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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2025 , Vol. 37 >Issue 2: 173 - 184

DOI: https://doi.org/10.7536/PC240409

Review

Efficient and Stable Metal Macrocyclic Molecular Catalyst for Electrocatalytic Reduction of CO2 to CO

  • Guilong Wang 1, 3 ,
  • Shanhe Gong 1, 3 ,
  • Mengxian Li 2 ,
  • Jun Liu 1 ,
  • Xiaomeng Lv , 3, *
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  • 1 School of Environmental and Safety Engineering, Jiangsu University, Zhenjiang 212013, China
  • 2 School of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China
  • 3 School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China
* email:

Received date: 2024-04-07

  Revised date: 2024-09-01

  Online published: 2025-02-07

Supported by

Development Project of Zhenjiang(GY2021004)

Innovation and Practice Fund for Industrial Centers of Jiangsu University(ZXJG2022007)

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Abstract

Electrocatalytic reduction of CO2 into value-added chemicals has been a research hotspot in recent years, among which electrocatalytic conversion of CO2 to CO is an industrial-related potential route. Among the electrocatalysts, metal macrocyclic molecular catalysts have attracted much attention due to their functional structure diversity, high conjugation structure, high chemical stability and great potential in electrochemical research. Herein, this paper reviews and introduces several main metal macrocyclic molecular catalysts, related reaction mechanisms and development progress. As to the problems of their low electrical conductivity and instability under long-term operation, the main strategies of heterogeneous systems on catalytic activity and stability were thoroughly discussed, including the introduction of the conductive carrier with high surface areas via non-covalence or covalence connection, building the polycondensation/ polymerization or COF skeleton structure, and modification of functional group with different effect. Finally, the challenges of catalytic activity and stability were analyzed and solving strategies were proposed, focusing on heterogeneous catalysts design, optimization of electrolyzer, and machine learning.

Contents

1 Introduction

2 Development history of metal macrocyclic molecular catalysts for electrocatalytic CO2 reduction

3 Research on metal macrocyclic molecular catalysts and related catalytic mechanism

4 Regulation of the activity and stability of CO2RR electrocatalyzed by metal macrocyclic molecular catalysts

4.1 Immobilization of a conductive carrier with a high surface area

4.2 Periodic skeleton structure formation

4.3 Combination with functional groups

5 Conclusion and prospect

Key words： macrocyclic molecular catalyst; electrocatalysis; carbon dioxide; stability; activity

Cite this article

Guilong Wang , Shanhe Gong , Mengxian Li , Jun Liu , Xiaomeng Lv . Efficient and Stable Metal Macrocyclic Molecular Catalyst for Electrocatalytic Reduction of CO2 to CO[J]. Progress in Chemistry, 2025 , 37(2) : 173 -184 . DOI: 10.7536/PC240409

1 Introduction

Carbon dioxide (CO2) is an essential substance in the natural carbon cycle, but the extensive combustion of fossil fuels after the Industrial Revolution has led to the large-scale emission of CO2, and the significant increase in atmospheric CO2 concentration has triggered a series of environmental problems that cannot be balanced by natural photosynthesis alone[1-2]. As an inexpensive, abundant, and non-toxic carbon source, CO2 can be converted into value-added chemicals and fuels powered by renewable energy sources such as wind, solar, or tidal energy, which not only helps to reduce CO2 emissions but also stores intermittent energy in chemical bonds[3]. In recent decades, multiple pathways for CO2 reduction reaction (CO2RR) have been developed, including thermochemical reduction[4], biological conversion[5], electrocatalytic reduction[6], photocatalytic reduction[7], etc. Among them, the electrocatalytic reduction method has been widely developed. Electrochemical CO2RR has the following advantages: (1) relatively mild operation, usually performed at ambient temperature and pressure; (2) different reduction products can be obtained through simple potential control; (3) great industrialization potential and suitable for large-scale applications, and its products can meet part of the fuel demand; (4) electricity as power can be obtained from renewable energy. However, CO2 is chemically inert and thermodynamically extremely stable with a standard enthalpy of formation of −393.5 kJ·mol−1, so CO2RR is generally thermodynamically unfavorable and requires external energy input[8]. Meanwhile, the reduction potential of CO2RR in aqueous systems is close to the hydrogen evolution reaction (HER), and the side reaction HER will be inevitable and more thermodynamically favorable than CO2RR, which will reduce the selectivity of CO2RR[9]. Therefore, designing an electrocatalyst that can effectively lower the energy barrier of CO2RR and possesses excellent selectivity and stability is a necessary condition for industrial application.
Integrating thermodynamics and reaction kinetics, the reaction of selectively reducing CO2 to carbon monoxide (CO) is more likely to occur compared to multi-carbon products. Moreover, CO itself, not only serves as an important chemical raw material for synthesizing various organic compounds such as plastics, pharmaceuticals, and other commodities[10], but also acts as a significant chemical intermediate that can be further converted into other fuels and chemicals. For instance, liquid fuels can be produced through Fischer-Tropsch synthesis[11]. Therefore, exploring efficient and stable electrocatalysts to achieve the transformation from CO2 to CO is crucial for promoting the circular economy of carbon, which is beneficial for advancing the realization of "carbon peak and carbon neutrality" goals, thereby alleviating the greenhouse effect (Figure 1)[12].
图1 可再生能源驱动的零碳网络[12]

Fig. 1 Zero-carbon network powered by renewable energy[12] Copyright 2022, Royal Society of Chemistry

