Novel quaternary ammonium functional addition-type norbornene copolymer as hydroxide-conductive and durable anion exchange membrane for direct methanol fuel cells

Abstract

Novel quaternary ammonium functional addition-type norbornene copolymers (Q_CnP(BN/PhBN), n = 1, 6, 10, 12) with different alkyl side chain length comb-shaped structures or different contents of 2-(4-phenyl-butoxymethy-lene)-5-norbornene (PhBN) (22–77%) are synthesized via copolymerization of functionalized norbornenes, and their corresponding hydroxide-conductive anion exchange membranes (AEMs) with effective hydrophilic–hydrophobic separation are prepared and confirmed by TEM or SEM. The achieved AEMs show high ion exchange capacity (1.83 mmol g⁻¹), as well as low methanol permeability (1.97–20.4 × 10⁻⁷ cm² s⁻¹), which are lower than that of Nafion®. The ionic conductivity increases with the operation temperature increasing and is observed up to 4.14 × 10⁻³ S cm⁻¹. The AEMs exhibit excellent dimensional stability with a swelling degree in plane between 0.9–3.3% and good chemical stability under 6 M NaOH solution even after a month. Membrane electrode assembly (MEA) is fabricated by using the alkalized Q_C12P(BN/PhBN)-77 as the AEM and tested in an alkaline direct methanol fuel cell. The open circuit voltage (OVC) of 0.54 V and the maximum current density of 66 mW cm⁻² are achieved at 80 °C, respectively.