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Dipl.-Ing. Sebastian Brokamp

group leader stack technology
department fuel cells and systems

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Conductivity measurement on bipolar plates

Characterisation of bipolar plates with respect to their effective conductivity in fuel cells has to respect stack pressures and temperatures within the fuel cell stack.

The conductivity, respectively the resistance, is measured by detecting the voltage drop above a sample, to which a constant current is applied. The sample is fixed between two gold coated poles (area = 4 cm2), that are pressed together by linear controlled forces. Previously to the testing gas diffusion layers (GDL) with known pressure-dependent properties are applied to the poles. Thereby the probe chain is comparable to the chain of resistances (GDL - bipolar plate - GDL) in a fuel cell. A galvanostat delivers a constant current that is lead in the complete probe section by the two poles. A needle is fed through each of the poles and the GDLs. The needles are spring mounted coaxial to each other and are used as passive voltage sensors. Due to this assembly the following terms of resistance can be determined:

Leitwertmessung / Conductivity
  • total resistance Rtot
    Describes the resistance of the complete section.
  • forwardresistance Rfor
    Describes the resistance of the sample (=without the resistance of the GDL)
  • bulkresistance RΩ
    Describes the resistance according to the drop above the passive needles, which penetrate the bulk material of the sample. Attention has to be paid to samples with low conductivity in lateral directions. In this case the passive needles possibly can not detect any voltage drop. Furthermore the bulk resistance contains the internal contact resistance of two half-plates, if multipart bipolar plates are tested.
  • contact resistance RC
    Describes the contact resistance between GDL and bipolar plate.

Different contact pressures (3 - 30 bar) are tested by using an adapted software. The voltage drops between the poles and the needles are detected at four different currents for each contact pressure. The currents can be adjusted to the expected conductivity of the probes. According to ohm´s law the mean values of the conductivity at each pressure are determined.

Messtechnik BPP
Messtechnik BPP

Because the increasing contact area of the probe at higher pressure values, the qualitative gradient of the resistance-pressure-curve is a parabola that approaches asymptotically to a limiting value.

Aufstellung der flächenspezifischen Einzelwiderstände
Last update:  17.02.2017