Elucidating the Facet-Dependent Selectivity for CO2 Electroreduction to Ethanol of Cu–Ag Tandem Catalysts

Cu-Ag interactions in bimetallic Cu-Ag catalysts enhance C2+ product formation during electrochemical CO reduction

The electroreduction of CO (CORR) is a promising alternative to the direct CO2 electroreduction reaction (CO2RR) to produce C2+ products. Cu-based electrocatalysts enable the formation of C-C bonds, leading to various C2+ hydrocarbon and oxygenate products. Herein, we investigated how the composition of bimetallic Cu-Ag catalysts impacted the nature of the Cu-Ag interactions and the product distribution of the CORR, aiming to improve the selectivity to C2+ products. Cu-Ag catalysts containing 1-50 mol% Ag were prepared by sol-gel synthesis. A Ag content of 10 mol% of Ag (Cu0.9Ag0.1) was optimum with respect to increasing the C2+ product selectivity and suppressing H2 evolution. Operando X-ray absorption spectroscopy and quasi-in situ X-ray photoelectron spectroscopy demonstrated the complete reduction of CuO to Cu during CORR. Electron microscopy (EM) and in situ wide-angle X-ray scattering (WAXS) revealed substantial restructuring during reduction. EM imaging showed the formation of Ag-Cu core-shell structures in Cu0.9Ag0.1, while separate Cu and Ag particles were predominant at higher Ag content. In situ WAXS revealed the formation of a Cu-Ag nanoalloy phase in the bimetallic Cu-Ag samples. The optimum Cu0.9Ag0.1 sample contained more Cu-Ag nanoalloys than samples with a higher Ag content. The Cu-Ag interfaces between the Ag-core and the Cu-shell in the bimetallic particles are thought to host the nanoalloys. The optimum CORR performance for Cu0.9Ag0.1 is likely due to the enhanced Cu-Ag interactions, as confirmed by a sample prepared with the same surface composition by galvanic exchange.

Additives-Modified Electrodeposition for Synthesis of Hydrophobic Cu/Cu2O with Ag Single Atoms to Drive CO2 Electroreduction

Copper-based electrocatalysts are recognized as crucial catalysts for CO2 electroreduction into multi-carbon products. However, achieving copper-based electrocatalysts with adjustable valences via one-step facile synthesis remains a challenge. In this study, Cu/Cu2O heterostructure is constructed by adjusting the anion species of the Cu ions-containing electrolyte during electrodeposition synthesis. Then, Cu/Cu2O with tuned nanoarchitectures ranging from dendrites to polyhedrons is achieved by introducing transition metal ions as additives, leading to an adjustable interfacial microenvironment for CO2/H2O adsorption on the Cu/Cu2O electrodes. Additionally, the polyhedral Cu/Cu2O catalysts are used as templates for depositing Ag single atoms (AgSA), which are known as synergistic active sites for promoting *CO to *COH toward C2+ products. The prepared AgSA-Cu/Cu2O catalyst is evaluated in a flow cell and exhibited a FEC2+ of 90.2% and a partial current density (jc2+) of 426.6 mA cm-2 for CO2 electroreduction. As revealed by in situ Raman spectra and density functional theory calculations, the introduction of Ag single atoms slows down the reduction of Cu+ during CO2 electroreduction, especially at a high current density. This work provides a promising paradigm for diverse control of the compositions and hydrophobicity of Cu-based catalysts for selective CO2 electroreduction to C2+ products.

Electrochemical CO2 Reduction Reaction: Comprehensive Strategic Approaches to Catalyst Design for Selective Liquid Products Formation

The escalating concern regarding the release of CO2 into the atmosphere poses a significant threat to the contemporary efforts in mitigating climate change. Amidst a multitude of strategies for curtailing CO2 emissions, the electrochemical CO2 reduction presents a promising avenue for transforming CO2 molecules into a diverse array of valuable gaseous and liquid products, such as CO, CH3OH, CH4, HCO2H, C2H4, C2H5OH, CH3CO2H, 1-C3H7OH and others. The mechanistic investigations of gaseous products (e. g. CO, CH4, C2H4, C2H6 and others) broadly covered in the literature. There is a noticeable gap in the literature when it comes to a comprehensive summary exclusively dedicated to coherent roadmap for the designing principles for a selective catalyst all possible liquid products (such as CH3OH, C2H5OH, 1-C3H7OH, 2-C3H7OH, 1-C4H9OH, as well as other C3-C4 products like methylglyoxal and 2,3-furandiol, in addition to HCO2H, AcOH, oxalic acid and others), selectively converted by CO2 reduction. This entails a meticulous analysis to justify these approaches and a thorough exploration of the correlation between materials and their electrocatalytic properties. Furthermore, these insightful discussions illuminate the future prospects for practical applications, a facet not exhaustively examined in prior reviews.

Cascade Catalytic Systems for Converting CO2 into C2+ Products

The excessive emission and continuous accumulation of CO2 have precipitated serious social and environmental issues. However, CO2 can also serve as an abundant, inexpensive, and non-toxic renewable C1 carbon source for synthetic reactions. To achieve carbon neutrality and recycling, it is crucial to convert CO2 into value-added products through chemical pathways. Multi-carbon (C2+) products, compared to C1 products, offer a broader range of applications and higher economic returns. Despite this, converting CO2 into C2+ products is difficult due to its stability and the high energy required for C-C coupling. Cascade catalytic reactions offer a solution by coordinating active components, promoting intermediate transfers, and facilitating further transformations. This method lowers energy consumption. Recent advancements in cascade catalytic systems have allowed for significant progress in synthesizing C2+ products from CO2. This review highlights the features and advantages of cascade catalysis strategies, explores the synergistic effects among active sites, and examines the mechanisms within these systems. It also outlines future prospects for CO2 cascade catalytic synthesis, offering a framework for efficient CO2 utilization and the development of next-generation catalytic systems.

