DaltonTransactions

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Cite this: Dalton Trans., 2021, 50,11331

Received 27th May 2021,Accepted 8th July 2021

DOI: 10.1039/d1dt01729b

rsc.li/dalton

Recent progress on pristine two-dimensionalmetal–organic frameworks as active componentsin supercapacitors

Yuxuan Guo,a Kuaibing Wang, *a Ye Hong,a Hua Wu a and Qichun Zhang *b,c

Two-dimensional (2D) metal–organic frameworks (MOFs) are a new generation of 2D materials that can

provide uniform active sites and unique open channels as well as excellent catalytic abilities, interesting

magnetic properties, and reasonable electrical conductivities. Thus, these MOFs are uniquely qualified for

use in applications in energy-related fields or portable devices because they possess fast charge and dis-

charge ability, high power density, and ultralong cycle life factors. There has been worldwide research

interest in 2D conducting MOFs, and numerous techniques and strategies have been developed to syn-

thesize these MOFs and their derivatives. Thus, this is the opportune time to review recent research pro-

gress on the development of 2D MOFs as electrodes in supercapacitors. This review covers synthetic

design strategies, electrochemical performances, and working mechanisms. We will divide these 2D

MOFs into two types on the basis of their conductive aspects: 2D conductive MOFs and 2D layered MOFs

(including pillar-layered MOFs and 2D nanosheets). The challenges and perspectives of 2D MOFs are also

provided.

1. Introduction

There is currently great interest in reducing CO2 emissionsinto the atmosphere to stop global warming. This could beaccomplished by using clean and eco-friendly energy insteadof traditional energy sources.1–6 Supercapacitors (SCs) areclean and environmentally friendly storage devices that havebeen widely considered as energy containers with high capaci-

Yuxuan Guo

Yuxuan Guo is a graduatestudent in the Department ofChemistry at the College ofSciences, Nanjing AgriculturalUniversity, P. R. China. Hereceived his bachelor’s degree inApplied Chemistry from NanjingAgricultural University.Currently, he is pursuing hismaster’s degree under Prof.Kuai-Bing Wang at the College ofSciences, Nanjing AgriculturalUniversity, P. R. China. Hisresearch interests focus on the

application of 2D conductive MOFs and pillar-layered-MOF-derived electrode materials in the fields of SCs and lithiumbatteries.

Kuaibing Wang

Kuaibing Wang is currently anAssociate Professor in theDepartment of Chemistry at theCollege of Sciences, NanjingAgricultural University,P. R. China. He received his Ph.D. in Chemistry and ChemicalEngineering in 2013 fromNanjing University under thesupervision of Prof. Zhilin Wang.He worked in Prof. QichunZhang’s group in 2019 as a visit-ing scholar at the School ofMaterials Science and

Engineering, Nanyang Technological University, Singapore. Hisresearch interests focus on the controllable fabrication and appli-cation of 2D MOFs, bulky/nano-MOFs, and MOF-derived inorganichybrid materials for energy storage applications.

aDepartment of Chemistry, College of Sciences, Nanjing Agricultural University,

Nanjing 210095, Jiangsu, P. R. China. E-mail: wangkb@njau.edu.cnbDepartment of Materials Science and Engineering, City University of Hong Kong, 83

Tat Chee Avenue, Hong Kong, SAR 999077, P. R. China.

E-mail: qiczhang@cityu.edu.hkcCenter of Super-Diamond and Advanced Films (COSDAF), City University of Hong

Kong, Hong Kong, SAR 999077, P. R. China

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tance but with lower voltage limits, and they could bridge thegap between traditional capacitors and rechargeablebatteries.7–13 Compared with batteries and traditional physicalcapacitors, SCs have several advantages, including high powerdensity, long endurance, and less environmentalpollution.14–22 SCs were well-known as sources of energy in the1990s, when they were used to supply power to open airplanedoors in emergencies.23,24 Subsequently, SCs were graduallyapplied in civilian devices, were involved in the automotivefield, and were even combined with lithium-ion batteries tobecome a hybrid energy source utilized in high-power densityprocesses such as braking and engine starting inautomobiles.25–28 Based on the developmental history and theworking mechanism, SCs are mainly divided into electricdouble-layer capacitors (EDLCs), pseudocapacitors (PCs),battery-type capacitors (BTCs), and hybrid supercapacitors(HSCs).29–33 Correspondingly, the active electrode materialswere also changed and simultaneously improved, where thesematerials were used in the early carbon electrodes displayingthe EDLC mechanism, metal oxides with PC or BTC workingmodes, and recent hybrid materials containing carbon, con-ducting polymers, and metal oxides.12,34–43 However, due tothe limitations of negative electrode materials and their draw-backs as electrodes, designing and searching for new electro-des with high electrochemical performances are still highlydesirable, and their research is still in progress.

