Designing N-Confused Metalloporphyrin-Based Covalent Organic Frameworks for Enhanced Electrocatalytic Carbon Dioxide Reduction

Boosting selective CO2 reduction via strong spin-spin coupling on dual-atom spin-catalysts

Achieving high selectivity in electrochemical conversion of carbon dioxide (CO2) into valuable products remains a significant challenge. This study investigates the influence of spin states on dual-atom catalysts within two-dimensional metal-organic frameworks (2D-MOFs) and zero-dimensional molecular metal complexes (0D-MMCs), emphasizing their role in the selective electrocatalytic reduction of CO2. Utilizing first-principles calculations, we systematically evaluate dual-atom spin-catalysts (DASCs) TM2S4(NH)2(C6H4)2 0D-MMC and TM2S4(NH)2C4 2D-MOF for CO2 reduction reactions (CO2RR) across various spin states: antiferromagnetic (AFM), ferromagnetic (FM), and non-magnetic (NM). Our analysis confirms that, beyond successfully designing and screening highly active catalysts, the selectivity for various C1 products in CO2 reduction can be readily adjusted by DASCs via spin-spin coupling. Specifically, Mn2 and Fe2 2D-MOF DASCs with an AFM ground state are more inclined to produce formic acid, while their FM counterparts favor the formation of methane, surpassing formic acid among others. Additionally, we demonstrate that 0D-MMCs, as molecular units of 2D-MOFs, achieve comparable catalytic performance. Combining theoretical insights with machine learning highlights the crucial role of electronic and geometric descriptors in the catalytic performance. Our work establishes the correlation between spin-spin coupling and highly selective CO2 reduction in DASCs, offering an effective strategy for designing tunable and efficient electrocatalysts.

Enhanced Electrocatalytic Carbon Dioxide Reduction Activity via Local Charge Environment Regulation of Active Sites with Rational Functionalization

Covalent organic frameworks (COFs) are a new emerging class of electrochemical catalysts for the CO2 reduction reaction (CO2RR) with fascinating structural tunability. In this work, to dig more detailed information about the effect of local charge environment regulation of active sites via structure modification on the catalytic performance of COFs for CO2RR, the Gibbs free-energy change (ΔG) of each elementary reaction step involved in the CO2RR and competitive hydrogen evolution reaction (HER) on COF366-Co and its derivatives were examined theoretically. It is observed that the valence band maximum (VBM) energy level of the COFs is increased by incorporation of electron-donating groups, and then the charge distribution on the Co center of COF366-Co is increased due to the increased charge-transfer amount from the electron-occupied N sp2 hybrid orbitals to the empty Co3d orbitals. For incorporating electron-withdrawing groups, the VBM energy level and the d-band center (ξd) of the Co atom are downshifted, and the d-band center gets closer to the occupied VBM energy level as the VBM is decreased to a larger extent than the ξd. As a result, electrosorption of the intermediate is facilitated and the CO2RR performance is enhanced by such a linker functionalization strategy, especially for electron-withdrawing groups. Our study highlights the key role that controlled local electrical environment via chemical structure modification of COFs can play in regulating the catalytic activity for its CO2RR applications.

Recent Progress of Covalent Organic Frameworks-Based Materials Used for CO2 Electrocatalytic Reduction: A Review

The excessive CO2 emissions from human activities severely impact the natural environment and ecosystems. Among the various technologies available, electrocatalytic CO2 reduction is regarded as one of the most promising routes due to its exceptional environmental friendliness and sustainability. Covalent organic frameworks (COFs) are crystalline, porous organic networks that are formed through thermodynamically controlled reversible covalent polymerization of organic linkers via covalent bonding. These materials exhibit high porosity, large surface area, excellent chemical and thermal stability, sustainability, high electron transfer efficiency, and surface functionalization capabilities, making them particularly effective in electrocatalytic CO2 reduction. First, this review briefly introduces the fundamental principles of electrocatalysis and the mechanism of electrocatalytic CO2 reduction. Next, it discusses the composition, structure, and synthesis methods of COF-based materials, as well as their applications in electrocatalytic CO2 reduction. Furthermore, it reviews the research progress in this field from the perspective of different types of COF-based catalysts. Finally, in light of the current research status, the development prospects of COF-based catalysts are explored, providing a reference for the development of more efficient and stable COF electrocatalysts for CO2 reduction.

