Electrodeposited Zn Dendrites with Enhanced CO Selectivity for Electrocatalytic CO₂ Reduction

Electrochemical CO₂ reduction is a key reaction for CO₂ conversion to valuable fuels and chemicals. Because of the high stability of the CO₂ molecule, a catalyst is typically required to minimize the energy input and improve reaction rates needed for device level commercialization. In this paper, we report a nanostructured Zn dendrite catalyst that is able to electrochemically reduce CO₂ to CO in an aqueous bicarbonate electrolyte with greatly enhanced properties. The catalytic activity is over an order of magnitude higher than that of bulk Zn counterparts, with a CO faradaic efficiency around 3-fold higher. The stability of the Zn electrode under realistic CO₂ electrolysis conditions was explored using scanning electron microscopy and in situ/operando X-ray absorption spectroscopy techniques. The results clearly demonstrate that nanostructured and bulk Zn catalysts are structurally stable at potentials more negative than −0.7 V versus RHE, whereas severe chemical oxidation occurs at more positive potentials

Environmental In Situ X‑ray Absorption Spectroscopy Evaluation of Electrode Materials for Rechargeable Lithium–Oxygen Batteries

Lithium–oxygen batteries have attracted much recent attention due their high theoretical capacities, which exceeds that of Li-ion batteries. Among all the metal oxides that have been investigated in oxygen cathodes, α-MnO₂ materials have shown unique electrochemical properties in rechargeable lithium oxygen batteries. Although extensive research has been performed to investigate the structure of α-MnO₂ upon lithium intercalation, its behavior upon reacting with lithium under an oxygen environment remains to be fully explored. Here, we performed a systematic study on the behavior of two forms of α-MnO₂ nanowires (i.e., potassium and ammonia versions) together with bulk α-MnO₂ in oxygen cathodes through environmental in situ X-ray absorption spectroscopy. The results show that the α-MnO₂ materials undergo lithium insertion/removal and lithium peroxide formation/decomposition simultaneously. The former causes a self-switching of the oxidation state of Mn during cycling. Additionally, we found that potassium-containing α-MnO₂ nanowires exhibit a suppression of Mn reduction until late in cell discharge under oxygen, retaining a higher degree of Mn⁴⁺ character for enhanced oxygen reduction activity than other, similar α-MnO₂ materials. During cell recharge along with oxygen evolution, the materials were found to return to their initial states at low overpotential

Mechanistic Insights into the Electrochemical Reduction of CO₂ to CO on Nanostructured Ag Surfaces

Electroreduction of CO₂ in a highly selective and efficient manner is a crucial step toward CO₂ utilization. Nanostructured Ag catalysts have been found to be effective candidates for CO₂ to CO conversion. In this report, we combine experimental and computational efforts to explore the electrocatalytic reaction mechanism of CO₂ reduction on nanostructured Ag catalyst surfaces in an aqueous electrolyte. In contrast to bulk Ag catalysts, both nanoparticle and nanoporous Ag catalysts show enhanced ability to reduce the activation energy of the CO₂ to COOH_ads intermediate step through the low-coordinated Ag surface atoms, resulting in a reaction mechanism involving a fast first electron and proton transfer followed by a slow second proton transfer as the rate-limiting step

The Central Role of Bicarbonate in the Electrochemical Reduction of Carbon Dioxide on Gold

Much effort has been devoted in the development of efficient catalysts for electrochemical reduction of CO₂. Molecular level understanding of electrode-mediated process, particularly the role of bicarbonate in increasing CO₂ reduction rates, is still lacking due to the difficulty of directly probing the electrochemical interface. We developed a protocol to observe normally invisible reaction intermediates with a surface enhanced spectroscopy by applying square-wave potential profiles. Further, we demonstrate that bicarbonate, through equilibrium exchange with dissolved CO₂, rather than the supplied CO₂, is the primary source of carbon in the CO formed at the Au electrode by a combination of in situ spectroscopic, isotopic labeling, and mass spectroscopic investigations. We propose that bicarbonate enhances the rate of CO production on Au by increasing the effective concentration of dissolved CO₂ near the electrode surface through rapid equilibrium between bicarbonate and dissolved CO₂

Asymmetric Synthesis of a Glucagon Receptor Antagonist via Friedel–Crafts Alkylation of Indole with Chiral α‑Phenyl Benzyl Cation

Development of a practical asymmetric synthesis of a glucagon receptor antagonist drug candidate for the treatment of type 2 diabetes is described. The antagonist consists of a 1,1,2,2-tetrasubstituted ethane core substituted with a propyl and three aryl groups including a fluoro-indole. The key steps to construct the ethane core and the two stereogenic centers involved a ketone arylation, an asymmetric hydrogenation via dynamic kinetic resolution, and an <i>anti</i>-selective Friedel–Crafts alkylation of a fluoro-indole with a chiral α-phenyl benzyl cation. We also developed two new efficient syntheses of the fluoro-indole, including an unusual Larock-type indole synthesis and a Sugasawa-heteroannulation route. The described convergent synthesis was used to prepare drug substance in 52% overall yield and 99% ee on multikilogram scales

Asymmetric Synthesis of a Glucagon Receptor Antagonist via Friedel–Crafts Alkylation of Indole with Chiral α‑Phenyl Benzyl Cation

Development of a practical asymmetric synthesis of a glucagon receptor antagonist drug candidate for the treatment of type 2 diabetes is described. The antagonist consists of a 1,1,2,2-tetrasubstituted ethane core substituted with a propyl and three aryl groups including a fluoro-indole. The key steps to construct the ethane core and the two stereogenic centers involved a ketone arylation, an asymmetric hydrogenation via dynamic kinetic resolution, and an <i>anti</i>-selective Friedel–Crafts alkylation of a fluoro-indole with a chiral α-phenyl benzyl cation. We also developed two new efficient syntheses of the fluoro-indole, including an unusual Larock-type indole synthesis and a Sugasawa-heteroannulation route. The described convergent synthesis was used to prepare drug substance in 52% overall yield and 99% ee on multikilogram scales