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Electrical conduction and mass transport properties of SrZr0.99Fe0.01O3 − δ
Solid State Ionics 181 (2010) 868–873
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Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Electrical conduction and mass transport properties of SrZr0.99Fe0.01O3 − δ Atsushi Unemoto a,⁎, Atsushi Kaimai b, Kazuhisa Sato c, Naoto Kitamura d, Keiji Yashiro c, Hiroshige Matsumoto e, Junichiro Mizusaki c, Koji Amezawa b, Tatsuya Kawada b a
Graduate School of Engineering, Tohoku University, 6-6 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Graduate School of Environmental Studies, Tohoku University, 6-6 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8579, Japan d Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda 278-8510, Japan e Inamori Frontier Research Center, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan b c
a r t i c l e
i n f o
Article history: Received 24 March 2010 Received in revised form 1 May 2010 Accepted 4 May 2010 Keywords: SrZrO3 Proton conductor Transition metal Mixed conduction Hydrogen evolution
a b s t r a c t The electrical conductivity of SrZr0.99Fe0.01O3 − δ was evaluated by a four-probe ac technique. The measurements were conducted in hydrogen and in oxygen containing atmospheres at 823 ≤ T / K ≤ 1273. It was found from the X-ray absorption spectroscopic measurements that Fe in the oxide is trivalent both in hydrogen and in oxygen. In order to determine the major carrier in the oxide, gas partial pressure dependences and isotope effect of hydrogen and deuteron on the electrical conductivity was investigated. In humidified hydrogen, it was found that proton conduction is predominant in the lower temperature region while oxide ion conduction starts to contribute to the total by increasing temperature of above 1173 K. In oxygen containing gas, the protonic conduction is found to be predominant at lower temperatures while the contribution of the electron hole conduction is significant at higher temperatures. Hydrogen evolution property was evaluated using the SrZr0.99Fe0.01O3 − δ disc as a solid electrolyte. Hydrogen evolution rate obeyed the Faraday's law in humidified hydrogen at 1173 K, suggesting that the transport number of ionic species is unity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Acceptor-doped perovskite-type oxides such as cerates [1], zirconates [2,3] and so on are known to significantly conduct protons in humidified atmospheres at high temperatures. Thus, this kind of materials provides various possible applications [4–6]. Zirconates such as SrZrO3, BaZrO3 and CaZrO3 are promising candidates as high-temperature solid electrolytes especially for hydrogen separation from syngas [2,3]. This is because the above oxides are known to have high tolerance against ambient gases such as water vapor and carbon dioxide contrary to proton conducting cerates [7–10]. Matsumoto and his co-workers have investigated the electrical hydrogen pumping properties using SrZrO3-based solid electrolytes [9–11], and been working for further decrease of the electrode resistances [8,11]. In recent years, Matsumoto et al. have also reported that an additional substitution of Ru-ion into B-site of Sr(Zr,Y)O3 allows the oxide to have mixed proton and electron hole conductivity, resulting in hydrogen permeation across the membranes [12]. In our recent work, ceramic membranes consisting of proton conducting and electron conducting oxide composites were proposed as a novel concept for hydrogen permeation membranes [13]. In order to prove the validity of the concept, we chose the system, SrZrO3–SrFeO3. ⁎ Corresponding author. Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Japan. Tel.: +81 22 217 5816; fax: +81 22 795 5211. E-mail address: [email protected] (A. Unemoto). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.05.002
Since Sasamoto and his co-workers have reported that the solubility of SrFeO3 into SrZrO3 is small [14], we expected that the SrFeO3 phase and SrZrO3 phase are separated. In those oxide composite membranes, it was expected that Sr(Zr,Fe)O3 phase and SrFeO3 phase individually provide pathways for protons and electrons, respectively. Although hydrogen concentration increased at the hydrogen permeation side as we expected, partial conductivity contributions of proton and oxide ion in Sr(Zr,Fe)O3 were not carefully examined both in that work and the literature [14]. Since the solubility limit of SrFeO3 into SrZrO3 at temperatures from 1923 to 2023 K was suggested to be at most 2.5 mol% [13], thus in this work, the electrical conductivity of a solid solution, SrZr0.99Fe0.01O3 − δ, has been evaluated with the four-probe ac technique. Based on the electrical conductivity as functions of gas partial pressures and temperatures, the electrical conduction properties have been discussed. X-ray absorption (XAS) measurements were also carried out to examine the valence of Fe in the oxide. At the last of this paper, the electrical hydrogen evolution has been demonstrated using SrZr0.99Fe0.01O3 − δ as an electrolyte. 2. Experimental 2.1. Preparation of sample powder Powder of SrZr0.99Fe0.01O3 − δ was prepared via a conventional solidstate reaction route. The starting powders, SrCO3 (99.99%, Rare Metallic Co., Ltd.), ZrO2 (TZ-0, Tosoh Corp.) and Fe2O3 (99.99%, Kojyundo Chemical
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Lab. Co., Ltd.), were precisely weighed and mixed by a planetary ball mill in ethanol at 300 rpm for an hour. They were calcined at 1473 K for 5h in air and crushed by a planetary ball mill in ethanol at 300 rpm for 3h. The calcination and milling were repeated for three times. The resultant powder was formed into discs, then, sintered at 2023 K for 5h in air. X-ray diffraction (XRD) patterns were taken for the calcined powder, sintered disc and the ones after annealing in unhumidified 1.0%H2 at 873 K and 1273 K. For microstructure observation by SEM (JSM-7001F, JEOL Ltd.) and composition analysis by EDX (Inca x-act, Oxford Instruments), the sintered compact was polished by a diamond paste of 3 μm, then, resintered at 1623 K for 3 h in air. 2.2. X-ray absorption spectroscopic measurements Powders of SrZr0.99Fe0.01O3 − δ were annealed individually in oxygen partial pressures of 0.21 atm (in air), 10− 15.73 atm (in 1.0%H2–1.9%H2O) and 10− 19.73 atm (in 98.1%H2–1.9%H2O) at 1173 K for 5h, and then, quenched down to room temperature. The each powder was mixed together with boron nitride (N 99%, Kojyundo Chemical Lab. Co., Ltd.) and then pressed into discs. As a reference, a disc consisting of commercial powder of Fe2O3 and boron nitride was also prepared. Spectra of the X-ray absorption near edge structure (XANES) of Fe K-edge were collected via a transmission mode at the beam line, BL-7C, of the synchrotron radiation facility, Photon Factory (High Energy Accelerator Research Organization, Institute of Materials Structure Science, Japan). 