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Catalysts 2021, 11, 482 2 of 19 and will be severely affected in the near future [2]. Due to the modular design, the scale-up of electrolysers for CO2 conversion can be accomplished by stacking individual cells. The main advantage of the electrochemical processes is water as a hydrogen source, which oth- erwise is produced with conventional technologies using fossil fuels. The water is present either in electrolyte solutions or humidified feed streams and is fundamental for the in-situ protons (H+) generation to convert CO2 into chemicals or fuels. The electrocatalytic process can be controlled by the used electrode potential and operative conditions: it enables oper- ation under mild conditions like temperature below 100 ◦C and pressure below 10 bars [3]. Nevertheless, some obstacles hinder the benefits of the electrochemical conversion of CO2; among these: large overpotentials, low exchange current densities, low selectivity, deac- tivation of electrodes, dependence on reaction environment. Despite its complexity, this innovative process has attracted considerable attention from researchers. Until some years ago, researchers focused their work on observing the effects of surface modification (e.g., nano-structuring and surface tailoring) on catalyst selectivity and activity. In this regard, like H-cell, a traditional configuration has been utilised as a platform for improvements (see Figure S1a). However, researchers have moved their attention towards continuous flow cells (see Figure S1b–d). Compared to their batch-type counterparts, continuous reactors have several benefits: among these, they allow to overcome mass-transport limitations and better control the residence time in the reactor [4]. In contrast to H-cell, the flow-cell set-up consists of different compartments. Thin layers constitute each compartment with a specific design, from whose overlapping gaseous and liquid pathways are defined. Due to the low solubility of CO2 and mass transfer limitations from the bulk to the electrode surface, current densities in aqueous-fed systems (where CO2 is dissolved in the electrolyte) are limited to 35–40 mA cm−2 [5]. It has prompted researchers toward gas-diffusion layer based-systems (where the catalyst is placed on porous conductive support) to supply gaseous CO2 directly to the catalyst layer, to overcome the main limits of liquid reduction processes, and to make the upscaling possible: shorter diffusion path- way and a higher CO2 concentration can lead to commercially-relevant current densities (≥100 mA cm−2). However, there is a trade-off between high energetic efficiencies and Faradaic efficiencies (FEs): Vennekoetter et al., who focused on the importance of the reac- tor design for a more energy efficient CO2 conversion, concluded that a zero-gap assembly (Membrane Electrode Assembly (MEA) or cell-like fuel system) would be theoretically the perfect electrochemical reactor [3]. Regarding the local environment, it has been evidenced that this parameter may alter the reaction course. Thus, it affects the catalytic activity, as well as influences the catalyst surface modification [5]. Several studies carried out either on Gas Diffusion Electrode- based (GDE) set-ups or in H-cell reported large pH shifts at the electrode surface during CO2 electroreduction. It has prompted the scientific community to pay attention to the role of pH in electrolysis since changes of pH close to the catalytic surface have been recognised to have a similar effect to that of surface modification on determining the reaction pathways [5]. It is known that the pH near the electrode surface, referred to as “local pH”, is usually higher than that measured in the bulk electrolyte due to the production of OH− ions (see Table S1) [6]. The high local pH might play a dual role [7]: on the one hand, favouring the production of carbonate and bicarbonate species (HCO32−, CO32−), which, in turn, tend to decrease the pH and reduce the concentration of CO2 at the catalytic surface [7,8], and on the other hand affects the surface coverage by adsorbed intermediates, suppressing the hydrogen evolution reaction (HER) because of a limited transport of protons [7,9,10]. These mechanisms seem to be quite contradictory and do not help to strengthen the considerations made to date. In this regard, it would be helpful to develop tools for monitoring pH changes during the reaction. This work aims to offer an insight into the issues encountered when operating a GDE-cell configuration and show the reader how targeted changes may allow this system to overcome the limitations of operation with dissolved CO2 and pursue commercially- relevant current densities. The electrochemical CO2 conversion was here performed onPDF Image | Gas Diffusion Electrode Systems for the Electro CO2 Conversion
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