Electrochemical CO2 Reduction into Chemical Feedstocks: From Mechanistic Electrocatalysis Models to System Design

Single-atom catalysts confined in shell layer achieved by a modified top-down strategy for efficient CO2 reduction

High-temperature pyrolysis is a primary method for synthesizing single-atom catalysts (SACs). However, this method accelerates the migration of metal atoms within the solid support, leading to low atom utilization. Herein, we report a novel top-down synthesis strategy wherein surface-sintered nickel sulfide (NiS2) nanoparticles (NPs) are in situ atomized into single atoms, achieving confinement of the single-atom catalyst within the shell layer and synthesizing a high-performance single-atom catalyst. Systematic investigations indicate that driven by strong interactions between metal atoms and the support, the NiS2 NPs on the surface of the support atomize into single Ni atoms, which are predominantly distributed on the support surface, thus enhancing the accessibility of the active sites. Furthermore, theoretical calculations indicate that introducing S atoms into the second coordination shell around Ni atoms significantly reduces the activation energy of the CO2 reduction reaction, thereby enhancing the catalytic performance of the single-atom catalyst. In the flow cell, the Ni single-atom catalyst achieving nearly 100% Faradaic efficiency for CO (FECO) over a wide potential range of -0.5 to -1.3 V versus reversible hydrogen electrode (vs. RHE). At -1.6 V vs. RHE, the partial current density for CO reaches a maximum of 709 mA cm-2 (turnover frequency of 28.67 s-1) with a FECO of 95.9%.

Photo-assisted thermal catalytic CO2 reduction over Ru-TiO2 catalysts

Photothermal catalysis is a promising technology to convert CO2 into high value-added products. Here, we show that loading Ru NPs on TiO2 achieved a remarkable photothermal synergistic effect and the Ru-TiO2 demonstrated a high efficiency for the photothermal conversion of low CO2 concentration to CH4 at the gas-solid interface. The photothermal activity of the Ru-TiO2 (217.9 µmol/(g·h)) was nearly 6 times higher than pure thermal activity (38.08 µmol/(g·h)), and nearly 20 times than the photocatalytic activity (10.9 µmol/(g·h)). We revealed that the light excitation could drive the generated electrons from TiO2 to Ru particles, beneficial to CO2 reduction, while external heating showed no influence on the charge separation of the Ru-TiO2. Hence, the photothermal synergy is not a heat-assisted photocatalytic process, but a photo-assisted thermal catalytic process. We finally demonstrated that the CO2 was firstly converted to CO, and the CO was further hydrogenated to CH4. The introduction of light could promote the activation of intermediate CO species at the Ru-Ti interface sites, thus greatly accelerating CO hydrogenation to CH4. This work contributes to further understanding of the mechanism of photothermal catalytic CO2 reduction.

Modulation of the electronic structure of nitrogen-carbon sites by sp3-hybridized carbon coupled to chloride ions improves electrochemical carbon dioxide reduction performance

The challenges remain to develop cost-effective carbon-based catalysts with high activity and selectivity. Here, we synergistically modulate carbon-based electrocatalysts through Cl doping with intrinsic defects in sp3-hybridized carbon and apply them to the electrochemical CO2 reduction reaction (CO2RR). The designed electrocatalyst achieved high selectivity over a wide potential range (-0.7 to -1.0 V), with a faraday efficiency of 96.3 % at -0.8 V for CO. In situ Fourier transform infrared spectroscopy, and analytical studies show pyrrole N to be the active site of CO2RR, and doping Cl increases the content of sp3-hybridized carbon in the carbon substrate, which synergistically accelerates the supply of hydrolysis dissociated protons and facilitates the protonation process of the intermediate products from *CO2 to *COOH. Density functional theory calculations show that Cl coupled sp3-hybridized carbon inhibits the adsorption of H* in the pyrrole N site and facilitates the desorption of *CO, thus promoting the whole process of CO2RR.

Size-Dependency of Electrochemically Grown Copper Nanoclusters Derived from Single Copper Atoms for the CO Reduction Reaction

Electrochemically grown copper nanoclusters (CuNCs: <3 nm) from single-atom catalysts have recently attracted intensive attention as electrocatalysts for CO2 and CO reduction reaction (CO2RR/CORR) because they exhibit distinct product selectivity compared with conventional Cu nanoparticles (typically larger than 10nm). Herein, we conducted a detailed investigation into the size dependence of CuNCs on selectivity for multicarbon (C2+) production in CORR. These nanoclusters were electrochemically grown from single Cu atoms dispersed on covalent triazine frameworks (Cu-CTFs). Operando X-ray absorption fine structure analysis revealed that Cu-CTFs containing 1.21 wt % and 0.41 wt % Cu (Cu(h)-CTFs and Cu(l)-CTFs, respectively) formed CuNCs of 2.0 and 1.1 nm, respectively, at -1.0 V vs. RHE. The selectivity for CORR products was particularly dependent on the size of CuNCs, with the Faraday efficiencies of C2+ products being 52.3 % and 32.7 % at -1.0 V vs. RHE with Cu(h)-CTFs and Cu(l)-CTFs, respectively. Spherical CuNCs modeling revealed that larger cluster sizes led to a greater proportion of a surface coordination number (SCN) of 8 or 9. Density functional calculations revealed that the CO dimerization reaction was more likely to proceed at SCNs of 8 or 9 compared to SCN of 7 because of the stability of the *OCCO intermediate.

Enhanced CO2 electrochemical reduction on single-atom catalysts with optimized environmental, central and axial chemical ambient

Single-atom catalysts (SACs) have received significant research interests for electrocatalytic CO2 reduction reaction (CO2RR) to produce valuable chemicals. Designing optimal SACs for CO2RR is a great challenge because of the strong scaling relationship among the many carbon-containing intermediates. In this study, we designed high-performance SACs, breaking the scaling relationship through changing environmental nonmetals, central atoms and axial nonmetals together via a series of density functional theory (DFT) calculations. After screening through configuration stabilities, CO and CO2 adsorption energy, limiting potential of H2, product adsorption energy, limiting potential of products, energy barrier of C-C coupling process and AIMD simulations, we finally observed ten optimal SACs (Ti-N4-B, Ti-N4-Si, Ti-CN3-Si, Ti-CN2O(1)-S, Sc-C2NO(1)-B, Sc-C2NO(1)-Si, Ti-BCN2(2)-N, Sc-CN2O(3)-Si, Ru-C2NO(3)-C and Ti-BONC-C) after considering 4311 possible configurations with high activity and selectivity for HCOOH, CH4 and C2H6O formation. Among them, Ti-N4-B, Ru-C2NO(3)-C, and Sc-C2NO(1)-B have the lowest overpotentials for producing HCOOH, CH4, and C2H6O with UL of -0.2 V, -0.29 V, and -0.51 V, respectively. Subsequently, electronic analysis is implemented to provide a more comprehensive explanation at the electronic level for the enhanced CO2RR performance of the discovered SACs. Our research demonstrates that the performance of SACs on CO2RR can be significantly enhanced and altered by the combination of environmental nonmetals, central atoms, and axial nonmetals in a rational design. Importantly, it also establishes a design principle for the rapid screening of prospective catalysts for CO2RR with high activity and selectivity.

