Stable hexaazatrinaphthylene-based covalent organic framework as high-capacity electrodes for aqueous hybrid supercapacitors

INTRODUCTION

Aqueous hybrid supercapacitors (AHCs) consist of a battery-like electrode, a capacitive electrode, and an aqueous electrolyte. They are competitive electrochemical energy storage devices for large-scale applications due to their merits of the potential coexistence for delivering high energy density, exceptional power density, excellent cyclability and good safety^[1-5]. Various inorganic battery-type electrode materials^[6], in particular, transition metal oxides^[7], transition metal sulfides^[8] and transition metal hydroxides^[9], have been extensively employed as high-capacity cathodes in AHCs^[10,11]. For example, Zn_0.5Cu_0.5Co₂O₄ with oxygen vacancies^[12] exhibits a specific capacity of 271 mA h g^-1 at 1 A g^-1. NiCo₂S₄ decorated in mesoporous carbon hydrogel^[13] shows a high specific capacity of 317.9 mAh g^-1 at 1 A g^-1. Mesoporous Ni(OH)₂ cubic nanocages intercalated with VO₄^3-[14] obtain a specific capacity of 280 mAh g^-1 (approaching the theoretical value) at 1 A g^-1. However, these traditional metal-based (Ni, Co, and Cu) cathode materials face limited transition metal element resources and inferior cycle performance due to their large structural changes upon repeated charge-discharge processes. By contrast, organic compounds linked by covalent bonds primarily comprise abundant elements such as C, H, O, N, and S. These compounds exhibit characteristics of resource sustainability, adjustable molecular constitution and flexible structure, making them suitable for sustainable application of AHCs^[15].

As a typical class of crystalline porous organic compounds, covalent organic frameworks (COFs) have garnered attention as promising electrode material candidates due to their abundant redox-active groups and periodic networks with highly cross-linked organic molecules^[16,17]. They also show unique physicochemical properties, including high crystallinity, porous structure and high surface area, which could be beneficial for electrolyte ion diffusion. In particular, π-conjugated imine (C=N)-based COFs, such as e-COF^[17], 4-(Dicyanomethylene)-2,6-dimethyl-4H-pyran (DDP)-based covalent triazine frameworks (CTFs)^[18], covalent organic framework-5 (COF-5)^[19] and hexamine dipyrazinoquinoxaline-based covalent organic framework (HADQ COF)^[20], have obtained remarkable success as electrode materials for both nonaqueous and aqueous batteries, as well as supercapacitors thanks to the structural benefits of rich redox centers and intrinsic electrical conductivity. Despite possessing a high theoretical capacity, imine-based COFs suffer from infertile redox active sites and poor structure stability, especially in aqueous acidic or alkaline electrolytes.

So far, only a few imine-linked COFs have been reported as supercapacitor electrodes in aqueous media. For example, a hexaazatrinaphthylene (HATN)-based COF containing an aza-fused π-conjugated framework displays a small surface area and hence a general specific capacitance of 663 F g^-1 at 1 A g^-1, while its retaining stability through 7,500 cycles test in 1 M H₂SO₄^[21]. A porphyrin-based COF linked by imine bonds shows an extensive surface area accompanied by an impressive specific capacitance of 1,443 F g^-1 at 1 A g^-1 but relatively short 3,000 cycles when tested in 0.5 M H₂SO₄ electrolyte^[22]. In this case, the π-conjugated HATN-based COFs could comparatively stabilize imine linkages in rigid aromatic frameworks while compromising the redox activity in acidic electrolytes^[23,24]. On the other hand, few publications have reported imine-based COFs for alkaline AHCs, showing ordinary specific capacitances (< 800 F g^-1 at 1 A g^-1)^[25,26]. Comprehensively speaking, the specific capacities demonstrated by COF electrodes are still lower than those of metal-based electrodes for AHCs on account of the existence of unusable active groups. Therefore, it is challenging to design stable and highly porous imine-based COFs with high-efficiency utilization of active sites for AHCs with competitive performance.

