SOFC Stacks for Mobile Applications

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					          SOFC Stacks for Mobile Applications
                                                                       Figure 2 shows the performance map of a 5-cell
M. Lang, T. Weckesser, C. Auer, P. Jentsch, A. Friedrich      short stack operated with humidified H2+N2 as fuel gas
          German Aerospace Center (DLR),                      and air. With this diagram, the optimum operating
       Institute for Technical Thermodynamics,                conditions in terms of fuel utilization and power density
   Pfaffenwaldring 38-40, 70569 Stuttgart, Germany            can be determined. Moreover, the influence of
                                                              temperature and fuel gas flow rate can be analyzed.
         Solid Oxide Fuel Cells (SOFCs) are gaining
more and more importance as auxiliary power units                      Moreover, the cells in the stacks were examined
(APU), e.g. for passenger cars, trucks and airplanes. In      by impedance spectroscopy. Figure 3 shows the
this context, the main challenge is the development of        impedance spectra of a cell in a 5-cell short stack,
SOFC stacks, which fulfill the strong requirements for        operated at 750°C with humidified H2+N2 and air as a
mobile applications. These are a low weight, a low            function of the current density. The Nyquist plot shows a
volume, high power densities with reformate gases and         strong decrease of the overall impedance in the low
low degradation rates under dynamic operating                 frequency range with increasing current density, whereas
conditions, e.g. thermal, redox and electrical current load   the high frequency impedance is nearly independent from
cycling.                                                      the current. Altogether, three frequency dependent
                                                              processes were detected. The polarization resistances of
        The paper presents results of investigations of       the anode and the cathode at high and middle frequencies
SOFC short stacks and stacks for mobile applications.         and a gas concentration impedance in the low frequency
Therefore, a light weight stack design (Figure 1) was         range.
developed in an industrial consortium (ElringKlinger,
Ceramtec, ThyssenKrupp) in cooperation with the
German Aerospace Center (DLR) in Stuttgart and the
Research Center Jülich (FZJ). This design is based on
stamped metal sheet bipolar plates into which anode
supported cells (ASC) are integrated.

                                                              Figure 3: Nyquist plot of cell 2 of a 5-cell short stack with
                                                                 H2+N2+5%H2O and air at different current densities

                                                                       The voltage losses at the different cell
                                                              components (Figure 4) were determined by fitting of the
                                                              spectra with the help of an equivalent circuit. The highest
  Figure 1: Light weight SOFC stack components in the         losses occur at the LSM cathode, whereas the polarization
                     cassette design                          losses of the anode are lower. Due to the high porosity of
          The SOFC stacks were operated at different          the anode substrate support, the gas concentration losses
temperatures, varying fuel gas compositions and different     are lowest. According to the theory, the ohmic losses
fuel gas flow rates. The stacks were electrochemically        increase linearly with increasing current.
characterized mainly by long-term measurements, by
current-voltage measurements and by impedance
spectroscopy. The uniformity inside the stacks was
analyzed. The performance maps (Figure 2) of the short
stacks with different fuel gases are presented.

                                                               Figure 3: Voltage losses of cell 2 of a 5-cell short stack
                                                                   operated at 750°C with H2+N2+5%H2O and air

                                                              Finally, the degradation rates were determined from the
                                                              long-term measurements. At 750°C and a current density
    Figure 2: Performance map of a 5-cell short stack,        of 300 mA/cm2 degradation rates of about 1-2%/1000 h
          operated with H2+N2+5%H2O and air                   were determined.