Quaternized poly(ether ether ketone) hydroxide exchange membranes for fuel cells

Chloromethylated PEEK (CMPEEK) was successfully synthesized for the first time by concentrated sulfuric acid as the solvent and its degree of chloromethylation (DC) was controlled (38–98%) by varying the chloromethylation time. The sulfonation of PEEK was confirmed to be negligible during the chloromethylation by 1H NMR analysis. CMPEEKs are soluble in high-boiling-point solvents, which makes it possible to prepare CMPEEK membranes. Subsequently, it was quaternized and ion-exchanged by NaOH to form quaternized PEEK hydroxide (QAPEEKOH) membrane. The ion exchange capacity of QAPEEKOH membranes ranges from 0.43 to 1.35 mmol g−1, and the corresponding degree of quaternization of chloromethyl group increases from 35% to 61%. QAPEEKOH membranes from DC of 38–70% have appropriate water uptake (≤145%) and moderate swelling ratio (≤27%) even at 60 °C. More importantly, their hydroxide conductivities, such as 12 mS cm−1 at 30 °C and 0.95 mmol g−1 of IEC, are higher than those of most other quaternized aromatic polymer membranes. TGA shows that QAPEEKOH has high thermal stability (TOD: 148 °C). All the above properties indicate that QAPEEKOH membrane is very promising for the potential application in hydroxide exchange membrane fuel cells.