the Pt(111)/Co2+ system (used as a representative of the 3d-TMs consider in this work) reveals that after potential cycling in solution containing ppm-levels of Co2+, nanoclusters (ca. 2 atomic layers thick) are uniformly distributed across the (111) terrace. Second, the adsorption of OHad (position of irreversible peaks) follows the trends in oxophilicity of the 3d elements, consistent with what was reported previously.23 To further probe the nature of clusters, we use the CO oxidation reaction, which is known to be strongly dependent on a nature and surface coverage by hydroxyl species. A key observation in Figure 1c is that the most active surface is covered with relatively high coverage of Co-hydr(oxy)oxide clusters (see STM image in Figure 1b and the polarization curve in Figure 1c). Moreover, there is STM evidence that deactivation in the subsequent sweeps is closely related to the disappearance of the very same clusters (Figure 1c); as evident from the 15th sweep recorded in CO saturated solution. It is therefore plausible that, during the CO oxidation, OH species associated with the 3d-TM clusters are consumed and, as a result, the remaining Co species become thermodynamically unstable and dissolve into the solution. We notice in passing, after ∼30 potential cycles, that Co(OH)2 clusters are barely present on the surface (see Figure S3). The underlying CO oxidation mechanism can also be extended to Pt−Co bimetallic systems; e.g., rather than affecting segregation of Pt to the surface in Pt−Co bimetallic alloys,24 a key role of CO is simply to “clean” the Pt−Co surface of the Cohydr(oxy)oxides species, which are formed when non-noble Co metal atoms are exposed to alkaline electrolytes and positive potentials. We conclude, therefore, that the anomalous behavior of CVs in the “butterfly” potential region (0.6 < E < 0.9 V) appears to be due to the presence of 3d-TM hydr(oxy)oxides

Figure 1. Comparison of STM and cyclic voltammograms for Pt(111) electrode. (a) STM image of as-prepared Pt(111) surface after the first sweep at RT. The CV for this surface is also shown. Also shown are the CVs after 25 sweeps in 0.1 M KOH at RT, 10 sweeps in 0.1 M KOH at 60 °C, and 15 sweeps in 1 M KOH at RT. CVs exhibit distortion of adsorption properties of both Hupd and OHad. Note the scan window was shortened for the 1 M KOH in the Hupd region due to reference electrode correction. (b) STM image of Pt(111) electrode cycled in 0.1 M KOH with 25 ppm of Co2+ cations. Also shown are the CVs after 25 sweeps in 0.1 M KOH in solutions containing different cations such as Ni2+, Co2+, and Cr2+. Fluka KOH electrolyte, ultra high purity, was used for these spiking measurements (c) STM image of the cycled electrode (Figure 1b) after cycling in CO saturated solution. Also shown are the polarization curves for the CO oxidation reaction for the fresh as-prepared electrode as well as the consecutive cycles of the “contaminated” electrode. CV at the end of “cleaning” protocol is shown in Figure S3. We note in passing that, holding the contaminated electrode in CO-saturated solution at 0.05 V does not show any 22233

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Figure 2. Polarization curves corresponding to the ORR and HOR on Pt surfaces. (a) Effect of cycling on the ORR and HOR activities of the Pt(111) surface in 0.1 M KOH as a function of cycling at room temperature. Activities are found to decrease with cycling, consistent with contamination of the surface by the adsorption of 3d-TM hydr(oxy)oxides. (b) Comparison of the ORR activity stability at 60 °C for Pt-poly surfaces. Pt(111) surface exhibits significant deactivation within few cycles at such high temperatures. Inset shows the voltammetric features’ evolution with cycling for the Pt-poly electrode at RT. Pt-poly exhibits deactivation of ∼200 mV for the half-wave potential with cycling at high temperature, which is higher, compared to ∼120 mV at 30 °C.

