Advances in Metal-Organic Frameworks (MOFs) for Rechargeable Batteries and Fuel Cells

Abstract

The growing demand for energy, coupled with the unsustainable nature of fossil fuels due to global warming and the greenhouse effect, have led to the advancement of renewable energy production concepts. Innovations such as photovoltaics, wind energy, and infrared energy harvesters are emerging as viable solutions. The challenge lies in the stochastic nature of renewable energy sources, which necessitates the implementation of electrical energy storage solutions, whether through batteries, supercapacitors, or hydrogen production. In this regard, innovative materials are essential to address the questions associated with these technologies. Metal-organic frameworks (MOFs) are crucial for achieving clean and efficient energy conversion in fuel cells and storage in batteries and supercapacitors. Metal-organic frameworks (MOFs) can be used as electrocatalytic materials, membranes for electrolytes, and energy storage materials. They exhibit exceptional design versatility, large surface, and can be functionalized with ligands with several charges and metallic centers. This article offers an in-depth examination of materials and devices utilizing metal-organic frameworks (MOFs) for electrochemical processes concerning the generation, transformation, and storage of electrical energy. This review specifically focuses on rechargeable batteries and fuel cells that incorporate MOFs. Finally, an outlook on the potential applications of MOFs in electrochemical industries is presented.

1. Introduction

Projections indicate that global energy demand may reach as high as 500 EJ annually by the year 2050. The imperative for clean and alternative energy sources stands as one of the most significant challenges facing 21st-century science [1]. The worldwide trend of urban growth, along with an increasing population, have led to a marked rise in the release of greenhouse gases, particularly carbon dioxide. There exists a consensus regarding the necessity to diminish greenhouse gas emissions and to transition energy generation towards environmentally sustainable and renewable sources, such as photovoltaics and wind energy. Given that these sources do not generate a constant flow of electrical energy, it becomes essential to implement a system for storing the energy produced that is not utilized immediately [2]. This can be achieved through the implementation of sophisticated batteries and supercapacitors. Considering the swiftly changing transportation requirements of contemporary society, lithium-sulfur batteries (LSBs) [3] are garnering interest as the next-generation power sources for diverse future aerial transportation modalities, encompassing urban air mobility and drones. This results from their competitive pricing and elevated potential energy density of 2600 Wh kg⁻¹ [4]. An alternative approach involves the production and storage of hydrogen, which can subsequently be utilized in fuel cells to generate electricity and heat precisely when needed. Metal-organic frameworks (MOFs) constitute an innovative class of materials that have garnered significant attention in the last twenty years. MOF materials offer considerable benefits owing to their large specific surface area and plentiful Lewis acid sites present on their surfaces. Metal-organic frameworks (MOFs) demonstrate considerable promise in improving battery performance. Thanks to their unique structure and characteristics, MOFs significantly improve the conductivity, stability, and cycle life of batteries, presenting new possibilities for future energy storage technologies, catalytic processes, and gas separation [5,6,7,8].

Metal-organic frameworks (MOFs) are characterized by open networks created from metal-centered secondary building units (SBUs) that are interconnected by organic components. This results in the development of extensive one-dimensional (1D), two-dimensional (2D), or three-dimensional (3D) structures [9,10]. The networks show a crystalline character, indicating long-range order. Uniform pores or channels are integrated throughout the framework, often absorbing moieties such as solvents incorporated during synthesis or counter ions that neutralize the entire charges on the entire structure due to the charged metal nodes [6,8,11]. In contrast to other porous substances like zeolites and carbon-based materials, the distinguishing characteristic of metal-organic frameworks (MOFs) is their ability to achieve a “directed” architecture by the meticulous selection of both metals and organic linkers [12,13,14].

In addition to the conventional hydrothermal preparation method [15], various alternative techniques such as electrochemistry [15,16], sonochemistry [17,18], microwaves [19], and mechanochemistry [20,21] have been employed, frequently leading to distinct properties of the same MOF [22], for the synthesis of MOFs and their derivatives.

MOFs provide an ideal environment for electrochemical processes because of their large surface area, rendering them promising options for electrochemistry, (electro)catalysis, and energy storage applications. In comparison to other porous materials like activated carbon and zeolites, MOFs exhibit enhanced architectures and functions. However, the low conductivity (below 10⁻¹⁰ S cm⁻¹) of the majority of MOFs hinders their practical electrochemical applications, highlighting the need for improvement. Materials with a high density of charge carriers can serve as either metallic conductors or semiconductors [23,24]. MOF materials present considerable promises for improving electronic and proton conductivity, attributed to their functional pore surfaces, elevated surface areas, and broad structural tunability (Figure 1). A comprehensive overview of the advancements in conductive MOFs is available in [25,26,27]. Zhang et al. [24] provide a review of extensive improvement strategies for electronically and ionically conductive MOFs, along with recent advancements in the field.

This manuscript seeks to deliver an extensive examination of the most recent advancements in the utilization of MOFs in electrochemical devices for the purposes of electrical energy production and conversion. The analysis includes considerable challenges and constraints regarding the large-scale applications of MOFs, and potential advancements in MOF-based conductors.

2. Metal-Organic Frameworks (MOFs) in Batteries

The continuous rise in worldwide demand for energy has necessitated that politicians and scientists continue to pursue more efficient and environmentally friendly energy options for the future.

Batteries are essential components in the advancement of energy conversion and storage projects for the future. However, challenges persist not only in power capacity and longevity, but primarily in the scalability and environmentally sustainable production methods of battery systems.

The battery’s cathode, serving as the source of the oxidizing agent, is essential to the functionality of both primary and secondary batteries. Modern cathodes require features such as structural and chemical stability, rechargeability, longevity, and safety. Researchers have extensively investigated resilient materials for electrode formation that can maintain the necessary specific capacity together with ionic and electronic conductivity during continuous cycling [28,29,30,31].

In this context, metal-organic frameworks (MOFs) have been recognized as a promising category of materials for the direct use in cathode electrodes or as precursors for the fabrication of cathode electrodes. Metal-organic frameworks (MOFs) can be manufactured by many scalable methods, from many educts [32,33,34] yielding nanomaterials with unique geometries and porosity that demonstrate beneficial electrochemical characteristics [35,36,37].

Scientists have extensively investigated resilient materials for electrodes that can maintain the necessary specific capacity and ionic and electronic conductivity, as well as catalytic activity throughout continuous cycling. In this context, metal-organic frameworks (MOFs) have been recognized as a promising category of materials for the fabrication of cathode electrodes or as precursors for cathode electrodes [38,39,40,41].

Metal-organic frameworks (MOFs) including ZIF-8 [42,43,44,45,46], ZIF-67 [47,48,49], MOF-74 [50], MOF-5 [51,52,53], MIL-53 [54], MIL-120 and MIL-122 [55], DUT-4, DUT-5 [55,56], and HKUST-1 [57,58], together with its derivatives, have been examined as prospective electrode materials for several battery types, including metal-ion, lithium, and metal-air batteries. The previously described battery types exhibit several distinctions in cathode activity, resulting in MOFs fulfilling different roles in each application. MOFs often offer an increased specific surface area with several mass diffusion channels for ions, enhancing the interaction between the electrolyte [28,59,60] and the entire active surface area through their inherent micro-porosity or the engineered macro-meso-porosity among the particles [61,62,63]. This approach can enhance the starting capacity of the electrode and expedite the kinetics of redox reactions.

A literature survey indicates that metal-organic frameworks (MOFs) are predominantly regarded as optimal precursors or sacrificial templates for generating carbonaceous materials or oxides post-thermal treatment. Consequently, the use of pristine MOFs in batteries is restricted or entirely absent for specific battery types, including lithium-selenium [64], sodium-ion [65,66], and zinc-air [67,68,69]. The pyrolysis of MOFs is an efficient method for producing carbon matrices with designed molecular structures, as MOF-derived carbons retain the morphological features of the used precursor materials [38,70,71,72].

As a result, researchers are able to leverage the intrinsic conductivity of carbon and explore various carbon network architectures to achieve the ideal property combination for each type of battery cathode.

2.1. Lithium-Ion Batteries

Lithium-sulfur [73,74,75] and lithium-selenium [64] batteries are highly promising technologies, providing substantial theoretical capacity (1 C = 1672 mAh g⁻¹), sustainability, and low production costs [76,77].

Sulfur acts as the active element of the cathode. However, it is incapable of functioning autonomously as an electrode, requiring a supporting material for its containment.

