Influence of the Supporting Matrix on the Electrochemical Properties of

Sep 10, 2008 - The composite with SiP2O7 matrix exhibited the highest conductivity ... to the interfacial interaction between CsH5(PO4)2 and the matri...
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J. Phys. Chem. C 2008, 112, 15532–15536

Influence of the Supporting Matrix on the Electrochemical Properties of CsH5(PO4)2 Composites at Intermediate Temperatures Hiroki Muroyama, Toshiaki Matsui,* Ryuji Kikuchi, and Koichi Eguchi Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan ReceiVed: May 16, 2008; ReVised Manuscript ReceiVed: July 26, 2008

Proton-conductive CsH5(PO4)2 composites with various pyrophosphate and silica matrices (e.g., SiP2O7, TiP2O7, and SiO2, etc.) were synthesized, and the composite effects were investigated at intermediate temperatures based on the electrochemical measurement, thermal analysis, and contact angle measurement. The melting and dehydration processes of CsH5(PO4)2 in composites depended on the matrix species. The composite with SiP2O7 matrix exhibited the highest conductivity among all composites in the temperature range investigated. This result indicates that the conductivity of composites was affected by the pyrophosphate unit and the metal species in the crystalline structure of the pyrophosphate. The conductivity of composites appears to correlate with the compatibility between the components examined by contact angle measurement. These differences in electrochemical and physical properties should be attributed to the interfacial interaction between CsH5(PO4)2 and the matrix. Introduction Systems composed of ionic conductor-nonionic conductor mixture, such as LiClO4/SiO2, CsHSO4/SiO2, and (Li, K)2CO3/ γ-LiAlO2, etc., have attracted much attention for application in electrochemical devices, and their thermal and electrochemical behavior have been investigated so far.1-18 The addition of nonionic conductor to ionic conductor gives rise to depression of the melting point of the conductive phase. This phenomenon is remarkable when the additive possesses a large surface area or occupies a large volume fraction in the system. Ionic conduction in a conductive phase is strongly influenced in the vicinity of the interface with a nonionic conductor. In such coexisting systems, a notable correlation can be often found between their conductivity and composition; e.g., the conductivity of composite system with an optimum composition exceeds that of the pure ionic conductor. This promoted conductivity is afforded by the disordered state in the ionic conductor or the formation of ionic defects at the interface. Furthermore, the electrochemical property of ionic conductor-oxide (SiO2, TiO2, and Al2O3) composite systems is dependent on the oxide species. The enhancement in conductivity was more noticeable for SiO2 than others.6-8 This result implies that the surface acid-base property of the oxide affects the conduction mechanism in the system. Thus, it is of primary importance to focus on the interfacial interaction between ionic conductor and nonionic conductor for understanding of the thermal and electrochemical properties of composite electrolytes. Recently, the composite based on CsH2PO4/SiP2O7 has been reported for the electrolyte employed in solid-state fuel cells operative at 200-300 °C. The operation in this intermediate temperature range is expected to combine advantageous properties of low- and high-temperature fuel cells.13,14 This composite exhibited the higher conductivity with different temperature dependence as compared with CsH2PO4 under a humidified atmosphere in the temperature range of 110-280 °C. In this * Corresponding author. Tel.: +81-75-383-2523. Fax: +81-75-383-2520. E-mail: [email protected].

