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Catalysts 2021, 11, 482 16 of 19 2 2− ∂ CO3 − − 2− 0=DCO2− 2 +k2f HCO3 OH −k2r CO3 , (7) 3 ∂x where Di terms account for species diffusivity, kir/kif account for the rate constants of forwardandreversereactions,andRxnCO2 andRxnOH− accountfortheconsumptionof CO2 and production of OH−, respectively. By implementing this non-linear system of ordinary differential equations (ODE) in Matlab, it is possible to obtain the trend of species concentrations in relation to the distance from the electrode surface and to estimate the surface (local) pH (see Supporting Information (SI) for calculation details). 4. Conclusions The electrochemical conversion of CO2 to added-value products was investigated in a gas diffusion electrode-based system. The GDE was fabricated by depositing different Cu-based catalytic inks on a porous conductive support through an airbrush, which was used to disperse the catalyst well and minimise the agglomeration of particles. Different products were obtained during the CO2 reduction process, specifically: hydrogen, carbon monoxide, formate, methanol, acetone, ethanol, 1-propanol, 2-propanol, and other com- pounds. Depending on the type of catalyst and the operating conditions, specific products were obtained in greater quantities than others. At the lowest applied potential, CO and formate formation was promoted on a CuO-based catalyst (Cu-06). The variation of the catalyst loading and Nafion content on the Cu-06 GDE structure affected the electrode per- formance. At the lowest Nafion ionomer loading (15%), the electrochemical CO2 reduction to CO drastically increases, achieving an H2/CO ratio of ~1, a suitable feedstock for further ethanol synthesis. On the contrary, with a Nafion content of 45%, the selectivity towards CO decreased by 80%, while the production of ethanol, 1-propanol, and 2-propanol was increased (FE < 5%). From these findings, it emerged that C-C coupling was not promoted at high overpo- tentials, as one would instead expect from literature and from primary experimental data in liquid-phase conditions. On the contrary, higher applied potentials seemed to promote the HER, as a consequence of mass-transport limitations occurring throughout the porous structure of the electrode, where the triple-phase boundary (TPB, catalyst-CO2-electrolyte) should be formed. This research shows that the liquid crossover severely impacts the GDE performance, which is affected by electrode-wetting. At high applied potentials, a higher flow rate of liquid crossing the GDE may be responsible for a worsening of the CO2 diffusion towards the active sites of the catalyst, hampering its conversion and favouring the hydrogen production. Further efforts to investigate the influence of the GDE struc- ture on electrolyte flooding are needed to support the optimisation of this challenging co-electrolysis system and move towards its industrial deployment. Moreover, the role of the pH on the CO2 reduction process was also studied: a higher pH on the electrode surface may promote the CO2 hydration to carbonate and bicarbonate species, thus lowering the CO2 concentration at the TPB, causing the deactivation of the catalyst and hindering the mechanisms for C2+ liquid products formation. As a general outcome, gaseous products were favoured over the liquids of most interest. It reflects the complexity of C2+ reaction pathways and the need to optimise the GDE-based co-electrolysis system further to achieve high current densities and Faradaic efficiencies (FE) towards liquid CO2 reduction products. Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/catal11040482/s1, Figure S1: Schematic concepts of (a) H-type cell, (b) two compartments cell, (c) Gas Diffusion Electrode cell and (d) Membrane Electrode Assembly for electrochemical CO2 reduction reactions, Figure S2: XRD patterns of CuZnAl-oxide based catalysts (a) Cu-06, (b) CuZ- 06-03, (c) CuZA-06-03-01, Figure S3: High resolution O1s (a) and Zn2p3/2 (b) XPS spectra of CuZ-06-03 and CuZA-06-03-01 catalysts and in the table the percentage of oxidation states of copperPDF Image | Gas Diffusion Electrode Systems for the Electro CO2 Conversion
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