18672
J. Phys. Chem. C 2007, 111, 18672-18678
Relationship between the Surface Coverage of Spectator Species and the Rate of Electrocatalytic Reactions Dusan S. Strmcnik,†,‡ Peter Rebec,‡ Miran Gaberscek,‡ Dusan Tripkovic,† Vojislav Stamenkovic,† Christopher Lucas,§ and Nenad M. Markovic´ *,† Materials Science DiVision, Argonne National Laboratory, UniVersity of Chicago, Argonne, Illinois 60439, National Institute of Chemistry, HajdrihoVa 19, 1000 Ljubljana, SloVenia, and OliVer Lodge Laboratory, Department of Physics, UniVersity of LiVerpool, LiVerpool, L69 7ZE, UK ReceiVed: July 17, 2007; In Final Form: September 4, 2007
Relationships between the surface coverage of spectator (blocking) species and the rate of the hydrogen oxidation reaction (HOR), the oxygen reduction reaction (ORR), and the bulk oxidation of dissolved CO on Pt(100) and Pt(111) single crystals in acidic electrolytes has been probed by cyclic voltammetry, in situ surface X-ray scattering (SXS), and ex situ scanning tunneling microscopy (STM) techniques. It is shown that the surface coverage by spectator species during the HOR and the ORR are the same as for the corresponding coverage obtained in the inert (Ar-saturated) environment. This observation is consistent with the proposition that the availability of active sites for H2 and O2 is determined almost entirely by the coverage of adsorbates from the supporting electrolyte and not by the active intermediates. Related electrochemicalSXS studies undertaken for bulk CO oxidation reveal that the maximum rate above the ignition potential is reached on a surface that is covered by ≈90% of an ordered CO adlayer. The nature of the active sites in this case is determined by a combination of electrochemical and STM results. It is found that the active sites in this potential region are steps, which appear to be active sites for OH adsorption. To get insight into the relationship between the diffusion-limiting current and the surface coverage by the inactive CO adlayer, we introduce the concept of a partially blocked electrode surface with active and inactive areas. On the basis of the calculations and experimental results, it is proposed that the active sites for given electrochemical reactions on Pt electrodes are arrays of adsorbate-free nanoscale patches embedded in an inactive adlayer of nonreactive molecular species.
1. Introduction Recent years have witnessed rapid advances in the microscopiclevel understanding of relationships between the geometry of surface atoms and the rate of electrochemical reactions.1 This has been driven in part by the emergence of spatial microscopic techniques applicable to metal-solution interfaces2-10 together with the development of reliable as well as straightforward means of utilizing rotating disk electrodes with single-crystal metal surfaces in a disk configuration.11 This approach has been extremely successful in finding that all bonding reactions, typified by the hydrogen evolution/oxidation reaction (HER/ HOR), the oxygen reduction reaction (ORR), and the oxidation of small organic molecules and CO on metal surfaces are structure sensitive.1 The structure-/potential-dependent surface coverages of spectator species (Θad), such as underpotentially deposited hydrogen (ΘHupd), anions from supporting electrolytes (ΘA), and oxygenated species (ΘOHad and ΘO), have been shown to be critical to the structure sensitivity of the HOR and the ORR on platinum single crystals. Moreover, it has been proposed that the structure sensitivity for the oxidation of dissolved CO (CO bulk oxidation) is controlled by a delicate balance between the adsorption of COad, OHad, and anions.12 Surface coverages of spectator adsorbates and COad (ΘCOad) in Ar-purged solutions on Pt(hkl) have routinely been discerned * Corresponding author. E-mail:
[email protected]. † University of Chicago. ‡ National Institute of Chemistry. § University of Liverpool.
