Pyrazine‐Embedded 2D Conjugated Metal–Organic Framework with Quasi‐Honeycomb Lattice for High‐Capacitance Lithium‐Ion Storage

Abstract

As a unique class of framework electronic materials, 2D conjugated metal–organic frameworks (2D c‐MOFs) exhibit intrinsic porosity, superior electrical conductivity, and abundant active sites. These properties endow them with great potential in electrochemical lithium‐ion storage. However, the development of 2D c‐MOF‐based capacitors has encountered a bottleneck in enhancing Li‐ion storage capacitance, and the design of high‐capacitance MOF electrode materials has remained a challenge. Herein, we synthesize a Cu‐OHDDQP (octahydroxy‐dibenzo[a,c]dibenzo[5,6:7,8]quinoxalino[2,3‐i]phenazine) 2D c‐MOF with a quasi‐honeycomb lattice by employing a nonplanar D₂‐symmetric conjugated ligand embedding redox‐active pyrazine moieties. The quasi‐honeycomb lattice features a dual‐porous tessellation of C₆‐symmetric and C₃‐symmetric pores. Notably, when utilized as active material for electrochemical lithium storage, Cu‐OHDDQP achieves a record‐high gravimetric specific capacitance among reported 2D c‐MOFs of 452 F g⁻¹ in aqueous lithium electrolyte, along with a decent cycling stability of 90% after 1000 cycles. Such high capacitance is attributed to both the quasi‐honeycomb lattice leading to higher surface area and the redox‐active pyrazine moieties offering extra lithium‐adsorption sites and associated pseudocapacitance. This work demonstrates that rational ligand design enables high‐capacitance MOF electrodes materials, highlighting the potential of conductive MOFs for electrochemical energy technologies.

A Cu‐OHDDQP 2D conjugated metal–organic framework (2D c‐MOF) was synthesized possessing a unique quasi‐honeycomb topology, from a pyrazine‐fused, D₂‐symmetric ligand. The achieved 2D c‐MOF electrode displayed a remarkable gravimetric capacitance up to 452 A g⁻¹ for electrochemical lithium storage. Supported by theory and the spectroscopic experiments, the role of the backbone‐embedded pyrazine moieties is unveiled, showing they can offer extra sites for both lithium adsorption and pseudocapacitive charge storage.

Introduction

Recent years have witnessed the rise of 2D conjugated metal–organic frameworks (2D c‐MOFs) as promising conductive framework materials for electronics and energy applications.^{[ 1 ]} 2D c‐MOFs comprise symmetric, highly π‐conjugated organic ligands with ortho‐substituted hydroxyl,^{[ 2 ]} amino,^{[ 3 ]} or thiol^{[ 4 ]} groups, chelating with transition metal ions (e.g., Cu(II),^{[ 5 ]} Ni(II),^{[ 4 ]} Co(II),^{[ 2 ]} Zn(II)^{[ 6 ]}) via square planar coordination linkages to form holey, π‐conjugated coordination sheets in hexagonal,^{[ 7 ]} square,^{[ 8 ]} honeycomb,^{[ 9 ]} rhombic,^{[ 10 ]} and occasionally Kagome^{[ 11 ]} topologies. Such monolayers further assemble into layer‐stacked bulks with polygonal channels via interlayer π–π interactions. The extensive delocalization of π‐electrons via both in‐plane conjugation and out‐of‐plane stacking enables explicit electrical conductivity.^{[ 1 ]} As a result, 2D c‐MOFs couple inherent porosity and accessible inner surfaces with intrinsic conductivity, offering substantial potential for electrochemical lithium storage.^{[ 12} , ¹³ , ¹⁴ , ^{15 ]}

Nonetheless, to date, two challenges have largely limited the lithium storage capacitance of 2D c‐MOFs. The major challenge lies on the inadequacy of functional surface sites for ion adsorption or charge accommodation as most 2D c‐MOFs are typically constructed from aromatic hydrocarbon cores such as triphenylene,^{[ 2 ]} trinaphthylene,^{[ 16 ]} and coronene.^{[ 7 ]} These organic linkers, lacking lithium‐binding or redox‐active heteroatoms, are inefficient for lithium storage via either electrical double layer (EDL) pathway or pseudocapacitive faradaic reactions.^{[ 17 ]} Our group has attempted to tackle this problem by integrating nitrogen‐rich phthalocyanine (Pc) as a 16‐membered conjugated macrocycle into 2D c‐MOFs, affording Ni₂[CuPcX₈] (CuPc = copper(II) phthalocyanine, X = NH^{[ 18 ]} or S^{[ 19 ]}). The Pc‐borne, redox‐active pyrrole moieties accommodate electrons through active faradaic reactions upon adsorbing electrolyte species, thus offering extra pseudocapacitance on top of the routine capacitive contribution by coordination linkages.^{[ 18 ]} The second limitation is the relatively low surface area of layer‐stacked 2D c‐MOFs, typically ranging from 200 to 500 m² g⁻¹, which hinders ionic accessibility and charge storage by the porous scaffolds.^{[ 20 ]} On this regard, the high porosity of 2D c‐MOFs could be achieved from hierarchical configurations^{[ 20} , ^{21 ]} or rational ligand engineering.^{[ 22} , ^{23 ]} For instance, a dual‐pore 2D c‐MOF, namely, Cu‐EP (EP = ethynylphenanthrene) was constructed in a rare Kagome topology from a macrocyclic conjugated ligand possessing an intrinsic pocket aperture, resulting in a record high surface area of 1502 m² g⁻¹.^{[ 22 ]} However, the reported gravimetric capacitance for lithium storage by 2D c‐MOFs remains limited (ranging from 100 to 400 F g⁻¹),^{[ 18} , ²⁴ , ²⁵ , ^{26 ]} requiring urgent input on rational ligand design to tune the active sites and porous structures to boost the ionic storage capacitance.