composite, a proton-conductive CsH5(PO4)2 phase was newly formed via chemical reaction between CsH2PO4 and SiP2O7. Then, the composite effect of SiP2O7 as a supporting matrix for CsH5(PO4)2 composites was examined in comparison with SiO2 matrix.15 The conductivity of the CsH5(PO4)2/SiP2O7 composite was higher by 1 order of magnitude than that of the CsH5(PO4)2/SiO2 composite. It was concluded that this phenomenon should be attributed to the difference in interfacial interaction between the proton conductor and the matrix. As the influence of the surface area of the matrix on the conductivity was not considered, no distinct effect originated from the difference in interaction between the two components was evinced. In the system of NH4PO3 composite, various pyrophosphates were selected as supporting matrices and the correlation between metal species in the pyrophosphate and the conductivity of the composites was studied.17,18 The surface acidity of pyrophosphate measured as the ζ-potential depended on their metal species. In this composite, HPO3 formed via the partial decomposition of NH4PO3 served as a proton conductor. The extent of this decomposition varied depending on pyrophosphates, resulting in the difference in carrier concentration. The protonic mobility in these composites, thus, could not be compared with each other considering the definition of ionic conductivity. In this study, then, we focused on the interfacial interaction to elucidate the composite effect of the matrix in CsH5(PO4)2-based composite. The influence of phosphate and oxides on the conduction behavior was investigated for composites with SiP2O7 and SiO2 matrices. The surface area of the matrix and CsH5(PO4)2 concentration in the composite were quantified. The compatibility between CsH5(PO4)2 and the matrix was also examined from wetting at the interface by contact angle measurement. The correlation between the protonic mobility in composite and the surface acid-base property of the matrix was studied for the composites with SiP2O7 and TiP2O7. The mixed pyrophosphate containing Si and Ti in the crystal was synthesized for optimization of its surface acidity from SiP2O7 and TiP2O7. This is because the composite oxide,

10.1021/jp8043362 CCC: $40.75  2008 American Chemical Society Published on Web 09/10/2008

Properties of CsH5(PO4)2 Composites

J. Phys. Chem. C, Vol. 112, No. 39, 2008 15533 TABLE 1: Specific Surface Area of Matrices

SiP2O7 TiP2O7 (Si, Ti)P2O7(alkoxide) (Si, Ti)P2O7(powder) SiO2(290) SiO2(3.4)

specific surface area/m2 g-1

specific surface area/m2 mol-1

1.0 2.2 3.5 1.8 290 3.4

2.0 × 102 4.9 × 102 7.4 × 102 3.8 × 102 1.7 × 104 2.0 × 102

TABLE 2: Volume Fraction and Concentration of CsH5(PO4)2 in Composites concn of vol fraction of CsH5(PO4)2/% CsH5(PO4)2/103 mol m-3 Figure 1. Synthetic procedure of various pyrophosphates for matrices.

SiO2-TiO2, possesses a larger number of sites with stronger acidity than both single-component oxides.19 Experimental Section Preparation of Materials. Cesium pentahydrogen diphosphate, CsH5(PO4)2, was synthesized from Cs2CO3 (Aldrich) and H3PO4 (Wako Pure Chemical Industries, guaranteed reagent) in aqueous solution and subsequently dried at ca. 100 °C. The X-ray diffraction (XRD) pattern of the resulting powder was identical to that of CsH5(PO4)2 in the literature.20 Several pyrophosphates were prepared as depicted in Figure 1. The oxides were synthesized by the hydrolysis of Si(OC2H5)4 (Wako Pure Chemical Industries) and Ti[OCH(CH3)2]4 (Kishida Chemical Co., Ltd.) and subsequent calcination at 500 °C for 3 h. The composite oxide of SiO2 and TiO2 is described as (Si, Ti)O2. The mixture of obtained oxide and H3PO4 was subjected to the stepwise annealing process.17 The resultant pyrophosphates, SiP2O7 and TiP2O7, were attributed to the cubic phase by XRD.21,22 The pyrophosphate synthesized from metal alkoxide solution with Si:Ti ) 1:1 is hereafter represented as (Si, Ti)P2O7(alkoxide). The powder mixture consisting of SiP2O7 and TiP2O7, (Si, Ti)P2O7(powder), was also prepared. Silica samples with different specific surface areas of 290 and 3.4 m2 g-1 were also used as matrices (abbreviated as SiO2(290) and SiO2(3.4), respectively). The SiO2(3.4), which can be expected to be less hydrophilic, was obtained by the calcination of hydrophilic SiO2(290) (Nippon Silica) at 1000 °C for 3 h. The composite electrolytes of CsH5(PO4)2 and the matrix were fabricated by the mechanical mixing of resultant powders in various molar ratios. The composite in the molar ratio of CsH5(PO4)2/matrix ) x (x ) 0.11, 0.25) is hereafter represented as xCsH5(PO4)2/matrix. Characterization. Thermogravimetry (TG) and differential thermal analysis (DTA) were conducted under flowing dry Ar with a heating rate of 5 °C/min (SII Nano Technology Inc., EXSTAR6000 TG/DTA 6300). The crystalline phases were identified by XRD (Rigaku RINT 1400 X-ray diffractometer). The specific surface area of matrices was measured by the BET method with N2 adsorption (Shimadzu, Gemini 2375), as summarized in Table 1. The true density of materials was measured by Archimedes’ method (Quantachrome Instruments, Ultrapycnometer 1000). The wetting property between CsH5(PO4)2 and the matrix was characterized by the contact angle measurement (ULVAC-RIKO Inc., WET-1200). The spectra for O 1s of the oxides were obtained by X-ray photoelectron spectroscopy (XPS) with Mg KR radiation (Shimadzu, ESCA-850). Each binding energy was referred to the C 1s peak (284.3 eV).

