Electrocatalytic CO2 Reduction Research Articles

Design of reaction-driven active configuration for enhanced CO2 electroreduction

Metal-nitrogen-carbon single-atom catalysts (SACs) have emerged as promising candidates for electrocatalytic CO2 reduction reaction. However, the perpendicular dz2 orbital within planar metal site mainly interacts with *COOH, resulting in inferior CO2 activation. Inspired by reaction-driven active configuration, here we propose to upshift nickel single-atom away from nitrogen-carbon substrate, prominently promoting the interaction between CO2 and other d orbitals besides dz2. Theoretical and experimental analyses reveal that upshifting nickel site away substrate induces dxz, dyz, and dz2 to hybridize with CO2, expediting CO2 conversion to *COOH. The planar and out-of-plane Ni-N sites are formed on carbon nanosheet (Ni1-N/CNS) and curved nanoparticle (Ni1-N/CNP), respectively, which is verified by X-ray absorption fine structure spectroscopy. Impressively, the Ni1-N/CNP presents CO Faradaic efficiency of 96.4 % at 500 mA cm−2 and energy conversion efficiency of 79.8 % in flow cell, outperforming Ni1-N/CNS and most SACs. This work highlights the simulation of reaction-driven active sites for efficient electrocatalysis.

Electrocatalytic reduction of CO2 to C2H4 by monometallic Cu4 cluster supported on CeO2(110) surface

Cu-based catalysts have attracted much attention in the field of electrocatalytic CO2 reduction reactions (CRR). In our current work, the electrocatalytic reduction of CO2 by Cu cluster supported on CeO2 is calculated according to the density functional theory (DFT), and some interesting results are obtained. Monometallic copper cluster electrocatalysis of CO2 produce C2 products at lower potentials. The overpotential for the generation of ethylene is much lower than those of other products. The modulation of the substrate CeO2 changed the electronic structure of the Cu clusters and constructed Cu0 and Cu+1 active sites on the catalyst surface, and the coordinated action of Cu0 and Cu+1 made the C–C coupling more likely to take place and facilitated the generation of C2 products. The results of this study help to further understand the role of Cu-based catalysts in the electrocatalytic reduction of CO2 from theoretical point of view and provide guidance for future electrocatalytic experiments based on CRR.

Efficient tuning of the selectivity of Cu-based interface for electrocatalytic CO2 reduction by ligand modification

The development of efficient strategies to tune the CO2RR selectivity of Cu-based catalytic interfaces, especially on specific domains, such as Cu (200) facets with high activity toward competitive hydrogenation evolution reaction (HER), remains a challenging task. In this work, Cu-based catalytic layers with thiocyanate (-SCN), cyanide (-CN), or ethylenediamine (-NH2R) coordination linkages are prepared on Cu nanocolumns arrays (Cu NCAs) with predominant (200) exposed facets. The coordination of these ligands induces more Cu+ species and inhibits the adsorption of H∗ on the Cu (200) facet, leading to enhanced CO2RR performance and substantially suppressing the competitive HER. The faradaic efficiency (FE) of Cu–SCN, Cu–CN, and Cu–NH2R NWAs for producing HCOOH, C2H4, and C1 mixture products (HCOOH and CO) reach to 66.5%, 21.1%, and 57.1%, respectively. In situ spectroscopic studies reveal Cu–SCN, Cu–CN, and Cu–NH2R exhibit more reasonable adsorption energy toward ∗OCHO, ∗CO, and ∗COOH intermediates, promoting the HCOOH, C2H4, and C1 mixture generation, respectively. This study might provide a new perspective for the development of high-performance Cu-based CO2RR catalytic electrodes based on the combination of various commercial free-standing Cu substrates and organic/inorganic ligands.

