The performance of proton exchange membrane fuel cells (PEMFC) is strongly determined by the structure and composition of the electrode layer.The interactions between the ionomer,carbon black particles,and solvent aff...The performance of proton exchange membrane fuel cells (PEMFC) is strongly determined by the structure and composition of the electrode layer.The interactions between the ionomer,carbon black particles,and solvent affect the suspension properties and thus the layer morphology.We analyze the effect of the ionomer-to-carbon (I/C) weight ratio for two different types of carbon black on the suspension and layer characteristics.Highly branched carbon blacks with a high surface area tend to form less cracked layers.As less branched carbons can pack together more closely,a smaller pore size results in a larger capillary pressure during drying and thus more cracks.The added ionomer adsorbs on the carbon particles and improves the colloidal stability of the carbon black particles.The carbon black aggregates are thus smaller,resulting in closer packing and thinner layers.Moreover,the addition of the ionomer increases the critical coating thickness (CCT) of the layers because drying stresses are dissipated by the deformation of the ionomer,preventing crack formation.An optimum I/C weight ratio is identified for optimal layer formation and minimized crack formation.展开更多
Molecular dynamics （MD） simulations of the consecutive compression-decompression cycles ot hexagonal zinc sulfide （wurtzite） nanoparticles predict an irreversible phase transformation to the cubic polymorph.The ph...Molecular dynamics （MD） simulations of the consecutive compression-decompression cycles ot hexagonal zinc sulfide （wurtzite） nanoparticles predict an irreversible phase transformation to the cubic polymorph.The phase transformation commences at the contact area between the particle and the inden- ter and proceeds with the number of compression cycles. Dislocations are visible for a particle size above 5nm. Results from wet grinding and dry powder compression experiments on a commercial wurtzite pigment agree qualitatively with MD simulation predictions. X-ray diffraction patterns reveal that the amount of cubic polymorph in the compressed samples increases with pressure applied to the powder. In comparison with powder compression, wet milling leads to a more pronounced phase transformation. This occurs because the particles are exposed to a large number of stress events by collision with the grinding media, which leads to the formation of defects and new surface crystallites by particle fracture. According to the MD simulations, phase transformation is expected to occur preferentially in surface crystallites because they experience the highest mechanical load. Because of the phase transformation, the wet ground and compressed samples exhibit a lower photo- luminescence intensity than the feed material. In comparison with powder compression, milling reduces the photoluminescence intensity more substantially. This occurs because a higher defect concentration is formed. The defects contribute to the phase transformation and photoluminescence quenching.展开更多
文摘The performance of proton exchange membrane fuel cells (PEMFC) is strongly determined by the structure and composition of the electrode layer.The interactions between the ionomer,carbon black particles,and solvent affect the suspension properties and thus the layer morphology.We analyze the effect of the ionomer-to-carbon (I/C) weight ratio for two different types of carbon black on the suspension and layer characteristics.Highly branched carbon blacks with a high surface area tend to form less cracked layers.As less branched carbons can pack together more closely,a smaller pore size results in a larger capillary pressure during drying and thus more cracks.The added ionomer adsorbs on the carbon particles and improves the colloidal stability of the carbon black particles.The carbon black aggregates are thus smaller,resulting in closer packing and thinner layers.Moreover,the addition of the ionomer increases the critical coating thickness (CCT) of the layers because drying stresses are dissipated by the deformation of the ionomer,preventing crack formation.An optimum I/C weight ratio is identified for optimal layer formation and minimized crack formation.
基金supported financially by Arbeitsgemeinschaft industrieller Forschungsvereinigungen(AiF)(Grant No.:IGF333ZN)
文摘Molecular dynamics （MD） simulations of the consecutive compression-decompression cycles ot hexagonal zinc sulfide （wurtzite） nanoparticles predict an irreversible phase transformation to the cubic polymorph.The phase transformation commences at the contact area between the particle and the inden- ter and proceeds with the number of compression cycles. Dislocations are visible for a particle size above 5nm. Results from wet grinding and dry powder compression experiments on a commercial wurtzite pigment agree qualitatively with MD simulation predictions. X-ray diffraction patterns reveal that the amount of cubic polymorph in the compressed samples increases with pressure applied to the powder. In comparison with powder compression, wet milling leads to a more pronounced phase transformation. This occurs because the particles are exposed to a large number of stress events by collision with the grinding media, which leads to the formation of defects and new surface crystallites by particle fracture. According to the MD simulations, phase transformation is expected to occur preferentially in surface crystallites because they experience the highest mechanical load. Because of the phase transformation, the wet ground and compressed samples exhibit a lower photo- luminescence intensity than the feed material. In comparison with powder compression, milling reduces the photoluminescence intensity more substantially. This occurs because a higher defect concentration is formed. The defects contribute to the phase transformation and photoluminescence quenching.