0.25CsH5(PO4)2/SiP2O7 0.25CsH5(PO4)2/SiO2(290) 0.25CsH5(PO4)2/SiO2(3.4) 0.11CsH5(PO4)2/SiO2(3.4)

33 56 54 33

2.8 4.7 4.5 2.8

Measurement of Conductivity. For electrochemical measurements, the composite was uniaxally pressed into a pellet (diameter; 10 mm, thickness; 1-3 mm) with Pt/C electrodes (E-TEK, 1.0 mg cm-2). In the case of CsH5(PO4)2, the glass cell with Pt electrodes was used. Proton conductivity was measured by ac impedance spectroscopy (Solartron 1260 frequency response analyzer). The applied frequency was in the range of 0.1 Hz to 1 MHz with voltage amplitude of 30 mV. The measurement was conducted under 30% H2O/Ar atmosphere, which was prepared by bubbling dry Ar through water at 70 °C. At each temperature, samples were kept for 30 min to achieve the steady state. Results and Discussion Influence of the Pyrophosphate Unit on the Conduction Behavior of Composites. The composite effect of matrix species on the conduction behavior of composite electrolytes was investigated. The volume fraction and molar concentration of CsH5(PO4)2 component in each composite were estimated from the true density of CsH5(PO4)2 and the matrix, as summarized in Table 2. In this evaluation, all composites were assumed to possess the equivalent relative density. The difference in concentration of the CsH5(PO4)2 component in 0.25CsH5(PO4)2/SiO2(290) and 0.25CsH5(PO4)2/SiO2(3.4) is considered to be negligible. The TG-DTA profiles of CsH5(PO4)2 and composites are shown in Figure 2. A large endothermic peak was observed in the temperature range between 146 and 156 °C in the DTA profiles of all samples, which can be assigned to the melting of CsH5(PO4)2.23-25 For composites, the melting point appeared at lower temperature than the original one, indicating that the matrix influenced the state of CsH5(PO4)2. In particular, 0.25CsH5(PO4)2/SiO2(290) exhibited the remarkable melting point depression as compared with other composites. This was attributed to the interaction between the two components at a larger contacting interface. The hydrophilicity of SiO2(290) was also one of the reasons for this melting point depression. It was reported that the melting of salt was affected by the hydrophilicity of the coexisting solid phase; R-alumina additive reduced the melting point of the salt, whereas no change in melting behavior was observed in the case of SiC.2,5 Since the CsH5(PO4)2 component melted at a slightly lower temperature in 0.25CsH5(PO4)2/SiP2O7 than in the composite with SiO2(3.4), the surface of SiP2O7 should be more hydrophilic than that of SiO2(3.4). As can be seen in Figure 2(a), the dehydration of

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Figure 2. TG-DTA profiles for CsH5(PO4)2 and composites under flowing dry Ar with a heating rate of 5 °C min-1.

