A Surface Reconstruction Route to High Productivity and Selectivity in CO2 Electroreduction toward C2+ Hydrocarbons

Atomic-Level Elucidation of Lattice-Hydrogens in Copper Catalysts for Selective CO2 Electrochemical Conversion toward C2 Products

Copper is the most efficient and practical electrocatalyst for the electrochemical reduction of carbon dioxide (ECR) to give multicarbon (C2+) products, but the mechanism by which such products are formed - though known to involve lattice-hydrogens - remains elusive, and the selectivity of the reaction is poor. Herein, we report the synthesis of [AuCu24(dppp)6H22]+, a copper hydride nanocluster bearing exposed Cu3H3 units in specific surface cavities, and our use of it to study the mechansim and selectivity of the reduction of CO2 to C2+ products. Results of in situ infrared spectroscopy and theoretical calculations showed that these Cu3H3 units can effectively lower the energy barrier to the formation of the *COCOH intermediate, which allowed the competition between the C1 and C2 pathways to be elucidated. Isotope labeling experiments and catalyst recrystallization studies corroborated the theoretical simulations, identifying the lattice-hydrogen (H-) in the Cu3H3 active unit as being indispensable for the formation of C2H4. The molecular design guidelines which this work has facilitated constitute a new approach towards the of copper-based catalysts that convert CO2 to C2+ products based on lattice-hydrogen engineering.

Heteroarchitectural Gas Diffusion Layer Promotes CO2 Reduction Coupled with Biomass Oxidation at Ampere-Level Current Density

Achieving high product selectivity at ampere-level current densities is essential for the industrial application of electrochemical CO2 reduction. However, the operational stability of CO2 electrolyzers at large current density has long been hindered by flooding of gas diffusion layer (GDL). Herein, a new heteroarchitectural GDL is designed to overcome flooding. Such GDL is constructed by sequentially sputtering the conductive silver and titanium boride (TiB2) onto a polytetrafluoroethylene substrate. Assembled with Cu catalyst in a flow cell, a maximum ethylene Faradaic efficiency of 64.7 % was achieved at a current density of 1.2 A cm-2 in 6 M KOH. Furthermore, the GDL is capable of stable operation for over 40 hours at 400 mA cm-2. Theoretical calculations and in situ experiments demonstrate enhanced intermediates adsorption on the TiB2-supported Cu surface, thereby reducing the energy barrier for C-C coupling. When coupling the CO2 reduction reaction with 5-hydroxymethylfurfural oxidation reaction, Faradaic efficiencies of 49.2 % for ethylene and 85.4 % for 2,5-furandicarboxylic acid were achieved at 1.2 A cm-2. This work provides a highly stable GDL for efficient CO2 conversion at ampere-level current density and paves the way for integrating biomolecules conversion in stack-level devices.

Controlling the Phase Composition of Pre-Catalysts to Obtain Abundant Cu(111)/Cu(200) Grain Boundaries for Enhancing Electrocatalytic CO2 Reduction Selectivity to Ethylene

The preparation of ethylene (C2H4) by electrochemical CO2 reduction (ECO2R) has dramatically progressed in recent years. However, the slow kinetics of carbon-carbon (C-C) coupling remains a significant challenge. A generalized facet reconstruction strategy is reported to prepare a 3-phase mixed pre-catalyst (Cu3N-300) of Cu3N, Cu2O, and CuO by controlling the calcination temperature and to obtain the derived Cu catalyst (A-Cu3N-300-0.5) enriched with Cu(111)/Cu(200) grain boundaries (GBs) by subsequent constant potential reduction. Its Faraday efficiency (FE) toward C2H4 at a low reaction potential of -1.07 V (vs reversible hydrogen electrode (RHE)) is 46.03%, which is much higher than the other 3 derived Cu catalysts containing single Cu(111) facets (24.89% and 24.52%) and Cu(111)/Cu(111) GBs (28.66%). Combining in situ experimental and theoretical computational studies, abundant Cu(111)/Cu(200) GBs is found to enhance CO2 activation and significantly promote the formation and adsorption of *CO intermediates, thereby lowering the activation energy barrier of C-C coupling and increasing the FE of C2H4.

Bridging activity gaps between batch and flow reactor configurations in the electroreduction of carbon dioxide

To date, the understanding of various modes of CO2 mass transport remains incomplete, impeding the transfer of catalysts identified in the more accessible electrochemical batch cells to high-performance flow cells. In this work, we demonstrate that the meniscus region formed between the electrode and the convex liquid level due to the electrowetting of the catalyst plays a vital role in the CO2RR in batch cells. CO2RR in the meniscus region in batch cells exhibits similar performance with that in flow cells, and the performance disparity between these two configurations largely disappears when conducting CO2RR primarily in the meniscus region. An assembled double-sided gas diffusion electrode with a gas channel is developed to maximize the meniscus-like region, achieving a CO2RR partial current density of 640 mA/cm2geo on commercial Cu in the KHCO3 electrolyte. This performance represents the highest CO2RR activity in neutral buffered media.

Exploring the influence of cell configurations on Cu catalyst reconstruction during CO2 electroreduction

Membrane electrode assembly (MEA) cells incorporating Cu catalysts are effective for generating C2+ chemicals via the CO2 reduction reaction (CO2RR). However, the impact of MEA configuration on the inevitable reconstruction of Cu catalysts during CO2RR remains underexplored, despite its considerable potential to affect CO2RR efficacy. Herein, we demonstrate that MEA cells prompt a unique reconstruction of Cu, in contrast to H-type cells, which subsequently influences CO2RR outcomes. Utilizing three Cu-based catalysts, specifically engineered with different nanostructures, we identify contrasting selectivity trends in the production of C2+ chemicals between H-type and MEA cells. Operando X-ray absorption spectroscopy, alongside ex-situ analyses in both cell types, indicates that MEA cells facilitate the reduction of Cu2O, resulting in altered Cu surfaces compared to those in H-type cells. Time-resolved CO2RR studies, supported by Operando analysis, further highlight that significant Cu reconstruction within MEA cells is a primary factor leading to the deactivation of CO2RR into C2+ chemicals.

