Quantitative analysis of solid oxide fuel cell anode microstructure change during redox cycles

• Performance was enhanced and TPB density increased after redox treatment.
• Performance degraded and TPB density decreased after discharge process.
• As redox cycles are repeated, gradual degradation was observed.
• TPB density increased as the redox cycles are repeated.
• Performance degradation cannot be explained from microstructure change.

In the present study, correlation between solid oxide fuel cell anode microstructure and electrochemical performance during redox cycles was investigated. Electrolyte-support cell with nickel/yttria stabilized zirconia composite anode was prepared and tested under discharge process with redox cycles. Redox treatment was basically conducted every 20 h during discharge process. Polarization resistance decreased just after redox treatment and increased during discharge process. Enhancement of cell performance after every redox cycles and faster degradation in the following discharge process were observed. Polarization resistance gradually increased as redox cycles were repeated. Focused ion beam-scanning electron microscopy (FIB-SEM) observation was conducted for reconstructing the three dimensional microstructures of the tested samples. From the three dimensional microstructure reconstruction, it is found that the shape of nickel particle got thinner and complicated after redox cycles. Triple phase boundary (TPB) length increased after redox treatment and decreased after discharge process. This TPB change was highly associated with Ni connectivity and Ni specific surface area. These microstructure changes are consistent with the change of cell performance enhancement after redox treatment and degradation after discharge process. However, TPB length density kept on increasing as redox cycles are repeated, which is inconsistent with the gradual degradation of anode performance.