A new layered barium cobaltite electrode for protonic ceramic cells

Abstract

Protonic ceramic cells (PCCs) hold significant promise as energy conversion devices operating at lower temperatures in comparison to traditional Solid Oxide Cells (SOCs). However, the widespread adoption of PCCs depends on developing innovative, high-performing, electrode materials that exhibit enhanced chemical compatibility with barium-based perovskite electrolytes. Here, a new cobaltite, Ba₂Co₉O₁₄ (BCO), is employed for the first time as an electrocatalyst for oxygen reactions in PCCs in contact with a BaZr_0.852Y_0.148O_3−δ (BZY15) + 4 mol% ZnO (sintering agent) electrolyte. BCO displays electrochemical performance comparable to the current state-of-the-art oxygen electrode under wet conditions (p_H2O ∼ 10⁻² atm). Furthermore, it demonstrates excellent chemical compatibility with the BZY15 electrolyte. Thermogravimetric experiments reveal no significant oxygen loss below 800 °C and no noticeable proton uptake. Conversely, X-ray photoelectron spectroscopy results highlight the formation of surface oxygen vacancies and mixed valent Co³⁺/Co²⁺ states, as corroborated by bond valence sum calculations from Rietveld refinement of the X-ray diffraction data. Consequently, due to the platelet-like morphology of the BCO electrode grains, and considering its poor bulk ionic conduction, the surface diffusion process becomes highly important in explaining the high-performing electrochemical behaviour. Moreover, the impedance spectroscopy data analysis brings to light the existence of electronic leakage within the electrolyte substrate, leading to a significant underestimation of the electrode polarisation resistance and misconception of the electrode mechanism. To address this issue, a data correction is applied, revealing that electrode kinetics is strongly rate-limited by oxygen diffusion on the surface of the BCO grains towards the triple-phase boundary, where proton transfer occurs, releasing water. In contrast, the adsorption and/or the oxygen dissociation steps are facilitated given the predominantly electronic character of the BCO material, which is suggested to originate from a small polaron hopping mechanism. Our results, thereby, introduce a new intergrowth series of cobaltites, which present an exciting avenue for exploration in the context of PCCs.