To tackle the above challenges for electrochemical lithium storage by 2D c‐MOFs, herein we report the synthesis of a highly crystalline, nitrogen‐rich bulk 2D c‐MOF with a novel quasi‐honeycomb lattice topology, named Cu‐OHDDQP (Figure 1a), using a pyrazine‐fused, D₂‐symmetric conjugated ligand of octahydroxy‐tetraphenyl‐pyrazino[2,3‐g]quinoxaline (OHTPPQ). The design of Cu‐OHDDQP integrates both the advantages of incorporating redox‐active, heteroatomic pyrazine moieties to boost lithium adsorption and constructing dual‐porous lattice toward high surface area to promote ion accessibility and diffusion. Specifically, the unique connection between OHTPPQ and Cu(II) nodes affords novel quasi‐honeycomb topology, featuring a dual‐pore tessellation of C₆‐symmetric regular hexagons and C₃‐symmetric truncated triangles (Figure S1), giving rise to higher surface area than the traditional honeycomb 2D c‐MOFs. In addition, the bi‐pyrazine‐fused OHTPPQ ligands endow the quasi‐honeycomb framework with lithium‐binding nitrogen atoms in an area‐dense fashion, which undergoes redox reactions to facilitate pseudocapacitive lithium storage during charge/discharge cycles. The backbone nitrogen dopants also serve to promote electron injection to tune the electronic and electrochemical properties of Cu‐OHDDQP toward more efficient lithium storage. Notably, the resultant Cu‐OHDDQP bulk crystals exhibit a high Brunauer–Emmett–Teller (BET) surface area of 610 m² g⁻¹. The 4‐probe measurements show that the Cu‐OHDDQP displays the semiconducting behavior with a room temperature electrical conductivity of 5.1 x 10⁻³ S cm⁻¹ and the time‐resolved terahertz spectroscopy (TRTS) analysis demonstrates a charge carrier mobility of around 22 cm²V⁻¹s⁻¹. Notably, the synergy between novel quasi‐honeycomb lattice and nitrogen‐rich skeleton provides Cu‐OHDDQP with a remarkable gravimetric capacitance up to 452 F g⁻¹ at 0.5 A g⁻¹, together with a potential window of 1.0 V (−0.5 to 0.5 V versus Ag/AgCl) in aqueous lithium sulfate electrolyte. The spectroscopic characterizations along with density functional theory (DFT) calculations, unveil the dual‐role of pyrazine moieties in electrochemical lithium storage by Cu‐OHDDQP, which offer extra ion‐adsorption sites and enhance pseudocapacitive charge storage by engaging in active surface redox reactions. This work demonstrates the feasibility of topological control and heteroatom doping by rational ligand design to tailor 2D c‐MOFs toward enhanced electrochemical lithium storage.

Figure 1.

a) Synthesis of Cu‐OHDDQP bulk crystal with quasi‐honeycomb topology, with pyrazine moieties colored in blue, C₆‐symmetric hexagonal pores in green, and C₃‐symmetric truncated triangular pores in yellow, respectively. b) Fourier transform infrared (FT‐IR) spectrum of Cu‐OHDDQP in comparison with the antecedent OHTPPQ ligand. c) Scanning electron microscopy (SEM) image of Cu‐OHDDQP bulk powders revealing rod‐like morphology. The scale bar represents 500 nm. d) X‐ray photoelectron spectroscopy (XPS) spectrum of Cu‐OHDDQP bulk powder showing the presence of Cu, C, N, and O.

Results and Discussion

Synthesis and Characterization

Prior to synthetic efforts, we carried out theoretical calculations on OHTPPQ and its fully cyclized derivative, octahydroxy‐dibenzo[a,c]dibenzo[5,6:7,8]quinoxalino[2,3‐i]phenazine (i.e., OHDDQP), to understand the effect of nitrogen incorporation on the electronic behavior of the envisaged ligands (Figure S2). As evidenced by the electrostatic potential maps (ESPs), the pyrazine‐fused conjugated ligands possess appreciably higher electron density on the nitrogen atoms, than their nitrogen‐free counterparts. This is likely due to the electronegative nitrogen dopants concentrating electrons around them. Also, a downshift of the HOMO–LUMO gap was observed for both N‐doped ligands. Altogether, the nitrogen incorporation improves the redox‐activity of 2D c‐MOFs and provides extra cation‐affinitive sites for lithium storage.

