Kosuke Nishida

Water Management and Experimental Diagnostics in Polymer Electrolyte Fuel Cell

Polymer electrolyte fuel cell (PEFC) is a promising candidate for mobile and vehicle applications and distributed power systems because of its high power density and low operating temperature. However, there are several technical problems to be solved in order to achieve practicability and popularization. Especially, water management inside a PEFC is essential for high performance operation. At high current densities, excessive water generated by the electrode reaction is rapidly condensed in the cathode electrode. When the open pores in the catalyst layer (CL) and gas diffusion layer (GDL) are filled with liquid water, oxygen cannot be supplied to the reaction sites. Furthermore, water migrates significantly through the electrolyte membrane from the anode to cathode owing to electroosmotic drag. Thus, the membrane dehydration occurs mainly on the anode side and causes the low proton conductivity during low-humidity operation. These phenomena known as “water flooding” and “dryout” are a critical barrier for high efficiency and high power density. To alleviate these issues, it is necessary to develop various diagnostic tools for understanding the fundamental phenomena of water transport between cathode and anode in PEFC.

In Situ Measurement of Flow Velocity in Gas Channel in a PEMFC by Laser Ablation Tagging Visualization

We developed a visualization system of gas flow behavior called laser ablation tagging visualization (LATV), which showed a potential to examine flow behaviors in gas channel in an operating proton exchange membrane fuel cells (PEMFC). We constructed LATV system with two pulsed lasers: one is for laser ablation tagging and the other is for excitation of the tag material for visualization. The system was validated and was also applied to an operating PEMFC. Flow velocity in channel at operating conditions was examined with variation of current density. Under low current density condition, gas flow velocity was consistent with theoretical values calculated from supplied volume flow rate at the inlet. On the other hand, result under high current density condition shows that flow velocity in the channel accelerated toward the downstream, possibly due to generation of vapor and enhancement of water transport across the MEA from the anode to the cathode.

Measurement of Water, Temperature and Current Distributions in Anode of Polymer Electrolyte Fuel Cell During Low Humidity Operation

In order to prevent membrane dryout in polymer electrolyte fuel cells (PEFCs) during low humidity operation, it is essential to understand the fundamental phenomena of the water transport and reaction distribution at the anode side in an operating fuel cell. In this study, the water vapor distribution along the anode flow channel of a PEFC under low humidity conditions was quantitatively evaluated by using humidity test paper (HTP), and the effects of flow configuration and inlet humidification on the water distribution in the anode were investigated. HTP is a test paper for detecting water vapor of 20–90% RH, which is coated with a blue surface. This test paper was inserted between the anode electrode and separator in the transparent fuel cell, and the discoloration of HTP was directly visualized using a digital CCD camera. Furthermore, the temperature and current distributions in the anode electrode were measured using IR thermography and segmented cell structure concept. It was found that the water vapor concentration on the anode side increases immediately after the startup because of the back diffusion of the product water from the cathode to anode. In the case of the co-flow configuration with the dry anode and cathode inlets, the water vapor concentration increases monotonically along the anode flow channel. In addition, the anode water distribution affects the temperature and current distributions in the fuel cell largely. The local temperature and current density at the dry anode inlet are lower than those in the downstream region because of the membrane dehydration and low proton conductivity. On the other hand, in the case of the counter-flow pattern, the distributions of water concentration, temperature and current density have the maximum points in the midstream region. The counter-flow configuration is effective in improving the membrane hydration and alleviating the anode dryout.Copyright

Visualization of Water Transport and Freezing Process in a Gas Diffusion Layer of PEFC During Low Temperature Operation

During low temperature operation of polymer electrolyte fuel cell (PEFC), water flooding in a cathode electrode is a critical barrier to achieve high power density, because liquid water condensed in the porous gas diffusion layer (GDL) blocks oxygen transport to the active reaction sites. Furthermore, during startup below the freezing point, liquid water in the cathode GDL or on the catalyst layer (CL) surface is frozen and the cell voltage is suddenly decreased. In this study, we experimentally investigate liquid water transport and freezing process in a cathode GDL of PEFC at low temperature operations using an optical diagnostic. The results showed that liquid water behavior and ice formation at the cathode electrode are largely affected by operating parameters such as current density and ambient temperature. Furthermore, in order to prevent water flooding, we propose the structure of cathode GDL that has several slits for water removal. With this cathode GDL structure, it was found that liquid water accumulated inside the cathode electrode is rapidly removed and cell voltage is improved.© 2007 ASME