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Electrochemical synthesis of metal phosphates by cathodic reduction
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Materials Research Bulletin 36 (2001) 2043–2050
Electrochemical synthesis of metal phosphates by cathodic reduction M. Dinamani, P. Vishnu Kamath* Department of Chemistry, Central College, Bangalore University, Bangalore 560 001, India (Refereed) Received 1 February 2001; accepted 1 May 2001
Abstract Acidified aqueous metal nitrate solutions containing dissolved phosphate ions yield the corresponding metal phosphates upon cathodic reduction. In alkaline earth metal containing electrolytes [phosphate source, (NH4)2HPO4], the phase obtained is MHPO4 (M ⫽ Sr, Ba). Controlled growth of adherent micrometer-thick coatings of SrHPO4 could be achieved on the cathode substrate. In Mg, Co, and Ni containing electrolytes, electrosynthesis yields the hydrates of the corresponding NH4MPO4 (M ⫽ Mg, Co, Ni) at low (25 mA cm⫺2) current densities, while at high (60 mA cm⫺2) current densities, X-ray amorphous metal orthophosphates are obtained. The use of Na3PO4 as the phosphate source also yielded X-ray amorphous products. In the Co system, the electrosynthesized phase, NH4CoPO4䡠H2O was different from that obtained by chemical precipitation. This study demonstrates the versatility of electrosynthesis. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Inorganic compounds
1. Introduction Metal phosphates are important inorganics as well as biomaterials. Both alkaline earth metals and transition metal phosphates find a wide variety of applications as heterogeneous catalysts [1,2], low thermal expansion ceramic materials [3], ion exchangers [4], and biocompatible coatings for metal endoprostheses [5]. Metal phosphates are conventionally prepared by chemical precipitation using H3PO4 in the presence of a base [6], molten salt flux
* Corresponding author. E-mail address: [email protected] (P.V. Kamath). 0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 1 ) 0 0 6 8 2 - 1
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synthesis [7], and the sol-gel method [8]. From all these methods the desired material can be obtained in the powder form in bulk. Electrosynthesis has emerged as a convenient and inexpensive technique for the synthesis of metal oxides and hydroxides [9,10]. The product can be obtained in bulk, as a thin/thick film or a coating. Electrosynthesis often yields metastable phases, which cannot be obtained by chemical synthesis. This technique has been successfully used for the synthesis of apatitic calcium phosphates [11,12] by cathodic reduction of an acidified aqueous solution of calcium nitrate containing dissolved phosphate ions. Cathodic reduction leads to electrogeneration of base resulting from a number of reactions [9,13], chief among which are the nitrate reduction and hydrogen evolution reactions [14]. In this article, we extend this technique to the synthesis of a wide variety of metal phosphates. We illustrate how the variation in deposition conditions can lead to product selectivity.
2. Experimental For bulk synthesis the electrolyte was prepared by adding a solution of (NH4)2HPO4 (2.4 g in 20 ml of deionized water) to the desired metal nitrate solution (1 M, 25 ml). The resulting slurry was dissolved in a minimum volume of dilute HNO3, and the clear solution was diluted to 100 ml. This final solution (0.25 M with respect to metal ion) was taken in the cathode chamber of a divided cell. Electrolysis was carried out with Pt flags (surface area, 2 cm2). A 0.25/M KNO3 solution was taken in the anode chamber. The synthesis was carried out galvanostatically (current density, 25– 60 mA cm⫺2) for 4 h. In all cases, the formation of a film could be visually observed within minutes of electrolysis. On continuing the deposition, the film is found to thicken and flake off. At the end of the synthesis, the material was filtered, washed, and dried at 65–70°C. In a few
Fig. 1. Powder XRD patterns of electrosynthesized BaHPO4 (a) and SrHPO4 (b).
M. Dinamani, P. Vishnu Kamath / Materials Research Bulletin 36 (2001) 2043–2050
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Fig. 2. Weight of the electrosynthesized coating of SrHPO4 as a function of concentration [time, 5 min; current density, 15 mA cm⫺2] (a) current density [time, 5 min; bath concentration, 0.25 M] (b), and time [bath concentration 0.25 M; current density, 15 mA cm⫺2] (c).
