Ligand Engineering in Nickel Phthalocyanine to Boost the Electrocatalytic Reduction of CO2

Designing and synthesizing efficient molecular catalysts may unlock the great challenge of controlling the CO2 reduction reaction (CO2RR) with molecular precision. Nickel phthalocyanine (NiPc) appears as a promising candidate for this task due to its adjustable Ni active‐site. However, the pristine NiPc suffers from poor activity and stability for CO2RR owing to the poor CO2 adsorption and activation at the bare Ni site. Here, a ligand‐tuned strategy is developed to enhance the catalytic performance and unveil the ligand effect of NiPc on CO2RR. Theoretical calculations and experimental results indicate that NiPc with electron‐donating substituents (hydroxyl or amino) can induce electronic localization at the Ni site which greatly enhances the CO2 adsorption and activation. Employing the optimal catalyst—an amino‐substituted NiPc—to convert CO2 into CO in a flow cell can achieve an ultrahigh activity and selectivity of 99.8% at current densities up to −400 mA cm−2. This work offers a novel strategy to regulate the electronic structure of active sites by ligand design and discloses the ligand‐directed catalysis of the tailored NiPc for highly efficient CO2RR.


Introduction
Electrocatalytic conversion of CO 2 into value-added chemicals using the surplus sustainable energy can be an appealing technology to mitigate environmental and energy issues. [1][2][3][4] The structural flexibility of phthalocyanine ring enables adjustable physical and chemical properties by tuning the periphery substituents of metal phthalocyanines. [22][23][24] In this sense, electron-donating substituents tend to provide more electrons to the π-conjugated system of the phthalocyanine, which is central to increase the electronic density and improve the catalytic performance of the metal site. [25,26] The amino and hydroxyl groups, with lone pair electrons from the 2p orbit of N or O, exhibit the prominent electron-donating properties via the p-π conjugation. [27,28] Compared with the hydroxyl group, a lower electronegativity of N atom from amino leads to an interior inductive effect, which enhances its electrondonating ability. [29] Furthermore, the Ni site is the active site for CO 2 RR, as such it directly determines the catalytic behavior of the electrochemical CO 2 conversion when employing NiPc as molecular catalysts. [30,31] Modifying NiPc with different electron-donating substituents can evoke the various electronic structures of active sites (Ni-N 4 ), which is beneficial to uncover the ligand-directed molecule catalysis of NiPc-based catalysts.
In this work, the ligand of NiPc was modified with amino or hydroxyl groups to improve its CO 2 RR performance and also to reveal the ligand-directed catalytic performance of tailored NiPc. Theoretical calculations and experimental results demonstrate that the electronic density at Ni site is positively correlated with the electron-donating capability of the substituents. Furthermore, an increased electronic density at Ni site facilitates the CO 2 adsorption and activation, enhancing the catalytic performance of the NiPc. In particular, the amino-modified NiPc shows the best performance for CO 2 RR compared with other NiPc molecular catalysts. This can be attributed to the stronger electron-donating property of amino groups compared to hydroxyl and hydrogen groups. Amino-substituted NiPc were tested in a gas flow cell, delivering a remarkable activity and ultrahigh selectivity of CO (>99.8%) for CO 2 RR across a large current range, expanding from −20 to −400 mA cm −2 . Phthalocyanine's ligands can regulate the electronic structure of the molecular catalyst in order to improve the CO 2 adsorption and activation, providing a new approach to modulate the electronic structure of active sites for enhanced CO 2 RR performance.

