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Review Paper

Applied Science and Convergence Technology 2024; 33(3): 53-61

Published online May 30, 2024

https://doi.org/10.5757/ASCT.2024.33.3.53

Copyright © The Korean Vacuum Society.

Recent Advances in Metal-Organic Frameworks-Based Materials for Supercapacitor Applications

Bhimanaboina Ramulu and Jae Su Yu*

Department of Electronics and Information Convergence Engineering, Institute for Wearable Convergence Electronics, Kyung Hee University, Yongin 17104, Republic of Korea

Correspondence to:jsyu@khu.ac.kr

Received: May 14, 2024; Accepted: May 23, 2024

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc-nd/4.0/) which permits non-commercial use, distribution and reproduction in any medium without alteration, provided that the original work is properly cited.

This review discusses cutting-edge research on using metal-organic frameworks (MOFs) and their derivatives to improve the properties of energy storage active materials, with particular emphasis on pseudocapacitors and hybrid supercapacitors. This study presents novel methodologies for synthesizing MOF-based nano/micromaterials with distinct morphologies and structures, including anion exchange strategies and co-axial electrospinning processes. This review introduces case studies that demonstrate the superior performance of these materials as anodes in various energy storage applications, including lithium-ion capacitors and batteries. Furthermore, this study investigated the potential of MOF-based materials for zinc-ion hybrid capacitors and supercapacitors, emphasizing favorable electrochemical characteristics. This study emphasizes the ability of MOF-based materials to overcome the constraints of traditional electrode materials, providing insights into enhancing the charge storage and diffusion characteristics for improved electrochemical performance. These results have significant potential for expanding energy storage technology, assisting renewable energy integration, supporting grid stabilization, and optimizing portable electronics. This review emphasizes the value of multidisciplinary collaboration in materials science, chemistry, and engineering for translating findings into practical applications and successfully addressing global energy concerns.

Keywords: Metal-organic frameworks, Pseudocapacitors, Hybrid supercapacitors, Energy storage properties

Supercapacitors (SCs) have gained significant attention as energy storage devices because of their high power densities, fast charge/discharge rates, and extended cycling lifetimes [1]. The performance of SCs relies on carefully selecting active materials for their electrodes. Carbon-based materials, noble metals, conducting polymers, and transition metal hydroxides/oxides have been extensively explored as potential active materials for SCs. Carbon-based materials, such as activated carbon, carbon nanotubes, and graphene, offer large surface areas, excellent electrical conductivity, and good chemical stability. They operate through non-faradaic charge storage mechanisms, enabling high power density but limited energy density [2]. Conducting polymers, such as polypyrrole and polyaniline, and transition metal hydroxides/oxides, such as manganese and nickel hydroxides, undergo reversible faradaic charge storage processes involving redox reactions with electrolyte ions, enabling higher energy storage capacities. However, issues such as poor crystallinity, agglomeration, and mechanical instability should be resolved [3]. In recent years, metal-organic frameworks (MOFs) have emerged as promising candidates for SCs. MOFs, composed of metal ions or clusters coordinated to organic ligands, exhibit well-ordered nanoporous structures, large specific surface areas, and adjustable chemical functionalities [4]. In addition, they have demonstrated satisfactory electrical conductivities and offered cost-effective synthesis routes. MOFs can achieve high capacitance and efficient charge transfer utilizing reversible faradaic reactions. Their regular pore structures also facilitate rapid ion diffusion, supporting high-rate performance [5]. To fully exploit the potential of MOFs in SCs, challenges such as optimizing synthesis methods, improving electrical conductivity, enhancing structural stability, and ensuring long-term durability must be addressed [6].

This review provides a comprehensive overview of the advancements and challenges of using MOFs as active materials for SCs for high-performance and sustainable energy storage systems.

The charge storage mechanism of MOFs in SCs encompasses different processes, such as double-layer capacitance and pseudocapacitance/ battery-type behavior. MOFs with high surface areas enhance double-layer capacitance by adsorbing electrolyte ions at their metal or coordination sites, forming an electrical double layer [7] and providing additional charge storage capacity. Several MOFs exhibit pseudocapacitance because of their redox-active metal centers or organic linkers. The reversible redox reactions between the MOF and electrolyte ions enable faradaic charge storage and enhance energy storage capacity beyond that of the double-layer capacitance. Furthermore, certain MOFs display battery-type behavior, reversibly intercalating or inserting ions into their crystal structures. This mechanism resembles that observed in batteries and provides an avenue for charge storage. MOF’s multiple charge storage mechanism makes them highly attractive for SC applications [8], as they offer high capacitance, improved energy storage performance, and tailored functionality. However, the charge storage mechanism can vary depending on factors such as MOF’s structure, composition, and electrolyte [9]. Further research is required to optimize the properties of MOFs, understand their electrochemical behavior, and design tailored MOFbased SCs with enhanced performance and efficiency. Figure 1 illustrates the charge-storage mechanisms of certain active materials. Electric double-layer capacitor (EDLC)- and pseudocapacitor (PC)-type materials are distinguished by their charge storage mechanisms [10]. EDLC-type materials, especially carbon-based materials, collect charge non-faradaically, resulting in cyclic voltammetry (CV) responses without clear oxidation-reduction peaks within a defined potential window [Figs. 1(a) and 1(b)]. Conversely, PC-type materials exhibit reversible faradaic reactions, demonstrating slight redox peaks in the forward and backward CV sweeps. The term ‘pseudocapacitance’ describes this capacitive behavior mimicking true capacitance. Despite the similarities observed in the CV and galvanostatic charge/discharge (GCD) profiles of the PC- and EDLC-type materials [Figs. 1(c) and 1(d)], the PC-type materials undergo reversible redox properties. The diffusion coefficient (b) for the EDLC- and PC-type materials is approximately 1. While the charge remains consistent within the specified potential range for EDLC- and PC-type materials, their electrochemical performance should be assessed in the capacitance unit of Farad (F). Moreover, recent studies have focused on battery-type transition metal oxides because of their outstanding characteristics, including high redox chemistry, electrical conductivity, and capacity performance. Furthermore, they exhibit distinct redox peaks in CV measurements and nonlinear charge–discharge curves, displaying plateau behavior in GCD measurements, as depicted in Figs. 1(e) and 1(f). The value of b for this behavior is approximately 0.5. Given that the charge generated during the measurement varies within the defined potential window, the capacity of battery-type materials should be computed in Coulomb (C) or ampere-hour (Ah) [11,12].

