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catalysts Article Investigation of Gas Diffusion Electrode Systems for the Electrochemical CO2 Conversion Hilmar Guzmán 1,2,*,† , Federica Zammillo 1,† , Daniela Roldán 1, Camilla Galletti 1 , Nunzio Russo 1 and Simelys Hernández 1,2,* Citation: Guzmán,H.;Zammillo,F.; Roldán, D.; Galletti, C.; Russo, N.; Hernández, S. Investigation of Gas Diffusion Electrode Systems for the Electrochemical CO2 Conversion. Catalysts2021,11,482. https:// doi.org/10.3390/catal11040482 Academic Editor: Bruno Fabre Received: 13 February 2021 Accepted: 6 April 2021 Published: 9 April 2021 1 2 * Correspondence: hilmar.guzman@polito.it (H.G.); simelys.hernandez@polito.it (S.H.) † These authors contributed equally to this work. Abstract: Electrochemical CO2 reduction is a promising carbon capture and utilisation technology. Herein, a continuous flow gas diffusion electrode (GDE)-cell configuration has been studied to convert CO2 via electrochemical reduction under atmospheric conditions. To this purpose, Cu-based electrocatalysts immobilised on a porous and conductive GDE have been tested. Many system variables have been evaluated to find the most promising conditions able to lead to increased production of CO2 reduction liquid products, specifically: applied potentials, catalyst loading, Nafion content, KHCO3 electrolyte concentration, and the presence of metal oxides, like ZnO or/and Al2O3. In particular, the CO productivity increased at the lowest Nafion content of 15%, leading to syngas with an H2/CO ratio of ~1. Meanwhile, at the highest Nafion content (45%), C2+ products formation has been increased, and the CO selectivity has been decreased by 80%. The reported results revealed that the liquid crossover through the GDE highly impacts CO2 diffusion to the catalyst active sites, thus reducing the CO2 conversion efficiency. Through mathematical modelling, it has been confirmed that the increase of the local pH, coupled to the electrode-wetting, promotes the formation of bicarbonate species that deactivate the catalysts surface, hindering the mechanisms for the C2+ liquid products generation. These results want to shine the spotlight on kinetics and transport limitations, shifting the focus from catalytic activity of materials to other involved factors. Keywords: gas diffusion electrode; CO2 reduction; electrocatalyst; copper; liquid fuels; mass trans- port limitations 1. Introduction Carbon dioxide (CO2) is an important trace gas in the Earth’s atmosphere, produced by natural processes and human activities (e.g., fossil fuels use as an energy source). With the aim of a transition towards the use of renewables energies, away from fossil fuels, CO2 can be regarded as a resource for beneficial processes. In such a context, several routes can be followed to obtain added-value products, namely: stochiometric, biochemical, photocatalytic, photoelectrochemical, electrochemical, and thermochemical. The last two might be deemed the more encouraging approaches to obtaining added-value products from CO2; however, improved reaction conditions and catalyst materials with high activity and stability are still required [1]. This research work is focused on the electrochemical route. Electrocatalysis represents a promising method to increase the penetration of re- newables into the fuels and chemicals industries, helping to close the carbon loop with carbon-neutral electricity sources. Hence, among the advantages, it offers a way to handle the growing world demand for resources, which greatly continues to depend on fossil fuels CREST Group, Department of Applied Science and Technology (DISAT), Politecnico di Torino, C.so Duca degli Abruzzi, 24, 10129 Turin, Italy; federica.zammillo@polito.it (F.Z.); daniela.roldan@polito.it (D.R.); camilla.galletti@polito.it (C.G.); nunzio.russo@polito.it (N.R.) Center for Sustainable Future Technologies (IIT@PoliTo), IIT—Istituto Italiano di Tecnologia, Via Livorno, 60, 10144 Turin, Italy Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Catalysts 2021, 11, 482. https://doi.org/10.3390/catal11040482 https://www.mdpi.com/journal/catalystsPDF Image | Gas Diffusion Electrode Systems for the Electro CO2 Conversion
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