High Selectivity Toward C2H4 Production over Cu Particles Supported by Butterfly-Wing-Derived Carbon Frameworks

Structural Regulating of Cu-Based Metallic Electrocatalysts for CO2 to C2+ Products Conversion

Electrochemical carbon dioxide reduction reaction (CO2RR) to highly value-added multi-carbon (C2+) fuels or chemicals is a promising pathway to address environment issues and energy crisis. In the periodic table, Cu as only the candidate can convert CO2 to C2+ products such as C2H4 and C2H5OH due to the suitable absorption energy to reaction intermediate. However, application of Cu is limited for its low activity and poor selectivity. The tandem catalytic strategy can effectively solve the problems caused by single copper catalyst. In tandem catalysis, how to promote the formation, transport, adsorption and coupling of the important intermediate CO is the key issue to improve the selectivity of C2+ products. Regulating the structure of Cu-based bimetallic can effectively promote these processes to Electrochemical CO2RR on account of its synergistic effect, electronic effect and interfacial interaction. In this review, we systematically summarized the relationship between structure of Cu-based bimetallic catalysts with performance of electrochemical CO2RR. More importantly, we reveal that different Cu-based bimetallic structures enhance the activity and selectivity of the catalysts by regulating the processes such as the transport and adsorption of the reaction intermediate CO. Then, we proposed well-effective strategies to rationally design Cu-based metallic catalysts. Finally, we put forward some challenges and opportunities that Cu-based bimetallic catalysts would face in the development of electrochemical CO2RR technology in the future.

Electrocatalytic CO2 Reduction to Ethylene: From Advanced Catalyst Design to Industrial Applications

The value-added chemicals, monoxide, methane, ethylene, ethanol, ethane, and so on, can be efficiently generated through the electrochemical CO2 reduction reaction (eCO2 RR) when equipped with suitable catalysts. Among them, ethylene is particularly important as a chemical feedstock for petrochemical manufacture. However, despite its high Faradaic efficiency achievable at relatively low current densities, the substantial enhancement of ethylene selectivity and stability at industrial current densities poses a formidable challenge. To facilitate the industrial implementation of eCO2 RR for ethylene production, it is imperative to identify key strategies and potential solutions through comprehending the recent advancements, remaining challenges, and future directions. Herein, the latest and innovative catalyst design strategies of eCO2 RR to ethylene are summarized and discussed, starting with the properties of catalysts such as morphology, crystalline, oxidation state, defect, composition, and surface engineering. The review subsequently outlines the related important state-of-the-art technologies that are essential in driving forward eCO2 RR to ethylene into practical applications, such as CO2 capture, product separation, and downstream reactions. Finally, a greenhouse model that integrates CO2 capture, conversion, storage, and utilization is proposed to present an ideal perspective direction of eCO2 RR to ethylene.

Trace Ionic Liquid-Assisted Orientational Growth of Cu2 O (110) Facets Promote CO2 Electroreduction to C2 Products

Cu2 O has great advantages for CO2 electroreduction to C2 products, of which the activity and selectivity are closely related to its crystal facets. In this work, density functional theory calculation indicated that the (110) facets of Cu2 O had a lower energy barrier for the C-C coupling compared to the (100) and (111) facets. Therefore, Cu2 O(110) facets were successfully synthesized with the assistance of trace amounts of the ionic liquid 1-butyl-3-methylimidazolium ([Bmim]BF4 ) by a sample wet-chemical method. A high faradaic efficiency of 71.1 % and a large current density of 265.1 mA cm-2 toward C2 H4 and C2 H5 OH were achieved at -1.1 V (vs. reversible hydrogen electrode) in a flow cell. The in situ and electrochemical analysis indicated that it possessed the synergy effects of strong adsorption of *CO2 and *CO, large active area, and excellent conductivity. This study provided a new way to enhance the C2 selectivity of CO2 electroreduction on Cu2 O by crystal structure engineering.

In-situ spectroscopic probe of the intrinsic structure feature of single-atom center in electrochemical CO/CO2 reduction to methanol

While exploring the process of CO/CO₂ electroreduction (CO_xRR) is of great significance to achieve carbon recycling, deciphering reaction mechanisms so as to further design catalytic systems able to overcome sluggish kinetics remains challenging. In this work, a model single-Co-atom catalyst with well-defined coordination structure is developed and employed as a platform to unravel the underlying reaction mechanism of CO_xRR. The as-prepared single-Co-atom catalyst exhibits a maximum methanol Faradaic efficiency as high as 65% at 30 mA/cm² in a membrane electrode assembly electrolyzer, while on the contrary, the reduction pathway of CO₂ to methanol is strongly decreased in CO₂RR. In-situ X-ray absorption and Fourier-transform infrared spectroscopies point to a different adsorption configuration of *CO intermediate in CORR as compared to that in CO₂RR, with a weaker stretching vibration of the C-O bond in the former case. Theoretical calculations further evidence the low energy barrier for the formation of a H-CoPc-CO^- species, which is a critical factor in promoting the electrochemical reduction of CO to methanol.

