Proton Conducting ZrO2-1.6P2O5 Electrolyte Hybridized with ZnO-2P2O5 for Intermediate Temperature Fuel Cells

Abstract

In recent years, fuel cells have been attracting much attention as clean energy sources with the progress of global warming. However, a large amount of platinum was required as a catalyst for the fuel cells operating at a low temperature range (~ 80°C) because not only the catalyst was poisoned by carbon monoxide but also platinum has insufficient activity for an oxygen reduction reaction. Intermediate temperature full cells (ITFC) operating at 150 to 300°C are one of the solutions that reduce the amount of platinum. We reported that ZrO2-1.6P2O5 electrolytes have a high proton conductivity in the temperature range of 150 to 300°C and that the performance of water resistance, as well as gas barrier property, was improved by the hybridization with proton conductive ZnO-2P2O5 glass. In this study, we report on the preparation and characterization of silane-treated ZrO2-1.6P2O5 electrolytes hybridized with ZnO-2P2O5 glass electrolyte. A ZrO2-1.6P2O5 electrolyte (abbreviated as EL) was synthesized by mixing (NH4) 2HPO4 and ZrCl2O·8H2O at a stoichiometric ratio, followed by a heat-treatment at 500°C for 45 min under 10 mol% H2O. CaO doped ZnO-2P2O5 glass powders (abbreviated as ZnO-2P2O5 + 0.3 M CaO) were prepared via the following process: After Zn powder was dissolved in 86 wt% phosphoric acid at various stoichiometric ratio, A CaO powder was added in the solution to be 0.3 mol/L with respect to the volume of the phosphoric acid. The suspension melted at 900°C was quenched by pressing on a copper plate at 300°C to vitrify the sample. The NaCl powder was grinded by a ball milling with pottery balls (10 mm in diameter) and classified by sieving. The glass powders was mixed with the milled a NaCl powder (55 vol%) and pressed into a pellet (12 mm in diameter) at 340 MPa, followed by a heat-treatment at 300°C for 30 min. The pellet was immersed in 300 mL of ultra-pure water to produce a porous glass matrix by removing the NaCl particles in the pellet (denoted as ZnO-2P2O5 + 0.3 M CaO) -EL (NaCl particle diameter)). The porous glass matrix was immersed in 1 wt% octadecyltrichlorosilane-toluene solution for 3 min, followed by the injection of 1 mL of the solution into the glass matrix on a filter bottle with an aspirator. The silane-treated porous glass matrix was filled with EL-86 wt% phosphoric acid suspension on a filter bottle with an aspirator. Then, the pellet was again immersed in the 1 wt% octadecyltrichlorosilane-toluene solution for 3 min (added -SP to a sample name). The proton conductivity of the hybrid electrolytes was measured by an AC impedance analyzer (HP-4192A, Yokogawa) with two-terminal configuration. The water resistance of the electrolyte was calculated from the change of pH in 200 mL Milli-Q water. The hydrogen gas permeability of the electrolytes was measured by gas chromatography (GC-8A, Shimadzu). Fig. 1 shows the result of water resistance tests for the electrolytes. The initial rate of dissolution for silane-treated samples using powders(φ=76 μm and 9.7 μm) were 4.12×10-8 mol cm-2 s-1 and 8.28×10-8 mol cm-2 s-1, respectively. These values are 30 and 43 times smaller than those of untreated ones; the silane-treated sample using NaCl (φ=76 μm) powder showed 64 times higher water resistance than that of EL. Acknowledgement:This study was partially supported by a Grant-in-Aid for Young Scientists (B) (No. 26820321) from Japan Society for the Promotion of Science. Figure 1