Copper/Polyaniline Interfaces Confined CO2 Electroreduction for Selective Hydrocarbon Production

Polyaniline (PANI) provides an attractive organic platform for CO2 electrochemical reduction due to the ability to adsorb CO2 molecules and in providing means to interact with metal nanostructures. In this work, a novel PANI supported copper catalyst has been developed by coupling the interfacial polymerization of PANI and Cu. The hybrid catalyst demonstrates excellent activity towards production of hydrocarbon products including CH4 and C2H4, compared with the use of bare Cu. A Faradaic efficiency of 71.8 % and a current density of 16.9 mA/cm2 were achieved at -0.86 V vs. RHE, in contrast to only 22.2 % and 1.0 mA/cm2 from the counterpart Cu catalysts. The remarkably enhanced catalytic performance of the hybrid PANI/Cu catalyst can be attributed to the synergistic interaction between the PANI underlayer and copper. The PANI favours the adsorption and binding of CO2 molecules via its nitrogen sites to form *CO intermediates, while the Cu/PANI interfaces confine the diffusion or desorption of the *CO intermediates favouring their further hydrogenation or carbon-carbon coupling to form hydrocarbon products. This work provides insights into the formation of hydrocarbon products on PANI-modified Cu catalysts, which may guide the development of conducting polymer-metal catalysts for CO2 electroreduction.

Unveiling the Interfacial Species Synergy in Promoting CO2 Tandem Electrocatalysis in Near-Neutral Electrolyte

The interfacial species-built local environments on Cu surfaces impact the CO2 electroreduction process significantly in producing value-added multicarbon (C2+) products. However, intricate interfacial dynamics leads to a challenge in understanding how these species affect the process. Herein, with ab initio molecular dynamics (AIMD) and finite element method (FEM) simulations, we reveal that the highly concentrated interfacial species, including the *CO, hydroxide, and K+, could synergistically promote the C-C coupling on the one-dimensional (1D) porous hollow structure regulated interfacial environment. The Cu-Ag tandem catalyst was then synthesized with the as-designed structure, exhibiting a high C2+ Faradaic efficiency of 76.0% with a partial current density of 380.0 mA cm-2 in near-neutral electrolytes. Furthermore, in situ Raman spectra validate that the 1D porous structure regulates the concentration of interfacial CO intermediates and ions to increase *CO coverage, local pH value, and ionic field, promoting the CO2-to-C2+ activity. These results provide insights into the design of practical ECR electrocatalysts by regulating interfacial species-induced local environments.

A Review on the Influence of Crystal Facets on the Product Selectivity of CO2RR over Cu Metal Catalysts

The electrocatalytic carbon dioxide reduction reaction (ECRR) is promising in converting environmentally harmful CO2 into useful chemicals, but the large-scale application of this technology is seriously limited by its low efficiency and selectivity. Cu-based electrocatalysts displayed attractive ability in converting CO2 to multiple products, and the product selectivity can be manipulated through various approaches. Among them, exposing specific crystal facets through crystal facet engineering has been proven to be highly effective in obtaining specific products and has attracted numerous researchers. However, to our knowledge, few reports have systematically summarized the relationship between the crystal facet control of Cu catalysts and the catalytic products. This review begins by outlining the general mechanism of CO2 electrocatalytic reduction on Cu-based catalysts, and then summarizes the preferences of low-index and high-index Cu facets regarding product selectivity and delves into the synergistic effects between facets (including different Cu facets and interactions between Cu and non-Cu facets) and their impact on CO2 reduction reaction (CO2RR). In addition, the study of the recently developed Cu single-atom catalysts in ECRR was also introduced. Finally, we provide an outlook on the development of high-performance Cu-based catalysts for applications in CO2RR. The purpose of this review is to provide a clear vein and meaningful guidance for the following studies over the crystal facet control of Cu-based electrocatalysts.

Strategies for overcoming challenges in selective electrochemical CO2 conversion to ethanol

The electrochemical conversion of carbon dioxide (CO2) to valuable chemicals is gaining significant attention as a pragmatic solution for achieving carbon neutrality and storing renewable energy in a usable form. Recent research increasingly focuses on designing electrocatalysts that specifically convert CO2 into ethanol, a desirable product due to its high-energy density, ease of storage, and portability. However, achieving high-efficiency ethanol production remains a challenge compared to ethylene (a competing product with a similar electron configuration). Existing electrocatalytic systems often suffer from limitations such as low energy efficiency, poor stability, and inadequate selectivity toward ethanol. Inspired by recent progress in the field, this review explores fundamental principles and material advancements in CO2 electroreduction, emphasizing strategies for ethanol production over ethylene. We discuss electrocatalyst design, reaction mechanisms, challenges, and future research directions. These advancements aim to bridge the gap between current research and industrialized applications of this technology.

Customizing pyridinic nitrogen coordination in Ni-N-C for electrocatalytic CO2reduction towards CO

Nickel anchored N-doped carbon electrocatalysts (Ni-N-C) are rapidly developed for the electrochemical reduction reaction of carbon dioxide (CO2RR). However, the high-performanced Ni-N-C analogues design for CO2RR remains bewilderment, for the reason lacking of definite guidance for its structure-activity relationship. Herein, the correlation between the proportion of nitrogen species derived from various nitrogen sources and the CO2RR activity of Ni-N-C is investigated. The x-ray photoelectron spectroscopy (XPS) spectrum combined with the CO2RR performance results show that pyridinic-N content has a positive correlation with CO2RR activity. Moreover, density functional theory (DFT) demonstrates that pyridinic-N coordinated Ni-N4sites offers optimized free energy and favorable selectivity towards CO2RR compared with pyrrolic-N. Accordingly, Ni-Na-C with highest pyridinic-N content (ammonia as nitrogen source) performs superior CO2RR activity, with the maximum carbon monoxide faradaic efficiency (FECO) of 99.8% at -0.88 V vs. RHE and the FECOsurpassing 95% within potential ranging of -0.88 to -1.38 V vs. RHE. The building of this parameter for CO2RR activity of Ni-N-C give instructive forecast for low-cost and highly active CO2RR electrocatalysts.

