Selectivity in Electrochemical CO2 Reduction

Atomic-level engineering Ni-N2O2 interfacial structure for enhanced CO2 electrocatalytic reduction efficiency

The precise atomic-scale preparation of single-atomic active sites with unique coordination structures in electrocatalysts for the carbon dioxide reduction reaction (CO2RR), coupled with the elucidation of their mechanisms at the atomic level, remains a formidable challenge. In this manuscript, a simple one-pot synthesis method was adopted to successfully synthesize an O-doped Ni single-atom catalyst (Ni-NOG), characterized by a distinct Ni-N2O2 symmetric coordination structure. The incorporation of Ni-O bonds alters the electronic configuration of the catalyst's central atoms within the catalyst, thereby boosting both the catalytic selectivity and efficiency during CO2RR. The synthesized electrocatalyst exhibited outstanding performance in the CO2RR process, achieving a Faraday efficiency (FE) of 97.4 % at a potential of -0.8315 V versus to reversible hydrogen electrode (vs. RHE). Furthermore, the selectivity remained consistently above 95 % throughout a 98-hour stability test, surpassing the performance of most advanced catalysts currently available. Theoretical simulations demonstrate that the Ni-N2O2 symmetric coordination structure shows a small activation barrier in the rate-limiting step, favoring the swift generation of intermediate species and demonstrating robust catalytic activity. This work not only offers a straightforward and approach method for the preparation of single-atom catalysts but also clarifies the pivotal role of O-element doping within the coordination environment in enhancing catalyst performance.

Stabilizing *COOH on Ni Single-Atom Sites by Dual-Phase Ni3ZnC0.7 and Ni for Enhanced Electrocatalytic CO2 Reduction

Metal-based nanoparticle-modulated single-atom catalysts have garnered significant attention in the field of electrocatalytic CO2 reduction reaction. However, the scaling relationships between the intermediates and their binding energies lead to unsatisfactory selectivity for specific products. Herein, Ni3ZnC0.7-Ni nanoparticles (NPs)-modulated Ni single-atoms (SAs) supported on N-doped carbon (NiSAs/Ni3ZnC0.7-NiNPs@NC) were constructed using a metal-organic framework as a template. Experiments and theoretical calculations reveal that the charge transfer from Ni3ZnC0.7 and Ni NPs to Ni SAs results in the formation of electron-enriched Ni SAs sites, which is conducive to stabilize the key *COOH intermediate. The catalyst shows a partial current density for CO of -345 mA cm-2 and a Faradaic efficiency for CO (FECO) of 99.3% at -0.8 V versus a reversible hydrogen electrode in a flow cell. Furthermore, the catalyst maintains FECO of above 94% after continuous electrolysis for 18 h, showcasing its remarkable long-term stability.

Structural Reconstruction of Copper-Based Catalysts in CO2 Electroreduction Reaction: A Comprehensive Review

The escalating concerns over climate change and environmental pollution have intensified the pursuit for sustainable solutions to mitigate CO2 emissions, with the electrochemical CO2 reduction reaction (CO2RR) emerging as a promising strategy to convert CO2 into valuable chemicals and fuels. Central to this process is the development of efficient electrocatalysts, where Cu-based catalysts have garnered significant attention due to their high activity towards multi-carbon products. However, understanding of structural reconstruction of Cu-based catalysts under operational conditions presents a substantial challenge, complicating the identification of real active sites and the elucidation of structure-performance relationships. Herein, we first highlight the fundamental principles governing the structural reconstruction in CO2RR, encompassing both thermodynamic and kinetic perspectives. We then introduce advanced Operando techniques employed to monitor the structural changes of catalysts. The review further delves into the dynamic evolution behaviors of Cu-based catalysts, including atomic rearrangement and morphology evolution, with a focus on correlating these behaviors with catalytic properties such as activity, selectivity, and stability. Finally, we discuss cases of emerging strategies, such as heteroatom doping and electrolyte engineering, that hold promise for manipulating the structural reconstruction of Cu-based catalysts, and we explore future opportunities in this rapidly evolving field.

Engineering Local Coordination and Electronic Structures of Dual-Atom Catalysts

Heterogeneous dual-atom catalysts (DACs), defined by atomically precise and isolated metal pairs on solid supports, have garnered significant interest in advancing catalytic processes and technologies aimed at achieving sustainable energy and chemical production. DACs present board opportunities for atomic-level structural and property engineering to enhance catalytic performance, which can effectively address the limitations of single-atom catalysts, including restricted active sites, spatial constraints, and the typically positive charge nature of supported single metal species. Despite the rapid progress in this field, the intricate relationship between local atomic environments and the catalytic behavior of dual-metal active sites remains insufficiently understood. This review highlights recent progress and major challenges in this field. We begin by discussing the local modulation of coordination and electronic structures in DACs and its impact on catalytic performance. Through specific case studies, we demonstrate the importance of optimizing the entire catalytic ensemble to achieve efficient, selective, and stable performance in both model and industrially relevant reactions. Additionally, we also outline future research directions, emphasizing the challenges and opportunities in synthesis, characterization, and practical applications, aiming to fully unlock the potential of these advanced catalysts.

Optimizing Coordinated Active Sites of Transition Metal Complexes: Exploring Metal-Molecule Interactions for Governing CO2-to-CO Conversion

Syngas (H2/CO) is an essential chemical feedstock for industrial products. In these focal points, electrocatalytic CO2 reduction has emerged as a desirable strategy for realizing effective syngas production to satisfy energy and environmental requirements. In this work, a metal-molecule hybrid electrode with inherent H2 generation favorability has been crafted by loading molecular Co(Ni)-bpy (bpy = 2,2'-bipyridine) complexes on Ag foil. The efficient and stable CO2-to-CO conversion with adjustable faradic efficiency from 13 to 98% was realized by optimizing the Co(Ni)-bpy complexes. The regulation of molecular catalysts with the merits of high electron affinity can provide a coordination environment that allows for the localization of Co/Ni active sites at optimal positions with lower binding energies, maintaining their monodisperse properties, and being beneficial for strengthening the CO2 binding and inhibiting competitive reactions. An in-depth understanding of surface and coordination status has been realized by FIB-HRTEM and EXAFS, which confirm that the intimate metal-molecular interaction and well-dispersed mononuclear Co/Ni active sites play vital roles in enhancing catalytic performance. The strong electron residual between the Ag surface and metal-coordinated molecular catalysts may also contribute to the dramatic CO2-to-CO conversion. This study highlights the beneficial role of metal-molecule interactions in electrocatalytic reactions and contributes to ongoing efforts toward achieving controllable selectivity in electrocatalytic reduction of CO2 to syngas using molecular catalysts.

Rational Construction of Cu Active Sites for CO2 Electrolysis to C2+ Product

Electrocatalytic CO2 reduction reaction (CO2RR) has emerged as a promising approach in advancing towards carbon neutrality and addressing renewable energy intermittency. Copper-based catalysts have received much attention due to their high catalytic activity to convert CO2 into high value-added C2+ products. However, CO2RR exhibits a diversity of reduction products and unavoidable hydrogen precipitation side reactions due to the moderate adsorption strength of *CO on the copper surface and the fact that the electrode potential for CO2 reduction is very close to that for hydrogen precipitation reduction. Here, we summarize recent advances in the structural design and active site construction of copper-based catalysts for CO2RR, and investigate their effects on the improvement of CO2RR performance, with the aim of deepening the understanding of catalyst structure and active sites, thereby facilitating the design of more efficient copper-based catalysts for the sustainable production of value-added chemicals.

