Boosting Electrocatalytic Ethylene Epoxidation by Single Atom Modulation.

The electrochemical synthesis of ethylene oxide (EO) using ethylene and water under ambient conditions presents a low-carbon alternative to existing industrial production process. Yet, the electrocatalytic ethylene epoxidation route is currently hindered by largely insufficient activity, EO selectivity, and long-term stability. Here we report a single atom Ru-doped hollandite structure KIr4O8 (KIrRuO) nanowire catalyst for efficient EO production via a chloride-mediated ethylene epoxidation process. The KIrRuO catalyst exhibits an EO partial current density up to 0.7 A cm-2 and an EO yield as high as 92.0 %. The impressive electrocatalytic performance towards ethylene epoxidation is ascribed to the modulation of electronic structures of adjacent Ir sites by single Ru atoms, which stabilizes the *CH2CH2OH intermediate and facilitates the formation of active Cl2 species during the generation of 2-chloroethanol, the precursor of EO. This work provides a single atom modulation strategy for improving the reactivity of adjacent metal sites in heterogeneous electrocatalysts.

Directing the Selectivity of CO Electrolysis to Acetate by Constructing Metal-Organic Interfaces.

Electrochemically converting CO2 to valuable chemicals holds great promise for closing the anthropogenic carbon cycle. Owing to complex reaction pathways and shared rate-determining steps, directing the selectivity of CO2 /CO electrolysis to a specific multicarbon product is very challenging. We report here a strategy for highly selective production of acetate from CO electrolysis by constructing metal-organic interfaces. We demonstrate that the Cu-organic interfaces constructed by in situ reconstruction of Cu complexes show very impressive acetate selectivity, with a high Faradaic efficiency of 84.2 % and a carbon selectivity of 92.1 % for acetate production, in an alkaline membrane electrode assembly electrolyzer. The maximum acetate partial current density and acetate yield reach as high as 605 mA cm-2 and 63.4 %, respectively. Thorough structural characterizations, control experiments, operando Raman spectroscopy measurements, and density functional theory calculation results indicate that the Cu-organic interface creates a favorable reaction microenvironment that enhances *CO adsorption, lowers the energy barrier for C-C coupling, and facilitates the formation of CH3 COOH over other multicarbon products, thus rationalizing the selective acetate production.

Assessment of the Degradation Mechanisms of Cu Electrodes during the CO2 Reduction Reaction.

Catalyst degradation and product selectivity changes are two of the key challenges in the electrochemical reduction of CO2 on copper electrodes. Yet, these aspects are often overlooked. Here, we combine in situ X-ray spectroscopy, in situ electron microscopy, and ex situ characterization techniques to follow the long-term evolution of the catalyst morphology, electronic structure, surface composition, activity, and product selectivity of Cu nanosized crystals during the CO2 reduction reaction. We found no changes in the electronic structure of the electrode under cathodic potentiostatic control over time, nor was there any build-up of contaminants. In contrast, the electrode morphology is modified by prolonged CO2 electroreduction, which transforms the initially faceted Cu particles into a rough/rounded structure. In conjunction with these morphological changes, the current increases and the selectivity changes from value-added hydrocarbons to less valuable side reaction products, i.e., hydrogen and CO. Hence, our results suggest that the stabilization of a faceted Cu morphology is pivotal for ensuring optimal long-term performance in the selective reduction of CO2 into hydrocarbons and oxygenated products.

Reaction-induced iodine adsorption on Cu surfaces facilitates electrocatalytic CO2 reduction.

The electrolyte effect has been key to the electrochemical CO2 reduction reaction (CO2RR) and has received extensive attention in recent years. Here we combined atomic force microscopy, quasi-in situ X-ray photoelectron spectroscopy, and in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) to study the effect of iodine anions on Cu-catalyzed CO2RR in the absence or presence of KI in the KHCO3 solution. Our results suggested that iodine adsorption caused coarsening of the Cu surface and altered its intrinsic activity for CO2RR. As the potential of the Cu catalyst became more negative, there was an increase in surface iodine anion concentration ([I-]), which could be connected to the reaction-enhanced adsorption of I- ions accompanying the increase in CO2RR activity. A linear relationship was observed between [I-] and current density. SEIRAS results further suggested that the presence of KI in the electrolyte strengthened the Cu-CO bond and facilitated the hydrogenation process, enhancing the production of CH4. Our results have thus provided insight into the role of halogen anions and aided in the design of an efficient CO2RR process.

