Mechanistic and fuel-cell implications of a tristable response in the electrochemical oxidation of methanol

Journal of Electroanalytical Chemistry 457 (1998) 221–228

Mechanistic and fuel-cell implications of a tristable response in theelectrochemical oxidation of methanol

Mark Schell *

Department of Chemistry, Southern Methodist Uni6ersity, Dallas, TX 75275, USA

Received 12 May 1998; received in revised form 28 July 1998

Abstract

The results of experiments and calculations on the oxidation of methanol are presented in this paper. The oxidation process wasconsidered under conditions of constant current. Experimental measurements reveal that the oxidation of methanol exhibits aresponse in which three different branches of steady-state potentials exist under the same conditions. Characteristics of themeasurements and the present understanding of the electrochemical mechanism are consistent with the hypothesis that the twolower branches coexist because an intermediate, surface bonded CO, can react with both surface water molecules and surfacebonded hydroxyl radicals. Calculations show that the coexistence of the two lower branches of states can exist because of the tworeactions. Calculations reveal also a feedback mechanism that causes the system to make a transition between the two branches.The latter instability has practical importance, as it can cause a methanol-air fuel cell potential to drop suddenly. © 1998 ElsevierScience S.A. All rights reserved.

Keywords: Methanol; Electrocatalytic oxidation; Adsorption; Fuel cell; Instabilities; Multistable

1. Introduction

In this paper results from experiments and calcula-tions on the oxidation of methanol [1–4], conductedunder current controlled conditions, are presented. It isshown that the oxidation of methanol exhibits atristable response, i.e. three different branches of steadystates exist under the same conditions. The highestpotential states and their coexistence with other stateshave been observed previously and discussed for otherprocesses [5–7]. A tristable response is a new observa-tion and contains information on the mechanism formethanol oxidation: Spectroscopic studies have pro-vided strong evidence that surface bonded carbonmonoxide is an intermediate during the oxidation ofmethanol [2,3,8–12]. Surface bonded carbon monoxidemust react with oxygen containing species to produceCO2. Surface bonded hydroxyl radicals [13,14] are pro-

posed most often but water molecules are also plausiblereactants. Our results suggest that both species arereactants. One branch of steady states has potentialvalues where a form of surface water has appreciableconcentration in the solution without methanol [15].Hydroxyl radicals are present for other potential valuesincluding values belonging to a second branch of states.

The results presented here may be important for fuelcell technology [4]. It is shown that instabilities associ-ated with the coexistence of states can cause a suddenlarge increase in the electrode potential. If this occurredin the methanol half-cell reaction in a fuel cell, the cellpotential would undergo a large drop.

Besides experimental results, calculations using amodel of methanol oxidation are presented also. Themodel is based on a conventional electrochemical mech-anism and the theory of Bockris and co-workers fordifferent states of surface water [15–17]. The calcula-tions reproduce the coexistence of the lowest and inter-mediate branches of states and reveal information onthe feedback mechanism that leads to instability.* Fax: +1 214 7684089; e-mail: [email protected]

0022-0728/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved.PII S0022-0728(98)00315-5

M. Schell / Journal of Electroanalytical Chemistry 457 (1998) 221–228222

2. Experimental

High purity water was obtained from a Milliporesystem. The purity was maintained by storing the wa-ter and cycling it through an Easypure Watersystem(Barnstead, Dubuque, IA). For solutions containing0.52 M HClO4, either Baker Analyzed Ultrex perchlo-ric acid (J.T. Baker, Phillipsburg, NJ) or redistilledperchloric acid, 99.999% (Aldrich, Milwaukee, WI)was used. For larger acid concentrations, A.C.S.reagent grade (Aldrich) was used. The methanol wasPurge and Trap grade (Fisher Chemical, Fisher Scien-tific, Fair Lawn, NJ).

A rotating disk, 7.6 mm diameter of polycrystallinePt, was employed as the working electrode (Pine In-struments, Grove City, PA, part no. AFDD20Pt). Un-less stated otherwise, the disk was stationary. A AgClreference electrode in which the reference elements aresurrounded by a Na2SO4 electrolyte (Fisher) was usedas a reference electrode. However, the reported poten-tials are relative to the standard hydrogen electrode(SHE). Potentials with respect to other reference elec-trodes can be determined from the voltammogramsshown in Fig. 1.

