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Research Article  |  Open Access  |  2 Apr 2024

Macrocycle-based covalent-organic-polymer as efficient oxygen electrocatalysts for zinc-air flow batteries

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Chem Synth 2024;4:20.
10.20517/cs.2023.64 |  © The Author(s) 2024.
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Covalent organic polymers (COPs), as emerging porous materials with well-defined architectures and high hydrothermal stability, have attracted extensive attention in the field of electrocatalysis. Herein, we report a rational design method for preparing oxygen reduction reaction electrocatalysts with the assistance of a predesigned macrocyclic COP model molecular. With the predesigned nitrogen position and structural features in macrocyclic chain-like COP-based materials, the obtained COPMCT-Co-900 catalyst provided excellent oxygen reduction performance, where the half-wave potential (E1/2) reaches 0.85 V (vs. RHE), comparable to commercial Pt/C. We also extended the strategy to similar macrocycle COPs and Fe-based and Ni-based metal sources and studied the oxygen reduction reaction performance of corresponding catalysts, proving the universality of the method. Interestingly, we assemble COPMCT-Co-900 catalyst as air electrode catalyst of the self-made rechargeable zinc-air flow batteries, which exhibit outstanding power density (155.6 mW·cm-2) and long cycle life (90 h, 270 cycles at 10 mA·cm-2). Our studies provide a new method for the development of high-performance oxygen electrodes applied in zinc-air flow battery devices.


Oxygen reduction, covalent organic polymers, electrocatalyst, zinc-air flow batteries


Over the last decades, efficient design and research of non-precious metal catalysts (NPMC) have made significant progress in oxygen reduction reaction (ORR) electrocatalytic activity and stability[1-6]. The current NPMC systems mainly include transition metal (TM) nitrogen-doped carbon[7-13], supported TM oxides[14,15], TM carbides[16-18], heteroatom-doped carbon materials, etc.[19-21]. Among them, macrocyclic compounds have received widespread attention due to their unique 2D topology structure, high conjugation, and redox-rich chemical properties, exhibiting great potential to replace platinum-based catalysts[22-24]. Macrocyclic compounds are composed of various ligands, such as tetraazamacrocyclic porphyrins and phthalocyanines, and corrode with four nitrogen atoms, which form coordination sites with stabilized TM-N4 compounds, enhancing their ORR catalytic activity. One of the most attractive features for macrocyclic compounds is that their electronic and catalytic properties can be improved by rationally designing and adjusting their ligand characters. Metal macrocycles, such as porphyrins and phthalocyanines doped with TMs (Fe, Co or Ni), are one of the most widely used precursors for preparing ORR electrocatalysts. In 1964, Jasinski et al. first observed that metal macrocycles could promote the ORR of fuel cells and conducted in-depth research on them as potential cathode catalysts in metal-air flow batteries [zinc-air flow batteries (ZAFBs)] and fuel cells[25]; the researchers found that metal atom occupies the center of the macrocycle cavity with maximum stability, which radically facilitates the catalytic activity of ORR electrocatalysts[26-31]. In addition, high ORR catalytic activity was found existing in macrocyclic containing TM-N4 centers (especially CoN4 or FeN4 moiety), such as porphyrins compounds (tetramethoxyphenyl-porphyrin, tetraphenyl-porphyrin, phthalocyanine compounds, and tetraazanthracene)[32,33]. Therefore, metal macrocycle-based catalysts have been extensively studied from synthetic routes to electrocatalytic mechanisms, especially for Fe-based or Co-based porphyrins[34]. However, relatively poor intrinsic conductivity of metallomacrocycles affects the electron transport during electrocatalysis and leads to their behindhand electrocatalytic performance.

The high-temperature pyrolysis for preparing carbon-based catalysts is crucial to improving the ORR catalytic activity of metal macrocyclic systems. The preparation of TM-doped nitrogen-carbon-based catalysts through carbonization of metallophthalocyanine/porphyrin macrocycles has been extensively studied in the field of electrocatalytic materials, and significant progress has been made in terms of oxygen reduction reactivity and lifetime[35]. For example, Choi et al. realized the direct construction of an ordered mesoporous three-dimensional iron-porphyrin catalyst by pyrolyzing the FeN4 complex (tetrapyridyl porphyrin iron) and SBA-15[36]. Compared with Fe-N-C catalysts prepared by carbonizing various precursors on carbon supports, porphyrin-like iron materials completely avoid the addition of additional carbon supports, and the Fe-Nx active sites are uniformly distributed in mesoporous graphite matrix with high specific surface areas. These structures effectively increase the density of effective catalytic sites and promote the transport property of oxygen in macropores and mesopores, accelerating the kinetic of ORR. These characteristics make them an excellent ORR catalytic material.

