Synthesis of Sulfonic Acid-Containing Polybenzoxazine for Proton Exchange Membrane in Direct Methanol Fuel Cells

Abstract

A novel sulfonic acid-containing benzoxazine monomer, 3-[4-(4-(6-sulfo-3,4-dihydro-2H-1,3-benzoxazine-3-yl)benzyl)phenyl]-3,4-dihydro-2H-1,3-benzoxazine-6-sulfonic acid (abbreviated as SHS-ddm), was synthesized via Mannich reaction and acidizing reaction using sodium 4-hydroxybenzenesulfonate, 4,4′-diaminodiphenylmethane, and paraformaldehyde as raw materials. The structure of SHS-ddm was characterized by Fourier transform infrared (FTIR) spectroscopy, proton nuclear magnetic resonance (1H NMR) spectroscopy, and electrospray ionization mass spectrometry (ESI–MS). The cross-linked sulfonic acid-containing polybenzoxazine [poly(SHS-ddm)] membrane was prepared through high temperature solution-casting method. Membrane properties including proton conductivity, methanol permeability, mechanical property, hydrolytic and oxidative stabilities were evaluated in detail. Our results showed that poly(SHS-ddm) membrane exhibited a high proton conductivity of 0.154 S cm–1 at 80 °C and a very low methanol permeability of 5.8 × 10–8 cm2 s–1. The calculated selectivity parameter was 2.655 × 106 S s cm–3, which was much higher than that of Nafion 117 (8.081 × 104 S s cm–3). Employing this membrane in the membrane-electrode assembly (MEA), high methanol concentration up to 7 M was used. The cell exhibited a stable open circuit voltage (VOC) of 0.76 V and a maximum power density of 66.5 mW cm–2. Accordingly, only relatively low voltage drop of 1.415 mV h–1 was observed after a continuous operation for 12 h. Overall, this novel polybenzoxazine exhibited special potential for proton exchange membrane with low methanol permeability and low cost in direct methanol fuel cells.