Surface Facets Reconstruction in Copper-Based Materials for Enhanced Electrochemical CO2 Reduction

The unavoidable and unpredictable surface reconstruction of metallic copper (Cu) during the electrocatalytic carbon dioxide (CO2) reduction process is a double-edged sword affecting the production of high-value-added hydrocarbon products. It is crucial to control the surface facet reconstruction and regulate the targeted facets/facet interfaces, and further understand the mechanism between activity/selectivity and the reconstructed structure of Cu for CO2 reduction. Based on the current catalyst design methods, a facile strategy combining chemical reduction and electro-reduction is proposed to achieve specified Cu(111) facets and the Cu(110)/(111) interfaces in reconstructed Cu derived from cuprous oxide (Cu2O). The surface facet reconstruction significantly boosted the electrocatalytic conversion of CO2 into multi-carbon (C2+) products comparing to the unmodified catalyst. Theoretical and experimental analyses show that the Cu(110)/(111)s interface between Cu(110) and a small amount of Cu(111) can tailor the reaction routes and lower the reaction energy barrier of C-C coupling to ethylene (C2H4). The work will guide the surface facets reconstruction strategy for Cu-based CO2 electrocatalysts, providing a promising paradigm to understand the structural variation in catalysts.

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.

Anionic Coordination Control in Building Cu-Based Electrocatalytic Materials for CO2 Reduction Reaction

Renewable energy-driven conversion of CO2 to value-added fuels and chemicals via electrochemical CO2 reduction reaction (CO2RR) technology is regarded as a promising strategy with substantial environmental and economic benefits to achieve carbon neutrality. Because of its sluggish kinetics and complex reaction paths, developing robust catalytic materials with exceptional selectivity to the targeted products is one of the core issues, especially for extensively concerned Cu-based materials. Manipulating Cu species by anionic coordination is identified as an effective way to improve electrocatalytic performance, in terms of modulating active sites and regulating structural reconstruction. This review elaborates on recent discoveries and progress of Cu-based CO2RR catalytic materials enhanced by anionic coordination control, regarding reaction paths, functional mechanisms, and roles of different non-metallic anions in catalysis. Finally, the review concludes with some personal insights and provides challenges and perspectives on the utilization of this strategy to build desirable electrocatalysts.

Recent advances in dynamic reconstruction of electrocatalysts for carbon dioxide reduction

Electrocatalysts undergo structural evolution under operating electrochemical CO2 reduction reaction (CO2RR) conditions. This dynamic reconstruction correlates with variations in CO2RR activity, selectivity, and stability, posing challenges in catalyst design for electrochemical CO2RR. Despite increased research on the reconstruction behavior of CO2RR electrocatalysts, a comprehensive understanding of their dynamic structural evolution under reaction conditions is lacking. This review summarizes recent developments in the dynamic reconstruction of catalysts during the CO2RR process, covering fundamental principles, modulation strategies, and in situ/operando characterizations. It aims to enhance understanding of electrocatalyst dynamic reconstruction, offering guidelines for the rational design of CO2RR electrocatalysts.

Enhancing C2+ product selectivity in CO2 electroreduction by enriching intermediates over carbon-based nanoreactors

Electrochemical CO2 reduction reaction (CO2RR) to multicarbon (C2+) products faces challenges of unsatisfactory selectivity and stability. Guided by finite element method (FEM) simulation, a nanoreactor with cavity structure can facilitate C-C coupling by enriching *CO intermediates, thus enhancing the selectivity of C2+ products. We designed a stable carbon-based nanoreactor with cavity structure and Cu active sites. The unique geometric structure endows the carbon-based nanoreactor with a remarkable C2+ product faradaic efficiency (80.5%) and C2+-to-C1 selectivity (8.1) during the CO2 electroreduction. Furthermore, it shows that the carbon shell could efficiently stabilize and highly disperse the Cu active sites for above 20 hours of testing. A remarkable C2+ partial current density of-323 mA cm-2 was also achieved in a flow cell device. In situ Raman spectra and density functional theory (DFT) calculation studies validated that the *COatop intermediates are concentrated in the nanoreactor, which reduces the free energy of C-C coupling. This work unveiled a simple catalyst design strategy that would be applied to improve C2+ product selectivity and stability by rationalizing the geometric structures and components of catalysts.

Rational Design of Local Reaction Environment for Electrocatalytic Conversion of CO2 into Multicarbon Products

The electrocatalytic conversion of CO2 into multi-carbon (C2+) products provides an attractive route for storing intermittent renewable electricity as fuels and feedstocks with high energy densities. Although substantial progress has been made in selective electrosynthesis of C2+ products via engineering the catalyst, rational design of the local reaction environment in the vicinity of catalyst surface also acts as an effective approach for further enhancing the performance. Here, we discuss recent advances and pertinent challenges in the modulation of local reaction environment, encompassing local pH, the choice of the species and concentrations of cations and anions as well as local reactant/intermediate concentrations, for achieving high C2+ selectivity. In addition, mechanistic understanding in the effects of the local reaction environment is also discussed. Particularly, the important progress extracted from in situ and operando spectroscopy techniques provides insights into how local reaction environment affects C-C coupling and key intermediates formation that lead to reaction pathways toward a desired C2+ product. The possible future direction in understanding and engineering the local reaction environment is also provided.

