*Article* **Polydopamine Coated CeO2 as Radical Scavenger Filler for Aquivion Membranes with High Proton Conductivity**

**Roberto D'Amato 1,†, Anna Donnadio 2,3,\*, Chiara Battocchio 4, Paola Sassi 1, Monica Pica 2,3, Alessandra Carbone 5, Irene Gatto <sup>5</sup> and Mario Casciola 1,3**


**Abstract:** CeO2 nanoparticles were coated with polydopamine (PDA) by dopamine polymerization in water dispersions of CeO2 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@CeO2, 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@CeO2, 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 CeO2, the PDA coated filler mitigates the conductivity drop occurring at increasing CeO2 loading especially at 110 ◦C and 50% relative humidity but does not alter the radical scavenger efficiency of bare CeO2 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.

**Keywords:** cerium oxide; polydopamine; radical scavenger; proton conductivity; chemical degradation