Recently, a series of new porous materials with covalently linked periodic frameworks and conjugated structures, i.e., macrocycle-based covalent organic polymers (COPs), have been developed[37-40]. They are equipped with superior tailorability in functional design and structure regulation[41-43], so that after pyrolysis, the riveting positions of doped heteroatoms and metal sites in COP precursors can be well predesigned[44]. For example, in recent studies, our group used the Yamamoto coupling reaction to controllably synthesize two-dimensional macrocycle-based COPs with quasi-phthalocyanine structures coordinating with Ni, Fe and Co metal ions and prepared non-noble metal-doped ORR catalysts by high-temperature pyrolysis towards COP precursors[45,46]. Due to the inherent rigid structural characteristics and flexible, adjustable structural design similar to the covalent organic framework (COF)[29], the predesigned catalytic site structure can be maintained to some extent during the high-temperature pyrolysis process, which guarantees the ideal ORR catalytic activity compared with the direct calcination towards carbon, nitrogen and metal-containing composites. Under alkaline conditions, the ORR catalytic activity of COP-based NPMCs achieved great performance improvement. Compared with Pt/C, the Co-doped catalyst showed a higher limiting current density, and its kinetic current is 1.4 times that of the Pt/C catalyst.

Inspired by the above studies, here, we utilized the molecular model of macrocyclic COP to design and prepare nitrogen-coordinated precursor structures and obtained COPMCT-Co-900 electrocatalysts with excellent oxygen reduction activity through subsequent high-temperature carbonization. We extended this strategy to two other COPs with different macrocycle structures and studied the effects of precursors, metal sources and carbonization temperature on the oxygen reduction performance of catalysts. The results show that the COPMCT-Co-900 catalyst exhibits the best oxygen reduction performance, and the half-wave potential (E1/2) reached 0.85 V [vs. reversible hydrogen electrode (RHE)] under alkaline test conditions, which is comparable to commercial Pt/C. After 50,000 s of chronoperometric testing, the current density of COPMCT-Co-900 decreased by 3.2% (vs. Pt/C decreased by 21.9%), indicating their stable durability. In addition, the self-made rechargeable ZAFB using COPMCT-Co-900 catalyst as the air cathode showed a superior power density (155.6 mW·cm-2) and superior cycle life (90 h, 270 cycles). The preparation strategy points out a new direction for high-performance oxygen electrodes.



Ferric nitrate hexahydrate (99%) (Alfa Aesar), Cobal nitrate hexahydrate (99+%) (ACROS Organics), Nickel nitrate hexahydrate (Alfa Aesar), benzene-1,2,4,5-tetracarbonitrile (BTC) (99%) (Bide Medical), 3,5-diamino-1,2,4-triazole (DTZ) (Sigma Andrich), 2,6-diaminopyridine (DPD), 1,3,4thiadiazole-2,5-diamine (TZD), methanol (99.8+%) (Fisher Chemical), and isopropanol (Fisher Chemical), 5.0 wt% Nafion (Du Pont), 20% Pt/C (JM).

Preparation of COPMCT-M

DTZ was employed as monomers to polymerize with BTC and generated the corresponding products, denoted as COPMCT-Co, respectively. Typically, BTC and corresponding monomers were added to 60 mL ethylene glycol under the molar ratio of 1:2.4, and then ultrasound for 5 min to obtain solution A. Additionally, 1.2 equivalent amounts of metal salt, such as iron, cobalt, and nickel salts, were added to 20 mL ethylene glycol, and homogenized solution B was obtained by ultrasound. Solution B was rapidly introduced into solution A, and the microwave reactor was used to react at 180 °C for 1 h. After centrifugation, washing with ethylene glycol, ethanol and water four times, dry overnight in a vacuum drying oven at 70 °C.

Preparation of COPMCT-M-900

COPMCT-M is ground into a powder, carbonized in a tube furnace for 2 h (900 °C), and naturally cooled to obtain COPMCT-M-900.

Physical characterization

The microstructure of the samples was obtained by scanning electron microscope (TESCAN, MAIA 3 XMU) and transmission electron microscope (Hitachi, HT7700). X-ray diffractometer (XRD) data were measured by XRD-7000. Sample compositions were measured by X-ray photoelectron spectroscopy (XPS) (Thermo Fischer, ESCALAB). The Fourier transform infrared spectroscopy (FT-IR) analysis was performed on a Nicolet 8700/Continuum XL. The valence structure of the sample was measured by an X-ray photoelectron spectrometer (THERMO VG, ESCALAB 250). The molecular structure information was obtained by a Raman spectrometer (HORIBA JOBIN YVON SAS, LabRAM HR Evolution), and the specific surface area and pore distribution were determined by BET-ASAP2460.

Electrochemical measurements

We used the CHI electrochemical instrument to perform electrochemical tests on the samples through a three-electrode system, in which the working electrode was coated with a glassy carbon electrode of the sample to be tested. Saturated calomel and graphite rod were used as the reference and counter electrodes, respectively. Furthermore, 5 mg catalyst is evenly dispersed in the mixed solution of ethanol (950 μL) and 5% Nafion (50 μL) by ultrasound, and the ink is coated on the surface of the glass carbon electrode. The working electrode with a load of 0.76 mg·cm-2 was obtained by natural drying (the load of Pt/C was 0.25 mg·cm-2). The hydrogen peroxide yield and the electron transfer number were obtained by rotating ring disk electrode (RRDE) and calculated according to the following Equations (1) and (2):

$$ \mathrm{n}=\frac{4 \mathrm{I}_{\mathrm{d}}}{\mathrm{N}\left(\mathrm{I}_{\mathrm{d}}+\mathrm{I}_{\mathrm{r}} / \mathrm{N}\right)} $$

$$ \mathrm{HO}_{2}^{-}(\%)=\frac{200 \mathrm{I}_{\mathrm{r}}}{\mathrm{N}\left(\mathrm{I}_{\mathrm{d}}+\mathrm{I}_{\mathrm{r}} / \mathrm{N}\right)} $$

