2D Conductive Metal–Organic Frameworks for Electrochemical Energy Application

Two-dimensional conductive metal–organic frameworks (2D c-MOFs) have attracted research attention, benefitting from their unique properties such as superior electronic conductivity, designable topologies, and well-defined catalytic/redox-active sites. These advantages enable 2D c-MOFs as promising candidates in electrochemical energy applications, including supercapacitors, batteries and electrocatalysts. This mini-review mainly highlights recent advancements of 2D c-MOFs in the utilization for electrochemical energy storage, as well as the forward-looking perspective on the future prospects of 2D c-MOFs in the field of electrochemical energy.Table of content:1 Introduction2 Design Principles of 2D c-MOFs3 Synthesis of 2D c-MOFs4 2D c-MOFs for Electrochemical Energy Storage4.1 Supercapacitors4.2 Metallic Batteries4.2.1 Lithium-Ion Batteries4.2.2 Sodium-Ion Batteries4.2.3 Zinc-Ion Batteries4.2.4 Sodium–Iodine Batteries4.2.5 Lithium–Sulfur Batteries4.2.6 Potassium-Ion Batteries5 2D c-MOFs for Electrochemical Energy Conversion6 Conclusions and Outlook

2 Design Principles of 2D c-MOFs 2D c-MOFs possess strong in-plane conjugation and weak out-of-plane π-π stacking, which is constructed between transition metal ions and organic linkers.The organic monomers typically possess ortho-substituted sites (e.g., -OH, -NH 2 , -SH, or -SeH) that exhibit high conjugation, while the chelating nodes prefer transition metals ions (e.g., Cu, Ni, Co, Fe, and Pd).The linkages between the metal ions and organic linkers are primarily formed in three types: metal-bis(dioxolane) (termed as MO 4 ), metal-bis(diamine) (MN 4 ), and metal-bis(dithiolene) (MS 4 ).The strong dπ-pπ orbital hybridization between metallic nodes and π-conjugated organic linkers in 2D c-MOFs promotes efficient electron delocalization and charge transport within the plane.Furthermore, the rigid conjugated organic linkers facilitate strong interlayer π-π stacking interaction and extended electron conjugation of the 2D plane.Presently, the geometric and topological structures of 2D c-MOFs can be flexibly tailored through the design of various linkers, resulting in three primary network types: triangle, square and hexagonal lattices.Among the reported 2D c-MOFs, most of the symmetric organic ligands are limited to certain geometries with C 6 , C 3 and D 2 .For instance, aromatic nuclei with C 6 or C 3 symmetry represented by benzene, triphenylene and coronene analogues tend to form honeycomb lattice structures when coordinated with metal ions.Notably, the resulting pore diameter of the network closely correlates with the geometry size of the organic linkers.2][43] Additionally, phthalocyanine [44][45][46] and dibenzo[g,p]chrysene 33 cores with D 2 symmetry, as well as phenanthrotriphenylene-based ligands 47 with D 2h symme-try, typically adopt tetragonal or rhombic lattices.Despite the promising advancements in 2D c-MOFs, the reported organic linkers are limited in the mentioned types featuring various ortho-substituted sites.Conversely, while traditional planar ligands often yield planar structures, the challenge of synthesis and purification arises from the poor solubility of larger π-conjugated planar molecules.In response, recently a novel strategy for synthesizing 2D c-MOFs using a series of flexible non-planar π-conjugated planar ligands has been proposed, including nonplanar octahydroxyl tetraphenylbenzene-based derivatives, 48,49 metallosalphen, 10 and biscarbazole ligand, effectively addressing the solubility issue associated with larger π-conjugated molecules.For example, through in situ Scholl reactions, fully conjugated frameworks were formed during the oxidative cyclodehydrogenation process of non-planar ligands. 48Furthermore, 2D c-MOFs could be assembled from non-planar Salphen ligands via metal coordination-induced planarization within the N 2 O 2 pocket. 10These ligands present a novel avenue for constructing innovative 2D c-MOFs by broadening the spectrum of available ligand species (Figure 2).
The stacking modes of layered 2D c-MOFs can significantly impact the band structures, electronic properties, and pore geometries.Specifically, AA eclipsed and AA inclined parallel stacking modes of layered 2D c-MOFs yield extended 1D pore channels, while staggered AB mode produces narrower 1D channels.Furthermore, the majority of 2D c-MOFs exhibit a preference for AA-eclipsed rather than staggered AB mode due to the π-π stacking of conjugated ligands between adjacent layers.
3 Synthesis of 2D c-MOFs 2D c-MOFs can be synthesized using either a bottom-up approach or a top-down exfoliation method.Generally, the bottom-up strategy refers to the direct synthesis of 2D c-MOFs through the coordination of metal ions and organic ligands.This approach offers the merit of low-cost and adjustability in obtaining desired products by selecting appropriate metal sources and organic ligands.Common bottomup methods include solvothermal synthesis, interface-assisted synthesis, on-surface synthesis, and surfactant-assisted synthesis, which usually produce 2D c-MOFs with diverse morphologies, such as bulk powders and films (Figure 3).
In recent years, wet-interface-assisted synthesis strategies, including liquid-liquid, liquid-air, and solid-air interfacial synthesis, have emerged as promising approaches for the fabrication of 2D c-MOF thin films.A notable advantage of these strategies is the ability to precisely control the thickness and number of layers in the resulting thin films by controlling the reaction time.Importantly, 2D c-MOFs in thin-film morphologies offer an increased surface area, exposing a greater number of active sites, and exhibit enhanced conductivity compared to their bulk powder counterparts, which are beneficial to the utilization for electrical devices.
In a general top-down strategy, 2D c-MOFs are exfoliated into ultrathin nanosheets (NSs), which exhibit intrinsic conductivity, porosity, and abundant exposed active sites.The most commonly employed methods for achieving this exfoliation are sonication or ball-milling.Initially, 2D c-MOFs are synthesized in bulk samples through a hydro/thermal method, followed by the exfoliation process using sonication or ball-milling exfoliation to obtain thin NSs.The effectiveness of this exfoliation process depends on the disruption of the weak interactions between adjacent layers in the 2D c-MOFs, resulting in the formation of single or multiple layers of 2D c-MOFs.Notably, Feng and co-workers successfully prepared Cu-HHB (HHB = hexahydroxybenzene) NSs employing a surfactant-assisted solution strategy. 50In contrast to the top-down exfoliation methods, the NSs were achieved through a one-pot way, where the organic ligands, metal sources, and surfactant were ultrasonicated together in solution.Despite the fascinating potential of the topdown exfoliation approach for obtaining 2D c-MOF NSs, precise control over the homogeneous lateral size and thickness remains a challenge.

