2D covalent organic frameworks: organic electrode materials for aqueous batteries

Abstract

Two-dimensional covalent organic frameworks (2D COFs) have swiftly gained prominence as versatile organic electrode materials for aqueous batteries, driven by their insoluble skeletons, one-dimensional (1D) porous structures, and exceptional structural tunability. This review consolidates recent progress in the development of 2D COFs as cathode materials, addressing design principles, synthetic strategies, and electrochemical performance. It further underscores the wide-ranging applicability of 2D COFs across diverse aqueous batteries, including proton, zinc-ion, calcium-ion, magnesium-ion, ammonium-ion, and hybrid acid–alkali systems. Finally, this review delineates critical challenges—such as conductivity enhancement, scalable synthesis, and interfacial stability—and proposes future research directions aimed at expediting the translation of 2D COF-based aqueous batteries from laboratory-scale prototypes to viable energy storage solutions, thereby catalysing innovation in sustainable battery technologies.

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Xiaoli Yan

Xiaoli Yan received her BSc from Zhengzhou University in 2016. In 2022 she received her PhD from Tianjin University under the supervision of Prof. Long Chen. From 2022 to 2024, she underwent postdoctoral training with Prof. Yi Liu and Lei Wang at Shenzhen University. She is currently a lecturer at the Institute of New Energy on Chemical Storage and Power Sources, College of Applied Chemistry and Environmental Engineering, Yancheng Teachers University. Her current research focuses on the design and synthesis of 2D conjugated porous polymers for photocatalysis and energy-related applications.

1. Introduction

The accelerating depletion of fossil fuel reserves coupled with escalating environmental challenges has intensified the global imperative for renewable energy deployment.¹ However, the inherent intermittency and variability of renewable sources such as wind and solar power necessitate the development of robust electrochemical energy storage systems to facilitate their reliable integration into electrical grids. Among these, rechargeable batteries have emerged as leading candidates in this context, owing to their extended cycle life and high energy conversion efficiency, making them pivotal for supporting renewable energy utilization.² While lithium-ion batteries (LIBs) currently dominate the markets for portable electronics and electric vehicles, their application in large-scale energy storage is constrained by safety concerns related to thermal runaway and the scarcity of lithium resources.^3–6 In contrast, aqueous batteries, which employ water-based electrolytes, inherently mitigate flammability risks and offer advantages including environmental benignity, cost-effectiveness, and efficient ionic conductivity.^7–9 As a result, aqueous batteries have garnered significant attention as promising platforms for grid-scale energy storage systems.

Aqueous batteries incorporate a diverse array of charge carriers, including H⁺, Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Zn²⁺, NH₄⁺ and OH⁻ (Fig. 1).^10–12 The intrinsic properties of these ions—such as the ionic radius, hydrated radius, charge density, and ionic mass—are intimately linked to their hydration energies, migration barriers, and solvation shell structures, collectively exerting a profound influence on ion transport kinetics and overall battery performance. Protons (H⁺), for example, possess an exceptionally small hydrated radius (∼2.8 Å) and minimal molar mass (1.008 g mol⁻¹), enabling ultrafast diffusion but rendering them susceptible to parasitic hydrogen evolution reactions.¹³ Conversely, multivalent ions such as Zn²⁺ and Mg²⁺ provide superior volumetric capacities (e.g., Zn²⁺ at 5855 mA h cm⁻³) attributable to their elevated charge density; yet, their pronounced solvation results in substantial desolvation energies, which in turn limit rate capabilities.¹⁴ The divergent electrochemical behaviours arising from these ionic characteristics fundamentally dictate the design criteria for next-generation electrode architectures.^15–18 Notably, organic electrode materials offer distinct advantages over their traditional inorganic counterparts within aqueous battery systems.^13,19,20 Derived either from renewable biomass or synthesized via tailored organic chemistry, these materials not only align with sustainability goals but also enable environmentally benign end-of-life pathways through combustion or biodegradation. Their charge storage is governed by reversible redox reactions between organic moieties and metal ions, circumventing the rigid lattice constraints of inorganic materials. This intrinsic structural flexibility enables effective accommodation of multivalent ions while alleviating ion diffusion limitations imposed by crystalline architectures.²¹ Furthermore, their tunable molecular structures permit the integration of multiple redox-active functionalities—such as quinones, pyrazines and anhydrides—thereby enabling multi-electron reactions to boost specific capacity.^22–24

Fig. 1 Schematic diagram of the ionic radius, hydrated ionic radius, and ionic weight for different carriers.

Despite their inherent merits, organic small-molecule electrode materials frequently encounter rapid capacity fading, primarily attributed to their dissolution in electrolytes and limited active site accessibility resulting from dense molecular packing.^25,26 To address these challenges, anchoring redox-active moieties into rigid, insoluble polymer frameworks has proven effective.²⁷ Among these, two-dimensional covalent organic frameworks (2D COFs), a class of crystalline porous polymers constructed from organic building blocks via dynamic covalent bonds, have recently gained prominence as ideal platforms for aqueous battery electrodes (Fig. 2).^28–31 Their distinctive structural attributes—including high surface area, intrinsic insolubility, low density, and well-ordered nanochannels—endow them with exceptional stability in harsh acidic/alkaline electrolytes. The periodic one-dimensional (1D) pores facilitate rapid ion diffusion while maximizing the accessibility of active sites, thereby enhancing redox kinetics and material utilization. Moreover, precise molecular engineering allows for the targeted incorporation of redox-active functionalities into the COF backbone, further augmenting capacity and energy density.

Fig. 2 Schematic picture showing 2D COFs and their intrinsic structural merits as electrodes for diverse aqueous battery systems.

Recent advances have underscored the significant potential of 2D COFs and their derivatives as electrode materials across a spectrum of aqueous battery systems, encompassing proton, zinc-ion, calcium-ion, magnesium-ion, ammonium-ion, and hybrid acid/alkali batteries, as summarized in Fig. 3.³² This review provides a systematic overview of these developments, offering comprehensive insights into design principles, synthetic strategies, and electrochemical performance. By highlighting the intrinsic advantages of 2D COFs—including structural tunability and facile interlayer exfoliation—this work aims to unravel fundamental structure–function correlations and furnish theoretical guidance for the rational design of high-performance energy storage systems. Finally, current challenges, including conductivity optimization, scalable synthesis, and interfacial stability, are critically examined, with proposed future research directions aimed at accelerating the practical deployment of 2D COF-based aqueous batteries and advancing innovations in sustainable energy storage technologies.

Fig. 3 Chronological overview of the initial reports detailing the incorporation of 2D COFs across diverse aqueous battery systems including aqueous zinc-ion batteries (AZIBs), aqueous proton batteries (APBs), aqueous calcium-ion batteries (ACIBs), aqueous magnesium-ion batteries (AMIBs) and aqueous ammonium-ion batteries (AAIBs).

