Modified Metal−Organic Frameworks for Electrochemical Applications

Metal−organic frameworks (MOFs) are neo‐type porous materials synthesized via organic ligands and metal ions, which have drawn much attention due to their unparalleled advantages such as high specific surface area, large and clear pore structures, and uniformly distributed active sites. In these years, modified MOFs are regarded as kinds of materials with excellent electrochemical properties, that overcome poor conductivity and stability of original MOFs and narrow the gap between the basic science of MOFs and their future applications. At the same time, it also provides an opportunity to elaborate the synergistic effect of the synthesis strategy of modified MOFs on the performance. This review focuses on the synthesis, structure characterization, and electrochemical adhibition of modified MOFs and discusses the challenges and application prospects of modified MOFs in energy storage and conversion.


Introduction
The ever-growing energy dilemma and subsequent environmental pollution have stimulated global efforts to develop clean and renewable energy resources. [1] Among the next-generation energy technologies, rechargeable batteries (i.e., lithium-oxygen batteries (LiÀO 2 ), sodium-ion batteries (SIBs), and lithium-ion batteries (LIBs)), supercapacitors (SCs), and fuel cells show excellent potential for electrochemical energy storage, [2][3][4][5][6] and water electrolysis is a representative electrochemical energy conversion system. As for distribution of active sites. [40][41][42][43][44][45][46][47][48][49] Recently, MOF-based materials have been proved to be more competitive than other porous materials as electroactive materials in electrochemical energy storage and conversion due to their high specific surface areas, huge and clear void structures, and uniform distribution of active sites. [50][51][52][53] According to the reported literature, MOF-based materials are mainly segmented into three grasshoppers, including pristine MOFs, MOF composites (e.g., metal nanoparticles [MNPs]@MOFs, MOFs/conductive substrates), and MOF-derived materials (e.g., metal oxides/nitrides/sulfides/phosphides). Thereinto, although pristine MOFs have attracted considerable interest as electroactive materials, their poor electrical conductivity and insufficient chemical stability severely impede their electrochemical applications. Nevertheless, the structural and compositional features enable MOFs to be versatile hosts, precursors, or sacrificial templates for designing MOF composites and MOF derivatives. What's more, the synthesis of these materials can be realized easily via combining MOFs with other advanced functional materials (e.g., conductive substrates and MNPs) and converting MOFs/MOF composites into inorganic micro-/ nanomaterials by direct annealing or indirect post-treatments. These strategies not only avoid the modified of pristine MOFs, but also introduce the advantages of advanced functional materials into the composites. [54][55][56][57] Meanwhile, the obtained modified MOFs often possess high specific surface area, favorable electrical conductivity, and superior chemical stability, which are conductive to improving the electrochemical process in specific energy technologies. Modified MOF means without destroying itself, its own structure under the condition of the MOF's skeleton structure does not change, for a series of processing, including but not limited to composite and other functional materials (such as carbon material, MNPs, metal oxides, CPs, etc.), thermal conversion and partial derivatives, etc. The advantages of modified MOF materials add other advantages, such as larger specific surface area, superior chemical stability, and so on.
Under these guidelines, considerable MOF-based electroactive materials have been exploited and summarized in these years. Nevertheless, few reviews concentrated on summarizing the design of highly active MOF-based materials on the premise of preserving MOF structures. In recent years, many articles have introduced the excellent electrochemical performance of modified MOF materials. The modified MOF materials still retain the advantages of the MOF base materials such as high specific surface area, large and clear pore structure, uniform distribution of active sites, as well as good electrical conductivity and excellent chemical stability. These advantages make modified MOF materials have good electrochemical application potential, but there are few reviews in this area. Therefore, a comprehensive review on modified MOFs for electrochemical applications is noteworthy, which will provide insightful guidance and inspiration for the exploitation of modified MOFs and their applications in energyrelated technologies. Herein, this review outlines the recent advances of modified MOFs, including their synthesis, structural characterization, and electrochemical applications, putting emphasis on the synthetic strategy of modified MOFs and their synergistic effects on their performance. Finally, the outlook and perspective of modified MOFs in energy-related technologies are presented, in the hope that this review will inspire further investigation of modified MOFs toward energy storage and conversion (Scheme 1).

Synthesis Methods for Modified MOFs
Over the past decades, several advanced methods have been developed to synthesize modified MOFs. Herein, we will summarize the synthesis strategies of modified MOFs, including the design of MOF composites, thermal transformation, and partial derivatization.

Synthesis Methods for MOF Composites
The controlled integration of MOFs with functional materials has induced brand new multifunctional composites that show splitnew characteristics betony individual components. MOF composites are composed of MOFs and one or more materials with different constituents, including but not limited to different MOFs, [58][59][60] metalÀNPs, [61][62][63] metal oxides, [64][65][66] and conductive carbon materials. [67][68][69] The integration combines the advantages of MOFs and functional materials, such as the structural adaptability and flexibility, high porosity and ordered structure of MOFs, and the unique advantages of the introduced functional materials. It is expected that the obtained MOF composites can exhibit enhanced electrochemical capability. [70] 2.1.