The Role of Interfacial Water in CO2 Electrolysis over Ni-N-C Catalyst in a Membrane Electrode Assembly Electrolyzer.

CO2 electrolysis is a promising route for achieving net-zero emission through decarbonization. To realize CO2 electrolysis toward practical application, beyond catalyst structures, it is also critical to rationally manipulate catalyst microenvironments such as the water at the electrode/electrolyte interface. Here, the role of interfacial water in CO2 electrolysis over Ni-N-C catalyst modified with different polymers is investigated. Benefiting from a hydrophilic electrode/electrolyte interface, the Ni-N-C catalyst modified with quaternary ammonia poly(N-methyl-piperidine-co-p-terphenyl) shows a Faradaic efficiency of 95% and a partial current density of 665 mA cm-2 for CO production in an alkaline membrane electrode assembly electrolyzer. A scale-up demonstration using a 100 cm2 electrolyzer achieves a CO production rate of 514 mL min-1 at a current of 80 A. In-situ microscopy and spectroscopy measurements indicate that the hydrophilic interface significantly promotes the formation of the *COOH intermediate, rationalizing the high CO2 electrolysis performance.

Coverage-driven selectivity switch from ethylene to acetate in high-rate CO2/CO electrolysis.

Tuning catalyst microenvironments by electrolytes and organic modifications is effective in improving CO2 electrolysis performance. An alternative way is to use mixed CO/CO2 feeds from incomplete industrial combustion of fossil fuels, but its effect on catalyst microenvironments has been poorly understood. Here we investigate CO/CO2 co-electrolysis over CuO nanosheets in an alkaline membrane electrode assembly electrolyser. With increasing CO pressure in the feed, the major product gradually switches from ethylene to acetate, attributed to the increased CO coverage and local pH. Under optimized conditions, the Faradaic efficiency and partial current density of multicarbon products reach 90.0% and 3.1 A cm-2, corresponding to a carbon selectivity of 100.0% and yield of 75.0%, outperforming thermocatalytic CO hydrogenation. The scale-up demonstration using an electrolyser stack achieves the highest ethylene formation rate of 457.5 ml min-1 at 150 A and acetate formation rate of 2.97 g min-1 at 250 A.

Asymmetric Oxo-Bridged ZnPb Bimetallic Electrocatalysis Boosting CO2 -to-HCOOH Reduction.

Electrochemical CO2 reduction (ECR) is one of the promising CO2 recycling technologies sustaining the natural carbon cycle and offering more sustainable higher-energy chemicals. Zn- and Pb-based catalysts have improved formate selectivity, but they suffer from relatively low current activities considering the competitive CO selectivity on Zn. Here, lead-doped zinc (Zn(Pb)) electrocatalyst is optimized to efficiently reduce CO2 to formate, while CO evolution selectivity is largely controlled. Selective formate is detected with Faradaic efficiency (FEHCOOH ) of ≈95% at an outstanding partial current density of 47 mA cm-2 in a conventional H-Cell. Zn(Pb) is further investigated in an electrolyte-fed device achieving a superior conversion rate of ≈100 mA cm-2 representing a step closer to practical electrocatalysis. The in situ analysis demonstrates that the Pb incorporation plays a crucial role in CO suppression stem from the generation of the Pb-O-C-O-Zn structure rather than the CO-boosted Pb-O-C-Zn. Density functional theory (DFT) calculations reveal that the alloying effect tunes the adsorption energetics and consequently modifies the electronic structure of the system for an optimized asymmetric oxo-bridged intermediate. The alloying effect between Zn and Pb controls CO selectivity and achieves a superior activity for a selective CO2 -to-formate reduction.

A Reconstructed Cu2 P2 O7 Catalyst for Selective CO2 Electroreduction to Multicarbon Products.