In the following, we use this rate expression to discuss the puzzling cycling-/temperature-induced activity variations in alkaline solutions at constant rotation rates. As shown in Figure 2a, the activities for the HOR and the ORR are found to decrease with potential cycling. Notice that the difference in activity between the first and tenth potential cycles is massive, consistent with 3d-TM impurity species blocking the sites required for H2 and O2 adsorption; thus affecting the reaction through a (1 − Θcov − Θnoncov) term. The deactivation for the reactions is large enough to prevent achieving the mass transport limited currents in some cases for both the HOR and the ORR. Because the activities of the ORR and the HOR on Pt(111) at high temperatures change dramatically (even ∼3−4 cycles exhibit complete deactivation), the temperature effects will be presented only for the Pt-poly electrodes, which we found to be less sensitive toward poisoning effects by the impurities in the electrolyte. The behavior exhibited by the Pt-poly surface at 60 °C is shown in Figure 2b. The half wave potential (potential at half the maximum current) is found to shift toward higher overpotentials much faster at higher temperatures: after 50 cycles, ∼100 mV at 30 °C compared to ∼200 mV at 60 °C. Thus, from the high-temperature ORR experiments in Figure 2b, we conclude that one should be very careful in deriving activation energies from Arrhenius plots in alkaline solutions. With the exception of the CO oxidation reaction, the main contribution of the 3d-TM impurities on surface activity between 0.05 to 1.0 V (the HOR and ORR) appears to be via a (1 − Θcov − Θnoncov) term. So far, the role of the impurities has been found to be generally independent of the chemical nature of the elements (except for the CO oxidation reaction). Also, the potential regions of interest, particularly for the ORR and the HOR, are CV visible and hence can be correlated with the observed CV behaviors of the cycled electrodes. However, the situation below 0.05 V is rather different, as revealed in Figure 3 for the HER in electrolytes with intrinsic differences in the content and nature of transition metal impurities. In particular, while in the case of one electrolyte with a higher content of Co and Ni, the HER activity is found to enhanced after extensive potential cycling, under the same experimental conditions the HER is substantially deactivated in

rather than the formation of Pt-oxide with yet unknown stoichiometry, as has been considered previously in the literature.21,22 Given that these experiments were conducted in a glass-free cell, the main source of contamination is the electrolyte itself, and the role of glass components is secondary in nature, contrary to what was thought previously.19,25 We would like to point out that the CO oxidation was studied here primarily as a probe reaction, and our results indicate that CO oxidation reaction is the least affected experiment with cycling in alkaline solutions. In fact, CO oxidation or cycling appears to be the possible protocol for cleaning the electrodes in alkaline solutions. It is important to note that the time required for the complete distortion of the CVs is also a function of hydrodynamic conditions such as stirring of electrolyte, i.e., the number of CVs required for complete distortion in “clean” 0.1 M KOH is less than ∼12 cycles. A similar effect is also observed with decreasing the sweep rates and/or increasing the potential hold times before experiments. The use of rotating disk electrode (RDE) enhances the mass transport of both reactants (H2, O2, H2O) and the 3d-TM impurities present in the bulk of the electrolyte. As we demonstrate further below, interplay between these two mass-transport-dependent processes will, in turn, govern the rate HOR/HER and ORR. In general, in alkaline solutions, the rate of electrochemical reaction (current density i) at a constant electrode potential (E) can be governed by the following rate expression:11 i@ E1 = nFK1cr[1 − Θcov − Θnoncov ] exp−(

ΔG * ) kT

(1)

where n is the number of electrons, K1 is a constant that incorporates all constant variables and the rate constants for a particular reaction, F is Faraday’s constant, cr is the concentration of reactant species in a solution (H2, O2 and H2O), Θcov and Θnoncov represent the fraction of the surface masked by site-blocking covalently and noncovalently bound “spectator” species. The exponential ΔG* term (ΔG = ΔG*cov + ΔG*noncov) corresponds to the standard Gibb’s free energy change required to form products from the reactants and the active intermediates. 22234