Precise Tuning of Functional Group Spatial Distribution on Porphyrin Rings for Enhanced CO2 Electroreduction Selectivity

Molecular catalysts play a critical role in regulating the selectivity of electrocatalytic CO2 reduction reaction (CO2RR), yet the understanding of ligand function is largely restricted to modulating the electronic structure of the metal and reaction kinetics. Herein, a hydroxyl (─OH) ligand is introduced into a sterically hindered amino-porphyrin (o-TAPP) to synthesize the atropisomers porphyrin-salicylimine-Cu (o-Cu-Por-Sa) with hydrogen-bonding interactions (O─H⋯O), enabling efficient selection of CO and CH4 under dual effects. Detailed analysis shows that the ─OH of o-Cu-Por-Sa (αβαβ) forms a noncovalent hydrogen bond with carbonate, characterized by a bond length of 2.01 Å and an angle of 27.6°, and this interaction reduces the reaction energy barrier, achieving a faradaic efficiency (FE) of 84% for CH4. Moreover, the steric hindrance effect of the symmetric distribution of ─OH facilitates protonation reactions by preventing C-C coupling. In contrast, ─OH aggregated on o-Cu-Por-Sa (αααα) forms a pocket-like hydrogen bond grid, which restricts free CO2 adsorption, and the rapid dissociation of *CO also interrupts the reaction. This work highlights the pivotal role of dual effects induced by ligand atropisomerization in regulating selectivity, offering new insights for the design of efficient molecular catalysts.

Application of COF Materials in Carbon Dioxide Electrocatalytic Reduction

COFs have become the most attractive frontier research area in heterogeneous catalysis. Since the geometry and electronic structure of COFs are largely determined by their microenvironment, which in turn determines the performance in electrocatalytic processes, the precise integration of atoms of COF building blocks to achieve pre-designed composition, components and functions is the core. This paper focuses on the structural design, synthesis, electrocatalytic mechanism and application of COFs in electrocatalytic CO2RR (types of COFs in electrocatalytic CO2RR, performance evaluation indicators of COFs in electrocatalytic CO2RR, and the relationship between the structure of COFs and electrocatalytic performance). In addition, we also explore the challenges faced by COFs in CO2RR and the corresponding solution strategies. Finally, by highlighting the prospects and challenges of COFs structural regulation, we hope to provide inspiration for the further development of COFs in electrocatalytic applications.

Combining the Active Site Construction and Microenvironment Regulation via a Bio-Inspired Strategy Boosts CO2 Electroreduction Under Ampere-Level Current Densities

Electrochemical reduction of CO2 reaction (CO2RR) is recognized as a complicated process involving multiple steps on the gas-electrode-solution interface. Hence, it is equally important to construct highly efficient active sites and regulate favorable microenvironments around the reaction interface. Herein, we propose a bio-inspired strategy to address both issues simultaneously in one catalytic system. We first evaporate isolated Au sites on the surface of the Cu layer to tune the intrinsic activity of the Cu catalyst, then fix hexanethiol (HEX) molecules onto the Au sites through Au─S bonds to regulate reaction microenvironments (CuAu-HEX). Specifically, those Au/Cu bimetallic active sites can decrease the energy barriers for the C─C coupling procedure and accelerate the generation of multicarbon products. More importantly, those stable and nondense HEX molecules on Au sites can ensure long-term hydrophobicity and high local concentration of CO2 around active sites, rather than block the channels for reactant transfer. Consequently, this unique structure is favorable for the pathways toward multicarbon products, generating >70% Faradaic efficiencies (FE) for multicarbon products even at 1 A cm-2. Intriguingly, this modification layer is very similar to animal hair follicles, which might present a new strategy to regulate the interfacial environments in various electrocatalytic reactions.

Direct Observation of Electron Donation onto the Reactants and a Transient Poisoning Mechanism During CO2 Electroreduction on Ni Single Atom Catalysts

Single atom catalysts (SACs) are an important class of materials that mediate chemical reduction reactions, a key subset of which is Ni within a carbon support for the electrochemical CO2 reduction reaction (CO2RR). However, how the metal atom/clusters and the carbon-based support act in concert to catalyze CO2RR is not well understood, with most reports attributing activity solely to the Ni-Nx/C moieties. To address this gap, we have undertaken a mechanistic investigation, employing in situ X-ray absorption spectroscopy (XAS) coupled with electrochemical studies and density functional theory (DFT) calculations to further understand how Ni single atoms work in conjunction with the nitrogen-doped carbon matrix to promote CO2RR to CO, and how the presence of impurities such as those present in CO2-containing waste flue gases (including NOx, and CN-) changes the catalyst upon reduction. In contrast to previous works, we do not find strong evidence for a purely metal-based reduction upon application of negative reductive potentials. Instead, we present evidence for an increase in the equatorial vs. axial splitting of Ni, consistent with electrons moving onto the reactants via the Ni single atom 3dz 2 orbital. In addition, we demonstrate a transient poisoning mechanism of the Ni SAC by nitrite and thiocyanate, explaining the recovery of activity during CO2RR. These insights can aid the design of practical CO2 valorization technologies.

Dynamic protonation of ligand sites in molecular catalysts enhances electrochemical CO2 reduction

Molecular catalysts with functional group decorations are promising for electrocatalytic CO2 reduction to produce valuable chemicals and fuels. Using nickel phthalocyanine derivatives with cyano, methoxy, and dimethylamino groups, this study unveils why decorating molecular catalysts with either electron-donating or electron-withdrawing groups can enhance their activity. Notably, the dimethylamino group-decorated catalyst demonstrated stable and nearly 100% CO2-to-CO reduction selectivity over a wide potential range and high CO partial current densities up to 300 milliamperes per square centimeter. Theoretical and in situ spectroscopic analyses revealed the critical role of dynamic protonation of ligand sites in activating the metal center, which can be facilitated by the decoration of electron-withdrawing groups. Conversely, electron-donating groups, although requiring higher energy for protonation, enhance the synergy between metal centers and protonated sites, favoring the formation of key *COOH intermediates and improving CO selectivity at higher bias. This study underscores the importance of dynamic protonation of ligand sites in optimizing functionalized molecular catalysts for enhanced CO2RR activity.

Integrated Capture and Electrocatalytic Conversion of CO2: A Molecular Electrocatalysts Perspective

The ever-increasing concentration of atmospheric CO2, primarily driven by anthropogenic activities, has raised urgent environmental concerns, spurring the development of carbon capture and utilization (CCU) technologies. This review focuses on the integrated capture and electrochemical conversion of CO2 (ICECC), a promising approach that combines carbon capture with its direct electroreduction into value-added products. By eliminating energy-intensive steps such as CO2 release, compression, and transportation, ICECC offers a more energy-efficient and cost-effective alternative to conventional CCU methods. In this review, particular attention is given to molecular electrocatalysts, which offer high tunability and selectivity in electrochemical CO2 reduction reaction (eCO2RR). The role of capturing agents, including both external and dual-functional molecular systems, is critically examined to understand their influence on CO2 binding and catalytic efficiency. Whereas ICECC has significant potential, research in this area remains underexplored compared to conventional CO2 reduction methods. The review discusses the mechanistic insights into ICECC processes, highlighting key challenges and potential future research directions for improving catalyst design, enhancing capture efficiency, and scaling up ICECC technologies. These developments can make ICECC a critical component in achieving carbon neutrality and addressing climate change.

Photoredox capacity expanded by the Cu site of CuFe-Mabiq

The monometallic Fe-Mabiq and bimetallic CuFe-Mabiq photoredox catalysts feature similar optical spectra and short excited-state lifetimes. Nevertheless, they exhibit markedly different photochemistry. Photoreduction proceeds significantly faster for the bimetallic complex, and uniquely generates the three-electron reduced form. These characteristics underpin the self-sensitized photocatalytic behaviour of the bimetallic complex.

