Enhancing CO2 Reduction Efficiency on Cobalt Phthalocyanine via Axial Ligation

Electrochemical reduction of carbon dioxide (CO2RR) to value‐added products is a promising strategy to alleviate the greenhouse gas effect. Molecular catalysts, such as cobalt (II) phthalocyanine (CoPc), are known to be efficient electrocatalysts that are capable of converting CO2 into carbon monoxide (CO). Herein, we report an axial modification strategy to enhance CoPc's CO2RR performance. After coordinating with axial ligands, the electron density of Co was depleted via π‐backbonding. This π‐backbonding weakened the Co‐CO bond, resulting in rapid desorption of CO. Also, the presence axial ligands elevated the Co dz2 orbital energy, resulting in a significantly enhanced CO selectivity, evidenced by an increased faradaic efficiency (FE) from 82 % (CoPc) to 91 % and 94 % with the presence of pyridine (CoPc‐py) and imidizal ligands (CoPc‐im), respectively, at −0.82 V vs. RHE. Density functional theory calculations reveal that axial ligation of CoPc can reduce the energy barrier for CO2 activation and facilitate the formation of *COOH.


Introduction
Effective conversion of carbon dioxide (CO 2 ) to value-added products is an attractive strategy to alleviate global warming by reducing atmospheric CO 2 concentration while providing renewable pathways to address long-term seasonal energy shortage and chemical production. [1] Electrochemical reduction of CO 2 has gained significant interest as it is a sustainable approach that operates under ambient conditions and is driven by renewable energy sources, such as wind and solar energy. [2] However, the progress of carbon dioxide reduction reaction (CO 2 RR) is currently hindered by the lack of cost-effective catalysts with satisfactory efficiency, selectivity, and long-term stability. [3] Metal complexes with abundant redox-active sites for CO 2 adsorption and activation have attracted substantial attention. [4] Among them, transition metal phthalocyanines with a tunable coordination structure and well-defined catalytic active site are deemed to be promising molecular catalysts, [5] especially cobalt phthalocyanine (CoPc). [6] For example, previous work showed that the efficiency and selectivity of CoPc can be accelerated by modifying its peripheral functional substituents to fine-tune the electron density of the metal center. [7] Alternatively, immobilized CoPc on various carbon supports [8] can enhance its CO 2 RR performance by facilitating electron transfer from the carbon supports to the catalytic active sites. Recently, axial ligation of CoPc has been proven to be an alternative means to escalate the performance of CoPc. For instance, Cruz et al. [9] demonstrated that the performance of CO 2 RR of axial ligands-modified CoPc is directly correlated with the σ-donor strength of the axial ligands. Although axial ligation of CoPc has been reported to be an efficient strategy to enhance its CO 2 RR performance, the function of the axial ligands, especially how they influence molecule packing, electronic structure, and redox properties of CoPc, is not clear. [10] Herein, we report an axial ligand modification strategy of CoPc to enhance the CO 2 RR performance. Structural characterization demonstrates that the molecular structures of modified CoPc were shifted from the original β-phase to α-phase and clear Davydov splitting was observed in the Ultraviolet-visible (UV-vis) spectrum when imidazole was employed as the axial ligand. Additionally, the Co center electron density was depleted through π-backbonding from the filled Co d orbital to the unoccupied π* orbitals of axial ligands. Electrochemical measurements show that pyridine-modified (CoPc-py) and imidazole-modified (CoPc-im) CoPc possess an enhanced CO generation Faradaic Efficiency (FE) up to 91 % (CoPc-py) and 94 % (CoPc-im) compared to 82 % (CoPc) at À 0.82 V vs. RHE. Moreover, the current density increased from 2.99 mA/cm 2 (CoPc) to 6.15 mA/cm 2 (CoPc-py) and 5.93 mA/cm 2 (CoPc-im). The structural characterization and density functional theory (DFT) calculations reveal that, firstly, axial ligation elevates the dz 2 orbital energy and decreases the electron density of cobalt center, leading to the one-electron reduced compounds ([CoPc-L] À , L is the axial ligand) to be more nucleophilic. This facilitates the adsorption of CO 2 and improves interactions between the reduced compounds and CO 2 . Additionally, the DFT calculations show the energy barrier of the rate-determining step in CO 2 activation decreased with the axial ligand modification, which further clarifies the role of axial ligand in enhancing the performance of CO 2 RR.

