Inconsistent responses of cells on operating conditions in a 5 kW proton exchange membrane fuel cell stack

The cell inconsistency investigation for large-scale proton exchange membrane fuel cell (PEMFC) stacks is significant. In this paper, the inconsistency between fault cells and normal ones is investigated in a 5 kW stack with 30 cells of the active area of 312 cm2. Considering typical operating conditions, the stack temperature and hydrogen/air stoichiometry are investigated to discuss the effects on the inconsistency. As comprehensive indexes, voltage performance, polarization curve, and electrochemical impedance spectrum (EIS) are presented to evaluate the above experiments. To enhance the EIS resolution, the distribution of relaxation times (DRT) is applied to extract the characteristic frequency points from the EIS ranged from 0.5 Hz to 10 kHz. In conclusion, fault cells are sensitive to operating conditions with poor performance and larger polarization resistance compared with the normal cells. Furthermore, the DRT can well quantify the inconsistency of impedances produced by oxygen diffusion (fault 0.5 ~ 4.0 mΩ and normal 0.3 ~ 1.0 mΩ) and oxygen reduction reaction (fault 0.7 ~ 1.5 mΩ and normal 0.5 ~ 0.6 mΩ), respectively. Meanwhile, the inconsistency strongly relates to the operating conditions. Moreover, the damaged structure of fault cells in membrane electrode assemblies further validates the inconsistency by scanning electron microscopy.