Carbon Deposition in Solid Oxide Cells during Co-Electrolysis of H 2 O and CO 2

Abstract

Carbon formation during co-electrolysis of H 2 Oand CO 2 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 (H 2 O + CO 2 )conversionofupto 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. © 2014 The Electrochemical Society. All rights reserved.