A robust and conductive 3D Fe(II) MOF as a durable cathode for aqueous zinc-ion batteries

Abstract

A highly robust 3D ultramicroporous Fe-MOF (abbreviated as Fe-MET), constructed from very inexpensive, abundant, and commercially available materials, exhibits high conductivity (σ = 0.19 S m⁻¹) and can sustain under various conditions exhibiting exceptional stability (pH 1–14, numerous organic solvents, over 1 year in air, and in 1 M Zn(CF₃SO₃)₂ solution), as confirmed by PXRD, FESEM, FTIR, etc. The high conductivity coupled with a high BET surface area of 413 m² g⁻¹ and ultramicroporous homogeneous pore size of 4.6 Å made Fe-MET a promising material for the faradaic process during the electrochemical process. The solid-state AZIB coin cell fabricated using Fe-MET as the cathode delivers a maxmimum specific capacity of 34 mA h g⁻¹ at 20 mA g⁻¹ as a standalone electrode, decent energy, and a power density of 54.4 W h kg⁻¹ and 5.64 W kg⁻¹, exhibiting over 60% capacitance retention after 2100 cycles with no significant loss in coulombic efficiency. With these advances, Fe-MET has been recognised as a promising, pyrolysis-free standalone electrode material for efficient energy storage applications.

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1. Introduction

Developing sophisticated rechargeable batteries and ascertaining effective power distribution and beneficial viability for their application in massive energy storage systems is gaining a lot of interest.^1–3 Rechargeable Aqueous Zinc Ion Batteries (AZIBs) are presently promising alternatives to commercial Lithium-Ion Batteries (LIBs) owing to their numerous practical advantages such as low toxicity, high energy density, greater safety, and cost-effectiveness.^4–6 In particular, aqueous electrolytes offer higher ionic conductivities than their nonaqueous counterparts, enabling higher-rate capabilities and improving operational safety.⁷ Despite these advantages, however, AZIBs are still in the early stages of development, and significant research is a prerequisite to enhance their electrochemical performance and ensure structural stability over extended charge–discharge cycles.^8,9 Therefore, ongoing research to optimize cathode materials and address performance limitations is crucial for advancing AZIB technology as a key solution for sustainable, large-scale energy storage.¹⁰

Metal–organic frameworks (MOFs), known for their versatile properties, have shown significant potential in various applications, especially for energy storage applications in recent times.^11,12 Their backbone and porous structures can accommodate redox-active functionalities, making them suitable for energy storage.^11,12 However, low stability and poor conductivity potentially thwart their extensive use in energy storage, particularly in metal-ion batteries. Sometimes, to enhance the inherent electrical conductivity as well as electrochemical activity, the porous MOF precursors are pyrolyzed to produce MOF-derived materials to provide higher efficiency. To date, different MOF-derived Ni₂Se₂/Co₃Se₄, Fe₂Mo₃O₈@C@MoS₂, Bi@SA Cu–C, manganese-based oxides, vanadium oxides, etc., have been explored in the literature.^13–15 Hence, the prime preference for MOFs to be applied in metal-ion batteries is the judicious and strategic choice of electrochemically active MOFs with intrinsic conductivity, microporosity, high surface area, and chemical robustness.

Pure MOFs as electrode materials are quite rare in the literature,¹⁶ as they are often combined with conductive materials such as polypyrrole, polythiophene, polyaniline, or graphene oxide to boost their conductivity, or they are pyrolyzed into metal oxides on carbon materials.^17,18 While adding conductive fillers improves conductivity, it also reduces the surface area of MOFs or there may a change in phase separation sometimes to reduce the performance.¹⁹

On the other hand, reports on 3D, robust, redox-active, cost-effective, and easily synthesizable conductive porous MOFs with decent capacity and excellent cycling stability as exclusive cathode materials for AZIBs without additional conducting composites or pyrolysis components remain scarce. Given these insights, this study presents an ultra-stable, Fe(II)-based, 3D conductive ultramicroporous triazolate MOF (Fe-MET) as a standalone cathode material for an effective AZIB. Fe-MET exhibits a maximum specific capacity of 34 mA h g⁻¹ at 20 mA g⁻¹ current density and retains more than ∼60% initial capacitance with no significant loss of coulombic efficiency (CE) even after 2100 successive charge–discharge cycles at 50 mA g⁻¹ current density. The AZIB also exhibits high energy and power densities of 54.4 W h kg⁻¹ at 5.64 W kg⁻¹, demonstrating an impactful battery application.

