Synergistic Donor‐Acceptor Effect in Conductive Covalent Organic Frameworks of Polyphthalocyanines for Aqueous Zn–I₂ Batteries

ABSTRACT

The development of materials with sufficient electrical conductivity and electroactivity is crucial for advancing their application in energy storage; however, integrating feasible electron transport properties and redox activity in a material remains a significant challenge. Here, we report two conjugated COFs constructed with fully aromatic linkages, namely poly‐zinc phthalocyanine (pZnPc) and poly‐zinc naphthalocyanine (pZnNPc), which exhibit distinct degrees of π‐conjugation, endowing them with good electrical conductivity. Iodine incorporation further induces remarkable conductivity increases up to two orders of magnitude with 1 S m⁻¹ for pZnPc and 0.15 S m⁻¹ for pZnNPc, along with a 200% enhancement in the electroactivity of iodine (I₂). Spectroscopic analyses and density functional theory (DFT) calculations reveal that the host‐guest charge‐transfer interaction between the COFs and I₂ allows the formation of an electron donor‐acceptor system, which synergistically facilitates electron transport to improve the electrical conductivity of the system and lowers the reaction barrier for the iodine redox reaction, consequently suppressing the shuttling of the I₃ ⁻/I₅ ⁻ intermediates. As a result, the host‐guest complex material I₂@pZnPc as a cathode in aqueous Zn–I₂ batteries delivers a high specific capacity of 234.4 mAh g⁻¹, close to the theoretical limit of I₂, meanwhile offering excellent cycling stability of over 40,000 cycles.

Incorporating iodine (I₂) in polyphthalocyanine‐based conductive COFs establishes effective charge‐transfer interaction between the COF hosts and I₂, which simultaneously boosts the conductivity of COFs to 1 S m⁻¹ and enhances I₂ electroactivity by 200%. This synergistic effect makes the host‐guest complex I₂@COFs excellent cathode materials in aqueous Zn–I₂ batteries, delivering a high capacity of 234.4 mAh g⁻¹ and stability over 40,000 cycles.

1. Introduction

The development of electroactive materials with excellent charge transport properties is critical for applications in sensing [1], catalysis [2], and energy storage [3, 4, 5]. Traditional inorganic electrode materials often suffer from limited capacity and poor structural tunability, making it difficult to control their architecture at the atomic level [6, 7, 8]. In contrast, organic electrode materials have garnered increasing attention due to their high abundance, low cost, environmental friendliness, high specific capacity, and tunable molecular design [9, 10, 11, 12]. However, their intrinsically low electrical conductivity and high solubility in electrolytes lead to poor interfacial interactions with conventional electrode surfaces, thereby hindering direct electrochemical reaction kinetics [13, 14, 15].

Covalent organic frameworks (COFs), a class of crystalline porous materials, have begun to show promise in applications relying on electron transport properties, including catalysis [16, 17, 18], sensing [19], and energy storage and conversion [20, 21, 22], attributed to their π‐conjugated constituent, tunable topologies, abundant active sites, intrinsic porosity, and excellent chemical stability [23, 24, 25]. Nevertheless, conventional COFs typically contain only locally π‐conjugated motifs without overall long‐range π‐delocalization, which limits efficient charge transport [26, 27]. Enhancing the electrical conductivity of COFs through rational molecular engineering is essential for enabling their application as high‐performance electrode materials, where charge transfer between redox active species is required. To address these intrinsic limitations, several structural engineering strategies have been proposed to enhance the electronic conductivity of COFs, primarily including topological engineering, conjugation design, and stacking modulation. Recent studies have demonstrated that topological control is an effective strategy for improving charge transport in COFs, among which Lieb lattice structures exhibit dispersive bands with a Dirac cone at the M point in the first Brillouin zone, giving rise to unusual behavior, such as effectively massless fermions [28, 29]. In addition, the deliberate construction of p–π and π–π conjugated systems can facilitate both through‐bond charge migration, improving carrier mobility [19, 23, 30, 31]. In parallel, precise control over interlayer π–π stacking can significantly increase orbital overlap between adjacent layers, resulting in more delocalized electronic states and the formation of symmetric band structures with reduced or even zero bandgap [32, 33, 34]. Metallophthalocyanine (MPc) units, featuring an extended 18 π‐electron conjugated system and tunable M‐N₄ coordination site through metal substitution, have shown promising potential as one of the few molecular building blocks that can endow electrical conductivity through intralayer d–π or π–π conjugation and interlayer π–π stacking [35]. Together, these strategies collectively contribute to efficient charge delocalization and transport, laying the foundation for the use of COFs as high‐performance electronic and energy‐storage materials.

