Metal-organic frameworks for direct electrochemical applications

Coordination Chemistry Reviews 376 (2018) 292–318

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Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

Metal-organic frameworks for direct electrochemical applications Yuxia Xu, Qing Li, Huaiguo Xue, Huan Pang ⇑ School of Chemistry and Chemical Engineering, Guangling College, Yangzhou University, Yangzhou 225009, Jiangsu, PR China

a r t i c l e

i n f o

Article history: Received 15 June 2018 Accepted 13 August 2018

Keywords: Metal-organic framework Battery Supercapacitor Electrocatalyst Electrochemical sensor

a b s t r a c t Metal-organic frameworks are a class of functional porous materials. In recent years, metal-organic frameworks have become a hot research topic in the field of electrochemistry because of their controllable morphology, abundant pores, high specific surface area and versatility. Herein, we summarize the latest developments of metal-organic frameworks and metal-organic framework composites as electrode materials or catalysts for electrochemical applications such as batteries, supercapacitors, electrocatalysts and electrochemical sensors. The morphological and electrochemical properties of these promising metal-organic framework materials for their future development are discussed. Finally, based on the reported literature, we propose the future direction of metal-organic frameworks and metal-organic framework composites in the field of electrochemistry. Ó 2018 Elsevier B.V. All rights reserved.

Contents 1. 2.

3.

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5.

6. 7.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MOFs for batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. MOFs for Li-ion batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Pure MOFs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. MOF composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. MOFs for Li-S batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Pure MOFs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. MOF composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Challenge and opportunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MOFs for supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Pure MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. MOF composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Challenge and opportunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MOFs for electrocatalysts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Pure MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. MOF composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Challenge and opportunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MOFs for electrochemical sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Pure MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. MOF composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Challenge and opportunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other electrochemical applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (H. Pang). URL: http://huanpangchem.wix.com/advanced-material (H. Pang). https://doi.org/10.1016/j.ccr.2018.08.010 0010-8545/Ó 2018 Elsevier B.V. All rights reserved.

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1. Introduction Metal-organic frameworks (MOFs) are a new class of porous crystalline materials constructed by the coordination of metal ions/clusters and organic bridging ligands in a three-dimensional (3D) space [1,2]. MOFs have aroused widespread concern in the past few years and have now become one of the most rapidly developing areas of research. To date, more than 20,000 MOFs with different compositions, crystal structures and morphologies have been reported [3,4]. Because of their tailorable structure and functionality, high porosity and large internal surface area, MOFs have great potential in a variety of applications, such as gas storage [5–7], separation [8,9], catalysis [10–12], sensors [13,14], biomedicine [15,16], and so forth. The applications of MOFs in electrochemistry have been reported by many researchers over the past few years because the specific surface area of MOFs usually ranges from 1000 to 10,000 m2 g
1, which is higher than the surface area of most conventional porous materials. The pore size can be adjusted by altering the length of the organic ligand, for a maximum pore size of 9.8 nm [17,18]. For instance, in summaries of previous studies, some researchers have outlined the different synthetic methods of MOFs and have used MOFs as precursors to synthesize nanomaterials with various morphologies, such as nanoparticles, nanosheets, nanorods, nanospheres, etc [19–26]. Additionally, many researchers have summarized the latest research on the applications of nanostructured MOFs and MOF-derived materials

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in batteries and supercapacitors [27–32]. Although the design and application of MOFs have made great progress, but until recently, the potential of electrocatalysts have attracted the attention of researchers. They mainly study MOF-based materials as catalysts in important energy conversion electrochemical reactions including oxygen reduction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) as well as MOFbased materials in lithium-air batteries and carbon dioxide reduction for new progress in electrochemical applications [33–35]. Among different MOFs applications, MOF-based electrochemical sensors are a hopeful application. In recent years, some researchers have discussed the application of MOFs and their derivatives in sensors [14,36]. In this review, we provide an overview and comment on the recent progress of MOFs in batteries (lithium-ion batteries (LIBs), lithium-sulfur (Li-S) batteries, lithium-oxygen (Li-O2) batteries, sodium-ion batteries (SIBs), etc.), supercapacitors, electrocatalysts, electrochemical sensors, and so on. A schematic diagram is shown in Scheme 1. Similarly, a few earlier reviews reported the applications of MOFs or their derivatives. For example, Morozan et al. [37] reviewed the applications of MOFs and MOF-derived materials in the field of electrochemistry. Moreover, Wang and co-workers also applied MOFs to energy storage (batteries, supercapacitors) [38,39]. In recent years, Xu’s group has focused on the development of MOFs and their derivatives for clean energy applications, including batteries, supercapacitor, catalysis, and so on [1,35,40–44]. In addition, some researchers have reported

Scheme 1. The schematic diagram of MOFs and MOF composites for electrochemical application.

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changing the conditions (synthetic methods, calcination temperature) to synthesize MOFs and MOF-derived materials with different tunable nanostructures, as well as the application of MOFs in all aspects [19–25,45,46]. However, MOFs used directly in the electrochemical applications are rarely reported. Here, we discuss the applications of MOFs and MOF composites in electrochemistry. We expect to promote knowledge transfer from previous studies to generate new thoughts for the future development of MOFs in electrochemical applications. 2. MOFs for batteries Lithium batteries are widely used in a variety of portable electronic devices because of their environmental friendliness, long cycle life and high energy density [47–49]. Due to their structural flexibility, low cost and redox activity, MOFs are good candidates for electrode materials [50]. However, the practical application of many MOFs is hampered by poor conductivity, resulting in poor cycle performance of the battery. Therefore, researchers have tried to develop MOFs and MOF composites with good properties (Table 1). 2.1. MOFs for Li-ion batteries In the past few years, rechargeable LIBs have become promising energy storage devices because of their advantages of good cycle performance and high energy density [56,85–87]. MOFs have become a new type of electrode material with great potential and introduction of special properties in LIBs due to their structural diversity, adjustable redox properties, simple synthesis methods,

and low cost [88,89]. Their high surface area and porosity can be favorable to interfacial charge transport, to adapt to the Li insertion/extraction strain. In addition, MOFs have a higher thermal stability than pure organic materials, therefore, more efficient use of metal ions. Recently, as a new choice for porous crystalline materials, MOFs have been studied as positive, negative and electrolyte materials for LIBs. 2.1.1. Pure MOFs Fe-based MOFs have been reported by researchers, owing to their excellent electrochemical properties [90]. Férey and coworkers reported the first use of MIL-53(Fe) as a rechargeable intercalation cathode in Li-based batteries as early as 2007, but the capacity (75 mA h g
1) and cycle stability were not ideal [88]. Subsequently, Combelles et al. [91] explained the redox mechanism of MIL-53(Fe) when it was tested as electrode of LIBs by combining the first principles calculation based on density function theory and the local chemical bond analysis. They further proved that MIL-53(Fe) showed a promising cycling life and rate capability [92]. Shin et al. [93] prepared MIL-101(Fe) as an electrode by using a modified procedure. However, it was found that its redox chemistry (Fe2+/Fe3+) was not perfectly reversible. Studies showed that an appropriately functionalized MIL-101(Fe) SBU can improve the reversibility of the Fe2+/Fe3+ redox reactions. Recently, Shen and co-workers reported Fe-MIL-88B with a high specific capacity of the functional conjugated carbonyl-based materials [51]. The morphological features are displayed in Fig. 1a,b. For a half-battery and full-battery, Fe-MIL-88B maintained a capacity of 744.4 and 86.8 mA h g
1 over 400 and 100 cycles, respectively. The polyhedral nanorod electrodes hold the metal organic skeleton together throughout the battery operation, further confirming that some

Table 1 MOFs for batteries. Samples

CC/DC

RC/rate (mA g
1)

Capacity retention/CN

CE%

Electrode

Refs.

Fe-MIL–88 B Co2(OH)2BDC Co-BDC S-Co-MOF CoCOP Co-BTC MOF CoBTC-EtOH Co-MOF Co2(DOBDC) MOF Cu(2,7-AQDC) [Cu3(BTC)2] MOF Cu-TCA Mn-LCP Mn-BTC MOF Ni–Me4bpz Ni-MOF Cd(II) MOFs Zn3(HCOO)6 [Pb(4,40 -ocppy)2]7H2O BMOF Fe/Co-BTC Fe3O4@MOF ZIF8-10 Co(L) MOF/rGO CoCGr-5 Ni3(HCOO)6/CNT-50 Fe-MOF/rGO Mn-MOF/rGO10 S/ZIF-8 S–Zn-MOF S@MOF-525(Cu) MIL-101(Cr)/S MOF@GO Zn-based MOF@GO L-Co2(OH)2BDC

949.9/1507.4 1005/1385 780.6/1963.6 1564/1946 1107/1620 622/1739 879/1790.3 440/600 785/1409 147/– 641/1497 –/102.2 –/1807 694/1717 –/320 1369/1984 45/45 693/1344 578/1522 –/– 568.5/859.1 918.7/1266.2 369.2/1125.7 518.8/951.8 –/2566 751/– 891.1/2055.9 732.6/1677.5 –/1055 –/1476 –/– –/715 –/1126 –/1118 450/742

744.5/60 650/50 1090/200 1021/100 573/1000 750/100 473/2000 400/50 526.1/500 105/– 474/383 39.9/0.5 C 390/– –/103 120/50 620/100 –/– 560/60 489/100 190/100 639.3/200 1002/100 349.2/500 206/500 1368/100 560/300 1010.3/500 715/100 553/0.5 C 609/0.2 C 700/0.5 C 695/0.1 C 855/1673 657/1673 131/1000

93%/400 –/100 70.7%/100 –/200 100%/1000 –/200 97.4%/100 84.2%/150 –/200 –/50 100%/50 22%/200 –/50 100%/100 –/100 –/100 >70%/50 –/60 96%/500 –/200 –/50 –/100 –/20 –/330 –/400 –/400 –/200 98%/100 75%/300 41.3%/200 –/200 95%/134 71%/1500 –/1000 –/600

100 72.8 99.46 80.4 100 79 100 99.8 99 – 98 96.5 – 97 >98 100 – – 98 100 95 – – 98 >99 58 43.3 100 – – 100 96.6 100 98.8 100

anode anode anode anode anode anode anode anode anode cathode anode cathode anode anode anode anode cathode anode anode anode anode anode anode anode anode – anode anode cathode cathode cathode cathode cathode – anode

[51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [50] [64] [65] [66] [67] [68] [69] [70] [71] [72] [73] [74] [75] [76] [77] [78] [79] [80] [81] [82] [83] [84]

CC: charge capacity (mA h g
1); DC: discharge capacity (mA h g
1); RC: reversible capacity (mA h g
1); CN: cycle number; CE: coulombic efficiency.

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Fig. 1. (a,b) SEM and TEM images of the Fe-MIL-88B. (c) Schematic diagram of CoCOP nanowires with the robust framework structure and abundant interpenetrating tunnels for Li-ion transport in lithiation-delithiation processes. (d) Selected cycles and (e) capacity vs. cycle number plot of Zn3(HCOO)6 [current density of 60 mA g
1 (0.11 C, 1 C = 520 mA g
1)] was used within 0.005–3 V. (f) Conceptual schematic presentation of crystal engineering of naphthalenediimide-based MOFs 1–4. (g) Cyclic voltammograms of Mn-BTC MOF at 0.1 mV s
1. (h) Galvanostatic discharge–charge curves of Mn-BTC MOF. (i) Cycle stability of Mn-BTC MOF. (a,b) Reproduced with permission [51]. Copyright 2017, Elsevier. (c) Reproduced with permission [55]. Copyright 2016, The Royal Society of Chemistry. (d,e) Reproduced with permission [67]. Copyright 2010, The Royal Society of Chemistry. (f) Reproduced with permission [66]. Copyright 2016, American Chemical Society. (g–i) Reproduced with permission [50]. Copyright 2015, American Chemical Society.

nanostructured MOFs are better suited for stable lithiation/delithiation processes than other structures. When nanostructured MOFs are used as electrode materials, their conductivity is increased, and high surface area and short diffusion path are provided for ion transport and electron conduction, and cycle performance can be further improved at a high rate. For LIBs, Co-MOFs as electrodes are also very common [52–55,58,59]. For example, Hu and co-workers obtained a Co-BDC MOF as an anode material through a one-pot method [53]. Test results showed that the materials exhibited high capacities of 1090 mA h g
1 at 200 mA g
1. Recently, they also synthesized shell-like S-Co-MOF by a surfactant-assisted solvothermal method using cobaltous nitrate and terephthalic acid as the metal source and organic ligands, respectively [54]. The structure of S-Co-MOF contains two types of edge-sharing CoO6 octahedra, with the deformation of the CoO6 octahedral site during the test, showing high cycling stability. However, the lithium-ion storage capacity was not high enough. Song et al. [55] used an easy

hydrothermal method to fabricate a 1D cobalt coordination polymer (CoCOP) nanowires. The lithium-ion transfer process is shown in Fig. 1c. CoCOP provided a capacity of over 1100 mA h g
1 at 20 mA g
1. Lately, Li’s group [57] synthesized Co-BTC coordination polymers (Co-BTC CPs) by a simple hydrothermal process, in which three types of solvents were chosen to form CPs with different morphologies. The test data showed that the cycling stabilities of the three CoBTC CPs at 100 mA g
1 have no significant distinction, however, the current densities increased to 2 A g
1, and the difference is very obvious. The optimized CoBTC-EtOH product retained a capacity of 473 mA h g
1 at 2 A g
1 after 500 cycles while maintaining nearly 100% coulombic efficiency. The convenient method can extend to various energy storage devices. In addition to Fe-based MOFs and Co-based MOFs, some other typical metal-based MOFs have also been researched as electrodes for LIBs, such as Cu-based MOFs [60–62,94]. Mn-based MOFs [50,63,95,96], Ni-based MOFs [64,65]. In 2014, the first microporous Cu(2,7-AQDC) (AQDC = Anthraquinone dicarboxylate) with

