Metal–Organic Framework-Derived Nanoporous Metal Oxides toward

Jun 14, 2017 - Transition metal oxides (TMOs) have attracted significant attention for energy storage applications such as supercapacitors due to thei...
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Metal−Organic Framework-Derived Nanoporous Metal Oxides toward Supercapacitor Applications: Progress and Prospects Rahul R. Salunkhe,†,§ Yusuf V. Kaneti,†,§ and Yusuke Yamauchi*,†,‡ †

International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Australian Institute of Innovative Materials (AIIM), University of Wollongong, North Wollongong, New South Wales 2500, Australia ABSTRACT: Transition metal oxides (TMOs) have attracted significant attention for energy storage applications such as supercapacitors due to their good electrical conductivity, high electrochemical response (by providing Faradaic reactions), low manufacturing costs, and easy processability. Despite exhibiting these attractive characteristics, the practical applications of TMOs for supercapacitors are still relatively limited. This is largely due to their continuous Faradaic reactions, which can lead to major changes or destruction of their structure as well phase changes (in some cases) during cycling, leading to the degradation in their capacitive performance over time. Hence, there is an immediate need to develop new synthesis methods, which will readily provide stable porous architectures, controlled phase, as well as useful control over dimensions (1-D, 2-D, and 3-D) of the metal oxides for improving their performance in supercapacitor applications. Since its discovery in late 1990s, metal−organic frameworks (MOFs) have influenced many fields of material science. In recent years, they have gained significant attention as precursors or templates for the derivation of porous metal oxide nanostructures and nanocomposites for next-generation supercapacitor applications. Even though these materials have widespread applications and have been widely studied in terms of their structural features and synthesis, it is still not clear how these materials will play an important role in the development of the supercapacitor field. In this review, we will summarize the recent developments in the field of MOF-derived porous metal oxide nanostructures and nanocomposites for supercapacitor applications. Furthermore, the current challenges along with the future trends and prospects in the application of these materials for supercapacitors will also be discussed. KEYWORDS: metal−organic frameworks, porous metal oxides, nanocomposites, supercapacitors, energy storage, MOF-derived oxides, hybrid materials, hollow nanomaterials

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discharge and conventional capacitors that are comparatively faster for charge−discharge (power density, PD) but have a lower energy storage capacity. Traditional supercapacitors known as electrochemical double-layer capacitors (EDLCs) typically employ carbonbased materials with high surface area and porosity. EDLCs store energy by forming an ionic double layer which diffuses very quickly, hence providing a very rapid discharge as well as higher power densities.3 However, the ED of EDLCs and the

enewable energy technologies, such as solar power, wind power, hydroelectricity, and biomass, have made significant advances in securing the energy future of mankind. The increased utilization of renewable energy resources will require a drastically increased ability to store charge. As such, the development of efficient energy storage devices has become increasingly crucial.1 The electrochemical capacitors, also called supercapacitors, can store energy in two closely spaced layers with opposing charges and are used to power up electrical vehicles, consumer electronics, memory backups, and military devices.2 Electrochemical supercapacitors bridge the gap between batteries that offer a high energy storage capacity (energy density, ED) but are slow in charge− © 2017 American Chemical Society

Received: April 22, 2017 Accepted: June 7, 2017 Published: June 14, 2017 5293

DOI: 10.1021/acsnano.7b02796 ACS Nano 2017, 11, 5293−5308

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Scheme 1. Schematic Illustration Showing the Fabrication of Porous Metal Oxide Nanostructures with Large Surface Area and High Porosity from Metal−Organic Frameworks (MOFs) Composed of Metal Ions and Organic Linkers as Precursors through a Two-Step Annealing in Nitrogen (N2) and Air, Respectively

1990s.15 Due to the variety of existing metal ions and organic linkers, limitless combinations of MOFs can be developed to suit the targeted application. Depending on the organic and inorganic units, structurally, MOFs can be prepared in onedimension (1-D), two-dimension (2-D), or three-dimension (3-D). This has led to the development of well over 20 000 MOFs in the past two decades.16 Furthermore, the pore size of MOFs can be tuned in order to improve their functional performance. Thus, depending on the need of a specific application, MOF materials can be designed with specific structures, textural characteristics, and dimensions to meet the demands of the applications. Because of these distinctive properties, MOFs have found applications in gas adsorption and desorption,17 drug delivery,18 optoelectronics,19 and catalysis.20 More interestingly, other than their direct use, MOFs can also be used as sacrificial templates to derive various porous nanomaterials by applying different thermal and/or chemical treatments. For instance, the heat treatment of MOFs in an inert atmosphere can produce highly porous carbons with high specific surface areas (∼3000 m2 g−1), given that chemical etching is conducted to remove the surface metal ions. Compared with carbonaceous materials fabricated using conventional precursors, MOF-derived carbons often exhibit controllable porous architectures, pore volumes, and surface areas. Typically, the carbonization of MOFs to produce porous carbons can be performed directly without (“direct carbonization”) or with secondary precursors (e.g., ethylenediamine,21 furfuryl alcohol,22 etc.) (“indirect carbonization”).23 The former is more advantageous as it is more facile and involves only a single calcination step. To date, various porous carbon materials with controlled morphologies from 0-D to 3-D have been successfully derived from the carbonization of MOFs, highlighting the versatility of MOFs as precursors.24−26 Furthermore, MOF-derived carbons can be doped with elements, such as nitrogen (N), phosphorus (P), and/or sulfur (S) to further enhance their functional performance.27−29 On the other hand, the direct heat treatment of MOFs in air can lead to their decomposition into their corresponding metal oxides (Scheme 1).8 Typically, the derived metal oxide nanostructures possess the same structures as those of the parent MOFs (with optimized heating conditions in nitrogen (N2) and/or air atmosphere), as well as higher surface areas compared to metal oxides produced by other methods, often with readily developed porous architectures. Moreover, the variations in annealing temperature and time can provide useful control over the composition, surface area, and pore size distribution of MOF-derived oxides.13,23Apart from pure metal