In the present study, we report a stable imine-linked HATN-COF with abundant pore structure as high capacity and long lifetime AHC organic electrodes, synthesized via a one-step solvothermal condensation process through reacting cyclohexanehexone and 2,3,6,7,10,11-hexaiminotriphenylene (HAT) in an ethanol solvent. The high crystalline and porous structures of π-conjugated HATN-COF not only facilitate electron and ion transport but also ensure a broader exposure of functional active sites conducive to faradaic reactions. Simultaneously, incorporating the ethanol molecules in frameworks, the HATN-COF presents high chemical stability in strong acids and bases because of the formation of hydrogen bond networks by ethanol molecules that can protect the imine bonds. Consequently, HATN-COF shows a remarkably large specific capacity (367 mAhg^-1 at 1 A g^-1) in alkaline conditions, exceeding the current state-of-the-art COF-based positive electrodes, while maintaining favorable rate performance and long-term cycling stability. Furthermore, an AHC assembled with HATN-COF and activated carbon (AC) as positive and negative electrodes, respectively, demonstrates superior energy density, high power density and remarkable cycling durability, thereby demonstrating its high potential as an organic electrode for future AHCs.

EXPERIMENTAL

RESULT AND DISCUSSION

As illustrated in the schematic diagram of Figure 1A, the synthesis of HATN-COF (C₂₀H₁₂N₆) involves a solvothermal condensation reaction between HKH and HAT in an acidic ethanol solution. Powder X-ray Diffraction (PXRD) analysis is initially collected to identify the structure and crystallinity of HATN-COF [Figure 1B]. Two sharp 2θ peaks at 7.3° and 26.3° correspond to (100) and (001) facets of HATN-COF, respectively, representing high crystallinity. This experimental pattern shows a good match with the simulated PXRD pattern of the AA-stacking structure by Materials Studio^[30], which has low residuals (R_p = 5.0% and R_wp = 8.3%). Specially, the peak at 7.3° corresponds to the periodic pore structure with a pore width of 11.6 Å, in good concordance with the simulated pore diameter of HATN-COF [Supplementary Figure 1], facilitating the efficient transfer of electrolyte ions within the electrode material to ensure the rapid reaction kinetics. Meanwhile, the broad peak at 26.3° is mainly ascribed to the extended π-conjugated laminated construction with an interlayer spacing of 3.4 Å and π-π stacking between the HATN-COF layers corresponds to the (001) plane^[31,32].

Figure 1. Synthesis and structural characterization of HATN-COF. (A) Synthetic scheme; (B) The experimental and simulated XRD patterns and corresponding space-filling model. HATN: Hexaazatrinaphthylene; COF: Covalent organic frameworks; XRD: X-ray diffraction.

SEM connected with EDS analysis and TEM techniques have been employed to gain further insight into microstructure and constituent of HATN-COF. A SEM image shows uniform microrod morphology featuring a length dimension approximating 2 µm × 200 nm [Figure 2A]. EDS elemental mapping images depict uniformly distributed C and N elements over the entire HATN-COF rod [Figure 2B and Supplementary Figure 2]. Furthermore, elemental analysis displays a C/N atomic ratio of 3.4, approaching the theoretical ratio of 3.3 [Supplementary Table 1], confirming the elemental composition of the HATN-COF. A TEM image manifests the rods consist of layered stacking [Figure 2C]. It is further confirmed by ultrasonic crushing of the HATN-COF powder in ethanol, where the multilayered stacking structure is more obviously visible in the SEM and TEM images [Supplementary Figures 3 and 4], ascribable to the robust interlayer π-π stacking interaction among neighboring layers of conjugated aromatic rings^[33]. Additionally, the Selected Area Electron Diffraction (SAED) pattern [Figure 2C] unveils two typical (100) and (001) crystalline facets of the HATN-COF crystal phase. These findings are in excellent agreement with the PXRD pattern. ¹³C SSNMR analysis further demonstrates the carbon skeleton of HATN-COF structure. The ¹³C SSNMR spectrum exhibits four obvious peaks at 105, 125, 140, and 150 ppm [Figure 2D], which are characteristic of the carbon signals of C-C, C=C, C-N and C=N in HATN-COF framework^[34]. The FTIR spectrum also identifies interrelated functional moieties within HATN-COF [Figure 2E]. The peaks at 1,646 and 1,432 cm^-1 are ascribed to the stretch vibrations of C=N^[35] and C-N^[36], respectively, signifying the formation of imine linkages. Simultaneously, the peaks at 1,510 and 1,284 cm^-1 are assigned to the stretch vibrations of C=C and C-C in HAT units, further confirming the formation of HATN polymers. Besides, the broad peak between 3,600 and 2,200 cm^-1 corresponds to the hydrogen bonding stretch from solvent ethanol molecules^[37]. This is also demonstrated by the TGA-DSC characterization, where some crystalline ethanol molecules could be present in the HATN-COF framework [Supplementary Figure 5]. The hydrogen-bonding networks have been reported to suppress the acid/base offense on nitrogen heterocycles^[38,39].