2 Development of Electrocatalytic CO2 Reduction by Metal Macrocycle Molecular Catalysts

In the 1930s, researchers discovered the role of metal macrocyclic compounds in molecular recognition, which made macrocyclic compounds highly significant in drug development and supramolecular chemistry. In 1964, Jasinski[13] first discovered that cobalt phthalocyanine (CoPc) has the catalytic ability for electrochemical oxygen reduction, opening the door to using transition metal macrocyclic compounds to catalyze redox reactions. Various phthalocyanines and porphyrin metal macrocyclic compounds have since been extensively studied by researchers. In 1974, Meshitsuka et al.[14] were the first to explore the application of transition metal macrocyclic compounds in electrocatalytic CO2 RR. In saturated CO2 aqueous solutions, phthalocyanine (CoPc or NiPc) was adsorbed onto graphite electrodes, and polarization curves of cobalt phthalocyanine or nickel phthalocyanine were observed, though no definitive reduction products were identified at the time. In 1984, Lieber et al.[15] experimentally showed that CoPc adsorbed on carbon cloth had a CO selectivity of 50%, but in citrate buffer, the total current density (jtotal) at an overpotential of 0.5 V was only about 1 mA·cm−2. To further enhance the current density in the material's electrocatalytic process, reduce the reaction overpotential, and improve product selectivity and stability, researchers have explored metal macrocyclic molecular catalysts in the direction of electrocatalytic CO2 reduction around heterogeneous catalysis, polymerization reactions, ligand modifications, etc., driving rapid progress in this field (Figure 2). In 2017, Li Yanguang’s team from Soochow University[16] prepared a composite material of carbon nanotube-supported poly-cobalt phthalocyanine (CoPPc) using an in-situ polymerization method. By leveraging the high conductivity and large specific surface area of carbon nanotubes, the synergistic effect generated after combining CoPPc with carbon nanotubes significantly improved the conductivity and number of electrocatalytic active sites of CoPPc and accelerated the kinetics of electrocatalytic CO2 reduction, thereby achieving better electrocatalytic performance. Additionally, the template-directed polymerization greatly enhanced the physicochemical robustness of CoPPc. In 2019, Berlinguette from the University of British Columbia in Canada and Robert from Paris University in France[17] immobilized commercial CoPc on gas diffusion electrodes, and in a flow cell using CO2 gas feed, achieved efficient activity (CO Faradaic efficiency (FECO) >95% at 150 mA·cm−2) and stability (maintained for over 100 hours at 50 mA·cm−2). In 2021, Yongye Liang’s research group from Southern University of Science and Technology[18] improved the electrode activity of cobalt tetraphenylporphyrin (CoTPP) and cobalt triphenylcorrole (CoTPC) loaded on carbon nanotubes (CNT) by optimizing the molecular loading drop-dry method and poly(4-vinylpyridine) bridging method, achieving FECO of approximately 80%. In 2022, Min Liu’s team from Central South University[19] developed a carbon nanotube-supported poly-nickel phthalocyanine (NiPPc/CNT). By designing and synthesizing NiPPc with extended conjugated structures, they adjusted the electron density of the Ni active sites, enhancing the adsorption and activation of CO2, demonstrating a new method for modulating the electron density of catalytic sites in macrocyclic molecular catalysts. In 2023, Gao Feng Zeng and Qing Xu’s team from the Chinese Academy of Sciences[20] constructed COFs with amine bonds and ionic frameworks, respectively enhancing the binding ability of CO2 molecules and framework conductivity, achieving high activity and selectivity in CO2 RR, providing a new synthetic strategy for developing covalent organic frameworks for CO2 reduction reactions.
图2 过渡金属大环分子催化剂高效稳定地实现CO2转化成CO的发展进程[13⇓⇓⇓⇓⇓⇓-20]

Fig. 2 Development of efficient and stable conversion of CO2 to CO using transition metal macrocyclic catalysts[13⇓⇓⇓⇓⇓⇓-20]

3 Investigation of Metal Macrocyclic Molecular Catalysts and Their Catalytic Mechanisms

Currently, in the research on the electrocatalytic CO2 RR with CO as the reduction product, a variety of catalysts have been developed, including noble metal catalysts[21], metal macrocyclic molecular catalysts[22], single-atom catalysts[23], and carbon-based catalysts[24], all of which have achieved efficient and stable CO2 RR. The research group previously constructed silver-based aerogel systems with abundant porous structures[25-26], nickel single-atom systems[27], nitrogen-doped carbon nanohollow spheres loaded with phthalocyanine cobalt and polyphthalocyanine cobalt systems[28-29], etc., and has conducted extensive fundamental research on the electrocatalytic conversion of CO2 RR to CO. It was found that metal macrocyclic molecular catalysts, such as porphyrin (Por)[30], phthalocyanine (Pc)[31], covalent organic frameworks (COFs)[32], etc., possess well-defined molecular centers (M-N4 units), which are beneficial for elucidating mechanisms at the molecular level and studying catalyst reactivity through kinetics. In addition, the d orbitals of the metal centers in transition metal macrocycles exhibit multivalent redox capabilities, thereby promoting the transition metal macrocycles to possess high redox activity and providing open binding sites to interact with small molecule CO2, achieving the electrocatalytic CO2 reduction reaction.
Porphyrin compounds are derivatives formed when the meso-positions and β-positions on the porphin ring are substituted by other functional groups, and porphin (Figure 3a1) consists of four pyrroles connected by four methine bridges, forming an 18-electron closed and continuous conjugated system. The four positions on the ring at 5, 10, 15, and 20 are the meso-positions, and the remaining eight carbon atom positions with hydrogen atoms are called β-positions, where these hydrogen atoms can be substituted by other groups through chemical reactions. When two hydrogen atoms on the nitrogen atoms of the porphyrin ring are replaced by a metal, it forms metalloporphyrin, such as iron porphyrin catalyst which is a commonly used porphyrin-based catalyst (Figure 3a2)[33]. Phthalocyanine is a compound with an 18-electron conjugated system composed of four isoindole units (Figure 3b1), whose large-ring conjugated system exhibits strong coordination ability and can undergo coordination reactions with many metals to form different types of metal complexes, known as metal phthalocyanines (Figure 3b2). Metal phthalocyanines have a large planar conjugated system, which can promote various types of catalytic reactions to occur axially on the metal phthalocyanine molecules and exhibit excellent catalytic activity. They are now widely used in catalytic materials for organic synthesis catalysis, electrocatalysis, and electrode catalysts for new batteries. COFs were discovered by Yaghi et al.[34] in 2005 as a class of porous crystalline organic materials, consisting of organic building blocks connected by covalent bonds to form periodic structures of porous skeletons with strong covalent forces between the skeletons. COFs have abundant active unit centers, adjustable pore sizes, large specific surface areas, highly ordered pore structures, ease of functional modification, and diverse synthesis methods. The research group of Lan Yaqian at South China Normal University[35] designed a series of structurally stable dioxin (TFPN)-linked metal phthalocyanine covalent organic frameworks (MPc-TFPN COF, M=Ni, Co, Zn) (Figure 3c). The crystalline and porous nature of COF catalysts effectively solves the problems faced by traditional catalysts during the reaction process, such as difficult product separation and difficulty in reusing the catalyst. Their role in catalysis cannot be ignored, and they are expected to become promising materials in the field of heterogeneous catalyst research.
图3 （a1）卟吩的结构和（a2）5，10，15，20-四（2，6-二羟基)苯基铁卟啉[33]；（b1）酞菁与（b2）金属酞菁的结构；（c）通过MPc-8OH和TFPN缩合合成MPc-TFPN共价有机框架[35]

