Tubular high-temperature proton conductors: transport numbers and hydrogen injection

Solid State Ionics 139 (2001) 95–104 www.elsevier.com / locate / ssi

Tubular high-temperature proton conductors: transport numbers and hydrogen injection T. Schober* ¨ Festkorperforschung ¨ ¨ ¨ , Forschungszentrum Julich , 52425 Julich , Germany Institut f ur Received 9 August 2000; received in revised form 10 October 2000; accepted 6 November 2000

Abstract Tubes of high-temperature proton conductors [Sr(Zr 0.8 Ce 0.2 ) 0.8 In 0.2 O 32x , BaZr 0.9 Y 0.1 O 32x and CaZr 0.9 In 0.1 O 32x ] were investigated in various gas atmospheres. Transport numbers for selected species were measured and compared with literature values. The electrochemical injection of hydrogen into inert gases was demonstrated. By increasing the proton current, titration curves were obtained analogous to those for strong acid–strong base titrations. The modelling of such curves is presented. Fuel cell operation and steam electrolysis was also demonstrated. Exposure of one side of the tubes to ethanol vapor was found to lead to proton pick-up of the tubes. More drastic effects were observed with acetone vapor, which also produced proton pick-up but also the deposition of undesirable carbonaceous layers.  2001 Elsevier Science B.V. All rights reserved. Keywords: Proton conduction; Hydrogen pumping; Perovskite; Steam electrolysis; Transport numbers

1. Introduction Mixed metal oxides having the ABO 3 perovskite structure are of considerable importance as oxygen and proton conductors since they tolerate changes in chemical doping at the A or B cation sites. For divalent A and tetravalent B atoms it is commonly accepted that doping with trivalent atoms on B sites leads to the formation of oxygen vacancies which are a prerequisite for the absorption of water by the compound at elevated temperatures. A case in point

*Tel.: 149-2461-614-415; fax: 149-2461-612-550. E-mail address: [email protected] (T. Schober).

is the compound BaZr 0.9 Y 0.1 O 2.95 [1]. If water is absorbed, the O vacancies are partly filled according to ? (H 2 O) gas 1 V ??O 1 O 3 O ⇔2OH O ,

(1)

resulting in a mobile protonic species responsible for proton conduction. Most previous impedance and electrochemical measurements on proton conductors were performed on flat, disc-shaped specimens. Many applications in the future require, however, tubular configurations as introduced for oxygen ion conduction in the Siemens–Westinghouse SOFC design, or for automotive, l-type oxygen sensors in the original Bosch design. The first commercial hydrogen sensor

0167-2738 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00822-5

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[2] introduced by TYK Co., Japan, consists of a tube of CaZr 0.9 In 0.1 O 32x cemented in a supporting tube of Si 4 N 3 or alumina. Finally, ammonia synthesis at atmospheric pressure was demonstrated with a tube of the classic proton conductor SrCe 0.95 Yb 0.05 O 32x closed at one end [3]. In the present work, results are presented on the transport number and various electrochemical measurements using tubes of Sr(Zr 0.8 Ce 0.2 ) 0.8 In 0.2 O 32x , referred to here as SZCI, of BaZr 0.9 Y 0.1 O 32x (BZY10), and of CaZr 0.9 In 0.1 O 32x (CZI10). SZCI was introduced in Ref. [4] where the proton content as a function of temperature and pH 2 O was presented. BZY10 was rediscovered in Ref. [1] to be a good proton conductor when sufficiently high (.17008C) sintering temperatures are used. Impedance data on such BZY10 appeared in Ref. [5], while transport numbers were given in Ref. [6]. TYK sensors are fabricated from CZI10 [2]. Special attention was given to the sealing problems. It was investigated, for instance, whether or not glass solder seals or ceramic cement seals held at high temperatures work equally well as designs with seals only at the cold ends of ceramic tubes using elastomers in compression fittings.

2. Experimental

ples were used. The ZrO 2 tube together with the SZCI sensor was connected to a glass manifold such that the two sides of the sensor tube could be exposed to different arbitrary gases. The experimental setup shown in Fig. 1 was used to demonstrate the injection of hydrogen into an inert gas stream. A PEM demonstration fuel cell was used to detect the presence of hydrogen in the gas stream.