2D c-MOFs for Electrochemical Energy Storage 4.1 Supercapacitors
Supercapacitors represent a promising class of electrochemical energy storage devices that feature high-power density, ultrafast charging/discharging process, and long service life. 51 In 2018, Baoʼs group developed two highly dense 2D c-MOFs, 53 namely Ni-HAB and Cu-HAB, by utilizing small sized hexaaminobenzene (HAB) as the organic ligand (Figure 4d-i).These HAB-based MOFs exhibited exceptional chemical stability in aqueous environments, making them suitable for use in aqueous electrolytes.Besides, both Ni-HAB and Cu-HAB demonstrated nearly identical cyclic voltammetry (CV) profiles, indicating a highly reversible electrochemical process characteristic of pseudo-capacitive materials.Notably, Ni-HAB, in submillimeter-thick pellets, achieved a volumetric capacitance of 760 F • cm −3 at a scan rate of 0.2 mV • s −1 for a 50-µm thick pellet, along with a gravimetric capacitance exceeding 400 F • g −1 for a 190-µm thick pellet.Moreover, by utilizing the small particle size of HAB MOFs, even with an increased pellet thickness of 360 µm, the areal capacitance still surpassed 20F • cm −2 .Moreover, the Ni-HAB electrode exhibited excellent cycling stability, with 90 % of the capacitance retained after 12,000 galvanostatic charge/discharge cycles at a current density of 10 A • g −1 .This study not only emphasizes the significance of ligand selection in the design of 2D c-MOFs, but also demonstrates the potential of 2D c-MOFs in the field of miniaturized capacitive energy storage.
In 2020, our group reported a conjugated copper(II) catecholate-based MOF (Cu-DBC, DBC: dibenzo[g,p]chrysene-2,3,6,7,10,11,14,15-octaol) with a high electrical conductivity of approximately 1.0 S • m −1 at room temperature. 33Attributed to its high conductivity and the excellent redox reversibility of both the ligand and copper centers, Cu-DBC exhibited superior capacitor performances with a gravimetric capacitance of up to 479 F • g −1 at a discharge rate of 0.2 A • g −1 (Figure 5a-f).In addition, the symmetric solid-state supercapacitor of Cu-DBC demonstrated a high areal capacitance of 879 mF • cm −2 , a volumetric capacitance of 22 F • cm −3 , a good energy density of 13.8 Wh • kg −1 , and remarkable cycling stability.Notably, the synergistic effect of both pseudo-capacitance and EDLC charge-storage mechanism contributed to the superior performance of Cu-DBC compared to most reported MOF-based supercapacitors, highlighting the potential applications of redox-active c-MOFs in energy storage.
Recently, our group has synthesized a novel 2D c-MOF (Cu-TBC) via coordinating hexahydroxyl tribenzocoronene (6OH-TBC) with Cu 2+ (Figure 5g-k). 54Cu-TBC displayed a moderate electrical conductivity of approximately 0.068 S • m −1 .Remarkably, Cu-TBC maintained its crystallinity even after immersion in 1 M H 2 SO 4 for 3 days, showcasing excellent chemical stability under acidic conditions, which is uncommon among 2D c-MOFs.The large conjugation of the ligand in Cu-TBC facilitated enhanced in-plane charge transport and out-of-plane π-π interactions, promoting efficient electron flow throughout the framework.On account of its moderate electrical conductivity, regular channels, and abundant redox active sites, the Cu-TBC-based electrode demonstrated an ultra-high gravimetric capacitance of 474.8 F • g −1 at a current density of 0.2 A • g −1 in 0.1 M H 2 SO 4 .Moreover, when Cu-TBC was utilized in an asymmetric supercapacitor configuration, an energy density of 18.89 Wh • kg −1 at a power density of 0.15 kW • kg −1 was achieved, leveraging both electric double layer charge-storage and pseudo-capacitance mechanisms.Notably, the energy density reached 7.4 Wh • kg −1 when the power density was increased to 6 kW • kg −1 , while maintaining good cycling stability, with approximately 83 % of the initial capacitance maintained after 5000 cycles at a large current density of 5 A • g −1 .Overall, this work not only expands the family of acid-stable 2D c-MOFs but also presents a promising avenue for utilizing 2D c-MOFs as ideal electrode materials in supercapacitors and other energy storage applications.
Apart from the traditional bulk 2D c-MOFs in the form of powder pellets, there is growing interest in exploring alternative morphologies such as crystalline nanowire arrays (NWAs) and NSs for energy storage devices.For example, Xu et al. successfully synthesized the conductive MOF Cu-CAT in the form of NWAs on carbon fiber paper, 55 which was employed as the sole electrode material for solid-state supercapacitors for the first time (Figure 6a-c).The Cu-CAT NWA electrode exhibited superior performance compared to its powder electrode, which could be attributed to the lower intrinsic resistance and charge transfer resistance at the electrode/electrolyte interface of the NWA electrode.This facilitated effective charge and electron transport at the interface of the NWA and electrolyte.Furthermore, the Cu-CAT NWA-based solid-state supercapacitor achieved a remarkably high surface area-normalized capacitance of approximately 22 µF • cm −2 , comparable to those of most carbon materials.This work not only presents a promising avenue for utilizing 2D c-MOFs as ideal electrode materials in supercapacitors but also underscores the significance of engineering the morphology of MOFs from irregularly shaped crystallites to NWAs to enhance electrochemical performance.
In  ing a NaCl-assisted ball milling method (Figure 6d-f). 56The resulting Ni 2 [CuPc(NH) 8 ] NSs exhibited a conductivity of 0.01 S • m −1 and demonstrated a p-type semiconducting behavior with a mobility of approximately 1.5 cm 2 • V −1 • s −1 at room temperature.The exceptional properties of intrinsic conductivity, porosity, high crystallinity, and ultrahigh active site utilization make the Ni 2 [CuPc(NH) 8 ] NSs highly suitable for the fabrication of micro-supercapacitors (MSCs).To construct flexible MSCs, the Ni 2 [CuPc(NH) 8 ] NSs were combined with electrochemically exfoliated graphene (EG) NSs as electrodes.The optimized weight ratio of graphene to Ni 2 [CuPc(NH) 8 ] was determined to be 2, denoted as Ni 2 [CuPc(NH) 8 ]/EG-2.This configuration yielded a remarkable areal capacitance of 18.9 mF • cm −2 at a current density of 0.04 mA • cm −2 , along with excellent cycling stability, retaining 91.4 % of initial capacitance after 5000 charge/discharge cycles.Moreover, the fabricated MSCs exhibited an ultrahigh areal power density of 168 mW • cm −2 at 1.1 µWh • cm −2 and an areal energy density of 1.7 µWh • cm −2 at 16 mW • cm −2 , comparable to those of carbon-based MSCs.Notably, the Ni 2 [CuPc(NH) 8 ]/EG-2-based MSCs maintained 86.2 % of their initial performance even after 3000 cycles, even when subjected to alternating flat and bent states.This observation underscores the superior mechanical flexibility and electrochemical stability of the device.Overall, this work presents a promising avenue for the top-down exfoliation of 2D c-MOF NSs, highlighting their immense potential, and demonstrates their successful integration into flexible electronic devices.
Generally, the EDLC is typically governed by a physical mechanism that relies on charge storage from high SSAs, with minimal involvement of the redox processes.Consequently, this mechanism tends to result in limited capacitance and low-energy density.In contrast, PCs store charge through reversible redox reactions, which can be categorized as either surface or intercalation pseudo-capacitance, depending on whether the reactions occur at the surface or within the bulk.As a result, the PCsʼ mechanism can offer higher capacitance compared to EDLCs.Despite the immense potential of 2D c-MOFs for applications in supercapacitors, the understanding of the charge storage mechanism in these materials remains limited.Therefore, further research is necessary to identify the factors that influence the electrochemical response and improve overall performance.
In 2020, Bao and co-workers conducted a study on a nickel-based 2D c-MOF known as NiHAB (HAB = hexaaminobenzene) to investigate its high capacitance mechanism (Figure 7a-c). 57In situ Raman spectroscopy revealed benzene-ring-related vibrations at different charge/discharge states, indicating partial reduction of the organic ligand in the NiHAB.Additionally, in situ X-ray absorption spectroscopy (XAS) confirmed that there were no changes in the Ni oxidation stage during the charge/discharge process at any of the tested potentials.This led to the conclusion that the pseudo-capacitive mechanism of the NiHAB was primarily governed by the redox reactions of the ligands.Furthermore, the NiHAB was observed to adopt a surface pseudo-   7d-g). 58Among the three Ni-MOFs, Ni 3 (HITP) 2 was chosen as an example, and samples with SSAs of 556, 641 and 732 m 2 • g −1 were obtained through different synthetic procedures, where products with the highest SSA exhibited enhanced crystallinity.The gravimetric capacitances of Ni 3 (HITP) 2 with SSAs of 556, 641 and 732 m 2 • g −1 were 58, 70 and 76 F • g −1 , respectively, with an enlarged voltage window from about 2.1 to 2.8 V.This confirmed that larger SSAs and improved crystallinity were favorable for increasing gravimetric capacitance and working voltage.Furthermore, the MOF-based supercapacitor exhibited a smaller equivalent series resistance than a similarly structured cell with commercial active carbon, indicating that the crystal structures and aligned channels of MOFs contributed to faster charge/ion transport than the amorphous topology of porous carbons.Overall, the experimental results were consistent with the modeling data, demonstrating that molecular simulations can accurately represent real MOF-based supercapacitor systems.This provides valuable guidelines for designing 2D c-MOF-based supercapacitors from a molecular perspective.