Research progress of transition metal catalysts for electrocatalytic EG oxidation

Ethylene glycol (EG) is a small-molecule alcohol with a low oxidation potential and is a key monomer in the production of polyethylene terephthalate (PET). The efficient oxidation of EG can further enable the recycling of waste PET. Currently, there are many studies on catalysts for EG oxidation, among which transition metal catalysts (including traditional non-precious metals such as Fe, Co, Ni and other noble metals such as Pt and Pd) have good prospects for application in EG oxidation reactions due to their unique electronic structures. In this study, the synthesis strategy of transition metal catalysts for the electrocatalytic oxidation of EG is summarized and the performance of different types of catalysts in the EG oxidation reaction is reviewed. Advanced characterization methods were used to understand the oxidation mechanism of EG and to control the conversion of EGOR intermediates into target products. Therefore, we need to further explore efficient catalysts for EG oxidation to achieve efficient reactions.

Rational Design of Rare Earth-Based Nanomaterials for Electrocatalytic Reactions

Rare earth-based nanomaterials hold great promise for applications in the electrocatalysis field owing to their unique 4f electronic structure, adjustable coordination modes, and high oxophilicity. As a cocatalyst, the location of rare earth elements can alter the intrinsic properties of support, including coordination environments, electronic structure, and structure evolution under applied potentials in a variable manner, to potentially impact catalytic performance with respect to their activity, stability, and selectivity. Therefore, a comprehensive understanding of the effects of rare earth elements' location on local structure and reaction mechanisms is a prerequisite for designing advanced rare earth-based nanomaterials. In this review, the rare earth-based nanomaterials have been categorized into three main groups based upon the location of rare earth elements in the support, namely lattice, surface, and interface structure. We initially discuss recent advances and representing breakthroughs to realize controllable synthesis of rare earth-based nanomaterials. Next, we discuss the state-of-the-art rare earth-based nanomaterials and the structure modulation strategy employed to enhance their catalytic performance. Combined with advanced characterizations, the role of rare earth elements in reaction mechanisms and structure evolution process is also discussed. Finally, we further highlight the future research directions and remaining challenges for the development of rare earth-based nanomaterials in practical applications.

Electroreduction of diluted CO2 to multicarbon products with high carbon utilization at 800 mA cm-2 in strongly acidic media

Acidic CO2 electroreduction using diluted CO2 (as in flue gas) as the feedstock can simultaneously circumvent the CO2 purification step and lower the carbon loss in conventional alkaline or neutral electrolyte, and thus is highly desired but has rarely been achieved thus far. Herein, we report a simple and general strategy using an imidazolium-based anion-exchange ionomer as the coating layer, which could enrich the diluted CO2 to generate a high local CO2 concentration, and simultaneously block the proton transport to the cathode surface to suppress the competing hydrogen evolution reaction. As a result, the ionomer-modified Cu catalyst can achieve an efficient electroreduction of diluted CO2 (15 vol% CO2) to multicarbon (C2+) products in strong acid (pH 0.8), with a high C2+ Faradaic efficiency of 70.5% and a high single-pass carbon efficiency of 73.6% at a current density of 800 mA cm-2, competitive with that obtained with pure CO2. These findings provide opportunity for the direct electrochemical conversion of flue gas into valuable products with high efficiency.

Efficient Bicarbonate Electrolysis to Formate Enabled via Ionomer Surface Modification in Cation Exchange Membrane Electrolyzers

Electrochemical CO2 reduction (CO2RR) is a promising method for converting CO2 into valuable chemicals, with formate being a particularly viable product. However, current gas-fed CO2RR systems rely on highly pure CO2 feed gases and are incompatible with point-source CO2 emissions without prior capture and concentration. Bicarbonate electrolysis offers a potential solution by bridging the gap between CO2 emissions and utilization. However, existing electrolyzer configurations, especially those using bipolar membranes (BPM), require high working voltages and suffer from poor energy efficiency. Here, we present a cation exchange membrane (CEM)-based membrane electrode assembly (MEA) incorporating a surface-modified bismuth cathode catalyst. The success of this approach is attributed to two key factors: the use of the positively charged ionomer PiperION for surface modification, which creates a favorable cathode microenvironment; the single CEM that enhances proton flux from the anode to the cathode while reducing ionic impedance. The CEM-based MEA demonstrates a formate faradaic efficiency of up to 80%, with a significant 1.5 V reduction in operating voltage compared to BPM-based MEAs at 300 mA cm-2. Additionally, the CEM-based MEA exhibits excellent tolerance to O2 impurities and maintains high performance even with simulated flue gas, making it suitable for direct CO2 utilization from point sources.

Highly selective CO2 electroreduction in an exsolution-induced flow cell using a hierarchical monolithic nano-Ag foam electrode

Electrochemical CO2 reduction driven by renewable energy offers a promising route to carbon neutrality. The flow-through induced dynamic triple-phase boundary cell (FTDT cell) addresses the key challenges of conventional gas diffusion electrodes (GDEs), including salt precipitation and electrode flooding, by enabling direct electrolysis of CO2-saturated solutions. However, the Ag catalyst with carbon cloth as a substrate in the FTDT cell exhibits the shortcomings of a few active sites and poor structural stability. Here, we reported a monolithic nano-Ag foam electrode featuring well-developed pores and a hierarchical nanostructure with a high electrochemically active surface area (ECSA), which is ten times that of the Ag NPs electrode, enhancing the CO2 electroreduction performance of the FTDT cell significantly. Classical nucleation theory (CNT) clarified that the nanostructures accelerate bubble nucleation, and visualization experiments confirmed that the periodically connected pore structure provides abundant dynamic gas-liquid-solid triple-phase boundaries (TPBs). At an industrial current density of 200 mA cm-2 and a cell voltage of 2.34 V, the nano-Ag foam electrode achieves a CO faradaic efficiency of 93% and an overall energy efficiency of 51.34%, presenting a promising approach for the commercialization of CO2 electrolysis.

Engineering Flow-Through Hollow Fiber Gas-Diffusion Electrodes for Unlocking High-Rate Gas-Phase Electrochemical Conversion

Designing advanced electrodes with efficient contact with gas, electrolytes, and catalysts presents significant opportunities to enhance the accessibility of concentrated gas molecules to the catalytic sites while mitigating undesirable side reactions such as the hydrogen evolution reaction (HER), which advances the gas-phase electrochemical reduction toward industrial-scale applications. Traditional planar electrodes face challenges, including limited gas solubility and restricted mass transport. Although commercial flow-by gas-diffusion electrodes can reduce mass transfer resistance by enabling direct diffusion of gas molecules to active sites, the reliance on diffusive gas flow becomes insufficient to meet the rapid consumption demands of gas reactants at high current density. Flow-through hollow fiber gas-diffusion electrodes (HFGDEs) or hollow fiber gas penetration electrodes (HFGPEs) provide a promising solution by continuously delivering convective gas flow to active sites, resulting in enhanced mass transport and superior gas accessibility near the catalytic sites. Notably, HFGDEs have demonstrated the ability to achieve current densities exceeding multiple amperes per square centimeter in liquid electrolytes. This review provides a comprehensive overview of the design criteria, fabrication methods, and design strategies for porous metallic HFGDEs. It highlights the state-of-the-art advancements in HFGDEs composed of various metals (e.g., Cu, Ni, Ag, Bi, Ti, and Zn), with a particular focus on their utilization in the electrochemical conversion of CO2. Finally, future research directions are discussed, underscoring the potential of porous metallic HFGDEs as a versatile and scalable electrode architecture for diverse electrochemical applications.