2.2. Tubes of BaZr0.9 Y0.1 O32 x ( BZY10) Again the mixed oxide route was used. As above, the maximum of the grain size distribution was pushed below the 1 mm limit. After manufacturing and sintering (HITEC Materials) of the external tubes they had an O.D. of 10 mm, an I.D. of 6 mm and a length of 120 mm. A considerable problem was leak tightness and cracks in the tubes. In the scope of the present work the parameters pertaining to optimized tube production were not elaborated. Nevertheless, a few crack-free tubes were obtained with a tolerable leak rate. The acceptable tubes were given Pt electrodes inside and outside and connected to a suitable glass manifold using compression fittings at the ends which were maintained near ambient temperature. As above, different gases could be applied to the inner and outer sides of the tubes. A commercial oxygen partial pressure gauge was installed directly behind the proton conductor tube.

2.1. Tubes of Sr( Zr0.8 Ce0.2 )0.8 In0.2 O32 x ( SZCI) 2.3. Tubes of CaZr0.9 In0.1 O32 x ( CZI10) Powder preparation was carried out using the mixed oxide route [4]. The calcined powder was milled until the maximum of the grain size distribution was below 1 mm. From that powder an external contractor (HITEC Materials, Karlsruhe, Germany) fabricated tubes with a hemispherical cap at one end with the following dimensions after final sintering: I.D. 4 mm, O.D. 6.7 mm, length 40 mm. After applying Pt electrodes and attaching Pt wires the tube was inserted into a larger ZrO 2 tube and sealed at 8508C using the glass solder 8422 (Schott). Heating was achieved by introducing commercial rod-like heating elements, used in Bosch-type automotive oxygen sensors operating at 12 V, into the SZCI tubes. For temperature control, thin thermocou-

Here, commercially available TYK sensors were used which had either Si 4 N 3 or Al 2 O 3 supporting tubes. After dismantling a sensor, the proton conducting tubes were found to have dimensions of 3.7 mm O.D., 2.4 mm I.D. and 20 mm length. A He leak test revealed, for a number of sensors, that the original bond between the proton conducting tube and the support tube was somewhat leaky, which is probably of no relevance for the original application of the sensors. For stringent requirements we sealed the proton conductor–support tube connection with the above glass solder which roughly matches the expansion coefficients of the tubes used. The overall configuration of the manifold is shown in Fig. 2.

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Fig. 1. Experimental setup to detect the injection of hydrogen into an inert gas stream using an internally heated SZCI tube. The latter is joined to a zirconia tube using glass solder. If a voltage of the right polarity is applied to the electrodes, protons are driven to the tube outside and recombine to form H 2 . The reducing Ar–H 2 mixture is now driven into the PEM fuel cell, acting as a pO 2 sensor, and produces a voltage of about 900 mV (moist Ar alone produces an EMF of about 50–70 mV).

3. Results and discussion

3.1. SZCI For the interpretation of Nernst voltages we use the following two equations [7]: RT p O9 2 RT p H9 2 O high pO 2 : U 5 (t O 1 t H )] ln] 2 t H ] ln]], 4F p O992 2F p 99 H 2O (2) RT p 9H 2 low pO 2 : U 5 2 (t O 1 t H )] ln] 2F p 99 H2 RT p 9H 2 O 2 t O ] ln]], 2F p 99 H 2O

(3)

pertaining to oxidizing and reducing conditions. Thus, by keeping the activity of one species constant on both sides and varying the activity of the other,

we can extract the relevant transport numbers. Fig. 3 shows the Nernst voltages of a water vapor concentration cell at a high pO 2 of about 10 24 bar. Here, 992 O ; pref ¯ 6.1 mbar, whereas the p 9O 2 5 p 99 O 2 ? pH water vapor pressure p 9H 2 O was varied. We first note that roughly a straight line was obtained as predicted by Eq. (2). Comparing the experimental curve with theory for t H 5 1 we deduce that t H ¯ 0.06560.025 in our experiment. In agreement with our previous work [8–10] this result is definite evidence for proton conduction with an overwhelming conductivity contribution from electronic holes, s DC h ? , since

s DC H t H 5 ]]]]]]] DC DC , s DC 1 s 1 s DC ? ?? 1 s e9 H h VO

(4)

where the symbols are self-explanatory. Apparently, under oxidizing conditions, too many electronic holes are present which masks proton conduction. The sum (t H 1 t O ) was determined at 7008C in a

