Uniformity Identification and Sensitivity Analysis of Water Content of Each PEM Fuel Cell Based on New Online High Frequency Resistance Measurement Technique

Water content estimation is a key problem for studying the PEM fuel cell. When several hundred fuel cells are connected in serial and their active surface area is enlarged for sufficient power, the difference between cells becomes significant with respect to voltage and water content. The voltage of each cell is measurable by the cell voltage monitor (CVM) while it is difficult to estimate water content of the individual. Resistance of the polymer electrolyte membrane is monotonically related to its water content, so that the new online high frequency resistance (HFR) measurement technique is investigated to identify the uniformity of water content between cells and analyze its sensitivity to operating conditions in this paper. Firstly, the accuracy of the proposed technique is experimentally validated to be comparable to that of a commercialized electrochemical impedance spectroscopy (EIS) measurement equipment. Secondly, the HFR of each cell of a 164-cell and 270 cm² stack is acquired by means of the re-designed CVM and it contributes to the uniformity identification of their water content. Thirdly, the sensitivity analysis of the HFR to operating conditions implies the importance of air excess ratio and chamber pressure in adjusting water content, and the dynamic experiment shows the difference of time constant between removal and accumulation of water in the fuel cell, namely the hysteresis phenomenon. The online HFR measurement technique for each cell of a large stack proposed in this study can be implemented to more situations like experimental research in laboratory, application in transportation area and even extension to battery research.