Vanadium‐based metal‐organic frameworks and their derivatives for electrochemical energy conversion and storage

Abstract

With the excessive consumption of nonrenewable resources, the exploration of effective and durable materials is highly sought after in the field of sustainable energy conversion and storage system. In this aspect, metal‐organic frameworks (MOFs) are a new class of crystalline porous organic‐inorganic hybrid materials. MOFs have recently been gaining traction in energy‐related fields. Owing to the coordination flexibility and multiple accessible oxidation states of vanadium ions or clusters, vanadium‐MOFs (V‐MOFs) possess unique structural characteristics and satisfactory electrochemical properties. Furthermore, V‐MOFs‐derived materials also exhibit superior electrical conductivity and stability when used as electrocatalysts and electrode materials. This review summarizes the research progress of V‐MOFs (inclusive of pristine V‐MOFs, V/M‐MOFs, and POV‐based MOFs) and their derivatives (vanadium oxides, carbon‐coated vanadium oxide, vanadium phosphate, vanadate, and other vanadium doped nanomaterials) in electrochemical energy conversion (water splitting, oxygen reduction reaction) and energy storage (supercapacitor, rechargeable battery). Future possibilities and challenges for V‐MOFs and their derivatives in terms of design and synthesis are discussed. Lastly, their applications in energy‐related fields are also highlighted.

INTRODUCTION

Over the last few decades, tremendous efforts have been devoted to exploring advanced electrochemical energy conversion and storage systems due to the rapid exhaustion of fossil fuels and the deterioration of global warming.^1–3 Electrochemical energy conversion systems have been proven as one of the cleanest and most sustainable ways to alleviate energy and environmental problems. Water splitting, fuel cells, metal-air batteries (MABs), and carbon/nitrogen fixation have shown effectiveness in the production and utilization of electro-fuels (e.g., H₂ or CO), as well as in the treatment of greenhouse gases (CO₂).^4–9 Electrocatalysis through the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) are indispensable routes for most of the energy conversion devices.^10–13 However, the respective electrocatalysts consume tremendous amount of electrical energy due to the large overpotential and slow kinetics. In addition, the catalysts are expensive and have relatively short lifespans. These shortcomings hamper the possibility for large-scale applications in commercial equipment which are typically subjected to harsh conditions.^14,15 On the other hand, supercapacitors (SCs), metal (lithium, sodium, and potassium) ion batteries, and lithium-sulfur batteries (LSBs) are promising electrochemical energy storage devices for portable electronics and electric vehicles.^16–18 Nevertheless, these energy storage devices are still limited by their inherent safety issues. These devices face uncontrolled growth of metal dendrites, serious shuttle effects, and drastic change in the volume of the electrode material which adversely affect the rate capability, cycle life, and security. Further rectifications are required to meet the multifaceted demands from the market.^19,20 In view of the above-mentioned issues, the development for more efficient yet durable electrocatalysts and electrode materials with desirable electrochemical properties are of the utmost importance. Carbon-based materials,^21–26 gel-based materials,²⁷ Prussian blue analogues (PBAs)­-based materials,²⁸ conducting-polymer-based materials,²⁹ transition metal dichalcogenide-based materials,^30–32 perovskite-type oxides,³³ metal sulfides,³⁴ layered double hydroxides (LDHs),³⁵ MXenes^36–40 are some of the materials that were investigated recently by researchers in hopes to meet the demands.

Apart from those, metal-organic frameworks (MOFs) are part of emerging porous crystalline materials that are being intensely researched in the past two decades, particularly in terms of design synthesis and modification.^41–45 MOFs are a class of organic-inorganic hybrid materials, consisting of metal nodes (ions or clusters) and various organic ligands, and they provides promising potential to be used in selective gas adsorption and separation, hydrogen storage, chemical sensors, heterogeneous catalysis, and drug delivery.^46–53 Research has also shown that MOFs with abundant metal sites, porous structures, and lager specific surface areas can offer more accessible active sites and faster mass/charge transfer, thus improving the electrochemical performance of electrocatalysts and electrode materials.^54–57 The pristine and modified forms can provide various functionalities and benefits in different applications. While MOFs composites are assembled with functional species, including graphene, carbon nanotubes (CNTs), metal nanoparticles, MXenes, covalent-organic frameworks (COFs), and even enzymes,^58,59 which exhibit synergistic effects and multifunctionality. Besides, MOFs derivatives provide high conductivity and robust structure.^60–64 The types of MOFs are manifold. This review will provide an overview of the rarely reported MOFs.

In comparison to MOFs based on divalent metals (Zn²⁺, Ni²⁺, Cu²⁺, etc.) coordination nodes, MOFs based on trivalent or tetravalent metals are rarely reported due to the complexity of their node clusters and the flexibilities of their coordination environments. Vanadium (V), a common transition metal, belongs to the fifth subgroup with 3d³4s² on its outer electron layer structure. This means that it has abundant valence electrons to allow a variety of redox behaviors.^65–67 Thus, V-based materials (vanadium oxide, vanadium nitride, vanadium sulfide, mixed metal vanadate, vanadyl phosphate) may be applied for future development of electrochemical energy conversion and storage technologies.^68–73 V-based MOFs (V-MOFs), specifically on those with V ions or clusters occupied in the vertex positions of its polyhedral structure, have recently gained traction due to their multiple valence states and open-framework crystal structure. These could then lead to high specific capacity, remarkable cycle stability, and considerable electrocatalytic performance.⁷⁴ Moreover, it is very encouraging that MOFs as precursors can not only provide new synthetic ideas for the design of V-based materials, but also endow them with unique structure, high specific surface area, excellent conductivity, and robust stability. These will then promote the rapid diffusion of electrolyte and improve electrochemical performance significantly.⁷⁵

The application of V-MOFs in energy-related fields has not yet been reviewed widely. This review will cover the applications, its recent advances, and challenges of using V-MOFs and their derivatives as electrocatalysts or electrode materials. This review will discuss the various kinds of V-MOFs and their derived materials that have been reported, followed by their present applications in electrochemical energy conversion (HER, OER, ORR) and storage (SCs, metal ion batteries) (Scheme 1). Finally, the future direction and challenges for the development of V-MOFs and their derivatives in terms of energy-related applications will be discussed.

Scheme 1. An overview of V-MOFs and their derivatives used in energy conversion and storage. V-MOF, vanadium-metal-organic frameworks

V-MOFS

The formation of V-MOFs is usually a process of self-assembly of vanadium ions and organic ligands to form a network structure. As an essential metal unit, vanadium trichloride (VCl₃), vanadyl sulfate (VOSO₄), and vanadium powder are often used as vanadium sources, which are usually connected with each other by the polytopic carboxylate linkers or nitrogen-containing heterocycles to form three dimensional frameworks with different topologies. The typical synthetic strategies, include hydro/solvothermal method, ultrasonic method, and coprecipitation method, provide layouts to design and prepare different morphologies of V-MOFs.^76–78 According to the composition and form of vanadium nodes, V-MOFs in this review are deliberately divided into three parts: (1) the pristine V-MOFs, (2) mixed-metal MOFs (V/M-MOFs, M refers to other metal nodes), and (3) polyoxovanadate-based MOFs (POV-MOFs).

The pristine V-MOFs

Although tremendous efforts have been made to synthesize crystalline V-MOFs, only a few crystalline V-MOFs, such as MIL-47,⁷⁹ MIL-68⁸⁰ and MIL-59⁸¹ (MIL stands for Materials of Institute Lavoisier), can be accurately determined by single-crystal X-ray diffraction (SXRD) based on their crystal structures. To overcome such ordeal, crystallographers can use either synchrotron radiation or powder X-ray diffraction (PXRD) to create desired MOFs structures for various applications. This results in having a wide variety of V-MOFs being reported in recent years (Figure 1).^78–89

Figure 1. Crystal structures of representative V-MOFs. COMOC-2,78 MIL-47,79 MIL-68,80 MIL-59,81 COMOC-3,82 MIL-60,83 MIL-71,84 MFM-300,85 MIL-88B,86 MIL-10088 and MIL-101.87 Color codes: V, green; C, grey; O, red

The two widely known structural motifs of building units for the connection form of V containing nodes are infinite trans chains of [V^III(OH)₂O₄] octahedra, and oxo-centered trimers of vanadium octahedra (Figure 1). In the case of the former, the synthetic form of V-MOFs contains octahedral [V^III(OH)(R-(CO₂)₂)] infinite long chains, and some of the representatives include MIL-47,⁷⁹ MIL-68,⁸⁰ COMOC-2,⁷⁸ COMOC-3,^82,88 (COMOC stands for Center for Ordered Materials, Organometallics and Catalysis, Ghent University) MIL-60,⁸³ and MIL-71.⁸⁴ Among them, MIL-47 has been widely studied because of its large pores (aperture sizes: 10.5 × 11.0 Å, taking into account the van der Waals radii), flexible and open structure, as well as the large specific surface area (S_BET = 930 m²/g, S_Langmuir = 1320 m²/g).^90–100 From a structural point of view, it is built from infinite chains of corner-sharing VO₆ octahedra and interconnected by terephthalate groups, creating a three-dimensional topology of orthorhombic structure. MIL-47 also exhibits the similar breathing effects to MIL-53 (Cr),^101,102 the guest molecules trapped in V-MOFs possessing {V − O − V}_∞ chains which can be removed by thermal activation to form empty pores. The cell volume will then increase by 40%, converting itself from the narrow pore form (NP) to the large pore form (LP). In addition, the V^III − OH bond can be oxidized to the vanadium-oxygen double bond, increasing the valence state of vanadium from +3 to +4, without affecting its original topological structure.⁷⁹ On the other hand, some V-MOFs (MIL-59,⁸¹ MIL-88B,⁸⁶ MIL-100,⁸⁸ and MIL-101⁸⁷) possessing the general formula [V₃O(OOCR)₆L₃]ⁿ⁺ (L = terminal ligand: H₂O or Cl⁻) contain oxo-centered vanadium trimers in the framework structure. As shown in the crystal structure of MIL-100 and MIL-101 in Figure 1, the trimers are interlinked by polycarboxylates to form the supertetrahedra (ST), which are further interconnected in three-dimensional (3D) fashion to produce the augmented zeolite mobil thirty-nine (MTN) type of framework. Therefore, both MIL-100 and MIL-101 networks exhibit cage-like structures, which is suitable as host materials for energy storage.^87,89

