Haile Group SURF2008

SURF 2009: Mixed conducting electrodes for solid-oxide fuel-cells

Direct Mentor: Dr. Yoshi Yamazaki <yamazaki@caltech.edu>

Course Requirements:
Introductory materials science course (MS 115a); Solid State Electrochemistry (MS 143); Chemistry lab course

Academic Standing:
Caltech students preferred, previous research experience required.

Background

Doped barium zirconate, BaZrO₃, has a high proton conductivity which renders it attractive as an electrolyte for fuel cell (and other) applications. The material becomes a proton conductor after the introduction of trivalent elements (e.g. Y, Gd) that substitute onto the Zr site and create, for reasons of charge balance, vacancies on the oxygen site. Upon exposure of the material to high humidity, water molecules are incorporated into the structure. Specifically, the water molecules dissociate into oxygen ions that fill up the formerly vacant oxyen sites and protons that associate themselves with oxygen ions in the structure to form internal hydroxyl groups. The protons can hop from one oxygen ion to the next, imparting high protonic conductivity to the material.

In addition to the electrolyte, a fuel cell requires electrodes. The electrodes must fulfill several functions, many of which are facilitated if the electrode material is capable of transporting both ionic and electronic charge. Introduction of transition metal ions (with variable valence) into oxides typically enhances their electronic conductivity.

Description

In order to develop mixed proton and electronically conducting materials, the student will begin with the proton conductor, BaZr0.8Y0.2O3-d, and systematically replace a portion of the Zr with Co, preparing the series, Ba(Zr0.8-xY0.2Cox)O3-d, and then measure the transport properties.

The student will synthesize the materials using chemical solution methods and confirm that the desired phase has been obtained by x-ray powder diffration. From loose powders, dense compacts will be prepared by standard ceramics processing methods (pressing and firing) and chemical analysis will be done by electron microprobe analysis. Electrical measurements will be performed by a.c. impedance spectroscopy under atmospheres with well-controlled water and oxygen partial pressures. An increase in conductivity with increasing water partial pressure is an indicator of proton conductivity, whereas an increase in conductivity with increasing oxygen partial pressre is an indicator of p-type electronic conductivity.

Evaluation of the performance of these electrodes in solid oxide fuel cells is considered beyond the scope of this ten week SURF project.

Additional Information

Interested students should review the following references prior to contacting either Prof. Haile or the research mentor, Dr. Yoshi Yamazaki. These papers will also be relevant for preparing the SURF research proposal.

M. A. Azimova, S. McIntosh, "Transport properties and stability of cobalt doped proton conducting oxides," Solid State Ionics (2009). doi:10.1016/j.ssi.2008.12.013.

Y. Yamazaki, P. Babilo and S. M. Haile, “Defect chemistry of yttrium-doped BaZrO3 – A thermodynamic analysis of defect incorporation,” Chem. Mater. 20, 6352-6357 (2008).

P. Babilo, T. Uda and S. M. Haile, “Processing of Yttrium Doped Barium Zirconate for High Proton Conductivity,” J. Mat. Res. 22, 1322-1330 (2007).

J. W. Phair and S. P. S. Badwal, "Review of Proton Conductors for Hydrogen Separation," Ionics 12, 1-3-115 (2006).

S. M. Haile, "Fuel Cell Materials and Components," Acta. Met. 51, 5981-6000 (2003).

K.-D. Kreuer, "Proton-Conducting Oxides," Annu. Rev. Mat. Res. 33, 333-359 (2003).

Additional papers may also be posted.

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