Theoretical Calculations
The structure models of the pristine NiPc and the NiPc with tetra hydroxyl (NiTHPc) and tetra amino (NiTAPc) substituents are illustrated in Figure 1a and Figure S1, Supporting Information. In order to explore the effect of the substituents on the NiPc system, the electronic density of the Ni site was analyzed. Hirshfeld and Mulliken charges of Ni ( Figure 1b) indicate that the electron can be localized on the Ni atom by electrondonating substituents of hydroxyl or amino groups, and that NiTAPc exhibits an apparent higher electronic localization than that of the NiTHPc due to the stronger electron-donating ability of the amino group. To study the effect of the electronic structure on the CO 2 adsorption, the adsorption energy of CO 2 on each NiPc molecule was examined (Figure 1c and Figure S2, Supporting Information). NiTAPc shows the best CO 2 adsorption ability (−0.228 eV) in contrast to those of NiTHPc (−0.22 eV) and NiPc (−0.218 eV). To understand the CO 2 RR performances of these molecular catalysts, the free energies of the CO 2 RR pathway were calculated (Figure 1d). According to the scheme of electrochemical reduction of CO 2 to CO, a proton-coupled electron transfer takes place at every reactionstep ( Figure S3, Supporting Information), [32] and the formation of *COOH is assumed as the first CO 2 activation step in the theoretical calculations. [33] The rate-determining step in each of these molecular catalysts is the CO 2 activation to *COOH, as shown in Figure 1d and Figure S4, Supporting Information. Since the *COOH can be adsorbed and hybridized with the d 2 z orbital of the metal in metal-complex catalysts, [34] the Ni d 2 z orbital level of different NiPc was investigated. The electrondonating substituents-hydroxyl or amino-can concentrate a higher electron density on the Ni site and thus raising it and facilitating the interaction with the lowest unoccupied molecule orbital (LUMO) of the COOH molecule ( Figure S5, Supporting Information), [35][36][37] which is reflected in the lower the free energy of the *COOH intermediate. NiTAPc has the lowest energy barrier for CO 2 activation, which manifests the rapid process for CO 2 RR in comparison to the NiTHPc and NiPc. According to these theoretical results, the electron-donating substituents of hydroxyl-or amino-modified NiPc can induce a higher electronic localization at the Ni site and thus enhances CO 2 adsorption and activation to boost the CO 2 RR.

Synthesis and Characterization of the Proposed Molecular Catalysts
To synthesize the proposed molecular catalysts, a facile solvothermal reaction was conducted by employing the substituted phthalonitrile and nickel acetate as reactants ( Figure S6, Supporting Information). [38] Matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) of as-prepared molecular catalysts (Figure 2a) shows the mass of the molecular ion at 630 and 634 from NiTHPc and NiTAPc, respectively, demonstrating the successful preparation of the proposed NiTAPc and NiTHPc. Moreover, 1 HNMR spectra of NiTHPc and NiTAPc (Figure 2b) in deuterated dimethyl sulfoxide (DMSO-d 6 ) further certify the chemical structure of the as-synthesized molecules. [39,40] As shown in Figure S7, Supporting Information, the peak area was carefully calculated. The hydrogen content of the amino substituent is closely twice compared with the hydrogen of the ring in NiTAPc, while the hydrogen content in the hydroxyl group is close to the amount of hydrogen atoms of the ring in NiTHPc. This further reflects that there are four substituents in both NiTAPc and NiTHPc. In summary, these results confirm the successful synthesis of the substituted NiPc. Fourier transform infrared spectroscopy (FTIR) spectra ( Figure 2c and Figure S8, Supporting Information) prove the presence of hydroxyl and amino groups in NiTHPc and NiTAPc, respectively, without the characteristic peak of phthalonitrile at around 2230 cm −1 . Ultraviolet-visible (UV-vis) spectroscopy was used to further characterize the NiPc, NiTHPc, and NiTAPc owing to the substitute-dependent optical response. As shown in Figure 2d, the substituted NiPc exhibits the characteristic Q band absorption at around 600-800 nm as well as the B band absorption, [41] without the absorption band of the nickel acetate ( Figure S9, Supporting Information). Furthermore, the Q band absorption is attributed to the π → π* electron transition from the highest occupied  orbital to the LUMO of the phthalocyanine ring. [42] The Q band absorption without splitting indicates the symmetrical substituent position of the synthesized molecules. [43] Moreover, there is a red shift in the Q band absorption among NiTHPc and NiTAPc due to the increased electron-donating ability of the amino substituent, [44,45] which highly matches the results of our theoretical calculations on the energy of the optical transitions in each case ( Figure S10, Supporting Information). X-ray powder diffraction (XRD) patterns ( Figure S11, Supporting Information) show similar signal peaks for all samples below 10°, suggesting an analogous molecular structure of the synthetized samples. Raman spectra of NiPc, NiTHPc, and NiTAPc ( Figure S12, Supporting Information) exhibit the same sensitive band at 1545 cm −1 , which can be attributed to the Ni interaction with the phthalocyanine ring. [46] Based on the abovementioned results, we have demonstrated the successful preparation of the target molecular catalysts: NiTHPc and NiTAPc.
