Quasi-Three-Dimensional Cyclotriphosphazene-Based Covalent Organic Framework Nanosheet for Efficient Oxygen Reduction

Highlights JUC-610-nanosheet exhibits highly efficient oxygen reduction reaction (ORR) catalytic activity in alkaline electrolyte with half-wave potential of 0.72 V versus reversible hydrogen electrode, which is one of the best covalent organic frameworks (COF)-based ORR electrocatalysts reported so far. It has been confirmed by experiments and density functional theory calculations that the abundant electrophilic structure in Q3CTP-COFs induces a highly density of carbon active sites, and the unique bilayer stacking facilitates the exposure of active carbon sites and accelerates the mass diffusion during ORR. JUC-610-nanosheet can also serve as a promising cathode for Zn-air batteries (power density of 156 mW cm–2 at 300 mA cm–2), which promotes the development of metal-free carbon-based electrocatalysts. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-023-01111-8.


HIGHLIGHTS
• JUC-610-nanosheet exhibits highly efficient oxygen reduction reaction (ORR) catalytic activity in alkaline electrolyte with half-wave potential of 0.72 V versus reversible hydrogen electrode, which is one of the best covalent organic frameworks (COF)-based ORR electrocatalysts reported so far.
• It has been confirmed by experiments and density functional theory calculations that the abundant electrophilic structure in Q3CTP- COFs induces a highly density of carbon active sites, and the unique bilayer stacking facilitates the exposure of active carbon sites and accelerates the mass diffusion during ORR.
• JUC-610-nanosheet can also serve as a promising cathode for Zn-air batteries (power density of 156 mW cm -2 at 300 mA cm -2 ), which promotes the development of metal-free carbon-based electrocatalysts.

Introduction
Oxygen reduction reaction (ORR) plays a significant role in clean and sustainable energy conversion, such as metal-air batteries and fuel cells [1][2][3][4].Pt-based materials are currently considered as most efficient electrocatalysts for ORR.However, their high-cost, scarcity, and instability in operation conditions restrict the future large-scale applications of these energy conversion devices [5][6][7].In the past decade, metal-free materials (MFMs), such as porous carbon and porous organic polymers, have exerted a great effect on the design of low-cost and efficient electrocatalysts for ORR [8][9][10][11][12][13][14][15].Notably, covalent organic frameworks (COFs) are an emerging class of ORR electrocatalysts due to their high surface areas, tunable porous structures, structural tunability, and well-defined building units [16][17][18][19][20]. Reasonably designing and synthesizing COF materials with chemically adjustable specific blocks can facilitate the development of MFM-based ORR electrocatalysts [21][22][23].During ORR electrocatalysis process, efficient active sites and fast kinetic mass diffusion of electrocatalysts are indispensable ingredients.Expectantly, it has been confirmed through both experiment and theory that the electronic redistribution of carbon (C) atoms in MFMs can optimize the adsorption and dissociation behaviors for reactant (O 2 ) and intermediates (OOH*, O*, and OH*) [24][25][26], inducing ORR catalytic activity.For example, Dai et al. [27] and Nakamura et al. [28] demonstrated that the C atoms with Lewis basicity (positive charge density) were the efficient ORR active sites.Thus, constructing electrophilic structures in COFs can induce positively charged carbon active sites.Furthermore, optimal pore structures to expose more active sites and excellent conductivity of electrocatalysts can accelerate mass diffusion and electron transport to facilitate the ORR catalytic process [29].Hence, controllably and precisely synthesizing COFs with highly dense and exposed carbon active sites from the perspective of customizing structures is of great potential to develop highly active metal-free ORR electrocatalysts [30].
Among COF materials, three-dimensional (3D) COFs exhibit hierarchical pore structures to expose numerous open sites [31] profitably, but the structural instability and poor conductivity limit their practical applications as electrocatalysis.Two-dimensional (2D) COFs with excellent interlayer regulation have significant advantages in stable structures and electron transport properties [32,33] but are restricted by mass diffusion due to their 2D stacking [34,35].In view of this situation, it may be possible to construct efficient 2D COF electrocatalysts with quasi-3D (Q-3D) structures and channel features through clever structural design.However, the emergence of structural units centered on the Q-3D 1 3 cyclotriphosphazene structure makes our hypothesis plausible.The distinctive structure of Q-3D COF makes its 2D planar structure regularly distort, thus creating a new vertical multi-channel to enhance the pore permeability and the mass diffusion [36].Moreover, the weak interlayered π-π interactions of Q-3D COFs can be easily exfoliated into nanosheets (NSs) [37].Due to large number of active sites, faster ion diffusion, and high conductivity of thin layers, COF NSs was expected to further improve the performance of electrocatalysts [38][39][40][41].Therefore, the active carbon sites and mass diffusion requirements of ORR electrocatalysts initiate us to judiciously design and accurately synthesize the novel Q-3D COF NSs with highly exposed carbon active sites for efficient ORR.
The FT-IR spectra of COFs showed that the C=N bonds still existed after treatment under 6 M KOH for 24 h (Figs.S17-S19).Interestingly, we elaborately selected CTP-6-CHO with the structure of six cross-side arms outside the central plane as the node module of COFs.The CTP-6-CHO has a unique stereoscopic structure in which the O-P-O plane (β plane) is perpendicular to the aromatic ring plane of N 3 P 3 (α plane, see Experimental Procedures) [37,49].In addition, the TEM images with elemental mappings verified that C, N, O, and P atoms were homogeneously distributed in Q3CTP-COFs (Fig. 2d-f).