Metal (Ca, Cu, Zn)-modified defect-rich Bi nanosheets for electrocatalytic reduction of CO2 to HCOOH at high current density

Bismuth-based materials have emerged as promising electrocatalysts for the electrochemical reduction of carbon dioxide (eCO2RR) to HCOOH. However, it remains challenges to meet the practical application requirements and identify the active sites of bismuth-based catalysts in eCO2RR. Herein, bismuthate (MBi2O4, M = Ca, Cu, Zn) derived nanosheet-like electrocatalysts with metal-modified edge defects are developed. During electrochemical processes, MBi2O4 pre-catalyst undergoes surface reconstruction to expose more active Bi0 sites and to form abundant metal-modified edge defects, which facilitate the modulation of charge state of adjacent Bi sites and improve the adsorption of *OCHO intermediate, thereby enhancing the selectivity of eCO2RR towards HCOOH. The Ca-modified defect-rich Bi catalyst exhibits a maximum HCOOH Faradaic efficiency of 94.3 % at an industrially relevant current density of 400 mA cm−2, together with high stability/reproducibility of 55 h. This work proposes a new strategy for construction of Bi-based catalysts and explores its structural reconstruction mechanism during the electrocatalytic process, opening a new window for design of eCO2RR catalysts.

Simultaneous Formate and Syngas Conversion Boosts Growth and Product Formation by Clostridium ragsdalei.

Electrocatalytic CO2 reduction to CO and formate can be coupled to gas fermentation with anaerobic microorganisms. In combination with a competing hydrogen evolution reaction in the cathode in aqueous medium, the in situ, electrocatalytic produced syngas components can be converted by an acetogenic bacterium, such as Clostridium ragsdalei, into acetate, ethanol, and 2,3-butanediol. In order to study the simultaneous conversion of CO, CO2, and formate together with H2 with C. ragsdalei, fed-batch processes were conducted with continuous gassing using a fully controlled stirred tank bioreactor. Formate was added continuously, and various initial CO partial pressures (pCO0) were applied. C. ragsdalei utilized CO as the favored substrate for growth and product formation, but below a partial pressure of 30 mbar CO in the bioreactor, a simultaneous CO2/H2 conversion was observed. Formate supplementation enabled 20-50% higher growth rates independent of the partial pressure of CO and improved the acetate and 2,3-butanediol production. Finally, the reaction conditions were identified, allowing the parallel CO, CO2, formate, and H2 consumption with C. ragsdalei at a limiting CO partial pressure below 30 mbar, pH 5.5, n = 1200 min-1, and T = 32 °C. Thus, improved carbon and electron conversion is possible to establish efficient and sustainable processes with acetogenic bacteria, as shown in the example of C. ragsdalei.

Porphyrin Aggregation under Homogeneous Conditions Inhibits Electrocatalysis: A Case Study on CO2 Reduction.

Metalloporphyrins are widely used as homogeneous electrocatalysts for transformations relevant to clean energy and sustainable organic synthesis. Metalloporphyrins are well-known to aggregate due to π-π stacking, but surprisingly, the influence of aggregation on homogeneous electrocatalytic performance has not been investigated previously. Herein, we present three structurally related iron meso-phenylporphyrins whose aggregation properties are different in commonly used N,N-dimethylformamide (DMF) electrolyte. Both spectroscopy and light scattering provide evidence of extensive porphyrin aggregation under conventional electrocatalytic conditions. Using the electrocatalytic reduction of CO2 to CO as a test reaction, cyclic voltammetry reveals an inverse dependence of the kinetics on the catalyst concentration. The inhibition extends to bulk performance, where up to 75% of the catalyst at 1 mM is inactive compared to at 0.25 mM. We additionally report how aggregation is perturbed by organic additives, axial ligands, and redox state. Periodic boundary calculations provide additional insights into aggregate stability as a function of metalloporphyrin structure. Finally, we generalize the aggregation phenomenon by surveying metalloporphyrins with different metals and substituents. This study demonstrates that homogeneous metalloporphyrins can aggregate severely in well-solubilizing organic electrolytes, that aggregation can be easily modulated through experimental conditions, and that the extent of aggregation must be considered for accurate catalytic benchmarking.

