Active Area Determination of Porous Pt Electrodes Used in Polymer Electrolyte Fuel Cells: Temperature and Humidity Effects

Journal of The Electrochemical Society, 157 �12� B1795-B1801 �2010� B1795

Active Area Determination of Porous Pt Electrodes Used inPolymer Electrolyte Fuel Cells: Temperature and HumidityEffectsRakel Wreland Lindström,*,z Katrin Kortsdottir, Maria Wesselmark,**Alejandro Oyarce, Carina Lagergren,* and Göran Lindbergh*

Applied Electrochemistry, School of Chemical Science and Engineering, Kungliga Tekniska Högskolan,SE-10044 Stockholm, Sweden

This paper discusses the proper measure of the electrochemically active area �ECA� of carbon supported Pt catalyst in polymerelectrolyte fuel cells employing in situ cyclic voltammetry. The charges of the hydrogen underpotential deposition �Hupd� and COstripping peaks obtained in situ are compared, and the influence of operation temperature �25–80°C� and relative humidity�40%–90%� is discussed. The results show that the charges of the Hupd decrease with rising temperature, while the correspondingcharges of the CO stripping peak are essentially independent of temperature, at least at high relative humidity. The unexpectedlysmall Hupd charges are explained by the significant overlap with the hydrogen evolution reaction in a fuel cell at elevatedtemperatures. According to our results, it is proposed that a more reliable value of Pt ECA is estimated from the CO strippingcharge. However, with decreasing humidity the charges of both Hupd and CO stripping peaks decrease, which is probably an effectof increasing blockage of Pt active sites by hydrophobic domains in the electrode ionomer. Some implications of varying cellconditions on the estimated Pt ECA and its correlation with fuel cell activity are discussed in an example from a fuel celldegradation test.© 2010 The Electrochemical Society. �DOI: 10.1149/1.3494220� All rights reserved.

Manuscript submitted May 7, 2010; revised manuscript received August 12, 2010. Published October 14, 2010.

0013-4651/2010/157�12�/B1795/7/$28.00 © The Electrochemical Society

In fuel cell experiments it is valuable to be able to determine thecatalyst active area in situ. With the increased focus on degradation,it is even more important to make an accurate determination of thechanges in active area before and after degradation tests. It is gen-erally considered that a good estimation of the platinum active areacan be made from the hydrogen underpotential deposition �Hupd�charge. The area over the two cathodic peaks, from the double layercharge to the minimum just before the onset of hydrogen evolution,is assumed to correspond to 77% of a full monolayer �mL� of ad-sorbed hydrogen on polycrystalline platinum, which averaged overthe three crystal base planes corresponds to about 210 �C cm−2.1,2

This method was originally defined in an acidic solution at roomtemperature. For porous electrodes used in fuel cells, the electro-chemically active area �ECA� is often expressed per milligram ofcatalyst nanoparticles and measured over the desorption peak�s� oras an average between the adsorption and desorption peak�s�.

The normal conditions of a running fuel cell are significantlydifferent from the case in acidic solution at room temperature. A fuelcell is generally operated at an elevated temperature, and instead ofan aqueous solution electrolyte, with free mobility of ions, a solidpolymer, in this case Nafion, with sulfonic acid �SO3

−� groups, situ-ated on a side chain of the polymer, is employed. These attractprotons and form a hydrophilic network in which water and protonscan be transported via nanometer wide water channels through themembrane.3 In contrast, the perfluorated backbone of Nafion ishighly hydrophobic and constitutes the framework for the membranechannel system. The proton conductivity of the membrane, which isrelated to the number of open channels through the membrane, isdetermined by the water content.

The gas diffusion electrodes consist of a noncompact mixture ofPt nanoparticles deposited on porous carbon support, carbon blackor other types of carbon structures, and about 30–40 wt % ionomer,i.e., polymer fibers that must wet the catalyst particles for electro-chemical activity. The optimum Nafion content for the cathode elec-trode has previously been shown to be 36 wt %.4 Further, for fullutilization of the platinum, electron conductivity, via the gas diffu-sion layer �GDL� and the carbon particle network, is required be-

* Electrochemical Society Active Member.** Electrochemical Society Student Member.

z E-mail: [email protected]

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tween the active platinum surfaces and the current collectors. Duringfuel cell operation, the reactants, hydrogen and oxygen, must becontinuously supplied via gas channels and pores in the GDL andthe electrodes to reach the active platinum surfaces. In addition, thegases contain a considerable amount of water vapor that will form athin water film over all solid matter in the cell.

