Electrochemical Impedance Study of MembraneâElectrode

J. Phys. Chem. B 1999, 103, 9645-9657

9645

Electrochemical Impedance Study of Membrane-Electrode Assemblies in PEM Fuel Cells. II. Electrooxidation of H2 And H2/Co Mixtures on Pt/Ru-Based Gas-Diffusion Electrodes Mariana Ciureanu,*,† Hong Wang,† and Zhigang Qi‡ HPower Enterprises of Canada, Inc., 1069 Be´ gin, Ville St. Laurent, Quebec H4R1V8, Canada, and HPower Corporation, 60 Montgomery Street, BelleVille, New Jersey 07109 ReceiVed: June 7, 1999; In Final Form: August 24, 1999

The electrochemical oxidation of H2/CO mixtures (100 ppm and 2% CO) on Pt/Ru gas-diffusion electrodes (GDE) is examined using in-situ electrochemical impedance spectroscopy (EIS). The impedance spectrum of the poisoned electrode is strongly concentration, and potential dependent, showing evidence for three states of activity of the surface: a) a state in the 0-0.3 V potential range, in which the impedance is very high and increases with the bias potential; a low-frequency loop in EIS is assigned to the rate-determining step (CO adsorption or diffusion), while a high-frequency arc provides evidence for a more rapid processsthe charge transfer on holes preexisting in the CO ads adlayer; (b) a state between 0.3 V and a critical potential (Vcrit), in which the impedance decreases with potential; the characteristic feature in this region is the appearance of a pseudo-inductive pattern, with a negative (inductive) loop at the lowest frequencies, assigned to the oxidation of Pt-COads by Ru-OHads. The time scale of this process may be evaluated form EIS, which enables a discussion of the effects of CO concentration, potential, and temperature; (c) a state at potentials higher than Vcrit, in which the diameters of the two loops in the EIS become equal. Vcrit may be used as a diagnostic criterion for CO tolerance, with a significance close to that of the ignition potential. The differences in Vcrit with respect to the values obtained on Pt-based GDE are assigned to the increase of the ratio between the rate of COads oxidation (Vox) and the rate of CO adsorption (Vads). Stripping voltammetry and polarization curves, recorded in situ, are used to support the conclusions obtained from impedance measurements.

1. Introduction A major problem in the practical use of proton exchange membrane fuel cells (PEMFC) for transportation and stationary applications is the deactivation of the Pt anode electrocatalyst by trace levels of carbon monoxide. Such traces are present in the H2 feed stream resulting from hydrocarbon or methanol fuel reforming even after a preliminary purification step. To alleviate this problem, intensive work has been devoted in the last 2 decades to finding electrocatalysts that are tolerant to CO in hydrogen at operating temperatures bellow 100 °C, which are characteristic for PEMFC. The best CO-tolerant electrocatalyst known to date is the Pt-Ru alloy with a 50% Ru atomic composition, as well as ternary alloys derived from the latter. The unusually high CO tolerance of the Pt-Ru alloy has been object to numerous studies, starting with the late 1960s.1,2 However, it is only in the past decade that careful kinetic evaluation of the chemistry behind the unique property of this surface has provided a better understanding of the processes responsible for CO tolerance.3-8 These studies have been performed both on well-defined surfaces4-6 and on bimetallic colloids supported on Vulcan XC727,8 and have correlated the behavior of Pt-Ru alloys with the enhanced facility toward CO oxidation, rather than to CO tolerance in the precise meaning of the term. It is the purpose of the present paper to examine the behavior of carbon supported Pt-Ru electrocatalysts using a different methodsthe electrochemical impedance spectroscopy (EIS). The first paper of this series has demonstrated that this * To whom correspondence should be addressed. † HPower Enterprises of Canada. ‡ HPower Corporation.

technique,9 which was never used previously to investigate the oxidation of H2/CO mixtures on gas diffusion electrodes (GDE), may provide interesting information on the mechanism of processes responsible for CO poisoning and for the opposite effectsCO removal from the surface. The advantage of EIS with respect to stripping voltammetry is that it enables direct in situ studies in the presence of both H2 and CO, i.e., in conditions close to those of practical interest; besides, EIS allows a separation in the frequency domain of the various rate processes contributing to overpotential.10-12 In part I of this series,9 we discussed the behavior of Pt-based GDE: it was shown that Pt surfaces poisoned with large (2%) and small CO contents (100 ppm) behave differently. For large CO concentrations, in the low overpotential range (below 0.3V), the electrode is practically blocked, while with 100 ppm some moderate charge transfer (hydrogen ionization) occurs at a rate which is inferior to that of CO adsorption. At about 0.3 V there is a sudden change in mechanism and the pattern of the EIS gets a “pseudo-inductive” aspect, which provides direct electrochemical evidence for the fact that COads starts being oxidized from the Pt surface. The onset of the process occurs at about 0.3 V, but the activity of the electrode is fully recovered only at a higher potential (Vcrit), which is concentration dependent. In view of the above results, several important questions emerge for PtRu, the binary alloy considered so far to have the best tolerance to CO poisoning: (a) whether the onset of COads oxidation on Pt/Ru appears at a potential lower than for Pt (as expected from the lower potential of appearance of oxygenated species on Ru as compared with Pt), or occurs at the same potential, as demonstrated by IR and MS data on the onset of CO2 evolution;3

10.1021/jp9918407 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/12/1999

9646 J. Phys. Chem. B, Vol. 103, No. 44, 1999

Ciureanu et al.

