Start up and freezing processes in PEM fuel cells

The isothermal operation of single PEM fuel cells below 0 degrees C is investigated by potentiostatic experiments and analyzed by physical modeling. Current decay is observed during potentiostatic operation, which is caused by the freezing of product water in the porous cathode layers. This paper describes the implementation of the ohmic losses and changes of the active cathode area as a function of the charge transfer density in a physical PEM fuel cell model. The model is comprised of a set of differential equations accounting for the electrochemical reaction and diffusion processes in the cathode of the fuel cell. The model describes the time-resolved current decay during freezing. Thus a better understanding of freezing processes at sub-zero conditions in PEM fuel cells is gained and mechanisms proposed from earlier experimental data and statistical based interpretations are confirmed. Furthermore, the transient water uptake behavior of the membrane is studied in situ at different temperatures.