Ligand Engineering of Co-N4 Single-Atom Catalysts for Highly-Active and Stable Acidic Oxygen Evolution

The development of stable and efficient single-atom catalysts (SACs) for the oxygen evolution reaction (OER) in acidic media remains challenging. This work reports a novel NH3-assisted pyrolysis strategy to synthesize Co-N4 SACs with controlled nitrogen coordination environments on crumpled graphene supports. The pyrrolic N4-coordinated Co sites demonstrate superior OER activity compared to their pyridinic counterparts, achieving an overpotential of 351 mV at 10 mA cm-2 in 0.5 m H2SO4. Combined density functional theory calculations and operando X-ray absorption spectroscopy reveal that the pyrrolic coordination environment facilitates enhanced OH- adsorption and subsequent OER kinetics due to its unique electronic structure and geometric flexibility. A multi-layered protective mechanism in the pyrrolic system enables exceptional stability during long-term acidic OER operation, stemming from higher defect formation energy of Co sites and strategic distribution of sacrificial nitrogen species in the graphene network. These findings provide fundamental insights into designing stable single-atom catalysts for challenging electrochemical applications.

Progress in Catalysts for Formic Acid Production by Electrochemical Reduction of Carbon Dioxide

Utilising renewable energy to drive the conversion of carbon dioxide into more valuable products can effectively alleviate the energy crisis and protect the environment while actively responding to the policy of "carbon peaking and carbon neutrality". Additionally, formic acid/formate is one of the most promising and commercially valuable products of the electrocatalytic CO2 reduction reaction (ECO2RR) as well as a nonhazardous material for hydrogen storage. With the continuous progress in the field of electrocatalytic CO2 reduction to formic acid/formate (ECO2RF), various electrocatalysts with excellent performance have been developed. In this paper, first, the reaction mechanism of ECO2RF is briefly summarised, and then the recent research progress for various catalysts for ECO2RF, including metal-based catalysts, carbon-based material catalysts, metal-organic framework catalysts, covalent organic framework catalysts, and molecular catalysts, is reviewed. Finally, the current challenges and future perspectives of ECO2RF are discussed and presented.

Well-defined asymmetric nitrogen/carbon-coordinated single metal sites for carbon dioxide conversion

Asymmetric nitrogen/carbon-coordinated single metal sites (M-NxC4-x) outperform symmetric M-N4 sites in carbon dioxide (CO2) electroreduction. However, the challenge of crafting well-defined M-NxC4-x sites complicates the understanding of their structure-catalytic performance relationship. In this study, we employ metallized N-confused tetraphenylporphyrin (M-NCTPP) to investigate CO2 conversion on M-N3C1 sites using both density functional theory and experimental methods. The optimal cobalt (Co)-N3C1 site (Co-NCTPP) achieves a current density of 500 mA cm-2 and a carbon monoxide Faraday efficiency exceeding 90 % at -1.25 V vs. the reversible hydrogen electrode, surpassing the performance of Co-N4 (Co-TPP). This research introduces a novel approach for designing and synthesizing high-activity heteroatom-anchored single metal sites, advancing fundamental understanding in the field.

High-Entropy Metal Sulfide Promises High-Performance Carbon Dioxide Reduction

The efficient conversion of carbon dioxide (CO2) requires the development of stable catalysts with high selectivity and reactivity within a wide potential range. Here, the high-entropy metal sulfide CuAgZnSnS4 is designed for CO2 reduction with excellent performance (FEcarbon products ≥ 90%) in whole test potential windows (600 mV) based on the synergistic effect of the high-entropy metal sulfide. In particular, CuAgZnSnS4 exhibits better single-product selectivity with the highest FEHCOOH/FECO value (29.03) at -1.28 versus reversible hydrogen electrode (RHE). In combination with in situ measurements and theoretical calculations, it is further revealed that the synergistic effect of CuAgZnSnS4 realizes the controllable regulation of the surface electronic structure at Sn active sites, strengthening orbital interactions between *OCHO and Sn active sites. As a result, the effective adsorption and activation of *OCHO instead of *H are obtained, improving the single-product selectivity of electrocatalytic CO2 reduction and inhibiting the competitive hydrogen evolution reaction significantly. Our findings may complete the understanding of the synergistic effect for high-entropy materials in catalysis and offer new insight into the design of efficient electrocatalysts with high catalytic activity.