(b) whether the good CO tolerance of Pt/Ru, as compared with that of Pt, is the result of a higher rate of the oxidative removal of COads, or rather is due to a higher value of the ratio between the rates of COads oxidation and CO adsorption (Vox/Vads); (c) whether the values of Vcrit may be compared with the values of the “ignition potential”, or with those of the peak maximum in stripping voltammetry, thus providing a new criterion for CO tolerance. Answering those questions might contribute to a better understanding of the factors responsible for CO tolerance and might provide a help in the search for new electrocatalysts with improved properties. 2. Experimental Section The membrane-electrode assemblies (MEA) were those of a typical fuel cell with a 1 cm2 surface area , employing Nafion 112 polymer electrolyte membrane from DuPont. The individual behavior of the anode was investigated using single cells of the type (H2/H2), with identical hydrogen supply in both compartments. The behavior of the poisoned electrode was subsequently studied in a H2/(H2+CO) cell, in which the feed of the working compartment was switched to gas mixtures with two different CO contents, namely, 2% and 100 ppm CO; these are the gas compositions which may be attained by methanol or hydrocarbon steam reforming, with and without a subsequent CO-removal process. A reference for the H2/(H2+CO) cell was made by replacing H2 with N2 in cells of the type H2/(N2+CO). Measurements were made in two-electrode arrangement to enable a direct extrapolation of data to fuel cell conditions. The working electrode was prepared by a procedure proprietary to H Power Corp. from an E-TEK catalyst containing 20% PtRu (1:1)/Vulcan XC-72, pressed on a gas-diffusion backing of Toray paper. Electrodes with different noble metal loadings (0.35, 0.5, and 3.2 mg/cm2) were examined. Unless otherwise specified, the data presented are for the highest electrocatalyst loading, which showed the best performance in fuel cell tests the presence of CO. To enable the comparison with the data obtained previously on Pt electrodes,28 the auxiliary electrode was kept the same as in the latter experiments, i.e., with 1.7 mg/cm2 Pt catalyst pressed on a gas-diffusion backing of Toray paper. The gas flows, cell temperature, and humidification were controlled with a fuel cell test station manufactured by Fuel Cell Technologies Inc. The cell temperatures were 50 and 75 °C, which are in the typical ranges for low and high power FC systems, respectively. Most data reported here are for the lowest temperature, where poisoning effects are more important. The gas streams were humidified by passage through sparging bottles at a temperature 15 °C higher than that of the cell. The electrode poisoning has practically no influence on the open cell voltage (OCP): the H2/(H2+100 ppm CO) cells were found to have the same open cell voltage as the symmetrical (H2/H2) cell (0.0 V), while a very small anodic shift (less than 10 mV) was found for H2/(H2+2% CO) cell. The bias voltages in the two-electrode arrangement were measured against the cathode, taken as reference; preliminary experiments performed with the symmetrical (H2/H2) cell demonstrated the reversibility of the cathode, so that the difference between IR-corrected potentials in the two- and three-electrode arrangement was relatively small. The ac impedance spectra were recorded using a Schlumberger 1250 frequency-response analyzer, controlled by a PAR 273A potentiostat-galvanostat (EG&G Instruments Corp.). The impedance spectra were measured in the constant voltage mode by sweeping frequencies over the 0.03 Hz-10 kHz range, recording 10 points/decade. Typically, a dc polarization curve

Figure 1. Time dependence of the phase angle (determined at 0.05 V and 100 Hz) after H2+CO admission in the anode compartment for Pt and Pt/Ru-based electrodes.

was recorded prior to impedance measurements to determine the dc current corresponding to each cell voltage. The maximum attainable current was 1 A. The constancy of the current measured before and after each impedance measurement was taken as a criterion for the stability of the cell during the measurement. The stripping voltammograms were recorded using an EG&G M273 potentiostat-galvanostat in same cell as in the EIS experiments, in which the reference compartment was fed with H2, while the working electrode compartment was fed first with the investigated gas mixture and subsequently flushed with N2 for 2 h. Two types of polarization curves were recorded. In the first, the effect of anode poisoning was investigated in the same cells as for EIS experiments (surface area 1 cm2), using H2+CO as anode gas and H2 as cathode gas, both at a flow rate of 80 cm3 pm. In the second type of experiment, the effect of anode poisoning was tested on the fuel cell itself: polarization curves were recorded for normal fuel cells (denoted as H2/air), in which the anode gas was H2 (or H2+CO mixtures) at 1 bar and 80 sccpm, while the cathode gas was air at 1 bar and 400 scc pm. In the latter case, single fuel cells with a surface area of 5 cm2 were examined. 3. Results The electrochemical impedance spectra were recorded comparatively for the (H2/H2), H2/(H2+100 ppm CO), and H2/ (H2+2% CO) cells, at OCP and several bias voltages. For the (H2/H2) cell, measurements could be performed only at OCP, because of the reversible behavior of both electrodes: even at very low voltages (0.1 V, IR corrected), the current exceeded the upper limit of the instrument (1A). For the remaining cases, the cell was operated with pure H2 in the cathodic compartment and a synthetic gas mixture with 100 ppm or 2% CO in the anodic compartment. Prior to impedance measurements, the anode was flushed continuously with the gas mixture for 2-4 h. To verify the attainment of a steady state, the impedance components were monitored continuously as a function of time

Membrane-Electrode Assemblies in PEM Fuel Cells

J. Phys. Chem. B, Vol. 103, No. 44, 1999 9647

Figure 2. Complex impedance plot for a (H2/H2+CO) cell at OCP: (a) Pt/Ru-based anode; insert presents the cell fed with H2+2% CO. (b) Pt-based anode, insert presents the cell fed with H2+2% CO. Legends present the gas composition in the anode compartment. Points: experimental. Lines: fitted with the equivalent circuit in Scheme 1b.

at a fixed frequency, arbitrarily chosen (100 Hz). Both components of the impedance, the modulus function |Z| and the phase angle φ, showed a considerable increase with respect to the unpoisoned electrode. Figure 1 presents the increase with time of the phase angle, a known effect of adsorption at the interface; results are compared with data obtained in similar conditions, using a Pt-based GDE (1.7 mg Pt/cm 2). It may be observed that, for the 2% CO mixture, there is a sudden increase in the value of the phase angle φ after the first 60 s, followed by oscillations around a relatively constant value. The latter is about half of that obtained with the Pt-based GDE,9 for which the φ angle, measured at the same frequency is close to 45°. The oscillations are also similar to those found for the Pt electrode and tentatively assigned9 to a redistribution of the adsorbed CO on the catalyst layer, which is slow on the time scale of the experiment. This explanation is in agreement with data reported by Motoo,13a who demonstrates that equilibration of the surface occurs with rearrangements between bridged and linear forms of CO, accompanied by sorption/desorption. If the monitoring frequency is decreased to 1 Hz or below, no oscillations were observed. For the mixture with 100 ppm CO, the phase angle increase was very slow, so that the duration of the poisoning was increased to 4 h. 3.1. EIS in Open Circuit Conditions. Figures 2a and 3a present typical complex impedance and phase shift plots obtained at OCP for an electrode with a high content of noble metal (3.2 mg/cm2). The complex impedance plot for the symmetrical unpoisoned cell (H2/H2) consists of two depressed arcs. The assignment of these arcs is similar to that made previously for the similar patterns obtained on Pt-based GDE:9 the arc at low frequencies (LF) may be assigned to the chemisorptive dissociation of the hydrogen molecules (H2 ) 2Hads); the arc at the highest frequency (HF) is due to a reversible charge-transfer process involving the adsorbed hydrogen atoms (Hads ) H+ + e). Changing the gas in the anodic compartment to a H2 + CO mixture produces considerable increase of the impedance, as a result of CO adsorption. The difference of the HF and LF intercepts with the abscissa Rl ) (R0 - R m) shows an increase with respect to the unpoisoned electrode but remains much smaller than the values found in similar conditions in the EIS of Pt-based GDE,9 which are presented for comparison in Figure 2b. In the case of the anode fed with 100 ppm CO gas mixture,

