Toward improved mechanical strength, oxidative stability and proton conductivity of an aligned quadratic hybrid (SPEEK/FPAPB/Fe₃O₄-FGO) membrane for application in high temperature and low humidity fuel cells

Iron oxide (Fe₃O₄) anchored functionalized graphene oxide (FGO) is applied as a magnetically active filler as well as a solid proton conductor to realize an aligned hybrid membrane architecture with blended polymer matrix consisting of fluorinated poly arylene propane biphenyl (FPAPB) and sulfonated poly ether ether ketone (SPEEK) for application in high temperature and low humidity fuel cells. The simple, but effective, functionalization of GO is performed with sulfonic acid (SO₃H) containing organosilane to increase the per unit volume of SO₃H groups and is then anchored by Fe₃O₄ nanoparticles to create the magnetic property. A constant magnetic field of 0.25 T is applied to perpendicularly align Fe₃O₄-FGO to the cross-section surface of membrane during solution casting process. The carbon skeleton of aligned Fe₃O₄-FGO and C-F bonds of FPAPB allow for an unprecedented approach to tailor the mechanical strength and oxidative stability of quadratic hybrid membrane, as evidenced by the results of dynamic mechanical analysis (DMA) and ex situ durability test. The incorporation of aligned Fe₃O₄-FGO improves water absorption, ion exchange capacity and proton conductivity of membrane by increasing the number of SO₃H groups per unit volume of membrane domain, while retaining considerable dimensional stability. The peak proton conductivity of aligned SPFSGF-5 membrane at 120 °C under 20% RH is 11.13 mS cm^-1, which is 1.44 fold better than that of pristine SP membrane (7.68 mS cm^-1). In contrast, under identical operating conditions, the Nafion-112 membrane exhibited a peak proton conductivity of 9.78 mS cm^-1, a 1.13 fold lower conductivity compared to aligned quadratic hybrid membrane. Furthermore, the aligned quadratic hybrid membrane exhibited lower H₂ and O₂ gas permeability compared to pristine SP and unaligned quadratic hybrid membranes.