Advances in Sn-Based Catalysts for Electrochemical CO2 Reduction

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.

Layered Sn-Au Thin Films for Increased Electrochemical ATR-SEIRAS Enhancement

Operando electrochemical attenuated total reflection surface-enhanced infrared absorption spectroscopy (EC ATR-SEIRAS) is a valuable method for a fundamental understanding of electrochemical interfaces under real operating conditions. The applicability of this method depends on the ability to tune the optical and catalytic properties of an electrode film, and it thus requires unique optimization for any given material. Motivated by the growing interest in Sn-based electrocatalysts for selective reduction of CO2 to formate species, we investigate several Sn thin-film synthesis routes for the resulting SEIRA signal response. We compare the SEIRA performance of thermally evaporated metallic Sn to a series of Sn-based films on top of a SEIRA-active Au substrate (metallic Sn, oxide-derived metallic Sn, and metal oxide SnOx). Using alkanethiol self-assembled monolayers as a probe, we find that electrodepositing metallic catalyst films on top of SEIRA-active Au substrates yield higher signal relative to thermal evaporation as well as higher signal than the independent SEIRA-active Au underlayer. These observations come despite the fact that thermally evaporated Sn has a significantly higher surface roughness (and thus higher adsorbate population), suggesting specific SEIRA-magnifying effects for the stacked films. Finally, we applied these films to observe the electrochemical conversion of CO2. Differences are observed in spectral features based on the composition of the electrode being either metallic or oxide-derived metallic Sn, implying differences in their respective reaction pathways.

Regulation Strategy of Nanostructured Engineering on Indium-Based Materials for Electrocatalytic Conversion of CO2

Electrochemical carbon dioxide reduction (CO2 RR), as an emerging technology, can combine with sustainable energies to convert CO2 into high value-added products, providing an effective pathway to realize carbon neutrality. However, the high activation energy of CO2 , low mass transfer, and competitive hydrogen evolution reaction (HER) leads to the unsatisfied catalytic activity. Recently, Indium (In)-based materials have attracted significant attention in CO2 RR and a series of regulation strategies of nanostructured engineering are exploited to rationally design various advanced In-based electrocatalysts, which forces the necessary of a comprehensive and fundamental summary, but there is still a scarcity. Herein, this review provides a systematic discussion of the nanostructure engineering of In-based materials for the efficient electrocatalytic conversion of CO2 to fuels. These efficient regulation strategies including morphology, size, composition, defects, surface modification, interfacial structure, alloying, and single-atom structure, are summarized for exploring the internal relationship between the CO2 RR performance and the physicochemical properties of In-based catalysts. The correlation of electronic structure and adsorption behavior of reaction intermediates are highlighted to gain in-depth understanding of catalytic reaction kinetics for CO2 RR. Moreover, the challenges and opportunities of In-based materials are proposed, which is expected to inspire the development of other effective catalysts for CO2 RR.

Carbon Catalysts Empowering Sustainable Chemical Synthesis via Electrochemical CO2 Conversion and Two-Electron Oxygen Reduction Reaction

Carbon materials hold significant promise in electrocatalysis, particularly in electrochemical CO2 reduction reaction (eCO2 RR) and two-electron oxygen reduction reaction (2e- ORR). The pivotal factor in achieving exceptional overall catalytic performance in carbon catalysts is the strategic design of specific active sites and nanostructures. This work presents a comprehensive overview of recent developments in carbon electrocatalysts for eCO2 RR and 2e- ORR. The creation of active sites through single/dual heteroatom doping, functional group decoration, topological defect, and micro-nano structuring, along with their synergistic effects, is thoroughly examined. Elaboration on the catalytic mechanisms and structure-activity relationships of these active sites is provided. In addition to directly serving as electrocatalysts, this review explores the role of carbon matrix as a support in finely adjusting the reactivity of single-atom molecular catalysts. Finally, the work addresses the challenges and prospects associated with designing and fabricating carbon electrocatalysts, providing valuable insights into the future trajectory of this dynamic field.

Impact of palladium/palladium hydride conversion on electrochemical CO2 reduction via in-situ transmission electron microscopy and diffraction

Electrochemical conversion of CO2 offers a sustainable route for producing fuels and chemicals. Pd-based catalysts are effective for converting CO2 into formate at low overpotentials and CO/H2 at high overpotentials, while undergoing poorly understood morphology and phase structure transformations under reaction conditions that impact performance. Herein, in-situ liquid-phase transmission electron microscopy and select area diffraction measurements are applied to track the morphology and Pd/PdHx phase interconversion under reaction conditions as a function of electrode potential. These studies identify the degradation mechanisms, including poisoning and physical structure changes, occurring in PdHx/Pd electrodes. Constant potential density functional theory calculations are used to probe the reaction mechanisms occurring on the PdHx structures observed under reaction conditions. Microkinetic modeling reveals that the intercalation of *H into Pd is essential for formate production. However, the change in electrochemical CO2 conversion selectivity away from formate and towards CO/H2 at increasing overpotentials is due to electrode potential dependent changes in the reaction energetics and not a consequence of morphology or phase structure changes.

