- Email: [email protected]
Study on current efficiency of steam electrolysis using a partial protonic conductor SrZr0.9Yb0.1O3−α
Solid State Ionics 138 (2001) 243–251 www.elsevier.com / locate / ssi
Study on current efficiency of steam electrolysis using a partial protonic conductor SrZr 0.9 Yb 0.1 O 32 a a, a a b Tetsuro Kobayashi *, Katsushi Abe , Yoshio Ukyo , Hiroshige Matsumoto a
b
Toyota Central R& D Labs., Inc., Nagakute, Aichi 480 -1192, Japan Center for Integrated Research in Science and Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464 -8603, Japan Received 15 February 2000; received in revised form 29 August 2000; accepted 5 October 2000
Abstract The current efficiency of steam electrolysis was measured in the temperature range from 460 to 6008C using a steam electrolysis cell constructed with a partial protonic conductor SrZr 0.9 Yb 0.1 O 32 a as an electrolyte and Pt cermet electrodes. The efficiency was increased with increasing partial pressure of water vapor and temperature. The results were considered in relation to the reaction rates at the anode and cathode. Under the operating conditions of steam electrolysis, the reaction rate of producing H 2 from protons at the cathode was found to be faster than that of oxidizing water vapor into protons and O 2 at the anode. Therefore, the average concentration of protons in the partial protonic conductor during electrolysis decreased. On the other hand, the average concentration of holes increased. This is considered to decrease the efficiency of steam electrolysis. It was found that the effective transport numbers of charge carriers in the partial protonic conductor were controlled by the reaction rates at the electrodes at relatively low temperatures at which the equilibria between the atmosphere and defects in the partial protonic conductor were difficult to obtain. 2001 Elsevier Science B.V. All rights reserved. Keywords: Protonic conductor; Steam electrolysis; Current efficiency; Hole; Transport number
1. Introduction Some perovskite-type oxides based on SrCeO 3 , BaCeO 3 , CaZrO 3 , SrZrO 3 and BaZrO 3 such as SrCe 0.95 Yb 0.05 O 32 a and SrZr 0.9 Yb 0.1 O 32 a , in which a part of the Ce or Zr is substituted with trivalent cations of IIIA, IIIB and rare earth elements, exhibit good protonic conduction (10 23 S cm 21 ) around 7008C [1]. These materials with protonic conduction *Corresponding author. Tel.: 181-561-63-5364; fax: 181-56163-6136. E-mail address: [email protected] (T. Kobayashi).
have been studied widely for their applications as electrolytes for hydrogen sensors [2–4], SOFCs [5– 7] and gas reactors [8,9]. These protonic conductors have oxide ion vacancies. The equilibria between oxide ion vacancies (V ??O ), protons (H ?i ), holes (h ? ), H 2 , O 2 and H 2 O can, for instance, be expressed according to Eqs. (1)–(4), K1 1 → H ?i ] H2 1 h? ← 2
(1)
K2 1 → 2h ? 1 O O3 ]O 2 1 V O?? ← 2
(2)
0167-2738 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00793-1
T. Kobayashi et al. / Solid State Ionics 138 (2001) 243 – 251
244 K3
→ 2H ?i 1 O 3 H 2 O 1 V ??O ← O
(3)
K4 1 → 2H i? 1 ]O 2 H 2 O 1 2h ? ← 2
(4)
where O 3 O and K denote the oxide ion in the normal lattice site and the equilibrium constant, respectively. Each equilibrium constant was obtained by Eqs. (5)–(8), where [O 3 O ] was assumed to be unity. [H ?i ] K1 5 ]]] [h ? ]P H1 /22
(5)
[h ? ] 2 K2 5 ]]] [V ??O ]P 1O/22
(6)
[H ?i ] 2 K3 5 ]]] [V ??O ]PH 2 O
(7)
? 2
1/2
[H i ] P O 2 K3 K4 5 ]]] 5] ? 2 [h ] PH O K2
(8)
2
Protons (H ?i ), holes (h ? ) and oxide ion vacancies (V ??O ) are the main charge carriers in these partial protonic conductors [10]. There are some reports on the transport numbers of the charge carriers in these partial protonic conductors. Yajima et al. have measured the transport numbers of the charge carriers in SrZr 0.9 Yb 0.1 O 32 a in air containing 9.2 Torr H 2 O by an ac impedance technique and shown that hole conduction in SrZr 0.9 Yb 0.1 O 32 a is dominant at temperatures higher than 7008C and that protonic conduction is predominant at temperatures lower than 7008C [11]. However, they have also shown that the transport number of protons in SrZr 0.9 Yb 0.1 O 32 a is almost unity in the high temperature range from 600 to 10008C by experiments on hydrogen permeation and hydrogen sensors [12]. Iwahara and co-workers have measured the transport numbers of the charge carriers in BaCe 12x Sm x O 32 a under the operating conditions for a fuel cell. They have shown that oxide ion conduction and protonic conduction in BaCe 12x Sm x O 32 a are dominant at temperatures higher and lower than 8008C, respectively [5]. They have also demonstrated that hydrogen sensors using BaCe 0.9 Nd 0.1 O 32 a show a theoretical electromotive force (EMF) and that the transport number of ions in
this material is estimated to be almost unity in the temperature range from 200 to 9008C [2]. Kurita et al. have reported the protonic conduction domain of CaZr 0.9 In 0.1 O 32 a in detail using an ac impedance technique. They have shown, for example, that protonic conduction is dominant at partial pressures of water vapor above 0.01 atm and temperatures below 6008C even under a partial pressure of oxygen of nearly 1 atm [13]. Summarizing the above studies, the transport number of protons in these partial protonic conductors is almost unity in atmospheres with hydrogen sources (water vapor or hydrogen gas) which are oxygen-free. In atmospheres containing oxygen, hole conduction or oxide ion conduction becomes dominant at temperatures higher than 7008C. However, even though atmospheres contain oxygen, it is thought that the transport number of protons becomes significant at temperatures lower than 7008C in the presence of a hydrogen source such as water vapor [5,11,13]. We have constructed a steam electrolysis cell using a partial protonic conductor SrZr 0.9 Yb 0.1 O 32 a . SrZr 0.9 Yb 0.1 O 32 a is chemically more stable than other partial protonic conductors based on SrCeO 3 and BaCeO 3 in an atmosphere containing CO 2 [14– 16], and its protonic conductivity is higher than that of partial protonic conductors based on CaZrO 3 and BaZrO 3 [17]. We have demonstrated using the steam electrolysis cell that hydrogen was produced by electrolyzing water vapor and that harmful nitrogen oxide (NO) was electrochemically reduced by the hydrogen as a reducing agent [18]. The temperature required for the operation of the steam electrolysis cell is from 400 to 6008C, which corresponds to the temperature of the exhaust gas of automobiles, although these partial protonic conductors are usually used at above 6008C at which they show high conductivity. From the results of previous reports [5,11,13], the protonic conduction is expected to be dominant in these partial protonic conductors even under O 2 -rich conditions at below 6008C. In this study, the current efficiency of steam electrolysis was measured in the temperature range from 460 to 6008C using a cell with SrZr 0.9 Yb 0.1 O 32 a as an electrolyte, and the effective transport numbers of the charge carriers in SrZr 0.9 Yb 0.1 O 32 a were examined under the oper-
T. Kobayashi et al. / Solid State Ionics 138 (2001) 243 – 251
ating conditions of steam electrolysis. In these experiments, the dependence of the current efficiency on the partial pressure of water vapor was examined when the partial pressure of O 2 gas was constant.
