Date of Completion

5-7-2013

Embargo Period

5-7-2013

Keywords

Solid Oxide Fuel Cell, Interconnect Coatings, Manganese Cobaltite, Spinel

Major Advisor

Mark Aindow

Associate Advisor

S. Pamir Alpay

Associate Advisor

Prabhakar Singh

Field of Study

Materials Science and Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

The implementation of improved electrolyte materials have led to modern solid oxide fuel cells (SOFCs) which operate at lower temperatures (600-800 °C) than previously possible. The lower operating temperatures enable the use of chromia forming metallic interconnects which are less expensive and more manufacturable than their ceramic counterparts. However, poor oxidation resistance and the evolution of volatile chromia species currently limit the lifetime of the SOFC stack. As such, protective conductive ceramic coatings such as Mn1.5Co1.5O4 (MCO) are necessary to protect the underlying alloy and prevent chromia species from poisoning the SOFC cathode.

In this thesis, a study of microstructural changes of bare and MCO coated Crofer 22 APU and Haynes 230 alloys is presented. Oxidation studies on bare Crofer 22 APU heat-treated at 1050 °C prior to oxidation showed that samples with a larger starting grain size had up to a 3.5X reduction in oxidation kinetics. In addition, heat treatment prior to oxidation was shown to promote the formation of an external MnCr2O4 protective spinel. MCO coated Crofer 22 APU was examined after various exposure times in air at 800 °C. Detailed Focused Ion Beam (FIB) and Transmission Electron Microscopy (TEM) studies revealed the presence of a thick (Mn,Co,Cr,Fe)3O4 cubic spinel reaction layer (RL).

Similarly, bare Haynes 230 oxidation kinetics was examined and shown to have a parabolic rate constant, kg, of 8.9x10-9 mg2cm-4s-1, which is nearly an order of magnitude slower than the oxidation kinetics of Crofer 22 APU. MCO coated Haynes 230 also forms a spinel RL, but this is approximately 3-5X thinner than RLs formed on MCO coated Crofer 22 APU exposed under identical oxidation conditions. Previously developed equivalent circuit models have shown that the RLs reported here can have a profound impact on the oxidation kinetics of the coated interconnect system. The formation mechanisms, RL crystal structure, chemistry and implications for long-term SOFC performance are discussed.

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