Comprehensive Insights and Advancements in Gel Catalysts for Electrochemical Energy Conversion

Continuous worldwide demands for more clean energy urge researchers and engineers to seek various energy applications, including electrocatalytic processes. Traditional energy-active materials, when combined with conducting materials and non-active polymeric materials, inadvertently leading to reduced interaction between their active and conducting components. This results in a drop in active catalytic sites, sluggish kinetics, and compromised mass and electronic transport properties. Furthermore, interaction between these materials could increase degradation products, impeding the efficiency of the catalytic process. Gels appears to be promising candidates to solve these challenges due to their larger specific surface area, three-dimensional hierarchical accommodative porous frameworks for active particles, self-catalytic properties, tunable electronic and electrochemical properties, as well as their inherent stability and cost-effectiveness. This review delves into the strategic design of catalytic gel materials, focusing on their potential in advanced energy conversion and storage technologies. Specific attention is given to catalytic gel material design strategies, exploring fundamental catalytic approaches for energy conversion processes such as the CO2 reduction reaction (CO2RR), oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and more. This comprehensive review not only addresses current developments but also outlines future research strategies and challenges in the field. Moreover, it provides guidance on overcoming these challenges, ensuring a holistic understanding of catalytic gel materials and their role in advancing energy conversion and storage technologies.

Efficient Cu2O Photocathodes for Aqueous Photoelectrochemical CO2 Reduction to Formate and Syngas

Photoelectrochemical carbon dioxide reduction (PEC-CO2R) represents a promising approach for producing renewable fuels and chemicals using solar energy. However, attaining even modest solar-to-fuel (STF) conversion efficiency often necessitates the use of costly semiconductors and noble-metal catalysts. Herein, we present a Cu2O/Ga2O3/TiO2 photocathode modified with Sn/SnOx catalysts through a simple photoelectrodeposition method. It achieves a remarkable half-cell STF efficiency of ∼0.31% for the CO2R in aqueous KHCO3 electrolyte, under AM 1.5 G illumination. The system enables efficient production of syngas (FE: ∼62%, CO/H2 ≈ 1:2) and formate (FE: ∼38%) with a consistent selectivity over a wide potential range, from +0.34 to -0.16 V vs the reversible hydrogen electrode. We ascribe the observed performance to the favorable optoelectronic characteristics of our Cu2O heterostructure and the efficient Sn/SnOx catalysts incorporated in the PEC-CO2R reactions. Through comprehensive experimental investigations, we elucidate the indispensable role of Cu2O buried p-n junctions in generating a high photovoltage (∼1 V) and enabling efficient bulk charge separation (up to ∼70% efficiency). Meanwhile, we discover that the deposited Sn/SnOx catalysts have critical dual effects on the overall performance of the PEC devices, serving as active CO2R catalysts as well as the semiconductor front contact. It could facilitate interfacial electron transfer between the catalysts and the semiconductor device for CO2R by establishing a barrier-free ohmic contact.

Tweaking Photo CO2 Reduction by Altering Lewis Acidic Sites in Metalated-Porous Organic Polymer for Adjustable H2 /CO Ratio in Syngas Production

Herein, we have specifically designed two metalated porous organic polymers (Zn-POP and Co-POP) for syngas (CO+H2 ) production from gaseous CO2 . The variable H2 /CO ratio of syngas with the highest efficiency was produced in water medium (without an organic hole scavenger and photosensitizer) by utilizing the basic principle of Lewis acid/base chemistry. Also, we observed the formation of entirely different major products during photocatalytic CO2 reduction and water splitting with the help of the two catalysts, where CO (145.65 μmol g-1  h-1 ) and H2 (434.7 μmol g-1  h-1 ) production were preferentially obtained over Co-POP & Zn-POP, respectively. The higher electron density/better Lewis basic nature of Co-POP was investigated further using XPS, XANES, and NH3 -TPD studies, which considerably improve CO2 activation capacity. Moreover, the structure-activity relationship was confirmed via in situ DRIFTS and DFT studies, which demonstrated the formation of COOH* intermediate along with the thermodynamic feasibility of CO2 reduction over Co-POP while water splitting occurred preferentially over Zn-POP.

Enrichment Strategies for Efficient CO2 Electroreduction in Acidic Electrolytes

Electrochemical CO2 reduction reaction (CO2 RR) has been recognized as an appealing route to remarkably accelerate the carbon-neutral cycle and reduce carbon emissions. Notwithstanding great catalytic activity that has been acquired in neutral and alkaline conditions, the carbonates generated from the inevitable reaction of the input CO2 with the hydroxide severely lower carbon utilization and energy efficiency. By contrast, CO2 RR in an acidic condition can effectively circumvent the carbonate issues; however, the activity and selectivity of CO2 RR in acidic electrolytes will be decreased significantly due to the competing hydrogen evolution reaction (HER). Enriching the CO2 and the key intermediates around the catalyst surface can promote the reaction rate and enhance the product selectivity, providing a promising way to boost the performance of CO2 RR. In this review, the catalytic mechanism and key technique challenges of CO2 RR are first introduced. Then, the critical progress of enrichment strategies for promoting the CO2 RR in the acidic electrolyte is summarized with three aspects: catalyst design, electrolyte regulation, and electrolyzer optimization. Finally, some insights and perspectives for further development of enrichment strategies in acidic CO2 RR are expounded.

SnS2/MWNTs/sponge electrode combined with plasma dielectric barrier discharge catalytic system: CO2 reduction and pollutant degradation

SnS2 nanosheets combined with multi-walled carbon nanotubes (MWNTs) were made into sponge electrodes which were used for CO2 reduction reaction (CO2RR) in dielectric barrier discharges (DBD) system. The amounts of formate and formaldehyde produced by CO2 reduction with SnS2/MWNTs/sponge electrode were 299.52 and 31.62 μmol h-1, which were higher than that of MWNTs/sponge electrodes. The addition of pollutants had different degrees of inhibitory effect on CO2 reduction, among which addition of bisphenol A (BPA) had the smallest effect that the degradation rate of BPA was 94.37% and the C1 products remained 204.43 μmol after 60 min discharge. The mechanism of CO2RR was studied by quencher experiment, and the main contribution order of the active substance in DBD system for CO2RR is: H+>e->·OH>·O2-. It was found that the degradation process of pollutants consumed H+ and e- in solution, thereby inhibiting CO2RR. Generally, the SnS2/MWNTs/sponge electrode provided a reference for the design of catalysts for CO2 reduction and pollutant degradation in plasma gas-liquid system.

