Abstract
The introduction of sulfonic acid modified silica in Nafion nanocomposite membranes is a good method of improving the Nafion performance at high temperature and low relative humidity. Sulfonic acid-modified silica is bifunctional, with silica phase expected to offer an improvement in membranes hydration while sulfonic groups enhance proton conductivity. However, as discussed in this paper, this may not always be the case. Proton conductivity enhancement of Nafion nanocomposite membranes is very dependent on silica particle size, sometimes depending on experimental conditions, and by surface modification. In this study, Sulfonated Preconcentrated Nafion Stober Silica composites (SPNSS) were prepared by modification of Stober silica particles with mercaptopropyltriethoxysilane, dispersing the particles into a preconcentrated solution of Nafion, then casting the membranes. The mercapto groups were oxidized to sulfonic acids by heating the membranes in 10 wt % hydrogen peroxide for 1 h. At 80 °C and 100% relative humidity, a 20%–30% enhancement of proton conductivity was only observed when sulfonic acid modified particle less than 50 nm in diameter were used. At 120 °C, and 100% humidity, proton conductivity increased by 22%–42% with sulfonated particles with small particles showing the greatest enhancement. At 120 °C and 50% humidity, the sulfonated particles are less efficient at keeping the membranes hydrated, and the composites underperform Nafion and silica-Nafion nanocomposite membranes.
Highlights
Proton conductivity of Nafion, still considered the benchmark for polymer electrolyte membranes in PEM fuel cells, drops with temperatures >100 ̋ C, limiting full exploitation of advantages of operatingPEM fuel cells at temperatures above the boiling point of water [1,2,3]
While hydration is improved and membrane failure is reduced at elevated temperatures, proton conductivity is often lower with metal oxide modified Nafion membranes due to volumetric dilution of sulfonic groups in membranes [6,7,8]
To influence of particle size on theon nanocomposite membrane properties, it is necessary necessary to prepare spherical with the size a narrow size to prepare spherical silica particlessilica with particles control over thecontrol size andover a narrow size and distribution
Summary
Proton conductivity of Nafion, still considered the benchmark for polymer electrolyte membranes in PEM fuel cells, drops with temperatures >100 ̋ C, limiting full exploitation of advantages of operatingPEM fuel cells at temperatures above the boiling point of water [1,2,3]. Proton conductivity of Nafion, still considered the benchmark for polymer electrolyte membranes in PEM fuel cells, drops with temperatures >100 ̋ C, limiting full exploitation of advantages of operating. While hydration is improved and membrane failure is reduced at elevated temperatures, proton conductivity is often lower with metal oxide modified Nafion membranes due to volumetric dilution of sulfonic groups in membranes [6,7,8]. This can be minimized by modifying the silica particles with sulfonic acid groups [9,10,11].
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