Metal-Organic-Framework-Derived Hollow N-Doped Porous Carbon with Ultrahigh Concentrations of Single Zn Atoms for Efficient Carbon Dioxide Conversion

Trifunctional Metal-Organic Framework Catalyst for CO2 Conversion into Cyclic Carbonates

Herein, we reported a facile strategy for the preparation of trifunctional ionic metal-organic frameworks (MOFs) incorporating imidazolium cation functionalities. This strategy exploits the Debus-Radziszewski reaction to create the cationic imidazole ring by postsynthetic modification, meanwhile introducing exchangeable counteranions. On the basis of this strategy, MIL-101-IMOH-Br^- has been synthesized, which combines Lewis acidic sites, Brønsted acidic sites, and nucleophilic centers to achieve catalysis for the carbon dioxide-epoxide cycloaddition into cyclocarbonate without any cocatalyst and solvent.

Cobalt and nitrogen co-doped mesoporous carbon for electrochemical hydrogen peroxide sensing: the effect of graphitization

In this study, a facile strategy for the scalable synthesis of cobalt and nitrogen co-doped mesoporous carbon (Co-N/C) is reported. Structural characterization demonstrated that Co and N were successfully co-doped in the highly porous carbon. Graphitization of porous carbon was achieved by the introduction of cobalt species. The degree of graphitization of Co-N/C could be further promoted by increasing the calcination temperature. By taking advantage of the excellent mass and electron transfer kinetics attributed to the high specific surface area, high porosity and high graphitization, the obtained Co-N/C exhibited good electrochemical activity towards H₂O₂ reduction and excellent sensing performance for the electrochemical detection of H₂O₂. The Co-N/C-950 catalyst obtained at 950 °C showed good electrochemical sensing performance with a detection limit of 2 μM and a wide linear response over the concentration range from 0.03 mM to 13 mM. Meanwhile, Co-N/C exhibited high selectivity toward the detection of H₂O₂ in the presence of possible interferences during the applications such as NaCl, glucose, ascorbic acid and so on. The results confirm that Co-N/C could be used as an efficient electrocatalyst to fabricate electrochemical sensing devices.

Selective Etching Quaternary MAX Phase toward Single Atom Copper Immobilized MXene (Ti3C2Clx) for Efficient CO2 Electroreduction to Methanol

Single atom catalysts possess attractive electrocatalytic activities for various chemical reactions owing to their favorable geometric and electronic structures compared to the bulk counterparts. Herein, we demonstrate an efficient approach to producing single atom copper immobilized MXene for electrocatalytic CO₂ reduction to methanol via selective etching of hybrid A layers (Al and Cu) in quaternary MAX phases (Ti₃(Al_1-xCu_x)C₂) due to the different saturated vapor pressures of Al- and Cu-containing products. After selective etching of Al in the hybrid A layers, Cu atoms are well-preserved and simultaneously immobilized onto the resultant MXene with dominant surface functional group (Cl_x) on the outmost Ti layers (denoted as Ti₃C₂Cl_x) via Cu-O bonds. Consequently, the as-prepared single atom Cu catalyst exhibits a high Faradaic efficiency value of 59.1% to produce CH₃OH and shows good electrocatalytic stability. On the basis of synchrotron-based X-ray absorption spectroscopy analysis and density functional theory calculations, the single atom Cu with unsaturated electronic structure (Cu^δ+, 0 < δ < 2) delivers a low energy barrier for the rate-determining step (conversion of HCOOH* to absorbed CHO* intermediate), which is responsible for the efficient electrocatalytic CO₂ reduction to CH₃OH.

In-situ synthesized porphyrin polymer/TiO2 composites as high-performance Z-scheme photocatalysts for CO2 conversion

The promising photocatalytic conversion of CO₂ into valuable fuel promotes the development of photocatalyst through various methods. In this work, TiO₂ nanoparticle was composited with covalent porphyrin polymers (COP-Ps) to fabricate composite photocatalysts. The resultant COP-Ps/TiO₂ composites by in situ hydrothermal method exhibit much improved photocatalytic activity for the conversion of CO₂ into CO relative to two components, and it is attributable to improved charge transfer between two moieties led by strong interaction. Especially, TiO₂ is composited more evenly with the sulfonated hollow COP-P (sh-COP-P). The resultant composite sh-COP-P/TiO₂ performs best with a CO production rate of 5.70 μmol·g^-1·h^-1, which is approximately 20.4 times as high as that of pure TiO₂ and 2.3 times of sh-COP-P polymer. For comparison, the simple physical mixture of sh-COP-P and TiO₂ (sm-sh-COP-P/TiO₂) was fabricated, and it performs more badly due to poor mixing uniformity. A Z-scheme photocatalytic mechanism was proposed for sh-COP-P/TiO₂ composite on the basis of energy band analysis and hydroxyl radical test. This study provides a new in situ strategy to fabricate organic polymer/metal oxide composites of high photocatalytic activity for CO₂ reduction.

