Synthesis and Characterization of Micro and Nano Ba 3-x K x H x (PO 4 2 and Ba 3-x Na x H x (PO 4 2

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Synthesis and Characterization of Micro and Nano Ba3-xKxHx(PO4)2 and Ba3-xNaxHx(PO4)2 Sun Hwi Bang1, Sam Chun1, and Adrian Highower1 1 Harvey Mudd College, 301 Platt Boulevard, Claremont, CA 91711, U.S.A. ABSTRACT Ba3-xKxHx(PO4)2 is a candidate solid-state proton conductor for solid acid fuel cells that is waterinsoluble. The measured conductivity of ~ 2.4 10-5 S cm-1 for the composition x=0.80 at 250°C is not competitive for solid acid fuel cell applications. This work investigates a methods for synthesizing solid acid electrolytes with the strategy of increasing proton conductivity by cation substitution and decreasing particle size. We report on the synthesis of nano Ba3-xKxHx(PO4)2 to a novel Ba3-xNaxHx(PO4)2 . X-ray diffraction was used to confirm the Ba3(PO4)2 crystal structure and measure lattice strain as a function of cation substitution. SEM confirmed the morphology of micro Ba3-xNaxHx(PO4)2 is substantially different from micro Ba3-xKxHx(PO4)2, suggesting that Ba3-xNaxHx(PO4)2 has a different growth kinetics. INTRODUCTION Solid acid fuel cells (SAFCs) are propose to address the issues of humidification and fuel permeation which currently plague proton exchange membrane fuel cells (PEMFCs). SAFCs operate at elevated temperature of 100 – 300°C, while PEMFCs are limited to temperatures ~100°C due to the critical role of water in their proton conduction mechanism. SAFC are based on solid acids proton conducting electrolytes with common stoichiometries MH2XO4 and M3H(XO4)2, where M = Cs, Rb, K, Na, or NH4, and X = P, S, Se, or As. In the superprotonic state, which the proton conductivity jumps by 3-4 orders of magnitude, conductivity increases in the range of 10-3 – 10-1 -1 cm-1 [1]. Above the superprotonic transition temperature, the hydrogen-oxygen bonds can be broken thermally, by the rotation of nearby PO4 (Figure 1). The proton hops to another interstitial site as the electron is accepted by the PO4. This creates a defect consisting of a H2PO4+1 and PO4-1. A proton on the H2PO4+1 hops to another interstitial site, causing movement of the defect through the crystal. Proton conduction occurs by defect movement through proton hopping and the repeated breaking and formation of hydrogen bonds. It has been shown that all known superprotonic conductors are decidedly water-soluble which ultimately decreases the cell efficiency by the means of highly humidified atmosphere on to the electrolytes. Therefore, the water insolubility of both Ba3(PO4)2 and BaHPO4 suggest that Ba3-xKxHx(PO4)2 (referred to as BKHP) will be highly insoluble in water. Despite its waterinsolubility, BKHP is not a competitive fuel cell material for, its low proton conductivity: Ba3-xKxHx(PO4)2:

H = 2.4  10-5 S cm-1 (x = 0.8, 250 °C) [2]

We have developed synthesis of novel sodium substituted, Ba3-xNaxHx(PO4)2 compounds with Ba3(PO4)2 crystal structures in order to improve proton conductivity. Barium occupies both M(1) and M(2) site of BKHP (Figure 1). Potassium has been found to substitute exclusively on the M(2) site with few hydrogen n