Figure 1. (a), (c), and (e) Schematic CV plots. (b), (d), and (f) GCD plots of different energy storage technologies.

Bian et al. [13] aimed to improve the performance of Li-ion capacitors (LICs) by employing a novel anion exchange strategy on MOF-embedded nanofibers [Fig. 2(a)]. The study addressed the challenges related to aggregation and structural failure in conversion and alloy-type nanomaterials, limiting their use as reliable electrodes. A three-dimensional (3D) hierarchical structure was developed utilizing a co-axial electrospinning-assisted method based on anion exchange. This innovative in-situ synthetic approach allows simultaneous compositional and structural transformations within aqueous solutions at room temperature. Consequently, dissolved nanostructures with various structures were anchored to a 3D framework. The hierarchical morphology and crystallinity of the composite material were confirmed via numerous characterizations, including X-ray diffraction (XRD) and scanning electron microscopy (SEM), as illustrated in Figs. 2(b)-(d). Moreover, they synthesized CoSnx@CPAN, a nanofiber composite derived from CoSn(OH)6@PAN through ligand exchange in a Na2SnO3 solution, to demonstrate the effectiveness of the anion exchange strategy. CoSnx@CPAN is an anode material with outstanding performance in LICs, demonstrating remarkable Li-storage capabilities with improved energy density, fast charging, and robust capacity. At 0.1 A g−1, the CoSnx@CPAN showed a specific capacity of 657.7 mAh g−1, surpassing the properties of pristine Co@CPAN (330.9 mAh g−1). The hierarchical structure of CoSnx@CPAN facilitates rapid charge transfer, and the material predominantly displays pseudocapacitive behavior [b values obtained from the CV study, as presented in Fig. 2(e)] during Li insertion and extraction. Furthermore, the CoSnx@CPAN demonstrated exceptional rate performance and cycling durability, as illustrated in Fig. 2(f). After 2,000 cycles at 1 A g−1, an anode retained a specific capacity of 280.0 mAh g−1, indicating outstanding cycling stability. The diffusion behavior and electron/charge-transfer capability of CoSnx@CPAN contributed substantially to its excellent performance. This anion-exchange strategy offers a versatile and energy-efficient approach for creating various MOF/polymer-derived hierarchical architectures and promises significant advancements in electrochemical applications. Yang et al. [14] focused on advancing energy storage technology, specifically Li-ion batteries (LIBs), by developing innovative materials. They investigated the synthesis of 3D bimetallic MOFs with a new hierarchical bundle-type hybrid structure. This pioneering approach involves a series of steps, including initial carbonization and subsequent selenization, forming porous hierarchical Ni-Co-Se nanoparticles decorated on a 3D carbon network. The resulting Ni-Co-Se@C hybrid morphology exhibited exceptional performance when used as an anode in LIBs. It demonstrated a good reversible Li-ion charge capacity, outstanding cycling ability (2,061 mAh g-1 after 300 cycles), and remarkable rate capability (493 mAh g-1 at 8 A g-1). Several key features can enhance the electrochemical properties of the Ni-Co-Se@C electrode. The combination of dual-metal selenides in the electrode leads to a synergistic effect, enhancing Li-storage capabilities. The well-fabricated porous hierarchical bundle-type structure provided numerous active sites for Li-storage and facilitated fast charge transfer. The unchanging carbon framework and reduced particle size during the first cycle alleviated the volume variation and enhanced the cycling stability of the electrode. Additionally, the electrode displayed pseudocapacitive behavior, contributing to the high energy storage capacity for Li ions. The synthesis involved a facile method utilizing a surfactant coprecipitation technique to obtain Ni-Co-BTC bundle-type morphologies. Subsequent thermal annealing and selenization converted the precursor into the desired Ni-Co-Se@C composite. Material characterization using SEM confirmed hierarchical bundle-like morphology of the composite, as illustrated in Figs. 2(g) and 2(h). Electrochemical analyses, including CV and GCD tests [Figs. 2(i)-(l)], demonstrated the impressive performance of the Ni-Co-Se@C electrode. The Ni-Co-Se@C composite exhibited a higher specific capacity than the corresponding theoretical value for Ni/Co-Se, and outstanding cycling stability, with a 325 % capacity increase after 300 cycles. Moreover, the electrode demonstrated excellent rate capability, maintaining high capacity at high current densities of up to 8 A g-1.