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T. Schober / Solid State Ionics 139 (2001) 95 – 104

Fig. 2. Experimental setup constructed from quartz components for the study of the CZI10 tubes produced by TYK Co. (not to scale!). Separate gases can be fed to the inside and outside of the tube. Painted Pt electrodes on the inside and outside of the tube can be contacted using Pt mesh and wire. Quartz tubes were sealed with compression fittings using elastomer rings.

similar experiment by keeping the reference side at a pO 2 of 1 bar and varying the pO 2 on the other side with constant humidity on both sides (Fig. 4). The experimental slope corresponds to t H 1 t O ¯ 0.0560.03. In view of the value determined for t H it is reasonable to assume that t O ¯ 0. Switching now to very low pO 2 (Eq. (2)) the sum (t H 1 t O ) was determined in a hydrogen concentration cell (Fig. 5) at 7008C. The reference hydrogen pressure was given by an Ar–4% H 2 mixture on one side, while the other was exposed to a variable pH 2 with constant humidity on both sides. Experimentally, we obtain t H 1 t O ¯ 0.360.04. In a separate, water vapor concentration experiment using a constant pH 2 , t O was found to be negligible. Thus, under very reducing conditions, t H ¯ 0.360.04. This result deviates dramatically from our previous transport number data for BCN18, BZY10 and BISO [8–10], where t H ¯ 1.0 was observed for hydrogen concentration cells. We assume for SZCI that the electronic conduction, s DC e9 , arising from H 2 1 2O O ⇔2OH O 1 2e9 3

Fig. 3. SZCI tube. Nernst voltages of a water vapor concentration cell at relatively high pO 2 . The theoretical curve for t H 5 1.0 is also shown.

?

(5)

is strongly competing with proton conduction (Eq. (4)).

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In order to obtain information on the total ionic transport number, t ion , averaged over |24 orders of magnitude of pO 2 , the fuel cell operation of a SZCI tube was studied. The outside of the tube was exposed to flowing Ar–4% H 2 , whereas the inside was exposed to synthetic air. The water vapor partial pressure was increased stepwise by adjusting the temperature of the water bath but was kept the same in the two gas mixtures. Using the mass action law of water equilibrium H 2 11 / 2O 2 ⇔H 2 O allowed us to calculate the oxygen partial pressure in the moist Ar–H 2 gas. Fig. 6 shows the experimental open cell voltage (OCV) at 7008C as a function of this calculated pO 2 and also the theoretically expected values for t ion 5 1 based on U 5 RT / 4F ln( p O9 2 /p O992 ), Fig. 4. SZCI tube. Nernst voltages of an oxygen concentration cell with constant pH 2 O . The theoretical curve for (t H 1 t O ) 5 1.0 is also depicted.

Fig. 5. SZCI tube. Nernst voltages for a hydrogen concentration cell with constant pH 2 O . The curve for t H 5 1 is also shown. Note that t O ¯ 0.

(6)

where the reference pO 2 is 0.205 bar. It can be seen that the ionic transport number is about 23% lower than the perfect value where t ion 5 1. Apparently, there are further conduction mechanisms than the ionic ones, such as hole conduction at high pO 2 and electron conduction at low pO 2 . Using the setup shown in Fig. 1, hydrogen in-

Fig. 6. SZCI tube, fuel cell operation. Details in text. The open cell voltage is shown as a function of pO 2 . The theoretical curve for t ion 5 t H 1 t O 5 1.0 is also included.

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jection into inert gas streams is demonstrated. If only moist Ar flows through the proton conductor cell, the PEM fuel cell, acting as an O 2 -concentration cell, yields an output of 50–70 mV according to Eq. (6), which is a measure of the pO 2 in the Ar stream. However, if the proton conductor cell is polarized such that the protons are driven to the outer side, the protons arriving at the surface may recombine and form hydrogen gas which is transported to the PEM cell. The latter will then display an EMF of about 900 mV after a few minutes which is typical of an air-dilute hydrogen fuel cell.