In addition to the structural differences caused by the secondary building units (SBUs), the design and modification of ligands also play important roles in the structural characteristics and functionality of V-MOFs. It is well known that the properties (pore size or chemistry) of a particular MOF can be systematically adjusted without changing the basic topological structure. This can be done by introducing functional groups (with different sizes, polarity, acidity, etc.) in the organic linkers. Vanpoucke et al.⁹⁰ investigated the effect of ligand functionalization to regulate the electronic structure of V-MOFs, revealing that the introduction of functional groups brought about a splitting of the π orbital on the chain due to their electron-withdrawing/donating nature. The organic ligands modified by diverse functional groups (X = −Cl, −Br, −CH₃, −CF₃, −OH, −OCH₃, etc.) can also be used as a linker to synthesize a series of V-MOFs whilst maintaining their phase purity.^103–105 Additionally, Yot et al.⁹⁷ reported that the large molecular weight functional groups (−Br or −CF₃) functionalized ligands can also improve the mechanical energy storage performances of these hybrid porous materials. In comparison with MIL-101 (V), the amino-modified MIL-101 (NH₂-MIL-101) do not only have a larger specific surface area, but also demonstrate higher CO₂/N₂ adsorption capacity and moderate CO₂ isosteric heat of adsorption.¹⁰⁶ Moreover, the steric resistance and physical property of the substituent for the ligands also have certain influence on the construction of MOFs.

In general, the abundant coordination environment of V ions enables the structural diversity of the pristine V-MOFs, where the porous character and coordinatively unsaturated metal sites offer the possibility of energy storage and conversion. And the control of the ligands also facilitates the modulation of the electronic structure of the electrocatalytic active site. In addition, it is worth looking forward to the richness of the pristine V-MOFs to offer more options for the design and synthesis of V/M-MOFs and V-MOFs derivatives.

V/M-MOFs

With the help of the modular building principle, different metal ions with similar structural roles in the construction of MOFs can be intentionally integrated into molecular scaffolds to produce mixed-metal MOFs.^107,108 The mixed metal nodes can self-assemble in an equivalent manner and occupy random positions in its final MOF skeleton. It is well known that MIL-53 (M) (M = Al, Fe, Cr, etc.) is a general term for a class of well-known homologous carboxylate-based MOFs, consisting of an infinite chains of corner-sharing MO₄(OH)₂ octahedra.^109,110 As an analogue of MIL-53, V-MOFs ([V^III(OH)bdc]_n) features a similar crystal structure. Therefore, a series of mixed-metal V/M-MOFs (M = Al,¹¹¹ Ni,¹¹² Fe,¹¹³) have been prepared by direct synthesis, obtaining same phase as MIL-53. Significantly, “V-doping” has a positive impact on the adsorption and electrocatalytic properties of MOFs materials due to the possible stabilization of the mixed-valence V^III/V^IV oxidation states in its pure activated phase. Also, based on the modular principle, Rocha et al.¹¹⁴ reported the construction of a series of modular trinodal mixed-metal V/La-MOFs via self-assembly of V-containing cyclic trinuclear anionic units with cationic {MO₈} or {MO₉} aqua-based lanthanide complexes. Unlike the relatively independent secondary building units, the heterobimetallic SBUs connect two different metal centers by a short bridge and have great potential for the construction of metal-organic polymers.¹¹⁵ Senchyk et al.¹¹⁶ successfully integrated the novel Ag^I/VOF (VOF stands for vanadium oxofluoride) heterobimetallic complexes into MOFs employing 1,2,4-triazole ligands with aliphatic linkers, whereby a series of novel Ag^I/V^V-MOFs were successfully synthesized by a simple one-pot hydrothermal methodology (Figure 2A).

Figure 2. (A) Ag/VOF building fragment as a node of the “square grid” framework topology with linear triazolyl linkers in the crystal structure. Reproduced with permission: Copyright 2012, American Chemical Society.116 (B) The structure of VIV-MFU-4l. Reproduced with permission: Copyright 2018, Wiley-VCH.117 (C) The structure of V2Cl2.8(btdd) and primary VII coordination sphere. Cyan, green, blue, red, and grey spheres represent V, Cl, N, O, and C, atoms, respectively. Reproduced with permission: Copyright 2021, American Chemical Society.118 (D) Schematic illustration of a two-step self-assembly process that is responsible for the formation of the VO(TCPP-Zn2-bpy). Color codes: green, V; orange, Zn; red, O; light blue, N; gray, C. Reproduced with permission: Copyright 2018, American Chemical Society119

Moreover, the post-modification strategy has also been proven to be an effective method for the synthesis of V/M-MOFs. MFU-4l ([Zn₅Cl₄(BTDD)₃], BTDD: bis(1H-1,2,3-triazolo[4,5-b],[4′,5′-i])dibenzo[1,4]dioxin) (MFU stands for Metal-organic Framework Ulm-University) can be modified post-synthetically via partial or complete substitution of peripheral metal sites, thus opening a route towards a large variety of functionalized MOFs.^120–122 Comito et al.¹¹⁷ reported the incorporation of vanadium into MFU-4l by cation exchange to prepare two benzotriazolate-based V/Zn-MOFs, and vanadium incorporation can be observed when each of the precursors was used in a modest excess (Figure 2B). When VCl₄ or VCl₃(THF)₃ (THF: tetrahydrofuran) was used as a precursor, the as-prepared V^IV/Zn-MOFs showed nearly identical edge energies and pre-edge features. By contrast, MFU-4l exchanged with VCl₂(py)₄ (py: pyridine) exhibited an edge energy most consistent with vanadium (II). The same ligand can also be used to synthesize monometallic MOF (V₂Cl_2.8(btdd)), with a structure containing one-dimensional hexagonal channels and vertices that are decorated by vanadium sites. It is worth noting that V₂Cl_2.8(btdd) is a mixed-valent, containing 40% V^III sites, charge-balanced by terminal chloride ligands, and 60% V^II sites in a square pyramidal geometry (Figure 2C).^118,123

The heterometallic MOFs can also be synthesized from metalloligands such as pyridine-2-carboxylates,¹²⁴ Schiff bases,^125,126 metallosalen,^127,128 and other ligands.^129,130 On the other hand, porphyrins are different from other metalloligands. The cavity inside porphyrins can adopt numerous metal elements, and the 3D stacking of porphyrin-based MOFs can be controlled by the coordination geometry of the porphyrin metal center to form the well-known porphyrin paddle-wheel frameworks (PPFs).¹³¹ When choosing different types of metallo-porphyrin and paddle-wheel clusters, various frameworks with the same topology can be obtained, while the pore size can be controlled by changing the columnar molecules that connect the two porphyrin layers. Besides, due to the presence of porphyrins, PPFs are expected to have a unique feature: possessing a large number of coordination unsaturated metal centers, which plays a key role in applications such as gas adsorption and catalytic conversion.¹³² Yamabayashi et al.¹¹⁹ have successfully synthesized a 3D porous vanadyl-based MOF, [VO(TCPP-Zn₂-bpy)] (TCPP: tetracarboxylphenylporphyrinate; bpy: 4,4′-bipyridyl), and investigated how such structural modifications influence qubits' performances. Similar to [Cu(TCPP)-Zn₂(H₂O)₂] reported by Urtizberea et al.,¹³³ the peripheral carboxyl substituents of the metalloporphyrin macrocycle interacted with Zn²⁺ to form two-dimensional (2D) layers, and then the pillar-like bpy ligands allow connection between the 2D layers to form a 3D MOF by further axial coordination between pairs of [Zn₂(COO)₄] subunits. The result shows that the introduction of vanadyl units in the 3D network has an obvious effect on the infrared-active low-energy vibrational modes (Figure 2D). In contrast, research on similar 2D planes without pillars are relatively scarce, Urtizberea et al.¹⁰³ have also successfully synthesized 2D-MOF [{VO(TCPP)}Zn₂(H₂O)₂]_∞ at the air-water interface, even allowing mono- and multiple-layer deposits to be transferred to various substrates. Compared with the pristine V-MOFs, V/M-MOFs have obvious regulatory advantages in terms of structure and composition, which provide more active sites for electrocatalytic applications. Besides, V/M-MOFs also enrich the kinds of precursors and the corresponding derivatives.