CsH5(PO4)2 was initiated soon after melting under dry condition.23 Different behavior of weight change was confirmed for composites above the melting point. The weight of 0.25CsH5(PO4)2/SiO2(3.4) decreased almost linearly with an increase in temperature as is the case of CsH5(PO4)2, whereas the other composites exhibited nonlinear weight change accompanied with an endothermic peak at ca. 190 °C. It is noted that for the composites with SiO2(3.4), the weight loss was changed by increasing the volume fraction of the matrix. These results suggest that the dehydration of CsH5(PO4)2 in composites is significantly affected by the matrix species. Since the chemical property of the ionic salt at the interface is different from that in the bulk due to the interaction with the matrix, the dehydration of CsH5(PO4)2 located in the vicinity of the matrix preferentially proceeds. Thus, it was concluded that the thermal behavior of composites was dependent on the amount and surface property of matrices. The contact angle of the molten CsH5(PO4)2 on matrix substrate was measured to evaluate the interfacial interaction. A small piece of CsH5(PO4)2 was placed on the substrate plates of SiP2O7 or SiO2(3.4). Then, the contact angle measurement was conducted during heating at a rate of 12 °C/min under a dry Ar atmosphere. The temperature was monitored with the thermocouple located on the substrate. The images of the CsH5(PO4)2 droplet on the substrate at 188 °C and the contact angle for CsH5(PO4)2 as a function of the substrate temperature are shown in Figures 3 and 4, respectively. On the SiP2O7 substrate, the CsH5(PO4)2 droplet spread rapidly after melting, resulting in the drastic decrease in the contact angle with an increase in temperature. In contrast, the spherical shape of the molten CsH5(PO4)2 on SiO2(3.4) was unchanged at every temperature investigated. Accordingly, the contact angle of CsH5(PO4)2 on the matrix strongly depended on the surface free energy of the matrix, and the matrix of SiP2O7 showed better wettability with CsH5(PO4)2 than SiO2(3.4). This phenomenon reflects the difference in interfacial interaction between CsH5(PO4)2 and the matrix. Figure 5 shows temperature dependence of the conductivity for CsH5(PO4)2 and composites under 30% H2O/Ar atmosphere. The conductivity of all the samples increased drastically with a rise in temperature between 120 and 150 °C due to the melting of CsH5(PO4)2. The enhancement in the conductivity signifi-

Muroyama et al.

Figure 3. Images of CsH5(PO4)2 droplet shape on (a) SiP2O7 and (b) SiO2(3.4) substrates at 188 °C under a dry Ar atmosphere.

Figure 4. Contact angle for CsH5(PO4)2 on SiP2O7 and SiO2(3.4) substrates as a function of the substrate temperature with a heating rate of 12 °C min-1 under a dry Ar atmosphere.

Figure 5. Temperature dependence of the conductivity for CsH5(PO4)2 and composites under 30% H2O/Ar atmosphere in heating process.

cantly depended on the matrix species. The conductivity of 0.25CsH5(PO4)2/SiP2O7 and 0.25CsH5(PO4)2/SiO2(290) was higher by 1.5 orders of magnitude than that of CsH5(PO4)2 at ca. 120 °C. It is well-known for oxoacid salts such as CsHSO4 and CsH2PO4 that the oxide additive leads to the formation of disordered phase at the contacting interface between two components and the resultant enhancement in conductivity.8-12 Thus, CsH5(PO4)2 in the vicinity of the matrix could be readily transformed into such a disordered or molten state from the