Highly Dispersed Single Clusters Supported Porphyrinic Metal-Organic Frameworks for Synergetic CO2 Electroreduction to CH4

The electrocatalytic CO2 reduction is a promising path toward the carbon-neutral goal but remains a huge challenge due to the high activation barrier for CO2 and poor selectivity. Herein, the highly dispersed triruthenium single cluster (Ru3-SCs) is confined into the nanospace of pyrrole-3-carboxylic acid (PyrA)-modified nickel-porphyrin-based metal-organic framework (Ni-PCN-222-PyrA) to form the composite (Ru3-SCs@Ni-PCN-222-PyrA) through the pre-coordination confinement strategy. The prepared Ru3-SCs@Ni-PCN-222-PyrA can accelerate the selective reduction of CO2 to CH4 via electrocatalysis. Under -1.0 V versus reversible hydrogen electrode (RHE), Ru3-SCs@Ni-PCN-222-PyrA affords CO2 electroreduction to CH4 with a high selectivity of 71.9% Faradaic efficiency. Mechanistic studies reveal that the superior reactivity can be attributed to the ensemble effect and synergistic catalysis of Ru3-SCs, in which one Ru atom is responsible for CO2 reduction to *CO and another Ru atom promotes the water splitting to generate *H, and then the two intermediates of *CO and *H coupled to form the key intermediate of *CHO in a thermodynamically favorable way.

A Transition Metal-Free Approach for the Conversion of Real-Life Cellulose-Based Biomass into Formate

Formic acid (FA) and its salt are recognized as valuable molecules for various industries such as textiles and pharmaceuticals. Currently, the global demand of FA and its salts stands at 1.137 million metric tons per year, necessitating the development of sustainable methods to meet the future demands. While numerous approaches are developed for the generation of FA but the requirement of harsh reaction conditions to achieve them is unavoidable. On the other hand, the world production of biomass is estimated at 146 billion metric tons per year and that can be considered as a prospective source of FA and their salts. Additionally, cellulose accounts for approximately 35-45% of the biomass composition. Considering this, a visible-light-mediated approach is presented to produce formate directly from biomass at room temperature as well as at atmospheric pressure. In this approach, selective generation of hydroxyl radical has been achieved which later converted sugars, cellulose, and hemicellulose into formate. Furthermore, the conversion of cellulose-rich daily-life materials such as discarded paper into the product through a series of flow experiments is demonstrated. Finally, mechanistic investigations including electron paramagnetic resonance (EPR) spectroscopy, and density functional theory (DFT) calculations are conducted to gain insights into the underlying reaction mechanism.

Efficacy of Redox-Active Cu(II) Dipyrrin Complexes toward Electrochemical Reduction of CO2

New D-A-type catalysts based on Cu (II) complexes (C1 and C2) including dipyrrin ligands with phenothiazine/carbazole as the meso-substituent have been described. The complexes have been thoroughly characterized by various methods (1H, 13C, ESI-MS, EPR, and UV-vis studies), and structures of both C1 and C2 unequivocally determined by X-ray single crystal analyses. The catalysts C1 and C2 are stable at room temperature and exhibit Faradaic efficiency values of ∼56% (C1) and ∼46% (C2) toward homogeneous reduction of CO2 to CO. The release of CO has been validated by gas chromatographic (GC) studies. Electron-rich phenothiazine and carbazole included in the catalysts facilitate proton transfer, enabling rapid and selective formation of CO over H2 with FEH2 values of ∼22% for C1 and ∼7% for C2 and turnover numbers (TON) of ∼46 for C1 and ∼21 for C2. Furthermore, the formation of formate ions has been affirmed by ion chromatography (C1, ∼16%; C2, ∼18%). Detailed electrochemical studies and product analyses suggested that C1 displays superior catalytic activity relative to C2 which has been further supported by theoretical studies.

Electrocatalytic CO2 reduction by a cobalt porphyrin mini-enzyme

Cobalt-mimochrome VI*a (CoMC6*a), a synthetic mini-enzyme with a cobalt porphyrin active site, is developed as a biomolecular catalyst for electrocatalytic CO2 reduction in water. The catalytic turnover number reaches ∼14 000 for CO production with a selectivity of 86 : 5 over H2 production under the same conditions. Varying the applied potential and the pK a of the proton donor was used to gain insight into the basis for selectivity. The protected active site of CoMC6*a is proposed to enhance selectivity for CO2 reduction under conditions that typically favor H2 production by related catalysts. CoMC6*a activity and selectivity change only marginally under air, indicating excellent oxygen tolerance.

Recent Advances in Nanostructured Perovskite Oxide Synthesis and Application for Electrocatalysis

Nanostructured materials have garnered significant attention for their unique properties, such as the high surface area and enhanced reactivity, making them ideal for electrocatalysis. Among these, perovskite oxides, with compositional and structural flexibility, stand out for their remarkable catalytic performance in energy conversion and storage technologies. Their diverse composition and tunable electronic structures make them promising candidates for key electrochemical reactions, including the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and carbon dioxide reduction (CO2RR). Nanostructured perovskites offer advantages such as high intrinsic activity and enhanced mass/charge transport, which are crucial for improving electrocatalytic performance. In view of the rapid development of nanostructured perovskites over past few decades, this review aims to provide a detailed evaluation of their synthesis methods, including the templating (soft, hard, colloidal), hydrothermal treatments, electrospinning, and deposition approaches. In addition, in-depth evaluations of the fundamentals, synthetic strategies, and applications of nanostructured perovskite oxides for OER, HER, and CO2RR are highlighted. While progress has been made, further research is needed to expand the synthetic methods to create more complex perovskite structures and improve the mass-specific activity and stability. This review offers insights into the potential of nanostructured perovskite oxides in electrocatalysis and provides potential perspectives for the ongoing research endeavor on the nanostructural engineering of perovskites.

Origins of HCOOH Selectivity Over CO Mediated by an Unusual Fe(I)-Porphyrin Bearing a β-Substituted Cation

Molecular metalloporphyrins have been commonly reported to efficiently catalyze electrochemical CO2-to-CO conversion. Unconventionally, Dey and coworkers reported that an iron-porphyrin analogue bearing a pendant amine binds with CO2 at the Fe(I) state and reduces CO2 into formic acid using water molecules as proton sources. However, the origins of HCOOH selectivity over the conventional CO product, as well as fundamental mechanistic details, are lacking. In the work, theoretical computations were employed to fundamentally investigate the reaction mechanisms. Our calculations reconfirmed that the formal Fe(I)-porphyrin would proceed with a direct CO2-binding step, and this behavior could be ascribed to the significant hydrogen bonding and through-space electrostatic interactions between the cationic N-H and [CO22-]-coordinated species. A two-electron transfer process in the key CO2-binding step is found, which is estimated to proceed consecutively with protonation and 1e-reduction to give rise to an Fe(III)-COOH and Fe(II)-COOH intermediate, respectively. The cationic N-H plays vital roles in the stabilization of C-protonation species to yield HCOOH. Moreover, the cationic N-H terminal could hinder the dissociation of CO. Our computational results are consistent with experimental observations. The origins of HCOOH selectivity are elucidated, and an insightful mechanistic understanding of the cooperative roles of second-sphere hydrogen bonding and cationic effects is provided.

Access to Heterobimetallic MII/CuI Complexes with a Multichelate Platform and Their Reactivity Studies in CO2RR

We describe the selective formation of heterobimetallic complexes, exploiting the coordination trends of the developed bis-terpyridyl trans-1,2-cyclohexadiamine platform (L). Following a stepwise addition, we first reacted ligand L toward tetrakisacetonitrile transition metal precursors, [M(MeCN)4][BF4]2 (where M = Fe or Ni), to generate the monometallic complexes 1 ([FeL][BF4]2) and 2 ([NiL][BF4]2). These species were later combined with the tetrakisacetonitrile precursor [Cu(MeCN)4][BF4], generating the corresponding heterobimetallic complexes 3 ([FeCuL(MeCN)2][BF4]3) and 4 ([NiCuL(MeCN)2][BF4]3). The four species obtained, in high yields, have been structurally characterized. Their cyclic voltammetry analysis revealed the impact of the CuI-atom presence on the heterobimetallic complexes under argon and carbon dioxide (CO2) atmospheres. Controlled potential electrolysis studies revealed the instability of complexes 1-4 toward CO2RR, generating the heterogeneous material in solution and on the electrode surface. In contrast, CO2 photoreduction studies revealed higher stability and photocatalytic activity for the FeII-based complexes (1 and 3), generating CO with 88% selectivity.

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.