Low Overpotential Electrochemical Reduction of CO2 to Ethanol Enabled by Cu/CuxO Nanoparticles Embedded in Nitrogen-Doped Carbon Cuboids

The electrochemical conversion of CO₂ into value-added chemicals is a promising approach for addressing environmental and energy supply problems. In this study, electrochemical CO₂ catalysis to ethanol is achieved using incorporated Cu/Cu_xO nanoparticles into nitrogenous porous carbon cuboids. Pyrolysis of the coordinated Cu cations with nitrogen heterocycles allowed Cu nanoparticles to detach from the coordination complex but remain dispersed throughout the porous carbon cuboids. The heterogeneous composite Cu/Cu_xO-PCC-0h electrocatalyst reduced CO₂ to ethanol at low overpotential in 0.5 M KHCO₃, exhibiting maximum ethanol faradaic efficiency of 50% at -0.5 V vs. reversible hydrogen electrode. Such electrochemical performance can be ascribed to the synergy between pyridinic nitrogen species, Cu/Cu_xO nanoparticles, and porous carbon morphology, together providing efficient CO₂ diffusion, activation, and intermediates stabilization. This was supported by the notably high electrochemically active surface area, rich porosity, and efficient charge transfer properties.

Copper-Based Catalysts for Electrochemical Carbon Dioxide Reduction to Multicarbon Products

Electrochemical conversion of carbon dioxide into fuel and chemicals with added value represents an appealing approach to reduce the greenhouse effect and realize a carbon-neutral cycle, which has great potential in mitigating global warming and effectively storing renewable energy. The electrochemical CO2 reduction reaction (CO2RR) usually involves multiproton coupling and multielectron transfer in aqueous electrolytes to form multicarbon products (C2+ products), but it competes with the hydrogen evolution reaction (HER), which results in intrinsically sluggish kinetics and a complex reaction mechanism and places higher requirements on the design of catalysts. In this review, the advantages of electrochemical CO2 reduction are briefly introduced, and then, different categories of Cu-based catalysts, including monometallic Cu catalysts, bimetallic catalysts, metal-organic frameworks (MOFs) along with MOF-derived catalysts and other catalysts, are summarized in terms of their synthesis method and conversion of CO2 to C2+ products in aqueous solution. The catalytic mechanisms of these catalysts are subsequently discussed for rational design of more efficient catalysts. In response to the mechanisms, several material strategies to enhance the catalytic behaviors are proposed, including surface facet engineering, interface engineering, utilization of strong metal-support interactions and surface modification. Based on the above strategies, challenges and prospects are proposed for the future development of CO2RR catalysts for industrial applications.

Graphical Abstract

Metal-Doped PdH(111) Catalysts for CO2 Reduction

PdH-based catalysts hold promise for both CO₂ reduction to CO and the hydrogen evolution reaction. Density functional theory is used to systematically screen for stability, activity, and selectivity of transition metal dopants in PdH. The transition metal elements Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ru, Rh, Ag, Cd, Hf, Ta, W, and Re are doped into PdH(111) surface with six different doping configurations: single, dimer, triangle, parallelogram, island, and overlayer. We find that several dopants, such as Ti and Nb, have excellent predicted catalytic activity and CO₂ selectivity compared to the pure PdH hydride. In addition, they display good stability due to their negative doping formation energy. The improved performance can be assigned to reaction intermediates forming two bonds consisting of one C-Metal and one O-Metal bond on the PdH surface, which break the scaling relations of intermediates, and thus have stronger HOCO* binding facilitating CO₂ activation.