In a liquid electrolyte, such as sulfuric or perchloric acid or al-kaline solution, the appearance of the Hupd on polycrystalline Pt orPt/C catalysts differs slightly between different electrolytes, both inthe peak positions and the relative size of the two peaks. It wasproposed by Markovic and co-workers5,6 that this is a consequenceof the adsorption of anions that are adsorbed in the double layerregion and are replaced by adsorbed �or replace desorbing� hydro-gen at the potentials of the peaks. Sulfate, for example, is stronglybonded to the Pt surface, resulting in defined Hupd peaks in thevoltammogram. Generally, the peak positions are shifted negativelywith increasing temperature, both in aqueous solution7-9 and in con-tact with Nafion.10 The Hupd charge on Pt, polycrystalline and singlecrystal, has been shown to be practically unaffected by temperaturein aqueous solution.7,11-15 However, in fuel cells operated at elevatedtemperatures, a significant decrease in the charge has been observedwhen the temperature is increased.16-18

Several studies have been performed to investigate the interac-tions between Nafion and the platinum surface and the influence onthe Hupd,10,19-22 where most of them were performed on Nafion-coated Pt in H2SO4 �aq�. Maruyama et al.19 reported that recastNafion did not reach the expected exchange current densities, eventhough they found a higher hydrogen solubility in Nafion comparedto 0.1 M HClO4. The lower electrochemical activity for Nafion wasexplained by blocking of the catalyst by electrochemically inactivefluorocarbon domains in the Nafion ionomer, even though these do-mains showed excellent hydrogen solubility. The same argumentwas used by Chou et al.,22 who compared the Hupd on a Nafion-coated Pt surface with one without a Nafion-coating. The Hupdcharge measured from cyclic voltammetry �CV� for the bare surfacein 0.5 M H2SO4 was 50% larger compared to the coated one atroom temperature. Also, Jiang and Kucernak20 observed a suppres-sion of the Pt hydrogen and oxygen electrochemistry by Nafionwhen comparing 0.5 M H2SO4 electrolyte with water saturatedNafion. Possible effects of ionic or organic impurities were confutedin their study. However, according to Strmcnik et al.,21 impuritiescan play an important role in the electrode activity. Liu et al.23 have

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B1796 Journal of The Electrochemical Society, 157 �12� B1795-B1801 �2010�B1796

studied porous electrodes in a fuel cell at varying relative humidity�RH� and found that the Hupd charge was nearly independent ofwater activity. Xu et al.,24 however, saw an increase in the Hupdcharge when increasing the relative humidity from 20% up to 72%,but observed no further increase above 72%.

Another common method for the determination of the active areais to use stripping of an adsorbed CO monolayer from a CO satu-rated aqueous solution. CO adsorbs with approximately 0.8 mL 25

on polycrystalline platinum, as revealed by electrochemical strip-ping and infrared spectroscopy in ultrahigh vacuum.26 Bett et al.2

determined the CO molecule/H atom ratio electrochemically as ap-proximately 0.9. The charge of the oxidation peak is generally takento be equivalent to two electrons per CO molecule, which results ina charge about twice that of the Hupd peak. Similar to Hupd, a linearnegative shift is observed for the CO stripping peak potential withincreasing temperature in aqueous electrolyte15,27 in contact withNafion10 as well as in a fuel cell.28 The CO oxidation charge hasgenerally been observed as fairly constant when increasingtemperature.10,15,17,27 Some studies have experienced a decrease ofthe CO oxidation charge at temperatures above 60°C and attributethe difference to thermal desorption of CO in the time period be-tween the adsorption and stripping procedure.10,15 Shinozaki et al.17

concluded that the most accurate determination of active platinumsurface area was made by CO adsorption and that the most reliableresult was obtained at a temperature of 40°C or lower. Ioroi et al.28

studied the effect of relative humidity on CO oxidation in a fuel celland found that the charge of the oxidation peak was almost indepen-dent of relative humidity, although the peak potential shifted nega-tively with increased humidification up to saturation.

In this paper we have compared the electrochemically active sur-face area determined from hydrogen adsorption/desorption with thatdetermined by CO monolayer oxidation peak charges for a conven-tional porous Pt/C electrode in a single fuel cell setup, at varyingtemperatures and relative humidity. We also give an example onhow the present cell conditions can influence the determination ofthe electrochemically active surface area and what information canbe obtained by using CO stripping as a diagnostic tool after anaccelerated fuel cell degradation test. This paper is an approach tocorrelate the results from simplified and well studied model andsingle crystal electrodes with the more realistic fuel cell system,where contributions from the solid electrolyte, the gas diffusionelectrodes, and humidified gas may contribute to the overall reactionbehavior.