Theoretical Study on the Mechanism of the Electrocatalytic CO2 Reduction to Formate by an Iron Schiff Base Complex

The iron(III) chloride compound 6,6'-di(3,5-ditert-butyl-2-hydroxybenzene)-2,2'-bipyridine (Fe(tbudhbpy)Cl) can effectively catalyze the electrochemical CO2 reduction in N,N-dimethylformamide. Density functional calculations were conducted to investigate the mechanism and unravel the governing factors of product selectivity. The results suggest that the initial catalyst, Fe(tbudhbpy)Cl (formally FeIII-Cl), undergoes two reduction steps, accompanied by the dissociation of Cl-, leading to the formation of the active ferrous radical intermediate 2 (formally FeI). Without phenol, 2 attacks CO2 to generate the FeIII-carboxylate intermediate FeIII-CO2, followed by a one-electron reduction to generate FeII-CO2, which reacts with another CO2 to produce CO. This aligns with the experimental result that CO is the main product when the phenol is absent. In contrast, when phenol is presented, the triple reduced species 3 is protonated at its ligand N site to yield 3pt(N) (formally Fe0-NH), which subsequently performs a nucleophilic attack on CO2 to afford formate. This process occurs via an orthogonal electron/proton transfer mechanism, where two electrons and one proton are transferred from the ligand to the CO2 moiety. The redox noninnocent nature of the ligand is thus crucial for formate formation, as it facilitates electron and proton shuttling, enabling 3pt(N) to attack CO2 through this unusual mechanism effectively.

Regulating the electrocatalytic active centers for accelerated proton transfer towards efficient CO2 reduction

The electrochemical CO2 reduction reaction (CO2RR) is an important application that can considerably mitigate environmental and energy crises. However, the slow proton-coupled electron transfer process continues to limit overall catalytic performance. Fine-tuning the reaction microenvironment by accurately constructing the local structure of catalysts provides a novel approach to enhancing reaction kinetics. Here, cubic-phase α-MoC1-x nanoparticles were incorporated into a carbon matrix and coupled with cobalt phthalocyanine molecules (α-MoC1-x-CoPc@C) for the co-reduction of CO2 and H2O, achieving an impressive Faradaic efficiency for CO close to 100%. Through a combination of in-situ spectroscopies, electrochemical measurements, and theoretical simulations, it is demonstrated that α-MoC1-x nanoparticles and CoPc molecules with optimized local configuration serve as the active centers for H2O activation and CO2 reduction, respectively. The interfacial water molecules were rearranged, forming a dense hydrogen bond network on the catalyst surface. This optimized microenvironment at the electrode-electrolyte interface synergistically enhanced water dissociation, accelerated proton transfer, and improved the overall performance of CO2RR.

Hinged Carboxylate in the Artificial Distal Pocket of an Iron Porphyrin Enhances CO2 Electroreduction at Low Overpotential

To efficiently capture, activate, and transform small molecules, metalloenzymes have evolved to integrate a well-organized pocket around the active metal center. Within this cavity, second coordination sphere functionalities are precisely positioned to optimize the rate, selectivity, and energy cost of catalytic reactions. Inspired by this strategy, an artificial distal pocket defined by a preorganized 3D strap is introduced on an iron-porphyrin catalyst (sc-Fe) for the CO2-to-CO electrocatalytic reduction. Combined electrochemical, kinetic, and computational studies demonstrate that the adequate positioning of a carboxylate/carboxylic group acting in synergy with a trapped water molecule within this distal pocket remarkably enhances the reaction turnover frequency (TOF) by four orders of magnitude compared to the perfluorinated iron-tetraphenylporphyrin catalyst (F20Fe) operating at a similar low overpotential. A proton-coupled electron transfer (PCET) is found to be the key process responsible for the unexpected protonation of the coordinating carboxylate, which, upon CO2 insertion, shifts from the first to the second coordination sphere to play a possible secondary role as a proton relay.

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.

Digital Descriptors in Predicting Catalysis Reaction Efficiency and Selectivity

Accurately controlling the interactions and dynamic changes between multiple active sites (e.g., metals, vacancies, and lone pairs of heteroatoms) to achieve efficient catalytic performance is a key issue and challenge in the design of complex catalytic reactions involving 2D metal-supported catalysts, metal-zeolites, metal-organic catalysts, and metalloenzymes. With the aid of machine learning (ML), descriptors play a central role in optimizing the electrochemical performance of catalysts, elucidating the essence of catalytic activity, and predicting more efficient catalysts, thereby avoiding time-consuming trial-and-error processes. Three kinds of descriptors─active center descriptors, interfacial descriptors, and reaction pathway descriptors─are crucial for understanding and designing metal-supported catalysts. Specifically, vacancies, as active sites, synergize with metals to significantly promote the reduction reactions of energy-relevant small molecules. By combining some physical descriptors, interpretable descriptors can be constructed to evaluate catalytic performance. Future development of descriptors and ML models faces the challenge of constructing descriptors for vacancies in multicatalysis systems to rationally design the activity, selectivity, and stability of catalysts. Utilization of generative artificial intelligence and multimodal ML to automatically extract descriptors would accelerate the exploration of dynamic reaction mechanisms. The transferable descriptors from metal-supported catalysts to artificial metalloenzymes provide innovative solutions for energy conversion and environmental protection.

Solid-State-Electrolyte Reactor: New Opportunity for Electrifying Manufacture

Electrocatalysis, which leverages renewable electricity, has emerged as a cornerstone technology in the transition toward sustainable energy and chemical production. However, traditional electrocatalytic systems often produce mixed, impure products, necessitating costly purification. Solid-state electrolyte (SSE) reactors represent a transformative advancement by enabling the direct production of high-purity chemicals, significantly reducing purification costs and energy consumption. The versatility of SSE reactors extends to applications such as CO2 capture and tandem reactions, aligning with the green and decentralized production paradigm. This Perspective provides a comprehensive overview of SSE reactors, discussing their principles, design innovations, and applications in producing pure chemicals-such as liquid carbon fuels, hydrogen peroxide, and ammonia-directly from CO2 and other sources. We further explore the potential of SSE reactors in applications such as CO2 capture and tandem reactions, highlighting their compatibility with versatile production systems. Finally, we outline future research directions for SSE reactors, underscoring their role in advancing sustainable chemical manufacturing.

Boosting Surface Coverage of CO Intermediates through Multimetallic Interface Interactions for Efficient CO2 Electrochemical Reduction

Given the inherent challenges of the CO2 electroreduction (CO2ER) reaction, solely from CO2 and H2O, it is desirable to develop selective product formation pathways. This can be achieved by designing multimetallic nanocomposites that provide optimal CO coverage, allowing for tunability in the product formation. In this work, Ag and Zn codoped-SrTiO3 (ZAST) composite immobilized carbon black (CB)-modified GCE working electrode (ZAST@CB/GCE) was developed for the electrochemical conversion of CO2 to multicarbon products. The complete reaction was carried out in a CO2-saturated aqueous system of 0.5 M KHCO3 electrolyte. A potential-dependent product selectivity was suggested based on the NMR results, wherein raising the potential value enhanced the formation of liquid products such as acetone and alcohols while suppressing competitive HER. The total Faradaic efficiency for liquid products reached an impressive 97% at a potential of -0.6 V vs. RHE. This represents a significant advancement in acetone production pathways and valorization of CO2ER technology.

