A LT-PEMFC Stack with Extended Operating Temperature Range up to 120 °C

Kurzfassung

Polymer electrolyte membrane fuel cell (PEMFC) stacks for automotive applications operate typically at temperature ranges between –20 °C and 80 °C [1, 2]. On the other hand, specific environmental or operative conditions require the possibility for the stack to operate in a wider temperature range as well. Among the possible scenarios, two cases are of particular relevance: vehicles running in hot areas like deserts, and long uphill drive. In the desert, the ambient temperature is already relatively high, and the cooling system efficiency is reduced due to the smaller temperature difference between stack and environment. During mountain driving, a higher power is required at lower vehicle speed. Consequently, the stack temperature increases, since the cooling of the system through air ventilation is reduced at lower uphill cruising speed. In this work we present the development and characterization of a 30-cell LT-PEMFC stack designed at the German Aerospace Center for operation in an extended temperature range up to 120 °C. The feasibility to temporarily operate the stack at higher temperature would also 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. The results presented in this contribution include the current-voltage characteristic curves measured on the 30-cell stack and the homogeneity obtained along the cells. Then, the stack behaviour at wide operating temperatures was investigated by performing a series of 20 thermal cycles from 90 to 120 °C, carried out in galvanostatic conditions at a controlled current of 0.5 A/cm2 (approx. 1.5 kW electrical stack power output). The outcome is promising, since only a slight stack voltage decrease was observed during the thermal cycling. Furthermore, the results of a 1200 h long-term stability test are reported, together with an end-of-life electrochemical characterization (cyclic voltammetry and hydrogen crossover measurements) performed on all the cells. The electrochemical analysis can help to identify the main stressors responsible for the observed stack performance loss. Therefore, the catalyst, electrode and membrane degradation are examined by the determination of hydrogen crossover rates and electrochemically active surface areas (EASAs) of each cell. [1] Wu, J.F.; Yuan, X.Z.; Martin, J.J.; Wang, H.J.; Zhang, J.J.; Shen, J.; Wu, S.H.; Merida, W.; J. Power Sources, 2008, 184 (1), 104-119. [2] Schießwohl, E.; von Unwerth, T.; Seyfried, F.; Brüggemann, D.; J. Power Sources, 2009, 193 (1), 107-115.