Emergent electrochemical functions and future opportunities of hierarchically constructed metal–organic frameworks and covalent organic frameworks

Yosuke Hara ab and Ken Sakaushi *b
aDepartment of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan
bCenter for Green Research on Energy and Environmental Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. E-mail: SAKAUSHI.ken@nims.go.jp

Received 29th December 2020 , Accepted 1st March 2021

First published on 1st March 2021


Designing spatial and architectural features across from the molecular to bulk scale is one of the most important topics in materials science which has received a lot of attention in recent years. Looking back to the past research, findings on the influences of spatial features denoted as porous structures on the applications related to mass transport phenomena have been widely studied in traditional inorganic materials, such as ceramics over the past two decades. However, due to the difficulties in precise control of the porous structures at the molecular level in this class of materials, the mechanistic understanding of the effects of spatial and architectural features across from the molecular level to meso-/macroscopic scale is still lacking, especially in electrochemical reactions. Further understanding of fundamental electrochemical functions in well-defined architectures is indispensable for the further advancement of key next-generation energy devices. Furthermore, creating periodic porosity in reticular structures is starting to be recognized as an emerging approach to control the electronic structure of materials. In this review, we focus on the investigations on preparing well-defined molecular-level crystalline porous materials known as metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) into hierarchically constructed architectures from molecular structures lower than the reticular frameworks to meso-/macroscopic scale structures. By connecting well-defined nanosized porous structures in MOFs/COFs and additional length-scale space or shapes, emergent electrochemical functions towards emerging devices, such as beyond Li-ion batteries including all-solid-state rechargeable batteries, are expected to be obtained. By summarizing recent advancements in synthetic strategies of hierarchically constructed MOF/COF based materials and fundamental investigation of their structural effect in a wide spectrum of electrochemical applications, we highlight the importance and future direction of this developing field of hierarchically constructed MOFs/COFs, while emphasizing the required chemical stability of the MOFs/COFs which meet the use in the game-changing electrochemical devices.

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Yosuke Hara

Yosuke Hara is a PhD student at the Graduate School of Science, Division of Chemistry, Kyoto University, Japan. Since 2019, he has been a Fellow of the Japan Society for the Promotion of Science (Research Fellowship for Young Scientist DC1). He received his B.Sc. in chemistry in 2017 from Kyoto University, and M.Sc. degree in 2019 from the same University under the supervision of Prof. Kazuki Nakanishi and Prof. Hiroshi Kitagawa. Since 2018, he has been working at the National Institute for Materials Science as a visiting student in Dr Ken Sakaushi's group on the key fundamental issues of electrochemical energy storage reactions by using a wide spectrum of model porous materials. Now, he also focuses on the synthesis of hierarchical metal–organic frameworks under the supervision of Prof. Shuhei Furukawa.

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Ken Sakaushi

Ken Sakaushi is a Senior Researcher at the National Institute for Materials Science (NIMS), Tsukuba, Japan. He studied physics (B. Sc., 2008) and materials chemistry (M. Sc., 2010) at Keio University, and electrochemistry at the National Institute of Advanced Industrial Science and Technology. As a Fellow of the Deutsche Akademische Austauschdienst (DAAD), he completed his Ph. D. in 2013 at the Leibniz Institute for Solid State and Materials Research, and TU Dresden. Then, he joined the Max Planck Institute of Colloids and Interfaces on a fellowship from the Max Planck Society. Since 2015, Ken has been a Tenured Member of NIMS. His research focuses on various fundamental aspects of electrode processes and emerging functional materials. Besides academic activities, he loves playing piano and football. Ken is recognized by awards including the 2016 ISE Travel Award for Young Electrochemists, the 2019 PCCP Prize, and the 70th CSJ Award for Young Chemist. Further information: https://sites.google.com/site/sakaushiken/.

1. Introduction

Spatial and architectural design of materials from the molecular level to meso-/macroscopic scale has attracted material scientists.1–4 To implement fundamental properties of materials in practical applications, it is highly important to design structural features in multiple length scales with optimal properties depending on the target applications. Referring to the past research, the concept of the spatial and architectural design of materials across the multiple length scales has been proven in the field of traditional porous inorganic materials, such as ceramics.3,5 Their morphological difference including porous structures offers a variety of functionalities in the fields of catalysis, separation, energy storage and conversion, life science, and other industrial applications.3,5 In addition to the research studies on structuring the materials for practical applications, precise control of hierarchical constructions including meso-/macroscopic shapes of the materials can lead us to the investigation of the mechanistic understanding of the effect of meso-/macroscopic sized materials on specific functions. However, focusing on this class of traditional inorganic materials, precise control of the porous structures and additional functional groups at the molecular level has been a challenge, and resulted in the lack of the mechanistic understanding of the effects of spatial and architectural features across from the molecular level to meso-/macroscopic scale.

One important concept to control the porous structures at the molecular level is reticular chemistry6 known as the chemistry linking molecular building blocks through strong bonds to porous crystalline framework structures. Metal–organic frameworks (MOFs)7,8 and covalent organic frameworks (COFs)9–11 have been extensively synthesized through reticular chemistry. The high chemical designability and accessibility of external guests into internal pore structures have been facilitating their use for a variety of applications including gas storage or separation,12 catalysis,13 energy storage and conversion.14–17 Furthermore, MOFs/COFs have a unique feature of their fundamental properties: these materials allow us to tune their pore size and periodicity of pores, and incorporation of heteroatoms/metal coordinating moieties in a fine periodic way.18–20 These features clearly distinguish MOFs/COFs from traditional materials because this feature allows us to tune electronic structures,21 which is key to give rise to a wide spectrum of functions.22 Recently, research studies on MOFs and COFs have gradually shifted to optimize the hierarchical construction of MOFs and COFs for their applications.4,18,23 Besides, focusing on the reticular network, the multiple-pore skeletons24 and multiple functional groups25,26 also are recognized as hierarchical constructions.

