Date of Award

2014

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Xingjian Xue

Abstract

Solid oxide fuel cells (SOFCs) are operated in high temperature conditions (750-1000 oC). The high operating temperature in turn may lead to very complicated material degradation issues, significantly increasing the cost and reducing the durability of SOFC material systems. In order to widen material selections, reduce cost, and increase durability of SOFCs, there is a growing interest to develop intermediate temperature SOFCs (500-750 oC). However, lowering operating temperature will cause substantial increases of ohmic resistance of electrolyte and polarization resistance of electrodes. This dissertation aimed at developing high-performance intermediate-temperature SOFCs through the employment of a series of layered perovskite oxides as novel cathode materials to minimize the potential electrode polarization on oxygen reduction reaction resulting from the unique crystal structure. The high performance of such perovskites under lower temperatures lies in the fact that a simple cubic perovskite with randomly occupied A-sites transforming into a layered compound with ordered lanthanide and alkali-earth cations may reduce the oxygen bonding strength and provide disorder-free channels for oxygen ion migrations. In order to compromise the cell performance and chemical and mechanical stability, the substitution of Fe in B site was comprehensively investigated to explore the effects of Fe doping on the crystal structure, thermal and electrical properties, as well as electrochemical performance. Furthermore, a platinum nanowire network was successfully developed as an ultrathin electrochemically efficient current collector for SOFCs. The unique platinum network on cathode surface can connect the oxygen reduction reaction (ORR) sites at the nano-scale to the external circuit while being able to substantially avoid blocking the open pores of the cathode. The superior electrochemical performance was exhibited, including the highly reduced electrode polarization resistance of 0.1 Ω cm2, improved power density of 1535 mW cm-2 at 650 oC in hydrogen and good thermal-cycle stability. Furthermore, this novel nano-scale platinum current collector can be extensively applied to other cathode materials and cell structures while showing the capability of being scaled up for mass productions due to the easily operated spraying process.

The SOFCs with metal oxide as anodes are usually electrolyte-supported design. This design requires relatively thick electrolyte of 300~500 µm to support the entire cell, leading to significant ohmic resistance. Accordingly, high temperatures (800-900 oC) are needed to reduce the ohmic loss for high power outputs. Anode-supported designs may effectively reduce the ohmic loss with thin electrolyte membrane while lowering the operating temperatures. However, the anode-supported designs with metal oxides as anode materials are difficult to fabricate. In general, high sintering temperature is needed to co-fire the anode substrate/electrolyte assembly to densify the thin electrolyte, which in turn may induce the densification of the porous anode substrate, resulting in anode porosity loss. In this work, a ceramic anode supported SOFC based on perovksite oxide of La0.75Sr0.25Cr0.5Mn0.5O3-δ (LSCM) has been prepared to evaluate the superiority of this new cell design. The cell exhibits the power density of 596 mW cm-2 and 381 mW cm-2 at 700 oC with wet hydrogen and methane as the fuel respectively, the highest performance up to date for the cells with metal oxide anodes at this temperature.

Rights

© 2014, Hanping Ding

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