Steering the Selectivity of Carbon Dioxide Electroreduction from Single-Carbon to Multicarbon Products on Metal-Organic Frameworks via Facet Engineering.

Although metal-organic frameworks (MOFs) have attracted more attention for the electrocatalytic CO2 reduction reaction (CO2RR), obtaining multicarbon products with a high Faradaic efficiency (FE) remains challenging, especially under neutral conditions. Here, we report the controlled synthesis of stable Cu(I) 5-mercapto-1-methyltetrazole framework (Cu-MMT) nanostructures with different facets by rationally modulating the reaction solvents. Significantly, Cu-MMT nanostructures with (001) facets are acquired using isopropanol as a solvent, which favor multicarbon production with an FE of 73.75% and a multicarbon:single-carbon ratio of 3.93 for CO2RR in a neutral electrolyte. In sharp contrast, Cu-MMT nanostructures with (100) facets are obtained utilizing water, promoting single-carbon generation with an FE of 63.98% and a multicarbon: single-carbon ratio of only 0.18. Furthermore, this method can be extended to other Cu-MMT nanostructures with different facets in tuning the CO2 reduction selectivity. This work opens up new opportunities for the highly selective and efficient CO2 electroreduction to multicarbon products on MOFs via facet engineering.

Interfacial Accumulation and Stability Enhancement Effects Triggered by Built-in Electric Field of SnO2/LaOCl Nanofibers Boost Carbon Dioxide Electroreduction.

Constructing a built-in interfacial electric field (BIEF) is an effective approach to enhance the electrocatalysts performance, but it has been rarely demonstrated for electrochemical carbon dioxide reduction reaction (CO2RR) to date. Herein, for the first time, SnO2/LaOCl nanofibers (NFs) with BIEF is created by electrospinning, exhibiting a high Faradaic efficiency (FE) of 100% C1 product (CO and HCOOH) at -0.9--1.1 V versus reversible hydrogen electrode (RHE) and a maximum FEHCOOH of 90.1% at -1.2 VRHE in H-cell, superior to the commercial SnO2 nanoparticles (NPs) and LaOCl NFs. SnO2/LaOCl NFs also exhibit outstanding stability, maintaining negligible activity degradation even after 10 h of electrolysis. Moreover, their current density and FEHCOOH are almost 400 mA cm-2 at -2.31 V and 83.4% in flow-cell. The satisfactory CO2RR performance of SnO2/LaOCl NFs with BIEF can be ascribed to tight interface of coupling SnO2 NPs and LaOCl NFs, which can induce charge redistribution, rich active sites, enhanced CO2 adsorption, as well as optimized Gibbs free energy of *OCHO. The work reveals that the BIEF will trigger interfacial accumulation and stability enhancement effects in promoting CO2RR activity and stability of SnO2-based materials, providing a novel approach to develop stable and efficient CO2RR electrocatalysts.

Single-Atom based Metal-Organic Framework Photocatalysts for Solar-Fuel Generation.

The growing demand for fossil fuels and subsequent CO2 emissions prompted a search for alternate sources of energy and a reduction in CO2. Photocatalysis driven by solar light has been found as a potential research area to tackle both these problems. In this direction, SAC@MOF (Single-atom loaded MOFs) photocatalysis is an emerging field and a promising technology. The unique properties of single-atom catalysts (SACs), such as high catalytic activity and selectivity, are leveraged in these systems. Photocatalysis, focusing on the utilization of Metal-Organic Frameworks (MOFs) as platforms for creating single-atom catalysts (SACs) characterized by metal single-atoms (SAs) as their active sites, are noted for their unparalleled atomic efficiency, precisely defined active sites, and superior photocatalytic performance. The synergy between MOFs and SAs in photocatalytic systems is meticulously examined, highlighting how they collectively enhance photocatalytic efficiency. This review examines SAC@MOF development and applications in environmental and energy sectors, focusing on synthesis and stabilization methods for SACs on MOFs and also characterization techniques vital for understanding these catalysts. The potential of SAC@MOF in CO2 Photoreduction and Photocatalytic H2 evolution is highlighted, emphasizing its role in green energy technologies and advances in materials science and Photocatalysis.

Synergistic Effect of Grain Boundaries and Oxygen Vacancies on Enhanced Selectivity for Electrocatalytic CO2 Reduction.