Kouteckye-Levich (K-L) Equations (3)-(5):

$$ \frac{1}{\mathrm{~J}}=\frac{1}{\mathrm{~J}_{\mathrm{l}}}+\frac{1}{\mathrm{~J}_{\mathrm{k}}}=\frac{1}{\mathrm{~J}_{\mathrm{k}}}+\frac{1}{\mathrm{~B} \omega^{1 / 2}} $$

$$ \mathrm{~B}=0.62 \mathrm{nFC}_{0} \mathrm{D}_{0}^{2 / 3} \mathrm{v}^{-1 / 6} $$

$$ \mathrm{~J}_{\mathrm{k}}=\frac{\mathrm{J} \times \mathrm{J}_{\mathrm{i}}}{\mathrm{J}_{\mathrm{L}}-\mathrm{J}} $$

Where Ir, Id, J, N, Jk, B, JL, ω, n, F, C0, ν, and D0 represent respectively the ring current, the disk current, the measured current density, the ring collection efficiency, the angular velocity of the electrode (rpm), the reciprocal slope of equation, the transferred electron number, the kinematic viscosity of the electrolyte, the kinetic and diffusion-limiting current density, the Faraday constant, the bulk concentration of O2, (1.93 × 10-5 cm2·s-1), and the O2 diffusion coefficient. The electrochemically active surface area of the sample was calculated using the following Equation (6):

$$ \mathrm{ECSA}=\frac{\mathrm{j} / \nu}{\mathrm{C}_{\mathrm{GC}}} $$

where ν: scan rate, CGC: double layer capacitance, j: measured current density.

Recharge ZAFBs assembly

The test steps of the rechargeable ZAFBs are as follows: (1) The electrolyte is configured. In addition, 6 M KOH and 0.2 M ZnO are dissolved in deionized water; (2) The air cathode is prepared and the sample is configured into a uniform ink and sprayed on carbon paper with a gas diffusion layer, and the sample load is 2mg·cm-2; (3) Anode preparation, polishing of 0.3 mm thick zinc sheet; (4) The above materials are assembled into ZAFBs, and the electrolyte flow rate is set at 5mL·min-1.


As shown in Scheme 1, we utilize BTC and DTZ with high symmetric functionalities as reaction monomers to polycondense into chain hemiporphyrocyanogen COP rich in CoN4 active centers, denoted as COPMCT-Co. Subsequently, the COPMCT-Co-900 catalyst was obtained by pyrolysis of COPMCT-Co precursor at high-temperature conditions of 900 °C.

Macrocycle-based covalent-organic-polymer as efficient oxygen electrocatalysts for zinc-air flow batteries

Scheme 1. Schematic diagram of the synthesis of COPMCT-Co-900 catalyst. COP: Covalent organic polymer.

The structural information of the COPMCT-Co precursor was confirmed by solid-state 13C nuclear magnetic resonance spectroscopy (NMR) [Figure 1A]. The characteristic peak at (a) 160.73 ppm is attributed to the carbon moiety on the five-membered ring connecting the nitrogen atom in the benzpyrole. The characteristic peak at (b) 153.7 ppm is ascribed to the carbon-containing moiety in the DTZ monomer; the characteristic peaks at (c) 140.74, (d) 126.0, (e) 115.47 ppm were attributed to the carbon atom of the benzene ring in the indole ring, respectively. We conducted high-resolution transmission electron microscopy (HRTEM) tests on the catalyst to further determine its composition and structure [Figure 1B and C]. The obvious cobalt nanoparticles were observed in Figure 1B. After further local magnification, we find that obvious lattice fringes with a lattice spacing of 0.20 nm [corresponding to the (111) crystal plane of cobalt nanoparticles] and 0.34 nm [corresponding to the (002) crystal plane of carbon matrix], respectively, indicating that Co-N-C catalyst loaded with Co nanoparticles has been successfully synthesized. Then, the elemental composition and chemical states of the COPMCT-Co-900 were tested by XPS [Figure 1D-F and Supplementary Figure 1]. COPMCT-Co is mainly composed of Co, N, O and C elements, of which C occupies the highest content, reaching 67.85%, followed by N, O and Co elements, accounting for 16.47%, 14.7% and 0.98%, respectively. The high-resolution Co 2p XPS spectra of COPMCT-Co-900 are deconvoluted into Co 2p3/2, Co 2p1/2, and Co-N4 moieties and other satellites. The high-resolution N 1s XPS spectra of COPMCT-Co-900 are deconvoluted into graphitic N (401.2 eV), pyrrolic N (398.8 eV), and Co-Nx (399.8 eV) moieties[47]. The high-resolution C 1s XPS spectra of COPMCT-Co-900 are deconvoluted into C-N, C=C, C=O, and C-O moieties. These results indicate that in the COPMCT-Co-900 catalyst, in addition to cobalt nanoparticles, another part of the Co atom is present in the COPMCT-Co-900 catalyst in the form of an active moiety of CoNx.