Promoting C-C coupling for CO2 reduction on Cu2O electrocatalysts with atomically dispersed Rh atoms

Cu2O doped with atomically dispersed Rh (Rh:Cu2O) is synthesized with a wet chemical method. It shows higher activity and faradaic efficiency at lower overpotential for reduction of CO2 to C2+ products, especially C2H4, than pristine Cu2O. We found that introducing Rh promotes CO2 adsorption, *CO hydrogenation to *CHO and their coupling to O*CCHO intermediates, which contributes to enhanced catalytic performance.

Efficient CO2 Electroreduction to Multicarbon Products at CuSiO3/CuO Derived Interfaces in Ordered Pores

Electrochemical CO2 conversion to value-added multicarbon (C2+) chemicals holds promise for reducing CO2 emissions and advancing carbon neutrality. However, achieving both high conversion rate and selectivity remains challenging due to the limited active sites on catalysts for carbon-carbon (C─C) coupling. Herein, porous CuO is coated with amorphous CuSiO3 (p-CuSiO3/CuO) to maximize the active interface sites, enabling efficient CO2 reduction to C2+ products. Significantly, the p-CuSiO3/CuO catalyst exhibits impressive C2+ Faradaic efficiency (FE) of 77.8% in an H-cell at -1.2 V versus reversible hydrogen electrode in 0.1 M KHCO3 and remarkable C2H4 and C2+ FEs of 82% and 91.7% in a flow cell at a current density of 400 mA cm-2 in 1 M KOH. In situ characterizations and theoretical calculations reveal that the active interfaces facilitate CO2 activation and lower the formation energy of the key intermediate *OCCOH, thus promoting CO2 conversion to C2+. This work provides a rational design for steering the active sites toward C2+ products.

Understanding the charge transfer effects of single atoms for boosting the performance of Na-S batteries

The effective flow of electrons through bulk electrodes is crucial for achieving high-performance batteries, although the poor conductivity of homocyclic sulfur molecules results in high barriers against the passage of electrons through electrode structures. This phenomenon causes incomplete reactions and the formation of metastable products. To enhance the performance of the electrode, it is important to place substitutable electrification units to accelerate the cleavage of sulfur molecules and increase the selectivity of stable products during charging and discharging. Herein, we develop a single-atom-charging strategy to address the electron transport issues in bulk sulfur electrodes. The establishment of the synergistic interaction between the adsorption model and electronic transfer helps us achieve a high level of selectivity towards the desirable short-chain sodium polysulfides during the practical battery test. These finding indicates that the atomic manganese sites have an enhanced ability to capture and donate electrons. Additionally, the charge transfer process facilitates the rearrangement of sodium ions, thereby accelerating the kinetics of the sodium ions through the electrostatic force. These combined effects improve pathway selectivity and conversion to stable products during the redox process, leading to superior electrochemical performance for room temperature sodium-sulfur batteries.

Recent advances in the role of interfacial liquids in electrochemical reactions

The interfacial liquid, situated in proximity to an electrode or catalyst, plays a vital role in determining the activity and selectivity of crucial electrochemical reactions, including hydrogen evolution, oxygen evolution/reduction, and carbon dioxide reduction. Thus, there has been a growing interest in better understanding the behavior and the catalytic effect of its constituents. This minireview examines the impact of interfacial liquids on electrocatalysis, specifically the effects of water molecules and ionic species present at the interface. How the structure of interfacial water, distinct from the bulk, can affect charge transfer kinetics and transport of species is presented. Furthermore, how cations and anions (de)stabilize intermediates and transition states, compete for adsorption with reaction species, and act as local environment modifiers including pH and the surrounding solvent structure are described in detail. These effects can promote or inhibit reactions in various ways. This comprehensive exploration provides valuable insights for tailoring interfacial liquids to optimize electrochemical reactions.

Facet-Defined Dilute Metal Alloy Nanorods for Efficient Electroreduction of CO2 to n-Propanol

Electroreduction of CO2 into liquid fuels is a compelling strategy for storing intermittent renewable energy. Here, we introduce a family of facet-defined dilute copper alloy nanocrystals as catalysts to improve the electrosynthesis of n-propanol from CO2 and H2O. We show that substituting a dilute amount of weak-CO-binding metals into the Cu(100) surface improves CO2-to-n-propanol activity and selectivity by modifying the electronic structure of catalysts to facilitate C1-C2 coupling while preserving the (100)-like 4-fold Cu ensembles which favor C1-C1 coupling. With the Au0.02Cu0.98 champion catalyst, we achieve an n-propanol Faradaic efficiency of 18.2 ± 0.3% at a low potential of -0.41 V versus the reversible hydrogen electrode and a peak production rate of 16.6 mA·cm-2. This study demonstrates that shape-controlled dilute-metal-alloy nanocrystals represent a new frontier in electrocatalyst design, and precise control of the host and minority metal distributions is crucial for elucidating structure-composition-property relationships and attaining superior catalytic performance.