Computational Modeling of Electrocatalysts for CO2 Reduction: Probing the Role of Primary, Secondary, and Outer Coordination Spheres

ConspectusIn the search for efficient and selective electrocatalysts capable of converting greenhouse gases to value-added products, enzymes found in naturally existing bacteria provide the basis for most approaches toward electrocatalyst design. Ni,Fe-carbon monoxide dehydrogenase (Ni,Fe-CODH) is one such enzyme, with a nickel-iron-sulfur cluster named the C-cluster, where CO2 binds and is converted to CO at high rates near the thermodynamic potential. In this Account, we divide the enzyme's catalytic contributions into three categories based on location and function. We also discuss how computational techniques provide crucial insight into implementing these findings in homogeneous CO2 reduction electrocatalysis design principles. The CO2 binding sites (e.g., Ni and "unique" Fe ion) along with the ligands that support it (e.g., iron-sulfur cluster) form the primary coordination sphere. This is replicated in molecular electrocatalysts via the metal center and ligand framework where the substrate binds. This coordination sphere has a direct impact on the electronic configuration of the catalyst. By computationally modeling a series of Ni and Co complexes with bipyridyl-N-heterocyclic carbene ligand frameworks of varying degrees of planarity, we were able to closely examine how the primary coordination sphere controls the product distribution between CO and H2 for these catalysts. The secondary coordination sphere (SCS) of Ni,Fe-CODH contains residues proximal to the active site pocket that provide hydrogen-bonding stabilizations necessary for the reaction to proceed. Enhancing the SCS when synthesizing new catalysts involves substituting functional groups onto the ligand for direct interaction with the substrate. To analyze the endless possible substitutions, computational techniques are ideal for deciphering the intricacies of substituent effects, as we demonstrated with an array of imidazolium-functionalized Mn and Re bipyridyl tricarbonyl complexes. By examining how the electrostatic interactions between the ligand, substrate, and proton source lowered activation energy barriers, we determined how best to pinpoint the SCS additions. The outer coordination sphere comprises the remaining parts of Ni,Fe-CODH, such as the elaborate protein matrix, solvent interactions, and remote metalloclusters. The challenge in elucidating and replicating the role of the vast protein matrix has understandably led to a localized focus on the primary and secondary coordination spheres. However, certain portions of Ni,Fe-CODH's expansive protein scaffold are suggested to be catalytically relevant despite considerable distance from the active site. Closer studies of these relatively overlooked areas of nature's exceptionally proficient catalysts may be crucial to continually improve upon electrocatalysis protocols. Mechanistic analysis of cobalt phthalocyanines (CoPc) immobilized onto carbon nanotubes (CoPc/CNT) reveals how the active site microenvironment and outer coordination sphere effects unlock the CoPc molecule's previously inaccessible intrinsic catalytic ability to convert CO2 to MeOH. Our research suggests that incorporating the three coordination spheres in a holistic approach may be vital for advancing electrocatalysis toward viability in mitigating climate disruption. Computational methods allow us to closely examine transition states and determine how to minimize key activation energy barriers.

Regulating Interfacial Hydrogen-Bonding Networks by Implanting Cu Sites with Perfluorooctane to Accelerate CO2 Electroreduction to Ethanol

Efficient CO2 electroreduction (CO2RR) to ethanol holds promise to generate value-added chemicals and harness renewable energy simultaneously. Yet, it remains an ongoing challenge due to the competition with thermodynamically more preferred ethylene production. Herein, we presented a CO2 reduction predilection switch from ethylene to ethanol (ethanol-to-ethylene ratio of ~5.4) by inherently implanting Cu sites with perfluorooctane to create interfacial noncovalent interactions. The 1.83 %F-Cu2O organic-inorganic hybrids (OIHs) exhibited an extraordinary ethanol faradaic efficiency (FEethanol) of ∼55.2 %, with an impressive ethanol partial current density of 166 mA cm-2 and excellent robustness over 60 hours of continuous operation. This exceptional performance ranks our 1.83 %F-Cu2O OIHs among the best-performing ethanol-oriented CO2RR electrocatalysts. Our findings identified that C8F18 could strengthen the interfacial hydrogen bonding connectivity, which consequently promotes the generation of active hydrogen species and preferentially favors the hydrogenation of *CHCOH to *CHCHOH, thus switching the reaction from ethylene-preferred to ethanol-oriented. The presented investigations highlight opportunities for using noncovalent interactions to tune the selectivity of CO2 electroreduction to ethanol, bringing it closer to practical implementation requirements.

Ferrocene-Functionalized Atomically Precise Metal Clusters Exhibit Synergistically Enhanced Performance for CO2 Electroreduction

The integration of organometallic compounds with metal nanoparticles can, in principle, generate hybrid nanocatalysts endowed with augmented functionality, presenting substantial promise for catalytic applications. Herein, we synthesize an atomically precise metal cluster (Ag9Cu6) catalyst integrated with alkynylferrocene molecules (Ag9Cu6-Fc). This hybrid catalyst design facilitates a continuous electron transfer channel via an ethynyl bridge and establishes a distinctive local chemical environment, resulting in remarkably enhanced catalytic activity in CO2 electroreduction. The Ag9Cu6-Fc catalyst achieves a record-high product selectivity of CO Faradaic efficiency of 100 % and an industrial-level CO partial current density of -680 mA/cm2, surpassing the performance of the Ag9Cu6 cluster (62 % and -230 mA/cm2, respectively) without ferrocene functionalization in a membrane electrode assembly cell. Operando experimental and computational findings offer valuable insights into the role of ferrocene functionalization in synergistically improving the catalytic performance of metal clusters, propelling the advancement of metallic-organometallic hybrid nanoparticles for energy conversion technologies.

Ni Single Atoms/Fe3N Nanoparticles Supported by N-Doped Carbon Hollow Nanododecahedras with Nanotubes on the Surface for Efficient Electro-Reduction of CO2 to CO

The transition metal single atoms (SAs)-based catalysts with M-NX coordination environment have shown excellent performance in electrocatalytic reduction of CO2, and they have received extensive attention in recent years. However, the presence of SAs makes it very difficult to efficiently improve the coordination environment. In this paper, a method of direct high-temperature pyrolysis carbonization of ZIF-8 adsorbed with Ni2+ and Fe2+ ions is reported for the synthesis of Ni SAs and Fe3N nanoparticles (NPs) supported by the N-doped carbon (NC) hollow nanododecahedras (HNDs) with nanotubes (NTs) on the surface (Ni SAs/Fe3N NPs@NC-HNDs-NTs). The synergistic effect between Ni SAs and Fe3N NPs can obviously improve the proton-coupled electron transfer step of CO2 reduction reaction and promotes the process of electrocatalytic reduction of CO2 to CO. The fabricated Ni SAs/Fe3N NPs@NC-HNDs-NTs exhibits a high CO selectivity of up to 94% in the potential range of -0.41--0.81 V versus Reversible Hydrogen Electrode (vs RHE), and an optimal CO Faraday efficiency (FECO) of ≈97.31% at -0.68 V (vs RHE) in the reduction reaction CO2 to CO. In the theoretical calculation results, due to the non-bonding synergy effect between Ni SAs and Fe3N NPs, the free energy of *COOH formation is greatly reduced and the adsorption of *CO is obviously improved, which will efficiently promote the conversion between the intermediates in the reaction step and accelerate electro-reduction process of CO2. This work will provide a new method for constructing a mutually optimized coordination environment between Ni SAs and Fe3N NPs to improve the catalytic performance of CO2RR by synergistic complementarity between the dual active sites.

The volcanic relationship of model phthalocyanine molecular catalysts in the CO2 reduction reaction

We have constructed a series of model metal phthalocyanines (MPc) for the carbon dioxide reduction reaction (CO2RR), constructed a volcano relationship through density functional theory (DFT) and experiments, and obtained cobalt phthalocyanine (CoPc) at the apex. The volcano diagram is conducive to the screening of catalysts and has a guiding role in the design of catalysts.