Macrocycle-based covalent-organic-polymer as efficient oxygen electrocatalysts for zinc-air flow batteries

Figure 1. (A) NMR spectra of COPMCT-Co; (B and C) HRTEM images of COPMCT-Co-900 at different scales; (D) High-resolution Co 2p XPS for COPMCT-Co-900; (E) High-resolution N 1s XPS for COPMCT-Co-900; (F) High-resolution C 1s XPS for COPMCT-Co-900. NMR: Nuclear magnetic resonance spectroscopy; COP: covalent organic polymer; HRTEM: high-resolution transmission electron microscopy; XPS: X-ray photoelectron spectroscopy.

We performed FT-IR tests on BTC and our as-synthesized COPMCT-Co polymer [Figure 2A]. The monomer BTC exhibits a strong characteristic peak at 2,245 cm-1, corresponding to the stretching vibration peak of -C≡N. However, the COPMCT-Co polymer showed almost no characteristic peak at 2,245 cm-1, indicating that BTC and DTZ monomers have been successfully polymerized. Besides, the COPMCT-Co exhibits the characteristic peaks at 1,646 and 1,536 cm-1, which are attributed to -C=N moiety and conjugate rings, respectively. These results indicate that BTC monomers reacted with DTZ and cobalt atoms to form COPMCT-Co. Further, we analyzed the specific surface area and pore distribution of COPMCT-Co-900 catalyst. The pore distribution shows that the COPMCT-Co-900 catalyst is dominated by mesoporous pores and contains a certain amount of macropores [Figure 2B]. This pore structure facilitates the diffusion of active substances and reduces the resistance to mass transfer. Figure 2C shows that the COPMCT-Co-900 has a large specific surface area (288.5 m2·g-1), facilitating full exposure with the active site. The catalyst was characterized by Raman spectra to investigate the graphitization degree of the COPMCT-Co-900 sample [Figure 2D]. Figure 2D shows that the ID/IG value of COPMCT-Co-900 is 0.92, indicating that the catalyst keeps a higher graphitization degree and conductivity, which is also confirmed by the HRTEM image in Figure 1C. We further performed the XRD spectra to analyze the chemical composition and crystal structure of COPMCT-Co and COPMCT-Co-900 [Figure 2E and F]. We found that this material has only a wide peak, which indicates that COPMCT-Co precursor exists in an amorphous form. Figure 2F shows the XRD pattern after carbonization. It can be seen from the Figure 2F that COPMCT-Co-900 has diffraction peaks at 26.31°, 44.2°, 51.5° and 75.9°. The diffraction peak at 26.31° corresponds to the (002) crystal face of carbon. The diffraction peaks of 44.3°, 53.5° and 76.9° correspond to the (111), (200) and (220) crystal faces of cobalt nanoparticles, respectively, proving the existence of cobalt nanoparticles in COPMCT-Co-900.

Macrocycle-based covalent-organic-polymer as efficient oxygen electrocatalysts for zinc-air flow batteries

Figure 2. (A) FT-IR spectra of BTC and COPMCT-Co; (B) The corresponding pore distribution of COPMCT-Co-900; (C) N2 adsorption and desorption isotherm of COPMCT-Co-900; (D) Raman spectra of COPMCT-Co-900; XRD spectra of (E) COPMCT-Co and (F) COPMCT-Co-900. FT-IR: Fourier transform infrared spectroscopy; BTC: Benzene-1,2,4,5-tetracarbonitrile; COP: covalent organic polymer; XRD: X-ray diffractometer.

We conducted electrochemical tests on COPMCT-Co at different carbonized temperatures to explore the effect of carbonization temperature on catalyst performance. Figure 3A shows the oxygen reduction polarization curves of catalysts carbonized at 800-1,000 °C. The results show that the oxygen reduction performance of the catalyst obtained by carbonization at 900 °C is optimal. The COPMCT-Co-900 has the best electrochemical performance, with an initial potential of 0.97 V, a limited current density of 6.1 mA·cm-2, and a E1/2 of 0.85 V. Figure 3B shows the E1/2 of the catalyst at these five carbonization temperatures. In the range of 800~1000 °C, the E1/2 of the catalyst first increases and then decreases with rising temperature, indicating 900 °C as the optimal carbonization temperature. Having fast oxygen reduction kinetics is very important for the catalyst performance index. As shown in Figure 3C, the oxygen reduction polarization curves of catalysts carbonized at various carbonization temperatures were converted into the Tafel slope. The COPMCT-Co-900 delivers a Tafel slope of 79.5 mV·dec-1. Therefore, COPMCT-Co-900 maintains the best oxygen reduction kinetics, and 900 °C is the optimal temperature for catalyst carbonization. We first performed cyclic voltammetry (CV) tests on the catalyst in an alkaline electrolyte (0.1 M KOH aqueous solution) [Figure 3D]. An obvious oxygen reduction peak for the catalyst appears at ~0.8 V (vs. RHE). By contrast, we tested its CV test in nitrogen-saturated 0.1 mol·L-1 KOH solution, and no peak appears in the range of 0-1.2 V (vs. RHE), indicating the superior ORR catalytic activity of COPMCT-Co-900. In addition, we explored the effects of diverse doping metals on oxygen reduction activity and synthesized COPMCT-Co-900, COPMCT-Ni-900, COPMCT-900, and COPMCT-Fe-900. The results showed that COPMCT-Co-900 maintains the best ORR activity and ORR kinetics compared to COPMCT-900 catalysts with other metal centers [Figure 3E and F].