The OHTPPQ ligand was prepared via a four‐step synthetic procedure (Scheme S1). The intermediate of tetramethoxy‐benzil 4 was obtained from the palladium‐catalyzed cross‐coupling of aryl iodide 1 and aryl alkyne 2 into diphenylethyne 3, followed by oxidation with potassium permanganate. The intermediate 4 subsequently underwent aldimine condensation with 1,2,4,5‐tetraaminobenzene to afford the methoxy‐protected precursor OMTPPQ 5, which was further deprotected by boron tribromide to yield OHTPPQ as an air‐sensitive blue powder. The structure of the as‐synthesized ligand was confirmed via nuclear magnetic resonance (NMR) and mass spectroscopy (Figures S3–S11). Following the screening of synthetic conditions (Figures S12 and S13), Cu‐OHDDQP was solvothermally obtained as a black crystalline powder from OHTPPQ and copper nitrate in a mixed solvent of water and N,N‐dimethylformamide (DMF, v/v = 2/1) at 85 °C for 72 h (Figure 1a). This synthesis is accomplished with the in situ intramolecular cyclodehydrogenative coupling (Scholl reaction^{[ 11 ]}) of nonplanar OHTPPQ during coordination polymerization with Cu(II), affording highly planar and conjugated quasi‐honeycomb lattice (Scheme S2). The in situ Scholl reaction was confirmed with acid digestion experiment and UV–visible (UV–vis) absorption spectroscopy (Figures S14 and S15; Scheme S3). Detailed synthetic procedures are available in Supporting Information.

The Fourier transformed infrared (FT‐IR) spectrum of Cu‐OHDDQP bulk powder (Figure 1b) was compared with those of its precursors, highlighting the disappearance of broad O─H vibrational stretching peaks at 3000–3500 cm⁻¹, indicating successful coordination between OHTPPQ and Cu ions. Moreover, the rise of a strong peak at 1250–1300 cm⁻¹ is attributed to the C─O─Cu coordination bond.^{[ 27 ]} The Raman spectrum shows three characteristic bands: two in the range of 1300–1400 cm⁻¹ and one at 1400–1600 cm⁻¹, assigned as the typical D and G bands for graphene‐like structures^{[ 5 ]} (Figure S16). Scanning electron microscopy (SEM) image (Figures 1c and S17) reveals a homogeneous morphology of rod‐shaped crystals, with lengths ranging from 300 to 3000 nm and widths from 50 to 100 nm. The sample was further probed with energy‐dispersive X‐ray spectroscopy (EDS), showing a uniform elemental distribution of copper, carbon, nitrogen, and oxygen throughout the sample crystals (Figure S18). X‐ray photoelectron spectroscopy (XPS) further confirms the composition of copper, carbon, nitrogen, and oxygen for Cu‐OHDDQP (Figure 1d). The high‐resolution O 1s spectrum shows two oxygen peaks for both C═O (533.4 eV) and C─O (531.9 eV) bonds, distinct from the pristine OHTPPQ ligand containing only the C─O moiety, indicative of a partially oxidized coordination linkage in the form of copper‐dioxolene‐semiquinone or copper‐bis(semiquinone) (Figure S19). The redox‐active CuO₄ linkage has been found to create free radicals as extra charge carriers, promoting π–d delocalization and enhancing the overall conductivity of the 2D c‐MOF framework.^{[ 1 ]} Such redox activity in Cu‐OHDDQP is further evidenced by the Cu 2p spectrum, showing a cuprous Cu(I) peak at 932.1 eV in addition to the expected cupric Cu(II) peak at 934.2 eV with a Cu(I)/Cu(II) ratio of 1:3. The presence of the Cu(I) oxidation state in the as‐synthesized bulk powder could be attributed to metal‐to‐ligand charge transfer for overall charge neutrality, in light of the partially oxidized semiquinone ligand. Inductively coupled plasma optical emission spectrometer (ICP‐OES) reveals a copper content of 16.84 wt%, close to the theoretical value predicted by structural modeling (Table S1). This, combined with elemental analysis (EA) of the sample powder, suggests a Cu/C mass ratio of 0.34, in good agreement with the theoretical Cu/C mass ratio derived from the proposed chemical formula.

The crystal structure of Cu‐OHDDQP was resolved by combining powder X‐ray diffraction (PXRD), high‐resolution transmission electron microscopy (HRTEM), and structural simulation by DFT. In principle, the D₂‐symmetric OHTPPQ can lead to in‐plane topology of either rhombic or quasi‐honeycomb patterns, depending on the connectivity between the ligand and the Cu nodes. To probe the actual lattice topology, five possible unit cells were modeled and optimized for XRD patterns (Figure S20). Furthermore, to resolve the crystal structures of the Cu‐OHDDQP with multiscaled diffraction techniques,^{[ 26} , ²⁸ , ²⁹ , ³⁰ , ³¹ , ^{32 ]} we performed synchrotron PXRD measurements, which reveal an appreciable match with the simulated quasi‐honeycomb lattice in an AA‐slipped stacking mode (Figure S21). Rietveld refinement against synchrotron PXRD converges to unit cell parameters of a = 34.77 Å, b = 34.66 Å, c = 3.42 Å and α = 83.5°, β = 98.4°, γ = 120.5°, with R _WP = 2.34% and R _P = 2.26% (Figure 2a). The refined diffraction pattern displays prominent peaks (2θ) at 5.1°, 10.3°, and 13.7°, corresponding to facets of (110), (220), and (410), respectively. The broad peak centered at 27.3° represents the (001) facet arising from the stacking of π‐conjugated multilayers. The TEM images also display the rod‐shaped morphology of Cu‐OHDDQP (Figures S22 and S23). Out‐of‐plane high‐resolution lattice image of the nanorods exhibits clear lattice fringes with a d‐spacing of 1.71 nm, indexed as the (110) facet, indicative of long‐range order along the interlayer‐stacked direction (Figures 2b and S24). Selected area electron diffraction (SAED) shows distinctive diffraction spots lined up in a columnar array, indicative of a typical layer‐stacked configuration of 2D coordination sheets^{[ 33 ]} (Figures 2c and S25). The geometrically optimized lattice is presented from both the in‐plane direction (Figure 2d) and the out‐of‐plane direction (Figure S26).