Experimental

MEA preparation.— A Nafion 115 membrane, purchased fromDuPont, was cleaned by boiling, first in 3% H2O2 �aq� solution for1 h, followed by 1 h in 0.5 M H2SO4, and finally boiled in threesuccessive Milli-Q baths for a few minutes. The membrane was thendried in an oven at 70°C for 15 min and pressed flat at 130°C justbefore applying the catalyst. The electrode ink was prepared bymixing the Pt/C powder �20 wt % Pt/Vulcan XC-72 from ETEK�with 5 wt % Nafion solution �from DuPont� using Milli-Q water andisopropanol in equal volumetric amounts as solvent, giving 36 wt %Nafion in the dry electrode. The ink was ultrasonicated and stirredfor 24 h before 20 �l was pipetted on the membrane placed on aheated table �90°C� for evaporation of the solvents. The resultingelectrode had an approximate area of 0.3 cm2 and a loading of0.17 mg Pt. A commercial gas diffusion electrode from ETEK �30%Pt/C on a GDL� with a diameter of 16 mm was used as a combinedreference and counter electrode. A GDL from Carbel, also 16 mm indiameter, was used for the working electrode �WE�. The membraneelectrode assembly �MEA�, including the GDLs, was hot pressed at130°C for 30 s.

The cell assembly.— The MEA, including the GDLs, wasmounted in a modified ElectroChem Inc standard fuel cell hardwarewith own design of the graphite plates using a spiral gas channel�1 mm wide and deep channels� over a circular area of 2.0 cm2, in


which the gas entered from the center. With an approximate cellvolume of 0.1 cm3 and gas flows around 1 mL/s, the residence timeof the gas was about 0.1 s over the electrode. The hardware wascoupled to the humidifiers as shown in the schematic sketch in Fig.1.

In this design it was possible to switch between pure humidifiednitrogen gas flow and a humidified carbon monoxide-containing gas�2% CO balanced with Ar�. To avoid hydrogen crossover the inletgas on the combined counter and reference electrode �CE� was 5%H2 balanced with Ar, giving rise to a potential shift of −0.045 Vversus the relative hydrogen electrode �RHE� according to Nernst’sequation. All graphs shown in this paper are corrected for this shift.

Electrochemical characterization.— Cyclic voltammetry curveswere recorded with a PAR273A potentiostat, controlled by Cor-rWare software. Before experiments the MEAs were activated at80°C and 90% RH according to the following protocol:

a� Cycling in N2 �50 cycles at 100 mV/s� between 0.1 and 1.2 Vversus RHE with 5% H2 on the CE.

b� Cycling in O2 �50 cycles at 100 mV/s, 5 cycles at 10 mV/sand 5 cycles at 1 mV/s� between 0.5 and 0.9 V versus RHE with H2on the CE.

c� A second cycling in N2 �50 cycles at 100 mV/s� between 0.1and 1.2 V versus RHE with 5% H2 on the CE.

Stripping curves were recorded with CV, in the potential windowbetween 0.05 and 1.2 V, at a sweep rate of 10 mV/s. After a cycle inhumidified N2 gas, the potential scan was stopped at 0.15 V and thegas flow shifted to CO-containing gas for CO adsorption during2 min. Thereafter, the inlet gas was shifted back to nitrogen for5 min, to flush out remaining CO in the cell, before subsequentstripping. Measurements were made at temperatures between 25 and80°C and at different levels of relative humidity �40%–100%�.

Figure 1. �Color online� Experimental setup: gas humidifiers and gas flowmeters �1�, heated stainless steel tube system with Swagelok connectionsincluding valves enabling a shift between pure and contaminated gas flow�2�, fuel cell hardware at controlled temperature connected to a potentiostat�3�.


B1797Journal of The Electrochemical Society, 157 �12� B1795-B1801 �2010� B1797

Further, a support degradation test was conducted, where theelectrode was activated over night by potential cycling in oxygen2000 times between 0.6 and 0.9 V versus RHE followed by degra-dation in a potentiostatic degradation test at 1.4 V versus RHE for3 h in nitrogen. Prior to and after the degradation tests, polarizationcurves in oxygen, cyclic voltammetry in nitrogen, as well as COstripping curves were performed according to the same protocol asdescribed above, except that the sweep rate used for CO strippingwas 20 mV/s. In addition, polarization curves were recorded with100% hydrogen on both working electrode and counter electrodeusing the slope to determine the resistance of the fuel cell. Thecurrent densities in the support degradation test example are all cor-rected for the cell resistance.

