Styrene-co-butyl acrylate copolymers with potential application as membranes in PEM fuel cell

Abstract

Styrene-butyl acrylate (St-BuA) copolymers were obtained by mass free radical copolymerization reactions, using BPO as initiator. A 70:30 (St-BuA) comonomer composition was selected. Three different molar concentrations of sulfonating agent (50, 100 and 150%) were considered to obtain the sulfonated copolymer (sSt-BuA) and membranes were casted from sSt-BuA copolymers. Copolymers were characterized with FTIR, TGA and NMR 1H and mechanically by TMA. The sSt-BuA sulfonation degree (SD) effect on the ion-exchange capacity (IEC) was evaluated by titration and water uptake (WU) by gravimetry. Microstructure was also observed using scanning electron microscopy (SEM), and its electrochemical properties were evaluated by electrochemical impedance spectroscopy (EIS). The chemical composition of St-BuA copolymer was confirmed by proton nuclear magnetic resonance (NMR 1H) spectroscopy. Fourier transformed infrared spectroscopy (FT-IR) and thermal gravimetric analysis (TGA) analysis also confirmed the existence of both comonomers and a successful sulfonation up to an actual 30% level, as well as a good thermal stability over 300 °C for the molecular structure and over 150 °C for sulfonated membranes. TMA indicate an increase in flexure modulus along degree of sulfonation. SEM images show a highly dense material with low pore size for the sulfonated copolymer. The IEC values obtained varied from 0.83 to 1.18 meq g−1, depending on sulfonation degree. The proton conductivity for St-BuA 50% membrane (9.77 × 10−5 S cm−1) was a two order of magnitude lower than the commercial Nafion membrane (4.53 × 10−3 S cm−1). Thermal, mechanical and electrochemical results demonstrate that these sSt:BuA copolymers are promising materials to be used as membranes in fuel cells applications.