Decorated and sculpted zeolitic imidazolate frameworks for electrocatalytic carbon dioxide reduction to formate

The production of value-added chemicals using electrocatalytic CO2 reduction (ECR) is an effective strategy to achieve “carbon neutrality”. However, highly formate-selective production with zinc-based catalysts in ECR systems remains a challenge. In this work, a macroporous hollow nanocage structure was successfully created through the etching of zeolitic imidazolate frameworks (ZIFs) using phytic acid (PA). The surface of the hollow nanocage was then adorned, leading to the formation of a hybrid superstructure product (ZIF-P5/In) with a substantial surface area, enabling effective ECR to formate. The Faraday efficiency of ZIF-P5/In for formate is 80.2 % at −1.26 VRHE. This feasible strategy is expected to provide new insights into the preparation of highly efficient ZIF-derived electrocatalysts.

Copper oxide foam with high specific surface area for electrocatalytic reduction of CO2 to C2+ products

AbstractThe carbon dioxide reduction reaction (CO2RR) for the synthesis of high‐energy‐density and high‐value multi‐carbon (C2+) products has demonstrated considerable potential for practical applications. In this work, we design a novel copper oxide foam (OD‐Cu foam) catalyst through a high‐temperature calcination process, characterized by a substantial specific surface area. The distinctive three‐dimensional structure of the OD‐Cu foam catalyst and the metal oxide particles covered on its surface provide abundant active sites. The total Faradaic efficiency of 57.3% for C2+ products over the OD‐Cu foam is achieved at −0.85 V versus reversible hydrogen electrode (RHE). Furthermore, the partial current density for C2+ products over the OD‐Cu foam reaches 44.1 mA cm−2 at −0.95 V versus RHE, surpassing significantly that both of Cu foam (3.4 mA cm−2) and copper oxide foil (OD‐Cu foil) (1.6 mA cm−2). In addition, the integrated structure of the OD‐Cu foam, which does not require complex preparation processes, facilitates its application in CO2RR. These results underscore the significance of three‐dimensional structure and high specific surface area, emphasizing the considerable potential of this catalyst for effective and sustainable CO2 conversion.

Polyoxometalates for carbon dioxide activation: Current progress and perspective

AbstractThe elevated level of carbon dioxide (CO2) in the troposphere is playing a crucial role in changing the earth's climatic system. Excess CO2 concentration is trapping a substantial proportion of heat imparting adverse range of environmental effects like extreme weather events, agricultural impacts and disruption of the ecosystem. Strategies such as CO2 capturing and storage are being pursued using polyoxometalates (POMs) to mitigate the elevated level of CO2 and their efficient conversion into useful chemicals. This review outlines the importance of CO2 reduction by electrocatalytic and photocatalytic reduction of CO2. POMs have several advantages for CO2 activation because of their promising catalytic activity, high selectivity and versatility for incorporating different metal ions and ligands. The efficient redox active metal center facilitates the conversion of CO2 by providing sufficient activation energy. Their robust structure also enables them resist harsh conditions, including high temperature and acidic or basic conditions. Owing to these properties, POMs can be a catalyst, co‐catalyst, photosensitizer or multielectron donor for CO2 reduction. Finally, we stated the challenges and future insight for CO2 activation using POMs.

Rapid Joule-heating synthesis of metal/carbon-based electrocatalysts for efficient carbon dioxide reduction

Carbon-loaded metal nanoparticles (NPs) are widely employed as functional materials for electrocatalysis. In this study, a rapid thermal shock method was developed to load various metal nanoparticles onto carbon supports. Compared to conventional pyrolysis processes, Joule heating enables rapid heating to elevated temperatures within a short period, effectively preventing the migration and aggregation of metal atoms. Simultaneously, the anchoring effect of defective carbon carriers ensures the uniform distribution of NPs on the carbon supports. Additionally, nitrogen doping can significantly enhance the electronic conductivity of the carbon matrix and strengthen the metal-carbon interactions, thereby synergistically improving catalyst performance. When used as electrocatalysts for electrocatalytic CO2 reduction, bismuth-, indium-, and tin/carbon-carrier-based catalysts exhibit excellent Faraday efficiencies of 92.8%, 86.4%, and 73.3%, respectively, for formate generation in flow cells. The influence of different metals and calcination temperatures on catalytic performance was examined to provide valuable insights into the rational design of carbon-based electrocatalysts with enhanced electrocatalytic activity.