Research studies focusing on this type of hierarchically constructed reticular material have been increasingly reported over the last decade. We consider that these series of research streams can be connected to the progress of the research field of hierarchically structured inorganic materials. In addition, it provides the opportunity to make up for the lack of mechanistic understanding of the effects of hierarchically constructed materials across from the molecular level, which aims to tune fundamental electronic properties, to meso-/macroscopic scale. In addition, molecular scale hierarchical constructions in the reticular network provide multiple functions with a fine periodic way. Furthermore, this type of hierarchically controlled material can control a variety of interactions with electronic properties of materials. We consider that the concept of hierarchically constructed reticular materials, from the additional functional structures lower than the reticular networks to the meso-/macroscopic constructions above the reticular networks, will open a new avenue to prepare modern functional materials with designer function (Fig. 1). In this review, we especially focus on the importance and future direction of the establishing synthetic strategy of hierarchically constructed materials, and the fundamental investigation of their structural effect in electrochemical applications toward the mechanistic understanding of the effects of spatial and architectural features across from the molecular level to meso-/macroscopic scale. In particular, we focus on the reticular-network-based hierarchically constructed MOFs/COFs. Furthermore, promising properties of MOFs/COFs, such as the ion-conducting functions toward solid-state rechargeable batteries as well as chemical stability for practical electrochemical devices, are discussed. We note here that the MOF/COF derived carbon materials and the hierarchically constructed materials as hybrids of MOFs/COFs and external components can be seen elsewhere.27–29

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Fig. 1 Schematics of the physical and electrochemical features of the hierarchically constructed materials across from the molecular level to meso-/macroscopic scale.

2. Effects of hierarchical construction in MOFs and COFs: a view from electrochemical functions

2.1 History of the hierarchically structured materials

Focusing on the meso-/macroscopic morphology of the inorganic materials from nano to micrometre scale, the morphology can be classified as 0D, 1D, 2D, and 3D depending on the geometrical dimensions.30 From the view from the hierarchically constructed materials, one important morphology is 3D porous structures.3,31–33 As a result of progress of the field of porous materials, pores on several length scales have been termed by IUPAC depending on their structural sizes: pores smaller than 2 nm are typically termed micropores, pores between 2 and 50 nm are termed mesopores, and pores larger than 50 nm are macropores. Especially in the field of energy storage, porous structures in conductive architectures affect the functions depending on their porous structural sizes: e.g. micro- (<2 nm)/mesoporous (2–50 nm) structures provide a high accessible surface area for surface electrochemical reactions,34–37,38,39 while meso-/macroporous structures contribute to facile mass transportations throughout the electrodes.40,41 From the view point of this definition, most of the previous reports on MOFs and COFs are regarded as microporous materials, and for some frameworks with large pore sizes are regarded as mesoporous materials.6

In the history of porous materials, the most studied compounds with control of multiple length scales are carbon materials42 and metal oxides.43 Considering the development of synthetic strategies to control hierarchical construction of the materials, the reports of hierarchical MOFs and COFs have been much less than those of carbon materials and metal oxides so far. On the other hand, very recently an increasing number of reports on hierarchical MOFs and COFs including their hierarchical composite materials have been reported. In this review, we first summarize the important findings which have been obtained with conventional carbon materials before discussing this state-of-the-art progress of MOFs and COFs.

