Performance degradation of proton-conducting ceramic electrolyzers for high-temperature water splitting

Abstract

The purpose of this thesis is to identify, quantify, and mitigate sources of degradation in proton-conducting ceramic electrochemical devices. The growing research in proton-conducting ceramics is allowing advances in electrochemical technology that are being harnessed to address societal challenges in electricity generation, energy storage, and fuel synthesis. Current electrochemical technology such as solid oxides have expensive material components and performance degradation issues, which is preventing the success of commercial kW-scale assemblies. The compatibility and stability of integration of novel perovskite materials with traditional-solid-oxide, chromium-based ferritic metals in operation of over 1000 hours is unclear. In addition, the operating conditions of proton-conductors are substantially different compared to oxygen-ion conductors, such as high-vapor content at the electrodes and an operating temperature that is less than that of solid oxide electrochemical devices. While operating at lower-temperatures should be beneficial, significant investigation of material stability is required for acceptable commercialization. Due to the complexity of multiple factors that could cause degradation, this thesis focuses on symmetric-supported proton-conducting ceramic electrochemical devices. The two electrolyte materials are BaCe0.7Zr0.1Y0.1Yb0.1O3-d (BCZYYb7111) and BaCe0.4Z0.4Y0.1Yb0.1O3-d (BCZYYb4411), and the steam electrode material is BaCo0.4Fe0.4Zr0.1Y0.1O3−δ (BCFZY). The interconnect metals are Ce/Co coated Sandvik 441 and uncoated Sandvik 441. The environmental conditions are 10% steam + air (shop air or, 21% O2 and 79% N2) and dry oxygen. The operating conditions are 100 SCCM mass flow rate and 550°C. In order to collect accurate and repeatable measurements of degradation, a next-generation test stand with a ceramic coupon apparatus was built, validated, calibrated, and commissioned. Degradation rates were captured and recorded through continuous and long-term Electrochemical Impedance Spectroscopy (EIS) measurements. Specifically, large volumes of EIS data is used to extract DC resistance and polarization resistance through a circle-tracing program written in RStudio, in order to determine area specific resistance (ASR), conductivities and degradation rates. Results showed drastic degradation of BCFZY steam electrode at 50% steam + air and acceptable stability at 10% steam + air. Electrolyte perovskite material BCZYYb4411 shows slightly higher stability compared to BCZYYb7111 in 10% steam + air. Additionally, a dry oxygen-rich environment shows high stability of BCZYYb4411|BCFZY. Ce/Co coated, untreated Sandvik 441 interconnects show a lower degradation rate compared to uncoated, untreated Sandvik 441 interconnects; however, significant degradation rates are still present for both the electrode and electrolyte. EDX analysis on interconnects showed an insulation layer of hematite formation. In conclusion, proton-conducting perovskite materials and integration of traditional-solid-oxide, Cr-based ferritic metals degrade in high-water vapor environments. An increase of stability can be improved through a change of perovskite material ratio composition, and an oxygen-rich environment shows lower degradation rates.