by integrating the charge under the i versus E voltametric curves. However, establishment of the Θad and ΘCOad values concurrently with the HOR, the ORR, and the bulk CO oxidation reactions is generally unachieved, primarily due to three reasons: (i) the relatively small current (in the range of µA) associated with the adsorption/desorption of spectator species in Ar-saturated solution is superimposed (e.g., “masked”) on a high faradic current (in the range of mA) of the electrocatalytic reaction under study; (ii) the surface coverages of reactive intermediates, which are formed in the course of faradic reactions, are not well defined; and (iii) the effect of spectator species on the energy of adsorption of intermediates is poorly understood. Because of these complexities, the relationships between coverages of spectator species and rates of the HOR, the ORR, and the CO bulk oxidation reaction on Pt (as well as other stable metal surfaces) remain unclear. In this paper, we describe a comparative examination of coupled electrochemical-surface X-ray scattering and scanning tunneling microscopy (SXS-STM) measurements of the adsorption of spectator species on Pt(100) and Pt(111), and the effects that these species have on the rates of polymer electrolyte membrane fuel cell (PEMFC) reactions in acidic aqueous media. To provide a broad assessment of adsorbate coverage effects in the HOR and the ORR, we compare cyclic voltammograms in an inert (Ar-saturated) environment with the corresponding curves obtained in a solution saturated with H2 and O2. In addition to these two reactions, we also show how the rate of
10.1021/jp0756146 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/28/2007
Surface Coverage of Spectator Species CO bulk oxidation is affected by the structure of CO and the nature of the available adsorption sites. The corresponding analysis of the polarization curves and cyclic voltamograms indicates that the maximum rate of these three reactions is reached on surfaces that are covered with more than 90% of inactive spectator species. It is therefore proposed that the active sites for given electrochemical reactions on Pt electrodes are arrays of adsorbate-free nanoscale patches embedded in an inactive adlayer of nonreactive molecular species. 2. Experimental Section 2.1. Electrochemical Measurements. Most of the experimental details have been described previously.7,11-14 Following the flame-annealing and hydrogen (or argon)-cooling method, Pt(111) or Pt(100) electrodes were transferred into the disk configuration of the rotating disk electrode (RDE). The cleanliness of the transfer and the electrolyte, even under sustained rotation at high rotation rates, was demonstrated in previous work.11 For the electrooxidation of a dissolved CO gas, the electrolyte (0.1 M HClO4, EMD ACS) was purged with CO for 15 min while the electrode was held at 0.05 V to allow for the complete poisoning of the electrode surface with COad. Potentiodynamic data were recorded by applying a positive sweep direction. In experiments for the HOR and ORR, the surface was covered either by a Br-, SO42-, or COad adlayer, and the electrolyte was saturated by molecular hydrogen (or oxygen). The current-potential curves were recorded with a sweep rate of 50 mV/s. For both the Pt(111) and Pt(100) systems, the surface coverage of adsorbates is referenced to the atomic density of the (111)-(1 × 1) and (100)-(1 × 1) surfaces, that is, 1.5 × 1015 and 1.2 × 1015 atoms /cm2, respectively. The pure gases were purchased from Matheson (Matheson purity: 4N CO, 5N Ar, and 6N H2). All potentials are referred to the reversible hydrogen electrode at room temperature, and current densities are based on the geometric surface area (A ) 0.283 cm2). 2.2. Surface X-ray Diffraction Measurements. In surface X-ray diffraction measurements, the electrochemical X-ray cell was purged either with nitrogen or with CO. The CO diffuses through the thin polypropylene film trapping the electrolyte and adsorbs (dynamically) onto the Pt(111) surface while under potential control. For more experimental details, see ref 12. The experimental protocol required for determining the surface structure of Brad and potential-dependent fractional coverages of Br are summarized in detail in our previous SXS7,15 and RRDE13 measurements, respectively. 2.3. STM Measurements. The experimental STM procedure involved several steps: (i) after the flame-annealing procedure,16 a drop of water was placed on the crystal to protect the surface during its transfer into the electrochemical cell; (ii) the electrode was immersed into electrolyte under potential control (0.05 V) and the cyclic voltammetry was recorded to establish the surface quality; (iii) CO was introduced into the cell while the electrode was held at 0.05 V and then the polarization curve for CO bulk oxidation was recorded in CO-saturated solution over the potential range 0.05 < E < 0.95 V; (iv) after the second potential cycle, the electrode was emersed from the electrolyte at 0.05 V and transferred into a glove box filled with carbon monoxide; (v) the mounted sample and microscope were then enclosed in an airtight cylinder filled with CO at 1 atm; (vi) The partial pressure inside the cylinder was varied by overpressuring the cylinder with a known amount of high-purity Ar or CO; and (vii) after the STM experiments, the electrode was again transferred to the electrochemical cell for post-STM