Fig. 3 (a1) Structures of Porphin and (a2) 5,10,15,20-tetra (2,6-dihydroxyl) phenyl iron porphyrin[33]. (b1) Structures of phthalocyanine and (b2) metal phthalocyanine. (c) Synthesis of MPc-TFPN covalent organic framework by condensation of MPc-8OH and TFPN[35]

When metal macrocyclic molecular catalysts are used for CO2 RR, they involve a series of complex electron transfer steps. The active sites on the catalyst surface adsorb CO2 molecules, and then achieve their reduction through stepwise proton-coupled electron transfer (PCET) steps (Fig. 4). An in-depth analysis of these steps helps to reveal the reaction mechanism of the entire CO2 RR[36]. For metal macrocyclic molecular catalysts, the central metal acts as the main medium for electron transfer and usually plays a key role. In the process of CO2 RR converting to CO, electrons start from the electrode surface, pass through the metal center, and reach the adsorbed CO2 molecule. This process involves multiple intermediates and free energy barriers, where *COOH and *CO are considered key intermediates. In the gradual electron transfer, the formation of the *COOH intermediate is the initial step, which further converts into the *CO intermediate. The latter undergoes another electron transfer and proton release, ultimately generating gaseous CO products. The specific reaction process will be influenced by three aspects: (1) Different central metals have different electronic structures, leading to varying interaction forces with reaction intermediates, causing changes in the activity and stability of the metal macrocyclic molecules. For example, among MePc (Me=Mn, Fe, Co, Ni, Cu) with a clear Me-N4 structure, CoPc has the optimal activity for forming CO because the *CO binding energy on the Co site is moderate, affecting the key reaction steps of *COOH formation and *CO desorption, which benefits the overall reaction thermodynamics[37]. (2) The properties of the heterogeneous catalyst support materials also significantly affect electron transfer. For instance, when large metal phthalocyanine molecules are bound to carbon carriers such as carbon nanotubes or graphene with high conductivity and suitable electronic structure, the carrier can promote electron transfer to CO2 molecules through π-π stacking interactions while stabilizing intermediates, thereby improving the overall catalytic efficiency[38]. (3) Functional group modifications of the macrocyclic molecular skeleton can also modulate the electronic structure of the metal center, affecting the interaction between the metal center and reaction intermediates. Therefore, by optimizing synthetic design, modulating the electronic structure of the metal center, and optimizing the interaction between the site and reactants and reaction intermediates, it is expected to greatly enhance the performance of CO2 RR, providing new avenues for designing efficient catalysts that convert CO2 into valuable chemicals.
图4 电催化CO2生成CO的路径图

Fig. 4 Path diagram of electrocatalytic CO2 to CO

This article reviews the key factors affecting the activity and stability of materials in terms of the immobilization methods and functional group modifications, based on the mechanism of electrocatalytic CO2 RR by metal macrocyclic molecules. Firstly, it briefly introduces the development process of metal macrocyclic molecular catalysts for electrocatalytic conversion of CO2 to CO; Secondly, focusing on optimizing the fixation methods of heterogeneous molecular catalysts and the impact of metal active centers and exocyclic functionalization on site activity, it summarizes recent strategies for improving the activity and stability of metal macrocyclic compounds; Finally, combined with the electrocatalytic mechanism, it analyzes the opportunities and challenges for the industrial prospects of metal macrocyclic molecule electrocatalytic CO2 RR.