charges which can be physically stored by EDLCs are quite low in aqueous as well as organic electrolytes.4 Two-dimensional graphene has been increasingly used to provide higher charge− discharge rates; however, it exhibits a lower volumetric capacitance compared to that of traditional porous carbons.5 In general, the most critical factors that determine the capacitive performance of EDLCs are conductivity, surface area, pore volume, pore size distribution, and size of the electrolyte ions in solvated and desolvated shells.6 On the other hand, electrochemically active materials such as transition metal oxides (TMOs)7−10 and sulfides11 employ a reversible Faradaic reaction to store charges. TMOs have shown higher capacitance capabilities as well higher energy densities by at least an order of magnitude than those of carbon-based materials. However, the microstructure of the metal oxide materials may become distorted, and its original phase may be changed as a result of these continuous reactions, which ultimately results in poor cycling stability for commercial applications.12 Thus, the development of highly porous metal oxide nanostructures with excellent electrochemical properties is crucial for achieving high ED and PD in EES. The capacitance of TMO materials is strongly governed by several parameters including porosity (to enable a fast access for the electrolyte), crystallinity (to ensure the deep diffusion of the electrolyte), the size of the redox active species, as well as the surface area of the electrode materials (to enhance the contact area between the electrode materials and the electrolyte).13 In order to achieve high ED and PD, it is highly desirable that the electrode material should be used in combination with a suitable electrolyte. However, to date, there is no known perfect electrolyte suitable for all types of electrodes for achieving highperformance next-generation supercapacitors.14 Therefore, there is a strong need to develop an easily scalable, costeffective method for synthesizing advanced functional materials which exhibit high stability, high electrical conductivity, and permanent porosity with tunable pore size and pore volume for high-performance next-generation energy storage applications. As of now, metal oxide nanostructures have been synthesized by a variety of chemical and physical methods. Some of these methods have shown promise as cost-effective methods for the synthesis of metal oxides; however, very few of them can be applied at an industrial-scale level. Hence, as of today, the majority of commercial supercapacitors still heavily rely on carbon-based supercapacitors which have a cycle life of up to 500 000 cycles.1 Metal−organic frameworks (MOFs) can be made by attaching inorganic metal ions to organic linkers via strong chemical bonds, as discovered by Yaghi and Li in the late 5294

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hybridized with various carbon-based materials, such as reduced graphene oxide (rGO), carbon nanotubes (CNTs), and 3-D graphene networks as conductive matrices to enhance their electrochemical performance for supercapacitor applications.30 The highly tunable properties of MOF-derived metal oxides along with the possibilities to combine them with highly conductive carbon supports make them attractive candidates for next-generation supercapacitors. In relation to this, we will discuss the recent developments in the fabrication of MOFderived nanoporous metal oxides and metal oxide nanocomposites for supercapacitor applications. Various MOF Precursors for the Synthesis of Porous Metal Oxides. The electrochemical performance of TMO materials in supercapacitors is strongly governed by the combination of various parameters including surface area, crystallinity, porosity, surface energy, chemical reactivity, and so on. In order to exploit the full potential of transition metal oxides (TMOs) in supercapacitors, it is important to develop porous TMO nanostructures with controlled nanoporous architectures and textural characteristics for achieving the desired electrochemical performance. So far, the synthesis of porous TMO materials is mainly accomplished using wet chemical routes; however, challenges still remain with regard to tailoring the morphology in an easily scalable way with a high degree of reproducibility.46,72,73 MOFs have emerged as attractive templates for the derivations of porous TMO materials due to their tunable composition, highly designable structures, large surface area, and controllable porosity.23 As MOFs are made up of metal ion centers coordinated to organic linkers, their controlled heating in air can give rise to porous TMO nanomaterials due to the oxidation of the metal ions and the release of gaseous CO2 and NO2 originating from the decomposition of organic linkers

oxide nanostructures, porous metal oxide nanocomposites can also be fabricated through the direct heat treatments of bimetallic MOFs or core−shell MOF composites at high temperatures in air or through a two-step annealing, first in N2, then in air (Scheme 2). The latter method is particularly Scheme 2. Strategic Concepts for the Preparation of Porous Nanostructured Metal Oxides from MOFs toward Supercapacitor Applications

beneficial for preserving the original structure of the parent MOFs and, therefore, for achieving higher surface area and porosity. In addition, MOF-derived oxides can be further

