| Summary: | 碩士 === 國立雲林科技大學 === 工業化學與災害防治研究所 === 94 === High proton conducting membranes have been made by using poly(vinyl alcohol), with sulfosuccinic acid as a crosslinking agent and poly(styrene sulfonic acid-co-maleic acid), called PVA/SSA/PSSA-MA membrane. The effect of PSSA-MA on the membrane properties such as water uptake, ion-exchange capacity (IEC), proton conductivity and methanol permeability was investigated. The membrane exhibits about two times higher proton conductivity than Nafion115 does when PSSA-MA content up to 80 wt%, and the methanol permeability is only one third of that for Nafion115. The results suggested that the semi-interpenetration membranes have not only higher conductivity but lower methanol permeability than Nafion under the same measurement conditions.
The degree of crosslinking was also discussed by using different crosslinking agent and heat-treatment time, respectively. The conductivity and water uptake decreased with the increasing of heat-treatment time. It is because that higher degree of crosslinking causes the membrane structure more tightly and the less space of water exists around sulfonic acid. Similarly, the conductivity and water uptake decreases with the increasing of crosslinking agent, SSA.
Water, methanol, and water/methanol self-diffusion coefficients were determined by PFG-NMR. By the result, water diffusion coefficients in hybrid membranes increase with the content of PSSA-MA. When PSSA-MA content is up to 80 wt%, water self diffusion coefficient is about one third of that in Nafion115 and methanol self diffusion coefficient is one order of magnitude lower than that in Nafion115. It seems that hybrid membranes have better methanol resistance.
The activation energy of proton conduction and methanol permeation are determined by Arrhenius equation. The activation energy of proton conduction of hybrid membranes decreased with the content of PSSA-MA due to the decreasing of distance between acidic groups. The increasing of water uptake and free water is also a factor of the activation energy of proton conduction. Proton transport becomes easier with the higher water uptake and the more free water in membranes causes proton transport mechanism to prefer to diffusion.
The activation energy of methanol permeation of hybrid membranes decreased with the content of PSSA-MA due to the increasing of water uptake and free water. The dielectric constant of water and methanol is 80 and 33, respectively. There is strong interaction between polymers in hybrid membranes, so the diffusion of methanol in hybrid membranes is harder than in Nafion.
In order to improve the mechanical property of hybrid membrane at fully hydrated state, PVA of higher molecular weight were used. With the increase of PVA molecular weights, water uptake and methanol permeability decreased modestly, but no obvious influence on proton conductivity with PVA molecular weights was found under fully hydrated conditions. It seems that the increase of PVA MW could reduce the swelling of hybrid membrane.
PSSA-MA, as a ionomer, was found to play an important role in increasing proton conductivity. By the single cell testing result, PVA/SSA/PSSA-MA series membranes are regarded suitable for PEMFC application due to its higher limit current (2 A/cm2) and power density (0.7 W/cm2) than those of Nafion115.
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