Composition effects of electrodeposited Cu-Ag nanostructured electrocatalysts for CO2 reduction

The electrochemical carbon dioxide reduction (CO2RR) on Cu-based catalysts is a promising strategy to store renewable electricity and produce valuable C2+ chemicals. We investigate the CO2RR on Cu-Ag nanostructures that have been electrodeposited in a green choline chloride and urea deep eutectic solvent (DES). We determine the electrochemically active surface area (ECSA) using lead underpotential deposition (UPD) to investigate the CO2RR intrinsic activity and selectivity. We show that the addition of Ag on electrodeposited Cu primarily suppresses the production of hydrogen and methane. While the production of carbon monoxide slightly increases, the partial current of the total C2+ products does not considerably increase. Despite that the production rate of C2+ is similar on Cu and Cu-Ag, the addition of Ag enhances the formation of alcohols and oxygenates over ethylene. We highlight the potential of metal electrodeposition from DES as a sustainable strategy to develop bimetallic Cu-based nanocatalysts for CO2RR.

Mo4+-Doped CuS Nanosheet-Assembled Hollow Spheres for CO2 Electroreduction to Ethanol in a Flow Cell

Electrocatalytic CO2 reduction reaction (CO2RR) to ethanol has been widely researched for potential commercial application. However, it still faces limited selectivity at a large current density. Herein, Mo4+-doped CuS nanosheet-assembled hollow spheres are constructed to address this issue. Mo4+ ion doping modifies the local electronic environments and diversifies the binding sites of CuS, which increases the coverage of linear *COL and produces bridge *COB for subsequent *COL-*COH coupling toward ethanol production. The optimal Mo9.0%-CuS can electrocatalyze CO2 to ethanol with a faradaic efficiency of 67.5% and a partial current density of 186.5 mA cm-2 at -0.6 V in a flow cell. This work clarifies that doping high valence transition metal ions into Cu-based sulfides can regulate the coverage and configuration of related intermediates for ethanol production during the CO2RR in a flow cell.

Review on strategies for improving the added value and expanding the scope of CO2 electroreduction products

The electrochemical reduction of CO2 into value-added chemicals has been explored as a promising solution to realize carbon neutrality and inhibit global warming. This involves utilizing the electrochemical CO2 reduction reaction (CO2RR) to produce a variety of single-carbon (C1) and multi-carbon (C2+) products. Additionally, the electrolyte solution in the CO2RR system can be enriched with nitrogen sources (such as NO3-, NO2-, N2, or NO) to enable the synthesis of organonitrogen compounds via C-N coupling reactions. However, the electrochemical conversion of CO2 into valuable chemicals still faces challenges in terms of low product yield, poor faradaic efficiency (FE), and unclear understanding of the reaction mechanism. This review summarizes the promising strategies aimed at achieving selective production of diverse carbon-containing products, including CO, formate, hydrocarbons, alcohols, and organonitrogen compounds. These approaches involve the rational design of electrocatalysts and the construction of coupled electrocatalytic reaction systems. Moreover, this review presents the underlying reaction mechanisms, identifies the existing challenges, and highlights the prospects of the electrosynthesis processes. The aim is to offer valuable insights and guidance for future research on the electrocatalytic conversion of CO2 into carbon-containing products of enhanced value-added potential.

Effect of crystal facets in plasmonic catalysis

While the role of crystal facets is well known in traditional heterogeneous catalysis, this effect has not yet been thoroughly studied in plasmon-assisted catalysis, where attention has primarily focused on plasmon-derived mechanisms. Here, we investigate plasmon-assisted electrocatalytic CO2 reduction using different shapes of plasmonic Au nanoparticles - nanocube (NC), rhombic dodecahedron (RD), and octahedron (OC) - exposing {100}, {110}, and {111} facets, respectively. Upon plasmon excitation, Au OCs doubled CO Faradaic efficiency (FECO) and tripled CO partial current density (jCO) compared to a dark condition, with NCs also improving under illumination. In contrast, Au RDs maintained consistent performance irrespective of light exposure, suggesting minimal influence of light on the reaction. Temperature experiments ruled out heat as the main factor to explain such differences. Atomistic simulations and electromagnetic modeling revealed higher hot carrier abundance and electric field enhancement on Au OCs and NCs than RDs. These effects now dominate the reaction landscape over the crystal facets, thus shifting the reaction sites when comparing dark and plasmon-activated processes. Plasmon-assisted H2 evolution reaction experiments also support these findings. The dominance of low-coordinated sites over facets in plasmonic catalysis suggests key insights for designing efficient photocatalysts for energy conversion and carbon neutralization.

Graphene Electrode for Studying CO2 Electroreduction Nanocatalysts under Realistic Conditions in Microcells

The ability to resolve the dynamic evolution of electrocatalytically induced processes with electrochemical liquid-phase electron microscopy (EM) is limited by the microcell configuration. Herein, a free-standing tri-layer graphene is integrated as a membrane and electrode material into the electrochemical chip and its suitability as a substrate electrode at the high cathodic potentials required for CO2 electroreduction (CO2ER) is evaluated. The three-layer stacked graphene is transferred onto an in-house fabricated single-working electrode chip for use with bulk-like reference and counter electrodes to facilitate evaluation of its effectiveness. Electrochemical measurements show that the graphene working electrode exhibits a wider inert cathodic potential range than the conventional glassy carbon electrode while achieving good charge transfer properties for nanocatalytic redox reactions. Operando scanning electron microscopy studies clearly demonstrate the improvement in spatial resolution but reveal a synergistic effect of the electron beam and the applied potential that limits the stability time window of the graphene-based electrochemical chip. By optimizing the operating conditions, in situ monitoring of Cu nanocube degradation is achieved at the CO2ER potential of -1.1 V versus RHE. Thus, this improved microcell configuration allows EM observation of catalytic processes at potentials relevant to real systems.