To further investigate the structure-dependent electronic effects of the synthetized molecules, X-ray photoelectron spectroscopy (XPS) and synchrotron X-ray absorption spectroscopy (XAS) were measured. From the XPS survey scan and N 1s spectra of NiPc, NiTHPc, and NiTAPc in Figure S13 and Discussion S1, Supporting Information, the amino and hydroxyl groups were successfully incorporated in NiTHPc and NiTAPc, respectively. [47] The C and N K-edge of the molecules were measured ( Figure S14 and Discussion S2, Supporting Information). The substituents have no measurable effects on the C K-edge spectra of the molecules. However, the N K-edge spectra of NiTHPc and NiTAPc can be greatly influenced by the electron-donating character of the substituents. Noticeably, the binding energy of Ni 2p (Figure 3a) has a negative shift in NiTHPc and NiTAPc compared with that of the pristine NiPc. Accordingly, XAS spectra of the Ni L-edge (Figure 3b) also show a negative shift in the absorption peaks of NiTHPc and NiTAPc in contrast to the pristine NiPc. NiTAPc displays a remarkable shift in the absorption energy compared with that of the NiTHPc owing to the increased electron-donating ability of the amino substituents. To further confirm the electronic localization at the central Ni atom, X-ray absorption near-edge spectra (XANES) were conducted. XANES spectra of Ni K-edge ( Figure 3c) shows a negative shift in the NiTHPc and in the NiTAPc compared to that of the traditional NiPc, demonstrating that the electron of phthalocyanine ligands could be localized on the Ni site by electron-donating substituents. NiTAPc shows the most apparent electronic localization among the NiPc-based molecules, in good agreement with the results of XPS, XAS of Ni L-edge, and theoretical calculations. In addition, extended X-ray absorption fine structure (EXAFS) spectra of Ni K-edge (Figure 3d), the wavelet transform plot ( Figure S15, Supporting Information), and EXAFS fitting ( Figure S16 and Table S1, Supporting Information) demonstrate the similar coordination number and structure of Ni in each catalyst. Thus, the electronic localization on the Ni site is derived from the peripheral substituents rather than the changed in the coordination number of Ni site. These results indicate that substituents with electron-donating ability can arouse the electronic localization on the central Ni atom, and that the degree of electronic localization is positively correlated with the electron-donating ability of the substituents. To disperse the molecules for the electrocatalytic CO 2 RR, NiPc, NiTHPc, and NiTAPc were supported on conductive carbon nanotubes to form the respective electrocatalysts: NiPc/ CNT, NiTHPc/CNT, and NiTAPc/CNT. Scanning electron microscope (SEM) images ( Figure S17, Supporting Information) and the elemental mapping of N ( Figure S18, Supporting Information) from the high-angle annular dark field-scanning transmission electron microscope (HAADF-STEM) demonstrate that the molecules can be well dispersed in the carbon nanotubes. The results of XRD and Raman from the composite catalysts ( Figure S19, Supporting Information) suggest the presence of carbon. [48,49] XANES Ni K-edge spectra of the molecular catalysts and its mixture with the conductive carbon ( Figure S20a-c, Supporting Information) show the stable electronic structure of the Ni site. Accordingly, EXAFS spectra of the Ni K-edge ( Figure S20d, Supporting Information) manifest the NiPc-like structure of all the electrocatalysts. It can be concluded that molecular catalysts can be well dispersed onto carbon nanotubes, and that they maintain the pristine electronic structure of the Ni sites in the composite catalysts.

