Novel isostructural iron-series-MOF calcined derivatives as positive and negative electrodes: A new strategy to obtain matched electrodes in a supercapacitor device

Abstract

The performance of asymmetric supercapacitors (ASCs) is strongly restricted by the capacity gap between the positive and negative electrodes. To address this issue, two new electrode materials deriving from Co- and Fe-based metal–organic frameworks (MOFs, Co-TAMBA-d, and Fe-TAMBA-d) through a single-step sintering method have been developed by considering the superiorities of the derivatives of MOFs including large surface areas, sufficient metal-atom-doping content, and extreme surface wettability to the bath solution. The as-prepared Co-TAMBA-d as a positive electrode delivers typical pseudocapacitive behavior with the improvement of capacity, which is better than those of pristine MOF materials, while Fe-TAMBA-d as negative electrodes displays better electrochemical behavior than those of activated carbon. ASCs based on these two electrodes exhibits excellent energy density and power density of 47 W h/kg and 1658 W/kg, respectively, where this device can maintain prominent cycling stability with capacity retention after 5000 cycles being about 75%. Furthermore, the capacity can feed a series of red light-emitting diodes, which gives solid evidence of the potential utilization. These results can afford the feasibility of isostructural MOF derivatives as promising electrodes in novel ASCs.

INTRODUCTION

As one of the key storage devices in energy management, environmentally friendly supercapacitors are receiving more and more attention from scientists. Especially, asymmetric supercapacitors (ASCs) with two different materials as positive and negative electrodes become a hot topic due to their several advantages including long life span, high specific energy, large power density, and rapid charging/discharging rate.^1–6 Typically, in ASC systems, electrodes can offer the active sites for redox reactions or the adsorption/desorption of chemicals, which plays a decisive role in the performance of supercapacitors. Currently, the electrode materials can be divided into several types including metallic oxide/hydroxide/sulfide,^7–11 metal–organic frameworks (MOFs),^12,13 and MOF derivatives.¹⁴ Due to the abundant reaction sites and viable accesses for ions deliveries,¹⁵ MOF derivatives are of high profiles among these electrode materials.^13,16–18 For example, He et al.¹⁹ reported a synthetic method of bimetallic hydroxide with good electrochemical performances from a MOF template. The assembled Co₃O₄@NiCo₂O₄ nanosheet synthesized by Tao et al.²⁰ from Co-MOF was used in an ASC device, which exhibited a high energy density of 36 W h/kg at a power density of 852 W/kg. Recently, Shi et al.²¹ fabricated Ni/C/rGO-n derived from Ni-MOF for supercapacitors, which displayed a great rate capacity with 86.4% retention from 8 to 20 A/g. These prominent electrochemical performances could be ascribed to the superiorities of MOF derivatives, such as large surface areas, sufficient metal atom doping content, and extreme surface wettability to the bath solution.

Although many signs of progress in the positive electrode materials of supercapacitors have been witnessed, the negative counterparts lag behind. In fact, the voltage window of most MOF derivatives is in the range of positive potential except for few MOF derivatives displaying well electrochemical performance within the negative potential.²² However, as for the ASC system, the innovative advance of negative electrode materials is as equally important as that of positive electrode materials, and they cooperate with each other to determine the total voltage window, capacities, specific energy, and power of ASC devices.²³ In a lot of earlier studies, many types of research focused on the development of promising negative electrode materials with good electrochemical properties, while the problem of the capacity gap between positive and negative electrodes has been ignored originating from the utilization of stable products (especially activated carbon) with inferior electrochemical performances for assembling ASC devices. Unfortunately, such an arrangement scarifies the capacity potential of those remarkable positive electrodes in this one-side method. The strategy of designing the matched positive and negative electrodes is significant to reduce the capacity gap and utilize the capacity potential to the greatest extent when assembling ASC devices.