Cation-Radius-Controlled Sn-O Bond Length Boosting CO2 Electroreduction over Sn-Based Perovskite Oxides

Despite the intriguing potential shown by Sn-based perovskite oxides in CO2 electroreduction (CO2 RR), the rational optimization of their CO2 RR properties is still lacking. Here we report an effective strategy to promote CO2 -to-HCOOH conversion of Sn-based perovskite oxides by A-site-radius-controlled Sn-O bond lengths. For the proof-of-concept examples of Ba1-x Srx SnO3 , as the A-site cation average radii decrease from 1.61 to 1.44 Å, their Sn-O bonds are precisely shortened from 2.06 to 2.02 Å. Our CO2 RR measurements show that the activity and selectivity of these samples for HCOOH production exhibit volcano-type trends with the Sn-O bond lengths. Among these samples, the Ba0.5 Sr0.5 SnO3 features the optimal activity (753.6 mA ⋅ cm-2 ) and selectivity (90.9 %) for HCOOH, better than those of the reported Sn-based oxides. Such optimized CO2 RR properties could be attributed to favorable merits conferred by the precisely controlled Sn-O bond lengths, e.g., the regulated band center, modulated adsorption/activation of intermediates, and reduced energy barrier for *OCHO formation. This work brings a new avenue for rational design of advanced Sn-based perovskite oxides toward CO2 RR.

Anion Exchange Facilitates the In Situ Construction of Bi/BiO Interfaces for Enhanced Electrochemical CO2 -to-Formate Conversion Over a Wide Potential Window

Electrochemical reduction of CO2 (CO2 RR) into value-added products is a promising strategy to reduce energy consumption and solve environmental issues. Formic acid/formate is one of the high-value, easy-to-collect, and economically viable products. Herein, the reconstructed Bi2 O2 CO3 nanosheets (BOCR NSs) are synthesized by an in situ electrochemical anion exchange strategy from Bi2 O2 SO4 as a pre-catalyst. The BOCR NSs achieve a high formate Faradaic efficiency (FEformate ) of 95.7% at -1.1 V versus reversible hydrogen electrode (vs. RHE), and maintain FEformate above 90% in a wide potential range from -0.8 to -1.5 V in H-cell. The in situ spectroscopic studies reveal that the obtained BOCR NSs undergo the anion exchange from Bi2 O2 SO4 to Bi2 O2 CO3 and further promote the self-reduction to metallic Bi to construct Bi/BiO active site to facilitate the formation of OCHO* intermediate. This result demonstrates anion exchange strategy can be used to rational design high performance of the catalysts toward CO2 RR.

Stannate-Based Materials as Anodes in Lithium-Ion and Sodium-Ion Batteries: A Review

Binary metal oxide stannate (M₂SnO₄; M = Zn, Mn, Co, etc.) structures, with their high theoretical capacity, superior lithium storage mechanism and suitable operating voltage, as well as their dual suitability for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs), are strong candidates for next-generation anode materials. However, the capacity deterioration caused by the severe volume expansion problem during the insertion/extraction of lithium or sodium ions during cycling of M₂SnO₄-based anode materials is difficult to avoid, which greatly affects their practical applications. Strategies often employed by researchers to address this problem include nanosizing the material size, designing suitable structures, doping with carbon materials and heteroatoms, metal-organic framework (MOF) derivation and constructing heterostructures. In this paper, the advantages and issues of M₂SnO₄-based materials are analyzed, and the strategies to solve the issues are discussed in order to promote the theoretical work and practical application of M₂SnO₄-based anode materials.

Recent Advances in Graphitic Carbon Nitride Based Electro-Catalysts for CO2 Reduction Reactions

The electrocatalytic carbon dioxide reduction reaction is an effective means of combating the greenhouse effect caused by massive carbon dioxide emissions. Carbon nitride in the graphitic phase (g-C₃N₄) has excellent chemical stability and unique structural properties that allow it to be widely used in energy and materials fields. However, due to its relatively low electrical conductivity, to date, little effort has been made to summarize the application of g-C₃N₄ in the electrocatalytic reduction of CO₂. This review focuses on the synthesis and functionalization of g-C₃N₄ and the recent advances of its application as a catalyst and a catalyst support in the electrocatalytic reduction of CO₂. The modification of g-C₃N₄-based catalysts for enhanced CO₂ reduction is critically reviewed. In addition, opportunities for future research on g-C₃N₄-based catalysts for electrocatalytic CO₂ reduction are discussed.

Kinetic Understanding of Catalytic Selectivity and Product Distribution of Electrochemical Carbon Dioxide Reduction Reaction

CO₂ can be electrochemically reduced to different products depending on the nature of catalysts. In this work, we report comprehensive kinetic studies on catalytic selectivity and product distribution of the CO₂ reduction reaction on various metal surfaces. The influences on reaction kinetics can be clearly analyzed from the variation of reaction driving force (binding energy difference) and reaction resistance (reorganization energy). Moreover, the CO₂RR product distributions are further affected by external factors such as electrode potential and solution pH. A potential-mediated mechanism is found to determine the competing two-electron reduction products of CO₂ that shifts from thermodynamics-controlled product formic acid at less negative electrode potentials to kinetic-controlled product CO at more negative electrode potentials. Based on detailed kinetic simulations, a three-parameter descriptor is applied to identify the catalytic selectivity of CO, formate, hydrocarbons/alcohols, as well as side product H₂. The present kinetic study not only well explains the catalytic selectivity and product distribution of experimental results but also provides a fast way for catalyst screening.

