Discloure Of The Internal Transport Characteristic Of An Air-cooled PEMFC

Proton Exchange Membrane Fuel Cell（PEMFC）is an energy-conversion device that generates electricity by the electrochemical reaction of hydrogen and oxygen.PEMFCs have plenty of advantages such as high efficiency,no pollution and low noise.For such reasons,they have great potential for the application in mobile power sources,automobile dynamics and stationary power plants etc.Among all kinds of PEMFCs,air-cooled PEMFCs are more suitable for low power fields like unmanned aerial vehicles and portable battery because no air compressor and humidifying devices are needed which could lower the system complexity and simplify the structural design.However,water management is a serve challenge because air-cooled PEMFCs are operated at such conditions that pure hydrogen is blown into the flow field and the cathode is directly exposed to the atmosphere.This paper is aimed at researching the internal transport characteristics of an air-cooled PEMFC by developing a three-dimensional,isothermal,two-phase,steady computational fluid dynamics（CFD）model.Experimental methods are also applied in this paper.Figuring out the internal transport phenomena helps to understand the factors that affect the performance of an air-cooled PEMFC and to provide guidance for the optimization of flow field.The main content of the paper is shown as follows:（1）First,we established the CFD model based on Base Case.The operating conditions were set as:H₂ stoichiometry=1.5,relative humidity of H₂=0%,air stoichiometry=150 and relative humidity of air=60%.The polarization curve obtained at the experiment platform was utilized to verify the numerical model.Liquid water flooding was observed in the cathode electrode and the part under the cathode rib showed the most sever water flooding phenomenon.Species on both the anode and cathode side presented the similar distribution law,i.e.,H₂,O₂ and H₂O were all periodically distributed in the H₂flow direction.The distribution of membrane water content had two features:1)The average water content for an air-cooled PEMFC was apparently lower than water-cooled PEMFCs.2)The water content at the membrane under the cathode rib was much larger than that under the cathode channel.This discovery indicated that the membrane wettability for an air-cooled PEMFC was terrible which led to the poor performance.On the other hand,internal stress existed between the part under the cathode rib and that under the cathode channel due to the enormous difference of water content at these two parts.This was an obstruction for the long-time operation of an air-cooled PEMFC.（2）Based on the research of Base Case,we also conducted parameters sensitivity analysis on the effects of the operating conditions（air stoichiometry,air relative humidity and cathode flow field）.The results showed that with the increase of air stoichiometry,the cell temperature decreased,the membrane water content rose up and the performance was thus improved.The membrane water content for different air stoichiometry displayed the same characteristic,i.e.,water content was periodically distributed,the minimum value showed at the part under the cathode channel while the maximum value showed at the part under the cathode rib.With the increment of the air relative humidity,the PEMFC performance was improved but the cathode water flooding was also server for the case of larger air relative humidity.Narrower cathode channel facilitated improving the uniformity of current density and increasing the membrane water content which resulted in better PEMFC performance.（3）In order to research the internal transport phenomena of an air-cooled PEMFC systematically,this paper analyze the single cell impedance characteristic by means of EIS tests at different operating conditions.It was clearly shown that smaller the air stoichiometry was,larger the polarization resistance was.The cell impendence increased with the reduction of air relative humidity.Especially,the single cell using dry air showed the largest ohmic resistance and the largest polarization resistance.The polarization resistance became lower with the decrease of the cathode channel width.However,the impact of the cathode channel geometry was not so much as the air stoichiometry.Therefore,the conclusions from the experimental research had the similar trend as the results from the theoretical model.