4 Activity and Stability Regulation of Electrocatalytic CO2 RR by Metal Macrocyclic Molecular Catalysts

In recent years, the research and development of metal macrocyclic molecular catalysts (Pc, Por, COFs, etc.) have continued to heat up, focusing on addressing the poor conductivity and stability issues of homogeneous metal macrocyclic molecular catalysts during prolonged electrochemical operation, with the aim of regulating and optimizing their electrocatalytic activity and stability (Table 1). The main design and regulation methods include: (1) introducing high surface area conductive carriers through non-covalent or covalent bonding; (2) constructing polymeric backbone structures via condensation/polymerization reactions and synthesizing one-dimensional to three-dimensional COF framework structures; (3) modifying the active macrocyclic molecules with exocyclic functional groups.
表1 金属大环分子催化剂优化调控方式的异同分析归纳表

Table 1 Analytical summary of similarities and differences in the optimized modulation of metal macrocyclic molecular catalysts

Optimizing regulation Introduction of conductive carriers with high surface area Periodic fixation techniques Introduction of functional groups
Similarity To regulate and optimize the electrocatalytic activity and stability of metal macrocyclic molecular catalysts.
Differences Non-covalent connection:
The advantage is that the pre-processing is simple, which is conducive for rapid screening.
The limitation is that the interaction depends on the nature of the carrier, which is easy to self-polymerization, and the molecule is easy to fall off due to weak interaction.
Covalent bonding:
The advantage is enhanced dispersion and stability, increased activity and utilization.
The limitation is low loading.		 The polymer frame structure was constructed to provide the active site, improve the overall activity and stabilize the electrocatalytic performance, which is expected to meet the long-term stability test. Reasonable introduction of functional groups to regulate physical and chemical properties and microstructure, through induction, stereoscopic, electrostatic, hydrogen bonding effects. For example, Electron-donating groups change the coordination environment, while electron-withdrawing groups promote metal-ion reduction.

4.1 Introduction of High Surface Area Conductive Supports

Macrocyclic metal molecular catalysts are widely used in the field of electrocatalysis, but it is difficult to maintain the macrocyclic conjugated structure during long-term electrocatalytic reactions; besides, the non-conductive nature of the macrocyclic molecules themselves hinders electron transfer[39]. Current research attempts to combine the catalysts with conductive substrates (such as graphene (G)[40], carbon aerogel (CA)[41], carbon nanotubes (CNTs)[42]) through physical adsorption or chemical bonding (Figure 5), fixing the macrocyclic molecules and utilizing the high surface area and conductive properties of the substrate to stabilize and enhance the CO2 RR activity and stability of the catalysts (Table 2).
图5 金属大环分子催化剂引入高表面积导电载体的方式

Fig. 5 Metallic macrocyclic molecular catalysts introduced by way of conductive carriers with high surface area

表2 通过非共价和共价键方式固载的多相金属大环分子催化剂的比较[45⇓⇓⇓⇓-50]

Table 2 Comparison of heterogeneous metal macrocyclic catalysts immobilized via non-covalent and covalent bonding[45⇓⇓⇓⇓-50]

Catalyst Cell type Electrolyte jco（mA·cm−2） FECO（max） （%） Stability Ref.
CoPc/GDY/G H 0.1 mol·L−1 KHCO3 ~9 96 24 h@−0.81 V 45
CoPc@N-CA-500 H 0.5 mol·L−1 KHCO3 21.72 92.4 20 h@−0.8 V 46
CoPc-py-CNT H 0.2 mol·L−1 NaHCO3 9.9 98 12 h@−0.63 V 47
CoPP@CNT H 0.5 mol·L−1 NaHCO3 25.1 98 12 h@−0.60 V 48
CoPc-COOH/CNT-NH2 H 0.5 mol·L−1 KHCO3 22.4 91 48 h@−0.58 V 49
CoTMAPc@CNT flow 1 mol·L−1 KOH 239 95.6 15 h@−0.4 V 50

4.1.1 Non-Covalent Bonding

Non-covalent immobilization techniques, such as π-π interactions, electrostatic interactions, and coordination effects, are commonly employed in the design and synthesis of heterogeneous molecular catalysts[43]. Due to the lack of need for complex pre-treatment methods, this technique exhibits unique advantages in the rapid screening of effective electrocatalysts for CO2 RR. Van der Waals π-π interactions are widely recognized as an effective method for directly immobilizing molecular catalysts onto substrates. Generally, the molecular catalysts used should be highly aromatic hydrocarbon structures with delocalized electrons to facilitate interactions with the substrate, thereby inducing electron transfer. For instance, porphyrin and phthalocyanine derivatives can be directly immobilized on substrates through π-π interactions[14,44]. The research group of Liming Zhang at Fudan University[45] designed a graphdiyne/graphene (GDY/G) heterostructure via non-covalent bonding as a two-dimensional conductive scaffold to anchor CoPc. In the CoPc/GDY/G sandwich structure, graphene serves as the conductive layer, GDY functions as the adsorption layer, and CoPc molecules act as the catalytic layer, with these three layers assembled together through van der Waals forces. Research findings indicate that the GDY layer provides specific chelating sites to stabilize CoPc molecules, suppress molecular aggregation, and achieve good dispersion, significantly enhancing the activity and stability of CO2 RR. In H-type electrolyzer tests, when the partial current density of CO (jco) reaches 12 mA·cm−2, FECO is as high as 96%. In flow cell tests, when jco is 100 mA·cm−2, FECO is 97%, and it can sustain operation for over 24 hours. The research group of Xiaomeng Lv at Jiangsu University used CoPc as a precursor and nitrogen-doped carbon aerogel (N-CA) as a carrier to synthesize an efficient catalyst CoPc@N-CA-500 through a pyrolysis strategy. Well-dispersed Co-NxC sites are stabilized on N-CA, and thanks to the porous system of carbon aerogel, the mass transfer of CO2 molecules is improved, exposing abundant Co-NxC active sites, accelerating the reaction kinetics of CO2, and enhancing activity (jco is 21.72 mA·cm−2, −0.8 V vs. RHE). Within a wide potential window of −0.5~−0.9 V vs. RHE, CO selectivity is maintained above 80%, with the maximum FECO reaching 92.48%, and the reaction can sustain operation for 20 hours (at −0.8 V vs. RHE)[46].
To address the poor dispersion and conductivity of metal macrocyclic molecular catalysts, researchers have combined the catalysts with carbon-based supports that exhibit good conductivity and large specific surface areas through non-covalent bonding, aiming to enhance the dispersion and electron transfer of molecular catalysts while increasing the density of active sites on the electrode. However, such interactions also have certain limitations, largely depending on the properties of the support itself, which requires a large specific surface area for the dispersion of molecular catalysts. But at higher catalyst loadings, surface molecules still tend to self-aggregate, reducing the number of effective active sites. On the other hand, this weak interaction can also lead to molecules detaching from the support or electrode surface under harsh electrocatalytic reaction conditions, significantly reducing the stability of the catalyst. Therefore, the non-covalent immobilization technique for such macrocyclic molecular catalysts has certain limitations.