Microwave-Assisted Synthesis of Tris-Anderson Polyoxometalates for Facile CO2 Cycloaddition

Four new tris-Anderson polyoxometalates (POMs), (NH₄)₄[ZnMo₆O₁₈(C₄H₈NO₃)(OH)₃]·4H₂O (1), (NH₄)₄[CuMo₆O₁₈(C₄H₈NO₃)(OH)₃]·4H₂O (2), (TBA)₃(NH₄)[ZnMo₆O₁₇(C₅H₉O₃)₂(OH)]·10H₂O (3) (TBA = n-C₁₆H₃₆N), and (NH₄)₄[CuMo₆O₁₈(C₅H₉O₃)₂]·16H₂O (4), were synthesized by a microwave-assisted method. Single-crystal X-ray diffraction revealed that 1 and 2 contained a tris (trihydroxyl organic compounds) ligand grafted on one side, while two tris ligands were grafted on two sides to form χ/δ and δ/δ isomers in 3 and 4, respectively. ¹H and ¹³C NMR spectra of the χ/δ isomer 3 were obtained for the first time, with six methylenes showing six peaks in the ¹H NMR spectrum and only four peaks in the ¹³C NMR spectrum. Mass spectrometry monitoring revealed that during the microwave-assistant process the tris ligand can graft onto POMs to form 1, while tris directly coordinates with metallic heteroatoms to form isopolymolybdates during the conventional reflux synthesis process. In addition, 1-4 can catalyze CO₂ with epoxides into cyclic carbonates with high selectivity and yields at an atmospheric pressure of CO₂, which is lower than the pressure of CO₂ in other catalysis using POMs as catalysts. Furthermore, 1-4 showed good catalytic stability and cycling properties. Mechanism studies substantiated POMs cocatalyzed with Br^- to improve the catalytic yields.

Rational Fabrication of Low-Coordinate Single-Atom Ni Electrocatalysts by MOFs for Highly Selective CO2 Reduction

Single-atom catalysts (SACs) have attracted tremendous interests due to their ultrahigh activity and selectivity. However, the rational control over coordination microenvironment of SACs remains a grand challenge. Herein, a post-synthetic metal substitution (PSMS) strategy has been developed to fabricate single-atom Ni catalysts with different N coordination numbers (denoted Ni-N_x -C) on pre-designed N-doped carbon derived from metal-organic frameworks. When served for CO₂ electroreduction, the obtained Ni-N₃ -C catalyst achieves CO Faradaic efficiency (FE) up to 95.6 %, much superior to that of Ni-N₄ -C. Theoretical calculations reveal that the lower Ni coordination number in Ni-N₃ -C can significantly enhance COOH* formation, thereby accelerating CO₂ reduction. In addition, Ni-N₃ -C shows excellent performance in Zn-CO₂ battery with ultrahigh CO FE and excellent stability. This work opens up a new and general avenue to coordination microenvironment modulation (MEM) of SACs for CO₂ utilization.

Single-Atom Iron and Doped Sulfur Improve the Catalysis of Polysulfide Conversion for Obtaining High-Performance Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are regarded as promising secondary energy storage devices for their high energy density and low cost. The electrochemical performance of Li-S batteries is mainly determined by the efficient and reversible conversion of lithium-polysulfides to Li₂S when discharging and to S when charging. Herein, a catalytic strategy is proposed to accelerate the reversible conversion of S and the discharge products in Li-S batteries. This reversible transformation is achieved with active sites of single-atom iron on nitrogen- and sulfur-doped porous carbon (FeNSC). We prove that the synergy between atomically dispersed iron and doped sulfur accelerates the reversible electrochemical conversion reactions in Li-S batteries. The FeNSC/S hybrid cathode exhibits superior long-term cycling stability even at a high current density of 1C, with only 0.047% capacity decay per cycle over 1000 cycles. This study demonstrates a novel method for improving the conversion of polysulfides based on electrocatalysis strategies to ultimately obtain high-performance Li-S batteries.