The electrode cleaning procedures, equipment forcontrolling the experiments, the electrochemical cell,and equipment for handling the data that were usedare the same as those listed previously [18,19]. Beforean experiment on a methanol solution, the cyclicvoltammogram in Fig. 1(c) for the base solution wasobtained. The temperature of the solution was main-tained at 25.090.2°C.

A comparison of our conditions with others can bemade by examining limiting cyclic voltammograms(CVs) in Fig. 1 that were recorded for solutions thatcontained 0.52 M perchloric acid. The lowest ampli-tude CV in Fig. 1(a) is for the base solution. The twoother CVs in Fig. 1(a) are for solutions that contained5.0×10−5 and 2.0×10−3 M methanol. The methanolCVs contain broad peaks that occur in the forwarddirection immediately before the peaks that occur inthe CV for the base solution; the latter peaks areaccepted generally to be associated with formation ofPtOH.

Fig. 1(b) contains a CV for a 0.50 M methanolsolution. Comparing this CV with the CV for the basesolution, reproduced in Fig. 1(c) for clarity, showsthat there is now a methanol peak above the PtOHpeaks. The origin of this peak is not just a simple shiftof the peaks in Fig. 1(a). The peak in Fig. 1(b) has aleft shoulder whose location is consistent with thepeaks in Fig. 1(a). The CV in Fig. 1(b) was obtainedin an experiment in which, starting with a low concen-tration, the concentration of methanol was increasedin small increments. Beginning with an initial concen-tration of 0.50 M methanol, the shoulder is often

smoothed out in the limiting CV. However, the shoul-der is observed easily in transient cycles.

The CVs suggest there may be an underlying di-chotomy in the oxidation of methanol. The dichotomyappears to involve two species that may be formedalso during application of cyclic voltammetry to thebase solution. One species is present at potential val-ues that precede those values where PtOH forms. Aform of surface water has a substantial concentrationin this potential range [15]. The other species appearsto be PtOH. The following section deals with resultsfrom experiments conducted under conditions of con-stant current that are consistent with the existence of adichotomy in methanol oxidation.

Fig. 1. Cyclic voltammograms. Current is plotted against the poten-tial. Sweep rate=100 mV s−1. [HClO4]=0.52 M. (a) 1: base solu-tion; 2: [CH3OH]=5.0×10−5 M; 3: [CH3OH]=2.0×10−3 M. (b)[CH3OH]=0.5 M. (c) base solution.

M. Schell / Journal of Electroanalytical Chemistry 457 (1998) 221–228 223

Fig. 2. Measured potential values plotted against the applied currentdensity. Solid circles are measurements made on increasing the cur-rent and solid triangles are measurements made on decreasing thecurrent. [HClO4]=0.52 M. (a) [CH3OH]=0.01 M. (b) [CH3OH]=0.05 M. (c) [CH3OH]=0.10 M.

Closed circles are measurements made while the currentwas increased and closed triangles are measurementsmade while the current was decreased.

Fig. 2(a) displays results for a solution containing0.01 M methanol; the data in Fig. 2(b and c) are forsolutions containing 0.05 and 0.10 M methanol, respec-tively. For each case, the lower branch of potentialvalues (closed circles) exhibits an abrupt increase to arelatively large value. These transitions are detectedeasily in experiments; an example of these transitions ina plot of measured potential versus time is shown inFig. 3(a). If the current was decreased immediately aftera transition occurred, a path was followed that differeddrastically from the path obtained on increasing thecurrent (Fig. 2). Continuing the decreases in currentwould lead eventually to a substantial decrease in thepotential. After this transition, potential values wereobtained that were close to those values recorded onthe forward path. After beginning an experiment, nohysteresis loop of significant size was obtained if thedirection of the current changes was reversed beforereaching the upward transition.

