Proton Conductivity of Nafion/Ex-Situ Sulfonic Acid-Modified Stöber Silica Nanocomposite Membranes As a Function of Temperature, Silica Particles Size and Surface Modification

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.

Interfacial Water Drives Improved Proton Transport in Siliceous Nanocomposite Nafion Thin Films

“…[70,67,66] If water uptake or ion conduction by surface silanols were the only mechanisms through which conductivity is enhanced, one would expect maximal enhancement for small particle sizes that maximize surfacea rea. Such ac learr elationship has not been found in the literature [29,36] or in our own experiments.The interconnected network hypothesise xplains why the effect of particles ize (and particle loading) is ambiguous.Abalancem ust be found between the size of Nafion channels and filler particles, the critical mass loading to obtain an interconnected network, water uptake, and IEC. The structure-directing effect of fillers is expected to be even more pronouncedint hin films.…”

“…[20,21] The design of composite membranes may follow three strategies:1 )increase water uptake by addingh ygroscopicf illers, such as silica, [22][23][24][25] metal oxides, [26,27] and zeolites; [28][29][30][31] 2) improve conductivity by adding acidic proton conductors, such as heteropolyacids; [32,33] or 3) influence the morphologyb ym embrane-filler interactions. [34][35][36][37] Although water uptake in Nafion is strongly correlated with ion conduction, increasingw ater uptake by using filler materials does not always lead to improved conductivity. [2] Siliceous filler materials have a high surface-area-to-volume ratio and, therefore, contain plenty of surface silanol groups (SiOH) that can readily physisorb water.…”

Molecular Dynamics Simulations of Substrate Hydrophilicity and Confinement Effects in Capped Nafion Films

“…Nanoparticles are added to Nafion to enhance water retention and proton conductivity. 7 , 57 Therefore, it is important to study the effect on water transport because of the nanoparticle hydrophilicity and filler fraction.…”

“…Nafion nanocomposites containing highly hydrophilic nanoparticles, such as modified silica, have shown higher proton conductivity as compared to that of bulk Nafion. 7 Crossover of methanol in DMFCs has been reduced by using Nafion nanocomposites. 17 Vanadium ion crossover has also been reduced by doping Nafion with nanoparticles.…”

pH- and thermo-sensitive MTX-loaded magnetic nanocomposites: synthesis, characterization, and in vitro studies on A549 lung cancer cell and MR imaging

“…The MSNs were simply synthesized according to a modified St€ ober technique [32]. The process was as follows: a mixture solution containing 1 ml of (9 M) ammonium hydroxide and 28 ml of anhydrous ethanol was prepared and stirred for 5 min at 600 rpm.…”