the impedance plot clearly results from a superposition of two arcs of comparable size, while for 2% CO, the HF arc is larger. Comparison of the phase shift plots in Figure 3b shows that the HF maximum is located in the same frequency range as that obtained for the H2/H2 cell, which supports its assignment to the charge transfer (hydrogen ionization) on the transient holes in the CO adlayer. The LF arc is assigned to the slow CO adsorption at the interface, which alters the structure of the Hads layer. An alternative assignment for the LF arc would be that found for Pt poisoned with 2% COsthe slow diffusion of gaseous CO in the electrode backing. This assignment was ruled out for Pt/ Ru, based on the dependence of the two components (Z′ and Z′′) on ω-1/2 (ω is the angular frequency ω ) 2πf) at OCP, which is different from that expected13b for a Wartburg diffusion .29 One should note, however, that this criterion cannot be absolutized, since on blocked electrodes, the nonlinear diffusion problem is mathematically the same as for linear diffusion coupled with a first order homogeneous reaction. It is mainly a comparison of the behavior of the impedance components in different potential ranges that seems to support the above idea (Figure 4). Comparison of the inserts in Figure 2a and b demonstrates that both the size and the pattern of the complex impedance plots are very different for Pt/Ru and Pt. The fact that, for 2% CO, the rate-determining process on Pt/Ru is CO adsorption, while for Pt is CO diffusion,9 explains the fact that, under identical conditions, the impedance in the LF range is almost 30 times smaller that that for the former. For 100 ppm CO, adsorption is rate determining for both electrocatalysts. From the Bode plots in Figure 3 a it may be seen that, on Pt/Ru, the HF maximum is more intense for the mixture with 100 ppm CO and relatively small for that with 2% CO. The HF maximum is higher for 2% CO, but half of that obtained in identical conditions with a Pt-based GDE (1.7 mg/cm2) (Figure 3c). 3.2. EIS Dependence on the Bias Potential. With increasing bias voltage, the pattern of the complex impedance and Bode plots changes, as seen in Figure 5 to 8. The effect of the bias voltage (IR corrected) on the real and imaginary components of the impedance, phase angle, and |Z| value, measured at an arbitrarily chosen frequency in the low-frequency range (0.125 Hz), are presented in Figure 9. It may be observed that there are two characteristic potentials: at V0 ) 0.3 V, Z′ starts


Ciureanu et al.

Figure 3. Bode plots for a (H2/H2+CO) cell at OCP: (a) Pt/Ru-based anode. (b) Magnified portion of Figure 3a. (c) Pt-based anode. Legends present the gas composition in the anode compartment.

Figure 4. Randles plots for a (H2/H2+2% CO) cell at several bias potentials (IR corrected): (1,1′) 0 V. (2.2′) 0.095 V. (3,3′) 0.6 V.

decreasing, while Z′′ and φ change sign; after Vcrit, φ, and Z′′ undergo a rapid decrease, while Z′ and |Z| approach a small, relatively constant value. Since it is known that the lowfrequency limit of the spectrum Z′ approaches the dc resistance of the interface, the region with low Z′ values may be considered as one in which the activity of the electrode has been restored. Unlike Pt-based GDE, where Vo and Vcrit are separated by 0.3 V for 2% CO, on Pt/Ru those potentials are not very distant: the difference Vcrit-Vo is about 0.15 V for 2% CO and 0.08 V for 100 ppm. On the basis of the plots in Figure 9, and on the pattern changes observed from Figures 5-8, the existence of three states of activity of the catalyst surface may be diagnosed, in a similar way as for Pt-based GDE.9 (a) In the potential range 0-0.3 V, for the electrode poisoned with 2% CO, only one large open arc is observed in the complex impedance plot (Figure 5a), with both Z′ and Z′′ increasing rapidly with the bias potential. The high values of the impedance in this range show that in this potential range the surface is partially blocked and thus inactivated. The observed loop might be assigned either to slow diffusion of CO in the porous

electrode or to slow CO adsorption. We consider the former assignment to be correct, based on the fact that in the frequency range of interest the plots of the two impedance components present the linear dependence on ω-1/2 expected for a process controlled by linear diffusion (Figure 4). In the Bode plot, there is only one large LF maximum observed (Figure 6a). In the same potential range, for the cell with 100 ppm CO, the complex impedance plot retains the pattern observed at OCP, characteristic for a system with two time constants. The presence of the HF loop in the complex impedance plot (Figure 7a) and a corresponding maximum in the Bode plot (Figure 8a) demonstrates that a residual charge transfer (hydrogen oxidation) is still possible on the holes present in the CO adlayer on the surface. The existence of a residual charge transfer, which is more rapid than CO adsorption, is of fundamental importance for understanding the reasons for which Pt-Ru electrodes retain an activity for hydrogen ionization at low CO concentrations. The LF loop in the impedance spectrum may be assigned to the rate-determining adsorption of CO on the holes in the preexisting adlayer. The LF limit of the EI spectrum increases with increasing potentials, but the effect is much smaller than that observed in Figure 5a for 2% CO. The characteristic frequencies of the two loops approach each other with increasing potentials and the two maximums coalescent at about 0.150.2 V. (b) At potentials close to Vo ) 0.3 V, both for the high and low CO concentration, the impedance plot changes to a pseudoinductive pattern, similar to that reported previously for Pt: 9 a large, positive loop at higher frequencies is accompanied at lower frequencies by a negative loop (located in the fourth quadrant of the complex impedance diagram). Such pseudoinductive patterns are known to characterize systems with adsorption intermediates, or with a transition between a passive and an active state 16-18. Conway et al.19 demonstrated that a semiinductive pattern appears in adsorption systems, in which the potential dependence of the coverage of the adsorbed species (dθ/dE) changes sign. Thus, the patterns observed in Figures 6b and 7b diagnose the appearance of a process of CO removal from the surface, presumably by an oxidative reaction. In experiments in which the H2+2% CO gas mixture was replaced by N2+2% CO, the negative loop was not observed. A similar observation was made for Pt-based GDE9 and was



Figure 5. Complex impedance plot for a (H2/H2+2% CO) cell at several bias potentials (IR corrected). (a) Low overpotential range. (b) High overpotential range. Points: experimental. Lines: fitted with the equivalent circuit in Scheme 1b.

interpreted as a proof that the negative loop is due to a coupling of the oxidative removal of CO with hydrogen oxidation. Other characteristic features of the impedance patterns in this range may be observed from Figures 5-8. (c) For 2% CO, at bias potentials close to 0.3 V, the diameter of the semicircle in the first quadrant denoted as (Ra - Rm) is larger than that of the semicircle in the fourth quadrant (Ra R0), respectively, so that the ratio γ ) (Ra - R0)/(Ra - Rm) is smaller than unity (with the notations presented in Figure 5a for the spectrum at 0.292 V). The diameters of both loops decrease with increasing potential, but the decrease of the positive loop is more rapid, so that γ increase. (d) For potentials higher than 0.3 V, there is a shift of both characteristic frequencies (f1 and f2) to higher frequencies with increasing potentials (Figure 6a). Up to Vcrit, the shift is more rapid for f1 (the characteristic frequency of the inductive loop) and rather small for f2; therefore, the two maximums in the phase shift plot (which are close, but not identical with f1 and f2) draw closer to each other as the potential increases. The modulus plot presents a maximum at the frequency where Z′′ changes sign (Figure 6b). The maximum value decreases with increasing potential, while its frequency shifts to larger values. It may also be observed that the |Z| vs frequency curves intersect each other at a frequency close to 1 Hz. (e) At a critical potential (Vcrit), γ approaches unity and remains constant for further potential increase, even if the diameters of the two semicircles continue to decrease. At Vcrit the LF resistance (R0) is very low, showing that the poisoning effects have been eliminated and the activity of the electrode was restored. Vcrit is 0.45 and 0.38 V for the electrodes fed with 2% CO and 100 ppm CO, respectively. Both values are smaller than those reported for similar experiments on Pt-based GDE9 (0.59 and 0.43 V). Vcrit could be used instead of the “ignition potential” (the foot of the wave in RDE experiments), as a criterion of the CO tolerance of the electrocatalyst. Table 1 compares the values of Vcrit at high and low CO concentrations with literature data, from RDE experiments on well-defined, or carbon-supported electrodes. While for Pt-based GDE, the characteristic frequency for the inductive loop is almost 1 order of magnitude smaller that that in the first quadrant, for Pt-Ru electrodes, the two characteristic

