Axial Coordination Modulation of FeN₄ Sites in Dioxin-Linked Covalent Organic Hybrid Catalysts for Enhanced ORR Activity and Zinc–Air Battery Application

Abstract

Effective regulation of the adsorption strength of oxygen reduction reaction (ORR) intermediates on active sites is the key to enhancing their catalytic performance. This study proposes an axial coordination modulation strategy by successfully anchoring the dioxin-linked FePcF₁₆-based covalent organic frameworks (COFs) onto amino-functionalized multi-walled carbon nanotubes (NH₂-MWCNTs), constructing a FePcF₁₆-COF/NH₂-MWCNT hybrid catalyst. Experimental results demonstrate that the catalyst exhibits outstanding ORR activity (E_1/2 = 0.901 V; J_L = 5.133 mA cm⁻²), outperforming commercial 20% Pt/C and most reported Fe-based non-precious metal catalysts. Furthermore, the robust dioxin-linked COF skeleton endows the catalyst with excellent electrochemical stability. A zinc–air battery using this catalyst as the cathode also demonstrates superior power density and cycling performance. This work provides a new strategy for designing highly efficient ORR catalysts through axial coordination environment engineering.

1. Introduction

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are a critical electrochemical process in renewable energy systems such as fuel cells and metal–air batteries [1,2,3,4,5,6]. However, its practical application is significantly hindered by sluggish kinetics and high overpotential, which substantially limit the energy conversion efficiency of these devices [7,8,9]. Although platinum catalysts represent the gold standard in ORR due to their exceptional activity, their scarcity, considerable expense, and poor poisoning tolerance severely restrict their large-scale electrochemical implementation [10,11,12,13]. Consequently, research efforts have shifted to focus on the development of economical and earth-abundant non-precious metal catalysts [14,15,16,17]. Among various candidates, iron–nitrogen–carbon (Fe-N-C) materials have exhibited ORR catalytic performance rivaling that of Pt [18,19,20]. Nevertheless, the precise construction of Fe-N-C structures that integrate structurally precise active sites with robust stability and exceptional conductivity remains a key challenge for achieving superior catalytic performance.

In the last several years, covalent organic frameworks (COFs) have risen as a new family of porous crystalline materials [21]. Their periodically regular channels; high specific surface area; and atomically precise designability of catalytic sites, such as metal-phthalocyanine units, make them ideal precursors or direct platforms for constructing high-performance M-N-C electrocatalysts [22,23,24,25,26]. For example, Huang et al. tuned the linkage microenvironment of FePc-based polymers to indirectly modulate the active centers, achieving enhanced ORR performance [27]. Li et al. constructed imine-linked NiPc-based COFs with donor–acceptor units to enhance charge transfer, demonstrating good ORR and HER bifunctional activity [28]. However, most conventional COFs face two major challenges: first, their limited π-conjugation often results in low intrinsic electrical conductivity, hindering efficient charge transfer [29,30,31]; second, insufficient chemical stability under high-temperature and strongly alkaline conditions may lead to structural degradation and consequent activity loss [32,33,34,35]. These shortcomings significantly limit the application of COFs in high-performance electrocatalysis.

To address these challenges, this work proposes a strategy centered on skeletal engineering coupled with carrier synergy. First, regarding skeletal engineering, the dioxin linkage was selected to construct the COFs skeleton due to its high bond energy and full conjugation. This linkage confers exceptional chemical and thermal stability to the material, enabling it to withstand harsh electrochemical environments [36,37,38]. Second, in terms of carrier synergy, aminated multi-walled carbon nanotubes (NH₂-MWCNTs) were innovatively employed as a conductive substrate for the COFs. The introduced amino groups enhance the interaction between the COFs and the carrier, which promotes the uniform growth and robust anchoring of the active COFs layer [39]. More importantly, the electron-donating propensity of these amino groups can further fine-tune the electronic configuration surrounding the active Fe center, consequently lowering the reaction energy barrier. Moreover, the resulting material showcases a hierarchical pore structure that simultaneously reveals numerous active sites and ensures the presence of efficient pathways for rapid reactant and product transport.