Operando Spectroscopic Analysis of Axial Oxygen-Coordinated Single-Sn-Atom Sites for Electrochemical CO2 Reduction

Sn-based materials have been demonstrated as promising catalysts for the selective electrochemical CO₂ reduction reaction (CO₂RR). However, the detailed structures of catalytic intermediates and the key surface species remain to be identified. In this work, a series of single-Sn-atom catalysts with well-defined structures is developed as model systems to explore their electrochemical reactivity toward CO₂RR. The selectivity and activity of CO₂ reduction to formic acid on Sn-single-atom sites are shown to be correlated with Sn(IV)-N₄ moieties axially coordinated with oxygen (O-Sn-N₄), reaching an optimal HCOOH Faradaic efficiency of 89.4% with a partial current density (j_HCOOH) of 74.8 mA·cm^-2 at -1.0 V vs reversible hydrogen electrode (RHE). Employing a combination of operando X-ray absorption spectroscopy, attenuated total reflectance surface-enhanced infrared absorption spectroscopy, Raman spectroscopy, and ¹¹⁹Sn Mössbauer spectroscopy, surface-bound bidentate tin carbonate species are captured during CO₂RR. Moreover, the electronic and coordination structures of the single-Sn-atom species under reaction conditions are determined. Density functional theory (DFT) calculations further support the preferred formation of Sn-O-CO₂ species over the O-Sn-N₄ sites, which effectively modulates the adsorption configuration of the reactive intermediates and lowers the energy barrier for the hydrogenation of *OCHO species, as compared to the preferred formation of *COOH species over the Sn-N₄ sites, thereby greatly facilitating CO₂-to-HCOOH conversion.

Cu-doped SnS2 nanosheets with superior visible-light photocatalytic CO2 reduction performance

Photocatalytic CO₂ reduction utilizing solar energy is a clean, environment-friendly strategy converting CO₂ into hydrocarbon fuels to solve the energy crisis and climate issues. Herein, we report the synthesis of Cu-doped SnS₂ nanosheets via a simple hydrothermal method. The prepared Cu-doped SnS₂ composite displays superior photocatalytic CO₂ reduction activity. The optimized CH₃OH yield of the composite is two times higher than that of pure SnS₂. The enhanced photocatalytic performance is attributed to effective charge separation resulting from the thinner nanosheets and delocalization of electrons from SnS₂ to Cu, high visible light utilization efficiency, enlarged S_BET, negative shift of the flat-band potential and reduced charge transfer resistance with the introduction of Cu atoms. This work suggests the potential application of Cu-doped SnS₂ in photocatalytic CO₂ reduction.

Heterogeneous Structure of Sn/SnO2 Constructed via Phase Engineering for Efficient and Stable CO2 Reduction

The electrochemical carbon dioxide reduction reaction (CO₂RR) catalyzed by Sn-based materials shows great potential for CO₂-to-formate conversion. The presence of tin species with different oxidation states can promote the catalytic performance, most likely due to the interfaces of metallic and oxide phases that induce a synergistic effect. Therefore, it is desirable yet challenging to synthesize a hybrid catalyst with abundant active heterogeneous interfaces. Herein, we synthesize a hybrid catalyst constructed by decorating nanosized SnS₂ in the SnO₂ matrix. The uniformly distributed SnS₂ nanoparticles are first reduced to metallic tin, which assists in the generation of abundant Sn/SnO₂ heterogeneous interfaces under the in situ reduction process. Because of the electronic modulation at the heterogeneous interfaces, the resulting catalyst delivers a high current density of 200 mA·cm^-2 at -0.86 V vs RHE, and the performance is stable for over 20 h. This work suggests a potentially powerful interface engineering strategy for the development of high-performance electrocatalysts for the CO₂RR.

Exfoliated 2D Layered and Nonlayered Metal Phosphorous Trichalcogenides Nanosheets as Promising Electrocatalysts for CO2 Reduction

Two-dimensional (2D) materials catalysts provide an atomic-scale view on a fascinating arena for understanding the mechanism of electrocatalytic carbon dioxide reduction (CO₂ ECR). Here, we successfully exfoliated both layered and nonlayered ultra-thin metal phosphorous trichalcogenides (MPCh₃ ) nanosheets via wet grinding exfoliation (WGE), and systematically investigated the mechanism of MPCh₃ as catalysts for CO₂ ECR. Unlike the layered CoPS₃ and NiPS₃ nanosheets, the active Sn atoms tend to be exposed on the surfaces of nonlayered SnPS₃ nanosheets. Correspondingly, the nonlayered SnPS₃ nanosheets exhibit clearly improved catalytic activity, showing formic acid selectivity up to 31.6 % with -7.51 mA cm^-2 at -0.65 V vs. RHE. The enhanced catalytic performance can be attributed to the formation of HCOO* via the first proton-electron pair addition on the SnPS₃ surface. These results provide a new avenue to understand the novel CO₂ ECR mechanism of Sn-based and MPCh₃ -based catalysts.

