By isothermal potentiostatic cold start measurements of
single fuel cells under different operating conditions we
found that water freezes in the porous structures of the cathode
electrode, microporous layer, and the GDL. Statistic evaluation
of the experiments showed that dryer membranes and
high air flowrates are beneficial for high charge transfer rates.
Furthermore, it was found that performance degradation
plays an important role on the charge transfer too. As pre stage
to physical modelswepresented statistic-based models which
explain the characteristics of the current density during cold
start-up as function of all operating parameters and estimates
their significant influence. From these results we conclude
that product water increases first the membrane humidity
even at−10 ◦C, and after it has reached its maximum, product
water floods the porous structures of the electrode, MPL and
GDL, and freezes. Therefore, the current density decays. This
assumption is proved by electrochemical impedance spectra
during isothermal potentiostatic cold start experiments,
which show that the membrane/contact resistance decreases,
whereas the charge transfer resistance increases over time.
For both values the charge transfer and the amount of product
water, respectively, plays a significant role. Ice formation
leads to a strong degradation of the cell performance and
the electrochemical active surface area of the cathode. The
membrane/contact resistance increases with the number of
cold start-up experiments and structural changes on the cathode
GDL and MPL were observed, leading to changes in
hydrophobicity. To prevent degradation, elaborated start-up
and shut-down procedures have to be developed in the future
to overcome the obstacle cold start-up of PEM fuel cells