Carbon formation during co-electrolysis of H2O and CO2 in Ni-YSZ supported Solid Oxide Electrolysis Cells (SOECs) may occur, especially at high current density and high conversion. In order to evaluate the carbon formation limits, five galvanostatic tests were performed in this work at electrolysis current densities from 1.5 to 2.25 A/cm2 and reactant (H2O + CO2) conversion of up to 67%. Delamination and carbon nano-fibers were observed at the Ni-YSZ|YSZ interface for two cells with a dense microstructure operated at electrolysis current densities of 2.0 and 2.25 A/cm2 and a conversion of 59% and 67% respectively. Based on the nominal gas composition, carbon formation only occurs, according to equilibrium thermodynamics, at a reactant conversion above 99%. Therefore, the observed carbon formation is ascribed to a change in gas composition due to the diffusion limitations within the Ni-YSZ support and the active Ni-YSZ electrode. Carbon nano-fibers were only observed close to the YSZ electrolyte, indicating a very reducing atmosphere and a large over-potential gradient in the active electrode, being highest at the interface to the bulk electrolyte and decreasing toward the Ni-YSZ support.
|Journal||Journal of the Electrochemical Society|
|Publication status||Published - 2014|
|Event||Pacific Rim Meeting on Electrochemical and Solid-State Science: 222nd Meeting of ECS — The Electrochemical Society and 2012 Fall Meeting of The Electrochemical Society of Japan - Hawaii Convention Center and the Hilton Hawaiian Village, Honolulu, United States|
Duration: 7 Oct 2012 → 12 Oct 2012
|Conference||Pacific Rim Meeting on Electrochemical and Solid-State Science|
|Location||Hawaii Convention Center and the Hilton Hawaiian Village|
|Period||07/10/2012 → 12/10/2012|