Two-dimensional (2D) materials are composed of single/fewlayers of atoms or molecules that are connected together bystrong covalent bonds or ionic bonds in the intralayer, whilethe interlayers stack together through van der Waalsforces.44–54 Since their discoveries, 2D materials have attractedsignificant research interest due to their extraordinary physico-chemical properties, such as large specific surface area, excel-lent optical transparency, and electrical and thermal conduc-

tivity. Compared with zero-dimensional nanoparticles, one-dimensional nanostructures, and bulk materials, the atomicthickness of 2D materials endow them with high mechanicalflexibility and optical transparency, making them ideal for flex-ible and transparent electronic devices.55–58 Moreover, 2Dmaterials are promising candidates for gas separation due totheir ultra-thin thickness and large lateral dimensions.Furthermore, large specific surface area volume enables rapidcontact between reactant molecules and active sites, therebyenhancing catalysis.59–63 Clearly, 2D materials have the poten-tial to be broadly applied due to such unique characteristics.Since the discovery of classic 2D graphene in 2004, 2Dmaterials including transition metal oxide nanosheets, tran-sition metal disulfide nanosheets, Mxenes, and other energystorage materials with satisfactory physical and chemical pro-perties have been reported.64–67

Metal–organic frameworks (MOFs) are 2D or 3D porouscoordination polymers formed through the self-assembly oforganic ligands typically containing nitrogen or oxygen groupswith metal ions.68–76 Unlike traditional inorganic porousmaterials, MOFs can be constructed by selecting and regulat-ing different organic building blocks at the molecular level.Additionally, MOFs also have higher porosity, larger specificsurface area, and diverse structures and functions. Comparedwith 3D MOFs, planar 2D MOFs undergo extended 2Dπ-conjugation and possess graphene-like structures that mightresult in one of the most conductive skeletal structures.77–87

Moreover, their lamellar structures can provide larger specificsurface areas and more accessible active sites, namely, (i) con-sidering that metal centers possess both octahedral coordi-nation and planar square coordination on the 2D surface, theunique surfaces and surface areas of 2D MOFs possess agreater number of open coordination and electroactive sitesfor molecular interaction; and (ii) the unique interlayersprovide a greater number of spaces for ions or electrons to beeasily transported, which thus increases the density of theactive redox sites and subsequently enhances the gravimetricor volume capacitances of the active energy storagematerials.57 Although there are various advantages to 2DMOFs over other dimensional materials, they do have severaldisadvantages as indicated in recent reports (Table 1).88 Forexample, Bi et al.88 indicated that developing conductive MOFswith 3D scaffolds might be advantageous over dense stacks of2D MOFs with quasi-1D pores because 2D MOFs cannotprovide ion/electron transmission paths in all directions,which simultaneously decreases cation/anion swapping as wellas energy and power densities.88 Therefore, the design andpreparation of 2D MOFs and further study on their electro-chemical working mechanisms are still crucial for the develop-ment of 2D MOF-based SC electrodes.

Additionally, there are many derivatives of 2D MOFs thathave made significant contributions to energy storage and con-version fields, and have been extensively reported and reviewedin recent years.89–93 In this regard, this review aims to summar-ize the recent progress in the production of 2D MOFs as SCelectrode materials from two aspects (2D conductive MOFs

Qichun Zhang

Qichun Zhang is a Professor atCity University of Hong Kong,China. Before he moved to HongKong, he was an AssistantProfessor (01/2009–02/2014)and an Associate Professor withtenure (03/2014–08/2020) atNanyang TechnologicalUniversity, Singapore. Hisresearch focuses on carbon-richconjugated materials and theirapplications. Currently, he is anassociate editor for the Journalof Solid State Chemistry, and is

a member of the advisory board for Materials ChemistryFrontiers, Chemistry-an Asian Journal, Journal of MaterialsChemistry C, and Aggregate and Inorganic ChemistryFrontiers. He is also a fellow of the Royal Society of Chemistry.He has published >420 papers and 5 patents (H-index: 86)

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and 2D layered MOFs containing pillar-layered MOFs and 2Dnanosheets). The classifications for 2D MOFs, which can beutilized as SC electrodes, are shown in Table 2, and a timelineof key events associated with 2D MOFs of SCs is shown inScheme 1. We hope that this review will offer some guidelinesfor designing and preparing novel 2D MOFs with greaterenergy storage in the future.