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Fig. 3. Scanning electron micrographs of SrHPO4 coatings at low (600⫻) (a) and high (3000⫻) (b) magnifications.
selected experiments, Na3PO4 was used as the phosphate source. For fabrication of coatings, electrolysis was carried out in an undivided cell using a stainless steel (SS 316) flag (surface area, 6 cm2) as the cathode and a cylindrical Pt mesh (geometric area, 28 cm2) as the counter. The deposition current (10 – 40 mA cm⫺2) and duration (1–30 min) were chosen suitably to obtain the product as an adherent coating. Prior to electrodeposition, the stainless steel flags were cleaned with detergent and electrochemically polished as described elsewhere [15]. All samples were characterized by powder X-ray diffraction (JEOL Model JDX8P powder diffractometer, Co K␣ source, ⫽ 1.79 Å), IR spectroscopy (Nicolet Model Impact 400D FTIR spectrometer, KBr pellets, 4 cm⫺1 resolution) and thermogravimetry (lab-built system, heating rate, 5°C min⫺1). Scanning electron micrographs were recorded using a JEOL Model JSM 840A scanning electron microscope fitted with a Link ISIS, Oxford Model EDX analyzer.
M. Dinamani, P. Vishnu Kamath / Materials Research Bulletin 36 (2001) 2043–2050
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Fig. 4. Powder XRD patterns of nickel phosphate electrosynthesized at high [60 mA cm⫺2] (a) and low [25 mA cm⫺2] (b) current densities.
3. Results and discussion On electrolysis of an aqueous metal nitrate solution, both nitrate reduction and hydrogen evolution reactions take place at the cathode leading to the generation of hydroxyl ions. The resulting increase in pH close to the electrode causes the precipitation of the metal ion as a hydroxide. In phosphate-containing baths, the metal is preferentially deposited as a phosphate owing to its lower solubility compared to other compounds [16]. As an illustration, we studied phosphate baths containing the alkaline earth metals. Electrolysis resulted in the deposition of the corresponding hydrogen-phosphates MHPO4 (M ⫽ Ca, Sr, and Ba). Although the detailed results of the Ca system have been published elsewhere [11], we show in Fig. 1 the powder XRD patterns of the materials obtained in the case of Sr and Ba. These patterns can be readily indexed according to the reported crystal structures of ␣-SrHPO4 (PDF: 33–1335) and BaHPO4 (PDF: 17–929), respectively. In Fig. 2 the controlled growth of SrHPO4 coatings as a function of deposition time, current density, and electrolyte concentration is shown. In all cases the coating weight increases with continued deposition until a limiting thickness is reached, after which the coating flakes off. The limiting thickness of the coating was estimated to be 14 m from the density of SrHPO4 (3.544 g cm⫺3) [17]. In Fig. 3 the scanning electron micrographs of the SrHPO4 coatings are shown. It is evident that the substrate is completely covered. At low
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Fig. 5. IR spectra of nickel phosphate electrosynthesized at low [25 mA cm⫺2] (a) and high [60 mA cm⫺2] (b) current densities.