POV-MOFs

Polyoxometalates (POMs) composed of transition metals (mainly vanadium, molybdenum, tungsten) and oxide cluster with high negative charge.¹³⁴ POMs have discrete anionic oxidation-bridged metal clusters, which have been widely studied in the field of energy materials research recently due to their stable structures and remarkable catalytic activities.¹³⁵ Addition of POMs into various pore supports, such as mesoporous silica,¹³⁶ and molecular sieves¹³⁷ is a commonly adopted strategy to enhance the stability and reusability of POMs. Alternatively, POM anions can also be used as inorganic units to construct a variety of POM-based MOFs.^138–140 As an important subclass of POMs, polyoxovanadates (POVs) have abundant redox behaviors due to the multiple accessible oxidation states (mainly based on the V^V/V^IV and more rarely V^IV/V^III redox couples). With that, POV-MOFs have gained traction because of their unique structures and electrochemical performances.¹⁴¹

A cubane (V₄O₁₆)-based MOFs expressed as Cu₂(phen)₂(V₄O₈)(PO₄)₄ [Cu₂V₄O₁₆-2D] (phen: phenanthroline) for transition metal (Cu^II) connections were reported for the first time by Dey et al.¹⁴² The bulk material exhibited two different morphologies, but both possessed the same structure (Figure 3A). From the perspective of the structure, four edge-sharing VO₆ octahedra were packed together to form one V₄O₈ cubane unit, and each V₄O₈ cubane was connected to four different Cu(phen)(O)₃ square pyramidal units (Figure 3B). Similarly, other clusters such as Wells-Dawson-type tungstovanadate clusters ([V₂W₁₈O₆₂]⁶⁻) were also used to construct POV-MOFs with 3D self-penetrated frameworks.¹⁴⁵ Liu's research team¹⁴³ has also made tremendous contribution in the design and synthesis of POV-MOFs in recent years. Two novel POV-MOFs, [Co(bib)]{V₂O₆} (V-Co-MOF) and [Ni(en)(bib)]{V₂O₆}·2H₂O (V-Ni-MOF) (bib: 1,4-bis(1H-imidazoly-1-yl)benzene; en: ethylenediamine) were synthesized under mild hydrothermal conditions, and V sites with both of them adopted the {VO₄} tetrahedral coordination geometry. It is worth noting that the Co centers in V-Co-MOF exhibited tetra-coordination twisted in tetrahedral configuration (coordination unsaturated metal sites, CUMS) (Figure 3C), and the Ni centers in V-Ni-MOF showed hexa-coordination octahedral geometry (coordination saturated metal sites, CSMS) (Figure 3D).¹⁴³ Under the same conditions, the catalytic performance of V-Co-MOFs was better than that of V-Ni-MOFs due to the synergistic effect of V^V sites and Co^II centers. Furthermore, [Ni(Bib)₂]{V₂O₆} ({V₆}-MOF) can also be synthesized by hydrothermal method.¹⁴⁴ Unlike the above two POV-MOFs containing 1D {V₂O₆}²⁻ polyanion chains, the {V₆}-MOF contained a hexa-nuclear vanadium anion cluster ({V₆O₁₈}⁶⁻), which only composed of two VO₅ tetragonal pyramids and four VO₄ tetrahedrons, connected by the apex sharing of O atoms (Figure 3E). Different from the pristine V-MOFs and V/M-MOFs materials, the most distinctive feature of POV-MOFs is that their building unit nodes are POVs rather than simple metal ions. And excellent stability, diverse valence states and rich composition of POVs can improve the catalytic activity and stability of V-MOFs, thus broadening their applications in electrochemistry.

Figure 3. (A) SEM image of the bulk Cu2V4O16-2D. (B) Interdigitation of 2D sheets of Cu2V4O16-2D via the phenanthroline ring in the ac-plane. Color code: black, C; green, Cu; red, O; blue, N; yellow, P; dark yellow, V. Reproduced with permission: Copyright 2011, American Chemical Society.142 (C) The 2D bimetallic oxide layers and 3D frameworks of the V-Co-MOF. (D) The 2D bimetallic oxide layers and 3D frameworks of the V-Ni-MOF. Reproduced with permission: Copyright 2020, Royal Society of Chemistry.143 (E) A schematic illustration of the crystal structures of {V6}-MOF: {V6O18}6− polyanion cluster, bimetallic decanuclear cluster {Ni4-V6}, 2D bimetallic oxide layer {Ni3V6O18} and 3D framework of {V6}-MOF. Reproduced with permission: Copyright 2021, American Chemical Society.144 2D, two-dimensional; 3D, three-dimensional; MOF, metal-organic framework; SEM, scanning electron microscopy

DERIVATIVES OF V-MOFS

When V-MOFs were used as precursors or templates, various forms of derivatives can be obtained, such as vanadium oxide, carbon-coated vanadium oxide, vanadium phosphate, vanadate and other vanadium doped nanomaterials (Figure 4).^146–154 The transformation pathway and theory of V-MOFs to related V-MOFs derived carbon, metal compounds and their composites are relatively complex. This is because they are drastically affected by the composition and structure characteristics of their precursors, pyrolysis temperature, calcination atmosphere and other transformation conditions.¹⁵⁵ Taking the calcination atmosphere as example, the heat treatment process can be divided into two cases: in air and in inert atmosphere. When annealed in air, organic components volatilize into CO₂ or retain a small amount of porous carbon, while vanadium metal units are being oxidized to form a variety of vanadium oxides with nanostructures. In addition, V-MOFs precursors also have a significant effect on the morphology and composition of their derivatives. On the one hand, the organic ligands are in-situ carbonized in the inert atmosphere to produce porous carbons with high specific surface areas, thus improving the stability and electrical conductivity of electrocatalysts or electrode materials. On the other hand, the V-MOFs derivatives usually retain the specific morphology of the precursors and provide highly tunable feature of spatial distribution of derived nanostructures, where the distribution of metal components and carbon is homogeneous, which is difficult to achieve with mixed carbon alone. More interestingly, due to the different mobilities of the atoms in the precursors of V-MOFs, derivatives with novel hollow structures can also be formed by the Kirkendall effect. Furthermore, the corresponding vanadium-based sulfides, selenides and phosphating materials can also be synthesized by annealing in the presence of sulfur, selenium, or sodium hypophosphite.

Figure 4. An illustration of the transformation of V-MOF to derivatives, and the electron microscopic images of some V-MOFs derivatives, including: (A) Vanadium oxide: shuttle-like V2O5, Reproduced with permission: Copyright 2017, Springer Nature Limited146; V2O5-500 °C, Reproduced with permission: Copyright 2020, Springer Nature Limited147; P-V2O5 (porous V2O5), Reproduced with permission: Copyright 2019, Elsevier B.V.148 (B) Vanadium oxide@C: c-V2O3@C (crystalline V2O3 and carbon composite), a-V2O5@C (amorphous V2O5 and carbon composite), Reproduced with permission: Copyright 2020, Wiley-VCH149; VOX/PCs, Reproduced with permission: Copyright 2018, Wiley-VCH.150 (C) LVP@M-101 (carbon-coated Li3V2(PO4)3 nanocomposites), Reproduced with permission: Copyright 2018, Elsevier B.V.151 (D) Co3V2O8 sponge, Reproduced with permission: Copyright 2016, American Chemical Society152; Co3V2O8 nanoroses, Reproduced with permission: Copyright 2017, Royal Society of Chemistry153; Co3V2O8 microspheres, Reproduced with permission: Copyright 2016, Royal Society of Chemistry154

During the pyrolysis process, most V-MOFs derivatives can maintain the original skeleton structure and morphology of V-MOF at low temperature (<600 °C), while endowing materials with controllable pore size and new morphology such as multi-layer core-shell structure.^146,149 For example, vanadium phosphate with multi-layer core-shell, nanosheet and nanowire can be prepared by ion exchange.¹⁵⁰ The composition and crystal phase of these derived materials, as well as the abundance of active metal sites and unsaturated organic sites do not only expand the electrochemical active surface area, but also increase the effective electron transport. In view of energy conversion and storage, V-MOFs can be directly used as electrocatalysts and electrode materials, or as sacrificial templates to prepare porous carbon matrix supported metal compounds to obtain various nanostructures and higher electrochemical performances. The following two sections will discuss the application of V-MOFs and derivatives in electrochemical energy conversion and energy storage.

ELECTROCHEMICAL ENERGY CONVERSION

Electrochemical energy conversion technology adopts clean process and has little impact on the environment, which plays a very important role in the sustainable development of energy.^156,157 Electrochemical water splitting is considered as one of the most promising energy conversion technologies for hydrogen production from water, which consists of two half reactions: OER at the anode and HER at the cathode.^158,159 Proton exchange membrane fuel cell (PEMFC), as an efficient alternative energy conversion device for direct chemical-to-electrical energy conversion, has also attracted widespread attention, among them, cathodic ORR is a multi-electron, multi-step reaction with sluggish kinetics, which then deemed as the critical limiting step, and consumed most catalyst materials.^160,161 Efficient electrocatalysts are prepared for these electrochemical reactions to reduce their overpotentials for practical applications, and a variety of V-MOFs and derivatives for electrocatalysis have been summarized in Table 1.

Table 1 Comparison of the performance of V-MOFs and their derivatives for electrocatalysis

[Table omitted. See PDF]

Abbreviations: HER, hydrogen evolution reaction; OER, oxygen evolution reaction; V-MoF, vanadium metal-organic framework.