2.3. Electrical conductivity and hydrogen permeation measurements Electrical conductivity measurements were carried out with fourprobe ac technique. Dense disc was cut into a parallelepiped, then, platinum-paste (TR-7907, Tanaka Precious Metals Co., Ltd.) was painted as electrodes. The specimen was put into a single chamber in which four probes are connected to the sets of electro-analytical equipments (Electrochemical Interface SI1287–Impedance/Gain-Phase Analyser SI1260, Solartron Analytical). Electrical hydrogen pumping was demonstrated using a dense disc with porous platinum as electrodes. Platinum-paste was painted onto both surfaces of the disc, then, set to the two-chamber type apparatus. A detailed configuration with an illustration is provided in Ref. [13]. 98.1% H2–1.9%H2O and 98.1%Ar–1.9%H2O were introduced into anode side and cathode side, respectively. Reference electrode was exposed to air. Hydrogen concentration of the outlet gas from the cathode was analyzed by a gas chromatograph (3000A Micro G.C., Agilent Technologies). Then, it was converted into hydrogen evolution rate. 3. Results and discussion 3.1. Specimens XRD patterns of the calcined powder (B) and the sintered disc (C) of SrZr0.99Fe0.01O3 − δ are displayed in Fig. 1. Those of unpolished surfaces of the sintered discs, which were annealed in unhumidified 1.0%H2 at 1273 K for 3 h (D) and at 873 K for 10 h (E), are also provided. As shown in the figure, the patterns are well assigned to be literature data of SrZrO3 with orthorhombic structure (A) [15]. This suggests that the specimen stays as a single phase even in unhumidified hydrogen at temperatures from 873 K to 1273 K. SrZr0.99Fe0.01O3 − δ is expected to be stable during the following electrochemical measurements performed in hydrogen containing gas. Fig. 2 shows SEM image of a SrZr0.99Fe0.01O3 − δ re-sintered surface at 1623 K in air. The compact is found to consist of grains with diameter of less than 5 μm. The chemical composition of the oxide was analyzed by EDX. Since the compact was sintered at high temperature such as 2023 K, thus, there might be an argument that the cation ratio of the oxide may deviate from the mixing ratio of starting powders because the
Fig. 1. X-ray diffraction patterns of (A) SrZrO3 of literature data [12], (B) calcined powder and (C) sintered disc of SrZr0.99Fe0.01O3 − δ, unpolished surfaces of the sintered disc annealed in: (D) unhumidified 1.0% H2 for 3 h at 1273 K and (E) unhumidified 1.0% H2 for 12 h at 873 K.
oxide contains elements having high vapor pressure such as Sr- and Feoxides. The investigation was performed by EDX point analysis with above ten points both grain interior and boundary. It was found that the oxide contains Sr, Zr, Fe and Hf. Detection of Hf is reasonable because ZrO2 generally contains it as impurity. The atomic ratios of Sr, Zr, Hf and Fe were 50.1%, 47.3%, 1.8%, 0.8%, respectively, in average. This suggests that significant deviation from the mixing ratio of starting powders was not observed both in grain interior and along grain boundary. The oxide keeps the A-site to B-site ratio to be approximately unity. It should be noted here that the ratio of Fe is small, the above values may contain an uncertainty to some extent. However, at least from this analysis, Fe and Sr ratios are found to be close to mixing ratio of starting powders. Thus, we took the nominal composition for the following analysis of the electrical conductivity. Lattice parameters of the synthesized oxide were calculated from the XRD patterns, appearing in Fig. 1, to be a=5.7860, b=5.8151 and c=8.1960. These are similar values to non-dope SrZrO3, appearing in the literature (a=5.7862, b=5.8151 and c=8.1960 [15]). This insignificant different may be due to the small concentration of dopant, Fe. 3.2. General tendency of electrical conduction behavior Fig. 3 shows the total conductivity of SrZr0.99Fe0.01O3 − δ as a function of inverse temperature in various gas compositions. The electrical conductivities of SrZrO3 [14] and SrZr0.9Y0.1O3 − δ [16] in the preceding papers are also provided. As seen in the figure, the electrical conductivity of SrZr0.99Fe0.01O3 − δ is higher than that of SrZrO3. This suggests that Fedoping into B-site of SrZrO3 obviously enhances electrical conductivity of the oxide.
Fig. 2. SEM image of a SrZr0.99Fe0.01O3 − δ surface re-sintered at 1623 K in air.
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In our previous work, emf (electron motive force) measurements of hydrogen concentration cell using SrZr0.99Fe0.01O3 − δ as an electrolyte were performed in humidified hydrogen. It was found that the transport number of ionic species is unity in the temperature range from 923 K to 1173 K [13]. As shown in Fig. 3, in hydrogen containing atmospheres, the electrical conductivity is found to be sensitive to partial pressure of water vapor. The electrical conductivity measured in humidified hydrogen is higher than that in unhumidified hydrogen. This suggests that protonic conduction rather than oxide ion conduction is significant in SrZr0.99Fe0.01O3 − δ. In order to confirm if the major carrier is proton, isotope effect of hydrogen and deuteron on the electrical conductivity was investigated. Fig. 4(a) shows total conductivity in 1.0%H2–1.9%H2O and that in 1.0% D2–1.9%D2O as a function of inverse temperature. It was found that the total conductivity in 1.0%D2–1.9%D2O is found to be smaller than that in 1.0%H2–1.9%H2O as displayed in Fig. 4(b) in the entire temperature region investigated. However, under the condition that the PH2/PH2O and PD2/PD2O ratios are kept to be a constant as we did, the equilibrium oxygen partial pressure is different between H2–H2O and D2–D2O atmospheres. Thus, the dependence of the electrical conductivity on oxygen partial pressure needs to be taken into consideration in order to discuss the isotopic effect of hydrogen and deuteron on the electrical conductivity. Total conductivities in H2–1.9%H2O and D2–1.9%D2O at 1173 K and 933 K as a function of oxygen partial pressure are given in Fig. 4. It was found that the total conductivity was independent on oxygen partial pressure while the difference in total conductivity measured in H2–H2O and in D2–D2O was observed. That is, the difference of the electrical conductivity in H2–1.9%H2O and that in D2– 1.9%D2O observed in Fig. 3 is essentially caused by the isotopic effect, not by the oxygen partial pressure dependence of the total conductivity. Means, major carrier in SrZr0.99Fe0.01O3 − δ are protons. As shown in Fig. 4(b) that the difference of the electrical conductivities in H2–H2O and in D2–D2O is 1.66–1.30 at 823–1023 K. The experimental value exceeded the expected one by the classical pffiffiffi theory ( 2) [17] in the lower temperature range. This may be due to the difference of mass dependent O–H and O–D stretching frequencies, namely semi-classical theory, resulting in difference of the activation energies [17]. In the case of Sr(Zr,Fe)O3, the activation energies of protonic conduction and deuteron ion conduction were calculated to be
Fig. 3. Total conductivity of SrZr0.99Fe0.01O3 − δ as a function of inverse temperature in various gas compositions. Literature data regarding SrZrO3 in air [11] and SrZr0.9Y0.1O3 − δ in humidified hydrogen [13] are also provided.
