**4. Conclusions**

Polymerization of dopamine in water suspensions of CeO2 allowed preparation of three types of PDA coated CeO2 nanoparticles containing 10, 25 and 50 wt% PDA. These materials were used as fillers of Aquivion membranes to improve their chemical stability towards hydroxyl and hydroperoxyl radicals while hindering as much as possible the reaction between the oxide surface and the sulfonic groups of the ionomer.

Composite membranes, loaded with 3 and 5 wt% of each filler, were characterized by conductivity measurements and accelerated degradation tests (based on the Fenton reaction) to assess the radical scavenger efficiency of the filler, and the results of these investigations were compared with those obtained, under the same conditions, for Aquivion membranes containing bare CeO2 or Bz@ CeO2. It was found that the decrease in conductivity associated with the increase in the CeO2 loading is appreciably less severe for membranes containing PDAx@CeO2 than for those containing bare CeO2, especially at 110 ◦C and 50% RH. Moreover, the FER of membranes filled with PDAx@CeO2 is significantly lower than that of membranes containing Bz@CeO2 and, for CeO2 percentage up to 4 wt%, it is coincident with that of membranes loaded with bare CeO2. Therefore, the PDA coating prevents to a large extent the decrease in conductivity without compromising the radical scavenger activity of CeO2. Interestingly, the radical scavenger efficiency of PDAx@CeO2 is nearly unaffected by the PDA content as also proved by the results of the OCV stress tests.

**Author Contributions:** Conceptualization, A.D.; Data curation, R.D., A.D., C.B., P.S., M.P., A.C., I.G. and M.C.; Investigation, R.D., C.B., P.S., M.P., A.C. and I.G.; Methodology, A.D.; Supervision, A.D. and M.C.; Writing—original draft, A.D. and M.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by MIUR grants AMIS and DELPHI through the program "Dipartimenti di Eccellenza 2018–2022".

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data underlying this article will be shared on reasonable request from the corresponding author.

**Conflicts of Interest:** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
