Engineered iron-doped MOF nanosheets: acid-induced lattice strain for enhanced rate performance in asymmetric supercapacitors

Abstract

The development of electrode materials is the key to realizing efficient energy storage. In order to solve the problems of low conductivity and poor cycling stability of existing metal–organic frameworks (MOFs), element doping and chemical etching strategies are effective strategies. In this work, we propose a strategy to modify the surface of MOFs via Fe doping and HCl etching strategies. The Jahn–Teller effect was induced and the electronic configuration of Co was optimized by doping Fe³⁺ ions with [Fe(CN)₆]³⁻. In addition, HCl etching induces lattice strain, enhances the interaction between Fe and Co, and provides a fast charge transfer rate. This synergistic effect enhances the conductivity of the Co MOF, introduces more electrochemically active sites, and further accelerates the electrochemical reaction kinetics. In particular, the specific capacity of e-Fe-MOF CNs-30 at 1 A g⁻¹ is as high as 1431 C g⁻¹, and the capacity retention rate is 84.2%. Additionally, an e-Fe-MOF CNs-30//AC asymmetric supercapacitor was assembled, which has a high energy density of 83.75 W h kg⁻¹ and a superior cycling stability of about 91.66% after 5000 cycles. The structural design of these MOFs significantly improves the low energy density and cycle life of MOF-based supercapacitors and provides insights into the electronic structure regulation and lattice strain engineering of low conductivity MOF electrodes.

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Introduction

With the advancement of science and technology, the energy crisis has become a global challenge, and the extensive combustion of fossil fuels has significantly impacted the environment. Consequently, the development of renewable and clean energy technologies has become crucial.^1,2 Nonetheless, the utilization of these energy sources is constrained by factors such as climate and geographical location.³ To overcome these limitations, the research and development of energy storage technologies, particularly supercapacitors (SCs), are of paramount importance. SCs combine the high energy storage capacity of batteries with the rapid charge–discharge capabilities of capacitors and have been applied across various domains,^4,5 such as portable electronic devices, hybrid vehicles, smart grids, and other fields.^6,7 Nonetheless, the relatively low energy density of SCs poses a constraint on their more extensive use. To advance the development of SCs, the key lies in designing efficient electrode materials to enhance their energy density.

Metal–organic frameworks (MOFs) are a type of crystal composed of metal ions and organic ligands connected through coordination bonds, characterized by high specific surface area, high porosity, and controllable topological morphology.^8,9 These characteristics enable their widespread use in electrochemical energy storage.^10–12 The limited conductivity and stability of most MOFs impede their use in SCs. Currently, there are many strategies to improve or compensate for these inherent defects of MOF materials. For example, Bai et al.¹³ prepared a high-capacity Co₃O₄/g-C₃N₄ p–n junction with enhanced conductivity and charge transfer by simulating solar irradiation. The Co₃O₄ phase undergoes lattice distortion after illumination, and the high single-ion magnetic anisotropy and low self-selection configuration of Co³⁺ ions enhance the covalent component for the Co–O σ bond. The covalent configuration of the bond further increases the number of reactive centers, which is beneficial for greatly improving the conductivity and charge transfer efficiency of the material. Meanwhile, Yao et al.¹⁴ obtained a NiMn-MOF@CoOOH composite electrode. The introduction of the NiMn-MOF resulted in multiple lattice strains on the surface of CoOOH, accelerating ion conduction and introducing additional electrochemically active sites. Thus, NiMn-MOF@CoOOH reaches an excellent specific capacity of 1771 F g⁻¹ at 1 A g⁻¹. It is worth noting that both lattice strain and doping engineering are effective methods for designing high-performance electrode materials, but they always exist in a single form. For instance, Zhou et al.¹⁵ constructed a porous and defect-rich Fe_xBi_2−xS₃ solid solution structure using Fe-doped Bi₂O₂CO₃ porous nanosheets, and the resulting composite phase exhibited significantly higher energy storage performance than its single phase. This can be attributed to Fe solutes being randomly dispersed in the Bi₂S₃ framework, which can effectively regulate the electronic structure of Bi elements and introduce rich defects due to the presence of Fe(II). These results provide a scientific basis for designing ideal performance MOF materials using synergistic effects.