J. Phys. Chem. C, Vol. 111, No. 50, 2007 18673 characterization. STM images were acquired with a Digital Instrumentations multimode dimension STM controlled by a Nanoscope III control station. All images were acquired using constant current mode. The set-point current was typically 4001200 pA, and the bias was 100-300 mV. The STM tip was Pt-Ir wire, sharpened by electrochemical etching. Voltammograms recorded before and after the STM measurements were identical, confirming that during STM experiments the electrode remained free of contamination. 2.4. Calculated Current Transients on RDE. To make the analysis of the experimental results more quantitative in the potential region where the rate of electrocatalytic reaction is controlled by mass transfer limitations, the i versus time response on a RDE has been calculated using Matlab software. As the problem of solving the current transients analytically on an array of closely spaced electrodes is too complex, numerical calculations were used in our analysis. The approach is similar to one that was described in detail in refs 17-22. First, a discrete twodimensional grid with dimensions ((15 µm in the x-direction and 1030 µm in the y-direction) was defined with number of rows I and columns J, so that the dimension of a single grid square was ∆x‚∆y ) x/I‚y/J. As will be discussed in Section 3.3.1, this two-dimensional grid is geometrically very similar to the STM image of the Pt(111) in the potential region where the CO oxidation reaction is under diffusion limitations. Second, concentration (Ci,j) changes in time (t) were calculated using the following conditions and equations:
t ) 0,
Ci,j ) 1 t>0
∆Ci,j (t) )
(
Ci,j(t + ∆t) ) Ci,j(t) - ∆Ci,j(t)
(Ci,j(t) - Ci+1,j(t))
∆t‚D
+
2
+
(∆x) (Ci,j(t) - Ci,j+1(t)) (∆y)2
(Ci,j(t) - Ci-1,j(t)) +
(∆x)2 (Ci,j(t) - Ci,j-1(t)) (∆y)2
)
(a)
(b)
In the following, three examples will be considered for discussing the so-called partially blocked electrode phenomena. The boundary conditions used are as follows: CI/2,15+n ) 0 (n ) {1,2,3...,999,1000}, ∆y ) 1 µm) for a totally active electrode; CI/2,15+10n ) 0 (n ) {1,2,3,...99,100}, ∆y ) 1 µm) for an electrode with 1 µm active holes; CI/2,150+10n ) 0 (n ) {1,2,3,...,999,1000}, ∆y ) 100 nm) for an electrode with 100 nm active holes. For all three cases ∆x was 100 nm. The values of ∆x and ∆y were chosen to give necessary spatial resolution in concentration changes along the electrode surface. Finally, setting the concentration to be constant 15 µm away from the electrode corresponds to the diffusion layer thickness observed at ca. 1000 rpm and was defined by the following boundary conditions: C1,j ) 1, CI,j ) 1, Ci,1 ) 1, Ci,J ) 1. On the basis of the calculated concentration profile the current on RDE is obtained using expression (c)
(dxdc)
iR
x)0
(c)
3. Results and Discussion 3.1. The HOR on Pt(100) and Pt(111) Modified by Spectator Adsorbates. For the purpose of demonstrating how
18674 J. Phys. Chem. C, Vol. 111, No. 50, 2007
Figure 1. HOR on Pt(100) 0.1 M HClO4 (gray line). HOR (first positive potential cycle to 0.7 V) on Pt(100) covered by monolayer of COad at 900 rpm (dark red line). First negative potential sweep recorded from 0.7 V (light red curve) and consecutive positive potential cycle from 0.05 to 0.7 V (dashed red line); after five consecutive sweeps between 0.05 and 0.7 V, the HOR accompanied with a stripping of the remaining COad is recorded above 0.7 V (dark red doted line). Stripping curve of COad on Pt(100) in Ar-purged electrolyte (blue curve) obtained on a freshly prepared surface. Sweep rate: 50 mV/s
