Substituent-induced electronic localization of nickel phthalocyanine with enhanced electrocatalytic CO2 reduction


 Designing efficient catalysts with high activity and selectivity is desirable and challenging for CO2 reduction reaction (CO2RR). Nickel phthalocyanine (NiPc) is a promising molecule catalyst for CO2RR. However, the pristine NiPc suffers from poor CO2 adsorption and activation due to its electron deficiency of Ni–N4 site, which leads to inferior activity and stability during CO2RR. Here, we develop a substituent-induced electronic localization strategy to improve CO2 adsorption and activation, and thus catalytic performance. Theoretic calculations and experimental results indicate that the electronic localization on the Ni site induced by electron-donating substituents (hydroxyl or amino) of NiPc greatly enhances the CO2 adsorption and activation, which is positively associated with the electron-donating abilities of substituents. Employing the optimal catalyst of amino-substituted NiPc to catalyze CO2 into CO in flow cell can achieve an ultrahigh activity and selectivity of 99.8% at the current densities up to 400 mA cm-2. This work offers a novel strategy to regulate the electronic structure of the active site by introducing substituents for highly efficient CO2RR.


Introduction
Electrochemical conversion of CO 2 into the value-added energy chemicals using surplus sustainable energy is an appealing route to mitigate environmental issues and energy crisis. [1][2][3] Catalysts are indispensable for the electrocatalytic CO 2 reduction reaction (CO 2 RR) to conquer the kinetic barrier of low electron a nity and inherent thermodynamic stability of CO 2 . [4][5][6][7] Molecule catalysts with redox-active sites for CO 2 activation/adsorption have attracted much attention. 8-10 Among them, nickel phthalocyanine (NiPc) is considered as a promising catalyst for CO 2 RR due to its proper active site. [11][12][13][14][15][16] Unfortunately, the pristine NiPc is subjected to inferior activity and stability owing to the poor CO 2 adsorption and activation rooted in the electron-de cient Ni site. [17][18][19][20][21] The electronic localization of active site with an increased electronic density is able to optimize the CO 2 adsorption/activation in traditional inorganic catalysts. [22][23][24][25][26] Inspired by this result, establishing electronic localization on the Ni site is expected as an effective strategy to improve CO 2 RR performance in the NiPc system.
The structural exibility of the phthalocyanine ring enables an adjustable electronic density of Ni site by tuning periphery substituents of NiPc. 27,28 The electron-donating substituents tend to denote more electrons into the π conjugation system of phthalocyanine ligand, which is bene cial to the electronic localization. 29,30 The amino and hydroxyl with lone pair electrons from the 2p orbit of N or O exhibit the prominent electron-donating property via p-π conjugation. 31,32 Compared with the hydroxyl, amino with a low electronegativity of N leads to interior inductive effect and thus shows preferable electron-donating ability. 33 As for the NiPc molecule catalyst, the electronic structure of the active site (Ni-N 4 ) directly determines the catalysis of electrochemical CO 2 conversion. 34,35 Modifying NiPc with different electron-donating substituents can evoke the various degree of electronic localization on the Ni site and simultaneously reveal an inherent law of NiPc-based molecule catalysts for CO 2 RR.
In this work, we demonstrate that the periphery substituents of NiPc can arouse the electronic localization on the Ni site, which facilitates the CO 2 adsorption and activation, and thus enhances the activity and stability of NiPc for CO 2 RR. Theoretic simulations demonstrate that the electronic localization in the substituted NiPc is positively correlated with the electron-donating capability of substituents in comparison to that of the pristine one. To realize this speculation, the amino and hydroxyl substituted NiPc were synthesized via a facile approach. As expected, the spectra of X-ray photoelectron spectroscopy (XPS) and synchrotron X-ray absorption spectroscopy (XAS) indicate the electronic density of the Ni site in substituted NiPc becomes concentrated by the substituent-induced electronic localization.
The amino-modi ed NiPc shows the superior performance of CO 2 RR compared with other NiPc catalysts, which can be attributed to the stronger electron-donating property of amino. NiPc substituted by amino on the gas diffusion layer delivers a remarkable activity and ultrahigh selectivity of CO (>99.8%) for CO 2 RR across a current range from -20 to -400 mA cm -2 . The substituents can regulate the electronic structures of molecule catalysts to improve the CO 2 adsorption and activation, which provides a new approach to modulate the electron structure of active site for enhanced CO 2 RR performance.

