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Copper oxides have been of considerable interest as electrocatalysts for CO2 reduction (CO2R) in aqueous electrolytes. However, their role as an active catalyst in reducing the required overpotential and improving the selectivity of reaction compared with that of polycrystalline copper remains controversial. Here, we introduce the use of selected-ion flow tube mass spectrometry, in concert with chronopotentiometry, in situ Raman spectroscopy, and computational modeling, to investigate CO2R on Cu2O nanoneedles, Cu2O nanocrystals, and Cu2O nanoparticles. We show experimentally that the selective formation of gaseous C2 products (i.e., ethylene) in CO2R is preceded by the reduction of the copper oxide (Cu2OR) surface to metallic copper. On the basis of density functional theory modeling, CO2R products are not formed as long as Cu2O is present at the surface because Cu2OR is kinetically and energetically more favorable than CO2R.

Resonant inelastic X-ray scattering and high-resolution X-ray absorption spectroscopy were used to identify the chemical state of a Co electrocatalyst in situ during the oxygen evolution reaction. After anodic electrodeposition onto Au(111) from a Co(2+)-containing electrolyte, the chemical environment of Co can be identified to be almost identical to CoOOH. With increasing potentials, a subtle increase of the Co oxidation state is observed, indicating a non-stoichiometric composition of the working OER catalyst containing a small fraction of Co(4+) sites. In order to confirm this interpretation, we used density functional theory with a Hubbard-U correction approach to compute X-ray absorption spectra of model compounds, which agree well with the experimental spectra. In situ monitoring of catalyst local structure and bonding is essential in the development of structure-activity relationships that can guide the discovery of efficient and earth abundant water splitting catalysts.

An in situ Raman spectroscopic investigation has been carried out to identify the composition of the active phase present on the surface of nickel electrodes used for the electrochemical evolution of oxygen. The electrolyte in all cases was 0.1 M KOH. A freshly polished Ni electrode oxidized upon immersion in the electrolyte and at potentials approaching the evolution of oxygen developed a layer of γ-NiOOH. Electrochemical cycling of this film transformed it into β-NiOOH, which was observed to be three times more active than γ-NiOOH. The higher activity of β-NiOOH is attributed to an unidentified Ni oxide formed at a potential above 0.52 V (vs Hg/HgO reference). We have also observed that a submonolayer of Ni oxide deposited on Au exhibits a turnover frequency (TOF) for oxygen evolution that is an order of magnitude higher than that for a freshly prepared γ-NiOOH surface and more than 2-fold higher than that for a β-NiOOH surface. By contrast, a similar film deposited on Pd exhibits a TOF that is similar to that of bulk γ-NiOOH. It is proposed that the high activity of submonolayer deposits of Ni oxide on Au is due to charge transfer from the oxide to the highly electronegative Au, leading to the possible formation of a mixed Ni/Au surface oxide. © 2012 American Chemical Society.

Tandem catalysis presents a promising strategy to improve the selectivity toward multicarbon products in the electrocatalytic carbon dioxide reduction reaction (CO2RR). For CO2RR, CO is a critical intermediate for producing multicarbon products. However, the management of CO localization and CO diffusion remains underexplored despite its critical role. Herein, a 3D tandem catalyst electrode with silver nanoparticles (Ag NPs) is designed to generate CO as an intermediate product within a copper (Cu) nanoneedle array. Via this nanostructured design, CO2 forms C2+ products with a high Faradaic efficiency (FEC2+) of 64% in an H‐cell and 70% in a flow cell with a current density of 350 mA cm−2. These figures‐of‐merit are currently among the top literature reports. More importantly, in situ Raman spectroscopy and finite‐element method calculations are employed to elucidate the origins of enhanced selectivity. These approaches reveal the crucial role of prolonging the CO diffusion path length for improving CO utilization during CO2 conversion with tandem catalyst systems. The favorable CO2RR FEC2+ in two distinct environments (H‐cell and flow cell) further corroborates that this effect is not limited to a particular reactor environment. Overall, this study provides new insights for designing tandem catalysts for improved CO2RR selectivity to C2+ products. [ABSTRACT FROM AUTHOR]

Recently discovered phosphate-derived Ni catalysts have opened a new pathway towards multicarbon products via CO2 electroreduction. However, understanding the influence of basic parameters such as electrode potential, pH, and buffer capacity is needed for optimized C3+ product formation. To this end, rigorous catalyst evaluation and sensitive analytical tools are required to identify potential new products and minimize increasing quantification errors linked to long-chain carbon compounds. Herein, we contribute to enhance testing accuracy by presenting sensitive 1H NMR spectroscopy protocols for liquid product assessment featuring optimized water suppression and reduced experiment time. When combined with an automated NMR data processing routine, samples containing up to 12 products can be quantified within 15 min with low quantification limits equivalent to Faradaic efficiencies of 0.1%. These developments disclosed performance trends in carbon product formation and the detection of four hitherto unreported compounds: acetate, ethylene glycol, hydroxyacetone, and i-propanol. Phosphate-derived nickel catalysts enable access to multicarbon products via CO2 electroreduction, but it remains unclear how operating conditions influence product formation. Here, refined 1H NMR spectroscopy protocols are introduced and combined with an automated NMR data processing routine, enabling quantification of carbon product formation as a function of various parameters. [ABSTRACT FROM AUTHOR]