the Θad values can be assessed independently from the concurrent faradic reaction as well as to explore the effect of spectator species on the rate of the HOR, three representative sets of polarization curves are summarized in Figure 1: (i) the HOR on a CO-free Pt(100) surface (gray curve); (ii) the HOR on Pt(100) covered by a full monolayer of CO (dark red curve); and (iii) the HOR on Pt(100) partially covered by CO (light red curve). In these experiments, CO adsorption (5 min at ≈0.05 V in CO saturated solution) is followed by purging the solution either with Ar or H2 and the subsequent recording of the CO-stripping curve (blue curve) and the H2 oxidation current on a CO-covered surface (red curves), respectively. Examination of the COstripping curve in Ar-saturated solution reveals that the onset of CO oxidation is accompanied by the formation of a so-called preignition wave over the potential region ≈ 0.15-0.7 V. (The preoxidation region is clearly seen if the current is magnified between 0.1-0.5 V. For clarity, however, the magnification of the preoxidation region is omitted.) Above 0.7 V, the stripping voltammetry is characterized by a sharp CO-stripping peak centered at ≈0.75 V. The total charge density under the COstripping peak (blue curve), which consists of the charge for CO oxidation coupled to the various anion/OH adsorption/ desorption processes,12,23 is 440 µC/cm2; which is equivalent to the stripping of a close-packed monolayer of COad. Figure 1 shows that a small yet clearly discernible H2 oxidation current between 0.15 < E < 0.5 V on the Pt(100)CO surface (dark red curve) is followed first by a fast increase in the reaction rate and then at ≈0.7 V by the diffusion-limiting current. Figure 1 also shows that if the potential sweep is reversed at ≈0.7 V then the current stays close to the diffusionlimiting value up to ≈0.3 V (light red curve). At more negative potentials, however, the current decays rapidly to essentially the same value as the one observed on a surface that is fully covered with COad. Given that the observed deactivation below 0.3 V coincides with the adsorption of Hupd on CO-free Pt sites, it appears that the Hupd also behaves as a blocking species in the HOR, as discussed previously in refs 24 and 25.
Strmcnik et al.
Figure 2. HOR on Pt(111) (gray curve), cyclic voltammetry of Pt(111) in 0.1 M HClO4 + 10-4 M Br- electrolyte (red curve) and corresponding HOR polarization curve (blue curve). The inset shows a model of the (3 × 3) bromide structure on the Pt(111) surface, revealed from in situ SXS measurements, which is stable in the potential window between 0.3 < E < 0.6 V. Sweep rate: 50 mV/s
Nevertheless, the activity during five consecutive potentials sweeps between 0.05 and 0.7 V (dashed light red curve) was roughly the same, suggesting that after the first sweep the remaining ΘCO is not affected by prolonged cycling within this potential range. Considering that the diffusion-limiting current of the HOR is achieved well below the potential where the main CO-stripping peak is observed in Ar-saturated solution (blue curve), the question arises: how many CO-free Pt sites are required for the HOR to reach the maximum diffusion-limiting rate? To evaluate the CO-stripping charge (ΘCO) in H2-saturated solution, after the fifth consecutive potential sweep in the preignition potential region the potential window was opened up to 0.9 V (dashed dark red curve). The observation of a single sharp peak centered at 0.75 V in H2-saturated solution (dark red dashed curve) is reminiscent of that observed in Ar-purged solution (blue curve). Note that the former is superimposed on the diffusion-limiting current for the HOR, consistent with CO electrooxidation during the H2 oxidation reaction. The surface coverage under this peak is ≈395 µC/cm2, which corresponds to ≈90% of the initial CO surface coverage. This in turn indicates that the diffusion-limiting current for the HOR on Pt(100) is reached on a surface that has only 10% of surface coverage by active “holes” in the CO adlayer. The observation that the diffusion-limiting current can be reached on a Pt surface almost completely covered by spectator species is confirmed by studying the HOR on Pt(111) modified by a Br adlayer (Brad) (Figure 2). We chose this system simply because the structure of Brad on Pt(111) has been examined extensively by our group7,15 and others.26 For example, by utilizing in situ SXS we have found that at 0.22 V Brad forms a (3 × 3) hexagonal adlayer structure (ΘBr ) 0.44 ML per surface Pt atom), which is covering ≈92% of the total number of Pt surface sites. At E > 0.22 V, however, the (3 × 3)Brad structure transforms into an incommensurate close-packed Br adlayer due to the continuous adsorption of Br-. Although the effect of H2 on the observed Brad structure/coverage is unknown, it is reasonable to suggest that neither H2 nor the reaction intermediate (the socalled overpotential deposited hydrogen, Hopd in Conway’s notation27) may affect the Br adlayer. This supposition is based on the fact that the Pt-Br interaction is much stronger than the Pt-Hopd interaction. Evidence for this is provided in Figure 2,
Surface Coverage of Spectator Species
Figure 3. Cyclic voltammetry of Pt(100) in 0.05 M H2SO4 (red curve) and the corresponding polarization curve for the ORR (blue curve). Sweep rate: 50 mV/s.