3.2. BZY10 All the pertinent transport numbers were recently determined using flat-plate geometry [6]. We confine ourselves here to experiments with proton injection into inert gases. Initially, moist Ar ( pH 2 O ¯ 23 mbar) was flushed through the proton conductor and the oxygen sensor behind it. The outside of the tube was exposed to humid air allowing BZY10 to pick up protons. Without any polarizing voltage at the BZY10 tube, the exit pO 2 at the oxygen sensor was |10 24 bar, typical for Ar with small amounts of O 2 stemming from tiny leaks, permeation at the seals and outgassing of the walls. This will be assumed in the following to form a constant background, p 0O 2 , at the entrance side. If the polarizing voltage is increased stepwise with the right polarity (Fig. 7) it can be seen that the exit pO 2 decreases gradually to the 10 26 bar range. This is produced by proton migration to the cathode and subsequent recombination and chemical reaction with the background oxygen. We emphasize here that this effect already occurs below the dissociation voltage of the H 2 O molecules, which is about 1.1 V. A further increase of the voltage leads to a precipitous drop of the 221 oxygen activity, eventually decreasing to the 10 bar level. Thus, Fig. 7 corresponds to a gas titration curve where the background oxygen is slowly consumed. This process is completely analogous to strong acid–strong base titrations and will be modelled below.

3.3. Modelling the gas titration curve 0

We assume that the initial Ar pressure is p Ar and the background oxygen level p 0O 2 . Further, for the

Fig. 7. Gas titration curve for a BZY10 tube. Outside of tube, moist air; inside, controlled flow of moist Ar. The dependence of pO 2 in the moist Ar measured behind the tube on the applied potential is shown.

reaction H 2 O⇔H 2 11 / 2O 2 the following mass action law is used [7]:

F

S

2.587 eV pO 2 5 906.6 exp 2 ]]] kT / eV

DG ( p 2

H 2O

/pH 2 )2 , (7)

where the pressures are given in bar. Modelling as a function of pH 2 is carried out in two different branches: to the left and to the right of the equivalence point in Fig. 7.

3.3.1. High-pO2 branch Here, background oxygen is consumed by hydrogen. Some oxygen is generated by the water splitting reaction and some extra water is produced. Thus 1 pO 2 5 p 0O 2 2 ] pH 2 2

F

S

2 2.587 eV 1 906.6 exp ]]]] kT / eV

p 1p DG S]]] D. p 2

0 H 2O

2

H2

H2

(8)

T. Schober / Solid State Ionics 139 (2001) 95 – 104

3.3.2. Low-pO2 branch Most of the oxygen is generated by the water splitting reaction. The effective hydrogen pressure is given by the initial hydrogen pressure minus the amount to neutralize the background oxygen level: pO 2

F

S

2 2.587 eV 5 906.6 exp ]]]] kT / eV

DG S 2

p 0H 2 O ]]]] ( pH 2 2 2p 0O 2 )

D

2

. (9)

In Fig. 8 the high-pO 2 branch (open squares) and the low-pO 2 branch (filled triangles) are superimposed. It can be seen that the essential features of Fig. 7 are reproduced. A one-to-one correspondence of the two figures may not be achieved since the hydrogen partial pressure generated in the tube is not a linear function of the applied voltage, and since water vapor electrolysis sets in at about 1.1 to 1.2 V increasing suddenly the supply of free protons at the anodic surface. In summary, proton conductor tubes are quite useful for producing low and ultralow pO 2 . Several authors have reported the production of atmospheres down to a pO 2 of 10 220 bar using ZrO 2 -based oxygen ion conductors (see, for instance, Ref. [11]) in which oxygen is continuously pumped

101

out of a given volume. Under those conditions it is debatable whether the very low pO 2 arises from the almost complete absence of oxygen molecules in the gas phase, or whether some hydrogen is liberated from spurious organic molecules or water vapor and effectively a hydrogen–water vapor atmosphere is present. With our proton conductor tubes we have a preset pH 2 O and a current controlled, adjustable pH 2 and, if a sample is at an elevated temperature, the reaction H 2 O⇔H 2 11 / 2O 2 will reach equilibrium and the desired pO 2 is established at the position of the sample.