Lithium-Ion Batteries
Lithium-ion batteries (LIBs) have gained widespread application in various energy storage devices owing to their high energy density and stable long-term cycling performance. 59owever, there are several key remaining challenges, including low specific capacity, poor rate performance, and volume expansion.The emergence of 2D c-MOFs offers a promising avenue to efficiently address these issues.2D c-MOFs possess rigid and expanded π-conjugated networks, abundant redox active sites, and well-defined channels, which contribute to reduced solubility in electrolytes and enhanced charge transport compared to inorganic/organic small-molecule electrode materials.2][63][64]65 In 2018, Nishiharaʼs group reported a metallically conductive bis(diimino)nickel framework (NiDI) which exhibited multiple redox states due to charge distribution between ligands and metal ions (Figure 8a-e). 60Benefitting from its excellent electric conductivity and unique redox property, NiDI was employed as a cathode material for LIBs and demonstrated excellent electrochemical properties.Specifically, NiDI exhibited a high specific capacity of 155 mA h • g −1 and an energy density of 434 Wh • kg −1 at a current density of 10 mA • g −1 , along with stable performance of 250 mA • g −1 up to 300 cycles, which were comparable to those of other commercially used cathode materials in LIBs.The microscopic energy storage mechanism of NiDI was investigated using a combination of electrochemical spectral techniques and density-function calculations, which involved the insertion/desertion of both the cation (Li + ) and anion (PF 6 − ) as electron carriers in the charge/discharge process.This work not only demonstrates the promising potential of NiDI for LIBs, but also lays the foundation for further exploration of 2D c-MOFs in electrochemical energy storage.In 2020, Zhang and colleagues presented a groundbreaking study on an intrinsically electron-conductive 2D Cu-BHT MOF, composed of Cu(II) ions and a benzenehexathiolate (BHT) linker, exhibiting an impressive electronic conductivity of 231 S • cm −1 at room temperature (Figure 8f-l). 61The unique structure of this MOF is characterized by a-Cu-S-kagome lattice framework, formed through the coordination between BHT connecters and Cu(II) ions, resulting in an exceptionally dense delocalized charge configuration.Furthermore, the strong 2D-chelating effect between the small BHT linkers and Cu(II) ions, facilitated by overlapping d-π orbital interactions, contributed to the remarkable chemical stability of Cu-BHT in both aqueous and organic media.Additionally, Cu-BHT exhibited exceptional thermal stability, withstanding temperatures up to 456 °C, which enabled its application in a wide range of chemical environment, including varying pH range and sophisticated electrolyte composition at elevated temperatures.Motivated by the aforementioned advantages, the utilization of Cu-BHT as a cathode material in LIBs was investigated to assess its lithium storage performance.Remarkably, the Cu-BHT cathode exhibited an impressive specific capacity of 232 mAh • g −1 at a current density of 50 mA • g −1 , coupled with an extended cycling life of up to 500 cycles and a low capacity decay rate of 0.048 % per cycle at a high current density of 300 mA • g −1 .Furthermore, the Cu-BHT cathode demonstrated a negligible selfdischarge rate of 0.00 025 V • h −1 and exceptional high-capacity retention of 98.1 % after a 10-day shelving time, confirming its outstanding electrode stability.Electrochemical characterization and density functional theory (DFT) theoretical analysis revealed that each Cu-BHT unit could accommodate up to 4 lithium ions within the potential range of 1.5 -3.0 V (vs Li + /Li).This study presents a practical approach for the development of high-power energy storage materials, offering promising prospects for advanced battery technologies.
In 2020, our collaborators and we 62 successfully developed a redox-active 2D copper-benzoquinoid MOF (Cu-THQ) by a simple solvothermal method, which was subsequently employed as the cathode material for LIBs (Figure 9).The 2D Cu-THQ MOF exhibited remarkable electrochemical performance attributed to its porous structures and abundant redox characters.It demonstrated a reversible capacity of up to 387 mA h • g −1 , along with a high specific energy density of 775 Wh • kg −1 and excellent cycling stability, retaining 340 mA h • g −1 after 100 cycles.Notably, the 2D Cu-THQ MOF also exhibited superior rate performance, delivering a high capacity of 310.5 mA h • g −1 at a higher current density of 50 mA • g −1 .The lithium storage mechanism of the 2D Cu-THQ MOF was comprehensively investigated using various spectroscopic techniques, including hard XAS, soft XAS, electron paramagnetic resonance (EPR), and Fourier transformed infrared (FTIR) spectra.The results revealed a threeelectron redox reaction per coordination unit and a one-electron redox reaction per copper ion mechanism, accompanied by the insertion/extraction of PF 6 − and Li + in the MOFʼs skeleton, involving both metal ions and ligands in the redox process.This comprehensive study provides insights into the underlying electrochemical Li + embedding mechanism in MOF-based cathodes, paving the way for the rational design and development of 2D c-MOFs for energy storage devices.
Recently, our group has designed a new pyrazine-based 2D c-MOF (TPQG-Cu-MOF), which was applied as the cathode in LIBs (Figure 10). 63Benefitting from the excellent conductivity (about 2.39 × 10 −4 S • m −1 at room temperature), large surface area (692 m 2 • g −1 ), as well as the abundant redox active sites, TPQG-Cu-MOF showed great potential for application as a cathode material for LIBs.Notably, TPQG-Cu-MOF showed a reversible specific capacity of   Recently, Bu and co-workers have developed a 2D c-MOF (Cu-HATN) with dual redox-active sites by incorporating both redox-active functional units hexaazatriphenylene (HATN) and CuO 4 units in the skeleton (Figure 11a-c). 64Cu-HATN exhibited a large surface area of 418 m 2 • g −1 and an appropriate pore diameter of ca.1.5 nm, which facilitated the exposure of redox-active sites and the facile transport of Li + ions, thereby enhancing the performance of batteries.Owing to its ordered channels, semiconducting feature, and the maximum number of exposed redox-active centers, Cu-HATN was applied as the anode material for LIBs and demonstrated exceptional Li storage performance.Notably, the Cu-HATN-based electrodes exhibited an impressive specific capacity of 763 mAh • g −1 at 300 mA • g −1 , coupled with longterm cycling stability (≈ 90 % capacity retention after 600 cycles with a Coulombic efficiency of ≈ 100 % at 300 mA • g −1 ), surpassing the performance of most recently reported 2D c-MOF-based electrodes.Furthermore, the Li storage mechanism of the Cu-HATN electrode was elucidated through the combination of diverse analytic techniques and density-functional calculations.This work provides a new perspective for the design of novel electrodes in LIBs through integrating multiple redox-active moieties into 2D c-MOFs.
The preparation of 2D c-MOFs in a bulk powder form via conventional solvothermal synthesis often results in the burial of a significant number of active sites, leading to hindered ion diffusion and low utilization of these sites.To overcome this limitation, the development of ultrathin NSs with higher surface areas and accessible active sites is highly desirable, as they offer shorter ion/electron migration length and the potential for enhanced electrochemical performance.In a recent study by Feng et al., a series of ultrathin 2D c-MOF NSs, including HHB-Cu (HHB = hexahydroxybenzene), HHB-Ni and HHTP-Cu (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene), were synthesized using a surfactant-assisted solution method (Figure 11d-h). 50Notably, HHB-Cu (Ni) NSs exhibited a thickness of 4 -5 nm (~8 -10 layers) and a lateral size of 0.25 -0.65 µm 2 , while single-crystalline HHTP-Cu NSs had a thickness of ~5.1 ± 2.6 nm (~10 layers) and a lateral size of 0.002 -0.02 µm 2 .Remarkably, HHB-Cu NSs demonstrated a Brunauer-Emmett-Teller (BET) surface area of 385 m 2 • g −1 , significantly higher than those of corresponding bulk samples (119 m 2 • g −1 ), highlighting the importance of increased surface area between layers.Motivated by the ultrathin nature and abundance of exposed redox active sites, HHB-Cu NSs were employed as cathode materials for LIBs.Specifically, the HHB-Cu NS electrode exhibited a specific capacity of 153 mAh • g −1 within the potential window of 1.3 -2.6 V, along with excellent rate capability (81 % charge within 3 min, 81 %, 1.0 A • g −1 vs. 0.1 A • g −1 ) and long-term cycling stability (90 % capacity retention after 1000 cycles), surpassing the performance of most reported MOF-based cathodes.In comparison, the bulk HHB-Cu exhibited a much lower capacity of 40 -50 mAh • g −1 and a significantly higher charge transfer resistance, further confirming the contribution of the ultrathin nature of HHB-Cu NSs to faster ion/electron kinetics and higher utilization efficiency of redox sites.Overall, this work not only presents a surfactant-assisted strategy for the development of crystalline 2D c-MOF NSs but also demonstrates the promising potential of 2D c-MOFs in electrochemical energy storage applications.