Properties of CsH5(PO4)2 Composites

Figure 6. O 1s spectra of (a) SiO2, (b) (Si, Ti)O2 (Si:Ti ) 1:1), (c) powder mixture of SiO2 and TiO2 (Si:Ti ) 1:1), and (d) TiO2. Broken lines show the peak positions of the O 1s binding energy in SiO2 and TiO2.

original crystal state in these samples, as can be supported by the melting point depression in Figure 2. On the other hand, both composites with SiO2(3.4) exhibited almost the same conductivity as CsH5(PO4)2 at 120-150 °C. These results indicated that the state of CsH5(PO4)2 at the interface in these composites was different from that in 0.25CsH5(PO4)2/SiP2O7 and 0.25CsH5(PO4)2/SiO2(290). In the temperature range between 150 and 280 °C, the temperature dependence of the conductivity for every sample was almost similar to each other. The proton-conduction mechanism in composites will be analogous to that in CsH5(PO4)2. The difference in conductivity was also confirmed for composites in this temperature range. In the case of 0.25CsH5(PO4)2/SiO2 composite, the conductivity of 0.25CsH5(PO4)2/SiO2(290) was lower than that of 0.25CsH5(PO4)2/SiO2(3.4). The difference may result from the surface properties of SiO2 such as surface area and hydrophilicity, etc. Mizuhata et al. reported for the liquid ionic conductor/SiO2 powder system that the conductivity decreased with an increase in the surface area of SiO2 powder at 30 °C because of the complicated conduction path in the pore of SiO2 powder.3 In the case of comparison between matrices with comparable surface area (Table 1), the conductivity of 0.25CsH5(PO4)2/ SiP2O7 and 0.25CsH5(PO4)2/SiO2(3.4) was almost the same value of 50 mS cm-1 at ca. 230 °C. The ionic conductivity is proportional to the product of charge carrier concentration and mobility. To clarify the influence of matrix species on mobility, the carrier concentration is required to be equivalent among composites. Thus, the conductivity of samples with same CsH5(PO4)2 concentration was compared. The composite of 0.25CsH5(PO4)2/SiP2O7 exhibited 1 order of magnitude higher conductivity than 0.11CsH5(PO4)2/SiO2(3.4). It was tentatively concluded that the protonic mobility was dependent on the matrix species due to interfacial interaction. The matrix of SiP2O7 showed better compatibility with CsH5(PO4)2 than SiO2 matrices, which was reflected by the wetting property of CsH5(PO4)2. Influence of Surface Acid-Base Property of Pyrophosphates on the Conductivity of Composites. The correlation between the conductivity and the surface acidity of the matrix was investigated by using various pyrophosphates as matrices. The XPS spectra for O 1s of the oxides synthesized from metal alkoxides are depicted in Figure 6. For comparison, the powder mixture of SiO2 and TiO2 was also measured. Broken lines show the peak positions of O 1s binding energy in SiO2 and TiO2. In

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Figure 7. XRD patterns of (a) (Si, Ti)P2O7(alkoxide) and (b) (Si, Ti)P2O7(powder) (Si:Ti ) 1:1).

Figure 8. Temperature dependence of the conductivity for 0.25CsH5(PO4)2/MP2O7 (M ) Si, (Si, Ti), Ti) under 30% H2O/Ar atmosphere in heating process.