The results are consistent with the hypothesis thattwo different states coexist under the same conditions.A somewhat similar coexistence was observed previ-ously in the oxidation of other oxygenated organics[5–7]. However, these latter processes involve stateswith much larger values for the potential. The coexis-tence of these high potential states and other states wasexplained in terms of oxygen evolution and an oxideformation feedback mechanism [6,7]. These high poten-tial states exist also in the oxidation of methanol.Examples of these high potential states are shown inFig. 2(c). They were obtained by holding the current atapproximately 25 mA cm−2 until the potential reacheda high value. The current was then decreased indecrements.

The closed circles in Fig. 2(c), except for the highestone, represent the averages of six experiments. Thestandard deviation for each set of measurements iswithin the diameter of each circle; the average standarddeviation is 22.9 mV and the largest standard deviationis 38.3 mV. The average value of the current where theupward transition occurred is 19.6 mA cm−2; the up-ward transition has a standard deviation of 2.8 mAcm−2.

Although the results in Fig. 2 can be reproduced anddemonstrate a general trend with increasing methanolconcentration, there is a distortion of ‘asymptotic be-havior.’ This distortion occurs because of a slow com-ponent in the relaxation. The slow component causesresults to depend on the timing of the experiments. Forexample, using the same procedure to obtain Fig. 2(c),a 0.50 M methanol solution made the first upwardtransition at the current value 64 mA cm−2. Using asolution with the same concentration, keeping the sys-

3. Results from current controlled experiments

3.1. Tristability

Measurements of the potential as a function of cur-rent are shown in Fig. 2. They are from experiments inwhich the current was changed in increments and thendecrements. The system was held at each current valuefor a fixed time, 15 min in the case of Fig. 2. Potentialvalues at the end of each time interval were recorded.


tem at open circuit conditions for 16 h, and thenincreasing the current every 45 min, led to a transitionat approximately 8.0 mA cm−2. This result is shown inFig. 3(c). The highest point obtained when increasingthe current on the intermediate branch in Fig. 3(c) wasmeasured after holding the current constant for 10 h.The next point was measured 10 h later and it was onthe highest branch of states. In another experiment, thecurrent was reversed on the middle branch and eachvalue of the current was held for 45 min, open trianglesin Fig. 3(c). The states in the region of coexistenceexhibited a change in the potential of less than 2 mVover the final 30 min.

3.2. Oscillations, effects of rotation rate and otherbeha6iors

The stated behavior changes with conditions andother types of behavior were observed. Results indicatethat the characteristics of the middle branch changewith ion concentration. In Fig. 3 (a) the transition tothe middle branch is shown for a large ion concentra-tion. The plot in this figure shows that the middlebranch extends to a current value well beyond the valuewhere the lower branch ends. Fig. 3(b) shows thetransition from the lower branch for a smaller ionconcentration. In this case there is either no middle

Fig. 3. (a) Measured potential is plotted against time. Current changes were applied where there is an abrupt increase in the plotted potential.Applied current values are located above the plot. [CH3OH]=0.10 M, [HClO4]=1.5 M. (b) The same as (a) except [HClO4]=0.3 M. (c)Measured potential values plotted against the applied current density. Solid circles are measurements made on increasing the current, solidtriangles (upper branch) and open triangles (middle branch) are measurements made on decreasing the current. All current values were held for45 min except the three values discussed in the text. [HClO4]=0.52 M, [CH3OH]=0.50 M.


branch where the system makes the transition or it hasmoved to a location close to the upper branch.

Often the system would undergo an oscillation in thepotential immediately before a transition. These werelarge amplitude oscillations of the same type observedfirst by Buck and Griffith [20]. Using other techniques,different oscillations were discovered by Krausa andVielstich [13]. These and all other observed oscillationswere transients.

Finally, application of rotation rates had no effect onlimiting behavior. If a large change in rotation rate wasapplied, \200 rpm, transients could appear but even-tually the system would relax to the same behavior thatit displayed before the change.

In the next section a model that describes the coexis-tence of the middle branch of states with the lowerbranch of states is dealt with.

4. Formulation of model

In this section a model that can explain the coexis-tence of the middle and lowest branches of statesdiscussed in the last section, Figs. 2 and 3(c), is devel-oped. The model does not include the high potentialstates, i.e. the upper most branch of states in Fig. 2(c)and Fig. 3(c).