V-MOFs as electrocatalysts

The homogenous dispersed metal ions (or clusters), along with the ultrahigh specific surface area of MOFs enable electrocatalysts to provide abundant active sites for water splitting. Furthermore, the poor electronic conductivity of MOFs can be improved to some extent by using nickel foam¹⁶⁷ or other carbon nanoparticles¹⁶⁸ as substrates. It is well known that most bimetallic and polymetallic-based electrocatalysts demonstrate significantly better performance than their monometallic-based counterparts, because the electronic configuration of monometallic electrocatalysts can be adjusted by introducing other metals, yielding significant improvement in their catalytic activities.¹⁶⁹ Taking advantage of the synergistic effect of multiple metals, some V/M-MOFs electrocatalysts have been developed for water splitting, where the M ions usually act as the active center of the electrocatalysis, and V ions with abundant oxidation states often play the role of regulating the electronic structure of the active sites.

Using nickel and vanadium as raw materials, the three-dimensional nickel-vanadium-based MOFs supported on nickel foam (Ni₂V-MOFs@NF) was synthesized by two-step of method: ultrasonic and hydrothermal at the ratio of 1:2 (Figure 5A).¹¹² Ni₂V-MOFs@NF attained the current density of 10 mA/cm² (η₁₀) at very low overpotentials of 244 and 89 mV with the low Tafel slopes of 38.1 and 98.3 mV/dec for OER and HER, respectively. Furthermore, this bifunctional electrocatalyst can also realize a cell voltage of 1.55 V at a current density of 10 mA/cm², which is comparable to the commercial electrocatalysis (Figure 5B). In addition, the chronopotentiometry also proves that the electrolytic cell for water electrolysis had excellent stability (Figure 5C). The superior performance of these new synthetic electrocatalysts would give the credit to the special morphology from the 2D ultrathin nanosheets self-assembling to a 3D nanoflower, as well as the electronic properties manipulation caused by the synergetic effect between Ni ions and V ions. Additionally, the incorporation of high-valent vanadium reduces the band gap of Ni₂V-MOFs and improves the electron conductivity. With the same strategy of morphological control and electronic regulation, Han et al.¹¹³ reported the preparation of well-defined arrays of V/Fe-MOF with controllable stoichiometry supported on NF through the ultrasound and solvothermal synthesis method (Figure 5D). Notably, V-MOF@NF and Fe-MOF@NF obtained by the same synthesis method depict different microscopic characteristics, and only the former can form array structure. This proves that the presence of V element played an important role in the formation of array. For the combination of V²⁺ and Fe²⁺ in bimetallic V/Fe-MOF@NF, the π-bond of Fe−O would increase due to the repulsion between O²⁻ and V²⁺, thus resulting in a portion charge transfer from V²⁺ to Fe²⁺ (Figure 5E). The morphological control and electronic regulation strategy for the synthesis of V/Fe-MOF@NF has led to a high electrocatalytic activity in alkaline electrolyte with a relatively low overpotential of 246 mV@10 mA/cm² for OER and 147 mV@10 mA/cm² for HER, respectively. V/Fe-MOF@NF can also provide a current density of 10 mA/cm² at a low cell voltage of 1.61 V, which was better than the single metal MOF catalysts (V-MOF@NF and Fe-MOF@NF) (Figure 5F). There was no obvious degradation for the current density after 3600 s of continuous water-splitting reaction, indicating the remarkable high electrochemical stability of the V/Fe-MOF@NF | | V/Fe-MOF@NF couple (Figure 5G).

Figure 5. (A) A schematic diagram for growing Ni2V-MOFs on a nickel foam substrate by ultrasound and solvothermal methods. (B) LSV curves for synthetic samples for overall water splitting. (C) Chronopotential curve of Ni2V-MOFs@NF at a current density of 10 mA/cm2 for 80 h and the inset is a photograph of the electronic cell. Reproduced with permission: Copyright 2020, American Chemical Society.112 (D) A schematic for the formation of V/Fe-MOF@NF. (E) A schematic illustration of the synergistic effect between V and Fe in V/Fe-MOF@NF. (F) LSV curves of V/Fe-MOF@NF as anode and cathode catalyst in 1 mol/L KOH for overall water splitting. (G) The current-time (I-t) curves of V/Fe-MOF@NF at the constant potential for 3600 s (inset: An optical photograph for overall water splitting reaction in a two-electrode configuration). Reproduced with permission: Copyright 2021, Elsevier B.V.113 MOF, metal-organic frameworks; Ni2V-MOFs@NF, nickel-vanadium-based MOFs supported on nickel foam

Hu et al.¹⁶² also investigated the role of POVs in electrocatalytic water oxidation reaction. Three bulk POV-MOFs materials, [Ni₂(BBTZ)(H₂O)₄]V₄O₁₂·2H₂O (1), [Co₂(BBTZ)(H₂O)₄]V₄O₁₂·2H₂O (2), [Ni(BBTZ)₂]V₂O₆·2H₂O (3) (BBTZ = 1,4-bis-(1,2,4-triazol-1-ylmethyl)benzene), were synthesized as electrocatalysts for OER. These MOFs can maintain structural stability when soaking in 1 mol/L KOH electrolyte, but the transformation of the structures of these MOFs only occurs on their electrode surfaces to form the V-doping metal oxyhydroxides as OER-active species during the electrocatalytic reaction. This proves that POVs were not active sites but precursors for the evolution of OER active species.

Overall, the electrocatalytic performance of the V/M-MOFs for HER and OER is superior to that of the monometallic V-MOFs, especially when combined with the active metal (M = Ni, Fe). The comparison between Ni₂V-M-MOFs and POV-MOFs reveals that V/M-M-MOFs with nanosheet morphology have better electrocatalytic performance. Therefore, in addition to the synergistic effect between metals, morphology control also has an important influence on electrocatalytic performance.

V-MOFs derivatives as electrocatalysts

Partial or full conversion of MOFs to metal carbon composites is a promising solution to enhance the conductivity and stability of electrocatalyst rather than using MOFs directly as electrocatalysts.^60,61,170 By controlling the heat treatment process of the original MOFs, the organic ligands can be graphitized to a certain extent, maintaining the original morphologies, and improving their conductivity dramatically. Moreover, the metal sites of the derivatives can achieve better catalytic activity and more exposed active regions when their ligands were removed.

As has been reported by Zhou et al.¹⁷¹ and Yang et al.,¹⁷² some V-doped electrocatalysts exhibit excellent electrocatalytic properties, in which V-doping can accelerate the electron transfer. For MOF-V-Ni₃S₂/NF, as a V-doped MOFs derivative, it requires low overpotentials of 268 mV@10 mA/cm² and 118.1 mV@10 mA/cm² for OER and HER, respectively.¹⁶³ Due to the inherently large surface areas, exposed active site rate and improved charge collection of MOF-V-Ni₃S₂/NF, its catalytic performance is superior to that of the directly prepared V-Ni₃S₂/NF. ZIF-67 (ZIF stands for zeolitic imidazolate frameworks) is one of the more popular MOFs precursors to synthesize heteroatom-doped porous nanocarbon skeletons.^173,174 CoVO/C¹⁶⁴ and V-Co_xP@NC¹⁶⁵ are V-doped ZIF-67 derivatives that were successfully synthesized by different materials design strategies. For the former, it was synthesized by just a two-step process. The process started with the use of molecular vanadium oxide clusters embedded in ZIF-67 to prepare crystalline Co-V-oxide nanoparticles embedded in a layered nitrogen doped porous carbon matrix (Figure 6A). This then delivered higher activity for OER with an overpotential of 350 mV (at 10 mA/cm²) and a Tafel slope of 75 mV/dec compared with the pyrolyzed pure ZIF-67 reference (Figure 6B,C).¹⁶⁴

Figure 6. (A) An illustration of the two-step fabrication process of CoVO/C composite. (B) LSV polarization curves and (C) Tafel plots of CoVO/C, and the pyrolyzed ZIF-67 reference in 1.0 mol/L aqueous KOH for the OER. Reproduced with permission: Copyright 2019, Royal Society of Chemistry.164 OER, oxygen evolution reaction

Whereas for the V-Co_xP@NC,¹⁶⁵ it can be obtained by low temperature phosphating of V-ZIF-67 synthesized by the partial ligand exchange of the as-prepared ZIF-67 and NaVO₃ (Figure 7A). The interaction between V and Co_xP can adjust the electronic structure of Co active sites and produce lattice distortions, which then promotes the catalysis of water splitting. The results showed that V-Co_xP@NC requires a small cell voltage of 1.54 V at current density of 10 mA/cm² to maintain the current density at 10 mA/cm² with slight fluctuation after the 20 h test (Figure 7B,C). Gopi et al.¹⁶⁶ reported that V-doped nickel-iron bimetallic nanoarrays (V-Ni_xFe_y-MOF@GO) supported by carbon materials exhibited excellent electrocatalytic performance for OER and HER in both acidic and alkaline media. For V-Ni_xFe_y-MOF@GO, the unique nanoparticles had been fabricated by carbonization of the as-prepared MOFs materials. The carbon in the structure can promote rapid ion diffusion, high conductivity and high catalytic activity, and V-doping can not only play a strong synergistic role with Ni/Fe, but also significantly increase the corrosion potential of electrocatalyst surface.