Co-based materials are the most profoundly developed positive electrode materials due to their preponderance in active redox properties and low toxicity.^24–27 Liu et al.²⁸ constructed Co-MOF-derived Co₉S₈@S,N-doped carbon materials, which displayed a high capacity value of 429 F/g at a current density of 2 A/g. In contrast, Fe-based materials are supposed to be the prospective candidates as negative electrode materials, and Hou et al.²⁹ assembled an ASC with Fe₃C/Fe@NC as the negative electrode, which could deliver a high energy density of 72 W·h/kg at a power density of 830 W/kg. Therefore, it is rational to prepare Co- and Fe-based materials as the positive and negative electrodes in the assembly of ASC devices.^30–34 Clearly, it is an efficient design philosophy to prepare novel MOF-derivative materials as electrodes,³⁵ especially the desired Co/Fe-based MOF derivatives. In this respect, Salunkhe et al.³⁶ have synchronously prepared positive and negative electrode materials from MOFs by adjusting the heat treatment conditions, in which this ASC reaches a high specific energy density of 36 W·h/kg and high specific power of 1600 W/kg. Moreover, we also reported a negative material from two-dimensional (2D) porous MOF nanosheet precursors that paired with other derivatives from the same MOF to assemble an ASC, which exhibited a high energy density of 33 W h/kg with a power density of 375.1 W/kg.³⁷ However, the isostructural MOF derivatives were employed as both positive and negative materials in the manufacture of ASC devices to observe the electrochemical performances toward supercapacitors, which has not been reported.

In this study, using a novel azole carboxylic acid ligand, we successfully prepared isostructural MOFs of Co-TAMBA and Fe-TAMBA and further converted them into two derivatives of Co-TAMBA-d and Fe-TAMBA-d through calcining them at an N₂ atmosphere. The as-obtained Co/Fe-MOFs derivatives possess many advantages such as similar structures, ample redox reaction sites, and comparatively large BET surface areas, which can benefit the transportation and diffusion of ions/electrons between two electrodes. Innovatively, when pairing Co-TAMBA-d with Fe-TAMBA-d in an ASC device of Co-TAMBA-d//Fe-TAMBA-d, this device performs much better than those ASC devices based on conventional activated carbon as negative electrodes. The ASC device realizes a maximum energy density of 47 W·h/kg at a specific power of 1658 W/kg and can even maintain an excellent specific energy of 21 W·h/kg when power density is added to 8494 W/kg. Such results can be ascribed to the analogical structures with the alike capacities. These results confirm that employing electrode materials from two isostructural MOFs as positive and negative electrodes in one ACS device is a pregnant strategy to get a well-matched positive–negative electrode pair to maximize its electrochemical properties.

RESULTS AND DISCUSSION Characterization and electrochemical performance of Co-TAMBA and Fe-TAMBA

Isostructural [M(TAMBA)₂(H₂O)₂]_n (M = Co or Fe), denoted as Co-TAMBA or Fe-TAMBA, has been successfully obtained and proven through single-crystal X-ray diffraction analysis. Since Co-TAMBA and Fe-TAMBA possess isostructural characteristics, only Co-TAMBA is represented meticulously. The structure of Fe-TAMBA is described in Supporting Information: Figure S1. The analysis shows that the Co^II ion in the asymmetric unit is six-coordinated with four TAMBA ligands and two water molecules to form a distorted octahedral geometry (Figure 1A). The adjacent Co^II ions are linked together by four TAMBA ligands to form one-dimensional (1D) chains (Figure 1B), which are further bridged by abundant intermolecular hydrogen bonds of O1W-H1A···N2, N1-H1N···O2, and N3-H20···O1 to form a pseudo-2D layer (Figure 1C). Moreover, these pseudo-2D layers are further linked by O1W-H1B···O1 to generate a three-dimensional (3D) network (Figure 1D).

Figure 1. (A) Coordination environment for CoII in Co-TAMBA. (B) One-dimensional chain with adjacent ring structures of Co-TAMBA. (C) Two-dimensional layer of Co-TAMBA. (D) Three-dimensional network of Co-TAMBA.

To verify the new isostructural characters of Co-TAMBA and Fe-TAMBA, the X-ray diffraction (XRD) spectra were tested and provided in Supporting Information: Figure S2, in which the peak positions of both coordinated complexes resembled each other. The experimental XRD patterns of both Co-TAMBA and Fe-TAMBA are matched well with the simulated XRD modes obtained from single-crystal data, indicating that the two MOFs are pure and have isostructures. The isostructural information of Co-TAMBA and Fe-TAMBA is further testified by FT-IR spectra, where both FT-IR spectra (Supporting Information: Figure S3) show the same specific peaks from the characteristic group in the HAMBA ligand, confirming their isostructures. Moreover, the broad peaks at 3450 cm⁻¹ symbolize the existence of –OH groups from the H₂O molecules. Close to this broad peak, several peaks centered around 3000 cm⁻¹ are responsible for the stretching vibration of C–H bands from TAMBA. Some other peaks at 3130 cm⁻¹ and 1600–1400 cm⁻¹ can be assigned to the vibrations of aromatic C–H and C=C bonds as well as the benzene skeleton from ligands. Moreover, the absorption bands at ~863 cm⁻¹ are associated with the ring vibrations of 1,4-substituted benzenes, meaning that the TAMBA ligands are coordinated with cobalt/iron ions in Co-TAMBA and Fe-TAMBA.