4.1.2 Covalent Connection

Compared with simple physical adsorption, anchoring molecular catalysts on the surface of carriers or electrodes through stronger covalent bonds or coordination bonds can effectively solve the self-aggregation problem of molecular catalysts caused by strong π-π stacking interactions between molecules. Specifically, metal macrocyclic compounds covalently modified electrodes can be obtained by coordinating with the metal center or through ligand grafting. Minghui Zhu's research group at East China University of Science and Technology[47] designed a cobalt phthalocyanine-based catalyst loaded on pyridine-functionalized carbon nanotubes (CoPc-py-CNT). Characterization results show that the pyridine functional group forms a new Co—N coordination bond with CoPc, and this stronger interaction can act as a driving force to make CoPc molecules diffuse more evenly on the surface of carbon nanotubes. This novel hybrid catalyst exhibits high electrochemical CO2 reduction activity (at −0.63 V vs. RHE, the turnover frequency for CO (TOFCO) is 34.5 s−1) and high selectivity (FECO >98%). The group also reported a method using O as an axial ligand to replace the Cl element in cobalt protoporphyrin chloride (CoPPCl), thereby achieving the grafting of CoPP onto CNTs (CoPP@CNT). At an overpotential of 490 mV, this catalyst has a jco of 25.1 mA·cm−2, and the corresponding FECO is 98.3%, surpassing the performance of most traditional non-covalently bonded catalysts reported in the literature[48].
Covalently grafting macrocyclic molecular catalyst ligands onto supports or electrodes is another method to achieve covalent modification. The research group of Chuan Shi from Dalian University of Technology[49] proposed a method of functionalizing catalysts by covalently linking carbon substrates and metal phthalocyanine molecules, thereby enhancing CO2 RR performance. By utilizing an amidation reaction, carboxyl (-COOH) functionalized CoPc (CoPc-COOH) was grafted onto amino (-NH2) modified carbon nanotube substrates to prepare the covalent catalyst CoPc-COOH/CNT-NH2. The unique conjugated interface and electron transfer effect between CoPc-COOH and CNT-NH2 resulted in this heterogeneous catalyst exhibiting a jco of 22.4 mA·cm-2 in CO2 RR, with FECO reaching up to 91% at -0.88 V vs. RHE, and maintaining stable operation for over 48 hours. The research group of Ruquan Ye from City University of Hong Kong[50] covalently grafted tetraaminophthalocyanine cobalt (CoTAPc) onto carbon nanotubes (CoTMAPc@CNT) via diazonium reaction, followed by complete methylation. At -0.72 V vs. RHE, jco increased by approximately 700% compared to physically mixed samples. In flow cell testing, at an overpotential of 590 mV and low molecular loading (0.069 mg·cm-2), it achieved industrial-level current density (239 mA·cm-2) with FECO of 95.6%.
Compared with the supported catalysts obtained by traditional physical mixing methods, the above strategy anchors macromolecular catalysts on the carrier surface by modifying specific functional groups on the carrier. This method not only enhances the dispersion ability of molecules on the carrier surface to achieve higher loading and improve overall catalytic activity and the utilization rate of active components, but also enhances the stability of molecules on the carrier surface through strong interactions to withstand harsher electrocatalytic reaction conditions. However, in the covalent connection strategy, the loading amount of metal macrocyclic molecular catalysts on the carrier depends on the number of functional groups modified on the carrier surface and the metal sites. Due to the limitation of the modified groups and metal sites on the carrier surface, the density of effective active components on the catalyst anchored by covalent bonds is often low. There is a direct link between the catalytic performance of the catalyst and the density of active components; the limited molecular loading also restricts the further improvement of catalytic activity. This presents both challenges and opportunities for further enhancing the long-term stable operation of molecular catalysts under high current.

4.2 Fixed Period Technology

By constructing polymer framework structures with high covalent bond energy through condensation or polymerization reactions, a large number of active sites can be provided, thereby enhancing the overall catalytic activity of the material (Table 3). The team of Yan-Guang Li from Soochow University[16] used 1,2,4,5-tetracyanobenzene (TCNB) as a precursor for preparing polyphthalocyanine. After mixing TCNB with cobalt chloride and carbon nanotubes, TCNB was polymerized in situ along the carbon nanotubes under microwave heating to form carbon nanotube-supported cobalt polyphthalocyanine (CoPPc/CNT). Experimental tests showed that the composite material had FECO higher than 90% at a small overpotential (500 mV) and could continuously operate stably for more than 24 hours (−0.54 V vs RHE). They believed that the extensive crosslinking of CoPPc on conductive carriers not only suppressed the aggregation of organic molecules, expanding their electrochemically active surface area, but also enhanced their physical and chemical stability. Subsequently, the team led by Rong Cao from the Chinese Academy of Sciences[51] developed a conjugated polyphthalocyanine framework stabilizing bimetallic active sites. By adjusting the local electron density of cobalt sites in a series of conjugated bimetallic polyphthalocyanine frameworks (CoxZnyPPc), they improved product selectivity and current density. Under optimal ratio conditions (Co∶Zn=3∶1), within a wide working potential window of −0.3~−0.9 V vs. RHE, FECO exceeded 90%, and jco reached 212 mA·cm−2, which were 1.7 times and 9.1 times those of CoPPc and ZnPPc, respectively.
表3 具有周期结构的金属大环分子催化剂的比较[16,51⇓-53]