Figure 7. (A) A schematic illustration of V-CoxP@NC NPs fabrication. (B) LSV curves of the V-CoxP@NC | | V-CoxP@NC and 20% Pt/C | | RuO2 electrolytic system in alkaline solution. (C) The i–t curve of V-CoxP@NC NPs. Reproduced with permission: Copyright 2020, Elsevier B.V.165

Apart from water splitting, the studies of V-MOFs derivatives on ORR have also been widely studied. Zhang et al.¹⁷⁵ reported the preparation of the porous Co-V-N/NC nanocomposite, which constitutes cobalt-vanadium nitride (Co-V-N) confined in nitrogen-doped porous carbon (NC) (Figure 8A). In addition to various accessible active sites at the phase boundaries, Co₄N, VN, and NC derived from ZIF-67 are also conducive to achieve effective charge transfer and mass transport. In addition, the electrical conductivity of electrocatalysts can be improved due to the incorporation of metallic Co₄N and VN (Figure 8B). The as-prepared Co-V-N/NC electrocatalysts exhibited excellent ORR performance with a limiting current density of 6.1 mA/cm², and the onset and half-wave potentials of 0.98 and 0.85 V, respectively due to the strong synergistic coupling effect among multi-components (Figure 8C). Moreover, Co-V-N/NC nanocomposites were more durable and methanol-resistant compared to the commercial Pt/C catalyst (Figure 8D,E). Muthurasu et al.¹⁷⁶ reported the synthesis of iron-doped metal-organic framework assisted cobalt vanadate integrated with cobalt oxide (Fe doped MOF CoV@CoO nanoflakes) via a facile synthetic approach. This approach synthesized a unique nanoflakes heterostructure, an abundant porous structure, high specific surface area, as well as extremely large active sites. Interestingly, the resulting Fe doped MOF CoV@CoO nanoflakes can be used trifunctional electrocatalyst for energy conversion such as ORR, OER, and HER, which produced very low overpotential and Tafel slope values. Electrocatalytic test results show that two-electrode water-splitting devices can exhibit 1.53 V at the current density of 20 mA/cm². Besides, portable rechargeable zinc-air battery was further assembled by employing specifically designed multifunctional electrocatalysts that can attain open-circuit voltage of 1.456 V, a power density of 138 mW/cm³, and excellent cyclic stability, which is superior to Pt/C systems.

Figure 8. (A) A schematic illustration of the synthetic process of Co-V-N/NC. (B) Nyquist plots of the Co-V-N/NC and VN/NC recorded in O2-saturated 0.1 mol/L KOH, inset is the equivalent circuit. (C) LSV curves of Co-V-N/NC and commercial Pt/C in an O2-saturated 0.1 mol/L KOH with a scan rate of 10 mV/s and rotating speed of 1600 r/min. (D) Current-time (i-t) chronoamperometric responses of Co-V-N/NC and Pt/C obtained at −0.35 V (vs. Ag/AgCl) in O2-saturated 0.1 mol/L KOH. (E) i–t curves of Co-V-N/NC and Pt/C at −0.35 V before and after adding 3 mol/L methanol. Reproduced with permission: Copyright 2019, Elsevier B.V.175

Compared with V-MOFs electrocatalysts, V-MOFs derivatives have the advantage of improving the electrocatalytic stability and conductivity, mainly due to the generation of some inorganic nanoparticles such as metal oxides or alloys during pyrolysis, as well as some highly conductive carbon materials. However, it is worth noting that the heat treatment of V-MOFs often leads to the aggregation of metal atoms, reducing the number of active sites in the catalyst to some extent. Therefore, the precise control of pyrolysis conditions is particularly important for the application of V-MOFs derivatives.

ELECTROCHEMICAL ENERGY STORAGE

In recent years, electrochemical energy storage devices have achieved great success for the small portable electronic devices due to its environmental friendliness.¹⁶⁴ It is considered as one of the more promising ways to deliver clean energy sources to mitigate environmental issues and energy crisis.¹⁷⁷ To meet the market demand, energy storage devices are expected to exhibit higher power/energy density, longer service life, higher safety with lower manufacturing cost than before. Ever since the emergence of lithium-ion batteries, more efforts have been devoted to mitigating its inherent safety issues. The research of MOFs as electrode materials has garnered attention and developed rapidly in two aspects, one is that MOFs can be used directly as electrode materials by integrating electrochemical redox centers, and the other is MOFs derivatives with unique structures can be used in supercapacitors, and Li-based batteries (Li-ion, Li-S, and Li-O₂ batteries).⁵⁹ In this section, various V-MOFs and their derivatives for electrochemical energy storage will be segregated into these two aspects (Table 2).

Table 2 Comparison of the performance of V-MOFs and their derivatives for energy storage

[Table omitted. See PDF]

Abbreviations: GFDW, glass fiber drawing wastewater; MOF, metal-organic framework; NDC, 1,4-naphthalene dicarboxylates; SLS, sodium ligninsulfonate.

V-MOFs as electrode materials MOF-47 based materials

As an emerging energy storage device, supercapacitors have the advantages of large power density, fast charge and discharge rates and long cycle life.^186–188 Yan et al.¹⁸⁹ reported for the first time that MIL-47 was synthesized by a facile one-step solvothermal strategy and was used as an electrode material for supercapacitors. With that, it could exhibit relatively high capacity, long cycle life and high energy density. The MIL-47 synthesized by that mean yielded uniform rods with a length of 0.5−3 µm and a width of 200 nm, and it exhibited a large specific surface area of 116 m²/g with an average pore diameter of 3.6 nm. With that, it then promotes better diffusion of ions and electrolytes, thus improving the electrochemical performance (Figure 9A,B). The MIL-47 electrode exhibited a maximum specific capacitance of 572.1 F/g at current density of 0.5 A/g and 520.8 F/g at 1 A/g, yet still able to retain 92.8% of the capacity after 1000 cycles (Figure 9C,D). The probable reactions can be described as the equation of V^IV (O) (bdc) + xNa⁺ + xe⁻ ↔ Na_xV^IV (O) (bdc). Furthermore, both aqueous and solid-state asymmetric supercapacitors can be successfully assembled by rod-like MIL-47 and activated carbon. After 10,000 charge–discharge cycles, the specific capacitance of aqueous and solid-state devices can be maintained at 92.1% and 93.6%, respectively, which revealed outstanding cycling stability (Figure 9E,F). For the solid-state device, it exhibited a remarkable energy density of 6.72 mW·h/cm³ at a power density of 70.35 mW/cm³. The superior performance proved that V-MOFs electrodes have the potential to be used as active materials in supercapacitors.

Figure 9. (A) Field emission scanning electron microscopy (FE-SEM) and (B) high resolution transmission electron microscope (HRTEM) images of as-prepared MIL-47. (C) The galvanostatic charge-discharge curves of rod-like MIL-47 electrode at different current densities. (D) Charge–discharge cycling curve at a current density of 1 A/g in 1.0 mol/L Na2SO4 electrolyte. (E) Charge–discharge cycling curve of the rod-like V-MOF//activated carbon aqueous device at a current density of 1 A/g. (F) Charge-discharge cycling curve of the rod-like V-MOF//activated carbon solid-state device at a current density of 2 mA/cm2. Reproduced with permission: Copyright 2018, Wiley-VCH.189 V-MOF, vanadium metal-organic frameworks

In 2019, He et al.¹⁷⁸ have reported the direct growth of 3D conductive MIL-47 nanowire-bundle arrays on carbon nanotube fibers (CNTFs) via a time-dependent solvothermal reaction. The products were synthesized with varying reaction times of 12, 24, 48, and 60 h, and were denoted as V-MOF-12, V-MOF-24, V-MOF-48, and V-MOF-60, respectively (Figure 10A). It was then used as the cathode for Zn-ion (V-MOFs//Zn) batteries. Based on their experimental results, the optimized V-MOF-48 obtained the highest volumetric capacity of 101.8 mA·h/cm³ at a current density of 0.1 A/cm² and achieved the best rate capability in an aqueous electrolyte amongst the variants (Figure 10B,C). In this study system, the performance of all-solid-state fiber-shaped V-MOF//Zn battery assembled by poly(vinyl alcohol) (PVA) electrolyte has also been investigated (Figure 10D), it can deliver a high energy density of 17.4 mW·h/cm³ while maintaining a high power density of 1.46 W/cm³. The galvanostatic charge-discharge curves at various current densities show that the specific capacity in the PVA electrolyte is similar to that in the aqueous electrolyte at a low current density (0.1 A/cm²), and it still retained 56.9% of initial capacity when the current density was increased 50-fold (Figure 10E,F). In addition to the applications mentioned above, Ru et al.¹⁷⁹ recently have focused on the use of MIL-47 in conventional aqueous button batteries. The layered MIL-47 nanorods synthesized via one-step hydrothermal method are utilized as the cathode material for the aqueous MOF-Zn battery, which show high initial specific capacity of 320 mA·h/g at 0.1 A/g, as well as the desirable rate capability and cycling performance.

