Electrocatalysts with bifunctional covalent organic frameworks with tunable p-band centers for rechargeable Zn-air batteries

Fine control of the physicochemical structure of carbon electrocatalysts is crucial for improving the slow oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in rechargeable zinc-air batteries. Covalent organic frameworks (COFs) are considered good candidates for carbon materials because their structures can be precisely controlled. However, conferring bifunctional electrocatalytic activity on COFs to ORFs and OERs remains a challenge. Herein, pyridine-linked triazine covalent organic frameworks (PTCOFs) with well-defined active sites and pores were easily prepared under mild conditions, and their electronic structures were tuned by incorporating Co nanoparticles (CoNP-PTCOFs), resulting in Induction of bifunctional electrocatalytically active ORR and OER. CoNP-PTCOF showed low overpotential for both ORR and OER with excellent stability. Computational simulations found that the charge transfer shifts the p-band center of CoNP-PTCOF down compared to pristine PTCOF, which facilitates the adsorption and desorption of oxygen intermediates on the pyridine carbon active sites during the reaction. Compared with batteries containing commercially available Pt/C and RuO2, the Zn-air battery assembled with bifunctional CoNP-PTCOF has a small voltage gap of 0. V and excellent durability of 720 cycles. This strategy of tuning COF electrocatalytic activity can be extended to design various carbon electrocatalysts.

1 Introduction

Water-based batteries are expected to meet the growing demand for safe and non-explosive energy storage devices in wearable electronics, mobile displays, medical devices, and electric vehicles. In particular, rechargeable Zn-air batteries have received increasing attention as aqueous energy storage systems due to their high energy density, superior durability, excellent safety, low cost, and environmentally benign properties. Zn-air batteries are driven by the reversible oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the cathode under an aqueous alkaline electrolyte during discharge and charge. However, the ORR and OER of Zn-air batteries show very slow reaction kinetics with high overpotentials, thus requiring efficient electrocatalysts to facilitate these reactions. Noble metal electrocatalysts such as Pt/C, IrO2 and RuO2 are widely used to promote ORR and OER due to their excellent catalytic activities. However, noble metal electrocatalysts exhibit several inherent limitations, including monofunctional activity for ORR or OER, poor durability, high cost, and low abundance.

To overcome the limitations of current noble metal electrocatalysts, heteroatom-doped or non-noble metal-bound carbon electrocatalysts have been extensively studied as bifunctional electrocatalysts to efficiently accelerate ORR and OER on air cathodes for Zn-air batteries. Many previous results have shown that the bifunctional electrocatalytic activity of carbon nanomaterials for ORR and OER strongly depends on their chemical and physical structures, including metal-N-C active sites, N doping, pore size, and surface area. Therefore, it is very important to finely construct well-defined carbon electrocatalyst structures and appropriately tune their electronic structures in a controllable manner to achieve excellent activities. However, to date, many developed carbon electrocatalysts have been produced by a trial-and-error approach, as the precise control of their active sites and electronic structures remains a great challenge. Therefore, for rechargeable Zn-air batteries, a simple and controllable strategy is required to rationally design bifunctional carbon electrocatalysts active for both ORR and OER.

Covalent organic frameworks (COFs) are an emerging class of crystalline carbon materials, which are generated by covalent bonding of many organic building blocks. The chemical and physical structures of COFs can be precisely tuned by changing the types of organic building blocks, bonds, and geometric structures during synthesis reactions. Furthermore, the heteroatoms (B, N, O, or S) are uniformly distributed and have well-defined pores throughout the COF’s framework, making it effective for other catalytic applications. Considering their outstanding features in precisely controlled structures, COFs would be excellent candidates for the rational design of bifunctional carbon electrocatalysts to promote ORR and OER in zinc-air batteries. Some COF-based electrocatalysts have been reported. However, they only promote ORR or only OER and do not show bifunctional electrocatalytic activity. To date, there are very few bifunctional COF electrocatalysts that can accelerate ORR and OER in zinc-air batteries. Therefore, designing bifunctional COF electrocatalysts by tailoring their electronic structures is crucial for promoting reversible oxygen redox reactions at the liquid-solid interface of Zn-air batteries.