Nanoengineering Metal-Organic Framework-Based Materials for Use in Electrochemical CO2 Reduction Reactions

Electrocatalytic reduction of carbon dioxide to valuable chemicals is a sustainable technology that can achieve a carbon-neutral energy cycle in the environment. Electrochemical CO₂  reduction reaction (CO₂ RR) processes using metal-organic frameworks (MOFs), featuring atomically dispersed active sites, large surface area, high porosity, controllable morphology, and remarkable tunability, have attracted considerable research attention. Well-defined MOFs can be constructed to improve conductivity, introduce active centers, and form carbon-based single-atom catalysts (SACs) with enhanced active sites that are accessible for the development of CO₂  conversion. In this review, the progress on pristine MOFs, MOF hybrids, and MOF-derived carbon-based SACs is summarized for the electrocatalytic reduction of CO₂ . Finally, the limitations and potential improvement directions with respect to the advancement of MOF-related materials for the field of research are discussed. These summaries are expected to provide inspiration on reasonable design to develop stable and high-efficiency MOFs-based electrocatalysts for CO₂ RR.

Metal-Free Bifunctional Ordered Mesoporous Carbon for Reversible Zn-CO2 Batteries

The fabrication of Zn-CO₂ batteries is a promising technique for CO₂ fixation and energy storage. Herein, nitrogen-doped ordered mesoporous carbon (NOMC) is adopted as a bifunctional metal-free electrocatalyst for CO₂ reduction and oxygen evolution reaction in the near-neutral electrolyte. The ordered mesoporous structures and abundant N-dopings of NOMC facilitate the accessibility and utilization of the active sites, which endow NOMC with excellent electrocatalysis performance and outstanding stability. Especially, a nearly 100% CO Faradaic efficiency is achieved at an ultralow overpotential of 360 mV for CO₂ reduction. When constructed as an aqueous rechargeable Zn-CO₂ battery using NOMC as the cathode, it yields a high peak power density of 0.71 mW cm^-2 , a good cyclability of 300 cycles, and excellent energy efficiency of 52.8% at 1.0 mA cm^-2 .

The in situ growth of Cu2O with a honeycomb structure on a roughed graphite paper for the efficient electroreduction of CO2 to C2H4

A Cu2O/NGP self-supporting electrocatalyst is used for the electrocatalytic reduction of CO2 to ethylene to solve environmental and energy problems.

Non-Innocent Role of Porous Carbon Toward Enhancing C2-3 Products in the Electroreduction of Carbon Dioxide

Selectivity for C-C coupled products remains a major challenge for electrochemical CO₂ reduction. Herein, we report a facile method by modifying a Cu foil surface with a layer of porous carbon. The structure of carbon has a major influence on C₁ and C_2,3 product selectivity. A carbon aerogel modifier leads to higher C_2,3 product formation than that of a carbon black modifier, demonstrating the non-innocent role of carbon materials. In both cases, major surface reconstruction on the Cu foil-such as pitting and particle formation-is observed during electrocatalysis. In addition, the restructured Cu surface shows distinctly lower activity toward CO₂ reduction when the carbon modifier is removed. This is likely due to the fact that the carbon modifiers influence the product distribution by (i) modulating the local pH and CO₂ concentration by serving as a highly porous and hydrophobic barrier, and (ii) restructuring the metal surface that generates more active sites. Our findings illustrate that the carbon in carbon-based catalysts can have an disproportionate role in directing product formation in electrocatalytic carbon dioxide reduction.

Single-unit-cell-thick layered electrocatalysts: from synthesis to application

Electrocatalysts are critical for water splitting, carbon dioxide reduction, and zinc-air battery. However, the low-exposed surface areas of bulk electrocatalysts usually limit the complete utilization of active sites. Ultrathin electrocatalysts have noteworthy advantages in maximizing the use of active sites. Among the pioneering works on such performing catalysts, the development of single-unit-cell-thick layered electrocatalysts (STLEs) has attracted extensive attention owing to their superior specific surface area and large number of vacancies, which can provide abundant available surface active sites. Therefore, this minireview provides recent advances in STLE synthesis and applications, which are helpful for electrocatalysis-oriented researchers. Finally, the future perspectives and challenges for developing high-performance STLEs are proposed.

New aspects of C2 selectivity in electrochemical CO2 reduction over oxide-derived copper

Persistent Cu₂O on ODCu plays an important role in C2 product selectivity due to its interactions with CO₂RR intermediates.

Oxygen Doping Induced by Nitrogen Vacancies in Nb4 N5 Enables Highly Selective CO2 Reduction

Surface vacancy engineering holds great promise for boosting the electrocatalytic activity for CO₂ reduction reaction; however, the vacancies are generally unstable and may degrade into the inactive phase during electrolysis. Stabilizing the vacancy-enriched structure by heteroatoms can be an effective strategy to get a robust and active catalyst. Herein, a nitrogen-vacancy enriched Nb₄ N₅ on N-doped carbons is constructed, which is thereafter stabilized by a self-enhanced oxygen doping process. This oxygen-doped complex is used as an effective CO₂ catalyst, which exhibits a maximum CO Faradaic efficiency of 91% at -0.8 V (vs reversible hydrogen electrode, RHE) and long-term stability throughout 30 h of electrocatalysis. Density function theory calculations suggest that the incorporation of oxygen in Nb₄ N₅ facilitates the formation of *COOH and thus promotes the CO₂ reduction.

