Organic molecules involved in Cu-based electrocatalysts for selective CO2 reduction to C2+ products

Electrocatalytic carbon dioxide (CO₂) reduction reaction (CO₂RR) is a promising process to mitigate the environmental issues caused by CO₂, as well as to produce valuable multicarbon (C₂₊) products. Significant progresses have been made to explore highly efficient Cu-based electrocatalysts for CO₂RR in recent years. Adding organic molecules into electrocatalytic systems can tune the CO₂ interaction with the electrocatalysts for CO₂RR, therefore, the final C₂₊ products, which are not solely achieved by inorganic modification. In this review, we will summarize the recent progress of the organic molecules participation in CO₂ electroreduction to C₂₊ products on Cu-based electrocatalysts. The applied organic molecules are reviewed based on the heteroatoms (N and S), with the emphasis on their roles in activity and selectivity toward C₂₊ products. A perspective on the application of organic molecules for efficient and selective CO₂RR has been provided.     

Electrochemical CO2 reduction reaction on cost-effective oxide-derived copper and transition metal–nitrogen–carbon catalysts

The electrochemical CO₂ reduction reaction (CO₂RR) either to generate multicarbon (C₂₊) or single carbon (C₁) value-added products provides an effective and promising approach to mitigate the high CO₂ concentration in the atmosphere and promote energy storage. However, cost-effectiveness of catalytic materials limits practical application of this technology in the short term. Herein, we summarize and discuss recent and advanced works on cost-effective oxide-derived copper catalysts for the generation of C₂₊ products (hydrocarbons and alcohols) and transition metal–nitrogen–doped carbon electrocatalytic materials for C₁ compounds production from CO₂RR. We think they represent suitable electrocatalyst candidates for scaling up electrochemical CO₂ conversion. This short review may provide inspiration for the future design and development of innovative active, cost-effective, selective and stable electrocatalysts with improved properties for either the production of C₂₊ (alcohols, hydrocarbons) or carbon monoxide from CO₂RR.     

Dynamic Changes in the Structure,Chemical State and Catalytic Selectivity of Cu Nanocubes during CO2 Electroreduction: Size and Support Effects

In situ and operando spectroscopic and microscopic methods were used to gain insight into the correlation between the structure, chemical state, and reactivity of size‐ and shape‐controlled ligand‐free Cu nanocubes during CO₂ electroreduction (CO₂RR). Dynamic changes in the morphology and composition of Cu cubes supported on carbon were monitored under potential control through electrochemical atomic force microscopy, X‐ray absorption fine‐structure spectroscopy and X‐ray photoelectron spectroscopy. Under reaction conditions, the roughening of the nanocube surface, disappearance of the (100) facets, formation of pores, loss of Cu and reduction of CuO_x species observed were found to lead to a suppression of the selectivity for multi‐carbon products (i.e. C₂H₄ and ethanol) versus CH₄. A comparison with Cu cubes supported on Cu foils revealed an enhanced morphological stability and persistence of Cu^I species under CO₂RR in the former samples. Both factors are held responsible for the higher C₂/C₁ product ratio observed for the Cu cubes/Cu as compared to Cu cubes/C. Our findings highlight the importance of the structure of the active nanocatalyst but also its interaction with the underlying substrate in CO₂RR selectivity.     

Dynamic Changes in the Structure,Chemical State and Catalytic Selectivity of Cu Nanocubes during CO2 Electroreduction: Size and Support Effects

In situ and operando spectroscopic and microscopic methods were used to gain insight into the correlation between the structure, chemical state, and reactivity of size‐ and shape‐controlled ligand‐free Cu nanocubes during CO₂ electroreduction (CO₂RR). Dynamic changes in the morphology and composition of Cu cubes supported on carbon were monitored under potential control through electrochemical atomic force microscopy, X‐ray absorption fine‐structure spectroscopy and X‐ray photoelectron spectroscopy. Under reaction conditions, the roughening of the nanocube surface, disappearance of the (100) facets, formation of pores, loss of Cu and reduction of CuO_x species observed were found to lead to a suppression of the selectivity for multi‐carbon products (i.e. C₂H₄ and ethanol) versus CH₄. A comparison with Cu cubes supported on Cu foils revealed an enhanced morphological stability and persistence of Cu^I species under CO₂RR in the former samples. Both factors are held responsible for the higher C₂/C₁ product ratio observed for the Cu cubes/Cu as compared to Cu cubes/C. Our findings highlight the importance of the structure of the active nanocatalyst but also its interaction with the underlying substrate in CO₂RR selectivity.     