Role of Water in Green Carbon Science

Within the context of green chemistry, the concept of green carbon science emphasizes carbon balance and recycling to address the challenge of achieving carbon neutrality. The fundamental processes in this field are oxidation and reduction, which often involve simple molecules such as CO2, CO, CH4, CHx, and H2O. Water plays a critical role in nearly all oxidation-reduction processes, and thus, it is a central focus of research in green carbon science. Water can act as a direct source of dihydrogen in reduction reactions or participate in oxidation reactions, frequently involving O-O coupling to produce hydrogen peroxide or dioxygen. At the atomic level, this coupling involves the statistically unfavorable proximity of two atoms, requiring optimization through a catalytic process influenced by two types of factors, as described by the authors. Extrinsic factors are related to geometrical and electronic criteria associated with the catalytic metal, involving its d-orbitals (or bands in the case of zerovalent metals and electrodes). Intrinsic factors are related to the coupling of oxygen atoms via their p-orbitals. At the mesoscopic or microscopic scale, the reaction medium typically consists of mixtures of lipophilic and hydrophilic phases with water, which may exist under supercritical conditions or as suspensions of microdroplets. These reactions predominantly occur at phase interfaces. A comprehensive understanding of the phenomena across these scales could facilitate improvements and even lead to the development of novel conversion processes.

Strategies for Enhancing Stability in Electrochemical CO2 Reduction

The electrochemical CO2 reduction reaction (CO2RR) holds significant promise as a sustainable approach to address global energy challenges and reduce carbon emissions. However, achieving long-term stability in terms of catalytic performance remains a critical hurdle for large-scale commercial deployment. This mini-review provides a comprehensive exploration of the key factors influencing CO2RR stability, encompassing catalyst design, electrode architecture, electrolyzer optimization, and operational conditions. We examine how catalyst degradation occurs through mechanisms such as valence changes, elemental dissolution, structural reconfiguration, and active site poisoning and propose targeted strategies for improvement, including doping, alloying, and substrate engineering. Additionally, advancements in electrode design, such as structural modifications and membrane enhancements, are highlighted for their role in improving stability. Operational parameters such as temperature, pressure, and electrolyte composition also play crucial roles in extending the lifespan of the reaction. By addressing these diverse factors, this review aims to offer a deeper understanding of the determinants of long-term stability in the CO2RR, laying the groundwork for the development of robust, scalable technologies for efficient carbon dioxide conversion.

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.

Electrochemical reduction of CO2 on pure and doped Cu2O(111)

Cu2O has been demonstrated to be effective for converting CO2 into value-added products. However, the mechanism of the carbon dioxide reduction (CO2R) on the most stable surface, Cu2O(111), is still under debate. Additionally, how to improve its activity and selectivity is a challenging issue too. In this work, we unravel that CO2R can occur before Cu2O reduction (Cu2O-R) when the applied potential is below -0.44 V and doping can improve its catalytic performance based on first-principles calculations. The pure Cu2O(111) surface shows high activity and selectivity for the production of formic acid (HCOOH). However, the performance of CO2R deteriorates on the reduced Cu2O(111). Doping p-block elements (Al, Ga, In, Tl, Sn, Pb, Bi) is proven to be a workable strategy to improve its catalytic performance by suppressing hydrogen evolution reaction (HER). Importantly, Ga-Cu2O exhibits the favorable bonding strength for *OCHO, which is responsible for the optimal catalytic activity (-0.18 V) among other p-block elements. Our calculations thus provide an insight into CO2 reduction mechanism of Cu2O(111), favoring rational design of Cu2O-based catalyst.

Progress in Cu-Based Catalyst Design for Sustained Electrocatalytic CO2 to C2+ Conversion

The electrocatalytic conversion of CO2 into valuable multi-carbon (C2+) products using Cu-based catalysts has attracted significant attention. This review provides a comprehensive overview of recent advances in Cu-based catalyst design to improve C2+ selectivity and operational stability. It begins with an analysis of the fundamental reaction pathways for C2+ formation, encompassing both established and emerging mechanisms, which offer critical insights for catalyst design. In situ techniques, essential for validating these pathways by real-time observation of intermediates and material evolution, are also introduced. A key focus of this review is placed on how to enhance C2+ selectivity through intermediates manipulation, particularly emphasizing catalytic site construction to promote C─C coupling via increasing *CO coverage and optimizing protonation. Additionally, the challenge of maintaining catalytic activity under reaction conditions is discussed, highlighting the reduction of active charged Cu species and materials reconstruction as major obstacles. To address these, the review describes recent strategies to preserve active sites and control materials evolution, including novel catalyst design and the utilization and mitigation of reconstruction. By presenting these developments and the challenges ahead, this review aims to guide future materials design for CO2 conversion.

Unfolding Electrolyzer Characteristics to Reveal Solar-to-Chemical Efficiency Potential: Rapid Analysis Method Bridging Electrochemistry and Photovoltaics

Development of photovoltaic-electrochemical (PV-EC) systems for energy storage and industry decarbonization requires multidisciplinary collaborative efforts of different research groups from both photovoltaic and electrochemical research communities. Consequently, the evaluation of the solar-to-chemical or solar-to-fuel efficiency of a new electrolyzer (EC) as a part of a PV-EC system is a time-consuming task that is challenging in a routine optimization loop. To address this issue, a new rapid assessment method is proposed. This method employs power balance requirements to unfold the input EC characteristics into the parameter space of PV-EC systems. The system parameters, composed with the EC output characteristics, yield the solar-to-chemical efficiency attainable by the electrolyzer in combination with any PV device under any irradiance at any relative PV-to-EC scaling and any mode of power coupling. This comprehensive overview is achieved via a mathematically simple conversion of the EC characteristics in any spreadsheet software. The method, designed to streamline the development and minimize the efforts of both the photovoltaic and electrochemical communities, is demonstrated via the analysis of CO2-reduction electrolyzer characteristics and verified with dedicated PV-EC experiments.

Single-Atom-Embedded Nitrogen-Doped Graphene as Efficient Electrocatalysts for the CO2 Reduction Reaction

Single-atom catalysts (SACs) have displayed unprecedented activity and selectivity for electrochemical CO2 reduction reaction (CO2RR). Herein, a series of metal single atoms embedded on nitrogen-doped graphene (M-N4G, where M = In, Tl, Ge, Sn, Pb, Sb, and Bi) is systematically evaluated as CO2RR electrocatalysts by density functional theory (DFT) calculations. The computational results show that most M-N4G exhibit better CO2RR selectivity over the hydrogen evolution reaction (HER). Ge/Pb-N4G exhibits excellent electrocatalytic performance in the generation of HCOOH from the CO2RR with low limiting potentials of -0.292 and -0.306 V, which surpass the performance of the vast majority of electrocatalysts. Adsorption energy of the key intermediate *HCOO can be used as an effective reactivity reaction descriptor to screen promising CO2RR catalysts. The results of this work highlight M-N4G as an ideal electrochemical for the electrocatalytic CO2RR.

Direct Electroreduction of Low-Concentration CO2: Progress and Perspective

The conversion of CO2 into carbon-based fuels and chemicals via the electrocatalytic CO2 reduction reaction (CO2RR) offers an attractive route to reducing the CO2 emission for carbon neutrality. Currently, high-purity CO2 gas has been widely used as the feedstock for most of the CO2RR studies, while CO2 sources with a typically low concentration impose the extra cost for CO2 capture and purification steps. The direct utilization of low-concentration CO2 for the CO2RR is a promising approach to substantially address this problem. In this Perspective, we first highlight the prominent advantages of direct electroreduction of low-concentration CO2. Then we focus on the summary of several important design strategies for CO2RR in diluted CO2 and gas impurities-containing CO2 atmosphere. Finally, we propose personal outlooks on future challenges and some opportunities for this fascinating research field.