Based on this rationale, a novel hybrid electrocatalyst, FePcF₁₆-COF/NH₂-MWCNT, has been successfully synthesized via a facile solvothermal method in this study. Electrochemical tests unambiguously demonstrated that the hybrid catalyst exhibits outstanding ORR performance through axial coordination modulation of atomically dispersed FeN₄ sites, achieving a half-wave potential (E_1/2) of 0.901 V (vs. RHE) and a limiting current density (J_L) of 5.133 mA cm⁻². Moreover, the specific COF structure with robust dioxin linkages further guarantees the stability of the catalyst under harsh conditions. When employed in zinc–air batteries, it achieves a maximum power density of 169.75 mW cm⁻², with remarkable cycle stability.

2. Results and Discussion

The surface morphology of the as-synthesized FePcF₁₆-COF/NH₂-MWCNT was probed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images revealed that, compared to pristine NH₂-MWCNTs (Figure 1a), FePcF₁₆-COF/NH₂-MWCNT retained the original morphology of NH₂-MWCNTs (Figure 1b) without significant agglomeration, and its surface was covered by a thin layered material (Figure 1c). Thickness analysis based on TEM images confirmed the successful formation of a thin COF layer, with a thickness of 0.74 ± 0.05 nm, on the surface of the NH₂-MWCNTs (Figure 1d–f). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was performed alongside energy-dispersive X-ray spectroscopy (EDS) mapping for C, N, O, F, and Fe (Figure 1g), verifying the even distribution of these elements within FePcF₁₆-COF/NH₂-MWCNT. This result directly indicates uniform dispersion of metal centers throughout the structure. These morphological observations collectively confirm the successful formation of the COF layer around the NH₂-MWCNTs.

FePcF₁₆-COF/NH₂-MWCNT was analyzed by Fourier-transform infrared spectroscopy (FT-IR) measurements so as to comprehensively ascertain its chemical structure and elemental composition (Figure S1). For FePcF₁₆-COF/NH₂-MWCNT, the acquired FT-IR spectrum revealed prominent vibration bands centered at 1267 cm⁻¹ and 1126 cm⁻¹, corresponding to the asymmetric and symmetric stretching vibrations, respectively, associated with the C–O–C moieties present in the dioxin linkage. Additionally, the peaks observed at 1520 cm⁻¹ and 1487 cm⁻¹ are attributed to the vibrational bands of the benzene rings within the phthalocyanine unit. Particularly striking was the complete disappearance of the O–H stretching vibration within the region of 3300–3550 cm⁻¹, providing compelling evidence for the quantitative transformation of hydroxyl moieties present in the starting materials into the targeted products. The emergence of the C–O–C stretching vibrations, together with the absence of the O–H stretching band, confirms the successful formation of the dioxin-linked structure in FePcF₁₆-COF/NH₂-MWCNT. To elucidate the role of NH₂-coordination in modulating the catalyst structure, a control sample FePcF₁₆-COF/MWCNT was prepared under identical synthetic conditions using pristine MWCNTs (without –NH₂ functionalization). Raman spectroscopy data were used to compare the degree of graphitization and defect density between the FePcF₁₆-COF/NH₂-MWCNT and FePcF₁₆-COF/MWCNT catalysts. Vibrational characterization through Raman spectroscopy was performed on both catalysts, with the resulting spectra presented in Figure S2. Quantitative assessment of carbon structural defects within these materials involved determining the peak intensity ratio of the D mode (~1310 cm⁻¹) relative to the G mode (~1590 cm⁻¹). The I_D/I_G ratios for FePcF₁₆-COF/NH₂-MWCNT and FePcF₁₆-COF/MWCNT are 2.04 and 2.01, respectively. This indicates that FePcF₁₆-COF/NH₂-MWCNT possesses a higher density of carbon defects compared to FePcF₁₆-COF/MWCNT, which is more favorable for enhancing ORR activity. The XRD pattern of FePcF₁₆-COF/NH₂-MWCNT showed a peak at 43.42°, characteristic of the C (101) plane of carbon nanotubes (Figure S3). No discernible diffraction signals for iron species could be detected, which is attributed to the homogeneous dispersion of iron within the catalyst. Notably, the XRD pattern of FePcF₁₆-COF/NH₂-MWCNT (Figure S3) only shows the C (002) and C (101) diffraction peaks of the carbon nanotubes, while no characteristic peaks of the COFs itself are observed. This is attributed to the limited long-range order of the COFs layer, resulting in weak diffraction signals, which may also be masked by the strong signals from the carbon nanotubes [40,41].