Fig. 4. (a) Total conductivity of SrZr0.99Fe0.01O3 − δ in 1.0% H2–1.9% H2O and 1.0% D2– 1.9% D2O as a function of inverse temperature. (b) Ratio of electrical conductivity in 1.0% H2–1.9% H2O to that in 1.0% D2–1.9% D2O.
0.64 eV and 0.66 eV, respectively, by using broken lines in Fig. 4 (a). This difference is in the range of the expected value by semi-classical theory as in oxides [17] and phosphate [18]. The activation energy of proton migration is found to be slightly larger than single crystal of Sr(Zr,Y)O3 (0.45 eV [19]), Ba(Zr,RE)O3 (RE = rare earth, 0.43 – 0.50 eV [20]) and Ba(Ce,Y)O3 (0.48 – 0.63 eV [21]). It was found from Fig. 4(b) that the ratio becomes smaller as temperature increased. This indicates that proton concentration decreased with increasing temperature and consequently protonic conductivity decreased. Since the transport number of ionic species is unity in humidified hydrogen [13], it can be expected that the oxide ion conduction started to contribute to the total as temperature increased. Actually, in unhumidified hydrogen, the gradient of log(σT) vs. 1/T in the higher temperature region, TN 1173, was larger than that in the lower temperature region, Tb 973. This implies that the carrier species is different between the higher and the lower temperature regions similar to SrZr0.9Y0.1O3 − δ [16]. In oxygen containing atmospheres, dependence of total conductivity on water vapor pressure is found to be less significant than that in hydrogen containing atmospheres as temperature increased. The behavior suggests that the defect species other than protons works as carrier species at higher temperatures. As discussed in the previous paragraph, the contribution of the oxide ion conductivity is expected to be small as seen in hydrogen containing atmospheres. Thus it cannot explain the difference in electrical conductivity between hydrogen and oxygen containing atmospheres by considering the contribution of oxide ion conductivity. In addition, the difference of the electrical conductivities between hydrogen and oxygen is remarkable as temperature increased. Thus, it can be expected that the electron hole
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conduction is predominant in oxygen containing atmospheres in the higher temperature region similar to the case of SrZr0.9Y0.1O3 − δ [16]. In the lower temperature region, however, dependence of the electrical conductivity on partial pressure of water vapor is significant. The electrical conductivity in humidified 1.0% O2 was higher than that in unhumidified 1.0% O2. This suggests that proton conduction is predominant in the lower temperature region in humidified oxygen as well as in humidified hydrogen. 3.3. Electrical conduction in reducing atmospheres In the previous section, we showed that the protonic conduction is predominant in humidified hydrogen. As was suggested by the results of the emf measurements in our previous work [13], predominant carrier species in the oxide is expected to be protons and oxide ions in humidified hydrogen. Thus, total conductivity, σ, can be separated into partial conductivity contributions as, σ = σHþ + σO2−
ð1Þ
where σH+ and σO2− are partial conductivities of proton and oxide ion, respectively. Dependence of the electrical conductivities on oxygen partial pressure and water vapor is displayed in Figs. 5 and 6(a), respectively. According to the reports by Mizusaki and his co-workers on (SrZrO 3 ) 1 − y(La 0.75 Sr 0.25 FeO 3 − δ ) y [22,23] and La 1 − x Srx FeO3 − δ [23,24], n-type conduction attributed to the mixed valence state of Fe2+ and Fe3+ appears in reducing conductions. Thus, the electrical conductivity is expected to be a function of oxygen partial pressure. However, as shown in Figs. 5 and 6(a), total conductivities are found to be independent of oxygen partial pressure while those strongly depended on partial pressure of water vapor. That is, oxygen vacancy among the possible positive defect, VO••, (OH)•O, Fe•Fe and h•, is considered to be the major positive defect in the oxides at 1173 K in humidified hydrogen. The oxygen partial pressure dependences of the electrical conductivity suggest that the valence state of Fe-ion in the oxide is fixed in oxygen partial pressure range of 10− 20.14–10− 15.86 atm. In order to examine if the valence of Fe in SrZr0.99Fe0.01O3 − δ, the XAS measurements were conducted. That is because the XANES spectra are known to sensitively reflect the electronic state of cations in the oxides [25–29]. As reported by Haas and his co-workers and by the authors regarding La1 − xSrxFeO3 − δ [28] and (La0.9Sr0.1FeO3 − δ)0.05(SrZrO3)0.95 [29], respectively, shifts in absorption peaks in XANES spectra of Fe Kedge is observed accompanying with reduction of Fe in the oxides. Fig. 7 shows XANES spectra of Fe K-edge of Fe2O3 and SrZr0.99Fe0.01O3 − δ, which was annealed in various oxygen partial pressures at 1173 K for 5 h and quenched to room temperature. Although a wide range of oxygen
Fig. 5. Total conductivity of SrZr0.99Fe0.01O3 − δ as a function of oxygen partial pressure at 1173 K and 933 K in H2–1.9%H2O and D2–1.9%D2O.
Figs. 6. (a) Total conductivity and (b) partial proton and deuteron ion conductivities of SrZr0.99Fe0.01O3 − δ in 1.0% H2–H2O and 1.0% D2–D2O as functions of roots of partial pressure of water vapor or heavy water vapor.
partial pressure was examined, no significant change in XANES spectra was observed for SrZr0.99Fe0.01O3 − δ. In addition, absorption peaks of SrZr0.99Fe0.01O3 − δ appeared similar X-ray energy to that of Fe2O3. This implies that the valence state of Fe is essentially fixed to be 3+ in oxygen partial pressure range of 10− 19.73–0.21 at 1173 K. By taking the constant valence state of Fe-ion into consideration, the equilibrium of water vapor with the oxide can be simply expressed by using the Kröger–Vink's notation [30] as, ••
•
H2 O þ VO þ OO ¼2ðOHÞO
ð2Þ
Fig. 7. Normalized Fe K-edge XANES spectra of Fe2O3 and SrZr0.99Fe0.01O3 − δ. The latter oxide was annealed at 1173 K in log(P02/atm) = − 0.68 (in air), − 15.73 (in 1.0% H2– 1.9% H2O) and −19.73 (in 98.1% H2–1.9% H2O) for 5 h and quenched to room temperature.
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Fig. 8. Electrical conductivity of SrZr0.99Fe0.01O3 − δ as a function of oxygen partial pressure in humidified and unhumidified gases at 1173 K. Literature data regarding SrZrO3 [11], (SrZrO3)0.9(La0.75Sr0.25FeO3 − δ)0.1 and (SrZrO3)0.75(La0.75Sr0.25FeO3 − δ)0.25 [15–17] are also provided.
