Studies on the performance improvement of PEM fuel cell in low temperature flooding condition by removing the condensed water inside GDL

Abstract

When a fuel cell vehicle is operated in low temperature right after starting up, performance drop due to flooding often occurs in the catalyst layers, micro porous layer(MPL), gas diffusion layers (GDL), and gas channels of the fuel cell. It blocks the reactant gas transfer to the catalyst layer and reduces power output from a fuel scell when high current is needed. So, flooding is a very serious problem needed to be solved. Among those accumulated water, the accumulated water inside GDL is very hard to remove unless a fuel cell is fully warmed up. So, in this study, the method of removing accumulated water droplets from GDL and MPL was studied with numerical simulation and experiments. New concept of water removal method was suggested. Its main concept is pushing the water droplet inside GDL with increased gas pressure from catalyst layer side. Inside GDL water droplets are trapped among the fibers of GDL. To push these water droplet toward gas channel, additional force, which can overcome the surface tension between the fiber of GDL and water droplets, is needed. By lowering a cell voltage artificially, this additional force can be obtained. This concept was named as voltage lowering method. If the cell voltage is lowered during its operation, the additional heat generated at the catalyst layer heats the GDL and the gas nearby the catalyst layer. So, the gas pressure is increased and push the water droplets toward gas channel. To realize this concept in real fuel cell system, two ideas, which were low air S.R. operation and hydrogen mixing operation, were derived. These two ideas used concentration loss to lower the cell voltage. Before test these ideas in real fuel cell system, the concept of voltage lowering method was verified by numerical simulation. Generally, the gas pressure distribution across GDL is not considered in calculating water transport inside GDL. However from the simulation results, it was found that the gas pressure distribution across GDL should not be neglected in calculating water transport inside GDL. From the simulation results including gas pressure across GDL, it was found that this method helped to increase pressure gradient across GDL and to remove accumulated water. Consequently, more oxygen was transferred to the catalyst layer. Therefore, it was concluded that this method is useful in removing accumulated water and recovering cell voltage in flooding condition. After verifying the effectiveness of the voltage lowering method using numerical simulation, this concept was verified by experiments. In this experiments, low air S.R. operation was mainly verified, and the hydrogen mixing operation was tested to compare the results with that of low air S.R. operation. The main idea of the low air S.R. operation was lowering the air flow rate supplied to the cathode while maintaining current output from the fuel cell. And the idea of hydrogen mixing operation was to supply a small amount of hydrogen into the air supplied to cathode while maintaining the current output from a fuel cell. From the test results including transparent fuelcell test, low air S.R. operation was more effective in removing accumulated water from GDL than hydrogen mixing operation. Hydrogen mixing operation emitted hydrogen gas which was not used during the operation. However, there was almost no hydrogen gas in the exhaust gas of low air S.R. operation. So, the low air S.R. operation is more appropriate method that is applicable to a fuel cell vehicle. Additionally, a degradation test for low air S.R. operation was conducted. From the test results, it was concluded that the low air S.R. operation didnt seriously affect the performance loss of a fuel cell. In conclusion, it was concluded that the low air S.R. operation is an effective method of removing accumulated water from GDL for a real fuel cell vehicles.