magnification granular particles of approximately 10 m size can be seen. At higher magnifications, the granules exhibit a fibrous morphology. The EDXA data yield a 1:1 ratio for Sr:P, as expected of the SrHPO4 phase. In Fig. 4 the powder XRD patterns of the nickel phosphates synthesized at different current densities are shown. At low (25 mA cm⫺2) currents, NH4NiPO4䡠6H2O (PDF: 21–34) was obtained, whereas at high (60 mA cm⫺2) currents, the product was X-ray amorphous. To characterize the X-ray amorphous phase, the infrared spectra of the two compounds were recorded and compared (see Fig. 5). The former exhibits a five-peak spectrum in the 1100 – 800 cm⫺1 region characteristic of HPO2⫺ anion. The latter 4 exhibits a single peak spectrum at 1040 cm⫺1 characteristic of the more symmetric orthophosphate group. Thermogravimetry as well as isothermal (900°C, 4 h) weight loss studies of this sample showed a 28% weight loss attributable to the dehydration of the Ni3(PO4)2䡠8H2O. It can, therefore, be concluded that electrodeposition at high currents results in the formation of an amorphous phase identifiable as Ni3(PO4)2䡠8H2O. Similar results are observed in the Mg system. Fig. 6 shows the powder XRD patterns of cobalt phosphate system. Electrosynthesis yielded a biphasic mixture comprising NH4CoPO4䡠H2O (PDF: 21–793) as the predominant
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Fig. 6. Powder XRD patterns of cobalt phosphate synthesized electrochemically (a) and chemically (b). Peaks marked by asterisk in curve (a) are impurities that correspond to the phase obtained by chemical synthesis as shown in curve (b).
phase. The impurity lines corresponded to Co3(PO4)2䡠8H2O (PDF: 33– 432). Chemical precipitation, on the other hand, yielded single-phase Co3(PO4)2䡠8H2O. In summary, both alkaline earth and transition metal phosphates can be synthesized electrochemically. Different products can be selected by varying the bath composition and deposition conditions. The products can be grown into thick coatings on the electrode or can be obtained as polycrystalline samples in bulk. Acknowledgments The authors thank the Department of Science and Technology, Government of India for financial support, the Solid State and Structural Chemistry Unit and the Department of Metallurgy, Indian Institute of Science for powder diffraction, and electron microscopy facilities, respectively. References [1] A. Legrouri, J. Lenzi, M. Lenzi, React. Kinet. Catal. Lett. 62 (1997) 313. [2] A. Legrouri, S.S. Romdhane, J. Lenzi, M. Lenzi, G. Bonel, J. Mater. Sci. 31 (1996) 2469.
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[3] R. Brochu, M. El-Yacoubi, M. Louer, A. Serghini, M. Alami, D. Louer, Mater. Res. Bull. 32 (1997) 15. [4] M.-H. Herzog-Cance, D.J. Jones, R.E. Mejjad, J. Roziere, J. Tomkinson, J. Chem. Soc. Faraday Trans. 88 (1992) 2275. [5] J.I. Huaxia, C.B. Ponton, P.M. Marquis, J. Mater. Sci. Mater. Med .3 (1992) 281. [6] A. Goni, J.L. Pizarro, L. M. Lezama, G. E. Barberis, M. I. Arriortua, T. Rojo, J. Mater. Chem. 6 (1996) 421. [7] A.V. Barabanova, A.O. Turakulova, V.V. Lunin, P. Afanasiev, J. Mater. Chem. 7 (1997) 791. [8] F.J. Perez-Reina, P. Olivera-Paster, E. Rodriguez-Castellon, A. Jimenez-Lopez, J.L.G. Fierro, J. Solid State Chem. 122 (1996) 231. [9] G.H.A. Therese, P.V. Kamath, Chem. Mater. 12 (2000) 1195. [10] I. Zhitomirsky, Am. Ceram. Soc. Bull. September (2000) 57. [11] G.H.A. Therese, P.V. Kamath, G.N. Subbanna, J. Mater. Chem. 8 (1998) 405. [12] L.-Y. Huang, K.-W. Xu, J. Lu, J. Mater. Sci. Mater. Med. 11 (2000) 667. [13] Y. Matsumoto, T. Morikawa, H. Adachi, J. Hombo, Mater. Res. Bull. 27 (1992) 1319. [14] J.A. Switzer, Am. Ceram. Soc. Bull. 66 (1987) 1521. [15] D.A. Corrigan, R.M. Bendert, J. Electrochem. Soc. 136 (1989) 723. [16] D. Dobos, Electrochemical Data, Elsevier Scientific, Amsterdam, 1975. [17] R.C. Weast, Ed., CRC Handbook of Chemistry and Physics, 67th ed., CRC Press, Boca Raton, FL, 1986.