In this work, we propose an innovative strategy to address the challenges of MOF materials in terms of conductivity and cycling stability: (1) the synergistic effect of Fe doping and HCl etching is utilized to “double enhance” the MOF conductivity. On the one hand, the [Fe(CN)₆]³⁻ coordinate introduced by Fe doping triggers the Jahn–Teller (J–T) effect, which induces the formation of empty orbitals by Co³⁺ ions, generating the free motion of additional electrons to enhance the MOF electrical conductivity. On the other hand, the negative effect of the J–T effect on the MOF nanosheet structure is mitigated and synergized by using the lattice strain generated by HCl etching. The lattice strain not only stabilizes the MOF nanosheet structure, but also changes the electron density around Co to enhance the MOF conductivity together with the J–T effect. (2) The lattice strain is utilized to modulate the MOF nanostructure, thus extending the cycle life of the MOF. The lattice strain generates compressive stresses that cause the MOF nanosheets to curl. This structure can significantly increase the specific surface area of the MOF, expose more active sites to promote electrochemical reactions, and alleviate the surface volume expansion caused by long-term cycling, thus prolonging the cycle life of MOF materials. The e-Fe-MOF CNs-30 prepared based on this strategy exhibited a specific capacity of up to 1431 C g⁻¹ at 1 A g⁻¹. The constructed asymmetric supercapacitor (ASC) achieved a maximum energy density of 83.75 W h kg⁻¹ at a power density of 1600 W kg⁻¹ and maintained a capacitance retention of 91.66% after 5000 cycles, which is a performance superior to those of most of the currently reported electrode materials. This work not only provides a new idea for the design of high energy density supercapacitors based on MOF-based materials, but also provides potential and application of lattice strain engineering in materials science.

Results and discussion

Morphology and structure

The MOF-derived e-Fe-MOF CNs materials were synthesized by the “coprecipitation–ion–exchange–HCl etching” method, as shown in Fig. 1. In comparison with the traditional synthesis methods, such as hydrothermal/solvent heating, microwave-assisted, ultrasound-assisted, mechanochemical, and electrochemical methods,¹⁶ this room-temperature synthesis method can drastically reduce the operation difficulty and minimize the energy loss. Firstly, Co MOF nanosheets were synthesized by coordinating Co²⁺ ions with 2-methylimidazole (2-MIM) ligands by the coprecipitation method at room temperature. Then, [Fe(CN)₆]³⁻ ions undergo an anion exchange reaction in the Co MOF, introducing an Fe-doped Co MOF. The electronic structure of Co was regulated by doping transition metal Fe ions with a radius similar to that of the Co ions. The e-Fe-MOF CNs were obtained by immersing in HCl solution at room temperature and adjusting the etching time at different intervals. The lattice strain engineering in etching was mainly reflected in two aspects: (1) the strong interaction between Fe and Co is generated after etching, and the rapid charge transfer further optimizes the electronic structure of Co; (2) the etching suppresses the negative impact of the J–T effect in doping and maintains the stability of the two-dimensional (2D) structure of the MOF, and the three-dimensional (3D) structure transformation is realized. During this process, the apparent properties of the NF substrate are changed. As shown in Fig. S1,† the initial Co MOF grown on NF appears bright purple. After Fe doping, the color of NF changes to dark green. The retention of this dark green color is dependent on the HCl etching time, with a longer etching time resulting in a lighter shade of dark green.

Fig. 1 Schematic diagram of the preparation of e-Fe-MOF CNs.