Voltage-Dependent Electrochemical Carbon Dioxide Reduction Mechanism Unveiled by Kinetic Monte Carlo Simulation

The voltage-dependent dynamic evolution of the electrocatalytic carbon dioxide reduction reaction (CO2RR) on Cu-based catalysts is still unclear. Herein, a Kinetic Monte Carlo (KMC) model that tracks the evolution of the CO2RR on the Cu (111)/(100) surface is developed. Using the Density Functional Theory calculated energetics of 178 elementary reactions in CO2RR toward C1-C2 multispecies production, the KMC model predicted CO2RR linear sweep voltammetry and potential-dependent product distribution that agree well with experimental observations. Degree of rate control analysis reveals that, on Cu (111), the primary hydrocarbon product is C1 species CH4, and as the working potential increases, its rate-determining step (RDS) changes from CO hydrogenation toward the CHO* formation step into the COH* formation step. The Cu (100) surface is more active toward C2H4 and CH3CH2OH production with CO* symmetric coupling step as RDS. This KMC model provides important insights into the CO2RR dynamics on Cu catalysts.

In situ Raman spectroscopic studies of CO2 reduction reactions: from catalyst surface structures to reaction mechanisms

The electrochemical CO2 reduction reaction (eCO2RR) has gained widespread attention as an important technology for carbon cycling and sustainable chemistry. In situ Raman spectroscopy, due to its molecular structure, sensitive advantage and real-time monitoring capability, has become an effective tool for studying the reaction mechanisms and structure-performance relationships in eCO2RR. This article reviews recent advancements in the application of in situ Raman spectroscopy in eCO2RR research, focusing on its critical role in monitoring reaction intermediates, analyzing catalyst surface states, and optimizing catalyst design. Through systematic studies of different catalysts and reaction conditions, in situ Raman spectroscopy has revealed the formation and transformation pathways of various intermediates, deeply exploring their relationship with the active sites of the catalysts. Furthermore, the review discusses the integration of in situ Raman spectroscopy with other characterization techniques to achieve a more comprehensive understanding of the reaction mechanisms. Finally, we summarize the current challenges and opportunities in this research area and look ahead to the future applications of in situ Raman spectroscopy in the field of eCO2RR.

Constraining CO2 Coverage on Copper Promotes CO2 Electroreduction to Multi-carbon Products in Strong Acid

Electrocatalytic CO2 reduction (CO2R) to multi-carbon (C2+) products in strong acid presents a promising approach to mitigate the CO2 loss commonly encountered in alkaline and neutral systems. However, this process often suffers from low selectivity for C2+ products due to the competing C1 (e.g., CO and HCOOH) formation and complex C-C coupling kinetics. In this work, we report a CO2 coverage constraining strategy by diluting CO2 reactant feed to modulate the intermediate distribution and C-C coupling pathways for an enhanced electrosynthesis of C2+ products in strong acid. Lowering the CO2 feed concentration reduces CO2 coverage on copper catalyst, enriching the surface coverage and optimizing the adsorption configuration of the key CO intermediate for C-C coupling. This approach efficiently suppresses the formation of undesired C1 products. By employing a 20 % CO2 feed, we achieved a significant improvement in C2+ Faradaic efficiency, reaching 68 % at 100 mA cm-2, approximately 1.7 times higher than the 41 % obtained using pure CO2. We demonstrated the direct electroreduction of a 30 % CO2 feed-representative CO2 concentration of typical industrial flue gases-in a full electrolyzer, achieving a C2+ selectivity of 78 % and an energy efficiency of 23 % at 200 mA cm-2.

Behavior, mechanisms, and applications of low-concentration CO2 in energy media

This review explores the behavior of low-concentration CO2 (LCC) in various energy media, such as solid adsorbents, liquid absorbents, and catalytic surfaces. It delves into the mechanisms of diffusion, adsorption, and catalytic reactions, while analyzing the potential applications and challenges of these properties in technologies like air separation, compressed gas energy storage, and CO2 catalytic conversion. Given the current lack of comprehensive analyses, especially those encompassing multiscale studies of LCC behavior, this review aims to provide a theoretical foundation and data support for optimizing CO2 capture, storage, and conversion technologies, as well as guidance for the development and application of new materials. By summarizing recent advancements in LCC separation techniques (e.g., cryogenic air separation and direct air carbon capture) and catalytic conversion technologies (including thermal catalysis, electrochemical catalysis, photocatalysis, plasma catalysis, and biocatalysis), this review highlights their importance in achieving carbon neutrality. It also discusses the challenges and future directions of these technologies. The findings emphasize that advancing the efficient utilization of LCC not only enhances CO2 reduction and resource utilization efficiency, promoting the development of clean energy technologies, but also provides an economically and environmentally viable solution for addressing global climate change.

Recent advances and developments in solar-driven photothermal catalytic CO2 reduction into multicarbon (C2+) products

Solar-driven catalytic conversion of carbon dioxide (CO2) into value-added C2+ chemicals and fuels has attracted significant attention over the past decades, propelled by urgent environmental and energy demands. However, the catalytic reduction of CO2 continues to face significant challenges due to inherently slow reduction kinetics. This review traces the historical development and current state of photothermal CO2 reduction, detailing the mechanisms by which CO2 is transformed into C2+ products. A key focus is on catalyst design, emphasizing surface defect engineering, bifunctional active site and co-catalyst coupling to enhance the efficiency and selectivity of solar-driven C2+ synthesis. Key reaction pathways to both C1 and C2+ products are discussed, ranging from CO, CH4 and methanol (CH3OH) synthesis to the production of C2-4 products such as C2-4 hydrocarbons, ethanol, acetic acid, and various carbonates. Notably, the advanced synthesis of C5+ hydrocarbons exemplifies the remarkable potential of photothermal technologies to effectively upgrade CO2-derived products, thereby delivering sustainable liquid fuels. This review provides a comprehensive overview of fundamental mechanisms, recent breakthroughs, and pathway optimizations, culminating in valuable insights for future research and industrial-scale prospect of photothermal CO2 reduction.

Electrochemical CO2 reduction to liquid fuels: Mechanistic pathways and surface/interface engineering of catalysts and electrolytes

The high energy density of green synthetic liquid chemicals and fuels makes them ideal for sustainable energy storage and transportation applications. Electroreduction of carbon dioxide (CO2) directly into such high value-added chemicals can help us achieve a renewable C cycle. Such electrochemical reduction typically suffers from low faradaic efficiencies (FEs) and generates a mixture of products due to the complexity of controlling the reaction selectivity. This perspective summarizes recent advances in the mechanistic understanding of CO2 reduction reaction pathways toward liquid products and the state-of-the-art catalytic materials for conversion of CO2 to liquid C1 (e.g., formic acid, methanol) and C2+ products (e.g., acetic acid, ethanol, n-propanol). Many liquid fuels are being produced with FEs between 80% and 100%. We discuss the use of structure-binding energy relationships, computational screening, and machine learning to identify promising candidates for experimental validation. Finally, we classify strategies for controlling catalyst selectivity and summarize breakthroughs, prospects, and challenges in electrocatalytic CO2 reduction to guide future developments.

Atomically Dispersed Ni-N-C Catalysts for Electrochemical CO2 Reduction

The atomic dispersion of nickel in Ni-N-C catalysts is key for the selective generation of carbon monoxide through the electrochemical carbon dioxide reduction reaction (CO2RR). Herein, the study reports a highly selective, atomically dispersed Ni1.0%-N-C catalyst with reduced Ni loading compared to previous reports. Extensive materials characterization fails to detect Ni crystalline phases, reveals the highest concentration of atomically dispersed Ni metal, and confirms the presence of the proposed Ni-Nx active site at this reduced loading. The catalyst shows excellent activity and selectivity toward CO generation, with a faradaic efficiency for CO generation (FECO) of 97% and partial current density for CO (jco) of -9.0 mA cm-2 at -0.9 V in an electrochemical H-type cell. CO2RR activity and selectivity are also studied by rotating disk electrode (RDE) measurements where transport limitations can be suppressed. It is expected that the utility of these Ni-N-C catalysts will lie with tandem CO2RR reaction schemes to multi-carbon (C2+) products.