2. Experimental section

2.1. Materials

All commercial chemicals were used as received unless stated otherwise. Iron(II) chloride (98%, anhydrous, Sigma Aldrich), 1H-1,2,3-triazole (>98%, TCI), N,N-dimethylformamide (DMF, ACS grade, Fisher Scientific), methanol (MeOH, HPLC grade, Fisher Scientific), Zn(CF₃SO₃)₂, Nafion and commercial Activated Carbon (AC) were purchased from Sigma-Aldrich. The aqueous Zn(CF₃SO₃)₂ solution used for the electrochemical studies was prepared using deionized water. Ti-foil (thickness 0.05 mm, size 150 × 150 mm, purity >99.6%) was purchased from Sigma Aldrich. Solvothermal syntheses were conducted in a round-bottom flask.

2.2. Methods

Synthetic procedures of Fe-MET. FeCl₂ (2.282 g) was placed in a round-bottomed flask, and the flask was repeatedly evacuated and refilled with N₂ three times. Then DMF (75 mL) was added to the flask under an N₂ atmosphere and stirred to dissolve FeCl₂ completely. DMF (25 mL) solution of 1H-1,2,3-triazole (3.16 mL) was slowly added, and the reaction solutions were heated to 120 °C for 48 h. After cooling to room temperature, the pink solid was collected by centrifugation, washed with DMF three times, immersed in MeOH for three days, and dried at 60 °C to afford the product powders.^20,21

2.3. General characterization

The PXRD spectra were recorded using a Rigaku Ultima IV X-ray diffractometer, keeping all sample parameters constant. A step width of 0.01 and a scan rate of 1° min⁻¹ from 5–40° (Cu K_α radiation, λ = 1.54 Å) were used during measurements. The UV-vis spectroscopic studies were performed using a JASCO V-670 spectrometer. An FT-IR spectrometer (Bruker-Alpha-II-Platinum ATR) was used to record the IR spectra of the samples. Solid samples were used directly to record the spectra. The morphological studies coupled with energy-dispersive X-ray (EDX) analysis of Fe-MET was performed by field emission scanning electron microscopy (FE-SEM) by drop-casting the materials on a silicon wafer and drying the samples at room temperature. An FEI Apreo LoVac instrument with an operating voltage of 20 kV was used to obtain the FESEM images. High-resolution transmission electron microscopy (HRTEM) images were recorded using a FEI Tecnai G² S-T win microscope with an accelerating voltage of 200 kV. A Microtrac Bel – BELSORP mini II model surface area analyzer was used to collect nitrogen adsorption isotherms. Before the N₂ measurements at 77 K, the crystals were activated at 90 °C in a vacuum oven for 24 h. BET surface area of the Fe-MET was obtained along with the pore diameter. XPS analysis of the frameworks was carried out using a Thermo Scientific K_alpha instrument. The source used for the XPS analysis is the Al Kα source [X-ray source 1486.8 eV]. The thermal stability of the MOF frameworks was analyzed using a Shimadzu DTG-60 TGA instrument in the 30 to 800 °C temperature range with a heating rate of 10 °C min⁻¹ under an N₂ atmosphere.

2.4. Coin cell fabrication and electrochemical measurements

The electrode was prepared using a slurry coating technique. In this process, Fe-MET MOF, carbon black, and polyvinylidene fluoride (PVDF) were mixed in an 80 : 10 : 10 mass ratio, ground together in N-methyl pyrrolidone (NMP), and then made into a slurry. This slurry was applied onto a porous titanium foil current collector with 1 × 1 cm dimensions. The electrode sheet was subsequently dried at 80 °C for overnight. The active material loading on the electrode sheet was ∼1.89 mg cm⁻².