The overall reaction for the electrochemical oxidationof methanol can be written as:

CH3OH+H2O�Pt

CO2+6H+ +6e− (1)

It was believed at least two reaction paths existed inthe oxidation of methanol. One path, the direct route,involves only relatively short lived intermediates. Theother path involves an intermediate with a long lifetimeat low potentials. The intermediate is now known to besurface bonded carbon monoxide, PtCO [2,3]. Investi-gations revealed that the path involving PtCO domi-nates the oxidation of methanol under a wide range ofconditions [9]. This led some to hypothesize that thedirect route may not exist at all [21]. There is evidencethat the direct route exists but that its efficiency is lowversus the PtCO path [22]. Following Krausa and Viel-stich [13] we consider only the PtCO route.

The pathway involving PtCO includes the adsorptionof methanol, dehydrogenation of methanol adsorbed,formation of PtCO, and reaction of PtCO with surfacebonded hydroxyl radicals, PtOH. Prior to the last reac-tion, PtOH is formed through the chemisorption ofwater. The chemical equations for these steps are writ-ten as follows:

CH3OH+uPt�Ptu [CH3OHads], (2)

Ptu [CH3OHads]�Pt6 [PtCH2OH]+PtH+ (u−6−2)Pt,(3)

Pt6 [PtCH2OH]+n2 Pt

�Ptw [PtCHOH]+H+ + (6+n2−w)Pt+e−, (4)

Ptw [PtCHOH]+n3 Pt

�Pty [PtCHO]+H+ + (w+n3−y)Pt+e−, (5)

Pty [PtCHO]+n4 Pt�PtCO+H+ + (y+n4)Pt+e−,(6)

PtH�H+ +Pt+e−, (7)

PtCO+PtOH�k3

CO2+H+ +2Pt+e− (8)

and

H2O+Pt Xk4

k−4H+ +PtOH+e− (9)

In Eqs. (2)–(9), mPt represents m vacant surfacesites, and Pti on the left side of the brackets representsi sites occupied through weak interactions. Each of theni ’s is equal to either zero sites or one site depending onwhether no sites are required for splitting off a hydro-gen, whether a site occupied already through weakinteractions can be used, or whether an additional siteis required. It is not known whether Eq. (7) is anintermediate step for Eqs. (4)–(6). Finally, the model aswritten accounts for only one form of surface bondedCO; linearly bonded CO.

Eqs. (2)–(9) form a conventional representation forthe mechanism of the oxidation of methanol. It is alsoplausible that PtCO reacts with water:

PtCO+PtH2O�k2

CO2+2H+ +2Pt+2e− (10)

We incorporate Eq. (10) into our model.The description of the oxidation process by Eqs.

(2)–(10) leads to a formidable model. Several approxi-mations are required to produce a simple model. Fol-lowing Gasteiger et al. [23] and assuming that the stepsin the dehydrogenation of methanol are always fastrelative to the oxidation of surface bonded carbonmonoxide Eqs. (2)–(7) are replaced with:

CH3OH+uPt�k1

PtCO+4H+ + (u−1)Pt+4e− (11)

Assuming the rate determining step for the overallreaction in Eq. (11) uses only one surface site, andusing Eqs. (8) and (10), the following differential equa-tion is derived easily for the rate of change of thesurface concentration of carbon monoxide, uCO:

d(uCO)/dt=k1S−k2uCOuwu−k3uCOuOH, (12)

where uwu and uOH represent the surface concentrationsof water and surface bonded hydroxyl radicals, respec-tively, and S represents the concentration of vacantsites, assuming throughout that each surface moleculeoccupies only one site. Appropriately scaled rate coeffi-cients are used so that each concentration is dimension-less and ranges from zero to one. Also, all ratecoefficients that appear in differential equations havethe form


ki=k0i exp [zi F(E−Ei)/(2RT)], (13)

where k0i is a constant, F is Faraday’s constant, E is thepotential, R is the gas constant, T is the temperature,and zi is the charge passed from the electrode. Weassume arbitrarily that all symmetry factors are equalto 1/2.

Not all water molecules on or near the electrodesurface will be reactants in Eq. (10). To divide watermolecules into groups, the three-surface-state theory ofBockris and co-workers [15–17] is used. In one surfacestate, the up state, the oxygen atoms point toward theelectrode surface and the hydrogen end of a watermolecule points towards the solution.

