The electrochemical reduction of CO2 has gained significant interest recently as it has the potential to trigger a sustainable solar-fuel-based economy. In this Perspective, we highlight several ...heterogeneous and molecular electrocatalysts for the reduction of CO2 and discuss the reaction pathways through which they form various products. Among those, copper is a unique catalyst as it yields hydrocarbon products, mostly methane, ethylene, and ethanol, with acceptable efficiencies. As a result, substantial effort has been invested to determine the special catalytic properties of copper and to elucidate the mechanism through which hydrocarbons are formed. These mechanistic insights, together with mechanistic insights of CO2 reduction on other metals and molecular complexes, can provide crucial guidelines for the design of future catalyst materials able to efficiently and selectively reduce CO2 to useful products.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
The catalytic activity and hydrocarbon selectivity in electrochemical carbon dioxide (CO2) reduction on cuprous oxide (Cu2O) derived copper nanoparticles is discussed. Cuprous oxide films with 100, ...110 and 111 orientation and variable thickness were electrodeposited by reduction of copper(ii) lactate on commercially available copper plates. After initiation of the electrochemical CO2 reduction by these oxide structures, the selectivity of the process was found to largely depend on the parent Cu2O film thickness, rather than on the initial crystal orientation. Starting with thin Cu2O films, besides CO and hydrogen, selective formation of ethylene is observed with very high ethylene-to-methane ratios (∼8 to 12). In addition to these products, thicker Cu2O films yield a remarkably large amount of ethane. Long term Faradaic efficiency analysis of hydrocarbons shows no sign of deactivation of the electrodes after 5 hours of continuous experiment. Online mass spectroscopy studies combined with X-ray diffraction data suggest the reduction of the Cu2O films in the presence of CO2, generating a nanoparticulate Cu morphology, prior to the production of hydrogen, CO, and hydrocarbons. Optimizing coverage, number density and size of the copper nanoparticles, as well as local surface pH, may allow highly selective formation of the industrially important product ethylene.
The development of active, cost-effective and stable oxygen-evolving catalysts is one of the major challenges for solar-to-fuel conversion towards sustainable energy generation. Iridium oxide ...exhibits the best available compromise between catalytic activity and stability in acid media, but it is prohibitively expensive for large-scale applications. Therefore, preparing oxygen-evolving catalysts with lower amounts of the scarce but active and stable iridium is an attractive avenue to overcome this economical constraint. Here we report on a class of oxygen-evolving catalysts based on iridium double perovskites which contain 32 wt% less iridium than IrO2 and yet exhibit a more than threefold higher activity in acid media. According to recently suggested benchmarking criteria, the iridium double perovskites are the most active catalysts for oxygen evolution in acid media reported until now, to the best of our knowledge, and exhibit similar stability to IrO2.
•Formulation of a Pd-Pt electrocatalyst with small overpotential for CO2 reduction.•Reversible catalysis of CO2 to formic acid and back.•Bicarbonate reduction to formic acid.
The electrochemical ...reduction of CO2 is a reaction of much current interest as a possible reaction for energy storage. In this paper, we show that on electrodeposited palladium on platinum, a good formic acid oxidation catalyst, the onset potential for CO2 reduction to formic acid is dramatically reduced in comparison to bulk palladium. Two different reaction pathways are observed; a pathway at low overpotential in which formic acid is produced from either direct bicarbonate reduction or from the reduction of CO2 generated from bicarbonate near the surface, and a pathway at more negative potentials where formic acid is produced from direct CO2 reduction. Furthermore, we show that reversible formic acid oxidation and CO2 reduction is possible on this catalyst, although unfortunately the processes are hindered by poisoning of the catalyst, most likely by CO.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Electrochemical CO2 reduction has received an increased amount of interest in the last decade as a promising avenue for storing renewable electricity in chemical bonds. Despite considerable progress ...on catalyst performance using nanostructured electrodes, the sensitivity of the reaction to process conditions has led to debate on the origin of the activity and high selectivity. Additionally, this raises questions on the transferability of the performance and knowledge to other electrochemical systems. At its core, the discrepancy is primarily a result of the highly porous nature of nanostructured electrodes, which are vulnerable to both mass transport effects and structural changes during the electrolysis. Both effects are not straightforward to identify and difficult to decouple. Despite the susceptibility of nanostructured electrodes to mass transfer limitations, we highlight that nanostructured silver electrodes exhibit considerably higher activity when normalized to the electrochemically active surface in contrast to gold and copper electrodes. Alongside, we provide a discussion on how active surface area and thickness of the catalytic layer itself can influence the onset potential, selectivity, stability, activity and mass transfer inside and outside of the three dimensional catalyst layer. Key parameters and potential solutions are highlighted to decouple mass transfer effects from the measured activity in electrochemical cells utilizing CO2 saturated aqueous solutions.
