**1. Introduction**

In recent years, the need to arrest the effects of climate change is pushing governments worldwide to plan and coordinate efforts to achieve a dramatic reduction in CO2 emissions. This requires a revolution in energy supply toward much more flexible renewable energy systems. Hydrogen offers several benefits for simultaneously decarbonizing transport, housing and industrial sectors. Among hydrogen-based technologies, proton exchange membrane (PEM) fuel cells have revealed promising for stationary and automotive applications. Because of the lifetime targets for large-scale stationary applications (≈40,000 h), as well as for automotive applications (>6000 h) [1], the proton exchange membrane durability represents a key element for the longevity of the device. However, several factors limit the membrane's long-term stability. A source of membrane degradation is due to chemical degradation caused by radical species such as H•, OH• and HOO• [2–6], which gives rise to a thinning of the membrane leading to short the lifetime of PEM fuel cells. A strategy to mitigate such radical attacks consists of the incorporation of radical scavengers.

**Citation:** D'Amato, R.; Donnadio, A.; Battocchio, C.; Sassi, P.; Pica, M.; Carbone, A.; Gatto, I.; Casciola, M. Polydopamine Coated CeO2 as Radical Scavenger Filler for Aquivion Membranes with High Proton Conductivity. *Materials* **2021**, *14*, 5280. https://doi.org/10.3390/ma14185280

Academic Editor: Arunas Ramanavicius

Received: 26 August 2021 Accepted: 10 September 2021 Published: 14 September 2021

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For example, the introduction of metal cations, such as Ce4+ and Mn2+, or their oxides, including CeO2 and MnO2, revealed to be effective in mitigating the chemical degradation of PFSA (PerFluoroSulfonic Acid) polymers [7–15] because, due to the multivalent oxidation state of the metals, they can act as catalysts for the decomposition of hydroxyl and hydroperoxyl radicals.

In a recent paper, it was reported that CeO2 nanoparticles dispersed in an Aquivion matrix undergo partial solubilization at relative humidity in the range 50–90%, when the temperature is increased from 80 to 110 ◦C [16]. As a consequence, for CeO2 loadings greater than 2 wt%, a decrease in the composite membrane conductivity was observed with increasing temperature, in such a way that the larger the CeO2 loading, the more severe the conductivity drop. It was also found that the formation of a protective shell on the oxide surface, made of fluorophosphonates bonded to cerium ions through the –PO3 groups, partially avoided the conductivity drop. However, the organically modified CeO2 nanoparticles show reduced radical scavenger activity in comparison with the pristine nanoparticles. A reasonable compromise between stable conductivity and improved membrane stability towards radical was reached by bonding a fluorobenzyl phosphonate (hereafter Bz) to the CeO2 surface. Based on these results and taking into account that the phosphonate can be hydrolyzed after long-term operation under conditions of high membrane hydration, it was of interest to coat the oxide surface with a polymeric film that could be hydrolytically more stable than the phosphonate coating.

To this aim, polydopamine (PDA, Scheme 1) was chosen for its strong and universal adhesion ability and the simple deposition process through self-polymerization in an alkaline aqueous solution [17–33].

**Scheme 1.** Structure of polydopamine.

The preparation of PDA-based materials has rapidly advanced in recent years with a significant expansion in their applications [21–34], becoming one of the most attractive areas within the materials field including surface modification, biosensing [35,36], nanomedicine [37] and systems for energy applications [38–43]. In particular, PDA allows obtainment of a beneficial and advantageous interface between CeO2 and PFSA improving the lifetime of PEM fuel cells [44].

This paper reports the formation of a PDA film on the surface of CeO2 nanoparticles by dopamine polymerization in a water suspension of CeO2, and the use of this composite material (PDA@CeO2) as a filler of membranes made of Aquivion.

Membranes containing 3 and 5 wt% filler loadings were characterized by conductivity measurements at 80 and 110 ◦C, in the RH range 50–90%, to test their stability at increasing temperature. These membranes were also subjected to accelerated ageing by using the Fenton reagent to assess the radical scavenger efficiency of the filler based on the fluoride emission rate (FER). Both conductivity and FER data collected in the present work are compared with the corresponding literature data for composite Aquivion membranes filled with pristine CeO2 and Bz@CeO2. The most stable membrane was also characterized by Open Circuit Voltage (OCV) stress tests coupled with hydrogen crossover determinations.

## **2. Materials and Methods**

#### *2.1. Materials*

Cerium (III) nitrate hexahydrate (Ce(NO3)3·6H2O) was from Carlo Erba. A 20 wt% Aquivion dispersion in water (D83-6A, ionomer equivalent weight = 830 g/equiv.) was kindly provided by Solvay Specialty Polymers, Italy. The citric acid (C6H8O7·H2O), dopamine and all other reagents were purchased from Sigma-Aldrich and used without purification.