Bi-Doped SnO Nanosheets Supported on Cu Foam for Electrochemical Reduction of CO2 to HCOOH

Design and fabrication of efficient electrocatalysts is essential for electrochemical reduction of carbon dioxide (CO₂). In this work, bismuth (Bi)-doped SnO nanosheets were grown on copper foam (Bi-SnO/Cu foam) by a one-step hydrothermal reaction method and applied for the electrochemical reduction of CO₂ to formic acid (HCOOH). The experimental results indicated that Bi doping stabilized the divalent tin (Sn²⁺) existing on the surface of the electrocatalyst, making it difficult to be reduced to metallic tin (Sn⁰) during the electrochemical reduction process. In addition, combining with density functional theory (DFT) calculations, it is found that Bi doping and electron transfer from the catalyst to the Cu foam substrate could enhance the adsorption of *OOCH intermediates. As such, the Bi-doped SnO electrocatalyst exhibited a superior faradaic efficiency of 93% at -1.7 V (vs Ag/AgCl) for the reduction of CO₂ to HCOOH, together with a current density of 12 mA cm^-2 and excellent stability in at least 30 h of operation.

Bio-inspired hydrophobicity promotes CO2 reduction on a Cu surface

The aqueous electrocatalytic reduction of CO₂ into alcohol and hydrocarbon fuels presents a sustainable route towards energy-rich chemical feedstocks. Cu is the only material able to catalyse the substantial formation of multicarbon products (C₂/C₃), but competing proton reduction to hydrogen is an ever-present drain on selectivity. Here, a superhydrophobic surface was generated by 1-octadecanethiol treatment of hierarchically structured Cu dendrites, inspired by the structure of gas-trapping cuticles on subaquatic spiders. The hydrophobic electrode attained a 56% Faradaic efficiency for ethylene and 17% for ethanol production at neutral pH, compared to 9% and 4% on a hydrophilic, wettable equivalent. These observations are assigned to trapped gases at the hydrophobic Cu surface, which increase the concentration of CO₂ at the electrode-solution interface and consequently increase CO₂ reduction selectivity. Hydrophobicity is thus proposed as a governing factor in CO₂ reduction selectivity and can help explain trends seen on previously reported electrocatalysts.

Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte

To date, copper is the only heterogeneous catalyst that has shown a propensity to produce valuable hydrocarbons and alcohols, such as ethylene and ethanol, from electrochemical CO₂ reduction (CO₂R). There are variety of factors that impact CO₂R activity and selectivity, including the catalyst surface structure, morphology, composition, the choice of electrolyte ions and pH, and the electrochemical cell design. Many of these factors are often intertwined, which can complicate catalyst discovery and design efforts. Here we take a broad and historical view of these different aspects and their complex interplay in CO₂R catalysis on Cu, with the purpose of providing new insights, critical evaluations, and guidance to the field with regard to research directions and best practices. First, we describe the various experimental probes and complementary theoretical methods that have been used to discern the mechanisms by which products are formed, and next we present our current understanding of the complex reaction networks for CO₂R on Cu. We then analyze two key methods that have been used in attempts to alter the activity and selectivity of Cu: nanostructuring and the formation of bimetallic electrodes. Finally, we offer some perspectives on the future outlook for electrochemical CO₂R.

Simple physical preparation of single copper atoms on amorphous carbon via Coulomb explosion

Metal single atom (MSA) materials exhibit excellent properties and are receiving widespread interest for their effectiveness in promoting a variety of catalytic reactions. The current strategies for preparing MSA catalysts involve complicated operation flows and suffer from low loading of the single atoms prepared, owing to the surface defect density of the substrate. In this paper, we report a simple physical method for preparing high-density copper single atom catalysts on amorphous carbon by Coulomb explosion. The results of the in situ observation showed that copper atoms on particle surfaces were emitted under electron beam irradiation and were captured by defects in a surrounding amorphous carbon film as isolated single atoms. By controlling the time and intensity of the Coulomb explosion, the ratio of copper single atoms to clusters aggregated from single atoms on the amorphous carbon can be controlled. Our work will provide new ideas for a universal simple physical preparation of MSA catalysts.