Novel proton-exchange membrane based on single-step preparation of functionalized ceramic powder containing surface-anchored sulfonic acid

A novel approach to the synthesis of a low-cost proton-exchange membrane (PEM) based on the single-step preparation of a functionalized ceramic powder containing surface-anchored sulfonic acid (SASA) and a polymer binder, is presented for the first time. The added value of this technique, compared with earlier work published by our group, is the adoption of a direct, single-step synthesis, as opposed to a multiple-step synthesis. The latter requires an oxidation step, in order to convert the thiol group into a sulfonic group. SASA powders of different compositions have been prepared and characterized by means of Brunaur-Emmet-Teller (BET), thermogravimetric analysis-differential thermal analysis (TGA-DTG), differential scanning calorimeter (DSC), Fourier transformation infrared (FT-IR), nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM) and electrochemical techniques. The lowest equivalent weight measured for SASA powders is 1281 g equiv.^-1. The ionic conductivity of a 100-μm-thick membrane is measured ex situ at room temperature (25 ± 3 °C) and the highest proton conductivity is 48 mS cm^-1. The typical pore size, for the SASA powders is less than 10 nm and ranges from 2 to 50 nm for the SASA-based membranes. The membranes are thermally stable up to 250 °C. Direct methanol fuel cells (DMFCs) are assembled with some of the membranes. Preliminary tests showed that the cell resistance for a ∼100-μm-thick membrane ranges between 0.29 and 0.19 Ω cm² from 80 to 130 °C, respectively, and that the maximum cell power density with a 1 M methanol solution is 127, 208 and 290 mW cm^-2 at 80, 110 and 130 °C, respectively, while the corresponding methanol crossover current density is 0.093, 0.238 and 0.281 A cm^-2.