In-situ construction of iron-modified nickel nanoparticles assisted by hexamethylenetetramine with the internal and external collaboration for highly selective electrocatalytic carbon dioxide reduction

Carbon dioxide (CO2) electroreduction provides a sustainable route for realizing carbon neutrality and energy supply. Up to now, challenges remain in employing abundant and inexpensive nickel materials as candidates for CO2 reduction due to their low activity and favorable hydrogen evolution. Here, the representative iron-modified nickel nanoparticles embedded in nitrogen-doped carbon (Ni1-Fe0.125-NC) with the porous botryoid morphology were successfully developed. Hexamethylenetetramine is used as nitrogen-doped carbon source. The collaboration of internal lattice expansion with electron effect and external confinement effect with size effect endows the significant enhancement in electrocatalytic CO2 reduction. The optimized Ni1-Fe0.125-NC exhibits broad potential ranges for continuous carbon monoxide (CO) production. A superb CO Faradaic efficiency (FECO) of 85.0 % realized at −1.1 V maintains a longtime durability over 35 h, which exceeds many state-of-the-art metal catalysts. Theoretical calculations further confirm that electron redistribution promotes the desorption of CO in the process for favorable CO production. This work opens a new avenue to design efficient nickel-based materials by considering the intrinsic structure and external confinement for CO2 reduction.

Promoting CO Electroreduction to C2+ Oxygenates by Distribution of Water Dissociation Sites.

The electrocatalytic CO2 or CO reduction reaction is a complex proton-coupled electron transfer reaction, in which protons in the electrolyte have a critical effect on the surface adsorbed *H species and the multi-carbon oxygenate products such as ethanol. However, the coupling of *H and carbon-containing intermediates into C2+ oxygenates can be severely hampered by the inappropriate distributions of those species in the catalytic interfaces. In this work, the controlled distribution of highly dispersed CeOx nanoclusters is demonstrated on Cu nanosheets as an efficient CO electroreduction catalyst, with Faradaic efficiencies of ethanol and total oxygenates of 35% and 58%, respectively. The CeOx nanoclusters (2-5 nm) enabled efficient water dissociation and appropriate distribution of adsorbed *H species on the Cu surface with carbon-containing species, thus facilitating the generation of C2+ oxygenate products. In contrast, pristine Cu without CeOx tended to form ethylene, while the aggregated CeOx nanoparticles promoted the surface density of *H and subsequent H2 evolution.

Co-Co and Co-Zn bimetallic complexes for electrocatalytic CO2 reduction: The role of interrelated intramolecular effects on activity

Co-Co and Co-Zn bimetallic complexes for electrocatalytic CO2 reduction: The role of interrelated intramolecular effects on activity

Pseudocapacitance Facilitates the Electrocatalytic Reduction of Carbon Dioxide

AbstractElectroreduction of CO2 to value‐added products is a promising strategy for CO2 reuse, where copper has a unique ability to produce oxygenates and C2+ products. Unfortunately, the electronic factors making copper so unique are unknown, which limits the ability to design improved catalysts. By combining in situ surface‐sensitive X‐ray resonant photoelectron spectroscopy with density functional theory calculations, the complex electronic structure of copper is uncovered during the electrocatalytic reduction of CO2. It is found that the pseudocapacitive charging of copper, produced by the incorporation of protons and electrons into the subsurface, facilitates the activation of CO2, while simultaneously increasing the barrier for H‐H coupling. The net result is that cathodic pseudocapacitive charge suppresses the hydrogen evolution reaction and promotes the production of hydrocarbons and oxygenated products on copper. These results represent a new paradigm in the understanding of CO2 reduction, highlighting the key role of pseudocapacitive charge in the reaction.