Tinware-Inspired Aerobic Surface-Initiated Controlled Radical Polymerization (SI-Sn0CRP) for Biocompatible Surface Engineering

Surface anchored polymer brushes prepared by surface-initiated controlled radical polymerization (SI-CRP) have raised considerable interest in biomaterials and bioengineering. However, undesired residues of noxious transition metal catalysts critically restrain their widespread biomedical applications. Herein, we present a robust and biocompatible surface-initiated controlled radical polymerization catalyzed by a Sn(0) sheet (SI-Sn⁰CRP) under ambient conditions. Through this approach, microliter volumes of vinyl monomers with diverse functions (heterocyclic, ionic, hydrophilic, and hydrophobic) could be efficiently converted to homogeneous polymer brushes. The excellent controllability of SI-Sn⁰CRP strategy is further demonstrated by the exquisite fabrication of predetermined block and patterned polymer brushes through chain extension and photolithography, respectively. Additionally, in virtue of intrinsic biocompatibility of Sn, the resultant polymer brushes present transcendent affinity toward blood and cell, in marked contrast to those of copper-based approaches. This strategy could provide an avenue for the controllable fabrication of biocompatible polymer brushes toward biological applications.

Nanostructure Engineering of Sn-Based Catalysts for Efficient Electrochemical CO2 Reduction

Excessive anthropogenic CO₂ emission has caused a series of ecological and environmental issues, which threatens mankind's sustainable development. Mimicking the natural photosynthesis process (i.e., artificial photosynthesis) by electrochemically converting CO₂ into value-added products is a promising way to alleviate CO₂ emission and relieve the dependence on fossil fuels. Recently, Sn-based catalysts have attracted increasing research attentions due to the merits of low price, abundance, non-toxicity, and environmental benignancy. In this review, the paradigm of nanostructure engineering for efficient electrochemical CO₂ reduction (ECO₂ R) on Sn-based catalysts is systematically summarized. First, the nanostructure engineering of size, composition, atomic structure, morphology, defect, surficial modification, catalyst/substrate interface, and single-atom structure, are systematically discussed. The influence of nanostructure engineering on the electronic structure and adsorption property of intermediates, as well as the performance of Sn-based catalysts for ECO₂ R are highlighted. Second, the potential chemical state changes and the role of surface hydroxides on Sn-based catalysts during ECO₂ R are introduced. Third, the challenges and opportunities of Sn-based catalysts for ECO₂ R are proposed. It is expected that this review inspires the further development of highly efficient Sn-based catalysts, meanwhile offer protocols for the investigation of Sn-based catalysts.

Near-Unity Electrochemical CO2 to CO Conversion over Sn-Doped Copper Oxide Nanoparticles

Bimetallic electrocatalysts have emerged as a viable strategy to tune the electrocatalytic CO₂ reduction reaction (eCO₂RR) for the selective production of valuable base chemicals and fuels. However, obtaining high product selectivity and catalyst stability remain challenging, which hinders the practical application of eCO₂RR. In this work, it was found that a small doping concentration of tin (Sn) in copper oxide (CuO) has profound influence on the catalytic performance, boosting the Faradaic efficiency (FE) up to 98% for carbon monoxide (CO) at -0.75 V versus RHE, with prolonged stable performance (FE > 90%) for up to 15 h. Through a combination of ex situ and in situ characterization techniques, the in situ activation and reaction mechanism of the electrocatalyst at work was elucidated. In situ Raman spectroscopy measurements revealed that the binding energy of the crucial adsorbed *CO intermediate was lowered through Sn doping, thereby favoring gaseous CO desorption. This observation was confirmed by density functional theory, which further indicated that hydrogen adsorption and subsequent hydrogen evolution were hampered on the Sn-doped electrocatalysts, resulting in boosted CO formation. It was found that the pristine electrocatalysts consisted of CuO nanoparticles decorated with SnO₂ domains, as characterized by ex situ high-resolution scanning transmission electron microscopy and X-ray photoelectron spectroscopy measurements. These pristine nanoparticles were subsequently in situ converted into a catalytically active bimetallic Sn-doped Cu phase. Our work sheds light on the intimate relationship between the bimetallic structure and catalytic behavior, resulting in stable and selective oxide-derived Sn-doped Cu electrocatalysts.

Hierarchical S-modified Cu porous nanoflakes for efficient CO2 electroreduction to formate

Electrochemical reduction of CO₂ into liquid fuels is a promising approach to achieving a carbon-neutral energy cycle but remains a great challenge due to the lack of efficient catalysts. Here, the hierarchical architectures assembled by ultrathin and porous S-modified Cu nanoflakes (Cu-S NFs) are designed and constructed as an efficient electrocatalyst for CO₂ conversion to formate with high partial current density. Specifically, when integrated into a gas diffusion electrode in a flow cell, Cu-S NFs are capable of delivering the ultrahigh formate current density up to 404.1 mA cm^-2 with a selectivity of 89.8%. Electrochemical tests and theoretical calculations indicate that the superior performance of the designed catalysts may be attributed to the unique structure, which can provide abundant active sites, fast charge transfer, and highly active edge sites.

Polyethyleneimine-reinforced Sn/Cu foam dendritic self-supporting catalytic cathode for CO2 reduction to HCOOH

In this work, a novel catalytic cathode of polyethyleneimine (PEI)-Sn/Cu foam with dendritic structure was prepared by electrodeposition and impregnation. It was used in the electrocatalytic reduction of CO₂ to HCOOH, and its performance in this process was evaluated. At -0.97 V vs. RHE, the faradaic efficiency and current density reached 92.3% and 57.1 mA cm^-2, respectively, in a 0.5 M KHCO₃ electrolyte. The HCOOH production rate reached 890.4 μmol h^-1 cm^-2, which exceeds those for most reported Sn catalysts. Density functional theory calculations showed that use of Sn/Cu foam is more conducive to HCOOH formation than use of Cu or Sn alone, and *OCHO is the main intermediate in HCOOH formation. The results of OH^- adsorption experiments confirmed that the introduction of PEI enhanced the catalytic capacity of the Sn/Cu foam, stabilized CO₂·^- intermediates, and promoted HCOOH generation. These results will provide an attractive strategy for developing efficient catalysts with excellent activities and stabilities for CO₂ electroreduction.