Experiment and Characterization of JUC-610-CON
In addition, given consisting of unique rhombus pores, large interlayer spacings, and the uniquely flexible structure of CTP-6-CHO units lead to the weakened interlayer conjugation and possess the larger c distance (Table S1) of JUC-610, and the bulk JUC-610 can be easily exfoliated into ultrathin NSs (denoted as JUC-610-CON) in large quantities (see Supporting Information) only 3 h by using sonication method.The ultrathin nature of JUC-610-CON was uncovered by TEM and AFM technologies.Tyndall effect was observed when a green laser went through the solution of JUC-610-CON (Fig. S20), confirming its colloidal nature.The atomic force microscopy (AFM) image revealed that the thickness of the obtained JUC-610-CON was ~ 4 nm, corresponding to ~ 8 layers and a flake size of ~ 800 nm (Fig. 2a-c and Table S7).Meanwhile, JUC-610-CON exhibited thin nanosheets by TEM and SEM (Figs. 2d and S21).To determine the crystal structure of JUC-610-CON, XRD and nitrogen adsorption-desorption analyses were performed.The results indicate the crystallinity and porosity of JUC-610-CON are identical to that of the bulk JUC-610, but the intensities of the first XRD peak (100) and N 2 adsorption isotherm are decreased (Figs.S22-S23).

ORR Catalytic Activity
To survey the ORR catalytic activity of prepared COFs and their nanosheets, the rotating disk electrode (RDE) experiments were performed in O 2 -saturated 0.1 M KOH electrolyte.In addition, JUC-612 was also synthesized by condensing 1,3,5-tri(1,3-Diformyl biphenyl)benzene (TBPB-6-CHO) and TAPT as a control sample for ORR performance (Figs.S24-S31).According to the linear sweep voltammetry (LSV) curves, JUC-610-CON and Q3CTP-COFs exhibited higher ORR performance than that of JUC-612, revealing that the existence of the Q-3D structures and abundant heteroatoms significantly induces positively charged carbon active sites and improves the ORR active sites (Figs.3a, S32-S36, and Scheme 2).The ORR catalytic activity of JUC-610 was superior to that of JUC-611, which indicates that N atoms in TAPT blocks also play an important role in improving the ORR performance.In addition, the half-wave potential of JUC-610-CON and the bulk JUC-610 were 0.72 and 0.69 V versus RHE, respectively (Fig. S37), suggesting that ultrathin nanosheet structure exposes more active sites and facilitates mass diffusion during ORR process, which can be comparable to previously reported COF materials and most metal-based ORR electrocatalysts (Fig. 3f and Tables S8-S9).The Tafel slope of JUC-610-CON (61.95 mV dec −1 ) is lower than those of JUC-610 (65.8 mV dec −1 ), JUC-611 (70.9 mV dec −1 ) and JUC-612 (79.7 mV dec −1 ), indicating the superior ORR kinetics of JUC-610-CON (Fig. 3b).The electrochemically active surface areas (ECSA) of prepared samples were conducted by electrochemical double-layer capacitance (C dl ) (Figs. S38-S41).The C dl of JUC-610-CON (19.2 mF cm −2 ) is larger than those of JUC-610 (17.8 mF cm −2 ), JUC-611 (7.6 mF cm −2 ), and JUC-612 (7.6 mF cm −2 , Fig. 3c).To further explore the intrinsic activity of the prepared COFs, the turnover frequency (TOF) was carried out at 0.7 V versus RHE, indicating that C atoms in adjacent N-O-P atoms are active sites, and the highly dense carbon active sites accelerate mass diffusion during ORR process.As shown in Fig. 3d, the TOF value of JUC-610-CON is 0.0035 s −1 , which has higher active site utilization efficiency than Q3CTP-COFs and JUC-612.The mass activity .These results thus reveal that the JUC-610-CON exhibits the most efficient ORR catalytic performance due to ultrathin nanosheet structure and the highly dense carbon active sites.Moreover, all the electron transfer numbers (n) of JUC-610-CON, JUC-610, JUC-611, and JUC-612 derived from Koutecky-Levich (K-L) plots at 0.2 V versus RHE (Fig. 3e) were closed to 4 (3.82,3.43, 3.42, and 3.67 respectively).Subsequently, a ZAB was assembled using the JUC-610-CON as the air cathode due to its excellent ORR activity in alkaline solutions.The current density of JUC-610-CON-based ZAB was about 87.2 mA cm −2 at 1.0 V, and the maximum peak power density at 0.60 V was about 0.15 W cm −2 (Fig. S42).In addition, the JUC-610-CONbased ZAB also exhibited potential plateaus of 1.16, 0.95, 0.76, and 0.64 V at discharge current densities of 25, 100, 200, and 300 mA cm −2 , respectively (Fig. 3g).Such ZAB could operate for 200 h with negligible voltage loss by regular replacement of electrolyte (6 M KOH) and zinc plate (Fig. 3h).Two JUC-610-CON-based ZABs in series were able to light a 2 V rated "COF" LED (Fig. 3h inset), which indicates this material is very promising as electrode material in metal-air batteries.