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a redox-active site was obtained by using a simple synthetic strategy [60]. The relative positions of the anthraquinone groups fluctuate periodically around the Cu2(Ac)4 cluster, leading to the zigzag structure of the 2D sheet and allowing interaction between weak p-p adjacent layers. When utilized as a cathode of LIBs, a very high battery reproducibility was achieved by controlling the working voltage window, indicating that the frame is robust. In addition, Cu-TCA (H3TCA = tricarboxytriphenyl amine) was fabricated, and it acted as a cathode for lithium batteries, such that the redoxactive metal co-existed with the ligand [62]. This MOF electrode material achieved a long cycle stability. The porous Cu-TCA maintained an excellent rate performance. Liu and his colleagues explored a new MOF material, 2D [Mn-(tfbdc) (4,40 -bpy)(H2O)2] (Mn-LCP), by using 2,3,5,6-tetrafluor oterephthalic acid (H2tfbdc) and 4,40 -bipyridyl (4,40 -bpy) as ligands and manganese(II) acetate tetrahydrate as a metal source [63]. However, the lithium storage capacity of Mn-LCP was not sufficiently high. To improve the capacity of MOFs, Maiti et al. [50] designed and prepared a Mn-1,3,5-benzenetricarboxylate MOF (Mn-BTC MOF). In Li-ion button batteries, the as-prepared MnBTC MOF anode delivered good electrochemical characteristics (Fig. 1g–i). Moreover, the existence of the aromatic core in conjugated carboxylates may have strong p-p interactions and may stabilize the 3D structure of MOFs. However, Zhang and co-workers found a novel electrochemical mechanism called the ‘‘bipolar charge” mechanism [95]. When Mn-MOFs with a flexible redox activity were used as the active cathode material, they exhibited excellent cycle stability. In addition, in a single redox cycle, not merely lithium ions but also bulky anions are inserted into the pores of the MOF, contributing to an increase in the total capacity. In the case of MOFs, further development can be achieved by substituting Mn(II) with trivalent metal cations and replacing anthraquinones with organic electron donors, resulting in new redox activity MOFs with higher energy density and enhanced discharge voltage. Due to the presence of reversible lithium storage, Cd-based MOFs with enhanced thermal stability have also been reported. Tian et al. [66] obtained two kinds of isostructural naphthalenediimide-based MOFs (i.e., Cd-MOF, Co-MOF) with different porosities by anion and thermodynamic control. The preparation process is shown in Fig. 1f. These MOFs were tested as cathodes for LIBs, the Cd(II) MOFs (MOFs 1 and 2) had a better capacity and cycling stability than the Co(II) MOFs (MOFs 3 and 4). The electrochemical characteristics of the MOF electrode are significantly affected by the metal nodes and porosity. In the early days, Li and co-workers studied Zn-MOFs (MOF-177), which showed good cycling capacity, but the capacity was relatively low [97]. Saravanan et al. [67] reported the successful application of Zn-MOFs for Li storage, and they synthesized and studied Zn3(HCOO)6, Co3(HCOO)6 and Zn1.5Co1.5(HCOO)6. Zn3(HCOO)6 exhibited the best electrochemical results (Fig. 1d,e). The substrate involved in the cycling process is lithium formate instead of the typical Li2O, which provides good support for ex situ FTIR results. Kaveevivitchai and co-workers reported a reflux method to synthesize microporous VIV(O)(bdc) [MIL-47] [98]. Li/V(O)(bdc) cells exhibited an excellent capacity performance (82 mA h g
1 at C/12) that was higher than that found for MIL-53(Fe) [88]. In another study, nanosized UiO-66 was synthesized via reducing the reaction temperature [99]. When tested as an electrode of LIBs, UiO-66 had little volume change during the uptake of lithium. Additionally, MOFs have been investigated as electrolytes for allsolid-state lithium batteries. By a simple hot-press method, Angulakshmi et al. [100] prepared magnesium-benzene tricarboxylate MOFs (Mg-BTC MOFs). The incorporation of Mg-BTC MOFs in the polymer matrix also dramatically improve the thermal stability, compatibility and elongation at break of the polymer film.

In 2017, Liu et al. [86] used MOFs and their molecular structure to promote the stable circulation of the Li metal anode. The use of NH2-MIL-125(Ti)-coated separators enabled long-lived Li|Cu and Li|Li batteries with dendrite-free Li deposition and long-term reversible Li plating/stripping. This study provides a novel method for Li anode protection using MOF materials. In addition, new functionalized MOFs with significant chemical and thermal stability were designed [69]. The functionalized porosity of the material showed good Li storage capacity and high coulombic efficiency. A high energy density battery was designed for improved safety. Ogihara et al. [101] reported intercalated MOFs (iMOFs, 2,6-Naph (COOLi)2) in high-voltage bipolar batteries. The skeleton structure of the iMOF electrode material was maintained during the intercalation Li, which allowed electrons and Li+ to be transported through molecular self-assembly, thus providing the observed favorable cycle stability. The electrode material is accompanied by very small volumetric strain (0.33%) during Li embedment. In addition, the practical application of iMOFs will help to design high-energy density batteries with high safety. In addition to single metal MOFs, some bimetallic MOFs as electrode materials of LIBs have been reported. For example, Xu and co-workers successfully synthesized uniform hierarchical Fe/Co-BTC nanotubes via a one-pot solvothermal process [70]. They chose sodium phosphotungstate (Keggin-type POM, NaPW12) to adjust the synthesis of MOF crystals. They chose sodium phosphotungstate (Keggin-type POM, NaPW12) to adjust the synthesis of MOF crystals. These nanotubes revealed not only a superb cycling stability as an anode material for LIBs but also a prominent catalytic performance for the detoxification of sulfur compounds with O2. Mn-BTC MOFs have poor cyclic stability, and Li and co-workers attempted to increase battery performance by doping cobalt in Mn-BTC [102]. MnCo-BTC displayed a high capacity of 901 mA h g
1 at 100 mA g
1 after 150 cycles. 2.1.2. MOF composites The conductivity of the pure MOF materials is not good. Therefore, researchers have sought to alter the electrochemical performance by combining MOFs with other conductive materials, such as metal oxides [71,72], conductive polymers [103], single metal [104,105], carbon materials [73,75–77], and so on. To enhance the conductivity of ZIF-8, Zheng et al. [72] successfully prepared a core/shell structure of Zn2SnO4 (ZTO)/ZIF-8 nanocomposites. When tested as anode materials, the synthesized nanocomposites showed larger charge capacities and fine cycle performance. Additionally, Han et al. [104] developed a novel MOF sandwich coating method (MOF-SC) to prepare mixed LIB electrodes. The structure of MOF-SC is shown in Fig. 2a. The areal capacity of the micro-Si MOF sandwich structure, C/Si/ZIF-8, reached 1700 lA h cm
2 at 265 lA cm
2 and maintained 850 lA h cm
2 over 50 cycles. To explore the applicability of the MOF-SC method, they also studied other representative MOFs (HKUST-1, MOF-5, ZIF-67, MIL-53, and NH2-MIL53). The crystal structures of different MOFs are shown in Fig. 2b. Moreover, the capacity behaviors of micro-Si and micro-Si coated by different MOFs are displayed in Fig. 2c. The MOF layer with large pore volume and high surface area can accommodate more electrolyte and allow for faster Li+ diffusion, which further increases the rate performance of the material and reduces the overall impedance. The novel MOF-SC structure is used as a protective cushion for an effective Si anode. In addition, this unique MOF-SC is also very simple and easy to operate, which are characteristics required for future industrial applications. Recently, Co-based and Cd-based MOFs are also promising candidates as anodes for LIBs. Dong and co-workers successfully obtained two multifunctional MOFs (Co(L) MOF and Cd(L) MOF, L = 5-aminoisophthalic acid) with the same coordination mode by a facile solvothermal route at 85 °C for 24 h [73]. Furthermore, MOF/reduced graphene oxide (rGO)

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297

Fig. 2. (a) Representative procedure of the coating method, the structure of the sandwich electrode and the possible protection mechanism during cycling. The red arrows represent the diffusion of Li+. (b) Crystal structures of different MOFs (HKUST-1, MOF-5, ZIF-8, ZIF-67, MIL-53, NH2-MIL-53). (c) Capacities of the sixth cycle and capacity retentions of micro-Si and micro-Si coated by different MOFs after 30 cycles. Si/MOF represents the structure with surface coated by different MOFs. (d) Schematic illustrating the time evolution of the growth process of Ni3(HCOO)6/CNT. (e) Cyclic stability of Co(L) MOF/rGO electrode at 500 mA g
1. (a–c) Reproduced with permission [104]. Copyright 2015, American Chemical Society. (d) Reproduced with permission [75]. Copyright 2017, The Royal Society of Chemistry. (e) Reproduced with permission [73]. Copyright 2017, American Chemical Society.

composites were prepared by ball milling, and they showed a better performance than that of the Co(L) MOF and Cd(L) MOF. After 120 cycles, the discharge capacity of 639 mA h g
1 for Co(L) MOF/rGO was maintained at 500 mA g
1 (Fig. 2e). Here, the relationship between the structure and electrochemical performance of MOFs was mainly explored. An effective organic moiety-dominated Li+ insertion/extraction mechanism is also important for improving the electrochemical performance. To improve the electrochemical properties of MOFs, the [Ni3(HCOO)6]/ CNT ellipsoids were prepared via controlling the reaction conditions [75]. The growth process of the Ni3(HCOO)6/CNTs is shown in Fig. 2d. Compared with pristine [Ni3(HCOO)6], the obtained composites displayed a significantly enhanced electrochemical activity in LIBs. MOF composites acting as electrolytes have also been researched. Gerbaldi et al. [103] described the successful dispersion of Al-BTC MOFs in a poly(ethylene oxide)-based polymer

matrix for use in all-solid-state lithium batteries. The prepared nanocomposite polymer electrolyte (NCPE) membranes are highly stable to lithium metal even after an extended storage time. Interface problems restrict the high energy density of the battery. Solid electrolyte is the key to the development of lithium based batteries. A new type of solid electrolyte based on ionic liquid impregnated MOF nanocrystals (Li-IL@MOF) was reported by Wang et al. [106], and showed high ionic conductivity at room temperature. This strategy can be further extended to other battery systems (such as Na, K, Al batteries). Shen et al. [107] reported a new type of solid-state electrolyte with a biomimetic ion channel, which was achieved by complexing the anion of the electrolyte to the open metal site of MOFs. This method produces six new superionic conductors. The high ambient conductivity and low activation energy properties give lithium-based batteries excellent cycle stability and rate performance. These have opened up new avenues for MOFs as new solid electrolytes in batteries.