In pursuit of a more comprehensive analysis regarding the catalyst’s surface elemental composition, investigations employing X-ray photoelectron spectroscopy (XPS) were carried out to scrutinize the surface chemistry in conjunction with the electronic structure exhibited by FePcF₁₆-COF/NH₂-MWCNT alongside FePcF₁₆-COF/MWCNT. The survey XPS spectra acquired from both catalytic materials verified the coexistence of C, N, O, F, plus Fe, in good agreement with the compositional information gleaned from EDS mapping analyses (Figure 2a). Subsequent quantitative analysis yielded elemental contents of C (90.68%), N (3.06%), O (3.02%), F (2.89%), and Fe (0.36%) (Figure S4). Inspection of the high-resolution N 1s profiles (Figure 2c) revealed that FePcF₁₆-COF/NH₂-MWCNT displays two distinct components centered at 399.5 eV and 400.9 eV, attributable to C=N and Fe–N coordination environments, in that order. Comparative analysis revealed a significant increase in the proportion of fitted Fe–N coordination within FePcF₁₆-COF/NH₂-MWCNT relative to FePcF₁₆-COF/MWCNT [42]. This result indicates that the Fe centers in FePcF₁₆-COF/NH₂-MWCNT engage in axial coordination with the –NH₂ groups of the NH₂-MWCNTs. Detailed examination of the Fe 2p region (Figure 2d) provides additional support for this interaction. For FePcF₁₆-COF/NH₂-MWCNT, photoelectron signals arising from Fe 2p_3/2 and Fe 2p_1/2 were detected at 709.6 eV and 723.3 eV, in that sequence. For FePcF₁₆-COF/MWCNT, the corresponding peaks appeared at 710.0 eV and 723.6 eV. This negative shift in binding energy observed for FePcF₁₆-COF/NH₂-MWCNT underscores the capacity of –NH₂ moieties to donate electron density toward the Fe center.

Investigation into the specific surface area along with porosity pertaining to FePcF₁₆-COF/NH₂-MWCNT and FePcF₁₆-COF/MWCNT was conducted through Brunauer–Emmett–Teller (BET) analysis (Figure S5). Quantitative porosity analysis revealed that FePcF₁₆-COF/NH₂-MWCNT possesses a 141.64 m² g⁻¹ specific surface area, 0.71 cm³ g⁻¹ cumulative pore volume, and 20.12 nm mean pore diameter, standing in marked contrast to the 128.17 m² g⁻¹, 0.69 cm³ g⁻¹, and 21.87 nm values manifested by FePcF₁₆-COF/MWCNT (Table S1). The larger specific surface area, alongside the smaller pore volume and pore size of FePcF₁₆-COF/NH₂-MWCNT, suggests more sufficiently exposed active sites and the presence of interconnected nanochannels. In contrast to most reported phthalocyanine-based COFs, which suffer from poor chemical stability, the dioxin-linked framework of FePcF₁₆-COF/NH₂-MWCNT exhibited superior thermal stability. This was confirmed by thermogravimetric (TG) analysis, which showed no significant mass loss for FePcF₁₆-COF/NH₂-MWCNT below 400 °C under a N₂ atmosphere or below 420 °C in air (Figure S6).