Materials Research Bulletin 36 (2001) 2043–2050
Electrochemical synthesis of metal phosphates by cathodic reduction M. Dinamani, P. Vishnu Kamath* Department of Chemistry, Central College, Bangalore University, Bangalore 560 001, India (Refereed) Received 1 February 2001; accepted 1 May 2001
Abstract Acidified aqueous metal nitrate solutions containing dissolved phosphate ions yield the corresponding metal phosphates upon cathodic reduction. In alkaline earth metal containing electrolytes [phosphate source, (NH4)2HPO4], the phase obtained is MHPO4 (M ⫽ Sr, Ba). Controlled growth of adherent micrometer-thick coatings of SrHPO4 could be achieved on the cathode substrate. In Mg, Co, and Ni containing electrolytes, electrosynthesis yields the hydrates of the corresponding NH4MPO4 (M ⫽ Mg, Co, Ni) at low (25 mA cm⫺2) current densities, while at high (60 mA cm⫺2) current densities, X-ray amorphous metal orthophosphates are obtained. The use of Na3PO4 as the phosphate source also yielded X-ray amorphous products. In the Co system, the electrosynthesized phase, NH4CoPO4䡠H2O was different from that obtained by chemical precipitation. This study demonstrates the versatility of electrosynthesis. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Inorganic compounds
1. Introduction Metal phosphates are important inorganics as well as biomaterials. Both alkaline earth metals and transition metal phosphates find a wide variety of applications as heterogeneous catalysts [1,2], low thermal expansion ceramic materials [3], ion exchangers [4], and biocompatible coatings for metal endoprostheses [5]. Metal phosphates are conventionally prepared by chemical precipitation using H3PO4 in the presence of a base [6], molten salt flux
* Corresponding author. E-mail address: [email protected] (P.V. Kamath). 0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 1 ) 0 0 6 8 2 - 1
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M. Dinamani, P. Vishnu Kamath / Materials Research Bulletin 36 (2001) 2043–2050
synthesis [7], and the sol-gel method [8]. From all these methods the desired material can be obtained in the powder form in bulk. Electrosynthesis has emerged as a convenient and inexpensive technique for the synthesis of metal oxides and hydroxides [9,10]. The product can be obtained in bulk, as a thin/thick film or a coating. Electrosynthesis often yields metastable phases, which cannot be obtained by chemical synthesis. This technique has been successfully used for the synthesis of apatitic calcium phosphates [11,12] by cathodic reduction of an acidified aqueous solution of calcium nitrate containing dissolved phosphate ions. Cathodic reduction leads to electrogeneration of base resulting from a number of reactions [9,13], chief among which are the nitrate reduction and hydrogen evolution reactions [14]. In this article, we extend this technique to the synthesis of a wide variety of metal phosphates. We illustrate how the variation in deposition conditions can lead to product selectivity.
2. Experimental For bulk synthesis the electrolyte was prepared by adding a solution of (NH4)2HPO4 (2.4 g in 20 ml of deionized water) to the desired metal nitrate solution (1 M, 25 ml). The resulting slurry was dissolved in a minimum volume of dilute HNO3, and the clear solution was diluted to 100 ml. This final solution (0.25 M with respect to metal ion) was taken in the cathode chamber of a divided cell. Electrolysis was carried out with Pt flags (surface area, 2 cm2). A 0.25/M KNO3 solution was taken in the anode chamber. The synthesis was carried out galvanostatically (current density, 25– 60 mA cm⫺2) for 4 h. In all cases, the formation of a film could be visually observed within minutes of electrolysis. On continuing the deposition, the film is found to thicken and flake off. At the end of the synthesis, the material was filtered, washed, and dried at 65–70°C. In a few
Fig. 1. Powder XRD patterns of electrosynthesized BaHPO4 (a) and SrHPO4 (b).
M. Dinamani, P. Vishnu Kamath / Materials Research Bulletin 36 (2001) 2043–2050
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Fig. 2. Weight of the electrosynthesized coating of SrHPO4 as a function of concentration [time, 5 min; current density, 15 mA cm⫺2] (a) current density [time, 5 min; bath concentration, 0.25 M] (b), and time [bath concentration 0.25 M; current density, 15 mA cm⫺2] (c).