Figure 10. (A) An illustration of the preparation process for the V-MOF@CNTF with hierarchical nanowire structure. (B) SEM image of V-MOF-48. (C) The galvanostatic charge-discharge curves at a current density of 0.1 A/cm2 for V-MOF-12//Zn, V-MOF-24//Zn, V-MOF-48//Zn, and V-MOF-60//Zn. (D) An illustration of the all-solid-state fiber-shaped V-MOF-48//Zn battery. (E) Galvanostatic charge-discharge curves and (F) specific capacities of the fiber-shaped V-MOF-48//Zn battery at various current densities. Reproduced with permission: Copyright 2019, Elsevier B.V.178

MOF-100 based materials

As one of the core battery technologies in the post-lithium-ion era, Li-S batteries (LSBs) have attracted a lot of research interests due to their high theoretical capacity (≈1675 mA·h/g for S and ≈3860 mA·h/g for metallic Li) and energy density (≈2500 W·h/kg considering complete reaction to Li₂S) in recent years.^190,191 However, the shuttle effect of polysulfides in their batteries and the reduction of Li₂S or Li₂S₂ products hampered their practical application, thus great efforts have been made to develop advanced cathode materials to alleviate these problems.^192,193 When employed in Li-S batteries, MOFs can be either as host materials to host the sulfur and confine the polysulfides, or as separators to prevent the migration of polysulfides to anode side, which can facilitate electron transport and capture of soluble polysulfide.^194–196 Hou et al.¹⁸⁰ reported for the first time that mesoporous MIL-100(V) and MIL-100(V)/reduced graphene oxide (rGO) composites can be used as sulfur cathode hosts for Li-S batteries. In the crystal structure of MIL-100(V), there is also a small percentage of V⁴⁺ in addition to a large amount of V³⁺ in vanadyl groups, which provide different Lewis acid sites and form strong interactions with sulfur and lithium polysulfides (Figure 11A). MIL-100(V) and MIL-100(V)/rGO are grounded with sulfur in a certain proportion, respectively. The mixture is then heated to obtain S@MIL-100(V) and S@MIL-100(V)/rGO, both of which show excellent electrochemical performance for Li–S batteries. When tested at 0.1 C, the initial charge and discharge capacities of S@MIL-100(V) were 879 mA·h/g and 849 mA·h/g, respectively. In addition, the capacity was maintained at 550 mA·h/g after 200 cycles (Figure 10B, C). This proves that the diffusion of lithium polysulfides has been effectively controlled. For S@MIL-100(V)/rGO, most of the sulfur-containing MIL-100(V) particles are attached to reduced graphene oxide flakes (Figure 11D), and the reversible capacity was maintained at 650 mA·h/g after 75 cycles at a current density of 0.1 C (Figure 11E). While, the specific capacity of S@MIL-100(V)/rGO attenuated slowly when the current density was increased from 0.05 to 0.5 C, and the specific capacity at 0.5 C was still 570 mA·h/g (Figure 11F). With the aforementioned results, it shows that the sulfur loading and cycling stability can be improved when V-MOFs are applied as sulfur cathode hosts for Li–S batteries.

Figure 11. (A) Crystal structure of MIL-100(V) (vanadium, carbon, and oxygen atoms are depicted as green, gray, and red spheres, respectively). (B) Galvanostatic discharge/charge profiles of S@MIL-100(V). (C) Cycling performance of S@MIL-100(V) at a current of 0.1 C. (D) TEM image of S@MIL-100(V)/rGO. (E) Cycling performance of S@MIL-100(V)/rGO at a current of 0.1 C. (F) Rate performance of S@MIL-100(V)/rGO at 0.05, 0.1, 0.2, and 0.5 C. Reproduced with permission: Copyright 2017, Springer Nature Limited.180 TEM, transmission electron microscope

PBAs-based materials

The discovery of Prussian blue can be traced back to 1704.¹⁹⁷ Early work by Neff et al. in the 1980s demonstrated the exceptional electrochemical activity and ionic permeability of Prussian blue crystals, which has led to the widespread use of PBAs in batteries to date.^198,199 Although the excellent crystal structure and simple diffusion kinetics of PBAs, the capacity delivered in aqueous systems has been limited to less than about 60 mA·h/g. The electrochemical process of PBAs is mainly controlled by the electrochemical process of metal ions inserting/extracting the ions. So, the optimization of the transition metal ions combination is particularly important. Lee et al.¹⁸¹ prepared a novel PBAs (Na_0.4(VO)₃[Fe(CN)₆]₂·12H₂O, V/Fe PBAs) by a facile co-precipitation method for aqueous rechargeable batteries, where V and Fe ions are conductive to the multiple-electron energy storage process (Figure 12A). With that, the V/Fe PBAs have exhibited capacity of 91 mA/h under a current density of 110 mA/g, even the capacity of the 250th cycle was still at 55 mA/h (Figure 12B). In addition, with its open structure and 3D hydrogen bond network, the V/Fe PBAs exhibited a relatively high capacity of 54 mA·h/g under an extremely high current density of 3520 mA/g (Figure 12C). Additionally, the specific capacity also exhibited highly stable performances. Long-term cyclic stability results show that the capacity retention was 80% and 79% after 250 cycles for current densities of 880 and 1760 mA/g, respectively (Figure 12D). However, the capacity decreased more rapidly in the initial cycle due to the dissolution of vanadium ions. Inhibition of the dissolution and enhancement of reaction kinetics of V ions are effective measures to circumvent the dramatic electrochemical performance deterioration. Additionally, those measures can also improve the electrochemical performance of V/Fe PBA, such as the introduction of mixed conductor coating. With the similar open framework structures and huge interstitial sites to PBAs, a series of metal-organic phosphate open framework (MOPOF), A₂[(VO)₂(HPO₄)₂(C₂O₄)] (A = Li, Na, or K) as cathodes for hybrid LIBs^200,201 and KIBs²⁰² have also been widely reported by Vittal's and Chen's group. The diffusion channels for Li or other alkali ions can deliver highly reversible storage capacities, and it may give a new direction to design and develop cathode materials.

Figure 12. (A) Low-magnification TEM image of the cathodes synthesized by co-precipitation (with HCl). Inset is the crystallographic structure of V/Fe PBA (yellow, blue, green, red, and purple spheres represent Na, V, Fe, C, and N, respectively). (B) Potential profiles of the cathodes synthesized by co-precipitation (with HCl), evolving over 250 cycles (potential range: 0.45–1.15 V versus Ag/AgCl, charge/discharge current density: 110 mA/g). (C) Specific capacity of the charge (filled circle) and discharge (open circles) process for the V/Fe PBA cathodes measured under various charge/discharge current densities ranging from 55 to 3520 mA/g. (D) Discharge capacity and Coulombic efficiency as a function of the cycle number with a charge/discharge current density of 880 and 1760 mA/g, respectively. Reproduced with permission: Copyright 2017, Wiley-VCH.181 PBA, Prussian blue analogues; TEM, transmission electron microscope

As mentioned above, V-MOFs as electrode materials can improve electrochemical energy storage performance in three aspects: (1) The high specific surface area of V-MOFs can improve ion transport property during energy storage processes; (2) Its open-framework crystal structure can also accommodate appropriate space for ion storage and electrochemical reactions; and (3) Vanadium ions with different valance states can provide various Lewis acid sites and form stronger chemical interactions with guest ions.

V-MOFs derivatives as electrode materials

In general, the direct usage of V-MOFs as electrode materials was rarely studied due to the poor electrical conductivity and their complicated energy storage mechanisms of MOFs. For MOFs, there are mainly two types of energy storage: pore reversible energy storage and reversible chemical reaction energy storage. For LIBs, the reversible de-intercalation of lithium ion in pores of MOFs belongs to the mechanism of pore reversible lithium storage, while the organic ligands in the MOFs structure provide insertion sites for lithium ions.²⁰³ It is noteworthy that the structure of MOFs would dissolve or collapses after the process of charging and discharging, the resulting solid then (which may be vanadium oxide or carbon) continues the subsequent lithium storage process. The lithium storage mechanism is then transformed into reversible chemical reaction, which refers to the occurrence of reversible chemical reaction at the electrode, and lithium ion replaces vanadium ion to form new vanadium-based compounds. This energy storage mechanism has inspired researchers to continuously develop and exploit the precursor V-MOFs, especially the MOFs derivatives in energy storage has received widespread attention.

Vanadium oxide/vanadium oxide@C

When V-MOFs are used as precursors, the self-templating method is proved to be a convenient and effective way to achieve the conversion of V-MOFs to vanadium oxides. Among the plentiful vanadium oxides, vanadium sesquioxide (V₂O₃) shows typical corundum-type structures with a spatially symmetric of R3̅c, the oxygen atoms occupy the six vertices of the vanadium atom centered octahedron, and every two VO₆ octahedrons share an edge, thus producing a special pore and a suitable pore diameter distribution. The resulting features then help in accelerating the de-intercalation of ions and improve the rate capability. In addition, the porous carbon derived from organic ligands coated on the surface can further improve the cycle life of the electrode material. Therefore, the carbon-coated V₂O₃ (V₂O₃@C) derived from V-MOFs has attracted much attention of many researchers due to the advantages in composition and structure.^146–150

When MIL-88B(V) was used as precursor, the porous shuttle-like vanadium oxides (V₂O₅, V₂O₃/C) were successfully prepared through a particular calcining process, in which V₂O₅ and V₂O₃/C were obtained by calcination in air and Ar atmosphere, respectively (Figure 13A).¹⁴⁶ In terms of morphology, both V₂O₅ and V₂O₃/C retained a prismatic structure like MIL-88B(V), whereas abundant micropores were formed during calcination. V₂O₅ and V₂O₃/C exhibited hollow structure and porous structure, respectively (Figure 13B,C). The in-situ carbon layers can not only enhance the electronic conductivity of vanadium oxides, but also allow full electrolyte penetration and rapid metal ion transport. In Figure 13D, V₂O₃/C exhibited excellent electrochemical properties when used as an anode material for SIBs. The specific discharge capacity can reach 181 mA·h/g at 2000 mA/g, and the capacity can still be retained at 133 mA·h/g, corresponding to a fading rate of 0.032% per cycle after 1000 cycles. Density functional theory (DFT) calculation results indicated that Na ions preferred to form Na−O chemical bond with O atoms, accompanied by obvious charge transfer. It can also be found that its good conductivity can be maintained after the insertion of Na ions (Figure 13E,F).¹⁴⁶