Compared with the above strategies that rely heavily on elaborate molecular design and synthetic complexity, the doping strategy based on host‐guest interactions provides a convenient and efficient approach to further enhance the electrical conductivity of COFs. Incorporating electron‐rich or electron‐deficient guest molecules, such as tetrathiafulvalene (TTF) [36], tetracyanoquinodimethane (TCNQ), [37] fullerene (e.g., C₆₀) [38], iodine (I₂) [30, 39], and others [40] into the COF backbone can lead to the formation of donor‐acceptor (D‐A) charge transfer complexes to provide efficient transport pathways or to improve charge carrier densities, thereby enhancing transport. Among these guest molecules, I₂, characterized by multiple redox states, a suitable redox potential, high theoretical capacity (211 mAh g⁻¹), and natural abundance, is widely employed in aqueous Zn–I₂ batteries [41, 42]. Furthermore, recent studies have demonstrated that multi‐electron iodine conversion can be achieved by introducing additional halogen species, enabling four‐electron or even six‐electron transfer processes to deliver capacities beyond the two‐electron process [42, 43, 44]. However, its poor conductivity, high volatility, and solubility often result in low utilization and serious shuttle effects [45]. Although porous carbon‐based materials, such as graphene and carbon nanotubes, have been widely employed as conductive hosts to improve the electrical conductivity of iodine electrode in Zn–I₂ batteries, their nonpolar surface usually shows weak affinity toward I₂ and iodine related species involved in the I₂ reduction reactions. Such weak interactions, consequently, have resulted in sluggish kinetics of the I₂/I⁻ two‐electron transfer redox reaction to give high overpotential, as well as low capacity [46]. In addition, the inefficient adsorption of the polyiodide intermediates is also a reason for severe capacity fading and poor cycling stability because of the unavoidable active materials shuttling between electrodes [47, 48].

Herein, we report a host‐guest strategy for producing electroactive materials with high electrical conductivity and excellent electroactivity using two conjugated COFs, poly‐zinc phthalocyanine (pZnPc) and poly‐zinc naphthalocyanine (pZnNPc), in which zinc porphyrazine units are respectively fused with benzene and naphthalene rings to form frameworks with Lieb lattice. The two COFs with different degrees of π‐conjugation exhibited decent intrinsic electrical conductivities of 2.94 × 10⁻² and 1.67 × 10⁻³ S m⁻¹. Upon I₂ doping, the electrical conductivity of the COFs is improved by nearly two orders of magnitude, reaching 1.03 S m⁻¹ for I₂‐doped pZnPc. Spectroscopic analysis and density functional theory (DFT) calculations reveal effective host‐guest charge‐transfer interactions between COFs and I₂, forming a donor‐acceptor system that enhances charge transfer to and reduces the energy barrier for the iodine redox reaction (IRR). The synergistic effect between the COFs and I₂ has led to a 200% enhanced electrochemical activity of I₂. When applied as cathode materials in Zn–I₂ batteries, I₂‐loaded COFs deliver high specific capacities that approach theoretical values, along with excellent rate performance. The shuttle effect of I₃ ⁻/I₅ ⁻ intermediates is also effectively suppressed, ascribed to the fast kinetics and the affinity from the COF host, further contributing to the long‐term cycling stability. This work presents, for the first time, an effective strategy for simultaneously improving the conductivity of the host framework and the redox activity of small guest molecules by harnessing their synergistic donor‐acceptor effect, conceptualizing a new approach for the development of high‐performance Zn–I₂ batteries.

2. Results and Discussion

2.1. Design Principles and Synthesis of the COFs

To effectively enhance the electrical conductivity of COFs for use as electrode materials, we adopted a combination of structural design strategies, including extending π‐conjugation, increasing the number of d‐π conjugated units, and introducing guest molecules. Building on our previous work demonstrating the excellent electronic properties of polymetallophthalocyanines COFs [49], we here synthesized two aromatically conjugated COFs containing differently fused phthalocyanine units, respectively named pZnPc and pZnNPc, via a quadruple intermolecular nucleophilic annulation reaction between ZnCl₂ and the readily available ligands tetracyanobenzene (TCB) and tetracyanonaphthalene (TCN) (see Scheme 1 and Figures S1–S4). ZnCl₂ was employed as the zinc source for its relatively low melting point (290 °C), which dissolves the nitrile monomers for scalable synthesis and its catalytic ability in the nucleophilic annulation in a sufficiently reversible manner for the formation of crystalline porous frameworks [50]. The fully aromatic linkages contribute to rigid Lieb scaffolds that ensure high structural stability and efficient charge delocalization, resulting in considerable bulk conductivity. Furthermore, the well‐defined microporous channels and abundant electron‐rich sites of these COFs render them ideal hosts for I₂ [51, 52]. Upon the incorporation of I₂ as a guest molecule, effective host–guest interactions are expected to be established between iodine species and the COF skeletons, enabling efficient charge transfer to further markedly enhance the overall charge transport capability of the material. Meanwhile, in contrast to carbon‐based materials, enhanced adsorption to I₂ and iodine related species from the COFs may also improve the kinetics of IRR and minimize the shuttling effect to improve the battery performance.

