The Proton and Oxygen Ion Conduction in a NaCl Based Composite Electrolyte

Abstract

Proton conducting and oxygen-ion conducting materials are widely studied for their applications in fuel cells, electrolysis cells, electrochemical reactors, and sensors [1±3]. Proton electrolyte materials, include water-containing materials (HUP, etc.), oxo acids and their salts (CsHSO4, etc.), and complex oxides (MCeO3, MZrO3, etc., where M ˆ Ca, Sr, Ba) [2]. The transportation of proton is based on the hydrogen-bond defects incorporated into the oxides or the rotation of tetrahedral XO4 [1±3]. In a word, most proton and=or oxygen-conducting materials are oxygen-containing materials. Furthermore, the well-known proton-conducting materials (MCeO3, MZrO3, etc.) and oxygen-conducting materials (YSZ, etc.) all operate at high temperatures (800± 1100 8C). Recently, in our experiments we found that a NaCl-based composite electrolyte showed distinct proton and oxygen ion conduction at intermediate temperatures. Moreover, pure NaCl belongs to the non-oxygen material systems, which are obviously different from the usual H‡ and O2y conductors. Compared with the reported proton and oxygen conductors, this material is very cheap and easily available, which will lower the operational cost of electrochemical devices using it as electrolyte. Sodium chloride was sintered at 850 8C for 2 h, pressed into tablets, coated with platinum paste on both sides, and calcined at 600 8C for 3 h to form porous electrodes before electrochemistry measurements. In this work, the H2=O2 fuel cell with pure NaCl as electrolyte (Fig. 1a) resulted in a current density of 70 mA cmy2 at a 100 mV terminal voltage, indicating that pure NaCl, which is a nonoxygen material, exhibits proton and=or oxygen ion conduction. To improve the structural strength and performance of the cells, a-Al2O3 was mixed with NaCl and this material was given the same experimental process as described previously. Of course, the NaCl-Al2O3 composite cannot be classi®ed as non-oxygen material system. The experimental results for both hydrogen and oxygen concentration cells indicate that the composite materials have both proton and oxygen conductivity between 650± 750 8C. In NaCl-Al2O3, composite solid electrolytes with a composition of 30 mol % Al2O3 have the highest conductivity in air [4, 5]. However, to our knowledge, no report on the proton or oxygen ion conduction of that material appears. The sintered tablets of pure inorganic salts usually have poor construction strength. The addition of a second phase (such as Al2O3, CeO2, SiO2) not only greatly improves the mechanical strength but it enhances the conductivity of the materials [6±8]. Therefore, in our experiments, we have mainly studied the properties of the hydrogen concentration cell (HCC), oxygen concentration cell (OCC), and the H2=O2 fuel cell of NaCl-Al2O3 composite electrolytes containing 30 mol % a-Al2O3. High-purity NaCl and a-Al2O3, with molar ratio of 7:3 were mixed