Mass transport. One notable electrochemical application of porous carbons is the electric double-layer capacitance (EDLC) owing to the rich microporous structures of the activated carbon.44–46 Although quite interesting behaviours are observed,47,48 most of the EDLC applications emphasize the influence of mass diffusion in hierarchically porous structures at the electrode process.49 To enhance the capacitance of the activated carbons, many researchers have reported the synthesis of meso-/macroporous carbons, and their performance in supercapacitors. The role of the meso-/macroporous structures in these applications is the enhancement of the permeation of electrolyte through the microporous structures and the mobility of ions.50 In macro-/meso-/microporous activated carbon based electrodes, the ions including Li+ can be quickly transported into micropores through the macro-/mesoporous structures (Fig. 2A), and resulted in better electric double layer capacitor performance whether at a low-mid or high rate.50 Without macro-/mesoporous structures, only the outermost surface layer of the activated carbon can contribute to the capacitance.51,52 Most of the obtained meso-/macroporous structures in activated carbons can be divided into several categories: activated carbons obtained by a (1) template process, (2) template-free process, and (3) biomass-derived process.49,53,54 These synthetic strategies to obtain meso-/macroporous structures facilitate the use of porous materials as electrocatalysts to improve the activity per mass.55–62 One promising synthetic process which can be applied to MOFs and COFs is the template process.63–66 Several reports of synthesis of hierarchical MOFs and COFs by the template process have been reported. It is worth noting that materials prepared by the template process have a similar physical morphology. There are many reports focusing on the effect of the pore formation and pore size of the activated carbon on EDLC and other fundamental reactions such as ORR and OER as a catalyst support. Therefore, we can estimate how meso-/macroporous structures of MOFs and COFs affect the performance from the view point of mass diffusion according to the literature of the history of hierarchically structured activated carbons. In addition, this insight would be important to reveal the nanoscopic structural effect on performance.
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Fig. 2 Schematics of the highlighted features of the 3D porous structures. (A) Meso-/macroporous structures enhance the permeation of the electrolyte and the mobility of the Li+ ions through the microporous structures. (B) Meso-/macroscopic structures including lattice-like and fiber-like stacked macroporous network impact on the mechanical properties of the materials. (C) Macroporous network structures provide the suitable space for reversible formation/decomposition of the solid discharge product during the electrode process. In the Li–O2 battery, Li+ ions and O2 gas electrochemically react to form Li2O2 during the discharge process.
Mechanical properties. In addition to the contribution to mass diffusion efficiency, it is also important that meso- or macroporous structures influence their mechanical properties.67–69 The presence of meso-/macroporous structures in porous materials dictates the physical morphology of the skeletal structures of the porous materials. Thus, considering the meso-/macroporous structures also means to consider the architectural shape of the skeletons, which is strongly related to the mechanical properties of the materials. For example, the meso-/macropore skeletons of the materials are 3D interconnected, and the mechanical strength becomes higher compared to the materials with similar meso-/macroporosity in mesoscopic and monolithic materials.67 (Fig. 2B) In addition, by tuning the architectural structures, the 3D network can show recoverable properties even in the ceramic materials.68 Furthermore, fiber-like stacked macropore skeletons increase the flexibility of the self-standing materials (Fig. 2B), and many research studies on metal, metal oxide, carbons, etc. have been reported.69 This insight would be important not only for the further investigation of the mechanical relationship between the reticular MOF/COF network and their bulky parent phase but also for the preparation of the MOF/COF based flexible and stiff devices.
Reaction space for reversible electrochemical formation/decomposition of solid products. It is also noteworthy that macroporous structures contribute to provide suitable space for reversible formation/decomposition of solid products during the electrode process.70,71–74,77 (Fig. 2C) This space is especially important for non-intercalation-based electrochemical energy storage chemistries, such as Li–O2 or Li–S chemistries. These chemistries are relying on electrochemical-formation/-decomposition of solid products on the electrode surfaces, whose mechanism is far different from the intercalation mechanism. Although a variety of investigations have tried to understand the detailed process of the nonaqueous rechargeable Li–O2 electrochemistry,72,74–76 the microscopic mechanism of this chemistry is still under debate. Deposition and accumulation of the intermediates and products, such as Li2O2, obstruct the small porous structures including meso-/microporous structures, and resulted in blocking the pathway for O2 diffusion and Li+ migration. Furthermore, as shown by the very recent work by using well-defined and stable model bimodal porous electrodes,77 an optimized porous structure is indispensable to improve energy efficiency and cycle life of the non-aqueous Li–O2 electrode process, especially the systems containing redox-mediators. Therefore, multiscale structured MOFs and COFs have the opportunity to be applied to the product formation/decomposition-based electrode processes.

2.2 Hierarchically constructed MOFs and COFs towards advanced electrochemical applications

Electrically conductive MOFs and COFs have attracted huge interest for a variety of applications, including electrocatalysis,78,79,80 chemiresistive sensing,81,82 and energy conversion and storage systems.17,80 One of the advantages for these types of reticular materials is well-defined periodicity of the arrangements of inorganic or organic moieties of the frameworks which tune the pore size and electrochemical properties. From the view point of the origin of electroconductivity of these types of materials and design principles, many other reviews have been provided recently.78,80–82 In this review, we especially focus on hierarchically constructed MOFs and COFs, and their electrochemical applications. Although the development of the electrochemical applications of hierarchically constructed MOFs and COFs has occurred within the last ten years, several strategies are available, and will be discussed in this review.

One of the important unique features of MOFs and COFs, which is different from the conventional porous ceramics, is the crystal engineering based well-designability of reticular networks at the molecular level. By introducing additional structural features below and above reticular networks, emergent functional materials with a designer function are expected (Fig. 1). In addition to the defined framework crystalline networks, MOFs and COFs have inner spaces within the frameworks. By introducing additional functional structures in inner spaces, there are opportunities to introduce ion conductivities through inner spaces.26 In addition, the introduction of active sites into the framework structures with targeted architectural locations and combination provides tailorable properties with well-defined structures.25 Furthermore, by designing meso-/macroscopic structural features and constructions, morphology derived functions including mass diffusion control,64,83 molecular selectivity,84 and utility as model materials85,86 have been provided.

