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Xinqi Wang, Lili Zhang, Ning Wang, Shaohui Sun, [Hao Wan](https://orcid.org/0000-0002-4487-3538), [Renzhi Ma](https://orcid.org/0000-0001-7126-2006), Wei Ma

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[Boosting electrocatalytic oxygen reduction of Fe-Co polyphthalocyanine via the synergy of metal component optimization and axial ligand modification](https://mdr.nims.go.jp/datasets/d9ba5ee4-e6b1-422d-b0e5-4f85c6e2532e)

## Fulltext

Boosting Electrocatalytic Oxygen Reduction of Fe-Co Polyphthalocyanine via the Synergy of Metal Component Optimization and Axial Ligand Modification Xinqi Wanga,† , Lili Zhanga,† , Ning Wang a, Shaohui Suna , Hao Wana,b,* , Renzhi Ma,c Wei Maa,*a School of Chemical Engineering, Zhengzhou University, Zhengzhou, 450001, P. R. China b Zhongyuan Critical Metals Laboratory, Zhengzhou University, Zhengzhou, 450001, P. R. Chinac Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan.* Corresponding authors. E-mail: wanhao@zzu.edu.cn (H. W.); mawei@zzu.edu.cn (W. M.)†These authors contributed equally to this work.Abstract: Conjugated metal polyphthalocyanine (MPPc) organic skeletons, a kind of novel porous materials containing inherent M-N4 highly active sites, are of great significance for oxygen reduction reaction (ORR). Herein, bimetallic Fe-Co phthalocyanine covalent polymers (denoted as FeCoPPc) were prepared as ORR electrocatalysts through a solvothermal process and subsequent oxidation treatment. Meanwhile, the effect of axial oxygen-containing ligands of metal centers on the catalytic kinetic processes was also investigated. Benefiting from the highly exposed bimetal active sites and the finely designed ligand structure, FeCoPPc with optimized Fe/Co ratio and ClO3− coordination (Fe0.9Co0.1PPc-ClO3) displayed an outstanding ORR electrocatalytic activity and superior durability compared to 20% Pt/C, manifested by an onset potential of 0.986 V vs. reversible hydrogen electrode (RHE) and a half-wave potential of 0.923 V vs. RHE in a 0.1 M KOH solution. This work may guide the rational design of high-performance electrocatalysts to regulate the catalytic activity.Graphical abstractA series of bimetallic Fe-Co phthalocyanine covalent polymers with different Fe/Co ratios and oxygen-containing axial ligands are constructed. Experimental and theoretical investigations reveal that with the synergy of highly exposed bimetal active sites and the finely designed ligand structure, polyphthalocyanine with optimized Fe/Co ratio and ClO3− coordination (Fe0.9Co0.1PPc-ClO3) displays an outstanding activity toward electrocatalytic oxygen reduction reaction.Keywords: bimetallic electrocatalysts; Fe-Co polyphthalocyanines; axial ligands; oxygen reduction reaction.1. IntroductionCathodic oxygen reduction reaction (ORR) offers a sustainable route from chemical energy to electric energy in sustainable energy devices such as fuel cells and metal-air batteries.[1, 2] In particular, Pt-group metal-based nanomaterials usually exhibit state-of-the-art ORR activity. However, their high cost, low availability and sustainability hamper the large-scale commercial applications. It is, therefore, essential to develop ORR catalysts based on non-precious metal elements for green energy conversion.[3, 4] In recent years, transition metal-based catalysts have been rapidly developed for electrochemical multifunctional applications. Metal polyphthalocyanine (MPPc) organic frameworks, as a novel kind of hyperconjugated layered framework materials with specific geometrical configurations and electronic effects, are spatially self-assembled between organic ligands and metal ions.[5] The modifiable functional ligands and the directional placement of metal atoms endow 2D MPPc catalysts containing active metal-nitrogen-carbon moieties with unique advantages over phthalocyanine monomers toward ORR, such as the more central active sites accessibility, ultra-high specific surface area and excellent structural stability.[6, 7] According to earlier studies, M-N4 moieties, especially Fe-N4 active units, with tetra-coordinated configuration are the main catalytic sites for MPPcs.[8, 9] However, the symmetry and delocalization of the electron distribution, which are induced by the symmetric planar structure, led to the chemical inertness on the active sites, endowing them with unsatisfactory catalytic performance compared with the commercial noble metal catalysts.[10] Recently, the ORR performance was significantly improved by introducing additional strong-field/weak-field ligands in the Fe-N4 axial direction for poly(iron phthalocyanine) (FePPc), which adjusted the 3d orbital configuration and broke the original active-site electronic structural symmetry.[11] Despite the catalytic performance of single-metal FePPc, it is challenging to improve the active site density of single metal atoms and intrinsic activity on the catalytic sites, which directly influence the electron transfer and proton transport and thus determine the ORR activity.Additionally, MPPcs with adjacent bimetallic sites in a fully conjugated framework can achieve synergistic catalysis to optimize ORR performance by generating long-range interactions.