The thermal stabilities of Co/Fe-TAMBA were evaluated by TGA (Supporting Information: Figure S4). A heavy mass loss at around 400 °C corresponds to the pyrolysis of organic ligands in Co/Fe-TAMBA. To obtain their derivatives, 800 °C was selected as the calcination temperature on the basis of the TG analyses.

To evaluate the electrochemical properties of as-prepared crystalline MOFs, the Co-TAMBA and Fe-TAMBA were used as working electrode materials under a three-electrode configuration in a 6 mol/L potassium hydroxide electrolyte. The loading of the effective constituent in a three-electrode system is about 2 mg. A series of tests such as galvanostatic charge–discharge (GCD) and cyclic voltammetry (CV) were executed. As shown in Supporting Information: Figure S5A,B, a couple of distinctly parted redox peaks are depicted in the CV curves of Co-TAMBA and Fe-TAMBA electrodes. For Co-TAMBA and Fe-TAMBA electrodes, the oxidation peaks exist in the range of 0.42–0.5 and −0.6 to −0.4 V, while the reduction peaks locate at around 0.25–0.35 and −1 to −0.9 V, respectively. During the charging procedure, the existence of two peaks from Co-TAMBA could be ascribed to the oxidizing reactions of Co^II into Co^III and Co^IV.³⁸ As for Fe-TAMBA, the occurrence of oxidizing peaks could belong to the oxidation reactions of Fe^II. Given the existence of redox peaks in the CV curves, these two MOF-based electrodes perform the evident battery-type behaviors.^39–41 Due to the differences in central metals, Co-TAMBA and Fe-TAMBA show potential redox capacities as positive and negative electrodes in an ASC system, respectively. Further, the gentle platforms in the GCD profiles (Supporting Information: Figure S5C,D) further confirm the features of the battery-type supercapacitors.⁴² Meanwhile, the GCD method is also trustworthy for calculating the specific capacity of electrode materials at different current densities. At a current density value of 2 A/g, the discharge capacities of the Co-TAMBA and Fe-TAMBA electrodes were calculated severally to be 168 and 192 C/g. It is obvious that the capacities of these two MOFs are comparatively lower compared with those in recent reports.⁴³ The cycling stabilities of Co-TAMBA and Fe-TAMBA electrodes were also investigated. As shown in Supporting Information: Figure S6A,B, after 2000 cycles, the capacity retentions of the Co-TAMBA and Fe-TAMBA electrodes are 76% and 26%, respectively. Potentially, the disadvantages such as diminutive surface areas, less exposed active sites, and poor inferior cycling stabilities affect the electrochemical performances of Co-TAMBA and Fe-TAMBA electrodes. To improve the capacities of electrode materials, one strategy of MOF derivatives instead of the crystalline Co-TAMBA and Fe-TAMBA has been adopted, anticipating that the MOF derivatives as electrode materials could benefit from the delivery of ions, the increase of specific areas, and the abundant active sites.³⁷ Hence, these two new isostructural MOFs are calcined to produce the MOF derivatives, named Co-TAMBA-d and Fe-TAMBA-d.

Characterization and electrochemical performance of Co-TAMBA-d and Fe-TAMBA-d

The obtained MOF precursors were calcined in an Ar atmosphere at 800 °C, finally producing Co-TAMBA-d/Fe-TAMBA-d nanospheres as confirmed later. To explore the morphologies of the calcined representatives, the SEM study is conducted. In Supporting Information: Figure S7A,B, Co-TAMBA-d samples show closely packed spheres with a diameter of 200 nm, which is similar to the Fe-TAMBA-d samples (Supporting Information: Figure S7C,D), suggesting that both Co-TAMBA-d and Fe-TAMBA-d have similar structural characteristics.