Hierarchically constructed 2D MOFs and COFs. We discuss 2D MOF and COF based materials. Among the conductive MOFs and COFs, 2D MOFs and COFs have noteworthy features while having the conductivity.22 Since the discovery of free-standing graphene in 2004, electronic properties of 2D materials have attracted interest.87–89 The remarkable point of 2D MOFs and COFs is their micro-/mesoporous 1D pore structures derived from the stacking sequence of the 2D layers85 (Fig. 3A and B). These features of 2D MOFs and COFs make them unique from the view point that the other crystalline inorganic compounds composed of 2D sheets like MXene, 2D transition metal dichalcogenides have non-microporous structures.90,91 In addition, by selecting the combination of units which construct the reticular network, MOFs and COFs have the opportunity to tune their functions.22
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Fig. 3 Hierarchically constructed reticular materials based on 2D framework structures. (A) Schematic of the conductive 2D MOF Cu3HHTT2 stacking lattice. The metal ion is Cu2+, and the organic linker is 2,3,7,8,12,13-hexahydroxytetraazanaphthotetraphene. (B) HRTEM image of a Cu3HHTT2 rod-like crystal imaged normal to the c direction. The inserted image is the Cryo-EM image of a Cu3HHTT2 plate-like crystal. (C) Conductive 2D MOF device of Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2 (Ni3(HITP)2). SEM image of a rod-like crystal of the Ni3(HITP)2 device with Ti/Pd contacts. (D) TEM image of the macroporous TpBpy type COF (denoted as macro-TpBpy). TpBpy was synthesized by reacting the organic linkers 1,3,5-triformylphloroglucinol (Tp) and 2,2′-bipyridine-5,5′-diamine (Bpy). OER performance of macro-TpBpy-Co. Macro-TpBpy-Co was post-synthesized by the coordination of cobalt acetate to the bipyridine amine linker. (E) OER polarization curves and (F) corresponding Tafel plots for macro-TpBpy, macroTpBpy-Co, TpBpy-Co, and a commercial RuO2 catalyst in an alkaline aqueous electrolyte (0.1 M KOH) using a typical three-electrode system. For comparison, the electrocatalytic activities of these catalysts were evaluated with the same mass loading of 0.25 mg cm−2. The catalysts were cast onto a rotating disk electrode (RDE). Then, macro-TpBpy-Co requires a much lower overpotential than macro-TpBpy and TpBpy-Co with the same anodic current density. In the corresponding Tafel slope, both macro-TpBpy-Co and TpBpy-Co exhibit a lower linear slope of 54 mV dec−1 and 58 mV dec−1, respectively. These values are much lower than that of macro-TpBpy (339 mV dec−1), and lower than that of RuO2 (79 mV dec−1), and indicating the favorable reaction kinetics in the OER process. (G) Scatter plot of jreal/site for macro-TpBpy-Co and TpBpy-Co, versus applied potential. A much higher jreal/site for macro-TpBpy-Co than that of TpBpy indicates an enhanced OER activity per active site due to the additional macroporous structures. (H) Schematic of the Li+ ion pathway through the glassy poly(ethylene oxide) moieties covalently reticulated in COF frameworks. The panels A and B are reprinted from ref. 85, copyright (2020), with permission from Nature Springer. The panel C is reprinted from the reference (https://pubs.acs.org/doi/abs/10.1021/acscentsci.9b01006),86 copyright (2019), with permission from American Chemical Society. Further permissions related to the material excerpted should be directed to the ACS. The panels D–G are reprinted from ref. 64, copyright (2019), with permission from American Chemical Society. The panel H is reprinted from ref. 26, copyright (2019), with permission from American Chemical Society.

Firstly, we discuss the 2D MOFs and COFs with 1D or 2D meso-/macroscopic structures, which features material shapes in 2D reticular materials. Considering the construction of 2D MOFs and COFs, because of the anisotropic three-dimensional stacking structures, the crystal shape tends to form rod- or plate shapes depending on the synthetic conditions.92 If network growth tendency is predominant in the 2D directions, formation of mesoscopic 2D COF/MOF plate-like crystals can be obtained. This type of 2D plate mesoscopic morphology of COF and MOFs is important to investigate the fundamental electrochemical properties. Recently, the study of anisotropic electrical transport in 2D MOFs was reported by using this type of 2D plate-like single crystal of MOFs.85,86 In addition, focusing on the meso-/macroscopic 2D thin sheet-like materials, 2D COF/MOF thin sheets can be obtained by a variety of approaches.93,94 The 2D thin film has attracted huge interest due to its large surface area and high surface-to-volume atom ratio. Most of the reported morphologies of the 2D MOFs and COFs are the 2D thin film, the thin film like morphology facilitates the fast ion diffusion of the guest molecule into the pore structures compared to the bulky morphology.95 The synthetic approaches can be divided into two categories: (1) 2D COF/MOF sheets grown on substrate surfaces, and (2) free-standing 2D COF/MOF sheets. Free-standing 2D COF/MOF sheets can also be transcribed on the substrate. This type of morphology is important for not only the practical electrochemical applications,38,96,97 but also important for the fundamental understanding of the effect of the structures at molecular level into the electrochemistry.98,99

Furthermore, anisotropic crystalline growth along the axial axis for rod like morphology of the 2D COFs and MOFs is also an important feature considering the mesoscopic morphology of 2D COFs and MOFs. During mass diffusion of the guest molecules through the materials, guest molecules should diffuse through 1D pores.100 Then, the diffusion rate of the molecule should vary not only with the size of the pore, but also with the distortion of the pores.100 In addition, mesocrystals of the 2D layered reticular materials with rod-like morphology also can be used as a model system, for example, measurement as a single crystal (Fig. 3C).85,86 In this context, independent control of reticular networks and mesoscopic morphology in a multiple-length scale is important to use COFs and MOFs as a model system.

In addition to the simply parallel or vertical growth tendency of 2D COFs and MOFs, some studies have attempted to construct additional porous structures to construct hierarchically porous materials. For example, focusing on the 2D COFs, several strategies including template-free, self-templated synthesis, and templating synthesis have been reported.64,101–103 Although most of these works have not focused on electrochemical applications, the diversity of these synthetic strategies suggests further advancement of hierarchically porous 2D COFs. In particular, synthesis and electrochemical characterization of macroporous COFs have been reported in 201964 (Fig. 3D–G). In the report, by using polystyrene spheres as a hard template, macroporous COFs have been successfully prepared (Fig. 3D). In addition, the authors proved much improved activity of macroporous COFs compared to only microporous COFs as oxygen evolution reaction (OER) catalysts (Fig. 3E–G). This study has proved that introducing macroporous structures within the microporous COFs enhances mass transportation into the active sites in the reticular networks by testing Co-coordinated bipyridine-based COFs as OER catalysts. Although this study shows that different mesoscopic structures result in a considerable difference in activity due to the mass diffusion limitation derived from mesoscopic morphology, the identification of the real active site is still challenging due to the conversion of the Co-coordinated bipyridine into cobalt hydroxide after the OER, as verified by XPS in the report.64 This issue of the instability of reticular materials has to be carefully taken into consideration in the electrochemical applications as discussed in section 2.3.