Water molecules in the up state will be taken as thereactive ones. An expression is required for the concen-tration of up-state water molecules, uwu, to use in Eq.(12) and subsequent expressions. One approximation isto consider the molecules in the up state to be in aquasi-equilibrium state. Within this approximation, thefraction of surface sites occupied by water molecules inthe up state is given by [16]:

uwu=S exp [–DG/(kBT)], (14)

where kB is Boltzmann’s constant and DG has threecontributions. One contribution to DG is the chemicalinteraction energy of the dipole with the metal, DGc

[16]. A second contribution is the electrical work, DGe,which can be expressed as [17]:

DGe= −m(E−Epzc)/d, (15)

where m is the dipole moment of water, Epzc is thepotential of zero charge, and d is the thickness of thedouble layer. The third contribution to DG arises fromlateral interaction of a water molecule in the up statewith other molecules. Only interactions with other wa-ter molecules in the same state are considered and thiscontribution is represented by Ucuwu [16], where U isthe interaction energy and c an average coordinationnumber, the following approximation is made

exp [−Ucuwu/(kBT)]:1−Ucuwu/(kBT). (16)

Using this approximation and the definitions

g=m/(dkBT), (17)

and

E0=Epzc+DGcd/m, (18)

the surface concentration of water molecules in the upstate can be written as

uwu=SKwu/[1+UcSKwu/(kBT)]; (19)

where

Kwu=exp [g(E−E0)]. (20)

The approximation, Eq. (19), corresponds to a pic-ture in which water molecules in the up state, afterreacting, are replaced immediately by molecules in thebulk solution or by molecules in other surface states sothat the quasi-equilibrium state is maintained. Thisapproximation breaks down at high potentials wherewater molecules in the up state are transformed tosurface bonded hydroxyl radicals which, in turn, blockformation of surface water molecules. Without modifi-cation, Eq. (19) predicts an excess of surface watermolecules in the up state at high potentials. To modifyEq. (19) the following reactions are considered

H2O+Pt Xkf

k rPtH2O, (21)

PtH2Ou Xk5

k−5H+ +PtOH+e− (22)

Using the law of mass action, setting the activity ofwater equal to one, and making the steady-state ap-proximation leads to the following expression for uwu:

uwu= (SKwu+uOHKa)/[1+Kb+UcSKwu/(kBT)]; (23)

where

Ka=k−5/kr, (24)

and

Kb=k5/kr. (25)

The following expressions are used for Ka and Kb

Ka=exp [−F(E−Ea)/(2RT)] (26)

and

Kb=exp [F(E–Eb)/(2RT)]. (27)

Eqs. (26) and (27) are based on the assumption that thecoefficient of the potential in kr is small compared toF/(2RT).

The differential equation describing the rate ofchange of the surface concentration of bonded hydroxylradicals is given by

d(uOH)/dt

=k4S−k−4uOH+k5uwu−k−5uOH−k3uCOuOH. (28)

The equations for the surface concentrations, Eqs.(12) and (28), are coupled to the following differentialequation describing the rate of change in the potential

CdE/dt= −QoSj zj rj+I, (29)

where C is the capacitance (assumed constant), Qo

corresponds to the charge transferred on depositing amonolayer of hydrogen atoms (220 mC cm−2), I is theapplied current density and

Si zi ri=4k1S+2k2uCOuwu+k3uCOuOH+k4S−k−4uOH

+k5uwu−k−5uOH.


Fig. 4. Calculations from model, Eqs. (12), (23), (28) and (29). For all quantities, the number 1 refers to the bottom branch of states, the number2 refers to the intermediate branch and the number 3 refers to the top branch. (a) Potential is plotted against current. Solid branches representstable steady states, dashed branch represents unstable steady states. (b) Concentration of surface bonded CO (solid curve) and concentration ofsurface sites (dashed curve) are plotted against current. (c) Concentration of water in the up state (solid curve) and concentration of PtOH (dashedcurve) are plotted against current. (d) Rate of the reaction between water in the up state and PtCO (solid curve) and rate of the reaction betweenPtOH and PtCO (dashed curve) are plotted against current.