The specific external surface areas of Co-TAMBA/Fe-TAMBA and Co-TAMBA-d/Fe-TAMBA-d were calculated by the BET gravimetric surface areas (Supporting Information: Figure S8A). In fact, the surface areas of Co-TAMBA/Fe-TAMBA are only 1.3 and 5.2 m²/g separately, suggesting inferior properties. In comparison, the surface areas of Co-TAMBA-d/Fe-TAMBA-d, also measured by the BET method, are 178 and 127 m²/g, respectively. The thoroughly collapsed structure obviously improves the surface area of Co/Fe-MOF-d materials. Moreover, the result was also proven by the BJH hole size distribution graph, as shown in Supporting Information: Figure S8B, which further affirms the existence of micro and mesopores. It has been reported that the capacity of electrode materials strongly relies on the surface area, total hole volume, and microdensity. Therefore, a large specific surface area and abundant micropores would directly promote charge transport at the electrolyte/electrode interfaces, which can facilitate the enhancement of capacity, leading to higher electrochemical performance for supercapacitors.

The information from the FT-IR spectra (Figure 2A) can verify the similar structural features of Co-TAMBA-d and Fe-TAMBA-d. There is a distinct peak at ~3440 cm⁻¹, indicating the existence of hydroxide groups from chemisorbed water molecules.⁴⁴ The peaks of the vibration of C–N and C–C bonds are located at 2800 and 1620 cm⁻¹. The absorption bands in the range of 940–1150 cm⁻¹ are attributed to the stretching bonds of C–O. The peaks in the range of 1000–1300 cm⁻¹ belong to the stretching and ring vibrations of C–C and C–N bonds. The characteristic peaks of TAMBA ligands (especially the peaks at 3000, 2800–3000, 1820, and 863 cm⁻¹) disappeared, indicating the collapsing structure of MOFs and the formation of new materials.

Figure 2. (A) FT-IR images of Co-TAMBA-d and Fe-TAMBA-d. (B) XRD images of Co-TAMBA-d and Fe-TAMBA-d. (C) TEM image of Co-TAMBA-d. (D) HRTEM image of Co-TAMBA-d. (E) TEM image of Fe-TAMBA-d. (F) HRTEM image of Fe-TAMBA-d. FT-IR, Fourier-transform infrared spectroscopy; HRTEM, high-resolution transmission electron microscope;  TEM, transmission electron microscope; XRD, X-ray diffraction.

Further characterization through XRD (Figure 2B) indicates that three diffraction peaks (44.216°, 51.522°, and 75.853°) are in well agreement with the (111), (200), and (220) crystal planes of the Co metal (PDF# 15-0806), while the two diffraction peaks centered on 44.675° and 65.026° correspond to the (110) and (200) facets of the Fe metal (PDF# 87-0721), suggesting the formation of metal elements. The original peaks of Co-TAMBA and Fe-TAMBA are no longer present, further indicating the formation of new composites. Transmission electron microscope (TEM) images shown in Figure 2C,E exhibited that Co-TAMBA-d/Fe-TAMBA-d had numerous Co⁰/Fe⁰ nanoparticles. The detailed information is further viewed by a high-resolution transmission electron microscope (Figure 2D,F); Co-TAMBA-d/Fe-TAMBA-d displays legible crystal lattice stripes with slice gaps of 0.21 and 0.15 nm, which could be indexed to the (111) lattice plane of Co⁰ and the (200) lattice plane of Fe⁰. These massages are in accord with the results of XRD.