2. Structural properties of 2D COFs

2.1 Introduction to 2D COFs

2D COFs represent a burgeoning class of crystalline porous polymers, assembled through the dynamic covalent linkage of organic building blocks.^33–35 Their distinctive architecture arises from the reversible nature of these covalent bonds combined with the precise geometric topology of the constituent units, conferring high surface areas, tunable pore sizes, ordered π-conjugated networks, and exceptional crystallinity.^36–38

Dynamic covalent bonds and crystallinity. The formation of 2D COFs is governed by a thermodynamically controlled self-correction mechanism, wherein reversible covalent bonds—such as boroxine (B–O) and boronate esters,³⁹ imines (C=N),⁴⁰ and hydrazones⁴¹—facilitate the assembly of highly ordered frameworks.^35,42 Imine linkages, for example, exhibit dynamic reversibility in organic solvents, enabling structural rearrangement into thermodynamically stable layered architectures; however, their susceptibility to hydrolysis under harsh conditions necessitates post-synthetic modifications to enhance stability.^43–45 In contrast, frameworks constructed from irreversible bonds, including triazine⁴⁶ and carbon–carbon double bonds (C=C)^47–49 formed via Aldol condensation, and benzimidazole linkages,⁵⁰ while more challenging to crystallize, offer superior chemical robustness. Notably, C=C-linked COFs (e.g., TFPM-PZI) demonstrate fully sp²-carbon conjugated structures that markedly improve charge transport and optoelectronic responses.^47,48

Topology design and interlayer stacking. The topological architecture of 2D COFs is fundamentally governed by the symmetry of their constituent monomers, which commonly exhibit C₁, C₂, C₃, C₄, or C₆ symmetry.^51–54 These monomers assemble into diverse lattice geometries, including hexagonal, Kagome, and square grids, among others. For instance, condensation between C₃- and C₂-symmetric monomers yields hexagonal pore structures with pore sizes finely tunable via monomer dimensions.³⁷ The combinations of [C₄ + C₂] and [C₄ + C₄] give rise to tetragonal 2D COFs,^38,55,56 whereas [C₆ + C₂] results in trigonal topologies.^57,58 Notably, [C₂ + C₂] pairings facilitate the formation of Kagome lattices.^59,60 Furthermore, these planar layers stack through π–π interactions or hydrogen bonding, forming one-dimensional nanochannels that facilitate ion and molecular transport.^31,61–63 The incorporation of non-planar units, such as those bearing amide side chains, can modulate interlayer hydrogen bonding, thereby enhancing mechanical stability.^64–67 Specifically, topological engineering plays a pivotal role in modulating the electron transfer properties of 2D COFs. Xu et al.⁶⁸ systematically designed and synthesized porphyrinic 2D COFs with distinct sql and bex topologies, demonstrating that the bex configuration markedly improves electron transfer efficiency relative to the sql counterpart. This work underscores the critical influence of topological design in tuning the electronic behavior and charge transport within 2D COFs.

Synthetic methods. The synthesis of 2D COFs requires balancing the reversibility of dynamic bonds and crystallinity control. Conventional solvothermal methods promote dynamic assembly of reversible bonds at elevated temperature and pressure,^47,69 while ionothermal synthesis employs low-volatility molten salts to achieve uniform nucleation.^46,70 Microwave-assisted synthesis and mechanochemical synthesis^71,72 have significantly shortened reaction times from days to hours. Zhang et al.^73–76 recently reported an emerging solvent-free synthesis using organic flux agents, enabling scalable COF production (from the gram to kilogram scale) and direct formation into foams or fibers, drastically reducing production costs (<$50/kg). Notably, 3D printing and continuous flow synthesis techniques are expanding the structural complexity and industrial application potential of COFs.^77–81

Collectively, the interplay of dynamic covalent chemistry, topological precision, and innovative synthetic methodologies underpins the structural sophistication of 2D COFs, positioning them as promising candidates for organic electrode materials in aqueous energy storage systems.^82,83

2.2 Distinguishing advantages of 2D COFs for electrodes

Highly tunable structures. The modularity of 2D COFs affords unparalleled tunability of their framework architecture, pore environment, and redox-active sites. Unlike conventional electrode materials such as metal oxides,⁸⁴ carbonaceous substances,⁸⁵ and organic polymers,^86,87 COFs offer highly customizable covalent networks and periodic porosity. Functional moieties (e.g., C=N, C=O, and –NH–) can be strategically incorporated to directly engage in redox reactions with charge carriers like Li⁺, Zn²⁺, and NH₄⁺.^88,89 For example, the integration of redox-active carbonyl groups (C=O) into pyrene-based monomers, followed by solvothermal condensation with 1,3,5-triformylbenzene (Tp), has yielded 2D COFs exhibiting high capacitance and remarkable cycling stability in supercapacitors.⁹⁰ Then, this COF was further extended to lithium-ion,⁹¹ hybrid acid/alkali,⁹² and zinc-ion batteries⁹³ due to their abundant redox-active sites.

Interlayer exfoliation. Despite the structural stability conferred by strong π–π stacking, the dense interlayer packing in highly crystalline 2D COFs can impede ion transport perpendicular to the layers. Exfoliation strategies that reduce layer thickness and stacking density could effectively shorten ion diffusion pathways and expose additional active sites, thereby enhancing electrochemical kinetics.^94–96 For example, Lu et al.⁹⁷ demonstrated that mechanical grinding combined with chemical exfoliation can produce DAAQ-COF nanosheets with controlled thicknesses ranging from 4 to 250 nm (Fig. 4a and b). Systematic evaluation of these nanosheets as sodium-ion battery anodes revealed a clear correlation between reduced thickness and enhanced electrochemical performance. As shown in Fig. 4c, the thinnest nanosheets (4–12 nm) show much higher stable capacity (420 mA h g⁻¹) than the 50–85 nm sample (310 mA h g⁻¹), 100–180 nm sample (208 mA h g⁻¹) and the 100–250 nm sample (154 mA h g⁻¹). Notably, the 4–12 nm sample retained a reversible capacity of 198 mA h g⁻¹ at a high current density of 5 A g⁻¹ following initial activation, and maintained 99% capacity retention over 10 000 cycles (Fig. 4d). These findings underscore the critical role of precise modulation of interlayer stacking and nanosheet thickness in simultaneously optimizing capacity, rate capability, and long-term cycling stability, thereby providing a promising avenue for the rational design of high-performance organic electrode materials.

Fig. 4 Exfoliation strategies to improve the electrochemical performance of 2D COF electrodes. (a) Schematic representation of sodium-ion transport pathways within DAAQ-COF. (b) Illustrations of exfoliated DAAQ-COF nanosheets with controlled thicknesses in the range of 4–12 nm, 50–85 nm, 100–180 nm, and 100–250 nm. (c) Cycle performance of the different thickness samples at 100 mA g⁻¹. (d) Cycle performance of the 4–12 nm thick sample at 5 A g⁻¹. Reproduced with permission from ref. 97 copyright© 2019, American Chemical Society.

Exfoliation of 2D COF nanosheets is predominantly achieved through two overarching strategies: physical and chemical methods. Physical approaches typically involve ultrasonic treatment of dispersed 2D COF powders or mechanical grinding followed by dispersion, effectively disrupting interlayer π–π interactions to yield few- or multilayer nanosheets.^98–101 Conversely, chemical exfoliation leverages post-synthetic modifications to introduce guest molecules within the 2D COF lattice, thereby attenuating interlayer forces and facilitating nanosheet delamination.^102,103 For example, Dichtel et al.¹⁰⁴ pioneered an acid-mediated exfoliation approach for imine-linked COFs. Protonation of the imine linkages by trifluoroacetic acid (TFA) generates electrostatic repulsion between BND-TFB COF layers, facilitating rapid delamination into nanosheets under stirring in a mixed acetonitrile–tetrahydrofuran solvent environment. The choice of exfoliation technique in practical contexts is generally dictated by the inherent structural attributes and targeted functionalities of the 2D COFs, enabling the tailored fabrication of nanosheets optimized for specific applications.

These unique features enable 2D COFs to harmonize high capacity, rapid ion transport, and enduring cycling stability, establishing innovative design principles and technical pathways for next-generation aqueous battery electrodes.^32,105

3. Electrochemical performance of 2D COFs

3.1 Aqueous proton batteries

Aqueous proton batteries (APBs), which employ protons as charge carriers, represent a promising advancement in energy storage technology.^106,107 Unlike conventional LIBs, which rely on the migration of Li⁺ through an organic electrolyte, APBs harness the migration of protons through water molecules, offering unique electrochemical properties and potential advantages:^{13,22,108,109} (1) cost-effectiveness: APBs utilize abundant (≈1400 ppm) and inexpensive proton sources, such as acids like sulfuric acid (H₂SO₄) or phosphoric acid (H₃PO₄). The high abundance of hydrogen not only enhances resource sustainability but also contributes to the relatively low cost of these batteries. (2) High ionic conductivity: protons exhibit high ionic conductivity in aqueous electrolytes, which originate from the prominent Grotthuss mechanism. This process facilitates rapid proton transport through the chains of hydrogen-bonded water molecules via concerted bond vibrations involving H⋯O hydrogen bonds and H–O covalent bonds.¹¹⁰ Consequently, APBs exhibit promising potential for high-power energy storage applications. (3) Wide operational temperature window: protons retain their mobility within water molecules even at subambient temperatures, enabling APBs to function effectively across a wide thermal range. This characteristic renders APBs particularly suitable for applications in cold environments where conventional battery chemistries often suffer from diminished performance. Collectively, these advantages underscore the potential of APBs as promising candidates for large-scale energy storage. Nevertheless, traditional inorganic electrode materials frequently undergo deleterious side reactions in acidic electrolytes, and the rapid proton insertion/extraction processes can induce structural degradation, thereby compromising cycling stability of APBs.⁹ Therefore, there is an urgent need to develop structurally stable electrode materials that can accommodate the fast proton transport without structural collapse for advancing aqueous proton battery technologies.