During the discharge process, lithium ions transfer from the electrolyte to the saturated host, leading to the synthesis of lithium sulfide via several redox reactions and intermediary compounds [78]. Despite the insolubility of sulfur and its reduction products, the polysulfide intermediates exhibit significant mobility and solubility in typical electrolytes, migrating towards the anode and generating short-chained polysulfides that subsequently diffuse back. This cyclical diffusion results in anode corrosion, contributing to self-discharge and diminished cycling stability [79,80]. As a result of the shuttle effect, scientists have exerted considerable effort to create innovative cathode materials capable of effectively encapsulating sulfur and restricting its migration [81,82,83]. Pristine MOFs and their derived materials have been examined as sulfur hosts because of their high surface area and pore structure, particularly carbon derivatives that demonstrate enhanced electrical conductivity and structural flexibility.

The earliest publication on using MOFs in a Li-S battery was MIL-100 (Cr) by Demir-Cakan et al. [84]. It is constructed from trimesic acid linkers and chromium octahedra, including mesoporous cages measuring 25–29 Å and a pore volume of approximately 1 cm³ g⁻¹. Liquid sulfur (melting point is 115 °C) is introduced into the porous structure at 155 °C; a minimum of 25 wt% carbon is required for the electrode preparation to enhance conductivity. Despite the insufficient attachment of polysulfide anions to the framework and the possible hindrance of sulfur at the inner tube surfaces by the carbon matrix, the cathode capacity stayed under 500 mAh g⁻¹ after 50 cycles. The vanadium modification was tested for over 200 cycles at 0.1 C, exhibiting a capacity of approximately 550 mAh g⁻¹. In contrast, a composite based on reduced graphene oxide showed slightly higher efficiency, achieving 650 mAh g⁻¹ at 0.1 C after 75 cycles and 450 mAh g⁻¹ for 300 cycles at 0.5 C, although these results are not directly comparable [85].

Hong et al. [86] synthesized Cu-TDPAT using the 2,4,6-tris(3,5-dicarboxylphenylamino)-1,3,5-triazine ligand in varying sizes by modifying the molecular weight. The incorporation of nitrogen-rich chemical groups from the triazine linker (basic sites) and the available metal sites of Cu₂(COO)₄ (acidic sites) has yielded an architecture including three types of cages for sulfur containment. Type A is analogous to the HKUST-1 cage, type B employs four ligands to form a tetrahedral cage measuring 14 Å, and type C uses eight ligands to build an octahedral structure of 20.3 Å. The capacities after 300 cycles at 0.5 C are 352, 436, 631, and 831 mAh g⁻¹ for particle sizes of 1000, 500, 200, and 100 nm, respectively.

HKUST-1 (Cu-BTC) is among the most renowned and extensively researched copper MOFs. Wang et al. [87] have reported a conventional approach of producing sulfur cathodes, which involves impregnating sulfur into the MOF pores through diffusion, to a straightforward mechanochemical mixing process. The confined environment of sulfur not only inhibits its dissolution into the electrolyte, but additionally delays its sublimation by 40 °C. The reversible interaction between sulfur and open Cu²⁺ sites enhances battery performance, attaining 500 mAh g⁻¹ after 170 cycles (at a charge/discharge rate of 0.1 C/0.05 C), compared to around 250 mAh g⁻¹ for the milled sample.

The milling procedure, now utilizing ethanol, was recently reexamined, yielding comparable findings [88]. The cell commenced with a maximum capacity of 620 mAh g⁻¹, stabilized at 280 mAh g⁻¹ after the 10th cycle, and concluded with 200 mAh g⁻¹ after 100 cycles at 0.1 C. To achieve elevated performance, additional fine tuning of sulfur and carbon black quantities is required. However, an increase in carbon black for enhanced conductivity may negatively impact sulfur loading.

Feng et al. have investigated Cu [89] and Co [90] metal centers using varying sulfur weight ratios (1:1, 1:2, and 1:3 MOF:S) according to the BTC linker. The chemical interactions between the functional groups of the MOF and sulfur have been investigated using XPS and DFT. In both MOFs, the 1:2 ratio yielded superior performance; nevertheless, the copper variant exhibited higher specific capacities (1051 mAh g⁻¹ after 300 cycles at 0.12 C) compared to the cobalt variant (600 mAh g⁻¹ after 100 cycles at 0.1 C).

The association between particle size and capacity was examined for HKUST-1, indicating a reduced capacitance for bigger particles [57]. Samples measuring 0.16, 1.6, and 5.9 mm have been prepared, resulting in capacities of 679, 540, and 480 mAh g⁻¹, with retentions of 64%, 60%, and 54%, after 20 cycles at 0.1 C. The utilization of sulfur could be enhanced due to the increased surface area and the shortened electron diffusion path for the reduction in sulfur groups within the framework. Zhou et al. [56] came to the same finding regarding the relation of particle size and reversible capacities in ZIF-8, alongside their investigation of HKUST-1, MIL-53 (Al), and NH2-MIL-53 (Al). Particles measuring 150 nm, 1 mm, and 3 mm have been tested for 100 cycles at a rate of 0.5 C, yielding capacities of 733, 556, and 491 mAh g⁻¹, respectively. A subsequent comprehensive analysis, including sizes of 2 mm, 800 nm, 200 nm, 70 nm, and 15 nm demonstrated capacities of 489, ~570, ~765, ~860, and 968 mAh g⁻¹, with retention rates of 45%, 57%, 75%, 50%, and 22% after 250 cycles at 0.5 C [91]. In conclusion, sulfur utilization enhances with decreased particle size. However, optimal cycling stability is attained at an intermediate size.

To achieve ZIF-8 with ordered micro- and macro-porosity, the PMMA template of nanosized spheres was used for synthesizing ZnO precursor material for the fabrication of S-3DOM ZIF [92]. In comparison to the standard ZIF-8 sample, it exhibits a superior capacity of 674 mAh g⁻¹ after 500 cycles at 2 C, with a retention of 84%, as opposed to 345 mAh g⁻¹ (52.3% retention). The enhanced efficiency can be attributed to the intensified interaction between 3DOM ZIF and polysulfides, as analytically confirmed by means of UV-Vis and XPS. Liu et al. [93] have prepared a ZIF-8@RGO free-standing film via suction filtration and subsequently freeze-dried using liquid nitrogen and additional carbonization at 800 °C for 3 h, resulting in a ZIF-8@RGO scaffold rich in N/Zn active sites (Figure 2).

When compared to other metal-organic frameworks, the maximum discharge capacities observed for ZIF-8, HKUST-1, NH2-MIL-53, and MIL-53 were 738, 431, 568, and 793 mAh g⁻¹, respectively, following 300 cycles at a rate of 0.5 C. At the end of the cycling procedure, the capacities decreased to 75%, 66%, 58%, and 44% of their maximum values, accordingly. The reduction in capacity of HKUST-1 is attributed to its significant particle size, which exceeds 10 mm [56]. Bai et al. [94] proposed a different approach to sulfur melt diffusion by generating in situ encapsulated S nanoparticles during the synthesis of the MOFs. A direct comparison of ZIF-8 at 800 nm, ZIF-67 at 1 µm, and HKUST-1 at 500 nm revealed suboptimal results for both initial capacities and after 1000 cycles at 0.2 C, with values of 255–170, 260–155, and 340–250 mAh g⁻¹, in each case.

Ge et al. [95] demonstrated the synthesis of ZIF-67 by incorporating tannic acid for 5 or 10 min as a modulator. It effectively shapes the structure by cleaving Co-N bonds and facilitating N-H bond formation, thereby adjusting polarity and limiting the solubility of polysulfide intermediates in the electrolyte. A further benefit is that the -OH groups of tannic acid can capture lengthy polysulfides, transforming them into insoluble polysulfide and thiosulfate; therefore, alleviating the shuttle effect [4,79]. A period of 5 min provides enhanced cycling stability over 100 cycles at a current density of 100 mA g⁻¹, yielding a capacity of 757 mAh g⁻¹ compared to 500 mAh g⁻¹ (10 min) and 422 mAh g⁻¹ (standard ZIF-67) [95]. The electrochemical efficiency of the ZIF-67-S, ZIF-67-5-S, and ZIF-67-10-S cathodes is presented in Figure 3. Also, a capacity of 521 mAh g⁻¹ during an extended cycling period (550 cycles) at a rate of 500 mA g⁻¹ is illustrated. Tannic acid has been utilized to synthesize a hollow ZIF-8/CNT composite exhibiting commendable cycling stability [96].