Sodium-Ion Batteries
In the realm of electrochemical energy storage, LIBs have gained significant popularity in commercial market.However, the scarcity of lithium resources on Earth poses a notable challenge.In light of this, sodium-ion batteries (SIBs) have emerged as a promising alternative due to the abundance of sodium and its low electrochemical potential (−2.71 V vs. standard hydrogen electrode (SHE)). 66,67It is important to note that the sodium ion possesses a larger radius and molar mass compared to the lithium ion, necessitating the exploration of suitable electrode materials distinct from those employed in LIBs.The emergence of 2D c-MOFs produces a promising way for the development of SIB electrodes with exceptional electrochemical performance.
In 2018, Baoʼ s group reported a 2D c-MOF named Co-HAB, comprising a redox-active HAB and Co(II), marking its pioneering application as a high-power SIB electrode material (Figure 12a-c). 68The selection of HAB as the organic linker was based on its ability to undergo six-electron redox reactions, forming a densely packed assembly of six amine groups in its fully oxidized state.Consequently, HAB exhib-ited a high density of redox centers, aligning with the design principle for energy-dense electrode materials.The choice of metal source also significantly influenced the performance of 2D c-MOFs.Previous studies predominantly employed Cu (II) and Ni(II) as the primary metal nodes due to their favorable D 4 h coordination symmetry when interacting with strong field ligands.When connected to a hexadentate ligand with D 3 h symmetry, the D 4 h coordination tended to yield a (2,3)-connected honeycomb 2D lattice, resulting in a highly conjugated system facilitated by effective π-orbital overlap between ligands and metal nodes.However, limited by the less preferred D 4 h coordination symmetry, there have been limited research studies focusing on cobalt-based 2D c-MOFs, which attracted interest to study their potential as anode materials, due to the weaker electron affinity than Cu(II) and Ni(II).Notably, to obtain pure Co-HAB without Co(OH) 2 impurities, HAB and Co(II) salts were initially mixed, followed by the addition of NH 4 OH.Through synthetic optimization, the highly crystalline Co-HAB with a high bulk conductivity of 1.57 S • cm −1 was obtained.Co-HAB electrodes demonstrated a remarkable specific capacity as high as 291 mAh • g −1 at 50 mA • g −1 , with a capacity retention of 226 mAh • g −1 at 500 mA • g −1 and a Coulombic efficiency approaching 100 % after more than 50 cycles.Furthermore, Co-HAB exhibited an impressive rate capability, delivering a specific capacity of 214 mAh • g −1 within 7 min or 152 mAh • g −1 in 45 s.These exceptional electrochemical performances provided evidence for a reversible three-electron redox reaction per HAB, enabling the storage of three electrons and Na + per HAB in organic electrolytes.This discovery highlights the promising potential of Co-HAB as a new electrode material for NIBs.The Co-HAB represents the first conductive MOF applied in sodium storage, thereby stimulating further investigations into cobalt-based 2D c-MOFs and their application prospects in energy storage devices.
In 2021, Wangʼs group reported two analogous 2D c-MOFs (Zn-HHTP and Cu-HHTP) employed in SIBs, and investigated the relationships between electrochemical performance and structures (Figure 12d-j). 69Zn-HHTP exhibited a high reversible capacity of ≈ 150 mAh • g −1 at 100 mA • g −1 and a superior capacity retention of 90 % after 1000 cycles.Moreover, Zn-HHTP could maintain 70 % of the initial capacity at a high current density of 1000 mA • g −1 even after 5000 cycles.The combination of systematical characterizations and theoretical calculations revealed that the redox reactions mainly occurred at the ligands for Zn-HHTP.On the contrary, Cu-HHTP possessed the redox process of both ligands and metal sources, which involved the storage of only Na + -cations, leading to poorer cyclability.Notably, benefitting from the successive storage of cations and anions by the redox reaction of only ligands, Zn-HHTP showed much higher cyclability than Cu-HHTP.

Zinc-Ion Batteries
Rechargeable aqueous zinc-ion batteries (ZIBs) have been identified as a highly desirable energy storage system due to their utilization of low-cost zincs, safe water-based electrolytes, low toxicity, high theoretical capacity (820 mAh • g −1 ), and low redox potential (−0.76 V vs. SHE). 70,71In contrast to the general organic electrolytes used in LIBs or NIBs, the utilization of aqueous electrolytes in ZIBs has been shown to achieve high ionic conductivity up to 1 S • cm −1 , while non-aqueous electrolytes limit the conductivities of ~10 −10 • cm −1 .This allows for more efficient ion/ charge transport, contributing to higher power density and energy density.2D c-MOFs, in comparison to traditional inorganic/organic-based electrodes, possess highly desirable porous structures and regular channels, remarkable electrical conductivities, and insolubility in common organic solvents.These characteristics benefit the ionsʼ transport and imbedding, demonstrating the promising potential of 2D c-MOFs as cathodes for ZIBs.
In 2019, Stoddart and colleagues conducted a pioneering study on the implementation of a 2D c-MOF (Cu 3 (HHTP) 2 ) as cathode materials in aqueous ZIBs (Figure 13a-e). 72The Cu 3 (HHTP) 2 cathode demonstrated a high reversible capacity of 228 mAh • g −1 at a current density of 50 mA • g −1 , along with excellent cycling stability, retaining 75 % of its capacity after 100 cycles at 500 mA • g −1 .Even at a higher current density of 4000 mA • g −1 , the Cu 3 (HHTP) 2 electrodes maintained 75 % of their initial capacity after 500 cycles, indicating their structural stability.The diffusion coefficient of Zn 2+ ions within Cu 3 (HHTP) 2 was calculated to be 1.2 × 10 −9 cm 2 • s −1 , confirming fast redox reactions.Electrochemical impedance spectroscopy measurement revealed interfacial resistance of 150 and 16,000 Ω cm 2 in aqueous and organic electrolytes, respectively, along with conductivities of 0.7 × 10 −2 and 0.6 × 10 −5 S • cm −1 .The presence of H 2 O was found to decrease the desolvation energy and Coulombic repulsion at the interface, resulting in lower interfacial resistance upon the insertion of hydrated Zn 2+ ions.Consequently, the combination of high Zn 2+ ion diffusion and low interfacial resistance contributed to the superior rate performance of Cu 3 (HHTP) 2 .XPS measurements and DFT calculations demonstrated that both the quinoid component of the HHTP ligand and copper ions served as the redox-active sites during the discharge and charge process, enhancing the specific capacity of ZIBs.In addition, reversible redox peaks observed in CV profiles indicated an intercalation pseudo-capacitance charge-storage mechanism for Cu 3 (HHTP) 2 , suggesting a capacitive-dominated kinetics.Notably, Cu 3 (HHTP) 2 exhibited a low selfdischarge rate of 0.003 V • h −1 and retained 83 % of its initial capacity after 5 days of storage, confirming its excellent stability.This work provides valuable insights into the utilization of 2D c-MOFs as cathodes for MOF-based ZIBs.
In 2023, Wong and Song successfully fabricated a heterostructure known as MOF/MX by alternately stacking a 2D c-MOF (Cu-HHTP) and a modified V 2 CT x MXene (Figure 13f-i). 73This composite material was employed as a cathode in ZIBs.The composite of MOF/MX exhibited the advantages of both 2D c-MOFs and MXene, which not only enhanced the conductivity for effective charge transport, but also prevented the aggregation of Cu-HHTP NSs.Consequently, the Cu-HHTP/MX exhibited a larger BET surface area and pore volume compared to Cu-HHTP alone, providing more active sites and an increased effective contact area for Zn 2+ transport.The EPR spectrum revealed that Cu-HHTP/ MX showed a similar g-value but with improved intensity compared to Cu-HHTP, indicating a higher concentration of oxygen vacancies.This oxygen vacancy enhancement facilitated fast Zn The excellent electrochemical performance of Cu-HHTP/MX could be attributed to its multilayered structure, which provided enhanced stability and faster charge transfer kinetics.Through ex situ electrochemical characterizations and theoretical calculations, a reversible intercalation mechanism dominated by capacitance-controlled behavior was revealed, elucidating the kinetics of Zn 2+ insertion and extraction.The lower charge transfer resistance, higher Zn 2+ diffusion coefficient, and low ohmic resistance of Cu-HHTP/MX further confirmed its superior electrical conductivity, ensuring outstanding electrochemical performance.This work expands the range of 2D c-MOF composites and advances the development of MOF-derived materials for high-performance ZIBs.
Recently, our group has developed an anthraquinonebased 2D c-MOF (Cu-TBPQ MOF) with integrated redox-active sites, serving as the cathode material for ZIBs. 74The incorporation of abundant redox-active quinone moieties and [CuO 4 ] coordination nodes in the Cu-TBPQ MOF facilitated remarkable redox behaviors, positioning it as a promising cathode material for ZIBs.Notably, the Cu-TBPQ MOF cathode exhibited a superior reversible specific capacity of 371.2 mAh • g −1 at a current density of 50 mA • g −1 .It also demonstrated exceptional rate performance, maintaining a capacity of 311 mAh • g −1 upon returning the current density to 50 mA • g −1 .Additionally, the Cu-TBPQ MOF cathode also showed impressive cycling stability, retaining a capacity of 120.3 mAh • g −1 at a high current density of 2.0 A • g −1 after 500 charge/discharge cycles.Comprehensive spectroscopic analyses indicated that the catechol and quinone sites served as redox active sites, contributing to the Zn 2+ storage in the Cu-TBPQ MOF cathode.Our study presents an efficient strategy to designing redox-active 2D c-MOFs by incorporating multiple active moieties into the framework, thereby establishing a foundation for the MOF-based materials in energy storage fields.