Tailoring Copper Nanocrystals towards C2 Products in Electrochemical CO2 Reduction

Favoring the CO₂ reduction reaction (CO2RR) over the hydrogen evolution reaction and controlling the selectivity towards multicarbon products are currently major scientific challenges in sustainable energy research. It is known that the morphology of the catalyst can modulate catalytic activity and selectivity, yet this remains a relatively underexplored area in electrochemical CO₂ reduction. Here, we exploit the material tunability afforded by colloidal chemistry to establish unambiguous structure/property relations between Cu nanocrystals and their behavior as electrocatalysts for CO₂ reduction. Our study reveals a non‐monotonic size‐dependence of the selectivity in cube‐shaped copper nanocrystals. Among 24 nm, 44 nm and 63 nm cubes tested, the cubes with 44 nm edge length exhibited the highest selectivity towards CO2RR (80 %) and faradaic efficiency for ethylene (41 %). Statistical analysis of the surface atom density suggests the key role played by edge sites in CO2RR.     

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

Electrocatalytic CO₂ reduction reaction (CO₂RR) to multi-carbon products (C₂₊) in acidic electrolyte is one of the most advanced routes for tackling our current climate and energy crisis. However, the competing hydrogen evolution reaction (HER) and the poor selectivity towards the valuable C₂₊ products are the major obstacles for the upscaling of these technologies. High local potassium ions (K⁺) concentration at the cathode's surface can inhibit proton-diffusion and accelerate the desirable carbon-carbon (C−C) coupling process. However, the solubility limit of potassium salts in bulk solution constrains the maximum achievable K⁺ concentration at the reaction sites and thus the overall acidic CO₂RR performance of most electrocatalysts. In this work, we demonstrate that Cu nanoneedles induce ultrahigh local K⁺ concentrations (4.22 M) – thus breaking the K⁺ solubility limit (3.5 M) – which enables a highly efficient CO₂RR in 3 M KCl at pH=1. As a result, a Faradaic efficiency of 90.69±2.15 % for C₂₊ (FE_C2+) can be achieved at 1400 mA.cm⁻², simultaneous with a single pass carbon efficiency (SPCE) of 25.49±0.82 % at a CO₂ flow rate of 7 sccm.     

Selective CO2 Reduction to Ethylene Mediated by Adaptive Small-molecule Engineering of Copper-based Electrocatalysts

Electrochemical CO₂ reduction reaction (CO₂RR) over Cu catalysts exhibits enormous potential for efficiently converting CO₂ to ethylene (C₂H₄). However, achieving high C₂H₄ selectivity remains a considerable challenge due to the propensity of Cu catalysts to undergo structural reconstruction during CO₂RR. Herein, we report an in situ molecule modification strategy that involves tannic acid (TA) molecules adaptive regulating the reconstruction of a Cu-based material to a pathway that facilitates CO₂ reduction to C₂H₄ products. An excellent Faraday efficiency (FE) of 63.6 % on C₂H₄ with a current density of 497.2 mA cm⁻² in flow cell was achieved, about 6.5 times higher than the pristine Cu catalyst which mainly produce CH₄. The in situ X-ray absorption spectroscopy and Raman studies reveal that the hydroxyl group in TA stabilizes Cu^δ+ during the CO₂RR. Furthermore, theoretical calculations demonstrate that the Cu^δ+/Cu⁰ interfaces lower the activation energy barrier for *CO dimerization, and hydroxyl species stabilize the *COH intermediate via hydrogen bonding, thereby promoting C₂H₄ production. Such molecule engineering modulated electronic structure provides a promising strategy to achieve highly selective CO₂ reduction to value-added chemicals.     

Recent advances in carbon-based materials for electrochemical CO2 reduction reaction

The increase of atmospheric CO₂ concentration has caused many environmental issues. Electrochemical CO₂ reduction reaction（CO₂RR） has been considered as a promising strategy to mitigate these challenges. The electrocatalysts with a low overpotential, high Faradaic efficiency, and excellent selectivity are of great significance for the CO₂RR. Carbon-based materials including metal-free carbon catalysts and metal-based carbon catalysts have shown great p...     