Customizing Ionic Liquids Functionalized MOFs Composites with Hydrophobic Interface for Electrochemical CO2 Reduction

The electrochemical carbon dioxide reduction reaction (CO2RR) to generate feedstocks for chemical products (e.g., carbon monoxide, CO) offers a highly attractive method for achieving the closure of the carbon cycle. Ionic liquids (ILs)-functionalized Cu-based catalyst Cu2O-HKUST-1/IL1/PTFE was developed, configuring metal-organic frameworks(MOFs) based materials with high adsorption and multiple active sites. The modified electrocatalysts exhibited high specific surface area, strong CO2 adsorption capacity, abundant active sites, and fast charge transfer rate. The nucleophilic active site of deprotonation at the C2 site in imidazole ILs further improved the selectivity of proton migration and CO product generation, which was verified through DFT calculations for the low Gibbs free energy of the generated intermediate interactions. In addition, the hydrophobic interface constructed by PTFE facilitated the inhibition of the hydrogen evolution reaction (HER) and significantly improved the efficiency of CO2 electroreduction. The Cu2O-HKUST-1/IL1/PTFE catalyst manifested a high C1 Faraday efficiency (FE) up to 96.5% and in particular 92.7% for FECO at -1.7 V vs RHE. The present work provides an efficient strategy for configuring ILs-functionalized MOFs-based materials with good hydrophobic interfaces to enhance the efficiency of CO2 electroreduction to C1 products.

Materials Containing Single-, Di-, Tri-, and Multi-Metal Atoms Bonded to C, N, S, P, B, and O Species as Advanced Catalysts for Energy, Sensor, and Biomedical Applications

Modifying the coordination or local environments of single-, di-, tri-, and multi-metal atom (SMA/DMA/TMA/MMA)-based materials is one of the best strategies for increasing the catalytic activities, selectivity, and long-term durability of these materials. Advanced sheet materials supported by metal atom-based materials have become a critical topic in the fields of renewable energy conversion systems, storage devices, sensors, and biomedicine owing to the maximum atom utilization efficiency, precisely located metal centers, specific electron configurations, unique reactivity, and precise chemical tunability. Several sheet materials offer excellent support for metal atom-based materials and are attractive for applications in energy, sensors, and medical research, such as in oxygen reduction, oxygen production, hydrogen generation, fuel production, selective chemical detection, and enzymatic reactions. The strong metal-metal and metal-carbon with metal-heteroatom (i.e., N, S, P, B, and O) bonds stabilize and optimize the electronic structures of the metal atoms due to strong interfacial interactions, yielding excellent catalytic activities. These materials provide excellent models for understanding the fundamental problems with multistep chemical reactions. This review summarizes the substrate structure-activity relationship of metal atom-based materials with different active sites based on experimental and theoretical data. Additionally, the new synthesis procedures, physicochemical characterizations, and energy and biomedical applications are discussed. Finally, the remaining challenges in developing efficient SMA/DMA/TMA/MMA-based materials are presented.

Electrocatalytic Reduction of Carbon Dioxide in Acidic Electrolyte with Superior Performance of a Metal-Covalent Organic Framework over Metal-Organic Framework

CO2 electroreduction (CO2RR) to generate valuable chemicals in acidic electrolytes can improve the carbon utilization rate in comparison to that under alkaline conditions. However, the thermodynamically more favorable hydrogen evolution reaction under an acidic electrolyte makes the CO2RR a big challenge. Herein, robust metal phthalocyanine(Pc)-based (M = Ni, Co) conductive metal-covalent organic frameworks (MCOFs) connected by strong metal tetraaza[14]annulene (TAA) linkage, named NiPc-NiTAA and NiPc-CoTAA, are designed and synthesized to apply in the CO2RR in acidic electrolytes for the first time. The optimal NiPc-NiTAA exhibited an excellent Faradaic efficiency (FECO) of 95.1% and a CO partial current density of 143.0 mA cm-2 at -1.5 V versus the reversible hydrogen electrode in an acidic electrolyte, which is 3.1 times that of the corresponding metal-organic framework NiPc-NiN4. The comparison tests and theoretical calculations reveal that in-plane full π-d conjugation MCOF with a good conductivity of 3.01 × 10-4 S m-1 accelerates migration of the electrons. The NiTAA linkage can tune the electron distribution in the d orbit of metal centers, making the d-band center close to the Fermi level and then activating CO2. Thus, the active sites of NiPc and NiTAA collaborate to reduce the *COOH formation energy barrier, favoring CO production in an acid electrolyte. It is a helpful route for designing outstanding conductive MCOF materials to enhance CO2 electrocatalysis under an acidic electrolyte.