Electrochemical Reduction of CO2 to CH3OH Catalyzed by an Iron Porphyrinoid

Designing catalysts for the selective reduction of CO2, resulting in products having commercial value, is an important area of contemporary research. Several molecular catalysts have been reported to facilitate the reduction of CO2 (both electrochemical and photochemical) to yield 2e-/2H+ electron-reduced products, CO and HCOOH, and selective reduction of CO2 beyond 2e-/2H+ is rare. This is partly because the factors that control the selectivity of CO2 reduction beyond 2e- are not yet understood. An iron chlorin complex with a pendent amine functionality in its second sphere, known to selectively catalyze CO2RR to HCOOH with a very low overpotential from its formal Fe(I) state, can catalyze CO2RR from its formal Fe(0) state by 6e-/6H+, forming CH3OH as a major product with a Faradaic yield of ∼50%. Mechanistic investigations using in situ spectro-electrochemistry indicate that the reactivity of a low-spin d7 FeI-COOH intermediate species generated during CO2RR is crucial in determining the product selectivity of this reaction. In weakly acidic conditions, C-protonation of this FeI-COOH species, which is also chemically prepared and spectroscopically characterized, leads to HCOOH. The O-protonation, leading to C-OH bond cleavage and eventually to CH3OH, is ∼3 kcal/mol higher in energy and can be achieved in more acidic solutions. Hydrogen bonding to the pendent amine in the catalyst stabilizes reactive intermediates formed in the CO2RR and enables 6e-/6H+ reduction of CO2 to CH3OH.

Dual Metallosalen-Based Covalent Organic Frameworks for Artificial Photosynthetic Diluted CO2 Reduction

Directly converting CO2 in flue gas using artificial photosynthetic technology represents a promising green approach for CO2 resource utilization. However, it remains a great challenge to achieve efficient reduction of CO2 from flue gas due to the decreased activity of photocatalysts in diluted CO2 atmosphere. Herein, we designed and synthesized a series of dual metallosalen-based covalent organic frameworks (MM-Salen-COFs, M: Zn, Ni, Cu) for artificial photosynthetic diluted CO2 reduction and confirmed their advantage in comparison to that of single metal M-Salen-COFs. As a results, the ZnZn-Salen-COF with dual Zn sites exhibits a prominent visible-light-driven CO2-to-CO conversion rate of 150.9 μmol g-1 h-1 under pure CO2 atmosphere, which is ~6 times higher than that of single metal Zn-Salen-COF. Notably, the dual metal ZnZn-Salen-COF still displays efficient CO2 conversion activity of 102.1 μmol g-1 h-1 under diluted CO2 atmosphere from simulated flue gas conditions (15 % CO2), which is a record high activity among COFs- and MOFs-based photocatalysts under the same reaction conditions. Further investigations and theoretical calculations suggest that the synergistic effect between the neighboring dual metal sites in the ZnZn-Salen-COF facilitates low concentration CO2 adsorption and activation, thereby lowering the energy barrier of the rate-determining step.

Controllable Regulation of CO2 Adsorption Behavior via Precise Charge Donation Modulation for Highly Selective CO2 Electroreduction to Formic Acid

The synthesis of value-added products via CO2 electroreduction (CO2ER) is of great significance, but the development of efficient and versatile strategies for the controllable selectivity tuning is extremely challenging. Herein, the tuning of CO2ER selectivity through the modulation of CO2 adsorption behavior is proposed. Using the constructed zeolitic MOF (SNNU-339), CO2 adsorption behavior is controllably changed from *CO2 to CO2* via the precise ligand-to-metal charge donation (LTMCD) regulation. It is confirmed that the high electronegativity of the coordinate ligand directly restricts the LTMCD, reduces the charge density on the metal sites, lowers the Gibbs free energy for CO2* adsorption, and leads to the transformation of CO2 adsorption mode from *CO2 to CO2*. Owing to the modulated CO2 adsorption behavior and regulated kinetics, SNNU-339 exhibits superior HCOOH selectivity (≈330% promotion, 85.6% Faradaic efficiency) and high CO2ER activity. The wide applicability of the proposed approach sheds light on the efficient CO2ER. This study provides a competitive strategy for rational catalyst design and underscores the significance of adsorption behavior tuning in electrocatalysis.

Convergence of Tandem Catalysis and Nanoconfinement Promotes Electroreduction of CO2 to C2 Products

An efficient electrocatalytic conversion of CO2 into valuable multicarbon (C2+) products requires enhanced C-C coupling of C1 intermediates. Herein, we combine a tandem effect with a confinement strategy to construct a hollow Cu2O@Ag nanoshell electrocatalyst with a well-defined porous structure to improve the *CO intermediate coverage on the catalyst surface. In CO2 electroreduction, in situ Raman spectroscopy shows that the introduction of Ag can not only promote the CO intermediate production but also improve the stability of Cu+ to capture the *CO intermediate due to a CO-tandem effect, and the fine-tuned hollowness degree and pore size of Cu2O@Ag create a spatially confined microenvironment for trapping CO2 as well as the enrichment of CO, which greatly facilitate subsequent C-C coupling for C2+ product. The optimized Cu2O@Ag-45 with a specific nanoconfinement exhibits an enhanced ethylene (C2H4) production under the wide potential range from -0.4 to -1.2 V (vs RHE), and a Faradaic efficiency of 55.4% for C2+ products could be achieved at -1.2 V (vs RHE). This study highlights a promising strategy for the electrochemical reduction of CO2 to C2+ products on efficient C-C coupling catalysts.

Steering the Selectivity of CORR from Acetate to Ethanol via Tailoring the Thermodynamic Activity of Water

The electrochemical conversion of carbon monoxide (CO) into oxygenated C2+ products at high rates and selectivity offers a promising approach for the two-step conversion of carbon dioxide (CO2). However, a major drawback of the CO electrochemical reduction in alkaline electrolyte is the preference for the acetate pathway over the more valuable ethanol pathway. Recent research has shed light on the significant impact of thermodynamic water activity on the electrochemical CO2 reduction reaction pathways, but less is understood for the electrochemical reduction of CO. In this study, we investigated how the water activity at the electrified interface can be enhanced to adjust the selectivity between acetate and ethanol. We employed an ionomer modifier to lower the local concentration of alkali ions (via Donnan exclusion), successfully enhancing ethanol production while suppressing acetate formation. We observed a remarkable improvement in the Faradaic efficiency of ethanol and alcohol (i. e. ethanol, propanol etc), which reached 42.5 % and 55.1 %, respectively, at a current density of 700 mA cm-2. The partial current densities of ethanol and alcohol reached 698 and 942 mA cm-2 at 2000 mA cm-2. Furthermore, we achieved a 3.7-fold increase in the ethanol/acetate ratio, providing clear evidence of our successful modulation of product selectivity.

Relationships between the Surface Hydrophilicity of a Bismuth Electrode and the Product Selectivity of Electrocatalytic CO2 Reduction

Two types of bismuth films (micro-Bi and nano-Bi) were prepared, and their electrocatalytic behavior was studied in terms of reduction current and product selectivity in a potential range of -0.776 to -1.376 V vs RHE. CO2 and H2O molecules competed with each other for reduction on the surfaces of both types of films, and formate and H2 were the respective major products of reductive reactions. Under the same conditions, nano-Bi exhibited lower selectivity for formate and higher selectivity for H2 compared to the respective micro-Bi cases with bismuth films of similar thickness. This can be attributed to the higher hydrophilicity of bismuth film surfaces of nano-Bi due to surface nanoscale roughness and lower surface-carbon content compared with those of micro-Bi. Our results suggest a new strategy for controlling the selectivity of electrocatalytic CO2 reduction under aqueous electrolytes through the use of surface engineering.

Transferring enzyme features to molecular CO2 reduction catalysts

Carbon monoxide dehydrogenases and formate dehydrogenases efficiently catalyze the reduction of CO2. In both enzymes, CO2 activation at the metal active site is assisted by proximate amino acids and Fe-S-clusters. Functional features of the enzyme are mimicked in molecular catalysts by redox-active ligands, acidic and charged groups in the ligand periphery, and binuclear scaffolds. These components have all improved the catalytic performance of synthetic systems. Recent studies impart a deeper understanding of the individual contributions of the various functionalities to reactivity and of their combined effects. New catalyst platforms reveal alternate pathways for CO2 reduction, unique intermediates, and strategies for switching selectivity. Design of a wider array of complexes that combine different functional elements is encouraged to further optimize catalysts for CO2 reduction, especially for product formation beyond CO. More diverse bimetallic catalysts are needed to better exploit metal-metal interactions for CO2 conversion.