Rational Design of Dual-Atom Catalysts for Electrochemical CO2 Reduction to C1 and C2 Products with High Activity and Selectivity: A Density Functional Theory Study

Carbon dioxide (CO2) electroreduction using renewable energy provides a sustainable solution to mitigate greenhouse effects and achieve carbon neutrality. Developing high-performance electrocatalysts for the CO2 reduction reaction (CO2RR) is key to promoting such a technology. Herein, we systematically explored the CO2RR catalytic activity of 325 dual-metal-site catalysts (DMSCs) through density functional theory (DFT) calculations. Among them, the Sc/Tc DMSC is particularly advantageous for HCOOH, CH4, and CH3CH2OH production, with limiting potentials of -0.45 V, -0.45 V, and -0.46 V, respectively. The Ti/Rh DMSC can selectively convert CO2 to CH3CH2OH at ultralow overpotentials (U L = -0.21 V). HCOOH is the preferred product of the CO2RR on the Mn/Fe site with a U L of -0.30 V. Mn/Fe presents the highest inhibitory effects on the side reaction, the hydrogen evolution reaction (HER), with a U L of -0.66 V. Moreover, electronic analysis was conducted to further explain the enhancement for the CO2RR of explored catalysts at the subatomic level. Our work offers a strategy for screening of high-performance DMSCs and reveals the mechanisms of the CO2RR to target products for selected catalysts, benefiting the further development of CO2RR electrocatalysts.

Promotion of C─C Coupling in the CO2 Electrochemical Reduction to Valuable C2+ Products: From Micro-Foundation to Macro-Application

The electrochemical CO2 reduction reaction (CO2RR) to valuable C2+ products emerges as a promising strategy for converting intermittent renewable energy into high-energy-density fuels and feedstock. Leveraging its substantial commercial potential and compatibility with existing energy infrastructure, the electrochemical conversion of CO2 into multicarbon hydrocarbons and oxygenates (C2+) holds great industrial promise. However, the process is hampered by complex multielectron-proton transfer reactions and difficulties in reactant activation, posing significant thermodynamic and kinetic barriers to the commercialization of C2+ production. Addressing these barriers necessitates a comprehensive approach encompassing multiple facets, including the effective control of C─C coupling in industrial electrolyzers using efficient catalysts in optimized local environments. This review delves into the advancements and outstanding challenges spanning from the microcosmic to macroscopic scales, including the design of nanocatalysts, optimization of the microenvironment, and the development of macroscopic electrolyzers. By elucidating the influence of the local electrolyte environment, and exploring the design of potential industrial flow cells, guidelines are provided for future research aimed at promoting C─C coupling, thereby bridging microscopic insights and macroscopic applications in the field of CO2 electroreduction.

Benchmarking Performance Indices of Electrochemical CO2 Reduction to Ethylene Based on Prospective Life Cycle Assessment for Negative Emissions

To mitigate global warming to the most ambitious targets, it is necessary to remove CO2 from the atmosphere and reduce fossil fuels use. The electrochemical conversion of CO2 to ethylene (C2H4) as a basic chemical is a promising technology that meets both requirements; however, its life cycle CO2 emissions remain inconclusive because of varying assumptions in the performance indices. This study aimed to set benchmarks for the four most sensitive indices to achieve -0.5 t-CO2/t-C2H4 by calculating net greenhouse gas (GHG) emissions through a prospective life cycle assessment of a model system including CO2 capture, CO2 enrichment, electrochemical conversion, CO2 recycling, and cryogenic separation. As a result, the electrochemical conversion process was the hotspot of life cycle emissions, and representative benchmarks were determined as follows: cell voltage, 3.5 V; C2H4 Faraday efficiency, 70 %; conversion rate, 20 %; and electrochemical CO2 recycling energy, 2.2 GJ/t-CO2. The gaps between the benchmarks and current top data of cell voltage and Faraday efficiency were <10 %, and suppressing the performance degradation for up to one year was found to be a critical requirement. These results can direct research towards the development of a year-round stable system, rather than further improving the performance indices.

Construction of an Indium-Based Coordination Polymer with Redox Non-Innocent Ligand for High-Efficient Electrochemical CO2 Reduction

Developing high-activity and long-term stable electrocatalysts for electrochemical CO2 reduction reaction (eCO2RR) to valuable products is still a challenge. An in-depth understanding of reaction mechanisms and the structure-function relationship is required for the development of an advanced catalytic eCO2RR system. Herein, a coordination polymer of indium(III) and benzenehexathiol (BHT) was developed as an electrocatalyst (In-BHT) for eCO2RR to HCOO-, which displayed an outstanding catalytic performance over the entire pH range. However, experimental results revealed significantly different catalytic pathways in the acid and neutral/alkaline solutions, which are attributed to the influence of redox non-innocent ligands on the rate-determining step (RDS). In the acid solution, the RDS is the formation of *OCOH intermediate through the proton transfer that originates from H2O in the solution, leading to relatively sluggish kinetics. But in the neutral or alkaline solution, the thiolate groups could be protonated during the catalytic process, and such proton can attack on carbon of absorbed CO2 via an intramolecular proton transfer, promoting the formation of *OCHO intermediate, resulting in faster kinetics. Our findings revealed the pivotal roles of the redox non-innocent ligands of metal active sites for eCO2RR, providing a new idea for designing highly efficient electrocatalysts.

Integrated "Two-in-One" Strategy for High-Rate Electrocatalytic CO2 Reduction to Formate

The electrochemical CO2 reduction reaction (ECR) is a promising pathway to producing valuable chemicals and fuels. Despite extensive studies reported, improving CO2 adsorption for local CO2 enrichment or water dissociation to generate sufficient H* is still not enough to achieve industrial-relevant current densities. Herein, we report a "two-in-one" catalyst, defective Bi nanosheets modified by CrOx (Bi-CrOx), to simultaneously promote CO2 adsorption and water dissociation, thereby enhancing the activity and selectivity of ECR to formate. The Bi-CrOx exhibits an excellent Faradaic efficiency (≈100 %) in a wide potential range from -0.4 to -0.9 V. In addition, it achieves a remarkable formate partial current density of 687 mA cm-2 at a moderate potential of -0.9 V without iR compensation, the highest value at -0.9 V reported so far. Control experiments and theoretical simulations revealed that the defective Bi facilitates CO2 adsorption/activation while the CrOx accounts for enhancing the protonation process via accelerating H2O dissociation. This work presents a pathway to boosting formate production through tuning CO2 and H2O species at the same time.