Figure 1. Progress in the synthesis of MOF-derived porous metal oxides. Bar graphs showing a variety of MOF precursors which have been used to derive metal oxide nanostructures (CuO,31 Co3O4,8,32−38 Co3V2O8,39 CeO2,40 Fe2O3,41−43 MgO,44 Mn2O3,45−47 NiO,48−50 TiO2,51−54 and ZnO55−57) and nanocomposites (CoFe2O4,58 CuCo2O4,59 Mn1.8Fe1.2O4,60 ZnCo2O4,61 ZnxCo3−xO4,62 ZnMn2O4,63 Co3O4@NiCo2O4,64 Co3O4/ZnFe2O4,65 CuO/Cu2O,66 CuO@NiO,67 Cu/Cu2O@TiO2,68 Cr2O3@TiO2,69 Fe2O3@TiO2,70 and Fe2O3/NiCo2O471) and the corresponding surface area values of the obtained metal oxide nanostructures and nanocomposites. (Note: PB, Prussian Blue; BTC, benzene1,3,5-tricarboxylic acid; ptcda, perylene-3,4,9,10-tetracarboxylic dianhydride; Cu-BDC:[Cu(tpa)(dmf)]; tpa, terephthalic acid; DMF, N,Ndimethylformamide; Mg-aph, Mg4(adipate)4(DMA)-(H2O)]·5DMA·2MeOH·4H2O; DMA, N,N-dimethylacetamide; MeOH, methanol). 5295

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Figure 2. “Two-for-one” concept for the preparation of nanoporous carbons (NPCs) and metal oxides. Nanoporous carbon and cobalt oxide have been prepared using ZIF-67 as a single precursor. Different heating conditions lead to the conversion of the ZIF-67 precursor into the resultant cobalt oxide and carbon materials. Reproduced from ref 8. Copyright 2015 American Chemical Society.

during the heat treatment. The resulting nanoporous oxide materials typically retain the shape and porosity of the parent MOFs, which are favorable for obtaining large surface area. To date, various porous TMO nanomaterials have been derived from MOF precursors, including cupric oxide (CuO),31,74 cobalt oxide (Co3O4),8,32−38,61,75 iron oxide (Fe2O3),41−43 magnesium oxide (MgO),44 manganese oxide (Mn2O3),45−47 nickel oxide (NiO),48−50 titanium dioxide (TiO2),51−54 zinc oxide (ZnO),55−57,76 and cerium oxide (CeO2),40 and recently, this has been expanded to mixed TMOs, such as ferrites (CoFe2O458 and Mn1.8Fe1.2O460) and cobaltites (CuCo2O4,59 ZnCo2O4,77 ZnxCo3−xO4,62 and ZnMn2O463). More interestingly, MOFs are not limited for the development of single metal oxides but can be useful for the preparation of various mixed metal oxides and metal oxide nanocomposites, such as Co 3 O 4 /NiCo 2 O 4 , 64 Co 3 O 4 / ZnFe2O4,65 CuO/Cu2O,66 CuO@NiO,67 Cu/Cu2O@TiO2,68 Cr2O3@TiO2,69 Fe2O3/Co3O4,73 Fe2O3@TiO2,70 Fe2O3/ NiCo2O4,71 NiFe2O4/Fe2O378 NiO@ZnO,79 ZnO@Co3O4,80 ZnO@ZnCo2O4,81 and ZnO/ZnFe2O4.82−84 Therefore, by involving multiple functionalities and high surface area, these materials can be designed for achieving high-performance electrodes for supercapacitors. The different precursors used to derive porous metal oxides and the corresponding surface areas are shown in Figure 1. These porous MOF-derived TMO nanomaterials have been employed for a wide variety of applications including lithium-ion batteries (LIBs), supercapacitors, gas sensors, and catalysis.13 However, for this review, we will focus solely on the development of MOFderived metal oxides for supercapacitor applications. In the next few sections, we will discuss the developments of each of these MOF-derived materials including their electrochemical properties and the various parameters which affect their supercapacitor applications. MOF-Derived Metal Oxide Nanostructures. Among TMOs, Co3O4 is one of the most widely used oxides for supercapacitor applications, due to their excellent electrochemical stability, high theoretical capacitance (3600 F g−1), low cost, and high abundance.8 Porous Co3O4 nanostructures have been obtained from the pyrolysis of various Co-based MOFs, such as PBA (nanocages), ZIF-67 (nanocages, polyhedrons), MOF-71 (nanoparticles), Co3L2(TPT)2·xG]n