Local reaction environment in electrocatalysis

Beyond conventional electrocatalyst engineering, recent studies have unveiled the effectiveness of manipulating the local reaction environment in enhancing the performance of electrocatalytic reactions. The general principles and strategies of local environmental engineering for different electrocatalytic processes have been extensively investigated. This review provides a critical appraisal of the recent advancements in local reaction environment engineering, aiming to comprehensively assess this emerging field. It presents the interactions among surface structure, ions distribution and local electric field in relation to the local reaction environment. Useful protocols such as the interfacial reactant concentration, mass transport rate, adsorption/desorption behaviors, and binding energy are in-depth discussed toward modifying the local reaction environment. Meanwhile, electrode physical structures and reaction cell configurations are viable optimization methods in engineering local reaction environments. In combination with operando investigation techniques, we conclude that rational modifications of the local reaction environment can significantly enhance various electrocatalytic processes by optimizing the thermodynamic and kinetic properties of the reaction interface. We also outline future research directions to attain a comprehensive understanding and effective modulation of the local reaction environment.

Designing Surface and Interface Structures of Copper-Based Catalysts for Enhanced Electrochemical Reduction of CO2 to Alcohols

Electrochemical CO2 reduction (ECR) has emerged as a promising solution to address both the greenhouse effect caused by CO2 emissions and the energy shortage resulting from the depletion of nonrenewable fossil fuels. The production of multicarbon (C2+) products via ECR, especially high-energy-density alcohols, is highly desirable for industrial applications. Copper (Cu) is the only metal that produces alcohols with appreciable efficiency and kinetic viability in aqueous solutions. However, poor product selectivity is the main technical problem for applying the ECR technology in alcohol production. Extensive research has resulted in the rational design of electrocatalyst architectures using various strategies. This design significantly affects the adsorption energetics of intermediates and the reaction pathways for alcohol production. In this review, we focus on the design of effective catalysts for ECR to alcohols, discussing fundamental principles, innovative strategies, and mechanism understanding. Furthermore, the challenges and prospects in utilizing Cu-based materials for alcohol production via ECR are discussed.

Potential Alignment in Tandem Catalysts Enhances CO2-to-C2H4 Conversion Efficiencies

The in-tandem catalyst holds great promise for addressing the limitation of low *CO coverage on Cu-based materials for selective C2H4 generation during CO2 electroreduction. However, the potential mismatch between the CO-formation catalyst and the favorable C-C coupling Cu catalyst represents a bottleneck in these types of electrocatalysts, resulting in low tandem efficiencies. In this study, we propose a robust solution to this problem by introducing a wide-CO generation-potential window nickel single atom catalyst (Ni SAC) supported on a Cu catalyst. The selection of Ni SAC was based on theoretical calculations, and its excellent performance was further confirmed by using in situ IR spectroscopy. The facilitated carbon dimerization in our tandem catalyst led to a ∼370 mA/cm2 partial current density of C2H4, corresponding to a faradic efficiency of ∼62%. This performance remained stable and consistent for at least ∼14 h at a high current density of 500 mA/cm2 in a flow-cell reactor, outperforming most tandem catalysts reported so far.

Breaking K+ Concentration Limit on Cu Nanoneedles for Acidic Electrocatalytic CO2 Reduction to Multi-Carbon Products

Electrocatalytic CO2 reduction reaction (CO2 RR) to multi-carbon products (C2+ ) in acidic electrolyte is one of the most advanced routes for tackling our current climate and energy crisis. However, the competing hydrogen evolution reaction (HER) and the poor selectivity towards the valuable C2+ products are the major obstacles for the upscaling of these technologies. High local potassium ions (K+ ) concentration at the cathode's surface can inhibit proton-diffusion and accelerate the desirable carbon-carbon (C-C) coupling process. However, the solubility limit of potassium salts in bulk solution constrains the maximum achievable K+ concentration at the reaction sites and thus the overall acidic CO2 RR performance of most electrocatalysts. In this work, we demonstrate that Cu nanoneedles induce ultrahigh local K+ concentrations (4.22 M) - thus breaking the K+ solubility limit (3.5 M) - which enables a highly efficient CO2 RR in 3 M KCl at pH=1. As a result, a Faradaic efficiency of 90.69±2.15 % for C2+ (FEC2+ ) can be achieved at 1400 mA.cm-2 , simultaneous with a single pass carbon efficiency (SPCE) of 25.49±0.82 % at a CO2 flow rate of 7 sccm.

Integration of Cobalt Phthalocyanine, Acetylene Black and Cu2 O Nanocubes for Efficient Electroreduction of CO2 to C2 H4

Suppressing side reactions and simultaneously enriching key intermediates during CO2 reduction reaction (CO2 RR) has been a challenge. Here, we propose a tandem catalyst (Cu2 O NCs-C-Copc) consisting of acetylene black, cobalt phthalocyanine (Copc) and cuprous oxide nanocubes (Cu2 O NCs) for efficient CO2 -to-ethylene conversion. Density-functional theory (DFT) calculation combined with experimental verification demonstrated that Copc can provide abundant CO to nearby copper sites while acetylene black successfully reduces the formation energies of key intermediates, leading to enhanced C2 H4 selectivity. X-ray photoelectron spectroscopy (XPS) and potentiostatic tests indicated that the catalytic stability of Cu2 O NCs-C-Copc was significantly enhanced compared with Cu2 O NCs. Finally, the industrial application prospect of the catalyst was evaluated using gas diffusion electrolyzers. TheF E C 2 H 4 ${{\rm { F}}{{\rm { E}}}_{{{\rm { C}}}_{{\rm { 2}}}{{\rm { H}}}_{{\rm { 4}}}}}$ of Cu2 O NCs-C-Copc can reach to 58.4 % at -1.1 V vs. RHE in 0.1 M KHCO3 and 70.3 % at -0.76 V vs. RHE in 1.0 M KOH. This study sheds new light on the design and development of highly efficient CO2 RR tandem catalytic systems.

Multiscale CO2 Electrocatalysis to C2+ Products: Reaction Mechanisms, Catalyst Design, and Device Fabrication

Electrosynthesis of value-added chemicals, directly from CO2, could foster achievement of carbon neutral through an alternative electrical approach to the energy-intensive thermochemical industry for carbon utilization. Progress in this area, based on electrogeneration of multicarbon products through CO2 electroreduction, however, lags far behind that for C1 products. Reaction routes are complicated and kinetics are slow with scale up to the high levels required for commercialization, posing significant problems. In this review, we identify and summarize state-of-art progress in multicarbon synthesis with a multiscale perspective and discuss current hurdles to be resolved for multicarbon generation from CO2 reduction including atomistic mechanisms, nanoscale electrocatalysts, microscale electrodes, and macroscale electrolyzers with guidelines for future research. The review ends with a cross-scale perspective that links discrepancies between different approaches with extensions to performance and stability issues that arise from extensions to an industrial environment.