Figure 2.

a) Overlay of synchrotron PXRD patterns with Rietveld refined plot of Cu‐OHDDQP bulk powder. b) HRTEM of Cu‐OHDDQP nano‐rod along the (001) direction. The two fringes are separated by 1.71 nm, corresponding to (110) facet. c) SAED pattern of a single crystal manifesting a typical layer‐stacked configuration. d) In‐plane display of the quasi‐honeycomb lattice in the Cu‐OHDDQP bulk crystal, with pore sizes of 14.5 nm and 15.3 nm. e) N₂ adsorption‐desorption isotherm of Cu‐OHDDQP bulk powder. Inset: pore size distribution calculated with non‐local density functional theory (NLDFT) model.

Thermogravimetric analysis (TGA) reveals the thermodynamic stability of Cu‐OHDDQP up to around 300 °C under an argon atmosphere (Figure S27). A weight loss of around 6% at 100 °C was observed likely due to water molecules trapped in sample pores, followed by another loss of 2% at 100–150 °C assigned to residual DMF. This, together with XPS, ICP‐OES, elemental analysis, and structural modeling, suggests a chemical formula of Cu₂(C₃₄H₁₀N₄O₈)(H₂O)₂ (i.e., Cu₂(OHDDQP)(H₂O)₂) for Cu‐OHDDQP. The electron paramagnetic resonance (EPR) spectrum of Cu‐OHDDQP displays a large asymmetric signal at g = 2.089 arising from unpaired electrons in the Cu(II) nodes and a narrow symmetric signal at g = 2.003 owing to the partially oxidized semiquinonate monoradical in the CuO₄ linkage^{[ 34 ]} (Figure S28 and Scheme S4). Measurement of magnetic susceptibility with a superconducting quantum interference device (SQUID) reveals negligible magnetic hysteresis for ferromagnetism from 5 to 400 K (Figure S29). The near‐linear dependence of reciprocal magnetic susceptibility (1/χ) on temperature indicates the paramagnetic nature of Cu‐OHDDQP.^{[ 5 ]}

The porosity of Cu‐OHDDQP was assessed using nitrogen gas adsorption–desorption experiments at 77 K. After thermal activation at 120 °C, the powder sample exhibited a notable BET surface area of 610 m² g⁻¹ with an average pore size of 1.46 nm (Figure 2e). The calculated pore size agrees with the BET‐measured data; however, it is challenging to distinguish the dual‐pore sizes due to their similar pore diameter values. The formation of such an exotic quasi‐honeycomb topology for Cu‐OHDDQP crystals might be a concerted result of the large steric hindrance imposed by the OHTPPQ ligand and the higher thermodynamic stability of the quasi‐honeycomb lattice compared to the rhombic lattice.^{[ 35 ]} The MOF powder was found to retain its lattice structure after thermal activation, as indicated by PXRD of the activated sample powder (Figure S30a). Moreover, the chemical stability of the sample was evaluated by submerging the powder into various solvents followed by PXRD analysis to assess its structural integrity after chemical treatment. The bulk powder exhibits decent tolerance to neutral solvents including water, isopropanol, and DMF, but degrades within 24 h when exposed to strong acid or base conditions (Figure S30b).

Electrical and Optoelectronic Properties

To examine the electrical properties of Cu‐OHDDQP, the bulk powder was pressed into a pellet for conductivity measurements using a van‐der‐Pauw (vdP) four‐probe configuration. The ohmic contact was confirmed by the linear current responses^{[ 36 ]} (Figure S31a). The variable‐temperature conductivity measurements reveal a positive correlation between conductivity and temperature over the range of 2 to 400 K in a concaved fashion, indicative of apparent semiconductive behavior^{[ 23 ]} (Figures 3a and S31b). A conductivity of 5.1 × 10⁻³ S cm⁻¹ could be determined at 298 K. The linear Arrhenius fit over 300–400 K indicates thermally activated charge transport behavior^{[ 5 ]} with a thermal activation energy of 0.12 eV (Figure S31c). Moreover, the conductivity curve over 100–300 K can be well described by the 3D Mott variable range hopping (VRH) model, suggesting hopping‐type intergrain charge transport^{[ 37 ]} (Figure S31d).

Figure 3.

a) Variable temperature conductivity curve of pelletized Cu‐OHDDQP via a van der Pauw four‐probe configuration. Inset: Arrhenius plot showing a thermal activation energy of 0.12 eV. b) UV–vis–NIR absorption of Cu‐OHDDQP bulk dispersion displaying a broad absorption band over the near IR region. Inset: Tauc plot showing an indirect optical bandgap of 0.44 eV. c) Calculated electronic band structure (left graph) for alpha (red) and beta (black) spins, and projected density of states (PDOS, right graph) differentiated by spin on atoms of Cu‐OHDDQP, in the form of a monolayer. d) Calculated electronic band structure (left graph) for alpha (red) and beta (black) spins, and projected density of states (PDOS, right graph) differentiated by spin on atoms of Cu‐OHDDQP, in the form of a multilayered bulk crystal in an AA‐slipped stacking mode. e) Time‐resolved terahertz (THz) photoconductivity of Cu‐OHDDQP. f) Frequency‐resolved complex THz photoconductivity of Cu‐OHDDQP measured at ∼0.5 ps after the maximum THz photoconductivity. The red and blue lines represent the Drude–Smith (DS) fits of the real and imaginary parts of the complex photoconductivity, respectively.