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Fig. 3. Scanning electron micrographs of SrHPO4 coatings at low (600⫻) (a) and high (3000⫻) (b) magnifications.
selected experiments, Na3PO4 was used as the phosphate source. For fabrication of coatings, electrolysis was carried out in an undivided cell using a stainless steel (SS 316) flag (surface area, 6 cm2) as the cathode and a cylindrical Pt mesh (geometric area, 28 cm2) as the counter. The deposition current (10 – 40 mA cm⫺2) and duration (1–30 min) were chosen suitably to obtain the product as an adherent coating. Prior to electrodeposition, the stainless steel flags were cleaned with detergent and electrochemically polished as described elsewhere [15]. All samples were characterized by powder X-ray diffraction (JEOL Model JDX8P powder diffractometer, Co K␣ source, ⫽ 1.79 Å), IR spectroscopy (Nicolet Model Impact 400D FTIR spectrometer, KBr pellets, 4 cm⫺1 resolution) and thermogravimetry (lab-built system, heating rate, 5°C min⫺1). Scanning electron micrographs were recorded using a JEOL Model JSM 840A scanning electron microscope fitted with a Link ISIS, Oxford Model EDX analyzer.
M. Dinamani, P. Vishnu Kamath / Materials Research Bulletin 36 (2001) 2043–2050
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Fig. 4. Powder XRD patterns of nickel phosphate electrosynthesized at high [60 mA cm⫺2] (a) and low [25 mA cm⫺2] (b) current densities.
3. Results and discussion On electrolysis of an aqueous metal nitrate solution, both nitrate reduction and hydrogen evolution reactions take place at the cathode leading to the generation of hydroxyl ions. The resulting increase in pH close to the electrode causes the precipitation of the metal ion as a hydroxide. In phosphate-containing baths, the metal is preferentially deposited as a phosphate owing to its lower solubility compared to other compounds [16]. As an illustration, we studied phosphate baths containing the alkaline earth metals. Electrolysis resulted in the deposition of the corresponding hydrogen-phosphates MHPO4 (M ⫽ Ca, Sr, and Ba). Although the detailed results of the Ca system have been published elsewhere [11], we show in Fig. 1 the powder XRD patterns of the materials obtained in the case of Sr and Ba. These patterns can be readily indexed according to the reported crystal structures of ␣-SrHPO4 (PDF: 33–1335) and BaHPO4 (PDF: 17–929), respectively. In Fig. 2 the controlled growth of SrHPO4 coatings as a function of deposition time, current density, and electrolyte concentration is shown. In all cases the coating weight increases with continued deposition until a limiting thickness is reached, after which the coating flakes off. The limiting thickness of the coating was estimated to be 14 m from the density of SrHPO4 (3.544 g cm⫺3) [17]. In Fig. 3 the scanning electron micrographs of the SrHPO4 coatings are shown. It is evident that the substrate is completely covered. At low
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Fig. 5. IR spectra of nickel phosphate electrosynthesized at low [25 mA cm⫺2] (a) and high [60 mA cm⫺2] (b) current densities.
