Electrocatalytic role of stabilized zirconia on the anodic current—over-potential behavior in hydrocarbon fuel cells

Solid State lonics 3/4 (1981) 447-452 North-Holland Publishing Company

ELECTROCATALYTIC ROLE OF STABILIZED ZIRCONIA ON THE ANODIC CURRENT-OVER-POTENTIAL BEHAVIOR IN HYDROCARBON

FUEL

CELLS

B.G. ONG

Faculty of Engineering, University of Malaya, Kuala Lumpur. Malaysia C,C. CHIzkNG

SRI International, Menlo Park, CA 944)25, USA and David M. MASON

Department of Chemical Engineering, Stanford University, Stanford. CA 94305. USA

The anodic current-over-potential behavior of various pure gases at 1 atm and 700-850°C was investigated in a fuel cell system using an 8 tool% SczO3-stabilized ZrO2 disc electrolyte. Air was used as a source of O2 at the cathode, while the individual fuel gases reacted at the anode were H:, CO, CHzOH, C2HsOH and CH~. The working and reference cathodic and anodic electrodes comprised either porous Pt or Au. Low current-over-potential measurements were used to obtain exchange currents as a function of temperature, permitting values of activation enthalpy of the anodic electrocatalytic process to be calculated, which for a given gas (except CH~) was found to be within 5 kcal/mole for either Pt or Au electrodes. Activation enthalpies ranged from 20-30 kcal/mole for the gases studied. This independence of the values of activation enthalpies on the electrode material strongly suggests that the electrolyte and not the electrode metal is playing the major electrocatalytic role, since Pt and Au (or their compounds with Zr) would be expected to behave catalytically quite differently. High-current over-potential measurements were made, with the electrolyte in two conditions: (1) a normal, unblackened state, and (2) a highly catalytic blackened state obtained by a specific current treatment. Anodic currents obtained using the blackened electrolyte were one to two orders of magnitude larger than those for the unblackened electrolyte. In both cases the fuel cell showed similar hydrocarbon reactivity at a given over-potential whether porous Pt or Au electrodes were used. This behavior together with the similarity in activation enthalpies observed for both Pt and Au supports the importance of the role the electrolyte surface plays, compared to the electrode, in the electrocatalytic process. The high reactivity of the surface of the electrolyte may be due to the presence of electrochemically active F' centers the population of which is increased in the blackened state.

I. Introduction In previous studies [1-6] of high-temperature f u e l c e l l s u s i n g s t a b i l i z e d Z r O 2 as t h e e l e c t r o l y t e , d a t a o n t h e t o t a l o v e r - p o t e n t i a l o f t h e cell v e r sus current have been presented but not the separate anodic and cathodic over-potential c u r v e s . B e c a u s e o f t h e i n t e r e s t in o x y g e n p u m p s and sensors, single-electrode over-potential studies have concentrated on the oxygen elect r o d e ( f u e l - c e l l c a t h o d e ) as p e r t h e i n v e s t i g a t i o n s o f F a b r y a n d K l e i t z [7], W a n g a n d N o w i c k [8], a n d I s a a c s e t al. [9]. T h e f u e l - c e l l 0167-2738/81/0000-0000/$02.50

© North-Holland

anodic over-potential-current behavior, which may be influenced by the structural, dittusional, and chemisorption properties of a given fuel gas and the nature of the electrode--electrolyte surface has not received much attention. Yet an understanding of the anodic electrocatalytic p r o c e s s e s is o f i m p o r t a n c e in f u e l - c e l l t e c h n o l o g y , p a r t i c u l a r l y at t e m p e r a t u r e s b e l o w t h e l e v e l o f 1 0 0 0 ° C b e i n g p r e s e n t l y u s e d [3]. T e m peratures well below 1000°C are much more hospitable to the lifetime of materials; however the concomitant reduced rates of various physical c h e m i c a l p r o c e s s e s a t l o w e r t e m p e r a t u r e s Publishing Company

B.G. Ong et al. I Stabilized zirconia and current-over-potential in hydrocarbon fuel cells"

