Molecularly Dispersed Heterogenized Metallomacrocycles: Molecular Structure Sensitivity of CO 2 Electrolysis

Open AccessCCS ChemistryRESEARCH ARTICLES10 Oct 2022Molecularly Dispersed Heterogenized Metallomacrocycles: Molecular Structure Sensitivity of CO2 Electrolysis Dong-Dong Ma, Shu-Guo Han, Sheng-Hua Zhou, Wen-Bo Wei, Xiaofang Li, Bo Chen, Xin-Tao Wu and Qi-Long Zhu Dong-Dong Ma State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (CAS), Fuzhou 350002, Fujian Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou 350108, Fujian University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Shu-Guo Han State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (CAS), Fuzhou 350002, Fujian Google Scholar More articles by this author , Sheng-Hua Zhou State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (CAS), Fuzhou 350002, Fujian University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Wen-Bo Wei State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (CAS), Fuzhou 350002, Fujian University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Xiaofang Li State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (CAS), Fuzhou 350002, Fujian Google Scholar More articles by this author , Bo Chen Department of Chemistry, City University of Hong Kong, Hong Kong 999077 Google Scholar More articles by this author , Xin-Tao Wu State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (CAS), Fuzhou 350002, Fujian Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou 350108, Fujian University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author and Qi-Long Zhu *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (CAS), Fuzhou 350002, Fujian Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou 350108, Fujian University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202202294 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Revealing the structure–performance relationships of the molecularly dispersed heterogenized metallomacrocycles (MDHMs) with π-conjugated heterojunctions is of great, yet largely unmet, significance for realizing the rational regulation of selectivity, Faradaic efficiency, and durability in the electrochemical CO2 reduction reaction (CO2RR). Herein, we describe new insights into the molecular-structure sensitivity of the electrochemical CO2RR over the MDHMs through peripherally functionalizing the heterogenized nickel phthalocyanines with diverse methylimidazole groups. As studied experimentally and theoretically, the heterointerfacial effect and self-built microenvironment of the MDHMs are significantly modulated by accurate structural clipping at the molecular level, predominately steering their electrocatalytic performance for CO2-to-CO conversion in both H-type and flow electrolytic cells. Particularly, the optimized MDHM, 2MIMCβNiPc/CNT, displays a significantly improved electrocatalytic performance for CO2RR with near-unity Faradaic efficiency and large current density. Furthermore, the paired electrolysis systems with the MDHMs as the multifunctional electrocatalysts was developed for making full use of the anodic and cathodic reactions, which shows a new and profitable prospect for energy optimization, pollutant regeneration, and green electrosynthesis in a modular form, and also provides guidance for the design and synthesis of novel efficient yet economical electrocatalysts. Download figure Download PowerPoint Introduction Under the “double carbon” target, the development of green energy has an important practical significance, which is simultaneously correlated with the emission reduction and efficient conversion of CO2.1–6 By comparison, the electrochemical conversion technique has long been recognized as a green and sustainable method in CO2 utilization. Meanwhile, among a variety of electrochemical CO2-conversion products, CO is a fundamental industrial feedstock that shows high techno-economic feasibility and flexible value-added scalability.7–13 It