Fabrication of -SO₃H-functionalized polyphosphazene-reinforced proton conductive matrix-mixed membranes

Proton exchange membranes (PEMs) have emerged as very promising membranes for automotive applications because of their notable proton conductivity at low temperatures. These membranes find extensive utilization in fuel cells. Several polymeric materials have been used, but their application is constrained by their expense and intricate synthetic processes. Affordable and efficient synthetic methods for polymeric materials are necessary for the widespread commercial use of PEM technology. The polymeric combination of hexachlorocyclotriphosphazene (HCCP) and 4,4-diamino-2,2-biphenyldisulfonic acid facilitated the synthesis of PP-(PhSO₃H)₂, a polyphosphazene with built-in -SO₃H moieties. Characterization revealed that it was a porous organic polymer with high stability. PP-(PhSO₃H)₂ exhibited a proton conductivity of up to 8.24 × 10^-2 S cm^-1 (SD = ±0.031) at 353 K under 98% relative humidity (RH), which was more than two orders of magnitude higher than that of its -SO₃H-free analogue, PP-(Ph)₂ (2.32 × 10^-4 S cm^-1) (SD = ±0.019) under identical conditions. Therefore, for application in a PEM fuel cell, PP-(PhSO₃H)₂-based matrix-mixed membranes (PP-(PhSO₃H)₂-MMMs) were fabricated by mixing them with polyacrylonitrile (PAN) in various ratios. The proton conductivity could reach up to 6.11 × 10^-2 S cm^-1 (SD = ±0.0048) at 353 K and 98%RH, when the weight ratio of PP-(PhSO₃H)₂ : PAN was 3 : 1, the value of which was comparable with those of commercially available electrolytes used in PEM fuel cells. PP-(PhSO₃H)₂-MMM (3 : 1) had an extended lifetime of reusability. Using phosphazene and bisulfonated multiple-amine modules as precursors, we demonstrated that a porous organic polymer with a highly effective proton-conductive matrix-mixed membrane for PEM fuel cells could be produced readily by an intuitive polymeric reaction.