Modification of Oxygen Electrodes for Solid Oxide Cells via Pre-Use Treatments

Solid oxide cells require high temperatures to drive thermally activated reactions. This calls for expensive auxiliary components, increasing the electrolyzer system cost, limiting a technology breakthrough. Developing new materials with high activity at lower temperatures is challenging, so research now also focuses on pre-use treatments to boost the performance of existing materials.

To improve the performance of well-known oxygen electrodes, seeking to facilitate a temperature reduction, three pre-use treatments have been investigated: infiltration, washing and redoxing. Symmetrical cells, mainly composites of Sr-doped lanthanum ferrites and Gd-doped ceria, were characterized electrochemically to evaluate the effect of the pre-use treatments. Selected electrodes were subjected to further material characterizations.

The Smith acidity of infiltrated binary oxides has been investigated as a performance descriptor. Basic oxides reduced the polarization resistance (R_p), while acidic oxides increased it. However, no direct scaling between the R_p and Smith acidity was identified. Furthermore, basic oxides of dielectric behavior penalized the R_p at higher loadings. In contrast to this, it was found that R_p scales equally well with the pH of the precursor solution. For instance, washing with a high pH precursor that leaves no secondary phases behind reduced the R_p more than infiltration, illustrating that the chemical environment has an impact on the electrode properties. This underlines the importance of considering whether the precursor pH or the secondary phases affect the performance most. Considering both, resulted in an even greater Rp reduction.

Redoxing improved all investigated electrodes by reducing the R_p and enhancing the durability of most electrodes, especially for Gd-doped ceria containing electrodes. The redoxing mechanism creates a complex multiphase system. It was proposed that the presence of Gd-doped ceria protects the rest of the electrode from contaminants during reduction.

Overall, pre-use treatments enabled temperature reductions of 50–75 °C without increasing the R_p. Proving that well-known materials can be improved.