MOF-5 is another viable material, having a maximal surface area of 3800 m² g⁻¹ [97]. The cathode synthesized at room temperature exhibited a starting capacity of 1476 mAh g⁻¹, stabilizing at around 624 mAh g⁻¹ after 10 cycles, and resulting after 200 cycles at 0.2 C to a capacity of 609 mAh g⁻¹ [98]. Wang et al. [99], prepared three variants of MOF-525 (contains Zr), utilizing porphyrin linkers, and evaluated them across 200 cycles at a rate of 0.5 C. Specifically, MOF-525(Cu) attained a capacity of 704 mAh g⁻¹, while MOF-525(FeCl) and MOF-525(2H) exhibited capacities of 616 and 402 mAh g⁻¹, respectively. This arrangement is prompted by the configuration offering two, one, and zero Lewis acidic sites, respectively, capable of interacting with sulfur and polysulfides. These sites likewise provide capacities exceeding 400 mAh g⁻¹ at a rate of 5 C. Liu et al. [100] have investigated 2,6-Zr-AQ-MOF and 1,4-Zr-AQ-MOF, which are zirconium metal-organic frameworks utilizing anthraquinone linkers. The high density of AQ groups inside the MOF architecture explains the improved performance of 1,4-Zr-AQ-MOF during 300 cycles at 4 C (403 vs. 300 mAh g⁻¹). The 1,4 configuration is directed into the pore, facilitating tighter contacts with neighboring AQs, while the AQs in 2,6-Zr-AQ-MOF are oriented along the framework, indicating that pore geometries play a vital role in regulating charge conduction (Figure 4). Lai et al. [101] prepared and tested cathodes for lithium-ion batteries based on MOFs with Anthraquinone-2,3-Dicarboxylate ligands.

Another important utilization for MOFs in Li-S batteries is in the field of separators [5]. Ngo et al. [76] have used glass fiber after modification with a bimetallic MOF as separator. This has enhanced the cycling efficiency of the new cell relative to the unmodified glass-fiber cell by a factor of two. A novel separator based on a 2D MOF/PP composite inhibited the polysulfide shuttle effect and increased the ionic conductivity leading to an enhancement of the sulfur cathode’s electrochemical performance [102].

NiCo-MOF is notably intriguing as an electrode material to investigate the electrolyte/electrode interface, owing to its inherent porosity and ultrathin nanomorphology, which provide a substantial surface area that enhances the interaction between the electrode and the electrolyte. The ultrathin design reduces ion transport paths, guaranteeing that interfacial reactions prevail over bulk effects, rendering it optimal for investigating interfacial phenomena. Moreover, NiCo-MOF-derived NiCo₂O₄ exhibits mixed-valence states of Ni and Co, which augment redox processes. The redox-active sites exhibit significant sensitivity to the interface, facilitating the investigation of ion transport and reaction processes. Recently, a dominating pseudocapacitive behavior at high-rate performance was revealed, overcoming the structural degradation issues often encountered in NiCo-based devices. The electrode outperformed previous research with its 83% pseudocapacitive contribution, demonstrating high-rate performance and quick, diffusion-independent charge storage [103].

Utilizing both theoretical and experimental approaches, deviations in conduction processes occurred between Ni-MOF and NiCo-MOF, along with their electrochemical performance as anode materials in rechargeable lithium-ion batteries. The average conductivity of NiCo-MOF has been determined at 0.058 mS cm⁻¹, demonstrating a significant increase, being 34 times higher than that of Ni-MOF (0.0017 mS cm⁻¹). Calculations using the density functional theory (DFT) suggest that the rise in conductivity results from the incorporation of Co²⁺, which enhances the electron transport pathways and markedly elevates carrier concentration. In contrast to Ni-MOF, NiCo-MOF exhibits an increase of 19 additional electrons at the Fermi level. The improved conductivity leads to a notable enhancement in electrochemical performance; under 2 C conditions, NiCo-MOF exhibits a specific capacity of 696.3 mAh g⁻¹ after 200 cycles, which is double that of Ni-MOF (299.9 mAh g⁻¹) [104].

Wei and Qin [105] proved with the example of a 2D Iron-Quinoid MOF that it is possible to macroscopically tune conductivity properties from an electronic structure using DFT calculations. The results from the DFT calculations indicated a narrow bandgap, while subsequent quantum chemical modeling of the bimetal unit within the Iron-Quinoid MOF indicated an increase in the bandgap upon gradual oxidation of the MOF structure.

Computational quantum mechanical methods typically yield detailed electronic structure information with adequate precision, offering valuable insights into the conductive behavior of MOFs. A novel design approach involves leveraging machine learning to enhance the efficiency of screening and discovering new conductive materials. Zanca et al. [106] provided a comprehensive overview of the prevalent quantum mechanical techniques utilized to characterize key band structure parameters of MOFs.

2.2. Zinc (Zn)-(Metal-Ion) Air Batteries

Metal-ion batteries, particularly lithium-ion batteries, have emerged as the most prevalent type, mostly due to their outstanding efficiency and energy density in electronic applications. Notwithstanding the prevalent application in commercial contexts, opportunities for enhancement persist by substituting transition metal oxide cathodes, such as LiMO₂, LiM₂O₄, and LiMPO₄ (M:Mn, Co, Ni, etc.) such as LiCoO₂, LiMn₂O₄, and LiFePO₄, with metal-organic frameworks and carbon/oxide-based materials. In this respect, a composite of Mn-based MOFs and carbon nanotubes (Mn-MOF/CNTs) was synthesized for zinc-ion batteries (ZIBs) cathodes. The constructed Mn-MOF/CNT//Zn battery demonstrates a notable specific capacity of 260 mAh g⁻¹ at a current density of 50 mA g⁻¹, along with impressive capacity retention of around 100% after 900 cycles at 1000 mA g⁻¹ [107].

The notable electrochemical efficiency can be linked to the elevated porosity and conductivity of Mn-MOF/CNT, along with the presence of the Mn²⁺ electrolyte additive. The extensive variety of pore and morphological characteristics, along with the capacity for storage of metallic ions, renders them excellent cathode materials.

The application of MOF-177 (Zn₄O(1,3,5-benzene-tribenzoate)₂) in lithium-ion batteries (LIBs) began almost 20 years ago with its utilization in the anode compartment [108]. Despite the results being unimpressive due to restricted cycle stability, this first assessment prompted further exploration of additional MOFs. Subsequently, the MIL group [6] was investigated as cathodes, commencing with the examination of Fe²⁺ and Fe³⁺ active sites as redox centers in MIL-53 [109] and MIL-68 [110]. MIL-53 may provide ca. 70 mAh g⁻¹ by accommodating 0.6 Li/Fe, while MIL-68 can retain 0.35 Li inside its architecture. Although either MOFs utilize terephthalic acid, the divergent reaction conditions result in dissimilar pore geometries, triangular in MIL-53 [54] and hexagonal in MIL-68, elucidating the challenges associated with lithium electrolyte diffusion into the MOF structure. Formulated using the identical linker, MIL-101 was evaluated, yielding unsatisfactory findings attributed to the stability of the oxidized form (Fe³⁺) of Fe²⁺, resulting in capacity decline and rapid electrode deterioration [111]. The metal-organic framework (MOF) was subsequently reevaluated, demonstrating a discharge capacity of 72 mAh g⁻¹ over 100 cycles at a rate of 0.2 C and an absorption of 0.62 Li/Fe [63].

Recently, Deng et al. [112] published results on the development of non-noble metal FeNi@NC-900 bifunctional air-cathode catalysts for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), which represent a crucial advancement in the development of practical rechargeable zinc-air batteries (ZABs). Another iron metal-organic framework, utilizing ferrocenedicarboxylate (Fe₂(DFc)₃), was synthesized, exhibiting a remarkable starting specific capacity of 172 mAh g⁻¹ at 50 mA g⁻¹ and exceptional stability, maintaining ca. 70 mAh g⁻¹ at 2000 mA g⁻¹ for 10,000 cycles [113]. Various materials for cathodes can be employed directly in aqueous ZN-Ni batteries by doping Ni-MOF-74 with M^x+ ions (Mn²⁺, Co²⁺, Cu²⁺, Zn²⁺, Al³⁺, Fe³⁺) to preserve the genuine architecture while modifying the quantity and type of metal ions [114]. Ni/Co bimetallic metal-organic frameworks (NCBMs) are excellent candidates for alkaline zinc batteries due to their exceptional physical and chemical characteristics. An alkaline zinc battery, utilizing MBene-NCBM as the cathode and zinc foam as the anode, has demonstrated a remarkable energy density of 280 Wh kg⁻¹ alongside a power density of 3306 W kg⁻¹. The cycling tests reveal that the device’s capacitance sustained a level of 79% following 1500 cycles [115].