ECA estimation.— The Hupd charge was calculated from the in-tegral over the desorption peaks in the Hupd region using the COstripping curve, for which the peaks are completely suppressed, as abaseline. It was considered that the charge of the adsorption peakswas more difficult to estimate due to the overlap with the onset ofhydrogen evolution. The integral of the area from the onset of thehydrogen evolution � � 0.1 V� to the maximum value in the doublelayer charge � � 0.35 V� was approximately the same for the ad-sorption and desorption, within an error of 10%. The charge of theCO stripping peak was estimated in a similar manner using thecorresponding base CV �in nitrogen gas� as a baseline.

Results and Discussion

The base CV of Pt/C MEA.— Figure 2 shows typical cyclic vol-tammetry curves obtained in flowing nitrogen humidified to 90%RH at 25–80°C. The two characteristic hydrogen adsorption anddesorption peaks typical for polycrystalline Pt in sulfuric acid solu-tion as well as carbon supported Pt are present in the curve obtainedat 25°C. The symmetric peaks �adsorption and desorption� at 0.25 V�II� shift negatively with increasing temperature in accordance withliterature on studies performed in solution6-9,11-14 and using Nafionmembranes as electrolyte.10,28 The peaks at 0.15 V �I� disappear asthe temperature rises, and at 80°C only one pair of peaks at 0.2 V isapparent.

The onset of the hydrogen evolution seen in our voltammogramsis shifted positively compared to those obtained in acidicsolutions.7,15 This shift is argued to be caused by the deviation fromstandard conditions in the inert nitrogen atmosphere resulting in amore positive equilibrium potential. This effect is dynamic andcauses a variation in the equilibrium potential due to the evolution

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and consumption of hydrogen during the potential sweep, as previ-ously suggested by Schneider et al.29 Further, mass transport fromthe electrode surface facilitates an increase in the hydrogen evolu-tion potential. Biegler et al.1 found that the onset of hydrogen evo-lution in liquid electrolyte started at more positive potentials for asmooth polycrystalline platinum electrode compared to a roughplatinum black electrode. An additional positive shift in the onsetpotential of the hydrogen evolution could also be revealed on thesmooth electrode upon stirring heavily, confirming that the hydrogenevolution was indeed mass transport controlled. Using a segmentedfuel cell at room temperature, Schneider et al.29 showed that theonset of hydrogen evolution in nitrogen humidified gas flow is de-pendent on the flow rate and on the position in the cell along theflow channel. The onset potential shifted more negatively with noflow or when it was measured deeper into the cell, leading to asimultaneous appearance of a well defined desorption peak �I�. Thisevidences that the off transport of evolved hydrogen counteracts theelectrode reaching its equilibrium, since the removal of evolved hy-drogen by inert gas must be compensated for by the formation ofhydrogen, causing an increase in the hydrogen evolution current andconsequently in the overlap of the hydrogen evolution and the Hupdregion. In a recent study, Schneider et al.30 found similar results atan elevated temperature �70°C�, although more pronounced com-pared to their previous study at room temperature.29

In our measurements the gas flow velocity was controlled beforethe gases were heated up in the humidifier and in the gas lines. Dueto the gas expansion with rising temperature in the cell, the actualflow velocity through the cell must also increase. It is thereforeargued that the disappearance of the weakly adsorbed/desorbed hy-drogen peaks �I� with temperature is partly due to an increasing offtransport of evolved hydrogen and so, an increasing overlap of thehydrogen evolution reaction with the hydrogen adsorption and de-sorption peaks. Varying the flow of nitrogen between 25 and140 mL/min had no effect on the size or shape of the peaks. How-ever, setting the flow too low resulted in unstable curves, whereeither crossover of hydrogen from the opposite site or permeation ofoxygen from the surrounding air influenced the curves. Stopping thegas flows completely resulted in the reappearance of a nicely re-solved desorption peak �I�.

The platinum oxide region also changes with temperature. Inaccordance with Jiang and Kucernak10 and Chaparro et al.,18 boththe oxidation and reduction currents increase with temperature,which may be explained by the combined effect of faster kinetics ofthe oxide growth and the presence of more water in the gas at highertemperatures. A similar observation can be made when altering thelevel of humidity. At 80°C �Fig. 3 and 5�, the platinum oxidationand reduction peaks increase as the level of humidity rises, similarto the increase of the hydrogen desorption and adsorption peaks. Thestrong dependence of humidity on the currents for Pt oxide forma-tion is generally accepted and well documented �see, e.g., Ref. 24�,while the behavior of the hydrogen adsorption and desorption peakswith variations in humidity is not as clear. The increase of the Hupdcharge with increased humidity is in accordance with observationsmade by Xu et al.,24 while Liu et al.23 found the Hupd charge nearlyindependent of relative humidity.

