σ(PDA50) > σ(PDA25) > σ(PDA10)

which indicates that the conductivity increases with decreasing the CeO2 content in the filler. Moreover, going from 80 to 110 ◦C, the conductivity evolution depends also on the CeO2 mass fraction in the filler in such a way that it increases in the presence of PDA50@CeO2 but keeps nearly constant in the presence of PDA10@CeO2.

**Figure 7.** Conductivity as a function of relative humidity, at 80 and 110 ◦C, for composite Aquivion membranes containing 5 wt% PDAx@CeO2. The conductivity of bare Aquivion is also reported.

A similar trend was already reported for Aquivion/CeO2 composite membranes in a recent work [16] where it was shown that the increase in temperature favors the acid-base reaction between cerium oxide and ionomer protons thus causing an IEC decrease which, depending on CeO2 content, offsets the expected increase in conductivity.

To get insight into the dependence of the conductivity on CeO2 loading, the conductivity of the PDAx@CeO2 composite membranes is plotted in Figures 8 and 9 as a function of CeO2 wt% in the membrane at constant temperature (80 and 110 ◦C) and RH (50 and 90%). For comparison, the conductivity of composite Aquivion membranes filled with bare CeO2 and Bz@CeO2 is also reported [16]. At 80 ◦C, the conductivity of the composite membranes filled with PDAx@CeO2 is weakly dependent on CeO2 loading and is close to the conductivity of Aquivion. On the other hand, at 110 ◦C, the composite membranes become progressively less conductive with increasing of the CeO2 loading so that the membrane with 5.4 wt% CeO2 is by a factor of about 2.5 less conductive than Aquivion both at 50 and 90% RH.

**Figure 8.** Conductivity as a function of CeO2 loading, at 80 ◦C, for composite Aquivion membranes filled with PDAx@CeO2 (x = 10, 25 and 50), as well as with the physical mixture PDA/CeO2 (see text). The conductivity of composite Aquivion membranes containing bare CeO2 and Bz@CeO2 (redrawn from ref. [16]) is reported for comparison.

**Figure 9.** Conductivity as a function of CeO2 loading, at 110 ◦C, for composite Aquivion membranes filled with PDAx@CeO2 (x = 10, 25 and 50), as well as with the physical mixture PDA/CeO2 (see text). The conductivity of composite Aquivion membranes containing bare CeO2 and Bz@CeO2 (redrawn from ref. [16]) is reported for comparison.

The CeO2 loading being the same, the conductivity of the PDAx@CeO2 membranes is similar to the conductivity of the Bz@CeO2 membranes except for 110 ◦C and 50% RH, where the PDAx@CeO2 membranes are more conductive by a factor of ~2 at the highest CeO2 loadings.

Moreover, the PDAx@CeO2 membranes are always more conductive than the corresponding membranes filled with bare CeO2 and the difference in conductivity increases with decreasing RH and with increasing filler loading and temperature. As a consequence, at 110 ◦C and 50% RH, the conductivity of the membrane with PDA10@CeO2 containing 4.5 wt% CeO2 is by one order of magnitude higher than that of the corresponding membrane containing bare CeO2.

It was of interest to prove that the better conductivity of the PDAx@CeO2 membranes, in comparison with the corresponding membranes loaded with bare CeO2, is indeed due to the presence of the PDA shell on the CeO2 surface. To this end, a composite Aquivion membrane containing the same amount of PDA and CeO2 as the membrane loaded with 5 wt% PDA10@CeO2 (i.e., 0.5 wt% PDA and 4.5 wt% CeO2) was prepared by mixing the Aquivion dispersion with a physical mixture of PDA and bare CeO2. The conductivity of this membrane, determined at 50 and 90% RH, first at 80 ◦C and then at 110 ◦C (the asterisk in Figures 8 and 9), was always lower than the conductivity of the membrane loaded with 5 wt% PDA10@CeO2, being in three cases even coincident with the conductivity of the membrane containing 4.5 wt% bare CeO2. These results show that it is the PDA coating that efficiently protects the cerium oxide particles against the acidic sulfonic groups of the ionomer, thus avoiding to a large extent the severe conductivity drop occurring with bare CeO2.