MOFs/PVA hybrid membranes with enhanced mechanical and ion-conductive properties

Nanomaterials have been treated as effective dopants for enhancing mechanical and ion-conductive properties of polymer membranes. Among various nanomaterials, metal–organic frameworks are attracting enormous attention from researchers because of their intriguing structural and functional properties. Here we report a gentle and simple synthesis method of ZIF-8 nanomaterials, which are applied as dopants for polyvinyl alcohol composite membranes. This nanomaterials display uniform size distribution and high purity through various structural investigations. The as-prepared polymer composite membranes present enhanced mechanical and ion-conductive properties compared to pristine samples. This work provides a novel ideal on the design of nanomaterial dopants for high-performance polymer membranes.

Rational design of the Z-scheme hollow-structure Co9S8/g-C3N4 as an efficient visible-light photocatalyst for tetracycline degradation

The development of photocatalysts with high catalytic activity that are capable of full utilization of solar energy is a challenge in the field of photocatalysis. Accordingly, in the present study, an efficient Z-scheme cage-structured Co₉S₈/g-C₃N₄ (c-CSCN) photocatalyst was constructed for the degradation of tetracycline antibiotics under visible-light irradiation. The Z-scheme charge-transfer mechanism accelerates the separation of photogenerated charge carriers and effectively improves photocatalytic activity. Moreover, c-CSCN has a hollow structure, allowing light to be reflected multiple times inside the cavity, thereby effectively improving the utilisation efficiency of solar energy. As a result, the photocatalytic activity of c-CSCN is 1.5-, 2.5-, and 5.8-times higher than those of sheet-type Co₉S₈/g-C₃N₄ (s-CSCN), c-Co₉S₈, and g-C₃N₄, respectively, for the degradation of tetracycline. c-CSCN maintains favourable photocatalytic activity over five consecutive degradation cycles, demonstrating its excellent stability. In addition, c-CSCN performs efficient tetracycline removal in different water substrates. Moreover, c-CSCN exhibits excellent ability to remove tetracycline under direct natural sunlight. This work fully demonstrates that c-CSCN has high catalytic activity and the potential for practical application as a wastewater treatment material.

Transition Metal and Nitrogen Co-Doped Carbon-based Electrocatalysts for the Oxygen Reduction Reaction: From Active Site Insights to the Rational Design of Precursors and Structures

Considering the urgent requirement for clean and sustainable energy, fuel cells and metal-air batteries have emerged as promising energy storage and conversion devices to alleviate the worldwide energy challenges. The key step in accelerating the sluggish oxygen reduction reaction (ORR) kinetics at the cathode is to develop cost-effective and high-efficiency non-precious metal catalysts, which can be used to replace expensive Pt-based catalysts. Recently, the transition metal and nitrogen co-doped carbon (M-N_x /C) materials with tailored morphology, tunable composition, and confined structure show great potential in both acidic and alkaline media. Herein, the mechanism of ORR is provided, followed by recent efforts to clarify the actual structures of active sites. Furthermore, the progress of optimizing the catalytic performance of M-N_x /C catalysts by modulating nitrogen-rich precursors and porous structure engineering is highlighted. The remaining challenges and development prospects of M-N_x /C catalysts are also outlined and evaluated.

Conversion of CO2 into cyclic carbonate catalyzed by an N-doped mesoporous carbon catalyst

Ammonium hydroxide is first used as a nitrogen source to synthesize N-doped mesoporous carbon spheres (N-MCSs). Using N-MCS800 as a catalyst, the TOF of the cycloaddition of CO2 with epichlorohydrin is 236 h−1.

Aminoethylimidazole ionic liquid-grafted MIL-101-NH2 heterogeneous catalyst for the conversion of CO2 and epoxide without solvent and cocatalyst

An aminoethylimidazole IL-functionalized MIL-101-NHIM-NH₂ catalyst efficiently catalyzes the cycloaddition reaction of CO₂ and epoxide without solvent and cocatalyst, owing to the synergistic effects of Cr, –NH₂ and Br⁻ active sites.

