(Invited) Evaluation of Steam Splitting (OER) Kinetics in Praseodymium-Based Perovskite Thin Film Electrodes for Efficient Intermediate-Temperature Water Electrolysis

Abstract

Water electrolysis at high temperatures (> 600 ˚C) provides both kinetic (no precious metals) and thermodynamic (lower voltage requirement) benefits for hydrogen generation. However, recently steam electrolysis at intermediate temperatures (300 - 600 ˚C) has attracted attention, given potentially faster startup times, ready access to waste heat, and mitigation of some degradation modes, while maintaining some of the high temperature benefits. Intermediate-temperature electrolysis can be accomplished with protonic ceramic electrolysis cells (PCECs), for which a primary challenge is the need for efficient, chemically stable, and long-term durable oxygen/steam electrodes. Electrode material candidates tend to be perovskite-based oxides that must be triple conductors, enabling transfer of oxide ions, protons, and electronic species for PCECs. While Barium (Ba) containing perovskite (ABO3) oxygen electrodes are an active focus due to their low hydration enthalpies enabled by the basicity of Ba, their poor long-term stability and non-optimized catalytic activity hinder the practical deployment of PCECs. Additionally, segregation of large cations (e.g., Ba, Sr) often takes place on electrode surfaces, which further hinders their catalytic performance and long-term stability. Thus, we have been investigating synthesis and performance of lower-Ba or Ba-free oxygen/steam electrodes and relating these to the evolution of the surface chemistry during operation. In this work, we fabricated Pr-based perovskite oxides, including Pr(Ga,Mg)O3-x, Ba(Pr,Y)O3-x, and others, as thin films with well-defined geometries and surface areas using pulsed laser deposition and compared their performance to the standard Ba(Co,Zr,Fe,Y)O3-x. Considering the multiple valence states (3+, 4+) of Pr and its large size, which may lead to surface segregation, the inclusion of Pr, or even replacement of Ba with Pr, is hypothesized to benefit catalytic activity and stability. We examined the surface water splitting/ hydrogenation coefficients (k-values) using electrical conductivity relaxation with pH2O steps, as a function of time and gas atmosphere. Further investigations of the role of crystalline quality, composition, and thickness have been made to understand the factors controlling the thin film electrode activity and stability. Our results of quantitative k-values on well-defined surfaces can guide design of high performance steam/oxygen electrodes and understanding of degradation mechanisms impacting performance of PCECs.