is remarkable, however, that the exploration of highly efficient electrocatalysts and the optimization of energy-saving catalytic systems are crucial yet still challenging. Currently, the molecularly dispersed heterogenized metallomacrocycles (MDHMs), an emerging kind of unique molecule-based electrocatalysts, show distinctive advantages in many electrocatalytic reactions (e.g., CO2 reduction reaction (CO2RR), O2 reduction reaction, N2 reduction reaction, etc.), including their well-defined and tunable molecular structures, activated intrinsic activity, maximum surface active-site density, and distinguished charge conduction capability.14–22 Indeed, for sustainable electrocatalytic CO2RR, the MDHMs catalysts, as a kind of π-conjugated heterojunction at the molecular level, remain the topic of active research, and many tactics and mechanisms have been proposed to promote the carbon appreciation and energy optimization.23–34 From a fabrication perspective, four factors are indispensable for the MDHMs with potential CO2RR performance: molecular engineering,29,33,35–37 substrate selection,35,38–40 heterojunction formation,35,36,38,40–44 and microenvironment construction.28,45–47 Typically, as for molecular engineering, our group previously reported a peculiar heterojunction-type electrocatalyst, the newly-designed twelve-azido-bearing nickel phthalocyanine (NiPc) anchored onto carbon nanotubes (CNT) at the single-molecular level, which exhibited ultrahigh selectivity for CO2-to-CO conversion with 100% Faradaic efficiencies (FEs) in a wide potential range compared with counterparts.41 Evidently, the heterojunction formation mentioned above is highly pivotal in the management of the MDHMs catalysts for electrocatalytic CO2 conversion. More recently, through designing a series of distinguishable MDHMs catalysts, we also verified the universality and significance of heterointerface engineering on boosting the electrocatalytic CO2RR activities.48 These previous studies provide a strong foundation for rational development of MDHMs in CO2 electro-conversion. However, the specific effects of structural geometries of active metallomacrocycle molecules (e.g., metallophthalocyanines) for energy-economic electrocatalytic CO2RR are still elusive, especially in the heterogeneous single-molecule form, requiring precise discovery of the structural sensitivity of the molecules in executing CO2-to-CO. In the long run, molecular-level insight into the scalability and general design of the MDHMs catalysts is conducive to the vigorous development of large-scale CO2 electrocatalytic systems, thus accelerating the global carbon cycle. Following the above guidance, here we innovatively designed and synthesized three NiPcs (2MIMβNiPc, 2MIMαNiPc, and 2MIMCβNiPc) peripherally functionalized with diverse methylimidazole groups, which can be self-adaptively anchor onto CNT through structure-induced heterointerfacial interactions, that serve as the paradigm catalysts to survey the molecular-structure sensitivity of electrochemical CO2RR. Concretely, 2MIMCβNiPc/CNT decorated with the flexible methylimidazole groups at the β positions of the phthalocyanine ring exhibits an extraordinary electrocatalytic performance for CO2RR with more competitive current densities and FEs at different potentials and excellent durability under general conditions, compared with 2MIMβNiPc/CNT and 2MIMαNiPc/CNT. Furthermore, based on the design considerations of 2MIMCβNiPc/CNT, we confirmed the influences of the self-built microenvironment, molecule dispersion, and surface active-site densities towards CO2-to-CO electroreduction. The combination of spectroscopic characterization and theoretical calculations revealed that the highly efficient CO2-to-CO activity and promising stability of 2MIMCβNiPc/CNT can be ascribed to the optimized heterointerfacial effect and local microenvironment engineering at the molecular level. More interestingly, guided by the results of