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predict the trends in the HER with cycling. This is often related to the variations in the relative concentrations of the cations. Therefore, significant precautions are necessary while measuring the HER activities in alkaline solutions. For fundamental studies, in order to avoid the contributions of the impurities, activities for the HER must be either derived from the very first potential scan, or the electrolyte purity must be significantly enhanced. For practical applications, however, stable activities are seldom established (in fact, it is customary to see deactivation of the HER), signaling that alkaline electrolytes are predominantly contaminated with undesired Fe-/Mn-type impurities. So far, we have demonstrated using the Pt extended surfaces, that it is indeed possible to understand the origin of anomalous activities in alkaline electrolytes. The relatively high sensitivity of these surfaces aids in the detection and characterization of the electrochemical signatures for the adsorbed species responsible. In order to apply these principles and to demonstrate that the behavior exhibited by such surfaces is completely transformational across various length scales, we compare the results for cycling of Pt nanoparticles (see Experimental Section for preparation) in alkaline electrolytes. Figure 4a,b shows the effect of cycling on both formation of adsorbates as well as the role of CO oxidation reaction based “cleaning” on recovering the original voltammogram. Unlike the case of extended surfaces, the surface contamination process is slower, due to the lower sensitivity and lack of welldefined adsorption sites; however, within the time frame of a reasonable long-term experiment (e.g., 100 cycles), the features similar to those in Figure 1 appear: (i) suppression of the Hupd and (ii) irreversible peaks in the oxide (hydroxide) region (E > 0.6 V). On subjecting this surface to the CO-cycling protocol discussed briefly in Figure 1c, we once again find that the surface adsorbed species are removed, restoring the original voltammogram. In order to confirm the difference between oxidation of the CO molecules through the hydroxyl groups on these 3d-hydr(oxy)oxides versus CO displacement of such species, we have shown the CO stripping data for the

Figure 3. Polarization curves comparing the HER activities for the Pt(111) surface cycled in two different batches of 0.1 M KOH. The first one is rich in Fe2+ and as shown, the activity for the HER decreases with cycling, and the second electrolyte was found to have a higher content of Co2+ compared to other TM cations and was found to exhibit higher activities for the HER. This explains the onset of the anomalous activities observed for the HER in alkaline solutions.

the other electrolyte, which had a higher concentration of Fe. Such behavior is not uncommon, and has been discussed in literature, for example, Petrii and Tsirilna,26 as well as others who have reported the deactivation of the HER activities as was compiled by Trasatti.9 Nevertheless, in our experiments, we found that solution used in the first experiment is rich in the concentrations of Co and Ni impurities, while in the second experiment with higher concentrations of Fe. As was shown recently,23 the nature of the transition element and hence its hydr(oxy)oxide on the surface plays a significant role in determining the activity of the surface for the HER. It was shown that the presence of less oxophilic hydroxides such as Co(OH)2 and Ni(OH)2 on the surface of the Pt can optimize the turnover frequency (TOF) of the water dissociation step and indeed to catalyze the HER.14,23 On the other hand, the TOF is attenuated on highly oxophilic Fe or Mn hydr(oxy)oxides. It must be noted that while these two cases presented here exhibit qualitatively distinct features, it is often difficult to

Figure 4. (a) Comparison of pristine Pt/C electrode and the one that was cycled for 100 cycles between 0.05 and 1.0 V at 50 mV/s. Slower scan rates (20 mV/s) were also tried, and similar behavior was obtained in fewer than 100 cycles. Also shown is the CV for the “cleaned” electrode, which was obtained after cycling in CO for 30 cycles at 50 mV/s. The surface essentially resembles that of the pristine electrode with small signatures for the contamination. (b) CO stripping curves for both pristine electrode and the cycled electrode recorded at 20 mV/s (corresponding CV in 4a). The cycled electrode clearly exhibits a prepeak at 0.45 V. The reduction curve exhibits the signature for the 3d-hydr(oxy)oxides present on the surface, indicating that CO does not displace these species from the surface, and the removal involves reaction of the OH groups present on these oxides with the adsorbed CO molecules. CV for the pristine electrode is also shown. 22235