Scrupulously, X-ray photoelectron spectroscopy (XPS) has been introduced to investigate the surface factors of Co-TAMBA-d and Fe-TAMBA-d samples. As shown in Figure 3A,B, the XPS spectra of both Co-TAMBA-d and Fe-TAMBA-d composites validate the presence of elemental M (M = Co or Fe), C, N, and O. In the Co 2p spectrum (Figure 3C), three pairs of spin–orbit couplings and two satellite peaks (786.9 and 804.8 eV) can be observed, where peaks located at 778.2 and 795 eV are deemed to Co⁰, the peaks at 780.9 and 791.9 eV are assigned to Co^II, and the peaks at 783.5 and 798.5 eV belong to Co^III.⁴⁵ The Fe 2p spectrum (Figure 3D) has two peaks of Fe 2p_1/2 and Fe 2p_3/2 and two satellite peaks (716.2 and 733.2 eV). Fe 2p_1/2 and Fe 2p_3/2 can further be split into three pairs of spin–orbit couplings, in which the peaks centered at 709.8 and 721.5 eV are deemed to Fe⁰,⁴⁶ the peaks at 711.1 and 724.8 eV are supposed to be Fe^II,⁴⁶ and the peaks at 715.5 and 727.1 eV attribute to Fe^III.⁴⁷ The high resolution of C 1s spectra (Supporting Information: Figure S9A,B) can be deconvoluted into two obvious peaks separately at 284.69 eV (Co; Fe: 284.68 eV, C–C/C–H) and 285.72 eV (Co; Fe: 286.02 eV, C–O/C–N).⁴⁸ These results are in agreement with the FT-IR analysis. In the N 1s region (Supporting Information: Figure S9C,D), three peaks are attributed to pyrrolic N (Co: 400.88 eV, Fe: 400.48 eV), pyridinic N (Co: 398.48 eV, Fe: 398.98 eV), and graphitic N (Co: 402.58 eV, Fe: 401.58 eV), proving that the N has been doped into the material framework with various patterns.⁴⁴ The N sites can supply lone-pair atoms, which could enhance the surface wettability and devote to the capacity. Moreover, the graphitic N could strengthen the ions’ diffusion of the as-prepared materials.^44,49,50 As shown in Supporting Information: Figure S9E,F, the O 1s region can be resolved into three peaks, corresponding to –OH (Co: 533.78 eV, Fe: 533.76 eV), C–O (Co: 531.88 eV, Fe: 531.85 eV), and metal–O (Co: 529.86 eV, Fe: 530.17 eV).^48,51,52

Figure 3. (A, B) XPS images of Co-TAMBA-d and Fe-TAMBA-d. (C) High-resolution XPS images of Co 2p for Co-TAMBA-d. (D) High-resolution XPS images of Fe 2p for Fe-TAMBA-d. XPS, X-ray photoelectron spectroscopy. 

Under a three-electrode configuration in a 6 mol/L potassium hydroxide electrolyte, the electrochemical performances have been tested. In the potential range of 0–0.5 V, the energy storage character of Co-TAMBA-d electrodes could be simply regarded as battery-type capacity due to its surface redox sites. As shown in Figure 4A, a couple of redox peaks display over the potential window from 0.2 to 0.5 V, where the oxidation peak is located at around 0.5 V and the reduction peak is centered at about 0.25 V. As for the Fe-TAMBA-d materials (Figure 4B), a pair of reversible redox peaks are observed from −0.5 to −1 V, and the reduction peak locates at probably −0.95 V. In contrast, the oxidation peak appears at approximately −0.6 V. The distinct redox peaks and the manifested profile outlines for both Co-TAMBA-d and Fe-TAMBA-d electrodes are not equal to those of a common electrical double-layer capacitor,⁵³ pointing out that the electric behavior is assigned to battery mode.³⁹ The distorted triangle of the charge–discharge profile (Figure 4C,D) indicates a metal-based faradaic characteristic owing to the intercalation/deintercalation of hydroxyl ions.⁵⁴ The OH− can immerse into the framework of MOF-derivative materials and transfer charge between the electrolyte and electrodes.^53,55 As shown in Scheme 1, the working mechanism is similar to those of the reports^53,55 about the theories of supercapacitors under an alkaline system. The redox reactions can be summed up by the following equations^56–58: [Image Omitted. See PDF] [Image Omitted. See PDF] [Image Omitted. See PDF] [Image Omitted. See PDF] [Image Omitted. See PDF] [Image Omitted. See PDF] [Image Omitted. See PDF] [Image Omitted. See PDF]

Figure 4. (A) CV curves of Co-TAMBA-d electrodes at different sweep rates. (B) CV curves of Fe-TAMBA-d electrodes at different sweep rates. (C) GCD curves of Co-TAMBA-d electrodes at different current densities. (D) GCD curves of Fe-TAMBA-d electrodes at different current densities. (E) Specific capacities at different current densities. Inset: EIS results of Co-TAMBA, Fe-TAMBA, Co-TAMBA-d, and Fe-TAMBA-d. (F) Endurance test for Co-TAMBA-d and Fe-TAMBA-d electrodes at a current density of 2 A/g. CV, cyclic voltammetry; EIS, electrochemical impedance spectroscopy; GCD, galvanostatic charge–discharge. 

