A PEMFC Stack with Extended Operating Temperature Range up to 120 °C and Related Water Management Considerations

Kurzfassung

Water management research is an important field that has to be taken into account during fuel cell development since membrane hydration and water at the catalyst surface have a direct influence on cell performance. The consumption of reactants H2 and O2 as fuel, from inlet to outlet, leads to a distribution in the reaction rate at the catalytic active area at both cathode and anode. This results in gradients in temperature and water production, which can have an influence on the reaction rate [1]. Also flooding can occur, when local partial pressure of water exceeds the saturation pressure at the local temperature [1]. Other effects include water transport distributions through the membrane due to electroosmosis and diffusion, as well as inhomogeneity in membrane conductivity due to changes in the local water activity [1]. The interaction of all these factors makes operation and understanding of phenomena occurring in a single cell already complex, and even more complicated in a stack. The work presented here concerns the characterization of a 30-cell polymer electrolyte membrane fuel cell (PEMFC) stack developed at the DLR designed for operation in an extended temperature range up to 120 °C. The normal operating temperature range of a PEMFC stack for automotive application lies between –20 °C and 80 °C. Nevertheless, the feasibility to operate in a broader temperature range can be of particular relevance in two possible scenarios: vehicles running in hot areas like deserts, and long uphill drive (e.g. during mountain driving) that requires higher power with simultaneous slow average speed, and thus with minimized air cooling. In this contribution we present the current-voltage characteristic curves measured on the 30-cell stack and the homogeneity obtained along the cells. Moreover, in order to investigate the stack behaviour at extended operating temperatures, we performed a series of 20 temperature cycles from 90 to 120 °C. The results are promising, since only a slight stack voltage decrease was observed. Furthermore, the results of a 1000 h long-term stability test are shown, together with an electrochemical characterization (CV and EIS) on the most relevant cells, before and after the long-term test, to find out possible causes of degradation due to catalyst or electrode degradation. In order to explore the stack water management behaviour, shorter stacks with the same design were used. Humidification measurements at gases inlet and outlet were performed, together with the use of a segmented board developed at DLR [2] at different stack positions, to follow performance changes depending on the operative conditions (in particular humidification). [1] S. Shimpalee, S. Greenway, J. W. Van Zee, J. Power Sources, 2006, 160, 398-406. [2] R. Lin, H. Sander, E. Gülzow, A. K. Friedrich, ECS Trans., 2010, 26, 229-236.