The electrochemical CO2 reduction reaction (CO2 RR) over Cu-based catalysts shows great potential for converting CO2 into multicarbon (C2+ ) fuels and chemicals. Herein, we introduce an A2 M2 O7 structure into a Cu-based catalyst through a solid-state reaction synthesis method. The Cu2 P2 O7 catalyst is electrochemically reduced to metallic Cu with a significant structure evolution from grain aggregates to highly porous structure under CO2 RR conditions. The reconstructed Cu2 P2 O7 catalyst achieves a Faradaic efficiency of 73.6 % for C2+ products at an applied current density of 350 mA cm-2 , remarkably higher than the CuO counterparts. The reconstructed Cu2 P2 O7 catalyst has a high electrochemically active surface area, abundant defects, and low-coordinated sites. In situ Raman spectroscopy and density functional theory calculations reveal that CO adsorption with bridge and atop configurations is largely improved on Cu with defects and low-coordinated sites, which decreased the energy barrier of the C-C coupling reaction for C2+ products.

High-Rate CO2 Electroreduction to C2+ Products over a Copper-Copper Iodide Catalyst.

Electrochemical CO2 reduction reaction (CO2 RR) to multicarbon hydrocarbon and oxygenate (C2+ ) products with high energy density and wide availability is of great importance, as it provides a promising way to achieve the renewable energy storage and close the carbon cycle. Herein we design a Cu-CuI composite catalyst with abundant Cu0 /Cu+ interfaces by physically mixing Cu nanoparticles and CuI powders. The composite catalyst achieves a remarkable C2+ partial current density of 591 mA cm-2 at -1.0 V vs. reversible hydrogen electrode in a flow cell, substantially higher than Cu (329 mA cm-2 ) and CuI (96 mA cm-2 ) counterparts. Induced by alkaline electrolyte and applied potential, the Cu-CuI composite catalyst undergoes significant reconstruction under CO2 RR conditions. The high-rate C2+ production over Cu-CuI is ascribed to the presence of residual Cu+ and adsorbed iodine species which improve CO adsorption and facilitate C-C coupling.

Benzoic Anhydride as a Bifunctional Electrolyte Additive for Hydrogen Fluoride Capture and Robust Film Construction over High-Voltage Li-Ion Batteries.

High-voltage LiNi0.8 Co0.1 Mn0.1 O2 (NCM811)-based Li-ion batteries (LIBs) with enhanced performance can be achieved by properly tailoring the electrolyte systems. Benzoic anhydride (BA) was proposed here as a promising bifunctional electrolyte additive that can not only construct a robust cathode-electrolyte interface (CEI) film on the electrode surface but also capture HF/H2 O in the electrolyte effectively. Compared to the cell without the BA additive, the capacity of Li/NCM811 half-cell with 1.0 wt % BA was increased from 128.5 to 149.6 mAh g-1 after 200 cycles at 1 C between 3.0 and 4.3 V. Even at a higher cut-off voltage of 4.5 V, the BA-containing Li/NCM811 half-cell delivered a capacity retention of 69 % after 200 cycles, much higher than that of the half-cell without the additive (56 %). Both theoretical calculation and experimental results verified that the BA additive could be preferentially oxidized to form a stable interface film with high conductivity that protected the NCM811 cathode and suppressed the decomposition of the electrolyte.

On the Activity/Selectivity and Phase Stability of Thermally Grown Copper Oxides during the Electrocatalytic Reduction of CO2.

Revealing the active nature of oxide-derived copper is of key importance to understand its remarkable catalytic performance during the cathodic CO2 reduction reaction (CO2RR) to produce valuable hydrocarbons. Using advanced spectroscopy, electron microscopy, and electrochemically active surface area characterization techniques, the electronic structure and the changes in the morphology/roughness of thermally oxidized copper thin films were revealed during CO2RR. For this purpose, we developed an in situ cell for X-ray spectroscopy that could be operated accurately in the presence of gases or liquids to clarify the role of the initial thermal oxide phase and its active phase during the electrocatalytic reduction of CO2. It was found that the Cu(I) species formed during the thermal treatment are readily reduced to Cu0 during the CO2RR, whereas Cu(II) species are hardly reduced. In addition, Cu(II) oxide electrode dissolution was found to yield a porous/void structure, where the lack of electrical connection between isolated islands prohibits the CO2RR. Therefore, the active/stable phase for CO2RR is metallic copper, independent of its initial phase, with a significant change in its morphology upon its reduction yielding the formation of a rougher surface with a higher number of underco-ordinated sites. Thus, the initial thermal oxidation of copper in air controls the reaction activity/selectivity because of the changes induced in the electrode surface morphology/roughness and the presence of more undercoordinated sites during the CO2RR.