Acidic Media Impedes Tandem Catalysis Reaction Pathways in Electrochemical CO2 Reduction

Electrochemical CO2 reduction (CO2 R) in acidic media with Cu-based catalysts tends to suffer from lowered selectivity towards multicarbon products. This could in principle be mitigated using tandem catalysis, whereby the *CO coverage on Cu is increased by introducing a CO generating catalyst (e.g. Ag) in close proximity. Although this has seen significant success in neutral/alkaline media, here we report that such a strategy becomes impeded in acidic electrolyte. This was investigated through the co-reduction of 13 CO2 /12 CO mixtures using a series of Cu and CuAg catalysts. These experiments provide strong evidence for the occurrence of tandem catalysis in neutral media and its curtailment under acidic conditions. Density functional theory simulations suggest that the presence of H3 O+ weakens the *CO binding energy of Cu, preventing effective utilization of tandem-supplied CO. Our findings also provide other unanticipated insights into the tandem catalysis reaction pathway and important design considerations for effective CO2 R in acidic media.

Evolution of Electrocatalytic CO2 Reduction Activity Induced by Charge Segregation in Atomically Precise AuAg Nanoclusters Based on Icosahedral M13 Unit 3D Assembly

The precise self-assembly of building blocks at atomic level provides the opportunity to achieve clusters with advanced catalytic properties. However, most of the current self-assembled materials are fabricated by 1/2D assembly of blocks. High dimensional (that is, 3D) assembly is widely believed to improve the performance of cluster. Herein, the effect of 3D assembly on the activity for electrocatalytic CO2 reduction reaction (CO2 RR) is investigated by using a range of clusters (Au8 Ag55 , Au8 Ag57 , Au12 Ag60 ) based on 3D assembly of M13 unit as models. Although three clusters have almost the same sizes and geometric structures, Au8 Ag55 exhibits the best CO2 RR performance due to the strong CO2 adsorption capacity and effective inhibition of H2 evolution competition reaction. The deep insight into the superior activity of Au8 Ag55 is the unique electronic structure attributed to the charge segregation. This study not only demonstrates that the assembly mode greatly affects the catalytic activity, but also offers an idea for rational designing and precisely constructing catalysts with controllable activities.

Multilayer-Cavity Tandem Catalyst for Profiling Sequentially Coupling of Intermediate CO in Electrocatalytic Reduction Reaction of CO2 to Multi-Carbon Products

Electrochemical CO2  reduction reaction (CO2 RR) is an effective approach to address CO2  emission, promote recycling, and synthesize high-value multi-carbon (C2+ ) chemicals for storing renewable electricity in the long-term. The construction of multilayer-bound nanoreactors to achieve management of intermediate CO is a promising strategy for tandem to C2+ products. In this study, a series of Ag@Cu2 O nanoreactors consisting of an Ag-yolk and a multilayer confined Cu shell is designed to profile electrocatalytic CO2 RR reactions. The optimized Ag@Cu2 O-2 nanoreactor exhibits a 74% Faradaic efficiency during the C2+ pathway and remains stable for over 10 h at a bias current density of 100 mA cm-2 . Using the finite element method, this model determines that the certain volume of cavity in the Ag@Cu2 O nanoreactors facilitates on-site CO retention and that multilayers of Cu species favor CO capture. Density functional theory calculations illustrate that the biased generation of ethanol products may arise from the (100)/(111) interface of the Cu layer. In-depth explorations in multilayer-bound nanoreactors provide structural and interfacial guidance for sequential coupling of CO2 RR intermediates for efficient C2+ generation.

Adjacent Copper Single Atoms Promote C-C Coupling in Electrochemical CO2 Reduction for the Efficient Conversion of Ethanol

The electrochemical CO2 reduction reaction (CO2RR) using renewable electricity is one of the most promising strategies for reaching the goal of carbon neutrality. Multicarbonous (C2+) products have broad applications, and ethanol is a valuable chemical and fuel. Many Cu-based catalysts have been reported to be efficient for the electrocatalytic CO2RR to C2+ products, but they generally offer limited selectivity and current density toward ethanol. Herein, we proposed a silica-mediated hydrogen-bonded organic framework (HOF)-templated approach to preparing ultrahigh-density Cu single-atom catalysts (SACs) on thin-walled N-doped carbon nanotubes (TWN). The content of Cu in the catalysts prepared by this method could be up to 13.35 wt %. It was found that the catalysts showed outstanding performance for the electrochemical CO2RR to ethanol, and the Faradaic efficiency (FE) of ethanol increased with the increase in Cu-N3 site density. The FE of ethanol over the catalysts with 13.35 wt % Cu could reach ∼81.9% with a partial current density of 35.6 mA cm-2 using an H-type cell, which is the best result for electrochemical CO2RR to ethanol to date. In addition, the catalyst could be stably used for more than 25 h. Experimental and density functional theory (DFT) studies revealed that the adjacent Cu-N3 active sites (one Cu atom coordinates with three N) were the active sites for the reaction, and their high density was crucial for the high FE of ethanol because the adjacent Cu-N3 sites with a short distance could promote the C-C coupling synergistically.

Nanoreactor Engineering Can Unlock New Possibilities for CO2 Tandem Catalytic Conversion to C-C Coupled Products

Climate change is becoming increasingly more pronounced every day while the amount of greenhouse gases in the atmosphere continues to rise. CO2 reduction to valuable chemicals is an approach that has gathered substantial attention as a means to recycle these gases. Herein, some of the tandem catalysis approaches that can be used to achieve the transformation of CO2 to C-C coupled products are explored, focusing especially on tandem catalytic schemes where there is a big opportunity to improve performance by designing effective catalytic nanoreactors. Recent reviews have highlighted the technical challenges and opportunities for advancing tandem catalysis, especially highlighting the need for elucidating structure-activity relationships and mechanisms of reaction through theoretical and in situ/operando characterization techniques. In this review, the focus is on nanoreactor synthesis strategies as a critical research direction, and discusses these in the context of two main tandem pathways (CO-mediated pathway and Methanol-mediated pathway) to C-C coupled products.