Dual-engineering involved of grain boundaries (GBs) and oxygen vacancies (VO) efficiently engineers the material's catalytic performance by simultaneously introducing favorable electronic and chemical properties. Herein, a novel SnO2 nanoplate is reported with simultaneous oxygen vacancies and abundant grain boundaries (V,G-SnOx/C) for promoting the highly selective conversion of CO2 to value-added formic acid. Attributing to the synergistic effect of employed dual-engineering, the V,G-SnOx/C displays highly catalytic selectivity with a maximum Faradaic efficiency (FE) of 87% for HCOOH production at -1.2 V versus RHE and FEs > 95% for all C1 products (CO and HCOOH) within all applied potential range, outperforming current state-of-the-art electrodes and the amorphous SnOx/C. Theoretical calculations combined with advanced characterizations revealed that GB induces the formation of electron-enriched Sn site, which strengthens the adsorption of *HCOO intermediate. While GBs and VO synergistically lower the reaction energy barrier, thus dramatically enhancing the intrinsic activity and selectivity toward HCOOH.

Synthesis and Electrocatalytic Applications of Layer-Structured Metal Chalcogenides Composites.

Featured with the attractive properties such as large surface area, unique atomic layer thickness, excellent electronic conductivity, and superior catalytic activity, layered metal chalcogenides (LMCs) have received considerable research attention in electrocatalytic applications. In this review, the approaches developed to synthesize LMCs-based electrocatalysts are summarized. Recent progress in LMCs-based composites for electrochemical energy conversion applications including oxygen reduction reaction, carbon dioxide reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, overall water splitting, and nitrogen reduction reaction is reviewed, and the potential opportunities and practical obstacles for the development of LMCs-based composites as high-performing active substances for electrocatalytic applications are also discussed. This review may provide an inspiring guidance for developing high-performance LMCs for electrochemical energy conversion applications.

Anode Reaction Control for a Single-Compartment Electrochemical CO2 Reduction Reactor with a Surface-Activated Diamond Cathode.

The electrochemical reaction of carbon dioxide (CO2 ) in aqueous electrolyte solutions is attracting increasing attention for sustainable chemical production. Boron-doped diamond (BDD) electrodes have been previously shown to be very effective for the stable electrochemical production of formic acid from CO2 . Typically, the electrochemical production of formic acid by CO2 reduction (CO2 R) reaction is performed with a dual-compartment flow reactor equipped with a membrane separator. The problems caused by the membrane separator, such as scaling-up, complicated operational control and materials costs can be solved using a membrane free single-compartment reactor. Here we demonstrate anode reaction control for a single-compartment CO2 R flow reactor using a surface-activated BDD cathode and achieve a Faradaic efficiency for formic acid production of over 70 %.

Photoelectrocatalytic Utilization of CO2 : A Big Show of Si-based Photoelectrodes.

CO2 is a greenhouse gas that contributes to environmental deterioration; however, it can also be utilized as an abundant C1 resource for the production of valuable chemicals. Solar-driven photoelectrocatalytic (PEC) CO2 utilization represents an advanced technology for the resourcing of CO2 . The key to achieving PEC CO2 utilization lies in high-performance semiconductor photoelectrodes. Si-based photoelectrodes have attracted increasing attention in the field of PEC CO2 utilization due to their suitable band gap (1.1 eV), high carrier mobility, low cost, and abundance on Earth. There are two pathways to PEC CO2 utilization using Si-based photoelectrodes: direct reduction of CO2 into small molecule fuels and chemicals, and fixation of CO2 with organic substrates to generate high-value chemicals. The efficiency and product selectivity of PEC CO2 utilization depends on the structures of the photoelectrodes as well as the composition, morphology, and size of the catalysts. In recent years, significant and influential progress has been made in utilizing Si-based photoelectrodes for PEC CO2 utilization. This review summarizes the latest research achievements in Si-based PEC CO2 utilization, with a particular emphasis on the mechanistic understanding of CO2 reduction and fixation, which will inspire future developments in this field.

Effects of the Carbon Support on Heterogeneous Molecular Catalysts for Carbon Dioxide Reduction.