Macrocycle-based covalent-organic-polymer as efficient oxygen electrocatalysts for zinc-air flow batteries

Figure 3. (A) The LSV curves of COPMCT-Co-X (X represent pyrolysis temperature) catalysts at different temperatures; (B) E1/2 of COPMCT-Co-X catalysts at different temperatures; (C) Tafel plots of COPMCT-Co-X; (D) The CV curves of COPMCT-Co-900; (E) The LSV curves of COPMCT-Co-900, COPMCT-Ni-900, COPMCT- 900 and COPMCT-Fe-900; (F) Tafel plots of COPMCT-Co-900, COPMCT-Ni-900, COPMCT- 900 and COPMCT-Fe-900. LSV: Linear sweep voltammetry; COP: covalent organic polymer; CV: cyclic voltammetry.

In order to prove the universality and versatility of the method, we have successfully prepared two other Co-based chain COP precursors using DPD and TDZ monomers, denoted as COPMCT-Co, COPMCP-Co and COPMCZ-Co [Supplementary Figure 2]. Linear sweep voltammetry (LSV) curve results show that the ORR catalytic performance of COPMCT-Co is the highest compared with other catalysts. Through quantitative calculation analysis of the precursor, we found that the electrostatic potential of COPMCT-Co is between COPMCZ-Co and COPMCZ-Co, and the adsorption strength of oxygen intermediates is moderate, which is conducive to both the oxygen adsorption process and the oxygen desorption process. Therefore, COPMCT-Co shows excellent ORR performance [Supplementary Figure 3]. After high-temperature pyrolysis of 900 °C towards COPMCP-Co and COPMCZ-Co precursors, the COPMCP-Co-900 and COPMCT-Co-900 catalysts were obtained, respectively. We evaluated their catalytic activity through LSV curves [Figure 4A]. All catalysts have demonstrated satisfactory ORR catalytic activity. Among them, COPMCT-Co-900 exhibits the highest catalytic activity, and its E1/2 reaches 847 V; COPMCP-Co-900 maintains the second highest catalytic activity, and its E1/2 reaches 798 V; the E1/2 of COPMCT-Co-900 is approximately 751 V. We continue to test the LSV curves of the samples at varying rotational speeds to further explore the catalytic process of the catalyst. Based on these curves, K-L equations at different voltages were derived. As shown in Figure 4B, the current increases proportionally as the rotation speed increases. Figure 4C exhibits the K-L equation based on the oxygen reduction polarization curve in Figure 4B. The number of electron transfers can be further obtained by fitting the slope of the curve of the K-L equation. According to the data, the electron transfer number of COPMCT-Co-900 is close to 4, indicating that COPMCT-Co-900 maintains a high selectivity for the ORR process. Good cycle stability is the basis of the commercial application of catalysts. We compared the stability of COPMCT-Co-900 with that of commercial Pt/C by a potentiostatic method. As can be seen from Figure 4D, after 50,000 s of chronoamperometry testing, the current density of COPMCT-Co-900 decreased by 3.2%, while the current density loss of Pt/C is more than 21.9%. Further, we evaluated the electron transfer number (Ne) and hydrogen peroxide yield of the catalyst ORR by RRDE [Figures 4E and F]. In the range of 0.2-0.8 V, the hydrogen peroxide yield of COPMCT-Co, COPMCT-Co-900 and the Pt/C reach 67%-93%, 1.4%-4.4% and 1.6%-3.6%, respectively. The low hydrogen peroxide yield proves the high selectivity for four-electron ORR. The Ne of COPMCT-Co is close to 2.5, while that of COPMCT-Co-900 and Pt/C is close to 4, which indicates that COPMCT-Co-900 and Pt/C follow a four-electron reaction.

Macrocycle-based covalent-organic-polymer as efficient oxygen electrocatalysts for zinc-air flow batteries

Figure 4. (A) ORR LSV curves of COPMCT-Co-900, COPMCP-Co-900 and COPMCZ-Co-900; (B) LSV of COPMCT-Co-900 at different rotating rates; (C) The corresponding K-L plots at various potentials, respectively; (D) I-t curves performed of COPMCT-Co-900 and Pt/C; (E) Hydrogen peroxide yield of COPMCT-Co, COPMCT-Co-900 and Pt/C; (F) Electron transfer numbers of COPMCT-Co, COPMCT-Co-900 and Pt/C. ORR: Oxygen reduction reaction; LSV: linear sweep voltammetry; COP: covalent organic polymer.