The UV–visible near infrared (UV–vis–NIR) absorption of Cu‐OHDDQP was compared to that of the OHTPPQ ligand, showing both redshift and broadening of absorption bands, manifesting expansive π‐conjugation over the coordination framework^{[ 35 ]} (Figure S32a). Two broad bands were observed for Cu‐OHDDQP at 314 nm for π–π* transition in the conjugated ligand and 600 nm for metal‐to‐ligand charge transfer^{[ 5 ]} (Figures 3b and S32b). Tauc plot of the absorption curve reveals an indirect optical bandgap of 0.44 eV (Figures 3b and S32c,d). Ultraviolet photoelectron spectroscopy (UPS) reveals information on the electronic band structure of Cu‐OHDDQP with respect to Fermi level (F_E ) (Figure S33). The measurement was performed under a bias of −5 V to enable a clear secondary electron cutoff (SECO).^{[ 38 ]} The binding energy of the SECO onset was determined to be −12.68 eV, corresponding to a work function (WF) of 3.64 eV, indicative of facile electron emission from Cu‐OHDDQP surface.

The electronic band structure of Cu‐OHDDQP was examined via first‐principles calculations by the Vienna Ab initio Simulation Package (VASP)^{[ 39} , ^{40 ]} for both the quasi‐honeycomb monolayer and the AA‐slipped bulk. The monolayer displays degenerate spin states of spin‐up (α) and spin‐down ( β) electrons across the Brillouin zone (K, Γ, M) (Figure 3c), indicating no spin splitting at the Fermi level (E − E _f = 0) and thus zero magnetism.^{[ 41 ]} An indirect bandgap of 0.09 eV was observed due to the vertically misaligned valence band maximum (VBM) and conduction band minimum (CBM), suggesting a change in momentum is required to activate the corresponding for electronic transition. This small bandgap classifies Cu‐OHDDQP monolayer as a semiconductor, discrepant from the majority of regular honeycomb 2D c‐MOFs possessing metallic monolayers (Table S2). This is considered as a concerted effect of the novel quasi‐honeycomb topology and nitrogen incorporation. Moreover, such semiconducting feature with narrow bandgap could provide Cu‐OHDDQP with built‐in electric field at the electrode/electrolyte interface to facilitate lithium transport and modulate its redox potential toward more stable faradaic reactions via band bending at active sites.^{[ 42} , ^{43 ]} The electron‐hole reduced effective mass was calculated from the band structure as m _e‐h * = 0.213 m ₀, where m ₀ is the free electron mass (Table S3). The calculated projected density of states (PDOS) shows sharp peaks indicative of discrete electron energy levels, which is typical in low‐dimensional materials due to quantum confinement effects.^{[ 44 ]} The symmetrical plots for both spin states reinforce the spin‐degeneracy of the monolayer. The multilayered bulk of Cu‐OHDDQP presents more pronounced band dispersion, characteristic of layer‐stacked materials with delocalized electronic states^{[ 5 ]} (Figure 3d). In contrast to the monolayer, the multilayered bulk displays multiple bands across the Fermi level, indicating metallic behavior. The transition from semiconductive monolayer to metallic multilayered bulk suggests a strong effect of interlayer electronic coupling^{[ 45 ]} giving rise to enhanced electron delocalization and continuous PDOS. However, the Cu‐OHDDQP powder behaves as an apparent semiconductor, due to grain boundaries and possible lattice defects.

We further employed time‐resolved terahertz spectroscopy (TRTS) to investigate the charge transport properties of Cu‐OHDDQP. Upon excitation with a 1.55 eV femtosecond pulsed laser, Cu‐OHDDQP shows a transient rise in conductivity due to the photoinjection of mobile carriers, consistent with its apparent semiconducting nature^{[ 38} , ^{46 ]} (Figure 3e). The photoconductivity decays over several picoseconds, attributed to free carrier localization or electron‐hole recombination. The frequency‐resolved complex photoconductivity, comprising both real and imaginary components, can be well described by the Drude–Smith (DS) model (Figure 3f). This phenomenological model (see details in Supporting Information) accounts for spatially confined charge transport influenced by backscattering effects. The degree of confinement is characterized by the backscattering parameter (c _DS), which ranges from −1 (for complete backscattering) to 0 (for isotropic scattering). The DS fit for Cu‐OHDDQP reveals a DS scattering time (τ_DS) of 90 fs and a c _DS of −0.97. Using $m_{\mathrm{e}-\mathrm{h}}^{\ast }=$ 0.213 m ₀ from DFT and e as the elementary charge, the charge carrier mobility μ = $\frac{e{\tau }_{\mathrm{DS}}}{m_{\mathrm{e}-\mathrm{h}}^{\ast }}(1+c_{\mathrm{DS}})$ amounts to ∼22 cm² V⁻¹s⁻¹.