Molecular catalysts stabilized on a support material, also called heterogeneous molecular catalysts, exhibit excellent performance in carbon dioxide reduction reaction (CO2 RR). Different support in these electrocatalysts can have a substantial influence on the activity, making support control one tool to enhance the CO2 RR performance. However, a systematic understanding of the support effects is lacking. Taking cobalt phthalocyanine (CoPc) immobilized onto different carbon materials as examples, we demonstrate that the surface area, pore structure and the morphology of the as-prepared heterogeneous molecular catalysts can influence the CO2 transfer and adsorption, and then change the CO2 RR activity. In contrast to the other four materials, CoPc/mesoporous carbon (MC) can efficiently convert carbon dioxide to carbon monoxide at minimal overpotential (-0.8 V vs. RHE) due to its special nanostructure and pore distribution. The results of this study suggest that the performance of electrocatalytic reduction of carbon dioxide can be improved by changing different substrates.

Solid Electrolytes for Low-Temperature Carbon Dioxide Valorization: A Review.

One of the most promising approaches to address the global challenge of climate change is electrochemical carbon capture and utilization. Solid electrolytes can play a crucial role in establishing a chemical-free pathway for the electrochemical capture of CO2. Furthermore, they can be applied in electrocatalytic CO2 reduction reactions (CO2RR) to increase carbon utilization, produce high-purity liquid chemicals, and advance hybrid electro-biosystems. This review article begins by covering the fundamentals and processes of electrochemical CO2 capture, emphasizing the advantages of utilizing solid electrolytes. Additionally, it highlights recent advancements in the use of the solid polymer electrolyte or solid electrolyte layer for the CO2RR with multiple functions. The review also explores avenues for future research to fully harness the potential of solid electrolytes, including the integration of CO2 capture and the CO2RR and performance assessment under realistic conditions. Finally, this review discusses future opportunities and challenges, aiming to contribute to the establishment of a green and sustainable society through electrochemical CO2 valorization.

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

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

Bioinspired Polyoxo-titanium Cluster for Greatly Enhanced Solar-Driven CO2 Reduction.

Developing artificial enzymes with excellent catalytic activities and uncovering the structural and chemical determinants remain a grand challenge. Discrete titanium-oxo clusters with well-defined coordination environments at the atomic level can mimic the pivotal catalytic center of natural enzymes and optimize the charge-transfer kinetics. Herein, we report the precise structural tailoring of a self-assembled tetrahedral Ti4Mn3-cluster for photocatalytic CO2 reduction and realize the selective evolution of CO over specific sites. Experiments and theoretical simulation demonstrate that the high catalytic performance of the Ti4Mn3-cluster should be related to the synergy between active Mn sites and the surrounding functional microenvironment. The reduced energy barrier of the CO2 photoreduction reaction and moderate adsorption strength of CO* are beneficial for the high selective evolution of CO. This work provides a molecular scale accurate structural model to give insight into artificial enzyme for CO2 photoreduction.

Synthesis and Electrochemical Study of Gold(I) Carbene Complexes.

In this work, we have prepared and characterized some gold compounds wearing a N-heterocyclic carbene (NHC) ligand as well as alkynyl derivatives with different substituents. The study of their electrochemical behavior reveals that these complexes show an irreversible wave at potentials ranging between -2.79 and -2.91 V, referenced to the ferrocenium/ferrocene pair. DFT calculations indicate that the reduction occurs mainly on the aryl-C≡C fragment. The cyclic voltammetry experiments under CO2 atmosphere show an increase in the faradaic current of the reduction wave compared to the experiments under argon atmosphere, indicating a possible catalytic activity towards the carbon dioxide reduction reaction (CO2RR).

Electrochemical Water Oxidation and CO2 Reduction with a Nickel Molecular Catalyst.

Mimicking the photosynthesis of green plants to combine water oxidation with CO2 reduction is of great significance for solving energy and environmental crises. In this context, a trinuclear nickel complex, [NiII3(paoH)6(PhPO3)2]·2ClO4 (1), with a novel structure has been constructed with PhPO32- (phenylphosphonate) and paoH (2-pyridine formaldehyde oxime) ligands and possesses a reflection symmetry with a mirror plane revealed by single-crystal X-ray diffraction. Bulk electrocatalysis demonstrates that complex 1 can homogeneously catalyze water oxidation and CO2 reduction simultaneously. It can catalyze water oxidation at a near-neutral condition of pH = 7.45 with a high TOF of 12.2 s-1, and the Faraday efficiency is as high as 95%. Meanwhile, it also exhibits high electrocatalytic activity for CO2 reduction towards CO with a TOF of 7.84 s-1 in DMF solution. The excellent electrocatalytic performance of the water oxidation and CO2 reduction of complex 1 could be attributed to the two unique µ3-PhPO32- bridges as the crucial factor for stabilizing the trinuclear molecule as well as the proton transformation during the catalytic process, while the oxime groups modulate the electronic structure of the metal centers via π back-bonding. Therefore, apart from the cooperation effect of the three Ni centers for catalysis, simultaneously, the two kinds of ligands in complex 1 can also synergistically coordinate the central metal, thereby significantly promoting its catalytic performance. Complex 1 represents the first nickel molecular electrocatalyst for both water oxidation and CO2 reduction. The findings in this work open an avenue for designing efficient molecular electrocatalysts with peculiar ligands.