The scanning electron microscopy (SEM) images in Fig. 2a–c and S2† show that samples with different etching times preserve the triangular nanosheet morphology of their precursors well. In particular, the triangular nanosheets of e-Fe-MOF CNs had undergone varying degrees of curling. The magnified images of e-Fe-MOF CNs-15, e-Fe-MOF CNs-30 and e-Fe-MOF CNs-60 confirmed this point well (insets of Fig. 2a–c). This indicates that HCl etching causes lattice strain, resulting in compressive stress acting from the edge to the center of the triangular nanosheets, resulting in this unique curly structure.¹⁷ Compared to 2D MOFs, this 3D structure significantly increases the specific surface area and exposes a greater number of active sites on the surface for redox reactions, further promoting the occurrence of electrochemical reactions.¹⁸

Fig. 2 Structural characterization of e-Fe-MOF CNs-15, e-Fe-MOF CNs-30, and e-Fe-MOF CNs-60. (a–c) SEM images (a₁, b₁, c₁: IFFT image, a₂, b₂, c₂: FFT image, and a₃, b₃, c₃: intensity profiles of the (211) crystal plane). (d) XRD pattern. (e) Elemental mapping images. (f) EDS spectrum of e-Fe-MOF CNs-30.

The structure of the etched samples was studied in more detail through a series of characterization processes. Transmission electron microscopy (TEM) images reveal that e-Fe-MOF CNs exhibit curling in random areas of the triangular nanosheets (Fig. S3a, b and d†). Furthermore, it is observed that as the etching time increases, the curling becomes more pronounced, which is in agreement with the SEM results. Moreover, the lattice fringes with crystal plane spacing of 0.473 nm, 0.482 nm, and 0.492 nm can be observed from high-resolution transmission electron microscopy (HRTEM) images (Fig. S3c†), which can be attributed to the (211) crystal plane of the Co MOF. It is worth noting that the standard spacing of the (211) crystal plane is 0.496 nm, and the sample plane spacing observed after 15, 30 and 60 min of HCl etching is smaller than the standard spacing. This implies the occurrence of lattice contraction, confirming the generation of lattice strain. Then, the selected area electron diffraction (SAED) pattern further reveals a low crystallinity of e-Fe-MOF CNs-30 (inset of Fig. S3c†). The X-ray diffraction (XRD) pattern shows the crystal structure of the sample before and after etching (Fig. 2d). After successfully creating the Co MOF precursor (Fig. S4†), the etched samples displayed diffraction peaks that matched the (100) and (111) planes of ZIF-67 (JCPDs PDF#43-0144) at 7.25° and 12.58° incidence angles.¹⁹ The absence of iron-related diffraction peaks in the Fe-doped Co MOF sample could be attributed to its minimal iron content.²⁰ In addition, the diffraction peak intensities in some planes of the samples at different etching times showed a nonlinear relationship with the etching time. The prolongation of etching time may increase the etching depth of the sample surface, but it may also be accompanied by irregular changes in the grain structure and arrangement in the sample, which affect the crystallinity of certain grain surfaces in the sample, and make the corresponding changes in the diffraction peak intensities show an irregular trend.^21,22 Notably, the characteristic reflections of e-Fe-MOF CNs-15, e-Fe-MOF CNs-30 and e-Fe-MOF CNs-60 at 17.86° on the (211) crystal plane increased to 18.84°, 18.42° and 18.22°, respectively. The lattice contraction rates were 4.8%, 2.7%, and 0.7%, which further prove the generation of lattice strain.²³ Additionally, the elements Co, Fe, C, N, and O are evenly dispersed throughout the entire area of the curled nanosheet (Fig. 2e and S5†). Among them, the Fe content in e-Fe-MOF CNs-30 is 4.89%, which proves the successful doping of the Fe element (Fig. 2f).

