1. Introduction

　CsH₂PO₄ (CHP) has been studied as a solid electrolyte for an intermediate-temperature fuel cell. It shows relatively high proton conductivity (10^–2 Scm^-1) in the intermediate temperature (100–300℃) range. However, humidification is necessary to maintain the crystal structure of CHP, and water resistance is extremely low. We reported that proton conducting glasses (CHP-PO₄) made from CHP and phosphoric acid shows high proton conductivity and excellent water resistance compared with CHP under dry N₂. However, the water resistance of the phosphate glasses needs to be improved for application to solid electrolytes. Metal oxides such as CaO and ZnO are often added for improving the water resistance of glass. Thus, in this study, we attempted to further improve the water resistance by adding the above metal oxide to CHP-PO₄. The effect on proton conductivity was also evaluated by AC impedance measurements.

2. Experimental

　CHP was synthesized according to the literature¹. A phosphate glass electrolyte was prepared by mixing CHP and 85 wt% H₃PO₄ at a molar ratio of 1: 1 and heating at 500 °C for 5 min, then raising the temperature to 900 °C for 15 min. The obtained melt was spread out on a copper plate heated at 180 °C, and quenched to obtain a phosphate glass electrolyte. Ca or Zn containing glasses (abbreviated as CHP-x(CaO or ZnO)-PO₄) were prepared as follows: CHP, H₃PO₄ and ZnO or CaO were mixed in Cs : (Zn or Ca) molar ratios of 1: x and melted and quenched under the same conditions as mentioned above. Raman spectra were measured using NRS3100 (JASCO Co. Ltd.). The proton conductivity of the electrolytes was measured by an AC two-terminal method under dry nitrogen using an impedance analyzer (Yokogawa, HP-4192A). The water resistance test was conducted by the following method: The sample was immersed in 200 mL of ultrapure water with stirring at 100 rpm, and the amount of eluted hydrogen ions per unit surface area of the sample was determined with a pH meter (CUSTOM Co. Ltd, IWC-5).

3. Results and discussion

The results of the water resistance test of phosphate glass electrolytes are shown in Fig. 1. It was revealed that increase in the ZnO or CaO ratio in the samples results in the improvement of the water resistance.

　The proton conductivities of the samples containing Zn or Ca were lowered to 10^–3–10^–4 S cm^–1. Raman spectroscopic analysis of the phosphate glass revealed that the peak derived from ν(PO₂)_syn of the phosphate glass in CHP-x(CaO or ZnO)-PO₄ shifted to the higher frequency by about 10 cm^-1, which is attributable to the shorter phosphate chain. The present result suggested that the metal ions cross-linked phosphate chains and contribute the improvement of water resistance for the electrolytes.

Reference

1. 1. Qing et al., Electrochim. Acta, 169, 219–226, (2015).