In 2023, Feng et al.¹¹¹ reported a novel 2D conjugated COF, denoted as poly(benzimidazobenzophenanthroline)-ladder-type (BBL-ladder-type 2D c-COF or 2DBBL-TP), engineered for rapid proton storage. This framework incorporates a distinctive redox-active site derived from the carbonyl dye perinone (Fig. 5a). Density functional theory (DFT) calculations reveal that the π-delocalization within perigone markedly diminishes the basicity of the C=O groups, thereby conferring a preferential affinity for protons over metal ions. This unique characteristic enables the 2DBBL-TP electrode to deliver exceptional electrochemical performance in both mild aqueous Zn-ion electrolyte (1 M ZnSO₄) and strong acidic media (0.5 M H₂SO₄). When hybridized with carbon nanotubes (2DBBL-TP/CNTs), the electrode exhibits an outstanding rate capability reaching 200 A g⁻¹, maintaining 67% of its capacity relative to 1 A g⁻¹ in H₂SO₄ electrolyte (Fig. 5b), surpassing the performance benchmarks of state-of-the-art conjugated polymers, COFs, and metal–organic frameworks. Furthermore, the electrode demonstrates remarkable cycling stability, with negligible capacity degradation after 10 000 charge–discharge cycles at 100 A g⁻¹ (Fig. 5c). This work underscores the promise of rationally designed 2D COFs in advancing high-performance APBs. Despite these advances, limitations arise from the reliance on a single active site (C=O) and the relatively low density of such sites within the material, constraining the discharge capacity to 76 mA h g⁻¹ at 1 A g⁻¹. Concurrently, our group developed a TPAD-COF cathode by integrating quinone redox centres with conductive aniline backbones to form aniline-fused quinonoid units (TPAD), achieving both long-term stability and high discharge capacity in APBs (Fig. 5d).¹¹² The abundant –NH– functional groups within the TPAD units impart exceptional hydrophilicity and intrinsic proton conductivity to the TPAD-COF, achieving a proton conductivity of up to 2.32 × 10⁻⁴ S cm⁻¹ at room temperature under 100% humidity conditions (Fig. 5e). Furthermore, the coexistence of –NH– and C=O redox-active sites facilitate six-electron transfer per repeating unit in 1.2 M H₂SO₄ electrolyte, yielding a high discharge capacity of 126 mA h g⁻¹ at 0.2 A g⁻¹ (Fig. 5g). The 2D COF's resistance to strong acids and bases, along with its intrinsic rigidity and insolubility, results in a capacity retention of up to 85% after 5000 charge–discharge cycles. By employing TPAD-COF as the cathode paired with an anthraquinone anode, a fully organic proton battery was successfully assembled, demonstrating the practical application of TPAD-COF in APBs (Fig. 5f). Recently, Zhang et al.⁷³ developed two polyimide-based 2D COF electrodes exhibiting exceptional cycling stability, retaining 99% of their capacity after 5000 cycles at a high rate of 5 A g⁻¹ (Fig. 5h and i). Additionally, Chen et al.¹¹³ constructed a fully conjugated COF (TABQ-COF) leveraging the synergistic effects of dual-active sites (C=O and C=N), achieving an enhanced specific capacity of 401 mA h g⁻¹ (Fig. 5j and k). The assembled TABQ-COF//MnO₂ full cell achieved a reversible capacity of 247 mA h g⁻¹ at 5 A g⁻¹, with a capacity retention of up to 100% after 10 000 cycles. These studies highlight the substantial potential of 2D COF materials as electrode platforms for improving the cycling stability and performance of APBs.

Fig. 5 Aqueous proton batteries with 2D COF electrodes. (a) Synthesis of 2DBBL-TP. (b) and (c) GCD and cycling stability of 2DBBL-TP/CNTs in 0.5 M H₂SO₄. Reproduced with permission from ref. 111 copyright© 2023, Wiley-VCH GmbH. (d) Synthesis of TPAD-COF. (e) Nyquist plots of TPAD-COF. (f) Illustration of the working mechanism for a TPAD-COF/AQ all-organic proton battery in a 1.2 M H₂SO₄ electrolyte. (g) Galvanostatic discharge/charge curves of TPAD-COF. Reproduced with permission from ref. 112 copyright© 2023, Wiley-VCH GmbH. (h) Synthesis of NKCOF-5 and -9. (i) Cycling stability of NKCOF-5 at 5 A g⁻¹. Reproduced with permission from ref. 73 copyright© 2024, Wiley-VCH GmbH. (j) GCD of TABQ-COF at 0.2 A g⁻¹ in 0.5 M H₂SO₄. (k) Design conception of TABQ-COF. Reproduced with permission from ref. 113 copyright© 2025, Wiley-VCH GmbH.

Despite notable advancements in the application of 2D COF electrode materials for APBs, several critical challenges persist.¹³ These challenges are primarily manifested in the following aspects: (1) voltage constraints: most 2D COF electrodes incorporating C=O and C=N redox-active sites currently operate at voltages below 0.5 V, thereby limiting the overall energy density achievable in these systems; (2) ambiguous structure–activity relationship: the complex interplay among redox-active moieties, electron density distribution, and pore architecture within 2D COFs exerts a synergistic influence on electrochemical performance. Furthermore, the transport pathways and kinetic mechanisms of protons within the channels of 2D COFs lack systematic investigation, leading to a bottleneck in the performance enhancement of current aqueous proton batteries.

3.2 Aqueous zinc-ion batteries

Aqueous zinc-ion batteries (AZIBs) have garnered considerable interest owing to the inherent merits of zinc metal anodes, including abundant natural abundance, high theoretical capacity (819 mA h g⁻¹), low redox potential (−0.76 V vs. SHE), and elevated hydrogen evolution overpotential.^114–116 The history of zinc-based batteries can be traced back to 1799 with its application in voltaic piles.¹¹⁷ However, the advancement of AZIBs was historically hindered by challenges such as uncontrolled zinc dendrite formation and the generation of insulating, irreversible byproducts in alkaline electrolytes, which collectively led to rapid capacity fading and poor coulombic efficiency. A pivotal breakthrough occurred in the 1980s when Shoji et al.^118,119 developed rechargeable Zn//MnO₂ batteries employing mildly acidic ZnSO₄ electrolytes. This foundational work was further substantiated by Kang's group¹²⁰ in 2012 through systematic studies on mildly acidic aqueous electrolytes.^114–116 Cathode materials, as critical determinants of AZIB performance, predominantly comprise inorganic compounds including manganese dioxide (MnO₂), vanadium pentoxide (V₂O₅), molybdenum trioxide (MoO₃), and zinc hexacyanoferrate (ZnHCF).^121–124 Nevertheless, these conventional cathodes suffer from inherent limitations: Zn²⁺ intercalation-induced lattice distortion and irreversible dissolution often result in sluggish ion diffusion/transport kinetics, low coulombic efficiency, and compromised cycling stability.^125,126

