A Novel Water‐Free Proton‐Conducting Solid Electrolyte based on an Organic/Inorganic Hybrid

Abstract

Proton-conducting polymers have attracted much attention due to their promising application as membranes in fuel cells. The most commonly used polymer has been a perfluorinated ionomer known by its trade name, Nafion. It is well known that, including Nafion, the proton-transport properties of water-swollen perfluorinated ionomers largely depend on their water content, which limits their application to temperatures and pressures below the dew point of water. [1,2] This puts several constraints on the use, as fuels, of hydrogen-rich reformats, which usually contain traces of carbon monoxide or methanol, as they require somewhat higher temperatures. Therefore, it is worthwhile to seek distinctly different proton-conducting materials. One approach is based on the complexation of basic polymers (e.g., polybenzimidazole) with oxo-acids (e.g., phosphoric acid).[3±5] Another approach is the substitution of water by heterocycles, such as imidazole, pyrazole, and benzimidazole.[6±10] They are known to present high-proton conductivity that is comparable to the value of hydrated membranes, especially at high temperatures ranging between 150 and 250 C. The approach of employing these heterocycles as proton solvents in fuel-cell membranes may require their immobilization to prevent the leakage of heterocycles during fuel-cell operation. The first attempt towards full immobilization of a heterocycle as a proton solvent in completely water-free electrolytes has been recently developed by Schuster et al. [9] They reported that structure diffusion, which consists of intermolecular proton transfer and structural reorganization by a hydrogen-bond breaking and forming process, is the dominant proton-conducting mechanism in the imidazole-terminated ethylene oxide oligomers. However, this material is not yet available for practical applications, mainly because of its liquid-like physical state. Following this model system, in this study, a new waterfree proton-conducting solid electrolyte based on an organic/ inorganic hybrid is presented. Since a siloxane oligomer functionalized with imidazole-terminated side chains (CImSS) [10] has been reported as a material with a good proton-transport ability as well as a good self-dissociation constant, this moiety is employed as a candidate for a proton transporter, although it shows a viscous liquid characteristic. The CImSS has been synthesized according to a literature procedure [10] (Fig. 1). As a mechanical framework, silicates prepared by a sol±gel synthesis of silane monomers have been chosen. Two silane monomers, such as tetraethoxysilane (TEOS) and methyltriethoxysilane (MTEOS), were employed after consideration of their different polarities. The polarity difference between the CImSS and the silane monomers could affect the phase separation during the blending accompanied by the sol±gel synthesis. The morphologies of the CImSS/silicate hybrids were investigated by field-emission scanning electron microscopy (FE-SEM) measurements (Fig. 2). The physical appearance of the CImSS/MTEOS-based...

Effect of Polymer Composition and Water Content on Proton Conductivity in Vinyl Benzyl Phosphonic Acid—4‐Vinyl Pyridine Copolymers

“…However, leaching of the mobile (e.g., liquid) molecules by water remains a severe problem when used in fuel cells. [5] More recently, proton conducting polymers with tethered phosphonic acid, [22] imidazole, [23] and benzimidazole [24] moieties have been prepared. Both, proton conductivities and the proton diffusion coefficients of phosphonic acid, imidazole, and their derivatives tethered to small molecules and various oligomers have been studied by Kreuer and coworkers.…”

SiO2 nanoparticles-coated poly(paraphenylene terephthalamide) nonwovens as reinforcing porous substrates for proton-conducting, sulfonated poly(arylene ether sulfone)-impregnated composite membranes

“…The SiO 2 solution was cast into the PPTA nonwoven via a dipcoating process. The SiO 2 solution-immersed nonwoven was dried at room temperature for 2 h to evaporate the solvent and was then cured by heating at 50, 100, 120, 140°C for 3 h at each temperature to complete sol-gel reaction of the TEOS-based binders [20,21]. The thickness of the SiO 2 /PPTA substrate was observed to be around 19 μm.…”

Anhydrous proton conducting composite membranes containing Nafion and triazole modified POSS

“…Thus, such proton solvents are generally required to be immobilized inside membranes for practical fuel cell applications. Immobilization of proton solvents on polymeric materials [17][18][19][20][21] or nano-structured inorganic materials [22,23] has been developed. For membranes incorporated with proton solvent modified nanostructured inorganic materials, the reduced mechanical properties are often encountered, which may in turn reduce the durability and reliability of assembled fuel cells [24].…”