Two pathways for the formation of ethylene in CO reduction on single-crystal copper electrodes.

Carbon monoxide is a key intermediate in the electrochemical reduction of carbon dioxide to methane and ethylene on copper electrodes. We investigated the electrochemical reduction of CO on two single-crystal copper electrodes and observed two different reaction mechanisms for ethylene formation: one pathway has a common intermediate with the formation of methane and takes place preferentially at (111) facets or steps, and the other pathway involves selective reduction of CO to ethylene at relatively low overpotentials at (100) facets. The (100) facets seem to be the dominant crystal facets in polycrystalline copper, opening up new routes to affordable (photo)electrochemical production of hydrocarbons from CO(2).

Production of Gas Diffusion Layers with Tunable Characteristics.

Gas diffusion electrodes (GDEs) allow electrochemical reactions to occur at higher rates by enhancing the mass transport of gaseous reactants to the catalyst. These electrodes are made of two layers: the catalyst layer and the gas diffusion layer (GDL). The catalyst layer is frequently studied for gas diffusion electrodes, and the GDL is rarely a focus. Consequently, no studies investigate interaction effects that may be present between these two layers. To study such interactions, it must be possible to obtain GDLs with various characteristics. This study uses a design of experiments to understand how multiple factors in the production method for GDLs can be adjusted to tune the characteristics of the GDL. These GDLs are particularly intended for the electrochemical reduction of CO2. The conductance through the GDL, surface conductivity, thickness, elasticity, hydrophobicity, and porosity are measured for the 26 synthesized electrodes, and the top influential production factors are identified for each characteristic.

Efficiency and selectivity of CO2 reduction to CO on gold gas diffusion electrodes in acidic media.

The electrochemical reduction of CO2 to CO is a promising technology for replacing production processes employing fossil fuels. Still, low energy efficiencies hinder the production of CO at commercial scale. CO2 electrolysis has mainly been performed in neutral or alkaline media, but recent fundamental work shows that high selectivities for CO can also be achieved in acidic media. Therefore, we investigate the feasibility of CO2 electrolysis at pH 2-4 at indrustrially relevant conditions, using 10 cm2 gold gas diffusion electrodes. Operating at current densities up to 200 mA cm-2, we obtain CO faradaic efficiencies between 80-90% in sulfate electrolyte, with a 30% improvement of the overall process energy efficiency, in comparison with neutral media. Additionally, we find that weakly hydrated cations are crucial for accomplishing high reaction rates and enabling CO2 electrolysis in acidic media. This study represents a step towards the application of acidic electrolyzers for CO2 electroreduction.

Catalysts and Reaction Pathways for the Electrochemical Reduction of Carbon Dioxide.

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.

Electrocatalytic reduction of carbon dioxide to carbon monoxide and methane at an immobilized cobalt protoporphyrin.

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.

Cellobiose hydrolysis and decomposition by electrochemical generation of acid and hydroxyl radicals.

This paper addresses the hydrolysis of cellobiose to glucose and its further decomposition with electrochemically generated acid (H(+)) on a platinum electrode, and with electrochemically generated hydroxyl radicals (OH(·)) on boron-doped diamond (BDD). The results are compared with the hydrolysis promoted by conventional acid (H(2)SO(4)) and OH(·) (from Fenton's reaction) and supported by product analysis by using online HPLC (for soluble products) and online electrochemical mass spectrometry (for CO(2)). Cellobiose hydrolysis follows a first-order reaction, which obeys Arrhenius' law over the temperature range from 25-80 °C with different activation energies for the acid- and radical-promoted reaction, that is, approximately 118±8 and 55±1 kJ mol(-1), respectively. The high local acidity with electrochemically generated H(+) on the Pt electrode increases the rate of glucose formation, however, the active electrode (PtO(x)) interacts with glucose and decomposes it further to smaller organic acids. In addition, O(2) formed during the oxygen evolution reaction (OER) lowers the selectivity of glucose by forming side-products. OH(·) generated on a BDD electrode first hydrolyzes the cellobiose to glucose, but rapidly attacks the aldehyde on glucose, which is further decomposed to smaller aldoses and finally formaldehyde, which is subsequently oxidized electrochemically to formic acid.