Porphyrin-based covalent organic frameworks from design, synthesis to biological applications

Covalent organic frameworks (COFs) constitute a class of highly functional porous materials composed of lightweight elements interconnected by covalent bonds, characterized by structural order, high crystallinity, and large specific surface area. The integration of naturally occurring porphyrin molecules, renowned for their inherent rigidity and conjugate planarity, as building blocks in COFs has garnered significant attention. This strategic incorporation addresses the limitations associated with free-standing porphyrins, resulting in the creation of well-organized porous crystal structures with molecular-level directional arrangements. The unique optical, electrical, and biochemical properties inherent to porphyrin molecules endow these COFs with diversified applications, particularly in the realm of biology. This review comprehensively explores the synthesis and modulation strategies employed in the development of porphyrin-based COFs and delves into their multifaceted applications in biological contexts. A chronological depiction of the evolution from design to application is presented, accompanied by an analysis of the existing challenges. Furthermore, this review offers directional guidance for the structural design of porphyrin-based COFs and underscores their promising prospects in the field of biology.

Strategies for Enhancing the Photocatalytic and Electrocatalytic Efficiency of Covalent Triazine Frameworks for CO2 Reduction

Converting carbon dioxide (CO2) into fuel and high-value-added chemicals is considered a green and effective way to solve global energy and environmental problems. Covalent triazine frameworks (CTFs) are extensively utilized as an emerging catalyst for photo/electrocatalytic CO2 reduction reaction (CO2RR) recently recognized for their distinctive qualities, including excellent thermal and chemical stability, π-conjugated structure, rich nitrogen content, and a strong affinity for CO2, etc. Nevertheless, single-component CTFs have the problems of accelerated recombination of photoexcited electron-hole pairs and restricted conductivity, which limit their application for photo/electrocatalytic CO2RR. Therefore, emphasis will then summarize the strategies for enhancing the photocatalytic and electrocatalytic efficiency of CTFs for CO2RR in this paper, including atom doping, constructing a heterojunction structure, etc. This review first illustrates the synthesis strategies of CTFs and the advantages of CTFs in the field of photo/electrocatalytic CO2RR. Subsequently, the mechanism of CTF-based materials in photo/electrocatalytic CO2RR is described. Lastly, the challenges and future prospects of CTFs in photo/electrocatalytic CO2RR are addressed, which offers a fresh perspective for the future development of CTFs in photo/electrocatalytic CO2RR.

Single-Atom Catalysts and Dual-Atom Catalysts for CO2 Electroreduction: Competition or Cooperation?

Developing highly active and selective electrocatalysts for electrochemical reduction of CO2 can reduce environmental pollution and mitigation of greenhouse gas emission. Owing to maximal atomic utilization, the atomically dispersed catalysts are broadly adopted in CO2 reduction reaction (CO2 RR). Dual-atom catalysts (DACs), with more flexible active sites, distinct electronic structures, and synergetic interatomic interactions compared to single-atom catalysts (SACs), may have great potential to enhance catalytic performance. Nevertheless, most of the existing electrocatalysts have low activity and selectivity due to their high energy barrier. Herein, 15 electrocatalysts are explored with noble metallic (Cu, Ag, and Au) active sites embedded in metal-organic hybrids (MOHs) for high-performance CO2 RR and studied the relationship between SACs and DACs by first-principles calculation. The results indicated that the DACs have excellent electrocatalytic performance, and the moderate interaction between the single- and dual-atomic center can improve catalytic activity in CO2 RR. Four among the 15 catalysts, including (CuAu), (CuCu), Cu(CuCu), and Cu(CuAu) MOHs inherited a capability of suppressing the competitive hydrogen evolution reaction with favorable CO overpotential. This work not only reveals outstanding candidates for MOHs-based dual-atom CO2 RR electrocatalysts but also provides new theoretical insights into rationally designing 2D metallic electrocatalysts.