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can become controlling and need to be thoroughly investigated and understood for the design of practical fuel cells. In the present study, reference electrodes were used for obtaining the anodic (fuel) overpotential characteristics. Porous Pt and Au electrodes which have quite different ordinary catalytic activity with respect to the species under study were used to ascertain the importance of the anode metal v i s a vis the electrolyte in the electrocatalysis. Pancharatnam et al. [10] found in the electrochemical decomposition of NO that for an applied voltage across a stabilized ZrO2 cell exceeding ! V, both porous Pt and Au electrodes yielded comparable decomposition rates. These electrochemical decomposition rates were about three orders of magnitude larger than the ordinary chemical decomposition rates obtainable on a pure Pt surface and about six to eight orders of magnitude larger than the chemical decomposition rates obtainable on a bare stabilized ZrO2 surface. Since Pt is catalytic for the decomposition of NO but Au is not, it was concluded that the electrochemical decomposition was being catalyzed by active sites on the electrolyte surface itself. The concentration of such sites was greatly increased when a voltage exceeding 1 V was applied across the cell. It was proposed that the active sites in the electrolyte were probably oxygen ion vacancies with two trapped elec-

trons, i.e. F' centers, as described by Kroger and co-workers [11]. The possible electrocatalytic enhancement of anodic fuel reactions by the electrolyte is further explored in the present paper.

2. Experimental The fuel-cell assembly is shown in fig. 1. Four similar semicircular paste electrodes, each with a working area of = 1.2 c m 2, w e r e painted on an electrolyte disc of 8 mol% Sc203-stabilized Z r O 2 , 2.5 cm in diameter and = 1.5 mm thick. The electrode material used was either #6926 unfluxed Pt ink or A-3156 unfluxed Au ink manufactured by Engelhard Industries. As shown in fig. 1 the working electrodes comprised the lower semicircular portion of the disc separated by a thin gap from the semicircular upper reference electrodes. The individual fuel gases reacted at the anode w e r e H2, C O , C H 3 O H , C 2 H s O H and CH4. Helium saturated with either CH3OH or C2HsOH vapor at 25°C provided these fuels at the anode. On the cathode side, air was used throughout all the studies. The total gas pressure in every case was 1 atm and the temperature ranged from 700-850°C. Gas flow rates were kept at 75 ml/min with the gases flowing normal to the disc in a stagnation-flow

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B.G. Ong et al. / Stabilized zirconia and current-over-potential in hydrocarbon fuel cells

configuration. It was found that higher gas flow rates did not affect the current-potential curves indicating gas-phase diffusion was not influential. Some m e a s u r e m e n t s were carried out at very low total currents (less than 1.0 mA). The lowcurrent data were used to determine exchange currents and activation enthalpies. Measurements at high currents were then carried out to obtain the full range of over-potential versus current curves. In the regime of higher currents with t e m p e r a t u r e cycling, changes in electrode morphology were noted as observed by Pizzini et al. [12], and good reproducibility was difficult to obtain. The over-potential 7/ (defined as positive) at each working electrode can be calculated from the formula.

3. Results and discussion

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where V and V0 are the potential difference between the working and reference electrodes at current I and at zero current, respectively. Typically V0 is very close to zero mV. IRac is the electrolyte ohmic drop across the whole cell, Rac being measured at 1 kHz with an impedance bridge. The factor 0.5 results from the symmetry on the disc arrangement, the IRac drop having an equal effect on both the anodic and cathodic value of ~7- When the m a x i m u m current under the fuel-cell m o d e of operation was attained (self-generated potential approaching zero), an external power supply was used to obtain higher currents. For both cases of unblackened and blackened electrolyte, for measurements at the highest currents, the applied voltage did not exceed 1.5 V, which is well below the decomposition voltage of the electrolyte (2.2-2.4 V). Blackening of the electrolyte was achieved by passing a current of 2.0 A through the electrolyte disc for ~ 1 min, first in one direction and then in the opposite direction. This procedure caused both sides of the disc to become blackened and was carried out each time a different gas was used at the anode. Typically, at 850°C, the applied voltage necessary to obtain 2.0 A and blackening was ~3.5 V which is above the decomposition voltage of the electrolyte.

449

3.1. The low-current range

The general form of the B u t l e r - V o l m e r equation, valid for multi-step reactions, is given by Bockris and Reddy [13]: I = Io[exp(aF71/RT) - e x p ( - f l F ~ / R T ) ] ,

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where a and/3 are empirical transfer coefficients for forward and reverse steps respectively. F is Faraday's constant, R is the universal gas constant and T is the absolute temperature. The exchange current I0 is best evaluated at low values of 77 where mass-transfer effects are minimized. Expanding the exponential terms for low values of r/ leads to the equation: (3)