Recent developments have underscored 2D COFs as highly promising cathode materials for AZIBs, owing to their large specific surface areas, tunable porous structure, and precise molecular-level structural designability.^105,127,128 These characteristics enable enhanced Zn²⁺ ion transport and reaction kinetics, thereby offering substantial improvements in both specific capacity and long-term cycling stability, and revitalizing the advancement of AZIB technologies. By strategically engineering molecular structures to incorporate high-density active sites, the electrochemical performance of 2D COFs as cathodes can be markedly elevated. Notably, C=O act as redox-active centres during charge–discharge processes, reversibly coordinating with Zn²⁺ ions to enable efficient ion storage. Khayum et al.¹²⁹ synthesized a β-ketoamine COF (HqTp-COF) via the condensation of 1,3,5-triformylbenzene with 2,5-diaminophenol dihydrochloride, marking the first investigation of 2D COFs as cathode materials for AZIBs (Fig. 6a). The C=O in HqTp-COF arise from keto–enol tautomerism and the electrochemical oxidation of phenolic linkages, with both C₃- and C₂-symmetric carbonyl groups actively participating in the redox storage reactions. The HqTp-COF cathode delivers a specific capacity of 276.0 mA h g⁻¹ at a current density of 125.0 mA g⁻¹, retaining 95% of its initial capacity after 1000 cycles, alongside a stable coulombic efficiency of approximately 98% throughout cycling (Fig. 6b–d). To further enhance electrochemical performance, Liu et al.¹³⁰ synthesized Tp-PTO-COF via the condensation of Tp with 2,7-diaminopyridine-4,5,9,10-tetraone (DAPTO), employing it as a cathode material for AZIBs (Fig. 6e). Experimental results indicate that Tp-PTO-COF achieves an impressive specific capacity of 301.4 mA h g⁻¹ at 0.2 A g⁻¹ and maintains a reversible capacity of 192.8 mA h g⁻¹ even at a high current density of 5 A g⁻¹ (Fig. 6f). Furthermore, it demonstrates exceptional cycling stability and well-defined charge–discharge plateaus, delivering 218.5 mA h g⁻¹ with 95% capacity retention after 1000 cycles at 2 A g⁻¹. This outstanding electrochemical performance is attributed to the high density of nucleophilic active sites (C=O), a precisely engineered porous structure, and the inherent chemical robustness of Tp-PTO-COF. Complementary experimental investigations and DFT calculations elucidate the structural evolution and zinc-ion intercalation mechanisms within the framework. Collectively, these insights provide a foundation for the rational design of organic cathode materials aimed at high-performance and sustainable AZIBs. Zheng et al.¹³¹ also demonstrated the promising potential of quinone-based COFs (BT-PTO COF) as high-performance cathode materials for AZIBs. This COF was synthesized via a Schiff-base solvothermal condensation reaction between benzenetricarboxaldehyde (BT) and DAPTO (Fig. 6g). The well-ordered channel structure of BT-PTO COF facilitates efficient ion transfer and enables a redox pseudocapacitance mechanism, resulting in superior electrochemical performance. The BT-PTO COF exhibits a high reversible capacity of 220 mA h g⁻¹ at 0.5 A g⁻¹ and delivers exceptional long-term cycling stability, retaining 98.0% of its capacity after 10 000 cycles at 5 A g⁻¹ (Fig. 6h). Notably, this study reveals the first direct evidence of the Zn²⁺ co-insertion mechanism, wherein Zn²⁺ ions preferentially insert over two H⁺ ions (Fig. 6i). At elevated current densities, this co-insertion behavior shifts to favour increased proton insertion, thereby endowing the COF with ultrafast ionic transport kinetics. As a result, the assembled AZIB achieves a remarkable power density of 184 kW kg⁻¹_(COF) and an energy density of 92.4 Wh kg_(COF)⁻¹, underscoring its rapid charge–discharge capabilities. The rationally designed BT-PTO COF not only demonstrates superior electrochemical performance but also provides new insights into charge-storage mechanisms involving Zn²⁺ co-insertion in organic cathode materials.

Fig. 6 Aqueous zinc-ion batteries with 2D COF electrodes. (a) Synthesis of HqTp-COF. (b) Diagrammatic representation of the aqueous Zn/HqTp COF battery. (c) GCD measurements at different current densities for HqTp-COF. (d) Cycling stability of HqTp-COF. Reproduced with permission from ref. 129 copyright© 2019, The Royal Society of Chemistry. (e) Synthesis of Tp-PTO-COF. (f) GCD measurements at different current densities for Tp-PTO-COF. Reproduced with permission from ref. 130 copyright© 2022, The Royal Society of Chemistry. (g) Synthesis of BT-PTO-COF. (h) GCD measurements at different current densities for BT-PTO-COF. (i) Proposed H⁺, Zn²⁺ co-insertion process of BT-PTO COF in 3 M Zn(CF₃SO₃)₂. Reproduced with permission from ref. 131 copyright© 2022, Wiley-VCH GmbH.

Beyond the well-established role of C=O as active sites for Zn²⁺ storage, C=N functionalities have also been recognized as critical contributors to electrochemical capacity. In 2020, Alshareef et al.¹³² synthesized a novel phthalocyanine-based covalent organic framework (PA-COF) through the condensation of 2,3,7,8-tetraaminophenazine with cyclohexane, endowing the material with a high density of nitrogen active sites and pioneering its application in zinc-ion supercapacitors (Fig. 7a). The PA-COF exhibited a notable capacity of 247 mA h g⁻¹ at 0.1 A g⁻¹ and demonstrated exceptional cycling stability, with a capacity decay rate of merely 0.38% per cycle over 10 000 cycles at 1.0 A g⁻¹ (Fig. 7b). To further augment electrochemical performance, recent efforts have focused on designing COFs incorporating dual active sites, such as combined C=O and C=N groups, as cathode materials for AZIBs. This dual-site strategy not only increases the density of coordination sites but also facilitates synergistic interactions between distinct redox centres, thereby enhancing overall electrochemical behaviour. For instance, Alshareef et al.¹³³ reported the functionalization of a C=N-only COF (HA-COF) with quinone groups to yield HAQ-COF (Fig. 7c). Electrochemical analyses coupled with computational studies revealed that quinone carbonyl incorporation substantially improved Zn²⁺ storage capacity and lowered the lowest unoccupied molecular orbital (LUMO) energy level, thereby effectively elevating the redox potential. Compared to HA-COF, HAQ-COF exhibited a higher discharge capacity of 344 mA h g⁻¹ at 0.1 A g⁻¹ (Fig. 7d), and maintained excellent cycling stability with a capacity retention of 85% after 10 000 cycles at 5 A g⁻¹ (Fig. 7e). In contrast, the HA-COF electrode exhibited a significantly diminished performance, with a maximum reversible capacity of only 81 mA h g⁻¹ within the first ten cycles at 5 A g⁻¹, declining further to 61 mA h g⁻¹ after 10 000 cycles. This comparative study highlights the pronounced enhancement in electrochemical performance afforded by COFs featuring both carbonyl and imine functionalities relative to those containing solely single active sites. Wang et al.¹³⁴ subsequently investigated a covalent triazine framework (CTF-TTPQ) incorporating dual active sites as an organic cathode for AZIBs (Fig. 7f). This study emphasized the enhancement of electrochemical performance driven by the high density of C=O and C=N functionalities within CTF-TTPQ. The molecular design strategically integrates quinoline and phenazine units linked via triazine nodes, forming an extended planar conjugated network. This architecture not only furnishes abundant C=O and C=N redox-active sites but also facilitates efficient ion intercalation and deintercalation within the electrolyte. Experimental investigations combined with ex situ characterization and DFT calculations elucidate a co-insertion mechanism for H⁺ and Zn²⁺ ions, wherein Zn²⁺ ions simultaneously coordinate with the densely packed carbonyl and imine redox centres (Fig. 7g). Exploiting these multiple electroactive sites, CTF-TTPQ achieves a high reversible capacity of 404 mA h g⁻¹ at 0.3 A g⁻¹ over a voltage window of 0.1–1.4 V vs. Zn²⁺/Zn, along with a high energy density of 432.28 Wh kg⁻¹ (Fig. 7h). Recently, Li et al.¹³⁵ designed and synthesized a covalent organic framework (TA-PTO-COF) featuring dual redox-active sites of C=O and C=N by the condensation of tris(4-formylbiphenyl)amine (TA) with DAPTO-NH₂. The conjugated structure, characterized by multiple active sites and regular channels, endows TA-PTO-COF with high electrical conductivity, abundant accessible active sites, and rapid ion diffusion pathways, collectively promoting accelerated reaction kinetics and enhanced capacity. These studies collectively underscore that the incorporation of multiple active sites within COFs markedly improves electrochemical performance relative to frameworks bearing a single type of active site.