Figure 2. (a) Synthesis steps: ZIF-67@PAN nanofiber, anion exchange, carbonization, and CoSnx nanoparticle formation. (b-f) XRD pattern, SEM images, and electrochemical properties of CoSnx@CPAN. Adapted with permission from [13], Copyright 2020, Elsevier. (g) and (h) SEM images of Ni-Co/C and Ni-Co-Se/C-600. (i-l) Charge storage properties of Ni-Co-Se/C-600. Adapted with permission from [14], Copyright 2019, Elsevier.

Li et al. [15] produced hierarchically nanostructured Mn3O4 by calcining a manganese-1,3,5-benzenetricarboxylate (Mn-BTC) MOF in the presence of reduced graphene oxide (rGO). This synthesis yielded an rGO/Mn3O4 composite that served as a good electrode material for SC applications. The preparation involved fabricating a porous rGO aerogel using a hydrothermal approach, followed by lyophilization to obtain the aerogel. The Mn-BTC MOF was synthesized using a PVP-modification method in a water/ethanol solvent. To achieve an effective mixture of Mn-BTC [Fig. 3(a)] and rGO, a simple ball milling method was employed, producing a composite with rod-type MOF structures embedded in a graphene matrix [Fig. 3(b)]. Upon calcination, the Mn-BTC-derived Mn3O4 presented a novel porous rod structure, including nanoparticles in the resulting rGO/Mn3O4 composite, as shown in the transmission electron microscopy (TEM) images in Figs. 3(c) and 3(d). The composite electrode of rGO/Mn3O4 demonstrated a remarkable specific capacitance of 420 F g−1 at 0.5 A g−1, surpassing that of other prepared electrodes, as illustrated in Figs. 3(e)-(h). Conversely, when utilized in an allsolid-state symmetric SC (SSC), the rGO/Mn3O4 composite achieved energy and power densities of 22.1 Wh kg−1 and 3.0 kW kg−1, respectively. The exceptional capacitive behavior of the rGO/Mn3O4 electrode was attributed to the efficient amalgamation of Mn-BTCderived manganese oxides featuring hierarchical structures within the rGO matrix, facilitating ion and electron transport during the electrochemical processes. Moreover, the study proposed a new strategy for preparing rGO/Mn3O4 composites from Mn-BTC MOFs as SC electrodes. The unique hierarchical Mn3O4 architecture and good electrochemical features of the rGO/Mn3O4 electrode can advance energy storage technology and meet the demands of energy-efficient applications. Li et al. [16] presented a novel Zn-ion hybrid capacitor (ZIHC) based on a pencil-type hierarchically porous carbon material derived from MOF. The unique structure of this hierarchically porous carbon, resembling sharpened pencils, exhibited superior electrochemical performance. The pencil-type hierarchically porous carbon preparation involves using MOF [Materials of Institute Lavoisier (MIL)-47] as a precursor, combined with the chemical activation method. The MIL-47 precursor was annealed at 800 °C in an argon atmosphere, followed by thorough etching with a 6 M HNO3 to eradicate metal ions [the entire synthetic approach is presented in Fig. 3(i)]. The resulting hierarchically porous carbon material, labeled MPC-x [where x represents the alkaline aqueous electrolyte (KOH) ratio], retained the structural characteristics of MIL-47 and exhibited a fairish pore size distribution and ample specific surface area, along with abundant oxygencontaining functional groups. The electrochemical behaviors of the MPC-x specimens were assessed using a three-electrode configuration immersed in a 6 M KOH aqueous solution. The CV plots exhibited a quasi-rectangular profile, suggesting pseudocapacitance stemming from oxygen-containing functional entities. Within the range of MPC-x variations, MPC-2 exhibited the highest specific capacitance, attributed to its increased surface oxygen content and the presence of quinone-type oxygen-containing functional groups (C=O). In Fig. 3(k), the specific capacitance reached 265.5, 326.0, and 297.5 F g−1 at a current density of 0.5 A g−1 for MPC-1, MPC-2, and MPC-3, respectively, as estimated from the GCD discharge times [Figs. 3(j) and 3(l)]. Moreover, the CV, GCD, and capacity of the fabricated ZIHCs were measured, as illustrated in Figs. 3(m)-(o). Figure 3(p) displays the kinetic behaviors of energy storage analyzed by calculating the b values from the CV plots, with peaks 1 and 2 exhibiting b values of 0.87 and 0.90, respectively. These results suggest that the capacitive behavior and diffusion-controlled processes contribute to energy storage, with dominating capacitive behavior measured at 5 mV s-1, as depicted in Fig. 3(q). The capacitive properties were progressively improved from 65.4 to 92.9 % as the scan rate altered from 2 to 50 mV s−1 [Fig. 3(r)], indicating fast kinetic behavior with a capacitive-driven feature for MPC-2. The ZIHC device depended on MPC-2, revealing a large voltage window of 0–1.8 V owing to the introduction of a Zn anode. This ZIHC device achieved a notable specific capacitance of 289.2 F g−1 at 0.2 A g−1, with 154.4 F g−1 retained at a high current density of 10 A g−1. The Ragone plot showed a high energy density of 130.1 and 59.0 Wh kg−1 at power densities of 180.3 W kg−1 and 7.8 kW kg−1, respectively, outperforming many reported ZIHCs. The ZIHC device also demonstrated excellent cycle stability, with 96.7 % capacity retention after 10,000 cycles at 10 A g−1.