Carbon-Based Materials@MOFs
Carbon has a variety of allotropes, such as graphite, nanotubes, diamonds, etc. The types of carbon materials are more diverse, like graphene, carbon cloth (CC), carbon nanotubes, and so on. [71,72] Carbon can exist in dimensions from 0D to 3D in various forms (e.g., particles, fibers, foams, fabrics, and composites). Taking graphene and carbon nanotubes as examples, carbon materials are held to be the most valuable structural fillers in MOF composites due to their great tensile strength, superlight weight, and distinguished chemical and thermal stability. [73,74] So far, composites of carbon-based materials and MOFs have been extensively used in many fields. [75][76][77][78][79] Graphene oxide (GO) is ionic conductive, and its incorporation can improve the ionic conductivity of most MOFs.  [80] Copyright 2013, Wiley-VCH. j) The synthetic illustration of ZnCo-ZIF@GO; k) TEM and l)SEM images of ZnCo-ZIF@GO. Reproduced with permission. [81] Copyright 2020, The Royal Society of Chemistry. m) Preparation course of PANI-ZIF-67-CC; SEM of n) the CC fibers, o) ZIF-67-CC, and p) PANI-ZIF-67-CC. Reproduced with permission. [82] Copyright 2015, American Chemical Society.
Loh's group combined Cu-MOFs with GO via hydrothermal methods, in which the 3D Cu-MOF was synthesized using paddle-wheel Cu 2 (COO) 4 (ted) 2 as the secondary building unit (SBU) (Figure 1a-c). [80] The group mixed different mass fractions of GO with Cu-MOF precursors. The morphologies of Cu-MOF, GO, and (GO X wt%, X ¼ 2, 4, or 8) Cu-MOF were demonstrated through scanning electron microscopy (SEM), which revealed that with the increase in GO concentration, the morphology of Cu-MOF changed significantly (Figure 1di). Especially, when 8 wt% GO was added to the Cu-MOF precursor, a cubic symmetry distortion displayed on the space group from P4/ncc (No. 130) to P4/mbm (No. 127). Wang's team grew the bimetal mixed ZnCo-ZIFs (mole ratio of Co to Zn is 4/6) on GO nanosheets via the in situ growth method to obtain ZnCo-ZIFs@GO (Figure 1j). [81] Transmission electron microscopy (TEM) and SEM images demonstrated that ZnCo-ZIF@GO showed well-defined ZIF-67 rhombohedral dodecahedrons and were dispersed on GO nanosheets uniformly, with a medial dimension of about 60 nm (Figure 1k-l).
In addition to combining graphene with MOFs to enhance the composites' ionic conductivity and thus improve their electrochemical performance, porous electrodes were constructed by pouring MOFs onto a CC with the assistance of CP. Wang and co-workers developed a two-step process to fabricate PANI-ZIF-67-CC electrode, (Figure 1m), in which a slurry containing ZIF-67, conductive super P, polyvinylidene fluoride (PVDF), and NMP was cast on CC to obtain ZIF-67-CC and then interconnected by conductive polyaniline (PANI) via electropolymerization. [82] The fabrication process did not change the MOF structure, and ZIF-67 crystals with a medial dimension of about 300 nm were distributed on the surface of CC fiber uniformly (Figure 1n,o). After electropolymerization of aniline, the ZIF-67 crystal's external surface displayed evident morphology changes. The formed PANI chains were coated on the surface of ZIF and served as bridges between the isolated MOF crystals ( Figure 1p).

Metal Oxides@MOFs
Metal oxides have been investigated in electrochemical energy storage and conversion extensively, because of the controllable shape, size, crystallinity, and functionality. [83][84][85][86][87][88][89][90][91] Nonetheless, their adhibitions are usually impeded via the low superficial areas and high surface energies. To further improve the function of metal oxides, hybridizing them into MOFs with large surface areas and well-organized pores is a promising approach, [92] which not only enhances the surface areas of metal oxides, but also integrates the advantages of both MOFs and metal oxides, so as to comprehensively improve the performance of the composites.
Xu and co-workers synthesized high-alkaline and stable metal oxide@MOF materials (Co 3 O 4 @Co-MOF) through a steerable one-pot hydrothermal method using ptcda (perylene-3,4,9,10tetracarboxylic dianhydride, C 24 H 8 O 6 ) as ligand and Co 2þ as the metal nodes under the pH range of 11À13 ( Figure 2a). As shown in SEM and TEM graphics, Co 3 O 4 @Co-MOF was leaf like (Figure 2b), and Co 3 O 4 nanocrystals with a medial dimension of 50 nm were evenly scattered on each side of Co-MOF (Figure 2c,d). [93] Keggin-type POM is a distinct polymetallic oxygen cluster, in which three or more transition metalÀoxygen anions share an oxygen atom, forming a closed 3D structure. Owing to the high polarity of the surface, Keggin-type POM is regarded as a potential multifunctional material. Pang and co-workers successfully introduced a typical Keggin-type POM (i.e., H 3 PW 13 O 40 ) to coat with ZIF-67 and then prepared the yolk/shell structure ZIF-67@POM hybrids (Figure 2e). [94] The POM was dispersed on the exterior surface of ZIF-67 evenly. It can be found in SEM images that the primitive ZIF-67 displays a typical dodecahedron shape with a mean dimension of around 500 nm (Figure 2f ), while 6-ZIF-67@POM (6 represents the time value in hours of the synthetic process) is irregular dodecahedron and has an average size of about 250 nm (Figure 2g). The inimitable yolk/shell structure of 6-ZIF-67@POM was further proved by TEM (Figure 2h).  [94] Copyright 2019, American Chemical Society.