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In summary, MOF electrode materials retain their framework structure during Li insertion with a significantly small volumetric strain, which allows the self-assembly of electrons and Li+ transport molecules and thus provides favorable cycling stability. Despite great progress, the insulating properties of MOFs appear to prevent their suitability for electrochemical intercalation. Under actual operating conditions, there is still room to achieve a high capacity, low cost, long life and easy expansion, which require both a better understanding of the Li storage mechanism and the design of MOFs and MOF composites with better electrochemical performance. 2.2. MOFs for Li-S batteries In recently years, Li-S batteries have attracted much attention for second-generation rechargeable batteries because of their high theoretical capacity (1675 mA h g
1), specific energy density (2600 W h kg
1), and natural abundance [108,109]. Furthermore, sulfur is abundant in nature. Although sulfur has many advantages, some sulfur compounds hinder the application of Li-S batteries from academic research to industrial applications, more specifically concerning the ‘‘shuttle phenomenon” of polysulfides [110–112]. MOFs have unparalleled synthetic flexibility and adjustable pore size, and thus, researchers have begun to use MOFs

to capture and immobilize sulfur by the weak host-guest interactions. 2.2.1. Pure MOFs The study of MOFs in Li-S battery is still in the early stages. To sufficiently research MOF hosts to obtain excellent battery characteristics, the pore structure and performance of MOFs must be coordinated. MOFs are promising cathode materials for Li-S battery systems. Demir-Cakan and co-workers presented a mesoporous chromium trimesate MOF (MIL-100(Cr)) as a host material for sulfur storage, and it significantly increased the capacity retention of the Li-S cathodes [113]. In addition, it was found that the insulating mesoporous MIL-100(Cr) and SBA-15 structures are more effective for the normal operation of the sulfur electrode than the mesoporous carbon, indicating that the electrode limitation is more important for the electrode conductivity. Outstanding performance is also related to the unique topology of MIL-100(Cr), balanced polarity characteristics and high chemical stability. This method is also extended to other mesoporous oxide structures. In addition, a fast cathode with good cycling ability based on sulfur and ZIF-8 nanocrystals was reported [78]. The reaction mechanism diagram and discharge process of the S/MOF electrode system are shown in Fig. 3a,b. When 30 wt% sulfur was loaded in the electrode, a

Fig. 3. (a) Factors for consideration in the rational design of an efficient cathode made from MOF and sulfur. (b) Graphical model of the S/MOF system in the discharge process. (c) Long-term cycle abilities of S/MOFs. (d) Cycling performance of S@MOF-525(Cu). (e) Cycle stabilities of Ni-MOF/S composite at 1.5–3.0 V; inset of panel (e) is the corresponding coulombic efficiency during cycling. (f) Crystal structure of Ni-MOF containing two different types of pores represented by dark yellow sphere and blue sphere: mesopore (yellow sphere showed pore volume; gray, C; red, O; green, Ni; blue, N); micropore (blue sphere showed the pore volume). (a–c) Reproduced with permission [78]. Copyright 2014, The Royal Society of Chemistry. (d) Reproduced with permission [80]. Copyright 2015, American Chemical Society. (e,f) Reproduced with permission [115]. Copyright 2014, American Chemical Society.

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significant discharge capacity was exhibited. The decay over more than 300 cycles at 0.5 C was 0.08% each cycle (Fig. 3c). Compared with the other three samples, the ZIF-8/S was outstanding in the long-term cycling of Li-S cells. The small particle size and small pore size of the MOF host can help achieve a high capacity and cycle stability. Therefore, the particle size of the materials plays an important role in LIBs. Subsequently, ZIF-8 was chosen as the prototypical host, and five ZIF-8 samples of different particle sizes from <20 nm to >1 mm were successively prepared as S@MOF cathodes [114]. The experimental test showed that the reduction in the particle size of ZIF-8 can significantly improve the sulfur utilization, but the best cycle performance was achieved with a medium size (200 nm for ZIF-8). Subsequently, Li et al. [110] designed a MOF material with an aromatic ring antennae, and the material served as the host to capture sulfur. Targeting aromatic ring antennae in the open and semi-open channels of the framework effectively offers p-p* conjugated substrates for charge transfer and the capture of guest molecules. These channels ensure a higher sulfur loading, excellent discharge capacity, and good cycling stability. This work is also worth using in other porous materials. Sulfurcontaining non-carbonized MOF (S-MOF) cathodes were prepared by Shanthi et al. [79], and they showed initial specific capacities of 1476 mA h g
1, which stabilized at 609 mA h g
1 with nearly no fading after 200 cycles. In 2015, Wang and co-workers developed a zirconium-metal porphyrin framework (MOF-525) to create novel sulfur hosts in Li-S batteries [80]. The particular structure of the MOF-525(Cu) enables each Cu2+ site to provide two Lewis acid sites, allowing it to be a very powerful MOF host for containing sulfur and polysulfides. In all reported sulfur/MOFs composites, the S@MOF-525(Cu) cathode exhibited the best performance, with a reversible capacity of approximately 700 mA h g
1 at 0.5C over 200 cycles. The high porosity of MOF-525(Cu) further enhances its ability to absorb more sulfur. The further development of design and synthesis of mixed-MOFs will result in better MOF host for sulfur limitation

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2.2.2. MOF composites To improve the capacity and cycle stability, the composites were studied as a substrate for sulfur fixation in Li-S batteries [81–83]. Zhao et al. [81] reported chrome-MOF [MIL-101(Cr)]/S composites as cathodes in Li-S batteries. Moreover, graphene sheets were wrapped around the MIL-101(Cr)/S composites to increase their electrical conductivity for electron transfer. One of the technical problems of Li-S batteries is the shuttling of soluble polysulfides between the electrodes, resulting in a fast decline in capacity. To solve the shuttle problem, a MOF-based battery separator was proposed [82]. The schematic of the Cu3(BTC)2@GO separators in Li-S batteries is shown in Fig. 4a. Moreover, the SEM image of the multilayered Cu3(BTC)2@GO separator is shown in Fig. 4c. Li-S batteries with MOF-based separators showed a lower capacity decay of 0.019% each cycle over 1500 cycles (Fig. 4d). This is due to the synergistic effect of the MOF particles and GO laminates, as well as the size effect obtained from the homogeneous porous separator frame. The MOF-based separators is used as ionic sieves in Li-S batteries, which selectively sieves Li+ ions, and effectively inhibits the migration of polysulfides. Similarly, Bai and coworkers [83] also fabricated a new Zn(II)-MOF@GO separator for Li-S batteries, as displayed in Fig. 4b. The battery with the Zn(II)based MOF@GO separator showed a high discharge capacity (Fig. 4e). However, the electrochemical stability was not as stable as previously reported Cu3(BTC)2@GO separators in the literature. When the insulating properties, shuttle effect and volume expansion of the sulfur electrode are well solved, Li-S batteries have been widely concerned by researchers because of their high energy density, low cost and environmental friendliness. Mao et al. [116] designed a MOF/CNT thin film with unique hierarchical porous structure for flexible and even foldable Li-S battery. The Li-S battery designed here maintains the electrical connection of the restricted active sulfur, allows large volume changes during the lithiation/delithiation process, and imparts great flexibility and integrity to the electrode through the CNT interpenetrating MOF.

Fig. 4. (a) Schematic of MOF@GO separators in Li-S batteries. The MOF@GO separator acts as an ionic sieve towards the soluble polysulfides. The enlarged image illustrates the MOF pore size (approximately 9 Å), which is obviously smaller than that of the polysulfides (Li2Sn, 4 < n  8). (b) Schematic of the Zn-based MOF@GO separator. The atomic stack (right) explains the micropores. (c) SEM image of the multilayered MOF@GO separator. The inset shows a digital photo along the MOF side. (d) Cycling stability at 1 C after 1500 cycles with MOF@GO separators and after 1000 cycles with GO separators. (e) Discharge capacity and coulombic efficiency at 1 C after 1000 cycles. (a,c,d) Reproduced with permission [82]. Copyright 2016, Nature. (b,e) Reproduced with permission [83]. Copyright 2016, Royal Society of Chemistry.

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In summary, the rational design and systematic tailoring of effective storage host MOFs to enhance sulfur storage, and the full limitations of the moving redox centers provide great opportunities for designing MOFs and MOF composites. In addition, the high porosity of MOFs ensures high sulfur loading. However, the particle size and conductivity of MOFs should be considered in Li-S batteries, and MOFs with different particle sizes can be obtained by controlling the synthesis conditions. 2.3. Others MOFs are also more widely used in other batteries, such as Li-O2 batteries [117], SIBs, and so on, because of their high porosity and adjustable framework components. The theoretical specific energy (5200 W h kg
1) of Li-O2 batteries is even higher than that of Li-S batteries [118,119]. For instance, Wu et al. [120] researched MOFs as cathodes of Li-O2 batteries. The crystal structures of different MOFs are shown in Fig. 5a. These MOFs provide a wide range of surface areas, many kinds of structural topologies and unique

metal locations. Among which Mn-MOF-74 has a regular 1D open channel and abundant open metal sites and showed the highest discharge capacity of 9420 mA h g
1 at 50 mA g
1 in O2 atmosphere. The Mn-MOF-74-based battery was subjected to six complete cycles at 200 mA g
1 (Fig. 5d). Accessible metal sites in the uniform channels increase the number of O2 molecules in the pores and helpe to increase the efficiency of the reaction, which may be due to the open metal site of M-MOF-74 (M = Mg, Mn, Co). In recent years, SIBs have also attracted great research interest due to the low production costs and abundant resources [121,122]. For instance, Dong et al. [73] prepared a Co(L) MOF and Cd(L) MOF with the same coordination mode by a facile and scalable solvothermal method. However, the electrochemical properties of MOFs/rGO composites synthesized by ball milling are higher than those of pure MOFs. When used as an anode of SIBs, Co(L) MOF/rGO showed an outstanding discharge capacity (206 mA h g
1 at 500 mA g
1 over 330 cycles). They studied the effects of different metal ions and coordination water on the electrochemical properties of MOFs, and revealed the sustainability of the structure after ball

Fig. 5. (a) Crystal structures of MOF-5, HKUST-1, M-MOF-74, and a view of the 1D channel (yellow cylinder) in M-MOF-74. Blue polyhedra and spheres, red spheres, and gray spheres represent metal, oxygen, and carbon atoms, respectively. (b) SEM images of MIL-125-NH2-modified GFs and (c) UiO-66-CH3-modified GFs. (d) Cycling response of the Mn-MOF-74 based battery at 200 mA g
1. (e) Schematic of the ZIB. (f) The core-shell structure of MOF-525/s-PT composite film coated on the carbon cloth. (a,d) Reproduced with permission [120]. Copyright 2014, WILEY-VCH. (b,c,e) Reproduced with permission [123]. Copyright 2016, American Chemical Society. (f) Reproduced with permission [124]. Copyright 2017, Elsevier.

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MOFs in redox flow batteries. Chen and co-workers synthesized MOF-525 nanoparticles as electrocatalysts for counter electrodes in dye-sensitized solar cells. They also combined MOF-525 with a conducting polymer and deposited it on carbon cloth (CC) (Fig. 5f) [124]. The CC as an electrically conductive core enhances the electrochemical performance of the MOF-525/s-PT composite.

milling, and proposed the possible electrochemical mechanism of the electrode. Recently, L-Co2(OH)2BDC was reported as an anode material in potassium ion batteries (PIBs), and it also showed an excellent storage performance [84]. In addition to being used as electrodes, MOF materials can also act as electrocatalysts in batteries. Two kinds of nanoporous materials, MIL-125-NH2 and UiO-66CH3, were synthesized and deposited on the surface of graphite felt (GF) electrodes and were used as electrocatalysts in zincpolyiodide redox flow batteries (ZIBs) [123]. The novel ZIB system is based on the following redox reactions:

Cathode : I3
þ 2e
$ 3I
ðEH ¼ 0:536 V vs SHEÞ

ð1Þ

Anode : Zn $ Zn2þ þ 2e
ðEH ¼
0:7626 V vs SHEÞ

ð2Þ

Overall : I3
þ Zn $ 3I
þ Zn2þ ðE ¼ 1:2986 V vs SHEÞ

ð3Þ

2.4. Challenge and opportunity According to previous reports, the application of MOFs in batteries shows great potential because they have special structures, high porosity, large specific surface area and other versatile properties. In order to use MOFs in commercial batteries, there are still many technical problems to be solved, such as: (1) how to improve the chemical and structural stability of MOFs, (2) how to improve the particle size of MOFs, and (3) how to improve the conductivity of MOFs. The chemical and structural stability of materials used in batteries is important for improving cycle stability. In recent years, researchers have synthesized a large number of stable MOFs, such as Co-MOF-74, MIL-101, ZIF-67, etc., which are relatively stable in aqueous solutions and organic solvents. In order to accurately control the particle size of MOFs, the nucleation and growth process of the isolated MOF is crucial for the controlled adjustment of the particle size of MOFs. In order to improve the electrical

Moreover, the mechanism of ZIB is shown in Fig. 5e. The SEM images of UiO-66-CH3-modified and MIL-125-NH2-modified GFs (Fig. 5b and c) indicate that most of the particle sizes are within the 100–500 nm range. In addition, it was demonstrated that MIL-125-NH2 better promoted the redox reaction of I
/I
3 compared to UiO-66-CH3. However, UiO-66-CH3 was more stable than MIL-125-NH2, resulting in a more stable cycling performance, which will promote the development and application of more Table 2 MOFs for SCs. Electrode material

Electrolyte

SA

CD

SC

CR/CN

Refs.