CO 2 Adsorption Measurements
According to thermogravimetric analysis of the samples ( Figure S21a, Supporting Information), the CO 2 absorption ability of each catalytic site was further identified by CO 2 temperatureprogrammed desorption (TPD) measurements. Compared with the TPD result of the phthalocyanine (H 2 Pc) without Ni site ( Figure S21b, Supporting Information), it can be determined that the CO 2 desorption peak before 120 °C originates from a physisorption contribution on the Ni site in the NiPc-based catalysts (Figure 4a). The CO 2 adsorption signal at 180 °C can be assigned to the CO 2 absorption on the amino. [50] The pristine NiPc with the electron-deficient Ni site, displays a negligible CO 2 adsorption with a low CO 2 desorption temperature ( Figure 4a). On the other hand, NiTHPc and NiTAPc with the electronic localization of Ni sites possess moderate and superior CO 2 desorption temperature, respectively. NiTAPc shows the optimal CO 2 adsorption due to its conspicuous electronic localization among the three of the NiPc-based catalysts. Furthermore, the CO 2 adsorption responses of the NiPc/CNT, NiTHPc/CNT, and NiTAPc/CNT were also explored ( Figure S22, Supporting Information). N 2 was used as control gas to study the CO 2 adsorption responses. NiTAPc/CNT exhibits a remarkable response in the CO 2 atmosphere (Figure 4b), outperforming the moderate response of NiTHPc/CNT and the inferior response of NiPc/CNT. These results indicate that the enhanced CO 2 adsorption is enabled by the electronic localization of Ni sites, in accordance with the results from the previous sections.

Electrocatalytic Reduction of CO 2 in an H-Type Cell
To evaluate the catalytic performance of developed catalysts, electrochemical measurements were carried out in the H-type cell. The current densities in this work are referred to geometric surface of the electrode. Linear sweep voltammetry (LSV) curves in the CO 2 and Ar-saturated 0.5 m KHCO 3 electrolyte (Figure 5a) demonstrate that the composite catalysts have apparent catalytic activities for CO 2 RR. However, the blank carbon nanotube shows no activity for CO 2 RR ( Figure S23, Supporting Information). H 2 Pc-without the Ni center-supported on carbon nanotubes (H 2 Pc/CNT) was employed as control sample, showing a negligible performance for CO 2 RR ( Figure S24, Supporting Information). This demonstrates that the Ni site of NiPc is the active site for the CO 2 RR. [13,51] To further confirm the importance of the Ni active site, the free energy of the rate-determining step-from CO 2 to *COOHon both the amino and hydroxyl substituted molecules was also investigated. From Figure S25, Supporting Information, the COOH cannot interact with the O and N from the amino and hydroxyl groups, respectively. This result indicates that the substituents by themselves do not possess catalytic activity for CO 2 RR. According to these results, the Ni site is the solely active site for CO 2 RR.
Electrochemical active surface area (ECSA) measurements were performed according to the electrochemical redox behavior of NiPc-based molecules. As displayed in Figure S26, Supporting Information, the reduction peaks around 0 V are ascribed to the reduction of the phthalocyanine ring in the molecular catalysts. [20] The area of the reduction peak (NiPc/ CNT > NiTHPc/CNT > NiTAPc/CNT) indicates a superior ECSA of NiPc/CNT, moderate ECSA of NiTHPc/CNT, and inferior ECSA of NiTAPc/CNT, which corresponds well with the results of inductively coupled plasma-optical emission spectrometry (ICP-OES) in Table S2, Supporting Information. From the LSV in the CO 2 -saturated electrolyte (Figure 5a), the aminosubstituted NiPc with superior electronic localization of Ni site exhibits the best performance of CO 2 RR compared to the other catalysts. The hydroxyl and amino electron-donating substituents can greatly improve the catalytic activity of NiPc. Tafel slopes from the corresponding LSV ( Figure S27, Supporting Information) demonstrate the fastest dynamic processes of NiTAPc for CO 2 RR among the NiPc-based molecular catalysts. Therefore, the catalytic performances result from the intrinsic activity of the Ni site from the molecular catalysts.