SCHEME 1.

Schematic representation of the synthesis of (a) pZnPc and (b) pZnNPc.

2.2. Characterizations of the COFs

Powder x‐ray diffraction (PXRD) confirmed the successful synthesis of the COFs pZnPc and pZnNPc. For pZnPc, diffraction peaks were observed at 2θ = 8.8°, 26.8°, 34.4°, and 44.0° (Figures 1a and S5), corresponding to the (100), (001), (410), and (411) crystal planes, respectively. In the case of pZnNPc, peaks appeared at 2θ = 6.8°, 13.7°, and 25.6°, which can be assigned to the (100), (200), and (001) crystal planes (Figures 1d and S6). Simulated PXRD patterns based on the P4/mmm space group with AA‐stacking arrangements of ZnPc (Figures 1b, c, S7, and Table S1) and ZnNPc units showed good agreement with the experimental results of pZnPc and pZnNPc, respectively (Figures 1e, f, S8, and Table S2). Scanning electron microscopy (SEM) images revealed that both COFs are composed of irregularly shaped nanoparticles (Figures S9 and S10). Energy‐dispersive x‐ray spectroscopy (EDX) mappings confirmed the uniform distribution of C, N, and Zn elements throughout the frameworks (Figures 1g, S11, and 12). Transmission electron microscopy (TEM) images further supported the formation of two‐dimensional nanosheets for both COFs. In particular, pZnPc exhibited well‐defined square‐like domains with an approximate side length of ∼1 nm (Figure 1h), which well matched the distance of the (100) plane in the theoretical model, indicating the successful construction of the targeted crystalline structures. Meanwhile, pZnNPc also showed lattice fringes with a regular spacing of ∼ 1.3 nm ascribed to the (100) planes (Figure S13).

FIGURE 1.

Structure and characterization of pZnPc and pZnNPc. Experimental PXRD and simulated PXRD from models in P4/mmm space group of (a) pZnPc and (d) pZnNPc, respectively. Top and side views of simulated structure models of (b and c) pZnPc and (e and f) pZnNPc with AA stacking, respectively (C, light gray; N, lilac; Zn, amaranth; H, white). (g) EDX mapping for Zn, C, and N elements and (h) TEM (inset of h: FFT analysis from the TEM image) images of pZnPc.

Fourier transform infrared (FT‐IR) spectroscopy was conducted to further validate the formation of COFs. The disappearance of the peak at 2245 cm⁻¹, attributed to the C≡N stretching vibration, indicates the consumption of the monomers (Figure S18). In addition, the characteristic C─N stretching modes at 1481 and 1256 cm⁻¹, corresponding to the pyrrole units of the phthalocyanine framework were observed supporting the successful construction of the phthalocyanine‐based COFs [52, 53, 54]. The solid‐state ¹³C cross‐polarization/magic angle spinning nuclear magnetic resonance spectra of both COFs showed peaks assigned to aromatic carbons in the newly formed polyphthalocyanines (Figure S19) [55, 56]. Inductively coupled plasma atomic emission spectroscopy (ICP‐AES) measurements showed Zn metal contents of 11.56% and 9.10% for pZnPc and pZnNPc, respectively, roughly matching the theoretical values of 15.5% and 12.53%. N₂ adsorption isotherms conducted at 77 K demonstrate that the specific surface areas of pZnPc and pZnNPc were 286 and 97 m²/g, respectively. Both COFs exhibited uniform pore distribution with pore size centered at 1.1 and 1.3 nm for pZnPc and pZnNPc, respectively (Figures S20 and S21). The lower surface area of pZnNPc was likely attributed to its limited crystallinity and the presence of guest molecules, such as tetraethylene glycol used in COF synthesis [57]. When scaling up the synthesis of pZnPc from milligram to gram scale (up to 1.2 g), the COF retained fair crystallinity and a BET surface area comparable to that of the small‐scale synthesis (Figures S22 and S23). Thermogravimetric analysis of both COFs exhibited a slight weight loss of ∼10% at up to 400 °C under a N₂ atmosphere, indicating their excellent thermal stability (Figure S24). After treatment in various organic solvents as well as under strongly acidic (6 M HCl) and basic (6 M NaOH) conditions for one week, both pZnPc and pZnNPc retained their crystallinity, suggesting their robust chemical stability derived from the aromatically conjugated skeleton (Figure S25).