Research progress on metal-organic framework compounds (MOFs) in electrocatalysis.

Metal-organic frameworks (MOFs) have favorable characteristics such as large specific surface area, high porosity, structural diversity, and pore surface modification, giving them great potential for development and attractive prospects in the research area of modern materials electrocatalysis. However, unsatisfactory catalytic activity and poor electronic conductivity are the main challenges facing MOFs. This review focuses on MOF-based materials used in electrocatalysis, based on the types of catalytic reactions that have used MOF-based materials in recent years along with their applications, and also looks at some new electrocatalytic materials and their future development prospects.

Biomimetic Phthalocyanine-Based Covalent Organic Frameworks with Tunable Pendant Groups for Electrocatalytic CO2 Reduction.

Electrocatalytic carbon dioxide reduction reaction (CO2RR) is an effective way of converting CO2 into value-added products using renewable energy, whose activity and selectivity can be in principle maneuvered by tuning the microenvironment near catalytic sites. Here, we demonstrate a strategy for tuning the microenvironment of CO2RR by learning from the natural chlorophyll and heme. Specifically, the conductive covalent organic frameworks (COFs) linked by piperazine serve as versatile supports for single-atom catalysts (SACs), and the pendant groups modified on the COFs can be readily tailored to offer different push-pull electronic effects for tunable microenvironment. As a result, while all the COFs exhibit high chemical structure stability under harsh conditions and good conductivity, the addition of -CH2NH2 can greatly enhance the activity and selectivity of CO2RR. As proven by experimental characterization and theoretical simulation, the electron-donating group (-CH2NH2) not only reduces the surface work function of COF, but also improves the adsorption energy of the key intermediate *COOH, compared with the COFs with electron-withdrawing groups (-CN, -COOH) near the active sites. This work provides insights into the microenvironment modulation of CO2RR electrocatalysts at the molecular level.

Quantitative Construction of Boronic-Ester Linkages in Covalent Organic Frameworks for the Carbon Dioxide Reduction.

Covalent organic frameworks (COFs) have been utilized for catalyzing the reduction of carbon dioxide (CO2RR) due to their atomic metal centers and controllable pore channels, which are facilitated by different covalent bonds. However, the exploration of boron-based linkages in these catalytic COFs has been limited owing to potential instability. Herein, we present the construction of boronic ester-linked COFs through nucleophilic substitution reactions in order to catalyze the CO2 RR. The inclusion of abundant fluorine atoms within the frameworks enhances their hydrophobicity and subsequently improves water tolerance and chemical stability of COFs. The content of boron atoms in the COF linkages was carefully controlled, with COFs featuring a higher density of boron atoms exhibiting increased electronic conductivity, enhanced reductive ability, and stronger binding affinity towards CO2 . Consequently, these COFs demonstrate improved activity and selectivity. The optimized COFs achieve the highest activity, achieving a turnover frequency of 1695.3 h-1 and a CO selectivity of 95.0 % at -0.9 V. Operando synchrotron radiation measurements confirm the stability of Co (II) atoms as catalytically active sites. By successfully constructing boronic ester-linked COFs, we not only address potential instability concerns but also achieve exceptional catalytic performance for CO2 RR.

Ruthenium Single Atomic Sites Surrounding the Support Pit with Exceptional Photocatalytic Activity.