In this study, pyridine-linked triazine covalent organic frameworks (PTCOFs) with well-defined active sites and pores can be easily synthesized by the nucleophilic substitution reaction of cyanuric chloride with 2-diaminopyridine under mild conditions. . Then, the electronic structure of PTCOF was tuned by incorporating Co nanoparticles (denoted as CoNP-PTCOF) to induce bifunctional electrocatalytic activity of ORR and OER, since cobalt is a widely used transition metal for oxygen electrocatalysis, and Incorporating it into the carbon structure can enhance the electrocatalytic activity of the pyridine-rich CoNP-PTCOF for oxygen redox reactions in a comprehensive manner [5, 6, 11]. Furthermore, computer simulations were performed to gain insight into the electronic structures of PTCOF and CoNP-PTCOF and the catalytically active centers of pyridine carbon for ORR and OER under alkaline conditions. Finally, a Zn-air battery was fabricated using bifunctional CoNP-PTCOF. The cell exhibits superior performance compared to cells with monofunctional Pt/C and RuO2 mixtures.

2 Results and Discussion

2. Synthesis of PTCOF with good structure and pores

Pyridine and its derivatives have polar and conjugated bonds, have coordination ability, and can form complexes with various metal ions, resulting in excellent electrocatalytic activity. Pyridine-rich PTCOFs with micropores and mesopores can be easily prepared by the nucleophilic substitution reaction of cyanuric chloride with 2,-diaminopyridine in acetonitrile (Scheme 1). The filtration-based PTCOF post-processing procedure is very simple and its yield is high (92%).

plan 1

Schematic illustration of the synthesis of pyridine-linked triazine covalent organic framework (PTCOF) and cobalt nanoparticle-embedded PTCOF oxygen electrocatalyst (CoNP-PTCOF).

The structure of the as-prepared PTCOF was characterized by various spectroscopic methods. As shown in transmission electron microscopy (TEM) images (Fig. 1a,b), the PTCOFs are triangular with layered stacking. Compared to imine-based COFs, PTCOFs exhibit relatively low crystallinity. Amorphous or low-crystalline carbon structures have been reported to be beneficial for facile ion migration and mass transport at solid-liquid interfaces. As shown in the energy dispersive X-ray spectroscopy (EDX) image (Fig. 1c), nitrogen atoms are uniformly dispersed on the carbon backbone of PTCOF. In addition, chlorine atoms are found in the entire region of PTCOF because the pyridyl group is protonated to form pyridinium chloride during the synthesis reaction.

Structural analysis of the ionic PTCOF. a) TEM, b) HR-TEM (inset: SAED mode) and c) elemental mapping images of PTCOF. d) Solid-state 13C-NMR and e) FT-IR spectra of PTCOF. f) Distribution map of pore size distribution of PTCOF using the NL-DFT model. g) Powder XRD pattern of PTCOF (inset: magnified pattern from 15° to 37°). h) Top and side views of the AA-stacked PTCOF.

The 13C-NMR spectrum of PTCOF clearly shows that the characteristic peaks of C atoms of pyridinium and triazine are at 110., 144., 162, respectively. and 170.ppm (Fig. 1d), while cyanuric chloride at 172 ppm in the peak spectrum of the CCl bond disappeared. In addition, characteristic vibrational modes of pyridinium and triazine groups, such as CN (1537 cm-1), CC (1437 cm-1), CN (1064 cm-1), NH stretching (3066) cm-1), NH bending (1628 cm-1) and N+H stretching (2782 cm-1) appear in the Fourier transform infrared (FT-IR) spectrum of PTCOF, while the same as CH The Cl bond of the cyanuric chloride at 847 cm-1 of the associated peak clearly disappeared (Fig. 1e; Fig. S1, Supporting Information).

The chemical structure of PTCOF was further confirmed by X-ray photoelectron spectroscopy (XPS). In the C1s XPS spectrum of PTCOF, the peaks for the CN bond of pyridinium and the NCN bond of triazine appear at 286. eV and 287. eV, respectively (Fig. S2, Supporting Information).

In addition, pyridinium N (400. eV) and NCN (398. eV) as well as chlorine (202., 200., 198. and 197. eV) were clearly observed in the N1s and Cl2p spectra of PTCOF. The content of chlorine atoms in PTCOF was measured by ion chromatography to be 12. wt%, almost the same as the theoretical amount (14. wt%). These results demonstrate that pyridine-rich PTCOF was successfully synthesized in a simple manner.

The pore size of the PTCOF was then measured by Brunauer–Emmett–Teller (BET) analysis. The PTCOF shows a type IV isotherm with a mixed H1-H3 hysteresis loop (Fig. S3, Supporting Information), corresponding to the layered stacking shown in the TEM image.

