Using B-site deficiency to trigger in-situ formation of a nano composite high entropy double perovskite oxide air electrode for protonic electrochemical cells

Abstract

Protonic ceramic electrochemical cells (PCCs) have recently demonstrated promising progress both in fuel cell mode for the efficient production of electric power and in electrolysis mode for the versatile production of hydrogen and other fuels. In both operating modes, the air/steam electrode plays a critical role in determining PCC performance. High-entropy perovskite oxides (HEPs) have recently gained attention for PCC air electrode application, as they can potentially enhance both catalytic activity and durability. Beyond single-phase systems, meanwhile, nanocomposite electrodes have also emerged as another way to enhance performance and lifetime. Here, we combine these two strategies to develop a B-site defficient HEP that re-structures in-situ during initial cell operation to form a multi-phase nanocomposite air electrode with enhanced performance and stability. The HEP nanocomposite is based on our prior development of the Ba(Ca0.2Gd0.2La0.2Pr0.2Sr0.2)Co1.5Fe0.5O6 (BaHEO) series of HEP oxides, which demonstrate high PCC catalytic activity. We introduce both Zn substitution and cation deficiency on the BaHEO B-site, yielding Ba(Ca0.2Gd0.2La0.2Pr0.2Sr0.2)Co1.35Fe0.45Zn0.1O6 (BaHEOZndefi). These modifications lead to the spontaneous segregation of a Zn-rich phase during initial air electrode operation, which enhances proton uptake/hydration and increases the oxygen vacancy concentration and catalytic activity of the host BaHEO phase. In laboratory-scale coin cell testing, PCCs featuring the BaHEOZndefi nanocomposite air electrode demonstrate superior electrochemical performance compared to PCCs based on the single-phase BaHEO, reaching 613 mW cm-2 at 650 oC in fuel cell mode, and -886 mA·cm-2 at 1.3V in electrolysis mode at the same temperature. Hybrid-distribution of relaxation times (hybrid-DRT) characterization reveals that the BaHEOZndefi nanocomposite electrode promotes significantly lower polarization resistance for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Furthermore, BaHEOZndefi-based PCCs exhibit stable operation under both fuel cell and electrolysis modes over >300 hrs cummulative testing. These findings establish the in-situ HEP nanocompositing strategy as a promising route to improve PCC air-electrode performance and durability.