Interfacial Chemistry in the Electrocatalytic Hydrogenation of CO2 over C-Supported Cu-Based Systems

Operando soft and hard X-ray spectroscopic techniques were used in combination with plane-wave density functional theory (DFT) simulations to rationalize the enhanced activities of Zn-containing Cu nanostructured electrocatalysts in the electrocatalytic CO2 hydrogenation reaction. We show that at a potential for CO2 hydrogenation, Zn is alloyed with Cu in the bulk of the nanoparticles with no metallic Zn segregated; at the interface, low reducible Cu(I)-O species are consumed. Additional spectroscopic features are observed, which are identified as various surface Cu(I) ligated species; these respond to the potential, revealing characteristic interfacial dynamics. Similar behavior was observed for the Fe-Cu system in its active state, confirming the general validity of this mechanism; however, the performance of this system deteriorates after successive applied cathodic potentials, as the hydrogen evolution reaction then becomes the main reaction pathway. In contrast to an active system, Cu(I)-O is now consumed at cathodic potentials and not reversibly reformed when the voltage is allowed to equilibrate at the open-circuit voltage; rather, only the oxidation to Cu(II) is observed. We show that the Cu-Zn system represents the optimal active ensembles with stabilized Cu(I)-O; DFT simulations rationalize this observation by indicating that Cu-Zn-O neighboring atoms are able to activate CO2, whereas Cu-Cu sites provide the supply of H atoms for the hydrogenation reaction. Our results demonstrate an electronic effect exerted by the heterometal, which depends on its intimate distribution within the Cu phase and confirms the general validity of these mechanistic insights for future electrocatalyst design strategies.

The ABC of Generalized Coordination Numbers and Their Use as a Descriptor in Electrocatalysis

The quest for enhanced electrocatalysts can be boosted by descriptor-based analyses. Because adsorption energies are the most common descriptors, electrocatalyst design is largely based on brute-force routines that comb materials databases until an energetic criterion is verified. In this review, it is shown that an alternative is provided by generalized coordination numbers (denoted by CN ¯ $\overline {{\rm{CN}}} $ or GCN), an inexpensive geometric descriptor for strained and unstrained transition metals and some alloys. CN ¯ $\overline {{\rm{CN}}} $ captures trends in adsorption energies on both extended surfaces and nanoparticles and is used to elaborate structure-sensitive electrocatalytic activity plots and selectivity maps. Importantly, CN ¯ $\overline {{\rm{CN}}} $ outlines the geometric configuration of the active sites, thereby enabling an atom-by-atom design, which is not possible using energetic descriptors. Specific examples for various adsorbates (e.g., *OH, *OOH, *CO, and *H), metals (e.g., Pt and Cu), and electrocatalytic reactions (e.g., O₂  reduction, H₂  evolution, CO oxidation, and reduction) are presented, and comparisons are made against other descriptors.

TM2 -B2 Quadruple Active Sites Supported on a Defective C3 N Monolayer as Catalyst for the Electrochemical CO2 Reduction: A Theoretical Perspective

Developing high-performance electrocatalysts for the CO₂ reduction reaction (CO₂ RR) holds great potential to mitigate the depletion of fossil feedstocks and abate the emission of CO₂ . In this contribution, using density functional theory calculations, we systematically investigated the CO₂ RR performance catalyzed by TM₂ -B₂ (TM=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu) supported on a defective C₃ N monolayer (V-C₃ N). Through the screening in terms of stability of catalyst, activity towards CO₂ adsorption, and selectivity against hydrogen evolution reaction, Mn₂ -, Fe₂ -, Co₂ -, and Ni₂ -B₂ @V-C₃ N were demonstrated to be a highly promising CO₂ RR electrocatalyst. Due to quadruple active sites, these candidates can adsorb two or three CO₂ molecules. Strikingly, different products, distributing from C₁ to C₂₊ , can be generated. The high activity originates from the synergistic effect of TM and B atoms, in which they serve as adsorption sites for the C- and O-species, respectively. The high selectivity towards C₂₊ products at the Fe₂ -, and Ni₂ -B₂ sites stems from moderate C adsorption strength but relatively weak O adsorption strength, in which a universal descriptor, that is, 0.6 ΔE_C -0.4 ΔE_O =-1.77 eV (ΔE_C /ΔE_O is the adsorption energy of C/O), was proposed. This work would offer a novel perspective for the design of high active electrocatalysts towards CO₂ RR and for the synthesis of C₂₊ compounds.

2D carbon nitride as a support with single Cu, Ag, and Au atoms for carbon dioxide reduction reaction

The electrochemical conversion of CO2 into value-added chemicals is an important approach to recycling CO2. In this work, we have combined the most efficient metal catalysts for this reaction, namely Cu, Ag, and Au, as single-atom particles dispersed on a two-dimensional carbon nitride support, with the aim of exploring their performance in the CO2 reduction reaction. Here, we report density functional theory computations showing the effect of single metal-atom particles on the support. We found that bare carbon nitride needed a high overpotential to overcome the energy barrier for the first proton-electron transfer, while the second transfer was exergonic. The deposition of single metal atoms enhances the catalytic activity of the system as the first proton-electron transfer is favored in terms of energy, although strong binding energies were found for CO adsorption on Cu and Au single atoms. Our theoretical interpretations are consistent with the experimental evidence that the competitive H2 generation is favored due to the strong CO binding energies. Our computational study paves the road to finding suitable metals that catalyze the first proton-electron transfer in the carbon dioxide reduction reaction and produce reaction intermediates with moderate binding energies, promoting a spillover to the carbon nitride support and thereby serving as bifunctional electrocatalysts.