Figure 2. Characterizations of HATN-COF. (A) SEM image; (B) EDS elemental mappings; (C) TEM image (inset, SAED image); (D) ¹³C SNMR spectrum and (E) FTIR spectrum. HATN: Hexaazatrinaphthylene; COF: Covalent organic frameworks; SEM: Scanning electron microscopy; EDS:Energy-dispersive X-ray spectroscopy; TEM: Transmission electron microscopy; SAED: Selected area electron diffraction; FTIR: Fourier transform infrared; SNMR: Solid-state nuclear magnetic resonance.

XPS is conducted to verify the HATN-COF chemical constitution. The XPS survey spectra clearly display the distinctive peaks associated with C, N, and O components within the HATN-COF [Figure 3A and Supplementary Table 2]^[40]. Moreover, the deconvoluted peaks in the C 1s spectrum in Figure 3B, located at 284.6, 285.0, 285.5 and 284.0 eV, are attributed to the C=C^[41], C-O-H, C=N^[42] and C-N stemmed from triphenylene, ethanol guest molecule and formed imine, respectively. The deconvoluted peaks of N 1s at 398.7 and 400.5 eV in Figure 3C are severally assignable to C=N^[35] bond and C-N-C linkage within the HATN unit^[43]. Additionally, the O 1s signals observed at 531.7, 532.8 and 533.6 eV are indicative of the C-O bond^[44], C=O and -OH group, respectively [Supplementary Figure 6]^[38], which are ascribed to the intercalated ethanol molecules in HATN-COF. UV-vis spectroscopy of HATN-COF reveals a wide absorbance ranging from 250 to 480 nm [Supplementary Figure 7], corresponding to the syncretic transition of π-π* and n-π* for HATN unit, which is a conjugated framework composed with C=C and C=N linkages^[45], also evidenced by the XPS, FTIR and ¹³C SSNMR results. Furthermore, the maximum absorption wavelength ends at 789 nm the intersection at the X-axis in Supplementary Figure 7, attributable to a small band gap of 1.6 eV, which indicates the semiconductive behavior. Besides, the Raman spectrum of HATN-COF shows two obvious tagged bands centered at 1,367 and 1,508 cm^-1[Figure 3D], corresponding to defective D band and E_2g mode G band^[46], respectively, which is attributed to the basic features of COFs with the extended π-conjugated structure^[47]. Notably, a broad peak observed at 2836 cm^-1 is characteristic of the presence of the 2D band^[40], indicating a significant level of graphitization achieved in HATN-COF^[40]. The above results indicate that the HATN-COF has been successfully synthesized.

Figure 3. Characterizations and acid-alkali tolerance test of HATN-COF. XPS analysis, (A) survey spectrum and the high-resolution (B) C 1s; and (C) N 1s spectra; (D) Raman spectrum; (E) XRD patterns and (F) FTIR spectrum of HATN-COF after soaking in 6 M HCl and 6 M KOH for seven days. HATN: Hexaazatrinaphthylene; COF: Covalent organic frameworks; XPS: X-ray photoelectron spectroscopy; XRD: X-ray diffraction; FTIR: Fourier transform infrared.