Synthesis and Characterization
The axial ligands modified CoPc ( Figure 1a) catalysts: CoPc-py ( Figure 1b) and CoPc-im ( Figure 1c) were prepared through a facile reaction by reacting CoPc and pyridine (or imidazole) to form the five-coordinated square pyramidal structure (Supporting Information).
CoPc powder exhibits clear diffraction peaks at 2θ = 7.108°a nd 2θ = 9.247° (Figure 2a), corresponding to the (100) and (102) crystal planes of the CoPc (Table S1), respectively. This indicates the presence of the β-CoPc ( Figure S1a-b, PDF# 14-0948) crystal structure (Table S2), in which the adjacent columns of molecules are almost perpendicular. Characteristic peaks of α-CoPc ( Figure S1c -d, PDF # 44-1994), such as 2θ = 16.233°and 16.703° (Table S1) are observed in CoPc-py and CoPc-im, respectively, which implies that incorporating the axial ligands change the preferential packing structure of CoPc. [11] UV-vis spectroscopy was used to further characterize the CoPc, CoPcpy, and CoPc-im owing to their substitute-dependent optical responses. The UV-vis spectrum of CoPc ( Figure 2b) exhibits two characteristic peaks: a weak B band (or Soret band), resulting from the transition between deeper π orbitals to the lowest unoccupied molecular orbital (LUMO) and an intense Q band, which is attributed to the π!π* electron transition from the highest occupied molecular orbital (HOMO) to the LUMO of the phthalocyanine ring. [12] The addition of the axial ligand caused a blue shift in the CoPc Q band (from 669 nm to 660 nm for CoPc-py and CoPc-im). This indicates a decrease in the electron density of the cobalt center on CoPc, attributed to the π-backdonation from the filled d orbital of Co to a π* orbital of the axial ligand. [13] Interestingly, the Q band of CoPc-im splits into two bands, between 660 nm and 740 nm, known as Davydov splitting. [14] The Davydov splitting usually occurs from the interaction between transition dipole moments of different orientations of molecules that are adjacent to each other at a close enough distance to give electronic transitions. [15] The strong Davydov splitting indicates that axial imidazole coordination leads to a herringbone arrangement in CoPc-im. [16] The change of molecular arrangement in CoPc-py and CoPc-im as indicated by UV-vis is in good agreement with XRD results.
Fourier transform infrared spectroscopy (FTIR) further confirms the structure of CoPc-py and CoPc-im. As shown in Figure 2c, characteristic CÀ H out-of-plane deformation is identified at 732 cm À 1 in CoPc, while the similar absorption bands for  CoPc-py and CoPc-im are located at 724 cm À 1 and 725 cm À 1 , respectively. For CoPc, the C=N in plane stretching vibration and CÀ H in plane deformation absorption are further identified at 780 cm À 1 and 1087 cm À 1 , respectively. In comparison, those two characteristic peaks in CoPc-py and CoPc-im shift to 774 cm À 1 , 1091 cm À 1 , 777 cm À 1 and 1090 cm À 1 , respectively ( Figure S3 and Table S3). The shift of characteristic peaks further suggests that the addition of axial ligands will lead to the molecular stacking model changes. Moreover, enhanced characteristic stretches of C=N [17] and CÀ N [18] at 1607 and 1250 cm À 1 are observed in CoPc-py and CoPc-im, respectively, which are ascribed to the presence of axial pyridine and imidazole on CoPc. In addition, Raman spectra ( Figure S4) show similar peak shifts which support the FTIR results. In summary, CoPc-py and CoPc-im exhibit a near α-phase crystal structure compared to βphase, which shows a distinguishable molecular stacking model and arrangement, as reflected in results from XRD, FTIR, and Raman spectroscopy.