DFT Calculations
To reveal the location of carbon active sites for ORR in our samples, the DFT calculations were performed (Fig. 4).All calculations were carried out with the Gaussian 09 package and Vienna Ab-initio Simulation Package (VASP).The natural population analysis (NPA) was performed on the theoretical level of B3LYP/6-311G (d, p) using the NBO program.The electrostatic potential was considered in Gaussian 09 to describe the charge distribution of two chemical systems [50,51].The average electrostatic potential of O atoms in CTP-6-CHO and N atoms in TAPT were smaller than those of their adjacent C atoms, indicating the relative electrophilicity of the N and O atoms.The NPA charge results illustrated that both C atoms adjacent to the electrophilic O atom in CTP-6-CHO and the electrophilic N atom in TAPT showed positive charge, which are 0.31 and 0.47, respectively (Fig. 4a, b).Moreover, for these active carbon sites in CTP-6-CHO and TAPT, the 3D charge densities of three

Conclusions
In summary, we have precisely and controllably synthesized the unique Q3CTP-COFs and their nanosheets using CTP-based blocks, which can act as efficient ORR electrocatalysts for Zn-air batteries.It was confirmed that the electrophilic structures in such Q3CTP-COFs induced abundant positively charged carbon ORR active sites to facilitate O 2 adsorption and reduction, which has been supported by DFT calculations.The unique bilayer stacking structures of Q3CTP-COFs promote the exposure of active carbon sites to accelerate ORR process and the mass (ions, O 2 and intermediate) diffusion efficiency during ORR.Furthermore, Q3CTP-COFs could be easily exploited into nanosheets, which improves their ORR catalytic activity (half-wave potential of 0.72 V vs. RHE in alkaline electrolyte) and can be applied for promising cathodes for Zn-air batteries (delivered power density of 156 mW cm -2 at 300 mA cm -2 ).Thus, this work provides a new way to fabricate metal-free ORR electrocatalysts with atomically definite carbon active sites and promotes their extensive application in clean energy conversion devices.

ABSTRACT
Metal-free carbon-based materials are considered as promising oxygen reduction reaction (ORR) electrocatalysts for clean energy conversion, and their highly dense and exposed carbon active sites are crucial for efficient ORR.In this work, two unique quasi-three-dimensional cyclotriphosphazene-based covalent organic frameworks (Q3CTP-COFs) and their nanosheets were successfully synthesized and applied as ORR electrocatalysts.The abundant electrophilic structure in Q3CTP-COFs induces a high density of carbon active sites, and the unique bilayer stacking of [6 + 3] imine-linked backbone facilitates the Jianhong Chang and Cuiyan Li contributed equally to this work.

Fig. 1 a
Fig. 1 a, b Powder XRD patterns of JUC-610 and JUC-611.c, d N 2 adsorption-desorption isotherms at 77 K and pore-size distribution (inset) based on the NLDFT model to the adsorption data for JUC-610 and JUC-611.e, f FT-IR spectra of JUC-610 and JUC-611

Fig. 2 a
Fig. 2 a AFM image of JUC-610-CON, and b the corresponding height curves for the selective areas in a. c Lateral size distribution histogram of JUC-610-CON, and Gaussian fit curve is colored in red.d TEM image of JUC-610-CON.e, f TEM and the related elemental mapping images of carbon, nitrogen, oxygen, and phosphorus for JUC-610 and JUC-611

Fig. 4 a
Fig. 4 a, b NPA charges and the corresponding surface electrostatic potential maps of CTP-6-CHO and TAPT calculated by DFT.c, d Side view of the 3D charge densities for the three ORR reaction steps of C site for CTP-6-CHO and TAPT.Gray, purple, red, blue and light pink balls represent C, P, O, N, and H atoms, respectively