Abstract: Quasi-spherical carbon nano-onions(CNOs) with a size of ca.5 nm were synthesized via thermal annealing of nanodiamonds. Subsequent oxidation using concentrated H2SO4/HNO3 introduced －COOH groups, yielding carboxylated CNOs(C-CNOs). Further functionalization through acylation and nucleophilic substitution produced highly phosphorylated CNOs (P-CNOs). High-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) confirmed the successful introduction of －COOH and －PO3H2 groups, with P-CNOs exhibiting an ion exchange capacity(IEC) of 1.85 mmol/g. Homogeneous, intact, and dense SPAES/P-CNOs composite membranes were prepared via solution casting by blending P-CNOs with sulfonated poly(arylene ether sulfone)(SPAES). Compared to pristine SPAES, the composite membranes show enhanced properties including water uptake/swelling, oxidative stability, and proton conductivity. This improvement stems from hydrogen-bonding networks between the －COOH/－PO3H2 groups in P-CNOs and the －SO3H groups in SPAES, forming a more stable network structure that bolsters mechanical properties and chemical stability while facilitating proton transfer. The SPAES/P-CNOs-1.5 membrane achieves a high proton conductivity of 220 mS/cm at 90 °C. At 80 °C and 100% RH, its maximum power density reaches 650 mW/cm2, 36% higher than that of SPAES. It also shows excellent mechanical property and high thermal-dimensional-chemical stability, indicating significant application potential.

Key words: Sulfonated poly(aryl ether sulfone), Phosphorylated carbon nano-onions, Organic-inorganic composite, Proton exchange membrane, Fuel cell