Electrocatalytic CO2 Reduction to C2+ Products in Flow Cells

Electrocatalytic CO2 reduction into value-added fuels and chemicals by renewable electric energy is one of the important strategies to address global energy shortage and carbon emission. Though the classical H-type electrolytic cell can quickly screen high-efficiency catalysts, the low current density and limited CO2 mass transfer process essentially impede its industrial applications. The electrolytic cells based on electrolyte flow system (flow cells) have shown great potential for industrial devices, due to higher current density, improved local CO2 concentration, and better mass transfer efficiency. The design and optimization of flow cells are of great significance to further accelerate the industrialization of electrocatalytic CO2 reduction reaction (CO2 RR). In this review, the progress of flow cells for CO2 RR to C2+ products is concerned. Firstly, the main events in the development of the flow cells for CO2 RR are outlined. Second, the main design principles of CO2 RR to C2+ products, the architectures, and types of flow cells are summarized. Third, the main strategies for optimizing flow cells to generate C2+ products are reviewed in detail, including cathode, anode, ion exchange membrane, and electrolyte. Finally, the preliminary attempts, challenges, and the research prospects of flow cells for industrial CO2 RR toward C2+ products are discussed.

In Situ Infrared Spectroscopic Evidence of Enhanced Electrochemical CO2 Reduction and C-C Coupling on Oxide-Derived Copper

The reaction mechanism of CO2 electroreduction on oxide-derived copper has not yet been unraveled even though high C2+ Faradaic efficiencies are commonly observed on these surfaces. In this study, we aim to explore the effects of copper anodization on the adsorption of various CO2RR intermediates using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) on metallic and mildly anodized copper thin films. The in situ SEIRAS results show that the preoxidation process can significantly improve the overall CO2 reduction activity by (1) enhancing CO2 activation, (2) increasing CO uptake, and (3) promoting C-C coupling. First, the strong *COO- redshift indicates that the preoxidation process significantly enhances the first elementary step of CO2 adsorption and activation. The rapid uptake of adsorbed *COatop also illustrates how a high *CO coverage can be achieved in oxide-derived copper electrocatalysts. Finally, for the first time, we observed the formation of the *COCHO dimer on the anodized copper thin film. Using DFT calculations, we show how the presence of subsurface oxygen within the Cu lattice can improve the thermodynamics of C2 product formation via the coupling of adsorbed *CO and *CHO intermediates. This study advances our understanding of the role of surface and subsurface conditions in improving the catalytic reaction kinetics and product selectivity of CO2 reduction.

Covalent Organic Framework Ionomer Steering the CO2 Electroreduction Pathway on Cu at Industrial-Grade Current Density

CO2 electroreduction holds great promise for addressing global energy and sustainability challenges. Copper (Cu) shows great potential for effective conversion of CO2 toward specific value-added and/or high-energy-density products. However, its limitation lies in relatively low product selectivity. Herein, we present that the CO2 reduction reaction (CO2RR) pathway on commercially available Cu can be rationally steered by modulating the microenvironment in the vicinity of the Cu surface with two-dimensional sulfonated covalent organic framework nanosheet (COF-NS)-based ionomers. Specifically, the selectivity toward methane (CH4) can be enhanced to more than 60% with the total current density up to 500 mA cm-2 in flow cells in both acidic (pH = 2) and alkaline (pH = 14) electrolytes. The COF-NS, characterized by abundant apertures, can promote the accumulation of CO2 and K+ near the catalyst surface, alter the adsorption energy and surface coverage of *CO, facilitate the dissociation of H2O, and finally modulate the reaction pathway for the CO2RR. Our approach demonstrates the rational modulation of reaction interfaces for the CO2RR utilizing porous open framework ionomers, showcasing their potential practical applications.

New Insights on Designing the Next-Generation Materials for Electrochemical Synthesis of Reactive Oxidative Species Towards Efficient and Scalable Water Treatment: A Review and Perspectives

Electrochemical water remediation technologies offer several advantages and flexibility for water treatment and degradation of contaminants. These technologies generate reactive oxidative species (ROS) that degrade pollutants. For the implementation of these technologies at an industrial scale, efficient, scalable, and cost-effective in-situ ROS synthesis is necessary to degrade complex pollutant mixtures, treat large amount of contaminated water, and clean water in a reasonable amount of time and cost. These targets are directly dependent on the materials used to generate the ROS, such as electrodes and catalysts. Here, we review the key design aspects of electrocatalytic materials for efficient in-situ ROS generation. We present a mechanistic understanding of ROS generation, including their reaction pathways, and integrate this with the key design considerations of the materials and the overall electrochemical reactor/cell. This involves tunning the interfacial interactions between the electrolyte and electrode which can enhance the ROS generation rate up to ~ 40% as discussed in this review. We also summarized the current and emerging materials for water remediation cells and created a structured dataset of about 500 electrodes and 130 catalysts used for ROS generation and water treatment. A perspective on accelerating the discovery and designing of the next generation electrocatalytic materials is discussed through the application of integrated experimental and computational workflows. Overall, this article provides a comprehensive review and perspectives on designing and discovering materials for ROS synthesis, which are critical not only for successful implementation of electrochemical water remediation technologies but also for other electrochemical applications.

Direct OC-CHO coupling towards highly C2+ products selective electroreduction over stable Cu0/Cu2+ interface

Electroreduction of CO2 to valuable multicarbon (C2+) products is a highly attractive way to utilize and divert emitted CO2. However, a major fraction of C2+ selectivity is confined to less than 90% by the difficulty of coupling C-C bonds efficiently. Herein, we identify the stable Cu0/Cu2+ interfaces derived from copper phosphate-based (CuPO) electrocatalysts, which can facilitate C2+ production with a low-energy pathway of OC-CHO coupling verified by in situ spectra studies and theoretical calculations. The CuPO precatalyst shows a high Faradaic efficiency (FE) of 69.7% towards C2H4 in an H-cell, and exhibits a significant FEC2+ of 90.9% under industrially relevant current density (j = -350 mA cm-2) in a flow cell configuration. The stable Cu0/Cu2+ interface breaks new ground for the structural design of electrocatalysts and the construction of synergistic active sites to improve the activity and selectivity of valuable C2+ products.