The Functionality of Surface Hydroxy Groups on the Selectivity and Activity of Carbon Dioxide Reduction over Cuprous Oxide in Aqueous Solutions

Carbon dioxide (CO₂) reduction in aqueous solutions is an attractive strategy for carbon capture and utilization. Cuprous oxide (Cu₂O) is a promising catalyst for CO₂ reduction as it can convert CO₂ into valuable hydrocarbons and suppress the side hydrogen evolution reaction (HER). However, the nature of the active sites in Cu₂O remains under debate because of the complex surface structure of Cu₂O under reducing conditions, leading to limited guidance in designing improved Cu₂O catalysts. This paper describes the functionality of surface‐bonded hydroxy groups on partially reduced Cu₂O(111) for the CO₂ reduction reaction (CO₂RR) by combined density functional theory (DFT) calculations and experimental studies. We find that the surface hydroxy groups play a crucial role in the CO₂RR and HER, and a moderate coverage of hydroxy groups is optimal for promotion of the CO₂RR and suppression of the HER simultaneously. Electronic structure analysis indicates that the charge transfer from hydroxy groups to coordination‐unsaturated Cu (Cu_CUS) sites stabilizes surface‐adsorbed COOH*, which is a key intermediate during the CO₂RR. Moreover, the CO₂RR was evaluated over Cu₂O octahedral catalysts with {111} facets and different surface coverages of hydroxy groups, which demonstrates that Cu₂O octahedra with moderate coverage of hydroxy groups can indeed enhance the CO₂RR and suppress the HER.     

From Biomimicking to Bioinspired Design of Electrocatalysts for CO2 Reduction to C1 Products

The electrochemical reduction of CO₂ (CO₂RR) is a promising approach to maintain a carbon cycle balance and produce value-added chemicals. However, CO₂RR technology is far from mature, since the conventional CO₂RR electrocatalysts suffer from low activity (leading to currents <10 mA cm⁻² in an H-cell), stability (<120 h), and selectivity. Hence, they cannot meet the requirements for commercial applications (>200 mA cm⁻², >8000 h, >90 % selectivity). Significant improvements are possible by taking inspiration from nature, considering biological organisms that efficiently catalyze the CO₂ to various products. In this minireview, we present recent examples of enzyme-inspired and enzyme-mimicking CO₂RR electrocatalysts enabling the production of C₁ products with high faradaic efficiency (FE). At present, these designs do not typically follow a methodical approach, but rather focus on isolated features of biological systems. To achieve disruptive change, we advocate a systematic design methodology that leverages fundamental mechanisms associated with desired properties in nature and adapts them to the context of engineering applications.     

Molecular Evidence for Metallic Cobalt Boosting CO2 Electroreduction on Pyridinic Nitrogen

Nitrogen‐doped carbon materials (N‐C_mat) are emerging as low‐cost metal‐free electrocatalysts for the electrochemical CO₂ reduction reaction (CO₂RR), although the activities are still unsatisfactory and the genuine active site is still under debate. We demonstrate that the CO₂RR to CO preferentially takes place on pyridinic N rather than pyrrolic N using phthalocyanine (Pc) and porphyrin with well‐defined N‐C_mat configurations as molecular model catalysts. Systematic experiments and theoretic calculations further reveal that the CO₂RR performance on pyridinic N can be significantly boosted by electronic modulation from in‐situ‐generated metallic Co nanoparticles. By introducing Co nanoparticles, Co@Pc/C can achieve a Faradaic efficiency of 84 % and CO current density of 28 mA cm^?2 at ?0.9 V, which are 18 and 47 times higher than Pc/C without Co, respectively. These findings provide new insights into the CO₂RR on N‐C_mat, which may guide the exploration of cost‐effective electrocatalysts for efficient CO₂ reduction.     

Lattice Compression Revealed at the ≈1 nm Scale

Lattice tuning at the ≈1 nm scale is fascinating and challenging; for instance, lattice compression at such a minuscule scale has not been observed. The lattice compression might also bring about some unusual properties, which waits to be verified. Through ligand induction, we herein achieve the lattice compression in a ≈1 nm gold nanocluster for the first time, as detected by the single-crystal X-ray crystallography. In a freshly synthesized Au₅₂(CHT)₂₈ (CHT=S-c−C₆H₁₁) nanocluster, the lattice distance of the (110) facet is found to be compressed from 4.51 to 3.58 Å at the near end. However, the lattice distances of the (111) and (100) facets show no change in different positions. The lattice-compressed nanocluster exhibits superior electrocatalytic activity for the CO₂ reduction reaction (CO₂RR) compared to that exhibited by the same-sized Au₅₂(TBBT)₃₂ (TBBT=4-tert-butyl-benzenethiolate) nanocluster and larger Au nanocrystals without lattice variation, indicating that lattice tuning is an efficient method for tailoring the properties of metal nanoclusters. Further theoretical calculations explain the high CO₂RR performance of the lattice-compressed Au₅₂(CHT)₂₈ and provide a correlation between its structure and catalytic activity.     