N₂ adsorption-desorption measurement further reveals the porosity of HATN-COF. Supplementary Figure 8 depicts the characteristic type-I and type-IV mixed isotherm, which demonstrates the presence of micro- and mesoporous with a specific surface area of approximately 410 m² g^-1 as determined by a Brunauer-Emmett-Teller method and a pore volume of 0.28 cm³ g^-1 by a density functional theory method. Simultaneously, the pore size distribution (PSD) is primarily located at 6-13 Å, where a PSD of 11.6 Å agrees with the simulated structure model [Supplementary Figure 1], while the smaller pore of 6-10 Å could result from the filled ethanol molecules on some pores. The small portion of mesopores of 34-40 Å could be due to the stacking of small nanosheet structures^[38]. Particularly, the chemical stability of HATN-COF is assessed by being immersed in high-concentration acidic and alkaline electrolytes. XRD patterns show the unaltered crystalline structure of HATN-COF after soaking in 6 M HCl and 6 M KOH for seven days [Figure 3E], where the position and strength of (100) and (001) facets are both retained. The unaltered C=N and C-N covalent linkages within the FTIR spectrum underscore the remarkable resilience of HATN-COF against acidic and alkaline conditions [Figure 3F], stemming from the shielding of imine linkages through hydrogen bonds that safeguard against H⁺ or OH^- attacking^[27,48]. Obviously, the stable HATN-COF is a potential electrode material to be used in alkaline hybrid supercapacitors.

Electrochemical performance of HATN-COF electrode material is first evaluated through a three-electrode setup, which operates within a potential range spanning 0~0.6 V in 6 M KOH electrolyte solution. As the scan rate is incremented from 5 to 100 mV s^-1, all cyclic voltammetry (CV) curves display a group of broad redox peaks while preserving a similar shape with escalated currents [Figure 4A], indicating a quasi-reversible reaction process derived from the redox-active imines. The relatively negligible currents in the neutral electrolyte further confirm the redox reaction process for HATN-COF material instead of the electrostatic adsorption [Supplementary Figure 9]. Generally, the dependency of the peak current (i) on the scan rate (v) adheres to the power law, expressed as^[49]: i=av^b, with a and b serving as variable constants. Upon correlating log (i) versus log (v), the b values for the oxidation and reduction peaks are determined to be 0.61 and 0.57, respectively, which suggests a typical battery-type behavior [Supplementary Figure 10]. Subsequently, performing the GCD assessment offers a profound understanding of the charge storage abilities exhibited by the HATN-COF electrode [Figure 4B and Supplementary Table 3]. All GCD curves show symmetric platforms at various current densities, which further confirms a battery-type characteristic of HATN-COF with nice reversibility. Impressively, the HATN-COF delivers an unexpected specific capacity of 367.3 mAh g^-1 (equivalent to 2,644.5 F g^-1) under 1 A g^-1, surpassing numerous reported COF materials [Supplementary Table 4]^[32], such as TaPa-Py-COF (209 F g^-1 at 0.5 A g^-1)^[50], anthraquinone-based covalent organic frameworks/graphene composite aerogel (DAAQ-COFs/GA) (378 F g^-1 at 1 A g^-1)^[51] and TDFP-1 (418 F g^-1 at 0.5 A g^-1)^[52], while demonstrating comparable performance to high-capacity Ni/Co-based inorganic materials [e.g., CF@(Ni, Co)Se-2, 188 mAh g^-1[53]; Ni-Co-LDH-4:1, 255 mAh g^-1[28]; Ni₉S₈, 278 mAh g^-1[54]. All measures at 1 A g^-1]. Upon escalation of current densities to 20 A g^-1, the HATN-COF maintains an impressive high specific capacity of 259.7 mAh g^-1 with 70.7% retention relative to 1 A g^-1 [Figure 4C], showing splendid rate performance. So, the conjugated HATN-COF with abundant pores can provide enough channels for a fast redox reaction. Furthermore, the behavior of charge transfer is investigated using EIS measurements. According to the Nyquist plot in Figure 4D, the HATN-COF exhibits both small equivalent series resistance (R_s) value at 0.46 Ω in conjunction with a charge transfer resistance (R_ct) value at 0.57 Ω, which signifies low resistance to ionic kinetics. What is more, after enduring 20,000 cycles at 6 A g^-1, the HATN-COF still presents a competitively large specific capacity of 344 mAh g^-1 (maintaining 97.8% of its original capacitance, Figure 4E), demonstrating its ultralong cycle life, which surpasses that of the most reported organic materials [Supplementary Table 4]^[52]. The excellent cycling performance in alkaline electrolytes could be attributed to the high alkaline tolerance for HATN-COF structure. Moreover, the HATN-COF electrode after cycling the GCD test is further characterized by utilizing SEM, XRD and FTIR [Supplementary Figure 11], where the changeless morphology, crystal, and chemical structure consistently reconfirm high electrochemical stability of HATN-COF.