2.3. I₂ Doping of pZnPc and pZnNPc

Subsequently, I₂ doping of COF was carried out via a thermal sublimation approach, wherein a physical mixture of COF and I₂ was heated at 100 °C for 8 h, allowing I₂ molecules to be introduced into the COF. SEM and EDX analyses showed that the overall morphology of the I₂ incorporated ZnPc and pZnNPc, respectively named I₂@pZnPc and I₂@pZnNPc, remained unchanged after I₂ incorporation, and that the iodine were uniformly distributed throughout the COF frameworks (Figure S14–S17). The electrical conductivities of cold‐pressed pellets of pZnPc, pZnNPc, I₂@pZnPc, and I₂@pZnNPc, measured by the standard four‐probe method at 298 K, were determined to be 2.94 × 10⁻², 1.67 × 10⁻³, 1.03, and 1.15 × 10⁻² S m⁻¹, respectively (Figures 2a, S26, and Table S3). Notably, compared to their pristine counterparts, the conductivity of the COFs increased by nearly two orders of magnitude after I₂ doping, with I₂@pZnPc ranking among the most conductive COFs reported to date [30, 58, 59].

FIGURE 2.

(a) Electrical conductivity values of pZnPc, pZnNPc, I₂@pZnPc, and I₂@pZnNPc using the four‐point probe method (n = 3; error bars represent standard deviation). Comparison of (b, c) UV‐vis‐NIR spectra and (d) Tauc plots of pZnPc, I₂@pZnPc, pZnNPc, and I₂@pZnNPc. High‐resolution XPS spectra of (e) I 3d for I₂@pZnPc and (f) N 1s for pZnPc and I₂@pZnPc.

To investigate the mechanism underlying the conductivity enhancement, solid‐state ultraviolet− visible−near infrared (UV‐vis‐NIR) absorption spectra were collected. After I₂ doping, both COFs exhibited a pronounced red shift in their maximum absorption peaks, with the position shifted from 602 to 615 nm for pZnPc and from 689 to 719 nm for pZnNPc (Figure 2b, c). Tauc plot analysis demonstrated that the optical bandgap was narrowed from 1.65 to 1.56 eV for pZnPc and from 1.25 to 1.20 eV for pZnNPc (Figure 2d). To further elucidate the interaction between I₂ and the COFs, x‐ray photoelectron spectroscopy (XPS) measurements were performed. XPS analysis of the COFs before and after doping revealed that the C 1s spectra remained largely unchanged (Figures S27 and S29), and the Zn 2p binding energies showed minimal shifts (Figure S31). Interestingly, the deconvolution of the I 3d_5/2 region revealed two peaks. The minor peak appeared at 620.4 eV, which corresponded to free I₂ (Figures 2e and S30b) [60]. The major peak at 619.05 eV, a binding energy lower than that of free I₂, was likely attributed to I₂ in a bound state. This shift in binding energy indicated partial electron transfer from the COF to the incorporated I₂ molecules [51]. Meanwhile, a distinct positive shift in the N 1s spectra of I₂@pZnPc was observed with a new peak emerging at 400.45 eV, indicative of electron transfer from the N atoms of the ZnPc moiety to I₂ molecules (Figure 2f). Similar changes were also observed in the XPS spectra of I₂@pZnNPc (Figure S30a) [51, 61]. Synchrotron soft x‐ray absorption spectroscopy (sXAS) of the COFs after I₂ incorporation demonstrated that the N K‐edge spectra of the COFs showed enhanced intensity, indicating electron transfer from the Pc unit to I₂ (Figure S32) [62, 63]. Meanwhile, the M‐edge spectra of iodine appeared at 644.81 eV (Figure S33), close to that of negatively charged iodine species [64, 65], which was consistent with the XPS results, providing further evidence of charge transfer between I₂ and the COFs. The effective adsorption of I₂ by COFs was also visualized by the color change upon immersing the COF in the I₂ solution (Figure S34). The I₂ uptake capacities of pZnPc and pZnNPc were quantitatively determined to be 31.23% and 20.5%, respectively, by UV–vis absorption spectroscopy (Figure S35). These results suggested that the effective host–guest interactions between the COFs and I₂ molecules and the formation of donor‐acceptor systems, which promoted charge‐transfer transitions and enhanced their electrical conductivity [66, 67, 68].