[12] For example, Fe and Co dual-site atoms in the Fe-N4/Co-N4 conjugated polyphthalocyanine generated a synergistic effect through spatial electron modulation, which altered the d-orbital energy levels of the Fe centers, thus optimizing the adsorption properties on oxygen-containing species and accelerating the ORR.[13] Inspired by this, the interaction of the two transition metals with the axial donor atoms leads to a special change in the electron configuration of the central metal, which drives the substrate-intermediates to reach moderate adsorption strength and is conducive to promoting the efficient ORR activity of MPPcs. Moreover, for traditional M-N4 sites, the unsaturated axial coordination is flexible. The oxygen-enriched monodentate ligands (e.g. ROn) may lead to five-coordination configurations with unusually stable local structures for MPPc molecules, and their distinctive ORR performance. However, the research in the relevant area is still vacant. In addition, how the synergy of these two positive factors, i.e., heteroatom and axial ligands, influences the ORR activity deserves to be unveiled for the high-performance MPPc catalysts. Herein, a series of Fe-Co polyphthalocyanine catalysts with different Fe/Co molar ratios and oxygen-containing axial ligand modifications (named as FexCo1-xPPc-AL, where AL stood for -ClO3, -BrO3 or -IO3) were prepared through a continuous oxidation and sequential ultrasound-assisted strategy for boosting alkaline ORR, which provided a catalytic platform for enabling spatial coordination on MPPc networks. In the conjugated MPPc framework, the introduction of heterometal atoms regulated the redistribution of electron cloud density of Fe-N4 active units while the field effect of axial coordination modification influenced the electronic states of the metal centers and the associated peripheral conjugated isoindole ring to promote the equilibrium adsorption of oxygenated species at the double-metal sites. Based on the synergy of the above two factors, the developed Fe0.9Co0.1PPc-ClO3 catalyst exhibited exceptional ORR catalytic activity and electrochemical stability in alkaline media with a high half-wave potential (E1/2) of 0.923 V vs. RHE, which was superior to 20% commercial Pt/C and congeneric Fe-Co polyphthalocyanines.2. Experimental Section2.1. MaterialsFerrous(Ⅱ) chloride hydrate (FeCl2·4H2O, Woka), Cobalt (Ⅱ) chloride hydrate (CoCl2·6H2O, Sinopharm Group Co. Ltd., China), 1,2,4,5-tetracyanobenzene (TCB, >98.0%, Shanghai Aladdin reagent Co. Ltd., China), n-pentanol (>98.0%, Shanghai Aladdin reagent Co. Ltd., China), 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU, >98.0%, Shanghai Aladdin reagent Co. Ltd., China), N, N-dimethylformamide (DMF, >99.0%, Aladdin), hydrogen peroxide (H2O2, 30%, Shanghai Aladdin reagent Co. Ltd., China), hydrochloric acid (HCl, >99.0%, Sinopharm Group Co. Ltd., China), sodium chlorate (NaClO3, >98%, Beijing innochem technology Co. Ltd., China), sodium bromate (NaBrO3, >98%, Beijing innochem technology Co. Ltd., China), sodium iodate (NaIO3, >98%, Beijing innochem technology Co. Ltd., China), potassium hydroxide (KOH, 85%, Sinopharm Group Co. Ltd., China), alcohol (AR, Sinopharm Group Co. Ltd., China), Methanol (AR, Sinopharm Group Co. Ltd., China), carbon black (Carbot-XC, 99%, America), Pt/C (20%, Shanghai Adamas reagent Co. Ltd., China).2.2. Synthesis of FeCoPPcFeCoPPc materials were prepared via a solvothermal method. Typically, TCB (2.5 mmol) and DBU (0.25 mL), serving as the ligand monomer and the catalyst respectively, were introduced into N2-saturated n-pentanol (30 mL). Then FeCl2·4H2O and CoCl2·6H2O were dissolved in the mixture by stirring for 30 min, transferred into a sealed Teflon-lined autoclave, and kept at 160 °C for 8 h. The resulting product was filtered and washed repeatedly with ethanol several times. Subsequently, the washed product was dispersed in a mixture containing 1 M H2O2 and 0.2 M HCl, and stirred for 6 h at room temperature. Collected by centrifugation, repeatedly washed with ethanol and water, and dried at 60 °C under vacuum, the precipitate FeCoPPc was obtained.To prepare PPc catalysts with different Fe/Co contents, the addition of Fe and Co sources with a total molarity of 1.0 mmol was controlled, while the rest of the conditions remained unchanged. The obtained products were denoted as FexCo1-xPPc (x = 0, 0.9, and 1, separately) according to the feed ratio.2.3 Synthesis of Fe0.9Co0.1PPc-ALFe0.9Co0.1PPc-AL was obtained by treating 50 mg of Fe0.9Co0.1PPc dispersed in 2M NaClO3, 2M NaBrO3 or 2M NaIO3 aqueous solutions, respectively, for 1 h under a 30 °C ultrasonication. The dispersion was filtered and washed, followed by drying at 60 °C in a vacuum oven overnight.2.4. Materials characterizationX-ray diffraction (XRD) data were collected by a X-ray diffractometer (Rigaku SmartLab SE) with Cu Kα radiation (λ = 1.5406 Å). The morphology information was characterized by an Auriga-45-63 scanning electron microscope (SEM). Transmission electron microscope (TEM), high-resolution TEM