Electrical conductivity of Ni–YSZ composites: Degradation due to Ni particle growth

The short-term changes in the electrical conductivity of Ni–YSZ composites (cermets) suitable for use in Solid Oxide Fuel Cells (SOFC) were measured by an in-situ 4-point DC technique. The isothermal reduction was carried out in dry, humidified or wet hydrogen at temperatures from 600 to 1000 °C. While the cermets reduced at 600 °C showed a stable conductivity of 1000–1200 S/cm, rapid initial conductivity loss was observed at elevated temperatures. At 1000 °C the conductivity degraded nearly instantaneously to about 800 S/cm, and continued to decline fast to about 400 S/cm. At 850 °C, the presence of steam did have an accelerating effect on the conductivity loss. Scanning Electron Microscopy of cermets reduced in different conditions showed increasing particle size and loss of metal-to-metal percolation in the samples reduced at higher temperatures. The short-term changes in conductivity were modelled using two different semi-empirical approaches. Thermodynamic calculations were carried out to assess the vaporisation of Ni in the conditions tested. The rate and mechanisms of conductivity degradation due to Ni particle growth are discussed in light of the measurements, modelling and literature data.

Research Highlights
► El. conductivity loss of Ni–YSZ composites was measured in different conditions.
► Stable 1000 S/cm at 600 °C, operation at 850 °C or above leads to fast degradation.
► At 600 °C no effect from steam, whereas at higher T steam accelerates degradation.
► Data were fitted with two time constants indicating at least two growth processes.
► Conductivity loss was related to Ni particle growth and loss of Ni percolation.