Worth mentioning in the context of active materials, MOFs and COFs also have attracted considerable interest in the field of electrolytes.104,105 The design of high ionic conductivity solid state is of great interest for replacement of solution electrolytes in high energy density batteries. Typically, Li+, Na+, and H+ have attracted interest as important carriers. Traditional ion conductive materials are based on organic polymers, which prefer amorphous or glassy phases to accumulate both dynamic and high concentration of functional groups for ion conduction.106,107 The amorphous or glassy features of the traditional organic polymer prevent the precise design and understanding of the structures. On the other hand, crystalline COFs and MOFs have the opportunity to design functional groups to provide the efficient ion conductive pathways combining with reticular networks. Because of the presence of the porous structures of reticular networks, the additional functional structures can be introduced within the reticular porous networks and/or reticular networks themselves (Fig. 1). In 2019, poly(ethylene oxide) (PEO) chain incorporated crystalline 2D COFs, which show high Li+ ion transportation, have been reported (Fig. 3H).26 In this context, the PEO chains provide the ion transport pathway, whereas COF networks determined the structural integrity. This type of hierarchically construction of reticular materials provides the understanding to the relationship between structures and the ion transportations.

In addition to the considerable attention in the energy storage and conversion applications of the MOFs and COFs, the sensitive detection related applications by using MOFs and COFs have attracted interest recently.81,82 Efficient sensor materials with high performance for selective, fast and sensitive detection of gases are strongly desired for human health and environmental protection. Although various conventional gas sensors have been widely used to detect industrial exhausts, further development toward novel smart sensing and switching materials is much required due to their inherent limitations.108–110 The MOFs and COFs have been investigated extensively for use as high performance sensors due to the multiple following advantages: (1) their permanent porosity which provides a large surface area and numerous active sites for accelerating surface host–guest adsorption–desorption, and (2) their tunable pore size, shape, and surface environment which enhance the sensing sensitivity.81,82 One major method for gas sensing is electrical sensing which is based on the principle that the electronic properties or resistance of the sensing materials changes during the adsorption and interaction with the guest gas.111 In this context, the unique properties of MOFs and COFs including a large surface area, tunable pore size, and conductivity provide the opportunity for a new platform for electrical sensing. From the view point of the hierarchically constructed materials, macroscopic heterostructuring of reticular networks is important to achieve unique sensing performance. In 2019, macroscopic heterostructured MOF-on-MOF thin films with different types of MOF layers were obtained using the van der Waals integration method (Fig. 4).84 Two types of MOF layers with different lattices, Cu-TCPP (TCPP = 5,10,15,20-tetrakis-(4-carboxyphenyl)porphyrin) and Cu-HHTP (HHTP = 2,3,6,7,10,11-hexahydrotriphenylene), were chosen to construct MOF-on-MOF films. Then, the Cu-TCPP layer provided the molecular sieving effect, while the Cu-HHTP layer acted as the chemiresistive sensing layer. Abundant coordination-unsaturated Cu ions of upper layer Cu-TCPP showed stronger interactions with NH3 than benzene, while lower layer Cu-HHTP acted as the sensing layer to produce a highly sensitive and selective gas sensor (Fig. 4). In this context, macroscopic sheet-like morphology of MOFs provides the usability of the reticular materials as sensing devices, while constructing macroscopic MOF sheets hierarchically above and below also imparts additional functions such as selectivity.

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Fig. 4 Illustration of the preparation of 2D MOF-on-MOF thin films with van der Waals integration and application of the films as highly selective benzene-sensing materials.84 The figure is reprinted from ref. 84, copyright (2019), with permission from John Wiley and Sons.
Hierarchically constructed 3D MOFs and COFs. We discuss 3D MOF and COF based materials. Conductive 3D MOFs and COFs are promising candidates as hierarchically structured model electrodes owing to the 3D pore structures in which guest molecules can access in an isotropic manner. However, it is still challenging to design these types of 3D reticular materials with intrinsic conductive nature.4,22 Although some state-of-the-art progress has been reported very recently, few research studies have attempted to control their hierarchical constructions of the conductive 3D MOFs and COFs. On the other hand, to use the unique features of 3D MOFs including a high accessible surface area and chemical tenability, some researchers reported composite materials of insulating MOFs in electrochemical applications.112,113 Hierarchical structuralization of 3D MOFs has been developed in the field of insulating MOFs, and a variety of physical morphologies have been reported including macro-/mesoporous,114 macroporous,66 mesoporous,63,115 (Fig. 5) and other mesoscopic materials with unique shapes and constructions.116–118 Therefore, the composite materials of insulating 3D MOFs and conductive additives provide an opportunity for emergent functionalities in electrochemical applications from the view point of the chemistry in multiple length scales. Although as this section focuses mainly on MOFs/COFs toward electrode materials, an electrically-conductive feature is indispensable. However, we note here that the insulating 3D MOFs/COFs having an ion-conducting property are a very intriguing feature for next-generation electrochemical energy technology as discussed in section 2.3.
image file: d0nr09167g-f5.tif
Fig. 5 Highlighted physical morphology of MOFs with highly controlled hierarchically porous structures. (A) Macro-/meso-/microporous crystalline UiO-66-NH2(Zr) type monolithic samples. The synthetic process is based on two-step approach (1) sol–gel synthesis of low crystalline UiO-66-NH2(Zr), and (2) solvothermal reorganization from low crystallinity to high crystallinity accompanied by the expansion of the mesopores in the macropore skeletons with targeted mesopore size. (B) Macro-/microporous single crystal ZIF-8(Zn) type MOF. The synthetic process is based on the hard template process using polystyrene nanospheres. The strategy relies on the strong shaping effects of a polystyrene nanosphere monolith template and a double-solvent-induced heterogeneous nucleation approach. (C) Meso-/microporous UiO-66-NH2(Zr). The synthetic process is based on the soft template process using amphoteric surfactants, which have a carboxylic anchor in the hydrophilic head. (D) Meso-microporous UiO-66(Ce). The synthetic process is based on the soft template process using common Pluronic surfactant F127. The synergitic effects based on triblock copolymer templates and the Hofmeister salting-in anions promote the nucleation of the UiO-66 type MOFs and the in situ crystallization of MOFs around triblock copolymer templates. The panel A is reprinted from the reference, copyright (2019), with permission from John Wiley and Sons.114 The panel B is reprinted from the reference, copyright (2018), with permission from American Association for the Advancement of Science.66 The panel C is reprinted from the reference, copyright (2018), with permission from John Wiley and Sons.63 The panel D is reprinted from the reference, copyright (2020), with permission from John Wiley and Sons.115