Ni-MOF-24 Ni3(HITP)2 ([Ni3(OH)2(C8H4O4)2(H2O)4]2H2O Ni-DMOF-ADC013 Ni(HOC6H4COO)1.48(OH)0.521.1H2O Co8-MOF-5 Co-LMOF Co-MOF Co-MOF Co-BPDC Cu–CAT NWAs {[Cu2Cl(OH)(L)2](CH3OH)4}n CIRMOF-3-950 UiO-66 Zr-MOF HP-UiO-66 Ni/Co–MOF Ni/Co-MOF Co/Ni-MOF Zn/Ni-MOF NiCo-NFA MOF-5Ni50% rGO50% 10 wt% rGO/HKUST-1 ZIF-8/GO ZIF-67/GO Ni-MOFs@GO-3 Ni-MOF/CNT-5 CNTs@Mn-MOF MOF-5/AC-C nsp 850 PANI-ZIF-67-CC PEDOT/H-15G-CNTF MOF/PANI POAP/ZIF-67 POAP/Cu(btec)0.5DMF POAP/Cu-bipy-BTC ZIF-PPy-2 Zn/Ni-MOF@PPy Ni2CO3(OH)2/ZIF-8 Ni3(NO3)2(OH)4@Zr-MOF NiC2O4/ZIF-67 MOF-2 MnOx–MHCF

6 M KOH 1 M TEABF4/CAN 3 M KOH 2 M KOH 6 M KOH 0.1 M TBAPF6 in C2H3N 1 M KOH 5 M KOH 1 M LiOH 0.5 M LiOH 3 M KCl 6 M KOH 1 M H2SO4 6 M KOH 6 M KOH 6 M KOH 3 M KOH 1 M LiOH 3 M KOH 3 M KOH – 1 M KOH 0.5 M Na2SO4 6 M KOH 6 M KOH 2 M KOH 6 M KOH 1 M Na2SO4 6 M KOH 3 M KCl 3 M KCl 1 M H2SO4 0.1 M HClO4 0.1 M HClO4 0.1 M HClO4 1 M Na2SO4 3 M KOH 6 M KOH 6 M KOH 6 M KOH 6 M KOH 1.0 M Na2SO4

– 630 117.42 – 186.8 2900 12.15 – – 138.35 540 302 553 – 1047 490 775 – 404.5 367.1 1103.7 – 1241 876 575 74.02 41.5 – 677 73 950 – – – 36.24 1168.1 – 313.8 1073 261.1 – 818

0.5 0.1 1.4 50 1 0.01 1 1 0.6 – 0.5 0.5 – – – 0.2 1 1 – – 1 – 1 – – 1 0.5 1 1.5 – – 1 – – – 0.5 1 – – – 0.25 10

1127 117 988 395 1698 0.3 2474 2564 206.76 179.2 120 1148 239 920 1144 849 1049 530.4 – – 129.8 758 385 400 252 2192.4 1765 203.1 300 371 – 477 724 241 422 554.4 160.1 851 992 1019.7 1620 1200

>90%/3000 90%/10000 96.5%/5000 >98%/16000 94.8%/1000 92%/1000 94.3%/2000 95.8%/3000 98.5%/1000 94.3%/1000 80%/5000 90%/2000 –/10000 –/5000 –/2000 68.8%/2000 97.4%/5000 99.75%/2000 –/5000 –/5000 94.1%/5000 –/500 –/4000 –/1500 76%/1500 85.1%/3000 95%/5000 88%/3000 91.5%/3000 >80%/2000 89.8%/2000 >90%/100 –/– 91%/1000 –/1000 90.7%/10000 91.8%/3000 –/5000 –/3000 73%/2000 >91%/3000 >94.7%/10000

[129] [130] [128] [125] [131] [132] [133] [134] [135] [136] [137] [138] [139] [140] [141] [142] [143] [144] [145] [145] [146] [147] [148] [149] [149] [150] [151] [152] [153] [154] [155] [156] [157] [158] [159] [160] [161] [162] [163] [164] [165] [166]

SA: BET Surface area (m2 g
1); CD: Current density (A g
1); SC: Specific capacitance (F g
1); CR: Capacity retention; CN: Cycle number.

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conductivity of MOFs, the researchers tried to design and synthesize conductive MOFs, and also tried to develop MOF composites to further improve their conductivity.

3. MOFs for supercapacitors Supercapacitors (SCs) have drawn increasing attention as a new type of energy storage device because they have faster chargedischarge rates, higher power densities, and a longer cycle life than traditional rechargeable batteries [118,125–127]. Due to the obvious surface area and the easily adjustable pore size, MOFs have attracted many eyeballs as potential electrodes for SCs [128]. However, the direct application of MOFs as SC electrode materials is mainly faced with poor conductivity and mechanical/chemical stability. In SCs, the use of MOFs has only been widely reported by researchers in recent years. Here, we mainly summarize the application of pure MOFs and MOF composites in SCs (Table 2).

In supercapacitors, some common good organic linkers are reacted with metal salts such as transition element (Co, Ni, Mn, Cu, Zn, etc.) to form MOFs materials, which help to improve the electrochemical properties of the materials. Common good organic linkers are 1,4-benzenedicarboxylic acid (1,4-H2bdc), 2methylimidazole, 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP), etc., as shown in Scheme 2. For example, the use of 1,4-H2bdc as a ligand to form special layered structures with conductive network frameworks in MOFs, which can play an important role in improving their electrochemical performance. Transition metals are coordinated with HHTP to form honeycomb-like conductive MOFs, showing excellent electrical conductivity and good charge transport properties. In order to improve the conductivity of MOFs, researchers also use the conjugated gust molecules with redox activity to permeate into MOFs, such as 7,7,8,8-tetracyanoquinodi methane, tetracyanoethylene, N,N’-dicyanoquinonediimine, and the permeable MOFs lead to an apparent increase in electrical conductivity, which is higher than that of the nonpermeable MOFs. 3.1. Pure MOFs

Scheme 2. Schematic representation of 1,4-H2bdc (a), C4H6N2 (b), C6H12N2 (c), C18H12O6 (d), H3BTC (e), C12H4N4 (f), 4,40 -H2bpc (g) and C10H4O4N2 (h).

MOFs can be directly used as novel electrode materials due to their distinct structure combined with their pseudocapacitive redox centers. The direct application of MOFs as SC electrodes is less studied (steric hindrance to ion insertion due to their pore size and the incompatibility of MOFs and electrolytes) and still faced many difficulties and challenges [129,138,140,167]. For instance, Yang et al. [129] prepared a hierarchical 2D structure of Ni-based MOF and served as an electrode material of SCs in alkaline electrolyte. The relationship between the intrinsic properties of Ni-based MOFs and their electrochemical performance were studied. Similarly, another unique 2D-layered structure of the Ni-MOF was reported by Jiao et al. [168], and it served as an electrode in alkaline battery-supercapacitor hybrid devices (Fig. 6a). The synergistic 3
effect of Ni-MOF and Fe(CN)4
significantly enhanced 6 /Fe(CN)6 the electrochemical performance, with a high energy density of 55.8 W h kg
1 and excellent power density of 7000 W kg
1

Fig. 6. (a) Schematic diagram of the ABSHD cell. (b) Relative size of pores, electrolyte Et4N+ and BF4
ions, and acetonitrile solvent molecules shown in a space-filling diagram of idealized Ni3(HITP)2. Green, lime, blue, grey, brown, and white spheres represent Ni, F, N, C, B, and H atoms, respectively. (c) FESEM and (d) TEM images of the accordionlike Ni-MOF. (e) Galvanostatic charge-discharge curves of the device at various current densities of Ni-MOF//CNTs-COOH ABSHD in 3 M KOH + 0.1 M K4Fe(CN)6 electrolyte. (f) Ragone plots of two fabricated types of ABSHD. (g) Molecular structure of Ni3(HITP)2. (h) Capacitance retention under repeated cycling at 2 A g
1 for 10,000 cycles. (a,e) Reproduced with permission [168]. Copyright 2016, The Royal Society of Chemistry. (c,d,f) Reproduced with permission [128]. Copyright 2016, The Royal Society of Chemistry. (b,g,h) Reproduced with permission [130]. Copyright 2017, Nature.

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(Fig. 6e). The proposed ‘‘synergistic effect” can be expanded to the potential development of other MOFs with special structures. A new type of solid accordion-like Ni-MOF ([Ni3(OH)2(C8H4O4)2 (H2O)4]2H2O)//activated carbon device was assembled by our group [128]. The SEM and TEM images of the accordion-like NiMOF are displayed in Fig. 6c,d, which is made up of nanosheetlike substructures. The electrochemical measurements showed a good asymmetric capacitance behavior and cycling stability (retaining 92.8% after 5000 cycles, Fig. 6f). The excellent supercapacitor performance is attributable to the special structure of the synthesized Ni-MOF, consisting of many layered microplates, which means that there are thousands of nanochannels in the layered structure that greatly improves the diffusion of ions and electrolytes. In addition to the hierarchical structure of Ni-based MOFs, other nanoscale materials were prepared [125,131]. In 2016, Qu et al. [125] successfully prepared nickel-based, pillared MOFs with a similar topology. When tested as electrodes for SCs, the Ni-DMOF-ADC delivered high stability and good cycling stability (>98% after 16,000 cycles at 10 A g
1) compared with those of Ni-DMOF-TM and the Ni-DMOF-NDC. The outstanding electrochemical performance is due to the conversion of DMOFs to functionalized nickel hydroxide, which inherits the high stability of DMOF-ADC while

303

maintaining the integrity of charging and discharging process. However, one of the disadvantages of MOFs is poor conductivity. Recently, Sheberla and colleagues paved the way for intrinsically conductive MOFs in SCs [130]. A conductive porous Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2 (Ni3(HITP)2) showed a bulk electrical conductivity of >5000 S m
1, which was higher than that of activated carbons and holey graphite (1000 S m
1), and it served as the sole electrode material for electrochemical doublelayer capacitors (EDLCs). As shown in Fig. 6b and g, Ni3(HITP)2 is composed of stacked p-conjugated 2D layers and penetrated via a 1D columnar channel of 1.5 nm diameter. Two-electrode batteries made from conductive Ni3(HITP)2 delivered a capacity retention of over 90% after 10,000 cycles (Fig. 6h). MOF-based devices exhibit areal capacitance exceeding most carbon-based materials. The large open channel allows for fast electrolyte movement, excellent conductivity eliminates the need for conductive additives, and high surface area allows for high double-layer capacitance. Therefore, conductive MOFs are promising active materials for EDLCs, suggesting the emergence of a new generation of SCs. The earlier use of Co8-MOF-5 as an electrode material for SCs was reported by Díaz et al. [132]. However, its electrochemical properties are restricted by the special MOF and electrolyte.

Fig. 7. (a) Construct for nMOF Supercapacitors. (b) Crystal structure of Cu-CAT viewed along the c-axis. (c) Schematic synthesis of the HP-UiO-66 and schematic of PC//HPUiO-66 ASC. (d) SEM image of the Cu-CAT NWAs growing on carbon fiber paper. (e) Comparison of the specific capacitances of the electrodes of different MOF materials. (a) Reproduced with permission [171]. Copyright 2014, American Chemical Society. (b,d,e) Reproduced with permission [137]. Copyright 2017, Wiley-VCH. (c) Reproduced with permission [142]. Copyright 2017, American Chemical Society.

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Liu et al. [133] reported the synthesis of 2D layered Co-LMOF ({[Co(Hmt)(tfbdc)(H2O)2](H2O)2}n, Hmt = hexamethylenetetramine; H2tfbdc = 2,3,5,6-tetrafluoroterephthalic acid) and employed it as an electrode material for SCs. They had a high specific capacitance (2474F g
1 at 1 A g
1) and excellent cyclic stability (94.3% over 2000 cycles). The inherent properties of Co-LMOF, nanosized particles and the sufficient space available for the storage and diffusion of the electrolytes promote its electrochemical performance. Nevertheless, its performance is not good enough. Similarly, Yang and coworkers synthesized Co-MOF nanosheets with higher capacitive behavior (2564 F g
1 at 1 A g
1) and outstanding cyclic stability (95.8% of initial value over 3000 cycles) [169]. In addition, other Co-based MOF electrodes for SCs have been reported [135,136,170]. To reveal the high capacitance and long cycle behavior of MOFs, a series of 23 different nanocrystal MOFs (nMOFs) with various organic functional groups and metal ions, different pore sizes and shapes, and various structural types were synthesized [171]. To overcome the poor conductivity of these nMOFs, the nMOFs were doped with graphene to fabricate coin-type devices (Fig. 7a). These nMOFs exhibited a high capacitance; in particular, a zirconium MOF had the stack and areal capacitance of 0.64 and 5.09 mF cm
2, which were approximately 6 times that of a supercapacitor made from commercial carbon materials. These findings are a key step toward developing high-capacitance SCs in the future. Very recently, Li et al. [137] prepared conductive MOF nanowire arrays (NWAs) and demonstrated their application as the sole electrode for solid-state SCs. The main purpose was to grow a conductive MOF (Cu-CAT) on carbon fiber to form crystalline NWAs. The crystal structure of Cu-CAT is shown in Fig. 7b. The SEM image of the Cu-CAT NWAs grown on carbon fiber paper shows that the nanowires have a uniform hexagonal-prism shape and hexagonal top facet (Fig. 7d). The MOF NWAs had the highest areal capacitance (22 mF cm
2), good specific capacitances (Fig. 7e), and best rate performance of all reported MOFs for SCs because of the nanostructured morphology, excellent electrical conductivity and full utilization of the high porosity. In addition, Zr-based MOF synthesized via a simple process has also been reported, and it was evaluated as an electrode material in SCs [140–142]. However, the low conductivity and the unique microporous structure greatly limit the use of MOFs in SCs. To overcome these shortcomings, Gao