Alloying as a Strategy to Boost the Stability of Copper Nanocatalysts during the Electrochemical CO2 Reduction Reaction

Copper nanocatalysts are among the most promising candidates to drive the electrochemical CO₂ reduction reaction (CO₂RR). However, the stability of such catalysts during operation is sub-optimal, and improving this aspect of catalyst behavior remains a challenge. Here, we synthesize well-defined and tunable CuGa nanoparticles (NPs) and demonstrate that alloying Cu with Ga considerably improves the stability of the nanocatalysts. In particular, we discover that CuGa NPs containing 17 at. % Ga preserve most of their CO₂RR activity for at least 20 h while Cu NPs of the same size reconstruct and lose their CO₂RR activity within 2 h. Various characterization techniques, including X-ray photoelectron spectroscopy and operando X-ray absorption spectroscopy, suggest that the addition of Ga suppresses Cu oxidation at open-circuit potential (ocp) and induces significant electronic interactions between Ga and Cu. Thus, we explain the observed stabilization of the Cu by Ga as a result of the higher oxophilicity and lower electronegativity of Ga, which reduce the propensity of Cu to oxidize at ocp and enhance the bond strength in the alloyed nanocatalysts. In addition to addressing one of the major challenges in CO₂RR, this study proposes a strategy to generate NPs that are stable under a reducing reaction environment.

Accelerated Transfer and Spillover of Carbon Monoxide through Tandem Catalysis for Kinetics-boosted Ethylene Electrosynthesis

Cu-based catalysts have been widely applied in electroreduction of carbon dioxide (CO₂ ER) to produce multicarbon (C₂₊ ) feedstocks (e.g., C₂ H₄ ). However, the high energy barriers for CO₂ activation on the Cu surface is a challenge for a high catalytic efficiency and product selectivity. Herein, we developed an in situ *CO generation and spillover strategy by engineering single Ni atoms on a pyridinic N-enriched carbon support with a sodalite (SOD) topology (Ni-SOD/NC) that acted as a donor to feed adjacent Cu nanoparticles (NPs) with *CO intermediate. As a result, a high C₂ H₄ selectivity of 62.5 % and an industrial-level current density of 160 mA cm^-2 at a low potential of -0.72 V were achieved. Our studies revealed that the isolated NiN₃ active sites with adjacent pyridinic N species facilitated the *CO desorption and the massive *CO intermediate released from Ni-SOD/NC then overflowed to Cu NPs surface to enrich the *CO coverage for improving the selectivity of CO₂ ER to C₂ H₄ .

Recent Progress in Surface-Defect Engineering Strategies for Electrocatalysts toward Electrochemical CO2 Reduction: A Review

Climate change, caused by greenhouse gas emissions, is one of the biggest threats to the world. As per the IEA report of 2021, global CO2 emissions amounted to around 31.5 Gt, which increased the atmospheric concentration of CO2 up to 412.5 ppm. Thus, there is an imperative demand for the development of new technologies to convert CO2 into value-added feedstock products such as alcohols, hydrocarbons, carbon monoxide, chemicals, and clean fuels. The intrinsic properties of the catalytic materials are the main factors influencing the efficiency of electrochemical CO2 reduction (CO2-RR) reactions. Additionally, the electroreduction of CO2 is mainly affected by poor selectivity and large overpotential requirements. However, these issues can be overcome by modifying heterogeneous electrocatalysts to control their morphology, size, crystal facets, grain boundaries, and surface defects/vacancies. This article reviews the recent progress in electrochemical CO2 reduction reactions accomplished by surface-defective electrocatalysts and identifies significant research gaps for designing highly efficient electrocatalytic materials.

Graphdiyne supported Ag-Cu tandem catalytic scheme for electrocatalytic reduction of CO2 to C2+ products

The electrochemical CO₂ reduction reaction (CO₂RR) to added-value C₂₊ products is a worthy way to effectively reduce CO₂ levels in the atmosphere. Cu nanomaterials have been proposed as efficient CO₂RR catalysts for producing C₂₊ products; however, the difficulties in controlling their efficiency and selectivity hinder their applications. Herein, we propose a simple routine to construct a graphdiyne (GDY) supported Ag-Cu nanocluster as a C₂₊ product-selective electrocatalyst and optimize the composition by electrochemical performance screening. The synthesized Ag-Cu nanoclusters are uniformly distributed on the surface of GDY with particle sizes constricted to 3.7 nm due to the strong diyne-Cu interaction. Compared to Cu/GDY, Ag-Cu/GDY tandem schemes exhibited superior CO₂RR to C₂₊ performance with a Faraday efficiency (FE) of up to 55.1% and a current density of 48.6 mA cm^-2 which remain stable for more than 33 hours. Theoretical calculations show that the adsorption energy of CO is much higher on Cu (-1.066 eV) than on Ag (-0.615 eV), thus promoting the drift of *CO from Ag to Cu. Moreover, the calculations indicate that the key C-C coupling reaction of *CO with *COH is more favored on Ag-Cu/GDY than on the original Cu/GDY which contributes to the formation of C₂₊ products. Our findings shed light on a new strategy of combining a GDY support with a tandem catalytic scheme for developing new CO₂RR catalysts with superior selectivity and activity for C₂₊ products.

The spatial distribution of cobalt phthalocyanine and copper nanocubes controls the selectivity towards C2 products in tandem electrocatalytic CO2 reduction

The coupling of CO-generating molecular catalysts with copper electrodes in tandem schemes is a promising strategy to boost the formation of multi-carbon products in the electrocatalytic reduction of CO₂. While the spatial distribution of the two components is important, this aspect remains underexplored for molecular-based tandem systems. Herein, we address this knowledge gap by studying tandem catalysts comprising Co-phthalocyanine (CoPc) and Cu nanocubes (Cu_cub). In particular, we identify the importance of the relative spatial distribution of the two components on the performance of the tandem catalyst by preparing CoPc-Cu_cub/C, wherein the CoPc and Cu_cub share an interface, and CoPc-C/Cu_cub, wherein the CoPc is loaded first on carbon black (C) before mixing with the Cu_cub. The electrocatalytic measurements of these two catalysts show that the faradaic efficiency towards C₂ products almost doubles for the CoPc-Cu_cub/C, whereas it decreases by half for the CoPc-C/Cu_cub, compared to the Cu_cub/C. Our results highlight the importance of a direct contact between the CO-generating molecular catalyst and the Cu to promote C-C coupling, which hints at a surface transport mechanism of the CO intermediate between the two components of the tandem catalyst instead of a transfer via CO diffusion in the electrolyte followed by re-adsorption.