Full text
Available for:
IJS, KILJ, NUK, UL, UM, UPUK
The electrochemical conversion of carbon dioxide and water into useful products is a major challenge in facilitating a closed carbon cycle. Here we report a cobalt protoporphyrin immobilized on a ...pyrolytic graphite electrode that reduces carbon dioxide in an aqueous acidic solution at relatively low overpotential (0.5 V), with an efficiency and selectivity comparable to the best porphyrin-based electrocatalyst in the literature. While carbon monoxide is the main reduction product, we also observe methane as by-product. The results of our detailed pH-dependent studies are explained consistently by a mechanism in which carbon dioxide is activated by the cobalt protoporphyrin through the stabilization of a radical intermediate, which acts as Brønsted base. The basic character of this intermediate explains how the carbon dioxide reduction circumvents a concerted proton-electron transfer mechanism, in contrast to hydrogen evolution. Our results and their mechanistic interpretations suggest strategies for designing improved catalysts.
Aqueous-phase electrochemical reduction of carbon dioxide requires an active, earth-abundant electrocatalyst, as well as highly efficient mass transport. Here we report the design of a porous hollow ...fibre copper electrode with a compact three-dimensional geometry, which provides a large area, three-phase boundary for gas-liquid reactions. The performance of the copper electrode is significantly enhanced; at overpotentials between 200 and 400 mV, faradaic efficiencies for carbon dioxide reduction up to 85% are obtained. Moreover, the carbon monoxide formation rate is at least one order of magnitude larger when compared with state-of-the-art nanocrystalline copper electrodes. Copper hollow fibre electrodes can be prepared via a facile method that is compatible with existing large-scale production processes. The results of this study may inspire the development of new types of microtubular electrodes for electrochemical processes in which at least one gas-phase reactant is involved, such as in fuel cell technology.
Display omitted
•Optimization for ultrasound-assisted fabrication of PVF/CNT electrodes was performed.•Ultrasonic treatment enhanced the stability of fabricated PVF/CNT electrodes.•Ultrasound had a ...pronounced effect on the surface morphology of PVF/CNT electrodes.•The Box–Behnken experimental design was applied to determine optimum active surface sites.•The efficiency of formate adsorption was improved by optimizing experimental conditions.
The selective separation of ions is a major technological challenge having far-ranging impacts from product separation in electrochemical production of base chemicals from CO2 to water purification. In recent years, ion-selective electrochemical systems leveraging redox-materials emerged as an attractive platform based on their reversibility and remarkable ion selectivity. In the present study, we present an ultrasound-intensified fabrication process for polyvinyl ferrocene (PVF)–functionalized electrodes in a carbon nanotube (CNT) matrix for selective electro-adsorption of formate ions. To this end, a response surface methodology involving the Box–Behnken design with three effective independent variables, namely, PVF to CNT ratio, sonication duration, and ultrasonic amplitude was applied to reach the maximum formate adsorption efficiency. The fabricated electrodes were characterized using cyclic voltammetry, X-ray diffraction, Raman spectroscopy, and scanning electron microscopy (SEM). SEM images revealed that an optimized ultrasonic amplitude and sonication time provided remarkable improvements in electrode morphology. Through a sedimentation study, we qualitatively demonstrate that the main optimized conditions improved PVF/CNT dispersion stability, consequently providing the highest number of active surface sites for adsorption and the highest adsorption efficiency. The highest percentage of active electrode surface sites and the maximum adsorption efficiency were 97.8 and 90.7% respectively at a PVF/CNT ratio of 3, ultrasonication time of one hour, and 50% ultrasonic amplitude.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The formation of ethylene in CO2 electroreduction over rough copper electrodes is often explained by the presence of specific surface crystal steps, edges and defects. We demonstrate that an ...identical electrode covered with copper nanoparticles can yield either predominantly ethylene or methane, depending on the electrolyte concentration and applied CO2 pressure. Calculations of the pH near the electrode surface suggest that ethylene formation is favored by a relatively high (local) pH. Furthermore, the conditions leading to the formation of significant amounts of methane result in rapid deterioration of hydrocarbon production rates, whereas electrode performance in conditions favoring ethylene production can be sustained for hours. This study substantially alters the mechanistic interpretation of formation of ethylene over rough copper surfaces and implies that applied process conditions inducing pH variations near the electrode surface need to be taken into consideration.
Structure or Kinetics? The hydrocarbon selectivity in CO2 reduction over rough copper surfaces is determined by electrolyte concentration and CO2 pressure. The dramatic effect of these process conditions on selectivity and electrode stability can be explained by pH variations at the electrode surface.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Electrocatalytic CO2 reduction to generate multicarbon products is of interest for applications in artificial photosynthetic schemes. This is a particularly attractive goal for CO2 reduction by ...copper electrodes, where a broad range of hydrocarbon products can be generated but where selectivity for C–C coupled products relative to CH4 and H2 remains an impediment. Herein we report a simple yet highly selective catalytic system for CO2 reduction to C≥2 hydrocarbons on a polycrystalline Cu electrode in bicarbonate aqueous solution that uses N-substituted pyridinium additives. Selectivities of 70–80% for C2 and C3 products with a hydrocarbon ratio of C≥2/CH4 significantly greater than 100 have been observed with several additives. 13C-labeling studies verify CO2 to be the sole carbon source in the C≥2 hydrocarbons produced. Upon electroreduction, the N-substituted pyridinium additives lead to film deposition on the Cu electrode, identified in one case as the reductive coupling product of N-arylpyridinium. Product selectivity can also be tuned from C≥2 species to H2 (∼90%) while suppressing methane with certain N-heterocyclic additives.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM, UPUK
PDF