The electronic effect of the axial ligand was further investigated by X-ray photoelectron spectroscopy (XPS). In the Co 2p spectrum, compared to the pristine CoPc, after axialligand modification, Co 2p peaks shifted to a higher binding energy, which indicates a decrease in electron density of the Co center ( Figure 2d). In addition, to differentiate Co II and Co III , the Co 2p 1/2 -2p 3/2 spin-orbital energy spacing is used, where a 16.0 eV difference indicates the existence of high-spin Co II whereas a 15.0 eV difference represents the presence of lowspin Co III . [19] The Co 2p 1/2 -Co 2p 3/2 spin-orbital energy spacings are 15.40 eV, 15.50 eV, and 15.10 eV in CoPc, CoPc-py and CoPcim, respectively. These values are between 15.0 eV and 16.0 eV, indicating the coexistence of Co II and Co III . After the deconvolution of Co 2p XPS spectra, a higher ratio of Co III /Co II (Table S4) was observed in axial ligands-modified CoPc (CoPc-py and CoPc-im) compared to CoPc. This phenomenon indicates a reduced electron density on the Co center upon the attachment of axially anchored ligands. Meanwhile, a relatively higher percentage of CoÀ N x species was observed from N 1s XPS spectra ( Figure S6 and Table S5) in the axial ligand-coordinated samples. In addition, in CoPc-im, pyrrolic N is identified, confirming the successful coordination of axial imidazole. [20] To further understand the local structure of CoPc-py and CoPc-im, X-ray absorption near-edge structures (XANES) and extended X-ray absorption fine structure (EXAFS) of Co were carried out. The Co K-edge XANES spectrum (Figure 2e) of CoPc exhibits two characteristic adsorption peaks that are the fingerprints of the Co-N 4 structure: a weak peak at 7709 eV, that is assigned to the dipole forbidden 1s to 3d transition; and a strong peak at 7716 eV, that is attributed to the 1s to 4p z shakedown satellite. [10b] Compared to that of CoPc, the 1s to 4p z peaks at 7716 eV are much weaker in CoPc-py and CoPc-im, which agrees with a transformation from a 4-coordinate square planar geometry (CoPc) to a 5-coordinate square-pyramidal structure (CoPc-py and CoPc-im). [10b] The first derivatives of the Co K-edge XANES ( Figure S7a) confirmed the change of Co valence state in CoPc-py and CoPc-im, where more Co III species were presented after coordinating with the axial ligand, consistent with the XPS results. In addition, the R distance (without phase correction) between Co and N at the first coordination sphere (CoÀ N 1 ) (1.47 Å) remained unchanged in CoPc-py and CoPc-im ( Figure S7b, Table S6) compared to CoPc. However, the bond distances of CoÀ C and CoÀ N 2 at the second coordination sphere were reduced from 2.55 Å (CoPc) to 2.52 Å (CoPc-py and CoPc-im), and from 3.62 Å (CoPc) to 3.59 Å (CoPcpy) and 3.56 Å (CoPc-im), respectively. The decreased CoÀ C and Co-N 2 distances in CoPc-py and CoPc-im suggest a distortion of phthalocyanine molecule that results from coordinating with the axial ligand. The apparent difference in k-space further confirmed this transformation (Figure 2f). The coordination number of the Co atom in CoPc-py and CoPc-im is determined to be five via the EXAFS fitting, indicating the formation of a square-pyramidal structure. The additional axial coordination (CoÀ N 3 ) is introduced from pyridine or imidazole to the Co center (Table S7). These results together suggested that CoPcpy and CoPc-im with square pyramidal coordination geometries were successfully synthesized by coordinating with the axial ligands.