Even though the value of the term a +/3 cannot be experimentally obtained separately it should not be very temperature dependent. By applying eq. (3) to the experimental data, an Arrhenius plot of log(a +/3)Io versus 1 / T permits the activation enthalpy for the anodic reaction to be obtained. Such Arrhenius plots were constructed for the gases under study using porous Pt and Au electrodes and the activation enthalpies (kcal/mole) calculated from the slopes of the resulting lines are in table 1. A p a r t from E R a the activation enthalpies using porous Pt and Au electrodes are on the average within 3 kcal/mole of each other. There is evidence that the high apparent activation enthalpy for methane using the Au anode is anomalous owing to observed carbon deposition Table 1 Activation enthalpies (kcal/mole) for Pt and Au porous anodes for several fuel gases Gas

Pt electrode (kcal/mole)

Au electrode (kcal/mole)

H2 CO CH3OH C2HsOH CH4

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B.G. Ong et al. / Stabilized zirconia and current-over-potential in hydrocarbon fuel cells

450

on the electrode at lower temperatures, blocking the pores of the electrode. On the basis of these activation enthalpies it is concluded that the role of the electrode metal on the electrocatalysis of the species is small compared to that of the electrolyte. Owing to changing electrode morphology there is a continual reduction of the exchange current with time, with (c~ + /3)/0 being diminished three-fold during a week's continuous operation. H o w e v e r the slopes of such curves, do not change and thus the activation enthaipy is not influenced by the electrode rearrangement.

3.2. The high-current range Of the many species investigated, fig. 2 shows representative shapes of anodic current versus over-potential curves for an a i r - C O fuel cell using Pt and Au electrodes. The electrolyte was IO0

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in its normal unblackened state. In general the anodic reactivity of all fuels were within 20% of one another except for H2 which displayed to ~about four to five times larger currents. The horizontal arrow on each curve indicates the maximum current which could be obtained in the fuel-cell m o d e of operation. External power had to be supplied to obtain the continuation of the current-potential curves. The carrents obtained using Pt electrodes are seen to be ~ 8 times those employing Au electrodes. On first sight, this might seem to be due to a higher catalytic activation of Pt over Au. However, the current obtainable is proportional to the gas-electrode--electrolyte triple-phase area, and scanning electron micrographs [14] show that for the porous electrodes used, the Pt particles are much smaller than the Au particles. Hence the Pt electrodes have a much larger triple-phase area for reaction than the Au electrodes. Thus it is believed a m a j o r portion of the extra current may be due to this factor rather than the greater catalytic activity of Pt. Fig. 3 shows current versus over-potential curves for the a i r - C O fuel-cell after the electrolyte had been blackened by the previously described current treatment. Curves resembling these were obtained for the other gases studied. The important features to note are: (i) the currents obtainable from the Au electrodes are as large as those from the platinum electrodes; (if) the currents obtainable after the electrolyte has been blackened are one to two orders of magnitude larger than those obtainable with the unblackened electrolytes; and (iii) the anodic over-potential curves for Pt and Au electrodes are practically indistinguishable from each other. The exact chemical nature of the blackened electrolyte is as yet unresolved, though the blackening phenomenon was observed as early as the days of the Nernst glower, and Weininger and Z e m a n y [15] have shown that blackening results only after considerable amounts of oxygen have evolved from the electrolyte. The subject has been dealt with by Wright et al. [16], Blumenthal [17], Kroger [11], Casselton [18], and Fabry and Kleitz [19]. The high reactivity of a blackened electrolyte towards 02 reduction has been attributed to

B.G. Ong et al. / Stabilized zirconia and current--over-potential in hydrocarbon fuel cells

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etc. are known to exist [17]. Weppner [24] has shown that under sufficiently reducing conditions, for example under conditions when the electrolyte is blackened, it is also possible for the zirconia electrolyte to form such intermetallic compounds directly with the cathode metal. Hence Steele has proposed that the enhanced reactivity may be due to the intermetallic compounds ZrPt3 or ArAu3 which could be catalytic. Again, it would seem unlikely that compounds containing Pt or Au would have similar catalytic activity and chemisorption properties with respect to the various hydrocarbons. However, it is possible to prepare such intermetallic compounds and use them as anode materials and directly check their catalytic activity towards hydrocarbon electro-oxidation. Transmission electron microscopy has shown the black substance to be a sub-oxide of Zr.