2.4. Zn–I₂ Batteries Based on I₂@COFs

The above I₂‐doping study demonstrated concurrent electronic regulations in both the COFs and I₂ mediated by their host–guest interactions, which yielded materials I₂@pZnPc and I₂@pZnNPc with satisfying conductivity. We further hypothesized that such interaction may synergistically facilitate the redox activation of I₂ species since the favorable D‐A type electron transfer pathways were created in these systems, holding promise for improving their electrochemical performance in I₂‐based batteries. With this in mind, we therefore developed Zn–I₂ batteries utilizing I₂@pZnPc and I₂@pZnNPc as cathode materials and evaluated their electrochemical performance for energy storage (Figures 3a).

FIGURE 3.

Electrochemical performance of COFs as Zn–I₂ battery electrodes. (a) Schematic of COFs as cathode materials for Zn–I₂ batteries. (b) CV curves, (c) Tafel plots, (d) EIS plots, and (e) Galvanostatic charge‐discharge profiles and (f) rate performance at current densities ranging from 0.5 to 10 A g⁻¹, (g) Cycling performance at 0.5 A g⁻¹, and (h) long‐term cycling stability at 10 A g⁻¹ for I₂@pZnPc, I₂@pZnNPc, and I₂@KB. (i) GCD curves and (j) cycling performance at 1 A g⁻¹ for the pouch cell tests. (k) Photographs showing three pouch cells connected in series lighting up an LED device (>3 V).

Zn–I₂ batteries were then assembled using I₂@pZnPc and I₂@pZnNPc as cathode materials, Zn foil as the anode, and 2 M ZnSO₄ as the electrolyte (see Supporting Information for details). Firstly, cyclic voltammetry (CV) measurements were conducted to assess the electrochemical behavior of I₂‐incorporated COFs, as well as an I₂ and KB composite (I₂@KB) serving as the comparison material. Both I₂@pZnPc and I₂@pZnNPc exhibited redox behaviors consistent with a typical I₂/I⁻ process (Figure 3b) [69, 70]. However, compared to I₂@KB, I₂@pZnPc and I₂@pZnNPc exhibited much higher currents at the same scan rate (Figures S36–S38). Moreover, both COFs showed highly symmetric oxidation and reduction peaks at different scan rates, suggesting excellent redox reversibility of the two‐electron I₂ reduction process. CV analysis of I₂@pZnPc revealed that surface‐controlled contributions were 46.19%, 51.09%, 58.05%, 67.57%, and 79.48% at scan rates of 0.5, 1, 2, 5, and 10 mV s⁻¹, respectively (Figure S39). Tafel slope analysis showed slope values of 68.78 and 111.25 mV dec⁻¹ for I₂@pZnPc and I₂@pZnNPc, respectively, much smaller than 141.69 mV dec⁻¹ for I₂@KB, demonstrating the crucial role of the COF hosts in promoting IRR kinetics (Figure 3c). Electrochemical impedance spectroscopy (EIS) results of I₂@pZnPc, I₂@pZnNPc, and I₂@KB showed a semicircular arc at high frequencies corresponding to charge‐transfer resistance (R_ct) and a diagonal line at low frequencies related to ion diffusion. The smaller R_ct values of I₂@pZnPc and I₂@pZnNPc indicate their enhanced charge‐transfer kinetics (Figures 3d and S40). To elucidate the dynamic evolution of interfacial charge transfer during the redox process, in situ EIS was conducted on I₂@pZnPc at different charge and discharge states (Figure S41a, c). The charge‐transfer resistance gradually decreased during discharge and increased upon charging, revealing the reversible nature of the interfacial electrochemical processes [71, 72]. The distribution of relaxation times (DRT) analysis of the in situ EIS spectra revealed three distinct relaxation processes with a relaxation time constant (τ) of 10⁻⁴, 10⁻², and 1 s (Figure S41b, d), which likely corresponded to the change of ohmic resistance of the electrode [73], the electron transfer of redox species, and interfacial charge exchange, respectively [48].