To construct a CR2032-type coin cell battery, the Fe-MET electrode was used as the cathode, paired with a zinc foil anode (>99.99% purity, 14 mm diameter). A Whatman filter paper was the separator, and 1 M Zn(CF₃SO₃)₂ aqueous solution was employed as the electrolyte. The Fe-MET electrode's cyclic voltammetry (CV) measurements were performed at scan rates of 0.4 to 2 mV s⁻¹ over a potential window of 0.2–1.8 V (vs. Zn²⁺/Zn) using coin-type batteries on a Biologic SP-150 electrochemical workstation. Galvanostatic discharge/charge tests were conducted for coin-type batteries using a NEWAREBTS-4000 battery testing system at different current densities within 20 mA g⁻¹ to 100 mA g⁻¹ (vs. Zn²⁺/Zn). Electrochemical impedance spectroscopy (EIS) was performed at open circuit potential, ranging from 0.1 Hz to 1 kHz. The detailed calculations for evaluating the battery performance are provided in the ESI.†

3. Results and discussion

3.1. Structural characterization

The brown-colored, highly microcrystalline Fe-MET was synthesized by a solvothermal process using anhydrous FeCl₂ as a metal precursor and 1,2,3-triazole (tz) as a small organic building unit.²⁰ As illustrated in Fig. 1a, each Fe²⁺ ion is ordered in an octahedral mode with six N atoms of adjoining tz units, where each tz unit is associated with three Fe²⁺ ions. The framework surrounds regular quasi-discreet 3D branched channels presented by the larger cavities associated with narrow pore windows (Fig. 1b and c). Each tz linker bridges three octahedrally coordinated Fe²⁺ centers in Fe-MET, forming a 3D network with diamond topology containing (–Fe–N–N–)_∞ chains, which is a possible reason for the intrinsic electrical conductivity.²¹ The regular quasi-discrete 3D structure in the skeleton branching channels of the same topology, demonstrated by Fig. 1b and c, shows narrow channel windows connecting bigger pocket-like cavities. The atoms exposed to the internal C and H atoms on the pore surface also create a significant steric obstruction that prevents the N atoms or Fe ions from accessing the guest molecules.

Fig. 1 Schematic representation of the synthetic approach of Fe-MET. The coordination environment of the Fe(II) metal center, (a) coordination environment, (b) packing diagram, and (c) pore surface structure in Fe-MET.

The powder X-ray diffraction (PXRD) study established the as-synthesized material's structural assembly and phase purity. The study revealed an unaltered peak position compared to the simulated version, as shown in Fig. 2a. The N₂ adsorption measurement at 77 K shows an overall uptake of 146 cm³ g⁻¹ at a pressure of 1 bar. Therefore, the BET surface area was estimated to be 413 m² g⁻¹ (Fig. 2b), and related pore size distribution calculations display a value of 4.6 Å in Fig. 2c, revealing the similarities with its crystal (Table S1 in the ESI†).²¹ The morphological analysis of the material through scanning and transmission electron microscopes (SEM and TEM) demonstrates the regular rhombus-shaped uniform particles (Fig. 2d and e), explicating the phase-consistent nature of the microstructures exhibiting excellent crystallinity (Fig. S1 in ESI†). The SEM energy-dispersive X-ray spectroscopy (EDX) and corresponding elemental mapping study reveal Fe, C, and N as the main constituents and their homogeneous distribution in Fe-MET (Fig. S2 in ESI†). Thermogravimetric (TG) analysis illustrates the thermal stability of Fe-MET up to 400 °C, after which the framework degrades (Fig. S3 in ESI†). Four-probe I–V measurement using an Fe-MET thin-film unveils a typical semiconductor characteristic with a high electrical conductivity of 0.19 S m⁻¹, higher than that of the recently reported MOFs (Fig. 2f and Table S2 in the ESI†). The Tauc plot reveals a low optical bandgap of 1.28 eV in Fe-MET, corroborating the high electrical conductivity of Fe-MET (Fig. 2g).