Fig. 7 Aqueous zinc-ion batteries with 2D COF electrodes. (a) Synthesis of PA-COF. (b) Cycling stability of PA-COF. Reproduced with permission from ref. 132 copyright© 2020, American Chemical Society. (c) Synthesis of HAQ-COF and HA-COF. (d) Capacity–voltage profiles of HAQ-COF. (e) Cycling stability of HAQ-COF and HA-COF. Reproduced with permission from ref. 133 copyright© 2021, Wiley-VCH GmbH. (f) CTF-TTPQ designed with high-density C=O and C=N redox sites for H⁺ and Zn²⁺ intercalation. (g) Calculated LUMO and HOMO energy levels for CTF-TTPQ repetitive units with their DFT optimized geometries in AZIBs. (h) GCD measurements at different current densities for CTF-TTPQ. Reproduced with permission from ref. 134 copyright© 2022, The Royal Society of Chemistry.

Currently, research on cathode materials for 2D COFs in AZIBs is predominantly centered on carbonyl and imine active sites; however, sulfur-containing thiazole electroactive groups have recently attracted increasing attention. In 2022, Peng et al.¹³⁶ reported a novel olefin-linked COF (COF-TMT-BT) designed as an AZIB cathode, distinguished by its incorporation of benzothiazole moieties that integrate sulfur and nitrogen atoms as electrochemically active centres (Fig. 8a and b). Experimental and computational results demonstrate that the nitrogen and sulfur atoms exhibit exceptional reversible binding affinity toward Zn²⁺, resulting in markedly enhanced electrochemical performance, with COF-TMT-BT achieving a high specific capacity of 283.5 mA h g⁻¹ at 0.1 A g⁻¹ (Fig. 8c). Further mechanistic insights elucidate that the charge-storage mechanism in COF-TMT-BT electrodes is predicated on the supramolecularly engineered reversible coordination of Zn²⁺ by the benzothiadiazole units. The highest occupied molecular orbitals (HOMOs) in COF-TMT-BT are predominantly localized within the benzothiadiazole segment without extensive continuous electron delocalization, while the LUMOs are similarly concentrated around these units, indicating that these moieties theoretically possess favorable electron affinity and elevated reduction potentials (Fig. 8d). Electrostatic potential surface simulations corroborate the role of benzothiadiazole moieties as electrochemically active centres for cation chelation, with electronegative regions localized near the corners of adjacent triazine cores and extending across the molecular plane boundaries of COF-TMT-BT, indicative of potential ionic adsorption sites within the framework (Fig. 8e).

Fig. 8 Aqueous zinc-ion batteries with 2D COF electrodes. (a) Synthesis of COF-TMT-BT. (b) Diagrammatic representation of the aqueous Zn/COF-TMT-BT unit cell. (c) GCD curves for COF-TMT-BT electrodes at various current densities. (d) HOMOs and LUMOs of COF-TMT-BT repetitive units. (e) Electrostatic potential surface of COF-TMT-BT repetitive units. Reproduced with permission from ref. 136 copyright© 2023, Wiley-VCH GmbH. (f) Design concept and working mechanism of the GDAQ//GDAQ-R SAOB. (g) Schematic diagram for the assembly process of a pouch-type SAOB. (h) Cycling stability of the GDAQ//GDAQ-R pouch-type SAOB at a current density of 5 A g⁻¹. Reproduced with permission from ref. 137 copyright© 2024, Wiley-VCH GmbH.

Conventional zinc anodes in AZIBs are fundamentally limited by dendritic growth and excessive zinc consumption, resulting in an unreasonably high negative-to-positive (N/P) capacity ratio that not only results in material wastage but also constrains improvements in energy density.^138–142 Fortunately, these challenges can be effectively addressed through the rational design of high-performance organic electrodes capable of dual functionality as both anodes and cathodes. Such an innovation facilitates the development of cost-effective and environmentally benign symmetric all-organic batteries (SAOBs), thereby enabling metal-free AZIB configurations.^143–145 Recently, Li et al.¹³⁷ developed a novel 2D COF electrode material enriched with C=N and C=O functional groups, which was integrated with graphene to assemble a full COF/graphene battery for the first time (Fig. 8f). Notably, this framework uniquely supports the concurrent storage of Zn²⁺ and H⁺ ions in mild aqueous electrolytes. The extensive porous network of the COF, combined with abundant reactive sites and enhanced electron delocalization induced by C=O modifications, underpins rapid proton storage kinetics. Furthermore, synergistic interactions between graphene and the COF's carbonyl groups amplify this effect. As a result, this SAOB exhibits a cycling stability and high-rate performance, sustaining over 15 000 cycles at 5 A g⁻¹ with a maintained capacity of 80 mA h g⁻¹ (Fig. 8g and h). This work represents a significant advancement towards aqueous organic batteries and marks a pivotal step in steering the field towards sustainable electrochemical energy storage solutions.

Collectively, these studies effectively demonstrate that 2D COFs are promising candidates for the development of stable AZIBs. Nonetheless, despite notable advancements, several critical challenges continue to limit the practical application of COF-based materials in AZIBs. Therefore, further research and exploration are essential prior to practical implementation, focusing on the following key areas: (1) the inherently low electrical conductivity of COFs remains a fundamental bottleneck. Conventional approaches such as incorporating conductive carbon additives or compositing with conductive substrates offer only partial mitigation and fail to resolve the issue at its core. Therefore, the rational design and synthesis of intrinsically high-conductivity COFs capable of fully harnessing their theoretical energy storage potential is imperative. (2) The electrochemical charge–discharge performance is intrinsically linked to the density of redox-active sites within the framework. Enhancing the number of active centres while minimizing their relative molecular weight can substantially elevate theoretical capacities. From a molecular engineering standpoint, COFs featuring multiple active sites coupled with low molecular mass represent a promising avenue toward high-capacity electrodes. However, achieving an optimal balance between structural stability and the incorporation of multiple active functionalities remains a formidable challenge. (3) A comprehensive understanding of the interplay between structural parameters of COFs—including composition, morphology, and crystallinity—and their electrochemical behaviour is essential. This necessitates the deployment of advanced in situ characterization techniques synergistically combined with DFT calculations to elucidate structure–property relationships. Such insights will be pivotal for guiding the rational design and scalable synthesis of 2D COF materials tailored for high-performance AZIBs.

3.3 Aqueous calcium-ion batteries

Aqueous calcium-ion batteries (ACIBs) have emerged as promising candidates for large-scale energy storage due to the earth's abundant calcium reserves, low cost, and their potential high volumetric/gravimetric capacity (2072 mA h mL⁻¹ and 1337 mA h g⁻¹).^146,147 Distinct from monovalent ions such as lithium and sodium, the divalent nature of Ca²⁺ ions confers a higher charge density per ion, thereby enabling elevated energy densities. However, the development of ACIBs is impeded by several critical challenges.^148–151 Primarily, the strong electrostatic interactions between divalent Ca²⁺ ions and host lattices severely restrict ion diffusion within inorganic crystalline frameworks, resulting in sluggish electrochemical kinetics. Additionally, the relatively large ionic radius of Ca²⁺ necessitates electrode materials with expansive diffusion channels to accommodate efficient ion transport, posing a significant barrier to achieving high-performance ACIBs. To address these limitations, extensive efforts have been directed towards the development of suitable electrode materials. Initial investigations predominantly focused on inorganic anode materials, such as graphite,¹⁵² silicon-based alloys,¹⁵³ tin-based alloys,¹⁵⁴ and sulfur/carbon composites.¹⁵⁵ Although these materials demonstrated certain electrochemical activity, their practical application was hindered by substantial volume changes during cycling, leading to rapid capacity degradation. In contrast, organic electrode materials have emerged as attractive alternatives due to their high specific capacities, intrinsic redox activity, and excellent reversibility, positioning them as promising anode candidates for ACIBs.

