catalysts Article Investigation of Gas Diffusion Electrode Systems for the Electrochemical CO 2 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 CO 2 Conversion. Catalysts 2021, 11, 482. https:// doi.org/10.3390/catal11040482 Academic Editor: Bruno Fabre Received: 13 February 2021 Accepted: 6 April 2021 Published: 9 April 2021 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/). 1 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.) 2 Center for Sustainable Future Technologies (IIT@PoliTo), IIT—Istituto Italiano di Tecnologia, Via Livorno, 60, 10144 Turin, Italy * Correspondence: hilmar.guzman@polito.it (H.G.); simelys.hernandez@polito.it (S.H.) † These authors contributed equally to this work. Abstract: Electrochemical CO 2 reduction is a promising carbon capture and utilisation technology. Herein, a continuous flow gas diffusion electrode (GDE)-cell configuration has been studied to convert CO 2 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 CO 2 reduction liquid products, specifically: applied potentials, catalyst loading, Nafion content, KHCO 3 electrolyte concentration, and the presence of metal oxides, like ZnO or/and Al 2 O 3 . In particular, the CO productivity increased at the lowest Nafion content of 15%, leading to syngas with an H 2 /CO ratio of ~1. Meanwhile, at the highest Nafion content (45%), C 2+ 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 CO 2 diffusion to the catalyst active sites, thus reducing the CO 2 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 C 2+ 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; CO 2 reduction; electrocatalyst; copper; liquid fuels; mass trans- port limitations 1. Introduction Carbon dioxide (CO 2 ) 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, CO 2 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 CO 2 ; 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 Catalysts 2021, 11, 482. https://doi.org/10.3390/catal11040482 https://www.mdpi.com/journal/catalysts