2. Experimental Fig. 1 shows a cross-section of the steam electrolysis cell. A closed end tube of sintered SrZr 0.9 Yb 0.1 O 32 a (supplied by TYK Co.) was used as a partial protonic conductor. The outside and inside diameters of the partial protonic conductor were 9 and 7 mm, respectively. The relative density of the sintered SrZr 0.9 Yb 0.1 O 32 a was about 96%. Pt cermet electrodes for the anode and cathode were prepared as follows. A fine powder of SrZr 0.9 Yb 0.1 O 32 a with a specific surface area of about 50 m 2 g 21 and Pt paste (U-3820, N.E. Chemcat Corp.) were mixed well and spread on the outside and inside of the partial protonic conductor and then calcined at 10008C for 3 h. The composition of these electrodes was 56 vol% of Pt and 44 vol% of SrZr 0.9 Yb 0.1 O 32 a . The height and area of the anode (outside) were 17.5 mm and about 5 cm 2 , respectively. The thickness of the electrodes was about 3 mm. The anode and cathode chambers were formed as shown in the figure. Water vapor was added to argon gas containing 0.4% O 2 by using two water-bubblers in series, and
Fig. 1. Cross-section of the experimental apparatus.
245
the mixture was then fed into the anode chamber at a rate of 30 ml min 21 . By controlling the temperature of the bubblers, the concentration of water vapor in the anode gas, which was measured by a humidity sensor, was controlled. All gas lines were heated higher than 1008C by ribbon heaters in order to prevent condensation of water vapor. Pure argon gas (the concentrations of O 2 and H 2 O as impurities were nominally less than 0.2 ppm) was fed to the cathode at a rate of 30 ml min 21 . When direct current was galvanostatically applied to the cell, water vapor was electrochemically oxidized into O 2 and protons at the anode, and protons were reduced to H 2 at the cathode. The concentrations of O 2 and H 2 were measured by gas chromatography. Experiments were carried out in the temperature range from 460 to 6008C under atmospheric pressure.
3. Results and discussions
3.1. Current efficiency of steam electrolysis Experimental runs of steam electrolysis were carried out at 4608C under the conditions with different partial pressures of water vapor (PH 2 O 5 0.032, 0.12 and 0.57 atm). The current density applied to the cells and the partial pressure of O 2 gas in the anode chamber were 2.4 mA cm 22 and 0.004 atm, respectively. Fig. 2 shows the dependence of the H 2 production rate on time during the application of direct current to the cell. The dotted line represents the theoretical H 2 production rate calculated using Faraday’s law when the current efficiency is assumed to be 100%. Although the H 2 production rate was nearly equal to the theoretical rate just after starting the electrolysis, it decreased with time and became almost constant after about 100 min. The H 2 production rates after becoming constant were larger under higher partial pressure of water vapor. Furthermore, the H 2 production rate was increased by changing the partial pressure of water vapor from 0.032 to 0.12 atm after the electrolysis was conducted under a partial pressure of 0.032 atm. It was found that the current efficiency of steam electrolysis, in other words, the effective transport number of protons, depended on the partial pressure of water vapor.
246
T. Kobayashi et al. / Solid State Ionics 138 (2001) 243 – 251
creased with increasing temperature. This result seems to contradict the results reported previously in which the protonic conduction becomes dominant at lower temperatures [5,11,13].
3.2. Analysis of non-efficiency of steam electrolysis
Fig. 2. Change in H 2 production rate during application of direct current to electrolysis cells under various partial pressures of water vapor. Temp., 4608C; and current density, 2.4 mA cm 22 . The dotted line represents 100% efficiency calculated by Faraday’s law.
About 100 min after direct current was applied to the cell, the H 2 production rate was measured in the temperature range from 460 to 6008C, when the partial pressures of O 2 and H 2 O in the anode gas were 0.004 and 0.12 atm, respectively. The results are shown in Fig. 3. The dotted line shows the theoretical H 2 production rate. The current efficiency decreased with increasing current densities and in-
Fig. 3. Change in H 2 production rate for steam electrolysis with current density at various temperatures. Partial pressure of water vapor: PH 2 O 50.12 atm. The dotted line represents 100% efficiency calculated by Faraday’s law.
Changes in the O 2 production rate at the anode and in the H 2 production rate at the cathode with time during steam electrolysis are shown in Fig. 4, when the temperature, current density and partial pressure of water vapor were 4608C, 2.4 mA cm 22 and 0.032 atm, respectively. The dotted line and dot-dashed line represent the theoretical H 2 and O 2 production rate, respectively, determined by Faraday’s law when the current efficiency is assumed to be 100%. The ratio of the amount of H 2 to that of O 2 produced should be 2:1. Less than 100 min after starting the electrolysis, however, the amount of O 2 produced was small compared with that of H 2 and the ratio was not 2:1. After more than 100 min, the ratio of H 2 to O 2 was almost 2:1. This result suggested that H 2 shown in the highlighted part in Fig. 4 was produced by a mechanism other than steam electrolysis. Fig. 5 shows the changes in the O 2 production rate at the anode and the H 2 production rate at the
Fig. 4. Changes in H 2 and O 2 production rate during application of direct current to the electrolysis cell. Temp., 4608C; and current density, 2.4 mA cm 22 . Partial pressure of water vapor: PH 2 O 5 0.032 atm. The dotted and dot-dashed lines represent 100% efficiency calculated by Faraday’s law.
T. Kobayashi et al. / Solid State Ionics 138 (2001) 243 – 251
2h ? 1 2e9 → 0
247
(15)
It is not certain at present if the hole current passes on the cathode side of the electrolyte where hydrogen is produced and the partial pressure of oxygen is very low. However, it can be assumed that the hole is in the partially frozen-in state [19–21], because the equilibria between the atmosphere and defects in the electrolyte are considered difficult to obtain at low temperatures such as 4608C. The anodic and cathodic current should be equal because the direct constant current was applied to the cell galvanostatically. Fig. 5. Changes in H 2 and O 2 production rate during application of direct current to the electrolysis cell in dry argon gas (not steam electrolysis) at 4608C.