Sodium-Iodine Batteries
The abundance of iodine and sodium on Earth has led to the development of sodium-iodine (Na-I 2 ) batteries as an alternative energy storage device to LIBs. 75However, Na-I 2 batteries involve a major redox reaction including I 2 ↔ NaI 3 ↔ NaI, where the polyiodide intermediate (NaI 3 ) tends to dissolve in the electrolyte, resulting in low capacity and hindering the application of Na-I 2 batteries. 76To address this issue, various porous carbon-based materials have been utilized as physical barriers to suppress polyiodide dissolution.However, the diffusion of polyiodides could not be completely restrained due to the poor affinity between nonpolar carbon materials and polar polyiodides.Fortunately, the emergence of 2D c-MOFs provides a feasible solution to this problem.Due to their extended π-conjugation, superior electrical conductivities, strong polarity of dense metal centers, and porous nanostructures, 2D c-MOFs present an appealing potential as an I 2 cathode for Na-I 2 batteries.
In 2020, Feng et al. demonstrated the use of a conjugated polyphthalocyanine copper MOF (Fe 2 -O 8 -PcCu) composited with I 2 serving as cathode material for a Na-I 2 battery (Figure 14). 27The Fe 2 -O 8 -PcCu/I 2 electrode exhibited a high specific capacity of 208 mA • g −1 at 0.3 A • g −1 , as well as excellent rate capability up to 2.5 A • g −1 .Moreover, a stable specific capacity of 150 mAh • g −1 was maintained even after 3200 cycles, surpassing the state-of-the-art electrodes for metal-I 2 batteries.Operando spectroeletrochemical measurements, electrochemical kinetics analyses, and density functional theory calculations revealed that the square planar Fe-O 4 centers in the Fe 2 -O 8 -PcCu were responsible for constraining polyiodide dissolution, thus contributing to the remarkable behavior in Na-I 2 batteries.Furthermore, the Fe 2 -O 8 -PcCu/I 2 electrode could be extended to other metal-I 2 batteries.When integrated into Zn-I 2 batteries in the aqueous electrolyte, the Fe 2 -O 8 -PcCu/I 2 electrode still exhibited a high discharge capacity of 124 mAh • g −1 even at an extremely high current density of 6.8 A • g −1 .Overall, this work presents a new way for designing 2D c-MOFs for high-performance metal-iodine batteries.

Lithium-Sulfur Batteries
Lithium-sulfur batteries (Li-S batteries) have garnered significant attention as a promising energy storage system due to the high theoretical capacity of sulfur (1675 mAh • g −1 ), impressive energy density (2600 Wh • kg −1 ), and abundance and cost effectiveness of sulfur. 77However, several challenges impede their further development.One major issue is the shuttling of polysulfide intermediates (Li 2 S n , 4 ≤ n ≤ 8) during the reversible redox reaction between lithium and sulfide, which leads to capacity fading and low Coulombic efficiency. 78While carbonaceous materials have been employed as sulfur hosts in Li-S batteries, their nonpolar sur-faces fail to provide sufficient affinity with polar polysulfides, resulting in incomplete dissolution.Conversely, most polar materials exhibit lower electrical conductivity compared to carbon materials, leading to suboptimal electrical behaviors.In this context, 2D c-MOFs hold great potential as sulfur host media due to their unique advantages, including high SSAs, porous structures with regular channels, abundant active sites, and excellent electrical conductivities.Notably, the high porosity and adjustable pore size of 2D c-MOFs enable them to accommodate significant amounts of sulfur within their pores, effectively suppressing the poly-sulfidesʼ shuttle effect and enhancing overall battery performance.][81] In 2018, Fangʼs group reported a 2D c-MOF Ni 3 (HITP) 2based large-area, crack-free and crystalline microporous membrane, and used it for the construction of Li-S batteries (Figure 15a-c). 82Ni 3 (HITP) 2 was selected due to its ultrahigh electrical conductivity (4000 S • m −1 ), honey-comb structures with regular 1D channels, and abundant polar sites, which made it an ideal candidate for binding polar polysulfides, and serving as a polysulfide barrier layer.The high-quality Ni 3 (HITP) 2 membranes, denoted as Ni 3 (HITP) 2 /PP, were grown directly on the surface of the commercial polypropylene (PP) separator, replacing the liquid-air interface with the liquid-solid interface to avoid membrane damage.The obtained Ni 3 (HITP) 2 /PP separator consisted of both conductive Ni 3 (HITP) 2 and insulating PP sides, with Ni 3 (HITP) 2 acting as a barrier layer to suppress polysulfide shuttling and the PP layer preventing short circuiting between the cathode and  anode.The thickness of the Ni 3 (HITP) 2 membrane could be tuned by varying the reaction time, ranging from 90 to 970 nm.The Ni 3 (HITP) 2 membrane showed a much higher conductivity (3720 S • m −1 ) than powder-compressed pellets (50 S • m −1 ) due to the less grain boundary.Furthermore, the Ni 3 (HITP) 2 membrane demonstrated excellent polysulfidecapturing ability and remarkable performance in suppressing polysulfide shuttling, as evidenced by the absence of polysulfide diffusion in the polysulfide permeation tests.The Li-S coin cells using Ni 3 (HITP) 2 /PP as a separator showed a high rate capability of 589 mAh • g −1 at 5 C, as well as a high capacity retention of 84.1 % and a low average capacity decay of 0.032 % after 500 cycles at 1 C. Furthermore, when the sulfur loading was increased to 8.0 mg • cm −2 and 70 wt%, a high areal capacity of 8.44 mAh • cm −2 and a capacity retention of 86 % after 200 cycles at 0.5 C were achieved.This work represents the first integration of a crystalline microporous membrane into Li-S batteries, demonstrating promising properties for suppressing polysulfide shuttling and promoting capacities, rate capabilities, and cycle stabilities.These findings will advance further development on 2D c-MOF materials for Li-S batteries.
In 2019, Hanʼs group conducted a study utilizing a highly conductive MOF material, Ni 3 (HITP) 2 , as the host for sulfur in Li-S batteries (Figure 15d-g). 83The Ni 3 (HITP) 2 material exhibited an impressive bulk electrical conductivity of approximately 198 S • m −1 , surpassing that of activated carbon.Additionally, the material possessed a high BET surface area of 628 m 2 • g −1 , indicating the presence of abundant polar sites that facilitate efficient interactions between sulfur species and Ni 3 (HITP) 2 .The pore size distribution of the material was centered at 0.8 and 2.1 nm, which served as physical barriers to impede the shuttling of polysulfides.The researchers successfully synthesized S@Ni 3 (HITP) 2 by infiltrating sulfur into Ni 3 (HITP) 2 under N 2 flowing conditions at 155 °C for 12 hours.Various characterization techniques, including scanning electron microscopy (SEM), elemental mapping, X-ray diffraction (XRD), thermogravimetry analysis, FTIR spectra, and XPS, confirmed the presence of sulfur in the S@Ni 3 (HITP) 2 composites.To construct matrix conduction networks, carbon nanotubes (CNTs) were employed instead of the traditional conductive additive acetylene black, resulting in the formation of S@Ni 3 (HITP) 2 -CNT cathodes.Notably, the S@Ni 3 (HITP) 2 -CNT cathodes delivered a high initial capacity of 1302.9 mAh • g −1 and a good capacity retention of 848.9 mAh • g −1 after 100 cycles at 0.2 C, with a coulombic efficiency exceeding 96.5 %.Moreover, excellent reversible discharge capacities of 807.4 mAh • g −1 and 629.6 mAh • g −1 were revealed at 0.5 C and 1 C after 150 and 300 cycles, respectively, demonstrating outstanding cycling stability.The synergistic effect of Ni 3 (HITP) 2 and CNTs prevented the nonuniform deposition of Li 2 S 2 /Li 2 S on the electrode surface, with CNTs providing long-range electron and ionic channels, along with Ni 3 (HITP) 2 ensuring short-range electron pathways, together enabling polysulfide shuttling inhibition.

