Water transport law of fuel cell membranes at different current densities

Abstract

Proton exchange membrane fuel cell (PEMFC) is considered to be one of the most promising technical for automotive power. Because it has high power density, low operating temperature and environmental friendliness. During the operation of the fuel cell, hydrogen protons accompanying water was transferred from the anode to the cathode by electro-osmotic drag. At the same time, the water will be produced at the cathode, and diffused from the cathode to the anode by a certain concentration difference. When the electro-osmotic drag is greater than the back diffusion, the anode needs to be continuously humidified to prevent the water loss in the membrane. When the back diffusion is greater than the electro-osmotic drag, the water generated in the cathode is continuously transferred to the anode. Therefore, the law of water transfer in the membrane is an important issue in fuel cell research. In the present work, it adapted such thicker homogeneous membranes such as Nafion 117. It was found that at low current densities (0.2 A/cm2), the anode was more likely to be flooded. While at high current densities, they believed that a higher electroosmotic drag will drag the anode water to the cathode, thereby reducing the anode water content. However, the used more and more thin membranes become a trend in fuel cell technology. For example, in the past, DuPont′s most thin membrane Nafion 211 was 25 μm, but now the 15 μm membrane represented by Gore was widely used, and Toyota has used 10 μm membrane. these thin membranes are different from the conventional thick membrane with uniform structure. Generally, the membrane was a sandwich structure with a perfluorosulfonic acid resin on both sides of the porous PTFE reinforced matrix. Therefore, the law of water transport was different from the homogeneous membrane. The water transport flux of the actual situation was tested by designing a dew point meter water balance test to analyze the water transfer law in the fuel cell. At the same time, the three-dimensional PEM fuel cell was simulated by using the fluid dynamics calculation software. The distribution of water content, anode water molar concentration and hydrogen mole fraction in the membrane was obtained. The results of experiment were compared to simulation results. The simulation results show that under the working condition of 75°C, 150 kPa, and cathode non-humidification conditions, the anode moisture of 15 μm membrane below 0.4 A/cm2 decreases with the increase of current. In this case, electroosmosis drags counteract partial back diffusion; The anode water above 0.4 A/cm2 increases with the increase of current, and the back diffusion played a leading role in transferring a large amount of water from cathode to the anode; the higher the current density, the more water was transferred to the anode. The actual test results showed that as the current increases, the more liquid water could be observed by the anode, which was consistent with the simulation results. When the membrane thickness was 15 μm, a thinner membrane increases the amount of water transferred from the cathode to anode. When the current was higher than 3.0 A/cm2, the anode water volume of 25 μm membrane will decrease due to electroosmotic drag. But the anode water of 15 μm membrane will still increase, and the effect of back diffusion will increase with the decrease of membrane thickness.