images (HR-TEM) and energy dispersive spectroscopy (EDS) elemental mapping were performed on a ZEISS LIBRA 200 with an accelerating voltage of 200 kV. Raman spectra were recorded on a LabRamHR Evolution with excitation from the 532 nm line of an Ar-ion laser. XPS measurements were performed on an ESCALAB 250Xi instrument (Thermo Electron) with an Al Kα radiation to detect the chemical composition and valence band information of the as-prepared samples. Fourier Transforms Infrared (FT-IR) spectra were recorded on a Thermo Scientific Nicolet iS50 Spectrometer to analyze the valence bond environment and chemical structure of materials. UV-Vis spectra were measured at a UV-vis spectroradiometer (Shimadzu UV-2600) to detect the internal structure of the as-prepared materials by electronic transition mechanism.2.5. Electrochemical measurementsAll electrochemical measurements were carried out on an electrochemical workstation (CHI 760E) coupled with a standard three-electrode system, in which a modified glassy carbon electrode, Pt sheet, and Ag/AgCl electrode filled with saturated KCl solution were employed as the working, counter, and reference electrode, respectively, while 0.1 M KOH aqueous solution was used as the electrolyte. To prepare the working electrode, 2 mg of the as-prepared catalyst and 2 mg of carbon black (Cabot XC-72) were dispersed into a mixture of ethanol, deionized water and Nafion solution (5 wt%) with a volume ratio of 128/7/2 to form a homogeneous ink. For comparison, a 20% Pt/C ink was also prepared by mixing 2 mg of commercial Pt/C (20 wt%) with a dispersion of the same proportion. 7.5 μL of the as-prepared ink was dripped onto the polished glassy carbon rotating disk electrode (RDE) (Ф = 3 mm) and dried at 75 °C for 12 h. The loading of catalysts is set as 0.21 mg cm-2. All measured potentials in this work were converted to the potentials versus the reversible hydrogen electrode (RHE) by the following equation:ERHE = E(Ag/AgCl) + pH × 0.059 + 0.197 VThe linear sweep voltammetry (LSV) curves with 95% iR compensation were evaluated at a scan rate of 5 mV s-1 in 0.1 M KOH under a rotation speed of 1600 rpm. The electrochemical active area (ECSA) was linearly proportional to the double-layer capacitance (Cdl). It was estimated from different CV curves within the non-faradaic potential windows between 1.067 and 1.167 V vs. RHE at the incremental scan rates of 20, 40, 60, 80 and 100 mV s−1, respectively. The slope of the curves obtained by plotting the current density difference (Δj = ja - jc) corresponding to the middle potential of CV curves versus the scan rate represents 2Cdl. Electrochemical impedance spectroscopy (EIS) data were recorded in the frequency range from 10,000 to 0.01 Hz at an applied voltage of 0.9V vs. RHE with a voltage amplitude of 5 mV. In addition, the mass activity (MA) of the catalysts could also be obtained by the ratio of current (I) to catalyst loading (S) at a voltage of 0.9 V vs. RHE. The equation is shown as follows:MA=IE = 0.9 VSTo estimate the electron-transfer number (n) during electrocatalytic ORR, the Koutecky-Levich (K-L) and rotating ring-disk electrode (RRDE) tests were employed. RDE measurements for the K-L equation were conducted at different rotating rates from 400 to 2025 rpm, and n was estimated as follows:1j = 1jk + 1jL = 1B ω-1/2 + 1jkB = 0.62nFCODO2/3υ-1/6where j is the measured current density while jL and jK are the diffusion limiting and kinetic current densities, respectively. ω stands for the angular velocity of the rotating disk and F represents the Faraday constant (96485 C mol–1). CO and DO are the volume concentration (1.2×10–3 mol L–1) and the diffusion coefficient of O2 in 0.1 M KOH electrolyte solution (1.9 × 10-5 cm2 s–1), respectively, while υ refers to the kinetic viscosity of the electrolyte (0.01 cm2 s–1). RRDE measurements were carried out on the ring-disk electrode with the same catalyst loading at 1.2 V vs. RHE. n value was calculated on the basis of the following equation:n = 4IDID + IRNwhere IR and ID were ring and disk current, respectively. N, the ring collection efficiency of the RRDE, was 37%. Furthermore, the yield of peroxide (denoted as HO2–%) was calculated based on the following equation:HO2-(%) = 200IR/NID + IR/N2.6 Theoretical calculation detailsTo further investigate the catalytic reaction mechanism, the density functional theory (DFT) was implemented by using Gaussian 16 (G16 A.01). Geometry optimization in the gas phase with default convergence conditions was carried out using the hybrid functional B3LYP and the D3BJ dispersion correction in conjunction with the basis sets 6-31+G(d, g) for C, O and N, and the LAN2DZ pseudopotential for Fe and Co atoms. Multiwfn 3.8 and VMD-Visual Molecular were used to analyze the Gaussian wave function files and visualize the results in the output. The change in Gibbs free energy (∆G) of reaction intermediates were calculated by the following equation:∆G = ∆E + ∆ZPE + T∆S – eUwhere ΔE, ΔZPE, and ΔS represented the total energy changes, the changes in zero-point energy (ZPE) and the change in entropy value (S) at each step of the