A summary of the peak shift and the total H desorption charges,as functions of temperature, is given in Fig. 3. A linear relationshipbetween the peak �II� position and the temperature is observed. Fur-ther, an increase in the temperature results in a strong decrease ofthe Hupd charge density, most likely due to the overlap with thehydrogen evolution reaction as discussed above. The Hupd is alsoaffected by the relative humidity. With rising relative humidity, thepeak potentials are shifted negatively, accompanied by a linear in-crease in the Hupd charges. The effect of humidity on the charges is,however, significantly smaller than that of temperature.

An attempt was made to analyze the effect of sweep rate on thebase CV, but due to the relatively high resistance in our cell system,the larger currents generated during the faster sweep rates gave riseto a significant hysteresis of the peaks. As a result a detailed inves-



tigation of the effect of the sweep rate on either the charge of thepeaks or the peak potential could not be made. The peak currentshould, however, be essentially unaffected by the distortion and wasas expected found to increase linearly with the square root of thesweep rate.

To summarize, a large deviation of the Hupd is found whenchanging the experimental conditions using the same MEA. An ob-servation that brings up the question of what is a proper measure ofthe platinum active area in a fuel cell and if the electrochemicallyactive surface area is changing with temperature and humidity. Forcomparison, the surface area was also determined employing COstripping.

CO monolayer oxidation.— In order to obtain a good measure ofthe active surface area at fuel cell relevant conditions, CO strippingwas performed directly in the fuel cell and not in the sulfuric orperchloric acid aqueous solutions, generally used as a standard. Thegaseous system has an advantage in the possibility to increase theCO concentration above the saturation level of CO in acidic watersolution. Further, CO adsorption is very rapid on the platinum sur-face, and after switching to inert gas flow �here nitrogen� the cell iseasily purged from any remaining CO. Figure 4 shows the CO strip-ping curves obtained at 90% RH for the same temperatures pre-sented in Fig. 2.

Upon CO adsorption, the hydrogen desorption peaks are sup-pressed, indicative of a saturated monolayer of CO on the platinumsurface. At high humidity and a potential sweep rate of 10 mV/s, themonolayer is oxidized to CO2 in a distinct peak between 0.6 and0.8 V, depending on the temperature. The negative shift in the peakpotential with increasing temperature is in accordance with observa-tions made in a fuel cell,28 water immersed Nafion,10 and in acidicsolution.15,27 In agreement with previous fuel cell studies,17,18,28 nodistinct prepeaks were observed in our experiments in contrast totypical CO stripping curves in acidic solution15 and for water im-mersed Nafion.10 The prepeak observed in the latter study10 is prob-ably due to higher pH, as the CO was supplied from a CO saturatedneutral water solution, as shown in Ref. 31. During the adsorptionthe displacement charge was monitored, but it was difficult to obtainreliable reproducible data as the displacement was very rapid andwithin the time frame of adjusting the valves, with the result of anoisy signal. In addition, crossover of hydrogen gas through the

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VVFigure 3. �Color online� The dependence of the Hupd charges �from thepositive desorption charge� on temperature and humidity, and peak potentialsfor the strongly bonded desorption peak II. The symbols are related to rela-tive humidity as follows: 90% RH �� 60% RH �� and 40% RH ��, filledsymbols are the charge densities and unfilled are the peak potentials.


MEA from the anode/reference electrode �H2 balanced with Ar�most likely interfered with the data. These complications hinderedany analysis of the displacement.

Figure 5 shows the dependence of relative humidity on the COstripping behavior. The positive shift in CO stripping potential withdecreased relative humidity is in accordance with Ioroi et al.,28 whofound that the shift in peak potential was linear with the partialpressure of water vapor. The potential shift and the broadening ofthe peaks are argued to be effects of a smaller amount of water, andconsequently less OHad on the surface, which is needed for the COoxidation to take place according to a Langmuir–Hinshelwood reac-tion mechanism. This effect can be compared to the effect of pH,since it is known that the potential of the CO stripping peak obtainedin alkaline solution is more negative than the one obtained in acidic

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Figure 4. �Color online� CO stripping curves obtained for a 20% Pt/Vulcanelectrode �0.17 mg Pt� in a nitrogen flow humidified to 90% RH, recorded ata sweep rate of 10 mV s−1, after CO monolayer adsorption for 2 min in 2%CO �Ar balanced� at 0.15 V followed by nitrogen purging for another 5 min.The temperatures in °C are indicated in the figure and the corresponding baseCVs are plotted in thin lines �see Fig. 2�.