Boosting the Electrocatalytic CO2 Reduction Reaction by Nanostructured Metal Materials via Defects Engineering

Electrocatalytic CO₂ reduction reaction (CO₂RR) is one of the most effective methods to convert CO₂ into useful fuels. Introducing defects into metal nanostructures can effectively improve the catalytic activity and selectivity towards CO₂RR. This review provides the recent progress on the use of metal nanomaterials with defects towards electrochemical CO₂RR and defects engineering methods. Accompanying these ideas, we introduce the structure of defects characterized by electron microscopy techniques as the characterization and analysis of defects are relatively difficult. Subsequently, we present the intrinsic mechanism of how the defects affect CO₂RR performance. Finally, to promote a wide and deep study in this field, the perspectives and challenges concerning defects engineering in metal nanomaterials towards CO₂RR are put forward.

Engineering Aspects for the Design of a Bicarbonate Zero-Gap Flow Electrolyzer for the Conversion of CO2 to Formate

CO₂ electrolyzers require gaseous CO₂ or saturated CO₂ solutions to achieve high energy efficiency (EE) in flow reactors. However, CO₂ capture and delivery to electrolyzers are in most cases responsible for the inefficiency of the technology. Recently, bicarbonate zero-gap flow electrolyzers have proven to convert CO₂ directly from bicarbonate solutions, thus mimicking a CO₂ capture medium, obtaining high Faradaic efficiency (FE) and partial current density (CD) toward carbon products. However, since bicarbonate electrolyzers use a bipolar membrane (BPM) as a separator, the cell voltage (V_Cell) is high, and the system becomes less efficient compared to analogous CO₂ electrolyzers. Due to the role of the bicarbonate both as a carbon donor and proton donor (in contrast to gas-fed CO₂ electrolyzers), optimization by using know-how from conventional gas-fed CO₂ electrolyzers is not valid. In this study, we have investigated how different engineering aspects, widely studied for upscaling gas-fed CO₂ electrolyzers, influence the performance of bicarbonate zero-gap flow electrolyzers when converting CO₂ to formate. The temperature, flow rate, and concentration of the electrolyte are evaluated in terms of FE, productivity, V_Cell, and EE in a broad range of current densities (10-400 mA cm^-2). A CD of 50 mA cm^-2, room temperature, high flow rate (5 mL cm^-2) of the electrolyte, and high carbon load (KHCO₃ 3 M) are proposed as potentially optimal parameters to benchmark a design to achieve the highest EE (27% is obtained this way), one of the most important criteria when upscaling and evaluating carbon capture and conversion technologies. On the other hand, at high CD (>300 mA cm^-2), low flow rate (0.5 mL cm^-2) has the highest interest for downstream processing (>40 g L^-1 formate is obtained this way) at the cost of a low EE (<10%).

Electrochemical Reduction of CO2 With Good Efficiency on a Nanostructured Cu-Al Catalyst

Carbon monoxide (CO) and formic acid (HCOOH) are suggested to be the most convenient products from electrochemical reduction of CO2 according to techno-economic analysis. To date, tremendous advances have been achieved in the development of catalysts and processes, which make this research topic even more interesting to both academic and industrial sectors. In this work, we report nanostructured Cu-Al materials that are able to convert CO2 to CO and HCOOH with good efficiency. The catalysts are synthesized via a green microwave-assisted solvothermal route, and are composed of Cu2O crystals modified by Al. In KHCO3 electrolyte, these catalysts can selectively convert CO2 to HCOOH and syngas with H2/CO ratios between 1 and 2 approaching one unit faradaic efficiency in a wide potential range. Good current densities of 67 and 130 mA cm−2 are obtained at −1.0 V and −1.3 V vs. reversible hydrogen electrode (RHE), respectively. When switching the electrolyte to KOH, a significant selectivity up to 20% is observed for C2H4 formation, and the current densities achieve 146 and 222 mA cm−2 at −1.0 V and −1.3 V vs. RHE, respectively. Hence, the choice of electrolyte is critically important as that of catalyst in order to obtain targeted products at industrially relevant current densities.

Recent progress of Bi-based electrocatalysts for electrocatalytic CO2 reduction

To mitigate excessively accumulated carbon dioxide (CO₂) in the atmosphere and tackle the associated environmental concerns, green and effective approaches are necessary. The electrocatalytic CO₂ reduction reaction (CO₂RR) using sustainable electricity under benign reaction conditions represents a viable way to produce value-added and profitable chemicals. In this minireview, recent studies regarding unary Bi electrocatalysts and binary BiSn electrocatalysts are symmetrically categorized and reviewed, as they disclose high faradaic efficiencies toward the production of formate/formic acid, which has a relatively higher value of up to 0.50 $·per kg and has been widely used in the chemical and pharmaceutical industry. In particular, the preparation methodologies, electrocatalyst morphologies, catalytic performances and the corresponding mechanisms are comprehensively presented. The use of solid-state electrolytes showing high economic prospects for directly obtaining high-purity formic acid is highlighted. Finally, the remaining questions and challenges for CO₂RR exploitations using Bi-related electrocatalysts are proposed, while perspectives and the corresponding strategies aiming to enhance their entire catalytic functionalities and boost their performance are provided.