Table 3 Comparison of metal macrocyclic molecular catalysts with periodic skeleton structure[16,51⇓-53]

Catalyst Cell type Electrolyte jco(mA·cm−2) FECO(max) (%) Stability Ref.
CoPPc/CNT H 0.5 mol·L−1 KHCO3 18.7 90 24 h@−0.54 V 16
Co3Zn1PPc flow 0.5 mol·L−1 KHCO3 212 90 9 h@−1.1 V 51
NiPc-COF H 0.5 mol·L−1 KHCO3 35 99.1 10 h@−0.7 V 52
CoPc-PI-COF-3 H 0.5 mol·L−1 KHCO3 31.7 96 20 h@−0.8 V 53
In addition, constructing COFs through surface reactions can maximize the provision of dense active sites, stabilizing molecular catalysts at the molecular level to achieve electrocatalytic CO2 RR performance, thereby potentially further meeting the long-term stability testing for high-current CO2 RR. The team of Huang Yuanbiao from the Chinese Academy of Sciences [52] synthesized a two-dimensional conductive nickel phthalocyanine-based COF (NiPc-COF) through a condensation reaction. The NiPc-COF connected by covalent pyrazine bonds can maintain CO2 RR activity for 10 hours (−0.7 V vs. RHE). This 2D-NiPc-COF nanosheet exhibits very high FECO (>93%) within a wide potential window of −0.6~−1.1 V vs. RHE, and the jco is 35 mA·cm−2 under −1.1 V vs RHE voltage conditions, benefiting from its large number of active sites and excellent electron transfer capability. Based on the synthesis of 2D COFs, Jiang Jianzhuang et al. from the University of Science and Technology Beijing [53] successfully developed a three-dimensional cobalt phthalocyanine polyimide (PI) COFs (CoPc-PI-COF-3) through a solvothermal reaction. Due to the three-dimensional porous structure of CoPc-PI-COF-3, its electrochemical surface concentration is as high as 183 nmol·cm−2, equivalent to 32.7% of the total metal phthalocyanine units acting as electrochemically active sites. Under −0.90 V vs RHE voltage conditions, jco is 31.7 mA·cm−2, and during continuous 20-hour testing, TOF is 0.6 s−1 (−0.8 V vs. RHE), indicating the bright prospects of 3D COFs as efficient electrocatalysts.

4.3 Introduction of Functional Groups

The modification of exocyclic functional groups on macrocyclic molecules can directly modulate the electronic structure of the metal center, thereby influencing the interaction between the metal center and reaction intermediates, and ultimately affecting the activity of the macrocyclic metal center. Therefore, the rational design of exocyclic substituents and the introduction of functional groups into macrocyclic compounds can regulate the reactivity and selectivity of molecular catalysts, enhancing the performance of materials for electrocatalytic CO2 RR (as shown in Table 4). Functional groups such as hydroxyl (—OH), amino (—NH2), and fluoro (—F) can influence the physicochemical properties and microstructure of the catalyst through induction effects, electrostatic interactions, hydrogen bonding, steric effects, and other mechanisms, enabling the rational design of macromolecular structures (as illustrated in Figure 6).
表4 引入官能团的金属大环催化剂的比较[42,54⇓⇓⇓⇓⇓-60]

Table 4 Comparison of metal macrocyclic catalysts with functional groups[42,54⇓⇓⇓⇓⇓-60]

Catalyst Cell type Electrolyte jco（mA cm−2） FECO（max） （%） Stability Ref.
CoPc/NH2-CNT flow 1 mol·L−1 KOH −225 100 100 h@−225 mA·cm−2 54
NiTAPc/CNT flow 1mol·L−1 KHCO3 −150 99.9 10 h@−150 mA·cm−2 55
NiPc-OME flow 1 mol·L−1 KOH −150 99.5 40 h@−150 mA·cm−2 56
COF-366-F-Co H 0.5 mol·L−1 KHCO3 65 mA·mg−1 87 — 57
CoFPc H 0.5 mol·L−1NaHCO3 6 93 12 h@−0.8 V 58
CoPc-CN/CNTs H 0.1 mol·L−1 KHCO3 ~15 98 — 59
CCG/CoPc-A H 0.1 mol·L−1 KHCO3 ~5 74 30 h@−0.69 V 42
(NHx)16-NiPc/CNTs flow 1 mol·L−1 KOH 305.4 100 13.9 h@−200 mA·cm−2 60
图6 金属大环分子催化剂引入环外官能团的效应

Fig. 6 Effects of the introduction of exocyclic functional groups into metal macrocyclic molecular catalysts

