Heterogeneous Cell Ageing in Polymer Electrolyte Fuel Cell Stacks

Polymer electrolyte fuel cell stacks, in the commonly used bipolar arrangement, consist of multiple stacked single cells in a filter-press-type arrangement. The bipolar arrangement connects the cells in series electrically and in parallel for the reactant and coolant flows; therefore, all cells have to carry the same current but they can receive different reactant mass flows. The reactant and the coolant supply may be different owing to statistically varying percolation resistances of the fluids and owing to the position of the cells in the stack. Therefore, the commonly made assumption that individual cells perform equally is valid neither for normal operation nor for the degradation of individual cells. Differences between cells can be of systematic or stochastic nature and translate into differences in the degradation rate under operation or start/stop conditions. The four main cases are discussed.