Assessment of electrocatalytic ORR performance for FePcF₁₆-COF/NH₂-MWCNT was carried out in 0.1 M KOH alkaline medium. Cyclic voltammetry (CV) profiles corresponding to the catalysts are depicted in Figure S7. Under O₂-saturated conditions, all catalysts exhibited well-defined redox features. Conversely, no such distinct peaks were apparent in N₂-saturated electrolyte. To acquire deeper understanding of ORR performance, linear sweep voltammetry (LSV) was conducted using a rotating disk electrode (RDE) within O₂-saturated 0.1 M KOH electrolyte (Figure 3a). The FePcF₁₆-COF/NH₂-MWCNT catalyst showed excellent ORR performance (E_onset = 0.984 V; E_1/2 = 0.901 V; J_L = 5.133 mA cm⁻²) (Table S2). These performance metrics exceeded those of the commercial 20% Pt/C (E_onset = 0.893 V; E_1/2 = 0.840 V; J_L = 4.766 mA cm⁻²) and outperformed most of the reported Fe-based non-precious metal catalysts (Figure 3h) [43,44,45,46,47,48,49,50,51,52,53,54]. Although FePcF₁₆-COF/MWCNT also demonstrated good ORR activity (E_onset = 0.968 V; E_1/2 = 0.871 V; J_L = 5.472 mA cm⁻²), its performance remained inferior to that of FePcF₁₆-COF/NH₂-MWCNT. This result indicates the favorable effect of NH₂-coordination on the catalyst’s ORR efficacy. To clarify the function of the COF layer formation in the ORR performance of the catalyst, a controlled experiment was conducted. For the controlled experiment, FePcF₁₆/NH₂-MWCNT was prepared through physical blending of the same reagents in identical ratios as for FePcF₁₆-COF/NH₂-MWCNT, but the mixture reacted at room temperature instead of under solvothermal conditions. LSV measurements revealed that FePcF₁₆/NH₂-MWCNT exhibited inferior electrocatalytic activity (E_onset = 0.957 V; E_1/2 = 0.858 V; J_L = 4.835 mA cm⁻²) compared to FePcF₁₆-COF/NH₂-MWCNT (Figure S8). This result unambiguously demonstrates that solvothermal treatment effectively restrained phthalocyanine monomer aggregation, yielding an in situ COF layer with ordered structure plus atomically dispersed active sites—both being indispensable to ORR performance improvement. The Tafel slope, a key parameter for assessing catalytic activity, was obtained from the corresponding LSV curves. The Tafel slope of FePcF₁₆-COF/NH₂-MWCNT (44.51 mV dec⁻¹) was lower than that of FePcF₁₆-COF/MWCNT (48.00 mV dec⁻¹), and both were appreciably below that of commercial 20% Pt/C (77.92 mV dec⁻¹), matching the activity trends revealed by the LSV measurements (Figure 3b).

The ORR pathway was investigated through determination of the electron-transfer number (n) via the Koutecký–Levich (K-L) approach, using LSV curves recorded at different rotation speeds (Figure S9a,b). For the FePcF₁₆-COF/NH₂-MWCNT catalyst, the calculated n value was approximately 4 across a wide potential range (Figure 3d), suggesting a dominant four-electron (4e⁻) transfer process. To more thoroughly evaluate the reaction selectivity, rotating ring-disk electrode (RRDE) measurements were conducted (Figure 3e). These measurements confirmed the superior selectivity of FePcF₁₆-COF/NH₂-MWCNT, which exhibited a hydrogen peroxide (H₂O₂) yield of less than 0.5% and a more favorable 4e⁻ pathway compared to the commercial 20% Pt/C benchmark.

In pursuit of a thorough evaluation of ORR performance exhibited by FePcF₁₆-COF/NH₂-MWCNT alongside FePcF₁₆-COF/MWCNT, quantification of electrochemical double-layer capacitance (C_dl) was achieved through electrochemical active surface area (ECSA) determinations, thereby enabling precise estimation of active site populations within these catalysts. Additionally, electrochemical impedance spectroscopy (EIS) was utilized to probe charge-transfer resistance (R_ct). First, the C_dl was determined by CV-based fitting (Figure S9c,d). As depicted in Figure S10, FePcF₁₆-COF/NH₂-MWCNT exhibits a C_dl of 1.35 mF cm⁻², surpassing the 0.86 mF cm⁻² value determined for FePcF₁₆-COF/MWCNT, suggesting a higher catalytically active surface area that contributes to enhanced catalytic performance. The EIS Nyquist plot (Figure S11) reveals that, compared with FePcF₁₆-COF/MWCNT (R_ct = 31.76 Ω), FePcF₁₆-COF/NH₂-MWCNT exhibits a lower charge-transfer resistance (R_ct = 28.09 Ω), suggesting better electron-transfer efficiency in FePcF₁₆-COF/NH₂-MWCNT. The above results indicate that the coordination between –NH₂ and FeN₄ sites enables –NH₂ to enhance the electron coupling across the support and the active sites through its electron-donating effect, improves the interfacial conductivity of the catalyst, enhances the electron-transfer efficiency during the oxygen reduction process, and thereby promotes the ORR reaction.