(tetrahedra), MOF-74-Co (ball-like), and Co-BTC (nanotubes, cuboids).23 Recently, Salunkhe et al.8 reported the synthesis of ZIF-67-derived Co3O4 polyhedra as an excellent electrode material for supercapacitor applications (Figure 2). The obtained Co3O4 product exhibited a polyhedral-like structure with a high specific surface area of ∼148 m2 g−1. In fact, the surface area of Co3O4 obtained by this method is much higher than those obtained by other traditional methods (e.g., hydrothermal method,85−87 chemical precipitation,88 and controlled precipitation89), which highlights the important advantage of this method. In order to obtain porous Co3O4 polyhedra without much destruction of the original shape of the parent MOF, the ZIF-67 particles were strategically heated through a two-step calcination method. First, the ZIF-67 polyhedra were calcined under N2 atmosphere at 500 °C, followed by a secondary annealing in air at 350 °C, at a heating rate of 5 °C min−1. The initial heat treatment in the N2 atmosphere was critical for obtaining a high surface area as well as for maintaining the polyhedral-like morphology. Wang and co-workers had previously proposed that the initial calcination of ZIF-67 in an inert atmosphere (argon (Ar) or N2) could minimize the generation of volatile gases (COx and NOx) which would have otherwise formed under a direct calcination in air, leading to the collapse of the frameworks.33 More importantly, a slow heating rate (between 1 and 5 °C min−1) is preferable during the calcination of ZIF-67 in air in order to maintain its structural integrity. This is because a very rapid heating rate may lead to the collapse of the ZIF-67 framework, as a result of the very fast removal of the organic component. Furthermore, a slower heating rate is also beneficial for promoting the formation of smaller pores, such as micropores (2500 F g−1), the real specific capacitance of NiO materials is still far below the theoretical value as a result of its intrinsically poor electrical conductivity.99 Zinc oxide (ZnO) has been considered as one of the most attractive materials for improving the electrochemical performance of NiO in supercapacitors due to its good electrical conductivity (which is useful for providing an electrically conducting pathway), as well as its high chemical stability and mechanical flexibility. For instance, NiO/ZnO hollow spheres derived from yolk−shell Ni−Zn MOF (Ni3(OH) 2(C 8H4O 4)2(H2O)4]·2H 2O) microspheres have been shown as promising electrode materials for supercapacitors (Figure 3a).79 To prepare the Ni−Zn MOF precursor, the metal salts (nickel and zinc nitrates) were 5300

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Figure 4. (a) Schematic illustration of the fabrication of three-dimensional graphene network and MOF-derived Mn2O3 composites (3DGN/ Mn2O3). (b) Specific capacitance as a function of the current density of the electrodes of Mn-BTC, Mn2O3, 3DGN/Mn-BTC, and 3DGN/ Mn2O3. (c) Cycling performance of the 3DGN/Mn2O3 electrode during 1800 charge−discharge cycles at 1 A g−1. Adapted with permission from ref 103. Copyright 2016 The Royal Society of Chemistry.

cycles at 1 A g−1. In such a composite electrode, the carbon component stored electrical energy from double-layer capacitance generated by charge separation at the interface between the electrode and liquid electrolyte. In comparison, the Cr2O3 nanoribbons acted as redox pseudocapacitor which stored electrical energy via fast and electrochemically reversible Faradaic redox reaction, while also contributing to the total capacitance through the double-layer effect. The outstanding capacitive performance of the Cr2O3/C composite electrode was attributed to several reasons including: (i) the enhanced electronic conductivity due to the presence of graphitic carbon, which facilitated the rapid transportation of electrons and ions, and (ii) the proper combination of micropores and mesopores together with polar functionalities within the carbon matrix which ensured maximum ionic wettability and storage during electrochemical tests. Apart from binary metal oxide/carbon composites, MOFs have also been employed as precursors to derive ternary metal/ metal oxide/carbon composites. This is possible because the metal nodes in MOFs can be transformed in situ into metal nanoparticles during the carbonization process, whereas the organic linkers are converted to porous carbon materials.23,102 For example, ternary MoO2@Cu@C composites have been synthesized from the thermolysis of [Cu2(BTC)4/3(H2O)2]6[POM](C4H12N)2·xH2O (POM = [PMo12O40]3−) (POM@ MOF) crystals.102 The POM@MOF crystals with polyhedral block-like morphology were obtained through the hydrothermal treatment of a mixture solution consisting of Keggintype phosphomolybdic acid (H3PMo12O40·nH2O), copper nitrate, H 3 BTC, and tetramethylammonium hydroxide ((CH3)4NOH) at 180 °C for 24 h. Following the heat treatment under N2 atmosphere at 600 °C, the POM@MOF crystals were successfully converted to MoO2@Cu@C, which exhibited a large BET surface area of 183.3 m2 g−1 and an average pore size of 6 nm. During calcination, the MoO2 and Cu derived from Mo-POM and Cu-MOF in POM@MOF crystals were embedded into the porous carbon matrix. In

for a limited distance (∼20 nm) inside their pores, unlike amorphous materials where the electrolyte can diffuse for a larger distance (∼50 nm).13,99 An effective strategy to overcome this problem is to combine the TMO material with a carbon-based material or to deposit the TMO material on a conductive carbon matrix. Carbon-supported metal oxide materials can achieve better stability compared to pure metal oxides in supercapacitor applications as the carbon matrix can effectively reduce mechanical stress by the synergic effects of pseudocapacitive and EDLC materials. Furthermore, the hybridization of metal oxides with carbon-based materials has been shown to be beneficial for enhancing their energy density for ASC by extending the potential window range.96,100 Recently, MOFs have been demonstrated as suitable precursors for the derivation of porous metal oxide−carbon composites due to the presence of metal nodes which can be oxidized to form metal oxides along with the existence of organic linkers which can be decomposed to form porous carbons.23 For instance, a composite material composed of graphitic carbon and chromium oxide (Cr2O3) nanoribbons with a large BET surface area of 438 m2 g−1 has been obtained from the carbonization of a mixture of polyfurfuryl alcohol and MIL-101 under Ar flow at 900 °C.101 In this reaction system, MIL-101(Cr) acted as both carbon and Cr2O3 sources as well as a template to accommodate furfuryl alcohol as the primary carbon source. During the calcination process, the polymerization of furfuryl alcohol occurred within the 3-D channels of MIL-101(Cr) at around 150 °C, whereas a further increase in temperature above 400 °C triggered the decomposition of MIL-101(Cr) and the conversion of polyfurfuryl alcohol to graphitic carbon. Galvanostatic charge/discharge tests of the Cr2O3/graphitic carbon composite electrode in the potential window of −0.5 to 0.1 V revealed that the composite electrode exhibited a maximum specific capacitance of 291 F g−1 at a current density of 0.25 A g−1. Furthermore, it showed a good electrochemical stability with a high capacitance retention of 95.5% after 3000 5301

