Triazole Functionalized Sol-Gel Membranes, Effect of Crosslink Density and Heterocycle Content on Water Free Proton Conduction and Membrane Mechanical Properties

Abstract

Freestanding, ion conducting, membranes were synthesized by incorporating triazole-containing tetracyclosiloxanes into a polyethylene glycol-tetraethyl orthosilicate (PEG-TEOS) based sol-gel matrix. These membranes show comparable or higher proton conductivities than their linear, liquid, polysiloxane analogs and fall within an order of magnitude of the target ion mobilities for use in proton exchange membrane fuel cells (PEMFC's). The absence of any unbound PEG or cyclic siloxane was confirmed by proton nuclear magnetic resonance (1H-NMR), while the chemical structure and composition of the membranes was corroborated by Fourier transform infrared (FTIR) spectroscopy. Thermogravimetric analysis (TGA) indicated that the membranes are stable up to 180°C and differential scanning calorimetry (DSC) analysis showed complete suppression of PEG crystallization after incorporation of the triazole-functionalized cyclosiloxanes. An increase in the molecular weight of the PEG chains used to create the sol-gel matrix produced membranes with increased flexibility and higher proton conductivities at temperatures below 100 °C. Pulse field gradient echo (PFG) NMR studies showed an increase in the apparent diffusion coefficient of the sol-gel threaded cyclosiloxane motifs compared to the linear polysiloxanes, indicating a significant reduction on the coupling between mechanical strength and ion transport capability.