Figure 13. (A) An illustration of preparation process for the shuttle-like porous V2O3/C and V2O5. TEM images of the shuttle-like (B) V2O5 and (C) V2O3/C. (D) The high-rate (2000 mA/g) cycling performance and corresponding Coulombic efficiency of V2O3/C. (E) Charge-density differences of V2O3 with one Na atom inserted. (F) Calculated density of states (DOS) for V2O3 after Na ion insertion. Reproduced with permission: Copyright 2018, Springer Nature Limited.146 TEM, transmission electron microscope

Kong et al.¹⁸² explored the effect of the introduction of ammonia treatment on vanadium matrix composites and their electrochemical properties in sodium ion capacitors (SICs). V₂O₃/N doped porous carbon (VNC-300) maintained the ultra-small size of V₂O₃ particles (~2.1 nm) in the process of low temperature nitridation, which was an excellent cathode material for sodium storage, and the optimized V₂O₃/N-doped porous carbon nanorods exhibited high specific capacity and excellent stability for sodium storage (Figure 14A). Most importantly, the SICs device based on active carbon as positive electrode and VNC-300 as negative electrode exhibited a good rate performance. The energy density values were from 63 to 38 W·h/kg at various current densities from 50 to 5.0 mA/g, and it exhibited an excellent energy density of 38.7 W·h/kg at a power density of 5805 W/kg (fast charge and discharge in 25 s) (Figure 14B). In 2018, this group also prepared vanadium oxide/porous carbon nanorods (VO_x/PCs) with oriented 2D pores by utilizing the thermally induced bond instability. The unstable bond between V and the coordination water was broken and a lot of slit-like pores were formed perpendicular to the direction of crystal growth (Figure 14C,D).¹⁵⁰ When used as anode materials for sodium storage, it displayed a discharge capacity of 481.5 mA·h/g at the low current density of 0.04 A/g and delivered the prominent capacity of 93.8 mA·h/g at the high rate of 10.0 A/g (Figure 14E). The results show that composites with hierarchical porous architectures contribute to an increase in reactive surface area, thus improving rate capability and cycling life of sodium storage. Most importantly, it is relatively easy to construct laminated composites from MOFs, which provides new ideas for the design and preparation of high-performance electrode materials.

Figure 14. (A) An illustration of the structural change process of VNC-p via ammonia treatment. (B) Cycling performance of VNC-300//AC device at 1.0 A/g (inset: rate capability at various current densities). Reproduced with permission: Copyright 2018, Elsevier B.V.182 (C) An illustration of the formation of hierarchical VOx/PCs. (D) TEM images showing structural changes at VM-500 °C, VM-600 °C, and VM-700 °C. (E) Rate performance of VM-600 °C and the disordered contrast. Inset: TEM images of VM-600 °C and the disordered contrast, VM stands for vanadium-based MOF with the structure [V(OH)(NDC)·xG]n (G = guest molecules, NDC = 1,4-naphthalene dicarbo-xylates). Reproduced with permission: Copyright 2018, Wiley-VCH.150 TEM, transmission electron microscope

V-MOFs derived V₂O₃/C and V₂O₅/C have also been reported as intercalation cathode for aqueous zinc-ion batteries (ZIBs). These V-MOFs derivatives have unique channel, appropriate pore size distribution and abundant valence states that can shorten the ion transport distance which in turn improves the conductivity, equates to good electrochemical performance. Interestingly enough, MIL-88B(V) derived crystalline c-V₂O₃@C can be further converted into the composite of amorphous V₂O₅ and conductive carbon (a-V₂O₅@C) for aqueous ZIBs by electrochemical oxidation strategy (Figure 15A).¹⁴⁹ Although a-V₂O₅@C showed a similar morphology to c-V₂O₃@C, the V₂O₃ nanoparticles smash to amorphous V₂O₅ during the electrochemical induction process. With amorphous V₂O₅ and conductive carbon, a-V₂O₅@C possessed more active sites and offered more isotropic zinc ion diffusion routes, yielding fast speed of zinc ion transport and high discharge capacity (Figure 15B). The discharge capacity of a-V₂O₅@C reached 620.2 mA·h/g at 0.3 A/g, and Zn/a-V₂O₅@C batteries displayed a superior rate performance that maintained at 72.8 mA·h/g at a high current density of 200 A/g (2571 C) and excellent cycling performance of 20000 cycles (91.4% capacity retention) at 40.0 A/g (Figure 15C,D). Furthermore, the flexible soft-packaged Zn/a-V₂O₅@C batteries have also been studied and have shown to be a great prospect for the practical application of flexible energy storage devices.

Figure 15. (A) Illustration of the fabrication process of vanadium oxide by calcination of MIL-88(V) precursor. (B) Illustration of Zn2+ diffusion and Zn2+ (de)intercalation energy in crystalline and amorphous V2O5 from first-principles calculations. Rate performance (C) and long-term cycle test (D) of a-V2O5@C. Reproduced with permission: Copyright 2020, Wiley-VCH149

Except for SIBs and ZIBs,^{146,148–150,182,204} the V-MOFs derived V₂O₅ and V₂O₃@C for LIBs or LSBs have also been carefully studied. Different temperatures in air atmosphere (400 °C, 450 °C, 500 °C, 550 °C, 600 °C, 650 °C) have considerable influence on the morphology of the target product V₂O₅, and the size of the product tends to decrease with the increase of temperature.¹⁴⁷ Smaller materials with larger specific surface areas can be obtained at an appropriate temperature, thus increasing the contact area between the material and the electrolyte, which was conducive to the de-intercalation of lithium ions. The results of charge-discharge cycle showed that the performance of the samples calcined at 600 °C was better than that at other temperatures. However, the insufficiency was that the capacity gradually decreased within 100 cycles, it may be due to the gradual decrease of V₂O₅ during the cycle which hindered the de-intercalation of lithium ions and the adverse reactions of electrode materials and electrolytes. Yang et al.¹⁸³ developed a synthetic approach to prepare V₂O₃@C hollow micro-cuboid with hierarchical lasagna-like structure through calcining MIL-47 precursor, and then the V₂O₃@C/S composite was further synthesized by mechanically mixed V₂O₃@C and sulfur S. When used as a cathode material for LSBs, it showed superior cycling stability (62.3% after 1000 cycles at 1 C) and rate performance (665.6 mA·h/g at 2 C) at a relatively high sulfur loading of 3.7 mg/cm².

Vanadate (mixed metal oxides [MMOs])

Transition metal oxides (TMOs) and MMOs as cathode materials for batteries have attracted much attention in recent years due to their high theoretical specific capacity, low price, good stability, as well as the synergistic influence of metal ions.^205–207 MMOs prepared by oxidative calcining of MOFs possess the superiorities of high energy density, high economic efficiency, and simple synthesis, greatly promoting the application in electrochemistry. In the past few years, some vanadium-centric, non-stoichiometric MMOs have shown potential applications in batteries, photocatalysis, and hydrogen storage.^208,209

Soundharrajan et al.¹⁵² successfully synthesized cobalt vanadate (Co₃V₂O₈) through pyrolysis of Co-V-MOFs network, which showed the well-defined sponge-like network morphology (Figure 16A). With the unique nanostructured morphology, the as-prepared Co₃V₂O₈ exhibited good cyclability of 700 cycles and specific capacity of 501 mA·h/g, respectively, at a current density 1000 mA/g (Figure 16B). While, in terms of stability, it can still retain an average capacity of 430 mA·h/g with Coulombic efficiency reaching close to 100% at the maximum current density of 3200 mA/g (Figure 16C). The research group also reported the preparation of Co₃V₂O₈ microspheres assembled with 20−50 nm particles from Co-V-MOFs precursor by one-pot method, which exhibited a higher discharge capacity and excellent reversible capacity for LIBs due to the synergistic effects between the metallic species.¹⁵⁴ Unlike Co-V-MOFs, nanoparticles were in situ generated and aggregated on the surface of the microspheres during the calcination process, and the large number of voids confirmed the porous structure of Co₃V₂O₈ (Figure 16D). When used as electrode material, it showed excellent cycling performance and rate performances under high current density (showing the specific discharge capacity of 940 mA·h/g after 100 cycles at 1 A/g with the Coulombic efficiency of nearly 100%) (Figure 16E). This study introduces a novel, typical and simple method for directly producing nanostructured materials in just a few minutes, similar to microwave digestion. Except for cobalt vanadate, nickel vanadate has attracted much attention due to the similar electrochemical properties.¹⁵³ The chemical kinship (ionic size, electronic structure, etc.) between nickel and cobalt implies that the two vanadate can be synthesized using the versatile methods. For instance, Sambandam et al.^210,211 also reported two MOF-derived Ni₃V₂O₈ used as excellent anode material for high-energy lithium-ion batteries.

Figure 16. (A) Scheme representation of Co3V2O8 sponge-like network formation obtained through Co-V-MOF intermediate and Ostwald ripening processes. (B) Cyclability curve for the Co3V2O8 at high current density of 1000 mA/g. (C) Rate performance of Co3V2O8 at various current densities. Reproduced with permission: Copyright 2016, American Chemical Society.153 SEM images (D) and corresponding cyclability (E) of Co3V2O8. Reproduced with permission: Copyright 2016, Royal Society of Chemistry.154 SEM, scanning electron microscope

From the literature discussed above, it can be concluded that polymetallic oxides derived from MOF materials have different morphological characteristics, and the synergistic effect amongst different metals promotes the energy storage capacity and improves the capacity, stability of electrode materials and high electronic conductivity.