Zeolitic imidazolate framework-derived composites with SnO2 and ZnO phase components for electrocatalytic carbon dioxide reduction

Zeolitic imidazolate framework (ZIF) and its derivatives have attracted a great deal of attention in the field of electrocatalysis. In this paper, a series of tin (Sn)-modified ZIF-based composites (ZSO-X/Y) are synthesized and used as catalysts for the electrochemical reduction of CO2 to produce low-carbon fuels. Among the catalysts obtained, ZSO-2/8 shows the best formate (HCOO-) selectivity compared with others. A faradaic efficiency of 76.70% and a catalytic current density of -9.81 mA cm-2 can be respectively achieved at a potential of -1.16 V vs. reversible hydrogen electrode (VRHE). The high catalytic performance can be attributed to the stable coexistence of two-phase components of SnO2/ZnO inside the catalyst. This work provides an insight into the development of high performance ZIF-based catalysts for the electrochemical reduction of CO2.

Operando Converting BiOCl into Bi2O2(CO3)xCly for Efficient Electrocatalytic Reduction of Carbon Dioxide to Formate

Bismuth-based materials (e.g., metallic, oxides and subcarbonate) are emerged as promising electrocatalysts for converting CO₂ to formate. However, Bi^o-based electrocatalysts possess high overpotentials, while bismuth oxides and subcarbonate encounter stability issues. This work is designated to exemplify that the operando synthesis can be an effective means to enhance the stability of electrocatalysts under operando CO₂RR conditions. A synthetic approach is developed to electrochemically convert BiOCl into Cl-containing subcarbonate (Bi₂O₂(CO₃)_xCl_y) under operando CO₂RR conditions. The systematic operando spectroscopic studies depict that BiOCl is converted to Bi₂O₂(CO₃)_xCl_y via a cathodic potential-promoted anion-exchange process. The operando synthesized Bi₂O₂(CO₃)_xCl_y can tolerate - 1.0 V versus RHE, while for the wet-chemistry synthesized pure Bi₂O₂CO₃, the formation of metallic Bi^o occurs at - 0.6 V versus RHE. At - 0.8 V versus RHE, Bi₂O₂(CO₃)_xCl_y can readily attain a FE_HCOO- of 97.9%, much higher than that of the pure Bi₂O₂CO₃ (81.3%). DFT calculations indicate that differing from the pure Bi₂O₂CO₃-catalyzed CO₂RR, where formate is formed via a ^*OCHO intermediate step that requires a high energy input energy of 2.69 eV to proceed, the formation of HCOO^- over Bi₂O₂(CO₃)_xCl_y has proceeded via a ^*COOH intermediate step that only requires low energy input of 2.56 eV.

Bi2O3/BiO2 Nanoheterojunction for Highly Efficient Electrocatalytic CO2 Reduction to Formate

Heterostructure engineering plays a vital role in regulating the material interface, thus boosting the electron transportation pathway in advanced catalysis. Herein, a novel Bi₂O₃/BiO₂ heterojunction catalyst was synthesized via a molten alkali-assisted dealumination strategy and exhibited rich structural dynamics for an electrocatalytic CO₂ reduction reaction (ECO₂RR). By coupling in situ X-ray diffraction and Raman spectroscopy measurements, we found that the as-synthesized Bi₂O₃/BiO₂ heterostructure can be transformed into a novel Bi/BiO₂ Mott-Schottky heterostructure, leading to enhanced adsorption performance for CO₂ and *OCHO intermediates. Consequently, high selectivity toward formate larger than 95% was rendered in a wide potential window along with an optimum partial current density of -111.42 mA cm^-2 that benchmarked with the state-of-the-art Bi-based ECO₂RR catalysts. This work reports the construction and fruitful structural dynamic insights of a novel heterojunction electrocatalyst for ECO₂RR, which paves the way for the rational design of efficient heterojunction electrocatalysts for ECO₂RR and beyond.

Accelerating Pd Electrocatalysis for CO2-to-Formate Conversion across a Wide Potential Window by Optimized Incorporation of Cu

Electrochemical reduction of carbon dioxide (CO₂) to formate is a viable way to reduce CO₂ emissions and realize a carbon-neutral energy cycle. Although Pd can convert CO₂ to formate with a high Faradaic efficiency at minimal overpotentials, it suffers from a limited and narrow potential window. Alloying is an important strategy for the catalyst design and tuning the electronic structures. Here, we report a series of PdCu bimetallic alloy catalysts with tunable compositions based on dendritic architectures. Optimal introduction of Cu atoms into the Pd matrix facilitates formate production and suppresses CO generation. In 0.1 M KHCO₃ aqueous solution, our best candidate, Pd₈₂Cu₁₈ catalyst, delivered a high formate Faradaic efficiency of 96.0% at -0.3 V versus RHE. More interestingly, the high selectivity (>90%) toward formate maintained an enlarged electrochemical potential window of 600 mV. The ensemble effect with electronic coupling between Pd and Cu upon alloying and its induced moderate surface O-containing configuration were found to enhance the formate formation and suppress CO poisoning during CO₂ reduction.

P-block atom modified Sn(200) surface as a promising electrocatalyst for two-electron CO2 reduction: a first-principles study

The P-block atom can effectively regulate the activity of two-electron CO2 reduction on Sn(200) surface.