Galvanostatic charge‐discharge (GCD) profiles at a current density of 0.5 A g⁻¹ revealed that I₂@pZnPc and I₂@pZnNPc afforded specific capacities (based on the mass of I₂) of 234.4 and 201.3 mAh g⁻¹, respectively, in comparison to the capacity of 117.9 mAh g⁻¹ from the composite I₂@KB (Figures 3e, S42, and S43). I₂@pZnPc exhibited a 200% enhancement in I₂ utilization compared to I₂@KB. The capacity of I₂@pZnPc slightly exceeded the theoretical capacity of 211 mAh g⁻¹ for I₂ due to the minor contribution from the capacitance of the conductive carbon black and pZnPc (Figure S44). In addition, I₂@pZnPc exhibited a much smaller polarization (ΔE = 54 mV) compared to I₂@pZnNPc (ΔE = 84 mV) and I₂@KB (ΔE = 153 mV), indicating more favorable kinetics for IRR. Without I₂ doping, the pristine pZnPc showed negligible capacity as expected (Figure S44). The rate performance was further evaluated over a current density range of 0.5–10 A g⁻¹. I₂@pZnPc delivered discharge capacities of 229.3, 202.9, 181.2, 160.2, and 148.9 mAh g⁻¹ at 0.5, 1, 2, 5, and 10 A g⁻¹, respectively. At 10 A g‐1, the discharge capacity of I₂@pZnPc and I₂@pZnNPc was approximately 180% and 134% higher than that of I₂@KB, respectively. Upon returning to 0.5 A g⁻¹, the capacity of I₂@pZnPc rapidly recovered to its initial value, demonstrating its good rate capability (Figure 3f). Cycling stability, another key criterion for electrode evaluation, was also assessed. I₂@pZnPc maintained a specific capacity of 220.9 mAh g⁻¹ over 200 cycles at 0.5 A g⁻¹ (Figures 3g and S45). After 200 cycles, EIS measurements showed that the I₂@pZnPc‐based Zn‐I₂ battery maintained a low interfacial charge‐transfer resistance, demonstrating interfacial stability (Figure S46). SEM showed no significant changes in the morphology of the COF electrodes after cycling, further confirming their structural stability (Figure S47). Long‐term cycling tests showed that I₂@pZnPc retained a high capacity of 108.6 mAh g⁻¹ after 40,000 cycles at 10 A g⁻¹, demonstrating excellent cycling durability (Figure 3h). Self‐discharge and post‐self‐discharge cycling tests further demonstrated that the host–guest interaction between pZnPc and I₂ enhanced the overall stability of the battery (Figure S48). These performances combined positioned I₂@pZnPc as one of the most optimal systems among porous carbon, inorganic cathode materials, and COFs reported so far for the two‐electron Zn–I₂ batteries cathode (Figure S49 and Table S4).

To further evaluate the practical applicability of I₂@pZnPc, a pouch cell with an active material loading of 2.95 mg cm⁻² was fabricated. At a current density of 1 A g⁻¹, the pouch cell delivered an initial discharge capacity of 213.9 mAh g⁻¹ and retained 195.1 mAh g⁻¹ after 500 cycles (Figure 3i and j). The rate performance was evaluated over a current density range of 0.5‐3 A g⁻¹. I₂@pZnPc delivered discharge capacities of 203.5, 189.1, 181.2, 179.1, and 163.4 mAh g⁻¹ at 0.5, 1.5, 2, and 3 A g⁻¹, respectively, demonstrating good rate capability (Figure S50). Moreover, the I₂@pZnPc‐based pouch cell exhibited a stable output voltage of 1.14 V at a current density of 1 A g⁻¹ (Figure 3i). We further assembled a pouch cell with a higher areal loading of 13.4 mg cm⁻². At a current density of 0.5 A g⁻¹, it maintained a high specific capacity of 203.8 mAh g⁻¹ and exhibited good cycling performance (Figure S51). Three such pouch cells connected in series successfully powered an LED module with an operating voltage above 3 V, demonstrating its capability in practical applications (Figure 3k). I₂@pZnPc also exhibited a relatively high specific capacity at −20 °C, which can be attributed to the catalytic effect of pZnPc on iodine redox reactions (Figure S52). I₂@pZnPc also delivered a high specific capacity of 442.5 mAh g⁻¹ in a four‐electron Zn–I₂ battery system (Figure S53). These results clearly demonstrate the crucial role of conjugated COFs as host matrices in facilitating iodine redox chemistry, as evidenced by the enhanced electrochemical performance of both I₂@pZnPc and I₂@pZnNPc compared to I₂@KB. In addition, I₂@pZnPc demonstrates superior capacity and reversibility relative to I₂@pZnNPc, which may originate from its higher conductivity and greater abundance of Zn–N₄ active sites. The Zn centers may act as the catalytic sites for iodine redox reactions, which was further supported by the observation that the I₂@pZnPc sample with Zn partially stripped exhibited a 30% decrease in capacity in the battery tests (Figure S54).