which shows the cyclic voltammogram for the Pt(111)-Brad system in Ar-purged solution (red curve) along with the corresponding polarization curve for the HOR (blue curve). As described in ref 13, in Ar-purged solution the potential region of the Hupd adsorption/desorption (0.05 E < 0.3 V) is accompanied by Br- desorption/adsorption. In order to study how this pseudocapacitance might be changed during the H2 oxidation reaction, the polarization curve for the HOR is recorded with relatively fast sweep rates. The representative example, summarized in Figure 2, reveals that in the potential range 0.15 E < 0.3 V a pseudocapacitance for Hupd/Brad adsorption/ desorption is clearly superimposed on top of the faradic current corresponding to the diffusion-limiting rate of the HOR. It is noteworthy that in the similar voltametric profiles observed in Ar-purged (red curve) and in H2-purged (blue curve) solutions, for example, within 0.15-0.3 V, there are no observable differences in the pseudocapacitance curves recorded in these two environments. This behavior is entirely consistent with the supposition that adsorption of Hupd and Brad are affected neither by H2 nor Hopd. Furthermore, the fact that the HOR is taking place only on spectator free sites (at 0.22 V < 10%) may also suggest that the coverage by reactive Hopd is most likely negligible in comparison with ΘBr. The fact that the theoretical value of the diffusion-controlled limiting current is observed at 0.22 V where the surface is covered by 92% of inactive Brad, implies that the maximum rate for the HOR can be reached on a surface with only 10% of active “holes” in the Br adlayer. Further inspection of Figure 1b reveals that by continuous deposition of Br- ions within the potential region 0.4 < E < 0.95 V (see adsorption isotherm in ref 13) the number of adsorbate-free Pt sites is decreasing and the rate of the HOR is attenuated monotonically below 0.4 V. This indicates that the rate of the HOR is completely limited by the availability of Pt surface sites that are not occupied by spectator Brad species. 3.2. The ORR on Pt(100) Modified by Spectator Adsorbates. In addition to the HOR, we have investigated how Θad may influence the ORR (Figure 3), a cathodic half-cell reaction in the PEMFC. Closely following the experimental procedure used in Figures 1 and 2, Θad in O2-free solution is determined from the cyclic voltammetry of the Pt(100) surface in 0.1 M H2SO4 in Ar-purged solution (red curve). In these experiments, to estimate the fractional surface coverages by sulfate anions we have used the so-called COad displacement methodology.28 From the charge integrated under the i versus time curves, we estimate that the surface is covered by ≈85% of (bi)sulfate
J. Phys. Chem. C, Vol. 111, No. 50, 2007 18675 anions. Again, the polarization curve for the ORR (blue curve) was recorded with a relatively fast sweep rate in order to be able to superimpose the pseudocapacitace for the formation of the (bi)sulfate and Hupd adlayers on top of the diffusion-limiting current for the ORR. Figure 3 shows that in the range 0.3-0.5 V almost identical voltametric curves are observed in Ar (red curve) and O2 (blue curve) saturated solutions. Therefore, similar conclusions for the ORR can be made to those for the HOR, namely: (i) the ORR is taking place only on spectator-free adsorption sites and can reach the diffusion-limiting current on a surface which is ≈90% covered by spectator species; (ii) the surface coverage by O2 and adsorbed intermediates that are formed in the course of the ORR (e.g., O2- and HO2- peroxo species, etc.) must be relatively small in comparison with Θad. Given that at E < 0.45 V desorption of sulfate ions is accompanied by the adsorption of Hupd, the decrease in the activity and change in the reaction pathway for the ORR in this potential region is controlled by Hupd, as suggested in ref 11. Notice that the deactivation in the HOR (Figure 1a) and the ORR is observed at almost the same potential, suggesting that in both reactions Hupd is the spectator species rather than an active intermediate. 