Fig. 2 (a) XRD patterns of as-synthesized bulk Fe-MET (green) and simulated Fe-MET (yellow), (b) the N₂ adsorption measurement at 77 K shows the BET surface area of the Fe-MET MOF, (c) the pore size distribution of Fe-MET, (d) FESEM image of Fe-MET (the inset shows a single crystal of Fe-MET), (e) TEM image of Fe-MET (HRTEM image in the inset) (the insets show the lattice fringe), and (f) I–V plot of the Fe-MET thin film. (g) Determination of the optical bandgap of a Fe-MET thin film (Tauc plot).

Furthermore, we proceed with characterization to thoroughly establish the robustness of Fe-MET using various characterization techniques. Notably, Fe-MET possessed excellent stability under various conditions such as acidic and basic, in water and various organic solvents, for a long time in a laboratory atmosphere, and in 1(M) Zn(CF₃SO₃)₂ solutions as verified by its impeccable crystallinity over an elongated period derived by BET, PXRD and FESEM studies (Fig. 3a–f). This claim is further supported by additional supplementary spectroscopic analyses such as PXRD, FESEM, and FT-IR (Fig. S4–S6 in the ESI†). The excellent robustness under harsh conditions, easy synthetic preparation, ultramicroporous nature, and intrinsic electrical conductivity of Fe-MET certainly sanction the adaptability of low-cost and easily handled electrolytes, making it a suitable electrode material for AZIBs.

Fig. 3 (a) N₂ adsorption isotherms of Fe-MET at 77 K under different conditions, (b) PXRD pattern of Fe-MET under different conditions, and FESEM images of Fe-MET (c) after 1 year of synthesis, (d) at pH 1, (e) at pH 14, and (f) soaked in 1 M Zn(CF₃SO₃)₂.

The X-ray photoelectron spectroscopy (XPS) studies were carried out to evaluate the surface composition, valence state, and bonding connectivity in Fe-MET. From the full survey spectrum, it is manifested that all three elements (Fe, N, and C) are present on the surface of the pristine Fe-MET MOF, coinciding with the earlier investigation by EDX elemental mapping (Fig. 4a). Fig. 4b indicates that Fe is present in the +2-oxidation state. Additionally, the corresponding energy diagram confirms the presence of only carbon (C) and nitrogen (N) peaks associated with the metal center.²¹ The detailed discussion and results of Fe-MET are available in our earlier report.²¹

Fig. 4 (a) XPS survey spectrum of Fe-MET. High-resolution (b) Fe 2p, (c) N 1s, and (d) C 1s XPS spectra of Fe-MET. The peak for oxygen (O 1s) was also detected in survey spectra because of surface oxidation when the sample was exposed to air.

3.2. Electrochemical performance of Fe-MET coin cells

The electrochemical performance of the Fe-MET electrode was evaluated using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) after assembling a Fe-MET//Zn coin cell (CR2032) consisting of Zn-foil as the anode, Fe-MET as the cathode and 1 M Zn(CF₃SO₃)₂ as the electrolyte as shown schematically in Fig. 5a.^22–25 The detailed fabrication procedure is discussed in the ESI.† The first four voltammograms of the Fe-MET//Zn battery in an electrochemical window of 0.2 to 1.8 V (vs. Zn²⁺/Zn) at 0.4 mV s⁻¹ are shown in Fig. 5b. The complete overlap of the curves over different cycles suggests a reversible electrochemical process. The distinct redox peaks in forward and reverse bias correspond to the redox transitions of iron (Fe²⁺/Fe³⁺) at 1.42/1.01 V (vs. Zn²⁺/Zn) for oxidation and reduction, respectively, associated with the reversible intercalation and (de)intercalation of Zn²⁺ ions. Furthermore, as the scan rate increased from 0.4 to 2 mV s⁻¹, the current response improved, indicating efficient and highly reversible Zn²⁺ ion insertion and extraction (Fig. 5c).

Fig. 5 Electrochemical performance of the Fe-MET//Zn coin cell. (a) Schematic of the Fe-MET//Zn CR 2032 coin cell, (b) CV study of the Fe-MET based coin cell in 1 M Zn(CF₃SO₃)₂ at 0.4 mV s⁻¹, (c) scan rate dependent CV study of the Fe-MET//Zn coin cell, (d) GCD profile of the FE-MET//Zn cell in the current density range of 20 mA g⁻¹ to 100 mA g⁻¹, (e) rate kinetics of the Fe-MET//Zn cell at different current rates, and (f) Nyquist plot of the cell with the corresponding equivalent circuit before and after the charge–discharge cycles.

