M.Sc Thesis


M.Sc StudentVictoria Zlotnik
SubjectPreparation, Application and Characterization of a Proton
Conductor for Dual Electrolyte SOFC
DepartmentDepartment of Chemical Engineering
Supervisor Professor Tsur Yoed
Full Thesis textFull thesis text - English Version


Abstract

In the current work, several methods were applied for lowering gadolinium doped barium cerates' (BCG), a proton-conducting electrolyte, sintering temperature. Its sintering temperature was decreased from about 1650 °C to below the sintering temperature of its porous Ni-CGO anode substrate (1300 °C). This procedure is essential for an efficient Double Electrolyte Fuel Cell (DEFC) preparation. The novel DEFC design, which consists of two different ion-conducting electrolytes separated by an anode body and connected each to a cathode layer on the opposite side, demands thin layer deposition and sintering on anode surface without causing any alternation of anode microstructure in order for it to reach high efficiencies. The lowering of proton-conductors' sintering temperature, therefore, is needed to prevent changes in anode pore size as a result of increased heating.

Electrolyte preparation and analysis had three stages: (1) electrolyte pellet preparation and analysis (2) electrolyte thin layer preparation and analysis (3) symmetric cell, consisting of an electrolyte sandwiched between two anodes, preparation and analysis. In the first stage powders were prepared by citrate-nitrate combustion (CNC) while NiO was added as a sintering aid. To achieve the second stage, a combined reactive sintering (RS) method was applied. In order to achieve the third stage two methods were attempted: symmetric cell preparation using the combined RS method and symmetric cell preparation using the flash sintering (FS) method.

SEM and density analysis of a 4 mol% Ni doped BCG electrolyte pellet sintered at 1250 °C revealed a highly dense sample with 95.4% of the theoretical density and an average grain size of 5 µm. Electrochemical analysis (EIS) of the sample in wet N2 and wet H2 revealed increased activation energy values for electrolytes' conductivity mechanisms, suggesting high associative behavior of the Ni dopant. Total conductivity at higher oxygen pressure was higher than the one at lower oxygen pressure indicating on possible change of nickel's oxidative state at higher oxygen pressure.

SEM and EDS analyses of the 4 mol% Ni doped BCG layer prepared by RS showed a dense, well attached and homogenous layer with ~86µm thickness after sintering at 1250 °C. XRD measurements were also performed.

A Symmetric cell was achieved by using the FS method. EIS analysis of the cell revealed higher grain boundary conductivity as compared to the bulk. Additionally, total conductivity values were smaller than expected, calling for preparation conditions fine-tuning.