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Author Contributions

nanoparticles in Figure 4b. Also shown is the comparison of CO stripping for “clean” (uncycled) Pt nanoparticle electrode. If the effect of addition of CO was to purely displace the oxide species, one would not expect any difference between the “clean” and the cycled electrodes for CO stripping. The existence of a prepeak in this region is a clear indication that CO oxidation proceeds through reaction with hydroxyl species adsorbed on the adsorbates on the surface. Furthermore, the preservation of the 3d-hydr(oxy)oxide peak after CO-stripping voltammetry (at 0.5 V in Figure 4b) is another clear indication that CO cleaning does not operate via a simple displacement of adsorbed species on the surface. This procedure also provides insight into the so-called CO-annealing in alkaline environments24 where the surface segregation of Pt in the Pt-M alloy nanoparticles was assumed to lead to the formation of Pt-rich surfaces. Given the higher oxophilicity of 3d elements as well as the similarity in the voltammetric features exhibited by these alloying elements to the ones in Figure 4a, for example, it is plausible that these alloying elements are removed from the Ptalloy nanoparticles through destabilization by CO molecules. The qualitative behavior of these nanocatalysts for the ORR, HOR, and the HER mimic that of the extended surfaces, where the former reactions are affected by a third-body effect of poisoning, and the latter depends on the nature of contaminant species present in the electrolyte. In conclusion, the anomalous adsorption and catalytic behavior in alkaline environments arise from the presence of 3d-TM impurities in the electrolyte. While using the FEP-based cells helps alleviate the problems that arise from reaction of the electrolyte with glass components of electrochemical cells, the intrinsic transition metal impurities present in the electrolyte play a bigger role in determining the electrochemical characteristics of metal electrodes. The use of Pt(111) as a probe was found to be very advantageous in determining the nature of 3d-TM impurities even at very low levels. We found that the ORR and HOR in KOH electrolytes are affected by a third-body effect where, irrespective of the nature of the 3d-TM elements, impurities simply act as spectators and block the surface sites (the 1 − Θ term). On the other hand, there are also desirable type of 3d-TM impurities, as in the case of Co(OH)2/Ni(OH)2 acting catalytically (the ΔG*) on the CO oxidation reaction and the HER. Lastly, this behavior is not restricted to Pt extended surfaces, and is found to affect the Pt nanoparticles system as well, albeit to a different degree. Taken together, our results demonstrated that extensive care must be taken while determining the adsorption and catalytic properties of metal surfaces in alkaline electrolytes, further necessitating the development of strategies to purify the alkaline electrolytes for a “real system”, and/or careful design of experimental protocols.

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R.S. and N.M. designed the experiments and wrote the manuscript. R.S., N.D., P.P.L., and D.S. did the electrochemical experiments. R.S. and D.T. prepared the samples and performed the STM measurements. R.S., N.M., and V.S. discussed the results. R.S. and N.M. prepared the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Office of Science, Office of Basic Energy Sciences, Division of Materials Science, U.S. Department of Energy, under contract DE-AC02-06CH11357. N.D. would like to thank the Chemical Sciences and Engineering Division at Argonne National Laboratory for funding. P.P.L. would like to thanks CAPES and FAPESP for financial support.

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ASSOCIATED CONTENT

S Supporting Information *

ICPMS results for various alkaline electrolytes, cleanliness of ultra-high purity electrolytes, the use of first scans for “control” for the alkaline measurements, and CO oxidation-induced cleaning of Pt(111) surfaces. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 22236

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp3075783 | J. Phys. Chem. C 2012, 116, 22231−22237