cathode while direct current was applied to the cell and dry pure argon gas was introduced to both the anode and cathode (steam electrolysis was not carried out). Although H 2 was also produced, the production of O 2 was not observed at all. This H 2 production was estimated to be caused by the evolution of H 2 formed from protons, which had already dissolved in SrZr 0.9 Yb 0.1 O 32 a , by applying direct current. Therefore, the amount of H 2 in the highlighted part of Fig. 4 was also thought to be that of H 2 evolved from the electrolyte. The reactions at the interface between the anode and electrolyte during steam electrolysis may be expressed as Eqs. (9)–(11). 1 H 2 O(gas) → 2H ?i 1 2e9 1 ]O 2 (gas) 2
(9)
1 O3 O (gas) 1 V ??O 1 2e9 O →] 2 2
(10)
0 → 2h ? 1 2e9
(11)
The reactions at the interface between the cathode and electrolyte are, e.g., Eqs. (12)–(15). 2H ?i 1 2e9 → H 2 (gas)
(12)
1 ]O 2 (gas) 1 V O?? 1 2e9 → O O3 2
(13)
H 2 O(gas) 1 V O?? 1 2e9 → H 2 (gas) 1 O O3
(14)
I9 1 I10 1 I11 5 I12 1 I13 1 I14 1 I15
(16)
where I denotes the value of current and the subscripts show the equation number of the reactions. When steam electrolysis is at a steady state, for example, after 100 min in Fig. 2, Eqs. (17)–(19) should hold. I9 5 I12
(17)
I10 5 I13 1 I14
(18)
I11 5 I15
(19)
Eqs. (17)–(19) correspond to the proton, oxide ion and hole currents in the electrolyte. In these experiments, however, I13 and I14 were estimated to be less 24 24 22 than 3310 and 1.5310 mA cm , respectively, because the concentration of O 2 and H 2 O contained as impurities in the cathode gas was nominally less than 0.2 ppm. Therefore, the oxide ion current might be ignored in these experiments, and the decrease in the efficiency of steam electrolysis might be attributed to the hole current. As shown in Fig. 4, just after applying the direct current, reaction (12) was dominant rather than reaction (15) at the cathode, because the H 2 -production rate was nearly equal to the theoretical value. At the anode, the O 2 -production rate was only 25% of the theoretical value; therefore, at most, 25% of the current applied to the cell was estimated to cause reaction (9) which shows ‘steam electrolysis’, even if the production of O 2 by reaction (10) is considered
248
T. Kobayashi et al. / Solid State Ionics 138 (2001) 243 – 251
to be zero. The rest of the current applied to the cell might be consumed by reaction (11) for producing holes at the anode. The O 2 -production rate slightly increased between 10 and 100 min as shown in Fig. 4 and reached only 38% of the theoretical value even in the steady state. On the other hand, the H 2 -production rate decreased rapidly in the transient state. Because the rate of reaction (9) for producing protons at the anode side of the electrolyte was slow, the rate of reaction (12) decreased and the rate of reaction (15) inversely increased. (The total current was always constant.) In other words, the rate of reaction (12) for producing H 2 gas from protons at the cathode was faster than that of reaction (9) for oxidizing water vapor into protons and O 2 at the anode and, therefore, holes were formed at the anode. Therefore, the average concentrations of protons and holes in the electrolyte decreased and increased, respectively. This is the reason why the efficiency of steam electrolysis is not 100%. This phenomenon is illustrated in Fig. 6 and seems to suggest that reaction (1) proceeds to the left side on the application of direct current. This figure also shows schematically the dissolution of water vapor into the electrolyte by reaction (3). When the atmosphere and defects in the electrolyte are at equilibrium, the concentration of protons is considered to be high, because the H 2 production rate was high just after starting the electrolysis. If reaction (4) proceeds fast, the concentration of protons may increase even though holes are produced by reaction (11). Therefore, the phe-
nomenon shown at the right in Fig. 6 can be caused at relatively low temperatures such as 4608C where the equilibria between the atmosphere and defects in the electrolyte may be difficult to obtain. The average concentrations of protons and holes under the steady state of steam electrolysis can be calculated roughly as follows. In this experiment, the volume of the electrolyte concerned with the steam electrolysis was 0.44 cm 3 . The theoretical density and molecular weight of SrZr 0.9 Yb 0.1 O 32 a were 5.6 g cm 23 and 234.22, respectively, and the relative density of the sintered electrolyte was 96%. Therefore, the amount of the electrolyte concerned with the electrolysis was calculated to be 0.01 mol. The amount of holes formed was estimated from the amount of evolved H 2 shown in the highlighted part of Fig. 4, according to Eq. (1). Dividing the amount of holes by the amount of the electrolyte gives the average concentration (mol mol 21 ) of holes formed in the electrolyte. Kreuer has reported that the concentration of protons dissolved in SrZr 0.9 Y 0.1 O 32 a is about 3 mol%, i.e., 0.03 mol mol 21 below 5008C [22]. When the electrolysis reached the steady state, the average concentration of protons contained in the electrolyte was calculated by subtracting the average concentration of holes formed from 0.03 mol mol 21 given by Kreuer [22]. These results are listed in Table 1. Furthermore, the current efficiency of steam electrolysis, i.e., the effective transport number of protons (t H ) in the steady state is expressed by Eq. (20), because the charge carriers were only protons and holes.
Table 1 Estimated values of effective transport number of protons t H , average concentration of holes [h] and protons [H], and mobility ratio of holes to protons mh /mH in SrZr 0.9 Yb 0.1 O 32 a at 4608C under various steam electrolysis conditions Steam electrolysis condition
Fig. 6. Illustrations of dissolution of water vapor into partial protonic conductor (left one) and replacement of protons with holes by application of direct current (right one).
PH 2 O
Current (mA cm 22 )
0.032 0.12 0.57 0.12
2.4 2.4 2.4 4.0
tH
[h] (mol mol 21 )
[H] (mol mol 21 )
mh /mH
0.41 0.55 0.68 0.47
0.0115 0.0084 0.0051 0.0107
0.0185 0.0216 0.0249 0.0193
2.31 2.10 2.35 2.03
T. Kobayashi et al. / Solid State Ionics 138 (2001) 243 – 251
sH [H] mH t H 5 ]]] 5 ]]]]] sH 1 sh [H] mH 1 [h] mh 1 5 ]]]] [h] mh 1 1 ]] [H] mH
(20)
where sH , [H] and mH are the conductivity, the concentration and the mobility of protons, respectively, and sh , [h], mh are those of holes. The ratio of the mobility of holes to that of protons ( mh /mH ) was calculated by Eq. (20) using the current efficiency and the average concentrations of holes and protons listed in Table 1. The results are also listed in Table 1. The values of ( mh /mH ) estimated under several experimental conditions were almost the same, about 2.2. The mobilities of protons and holes in SrCe 0.95 Yb 0.05 O 32 a [23,24] and in SrZr 12xYb x O 32 a reported previously are shown in Fig. 7. The mobility of protons in SrZr 12xYb x O 32 a was calculated using the conductivity and the concentration of protons reported by Yajima et al. [11] and Hibino et
Fig. 7. Mobilities of protons and holes in SrCe 0.95 Yb 0.05 O 32 a and SrZr 0.9 Yb 0.1 O 32 a .
249
al. [25]. The value of ( mh /mH ) estimated in this study is shown by the open circles in the figure and was nearly equal to the data reported previously by other research groups. Therefore, the interpretation that H 2 is evolved from the inside of the electrolyte and that holes were formed during steam electrolysis seems to be reasonable.
3.3. Dependence of current efficiency on partial pressure of water vapor and temperature In order to estimate the dependence of the current efficiency of steam electrolysis on the partial pressure of water vapor, reaction (4) was considered instead of the reactions in the electrodes, because the current of the oxide ion could be ignored in these experimental conditions. Eq. (21) was obtained by inserting Eq. (20) into Eq. (8) which gives the equilibrium constant K4 of reaction (4). 1 t H 5 ]]]]]] ]] 1/2 PO2 mh 1 1 ] ]] mH K4 PH 2 O
S œ D
(21)
In this study, K4 means the equilibrium constant determining the average concentrations of charge carriers under the steam electrolysis conditions. Because the K4 is different from the equilibrium constant under the static state, it is called the biasedequilibrium parameter K 49 in this paper. t H is the effective transport number of protons and corresponds to the current efficiency of steam electrolysis. Therefore, the dependence of the current efficiency of steam electrolysis on the partial pressure of water vapor can be calculated using Eq. (21). The wet argon gases containing 0.4% O 2 were fed to the anode in these experiments, and the change in the concentration of O 2 due to O 2 -production at the anode by steam electrolysis was small (less than 0.1%). When PO 2 and mh /mH are 0.004 and 2.2 (see Table 1), the results obtained by the calculation using Eq. (21) with various values of K 49 are shown in Fig. 8. The calculated line with K 49 of 3.4 is in good agreement with the experimental data at the current density of 2.4 mA cm 22 shown by the open circles in the figure. As described above, when steam electrolysis was conducted at relatively low temperatures, the average
250
T. Kobayashi et al. / Solid State Ionics 138 (2001) 243 – 251
Fig. 8. Dependence of current efficiency of steam electrolysis on partial pressure of water vapor. Temp., 4608C; and current density: 2.4 mA cm 22 . Open circles, experimental data; lines, calculated results using Eq. (21).
