In addition to their conventional uses, metal-organic frameworks (MOFs) have recently emerged as an interesting class of functional materials and precursors of inorganic materials for electrochemical energy storage and conversion technologies. This class of MOF-related materials can be broadly categorized into two groups: pristine MOF-based materials and MOF-derived functional materials. Although the diversity in composition and structure leads to diverse and tunable functionalities of MOF-based materials, it appears that much more effort in this emerging field is devoted to synthesizing MOF-derived materials for electrochemical applications. This is in view of two main drawbacks of MOF-based materials: the low conductivity nature and the stability issue. On the contrary, MOF-derived synthesis strategies have substantial advantages in controlling the composition and structure of MOF-derived materials. From this perspective, we review some emerging applications of both groups of MOF-related materials as electrode materials for rechargeable batteries and electrochemical capacitors, efficient electrocatalysts, and even electrolytes for electrochemical devices. By highlighting the advantages and challenges of each class of materials for different applications, we hope to shed some light on the future development of this highly exciting area.

MOF-based materials with different functionalities by tuning the constituent components: (left to right) electrochemical charge storage, electrocatalytic generation of fuels, and ionic conductivity. MOF-derived materials with different compositions, structures, and functionalities: (left to right) porous carbon with electric double-layer capacitance, hollow structure for charge storage, and carbon-supported composite for electrocatalysis. These MOF-related functional materials enable the storage and utilization of electricity from renewable energy sources.

Development of MOF-related materials for electrochemical energy storage and conversion has been a rapidly expanding research area in the past decade. Several excellent reviews have summarized recent advances in this field mostly focusing on specific aspects, such as MOF-related materials for specific applications (for example, photocatalysis/electrocatalysis and energy storage devices) or functional/nanostructured materials derived from MOFs ( 8 – 10 , 13 – 19 ). From the perspective of materials science, all these aspects are interrelated and essentially deal with the creation and modulation of electrochemical properties for energy-related applications. Therefore, an overview of this exciting research field is highly desirable. In this perspective, we aim to provide an overview for this highly interdisciplinary area and discuss the significant breakthroughs that MOF-related materials have brought to the field of electrochemical energy storage and conversion ( Fig. 1 ). Some coordination polymers and open-framework materials that might not be strictly defined as MOFs are also included for discussion. In particular, we will discuss how different functionalities are realized from the perspective of materials design and synthesis. On the basis of their chemical nature, MOF-related materials can be categorized into two groups: MOF-based materials refer to pristine/modified MOFs or composites that contain MOF moieties, and MOF-derived materials include various inorganic functional materials that are synthesized using MOFs as precursors and/or templates. These MOF-related functional materials offer unprecedented opportunities to both existing and emerging energy-related technologies. By reviewing recent advances, we hope to provide some future directions for the development of MOF-related functional materials, especially toward renewable energy–related applications.

As a relatively young but quickly growing family of porous materials, metal-organic frameworks (MOFs) have generated a tremendous amount of interest from researchers in widespread areas ( 4 , 5 ). With different metal-containing nodes, organic ligands, and connectivity, more than 20,000 different MOFs have been reported by the year 2013 and the number continues to grow ( 4 ). The composition and pore structure of MOFs can be modulated by tuning the precursors and synthetic conditions or by postsynthetic modifications. In addition, MOFs can be synthesized as nanoparticles and can also form nanocomposites with additional active components. The diversity of composition and structure leads to diverse and tunable functionalities of MOFs. The remarkably high porosity and surface area of MOFs are especially suitable for applications involving storage and interaction with guest species (for example, gas storage/separation and catalysis) ( 4 , 6 ). These features also trigger extensive research interests to explore the adaptation of MOF-related materials for electrochemical energy storage and conversion ( 7 – 10 ). Similar to conventional inorganic materials, MOFs containing redox-active metal centers, typically first-row transition metals (Fe, Co, Ni, Mn, etc.), are of particular interest for delivering electrochemical activity. Compared with both conventional inorganic and polymeric functional materials, MOFs might inherit their advantages (for example, electrochemically active metal centers and organic functional groups). Some additional benefits are also apparent, such as fully accessible organic molecule–coordinated metal sites and easily tunable pore structures. By tuning the metal and organic constituent components and/or constructing composites, MOFs have been successfully demonstrated as electrode materials for rechargeable batteries and electrochemical capacitors, efficient electrocatalysts for fuel production and utilization, and even electrolytes for electrochemical devices ( 9 ). Alternatively, to overcome the limitations of insufficient electronic conductivity and chemical stability of most pristine MOFs, converting MOFs into metal compounds, carbonaceous materials, or their composites has also been extensively explored ( 11 – 15 ). These MOF-derived functional materials usually exhibit remarkable advantages originating from their microstructures/nanostructures, showing great potential for energy-related technologies.