et al. [142] successfully prepared layered porous Zr-MOFs (HPUiO-66) using bimetallic Zn/Zr MOFs as precursors, which were tested as electrodes for aqueous asymmetric supercapacitor (ASC) (Fig. 7c). The specific capacitance (849 F g
1 at 0.2 A g
1) of porous HP-UiO-66 is 8.36 times that of the bare UiO-66. The layered porous structure, sufficient surface defects, and higher specific surface area and pore volume contribute to the excellent properties of the material. Bimetallic MOFs, which consist of two different central metal ions incorporated into the same framework, will provide an additional degree of structural stability and have drawn attention in recent years. Through a hydrothermal method, hollow structured 2D Ni-MOF nanoflakes and Ni/Co-MOF nanoflakes were obtained by researchers [144]. The capacitance of Ni/Co-MOF nanoflakes was significantly higher than that of pure Ni-MOFs (Fig. 8d). The lack of capacity of most MOFs has greatly hindered their application. Increasing the conductivity of MOFs by replacing Ni2+ in NiMOF with Co2+ or Zn2+ was reported by Zhang et al. [145]. It is clearly observed from the SEM images of Co/Ni-MOF and Zn/NiMOF that the Co/Ni-MOF flower-like structure is composed of many nanosheets with an average thickness of 30 nm and with relatively smooth surfaces, and the Zn/Ni-MOFs have a similar morphology (Fig. 8a,b). The abstract illustration of the mixed-metal organic frameworks (M-MOFs) and CNTs-COOHs as the electrodes of hybrid SCs is shown in Fig. 8c. M-MOFs exhibited a better electrochemical performance than Ni-MOFs (Fig. 8e). In short, the enhanced performance of M-MOFs can be attributed to the synergy contributed by improved electron conductivity, higher specific surface area and a pore size increase, which helps to increase the contact area between the electrodes and the electrolytes and to shorten the ion transport distance. 3.2. MOF composites When using pure MOFs as electrode materials for SCs, there are generally two key problems: lack of electrical conductivity and chemical stability, which both can be improved by combining MOFs with carbon materials, conductive polymers and so on [152,153,172]. Banerjee and co-workers prepared a Ni-doped MOF-5 and rGO composite [147], and the diagram is shown in

Fig. 8. (a) SEM images of the Co/Ni-MOF and (b) Zn/Ni-MOF. (c) Illustration of hybrid SCs containing M-MOFs and CNTs-COOHs as a positive and negative electrode, respectively. (d) Specific capacitances of Ni/Co-MOF nanoflakes, Ni-MOF nanoflakes, and ZIF-67 electrodes at 0.5 A g
1. (e) Cycling stabilities of the materials. (a–c,e) Reproduced with permission [145]. Copyright 2016, The Royal Society of Chemistry. (d) Reproduced with permission [144]. Copyright 2017, Springer.

Y. Xu et al. / Coordination Chemistry Reviews 376 (2018) 292–318

Fig. 9c. A composite of redox-active metals-doped MOFs and rGO exhibited a power density of 37.8 W h kg
1 at 227 W kg
1. This reaction also significantly increased the charge storage capacity of MOFs. By the ultrasonic mixing method, a composite of rGOHKUST-1 was prepared and tested as an electrode material of solid-type SCs [148]. The morphologies of rGO-HKUST-1 are displayed in Fig. 9a,b. Loading rGO onto HKUST-1 resulted in an average pore size of 8.2 nm with a mesoporous structure that exhibited a high specific surface area of 1241 m2 g
1. Electrochemical tests showed that 10 wt% rGO/HKUST-1 coated on carbon fiber paper exhibited a good cycle stability (Fig. 9d) and had a high specific capacitance, whereas pure HKUST-1 had a very low storage capability. Graphene as a carbon material can also significantly improve the performance of MOF composites [149,150]. Zhou and coworkers prepared a series of Ni-MOFs and Ni-MOFs@GO by a simple hydrothermal method. The in situ compound of the Ni-

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MOFs and GO nanosheets is illustrated in Fig. 9i. When tested as electrodes in 2 M KOH electrolyte, Ni-MOFs@3 wt% GO demonstrated a desirable capacitance (2192.4 F g
1 at 1 A g
1) and outstanding cycling endurance (85.1% capacitance retention over 3000 cycles, Fig. 9g). The Ni-MOF/CNT composite is also considered to be a promising positive electrode for asymmetric SCs, and the charge and discharge process is shown in Fig. 9h [151]. Ni-MOF/ CNT-X (X = 0, 2.5, 5, 10, different contents of CNTs) exhibited a different specific capacitance in 6 M KOH (Fig. 9e). However, Ni-MOF/ CNT-5 had the best electrochemical performance (Fig. 9f). MOFs and conductive polymers are combined to overcome the insulating characteristic of pure MOFs and to realize high performance SCs [154–158]. As a good example, Wang’s group developed an effective strategy to synthesize ZIF-67 onto carbon cloth (CC) and further electrodeposited PANI to obtain a flexible conductive porous electrode (PANI-ZIF-67-CC) [154]. This process reduced

Fig. 9. (a,b) SEM and TEM images of rGO-HKUST-1 composite. (c) The diagram for the formation of a composite of rGO and Ni-doped MOF-5. (d) Capacity retention after 4000 cycles of a single cell supercapacitor. (e) Specific capacitance of different Ni-MOF/CNTs. (f) Charge-discharge curves of Ni-MOF/CNT-5 at 0.5–10 A g
1. (g) Capacitance cycling performance of Ni-MOF-HCl-180 and Ni-MOFs@GO-3 at 10 A g
1. (h) The storage and release of electrons in the Ni-MOF/CNT composite. (i) Illustration of in-situ compound of Ni-MOFs with GO nanosheets. (a,b,d) Reproduced with permission [148]. Copyright 2015, Elsevier. (c) Reproduced with permission [147]. Copyright 2015, American Chemical Society. (e,f,h) Reproduced with permission [151]. Copyright 2015, The Royal Society of Chemistry. (g,i) Reproduced with permission [150]. Copyright 2016, American Chemical Society.

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the bulk resistance of the MOF without changing the underlying structure. Experimental data exhibited that the PANI-ZIF-67-CC electrode had a super-high areal capacitance of 2146 mF cm
2 in 3 M KCl solution, which was higher than many SCs reported in the literature (Fig. 10a). The deposited PANI effectively improves the conductivity of MOFs and enhances the Faraday process at the interface. The outstanding performance also comes from the high EDLC capacitance in the MOF inner surface area and the effective pseudo capacitance produced by PANI. This strategy may provide new ways to develop flexible solid-state SCs. More recently, researchers also used conductive polypyrrole (PPy) tubes as carriers to grow MOF particles. In order to improve the conductivity of the material, Wang et al. [159] constructed MOFs/PPy hybrids in the presence of dopamine by one-pot electrodeposition. The all-solid-state fabric fiber supercapacitors based on this mixture exhibit a high capacitance (206 mF cm
2), excellent mechanical flexibility and stable cycle life. The higher capacitance comes from the synergy of the porosity of UiO-66 and the conductivity and the pseudo capacitance of PPy. In addition, it shows great potential in wearable textile electronics. A series of ZIF-PPy-n (n = 1,2,3,5) with continuous microstructures were prepared by Xu et al. [160], as shown in Fig. 10c. The resulting ZIF-PPy-2 showed that MOF particles pass through a highly conductive PPy tube (Fig. 10e,f), which

increases electron transfer between the MOF particles and maintains a high effective porosity of the MOF. The ZIF-PPy-based flexible SC device exhibited a higher specific capacitance than the original ZIF-67, and the specific capacitance of ZIF-PPy-2 reached 554.4F g
1 (Fig. 10d). In this hybrid structure, PPy tube is used as adhesive and dispersant for MOF particles to form a ‘‘MOF-toPPy-to-MOF” conductive network. The hybrid materials show high double-layer capacitance from MOF and provide additional pseudo capacitance from PPy. Similarly, Jiao et al. [161] also introduced conductive PPy into a bimetallic organic framework and obtained a Zn/Ni-MOF@PPy electrode material. To demonstrate the potential of the designed hybrid supercapacitors (HSCs), two coin-type HSCs were connected in series to obtain a higher operating voltage (Fig. 10g and h). The assembled Zn/[email protected]//CNTsCOOH hybrid supercapacitors exhibited an energy density of up to 50.9 W h kg
1, a power density of 1338 W kg
1, and a remarkable cycle performance (Fig. 10b). Therefore, these SCs have many advantages, such as high flexibility, and have great potential for applications in flexible/wearable electronic devices. In recent years, nickel hydroxyl compounds have been commonly used as electrochemical capacitor electrode materials, and researchers have reported Ni2CO3(OH)2/ZIF-8 and Ni3(NO3)2(OH)4@Zr-MOF (UiO-66) nanocomposites demonstrating

Fig. 10. (a) Areal capacitances of the literature reported supercapacitors and that of PANI-ZIF-67-CC presented in this communication. The PANI-CC and ZIF-67-CC are control experiments. Co-Al LDH-NS/GO: Co-Al layered double hydroxide nanosheets (Co-Al LDH-NS) and GO. (b) Cycling stability of the HSC device at 10 A g
1. (c) A schematic of the preparation of ZIF-PPy. (d) GCD curves at 0.5 A g
1 of PPy, ZIF-67, and ZIF-PPy hybrids. (e,f) FESEM and TEM images of ZIF-PPy-2. (g) Digital photo of two coin-type HSCs. (h) Charge-discharge curves of a single coin-type HSC and two coin-type HSCs connected in series at 2 A g
1 (a) Reproduced with permission [154]. Copyright 2015, American Chemical Society. (b,g,h) Reproduced with permission [161]. Copyright 2017, The Royal Society of Chemistry. (c–f) Reproduced with permission [160]. Copyright 2017, American Chemical Society.

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outstanding electrochemical properties [162,163]. In addition, NiC2O4/ZIF-67 as a SC electrode was researched by Gao and coworkers [164]. Different from Gao and co-workers, Yang’s group fabricated Zn-doped Ni-MOFs with flower-like microspheres that delivered a large capacitance of 1620 F g
1 at 0.25 A g
1 [165]. The inherent properties of the Zn-doped Ni-MOF materials and the extended interlayer distance significantly improved the electrochemical performance of the material. Further, increasing attention has been paid to MOFs/metal oxides as SC electrodes [166]. Zhang et al. [166] studied the electrochemical performance of MnOx-MHCF and activated carbon. The MnOx-MHCF nanocube electrode showed a high specific capacitance of 1200 F g
1 at 10 A g
1 in 1.0 M Na2SO4 electrolyte, an ultrahigh cyclic stability with >94.7% capacitance retention over 10,000 cycles, and a high specific surface area of 818 m2 g
1. The application of MOF composites prepared via a facile in situ self-transformation approach in solid flexible SCs is of great significance for improving the electrochemical properties. In summary, MOFs offer the opportunity to tune monoatomic active metal centers, which can reduce mass consumption and increase the electrode–electrolyte interface contact. The tunable pore size in the micropore range and the ability to incorporate pseudocapacitive redox metal centers demonstrate the potential of MOFs as possible electrode materials for SCs. However, the lack of conductivity and chemical stability are two problems that can be overcome by combining MOFs with conductive materials.

3.3. Challenge and opportunity According to the research of MOFs in SCs, because MOFs have large specific surface area, it is very promising as electrode. However, MOFs still have some problems to be solved in the research of SCs, such as: (1) how to improve the conductivity and stability of MOFs, and (2) how to effectively increase the capacitance of MOFs. The low electron conductivity of most MOFs is the biggest drawback of their electrode materials as SCs. The conductivity of MOFs can be improved by infiltrating conjugates. Researchers also try to combine MOFs with conductive carbon materials, which increase electronic conductivity and enhance charge transfer char-

acteristics and stability of the materials. In fact, MOFs usually have some pseudo-capacitance because the valence variability of metal ions can serve as a redox center, but by introducing pseudocapacitive materials in the MOF system to increase the pseudocapacitance is more effective.

4. MOFs for electrocatalysts As an important class of catalysts for electrochemical energy conversion reactions, including the OER, ORR and HER, MOFs have recently attracted researchers’ attention for their potential applications in electrocatalysis (Table 3) [33,173–175]. This also meets the urgent need for clean and sustainable energy storage and conversion technologies. MOFs with high surface area and good porous structure can provide abundant metal active sites for electrocatalytic reactions as well as excellent electron transfer and rapid mass transfer [18,176]. However, the poor conductivity of MOFs hinder their further development, and therefore requires further investigation and improvement.

4.1. Pure MOFs Some Cu-based MOFs have exhibited electrocatalytic activity when directly used as active materials, and they include Cu-bipyBTC MOF [191,192], [Cu(adp)(BIB)(H2O)]n (BIB = 1,4bisimidazolebenzene; H2adp = adipic acid) [193] and so on [194,195]. Mao and co-workers synthesized a water-soluble Cubipy-BTC MOF and investigated its ORR electrocatalytic activity in phosphate buffer (pH = 6) [191]. The as-prepared sample showed two distinct redox peaks at approximately
0.15 V. The presence of O2 in the buffer significantly increased the peak current of the reduction while reducing the reverse oxidation peak current of the redox wave. Moreover, Cu-bipy-BTC was also prepared in a heterogeneous catalytic system, and its electrocatalytic properties in the oxidative carbonylation of methanol has been investigated [192]. Cu-based MOFs as modified electrodes have high catalytic activity toward H2O2 oxidation and the electrocatalytic oxidation of glucose.