Enhancing CO2 Electroreduction to Methane with a Cobalt Phthalocyanine and Zinc-Nitrogen-Carbon Tandem Catalyst.

Developing copper-free catalysts for CO2 conversion into hydrocarbons and oxygenates is highly desirable for electrochemical CO2 reduction reaction (CO2 RR). Herein, we report a cobalt phthalocyanine (CoPc) and zinc-nitrogen-carbon (Zn-N-C) tandem catalyst for CO2 RR to CH4 . This tandem catalyst shows a more than 100 times enhancement of the CH4 /CO production rate ratio compared with CoPc or Zn-N-C alone. Density functional theory (DFT) calculations and electrochemical CO reduction reaction results suggest that CO2 is first reduced into CO over CoPc and then CO diffuses onto Zn-N-C for further conversion into CH4 over Zn-N4 site, decoupling complicated CO2 RR pathway on single active site into a two-step tandem reaction. Moreover, mechanistic analysis indicates that CoPc not only generates CO but also enhances the availability of *H over adjacent N sites in Zn-N4 , which is the key to achieve the high CH4 production rate and understand the intriguing electrocatalytic behavior which is distinctive to copper-based tandem catalysts.

Revealing the Active Phase of Copper during the Electroreduction of CO2 in Aqueous Electrolyte by Correlating In Situ X-ray Spectroscopy and In Situ Electron Microscopy.

The variation in the morphology and electronic structure of copper during the electroreduction of CO2 into valuable hydrocarbons and alcohols was revealed by combining in situ surface- and bulk-sensitive X-ray spectroscopies with electrochemical scanning electron microscopy. These experiments proved that the electrified interface surface and near-surface are dominated by reduced copper. The selectivity to the formation of the key C-C bond is enhanced at higher cathodic potentials as a consequence of increased copper metallicity. In addition, the reduction of the copper oxide electrode and oxygen loss in the lattice reconstructs the electrode to yield a rougher surface with more uncoordinated sites, which controls the dissociation barrier of water and CO2. Thus, according to these results, copper oxide species can only be stabilized kinetically under CO2 reduction reaction conditions.

In Situ Investigation of Reversible Exsolution/Dissolution of CoFe Alloy Nanoparticles in a Co-Doped Sr2 Fe1.5 Mo0.5 O6- δ Cathode for CO2 Electrolysis.

Reversible exsolution and dissolution of metal nanoparticles in perovskite has been investigated as an efficient strategy to improve CO2 electrolysis performance. However, fundamental understanding with regard to the reversible exsolution and dissolution of metal nanoparticles in perovskite is still scarce. Herein, in situ exsolution and dissolution of CoFe alloy nanoparticles in Co-doped Sr2 Fe1.5 Mo0.5 O6-δ (SFMC) revealed by in situ X-ray diffraction, scanning transmission electron microscopy, environmental scanning electron microscopy, and density functional theory calculations are reported. Under a reducing atmosphere, facile exsolution of Co promotes reduction of the Fe cation to generate CoFe alloy nanoparticles in SFMC, accompanied by structure transformation from double perovskite to layered perovskite at 800 °C. Under an oxidizing atmosphere, spherical CoFe alloy nanoparticles are first oxidized to flat CoFeOx nanosheets, and then dissolved into the bulk with structure evolution from layered perovskite back to double perovskite. Electrochemically, CO2 electrolysis performance can be retrieved during 12 redox cycles due to the regenerative ability of the CoFe alloy nanoparticles. The anchoring of the CoFe alloy nanoparticles in SFMC perovskite via reduction shows enhanced CO2 electrolysis performance and stability compared with the parent SFMC perovskite.

In Situ Reconstruction of a Hierarchical Sn-Cu/SnOx Core/Shell Catalyst for High-Performance CO2 Electroreduction.