3.4. A remark on quartz tubes Quartz tubes were used as dummies and to test the leak tightness of the system. If they were coated with Pt electrodes on the inside and outside, flushed with Ar inside and with humid air on the outside, and polarized with about 1.1 V, a decrease of the pO 2 in the Ar stream at the exit side by roughly a factor of 2 could be observed. Apparently, it is also possible to introduce protons into the quartz and to transport them to the cathode where recombination will produce some H 2 , lowering the effective pO 2 of the gas stream.

3.5. CZI10 tubes 3.5.1. Water vapor concentration cell — high pO2 For one side, synthetic air with a pH 2 O of 6.1 mbar (ice bath in a bubbler) was used, and for the other side the same air but with a variable pH 2 O at a temperature of 6408C. The result is shown in Fig. 9. Analysis on the basis of Eq. (2) leads to t H 5 0.360.03. This result is in line with other systems [6,9]. Apparently, there is not only proton conduction but also electronic conduction (see also Eq. (4)).

Fig. 8. Modelled gas titration curve for the case studied in Fig. 7. X-axis: hydrogen pressure. We use here arbitrarily p 0O 2 5 5 3 10 25 bar. Note qualitative correspondence with Fig. 7.

3.5.2. Oxygen concentration cell — high pO2 For the reference side, synthetic air with a pH 2 O of 23 mbar was used; on the other side the pO 2 was varied but pH 2 O kept at 23 mbar. From the observed EMF at 6408C we conclude that t H 1 t O 5 0.560.03. This signifies that t O ¯ 0.2, which is the highest value observed in these systems [6,9].

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Fig. 9. CZI10 tube. Nernst voltages of a water vapor concentration cell at relatively high pO 2 . The theoretical curve for t H 5 1.0 is also shown.

Fig. 10. CZI10 tube. Nernst voltages for a hydrogen concentration cell with a constant pH 2 O . Deviations at high values of p /pref are ascribed to permeation and leakage effects and are disregarded.

3.5.3. Hydrogen concentration cell — very low pO2 Flushing the inside of the tube with a moist Ar–4% H 2 and the outside with a moist Ar–H 2 mixture of variable, but higher, pH 2 yielded, at T 5 6708C, the sum t H 1 t O 5 0.9760.03 (Fig. 10), a value which is fully in line with most previous measurements under very reducing conditions. We note a deviation from linearity for more elevated values of p /pref and ascribe this to undesirable permeation or leakage effects.

theoretically expected voltage according to Eq. (6) is also close to 1 V. We therefore conclude that the ionic transport number, t ion 5 (t H 1 t O ), is close to unity in this system. Apparently, hole conduction in the high-pO 2 regime as in the compounds BCN18 and BZY10 [6,9] is of less importance here.

3.5.4. Oxygen concentration cell — very low pO2 Exposing both sides to an Ar–4% H 2 stream with differing water vapor levels results in t O ¯ 0.02. This signifies that the t H value in the preceding section is about 0.9560.05. Thus, under reducing conditions, CZI10 behaves as a near perfect proton conductor with no noticeable electronic conduction. 3.5.5. Fuel cell operation Directing moist synthetic air at the outside and moist Ar–4% H 2 into the inside of the CZI10 tube at constant pH 2 O resulted in a Nernst voltage of 1.036 V of this oxygen concentration cell (T 5 6608C). The

3.5.6. Hydrogen injection into inert gas streams We first exposed the inside of a CZI10 tube to a moist Ar–4% H 2 mixture and the outside to a moist Ar flow which then continues further to the pO 2 gauge. If the system is leaktight, a pO 2 of about 5310 25 bar can be measured in the Ar gas. Increasing the voltage applied to the electrodes of the tube stepwise with correct polarity, but staying below the dissociation voltage for water, again allows the injection of hydrogen into the Ar and measurement of the gas titration curve. At an applied voltage of 700 mV, a pO 2 of 10 217 bar is readily reached. 3.5.7. Steam electrolysis Directing a stream of moist Ar at the outside and a separate one to the tube inside, from which it continues to the pO 2 gauge, allows us to observe