To illustrate the composition and internal functional group changes of the etching samples, we obtained the Fourier-transform IR (FTIR) spectra, as shown in Fig. 3a and S6a.† The Raman spectra in Fig. 3b and S6b† reveal two significant peaks at 2105 cm⁻¹ and 2150 cm⁻¹, corresponding to the CN vibration in Fe^3+/2+-CN-Co^3+/2+ of the etching samples. These peaks, which are characteristic of the CN groups in K₃[Fe(CN)₆] at 2085 cm⁻¹, shifted to 2100 cm⁻¹ and 2139 cm⁻¹ due to the influence of compressive stress.^24–26 Besides, a splitting peak appears at 2083 cm⁻¹, which is responsible for the introduction of hydrogen bonds by HCl etching, caused by hydrogen bond vibration. The UV-Vis spectra in Fig. S6c† for the Co MOF, Fe-doped Co MOF, and e-Fe-MOF CNs-30 indicate that the characteristic absorption peaks for the Co MOF are at 541 and 589 nm. The introduction of Fe and HCl etching do not alter these precursor peaks. Additionally, the specific surface area is crucial for the electrochemical performance of materials. After doping and etching, the thicknesses of multiple nanosheets in the samples were estimated, and their average thicknesses were reduced from ∼200 nm to ∼2 nm, which resulted in a size effect and improved the surface area to volume ratio of the MOF. Consequently, the specific surface area for e-Fe-MOF CNs-30 rose to 678 m² g⁻¹, a significant jump from the initial 71 m² g⁻¹ of the Co MOF. In addition, the pore structure of the MOF was significantly changed. Abundant mesopores and macropores mainly existed in the pristine Co MOF, and the pore distribution was mainly concentrated in 13.04 nm mesopores and 115.36 nm macropores. However, the modified e-Fe-MOF CNs-30 exhibits a more complex porous structure, which contains three types of pores, namely, micropores, mesopores, and macropores, with 1.4 nm micropores and 3.8 nm mesopores showing the main distribution (Fig. 3c and S7†). In this porous structure, micropores can provide sufficient active sites for ion storage, mesopores act as channels for ion migration and transport, and macropores provide the necessary space for ion buffering. This variety of porous structure gives e-Fe-MOF CNs-30 an excellent specific capacity.

Fig. 3 Characterization data of the Co MOF, Fe-doped Co MOF, e-Fe-MOF CNs-15, e-Fe-MOF CNs-30, and e-Fe-MOF CNs-60. (a) FTIR spectra. (b) Raman spectra. (c) N₂ adsorption–desorption isotherm curves of e-Fe-MOF CNs-30. (d) Possible crystal field study before and after Fe³⁺ doping in Co MOF. XPS spectra of (e) Co 2p and (f) Fe 2p. (g) Static contact angle.

To further investigate the chemical valence state changes of the materials after Fe doping and HCl etching, X-ray photoelectron spectroscopy (XPS) measurements were examined. As shown in Fig. S8a,† the binding energy peak of Fe doping can be clearly observed. In addition, the full survey scan confirmed that Co, Fe, C, N and O still exist in e-Fe-MOF CNs-30 after further etching (Fig. S8b†). As seen in Fig. S9,† the Co MOF doped with Fe³⁺ ions in K₃[Fe(CN)₆] does not cause any changes in the positions of Co²⁺ and Co³⁺ ions binding energies, but the peak binding energy of Co³⁺ ions increased significantly after doping. This indicates that more Co²⁺ ions are converted into Co³⁺ ions. At this point, the 3d orbital electrons of Co²⁺ and Co³⁺ are 3d⁷ and 3d⁶, respectively, with corresponding electron configurations of t_2g⁶e_g¹ and t_2g⁶e_g⁰. This can be attributed to the 3d orbital electron number of the central ion [Fe(CN)₆]³⁻ in the complex of 3d⁵ with an electron configuration of t_2g⁵e_g⁰.²⁷ The electron distribution in the ion is asymmetric, causing J–T distortion. Due to the J–T effect, the e_g orbital of Co³⁺ generates an empty orbital, which promotes the formation of free electrons in the original e_g orbital of Co³⁺, thereby enhancing the conductivity (Fig. 3d).²⁸ This may also be the reason for the slight change in the microstructure of the Co MOF after doping (Fig. S10†).²⁹