Sulfur Changes the Electrochemical CO2 Reduction Pathway over Cu Electrocatalysts

Electrochemical CO₂ reduction to value-added chemicals or fuels offers a promising approach to reduce carbon emissions and alleviate energy shortage. Cu-based electrocatalysts have been widely reported as capable of reducing CO₂ to produce a variety of multicarbon products (e.g., ethylene and ethanol). In this work, we develop sulfur-doped Cu₂O electrocatalysts, which instead can electrochemically reduce CO₂ to almost exclusively formate. We show that a dynamic equilibrium of S exists at the Cu₂O-electrolyte interface, and S-doped Cu₂O undergoes in situ surface reconstruction to generate active S-adsorbed metallic Cu sites during the CO₂ reduction reaction (CO₂RR). Density functional theory (DFT) calculations together with in situ infrared absorption spectroscopy measurements show that the S-adsorbed metallic Cu surface can not only promote the formation of the *OCHO intermediate but also greatly suppress *H and *COOH adsorption, thus facilitating CO₂-to-formate conversion during the electrochemical CO₂RR.     

Metal‐Doped Nitrogenated Carbon as an Efficient Catalyst for Direct CO2 Electroreduction to CO and Hydrocarbons

This study explores the kinetics, mechanism, and active sites of the CO₂ electroreduction reaction (CO2RR) to syngas and hydrocarbons on a class of functionalized solid carbon‐based catalysts. Commercial carbon blacks were functionalized with nitrogen and Fe and/or Mn ions using pyrolysis and acid leaching. The resulting solid powder catalysts were found to be active and highly CO selective electrocatalysts in the electroreduction of CO₂ to CO/H₂ mixtures outperforming a low‐area polycrystalline gold benchmark. Unspecific with respect to the nature of the metal, CO production is believed to occur on nitrogen functionalities in competition with hydrogen evolution. Evidence is provided that sufficiently strong interaction between CO and the metal enables the protonation of CO and the formation of hydrocarbons. Our results highlight a promising new class of low‐cost, abundant electrocatalysts for synthetic fuel production from CO₂.     

Electrochemical CO2 Activation and Valorization on Metallic Copper and Carbon-Embedded N-Coordinated Single Metal MNC Catalysts

The electrochemical reductive valorization of CO₂, referred to as the CO2RR, is an emerging approach for the conversion of CO₂-containing feeds into valuable carbonaceous fuels and chemicals, with potential contributions to carbon capture and use (CCU) for reducing greenhouse gas emissions. Copper surfaces and graphene-embedded, N-coordinated single metal atom (MNC) catalysts exhibit distinctive reactivity, attracting attention as efficient electrocatalysts for CO2RR. This review offers a comparative analysis of CO2RR on copper surfaces and MNC catalysts, highlighting their unique characteristics in terms of CO₂ activation, C₁/C₂₍₊₎ product formation, and the competing hydrogen evolution pathway. The assessment underscores the significance of understanding structure–activity relationships to optimize catalyst design for efficient and selective CO2RR. Examining detailed reaction mechanisms and structure-selectivity patterns, the analysis explores recent insights into changes in the chemical catalyst states, atomic motif rearrangements, and fractal agglomeration, providing essential kinetic information from advanced in/ex situ microscopy/spectroscopy techniques. At the end, this review addresses future challenges and solutions related to today's disconnect between our current molecular understanding of structure–activity-selectivity relations in CO2RR and the relevant factors controlling the performance of CO₂ electrolyzers over longer times, with larger electrode sizes, and at higher current densities.     

