| The internal gas-liquid two-phase flow mechanism and mass transport characteristics of proton exchange membrane fuel cell（PEMFC）have an important impact on its efficiency and reliability.As the direct carrier of gas-liquid transmission in PEMFC,the surface structure of gas diffusion layer,flow channel and proton exchange membrane have an important impact on the two-phase flow and mass transport characteristics.Therefore,the optimized design of the surface structure of key components has always been the top priority of PEMFC performance improvement.In this dissertation,the research on the surface structure of key components of PEMFC was extended to the micron scale.Aiming at the active regulation of two-phase flow and mass transport characteristics,taking the optimized design of surface functional microstructure of key components as the means,and based on the laser microstructure flexible machining technology accumulated in the early stage of the research group,a new method for the design of surface differentiated functional microstructure of key components of PEMFC was proposed.The combination of visualized test and numerical calculation clarified the mapping relationship between the functional microstructure control interface and the gas-liquid two-phase flow and mass transport characteristics,mastered the microstructure design scheme to improve the electrochemical performance of fuel cells,and finally laid a theoretical and scientific foundation for the independent development of the key components of the new generation of efficient and reliable PEMFC.The main research contents of this paper are as follows:（1）In order to meet the demand of GDL for rapid water transport and exhaust,a micropore functional structure with a diameter larger than the diameter of carbon fiber was designed to provide a channel for water and gas to pass through GDL quickly.The blind hole and through hole structure were processed on the surface of GDL by femtosecond ultrafast laser processing technology.The through hole diameter is 102.34μm,the blind hole diameter is 93.22μm and the blind hole depth is 84μm.The contact angles of unprocessed GDL surface,through-hole GDL surface and blind GDL surface are 125°,128°and 130°,respectively.In order to meet the requirements of drag reduction and drainage and pressure drop control,the hydrophobic structure of micro-bulges on the flow channel surface was designed based on the principle of lotus leaf micro-bulges.Micro-bulges with regular distribution on the flow channel surface were processed by millisecond laser textured processing technology.The bulge height is 194μm,the bulge diameter is 286μm and the distance between the two adjacent bulges is 86μm.The contact angle of the unprocessed channel surface is 115°and that of the micro-structured channel surface is 132°under the same hydrophobic treatment.（2）A GDL liquid water flow visualization test-bed was built to observe the effect of micropore structure on the transmission velocity of liquid water on the surface of GDL.The results showed that the water transfer speed of microporous GDL is significantly faster than that of unprocessed GDL,and the water transfer speed of through-hole GDL is faster than that of blind hole GDL.Based on the VOF method,a three-dimensional porous water gas transport model of GDL was established to further study the influence of the change of microporous morphology parameters on liquid water transport.The results showed that the water transport speed and liquid water saturation of through-hole GDL are significantly higher than that of blind hole GDL.In a certain range,the larger the size and number of micropores,the more obvious the change effect of water transport.（3）The visual test-bed of two-phase flow in the side channel of the polar plate was built,and the comparison test of water and gas flow between the channel with micro convex structure and the channel with smooth surface was carried out by using high-speed photography technology.The effect of micro bulged structure on strengthening water transmission in the channel was verified.Under different inlet pressure and inlet velocity,the water content in the microstructure channel is significantly less than that in the smooth channel,and the average velocity of droplets in the microstructure channel is significantly higher than that in the smooth channel.With the three-dimensional model of water gas two-phase flow in lotus-like leaf micro bulged channel was established and based on VOF method,the effects of lotus-like leaf micro bulged channel with different height,radius and spacing on water transmission and water removal were studied.The results showed that the flow velocity of droplets in lotus-like leaf micro bulged structure channel is significantly faster than that in direct channel.The change of micro bulge height has the most obvious impact on the liquid water flow.Within the designed parameters,the larger the micro bulge height is,the faster the liquid water velocity is,and the greater the peak value of maximum pressure drop is.The microstructure with appropriate height can effectively reduce the water coverage of the upper wall of the microstructure channel.The radius and spacing of microstructure have a certain effect on the flow of liquid water,but the effect of height change is not significant.The microporous structure was designed on the side of GDL channel,and the effects of microporous GDL channel with different depth,radius and spacing on the transport and removal of liquid water were studied.The results showed that the effect of micropore depth on liquid flow is the most obvious.Within the designed parameters,with the increase of micropore depth,the movement speed of droplets in microporous GDL channel gradually accelerates;With the increase of micropore depth,when the droplet returns to GDL,the peak drag reduction first increases and then decreases;With the increase of micropore depth,the peak pressure drop tends to increase.The change of pore radius and liquid phase spacing has no obvious effect on the flow.（4）A single cell model of proton exchange membrane fuel cell was established.Different functional microstructures were designed on the surface of proton exchange membrane（PEM）.The effects of microstructure morphology and parameters on the mass transfer characteristics of PEMFC were studied.The results showed that the surface functional microstructure can improve the performance of fuel cell,and the effect of microporous structure is better than that of convex structure.With the increase of micropore radius,the water storage capacity of micropores becomes stronger,the water content on the surface of PEM also increases gradually,and the membrane surface area also increases gradually,so as to improve the reaction efficiency.The number of micropores has little effect on the content of H2O and O2 on the cathode side.The water content on the cathode side of PEM at different intervals is almost the same,but the average current density of the catalyst layer increases with the increase of the number of micropores.（5）Based on the single cell model,the effects of GDL and channel surface functional microstructure on the mass transfer characteristics of PEMFC were studied.The results showed that compared with traditional GDL,the surface water content of microporous GDL is significantly reduced,and the existence of micropores greatly increases the low water content area of GDL.Different GDL pore sizes mainly affect the water content and oxygen content on the cathode side.Compared with depth and spacing,the pore size has the most obvious effect on the battery polarization curve,different pore depths mainly affect the mass fraction of liquid water on the surface of GDL.With the increase of pore depth,the mass fraction of liquid water on the surface of GDL decreases,while the mass fraction of oxygen increases;Micropore spacing mainly affects the mass fraction of water and oxygen on the surface of GDL,but has little effect on the electrochemical performance.Compared with the traditional flow channel,the micro convex flow channel can improve the battery performance and the distribution of water and oxygen content on the cathode side,but the effect is not obvious compared with setting microstructure on the surface of GDL and PEM.The main reason is that the flow channel does not directly participate in the reaction in PEMFC,and its main function is to provide channel for water and gas transport. |
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