Manuscript submitted December 9, 1998; revised manuscript received June 1, 1999.
There is worldwide interest in the development and commercial-ization of proton exchange membrane fuel cells (PEMFC) for vehic-ular and stationary applications. Such power devices operate at highefficiencies when using pure hydrogen, but fail when using hydro-gen obtained from hydrocarbon or methanol processing. The reasonfor this behavior is the capacity of CO, present in the reformed gasin non-negligible concentrations (0.01-2%), to act as a poison of thePt electrocatalyst in the anode. Adsorbed CO not only affects thereactivity of the accessible electrode surface by preventing H2 ad-sorption by site exclusion, but also lowers the reactivity of the re-maining uncovered sites through dipole interactions and electroncapture. Early studies on CO adsorption on Pt have been performedby various techniques, such as IR spectroscopy,1 or electrochemical-ly modulated IR reflectance spectroscopy,2 X-ray photoelectronspectroscopy (XPS) combined with thermal desorption spectroscopy(TDS),3 mass spectroscopy,1b,4 and radiochemical techniques.5 Theelectrochemical behavior of CO has been the object of an impressiveamount of work, with special emphasis on the estimation of the COcoverage and the discussion of the mechanistic aspects of oxida-tion.6-12 Most of these studies used potentiodynamic, potentiostatic,and galvanostatic techniques, and were performed mainly on Pt andPt alloys in solution, aiming to explain the mechanism of CO poi-soning, as well as to determine factors which may contribute to in-creased CO tolerance. No attempts have been reported to date of us-ing in situ experiments of electrochemical impedance spectroscopy(EIS) to characterize processes occurring on CO adsorption at theinterface, despite of the fact that this technique was demonstrated toprovide valuable information for the diagnostic and characterizationof PEMFC components and operating conditions.13-18
It is the purpose of the present paper to get in situ information onthe oxidation of hydrogen and H2/CO mixtures in PEMFC, usingEIS. Results are presented for membrane/electrode assemblies usedin PEMFC. The electrodes are gas-diffusion electrodes (GDE) ob-tained from carbon-supported electrocatalysts (Pt/C) backed by acarbon paper.
ExperimentalThe fuel cell tested was a 1 cm2 single cell employing Nafion 112
polymer electrolyte membrane. The electrodes consisted of a thin-
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film catalyst layer of 1.7 mg Pt/cm, 2 pressed on a gas-diffusion back-ing. Measurements were made both in two- and three-electrode ar-rangement. The gas flows, cell temperature, and humidification werecontrolled with a test station manufactured by Fuel Cell TechnologiesInc. The temperature of the cell was 508C and the gas streams werehumidified by passage through sparging bottles at 658C.
The ac impedance spectra were recorded using a Schlumberger1250 frequency-response analyzer, controlled by a PAR 273A poten-tiostat-galvanostat (EG&G Instruments Corp.). The impedance spec-tra were measured in the constant voltage mode by sweeping fre-quencies over the 0.01 Hz to 10 kHz range, and recording 10points/decade. Typically, a dc polarization curve was recorded priorto impedance measurements, to determine the dc current corre-sponding to each cell voltage. The maximum attainable current den-sities were 1 A/cm2.
Experiments have been performed both in two- and in three-electrode mode. In the latter case, the reference was an electrodewith a 3 mm2 surface area, having the same composition as the aux-iliary electrode and placed in the same compartment. The constancyof the current measured before and after each impedance measure-ment was taken as a criterion for the stability of the cell during theexperiment.
Several types of EIS experiments have been performed. In thefirst, a cell was used (hereafter denoted as H2/air), having in theanodic compartment H2 at atmospheric pressure and a flow rate of80 standard cubic centimeters per minute (sccm); the cathode gaswas air at atmospheric pressure and 400 sccm. Alternatively, in orderto test the effect of anode poisoning by CO on the impedance pat-terns, H2 was replaced by the mixture H2 1 2% CO.
In a second series of experiments, the individual behavior of theanode and cathode was investigated in symmetrical cells, operatedwith identical gas supply in both compartments: hydrogen (H2/H2),or air (air/air). For the (H2/H2) cell, experiments performed with thethree-electrode arrangement at several bias voltages demonstratedthe reversibility of the interface: the current increases very rapidlywith potential and attains the upper limit of the instrument (1 A) at0.1 V (IR-corrected).
The behavior of the poisoned electrode was subsequently studiedin H2/(H2 1 CO) cells, in which the gas in the working compartmentwas switched to gas mixtures with different CO contents, 2% and100 ppm, i.e., the concentrations of CO normally present in reformer
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gases, before and after purification. Since the auxiliary electrode be-haved reversibly, it was possible to perform all experiments with theH2/(H2 1 CO) cells in the two-electrode mode, which enables easi-er comparisons with actual FCs.
The cyclic voltammograms were recorded using an EG&G M273potentiostat-galvanostat using the same cell as in the impedance ex-periments, in which the reference compartment was fed with hydro-gen, while the working electrode compartment was fed with nitrogen.
ResultsElectrochemical impedance of PEM fuel cells.— A first series of
EIS experiments were performed on (H2/air) PEMFC. These exper-iments have reproduced data reported by Springer et al.,13 demon-strating that the main characteristics of the spectra are determined bythe cathode, due to the considerably higher impedance at the cath-ode/electrolyte interface. In this case, the spectra contain practicallyno information on the anodic process.