A key point of composite materials of insulating MOFs in electrochemical applications is that 3D MOFs have tunable pore geometries and apertures which can facilitate ion diffusion. One promising example for this type of application is the use of 3D MOFs in lithium–sulphur (Li–S) batteries (Fig. 6A). In the Li–S battery, encapsulation of sulfur in the porous materials is considered as a main approach to obtain improved electrochemical properties for more than a decade.119–127 Although the Li–S battery is one of the promising next-generation batteries,128 the intermediate polysulfide species dissolve in various electrolytes, and this resulted in poor cycle life. To solve this issue, encapsulation of sulfur in ordered mesoporous carbon composites was attempted in 2009,119 and successfully improved the electrochemical properties of the Li–S battery by enhancing the charge–discharge cycling performance and specific capacities owing to the sorption of polysulfides and ensuring the electronic contact with sulfur.129 However, it was still insufficient to reach to the criteria of electrochemical properties of the next-generation batteries. From the view point of material choice toward this issue in the Li–S battery, 3D MOFs have promising features for the further advancement of sulfur and intermediate polysulfide encapsulation owing to the designability of microporous structures,83,130 and chemical adsorption sites.131–133 In addition, adding the conductive additives into framework structures also tunes the conductivity of 3D MOFs toward Li–S batteries.83 Thus, hierarchical construction of spaces has been controlled by using MOFs, and resulted in the efficient use of chemical and spatial properties.83 For example, in 2018, the critical role of ion diffusion to high rate performance in Li–S batteries has been reported by using MOF based cathode materials which were prepared by the hybridization of the electro-conductive polymer polypyrrole and sulphur in well-defined mesoscopic MOF crystals including PCN-244, MIL-53, and MIL-101 type MOFs (Fig. 6B and C).83 In this context, the hierarchical relationship can be found not only between the reticular networks and mesoscopic material shape of the MOFs but also in the mesoscopic hybrid constructions between mesoscopic crystals and polypyrrole and sulphur. In this report, comparing the capacity retention of the cathode materials using MIL-53, MIL-101, PCN-224, and porous carbon, the capacities of the MOF based cathodes were in the following order: ppy-S-in-PCN-224 > S-in-carbon > ppy-S-in-MIL-53 > ppy-S-in-MIL-101 (Fig. 6D). Because the electrical conductivity was similar between the MOF based cathode materials, the difference of the capacity retention was regarded to be derived from the geometry of the reticular porous networks (Fig. 6B). In this context, PCN-244 based cathodes which have the shortest cross-linked geometrical ion diffusion pathway showed much higher capacity than the MIL-53 based cathodes which have a discrete smaller channel, and the MIL-101 based cathodes which have 3D hierarchical nano-cage pathways (Fig. 6B and D).

image file: d0nr09167g-f6.tif
Fig. 6 Highlighted approaches using 3D framework structured MOFs to improve Li–S battery performances. (A) Schematic of the Li–S battery. (B) Reticular pore structures with different geometries and aperture sizes affect mass diffusion. (C) Schematic of the synthetic procedure of polypyrrole (ppy)-Sulfur(S)-in-MOF constructs. (D) Cycling performance of ppy-S-in-PCN224, ppy-S-in-MIL-53, ppy-S-in-MIL-101, and S-in-carbon electrodes at 5.0 C. Cage-like pores connected via channels showed higher performance. The panels B-D are reprinted from the reference, copyright (2018), with permission from John Wiley and Sons.83

Overall, reticular materials have attracted interest both in terms of electronic structures and physical structures, and resulted in the unique features in electrochemical applications. In addition, for their efficient use and fundamental investigations, hierarchical construction and the shape as mesoscopic materials play a pivotal role.

2.3 Future vision on MOFs/COFs as model electrochemical systems

From the viewpoint of electrochemistry history, a porous electrode is a key topic because this system shows largely different properties compared to flat electrodes.134 As is well known, porous structures exponentially increase the complexity of electrode processes, which is already highly complicated even in flat model surfaces;135–142 therefore fundamental understanding on the systems, even on well-defined model systems, is not enough so far.143 However recent rigorous works using a variety of meso-/macroporous metallic electrodes have shown that meso-/macoroporosity can control the electrode process.40,77,144,145 Furthermore, a confinement environment is shown as an alternative approach to control (electro)catalytic reactions.40,146 MOFs/COFs are recently used as building-blocks to construct novel-type heterojunction electrocatalysts, which interestingly suggested not only to improve activities but also selectivity at the same time by taking advantage of molecular-level structural designability.147–150 Considering that the meso-/macroporosity of the metallic electrodes plays a pivotal role in control of the electrode process,40,77,144,145 designing hierarchical constructions of the electrodes with MOFs/COFs as building-blocks can provide further control of the electrode process including the activity and the selectivity.