Meanwhile, the characteristic peaks at 783.4 eV and 798.6 eV corresponding to Co(II) 2p_3/2 and Co(II) 2p_1/2 and 781 eV and 796.4 eV corresponding to Co(III) 2p_3/2 and Co(III) 2p_1/2 can be observed in the Co 2p spectrum of e-Fe-MOF CNs-30. The characteristic peaks located at 788.9 eV and 776.8 eV were satellite peaks of Co 2p and Co(0). It may be due to the reducibility of HCl, which resulted in some Co²⁺/Co³⁺ ions being reduced to Co⁰ ions. Moreover, the etched material exhibits a shift towards higher binding energy, indicating a decrease in electron density near Co²⁺/Co³⁺ ions (Fig. 3e). As shown in Fig. 3f, the characteristic peaks at 708.3 eV and 720.9 eV in the Fe 2p spectrum correspond to the satellite peaks of Fe(III) 2p_3/2 and Fe(II) 2p_1/2 at 712.5 eV and 723.9 eV, respectively. Among them, most of the characteristic peaks in the Fe 2p spectrum of e-Fe-MOF CNs-30 shift towards lower binding energies. The phenomenon indicates an increase in electron density near Fe²⁺/Fe³⁺ ions. The above results confirm that lattice strain leads to electron transfer from Co to Fe, and the strong interaction between Fe and Co enhances electron mobility and further enhances conductivity.^30,31 The lattice strain seems to effectively suppress the negative effects of the J–T effect. Therefore, after Fe doping caused a structural change in the Co MOF, HCl etching also maintained the nanosheet structure of the Co MOF. The fitted peaks of O 1s at 531.04, 531.74, and 532.74 eV can be attributed to the characteristic peaks of M–O, O–H, and absorbed O. Moreover, the peaks observed at 528.8 eV in the Fe-doped Co MOF are likely due to the presence of lattice O (O_L) (Fig. S11†).³² With the creation of an acidic environment after etching, the O_L disappears in e-Fe-MOF CNs-30.³³ The corresponding C 1s, N 1s and O 1s of the Co MOF, Fe-doped Co MOF, and e-Fe-MOF CNs-30 are shown in Fig. S12 and S13.†

In order to analyze the hydrophilicity of the as-prepared materials, static contact angle tests were conducted (Fig. 3g). The initial contact angles of the Co MOF, Fe-doped Co MOF and e-Fe-MOF CNs-30 were 90°, 82°, and 61°, respectively. After 2 s, the contact angles of the Fe-doped Co MOF and e-Fe-MOF CNs-30 decreased to 68° and 49°, respectively, while the contact angle of the Co MOF remained unchanged. All these results imply that e-Fe-MOF CNs-30 has the best hydrophilicity. Due to the polarity of HCl molecules, rapid ion diffusion within the electrode is guaranteed, making it easier to come in contact with the active sites.³⁴

