Automotive vehicles powered by the Proton Exchange Membrane Fuel Cell(PEMFC)play an important role in the new energy vehicle sector.Among all research topics about the PEMFC,the formulation and fabrication technology of the catalyst layer are crucial.These ingredients are first mixed and dispersed to become an ink,which is then coated to form the catalyst layer.The complex interactions among these components determine the microstructure and electrochemical performance of the catalyst layer.The impact of these interactions on the microstructure and performance of the catalyst layer is not yet well understood.The present research consists of four themes,i.e.,the fabrication processes,ink microstructure,crack morphology,and performance of the catalyst layer.The effects on ink dispersity due to different types of catalysts,presence of polymer ionomer,dispersion methods and parameters during the preparation processes are investigated.The influence of ink dispersion methods on the crack morphology after coating and drying is studied.The impact of coating parameters on crack morphology is also explored.Finally,the performance of the catalyst layer is measured and the effects of crack morphology on cell performance is analyzed.This research aims to correlates ink dispersity and catalyst layer microstructure with the cell performance so as to gain understanding to these factors and to optimize the cell performance.The contents of investigations and some major conclusions drawn from this work are summarized as follows:1.From the measurements of catalyst layer ink’s rheological characteristics,particle size distribution by the DLS technology,SEM surface characterization and contact angle,the effects of catalyst layer components on the ink’s dispersity and stability are investigated.The results show that:(1)Owning to its high hydrophilicity,Nafion polymer may change the surface charge of catalyst particles,thus improve the electrostatic stability between particles and help maintain the stability of the catalyst layer ink.After Nafion is added,the ink exhibits pseudoplastic fluid characteristics,and its viscosity tends to be stable with the increase of shear rate.(2)The Dupont D520 Nafion solution appears to has Newtonian fluid characteristics,indicating that the Nafion polymer exists in a free state in the solution and has good electrostatic stability.(3)Under the standard formula,the platinum particles on the surface of the Vulcan XC-72 carbon support may increase the viscosity of the ink.The viscosity of the 60 wt.%Pt-HSC ink is lower than that of the ink made from the 30 wt.%Pt-Vulcan XC-72.The particle size of the 60 wt.%Pt-HSC ink is larger than the 30wt.%Pt-Vulcan XC-72 ink.The former ink’s particle distribution range is wider than the latter,and the absolute value of its Zeta potential of the former ink is smaller.(4)The 60 wt.%Pt-HSC has a higher platinum content,and the distribution of platinum particles on the surface is less than that of the 30 wt.%Pt-Vulcan XC-72 ink due to its higher porosity.2.The inks prepared by 60 wt.%Pt-HSC and 30 wt.%Pt-Vulcan XC-72catalysts are dispersed by sonication and ball-milling.The effects of dispersion method and parameters on the dispersity and stability of both inks are investigated.The influence of these parameters is characterized and analyzed using data of rheological properties and particle size distribution.Experimental results have shown that:(1)Ball-milling and sonication both have a greater impact on the dispersibility of 60 wt.%Pt-HSC catalyst ink than on the 30 wt.%Pt-Vulcan XC-72 ink.The ink viscosity characteristics,thixotropic reaction,particle size distribution and Zeta potential obtained by ball milling dispersion are different with those obtained using ultrasonic dispersion.The particle size distribution of the ink using sonication shows agglomerates as large as 8,000 nm.(2)Ball-milling and sonication appear to have negligible effect on the dispersity of the 30 wt.%Pt-Vulcan XC-72 catalyst ink.The rheological beahviors and particle size distributions of both inks are found to be similar.(3)The ink viscosity is found to first increase and then stabilize as the dispersion time increases.Furthermore,the average viscosity of the ink drops when the milling ball diameter is less than 2 mm.Ball-milling speed and the amplitude of sonication have negligible effect on ink viscosity.(4)The ink viscosity is sensitive to the solid content ratio of the ink.The average viscosity is found to increase with the increase of solid content ratio of the ink.The dispersion energy required by the ball-milling dispersion increases with the increase of ink’s solid content,while it does not change for sonication dispersion.3.Four ink cases using two dispersion methods and two catalyst types are prepared and corresponding catalyst layers are made using the squeegee coating.The crack morphology of these catalyst layers is observed with an optical microscope.The effects of the squeegee speed,coating thickness and drying temperature on the crack morphology are studied.The hardness and Young’s modulus of these catalyst layers are measured by a nanoindentation system.The effects of ink dispersity on crack morphology and catalyst layer’s mechanical properties are analyzed.The results show that:(1)Based on the present preparation process,cracks of different patterns appear on the dried catalyst layer,including pinholes,I-shaped,T-shaped,Y-shaped,and O-shaped cracks.(2)Ink dispersity has a great influence on catalyst layer’s crack morphology.The dispersion method has a great impact on the crack morphology of the catalyst layer made of 60 wt.%Pt-HSC catalyst.Large particle agglomerates are observed in the ink catalyst layer prepared by sonication where cracks of the penetration type are apparent.Catalyst layers made from ink using ball-mill dispersion has fewer cracks.The catalyst layer crack morphology of the 30 wt.%Pt-Vulcan XC-72 is insensitive to dispersion method.(3)The scraping speed,coating thickness and drying temperature of the catalyst layer ink all contribute to the crack morphology on the catalyst layer.It is found that number of cracks decreases with increasing scraping speed.With the increase of layer thickness,the crack morphology gradually changes from pinholes to strip cracks.The area ratio of the cracks decreases and then increases over time.The area of??cracks first increases with respect to drying temperature,and then it decreases.The melting of ionomer in the catalyst layer may reduce the occurrence of cracks.(4)The hardness and Young’s modulus distribution of the ink catalyst layer after drying are not uniform,and the scattering of data is high for the catalyst layers with poor dispersion.The estimated crucial crack thickness determined by the Griffith’s fracture law is smaller for high dispersity inks.4.Four membrane electrode assemblies prepared using the afore-mentioned four catalyst layer cases are made using by a decal transfer method.Polarization curves and electrochemical active area(ECSA)measured by cyclic voltammetry are used to characterize their performance.The effects of crack morphology on performance is analyzed.Results have shown that:(1)The catalyst layer’s cracks have minor effects on the polarization curve,and catalyst layers with large particle agglomerates tend to have higher concentration polarization.(2)The thickness of the catalyst layer and the catalyst type have a great effect on the polarization curves.The maximum power density of the 60 wt.%Pt-HSC catalyst layer is up to 1.5 W/cm~2,which is more suitable for the cathode catalyst layer.(3)The crack morphology of the catalyst layer appears to have only slight effect on the ECSA and platinum utilization.The platinum utilization of these catalyst layers is found to be as low as only about 50%.A comparative study and analysis are carried out in the present study.The linkage between the physical parameters during the fabrication of PEMFC catalyst layer,microstructure,and performance are elucidated.This research provides guidelines for fabrication parameters needed for optimization in engineering applications. |