Electrochemical Measurements
The electrochemical properties and catalytic performance of the axial ligand-modified CoPc were accessed. Herein, cyclic voltammograms (CV) measurements were obtained in the Arsaturated 0.5 M KHCO 3 (pH 8.94) (Figure 3a and Figure S8a). The reversible redox peak (R 1 ), corresponding to Co II /Co I , at around E pc = À 0.58 V (vs. Ag/AgCl), was revealed. [21] In addition, the peak current intensity increased as a function of the scan rate. (Figure S9). In addition, the irreversible peak appearing at E pa = 0.70 V (vs. Ag/AgCl, Ox 2 ), is attributed to Co II /Co III . Additionally, in CoPc-im, the peak present at E pa = 0.79 V (vs. Ag/AgCl, Ox 3 ) in the anodic scan is assigned to the oxidation of the phthalocyanine macrocycle (Table S8). [22] As shown in Figure S8b, no extra waves were observed from imidazole and pyridine CV scans, which suggests the redox peaks are from the CoPc molecules instead of the organic ligands. The enhancement of the Co II /Co III peak in CoPc-py and CoPc-im aligns well with the XPS results that axial ligands induce the formation of more Co III species. The increased Co III species in CoPc-py and CoPc-im are attributed to the cobalt-to-axial ligand π-backbonding charge transfer.
To evaluate the electrocatalytic performance of the developed catalysts, electrochemical measurements were carried out in a gas-tight H-type cell with a standard three-electrode configuration. Initially, the cyclic voltammetry (CV) was measured in 0.5 M KHCO 3 solution saturated with CO 2 or Ar. CV of CoPc-py and CoPc-im exhibited a broad irreversible cathodic wave (at~À 1.50 V vs. Ag/AgCl) in the presence of CO 2 , which is attributed to the increased CO 2 RR catalytic activity of catalysts. In contrast, there is no peak at a similar position in the presence of Ar (Figure 3b and Figure S10). Linear sweep voltammetry (LSV) was also recorded in a CO 2 -saturated KHCO 3 solution (0.5 M, pH 7.30). Compared to CoPc, enhanced cathodic current density and positively shifted onset potentials of CoPc-py and CoPC-im (Figure 3c) were noticed. These results show that when CoPc is modified by axial ligands, its catalytic activity for CO 2 RR is improved.
To quantify the activity and selectivity of the modified CoPc towards CO 2 RR, the controlled-potential electrolysis was operated in a CO 2 -saturated KHCO 3 solution (0.5 M, pH 7.30). First, the gas and liquid products were identified by gas chromatography (GC) and 1 H nuclear magnetic resonance spectroscopy ( 1 H NMR), respectively. Carbon monoxide (CO) was identified as the major product, accompanied by H 2 as the byproduct. In addition, no liquid products were detected, as confirmed by the 1 H NMR ( Figure S11). The FE of CO and the current density ( Figure 3d, 3 e) showed escalated values on CoPc-py and CoPcim compared to CoPc in the potential range from À 0.52 V to À 1.02 V. For CoPc catalysts, the optimal FE of 82 � 1.2 % was obtained at À 0.82 V vs. RHE. At the same potential, CoPc-py and CoPc-im exhibited enhancements of FE which were 91 � 0.9 % and 94 � 0.8 %, respectively. Moreover, CoPc-py and CoPcim achieved about twice current densities of 6.15 mA/cm 2 and 5.93 mA/cm 2 , respectively, relative to 2.99 mA/cm 2 of CoPc. Meanwhile, the turnover frequency (TOF) for CO was calculated based on the total moles of molecular catalysts immobilized on carbon fiber paper (CFP). It is worth noting that in this work, the active sites were not quantified by integrating the CV peak for Co II to Co I , due to the difficulty in accessing an accurate result from the low loading amount of catalysts on CFP. The TOFs for CO production of CoPc-py and CoPc-im was calculated to be 597 h À 1 and 565 h À 1 , respectively, which was approximately 2.3 times higher than that of CoPc (255 h À 1 ) at À 0.82 V (Figure 3f). Additionally, CoPc-py and CoPc-im exhibit higher electrochemical active surface area (ECSA) than that of CoPc, which is directly proportional to an increased double-layer capacitance (C dl ) as shown in Figure S13 and S14a. The ECSAnormalized CO partial current density was demonstrated in Figure S14b. Lastly, stability is an important criterion to describe the performance of CO 2 RR catalysts. To determine the stability of CoPc-py and CoPc-im, long-term electrolysis at the potential of À 0.82 V vs. RHE ( Figure S16) was conducted. Both CoPc-py and CoPc-im showed stable current densities with small decays for around 5 h and FEs remained above 90 %, indicating that both catalysts have excellent durability under these operating conditions. Meanwhile, Co 2p XPS spectra were further performed to investigate the stability. As shown in Co 2p XPS spectra ( Figure S17), after electrolysis (À 0.82 V vs. RHE) for 5 hours, Co 2p spectra of CoPc-py and CoPc-im exhibit 0.43 eV and 0.61 eV binding energy shift compared to bare CoPc, respectively. The positive binding energy shift of Co center indicates the presence of axial ligands after long-term electrolysis. However, compared to the original CoPc-py and CoPc-im, after long-term electrolysis, the binding energy shift of CoPc-py (or CoPc-im) versus CoPc becomes smaller, which suggests the axial ligands partial dissociation during the CO 2 RR, agreeing well with the current density decay in the stability tests.