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Figure 5. �Color online� CO stripping curves obtained for a 20% Pt/Vulcanelectrode �0.17 mg Pt� in a nitrogen flow at 80°C, recorded at a sweep rate of10 mV s−1, after CO monolayer adsorption for 2 min in 2% CO �Ar bal-anced� at 0.15 V followed by nitrogen purging for another 2 min. The dif-ferent levels of relative humidity are indicated in the figure and the corre-sponding base CVs are plotted in thin lines.



solution.31 The observed tilt in the CV shows that the cell resistanceincreases with decreasing amount of water in the system, mainly dueto losses in ion conductivity in the membrane. However, an in-creased contact resistance as the pressure over the cell decreaseswith less swelling of the membrane at low humidity cannot be ruledout.

Figure 6 summarizes the effect of temperature and humidity onthe CO stripping charges and peak potentials. The most strikingobservation is that in comparison to the Hupd, the charge of the COadoxidation peak is independent of temperature for a well humidifiedcell. At the same relative humidity, using the same adsorption andpurging periods, the charge of the stripping peak is almost constant.These results are in contrast to Jiang and Kucernak,10 who found adecrease in the charge of the CO stripping peak as the Hupd chargedecreased at higher temperatures. Comparing the charges of the Hupdand the CO stripping peak at 80°C and 90% RH, we were at firstsurprised that the CO oxidation peak appeared to be much too largein comparison to the Hupd area. Only at 25°C did the CO oxidationpeak have a charge corresponding to that expected from the Hupd.

In order to ensure that the inert gas purging time was sufficient inour setup, its effect on the CO stripping peak was investigated.Prolonging the time caused a decrease in the size of the peak, but nosteady state value was obtained. After 1 h of nitrogen purging, 90%of the original peak remained, which is still three times that ex-pected from the Hupd. Purging over night in humidified nitrogen flowat 0.15 V, followed by a positive potential sweep, resulted in anabsent CO stripping peak. There was, however, a slight increase inthe positive sweep current at high potentials, probably due to oxida-tion of contaminants adsorbed during the night. Based on these ob-servations we concluded that the removal of nonadsorbed CO was infact very rapid in our setup and the gradual loss of adsorbed COwith time was due to oxidation with traces of oxygen in the system.

Lower relative humidity decreases the charges of the CO strip-ping peak especially for the higher temperatures, as can be seen inFig. 6. The same influence of humidity is also found for the chargesof the Hupd �Fig. 3�. The decreased charges are proposed to be due tolower wetting, and consequently, less active area available in theelectrode under drier conditions. In order to investigate if theamount of ionomer in the electrode would influence this behavior,experiments were also performed with an electrode with only26 wt % Nafion instead of the optimized composition of 36 wt %

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Figure 6. �Color online� The dependence of the CO monolayer oxidationcharges on temperature and humidity �subtracted from the correspondingbase CV� and peak potentials for the CO stripping peak. The symbols arerelated to relative humidity as follows: 90% RH �� 60% RH �� and 40 %RH ��, filled symbols are the charge densities and unfilled are the peakpotentials.


used in the rest of the study. Similar results were obtained for thiselectrode as the previous one. The CO and Hupd peak positions wereshifted with temperature and relative humidity, and the charges ofthe Hupd peak were decreased with both increased temperature anddecreased relative humidity. The CO stripping peak was shifted tomore negative potential with increased temperature and humidity ina similar manner as observed for the optimized electrode, and thecharge was essentially independent of temperature at high humidity,while a decrease was seen at lower humidity. However, this decreaseof the CO stripping charge was significantly larger for the lowerNafion content in the electrode than for the higher Nafion content,which supports the hypothesis that the decreased charges of the COstripping peak with decreased relative humidity are caused by lowerwetting of the catalyst at drier conditions.