frequencies are rather close, in particular at potentials near Vcrit. This becomes obvious from the phase shift plots, where there is a very abrupt switch between very large positive and negative values of the phase angle. Unlike Pt-based GDE, in the present experiments, the scattering of the points was considerable and was found to increase at lower CO content and at higher potentials. For the electrode fed with 100 ppm CO, the impedance switches to a rather unusual behavior at 0.53 V, with two arcs in the second and third quadrant of the spectrum (Figure 7b). For Pt-based GDE an analogous behavior could be observed only at potentials larger than 0.9 V.9 As the impedance plot switches to the second and third quadrant, there is also an unusual behavior in the Bode plot, with a sudden jump of the phase angle between the two possible extreme values (180° and -180°) (Figure 8b). 3.3. Effect of Temperature and Catalyst Loading on EIS. Comparison of the impedance spectra recorded in similar conditions at different temperatures (50 and 75° C) revealed different types of behavior in the three potential ranges mentioned before (Figure 10 a-c). In the low overpotential range, there is an enormous decrease of the LF limit of the spectrum with higher temperatures (plots 1 and 2 in Figure 10a), showing that the electrode is activated. This behavior is due to the acceleration of the rate-determining step (adsorption, or diffusion-controlled adsorption), which appears clearly from the shift of the spectrum to higher frequencies (Figure 10c). The value of Vcrit was found to be unaffected by the temperature for the electrode with high electrocatalyst content (3.2 mg/cm2). However, for electrodes with low loading (0.35 mg/cm2), the temperature increase resulted in a decrease of Vcrit, as demonstrated for the case presented in Figure 10b. For potentials higher than Vcrit, the temperature effect was minimal (plots 3 and 4 in Figure 10a): there is a slight decrease in the diameters of the two loops observed, but the LF limit of the spectrum remains practically unchanged. The reason for this behavior may be that both the oxidative removal of CO and the opposed processs readsorption of COsare accelerated by the temperature increase, as suggested by the equal shifts of the two characteristic frequencies (Figure 10c). Comparison of EIS obtained in similar conditions and


Ciureanu et al.

Figure 6. Bode plots for a (H2/H2+2% CO) cell at several bias potentials (IR corrected): (a) phase angle vs frequency. (b) |Z| vs frequency, inserts in a and b present the low overpotential range.

identical potentials, but with anodes with different electrocatalyst loading shows impedance values which increase with decreasing catalyst loading in all potential ranges. This observation is in agreement with the activity decrease resulting from polarization curves, which will be discussed below. The value of Vcrit remained unchanged for electrodes containing 3.2 and 0.5 mg/ cm2 noble metal, but increased (from 0.45 to 0.6 V, for a gas feed with 2% CO), as the catalyst loading was further lowered to 0.35 mg/cm2 . At potentials beyond Vcrit, the change of the impedance pattern from the quadrants I-IV to II-III was observed to appear at lower potentials for lower catalyst loadings (Figure 11).

3.4. Polarization Curves and Voltammetric Data. Stripping voltammograms and polarization curves have been recorded in situ, in a two electrode arrangement, to enable easier comparison with EIS experiments. Unlike experiments in liquid electrolytes with hydrogen bubbled through the solution, data obtained in situ on gas diffusion electrodes do not show evidence for a limiting diffusion step. This conclusion supported our impedance results obtained with the H2/H2 cell. To avoid currents larger than 1A, the voltammograms were recorded for a cell in which the working electrode was exposed to nitrogen. Figure 12 presents the results of a stripping voltammetry experiment: the working



Figure 7. Complex impedance plot for a (H2/H2+ 100 ppm CO) cell at several bias potentials (IR corrected). (a) Low overpotential range. (b) High overpotential range.

Figure 8. Phase angle plots for a (H2/H2+ 100 ppm CO) cell at several bias potentials (IR corrected). (a) Low overpotential range. (b) High overpotential range.

electrode was first equilibrated with a 2% CO-H2 mixture at 0.05V and subsequently was purged for 2 h with nitrogen to remove all gas-phase CO and CO2 before the electrochemical experiments. A clear CO oxidation peak is observed at 0.465 V, a potential lower than that obtained in similar experiments with Pt-based GDE (0.63 V).9 This value is lower than that obtained by Schmidt et al. by ex situ experiments on Pt-Ru coloid/Vulcan electrodes in liquid electrolytes (0.58 V)7 but is close to the value obtained with Pt50Ru50 bulk alloy electrodes (0.49 V). It may be observed that the values of the stripping potentials obtained in our experiments are rather close to the value of Vcrit. If, instead of typical conditions for stripping experiments (i.e., with gas-phase CO removed by nitrogen), the experiment was performed with CO present in the gas phase,

the oxidation maximum shifted by 300 mV to anodic potentials, due to the negative reaction order for CO oxidation. The charge corresponding to stripping the CO monolayer may be evaluated from the area below the CO peak; for adsorption from a gas mixture with 2% CO, a comparison with the corresponding value below the H desorption peak provides, after correcting for the double layer, the value of the coverage by adsorbed CO: θ ) 0.62 ( 0.05. Due to the broad peaks in the hydrogen adsorption/desorption region and large capacitive contribution, the error in θ is larger than that in similar experiments using Pt-based GDE. However, the difference from the latter value (θ ) 0.85) is higher than the experimental error. Figure 13 presents the steady state polarization curves recorded in the same conditions as in EIS experiments, i.e., with


Ciureanu et al.

Figure 9. Potential dependence of Z′, Z′′, |Z|, and φ determined at 0.125 Hz for a (H2/H2+2% CO) cell.

