Stability and Conductivity of Gd 2 ((Mo 1/3 Mn 2/3 ) x Ti 1-x ) 2 O 7
- PDF / 401,043 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 87 Downloads / 238 Views
J.J. Sprague, 0. Porat, H. L, Tuller Crystal Physics and Electroceramics Laboratory, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA ABSTRACT
A composite solid state electrochemical device, with (GdlxCa×) 2Ti 2O7 serving as the electrolyte and Gd 2(Ti1 .XMo•) 20 7 (GT-Mo) as the anode has recently been proposed. The latter exhibits high levels of mixed conduction under reducing atmospheres, but decomposes at high P0 2. We have recently succeeded in extending the stability limits of the GT-Mo to higher P0 2 with the addition of Mn. In this study, we report on the conductivity and stability of Gd 2((Mo1 /3Mn2/3)XTi1 X)20 7 (GMMT) as a function of P0 2, T, and composition utilizing impedance spectroscopy and x-ray diffraction. The addition of Mn extends the stability region of the material to P0 2 = 1atm with little change in the magnitude of the conductivity. Defect models explaining the dependence of the conductivity on oxygen partial pressure are presented. Preliminary results from the use of an electronic blocking sandwich cell used to isolate the ionic conductivity of GMMT are also presented. INTRODUCTION
In recent years, the development of a monolithic solid oxide fuel cell has received considerable attention [1-3]. In the monolithic design, a specific crystalline structure and phase serves as a template for the cell. The composition is then spatially modulated to achieve the desired functionality for each component of the cell. This kind of cell offers enhanced thermal, mechanical, and chemical stability with respect to traditional multi-phase cells. Past work [4-6] has shown that the pyrochlore system, Gd 2Ti 20 7 (GT), exhibits the versatility needed to make this monolithic cell. It has been shown that acceptor doped GT (with Ca for example) shows high ionic conductivity (ai > 10.2 at 1000 'C) with negligible electronic conductivity over a very wide range of oxygen partial pressure (Po2) and temperature [4], making it suitable as the electrolyte. On the other hand, recent work on Mo doped GT (GT:Mo) indicates that this material exhibits high electronic conductivity (ae > 1015 at 1000 'C) in addition to high ionic conductivity (a>i- 10i1 at 1000 °C) [6]. Mixed ionic and electronic conduction (MIEC) has been found to be desirable for achieving good electrode performance. GT:Mo was found to be stable under reducing atmospheres (P02< 10-15 atm), making it a suitable choice as anode. However, it decomposed under the oxidizing conditions necessary for the cathode. This decomposition was attributed to the variable valent Mo oxidizing from +4 to +6 accompanied by a sharp increase in oxygen interstitial concentration. The goal of this work is to fabricate and investigate stable pyrochlore materials with adequate levels of MIEC that remain stable in oxidizing environments and are suitable as cathode materials. With GT:Mo as the starting material, we co-dope with variably valent Mn to stabilize the Mo oxidation at high Po2. The goal is to compensate the
Data Loading...
