• Nissen, Jens
• Hölzle, Markus
• Schwämmlein, Jan Nicolas

Abstract

Gross fuel starvation is a harmful degradation stressor to polymer electrolyte membrane fuel cell (PEMFC) stacks. However, it is extremely difficult to fully avoid these events for integration level of fuel cell system and vehicle. In stack setups, properly reactant-supplied cells can apply their voltage to an undersupplied cell due to their electric serial connection. This can cause drastic local negative cell voltages, leading to substitutional reactions like the harmful carbon oxidation reaction.The presented study uses a 3-cell stack to illustrate how properly media supplied cells can apply their cell voltages to a starved cell. This stack consists of state-of-the-art automotive-sized membrane electrode assemblies and carbon composite bipolar plates. Fuel starvation of one cell was generated by an additional flow resistance in the hydrogen media inlet port. The local current density distribution of the starved cell showed two distinct regimes. These regimes suppose the occurrence of different electrochemical reactions along the flow direction. The progression of each fuel cells voltage along the flow field were measured by cell voltage monitoring measurements at eight positions. The voltage of the starved cell was found to be strongly inequal over the flow direction with intense negative voltages towards the fuel outlet. The occurrence of this non-equal cell voltages was explained by local measurements of the flow field plate potentials.A mechanism is proposed, which results in an increased carbon oxidation of the anode catalyst layer towards the fuel outlet during fuel starvation. This mechanism is unique to multi-cell fuel cells because it is based on the mutual distortion of cell voltages caused by electric interaction of adjacent fuel cells during gross fuel starvation.

Topics

• density
• impedance spectroscopy
• polymer
• Carbon
• composite
• Hydrogen
• current density