Fig. 10. Terminal voltages between anode (working electrode)–reference electrode and cathode (counter electrode)–reference electrode during the electrical hydrogen pumping.
and the equilibrium constant, K1, 1 1 1 ðOH Þ•o 2 • •• 1 or ðOHÞO = K12 VO 2 ½OO 2 p2H O K1 = 2 PH2 O ½Vo•• ½Oo
ð3Þ
where PH2O represents partial pressure of water vapor. As discussed above, oxygen vacancy is expected to be a major positive defect, i.e. the charge neutrality condition, [Fe′Zr] = 2[VO••], holds. When the variable in equation representing the electrical conductivity is assumed to be only the carrier concentration, Eq. (1) can be rewritten 1 as a linear function of p2H2 O . That is, σ = σH∘ þ p2H 1
2O
+ σO2−
ð4Þ
Fig. 6(a) shows σ in 1.0%H2–H2O and in 1.0%D2– where σH∘ þ is a constant. 1 1 D2O as a function of p2H O or p2D O . The segments of linear approximations 2 2 represent oxide ion conductivity when Eq. (4) holds. The increases from the segments represent the contribution of proton or deuteron ion conductivity. As seen in Fig. 6(a), oxide ion conductivity was found to be log(σ02−/S cm− 1) = −4.13. Partial proton and deuteron ion conductivities, which are determined by taking the difference of total1conductivity 1 and oxide ion conductivity, are displayed as a function of p2H O or p2D O in 2 2 Fig. 6(b). Transport number of proton was evaluated to be 0.75 in H2– 1.9%H2O at 1173 K from the dependence of the electrical conductivity on
partial pressure of water vapor, i.e. Eq. (4), Fig. 6(a) and (b). Although the transport number of proton is high such as 0.75 in1 humidified hydrogen, the partial protonic conductivity depended on p2H O based on 2 the defect equilibrium (3) with keeping the charge neutrality condition, •• [Fe′Zr] = 2[VO ]. This may imply that the mobility of proton is vastly larger than that of oxide ion in the oxide. The oxide ion conductivity estimated from Eq. (4) and Fig. 6(a) was plotted in Fig. 3 with an open diamond. The point was found to be on the straight line, which was extrapolated from two values of the conductivity in the higher temperature region in unhumidified hydrogen. This suggests that increase in the activation energy in the higher temperature region compared to that in the lower temperature regions is attributed to the change in major carrier from protons to oxide ions. The activation energy of migration of oxygen vacancy in SrZr0.99Fe0.01O3 − δ was calculated to be 0.91 eV. This is comparable to typical proton conducting zirconates such as Sr(Zr,Y)O3 (0.83 eV [16]) and Ba(Zr,RE)O3 (0.87–1.04 eV [20]). 3.4. Electrical conduction in oxidizing atmospheres Total conductivities in humidified and in unhumidified oxygen at 1173 K are shown in Fig. 8. It was found from the figure that the total conductivity in humidified oxygen is higher than that in unhumidified oxygen in lower oxygen partial pressure. This suggests that the protonic conductivity remains in that condition. It was also found that the total conductivities in both atmospheres seem to approximately depend 1 on p4O2 . For (SrZrO3)1 − y(La0.75Sr0.25FeO3 − δ)y and La1 − xSrxFeO3 − δ, p-type conduction appears in oxidizing atmospheres, in which the electronic defect is Fe•Fe [22–24,31]. However, this may not be the case of SrZr0.99Fe0.01O3 − δ. As discussed above, the valence of Fe is 3+ in SrZr0.99Fe0.01O3 − δ in the oxygen partial pressure range examined. Thus, the electronic defect may be induced by the following defect equilibrium, ••
VO þ 1=2O2 ¼ OO þ 2h
•
ð5Þ
By using the equilibrium constant of Eq. (5), K2, concentration of electron hole, [h•], is expressed by, • h =
•• 12 1 V K2 O p4O 2 ½OO
ð6Þ
Thus, the partial conductivity of electron hole is expected to linearly Fig. 9. Hydrogen evolution ratio (HER) at 1173 K as a function of current density. 98.1% H2–1.9% H2O and 98.1% Ar–1.9%H2O were supplied to anode and cathode, respectively. A broken line presents theoretical hydrogen evolution ratio based on the Faraday's law.
1
depend on p4O2 when major positive defect is oxygen vacancy and the variable of the electrical conductivity is only the carrier concentration. By considering the partial conductivity contributions of proton and
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oxide ion, the total conductivity under constant partial pressure of water vapor is expressed by, σ = σHþ + σO2− + σh∘ pO 1 4
2
ð7Þ
where σh∘ is a constant. The solid line in Fig. 8 was drawn by using Eq. (7). As seen there, it is in good agreement to the PO2 dependence of the electrical conductivity at 1173 K under constant partial pressure of water vapor in the entire region of the oxygen partial pressure. 3.5. Evaluation of hydrogen evolution ratio Fig. 9 shows hydrogen evolution rate (HER) at 1173 K as a function of current density when SrZr0.99Fe0.01O3 − δ was used as an electrolyte. The broken line in the figure represents the theoretical HER, which was calculated by the Faraday's law. It was found that the HER across the SrZr0.99Fe0.01O3 − δ electrolyte observed in the present study could be well described by the theoretical values. This suggests that transport number of ionic species is unity. Fig. 10 shows terminal voltages between anode–reference and cathode–reference electrodes during hydrogen evolution. Lines represent , open circuit voltages. It was found that the electrode resistance, RE = ∂E ∂j was found to be increased as current density increased. Since porous Pt was employed as electrodes as did Matsumoto et al. [7], the interfacial resistance is found to be similar to their report. 4. Conclusions Electrical conductivity of SrZr0.99Fe0.01O3 − δ was evaluated in this work. From the XAS measurements, it was found that Fe-ion in the oxide is trivalent both in oxygen and in hydrogen. In humidified hydrogen at temperatures from 823 K to 1273 K, the electrical conductivity strongly depended on partial pressure of water vapor. In addition, hydrogen and deuteron isotope effect on the electrical conductivity was observed. These results suggested that the proton conduction is predominant. The hydrogen evolution ratio across the SrZr0.99Fe0.01O3 − δ electrolyte could be well described by the Faraday's raw, suggesting that the transport number of ionic species is unity. In oxygen gas, electron hole conduction is found to be predominant in the higher temperature region. At lower temperatures, protonic conduction was found to significantly contribute to the total.