2. 2D conductive MOFs as electrodematerials for SCs

In the early stages of MOF development, most MOFs are non-conductive, and several reports indicate that the extremely lowconductivities of MOFs have limited their application inenergy storage systems. In 2009, the Kitagawa group first

reported a conductive MOF, Cu[Cu(pdt)2], with a relativelyhigh electrical conductivity of 6 × 10−4 S cm−1 at 300 K.94

Then, in 2012, Yaghi et al. reported a novel 2D planar gra-phene analogue MOF with π–d conjugation through the use ofconjugated hexahydroxy triphenylene (H6HHTP) as an organicbuilding unit with transition metal ions (CoII, NiII, CuII)

(Fig. 1a).95 The Co- and Ni-based MOFs displayed permanentporosity with Brunauer–Emmett–Teller (BET) surface areas of490 and 425 m2 g−1, respectively.96 Notably, single crystals ofthe Cu-based MOF exhibited a conductivity of 0.2 S cm−1. Afterthat, the research on 2D conductive MOFs experienced rapiddevelopment. In 2013, Kombe and his coworkers synthesized a2D π-conjugated nanosheet (Ni3(BHT)2) through the gas–liquidinterfacial method by reacting benzenehexathiol in the organicphase with nickel acetate in the water phase at the interface(Fig. 1b). Although the appearance of the Ni3(BHT)2 presented

Table 1 Comparison of the advantages and disadvantages of 2D MOFs and 3D MOFs

2D MOFs 3D MOFs

Advantages 1. Excellent cycle stability 1. Excellent surface areas and porosities2. Outstanding conductivity 2. Higher specific capacitance and more negligible impedance3. High porosity and large surface area 3. It is easy to prepare single crystals of 3D MOFs4. Easier to hybridize with inorganic materials forimproved electrochemical performance

Disadvantages 1. It is challenging to obtain crystals of 2Dconductive MOFs

1. Poor cycle stability

2. Lower energy and power density 2. Materials obtained by combining 3D MOFs with inorganic materialstend to deteriorate the electrochemical performance due to the clogging ofholes in the material

3. The conductive mechanism is still beingexplored

Table 2 Selected MOF-based electrodes for SCs

Sample name Electrolyte Measurements Specific capacitance (F g−1) Areal capacitance (mF cm−2) Ref.

2D conductive MOFsNi3(HITP)2 1 M TEABF4/can CP(0.05 A g−1) 111 — 109Cu-CAT NWs SSCs 3 M KCl CP(0.5 A g−1) 202 — 110

PVA/KCl CP(0.5 A g−1) 120 0.022Cu-/Ni-HAB 1 M KOH CV(0.2 mV s−1) 427 2000 111Ni3(HITP)2 nanosheets 1 M Na2SO4 CP(0.1 mA cm−2) — 15.69 112CNF@Ni-CAT 2 M KOH CP(0.5 A g−1) 502.95 — 56Cu-DBC 1 M NaCl CP(0.2 A g−1) 479 879 113Ni//Cu MOF array 1 M KOH CP(2A g−1) 1424 — 116Ni-CAT NWAs 3 M KCl CV(5 mV s−1) — 40.5 1172D layered MOFsNi-DMOF-ADC 2 M KOH CP(1 A g−1) 525 — 145PiCBA-based MSCs H2SO4-PVA gel CV(50 mV s−1) — 34.1 F cm−3 146Ni-MOF 3 M KOH CP(2 A g−1) 1668.7 — 147Co-MOF NS 3 M KOH CP(0.5 A g−1) 1159 — 148CNF@c-MOF 3 M KCl CP(0.33 A g−1) 125 — 149Ni-MOF/C-CNTs 3 M KOH CP(1 A g−1) 680 C g−1 — 150Co-MOF/rGO-40 1 M H2SO4 CP(1 mA cm−2) — 656.6 F cm−2 14Cu-MOF@δ-MnO2 1 M Na2SO4 CP(1 A g−1) 340 — 151CoMn-LDH-SO4 1 M KOH CP(2 mA cm−2) — 582.07 mC cm−2 152NAU-1 4 M KOH CP(1 A g−1) 800 — 153NAU-2 4 M KOH CP(1 A g−1) 828 — 153CoFRS 6 M KOH CP(1 A g−1) 131 C g−1 — 154NiFRS 6 M KOH CP(1 A g−1) 198 C g−1 — 154FeSC1 6 M KOH CP(1 A g−1) 683.2 C g−1 — 156FeSC2 6 M KOH CP(1 A g−1) 514 C g−1 — 156