Metal nanoparticles@MOFs
MNPs have drawn extensive attention in many fields, because of their special electronic structure and physiochemical features. However, MNPs have a high superficial area to bulk factor, and the excellent exterior energy usually brings about MNPs' aggregation and fuse, which is adverse to maximizing their activities in specific applications. [95][96][97] Assembling MNPs into porous materials (e.g., zeolites, mesoporous carbon, and metal oxides) could significantly suppress the accumulation of MNPs and then stabilize MNPs in a confined space. [98] MOFs, as new porous materials which possess a large superficial area and well-organized pores and cavities, can serve as compatible supports to incorporate with MNPs to maximize their advantages.
Pang et al. successfully loaded Au NPs on ZIF-67 through a bottle-around-ship tactics, obtaining Au@ZIF-67 ( Figure 3a). Thereinto, the constraint of ZIF-67 effectively suppressed the aggregation of Au NPs and then induced their uniform distribution. The synergetic effect between Au NPs and porous ZIF-67 in Au@ZIF-67 deeply enhanced their individual advantage and then improved the electrocatalytic activity. Even conducting thermal treatment on Au@ZIF-67, the Au NPs still dispersed homogeneously on the pyrolyzed ZIF-67, as presented in Figure 3b,c. [99] Xu's group reported a composite (AuNRs@ZIF-8) synthesized via nucleus coalescence or epitaxial growth, in which the highly dispersed gold nanorods were encapsulated inside ZIF-8 with flying colors (Figure 3d). [100] The SEM image exhibited that the surface of the monodispersed ZIF-8 particles is smooth (Figure 3e). The TEM image clearly showed that the synthesized ZIF-8 had a polyhedral morphology with a mean dimension of 230 nm (Figure 3f ). With the addition of AuNRs, the particle size range of AuNRs@ZIF-8 increased to about 400 nm, and the shape of the sample became atactic, along with the appearance of distinct dark nanorods, making clear the formation of coreÀshell nanostructures ( Figure 3g). As displayed in the inset of Figure 3h, the medial length of AuNRs' core was around 160 nm, and the width radius was 36 nm.
conductivity. [101] CPs are usually prepared by chemical/electrochemical polymerizations of monomer. [102,103] In view of the unique features of CPs, such as high thermal and chemical stability, high mechanical flexibility, and low density, the combination of CPs and MOFs is expected to produce new materials with high conductivities and activities.
Chen's group first incorporated conductive polypyrrole (PPY) into bimetallic organic framework (Zn/Ni-MOF) via an "oxidantfree additive" to construct SC electrodes (Zn/Ni-MOF@PPY) ( Figure 4a). [104] In Zn/Ni-MOF, the terephthalic acid and water molecules were coordinated with central metal Ni/Zn (Figure 4b). During the process of constructing Zn/ Ni-MOF@PPY, Zn/Ni-MOF assisted the formation of PPY owing to the characteristics of Lewis acid, while PPY increased the interlamellar spacing of Zn/Ni-MOF and facilitated the charge transfer. What's more, the incorporation of PPY did not alter the flower structure of Zn/Ni-MOF, except that the surface of Zn/Ni-MOF@PPY became rougher (Figure 4c,d).
Duan and co-workers developed a viable one-pot electrodeposition method to design UiO-66/PPY hybrid which was coated on carbon fibers under the presence of dopamine ( Figure 4e). [105] The uniform coating of UiO-66/PPY on the carbon fiber was realized, as presented in Figure 4f,g. The PPY coated on the surface of UiO-66 was well served as the conductive link filled into the interparticle of UiO-66.

MOFs@MOFs
In view of the unique structural features of MOFs, covering one MOF on another MOF (namely assembling MOF@MOF composites) is a promising strategy to enhance the conductivity, hierarchical porosity, activity, and stability of MOF composites. This strategy will greatly expand the structural complexity and induce the synergetic effect beyond individual MOFs. For this, coreÀshell and hollow structures have attracted considerable attention due to the promising properties. [106] Yamauchi and co-workers designed and prepared coreÀshellstructured ZIF-8@ZIF-67 using a seed-mediated growth method ( Figure 5a). [107] In the coreÀshell structure, white ZIF-8 crystals with good dispersity and uniform diameter were selected as the seeds. With the addition of Co 2þ and 2-methylimidazole, bright purple ZIF-67 shells were covered outside ZIF-8 crystals, obtaining the uniformly dispersed ZIF-8@ZIF-67 with rhomboid dodecahedron (Figure 5b  Reproduced with permission. [104] Copyright 2017, The Royal Society of Chemistry. e) Instructions for synthesis of UiO-66/PPY; the SEM images of f ) carbon fibers and g) UiO-66/PPY-coated carbon fibers. Reproduced with permission. [105] Copyright 2018, American Chemical Society.
www.advancedsciencenews.com www.small-structures.com identical protocol, but no coreÀshell structures were obtained, likely because of the rapid nucleation reaction of ZIF-8. Oh et al. prepared a leaf-shaped coreÀshell hybrid MOF (ZIF-L@ZIF-67) using ZIF-L as the seeds (Figure 5e). The growth of ZIF-67 shells on the external surface of ZIF-L did not alter the shape of ZIF-L, except that the thin flats became rougher (Figure 5f,g). [108] Huang et al. chose a simple ultrasonicationassisted stepwise synthesis method to prepare Ni-MOF@ Fe-MOF hybrids ( Figure 5h). The flexible ultrathin Ni-MOF nanosheets were prepared for the first time, whose morphology and good dispersity enable its further decoration with Fe-MOF nanosheets by the in situ coordination of Fe 3þ and terephthalic acid (H 2 BDC) (Figure 5i,j). [90] The introduction of Fe-MOF NPs on the ultrathin 2D Ni-MOF nanosheets brought about the enhanced activity of Ni-MOF@Fe-MOF toward OER.