Table 3 Electrocatalytic performance of different MOFs.

a b

Material

Testing condition

Eonset [V]a (V vs. RHE)

Ej=10 [v]b (V vs. RHE)

Tafel slope (mV dec
1)

Refs.

OER UTSA-16 Co-MOF/NF NiPc-MOF NiCo-UMOFNs NiFe-MOF MOF(Fe1-Co3)550N Fe/Ni2.4/Co0.4-MIL-53 ZIF-67 CUMSs-ZIF-67 CoCd-BNN (GO 8 wt%) Cu-MOF Co-MOF@CNTs (5 wt%) Ti3C2Tx-CoBDC

1.0 M 1.0 M 1.0 M 1.0 M 0.1 M 0.1 M 1.0 M 0.5 M 0.5 M 0.1 M 0.5 M 1.0 M 0.1 M

KOH KOH KOH KOH KOH KOH KOH KBi KBi KOH H2SO4 KOH KOH

1.6 1.61 1.48 1.42 – – – 1.58 1.50 – 1.19 1.51 1.51

1.638 1.541 1.58 1.48 1.47 1.62 1.449 1.78 1.64 1.583/1 mA cm
2 1.34/2 mA cm
2 1.57 1.64

77 77 74 42 34 72.9 52.2 74.9 53.7 110 65 69 48.2

[177] [178] [179] [180] [10] [181] [182] [183] [183] [184] [185] [186] [187]

ORR Ni/Co-MOF Fe-MOF@CNTs-G Ɛ-MnO2/MOF(Fe)

0.1 M KOH 0.1 M HClO4 0.1 M KOH

0.76 0.839 0.84

Half-wave potential (E1/2) 0.82 0.715 0.64

– 121.7
117

[144] [188] [189]

HER (GO 8 wt%) Cu-MOF NENU-500

0.5 M H2SO4 0.5 M H2SO4


0.087 1.41

– –

84 96

[185] [190]

Eonset for onset potential. Ej = 10 for overpotential required for the current density of 10 mA cm
2.

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Fe-, Co-, Ni-based MOFs show great potential for non-noble metal catalysis in the fields of electrochemical research [33,178,196–198]. For example, Hod et al. [199] investigated Fe_MOF-525 films as catalyst immobilizers for the electrochemical reduction of CO2. They obtained sample films by electrophoretic deposition yield an efficient surface coverage for the electrochemically active catalytic sites (approximately 1015 sites/cm2). The choice of MOF-525 as a catalyst immobilizer is due to its good molecular-grade porosity and outstanding chemical and structural stabilities. Furthermore, Fe-based MOFs can also be used as heterogeneous catalysts for ORR. Usov’s group constructed porphyrinic PCN-223-Fe frameworks by Zr6 oxo clusters and Fe(III) porphyrin linkers and grown on a conductive fluorine-doped tin oxide (FTO) substrate [200]. The PCN-223-Fe films displayed a high catalytic current when the cathode potential is applied, and obtained high H2O/H2O2 selectivity. This research opens a new path for optimizing MOF-based ORR catalysts by selecting proton sources that are better suited for the internal pore environment. Owing to the intrinsic open pore structure, the cobalt-citrate MOFs (UTSA-16) synthesized by a simple solvothermal method exhibited excellent electrocatalytic OER performance in alkaline media [177]. The overpotential of UTSA-16 is 408 mV at 10 mA cm
2, which is superior to most MOF-based electrocatalysts and standard Co3O4 counterparts. The researchers attributed the superior OER performance of UTSA-16 to its favorable structure, which has the inherent porosity of the MOF and the uniform distribution of the electroactive cobalt center in the framework. In addition, the large accessible surface and the ordered porous structure provide an open channel for effective ion diffusion, electronic motion and gas transmission, which is beneficial to the electrochemical catalysis. More

recently, Zhang et al. [178] demonstrated that Co-MOF nanosheet arrays on Ni foam shows better OER performance than UTSA-16. The Co-MOF/NF demanded an overpotential of only 311 mV at 50 mA cm
2 and exhibited a long-term electrochemical durability of at least 105 h. To improve the ORR electrocatalytic activity, Wang et al. [201] clarified the mechanism of ORR and conductive Ni-based MOF (Ni3(HITP)2) through experimental and computational data. The effects of the redox activity and the pKa of the organic ligand on the catalytic performance were studied. The 2D structure and excellent conductivity also contribute to improving the electrocatalytic performance. Through a bottom-up approach, a novel, p-conjugated conductive nickel phthalocyanine-based 2D MOF (NiPc-MOF) was designed by Jia et al. [179]. The electrochemical experiments showed that the NiPc-MOF films deposited on FTO had lower initial potentials (<1.48 V, overpotential <250 mV), high TOF values (2.5 s
1) and excellent catalytic durability. The reason non-noble metal materials exhibit outstanding electrocatalytic properties may be attributed to the electrocatalytic active sites of metal atoms. This promotes further development of MOFs in the field of new energy. In addition, 2D MOFs nanosheet-based mixtures were first synthesized by researchers and used in OER catalysts. Rui and colleagues introduced electrochemically inert Fe-MOF nanoparticles onto active 2D MOF nanosheets, demonstrating a significant increase in catalytic activity [202]. The mixed Ni-MOF@Fe-MOF catalyst showed a low overpotential (265 mV) in 1 M KOH. More importantly, the actual catalytic activity of the mixed Ni-MOF@Fe-MOF catalyst has also been revealed. In addition to the Fe-, Co-, Ni-based MOFs, bimetallic MOFs composed of those elements can also be used as good electrocatalytic materials [11,203]. Recently, Tang and his colleagues have

Fig. 11. (a) TEM image of NiCo-UMOFNs. The inset shows the Tyndall light scattering of NiCo-UMOFNs in an aqueous solution. (b) AFM image of as-prepared NiCo-UMOFNs. (c) Synthetic process of MOF nanosheet array. (d) An optical image (size: 1 cm  3 cm  1 mm) and (e) SEM images (scale bars are 300 mm) of NiFe-MOF electrodes. (f) LSV plots of NiFe-MOF, Ni-MOF, bulk NiFe-MOF and IrO2 for OER at 10 mV s
1 in 0.1 M KOH. (g) Tafel slopes of NiFe-MOF, Ni-MOF and bulk NiFe-MOF. (h) Stability test of NiFeMOF for 20,000 s at 1.42 V (vs RHE) in 0.1 M KOH. (a,b) Reproduced with permission [11]. Copyright 2016, Nature. (c–h) Reproduced with permission [10]. Copyright 2017, Nature.

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made a new breakthrough. They fabricated Ni-Co bimetal-organic framework nanosheets (NiCo-UMOFNs) with a simple ultrasonic method to achieve a 250 mV overpotential at 10 mA cm
2 [11]. The morphology of as-prepared NiCo-UMOFNs is an ultrathin sheet-like structure with a thickness of approximately 3 nm (Fig. 11a,b). The ex situ extended X-ray absorption fine structure data showed that the metal centers in the NiCo-UMOFNs are coordinatively unsaturated with the bulk crystals. The coordinatively unsaturated metal atoms are the main active centers, and the coupling effects between Ni and Co metals are essential for improving the OER activity. Not long ago, a study showed that a MOF nanosheet array could grow on the surface of different substrates by a dissolution-crystallization mechanism (Fig. 11a) [10]. The optical and SEM images of NiFe-MOF are displayed in Fig. 11d,e. As shown in Fig. 11f–h, the NiFe-MOF electrode delivered superior

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OER properties (overpotential of 240 mV at 10 mA cm
2, small Tafel plots of 34 mV dec
1, and excellent stability of 12000 s) compared to bulk NiFe-MOF and Ni-MOF. In addition, NiFe-MOF is also highly active for HER. The exposed active molecular metal sites on ultrathin nanosheets enhanced the conductivity and combined the hierarchical porosity, resulting in an improved OER and HER performance. It can be seen that the synthesis of unsaturated 2D MOF nanosheets with unsaturated coordination is expected to be a highly efficient electrocatalyst, which is crucial for the improvement of electrocatalytic performance. The performance of Fe/Ni-based MOFs can be further improved by the formation of trimetal MOFs. A series of Fe/Ni-based trimetallic MOFs (Fe/Ni/Co(Mn)-MIL-53) with high activity and stability were obtained via a simple solvothermal process and used directly as OER catalysts (Fig. 12c) [182]. The SEM of

Fig. 12. (a) The structure of NENU-501. (b) Polarization curves of the prepared catalysts in 0.5 M H2SO4 electrolyte. (c) Schematic diagram of the preparation of Fe/Ni/Co(Mn)MIL-53 and Fe/Ni/Co(Mn)-MIL-53/NF, and their direct utilization for OER. (d) The corresponding overpotential and current density of different catalysts at 10 mA cm
2 and 1.5 V vs the RHE, respectively. (e) SEM image of Fe/Ni2.4/Co0.4-MIL-53. (f) OER polarization curves of Fe/Ni2.4-MIL-53 and Fe/Ni2.4/Mx-MIL-53 (M = Co, Mn; x = 0.2, 0.4). (g) Polarization curves of Fe/Ni2.4/Co0.4-MIL-53 before and after 1000 cycles. (a,b) Reproduced with permission [190]. Copyright 2015, American Chemical Society. (c–g) Reproduced with permission [182]. Copyright 2017, Wiley-VCH.

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Fe/Ni2.4/Co0.4-MIL-53 is shown in Fig. 12e. Moreover, the optimized Fe/Ni2.4/Co0.4-MIL-53 showed excellent electrochemical performance toward the OER with an overpotential of 219 mV at 10 mA cm
2 as well as high durability, better than the other proportions of trimetal MOFs (Fig. 12d,f,g). The Ni species in MOFs act as active sites to promote electrolyte diffusion and protoncoupled electron transfer, while Fe, Co or Mn can regulate the intrinsic properties of the catalyst. High electrocatalytic properties are mainly attributed to the distinct structure and porosity of the material, as well as the synergistic effect of the mixed metals. However, so far only a few studies have reported inkjet-printed MOF films. Su et al. [204] prepared zirconium-based porphyrinic MOFs (MOF-525) with a similar surface area of 2500 m2 g
1 and a crystal size of 100 to 700 nm by an inkjet printing technology. The effects of film thickness and crystal size on the morphology of the printed film and electrocatalytic activity are studied in detail. The functionalization of MOFs has an influence on the OER thermodynamics. Musho et al. [205] designed UiO-66 (Zr) MOFs, and predicted the thermodynamic barriers of OER for three functionalized MOFs using density functional theory (DFT). However, beyond that, polyoxometalate-based MOFs (POMOFs) have been studied as hopeful electrocatalysts for HER [190,206]. Using POM fragments as the nodes and benzene tribenzoate as the linkers, two new POMOFs, NENU-500 and NENU-501, with good stability were prepared [190]. The structure of NENU-501 is shown in Fig. 12a. Notably, NENU-500 exhibited an initial overpotential of 180 mV and an overpotential of 237 mV at 10 mA cm
2 in acidic solution, which was better than other similar materials (Fig. 12b). NENU-500 and NENU-501 retained their HER catalytic activity over 2000 cycles. The POM-based MOF combines the redox properties of the POM part and the porosity of the MOF structure, which may contribute to the generation of hydrogen in electrocatalysis. The results of this experiment offer new ideas for the construction of high-porosity and stable POMOFs for the application of novel hydrogen-evolving electrocatalysts. At present, the coordination of unsaturated metal sites (CUMSs) as the catalytic center of the OER is also very popular in the field of

research, but it is challenging to control CUMSs in MOFs. Tao et al. [183] adjusted the coordination geometry of ZIF-67 to form CUMSs-ZIF-67, which provides a fast 4-elecron reaction with comparable catalytic activity to that of commercial RuO2. Similarly, by doping Co(II) into inactive Cd-MOFs, Maity and co-workers reported electrochemically active CoCd-MOF [184]. The overpotential required to generate 1 mA cm
2 using CoCd-BNN was found to be only 353 mV under alkaline conditions and maintained exceptional stability over 1000 cycles. This study highlights a simple, green approach to preparing water oxidation electrocatalysts. 4.2. MOF composites The inherent conductivity of most MOFs is less than ideal, and researchers have developed some MOF composites to improve the performance [186,207]. For example, Jahan and co-workers studied a GO-incorporated Cu-MOF composite as a tri-functional catalyst for the HER, OER, and ORR [185]. The composites showed higher currents and smaller overpotentials in the three electrocatalytic reactions and better stability in 0.5 M H2SO4 electrolyte compared to pure MOF. The (GO 8 wt%) Cu-MOF composite exhibited the best HER performance among all the obtained materials (Fig. 13f). This study solves the stability of the copper-based complexes in acidic media and improves the stability of the skeleton. A nanocomposite made of porphyrinic MOF-525/graphene nanoribbons (GNRs) were successfully prepared by Kung and co-workers [208]. Among them, interconnected GNR acts as a conductive bridge, providing easy charge transfer. Films of MOF-525/GNR nanocomposites showed better electrocatalytic activity for nitrite oxidation. In addition, CNTs also improve electron transport. In the study of Fang et al., non-noble metal-based Co-MOF@CNTs with high activity and strong durability for OER and ORR bifunctional catalysts was synthesized by the self-assembly method, as shown in Fig. 13a [186]. The Co-MOF@CNTs (5 wt%) have a comparable OER and ORR catalytic activity with RuO2 and 20 wt% Pt/C catalysts (Fig. 13b,c,e). The outstanding catalytic activity is usually ascribed to the distinct 3D layered structure and the synergy