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Acknowledgements This study was supported by the Grant-in-Aids for Scientific Research on Priority Area, “Nanoionics (439)”, and for JSPS (Japan Society of the Promotion of Science) Fellows. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
[31]
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Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Electrical conduction and mass transport properties of SrZr0.99Fe0.01O3 − δ Atsushi Unemoto a,⁎, Atsushi Kaimai b, Kazuhisa Sato c, Naoto Kitamura d, Keiji Yashiro c, Hiroshige Matsumoto e, Junichiro Mizusaki c, Koji Amezawa b, Tatsuya Kawada b a
Graduate School of Engineering, Tohoku University, 6-6 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Graduate School of Environmental Studies, Tohoku University, 6-6 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8579, Japan d Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda 278-8510, Japan e Inamori Frontier Research Center, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan b c
a r t i c l e
i n f o
Article history: Received 24 March 2010 Received in revised form 1 May 2010 Accepted 4 May 2010 Keywords: SrZrO3 Proton conductor Transition metal Mixed conduction Hydrogen evolution
a b s t r a c t The electrical conductivity of SrZr0.99Fe0.01O3 − δ was evaluated by a four-probe ac technique. The measurements were conducted in hydrogen and in oxygen containing atmospheres at 823 ≤ T / K ≤ 1273. It was found from the X-ray absorption spectroscopic measurements that Fe in the oxide is trivalent both in hydrogen and in oxygen. In order to determine the major carrier in the oxide, gas partial pressure dependences and isotope effect of hydrogen and deuteron on the electrical conductivity was investigated. In humidified hydrogen, it was found that proton conduction is predominant in the lower temperature region while oxide ion conduction starts to contribute to the total by increasing temperature of above 1173 K. In oxygen containing gas, the protonic conduction is found to be predominant at lower temperatures while the contribution of the electron hole conduction is significant at higher temperatures. Hydrogen evolution property was evaluated using the SrZr0.99Fe0.01O3 − δ disc as a solid electrolyte. Hydrogen evolution rate obeyed the Faraday's law in humidified hydrogen at 1173 K, suggesting that the transport number of ionic species is unity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Acceptor-doped perovskite-type oxides such as cerates [1], zirconates [2,3] and so on are known to significantly conduct protons in humidified atmospheres at high temperatures. Thus, this kind of materials provides various possible applications [4–6]. Zirconates such as SrZrO3, BaZrO3 and CaZrO3 are promising candidates as high-temperature solid electrolytes especially for hydrogen separation from syngas [2,3]. This is because the above oxides are known to have high tolerance against ambient gases such as water vapor and carbon dioxide contrary to proton conducting cerates [7–10]. Matsumoto and his co-workers have investigated the electrical hydrogen pumping properties using SrZrO3-based solid electrolytes [9–11], and been working for further decrease of the electrode resistances [8,11]. In recent years, Matsumoto et al. have also reported that an additional substitution of Ru-ion into B-site of Sr(Zr,Y)O3 allows the oxide to have mixed proton and electron hole conductivity, resulting in hydrogen permeation across the membranes [12]. In our recent work, ceramic membranes consisting of proton conducting and electron conducting oxide composites were proposed as a novel concept for hydrogen permeation membranes [13]. In order to prove the validity of the concept, we chose the system, SrZrO3–SrFeO3. ⁎ Corresponding author. Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Japan. Tel.: +81 22 217 5816; fax: +81 22 795 5211. E-mail address: [email protected] (A. Unemoto). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.05.002
Since Sasamoto and his co-workers have reported that the solubility of SrFeO3 into SrZrO3 is small [14], we expected that the SrFeO3 phase and SrZrO3 phase are separated. In those oxide composite membranes, it was expected that Sr(Zr,Fe)O3 phase and SrFeO3 phase individually provide pathways for protons and electrons, respectively. Although hydrogen concentration increased at the hydrogen permeation side as we expected, partial conductivity contributions of proton and oxide ion in Sr(Zr,Fe)O3 were not carefully examined both in that work and the literature [14]. Since the solubility limit of SrFeO3 into SrZrO3 at temperatures from 1923 to 2023 K was suggested to be at most 2.5 mol% [13], thus in this work, the electrical conductivity of a solid solution, SrZr0.99Fe0.01O3 − δ, has been evaluated with the four-probe ac technique. Based on the electrical conductivity as functions of gas partial pressures and temperatures, the electrical conduction properties have been discussed. X-ray absorption (XAS) measurements were also carried out to examine the valence of Fe in the oxide. At the last of this paper, the electrical hydrogen evolution has been demonstrated using SrZr0.99Fe0.01O3 − δ as an electrolyte. 2. Experimental 2.1. Preparation of sample powder Powder of SrZr0.99Fe0.01O3 − δ was prepared via a conventional solidstate reaction route. The starting powders, SrCO3 (99.99%, Rare Metallic Co., Ltd.), ZrO2 (TZ-0, Tosoh Corp.) and Fe2O3 (99.99%, Kojyundo Chemical
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Lab. Co., Ltd.), were precisely weighed and mixed by a planetary ball mill in ethanol at 300 rpm for an hour. They were calcined at 1473 K for 5h in air and crushed by a planetary ball mill in ethanol at 300 rpm for 3h. The calcination and milling were repeated for three times. The resultant powder was formed into discs, then, sintered at 2023 K for 5h in air. X-ray diffraction (XRD) patterns were taken for the calcined powder, sintered disc and the ones after annealing in unhumidified 1.0%H2 at 873 K and 1273 K. For microstructure observation by SEM (JSM-7001F, JEOL Ltd.) and composition analysis by EDX (Inca x-act, Oxford Instruments), the sintered compact was polished by a diamond paste of 3 μm, then, resintered at 1623 K for 3 h in air. 2.2. X-ray absorption spectroscopic measurements Powders of SrZr0.99Fe0.01O3 − δ were annealed individually in oxygen partial pressures of 0.21 atm (in air), 10− 15.73 atm (in 1.0%H2–1.9%H2O) and 10− 19.73 atm (in 98.1%H2–1.9%H2O) at 1173 K for 5h, and then, quenched down to room temperature. The each powder was mixed together with boron nitride (N 99%, Kojyundo Chemical Lab. Co., Ltd.) and then pressed into discs. As a reference, a disc consisting of commercial powder of Fe2O3 and boron nitride was also prepared. Spectra of the X-ray absorption near edge structure (XANES) of Fe K-edge were collected via a transmission mode at the beam line, BL-7C, of the synchrotron radiation facility, Photon Factory (High Energy Accelerator Research Organization, Institute of Materials Structure Science, Japan). 