Studies on the Performance Improvement of Polymer Electrolyte Membrane Fuel Cell by Modifying Cathode Flow Field

Abstract

Developing high performance polymer electrolyte membrane fuel cell (PEMFC) is now considered as the one of the most important key to make PEMFC commercialized. Although three losses which are activation losses, ohmic losses, and concentration losses occur when PEMFC operates, mitigating the concentration losses has a great effect on the overall performance of PEMFC because it is dominant losses in high load region. Since the concentration losses of PEMFC are caused by two factors oxygen depletion and flooding at the cathode side, keeping PEMFC away from those problems can be a solution for reducing the concentration losses. PEMFC is composed of many components such as membrane electrode assembly (MEA), gas diffusion layer (GDL), gasket, bipolar plate, and end plate. Among the components, bipolar plate has a great effect on the performance related with concentration losses because it has a flow field where providing gases and produced water flow. Therefore, developing novel flow field on the bipolar plate should be conducted to make performance improved. In this study, two novel flow fields were suggested to get a better performance. The performance improvement was verified and the factors of enhancement were analyzed. The characteristics and analysis of those two flow fields will be discussed. First of all, an inclined channel which has a gradually tapered depth was suggested as a new flow field. Unlike typical channel, the inclined channel has a different inlet and outlet depth. We designed three types of inclined channels which has the same inlet depth but different outlet depth. Therefore, the inclined channels which have different steepness level of channel depth were used as a new flow field at cathode side. Then, the performance of PEMFC was evaluated by using polarization curves and power curves. Moreover, electrochemical impedance spectroscopy (EIS) method also was used to investigate the losses of PEMFC when it operated with the inclined channels. Pressure drop between inlet and outlet of fuel cell is one of parameters we should consider when fuel cell operates because it is related with power consumption of balance of plant (BOP) in PEMFC system. Therefore, pressure drop of PEMFC with inclined channels also measured and compared with conventional channel. Then, we verified that the performance of PEMFC was improved by about 27.9% with inclined channel. Furthermore, numerical analysis was conducted to investigate the effects of inclined channel to performance of PEMFC. To verify the effect of inclined channel on the mitigating flooding, numerical analysis for droplet dynamics was conducted. Moreover, numerical analysis for comparing oxygen concentration along the different channels was also done to explain the effect of inclined channel on the concentration losses decreasing. As a result, we could verify the effect of inclined channel on the PEMFC performance and explain how the inclined channel make PEMFC improved. The other novel flow field suggested for developing high performance PEMFC is porous flow field by using metal foam as a cathode flow field. In case of conventional flow field, gases and liquid water are supposed to flow in certain channel. However, this conventional channel has problems such as flooding and oxygen depletion since produced liquid water can block the channel and diffusion area where gases diffuse to MEA. To solve these problems, several types of metal foams ware inserted into cathode bipolar plate. Then, the effect of metal foam to the performance of PEMFC and characteristics of each metal foam were investigated. As a result, we found that the maximum power of PEMFC increased about 50.6% when we replaced conventional channel with proper metal foam as a cathode flow field for PEMFC. Moreover, operating characteristics of PEMFC with different metal foams was investigated by polarization curve, EIS method, and stability test. Eventually, novel flow field called mixed metal foam flow field was suggested through the results of investigation. The mixed metal was made by combining two different metal foams which have different cell size. Since metal foams show different advantage with respect to electric conductivity and oxygen diffusion, we placed a metal foam which has a large contact surface area to the upstream region of flow field. Then, metal foam which has a large diffusion region was placed to the downstream region. Finally, we could get about 60.1% of maximum power increase when novel mixed metal foam was used as a flow field to the cathode side of PEMFC. Furthermore, additional experiments were conducted to verify the effect of metal foam on the water management in PEMFC and compare diffusion between conventional channel and metal foam flow field.