Christopher A. Coyle

Rupture Testing as a Tool for Developing Planar Solid Oxide Fuel Cell Seals

One of the critical issues in designing and fabricating high-performance planar solid oxide fuel cell (pSOFC) stacks is the ability to hermetically seal adjacent metal and ceramic components. In our pSOFC development program, we have designed a testing technique that allows us to screen through the numerous variables involved in developing glass seals. Using this test for example, we have found that the composition of the metal component plays an important role in the strength of the seal. Microstructural analysis of as-sealed specimens revealed that an interfacial reaction zone forms during joining, and it appears that the thickness and composition of this layer are the dominant parameters that control joint strength. In this paper the details of the seal test are reported. The results have proven particularly significant in the development of the next-generation stack design. Supporting microstructural and chemical analyses collected on the test specimens are also presented and used to interpret the seal test results in an effort to identify the necessary steps toward improving glass pSOFC seals.

Alternative planar SOFC sealing concepts

Abstract One of the challenges in manufacturing planar solid oxide fuel cells (pSOFCs) is in hermetically sealing the ceramic and metallic components such that the resulting joint remains rugged and stable over the lifetime of the stack. Traditionally, glass joining or compressive sealing has been used. While short-term success has been achieved with these techniques, it is apparent that to meet the long-term operational needs of stack designers, alternative sealing concepts will need to be conceived. At Pacific Northwest National Laboratory we have been developing two such alternatives, air brazing and bonded compliant sealing, the details of which are outlined here.

Ni/YSZ Anode Interactions with Impurities in Coal Gas

Performance of solid oxide fuel cell (SOFC) with nickel/zirconia anodes on synthetic coal gas in the presence of low levels of phosphorus, arsenic, selenium, sulfur, hydrogen chloride, and antimony impurities were evaluated. The presence of phosphorus and arsenic led to the slow and irreversible SOFC degradation due to the formation of secondary phases with nickel, particularly close to the gas inlet. Phosphorus and antimony surface adsorption layers were identified as well. Hydrogen chloride and sulfur interactions with the nickel were limited to the surface adsorption only, whereas selenium exposure also led to the formation of nickel selenide for highly polarized cells.

Effect of coal gas contaminants on solid oxide fuel cell operation

The operation of solid oxide fuel cells (SOFC) was evaluated on simulated coal gas in the presence of several coal gas impurities that are expected to remain in low concentration after warm gas cleanup. Phosphorus, arsenic and sulfur were considered in this study. The presence of phosphorus and arsenic in low, 1-2 ppm, concentrations led to the slow and irreversible SOFC degradation due to the formation of the secondary phases with nickel in the upper part of the nickel-based anode close to the gas inlet. Sulfur interactions with the nickel were limited to the surface only. Cell performance losses due to sulfur exposure were reversible and independent of the presence of other impurities.

SOFC Ohmic Resistance Reduction by HCl-Induced Removal of Manganese at the Anode/Electrolyte Interface

The ohmic resistance of anode-supported solid oxide fuel cells having a manganese-based cathode was lowered when operated in synthetic coal gas containing hydrogen chloride. This effect was not observed for cells with cathodes that did not contain manganese. Substantial amounts of Mn were found throughout the grain boundaries of the 8 mole% yttria-stabilized zirconia (8YSZ) electrolyte. Exposure to HCl partially removed Mn near the anode/electrolyte interface, presumably by volatilization as MnCl2(g). This work suggests that one of the underlying causes of higher than expected electrolyte resistance in anode-supported SOFCs is a lowering of the ionic conductivity of 8YSZ by incorporation of manganese.

The Effect of Composition on the Wetting Behavior and Joint Strength of the Ag-CuO Reactive Air Braze

One of the challenges in manufacturing solid-state electrochemical devices is in joining the ceramic and metallic components such that the resulting joint is rugged, hermetic, and stable under continuous high temperature operation in an oxidizing atmosphere. A well proven method of joining dissimilar materials is by brazing. Unfortunately many of the commercially available ceramic-to-metal braze alloys exhibit oxidation behavior which is unacceptable for potential use in a high temperature electrochemical device. An alternative braze alloy composition designed for oxidation resistance has been developed to join ferritic stainless steel to a variety of electrochemically active ceramic membranes including YSZ, nickel oxide, and mixed conducting perovskite oxides. The results of this study to date will be discussed.

Effects of trace metals and organic additives on porosity and dielectric constant of high purity mesoporous silica films

The beneficial effects that alkali metal and alkylammonium salt additions to molecularly templated silica sols have on the resulting mesoporous silica films formed from evaporative-coating methods with respect to porosity, elastic modulus, dielectric constant, and film surface uniformity were investigated and identified.