the spectrum for the powder mixture, the peak positions were consistent with those of SiO2 and TiO2 (Figure 6(c)). For (Si, Ti)O2, the two peaks shifted to low and high binding energies from the intrinsic positions of SiO2 and TiO2, respectively (Figure 6(b)). Accordingly, metal atoms were partially substituted in the main oxide framework, though a homogeneous solid solution was not obtained due to low solid solubility.26,27 The pyrophosphate containing both Si and Ti elements, (Si, Ti)P2O7(alkoxide), was fabricated from this composite oxide. Figure 7 shows the XRD pattern of the pyrophosphates containing both Si and Ti elements. For the powder mixture of (Si, Ti)P2O7(powder), the pattern consisted of SiP2O7 and TiP2O7 phases. In contrast, (Si, Ti)P2O7(alkoxide) exhibited three groups of peaks in the 2θ range of 22-30° and each was composed of triplets. The peaks at the highest and lowest 2θ in each group virtually corresponded to those of SiP2O7 and TiP2O7, respectively. The new diffraction peak appeared between these two peaks, indicating that both Si and Ti atoms coexist in the crystalline structure of (Si, Ti)P2O7(alkoxide). The temperature dependence of the conductivity for CsH5(PO4)2/pyrophosphate composites under 30% H2O/Ar atmosphere is shown in Figure 8. In this experiment, the specific surface areas of the matrix little affected the conductivity of composites because of their comparable values (1.0-3.5 m2 g-1). Additionally, all composites possessed the same molar concentration of CsH5(PO4)2 based on the assumption that their relative densities were equivalent. Thus, the conductivity of the

15536 J. Phys. Chem. C, Vol. 112, No. 39, 2008 composites is expected to reflect the influence of surface acidity of pyrophosphate on protonic mobility in the composite. Although the conduction behavior of composites was quite analogous between 140 and 260 °C, the conductivity was dependent on the metal species in the pyrophosphate and was in the following sequence in almost the whole temperature range investigated:

0.25CsH5(PO4)2/SiP2O7 > 0.25CsH5(PO4)2/(Si, Ti)P2O7(powder) > 0.25CsH5(PO4)2/(Si, Ti)P2O7(alkoxide) > 0.25CsH5(PO4)2/TiP2O7 (1) The pyrophosphate containing both metal elements, (Si, Ti)P2O7(alkoxide), was located at intermediate between the two single pyrophosphates. Furthermore, 0.25CsH5(PO4)2/(Si, Ti)P2O7(powder) exhibited higher conductivity than 0.25CsH5(PO4)2/ (Si, Ti)P2O7(alkoxide) despite the same Si/Ti ratio. Thus, the increasing conductivity of composites with the amount of SiP2O7 indicates that the surface of SiP2O7 was the most effective to enhance the protonic mobility among matrix materials. Consequently, it was revealed that the protonic mobility in the composite was significantly affected by the metal species in the crystalline structure of the pyrophosphate, which should determine the surface property of the material.18 Conclusions The CsH5(PO4)2 composite electrolytes with various pyrophosphate and silica matrices were synthesized, and their electrochemical and other properties were investigated at intermediate temperatures to reveal the composite effects of the matrix. The matrices of SiP2O7 and SiO2 with different surface area were selected to study the effect of the pyrophosphate unit on the conduction behavior of composites. It is noted that the matrix significantly affected the conductivity and the thermal behavior of composites. The matrix of SiP2O7 exhibited good compatibility with CsH5(PO4)2, resulting in high proton conductivity and low contact angle between these two components. Thus, the wetting property between the ionic conductor and supporting matrix may be one of the useful methods to select components for the development of highly proton-conductive electrolyte. The pyrophosphates with various metal species were used as matrices to examine the influence of their surface acidity on the proton conductivity in CsH5(PO4)2 composites. The conductivity of composites depended on the metal species in the crystalline structure of the pyrophosphate, indicating that the protonic mobility correlated with the surface property of material. Thus, it was concluded that these results originated from the interfacial interaction between the conduction phase