2.5. Mechanistic Study

To investigate the mechanisms of the host–guest interactions between the COFs and I₂ leading to their enhanced electronic properties for the electrochemical performance, a series of experimental and theoretical studies have been performed. In situ Raman spectroscopy revealed distinct peaks at 110 and 160 cm⁻¹ for the I₂@KB cathode, corresponding to I₃ ⁻ and I₅ ⁻ species, which intensified during charging and diminished upon discharging (Figures 4a and S55) [74]. In contrast, the I₂@pZnPc electrode exhibited only weak signals of I₃ ⁻ and I₅ ⁻ throughout the entire cycle (Figure 4b). Further insights into the diffusion of iodine species during charging were obtained through in situ UV–vis spectroscopy. For the I₂@KB cathode, characteristic absorption peaks at 288 and 351 nm appeared, corresponding to dissolved I₃ ⁻ in the electrolyte (Figure 4c and S56) [74]. By contrast, no noticeable I₃ ⁻ signal was detected for the I₂@pZnPc electrode throughout the charging process (Figure 4d). High‐resolution I 3d XPS spectra showed that, when the I₂@pZnPc‐based Zn‐I₂ cell was discharged to 1.2 V, peaks shifted to lower binding energies, attributed to the gradual accumulation of I⁻. This observation was consistent with the I₂→I₃ ⁻→I⁻ conversion process (Figure S57 and Table S5). These in situ Raman and UV–vis results indicated that the formation of an electron donor–acceptor system between the COF and iodine enhances the affinity between the COF framework and the iodine species, thereby effectively confining polyiodides, suppressing the shuttle effect, and enabling I₂@pZnPc to achieve excellent cycling stability in two‐electron Zn‐I₂ batteries.

FIGURE 4.

Mechanistic investigation of I₂ doping and polyiodide species evolution. In situ Raman spectra illustrating the I₃ ⁻/I₅ ⁻ electrochemical conversion during charge–discharge cycling of Zn–I₂ batteries with (a) I₂@KB and (b) I₂@pZnPc electrodes. In situ UV–vis spectra recorded during the charging process for the electrolytes using (c) I₂@KB and (d) I₂@pZnPc cathodes. (e) Electrostatic potential map of pZnPc and schematic of possible I₂ adsorption sites (isovalue: 0.3). (f) The comparison of optimized adsorption energies of I₂ at different sites in pZnPc. (g) Electron density difference plot of I₂@pZnPc. Yellow, charge depletion, blue, charge accumulation (isovalue: 0.01). Band structures and partial density of states of (h) pZnPc, (i) I₂@pZnPc‐I, and (j) I₂@pZnPc‐IV. (k) Free energy diagram and (l) Gibbs free energy values for the I₂‐I⁻ reduction reaction mediated by I₂@KB, I₂@pZnNPc, and I₂@pZnPc.

DFT calculations were employed to explore the adsorption of I₂ on COFs. Based on previous studies on interactions between I₂ and small molecules [42, 75], four possible adsorption sites have been proposed and assessed for pZnPc (Figure 4e), including the Zn site in the Zn–N₄ unit (I), the N site around the Zn–N₄ unit (II), the N atom in the bay area of ZnPc unit (III), and the pore channel region (IV) [45, 69]. In these proposed adsorption modes, the first three can be recognized as two‐dimensional layer‐dominated adsorption, while the last one is channel‐dominated. The adsorption energy of I₂ on sites I‐IV were −0.517, −0.386, −0.403, and −0.427 eV, respectively (Figure 4f). The most negative binding energy suggested that adsorption on site I was the most thermodynamically favorable. Compared to that of the free I₂ molecule, the I‐I bond length of I₂ binding at site I was elongated to 2.91 Å, indicating stronger adsorption and a higher degree of activation of I₂ at this site. Charge density distribution analysis showed that after the adsorption of I₂ on site I, the net charge on the I₂ molecule became −0.02 (Figure S58). Furthermore, the differential charge density revealed charge transfer from the COF framework to the I₂ molecule at site I, highlighting a strong host‐guest charge transfer interaction that is favorable for enhancing charge transport (Figure 4g). Besides the Zn site, the relatively negative adsorption energy and large I─I bond length of I₂ suggested that the adsorption within the COF channel could also be possible, considering that adsorption in the channel could be kinetically favorable. Calculation also indicated that the favorable adsorption modes of I₃ ⁻ and I⁻ were consistent with those of I₂, with adsorption at site I being the most thermodynamically favorable (Figures S59 and S60). Band structure calculations showed that the pristine pZnPc had a band gap of 0.291 eV (Figure 4h). After I₂ doping at I and IV sites, smaller band gaps of 0.09 and 0.178 eV were observed (Figures 4i and j). In addition, the energy gap from the Fermi‐level to the energy level at approximately 1.48 eV also became smaller, which corresponds to the transition process from 1.65 to 1.56 eV observed in the COF. Partial density of states (PDOS) further showed that, after I₂ incorporation, intensive states contributed from the p orbitals of I₂ were found at 0.16 eV near the Fermi level, which may contribute to the more favorable electron‐transfer pathways in I₂@pZnPc COF to enhance the conductivity (Figure S61). As shown in Figure 4i, in I₂@pZnPc COF, the highest occupied bands from the incorporated I₂ were around −0.98 eV, and the lowest unoccupied bands from I₂ were distributed at about 0.16 eV. The resulting gap was 1.1 eV, which is much lower than that for the free I₂ molecule (1.8 eV), suggesting that the interaction with the COF framework modulates the electronic structure of I₂ and enhances its reactivity [76]. pZnNPc also exhibited comparable band gap narrowing upon doping (Figures S61–S65). These results were in agreement with the UV‐Vis‐NIR spectra and confirmed the enhanced electrical conductivity of the COFs after I₂ doping.