To obtain the product distribution of the CO 2 RR at different potentials, chronoamperometry measurements ( Figure S28, Supporting Information) were carried out. Carbon monoxide (CO) can be detected as the only product from the results of gas chromatography ( Figure S29, Supporting Information) and 1 HNMR spectroscopy ( Figure S30, Supporting Information). Figure 5b shows the Faradaic efficiency of CO (FE CO ). The FE of hydrogen (FE H2 ) is also displayed in Figure S31, Supporting Information. It can be seen that the FE CO of the modified catalysts maintain a level of above 90% in the potential range from −0.58 to −0.90 V. NiTHPc/CNT and NiTAPc/CNT demonstrates an enhanced selectivity of CO for CO 2 RR in comparison to the NiPc/CNT, which can be attributed to electron-donating substituents that induce electronic localization of the Ni site. Although a high selectivity of CO in NiPc/CNT can be achieved, the stability (Figure 5c) is much lower than those of NiTHPc/CNT and NiTAPc/CNT, undergoing a fast degradation during the continuous CO 2 RR catalysis. NiTAPc/CNT can maintain a great stability. Impressively, NiTAPc/CNT shows a remarkable CO selectivity of almost 100% and a high activity (−12.2 mA cm −2 at −0.58 V and −16.7 mA cm −2 at −0.62 V), exceeding the performances of not only all the other molecular catalysts synthetized here but also most of the ones reported in the literature so far (Table S3, Supporting Information). This can be attributed to the stronger electron-donating capability of amino in NiTAPc than those of other substituents of NiPc in the reported works. Electrochemical impedance spectroscopy (EIS) was employed to explore the reaction dynamics of the catalysts during CO 2 RR. As exhibited in Figure 5d, the charge transfer resistance of the catalysts can be ranked in the following order: NiPc/ CNT > NiTHPc/CNT > NiTAPc/CNT, which demonstrates the fastest reaction dynamics of NiTAPc/CNT. The substituted NiPc with electron-donating groups encourages the electronic localization at the Ni atom and therefore promotes CO 2 RR performance in terms of activity, selectivity, and stability.

Electrocatalytic Reduction of CO 2 in a Flow Cell
To study the CO 2 RR performance at higher current densities, the composite molecular catalysts were supported on the gas diffusion layer (GDL) to form gas diffusion electrodes. In this way, a fast CO 2 diffusion can be obtained via the gas phase to the catalytic sites, which overcomes the limited mass transfer in the H-cell. The schematic illustration of the flow cell is displayed in Figure S32, Supporting Information. The pristine NiPc/ CNT loaded on the GDL shows a stable CO 2 RR performance from the LSV measurements in CO 2 ( Figure S33a, Supporting Information). However, this catalyst suffers from serious degradation for CO 2 RR due to the rupture of Ni-N 4 structure after LSV tests in Ar atmosphere ( Figure S33a, Supporting Information). [20] Furthermore, NiPc/CNT with the rupture of Ni-N 4 structure displays extremely low CO selectivity for CO 2 RR ( Figure S33b, Supporting Information), indicating that the stable structure of NiPc is the prerequisite for catalytic conversion of CO 2 into CO. Importantly, the original NiPc/CNT with adequate CO 2 in the flow cell displays a stable performance at a low current density of −60 mA cm −2 ( Figure S33c, Supporting Information), which stands in stark contrast to the poor stability of NiPc/CNT in the H-cell due to the limited CO 2 mass transfer. These situations demonstrate that the inferior stability of NiPc/CNT is aroused by its poor CO 2 adsorption with the low local concentration of CO 2 . When the current density increased at −200 mA cm −2 (Figure 6a), the performance of NiPc/CNT is dramatically declined due to limited CO 2 adsorption (Figure 6b). In contrast, the GDL coated with NiTAPc/CNT or NiTHPc/CNT can be operated at current densities from −20 to −500 mA cm −2 at applied potentials less than −0.80 V versus reversible hydrogen electrode (RHE) (Figure 6a). The EIS spectrum of the NiTAPc/CNT GDL ( Figure S34, Supporting Information) shows the relatively stable resistance after chronopotentiometry measurements at different current densities. An ultrahigh current density of −500 mA cm −2 can be obtained from NiTAPc/ CNT with the FE CO of 94.6%. NiTAPc/CNT displays a remarkable performance with high FE CO >99.8% between current densities of −20 and −400 mA cm −2 (Figure 6b), which is much better than other NiPc counterparts and even other reported catalysts (Table S3, Supporting Information). The operando Raman spectra ( Figure S35a-c, Supporting Information) prove the structure stability of Ni−N at around 250 cm −1 in molecule catalysts during the CO 2 electrolysis with adequate CO 2 . [15,52] The Ni 2p XPS spectra of molecule catalysts demonstrate the table binding energy of Ni after CO 2 electrolysis in H-cell also ( Figure S35d, Supporting Information, and Figure 3a). The operando FTIR spectra in Figure S36, Supporting Information, further confirm the reaction intermediates of *COOH from NiTAPc as well as the theoretical results. [53] Since the molecular catalysts are well dispersed onto the carbon nanotubes in the composite catalysts, we assumed that Ni-determined from the results of ICP-OES (Table S1, Supporting Information)-can act as the active sites. The turnover frequency of CO (TOF CO ) from various catalysts (Figure 6c) can then be calculated. The optimal catalyst, NiTAPc/CNT, can achieve a remarkable TOF CO of 41.9 s −1 , outperforming that of NiTHPc/CNT (28.0 s −1 ) and NiPc/CNT (10.0 s −1 ). To obtain the stability measurement of NiTAPc/CNT, the CO 2 RR performance was conducted at −150 mA cm −2 . The catalyst can maintain 10 h with the selectivity of CO over 99.8% (Figure 6d). The electrolysis time largely relies on the hydrophobicity of the GDL, which is influenced by the applied potential for electrolysis. [54] When the current density is decreased to −100 mA cm −2 , the stability of the NiTAPc/CNT electrode can be extended to 30 h with a high selectivity for CO of over 99.8% ( Figure S37a, Supporting Information). Conversely, the NiTAPc/CNT can only maintain 3 h at the current density of −400 mA cm −2 with CO selectivity above 90% ( Figure S37b, Supporting Information), which can be attributed to its more negative potential leading to decreased hydrophobicity of the GDL. On the basis of these data, the NiTAPc/CNT delivers most superb and stable performance for CO 2 RR among the molecular catalysts tested in here.