Although less prevalent than iron-based MOFs, there have been several initiatives involving copper for lithium-ion batteries. Peng et al. [116] employed Cu-TCA to harness redox activity from Cu/Cu²⁺ centers and the tricarboxytriphenyl amine organic linker (N/N⁺). By cycling at 0.5 C, significant capacity degradation occurred (starting at 102 mAh g⁻¹ with around 39% retention after 200 cycles), likely due to the loss of redox activity of Cu ions. In contrast, at elevated C-rates (1 C/2 C), retention rates improved markedly (approximately 71%) as the electrolyte addition (hexamethylene diisocyanate) enhanced system stability and augmented discharge capacity at 2 C by 11 mAh g⁻¹. Nagatomi et al. [117] utilized a phthalocyanine linker including copper and copper ions as the metal center to construct the 2D Cu-CuPC. The integration of micropores (1.4 nm) and mesopores (11 nm) alongside the framework’s moderate electrical conductivity resulted in a capacity of 52 mAh g⁻¹ after 200 cycles at 0.4 C. Gu et al. [118] have reported encouraging findings with Cu₃(HHTP)₂ (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene). The reversible capacity of 95 mAh g⁻¹ from the 20th to the 60th cycle (with an 8% degradation by the end of 100 cycles) may indicate an intercalation ratio (Li:Cu) approaching 100%, linked to the layered MOF structure and its inherent conductivity.

In situ transmission electron microscopy (TEM) serves as an effective method for examining the structure and changes in composition at the atomic level during electrochemical reactions, offering a comprehensive understanding of structural degradation. During charge–discharge, virtually all metal oxides (MOs) face structural disintegration and phase separation, in contrast to graphite. The nanoscale morphological modifications that occur during charge–discharge have been investigated using transmission electron microscopy (TEM). Ordered mesoporous nickel oxide (OMNiO) was prepared and employed as the model MO. A connection between TEM and electrochemical investigations to elucidate the underlying cause of the rapid capacity decline that is prevalent in MOs in general and NiO in particular was established [119]. A comprehensive review by Xia et al. [120] examines advancements in the in situ transmission electron microscopy (TEM), emphasizing its contribution to understanding the degradation mechanisms of MOF structures in interfaces between electrodes and electrolyte, electrode surfaces, particle interior, and thermal effects. The insights obtained from in situ TEM, which is rapidly advancing, are unique and anticipated to inform the design of improved layered cathode materials.

Tian et al. [121] have created isostructural MOFs with one-dimensional and two-dimensional configurations utilizing Cd or Co as metallic centers and naphthalenediimide ligands. The two-dimensional MOFs include quadratic cavities around 20 × 20 Å resulting in superior diffusion coefficients compared to the one-dimensional variants, attributable to their greater porosity. Despite the Co version demonstrating superior initial specific capacity, the MOF displayed instability and inadequate cycle performance. Conversely, Cd exhibited satisfactory performance with 47 mAh g⁻¹ over 50 cycles at a current density of 100 mA g⁻¹.

Finally, in situ synthesis of MOF-74 with a sophisticated surface modification led recently to higher stability of Zn-ion battery anodes [122]. An intriguing method involves the application of MOFs οn catalyst surfaces, for example, by utilizing Mn-MOF-74 to modify the surface of the Li-rich layered Li(Li_0.17Ni_0.20Co_0.05Mn_0.58)O₂ with a 2–3 nm thick layer [123]. This approach enhances oxygen storage capacity and increases the number of surface-active sites, and at the same time, increases ion diffusion through the structure channels. Specifically, after 100 cycles, the modified composite provided a capacity of 284 mAh g⁻¹ as compared to 236 mAh g⁻¹ at 0.1 C, with a capacity retention of approximately 87% and overall enhanced efficiency for all C-rates up to 5 C.

2.3. Zinc (Zn)-Air (Metal-Air) Batteries

Metal-air batteries (MABs) have drawn significant interest because of their comparatively high energy density and eco-friendliness relative to other battery technologies. Zinc and lithium electrodes represent the most appealing possibilities for metal anodes [124,125,126]. However, their cathodes are more intricate components that permit the ingress of ambient oxygen into the system. The entire efficiency of the cell is significantly contingent upon the kinetics of oxygen electrochemistry, specifically, the OER (oxygen evolution reaction) taking place at the cathode during charging, and during discharging the corresponding oxygen reduction reaction (ORR). The primary problem concerning MABs is the creation of suitable non-noble catalysts that promote both ORR and OER while preventing premature corrosion under cycling conditions [127].

Recently, many alternatives to non-precious electrocatalysts have been investigated, including single atom catalysts [128], carbon-based materials, transition metal oxides, metal alloys, and sulfides. The inherent characteristics of these electrocatalysts, determined by the compound, are complemented by parameters such as crystalline structure, morphological characteristics, and heterostructure interfaces. These elements can greatly impact the efficiency of an electrocatalyst by improving molar diffusion pathways and increasing the availability of active sites. Metal-organic frameworks (MOFs) demonstrate structural flexibility, uniform porosity, and high surface area, indicating considerable potential for application as electrocatalytic components in MAB cathodes.

Preliminary findings on MOFs in Li-O₂ batteries were disclosed in 2014, encompassing MOF-5, HKUST-1, and MOF-74 (Co, Mg, and Mn) [51]. MOF-5 and HKUST-1 exhibited primary capacities of 1780 and 4170 mAh g⁻¹, respectively, at a current density of 50 mA g⁻¹. The open metal sites of MOF-74 confer advantages resulting in capacities of 3630 mAh g⁻¹ for cobalt, 4560 mAh g⁻¹ for magnesium, and 9420 mAh g⁻¹ for Mn, with manganese-based catalysts demonstrating exceptional performance in electrochemical applications [129,130]. However, after six consecutive cycles at 200 mA g⁻¹, a capacity decline is revealed from around 5600 mAh g⁻¹ in the first cycle to 1300 mAh g⁻¹ in the last one, which is a common occurrence as demonstrated in subsequent reports. Yan et al. [131] examined the influence of Co-MOF-74 particle size. The rod dimensions (length × breadth) were altered by choosing various ratios of DMF:H₂O, specifically 1:4 (9.5 µm × 1400 nm), 1:1 (6.5 µm × 800 nm), and 4:1 (about 240 × 20 nm). The rod structure was maintained by using salicylic acid as a regulator, which can compete with the ligand, and therefore hinder the coordination bond between metal centers and organic linkers, which affects crystal growth and size.

Lv et al. [132] reported the synthesis of “MOF-plus-MOF” single atom catalysts for Zn-air batteries. They prepared cobalt atoms stabilized by N-doped carbon using a pyrolysis step, exhibiting 3D interconnected structures, a large specific surface area, and abundant N-active species. The Co-N₃C₁ single-atom configuration contributes to the exceptional ORR/OER performance of CoSAs@NC-920, surpassing that of commercial noble metal catalysts. Furthermore, it exhibited a greater specific capacity, enhanced discharge power density, and remarkable cycling stability than actual rechargeable zinc-air batteries.

Salicylic acid interacts analogously with 2,5-dihydroxy terephthalic acid and hence, terminating the directed chain formation and maintaining the crystal size by varying the modifier concentration. The small particles of 20 nm resulted in initial cycle capacities of 11,350 mAh g⁻¹ at 100 mA g⁻¹ and 6440 mAh g⁻¹ at 500 mA g⁻¹, significantly surpassing the capacities of 800 nm (4880 mAh g⁻¹) and 1400 nm (4710 mAh g⁻¹) at 100 mA g⁻¹. A subsequent endeavor to synthesize MOF-74 involved the creation of a bimetallic MnCo catalyst in a 1:4 ratio [133]. In contrast to mono-metallic MOFs, the synergistic effect enhances efficiency and reversibility, exhibiting an initial capacity of 11,150 mAh g⁻¹ at 200 mA g⁻¹ and stability over 44 cycles at a threshold of 1000 mAh g⁻¹, while other samples demonstrate lower capacities of 7767 mAh g⁻¹ (Mn and Co simple mixture), 6040 mAh g⁻¹ (Mn-MOF-74), and 5630 mAh g⁻¹ (Co-MOF-74). Nickel has also been investigated in an architecture comprising bipyridine and trimesic acid [134]. Following six cycles at 0.36 mA cm⁻², the capacity declines to approximately 1200 mAh g⁻¹, reflecting an 80% reduction from the initial cycle, but with a constant capacity of 600 mAh g⁻¹, it can work for 170 cycles at 0.6 mA cm⁻².

2.4. Metal-Organic Framework (MOF)-Based Composite Materials

2.4.1. Metal-Air Batteries (MABs)

As previously indicated, MOFs can exhibit adjustable particle sizes and hollow structures, which are maintained following their processing to generate carbon-based materials or their nanocomposites. These materials exhibit mixed ionic/electronic conductivity, making them exemplary materials for possible application in battery cathodes. Furthermore, the complete availability of their active sites is an essential characteristic for their application in MABs; thus, various MOF-derived components have been employed in gas-diffusion electrodes, mainly in zinc-air batteries (ZABs).