concentrations of the charge carriers in the electrolyte were controlled by the difference in the rates of reactions for producing protons and holes at the anode rather than the equilibria between the atmosphere and defects in the electrolyte. However, K 49 was constant at various partial pressures of water vapor even under the steam electrolysis conditions with the constant current used in these experiments. It is very interesting that Eq. (8) can also hold even under steam electrolysis conditions at low temperatures. The temperature dependence of K 49 was obtained by inserting data at 2.4 mA cm 22 of Fig. 3 into Eq. (21). In this calculation, the mobilities mH and mh at each temperature were estimated as shown in Fig. 7. The results are shown in Fig. 9, and K 49 increased with increasing temperature. This result also shows that the average concentration of protons [H i? ] under steam electrolysis condition increases with increasing temperature, because the reaction rate of Eqs. (9) or (4) increases with temperature. It was found in this study that the effective transport number of protons is controlled by the reaction rates at the electrode under steam electrolysis conditions at low temperatures, although the number is almost unity in the static state such as under ac impedance measurement at below 6008C [11,13]. The above consideration is very important for
Fig. 9. Arrhenius plot of biased-equilibrium parameter K 94 for SrZr 0.9 Yb 0.1 O 32 a under steam electrolysis conditions.
practical use of the cell developed in our study. Although the dependence of the current efficiency of steam electrolysis on the partial pressure of water vapor was considered in this paper, it is necessary to consider the effect of the partial pressure of O 2 on the current efficiency. Furthermore, if the partial pressure of water vapor is 1 atm, the current efficiency is estimated to be about 76% for the cell used in these experiments. The cell with Pt-plated electrodes reported previously [18], however, has shown an efficiency higher than 90% when PH 2 O is about 1 atm. The rate of reaction (9) and the parameter K 49 depend strongly on the characteristics of the electrodes as well as the electrolyte. Extensive research on electrodes for promoting the oxidation of water vapor into protons and O 2 would be necessary for improvement of the current efficiency of the steam electrolysis.
4. Conclusions A steam electrolysis cell was constructed using a partial protonic conductor SrZr 0.9 Yb 0.1 O 32 a as an electrolyte and Pt cermet electrodes which were
T. Kobayashi et al. / Solid State Ionics 138 (2001) 243 – 251
made from Pt pastes and a fine powder of SrZr 0.9 Yb 0.1 O 32 a . The current efficiency of steam electrolysis was measured using this cell at relatively low temperatures (460–6008C). The cause of nonefficiency was considered. The following results were obtained. 1. The current efficiency of steam electrolysis increased with increasing partial pressure of water vapor and temperature and decreased with increasing current density. 2. By applying direct current to the cell in dry pure argon gas (not steam electrolysis), it was found that protons dissolved in the partial protonic conductor were converted into H 2 gas and that holes were produced. 3. Under steam electrolysis conditions at low temperatures, the reaction rate of producing H 2 gas from protons at the cathode was found to be faster than that of oxidizing water vapor into protons and O 2 at the anode, and holes were formed at the anode. Therefore, the average concentrations of protons and holes in the partial protonic conductor decreased and increased, respectively. This is the reason for the non-efficiency of steam electrolysis. 4. The effective transport numbers of charge carriers in the partial protonic conductor were estimated to be controlled by the reaction rates at the electrodes under steam electrolysis conditions at relatively low temperatures where the equilibria between the atmosphere and defects in the partial protonic conductor were difficult to obtain. 5. When the dependence of the current efficiency of steam electrolysis on the partial pressure of water vapor was calculated, the experimental data were in good agreement with the calculated data using the biased-equilibrium parameter, which was controlled by the reaction rates at the electrodes.
Acknowledgements The authors are grateful to TYK Corp. for supplying the electrolytes.
251
References [1] H. Iwahara, Solid State Ionics 86–88 (1996) 9. [2] H. Iwahara, H. Uchida, K. Ogaki, H. Nagato, J. Electrochem. Soc. 138 (1991) 295. [3] T. Yajima, K. Koide, H. Takai, N. Fukatsu, H. Iwahara, Solid State Ionics 79 (1995) 333. [4] N. Fukatsu, N. Kurita, T. Ohashi, K. Koide, Solid State Ionics 113–115 (1998) 219. [5] H. Iwahara, T. Yajima, T. Hibino, H. Ushida, J. Electrochem. Soc. 140 (1993) 1687. [6] N. Bonanos, B. Ellis, M.N. Mahmood, Solid State Ionics 44 (1991) 305. [7] N. Taniguchi, E. Yasumoto, T. Gamo, J. Electrochem. Soc. 143 (1996) 1886. [8] S. Hamakawa, T. Hibino, H. Iwahara, J. Electrochem. Soc. 141 (1994) 1720. [9] T. Hibino, S. Hamakawa, T. Suzuki, H. Iwahara, J. Appl. Electrochem. 24 (1994) 126. [10] H. Uchida, N. Maeda, H. Iwahara, Solid State Ionics 11 (1983) 117. [11] T. Yajima, H. Suzuki, H. Iwahara, The Electrochem. Soc. Japan, 58th Meeting (Japan) (1991) 247. [12] T. Yajima, H. Suzuki, T. Yogo, H. Iwahara, Solid State Ionics 51 (1992) 101. [13] N. Kurita, N. Fukatsu, K. Ito, T. Ohashi, J. Electrochem. Soc. 142 (1995) 1552. [14] M.J. Scholten, J. Schoonman, J.C. Miltenburg, H.A.J. Oonk, The Electrochem. Soc. 183rd Meeting (Honolulu), 93(1) (1993) 1625. [15] M.J. Scholten, J. Schoonman, J.C. Miltenburg, H.A.J. Oonk, Solid State Ionics 61 (1993) 83. [16] K.D. Kreuer, Solid State Ionics 97 (1997) 1. [17] H. Iwahara, T. Yajima, T. Hibino, K. Ozaki, H. Suzuki, Solid State Ionics 61 (1993) 65. [18] T. Kobayashi, S. Morishita, K. Abe, H. Iwahara, Solid State Ionics 86–88 (1996) 603. [19] K. Sasaki, J. Maier, J. Appl. Phys. 86 (1999) 5422. [20] K. Sasaki, J. Maier, J. Appl. Phys. 86 (1999) 5434. [21] K. Sasaki, J. Claus, J. Maier, Solid State Ionics 121 (1999) 51. [22] K.D. Kreuer, Solid State Ionics 125 (1999) 285. [23] T. Yajima, H. Iwahara, Solid State Ionics 53–56 (1992) 983. [24] H. Uchida, H. Yoshikawa, T. Esaka, S. Ohtsu, H. Iwahara, Solid State Ionics 36 (1989) 89. [25] T. Hibino, K. Mizutani, T. Yajima, H. Iwahara, Solid State Ionics 57 (1992) 303.