TABLE 1: Comparison of Vcrit with Literature Data on the Criteria of CO Tolerance for Anode Electrocatalysts in PEM Fuel Cells electrode Pt/(H2+2% CO) Pt/(H2+1000 ppm CO) Pt/(H2+100 ppm CO) Pt-Ru/(H2+2% CO) Pt-Ru/(H2+1000 ppm CO) Pt-Ru/(H2+100 ppm CO)

V0.2 (V)a (ref 6)

Vcrit (V) (present work and ref 9)

Vign(V)

0.59

0.63d, 0.72

0.68 0.46

0.43 0.45

0.48b, 0.44c

0.46 0.36

0.38

a

Potential at which the current density attains 0.2 mA/cm2 in RDE experiments at 2500 rpm. Data presented are for well-defined electrodes. b Smooth surface of a well-defined Pt c Pt Ru 50 Ru50 alloy, ref 8. 50 50 alloy colloid supported on Vulcan XC72, potentiodynamic RDE experiments at 2500 rpm, ref 8. d ETEK Pt/Vulcan, potentiodynamic RDE experiments at 2500 rpm, ref 8.

gas mixtures containing 2% or 100 ppm CO. For 2% CO the current starts increasing abruptly at about 0.4 V, which is close to the value found for Vcrit (0.45 V) for the mixture containing 2% CO. For the mixture containing 100 ppm CO, the current was rather high, even in the low overpotential range, which is in agreement with the supposition that direct H2 oxidation is still possible, due to the presence of a large number of holes in the CO adlayer. There was no sudden increase of current observed in the investigated potential range, except for a change of slope at about 0.3 V. For both CO concentrations, a limiting current is attained at high potentials. Figure 14 compares typical current-voltage curves obtained in an (air/H2) fuel cell with a poisoned cell, air/(H2+CO), obtained with the same membrane-electrode assembly as that used in the EIS experiments. The results are similar to those found by other authors in this type of experiments21 and point out a considerable overvoltage increase in the presence of CO, which is augmented with the CO concentration. The catalyst loading was found to have no effect on the polarization curve in the presence of pure H2, so that only one curve was presented for the unpoisoned cell. In the presence of CO, there is a considerable difference between anodes with different catalyst loadingssthe anode with higher noble metal loading showing a lower overvoltage and a different pattern of the curve. It may be seen that, at low catalyst loading, in the 0.9-0.5 V region, the cell with poisoned anode (100 ppm CO), shows a dramatic

Figure 10. (a) Effect of temperature on the complex impedance plot for a H2/(H2+2% CO) cell; Pt/Ru anode with a loading of 3.2 mg/cm2. (b) Same as a, with a Pt/Ru anode with 0.35 mg/cm2. (c) Phase shift plot for a (H2/H2+2% CO) cell; Pt/Ru anode with a loading of 3.2 mg/cm2. Legend specifies the bias voltage and temperature of the experiment.

drop of current with respect to the corresponding unpoisoned cell; at voltages below 0.4 V, the current-voltage curve becomes parallel with that of the unpoisoned system, which points out that the effect of anode poisoning was minimized or completely annihilated. In the latter region, at a current of 0.11 A/cm2, the voltage difference of the polarization curves on the unpoisoned electrode and poisoned with 100 ppm CO is about 0.35 V, i.e., close to the 0.34 V value found for Vcrit. The same type of behavior was found in experiments with Pt-based GDE, with a much higher catalyst loading. However, for the latter, the current drop in the 0.9-0.4 V region was more important than for Pt/Ru electrodes; thus, even the least performant Pt/ Ru electrodes perform better than those containing Pt only. The performance of the electrode is considerably improved as the amount of Pt/Ru increases. At high catalyst loading the difference of the voltage at a given current vs the curve obtained with pure H2 is small and remains inferior to 0.35 V; the


Figure 11. Effect of catalyst loading on the complex impedance plot at potentials higher than Vcrit for a (H2/H2+2% CO) cell (V ) 0.6 V, IR corrected).

Figure 12. In situ stripping voltammograms on Pt/Ru (a1) first cycle, (a2) second cycle) and Pt (c: first cycle). Scan rate: 20 mV/s. Prior to the experiment, the electrodes were kept for 2 h at 0.05 V in a flow of H2+2% CO gas mixture and subsequently flushed with pure nitrogen for 2 h. The auxiliary electrode compartment was continuously flushed with hydrogen.

current-voltage curve is almost linear, a pattern which is similar to the polarization curve of the anode (Figure 13) and consistent with the moderate increase of the resistance observed in Figure 7a. This observation provides support for the idea that for catalysts with acceptable performance, the region which is relevant for practical applications is that with an oxidation overvoltage below 0.3 V, i.e., a region where the oxidative removal of COads is unimportant. For both catalyst loading, there is an overvoltage decrease on increasing temperature of the cell, which is consistent with the observed decrease of impedance at higher temperatures. 3.5. Simulation of EIS. The behavior of the interface at OCP in the presence of adsorbed CO can be modeled with an equivalent circuit typical for electrodes with adsorption phenomena (Scheme 1). In our case, the high-frequency circuit can be assigned to the charge transfer at the interface: Cdl is the capacitance of the double layer and R is the resistance associated with the charge transfer across the charged interface with a “frozen CO configuration”.15,16 The remaining low-frequency circuit (R1, C1) can be assigned to the capacitance and resistance of the adsorbed species. In Scheme 1b, Cdl and C1 were replaced by the constant phase elements Tdl and T1, to account for the


Figure 13. Polarization curves for (H2/H2+CO) cells with gas mixtures containing 100 ppm CO ((1) noncorrected potentials, (2) IR-corrected potentials) and 2% CO (3) noncorrected potentials, (4) IR-corrected potentials).

nonhomogeneity of the surface for both charge transfer and adsorption. Scheme 1b was found to provide a better fit of the experimental EIS patterns. Typical sets of parameters obtained from fits with the equivalent circuit of Scheme 1b is presented in Table 1, where data obtained on Pt/Ru GDE are compared with those previously reported for Pt-based electrodes. The fitted curves are presented in Figures 1 and 3 with continuous lines superimposed over experimental data points. Only cases in which diffusion does not appear to be determinant were considered. The reason for this is that, according to Armstrong,15 in systems with adsorption at the interface, the effect of diffusion is more complex and cannot be taken into account simply by introducing a diffusion impedance in the circuit. As seen form Table 2, at OCP, there is an increase of the absolute values of R (the charge-transfer resistance) and R1 with increasing CO concentration and on going from Pt/Ru to Pt. For T1 there is a considerable decrease with increasing the CO concentration between 100 ppm and 2% CO, or on changing Pt/Ru with Pt. Simulation of the semiinductive behavior can be performed either by introducing an inductance, or a negative capacitance coupled with a negative resistance. While for the former there is no acceptable physical meaning, the physical significance of negative parameters for systems presenting adsorbed intermediates was discussed by Armstrong15,16 and Conway et al.18,19 A negative resistance appears when the current decreases with increasing voltage, while a negative capacitance appears when there is a decrease of coverage of the adsorbed intermediates with increasing potential. Two different equivalent circuits were used to fit the pseudoinductive behavior using negative capacitances and resistances. The first option was to use the same circuit as in the low overpotential range (Scheme 1b), which takes into account the overall changes of the adsorption resistance and capacitance. This option resulted in the fitted curves presented by continuous lines in Figures 4b and 5b. The fit is very good for Pt and acceptable for Pt/Ru, where the proximity of the two characteristic frequencies is expected to produce complications. It is, however, difficult to assign a clear physical significance to the


Ciureanu et al.

