Experimental Characterization and Computational Modeling On Self Air-Breathing, Dead End Anode PEM Fuel Cell Performance

Air breathing PEM fuel cells, by virtue free convection driven, are typically characterized by low output power densities as compared to the conventional mechanical-assisted, forced convection type. However, this self air breathing design of the cathode is still considered as an ideal design choice for portable power application, as the simplicity of free convection air delivery can out-weight the cost, limited lifetime, reliability, complexity, noise, volume, weight and parasitic power consumption of an auxiliary fan. In this paper, we present the experimental characterization on the performance of the fuel cell in relation to various functional parameters such as temperature, air and hydrogen flow rates, humidity, and structural parameters such as parallel, triple serpentine or grids anode bipolar plate. It was found that dehumidifying the anode H₂ gas using silica gel would have negligible effect on the fuel cell performance, as the membrane could be dehydrated and result in low proton conductivity. Prior research works also found that inadequate supply of H₂ (with NaBH₄ and distilled water) would exhibit an unusual U-bend phenomenon (hairpin shape) after the polarization curve reaches its’ limiting current density. This is further confirmed in the current experimental setup by varying the gas regulator from the PEM electrolyzer; as the low H₂ from the supply would still result in hairpin polarization curve, but increasing the H₂ supply through the regulator would alleviate the H₂ starvation problem and hence remove the hairpin curve. Once the H₂ threshold is reached, increasing O₂ flow rate using suction fan would bring up the fuel cell performance to another level, by about 25% increase in power output. For this self air breathing PEMFC, it is also interesting to note that the hysteresis (hairpin curve beyond limiting current density) would reappear again as the air temperature is heated up from 22 to 60⁰C, mainly due to the O₂ starvation and/or membrane dehydration. Maximum power output is obtained at the intermediate 40⁰C. Lastly, it is recognized that any flow configuration that appear symmetric would typically have rooms for improvement in terms of heat and mass transport aspects. Asymmetric flow distributor design is in favor to the performance enhancement in terms of flow and reactant distribution. The hybrid triple-serpentine and grid anode bipolar plate is modeled through CFD (Computational Fuel Cell Dynamics) tools, verified and proposed as the optimum design to ensure reactants passing through all the MEA area below the footprint of the channel, and yet increasing the surface area available for PEMFC electrochemical reaction.