multimode paired electrolysis systems with 2MIMCβNiPc/CNT applied at both electrodes, the structure–performance relationships of the MDHMs were further established to optimize the design of the MDHMs catalysts, expecting to provide references for CO2 conversion, pollution control, green electrosynthesis, and other applications. Experimental Methods Synthesis Synthesis of 2MIMβ-CN2 The compound 2MIMβ-CN2 was synthesized by a slight modification of the reported literature method.49 2-Methylimidazole (328 mg, 4 mmol) and anhydrous K2CO3 (829 mg, 6 mmol) were stirred in dry N,N-dimethylformamide (20 mL) at room temperature for 5 h, then 4-nitrophthalonitrile (519 mg, 3 mmol) was rapidly added and unceasingly stirred for 72 h. Then, the reaction mixture was poured into 150 mL of deionized water, and the obtained precipitate was collected and washed many times with deionized water. Yield: 72.5%. 1H NMR (Dimethyl sulfoxide (DMSO)-d6, δ): 8.34 (s, 1H, Ar-H), 8.29–8.27 (d, 1H, Ar-H), 8.03–8.01 (d, 1H, Ar-H), 7.44 (s, 1H, Im-H), 6.95 (s, 1H, Im-H), 2.35 (s, 3H, –CH3). Synthesis of 2MIMα-CN2 2-Methylimidazole (1.64 g, 20 mmol) and anhydrous K2CO3 (2.76 g, 20 mmol) were stirred in dry DMSO (20 mL) at room temperature for 5 h, then 3-nitrophthalonitrile (1.73 g, 10 mmol) was rapidly added and unceasingly stirred for 72 h. Then, the reaction mixture was poured into 100 mL of NH4Cl saturated deionized water. After extracting with dichloromethane, the obtained organic phase was washed three times with NH4Cl saturated deionized water (50 mL). Then the crude product was purified by SiO2 (200–300 mesh) column chromatography using dichloromethane/methanol (10/1) as eluents. Yield: 24.1%. 1H NMR (DMSO-d6, δ): 8.27–8.22 (m, 1H, Ar-H), 8.06–8.01 (m, 2H, Ar-H), 7.41 (s, 1H, Im-H), 6.97 (s, 1H, Im-H), 2.20 (s, 3H, –CH3). Synthesis of 2MIMCβ-CN2 First, 2-(2-methyl-1H-imidazol-1-yl)ethanol was synthesized according to the reported literature method.50 Second, 2MIMCβ-CN2 was synthesized according to the above method. The crude product was purified by SiO2 (200–300 mesh) column chromatography using dichloromethane/methanol (50/1–30/1) as eluents. Yield: 48.9%. 1H NMR (DMSO-d6, δ): 8.06–8.04 (d, 1H, Ar-H), 7.78 (s, 1H, Ar-H), 7.45–7.42 (m, 1H, Ar-H), 7.10 (s, 1H, Im-H), 6.71 (s, 1H, Im-H), 4.42–4.40 (t, 2H, –CH2–), 4.31–4.29 (t, 2H, –CH2–), 2.31 (s, 3H, –CH3). Synthesis of 2MIMβNiPc 2MIMβ-CN2 (208 mg, 1.0 mmol), nickel (II) acetate tetrahydrate (75 mg, 0.3 mmol), and dried 1-pentanol (5 mL) were added and stirred at 120 °C for 30 min under Ar atmosphere. Then 0.5 mL of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was added and stirred at 140 °C for 12 h. After the mixture was cooled to room temperature, the black-green product was precipitated and washed several times with n-hexane. The dried crude product was dissolved in dichloromethane and purified by neutral aluminum oxide column chromatography using 4/1 dichloromethane/methanol (v/v) as eluent to give a dark violet solid. Yield: 75.4%. IR (KBr, υ, cm−1): 1615, 1531, 1500, 1414, 1334, 1297, 1171, 1139, 1093, 986, 930, 884, 832, 751. Elem. Anal. Calcd for C48H32N16Ni·5H2O: C, 58.73; H, 4.31; N, 22.83. Found: C, 58.45; H, 4.33; N, 22.31. High-resolution mass spectrometry (HRMS): [M+H]+ calcd for C48H32N16Ni, 891.2349; found, 891.2434. UV–vis (N-methyl-2-pyrrolidone (NMP), λ, nm): 334, 605, 670. Synthesis of 2MIMαNiPc 2MIMα-CN2 (250 mg, 1.2 mmol), nickel (II) acetate tetrahydrate (75 mg, 0.3 mmol), and dried 1-pentanol (5 mL) were added and stirred at 120 °C for 30 min under Ar atmosphere. Then 0.5 mL of DBU was added and stirred at 140 °C for 12 h. After the mixture was cooled to room temperature, the black-green product was precipitated and washed several times with n-hexane. The dried crude product was dissolved in dichloromethane and purified by neutral aluminum oxide column chromatography using dichloromethane/methanol (v/v = 50/1–25/1) as eluent to give a dark violet solid. Yield: 