2D COFs have emerged as a compelling class of organic electrode materials, attributed to their tunable intrinsic porous structures. Recent studies have demonstrated the potential of 2D COFs as electrode materials for ACIBs. In 2022, Li et al.¹⁵⁶ reported the pioneering application of HqTp COF—synthesized via the condensation of 2,5-diaminohydroquinone dihydrochloride (Hq) and Tp—as an anode material for ACIBs (Fig. 9a). The HqTp COF exhibited a high specific capacity of up to 119.5 mA h g⁻¹ at 1 A g⁻¹ and demonstrated exceptional rate performance, retaining 78.7 mA h g⁻¹ even at an ultrahigh current density of 50 A g⁻¹ (Fig. 9b). Mechanistic studies revealed that the carbonyl groups functioned as reversible active sites for Ca²⁺ insertion and extraction, with proton co-insertion. Furthermore, a full battery assembled with HqTp COF as the anode and activated carbon as the cathode demonstrated stable cycling performance with a capacity retention of 73.7% after 1600 cycles. To further enhance ACIB performance, Shi et al.¹⁵⁷ introduced a nitrogen-rich COF, TB-COF, featuring multiple carbonyls as an anode (Fig. 9c). TB-COF demonstrated a high reversible capacity of 253 mA h g⁻¹ at 1 A g⁻¹ and a cycling stability with minimal capacity decay after 3000 cycles (Fig. 9d). Mechanistic investigations identified C=O and C=N bonds as primary active sites for calcium storage, while also revealing a novel C=C site contributing to ion accommodation. Combined computational and experimental analyses indicated that each repeating unit of TB-COF could reversibly host up to nine Ca²⁺ ions, underscoring its remarkable storage capability. Building upon this, Lv et al.¹⁵⁸ recently reported PTHAT-COF, incorporating repeated pyrazine and pyridinamine units, as a low-potential anode for ACIBs (Fig. 9e). PTHAT-COF displayed a flat ultra-low potential plateau ranging from −0.6 to −1.05 V (vs. Ag/AgCl) and outstanding rate performance, maintaining a specific capacity of 152.3 mA h g⁻¹ at 1 A g⁻¹ (Fig. 9f). DFT calculations attributed the ultralow potential plateau to the elevated LUMO level (Fig. 9g). The anode also showed exceptional long-term cycling stability, with minimal capacity loss after 10 000 cycles. Mechanistic studies confirmed that reversible Ca²⁺ ion trapping at C=N active sites underpinned the superior electrochemical performance of PTHAT-COF. When paired with a manganese-based Prussian blue cathode, the full cell demonstrated remarkable durability, retaining 83.6% of its capacity after 10 000 cycles within a voltage window of 2.2 V (Fig. 9h and i).

Fig. 9 Aqueous calcium-ion batteries with 2D COF electrodes. (a) Synthesis and activation process of HqTp COF. (b) Rate performance of HqTp COF. Reproduced with permission from ref. 156 copyright© 2022, The Royal Society of Chemistry. (c) The mechanism diagram of one-layer of TB-COF. (d) Rate performance of TB-COF. Reproduced with permission from ref. 157 copyright© 2023, American Chemical Society. (e) Ball-and-stick structure model of PTHAT-COF. (f) GCD of the PTHAT-COF anode at different current densities. (g) Calculated LUMO and HOMO energy levels of PTHAT-COF repetitive units. (h) Full ACIB cell based on a PTHAT-COF anode and Mn-PBA cathode. (i) Cycling stability of this full ACIB cell. Reproduced with permission from ref. 158 copyright© 2023, Wiley-VCH GmbH.

Research on 2D COFs as anode materials for ACIBs has evolved rapidly, with significant advancements in capacity, rate performance, and cycling stability. The designable porous size and functional groups of COFs confer unique advantages for Ca²⁺ ion storage, positioning them as promising candidates for ACIBs. Future efforts prioritize the rational optimization of COF structural and compositional parameters to further enhance performance and explore their integration into full-cell architectures.

3.4 Aqueous magnesium-ion batteries

Magnesium ions (Mg²⁺) are characterized by a small ionic radius (0.65 Å) and a low reduction potential of −2.37 V versus the normal hydrogen electrode (NHE), endowing them with a high theoretical capacity.^159,160 However, akin to Ca²⁺, Mg²⁺ ions exhibit strong electrostatic interactions with host materials, which significantly hinder their diffusion kinetics and restrict the range of viable electrode materials.^161–164 Consequently, traditional inorganic materials commonly employed in lithium-ion batteries often demonstrate limited efficacy for Mg²⁺ storage due to their sluggish solid-state diffusion. To circumvent these limitations, attention has increasingly shifted towards organic electrode materials.^165–167

The application of 2D COFs in Mg-ion batteries (MIBs) was initially reported by Wang et al.¹⁶⁸ in 2020. They introduced a triazine-based porous COF as a cathode material, achieving a high-power density of 2.8 kW kg⁻¹, a specific energy density of 146 Wh kg⁻¹, and an ultralong cycle life of 3000 cycles with a minimal capacity fade of 0.0196% per cycle (Fig. 10a and b). This seminal work demonstrated the feasibility of employing COFs in MIBs. Building on this foundation, subsequent studies have focused on elucidating the underlying mechanisms of Mg²⁺ storage within COFs and optimizing their electrochemical properties. In 2023, Alshareef et al.¹⁶⁹ reported a symmetric aqueous magnesium ion supercapacitor based on HAQ-COF (Fig. 10c). HAQ-COF exhibits a specific capacity of 177.4 mA h g⁻¹ at 0.2 A g⁻¹ as an anode and 118.5 mA h g⁻¹ at the same current density as a cathode (Fig. 10d and e). Notably, the symmetric full battery assembled from HAQ-COF electrodes delivered a high operating voltage of 1.5 V, an energy density of 26.5 Wh kg⁻¹, and a power density of 3750 W kg⁻¹, outperforming those in recent reports in the field (Fig. 10f).^170–173 Charge-storage kinetics analyses revealed that the superior performance arises from pseudocapacitive Mg²⁺ ion insertion within the COF matrix, effectively mitigating the sluggish solid-state diffusion typically associated with divalent ions. Integrating electrochemical measurements, spectroscopic characterization, and theoretical simulations, the study provided a comprehensive understanding of the Mg²⁺ storage mechanism in HAQ-COF, highlighting a synergistic charge storage process involving coordination of Mg²⁺ ions with both nitrogen and oxygen atoms in the COF framework (Fig. 10g).

Fig. 10 Aqueous magnesium-ion batteries with 2D COF electrodes. (a) Chemical structure and possible electrochemical redox mechanism of the triazine-based COF. (b) Schematic illustrations of the reaction mechanism and equations of 2D COF cathodes in chloride-free electrolytes. Reproduced with permission from ref. 168 copyright© 2020 American Chemical Society. (c) The structure of HAQ-COF. GCD of the HAQ-COF (d) anode and (e) cathode. (f) Schematic illustration of the HAQ-COF-based symmetric Mg²⁺ ion battery. (g) Theoretical simulation of the Mg²⁺ ion coordination process for HAQ-COF. Reproduced with permission from ref. 169 copyright© 2023, Wiley-VCH GmbH.