Potassium-Ion Batteries
5][86] Compared with LIBs, PIBs benefit from abundant potassium resources, offering advantages such as cost efficiency and enhanced safety, thereby presenting promising applications in large-scale energy storage.Nonetheless, the larger ionic radius of potassium ions may pose challenges in terms of cycling stability and rate capacity of electrode materials.Consequently, the development of advanced cathode materials to enhance the performance of PIBs remains a pressing issue.In the current landscape, 2D c-MOFs have captivated researchers and emerged as a promising candidate for energy storage applications.The integration of metal node d orbitals and conjugated ligand π-orbitals in the 2D c-MOF system facilitates electron delocalization, thereby bolstering its stability.Furthermore, the coordination units within these frameworks can serve as electron acceptors or donors in the redox process.Therefore, 2D c-MOFs exhibit enormous potential as electrode materials of PIBs.
In 2024, Wangʼs group successfully synthesized a hexaazanonaphthalene (HATN)-based 2D c-MOF (Cu-HATNH) featuring dual-active sites (Figure 16). 87This material was further hybridized with CNTs to create the cathode material Cu-HATNH@CNT for PIBs for the first time. 86The incorporation of CNTs proved to be highly effective in facilitating charge transfer and enhancing the utilization of active sites, thereby significantly improving the electrochemical performance for 2D c-MOF cathode materials in PIBs.Cu-HA-TAH@CNT displayed an impressive initial capacity of 317.5 mAh • g −1 at 0.1 A • g −1 and an outstanding rate capacity of 147.1 mAh • g −1 at 10 A • g −1 .Moreover, a remarkable longtime cycling stability was also achieved with the capacity retention of 96.8 % at 5 A • g −1 even after 2200 cycles.Experimental characterization and theoretical calculations revealed that both the C=N sites of ligands and unsaturated [CuO 4 ] units played crucial roles in the multi-electron redox reactions.This study offers valuable insights into the design of 2D c-MOFs with multiple active sites serving as cathode materials in PIBs.
In addition to being used as the cathode materials in PIBs, 2D c-MOFs have shown promise as anode materials as well.A recent study by Li and Bin introduced a 2D c-MOF named HAN-Cu-MOF (HAN = hexaazatrinaphthalene), utilizing Nrich HAN units, for potassium storage as anode materials in PIBs under high temperature conditions. 88Leveraging the advantages of multiple redox-active C=N sites, CuO 4 units, and robust π-d conjugated structures, HAN-Cu-MOF exhibited significant potential as a PIB anode at elevated temper-atures.Remarkably, the HAN-Cu-MOF anode demonstrated extraordinary performance at a high working temperature of 60 °C, showcasing an impressive initial reversible specific capacity of 455 mAh • g −1 at 50 mA • g −1 and an outstanding rate capability of 161 mAh • g −1 at an ultrahigh current density of 2000 mA • g −1 .Furthermore, the anode exhibited remarkable cyclability, maintaining a capacity retention of 96.7 % at 1000 mA • g −1 even after 1600 cycles.Experimental analyses and theoretical calculations further underscored the significance of redox-active sites, particularly involving C=N groups and CuO 4 units, in facilitating high-performance potassium storage.This work provides novel insights into the design of redox-active 2D c-MOFs as anode materials for high-temperature potassium storage in PIBs.

2D c-MOFs for Electrochemical Energy Conversion
Besides their applications in electrochemical energy storage, 2D c-MOFs have demonstrated their potentials as electrocatalysts in various gas-involved reactions, such as the hydrogen evolution reaction (HER), 19,38,89 oxygen evolution reaction (OER), [90][91][92] oxygen reduction reaction (ORR), [93][94][95] and carbon dioxide reduction (CO 2 RR). 45,96,97Unlike conventional carbon catalysts, which often lack dense active sites, 2D c-MOFs offer the advantage of tunable catalytic selectivity due to their well-defined conjugated networks with adjust-able metal sources and ligands.This unique feature enables precise control over the catalytic performance of 2D c-MOFs in gas-involved reactions.
In 2015, Fengʼs group synthesized single-layer sheets of 2D c-MOFs through the Langmuir-Blodgett method (Figure 17a-c). 98These sheets were composed of nickel bis (dithiolene) complexes, denoted as THTNi (THT = triphenylenehexathiol).The resulting THTNi ultrathin sheets were approximately 0.7 -0.9 nm thick and could be uniformly dispersed on glassy carbon electrodes with an area of approximately 20 mm 2 .The large number of exposed nickel bis(dithiolene) moieties on the THTNi sheets contributed significantly to their electrocatalytic performance.Remarkably, the THTNi sheets delivered highly efficient electrocatalytic activity for the HER, with a Tafel slope of 80.5 mV • decade −1 and an overpotential of 333 mV at 10 mA • cm −2 in a 0.5 M H 2 SO 4 solution.This performance surpassed those of previously reported molecular catalysts supported by CNTs and heteroatom-doped graphene catalysts.
In 2019, Feng and Dong developed a copper phthalocyanine-based 2D c-MOF with Co-O 4 linkages, named as PcCu-O 8 -Co, which was combined with CNTs to create an electrocatalyst for the ORR (Figure 17d-f). 99The PcCu-O 8 -Co/CNTs composite demonstrated exceptional electrocatalytic performance for the ORR in alkaline media, owing to its welldefined Co-O centers with single occupied orbitals in the Co nodes, highly crystalline structure, and ordered porous framework.The composite exhibited an ORR half-wave potential (E 1/2 ) of 0.83 V vs. reversible hydrogen electrode  (RHE), an electron transfer number of (n) of 3.93, and a limiting current density (j L ) of 5.3 mA • cm −2 .Notably, even after 5000 ORR cycles, the morphology and structure of PcCu-O 8 -Co/CNT were retained, as confirmed by XRD and SEM measurements, indicating its remarkable stability during ORR tests.DFT calculations and in situ Raman spectro-electrochemistry measurement confirmed that the Co-O units served as the active sites for the ORR.Furthermore, when employed as an air electrocatalyst in primary zinc-air batteries, PcCu-O 8 -Co/CNT showed superior performance, with a discharge current density of 120 mA • cm −2 at 0.8 V and an excellent power density of 94 mW • cm −2 , surpassing commercial Pt/C systems (discharge current density of 95.5 mA • cm −2 and power density of 76.4 mW • cm −2 ).These findings indicate the feasibility of PcCu-O 8 -Co/CNT as an efficient ORR electrocatalyst and provide valuable insights for the development of 2D c-MOFs with tunable electronic structures for effective ORR electrocatalysis.
In the realm of OERs, Duʼs group reported a nickel phthalocyanine-based 2D c-MOF (NiPc-MOF) designed for electrochemical OER applications (Figure 18a-c). 92The NiPc-MOF film was generated by growing on a fluorine-doped tin oxide (FTO) substrate, with a thickness of approximately 300 nm.Notably, the NiPc-MOF film displayed a high electrical conductivity of around 0.2 S • cm −1 , a favorable attribute for OER electrocatalyst.Impressively, the NiPc-MOF film showed outstanding OER performance in 1.0 M KOH, showcasing a very low onset potential of 1.48 V, a high mass activity of 888.3A • g −1 , and a high highest turnover frequency (TOF) value of 2.5 s −1 .Moreover, the NiPc-MOF film also showed robust catalytic durability under alkaline conditions, with the catalytic potential remaining nearly constant even after prolonged testing.These results confirmed that the NiPc-MOF film was an efficient catalyst for the OER activity.
In addition, the modulation of the electronic structure of 2D c-MOFs by Fe doping is also an effective strategy for enhancing the OER performance.For example, Songʼs group investigated a series of bimetallic Fe-doped NiPc-NiFe x (x = 0.05, 0.09, 0.20) 2D c-MOFs for electrocatalytic OER (Figure 18d-f). 91 Furthermore, Cu 3 (HHTQ) 2 performed the lowest overpotential of −0.4 V, showcasing excellent energy efficiency in the CO 2 RR process.Importantly, Cu 3 (HHTQ) 2 maintained its CO 2 RR performance even after 10 hours at −0.4 V vs. RHE, indicating its remarkable catalytic durability.Through DFT calculations, a reasonable catalytic mechanism for the CO 2to-CH 3 OH conversion was proposed for Cu 3 (HHTQ) 2 , involving a six-proton/six-electron transfer process.This mechanistic understanding provides valuable insights into the role of Cu 3 (HHTQ) 2 in efficient electrochemical CO 2 RR.This work highlights the interplay between metal species and organic Moreover, the utilization of bimetallic centers in 2D c-MOFs has shown promise in enhancing the electrocatalytic activity for CO 2 RR.Notably, Feng and co-workers have recently reported a novel 2D c-MOF electrocatalyst with bimetallic centers for efficient CO 2 RR (Figure 19e-h). 100The bimetallic PcCu-O 8 -Zn/CNTs composite exhibited remarkable performance, including a high selectivity of 88 % for CO 2 -to-CO conversion, an excellent turnover frequency of 0.39 s −1 at −0.7 V vs. RHE, and exceptional long-term stability exceeding 10 hours.These achievements surpass those of previously reported MOF-based electrocatalysts.Furthermore, the introduction of different metal species and applied potentials allowed for the tunability of the molar H 2 / CO ratio, ranging from 1 : 7 to 4 : 1. Spectroscopic measurements and theoretical calculations revealed that the ZnO 4 units within the PcCu-O 8 -Zn linkages played a crucial role in enhancing the electrocatalytic CO 2 RR activity, while the CuN 4 units facilitated the efficient transfer of protons and electrons during the reaction process.Consequently, the synergistic effect of these bimetallic active sites contributed significantly to the overall CO 2 RR process.This study presents a promising avenue for the design of bimetallic 2D c-MOFs as highly efficient electrocatalysts, enabling synergistic catalysis of CO 2 RR.