35.5%. IR (KBr, υ, cm−1): 1621, 1531, 1502, 1408, 1304, 1256, 1191, 1148, 1110, 1092, 1040, 984, 915, 860, 817, 767, 751, 672. HRMS: [M+H]+ calcd for C48H32N16Ni, 891.2349; found, 891.2433. UV–vis (NMP, λ, nm): 335, 604, 670. Synthesis of 2MIMCβNiPc 2MIMCβ-CN2 (504 mg, 2.0 mmol), nickel (II) acetate tetrahydrate (150 mg, 0.6 mmol), and dried 1-pentanol (5 mL) were added and stirred at 120 °C for 30 min under Ar atmosphere. Then 0.9 mL of DBU was added and stirred at 140 °C for 12 h. After the mixture was cooled to room temperature, the black-green product was precipitated and washed several times with n-hexane. The dried crude product was dissolved in dichloromethane and purified by alkaline aluminum oxide column chromatography using dichloromethane/methanol (v/v = 20/1–10/1) as eluent to give a dark violet solid. Yield: 65.7%. IR (KBr, υ, cm−1): 1612, 1531, 1485, 1463, 1421, 1350, 1283, 1241, 1129, 1097, 1066, 961, 822, 751, 678. HRMS: [M+H]+ calcd for C56H48N16NiO4, 1067.3398; found, 1067.3476. UV–vis (NMP, λ, nm): 332, 388, 613, 678. Synthesis of ThCβNiPc The synthesis was performed following the same procedures as 2MIMCβNiPc. Integrated with anode oxidation The working electrodes for anode were prepared as the cathode. The 30 mL of electrolyte (0.5 M KHCO3) with 1 mmol potassium iodide (KI) or N,N-diethylhydroxylamine (DEHA) in the anodic compartment and 30 mL of electrolyte (0.5 M KHCO3) in the cathodic compartment was used for the two-electrode system under a certain atmosphere. The scan rate of the linear sweep voltammetry (LSV) curves was 10 mV s−1. Anodic electrolyte were analyzed by quantitative nuclear magnetic resonance (NMR) using DMSO as an internal standard after electrolysis for the total passing charge of ∼192.5 C (500 μL of electrolyte was mixed with 100 μL of D2O containing 0.25 μL of DMSO). Results and Discussion Synthesis and characterization of the MDHM catalysts The NiPcs 2MIMβNiPc, 2MIMαNiPc, and 2MIMCβNiPc with diverse methylimidazole groups were rationally designed based on different grafting patterns, as shown in Figure 1a. From a structural perspective, 2MIMαNiPc is derived from 2MIMβNiPc via splicing the methylimidazole groups from the β positions to α positions of the phthalocyanine ring. By introducing alkyl chains, the flexible methylimidazole groups are grafted around the phthalocyanine ring of 2MIMCβNiPc. Such structure design with consistent Ni–N4 active sites and peripheral functional groups provides a rational platform to expectantly explore the molecular structure sensitivity of electrochemical CO2RR over the MDHMs, that is, the specific roles of the methylimidazole groups in modulating the heterointerfacial effect and electron transfer capacity at the molecular level. Accordingly, 2MIMβNiPc, 2MIMαNiPc, and 2MIMCβNiPc were first characterized by the 1H NMR spectra, HRMS, and Fourier-transformed infrared (FT-IR) spectroscopy ( Supporting Information Figures S1–S7), verifying the successful synthesis of the designed structures. In the UV–vis spectra, compared with 2MIMβNiPc and 2MIMαNiPc, the red-shift of the characteristic Q-band of 2MIMCβNiPc is about 8 nm ( Supporting Information Figure S8), indicating that the 1a1u→1eg* electronic transition in 2MIMCβNiPc is easier, which may be due to the modulating effect of the alkyl chains.51,52 Furthermore, as revealed in Supporting Information Figure S9, from 2MIMαNiPc to 2MIMβNiPc to 2MIMCβNiPc, the Ni 2p3/2 spectra plotted by X-ray photoelectron spectroscopy (XPS) successively shift to lower energies, indicating a difference in the electron density of the central Ni sites due to the regulation of peripheral functional groups. All the above results indicate that the differentiated structures lay a good foundation for follow-up exploration. Figure 1 | (a) Molecule structures of 2MIMβNiPc, 2MIMαNiPc, and 2MIMCβNiPc; (b) loading 2MIMCβNiPc to the CNT; (c) local heterointerfacial interactions including π–π stacking and charge-transfer