2.3. Electrical conductivity and hydrogen permeation measurements Electrical conductivity measurements were carried out with fourprobe ac technique. Dense disc was cut into a parallelepiped, then, platinum-paste (TR-7907, Tanaka Precious Metals Co., Ltd.) was painted as electrodes. The specimen was put into a single chamber in which four probes are connected to the sets of electro-analytical equipments (Electrochemical Interface SI1287–Impedance/Gain-Phase Analyser SI1260, Solartron Analytical). Electrical hydrogen pumping was demonstrated using a dense disc with porous platinum as electrodes. Platinum-paste was painted onto both surfaces of the disc, then, set to the two-chamber type apparatus. A detailed configuration with an illustration is provided in Ref. [13]. 98.1% H2–1.9%H2O and 98.1%Ar–1.9%H2O were introduced into anode side and cathode side, respectively. Reference electrode was exposed to air. Hydrogen concentration of the outlet gas from the cathode was analyzed by a gas chromatograph (3000A Micro G.C., Agilent Technologies). Then, it was converted into hydrogen evolution rate. 3. Results and discussion 3.1. Specimens XRD patterns of the calcined powder (B) and the sintered disc (C) of SrZr0.99Fe0.01O3 − δ are displayed in Fig. 1. Those of unpolished surfaces of the sintered discs, which were annealed in unhumidified 1.0%H2 at 1273 K for 3 h (D) and at 873 K for 10 h (E), are also provided. As shown in the figure, the patterns are well assigned to be literature data of SrZrO3 with orthorhombic structure (A) [15]. This suggests that the specimen stays as a single phase even in unhumidified hydrogen at temperatures from 873 K to 1273 K. SrZr0.99Fe0.01O3 − δ is expected to be stable during the following electrochemical measurements performed in hydrogen containing gas. Fig. 2 shows SEM image of a SrZr0.99Fe0.01O3 − δ re-sintered surface at 1623 K in air. The compact is found to consist of grains with diameter of less than 5 μm. The chemical composition of the oxide was analyzed by EDX. Since the compact was sintered at high temperature such as 2023 K, thus, there might be an argument that the cation ratio of the oxide may deviate from the mixing ratio of starting powders because the
Fig. 1. X-ray diffraction patterns of (A) SrZrO3 of literature data [12], (B) calcined powder and (C) sintered disc of SrZr0.99Fe0.01O3 − δ, unpolished surfaces of the sintered disc annealed in: (D) unhumidified 1.0% H2 for 3 h at 1273 K and (E) unhumidified 1.0% H2 for 12 h at 873 K.
oxide contains elements having high vapor pressure such as Sr- and Feoxides. The investigation was performed by EDX point analysis with above ten points both grain interior and boundary. It was found that the oxide contains Sr, Zr, Fe and Hf. Detection of Hf is reasonable because ZrO2 generally contains it as impurity. The atomic ratios of Sr, Zr, Hf and Fe were 50.1%, 47.3%, 1.8%, 0.8%, respectively, in average. This suggests that significant deviation from the mixing ratio of starting powders was not observed both in grain interior and along grain boundary. The oxide keeps the A-site to B-site ratio to be approximately unity. It should be noted here that the ratio of Fe is small, the above values may contain an uncertainty to some extent. However, at least from this analysis, Fe and Sr ratios are found to be close to mixing ratio of starting powders. Thus, we took the nominal composition for the following analysis of the electrical conductivity. Lattice parameters of the synthesized oxide were calculated from the XRD patterns, appearing in Fig. 1, to be a=5.7860, b=5.8151 and c=8.1960. These are similar values to non-dope SrZrO3, appearing in the literature (a=5.7862, b=5.8151 and c=8.1960 [15]). This insignificant different may be due to the small concentration of dopant, Fe. 3.2. General tendency of electrical conduction behavior Fig. 3 shows the total conductivity of SrZr0.99Fe0.01O3 − δ as a function of inverse temperature in various gas compositions. The electrical conductivities of SrZrO3 [14] and SrZr0.9Y0.1O3 − δ [16] in the preceding papers are also provided. As seen in the figure, the electrical conductivity of SrZr0.99Fe0.01O3 − δ is higher than that of SrZrO3. This suggests that Fedoping into B-site of SrZrO3 obviously enhances electrical conductivity of the oxide.
Fig. 2. SEM image of a SrZr0.99Fe0.01O3 − δ surface re-sintered at 1623 K in air.
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In our previous work, emf (electron motive force) measurements of hydrogen concentration cell using SrZr0.99Fe0.01O3 − δ as an electrolyte were performed in humidified hydrogen. It was found that the transport number of ionic species is unity in the temperature range from 923 K to 1173 K [13]. As shown in Fig. 3, in hydrogen containing atmospheres, the electrical conductivity is found to be sensitive to partial pressure of water vapor. The electrical conductivity measured in humidified hydrogen is higher than that in unhumidified hydrogen. This suggests that protonic conduction rather than oxide ion conduction is significant in SrZr0.99Fe0.01O3 − δ. In order to confirm if the major carrier is proton, isotope effect of hydrogen and deuteron on the electrical conductivity was investigated. Fig. 4(a) shows total conductivity in 1.0%H2–1.9%H2O and that in 1.0% D2–1.9%D2O as a function of inverse temperature. It was found that the total conductivity in 1.0%D2–1.9%D2O is found to be smaller than that in 1.0%H2–1.9%H2O as displayed in Fig. 4(b) in the entire temperature region investigated. However, under the condition that the PH2/PH2O and PD2/PD2O ratios are kept to be a constant as we did, the equilibrium oxygen partial pressure is different between H2–H2O and D2–D2O atmospheres. Thus, the dependence of the electrical conductivity on oxygen partial pressure needs to be taken into consideration in order to discuss the isotopic effect of hydrogen and deuteron on the electrical conductivity. Total conductivities in H2–1.9%H2O and D2–1.9%D2O at 1173 K and 933 K as a function of oxygen partial pressure are given in Fig. 4. It was found that the total conductivity was independent on oxygen partial pressure while the difference in total conductivity measured in H2–H2O and in D2–D2O was observed. That is, the difference of the electrical conductivity in H2–1.9%H2O and that in D2– 1.9%D2O observed in Fig. 3 is essentially caused by the isotopic effect, not by the oxygen partial pressure dependence of the total conductivity. Means, major carrier in SrZr0.99Fe0.01O3 − δ are protons. As shown in Fig. 4(b) that the difference of the electrical conductivities in H2–H2O and in D2–D2O is 1.66–1.30 at 823–1023 K. The experimental value exceeded the expected one by the classical pffiffiffi theory ( 2) [17] in the lower temperature range. This may be due to the difference of mass dependent O–H and O–D stretching frequencies, namely semi-classical theory, resulting in difference of the activation energies [17]. In the case of Sr(Zr,Fe)O3, the activation energies of protonic conduction and deuteron ion conduction were calculated to be
Fig. 3. Total conductivity of SrZr0.99Fe0.01O3 − δ as a function of inverse temperature in various gas compositions. Literature data regarding SrZrO3 in air [11] and SrZr0.9Y0.1O3 − δ in humidified hydrogen [13] are also provided.
