Hydrolytically Stable Phosphorylated Hybrid Silicas for Proton Conduction

Graphical Abstract

A series of novel proton conductors with hydrolytically stable phosphonic acid functionalization have been newly prepared from the sol–gel derived hybrid silicas, which show practically important and promising characteristics for the application to intermediate temperature proton exchange membrane fuel cells, especially under near saturated humidity conditions.

Abstract

A new approach to the synthesis of fully immobilized phosphorus functionalized hybrid proton conductive gels based on phosphonic acid grafting is presented in this paper. The hybrid silicas with different amounts of phosphonic acid have been prepared and characterized using Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller surface area analysis, thermogravimetric analysis, and electrochemical techniques. The proton conductivity of the materials depend strongly on hydration, which increases by four orders of magnitude over the relative humidity (RH) range of 20 to 100 %, up to a maximum of 0.027 S cm^–1 at 100  °C and 100 % RH. For the reported samples, proton conduction is believed to occur within a dynamic hydrogen-bond network formed by functionalized P–OH groups and water molecules by the Grotthuss mechanism. However, the proton conductive sites (P–OH) are likely to be partially immobilized by strong protonic receptors (N atoms in amines), which reduces the free P–OH groups and restricts proton transfer. Hydration may cause a bonding structural rearrangement, which results in more free P–OH groups as active proton conductive sites and, therefore, greatly increased proton conductivity is observed.