Carbon dioxide adsorption and activation on ionic liquid decorated Au(111) surface: A DFT study

We use first principle approaches to study the adsorption and catalytic activation mechanism of CO₂ on ionic liquids (ILs, [C_nMIm]⁺[Cl]^- (n = 0-6)) attached to a Au(111) surface. The adsorption of CO₂ at this liquid-solid model interface occurs via either (i) parallel π-stacking mode or (ii) CO₂ oxygen lone pair (lp)···π interaction. These CO₂ physisorption modes, which depend on the CO₂ landing angle at this interface, are identified as an efficient way to activate CO₂ and its further conversion into value-added products. For illustration, we discuss the conversion of CO₂ into formic acid where the ILs@Au(111) decorated interface allows reduction of the activation energy for the CO₂ + H₂ → HCOOH reaction. In sum, our electrode/electrolyte based interface model provides valuable information to design novel heterogeneous catalysts for CO₂ conversion. Indeed, our work establishes that a suitable interface material is enough to activate CO₂.

Regulating the Electron Localization of Metallic Bismuth for Boosting CO2 Electroreduction

Electrochemical reduction of CO₂ to formate is economically attractive but improving the reaction selectivity and activity remains challenging. Herein, we introduce boron (B) atoms to modify the local electronic structure of bismuth with positive valence sites for boosting conversion of CO₂ into formate with high activity and selectivity in a wide potential window. By combining experimental and computational investigations, our study indicates that B dopant differentiates the proton participations of rate-determining steps in CO₂ reduction and in the competing hydrogen evolution. By comparing the experimental observations with the density functional theory, the dominant mechanistic pathway of B promoted formate generation and the B concentration modulated effects on the catalytic property of Bi are unravelled. This comprehensive study offers deep mechanistic insights into the reaction pathway at an atomic and molecular level and provides an effective strategy for the rational design of highly active and selective electrocatalysts for efficient CO₂ conversion.

Transition Metal-Modified Co4 Clusters Supported on Graphdiyne as an Effective Nitrogen Reduction Reaction Electrocatalyst

Conversion of N₂ into NH₃ through the electrochemical nitrogen reduction reaction (NRR) under ambient conditions represents a novel green ammonia synthesis method. The main obstacle for NRR is lack of efficient, stable, and cost-effective catalysts. In this work, by using density functional theory calculations, 16 transition metal-modified Co₄ clusters supported on graphdiyne (GDY) as potential NRR catalysts were systematically screened. Through the examinations of stability, N₂ activation, selectivity, and activity, Ti-, V-, Cr-, Mn-, and Zr-Co₃@GDY were identified as the promising candidates toward NRR. Further explorations on the NRR mechanisms and the Pourbaix diagrams suggest that Ti-Co₃@GDY was the most promising candidate catalyst, as it has the lowest limiting potential and high stability under the working conditions. The high activities originate from the synergy effect, where the Co₃ cluster acts as the electron donor and the heteroatom serves as the single active site throughout the NRR process. Our results offer a new perspective for advancing sustainable NH₃ production.

Effects of Benzyl-Functionalized Cationic Surfactants on the Inhibition of the Hydrogen Evolution Reaction in CO2 Reduction Systems

Cationic surfactants, mainly hexadecyl cetrimonium bromide (CTAB), are widely used in electrocatalysis to affect the selectivity of the reaction, specifically to inhibit the hydrogen evolution reaction (HER) in CO₂ reduction (CO₂R) systems. However, little research has been done on the modification of the functional groups present in such surfactants in order to promote this HER-inhibiting effect. In this work, the effectiveness of CTAB was promoted by substituting a methyl group of the quaternary amine for a benzyl group. This cationic surfactant, cetalkonium chloride (CKC), increased the hydrophobicity of the surface of the electrode, promoting the HER inhibition and the CO₂R when HCO₃^- is used as a carbon source, which allows combining capture and conversion in one and the same medium, making it industrially highly attractive. By performing a detailed electrochemical characterization, we proved that the benzyl group formed an enhanced hydrophobic layer on the surface of the electrode in addition to the alkyl chain of the surfactant, showing higher effectiveness compared to CTAB. In fact, the Faradaic efficiency of the CO₂R increased from 39 to 66% in saturated HCO₃^- electrolytes by using CKC instead of CTAB as the HER inhibitor. This opens up a wide range of avenues for research on the application of surfactants in the field of electrocatalysis, because, as proven, a selective modification of it can tune the selectivity of the reaction, adding a new variable in the design of an efficient carbon capture and utilization system.

Recent Advances in Interface Engineering for Electrocatalytic CO2 Reduction Reaction

Electrocatalytic CO₂ reduction reaction (CO₂RR) can store and transform the intermittent renewable energy in the form of chemical energy for industrial production of chemicals and fuels, which can dramatically reduce CO₂ emission and contribute to carbon-neutral cycle. Efficient electrocatalytic reduction of chemically inert CO₂ is challenging from thermodynamic and kinetic points of view. Therefore, low-cost, highly efficient, and readily available electrocatalysts have been the focus for promoting the conversion of CO₂. Very recently, interface engineering has been considered as a highly effective strategy to modulate the electrocatalytic performance through electronic and/or structural modulation, regulations of electron/proton/mass/intermediates, and the control of local reactant concentration, thereby achieving desirable reaction pathway, inhibiting competing hydrogen generation, breaking binding-energy scaling relations of intermediates, and promoting CO₂ mass transfer. In this review, we aim to provide a comprehensive overview of current developments in interface engineering for CO₂RR from both a theoretical and experimental standpoint, involving interfaces between metal and metal, metal and metal oxide, metal and nonmetal, metal oxide and metal oxide, organic molecules and inorganic materials, electrode and electrolyte, molecular catalysts and electrode, etc. Finally, the opportunities and challenges of interface engineering for CO₂RR are proposed.