To understand the catalytic role of COFs in the IRR, the free energy changes associated with the formation of key intermediates involved in the I₂ to I⁻ reduction process were evaluated. The I₃ ⁻ to I⁻ conversion was identified as the rate‐determining step [69, 77]. The Gibbs free energies (ΔG) of this step for I⁻@pZnPc, I⁻@pZnNPc, and I⁻@KB were calculated to be −5.94, −5.83, and −1.89 eV, respectively (Figures 4k, 4l, S66–S68, and Tables S6–S8). The much more negative ΔG values indicated that COFs were likely more favorable for catalyzing the IRR, consistent with the enhanced electrochemical performance of Zn–I₂ batteries based on COFs. Among them, pZnPc exhibits the lowest activation barrier and the most effective catalytic behavior, which can be attributed to its higher electrical conductivity and the greater abundance of Zn–N₄ active sites that collectively accelerate reaction kinetics and promote more efficient I₂ reduction. Gibbs free energy calculations for site IV revealed a relatively negative value of −3.01 eV (Figures S69, S70, and Table S9), suggesting that the pore environment can effectively promote the IRR. The observed enhancement is attributed to non‐covalent interactions between I₂ molecules and the hydrogen atoms on the COF pore walls, including weak C─H···I hydrogen bonds, which can polarize the I₂ molecules and promote charge transfer within the host–guest system [78, 79].

Taken together, spectroscopic characterizations and DFT calculations support that the incorporation of I₂ into COFs formed an electron donor‐acceptor system, which enhanced charge transport by narrowing the bandgap and lowering the energy barrier for I₂ reduction, thereby collectively improving the conductivity of pZnPc and pZnNPc as well as the electrochemical performance of Zn–I₂ batteries.

3. Conclusion

In summary, we have successfully synthesized two novel fully conjugated COFs, pZnPc and pZnNPc, establishing a new class of intrinsically conductive COFs with extended π‐conjugation. Subsequent I₂ doping significantly enhances their electrical conductivity, reaching up to 1 S m⁻¹, by forming charge‐transfer complexes between the COFs and I₂. When applied as the cathode material for aqueous Zn–I₂ batteries, I₂@pZnPc exhibited a significant 200% increase in specific capacity compared to the I₂@KB, reaching 234.4 mAh g⁻¹, along with excellent rate capability and cycling stability of as long as 40,000 cycles.

This work presents a new modular and scalable strategy for constructing highly conductive COFs through rational structural design and guest molecule integration, enabling simultaneous enhancement of framework conductivity and guest electroactivity. Although reticular materials have recently attracted attention as I₂ hosts [45, 67], the development of COFs with both structural robustness and electrical conductivity, as demonstrated in this work, marks an important step toward practical applications. It is anticipated that our strategy, through rational modular design and the integration of electron donor–acceptor systems, will enable the construction of new highly conductive COFs and open new avenues for advancing electronic applications beyond the Zn–I₂ battery system demonstrated here.

Author Contributions

Yi Zhang: methodology, data curation, formal analysis, validation, investigation, writing – original draft, and writing – review and editing. Peiyan Tong: data curation, validation, formal analysis, writing – review and editing, and investigation. Wei Chen: methodology, supervision, validation, writing – review and editing, and funding acquisition. Zheng Meng: conceptualization, methodology, supervision, writing – original draft, writing – review and editing, investigation, and funding acquisition.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: anie72905‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.