Bipolar Plate-Supported Solid Oxide Fuel Cells for Auxiliary Power Units
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Bipolar Plate-Supported Solid Oxide Fuel Cells for Auxiliary Power Units J. David Carter, Terry A. Cruse, Joong-Myeon Bae, James M. Ralph, Deborah J. Myers, Romesh Kumar, and Michael Krumpelt Chemical Technology Division, Argonne National Laboratory Argonne, IL 60439-4837, U.S.A. ABSTRACT This paper presents an advanced design and fabrication concept for a solid oxide fuel cell (SOFC). The concept is based on a laminate repeat unit comprised of a thin electrolyte, cermet anode, metallic gas flow fields, and a metallic bipolar plate. The laminate is sintered in a singlestep process in a controlled atmosphere, and the cathode is applied and sintered in situ during the initial heating of the cell (or stack). Observations about the types of cracks that formed in the electrolyte during the sintering process guided the development of the sintering protocol to yield the desired product. Cells with power densities exceeding 250 mW/cm2 have been tested. INTRODUCTION Recently, 5 kW SOFCs have been proposed for use in automotive and heavy vehicle auxiliary power units (APUs) that run on reformed gasoline or diesel [1]. Solid oxide fuel cells have an advantage over other fuel cell types because of their fuel flexibility, which allows for simple fuel reforming systems. In APU applications, the SOFC stack must also be able to withstand mechanical shock and vibration. However, most stack designs are subject to brittle failure because of their ceramic components. Some researchers have sought to improve the mechanical properties of the SOFC by supporting cells on metal substrates by using various fabrication methods [2, 3]. The concept being developed at Argonne, called TuffCell, is shown in Figure 1. It is an SOFC stack unit consisting of a ceramic electrolyte, cermet anode, metallic flow fields and a metallic bipolar plate. The brittle ceramic components are bonded to tough metallic layers that give the laminate improved mechanical strength. Each functional layer is made by casting oxide or metal slurries into films. The dried films are laminated and sintered together in a single high-temperature process combining traditional ceramic and powder metallurgy sintering techniques. The cathode is then applied during the stack building process and is sintered in situ during the initial heating of the stack. Because layers of the TuffCell are bonded together during sintering, an unbroken electrical current path is formed between the electrolyte, anode, flow fields and bipolar plate. This approach leaves only a single electrical contact layer for each repeat unit between the air flow field and the cathode. Conventional fuel cells have two or more contact areas, which results in a loss of power from individual cells as they are stacked together. This fabrication approach also allows flexibility in the composition of each layer. For example, a multilayered bipolar plate consisting of different alloys in the anode and cathode environments can easily be incorporated, allowing designers to choose materials best suited to each environm
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