3.3. The CO Oxidation Reaction on Pt(111) and the Concept of Partially Blocked Electrodes. To further explore the relationships between coverage of spectator species and the rate of PEMFC reactions, the rate of CO bulk oxidation on Pt(111) is correlated to the well-established p(2 × 2)-3CO T (x19xx19)R23.40 - 13CO phase transition. The results obtained from electrochemical and in situ SXS results are summarized in Figure 4. (As discussed previously,1,12 the experimental conditions for measuring the rate of the CO oxidation reaction in the rotation disk configuration and monitoring the phase transition in CO adlayer structures in the SXS electrochemical cell are almost identical; i.e., in both experiments there is no mass transfer limitation for CO adsorption.) The upper part of Figure 4 displays polarization curves for the oxidation of dissolved CO on Pt(111) in 0.1 M HClO4. Notice that a slow rate of CO oxidation in the preignition potential region (0.3 < E < 0.9 V) is followed by fast oxidative removal of CO at E ) 0.9 V (the ignition potential) and finally at E > 0.9 V the CO oxidation rate becomes entirely mass transfer limited. Although the rate of CO oxidation changes with electrode potential, in both the preignition potential region, as well as at/above the ignition potential, the mechanism for CO oxidation obeys a Langmuir-Hinshelwood (L-H) type reaction in which adsorbed CO reacts with adsorbed OH:12
COad + OHad ) CO2 + H+ e-
(1)
In agreement with previous SXS results,12,29-31 the bottom part of Figure 4 shows that with a continuous supply of CO to the X-ray cell at 0.05 V a diffraction pattern consistent with a p(2 × 2)-3CO structure (with a CO coverage of ΘCO ) 0.75 ML per surface Pt atom) is observed. As further demonstrated in Figure 4, the potential range of stability of the p(2 × 2)3CO phase is strongly affected by the oxidation of a small fraction (≈10%) of CO in the preignition potential region (see Figure 1). In this potential region, the current density is proportional to the number of the CO-free Pt sites, which based on our previous analysis are the active sites for OH adsorption. Figure 4 shows that the disappearance of the p(2 × 2)-3CO structure at 0.7 V is accompanied by the formation of a (x19xx19)R23.40 - 13CO structure (hereafter denoted as the “x19” structure with ΘCO ) 0.685 ML per surface Pt atom),
18676 J. Phys. Chem. C, Vol. 111, No. 50, 2007
Strmcnik et al.
Figure 5. Simulated current-time response of a 1 mm RDE with 100% active surface area (dark green curve), 10% active surface area with 1 µm holes (blue curve), and 10% active surface area with 100 nm holes (light green curve). Theoretical 100 and 10% Cotrell responses of a 1 mm electrode with no rotation are shown as black and dark blue dashed straight lines, respectively. Insets a-c: concentration contour plots (assessed from the blue curve) after (a) 10-5, (b) 5 × 10-3, and (c) 10-1 s.
Figure 4. (Top) The polarization curve for CO oxidation on Pt(111) in CO-saturated 0.1 M HClO4 solution at 298 K (sweep rate 2mV/s). (Inset) Magnification of the preoxidation potential region. (Bottom) X-ray voltammetry measured at the (1/2, 1/2, 0.12) and (3/19, 14/19, 0.12) positions where X-ray scattering arises due to the p(2 × 2)-3CO and (x19 x (x19)-13CO structures respectively. Schematics of the CO structures, indicating the unit cells, are shown between the two panels.