Study on current efficiency of steam electrolysis using a partial protonic conductor SrZr 0.9 Yb 0.1 O 32 a a, a a b Tetsuro Kobayashi *, Katsushi Abe , Yoshio Ukyo , Hiroshige Matsumoto a
b
Toyota Central R& D Labs., Inc., Nagakute, Aichi 480 -1192, Japan Center for Integrated Research in Science and Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464 -8603, Japan Received 15 February 2000; received in revised form 29 August 2000; accepted 5 October 2000
Abstract The current efficiency of steam electrolysis was measured in the temperature range from 460 to 6008C using a steam electrolysis cell constructed with a partial protonic conductor SrZr 0.9 Yb 0.1 O 32 a as an electrolyte and Pt cermet electrodes. The efficiency was increased with increasing partial pressure of water vapor and temperature. The results were considered in relation to the reaction rates at the anode and cathode. Under the operating conditions of steam electrolysis, the reaction rate of producing H 2 from protons at the cathode was found to be faster than that of oxidizing water vapor into protons and O 2 at the anode. Therefore, the average concentration of protons in the partial protonic conductor during electrolysis decreased. On the other hand, the average concentration of holes increased. This is considered to decrease the efficiency of steam electrolysis. It was found that the effective transport numbers of charge carriers in the partial protonic conductor were controlled by the reaction rates at the electrodes at relatively low temperatures at which the equilibria between the atmosphere and defects in the partial protonic conductor were difficult to obtain. 2001 Elsevier Science B.V. All rights reserved. Keywords: Protonic conductor; Steam electrolysis; Current efficiency; Hole; Transport number
1. Introduction Some perovskite-type oxides based on SrCeO 3 , BaCeO 3 , CaZrO 3 , SrZrO 3 and BaZrO 3 such as SrCe 0.95 Yb 0.05 O 32 a and SrZr 0.9 Yb 0.1 O 32 a , in which a part of the Ce or Zr is substituted with trivalent cations of IIIA, IIIB and rare earth elements, exhibit good protonic conduction (10 23 S cm 21 ) around 7008C [1]. These materials with protonic conduction *Corresponding author. Tel.: 181-561-63-5364; fax: 181-56163-6136. E-mail address: [email protected] (T. Kobayashi).
have been studied widely for their applications as electrolytes for hydrogen sensors [2–4], SOFCs [5– 7] and gas reactors [8,9]. These protonic conductors have oxide ion vacancies. The equilibria between oxide ion vacancies (V ??O ), protons (H ?i ), holes (h ? ), H 2 , O 2 and H 2 O can, for instance, be expressed according to Eqs. (1)–(4), K1 1 → H ?i ] H2 1 h? ← 2
(1)
K2 1 → 2h ? 1 O O3 ]O 2 1 V O?? ← 2
(2)
0167-2738 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00793-1
T. Kobayashi et al. / Solid State Ionics 138 (2001) 243 – 251
244 K3
→ 2H ?i 1 O 3 H 2 O 1 V ??O ← O
(3)
K4 1 → 2H i? 1 ]O 2 H 2 O 1 2h ? ← 2
(4)
where O 3 O and K denote the oxide ion in the normal lattice site and the equilibrium constant, respectively. Each equilibrium constant was obtained by Eqs. (5)–(8), where [O 3 O ] was assumed to be unity. [H ?i ] K1 5 ]]] [h ? ]P H1 /22
(5)
[h ? ] 2 K2 5 ]]] [V ??O ]P 1O/22
(6)
[H ?i ] 2 K3 5 ]]] [V ??O ]PH 2 O
(7)
? 2
1/2
[H i ] P O 2 K3 K4 5 ]]] 5] ? 2 [h ] PH O K2
(8)
2
Protons (H ?i ), holes (h ? ) and oxide ion vacancies (V ??O ) are the main charge carriers in these partial protonic conductors [10]. There are some reports on the transport numbers of the charge carriers in these partial protonic conductors. Yajima et al. have measured the transport numbers of the charge carriers in SrZr 0.9 Yb 0.1 O 32 a in air containing 9.2 Torr H 2 O by an ac impedance technique and shown that hole conduction in SrZr 0.9 Yb 0.1 O 32 a is dominant at temperatures higher than 7008C and that protonic conduction is predominant at temperatures lower than 7008C [11]. However, they have also shown that the transport number of protons in SrZr 0.9 Yb 0.1 O 32 a is almost unity in the high temperature range from 600 to 10008C by experiments on hydrogen permeation and hydrogen sensors [12]. Iwahara and co-workers have measured the transport numbers of the charge carriers in BaCe 12x Sm x O 32 a under the operating conditions for a fuel cell. They have shown that oxide ion conduction and protonic conduction in BaCe 12x Sm x O 32 a are dominant at temperatures higher and lower than 8008C, respectively [5]. They have also demonstrated that hydrogen sensors using BaCe 0.9 Nd 0.1 O 32 a show a theoretical electromotive force (EMF) and that the transport number of ions in
this material is estimated to be almost unity in the temperature range from 200 to 9008C [2]. Kurita et al. have reported the protonic conduction domain of CaZr 0.9 In 0.1 O 32 a in detail using an ac impedance technique. They have shown, for example, that protonic conduction is dominant at partial pressures of water vapor above 0.01 atm and temperatures below 6008C even under a partial pressure of oxygen of nearly 1 atm [13]. Summarizing the above studies, the transport number of protons in these partial protonic conductors is almost unity in atmospheres with hydrogen sources (water vapor or hydrogen gas) which are oxygen-free. In atmospheres containing oxygen, hole conduction or oxide ion conduction becomes dominant at temperatures higher than 7008C. However, even though atmospheres contain oxygen, it is thought that the transport number of protons becomes significant at temperatures lower than 7008C in the presence of a hydrogen source such as water vapor [5,11,13]. We have constructed a steam electrolysis cell using a partial protonic conductor SrZr 0.9 Yb 0.1 O 32 a . SrZr 0.9 Yb 0.1 O 32 a is chemically more stable than other partial protonic conductors based on SrCeO 3 and BaCeO 3 in an atmosphere containing CO 2 [14– 16], and its protonic conductivity is higher than that of partial protonic conductors based on CaZrO 3 and BaZrO 3 [17]. We have demonstrated using the steam electrolysis cell that hydrogen was produced by electrolyzing water vapor and that harmful nitrogen oxide (NO) was electrochemically reduced by the hydrogen as a reducing agent [18]. The temperature required for the operation of the steam electrolysis cell is from 400 to 6008C, which corresponds to the temperature of the exhaust gas of automobiles, although these partial protonic conductors are usually used at above 6008C at which they show high conductivity. From the results of previous reports [5,11,13], the protonic conduction is expected to be dominant in these partial protonic conductors even under O 2 -rich conditions at below 6008C. In this study, the current efficiency of steam electrolysis was measured in the temperature range from 460 to 6008C using a cell with SrZr 0.9 Yb 0.1 O 32 a as an electrolyte, and the effective transport numbers of the charge carriers in SrZr 0.9 Yb 0.1 O 32 a were examined under the oper-
T. Kobayashi et al. / Solid State Ionics 138 (2001) 243 – 251
ating conditions of steam electrolysis. In these experiments, the dependence of the current efficiency on the partial pressure of water vapor was examined when the partial pressure of O 2 gas was constant.
