Proton conductivity and microstructures of the core-shell type solid electrolytes in the MO₂-In₂O₃-P₂O₅ (MTi, Sn, Zr) systems

Abstract

The proton conducting 0.9MO₂·0.05In₂O₃·1.3P₂O₅ (MTi, Sn, Zr) electrolytes based on a core-shell structure were synthesized by a ball milling method. The core-shell type electrolytes showed the proton conductivities ranging from a higher value than those of Nafion membranes to 10^− 5 Scm^− 1 at intermediate temperatures of 150–200 °C, depending on the heat-treatment conditions. The samples with high conductivity were proved to adopt a core-shell structure by SEM observation, powder XRD analysis and ³¹P MAS-NMR measurements.

Introduction

Polymer electrolyte membrane fuel cells have the potential to be a clean and efficient means of electrical energy generation. However, they also have the following problems to be solved for their widespread use: (1) cost and CO-poisoning of Pt catalysts, (2) depletion of Pt resource, (3) cost, long-term durability and thermal properties of perfluoro-sulfonated polymer electrolytes, (4) humidity control, and (5) low-quality thermal source. As one of the solutions of these problems, extensive efforts have been devoted to develop intermediate temperature fuel cells that operate at temperatures of 150–300 °C. A key component of the fuel cells is the electrolyte. Inorganic proton (H⁺) conducting solid electrolytes (HCEs) [1] such as CsH₂PO₄ [2], [3], NH₄PO₃ [4], [5], [6] are the promising candidates, taking into account the low production cast and thermal durability.

We have recently developed the core-shell type HCEs in the TiO₂-In₂O₃-P₂O₅ system, which shows the high proton conductivity (σ_H) of 10^− 1–10^− 2 Scm^− 1 at intermediate temperatures around 100 °C–200 °C [7], [8]. They are among the best inorganic HCEs for intermediate-temperature fuel cells. We have proposed the “core-shell model” that the electrolyte consists of the core particles of non-conductive crystalline diphosphate MP₂O₇ or its solid solutions and the shells of an amorphous phosphate phase around the cores. Fig. 1 shows the shimatic image of the core-shell structure. Protons transport in the shells (or along the surface of the shells) connected with each other. They were easily prepared at low cost by a ball milling [7] or a sol-gel method [8]. On the other hand, Nagao and co-workers have recently discovered the crystalline Sn_1−xIn_xP₂O₇ (x = 0.1) electrolytes [9], [10], [11]. High conductivities of over 10^− 1 Scm^− 1 at temperatures of 150–300 °C were reported even in a dry atmosphere. They have insisted that the proton conductivity results from hoppings of protons introduced into the bulk of Sn_0.9In_0.1P₂O₇ thorough a reaction between water vapor, and electron holes and oxygen vacancies, as with the case of the densified sintered bodies of high temperature proton conducting ceramics (ex. Yb-doped SrCeO₃) [12]. The powdered Sn_0.9In_0.1P₂O₇ sample was prepared by a low temperature sol-gel process, and the green compact, which presumably had a rather low density, was used without densification (sintering) for the conductivity measurements. As far as we know, such a high conductivity has never been found in any green compact of hard ceramic oxide powders without sintering. In addition, amorphous materials sometimes tend to remain in samples prepared through the sol gel process of phosphate-containing ceramics. More recently, Nagao et al. [11] have also reported that the P-excess, non-stoichiometric composition Sn_0.9In_0.1P_2.4O₇ (the charge is not balanced) shows a much higher conductivity than the stoichiometric composition Sn_0.9In_0.1P₂O₇; however they have not yet explained the mechanism for the enhancement, the role of the excess P content and the unbalanced charge. Thus, the proton conduction mechanism has been not yet fully elucidated.

In this study, we prepared the core-shell type electrolytes with the composition 0.9MO₂·0.05In₂O₃·1.3P₂O₅ (MTi, Sn, Zr) by ball milling with H₃PO₄, followed by heat-treatments, and the sintered bodies of 0.9MO₂·0.05In₂O₃·P₂O₅ and TiO₂·P₂O₅. This research has been undertaken in connection with the following purposes: 1) The establishment of the preparation process of the core-shell type electrolytes. 2) The evaluation of the proton conductivity of the core-shell electrolytes and the sintered bodies (i.e. the core parts). 3) Detailed microstructure analysis on the core-shell electrolytes by scanning electron microscopic observation, powder X-ray diffraction analysis and solid state ³¹P NMR spectroscopy. 4) The examination of effects of the reheat-treatments on the microstructures and the conductivity to elucidate the cause of the high conductivity in the diphosphate-based electrolytes (7–11).