interactions; (d) TEM image; (e) AC-HAADF-TEM image; and (f) HS-LEIS spectrum of 2MIMCβNiPc/CNT. Download figure Download PowerPoint The fabrication of MDHMs was achieved by the structure-induced self-adaptive heterointerfacial interactions between NiPcs and CNT in NMP, which include π–π stacking and charge-transfer interaction as vividly shown in Figure 1b,c. The powder X-ray diffraction (PXRD) patterns and Raman spectra of 2MIMβNiPc/CNT, 2MIMαNiPc/CNT, and 2MIMCβNiPc/CNT are consistent with the feature of the CNT ( Supporting Information Figures S10 and S11), suggesting the high dispersion of NiPcs onto the CNT. This was further elucidated by transmission electron microscopy (TEM) images and energy-dispersive X-ray elemental mappings (Figure 1d and Supporting Information Figures S12–S15), in which the multiwalled structures of CNT with ∼15 layers are clearly identified without discernable molecular aggregates. The FT-IR spectra of the relevant catalysts show discernible absorption peaks of C=N and –CH3, implying the implantation of NiPcs ( Supporting Information Figure S16). Further, the inductively coupled plasma atomic emission spectroscopy affirmed the Ni content in MDHMs. As displayed in Supporting Information Table S1, only the experimental Ni content of 2MIMCβNiPc/CNT is consistent with the theoretical value, which may be due to the stronger heterointerfacial interactions in 2MIMCβNiPc/CNT than the others, indicating the importance of molecular structure in construction of MDHMs. More definitively, the aberration-corrected high-angle annular dark-field scanning TEM (AC-HAADF-STEM) image of 2MIMCβNiPc/CNT is presented in Figure 1e, in which the identified Ni sites are on the walls of the CNT, elucidating the dispersion of 2MIMCβNiPc at the molecular level. To gain insight into surface anchoring of the NiPcs in the MDHMs, the surface analysis technology high-sensitivity low-energy ion scattering (HS-LEIS) spectroscopy was employed.53 As exhibited in Figure 1f, the HS-LEIS spectra of 2MIMCβNiPc/CNT and CNT indicate that the 2MIMCβNiPc molecules are fully exposed on the external surface of the CNT. Thus, all these results indicate that the MDHM catalysts were successfully fabricated based on the desired target. Electrocatalytic CO2RR performances The CO2 electro-conversion activities of the 2MIMβNiPc/CNT, 2MIMαNiPc/CNT, and 2MIMCβNiPc/CNT catalysts were evaluated by integrating a continuous on-line analysis system using a typical H-type cell. LSV curves for 2MIMβNiPc/CNT, 2MIMαNiPc/CNT, and 2MIMCβNiPc/CNT were obtained (Figure 2a), which reveal similar onset potentials at −0.40 V but distinguishing current densities of 70–100 mA cm−2 at −1.20 V. More evidently, as displayed in Figure 2b, 2MIMCβNiPc/CNT exhibits a much larger current density and better stability in a potential range of −0.53 to −0.88 V, as compared with 2MIMβNiPc/CNT and 2MIMαNiPc/CNT, demonstrating the highest electrocatalytic CO2RR activity. Meanwhile, the reduction products were analyzed by gas chromatography (GC) and NMR spectroscopy ( Supporting Information Figure S17). As clearly illustrated in Figure 2c, 2MIMβNiPc/CNT and 2MIMCβNiPc/CNT exhibit a similarly high FECO of ∼96% at −0.73 V, outperforming a maximum of ∼72% for 2MIMαNiPc/CNT. In contrast, the pure CNT mainly produced hydrogen in the same potential range ( Supporting Information Figure S18), revealing that the origin of electrocatalytic CO2RR activity is the Ni–N4 sites. Furthermore, the partial current densities of CO (jCO) for 2MIMβNiPc/CNT, 2MIMαNiPc/CNT, and 2MIMCβNiPc/CNT are depicted in Figure 2d. Straightforwardly, the jCO of 2MIMCβNiPc/CNT increases significantly with the more negative potentials and reaches 38 mA cm−2 at −0.88 V, which is 2.2 and 11.0 times higher than those of 2MIMβNiPc/CNT and 2MIMαNiPc/CNT, respectively. To further verify the relevance of the surface active-sites to FECO and jCO, the electrocatalytic CO2RR performances of the above MDHMs