Figure 3. SEM images of (a) rGO/MB1:1 and (b) rGO/MO1:1 (inset: EDX analysis). (c, d) TEM images of rGO/MO1:1. (e, g) CV and GCD curves of rGO/MO3:2, rGO/MO1:1, and rGO/MO2:3. (f, h) CV and GCD curves of rGO, Mn-BTC, rGO/MB1:1, and rGO/MO1:1. Adapted with permission from [15], Copyright 2022, Elsevier. (i) MPC preparation and Zn//MPC-x ZIHC construction. (j) GCD curves and (k) rate capability of MPC-2. (l) GCD curves of SSC. (m-r) Electrochemical assessment of Zn//MPC-2 ZIHC: CV and GCD curves, rate performance, log plot of peak current vs. scan rate, capacitive contribution, and contribution ratios at 2–50 mV s−1. Adapted with permission from [16], Copyright 2022, Elsevier.

Sekhar et al. [17] introduced a versatile and efficient binder-less electrode for LIBs and hybrid SCs (HSCs). This innovative electrode was based on a unique MOF-derived architecture, combining CuV2O6 and Co3V2O8 composite materials. They employed a solution processing technique, followed by thermal treatment in an inert atmosphere, to create core-shell-like structures of CuV2O6 and Co3V2O8 on a copper foam substrate [Fig. 4(a)]. This approach successfully addressed issues related to nonconductive binders and sluggish charge-transfer rates. Figure 4(b) illustrates the design of the binder-free CuV2O6-Co3V2O8 composite with a core-shell-like architecture. The resulting CuV2O6-Co3V2O8 composite exhibited exceptional electrochemical performance in LIBs and SCs due to the inherent advantages of the vanadium-incorporated mixed metal oxides with versatile valence states and excellent redox activity. Additionally, when employed as anode materials for SCs, hollow carbon particles derived from MOFs enhance the energy storage capabilities of SC. The HSC constructed using CuV2O6-Co3V2O8 and MOF-derived hollow carbon particles demonstrated impressive energy storage performance. Furthermore, the study presented the morphology of the copper oxide (CuO) material, which indicated a uniform coverage of the copper foam stem with the CuO nanorod arrays. CuO-Co3O4 and CuO electrodes are compared in Fig. 4(c) (i,ii). The CuV2O6-Co3V2O8 composite displayed superior redox reactions and chargedischarge characteristics. Figure 4(c) (iii-v) shows the CV and current contribution of the CuV2O6-Co3V2O8 electrode, which exhibited mixed capacitive and battery-type behaviors. At various current densities, the CuV2O6-Co3V2O8 electrode demonstrated good rate capability, even at high current densities [Fig. 4(c) (vi)]. Furthermore, the practical applicability of HSC was demonstrated by successfully operating electronic devices using the stored energy. To explore its real-world feasibility, researchers have assembled a solid-state HSC (SSHSC) and harnessed the dynamic energy from a bicycle using a direct current generator. Charged SSHSC was found to effectively power various electronic components. Furthermore, the composite material demonstrated excellent rate performance and cycling stability in SC studies, delivering a high areal capacitance and energy storage efficiency. Moreover, an SSHSC was assembled to address the safety concerns related to electrolyte leakage, exhibiting outstanding rate capability and redox reversibility, as shown in Fig. 4(d) (i-iii). Li et al. [18] focused on fabricating layered Co(OH)2 nanostructures with controllable morphological properties to improve their energy storage performance as positive electrode candidates for HSCs. Hydrolyzing cobalt zeolitic imidazolate frameworks (ZIFs)-67 in appropriate pH-regulating additives helped successfully obtain robust α-Co(OH)2-A nanostructures with unique morphologies and properties. The morphological features of the as-fabricated coated Co(OH)2 materials were extensively characterized using SEM and TEM analyses, as illustrated in Figs. 4(e)-(g). The α-Co(OH)2-A nanosheets more electrochemically active surfaces compared to other phases. Additionally, the α-Co(OH)2-A phase had a higher interlayer arrangement of approximately 8.0 Å, required to store ions in active electrode materials. This larger spacing facilitates faster and more efficient diffusion of ions to active sites, contributing to the excellent electrochemical performance of α-Co(OH)2-A in HSCs. Controlled nanostructures of layered Co(OH)2 were achieved by carefully changing the pH of the reactant solvents and the hydrolysis methodologies of the ZIF-67 precursors. The appropriate coordination of anions with larger sizes, such as 2-methylimidazole anions (MIA), was critical in determining the morphology and crystalline phase of the final product. The presence of MIA groups led to the formation of interlayer crystal water, stabilizing the structure and facilitating charge transfer. The electrochemical properties of the α-Co(OH)2-A nanostructures were evaluated through CV and GCD tests. As illustrated in Fig. 4(h) (i,ii), the CV and current contribution profiles exhibited general combined capacitive and redox performances, signifying battery-type and capacitive charge storage behavior. The α-Co(OH)2-A phase presented two noticeable oxidation and reduction peaks accredited to the reversible faradaic redox procedures of Co2+/Co3+ and Co3+/Co4+. The specific capacity of α-Co(OH)2-A was significantly higher at 87.1 mAh g−1 compared to other phases, as presented in Fig. 4(h) (iii-v), as estimated from the discharge time of the GCD profiles. Moreover, the α-Co(OH)2-A electrode demonstrated excellent rate capability with 77 % capacity retention at a high current density of 20 A g−1. Most impressively, the α-Co(OH)2-A phase exhibited remarkable cycle stability, retaining over 100 % capacity after 200,000 charge-discharge cycles [Fig. 4(h) (vi)]. This exceptional stability was not observed in previously reported HSC electrode materials.