Muroyama et al. and the matrix, which could be a new approach to fabricate attractive proton-conductive materials. Acknowledgment. This work was supported by the Research and Development of Polymer Electrolyte Fuel Cells group of the New Energy and Industrial Technology Development Organization (NEDO) of Japan and by a JSPS Fellows from the Japan Society for the Promotion of Science. References and Notes (1) Deki, S.; Nakamura, S.; Kajinami, A.; Kanaji, Y. J. Chem. Soc., Faraday Trans. 1993, 89, 3805. (2) Deki, S.; Nakamura, S.; Kanaji, Y. J. Chem. Soc., Faraday Trans. 1993, 89, 3811. (3) Mizuhata, M.; Kitamura, M.; Deki, S. Electrochemistry 2003, 71, 1093. (4) Mizuhata, M.; Harada, Y.; Cha, G.; Be´le´ke´, A. B.; Deki, S. J. Electrochem. Soc. 2004, 151, E179. (5) Mizuhata, M.; Sumihiro, Y.; Deki, S. Phys. Chem. Chem. Phys. 2004, 6, 1944. (6) Bhattacharyya, A. J.; Maier, J. AdV. Mater. 2004, 16, 811. (7) Edwards, W. V.; Bhattacharyya, A. J.; Chadwick, A. V.; Maier, J. Electrochem. Solid-State Lett. 2006, 9, A564. (8) Ponomareva, V. G.; Lavrova, G. V. Solid State Ionics 2001, 145, 197. (9) Ponomareva, V. G.; Lavrova, G. V.; Simonova, L. G. Solid State Ionics 1999, 119, 295. (10) Shigeoka, H.; Otomo, J.; Wen, C.-j.; Ogura, M.; Takahashi, H. J. Electrochem. Soc. 2004, 151, J76. (11) Muroyama, H.; Matsui, T.; Kikuchi, R.; Eguchi, K. J. Electrochem. Soc. 2006, 153, A1077. (12) Otomo, J.; Minagawa, N.; Wen, C.-j.; Eguchi, K.; Takahashi, H. Solid State Ionics 2003, 156, 357. (13) Matsui, T.; Kukino, T.; Kikuchi, R.; Eguchi, K. Electrochem. SolidState Lett. 2005, 8, A256. (14) Matsui, T.; Kukino, T.; Kikuchi, R.; Eguchi, K. J. Electrochem. Soc. 2006, 153, A339. (15) Matsui, T.; Kukino, T.; Kikuchi, R.; Eguchi, K. Electrochim. Acta 2006, 51, 3719. (16) Muroyama, H.; Kudo, K.; Matsui, T.; Kikuchi, R.; Eguchi, K. Solid State Ionics 2007, 178, 1512. (17) Matsui, T.; Takeshita, S.; Iriyama, Y.; Abe, T.; Ogumi, Z. J. Electrochem. Soc. 2005, 152, A167. (18) Matsui, T.; Kazusa, N.; Kato, Y.; Iriyama, Y.; Abe, T.; Kikuchi, K.; Ogumi, Z. J. Power Sources 2007, 171, 483. (19) Itoh, M.; Hattori, H.; Tanabe, K. J. Catal. 1974, 35, 225. (20) JCPDS File card no. 34-0130; Joint Committee on Powder Diffraction Standards: Swarthmore, PA. (21) JCPDS File card no. 22-1274; Joint Committee on Powder Diffraction Standards: Swarthmore, PA. (22) JCPDS File card no. 38-1468; Joint Committee on Powder Diffraction Standards: Swarthmore, PA. (23) Muroyama, H.; Matsui, T.; Kikuchi, R.; Eguchi, K. J. Electrochem. Soc., submitted for publication. (24) Norbert, A.; Andre´, D. C. R. Acad. Sci. 1970, 270, 1781. (25) Lavrova, G. V.; Burgina, E. B.; Matvienko, A. A.; Ponomareva, V. G. Solid State Ionics 2006, 177, 1117. (26) Stakheev, A. Y.; Shpiro, E. S.; Apijok, J. J. Phys. Chem. 1993, 97, 5668. (27) Pabo´n, E.; Retuert, J.; Quijada, R.; Zarate, A. Microporous Mesoporous Mater. 2004, 67, 195.

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