Electrocatalytic reduction of CO2 to value-added products

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CO2 valorisation towards alcohols by Cu-based electrocatalysts: challenges and perspectives

Developing efficient technologies to decrease CO2 emissions and dealing with climate change issues are among the most critical challenges in worldwide research. This review discusses the most recent advances on the electrochemical transformation of CO2 to alcohols, mainly methanol, ethanol and n-propanol, as a promising way to produce renewable liquid fuels. The main focus is given to copperbased electrocatalyst with different structures (Cu nanoparticles, oxide-derived Cu, and Cu composites) because Cu is up to now the heterogeneous catalyst with the most relevant activity for producing valuable C1+ hydrocarbons and alcohols via CO2 co-electrolysis. Several factors that impact the reaction activity and selectivities, such as the catalyst morphology, composition, surface structure, electrolyte effects and the electrocatalytic cell design (including liquid-phase and catholyte-free systems) are considered and analysed. This review reports an overview of the state-of-the-art with the most recent investigation highlights. It aims to provide guidance on the best experimental practices, new research directions, and strategies to develop efficient electrocatalysts. An outlook about the main challenges to be still resolved for a future practical application of this technology is also provided, toward a future based on sustainability and independence from fossil fuels

Strategies for improving GDE performance by a uniform dispersion of catalyst nanoparticles and an optimal Nafion content

In the context of the strategies needed to mitigate CO2 emissions and combat climate change, the electrochemical CO2 reduction represents a promising alternative. Among the different reactors, GDE-based ones are widely studied systems: here, the limitations shown by configurations with CO2 dissolved in electrolyte solutions can be overcome by feeding CO2 directly in gaseous form. In this work, the manufacturing process of the Cu-based gas diffusion electrode, namely the catalytic ink deposition on a porous carbon paper support, was carried out both by airbrushing (manual) and by spray-coating (automated) techniques. The characterization of the electrodes was performed by using X-ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM) techniques. To assess electrodes behavior, cyclic and linear sweep voltammetry techniques were conducted. When comparing the achieved current densities at the highest applied potential, the electrode obtained with the spray coater displayed a better electrocatalytic activity (~10 mA/cm2 higher at about -2.25 V vs Ag/AgCl), with respect to that fabricated with the airbrush. A thorough study of the GDEs performance was accomplished, testing the so obtained electrodes and thereby evaluating the effect of a variation of Nafion content in the productivity and selectivity results toward the desired products. The catalyst layer dispersion is a critical aspect of electrochemical CO2 reduction and, confirming previous studies on the different deposition methods, a more uniform distribution of the catalyst particles enabled the spray coated GDEs to outperform the hand-made ones. Furthermore, the variation of Nafion content on the GDE structure had a relevant effect on the electrode performance, allowing to considerably reduce the side-production of hydrogen and increasing at the same time the CO generation

Investigation of Gas Diffusion Electrode systems for the electrochemical CO2 conversion

In the context of climate change and carbon management, electrochemical CO2 reduction represents a promising solution. In this study, the electrochemical conversion of CO2 under atmospheric conditions has been performed in a continuous flow gas diffusion electrode (GDE)-based cell configuration. A porous and conductive support has been employed to this end, where a Cu-based catalyst has been manually deposited in a GDE by means of an airbrusher. With the aim to increase the production of CO2 reduction liquid products, several variables of the studied system have been assessed. The most promising conditions have been explored among the applied potential, catalyst loading, Nafion content, KHCO3 electrolyte concentration and the presence of metal oxides, like ZnO or/and Al2O3. In particular, it has been found that the binder content has affected the production of CO, leading to syngas with a H2/CO ratio of ⁓1 at the lowest Nafion content (15%). In contrast, the highest Nafion content of 45% has led to an increase of C2+ products formation and a decrease of CO selectivity by 80%. The obtained results revealed that liquid crossover affects the GDE performance by severely compromising the CO2 transport to the active sites of the catalyst, thus reducing the CO2 conversion efficiency. A mathematical model confirmed the role of a high local pH, combined with electro-wetting, in promoting the formation of bi-carbonate species: salts formation may cause the catalyst deactivation and hinder the mechanisms for C2+ liquid products. The ultimate intent of this work is to direct the attention of the scientific community to other involved factors of the CO2 reduction process rather than the catalytic activity of the materials, which can impact on both kinetics and mass transport and in turns on the final efficiency of this kind of devices

Standardization of Cu2O nanocubes synthesis: Role of precipitation process parameters on physico-chemical and photo-electrocatalytic properties

A facile, reproducible, and scalable wet precipitation method was optimized to synthetise Cu2O nanocubes with tuneable morphology and photocatalytic properties. The synthesis process was standardized by controlling the flow rate of addition of the reducing agent. This allowed to control the Cu2O crystallites size, which decreased from 60 nm to 30 nm by increasing the L-ascorbic acid flow rate, while maintaining a high yield (ranging from 87% to 97%) and reproducibility, as confirmed by X-Ray diffraction, scanning electron microscopy, and X-Ray photoelectron spectroscopy analyses. Moreover, the role of the synthesis conditions on the Cu2O nanocubes specific surface area and electrochemical surface area (ECSA) were investigated and correlated to their photo-electrocatalytic activity for the reduction of water and CO2 under ambient conditions, on electrodes made by air brushing. Decreasing of the Cu2O crystallites size enhanced the photo-electrocatalytic activity most probably due to a superior surface area, ECSA and an optimum valence and conduction band positions, which improves the charge transfer properties of the photocatalyst. The here proposed methodology and outcomes are very promising for the scale-up of the precipitation synthesis, not only of Cu2O but also of other nanostructured metal oxides to be exploited as photo-catalysts for environmental and energy applications