Fig. 13. (a) The preparation of the catalyst Co-MOF@CNTs. (b) LSV curves of CNTs, Co-MOF, Co-MOF@CNTs (15 wt%), Co-MOF@CNTs (10 wt%), Co-MOF@CNTs (5 wt%) and CoMOF@CNTs (1 wt%) in 1.0 M KOH electrolyte at 5 mV s
1. (c) Durability test for the Co-MOF@CNTs (5 wt%) over 1000 cycles (inset: TEM of the Co-MOF@CNTs (5 wt%) after the stability test). (d) Illustration of the synthesis of Fe-MOF@CNTs-G. (e) ORR LSV curves of the CNTs, Co-MOF, Co-MOF@CNTs (5 wt%) and 20 wt% Pt/C at 5 mV s
1 (rotation rate: 1600 rpm). (f) Polarization curves of Pt/C, (GO 2, 4, 6, and 8 wt%) Cu-MOF, and Cu-MOF in N2-saturated 0.5 M H2SO4 at 2 mV s
1. (a–c,e) Reproduced with permission [186]. Copyright 2016, Elsevier. (d) Reproduced with permission [188]. Copyright 2017, The Royal Society of Chemistry. (f) Reproduced with permission [185]. Copyright 2013, Wiley-VCH.

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between the redox center Co(II), organic ligands and CNTs. Recently, Wang et al. [188] reported efficient ORR catalysts of 3D Fe-N doped MOF@CNTs/graphene (Fe-MOF@CNTs-G) hybrids. The preparation process of the hybrids is shown in Fig. 13d. For the zinc-air batteries, the catalyst exhibited outstanding activity and charge-discharge stability. Although some efforts have been made to use non-conductive MOFs or their composites with carbon nanotubes and graphene oxide as electrocatalysts, their performances were not very good. To develop an efficient MOF-based electrocatalyst, Cho et al. [209] selected CuS as a pore inclusion to obtain nano-CuS(x wt%) @Cu-BTC composites with higher ORR catalytic activity than the single component. The field emission-SEM (FE-SEM) and TEM images of the nano-CuS(x wt%)@Cu-BTC composites are shown in Fig. 14a–c. In the experiment, the conductivity and porosity of the material were adjusted by the amount of nano-CuS. All nano-CuS(x wt%)@Cu-BTC provided an increased kinetic current density value compared to Cu-BTC and nano-CuS(99 wt%), as shown in Fig. 14d. Moreover, CuS(28 wt%)@Cu-BTC showed the highest activity (initial potential of 0.91 vs. RHE). This is due to

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the synergistic effect of two different materials, Cu-BTC and CuS provide porosity and conductivity, respectively. The unique ƐMnO2/MOF(Fe) composite with a large specific surface area and high porosity also has a good ORR catalytic activity [189]. The strong interaction between Ɛ-MnO2 and MOF(Fe) stabilizes the structure of the ORR catalytic sites, resulting in better stability. By an inter-diffusion reaction, Zhao et al. [187] obtained 2D cobalt 1,4-benzenedicarboxylic acid (CoBDC)-Ti3C2Tx nanosheet hybrid material for OER applications. The preparation process is shown in Fig. 14e. The hybrid material achieved an excellent doublelayer capacitance and a Tafel slope of 48.2 mV dec
1 in 0.1 M KOH electrolyte, which was significantly better than the standard IrO2-based catalysts (Fig. 14f,g). Hybrid nanosheets were further used as cathodes of rechargeable zinc-air batteries, and they exhibited a good charge-discharge cycle performance (Fig. 14h,i). In addition, some other new MOF composites have been reported by researchers because of their excellent catalytic performance [210–212]. These catalysts can be used as metal-air batteries and fuel cells as efficient non-noble metal catalysts for alkaline electrolytes.

Fig. 14. FE-SEM images of (a) nano-CuS(5.3 wt%)@Cu-BTC and (b) nano-CuS(28 wt%)@Cu-BTC. (c) TEM images of nano-CuS(28 wt%)@Cu-BTC. (d) Kinetic current density at 0.40 V and 0.55 V vs. RHE for Cu-BTC (I), nano-CuS(1.4 wt%)@Cu-BTC (II), nano-CuS(5.3 wt%)@Cu-BTC (III), nano-CuS(8.8 wt%)@Cu-BTC (IV), nano-CuS(28 wt%)@Cu-BTC (V), nano-CuS(56 wt%)@Cu-BTC (VI), nano-CuS(99 wt%) (VII), and commercial 20 wt% Pt/C (VIII). (e) Schematic of the synthesis of Ti3C2Tx-CoBDC composite for OER. (f) Tafel plots of various electrodes modified by Ti3C2Tx, CoBDC, IrO2, and Ti3C2Tx-CoBDC hybrid in N2-saturated 0.1 M KOH (scan rate: 1 mV s
1). (g) Current density difference at 1.13 V plotted against scan rate to give the double-layer capacitance (Cdl) for CoBDC, Ti3C2Tx, and Ti3C2Tx-CoBDC composite. (h) Schematic of a rechargeable Zn-air battery. (i) Charge-discharge curves of the rechargeable Zn-air batteries at 0.8 mA cm
2. (a–d) Reproduced with permission [209]. Copyright 2016, Wiley-VCH. (e–i) Reproduced with permission [187]. Copyright 2017, American Chemical Society.

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The electrocatalytic activity generally depends on the accessible active center, conductivity, and electrode geometry. MOFs have abundant intrinsic metal active sites but are rarely used in electrocatalysis due to their poor conductivity and small pore size. This needs to be further overcome. For example, 2D MOF nanosheets with a thickness of several nanometers are expected to provide most of the exposed active atoms and rapid mass and ion transfer. Combining them with conductive nanostructures is expected to further enhance their performance in various electrocatalytic processes.

4.3. Challenge and opportunity According to the above studies, most MOFs are efficient catalysts in electrocatalysis due to their high surface area and permanent porosity. However, in the research, there are still some challenges, such as: (1) how to overcome the poor conductivity of pure MOFs, and (2) how to effectively control the pore structure of MOFs. Researchers can load catalytic nanoparticles or object materials into the MOF, in which MOFs act as porous templates, thus making the material higher conductivity and enhanced electrocatalytic activity. In addition, the coexistence of micropores-mesoporesmacropores in MOFs is very necessary. The development of MOFs with multiple structure, composed of hierarchical pores and 2D nanosheets, will provide more active sites for the material, which is of great significance for improving the catalytic performance.

5. MOFs for electrochemical sensors Recently, due to the development of nanoscience and nanotechnology, nanostructured MOFs have also made important progress in the application of electrochemical sensors. The redox and catalytically active sites introduced through the use of active metal ions and/or ligands endow MOFs with electrochemical sensing capabilities [36,213–215]. Therefore, the latest progress of MOFs has made outstanding achievements in the detection of electrochemical sensing in H2O2, glucose, heavy metal ions, and so on (Table 4) [216,217].

5.1. Pure MOFs In the past, MOFs were rarely used for conductivity-based sensing applications because most MOFs are insulating materials. However, the application of MOFs in non-enzymatic electrochemical sensors has aroused widespread concern in recent years [14,237]. Zhao et al. [218] synthesized ZIF-67 crystallites with porous {1 1 0} crystal planes by a simple and rapid method and were used to detect glutathione (GSH). The prepared ZIF-67-modified electrode can significantly increase the GSH signal during testing. In addition, Co-MOFs can also be used as the active substance of high efficiency hydrogen peroxide (H2O2) sensors [219,238]. The modified electrode showed a high electrocatalytic activity (low detection limit, wide linear range, and high sensitivity) for the reduction of H2O2. Therefore, Co-MOFs have great potentials for application in H2O2 electrochemical detection. Furthermore, The novel Cu-MOF [Cu(adp)(BIB)(H2O)]n also has a certain response to H2O2 in alkaline solutions with a linear range of 0.1–2.75 lM and a detection limit of 0.068 Μm [193]. Cu-MOF also showed high non-enzymatic electrocatalytic activity for H2O2 and glucose (GCE) [220,221]. For example, our group synthesized [Cu3(btc)2] nanocrystals with different shapes (nanocubes S1, truncated cubes S2, cuboctahedrons S3, and octahedrons S4) [221]. The FE-SEM images of the obtained [Cu3(btc)2] nanocubes are shown in the Fig. 15a,b. Compared to other electrodes, [Cu3(btc)2] nanocubes showed the best non-enzymatic electrocatalytic glucose activity as well as excellent sensitivity due to the synergistic effect of the porous nanocubes (Fig. 15c,f). In addition to detecting H2O2 and glucose in Cu-MOFs, 2D Cu-MOF nanosheets have also been used as effective enzyme inhibitors. Xu et al. [239] demonstrated that the activity of a-chymotrypsin (ChT) can be effectively inhibited with 96.9% inhibition by 2D Cu(Bpy)2(OTf)2 nanosheets, while Zn2 (bim)4 nanosheets did not significantly inhibit ChT. The results show that the Cu(II) center of the 2D nanosheet is combined with the 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid molecule in the buffer, resulting in electrostatic interaction between the nanosheet and the enzyme. Porphyrin MOFs constructed by porphyrin or metalloporphyrin linkers have been reported by researchers [222]. By using the solvothermal method, Kung et al. [222] grew uniform MOF-525

Table 4 Comparison of electrochemical performance of different MOFs sensors. MOFs

Modified electrode

Detection limit/mM

Linear range/lM

Sensitivity/mA mM
1 cm
2

Test sample

Refs.

ZIF-67 Co-MOF Cu-MOF Cu-MOF Cu-MOF Zr-MOF (Fe-P)n-MOF TMU-16-NH2 Cu-MOF Cu-MOF Cu-MOF Cu-based MOF-199 Cu-based MOF-199 {[Cu2(bep)(ada)2]H2O}n HKUST-1 Cu-TDPAT MOF-5 MOF-5 Zn-TSA Zn-TSA Zn-TSA ZIF-8 UiO-66-NH2 Ni-MOF PCN-333 (Al)

ZIF-67 Co(pbda)(4,4-bpy)2H2O]n Cu(adp)(BIB)(H2O)]n MOF-14 [Cu3(btc)2] MOF-525 (Fe-P)n-MOF-Au TMU-16-NH2 Cu-MOF–GN-3/GCE Cu-MOF–GN-3/GCE Cu-hemin MOFs/CS-rGO MOF-ERGO-5/GCE MOF-ERGO-5/GCE Cu-MOF/AB-2%/GCE SGO@HKUST-1 Cu-TDPAT-nERGO GC/Au-MOF-5 GC/Au-MOF-5 IL/Mb/Ag@Zn-TSA-CPE IL/Mb/Ag@Zn-TSA-CPE IL/GOx/Ag@Zn-TSA-CPE CuxO NPs@ZIF-8 UiO-66-NH2@PANI/GCE Ni-MOF/MWCNTs 3D-KSC/PCN-333 (Al)@MP-11

– 3.76 0.068 1 – 2.1 0.02  10
3 0.2 mg L
1 2 0.02 0.019 0.1 0.1 0.014 0.49 0.17 1 15.3 0.15 0.5 0.8 0.15 0.3 mg L
1 3 0.127

0.125–400 5–9000 0.1–2.75 1–900 0.125–2250 20–800 0.03  10
3–1 0.7–120 mg L
1 10–11180 0.5–6965.5 0.065–410 0.1–566 0.1–476 0.05–3 1.0–5600 4–12000 – – 0.3–20000 1.3–133000 2.0–1022 1.5–21442 0.5–600 mg L
1 10–1120 0.387–1725

21.5 83.1 – – 549 95 – – 57.73 – 14.5 – – – 135.4 – 0.23 0.43 1.27 – – 178 – 685 168

Glutathione H2O2 H2O2 H2O2 glucose nitrite Pb2+ Cd2+ H2O2 ascorbic acid H2O2 catechol hydroquinone H2O2 H2O2 H2O2 nitrite nitrobenzene H2O2 NO
2 glucose H2O2 Cd2+ urea H2O2

