**4. Conclusions**

In the present paper, a composite membrane based on a Nafion polymer matrix incorporating a CaTiO3−<sup>δ</sup> additive is proposed and investigated. Di fferent filler contents, namely two concentrations of 5 and 10 wt.% of the CaTiO3−<sup>δ</sup> additive, with respect to the dry Nafion content, were considered. The membrane with the lower amount of additive displayed better properties in terms of both water affinity and conductivity. Indeed, our results sugges<sup>t</sup> that a too-high content of additive can be detrimental for these particular properties. However, our results indicate that perovskite oxides can be useful as a water-retention and reinforcing additive in low-humidity proton-exchange membranes.

**Author Contributions:** Conceptualization, L.M., O.P., and M.A.N.; methodology, O.P., M.A.N., A.M., and A.P.; formal analysis, L.M., F.T., and K.E.; investigation, L.M., F.T., and K.E.; writing—original draft preparation, L.M., O.P., M.A.N., and A.M.; writing—review and editing, all the authors.

**Funding:** A.M. and K.E. acknowledge funding from the Swedish Foundation for Strategic Research (SSF, FFL-6 program, gran<sup>t</sup> no FFL15-0092) and from the Knut & Alice Wallenberg Foundation (Academy Fellows program, gran<sup>t</sup> no. 2016–0220).

**Acknowledgments:** L.M. acknowledges Erasmus+ Programme—Student Mobility for Traineeship—Project Unipharma-Graduates 2018/2019 of Sapienza University of Rome for the travel gran<sup>t</sup> allowing the traineeship at Chalmers University of Technology, Gothenburg, Sweden.

**Conflicts of Interest:** The authors declare no conflicts of interest.