2. Experimental Fig. 1 shows a cross-section of the steam electrolysis cell. A closed end tube of sintered SrZr 0.9 Yb 0.1 O 32 a (supplied by TYK Co.) was used as a partial protonic conductor. The outside and inside diameters of the partial protonic conductor were 9 and 7 mm, respectively. The relative density of the sintered SrZr 0.9 Yb 0.1 O 32 a was about 96%. Pt cermet electrodes for the anode and cathode were prepared as follows. A fine powder of SrZr 0.9 Yb 0.1 O 32 a with a specific surface area of about 50 m 2 g 21 and Pt paste (U-3820, N.E. Chemcat Corp.) were mixed well and spread on the outside and inside of the partial protonic conductor and then calcined at 10008C for 3 h. The composition of these electrodes was 56 vol% of Pt and 44 vol% of SrZr 0.9 Yb 0.1 O 32 a . The height and area of the anode (outside) were 17.5 mm and about 5 cm 2 , respectively. The thickness of the electrodes was about 3 mm. The anode and cathode chambers were formed as shown in the figure. Water vapor was added to argon gas containing 0.4% O 2 by using two water-bubblers in series, and
Fig. 1. Cross-section of the experimental apparatus.
245
the mixture was then fed into the anode chamber at a rate of 30 ml min 21 . By controlling the temperature of the bubblers, the concentration of water vapor in the anode gas, which was measured by a humidity sensor, was controlled. All gas lines were heated higher than 1008C by ribbon heaters in order to prevent condensation of water vapor. Pure argon gas (the concentrations of O 2 and H 2 O as impurities were nominally less than 0.2 ppm) was fed to the cathode at a rate of 30 ml min 21 . When direct current was galvanostatically applied to the cell, water vapor was electrochemically oxidized into O 2 and protons at the anode, and protons were reduced to H 2 at the cathode. The concentrations of O 2 and H 2 were measured by gas chromatography. Experiments were carried out in the temperature range from 460 to 6008C under atmospheric pressure.
3. Results and discussions
3.1. Current efficiency of steam electrolysis Experimental runs of steam electrolysis were carried out at 4608C under the conditions with different partial pressures of water vapor (PH 2 O 5 0.032, 0.12 and 0.57 atm). The current density applied to the cells and the partial pressure of O 2 gas in the anode chamber were 2.4 mA cm 22 and 0.004 atm, respectively. Fig. 2 shows the dependence of the H 2 production rate on time during the application of direct current to the cell. The dotted line represents the theoretical H 2 production rate calculated using Faraday’s law when the current efficiency is assumed to be 100%. Although the H 2 production rate was nearly equal to the theoretical rate just after starting the electrolysis, it decreased with time and became almost constant after about 100 min. The H 2 production rates after becoming constant were larger under higher partial pressure of water vapor. Furthermore, the H 2 production rate was increased by changing the partial pressure of water vapor from 0.032 to 0.12 atm after the electrolysis was conducted under a partial pressure of 0.032 atm. It was found that the current efficiency of steam electrolysis, in other words, the effective transport number of protons, depended on the partial pressure of water vapor.
246
T. Kobayashi et al. / Solid State Ionics 138 (2001) 243 – 251
creased with increasing temperature. This result seems to contradict the results reported previously in which the protonic conduction becomes dominant at lower temperatures [5,11,13].
3.2. Analysis of non-efficiency of steam electrolysis
Fig. 2. Change in H 2 production rate during application of direct current to electrolysis cells under various partial pressures of water vapor. Temp., 4608C; and current density, 2.4 mA cm 22 . The dotted line represents 100% efficiency calculated by Faraday’s law.
About 100 min after direct current was applied to the cell, the H 2 production rate was measured in the temperature range from 460 to 6008C, when the partial pressures of O 2 and H 2 O in the anode gas were 0.004 and 0.12 atm, respectively. The results are shown in Fig. 3. The dotted line shows the theoretical H 2 production rate. The current efficiency decreased with increasing current densities and in-
Fig. 3. Change in H 2 production rate for steam electrolysis with current density at various temperatures. Partial pressure of water vapor: PH 2 O 50.12 atm. The dotted line represents 100% efficiency calculated by Faraday’s law.
Changes in the O 2 production rate at the anode and in the H 2 production rate at the cathode with time during steam electrolysis are shown in Fig. 4, when the temperature, current density and partial pressure of water vapor were 4608C, 2.4 mA cm 22 and 0.032 atm, respectively. The dotted line and dot-dashed line represent the theoretical H 2 and O 2 production rate, respectively, determined by Faraday’s law when the current efficiency is assumed to be 100%. The ratio of the amount of H 2 to that of O 2 produced should be 2:1. Less than 100 min after starting the electrolysis, however, the amount of O 2 produced was small compared with that of H 2 and the ratio was not 2:1. After more than 100 min, the ratio of H 2 to O 2 was almost 2:1. This result suggested that H 2 shown in the highlighted part in Fig. 4 was produced by a mechanism other than steam electrolysis. Fig. 5 shows the changes in the O 2 production rate at the anode and the H 2 production rate at the
Fig. 4. Changes in H 2 and O 2 production rate during application of direct current to the electrolysis cell. Temp., 4608C; and current density, 2.4 mA cm 22 . Partial pressure of water vapor: PH 2 O 5 0.032 atm. The dotted and dot-dashed lines represent 100% efficiency calculated by Faraday’s law.
T. Kobayashi et al. / Solid State Ionics 138 (2001) 243 – 251
2h ? 1 2e9 → 0
247
(15)
It is not certain at present if the hole current passes on the cathode side of the electrolyte where hydrogen is produced and the partial pressure of oxygen is very low. However, it can be assumed that the hole is in the partially frozen-in state [19–21], because the equilibria between the atmosphere and defects in the electrolyte are considered difficult to obtain at low temperatures such as 4608C. The anodic and cathodic current should be equal because the direct constant current was applied to the cell galvanostatically. Fig. 5. Changes in H 2 and O 2 production rate during application of direct current to the electrolysis cell in dry argon gas (not steam electrolysis) at 4608C.