Another advantage of MOFs/COFs is that these materials can be applied as solid-state electrolytes by using internal pores of the reticular networks as ion pathways.26,104,105,151 In addition to the designing of ion-conducting functions derived from the reticular networks or additional functional groups at the molecular level, control of the meso-/macroscopic constructions of these materials and formation of composites with different types of liquid-based electrolytes could result in superionic conduction with the concept of nanoionics.152,153 The ions, which are expected to conduct through the materials, are not only for typical Li+ or Na+, but also for multivalent cations, such as Zn2+, Mg2+, and Al3+, or anions, such as PF6 and F. This ion-conducting feature of MOFs/COFs could lead to provide game-changing materials for electrochemical energy technology: for a solid-state electrolyte, which is the key component in all-solid-state-batteries being of enormous interest owing to their potential in improving the safety and achieving both high power and high energy densities,154–156 a material needs to have a high ionic conductivity for certain ions (for example Li+ or Na+) but be electrically insulating to prevent short-circuit. As most 3D MOFs/COFs are electrical insulators and highly porous, it thus becomes desirable to design such structures for solid-state electrolytes. Although there are already several reports addressing the potential application of MOFs/COFs as the solid-state electrolyte,26,151,157,158 the instability of these classes of materials at both the high/low potentials versus Li+/Li should be solved for the practical battery applications to achieve a high energy density and a long cycling life, and the instability issue will be a grand challenge for MOFs/COFs (Fig. 7). For instance, a solid-state-electrolyte for lithium batteries with a metallic lithium anode should be tolerant against the aggressive chemistry of lithium (−3.05 V vs. standard electrode potential). In addition, mechanical robustness will be highly important at the Li metal anode/MOF or COF-based solid-state electrolyte because MOFs/COFs will suffer damage from the dendrite formation and the mechanical pressure at the cell manufacturing processes. However, as the three-dimensionally ordered macroporous (3DOM) structure can improve electrochemical properties of batteries consisting of ceramic solid-electrolytes, such as Li1.5Al0.5Ti1.5(PO4)3 and Li7La3Zr2O12,159–161 hierarchically constructed MOFs/COFs have great potential to be a high-performance/less-toxic solid-state electrolyte alternative to the inorganic sulphide materials, which is the present standard for the commercially available all-solid-state batteries. This issue is the same for the use of MOFs/COFs as cathode materials in rechargeable batteries because the chemical stability at a high potential (= a harsh oxidative condition) is key to obtain a large cell voltage to achieve high volumetric/gravimetric energy densities (Fig. 7). For instance, the high-voltage cathodes, such as LiCoPO4 and Li2NiPO4F,162,163 can respectively provide 4.8 and 5.3 V vs. Li+/Li, which is quite beneficial for designing high-voltage cells. However, at the same time, electrolytes facing these cathodes will suffer due to the highly oxidative atmosphere. This stability issue of the solid-electrolyte/cathode interface is a key in the nonaqueous Li–O2 batteries as well.164

image file: d0nr09167g-f7.tif
Fig. 7 Schematic illustration for stability issues arising in MOF or COF based electrolytes in all-solid-state rechargeable batteries with a variety of next-generation Li-based energy storage systems. At the Li metal anode/electrolyte interface, not only a highly reductive atmosphere to decompose MOFs/COFs but also mechanical damage owing to the Li dendrite formation during charge/discharge will be another key issue. At the electrolyte/cathode interface, the highly oxidative atmosphere is a main issue. For example, LiNi0.8Mn0.1Co0.1O2 (NMC811), which is one of the most promising intercalating-cathodes for a commercially available high-energy-density Li battery, shows an average electromotive force (EMF) of 4.0 V vs. Li+/Li. This value is already quite high enough to oxidize several MOFs/COFs. Some high-voltage cathodes, such as LiCoPO4 and Li2NiPO4F, show respectively an EMF of 4.8 V and 5.3 V vs. Li+/Li. The interfaces with these cathodes are under a high oxidative atmosphere and therefore the electrolyte should be highly stable. The nonaqueous Li-O2 electrode process (O2 + 2Li+ + 2e ⇄ Li2O2) shows a relatively low EMF of ca. 3.0 V vs. Li+/Li; however the corrosion issue is well known because of the formation of oxygen radical species during the reaction. The Li-sulphur system (S + 2Li+ + 2e ⇄ Li2S) shows a low EMF of ca. 2.1 V vs. Li+/Li therefore electrolyte/cathode interface is under a mild oxidative atmosphere compared to other systems.

As already discussed in the previous section, the chemical stability of MOFs/COFs is important for the application as electrocatalysts as well because the thermodynamic potentials of the key electrochemical energy conversion reactions, such as the hydrogen evolution reaction (0 V vs. reversible hydrogen electrode; RHE) and oxygen evolution reaction (1.23 V vs. RHE), are located in the highly reductive or oxidative positions (Fig. 8), and of course overpotentials are not negligible but always considerable values. For example, one of the best electrocatalysts for the oxygen evolution reaction (OER) is IrO2; however a considerably high overpotential of 0.33 V is required to produce dioxygen from water even with IrO2 (the condition = 0.1 M NaOH at 10 mA cm−2).165 From this point, the best-performing MOF/COF based electrocatalysts probably need at least the same overpotential of 0.33 V to produce O2 with the same criteria with IrO2. Therefore, a design principle to obtain highly chemically stable MOFs/COFs should be a break-through to bring MOFs/COFs into our society as practical electrochemical devices.

