Revealing nanostructure-driven oxidative protection in thiourea-functionalized sPEEK hybrid membranes for PEM Fuel Cells

Abstract

Achieving long-term oxidative stability in proton exchange membrane fuel cells (PEMFCs) remains a major challenge. Here, we investigate the morphological mechanisms underlying the oxidative stability of thiourea-functionalized sol-gel (SG)/sulfonated poly(ether ether ketone) (sPEEK) hybrid membranes, previously developed using two organosilane precursors, N,N′-bis [3-(triethoxysilyl)propyl] thiourea (HTU) and N-phenyl,N′-[3-(triethoxysilyl)propyl] thiourea (TTU). These thiourea groups, introduced as sacrificial antioxidants, are used to tune the nanoscale organization of the SG phase within the polymer matrix. Contrast-variation small-angle neutron scattering (CV-SANS) shows that the SG phase is distributed within ionic nanochannels and interbundle regions. Although TEM and AFM indicate homogeneous dispersion at the microscale, sub-resolution fluctuations likely account for the SANS signal. HTU forms well-defined spherical hybrid domains (~5-6 nm), whereas TTU produces larger, less organized structures (~18-22 nm). Under oxidative conditions, pristine sPEEK swells and degrades above 0.1 wt%, while hybrid membranes preserve structural integrity and conductivity up to 0.3 wt%. Hybrid membranes retain conductivity as long as SG within ionic domains is preserved; its consumption triggers secondary protection via SG in interbundle regions. This two-stage mechanism governs membrane integrity and proton transport under oxidative stress.