Others
Apart from designing MOF composites, thermal transformation, and partial derivative strategy of MOFs as described earlier, many other methods can also be used to enhance the electrochemical performance of MOFs. For example, one can combine MOFs with functional materials different from those mentioned earlier.
Su's group prepared a heterogeneous MOF, A 2.7 B-MOF-FeCo 1.6, through assembling metal, terephthalic acid (A), and 2-amino-terephthalic acid ligands (B) (Figure 6a). [109] The team chose MIL-88B-Fe and benzoic acid ligands with 1D channels and good stability as the matrix and substituted Fe 3þ in MIL-88B with Co 2þ to synthesize bimetallic MOFs. The morphology of A 2.7 B-MOF-FeCo 1.6 was represented via SEM and TEM, and the uniform rod-like morphology was found (Figure 6b,c).
In addition, MOFs can also be combined with Co(OH)F to improve the electrochemical performance. Luo's group synthesized Co(OH)F precursors by hydrothermal method, converted Co(OH)F by vapor phase, and generated Co(OH) 2 coating on Co(OH) 2 surface by in situ cathodic electrodeposition ( Figure 6d). [110] SEM images of Co(OH)F showed smooth needle-like and brush-like structures. After annealing with imidazole at 300 C in Ar atmosphere, Co(OH)F was successfully transformed into Co(MOF). The SEM images showed that the nanorods were replaced by smooth nanorods, and the brush-like structure was maintained well (Figure 6e). TEM images further demonstrated the smooth nanorod structure of Co-MOF. SEM ( Figure 6f ) and TEM (Figure 6g-i) images showed that Co(OH) 2 nanosheets were coated on the appearance of Co-MOF.

Thermal Transformation
MOFs are characterized by the large specific surface areas, high porosity, and tunable functionality. However, in the specific applications, the performances of MOFs are restricted due to the limited active sites. To this end, rational thermal transformation was applied on MOFs on the premise of preserving MOF structures, which could remove the solvent/water molecules or partial ligands and bring about more exposed active sites. As a result, controlled calcination strategies are performed on MOFs, and the end-products with more active sites and high porosity are promising materials for facilitating the electrochemical process. [111]   Pang et al. assembled straw-bundle-like quasi-Ce-MOF composites via a rational two-step method (Figure 7a). [112] In this process, straw-bundle Ce-MOF precursor was prepared via one-step precipitation, and Ce-MOF was directly heated in nitrogen atmosphere at 350 o C for 1 h. Partial pyrolysis of Ce-MOF was achieved and the quasi-MOF material (Ce-MOF-350) was formed through the calcining process. As shown in Figure 7b,c, Ce-MOF-350 maintained the morphology of Ce-MOF without agglomeration or cracking. With the guidance of this strategy, Pang's team also synthesized a series of quasi-ZIF-67 via calcining ZIF-67 under low temperature (Figure 7d). [113] In the pristine ZIF-67, two facets of (002) and (011) were presented, while only (002) can be observed in quasi-ZIF-67-350 (Figure 7e-h), indicating the formation of more exposed metal sites, which could be ascribed to the loss of sectional ligands during the process of calcine.

Partially Derivative Strategy
MOF derivatives refer to hybrid materials where MOFs, as precursors/templates, are transformed into metal compounds (or metals) and other materials through simple annealing and other processes. In this part, it is introduced that only part of MOFs is transformed by simple annealing and other strategies, while the remaining part still keeps the structure of MOF material unchanged.
Wang and co-workers first designed coordinately unsaturated metal sites (CUMSs) in ZIF-67 via dielectric barrier discharge (DBD) plasma-etching method (Figure 8a). [114] As shown in Figure 8b-e, the dodecahedral ZIF-67 possesses smooth surface. After the treatment of DBD plasma etching, CUMSs-ZIF-67 maintains the morphology of ZIF-67, except that its surface became rougher and draped. Recently, Yu's group found that carbonÀnitrogen vacancies (V CN ) can be achieved by N 2 plasma bombardment, where ionized nitrogen plasma can break the FeÀCNÀNi units in the NiÀFe Prussian blue analogue (PBA), creating an unconventional carbonÀnitrogen vacancy (V CN ) ( Figure 8f ). [115] In the process of creating V CN , NiMoO 4 nanorods were used as templates to construct 1D K 2 NiFe(CN) 6 PBAs. After N 2 plasma bombardment, PBA-60 displayed a rod-like structure with jagged edges and was composed of a number of interconnected cubes (Figure 8g).
In addition to the above partially derivative methods, the electrochemical properties of MOFs can also be improved by partial phosphating. Yin and co-workers proposed steerable fractional phosphorylation tactics for generating CoP species in Co-based MOFs. [116] The end-product CoP/Co-MOF/CF was synthesized via sequential process, involving the hydrothermal deposition of Co(OH)F, transformation of Co(OH)F to Co-MOF, and partial phosphorylation of Co-MOF ( Figure 8h). As shown in SEM images, CoP/Co-MOF maintains the microstructure of nanorods, whereas the surface was rougher than that of Co-MOF (Figure 8i).
To improve the capacitance, Wei and co-workers synthesized a sequence of layered structural Zn-doped Ni-MOF with diverse dosages of ZnCl (i.e., MOF-0, MOF-1, MOF-2, MOF-3) (Figure 9a). [117] As presented in SEM and energy-dispersive Xray(EDX) spectroscopy, with the addition of Zn 2þ ions, the flower-like microspheres were prepared, and the dimension of microspheres grew with the increase in Zn 2þ ions (Figure 9b-i).
On the basis of zinc doping, Chen's group reported a method to partially replace Ni 2þ in Ni-MOF with Co 2þ or Zn 2þ to improve the conductivity of MOFs (Figure 9j). [118] Both Co/Ni-MOF and Zn/Ni-MOF retained the original crystal topology of Ni-MOF. It can be clearly observed by SEM images that the flower-like structure of Co/Ni-MOF and Zn/Ni-MOF consists of many nanosheets with relatively smooth surfaces (Figure 9k-l).