cathode while direct current was applied to the cell and dry pure argon gas was introduced to both the anode and cathode (steam electrolysis was not carried out). Although H 2 was also produced, the production of O 2 was not observed at all. This H 2 production was estimated to be caused by the evolution of H 2 formed from protons, which had already dissolved in SrZr 0.9 Yb 0.1 O 32 a , by applying direct current. Therefore, the amount of H 2 in the highlighted part of Fig. 4 was also thought to be that of H 2 evolved from the electrolyte. The reactions at the interface between the anode and electrolyte during steam electrolysis may be expressed as Eqs. (9)–(11). 1 H 2 O(gas) → 2H ?i 1 2e9 1 ]O 2 (gas) 2
(9)
1 O3 O (gas) 1 V ??O 1 2e9 O →] 2 2
(10)
0 → 2h ? 1 2e9
(11)
The reactions at the interface between the cathode and electrolyte are, e.g., Eqs. (12)–(15). 2H ?i 1 2e9 → H 2 (gas)
(12)
1 ]O 2 (gas) 1 V O?? 1 2e9 → O O3 2
(13)
H 2 O(gas) 1 V O?? 1 2e9 → H 2 (gas) 1 O O3
(14)
I9 1 I10 1 I11 5 I12 1 I13 1 I14 1 I15
(16)
where I denotes the value of current and the subscripts show the equation number of the reactions. When steam electrolysis is at a steady state, for example, after 100 min in Fig. 2, Eqs. (17)–(19) should hold. I9 5 I12
(17)
I10 5 I13 1 I14
(18)
I11 5 I15
(19)
Eqs. (17)–(19) correspond to the proton, oxide ion and hole currents in the electrolyte. In these experiments, however, I13 and I14 were estimated to be less 24 24 22 than 3310 and 1.5310 mA cm , respectively, because the concentration of O 2 and H 2 O contained as impurities in the cathode gas was nominally less than 0.2 ppm. Therefore, the oxide ion current might be ignored in these experiments, and the decrease in the efficiency of steam electrolysis might be attributed to the hole current. As shown in Fig. 4, just after applying the direct current, reaction (12) was dominant rather than reaction (15) at the cathode, because the H 2 -production rate was nearly equal to the theoretical value. At the anode, the O 2 -production rate was only 25% of the theoretical value; therefore, at most, 25% of the current applied to the cell was estimated to cause reaction (9) which shows ‘steam electrolysis’, even if the production of O 2 by reaction (10) is considered
248
T. Kobayashi et al. / Solid State Ionics 138 (2001) 243 – 251
to be zero. The rest of the current applied to the cell might be consumed by reaction (11) for producing holes at the anode. The O 2 -production rate slightly increased between 10 and 100 min as shown in Fig. 4 and reached only 38% of the theoretical value even in the steady state. On the other hand, the H 2 -production rate decreased rapidly in the transient state. Because the rate of reaction (9) for producing protons at the anode side of the electrolyte was slow, the rate of reaction (12) decreased and the rate of reaction (15) inversely increased. (The total current was always constant.) In other words, the rate of reaction (12) for producing H 2 gas from protons at the cathode was faster than that of reaction (9) for oxidizing water vapor into protons and O 2 at the anode and, therefore, holes were formed at the anode. Therefore, the average concentrations of protons and holes in the electrolyte decreased and increased, respectively. This is the reason why the efficiency of steam electrolysis is not 100%. This phenomenon is illustrated in Fig. 6 and seems to suggest that reaction (1) proceeds to the left side on the application of direct current. This figure also shows schematically the dissolution of water vapor into the electrolyte by reaction (3). When the atmosphere and defects in the electrolyte are at equilibrium, the concentration of protons is considered to be high, because the H 2 production rate was high just after starting the electrolysis. If reaction (4) proceeds fast, the concentration of protons may increase even though holes are produced by reaction (11). Therefore, the phe-
nomenon shown at the right in Fig. 6 can be caused at relatively low temperatures such as 4608C where the equilibria between the atmosphere and defects in the electrolyte may be difficult to obtain. The average concentrations of protons and holes under the steady state of steam electrolysis can be calculated roughly as follows. In this experiment, the volume of the electrolyte concerned with the steam electrolysis was 0.44 cm 3 . The theoretical density and molecular weight of SrZr 0.9 Yb 0.1 O 32 a were 5.6 g cm 23 and 234.22, respectively, and the relative density of the sintered electrolyte was 96%. Therefore, the amount of the electrolyte concerned with the electrolysis was calculated to be 0.01 mol. The amount of holes formed was estimated from the amount of evolved H 2 shown in the highlighted part of Fig. 4, according to Eq. (1). Dividing the amount of holes by the amount of the electrolyte gives the average concentration (mol mol 21 ) of holes formed in the electrolyte. Kreuer has reported that the concentration of protons dissolved in SrZr 0.9 Y 0.1 O 32 a is about 3 mol%, i.e., 0.03 mol mol 21 below 5008C [22]. When the electrolysis reached the steady state, the average concentration of protons contained in the electrolyte was calculated by subtracting the average concentration of holes formed from 0.03 mol mol 21 given by Kreuer [22]. These results are listed in Table 1. Furthermore, the current efficiency of steam electrolysis, i.e., the effective transport number of protons (t H ) in the steady state is expressed by Eq. (20), because the charge carriers were only protons and holes.
Table 1 Estimated values of effective transport number of protons t H , average concentration of holes [h] and protons [H], and mobility ratio of holes to protons mh /mH in SrZr 0.9 Yb 0.1 O 32 a at 4608C under various steam electrolysis conditions Steam electrolysis condition
Fig. 6. Illustrations of dissolution of water vapor into partial protonic conductor (left one) and replacement of protons with holes by application of direct current (right one).
PH 2 O
Current (mA cm 22 )
0.032 0.12 0.57 0.12
2.4 2.4 2.4 4.0
tH
[h] (mol mol 21 )
[H] (mol mol 21 )
mh /mH
0.41 0.55 0.68 0.47
0.0115 0.0084 0.0051 0.0107
0.0185 0.0216 0.0249 0.0193
2.31 2.10 2.35 2.03
T. Kobayashi et al. / Solid State Ionics 138 (2001) 243 – 251
sH [H] mH t H 5 ]]] 5 ]]]]] sH 1 sh [H] mH 1 [h] mh 1 5 ]]]] [h] mh 1 1 ]] [H] mH
(20)
where sH , [H] and mH are the conductivity, the concentration and the mobility of protons, respectively, and sh , [h], mh are those of holes. The ratio of the mobility of holes to that of protons ( mh /mH ) was calculated by Eq. (20) using the current efficiency and the average concentrations of holes and protons listed in Table 1. The results are also listed in Table 1. The values of ( mh /mH ) estimated under several experimental conditions were almost the same, about 2.2. The mobilities of protons and holes in SrCe 0.95 Yb 0.05 O 32 a [23,24] and in SrZr 12xYb x O 32 a reported previously are shown in Fig. 7. The mobility of protons in SrZr 12xYb x O 32 a was calculated using the conductivity and the concentration of protons reported by Yajima et al. [11] and Hibino et
Fig. 7. Mobilities of protons and holes in SrCe 0.95 Yb 0.05 O 32 a and SrZr 0.9 Yb 0.1 O 32 a .
249
al. [25]. The value of ( mh /mH ) estimated in this study is shown by the open circles in the figure and was nearly equal to the data reported previously by other research groups. Therefore, the interpretation that H 2 is evolved from the inside of the electrolyte and that holes were formed during steam electrolysis seems to be reasonable.
3.3. Dependence of current efficiency on partial pressure of water vapor and temperature In order to estimate the dependence of the current efficiency of steam electrolysis on the partial pressure of water vapor, reaction (4) was considered instead of the reactions in the electrodes, because the current of the oxide ion could be ignored in these experimental conditions. Eq. (21) was obtained by inserting Eq. (20) into Eq. (8) which gives the equilibrium constant K4 of reaction (4). 1 t H 5 ]]]]]] ]] 1/2 PO2 mh 1 1 ] ]] mH K4 PH 2 O
S œ D
(21)
In this study, K4 means the equilibrium constant determining the average concentrations of charge carriers under the steam electrolysis conditions. Because the K4 is different from the equilibrium constant under the static state, it is called the biasedequilibrium parameter K 49 in this paper. t H is the effective transport number of protons and corresponds to the current efficiency of steam electrolysis. Therefore, the dependence of the current efficiency of steam electrolysis on the partial pressure of water vapor can be calculated using Eq. (21). The wet argon gases containing 0.4% O 2 were fed to the anode in these experiments, and the change in the concentration of O 2 due to O 2 -production at the anode by steam electrolysis was small (less than 0.1%). When PO 2 and mh /mH are 0.004 and 2.2 (see Table 1), the results obtained by the calculation using Eq. (21) with various values of K 49 are shown in Fig. 8. The calculated line with K 49 of 3.4 is in good agreement with the experimental data at the current density of 2.4 mA cm 22 shown by the open circles in the figure. As described above, when steam electrolysis was conducted at relatively low temperatures, the average
250
T. Kobayashi et al. / Solid State Ionics 138 (2001) 243 – 251
Fig. 8. Dependence of current efficiency of steam electrolysis on partial pressure of water vapor. Temp., 4608C; and current density: 2.4 mA cm 22 . Open circles, experimental data; lines, calculated results using Eq. (21).
