Electro-Chemo-Mechanical Failure in a High-Performance Solid Oxide Cell

Abstract

Fuel electrode-supported solid oxide cells (FE-SOC) offer the possibility of low operation temperatures and / or high power density through the incorporation of a thin, supported electrolyte between two highly active electrodes. However, integrating fuel electrodes with an ionic conductor that is different from that of the electrolyte into FE-SOC remains a challenge due to the interdiffusion phenomena during co-sintering of the half-cell. As a result, most FE-SOC use cermet fuel electrodes made from Ni and stabilized zirconia.Here, we present a FE-SOC based on a Ni-Gd0.1Ce0.9O2- d (GDC) fuel electrode, a three-layer GDC / zirconia / GDC electrolyte and an La0.58Sr0.4CoO3- d (LSC) air electrode. The electrolyte is fabricated by co-sintering a thin, screen-printed layer of GDC with the fuel electrode and support. Subsequent electrolyte layers are deposited via physical vapor deposition (PVD), and the air electrode is fabricated via screen printing. The total electrolyte thickness is less than 5 µm, enabling an area-specific ohmic resistance < 10 mΩ cm² at 700 °C. A current density of 1.6 Acm-² is achieved at a cell voltage of 800 mV and a temperature of 700 °C, using 10% H2O/H2 as fuel and air as oxidant. This performance is about twice as high as that of a conventional Jülich cell using Ni-YSZ and LSCF electrodes and a 10 µm YSZ electrolyte under similar conditions. An electrolysis current of 2 A cm-² was achieved at a cell voltage of 1.11 V, at a temperature of 800 °C and using 50% H2O/H2 and air. The conventional Jülich cell requires a cell voltage above 1.25 V under similar conditions. Under these conditions, the new cell achieves a cell efficiency of 84 %, compared to 73 % for the conventional cell.However, running high electrolysis currents at temperatures below 750 °C resulted in catastrophic cell failure, visible as large voltage jumps in the I-V curve. In addition, a decreasing OCV value with decreasing temperature indicated damage to the cell before the catastrophic failure. Post-test analysis found a large number of cracks in the electrolyte, nucleating at the fuel electrode / electrolyte interface and running toward the air electrode. We show that the likely cause for this failure is the electrochemical reduction and subsequent chemical expansion of the doped ceria electrolyte layer, necessitating a detailed study of the local mechanical stresses in the cell during operation.