Figure 4. (a) Fabrication process of the CuV-CoV composite. (b) SEM and TEM images of the CuV-CoV composite. (c) (i, ii) Comparative CV and GCD curves of the CuO, CuO-Co3O4, and CuV-CoV electrodes. (c) (iii-vi) CV curves and current contribution at various scan rates and GCD curves at various current densities for the CuV-CoV electrode. (d) electrochemical properties of the CuV-CoV electrode-based solid-state HSCs. Adapted with permission from [17], Copyright 2020, Wiley. (e-h) SEM, TEM, and HR-TEM images, and electrochemical properties of α-Co(OH)2-A, α-Co(OH)2–C, and β-Co(OH)2–W electrodes. Adapted with permission from [18], Copyright 2020, Elsevier.

Rabani et al. [19] employed a blend of SEM and electrochemical studies to investigate the structural engineering and performance evaluation of Ru-decorated ZIFs for hydrogen evolution reactions and SCs. Using a solvothermal technique, they effectively produced nanocomposites of Ru-encapsulated ZIF-8 and ZIF-67. Figures 5(a)–(c) illustrate SEM images of the prepared materials. The SEM images of the ZIF surface incorporated with nanosized Ru particles exhibited a retained hexagonal morphology with noticeably rougher edges. The uniform distribution of the Ru particles on the ZIF surface confirmed the successful synthesis of the nanocomposites. In addition, electrochemical studies evaluated the performance of the as-synthesized electrocatalysts for hydrogen evolution reactions. Ru/ZIF-67 displayed superior performance, with a low overpotential of 95 mV at 10 mA cm–2 and a low Tafel slope of 61 mV dec-1 in basic media, outperforming ZIF-8, ZIF-67, and Ru/ZIF-8. Furthermore, the charge storage capacity of the nanocomposites in the SCs was evaluated using CV and GCD tests [Fig. 5(d) (i,ii)]. The Ru/ZIF-67 nanocomposite electrode demonstrated a considerably higher specific capacitance (Cs) of 1503.33 F g-1 at 1 A g-1 current density compared to that of the ZIF-8, ZIF-67, and Ru/ZIF-8 electrodes. This excellent performance is attributed to the pseudocapacitive redox activity occurring at the oxygen-containing functional groups along with the enhanced wetting properties of the electrolytes, promoting the accessibility of mobile ions on the electrode surface. To broaden the applicability of these nanocomposites, they fabricated high-efficiency SSCs by employing Ru/ZIF-8 and Ru/ZIF-67 as the negative and positive electrodes, respectively. The Ru/ZIF-67 SSC device exhibited a twofold increase in charge storage capacity (Cs = 278 F g-1) and energy density (68 Wh kg-1 at 1,260 W kg-1 power density) compared to that of the Ru/ZIF-8 SSC device [Fig. 5(e) (i-vi)]. Additionally, the Ru/ZIF-67 SSC device demonstrated remarkable cycling stability with 92 % capacity retention after 7,000 charge-discharge cycles, highlighting its excellent electrochemical stability. Zhang et al. [20] presented a remarkable breakthrough in energy storage materials by developing two-dimensional (2D) conjugated MOFs (c-MOFs) with dual redox sites, exhibiting exceptional pseudocapacitance and a wide potential window. The morphology of the synthesized 2D c-MOFs was characterized as a highly conductive and porous structure comprising quasione-dimensional aligned pore arrays. This unique morphology provided an interconnected network of conjugated frameworks, ensuring efficient charge transport and improved electron mobility throughout the material. Metallophthalocyanine (MPc) monomers have been utilized to fabricate 2D metal-bis(iminobenzosemiquinoid) (MN4)-linked c-MOFs, which offer additional redox-active sites compared to standard benzene or triphenylene blocks. Cu(II) 2,3,9,10,16,17,23,24-octaaminophthalocyanine (CuPc(NH2)8) was synthesized to construct MN4-linked M2[CuPc(NH)8] (M = Ni or Cu) on carbon cloth [Fig. 5(f) (i)]. Ni2[CuPc(NH)8] has a hierarchical porous structure composed of small crystalline grains (50–100 nm diameter) [Fig. 5(f) (ii)]. High-resolution TEM (HR-TEM) images with square unit cells of a = b ≈ 1.7 nm, aligned with the (100) plane of the proposed structure, are presented in Fig. 5(f) (iii). The electrochemical behavior of the M2[CuPc(NH)8] electrode was studied using a 1 M Na2SO4 threeelectrode system [Fig. 5(g)]. The CV curves revealed redox peaks, and the Ni2[CuPc(NH)8] electrode displayed a higher current response than the Cu2[CuPc(NH)8] electrode [Fig. 5(g) (i)]. The specific capacitances of the Ni2[CuPc(NH)8] and Cu2[CuPc(NH)8] electrodes at 2 mV s-1 were 310.3 and 259.1 F g-1, respectively. The redox peaks of CV curves were analyzed for capacitive contribution, with b values close to 1 (0.96), indicating faradaic capacitive behavior [Fig. 5(g) (iiv)]. The GCD curves exhibited symmetric shapes with small plateaus [Fig. 5(g) (vi)]. When assembled into quasi-solid-state SSCs [Fig. 5(h) (i-vi)], the Ni2[CuPc(NH)8] electrode demonstrated state-of-the-art energy density of 51.6 Wh kg−1 and a peak power density of 32.1 kW kg−1. Ni2[CuPc(NH)8]-SSCs exhibited 90.3 % capacitance retention after 5,000 cycles at 10 A g-1. These results open new possibilities when designing advanced energy storage devices and support future innovations in electrochemical systems.