reaction, respectively. ZPE could be obtained from vibrational frequency calculation. S was obtained with reference to thermodynamic databases. T is the temperature (298.15 K). Moreover, the influence of the solvent (H2O) for ∆G at each adsorption species was corrected by eliminating the solvation energy. The ∆G of the different reaction intermediates to the surface were defined as:ΔG*O= G*O - G* - (GH2O- GH2)ΔG*OH= G*OH -  G* - (GH2O- 1/2GH2)ΔG*OOH= G*OOH - G* - (2GH2O- 3/2GH2)where G* represented the energy of catalyst substrate, and G*OOH, G*O, and G*OH represented the optimal energies of the catalyst after the intermediate adsorption, respectively. GH2O and GH2 were the energies of an individual H2O and H2 molecule, respectively.The ORR free energy change ∆G1 ~ ∆G4 for each subsequent step could be obtained from the following expression: ΔG1 = ΔG*OOH - 4.92 VΔG2 = ΔG* O - ΔG*OOHΔG3 = ΔG*OH - ΔG*OΔG4 = -ΔG*OH3. Results and discussion3.1. Synthesis and physical characterization of FeCoPPc-AL catalystsIn a typical process, the metal polyphthalocyanines (MPPcs, M = Fe, Co) were prepared via solvothermal reaction using 1,2,4,5-tetracyanobenzene (TCB) as organic ligand, 1,8-diazabicyclo [5.4.0] undec-7-ene (DBU) as reaction catalyst and n-pentanol as a solvent system while FeCl2·4H2O and CoCl2·4H2O with designed ratios served as the metal resources. Subsequently, the bimetallic FexCo1-xPPc (x = 0, 0.5, 0.7, 0.8, 0.9 or 1) was obtained by H2O2 oxidation treatment. The oxidation process provides a favorable condition for axial coordination of metal sites. Finally, FexCo1-xPPc linked with an axial halate ligand, i.e. FexCo1-xPPc-AL, was synthesized through immersing FexCo1-xPPc in the corresponding inorganic halate solution under ultrasonication, as illustrated in Scheme 1. Scanning electron microscopy (SEM) was adopted to investigate the microstructure information of the as-prepared nanomaterials. Figure S1 displays a distinctly irregular 2D layer-stacked structure for Fe0.9Co0.1PPc-ClO3. Combined with the high-resolution transmission electron microscopy (HRTEM) image, the high crystallinity and surface structures are further examined, as shown in Figure 1a-b. The lattice fringe width of ~0.34 nm, as shown in the selected area electron diffraction (SAED) pattern (Figure 1b), corresponds to the (310) crystal planes of Fe0.9Co0.1PPc-ClO3, which is consistent with the XRD pattern.[14] The distribution information of Fe, Co, N and Cl elements is further obtained by energy-dispersive X-ray spectroscopy (EDX), in which all the elements are homogeneously dispersed in Fe0.9Co0.1PPc-ClO3 (Figure 1c). Besides, similarly, the apparent lattice fringes revealed by TEM images in FePPc-ClO3 and CoPPc-ClO3 also display the layered stacking structure (Figure S2). Crystal phase compositions of the as-prepared nanomaterials were examined by X-ray diffractometry (XRD). As shown in Figure 1d and Figure S4a, a series of FexCo1-xPPc and FexCo1-xPPc-AL samples possess similar diffraction peaks, consistent with PPc-based materials reported in the literature, indicating the successful synthesis of polyphthalocyanine skeletons.[15-18] The XRD patterns of the resulting samples can be deconvoluted into two π-π stacked crystal structures, i.e. the aligned alpha- and beta-forms (labeled as α-MPPc and β-MPPc, respectively). The main peaks in all samples observed at 26.2° and 29.4° are indexed to be the (310) and (001) diffraction planes of α-MPPc while those at 18.9° and 35.4° correspond to (101) and (102) planes of β-MPPc, respectively. The Fourier transform infrared spectroscopy (FT-IR) analysis was performed to investigate the characteristic functional groups and molecular structures of the conjugated MPPc catalysts (Figure 2a and Figure S4b). The FT-IR spectroscopy of TCB, as depicted in Figure S3, showed apparent peak signals at ca. ~918 cm–1 and ~508 cm–1, in which no clear signals for polyphthalocyanine were observed, indicating that the TCB ligands were successful in polymerization. The typical peak signals at ca. 1065 cm–1, 1383 cm–1 and 1660 cm–1 further confirm the successful synthesis of MPPc frameworks.[16, 19, 20] The peaks at ca. 1311 and 1157 cm–1 correspond to stretching vibrations of the C-C bonds and C-N bonds, while those located at ca. 682-768 cm–1 represent the wagging and torsional vibrations of the C-H groups in isoindole ring.[21] In particular, the peaks that emerged at ca. 913 cm–1 were associated with the vibration of the metal-nitrogen (M-N) coordination bonds, indicating the successful coordination of the M-N bond in the MPPc. In addition, compared to Fe0.9Co0.1PPc without axial ligand, Fe0.9Co0.1PPc-AL shows weak peak signals at 475 cm–1, which may be attributed to the vibration of inorganic ions. Figure 2b and Figure S4c show the Raman spectra in which several samples show A1g mode characteristic signal peaks near 681 cm–1, 1127 cm–1 and 1346 cm–1, while B1g and B2g mode peak signals can be found at 1530 cm–1 and 1430 cm–1, respectively, corresponding to in-plane vibration of metal phthalocyanine rings. Fe-O bond characteristic vibration peak may exist at 681 cm–1, however, it may be covered by the C-C stretching vibration peak. These results are consistent with those in the literature, thus indicating the successful formation of FeCoPPc samples.