concentrations of the charge carriers in the electrolyte were controlled by the difference in the rates of reactions for producing protons and holes at the anode rather than the equilibria between the atmosphere and defects in the electrolyte. However, K 49 was constant at various partial pressures of water vapor even under the steam electrolysis conditions with the constant current used in these experiments. It is very interesting that Eq. (8) can also hold even under steam electrolysis conditions at low temperatures. The temperature dependence of K 49 was obtained by inserting data at 2.4 mA cm 22 of Fig. 3 into Eq. (21). In this calculation, the mobilities mH and mh at each temperature were estimated as shown in Fig. 7. The results are shown in Fig. 9, and K 49 increased with increasing temperature. This result also shows that the average concentration of protons [H i? ] under steam electrolysis condition increases with increasing temperature, because the reaction rate of Eqs. (9) or (4) increases with temperature. It was found in this study that the effective transport number of protons is controlled by the reaction rates at the electrode under steam electrolysis conditions at low temperatures, although the number is almost unity in the static state such as under ac impedance measurement at below 6008C [11,13]. The above consideration is very important for
Fig. 9. Arrhenius plot of biased-equilibrium parameter K 94 for SrZr 0.9 Yb 0.1 O 32 a under steam electrolysis conditions.
practical use of the cell developed in our study. Although the dependence of the current efficiency of steam electrolysis on the partial pressure of water vapor was considered in this paper, it is necessary to consider the effect of the partial pressure of O 2 on the current efficiency. Furthermore, if the partial pressure of water vapor is 1 atm, the current efficiency is estimated to be about 76% for the cell used in these experiments. The cell with Pt-plated electrodes reported previously [18], however, has shown an efficiency higher than 90% when PH 2 O is about 1 atm. The rate of reaction (9) and the parameter K 49 depend strongly on the characteristics of the electrodes as well as the electrolyte. Extensive research on electrodes for promoting the oxidation of water vapor into protons and O 2 would be necessary for improvement of the current efficiency of the steam electrolysis.
4. Conclusions A steam electrolysis cell was constructed using a partial protonic conductor SrZr 0.9 Yb 0.1 O 32 a as an electrolyte and Pt cermet electrodes which were
T. Kobayashi et al. / Solid State Ionics 138 (2001) 243 – 251
made from Pt pastes and a fine powder of SrZr 0.9 Yb 0.1 O 32 a . The current efficiency of steam electrolysis was measured using this cell at relatively low temperatures (460–6008C). The cause of nonefficiency was considered. The following results were obtained. 1. The current efficiency of steam electrolysis increased with increasing partial pressure of water vapor and temperature and decreased with increasing current density. 2. By applying direct current to the cell in dry pure argon gas (not steam electrolysis), it was found that protons dissolved in the partial protonic conductor were converted into H 2 gas and that holes were produced. 3. Under steam electrolysis conditions at low temperatures, the reaction rate of producing H 2 gas from protons at the cathode was found to be faster than that of oxidizing water vapor into protons and O 2 at the anode, and holes were formed at the anode. Therefore, the average concentrations of protons and holes in the partial protonic conductor decreased and increased, respectively. This is the reason for the non-efficiency of steam electrolysis. 4. The effective transport numbers of charge carriers in the partial protonic conductor were estimated to be controlled by the reaction rates at the electrodes under steam electrolysis conditions at relatively low temperatures where the equilibria between the atmosphere and defects in the partial protonic conductor were difficult to obtain. 5. When the dependence of the current efficiency of steam electrolysis on the partial pressure of water vapor was calculated, the experimental data were in good agreement with the calculated data using the biased-equilibrium parameter, which was controlled by the reaction rates at the electrodes.
Acknowledgements The authors are grateful to TYK Corp. for supplying the electrolytes.
251
References [1] H. Iwahara, Solid State Ionics 86–88 (1996) 9. [2] H. Iwahara, H. Uchida, K. Ogaki, H. Nagato, J. Electrochem. Soc. 138 (1991) 295. [3] T. Yajima, K. Koide, H. Takai, N. Fukatsu, H. Iwahara, Solid State Ionics 79 (1995) 333. [4] N. Fukatsu, N. Kurita, T. Ohashi, K. Koide, Solid State Ionics 113–115 (1998) 219. [5] H. Iwahara, T. Yajima, T. Hibino, H. Ushida, J. Electrochem. Soc. 140 (1993) 1687. [6] N. Bonanos, B. Ellis, M.N. Mahmood, Solid State Ionics 44 (1991) 305. [7] N. Taniguchi, E. Yasumoto, T. Gamo, J. Electrochem. Soc. 143 (1996) 1886. [8] S. Hamakawa, T. Hibino, H. Iwahara, J. Electrochem. Soc. 141 (1994) 1720. [9] T. Hibino, S. Hamakawa, T. Suzuki, H. Iwahara, J. Appl. Electrochem. 24 (1994) 126. [10] H. Uchida, N. Maeda, H. Iwahara, Solid State Ionics 11 (1983) 117. [11] T. Yajima, H. Suzuki, H. Iwahara, The Electrochem. Soc. Japan, 58th Meeting (Japan) (1991) 247. [12] T. Yajima, H. Suzuki, T. Yogo, H. Iwahara, Solid State Ionics 51 (1992) 101. [13] N. Kurita, N. Fukatsu, K. Ito, T. Ohashi, J. Electrochem. Soc. 142 (1995) 1552. [14] M.J. Scholten, J. Schoonman, J.C. Miltenburg, H.A.J. Oonk, The Electrochem. Soc. 183rd Meeting (Honolulu), 93(1) (1993) 1625. [15] M.J. Scholten, J. Schoonman, J.C. Miltenburg, H.A.J. Oonk, Solid State Ionics 61 (1993) 83. [16] K.D. Kreuer, Solid State Ionics 97 (1997) 1. [17] H. Iwahara, T. Yajima, T. Hibino, K. Ozaki, H. Suzuki, Solid State Ionics 61 (1993) 65. [18] T. Kobayashi, S. Morishita, K. Abe, H. Iwahara, Solid State Ionics 86–88 (1996) 603. [19] K. Sasaki, J. Maier, J. Appl. Phys. 86 (1999) 5422. [20] K. Sasaki, J. Maier, J. Appl. Phys. 86 (1999) 5434. [21] K. Sasaki, J. Claus, J. Maier, Solid State Ionics 121 (1999) 51. [22] K.D. Kreuer, Solid State Ionics 125 (1999) 285. [23] T. Yajima, H. Iwahara, Solid State Ionics 53–56 (1992) 983. [24] H. Uchida, H. Yoshikawa, T. Esaka, S. Ohtsu, H. Iwahara, Solid State Ionics 36 (1989) 89. [25] T. Hibino, K. Mizutani, T. Yajima, H. Iwahara, Solid State Ionics 57 (1992) 303.