Quaternary phosphonium-functionalized poly(ether ether ketone) as highly conductive and alkali-stable hydroxide exchange membrane for fuel cells

A series of quaternary phosphonium-functionalized poly(ether ether ketone)s (PEEK-QPOHs) have been synthesized by phosphorization reaction, and their membranes have been prepared by a solution casting method. Ion exchange capacity (IEC) of PEEK-QPOH membranes ranges from 0.89 to 1.19 mmol g−1 with the degree of chloromethylation from 70% to 126%. Not only are PEEK-QPOH polymers soluble in typical high-boiling-point membrane-forming solvents, but also they are soluble in some low-boiling-point water-miscible solvents (low alcohols), implying their ability to serve as both membranes and ionomers. PEEK-QPOH membranes showed high hydroxide conductivity, e.g., a PEEK-QPOH 126% membrane (with IEC of 1.19 mmol g−1) exhibited 61 and 89 mS cm−1 at 20 °C and 60 °C, respectively. Based on the same polymer backbone of PEEK, quaternary phosphonium-functionalized PEEK membranes are more hydroxide-conductive and more alkali-stable than quaternary ammonium- and imidazolium-functionalized counterparts. All these properties indicate PEEK-QPOH is one of the most promising HEM materials for potential applications in alkaline membrane fuel cells.