| The mass transfer and heat/electric conduction characteristics within the proton exchange membrane fuel cell(PEMFC)are the key factors affecting the power density enhancement of the fuel cell.The gas diffusion layer and the flow field of the plate are the keys to determine the mass transfer performance and heat/electric conduction performance of the fuel cell,so enhancing the mass transfer performance of the diffusion layer and the flow channel(especially the liquid water discharge behavior)is the main research direction to enhance the performance of the fuel cell,and it is also PEMFCs face important technical challenges.In order to enhance the drainage performance of the diffusion layer composed of carbon fibers,the diffusion layer carbon paper is usually treated with polytetrafluoroethylene(PTFE)for hydrophobicity.However,the addition of PTFE increases the internal pores and surface hydrophobicity of the carbon paper,but also leads to an increase in the electrical and thermal resistance of the carbon paper.At the same time,the surface properties and structure of carbon paper will affect the initial state and growth motion process of droplets in the flow channel,thus affecting the dynamics and discharge performance of droplets in the flow channel.In this dissertation,a study on the dynamics characteristics of the initial state and growth process of PTFE-treated diffusion layer on droplets was carried out by using numerical modeling methods such as hydrodynamic model and graph theory model,combined with experimental verification means,and a flow channel simulation model with different flow channel sizes and different cyclic reducer structure characteristics was established to study the mechanism of enhanced mass transfer performance of fine flow channels with different microstructure characteristics;a graph theory model-based a prediction model for the electrical/thermal conductivity of diffusion-layer carbon paper was established,and the intrinsic mechanisms of the effects of PTFE content,PTFE distribution and pressure changes on the electrical/thermal conductivity of carbon paper were investigated.The main research contents and results of this dissertation are as follows.(1)This dissertation investigates the surface morphology,cross-sectional morphology,pore distribution characteristics and surface hydrophobicity of carbon paper by means of imaging and experimental tests,and finds a strong correlation between the pore size and surface hydrophobicity of the characteristic micro-pores of the carbon paper substrate and the initial state and growth behavior of droplets on the surface of carbon paper.The simulation results show that when the flow channel size(width × height)is reduced from 1 mm × 1 mm to 0.5 mm × 0.35 mm,the differential pressure driving the droplet increases from 200 Pa to 1120 Pa,an increase of 560%,and with the reduction of the flow channel size,the time required for the droplet to obtain a larger pressure difference in the flow channel is significantly reduced,and the driving force of the droplet motion is greatly enhanced.Under conditions of no humidification of the cathode,the experimental results show that when the cathode plate flow channel size(width × height)is reduced from 1 mm × 1 mm to 0.5 mm × 0.35 mm,the maximum power density of the cell increases from 986 mW·cm-2 to 1460 mW·cm-2,an increase of 48%,further confirming that the micro-flow channel design can significantly improve PEMFC electrochemical performance at high current densities.(2)In this dissertation,the effect of the cyclic reducer structure on the droplet transport performance is investigated by developing three kinds of simulation models for the flow channel:no reducer,circular bilateral reducer and circular bottom reducer.The results of the study show that the bilateral reductions can increase the droplet pressure differential from 12 Pa in the non-reduced flow channel to about 120 Pa,which is nearly 10 times higher.The mechanism of the reducer design is similar to the construction of a reducing and expanding nozzle in a flow channel.The repetitive construction of the reducer unit creates periodic fluctuations in flow rate and pressure in the flow channel,creating a pulsating gas-liquid two-phase flow.This periodic pressure difference fluctuation effectively enhances the movement of the gas-liquid two-phase fluid in the three-dimensional direction,which enhances the diffusion performance of the gas flow in the carbon paper on the cathode side and significantly improves the discharge performance of liquid water from the carbon paper on the cathode side(especially under the ridge).On this basis,the gas-liquid transfer characteristics of the circulating double-sided secondary reduction flow channel are investigated.It is found that the secondary reduction structure can increase the gas flow pulse frequency by a factor of one compared to the circulating double-sided single reduction structure,and can effectively suppress the generation of vortices and improve the mass transfer performance.(3)In this dissertation,the physical structure of carbon paper before and after PTFE modification is analyzed by microscopic imaging and energy dispersion spectroscopy,and the typical contact states inside the carbon paper before and after PTFE modification are also analyzed,and the assumption that the structure state of carbon paper can be described in the physical model is proposed.On this basis,a physical model of the carbon paper structure is proposed,together with a typical impedance calculation method in the physical model.A three-dimensional structural circuit model of charcoal paper is then derived,and the minimum repetition unit in the carbon paper circuit model is resolved,providing an important way to transform the three-dimensional circuit model into a graph theory model.Finally,the encoding rules required to transform the circuit model into a graph theory model are established,completing the entire structure from the establishment of the graph theory model to its solution,and realizing the theoretical method and analytical means to predict the thermal/electrical conductivity of carbon paper based on the graph theory model.(4)A graph theory prediction model of the electrical/thermal conductivity of carbon paper was developed based on a graph theory modeling approach.The accuracy and physical significance of this model were verified by experimental test results simultaneously.The results show that the type and density of contact sites within the carbon paper are intrinsically responsible for the thermal/electrical conductivity in the through-plane direction.The contact resistance and thermal resistance caused by PTFE on the surface of the carbon paper account for a small proportion.The simulation on the sensitivity of PTFE distribution showed that a carbon paper could reduce its resistance by 50%by reducing the PTFE aggregation with the same PTFE content.Finally,the combined influence of pressure,PTFE content and PTFE distribution on the thermal/electrical conductivity of carbon paper were investigated systematically through simulation models.The mechanism of the decreasing marginal effect of pressure on the electrical/thermal conductivity of carbon paper is analyzed,and a PTFE treatment modification method to enhance the thermal/electrical conductivity of carbon paper is proposed.By optimizing the PTFE distribution,the GDL area resistance of PTFE treatment can be reduced to 22 mΩ·cm2,a 90%reduction,and the electrical conductivity can be increased by approximately 5 times.In summary,this research has carried out the modelling simulations and experimental studies on the mass transfer and electrical/heat conduction characteristics in the diffusion layer and flow channel of PEMFC.The main findings can be summarized as follows:(1)the size reduction of the flow channel can significantly improve the droplet driving pressure difference in the flow channel,which can improve the drainage performance of the plate flow channel in cathode side,and significantly improve the ultimate current density and maximum power density of the fuel cell;(2)the cyclic reducer structure can generate the periodic fluctuations of both fluid flow velocity and pressure in the flow channel,which can effectively improve the mass transfer performance in the diffusion layer and the flow channel,and significantly improve the performance of the cathode side of the fuel cell;(3)The minimum repetition unit and coding rules of the carbon paper in the diffusion layer are resolved,and a graph theory prediction model of the electrical/thermal conductivity of the carbon paper is established;(4)The density of contact sites in the carbon paper of the diffusion layer is the key to its electrical/thermal conductivity,and the electrical/thermal conductivity of the carbon paper can be significantly improved by optimizing the PTFE distribution.These findings provide an important theoretical basis and solution for the design and development of high-power and high-current density proton exchange membrane fuel cells. |
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