Single-metal atomic sites and vacancies can accelerate the transfer of photogenerated electrons and enhance photocatalytic performance in photocatalysis. In this study, a series of nickel hydroxide nanoboards (Ni(OH)x NBs) with different loadings of single-atomic Ru sites (w-SA-Ru/Ni(OH)x) were synthesized via a photoreduction strategy. In such catalysts, single-atomic Ru sites are anchored to the vacancies surrounding the pits. Notably, the SA-Ru/Ni(OH)x with 0.60 wt % Ru loading (0.60-SA-Ru/Ni(OH)x) exhibits the highest catalytic performance (27.6 mmol g-1 h-1) during the photocatalytic reduction of CO2 (CO2RR). Either superfluous (0.64 wt %, 18.9 mmol g-1 h-1; 3.35 wt %, 9.4 mmol-1 h-1) or scarce (0.06 wt %, 15.8 mmol g-1 h-1; 0.29 wt %, 21.95 mmol g-1 h-1; 0.58 wt %, 23.4 mmol g-1 h-1) of Ru sites have negative effect on its catalytic properties. Density functional theory (DFT) calculations combined with experimental results revealed that CO2 can be adsorbed in the pits; single-atomic Ru sites can help with the conversion of as-adsorbed CO2 and lower the energy of *COOH formation accelerating the reaction; the excessive single-atomic Ru sites occupy vacancies that retard the completion of CO2RR.

Spontaneous α-C-H Carboxylation of Ketones by Gaseous CO2 at the Air-water Interface of Aqueous Microdroplets.

We present a catalyst-free route for the reduction of carbon dioxide integrated with the formation of a carbon-carbon bond at the air/water interface of negatively charged aqueous microdroplets, at ambient temperature. The reactions proceed through carbanion generation at the α-carbon of a ketone followed by nucleophilic addition to CO2. Online mass spectrometry reveals that the product is an α-ketoacid. Several factors, such as the concentration of the reagents, pressure of CO2 gas, and distance traveled by the droplets, control the kinetics of the reaction. Theoretical calculations suggest that water in the microdroplets facilitates this unusual chemistry. Furthermore, such a microdroplet strategy has been extended to seven different ketones. This work demonstrates a green pathway for the reduction of CO2 to useful carboxylated organic products.

Elaborate Modulating Binding Strength of Intermediates via Three-component Covalent Organic Frameworks for CO2 Reduction Reaction.

The catalytic performance for electrocatalytic CO2 reduction reaction (CO2RR) depends on the binding strength of the reactants and intermediates. Covalent organic frameworks (COFs) have been adopted to catalyze CO2RR, and their binding abilities are tuned via constructing donor-acceptor (DA) systems. However, most DA COFs have single donor and acceptor units, which caused wide-range but lacking accuracy in modulating the binding strength of intermediates. More elaborate regulation of the interactions with intermediates are necessary and challenge to construct high-efficiency catalysts. Herein, the three-component COF with D-A-A units was first constructed by introducing electron-rich diarylamine unit, electron-deficient benzothiazole and Co-porphyrin units. Compared with two-component COFs, the designed COF exhibit elevated electronic conductivity, enhanced reducibility, high efficiency charge transfer, further improving the electrocatalytic CO2RR performance with the faradic efficiency of 97.2 % at -0.8 V and high activity with the partial current density of 27.85 mA cm-2 at -1.0 V which exceed other two-component COFs. Theoretical calculations demonstrate that catalytic sites in three-component COF have suitable binding ability of the intermediates, which are benefit for formation of *COOH and desorption of *CO. This work offers valuable insights for the advancement of multi-component COFs, enabling modulated charge transfer to improve the CO2RR activity.

Engineering Ultrathin CuxS Layer on Planar Sb2S3 Photocathode to Enhance Photoelectrochemical Transformation.

Sb2S3 has been extensively used as light absorber for photoelectrochemical cell. However, its p-type nature may result in the formation of Schottky junction with substrates, thus hindering the collection of photogenerated holes. Herein, an ultrathin CuxS layer is successfully engineered as the bottom junction for Sb2S3 for the first time. Capitalizing on its impressive electrical properties and superior optical properties, the CuxS layer exhibits a high work function of 4.90 eV, which causes the upward band bending of p-type Sb2S3, forming a hole-transparent structure with ohmic contact. The transparency of the ultrathin CuxS layer enables back-illumination of the Sb2S3/CuxS platform, facilitating the integration of intricate catalyst layers for photoelectrochemical transformation. When modified with Pt nanoparticles, the photocurrent density reaches -5.38 mA cm-2 at 0 V vs. RHE, marking a fourfold increase compared to the photocathode without CuxS layer. When introducing a molecular hybrid TC-CoPc@carbon black, a remarkable average photocurrent density of -0.44 mA cm-2 at the overpotential of 0 V is obtained for CO2 reduction reaction, while the photocurrent density is less than -0.03 mA cm-2 without CuxS.