From CV analyses of CoPc-py and CoPc-im, the Co II /Co III peaks (at 0.70 V and 0.57 V vs. Ag/AgCl for CoPc-py and CoPcim, respectively) were found to be irreversible (Figure 3a). In addition, the redox peaks of Co II /Co I (at~À 0.58 V vs. Ag/AgCl) can be identified when scanning from the anodic direction. This result, together with previously reported work, [8b,23] strongly suggests that the catalytic reaction was triggered by the reduction of Co II to Co I in all samples even though CoPc-py and CoPc-im contain more Co III species. Therefore, one-electron reduced CoPc-L is the active species for CO 2 RR (Scheme S1).

Density Functional Theory Calculations
Further, DFT calculations were employed to investigate the relative free energies and molecular orbitals of the metal complexes before (neutral form) and after one-electron reduction. In the neutral form (Table S10), the dz 2 orbital is found to be the HOMO-14 orbital in CoPc (Co II d 7 ). In contrast, the dz 2 becomes the LUMO in CoPc-py and CoPc-im, because the Co center donates electrons to an axial ligand π* orbital through π-back donation, leading to the formation of more Co III (Co d 6 ) species. In the one-electron reduced form ([CoPc-L] À , Figure 4a and Table S11), the dz 2 orbital in [CoPc] À occupies relatively lower energy (HOMO-12) compared to the ligated compounds. Upon axial ligation, the dz 2 orbital is elevated to HOMO-1 in [CoPc-py] À and [CoPc-im] À . At the same time, the Co center coordination geometry changes from square planar to square pyramidal. Hypothetically, the elevated dz 2 orbital energy in CoPc-L leads to easier electron delocalization from the Co dz 2 orbital to the CO 2 π* orbital, increasing the nucleophilicity of the Co center and enhancing the Co center's ability to coordinate and activate the Lewis carbon of CO 2 . [10a] It is expected that the enhanced interaction between the Co center and CO 2 lowers the activation energy barrier for converting CO 2 into CO. Additionally, the Co center electron density is reduced through Co to axial ligand π-back donation after axial ligation; therefore, the CoÀ CO π-backbonding is weakened, resulting in rapid CO desorption. To confirm this hypothesis, we have also calculated the energetics of the intermediate reaction pathways (Computational Methods in Support Information). The formation of *COOH is the rate-determining step (RDS) for all catalysts because the formation of *COOH is the most endergonic step. As shown in Figure 4b, the required free energy change from CO 2 to the adsorbed *COOH for CoPc-py and CoPc-im is 1.66 eV and 1.54 eV, respectively, lower than that of CoPc (1.74 eV), confirming the beneficial energetics in modifying CoPc with an axial ligand. In addition, after forming adsorbed *COOH, for the second proton-coupled electron-transfer step, the free energy pathway was thermodynamically downhill to form the adsorbed *CO. While CoPc-im still needs to overcome an energy barrier (0.3 eV) for *CO desorption, its *CO desorption process is endergonic compared to the exothermic process of CoPc-py and CoPc, suggesting the CoPc-im catalyst might be sensitive to be poisoning by strong CO adsorption. [24] This phenomenon might be attributed to the fact that imidazole is a stronger sigma donor, which increases the electron density of the Co-CO species. Overall, the RDS energy barriers of CO 2 RR on CoPc-py and CoPc-im are much lower than that of CoPc, explaining our experimental observations that CoPc-py and CoPc-im are more efficient and selective than pristine CoPc. The optimized structures of all three catalysts for each CO 2 reduction step were displayed in Figure S18a-c. The results of DFT calculation confirm that the elevated energetics of the dz 2 orbital via axial ligation in CoPc molecular catalysts can decrease the CO 2 reduction RDS energy barrier, facilitating the formation of *COOH intermediate. This result correlated well with the hypothesis that coordination with the axial ligand would increase the nucleophilicity of the Co center.