Design of Binary Cu-Fe Sites Coordinated with Nitrogen Dispersed in the Porous Carbon for Synergistic CO2 Electroreduction

To relieve the green gas emission and involve the carbon neutral cycle, electrochemical reduction of CO₂ attracts more and more attention. Herein, a biatomic site catalyst of Cu-Fe coordinated with the nitrogen, which is doped in the carbon matrix (denoted as Cu-Fe-N₆ -C), is designed. The as-obtained Cu-Fe-N₆ -C exhibits higher performance than that of Cu-N-C and Fe-N-C, owing to bimetallic sites, proving synergistic functions based on different molecules and their interfaces. Cu-Fe-N₆ -C shows high selectivity toward CO, with high Faradaic efficiency (98% at -0.7 V), and maintaining 98% of its initial selectivity after 10 h electrolysis. The experimental results and theoretical calculations reveal that the synergistic catalysis of different metallic sites enlarges the adsorption enthalpy of CO₂ , reducing the activation energy result in generating high selectivity, activity, stability, and low impedance.

Hollow Carbon-Based Nanoarchitectures Based on ZIF: Inward/Outward Contraction Mechanism and Beyond

Hollow carbon-based nanoarchitectures (HCAs) derived from zeolitic imidazolate frameworks (ZIFs), by virtue of their controllable morphology and dimension, high specific surface area and nitrogen content, richness of metal/metal compounds active sites, and hierarchical pore structure and easy exposure of active sites, have attracted great interests in many fields of applications, especially in heterogeneous catalysis, and electrochemical energy storage and conversion. Despite various approaches that have been developed to prepare ZIF-derived HCAs, the hollowing mechanism has not been clearly disclosed. Herein, a specialized overview of the recent progress of ZIF-derived HCAs is introduced to provide an insight into their preparation strategy and the corresponding hollowing mechanisms. Based on the fundamental understanding of the structural evolution of ZIF nanocrystals during the high-temperature pyrolysis process, the hollowing mechanisms of ZIF-derived HCAs are classified into four categories: i) inward contraction of core-shell template@ZIF composites or hollow ZIFs, ii) outward contraction of ZIF@shell composites, iii) special outward contraction of ZIF arrays, and iv) mechanism beyond inward/outward contraction of pure ZIF nanocrystals. Finally, an outlook on the development prospects and challenges of HCAs based on ZIF precursors, especially in terms of controlled synthesis and future electrochemical application, is further discussed.

Zinc oxide rod/peanut shell-derived porous carbon composites for cooperative CO2 chemical fixation

A ZnO/biowaste-derived porous carbon composite exhibits admirable activity and selectivity in the cycloaddition of epoxides with CO₂ under mild conditions.

High performance cobalt nanoparticle catalysts supported by carbon for ozone decomposition: the effects of the cobalt particle size and hydrophobic carbon support

Catalytic gaseous ozone decomposition under high humidity is not only an urgent need but also a significant challenge because of the low stability over the available catalysts.

Rational Design of a ZnII MOF with Multiple Functional Sites for Highly Efficient Fixation of CO2 under Mild Conditions: Combined Experimental and Theoretical Investigation

The development of efficient heterogeneous catalysts suitable for carbon capture and utilization (CCU) under mild conditions is a promising step towards mitigating the growing concentration of CO₂ in the atmosphere. Herein, we report the construction of a hydrogen-bonded 3D framework, {[Zn(hfipbba)(MA)]⋅3 DMF}_n (hfipbba=4,4'-(hexaflouroisopropylene)bis(benzoic acid)) (HbMOF1) utilizing Zn^II center, a partially fluorinated, long-chain dicarboxylate ligand (hfipbba), and an amine-rich melamine (MA) co-ligand. Interestingly, the framework possesses two types of 1D channels decorated with CO₂ -philic (-NH₂ and -CF₃ ) groups that promote the highly selective CO₂ adsorption by the framework, which was supported by computational simulations. Further, the synergistic involvement of both Lewis acidic and basic sites exposed in the confined 1D channels along with high thermal and chemical stability rendered HbMOF1 a good heterogeneous catalyst for the highly efficient fixation of CO₂ in a reaction with terminal/internal epoxides at mild conditions (RT and 1 bar CO₂ ). Moreover, in-depth theoretical studies were carried out using periodic DFT to obtain the relative energies for each stage involved in the catalytic reaction and an insight mechanistic details of the reaction is presented. Overall, this work represents a rare demonstration of rational design of a porous Zn^II MOF incorporating multiple functional sites suitable for highly efficient fixation of CO₂ with terminal/internal epoxides at mild conditions supported by comprehensive theoretical studies.