In a second series of experiments, the (H2/air) cell was poisoned,by introducing a small concentration of CO (2%) in the anode gas.This resulted in a considerable change in the impedance spectra,which may be assigned to the significant contribution from the loss-es at the anode due to electrocatalytic activity deterioration. Figure 1presents a comparison of the impedance pattern recorded in the two-electrode mode at two bias potentials for a PEM fueled with purehydrogen (Fig. 1a and 2a) and with a H2 1 CO gas mixture con-taining 2% CO (Fig. 1b and 2b). It may be observed that in the lat-ter case, the impedance increases by an order of magnitude, whichdemonstrates that in this case, the cell behavior is determined byboth the cathodic and anodic processes. The practical consequenceof the considerable anodic impedance increase is the drastic en-hancement of the fuel cell overvoltage, to the point that a completepower loss occurs. EIS provides a simple way to separate the contri-bution of CO poisoning to FC losses and to evaluate the influence ofcell voltage and partial pressure of CO, as discussed below.
Electrode-electrolyte interface in the absence of CO.—In orderto investigate separately the processes occurring on the anode, thecell was further operated in the “symmetrical mode”,15 i.e., withhydrogen fed in both compartments. Experiments were performedboth in a two- and in a three-electrode arrangement. The advantage
Figure 1. (a, top) EIS spectrum for a H2/air PEMFC at (1) open cell voltage,(2) 0.3 V. (b, bottom) EIS spectrum of a (H2 1 2% CO)/air PEMFC at (1)open cell voltage; (2) 0.3 V.
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of the two-electrode arrangement (H2/H2) is that at the open cellpotential (OCP), it provides data resulting from a series combinationof two identical interfaces, which simplifies considerably the inter-pretation of data.
The phase shift and complex impedance plots obtained at OCP arepresented in Fig. 2a and b. The phase shift plot (Fig. 2b) is that of asystem with two different time constants, corresponding to the twomaxima at f1 5 1000 and f2 5 1 Hz. If the geometric capacitance ofthe polymer membrane is negligible, the electrical behavior of theentire cell can be described by a series combination of the resistanceof the polymer film (Rm) and two identical circuits, one for each elec-trode-membrane interface. Each of the two interfaces can be modeledwith an equivalent circuit typical for electrodes with adsorption phe-nomena, as presented in Fig. 3a. The system is similar to that madefor Pt interface with aqueous solutions.19-22 Here, R1, C1 are thecharge transfer resistance and the capacitance of the double layer(Cdl), and (R2, C2) are the adsorption resistance and capacitance. Withthis circuit, the high frequency (hf) arc in the complex impedance plotcan be assigned to charge transfer across the interface (Ha 5 H1 1e), while the low frequency (lf) arc is assigned to the the dissociativechemisorption of hydrogen at the electrode (H2 5 2Hads). Alterna-tively, the lf arc could be assigned to the slow diffusion of gas due tolow gas flow and limited porosity of the backing. In our experiments,the effect of limited gas supply was avoided by working in pure hy-drogen, after attainment of a steady state. Experiments performedwith different gas flows demonstrated a relative insensitivity of the lfarc to changes in flow between 20 and 120 sccm, showing that slowdiffusion of H2 in the porous electrode is probably not the processresponsible for the appearance of the second semicircle. Also, the plotof Z9 and Z0 vs. (v)21/2 does not reproduce the linear symmetricaldependence expected for diffusion control.
An observation resulting from Fig. 2 is the depression of botharcs in the Nyquist plot, which suggest that the electrode underinvestigation is nonhomogeneous for both C1 and C2. 19-22 Toaccount for this observation, we used the constant phase element(CPE) model, in which the two capacitances, (C1 and C2) are re-placed by the corresponding CPE, defined as
Figure 2. Impedance plots of a symmetrical H2/H2 cell at 0.0 V: (a, top)Nyquist plot; (b, bottom) Phase shift plot. Points are experimental data, linesare curves fitted with the equivalent circuit of Fig. 3b.
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Several fits were made for an equivalent circuit containing theresistance Rm, in series with two identical circuits, one for each elec-trode. The latter are presented in Fig. 3b. The parameters of theequivalent circuit obtained from the fit are presented in Table I. Aspecial comment should be made with respect to the very high val-ues found from the fitting for both T1 and T2. The values of T1 weretwo orders of magnitude larger than the product of the known valueof Cdl (25 mF) with the real surface area of the electrode; the valueof the latter was determined from cyclic voltammetry (CV) experi-ments to be 200 cm2. Some of the difference could be accounted forconsidering that T1 approaches Cdl only as a is equal to 1. Also, thevalue of Cdl may contain a contribution from the activated carbonsupport: EIS data have provided double layer capacitance values inthe range found in our experiments for flooded porous activated car-bon electrodes.23a Comparable T2 values were found for highlyporous electrodes in electrolyte solutions.23b At present, there are noreports on the magnitude of Hads pseudocapacitance in the triphasicsystem, hydrogen/porous electrode/solid electrolyte, and more stud-ies are required to establish if the present data reflect a general char-acteristic of such interfaces.
The hf intercept of the impedance plot in Fig. 2 with the abscis-sa provides a value of the ohmic resistance of the proton exchange
Figure 3. Equivalent circuits for an electrode with adsorbed species at theinterface: (a, top) when only pure resistors (R1, R2) and capacitors (C1 andC2) are involved; (b, bottom) when two CPE are involved (T1 and T2), corre-sponding to the double layer and one or more adsorbed species.