Concurrently, efforts have been directed towards optimizing the performance of COF-based MIBs through strategic electrolyte selection and electrode engineering. In 2023, Souto et al.¹⁷⁴ systematically investigated the influence of electrolyte composition and binder choice on the electrochemical behaviour of an anthraquinone-derived COF (DAAQ-TFP-COF) cathode in both Li- and Mg-ion battery systems. Specifically, the DAAQ-TFP-COF cathode was evaluated in MIBs employing two distinct electrolytes: a chloride-containing electrolyte (0.6 M MgTFSI₂–1.2 M MgCl₂ in dimethoxyethane, DME) and an electrolyte with weakly coordinating anions (0.2 M Mg(B(hfip)₄)₂ in DME). The study revealed that electrolyte composition exerts a pronounced influence on the electrochemical performance of the COF cathode. Notably, the chloride-based electrolyte induced higher overpotentials yet afforded superior cycling stability. These findings underscore the critical importance of tailoring both 2D COF architectures and electrolyte formulations to address the intrinsic challenges associated with accommodating divalent Mg²⁺ ions within the COF framework, thereby enhancing battery performance.

Despite substantial advancements in the deployment of COFs for MIBs, the rational design of novel COF structures that facilitate improved Mg²⁺ diffusion kinetics and deliver elevated specific capacities remains imperative. Moreover, the development of compatible electrolytes capable of stabilizing Mg²⁺ ions while suppressing parasitic side reactions is essential for optimizing electrochemical performance.

3.5 Aqueous ammonium-ion batteries

Although ammonium ions (NH₄⁺) have been relatively underexplored as charge carriers in aqueous energy storage systems, they offer distinct advantages over conventional ions such as Li⁺, Na⁺, K⁺, Zn²⁺, Mg²⁺, Ca²⁺, and Al³⁺.^175,176 Notably, the ultralow molar mass of NH₄⁺ (18 g mol⁻¹) contributes to significantly enhanced power densities in energy storage devices. Furthermore, NH₄⁺ ions are inherently sustainable and non-toxic, as they can be synthesized from abundant atmospheric nitrogen and hydrogen.^177–182 Despite possessing a relatively large ionic radius of 1.48 Å, NH₄⁺ exhibits a smaller hydrated radius (∼3.31 Å) compared to many metal cations, facilitating rapid diffusion within aqueous electrolytes.¹⁷⁹ Crucially, the tetrahedral geometry of NH₄⁺ enables unique topological chemistry within host materials, allowing intercalation via covalent-ionic bonding.¹⁸³ During (de)intercalation, NH₄⁺ ions promote charge transfer through the dynamic formation and cleavage of hydrogen bonds, resulting in enhanced energy storage capacity and superior energy density relative to those of their metal ion counterparts. Additionally, NH₄⁺-based systems offer significant advantages in recyclability over metal-ion batteries, owing to the water solubility and thermal decomposability of ammonium salts. Furthermore, NH₄⁺ electrolytes exhibit reduced corrosiveness and lower polarization during hydrogen evolution compared to H⁺/H₃O⁺ systems.¹⁸⁴ Collectively, these attributes underscore the practical significance of developing safe, cost-effective, and high-energy aqueous NH₄⁺ storage systems characterized by rapid kinetics and recyclability. Current research efforts are directed towards advancing electrode materials that offer high specific capacity, as NH₄⁺-based technologies continue to progress through foundational development stages.

2D COFs have garnered significant attention as electrode materials for aqueous ammonium-ion batteries (AAIBs), primarily due to their unique ability to form hydrogen bonds with NH₄⁺ ions—a critical factor that enhances both ion storage capacity and cycling stability. A seminal study by Alshareef et al.¹⁸⁵ in 2021 demonstrated the exceptional NH₄⁺ storage performance of HAQ-COF (Fig. 11a) which delivered a remarkable specific capacity of 220.4 mA h g⁻¹ at 0.5 A g⁻¹, substantially outperforming conventional electrode materials such as metal oxides (MnO_x: 176 mA h g⁻¹, V₂O₅: 88 mA h g⁻¹, and MoO₃: 100 mA h g⁻¹) and Prussian blue analogues (<90 mA h g⁻¹) (Fig. 11b).^186–193 The research elucidated a universal mechanism involving hydrogen bonding between nitrogen and oxygen atoms, which facilitated the formation of stable bi-diagonal coordinated structures during NH₄⁺ ion storage. Building upon these foundational insights, subsequent investigations have further advanced the mechanistic understanding of NH₄⁺ storage within COFs. Notably, Bao et al.¹⁹⁴ recently reported an anthraquinone carbonyl-containing COF as an anode material for AAIBs (Fig. 11c). This COF demonstrated a specific capacity of 141 mA h g⁻¹ at 0.1 A g⁻¹ and it retained 90% of its initial capacity after 8000 cycles at 6 A g⁻¹. Importantly, this study identified the superconjugated anthraquinone carbonyl groups as optimal sites for reversible NH₄⁺ storage via hydrogen bonding, whereas other carbonyl groups prone to enol–keto tautomerism adversely affected ion storage (Fig. 11d). Recent advancements have concentrated on enhancing the cycling stability and rate performance of COF-based electrodes for AAIBs. Alshareef et al.¹⁹⁵ introduced an aza-based COF (HATP-PT COF) as an ultra-stable anode (Fig. 11e), which exhibited remarkable cycling durability, maintaining its performance over 20 000 cycles without capacity degradation at 1.0 A g⁻¹ (Fig. 11f). When paired with a Prussian blue cathode, the complete battery demonstrated a capacity retention of 89% over 20 000 cycles, outperforming all previously reported AAIBs. This outstanding performance was attributed to the highly reversible hydrogen bond formation between NH₄⁺ ions and –C=N– bonds within the COF structure.

Fig. 11 Aqueous ammonium-ion batteries with 2D COF electrodes. (a) HAQ-COF for NH₄⁺ ion storage. (b) GCD of HAQ-COF in 0.5 M (NH₄)₂SO₄ electrolyte. Reproduced with permission from ref. 185 copyright© 2021 American Chemical Society. (c) Schematic representation of the keto–enol tautomerism in DAAQ-TP-COF. (d) Schematic representation of the coordination products of DAAQ-TP-COF with NH₄⁺. Reproduced with permission from ref. 194 copyright© 2025, Wiley-VCH GmbH. (e) Synthesis of HATP-PT COF. (f) Cycling performance of HATP-PT COF at 1.0 A g⁻¹. Reproduced with permission from ref. 195 copyright© 2024, Wiley-VCH GmbH.

2D COFs have emerged as highly promising electrode materials for AAIBs, offering substantial storage capacities, excellent cycling stability, and rapid ion diffusion kinetics. Their intrinsic ability to form robust hydrogen bonds with NH₄⁺ ions, combined with tunable structures and high surface areas, positions COFs as ideal candidates for advancing AAIB technology. Future research should prioritize the rational design and structural optimization of COFs to further enhance electrochemical performance, the development of scalable synthetic methodologies, and the integration of COF-based electrodes into practical battery configurations.

3.6 Hybrid acid/alkali all covalent organic framework batteries

The employment of aqueous electrolytes offers ion conductivities that surpass those of non-aqueous systems by approximately two orders of magnitude, presenting a compelling advantage for energy storage applications. However, the inherently narrow electrochemical stability window of water—limited by its decomposition potential near 1.23 V—imposes a fundamental constraint on the achievable voltage and thus the energy density of aqueous batteries.^12,196 To prevent water electrolysis, the operational potentials of electrode materials must be confined within this threshold, inherently restricting the overall cell voltage. In response, innovative battery architectures have been explored to extend the electrochemical stability window of aqueous systems. Among these, the hybrid acid/alkali battery emerges as a particularly promising strategy, wherein a cathode immersed in an acidic electrolyte is paired with an anode operating in an alkaline medium.⁹² This configuration effectively broadens the cell voltage window and enhances energy density. The integration of a membrane electrode assembly (MEA) design—anchoring the cathode and anode onto anion and cation exchange membranes (AEM and CEM), respectively—yields a compact device architecture that markedly improves electrochemical kinetics relative to conventional two- or three-chamber hybrid cells. Notably, certain 2D COFs employed as electrode materials exhibit pronounced pH-dependent behaviour, offering valuable tunability in the rational design of COF-based cathodes and anodes tailored for high-voltage, all-COF aqueous batteries.