Figure 14. Current-voltage curves for an unpoisoned (air /H2) fuel cell, and the corresponding (air/H2+100 ppm CO) cell at different temperatures. The GDE were based on Pt/Ru, except for one, which contained Pt only, presented for comparison. Legend specifies catalyst loading of anode, the temperature and CO content in the gas mixture. For the poisoned cell, all data points were recorded after flushing the anodic compartment with the (H2 +CO) gas mixture for 120 min. Between measurements, the voltage was kept at 0.5 V.

SCHEME 1

parameters C1 and R1, since they are influenced both by adsorption and the oxidative removal of CO. Therefore, only a few typical sets of parameters were presented in Table 2. The most interesting feature is the appearance of negative values for R2 and C2, a characteristic of systems with adsorbed intermediates. Another attempt to simulate the pseudo-inductive behavior was made using the equivalent circuit in Scheme 1c. Here, the circuit (C1, R1) accounts for the changes produced at the interface by the adsorption process, while a new (C2, R2) circuit accounts for the process of oxidative removal of CO. The time constants of the two circuits (τ1 and τ2) may be related with the time scale of the two processes at the adsorbed interface. Both τ1 and τ2 are large with respect to τ ) CdlR, so that the theory predicts the existence of a third loop (very small) at high frequencies; this should account for charge transfer on the surface with a “frozen” CO configuration, resulting from a

balance of the slower processes. Several sets of parameters thus obtained with the equivalent circuit in Scheme 1c are presented in Table 3. It also may be observed that the capacitance C2 is negative, which is what is expected for a system in which the coverage of CO is lowered as the potential increases. The absolute value of C2 is almost 1 order of magnitude larger than C1 and both decrease with increasing potentials. The new resistance, R2, is negative, which accounts for the fact that the total faradaic resistance in the low-frequency limit of the spectrum (R0 ) R + R1 + R2) is lowered in the systems with pseudo-inductive behavior. The absolute values of R1 and R2 decrease and draw closer to each other with increasing potentials. Therefore, the activity of the electrode is fully recovered when the absolute values of the two resistances are equal, so that R0 ≈ R at V g Vcrit. Owing to the larger value of C2, the time constant τ2 is always higher than τ1 (even if R1 and R2 are almost equal) and decreases with increasing potentials. Thus, the shift to higher frequencies observed for the inductive loop on increasing potentials may be interpreted by an acceleration of the oxidative removal of COads. From the examples provided at 0.3 V, it may be seen that the two resistances are smaller and the capacitances higher for Pt/Ru for those for Pt. 4. Discussion The observed features of the impedance pattern may be discussed in terms of a competition between several processes occurring on the electrode. The process of practical interest, hydrogen ionization, is a relatively rapid process, occurring by Tafel-Volmer mechanism, in which the rate determining step is a heterogeneous reactionsthe chemisorptive dissociation of H2;

H2 a Ha

(1)

Ha f H + + e

(2)

In the presence of CO, the rate of the process is determined by the magnitude of the CO coverage (θ), which results from



TABLE 2: Parameters Evaluated from a Fit of the Complex Impedance Plot with the Equivalent Circuit of Scheme 1b cell

GDE

bias voltage (V)

Rm (Ω cm2)

R (Ω cm2)

Tdl (F/cm2)R

R

R1 (Ω cm2)

T1 (F/cm2)R1

R1

H2/H2+102 ppm CO H2/H2+102 ppm CO H2/(H2+2% CO) H2/(H2+2% CO) H2/(H2+2% CO)

a

0.0 0.0 0.0 0.587 0.62

0.203 0.201 0.220 0.202 0.223

0.580 0.108 0.229 6.155 3.883

0.026 0.130 0.071 0.044 0.076

0.69 0.65 0.65 0.60 0.65

0.929 0.0295 1.257 -5.573 -3.499

0.035 3.444 0.1921 -0.000924 -0.00969

0.92 0.56 0.91 1.0 0.86

a

Pt Pt/Ru Pt/Ru Pta Pt/Ru

Reference 9.

TABLE 3: Parameters Evaluated by Simulating the Complex Impedance Plots with the Equivalent Circuit of Scheme 1c cell

GDE

bias (V)

R (Ω cm2)

Cdl (F/cm2)

R1 (Ω cm2)

C1 (F/cm2)

τ1 (s)

R2 (Ω cm2)

C2 (F/cm2)

τ2 (s)

H2/(H2+2% CO) H2/(H2+2% CO) H2/(H2+2% CO) H2/(H2+2% CO)

Pt Pt Pt/Ru Pt/Ru

0.300 0.598 0.292 0.60

0.400 0.203 0.238 0.208

0.00080 0.00080 0.000885 0.000819

49.1 2.15 9.504 1.489

0.0095 0.0026 0.0584 0.01067

0.466 0.0056 0.555 0.0158

-40 -1.90 -5.35 -1.271

-0.053 -0.0126 -0.526 -0.0918

2.12 0.024 2.81 0.117

the competition of three processes: (a) CO diffusion to the electrode, (b) CO adsorption and/or readsorption, (c) removal of CO by oxidation by oxygenated CO-adsorbed species. In the low overpotential range (0 < < 0.3 V), the ionization of hydrogen on preexisting holes is determined by the competition between (a) and (b): for low concentrations, the CO adsorption is rate determining, while at high concentrations the adsorption rate is high and the rate-determining process is CO diffusion. The latter case appears for both Pt and Pt-Ru in the presence of 2% CO, for which the observed impedance is very large between 0 and 0.3 V, so that electrodes are practically inactivated. This situation may be avoided by lowering the CO concentration to the point where adsorption becomes rate determining and/or changing the anode material to one for which the equilibrium CO coverage is lowered. The abrupt change of the impedance pattern which occurs at about 0.3 V reflects a mechanistic change: the pseudo-inductive loop points out that a new process appears, which lowers θ as the potential increases: the oxidative removal of COads, which creates holes on which H2 is oxidized. The onset of CO oxidation at 0.3 V is corroborated by independent research by infrared and mass spectrometry,3 which reveals the onset of CO2 evolution at this potential. The oxidative process may be explained in terms of a bifunctional mechanism: k1

CO(gas) + Pt {\ } Pt-COads k -1

(3)

k2

Pt-COads + Ru-OH ads 98 Pt + Ru + CO2 + H+ + e(4) This fundamental process was established by Gasteiger from studies on well-defined electrodes4-6 and has been generalized to provide a basis for understanding the CO tolerance of the electrocatalyts for H oxidation. On the basis of this mechanism, the CO tolerance is generally correlated with the peak potential for CO oxidation in stripping voltammetry, or with the “ignition” potential.22 With eq 4 accounting for the oxidative removal of COads, we may explain most of the experimental observations presented in the previous chapter. The fact that the inductive loop appears at the lowest frequencies suggests that COads oxidation is less rapid than CO readsorption. The characteristic frequency of the inductive loop shifts rapidly to higher frequencies with increasing potential, suggesting that the rate of the oxidative removal of COads increases with increasing potentials. Also, the characteristic frequency of the inductive loop increases with increasing temperatures (Figure 10c), suggesting that there is an increase in the rate of reaction 4.