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membrane (Rm) which is in the range 0.17 6 0.05 V cm2. The oscil-lations of the membrane resistance are due to the changes in themembrane humidification on long term cycling of the cell. A mem-brane over-drying causes the resistance of the membrane to shift tohigher values, while the lower limit at 0.12 V cm2 appears as an opti-mal humidification is attained.
The CPE related to the double layer capacitance (T1) was foundto be very sensitive to the changes in the resistance of the polymerelectrolyte membrane, an observation similar to that made forporous electrodes in liquid electrolytes21: as Rm is lowered, a1 in-creases and T1 decreases. This shows that optimizing humidification,the main factor affecting the value of Rm, will help “unfolding” thesurface, thus contributing to a better catalyst utilization.
The fit provides the value of the charge transfer resistance R1 5Rct from which an apparent value of the exchange current density i0could be determined as (i0)H2apar 5 RT/(zFRct) 5 1.4A/cm2. Thisvalue was evaluated using the geometrical surface area of the elec-trode. The real surface area of the porous electrode was estimated tobe 200 cm2, using in situ cyclic voltammetry experiments, as dis-cussed below. With this value of the surface area, the actual io forhydrogen oxidation is found to be about 7 mA/cm2, which is com-parable with the value obtained by Stonehart et al.24 for porous car-bon supported Pt electrodes in a 1 M H2SO4 solution (18 mA/cm2).Since io provides a measure of the electrocatalytic activity of theanode, while (io)H2apar also includes the effect of catalyst dispersionand utilization, Rct can be directly used as a criterion for the electro-catalyst efficiency for FC applications.
It is interesting to compare the above value of the exchange cur-rent with that obtained for oxygen reduction. Using a symmetricalcell (air/air), operated with air in both compartments, we have foundthe apparent exchange current density for the latter to be (i0)O2 apar 58.6 3 1025 A/cm2. The fact that the exchange current for oxygenreduction is smaller by four orders of magnitude than that for hydro-gen oxidation is in agreement with the experimental observationsdemonstrating that most of the overvoltage of the fuel cell is due tothe cathodic overpotential and that the EIS pattern of the PEMFC isdetermined to an overwhelming extent by the cathodic process.13-18
The values obtained for the two apparent io are higher than those ob-tained in similar experiments by Gülzow et al.,15 which demon-strates that preparation procedures and the physical/geometricalcharacteristics of the electrodes can significantly affect the value ofio and thus the overvoltage.
The EIS patterns obtained at E 5 0.0 V with the two electrodemode were similar with those obtained with three electrodes. Fig-ure 4 presents EIS patterns obtained with the three-electrode ar-rangement at several bias potentials. The increase of the lf loop withthe bias voltage is in agreement with the expected decrease of T2 asHads are depleted by oxidation. Since both electrodes behave revers-ibly, the current increased to the upper limit of the instrument (1 A)at a rather low potential (0.1 V).
EIS of CO-poisoned interfaces at OCP.—In order to investigateseparately the effects due to the anode poisoning, impedance exper-iments were performed on cells of the (H2 1 CO)/H2 type, usingsynthetic gas mixtures containing 100 ppm and 2%CO. With pureH2 in the cathodic compartment and a H2/CO mixture in the anodic(working electrode) compartment, the pattern of the EI spectrum
Table I. Parameters evaluated from a fit of EIS with the equivalent circuit of Fig. 3b.
Bias Rm R1 T1 R2 T2Cell voltage (V) (V) (V cm2) (F/cm2)a1 a1 (V cm2) (F/cm2)a2 a2
changes with respect to Fig. 2, with a dramatic increase of both realand imaginary components of the impedance. The impedance in-crease is responsible for the power loss of fuel cells fed with CO-containing reformed gases.25
To verify the attainment of the stationary state, the impedancecomponents at a fixed frequency (100 Hz) were monitored as a func-tion of time. Figure 5 presents the change of the phase angle as afunction of time. The period required to attain relatively stationaryvalues of f was 200 s for the mixture containing 2% CO, while itwas 400 s for that with 100 ppm CO; for the latter, a large oscilla-tion of f was observed in the first hour, showing that a redistributionof the adsorbed CO follows after a primary adsorption at the outersuperficial layer. A tentative explanation could be made in terms ofdata reported by Motoo;6b these authors have demonstrated that re-arrangements between two CO forms, bridged and linear, occur inthe first minutes after adsorption and that in the steady state the ratioof the two adsorbed forms is different, depending on the CO con-centration; the equilibration of the two forms may be accompanied
Figure 4. Three-dimensional plots of complex impedance patterns for the(H2/H2) cell, recorded at several bias potentials (IR-corrected) in a three-elec-trode arrangement.
Figure 5. Time dependence of the phase shift angle determined at E 5 0.0 Vand 100 Hz after CO admission in the anode compartment. (a, top) H2 1 2%CO; (b, bottom) H2 1 100 ppm CO.
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by the desorption of the excess CO, which could explain the de-crease in phase angle observed after 3200 s. Due to rearrangementsduring adsorption, steady reproducible impedance patterns can beobtained for the smaller concentration only after several hours.
From the steady state patterns at OCP presented in Fig. 6, itappears that there are important differences in the impedance plots,depending on whether the gas mixture contained 100 ppm CO, or 2%CO. In view of the considerable changes observed with respect to thesymmetrical H2/H2 cell, it may be inferred that the impedance of thecell is practically entirely due to the working (poisoned) electrode.
