Proton surface exchange kinetics of perovskite triple conducting thin films for protonic ceramic electrolysis cells: BaPr0.9Y0.1O3-δ (BPY) vs. Ba1-xCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY)

Protonic ceramic electrolysis cells (PCECs) are an attractive green H-2 production technology, given their intermediate-temperature operating range and ability to produce dry H-2. However, PCECs will benefit from development of more efficient and durable "triple conducting" anodes where steam is split, H incorporated, and oxygen evolved. In this work, we evaluated the kinetics of the steam-splitting/H incorporation reaction on BaPr0.9Y0.1O3-delta (BPY) in comparison to the benchmark Ba1-xCo0.4Fe0.4Zr0.1Y0.1O3-delta (BCFZY) composition, replacing most of the transition metal elements (Co, Fe, Zr) with the lanthanide Pr. We prepared geometrically well-defined perovskite BPY and BCFZY thin films by pulsed laser deposition and performed simultaneous optical transmission relaxation and electrical conductivity relaxation measurements at 400-500 degrees C in 0.21 atm O-2 during switching of the steam partial pressure to isolate and compare their proton surface exchange coefficients (k). The k values of BPY were comparable to those of BCFZY and more stable over time. According to angle-resolved XPS and STEM-EDS mapping of FIB cross-sections, the surface of BPY exhibited Ba enrichment, Pr deficiency, and Si contamination. In contrast, BCFZY exhibited Ba deficiency throughout, no obvious surface segregation, and less Si contamination. The Ba segregation on the BPY film appears to have promoted steam splitting/H incorporation kinetics even though the more basic surface reacted with the acidic environmental SiOxHy. Faster kinetics observed on stoichiometric BCFZY vs. Ba-deficient BCFZY confirmed the benefit of a high A-site Ba concentration. This result contrasts with most work on perovskites applied in solid oxide electrolysis cell anodes, in which A-site segregation is considered deleterious for surface reaction kinetics.