[218] [219] [193] [220] [221] [222] [223] [224] [225] [225] [226] [227] [227] [228] [229] [230] [231] [231] [232] [232] [232] [233] [234] [235] [236]

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Fig. 15. (a,b) FE-SEM images of the prepared [Cu3(btc)2] nanocubes. (c) Current-time responses at 0.60 V after successive injections of different amounts of glucose into stirring 0.1 M NaOH solution. (d) Crystal structure of MOF-525. (e) Fluorescence intensity of compounds in DMA with the introduction of diverse solvents (blue) and introduction of NB (red). (f) Plot of electrocatalytic current of glucose versus its concentrations within 0.125–2250 mM, (inset, 0.125–150 mM). (a–c,f) Reproduced with permission [221]. Copyright 2014, The Royal Society of Chemistry. (d) Reproduced with permission [222]. Copyright 2015, Elsevier. (e) Reproduced with permission [240]. Copyright 2017, American Chemical Society.

films on a conductive glass substrate, and the crystal structure is shown in Fig. 15d. When the membrane was used as a nitrite sensor, the linear range, sensitivity and detection limit were 20–800 mM, 95 mA mM
1 cm
2 and 2.1 mM, respectively. The study found that MOF-525 film has electrochemical addressability and stability in aqueous solution, and is suitable for electrochemical sensing applications. Wang et al. [223] also designed a new DNAfunctionalized iron-porphyrinic MOF as a highly selective electrochemical sensor for Pb2+. In addition to the detection of Pd2+, researchers fabricated TMU-16-NH2 ([Zn2(NH2-BDC)2(4-bpdh)] 3DMF) to determine Cd2+ by a simple electrochemical method [224]. MOF was used to improve the sensitivity to the cadmium ion. Very recently, a few researchers have also used synthetic MOFs as electrochemical chiral sensors and highly selective and sensitive bifunctional luminescent sensors [240,241]. In 2017, Yang’s group prepared two MOFs, e.g., [Ni(DTP)(H2O)]n (I) and [Cd2(DTP)2 (bibp)1.5]n (II) by using a novel electron-rich dicarboxylic acid

ligand H2DPT [240]. The test showed that Ni-MOF(I) showed an antiferromagnetic and electrochemical performance. Cd-MOF(II) exhibited good sensitivity and selectivity for nitrobenzene, and the effect of organic solvents on the anti-interference ability of Cd-MOF(II) was also studied (Fig. 15e). The results showed that the coordination polymer II may be a promising bifunctional luminescent sensor for rapid detection. Our research group formed ultrathin Ni-MOF (Ni20(C5H6O4)20(H2O)8]40H2O) nanobelts for the detection of glucose, which proved to be a highly efficient catalytic electrode [242]. 5.2. MOF composites At present, the research on pure MOFs in electrochemical sensors is still in its infancy, so people are trying to develop some MOF composites that respond to H2O2 and glucose to improve their performance. The nanocomposites of copper-based MOFs and

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carbon materials have always been the focus of researchers [225– 228]. Wang et al. [229] designed a non-enzymatic sensor by using a unique flower-like solvothermal-reduced GO (SGO)@HKUST-1 as the matrix for H2O2 detection. The preparation process and its application are displayed in Fig. 16a. The SEM image shows that the synthesized HKUST-1 has a regular octahedral configuration, as shown in Fig. 16c. GO is an effective structure-directing agent, when the added amount reaches 0.13 mg mL
1, all the octahedral HKUST-1 particles turned into layered flower-like SGO@HKUST-1 (Fig. 16d). This morphological change may result in a larger effective surface area and a higher mass transfer. The electroactive nanocomposite had a high selectivity for H2O2 reduction and retained at least 97% of its initial response after 15 days. The synergistic effect of SGO with high electrical conductivity and

HKUST-1 with suitable structure and electrochemical response promotes the electrochemical performance of the materials. Li and co-workers also reported bifunctional rht-type MOFs (CuTDPAT) and a nanosized electrochemically reduced graphene oxide (n-ERGO) sensor that is sensitive and selective for H2O2 [230]. Cu-TDPAT-nERGO exhibited a significantly enhanced electrocatalytic activity compared to that of Cu-TDPAT, highlighting the importance of n-ERGO in improving its performance. Recently, some Au nanoparticles (Au NPs) or Ag have been combined with MOFs with large inner surface areas and special structural features to improve the electrochemical performance [231,232]. Nitrite and nitrobenzene are considered harmful pollutants. Yadav’s group reported the sensitive determination of nitrite and nitrobenzene by Au-MOF-5(Zn) and Ag@ MOF-5(Zn) [231,243].

Fig. 16. (a) Illustration of the synthesis of SGO@HKUST-1 and its application. (b) Variation of the CS-SGO@HKUST-1/GCE response currents in the presence of 0.2 mM H2O2 in PBS at pH 7.0 tested every 3 days over 15 days. SEM images of (c) SGO@HKUST-1 (d) addition of GO during synthesis was 0.13 mg mL
1. (e) Amperometric response curves of CuxO NPs@ZIF-8/GCE upon successively adding H2O2 at 0.7 V. Left inset: amperometric response of H2O2 at lower concentration; right inset: the relationship between current signal and H2O2 concentration. (f) Illustration of the synthesis of CuxO NPs@ZIF-8. (g) Current response of CuxO NPs@ZIF-8/GCE to 50 lM H2O2, glucose, D-fructose, sucrose, alactose, L-cysteine, AA, UA and DA. (a–d) Reproduced with permission [229]. Copyright 2016, American Chemical Society. (e–g) Reproduced with permission [233]. Copyright 2016, American Chemical Society.

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The proposed sensor also has a low detection limit and high selectivity. Based on the different thermal stability of two MOFs, CuxO NPs@ZIF-8 polyhedra with core-shell heterostructures were also prepared by pyrolysis of nHKUST-1@ZIF-8 core-shell complexes (Fig. 16f) [233]. When used to build H2O2 sensors, the linear range of the composite was 1.5–21442 lM and showed high sensitivity (178 lA mM
1 cm
2) and selectivity (Fig. 16e,g). The materials presented here provide a new technique to increase the selectivity of non-enzymatic electrochemical sensors. Very recently, a few studies have applied porphyrin MOFs in electrocatalytic sensors [208,234,244]. Streptavidin-functionalized zirconium-porphyrin MOFs (PCN-222@SA) synthesized by covalent bonding were used as a signal nanoprobe in DNA sensors for signal transduction [244]. Kun et al. [208] also used a synthetic porphyrinic MOF/graphene nanoribbon (MOF-525/GNR) nanocomposite film for nitrite oxidation. In addition, a new type of conductive electrochemical sensor, PANI was uniformly coated on the surface of UiO-66-NH2 to obtain UiO-66-NH2@PANI and used for the reliable detection of Cd2+ ions [234]. This method of coating MOFs is used to construct highly sensitive electrochemical sensors to detect heavy metal ions. Recently, Ni-MOF composites have been examined in non-enzymatic urea detection and glucose oxidation. For instance, porous Ni-MOF and multiwalled carbon nanotube composites with very high sensitivity (685 mA mM
1 cm
2) used as non-enzymatic sensors for urea detection were reported by Quynh et al. [235]. By adopting a highly efficient one-step calcination method, our group combined Ni, NiO, and a carbon frame with Ni-MOF to obtain Ni-MOF/Ni/NiO/C composites, which was coated on glassy carbon to construct highly efficient non-enzymatic glucose and H2O2 electrochemical sensors [245]. Compared with NiMOF, the obtained nanocomposites greatly improved the electrochemical performance of the sensor. The researchers constructed a ternary metal nanoparticles@Y-1,4-NDC-MOF/ERGO (Metal = Ag, Cu) complex through the cation exchange strategy, showing low detection limits, wide linear range, high stability, and the sensor is used to measure H2O2 released by living cells [246]. In addition, some other researchers also reported the application of MOF composites in electrochemical sensors [236]. Due to advances in nanoscience and nanotechnology, MOFbased sensors have significant potential for the development for clinical, environmental and industrial applications. The use of one-component MOFs as electrodes in electrochemical sensors often results in narrow linear ranges, a low sensitivity, and poor stability due to their low electron conductivity and poor mechanical properties. In contrast, MOF composites can overcome the drawbacks of one-component MOFs to further improve the electrochemical properties.

5.3. Challenge and opportunity According to the above research, MOFs are becoming a very effective tool for electrochemical sensing applications. However, there are still some challenges, such as: (1) how to increase the signal transduction of MOFs, and (2) how to control the size of MOFs. Most MOF materials are insulators or semiconductors, and they still need to improve their electrical conductivity and redox activity to enhance their performance. The porosity of the material by combining MOFs with a variety of functional materials, such as carbon nanostructures, metal nanoparticles and polymers, facilitates the introduction of nonnative conductivity. In addition, the nanoscale size of MOFs are synthesized by changing the synthetic conditions, because large-size MOFs cannot ensure the rapid absorption and balance of analytes, resulting in longer response time for transportation.

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6. Other electrochemical applications In addition to outstanding performance in electrochemical energy storage, electrocatalysis, electrochemical sensing, etc., MOFs are also used in other electrochemical fields [247–249]. For instance, Kung et al. [247] grew a new MOF (NU-901) film on a conductive glass substrate and showed fast and reversible electrochromism. The rigid MOF stabilizes the radical cation of the strongly colored pyrene linker, resulting in a reversible color change of the MOF film. The electrochromic behavior is promoted by the nanoscale pores of the MOF and its unusual morphology. However, Li et al. [248] electrodeposited Zn4O(1,4benzenedicarboxylate)3 (MOF-5) with Zn4O(O2C-)6 as the structural unit under cathodic bias. This discovery revealed a relatively complex variety of electrochemical processes responsible for the crystallization of MOF-5, further emphasizing the importance of forming secondary Zn4O (O2C-R) building units from hydroxide intermediates in this landmark material. 7. Conclusion and perspective In summary, this review outlined the latest developments in the applications of MOFs and MOF composites in electrochemical devices, such as batteries (Li-ion batteries, Li-S batteries, Li-O2 batteries, etc.), supercapacitors, electrocatalysis, electrochemical sensing and so on, because of the high surface area, tunable porosity and uniform channels, constructed by metal ions and organic ligands. It is well known that mesoporous materials with large surface areas can enhance the diffusion kinetics by reducing the path of electron and ion transport in the batteries. However, the applications of many MOFs are limited by critical drawbacks, such as low stability in air and water. Therefore, it is very important to explore stable MOFs for electrochemical applications. Compared with that of transition metal oxides, the capacitance of MOFs is usually lower, which may be related to their poor conductivity, and the pore size is not suitable for electrolyte diffusion. To solve the problems of these MOF electrode materials: (1) The particle size is reduced to the nanoscale by a suitable method, or a new type of layered nanostructure is constructed, which will lead to an obvious decrease in the diffusion distance of the electrolyte ions and an increase in the specific surface area of the active material. Thus, the electrochemical characteristics will be significantly improved, such as the increase in the specific capacitance, and so on. (2) High conductivity MOFs are expected to be developed and used in electrochemical devices. The conductive MOFs have much higher the surface area than conventional MOFs fabrication under mild synthetic conditions, and their conductivity is very close to that of graphite. However, the use of common organic linkers to build conductive MOFs remains a challenge. (3) MOF composites help increase their conductivity and greatly improve the conductivity of the electrode. Most pure MOFs have poor conductivity, and therefore, researchers sought to improve their electrochemical properties by combining MOFs with conductive materials, such as carbon materials, conductive polymers, etc. In summary, MOFs and MOF composites show great potential in the electrochemical field because of their unique properties. Nevertheless, there are still problems and room for improvement in the synthesis and electrochemical performance of MOFs and MOF composites. Creating an effective method for the large-scale

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production of MOFs and MOF composites with small size and high conductivity will facilitate their rapid development in practical electrochemical applications. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC-21671170, 21673203, and 21201010), the Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP), Program for New Century Excellent Talents of the University in China (NCET-13-0645), the Six Talent Plan (2015-XCL-030), and Qinglan Project. We also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions and the technical support we received at the Testing Center of Yangzhou University.

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Yuxia Xu is now a graduate student under Prof. Pang and Xue’s supervision, Yangzhou University of chemistry and chemical engineering, China. Her research mainly focuses on the field of electrochemical energy storage materials and their applications for supercapacitors.

Huaiguo Xue received his Ph.D. degree in polymer chemistry from the Zhejiang University in 2002. He is currently a professor of physical chemistry and the dean of the College of Chemistry and Chemical Engineering at the Yangzhou University. His research interests focuses on electrochemistry, functional polymer and biosensors.

Huan Pang received his Ph. D. degree from Nanjing University in 2011. He now is a university distinguished professor in Yangzhou University. In the past 10 years, his group has been engaged in the design and synthesis of functional nanomaterials, especially for MOF-based materials. He has published more than 200 papers in peer-reviewing journals including Chemical Society Reviews, Advanced Materials, Energy Environ. Sci., with 5300 citations (H-index = 40). His research interests include the development of inorganic nanostructures and their applications in nanoelectrochemistry with a focus on energy devices.