Achieving Near-Unity CO Selectivity for CO2 Electroreduction on an Iron-Decorated Carbon Material

A straightforward procedure has been developed to prepare a porous carbon material decorated with iron by direct pyrolysis of a mixture of a porous polymer and iron chloride. Characterization of the material with X-ray diffraction, X-ray absorption spectroscopy, and electron microscopy indicates the presence of iron carbide nanoparticles encapsulated inside the carbon matrix, and elemental mapping and cyanide poisoning experiments demonstrate the presence of atomic Fe centers, albeit in trace amounts, which are active sites for electrochemical CO₂ reduction. The encapsulated iron carbide nanoparticles are found to boost the catalytic activity of atomic Fe sites in the outer carbon layers, rendering the material highly active and selective for CO₂ reduction, although these atomic Fe sites are only present in trace amounts. The target material exhibits near-unity selectivity (98 %) for CO₂ -to-CO conversion at a small overpotential (410 mV) in water. Furthermore, the material holds potential for practical application, as a current density over 30 mA cm^-2 and a selectivity of 93 % can be achieved in a flow cell.

Preparation of MOF Film/Aerogel Composite Catalysts via Substrate-Seeding Secondary-Growth for the Oxygen Evolution Reaction and CO2 Cycloaddition

Substrate-supported metal-organic frameworks (MOFs) films are desired to realize their potential in practical applications. Herein, a novel substrate-seeding secondary-growth strategy is developed to prepare composites of uniform MOFs films on aerogel walls. Briefly, the organic ligand is "pre-seeded" onto the aerogel walls, and then a small amount of metal-ion solution is sprayed onto the prepared aerogel. The sprayed solution diffuses along the aerogel walls to form a continuous thin layer, which confines the nucleation reaction, promoting the formation of uniform MOFs films on the aerogel walls. The whole process is simple in operation, highly efficient, and eco-friendly. The resulting hierarchical MOFs/aerogel composites have abundant accessible active sites and enable excellent mass transfer, which endows the composite with outstanding catalytic activity and stability in both liquid-phase CO₂ cycloaddition and electrochemical oxygen evolution reaction (OER) process.

Evolution of Zn(II) single atom catalyst sites during the pyrolysis-induced transformation of ZIF-8 to N-doped carbons

The pyrolysis of zeolitic imidazolate frameworks (ZIFs) is becoming a popular approach for the synthesis of catalysts comprising porphyrin-like metal single atom catalysts (SACs) on N-doped carbons (M-N-C). Understanding the structural evolution of M-N-C as a function of ZIF pyrolysis temperature is important for realizing high performance catalysts. Herein, we report a detailed investigation of the evolution of Zn single atom catalyst sites during the pyrolysis of ZIF-8 at temperatures ranging from 500 to 900 °C. Results from Zn L-edge and Zn K-edge X-ray absorption spectroscopy studies reveal that tetrahedral ZnN₄ centers in ZIF-8 transform to porphyrin-like ZnN₄ centers supported on N-doped carbon at temperatures as low as 600 °C. As the pyrolysis temperature increased in the range 600-900 °C, the Zn atoms moved closer to the N₄ coordination plane. This subtle geometry change in the ZnN₄ sites alters the electron density on the Zn atoms (formally Zn²⁺), strongly impacting the catalytic performance for the peroxidase-like decomposition of H₂O₂. The catalyst obtained at 800 °C (Zn-N-C-800) offered the best performance for H₂O₂ decomposition. This work provides valuable new insights about the evolution of porphyrin-like single metal sites on N-doped carbons from ZIF precursors and the factors influencing SAC activity.

The synthetic strategies for single atomic site catalysts based on metal-organic frameworks

Metal-organic frameworks (MOFs) are a good platform for the fabrication of single atomic site catalysts (SACs) due to their large specific surface area, rich pore structure, large number of unsaturated coordination metal sites and their intriguing and controllable structures. The influencing factors of each strategy used to synthesize SACs based on MOFs, such as the finetuning ligand strategy, heteroatom doping (N, P, S) strategy, space restriction strategy, bimetallic strategy, metal cluster defect strategy, substrate to capture strategy, and various post-treatment strategies have not been discussed. Here, we will discuss the influencing factors of each strategy and the relationship between the different methods, which are used to synthesize SACs based on MOFs.