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ACS Nano Table 1. Electrochemical Performances of Previously Reported MOF-Derived Metal Oxide Nanostructures and Nanocomposites for Supercapacitor Applications metal oxide materials CeO2 dumbbell-like Co3O4 polyhedra Co3O4 nanoparticles Mn2O3 nanobars NiO polyhedra ZnCo2O4 nanoparticles Co3O4/ZnO heterostructure Co3O4/ZnFe2O4 Fe3O4/C spindle-like NiO/ZnO double hollow spheres Cr2O3@C nanoribbons Mn2O3/3D-graphene network RGO@Co3O4 Co3O4/3D-graphene network/Ni foam

MOF precursor used

mass loading (mg cm−2)

Ce-BTC ZIF-67 Co-H3ABTC-bpy Mn-BTC Ni-MOF JUC-155 ZIF-8@ZIF-67 FeIII-MOF-5 Fe-MIL-88B NH2 Ni-Zn-MOF

5.0 2.0 n/a 2.0 5.0 3.0 n/a n/a n/a 4.0

2 3 2 3 6 6 6 6 1 3

MIL-101(Cr) Mn-BTC ZIF-67 ZIF-67

10 n/a n/a 0.6

6 M KOH n/a 6 M KOH 6 M KOH

electrolyte M M M M M M M M M M

KOH KOH KOH KOH KOH KOH KOH KOH KOH KOH

(4)

MoO2 + 4OH− ⇆ MoO4 2 − + 2H 2O + 2e−

(5)

specific capacitance (F g−1)

capacitance retention (%)

no. of cycles

ref

5 2 3 5 10 20 7 5 5 13.3

184 101 119 221 244 368 324 211 74 298

91 89 ∼100 88 ∼100 97.9 93.2 80.7 88 ∼100

10000 2000 3400 1000 1000 1500 1000 1000 1000 1200

40 8 105 46 50 77 98 65 106 79

2 5 5 5

103 270 496 307

95.5 ∼100 90 88

3000 1800 10000 2000

101 103 104 107

on the 3DGN (Figure 4a). Then, the Mn-BTC/3DGN hybrid was calcined in air at 400 °C to convert it to Mn2O3/3DGN. The deposited Mn2O3 particles exhibited a nanowire stacking flower-like morphology with a diameter of several micrometers and a rough surface contributed by the release of gaseous molecules during calcination. In terms of electrochemical performance, the Mn2O3/3DGN electrode exhibited high specific capacitances of 471.1, 403.1, 374.2, 326.5, and 270.0 F g−1 at current densities of 0.2, 0.5, 1, 2, and 5 A g−1, respectively (Figure 4b). The decrease in specific capacitance with increasing discharge current densities was attributed to the increased resistance in Mn2O3 and the relatively insufficient Faradaic redox reaction at higher current densities. The excellent rate capability of the Mn2O3/3DGN electrode was contributed by the hierarchically interconnected and porous structure of the materials, which could enable the facile penetration of electrolytes to promote the redox reactions and diffusion of ions during the rapid charge−discharge process. Furthermore, as depicted in Figure 4c, the specific capacitance of the Mn2O3/3DGN electrode remained unchanged even after 1800 charge−discharge cycles, indicating its excellent cycling stability. The good electrochemical stability of this composite electrode was attributed to the macroporous 3DGN which provided an easy access for the electrolyte and prevented the active materials from falling off into the electrolyte, as well as serving as a structural buffer for the large volume expansioncontraction during the redox reaction. In another example, rGO has been used as a carbon support for the deposition of ZIF-derived Co3O4 to form rGO@Co3O4 and sandwich-structured Co3O4-rGO-Co3O4 hybrids.104 To obtain the Co3O4-rGO-Co3O4 hybrid, sandwich-structured ZIF-67-GO-ZIF-67 hybrids were first synthesized by dispersing the GO solution in a methanol solution containing PVP, cobalt(II) nitrate, as well as 2-methylimidazole and reacting them for 6 h. Next, the obtained ZIF-67-GO-ZIF-67 hybrid was first calcined in Ar atmosphere at 350 °C for 2 h and then in air at the same temperature to achieve the porous Co3O4-rGOCo3O4 hybrid. In comparison, the rGO@Co3O4 particles were obtained by heating the GO@ZIF-67 particles under the same conditions as ZIF-67-GO-ZIF-67. The formation of the sandwich structure in the case of ZIF-67-GO-ZIF-67 was attributed to the addition of water as a solvent, which promoted