Renewable energy sources, such as solar and wind power, are taking up a growing portion of total energy consumption of human society. Owing to the intermittent and fluctuating power output of these energy sources, electrochemical energy storage and conversion technologies, such as rechargeable batteries, electrochemical capacitors, electrolyzers, and fuel cells, are playing key roles toward efficient and sustainable energy utilization ( 1 , 2 ). For example, electricity generated from solar and wind power can be efficiently stored in and released from rechargeable batteries and electrochemical capacitors, or converted into fuels by electrolyzers and further regenerated by fuel cells. Despite their different working principles, these electrochemical devices include the following key functional components ( 3 ): two electrodes (cathode and anode), where the major electrochemical processes take place, such as charge storage in batteries/capacitors and electrocatalytic reactions in electrolyzers/fuel cells, and an electrolyte that allows the transport of ions and blocks electronic conduction to complete the electric circuit. In principle, the physical (for example, electronic and ionic conductivity) and electrochemical (for example, redox and catalytic activity) properties of functional materials used in these components govern the performance of devices. Therefore, seeking better materials has been a primary quest for the development of future electrochemical energy–related technologies.

MOF-BASED MATERIALS

Electrochemical charge storage Rechargeable batteries and electrochemical capacitors are two primary types of electrochemical energy storage devices. Batteries, such as lithium-ion and sodium-ion batteries (LIBs and SIBs), rely on reversible shuttling of lithium/sodium ions between two electrodes, offering high energy density and moderate power density (20). In a typical LIB, Li+ ions de-intercalate from a cathode (for example, LiCoO 2 ), transport through an electrolyte, and intercalate into an anode (for example, graphite) when charging the cell, and this process reverses during discharging. In 2007, Férey and co-workers (21) reported the electrochemical lithium insertion in an iron-based MOF, MIL-53(Fe), with a limited specific capacity of 75 mAh g−1 based on the FeIII/FeII redox couple. Similar lithium storage properties have been recently reported in MIL-101(Fe) with limited reversibility (22). In some cases, high reversible capacity can be obtained via possible conversion and/or alloying reactions at low potential, whereas detailed investigations would be necessary to reveal the mechanism (23, 24). A feasible solution to increase the reversible capacity of MOFs is to introduce redox-active ligands, providing both cationic (metal centers) and anionic (ligands) redox activity. This was first exemplified by lithium storage in a two-dimensional (2D) MOF containing a redox-active bridging ligand (Fig. 2A) (25). The charge-discharge voltage profiles of the resulting MOF exhibit two distinct stages (Fig. 2B): a high-potential plateau attributed to the CuII/CuI redox couple and a low-potential plateau originated from anthraquinone groups in the ligands. With a total transfer of three electrons per formula unit, a high specific capacity of 147 mAh g−1 was reported, despite some gradual capacity fading (Fig. 2C). A similar strategy has been adopted to fabricate MOF-based cathode materials for SIBs (26). Fig. 2 MOF-related materials for charge storage. (A to C) A redox-active MOF Cu(2,7-anthraquinonedicarboxylate) [Cu(2,7-AQDC)] for lithium batteries: (A) structural schematic, (B) charge-discharge profiles, and (C) cycling performance [(A) to (C), adapted with permission from Zhang et al. (25)]. (D) Schematic of electrochemical Na storage in Prussian blue crystal [(D), adapted with permission from You et al. (28)]. (E and F) Electrochemical capacitors fabricated with nanocrystals of MOFs (nMOFs): (E) structure of nMOF electrochemical capacitor and (F) comparison of energy and power densities for electrochemical capacitors made from nMOF-867 and activated carbon [(E) and (F), adapted with permission from Choi et al. (34)]. (G to I) Electronic conductive MOF for electrochemical capacitors: structural schematics of (G) conductive MOF Ni 3 (2,3,6,7,10,11-hexaiminotriphenylene) 2 [Ni 3 (HITP) 2 ] and (H) electrolyte components in Ni 3 (HITP) 2 ; (I) cyclic voltammetry at a scan rate of 10 mV s−1 at different cell voltages [(G) to (I), adapted with permission from Sheberla et al. (36)]. (J to L) MOFs as sulfur host for lithium-sulfur (Li-S) batteries: (J) schematic showing the interaction between polysulfides and MOF scaffold, (K) comparison of binding energy of lithium polysulfides to Ni-MOF or Co-MOF, and (L) charge-discharge profiles of MOF/S composite cathodes [(J) to (L), adapted with permission from Zheng et al. (38)]. Prussian blue and its analogues, with a general formula of A x M[M′(CN) 6 ] (A, mobile cations; M and M′, transition metal cations), appear as very promising open-framework materials for electrochemical ion insertion. Unlike the limited ion insertion in typical MOFs, two or even more alkaline cations, such as Li+ and Na+, can be accommodated per formula unit under optimized conditions (Fig. 2D), leading to high specific capacity (for example, about 170 mAh g−1 based on Na 2 Fe[Fe(CN) 6 ]) with fast kinetics (27–31). Of particular interest is the reversible uptake of large cations, such as K+ and Rb+, or even multivalent cations (32), which is difficult for conventional inorganic compounds. This exceptional capability to host large guest molecules in open frameworks is further demonstrated by the oxidative insertion of anions in a Fe-based MOF, enabling the development of dual-ion batteries (33). Electrochemical capacitors, also known as supercapacitors, store charge by either electrical double-layer capacitance or pseudocapacitance, and hence are able to deliver high power density and long life span (20). Double-layer capacitance refers to electrical charge storage by adsorption of ions at the interface between an electrode (typically porous carbon) and a liquid electrolyte without redox process. Pseudocapacitance, on the other hand, involves ultrafast redox reactions at the (near-)surface region or even in the bulk of an electrode (typically transition metal oxides). High surface area and redox-active metal centers of MOFs can potentially offer high double-layer capacitance and pseudocapacitance, respectively. To fabricate electrochemical capacitors, a series of 23 different MOFs in nanocrystalline form has been recently explored by Choi et al. (34) (Fig. 2E). As expected, the electrochemical performance (capacitance, cycle life, charge-discharge profiles, etc.) varies notably among different MOFs, revealing their composition/structure-dependent properties. The best-performing MOF delivers energy and power densities notably higher than those of the benchmark activated carbon (Fig. 2F). The insulating nature of most reported MOFs appears to be a major obstacle for their electrochemical applications, especially for electrochemical capacitors with high power output. Adding a large amount of conductive additives or using thin-film electrodes could be adopted as compromised solutions (25, 34). Meanwhile, developing MOFs with high electronic conductivity would address the issue on a fundamental basis (35). For example, Sheberla and co-workers (36) recently demonstrated that a nickel-based MOF, Ni 3 (HITP) 2 , with a high electronic conductivity over 5000 S m−1 and sufficient pore size for accommodating electrolyte species, could be used as an active material for electrochemical capacitors (Fig. 2, G and H). With a voltage window of 1.0 V, a symmetric electrochemical capacitor based on Ni 3 (HITP) 2 exhibits a nearly ideal double-layer capacitive behavior (Fig. 2I). With an extended voltage window, a Faradaic process presumably attributed to quasi-reversible oxidation of Ni 3 (HITP) 2 is observed, implying the possibility of introducing pseudocapacitance in MOF-based electrochemical capacitors. MOF-based materials could also function as scaffolds to host active components, for example, O 2 /Li 2 O 2 in lithium-air batteries and S/Li 2 S in Li-S batteries (37, 38). Unlike porous carbon materials with relatively inert surfaces, the presence of open metal sites (OMSs) in many MOFs would interact with active species in electrodes and improve the electrochemical performance. Zheng et al. (38) reported a stable sulfur cathode based on a Ni-MOF/S composite. The strong binding between polysulfides (intermediates of reduced sulfur) and NiII centers of the Ni-MOF host plays an important role in preventing the loss of active materials from the electrode (Fig. 2J). This is further verified by a Co-MOF with identical structure but weaker interaction with polysulfides (Fig. 2K), which produces Co-MOF/S composite with lower specific capacity (Fig. 2L). In addition, the interaction between MOFs and sulfur species can be tuned by altering the OMSs or the pore structures (39, 40). Thus, better sulfur hosts could be developed, combining theoretical prediction and experimental verification.