The electrochemical performances of the prepared electrodes were tested in a three-electrode system to explore their electrochemical behavior. As shown in Fig. 4a, all of these CV curves exhibit two pairs of redox peaks, indicating that the electrode undergoes two reversible redox reactions during energy storage. Among them, the electrode etched for 15 min is conspicuous by the presence of oxidation peaks due to its similar current density of oxidation peaks in both reactions. The redox peaks at lower and higher potentials (O₁/R₁ and O₂/R₂) correspond to the redox reactions of Fe²⁺/Fe³⁺ ions and Co²⁺/Co³⁺ ions, respectively. However, with the increase of etching time, the electrode morphology changed slightly, resulting in small shifts in their oxidation and reduction potentials, respectively.^35,36 In addition, the enhancement of the redox peak current densities for Fe and Co can be attributed to the increased exposure of a specific crystalline plane in the electrodes and the expansion of the electrochemically active surface area.^37,38 Meanwhile, the e-Fe-MOF CNs-30 electrode has a larger CV integrated area compared to e-Fe-MOF CNs-15 and e-Fe-MOF CNs-60 at 2 mV s⁻¹, which means that the active substance in the e-Fe-MOF CNs-30 electrode is maximally oxidized or reduced, and the amount of charge generated during the reaction is maximized to obtain the largest specific capacity. Then, we also conducted the CV tests of the Co MOF and Fe-doped Co MOF, and the results were as expected; the CV curve area of the e-Fe-MOF CNs-30 electrode is significantly larger than those of the other two electrodes (Fig. S14†). As the scan rate increases, there is no significant distortion in the shape of the CV curves of all the electrodes, indicating a faster transport rate of ions and protons (Fig. S15 and S16†). Furthermore, we evaluated the specific capacity values of all electrodes through GCD tests. In Fig. 4b, a clear discharge plateau is observed in the GCD curves of all electrodes, indicating their battery-type behaviors.³⁹ The e-Fe-MOF CNs-30 electrode has the longest discharge time at 1 A g⁻¹ compared to the other electrodes, which is consistent with the results of the CV curve analysis. Based on the discharge time (Fig. 4c), the specific capacities of the e-Fe-MOF CNs-15, e-Fe-MOF CNs-30 and e-Fe-MOF CNs-60 electrodes were 940 C g⁻¹, 1431 C g⁻¹, and 1048 C g⁻¹, respectively.

Fig. 4 Comparison of the electrochemical performances of e-Fe-MOF CNs-15, e-Fe-MOF CNs-30, and e-Fe-MOF CNs-60. (a) CV curves at 2 mV s⁻¹. (b) GCD curves at 1 A g⁻¹. (c) Specific capacity at 1 A g⁻¹. (d) Nyquist plots (inset: magnified high-frequency region and circuit diagram). The e-Fe-MOF CNs-30 electrode: (e) cycling stability after 5000 charge–discharge cycles, (f) linear relationships between log(i) and log(v), (g) capacitive contribution at 10 mV s⁻¹, and (h) contribution ratios at various scan rates.

The charge transfer ability of electrodes was further analyzed by electrochemical impedance spectroscopy (EIS), as shown in Fig. 4d. e-Fe-MOF CNs-30 has the smallest charge transfer resistance (R_ct) value of 5.743 Ω. This suggests that properly curled MOF nanosheets provide the maximum number of ion/electron transport channels. The detailed values are shown in Table S1.† Subsequently, the cycling stability of three electrodes with different etching times was tested. The capacitance retention of the e-Fe-MOF CNs-30 electrode was 84.2% at a current density of 50 A g⁻¹, and also achieved a high coulombic efficiency of 97.95% (Fig. 4e). The coulombic efficiency curves showed small fluctuations due to the trace amount of HCl that may still remain inside the e-Fe-MOF CNs-30 electrode. During a long charge/discharge cycle test in an alkaline KOH electrolyte, KOH and HCl will undergo a neutralization reaction with the equation KOH + HCl → KCl + H₂O. This reaction will continuously deplete the active ions in the electrolyte. During discharge, if there are insufficient active ions in the electrolyte, the electrode may not be able to fully release the stored charge, resulting in fluctuations in coulombic efficiency.^40,41 Meanwhile, the capacitance retention rates of the other two electrodes of e-Fe-MOF CNs-15 and e-Fe-MOF CNs-60 were 77.77% and 84.2%, respectively, at the same current density (Fig. S17†). The specific capacity of e-Fe-MOF CNs-30 exceeds those of most MOF-based electrode materials reported in alkaline electrolytes (Table S2†). Furthermore, we investigated the morphology of the e-Fe-MOF CNs-30 electrode after the cycling stability test. As shown in Fig. S18,† the curled nanosheet structure was well maintained after 5000 charge–discharge cycles. This configuration offers ample room to accommodate volume fluctuations during charging and discharging, thereby preserving the stability of the electrode structure.