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a large structure at the micron level, the powder X-ray diffrac-tion (PXRD) measurement suggested that the crystalline struc-ture of Ni3(BHT)2 was composed of interlaced nanosheets. Theas-obtained single-layer π-conjugated structure was confirmedby atomic force microscopy (AFM) and scanning electronmicroscopy (SEM) technologies. Other evidence further con-firmed its high conductivity (up to 160 S cm−1) and illustrated

that the doping level would affect the activation energy andconductivity of Ni3(BHT)2.

97

In 2014, Cui and coworkers synthesized Pt3(HTTP)2 byreacting the H6HTTP ligand with PtCl2, which presents a 2Dlayered interlaced structure (Fig. 1c). Unlike other 2D conduc-tive MOFs, the balance of the HTTP-Pt framework requires theparticipation of sodium ions, indicating that platinum is not

Scheme 1 Timeline of critical events for 2D MOFs in the SC field.

Fig. 1 (a) Space-filling drawings of the single-crystal structure of Co-CAT-1 viewed from different directions. Reproduced with permission.95 (b)Schematic illustration and chemical structure of a monolayer nickel bis(dithiolene) complex nanosheet. Reproduced with permission.97 (c) A sche-matic drawing of the honeycomb net of HTT-Pt. Reproduced with permission.98 (d) A schematic drawing of the molecular structure of Ni3(HITP)2.Reproduced with permission.99 (e) Schematic illustration of palladium bis(dithiolene) nanosheet (PdDt). Reproduced with permission.100 (f )Scheme of the formation of Cu-BHT film. Reproduced with permission.101

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entirely oxidized during coordination. The HTTP-Pt frameworkcan be wholly oxidized to neutral materials with I2. As a result,the corresponding bulk conductivity (pressed pellet, two-probemeasurement) was only found to be 10−6 S cm−1. The authorsindicated that it was likely that the lower conductivity was dueto the presence of grain boundaries, which generally exist inpolycrystalline pellets within the Pt-MOF.98 In the same year,Dincă et al. reported a graphene analogue structure denoted asNi3(HITP)2 (Fig. 1d), which exhibited conductivity values of 2 Scm−1 (pressed pellet, four-probe) and 40 S cm−1 (pressed film,four-probe). The maximum conductivity was superior to thatof carbon-based materials.99

Then, in 2015, Pd3(BHT)2 synthesized by Pal et al. exhibiteda maximum conductivity of 2.8 × 10−2 S cm−1 (pressed pellet,four-probe, Fig. 1e).100 Similarly, Huang et al. synthesizedCu3(BHT)2 using a liquid–liquid interface reaction and variouscharacterizations, which illustrated that the Cu3(BHT)2 filmwas piled up by the 2D lattice of [Cu3(C6S6)]n (Fig. 1f). Thiscopper organic framework (COF) also displayed an excellentconductivity of 1580 S cm−1 (thin film, four-probe).101

Numerous semiconducting metal–organic graphene analogues(namely, s-MOGs by the Dincă group) were consecutivelyapplied in various fields, including electrocatalysis, thermo-electric effect, and gas separation.102–108

In 2017, Dincă and coworkers utilized a 2D conductive MOF(Ni3(HITP)2) as the active component material in the SC fieldfor the first time, although Ni3(HITP)2 was first reported in 2014by Sheberla et al. through the reaction between 2,3,6,7,10,11-hexaaminotriphenylene hexahydrochloride (HATP·6HCl) andammoniacal NiCl2.

99 Its structure is composed of stackedπ-conjugated 2D layers, endowing it with a greater specific

surface area (630 m2 g−1) and a more extensive pore size distri-bution (1.5 nm). Ni3(HITP)2 exhibits a fabulous bulk electricalconductivity of more than 5000 S m−1, far beyond the reportedcarbon-based materials. This material was used as a cathodematerial (similar to carbon-based electrode materials) andbehaved as an electrochemical double-layer capacitor (EDLC).Further electrochemistry tests proved that the hybrid symmetricSC device exhibited a high areal capacitance of 18 μF cm−2 at adischarge rate of 0.05 A g−1 and excellent capacity retention of90% over 10 000 cycles at a current density of 2 A g−1.109 Thisresearch result proves that 2D conductive MOFs are excellentactive materials for SCs, and also establishes a solid foundationfor the application of 2D conductive MOFs in the SC field.

