Molecularly designed, solvent processable tetraalkylammonium-functionalized fluoropolyolefin for durable anion exchange membrane fuel cells

Abstract

A new generation of solvent processable fluoropolyolefin was prepared and explored for durable anion exchange membrane fuel cells. By one-pot Ziegler-Natta copolymerization of a fluorinated monomer (1-(but-3-en-1-yl)-4-fluorobenzene) with 11-bromo-1-undecene and subsequent quaternization with trimethylamine, a series of tetraalkylammonium-functionalized fluoropolyolefin was synthesized by varying the molar ratio between monomers. The excellent solubility of the quaternized fluoropolyolefins provided by fluorinated moieties enabled the solvent processability to afford transparent and robust AEMs (PBFB-QA). Compared to heterogeneous quaternized membranes (H-PBFB-QA) and the corresponding non-fluorinated AEMs counterparts, the homogeneous quaternized PBFB-QA membrane showed lower water uptake due to the high hydrophobicity of fluorine atoms and more compact entanglement of polymer chains during solvent casting process. It was found that PBFB-QA membrane with an IEC of 1.47 meq/g showed the highest hydroxide conductivity of 29.2 mS/cm at 20 °C in spite of its low water uptake (11.9 wt%) and hydrated number (λ = 4.4). Thus, fluoropolyolefin AEMs standed out from the other reported polyolefin-based AEMs with the highest λ-normalized hydroxide conductivity, indicating the efficient ion transport in AEMs with the aid of less water. In addition to high ionic conductivity, fluoropolyolefin AEMs also exhibited excellent alkaline stability, and the retention of the conductivity of PBFB-QA-18 membrane was 89.5% after 700 h of aging in 1 M NaOH at 80 °C. More importantly, in contrast to the non-fluorinated polyolefin-based AEMs, fluoropolyolefin AEM showed better initial fuel cell device performance with a maximum power density of 133 mW/cm2 at 60 °C, and impressive device durability of alkaline fuel cell using the fluoropolyolefin AEM was observed with no loss of performance over 80 h of operation at 60 °C at a constant cell voltage of 0.3 V.