Enhancing Electrocatalytic CO2-to-CO Conversion by Weakening CO Binding through Nitrogen Integration in the Metallic Fe Catalyst.

Metallic iron (Fe) typically demonstrates the unfavorable catalytic activity for the CO2 reduction reaction (CO2RR), mainly attributed to the excessively strong binding of CO products on Fe sites. Toward this end, we employed an effective approach involving electronic structure modulation through nitrogen (N) integration to enhance the performance of the CO2RR. Here, an efficient catalyst has been developed, composed of N-doped metallic iron (Fe) nanoparticles encapsulated in a porous N-doped carbon framework. Notably, this N-integrated Fe catalyst displays significantly enhanced performance in the electrocatalytic reduction of CO2, yielding the highest CO Faradaic efficiency of 97.5% with a current density of 6.68 mA cm-2 at -0.7 V versus the reversible hydrogen electrode. The theoretical calculations, combined with the in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy study, reveal that N integration modulates the electron density around Fe, resulting in the weakening of the binding strength between the Fe active sites and *CO intermediates, consequently promoting the desorption of CO and the overall CO2RR process.

In Situ Reconstruction of Scalable Amorphous Indium-Based Metal-Organic Framework for CO2 Electroreduction to Formate over an Ultrawide Potential Window.

Amorphous metal-organic frameworks (aMOFs) are highly attractive for electrocatalytic applications due to their exceptional conductivity and abundant defect sites, but harsh preparation conditions of "top-down" strategy have hindered their widespread use. Herein, the scalable production of aMIL-68(In)-NH2 was successfully achieved through a facile "bottom-up" strategy involving ligand competition with 2-methylimidazole. Multiple in situ and ex situ characterizations reveal that aMIL-68(In)-NH2 evolutes into In/In2O3-x as the genuine active sites during the CO2 electrocatalytic reduction (CO2RR) process. Moreover, the retained amino groups could enhance the CO2 adsorption. As expected, the reconstructed catalyst demonstrates high formate Faradaic efficiency values (>90%) over a wide potential range of 800 mV in a flow cell, surpassing most top-ranking electrocatalysts. Density functional theory calculations reveal that the abundant oxygen vacancies in aMIL-68(In)-NH2 induce more local charges around electroactive sites, thereby promoting the formation of HCOO* intermediates. Furthermore, 16 g of samples can be readily prepared in one batch and exhibit almost identical CO2RR performances. This work offers a feasible batch-scale strategy to design amorphous MOFs for the highly efficient electrolytic CO2RR.

Modulating the electrocatalytic reduction of CO2 to CO via surface reconstruction of ZnO nanoshapes

The electrocatalytic conversion of carbon dioxide (CO2) into valuable chemicals presents a promising strategy for closing the carbon cycle. In this study, we synthesized zinc (Zn) catalysts through hydrothermal methods using either polyvinylpyrrolidone (PVP) or cetyltrimethylammonium bromide (CTAB) as stabilizing agents. These catalysts proved highly efficient in converting CO2 into carbon monoxide (CO). Our findings revealed that ZnO, synthesized with different morphologies—namely, nanoneedles (ZnO-NN) and nanorods (ZnO-NR)—underwent significant electro-reconstruction, ultimately leading to the formation of hexagonal metallic Zn crystals, regardless of their initial characteristics. Utilizing ex-situ operando techniques, we elucidated that metallic Zn serves as the active phase for the CO2-to-CO conversion process. In a comparison, ZnO-NN catalysts demonstrated superior selectivity and stability, achieving 91.3% CO selectivity at a potential of −0.88 V vs. RHE (Reversible Hydrogen Electrode) due to the facile transformation of ZnO to metallic Zn. Remarkably, these catalysts maintained this level of performance for more than 17 h. Conversely, ZnO-NR catalysts exhibited a lower CO selectivity of 62.5% at a relatively higher potential of −0.98 V vs RHE.

