Development of a Support-Cathode-Electrolyte Structure for Direct Carbon Fuel Cell

Jacek, Steven

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Development of a Support-Cathode-Electrolyte Structure for Direct Carbon Fuel Cell

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Electricity is an increasingly important resource in the world’s energy system, but conventional methods of energy production are based on technology which is neither efficient nor sustainable. Fuel cells are a known technology for electricity production, but suffer from poor operating life and manufacturability. A high-energy, high-efficiency direct carbon fuel cell (DCFC) has been proposed by Davis et al. which consists of a liquid metal anode, into which is submerged a tubular cathode-electrolyte structure. This structure is a three-layer ceramic composite consisting of a coarsely porous support, finely porous cathode, and fully dense electrolyte. Its development is described herein. The support layer is made of calcium-doped lanthanum manganite (LCM), the cathode active layer from a blend of LCM and yttria-stabilized zirconia (YSZ), and the electrolyte from YSZ. The support is slip cast; the cathode and electrolyte are dip-coated onto the support and dried before sintering. The support, which also functions as the current collector, is made of LCM and has a porosity of about 35 vol % 20µm pores. It is fabricated to a thickness of 2mm by slip casting with a casting time of 5 minutes. The cathode layer is made of an equal mass blend of LCM and YSZ so both its shrinkage upon sintering and thermal expansion coefficient are compatible with those of both the support and the electrolyte, preventing delamination during fabrication or operation. It has a fired porosity of about 35 vol% 5µm pores, and a thickness of about 40µm. It is applied to the support by dip coating. The electrolyte is made of zirconia doped with 8 mol % yttria to maximize oxygen ion vacancies. It is applied to the structure by dip coating on top of the dried cathode layer and has a thickness of about 20µm with closed porosity well below 10 vol%. The structure is co-fired for 2 hours at 1300°C. It is fired horizontally to maintain even sintering throughout the length. The structure was tested in conjunction with a molten alloy anode as a complete fuel cell. The cathode-electrolyte structure did not display damage or significant degradation after about 5 hours in contact with the molten anode at about 1000°C. A promising result from electrical testing is that the initial open-circuit voltage nearly reaches the theoretically-predicted value and decreases as the cell operates. This thesis focuses on the fabrication process of the cathode-supported electrolyte structure. In Chapter 1, context, motivation, and basic theory relevant to the process steps developed are presented. In Chapter 2, similar work is critically examined. Chapter 3 provides details of prototyping and characterization techniques which were used repeatedly in the development process, in the hope that other researchers may replicate and improve upon them. The development process itself is described in Chapter 4; discussion of design decisions and possible alternatives is here as well. Chapters 5 and 6 summarize the most valuable outcomes of this work, and Chapter 7 suggests strategies for improving this technology in future work. Learnings from this work will contribute to the technical literature, particularly in the areas of formulation and processing, advancing the future of DCFC technology.

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