Intermediate-Temperature Proton Conductors

Tliis class of material comprises structures with D = 3 or 2 (thus, more thermally stable than the D = 0 structures discussed in Section 7.6.2) containing intrinsic protonic species (H, OH, H3O, or NH4), in contrast to extrinsic proton conductors (as discussed in Section 7.6.3). Therefore, their protonic conductivities are independent of the surrounding atmosphere, although their decomposition temperatures (as listed below in this section) should be increased at high partial pressures of water and/or ammonia. In addition, they do not contain basic components such as SrO or BaO that cause the instability of perovskites in CO2 and steam. 

Unprecedented high protonic conductivities, of 0.1-0.2 S cm at 200-350 °C in unhumidified air, were recently reported [308, 309] for SnP2O7 with trivalent dopants (Al or In). The published formulas, Sni-xM + PzO . are, however, not charge-balanced, and the presence of x protons is implied. It is likely that the observed high conductivities might be due to residual liquid phosphoric acid used in excess for the synthesis these materials require careful re-examination. 

1 Burmakin, E.I. (1992) Alkali Ion Cond-ucting Solid Electrolytes, Nauka, Moscow (in Russian). 

2 Ivanov-Schitz, A.K. and Murin, LV. (2000) lonika Tverdogo Tela (Solid State Ionics), Vol. 1, St. Petersburg University Press, Russia (in Russian). 

3 Mehrer, H. (2007) Diffusion in Solids, Springer-Verlag, Berlin-Heidelberg, 

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Conductivity of Nation in comparison to some intermediate-temperature proton conductors and the oxide ion conductivity of YSZ (yttria-stabilized zirconia). (From Kreuer, K. D. et al 2004. Chemical Reviews 104 4637-4678.)... 

Steininger, H., Schuster, M., Kreuer, K. D., Kaltbeitzel, A., Bingol, B., Meyer, W. H., Schauff, S., Brunklaus, G., Maier, J. and Spiess, H. W. 2007. Intermediate temperature proton conductors for PEM fuel cells based on phosphonic acid as protogenic group A progress report. Physical Chemistry Chemical Physics 9 1764-1773. 

Steininger H, Schuster M, Kreuer KD, Maier J (2006) Intermediate temperature proton conductors based on phosphonic acid fimctimialized oligosiloxanes. Solid State Itmics 177 2457-2462... 

Ammonium polyphosphate composite-based proton conductors for the intermediate temperature range (200°C-300°C) have been found to possess good proton conductivity (0.1 Scm at 300°C) under humidified conditions [147], This temperature range would not have any kinetics or CO poisoning problem, but improving stability of the proton conductor in fuel cell conditions is still a challenge [148-151]. 

Nagao, M., Namekata, Y., Hibino, T., Sano, M. and Tomita, A. (2006) Intermediate-temperature NOx sensor based on an In +-doped SnP2O7 proton conductor. Electrochem. Solid-State Lett.,... 

Nagao M, Namekata Y, Hibino T, Sano M, Tomita A (2006) Intermediate-temperature NO sensor based on an ltf+-doped SnP Oj proton conductor. Electrochem Solid State Lett 9 H48-H51 Narita H, Zhang YC, Mizusaki J, Tagawa H (1995) Sohd state COj sensor using an electrolyte in the system Li2C03-Li3P0 -Al303. Solid State Ionics 79 349-353... 

Ito N, lijima M, Kimura K, Iguchi S (2005) New intermediate temperature fuel cell with ultra-thin proton conductor electrolyte. J Power Sources 152 200-203... 

FabbriE, D Epifanio A, Di Bartolomeo E,LicocciaS, Traversa E (2008) Tailoring the chemical stability of Ba(Ceo.8-xZrx)Yo.203 5 protonic conductors for intermediate temperature solid oxide fuel cells (IT-SOFCs). Solid State Ionics 179 558-564... 

E., Licoccia, S., and Traversa, E. (2008) Design of BaZro,8Yo,203 a protonic conductor to improve the electrochemical performance in intermediate temperature solid oxide fuel cells (TF-SOFCs). Fuel Cells, 8 (1), 69 76. 

Acceptor-doped LaBOa also displays modest proton conductivity [64]. Recently, it was found that lanthanum oxyborate, La26027(B03)s> contains 1 vacant oxygen position among the 28 and that this can be hydrated [65]. The material thus becomes a proton conductor at intermediate and low temperatures. 

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