Life Testing of Ni-YSZ Fuel Electrode Under Electrolysis and Fuel Cell Operation in High Reducing Environment

Abstract

An electrode supported Ni-YSZ/YSZ/Ni-YSZ symmetric cell was studied with direct-current operation, with one electrode operating in electrolysis mode, and another electrode in fuel cell mode. The 1000 h life tests were carried out at different current densities in 97% H2 / 3% H2O. At a current density ≤ 0.4 A/cm2, the cell total resistance value was reasonably constant, with a slight increase in ohmic resistance; no changes in microstructure were observed relative to the as-prepared cell. At current densities ≥ 0.6 A/cm2, there was a steady increase in ohmic resistance that was offset by a decrease in polarization resistance, resulting in a total resistance that was approximately stable. However, these higher current densities resulted in significant damage to the electrolyte microstructure. First, fracture was observed within the electrolyte near the electrolysis operation side. Second, fracture SEM images showed pronounced intergranular fracture that resulted because voids had formed at grain boundaries during the life tests. These changes presumably explain the observed ohmic resistance increase. In addition, nanoparticles formed in the electrode after electrolysis operation, probably explaining the decrease of polarization resistance. The constant-current results are compared with those from similar cells life tested in reversing-current operation. The critical current density values for onset of signification degradation were similar: between 0.4 and 0.6 A/cm2. At the same values of current densities ≥ 0.6 A/cm2, the rate of increase in ohmic resistance were comparable during direct-current and reversing-current operation, while the rate of decrease in polarization resistance with direct-current operation were less than half of that with reversing-current operation. The microstructure changes after life tests at higher current densities were different: First, reversing-current operation resulted in damage to the electrode/electrolyte interface, but the electrolyte appeared to be intact. Second, highly concentrated Ni and YSZ nanoparticles were observed at both side of electrodes after reversing-current operation. Numerical analysis was carried out to determine the effective oxygen partial pressure versus position within the electrolyte. The results show that the oxygen partial pressure near the electrolysis operation side decreased with increasing current density. The values may be low enough to reduce zirconia and form a Ni-Zr alloy. Subsequent oxidation of zirconium presumably causes the observed structural changes near the Ni-YSZ/YSZ interface.