5. Results from model calculations

Eqs. (12), (23), (28) and (29) form a closed set ofequations that can be reduced to an equation in whichthe potential is the only unknown. We solve the latterequation numerically. Results for the potential-currentcurve are shown in Fig. 4(a). Branches of the curve arelabelled for future reference: The bottom branch islabelled by the number 1, the intermediate branch andthe top branch are labelled by the numbers 2 and 3,respectively. The two solid branches represent stablestationary states and the middle branch, dashed, repre-sents unstable states. Parameter values used to obtainthese results are listed as follows: all k0i=0.10 s−1

except k01=0.00002 s−1; F/(2RT)=19.5, E1=0.45 V,E2=0.68 V, E3=0.822 V, E4=0.887 V, E5=Ea=Eb=0.580 V, g=5.1 V−1, E0=0.44 V and Uc/(kBT)=2.5. The model reproduces only qualitativebehavior, e.g. the top branch is about 200 mV too high.

The concentration of surface bonded carbon monox-ide, uCO, solid curve, and the concentration of vacantsites, S, dashed curve, are plotted against the current inFig. 4(b). The numbers 1 and 2 are beside branches ofcurves which are associated with the bottom and inter-mediate branches of Fig. 4(a), respectively. The num-bers in italics are for the dashed curve, the curve for S.A cusp-like feature separates the concentration on the

bottom and intermediate branches for the uCO curve. Adotted curve separates the concentration on the sametwo branches for the S curve. The concentrations asso-ciated with the upper branch of states are not shown.For the upper branch of states, uCO remains between10−7 and 10−6 and S remains between 10−6 and 10−5.

The concentration of water in the up state, uwu, solidcurve, and the surface concentration of bonded hy-droxyl radicals, uOH, dashed curve, are plotted againstthe current in Fig. 4(c). The number 3 is beside the partof the uOH curve associated with the upper branch inFig. 4(a). The numbers 2 and 3 have the same generalmeaning as in Fig. 4(b). The dividing point betweenconcentrations associated with the different branches inFig. 4(a) are either denoted by dotted curves or by cusplike features. The concentration of water in the up stateon the top branch is between 10−11 and 10−10 and isnot shown in Fig. 4(c).

Fig. 4(c) reveals that uOH increases rapidly as the endof the bottom branch is approached whereas all otherconcentrations, Fig. 4(b and c), decrease. The resultsreveal the main ingredient of the feedback mechanismfor instability. Increasing the current increases the po-tential which, in turn, increases the amount of surfacebonded hydroxyl radicals. On further increases, a pointis reached eventually where hydroxyl radicals cover asufficient amount of the electrode surface that the reac-


tion between water and CO is inhibited severely. At thispoint the system must make a transition to a higherpotential value so that other reactions can take placeand satisfy the applied current. The model is limited tothe reaction between hydroxyl radicals and CO.

These points are further substantiated by Fig. 4(d).In Fig. 4(d) the rate of the reaction between hydroxylradicals and CO, Eq. (8), dashed curve, and the rate ofthe reaction between water in the up state and CO, Eq.(10), solid curve, are plotted against the current. Theplots reveal that the rate of the water reaction begins todecrease at the end of the bottom branch and the rateof the hydroxyl reaction is relatively large and increaseson the top branch.

6. Conclusions

Results from current controlled experiments showthat the oxidation of methanol at Pt in acid solutionexhibits a response in which three branches of steadystates coexist. These results and the results of calcula-tions on a model of methanol oxidation are consistentwith the intuitive idea that the intermediate, surfacebonded carbon monoxide, PtCO, reacts with water atlow potentials and reacts with surface bonded hydroxylradicals, PtOH, at higher potentials: PtCO is known tobe produced and react over the potential ranges of thetwo lowest branches of states. Either no other reactionpathway [21] or only a pathway of low efficiency [22] isbelieved to exist besides the pathway containing PtCO.Therefore, reaction of PtCO must contribute to bothbranches of states. Since the two branches have acoexistence region, a different reactant dominates thereaction with PtCO on each branch. The higher branchhas potential values consistent with the presence ofPtOH but not surface water. The lowest branch ofstates has potential values where the concentration of aform of surface water (oxygen end pointing towards thesurface and hydrogen atoms pointing towards the solu-tion [15]) has substantial values.

