| Summary: | CeO<sub>2</sub> nanoparticles were coated with polydopamine (PDA) by dopamine polymerization in water dispersions of CeO<sub>2</sub> and characterized by Infrared and Near Edge X-ray Absorption Fine Structure spectroscopy, Transmission Electron Microscopy, Thermogravimetric analysis and X-ray diffraction. The resulting materials (PDAx@CeO<sub>2</sub>, with x = PDA wt% = 10, 25, 50) were employed as fillers of composite proton exchange membranes with Aquivion 830 as ionomer, to reduce the ionomer chemical degradation due to hydroxyl and hydroperoxyl radicals. Membranes, loaded with 3 and 5 wt% PDAx@CeO<sub>2</sub>, were prepared by solution casting and characterized by conductivity measurements at 80 and 110 °C, with relative humidity ranging from 50 to 90%, by accelerated ex situ degradation tests with the Fenton reagent, as well as by in situ open circuit voltage stress tests. In comparison with bare CeO<sub>2</sub>, the PDA coated filler mitigates the conductivity drop occurring at increasing CeO<sub>2</sub> loading especially at 110 °C and 50% relative humidity but does not alter the radical scavenger efficiency of bare CeO<sub>2</sub> for loadings up to 4 wt%. Fluoride emission rate data arising from the composite membrane degradation are in agreement with the corresponding changes in membrane mass and conductivity.
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