To evaluate the membrane resistance towards radical species generated by the decomposition of hydrogen peroxide, ex situ degradation tests were performed by treating the composite membranes with the Fenton solution (see Experimental section). The results of these tests are expressed in terms of fluoride emission rate, FER, defined as the ratio between the mass of released fluoride ions and the initial mass of the anhydrous membranes. Figure 10 shows the FER values of the PDAx@CeO2 composite membranes as a function of CeO2 loading and, for comparison, the FER values of composite Aquivion membranes filled with bare CeO2 and Bz@CeO2. The FER values obtained with PDAx@CeO2 are significantly lower than those of Bz@CeO2. Like Bz, the PDA coating prevents to a large extent the decrease in conductivity, but, unlike Bz, it does not compromise the radical scavenger activity of CeO2. The radical scavenger efficiency of PDAx@CeO2 is nearly coincident with that of membranes loaded with bare CeO2, for CeO2 percentage up to 4 wt%, and similar to that for higher loadings: a FER value of 10−<sup>3</sup> is indeed obtained with

5.4 wt% of PDA coated CeO2 and with 4.7 wt% of bare CeO2. It can also be observed that the membranes loaded with 5 wt% PDA50@CeO2 and with 3 wt% PDA10@CeO2 have the same FER and close CeO2 content (2.5 and 2.7 wt%, respectively) but very different PDA content (2.5 and 0.3 wt%, respectively). Thus, the radical scavenger properties of PDAx@CeO2 are mainly dependent on the CeO2 weight percentage, while the PDA coating does not shield significantly the radical scavenger activity of CeO2.

**Figure 10.** Fluoride emission rate (FER) as a function of CeO2 content for composite Aquivion membranes filled with PDA10@CeO2, PDA25@CeO2 and PDA50@CeO2. The FER of composite Aquivion membranes containing bare CeO2 and Bz@CeO2 (redrawn from ref. [16]) is reported for comparison.

After the Fenton test, the membranes with 3 wt% PDAx@CeO2 and the membrane with 6 wt% PDA10@CeO2 were washed with 1 M HCl and water, dried at 120 ◦C and weighed. The percentage weight loss concerning the initial weight of the anhydrous membrane (Table 2) decreases with increasing the CeO2 loading in the composite membrane, going from 25.4% for the membrane with 3 wt% PDA50@CeO2 to 5.9% for the membrane with 6 wt% PDA10@CeO2.

**Table 2.** Dry weight and conductivity percentage changes (%Δw and %Δσ) for bare Aquivion and Aquivion composite membranes loaded with PDAx@CeO2 after the Fenton test.


The conductivity of the aged membranes was also determined at 80 ◦C and 90% RH (Table 2). The percentage decrease in conductivity with respect to the initial membrane conductivity reflects qualitatively the trend of the weight changes. Thus, both the weight and conductivity of the aged membranes are consistent with the FER data.

Based on the results of ex situ characterization, the membrane loaded with 6 wt% PDA10@CeO2 (hereafter AQ-PDA@CeO2) was selected for MEA realization and in situ characterized by OCV stress tests. Hydrogen crossover measurements were carried out before and during the stress tests after 24 and 47 h from the beginning to check the stability of the membrane. Figure 11 shows the OCV vs. time curves for the bare Aquivion membrane (AQ) and a composite membrane containing 5 wt% CeO2 (AQ-CeO2). In all cases, after a non-linear drop during the first 3 h, the cell potential decays linearly as a function of time. The same trend is also observed in the time interval between hours 24 and 47 for AQ-PDA@CeO2 and AQ after the second hydrogen crossover determination (AQ-CeO2 stopped working).

**Figure 11.** OCV vs. time curves for AQ, AQ-PDA@CeO2 and AQ-CeO2 membranes.

The OCV decay rate in the linear regions (Table 3) is lower for AQ-PDA@CeO2 than for AQ both in the first and in the second time interval of the stress test; as a consequence, the overall OCV decay is ~15% for AQ-PDA@CeO2 and ~25% for AQ. Moreover, during the first interval, the decay rate of AQ-PDA@CeO2 and AQ@CeO2 is similar, thus confirming that the PDA coating does not compromise the radical scavenger activity of CeO2.


**Table 3.** OCV decay rate for the indicated membranes at 80 ◦C and 50% RH.

Consistently with the evolution of the OCV decay rate, the increase in the hydrogen crossover during the stress test (Table 4) is much larger for AQ (~23 times) than for AQ-PDA@CeO2 (~2 times).

**Table 4.** Hydrogen crossover for the indicated membranes at 80 ◦C and 50% RH.

