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Catalysts 2021, 11, 482 4 of 19 Al is also present in the powder), which has 5-fold the surface area of the Cu-06 catalyst. Therefore, the incorporation of ZnO and Al2O3 metal oxides seems to be responsible for the increase in surface area of the Cu-based catalyst, with a significant impact in the case of the Al2O3 addition. The XRD patterns of the employed powders can be seen in Figure S2. The diffraction peaks on Cu-06 can be assigned to the monoclinic CuO crystalline phase. Similar, but broader, CuO diffraction peaks are noticed in the CuZ-06-03 and CuZA-06-03-01 catalysts, which also present the ZnO diffraction peaks. Indeed, the incorporation of Zn and Al seems to influence the growth of the final crystals, leading to 40% and 50% smaller CuO crystallite size, respectively (see values calculated from Scherrer equation reported in Table S3). The diffraction peaks relative to aluminium oxide are absent, indicating that it is probably present as an amorphous phase. Additionally, the Electrochemically Active Surface Area (ECSA) values of the elec- trodes were determined (see Supporting Information (SI) for calculation details). As reported in Table S4, the highest ECSA values were obtained for CuZ-06-03 and CuZA-06- 03-01 catalysts. This finding is ascribed to their smaller crystallites with respect to Cu-06 (see Table S3). 2.2. Electrochemical Behaviour of the Employed Catalysts The electrochemical characterisation of the Cu-based catalysts was performed in 1 M KHCO3 at room temperature and atmospheric pressure by cyclic and linear sweep voltammetry. The Cyclic Voltammetry (CV) was carried out at 30 mV s−1 with N2 to study the electrochemical behaviour of the catalyst in the working solution, which can be considered as blank voltammograms (see the red curve in Figure 2a). The electrocatalytic activity under N2 flow can be attributed to the HER or the reduction of the catalyst [19]. In view of the investigations of hydrogen adsorption conducted on palladium and platinum catalysts, the redox peaks appreciated for Cu-06, in Figure 2a, at around −1 V vs. Ag/AgCl may be ascribed to the formation of adsorbed hydrogen [20,21]. On the other hand, the observed peaks can be assigned to reduce Cu+2 to Cu+1 or Cu0 [22]. Considering the voltammograms recorded under CO2 flow, a reduction peak appeared at a lower applied potential of ca. −0.5 V vs. Ag/AgCl, which can be attributed to the electron transfer process associated with the adsorbed intermediates during the CO2 reduction, as previously reported by Hori et al. [23]. Slightly higher activity is observed in the presence of CO2 than in N2, in terms of current densities measured at the same potential values (starting from ca. −1.5 V vs. Ag/AgCl). This improved result may be associated to the electrochemical CO2 reduction. In this regard, reference is made to C-containing products analysis results (see Section 2.3), which confirm this hypothesis. On the other hand, a mass transport phenomenon can be appreciated through the entwined curve over a particular potential range (between −1.625 and −1.875 V vs. Ag/AgCl) in Figure 2a. It is a negative differential resistance (NDR) behaviour (increased resistance is obtained with increasing voltage), typically observed in conventional semiconductor materials [24]. Besides, the capacitive behaviour of the curves may be attributed to charges accumulation. Figure 2b displays the Linear Sweep Voltammetry (LSV) results under CO2 flow for all the studied Cu-based electrocatalysts. A lower onset potential is noticed for the catalysts containing ZnO and Al2O3, which confirms the role of these metal oxides in promoting catalytic activity. At higher applied potentials, the catalytic activity does not appear to increase, although the CuZ-06-03 shows a current density value that is ~5 mA cm−2 greater than the other two catalysts, the maximum applied potential of −2 V vs. Ag/AgCl. The maximum activity observed in these cases (reduction currents < 40 mA cm−2) may be attributed to mass transfer limitations. In fact, the CuZA catalyst is capable of generating higher reduction current densities (~90 mA cm−2) when there are no mass transfer lim- itations, as it can be observed in the LSV carried out on this catalyst in a Rotating Disk Electrode (RDE) System (see Figure S5, Supporting Information (SI)).PDF Image | Gas Diffusion Electrode Systems for the Electro CO2 Conversion
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