terms of electrochemical performance, the MoO2@Cu@C hybrid electrode showed a high specific capacitance of 28.56 mA h g−1 at a current density of 0.5 A g−1. Furthermore, the symmetric supercapacitor device fabricated using these composites showed an excellent cycling stability up to 5000 charge−discharge cycles with 91% capacitance retention. The good electrochemical performance of the MoO2@Cu@C electrode was attributed to two main factors: (i) the abundant presence of micro- and mesopores, which provided pathways for transporting electrons and afforded spaces for storing K+, thereby increasing the electrolyte diffusion and supplies for ions consumed in Faradaic reactions, respectively, and (ii) the good dispersion of the MoO2 phase into the highly conductive phases of Cu and carbon, which reduced the intrinsic resistance of the composite. Moreover, the CV tests revealed that the charge transfer mechanism of the MoO2@Cu@C electrode is similar to that of previously reported Mo4+ material tested in alkaline electrolyte and can be expressed as MoO2 + x K+ + xe− ⇆ MoO2 − x (OK)x

rate (A g−1)

From the above examples, it is evident that the presence of a carbon matrix with a large surface area can lead to an improved dispersion of the metal oxide nanoparticles, which in turn promotes a better synergistic effect between the two materials. This synergistic effect is largely responsible for enhancing the electrochemical performance of MOF-derived metal oxide/ carbon composites for supercapacitor applications. Apart from directly deriving the porous metal oxide/carbon composites from MOFs, an alternative way to enhance the electrochemical performance of MOF-derived oxides for supercapacitors is to deposit them on external conductive carbon matrices (e.g., graphene oxide (rGO), graphene network, carbon nanotubes) through in situ growth processes. For example, a Mn2O3/3-D graphene network (3DGN) hybrid derived from Mn-BTC/3DGN has shown high electrochemical performance for free-standing supercapacitors.103 To prepare this hybrid material, the 3DGN was first immersed in the MnBTC growth solution (contained Mn(CH3COO)2, polyvinylpyrrolidone (PVP), and H3BTC) and aged for 24 h at room temperature to promote the in situ growth of Mn-BTC crystals 5302

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ACS Nano faster depletion of ions and molecules that fed the particles at the particle/solution interface in different solvents. In the case of the Co3O4-rGO-Co3O4 hybrid, ZIF-67 particles with diameters ranging from 700 to 1000 nm were homogeneously deposited on both sides of GO sheets to form a sandwich-like structure, as a result of the electrostatic attraction between the Co3O4 particles and rGO. The BET specific surface areas of the ZIF-67-derived Co3O4, rGO@Co3O4, and Co3O4-rGO-Co3O4 were measured to be 32.54, 198.54, and 494.54 m2 g−1, respectively, indicating the increase in specific surface area with the addition of rGO. When tested for supercapacitors, the rGO@Co3O4 electrode displayed specific capacitances of 546, 536, 526, and 496 F g−1 at current densities of 0.5, 1, 2, and 5 A g−1, respectively. Furthermore, the rGO@Co3O4 electrode was highly stable, showing a high capacitance retention of 90% after 10 000 cycles at 5 A g−1. The outstanding electrochemical performance of the rGO@Co3O4 hybrid for supercapacitors was attributed to (i) the porous nature of the hybrid which could improve the penetration of electrolyte and shorten the diffusion distance of electron/ion, but also mediated the volume change during charge−discharge cycles; (ii) the large specific surface area which could increase the contact area between the electrolyte and the electrode material; and (iii) the presence of an rGO layer which could effectively enhance the specific surface area and improved the electrical conductivity of the hybrid material by enhancing the electron transfer rate. The electrochemical properties of previously reported MOF-derived metal oxide nanostructures and metal oxide/carbon composites for supercapacitor applications are tabulated in Table 1. Based on the above studies, it is evident that MOFs are highly attractive precursors or templates for obtaining porous metal oxide/carbon composites, due to their large surface area, controllable porosity, and tunable composition and morphology. However, to date, there are only a few reports on the use of MOF-derived oxide/carbon composites for supercapacitor applications. This is largely due to some problems associated with the derivation of porous metal oxide/carbon composites for supercapacitor applications, including the following.13 (i) When carbonized at lower temperatures, the carbon component in the metal oxide/carbon composites is poorly graphitized and highly resistive, leading to a poor electrochemical performance. (ii) At higher carbonization temperatures, the carbon component is highly conductive; however, the metal oxide component may be reduced to metal nanoparticles which have poor capacitance. Electrolytes for MOF-Derived Metal Oxides. Depending on the pore size and specific surface area of the MOF-derived metal oxides, the selection of a proper electrolyte is critical for achieving a high capacitive performance. Figure 5 demonstrates the variations of different electrolyte ion size with hydrated ion size for aqueous electrolytes. Furthermore, the electrical conductivity values of these ions are shown with the color mapping of these points. Hence, if the electrolyte ion size matches the pore size of the metal oxide, it can lead to a significant improvement in the capacitive performance. The aqueous electrolytes for metal oxides are inexpensive, highly conductive, and easy to handle, thus making them highly favorable as electrolytes at a laboratory scale. Most metal oxides are highly active in aqueous electrolytes due to their Faradaic oxidation and reduction reactions with aqueous electrolytes for