Scheme 1. Speculated working mechanism of Co-/Fe-TAMBA-d under the alkaline system

At a constant current density of 2 A/g, the capacity values of Co-TAMBA-d and Fe-TAMBA-d electrodes are 304 and 392 C/g, which are calculated from discharge branches of the GCD curves, slightly higher than those of MOF-based electrodes. After a fivefold increase in current density (Figure 4E), the capacities of Co-TAMBA-d and Fe-TAMBA-d electrodes still retain 200 and 274 C/g, further indicating better rate capacities of Co-TAMBA-d and Fe-TAMBA-d electrodes than those of MOF-based electrodes.

The extent of contact between electrodes and electrolytes and the diffusion/emergence of charge and electrolyte ions are significant factors to determine the properties of materials. Some of these data (i.e., electrochemical impedance spectroscopy [EIS] analysis) are also collected. The Nyquist plots for Co-TAMBA-d and Fe-TAMBA-d electrodes are shown in Figure 4E inset and Supporting Information: Figure S10. In the high-frequency region, the equivalent series resistance (R_s) of Co-TAMBA-d and Fe-TAMBA-d electrodes is obtained from the intersection of the x-axis and the semicircle branch of the Nyquist plot, which are 0.47 and 0.54 Ω separately, representing the internal resistance of the electrode material, the electrolyte resistance, and the resistance of the interface between electrodes and electrolyte. The charge transfer resistance (R_ct) for Co-TAMBA-d and Fe-TAMBA-d electrodes, determined from semicircle diameter, is 0.40 and 0.45 Ω, respectively. These results provide an account of the brilliant performance of charge/proton transformation for Co-TAMBA-d and Fe-TAMBA-d electrodes. In the low-frequency region, the slope of the profiles indicates the diffusion resistance of the ions from the surface of the electrode to the electrolyte. The linear slopes of Co-TAMBA-d and Fe-TAMBA-d electrodes are high, suggesting excellent diffusion of ions and fast redox reactions. The Z’ versus ω^−1/2 curves are presented in Supporting Information: Figure S11, from which the Warburg coefficient value (σ) and the diffusion coefficient value of OH⁻ (D) can be calculated in line with the equations below⁵⁹: [Image Omitted. See PDF] [Image Omitted. See PDF]where R is the gas constant (J/(mol·K)); T represents the temperature (K); A is the area of the working electrode surface (cm²); n refers to the transferred electrons; F is Faraday's constant (95,400 C/mol); C is the concentration of hydroxyl ions (mol/cm³); σ is the Warburg coefficient; and ω is the angular frequency (Hz). The relevant σ values of Co-TAMBA, Co-TAMBA-d, Fe-TAMBA, and Fe-TAMBA-d electrodes are calculated to be 19.94, 18.17, 69.42, and 67.96, and consequently, their diffusion coefficients of OH⁻ (D) are 4.89 × 10⁻¹³, 5.87 × 10⁻¹³, 4.02 × 10⁻¹⁴, and 4.20 × 10⁻¹⁴ cm²/s, respectively. The large D values for Co-TAMBA-d and Fe-TAMBA-d mean a fast electronic transfer process.⁶⁰

Meanwhile, the different surface factors are also an influential part of the electrochemical performances of these materials. The larger surface areas and high micro- and mesopore volumes imply that Co/Fe-MOF-d can not only offer more electron transport channels and diffusion paths but also provide more active sites, thereby enhancing electrochemical performance for supercapacitors.^57,61,62 Detailed analyses of CV curves are conducted through the introduction of the power-law equation to explain the reason for the significance of the surface factor.³⁸ This equation of current (i) and sweep rate (v) is proposed by the Dunn group³⁸: $$i=a{v}^{b}$$, where a and b are constant terms, the b value can be determined from the slope of log i versus log v. The b values of Co-TAMBA, Fe-TAMBA, Co-TAMBA-d, and Fe-TAMBA-d are 0.62, 0.59, 0.69, and 0.53 correspondingly, which fall in between 0.5 (diffusion-controlled course) and 1.0 (surface capacitive controlled course). It suggests that the battery-type behaviors for these four electrodes combine two different mechanisms simultaneously. The contributions from both processes to these materials can be distinguished by using the following equation³⁸: $$i={k}_{1}v+{k}_{2}{v}^{1/2}$$, where k₂v^1/2 and k₁v represent the diffusion-controlled and capacitive-controlled fractions, respectively. The data of these two parts are shown in Supporting Information: Figure S12A–D and Figure 5A,B through quantified calculation. For Co-TAMBA, Fe-TAMBA, Co-TAMBA-d, and Fe-TAMBA-d, at a scan rate of 2 mV/s, the capacitive-controlled fraction in the total stored charge is 30%, 30%, 41%, and 41%, respectively. The higher capacitive fractions of Co-TAMBA-d/Fe-TAMBA-d at all scan rates reveal their fast electron and electrolyte ion transport. Superior properties of the produced Co-TAMBA-d/Fe-TAMBA-d are strongly attributed to their structural characteristics, such as large specific surface areas and abundant micropores.