Electrochemical Lithium Storage

The microporous configuration, marked surface area, intrinsic conductivity, and nitrogen‐rich skeleton of Cu‐OHDDQP render it competent for electrochemical lithium storage as an active electrode material. To enhance electrochemical interfacial mass transfer, here Cu‐OHDDQP was prepared into a working electrode via in situ crystal growth onto a carbon cloth substrate (Figures S34 and S37), using the solvothermal setup in a binder‐free manner.

Afterward, Cu‐OHDDQP was examined for its lithium storage performance in a typical three‐electrode cell assembly. The as‐prepared Cu‐OHDDQP electrode was employed as a working electrode (WE), with platinum foil as a counter electrode (CE) and Ag/AgCl as a reference electrode (RE). Various electrolytes were screened among common alkali metal or alkaline–earth metal salts as 1.0 M aqueous solutions via linear scan voltammetry (LSV) and cyclic voltammetry (CV), revealing lithium sulfate (Li₂SO₄) as the optimal electrolyte for Cu‐OHDDQP under a potential window of −0.5∼0.5 V (Figures S38 and S39). The CV curve of Cu‐OHDDQP at a scan rate of 50 mV s⁻¹ shows a nearly rectangular shape with a pair of redox peaks at ±0.22 V, manifesting a relatively large capacitance along with reversible redox reactions under the designated potential window (Figure 4a, blue curve). The occurrence of redox peaks validates pseudocapacitive contribution to the overall energy storage capacitance, differing the Cu‐OHDDQP from a typical electrical double‐layer capacitor (EDLC) that accumulates charge through purely electrostatic adsorption onto the electrode–electrolyte interface.^{[ 47 ]} On the contrary, blank carbon cloth under the same CV condition induces a negligible current response, thus excluding capacitive contribution of electrode substrate (Figure 4a, black curve).

Figure 4.

a) CV curve of Cu‐OHDDQP electrode at 50 mV s⁻¹, in comparison to a blank substrate of carbon cloth showing marginal current contribution. b) CV curves collected at various scan rates from 5 to 100 mV s⁻¹. c) Logarithm of current response versus that of applied voltage for each redox peak in CV curves. d) Capacitive contribution (orange) to overall capacitance at 50 mV s⁻¹. e) Fraction of capacitive contribution and diffusion‐controlled contribution to overall electrochemical capacitance at various scan rates. f) GCD curves for Cu‐OHDDQP electrode at various current densities. A gravimetric capacitance of 452 F g⁻¹ could be achieved at 0.5 A g⁻¹. g) Performance comparison of Cu‐OHDDQP to typical conjugated MOFs and other representative carbon‐rich conductive materials, with respect to specific capacitance and potential window. h) Cycling performance of Cu‐OHDDQP electrode highlighting retention of specific capacitance and coulombic efficiency after 1000 cycles at 5.0 A g⁻¹. i) XRD patterns of the pristine Cu‐OHDDQP electrode and those after cycling.

To gain more insight into the lithium‐storage kinetics of Cu‐OHDDQP, CV curves of the working electrode were further collected at various scan rates from 5 to 100 mV s⁻¹ (Figure 4b). As the scan rate increases, the CV curves retain a roughly rectangular shape with symmetric redox peaks, reflecting appreciable electrochemical stability and reversibility of Cu‐OHDDQP under designated potential window.^{[ 48 ]} From the set of CV curves, the characteristics of corresponding charge storage behavior could be assessed. The measured peak current (i _p, A) of a CV curve is related to its sweep rate (v, mV s⁻¹) via Equation (1), where the exponential b value serves to differ the current related to surface‐controlled process from the current related to diffusion‐controlled process. Specifically, b value of 1 indicates fast surface‐controlled, capacitive processes such as electrostatic charge adsorption or redox‐active faradaic reactions, whereas b value of 0.5 arises from slow diffusion‐controlled process in electrolyte bulk.^{[ 47 ]} Here, both redox peaks of anodic and cathodic scan of CV curves were calculated and fitted into a linear relationship, with the slopes (b value) being 0.83 and 0.84, respectively (Figure 4c). Therefore, the energy storage capacitance of the Cu‐OHDDQP electrode is dictated by capacitive processes including electrolyte adsorption onto the framework and faradaic reactions on redox‐active surface sites. The capacitive contribution was further calculated with Equation (2) (i.e., the Dunn method^{[ 49 ]}), where the current response to applied voltage, i(V), is deconvoluted into the current required to cause charge accumulation at the electrolyte interface or to induce faradaic reactions at the electrode surface (i.e., capacitive contribution, k ₁ v) and current related to diffusive processes (i.e., diffusion‐controlled contribution, k ₂ v ^1/2).^{[ 47 ]} As shown in Figure 4d, capacitive contribution constitutes a fraction of 60.6% to the overall current response under voltage sweeping at 50 mV s⁻¹, versus a contribution of around 40% by diffusion‐controlled process. Such diffusive contribution to overall lithium storage could result from the microporous, nitrogen‐rich nature of Cu‐OHDDQP, allowing for facile electrolyte diffusion and adsorption.^{[ 47 ]} The capacitive contribution overall shows a positive correlation with scan rate, reaching over 90% at 100 mV s⁻¹ (Figure 4e), manifesting faster ion diffusion under elevated current density.

$$i_{\mathrm{p}}=a{v}^b$$ (1)

$$i\left(V\right)=k_{1}v+k_2{v}^{1/2}$$ (2)

