Development of Cost-Effective YSZ Coating Methods for SOFC Interconnects

Solid oxide fuel cells (SOFCs) require robust seals that can prevent intermixing of air and fuel, remain inert in reducing and oxidizing environments while in contact with SOFC materials, and maintain their effectiveness through repeated thermal cycles. Recent research has focused on compliant (or viscous) glasses that remain vitreous over time in the SOFC stack operating environment, and are able to tolerate relative motion between the surfaces being sealed without the development of permanent leaks. Certain glasses (under investigation by MO-SCI and others) considered for this sealing application have broadly desirable thermo-mechanical properties and thermo-chemical characteristics, but have been found to chemically react with both bare and aluminized stainless steel SOFC interconnects, consequently forming phases that may adversely affect the integrity of the seal. On the other hand, these glasses do not react with the yttria-stabilized zirconia (YSZ) electrolyte used in most SOFC designs. Thus, YSZ could be an attractive barrier layer between the metallic SOFC interconnect and the sealing glass. The main goal of the proposed nine month, STTR Phase I program is to develop a dense and well-bonded YSZ coating on an SOFC interconnect alloy. The coating method proposed in this project utilizes the two-step YSZ powder/polymer process that has successfully deposited YSZ coatings on SOFC electrodes. This two-step YSZ infiltration method does not require special equipment such as plasma or laser deposition source, and the process can be scaled up and is suitable for coating larger and more complex surfaces. Our preliminary studies have already demonstrated proof-of- concept. During the Phase 1 program, we will (1) determine the relationships between processing parameters and the YSZ coating thickness; (2) determine how these processing conditions affect the interactions between the YSZ coatings and the viscous sealing glass; and (3) characterize the long-term interfacial reactions between optimized coating and the underlying stainless steel as well as the viscous sealing glass. Commercial Applications and Other Benefits: This STTR project will assist the nations SOFCs program in meeting its cost and performance targets by ensuring a stable barrier layer between SOFC interconnects and seals, and consequently achieving reliable seal operation over an extended operating life. The program will ultimately enable fuel cell-based near-zero emission coal plants with greatly reduced water requirements and the capability of capturing 97 percent or greater of carbon at costs not exceeding the typical cost of electricity available today. Achieving this goal will significantly impact the nation given the size of the market, expected growth in energy demand, and the age of the existing power plant fleet. It will also provide the technology base to enable grid-independent distributed generation applications.