Figure 5. Variations of hydrated ions size with bare ion size. The color map shows ionic conductivity range of individual ion.

charge storage.108 These Faradaic reactions can lead to a high capacitive performance (∼1000 F g−1) within a small or limited potential window compared to carbon materials in nonaqueous electrolytes using organic/ionic electrolytes. The limited potential window is the major limitation for metal oxides, as gas evolution occurs if the potential window exceeds 1.2 V, which leads to both decreased performance and safety concerns. The most commonly used electrolytes for metal oxides are H2SO4, KOH, NaOH, Na2SO4, KCl, etc. We expect that by optimizing the heating conditions of MOF-derived metal oxides as well by using different MOF precursors, it will become possible to match the pore-size of MOF-derived metal oxides with the targeted ion size of the electrolytes. In this way, we can design high-performance MOF-derived metal oxides for next-generation energy storage applications.

CONCLUSIONS In recent years, MOFs have emerged as highly attractive templates or precursors for the derivations of porous transition metal oxide nanomaterials due to their highly designable structures, tunable composition, large surface area, and controllable porosity. By carefully controlling the calcination conditions (e.g., atmosphere, calcination temperature, and time), MOFs which are made up of metal ion centers coordinated to organic linkers can be converted into porous metal oxide nanostructures or nanocomposites. Present Obstacles and Possible Solutions. MOFderived metal oxide nanostructures have shown some good potential as electrode materials for supercapacitor applications; however, they still suffer from some major drawbacks. Here, we will provide our perspectives on some key problems in this field. These problems are (i) poor electrical conductivity at higher charge−discharge rates due to poor bulk electron conduction, leading to an overall increase in resistance (ii) low electrode surface area, resulting in a small contact area between the electrode material and the electrolyte (iii) short cycle life at higher rates due to the increase in resistance and insufficient Faradaic redox reactions at higher current densities (iv) limitations in the diffusion distance of the electrolyte inside the pores of the metal oxides due to the high crystallinity of MOF-derived metal oxides. 5303

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ACS Nano In order to improve the electrochemical performance of MOF-derived metal oxides for supercapacitor applications, we propose the following strategies to address each of these issues including: (i,iii) The electrical conductivity can be improved by combining the MOF-derived metal oxides with highly conductive carbon substrates (e.g., rGO, carbon nanotubes, and graphene networks) together with a secondary metal oxide component. (ii) The large increase in surface area of MOF-derived metal oxides can be achieved by preheating the MOF precursors under N2 atmosphere before heat treating them in air. This is beneficial for preventing the rapid release of volatile gases (e.g., COx and NOx) which would have caused the collapse of the frameworks. (iv) The improvement in the distance at which the electrolyte can diffuse inside the pores of the metal oxides can be obtained by reducing the crystallinity of the MOFderived metal oxide materials. Previously, it has been shown that electrolyte could only diffuse for a limited distance (∼20 nm) inside the pores of crystalline oxide materials, whereas in amorphous oxide materials, the same electrolyte could diffuse for a larger distance (∼50 nm). Furthermore, this problem may also be solved by matching the pore size of the MOF-derived materials with the electrolyte ion size through optimization of the heating conditions. Versatile Design of MOF-Derived Metal Oxides. There is a huge scope for the development of phase-controlled, highly conductive MOF-derived metal oxides for supercapacitor applications. We expect that in the near future, the functionalization of MOF-derived metal oxides through heteroatomic doping or hybridization with carbonaceous materials will help to realize the potential use of MOF-derived metal oxides for high-performance supercapacitors. More interestingly, the parent MOF can be designed in 1-D, 2-D, and 3-D depending on the organic and inorganic constituents involved in the synthesis process. Thus, by optimizing the heating conditions, nanoporous metal oxide materials which retained the original shape and the high surface area of the parent MOFs can be obtained. Such a flexible control is very important considering the different requirements needed for different applications. For instance, some applications, such as frequency regulators, require materials with high power densities. The 1-D and 2-D materials can provide easy access for ion intercalation and deintercalation, thus providing rapid charge−discharge and ultrahigh reversibility (high power density, Scheme 3). In comparison, high energy density applications, such as mass rapid transit, need a higher charge storage capacity. This may be achieved through the use of 3-D materials which can provide many reaction sites in their 3-D networks. However, very few conventional methods offer the options for controlling the dimensionalities of the obtained metal oxide products for the desired applications. This clearly highlights the advantage of MOF-derived materials and their future significance. Furthermore, with respect to the commercialization of MOFderived metal oxides, it is highly important that they are tested thoroughly through robust charge−discharge tests (at least >200 000 cycles) to match the practical standard of commercial carbon electrodes. Hence, in the future, significant research efforts should be carried out for the synthetic method modifications of MOF-derived metal oxides and to increase their processability for commercial applications. Finally, a