Summary
Several advanced synthetic strategies of modified MOFs in recent years, including MOF composites, thermal transformation, and partial derivatization, were comprehensively introduced earlier. Reproduced with permission. [110] Copyright 2021, Elsevier.  Reproduced with permission. [113] Copyright 2020, American Chemical Society. Reproduced with permission. [114] Copyright 2017, Elsevier. f ) Schematic illustration of V CN -mediated Ni-Fe PBA. g) TEM image of PBA-60. Reproduced with permission. [115] Copyright 2019, Springer Nature. h) Synthetic scheme of CoP/Co-MOF/CF. i) SEM image of CoP/Co-MOF/CF. Reproduced with permission. [116] Copyright 2019, Wiley-VCH. These methods improve the electrochemical performance of MOF materials by increasing the exposure of active sites, while keeping the structure of MOFs unchanged.

Batteries
Batteries, including LiÀO 2 batteries, lithium-ion batteries (LIBs), [148,149] and lithiumÀsulfur batteries (LSBs), [127,150] are investigated in portable electronic devices as a result of their environmental protection, high energy density, and long cycle life. MOFs are identified as potential candidate materials for electrode because of their flexible structure, low cost, and strong redox activity. In this section, we will introduce the excellent properties of modified MOF materials used as battery electrodes. Wu's group prepared a composite membrane MOF/PAN by electrostatic spinning which uses polyacrylonitrile (PAN) and MOF materials as raw materials and demonstrated that Co-SIM-1/PAN can be used to improve the efficiency of lithiumion batteries. [151] Co-SIM-1 belongs to ZIF materials and possesses an average particle size of 0.76 μm (Figure 10a). The SEM image shows that the electrospun PAN fiber is relatively smooth (Figure 10b), while the 10% Co-SIM-1/PNA composite fiber is much rougher. The MOF loaded with PAN has two states: one is wrapped around PAN to increase the diameter of fiber and the other is embedded in the fibers (Figure 10c). The cross section of 10% Co-SIM-1/PNA film shows that Co-SIM-1 is embedded in the film (Figure 10d). After the addition of MOF material, lithium-ion transference number (t Liþ ) of the composite increases because MOF can adsorb more electrolytes in the surfaces and pores and fix the anions by the unsaturated metal sites at the same time (Figure 10e). After using four different separators to package NCM811||separator||Li batteries, the cycles and rate performances demonstrated that NCM811|| separator||Li battery packaged with 10% Co-SIM-1/PNA has higher capacity than any other separators (Figure 10f ). The discharge capacity of batteries assembled by 10% Co-SIM-1/PNA decreases during the process of cycling (Figure 10g). Meanwhile, the battery has distinguished cyclic stability after 250 cycles (Figure 10h).  [117] Copyright 2014, The Royal Society of Chemistry. j) Schematic illustration of M-MOFs; the SEM images of k) Co/Ni-MOF and l) Zn/Ni-MOF. Reproduced with permission. [118] Copyright 2017, The Royal Society of Chemistry. Except for the Li-ion battery, LiÀS battery is also a potential rechargeable energy conversion and storage system due to its high theoretical specific capacity and richness of sulfur on the earth. However, the adhibition of LiÀS batteries is hindered via the shuttle effect, which is generated at the cathode, usually bringing about the leakage of active material and decrease of cycle life. [152] For the sake of resolving the problem, Pang et al. proposed tactics to introduce sulfur-friendly metal ions (Cu 2þ ) into Al-MOF (i.e., (Al)MIL-53), thus constructing bimetallic MOF (i.e., Al/Cu-MOF) as the host material for sulfur in LiÀS batteries (Figure 11a). The distinct reduction peaks on cyclic voltammetry (CV) curves are related to the conversion of S 8 to polysulfides (Figure 11b). The comparison of galvanostatic charge/ discharge (GCD) curves among S-Super P, Al-MOF-S, and Al/ Cu-MOF-5-S electrodes indicated that the specific capacity of Al/Cu-MOF-S is higher than that of Al-MOF-S (Figure 11c,d). Further, the high performance of Al/Cu-MOF-S in LiÀS battery was confirmed by the high rate performance and small chargetransfer resistance (Figure 11e-g). The incorporation of Cu 2þ demonstrated that chemical fixation is a favorable means for the adsorption of polysulfide in LiÀS batteries.
MOF materials with excellent oxygen accessibility and abundant open metal sites are expected to strengthen the efficiency of LiÀO 2 batteries. [153] Lee's group designed a bimetallic MnCo-MOF-74 that can as a cathode catalyst for LiÀO 2 batteries.
Compared with Co-MOF-74 and Mn-MOF-74, the bimetallic MnCo-MOF-74 exhibits higher efficiency and reversibility in chargeÀdischarge cycles. This is mainly due to the porous structure of MOFs and the complementary contributions of CoÀ and MnÀmetal clusters. The microstructure of M-MOF-74 was characterized by SEM, and it was found that the samples of M-MOF-74 were hexagonal crystals, which grew straight in one orientation and the bimetallic MnCo-MoF-74 had the same nanorod structure with the single-metal Co-MOF-74 and Mn-MOF-74 (Figure 12a-c). The curves of charge and discharge of MnCo-MOF-74 at 200 mA g À1 show that the charge and discharge capacity of MnCo-MOF-74 is 11 150 mA h g À1 , which is much higher than that of Mn-MOF-74, Co-MOF-74, and mixture-MOF-74 ( Figure 12d). As shown in Figure 12e, compared with other electrodes, MnCo-MOF-74 maintained the minimum charging terminal voltage and the maximum discharge terminal voltage indicating that OER and ORR dynamics were improved. In the cycle test, MnCo-MOF-74 provided the termination discharge capacity of 1000 mA h g À1 for 44 times at the rate of 200 mA g À1 stably, which was significantly longer than Co-MOF-74 and Mn-MOF-74 whose cycle index was 22 and 18 times severally (Figure 12f-g). The performance of bimetal MnCo-MOF-74 is better than that of single-metal Co-MOF-74 and Mn-MOF-74. This is due to the synergism of CoÀ and MnÀmetal clusters. MnCo-MOF-74 improves the Figure 10. SEM images of a) Co-SIM-1, b) PAN, c) 10% Co-SIM-1/PAN, and d) cross section of the 10% Co-SIM-1/PAN; e) ion conduction and anionic adsorption processes of MOF in Co-SIM-1/PAN separator; f ) rate performance of the NCM811||separator||Li batteries with diverse separators; g) discharge curves of batteries using 10% Co-SIM-1/PAN as the separator; h) long-term cycle lifetime and coulombic efficacy of NCM811||separa-tor||Li batteries with diverse separators at 5 C. Reproduced with permission. [151] Copyright 2021, Elsevier. conformation of ORR discharge products, which are hastily resolved in OER discharge. This synergistic effect improves the efficiency and reversibility of LiÀO 2 cathode. The feasibility of regulating catalytic activity by multifunctional MOFs is thus proved.