Figure 6. (a) Phase shift plots at 0.0 V for a H2/H2 cell (curve i), comparedwith a H2/(H2 1100 ppm CO) cell ( curve ii ) and a H2/(H2 1 2% CO) cell(curve iii). (b) Impedance patterns at 0.0 V for a H2/(H2 1100 ppm CO) cell(curve ii) and a H2/(H2 1 2% CO) cell ( high frequency range, curve iii). (c)Impedance patterns at 0.0 V for a H2/(H2 1 2% CO) cell.
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The complex impedance plot in Fig. 6b presents two superim-posed semicircles, with a smaller hf one and a larger one at low fre-quencies. The impedance patterns are characteristic for a systemwith two time constants, with the characteristic frequencies depend-ing on CO concentration. The existence of two time constants is veryobvious from the corresponding phase shift plot, with two clearlydefined maxims in the phase angle graph. The hf arc, with a charac-teristic frequency in the same frequency range as for pure H2 can beassigned to a residual charge transfer process, the ionization ofhydrogen on the holes in the CO adlayer; the lf arc can be assignedto a rate-determining CO adsorption.
In the case the mixture containing 2% CO (Fig. 6c), there is onesingle, very large, open arc in the complex impedance spectrum andone broad maximum in the Bode plot. Based on the characteristicplots of the impedance components vs. v21/2 (linear, symmetricaldependencies), we assume that in this case, the observed arc is dueto the limited rate of CO diffusion in the GDE backing, rather thanto slow CO adsorption. The difference in the behavior for the elec-trodes poisoned with 100 ppm and 2% CO is due to the differentrates of adsorption: for the latter, this rate is much higher than for theformer, approaching equilibrium conditions, where diffusion effectsare likely to become important.26
A discussion of the observed impedance patterns can be made interms of the equivalent circuit presented Fig. 3b, defining the inter-face Pt/(H2 1CO). Table I presents the values of the parametersevaluated from the fit for the case of an electrode poisoned with100 ppm and the fitted spectrum was superimposed in Fig. 6a and bover the experimental data points. No attempts were made to fit thespectrum obtained with 2% CO, since according to Armstrong, dif-fusion effects in systems with adsorption intermediates cannot beaccounted for simply by adding a Warburg impedance.26
The fact that the process responsible for the impedance changesis reversible is supported by experiments in which electrodes werefirst left to attain a stationary state in the presence of CO and subse-quently were flushed with H2, while recording the impedance spec-trum (Fig. 7). Flushing with pure H2 resulted in the gradual dimin-ishment of the lf semicircle and the gradual transition to a patternclose to that presented in Fig. 2. A recovery of the plot in Fig. 2could be obtained only after 18 h.
Dependence of EIS on bias voltage for CO poisoned elec-trodes.—Figures 8 to 11 present the complex impedance and phase
Figure 7. Complex impedance plots at E 5 0.0 V for a H2/(H2 1 2% CO)cell after flushing with pure hydrogen for different periods of time.
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shift plots at several bias voltages1 in experiments performed with thetwo CO concentrations. At all bias voltages, the impedance was foundto be considerably larger for the mixture containing 2% CO than forthat with 100 ppm CO, which explains the well-known difference inthe behavior of fuel cells operated with the two gas mixtures.
Gas mixture containing 2% CO.—In the presence of a high CO con-centration, the impedance pattern at low potentials (0.05-0.2 V) pre-sents the characteristic features of a system dominated by diffusion:a plot of the two impedance components vs. v21/2 yields two straightlines, as expected for diffusion control. Both Z9 and Z 0 undergo aconsiderable increase with increasing potential. The lf limit of theimpedance attains enormous values, showing that the electrode ispractically completely inactivated.
At potentials higher than 0.3 V (Fig. 9a and b), the lf impedancedecreases abruptly and the pattern of the spectrum changes: the scat-tering associated with slow gas diffusion disappears and two well-defined semicircles appear, with diameters which decrease rapidlywith increasing voltage; in each case, a larger loop in the 10-20 Hzrange is accompanied at lf by a smaller loop in the fourth quadrantof the complex plane diagram, i.e., a typical pseudoinductive behav-ior.25-27 Such patterns are characteristic for systems with adsorbedintermediates,26 or for showing a transition between passive andactive states.27 Conway et al.,28-30 have demonstrated that a semi-in-ductive behavior in systems with adsorbed species is associated witha change in the sign of the coverage dependence on potential. Thus,the appearance of a semi-inductive pattern at 0.3 Va is a proof thatthe CO coverage starts decreasing at this potential. The process re-sponsible for this behavior is the oxidation of CO by oxygenatedspecies on the interface. Other characteristic features of the imped-ance patterns presented in Fig. 8 and 9 are the following.
1. As the bias potential increases from about 0.3, the diameters ofthe two semicircles in the fourth and first quadrants [denoted as (R-R0) and (R-R`), respectivelyb] decrease rapidly. At a critical poten-tial, Vcrit 5 0.59 V, the value of the resistance in the lf limit of thespectrum (R0) becomes close to the value expected for unpoisonedelectrodes. R0 may approximate the dc resistance of the interface,thus provide a measure of the electrode activity, thus, the low R0value obtained at Vcrit points out that the activity of the electrode wasrestored. This idea is supported by the polarization curves recordedin identical conditions, which show a sudden current increase in the
Figure 8. Influence of the bias voltage (IR-corrected) on the phase shift plotfor a H2/(H2 1 2% CO) cell. Points are experimental data, lines and fittedcurves.
a All bias voltages are corrected for IR drop.b The notations are those presented in Fig. 9b.
