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

Hydrodynamics and Oxygen Bubble Characterization of Catalytic Cells Used in Artificial Photosynthesis by Means of CFD

Miniaturized cells can be used in photo-electrochemistry to perform water splitting. The geometry, process variables and removal of oxygen bubbles in these cells need to be optimized. Bubbles tend to remain attached to the catalytic surface, thus blocking the reaction, and they therefore need to be dragged out of the cell. Computational Fluid Dynamics simulations have been carried out to assess the design of miniaturized cells and their results have been compared with experimental results. It has been found that low liquid inlet velocities (~0.1 m/s) favor the homogeneous distribution of the flow. Moderate velocities (0.5–1 m/s) favor preferred paths. High velocities (~2 m/s) lead to turbulent behavior of the flow, but avoid bubble coalescence and help to drag the bubbles. Gravity has a limited effect at this velocity. Finally, channeled cells have also been analyzed and they allow a good flow distribution, but part of the catalytic area could be lost. The here presented results can be used as guidelines for the optimum design of photocatalytic cells for the water splitting reaction for the production of solar fuels, such as H2 or other CO2 reduction products (i.e., CO, CH4, among others)

Application of reverse micelle sol-gel synthesis for bulk doping and heteroatoms Surface Enrichment in Mo-Doped TiO 2 nanoparticles

TiO 2 nanoparticles containing 0.0, 1.0, 5.0, and 10.0 wt.% Mo were prepared by a reverse micelle template assisted sol-gel method allowing the dispersion of Mo atoms in the TiO 2 matrix. Their textural and surface properties were characterized by means of X-ray powder diffraction, micro-Raman spectroscopy, N 2 adsorption/desorption isotherms at -196 °C, energy dispersive X-ray analysis coupled to field emission scanning electron microscopy, X-ray photoelectron spectroscopy, diffuse reflectance UV-Vis spectroscopy, and ζ-potential measurement. The photocatalytic degradation of Rhodamine B (under visible light and low irradiance) in water was used as a test reaction as well. The ensemble of the obtained experimental results was analyzed in order to discover the actual state of Mo in the final materials, showing the occurrence of both bulk doping and Mo surface species, with progressive segregation of MoO x species occurring only at a higher Mo content

A model for electrode effects based on adsorption theory

A model to describe the electrode effects based on the adsorption theory is proposed. We assume that the coverage (i.e by gas bubbles, electrodeposition of compounds, etc) of the electrodes is governed by a kinetics equation where the adsorption term is proportional to the bulk current density, and the desorption term to the actual coverage. The adsorption can take place only on the uncovered part of the electrode. We show that the coverage is responsible for a variation of the interface properties of the electrode. The time dependence of the electric response of the cell, submitted to an external voltage, is determined by solving the differential equation for the coverage. We show that two regimes are expected. One, in the limit of small time, controlled by the charging of the surface interface, and one related to the coverage. The theoretical predictions are in reasonable agreement with the experimental data concerning the time dependence of the current and the current-voltage characteristics of a home-made photo-electrolyzer constituted by a BiVO4 photoanode and a Pt cathode. Moreover, a normalized current-voltage curve was obtained, which fit also literature data based on (i) electrolysis on cylindrical stainless-steel electrodes in NaOH electrolyte and (ii) electrolytic plasma nitrocarburizing of AISI 1020 steel discs in an Urea-based aqueous solution, demonstrating the versatility and broad range of application of the here proposed model

ZnO Materials as Effective Anodes for the Photoelectrochemical Regeneration of Enzymatically Active NAD+

This work reports the study of ZnO-based anodes for the photoelectrochemical regeneration of the oxidized form of nicotinamide adenine dinucleotide (NAD+). The latter is the most important coenzyme for dehydrogenases. However, the high costs of NAD+ limit the use of such enzymes at the industrial level. The influence of the ZnO morphologies (flower-like, porous film, and nanowires), showing different surface area and crystallinity, was studied. The detection of diluted solutions (0.1 mM) of the reduced form of the coenzyme (NADH) was accomplished by the flower-like and the porous films, whereas concentrations greater than 20 mM were needed for the detection of NADH with nanowire-shaped ZnO-based electrodes. The photocatalytic activity of ZnO was reduced at increasing concentrations of NAD+ because part of the ultraviolet irradiation was absorbed by the coenzyme, reducing the photons available for the ZnO material. The higher electrochemical surface area of the flower-like film makes it suitable for the regeneration reaction. The illumination of the electrodes led to a significant increase on the NAD+ regeneration with respect to both the electrochemical oxidation in dark and the only photochemical reaction. The tests with formate dehydrogenase demonstrated that 94% of the regenerated NAD+ was enzymatically active