Figure 5. SEM images of (a) ZIF-8, (b) ZIF-67, (c) (i, ii) Ru/ZIF-8, and (c) (iii) Ru/ZIF-67. (d) CV and GCD curves of the electrodes. (e) SSC performance for Ru/ZIF-8 and Ru/ZIF-67: CV curves, GCD curves, scan rates, current densities, Cs values, and electrochemical impedance spectroscopy plots. Adapted with permission from [19], Copyright 2019, Elsevier. (f) Schematic of M2[CuPc(NH)8] growth, SEM image, and HR-TEM image of Ni2[CuPc(NH)8]. (g) Electrochemical properties of M2[CuPc(NH)8] electrodes. (h) Electrochemical behavior of Ni2[CuPc(NH)8]-SSCs: Schematic illustration of the device, CV and GCD curves, specific capacitances, Ragone plots, cycling stability, and Coulombic efficiency. Adapted with permission from [20], Copyright 2023, American Chemical Society.

In their pioneering study, Tian et al. [21] reported a groundbreaking approach for fabricating NiCo-hydroxide@Ni-MOF hollow prisms using the room-temperature nanoarchitectonics method. The synthesis involved two critical steps: precipitation of uniformly sized NiCo-hydroxide prisms and growth of vertically aligned Ni-MOF nanosheets on the prism surface [Fig. 6(a) (i-iii)], as illustrated in the TEM images of the NiCo-hydroxide@Ni-MOF. Moreover, using energy-dispersive X-ray (EDX) spectroscopy, as shown in Fig. 6(b) (iiv), the color-mapping images were validated, and the elements (Ni, Co, C, O, and N) were identified. This innovative design results in a hollow prismatic structure, offering numerous advantages for SC applications. The hollow structure of the NiCo-hydroxide@Ni-MOF enhances their electrochemical properties. By inhibiting nanosheet aggregation, this unique morphology maximizes the exposure of active sites, increasing the number of reaction sites for the electrolyte ions. Consequently, ion diffusion within the material is considerably accelerated, improving electrochemical performance. Electrochemical analyses of the NiCo-hydroxide@Ni-MOF and control samples were performed using a 6 M KOH three-electrode system. CV curves across 5 to 80 mV s−1 [Fig. 6(c) (i)] revealed consistent shape changes, which reflects excellent electrochemical reversibility. The GCD curves at current densities of 1 to 10 A g−1 [Fig. 6(c) (ii)] demonstrated high symmetry and good reversibility. At 1 A g−1, the NiCo-hydroxide@Ni-MOF exhibited 1,610 F g−1, as evidenced by GCD and CV results. Furthermore, the hollow and mesoporous structure of NiCo-hydroxide@Ni-MOF considerably enhanced charge aggregation and transport efficiency [Fig. 6(c) (iii,iv)]. Furthermore, NiCo-hydroxide@Ni-MOF demonstrated exceptional stability and reversibility during charge-discharge cycling. The symmetric CV curves and GCD profiles indicate outstanding Coulombic efficiency and high reversibility, which are crucial for long-term cycling stability. Remarkably, the asymmetric SC constructed using NiCo-hydroxide@Ni-MOF as the cathode displayed remarkable energy density (49.4 Wh kg−1 at 825 W kg−1) and retained 84 % of its initial capacity even after 5,000 cycles, affirming its suitability for practical applications. In another study, MOF-derived nickel-cobalt terephthalate hydroxides (MOF-D Ni-Co THs) were successfully synthesized using a simple and cost-effective solvothermal method [22]. The SEM and TEM analyses revealed the surface and intrinsic structural features of the active materials. The MOF-D Ni-Co THs were directly deposited on a nickel foam (NF) scaffold without binders, which enhanced their redox chemistry and allowed for high-mass loading (~8 mg cm−2). The SEM images in Fig. 6(d) (i-iii) show uniform deposition of Ni-Co TH-160 product on NF, exhibiting hybrid structures at low magnification. Furthermore, the magnified SEM images revealed the 2D sheet-like structure of the fabricated material. The thickness of the triangular Ni-Co TH-160 sheet was approximately 176.5 nm. Figure 6(d) (iv) depicts the elemental composition of Ni-Co TH-160, as confirmed by EDX spectroscopy. The color-mapping images in Fig. 6(e) (i-iv) demonstrated the uniform distribution of elements (Ni, Co, C, and O), with negligible N, likely from the organic solvent. The TEM analysis [Figs. 6(f) and 6(g)] confirmed the triangular microsheets. The HRTEM images revealed lattice fringes (3.2 nm, C8H6Co2O6; 3.4 nm, C8H12NiO8). These two phases synergistically contribute to performance improvement. The SAED pattern displayed bright spots owing to its mixed crystalline nature. The electrochemical performance of the prepared electrode materials was investigated using a conventional three-electrode system [Fig. 6(h) (i)]. The SEM results demonstrated that the unique morphological structures of the MOF-based Ni-Co THs on the NF substrate provide a large surface area for redox reactions, contributing to its superior areal capacity of 2,087 μAh cm–2 and a specific capacity of 200.68 mAh g-1 at a current density of 3 mA cm–2. The CV and GCD profiles [Fig. 6(h) (ii-v)] revealed improved redox reactions in the Ni-Co TH-160/9 material. The excellent performance is attributed to the unique morphological structures of the Ni-Co THs on the NF substrate, providing a large surface area for redox reactions. Employing the outstanding charge storage properties of the Ni-Co TH-160/9h electrode, they fabricated an electrochemical cell that exhibited high areal capacity, energy density, and power density of 1,678.6 μAh cm–2, 1.3 mWh cm–2, and 48.6 mW cm–2, respectively, along with high cycling performance (99.9 %) even after 35,000 cycles. The fabricated electrochemical cell demonstrated its potential for practical energy storage applications in powering various portable electronic devices. MOF-D Ni-Co THs with large mass loadings and outstanding electrochemical properties [Fig. 6(h) (vi)] make them promising candidates for durable and efficient electrochemical energy storage systems.

