(Invited) Proton Transport in Inorganic Phosphates

The search for suitable electrolytes for intermediate temperature fuel cells remains a formidable challenge.   Intermediate-temperature (100-400 °C) devices would enable the development of simple, low cost fuel cells, free from the water management and fuel flexibility issues of polymer membrane fuel cells and without the system and materials challenges of high temperature fuel cell systems ¹.

Proton transport in inorganic phosphate crystalline compounds and glasses has been reported for many systems such as LaPO₄, CsHPO₄, metal pyrophosphates (MP₂O₇) and a variety of composite glass ceramic materials.

Nagao et. al ² reported in 2006 that the anhydrous proton conductor In doped SnP₂O₇ exhibited ionic conductivity on the order of 0.1 S/cm at temperatures of 150-350°C in water-free atmospheres.  Attempts to reproduce the high conductivity material have had varying degrees of success ^3-6.  Since the initial reports, many new publications on pyrophosphates have appeared. The authors of these reports attribute the proton conductivity of these materials to the generation of oxygen vacancies via aliovalent doping with concurrent hydrolysis of water filling the vacant sites and thus creating mobile protons.  The mechanism cannot reconcile the high conductivities of the parent un-doped MP₂O₇materials and the high conductivity of materials with phosphate to metal ratios greater than 2.  

Other investigators have hypothesized that amorphous, phosphate rich phases derived from the material synthesis precursors may remain in grain boundaries and serve to enhance conductivity, as suggested for proton conducting LaPO₄ materials⁷, Sn_0.9In_0.1P₂O₇⁸ and also demonstrated for un-doped SnP₂O₇⁶. 

We have extensively studied the temperature and atmosphere dependent conductivity, thermal behavior, crystallography^{9, 10} and hydrogen vibration spectra of In³⁺-doped SnP₂O₇systems, varying the In concentration, and the metal to phosphate ratios.

Inelastic neutron scattering on dehydrated, hydrated and deuterated samples was particularly useful for probing the dynamics of hydrogen within the material and for identification of the hydrogen bonding within the materials.  A complex picture of low temperature proton transport dominated by an amorphous polyphosphate intergranular material and a high temperature conductivity that is influenced by indium doping has arisen.

Our study provides insights towards the proper investigation of phosphate materials and perhaps, new strategies to produce composite ceramic-glass proton conductors. The effects of crystalline/amorphous interfacial interactions also cannot be discounted.

References

1.         O. Paschos, J. Kunze, U. Stimming and F. Maglia, J Phys Condens Matter 23(23), 234110 (2011).

2.         M. Nagao, A. Takeuchi, P. Heo, T. Hibino, M. Sano and A. Tomita, Electrochemical and Solid-State Letters 9(3), A105 (2006).

3.         S. R. Phadke, C. R. Bowers, E. D. Wachsman and J. C. Nino, Solid State Ionics 183(1), 26-31 (2011).

4.         S. Tao, Solid State Ionics 180(2-3), 148-153 (2009).

5.         Y. Sato, Y. Shen, M. Nishida, W. Kanematsu and T. Hibino, Journal of Materials Chemistry 22(9), 3973 (2012).

6.         X. Xu, S. Tao, P. Wormald and J. T. S. Irvine, Journal of Materials Chemistry 20 (36), 7827 (2010).7.    G. Harley, R. Yu and L. Dejonghe, Solid State Ionics 178(11-12), 769-773 (2007).

8.         M. L. Einsla, R. Mukundan, E. L. Brosha and F. H. Garzon, ECS Transactions 11(1), 347-355 (2007).

9.         C. R. Kreller, M. S. Wilson, R. Mukundan, E. L. Brosha and F. H. Garzon, ECS Electrochemistry Letters 2(9), F61-F63 (2013).

10.        C. R. Kreller, M. S. Wilson, R. Mukundan, E. L. Brosha and F. H. Garzon, ECS Transactions 57(1), 1009-1018 (2013).

 Acknowledgements

The authors would like to acknowledge the financial support of the US Department of Energy, Los Alamos Internal Directed Research Program (LANL-LDRD) for financial support