Transition metal compounds: From properties, applications to wettability regulation

Transition metal compounds (TMCs) have the advantages of abundant reserves, low cost, non-toxic and pollution-free, and have attracted wide attention in recent years. With the development of two-dimensional layered materials, a new two-dimensional transition metal carbonitride (MXene) has attracted extensive attention due to its excellent physicochemical properties such as gas selectivity, photocatalytic properties, electromagnetic interference shielding and photothermal properties. They are widely used in gas sensors, oil/water separation, wastewater and waste-oil treatment, cancer treatment, seawater desalination, strain sensors, medical materials and some energy storage materials. In this view, we aim to emphatically summarize MXene with their properties, applications and their wettability regulation in different applications. In addition, the properties of transition metal oxides (TMOs) and other TMCs and their wettability regulation applications are also discussed.

Facile and Stable CuInO2 Nanoparticles for Efficient Electrochemical CO2 Reduction

Searching for electrocatalysts for the electrochemical CO2 reduction reaction (e-CO2RR) with high selectivity and stability remains a significant challenge. In this study, we design a Cu-CuInO2 composite with stable states of Cu0/Cu+ by electrochemically depositing indium onto CuCl-decorated Cu foil. The catalyst displays superior selectivity toward the CO product, with a maximal Faraday efficiency of 89% at -0.9 V vs the reversible hydrogen electrode, and maintains impressive stability up to 27 h with a retention rate of >76% in Faraday efficiency. Our systematical characterizations reveal that the catalyst's high performance is attributed to CuInO2 nanoparticles. First-principles calculations further confirm that CuInO2(012) is more conducive to CO generation than Cu(111) under applied potential and presents a higher energy barrier than Cu(111) for the hydrogen evolution reaction. These theoretical predictions are consistent with our experimental observations, suggesting that CuInO2 nanoparticles offer a facile catalyst with a high selectivity and stability for e-CO2RR.

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.

Atomically Precise Copper Nanoclusters for Highly Efficient Electroreduction of CO2 towards Hydrocarbons via Breaking the Coordination Symmetry of Cu Site

We propose an effective highest occupied d-orbital modulation strategy engendered by breaking the coordination symmetry of sites in the atomically precise Cu nanocluster (NC) to switch the product of CO2 electroreduction from HCOOH/CO to higher-valued hydrocarbons. An atomically well-defined Cu6 NC with symmetry-broken Cu-S2 N1 active sites (named Cu6 (MBD)6 , MBD=2-mercaptobenzimidazole) was designed and synthesized by a judicious choice of ligand containing both S and N coordination atoms. Different from the previously reported high HCOOH selectivity of Cu NCs with Cu-S3 sites, the Cu6 (MBD)6 with Cu-S2 N1 coordination structure shows a high Faradaic efficiency toward hydrocarbons of 65.5 % at -1.4 V versus the reversible hydrogen electrode (including 42.5 % CH4 and 23 % C2 H4 ), with the hydrocarbons partial current density of -183.4 mA cm-2 . Theoretical calculations reveal that the symmetry-broken Cu-S2 N1 sites can rearrange the Cu 3d orbitals withd x 2 - y 2 ${d_{x^2 - y^2 } }$ as the highest occupied d-orbital, thus favoring the generation of key intermediate *COOH instead of *OCHO to favor *CO formation, followed by hydrogenation and/or C-C coupling to produce hydrocarbons. This is the first attempt to regulate the coordination mode of Cu atom in Cu NCs for hydrocarbons generation, and provides new inspiration for designing atomically precise NCs for efficient CO2 RR towards highly-valued products.

Effects of Iron Species on Low Temperature CO2 Electrolyzers

Electrochemical energy conversion devices are considered key in reducing CO2 emissions and significant efforts are being applied to accelerate device development. Unlike other technologies, low temperature electrolyzers have the ability to directly convert CO2 into a range of value-added chemicals. To make them commercially viable, however, device efficiency and durability must be increased. Although their design is similar to more mature water electrolyzers and fuel cells, new cell concepts and components are needed. Due to the complexity of the system, singular component optimization is common. As a result, the component interplay is often overlooked. The influence of Fe-species clearly shows that the cell must be considered holistically during optimization, to avoid future issues due to component interference or cross-contamination. Fe-impurities are ubiquitous, and their influence on single components is well-researched. The activity of non-noble anodes has been increased through the deliberate addition of iron. At the same time, however, Fe-species accelerate cathode and membrane degradation. Here, we interpret literature on single components to gain an understanding of how Fe-species influence low temperature CO2 electrolyzers holistically. The role of Fe-species serves to highlight the need for considerations regarding component interplay in general.

Electrochemical Carbon Dioxide Reduction to Ethylene: From Mechanistic Understanding to Catalyst Surface Engineering

Electrochemical carbon dioxide reduction reaction (CO2RR) provides a promising way to convert CO2 to chemicals. The multicarbon (C2+) products, especially ethylene, are of great interest due to their versatile industrial applications. However, selectively reducing CO2 to ethylene is still challenging as the additional energy required for the C-C coupling step results in large overpotential and many competing products. Nonetheless, mechanistic understanding of the key steps and preferred reaction pathways/conditions, as well as rational design of novel catalysts for ethylene production have been regarded as promising approaches to achieving the highly efficient and selective CO2RR. In this review, we first illustrate the key steps for CO2RR to ethylene (e.g., CO2 adsorption/activation, formation of *CO intermediate, C-C coupling step), offering mechanistic understanding of CO2RR conversion to ethylene. Then the alternative reaction pathways and conditions for the formation of ethylene and competitive products (C1 and other C2+ products) are investigated, guiding the further design and development of preferred conditions for ethylene generation. Engineering strategies of Cu-based catalysts for CO2RR-ethylene are further summarized, and the correlations of reaction mechanism/pathways, engineering strategies and selectivity are elaborated. Finally, major challenges and perspectives in the research area of CO2RR are proposed for future development and practical applications.