The charge/discharge cycles maintain a consistent shape corresponding to redox reactions. The capacity stabilizes with nearly 100% coulombic efficiency during the cycles, indicating high reaction reversibility (Fig. 5d and e). The electrode's rate performance was tested across various current densities, ranging from 20 mA g⁻¹ to 100 mA g⁻¹. A 34 mA h g⁻¹ discharge capacity at 20 mA g⁻¹ is achieved. Even at a high current density of 100 mA g⁻¹, the electrode delivers a decent capacity with a respectable recovery, indicating excellent rate performance suitable for both high capacity and fast charge/discharge operations (Fig. 5e). Additionally, its specific capacity remains quite consistent as the current density increases from 50 mA g⁻¹ to 100 mA g⁻¹, including a decent energy and power density of 54.4 W h kg⁻¹ and 5.64 W kg⁻¹ respectively highlighting its potential as an effective cathode material for AZIBs. The transport properties of the Fe-MET MOF-based cathode were assessed using electrochemical impedance spectroscopy (EIS). The Nyquist plot for the Fe-MET//Zn cell, recorded across the frequency range from 0.1 Hz to 1 kHz (Fig. 5f), reveals two semi-circular features in the high-frequency region, corresponding to two charge transfer resistances R_CT1 and R_CT2 at 1.25 Ω, and 1.2 Ω, respectively, with a very low existing solution resistance (R_S) of 0.1 Ω. These results signify the robust interaction between the electrolyte and electrode materials. At lower frequencies, a straight line in the plot suggests Warburg impedance.^26–31 The combination of low R_CT and a phase angle of around 45° indicates efficient charge transfer and enhanced ion diffusion in the electrolyte, likely due to the intrinsic ultra-micropores contributing to the effectual battery-like behavior of the Fe-MET containing AZIB.^32,33 After 2100 consecutive charging–discharging cycles, the Nyquist plot (Fig. 5f) reveals a slight increment in series resistance with R_CT1 and R_CT2 at 1.55 Ω and 1.3 Ω, respectively. The result indicates that the material retains the transport properties and does not significantly deteriorate after the cycling study.

Fig. 6a illustrates the detailed mechanism, providing a clear view of the charging and discharging processes during the oxidation and reduction phases, which occur through Zn²⁺ ion intercalation and (de)intercalation. Including simple insertion processes like ion batteries, the conversion process is also involved as the metal centers within the MOF change the oxidation state, leading to a conversion reaction that converts Fe²⁺ to Fe³⁺ or vice versa, depending on the charging and discharging.^34–37 Meticulous overlap of charge/discharge cycles at higher current density shows the extraordinary stability of Fe-MET for AZIBs.^34–37

Fig. 6 (a) Graphical representation of the zinc ion battery mechanism indicating the charging–discharging stability plot of the Fe-MET//Zn coin cell battery at higher current density (100 mA g⁻¹), (b) lighting experiment using three-coin cells in a series connection (inset-assembly of a coin cell), and (c) cycling stability of the Fe-MET//Zn coin cell at 50 mA g⁻¹ current density.

3.3. Applications of Fe-MET AZIB coin cells

To demonstrate a state-of-the-art device, we engineered the coin cells as part of an innovative approach, adding value for practical, real-world applications. We assembled three-coin cells and connected them in series, using Fe-MET as the cathode and zinc as the anode. After charging the cells with a potentiostat, they successfully powered a series of light-emitting diode (LED, ∼1.8 V) bulbs for several minutes (Fig. 6b). The cycling performance of Fe-MET electrodes demonstrates a reversible capacity with a retention rate of 60% after 2100 cycles and coulombic efficiency of nearly 100% at a current density of 50 mA g⁻¹ (Fig. 6c). Fe-MET establishes superior cycling stability owing to its sustained morphological and chemical structure robustness. The performance of the Fe-MET-based battery is compared with recently reported MOF-based and well-known electrode materials in Table S3 in the ESI.†