Furthermore, we evaluated the performance of COPMCT-Co-900 in energy conversion devices ZAFBs, fabricated COPMCT-Co-900 as an air cathode catalyst of ZAFBs, and investigated its performance in ZAFBs [Figure 5A]. When COPMCT-Co-900 was used as the cathode catalyst for ZAFBs, the power density of 155.6 mW·cm-2 was significantly higher than that of the Pt/C catalyst (101.7 mW·cm-2) under the same test conditions [Figure 5B]. This result can be explained by the discharge polarization curves of the two, which show that the voltage of COPMCT-Co-900 drops more slowly as the current density increases. The polarization curve of COPMCT-Co-900 declines more slowly than that of Pt/C, indicating a lower internal resistance. Figure 5C shows that ZAFBs assembled with the COPMCT-Co-900 maintain a high open circuit voltage of around 1.43 V for a long time, which is superior to the Pt/C catalyst, demonstrating a lower impedance in the COPMCT-Co-900-powered ZAFBs. Notably, at current densities of 5, 10, 20, 40, or even 50 mA·cm-2, sustained discharge performance measurements show only slight attenuation for ZAFBs assembled with COPMCT-Co-900 catalyst after successive alternating cycles [Figure 5D]. We then tested the cyclic stability of ZAFBs by setting a constant charge and discharge current. The cycle life of the ZAFBs based on COPMCT-Co-900 exceeds 90 h, demonstrating its excellent charge-discharge performance and great application potential [Figure 5E].

Macrocycle-based covalent-organic-polymer as efficient oxygen electrocatalysts for zinc-air flow batteries

Figure 5. (A) A schematic configuration of the homemade Zn-air battery; (B) Comparison of polarization and power density curves using Pt/C and COPMCT-Co-900 as catalysts; (C) Open circuit plot of the Zn-air battery using COPMCT-Co-900 and Pt/C as catalysts; (D) Cycle discharge curves of COPMCT-Co-900-driven Zn-air batteries at periodically changed current density; (E) Discharge/charge cycling performance of Zn-air batteries with COPMCT-Co-900 as catalyst at a current density of 10 mA·cm-2. COP: Covalent organic polymer.


In conclusion, we demonstrate a rational design method for preparing ORR electrocatalysts with the assistance of a predesigned macrocyclic COP model molecular. Three kinds of metal macrocyclic COP materials with metal-N4 structures were prepared by a simple microwave-assisted method. The non-precious metal N/C electrocatalyst was obtained by carbonizing the metal macrocyclic COP precursor at high temperatures. Thanks to the high electrical conductivity, uniformly dispersed active sites and excellent robustness of the substrate, the synthesized COPMCT-Co-900 has excellent ORR performance, long lifetime and high tolerance to methanol. The E1/2 reaches 0.85 V (vs. RHE) under alkaline test conditions, and the current density of the catalyst only decreased by 3.2% after 50,000 s of chronoamperometry testing. In addition, COPMCT-Co-900 exhibited excellent power density (155.6 mW·cm-2) and exceptional stability (90 h) when used as a cathode catalyst in ZAFBs. This study not only provides a promising non-valuable ORR electrocatalyst to replace Pt but also points out a new direction for developing high-performance oxygen electrodes.


Authors’ contributions

Performed the synthesis, structural characterizations and electrochemical tests: Li X

Made substantial contributions to the conception and design of the study, performed data analysis and interpretation, and wrote the original draft: Leng Y

Supervised and led this project and revised the manuscript repeatedly: Xiang Z, Li X

Performed partial electrochemical tests and ZAFB device assembly: Chen T, Yin Y, Li J

All authors provided critical feedback and helped shape the research and paper. All authors commented on the paper.

Availability of data and materials

Not applicable.

Financial support and sponsorship

This work was supported by the National Key Research and Development Program of China (2022YFB3807500); the NSF of China (22220102003); the Beijing Natural Science Foundation (JL23003); “Double-First-Class” construction projects (XK180301, XK1804-02); China Postdoctoral Science Foundation (2023TQ0020) and Postdoctoral Fellowship Program of CPSF (GZC20230199).

Conflicts of interest

All authors declared that there are no conflicts of interest.

Ethical approval and consent to participate

Not applicable.

Consent for publication

Not applicable.


© The Author(s) 2024.

Supplementary Materials


1. King LA, Hubert MA, Capuano C, et al. A non-precious metal hydrogen catalyst in a commercial polymer electrolyte membrane electrolyser. Nat Nanotechnol 2019;14:1071-4.

2. Varnell JA, Tse EC, Schulz CE, et al. Identification of carbon-encapsulated iron nanoparticles as active species in non-precious metal oxygen reduction catalysts. Nat Commun 2016;7:12582.

3. Malko D, Kucernak A, Lopes T. In situ electrochemical quantification of active sites in Fe-N/C non-precious metal catalysts. Nat Commun 2016;7:13285.

4. Bashyam R, Zelenay P. A class of non-precious metal composite catalysts for fuel cells. Nature 2006;443:63-6.

5. Leng Y, Yang B, Zhao Y, Xiang Z. Fluorinated bimetallic nanoparticles decorated carbon nanofibers as highly active and durable oxygen electrocatalyst for fuel cells. J Energy Chem 2022;73:549-55.