Numerous studies confirm that exclusive heteroatom doping of MOF-derived three-dimensional carbonaceous materials can enhance the oxygen reduction reaction (ORR) and demonstrate favorable outcomes in primary alkaline zinc-air batteries (ZABs) [61,135,136]. NaCl has been utilized as a confining agent for ZIF-8 to maintain a consistent polyhedral structure post-carbonization. The produced catalysts demonstrated commendably stable performance when used in zinc-air battery systems at laboratory scale [61,135]. Yang et al. [136] introduced a silica-assisted template technique to enhance the accessibility of graphitic N-containing sites in a ZIF-8-based catalyst, attaining an exceptional specific surface area of 254 m² g⁻¹ via a synergistic structure comprising a macroporous structure and micromesoporous “walls”. The engineered material exhibited an exceptional specific capacity of 770 mAh g⁻¹ Zn at a challenging current density of 120 mA cm⁻² and a maximum power density of 197 mW cm⁻². In addition to heteroatom doping, the incorporation of metals such as Fe and Co presents a compelling approach for defining active regions within carbon matrices. The thermal treatment of MOFs often results in doping with metals or the uniform decorating of carbon-based materials with heteroatoms as well as metal atoms. ZIF-8 has been widely utilized for synthesizing Fe-N-C electrocatalysts with a certain pore hierarchy for the oxygen reduction reaction (ORR). Xu et al. [42] synthesized an Fe-N-C catalyst via the straightforward pyrolytic treatment of Fe-ZIF, achieving commendable ORR metrics (E_1/2 = 0.881) and effectively utilizing it in a ZAB. They demonstrated that Fe doping not only improves the electronic conductivity of carbon structures, but also enhances the wettability of the electrocatalyst with water, a trait that may facilitate the ORR by augmenting surface exposure to the electrolyte. Ferrocene (Fe(C₅H₅)₂) [137] and iron carbonyl (Fe(CO)₅) [138] have been utilized as starting materials to modify ZIF-8, resulting in the formation of carbon compounds containing iron moieties. In all studies, the three-dimensional porous Fe-N-C electrocatalytic materials, utilized as cathode materials in zinc-air batteries, achieved steady voltage during discharge and exhibited significantly high peak power densities (>200 mW cm⁻²), surpassing Pt-decorated carbon cathodes. Under specific conditions, ZIF-8 serves as a suitable and economical precursor for the synthesis of Fe-N-doped carbon nanotubes, which have remarkable ORR catalytic activity making them attractive as catalytic materials for ZABs [139,140].

Similar treatment approaches for ZIF-8 have been utilized to synthesize Fe-N-doped C-catalysts, which can improve OER kinetics, and are therefore appropriate for use in rechargeable liquid ZABs [141,142]. Specifically, Chen et al. [141] constructed a catalytic material featuring a dodecahedral structure and refined pore distribution, resulting in an extensive surface area and facilitating exceptional ORR performance. The ZAB system not only provides a high specific capacity of 801 mAh g⁻¹ and a peak power density of 184 mW cm⁻², but also withstands 600 continuous galvanostatic cycles of 10 min each at a current density of 10 mA cm⁻².

Recently, co-doped MOF-derived catalysts demonstrated significant efficacy in the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), thereby enhancing rechargeable zinc-air batteries (ZABs). They can be produced by direct pyrolysis of cobalt-containing metal-organic frameworks (MOFs), such as ZIF-67, in an inert environment [47,143]. ZIF-8 was pyrolyzed into a nitrogen-doped carbon structure (N/C) and subsequently reacted with a Co solution to produce a catalyst working effectively in both aqueous as well as solid-state zinc-air batteries [144]. The resulted catalyst demonstrated remarkable ORR and OER electrochemical characteristics (ΔE_(OER-OER) = 0.65 V) and are extraordinarily stable, sustaining prolonged cycles of two hours per cycle at 50 mA cm⁻². Similarly, other morphologies of Co-NeC generated from MOFs, such as nanorods [145], nanoflake arrays [146], and hexagonal structures [147], have been investigated. Zhang et al. [147] examined the influence of MOF precursors and pyrolysis conditions using the BTC, revealing that a Co-NeC catalyst, termed Co-BTC-bipy-700, demonstrated exceptional electrocatalytic properties when employed in ZABs.

The synergetic interaction of Fe and Co atoms inside N-doped carbon-based nanostructures effectively enhances the electrocatalytic ORR and OER efficiency of the MOF derivative material. Zhong et al. [148,149] pyrolyzed FeCo@ZIF to produce dual-doped carbon polyhedral (Figure 5), which demonstrated superior ORR activity relative to single atom-doped ZIF-derived catalysts, as indicated by DFT calculations and electrochemical ORR assessments (Figure 6) [148]. Duan et al. [149] conducted a comparable DFT analysis, revealing that the FeCo-N-C catalyst exhibited bifunctional electro-catalytic capabilities, substantiated by its outstanding performance as an air electrode in a rechargeable zinc-air battery. The cathode achieved a maximum power density of 150 mW cm⁻² and withstood 360 cycles of 15 min at a current density of 1 mA cm⁻².

Carbon compounds generated from MOFs have been investigated as prospective cathodes for MABs using a lithium anode. MOF-5 has been chemically etched and pyrolyzed to eliminate the Zn-containing structure, resulting in a material with BET surface area of 1255 m² g⁻¹, which demonstrated an initial capacity of 1437 mAh g⁻¹ [150]. Ge et al. [151] introduced a templated approach for the synthesis of carbon-based cathode materials produced from ZIF-8. Polystyrene beads served as a self-sacrificial template to create macropores in addition to the micropores of the ZIF-nanomaterial, significantly enhancing the specific capacity of the cathode (5082 mAh g⁻¹).

Yu et al. [152] synthesized a molybdenum-based metal-organic framework (MOF) that, upon thermal treatment in an argon environment, yielded nitrogen-doped porous molybdenum carbide (MoC_1−x) characterized by extensive meso- and macroporous channels, which created active sites conducive to oxygen reactions. The constructed battery exhibited a complete discharge capacity of 20,212 mAh g⁻¹ at a stable voltage of 2.62 V under a current of 200 mA g⁻¹, while it yielded 1000 mAh g⁻¹ over 100 cycles.

In addition to carbonaceous cathode materials, S-containing compounds obtained from Ni-MOF [153] and ZnCo-MOF [154] through hydrothermal sulphuration methods demonstrated potential efficacy as cathodes in rechargeable Li-O₂ batteries. Ni₃Se₂ and ZnCo₂S₄ exhibit distinctive layered architectures, contributing to the MAB longer cycle life of 120 cycles at 300 mA g⁻¹ and 90 cycles at 100 mA g⁻¹, respectively.

2.4.2. Lithium Batteries

Regarding carbon MOF-derived materials, it is evident that, in addition to the feasible doping of carbon matrices, the diverse pore sizes and structures of MOF-derived carbons have the potential to directly encapsulate molecules within the pores. A significant aspect is the formulation of active electrode materials for Li-ion batteries, which can be customized to include various metals, oxides, or chalcogenides. These features satisfy the criteria for an optimally conductive carbon matrix to confine sulfur, facilitating the formation of cathodes for lithium-sulfur batteries [155].

The extensive study of ZIF-8 as a metal-organic framework precursor for lithium-sulfur materials, and examination of many distinctive nanoparticle structures of the resultant carbon, including regular rhombic dodecahedrons [156,157], spheres [158], and nanosheets [159] are reported. Nitrogen-doped two-dimensional C-nanosheets with hierarchical porosity demonstrated effective conductivity as hosts for the immobilization of sulfur. The synthesized lithium-sulfur battery exhibited a starting capacity of 1226 mAh g⁻¹ and underwent comprehensive testing at different discharge rates. Furthermore, the air electrode demonstrated the ability to withstand 300 cycles, with a retention of 65.2% at 0.2 C after 100 cycles [159]. Hao et al. [160] synthesized two-dimensional graphene-like carbon architectures via thermal exfoliation of Cu containing 4,40-bipyridine MOFs, emphasizing the hydrophilicity of the 2D-sheet walls. The results of water vapor adsorption validate the favorable surface polarity of the carbon matrix, which attracts polysulfides and improves the kinetics of redox chemistry. The constructed Li-S battery retained 99% of its capacity for 250 cycles at 0.5 C.

Kim et al. [161] introduced dual heteroatom doping of mesoporous hexagonal rod-shaped carbon nanoparticles derived from S-modified MOF-74, demonstrating that this approach enhances the capacity retention of Li-S batteries. Thermal treatment at reducing conditions and phosphorization of ZIF-67 were proposed to adorn the carbon NPs with CoP, which reduces the solubility of the polysulfides and inhibits them from escaping from the pores of the carbon-based material.