[22, 23] The ultraviolet-visible absorption spectroscopy (UV-Vis) is caused by the valence electron transition in the outer layer of the molecule and expressed as characteristic peaks of the molecule to provide basic structural information about the material. All the polyphthalocyanine samples show obvious characteristic absorption peaks in the ranges of 300-400 nm (B band) and 600-800 nm (Q band), which can be attributed to the transition of π(a1u)-π*(eg) and π(a2u)-π*(eg), respectively, as shown in Figure 2c. Especially, the B and Q bands of polyphthalocyanines show a slight red shift after the introduction of Co, which may be ascribed to the synergistic interaction between the bimetallic elements. In addition, as can be seen in the UV-Vis diffuse reflectance absorption spectra (Figure S4d), compared with Fe0.9Co0.1PPc, Fe0.9Co0.1PPc-AL shows an obvious negative-shift absorption peak with Davydov splitting in the range of 600-700 nm, which is closely related to the axial ligand effect. This may be attributed to the modulation of the occupied d orbital electrons cloud density caused by the axial coordination of metal atoms, resulting in the change in the absorption intensity of electron transitions.[24, 25] The pore structure characteristics of catalysts were evaluated by a low-pressure N2 adsorption-desorption measurement at 77 K. N2 adsorption/desorption isotherm suggests that Fe0.9Co0.1PPc-ClO3 catalyst has a fully conjugated skeleton with a large Brunauer-Emmett-Teller (BET) surface area of 30.99 m2/g and a mesopore-dominant structure compared to FePPc-ClO3 and CoPPc-ClO3 (Figure S5), which is favorable to the adsorption of oxygen molecules for accelerating ORR. The BET surface area, pore volume and pore size of catalysts are summarized in Table S1.X-ray photoelectron spectroscopy (XPS) analysis was employed to investigate the chemical composition and valence states of the catalysts. The full spectrum analysis results also confirm the co-presence of the elements C, N, O, Fe and Co in the Fe0.9Co0.1PPc-AL, Fe0.9Co0.1PPc, FePPc-ClO3 and CoPPc-ClO3 (Figure S6). The elemental content of Fe0.9Co0.1PPc-AL is shown in Table S2. In the C 1s spectra, the broad peak at ~284.7 eV corresponds to the C=C/C-C species, while those located at ~286.9 eV and ~288.1 eV are assigned to C=N/C-N and C=O bonds, respectively (Figure S7a).[20, 26] The M-N coordination configurations of several MPPc frameworks are evidenced by XPS N 1s spectra, as shown in Figure S7b. The fitting peaks at ~398.4 eV, ~399.4 eV and ~400.2 eV were attributed to metal-site-linked N species (Nα), four isoindole ring-linked N species (Nβ) and M-N bonds, respectively.[27, 28] For Fe0.9Co0.1PPc-ClO3, the N 1s spectrum shows an obvious negative change compared to those of FePPc-ClO3 and CoPPc-ClO3, indicating that the electron migration occurs at the N atoms on the macrocycle. In the high-resolution Fe 2p spectra, two dominant peaks in Fe0.9Co0.1PPc-ClO3 assigned to Fe 2p3/2 and Fe 2p1/2, respectively, appear at ~710.3 eV and ~723.6 eV, separately. Fe 2p3/2 can be deconvoluted into three doublets at ~707.9 eV, ~710.0 eV and ~711.5 eV, while Fe 2p1/2 is divided into those at ~721.0 eV, ~723.2 eV and ~724.7 eV, corresponding to Fe-N, Fe2+-O and Fe3+-O, respectively (Figure 2d and Figure S8a).[29, 30] Compared with those of both FePPc-ClO3 and Fe0.9Co0.1PPc, the content of Fe2+ in Fe0.9Co0.1PPc-ClO3 is significantly increased, indicating that a change of the electron cloud density around the Fe atom which originates from the introduction of the second metal Co and the axial ligand. In addition, for Fe0.9Co0.1PPc-ClO3, two double peaks of the Co 2p spectrum are assigned to Co 2p3/2 located at ~780.6 eV and Co 2p1/2 at ~796.0 eV, which are deconvoluted to Co2+ (~780.6 eV, ~796.0 eV) and Co3+ (~776.6 eV, ~792.0 eV), respectively (Figure 2e and Figure S8b).[17, 31] It should be noted that compared with CoPPc-ClO3, the Co 2p3/2 peak of Fe0.9Co0.1PPc-ClO3 has a higher binding energy position, suggesting a strong synergistic interaction between Co and Fe. Similarly, it is observed that the Co 2p3/2 peak of Fe0.9Co0.1PPc-ClO3 shows a remarkably negative shift compared to that of Fe0.9Co0.1PPc, which is ascribed to the charge transfer between the center metal Co and axial oxygenated ligand.[32, 33] The variation of Fe and Co elemental content obtained from the integration of XPS-fitted curves is shown in Table S3, implying a possible change in electronic state distribution. High-resolution O 1s spectrum in Fe0.9Co0.1PPc-ClO3 can be deconvoluted into four characteristic peaks at ~532.0 eV, ~531.5 eV, ~530.8 eV and ~529.6 eV, assigned to adsorbed oxygen (Oads), -O-Cl, -O-H and M-O (Fe-O/Co-O) bonds, respectively, suggesting that the axial oxygen-containing ligands are coordinated to central metal sites (Figure 2f).