Semester

Summer

Date of Graduation

2022

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Xingbo Liu

Committee Co-Chair

Mike Brady

Committee Member

Wenyuan Li

Committee Member

Harry Finklea

Committee Member

Kostas Sierros

Committee Member

Terence Musho

Abstract

A solid oxide fuel cell (SOFC) is a clean and efficient energy conversion device. The development of intermediate-temperature SOFCs has made it preferable to use metallic interconnects (MICs) to greatly reduce the cost and significantly increase the efficiency compared to ceramic interconnect materials. However, gaseous chromium species will evaporate from the chromium-containing layer formed on the surface of commonly used MICs and balance of plant (BoP) components. Volatile chromium species have been shown to form solid deposits which poison the cathodes of SOFCs, causing drastic cell performance degradation and thereby limiting commercialization. In order to alleviate the Cr poisoning and achieve long-term high performance of SOFC stacks, various Al2O3-forming austenitic (AFA) stainless steels applied at different temperatures are evaluated in this work. It is shown that on the AFAs, an alumina-based protective layer forms under high temperature that is invulnerable to water vapor effects and suppresses the diffusion of chromium and manganese which can prevent the generation of spinels on the alloy surface. The chromium (Cr) evaporation behavior of several different types of iron (Fe)-based AFA alloys and benchmark Cr2O3-forming Fe-based 310 and Ni-based 625 alloys was investigated for 500 h exposures at 800 °C to 900 °C in air with 10% H2O. The Cr evaporation rates from alumina-forming austenitic (AFA) alloys were ~5 to 35 times lower than that of the Cr2O3-forming alloys depending on alloy and temperature. The Cr evaporation behavior was correlated with extensive characterization of the chemistry and microstructure of the oxide scales, which also revealed a degree of quartz tube Si contamination during the test. Long-term oxidation kinetics were also assessed at 800 to 1000 °C for up to 10,000 h in air with 10% H2O to provide further guidance for SOFC BOP component alloy selection. Besides the lower Cr evaporation rates and better oxidation resistance of AFAs than benchmark alloys after short-term (500 h) operation, AFAs also possess the sturdy and compact alumina layer after a long-term operation (5000 h). The Cr evaporation and high-temperature oxidation behaviors of AFA alloys are systematically investigated in air + 10% H2O at 800 °C after various durations compared to commercial alloy 310S. The Cr evaporation rates of 310S are about 35 times higher than the AFA alloys after the entire test. Breakaway oxidation and spallation are observed on 310S after only one cycle, while the AFA alloys show high oxidation resistance. It is found that there are no voids formation, and the formation of a continuous alumina layer stays compact and stable during the entire test which greatly reduces the Cr 3 evaporation. The long-term oxidation and chromium evaporation behaviors of chromia-forming alloy 625 and AFA alloys were evaluated by transpiration tests, weight gain tests, X-ray diffraction technique, scanning electron microscopy coupled with energy dispersive X-ray analysis, and scanning transmission electron microscopy coupled with energy dispersive X-ray analysis. Results indicated that the 625 exhibits a 28-and 56-times higher evaporated Cr amounts than OC11 and OC11LZ, respectively. Different behaviors between OC11 and OC11LZ are resulted from the formed oxides scale variation during the long-term operation. Moreover, the effect of reactive elements on the long-term oxidation and chromium evaporation behaviors was discussed. Cr evaporation from BoP components in high-temperature environment could severely deteriorate the electrochemical performance of SOFC. Several methods were applied to evaluate the Cr evaporation rates of BoP components after 500 h exposure at 800 °C to 900 °C in air with 10% H2O. An optimal method was designed to exclude the effect of silicon (Si) deposits from quartz tube and sodium (Na) deposits from the sodium carbonate on the oxidation process and the chemical interaction between Cr gaseous species and alumina tube which could provide further quantitative correlation of the evaporated Cr species quantities and degradation rates of SOFC. Based on the great performance of AFAs after long-term operation, AFAs are assembled with Anode-supported cells (ASC) to investigate the the Cr deposition of anode-supported cell under a constant current density of 0.5 A cm−2 at 800 °C with AFA alloys compared with commercial alloys which was analyzed by the distribution of relaxation times (DRT) and different equivalent circuit model methods. The performance deterioration of ASCs was mainly attributed to the increased polarization resistances of oxygen surface exchange and diffusion processes in the cathode region. The superior performance of ASC coupled with AFA alloys was due to the formed continuous alumina layer which can vastly decrease the evaporated gaseous Cr species, thus alleviating the Cr poisoning on the cathode region.

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