with the same Ni contents on carbon fiber paper (CFP) were strictly measured based on the same system. From the result given in Supporting Information Figure S19, 2MIMCβNiPc/CNT maintains significant advantages for FECO and current density in a wide potential window, which is also highlighted by the turnover frequencies (TOFs) of 2MIMβNiPc/CNT, 2MIMαNiPc/CNT, and 2MIMCβNiPc/CNT for CO production (Figure 2e), indicating that flexibly grafting methylimidazole groups at the β positions of the phthalocyanine ring generates a significant promotion of electrocatalytic CO2RR properties. In addition, the much-lowered Tafel slope and smaller electrochemical impedance for 2MIMCβNiPc/CNT (Figures 2f,g and Supporting Information Table S2) indicate the more favorable electron kinetics for the CO2-to-CO conversion in comparison with 2MIMβNiPc/CNT and 2MIMαNiPc/CNT.54–57 Similarly, the electrochemically active surface area evaluated by double-layer capacitance (Cdl) of 2MIMCβNiPc/CNT is larger than those of 2MIMβNiPc/CNT and 2MIMαNiPc/CNT ( Supporting Information Figure S20 and Figure 2h), featuring the electrocatalytic CO2RR ability of 2MIMCβNiPc/CNT.58–60 Figure 2 | Electrocatalytic CO2RR performances of 2MIMβNiPc/CNT, 2MIMαNiPc/CNT, and 2MIMCβNiPc/CNT. (a) LSV curves at the scan rate of 10 mV s−1; (b) total current densities at different potentials; (c) FEs of CO; (d) partial CO current densities; (e) TOFs; (f) Tafel slopes; (g) Nyquist plots; (h) capacitance values; (i) durability test at the potential of −0.73 V; (j) comparison of j, FECO, and CEE after 12 h stability test; and (k) durability test of 2MIMCβNiPc/CNT at 0.2 A cm−2 in a liquid flow cell. Download figure Download PowerPoint Under the guidance of requirements for practical applications, the stable operation of the designed electrocatalyst is pivotal. As revealed in Figure 2i, in comparison with 2MIMβNiPc/CNT and 2MIMαNiPc/CNT, 2MIMCβNiPc/CNT maintains a stable current density and FECO at −0.73 V for 12 h of operation. This is further highlighted by the relative j, FECO, and half-cell cathodic energy efficiency (CEE) after the stability test ( Supporting Information Figure S21 and Figure 2j). Furthermore, the selectivity and durability of 2MIMCβNiPc/CNT were evaluated at different potentials in a liquid flow cell ( Supporting Information Figure S22), where no obvious change in current density and FECO of 2MIMCβNiPc/CNT was found after 20 h of electrocatalysis at 200 mA cm−2 (Figure 2k). The TEM image ( Supporting Information Figure S23), PXRD pattern ( Supporting Information Figure S24), and UV–vis spectrum ( Supporting Information Figure S25) of the tested 2MIMCβNiPc/CNT show that the catalyst structure remained intact during the electrocatalysis. Therefore, 2MIMCβNiPc/CNT with superior structural advantages exhibits remarkable activity and impressive stability for the electroreduction of CO2 to CO, which is not only one of the most competitive electrocatalysts ( Supporting Information Table S3) but also of great value for understanding the inherent relationships between the structures and catalytic properties of MDHMs. Activity trace Based on the design and preparation of 2MIMCβNiPc/CNT, the influences of the functional groups-involved self-built microenvironment, molecule dispersion, and surface active-site densities on the CO2RR performance were further studied to form an in-depth understanding of the activity origin.15,61 As can be seen in Figure 3a–c, after replacing methylimidazoles with thiophenes (Th), the generated ThCβNiPc/CNT shows inferior current densities as compared with 2MIMCβNiPc/CNT, whereas the corresponding FEs of CO were not suppressed, suggesting that the methylimidazole groups-involved self-built microenvironment in 2MIMCβNiPc/CNT could be beneficial to the CO2 reactant enrichment and proton transport for promoting electrochemical CO2RR ( Supporting Information Figure S26).28,45 To reveal the influence of the molecule dispersion on the