Identifying Promising Ionic Liquids for Electrochemical CO2 Reduction

Electrochemical CO2 reduction (CO2R) is a promising technology to reduce CO2 atmospheric concentrations by simultaneously storing renewable energy and generating high added-value products.1 Among the many possible reaction products, the generation of syngas, i.e. a mixture of carbon monoxide and hydrogen, is particularly considered as it requires low energetic consumption, yet this product ensures a broad market share.2 Such process usually occurs on weak CO binding catalysts, such as Au and Ag,3 and it can be particularly enhanced using ionic liquids (IL) as co-catalysts in the electrolyte.4,5 Earlier computational studies indicate that ionic liquids can either stabilize the CO2 adsorbate via electric interaction 6 or poison the electrocatalytic surface,7 thus blocking CO2 reduction and enhancing water reduction. In our group, we recently carried out a systematic assessment of the role of different EMIM+/BMIM+-based ionic liquids for the electrochemical reduction of CO2 on silver electrodes. Such study resulted in a joint experimental/modeling work,8 where some of us demonstrated that IL anions tune the ratio between the concentration of cations (EMIM+ or BMIM+) and the carbene species (EMIM:/BMIM:) in the electrolyte. Such effect can be rationalized by using few thermodynamic descriptors, such as the formation energy of EMIM:/BMIM: species, their adsorption energy, and their ability to trap CO2 in solution. Consequently, the ratio of cations and carbenes rules the CO2 capture and electrochemical conversion properties of imidazolium based ILs. Herein, we carried out a follow up of the previous study,8 generalizing the previously suggested descriptors to provide predictive guidelines for experimental synthesis. Screening among different IL, we confirmed that the formation energy of EMIM:/BMIM: species is the primary driving force for enhancing water reduction. In fact, such carbenes either increase the local availability of protons to sustain hydrogen evolution (HER) or block the surface, hindering adsorption of CO2 at the surface and thus allowing only HER to occur. Such surface blocking effect was further confirmed by in-house measurements of electrochemically active surface areas (ECSA) in presence of different IL. In addition to surface poisoning, EMIM:/BMIM: can also trap CO2 in solution, further hindering CO2 reduction. Among the considered anions, acetate anion determines the lowest energy for EMIM+/BMIM+ deprotonation, consequently leading to high H2 partial current densities and low ECSA values. Instead, triflate anion prevents the formation of carbenes and thus hinders any surface poisoning effect, enabling Faradaic efficiency toward CO behind 90%. Overall, by generalizing the insights from the previous work,8 we here provide guidelines to identify the best ionic liquids out of simple thermodynamic properties. By extrapolating our results to IL not yet tested, it is possible to predict HER and CO2R activities on silver, thus enabling a direct pathway for the experimental design of IL for CO2R

Facile and scalable synthesis of Cu2O-SnO2 catalyst for the photoelectrochemical CO2 conversion

CO2 conversion into high-value-added products is becoming increasingly attractive to find substitutes for fossil-based ones and tackle the environmental crisis. Herein, a noble, simple, reproducible, and scalable Cu2O-SnO2 photo-electrocatalyst was synthesized and characterized. Coupling cuprous oxide with tin oxide allowed for protecting unstable Cu+1 species from photo-corrosion. Evidence of the SnO2 stabilization role were found via chronoamperometry tests under chopped light and XPS analysis. An optimized catalytic ink was developed to prepare the photocathodes. The CO2 photo-electroreduction tests demonstrated a prevalent production of CO and formate with Faradaic efficiencies of 35.47 % and 19.58 %, respectively, and a good system stability. Sunlight illumination demonstrated to play a major role to hinder H2 evolution and promote ≥C1+ products formation

Cu2O/SnO2 Heterostructures: Role of the Synthesis Procedure on PEC CO2 Conversion

Addressing the urgent need to mitigate increasing levels of CO2 in the atmosphere and combat global warming, the development of earth-abundant catalysts for selective photo-electrochemical CO2 conversion is a central and pressing challenge. Toward this purpose, two synthetic strategies for obtaining a Cu2O–SnO2 catalyst, namely co-precipitation and core–shell methods, were compared. The morphology and band gap energy of the synthesized materials were strongly different. The photoactivity of the core–shell catalyst was improved by 30% compared to the co-precipitation one, while its selectivity was shifted towards C1 products such as CO and formate. The stability of both catalysts was revealed by an easy and fast EIS analysis, indicating how the effective presence of a SnO2 shell could prevent the modification of the crystalline phase of the catalyst during PEC tests. Finally, directing the selectivity depending on the synthesis method used to produce the final Cu2O–SnO2 catalyst could possibly be implemented in syngas and formate transformation processes, such as hydroformylation or the Fischer–Tropsch process