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Figure 9. Influence of the bias voltage (IR-corrected) on the complex imped-ance plot for a H2/(H2 1 2% CO) cell. (a, top) Low overvoltage range, (b,center) intermediate potential range, and (c, bottom) high overpotentialrange. Points are experimental data, lines are fitted curves.
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domain at 0.6 V, as discussed below. The ratio (R-R0)/(R-R`) ap-proaches unity at Vcrit (Fig. 9b).
2. At potentials larger than 0.59 V, the diameters of the two semi-circles remain practically equal and Ro undergoes negligiblechanges (Fig. 9c). At very high potentials, an unusual impedancepattern is observed, with two semicircles in the second and thirdquadrant (Fig. 9c).
3. For the entire range between 0.3 and 0.94 V, the phase shiftplots show a hf shift with increasing voltage, both for the positiveand negative maxims (Fig. 8). The shift is more rapid for the nega-tive loop, so that the two maxims draw closer at higher voltage. Thissuggests that the rate of the oxidative removal of CO and that of COreadsorption are getting closer to each other. As the complex imped-ance plot switches to the second and third quadrants, the phase shiftplot shows an abrupt jump between positive and negative values off, showing that the two characteristic frequencies are equal.
Gas mixture containing 100 ppm CO.—For electrodes poisoned withthe lower CO concentration, at potentials below 0.3 V, the compleximpedance plot is similar with that at OCP, with a hf semicircle in thesame frequency range as for unpoisoned electrodes. This seems topoint out that at low CO concentrations, a residual charge transfer isstill possible, due to holes in the CO adlayer. The lf limit of theimpedance spectrum undergoes an increase with increasing potentials(Fig. 11a), but this is much smaller than that observed for 2% CO.
At potentials close to 0.3 V, a semi-inductive pattern of the spec-trum is obtained, similar with that observed at the higher CO con-centration. The decrease of the diameters of the two loops with in-creasing potentials is also similar, but the rate at which they ap-proach unity is higher: the value of Vcrit is 0.43 V, i.e., a potential0.15 V lower than obtained with 2% CO.
DiscussionA qualitative explanation of the observed impedance patterns for
the two concentrations at potentials higher than 0.3 V can be madein terms of the competion between the three processes responsiblefor the COads coverage: (i ) COads removal by oxidation, (ii ) COreadsorption on the holes thus created, and (iii ) diffusion of CO gasto the interface.
For gas with a larger CO partial pressure, at low potentials, thereis no oxidation reaction and CO adsorption is rapid, so that the rate-
Figure 10. Influence of the bias voltage (IR-corrected) on the phase shift plotfor a H2/(H2 1 100 ppm CO) cell.
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determining step is CO diffusion. As the potential increases beyond0.3 V, the oxidation of COads appears and becomes rate-determining.This sudden change in mechanism may be due to a change in theenergetics of the process at potentials close to 0.3 V, as a conse-quence of the change in the character of the surface: at potentials inthe range 0.05-0.1 V, CO is adsorbed onto a surface covered with Pt-Hads, while at 0.4 V and higher potentials, all Hads are depleted andCO is adsorbed onto the bare Pt. The two types of COads (weakly andstrongly bound CO) are oxidized at different potentials25 and pre-sumably at different rates. Thus, the change in the mechanism at0.3 V reflects the gradual change of the adsorbing surface, from Pt-Hads to bare Pt.
The mechanism of oxidative removal of CO was discussed byGasteiger et al., for well-characterized Pt electrodes.8a,b,c in terms ofa Langmuir-Hinshelwood reaction mechanism between COads andoxygen-containing species, nucleated on Pt surface at positive poten-tials.8a,b,c According to those authors,8c the oxidized species shouldbe of the Pt-OHads type, so that the oxidation of COads occurs as
COads 1 Pt-OHads 5 Pt 1 CO2 1 H1 1 e2 [2]
Equation 2 might provide a qualitative explanation for the ap-pearance of the inductive loop in the EIS at 0.3 V, as well as for inde-pendent studies demonstrating that the onset of CO2 evolution ap-pears indeed in the 0.3 V range (0.3 V from in situ mass spectrome-try and at 0.33 V from infrared spectroscopy1b). However, this po-tential is too low for Pt-OHads or any other form of oxidized Pt toappear. Therefore, we assume that the species responsible for thisCOads oxidation is more likely to be Pt-H2Oads
Pt-COads 1 Pt-H2Oads 5 2Pt 1 CO2 12H1 1 2e2 [3]
The oxidation results in formation of CO2, which is desorbedimmediately. Thus, new holes are created in the adsorbed adlayer, onwhich hydrogen can be oxidized.
The oxidation of COads occurs with water molecules which areoriented by adsorption into a sterically favorable orientation; thisidea is in agreement with previous theories on potential-dependent“nucleation” of oxidizing species,8a,b,c with the difference that theoxidizing species in this case is Pt-H2Oads. Quartz electrochemicalbalance experiments on Pt single crystals have evidenced in the
Figure 11. Influence of the bias voltage (IR-corrected) on the compleximpedance plot for a H2/(H2 1 100 ppm CO) cell. (a) Low overvoltageregion, (b) high overvoltage range.