and this is stable up to 1.15 V. The coherent domain size of the “x19” structure, inferred from SXS measurements, is relatively high, ≈15 nm, suggesting that the (111) terrace sites are almost fully covered by the “x19” structure. By comparing electrochemical and SXS data, three key observations can be inferred: (i) in the p(2 × 2)-3CO structure, COad is completely “inert” on the surface, supporting many examples from heterogeneous catalysis that ordered adlayer structures are in fact inactive systems, (ii) the appearance of the “x19” structure is initially mirrored by a slow rate of CO oxidation, and (iii) at higher potentials the “x19” structure is stable even in the potential region where CO oxidation is under pure mass transfer limitations. Therefore, the diffusion-limiting current (id)
id ) 0.620nFAD2/3ω1/2ν-1/6C*
(2)
for CO bulk oxidation is observed on the surface that is ≈90% covered by COad (where n ) 2 is the number of electrons, F is the Faraday constant, A ) 0.283 cm2 is the geometric surface area of electrode, D ) is the diffusion coefficient, ω is the rotation rate, υ ) is the kinematic viscosity, and C* ) is the concentration of CO). To get insight into the relationship between the diffusionlimiting current and the surface coverage by inactive CO, we introduce the concept of a so-called partially blocked electrode
with active and inactive areas on its surface.17-22 Along these lines, we propose that on the surface where the diffusion-limiting current is observed two different domains must coexist: (i) an array of Pt active nanopatches at which OHad may access the electrode surface and react with neighboring COad; and (ii) an inactive CO adlayer with a stable “x19” structure. 3.3.1 Concept of the Partially Blocked Electrode. In an attempt to shed further light on the importance of the concept of the partially blocked electrode with active and inactive areas on the CO-covered Pt(111), as well as to make the analysis more quantitative, the id versus time response of a RDE has been calculated using Matlab software. This analysis will enable some overall mechanistic deductions to be made regarding the observed id versus ΘCO relationships above the ignition potential. In this analysis, depending on the diffusion constant (D), the radius of patch (r), and the distance between the centers of two adsorbate-free patches (L), three possibilities are considered. As summarized in Figure 3 and discussed by Gileady in detail,32 the response of the current after the potential step to the diffusion-limited region on the highly blocked surface could in general be discussed in terms of the ratio between the Nernst layer thickness, δ, and the radius of the adsorbate-free patch and the ratio between δ and the distance between two patches. Three specific cases are considered: 1. The entire electrode (with a surface geometry A ) 0.283 cm2) is active. Figure 5 shows that at a short time the currenttime response on the RDE (dark green curve) follows a typical Cotrell current decay32 (black dashed line) characteristic for the planar electrode (eq 3, e.g.
id ) nFADC0/(πDt)1/2
(3)
In contrast to the planar stationary electrode, however, on the RDE at t ) τd the current levels off to a constant diffusionlimited value characteristic for the RDE (eq 2). 2. The surface of the macroscopic electrode is 90% covered by a nonactive adlayer and 10% by an ensemble of active micropatches, ∼1 µm in diameter. Under these conditions, the current-time response (light blue curve in Figure 5) has three distinctive regions: (i) At a short time t < τ1 the logarithm of current is a linearly decreasing function of logarithm time and follows eq 2 (dark blue dashed curve). Note that in a comparison with case 1, the current density under these conditions is
Surface Coverage of Spectator Species
J. Phys. Chem. C, Vol. 111, No. 50, 2007 18677
attenuated by a factor of 10, consistent with the 10-fold smaller active surface area; for example, the current is described by eq 432
id ) (nFADC*/(πDt)1/2)(r/L)
(4)
The corresponding concentration contours are schematically shown in the inset (Figure 5a). (ii) At τ1 < t < τ2, when the diffusion field around the individual electrode becomes spherical but there is still no significant overlap of the neighboring diffusion fields (Figure 5, inset b), the current is governed by eq 532
id ) (nFADC*/r)(r/L)
(5)
Note that the current response is decreasing with time due to the increasingly stronger interaction among neighboring diffusion fields. (iii) At t > τ3, when the diffusion fields that are completely overlapping the ensemble/array of active sites has a tendency to approach the Cotrell behavior (eq 3 and inset c in Figure 5). Finally, in (iv) the curve levels off as convection becomes predominant. Our calculations have predicted that even for relatively large (≈1 µm in diameter) active patches on a surface with 10% of active sites the current approaches ≈70% of the diffusion-limiting current. 