Thereafter, the long-term stability and methanol tolerance of the FePcF₁₆-COF/NH₂-MWCNT catalyst were assessed and benchmarked against commercial 20% Pt/C via chronoamperometric (i-t) testing. After 30,000 s of testing, FePcF₁₆-COF/NH₂-MWCNT maintained 91.2% of its initial current density, in stark contrast to the 21.8% current density loss observed for commercial 20% Pt/C, as depicted in the i-t curves (Figure 3f). To evaluate methanol tolerance, a 20 mL aliquot of methanol was introduced into the electrolyte at the 300 s mark. Upon methanol addition, the FePcF₁₆-COF/NH₂-MWCNT electrode exhibited only a minor and transient fluctuation in current (likely due to the concentration gradient caused by the instantaneous injection), and rapidly stabilized thereafter. In contrast, the 20% Pt/C catalyst showed an immediate and sharp current drop after methanol injection (Figure 3g). These results indicate that, in comparison with the 20% Pt/C benchmark, FePcF₁₆-COF/NH₂-MWCNT demonstrates superior durability and methanol tolerance. We performed SEM characterization of FePcF₁₆-COF/NH₂-MWCNT after the stability test. The results show that the catalyst largely retains its original morphology after the long-term ORR stability test (Figure S14), demonstrating its structural stability under the reaction conditions. This enhanced stability is primarily attributed to the robust dioxin linkages within the COF framework that ensure structural integrity under harsh electrochemical conditions. Furthermore, we have performed density functional theory (DFT) calculations to gain deeper insight into the effect of axial coordination between NH₂-MWCNT and the FeN₄ sites. Compared with FePcF₁₆-COF/MWCNT, FePcF₁₆-COF/NH₂-MWCNT exhibits a lower overpotential (η = 0.30 V), indicating a more favorable thermodynamic driving force for the ORR (details in SI).

The excellent ORR activity of FePcF₁₆-COF/NH₂-MWCNT renders it a promising candidate for zinc–air battery deployment. In pursuit of assessing this catalyst’s capabilities within zinc–air batteries, in-house fabricated zinc–air batteries utilizing FePcF₁₆-COF/NH₂-MWCNT, alongside commercial 20% Pt/C as cathodic catalysts, were constructed (Figure 4a) and then tested under identical conditions for performance comparison. The initial open-circuit voltage (OCV) measurement revealed that at 5 mA cm⁻², FePcF₁₆-COF/NH₂-MWCNT delivered 1.46 V, outperforming commercial 20% Pt/C (1.41 V) (Figure 4b). Galvanostatic discharge–charge profiles, together with the power density curves pertaining to both batteries (Figure 4c,d), were further tested using an electrochemical workstation. As shown, the charge–discharge voltage gap for the FePcF₁₆-COF/NH₂-MWCNT-based battery falls below that exhibited by the Pt/C-based battery. In addition, FePcF₁₆-COF/NH₂-MWCNT delivered a maximum power density of 169.75 mW cm⁻², demonstrating superior performance to commercial 20% Pt/C (143.23 mW cm⁻²), indicating favorable mass transport kinetics. Additionally, long-term cycling tests over 110 h were performed to assess the discharge–charge stability of the catalysts in zinc–air batteries. As presented in Figure 4e, the FePcF₁₆-COF/NH₂-MWCNT-based battery maintained stable discharge and charge voltages after 110 h of cycling, while the Pt/C-based battery exhibited pronounced voltage decay. The round-trip efficiency of the FePcF₁₆-COF/NH₂-MWCNT-based battery decreased from 61.3% to 51.9%, a drop of 9.4%, in contrast to the commercial 20% Pt/C-based battery, whose efficiency declined from 50.9% to 29.2%, a reduction of 21.7% (Figure S12). This demonstrates that the FePcF₁₆-COF/NH₂-MWCNT-based battery possesses superior discharge–charge stability. Overall, FePcF₁₆-COF/NH₂-MWCNT shows considerable application prospects as a Pt-free cathode catalyst in zinc–air batteries.