[34] In comparison with FePPc-ClO3 and CoPPc-ClO3, Fe0.9Co0.1PPc-ClO3 presents a negative energy shift at the O 1s peaks (especially in the M-O bond), indicating that the copolymerization of FeN4 and CoN4 units induces a charge transfer between the central metal and the axial O, which regulates the electron cloud density. Compared with Fe0.9Co0.1PPc, as shown in Figure S8c, after the introduction of -ClO3 ligand, the XPS peak signal of -OH shows a clear shift toward lower binding energy, and the content is greatly reduced, which further proves the electron can be more easily localized on the axial O sites of Fe0.9Co0.1PPc-ClO3 via the electron transfer process between the central metal and axial O. 3.2. Electrochemical activity and selectivity of the catalysts for ORRThe ORR performance of all the as-prepared samples was evaluated in a three-electrode device with 0.1 M KOH electrolyte. The impressive reduction peaks of Fe0.9Co0.1PPc-ClO3, FePPc-ClO3 and CoPPc-ClO3 were observed in O2-saturated solution in contrast to the cyclic voltammogram (CV) curves without sharp peaks in N2-saturated counterpart within the range from 0 to 1.2 V vs. RHE (Figure S9). Meanwhile, the higher cathodic peak potential of Fe0.9Co0.1PPc-ClO3 indicates that the heterometallic catalyst possesses enhanced ORR electrocatalytic activity in alkaline media. Subsequently, the electrocatalytic ORR performance of several samples was evaluated by linear sweep voltammetry (LSV). Notably, Fe0.9Co0.1PPc-ClO3 with bimetallic sites presents outperformed ORR activity compared to other tested samples and the frontier MPPc-based catalysts, exhibiting an onset potential (Eonset) of 0.986 V vs. RHE and E1/2 of 0.923 V vs. RHE (Figure 3a and Table S4). The corresponding Tafel plots suggest that Fe0.9Co0.1PPc-ClO3 contains promoted reaction kinetics with the smallest Tafel slop of 61 mV dec-1, exceeding that of the Pt/C catalyst (125 mV dec−1) (Figure 3b). It could be found that an excess of Fe sites is more likely to hinder the catalytic activity, while the activity of CoPPc-ClO3 with Co sites only is also limited. Thus, the synergy of Fe and Co sites leads to excellent catalytic activity. The electrochemical active area (ECSA) values of several materials, represented by the double-layer capacitance (Cdl), are shown in Figure S10a-c and Figure 3c, in which 17.71 mF·cm–2 for Fe0.9Co0.1PPc-ClO3 is superior to those for the control groups, revealing a larger surface area accessible in the electrochemical catalytic reaction. To further investigate the interfacial kinetics at the interface between electrolyte and catalyst, electrochemical impedance spectroscopy (EIS) measurements were carried out. The Nyquist plots, as shown in Figure 3d, present low transfer control resistance and fast electron transfer of Fe0.9Co0.1PPc-ClO3. Meanwhile, the catalytic performance of FexCo1-xPPc-ClO3 composites with different metal content is shown in Figure S8d-f and Figure S11. MA of FexCo1-xPPc-ClO3 catalysts with a series of Fe/Co contents was calculated to further evaluate their ORR performance. As shown in Figure S12, Fe0.9Co0.1PPc-ClO3 shows the highest mass activity of 10.18 A g-1 at 0.9 V vs. RHE, suggesting the catalyst possesses more catalytic active sites to enhance ORR. Therefore, the outstanding ORR catalytic activity of Fe0.9Co0.1PPc-ClO3 in all samples is due to the co-introduction and regular arrangement of Fe and Co metals at the adjacent sites. To estimate the selectivity during ORR, the kinetic process of Fe0.9Co0.1PPc-ClO3 was elucidated via two pathways. Rotating ring disk electrode (RRDE) measurements are conducted with a speed of 1600 rpm in O2-saturated 0.1 M KOH, as depicted in Figure 3e. The calculated electron transfer number (n) for Fe0.9Co0.1PPc-ClO3 is approximately the theoretical value of 4.0 while H2O2 yield is below 2%, implying that the ORR proceeded primarily via a four-electron pathway (Figure 3f). Furthermore, n value of ~4 can also be obtained by the Koutecky–Levich (K-L) equation which investigated charge transfer capability based on the increased current density with the increment of rotation rate from 400 to 1600 rpm (Figure S13). The corresponding kinetic current density (jk) is 17.84 mA cm−2, which is much higher than 2.73 mA cm−2 for the commercial Pt/C, indicating its intrinsic excellent ORR activity (Figure S14). All of the above results prove that Fe0.9Co0.1PPc-ClO3 catalyst with well-defined binary Fe/Co active sites has excellent catalytic selectivity toward ORR.To elucidate the effect of axial ligands on the electrocatalytic ORR activity, the relationship between intrinsic structure and ORR activity of Fe0.9Co0.1PPc-AL was deeply investigated by comparing with that of Pt/C. It was found that the axial ligand structure is favorable to the ORR catalytic performance compared to the pristine Fe0.9Co0.1PPc, which coincides with the ECSA (Figure S15a-b and Figure S16). Meanwhile, the catalytic kinetic properties affected by the axial ligands exhibit significant differences, including Tafel and EIS measurements (Figure S15c-d). The interaction of the axial oxygen-containing ligand with the central metal breaks the symmetry of the electron cloud density around the reactive sites during the reaction, which may change the electronic state of the central metal and thus effectively accelerate the ORR process.