A discussion of experimental results based on eqs 3 and 4 provides information on other points of interest for evaluating the CO tolerance. 4.1. Identity of the Potential for Onset of CO2 Evolution on Pt and Pt/Ru. In terms of eq 4 it is hard to understand the fact that the onset of the surface activation on Pt is the same as for Pt/Ru, namely, 0.3 V, as demonstrated by the appearance of the pseudo-inductive behavior in EIS9 and by infrared and mass spectrometry evidence of CO2 evolution.3 Since, at 0.3 V, the formation of Pt-OHads is out of question, we assumed in part 1 that oxidation of CO on Pt occurs by interaction with adsorbed water molecules, according to the equation:

Pt-COads + Pt-H2 Oads ) 2Pt + CO2 + 2H+ + 2e- (5) With this assumption, the species responsible for the oxidative removal of CO should be different for Pt and Pt/Ru at 0.3 V, yet CO2 starts evolving from both surfaces at the same potential. What is common in the two cases is that CO adsorption occurs on Pt,30 since for Ru the adsorption sites are mainly occupied by OHads. On Pt, in the potential range between 0.05 and 0.4 V, the hydrogen adsorption range, there are two types of COads: with CO adsorbed atop of adsorbed H molecules (“weakly adsorbed CO”) and with CO adsorbed on bare Pt surface (“strongly adsorbed CO”).23 The appearance of the pseudo-inductive pattern at 0.3 V (a potential where depletion of Hads on Pt becomes important and CO adsorbs on bare Pt) suggests that the onset of activation appears when “strongly adsorbed CO” starts being oxidized, a hypothesis advanced many years ago by Leiva et al. from potentiodynamic experiments.23 On Pt/Ru, the same should hold, since adsorption of CO occurs preferentially on Pt, which explains why the onset of the activation stage is the same on Pt and Pt/Ru. If this hypothesis were correct, then for other Pt-based electrocatalysts the onset of CO oxidation should appear also at potentials close to 0.3 V, a point which still remains to be demonstrated. The onset of CO2 evolution cannot be related directly to the polarization curves on Pt, since the current at 0.3 V is very low. This is in agreement with impedance data, which show that the pseudo-inductive behavior is observed for both Pt and Pt/Ru, but for the former, R0 is considerably higher than for the latter. 4.2. Origin of the Difference of Activity of Pt and Pt/Ru in the Low Overpotential Range. Neither for Pt nor for Pt/ Ru, the loops due to direct charge transfer on holes in the adlayer and on holes obtained by COads removal were not observed simultaneously, even if EIS evidence for such simultaneous processes has been provided for other catalytic poisons.24 Thus, hydrogen ionization occurs at a rate which is either higher or


Ciureanu et al.

Figure 15. Comparison of Bode plots recorded with a (H2+2% CO) gas mixture on Pt- and Pt/Ru-based GDE. Legend specifies the anode material and the potential (IR corrected).

slower than that of CO adsorption, depending on the potential range. A good tolerance in the low overpotential range should be correlated with a decreased rate of CO adsorption on Pt in the Pt/Ru catalyst. This idea is supported by the observations on differences in the time scales of CO readsorption on Pt and Pt/Ru. A possible cause for this decrease might be the siteblocking and electronic effects due to oxygenated species adsorbed on the second element, rather than the nature of those species. Also, the presence of Ru atoms in the immediate vicinity of Pt determines an increase in the d band vacancy and lattice parameters of Pt,25 factors which might affect the CO adsorption capacity. This observation is of particular importance for the design of new CO-tolerant electrocatalysts with good behavior in the low overpotential range, which are the most interesting for practical applications. Based on this idea, we may explain the reported similitude in the behavior of Pt-Ru (50% at Ru) and Pt-Mo (23% Mo) in the low overpotential range, despite of the disparate chemistry of Ru and Mo.26,27 4.3. Origin of the Difference in the Activity of Pt and Pt/ Ru Electrodes at Potentials Higher than 0.3 V. The excellent activity of Pt/Ru as compared to Pt is well documented by literature data4-8,21 and is supported by the polarization curves presented. Gasteiger et al.4 have formulated the hypothesis that this difference should be assigned to the larger oxidation rate constant on Pt/Ru as compared with pure Pt. Another hypothesis was formulated by Grgur et al. in the case of Pt-Mo:27 they pointed out that the steady-state coverage by COads, which ultimately decides the surface area available for H oxidation, is determined by the balance between the rate of adsorption (2) and the rate of CO removal by oxidation (3). Thus, according to those authors, it is the ratio Vox/Vads which determines the magnitude of the coverage, rather than the absolute values of the two rates. EIS measurements provide arguments to decide between the two hypothesis. At 0.3 V the characteristic frequency of the inductive loop is the same for both Pt and Pt/Ru, which points out that the time scale for CO oxidation is roughly the same (plots 1 and 2 in Figure 15). On the contrary, the characteristic frequency of the positive loop is higher on Pt than on Pt/Ru by almost 1 order

of magnitude, suggesting that it is the difference in the time scale of CO readsorption that determines the different behavior of the two electrocatalysts. In the high overpotential range the comparison presented in Figure 15 (plots 3,4 and 5,6) shows that the time scale for CO oxidation is shorter on Pt than on Pt/Ru-based electrodes. However, the two characteristic frequencies f1 and f2 are closer to each other for Pt/Ru than for Pt, which suggests that the absolute rates of the two processes (CO oxidation and adsorption) are closer. Thus, the impedance data presented seem to favor the idea that the better tolerance of Pt/Ru is due to a higher ratio between Vox and Vads, rather than to a larger Vox. A simple kinetic model may explain the dependence of the absolute value of θ on the ratio Vox/Vads: The rates of CO adsorption and desorption may be expressed in terms of θ, and CO pressure P, as

Vads ) k1(1 - θ)P

(6)

Vdes ) k-1θ

(7)

The rate of oxidation may be expressed as4

Vox ) k2θ(1 - θ)

(8)

The steady state value of θ may be obtained from the stationarity principle, dθ/dt ) 0.

θ)

k 1P k1P + k-1 + k2(1 - θ)

)

P P + K + k2(1 - θ)/k1

(9)

where K ) k-1/k1. From eq 9, an analytical expression of θ as a function of the ratio k2/k1 may be derived as

θ ) (k1/2k2){P + K + k2/k1 ( [(P + K + k2/k1)2 - 4Pa)]1/2} (10)



A more useful expression may be obtained by rearranging eq 9:

θ)