3. Experimental

3.1. Chemicals

Iron(II) phthalocyanine (FePcF₁₆, 95%) was purchased from Zhongke Yanshen Technology Co., Ltd. (Changchun, China). Multi-walled carbon nanotubes (MWCNTs), amino-functionalized multi-walled carbon nanotubes (NH₂-MWCNTs), and tetrahydroxy-1,4-benzoquinone were supplied by Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). N,N-Dimethylformamide (AR), absolute ethanol (AR), acetone (AR), and tetrahydrofuran (AR) were obtained from Hangzhou Gaojing Fine Chemical Co., Ltd. (Hangzhou, China). Commercial platinum-loaded carbon (20 wt% Pt/C) was provided by Johnson Matthey Chemicals Ltd. (Shanghai, China). All chemicals and solvents were used as received, without further purification.

3.2. Synthesis of FePcF₁₆-COF/NH₂-MWCNT and FePcF₁₆-COF/MWCNT

A thin layer of COF was successfully deposited onto NH₂-MWCNTs via a straightforward solvothermal method. This novel hybrid catalyst was designated as FePcF₁₆-COF/NH₂-MWCNT, as illustrated in Figure 5. Specifically, hexadecafluoro iron phthalocyanine (FePcF₁₆, 20 μmol) and NH₂-MWCNTs (68.48 mg) were stirred in a mixed solvent of DMF and Et₃N for 12 h at room temperature. Subsequently, tetrahydroxy-1,4-benzoquinone (THBQ, 40 μmol) was added to the mixture and stirred for 6 h at room temperature. The mixture was then charged into a Pyrex tube, degassed via three freeze–pump–thaw cycles, and heated at 100 °C for 5 days. The product was collected and rinsed with acetone, tetrahydrofuran, and ethanol in sequence, and then dried under vacuum at 60 °C overnight. For comparison, FePcF₁₆-COF/MWCNT was prepared following the same procedure, using pristine multi-walled carbon nanotubes (MWCNTs) (68.48 mg) instead of NH₂-MWCNTs.

3.3. Materials’ Characterization

The surface morphology of the as-prepared catalysts was initially examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fourier-transform infrared spectroscopy (FT-IR, VERTEX 70 mid, Bruker, Billerica, MA, USA) was employed to obtain information on the functional groups present in the synthesized catalysts. In situ Raman spectroscopy was performed on a Nicolet iS20 instrument (Thermo Scientific, Waltham, MA, USA) with an excitation wavelength of 532 nm. Crystal structure and phase composition of the catalysts were analyzed by X-ray diffraction (XRD). The surface elemental composition and chemical states were investigated by X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Scientific). Specific surface area, pore-size distribution, and pore volume of the catalysts were determined using an automatic surface area and porosity analyzer (Micromeritics ASAP 2460, Norcross, GA, USA).

3.4. Electrochemical Measurements

All electrochemical measurements were performed at room temperature using a CHI-760E electrochemical workstation. Tests were carried out in a 0.1 M KOH electrolyte employing a conventional three-electrode configuration. The catalyst ink was prepared by dispersing 5 mg of the sample in a mixture of 970 μL absolute ethanol and 30 μL Nafion solution (5 wt%), followed by ultrasonication for 60 min to obtain a homogeneous dispersion. A 20 μL aliquot of the catalyst ink was evenly drop-cast onto a glassy carbon electrode with a diameter of 5 mm in four separate steps, resulting in a catalyst loading of 0.51 mg cm⁻². Commercial 20% Pt/C was tested under exactly the same mass loading and Nafion ratio as our as-prepared catalyst. All potentials were converted to the reversible hydrogen electrode (RHE) scale according to Equation (1).