The Cu‐OHDDQP electrode was further examined using galvanostatic charge–discharge (GCD) measurements. The charge curve undergoes a voltage plateau between −0.2 to 0 V at low current density (Figure 4f), reflecting pseudocapacitive redox processes on the active electrode material.^{[ 47 ]} According to the discharge curve, Cu‐OHDDQP electrode exhibits a remarkable gravimetric specific capacitance of 452 F g⁻¹ over the potential window of −0.5 to 0.5 V, at a current density of 0.5 A g⁻¹. This value lies among the highest reported for planar 2D c‐MOFs as supercapacitors and is even superior to most other porous carbon‐rich materials (e.g., classic 3D MOFs, 2D COFs, graphene, MXene) used for capacitive energy storage (Figure 4g; Tables S4 and S5). In addition, the Cu‐OHDDQP electrode achieves an energy density of 62.8 Wh kg⁻¹ and a power density of 250.0 W kg⁻¹ at 0.5 A g⁻¹. The specific capacitance exhibits a negative correlation with current density, and a capacitance retention of 40% is observed with the current density increasing from 0.5 to 10.0 A g⁻¹ (Figure S40). On the other hand, coulombic efficiency shows a positive trend with the current density increasing from 88% at 0.5 A g⁻¹ to 96% at 10 A g⁻¹. Potentiostatic electrochemical impedance spectroscopy (EIS) profile of Cu‐OHDDQP shows a nearly vertical low‐frequency region for typical pseudocapacitive behavior and a low ionic resistance of 1.26 Ω in the high‐frequency region, implying facile access of lithium ions to the adsorption sites on the Cu‐OHDDQP surface^{[ 50 ]} (Figure S41). Furthermore, the Cu‐OHDDQP electrode retains 90% of its initial capacitance after 1000 cycles in a current density of 5.0 A g⁻¹ (Figure 4h). The post‐cycled lattice integrity was significantly demonstrated by the preservation of XRD patterns (Figure 4i). The cycling stability of Cu‐OHDDQP was also evidenced with multiple characterizations on the postcycled samples by SEM, TEM, XRD, BET, FT‐IR, and element analysis (Figures S42–S48) and a comparison to typical MOF‐based electrodes (Table S6).

Lithium Storage Mechanism

Investigations were extended to understand the lithium storage mechanism adopted by Cu‐OHDDQP. Potential‐dependent ATR‐FTIR was performed for the Cu‐OHDDQP electrode at various charge–discharge states, revealing the structural evolution of the carbon skeleton during GCD cycles (Figure 5a). As the Cu‐OHDDQP electrode loses electrons upon charging, peaks for the aryl core at 1200–1600 cm⁻¹ vanish, whereas a strong carbonyl peak emerges at around 1650 cm⁻¹, likely due to oxidation of the CuO₄ linkage from copper‐bis(dicatecholate) into the form of copper‐bis(quinone).^{[ 12 ]} Such a process is reversed to restore the copper‐bis(dicatecholate) form during reduction by discharging. Potential‐dependent XPS spectra further manifest the structural evolution of the Cu‐OHDDQP electrode during GCD. For the copper node, its oxidation state cycles between Cu(II) and Cu(I) during GCD due to active redox processes, as shown by the variable Cu(II)/Cu(I) molar ratios in the Cu 2p spectra, from nearly 1/1 at the fully discharged state (−0.5 V) to over 4/1 at the fully charged state (0.5 V) (Figures 5b and S49; Table S7). The redox activity was also observed in the N 1s spectra (Figure 5c). The fraction of pyridinic N (C─N═C) increases with applied potential from −0.5 to 0.5 V, indicating charging‐induced oxidation of the pyrazine moiety. On the contrary, upon reduction by discharging back to −0.5 V, the pyrrolic N (C─N─C) becomes the predominant proportion. This validates our design that the pyrazine moiety can serve as extra redox‐active sites for pseudocapacitive energy storage. The O 1s spectra further underpin the oxidation of CuO₄ linkage into copper‐bis(quinone) with increasing carbonyl fraction and vice versa (Figure 5d). Moreover, the S 2p spectra show a higher sulfur content at 0.5 V than at −0.5 V, suggesting the role of sulfate anions as adsorbate onto Cu‐OHDDQP during charging (Figure S50). This was also confirmed by SEM and EDS images of the Cu‐OHDDQP electrode at the fully charged state displaying a stronger sulfur signal than that at fully discharged state (Figure S51).

Figure 5.

a) Potential‐dependent ATR‐FTIR of Cu‐OHDDQP electrodes at different charge or discharge states, revealing structural evolution of carbon skeleton during GCD. b) Potential‐dependent high‐resolution XPS spectra on Cu 2p orbitals of Cu‐OHDDQP electrode, displaying Cu 2p_3/2 peaks and Cu 2p_1/2 peaks over 930–940 eV and 950–960 eV, respectively. Annotations are largely omitted for clarity (complete version available as Figure S49). Potential‐dependent high‐resolution XPS spectra on c) N 1s and d) O 1s orbitals. e) Calculated ESP of the basic repeating unit of Cu‐OHDDQP, revealing higher electron density at the oxygen atoms and the nitrogen atoms by more negative electrostatic potential (in red) and relatively low electron density at the copper nodes by more positive electrostatic potential (in blue). f) Possible active sites for the adsorption of electrolyte cations (Li⁺) and anions (SO₄ ²⁻) in a single repeating unit of Cu‐OHDDQP, with calculated adsorption energy annotated aside. g) Proposed energy storage mechanism for the Cu‐OHDDQP electrode in aqueous Li₂SO₄ electrolyte, where the repeating unit of Cu‐OHDDQP is simplified into the complex of copper‐bis(dihydroxy‐quinoxaline) for clearer illustration. As suggested, totally four electrons are transferred during each charge–discharge cycle, corresponding to a theoretical capacitance of 529 F g⁻¹ under a potential window of −0.5 to 0.5 V.