image file: d0nr09167g-f8.tif
Fig. 8 Representative issues in COF/MOF-based electrocatalysts showing in a model electrochemical device: an electrolyser directly coupled with photovoltaic system to produce green hydrogen and oxygen gases from aqueous solutions. In principle, MOF/COF electrocatalysts can be applied to a wide spectrum of electrocatalytic reactions. The combination of an optimum pore-structure and a high catalytic activity is key to improve device performance. However, a stability issue is crucially important in the application of MOFs/COFs as electrocatalysts. For example, IrO2 is the bench-marking oxygen evolution reaction (OER: 2H2O → O2 + 4H+ + 4e in acidic conditions) electrocatalyst so far, requiring an overpotential of 0.33 V to produce O2 with 10 mA cm−2 in the 0.1 M NaOH solution. From this point, the best MOF/COF-based OER electrocatalyst would need an overpotential equivalent to or more than IrO2, in which the potential region has a considerably high oxidative condition because the thermodynamic potential of OER is 1.23 V vs. standard electrode potential (E°). Hydrogen evolution reaction (HER: 2H+ + 2e → H2) would require less overpotentials compared to the OER; however 0 V vs. E° could be a relatively high reductive condition for MOFs/COFs. Furthermore, in general, electrocatalytic activities are often sensitive to pH values. A strategy to enhance chemical stability of MOFs/COFs to perform electrocatalytic reactions under harsh conditions (oxidative/reductive potentials, high/low pH values) will be a main research direction to bring these material classes into practical electrochemical devices having a durability to meet the demands from markets.

Although we can see a variety of remaining challenges, the hierarchically constructed MOFs/COFs have great potential to improve electrochemical performance derived from the meso-/macroscopic structures which contribute to the fast mass transportation,50,64 meso-/macroscopic structures which realize flexible and/or mechanically strong electrodes,67,69 the reaction space for reversible formation/decomposition of solid product derived from the meso-/macropores,70,71,72–74,77 heterostructuring84 and hybridization83 derived molecular selectivity. In addition, introducing additional functional groups within reticular networks and/or reticular networks themselves25,26 also provide the opportunity to improve the electrochemical performances. Furthermore, by designing material shapes as meso-/macroscopic well-defined arrangement, the hierarchically constructed MOFs and COFs can be used as a model system as the playground for a wide spectrum of proof-of-concepts crossing chemistry, physics, and biology.166–172 Then the knowledge acquired at the model systems would be transferred to practical technologies, for example as a high volumetric energy density cathode for battery electric vehicles (EVs), ultralow-overpotential electrocatalysts for electrolysers and fuel-cell EVs, as this approach is quite successful in the solid/gas heterogeneous catalysis,173,174 if the chemical stability issue is solved. Because MOFs/COFs are two of the most complicated classes of electrode materials, model electrochemical systems based on the two material classes are still necessary to unveil microscopic electrode processes shown by these materials, and obtain reliable pictures. However recent efforts on collaborative research studies of experiment and computational science suggest that computers are powerful tools to understand observations, which are difficult to reveal just by experimental analysis.175–177

3. Outlook

Over the last few decades, material design has been developed in multiple-length scales across from the molecular level to meso-/macroscopic scale. As a result of the progress of the field of inorganic materials especially in ceramic materials, the effect of the meso-/macroscopic scale in electrochemical properties has been well familiar. However, in conventional ceramic materials, it is difficult to design the molecular-level porous structures with well-defined locations. Recent progress in the field of reticular chemistry including the design of MOFs and COFs in electrochemistry provided the opportunity to control atomic locations and pore structures at the molecular level. In addition, by introducing additional functional groups in internal pores or reticular networks, emergent functions are expected. Although focusing on the insulating reticular materials, various types of meso-/macroscopic morphological control strategies have been reported, high-level structural control of meso-/macroscopic structures of electroconductive MOFs and COFs still remains key tasks in spite of the expectation of the emergent functions as discussed in section 2.3. The most important issue for the meso-/macroscopic hierarchical constructions of MOFs and COFs is their stability. For the removal of templates and further appreciation of the crystallization of the frameworks, high stability of the frameworks is needed for the application to practical electrochemical devices as discussed in section 2.3. As recent developments in this field have successfully achieved metallically conductive MOFs,166,178–180 thus the development of the chemically stable reticular frameworks of MOFs/COFs, for instance stable even at 0 or 5.0 V vs. Li+/Li, is a next grand challenge, and this chemical stability is necessary for the development of hierarchically constructed COFs/MOFs as well. Of course, the design strategy to obtain these highly stable MOFs/COFs should not sacrifice the interesting functions and promising properties such as electron/ion conduction and dynamic flexibility.181–183 In addition, a precise structural analysis is a key to understand fundamental functions of MOFs/COFs.184–190 Furthermore, compared to the conventional porous materials including ceramics, COFs and MOFs can control their electronic structures and pore structures combined with additional functional structures at the molecular level with periodicity. Therefore, these classes of materials are already fascinating model electrode systems to study a wide spectrum of microscopic electrode processes. From these viewpoints, the hierarchical construction of these types of reticular materials based on stable MOFs/COFs will provide the opportunity not only to achieve higher properties than those of traditional materials, such as oxides, to provide practical electrode materials for electrochemical devices but also further to investigate the fundamental properties of the molecular structures in a macroscopic set-up.

Author contributions

Y. H. and K. S. decided the theme of the review, and wrote the manuscript.

Conflicts of interest

There are no conflicts to declare.


This work was partially supported by JSPS KAKENHI (19K15527 and 19H05460), and a Grant-in-Aid for JSPS Fellow (19J22552). The authors thank Prof. Shuhei Furukawa, Prof. Hiroshi Kitagawa, Dr Kazuyoshi Kanamori (Kyoto University), and Prof. Kazuki Nakanishi (Nagoya University) for continuous support.


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