From these results we conclude
that product water increases first the membrane humidity
even at−10 ◦C
For portable fuel cell systems a multitude of applications have been presented over the past few years. Most of these applications were
developed for indoor use, and not optimised for outdoor conditions. The key problem concerning this case is the cold start ability of the polymer
electrolyte membrane fuel cell (PEMFC). This topic was first investigated by the automotive industry, which has the same requirements for
alternative traction systems as for conventional combustion engines.
The technical challenge is the fact that produced water freezes to ice after shut-down of the PEMFC and during start-up when the temperature
is below 0 ◦C.
To investigate the basic cold start behaviour isothermal, potentiostatic single cell experiments were performed and the results are presented.
The cold start behaviour is evaluated using the calculated cumulated charge transfer through the membrane which directly corresponds
with the amount of produced water in the PEMFC. The charge transfer curves were mathematically fitted to obtain only three parameters
describing the cold start-up with the cumulated charge transfer density and the results are analysed using the statistical software Cornerstone
4.0.
The results of the statistic regression analyses are used to establish a statistic-based prediction model of the cold start behaviour which
describes the behaviour of the current density during the experiment. The regression shows that the initial start current mainly depends on the
membrane humidity and the operation voltage. After the membrane humidity has reached its maximum, the current density drops down to
zero. The current decay also depends on the constant gas flows of the reactant gases.
Ionic conductivity of the membrane and charge transfer resistance were investigated by a series of ac impedance spectra during potentiostatic
operation of the single cell at freezing temperatures. Cyclic voltammetry and polarisation curves between cold start experiments show
degradation effects by ice formation in the porous structures which lead to significant performance loss.
© 2005 Elsevier B.V. All rights reserved
In this paper we present the results of potentiostatic single
cell cold start-up measurements under isothermal conditions
at−10 ◦C. The current density is increasing very fast after initial
start-up and later it decays towards zero. This behaviour
is ascribed to freezing of product water in the cathode to find out the main influencing operational parameters on the
cold start behaviour, statistic methods are used. Finally, a
statistic-based prediction model of the current decay during
isothermal cold start was developed
To get a deeper insight into the dynamic processes in
the cathode during cold start-up dynamic electrochemical
impedance spectroscopic analyses were done, which show
that the membrane/contact resistance as well as the charge
transfer resistance is changing with the amount of produced
water