3.4. Structural stability and Zn²⁺ ion intercalation/deintercalation

The structural and morphological evolution during cycling is examined to gain insight into the discharge/charge through the Zn²⁺ ion intercalation and deintercalation mechanism in the Fe-MET cathode. After complete discharging, the ex situ XPS analysis of the used Fe-MET cathode after 2100 charge–discharge cycles reveals the presence of Zn 2p peaks alongside the regular peaks for Fe 2p, C 1s, and N 1s (Fig. 7a and S7 in the ESI†). The appearance of Zn 2p peaks in the spent cathode materials after discharging for 2100 cycles confirms our mechanistic hypothesis of intercalation and deintercalation of Zn²⁺ ions inside the Fe-MET framework depending on the discharging and charging process of the prepared AZIB. The FESEM images of the Fe-MET electrode after the 2100 charging–discharging cycles reveal the retention of the overall morphology of Fe-MET (Fig. 7b and c), confirming its chemical robustness during the long-run cycle application process.

Fig. 7 (a) XPS survey scan of the Fe-MET framework before ZIB application (black) and after the cycling study of 2100 cycles (pink). (b) and (c) The SEM images of the used Fe-MET cathode after 2100 charge–discharge cycles in different resolutions.

The comparison of EDX spectra and elemental mapping of pristine Fe-MET and Fe-MET after discharging demonstrates the appearance of additional peaks for Zn and the presence of Zn in the used discharged Fe-MET cathode (Fig. 8). The appearance of Zn in EDX and elemental mapping of the used Fe-MET electrode further confirmed the intercalation and deintercalation mechanism of Zn²⁺ ions in Fe-MET. Again, the SEM image after 2100 cycles shows very minimal dendrite formation in the anode (Fig. S8 in the ESI†), resulting in better Fe-MET-based AZIB cycle stability. The Fe-MET's robustness and high cycle stability arise from the redox-stable Fe centers and the tz ligand, whose N-centers ensure complete Fe coordination, leaving no sites for further chemical interaction.

Fig. 8 Selective area EDX study of the Fe-MET electrode (a) and (b) before ZIB application, and (g) and (h) after the 2100 cycle charge–discharge application of the AZIB. Selective area elemental mapping using SEM of the Fe-MET electrode for (c) overall elements, (d) N, (e) C, and (f) Fe. Similar elemental mapping of the used Fe-MET electrode for (i) overall elements, (j) C, (k) N, (l) Fe, and (m) Zn after 2100 cycle charge–discharge application of the AZIB.

4. Conclusion

In summary, an extremely robust, conducting, ultramicroporous, redox-active, 3D triazolate Fe-MOF has been constructed as a standalone cathode material for AZIBs. The Fe-MET MOF exhibits high conductivity and exceptional stability under diverse harsh conditions confirmed by various analytical techniques. The solid-state AZIB coin cell fabricated using Fe-MET as the cathode delivers a good specific capacity and a decent energy and power density of 54.4 W h kg⁻¹ and 5.64 W kg⁻¹, exhibiting over 60% capacitance retention after 2100 cycles with no significant loss in coulombic efficiency. The analysis of used Fe-MET electrodes reveals the structural robustness and evidence for Zn²⁺ intercalation/deintercalation during the long charging–discharging cycles. This report inaugurates using the 3D ultra-robust conductive MOF material for AZIB's durable cathode materials.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

C. C. and S. K. thank the BITS Pilani Hyderabad Campus for the instrumental facilities. S. K. is indebted to BITS Pilani for a fellowship. C. C. thanks the Anusandhan National Research Foundation (ANRF), formerly known as Science & Engineering Research Board (SERB), for the SERB-CRG (CRG/2023/002310) project. S. C. acknowledges the Ramanujan Fellowship (file no. RJF/2023/000062), ANRF, Government of India, for the funding support.

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Footnote

† Electronic supplementary information (ESI) available: Electrochemical calculations, SAED, FESEM, EDXS, PXRD, FTIR, TGA, post-mortem studies, and comparison table for the conductivity of 3D MOFs. See DOI: https://doi.org/10.1039/d5se00273g

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