Interfacial Engineering of Bimetallic Carbide and Cobalt Encapsulated in Nitrogen-Doped Carbon Nanotubes for Electrocatalytic Oxygen Reduction

Heterojunction engineering is a fundamental strategy to develop efficient electrocatalysts for the oxygen reduction reaction by tuning electronic properties through interfacial cooperation. In this study, a heterojunction electrocatalyst consisting of bimetallic carbide Co₃ ZnC and cobalt encapsulated within N-doped carbon nanotubes (Co₃ ZnC/Co@NCNTs) is synthesized by a facile two-step ion exchange-thermolysis pathway. Co₃ ZnC/Co@NCNTs effectively promotes interfacial charge transport between the different components with optimizes adsorption and desorption of intermediate products at the heterointerface. In situ-grown N-doped carbon nanotubes (NCNTs) not only improve the electrical conductivity but also suppress the oxidation of transition metal nanoparticles in alkaline media. Moreover, the abundant nitrogen types (pyridinic N, Co-N_x , and graphitic nitrogen) in the carbon skeleton provide more active sites for oxygen adsorption. Benefitting from this optimized structure, Co₃ ZnC/Co@NCNTs hybrid not only demonstrates excellent oxygen reduction activity, with a half-wave potential of 0.83 V and fast mass transport with limited current density of 6.23 mA cm^-2 , but also exhibits superior stability and methanol tolerance, which surpass those of commercial Pt/C catalysts. This work provides an effective heterostructure for interfacial electronic modulation to improve electrocatalytic performance.

Hollow Carbon Sphere Nanoreactors Loaded with PdCu Nanoparticles: Void-Confinement Effects in Liquid-Phase Hydrogenations

Nanoreactors with hollow structures have attracted great interest in catalysis research due to their void-confinement effects. However, the challenge in unambiguously unraveling these confinement effects is to decouple them from other factors affecting catalysis. Here, we synthesize a pair of hollow carbon sphere (HCS) nanoreactors with presynthesized PdCu nanoparticles encapsulated inside of HCS (PdCu@HCS) and supported outside of HCS (PdCu/HCS), respectively, while keeping other structural features the same. Based on the two comparative nanoreactors, void-confinement effects in liquid-phase hydrogenation are investigated in a two-chamber reactor. It is found that hydrogenations over PdCu@HCS are shape-selective catalysis, can be accelerated (accumulation of reactants), decelerated (mass transfer limitation), and even inhibited (molecular-sieving effect); conversion of the intermediate in the void space can be further promoted. Using this principle, a specific imine is selectively produced. This work provides a proof of concept for fundamental catalytic action of the hollow nanoreactors.

Design of a Single-Atom Indiumδ+ -N4 Interface for Efficient Electroreduction of CO2 to Formate

Main-group element indium (In) is a promising electrocatalyst which triggers CO₂ reduction to formate, while the high overpotential and low Faradaic efficiency (FE) hinder its practical application. Herein, we rationally design a new In single-atom catalyst containing exclusive isolated In^δ+ -N₄ atomic interface sites for CO₂ electroreduction to formate with high efficiency. This catalyst exhibits an extremely large turnover frequency (TOF) up to 12500 h^-1 at -0.95 V versus the reversible hydrogen electrode (RHE), with a FE for formate of 96 % and current density of 8.87 mA cm^-2 at low potential of -0.65 V versus RHE. Our findings present a feasible strategy for the accurate regulation of main-group indium catalysts for CO₂ reduction at atomic scale.

Hollow Mesoporous Carbon Sphere Loaded Ni-N4 Single-Atom: Support Structure Study for CO2 Electrocatalytic Reduction Catalyst

Single-atom catalysts have become a hot spot because of the high atom utilization efficiency and excellent activity. However, the effect of the support structure in the single-atom catalyst is often unnoticed in the catalytic process. Herein, a series of carbon spheres supported Ni-N₄ single-atom catalysts with different support structures are successfully synthesized by the fine adjustment of synthetic conditions. The hollow mesoporous carbon spheres supported Ni-N₄ catalyst (Ni/HMCS-3-800) exhibits superior catalytic activity toward the electrocatalytic CO₂ reduction reaction (CO₂ RR). The Faradaic efficiency toward CO is high to 95% at the potential range from -0.7 to -1.1 V versus reversible hydrogen electrode and the turnover frequency value is high up to 15 608 h^-1 . More importantly, the effect of the geometrical structures of carbon support on the CO₂ RR performance is studied intensively. The shell thickness and compactness of carbon spheres regulate the chemical environment of the doped-N species in the carbon skeleton effectively and promote CO₂ molecule activation. Additionally, the optimized mesopore size is beneficial to improve diffusion and overflow of the substance, which enhances the CO₂ adsorption capacity greatly. This work provides a new consideration for promoting the catalytic performance of single-atom catalysts.