The effect of operating pressure on the performance during power generation and water purging of PEM fuel cells

Abstract

Polymer electrolyte membrane fuel cells (PEMFCs) are emerging due to their advantages of high energy density, high energy efficiency, low operation temperature and zero emission characteristics. Because these merits are good for automobile, so the fuel cell is spotlighted as a power source for vehicle. To generate more power within limited size condition, pressurized system is considered, but the previous studies did not explain the characteristics of pressurized system for various operating condition. Additionally, there is no effective purging method which can be applied to the pressurized system. To adapt the pressurized system for fuel cell in real world, an in-depth study is necessary. In this study, to analyze the performance of segmented fuel cell, 25 times segmented model is adapted. To evaluate the internal water distribution through the gas diffusion later and the channel, saturation profile model and water diffusion model in membrane is applied. From the simulation results, the internal water distribution is analyzed. For low RH condition, high pressure helps more vapor condensed, so fuel cell is humidified and current density becomes more homogeneous. For high RH condition, on the other hand, amount of water supplied to the fuel cell is decreased, so flooding problem can be solved. It is expected that experimental can be well analyzed using numerical analysis. To adapt the fuel cell as a power source for various applications, it is important to understand electrochemical characteristics such as current distribution and Ohmic resistance distribution were observed in order to validate the effect of operating pressure for various operating temperature, humidity, and flow rate of reactant gases conditions. 25 times segmented fuel cell is used to measure local current distribution and Ohmic resistance. The test was conducted in a constant current operation mode, and uneven current distribution was observed at high pressure condition. When the fuel cell was operated in a constant voltage operation mode, more electricity is generated for whole area because of high partial pressure of reactant gases. But current density distribution becomes inhomogeneous even at high temperature. Additionally, the results show that RH and flow rate is the main factor for current distribution. High flow rate with lower RH reactants make membrane of inlet area more dried out, so less electrochemical reaction occurs. When flow rate is low, capacity of evaporation is decreased, and effect of inlet RH condition is minimized. On the other hand, current distributions for various RH and flow rate conditions at 3 bar are relatively similar with each other. This is because pressurized volume acts as a damper, so effect of external conditions rarely affect the performance of fuel cell. When operating pressure is increased, relatively homogeneous current distribution is observed at low RH condition. That is, when the fuel cell is operated in high pressure, there is no need to supply lots of water to the fuel cell. Using the test results, dry start-up process is tested for normal pressure and high pressure. The results show that dry start-up performance is superior with high operating pressure condition. As polymer electrolyte membrane fuel cells are used for the power source of automobile, it is important to endure harsh conditions like sub-zero temperature. For successful cold start, the residual water in the cell should be minimized before start up. In this study, new effective purge method using a sudden pressure reduction is introduced. The internal ohmic resistance measurement using current interrupt method, dew point temperature measurement of exhausted gas in the cathode were carried out to verify the effectiveness of pressure reduction purge method. In addition, images of micro porous layer surface were obtained by microscope, and cold start experiments were done to check the usefulness of this purge method. The results show that most of water at catalyst layer and membrane electrolyte assembly is removed during pressure reduction purge process, compared with normal gas blowing purge process. Durability test was also conducted and no significant degradation during pressure reduction purge process was observed.