SCs
SCs, which are known as electric double-layer SCs, are electrochemical components that store energy through polarized electrolytes. They have high power density and can be used for regenerative braking, short-term energy collection, and burst power supply, [131][132][133][134]154] where carbon is usually used. However, SCs have low physical energy density and charge storage. In the past several years, modified MOFs have revealed outstanding advantages and potential in this field because of their outstanding conductivity, high surface specific area, and uniform pore size distribution. [135] Chen's group combined conductive polypyrrole (PPY) with a bimetallic organic framework to fabricate a high-property electrode (Zn/Ni-MOF@PPY). [104] A coin-type hybrid SC (HSC) was assembled with Zn/Ni-MOF@PPY as the anode, while CNTs-COOH was the cathode. The 2D layered structure of Zn/Ni-MOFs furnishes adequate space for transferring OH À . The PPY chains raise the spacing between Zn/Ni-MOFs layers and also offer a charge transport channel (Figure 13a). The HSC is evaluated at various potential windows, and the results indicate that potential windows can attain 1.4 V without significant polarization (Figure 13b). The shape of CV curves was maintained even at high scan rates, revealing that the system had small resistance and excellent rate performance (Figure 13c). The low internal resistance of HSC was confirmed    by low IR drops displayed in the chargeÀdischarge curves ( Figure 13d). In addition, the Ragone graphic of the HSC illustrated the relation between power density and energy density (Figure 13e) when the power density reached 699 W kg À1 ; the great potential of the designed hybrid SC is further demonstrated. Duan's group designed CFs@UiO-66/PPY composites to assemble all-solid-state fiber SCs. When the scanning rates increased from 5 to 100 mV, the length capacitance of CFs@UiO-66/PPY decreased from 15 to 8 mF cm À1 (Figure 13f ) and the weight capacitance was reduced from 90 to 50 g F À1 . [105] The chargeÀdischarge curves at diverse current densities ranging from 50 to 400 μA cm À1 presented the capacitive behavior of CFs@UIO-66/PPY (Figure 13g). At the same time, the CFs@UiO-66/PPY fiber electrode also showed stabilized cycling performance after 1000 cycles (Figure 13h).