Electrocatalysis for energy conversion Electrocatalysts are the central component of electrochemical energy conversion systems, which efficiently catalyze reactions to convert electricity into fuels for storage/transport or, in the opposite way, to regenerate electricity for on-site utilization (2). Specifically, electrochemical water splitting using electrolyzers produces hydrogen fuel by hydrogen evolution reaction (HER) coupled with oxygen evolution reaction (OER). Hydrocarbon fuels or other useful chemicals can be produced from carbon dioxide (CO 2 ) reduction, aiming for a carbon-neutral economy. In the meantime, electrochemical oxidation of fuels (for example, hydrogen for proton exchange membrane fuel cells) and oxygen reduction reaction (ORR) occur in fuel cells to generate electricity. Electrocatalysts are loaded on supports to form electrodes, allowing access of reactants and release of products in liquid and/or gas phases. An ideal electrocatalyst should exhibit features such as low overpotential during operation, high selectivity for desirable reactions, high durability, and low cost. Noble metals are highly efficient for many above-mentioned reactions (for example, platinum for HER and ORR). However, for large-scale deployment of these energy conversion technologies, low-cost noble metal–free catalysts are highly demanded (41). MOFs can be considered as polymerized forms of molecular catalysts, offering highly exposed coordinated metal centers as active sites. For example, transition metal porphyrins, a class of efficient molecular catalysts, can be transformed to heterogeneous catalysts by incorporating into MOFs as linkers for ORR (Fig. 3A) (42) or CO 2 reduction (43). Metal nodes in MOFs with OMSs could also potentially serve as active sites for various electrocatalytic reactions (44–46). However, regardless of targeted reactions, adaptation of MOF-based materials as efficient electrocatalysts has been generally hampered by low electronic conductivity, limited accessibility of active sites, and insufficient chemical stability. Fig. 3 MOF-related materials for electrocatalysis. (A) Schematic of a Zr-based MOF with FeIII porphyrin linkers as a heterogeneous catalyst for ORR [(A), adapted with permission from Usov et al. (42)]. (B) Polarization curves of Ni 3 (HITP) 2 under N 2 and O 2 atmosphere in 0.1 M KOH aqueous electrolyte at a scan rate of 5 mV s−1 and a rotation rate of 2000 rpm [(B), adapted with permission from Miner et al. (45)]. (C to E) UMOFNs as an electrocatalyst for OER: (C) crystal structure and (D) transmission electron microscopy (TEM) image of NiCo-UMOFNs and (E) polarization curves of various OER catalysts in O 2 -saturated 1 M KOH solution at a scan rate of 5 mV s−1 [(C) to (E), adapted with permission from Zhao et al. (46)]. (F and G) A Co-based MOF, MAF-X27-OH, for OER: (F) structure of MAF-X27-OH and (G) polarization curves of various Co-based catalysts at pH = 14 [three MAF-X27-OH(Cu) samples refer to the MOF catalyst directly grown on the Cu substrate] [(F) and (G), adapted with permission from Lu et al. (53)]. (H and I) A Ni-S electrocatalyst deposited on the fluorine-doped tin oxide (FTO) substrate with an array of NU-1000 rods for HER: (H) schematic of the creation of NU-1000_Ni-S hybrid system and (I) polarization curves of various catalysts in 0.1 M HCl aqueous electrolyte [(H) and (I), adapted with permission from Hod et al. (54)]. GCE, glassy carbon electrodes; RHE, reversible hydrogen electrode. The emerging family of conductive MOFs also brings opportunities to the development of efficient