Date of Completion

8-10-2020

Embargo Period

8-10-2021

Keywords

solid oxide fuel cell, SOFC, getter, alkaline earth metal, strontium manganese oxide, chromium capture, sulfur capture, air electrode, Cr poisoning, S poisoning

Major Advisor

Prabhakar Singh

Associate Advisor

Avinash M. Dongare

Associate Advisor

Seok-Woo Lee

Associate Advisor

Amit Pandey

Associate Advisor

Boxun Hu

Field of Study

Materials Science and Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

High-temperature electrochemical devices including solid oxide fuel cells (SOFC) have been recognized as promising candidates for next-generation devices for energy conversion, storage, and gas separation. These devices, however, remain susceptible to the electrocatalytic poisoning of air-electrodes from trace airborne contaminants such as Cr/Si/B vapors, SO2, CO2, and water species. The long-term exposure of air-electrodes to air flow at high operating temperatures leads to the accumulation of airborne gas impurities, and the formation of undesired compounds, degrading the electrochemical catalytic activity of air-electrodes. To date, most efforts to mitigate the electrode poisoning have focused on developing low-temperature/impurity-tolerant electrodes, and oxidation-resistant alloys. However, such methods possess their own drawbacks owing to thermal expansion discrepancy and chemical incompatibility between adjacent materials.

In developing a comprehensive strategy to prevent the electrode poisoning, it is of interest to elucidate how the gaseous contaminants interact with the electrodes under electrochemical operating conditions. Here, the adverse effects of Cr/S contaminants on established air-electrodes have been investigated by electrochemical/post-test characterizations. Chromium poisoning is mainly attributed to electrochemical reduction of Cr vapors at catalytic active sites while sulfur poisoning is primarily due to the strong chemical affinity of alkaline-earth elements with SO2. It is found that Cr/S poisoning behaviors vary with electrode, temperature, Cr/S concentrations, and polarization conditions.

As a novel approach to mitigate the electrode poisoning, absorbent materials (or getters), which capture airborne impurities, have been developed. The use of getters offers a cost-effective pathway for alleviating electrode contamination without the need to replace the established component materials. Considering the thermodynamics of related solid-gas reactions, the combination of alkaline-earth and transition-metal oxides such as Sr–Ni–O and Sr–Mn–O systems have been selected as Cr and S co-absorbent materials. Their efficacy for capturing Cr/S species was validated by electrochemical tests. Post-test characterization of the getter observed the elongation of absorbent particles (forming SrSO4 and SrCrO4) during the Cr/S impurity absorption, indicating the high mutual affinity of alkaline-earth-metals and Cr/S species. The Sr–Mn–O system showed high thermal and hydrolytic stability whereas Sr–Ni–O system suffered from hydrolytic reactions accompanied by Sr-segregation and phase transformation.

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