Light Control-Induced Oxygen Vacancy Generation and In Situ Surface Heterojunction Reconstruction for Boosting CO2 Reduction

The weak adsorption of CO₂ and the fast recombination of photogenerated charges harshly restrain the photocatalytic CO₂ reduction efficiency. The simultaneous catalyst design with strong CO₂ capture ability and fast charge separation efficiency is challenging. Herein, taking advantage of the metastable characteristic of oxygen vacancy, amorphous defect Bi₂O₂CO₃ (named BO_vC) was built on the surface of defect-rich BiOBr (named BO_vB) through an in situ surface reconstruction progress, in which the CO₃^2- in solution reacted with the generated Bi^(3-x)+ around the oxygen vacancies. The in situ formed BO_vC is tightly in contact with the BO_vB and can prevent the further destruction of the oxygen vacancy sites essential for CO₂ adsorption and visible light utilization. Additionally, the superficial BO_vC associated with the internal BO_vB forms a typical heterojunction promoting the interface carriers' separation. Finally, the in situ formation of BO_vC boosted the BOvB and showed better activity in the photocatalytic reduction of CO₂ into CO (three times compared to that of pristine BiOBr). This work provides a comprehensive solution for governing defects chemistry and heterojunction design, as well as gives an in-depth understanding of the function of vacancies in CO₂ reduction.

Localized Alkaline Environment via In Situ Electrostatic Confinement for Enhanced CO2-to-Ethylene Conversion in Neutral Medium

Electrocatalytic CO₂ reduction reaction (CO₂RR) is one of the most promising routes to facilitate carbon neutrality. An alkaline electrolyte is typically needed to promote the production of valuable multi-carbon molecules (such as ethylene). However, the reaction between CO₂ and OH^- consumes a significant quantity of CO₂/alkali and causes the rapid decay of CO₂RR selectivity and stability. Here, we design a catalyst-electrolyte interface with an effective electrostatic confinement of in situ generated OH^- to improve ethylene electrosynthesis from CO₂ in neutral medium. In situ Raman measurements indicate the direct correlation between ethylene selectivity and the intensities of surface Cu-CO and Cu-OH species, suggesting the promoted C-C coupling with the surface enrichment of OH^-. Thus, we report a CO₂-to-ethylene Faradaic efficiency (FE) of 70% and a partial current density of 350 mA cm^-2 at -0.89 V vs the reversible hydrogen electrode. Furthermore, the system demonstrated a 50 h stable operation at 300 mA cm^-2 with an average ethylene FE of ∼68%. This study offers a universal strategy to tune the reaction micro-environment, and a significantly improved ethylene FE of 64.5% was obtained even in acidic electrolytes (pH = 2).

Lewis Acidic Support Boosts C-C Coupling in the Pulsed Electrochemical CO2 Reaction

Copper-oxide electrocatalysts have been demonstrated to effectively perform the electrochemical CO₂ reduction reaction (CO₂RR) toward C₂₊ products, yet preserving the reactive high-valent CuO_x has remained elusive. Herein, we demonstrate a model system of Lewis acidic supported Cu electrocatalyst with a pulsed electroreduction method to achieve enhanced performance for C₂₊ products, in which an optimized electrocatalyst could reach ∼76% Faradaic efficiency for C₂₊ products (FE_C2+) at ∼-0.99 V versus reversible hydrogen electrode, and the corresponding mass activity can be enhanced by ∼2 times as compared to that of conventional CuO_x. In situ time-resolved X-ray absorption spectroscopy investigating the dynamic chemical/physical nature of Cu during CO₂RR discloses that an activation process induced by the KOH electrolyte during pulsed electroreduction greatly enriched the Cu^δ+O/Zn^δ+O interfaces, which further reveals that the presence of Zn^δ+O species under the cathodic potential could effectively serve as a Lewis acidic support for preserving the Cu^δ+O species to facilitate the formation of C₂₊ products, and the catalyst structure-property relationship of Cu^δ+O/Zn^δ+O interfaces can be evidently realized. More importantly, we find a universality of stabilizing Cu^δ+O species for various metal oxide supports and to provide a general concept of appropriate electrocatalyst-Lewis acidic support interaction for promoting C₂₊ products.

(2×1) Reconstruction Mechanism of Rutile TiO2(011) Surface

Understanding the reconstruction kinetics of solid surfaces involving an ensemble of atomic movements is practically important but challenging due to the complexity of high-dimensional potential energy surfaces. Herein, we develop a step-deciding technique incorporated with the nudged elastic band method, which enables multidirection pathway sampling and ensures the capture of a minimum energy path (MEP). Using this approach, the (2×1) reconstruction mechanism of a rutile-TiO₂(011) surface, a classic and long-standing open problem in the fields of surface science and heterogeneous catalysis, is quantified, and the MEP is explicitly identified and explained. Following the least-bond-breaking rule, it gives a stepwise Ti-O bond cleavage mechanism with a collection of decoupled local structural relaxation modes at an overall barrier of 1.25 eV critically affected by initial Ti-O bond opening, which is much lower than the common synergy mechanism. Moreover, the adsorption-induced reconstruction is rationalized considering practical reaction conditions, where H atom adsorbate is shown to effectively stabilize the labile one-fold O_1c intermediate and promote the reconstruction kinetics. This work reveals the reconstruction mechanism regarding multiatom movements and provides a general method for the structural exploration of other complicated systems.