TM and P dual sites on polymeric carbon nitride enable highly selective CO reduction to C2 products with low potentials: A theoretical perspective

The CO reduction reaction (CORR) for the production of high-value-added multi-carbon (C2+) products is currently being actively investigated, where searching for high-efficiency catalysts with moderate CO intermediate binding strength and low kinetic barrier for C-C coupling poses a significant challenge. In this study, we employed density functional theory computations to design four synergistic coupling dual sites catalysts for CORR to C2 products, namely, TM-P@melon, by co-doping transition metals (TM = Mn, Fe, Co, and Ni) and phosphorus (P) into the polymeric carbon nitride (i.e., melon-CN). Mn-P@melon and Ni-P@melon exhibit higher selectivity toward C2H5OH and C2H6, respectively, with limiting potentials (C-C coupling kinetic energy barriers) of -0.43 V (0.52 eV) and -0.17 V (0.26 eV), respectively. The introduction of TM and P atoms not only narrows the band gap of melon-CN but also favors the coupling of CO and *CHO, providing an active site for C-C coupling, thus facilitating the catalytic reaction. Our work provides rational insights for the design of stable, low-cost, and efficient CORR dual sites catalysts that facilitate the sustainable production of high-value C2 chemicals and fuels.

Non-Noble-Metal-Based Electrocatalysts for Acidic Oxygen Evolution Reaction: Recent Progress, Challenges, and Perspectives

The oxygen evolution reaction (OER) plays a pivotal role in diverse renewable energy storage and conversion technologies, including water electrolysis, electrochemical CO2 reduction, nitrogen fixation, and metal-air batteries. Among various water electrolysis techniques, proton exchange membrane (PEM)-based water electrolysis devices offer numerous advantages, including high current densities, exceptional chemical stability, excellent proton conductivity, and high-purity H2. Nevertheless, the prohibitive cost associated with Ir/Ru-based OER electrocatalysts poses a significant barrier to the broad-scale application of PEM-based water splitting. Consequently, it is crucial to advance the development of non-noble metal OER catalysis substance with high acid-activity and stability, thereby fostering their widespread integration into PEM water electrolyzers (PEMWEs). In this review, a comprehensive analysis of the acidic OER mechanism, encompassing the adsorbate evolution mechanism (AEM), lattice oxygen mechanism (LOM) and oxide path mechanism (OPM) is offered. Subsequently, a systematic summary of recently reported noble-metal-free catalysts including transition metal-based, carbon-based and other types of catalysts is provided. Additionally, a comprehensive compilation of in situ/operando characterization techniques is provided, serving as invaluable tools for furnishing experimental evidence to comprehend the catalytic mechanism. Finally, the present challenges and future research directions concerning precious-metal-free acidic OER are comprehensively summarized and discussed in this review.

Electrocatalytic Conversion of CO2 to Formic Acid: A Journey from 3d-Transition Metal-Based Molecular Catalyst Design to Electrolyzer Assembly

ConspectusElectrochemical CO2 reduction to obtain formate or formic acid is receiving significant attention as a method to combat the global warming crisis. Significant efforts have been devoted to the advancement of CO2 reduction techniques over the past few decades. This Account provides a unified discussion on various electrochemical methodologies for CO2 to formate conversion, with a particular focus on recent advancements in utilizing 3d-transition-metal-based molecular catalysts. This Account primarily focuses on understanding molecular functions and mechanisms under homogeneous conditions, which is essential for assessing the optimized reaction conditions for molecular catalysts. The unique architectural features of the formate dehydrogenase (FDH) enzyme provide insight into the key role of the surrounding protein scaffold in modulating the active site dynamics for stabilizing the key metal-bound CO2 intermediate. Additionally, the protein moiety also triggers a facile proton relay around the active site to drive electrocatalytic CO2 reduction forward. The fine-tuning of FDH machinery also ensures that the electrocatalytic CO2 reduction leads to the production of formic acid as the major yield without any other carbonaceous products, while limiting the competitive hydrogen evolution reaction. These lessons from the enzymes are key in designing biomimetic molecular catalysts, primarily based on multidentate ligand scaffolds containing peripheral proton relays. The subtle modifications of the ligand framework ensure the favored production of formic acid following electrocatalytic CO2 reduction in the solution phase. Next, the molecular catalysts are required to be mounted on robust electroactive surfaces to develop their corresponding heterogeneous versions. The surface-immobilization provides an edge to the molecular electrocatalysts as their reactivity can be scaled up with improved durability for long-term electrocatalysis. Despite challenges in developing high-performance, selective catalysts for the CO2 to formic acid transformation, significant progress is being made with the tactical use of graphene and carbon nanotube-based materials. To date, the majority of the research activity stops here, as the development of an operational CO2 to formic acid converting electrolyzer prototype still remains in its infancy. To elaborate on the potential future steps, this Account covers the design, scaling parameters, and existing challenges of assembling large-scale electrolyzers. A short glimpse at the utilization of electrolyzers for industrial-scale CO2 reduction is also provided here. The proper evaluation of the surface-immobilized electrocatalysts assembled in an electrolyzer is a key step for gauging their potential for practical viability. Here, the key electrochemical parameters and their expected values for industrial-scale electrolyzers have been discussed. Finally, the techno-economic aspects of the electrolyzer setup are summarized, completing the journey from tactical design of molecular catalysts to their appropriate application in a commercially viable electrolyzer setup for CO2 to formate electroreduction. Thus, this Account portrays the complete story of the evolution of a molecular catalyst to its sustainable application in CO2 utilization.

The Reconstruction of Bi2Te4O11 Nanorods for Efficient and pH-universal Electrochemical CO2 Reduction

The electrochemical CO2 reduction reaction (CO2RR) to generate chemical fuels such as formate presents a promising route to a carbon-neutral future. However, its practical application is hindered by the competing CO production and hydrogen evolution reaction (HER), as well as the lack of pH-universal catalysts. Here, Te-modified Bi nanorods (Te-Bi NRs) were synthesized through in situ reconstruction of Bi2Te4O11 NRs under the CO2RR condition. Our study illustrates that the complex reconstruction process of Bi2Te4O11 NRs during CO2RR could be decoupled into three distinct steps, i.e., the destruction of Bi2Te4O11, the formation of Te/Bi phases, and the dissolution of Te. The thus-obtained Te-Bi NRs exhibit remarkably high performance in CO2RR towards formate production, showing high activity, selectivity, and stability across all pH conditions (acidic, neutral, and alkaline). In a flow cell reactor under neutral, alkaline, or acidic conditions, the catalysts achieved HCOOH Faradaic efficiencies of up to 94.3 %, 96.4 %, and 91.0 %, respectively, at a high current density of 300 mA cm-2. Density functional theory calculations, along with operando spectral measurements, reveal that Te manipulates the Bi sites to an electron-deficient state, enhancing the adsorption strength of the *OCHO intermediate, and significantly suppressing the competing HER and CO production. This study highlights the substantial influence of catalyst reconstruction under operational conditions and offers insights into designing highly active and stable electrocatalysts towards CO2RR.

Amino-Induced CO2 Spillover to Boost the Electrochemical Reduction Activity of CdS for CO Production

A considerable challenge in CO2 reduction reaction (CO2RR) to produce high-value-added chemicals comes from the adsorption and activation of CO2 to form intermediates. Herein, an amino-induced spillover strategy aimed at significantly enhancing the CO2 adsorption and activation capabilities of CdS supported on N-doped mesoporous hollow carbon sphere (NH2-CdS/NMHCS) for highly efficient CO2RR is presented. The prepared NH2-CdS/NMHCS exhibits a high CO Faradaic efficiency (FECO) exceeding 90% from -0.8 to -1.1 V versus reversible hydrogen electrode (RHE) with the highest FECO of 95% at -0.9 V versus RHE in H cell. Additional experimental and theoretical investigations demonstrate that the alkaline -NH2 group functions as a potent trapping site, effectively adsorbing the acidic CO2, and subsequently triggering CO2 spillover to CdS. The amino modification-induced CO2 spillover, combined with electron redistribution between CdS and NMHCS, not only readily achieves the spontaneous activation of CO2 to *COOH but also greatly reduces the energy required for the conversion of *COOH to *CO intermediate, thus endowing NH2-CdS/NMHCS with significantly improved reaction kinetics and reduced overpotential for CO2-to-CO conversion. It is believed that this research can provide valuable insights into the development of electrocatalysts with superior CO2 adsorption and activation capabilities for CO2RR application.