MOF-derived materials can fulfill comparable charge capacity requirements in Li-Se battery cathodes. ZIF-8 and MOF-5 were among the initial metal-organic frameworks utilized to generate porous carbon structures by direct pyrolysis for the purpose of accommodating polyselenides in lithium-selenium [162] batteries. A comparable method was employed to produce from an Al-MOF, rod-shaped N-doped carbon sponge characterized by a porous architecture capable of facilitating bulk transition of Se ions [163]. Researchers highlight the significance of doping with hetero atoms and the introduction of mesopores in layered carbonaceous materials for maintaining stable electrochemical performance of the cathode across 200 cycles, achieving a consistent capacity of 443.2 mAh g⁻¹ at 0.5 C.

Jin et al. [164] recently synthesized distinctive carbon nanosheets featuring pores measuring between 0.58 and 150 nm using the carbonization of SAF (Figure 7). The resulting nanomaterial offered a highly conductive matrix able to inhibit volume expansion due to Se/Li interactions, and the PCNS/Se cathode achieved up to 200 cycles at current rates ranging from 0.25 to 2 C.

The thermal treatment of MOFs has been documented as an effective method for synthesizing cathodes for metal-ion batteries based on metal oxides. As previously stated, in metal-ion batteries, the cathode in lithium-ion batteries is the electrode that contains lithium. Yin et al. [165] synthesized different LiNi_0.5Mn_1.5O₄ electrodes initially by a MOF thermal oxidation technique, subsequently employing conventional methods to evaluate their performance. The resulting M-LNMO exhibited improved structural characteristics that significantly boosted Li-ion cell efficiency. This behavior of the enhanced electrode is ascribed to the uniform distribution of materials, elevated crystallinity, and the removal of contaminations. Metal oxides generated from MOFs (Mn₂O₃ and V₂O₅) have been utilized in cathodes for Zn-ion batteries to enhance the capacity for charge storage and rechargeability of the resulting devices [72,166].

2.4.3. Solid-State Lithium Batteries

Due to their potential for superior safety [167] and energy density, all-solid-state batteries are garnering growing attention as a leading candidate for future battery storage. Nonetheless, their extensive uses are constrained by several technological obstacles, such as low-conductivity electrolytes, dendritic formation [168], and inadequate cycle and/or rate performance [169]. They provide extremely high specific capacities at an increased electrochemical window, and thus enable very high energy densities. Despite the benefits provided by solid-state batteries, the used polymer electrolytes suffer from sluggish ion kinetics and inadequate selectivity in Li+ diffusion. Furthermore, extensive research is necessary to fully understand how lithium metal anodes behave dependent on the operating conditions [170]. Polymer electrolytes, including polyethylene oxide (PEO) and polyvinylidene fluoride (PVDF), are advantageous because of their affordability, versatility in preparation, and superior interfacial wetting capacity. Nonetheless, the ionic conductivity remains constrained to the range of 10⁻⁶–10⁻⁷ S cm⁻¹ at ambient temperature [171].

MOFs are suggested as novel ingredients for solid-state electrolytes [172]; nonetheless, the development of functionalized MOFs and the comprehension of their impact on ion transportation have presented significant challenges [173]. The specific benefits of MOFs can be used to control diffusion paths by their high specific surface area as cation hopping centers are very close to each other, thereby enabling a fast cation movement at low activation energies [174]. Furthermore, the high ordering in MOF-crystals leads to the uniform distribution of metallic cations, and thus inhibits the formation of dendrites [175].

Wu and Guo [176] reported the preparation and utilization of UiO/Li-IL solid electrolytes, by incorporating Li-IL into the nanopores of UiO-66 metal-organic frameworks in LiFePO₄/Li solid-state batteries. This type of solid electrolyte facilitates rapid lithium-ion migration in solid-state lithium batteries due to their high ionic conductivity (3.2 × 10⁻⁴ S cm⁻¹) and low interface resistances between the electrode and the solid electrolyte. The prepared batteries sustain 130.4 mAh g⁻¹ after 100 cycles at a rate of 0.2 C. Additionally, the initial discharge capacity recorded is 119 mAh g⁻¹, with a retention rate of 94% after 380 cycles at a rate of 1 C. To tackle the interface challenge and enhance energy density, Wang et al. [177] prepared an innovative solid-like electrolyte (SLE) utilizing ionic-liquid-impregnated MOF nanocrystals (Li-IL@MOF). This material exhibits remarkable electrochemical characteristics, featuring an excellent ionic conductivity of 3.0 × 10⁻⁴ S cm⁻¹, and strong compatibility between Li metal and active electrodes, resulting in low interfacial resistance.

Wang et al. [178] combined also ionic liquids but with 1,3,5-benzenetricarboxylic acid and copper complexes (HKUST-1) to create MOF arrangements, which were then incorporated into a PEO-based polymer, subsequently yielding CPE films by the solution casting process. The ionic conductivity of the synthesized CPEs film at 30 °C reached 1.20 × 10⁻⁴ S cm⁻¹, significantly surpassing that of the original PEO. Two-dimensional-copper p-dibenzoate (Cu(BDC)) MOFs, which possess sufficient open metal sites, have been used by Yu et al. [179] to prepare composite polymer electrolytes (CPEs) in a demonstration of engineering of fast conduction paths for lithium ions.

3. Metal-Organic Frameworks (MOFs) in Fuel Cells

Hydrogen fuel cells represent a superior alternative to fossil fuels, providing a clean and carbon-free means of transforming chemical energy directly into electrical energy [180]. Furthermore, they are almost twice as efficient as fossil fuels (more than 50% electrical efficiency for fuel cells versus ca. 30–35% for fossil fuels because of the losses of the Carnot cycle) [181,182]. They comprise two electrodes separated by a solid electrolyte. The electrodes must be catalytically active (especially in low temperature fuel cells) and possess a certain morphology and porosity that allows for the transport of reactants as close as possible to the electrode/electrolyte interface, and at the same time, the removal of reaction products (mostly water). Two significant obstacles impede the broad implementation of fuel cells. The first is related to mass transport limitations during the operation and the second is related to the risk of metal leaching from the catalytic materials used in the electrodes, which can result in a decreased lifespan [183]. A catalyst layer’s elevated porosity is essential to optimize reactant accessibility to the active sites located on the surface of the micropores of the gas diffusion electrode as well as of the active (catalyst loaded) electrode. Mesoporous templates can facilitate the attainment of a more uniform pore size and an organized mesoporous structure, enhancing mass transfer and resulting in superior PEMFC performance. The morphology and pore structure of the catalyst significantly affect its overall efficiency in proton exchange membrane fuel cells (PEMFCs). It is important to reach a balance between the presence of micropores, where catalytic sites are situated, and mesopores that help in mass transport [184]. Carbonization of MOFs is an interesting and viable option to produce carbonaceous materials with high porosity [185]. The carbonization of metal-organic frameworks (MOFs) resulted in the formation of well-structured networks inside their framework, and hence enhancing mechanical and chemical stability [186]. A complex but controlled pore structure is essential for effective diffusion of reactants and products, reducing mass transport limitations, and thus extending PEMFCs long-term operability reaching practical applications [187].

Metal-organic frameworks (MOFs) have numerous benefits for electrocatalysis, electrolyte membranes, and hydrogen storage materials; they exhibit exceptional design flexibility, extensive surface-to-volume ratios, and permit surface and bulk modification by different functional groups with multivalent linkers and metallic centers to enhance affinity for fuel cell applications. Significant attempts have been undertaken to harness the distinctive features of MOFs as materials for high-performance fuel cells [12].

PEMFCs are regarded as efficient power sources for fuel cell electric cars due to their clean emissions, air purification capabilities, and superior durability [188]. The improvement in the fabrication of metal-organic frameworks (MOFs) for fuel cell applications can be classified into three distinct approaches: (1) oxygen reduction reaction (ORR) catalyst material [39,189,190,191,192,193], (2) hydrogen storage material [194,195,196,197], and (3) inorganic filler for proton exchange membranes (PEMs) [198,199,200,201] or stand-alone electrolyte membranes [202,203] even for high-temperature PEM fuel cells with polybenzimidazole as electrolyte material [204].

In the anode compartment of PEMs, the supplied hydrogen is oxidized to protons and delivers electrons, which are transferred to the cathode side where they are needed for the reduction in the oxygen (Figure 8).