Scheme 3. Schematic Representation Showing the Flexibility of MOF-Derived Metal Oxide Design for Obtaining the Desired Performance in Terms of Energy and Power Densitiesa

a

By selecting suitable MOF precursors, it is possible to design porous metal oxide materials with the desired dimension (1-D,43 2-D,63 or 3D8). Controlling the dimension of the metal oxide materials is crucial for optimizing the contact area between the metal oxide electrode and the electrolyte for achieving high energy and power densities.30

deeper understanding of the charge−discharge mechanisms of MOF-derived metal oxide nanostructures is also necessary in order to optimize their electrochemical performance for supercapacitors and this may be accomplished by performing more numerical simulation studies in the future. Truly Innovative Materials for the Future. Over the years, transition metal oxides have been studied for their structural, electrical, and optical properties. They possess interesting electrochemical properties in combination with the proper electrolytes. Although transition metal oxides have attracted huge interests for academic research, they were still relatively limited in terms of their practical applications. Though several refinements have been suggested, the common problem was their synthesis by traditional synthetic ways which provide little control over the design of their microstructures. Their true potential for electrochemical applications was inhibited by our ability to control their microstructure, phase and porosity. MOFs on the horizon are poised to take care of these problems. Now, it is possible to design and develop shape, size, porosity, phase, and/or conductivity-controlled nanoporous transition metal oxide materials by selecting the proper organic and inorganic constituents of the MOF precursors and controlling the heat treatment conditions. The optimization of the properties of the parent MOF precursors is highly crucial for obtaining the desired properties of the derived metal oxide materials. The development of cost-effective methods for the synthesis of these materials may be challenging, however, not impossible, as some MOFs are already commercially available. Therefore, it is recommended that future works on MOF-derived oxide materials are aimed at addressing the challenges in this field to fully unlock their true potential for supercapacitor applications. 5304

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ACS Nano

AUTHOR INFORMATION

EDLC type capacitors; Asymmetric supercapacitors (ASCs), assembled using two dissimilar electrode materials with complementary working potential windows for the purpose of enlarging the cell voltage and energy density; traditionally, the negative electrode is composed of a carbonaceous material such as graphene, porous carbon, or carbon nanotubes, whereas the positive electrode may consist of a carbonaceous material, metal oxide, or metal sulfide; ASCs show several main advantages such as high energy density and large cell voltage; however, they exhibit low power density and short cycling life; in ASCs, the selection of a proper electrolyte is vital for achieving the desired device performance; Symmetric supercapacitors, supercapacitors which are assembled using two similar electrode materials; the working voltage of a symmetric supercapacitor is often limited to less than 1.0 V as a result of the thermodynamic breakdown potential of water molecules when aqueous electrolytes are employed; however, the working voltage of SCs can be improved beyond 2.5 V by replacing aqueous electrolytes with organic electrolytes

Corresponding Author

*E-mail: [email protected]. ORCID

Yusuke Yamauchi: 0000-0001-7854-927X Author Contributions §

R.R.S. and Y.V.K. equally contributed to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Y.V.K. thanks the Japan Society for Promotion of Science (JSPS) for providing the JSPS standard postdoctoral fellowship. This work was partly supported by an Australian Research Council (ARC) Future Fellow (FT150100479), the AIIMMANA 2016 grant, and JSPS KAKENHI Grant Number 17H05393 (Coordination Asymmetry). The authors would like to thank TOC Capacitor Co., Ltd. and Thank-Metal Co., Ltd. for their helpful suggestions and discussions on supercapacitors.

REFERENCES

VOCABULARY Metal−organic frameworks (MOFs), class of crystalline porous materials made by attaching inorganic metal ions to organic linkers via strong chemical bonds; depending on the organic and inorganic constituents, structurally, MOFs can be prepared in one-dimension (1-D), two-dimension (2-D), or three-dimension (3-D); this has led to the development of more than 20000 MOFs in the past two decades; MOFs can be designed with specific structures, textural characteristics, and dimensions, and as a result of these unique properties, MOFs have been employed in various applications, such as gas adsorption and desorption, drug delivery, optoelectronics, energy storage and conversion, and catalysis; Electrochemical capacitors or supercapacitors, an energy storage device which is constructed using two electrodes (anode (negative electrode) and cathode (positive electrode)) separated by an electrolyte (aqueous or organic) and a separator which enables the transfer of ions while keeping the electrodes electrically insulated from each other; supercapacitors exhibit several advantages over lithium-ion batteries (LIBs), such as faster charge−discharge, higher power density, and better reliability as they do not lose their storage capabilities over time; however, they possess a lower energy density compared to LIBs, meaning that they can store less amount of energy per unit weight. Recent developments in the fabrication of supercapacitor electrodes, including improvements in material production methods, and the use of various organic and aqueous electrolytes may soon bridge the energy density gap between supercapacitors and LIBs for some commercial applications; Electrostatic doublelayer capacitors (EDLCs), supercapacitors which store charges by means of surface adsorption of the ions from the electrolyte as a result of electrostatic attraction, leading to the formation of two charged layers (double layer); EDLCs are typically assembled using carbon-based electrode materials (porous carbons, graphene, and carbon nanotubes) as they possess large surface area, tunable porosity, good electronic conductivity, and low toxicity; the main critical parameters which determine the capacitive performance of EDLCs are conductivity, surface area, pore volume, pore size distribution, and size of the electrolyte ions in solvated and desolvated shells; due to the advantages of having high capacitance retention (up to 500000 cycles) and safe operation, most of the commercial supercapacitors are the

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