Pore Network Model And Similarity Experiment For Drying Of Gas Diffusion Layer Of PEMFC

The fuel cells,which can generate electricity directly from the chemical reaction,have the advantages of high efficiency,fuel diversity and no pollution,and hence are considered as the most promising energy suppliers in the fields of vehicles,power stations,and spacecraft.Among the various fuel cell,the proton exchange membrane fuel cell is regarded as one of the most promising power sources for a wide range of applications because of its low operating temperature,ease of start-up,and compactness.A typical proton exchange membrane fuel cell consists of bipolar plate,gas diffusion layer,catalyst layer,and proton exchange membrane.In gas diffusion layer,the continuously generated water is prone to condense to form liquid water.And liquid water can have damaging effects for battery performance.In a practical application,the dry gas is purged into the gas channel of PEMFC to remove the liquid water in gas diffusion layer through the drying mechanism.In order to reduce the pump power for the gas purge,it is needed to deeply understand the evaporation process in the gas diffusion layer so as to optimize the gas purge process.The gas diffusion layer is porous media,most of current studies are based on the macroscopic method,which therefore cannot elucidate the effects of the porous layer microstructures and now the experimental research on drying process for porous media are scarce.In this work,similarity experiment and pore network models are employed to disclose the drying process in the gas diffusion layer during the gas purge.Firstly,a two-dimensional pore network model is employed to investigate the influence of wetting properties on evaporation behavior in different structural porous networks.In this study,the viscous and gravity effects are ignored,and the capillary forces dominate the drying process.The parameters studies are explored by altering pore structure,i.e.the pore size distribution and the throat-pore ratio.The simulation results indicate that the drying fronts retain irregular in hydrophilic porous media,but in the hydrophobic case,the drying fronts depend on the network structures.The overall drying time is associated with evaporation rate,pore structure and wetting properties.In the wide PSD network,the overall drying time in hydrophilic porous media is less than that in the hydrophobic case.To validate the pore network model,a visual experimental study on drying of the porous media with gas purging boundary is designed.An image-processing program compiled by Matlab,consisted of four parts of background image preprocessing,boundary extraction,image matching and saturation calculation,is used to analyze drying pictures.The experimental results show that the image-processing program based on Matlab realizes in situ identification of drying characteristics on porous media.The image information indicate that the influence of purge gas flow for drying rate is dramatically during the initial period of drying,but its effect is not obvious in the later stage.Eventually,a large pore network visual experimental study is used to investigate the drying characteristics with large gas purging flows.The results of experiments indicated that,the drying process of porous media on large gas purging flows consists of four stages: liquid flow caused by gas purging and evaporation.The results of the overall drying time indicated that the proton exchange membrane fuel cell has better performance when the microporous layer exists and gas flow conditions is suitable.