The created protons traverse the polymeric membrane towards the cathode, where they react with the supplied O₂ gas to form water. The generated water is expelled from the catalyst surface into the flow-field channel due to capillary pressure differentials. Notwithstanding this water generation, the volume of water back-diffusion in a conventional PEM fuel cell is typically low, as the produced water is expelled externally (Figure 8b). MOFs provide superior water-storage capacity and demonstrate consistent performance due to their porous architecture and extensive surface [205]. Choi et al. [188] presented a water-stable Zr-based UiO-66-type MOF incorporating benzene-1,2,4,5-tetracarboxylate ligands, henceforth designated as UiO-66(Zr)-(COOH)₂, as a functional water-storage material for the cathode catalyst layer of membrane electrode assemblies (MEAs) (Figure 8c). UiO-66(Zr)-(COOH)₂ exhibits thermal and electrochemical stability, hence ensuring robustness and endurance throughout the PEMFC operation. In this respect, Du et al. [206] have fabricated a series of composites by incorporating ionic liquid (IL) into chromium terephthalate MIL-101 using the ship-in-bottle method (IL@MIL-101). The resulting IL@MIL-101 exhibited high water retention, which is crucial for proton conduction in various energy-related applications at low relative humidity.

Besides the above-described fuel cells, microbial fuel cells (MFCs) are devices that generate electric energy while simultaneously treating wastewater, utilizing microorganisms as biocatalysts [207,208]. The microbial fuel cell (MFC) represents a technology that has demonstrated the capability to process a diverse range of substrates [209]. MFCs employ electrochemically active bacteria that oxidize an organic substrate while reducing an electron acceptor, leading to concurrent wastewater treatment and current generation [210]. Nonetheless, the utilization of MFCs is limited by the cathodic oxygen reduction reaction (ORR) because of its elevated energetic barrier and slow kinetics at room temperature [211]. Consequently, achieving elevated electrode potential and optimal energy output necessitates the development of highly efficient and stable metal-based ORR electrocatalysts, which is essential for the practical implementation of MFCs [212].

Besides the enzymatic anodic reaction, the most important and difficult reaction in microbial fuel cells is the cathodic oxygen reduction reaction (ORR). This reaction is similarly important for all fuel cells working at low temperatures. At present, platinum group metals are recognized as the leading catalysts for ORR. However, the scarcity of these resources and their associated high costs necessitate the exploration of alternative, cost-effective electrocatalysts [213].

Metal-organic frameworks (MOFs) combined with activated carbon demonstrated a significant enhancement in power density, surpassing activated carbon by 60% and platinum by 160% [214]. The nitrogen groups in the MOFs, specifically pyridine-N and pyrrole-N, contributed to an increase in active sites and a reduction in charge transfer impedance, thereby facilitating improved oxygen reduction reaction (ORR). Moreover, metal-organic frameworks (MOFs) diminish diffusion limitations by enhancing conductivity and charge transfer, thereby increasing electron transfer efficiency and improving power production. The addition of reduced graphene oxide (rGO) in various amounts to carbon black (CB) significantly improved cathodic performance. This composite demonstrated superior capacitance, electron transport, and cathodic polarization resistance [215,216].

In the following paragraphs, some recent works on the development of MOFs in PEM membranes, and ORR catalysts for PEM fuel cells as well as for microbial fuel cells are given.

3.1. Metal-Organic Frameworks (MOFs) for Proton Exchange Membrane Fuel Cell (PEMFC) Electrolyte Membranes

The polymer electrolyte is considered the key component of PEMFCs, as it facilitates proton conduction, separates the cathode and anode compartments, and inhibits the transport of electrons within the cell.

The incorporation of a proton-conducting metal-organic framework (PCMOF) by hybrid assembly with an ionomer can enhance membrane performance in proton exchange membrane fuel cell (PEMFC) applications [217].

Nafion^® (DuPont Fuel Cells, Wilmington, NC, USA), a perfluorosulfonated polymer, is an excellent proton-conducting membrane, but suffers from the necessity to be watered. Because of the sulfonic groups, Nafion is hydrophilic and supports the H⁺ transport by promoting the Grotthuss proton hopping mechanism as the acidic groups act as a proton carrier [218]. Water absorption and ion-exchange capacity (IEC) are fundamental chemical properties of polymer electrolyte membranes.

To enhance the electrochemical performance of a Nafion membrane, Hou et al. [219] incorporated MOFs into the Nafion matrix. Zn-MOF and amino-modified Zn-MOF served as fillers in the produced hybrid membranes, with a content of 5% MOFs. At 80 °C and 58% relative humidity, the proton conductivities of the resulting Zn-MOF/Nafion-5 and Zn-MOF-NH₃/Nafion-5 were 7.29 × 10⁻³ S cm⁻¹ and 2.13 × 10⁻² S cm⁻¹, outperforming Nafion by 1.87 times and 5.47 times, respectively. The proton conductivity of hybrid MOF-Nafion electrolyte membranes could be substantially increased using sulfonated MOFs in the polymer, imitating the same environment for the proton conduction mechanism [220].

Another interesting polymer for electrolyte membranes in so-called high-temperature PEMFCs is polybenzimidazole (PBI). It works in high-temperature PEMFCs (T > 100 °C) because of its elevated glass transition temperature [221]. As the working temperature is above 100 °C, it cannot be wetted with liquid water, and therefore phosphoric acid is used [222].

Summarizing this subsection, it can be stated that composite membranes created from metal-organic frameworks and polymers exhibit outstanding performance and innovative design concepts, indicating significant potential for commercial applications.

3.2. Oxygen Reduction Reaction (ORR) Catalysts for Proton Exchange Membrane (PEM) Fuel Cells

On the basis of previous works of Li et al. [223] and Wang et al. [224] on CoO/N/C (CoO supported on nitrogen-doped carbon), Zhang et al. [225] synthesized nano-catalysts based on N-doped C-encapsulated metal cobalt nanoparticles with a core-shell structure (Co@N/C-Joule) through pyrolysis of ZIF-67 in an argon atmosphere. An N-doped carbonaceous material sheet is applied to the surface of Co nanoparticles, creating a core-shell configuration. The slender carbon sheet protects the cobalt nanoparticles, thereby enhancing catalytic activity. By comparing samples synthesized via conventional furnace pyrolysis and those synthesized using CTS in either Ar or Air atmosphere, it was shown that the latter demonstrated a reduction in pyrolysis time by about four orders of magnitude and a decrease in carbon shell thickness from 3.3 nm to 1.3 nm, along with an improvement in electrochemical performance. The enhanced catalytic efficiency is ascribed to the nanoparticle size of the catalysts and the tiny nitrogen-doped carbon shell.

3.3. Oxygen Reduction Reaction (ORR) Catalysts for Microbial Fuel Cells

Catalysts in MFCs can be categorized as metal-free catalysts, noble-metal catalysts, non-noble metal catalysts (comprising metal-organic frameworks (MOFs) and covalent organic frameworks (COFs)), single atom catalysts, and biocatalysts (Figure 9) [226].

Jiang et al. [211] established a mesoporous silica (mSiO₂)-assisted method for synthesizing highly distributed CuCo nanoalloys integrated into nitrogen-doped carbon Cu/Co-NC@mS catalyst, employing the zeolitic imidazolate framework (ZIF) as a source of carbon and nitrogen. The results indicate that protecting by mSiO₂ allows for preventing the aggregation of CuCo nanoalloys, thereby facilitating the formation of highly dispersed CuCo nanoparticles. The catalyst obtained by pyrolyzing at 800 °C (Cu/Co-NC@mS-800) demonstrates superior ORR performance compared to other Cu/Co-NC@mS-T catalysts, exhibiting a maximum power density of 1000 mW m⁻² when utilized as an air electrode in MFCs. The increased ORR performance of Cu/Co-NC@mS-800 was credited by the authors to several factors: (a) the presence of highly dispersed CuCo nanoalloy particles and an increased mesoporous surface area due to mSiO₂ protection; (b) the formation of a concave regular dodecahedron mesoporous structure supported by a thin graphitic carbon layer; (c) the synergetic effect observed between CuCo nanoalloys.

Based on density functional theory (DFT) calculations indicating that the interaction between Cu-BTC and MoS₂ markedly decreases charge transfer resistance and improves the oxygen reduction reaction (ORR), MoS₂ flower-like nanosheets were effectively incorporated onto a copper MOF (Cu-BTC) through a straightforward hydrothermal synthesis method, resulting in the formation of a Cu-BTC@MoS₂ composite material that was utilized as a cathode catalyst in microbial fuel cells (MFCs). The Cu-BTC@MoS₂ demonstrates a high limiting current density and exchange current density in the ORR, adheres to a four-electron transfer pathway, and exhibits significantly greater methanol tolerance compared to commercial Pt/C catalysts. The Cu-BTC@MoS₂-4 attained a peak power density of 241.20 mW m⁻². This performance is 9.89 times greater than that of pristine carbon cloth (CC), 3.58 times that of Cu-BTC, and 2.49 times that of MoS₂, while sustaining 328 mV during the entire operating time. The catalytically active material achieved a chemical oxygen demand (COD) removal of 94.89%, demonstrating considerable prospects for wastewater treatment applications [227]. Zeolitic imidazolate framework-67 (ZIF-67)-coated MoS₂ has been used as a low-price ORR catalyst in the cathode of microbial fuel cells exhibiting exceptional stability as well as electrocatalytic activity, mainly due to its high specific surface area of up to 1627.4 m²/g [228].