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applied with the right polarity, the pO 2 drops within minutes to 10 216 bar and eventually reaches 10 218 bar for a voltage of 600 mV. Apparently, protons are transferred to the cathode and recombine there to form H 2 . As before, H 2 consumes the background O 2 and leads to the formation of a reducing, moist Ar–H 2 mixture. There are two likely mechanisms for the formation of protons on the outer surface of the tube: • Dissociation of organic molecules by the dangling bonds of the transition metal oxides of the perovskite structure [13] (it has been known for about 50 years that such oxides have very good catalytic properties). Schematically, we may write one of many possibilities: mC 2 H 5 OH ⇒ xH 1 1 yOH 2 1 zCO 1 uH 2 O 1 Fig. 11. CZI tube. T 5 7008C. Experimental gas titration curve: pO 2 in the exit stream versus applied potential. Details in text. Steam electrolysis should set at about 1 V. An overvoltage should, however, be added to that value taking into account polarization effects.

steam electrolysis and the injection of H 2 into the gas stream passing through the tube inside. The pertinent gas titration curve is shown in Fig. 11. As before, some hydrogen injection is already noted below the dissociation voltage for water. Above that voltage the pO 2 drops precipitously. Using moist air rather than moist Ar on the outside, results, in principle, in the same positive result. It takes, however, a somewhat higher applied voltage (|100 to 200 mV) to reach the same objective. Presumably, this is a consequence of the changed transport numbers since the outside is now in the high-pO 2 regime.

3.5.8. Exposure to ethanol vapor Work on the reaction of acetone vapor with the surface of the perovskite KNbO 3 [12] motivated us to perform some cursory experiments using ethanol and acetone. If the outer side of the tube is exposed at 7008C to Ar saturated at 228C with ethanol and the inside to moist Ar, which subsequently flows to the pO 2 gauge, a pO 2 of about 2.5310 25 bar is observed after equilibration without a polarizing voltage at the CZI10 tube. If, however, only 200 to 250 mV are

vCH 4 1 . . . . • Dissociation at the Pt electrode on the outside of the tube. We cannot distinguish here between these two possibilities, but favor the first intuitively.

3.5.9. Exposure to acetone vapor As pointed out in Ref. [12] acetone vapor has a very reducing effect on a bare KNbO 3 surface. We used the same arrangement as for the ethanol vapor experiment: on the outside of the tube was Ar saturated with acetone. The inside was flushed with moist Ar which was then fed to the pO 2 gauge. The temperature was 7008C. To our surprise, the pO 2 level in the inside of the tube stabilized without any applied voltage a few minutes after the start of the experiment at around 10 219 bar. This we interpret by the assumption of massive hydrogen production on the outside of the tube by catalytic dissociation of acetone and subsequent diffusion of protons to the inside. Evolving hydrogen on the inside is then the reason for the low pO 2 level observed. Repeating this experiment at lower temperature down to 4008C gave qualitatively the same result. The only difference was that the final pO 2 was 10 218 bar at 4008C rather 219 than 10 bar at 7008C. With an applied voltage of the correct polarity the final pO 2 level could barely be lowered beyond the level observed without voltage.

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In summary, the experiment shows that acetone has a drastic effect on CZI10 (and presumably also on the other proton conductors) and creates a hydrogen-rich atmosphere at the surface by dissociation effects.

Acknowledgements Helpful discussions with K. Szot and J.B. Condon are acknowledged. Experimental assistance was provided by J. Friedrich and D. Triefenbach.

4. Conclusions • Transport number measurements show that SZCI, BZY10 and CZI10 are good proton conductors under reducing conditions and mediocre proton conductors in oxidizing atmospheres. • All three compositions are suitable for the injection of hydrogen into inert gas streams, either from Ar–H 2 mixtures, or by steam electrolysis. BZY10 is our first choice. • Sealing of the tubes is a very critical aspect. We favor compression fittings at the cold ends using elastomer rings, or else, glass solder seals with a matched coefficient of expansion between proton conductor, glass solder and support tube in the hot zone. • Exposure of the above proton conductor tubes to vapors of ethanol and acetone at |7008C leads to proton pick-up by the dissociation of the organic molecules at the catalytically active surface.

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