Figure 5. (A, B) Voltammetric response for the as-prepared Co-TAMBA-d and Fe-TAMBA-d electrodes at a sweep rate of 10 mV/s. (C) High-resolution XPS images of Co 2p of Co-TAMBA-d after cycling. (D) High-resolution XPS images of Fe 2p of Fe-TAMBA-d after cycling. XPS, X-ray photoelectron spectroscopy.

The cycling stabilities of Co-TAMBA-d and Fe-TAMBA-d electrodes were also investigated. As shown in Figure 4F, after 5000 cycles, the Co-TAMBA-d and Fe-TAMBA-d electrodes exhibited brilliant cycling stabilities by comparing the first loop, and the retentions of capacity are 84% and 90%, respectively. These results imply the electrochemical stabilities of these two materials, suggesting that the one-step decomposition of pure MOFs by calcining is effective.

The structural and constituent modifications of Co-TAMBA-d and Fe-TAMBA-d samples after continuous exposure in 6 mol/L KOH have been characterized through SEM, XRD, FT-IR, XPS, and TEM methods. After 5000 cycles, except for the aggregated cluster motif through adding conductive improvers and binders, SEM images (Supporting Information: Figure S13A,D) of both Co-TAMBA-d and Fe-TAMBA-d electrodes did not show significant morphological change. It is attractive that the as-obtained Co-TAMBA-d and Fe-TAMBA-d materials have not disintegrated anymore, even under the caustic alkalic electrolytes. These phenomena illustrate the high cycling stability of the calcinated materials. FT-IR spectra (Supporting Information: Figure S14A,B) show that the derivative structures are well retained except for the slight shift of the hydroxyl-group stretching vibration. Interestingly, their ex situ TEM images, XRD spectra, and XPS patterns (Supporting Information: Figures S13–15) confirm that the crystal phase and the valance states of these materials were altered due to the hydrolysis in the alkalic aqueous electrolyte during the charge–discharge process. Given the results indicated above, we can infer that the redox reactions of the Co⁰/Co^II/Co^III and Fe⁰/Fe^II/Fe^III happened in the charging/discharging process after 5000 continuous cycles.

An ASC device, denoted as Co-TAMBA-d//Fe-TAMBA-d, was fabricated with Co-TAMBA-d as a positive electrode and Fe-TAMBA-d as a negative counterpart in this study. After testing anomalous polarization outlines of CV curves at the different potential windows, the working voltage for the Co-TAMBA-d//Fe-TAMBA-d device was finally laid within the limits of 0–1.5 V (Figure 6A).

Figure 6. Electrochemical performance. (A) Performance of the Co-TAMBA-d//Fe-TAMBA-d device in the different potential windows at a sweep rate of 50 mV/s. (B) CV curves of the Co-TAMBA-d//Fe-TAMBA-d device at different sweep rates. (C) GCD curves of Co-TAMBA-d//Fe-TAMBA-d devices at different current densities. (D) Endurance test at a current density of 3 A/g. Inset: GCD plots at the first nine cycles. (E) Ragone plot of power density and energy density for the Co-TAMBA-d//Fe-TAMBA-d device. (F) Several red light-emitting diodes in the parallel display. CV, cyclic voltammetry; GCD, galvanostatic charge–discharge.