Conclusions and Outlook
As a burgeoning category of crystalline coordination polymers, 2D c-MOFs have garnered significant attention due to their distinctive lamellar structures, fully conjugated networks, well-defined channels, inherent redox-active sites, and adjustable properties.These inherent characteristics establish a solid groundwork for exploring various electrochemical applications.In this review, we present an overview of the design principles, synthesis methodologies, and electrochemical storage applications of 2D c-MOFs.
It is noteworthy that the majority of reported 2D c-MOFs have thus far exhibited limited structural diversity, primarily relying on a subset of π-conjugated organic ligands, including benzene, triphenylene, trinaphthylene, truxene, coronene, phthalocyanine, porphyrin and dibenzo[g,p]chrysene with ortho-substituted groups (-OH, -NH 2 , -SH and -SeH).The larger π-conjugated planar molecules often result in the poor solubility and challenge of synthesis and purification, which has hindered the exploration of novel architectures.To address this limitation, an innovative synthetic strategy for constructing 2D c-MOFs from non-planar ligands has been devised.For example, an in situ Scholl reaction was employed to realize 2D c-MOFs from nonplanar ligand precursors, represented by the tetraphenylbenzenebased derivatives.Meanwhile, the non-planar Salphen ligands via metal coordination-induced planarization within the N 2 O 2 pocket can yield the planar counterpart.These newly developed non-planar ligands will simplify the synthesis and expand the repertoire of 2D c-MOFs with unique charge transport properties.Importantly, the structural characteristics of 2D c-MOFs can be finely tuned by adjusting both the organic ligands and metal ions employed in their synthesis.This tunability enables the tailoring of various features, including crystallinity, porosity, stability and electrical conductivity, Consequently, these advancements hold great promise for the application of 2D c-MOFs in electrochemical energy storage.
On the contrary, the majority of 2D c-MOFs are typically synthesized as bulk powders using hydro/solvothermal methods.While this approach has proven to be a straightforward and high-yielding strategy for obtaining 2D c-MOFs, the resulting products often consist of polycrystalline samples with inherent boundaries and defects.These structural characteristics impose limitations on effective charge transport and electrical conductivity, thereby hindering their potential for high-performance electrical energy storage applications.Consequently, it becomes imperative to develop high-quality thin films or ultrathin NSs of 2D c-MOFs, which not only offer larger surface areas and ample accessible active sites but also alleviate issues related to sluggish ion diffusion and low utilization of active sites.Several methods, such as ultrasonic stripping and mechanical exfoliation, have been employed for the top-down preparation of ultrathin NSs of 2D c-MOFs.However, precise control over the thickness of these NSs remains challenging.To overcome this limitation, a bottom-up strategy has been proposed, involving the adjustment of organic ligand structures, metal cluster types, and optimization of synthetic conditions.This approach enables the direct synthesis of 2D c-MOF NSs at the molecular level.Nonetheless, significant challenges persist in the construction of 2D c-MOF thin films or NSs with desirable mechanical strength, controlled thickness, and lateral size.Addressing these challenges is crucial for advancing the practical applications of 2D c-MOFs in various fields.
Benefitting from their exceptional conductivities, abundance of redox-active sites, and rigid frameworks, 2D c-MOFs have demonstrated remarkable performance in energy storage, encompassing supercapacitors, LIBs, SIBs, ZIBs, Li-S batteries, and sodium-iodine batteries.Despite these advancements, the research on energy storage utilizing 2D c-MOFs is still in its infancy, and several key challenges remained.For instance, in the case of supercapacitors, 2D c-MOFs require superior electrical conductivity to facilitate rapid charge transport.Additionally, the presence of a large SSA and ample redox sites is crucial for efficient contributions to EDLCs and PCs, respectively.The synergistic combination of these factors contributes to superior capacitor performance.In the context of LIBs/SIBs, the availability of abundant redox-active sites in 2D c-MOFs is essential.Therefore, careful selection and design of redox-active organic ligands are desirable to enhance the electrochemical performance of these batteries.Furthermore, since ZIBs employ aqueous electrolytes as a replacement for organic electrolytes, it is imperative for 2D c-MOFs to exhibit stability under aqueous conditions.Moreover, in Li-S batteries and sodium-iodine batteries, where 2D c-MOFs typically serve as host materials, achieving a large pore volume and high SSA is crucial to enable high loading of active components.Addressing these challenges and optimizing the properties of 2D c-MOFs will pave the way for their practical applications in energy storage.
In summary, we anticipate that future research will delve deeper in the charge storage mechanism of 2D c-MOFs, thereby offering valuable insights into the structure-property correlations that underpin their design.This enhanced understanding will serve as a guiding principle for the development of novel 2D c-MOFs with tailored functions.We firmly believe that 2D c-MOFs will assume a blossoming achievements and development in energy storage and conversion.
Key words: 2D conductive metal-organic frameworks, energy storage, capacitors, batteries, electrocatalysts ties of 2D c-MOFs.Additionally, it highlights recent advancements in the utilization of 2D c-MOFs for electrochemical energy storage, specifically in the context of supercapacitors and metal batteries, as well as their application in electrocatalysis for energy conversion.Furthermore, this review presents a forward-looking perspective on the future prospects of 2D c-MOFs in the field of electrochemical energy.It addresses current research gaps, challenges, and emphasizes the commitment to advancing the development of next-generation energy storage devices based on 2D c-MOFs.

Figure 2
Figure 2 The reported (a) planar and (b) non-planar organic ligands for the synthesis of 2D c-MOFs.

Figure 3
Figure 3 (a) Schematic illustration of the one-pot hydro-/solvothermal synthesis for 2D c-MOFs.Reprinted with permission from Ref. 48.Copyright 2023 American Chemical Society.(b) Schematic illustration of the layer-by-layer growth procedure of Cu 3 (HHTP) 2 film.Reprinted with permission from Ref. 22.Copyright 2020 Wiley-VCH.(c) Schematic illustration of the exfoliation of bulk 2D c-MOFs into nanosheets.Reprinted from Ref. 56 published under a creative commons license (CC BY).(d) Synthesis of 2D c-MOF nanosheets by surfactant-assisted solution method.Reprinted with permission from Ref. 50.Copyright 2020 Royal Society of Chemistry.(e) Schematic illustration of MOF-on-MOF thin films by Van der Waals forces.Reprinted with permission from Ref. 12.Copyright 2019 Wiley-VCH.
Based on the energy storage mechanism, supercapacitors can be classified into electric double-layer capacitors (EDLCs) and pseudo-capacitors (PCs).EDLCs are typically formed by the electrolyte ions on the surface of the electrode, and therefore require a large SSA.In contrast, PCs store charges through redox reactions involving interfacial electron transfer between the solution and the electrode.Both EDLCs and PCs require fast charge transport properties, large SSAs, high porosity, and abundant redox-active sites.Notably, 2D c-MOFs are potential candidates for fabricating supercapacitors due to their unique layered structures and appealing electrical conductivity.The use of 2D c-MOFs provides the possibility of achieving ideal supercapacitors with superior electrochemical performance.In 2017, Dincăʼs group reported the first example of 2D c-MOFs, specifically Ni 3 (HITP) 2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene),52 which served as the sole electrode material in an EDLC without the need for extra conductive additives or binders (Figure4a-c).Ni 3 (HITP) 2 exhibited an ultrahigh bulk electrical conductivity exceeding 5000 S • m −1 , which was much higher than those of carbons and holey graphite (~1000 S • m −1 ) and even comparable to graphite.Additionally, Ni 3 (HITP) 2 possessed a large surface area of 630 m 2 • g −1 and open channels of ~1.5 nm diameter, which enabled it to accommodate large electrolyte ions commonly used in EDLCs, such as TEA + (0.68 nm), TEA + •7ACN (1.3 nm), BF 4 − (0.46 nm), BF 4 − •9ACN (1.16 nm).The high surface area allowed for a high double-layer capacitance, the excellent electrical conductivity facilitated the charge transport, and the large open channels were conducive to electrolyte movement.As a result, Ni 3 (HITP) 2 displayed promising potential as an active electrode material for EDLCs.Significantly, the Ni 3 (HITP) 2 -based device achieved a very high surface area-normalized capacitance of ~18 µF • cm −2 , exceeding that of most carbon-based materials, and showed a capacity retention greater than 90 % after 10,000 cycles.This work establishes 2D c-MOFs as a new family of active materials for EDLCs and demonstrates that active electrode materials can be rationally tuned at the molecular level.