Figure 4. Electrochemical performances of HATN-COF electrode. (A) CV profiles at varying scan rates; (B) GCD profiles at various current densities; (C) Rate capability performance and coulomb efficiency; (D) Nyquist plots, an inset showing enlarged portion; (E) Cycling performance at 6 A g^-1. The inset provides the GCD profiles for both the initial and the final five cycles. HATN: Hexaazatrinaphthylene; COF: Covalent organic frameworks; CV: Cyclic voltammograms; GCD: Galvanostatic charge-discharge.

To further evaluate the potential of HATN-COF for practical use, an AHC is constructed using HATN-COF as the positive electrode material and AC as the negative electrode material [Figure 5A]. In contrast to HATN-COF, AC displays rectangular CV and linear GCD curves [Supplementary Figure 12], which is a distinctive feature associated with the electrochemical double-layer behavior. The specific capacity for AC is 300 F g^-1 at 1 A g^-1, while retaining a respectable 240 F g^-1 at 20 A g^-1. To ensure equilibrium in charge distribution between the positive electrode and negative electrode of the AHC, a specific mass ratio of HATN-COF: AC = 1:6 is employed. As shown in Figures 5B and Supplementary Figure 13, HATN-COF and AC operate within opposite potential ranges, while the cell maintains a stable voltage window of 1.5 V. Upon escalation from 5 to 100 mV s^-1, CV curves of HATN-COF//AC AHC exhibit a well-maintained shape, suggesting good reaction reversibility [Figure 5C]. Calculated from the GCD test [Figure 5D], at 0.5 A g^-1, the specific capacity of HATN-COF//AC AHC reaches a high value of 215.4 F g^-1. Correspondingly, the device of HATN-COF//AC AHC provides the supreme power-density with 7,500 W kg^-1 and maximum energy density with 67.3 Wh kg^-1 at 375 W kg^-1, outperforming most COF-based supercapacitors [Figure 5E and Supplementary Table 5], such as RuO₂//Hex-COF (23.3 W h kg^-1 at 661.2 W kg^-1)^[21], FCTF//AC (46.3 W h kg^-1 at 975 W kg^-1)^[55] and TpTa-Py//TpTa-Py (9.06 W h kg^-1 at 100 W kg^-1)^[50]. Furthermore, at 6 A g^-1, the HATN-COF//AC AHC delivers a capacity retention of 97.3% after 20,000 cycles [Figures 5F and Supplementary Figure 14], surpassing most reported COF-based supercapacitors, confirming the excellent cyclic stability. In summary, the HATN-COF, with its stable porous HATN structure, serves as electrode material that enables the AHC full device to achieve superior energy storage performance.

Figure 5. Electrochemical performances of HATN-COF//AC AHC. (A) Schematic illustrations of AHC construction; (B) CV curves at 10 mV s^-1 of the HATN-COF and AC serving as positive and negative electrodes, respectively, within a three-electrode configuration utilizing 6 M KOH electrolyte; (C) CV curves of HATN-COF//AC at diverse scan rates; (D) GCD curves of HATN-COF//AC system at various current densities; (E) Ragone plots comparing the energy- and power-density of assembled device with other materials reported in literature; (F) Cycling stability performance. HATN: Hexaazatrinaphthylene; COF: Covalent organic frameworks; CV: Cyclic voltammograms; AHCs: Aqueous hybrid supercapacitors; GCD: Galvanostatic charge-discharge; AC: Activated carbon.

CONCLUSION

In summary, an imine-linked HATN-COF with high acid-base endurance is successfully synthesized and applied as the electrode material in AHC. The porous structure, robust HATN skeleton and hydrogen-bonding protection make HATN-COF possess highly efficient and extremely stabilized redox-active sites. Consequently, the HATN-COF reveals a remarkably elevated specific capacity reaching 367.3 mAh g^-1 under 1 A g^-1, impressive rate capability retaining 259.7 mAh g^-1 even at 20 A g^-1, and displays superb cyclability retaining 97.8% retention of its initial capacity post 20,000 cycles at 6 A g^-1. Moreover, the HATN-COF//AC AHC device achieves an outstanding energy density of 67.3 W h kg^-1 at 375 W kg^-1 plus a long cycling stability of up to 20,000 cycles without obvious capacity fading. This work presents a promising method to develop strong COF electrode materials for high-performance AHCs.