magnification granular particles of approximately 10 m size can be seen. At higher magnifications, the granules exhibit a fibrous morphology. The EDXA data yield a 1:1 ratio for Sr:P, as expected of the SrHPO4 phase. In Fig. 4 the powder XRD patterns of the nickel phosphates synthesized at different current densities are shown. At low (25 mA cm⫺2) currents, NH4NiPO4䡠6H2O (PDF: 21–34) was obtained, whereas at high (60 mA cm⫺2) currents, the product was X-ray amorphous. To characterize the X-ray amorphous phase, the infrared spectra of the two compounds were recorded and compared (see Fig. 5). The former exhibits a five-peak spectrum in the 1100 – 800 cm⫺1 region characteristic of HPO2⫺ anion. The latter 4 exhibits a single peak spectrum at 1040 cm⫺1 characteristic of the more symmetric orthophosphate group. Thermogravimetry as well as isothermal (900°C, 4 h) weight loss studies of this sample showed a 28% weight loss attributable to the dehydration of the Ni3(PO4)2䡠8H2O. It can, therefore, be concluded that electrodeposition at high currents results in the formation of an amorphous phase identifiable as Ni3(PO4)2䡠8H2O. Similar results are observed in the Mg system. Fig. 6 shows the powder XRD patterns of cobalt phosphate system. Electrosynthesis yielded a biphasic mixture comprising NH4CoPO4䡠H2O (PDF: 21–793) as the predominant
M. Dinamani, P. Vishnu Kamath / Materials Research Bulletin 36 (2001) 2043–2050
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Fig. 6. Powder XRD patterns of cobalt phosphate synthesized electrochemically (a) and chemically (b). Peaks marked by asterisk in curve (a) are impurities that correspond to the phase obtained by chemical synthesis as shown in curve (b).
phase. The impurity lines corresponded to Co3(PO4)2䡠8H2O (PDF: 33– 432). Chemical precipitation, on the other hand, yielded single-phase Co3(PO4)2䡠8H2O. In summary, both alkaline earth and transition metal phosphates can be synthesized electrochemically. Different products can be selected by varying the bath composition and deposition conditions. The products can be grown into thick coatings on the electrode or can be obtained as polycrystalline samples in bulk. Acknowledgments The authors thank the Department of Science and Technology, Government of India for financial support, the Solid State and Structural Chemistry Unit and the Department of Metallurgy, Indian Institute of Science for powder diffraction, and electron microscopy facilities, respectively. References [1] A. Legrouri, J. Lenzi, M. Lenzi, React. Kinet. Catal. Lett. 62 (1997) 313. [2] A. Legrouri, S.S. Romdhane, J. Lenzi, M. Lenzi, G. Bonel, J. Mater. Sci. 31 (1996) 2469.
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[3] R. Brochu, M. El-Yacoubi, M. Louer, A. Serghini, M. Alami, D. Louer, Mater. Res. Bull. 32 (1997) 15. [4] M.-H. Herzog-Cance, D.J. Jones, R.E. Mejjad, J. Roziere, J. Tomkinson, J. Chem. Soc. Faraday Trans. 88 (1992) 2275. [5] J.I. Huaxia, C.B. Ponton, P.M. Marquis, J. Mater. Sci. Mater. Med .3 (1992) 281. [6] A. Goni, J.L. Pizarro, L. M. Lezama, G. E. Barberis, M. I. Arriortua, T. Rojo, J. Mater. Chem. 6 (1996) 421. [7] A.V. Barabanova, A.O. Turakulova, V.V. Lunin, P. Afanasiev, J. Mater. Chem. 7 (1997) 791. [8] F.J. Perez-Reina, P. Olivera-Paster, E. Rodriguez-Castellon, A. Jimenez-Lopez, J.L.G. Fierro, J. Solid State Chem. 122 (1996) 231. [9] G.H.A. Therese, P.V. Kamath, Chem. Mater. 12 (2000) 1195. [10] I. Zhitomirsky, Am. Ceram. Soc. Bull. September (2000) 57. [11] G.H.A. Therese, P.V. Kamath, G.N. Subbanna, J. Mater. Chem. 8 (1998) 405. [12] L.-Y. Huang, K.-W. Xu, J. Lu, J. Mater. Sci. Mater. Med. 11 (2000) 667. [13] Y. Matsumoto, T. Morikawa, H. Adachi, J. Hombo, Mater. Res. Bull. 27 (1992) 1319. [14] J.A. Switzer, Am. Ceram. Soc. Bull. 66 (1987) 1521. [15] D.A. Corrigan, R.M. Bendert, J. Electrochem. Soc. 136 (1989) 723. [16] D. Dobos, Electrochemical Data, Elsevier Scientific, Amsterdam, 1975. [17] R.C. Weast, Ed., CRC Handbook of Chemistry and Physics, 67th ed., CRC Press, Boca Raton, FL, 1986.