Preparation and Proton Conductivity of Fuel Cell Membranes Based On Polymer-Modified Silica Colloidal Crystals

We will describe the preparation and study of a new class of proton-conducting membrane materials, namely, nanoporous colloidal membranes whose proton conductivity results from the nanopore surface modification with polymer molecules carrying acid functionalities.  Our design has a number of unique features that are fundamentally different from the traditional polyelectrolyte or composite organic-inorganic membranes.  In it, the highly ordered silica colloidal crystal with the continuous network of nanopores serves as a matrix providing mechanical stability and water retention and supporting the proton-conducting polymers, therefore eliminating the need to optimize simultaneously the mechanical and proton-conducting properties of the polymer.  The inorganic matrix also allows achieving high degrees of acid functionalization of the polymer without the danger for the membrane to become water soluble, and maintaining mechanical stability under the oxidative conditions and high temperature.  We prepared sintered self-assembled nanoporous silica colloidal crystals modified with poly(3-sulfopropyl-methacrylate) and poly(stryrenesulfonic acid) brushes covalently attached to the nanopore surface.  The resulting robust membranes possess temperature and humidity-dependent proton conductivity of ~2 × 10^-2 S cm^-1 at 30 °C and 94% R.H., ~1 × 10^-2 S cm^-1 at 85 °C and 60% R.H., and water uptake of ca. 20 wt% at room temperature.  We will discuss the relationship between the degree of sulfonation and proton conductivity, as well as performance of fuel cells utilizing out membranes.