The question remains of how to interpret the discrepancy be-tween the CO stripping and Hupd charges with increasing tempera-ture while both charges decrease with lower humidity. It may beargued that the CO adsorption is not an electrochemical process andmay take place at all Pt surface independent of whether the platinumparticle is in contact with the polymer electrolyte or not, while theHupd forms from protons or water and is only possible in contactwith the electrolyte. However, protons are formed during the oxida-tion of the CO adlayer and thus the surface must be in contact withthe electrolyte, electrical contact alone is insufficient. The counterions in Nafion are the sulfonic acid groups, bonded in the membraneframework, and in contrast with liquid electrolyte, they cannot mi-grate in the liquid surface film. Consequently, the charge balancecan only be maintained as long as the protons are formed in thevicinity of the Nafion solid electrolyte. However, due to the rela-tively fast surface diffusion of adsorbed CO �and possibly adsorbedH2� from isolated Pt sites to sites in contact with proton conductiveionomer, the electrochemical reactions can possibly proceed. In con-clusion, if the same physical requirements must be fulfilled by thetwo reactions, it seems more likely that the measured Hupd in a fuelcell is an underestimation of the electrochemically active surfacearea. The decreasing Hupd charge is proposed to be due to an in-creasing overlap with the hydrogen evolution reaction, which in-creases as the peak positions of the hydrogen adsorption and desorp-tion peaks shift negatively with increasing temperature incombination with a very slight positive shift for the hydrogen reduc-tion peak. If so, the active area determination measured from COmonolayer would be a more accurate one. The slightly decreasingcharges for both the Hupd and the CO stripping peak with decreasinghumidity support the hypothesis that with less water in the Nafion,the relative blocking of Pt active surface by hydrophobic domains inthe Nafion increases.

Example—degradation test at 1.4 V.— As mentioned in the in-troduction, the loss of electrochemically active Pt area is often usedas a measure of the degradation of the Pt/C electrodes. However, theapparent loss of Pt active area is not always connected to a loss inperformance. Figure 7a shows the polarization curves of a conven-tional E-tek electrode, before and after an accelerated carbon sup-port degradation test, constituted of a potentiostatic hold at 1.4 V for3 h at 80°C and 90% RH in nitrogen gas flow. According to theinset voltammograms, the double layer capacitance increases drasti-cally during the test, most likely due to the formation of surfaceoxides on the carbon, and the Hupd charge decreases. However, asindicated by the polarization curve, the performance is only de-creased in the high current region, presumably due to transport limi-tations in the electrode.32 Interestingly, after the cell was restartedafter having been switched off and left at rest during the night atroom temperature, without any gas flowing through, an increase inperformance was seen, with higher current densities in almost thewhole potential range. In addition, the Hupd charges are regained,whereas the increase in double layer capacitance remains, as can beseen in the inset in Fig. 7a. Obviously, the active area was not lostby the test, even though the Hupd appeared to disappear in the CV.To better analyze this, CO stripping was performed after each stage.



The corresponding CO stripping curves �20 mV/s� recorded beforedirectly after the potentiostatic test and after rest during the night aredisplayed in Fig. 7b.

The most interesting observation is the positive shift in the COoxidation peak potential for the curve taken directly after the test.The shift and the broadening of the peak resemble the shift observedfor lower humidity in Fig. 5. We propose that the observed shift inFig. 7b is a consequence of drying of the electrode during the test,since water is readily oxidized to oxygen gas at 1.4 V. The dryconditions decrease the proton conduction of the ionomer as waterchannels are contracted, which generally leads to increased resis-tance and decreased performance for oxygen reduction on theelectrode.23 However, the polarization curves are all iR-corrected bymeasurements with a symmetric H2/H2-cell after each polarizationcurve, and the resistance cannot explain the lowered performance athigher current densities. Another consequence of the increased re-sistance is an increasing hysteresis of the peaks in the CV, i.e., in thepositive going sweep direction a positive shift is found for the peakpotentials. After the stop and when the original conditions are reset,the CO stripping peak potential is shifted back to the original posi-tion. This may be partly explained by the regained Nafion structure

10-4 10-3 10-2 10-1 10 00.6

0.7

0.8

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Current density / A (mg Pt)

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nti

al/

Vvs

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i

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Potential / V vs RHE

Cu

rre

nt

de

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ty/

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-1

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Potential / V vs RHE

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tD

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ty/

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t)-1

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Figure 7. �Color online� Example:degradation test at 1.4 V with a 20%Pt/Vulcan electrode �0.16 mg Pt� at 80°C and 90% relative humidity, �a�Polarization curves in oxygen and cyclic voltammetry in nitrogen in the inset�i� before, �ii� after 3 h at 1.4 V, and �iii� after 12 h at rest at room tempera-ture. Scan rate in oxygen 50 mV s−1 and in nitrogen 100 mV s−1. �b� COstripping curves in a nitrogen flow recorded at a sweep rate of 10 mV s−1,after CO monolayer adsorption for 2 min in 2% CO �Ar balanced� at 0.15 Vfollowed by nitrogen purging for another 2 min �i� before, �ii� after 3 h at1.4 V, and �iii� after 12 h at rest at room temperaure.