Optimization of BiVO4 photoelectrodes made by electrodeposition for sun-driven water oxidation

In this work, the synthesis of cheap BiVO4 photoanodes for the photoelectrochemical water splitting reaction was optimized via the scalable thin film electrodeposition method. Factors affecting the photoelectrochemical activity, such as the electrodeposition time, the ratio of the Bi-KI to benzoquinone-EtOH in the deposition bath, and the calcination temperature, have been investigated by using the Central Composite Design of Experiments. Pristine monoclinic scheelite BiVO4 photoanodes having a photocurrent density of 0.45 ± 0.05mA/cm2 at 1.23 V vs RHE have been obtained. It was shown that a high photocurrent density is generally dictated by the following physico-chemical properties: a higher crystallite size, optimal thickness and a porous morphology, which give rise to a low charge transfer resistance, low onset potential and a high donor density. Moreover, to the best of our knowledge, this is the first report on the depth profile XPS analysis performed in BiVO4 photoanodes made by electrodeposition technique, from which it was concluded that the surface V species exist as V4+ while the bulk V species are V5+. The V4+ induces a higher amount of surface oxygen vacancies, which was found to be beneficial for the photoactivity

Electro-oxidation of phenol over electrodeposited MnOx nanostructures and the role of a TiO2 nanotubes interlayer

More and more attention has recently been paid to the electrochemical treatment of wastewater for the degradation of refractory organics, such as phenol and its derivatives. The electrodeposition of different types of manganese oxides (MnOx) over two substrates, namely metallic titanium and titania nanotubes (TiO2-NTs), is reported herein. X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS) analyses have confirmed the formation of different oxidation states of the manganese, while Field Emission Scanning Electronic Microscopy (FESEM) analysis has helped to point out the evolutions in the morphology of the samples, which depends on the electrodeposition parameters and calcination conditions. Moreover, cross section FESEM images have demonstrated the penetration of manganese oxides inside the NTs for anodically deposited samples. The electrochemical properties of the electrodes have been investigated by means of cyclic voltammetry (CV) and linear sweep voltammetry (LSV), both of which have shown that both calcination and electrodeposition over TiO2-NTs lead to more stable electrodes that exhibited a marked increase in the current density. The activity of the proposed nanostructured samples toward phenol degradation has been investigated. The cathodically electrodeposited manganese oxides (α-MnO2) have been found to be the most active phase, with a phenol conversion of 26.8%. The anodically electrodeposited manganese oxides (α-Mn2O3), instead, have shown higher stability, with a final working potential of 2.9 V vs. RHE. The TiO2-NTs interlayer has contributed, in all cases, to a decrease of about 1–1.5 V in the final (reached) potential, after a reaction time of 5 h. Electrochemical impedance spectroscopy (EIS) and accelerated life time tests have confirmed the beneficial effect of TiO2-NTs, which contributes by improving both the charge transfer properties (kinetics of reaction) and the adhesion of MnOx films

Syngas Production from Electrochemical Reduction of CO2: Current Status and Prospective Implementation

The CO2 that comes from the use of fossil fuels accounts for about 65% of the global greenhouse gas emission, and it plays a critical role in global climate changes. Among the different strategies that have been considered to address the storage and reutilization of CO2, the transformation of CO2 into chemicals or fuels with a high added-value has been considered a winning approach. This transformation is able to reduce the carbon emission and induce a “fuel switching” that exploits renewable energy sources. The aim of this brief review is to gather and critically analyse the main efforts that have been made and achievements that have been made in the electrochemical reduction of CO2 for the production of CO. The main focus is on the prospective of exploiting the intrinsic nature of the electrolysis process, in which CO2 reduction and H2 evolution reactions can be combined, into a competitive approach, to produce syngas. Several well-established processes already exist for the generation of fuels and fine-chemicals from H2/CO mixtures of different ratios. Hence, the different kinds of electrocatalysts and electrochemical reactors that have been used for the CO and H2 evolution reactions have been analysed, as well as the main factors that influence the performance of the system from the thermodynamic, kinetic and mass transport points of view