Investigation of Hydrogen Crossover Phenomena during Operation of Proton Exchange Membrane Fuel Cell Systems

Abstract

In this study, the crossover phenomenon according to various parameters that have not been covered yet and during operation of proton exchange membrane fuel cell system were investigated to provide practical information and better understanding on thereof. Furthermore, the effect of the pinhole formation and the location of pinhole on hydrogen crossover were analyzed. For successful landing of proton exchange membrane fuel cell system on markets, durability is imperative issue to be improved. Since the crossover phenomenon is strongly related to degradation process of materials, in this sense, conducting the researches for more practical parameters and conditions where the real system operates are needed. However, the most of previous researches were limited because it just had concentrated on the relationship between operating parameters and gas crossover that difficult to know even reflect on system operation. Therefore, in this study, we are focused on the useful parameters and conditions to apply its results on real proton exchange membrane fuel cell system. Firstly, effects of the bipolar plate designs, relative humidity condition difference between anode and cathode and flow direction (co-flow, counter-flow) on hydrogen crossover rate under unloaded condition are experimentally investigated to complements previous researches by dealing with new topics. Through this research, it was found that the bipolar plate design can have an influence both on performance and crossover rate due to the pressure in channel. To identify the effect of the relative humidity condition precisely, not only effect of the relative humidity condition difference between anode and cathode but also the behavior under flooding is analyzed. The relative humidity of air has more effect on hydrogen crossover because the amount of supplied air is higher than that of hydrogen, and higher relative humidity accelerates more crossover rate. However, too much water contributes to blocking the porous of gas diffusion layer and leads to prevent gas crossover. Also, the influence of the flow direction on crossover rate is studied. From the result, even if the performance under the fully humidified condition has little difference between the co and counter-flows, the hydrogen crossover rate greatly differs between both cases due to the gas distribution in the cell. Furthermore, the effect of the clamping pressure, relative humidity condition, flow direction and stoichiometric ratio on hydrogen crossover rate are analyzed under the real system operation condition. The effect of the relative humidity condition, flow direction and stoichiometric ratio with specific condition is totally changed as the current density is increased. Through this research, the importance of measuring crossover rate under loaded condition will give an insight to the further research on crossover phenomenon. Lastly, effect of the pinhole formation on crossover rate is studied. It was found that even if the size of a pinhole is too small to detect its existence by the performance change, it can be confirmed by detecting hydrogen crossover rate under various current densities. In addition, the location of the damage of MEA can be analyzed through the hydrogen crossover rate pattern for loaded and unloaded condition. Greatly scattered hydrogen crossover pattern was measured for blemish at inlet and higher hydrogen crossover rate was detected for pinhole at outlet. Based on this result, it is determined that the membrane of anode inlet and outlet side have an important role in performance and hydrogen crossover. This can suggest the guideline for manufacturing process of MEA that which part should be made carefully and strongly. Furthermore, judgement method for pinhole existence with four different parameters was proposed to provide detecting technique for MEA and stack manufacturers.