Hsiu-Li Lin

Weak Gel Behaviour of Poly(vinyl alcohol)-Borax Aqueous Solutions

Thermal transition of PVA-borax aqueous gels with a PVA concentration of 60 g/L and a borax concentration of 0.28 M was investigated at temperatures ranging from 15 to 60 ○C using static light scattering (SLS), dynamic light scattering (DLS), and dynamic viscoelasticity measurements. Three relaxation modes, i.e. two fast and one slow relaxation modes, were observed from DLS measurements. Two fast relaxation modes located around 10−3∼101 sec, with one fast mode (τf1) being scattering vector q-dependent and the other fast mode (τf2, with τf2>τf1) being q-independent. The τf1 mode was attributed to the gel mode whilst the τf2 mode could be due to the hydrodynamics of intra-molecular hydrophobic domains formed by uncharged segments of polymer backbones. The slow relaxation mode with relaxation time located around 101∼103 sec in DLS data was due to the motion of aggregated clusters and was observed only at temperatures above 40 ○C. The amplitude and relaxation time of slow mode decrease as temperature is increased from 40 to 60 ○C. At temperatures below 40 ○C, no slow relaxation mode was observed. The SLS measurements showed PVA-borax-water system had fractal dimensions Df∼2.4 and Df≤2.0 as temperature was below and above 40 ○C, respectively. The simple tilting test indicated gel behaviour for the PVA-borax aqueous system at temperatures below 40 ○C with a creep flow after a long time exposure in the gravity field. But the dynamic viscoelasticity measurements demonstrated a solution behaviour for PVA/borax/water at temperatures below 40 ○C, the critical gel point behaviour for G′(ω) and G″(ω) was not observed in this system as those reported for chemical crosslinked gels. These results suggest that the PVA-borax aqueous system is a thermoreversible weak gel.

Structures of Membrane Electrode Assembly Catalyst Layers for Proton Exchange Membrane Fuel Cells

In this paper, we modify the conventional 5-layer membrane electrode assembly (MEA, in which a proton exchange membrane (PEM) is located at its center, two Pt-C-40 (Pt on carbon powder support, Pt content 40 wt.%) catalyst layers (CLs) are located on the surfaces of the both sides of the PEM and two gas diffusion layers (GDLs) are attached next on the outer surfaces of two Pt-C-40 layers) and propose 7-layer and 9-layer MEAs by coating thin Pt-black CLs at the interfaces between the Pt-C-40 layer and the GDL and between the PEM and the Pt-C-40 layer and reducing the Pt-C-40 loading. The reduced Pt loading quantity of the Pt-C-40 layer is equal to the increased Pt loading quantity of the Pt-black layer, thus the total amount of Pt loadings in the unmodified conventional MEA and the modified MEAs are at a fixed Pt loading quantity. These modified MEAs may complicate the manufacture process. The main advantage of these 7- and 9-layer MEAs is the thinner CL thickness and thus lower CL proton transport resistance. Because of the thin Pt-black layer thickness in MEA, we avoid agglomeration of the Pt-black particles and maintain high Pt catalytic activity. We show these new CL structure MEAs have better fuel cells performance than the conventional 5-layer MEA.