Correlation between hydration properties and electrochemical performances on Ln cation size effect in layered perovskite for protonic ceramic fuel cells

PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (PrBSCF) has attracted much research interest as a potential triple ionic and electronic conductor (TIEC) electrode for protonic ceramic fuel cells (PCFCs). The chemical formula for PrBSCF is AA’B₂O_5+δ, with Pr (A-site) and Ba/Sr (A’-site) alternately stacked along the c-axis. Due to these structural features, the bulk oxygen ion diffusivity is significantly enhanced through the disorder-free channels in the PrO layer; thus, the A site cations (lanthanide ions) play a pivotal role in determining the overall electrochemical properties of layered perovskites. Consequently, previous research has predominantly focused on the electrical properties and oxygen bulk/surface kinetics of Ln cation effects, whereas the hydration properties for PCFC systems remain unidentified. Here, we thoroughly examined the proton uptake behavior and thermodynamic parameters for the hydration reaction to conclusively determine the changes in the electrochemical performances depending on LnBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (LnBSCF, Ln=Pr, Nd, and Gd) cathodes. At 500 °C, the quantitative proton concentration of PrBSCF was 2.04 mol% and progressively decreased as the Ln cation size decreased. Similarly, the Gibbs free energy indicated that less energy was required for the formation of protonic defects in the order of PrBSCF < NdBSCF < GdBSCF. To elucidate the close relationship between hydration properties and electrochemical performances in LnBSCF cathodes, PCFC single cell measurements and analysis of the distribution of relaxation time were further investigated.