Examining the Importance of Hydrogen Bonding and Proton Transfer in Iron Porphyrin-Mediated Carbon Dioxide Upconversion

ConspectusThe title should give a sense of the "big picture" of this Account, but what is it really about? An unexpected change in research direction? A series of courageous and creative students? A team taking on challenging problems in chemistry? The answer is a definite "yes" to all of the above. More specifically, the problem in which we are interested is the upconversion or valorization of carbon dioxide. This problem has captured the attention of a great many chemists in earnest following the gas crisis of the 1970s and more recently galvanized due to climate concerns arising from the ongoing release of anthropogenic carbon. Addressing the problem of atmospheric carbon accumulation requires effort in two very broad areas: capture and conversion. Storage is an alternative to conversion, but this eliminates the opportunity to use what might be otherwise a waste product. Our group has investigated a series of modified versions of iron(III)-5,10,15,20-tetraphenylporphyrin (FeTPP) that can convert CO2 to carbon monoxide, which is a versatile and useful precursor for other syntheses. Following pioneering work from Savéant and his colleagues in the 1990s and thereafter, we started with a simple question: how many pendent ancillary groups that can donate H-bonds or protons are needed to support efficient CO2-to-CO conversion? Using a molecule with only one 2-hydroxylphenyl group, we demonstrated that the single prepositioned -OH group gave rise to efficient turnover, but only when experiments were carried out in a weakly H-bond-accepting solvent system. In other words, the ability of a solvent to accept H-bonds can impede CO2 reduction. We followed up with a deeper investigation of the influence of H-bonding interactions with external acids in FeTPP-mediated CO2 reduction. Savéant's framework mechanism appears to be independent of solvent, and rate differences can be approximated by considering H-bonding equilibria. Following that work, we sought to better understand the minimum catalyst design requirements with respect to internal H-bond/proton donors. To that end, we produced all possible isomers of tetraarylpoprhyrins with 2,6-dihydroxyphenyl + phenyl groups. All else being equal, the complexes with a formally trans orientation of the 2,6-dihydroxyphenyl groups performed the best. Most recently, we surveyed the roles of internal and external Brønsted acids with different pKa values. Surprisingly, the best-performing catalysts have more weakly acidic internal groups. Overall, our work has demonstrated that CO2 reduction mediated by porphyrin catalysts can be improved by considering solvent H-bonding, the orientation of internal H-bonding groups, and the balance of the pKa values of internal and external acids. The future for molecular electrocatalysts is promising as more ideas emerge about how to design molecules and conditions for CO2 reduction.

Regulating Cu Oxidation State for Electrocatalytic CO2 Conversion into CO with Near-Unity Selectivity via Oxygen Spillover

Cu-based catalysts are the most intensively studied in the field of electrocatalytic CO2 reduction reaction (CO2RR), demonstrating the capacity to yield diverse C1 and C2+ products albeit with unsatisfactory selectivity. Manipulation of the oxidation state of Cu sites during CO2RR process proves advantageous in modulating the selectivity of productions, but poses a formidable challenge. Here, an oxygen spillover strategy is proposed to enhance the oxidation state of Cu during CO2RR by incorporating the oxygen donor Sb2O4. The Cu-Sb bimetallic oxide catalyst attains a remarkable CO2-to-CO selectivity approaching unity, in stark contrast to the diverse product distribution observed with bare CuO. The exceptional Faradaic efficiency of CO can be maintained across a wide range of potential windows of ≈700 mV in 1 m KOH, and remains independent of the Cu/Sb ratio (ranging from 0.1:1 to 10:1). Correlative calculations and experimental results reveal that oxygen spillover from Sb2O4 to Cu sites maintains the relatively high valence state of Cu during CO2RR, which diminishes the binding strength of *CO, thereby achieving heightened selectivity in CO production. These findings propose the role of oxygen spillover in CO2RR over Cu-based catalysts, and shed light on the rational design of highly selective CO2 reduction catalysts.

Progress and challenges in nitrous oxide decomposition and valorization

Nitrous oxide (N2O) decomposition is increasingly acknowledged as a viable strategy for mitigating greenhouse gas emissions and addressing ozone depletion, aligning significantly with the UN's sustainable development goals (SDGs) and carbon neutrality objectives. To enhance efficiency in treatment and explore potential valorization, recent developments have introduced novel N2O reduction catalysts and pathways. Despite these advancements, a comprehensive and comparative review is absent. In this review, we undertake a thorough evaluation of N2O treatment technologies from a holistic perspective. First, we summarize and update the recent progress in thermal decomposition, direct catalytic decomposition (deN2O), and selective catalytic reduction of N2O. The scope extends to the catalytic activity of emerging catalysts, including nanostructured materials and single-atom catalysts. Furthermore, we present a detailed account of the mechanisms and applications of room-temperature techniques characterized by low energy consumption and sustainable merits, including photocatalytic and electrocatalytic N2O reduction. This article also underscores the extensive and effective utilization of N2O resources in chemical synthesis scenarios, providing potential avenues for future resource reuse. This review provides an accessible theoretical foundation and a panoramic vision for practical N2O emission controls.

Spin states of metal centers in electrocatalysis

Understanding the electronic structure of active sites is crucial in efficient catalyst design. The spin state, spin configurations of d-electrons, has been frequently discussed recently. However, its systematic depiction in electrocatalysis is lacking. In this tutorial review, a comprehensive interpretation of the spin state of metal centers in electrocatalysts and its role in electrocatalysis is provided. This review starts with the basics of spin states, including molecular field theory, crystal field theory, and ligand field theory. It further introduces the differences in low spin, intermediate spin, and high spin, and intrinsic factors affecting the spin state. Popular characterization techniques and modeling approaches that can reveal the spin state, such as X-ray absorption microscopy, electron spin resonance spectroscopy, Mössbauer spectroscopy, and density functional theory (DFT) calculations, are introduced as well with examples from the literature. The examples include the most recent progress in tuning the spin state of metal centers for various reactions, e.g., the oxygen evolution reaction, oxygen reduction reaction, hydrogen evolution reaction, carbon dioxide reduction reaction, nitrogen reduction reaction, nitrate reduction reaction, and urea oxidation reaction. Challenges and potential implications for future research related to the spin state are discussed at the end.

Bifunctional Catalysts for CO2 Reduction and O2 Evolution: A Pivotal for Aqueous Rechargeable Zn-CO2 Batteries

The quest for the advancement of green energy storage technologies and reduction of carbon footprint is determinedly rising toward carbon neutrality. Aqueous rechargeable Zn-CO2 batteries (ARZCBs) hold the great potential to encounter both the targets simultaneously, i.e., green energy storage and CO2 conversion to value-added chemicals/fuels. The major descriptor of ARZCBs efficiency is allied with the reactions occurring at cathode during discharging (CO2 reduction) and charging (O2 evolution) which own different fundamental mechanisms and hence mandate the employment of two different catalysts. This presents an overall complex and expensive battery system which requires a concrete solution, while the development and application of a bifunctional cathode catalyst toward both reactions could reduce the complexity and cost and thus can be a pivotal for ARZCBs. However, despite the increasing research interest and ongoing research, a systematic evaluation of bifunctional catalysts is rarely reported. In this review, the need of bifunctional cathode catalysts for ARZCBs and associated challenges with strategies have been critically assessed. A detailed progress examination and understanding toward designing of bifunctional catalyst for ARZCBs have been provided. This review will enlighten the future research approaching boosted performance of ARZCBs through the development of efficient bifunctional cathode catalysts.