Figure 4
Figure 4 (a) The structure of 2D c-MOF: Ni 3 (HHTP) 2 .(b) The simulated diagram of different sizes of pores including electrolyte Et 4 N + and BF 4 − ions, and acetonitrile solvent molecules filled in Ni 3 (HHTP) 2 .(c) Comparison of areal capacitances of Ni 3 (HHTP) 2 with those of other materials.(ac) Reprinted with permission from Ref. 52.Copyright 2017 Springer Nature.(d) CV curves at different scan rates for Cu-HAB.(e) CV curves at different scan rates for Ni-HAB.(f) Gravimetric and volumetric rate performances of Ni-HAB electrodes with different weight loadings.(g) Areal rate performance of Ni-HAB electrodes with different weight loadings.(h) Cycling stability performance of Ni-HAB at 10 A • g −1 .(i) Comparison of volumetric and areal capacitance of Ni-HAB with those of other materials.(di) Reprinted with permission from Ref. 53.Copyright 2018 Springer Nature.

Figure 5
Figure 5 (a) Synthesis schematic diagram of Cu-DBC.(b) CV curves of Cu-DBC at different scan rates.(c) Galvanostatic charge-discharge profiles of Cu-DBC at different current densities.(d) Rate performance of areal and volumetric capacitances of Cu-DBC.(e) Comparison of energy density and power density of Cu-DBC with those of other reported materials.(f) Capacitance retention of Cu-DBC after different cycles at 5 A • g −1 .(a-f) Reprinted with permission from Ref. 33.Copyright 2020 Wiley-VCH.(g) Synthesis of Cu-TBC.(h) CV profiles at different scan rates.(i) Galvanostatic charge-discharge profiles at different current densities.(j) Cycling stability test of Cu-TBC.(k) Comparison of energy density and power density of Cu-TBC with those of other reported materials.(gk) Reprinted with permission from Ref. 54.Copyright 2023 Royal Society of Chemistry.

Figure 6
Figure 6 (a) Rate performance of Cu-CAT NWAs and Cu-CAT powder.(b) Galvanostatic charge-discharge profiles of Cu-CAT at different current densities.(c) Comparison of surface area normalized capacitance of Cu-CAT with those of other carbon materials.(ac) Reprinted with permission from Ref. 55.Copyright 2017 Wiley-VCH.(d) CV profiles of Ni 2 [CuPc(NH) 8 ]/EG-2-based MSC at different scan rates.(e) Galvanostatic charge-discharge profiles of Ni 2 [CuPc(NH) 8 ]/EG-2-based MSC at different current densities.(f) Comparison of energy density and power density of Ni 2 [CuPc(NH) 8 ]/EG-2-based MSC with those of other reported materials.(df) Reprinted with permission from Ref. 56.Copyright 2020 Wiley-VCH.

Figure 7
Figure 7 (a) Electrochemical in situ Raman spectra of NiHAB at different charge/discharge states.(b) Ni K-edge X-ray absorption nearedge structure spectra of NiHAB at different potentials.(c) CV profiles of NiHAB in solutions with pH ranging from 7 to 14. (ac) Reprinted with permission from Ref. 57.Copyright 2020 American Chemical Society.(d) The simulated system of MOF-based supercapacitor.(eg) Capacitive performance of Ni 3 (HITP) 2 predicted for practical cell-size supercapacitors.(dg) Reprinted with permission from Ref. 58.Copyright 2020 Springer Nature.

Figure 8
Figure 8 (a) Structure and redox progress occurred at ligands and metals ions.(b) Chemical structure of NiDI.(c) Diagram of redox reactions between NiDI and counter ions.(d) Charge-discharge tests of NiDI at different current densities.(e) Cycling stability test and coulombic efficiency of NiDI at 250 mA • g −1 .(a-e) Reprinted with permission from Ref. 60.Copyright 2018 Wiley-VCH.(f) Scheme of the Li-ion storage process of Cu-BHT.(g) Scheme of the LIB based on Cu-BHT cathode.(h) The mechanism of Li-ion storage for Cu-BHT cathode.(i) Cycling performance of Cu-BHT cathode at 300 and 1000 mA • g −1 .(j) Charge-discharge profiles of Cu-BHT at 300 mA • g −1 .(k) Rate capability of Cu-BHT from 100 to 2000 mA • g −1 .(l) Discharge curves of Cu-BHT at different current densities.(f-l) Reprinted with permission from Ref. 61.Copyright 2020 American Chemical Society.

Figure 10 (
Figure 10 (a) Synthesis of TPQG-Cu-MOF and its application as a cathode in LIBs.(b) Galvanostatic charge/discharge profiles of TPQG-Cu-MOF at various current densities.(c) The rate performance of TPQG-Cu-MOF at different current densities.(d) Cycling performance and coulombic efficiency of TPQG-Cu-MOF at 20 mAh • g −1 .(a-d) Reprinted with permission from Ref. 63.Copyright 2023 Wiley-VCH.
150.2 mAh • g −1 at a current density of 20 mAh • g −1 with a Coulombic efficiency of nearly 100 %.In addition, the capacity retention of 82.6 % could be reached after 500 cycles at a high current density of 1 A • g −1 , indicating the superior long cycle stability of TPQG-Cu-MOF.Moreover, both ex situ FTIR and X-ray photoelectron spectroscopy (XPS) measurements revealed the storage mechanism, involving the reversible redox reaction of pyrazine units and CuO 2 nodes during the Li + insertion/extraction process.Our research on integrating the redox unit of pyrazine into the 2D c-MOF not only promotes the understanding of the relationship between structure and property, but also provides new guidance for designing 2D c-MOF-based cathode materials in LIBs.

Figure 11 (
Figure 11 (a) Cycling performance of Cu-HATN at 300 mA • g −1 .(b) The simulated lithiation pathway of Cu-HATN.(c) The Li + storage mechanism of Cu-HATA during lithiation/delithiation process.(ac) Reprinted with permission from Ref. 64.Copyright 2023 Wiley-VCH.(d) Comparison of bulk HHB-Cu and HHB-Cu NSs using as cathodes in LIBs.(e) CV profiles of HHB-Cu NSs at 1 mV • s −1 .(f) Charge-discharge profiles of HHB-Cu NSs at various current densities.(g) The rate performance of HHB-Cu NSs at different current densities.(h) Cycling performance of HHB-Cu NSs at 1 A • g −1 .(d-h) Reprinted with permission from Ref. 50.Copyright 2020 Royal Society of Chemistry.
2+ insertion/extraction. Notably, the Cu-HHTP/ MX cathode demonstrated a high reversible specific capacity of 260.1 mAh • g −1 at 0.1 A • g −1 and exceptional rate capability of 173.1 mAh • g −1 at 4 A • g −1 .Additionally, it exhibited an excellent energy density of 200.96Wh • kg −1 at 0.1A • g −1 and a power density of 2729.6 W • kg −1 at 4 A • g −1 , surpassing the performance of Cu-HHTP.In addition, the Cu-HHTP/MX cathode exhibited remarkable long-term cycling stability, retaining 92.5 % of its capacity after 1000 cycles at 4 A • g −1 .

Figure 17 (
Figure 17 (a) Synthesis of THTNi single-layer sheet using the Langmuir-Blodgett method.(b) HER polarization plots of the THTNi sheet.(c) HER polarization plots of the THTNi sheet in different electrolyte solutions.(ac) Reprinted with permission from Ref. 98.Copyright 2015 Wiley-VCH.(d) ORR polarization profiles of PcCu-O 8 -Co/CNT at various rotating speeds.(e) ORR polarization curves of PcCu-O 8 -Co/CNT before and after CV cycles.(f) The reaction mechanism of PcCu-O 8 -Co/CNT during the ORR process.(df) Reprinted with permission from Ref. 99.Copyright 2019 Wiley-VCH.

Figure 19 (
Figure 19 (a) Synthesis of Cu 3 (HHTQ) 2 and Ni 3 (HHTQ) 2 .(b) The selectivity for CH 3 OH and H 2 for Cu 3 (HHTQ) 2 at various potentials during the CO 2 RR process.(c) Free-energy profiles and catalytic mechanism of Cu 3 (HHTQ) 2 for reduction of CO 2 to CH 3 OH.(d) Structures of intermediates in the mechanism of CO 2 RR process on Cu 3 (HHTQ) 2 .(a-d) Reprinted with permission from Ref. 97.Copyright 2021 Wiley-VCH.(e) Schematic structure of PcCu-O 8 -Zn.(f) Faradaic efficiency of CO and H 2 .(g) Free-energy profiles for HER and CO 2 RR.(h) Schematic illustration of HER and CO 2 RR reaction process on PcCu-O 8 -Zn.(eh) Reprinted from Ref. 100 published under a creative commons license (CC BY).