Ph.D Thesis


Ph.D StudentJeremy Rutman
SubjectInvestigation of Cathode Processes in Solid Oxide Fuel Cells
DepartmentDepartment of Physics
Supervisor Professor Emeritus Riess Ilan
Full Thesis textFull thesis text - English Version


Abstract

Various materials of possible use for solid-oxide fuel cells are synthesized and investigated in this work.  Materials studied include the ionic conductors yttrium stabilized zirconium and Mn- and Co-doped BiMeVOx.  A method for determining conductivity for our particular sample geometry was developed.  Ionic transference  number was investigated using two methods, one the Hebb-Wagner polarization method for blocking ionic conductivity, and the other a concentration-cell method measuring the extent of electronic short-circuiting of an electrolyte.

            The geometric factor relating conductivity to I/V as measured in a rectangular sample was determined in chapter 2.  The conformal map ez is used to project our rectangular geometry into the plane, where the geometric factor is known.   The map uses the approximation that the current lines for the infinite strip will closely resemble the current lines for the rectangle.

            In chapter 3 we describe coprecipitation of yttrium-stabilized zirconium which was carried out to synthesize fine powders of grain size in the tens of nanometers.  Fine-grained powders are a first step towards production of thin-film electrolyte layers of interest for lower temperature fuel cells, which will show decreased resistance in proportion to the thinness of the electrolyte layer. Grainsize is determined through the extent of peak-broadening in X-ray diffraction studies.  The grain size is studied as a function of preparation parameters.

Cobalt- and manganese-doped Bi4V2O11 are synthesized as part of a search for mixed ionic-electronic conductors suitable for fuel-cell cathodes.  Mixed ionic-electronic conductors have increased active area for the electrode reactions of fuel cells and hence introduce lower resistances into the total internal resistance of the fuel cell.  These samples are studied using SEM, EDS, Hebb-Wagner polarization, concentration cell measurements, four-point dc conductivity measurements, and ac impedance spectroscopy.  Results of conductivity measurements on Co- and Mn- doped BiMeVOx bulk can be explained in terms of solely ionic conductivity.  Hebb-Wagner polarization experiments designed to reveal electronic conductivity did not indicate any.  Open circuit voltage measurements on a concentration cell show a high ionic transference number t> ~ 0.7, with t=1 not ruled out.