M.Sc Thesis


M.Sc StudentShiri Tomer
SubjectHighly Porous Anode for Application in Double
Electrolyte Fuel Cell
DepartmentDepartment of Chemical Engineering
Supervisor Professor Tsur Yoed
Full Thesis textFull thesis text - English Version


Abstract

In this work, a highly porous anode that can be used in Double Electrolyte Fuel Cell (DEFC) or other electrochemical devices has been fabricated. The construction of a symmetric cell and its electrochemical characterization was an additional objective.  A novel stack design of a fuel cell, DEFC, is based on two types of electrolytes with different ionic conducting species. In this design, each electrolyte is placed at one side of the anode where the fuel is injected directly into the anode. Hence, the anode must be highly porous to allow the fuel to permeate through and reach to the electrolytes’ interfaces.

Highly porous anode was fabricated using the direct foaming technique in which foaming of colloidal suspensions is used for the fabrication of porous ceramic structures.

NiO-gadolinia doped ceria (CGO) was chosen as anode material and has been prepared using a chemical co-precipitation process (the NiO is later reduced to Ni). The NiO-CGO was foamed and dried at room temperature followed by sintering in air and reduction in humidified hydrogen. The fabricated anode has a porosity of about 80%. The anode has high strength and can be sliced, polished and reduced without breaking or cracking. The microstructure was investigated using HR-SEM. The pores are open and their sizes distribute from nanometers to tens of microns.

Electrochemical impedance spectroscopy (EIS) measurements were performed in order to check the feasibility of the pellets as anodes and the porosity influence on the electrical conductivity. Two kinds of experiments were done. In the first, both porous non reduced samples and dense non reduced samples were characterized at low and intermediate temperatures. At low temperatures both samples exhibit purely electronic conduction. At higher temperatures the measured conductivity of the porous sample was smaller than the dense one but with the same activation energy, meaning that the charge transfer does not change despite the different microstructure. Hence, the NiO phase remains continuous at the porous sample

The second experiment was performed on a symmetric cell comprised of porous anode |electrolyte| porous anode, at humidified hydrogen atmosphere. The influences of temperature and gas flow on the EIS measurements were studied. The sample showed low resistance and good electronic conductance. Hence it could be used as anode.