Figure 6. (a) TEM images, (b) elemental mapping images, and (c) (i, ii) CV and GCD curves of NiCo-hydroxide@Ni-MOF. (c) (iii, iv) GCD profiles and capacitance values of NiCo-hydroxide, Ni-MOF, and NiCo-hydroxide@Ni-MOF. Adapted with permission from [21], Copyright 2023, Elsevier. (d-g) SEM images, EDX spectrum, TEM images, lattice fringes, and SAED pattern of Ni-Co TH-160/9h. (h) (i-v) Schematic of electrochemical setup and comparative electrochemical properties of Ni-Co TH-based electrodes. (h) (vi) Mass loading of active materials on the NF substrate. Adapted with permission from [22], Copyright 2023, Elsevier.

Furthermore, researchers have introduced a groundbreaking method for converting MOFs into selenium/selenide/carbon composites with exceptional Na-storage capacity [23]. The study focused on Se-based materials for Na-ion batteries, considering their fast kinetics and excellent cyclability. However, achieving high Se content in these composites, which is crucial for maintaining a competitive edge over other electrode materials, has been a challenge. The confined annealing method developed in their study allowed the direct conversion of pristine MOFs, specifically ZIF-67, into selenium/selenide/carbon composites [Fig. 7(a)]. The process involved sealing the ZIF-67 crystals with Se powder in a glass vessel and annealing them at 600 °C for 9 h in a muffle furnace. This one-step approach resulted in simultaneous carbonization, selenization, and Se vapor deposition, forming selenium/selenide/carbon composites. Figure 7(b) (i-iv) presents SEM images of ZIF-67 and Se-ZIF at different magnifications. The resulting Se-ZIF composite exhibited outstanding electrochemical performance, including enhanced capacity and rate capabilities. The rate performance of Se-ZIF in Fig. 7(c) (i) revealed a remarkable reversible specific capacity across various current rates (0.1 to 5.0 A g−1), ranging from 490 to 228 mA h g−1. The composite demonstrated highcapacity retention of 78 % when the current density increased from 0.1 to 2.0 A g−1, as evidenced in Fig. 7(c) (ii). A comprehensive comparison in Fig. 7(c) (iii) underscored the superior performance of the Se-ZIF compared to previously utilized transition metal selenides, even surpassing pure transition metal selenides when considering Se weight percentage. The exceptional rate capability of Se-ZIF electrodes was further explored by analyzing the CV peak currents at different scan rates [Fig. 7(c) (iv)]. The slopes indicate mixed capacitive and diffusion-limited modes, with a substantial pseudocapacitive contribution of 77 % within the cut-off voltage range [Fig. 7(c) (v,vi)]. These findings collectively emphasize the outstanding electrochemical performance of Se-ZIF due to its high Se weight ratio, suggesting its potential as a promising electrode for advanced energy storage devices. Nagaraju et al. [24] explored a strategic combination of MOFs with diverse nanogeometries to enhance their electrochemical performance for advanced energy storage applications. A binder-free ternary Ni–Co–Mn-based MOF (NCM-based MOF) featuring hierarchical/dual-layered structures on NF was synthesized via a polarity-induced solution-phase method, as illustrated in Fig. 7(d) (i). They seamlessly integrated dual-layered NCM-based MOFs with a bottom layer of 2D nanosheets and a top layer of randomly distributed 3D nanoflowers on NF, ensuring strong adhesion through precise growth time control. The synergistic characteristics of these duallayered NCM-based MOFs demonstrated improved electrochemical properties from increased electroactive sites, efficient electrolyte diffusion paths, and enriched redox reactions in a KOH. SEM analysis revealed hierarchically assembled NCM-based MOF nanostructures with densely packed and interconnected 2D nanosheets covering the NF substrate, reaching a height of 1.8 μm on NF. Additionally, selfassembled 3D nanoflowers of various sizes were firmly attached to the nanosheets, forming dual-layered nanostructures, as illustrated in Fig. 7(d) (ii-iv). The 3D-on-2D NCM-based MOF/NF configuration is a promising electrode for SCs because of its larger electroactive surface area, enhanced electrochemical activity, and increased interspace for electrolyte ion penetration, improving redox reactions and charge storage properties. Electrochemical testing of the NCM-based MOF-5 sample revealed dominant redox peaks during the anodic and cathodic scans, indicating battery-type charge storage behavior, as illustrated in Fig. 7(e) (i). The material exhibited good reversibility and electrochemical kinetics, suggesting a diffusion-controlled charge storage mechanism [Fig. 7(e) (ii-iv)]. Quantifying the charge storage behavior highlighted the importance of the diffusion-controlled capacity to the overall charge storage properties of multi-layered NCM MOF-5. The electrode exhibited a higher redox performance and areal capacity than the other prepared electrode materials [Fig. 7(f) (i-iii)]. Moreover, NCM-based MOF-5 was employed as the positive electrode in a supercapacitor configuration, whereas nitrogen-oxygen-rich activated carbon was utilized as the negative electrode, with filter paper serving as the separator and KOH as the electrolyte. The resulting supercapattery exhibited elevated energy and power densities of 1.21 mWh cm−2 and 32.49 mW cm−2, respectively, positioning it as a compelling contender for advanced energy storage applications. Additionally, researchers have integrated a supercapacitor with a renewable solar power harvester, creating a self-charging station for various portable electronic applications, as illustrated in Figs. 7(g) and 7(h) (i-iii). The solar-driven supercapacitor system efficiently stored solar energy for extended use, demonstrating its potential to meet future energy challenges. Several MOF-based/derived materials exhibited dual capacitive and battery-like behaviors, as indicated by their b values. This mixed electrochemical nature highlights their potential for versatile energy storage applications, combining rapid capacitive charge/discharge with sustained energy-delivery characteristics of batteries/SCs.