electrocatalysts. The conductive Ni 3 (HITP) 2 MOF (36) has been demonstrated as an active ORR electrocatalyst (Fig. 3B) (45). The nitrogen-coordinated Ni centers are structurally analogous to M-N x (M = Fe, Co, Ni, etc.) units that have been actively explored as noble metal–free ORR catalysts (47). Despite the competitive ORR onset potential, the predominant two-electron reduction, rather than the more desirable four-electron reduction process observed for the Ni 3 (HITP) 2 catalyst, suggests the need to develop more effective active centers. Meanwhile, thin films of cobalt-dithiolene–based MOFs with 1D or 2D structures have been explored as electrocatalysts for HER (44, 48), where the combination of active cobalt-dithiolene sites, high electronic conductivity (49), and robust attachment to the electrode surface would contribute to the high activity. To increase the amount of easily accessible active sites on the surface, the construction of 2D nanostructures has been demonstrated for MOF-based catalysts by Zhao and co-workers (46). The small thickness of 3.1 nm of ultrathin Ni-Co MOF nanosheets (NiCo-UMOFNs) corresponds to only four metal coordination layers (Fig. 3, C and D). With exposure of a large amount of OMSs on both surfaces and the coupling effect between Co and Ni, NiCo-UMOFNs exhibit an onset potential for OER notably lower than that of bulky and mono-metal counterparts (Fig. 3E). In another attempt to develop advanced OER electrocatalysts, paddle wheel–type Co-based clusters are bound to a highly stable Fe-based MOF by a delicately designed postsynthetic approach, delivering both high activity and good stability (50). Alternatively, Manna et al. (51) immobilized a mononuclear CoII complex in the cavity of a MOF, producing an efficient OER electrocatalyst with a “ship-in-a-bottle” hybrid structure. The stability issues of MOF-based electrocatalysts during operation in highly acidic or basic solutions should not be overlooked (52). In addition, the true structure of the catalyst surfaces generally remains uncertain, raising serious difficulties for mechanistic studies. As an example, a recent work (53) on a Co-based MOF (MAF-X27-Cl) as a candidate for OER catalysts reveals an in situ replacement of Cl− ligands with OH− in an alkaline electrolyte, producing a new MOF of MAF-X27-OH (Fig. 3F). The OER activity of MAF-X27-OH has been dramatically boosted by OH− coordinated to the Co-based OMSs, with an electrocatalytic activity notably higher than that of Co(OH) 2 and Co 3 O 4 (Fig. 3G). In addition, growing MOF catalysts directly on a conductive substrate can further improve the performance, highlighting the importance of electronic conduction. Other than directly functioning as electrocatalysts, some MOFs can work as supporting components in electrocatalytic systems. As reported by Hod and co-workers (54), a Ni-S electrocatalyst for HER is electrodeposited on the conductive glass substrate (FTO) with an array of NU-1000 (a Zr-based MOF) rods. Although a flat layer of Ni-S is deposited on the substrate (Fig. 3H), rather than on the surface of NU-1000 rods, notably improved electrocatalytic activity has been observed in an acidic electrolyte compared with Ni-S deposited on a bare substrate (Fig. 3I). Considering the inactive nature of NU-1000 for HER, the authors reasoned that the high activity of the NU-1000/Ni-S hybrid system can be attributed to the promoted local proton delivery and/or transport (54). This work highlights the roles of MOFs as ionic conductors, which will be further discussed in detail in the following section.