Highly dispersed nickel-copper bimetallic catalysts prepared by monolayer NiCu-layered double hydroxide nanocomposites for CO2 electrocatalytic reduction

The design of novel electrocatalyst for CO2 reduction reaction to available chemical substance is a meaningful approach to mitigate the CO2 emission problem. However, it is challenging to achieve uniform dispersion of the metals in the carbon substrate to enhance catalytic performance. Surprisingly, layered double hydroxide nanosheets have uniform metal distribution, which are appropriate metal negative supports for CO2RR catalyst. Herein, we have successfully prepared a simple, green and efficient novel CO2RR metal catalyst using monolayer NiCu-LDH nanocomposites as precursor. Based on solid phase exfoliation, monolayer NiCu-LDH nanocomposite material was obtained, and then the ultra-thin two-dimensional carbon substrate with homogeneous metals dispersion was formed by in-situ nitridation. The obtained Ni2Cu1-CN shows excellent CO2RR performance with CO current density of 12.65 mA cm−2 and great CO faraday efficiency of 96.9 % at −0.8 V (vs. RHE). But metals formed nanoparticles due to vibration migration at high temperature, which caused active sites to be obscured. Surprisingly, introduction of Zn prevented Cu and Ni metals from agglomerating and exposed more active sites. The as‐prepared Ni2Cu1Zn1-CN shows excellent catalytic performance with the greatest CO faraday efficiency of 98.1 % and high CO current density of 16.6 mA cm−2 at −0.8 V (vs RHE) and remarkable stability.

Mo4+-Doped CuS Nanosheet-Assembled Hollow Spheres for CO2 Electroreduction to Ethanol in a Flow Cell.

Electrocatalytic CO2 reduction reaction (CO2RR) to ethanol has been widely researched for potential commercial application. However, it still faces limited selectivity at a large current density. Herein, Mo4+-doped CuS nanosheet-assembled hollow spheres are constructed to address this issue. Mo4+ ion doping modifies the local electronic environments and diversifies the binding sites of CuS, which increases the coverage of linear *COL and produces bridge *COB for subsequent *COL-*COH coupling toward ethanol production. The optimal Mo9.0%-CuS can electrocatalyze CO2 to ethanol with a faradaic efficiency of 67.5% and a partial current density of 186.5 mA cm-2 at -0.6 V in a flow cell. This work clarifies that doping high valence transition metal ions into Cu-based sulfides can regulate the coverage and configuration of related intermediates for ethanol production during the CO2RR in a flow cell.

Tuning the Microenvironment of Co−N4 Ensemble for Co Single‐Atom Catalysts for Electrocatalytic CO2 Reduction

AbstractSingle‐atom catalysts (SACs), with their well‐defined structure, serve as an efficient catalyst model for studying the structure‐performance relation in the electrocatalytic CO2 reduction reaction (CO2RR). Current CO2RR research primarily focus on the coordination interactions between metal single atoms and coordinated ligands, while the impact of the metal SAC′s microenvironment remains largely unexplored. Herein, we generate three microenvironments for a CoN4 ensemble by using different nitrogen sources to prepare Co SACs, i. e. pyridine‐type Co−N4 (Co−N4 SACPhen), the pyrrole‐type Co−N4 (Co−N4 SACDp) and the mixed‐type Co−N4(Co−N4 SACMm). It reveals that the Co−N4 SACPhen contributes to CO generation with 99.5 % selectivity at −0.76 vs RHE, whereas the Co−N4 SACDp accelerates a severe hydrogen evolution reaction. Experimental studies confirmed that the Co−N4 SACPhen possesses distinctive electrochemical properties and electronic structure, with the pyridine‐N being more conducive to electron accumulation and exhibited a higher electron delocalization, thereby promoting the generation of CO. This study deepens our understanding of the cobalt SAC′s microenvironment‘s influence on CO2RR.