The energy storage mechanism of electrodes is crucial, so the CV curves of all samples are analyzed using the relationship between the current (i) and scan rate (v), and the equations are as follows:⁴²

i = av^b (1)

log i = b log v + log a (2)

where i and v are the cathodic/anodic peak current (A) and scan rate (mV s⁻¹), respectively. In the case of small changes in scan rates, a and b can be considered as constants. The range of b values is usually between 0.5 and 1. When the b value is close to 0.5, the charge storage behavior is controlled by the diffusion-controlled process. If the b value is close to 1, capacitive controlled behavior dominates.⁴³ For e-Fe-MOF CNs-30, the b values at the anodic and cathodic peaks are 0.5 and 0.41, respectively (Fig. 4f), indicating a diffusion-controlled charge storage mechanism in electrodes. Subsequently, the capacitive contribution ratio is calculated using the following equation:⁴⁴

i (V) = k₁v + k₂v^½ (3)

where i is the current under a constant potential (V) and k₁ and k₂ are adjustable parameters. The purple area indicates a diffusion-controlled proportion of 94% at 10 mV s⁻¹ (Fig. 4g). With the increase of the scan rate, the proportion of capacitance control (depicted in pink) steadily rises, culminating at 22% at a scan rate of 20 mV s⁻¹. This suggests that the energy storage mechanism in e-Fe-MOF CNs-30 is predominantly governed by the diffusion-controlled process, as illustrated in Fig. 4h. The capacitive contributions for the other electrodes are detailed in Fig. S19,† demonstrating similar dynamic behavior.

Electrochemical performances of the assembled ASC

The feasibility of e-Fe-MOF CNs-30 electrodes for practical applications was further evaluated by assembling an ASC. The ASC was assembled using e-Fe-MOF CNs-30, activated carbon (AC), and cellulose as the positive electrode, negative electrode, and separator, respectively (Fig. 5a). Firstly, we tested the electrochemical properties of the AC negative electrode in a 1 M KOH electrolyte (Fig. S20†). The CV curves of the AC electrode display a shape close to a rectangle within the −1–0 V potential window, signifying double layer characteristics. According to the discharge time in the GCD curves, the specific capacitance of the AC electrode is calculated to be 204 F g⁻¹ at 1 A g⁻¹. Secondly, we calculated the mass of the AC negative electrode according to the eqn (S2)† and tested the electrochemical properties of the assembled ASC. The potential windows of e-Fe-MOF CNs-30 and AC electrodes are 0–0.5 V and −1–0 V at 10 mV s⁻¹, respectively (Fig. 5b). Then, the CV curves at various voltages were tested at a scan rate of 20 mV s⁻¹ (Fig. 5c). As the voltage window increases from 0–1.6 V to 0–1.8 V, an obvious polarization phenomenon occurs. Therefore, the working voltage window of the ASC is set to 0–1.6 V. Moreover, there is no significant distortion in the curve at different scan rates (Fig. 5d), which indicates that the ASC has good rate capability. According to the GCD curves at different current densities (Fig. 5e), the maximum specific capacitance of the ASC can reach 235.5 F g⁻¹ at 2 A g⁻¹ and maintain 126.25 F g⁻¹ at 20 A g⁻¹ (Fig. 5f).

Fig. 5 Electrochemical performance of e-Fe-MOF CNs-30//AC ASC. (a) The composition of the ASC device. (b) CV curves were collected for e-Fe-MOF CNs-30 and AC at 10 mV s⁻¹. (c) CV curves at different voltages at 20 mV s⁻¹. (d) CV curves at various scan rates. (e) GCD curves at various current densities. (f) The specific capacitance at different current densities. (g) Rate capability. (h) Cycling stability after 5000 charge–discharge cycles. (i) Ragone plots.