Ligand of NiPc Modified with Electron-Withdraw Substituents for CO 2 Reduction Reaction
To further verify the significance of the electronic localization at the Ni site for CO 2 RR, a tert-butyl substituted NiPc (NiTBPc) was synthesized ( Figure S38, Supporting Information). Tertbutyl is considered as a typical electron-withdraw substituent in phthalocyanines. [25,55] MALDI-TOF MS spectrum, FTIR spectra, UV-vis spectrum, and Raman spectrum prove the successful preparation of NiTBPc ( Figure S39, Supporting Information). SEM and HAADF-STEM characterizations ( Figure S40, Supporting Information) suggest that NiTBPc can be well dispersed onto the carbon nanotubes. Even the NiTBPc possesses an interior electron-withdraw ability due to the tert-butyl substituent; there is no obvious shift in the binding energy of Ni 2p and the absorption energy of Ni L-edge ( Figure S41a,b, Supporting Information). However, the XANES spectra of Ni K-edge in NiTBPc ( Figure S41c, Supporting Information) shows a positive shift of near-edge absorption compared with that of the NiPc, and this shift can be even more obvious ( Figure S41d, Supporting Information) after dispersing the molecular catalyst onto carbon nanotubes, indicating that there is an electronic delocalization on the Ni site in NiTBPc/CNT when compared to NiPc/CNT. EXAFS spectrum and wavelet transform plot of NiTBPc ( Figure S42, Supporting Information) demonstrate a similar structures to the NiPc. As expected, from the H-cell measurements of CO 2 RR, NiTBPc/CNT shows poor stability and exhibits low activity and selectivity in contrast to NiPc/ CNT ( Figure S43, Supporting Information). In the flow cell, the activity and selectivity of NiTBPc/CNT are lower than all the other composite counterparts ( Figure S44, Supporting Information) due to its electron-withdraw tert-butyl substituent. Thus, the electron-withdraw substituent that induces the electronic delocalization of Ni site can inhibit the catalytic activity of NiPc for CO 2 RR. This final control experiment further demonstrates that the electronic localization of the Ni site plays a significant role in boosting the CO 2 RR.

Conclusion
In summary, we developed a series of electron-donating substituents to modify peripheral ligand of NiPc, which can induce a remarkable electronic localization to increase the electronic density at the metal active site in this system. Theoretical calculations, XPS, and XAS results demonstrate the electronic localization at the Ni site induced by the ligand replacement. This electronic tuning of the active site is able to enhance the CO 2 adsorption and activation, and thus to improve the activity and stability of NiPc for CO 2 RR. Conversely, the electronwithdraw substituent triggers the electronic delocalization of the Ni site, resulting in a worse performance for the CO 2 RR. NiTAPc/CNT-with a significant electronic localization of the Ni site-in GDL delivered an ultrahigh activity and CO selectivity of 99.8% even at a high current density of −400 mA cm −2 . This work offers a new approach to tune the electronic density of the catalytic sites for enhanced CO 2 RR and it presents a valuable guidance to develop efficient molecular catalysts for future electrocatalytic processes.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.