Atomic Ordering-Induced Ensemble Variation in Alloys Governs Electrocatalyst On/Off States

The catalytic behavior of a material is influenced by ensembles─the geometric configuration of atoms on the surface. In conventional material systems, ensemble effects and the electronic structure are coupled because these strategies focus on varying the material composition, making it difficult to understand the role of ensembles in isolation. This study introduces a methodology that separates geometric effects from the electronic structure. To tune the Pd ensemble size on the catalyst surface, we compared the reactivity of structurally different but compositionally identical Pd3Bi intermetallic and solid solution alloys. Pd3Bi intermetallics display no reactivity for methanol oxidation (MOR), while their solid solution counterparts show significant reactivity (0.5 mA cmPd-2). Intermetallics form smaller ensembles (1, 3, 4, and 5 atoms across all low-energy facets), whereas solid solution Pd3Bi has several facets that support larger Pd ensembles, with an average size of 5.25 atoms and up to 6 atoms. A partially ordered Pd3Bi (a mixed phase of intermetallic and solid solution) alloy shows intermediate MOR activity (0.1 mA cmPd-2), confirming that methanol oxidation activity tracks with the average ensemble size. All Pd3Bi alloys maintained similar electronic structures, as confirmed by X-ray photoelectron spectroscopy (XPS) valence band spectroscopy and X-ray absorption near edge structure (XANES) measurements, indicating that reactivity differences arise from variations in the ensemble size induced by differences in the atomic ordering. Our findings offer an approach for designing materials with controllable active site configurations while maintaining the catalyst's electronic structure, thereby enabling more efficient catalyst design.

Multiscale Modeling of CO2 Electrochemical Reduction on Copper Electrocatalysts: A Review of Advancements, Challenges, and Future Directions

Although CO2 contributes significantly to global warming, it also offers potential as a raw material for the production of hydrocarbons such as CH4, C2H4 and CH3OH. Electrochemical CO2 reduction reaction (eCO2RR) is an emerging technology that utilizes renewable energy to convert CO2 into valuable fuels, solving environmental and energy problems simultaneously. Insights gained at any individual scale can only provide a limited view of that specific scale. Multiscale modeling, which involves coupling atomistic-level insights (density functional theory, DFT) and (Molecular Dynamics, MD), with mesoscale (kinetic Monte Carlo, KMC, and microkinetics, MK) and macroscale (computational fluid dynamics, CFD) simulations, has received significant attention recently. While multiscale modeling of eCO2RR on electrocatalysts across all scales is limited due to its complexity, this review offers an overview of recent works on single scales and the coupling of two and three scales, such as "DFT+MD", "DFT+KMC", "DFT+MK", "KMC/MK+CFD" and "DFT+MK/KMC+CFD", focusing particularly on Cu-based electrocatalysts as copper is known to be an excellent electrocatalyst for eCO2RR. This sets it apart from other reviews that solely focus exclusively on a single scale or only on a combination of DFT and MK/KMC scales. Furthermore, this review offers a concise overview of machine learning (ML) applications for eCO2RR, an emerging approach that has not yet been reviewed. Finally, this review highlights the key challenges, research gaps and perspectives of multiscale modeling for eCO2RR.

Electrochemical CO2 Reduction to Multicarbon Products on Non-Copper Based Catalysts

Electrochemical CO2 reduction reaction (eCO2RR) to value-added multicarbon (C2+) products offers a promising approach for achieving carbon neutrality and storing intermittent renewable energy. Copper (Cu)-based electrocatalysts generally play the predominant role in this process. Yet recently, more and more non-Cu materials have demonstrated the capability to convert CO2 into C2+, which provides impressive production efficiency even exceeding those on Cu, and a wider variety of C2+ compounds not achievable with Cu counterparts. This motivates us to organize the present review to make a timely and tutorial summary of recent progresses on developing non-Cu based catalysts for CO2-to-C2+. We begin by elucidating the reaction pathways for C2+ formation, with an emphasis on the unique C-C coupling mechanisms in non-Cu electrocatalysts. Subsequently, we summarize the typical C2+-involved non-Cu catalysts, including ds-, d- and p-block metals, as well as metal-free materials, presenting the state-of-the-art design strategies to enhance C2+ efficiency. The system upgrading to promote C2+ productivity on non-Cu electrodes covering microbial electrosynthesis, electrolyte engineering, regulation of operational conditions, and synergistic co-electrolysis, is highlighted as well. Our review concludes with an exploration of the challenges and future opportunities in this rapidly evolving field.

Tackling CO2 Loss in Electrocatalytic Carbon Dioxide Reduction by Advanced Material and Electrolyzer Design

Electrocatalytic CO2 reduction (ECO2R) has been considered as a promising approach to convert CO2 into valuable chemicals and fuels. CO2 loss in conventional alkaline electrolyzers has been recognized as a major obstacle that compromising the efficiency of the ECO2R system. This review firstly conducts an in-depth assessment of the origin and influence of CO2 loss. On this basis, this work summarizes electrolyzer configurations based on novel material and structure design that are capable of tackling CO2 loss, including acidic electrolyzer, bipolar membrane (BPM) derived electrolyzer, cascade electrolyzer, liquid-phase-anode electrolyzer, and liquid-fed electrolyzer. The design strategies and challenges of these carbon efficient electrolyzers have been deliberated in detail. By comparing and analyzing the advantages and limitations of various electrolyzer designs, this work aims to provide some guidelines for the development of efficient ECO2R technology toward large-scale industrial application.

Addressing the Carbonate Issue: Electrocatalysts for Acidic CO2 Reduction Reaction

Electrochemical CO2 reduction reaction (CO2RR) powered by renewable energy provides a promising route to CO2 conversion and utilization. However, the widely used neutral/alkaline electrolyte consumes a large amount of CO2 to produce (bi)carbonate byproducts, leading to significant challenges at the device level, thereby impeding the further deployment of this reaction. Conducting CO2RR in acidic electrolytes offers a promising solution to address the "carbonate issue"; however, it presents inherent difficulties due to the competitive hydrogen evolution reaction, necessitating concerted efforts toward advanced catalyst and electrode designs to achieve high selectivity and activity. This review encompasses recent developments of acidic CO2RR, from mechanism elucidation to catalyst design and device engineering. This review begins by discussing the mechanistic understanding of the reaction pathway, laying the foundation for catalyst design in acidic CO2RR. Subsequently, an in-depth analysis of recent advancements in acidic CO2RR catalysts is provided, highlighting heterogeneous catalysts, surface immobilized molecular catalysts, and catalyst surface enhancement. Furthermore, the progress made in device-level applications is summarized, aiming to develop high-performance acidic CO2RR systems. Finally, the existing challenges and future directions in the design of acidic CO2RR catalysts are outlined, emphasizing the need for improved selectivity, activity, stability, and scalability.

NiNC Catalysts in CO2-to-CO Electrolysis

CO2-to-CO electrolyzer technology converts carbon dioxide into carbon monoxide using electrochemical methods, offering significant environmental and energy benefits by aiding in greenhouse gas mitigation and promoting a carbon circular economy. Recent study by Strasser et al. in Nature Chemical Engineering presents a high-performance CO2-to-CO electrolyzer utilizing a NiNC catalyst with nearly 100% faradaic efficiency, employing innovative diagnostic tools like the carbon crossover coefficient (CCC) to address transport-related failures and optimize overall efficiency. Strasser's research demonstrates the potential of NiNC catalysts, particularly NiNC-IMI, for efficient CO production in CO2-to-CO electrolyzers, highlighting their high selectivity and performance. However, challenges such as localized CO2 depletion and mass transport limitations underscore the need for further optimization and development of diagnostic tools like CCC. Strategies for optimizing catalyst structure and operational parameters offer avenues for enhancing the performance and reliability of electrochemical CO2 reduction catalysts.