Fig. 4. (a) Total conductivity of SrZr0.99Fe0.01O3 − δ in 1.0% H2–1.9% H2O and 1.0% D2– 1.9% D2O as a function of inverse temperature. (b) Ratio of electrical conductivity in 1.0% H2–1.9% H2O to that in 1.0% D2–1.9% D2O.
0.64 eV and 0.66 eV, respectively, by using broken lines in Fig. 4 (a). This difference is in the range of the expected value by semi-classical theory as in oxides [17] and phosphate [18]. The activation energy of proton migration is found to be slightly larger than single crystal of Sr(Zr,Y)O3 (0.45 eV [19]), Ba(Zr,RE)O3 (RE = rare earth, 0.43 – 0.50 eV [20]) and Ba(Ce,Y)O3 (0.48 – 0.63 eV [21]). It was found from Fig. 4(b) that the ratio becomes smaller as temperature increased. This indicates that proton concentration decreased with increasing temperature and consequently protonic conductivity decreased. Since the transport number of ionic species is unity in humidified hydrogen [13], it can be expected that the oxide ion conduction started to contribute to the total as temperature increased. Actually, in unhumidified hydrogen, the gradient of log(σT) vs. 1/T in the higher temperature region, TN 1173, was larger than that in the lower temperature region, Tb 973. This implies that the carrier species is different between the higher and the lower temperature regions similar to SrZr0.9Y0.1O3 − δ [16]. In oxygen containing atmospheres, dependence of total conductivity on water vapor pressure is found to be less significant than that in hydrogen containing atmospheres as temperature increased. The behavior suggests that the defect species other than protons works as carrier species at higher temperatures. As discussed in the previous paragraph, the contribution of the oxide ion conductivity is expected to be small as seen in hydrogen containing atmospheres. Thus it cannot explain the difference in electrical conductivity between hydrogen and oxygen containing atmospheres by considering the contribution of oxide ion conductivity. In addition, the difference of the electrical conductivities between hydrogen and oxygen is remarkable as temperature increased. Thus, it can be expected that the electron hole
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conduction is predominant in oxygen containing atmospheres in the higher temperature region similar to the case of SrZr0.9Y0.1O3 − δ [16]. In the lower temperature region, however, dependence of the electrical conductivity on partial pressure of water vapor is significant. The electrical conductivity in humidified 1.0% O2 was higher than that in unhumidified 1.0% O2. This suggests that proton conduction is predominant in the lower temperature region in humidified oxygen as well as in humidified hydrogen. 3.3. Electrical conduction in reducing atmospheres In the previous section, we showed that the protonic conduction is predominant in humidified hydrogen. As was suggested by the results of the emf measurements in our previous work [13], predominant carrier species in the oxide is expected to be protons and oxide ions in humidified hydrogen. Thus, total conductivity, σ, can be separated into partial conductivity contributions as, σ = σHþ + σO2−
ð1Þ
where σH+ and σO2− are partial conductivities of proton and oxide ion, respectively. Dependence of the electrical conductivities on oxygen partial pressure and water vapor is displayed in Figs. 5 and 6(a), respectively. According to the reports by Mizusaki and his co-workers on (SrZrO 3 ) 1 − y(La 0.75 Sr 0.25 FeO 3 − δ ) y [22,23] and La 1 − x Srx FeO3 − δ [23,24], n-type conduction attributed to the mixed valence state of Fe2+ and Fe3+ appears in reducing conductions. Thus, the electrical conductivity is expected to be a function of oxygen partial pressure. However, as shown in Figs. 5 and 6(a), total conductivities are found to be independent of oxygen partial pressure while those strongly depended on partial pressure of water vapor. That is, oxygen vacancy among the possible positive defect, VO••, (OH)•O, Fe•Fe and h•, is considered to be the major positive defect in the oxides at 1173 K in humidified hydrogen. The oxygen partial pressure dependences of the electrical conductivity suggest that the valence state of Fe-ion in the oxide is fixed in oxygen partial pressure range of 10− 20.14–10− 15.86 atm. In order to examine if the valence of Fe in SrZr0.99Fe0.01O3 − δ, the XAS measurements were conducted. That is because the XANES spectra are known to sensitively reflect the electronic state of cations in the oxides [25–29]. As reported by Haas and his co-workers and by the authors regarding La1 − xSrxFeO3 − δ [28] and (La0.9Sr0.1FeO3 − δ)0.05(SrZrO3)0.95 [29], respectively, shifts in absorption peaks in XANES spectra of Fe Kedge is observed accompanying with reduction of Fe in the oxides. Fig. 7 shows XANES spectra of Fe K-edge of Fe2O3 and SrZr0.99Fe0.01O3 − δ, which was annealed in various oxygen partial pressures at 1173 K for 5 h and quenched to room temperature. Although a wide range of oxygen
Fig. 5. Total conductivity of SrZr0.99Fe0.01O3 − δ as a function of oxygen partial pressure at 1173 K and 933 K in H2–1.9%H2O and D2–1.9%D2O.
Figs. 6. (a) Total conductivity and (b) partial proton and deuteron ion conductivities of SrZr0.99Fe0.01O3 − δ in 1.0% H2–H2O and 1.0% D2–D2O as functions of roots of partial pressure of water vapor or heavy water vapor.
partial pressure was examined, no significant change in XANES spectra was observed for SrZr0.99Fe0.01O3 − δ. In addition, absorption peaks of SrZr0.99Fe0.01O3 − δ appeared similar X-ray energy to that of Fe2O3. This implies that the valence state of Fe is essentially fixed to be 3+ in oxygen partial pressure range of 10− 19.73–0.21 at 1173 K. By taking the constant valence state of Fe-ion into consideration, the equilibrium of water vapor with the oxide can be simply expressed by using the Kröger–Vink's notation [30] as, ••
•
H2 O þ VO þ OO ¼2ðOHÞO
ð2Þ
Fig. 7. Normalized Fe K-edge XANES spectra of Fe2O3 and SrZr0.99Fe0.01O3 − δ. The latter oxide was annealed at 1173 K in log(P02/atm) = − 0.68 (in air), − 15.73 (in 1.0% H2– 1.9% H2O) and −19.73 (in 98.1% H2–1.9% H2O) for 5 h and quenched to room temperature.
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Fig. 8. Electrical conductivity of SrZr0.99Fe0.01O3 − δ as a function of oxygen partial pressure in humidified and unhumidified gases at 1173 K. Literature data regarding SrZrO3 [11], (SrZrO3)0.9(La0.75Sr0.25FeO3 − δ)0.1 and (SrZrO3)0.75(La0.75Sr0.25FeO3 − δ)0.25 [15–17] are also provided.
