Research On Load Changing Characteristics Of Proton Exchange Membrane Fuel Cell

The proton exchange membrane fuel cell（PEMFC）is an energy conservation device that directly converts the chemical energy of the fuel into electrical energy without burning.As is not limited by the Carnot cycle,it has the significant advantages of high energy conservation efficiency.And more importantly,its by-product is only water without polluting environment.The PEMFC qualified many advantages is regarded as the promising next-generation mechanical power source and the world is devoted to developing and popularizing it.When PEMFC is used as a mechanical power source,it inevitably undergoes dynamic loading,which can result in undershoot that means the response voltage becomes lower than steady-state voltage.Unreasonable loading conditions can increase the undershoot and may even lead to a failure,which can act as a disadvantage of PEMFC and limit its application.Therefore,it is of great practical significance to study the load changing characteristics of PEMFC and improve its operational stability.The transient responses of PEMFC under different initial working conditions were studied.The experimental results showed that the fuel cell cannot be successfully loaded at start-up when the current step is increased from 0 A·cm^-2to 0.8A·cm^-2.It could be loaded successfully with a step current density of 1.5A·cm^-2after activation.Two kinds of undershoot may occurred during the loading process of PEMFC with a large step current density.The first undershoot occurred immediately with the successful response to a step current density as the water content in the membrane couldn’t change quickly.The second undershoot may happened after the current step because of the anode dehydration and could result in a load failure.Finally,the load strategy of PEMFC without activation was optimized and the loading time was reduced by 32.65%.The load changing performance of hydrogen-air and hydrogen-oxygen PEMFC with activation were studied at varied operating conditions and the electrochemical impedance spectroscopy was used to analyze their intrinsic mechanisms.The experimental results showed that the total polarization resistance of the hydrogen-air fuel cell and the ohmic resistance of the hydrogen-oxygen fuel cell could be decreased tremendously with the increased properly operating temperature and thus the load changing performance improved.However,the ohmic and polarization resistance may be increased aroused by the dehydration of the membrane electrode assembly due to the over-elevated operating temperature.When the working temperature was increased to 60℃,the response voltage of the hydrogen-air fuel cell decreased by 0.0638V compared with that of 30℃.The adverse effects could be eliminated by the increased relative humidity.When the relative humidity was increased from20%to 60%,the total polarization resistance decreased by 5.561m Ohm and 18.75%of the increment of the response voltage was found.With the increased stoichiometry,the performance of the hydrogen-oxygen would be depressed as more water was brought away with the faster gas flow,whereas it could be improved as the total polarization resistance of the hydrogen-air fuel cell could be extremely reduced.Nevertheless,with the increased working pressure,the performance of hydrogen-air and hydrogen-oxygen fuel cells both got better due to the increased partial pressure of the gaseous reactant and the decreased reaction resistance.Finally,the hydrogen-oxygen fuel cell had more stable dynamic characteristics than the hydrogen-air fuel cell under continuous operation due to smaller mass transfer resistance.A novel 3D flow field patterned with built-in obstacles and flow channel interruption was designed and analyzed numerically and experimentally.The simulation results showed that the 3D flow field transformed the traditional diffusion mass transfer into the optimized combination of diffusion and convection mass transfer and thus the capacity of mass transfer of the novel flow field was considerably enhanced.The flow speed was also increased by the3D flow field.Its purging effects on water improved and water partial pressure decreased tremendously.The experimental results showed that the steady-state output power and transient output power of the new 3D flow field were increased by 1.185W and 2.382W at0.3A·cm^-2,respectively.Better load changing characteristics were observed in the fuel cell with the new designed flow field.