The calculations show how the instability associatedwith the coexistence of the two lowest branches canoccur. As the current is increased the potential increasesso that the rates of reaction increase. Increasing thepotential increases the amount of PtOH which inhibitsthe reaction between water and PtCO. Eventually, anincrease in current will cause the system to jump to ahigher potential so that other reactions take place thatsatisfy the current, see Fig. 4.

The results of the current-controlled experiments maybe also important in fuel-cell technology. It is knownthat at high current densities limitations such as lowrates of supply of products to the reactive sites or lowrates of removal of products can cause a sudden dropin a fuel-cell potential [4]. The results here show thatinstabilities can cause also a sudden drop in the poten-tial of a methanol-air fuel cell. The possibility of chem-ical instabilities should be examined in designing fuelcells.

Acknowledgements

This research was supported by the National ScienceFoundation, Grant No. CHE-9731060. Yuanhang Xuis thanked for initial measurements.

References

[1] W. Vielstich, Fuel Cells, Wiley, New York, 1970.[2] R. Parsons, T. VanderNoot, J. Electroanal. Chem. 257 (1988) 9.[3] B. Beden, J.-M. Leger, C. Lamy, in: J.O’M. Bockris, B.E.

Conway, R.E. White (Eds.), Modern Aspects of Electrochem-istry, vol. 22, Plenum Press, New York, 1992.

[4] S. Srinivasan, in: O.J. Murphy, S. Srinivasan, B.J. Conway(Eds.), Electrochemistry in Transition, Plenum Press, New York,1992, pp. 577–602.

[5] D. Gilroy, B.E. Conway, Can. J. Chem. 46 (1968) 875.[6] Y. Xu, M. Schell, J. Phys. Chem. 94 (1990) 7137.[7] F.N. Albahadily, M. Schell, J. Electroanal. Chem. 308 (1991)

151.[8] M.I.S. Lopes, B. Beden, F. Hahn, J.M. Leger, J. Electroanal.

Chem. 313 (1991) 323.[9] D.S. Corrigan, M.J. Weaver, J. Electroanal. Chem. 241 (1988)

143.[10] M.I. Lopes, I. Fonseca, P. Olivi, B. Beden, F. Hahn, J.M. Legen,

C. Lamy, J. Electroanal. Chem. 346 (1993) 415.[11] T. Iwasita, F.C. Nart, J. Electroanal. Chem. 317 (1991) 291.[12] B. Beden, F. Hahn, C. Lamy, J.M. Legen, N.R. deTacconi, R.O.

Lezren, A.J. Arvia, J. Electroanal. Chem. 261 (1989) 401.[13] M. Krausa, W. Vielstich, J. Electroanal. Chem. 399 (1995) 7.[14] S.G. Sun, J. Clavilier, A. Bewick, J. Electroanal. Chem. 240

(1988) 147.[15] M.A. Habib, J.O’M. Bockris, Langmuir 2 (1986) 388.[16] J.O’M. Bockris, A.K.N. Reddy, Modern Electrochemistry, vol.

2, Plenum Press, New York, 1973, pp. 779–790.[17] J.O’M. Bockris, K.T. Jeng, J. Electroanal. Chem. 330 (1992) 541.[18] Y. Xu, A. Amini, M. Schell, J. Phys. Chem 98 (1994) 12759.[19] Z. Zdraveski, Y. Xu, A. Amini, M. Schell, J. Chem. Soc,

Faraday Trans. 92 (1996) 395.[20] R.P. Buck, L.R. Griffith, J. Electrochem. Soc. 109 (1962) 1005.[21] W. Vielstich, X.H. Xia, J. Phys. Chem. 99 (1995) 10421.[22] E. Herrero, W. Chrzanowski, A. Wieckowski, J. Phys. Chem. 99

(1995) 10423.[23] H.A. Gasteiger, N. Markovic, P.N. Ross Jr., E.J. Cains, J. Phys.

Chem. 97 (1993) 12020.

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Mechanistic and fuel-cell implications of a tristable response in the electrochemical oxidation of methanol