Surface Site-Specific Replacement for Catalysis Selectivity Switching

Surface atom replacement in materials without other composition/structure changes is challenging but is important for fundamental scientific research and for practical applications. In particular, for nanoparticles including nanoclusters, surface metal site-specific replacement with atomic precision has not yet been achieved. In this study, we for the first time achieved surface site-specific antigalvanic replacement with the remaining composition/structure and surface replacement-dependent selectivity in the electrocatalytic reduction of CO₂. Density functional theory (DFT) calculations describe the catalysis selectivity switch induced by replacing Ag with Cu and explain why Cu replacement facilitates C₂ production. Also, CO₂ electroreduction to C₂ on well-defined metal nanoclusters is first reported in this study.

Design strategy of a Cu-based catalyst for optimizing the performance in the electrochemical CO2 reduction reaction to multicarbon alcohols

The electrochemical CO₂ reduction reaction (ECRR) is a promising method to reduce excessive CO₂ emissions and achieve a sustainable carbon cycle. Due to the high reaction kinetics and efficiency, copper-based catalysts have shown great application potential for preparing multicarbon (C₂₊) products. C₂₊ alcohols have high economic value and use-value, playing an essential role in modern industry. Therefore, we summarize the latest research progress of the ECRR to synthesize C₂₊ alcohols on Cu-based catalysts and discuss the state-of-the-art catalyst design strategies to improve CO₂ reduction performance. Moreover, we analyzed in detail the specific reaction pathways for the conversion of CO₂ to C₂₊ alcohols based on DFT calculations. Finally, we propose the problems and possible solutions for synthesizing C₂₊ alcohols with copper-based catalysts. We hope that this review can provide ideas for devising ECRR catalysts for C₂₊ alcohols.

Tandem electrocatalytic CO2 reduction with Fe-porphyrins and Cu nanocubes enhances ethylene production

Copper-based tandem schemes have emerged as promising strategies to promote the formation of multi-carbon products in the electrocatalytic CO2 reduction reaction. In such approaches, the CO-generating component of the tandem catalyst increases the local concentration of CO and thereby enhances the intrinsic carbon-carbon (C-C) coupling on copper. However, the optimal characteristics of the CO-generating catalyst for maximizing the C2 production are currently unknown. In this work, we developed tunable tandem catalysts comprising iron porphyrin (Fe-Por), as the CO-generating component, and Cu nanocubes (Cucub) to understand how the turnover frequency for CO (TOFCO) of the molecular catalysts impacts the C-C coupling on the Cu surface. First, we tuned the TOFCO of the Fe-Por by varying the number of orbitals involved in the π-system. Then, we coupled these molecular catalysts with the Cucub and assessed the current densities and faradaic efficiencies. We discovered that all of the designed Fe-Por boost ethylene production. The most efficient Cucub/Fe-Por tandem catalyst was the one including the Fe-Por with the highest TOFCO and exhibited a nearly 22-fold increase in the ethylene selectivity and 100 mV positive shift of the onset potential with respect to the pristine Cucub. These results reveal that coupling the TOFCO tunability of molecular catalysts with copper nanocatalysts opens up new possibilities towards the development of Cu-based catalysts with enhanced selectivity for multi-carbon product generation at low overpotential.

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

Concentration Optimization of Localized Cu0 and Cu+ on Cu-Based Electrodes for Improving Electrochemical Generation of Ethanol from Carbon Dioxide

Copper-based electrodes can catalyze electroreduction of CO₂ to two-carbon products. However, obtaining a specific product with high efficiency depends on the oxidation state of Cu for the Cu-based materials. In this study, Cu-based electrodes were prepared on fluorinated tin oxide (FTO) using the one-step electrodeposition method. These electrodes were used as efficient electrocatalysts for CO₂ reduction to ethanol. The concentration ratio of Cu⁰ and Cu⁺ on the electrodes was precisely modulated by adding monoethanolamine (MEA). The results of spectroscopic characterization showed that the concentration ratio of localized Cu⁺ and Cu⁰ (Cu⁺/Cu⁰) on the Cu-based electrodes was controlled from 1.24/1 to 1.54/1 by regulating the amount of MEA. It was found that the electrode exhibited the best electrochemical efficiency and ethanol production in the CO₂ reduction reaction at the optimal concentration ratio Cu⁺/Cu⁰ of 1.42/1. The maximum faradaic efficiencies of ethanol and C₂ were 48% and 77%, respectively, at the potential of -0.6 V vs. a reversible hydrogen electrode (RHE). Furthermore, the optimal concentration ratio of Cu⁺/Cu⁰ achieved the balance between Cu⁺ and Cu⁰ with the most favorable free energy for the formation of *CO intermediate. The stable existence of the *CO intermediate significantly contributed to the formation of the C-C bond for ethanol production.

Near infrared optically responsive Ag-Cu bimetallic 2D nanocrystals with controllable spatial structures

The optical properties of cost-effective Ag-Cu bimetallic nanocrystals, with synergistically enhanced catalytic and biological activities, are limited within ultraviolet-visible region due to lack of morphology control. In order to overcome this constraint, two-dimensional (2D) Ag-Cu bimetallic heterostructures were designed and synthesized by a seed-mediated colloidal growth method. The conformal Cu domain was epitaxially deposited on Ag nanoplates with different spatial configuration under retention of their 2D shape. Both of the 2D Ag-Cu core@shell and Janus structures display tunable localized surface plasmon resonance from visible to near infrared regions. The results of catalytic reduction of 4-nitrophenol show that the 2D Ag-Cu core@shell structure has better synergistic catalytic performance than Janus structure and Ag plates. In addition to surface-related synergistically enhanced bactericidal performance, their antibacterial effect can also be significantly enhanced by near infrared light irradiation. These results indicate that 2D Ag-Cu heterostructures can benefit from both synergistically improved surface activity and great optical responsive characteristics.