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several mechanisms including the possibility of the presence of the high concentrations of catalytic F' centers contributed by oxygen atoms chemisorbed on the electrolyte surface itself, either on metallic zirconium particles or on sites in the unreduced zirconia [21]. The major role played by the reduced electrolyte surface has also been proposed by Pancharatnam et al. [10] and Gur [22] in their studies of the electrochemical decomposition of NO using a stabilized zirconia electrolyte cell. As far we know, the current work is the first to stress the role of the electrolyte in catalyzing hydrocarbon electro-oxidations. The vastly different chemisorption properties [20] of Pt and Au towards the hydrocarbons studied corroborates the premise that these electrode metals are not likely to be the electrocatalysts. An alternative explanation for the high reactivity obtained after electrolyte blackening has been suggested by Steele [23]. Intermetallic compounds between zirconium metal and other metals such as gold, platinum, copper, silver,

4. Conclusions Results of this study indicate that the activation enthalpies of the hydrocarbon electro-oxidations studied depend only weakly on the nature of the electrode metal, with Pt and Au electrodes giving activation enthalpies which are close to each other even though their ordinary non-electrochemical catalytic activities and chemisorption characteristics [20] are widely different. When the electrolyte is in a highly reduced form caused by current blackening, the catalytic activity of its surfaces is greatly increased. The high currents obtained do not depend on the nature of the electrode material, indicating again that the electrolyte surface is playing the major role in the electrochemical reaction. It is plausible that the increased reactivity is caused by the presence of high concentrations of F' centers which react with the chemisorbed species on the electrolyte surface.

Acknowledgement The major support for this research was made available by the Department of Energy, Division of Chemical Sciences, Office of Basic

452

B.G. Ong et al. / Stabilized zirconia and current-over-potential in hydrocarbon fuel cells

Energy Sciences (Grant No. 9864-AC7). Partial support of this research was made possible by the donors of the Petroleum Research Fund. administered by the American Chemical Society. The grant of sabbatical leave by the University of Malaya made it possible for B.G. Ong to engage in this project. All of these sources of support are gratefully acknowledged.

References [1] J. Weissbart and R. Rukm J. Electrochem. Soc. 1119 (1962) 723. [2] H. Binder, A. Kohling. H. Krupp, K. Richter and G. Sandstede, Electrochim. Acta 8 (1963) 781. [3] Final Report, Project Fuel Cell, Westinghouse Research and Development Report No. 57 (Department of Interior, Washington, 1970) Contract no. 14-(110(101-303. [4] T.H. Etsell and S.N. Flcngas, J. Electrochem. Soc. 11~ (1971) 1890. [5] C.J. Wen and D.M. Mason, J. Appl. Electrochcm. 8 [61 R.A. Goffe and D.M. Mason, ,1. Appl. Electrochem. (1981). to be published. I7] P. Fabry and M. Kleitz, J. Electroanal. Chem. 57 (19741 165. [8] D.Y. Wang and A.S. Nowick, J. Electrochem. Soc. 126 (1979) 1155.

[9] H.S. Isaacs, L.J. Olmer and S. Srinivasan, National Fuel Cell Seminar Abstracts 1978, p. 154. [l(I] S. Pancharatnam, R.A. Huggins and D.M. Mason, J. Electrochem. Soc. 122 (1975) 869. [11] H. Yanagida. R.J. Brook and F.A. Kroger, J. Electrochem. Soc. 117 (19701 593. [12] S. Pizzini, M. Bianchi, P. Colombo and S. Torchio, J. Appl. Electrochem. 3 (1973) 153. 113] J.O'M. Bockris and A.K.N. Reddy, Modern electrochemistry', Vol. 2 (Plenum Press. New York, 197(I) p. 1t107. [14] S. Pancharatnam, Ph.D. Thesis, Department of Chemical Engineering, Stanford University (1974). {15] J . L W e i n i n g e r a n d P.D. Zemany, J. Chem. Phys. 22 (1954) 1469. 116] D.A. Wright, J.S. Thorp, A. Aypar and H.P. Buckley, J. Mat. Sci. 8 (19731 876. [17] W.B. Blumenthal. The chemical behavior of zirconium (Van Nostrand, Princeton, 1958) pp. 152, 98. [18] R.E.W. Casselton, J. Appl. Electrochem. 4 (19741 25. [19] P. Fabry and M. Kleitz, in: Electrode processes in solid state ionics, eds. M. Kleitz and J. Dupuy (Reidel. Dordrecht. 1976) p. 331. I211] G. Wcdler, Chemisorption: tin experimental approach (Butterworths, London, 1976) p. 199. [21] T. Smith, J. Electrochem. Soc. 111 (19(~4) 1(12(t. {22] T.M. Gur, Ph.D. Thesis, Department of Materials Science and Engineering, Stanford University (19761. [23] B.C.H. Steele, in: Electrode processes in solid state ionits, eds. M. Kleitz and J. Dupuy (Rcidel, Dordrecht, 1976) p. 367. [24] W. Weppner, J. Electroanal. Chem. 84 (19771 339.