$$E_{RHE}=E_{Ag/AgCl}+0.197+0.059\times pH$$

(1)

CV and LSV for the ORR were conducted in both N₂- and O₂-saturated 0.1 M KOH solutions at scan rates of 50 mV·s⁻¹ and 5 mV·s⁻¹, respectively. All LSV curves were background-subtracted using the currents measured in N₂-saturated 0.1 M KOH solution. The ORR polarization curves were recorded over a potential range of 0.2 to 1.1 V (vs. RHE) at rotation speeds ranging from 400 to 2500 rpm, with 80% iR compensation applied. Koutecký–Levich (K-L) plots were derived from the LSV data at various rotation speeds using Equations (2) and (3).

$${\displaystyle \frac{1}{J}}={\displaystyle \frac{1}{J_L}}+{\displaystyle \frac{1}{J_K}}={\displaystyle \frac{1}{B{w}^{1/2}}}+{\displaystyle \frac{1}{J_K}}$$

(2)

$$B=0.62nF{C}_0{\left(D_0\right)}^{{\displaystyle \frac{2}{3}}}{v}^{-{\displaystyle \frac{1}{6}}}$$

(3)

Chronoamperometric (I-t) tests were performed at 0.7 V vs. RHE in O₂-saturated 0.1 M KOH at a rotation rate of 1600 rpm. The hydrogen peroxide yield (%) and electron-transfer number (n) were determined using a rotating ring-disk electrode (RRDE), and were calculated based on Equations (4) and (5). Here, I_D is the disk current, I_R is the ring current, and N is the collection efficiency of the Pt ring (N = 0.37 in this work).

$$n={\displaystyle \frac{4I_D}{I_D+\frac{I_R}{N}}}$$

(4)

$${HO}^{-}={\displaystyle \frac{2I_R}{{NI}_D+I_R}}\times 100\%$$

(5)

3.5. Zinc–Air Battery Measurements

The performance of the zinc–air batteries was evaluated in a homemade electrochemical cell using a LANHE CT-2001A battery testing system. A polished zinc plate served as the anode, and the electrolyte consisted of 6 M KOH + 0.2 M Zn(OAc)₂. Oxygen was supplied from ambient air. The air cathode was composed of a catalyst layer, a current collector layer, and a gas diffusion layer. The catalyst layer was prepared as follows: 5 mg of the catalyst was dispersed in 30 µL of 5 wt% Nafion solution and 970 µL of ethanol under prolonged stirring, followed by ultrasonication for 1 h to obtain a homogeneous catalyst ink. The ink was then uniformly drop-cast onto a clean carbon paper substrate (1.0 cm²) and dried naturally at room temperature, resulting in a catalyst loading of 1 mg cm⁻². For comparison, Pt/C-based zinc–air batteries were assembled under identical conditions. Galvanostatic discharge curves were recorded by LSV at a scan rate of 10 mV s⁻¹ without iR compensation. Current and power densities were normalized to the effective area of the air electrode (1 cm²).

4. Conclusions

We have successfully polymerized and anchored the FePcF₁₆-based COFs onto NH₂-MWCNTs with axial coordination modulation for the ORR. Experimental results demonstrated that the FePcF₁₆-COF/NH₂-MWCNT catalyst exhibits outstanding ORR performance (E_onset = 0.984 V; E_1/2 = 0.901 V), outperforming commercial 20% Pt/C alongside the majority of Fe-based non-precious metal catalysts documented earlier. The catalyst also demonstrated superior ORR performance in zinc–air batteries, attaining elevated open-circuit voltage, as well as enhanced power density relative to commercial 20% Pt/C. Moreover, the dioxin-linked COFs structure ensures excellent stability under harsh conditions. In summary, this work presents a novel strategy for effectively reinforcing catalyst activity through precise regulation of active sites, developing a new and feasible electrocatalytic system with exceptional ORR performance.