Figure 7. (a) ZIF-67 to Se-based composite pathways, (b) SEM images of ZIF-67 and Se-ZIF. (c) (i-vi) Rate performance, voltage profiles, comparison with Se/C and metal selenide, CV curves at different scan rates, and pseudocapacitive contribution for Se-ZIF. Adapted with permission from [23], Copyright 2019, Elsevier. (d) Schematic and SEM images of multi-layered Ni–Co–Mn-based MOF on NF. (e, f) Electrochemical properties and capacitive/diffusion-controlled current contributions. (g, h) Charging, discharging, motor fan operation, and colored LED illumination with solar energy-assisted supercapattery. Adapted with permission from [24], Copyright 2020, Springer Nature.

This review explored cutting-edge research on energy storage materials, focusing on MOFs and their derivatives. Various studies have addressed the limitations of traditional electrode materials used in LIBs, LICs, and HSCs. The authors proposed innovative approaches to improve the electrochemical properties of these energy storage applications. Several studies have investigated the synthesis and design of MOF-based nanomaterials with unique morphologies to improve their energy storage capabilities. This study covers topics such as hierarchical nanostructures and MOF-derived carbon composites and found that novel anion exchange strategies, co-axial electrospinning, and solvothermal techniques are promising methods for creating high-performance energy storage materials. The key findings include successfully synthesizing CoSnx@CPAN nanofiber composites and hierarchical porous Ni-Co-Se nanoparticles embedded in a 3D carbon network. These materials display outstanding performance as anode materials for LICs and LIBs, exhibiting high energy densities, fast charging rates, and robust Li-storage capacities. These innovative hierarchical structures facilitated rapid charge transfer and predominantly exhibited pseudocapacitive behavior during Li insertion and extraction. Researchers have explored novel ZIHCs based on hierarchical porous carbon materials derived from MOFs. These sharpened pencil-like carbon materials demonstrated remarkable energy density and cycling stability, enhancing their potential for energy storage applications. This review also discussed the synthesis of 2D c-MOFs with dual redox sites, which exhibit remarkable pseudocapacitance and an exceptionally wide potential window, contributing to their outstanding performance as SC electrodes. Furthermore, this study focused on using MOF-derived Ni-Co TH, NiCoMn-MOF, and CoV-CuV for SCs, revealing their promising electrochemical performances with high specific capacitance, rate capability, and cycling stability.

The research presented in the review offers valuable insights into the potential of MOFs and their derivatives in advancing energy storage technologies. Innovative synthesis methods and design strategies can overcome the limitations of conventional electrode materials, such as low capacity, slow charge transfer, and poor cycling stability. The unique morphologies and structures of MOF-based materials provide a platform for optimizing the charge storage and diffusion behaviors for enhanced electrochemical performance. These findings provide a foundation for future research and development in the field of energy storage materials. Further investigations into synthesizing MOFs with tailored structures and properties could provide new potential for enhanced energy storage applications. Moreover, exploring the scalability and cost-effectiveness of these materials is crucial for their practical applications in real-world energy storage devices. The perspective offered by this review indicates that the field of MOF-based energy storage materials is rapidly evolving, with significant potential for improving the efficiency, sustainability, and reliability of energy storage technologies. This study emphasizes the importance of continued interdisciplinary collaboration between materials science, chemistry, and engineering to translate these discoveries into practical applications and address the global challenges of energy storage and utilization. By harnessing the unique properties of MOFs and their derivatives, researchers can pave the way for a new generation of high-performance energy storage devices with broad applications in renewable energy integration, grid stabilization, and portable electronics.

This work was supported by the National Research Foundation of Korea (NRF) grant sponsored by the Korean Government (MSIP) (No. 2018R1A6A1A03025708).

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