[34, 35] Moreover, the difference in catalytic performance is also closely related to the spatial selectivity and structure of the axial ligands, e.g. suitable spatial coordination resistance, high selectivity and axial ligand saturation.3.3. Electrochemical stability of Fe0.9Co0.1PPc-ClO3 for ORRIn addition to its activity, the durability of a catalyst is even more critical for its practical applications. Fe0.9Co0.1PPc-ClO3 catalyst displays excellent long-term durability, as manifested in 5,000 cycles CV measurements in an O2-saturated 0.1 M KOH aqueous solution with a negative E1/2 shift of only 10.0 mV which is much lower than that of the commercial Pt/C (37.0 mV), as can be seen from Figure 4a. After a chronoamperometric test for 40,000s, Fe0.9Co0.1PPc-ClO3 exhibits considerably higher current retention compared with Pt/C (Figure S17). In addition, the Cdl values of catalysts after the durability test were estimated, as shown in Figures S18 and S19. In particular, the obtained Cdl value of 12.84 mF cm–2 for Fe0.9Co0.1PPc-ClO3 indicated that the catalyst maintained a large electrochemically active surface area. Meanwhile, the Nyquist plots reveal that no obvious change is observed in the charge transfer ability of Fe0.9Co0.1PPc-ClO3 catalyst before and after the long-term durability test. In addition, the methanol tolerance of Fe0.9Co0.1PPc-ClO3 also was examined. As shown in Figure 4b, only a slight change of current density is detected for Fe0.9Co0.1PPc-ClO3 with the addition of methanol solution at t = 200 s, considerably superior to the Pt/C electrocatalyst. Furthermore, even in a mixed solution containing 0.5 M CH3OH, the current response caused by Fe0.9Co0.1PPc-ClO3 almost did not degrade, while a distinct methanol oxidation peak (~0.75 V) was observed for 20% Pt/C under the same conditions (Figure S20). All these results indicate an outstanding resistance for Fe0.9Co0.1PPc-ClO3 toward methanol toxicity.To further elucidate the structure stability of the catalysts, XRD and SEM measurements after the durability testing were carried out. The XRD pattern of Fe0.9Co0.1PPc-ClO3 after the 5000 cycles CV, as presented in Figure S21, showed the two diffraction peaks, which were ascribed to (101) and (001) planes of catalysts, respectively (the intense signals noted as CP corresponded to the carbon paper substrate). Although the positions of the major diffraction peaks of catalysts did not significantly change, the weakening of the peak intensity and the disappearance of some small peak signals were still observed, which might be attributed to the decreased crystallinity of the catalysts after the stability test. Meanwhile, SEM images after the stability test, as depicted in Figure S22, showed that Fe0.9Co0.1PPc-ClO3 still retains the 2D-layered stacking features, further demonstrating the outstanding stability of catalysts.3.4. ORR mechanism analysis of Fe0.9Co0.1PPc-ClO3To further investigate the origin of the catalytic activity of Fe0.9Co0.1PPc-ClO3 and the possible mechanism responsible for its enhanced ORR performance, the rate-determining step (RDS) of the ORR at the active sites was studied using in situ Raman spectroscopy, as shown in Figure 5a. The signal peak of stretching vibration belonging to v(Fe-N/O) exists at the open-circuit voltage, and the peak intensity significantly changes with voltage, which indicates that the catalytic active site is located at the vicinity of Fe, and the RDS is the desorption of oxygen intermediate species.[36] In addition, according to previous literature reports, the Tafel slope is closely related to the reaction mechanism. The fitting results show that the Tafel slopes of all the Fe0.9Co0.1PPc-AL samples are about 60 mV dec–1 whereas those of Fe0.9Co0.1PPc without axial ligands are about 90 mV dec–1, which suggests that the second step of the electron transfer step is a possible RDS for Fe0.9Co0.1PPc-AL, that is, the desorption of the intermediate *OOH, as shown in Figure 5b.[37, 38]DFT calculations were conducted for a better understanding of the catalytic mechanism of Fe0.9Co0.1PPc-ClO3 electrocatalyst. For ease of calculation, here FeCoPPc-ClO3, FePPc-ClO3, CoPPc-ClO3 and FeCoPPc are simplified as conjugated dimeric iron cobalt phthalocyanine (d-FeCoPc-ClO3), dimeric iron phthalocyanine (d-FePc-ClO3) and dimeric cobalt phthalocyanine (d-CoPc-ClO3) containing axial -ClO3 coordination, as well as d-FeCoPc without -ClO3 axial ligand, as displayed in Figure 5c and Figure S23. The geometric structure models of FeCoPPc-ClO3 with FeN4-O sites and CoN4-O sites as possible active centers are denoted as 1-Fe and 1-Co, respectively. As illustrated in Figure 5d, the electrostatic potential (ESP) analysis reveals that the negatively charged green region mainly originates from the peripheral conjugated carbon framework, while the positively charged blue region is predominantly associated with the central metals Fe and Co. After the introduction of the metal Co, the Co-N4 metal units undergo electronic interactions with the neighboring Fe sites, leading to a change in the density of the electron cloud, which may promote the electron transfer from the phthalocyanine unit but also the redistribution of the electron cloud. In addition, as shown in Figure 5e and Figure S24, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the four phthalocyanine structures possess different electron cloud distribution states, indicating the influence of the central metal and axial ligand on the electron density of the MPPc structure. Based on frontier molecular orbital distribution, the HOMO-LUMO energy gap is closely related to electron conduction, i.e., a narrower HOMO-LUMO gap corresponds to a higher electronic conductivity.