In this working voltage, no distinct polarizations appeared along with the growth of the scan rates, suggesting the brilliant reversibility of the Co-TAMBA-d//Fe-TAMBA-d device (Figure 6B). Utilizing Co-TAMBA-d as positive electrode materials to pair with Fe-TAMBA-d materials could reach an excellent capacity value (226 C/g at 1 A/g, Figure 6C). At current densities of 2–10 A/g, the capacities of the ASC device are 226, 195, 163, 140, 128, and 102 C/g. The Co-TAMBA-d//Fe-TAMBA-d device obtains a specific energy of 47 W·h/kg at a power density of 1658 W/kg (Figure 6E, remained 21 W·h/kg at 8494 W/kg). As for negative electrodes fabricated in ASCs, the Fe-TAMBA-d represents superior properties to those utilizing activated carbon (Supporting Information: Table S1). Consequently, the assembling of ASC devices pairing two isostructural MOF-derivative electrodes would improve the energy and power densities effectively. Moreover, two devices in a series could power several red light-emitting diodes (LEDs) in parallel, illustrating the feasibility of the device (Figure 6F). These conclusions further indicate that the concurrent improvement of derivatives from isostructural MOFs as the positive or negative electrode is promising and effective.

The cycling stability can further prove the success of the simultaneous enhancement in the positive–negative systems. As shown in Figure 6D, the Co-TAMBA-d//Fe-TAMBA-d device exhibited excellent cycling endurance during all 5000 cycles and the graph nearly maintains a straight line after the first 1000 cycles. According to the comparison with the first cycle, the capacity retention rate is 75% (Figure 6D, inset).

In this study, the as-prepared Fe- and Co-TAMBA derivatives can work as prospective electrode materials for ASC devices, in which their high surface areas, large pore volumes, and capacitive matching effects may afford a good opportunity for this high performance. Meanwhile, graphitic, pyridinic, and pyrrolic N could effectively facilitate capacity and increase redox-active sites for energy storage. In comparison with conventional activated carbon as the negative electrode material, the Fe-TAMBA-d materials possess abundant metallic sites on the surface. Therefore, the Fe-TAMBA-d materials performed larger capacities (the maximum value: 392 C/g) than the conventional activated carbon in the same potential range of −1 to 0 V (the value of 100 C/g, Supporting Information: Figure S16). Employing the same synthesis process, isostructural Co/Fe-TAMBA precursors were obtained. Isostructural MOF precursors are the key factors to create a similar morphology during the conversion steps. Similar morphologies can provide analogical surface areas, surficial active sites, and wettability for derivatives, implying that isostructural MOF derivatives can be matched well. It speculates that narrowing the capacity gap between negative and positive electrodes plays a crucial role in the ASC device. Further, decreasing capacity disparity can improve the energy density and power density effectively.

CONCLUSION

The Co-TAMBA-d and Fe-TAMBA-d materials have been prepared from two isostructural MOF precursors and employed as positive and negative electrodes, respectively. The Co-TAMBA-d positive materials display the battery-type performance with the excellent capacity of 304 C/g at a current density of 2 A/g, and the Fe-TAMBA-d as independent negative electrode materials exhibits greater capacities (392 C/g at 2 A/g) than the conventional activated carbon and pure MOF materials among the same voltage range of −1 to 0 V. When the two isostructural MOF-derivative materials were fabricated into a total ASC device Co-TAMBA-d//Fe-TAMBA-d, the system presented brilliant electrochemical performance verifying by lighting a series of red LEDs, especially decent capacity (226 C/g) and great specific energy and power density (47 W·h/kg, 1658 W/kg). This study provides a reference that two isostructural MOF derivatives can be acted as promising candidates for the application in the ASC devices, which would improve the electrochemical properties by their analogical surface areas, surficial active sites, and wettability.

ACKNOWLEDGMENTS

This study was supported by the National Natural Science Foundation of China (Nos. 22279061, 21901120, 21371098), the Fundamental Research Funds for the Central Universities, the Natural Science Foundation of Jiangsu Province (Nos. BK20180514, BK20190503, BK20131314), and the Qing Lan Project of Jiangsu Province. Qichun Zhang thanks the funding support from City University of Hong Kong (Nos. 9380117, 7005620, and 7020040) and Hong Kong Institute for Advanced Study, City University of Hong Kong, China.

CONFLICTS OF INTEREST

The authors declare no conflicts of interest.

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available in the supplementary material of this article.