Cu catalysts are most apt for reducing CO(2) to multi‐carbon products in aqueous electrolytes. To enhance the product yield, we can increase the overpotential and the catalyst mass loading. However, these approaches can cause inadequate mass transport of CO(2) to the catalytic sites, which will then lead to H2 evolution dominating the product selectivity. Herein, we use a MgAl LDH nanosheet 'house‐of‐cards' scaffold to disperse CuO‐derived Cu (OD‐Cu). With this support‐catalyst design, at −0.7 VRHE, CO could be reduced to C2+ products with a current density (jC2+) of −1251 mA cm−2. This is 14× that of the jC2+ shown by unsupported OD‐Cu. The current densities of C2+ alcohols and C2H4 were also high at −369 and −816 mA cm−2 respectively. We propose that the porosity of the LDH nanosheet scaffold enhances CO diffusion through the Cu sites. The CO reduction rate can thus be increased, while minimizing H2 evolution, even when high catalyst loadings and large overpotentials are used. [ABSTRACT FROM AUTHOR]

Cationic ammonium surfactants can be used together with a suitable catalyst to enhance the electroreduction of carbon dioxide (CO2RR). However, the underlying reasons for the improvements are not yet well understood. In this study, it is shown that didodecyldimethylammonium bromide (DDAB; [(C12H25)2N(CH3)2]Br), when added to the catholyte, can increase the rate of CO2 reduction to CO on silver electrodes by 12‐fold at −0.9 V versus reversible hydrogen electrode. More importantly, electrochemical impedance spectroscopy revealed that DDAB lowers the charge transfer resistance (RCT) for CO2RR on silver, and these changes can be correlated with enhancements in partial current densities of CO. Interestingly, when DDAB is added onto two other CO‐producing metals, namely, zinc and gold, the CO2RR charge transfer kinetics are improved only on Zn, but not on Au electrodes. By means of a semiempirical model combining density functional theory calculations and experimental data, it is concluded that DDAB generally strengthens the adsorption energies of the *COOH intermediate, which leads to enhanced CO production on silver and zinc, but not on gold. [ABSTRACT FROM AUTHOR]

Intensive research in electrochemical CO2 reduction reaction has resulted in the discovery of numerous high-performance catalysts selective to multi-carbon products, with most of these catalysts still being purely transition metal based. Herein, we present high and stable multi-carbon products selectivity of up to 76.6% across a wide potential range of 1 V on histidine-functionalised Cu. In-situ Raman and density functional theory calculations revealed alternative reaction pathways that involve direct interactions between adsorbed histidine and CO2 reduction intermediates at more cathodic potentials. Strikingly, we found that the yield of multi-carbon products is closely correlated to the surface charge on the catalyst surface, quantified by a pulsed voltammetry-based technique which proved reliable even at very cathodic potentials. We ascribe the surface charge to the population density of adsorbed species on the catalyst surface, which may be exploited as a powerful tool to explain CO2 reduction activity and as a proxy for future catalyst discovery, including organic-inorganic hybrids. Tuning CO2 electrocatalysis to achieve multi-carbon products is interesting yet challenging. Here the authors report a histidine-functionalized Cu catalyst for CO2 reduction to multi-carbon products and reveal the correlation between catalyst surface charge and catalytic performance. [ABSTRACT FROM AUTHOR]

We have employed in situ electrochemical shell‐isolated nanoparticle‐enhanced Raman spectroscopy (SHINERS) and density functional theory (DFT) calculations to study the CO reduction reaction (CORR) on Cu single‐crystal surfaces under various conditions. Coadsorbed and structure‐/potential‐dependent surface species, including *CO, Cu−Oad, and Cu−OHad, were identified using electrochemical spectroscopy and isotope labeling. The relative abundance of *OH follows a "volcano" trend with applied potentials in aqueous solutions, which is yet absent in absolute alcoholic solutions. Combined with DFT calculations, we propose that the surface H2O can serve as a strong proton donor for the first protonation step in both the C1 and C2 pathways of CORR at various applied potentials in alkaline electrolytes, leaving adsorbed *OH on the surface. This work provides fresh insights into the initial protonation steps and identity of key interfacial intermediates formed during CORR on Cu surfaces. [ABSTRACT FROM AUTHOR]

Electrocatalytic carbon monoxide reduction has been previously reported to yield a range of carbonaceous products including alcohols, hydrocarbons and carboxylic acids. However, esters, an important family of organic compounds, have not been formed. Herein, we report the electrosynthesis of C3 to C6 acetate esters (H3C−(C=O)−O−R) from carbon monoxide using copper catalysts in a membrane electrode assembly cell. Ethyl acetate and propyl acetate could be produced with an unprecedented total Faradaic efficiency (FE) of ∼22 % and with a current density of up to −55 mA cm−2, alongside minor quantities of methyl acetate and butyl acetate. The esters are produced via the addition reaction of ethenone (H2C=C=O) and alcohols produced during CO reduction. We show that the near water‐free reaction conditions and the high local pH play key roles in the formation of the esters. [ABSTRACT FROM AUTHOR]