Results
Theoretic calculations. The structure models of the pristine NiPc and NiPc with substituents of hydroxyl (NiTHPc) and amino (NiTAPc) are illustrated in Fig. 1a. In order to explore the substituent effect on the NiPc system, the electronic density of Ni was analyzed ( Supplementary Fig. 1). Hrishfeld and Mulliken charge of Ni (Fig. 1b) indicates that the electron can be localized on Ni atom by the electron-donating substituents of hydroxyl or amino, and NiTAPc exhibits more apparent electronic localization than that of NiTHPc due to the stronger electron-donating ability of amino. The CO 2 adsorption energy of molecules was also examined ( Fig. 1c and Supplementary Fig. 2). 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 those molecule catalysts, the free energies of the CO 2 RR pathway were calculated (Fig.   1d). The rate-determining step on all of the molecule catalysts is the step of CO 2 activation to *COOH.
The substituted NiPc with an electron-donating substituent of hydroxyl or amino can lower the free energy of *COOH. NiTAPc with the lowest energy barrier for CO 2 activation manifests the rapid process for CO 2 RR in comparison to the NiTHPc and NiPc. According to these theoretic results, the electrondonating substituent of hydroxyl or amino-modi ed NiPc can induce the electronic localization of Ni site and thus enhances CO 2 adsorption and activation to boost CO 2 RR.
Molecule catalysts synthesis and characterization. To synthesize the proposed molecule catalysts, a facile solvothermal reaction was conducted by employing the substituted phthalonitrile and nickel acetate as the reactants (see Methods for details). Matrix-assisted laser desorption/ionization time of ight mass spectrometry (MALDI-TOF MS) of as-prepared molecule catalysts (Fig. 2a) shows the mass of molecule ion at 630 and 634 from NiTHPc and NiTAPc, respectively. This result con rms the successful synthesis of the substituted NiPc (Fig. 1a). Fourier transform infrared (FTIR) spectra ( Fig. 2b and Supplementary Fig. 3) prove the presence of hydroxyl and amino in NiTHPc and NiTAPc, respectively, without a characteristic peak of phthalonitrile at around 2230 cm -1 . Ultraviolet-visible (UV-Vis) spectroscopy was used to characterize the NiPc, NiTHPc and NiTAPc owing to the special optical property of corresponding substituted NiPc. As shown in Fig. 2c, the substituted NiPc exhibits the characteristic Q band absorption at around 600-800 nm and B band absorption, 36 without the absorption of nickel acetate ( Supplementary Fig. 4). Furthermore, the Q band absorption of π→π* is attributed to the electron transition from the highest occupied molecule orbital (HOMO) to the lowest occupied molecule orbital (LOMO) of the phthalocyanine ring. 37 The Q band absorption without splitting indicates the symmetrical substituent position of the synthesized molecule. 38 There is a redshift in the Q band absorption among NiTHPc and NiTAPc due to the increased electron-donating ability of amino, [39][40][41] which is highly matches the results of theoretic calculations ( Supplementary Fig. 5). X-ray powder diffraction (XRD) patterns (  Fig. 9) demonstrate the same coordination number of Ni in those catalysts. Thus, the electronic localization on the Ni site is derived from the peripheral substituents rather than the changed coordination number of Ni. These results indicate that the substituents of hydroxyl or amino with electron-donating ability can arouse the electronic localization on the central Ni atom of molecules, and the degree of electronic localization is positively correlated with the electron-donating ability of substituents.
To improve the conductivity of molecules for electrocatalytic CO 2 RR, NiPc, NiTHPc and NiTAPc were dispersed onto carbon nanotube to form catalysts of NiPc/CNT, NiTHPc/CNT and NiTAPc/CNT. Scanning electron microscope (SEM) in Supplementary Fig. 10 and the elemental mapping ( Supplementary Fig. 11) from high-angle annular dark eld-scanning transmission electron microscope (HAADF-STEM) demonstrate that the molecules can be well dispersed by carbon nanotubes. The results of XRD and Raman from the composite catalysts ( Supplementary Fig. 12) suggest the presence of carbon. 44 Fig. 14) and normalized by the surface area from N 2 adsorption/desorption analysis (Supplementary Table 1). The pristine NiPc with the electron-de cient Ni site displays negligible CO 2 adsorption (Fig. 4a). On the other hand, NiTHPc and NiTAPc possess the electronic localization of Ni sites that are induced by the electron-donating substituents, and thus both show a signi cantly enhanced ability of CO 2 adsorption. NiTAPc shows the optimal CO 2 adsorption due to its conspicuous electronic localization among the three of NiPc-based catalysts. Furthermore, the CO 2 adsorption responses of NiPc/CNT, NiTHPc/CNT and NiTAPc/CNT were also explored ( Supplementary Fig. 15). N 2 was used as the control gas to study the CO 2 adsorption responses. NiTAPc/CNT exhibits a remarkable response in the CO 2 atmosphere (Fig. 4b) Fig. 17). Electrochemical active surface area (ECSA) measurements ( Supplementary Fig. 18) indicate a similar active surface of composite catalysts. Therefore, the catalytic performance results from the intrinsic activity of molecule catalysts. From the LSV in the CO 2 -saturated electrolyte (Fig. 3a), the amino-substituted NiPc with superior electronic localization of Ni site exhibits the best performance of CO 2 RR than those of other catalysts. The electron-donating substituents of hydroxyl and amino can greatly improve the catalytic activity of NiPc. To obtain the product distribution at different potentials of catalysts, chronoamperometry measurements ( Supplementary Fig. 19) were carried out to detect the products of CO 2 RR. Carbon monoxide (CO) can be detected as the only product from the results of gas chromatograph (Supplementary Fig. 20) and 1 HNMR spectroscopy ( Supplementary Fig. 21). Fig. 3b shows the faradaic e ciency of CO (FE(CO)). As a byproduct of CO2RR, the FE of hydrogen (H 2 ) is also displayed in Supplementary Fig. 22. It can be seen that the FE(CO) of catalysts maintain a level of above 90% in the potential range of 0.58-0.90 V. NiTHPc/CNT and NiTAPc/CNT demonstrates the enhanced selectivity of CO for CO 2 RR in comparison to the NiPc/CNT, which can be attributed to electron-donating substituents induced electronic localization of Ni site. Although a high selectivity of CO in NiPc/CNT can be achieved, the stability (Fig. 3c) is much lower than those of NiTHPc/CNT and NiTAPc/CNT, undergoing a fast degradation during the continuous CO 2 RR catalysis.
NiTAPc/CNT shows a high activity of −12.2 mA cm -2 at −0.58 V and remarkable CO selectivity of almost 100%, exceeding the other molecule catalysts and even the most performances of representative in literatures (Supplementary Table 2). The electrochemical impedance spectrum (EIS) was employed to explore the reaction dynamics of catalysts during CO 2 RR. As exhibited in Fig. 3d, the charge transfer resistance of catalysts is ranked as the 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 of the Ni atom and therefore promotes CO 2 RR performance in terms of activity, selectivity and stability.
To study CO 2 RR performances at high current density, the composite molecule 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 H-cell. The schematic illustration of the ow cell is displayed in Supplementary Fig. 23. The pristine NiPc/CNT loaded on the GDL shows the stable CO 2 RR performance from LSV measurements in CO 2 , but this catalyst suffers from serious degeneration during LSV tests in the Ar without CO 2 atmosphere ( Supplementary Fig. 24a). Furthermore, NiPc/CNT with adequate CO 2 in the ow cell displays a stable potential at a low current density of −60 mA cm -2 ( Supplementary Fig. 24b). When the current density increased at −200 mA cm -2 , the performance of NiPc/CNT is dramatically declined due to limited CO 2 adsorption (Fig. 4a). Thus, the inferior stability of the pristine NiPc is rooted in the poor CO 2 adsorption of the active site. The GDL coated with NiTAPc/CNT or NiTHPc/CNT can be operated at the current densities from −20 to −500 mA cm -2 at applied potentials less than −0.80 V vs. RHE (Fig. 4a). 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 the current densities from -20 to -400 mA cm -2 (Fig. 4b), which is much better than the counterparts of other NiPc catalysts and even the reported catalysts (Supplementary Table 3). Accordingly, the turnover frequency of CO (TOF CO ) from various catalysts (Fig. 6c) is calculated according to the results of inductively coupled plasma-optical emission spectrometry (ICP-OES) in Supplementary Table 4. The optimal catalyst of 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 catalysts can maintain 12, 000 s with the selectivity of CO over 99.8% (Fig. 6d). On the basis of these electrochemical data, the NiTAPc/CNT delivers most superb performance for CO 2 RR among the molecule catalysts.
To further verify the signi cance of the electronic localization of Ni site for CO 2 RR, the tert-butyl, a typical electron-withdraw substituent in the phthalocyanine, 29,45 to substituted NiPc (NiTBPc) was synthesized ( Supplementary Fig. 25). MALDI-TOF MS spectrum, FTIR spectra, UV-Vis spectrum and Raman spectrum of NiTBPc (Supplementary Fig. 26) Fig. 30). In ow cell, the activity and selectivity of NiTBPc/CNT are lower than the counterparts of NiPc/CNT ( Supplementary Fig. 31) due to its electronwithdraw substituent of tert-butyl. Thus, the electron-withdraw substituent that induces the electronic delocalization of Ni site can inhibit the catalytic activity of NiPc for CO 2 RR. These conclusions further demonstrate that the electronic localization of the Ni site plays a signi cant role in boosting CO 2 RR.