OER & HER&ORR Electrocatalyst
ORR, OER, and HER are three crucial reactions in the evolution of green and sustainable energy systems. ORR is the cathode reaction occurring in hydrogen fuel cells and metalÀair cells, OER is the reverse process of ORR, and HER is a typical reaction of double-electron transfer reaction. Drawn on the advantages of both original MOF materials and typical electrocatalytic materials, modified MOFs have exhibited unique structures and low overpotentials, which make it the most ideal materials with the greatest potential in the field of electrocatalysis.
Pang's group reported a high-efficiency ZIF-67@POM electrocatalyst with yolk/shell structure (Figure 14a). [94] The distinctive yolk/shell structure and the synergistic effect of structures and components endow ZIF-67@POM with high activity for OER. As shown in Figure 14b, ZIF-67@POM exhibits a lower onset potential and a fast increase in current density, and its Tafel slope (58 mV dec À1 ) is among the smallest of all the hybrids (Figure 14c). Therefore, these results are even comparable with those of commercial RuO 2 . In addition to improving activity, the yolk/shell structure also heightens the long-term durability of ZIF-67@POM and its activity slightly decreases after 1000 CV cycle (Figure 14d). In addition, Au NPs loaded with ZIF-67 (i.e., Au@ZIF-67) were also constructed by Pang et al. and the composite showed improved OER activity compared with ZIF-67. [99] Furthermore, taking Au@ZIF-67 as a precursor, thermal treatments were carried out under different temperatures, and a series of Au@ZIF-67-T (T denotes the temperature) were obtained. Among them, Au@ZIF-67-500 achieves remarkable OER performance with smaller Tafel slope, lower overpotential, and lower charge transfer resistance (R ct ) (Figure 14e-h). The improved OER catalytic activity is attributed to Au NP loading, porous frameworks, and induced Co reduction thermally.
Besides OER, modified MOFs are expected to present excellent electrocatalytic performance for HER. Recently, Yin's group exploited staged phosphating tactics to coat CoP species on top of Co-based MOF (Co-MOF) outer coatings. (Figure 15a,b). The obtained CoP/Co-MOF exhibits remarkable HER activity with Figure 14. a) HAADFÀSTEM image of 6-ZIF-67@POM, b) IR-corrected OER polarization curves of ZIF-67@POM; c) the Tafel plots of ZIF-67@POM and RuO 2 obtained by OER curves; and d) the durability test for 6-ZIF-67@POM. Reproduced with permission. [94] Copyright 2019, American Chemical Society. e) TEM image of Au@ZIF-67; f ) linear sweep voltammetry (LSV) polarization curves of catalysts for OER, g) Tafel plots measured from the LSV, and h) electrochemical impedance spectroscopy of ZIF-67-500, Au@ZIF-67-350, Au@ZIF-67-400, Au@ZIF-67-450, Au@ZIF-67-500, and Au@ZIF-67-550. Reproduced with permission. [99] Copyright 2020, Springer Nature. an overpotential of 49 mV (@10 mA cm À2 ) in neutral media (PBS, pH ¼ 7.0) (Figure 15c). Furthermore, the high efficiency of CoP/Co-MOF was demonstrated by a smaller Tafel slope (Figure 15d), lower R ct (Figure 15e), and larger double-layer capacitance (C dl ) (Figure 15f ). The long-term stabilization of CoP/Co-MOF was presented via repeated CV cycles  www.advancedsciencenews.com www.small-structures.com ( Figure 15g) and chronopotentiometry (Figure 15h). [116] The excellent HER property of CoP/Co-MOF was put down to the distinct porous structure of Co-MOF and efficient synergy between CoP and Co-MOF, which not only furnished with more active sites, but also facilitated the release of generated gas. Yin's group designed and synthesized ε-MnO 2 /MOF(Fe) carrier via combining ε-MnO 2 with MOF(Fe) carrier. [162] The pristine MOF(Fe) presented a particle microstructure with a mean dimension of about 400À500 nm (Figure 16a). After introducing ε-MnO 2 , the morphology of MOF(Fe) remained intact, and new protruding nanorods emerged with one anchoring on the MOF(Fe) substrate firmly (Figure 16b,c). Using ε-MnO 2 /MOF(Fe) as ORR catalyst in alkaline electrolyte, the protruding nanorods, high specific surface area, and ample micropores synergistically facilitated the diffusion of oxygen and its close contact with ε-MnO 2 in the ORR process, which demonstrated that ORR activity and stability is better than those of ε-MnO 2 and ε-MnO 2 þ MOF(Fe). Specifically speaking, ε-MnO 2 /MOF(Fe) with Mn content of 9.57% displayed the best ORR performance, as proved by CV curves and ORR polarization curves (Figure 16d-f ).

Multifunctional Electrocatalyst
HER, OER, and ORR are fundamental half reactions in many energy technologies, for example, electrocatalytic water splitting, fuel cells, and metalÀair batteries. Given the good performance of modified MOFs in HER/OER/ORR, it is greatly expected that they can be used as bifunctional/trifunctional electrocatalysts, which will not only simplify the material preparation process and reduce costs, but also promote energy conversion and device integration.
Wang's group synthesized the bimetallic ZnCo-ZIF@GO (Figure 17a) via the in situ growth method, which shows good ORR and OER electrocatalytic activity in alkaline solution. [81] The property of ZnCo-ZIF@GO in ORR and OER was evaluated in 0.1 and 1.0 M KOH severally. The half-wave potential of ZnCo-ZIF@GO was found to be 0.76 V, which is more positive than that of ZnCo-ZIF (0.57 V) and GO (0.71 V) (Figure 17b). The timeÀcurrent test was used to study its electrochemical stability. After 30 000 s, the initial value of ZnCo-ZIF@GO catalyst was found to decrease slightly, while that of ZnCo-ZIF catalyst decreased significantly, which indicates that the electrochemical stability of ZnCo-ZIF@GO was better. The interaction of ZnCo-ZIF and graphene oxide flakes can improve the durability of the material (Figure 17c). In addition to good ORR properties, LSV curves of ZnCo-ZIF@GO, ZnCo-ZIF, and GO at 90% IR compensation show that the potential (E j¼10 ) required by ZnCo-ZIF@GO is only 1.66 V to reach the current density of 10 mA cm À2 , which is lower than the potential required for ZnCo-ZIF and GO (Figure 17d). Meanwhile, ZnCo-ZIF@GO also shows excellent OER performance with lower overpotential (at 10 mA cm À2 ) of 430 mV and smaller Tafel slope of 83 mV dec À1 (Figure 17e).  Loh and co-workers found that composite materials of GO and MOFs (Figure 17f,g) showed outstanding catalytic properties in ORR, OER, and HER. [80] According to the CV curves, the incorporation of GO increased the electroactive surface area of the composites and then efficiently enhanced its charge transfer kinetics (Figure 17h,j). In addition, as evidenced by the N 2 adsorption isotherms in Figure 17i, the addition of GO also increased the specific surface area of the composites. The trifunctional activity of GO-incorporated Cu-MOF composite in HER, OER, and ORR was estimated in acidic media, as shown in Figure 17k-n. The smaller the overpotentials, the greater the current and the better the stability of the composite materials, which are as a result of the untouchable porous structure, enhanced charge transfer capacity, and the cooperative effects between GO and Cu-MOF.

