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

학위논문 (박사)-- 서울대학교 대학원 : 기계항공공학부, 2016. 8. 김민수.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.Chapter 1. Introduction 1 1.1 Background of the study 1 1.2 Literature survey 4 1.3 Objectives and scopes 8 Chapter 2. Measurement of crossover rate under unloaded condition 10 2.1 Introduction 10 2.2 Gas crossover mechanism 11 2.3 Experimental apparatus and test procedure 14 2.3.1 Single cell preparation 14 2.3.2 Experimental apparatus 14 2.3.3 Mass spectrometer 17 2.3.4 Test procedure 17 2.4 Effect of the land/channel width ratio of bipolar plates 19 2.4.1 Bipolar plates 19 2.4.2 The electrochemical performance of PEMFCs 23 2.4.3 Hydrogen crossover rate of PEMFCs 28 2.4.4 Anode inlet pressure of PEMFCs 33 2.5 Effect of the relative humidity condition 34 2.5.1 Hydrogen crossover rate under different relative humidity conditions 34 2.5.2 Effect of the relative humidity condition difference between anode and cathode 37 2.5.3 Analysis on behavior of crossover rate under flooding 42 2.6 Effect of the flow direction 45 2.7 Summary 47 Chapter 3. Analysis on crossover phenomenon for various current densities 49 3.1 Introduction 49 3.2 Effect of the clamping pressure 49 3.3 Effect of the relative humidity condition 54 3.4 Effect of the flow direction 58 3.5 Effect of the stoichiometric ratio 62 3.6 Numerical analysis of crossover phenomenon under various current densities 68 3.7 Summary 69 Chapter 4. Effect of pinhole on the crossover at the membrane 72 4.1 Introduction 72 4.2 Pinhole formation processes 73 4.3 Preparation of materials 74 4.3.1 Preparation of the perforated MEA 74 4.3.2 Test procedure 75 4.4 Effect of formed pinhole on the membrane 75 4.4.1 The electrochemical performance of the perforated MEA 75 4.4.2 Hydrogen crossover rate of the perforated MEA 79 4.5 Effect of the location of pinhole on gas crossover 87 4.5.1 The electrochemical performance according to the location of pinhole 87 4.5.2 Hydrogen crossover rate according to the location of pinhole 92 4.6 Development for judgement method of pinhole existence 95 4.6.1 Open circuit voltage 95 4.6.2 Crossover rate 95 4.6.3 Overshooting under loaded conditions 96 4.6.4 High frequency ratio 96 4.7 Summary 98 Chapter 5. Concluding remarks 99 References 102 Abstract (in Korean) 112Docto