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range 0.3-0.4 V a process of adsorption/desorption involving onemolecule of water for two atoms of Pt.31
To support the assignment of the two loops in the impedance spec-trum, impedance patterns have been recorded also for a cell of thetype H2/(N2 1 2% CO), in which the gas mixture contained nitrogeninstead of hydrogen. As seen from Fig. 12, at low bias voltage, theimpedance pattern obtained for this cell is similar with that observedin Fig. 9, while at more positive potentials, there is a drastic drop ofimpedance (R0 particularly), but the inductive behavior is not appar-ent. The decrease of R0 suggests that there may be a change of COcoverage by reaction 3. Thus, even if the reaction between H2Oads andCOads still occurs, there is no H2 to compete with CO for the holesthus created and CO is readsorbed from the gas. It may be concludedthe inductive loop is due to the oxidation of COads coupled withhydrogen oxidation on the holes created in the COads adlayer.
With the above assignment of the inductive loop to the oxidationprocess described by Eq. 3, we can rationalize all the experimentalobservations. The oxidative removal of COads appears in a very nar-row range of potentials, so that the pattern characterizing the transi-tion from an inactive to an active state switches appearance veryabruptly. The appearance of the semi-inductive pattern can be usedas a diagnostic criterion for the onset of COads oxidation at 0.3 V andprovides the first direct electrochemical support for the onset of CO2evolution on Pt in the potential range as found by in situ mass spec-troscopy (MS) and IR.1b
An interesting feature results from the comparison of the diame-ters of the loops obtained at potentials larger than 0.3 V in Fig. 9b. At0.3 V (uncorrected), the value R0 for the unpoisoned electrode (R00)is 0.4 V cm2, as compared with 14.1 V cm2 for an electrode poisonedwith 2% CO (or 4 V cm2 for the electrode poisoned with 100 ppmCO). Even if CO oxidation starts at about 0.3 V, R0 is still very high,so that we conclude that the onset of COads oxidation does not resultautomatically in the full activation of the poisoned surface. However,starting with 0.3 V, impedance values are progressively lowered withincreasing potential, instead of being enhanced. The ratio R00/R0 ofthe two values could be used as a quantitative criterion for definingthe CO tolerance of an electrocatalyst at a given voltage. Alternative-ly, the potential Vcrit at which this ratio approaches unity, could beused as a criterion instead of the “ignition potential”. Unfortunately,
Figure 12. Influence of the bias voltage (IR-corrected) on the complex im-pedance plot for a H2/(N2 1 2% CO) cell. (a) Low overvoltage region, (b)high overvoltage range.
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the values of R00 at high voltage could not be measured for pure H2,since the currents exceeded the limit of 1 A. Therefore, a simpleroption would be to use as diagnostic tool the ratio between the diam-eters of the two loops in the pseudoinductive pattern (R-R0)/(R-R`).
The shift to higher frequencies of the LF loop shows that thetime-scale of rate-determining process, CO oxidation, decreaseswith increasing voltage, in agreement with the observations madefrom chronoamperometric experiments by Gasteiger et al.,8c onwell-defined Pt surfaces. The fact that below 0.9 V f1 is always lowerthan f2, suggests that CO readsorption is more rapid than CO oxida-tion; this explains the fact that the effect of poisoning is still impor-tant at potentials in the range 0.3-0.4 V. Since the f2 increase withpotential is more rapid than for f1, the rates of the two processesapproach with each other at high potentials; it may be inferred thatthe condition for the recovery of the surface activity seems to ap-proach the condition for CO tolerance formulated by Bellowset al.,25 requiring that vox $ vads. We should however note, that theequality was fulfilled for the high CO concentration only at 0.92 V,while the surface activity was recovered at about 0.6 V.
A comparison of the characteristic frequencies f1 at the samevoltage for the electrodes poisoned with 2% CO and 100 ppm COsuggests that the oxidation is more rapid at lower concentrations(Fig. 13). This is in agreement with the fact that the reaction orderfor CO oxidation on Pt is negative, a feature which is responsible, atleast in part, for the fact that at lower CO concentration the activa-tion appears at smaller potentials.
At potentials in the 0.9 V range, where oxygenated species of thePt-OHads type replace Pt-H2Oads, the oxidation is likely to occur viareaction 2 instead of 3. The following tentative explanation can ratio-nalize the observed features. The rate reaction 2 is likely to be high-er than that of 3 and may overcome that of CO readsorption, so thatthe surface is first rapidly cleaned of the pre-existing COads via reac-tion 2, with subsequent the readsorption of CO. These considerationsmay provide a qualitative explanation of the reversed impedance pat-tern presented in Fig. 9c with the semicircle in the second quadrantassigned to CO oxidation and that in the third quadrant to CO read-sorption. From the inset in Fig. 8 note that the characteristic fre-quencies at 0.92 V are equal and exceed those observed at 0.3 V byalmost three orders of magnitude.
The impedance spectra obtained at various bias potentials couldbe fitted perfectly using the equivalent circuit in Fig. 3b. The fittedpatterns are compared with the experimental ones in Fig. 5a, b, and8, 9. Only systems where diffusion effects appeared to be unimpor-tant were fitted. Typical sets of fitted parameters are presented inTable I. As seen from this table, there are major changes occurringin the magnitude of the parameters characterizing the overall contri-bution of CO poisoning: at 0 V, for the H2/(H2 1 100 ppm CO) sys-tem, T2 undergoes a three orders of magnitude decrease, while R1, R2
Figure 13. Influence of CO concentration on the Bode plots at 0.3 V.
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undergo an increase by one order of magnitude. The increase of R1is due to the blocking effect of CO, which prevents hydrogen ion-ization. The dramatic decrease of T2 (as compared to T1) might bedue in principal to the change in the structure of the adsorbed layer,but also (to a smaller extent) to the considerable increase of the fac-tor a2. The latter becomes rather close to unity for all pseudoinduc-tive patterns, so that C2 values can be used instead of T2.