over the night. At the same point, a better performance than beforethe degradation test is seen in the polarization curves, indicating thatthe high potential hold in the test resulted in an improvement of thecell, possibly due to changes of the morphology of the electrode.These phenomena will be discussed in a forthcoming paper.33

Comparing the charges of the CO stripping peak before, directlyafter, and after the stop, only a small variation was evident; thevariation was from 25 through 22 to 23 mC. The charge of the Hdesorption peak, however, varied from 9.6 through 6.5 to 6.4 mC�using the stripping curve as baseline� during the test. In accordancewith the previous results at 80°C, the charge of the CO oxidationpeak is much larger than expected from the 1:2 relation. This can beunderstood by the underestimation of the Hupd at this temperature ina gaseous flow cell, as has been discussed. The loss of charge di-rectly after the test, which remains after the stop, shows that the testdoes result in some loss of platinum active area. However, it shouldbe noted that the hydrogen desorption peak smears out directly afterthe test, and without the CO stripping curve as baseline the Hupdcharges would have been estimated as much smaller. The large lossof Hupd apparent in the cyclic voltammogram recorded at a fastsweep rate directly after the test, shown in the inset in Fig. 7a, isargued to be due to increased resistance and a decrease of the inter-face between catalyst and ionomer in the cell at lower humidity. Asa result, the peaks are smeared out and the overlap with the in-creased double layer capacitance caused by carbon surface oxides.To conclude, the risk for overlap with hydrogen evolution anddouble layer charges when determining the Hupd charge suggeststhat CO stripping is better to use for determination of the platinumactive area in a fuel cell.

This example clearly shows that the active area of carbon-supported platinum in contact with Nafion electrolyte is not easy todetermine and is strongly dependent on the present conditions in thecell. For a correct comparison of electrodes or the effect of varioustreatments, it is important that the experimental conditions are keptthe same, especially the level of humidity. The example also dem-onstrates that CO stripping experiments can be used as a diagnostictool to follow the effect of different electrochemical treatments usedfor instance in accelerated degradation tests.

Conclusions

The results show that there are considerable differences betweenexperiments in a fuel cell, with a gas diffusion electrode and Nafionelectrolyte, and the classical measurements in acidic or alkalineaqueous solutions. The most striking observation is that the hydro-gen region changes significantly with temperature as well as withhumidity. Compared to sulfuric acid solution where increased tem-perature only affects the peak positions but not the peak areas,7 thecharges in the fuel cell setup decrease strongly with temperature.However, the peak charges for the COad oxidation are about thesame, independent of temperature in the fuel cell and, similar toexperiments in sulfuric acid solution, the CO stripping potential isshifted linearly with temperature.10,15,27,28

It is suggested that the decreased charge of the Hupd with in-creased temperature is connected to the overlap with the hydrogenevolution reaction in a gas flow cell. With increasing temperature,the overlap is more pronounced since the hydrogen peaks are shiftedto more negative potential, while the onset of hydrogen evolution isalmost constant. The small decrease in both charges, Hupd and COstripping peak, at lower humidity confirms that with less water in theNafion ionomer, the blocking of the electrochemically active Pt sur-face by hydrophobic domains in the polymer electrolyte increases.

From the strong influence of temperature and humidity on theHupd in a fuel cell and as clearly visualized in our example of apotentiostatic corrosion test at 1.4 V, we conclude that determina-tion of the electrochemically active surface area of platinum usingthe charge of the Hupd region is not a good measure of the Pt lossafter degradation of a fuel cell. Both the parameter study and theaccelerated degradation test show that CO stripping is less affectedby, and better defined at, various fuel cell conditions than the H .
upd


As the Hupd is close to the potential for the onset of hydrogen evo-lution, an overlap between these reactions cannot be avoided. Wetherefore propose that CO stripping is a more accurate measure ofthe active area in a fuel cell. If the Hupd area will be used, a rela-tively good estimation is achieved when the temperature is de-creased to room temperature and water saturated gases at a very lowor, preferably, zero flow rates are used, as suggested in Ref. 29.

Acknowledgments

The work was supported by the Swedish Research Council, theSwedish Energy Agency, and the MISTRA fuel cell program.

Kungliga Tekniska Högskolan assisted in meeting the publication costs ofthis article.

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Active Area Determination of Porous Pt Electrodes Used in Polymer Electrolyte Fuel Cells: Temperature and Humidity Effects