Discussion
In summary, we developed the electron-donating substituents induced electronic localization strategy to regulate the electronic density of the catalytic site in NiPc system. XPS and XAS results demonstrated the electronic localization of Ni site induced by tailoring the peripheral substituent of the phthalocyanine ring. When the eletron-donating capabilities of substituents were increased, the electron of Ni in substituted NiPc became more localized than that of the pristine NiPc. The electron localized Ni site of NiPc helps to enhance CO 2 adsorption/activation and thus to improve the activity and stability of NiPc for CO 2 RR.
Conversely, the electron-withdraw substituent triggers the electronic delocalization of the Ni site, resulting in the worse performance for CO 2 RR. NiTAPc/CNT with a signi cant electronic localization of the Ni site in GDL delivered an ultrahigh activity and selectivity, which achieved at 99.8% even at a high current density of -400 mA cm -2 . This work offered a new approach to tune electronic localization of catalytic site for enhanced catalytic performances and it presented valuable guidance to develop e cient molecule catalysts for future electrochemical techniques. In the H-type cell, two chambers were separated by the proton exchange membrane of Na on 117. The three-electrode system consisted of a modi ed carbon paper (Toray, TGP-H-060) as a working electrode, a platinum plate as a counter electrode and an Ag/AgCl lled with saturated KCl electrode as a reference electrode. To prepare the working electrode, 4 mg of catalyst was dispersed into 1 mL of aqueous alcohol (50 vol%) and 60 μL of Na on (5 wt%) was employed as the binder. The prepared ink was dropped onto carbon paper with a loading of 0.4 mg cm -2 . The 0.5 M KHCO 3 saturated with high-purity CO 2 (99.999%) was used as the electrolyte for electrocatalytic CO 2 reduction. The ow rate of high-purity CO 2 was set at 20 sccm. The potentials in H-cell were transformed to the RHE with iR compensations by the equation:

Methods
where i is the current andis the solution resistance.
The products of CO 2 RR in cathodic were analyzed by an on-line gas chromatograph (Shimadzu, 2014C) and 1 HNMR. High-purity N 2 (99.999%) was used as the carrier gas. A thermal conductivity detector was employed to measure the H 2 fraction and a ame ionization detector was equipped with a nickel conversion furnace to analyze the CO fraction. The faradaic e ciency of products was calculated from gas chromatograph chromatogram peak according to the owing equation: where is the fraction, is the ow rate of CO 2 , is faraday constant (96485 C mol -1 ), is the normal atmosphere (101325 Pa), i is the applied current, is the gas constant (8.314 J·mol -1 ·K -1 ) and is the room temperature (298 K).
The ow cell measurements were performed on a home-made cell including sandwich of ow frames, gaskets and an anion-exchange membrane (Selemion DSVN  H, O, N). 48-50 The geometry optimization and frequencies calculations were performed. Kohn-Sham molecular orbital analysis and atomic charge analysis by Mulliken atom population and Hirshfeld atomic charge were carried out via using Multiwfn 3.8 software. [51][52][53][54] The 3D models of all the reaction intermediates were visualized by using the VMD 1.9 program. 55 The pathways for reduction of CO 2 into CO are listed as the following elementary steps: where * is the active site of the catalyst surface and * and * were two key intermediates during CO 2 RR.
The computational hydrogen electrode (CHE) model was used to describe the free energy of protonelectron pairs in the proton-electron transfer steps by using the free energy of hydrogen at the potential of 0 V. The effect of applied bias on a proton-electron transfer step is represented by adding a -eU term to the standard .

Declarations
Data availability The data that support the ndings of this study are available from the corresponding author on reasonable request.  Characterizations of NiPc, NiTHPc and NiTAPc. a MALDI-TOF MS spectra. b FTIR spectra. c UV-Vis spectra in dimethyl formamide. d XRD patterns.
Page 18/20 Figure 3 Electronic localization characterizations. a High-resolution XPS of Ni 2p spectra. b XAS spectra of Ni Ledge. c XANES spectra of Ni K-edge. d EXAFS spectra of Ni K-edge. Figure 4 CO2 adsorption ability. a The CO2 adsorption isotherms of NiPc, NiTHPc and NiTAPc normalized by the surface area from the N2 adsorption/desorption analysis. b The CO2 adsorption responses of NiPc/CNT, NiTHPc/CNT and NiTAPc/CNT at varied potential.

Supplementary Files
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