As shown in Figure 1f. PTCOF has mesopores of 3., 5 and 9. nm with a surface area of ​​55. m2 g-1. Micropores smaller than 2 nm were not observed in the pore distribution of PTCOF because the large amount of counter anions (Cl-) in protonated PTCOF (pyridine chloride) might block the micropores. As expected, micropores clearly appeared in the BET pore distribution after removal of HCl from protonated PTCOF by annealing (Figs S4 and S5, Supporting Information). Note that mesopores can facilitate mass transport at the liquid-solid interface, while micropores facilitate easy electron transfer during ORR and OER.

The crystal structure of PTCOF was further confirmed by X-ray diffraction (XRD) as well as Pawley refinement (Fig. 1g). In the XRD pattern of PTCOF, a broad diffraction peak was observed at ≈5°, corresponding to the (100), (200) and (400) planes. Furthermore, the (880) and (001) facets clearly appear at 19.° and 26.°, indicating that the interlayer spacing in the layered stack structure of PTCOF is 3.Å. The XRD pattern is in good agreement with the pattern obtained by simulation (black line). Based on the XRD pattern, the PTCOF has a triangular periodic cell with a dim AA stack (Fig. 1h; Fig. S6, Supporting Information). The corresponding micropores and mesopores were clearly observed in the BET analysis.

2. Modulation of electronic structure on PTCOF

The electronic structure of the PTCOF is tuned by incorporating Co nanoparticles (NPs) within the framework, which endows the ORR and OER bifunctional electrocatalytic activity. After formulating Co(NO3)2 into PTCOF, Co atoms are uniformly dispersed throughout the framework (Fig. S7, Supporting Information).

The co-formulated PTCOFs were then annealed at 800 °C for 2 h under Ar atmosphere to form CoNP-PTCOFs. As shown in Fig. 2a,b, Co NPs with an average size of 12 nm were embedded in the framework without aggregation. Selected area electron diffraction (SAED) patterns of CoNP-PTCOFs reveal the formation of crystalline Co NPs. From its dark-field images and EDX mapping data, Co NPs of CoNP-PTCOF with N and C atoms can be clearly observed.

Structural analysis of CoNP-PTCOF electrocatalysts. a) TEM, b) HR-TEM (inset: SAED mode), and c) elemental mapping images of CoNP-PTCOF. d) Powder XRD pattern of CoNP-PTCOF. e) N adsorption-desorption isotherms, f) CoNP-PTCOF pore size distribution calculated based on the NL-DFT method.

X-ray diffraction further characterized the CoNP-PTCOF. The peaks of the (111) and (200) facets of Co NPs appear at 44.° and 51.° (Fig. 2d), which are in good agreement with the SAED pattern. Note that the XRD pattern of CoNP-PTCOF is almost identical to that of pristine PTCOF, except for the new peaks of Co NPs and the peak shift of interlayer spacing is small (Fig. S8, Supporting Information), indicating that upon incorporation of Co NPs, the framework changes from 0. increased to 0. nm (22.°).

The surface area and pore size of CoNP-PTCOF were then measured by BET analysis. As shown in Fig. 2e, CoNP-PTCOF exhibited type IV isotherms with a distinct H3-type hysteresis loop and increased surface area (309. m2 g-1) compared to pristine PTCOF (55. m2 g-1) ). This increased surface area can be attributed to the removal of HCl from the pyridinium chloride moiety of the PTCOF during annealing, leading to the appearance of inherent micropores as well as the formation of other pores. Micropores (0.1 and 1.0 nm) of the framework were observed in CoNP-PTCOF (Fig. 2f), almost identical to those of annealed PTCOF (Fig. S5, Supporting Information). These results show that the framework structure is well preserved in CoNP-PTCOF.

The chemical structure of CoNP-PTCOF was identified by FT-IR spectroscopy and XPS. After cobalt nitrate is coordinated in PTCOF, the vibrational mode of nitrate anion appears at 1322 cm-1 in the FT-IR spectrum, while the pull of the N+H bond in the pyridinium moiety of PTCOF at 2782 cm-1 The extension mode disappeared (Fig. S9, Supporting Information).

Furthermore, the peak of the CN vibrational mode shifts from 1537 cm-1 to a lower frequency (1485 cm-1) in the spectrum, indicating that Co ions are bonded to the pyridine and triazine moieties of PTCOF. Coupling of Co NPs with the pyridine and triazine groups of PTCOF was also observed in the XPS spectra of CoNP-PTCOF. In the C1s XPS spectrum of CoNP-PTCOF, CN (from 286. to 285. eV) and NCN