The electrochemical CO2 reduction reaction (CO2 RR) to give C1 (formate and CO) products is one of the most techno-economically achievable strategies for alleviating CO2 emissions. Now, it is demonstrated that the SnOx shell in Sn2.7 Cu catalyst with a hierarchical Sn-Cu core can be reconstructed in situ under cathodic potentials of CO2 RR. The resulting Sn2.7 Cu catalyst achieves a high current density of 406.7±14.4 mA cm-2 with C1 Faradaic efficiency of 98.0±0.9 % at -0.70 V vs. RHE, and remains stable at 243.1±19.2 mA cm-2 with a C1 Faradaic efficiency of 99.0±0.5 % for 40 h at -0.55 V vs. RHE. DFT calculations indicate that the reconstructed Sn/SnOx interface facilitates formic acid production by optimizing binding of the reaction intermediate HCOO* while promotes Faradaic efficiency of C1 products by suppressing the competitive hydrogen evolution reaction, resulting in high Faradaic efficiency, current density, and stability of CO2 RR at low overpotentials.

Selective CO2 Electroreduction to Ethylene and Multicarbon Alcohols via Electrolyte-Driven Nanostructuring.

Production of multicarbon products (C2+ ) from CO2 electroreduction reaction (CO2 RR) is highly desirable for storing renewable energy and reducing carbon emission. The electrochemical synthesis of CO2 RR catalysts that are highly selective for C2+ products via electrolyte-driven nanostructuring is presented. Nanostructured Cu catalysts synthesized in the presence of specific anions selectively convert CO2 into ethylene and multicarbon alcohols in aqueous 0.1 m KHCO3 solution, with the iodine-modified catalyst displaying the highest Faradaic efficiency of 80 % and a partial geometric current density of ca. 31.2 mA cm-2 for C2+ products at -0.9 V vs. RHE. Operando X-ray absorption spectroscopy and quasi in situ X-ray photoelectron spectroscopy measurements revealed that the high C2+ selectivity of these nanostructured Cu catalysts can be attributed to the highly roughened surface morphology induced by the synthesis, presence of subsurface oxygen and Cu+ species, and the adsorbed halides.

Structure- and Electrolyte-Sensitivity in CO2 Electroreduction.

The utilization of fossil fuels (i.e., coal, petroleum, and natural gas) as the main energy source gives rise to serious environmental issues, including global warming caused by the continuously increasing level of atmospheric CO2. To deal with this challenge, fossil fuels are being partially replaced by renewable energy such as solar and wind. However, such energy sources are usually intermittent and currently constitute a very low portion of the overall energy consumption. Recently, the electrochemical conversion of CO2 to chemicals and fuels with high energy density driven by electricity derived from renewable energy has been recognized as a promising strategy toward sustainable energy. The activation and reduction of CO2, which is a thermodynamically stable and kinetically inert molecule, is extremely challenging. Although the participation of protons in the CO2 electroreduction reaction (CO2RR) helps lower the energy barrier, high overpotentials are still needed to efficiently drive the process. On the other hand, the concurrent hydrogen evolution reaction (HER) under CO2RR conditions leads to lower selectivity toward CO2RR products. Electrocatalysts that are highly active and selective for multicarbon products are urgently needed to improve the energy efficiency of CO2RR. The reduction of CO2 involves multiple proton-electron transfers and has many complex intermediates. Recent reports have shown that the relative stability of the intermediates on the surface of catalysts determines final reaction pathways as well as the product selectivity. Furthermore, this reaction displays a strong structure-sensitivity. The atomic arrangement, electronic structure, chemical composition, and oxidation state of the catalysts significantly influence catalyst performance. Fundamental understanding of the dependence of the reaction mechanisms on the catalyst structure would guide the rational design of new nanostructured CO2RR catalysts. As a reaction proceeding in a complex environment containing gas/liquid/solid interfaces, CO2RR is also intensively affected by the electrolyte. The electrolyte composition in the near surface region of the electrode where the reaction takes place plays a vital role in the reactivity. However, the former might also be indirectly determined by the bulk electrolyte composition via diffusion. Adding to the complexity, the structure, chemical state and surface composition of the catalysts under reaction conditions usually undergo dynamic changes, especially when adsorbed ions are considered. Therefore, in addition to tuning the structure of the electrocatalysts, being able to also modify the electrolyte provides an alternative method to tune the activity and selectivity of CO2RR. In situ and operando characterization methods must be employed to gain in depth understanding on the structure- and electrolyte-sensitivity of real CO2RR catalysts under working conditions. This Account provides examples of recent advances in the development of nanostructured catalysts and mechanistic understanding of CO2RR. It discusses how the structure of a catalyst (crystal orientation, oxidation state, atomic arrangement, defects, size, surface composition, segregation, etc.) influences the activity and selectivity, and how the electrolyte also plays a determining role in the reaction activity and selectivity. Finally, the importance of in situ and operando characterization methods to understand the structure- and electrolyte-sensitivity of the CO2RR is discussed.