Cu-Sn Aerogels for Electrochemical CO2 Reduction with High CO Selectivity

This work reports the synthesis of Cu_xSn_y alloy aerogels for electrochemical CO₂ reduction catalysts. An in situ reduction and the subsequent freeze-drying process can successfully give CnxSny aerogels with tuneable Sn contents, and such aerogels are composed of three-dimensional architectures made from inter-connected fine nanoparticles with pores as the channels. Density functional theory (DFT) calculations show that the introduction of Sn in Cu aerogels inhibits H₂ evolution reaction (HER) activity, while the accelerated CO desorption on the catalyst surface is found at the same time. The porous structure of aerogel also favors exposing more active sites. Counting these together, with the optimized composition of Cu₉₅Sn₅ aerogel, the high selectivity of CO can be achieved with a faradaic efficiency of over 90% in a wide potential range (-0.7 V to -1.0 V vs. RHE).

Micromodel of a Gas Diffusion Electrode Tracks In-Operando Pore-Scale Wetting Phenomena

Utilizing carbon dioxide (CO₂ ) as a resource for carbon monoxide (CO) production using renewable energy requires electrochemical reactors with gas diffusion electrodes that maintain a stable and highly reactive gas/liquid/solid interface. Very little is known about the reasons why gas diffusion electrodes suffer from unstable long-term operation. Often, this is associated with flooding of the gas diffusion electrode (GDE) within a few hours of operation. A better understanding of parameters influencing the phase behavior at the electrolyte/electrode/gas interface is necessary to increase the durability of GDEs. In this work, a microfluidic structure with multi-scale porosity featuring heterogeneous surface wettability to realistically represent the behavior of conventional GDEs is presented. A gas/liquid/solid phase boundary was established within a conductive, highly porous structure comprising a silver catalyst and Nafion binder. Inoperando visualization of wetting phenomena was performed using confocal laser scanning microscopy (CLSM). Non-reversible wetting, wetting of hierarchically porous structures and electrowetting were observed and analyzed. Fluorescence lifetime imaging microscopy (FLIM) enabled the observation of reactions on the model electrode surface. The presented methodology enables the systematic evaluation of spatio-temporally evolving wetting phenomena as well as species characterization for novel catalyst materials under realistic GDE configurations and process parameters.

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

Tensile-Strained Palladium Nanosheets for Synthetic Catalytic Therapy and Phototherapy

Palladium nanosheets (Pd NSs) are well-investigated photothermal therapy agents, but their catalytic potential for tumor therapy has been underexplored owing to the inactive dominant (111) facets. Herein, lattice tensile strain is introduced by surface reconstruction to activate the inert surface, endowing the strained Pd NSs (SPd NSs) with photodynamic, catalase-like, and peroxidase-like properties. Tensile strain promoting the photodynamic and enzyme-like activities is revealed by density functional theory calculations. Compared with Pd NSs, SPd NSs exhibit lower photothermal effect, but approximately five times higher tumor inhibition rate. This work calls for further study to activate nanomaterials by strain engineering and surface reconstruction for catalytic therapy of tumors.

Cu-based Organic-Inorganic Composite Materials for Electrochemical CO2 Reduction

Electrochemical CO2 reduction reaction (CO2RR) is an attractive pathway to convert CO2 into value-added chemicals and fuels. Copper (Cu) is the most effective monometallic catalyst for converting CO2 into multi-carbon products, but suffers from high overpotentials and poor selectivity. Therefore, it is essential to design efficient Cu-based catalyst to improve the selectivity of specific products. Due to the combination of advantages of organic and inorganic composite materials, organic-inorganic composites exhibit high catalytic performance towards CO2RR, and have been extensively studied. In this review, the research advances of various Cu-based organic-inorganic composite materials in CO2RR, i.e., organic molecular modified-metal Cu composites, Cu-based molecular catalyst/carbon carrier composites, Cu-based metal organic framework (MOF) composites, and Cu-based covalent organic framework (COF) composites are systematically summarized. Particularly, the synthesis strategies of Cu-based composites, structure-performance relationship, and catalytic mechanisms are discussed. Finally, the opportunities and challenges of Cu-based organic-inorganic composite materials in CO2RR are proposed.

Preparation of trimetallic electrocatalysts by one-step co-electrodeposition and efficient CO2 reduction to ethylene

Use of multi-metallic catalysts to enhance reactions is an interesting research area, which has attracted much attention. In this work, we carried out the first work to prepare trimetallic electrocatalysts by a one-step co-electrodeposition process. A series of Cu-X-Y (X and Y denote different metals) catalysts were fabricated using this method. It was found that Cu10La1Cs1 (the content ratio of Cu2+, La3+, and Cs+ in the electrolyte is 10 : 1 : 1 in the deposition process), which had an elemental composition of Cu10La0.16Cs0.14 in the catalyst, formed a composite structure on three dimensional (3D) carbon paper (CP), which showed outstanding performance for CO2 electroreduction reaction (CO2RR) to produce ethylene (C2H4). The faradaic efficiency (FE) of C2H4 could reach 56.9% with a current density of 37.4 mA cm-2 in an H-type cell, and the partial current density of C2H4 was among the highest ones up to date, including those over the catalysts consisting of Cu and noble metals. Moreover, the FE of C2+ products (C2H4, ethanol, and propanol) over the Cu10La1Cs1 catalyst in a flow cell reached 70.5% with a high current density of 486 mA cm-2. Experimental and theoretical studies suggested that the doping of La and Cs into Cu could efficiently enhance the reaction efficiency via a combination of different effects, such as defects, change of electronic structure, and enhanced charge transfer rate. This work provides a simple method to prepare multi-metallic catalysts and demonstrates a successful example for highly efficient CO2RR using non-noble metals.