Vanadium phosphate

M₃V₂(PO₄)₃ (M = Li or Na) is an excellent cathode material that has attracted much attention in recent years in both lithium-ion batteries and sodium-ion batteries. For instance, Li₃V₂(PO₄)₃ (LVP) has high average potential (4.0 V) and specific capacitance (197 mA·h/g), as well as benign ion mobility and excellent thermal stability.²¹² Unfortunately, the critical defect of LVP proves its exceedingly poor intrinsic electron transfer efficiency, which severely limits its high rate of charge and discharge and long circulation performance.²¹³ To improve the electrical conductivity of electrode materials, carbon from various sources is widely used in the coating of electrode materials, such as starch, polyvinylidene difluoride (PVDF), glucose, citric acid, and so on.²¹⁴ Interestingly, under controlled pyrolysis conditions, MOFs precursors provide significant carbon sources while being used to synthesize LVP.

Wang et al.¹⁸⁴ prepared Li₃V₂(PO₄)₃/phosphorus-doped carbon (LVP/P-C) nanocomposites with multilevel structures via one-pot in-situ synthesis using hybrid V-MOFs as precursor. It is worth noting in this study that glass fiber drawing wastewater (GFDW) containing starch, hydroquinone, formaldehyde and various surfactants was used as ligands in the synthesis of hybrid V-MOFs, and the soft templates in GFDW played a role in synthesizing the LVP/P-C nanocomposites with multilevel structures (Figure 17A–C). Compared with LVP and LVP/C, the best performance was achieved by LVP/P-C, leading to a discharge specific capacity of 138 and 65 mA·h/g at 0.1 and 10 C, respectively, and its capacity retention can reach 90% after 1100 cycles (Figure 17D). Most importantly, the waste utilization of GFDW not only improved the performance of lithium-ion batteries, but also alleviated its impact on the environment to a certain extent. MIL-101(V) with molecular formula of [V₃O(BDC)₃(H₂O)₂Cl_0.7(HBDC)_0.3] ·2H₂O·0.5EtOH can also be used as both carbon source and vanadium source to prepare the carbon-coated Li₃V₂(PO₄)₃ composite material (LVP/M-101).¹⁵¹ In Figure 17E,F, it depicts that the as-prepared LVP/M-101 has laminated structures with some hollow bubbles inside and the residual carbon outside. Compared with LVP prepared using V₂O₅, LVP@M-101 electrode exhibited superior behavior at various current densities from 0.1 to 10 C, and the high capacity of 126.6 mA·h/g was recovered when the current density was reversed back to 0.1 C, indicating the stability of the MOFs derived structure (Figure 17G).

Figure 17. Possible synthesis mechanism of the LVP/P-C nanocomposite with different morphologies. Formation schematic diagrams of (A) flower-like structure, (B) spheroidal foam structures and (C) prism structures. (D) Capacity retention and Coulombic efficiency of the LVP/P-C-3 sample for 1100 cycles at 10 C. Reproduced with permission: Copyright 2017, Elsevier B.V.184 (E-F) TEM images of LVP@M-101. (G) Rate capabilities of LVP@M-101 and LVP electrode at different current densities from 0.1 C to 10 C in the range of 3.0−4.8 V versus Li/Li+. Reproduced with permission: Copyright 2018, Elsevier B.V.151

Similar to LVP, Na₃V₂(PO₄)₃ (NVP) is also considered as a very promising cathode material in SIBs due to its excellent cycling life and rate performances.^215–217 NVP not only provides an effective space and path for the storage and transport of Na⁺ and electrons, but also can effectively restrain the volume change and prevent the agglomeration of electrode materials in the processes of charge and discharge. A new Na₃V₂(PO₄)₃ ultrathin sheet/N-doping hard carbon ultrathin sheet (NVPUS/NHCUS) with stable sandwich framework was synthesized by using V-MOFs as precursors, and the multifunctional sodium ligninsulfonate (SLS) was both an organic ligand of V-MOFs and a carbon source (Figure 18A).¹⁸⁵ Thanks to the collective and synergistic effects of electrochemical reactions at the nanoscale, the optimized NVPUS/NHCUS nanocomposite cathode shows the excellent electrochemical property (117.3 mA·h/g at the 0.1 C and remains 40 mA·h/g at 200 C after 6000 cycles) (Figure 18B,C). However, for traditional NVP spheroidal particle it has a relatively poor electron/sodium transport pathway as the phase transition reaction only takes place on the outer surface of the particle. In contrast, the NVPUS/NHCUS shows a highly efficient electron/sodium transport pathway for rapid insertion of sodium ions and provides a large electrode/electrolyte interface area as well (Figure 18D).

Figure 18. (A) Schematic diagram of the synthesis of NVPUS/NHCUS with framework structure derived from polymer sodium lignosulfonate. (B) Rate capabilities of samples from 0.1 to 250 C. (C) Long-term cycling stability and Coulombic efficiency of NVPUS/NHCUS cathode for 6000 cycles at 200 C. (D) Schematic representation exhibiting the high-efficiency electron/Na ion transport pathway for fast sodium insertion. Reproduced with permission: Copyright 2018, Wiley-VCH.185 NVPUS/NHCUS, Na3V2(PO4)3 ultrathin sheet/N-doping hard carbon ultrathin sheet

CONCLUSIONS AND PERSPECTIVES

In conclusion, this paper reviewed the structural properties and the electrochemical energy conversion and storage applications, including electrocatalysis, supercapacitors and rechargeable batteries, of various V-MOFs and their derivatives. V-MOFs have attracted extensive attention in these fields due to the porous structure and various redox behaviors caused by abundant chemical valences of V-containing nodes. Furthermore, a variety of V-MOF derived materials have also been used as electrocatalysts or electrodes. V-MOFs derivatives with good electrochemical performances can be obtained by choosing the appropriate V-MOFs precursor and controlling the temperature and atmosphere of calcination. During the calcination process, the porous structure and channels are maintained for electrolyte penetration and electron transport by the removal of organic ligands, and the carbon layer formed in situ under an inert atmosphere greatly increased the conductivity and stability of catalyst or electrode material. In addition, the introduction of heteroatoms (O, C, P, etc.) can also provide more active sites, thus improving the electrochemical activity of materials.

Some promising development strategies of V-MOFs and their derivatives for energy conversion and storage applications have also been represented in the following aspects: (1) V-MOFs can be combined with functional materials (graphene, metal nanoparticles, and functional molecules) to form multifunctional composites with improved electrochemical properties through the synergistic effects; (2) Precise control of MOFs growth can also be used to design and synthesize V-MOFs nanostructures, such as recrystallization, surface-control, and size-confinement; (3) For V-MOFs derivatives, the doping of heteroatoms can be effective to achieve high electrical conductivity and high affinity to adsorb intermediates, and porous carbon skeletons with various doping heteroatoms can be readily obtained by pyrolysis of MOFs with wisely selected organic ligands and encapsulated additives under specific atmosphere such as NH₃, PH₃, and H₂S gases. Although optimistic results have been achieved, research on the application of V-MOFs and its derivatives in energy-related fields deserves further study:

• (1)

  Despite the rich physical and chemical properties of V-MOFs materials, the coordination conditions of vanadium atoms with various organic ligands are relatively strict, which makes periodic V-MOFs materials rarely reported. Therefore, the exploration of V-MOFs that are easier to be prepared will encourage us to develop more vanadium-based materials.

• (2)

  Although various V-MOF derivatives show excellent electrochemical properties, few synthesis strategies have been reported. Thermal treatment is the most widely used preparation method, which is faced with high energy consumption, easy to produce greenhouse gases and harmful gases, structure collapse at high temperature, difficult to accurately control, as well as low yield. Therefore, the green, efficient, and simple strategies of transforming V-MOFs into inorganic nanomaterials are extremely necessary. More importantly, size, morphology and yield of V-MOFs derivatives also need to be precisely controlled.

• (3)

  The open framework and porous structure of MOF have great potential to solve the problems of low capacity or poor cyclic stability of traditional electrode materials when large radius ions (potassium ions) enter the lattice structure. However, as far as the current research results are concerned, there are only a few reports on V-MOFs in potassium ion batteries (PIBs).

Finally, V-MOFs and their derived materials play an increasingly important role in the development of various electrochemical devices. Although some challenges remain, the rapid growth of V-MOFs in recent years has paved the way for the development of more and more novel functional materials for energy conversion and storage.

ACKNOWLEDGMENTS

J. Z., and X. Y. C., contributed equally to this study. The authors gratefully acknowledge the National Natural Science Foundation of China (Nos. 51972067, 22001021, 51802044, 51902062 and 51802043), Guangdong Natural Science Funds for Distinguished Young Scholar (No. 2019B151502039), Natural Science Foundation of Jiangsu Province (No. BK20201048), Natural Science Research Project of Higher Education Institutions in Jiangsu Province (No. 20KJB150008), Singapore MOE AcRF Tier 1 (No. 2020-T1-001-031), the Open Fund of State Key Laboratory of Tea Plant Biology and Utilization (No. SKLTOF20190119), and the National Research Foundation of Singapore (NRF) Investigatorship (No. NRF2016NRF-NRFI001-22).

CONFLICTS OF INTEREST

The authors declare no conflicts of interest.

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.