Results for a New SOFC-Stack Design with Parallel-Connected Cells

The TU Clausthal and CUTEC institute have teamed up to design, build and test a new stack architecture, based on repeating units with cells connected in parallel. This approach allows the use of electrically conducting seals like brazes and can be realized without any glass sealing.

Unlike conventional (serially connected) stacks, where all cells operate at the same current but individual voltage, in this setup each cell contributes as much current as possible at a common voltage. Thereby accelerated degeneration caused by undervoltage and the associated oxidation of the nickel catalyst can be completely overcome.

The concept uses two cells in electrical parallel connection with the anodes facing each other to form a repeating unit. Two designs have been realized:  An all ceramic housing made from 3YSZ, sealed with reactive air brazing (RAB) and a metallic one with a Crofer 22 APU housing, joined by laser beam welding. The housing materials have been chosen with respect to their thermal expansion coefficient (TEC) matching the TEC of the used ESC cells. Special interconnectors have been produced via hydroforming to create an even flow field while providing sufficient contact area.

The paper will focus on the test results of the repeating units under various operation conditions and the analysis of the degradation of components and joints.

Polarization curves for the twincell repeating unit and the individuell cells under H₂ and air at 880°C are depicted in figure one.