activity, we substituted NMP with methanol as the solvent for anchoring 2MIMCβNiPc, which led to some aggregation of the 2MIMCβNiPc molecules on the CNT ( Supporting Information Figure S27). Under the same experimental conditions, the current densities of 2MIMCβNiPc/CNT prepared using methanol are lower compared with that prepared in NMP, as presented in Figure 3d,e, which could be due to the molecule aggregation leading to a decrease in the exposed Ni–N4 active sites and heterointerfacial interaction. Moreover, it seems that the formed 2MIMCβNiPc aggregates are unfavorable for achieving the higher CO selectivity in a wide potential window (Figure 3f). Furthermore, by controlling the loading of the 2MIMCβNiPc molecules onto the CNT, we unveiled the effect of surface active-site densities on the performance for electrochemical CO2RR (Figure 3g). With the same Ni contents on the CFP electrode, the as-designed electrocatalysts with different site densities delivered analogous current densities and stabilities at different potentials (Figure 3h), whereas the 2MIMCβNiPc/CNT with higher site density delivered higher FEs of CO, as illustrated in Figure 3i. As explored above, the introduction of suitable functional groups could provide a helpful microenvironment for CO2 enrichment and proton transport during CO2RR. In addition, tuning the dispersion and density of the surface active-sites may be a feasible strategy for MDHMs in enhancing the activity and selectivity of products. Figure 3 | Effect of functional groups on electrochemical CO2RR performance of MDHMs: (a) LSV curves (inset shows the molecular structures of NiPcs); (b) total current densities; (c) FEs of CO; effect of molecule dispersion on CO2RR performance of 2MIMCβNiPc/CNT: (d) LSV curves; (e) total current densities; (f) FEs of CO; effect of surface active-site densities on CO2RR performance of 2MIMCβNiPc/CNT: (g) LSV curves (inset shows surface active-site densities on CNT); (h) total current densities; and (i) FEs of CO. All electrodes have the same Ni content. Download figure Download PowerPoint Heterointerfacial effects To deeply grasp the structure–performance relationship of the investigated catalysts in electrochemical CO2RR, the spectroscopic analysis and density functional theory (DFT) calculations were carried out. As shown in Figure 4a and Supporting Information Figure S28, compared with 2MIMCβNiPc molecules, the Q-band absorption peak of 2MIMCβNiPc/CNT is red-shifted to 710 nm with ∼32 nm movement, which is close to that of 2MIMβNiPc/CNT and much larger than that of 2MIMαNiPc/CNT. The conspicuous shift observed for 2MIMCβNiPc/CNT declares the formation of a highly coupled heterointerface between 2MIMCβNiPc molecules and CNT substrate, which tunes the electronic structure of the active centers for achieving efficient CO2-to-CO activity. Meanwhile, the molecularly interfacial electron communication capacity in 2MIMCβNiPc/CNT is further verified by the XPS tests, as given in Supporting Information Figure S29 and Figure 4b. Upon anchoring onto CNT, the Ni 2p3/2 binding energy of 2MIMCβNiPc/CNT is positively shifted by ∼0.49 eV, exceeding those for 2MIMβNiPc/CNT (∼0.36 eV) and 2MIMαNiPc/CNT (∼0.13 eV) under the same conditions. The results demonstrate that, when the methylimidazole groups are flexibly grafted around the phthalocyanine ring, the charge transfer between the phthalocyanine ring and CNT will be obviously enhanced probably due to the smaller steric hindrance. Consequently, 2MIMCβNiPc/CNT displays the strongest heterointerfacial effect among these MDHMs, which could be conducive to the electrocatalytic activity. Furthermore, the heterointerface geometries of the designed electrocatalysts optimized by the DFT calculations are shown in Figure 4c–e. In detail, the predicted heterointerfacial distance between 2MIMCβNiPc and CNT is 3.54 Å, which is smaller than those of 2MIMαNiPc/CNT and 2MIMβNiPc/CNT, indicating its stronger heterointerfaci