Low-temperature fuel cells operating with contaminated feedstock

This work concerns the analysis and modeling of the dynamic and static behavior of Polymer Electrolyte Membrane Fuel Cells (PEMFC). Three fundamentally different measurement methods are used to determine the static, the large-signal, and the small-signal dynamic behavior of a fuel cell system. By combining the results of the three types of measurements, a joint dynamic and static (or dynastatic) model of the fuel cell is proposed. This model covers all three operational states of the fuel cell, while keeping close to the electrochemical aspects of the system. The dynastatic model describes the electrical behavior of the fuel cell to within 5%. The remaining part of this study is concerned with the integration of biomass energy generation systems together with fuel cells with the aim to convert biogas to electricity. With the conversion of biomass to hydrogen rich gas, polluting gasses (from the viewpoint of the fuel cell) are released. One of the major pollutions for the PEM fuel cell platinum catalyst is carbon monoxide (CO). Trace amounts of this gas, as low as 30 ppm, are responsible for lower output voltages of PEM fuel cells. This phenomenon is called CO-poisoning, and is related to the adsorption of CO to the platinum catalysts. To cope with this pollution, this work introduces an electrical regeneration procedure. The regeneration is done by pulsing the CO-poisoned cells with negative voltage, in order to electro-oxidize the adsorbed CO to carbon dioxide (CO2). In this way, carbon dioxide will disconnect itself from the platinum catalyst, making the occupied position free for the hydrogen reaction. Measurements show that the electrical regeneration of the PEM fuel cell is effective for gas mixtures containing up to 100 ppm CO. Compared with the same fuel cell operating with pure hydrogen, the regeneration method consumes only 2% of the produced electrical energy. This kind of performance has not been previously reported for fuel cell stacks with pure platinum catalysts. This study shows that the behavior of the individual cells in a fuel cell series stack differs as a consequence of a cell’s electrical position within the stack. It is demonstrated that the cells at the higher potential position in the stack can be electrically regenerated, while the cells in the lower potential position can not. It is also observed that the cell at the highest potential position in a four membrane stack can show "self-oxidizing" behavior in order to electro-oxidize the adsorbed CO to CO2. This self-oxidizing behavior was not reported before in the literature for fuel cells with pure platinum catalysts.