Summary
The increasing demand for renewable and clean energy has prompted the world to seek efficient energy technologies, which is expected to be both economically sustainable and environmentally friendly. HER is a significant kinetic process for production of hydrogen, which has the potential to replace fossil fuels. In the same way, efficient ORR and OER are essential for renewable energy platforms, especially for rechargeable batteries and fuel cells. Currently, the main task is to develop a practical, economic, and feasible electrocatalyst with enhanced performance and high stability. The excellent performances of MOFs as a reaction catalyst have been introduced hereinabove. It can be used not only as a certain reaction catalyst, but also as a catalyst for many reactions at the same time through some designs, so as to achieve the effect of high efficiency and cleaning ( Table 1).

Conclusion and Outlook
In conclusion, this article reviews an overview of the recent development of modified MOFs on the rational design and their corresponding electrochemical adhibitions in batteries, SCs, and electrocatalysis. The electrochemical properties and applications of MOFs are reviewed from the prospects of composites, functional materials, heat treatment, and partial derivation. In recent decades, the applications of MOFs as electrode materials in energy fields, for example, batteries and SCs, have become a flash topic in material science. Literature has introduced how to enhance the electrochemical performance of MOFs via combining MOFs with functional materials like carbon base, MNPs, metal oxides or CPs, or heat treatment or partial derivatives of MOFs materials, so as to solve the social problems of environmental pollution and resource shortage. Generally speaking, by combining the advantages of functional materials with the disadvantages of MOF materials, MOF composites obtain new physical and chemical performance and ameliorate properties that cannot be achieved by single components. In addition, heat treatment of MOF materials usually improves the electrochemical activity while retaining the porosity to achieve mass transfer. In the case of incomplete decomposition of the skeleton, modification of MOFs can reveal catalytic accelerate electron transfer and active metal sites, thus ameliorating the electrochemical function of the materials. The partial derivation of MOF materials is usually achieved by changing part of the internal structure of MOFs, so as to improve the electrochemical property.
MOF materials can also ameliorate the electrochemical capability of materials without derivatives or changing their structure, which have already been applied to the electrodes of batteries and SCs and been used as catalysts for HER, OER, and ORR. Improving the electrochemical performance of materials based on nonderivative MOFs remains in its initial phase and many challenges remain to be solved. The future researches are as follows. 1) MOF materials can be combined with MNPs, metal oxides, carbon-based materials, CPs, and other functional materials so as to obtain modified MOFs that will have a broad application in the future. 2) Alternative methods for large-scale syntheses of chemically and thermally stable porous MOFs are 3) The properties of functional materials with MOFs are of great importance to the construction of composite structures and the properties of composite materials. 4) Further research is needed to be done in combining more multifunctional materials with MOFs. 5) Exposure to active sites helps to enhance the electrochemical capability of MOF materials, but how to increase it while not changing the structures of MOF materials remains a challenge. 6) After some processing, MOF materials can be used as electrode material to raise the electrochemical properties, but the electrochemical stability of the material is poor. Repeated charge and discharge cycles can lead to the destruction of the structure and the decrease in electrochemical stabilities. Therefore, exploration and evolvement of new electrode materials with high-capacity function is still a topic worth thinking about. 7) After certain treatments, MOF materials can be used as a high-performance catalyst in ORR, HER, or OER. However, to efficiently use MOF materials, more researches are needed to be done before MOFs can be used as multifunctional catalysts for all the three reactions. 8) In SCs, the poor conductivity of MOF materials leads to low specific capacitance, low power density, and poor cycle stability problems. Therefore, it is imperative to raise the conductivity of MOF materials. 9) The most attractive aspects of MOF materials are their high porosity and high specific appearance area, which can promote electrolyte penetration, reduce volume conversion, and improve the forthputting of active materials in batteries. Nonetheless, in practical adhibition, due to the high porosity of the electrode material and low volume energy density and coulombic efficiency, adjusting the porosity of the MOFs electrode is of great significance to optimize the volume energy density, coulombic efficiency, and rate capacity. 10) It is a feasible method to combine micro-/nano-MOFs with special morphology with conductive materials such as functional carriers, including graphene and metal foams. Such carriers can quicken electron carriage, promote electrolyte diffusion, and enhance the durability and reliability of MOF materials. 11) The stability and conductivity of MOF electrode materials could also be enhanced through functional modification, which is to graft desired groups/atoms onto organic linkers metal or ions/clusters. Through researchers' efforts, some new electronically conductive MOFs have been produced, which will greatly promote the direct use of MOFs as electrochemical electrodes. In recent decades, MOF materials, as a kind of star-based materials, have shown great momentum of growth in electrochemistry despite various challenges. However, as for the practical applications of non-derived MOF materials, such as being used in batteries, ultracapacitors, or as the catalysts in OER, HER, and ORR, there remains a long way to go.