Investigation Of Flow Field Design Effect Based On X-Ray Visualization And Electrochemical Analysis For PEMFC

Energy,as an important power for the development of the world economy and human society,is an important material foundation for the survival and development of human society.At present,some developing countries are entering the stage of industrialization,which will inevitably lead to the rapid growth of energy consumption.However,the reserve of fossil energy in the world is limited,coupled with the increasing attention to environmental pollution.To develop efficient and clean decarbonized energy sources has become an effective method for governments around the world.The fuel cell is a device that directly converts chemical energy into electrical energy through an electrochemical reaction.The reaction product is only water and is not limited by the Carnot cycle.Moreover,it can achieve high energy conversion efficiency.With the advantages of high power density,low operating temperature,and stable working process,Proton Exchange Membrane Fuel Cell(PEMFC),has received extensive attention in the research and application of fuel cells.For PEMFC,the internal gas-liquid two-phase flow is important for water and heat management.On the one hand,membrane needs proper wetting to reduce the membrane resistance and improve proton conductivity.On the other hand,excessive water may lead to flooding,increasing the mass transfer loss and polarization loss,which will cause the decline of fuel cell performance.It can be seen that optimizing the water and heat management has a positive effect on improving the performance and durability of fuel cells.Therefore,it is particularly important to obtain water distribution online by nondestructive testing technology.In this study,based on the micro-focus X-ray tomography equipment integrated with the fuel cell test system,a fuel cell X-ray visualization platform is built to accomplish the online observation of liquid water in the fuel cell flow channel.Combining performance evaluation,electrochemical characterization,and visualization results innovatively,the process of liquid water generation in the flow field is analyzed and effective flow field optimization strategies are proposed.The attenuation coefficient of liquid water under a fixed beam intensity is calibrated,and the quantitative analysis of liquid water in the flow field is realized.The performance and the distribution of liquid water at different positions under different operating conditions are compared.Combined with electrochemical analysis methods,the results are verified with the visualization results to summarize the heat and mass transfer mechanism inside the fuel cell.The results show that the liquid water in the serpentine flow field is mainly concentrated at the exit position of the flow field.The reason for this phenomenon is that the pressure drop of the serpentine flow field is large,which is good for drainage but will result in a low membrane water content.The liquid water in the parallel flow field is easy to accumulate in the middle of the flow field.The main reason is that the pressure drop of the parallel flow field is small.As a result,the drainage is difficult and the flooding problem is easy to occur.Moreover,the parallel flow field is also prone to liquid water reflux.The problem increases the difficulty of removing water,resulting in a decrease of performance for PEMFC.The accumulation of liquid water in parallel flow fields will further exacerbate the uneven distribution of water and heat inside the fuel cell.Based on the experiment of the gas-liquid flow characteristics,the serpentine and parallel flow fields are optimized separately.For serpentine flow field,serpentine flow field with different channel ridge ratios and different channel depth are designed and manufactured to investigate the performance,pressure drop,and internal water management of different designed flow fields.It is found that the flow fields with smaller channel ridge ratio and smaller depth of the flow depth show bigger pressure drop,better performance,lower activation loss,and lower mass transfer loss.When the channel ridge ratio is small enough,the liquid water distribution in the flow channel is more uniform caused by the capillary force.For parallel flow field,the parallel flow channel of the cathode with hydrophobic treatment and using anode serpentine flow field are considered.The results show that both methods effectively improved the drainage capacity of parallel flow field.The combination of anode serpentine flow field and cathode parallel flow field indirectly enhances the water removal capacity of the cathode flow field by enhancing the back diffusion of liquid water.The parallel flow channel after hydrophobic treatment increases the contact angle of channel,thereby enhancing the drainage of parallel flow fields.