ICP-MS Assisted EDX Tomography: A Robust Method for Studying Electrolyte Penetration Phenomena in Gas Diffusion Electrodes Applied to CO2 Electrolysis

A carbon paper-based gas diffusion electrode (GDE) is used with a bismuth(III) subcarbonate active catalyst phase for the electrochemical reduction of CO2 in a gas/electrolyte flow-by configuration electrolyser at high current density. It is demonstrated that in this configuration, the gas and catholyte phases recombine to form K2CO3/KHCO3 precipitates to an extent that after electrolyses, vast amount of K+ ions is found by EDX mapping in the entire GDE structure. The fact that the entirety of the GDE gets wetted during electrolysis should, however, not be interpreted as a sign of flooding of the catalyst layer, since electrolyte perspiring through the GDE can largely be removed with the outflow gas, and the efficiency of electrolysis (toward the selective production of formate) can thus be maintained high for several hours. For a full spatial scale quantitative monitoring of electrolyte penetration into the GDE, (relying on K+ ions as tracer) the method of inductively coupled plasma-mass spectrometry (ICP-MS) assisted energy dispersive X-ray (EDX) tomography is introduced. This new, cheap and robust tomography of non-uniform aspect ratio has a large planar span that comprises the entire GDE surface area and a submicrometer depth resolution, hence it can provide quantitative information about the amount and distribution of K+ remnants inside the GDE structure, in three dimensions.

Enhancing CO2 Electroreduction Precision to Ethylene and Ethanol: The Role of Additional Boron Catalytic Sites in Cu-Based Tandem Catalysts

The electrocatalytic conversion of carbon dioxide (CO2) into valuable multicarbon (C2+) compounds offers a promising approach to mitigate CO2 emissions and harness renewable energy. However, achieving precise selectivity for specific C2+ products, such as ethylene and ethanol, remains a formidable challenge. This study shows that incorporating elemental boron (B) into copper (Cu) catalysts provides additional adsorption sites for *CO intermediates, enhancing the selectivity of desirable C2+ products. Additionally, using a nickel single-atom catalyst (Ni-SAC) as a *CO source increases local *CO concentration and reduces the hydrogen evolution reaction. In situ experiments and density functional theory (DFT) calculations reveal that surface-bound boron units adsorb and convert *CO more efficiently, promoting ethylene production, while boron within the bulk phase of copper influences charge transfer, facilitating ethanol generation. In a neutral electrolyte, the bias current density for ethylene production using the B-O-Cu2@Ni-SAC0.05 hybrid catalyst exceeded 300 mA cm-2, and that for ethanol production with B-O-Cu5@Ni-SAC0.2 surpassed 250 mA cm-2. This study underscores that elemental doping in Cu-based catalysts not only alters charge and crystalline phase arrangements at Cu sites but also provides additional reduction sites for coupling reactions, enabling the efficient synthesis of distinct C2+ products.

Delocalization State-Stabilized Znδ+ Active Sites for Highly Selective and Durable CO2 Electroreduction

Zinc (Zn)-based materials are cost-effective and promising single-metal catalysts for CO2 electroreduction to CO but is still challenged by low selectivity and long-term stability. Undercoordinated Zn (Znδ+) sites have been demonstrated to be powerful active centers with appropriate *COOH affinity for efficient CO production However, electrochemical reduction conditions generally cause the inevitable reduction of Znδ+, resulting in the decline of CO efficiency over prolonged operation. Herein, a Zn cyanamide (ZnNCN) catalyst is constructed for highly selective and durable CO2 electroreduction, wherein the delocalized Zn d-electrons and resonant structure of cyanamide ligand prevent the self-reduction of ZnNCN and maintain Znδ+ sites under cathodic conditions. The mechanism studies based on density functional theory and operando spectroscopies indicate that delocalized Znδ+ site can stabilize the key *COOH intermediate through hard-soft acid-base theory, therefore thermodynamically promoting CO2-to-CO conversion. Consequently, ZnNCN delivers a CO Faradaic efficiency (FE) of up to 93.9% and further exhibits a remarkable stability lifespan of 96 h, representing a significant advancement in developing robust Zn-based electrocatalysts. Beyond expanding the variety of CO2 reduction catalysts, this work also offers insights into understanding the structure-function sensitivity and controlling dynamic active sites.

Direct low concentration CO2 electroreduction to multicarbon products via rate-determining step tuning

Direct converting low concentration CO2 in industrial exhaust gases to high-value multi-carbon products via renewable-energy-powered electrochemical catalysis provides a sustainable strategy for CO2 utilization with minimized CO2 separation and purification capital and energy cost. Nonetheless, the electrocatalytic conversion of dilute CO2 into value-added chemicals (C2+ products, e.g., ethylene) is frequently impeded by low CO2 conversion rate and weak carbon intermediates' surface adsorption strength. Here, we fabricate a range of Cu catalysts comprising fine-tuned Cu(111)/Cu2O(111) interface boundary density crystal structures aimed at optimizing rate-determining step and decreasing the thermodynamic barriers of intermediates' adsorption. Utilizing interface boundary engineering, we attain a Faradaic efficiency of (51.9 ± 2.8) % and a partial current density of (34.5 ± 6.4) mA·cm-2 for C2+ products at a dilute CO2 feed condition (5% CO2 v/v), comparing to the state-of-art low concentration CO2 electrolysis. In contrast to the prevailing belief that the CO2 activation step (C O 2 + e - + * → C O 2 - * ) governs the reaction rate, we discover that, under dilute CO2 feed conditions, the rate-determining step shifts to the generation of *COOH (C O 2 - * + H 2 O → C * O O H + O H - ( a q ) ) at the Cu0/Cu1+ interface boundary, resulting in a better C2+ production performance.

High-Entropy Metal Sulfide Promises High-Performance Carbon Dioxide Reduction

The efficient conversion of carbon dioxide (CO2) requires the development of stable catalysts with high selectivity and reactivity within a wide potential range. Here, the high-entropy metal sulfide CuAgZnSnS4 is designed for CO2 reduction with excellent performance (FEcarbon products ≥ 90%) in whole test potential windows (600 mV) based on the synergistic effect of the high-entropy metal sulfide. In particular, CuAgZnSnS4 exhibits better single-product selectivity with the highest FEHCOOH/FECO value (29.03) at -1.28 versus reversible hydrogen electrode (RHE). In combination with in situ measurements and theoretical calculations, it is further revealed that the synergistic effect of CuAgZnSnS4 realizes the controllable regulation of the surface electronic structure at Sn active sites, strengthening orbital interactions between *OCHO and Sn active sites. As a result, the effective adsorption and activation of *OCHO instead of *H are obtained, improving the single-product selectivity of electrocatalytic CO2 reduction and inhibiting the competitive hydrogen evolution reaction significantly. Our findings may complete the understanding of the synergistic effect for high-entropy materials in catalysis and offer new insight into the design of efficient electrocatalysts with high catalytic activity.

Carbon encapsulated nanoparticles: materials science and energy applications

The technological implementation of electrochemical energy conversion and storage necessitates the acquisition of high-performance electrocatalysts and electrodes. Carbon encapsulated nanoparticles have emerged as an exciting option owing to their unique advantages that strike a high-level activity-stability balance. Ever-growing attention to this unique type of material is partly attributed to the straightforward rationale of carbonizing ubiquitous organic species under energetic conditions. In addition, on-demand precursors pave the way for not only introducing dopants and surface functional groups into the carbon shell but also generating diverse metal-based nanoparticle cores. By controlling the synthetic parameters, both the carbon shell and the metallic core are facilely engineered in terms of structure, composition, and dimensions. Apart from multiple easy-to-understand superiorities, such as improved agglomeration, corrosion, oxidation, and pulverization resistance and charge conduction, afforded by the carbon encapsulation, potential core-shell synergistic interactions lead to the fine-tuning of the electronic structures of both components. These features collectively contribute to the emerging energy applications of these nanostructures as novel electrocatalysts and electrodes. Thus, a systematic and comprehensive review is urgently needed to summarize recent advancements and stimulate further efforts in this rapidly evolving research field.

