Degradation of H₃PO₄/PBI High Temperature Polymer Electrolyte Membrane Fuel Cell under Stressed Operating Conditions: Effect of Start/Stop Cycling, Impurities Poisoning and H2 Starvation

The Polymer electrolyte membrane (PEM) fuel cells are promising fuel cell technology which can convert the chemical energy in for example hydrogen into electricity efficiently and environmentally friendly. In this work, some degradation issues of the HT-PEM fuel cell are experimentally investigated. Given the current challenges for production and storage of the H2, it is more practical to use a liquid fuel such as methanol as the energy carrier. However, the reformate gas produced from methanol contains impurities such as CO, CO2 and unconverted methanol. For stationary applications, especially for HT-PEM fuel cell based micro-CHP units for households, the daily startup/shutdown operation is necessary. Moreover, the faults in the H2 supply system or in controlling the reformer can cause the H2 starvation of the HT-PEM fuel cell. The effects of these operating conditions to the degradation of the HT-PEM fuel cell are studied in the current work. Both in-situ and ex-situ characterization techniques are conducted to gain insight into the degradation mechanisms of the HT-PEM fuel cell under these operating conditions.

The experimental results in this work suggest that the presence of methanol results in the degradation in cell performance of the HT-PEM fuel cell by increasing the charge transfer resistance and mass transfer resistance. The CO with volume fraction of 1% – 3% can cause significant performance loss to the HT-PEM fuel cell at the operating temperature of 150 oC. The cell performance loss caused by CO poisoning can be alleviated by the presence of water vapor. The CO oxidation via the water gas shift reaction is the main reason for the mitigated CO poisoning with the presence of water vapor. Meanwhile, the CO poisoning can deteriorate with the presence of CO2, although the CO2 alone does not affect the cell performance. H2 starvation results in reversal in the cell polarity. The carbon corrosion and water electrolysis reactions occur in the anode under H2 starvation conditions as confirmed by the presence of CO2 and O2 in the anode exhaust. The current density distribution becomes uneven under H2 starvation conditions, with high current density values in upstream regions and low current density values in downstream regions. The cell reversal and uneven current density distribution become more severe under lower H2 stoichiometry and higher current load conditions. The rapid decay in cell performance during a H2 starvation degradation test reveals that the H2 starvation can cause severe damage to the HT-PEM fuel cell. The degradation under H2 starvation conditions occur in the catalyst layer, mainly in the anode, while the membrane is not affected. The carbon corrosion in the anode and consequently the decrease in ECSA is the main reason for the degradation under H2 starvation conditions.