Catalytic Reduction of CN−, CO,and CO2 by Nitrogenase Cofactors in Lanthanide‐Driven Reactions

Nitrogenase cofactors can be extracted into an organic solvent to catalyze the reduction of cyanide (CN⁻), carbon monoxide (CO), and carbon dioxide (CO₂) without using adenosine triphosphate (ATP), when samarium(II) iodide (SmI₂) and 2,6‐lutidinium triflate (Lut‐H) are employed as a reductant and a proton source, respectively. Driven by SmI₂, the cofactors catalytically reduce CN⁻ or CO to C₁–C₄ hydrocarbons, and CO₂ to CO and C₁–C₃ hydrocarbons. The C C coupling from CO₂ indicates a unique Fischer–Tropsch‐like reaction with an atypical carbonaceous substrate, whereas the catalytic turnover of CN⁻, CO, and CO₂ by isolated cofactors suggests the possibility to develop nitrogenase‐based electrocatalysts for the production of hydrocarbons from these carbon‐containing compounds.     

Catalytic Reduction of CN−, CO,and CO2 by Nitrogenase Cofactors in Lanthanide‐Driven Reactions

Nitrogenase cofactors can be extracted into an organic solvent to catalyze the reduction of cyanide (CN^?), carbon monoxide (CO), and carbon dioxide (CO₂) without using adenosine triphosphate (ATP), when samarium(II) iodide (SmI₂) and 2,6‐lutidinium triflate (Lut‐H) are employed as a reductant and a proton source, respectively. Driven by SmI₂, the cofactors catalytically reduce CN^? or CO to C₁–C₄ hydrocarbons, and CO₂ to CO and C₁–C₃ hydrocarbons. The C? C coupling from CO₂ indicates a unique Fischer–Tropsch‐like reaction with an atypical carbonaceous substrate, whereas the catalytic turnover of CN^?, CO, and CO₂ by isolated cofactors suggests the possibility to develop nitrogenase‐based electrocatalysts for the production of hydrocarbons from these carbon‐containing compounds.     

Self-assembly of single metal sites embedded covalent organic frameworks into multi-dimensional nanostructures for efficient CO2 electroreduction

Morphology-controlled electrocatalysts with the ability of CO₂adsorption/activation, mass transfer, high stability and porosity are much desired in electrochemical CO₂reduction reaction（CO₂RR）. Here, three kinds of multi-dimensional nanostructures（i.e., hollow sphere, nanosheets and nanofibers） have been successfully produced through the modulation of porphyrin-based covalent organic frameworks（COFs）with various modulators. The obtained nanostructures with high-s...     

Electrochemical Conversion of CO2 to Syngas with Controllable CO/H2 Ratios over Co and Ni Single‐Atom Catalysts

The electrochemical CO₂ reduction reaction (CO₂RR) to yield synthesis gas (syngas, CO and H₂) has been considered as a promising method to realize the net reduction in CO₂ emission. However, it is challenging to balance the CO₂RR activity and the CO/H₂ ratio. To address this issue, nitrogen‐doped carbon supported single‐atom catalysts are designed as electrocatalysts to produce syngas from CO₂RR. While Co and Ni single‐atom catalysts are selective in producing H₂ and CO, respectively, electrocatalysts containing both Co and Ni show a high syngas evolution (total current >74 mA cm^?2) with CO/H₂ ratios (0.23–2.26) that are suitable for typical downstream thermochemical reactions. Density functional theory calculations provide insights into the key intermediates on Co and Ni single‐atom configurations for the H₂ and CO evolution. The results present a useful case on how non‐precious transition metal species can maintain high CO₂RR activity with tunable CO/H₂ ratios.     

Electrochemical Conversion of CO2 to Syngas with Controllable CO/H2 Ratios over Co and Ni Single-Atom Catalysts

The electrochemical CO₂ reduction reaction (CO₂RR) to yield synthesis gas (syngas, CO and H₂) has been considered as a promising method to realize the net reduction in CO₂ emission. However, it is challenging to balance the CO₂RR activity and the CO/H₂ ratio. To address this issue, nitrogen-doped carbon supported single-atom catalysts are designed as electrocatalysts to produce syngas from CO₂RR. While Co and Ni single-atom catalysts are selective in producing H₂ and CO, respectively, electrocatalysts containing both Co and Ni show a high syngas evolution (total current >74 mA cm⁻²) with CO/H₂ ratios (0.23–2.26) that are suitable for typical downstream thermochemical reactions. Density functional theory calculations provide insights into the key intermediates on Co and Ni single-atom configurations for the H₂ and CO evolution. The results present a useful case on how non-precious transition metal species can maintain high CO₂RR activity with tunable CO/H₂ ratios.