6. Li X, Liu Q, Yang B, Liao Z, Yan W, Xiang Z. An initial covalent organic polymer with closed-F edges directly for proton-exchange-membrane fuel cells. Adv Mater 2022;34:e2204570.

7. Bates JS, Johnson MR, Khamespanah F, Root TW, Stahl SS. Heterogeneous M-N-C catalysts for aerobic oxidation reactions: lessons from oxygen reduction electrocatalysts. Chem Rev 2023;123:6233-56.

8. Liu J, Wan X, Liu S, et al. Hydrogen passivation of M-N-C (M = Fe, Co) catalysts for storage stability and ORR activity improvements. Adv Mater 2021;33:e2103600.

9. Patniboon T, Hansen HA. Acid-stable and active M-N-C catalysts for the oxygen reduction reaction: the role of local structure. ACS Catal 2021;11:13102-18.

10. Shi Q, He Y, Bai X, et al. Methanol tolerance of atomically dispersed single metal site catalysts: mechanistic understanding and high-performance direct methanol fuel cells. Energy Environ Sci 2020;13:3544-55.

11. Sun K, Dong J, Sun H, et al. Co(CN)3 catalysts with well-defined coordination structure for the oxygen reduction reaction. Nat Catal 2023;6:1164-73.

12. Sun Y, Silvioli L, Sahraie NR, et al. Activity-selectivity trends in the electrochemical production of hydrogen peroxide over single-site metal-nitrogen-carbon catalysts. J Am Chem Soc 2019;141:12372-81.

13. Zhao CX, Li BQ, Liu JN, Zhang Q. Intrinsic electrocatalytic activity regulation of M-N-C single-atom catalysts for the oxygen reduction reaction. Angew Chem Int Ed Engl 2021;60:4448-63.

14. Singh SK, Kashyap V, Manna N, et al. Efficient and durable oxygen reduction electrocatalyst based on CoMn alloy oxide nanoparticles supported over N-doped porous graphene. ACS Catal 2017;7:6700-10.

15. Chen Z, Higgins D, Yu A, Zhang L, Zhang J. A review on non-precious metal electrocatalysts for PEM fuel cells. Energy Environ Sci 2011;4:3167.

16. Ratso S, Kruusenberg I, Käärik M, et al. Highly efficient transition metal and nitrogen co-doped carbide-derived carbon electrocatalysts for anion exchange membrane fuel cells. J Power Sources 2018;375:233-43.

17. Yu Y, Zhou J, Sun Z. Novel 2D transition‐metal carbides: ultrahigh performance electrocatalysts for overall water splitting and oxygen reduction. Adv Funct Mater 2020;30:2000570.

18. Das TK, Jesionek M, Çelik Y, Poater A. Catalytic polymer nanocomposites for environmental remediation of wastewater. Sci Total Environ 2023;901:165772.

19. Feng X, Bai Y, Liu M, et al. Untangling the respective effects of heteroatom-doped carbon materials in batteries, supercapacitors and the ORR to design high performance materials. Energy Environ Sci 2021;14:2036-89.

20. Cheon JY, Kim JH, Kim JH, Goddeti KC, Park JY, Joo SH. Intrinsic relationship between enhanced oxygen reduction reaction activity and nanoscale work function of doped carbons. J Am Chem Soc 2014;136:8875-8.

21. Liu M, Sun T, Peng T, et al. Fe-NC single-atom catalyst with hierarchical porous structure and P−O bond coordination for oxygen reduction. ACS Energy Lett 2023;8:4531-9.

22. Wang Y, Li K, Cheng R, et al. Enhanced electronic interaction between iron phthalocyanine and cobalt single atoms promoting oxygen reduction in alkaline and neutral aluminum-air batteries. Chem Eng J 2022;450:138213.

23. Madhavachary R, Abdelraheem EMM, Rossetti A, et al. Two-step synthesis of complex artificial macrocyclic compounds. Angew Chem Int Ed Engl 2017;56:10725-9.

24. Luo Y, Chen Y, Xue Y, et al. Electronic structure regulation of iron phthalocyanine induced by anchoring on heteroatom-doping carbon sphere for efficient oxygen reduction reaction and Al-Air battery. Small 2022;18:e2105594.

25. Jasinski R. A new fuel cell cathode catalyst. Nature 1964;201:1212-3.

26. Shao M, Chang Q, Dodelet JP, Chenitz R. Recent advances in electrocatalysts for oxygen reduction reaction. Chem Rev 2016;116:3594-657.

27. Luo M, Zhao Z, Zhang Y, et al. PdMo bimetallene for oxygen reduction catalysis. Nature 2019;574:81-5.

28. Wang HF, Chen L, Pang H, Kaskel S, Xu Q. MOF-derived electrocatalysts for oxygen reduction, oxygen evolution and hydrogen evolution reactions. Chem Soc Rev 2020;49:1414-48.

29. Li X, Xiang Z. Identifying the impact of the covalent-bonded carbon matrix to FeN4 sites for acidic oxygen reduction. Nat Commun 2022;13:57.