4.3.1 Inductive Effect

4.3.1.1 Electron-donating group

Metal macrocyclic molecular catalysts, despite having well-defined M-N4 catalytic sites, exhibit poor activity and stability in electrocatalytic CO2 RR. Some substituted or functionalized substituents modified metal macrocyclic compounds can display higher activity or stability, but the catalytic performance caused by the different electronic densities of the metal sites induced by the peripheral ligands of the substituents is often overlooked. The research group of Professor Lai Jianping from Qingdao University of Science and Technology[54] prepared carbon nanotube immobilized CoPc catalysts functionalized with -NH2, -OH, and -COOH groups through a coordination engineering strategy. Compared with no group, -OH group, and -COOH group, the -NH2 group can effectively change the coordination environment of the central metal Co, thereby significantly improving the TOF value (31.4 s−1, −0.6 V vs. RHE) and FECO (92.2%, −0.6 V vs. RHE) of CoPc/NH2-CNT. Under flow cell conditions, the CoPc/NH2-CNT catalyst can stably operate for 100 h at −225 mA·cm−2. To further explore the impact brought by electron-donating groups, the Liu Min team from Central South University reported a ligand modulation strategy to enhance NiPc catalytic performance and reveal the influence of ligand effects on CO2 RR. By modifying NiPc molecules with typical electron-donating groups: -OH or -NH2 and loading them onto CNTs, in flow cell tests, -NH2 substituted NiPc (NiTAPc) shows ultra-high FECO (>99.8%) in the large current range of −400~−20 mA·cm−2 and stable operation for 10 h (FECO >99.8%) under high current conditions of −150 mA·cm−2. Compared with -OH modified NiPc (NiTHPc), the amino group in NiTAPc has stronger electron-donating properties, promoting electron localization on Ni atoms, thus enhancing the performance of CO2 RR in terms of activity, selectivity, and stability[55]. Recently, the research group of Professor Liang Yongyan from Southern University of Science and Technology and their collaborators introduced an electron-donating methoxy group (-OMe) to modify NiPc, obtaining NiPc-OMe. Compared with the original NiPc and electron-withdrawing group (-CN) modified NiPc, this electron-donating group (-OMe) significantly optimized the stability of the NiPc-based catalyst (stable operation for 40 h at 150 mA·cm−2), maintaining the selectivity for CO above 99.5% within a wide current window (−10 ~ −350 mA·cm−2) over 40 hours[56].

4.3.1.2 Electron-Withdrawing Groups

Electron-withdrawing groups, while reducing the electron density of the active metal center, can promote the reduction of the metal center ion and decrease the adsorption of the active metal center with the product CO, facilitating product desorption, thereby enhancing catalytic performance and achieving high selectivity of the product. For instance, Diercks' team at the University of California, Berkeley[57] attempted to introduce the electron-withdrawing group fluorine (—F) into cobalt porphyrin-derived covalent organic frameworks. The resulting —F-modified COFs (COF-366-F-Co) achieved a current density of 65 mA·mg−1 at an overpotential of 550 mV, along with 87% FECO and 12 hours of operational stability. Compared to unmodified COFs, its catalytic performance was greatly enhanced. The study found that controlling different numbers of electron-withdrawing groups might not yield ideal results due to direct modification by inductive effects, as the benefits of lower overpotential were offset by significantly reduced TOF. Rodionov et al. from King Abdullah University of Science and Technology[58] studied —F-functionalized CoPc (CoFPc). Compared to non-functionalized CoPc, the Co(II)/Co(I) redox peak of CoFPc shifted positively by ~0.2 V, which facilitated the formation of active site Co(I). Consequently, when the voltage was −0.5 V vs. RHE, product selectivity reached 93%; while at −0.8 V vs. RHE, FECO remained at 93%, with the ability to operate continuously for 12 hours. Hai-Liang Wang's research group at Yale University and their collaborators[59] introduced the electron-withdrawing group cyano (—CN) onto carbon nanotubes with CoPc molecules, yielding high-performance CO2 RR catalysts (CoPc-CN/CNTs). Results showed that the addition of —CN promoted the positive shift of the Co(II)/Co(I) redox peak, decreased the electron density around the Co center, and facilitated the reduction of Co(II) to Co(I), thus optimizing CO2 RR activity. The obtained composite material exhibited FECO >95%, and a current density of 15.0 mA·cm−2 at a voltage of −0.52 V vs. RHE.

4.3.2 Steric Effect

The introduction of long-chain groups outside the macrocyclic ring can inhibit aggregation between sites, thereby promoting the exposure of molecular sites. Officer et al. from the University of Wollongong in Australia[42] used octa-alkoxy modified CoPc as the catalytic unit and supported it on graphene to obtain the CCG/CoPc-A hybrid catalyst. Compared with similar cobalt phthalocyanine/graphene (CCG/CoPc) catalysts, the alkoxy substitution helps to suppress the aggregation of phthalocyanines on graphene sheets, leading to a significant enhancement in the catalytic activity of individual phthalocyanine molecules. At an overpotential of 480 mV, the TOF is 5 s−1, and it operates stably for 30 h. Long-chain amino groups have also been shown to stabilize CO2 reaction intermediates. Zhu Qilong and his collaborators from the Chinese Academy of Sciences[60] designed and developed a single-molecule heterojunction electrocatalyst ((NHx)16-NiPc/CNTs), which has well-defined Ni sites and an amino-rich local microenvironment. (NHx)16-NiPc/CNTs exhibit excellent electrocatalytic activity and industrial-level current density for electrocatalytic CO2 reduction. Mechanistic analysis shows that this unique local microenvironment structure provides an effective "proton shuttle" effect to improve local alkalinity and proton transfer, and stabilizes CO2•− on the site by forming hydrogen bonds between H on —NH2 and O on CO2, ultimately achieving efficient operation of CO2 RR.

4.3.3 Electrostatic Interaction

Substituents with positive charges can stabilize the M-COO− structure through spatial electrostatic interactions, such as Fe-p-TMA, where the trimethylammonium group at the para position of the four phenyl rings of FeTPP promotes a more positive reduction potential due to its own electron-withdrawing effect. Activity analysis reveals that the positively charged ammonium in space helps stabilize the critical LxFe−CO2 intermediate, thereby promoting CO2RR. When the trimethylammonium group is introduced at the ortho position of the four phenyl rings of FeTPP, this effect is further enhanced due to the shortened distance between the group and the site[61].