In order to solve the problem of the 2D MOFs usually deli-vering low gravimetric capacitances, in the same year, Li et al.tried to synthesize 2D conductive MOFs with nanosized mor-phologies. The nanowire-type 2D MOF, namely Cu-CAT, wasfabricated by reacting copper acetate monohydrate with HTTPligand. Furthermore, it was grown on carbon fiber paper (CFP)in the form of an array by immersing CFP into the reactionsolution (Fig. 2a). Electrochemical tests showed that thespecific capacitance of Cu-CAT nanowire arrays (NWAs) wasonly attenuated by 34% when the current density increasedfrom 0.5 to 10 A g−1, and 80% of the initial capacitance wasretained after 5000 cycles at 800 mV s−1. Furthermore, thePXRD pattern of Cu-CAT NWAs displayed no change beforeand after the cycle. In addition, the Cu-CAT NWAs exhibited ahigh areal capacitance of 22 mF cm2, which supplied power toa red light-emitting diode (LED) for more than 1 min. Theseresults suggest the feasibility of fabricating nanostructured 2Dconductive MOFs as electrodes for SCs.110

Fig. 2 (a) The crystal structure and SEM images of Cu-CAT NWs. Reproduced with permission.110 (b) A schematic illustration of Cu-/Ni-HAB MOFs.Reproduced with permission.111 (c) A schematic drawing of the preparation of a Ni3(HITP)2 SC electrode using the EPD method. Reproduced withpermission.112

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In 2018, the Bao group reported a 2D conductive MOF withhigh redox activity and utilized it as the electrodes for SCs. Theauthors found that the capacity of this MOF was obtainedfrom pseudocapacitance rather than from electric double-layercharges. In order to increase the activity of the redox center,an ultra-small hexaaminobenzene (HAB) linker was selected toconstruct 2D conductive MOFs (Fig. 2b). Because the HABlinkers could coordinate with metal species (d8 and d9) toform right-angle-planar coordination geometry, sub-nanoporeswould be generated in the unique architecture. This featureresults in high volume and large area capacitance, and thus,the linkers were further assembled into sub-millimeter-thickelectrochemical capacitors. Electrochemical tests illustratedthat this 2D MOF material exhibited satisfactory chemicalstability in alkaline surroundings. Additionally, the selectedsmall HAB ligand was conducive to the synthesis of high-density frameworks and also provided an ultra-high volumecapacitance performance (760 F cm−3) with stable redox behav-ior and a mass capacitance of 400 F g−1. When the thicknessof the electrode was increased up to 360 μm, the area capaci-tance also reached 20 F cm−2. Additionally, after 12 000 cycles,the capacitance retention reached 90% of the original value.111

In 2019, different from the Dincă research group, the elec-trophoretic deposition method was first employed to fabricateNi3(HITP)2 electrodes with nanosized morphologies (Fig. 2c).The as-obtained 2D Ni3(HITP)2 nanosheets were further de-posited on nickel foam substrates for implementation in SCs.The MOF-based symmetric SCs exhibited an areal specificcapacitance of 15.69 mF cm−2 at a current density of 0.1 mAcm−2 and exceptional capacitance retention of 84% after100 000 cycles in a neutral electrolyte (Na2SO4). These excellentelectrochemical properties are attributed to the 2D conjugatedstructure of Ni3(HITP)2 and the application of electrophoreticdeposition in supercapacitor electrode fabrication.112 In thesame year, Han et al. synthesized a hybrid core–shell materialbased on CNF and a 2D conductive MOF Ni-CAT by the strategyof combining electrospinning technology with hydrothermaltreatment. Han et al. subsequently tested a double-electrodesystem composed of CNF@Ni-CAT and commercial activatedcarbon (AC). The results showed that the hybrid supercapacitordelivered a maximum energy density of 18.67 W h kg−1 at apower density value of 297.12 W kg−1 and maintained106.19% of the original specific capacitance after 5000 cycles,indicating its excellent electrochemical cycle stability. A bulbexperiment demonstrated that the material was able to illumi-nate a blue LED bulb for more than 60 seconds.56 Similarly,Liu and coworkers reported a 2D conductive MOF Cu-DBC(DBC is an organic ligand with the structure of D2-symmetriccatechol-based linker dibenzo-[g,p]chrysene-2,3,6,7,10,11,14,15-octaol). The electrochemical test suggestedthat Cu-DBC exhibited high areal (879 mF cm−2) and volu-metric (22 F cm−3) capacitances. In addition, Cu-DBC exhibi-ted a specific capacitance of 479 F g−1 at a current density of0.2 A g−1 in a 1.0 M NaCl aqueous solution.113