Furthermore, the rate performance was a significant parameter for the ASC. We also tested the ASC under a series of current densities in 50 cycles (Fig. 5g). After the current density increases from 2 to 12 A g⁻¹ and then returns to 2 A g⁻¹, the specific capacitance has no significant decrease and exhibits excellent electrochemical reversibility. After 5000 cycles, the capacitance retention is 91.66% of the initial specific capacitance. Besides, the coulombic efficiency remains at 91.67% after the cycling process (Fig. 5h). Due to the repeated shuttle of K⁺ or OH⁻ in the electrolyte between the positive and negative electrodes during a long-term charge–discharge cycle at high current density, the electrode volume will inevitably expand, which will greatly shorten the cycle life of the device. Thus, the 3D curled NS structure of e-Fe-MOF CNs-30 can effectively alleviate volume expansion and greatly extend the cycling life of the device. Finally, the energy and power density of the e-Fe-MOF CNs-30//AC ASC were determined at different densities. The ASC device achieves a peak energy density of 83.75 W h kg⁻¹ at a power density of 1600 W kg⁻¹. Furthermore, it sustains an energy density of 44.88 W h kg⁻¹ at an elevated power density of 16 kW kg⁻¹, outperforming many previously reported devices (Fig. 5i and Table S3†).^45–56

Considering the practical application of the SCs as an energy storage power source, we connected the device in series and parallel to observe its power supply to microelectronic devices. Fig. 6a and b show the CV and GCD curves of the series and parallel devices. The working voltage window of a series device is twice that of a single device, while the discharge time of a parallel device is twice that of a single device. The two devices were connected in series to drive a pink light emitting diode (LED) successfully. Meanwhile, the electronic watch with a working voltage of 1.5 V can also work normally (Fig. 6c and d). This further substantiates the viability of e-Fe-MOF CNs-30 as a positive electrode material for SCs in practical applications.

Fig. 6 The practical applications of the ASC device. (a and b) CV and GCD curves of a single device and two devices connected in series and parallel at 20 mV s⁻¹ and at 4 A g⁻¹. (c) Optical image of two devices connected in series driving an LED. (d) Optical images of a single device driving an electronic watch in 10 min.

Conclusions

In conclusion, we have successfully improved the inherent defects of Co MOFs by Fe doping and HCl etching strategies. On the one hand, HCl etching realizes the transformation of the MOF from a 2D to 3D structure by regulating and triggering the lattice strain of the MOF nanostructure, which provides a large specific surface area and increases the number of surface accessible active sites. The curled nanosheet structure mitigated the volume expansion of the MOF during the cycling process and prolonged the cycle life. In particular, the MOF structure changes upon Fe doping and is restored upon HCl etching. The lattice strain plays a dual role in regulating and stabilizing the structure. On the other hand, Fe doping in the MOF optimizes the electronic structure of Co, generating the J–T effect. This effect induces empty orbitals after the conversion of Co²⁺ ions to Co³⁺ ions, and the electrons in the original orbitals become free electrons, ultimately improving the electrical conductivity. The lattice strain induced by HCl etching facilitates charge transfer between Fe and Co, further promoting the enhancement of electrical conductivity. In both aspects, Fe doping and HCl etching show a synergistic effect. Therefore, the specific capacity of e-Fe-MOF CNs-30 reaches up to 1431 C g⁻¹ at 1 A g⁻¹. It also has excellent cycling stability, maintaining 84.2% of its original capacitance after 5000 cycles. In addition, the assembled e-Fe-MOF CNs-30//AC ASC can reach a maximum energy density and power density of 83.75 W h kg⁻¹ and 16 kW kg⁻¹, respectively. This work provides not only a valuable scientific basis for modified MOF materials, but also a precious reference value for their application in future energy storage and conversion technologies.

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

This work is supported by the National Natural Science Foundation of China (NSFC, No. 52402100), the China Postdoctoral Science Foundation (No. 2023M740552), and the Heilongjiang Postdoctoral Foundation (No. LBH-Z23067). Y. Z. acknowledges the technical support from the Analysis and Testing Center of NEFU.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi01670j

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