*3.2. Composite Membranes*

The IEC values (in milliequivalents per gram) of the composite membranes with 3 and 5 wt% PDAx@CeO2 loadings (Table 1) are at most by 4% lower than those calculated based on the ionomer weight percentage. In principle, this could be due to the protonation of nitrogen atoms of PDA and/or of surface oxide ions of CeO2. The fact that, for the same PDAx@CeO2 loading, the IECs show the sequence:

$$\text{IEC(PDA50)} > \text{IEC(PDA25)} > \text{IEC(PDA10)}$$

indicates that the protonation of the oxide ions is mainly responsible for the IEC decrease because the amount of cerium oxide in the filler is minimum for PDA50 and maximum for PDA10.

**Table 1.** IEC values (meq g−1) of PDAx@CeO2 composite membranes. The IEC values calculated based on the Aquivion wt% (AQcalc) are also reported.


Standard deviation = 0.0032.

The conductivity (σ) of membranes containing 3 and 5 wt% PDAx@CeO2 (with x = 10, 25 or 50), as well as the conductivity of a membrane containing 6 wt% PDA10@CeO2, was determined for RH increasing in the range 50–90%, first at 80 ◦C and then at 110 ◦C. In all cases, the plot of logσ as a function of RH is linear. As an example, Figure 7 displays the conductivity of composite membranes containing 5 wt% filler together with that of bare Aquivion. At both temperature and for each RH value, the following conductivity sequence is observed:
