Introduction

The design, materials and fabrication processes for the earlier technology Westinghouse tubular geometry cell have been described in detail previously (1). In that design, the active cell components were deposited in the form of thin layers on a ceramic porous support tube (PST). The tubular design of these cells and the materials used therein have been validated by successful electrical testing for over 65,000 h (>7 years). In these early technology PST cells, the support tube, although sufficiently porous, presented an inherent impedance to air flow toward air electrode. In order to reduce such impedence to air flow, the wall thickness of the PST was first decreased from the original 2 mm (the thick-wall PST) to 1.2 mm (the thin-wall PST). The calcia-stabilized zirconia support tube has now been completely eliminated and replaced by a doped lanthanum manganite tube in state-of-the-art SOFCs. This doped lanthanum manganite tube is extruded and sintered to about 30 to 35 percent porosity, and serves as the air electrode onto which the other cell components are fabricated in thin layer form. These latest technology cells are designated as air electrode supported (AES) cells.

In addition to eliminating the calcia-stabilized zirconia support tube, the active length of the cells has also been continually increased to increase the power output per cell. The active length has been increased from 30 cm for pre-1986 thick-wall PST cells to 150 cm for today's commercial prototype AES cells. Additionally, the diameter of the tube in longer length AES cells has been increased from 1.56 cm to 2.2 cm to accommodate larger pressure drops encountered in longer length cells. The power output of such an AES cell is 210 W at 1 atm pressure and 280 W at 10 atm pressure at 1000°C and 85% fuel (89% H2 + 11% HjO) utilization.

The fabrication of PST cells involved three electrochemical vapor deposition (EVD) steps, one each for the doped LaCr03 interconnection, the yttria-stabilized zirconia (YSZ) electrolyte, and the Ni-YSZ fuel electrode. Though EVD provides very high quality thin films (2), it requires capital intensive equipment making the process rather expensive. Investigations on alternate processing techniques have been underway at Westinghouse for several years to replace one or more EVD steps with a more cost-effective approach. The interconnection is now deposited by plasma spraying calcium alumínate containing lanthanum chromite powder over porous, doped lanthanum manganite air electrode tube; calcium alumínate facilitates densification during plasma spraying and subsequent heat treatment. These interconnections have a thermal expansion coefficient much better matched to that of the electrolyte than the previously-used EVD Mg-doped lanthanum chromite interconnection. Plasma spraying of interconnections has now been implemented in the manufacturing of all AES SOFCs. This has resulted in reduced process cycle time, increased yield, and a major reduction in cell fabrication cost. The materials and fabrication processes for the state-of-the-art AES cells are summarized in Table 1.

Table 1. Materials and fabrication processes for state-of-the-art AES cells.

Component

Material

Thickness

Fabrication Process

Air electrode tube

Doped LaMnOj

2.2 mm

Extrusion-sintering

Electrolyte

Zr0,(Y203)

40 pm

0 0

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