Polymer Fuel Cell Stack based on Sulfonic Acid Membranes with Extended Operating Temperature Range up to 120 °C

Kurzfassung

Low temperature polymer electrolyte membrane fuel cell (LT-PEMFC) stacks for automotive applications are typically operated at temperatures between -20 and 80 °C. The possibility to work at higher temperatures is limited by water management issues associated to the used membrane. The feasibility to operate in a wider temperature range can be of particular interest in specific environmental or operative conditions, when temporary temperature increase takes place, caused by an enhanced power demand and/or limited heat removal (e.g. driving in hot regions, or uphill on the mountain). The possibility for the stack to transiently operate at higher temperatures up to 120 °C would contribute to the improvement of cooling system components with scaled down dimensions, which in turn would lead to a reduced vehicle weight and thus to an overall lower fuel consumption. This contribution presents the results obtained on a 30-cell stack (2.5 kWel) developed at the German Aerospace Center to work at higher temperature. It was used to perform a short-term operation at temperatures up to 120 °C, and a long-term test at 80 °C and typical automotive conditions. The former test consisted of 20 temperature cycles between 90 and 120 °C, carried out in galvanostatic mode (0.5 A/cm2) and without adjustment of gas humidification. It demonstrated a fully reversible performance loss of 21 ± 1 % during each thermal cycle, and an irreversible degradation rate about 6-times higher than the one determined at 80 °C. The long-term stability test was carried out for 1200 h at constant load (0.5 A/cm2) and 80 °C, under typical automotive conditions, and was followed by an electrochemical characterization of the 30 cells. The stack exhibited linear stack voltage decay with acceptable degradation rates. The end-of-life electrochemical analysis was aimed to investigate catalyst, electrode and membrane degradation. To identify the main stressors that are responsible for the observed stack performance loss, hydrogen crossover rates and electrochemically active surface areas of each cell were determined. Thereby, the carbon corrosion and resulting effects on the catalyst layer structure as well as the electrocatalytic surface was identified to be the main cause for performance losses during the test, amplified by a malfunction of the used air compressor.