In 2023, Chen et al.⁹² successfully developed a hybrid acid/alkali all-COF battery by pairing a pyrene-4,5,9,10-tetraone-based COF cathode with an anthraquinone-based COF anode (Fig. 12a and b). Within this configuration, the cathode operates effectively under acidic conditions at a relatively positive potential, while the anode demonstrates optimal performance in an alkaline environment at a relatively negative potential. Electrochemical evaluation revealed that the all-COF battery delivered a high discharge capacity of 92.97 mA h g⁻¹ across an expanded voltage window of 2.0 V, achieving an energy density of up to 74.2 Wh kg⁻¹, with stable performance maintained over 300 cycles (Fig. 12c–f). The study offers valuable insights for the design of high-energy-density aqueous battery systems. Furthermore, symmetric all-organic aqueous batteries, which utilize identical materials for both the cathode and anode, offer streamlined manufacturing processes and reduced production costs, thereby promoting the advancement of battery systems.¹⁹⁷ Future efforts should prioritize the development of 2D COF electrode materials compatible with both acidic and alkaline aqueous electrolytes which hold significant promise for the realization of symmetric hybrid acid/alkali batteries.¹⁹⁸

Fig. 12 Hybrid acid/alkali all-COF battery. Synthesis of (a) 4KT-Tp COF and (b) DAAQ-Tp COF. (c) Schematic illustration of the working mechanism and (d) CV curves at 5 mV s⁻¹ of the acid 4 KT Tp COF/rGO cathode, the alkali DAAQ Tp COF anode and the hybrid acid/alkali all-COF battery. (e) GCD curves at different current densities and (f) cycling performance at 3 A g⁻¹ of the hybrid acid/alkali all-COF battery. Reproduced with permission from ref. 92 copyright© 2023 Wiley-VCH GmbH.

4. Conclusions and outlook

2D COFs, as crystalline porous organic materials with highly ordered periodic structures, have fundamentally transformed the landscape of aqueous battery electrode design. Their unique combination of structural tunability, high surface area, intrinsic insolubility, and environmental sustainability underpins their exceptional performance. The well-defined nanochannels facilitate efficient ion transport, while the modular frameworks allow precise incorporation of multiple redox-active sites, collectively enhancing both energy density and reaction kinetics. Compared to traditional inorganic electrodes, 2D COFs inherently accommodate multivalent ions such as H⁺, Zn²⁺, and Mg²⁺, circumventing the ion diffusion limitations imposed by rigid crystalline lattices. Moreover, the adoption of renewable feedstocks and environmentally benign synthetic routes further elevates their promise for sustainable energy storage technologies.

This review has provided a comprehensive overview of recent advancements in the deployment of 2D COFs as cathode materials across a wide range of aqueous battery systems, encompassing proton, zinc-ion, calcium-ion, magnesium-ion, ammonium-ion, and hybrid acid/alkali batteries. The key design principles, synthetic strategies, and electrochemical performance of 2D COFs have been discussed in detail, highlighting their potential to address the limitations of traditional inorganic electrode materials.

Despite the significant progress, several critical challenges remain to be addressed for the practical application of 2D COFs in aqueous batteries:

(1) Complex synthesis protocols: the scalability of COF synthesis is critical for their practical application. Conventional solvothermal methods require stringent conditions, including strictly anhydrous and oxygen-free environments, as well as exceptionally high monomer purity, making it challenging to maintain laboratory-level precision during industrial-scale production. Moreover, the use of toxic organic solvents in traditional solvothermal synthesis not only elevates costs but also poses significant environmental concerns, conflicting with principles of green manufacturing. Although emerging green technologies such as solvent-free synthesis^73–76 and 3D printing^77–81 have recently been developed to enable large-scale production of 2D COFs, the general applicability and robustness of these approaches remain to be thoroughly validated.¹⁹⁹

(2) Intrinsic conductivity constraints: the semiconducting nature of 2D COFs and extended ion transport pathways result in relatively low ionic and electronic conductivities. Strategies including the integration of conjugated backbones, doping with conductive additives (e.g., graphene and conductive polymers), and construction of heterointerfaces are vital to enhance charge transport kinetics.²⁰⁰

(3) Limited active site accessibility: although abundant redox-active groups exist, strong interlayer π–π stacking (interlayer spacing <3.4 Å) restricts active site exposure and impedes ion diffusion. Approaches such as exfoliation into few-layer nanosheets or in situ hybridization with conductive matrices (e.g., graphene and carbon nanotubes) have demonstrated efficacy in improving active site utilization and electrochemical performance.^97,98,201

Looking forward, research efforts could focus on the following transformative directions:

(1) Green synthesis and intelligent manufacturing: employ artificial intelligence-guided synthesis, including AI-driven monomer selection, to realize solvent-free, energy-efficient continuous production. Emphasize bio-derived monomers and degradable 2D COFs to enable closed-loop material lifecycles aligned with circular economy principles.^202–204

(2) Multiscale structural engineering: utilize machine learning models, such as graph neural networks for pore design, to fabricate hierarchically porous 2D COFs with gradient micro- to mesoporous architectures, facilitating selective ion transport tailored to specific ion sizes. Advanced in situ techniques like transmission electron microscopy (TEM) should be leveraged to visualize dynamic ion intercalation within sub-nanometer channels.^27,205

(3) Synergistic conductivity enhancement: design fully conjugated 2D COFs or hybridize with two-dimensional conductive materials (e.g., MXenes) to establish dual electron and ion transport networks. Investigate the influence of dynamic covalent bonds on carrier mobility during electrochemical cycling.

(4) Interface–electrolyte compatibility: engineer surface functional groups (e.g., sulfonic acid moieties) to optimize electrode–electrolyte interfaces and suppress parasitic side reactions. Develop novel aqueous electrolytes, including superconcentrated salts and hydrated ionic liquids, tailored to 2D COF pore environments to minimize desolvation energy barriers.^206,207

(5) Exploration of novel battery systems: the distinctive attributes of 2D COFs render them promising candidates for the development of novel aqueous battery systems, including symmetric/asymmetrical all-organic batteries wherein both the cathode and anode are constructed entirely from COFs, such as hybrid acid/alkali systems that offer expanded voltage windows and enhanced energy densities.⁹² Engineering COF electrodes that maintain high electrochemical performance across both acidic and alkaline media would markedly expand their applicability. Furthermore, precise molecular-level modulation of HOMO/LUMO energies of COFs via the deliberate incorporation of electron-donating and electron-withdrawing substituents offers a promising strategy to fine-tune electrode properties in metal-free all-COF batteries.^169,208,209

In summary, the inherently “structure-programmable” nature of 2D COFs offers unprecedented opportunities to surmount existing performance limitations in aqueous batteries. Through interdisciplinary collaboration—integrating computational materials science, operando characterization, and advanced engineering techniques—these materials are poised to transition from laboratory-scale prototypes to scalable, practical energy storage solutions. Such progress will be instrumental in advancing carbon-neutral energy storage technologies, thereby contributing significantly to a sustainable energy future.

Data availability

No primary research results, software or code have been included and no new data were generated or analyzed as part of this review.

Author contributions

Xiaoli Yan, Rong Xing and Heng-Guo Wang conceived the original idea and outlined the manuscript structure. Xiaoli Yan wrote the original draft and plotted the figures. Manrong Li, Lu Zhang and Tianxue Wan plotted the figures. Yingna Chang, Jindi Wang, Kefan Song, Yu Liu, Yuzhen Sun, Huayu Wu, Rong Xing and Heng-Guo Wang edited and reviewed the article prior to submission.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for financial support from the Jiangsu Natural Science Foundation Youth Fund Project (No. BK20220700), Jiangsu Province Industry University Research Cooperation Project (No. BY20221063), Key R&D Plan (Social Development) Project of Yancheng Science and Technology Bureau (No. YCBE202243), Project of Natural Science Research in Colleges and Universities of Jiangsu Province (23KJB480010), and the Yancheng City Basic Research Program Project (YCBK2023055).

Notes and references

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