*3.2. Electrocatalytic Performances*

The electrocatalytic activity for aerogels samples were evaluated using cyclic voltammetry (CV). First, electrochemical profiles were obtained in a 0.5 M H2SO4 aqueous solution at ambient conditions with a sweep rate of 20 mVs−<sup>1</sup> (Figure 7a). The peaks detected are attributed to (i) the hydrogen desorption in the 0.1–0.25 V range, (ii) hydrogen adsorption at 0.23 V, (iii) reduction of Pd (II) oxide at 0.65–0.75 V, and (iv) formation of Pd (II) oxide at 1–1.2 V. All these phenomena are present in the cyclic voltammograms of all Pd aerogels. However, the use of microwave radiation during any of the synthesis steps clearly improves the electrochemical activity of the materials.

**Figure 7.** *Cont*.

**Figure 7.** Electrochemical performance. (**a**) Pd aerogels profiles in 0.5 M H2SO4; (**b**) comparison between the aerogels in FAO; (**c**) stability performance on Pd-MMT vs. Pd/C; (**d**) PdA-MMT CV before and after the CA for 24 h; (**e**) FAO performance between PdA-CON, PdA-MMT, and Pd/C; and (**f**) MFC performance for the best aerogels obtained (PdA-MMT), a commercial catalyst (Pd/C) and an in-lab catalyst obtained by conventional procedure (PdA-CON).

To quantify this improvement, the electrochemical active surface area (*ECSA*) was evaluated on the electrode surface of each catalyst. The values of *ECSA* for the samples studied in this work were estimated from the cyclic voltammograms (i.e., Figure 7a) by using the reduction charge of Pd (II) oxide according to the following Equation (2):

$$ECSA = \frac{Q\_m}{m\_{Pd}ed\_m}$$

where, *Qm* denotes coulombic charge (*Q* per μCcm−2) for the reduction of Pd (II) oxide achieved by integrating the charges related to the reduction of Pd (II) oxide for the different samples; *mPd* is the mass amount of Pd loaded (g cm−2) on the GC electrode surface and *edm* is a constant (424 μC cm−2), which corresponds to the reduction of a Pd (II) oxide monolayer [37].

The *ECSA* values of the Pd aerogels samples depicted in Figure 7a are shown in Table 2. *ECSA* values for PdA-CON and the commercial Pd/C catalyst are also included for comparative purposes. As expected from the cyclic voltammograms, there is a great increase in the electrochemical active surface area for the samples prepared using MW. The lower particle size detected by TEM, the lower size of the crystals detected by XRD, and the higher content of the Pd0 evaluated by XPS, in samples obtained using microwave heating show clearly that this process has a huge impact on the electrochemical performance of the resulting Pd aerogels. In other words, by means of microwave heating instead of conventional heating for the synthesis of Pd aerogels, the innovation presented in this work, not only the is the processing time reduced but also the electrochemical behavior of these materials is notably enhanced.

**Table 2.** Electrochemical active surface area (*ECSA*) values for the samples studied.


This is further corroborated when performing the CV experiments in an electrolyte containing formic acid (i.e., Figure 7b). The evaluation of FAO was carried out in the same range of potential as the CV tests (0.0–1.4 V vs. RHE, see Figure 7a). Comparison of the electrochemical profiles with FAO curves recorded by GC electrodes clearly demonstrated that Pd aerogels offered strong peaks for the electro-oxidation at room temperature conditions [38]. The maximum current values during FAO occur at 0.4 V vs. RHE. Again, the effect of using microwave heating for the synthesis of the Pd aerogels clearly increases their activity in the electro-oxidation of formic acid. Thus, a maximum mass current (J) of 1750 mA mg−<sup>1</sup> was for the PdA-MMT sample; in the case of PdA-MM and PdA-MC materials, values of 1200 and 1190 mA mg−<sup>1</sup> were respectively obtained, being the lowest performance for a PdA-CC sample with 700 mA mg−1. On the other hand, it seems clear that increasing the temperature of the reduction step from 45 ◦C to 67 ◦C makes a difference in the formic acid oxidation activity (see Figure 7b).

As for the formic acid electro-oxidation mechanism with these Pd aerogels, the reaction occurs following two parallel paths, one giving rise to CO2 at reasonably low overpotentials through the so-called active intermediate and a chemical dehydration step leading to adsorbed CO, which will be oxidized to CO2 at higher potentials [39]. The peaks between 0.2 and 0.6 V for all Pd catalysts (Figure 7d) represent a direct pathway (HCOOH → COOHads/HCOOads + H+ + e<sup>−</sup> → CO2 + 2H+ + 2e−), while the peaks ranging from 0.7 to 0.9 V represent an indirect pathway (HCOOH → COads + H2O → CO2 + 2H+ + 2e−) [40]. Since the maximum electrochemical activity of all Pd aerogels studied was measured at ca. 0.4 V, the direct pathway is clearly favored in this case.

The best sample, PdA-MMT, obtained in this work was also compared with the Pd/C commercial catalyst (20 wt%) and PdA-CON in order to show the benefits of using the novel synthesis presented in this work (i.e., microwave heating during the synthesis and lyophilization as the drying procedure). To analyze the catalytic activity of PdA-MMT versus Pd/C, the maximum current intensity shown in Figure 7e should be compared. It can be seen that a current density near to 1900 mA mg−<sup>1</sup> was observed for the aerogel against almost 300 mA mg−<sup>1</sup> for Pd/C. Whilst in the case of the behavior of the PdA-MMT vs. the PdA-CON sample, a significant increase in electrochemical activity was detected for the PdA-MMT sample (see Figure 7e).

In order to characterize not only the activity but also the stability of the PdA-MMT sample, a test was carried out for 24 h on this sample and the commercial catalyst Pd/C (Figure 7c,d). The results reveal that the great activity of the sample obtained in this work (PdA-MMT) is totally maintained and stable with time.