In

Table 1. Indicative performance metrics of representative MOF-based materials and derivatives for rechargeable batteries and fuel cells.

MOFs	Derivatives/Devices	Discharge Capacity [mAh g−1 @ mA g−1 or @ C]	Cycle Performance [mAh g−1 @ Cycles]	Ref.
Ni/Co-MOF	NiFe2O@NiCo-LDH	891.2 @ 500	636.9 @ 100	[229]
Ni-Co-BTC MOF	NixCo3−xO4	1619.2 @ 1000	832 @ 673	[230]
Zn-Co PBA	Co3O4/NiO-C	864 @ 1000	776 @ 1000	[231]
ZIF-67, red P	CoxP-NC-800	2450 @ 1000	550 @ 100	[232]
MOF-74	MnCo-MOF	11,150 @ 200	1000 @ 44	[133]
ZIF-67	NiCo2O4/NiO	1535 @ 200	1492 @ 100	[233]
[Mn(phen)H2O] [V2O6]	MnV2O4/C	400 @ 100	377 @ 1000	[234]
MIL-125(Ti)	LiNi0.5Co0.2Mn0.3O2	186.7 @ 1000	145.3 @ 100	[235]
ZIF-8	S@S-NOHPC	927.7 @ 200	367 @ 200	[236]
CoPC	CoPC@GO	938 @ 1 C	919 @ 250	[237]
ZIF-8	Co-NC@AZO/S	1206.2 @ 200	950.03 @ 500	[238]
ZIF-8@ZIF-67	ZnSe@CoSe@CN	2739 @ 100	1500 @ 100	[239]
Zn-Co PBA	ZnO/Co3ZnC	1163.9 @ 200	1162.4 @ 300	[240]
Mn-MOF	MnO@rGO	1412.2 @ 2000	920 @ 160	[241]
Ni-Co NTA	NiCo2O4	3158 @ 100	1310 @ 100	[242]
ZIF-8	S@CuIr/NC	1288 @ 0.2 C	689 @ 500	[243]
ZIF-8	CNT-NC@GC/S	1498 @ 0.1 C	743 @ 500	[244]
UiOSOL	LiFePO4/Li metal	161 @ 0.1 C	155 @ 600	[245]
MOF-801	NCM811/Li metal	179 @ 0.5 C	113 @ 400	[246]
[NH2 (CH3)2] [FeIII FeII(HCOO)6]	Fe3O4@C	1714 @ 100	1041 @ 50	[247]
Co-BDC nanoplates	Co3O4 nanosheets	961 @ 1000	775 @ 200	[248]
Fe-Ni-BDC MOF	NixFe3−xO4 nanotubes	1466 @ 250	1184 @ 200	[249]
Fe-NTA	V-FeP	1228.3 @ 100	590.7 @1000	[250]
CAU-1-NH2	CAU-1-NH2-PMMA	1480 @ 200	450 @ 66	[251]

, indicative performance metrics of representative MOF-based materials for rechargeable batteries and fuel cells are tabulated.

4. Limitations and Outlook

Traditional MOFs exhibit significant limitations, notably in terms of conductivity and structural integrity, which restrict their utilization in electrochemical applications. Specific challenges associated with MOFs must be addressed to facilitate their wider commercialization. The limited electrochemical conductivity significantly constrains their potential applications, leading to the design of various nanocomposites with diverse structural forms across different dimensionalities. A viable alternative is to integrate MOFs with carbon materials, such as carbon nanotubes with superior conductivity, or to create binary metal oxides.

The functionalization of MOF variants remains limited; nonetheless, substantial potential exists to enhance their electrochemical properties. This can be accomplished by enclosing functional groups and various species within their frameworks, with the objective of developing high-performance energy storage devices through the identification of suitable MOF derivatives for both anodes and cathodes in electrochemical devices. MOF derivatives have emerged as effective materials to overcome these limitations and investigate applications across diverse electrochemical domains. Those substances perform as powerful templates, allowing for accurate control over the shape, chemical manipulation, heteroatom doping, increased surface area, and structural alterations. Significant potential exists for future applications, warranting increased interest from the academic community.

Future exploration in metal-organic frameworks (MOFs) may shift the focus from conventional applications like storage and separation to more critical concerns, including the production of clean energy. Significant advancements have been achieved in enhancing the performance of MOF-based components to meet industrial and commercial standards. Further progress necessitates that MOFs exceed the benchmarks established by conventional materials.

We do not yet have a clear grasp of the nature of conductivity in MOFs. Understanding how low-energy charge transport channels may be established in MOFs at the atomistic level will significantly speed up the creation of exceptional conductive MOFs. Computational quantum mechanical techniques can generate detailed information on the electronic structure in this situation with enough precision to shed light on the conductive behavior of MOFs. Using machine learning to speed up the screening and discovery of novel conductive materials is a new alternative design technique.

The integration of MOFs in energy production devices stands for a growing frontier, leading to bright outcomes. However, there are significant obstacles that require attention, such as the Ion Conduction Mechanism, the mechanistic investigation of which remains in the early stages, requiring comprehensive theoretical and fundamental analysis.

Furthermore, MOFs exhibit adjustable characteristics, including pore dimensions, structural configurations, crystal-size, and crystal-structure [252]. A thorough examination of the systematic relationships between these changes and their corresponding electrochemical characteristics is necessary. In this respect, the four-dimensional analysis system of “structure-performance-mechanism-application” [253,254,255,256,257,258,259,260] can be applied to improve the quality and efficiency of the developed MOF-based composite materials. An excellent overview on the application of this analysis system on lithium-ion batteries is given by Ji et al. [261].

Highlighting the use of inexpensive raw materials and implementing economically viable synthesis methods can improve the commercial feasibility of MOF-sourced solid electrolytes [28].

Yuan et al. [32] highlighted the absence of systematic research about the correlation between the synthesis process and the composition and structure of MOFs and their associated materials, complicating the understanding of the physicochemical alterations that transpire during synthesis. Using high-temperature in situ methodologies may be a viable pathway to examine this issue.

The materials derived from MOFs exhibit significant potential and various benefits for use in energy production and storage systems. Despite the numerous complex challenges that are difficult to address in the short term, there is a strong conviction that a promising future lies ahead for these innovative materials based on MOFs in real-world applications.

5. Summary

This review concludes with recent advancements in MOF-based materials for rechargeable batteries and fuel cells. This study emphasizes the impact of various synthesis strategies on the morphologies and sizes of these materials, demonstrating the significant potential of metal-organic frameworks (MOFs) to improve electrochemical devices for energy storage, such as rechargeable batteries, and energy production, including fuel cells. The relationship between electrochemical performance and the morphology of electrode active materials is emphasized. The primary challenges and perspectives concerning materials derived from MOFs for practical and sustainable electrochemical energy conversion applications are succinctly discussed.

Metal-organic frameworks (MOFs) demonstrate significant versatility, enabling systematic modifications designed to address diverse emerging requirements. In this context, metal-organic frameworks (MOFs) can significantly contribute to the hydrogen economy through their applications in electrocatalysis for hydrogen production and oxygen reduction, proton exchange membranes (PEMs), and energy storage, including rechargeable batteries and supercapacitors.

MOFs are compelling alternatives for fuel cell technologies due to their adaptable structures, diverse chemistry, highly uniform pores, adjustable pore morphologies, and high surface areas (up to 5.0 × 10³ m² g⁻¹). Additionally, the rational tuning of MOF properties has been facilitated by the meticulous design and functionalization of MOFs, which has been made possible by the use of multifunctional bridging ligands and post-synthetic modifications. The potential for the application of MOFs in fuel cells is indicated by the extensive variety of possible combinations of metal centers and organic linkers. The potential for a systematic investigation into the impact of structural parameters and chemical compositions on the performance of MOF-based electrochemical devices and the ability to manipulate pore structures in conjunction with surface engineering could provide significant fundamental insights.

A substantial body of research on MOFs and their derivatives has confirmed the reliability of their structures and chemical characteristics for contemporary electrochemical applications at the laboratory scale. However, further developments are necessary, as several problems with MOF-based electrochemical devices are presently being faced and require immediate attention for a successful upscaling of the use of MOFs in electrochemical energy production and storage devices.