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.