At higher bias potentials, the most interesting feature is theappearance of negative values for R2 and T2. The physical signifi-cance of negative parameters for systems presenting adsorbed inter-mediates was discussed by Armstrong27 and Conway.20,28,29 A neg-ative resistance appears when the current decreases with increasingvoltage; a negative capacitance appears when there is a decrease ofcoverage with increasing potential for one of the adsorbed interme-diates and the pseudoinductive behavior appears in the potentialrange where the above derivative changes sign. An behavior analo-gous with that presented in Fig. 9 was found by Barber and Conwayin liquid phase EIS of hydrogen evolution in the presence of sulfur-containing “catalyst poisons”.30
From Table I it may be seen that the order of magnitude of the T2is three to four orders of magnitude smaller than for the unpoisonedelectrode, while T1 undergoes only a moderate decrease. R1, R2 aswell as the difference R1-R2, decrease with increasing bias voltage.As expected, there is no correspondence between the measurable val-ues, Ro, R`, R, and the calculated values R1, R2. However, the differ-ence Ro-R` is rather close to the algebraic sum of calculated R1, R2.
Comparison of EIS with polarization curves and voltammetricdata.—Stripping voltammograms and polarization curves have beenrecorded in situ, in a two electrode arrangement, to enable easiercomparison with data obtained in EIS experiments. Unlike experi-ments in liquid electrolytes with H2 bubbled through the solution,data obtained in situ on gas diffusion electrodes do not show evi-dence for a limiting diffusion step. This conclusion supported ourimpedance results obtained with the H2/H2 cell. To avoid currentslarger than 1 A, the stripping voltammograms in Fig. 14a and b wererecorded for a cell in which the working electrode was exposed tonitrogen. The working compartment was first equilibrated witheither a N2 1 2% CO or a H2 1 2% CO mixture at 0.05 V and sub-sequently was purged for 2 h with N2 to remove all gas phase COand CO2. In experiments in which the CO was not purged, the oxi-dation maximum shifted by 300 mV to anodic potentials, a featurerelated to the negative reaction order for CO oxidation.8a
Figure 14a shows that the CV profile in the absence of CO has asimilar pattern with that found in three-electrode systems with liquidacid electrolyte using polycrystalline electrodes. This provides asimple method of determining the real surface area of the electrodeusing the standard procedure,18 by integrating the area under theHads peaks in the 0.05-0.4 V region and subtracting the value due tothe double layer charging. The charge thus obtained (42 mC) wascompared with the standard value corresponding to adsorption of ahydrogen monolayer on bright Pt (210 mC/cm2), to obtain the realsurface area of the electrode. The resulting value (200 cm2 Pt/cm2)corresponds to a normalized surface of 14 m2 Pt/g of Pt electrocata-lyst. An internal check for the value thus estimated was provided byintegrating the surface area below the peak corresponding to COadsorption in the 0.5-0.75 V range, which yielded 82 mC. Using thecharge required to oxidize a monolayer of CO on bright Pt(484 mC/cm2) yields a surface area of 169 cm2. Comparison with thearea determined from hydrogen adsorption peaks provides a value ofu 5 0.85 for the CO saturation coverage.
One should note that the platinum surface area thus determinedin situ is a direct measure of the platinum surface area which is indirect contact with the polymer electrolyte and is potentially availablefor reaction at the interface. Unlike the total electrochemical area(ECA), which may be determined ex situ from experiments on flood-ed electrodes,18b it includes the effect of limited catalyst utilization.
The stripping voltammogram of the fully poisoned electrode inFig. 14b provides a basis for rationalizing data obtained from EIS
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spectra at various bias voltages. It appears that on the poisoned elec-trode the hydrogen desorption maxima disappear, as a result of thedramatic increase of Rct. The CO oxidation is characterized by astripping peak centered at 0.63 V. This value is lower than that ob-tained by Schmidt et al.10 by ex situ experiments on 20% Pt/Vulcan(E-TEK) electrodes in liquid electrolytes, but close to the valueobtained by Ianiello et al.1b It is interesting to note that this potentialappears at more negative potentials that the peak for the reduction ofPt oxides on the surface on the cathodic scan. This supports the ideathat the process responsible for CO oxidation in the 0.3-0.7 V rangeis described by Eq. 3 rather than 2. The value of the peak maximumis located at a potential slightly higher than Vcrit. The oxidation of Ptappears at potentials starting with 0.87 V, i.e., in a range where thereversal of the impedance pattern was observed.
The polarization curves in Fig. 15 were recorded in conditionssimilar with those used in EIS experiments, with the H2 1 CO gasmixtures present in the working compartment. The potentials atwhich the current starts increasing abruptly, are practically the sameas found by EIS: 0.6 V for the mixture containing 2% CO and 0.45 Vfor the mixture containing 100 ppm CO. For the latter mixture, thecurrent at 0.5 V increases beyond 1 A, while for the former, a limit-ing current, is reached after 0.7 V.
Figure 16 compares typical current-voltage (I-V) curves obtainedin an (air/H2) PEMFC with a poisoned cell, air/(H2 1 100 ppm CO),which both use the same electrode-membrane assemblies as in theEIS experiments. It may be seen that for the poisoned cell, there is adramatic drop of current at higher voltages, followed by region inwhich the I-V curve becomes parallel with that of the unpoisonedsystem; the two parallel lines are separated by about 0.45 V, a valuenearly identical with the value found by EIS as the potential at whichthe electrode becomes CO tolerant.