3. The surface of the macroscopic electrode is 90% covered by the nonactive adlayer and 10% by an ensemble of nanopatches with ∼100 nm in diameter. The calculations show that although the log(i) - log(t) response of an array of nanopatches (light green curve) is quite similar to the case of an array of micropatches (light blue curve), there are two main differences between these two systems: (i) for an array of nanopatches the transition from a planar to a radial and back to a planar diffusion field occurs on a much shorter time scale, and (ii) as a consequences, this ensemble will exhibit current close to the theoretical diffusion-limiting current characteristic for the rotating disk electrode with active surface area A. For patches that are smaller than 100 nm, the active surface area required to produce the diffusion-limiting current that corresponds to the geometric surface area of the disk electrode may even be smaller than 1%. Returning to Figure 2 and applying the conclusions from Figure 5 to determine the relationship between the “x19” structure and the diffusion-limiting current, we suggest that the diffusion-limiting current for CO oxidation observed on the Pt(111) surface covered by ≈91% of CO may be understood by the model of a partially blocked electrode with active and inactive areas on its surface. We suggest that active nanopatches above the ignition potential are located at step/edge atoms on which OH adsorption is taking place. This proposition is consistent with the ex situ STM results summarized in Figure 6 as well as with refs 12, 33, and 34. In STM experiments, depending on the experimental conditions (see Experimental Section) either a (2 × 2) or “x19” CO structure was observed. At 1 atm CO partial pressure, the (2 × 2)-3CO structure (not shown) was observed. The “x19” structure was observed by significantly diluting the CO atmosphere with Ar (CO content was ≈50% or less). The total pressure was kept at 1 atm. The inset in Figure 4 shows an unfiltered real-space image of the “x19” structure (10 × 10 nm), containing 13 CO molecules, which is readily identified even in the large scale STM image. Figure 6 reveals that terraces of average size of ≈12 nm are completely covered by the x19 structure. Another interesting feature in Figure 6a is the presence
Figure 6. (Left) A large area “constant-current” STM image of Pt(111) covered by CO obtained after sweeping the potential from 0.05 to 0.95 V. The image illustrates the presence of steps on the surface in solution saturated with CO and x19-CO domains on terraces. The step/edges run along the [1 1-2] direction. (Right) Nanopatch model revealed from STM used to simulate the diffusion profiles in Figure 5.
of steps, which are running parallel to each other. The terrace width and distribution of steps revealed from the morphological profile in Figure 6 are in very good agreement with the nanopatch model used to simulate the diffusion profiles in Figure 5. Considering that the x19 structure is stable on terraces, it is plausible that the step sites are more active for OH adsorption than the terrace sites. It should be noted, however, that some or even the majority of step sites could still be covered by CO. The major difference between the nanopatch model discussed in Section 3.3.1 and the “STM-model” depicted in Figure 6 is that the values of r and L used in calculations are substantially larger than those inferred from the STM image. However, note that r and L chosen in eq 3 in Section 3.3.1 are the upper limit needed to observe theoretical diffusion-limiting current. By decreasing r and L, as appears to be the case in Figure 6, the condition to the observed theoretical diffusion-limiting current will be fulfilled even at the shorter time scale, and thus it will be even more applicable to CO bulk oxidation above the ignition potential where Pt(111) is ≈90% covered by COad. Although the same concept may also be applied in the cases represented in Figures 1-3, there are some differences between the CO oxidation reaction and the HOR and the ORR. Notably, in the case of the HOR and the ORR the spectator species are not involved in the electrocatalytic reactions, they are only blocking species. Further studies along these lines, including electrochemical-SXS-STM measurements of the nature of active sites during the HOR, the ORR, and the CO bulk oxidation reaction in the so-called “kinetically controlled” potential regions are planned. These studies will provide a new perspective on the nature of active sites required for the development of high surface area catalysts for efficient energy conversion and storage systems. 4. Conclusions The aim of this report was to elucidate relationships between the rates of PEMFC reactions and the surface coverage by spectator species and adsorbed intermediates. We found that the HOR and the ORR occur on an electrode surface that is always covered by spectator species. Fractional coverages (Θad) of spectator adsorbates were determined from cyclic voltammetry recorded in Ar-purged solution because the coverage by reaction intermediates was relatively small. During bulk CO oxidation, ΘCO is uniquely determined from SXS measuements. The precise nature of the active sites is, however, determined from STM images. For all three reactions, we found that the
18678 J. Phys. Chem. C, Vol. 111, No. 50, 2007 diffusion-limiting currents were reached on surfaces ≈90% covered by inactive adlayers. We suggest that the active sites on the 90% inactive electrode are in an array of nanopatches (with diameter