The Li‐ion storage behavior of Cu‐OHDDQP was also probed by chronoamperometry (Figure S52). The current responses display a sharp spike followed by exponential decay, indicative of capacitive charge storage. The discrepancy of peak current values between positive and negative potentials suggests different oxidation and reduction kinetics,^{[ 51 ]} which reveals the occurrence of faradaic redox reactions involving intercalation and deintercalation of lithium ions along electron transfer at the pyrazine moieties. The switching chronoamperometry profile further exhibited symmetric, repeatable current responses (Figure S52c), which indicated high electrochemical reversibility and durability of Cu‐OHDDQP for lithium ion storage.^{[ 52 ]} The redox activity of Cu‐OHDDQP was also evidenced by EPR spectra, where the signal declined from 0.5 to −0.5 V due to discharge‐induced reduction of paramagnetic Cu(II) into diamagnetic Cu(I) nodes (Figure S53).

We further employed DFT calculations to probe the electrolyte adsorption behavior of Cu‐OHDDQP. ESP result of a single structural repeating unit displays appreciably higher electron density at the pyrazine N and linkage O atoms due to their electronegative nature, inferring pronounced affinity toward Li ions in the electrolyte; in contrast, the copper node is relatively electron‐deficient, therefore more labile to sulfate adsorption (Figure 5e). The same point could be drawn from the calculated electron density distribution map (Figure S54). Calculation of adsorption energy was subsequently performed treating sulfate anion and lithium cation as active adsorbates for the charging and discharging process, respectively (Figures 5f and S55). Both the pyrazine nitrogen and linkage oxygen atoms could act as active sites for Li‐adsorption during discharging, with comparable adsorption energies of −1.43 eV for N and −2.03 eV for O. In addition, the copper node could adsorb SO₄ ²⁻ with an adsorption energy of −1.55 eV during charging (Table S8). Altogether, we formulated a mechanism for the capacitive energy storage by Cu‐OHDDQP (Figure 5g). During the discharge process, one repeating unit in the sulfate‐adsorbed Cu‐OHDDQP framework 1 at 0.5 V gains an electron from the external circuit to reduce the Cu(II) node into Cu(I), releasing the adsorbed SO₄ ²⁻ into the electrolyte. The copper‐quinone coordination linkage 2 then undergoes consecutive single electron reduction into monoradical copper‐semiquinonate species 3, and then into copper‐catecholate species 4, adsorbing two Li⁺ ions onto O atoms. This is followed by the reduction of one pyridinic N in the pyrazine moiety into pyrrolic N 5 binding another Li⁺, until fully discharged to −0.5 V. For the charging process, the lithiated form 5 undergoes a fourfold single electron oxidation to release adsorbed Li⁺ and bind electrolyte SO₄ ²⁻ to afford the charged intermediate 1. Overall, a single GCD cycle involves the transfer of four electrons for the Cu‐OHDDQP unit cell, leading to a theoretical specific capacitance of 529 F g⁻¹, which is slightly higher than the experimentally determined specific capacitance of 452 F g⁻¹. Such a gap could be attributed to factors such as crystal defects, incomplete material utilization, irreversible structural changes, and nonideal electrochemical reactions.^{[ 17} , ^{53 ]}

Conclusion

In this work, we constructed a Cu‐OHDDQP 2D c‐MOF possessing a novel quasi‐honeycomb lattice topology and pyrazine‐embedded framework. The Cu‐OHDDQP exhibits high crystallinity, a large surface area of 610 m² g⁻¹, and a room temperature conductivity of 5.1 × 10⁻³ S cm⁻¹. The quasi‐honeycomb lattice of Cu‐OHDDQP features a dual‐porous tessellation of C ₆ ‐symmetric hexagonal pores and C₃‐symmetric truncated triangular pores. Notably, serving as an active electrode for electrochemical lithium storage, Cu‐OHDDQP achieved an exceptional gravimetric capacitance of 452 F g⁻¹ within a window of −0.5∼0.5 V in aqueous Li₂SO₄ electrolyte, superior to the reported planar 2D c‐MOFs and most of the other porous carbon‐rich materials. Mechanistic investigation disclosed the beneficial role of N‐incorporation on the capacitive energy storage of Cu‐OHDDQP, by both offering extra Li‐adsorption sites and contributing to the overall pseudocapacitive activity. These findings shed light on the future development of redox‐active 2D c‐MOFs via rational ligand design as appealing electrode materials for high‐performance electrochemical energy storage.

Conflict of Interests

The authors declare no conflict of interest.

Supporting information

Supporting Information

Li X., Feng Y., Fu S., Wu T., Liang P., Ma X., Iqbal R., Qian Y., Ma Y., Bonn M., Wang H., Dai H., Hao J., Dong R., Angew. Chem. Int. Ed.. 2025, 64, e202502988. 10.1002/anie.202502988

Data Availability Statement

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