P(1 - δ) P(1 - δ) + K

(11)

where δ ) Vox/Vads. While eq 11 only provides an implicit θ dependence, it may still be useful to explain the change of θ with δ. It may be seen that θ f 0 as δ f 1, which explains why at large potentials the oxidation current approaches the value on the unpoisoned electrode; an unmodified Langmuir dependence on P is obtained for δ f 0. In the intermediate range, θ decreases with increasing δ, which in turn, increases with the anodic potential. Since the oxidation current increases with (1 - θ)2, eq 11 provides a qualitative explanation for the fact that the oxidation current on poisoned Pt surfaces is lower than that for Pt/Ru surfaces at the same potentials. 5. Conclusions EIS studies on electrode-membrane assemblies demonstrate the potentialities of this method as a tool for investigating processes which occur in PEM fuel cells and for the characterization of the influence of operating conditions. In situ impedance data are presented on the anodic process in PEM FC in the presence of CO poisoning. The major advantage of EIS with respect to other techniques is the capacity of resolution in the frequency domain of the factors contributing to overvoltage losses. Analysis of the potential dependence of EIS of CO-poisoned anodes reveals that three states can be evidenced on the electrode surface. (a) A state of low or no activity, between 0 and 0.3 V, for which the impedance loss increases with increasing potentials; in this region the rate-determining process is different, depending on the CO concentration, and may be diffusion or adsorption. In the latter case, which occurs at low CO concentrations, evidence is found for a residual charge transfer occurring at a rate superior to that of CO adsorption. Since in this region there is no CO oxidation, the charge transfer occurs on holes in the CO adlayer. The low values of θ, responsible for this charge transfer, are determined by the CO adsorption/desorption, thus are favored by a low value of k1 and/or a low pressure P. Therefore, the search for good CO-tolerant electrocatalysts in this region should focus on finding systems with large K values. The upper limit of the region (V0) is found to be similar for Pt/Ru as for Pt, but more studies are needed to establish whether this represents a general feature for all Pt-based electrocatalysts. (b) A state in which the surface activity increases with increasing potentials, in which adsorbed Pt-COads is oxidized by Ru-OHads, in a rate-determining step. This state is between V0 and a critical potential Vcrit, which is concentration-dependent and may serve as a criterion of CO tolerance. Vcrit - V0 is smaller for Pt/Ru than for Pt-based GDE, especially at low CO concentrations. There are arguments to support the idea that the electrode activity is determined by the value of the ratio Vox/Vads, rather than by the absolute value of Vox. (c) A state at potentials higher than Vcrit, in which the electrode is activated and may attain a performance comparable with that of unpoisoned electrodes. Thus, the value of Vcrit, the potential at which the inductive loop becomes equal in diameter with the loop in the first quadrant, may be used as a diagnostic criterion of CO tolerance. Like other authors, we consider that the oxygenated species on Ru are the main cause of the improved performance of Pt/

Ru as compared with Pt. However, we consider that in the low overpotential range, their activity is manifested by steric and electronic effects determining the blocking of CO adsorption, rather than via the oxidative removal of COads. Acknowledgment. The authors gratefully acknowledge the contribution of Dr. Raymond Roberge of H Power Enterprises of Canada and thank him for the careful revision of the manuscript and for many discussions during the preparation of this work. References and Notes (1) Niedrach, L. W.; McKee, D. W.; Paynter, J.; Danzig, I. F. Electrochem. Technol. 1967, 4, 318. (2) Ross, P. N.; Kinoshita, K.; Scarpellino, A. J.; Stonehart, P. J. Electroanal. Chem. 1975, 63, 97. (3) Ianiello, R.; Schmidt, V. M.; Stimming, U.; Stumper, J.; Wallau, A. Electrochim. Acta 1994, 39, 1863. (4) Gasteiger, H. A.; Markovic, N. M.; Ross, P. N.; Cairns, E. J. J. Phys. Chem. 1994, 98, 617. (5) Gasteiger, H. A.; Markovic, N. M.; Ross, P. N. J. Phys. Chem. 1995, 99, 8290. (6) Gasteiger, H. A.; Markovic, N. M.; Ross, P. N. J. Phys. Chem. 1995, 99, 16757. (7) Schmidt, T. J.; Noeske, M.; Gasteiger, H. A.; Behm, R. J. Langmuir 1997, 10, 2591. (8) Schmidt, T. J.; Noeske, M.; Gasteiger, H. A.; Behm, R. J.; Britz, P.; Bo´nnemann, H. J. Electrochem. Soc. 1998, 145, 925. (9) Ciureanu, M.; Wang, H. J. Electrochem. Soc. 1999. In press. (10) Springer, T. E.; Zawodinski, T. A.; Wilson, M. S.; Gottesfeld, S. Electrochemical Soc. Proc. 1995, 95-23, 137. (11) Springer, T. E.; Zawodinski, T. A.; Wilson, M. S.; Gottesfeld, S. J. Electrochem. Soc. 1996, 143, 587. (12) Gu¨lzow, E.; Helmbold, A.; Wagner, N.; Schulze, M.; Schnurnberger, W. Proceedings of the 12th World Hydrogen Energy Conference, 12th World Hydrogen Energy Conference, Buenos Aires, 1998; Bolcich, J., Veziroglu, T. N., Eds. (13) (a) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1986, 206, 197. (b) Breiter, M. W. Electrochim. Acta 1984, 29, 1725. (14) Vetter, K. J. Z. Electrochem. 1955, 59, 435. (15) Vogel, W.; Lundquist, J.; Ross, P.; Stonehart, P. Electrochim. Acta 1975, 20, 79. (16) Armstrong, R. D. Electroanal. Chem. 1972, 34, 387. (17) Armstrong, R. D.; Henderson, M. J. Electroanal. Chem. 1972, 39, 81. (18) Keddam, M.; Mattos, O. R.; Takenouti, H. J. Electrochem. Soc. 1981, 128, 257. (19) (a) Bai, L.; Conway, B. E. J. Electrochem. Soc. 1990, 137, 3737; (b) Electrochim. Acta 1993, 38, 1803; (c) J. Electrochem. Soc. 1991, 138, 2897. (20) Gao, L. J.; Conway, B. E. J. Electroanal. Chem. 1995, 395, 261. (21) Oetjen, H. F.; Schmidt, V. M.; Stimming, U.; Trila, F. J. Electrochem. Soc. 1996, 143, 3838. (22) Grgur, B. N.; Markovic, N. M.; Ross, P. N. Electrochim. Acta 1998, 43, 3631. (23) Leiva, E. P. M.; Santos, E. A.; Iwashita, T. J. Electroanal. Chem. 1986, 215, 357. (24) Barber, J. H.; Conway, B. E. J. Chem. Soc., Faraday Trans. 1996, 92, 3709. (25) Mukerjee, S.; McBreen, J. J. Electrochem. Soc. 1999, 146, 600. (26) Grgur, B. N.; Zhuang, G.; Markovic, N. M.; Ross, P. N., Jr. J. Phys. Chem. B 1997, 101, 3910. (27) Grgur, B. N.; Markovic, N. M.; Ross, P. N., Jr. J. Phys. Chem. B 1998, 102, 2494. (28) The Pt/Ru-based GDE with a loading of 3.2 mg/cm2 electrocatalyst contains 2 mg/cm2 Pt. (29) One should note however, that this criterion cannot be absolutized, since on blocked electrodes, the nonlinear diffusion problem is mathematically the same as for linear diffusion coupled with a first-order homogeneous reaction. It is mainly a comparison of the behavior of the impedance components in different potential ranges that seems to support the above idea. (30) According to Gasteiger et al.,4-6 there is no difference in the CO adsorption affinity on Pt and Ru, but on Ru, the formation of oxygenated species is energetically favored over CO adsorption.

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