Structural Design And Mass-Transfer Optimization Of Gas Diffusion Layer For Proton Exchange Membrane Fuel Cell

Proton Exchange Membrane Fuel Cell(PEMFC)is a device that converts chemical energy directly into electrical energy.It has a broad application in the fields of new energy vehicles and backup power.In the process of PEMFC,the gas diffusion layer(GDL)is needed for mass transfer.Therefore,optimizing the mass transfer efficiency of the gas diffusion layer plays a key role in improving the actual output performance and prolonging the service life of the PEMFC.The scientific problem to improve the mass transfer efficiency of gas diffusion layer is how to realize the efficient and stable transfer of water and gas in the gas diffusion layer.In this paper,the internal and interface mass transfer characteristics of the gas diffusion layer were studied.The off-line accelerated aging experiments of the gas diffusion layer were carried out,and the strategies to mitigate the performance degeneration of the gas diffusion layer were explored.The main research contents and results are as follows:(1)The structure design and physical properties of the microporous layer(MPL)in the gas diffusion layer will directly affect the diffusion of reaction gas and the removal of generated water.The water management ability of microporous layer needs to be further improved under wide humidity conditions.In this paper,different kinds of carbon powder were used to prepare microporous layer.It was found that the difference of hydrophobicity would cause the change of microporous layer morphology.The surface of microporous layer prepared by hydrophilic carbon powder is dense and accompanied by cracks,which is more suitable for low humidity conditions.The surface of microporous layer prepared by hydrophobic carbon powder is loose and porous,and the mass transfer efficiency is better under high humidity condition.When the hydrophilic and hydrophobic gradient structure design is adopted,the proton exchange membrane fuel cell can obtain higher and more stable mass transfer efficiency under wide humidity conditions.(2)The interfacial coordination between the microporous layer and the catalyst layer will affect the actual efficiency of the catalyst.Therefore,the interface structure design and composition optimization of microporous layer are the key to improve the interface mass transfer efficiency.In this paper,different metal oxides were used to doping the microporous layer slurry to construct a transition layer at the interface between microporous layer and catalyst layer.It is found that the limiting current density of the membrane electrode assembly is affected by the oxygen concentration gradient at low oxygen concentration,and the limiting current density is affected by the retardation of water at high oxygen concentration.By analyzing the oxygen mass transfer resistance values under different pressure conditions,it is found that the oxygen mass transfer resistance in the diffusion layer doped with cerium oxide is the least,which is due to the abundant oxygen vacancy in cerium oxide which promotes the oxygen reduction reaction.(3)The pore structure of the macroporous substrate in gas diffusion layer affects the stability of water and gas mass transfer in the membrane electrode assembly.However,the conventional hydrophobic treatment of substrate is easy to cause pore blockage,which is not conducive to rapid mass transfer.In this paper,by optimizing the hydrophobic treatment scheme of the macroporous substrate,the pore structure is effectively retained and the diffusion efficiency of gas is enhanced.Meanwhile,by wrapping the hydrophobic carbon layer in the porous substrate,the drainage efficiency under high humidity conditions is effectively improved,and the mass transfer optimization of the gas diffusion layer is realized.(4)At the interface between macroporous substrate and flow channel,the flow state of reaction gas and generated water is greatly affected by flow field structure.The mass transfer efficiency in gas diffusion layer can be enhanced by effective use of subridge convection between flow channels.In this paper,the effects of single-channel or four-channel serpentine flow field and their derived flow fields on mass transfer efficiency of gas diffusion layer are discussed.It is found that the subridge convection is weak in the four-channel serpentine flow field.In the single-channel serpentine flow field,the convection under ridge is strong,but the pressure difference between inlet and outlet is too large.The mixed flow field derived from single-channel serpentine flow field has an advantage in mass transfer efficiency under high humidity and high back pressure conditions.In the four-serpentine wave flow field derived from the fourchannel serpentine flow field,the distribution of reaction gas and generated water is the most uniform,and the convection effect under the ridge is also very significant,so the gas diffusion layer has a good mass transfer effect in this flow field.(5)In the use of proton exchange membrane fuel cells,unsteady conditions tend to cause high local potential,which will lead to corrosion of carbon materials.In order to study the corrosion mechanism of gas diffusion layer,the off-line accelerated aging experiments of the gas diffusion layer were carried out.It was found that the electrochemical corrosion of commercial gas diffusion layer Sigracet 29 BC mainly occurs in the microporous layer.By adjusting the surface morphology and composition of the home-made gas diffusion layer,the corrosion area is transferred to the macroporous substrate.Compared with commercial products,the performance degeneration of home-made gas diffusion layer has been effectively mitigated.Moreover,the performance degeneration of the gas diffusion layer can be further mitigated by the strong subridge convection in the four-serpentine wave flow field.In summary,the mass transfer efficiency of the gas diffusion layer was enhanced by adjusting the physical and chemical properties and microstructure of the gas diffusion layer.At the same time,the corrosion resistance difference between commercial products and home-made samples is discussed,and a scheme to mitigate the performance degeneration of gas diffusion layer is proposed.These results not only systematically explain the mass transfer mechanism of gas diffusion layer,but also help to further improve the output performance and durability of fuel cell.It contributes to the further development of fuel cell industry.