Dynamic Changes in the Structure, Chemical State and Catalytic Selectivity of Cu Nanocubes during CO2 Electroreduction: Size and Support Effects.

In situ and operando spectroscopic and microscopic methods were used to gain insight into the correlation between the structure, chemical state, and reactivity of size- and shape-controlled ligand-free Cu nanocubes during CO2 electroreduction (CO2 RR). Dynamic changes in the morphology and composition of Cu cubes supported on carbon were monitored under potential control through electrochemical atomic force microscopy, X-ray absorption fine-structure spectroscopy and X-ray photoelectron spectroscopy. Under reaction conditions, the roughening of the nanocube surface, disappearance of the (100) facets, formation of pores, loss of Cu and reduction of CuOx species observed were found to lead to a suppression of the selectivity for multi-carbon products (i.e. C2 H4 and ethanol) versus CH4 . A comparison with Cu cubes supported on Cu foils revealed an enhanced morphological stability and persistence of CuI species under CO2 RR in the former samples. Both factors are held responsible for the higher C2 /C1 product ratio observed for the Cu cubes/Cu as compared to Cu cubes/C. Our findings highlight the importance of the structure of the active nanocatalyst but also its interaction with the underlying substrate in CO2 RR selectivity.

Electrochemical promotion of catalysis over Pd nanoparticles for CO2 reduction.

Electrochemical promotion of catalysis (EPOC) has been shown to accelerate the rate of many heterogeneous catalytic reactions; however, it has rarely been reported in low-temperature aqueous electrochemical reactions. Herein, we report a significant EPOC effect for the CO2 reduction to generate formate over Pd nanoparticles (NPs) in a 1 M KHCO3 aqueous solution. By applying a negative potential over differently-sized Pd NPs, the rate of formate production is greatly improved as compared to that at an open-circuit voltage, with a rate enhancement ratio ranging from 10 to 143. The thermocatalytic and electrocatalytic reduction of CO2 compete with each other and are promoted by the applied negative potential and H2 in the feeds, respectively. Inspired by the EPOC effect, a composite electrode containing Pd/C and Pt/C catalysts on different sides of a carbon paper was constructed for catalyzing the CO2 reduction without adding H2 to the feeds. Water electrolysis over Pt NPs generates H2, which then effectively promotes formate production over Pd NPs.

Plasma-Activated Copper Nanocube Catalysts for Efficient Carbon Dioxide Electroreduction to Hydrocarbons and Alcohols.

Carbon dioxide electroreduction to chemicals and fuels powered by renewable energy sources is considered a promising path to address climate change and energy storage needs. We have developed highly active and selective copper (Cu) nanocube catalysts with tunable Cu(100) facet and oxygen/chlorine ion content by low-pressure plasma pretreatments. These catalysts display lower overpotentials and higher ethylene, ethanol, and n-propanol selectivity, resulting in a maximum Faradaic efficiency (FE) of ∼73% for C2 and C3 products. Scanning electron microscopy and energy-dispersive X-ray spectroscopy in combination with quasi-in situ X-ray photoelectron spectroscopy revealed that the catalyst shape, ion content, and ion stability under electrochemical reaction conditions can be systematically tuned through plasma treatments. Our results demonstrate that the presence of oxygen species in surface and subsurface regions of the nanocube catalysts is key for achieving high activity and hydrocarbon/alcohol selectivity, even more important than the presence of Cu(100) facets.