Effects of the Catalyst Dynamic Changes and Influence of the Reaction Environment on the Performance of Electrochemical CO2 Reduction

Electrochemical reduction of carbon dioxide (CO₂ ) is substantially researched due to its potential for storing intermittent renewable electricity and simultaneously helping mitigating the pressing CO₂ emission concerns. The major challenge of electrochemical CO₂ reduction lies on having good controls of this reaction due to its complicated reaction networks and its unusual sensitivity to the dynamic changes of the catalyst structure (chemical states, compositions, facets and morphology, etc.), and to the non-catalyst components at the electrode/electrolyte interface, in another word the reaction environments. To date, a comprehensive analysis on the interplays between the above catalyst-dynamic-changes/reaction environments and the CO₂ reduction performance is rare, if not none. In this review, the catalyst dynamic changes observed during the catalysis are discussed based on the recent reports of electrochemical CO₂ reduction. Then, the above dynamic changes are correlated to their effects on the catalytic performance. The influences of the reaction environments on the performance of CO₂ reduction are also discussed. Finally, some perspectives on future investigations are offered with the aim of understanding the origins of the effects from the catalyst dynamic changes and the reaction environments, which will allow one to better control the CO₂ reduction toward the desired products.

Preanodized Cu Surface for Selective CO2 Electroreduction to C1 or C2+ Products

The electrochemical CO₂ reduction over Cu catalysts has shown great potential in producing a wide range of valuable chemicals, but it is still plagued by a poor controllability on product distribution. Herein, we demonstrate an effective regulation of CO₂ reduction paths through a preanodization treatment of Cu foil electrodes in different electrolytes. The Cu electrode exhibits a superior C₁ and C₂₊ product selectivity after being preanodized in NaClO₄ (Cu-NaClO₄) and Na₂HPO₄ electrolyte (Cu-Na₂HPO₄), respectively. Combined with in situ electrochemical Raman, ATR-SEIRAS, and SEM characterizations, the preferential C₁ path is due to the deposition of many Cu nanocrystals with dominant Cu(111) facets on the Cu-NaClO₄ electrode. In contrast, the preferential C₂₊ path over the Cu-Na₂HPO₄ is attributed to formation of a unique Cu nanodendritic morphology, which strengthens the *CO intermediate adsorption and induces an environment of low local H₂O/CO₂ stoichiometric ratio, thus facilitating C-C coupling for C₂₊ production. Our findings may shed light on the rational control of the CO₂ reduction path through engineering of the Cu surface structure.

Modeling Operando Electrochemical CO2 Reduction

Since the seminal works on the application of density functional theory and the computational hydrogen electrode to electrochemical CO₂ reduction (eCO₂R) and hydrogen evolution (HER), the modeling of both reactions has quickly evolved for the last two decades. Formulation of thermodynamic and kinetic linear scaling relationships for key intermediates on crystalline materials have led to the definition of activity volcano plots, overpotential diagrams, and full exploitation of these theoretical outcomes at laboratory scale. However, recent studies hint at the role of morphological changes and short-lived intermediates in ruling the catalytic performance under operating conditions, further raising the bar for the modeling of electrocatalytic systems. Here, we highlight some novel methodological approaches employed to address eCO₂R and HER reactions. Moving from the atomic scale to the bulk electrolyte, we first show how ab initio and machine learning methodologies can partially reproduce surface reconstruction under operation, thus identifying active sites and reaction mechanisms if coupled with microkinetic modeling. Later, we introduce the potential of density functional theory and machine learning to interpret data from Operando spectroelectrochemical techniques, such as Raman spectroscopy and extended X-ray absorption fine structure characterization. Next, we review the role of electrolyte and mass transport effects. Finally, we suggest further challenges for computational modeling in the near future as well as our perspective on the directions to follow.

Electrochemical CO2 reduction - The macroscopic world of electrode design, reactor concepts & economic aspects

For the efficient electrochemical conversion of CO2 into valuable chemical feedstocks, a well-coordinated interaction of all electrolyzer compartments is required. In addition to the catalyst, whose role is described in detail in the part "Electrochemical CO2 Reduction toward Multicarbon Alcohols - The Microscopic World of Catalysts & Process Conditions" of this divided review, the general cell setups, design and manufacture of the electrodes, membranes used, and process parameters must be optimally matched. The authors' goal is to provide a comprehensive review of the current literature on how these aspects affect the overall performance of CO2 electrolysis. To be economically competitive as an overall process, the framework conditions, i.e., CO2 supply and reaction product treatment must also be considered. If the key indicators for current density, selectivity, cell voltage, and lifetime of a CO2 electrolyzer mentioned in the techno-economic consideration of this review are met, electrochemical CO2 reduction can make a valuable contribution to the creation of closed carbon cycles and to a sustainable energy economy.

Electrochemical CO2 reduction toward multicarbon alcohols - The microscopic world of catalysts & process conditions

Tackling climate change is one of the undoubtedly most important challenges at the present time. This review deals mainly with the chemical aspects of the current status for converting the greenhouse gas CO2 via electrochemical CO2 reduction reaction (CO2RR) to multicarbon alcohols as valuable products. Feasible reaction routes are presented, as well as catalyst synthesis methods such as electrodeposition, precipitation, or sputtering. In addition, a comprehensive overview of the currently achievable selectivities for multicarbon alcohols in CO2RR is given. It is also outlined to what extent, for example, modifications of the catalyst surfaces or the use of bifunctional compounds the product distribution is shifted. In addition, the influence of varying electrolyte, temperature, and pressure is described and discussed.