Electrochemical CO2 Conversion Commercialization Pathways: A Concise Review on Experimental Frontiers and Technoeconomic Analysis

Technoeconomic analysis (TEA) studies are vital for formulating guidelines that drive the commercialization of electrochemical CO2 reduction (eCO2R) technologies. In this review, we first discuss the progress in the field of eCO2R processes by providing current state-of-the-art metrices (e.g., faradic efficiency, current density) based on the recent heterogeneous catalysts' discovery, electrolytes, electrolyzers configuration, and electrolysis process designs. Next, we assessed the TEA studies for a wide range of eCO2R final products, different modes of eCO2R systems/processes, and discussed their relative competitiveness with relevant commercial products. Finally, we discuss challenges and future directions essential for eCO2R commercialization by linking suggestions from TEA studies. We believe that this review will catalyze innovation in formulating advanced eCO2R strategies to meet the TEA benchmarks for the conversion of CO2 into valuable chemicals at the industrial scale.

Alkali metal cations enhance CO2 reduction by a Co molecular complex in a bipolar membrane electrolyzer

The electrochemical reduction of CO2 is a promising pathway for converting CO2 into valuable fuels and chemicals. The local environment at the cathode of CO2 electrolyzers plays a key role in determining activity and selectivity, but currently some mechanisms are still under debate. In particular, alkali metal cations have been shown to enhance the selectivity of metal catalysts, but their role remains less explored for molecular catalysts especially in high-current electrolyzers. Here, we investigated the enhancement effects of cations (Na+, K+, Cs+) on Co phthalocyanine (CoPc) in a state-of-the-art reverse-biased bipolar membrane electrolyzer. When added to the anolyte, these cations increased the Faradaic efficiency for CO, except in the case of Na+ in which the effect was transient, but the effects are convoluted with the transport process through the membrane. Alternatively, these cations can also be added directly to the cathode as chloride salts, allowing the use of a pure H2O anolyte feed, leading to sustained improved CO selectivity (61% at 100 mA cm-2 after 24 h). Our results show that cation addition is a simple yet effective strategy for improving the product selectivity of molecular electrocatalysts, opening up new avenues for tuning their local environment for CO2 reduction.This article is part of the discussion meeting issue 'Green carbon for the chemical industry of the future'.

Electrochemical Formation of C2+ Products Steered by Bridge-Bonded *CO Confined by *OH Domains

During the electrochemical CO2 reduction reaction (eCO2RR) on copper catalysts, linear-bonded CO (*COL) is commonly regarded as the key intermediate for the CO-CO coupling step, which leads to the formation of multicarbon products. In this work, we unveil the significant role of bridge-bonded *CO (*COB) as an active species. By combining in situ Raman spectroscopy, gas and liquid chromatography, and density functional theory (DFT) simulations, we show that adsorbed *OH domains displace *COL to *COB. The electroreduction of a 12CO+13CO2 cofeed demonstrates that *COB distinctly favors the production of acetate and 1-propanol, while *COL favors ethylene and ethanol formation. This work enhances our understanding of the mechanistic intricacies of eCO(2)RR and suggests new directions for designing operational conditions by modifying the competitive adsorption of surface species, thereby steering the reaction toward specific multicarbon products.

Atomic Design of Copper Active Sites in Pristine Metal-Organic Coordination Compounds for Electrocatalytic Carbon Dioxide Reduction

Electrocatalytic carbon dioxide reduction reaction (CO2RR) has emerged as a promising and sustainable approach to cut carbon emissions by converting greenhouse gas CO2 to value-added chemicals and fuels. Metal-organic coordination compounds, especially the copper (Cu)-based coordination compounds, which feature well-defined crystalline structures and designable metal active sites, have attracted much research attention in electrocatalytic CO2RR. Herein, the recent advances of electrochemical CO2RR on pristine Cu-based coordination compounds with different types of Cu active sites are reviewed. First, the general reaction pathways of electrocatalytic CO2RR on Cu-based coordination compounds are briefly introduced. Then the highly efficient conversion of CO2 on various kinds of Cu active sites (e.g., single-Cu site, dimeric-Cu site, multi-Cu site, and heterometallic site) is systematically discussed, along with the corresponding catalytic reaction mechanisms. Finally, some existing challenges and potential opportunities for this research direction are provided to guide the rational design of metal-organic coordination compounds for their practical application in electrochemical CO2RR.

Simultaneous production of CO and H2O2 by paired electrolysis coupling CO2 reduction and water oxidation

Here, a novel paired electrolysis system is constructed, where fluorine-doped tin oxide glass serves as the anode for the water oxidation reaction to produce hydrogen peroxide (H2O2), and cobalt phthalocyanine (CoPc)/carbon nanotube (CNT) loaded carbon paper as the cathode for CO2 reduction to generate CO. This system demonstrates a high overall energy efficiency of 34%, where a faradaic efficiency exceeding 90% for CO2 reduction and 60% for water oxidation to H2O2 have been achieved, demonstrating significant energy savings of nearly 40% compared to the respective half-reaction systems.

Rational Strategies for Preparing Highly Efficient Tin-, Bismuth- or Indium-Based Electrocatalysts for Electrochemical CO2 Reduction to Formic acid/Formate

Electrochemical carbon dioxide reduction reaction (CO2RR) is an environmentally friendly and economically viable approach to convert greenhouse gas CO2 into valuable chemical fuels and feedstocks. Among various products of CO2RR, formic acid/formate (HCOOH/HCOO-) is considered the most attractive one with its high energy density and ease of storage, thereby enabling widespread commercial applications in chemical, medicine, and energy-related industries. Nowadays, the development of efficient and financially feasible electrocatalysts with excellent selectivity and activity towards HCOOH/HCOO- is paramount for the industrial application of CO2RR technology, in which Tin (Sn), Bismuth (Bi), and Indium (In)-based electrocatalysts have drawn significant attention due to their high efficiency and various regulation strategies have been explored to design diverse advanced electrocatalysts. Herein, we comprehensively review the rational strategies to enhance electrocatalytic performances of these electrocatalysts for CO2RR to HCOOH/HCOO-. Specifically, the internal mechanism between the physicochemical properties of engineering materials and electrocatalytic performance is analyzed and discussed in details. Besides, the current challenges and future opportunities are proposed to provide inspiration for the development of more efficient electrocatalysts in this field.

Efficient Photoelectrocatalytic Reduction of CO2 to Selectively Produce Ethanol Using FeS2/TiO2 p-n Heterojunction Photoelectrodes

Herein, the FeS2/TiO2 p-n heterojunction was first utilized as a photoelectrode for the PEC reduction of CO2 to selectively produce ethanol. The FeS2/TiO2 photoelectrode was fabricated through electrochemical anodization, electrodeposition, and vulcanization methods. The impact of the FeS2 loading amount and applied bias on the PEC performance was investigated. The behavior of photocurrent polarity reverse is observed depending on the FeS2 loading amount, which is related to the energy band structure of the semiconductor/electrolyte interface. The active sites for ethanol production were identified on TiO2 nanotubes rather than on the FeS2 surface. Incorporation of FeS2 not only broadened the visible light absorption range but also formed a p-n heterojunction with TiO2. FeS2/TiO2 with an electrodeposition time of 15 min exhibits the highest ethanol yield of 1170 μmol L-1 cm-2 for 3.5 h of reaction under ultraviolet-visible (UV-Vis) illumination at an applied bias of -0.7 V. Compared to TiO2, FeS2/TiO2 showed significantly higher ethanol yield due to its appropriate loading amount of FeS2 and the synergistic effect of strong UV-Vis light absorption and efficient separation and transfer of charge carriers at the p-n junction.