[39, 40] The HOMO-LUMO gap (0.63 eV) of d-FeCoPc-ClO3, much smaller than 1.07 eV for d-FePc-ClO3 and 1.74 eV for d-CoPc-ClO3, suggests that d-FeCoPc-ClO3 possesses the best electronic conductivity and therefore exhibits the best ORR performance, as depicted in Figure 5e. The free energy profile of FeCoPPc-ClO3 for the intermediate steps at a potential of 0 V is shown in Figure 5f. At the Fe sites, the free energy change from *OOH to *O species involving the larger energy barrier on FeCoPPc-ClO3 is nonspontaneous (ΔG > 0) under equilibrium potential conditions, which leads to the fact that the desorption of *OOH is difficult, therefore the desorption process of *OOH can be regarded as the RDS for ORR.[41, 42] While for the Co active centers, the Gibbs free energy change of the reaction process is highly variable, which is thermodynamically unfavorable. In addition, the free energy of the oxygenated species at the Co active sites is either strong or weak, which is not favorable to the ORR kinetic process. Therefore, the excellent ORR catalytic activity of the Fe0.9Co0.1PPc-ClO3 catalyst can be attributed to the interaction of the adjacent Fe-N4 and Co-N4 units. The introduction of metal Co sites modulates the electronic state density of the active center metal Fe, which facilitates the optimization of the binding energy for the desorption process of *OOH intermediate and thus achieves the optimal ORR kinetic process. Meanwhile, the oxygenated axial ligands can break the symmetric local charge microenvironment of the central atoms to optimize electrocatalytic activity and stability.4. ConclusionIn summary, MPc-based covalent organic polymers FeCoPPc electrocatalysts were prepared through a facile continuous oxidation process using H2O2, and FeCoPPc-AL with different spatial ligand structures were further synthesized by an ultrasound-assisted axial coordination strategy for enhanced ORR. FeCoPPc-ClO3 with an optimized Fe/Co ratio of 9 (i.e. Fe0.9Co0.1PPc-ClO3 sample) exhibited the best ORR activity of a high E1/2 potential up to 0.923 V vs. RHE with a small Tafel slope of 61.0 mV dec−1 in alkaline electrolyte, which remarkable outperformed commercial Pt/C electrocatalyst. A mechanistic study, using in-situ Raman spectroscopy and DFT analyses, revealed that the synergistic effect of Fe active sites neighboring Co was the main factors for the improvement of catalytic performance. Meanwhile, the introduction of axial O-coordination in Fe0.9Co0.1PPc further facilitated the breaking of the symmetric electronic structure, modulating the spatial electron cloud density. Therefore, the optimal binding strength for *OOH was achieved. This work offers a new thought to accelerating electrocatalytic ORR through the synergy of heterogeneous metal modification and spatial coordination modification.Author contributionThe manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript.AcknowledgementsThis work is financially supported by the National Natural Science Foundation of China (22208322). H.W. acknowledges support from the Project of Zhongyuan Critical Metals Laboratory (GJJSGFYQ202336).Formatting of funding sourcesThis work was supported by the National Natural Science Foundation of China (22208322).Scheme 1. Schematic illustration of the synthesis of FeCoPPc-AL catalysts.Figure 1 Micro-/Nanostructural characterizations of Fe0.9Co0.1PPc-ClO3. (a) TEM and (b) HRTEM images (inset: SAED pattern). (c) Elemental mapping images of Fe, Co, N, O and Cl. (d) XRD patterns of Fe0.9Co0.1PPc-ClO3, FePPc-ClO3 and CoPPc-ClO3 catalysts.Figure 2 Compositional and structural characterizations of the as-prepared Fe0.9Co0.1PPc-ClO3, FePPc-ClO3 and CoPPc-ClO3 catalysts. (a) FT-IR spectra, (b) Raman spectra and (c) UV-Vis spectra (using DMF as solvent). High-resolution XPS spectra of (d) Fe 2p, (e) Co 2p and (f) O 1s.Figure 3 Electrocatalytic ORR tests. (a) LSV curves with 95% iR compensation, (b) Tafel plots, (c) Cdl obtained by plotting current density difference against scan rate. (d) Nyquist plots. (e) RRDE tests for disk and ring current curves and (f) estimated HO2− yield (%) and electron transfer number (n) derived from the RRDE curves of Fe0.9Co0.1PPc-ClO3 and 20% Pt/C. Figure 4 Electrochemical stability and methanol resistance of Fe0.9Co0.1PPc-ClO3 and 20% Pt/C. (a) LSV curves with iR compensation of catalysts before and after 5000 cycles of CV with a scan rate of 50 mV s−1 toward ORR in O2-saturated 0.1 M KOH. 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