Durability of the Solid Oxide Cells for Co-Electrolysis of Steam and Carbon Dioxide under High Current Densities

Abstract

Production of hydrogen and syngas (CO + H2) using solid oxide electrolysis cells (SOECs) has become increasingly attractive due to high oil price, the capability for conversion and storage of intermittent energy from renewable sources and the general interest in hydrogen energy and carbon-neutral energy sources. Long-term stability of SOECs for high fuel productivity is crucial for the application of this technology. In this work, a series of galvanostatic durability tests were performed at high current densities (|i| = 1.5 or 2.0 A/cm2), 850 oC for up to about 700 hours for co-electrolysis of steam and CO2. Two types of Ni-YSZ supported cells, the LSM cell and LSCF cell, respectively with a LSM-YSZ or a LSCF-CGO oxygen electrode were tested using a setup with albite glass as sealing material, and using the gasses as received or in an setup using a glass with less known impurities, and cleaning the inlet gasses. The feed gas to the Ni-YSZ electrode consisted of 45 % H2O + 45 % CO2 + 10 % H2 and the reactant conversion was 45 % or 60 %. Electrochemical and the microstructural analysis were performed to investigate the degradation mechanisms of the tested SOECs. A significant increase of the ohmic resistance and Ni-YSZ TPB reaction resistance accounted for the main degradations of the SOECs. In contrast to LSCF cells, the LSM cells showed a faster and larger increase of ohmic resistance, indicating some kind of relation between YSZ degradation and type of oxygen electrode. The oxygen electrode itself showed no degradation or only limited degradation. Oxygen electrode delamination from the electrolyte was not observed for any of the tested cells. However, parallel cracks were observed in the electrolyte, which could be ascribed to the internal stress due to a large thermal gradient in YSZ electrolyte perpendicular to cell plane. Severe percolation loss of Ni occurred for the Ni-YSZ electrode adjacent to the YSZ electrolyte, contributing to the increase of TPB resistance due to decrease of active Ni-YSZ TPB length. The large cathodic polarization of Ni-YSZ electrode led to the more severe percolation loss of Ni particles. The blocking of the Ni-YSZ TPBs by impurities (e.g. SiOx) also contributed to the fast degradation of SOECs in the initial test period. However, the post-test observation revealed dominating SiOx inclusions inside the Ni grain close to the electrolyte, instead of segregation at the TPBs or Ni-YSZ interface. A reduction-oxidation process was proposed for the formation of SiOx inclusions in Ni under large cathodic polarizations. Formation of zirconia nanoparticles were observed for most of the tested cells close to the TPBs and Ni|YSZ interface for the innermost a few microns thick Ni-YSZ electrode. The nanoparticles are similar to the bulk YSZ in crystal structure and composition. The zirconia nanoparticles were probably formed by direct structure decomposing of YSZ under strong cathodic polarizations, rather than by a complete reduction to metallic Zr. The nanoparticle formation could impair the Ni-YSZ contact, leading to an increased ohmic and oxygen ion transfer resistance. Further, cells with different porosity were tested, for relative denser Ni-YSZ structure, carbon (CNT) formation was observed at the Ni-YSZ|YSZ interface, where fuel electrode delamination occurred during co-electrolysis of steam and CO2 at |i| ≥ 2.0 A/cm2. Gas diffusion limitations contribute to the dramatic increase of cell voltage and a very reducing atmosphere at the interface.