Conclusions
In summary, we reported an efficient axial ligation strategy with pyridine and imidazole to modify CoPc's coordination environment, enhancing its electrochemical CO 2 -to-CO performance. XRD and FTIR show a clear phase change of CoPc from β-phase to near α-phase, and stronger Davydov splitting of the Q band Summaries of the orbital energies and assignments are shown in Table S10 and Table S11. (b) Free energy diagram of CO 2(g) electroreduction to CO (g) on CoPc, CoPc-py, and CoPc-im. All energies are referred to CO 2(g) and proton-electron pairs. is observed in UV-vis spectra after axial ligation. XAS fitting results indicate the formation of the square-pyramidal structure after pyridine and imidazole axially coordinate with CoPc. XPS spectra elucidate a clear binding energy shift of Co 2p orbital, suggesting the existence of a π-backbonding effect from Co center filled d orbital to ligand π* orbital. In CO 2 RR experiments, CoPc-py and CoPc-im achieved an enhanced FE and current density of CO, up to 91 % (6.15 mA/cm 2 ) and 94 % (5.93 mA/ cm 2 ) in comparison with 82 % (2.99 mA/cm 2 ) of CoPc, respectively. DFT calculations confirmed that the dz 2 energy was elevated at the Co center in the axial ligand modified CoPc. The dz 2 energy elevation is the origin of the improved CO 2 RR since it significantly decreases the formation energy of *COOH intermediates. In addition, comparing CoPc-py to CoPc, the stronger π-backbonding from the Co center to the axial ligands can weaken the Co-CO bond, which is beneficial to CO desorption, while CoPc-im catalyst might be sensitive to be poisoining by strong CO adsorption. This work demonstrates the importance of axial direction modification of molecular catalysts, providing an alternative strategy to modify transition metal macrocycles (e. g., porphyrin, corrole, phthalocyanine, and so on) complexes for more efficient CO 2 electrochemical conversion.

Experimental Section
Synthesis of CoPc-py: 80 mg cobalt (II) phthalocyanine (CoPc) was put into a round-bottom flask and heated at 135°C for 3 hours under a reduced pressure in oil-bath with stirring to remove moisture. After the CoPc solid was cooled down to 75°C, the solution of 5 ml pyridine in 10 ml N,N-Dimethylformamide was added into flask. Subsequently, the mixed solution was heated at 75°C for 24 hours. After cooling down to room temperature, the mixture was centrifuged, in which the precipitate was washed with ethanol five times to remove the unreacted starting chemicals. Finally, the precipitate, denoted as CoPc-py (py refers to pyridine) was dried at 60°C in a vacuum oven for 24 hours.

Synthesis of CoPc-im:
CoPc-im was synthesized using a similar procedure as for CoPc-py with imidazole (81 mg) as the precursor. The large excess of axial ligands was added in the synthesis to ensure that equilibrium would favor the formation of axial ligand coordinated metal complex.