Experimental investigation of parameters influencing the freeze start ability of a fuel cell system

To improve the freeze start ability of a fuel cell system some significant influencing parameters are defined and investigated. Experiments with a fuel cell test system are carried out in a climate chamber at various conditions. The time interval until fuel cell stack power equals 50% of its maximum power is defined as an indicator for a successful freeze start as well as a value for comparison and evaluation of the results. The target of this work is the minimization of this freeze start time by avoiding the freezing of process water on the catalyst layer of the Membrane Electrode Assembly (MEA), since this leads to temporary performance losses.The shut down strategy of the fuel cell system is identified to be one of the main parameters influencing the freeze start. It is found that a higher degree of dryness in the stack leads to a significant improvement in the freeze start performance, since the water absorbing capacity of the membrane increases and therefore also the time until its saturation. If this saturation takes place after the temperature of the MEA reached 0 °C, no significant ice-formation occurs. It is shown that by improving the shut down strategy of the fuel cell system at T_Start = −6 °C a start without performance loss can be realized. At temperatures lower than that temporary performance losses occur.Even if a lower voltage leads to a higher current and therefore to a higher water production rate, its effect on the freeze start due to the increased heat of reaction is positive. Further investigated parameters, for example the volume of the coolant loop, also affect the freeze start ability, but it can be concluded that the shut down strategy is of main importance.