Fig. 10. Terminal voltages between anode (working electrode)–reference electrode and cathode (counter electrode)–reference electrode during the electrical hydrogen pumping.
and the equilibrium constant, K1, 1 1 1 ðOH Þ•o 2 • •• 1 or ðOHÞO = K12 VO 2 ½OO 2 p2H O K1 = 2 PH2 O ½Vo•• ½Oo
ð3Þ
where PH2O represents partial pressure of water vapor. As discussed above, oxygen vacancy is expected to be a major positive defect, i.e. the charge neutrality condition, [Fe′Zr] = 2[VO••], holds. When the variable in equation representing the electrical conductivity is assumed to be only the carrier concentration, Eq. (1) can be rewritten 1 as a linear function of p2H2 O . That is, σ = σH∘ þ p2H 1
2O
+ σO2−
ð4Þ
Fig. 6(a) shows σ in 1.0%H2–H2O and in 1.0%D2– where σH∘ þ is a constant. 1 1 D2O as a function of p2H O or p2D O . The segments of linear approximations 2 2 represent oxide ion conductivity when Eq. (4) holds. The increases from the segments represent the contribution of proton or deuteron ion conductivity. As seen in Fig. 6(a), oxide ion conductivity was found to be log(σ02−/S cm− 1) = −4.13. Partial proton and deuteron ion conductivities, which are determined by taking the difference of total1conductivity 1 and oxide ion conductivity, are displayed as a function of p2H O or p2D O in 2 2 Fig. 6(b). Transport number of proton was evaluated to be 0.75 in H2– 1.9%H2O at 1173 K from the dependence of the electrical conductivity on
partial pressure of water vapor, i.e. Eq. (4), Fig. 6(a) and (b). Although the transport number of proton is high such as 0.75 in1 humidified hydrogen, the partial protonic conductivity depended on p2H O based on 2 the defect equilibrium (3) with keeping the charge neutrality condition, •• [Fe′Zr] = 2[VO ]. This may imply that the mobility of proton is vastly larger than that of oxide ion in the oxide. The oxide ion conductivity estimated from Eq. (4) and Fig. 6(a) was plotted in Fig. 3 with an open diamond. The point was found to be on the straight line, which was extrapolated from two values of the conductivity in the higher temperature region in unhumidified hydrogen. This suggests that increase in the activation energy in the higher temperature region compared to that in the lower temperature regions is attributed to the change in major carrier from protons to oxide ions. The activation energy of migration of oxygen vacancy in SrZr0.99Fe0.01O3 − δ was calculated to be 0.91 eV. This is comparable to typical proton conducting zirconates such as Sr(Zr,Y)O3 (0.83 eV [16]) and Ba(Zr,RE)O3 (0.87–1.04 eV [20]). 3.4. Electrical conduction in oxidizing atmospheres Total conductivities in humidified and in unhumidified oxygen at 1173 K are shown in Fig. 8. It was found from the figure that the total conductivity in humidified oxygen is higher than that in unhumidified oxygen in lower oxygen partial pressure. This suggests that the protonic conductivity remains in that condition. It was also found that the total conductivities in both atmospheres seem to approximately depend 1 on p4O2 . For (SrZrO3)1 − y(La0.75Sr0.25FeO3 − δ)y and La1 − xSrxFeO3 − δ, p-type conduction appears in oxidizing atmospheres, in which the electronic defect is Fe•Fe [22–24,31]. However, this may not be the case of SrZr0.99Fe0.01O3 − δ. As discussed above, the valence of Fe is 3+ in SrZr0.99Fe0.01O3 − δ in the oxygen partial pressure range examined. Thus, the electronic defect may be induced by the following defect equilibrium, ••
VO þ 1=2O2 ¼ OO þ 2h
•
ð5Þ
By using the equilibrium constant of Eq. (5), K2, concentration of electron hole, [h•], is expressed by, • h =
•• 12 1 V K2 O p4O 2 ½OO
ð6Þ
Thus, the partial conductivity of electron hole is expected to linearly Fig. 9. Hydrogen evolution ratio (HER) at 1173 K as a function of current density. 98.1% H2–1.9% H2O and 98.1% Ar–1.9%H2O were supplied to anode and cathode, respectively. A broken line presents theoretical hydrogen evolution ratio based on the Faraday's law.
1
depend on p4O2 when major positive defect is oxygen vacancy and the variable of the electrical conductivity is only the carrier concentration. By considering the partial conductivity contributions of proton and
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oxide ion, the total conductivity under constant partial pressure of water vapor is expressed by, σ = σHþ + σO2− + σh∘ pO 1 4
2
ð7Þ
where σh∘ is a constant. The solid line in Fig. 8 was drawn by using Eq. (7). As seen there, it is in good agreement to the PO2 dependence of the electrical conductivity at 1173 K under constant partial pressure of water vapor in the entire region of the oxygen partial pressure. 3.5. Evaluation of hydrogen evolution ratio Fig. 9 shows hydrogen evolution rate (HER) at 1173 K as a function of current density when SrZr0.99Fe0.01O3 − δ was used as an electrolyte. The broken line in the figure represents the theoretical HER, which was calculated by the Faraday's law. It was found that the HER across the SrZr0.99Fe0.01O3 − δ electrolyte observed in the present study could be well described by the theoretical values. This suggests that transport number of ionic species is unity. Fig. 10 shows terminal voltages between anode–reference and cathode–reference electrodes during hydrogen evolution. Lines represent , open circuit voltages. It was found that the electrode resistance, RE = ∂E ∂j was found to be increased as current density increased. Since porous Pt was employed as electrodes as did Matsumoto et al. [7], the interfacial resistance is found to be similar to their report. 4. Conclusions Electrical conductivity of SrZr0.99Fe0.01O3 − δ was evaluated in this work. From the XAS measurements, it was found that Fe-ion in the oxide is trivalent both in oxygen and in hydrogen. In humidified hydrogen at temperatures from 823 K to 1273 K, the electrical conductivity strongly depended on partial pressure of water vapor. In addition, hydrogen and deuteron isotope effect on the electrical conductivity was observed. These results suggested that the proton conduction is predominant. The hydrogen evolution ratio across the SrZr0.99Fe0.01O3 − δ electrolyte could be well described by the Faraday's raw, suggesting that the transport number of ionic species is unity. In oxygen gas, electron hole conduction is found to be predominant in the higher temperature region. At lower temperatures, protonic conduction was found to significantly contribute to the total.
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Acknowledgements This study was supported by the Grant-in-Aids for Scientific Research on Priority Area, “Nanoionics (439)”, and for JSPS (Japan Society of the Promotion of Science) Fellows. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
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