30. Li X, Liu D, Liu Q, Xiang Z. A pyrolysis-free method toward large-scale synthesis of ultra-highly efficient bifunctional oxygen electrocatalyst for zinc-air flow batteries. Small 2022;18:e2201197.

31. Li X, Liu Y, Xiang Z. Dithiine bridged phthalocyanine-based covalent organic frameworks for highly efficient oxygen reduction reaction. J Phys Chem C 2022;126:4008-14.

32. Yang S, Yu Y, Gao X, Zhang Z, Wang F. Recent advances in electrocatalysis with phthalocyanines. Chem Soc Rev 2021;50:12985-3011.

33. Li X, Chen T, Yang B, Xiang Z. Fundamental understanding of electronic structure in FeN4 site on electrocatalytic activity via dz2-orbital-driven charge tuning for acidic oxygen reduction. Angew Chem Int Ed Engl 2023;62:e202215441.

34. Liang Z, Guo H, Zhou G, et al. Metal-organic-framework-supported molecular electrocatalysis for the oxygen reduction reaction. Angew Chem Int Ed Engl 2021;60:8472-6.

35. Wang X, Wang B, Zhong J, et al. Iron polyphthalocyanine sheathed multiwalled carbon nanotubes: a high-performance electrocatalyst for oxygen reduction reaction. Nano Res 2016;9:1497-506.

36. Choi J, Kim J, Wagner P, et al. Highly ordered mesoporous carbon/iron porphyrin nanoreactor for the electrochemical reduction of CO2. J Mater Chem A 2020;8:14966-74.

37. Côté AP, Benin AI, Ockwig NW, O’Keeffe M, Matzger AJ, Yaghi OM. Porous, crystalline, covalent organic frameworks. Science 2005;310:1166-70.

38. Liu Y, Diercks CS, Ma Y, et al. 3D covalent organic frameworks of interlocking 1D square ribbons. J Am Chem Soc 2019;141:677-83.

39. Lyle SJ, Osborn Popp TM, Waller PJ, Pei X, Reimer JA, Yaghi OM. Multistep solid-state organic synthesis of carbamate-linked covalent organic frameworks. J Am Chem Soc 2019;141:11253-8.

40. Lyu H, Diercks CS, Zhu C, Yaghi OM. Porous crystalline olefin-linked covalent organic frameworks. J Am Chem Soc 2019;141:6848-52.

41. Diercks CS, Lin S, Kornienko N, et al. Reticular electronic tuning of porphyrin active sites in covalent organic frameworks for electrocatalytic carbon dioxide reduction. J Am Chem Soc 2018;140:1116-22.

42. Nguyen HL, Hanikel N, Lyle SJ, Zhu C, Proserpio DM, Yaghi OM. A porous covalent organic framework with voided square grid topology for atmospheric water harvesting. J Am Chem Soc 2020;142:2218-21.

43. Zhang B, Wei M, Mao H, et al. Crystalline dioxin-linked covalent organic frameworks from irreversible reactions. J Am Chem Soc 2018;140:12715-9.

44. Gropp C, Ma T, Hanikel N, Yaghi OM. Design of higher valency in covalent organic frameworks. Science 2020;370:eabd6406.

45. Yamamoto T, Hayashi Y, Yamamoto A. A novel type of polycondensation utilizing transition metal-catalyzed C-C coupling. I. preparation of thermostable polyphenylene type polymers. Bull Chem Soc Jpn 1978;51:2091-7.

46. Zhou Z, Yamamoto T. Research on carbon - carbon coupling reactions of haloaromatic compounds mediated by zerovalent nickel complexes. Preparation of cyclic oligomers of thiophene and benzene and stable anthrylnickel(II) complexes. J Organomet Chem 1991;414:119-27.

47. He Y, Guo H, Hwang S, et al. Single cobalt sites dispersed in hierarchically porous nanofiber networks for durable and high-power PGM-free cathodes in fuel cells. Adv Mater 2020;32:e2003577.

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Leng Y, Chen T, Yin Y, Li J, Li X, Xiang Z. Macrocycle-based covalent-organic-polymer as efficient oxygen electrocatalysts for zinc-air flow batteries. Chem Synth 2024;4:20.

AMA Style

Leng Y, Chen T, Yin Y, Li J, Li X, Xiang Z. Macrocycle-based covalent-organic-polymer as efficient oxygen electrocatalysts for zinc-air flow batteries. Chemical Synthesis. 2024; 4(2): 20.

Chicago/Turabian Style

Leng, Yiming, Tengge Chen, Yuanyuan Yin, Jizhen Li, Xueli Li, Zhonghua Xiang. 2024. "Macrocycle-based covalent-organic-polymer as efficient oxygen electrocatalysts for zinc-air flow batteries" Chemical Synthesis. 4, no.2: 20.

ACS Style

Leng, Y.; Chen T.; Yin Y.; Li J.; Li X.; Xiang Z. Macrocycle-based covalent-organic-polymer as efficient oxygen electrocatalysts for zinc-air flow batteries. Chem. Synth. 2024, 4, 20.

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