Figure 14. (a, top) Cyclic voltammograms after 1 h cycling (scan rate,20 mV/s). The working electrode compartment contained pure N2; (b, bot-tom) Stripping voltammogram (scan rate, 20 mV/s). The working electrodewas poisoned for 2 h with a H2 1 2% CO mixture, and subsequently flushedwith N2 for 2 h.
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ConclusionsEIS studies on electrode-membrane assemblies demonstrate the
potentialities of this method as a tool for investigating processeswhich occur in PEM fuel cells and for the characterization of theinfluence of operating conditions.
In situ impedance data are presented for the first time on theanodic process in PEMFC-hydrogen oxidation and the influence ofCO poisoning is examined. The data are complementary with thosealready existing13,14,18 for the cathodic process and together providea complete image of the factors influencing the performance of thosedevices. The major advantage of EIS with respect to other tech-niques is the capacity of resolution in the frequency domain of thefactors contributing to overvoltage losses.
The impedance pattern in open cell conditions for H2/H2 cellsshow evidence for two processes occurring at the interface: the firstof them (at high frequencies) is associated with the charge transferand the second is assigned to the the pseudocapacitance resultingfrom the presence of adsorbed hydrogen species. Thus, for operationswith pure hydrogen, diffusion is not rate determining for the processat the interface, as happens with electrodes in liquid electrolytes. The
Figure 15. Polarization curves for the (H2/H2 1 CO) cells for gas mixtureswith 100 ppm CO and 2% CO.
Figure 16. I-V curves for unpoisoned (H2/air) fuel cells, compared with a(H2 1 100 ppm CO/air) cell.
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value of Rct, determined from the impedance spectrum provides adirect measure of the electrocatalyst activity for H2 oxidation.
In the presence of H2 1 CO mixtures, the impedance patternchanges dramatically and may provide exhaustive information onthe mechanism of electro-oxidation of H2/CO mixtures on Pt-basedGDE. In this case, the electrode surface in the anodic range is char-acterized by three states of activity.
1. A state of very low or no activity at potentials below 0.3 V, forwhich the impedance increases with the bias potential. This state isdifferent for gases containing 100 ppm and 2% CO, which demon-strates the importance of the time scale of CO adsorption from gasphase: for gas mixtures with 2% CO, the rate-determining process isCO diffusion and the surface is completely blocked for charge trans-fer; for gases with 100 ppm CO, the rate-determining process is COadsorption. In the latter case, a residual charge transfer is still possi-ble on the holes in the CO adlayer.
2. A state at potentials in which the surface is activated stepwiseby potential increase. This state is between 0.3 V and a critical po-tential depending on CO concentration. The oxidative removal ofCO, which becomes the rate-determining step of the electrode pro-cess, is evidenced by the appearance of a typical pseudoinductive be-havior, with a loop in the fourth quadrant of the complex impedanceplot. The appearance of the characteristic pseudoinductive patterndemonstrates that the CO oxidation on Pt may start at potentials aslow as 0.3 V, a conclusion supported by independent evidence for theonset of CO2 evolution, obtained by IR and MS experiments.1b Toaccount for the onset of CO oxidation at such potentials, we consid-er that the species responsible for the reaction is H2Oads instead of Pt-OHads. At potentials close to 0.3 V, the impedance is still very highfor both gas mixtures, which shows that the effects of poisoning arenot automatically removed by the onset of CO oxidation. However,starting with this potential, CO removal is accelerated by increasingpotentials and occurs on a progressively larger fraction of the surfacearea. This is demonstrated by the diameters of the two loops in thepseudoinductive pattern, which are not equal until a critical potentialis attained. The value of this potential was found to be 0.43 and0.59 V for electrodes poisoned with 100 ppm and 2% CO, respec-tively. The differences in the critical potential are due to the acceler-ation of CO oxidation at lower concentration (negative reactionorder) and probably also to the different CO saturation coverage.
3. A highly active state, at potentials higher than the criticalvalue. In this state, the diameters of the two loops are almost identi-cal and the low frequency limit of the spectrum is practically coinci-dent with that in the absence of CO, showing that the effects of poi-soning are annihilated. In the high potential region for this state,where the Pt-OHads are known to exist on the surface, the impedancespectrum shows a sudden reversal to the second and third quadrants.This very unusual behavior was assigned to an increase in the rate ofCO oxidation (which probably occurs at a higher rate with Pt-OHadsthan with Pt-H2Oads), to the point where it becomes higher than thatof CO adsorption.
The above considerations show that EIS is a powerful tool in pro-viding information on the rate-determining processes on poisonedelectrodes and thus, in making the first step toward finding the pro-cedures for obtaining the desired performance. The results obtainedwith Pt-based gas diffusion electrodes confirm the fact that, even forCO concentrations in the 100 ppm range, the overvoltage increasedue to poisoning is 0.43 V, i.e., too high for practical applications infuel cells. However, such data may provide a standard for evaluatingthe behavior of more tolerant alloys. Work is now in progress on thebehavior of Pt-Ru-based gas-diffusion electrodes.
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Acknowledgments
The authors wish to thank Professor Brian Conway of the OttawaUniversity and Professor G. Jerkiewicz of the University of Sher-brooke for helpful discussions during the course of this work. Theauthors are also indebted to Dr. Raymond Roberge and Dr. MartenTernan of H Power Canada, Inc. for the critical revision of themanuscript.
H Power Enterprises assisted in meeting the publication costs of thisarticle.
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