4.3.4 Hydrogen Bonding Interaction

The electrocatalytic CO2 reduction involves multiple proton-coupled and electron transfer processes. Increasing the proton concentration around the site can enhance the activity of CO2 RR. Nervi and his team from the University of Turin, Italy [62] studied the effect of local proton sources on the CO2 RR performance of manganese (Mn)-based molecular catalysts by introducing phenolic hydroxyl groups around the Mn-based macrocycle. The phenolic-OH groups within the molecule can provide protons for the reaction. Even in the absence of an external proton source, the molecular catalyst can reduce CO2 to CO. This proton-assisted catalysis helps to increase the current of CO2 RR. In addition, apart from proton donors, Rochford and his collaborators from the University of Massachusetts [63] found that Lewis base side groups would promote C—O bond cleavage. In the presence of phenol, the O of the methoxy group on the Mn-based macrocycle acts as a hydrogen bond acceptor, which can attack the H of the *COOH intermediate at the site, thereby promoting C—O bond cleavage. This significantly reduces the activation energy barrier for C—O bond cleavage and facilitates the CO2 RR reaction.
The above research shows that the effects and methods of modifying different exocyclic functional groups on optimizing the site activity and stability of molecular catalysts vary. According to the properties of the active sites of different metal macrocyclic catalysts, the rational introduction of functional groups with different physicochemical properties will help optimize the electrocatalytic reaction of metal sites for CO2, which is instructive for the rational design of highly efficient and stable metal macrocyclic molecular catalysts.

5 Conclusion and Prospect

After years of research and development, the excellent performance of metal macrocyclic molecular catalysts in the field of reducing CO2 to CO has attracted increasing attention. To further achieve the process of highly stable and active electrocatalytic CO2 reduction, future research[64⇓⇓-67] can be carried out in the following three aspects.
(1) Catalyst design: In terms of material design, the development of highly efficient and stable macrocyclic molecular catalysts suitable for high current densities remains a key research focus for the future. This paper explores the feasibility of enhancing the performance of the catalyst itself by considering the immobilization methods of macrocyclic molecules, the modification of functional groups, and the contraction/polymerization of backbone or framework structures, aiming to improve the catalytic activity and stability of the active sites of macrocyclic molecular catalysts to meet the practical requirements of low energy consumption, high yield, and large-scale applications. Research targeting the regulation of the metal center active sites of molecular catalysts to enhance the material's electrocatalytic CO2 RR activity and stability presents both challenges and opportunities. For instance, regulating the spin state of the active sites of transition metal centers, reducing the bandgap energy between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), also facilitates in-plane electron migration, thereby improving the activity and stability of macromolecular catalytic materials.
(2) Electrochemical reactor: The electrochemical reduction of CO2 reactor is the core of CO2 RR technology, achieving the transformation of CO2 into renewable fuels within the reactor. This process involves the catalytic reaction of CO2 and the multiphase transport of substances such as CO2, electrons, electrolyte ions, and products. Its structural form greatly influences the performance of CO2 RR. Currently, the commonly used setup in laboratory research is the H-cell, which immerses the cathode part entirely in the electrolyte to achieve CO2 catalytic conversion by applying a reduction potential, mainly for studying CO2 RR catalysts to evaluate their CO2 RR performance. The H-cell is limited by factors such as the distance between the membrane and cathode, local CO2 concentration at the cathode, low solubility of CO2 in aqueous solutions, pH value, and CO2 transport. Gas diffusion electrode (GDE) flow cells expose one side of the cathode to feed gas, allowing both liquid and gas phases to coexist within the catalyst layer. This structure not only effectively addresses the mass transfer issue of CO2 but also allows experiments to be conducted in highly alkaline electrolytes while achieving industrial-level current densities. In contrast, membrane electrode assembly (MEA) electrolyzers utilize the GDE configuration of flow cells and reduce the overall system resistance by directly contacting the catalyst layer with the ion exchange membrane, thereby enhancing the stability of the catalytic system. With advantages such as high current density (> 300 mA·cm−2) and low voltage (< 2.5 V), it holds significant promise for commercial applications[68]. However, the configuration of the electrolyzer and its operating conditions largely determine the local reaction environment near the electrode, thus modulating its catalytic performance. This is specifically reflected in the synergistic regulation of six key performance parameters: current density, Faradaic efficiency, energy efficiency, stability, CO2 utilization, and cell voltage[69]. Most previous studies adopted alkaline or neutral media for the gas-phase CO2 reduction to CO production process to achieve higher Faradaic efficiency. However, in alkaline or neutral media, CO2 RR easily generates carbonates and bicarbonates, leading to issues like salt precipitation, water intrusion, and higher anodic overpotential, severely limiting further development prospects[70]. Investigating CO2 RR catalysts in acidic solutions has become a new research direction. However, under acidic conditions, HER is more favorable compared to CO2 RR[71]. Therefore, achieving high selectivity, large current density, and long-term stability for electrocatalytic CO2 RR under acidic medium conditions will be a significant challenge for future research.
(2) The MEA electrolytic cell directly samples CO2 gas, which cannot avoid the energy-intensive steps in the CO2 preparation process. The MEA flow cell based on CO2 capture solution can achieve in-situ conversion of the CO2 capture solution, having the potential to reduce energy consumption and costs[72]. Using molecular catalysts as cathode catalytic materials to construct an integrated electrolytic cell device for CO2 capture and electrocatalytic conversion is expected to promote the industrial development process of electrocatalytic CO2 while experimentally achieving efficient and stable transformation of catalytic materials.
(3) The combination of machine learning and electrocatalytic CO2 RR: With the rapid development of computer artificial intelligence technology, machine learning is innovatively utilized to quickly screen and synthesize molecular catalytic materials, and help deepen the discovery and understanding of the universal rules of CO2 RR under big data, which can not only liberate manpower, but also obtain efficient and stable molecular catalytic materials, promoting the development of electrocatalytic CO2 RR.
The field of stable and efficient catalytic CO2 reduction to CO achieved by metal macrocyclic compounds is a highly promising area of research. By employing a combination of theoretical and experimental research methods, understanding of this reaction domain can be enhanced. It is expected that, combined with the development of electrolytic cells, the efficient and stable transformation of electrocatalytic CO2 can be realized, promoting the industrialization process of carbon neutrality and carbon peak.

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