Although 2D conductive MOFs have been developed forseveral years, their conductive mechanisms are still unclear. In

2020, Zuo et al. reported a new type of 2D material with redoxactivity and high proton conductivity.114 They proposed theconduction mechanism of ‘in-plane proton conduction/inter-face pseudocapacitive coupling’. On the basis of topologicallyguided synthesis, they used tetrathiafulvalene octacarboxylicacid ligand (H8TTFOC) to fabricate a 2D MOF-based materialTTFOC (1.30 × 10−2 S cm−1, 303 K, 98% RH) with high protonconductivity through the spatial arrangement of carboxylgroups (Fig. 3). Through single-crystal X-ray diffraction andvariable-temperature proton conductivity studies, the higherproton conductivity in In-TTFOC was proven to come from thesynergic effect among a large number of water molecules, di-methylammonium cations, and uncoordinated carboxylgroups in the pores, with the corresponding proton conduc-tion resulting from the Grotthuss mechanism. When theauthors further studied the electronic conduction behavior ofIn-TTFOC, the proton conduction of the material was found tobe completely converted into the electronic conduction of theoverall circuit. Under normal circumstances, the contact inter-face between ion-conductive materials (such as PEDOT) andmetal electrodes would limit the further conduction ofprotons. Therefore, further interface charge transfer is necess-ary for pure ion conductivity through a unique mechanism tofinally transform into the overall circuit conduction behavior.Through solid electrochemical studies and the potential corre-lation of nonlinear IV curves, the research team noted that theTTF ligand with redox activity in the In-TTFOC structureplayed an essential role in the interface charge transportprocess. That is, the electrochemically active TTF buildingelement provided faradaic current through its own redox at theinterface between the gold electrode and the sample (ligand-induced pseudocapacitance). Based on these findings, theyproposed a novel mechanism called ‘ionic conduction/pseudo-capacitance coupling’ that provides the theoretical guidancefor the design of future conductive complex porous materialsand devices.114

In regards to the charge-transfer mechanism for conductiveMOFs, Meng and coworkers proposed that intrinsically con-ductive MOFs usually delivered two charge-transport modes:hopping transport and band transport. Similar to the elec-tronic transmission of semiconductors, hopping transportrelies on the movement of charge from donor and acceptormolecules. Charges with discrete energy levels in favorableconditions at specific locations (for example, small spatial dis-tance and energy difference) can complete the hopping con-nection between adjacent units. Band transport is the continu-ous coordination or covalent bond transport of charge carriersin conductive materials. Band transport relies on the delocali-zation of charge carriers through the valence band or conduc-tion band.115

A clear single-crystal structure is the basis for studying allits physical properties. For materials chemistry, the propertiesof materials are closely related to their structures, where minorstructural divergences will significantly affect the macro-physi-cal properties of the materials. Therefore, obtaining high-quality crystals is the key to generating high-performance

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materials. However, there are few reports on the arrangementof 2D conductive coordination polymer single crystals becausethey are a new class of electronic materials, and this greatlylimits their in-depth research and application. The difficulty ofthe research lies in the complicated and unbalanced inter-action forces inside the material during the coordination crys-tallization process. At the end of 2020, Sun et al. reported thesingle crystal structure of a 2D coordination polymer bydesigning a new class of electron-deficient organic ligandmolecules.55 The authors observed the moiré pattern of 2Dcoordination polymers for the first time, and this establisheda strong foundation for the basic research and future appli-cations of new electronic materials. They found that the size ofthe single-crystal structure of the 2D coordination polymer (upto 200 microns) could be adjusted by enhancing the interlayerπ–π interaction and increasing the reversibility of the coordi-nation bond in the layer. Synchrotron radiation single-crystaldiffraction and cryo-electron microscopy (cryo-EM) technologyshowed that the 2D porous material was susceptible to elec-tron beams. Additionally, the photo resolution of high-resolu-tion transmission electron microscopy (HRTEM) can reach 1.9angstroms, which clearly revealed the arrangement of the con-ductive 2D coordination polymer material under the crystaland the bonding mode of the molecular ligands and metalions in the solid space. Benefiting from the acquisition ofhigh-quality crystals, electron beam etching (EBL) was used toconstruct single-crystal devices, and the electronic propertiesand magnetic origins of 2D coordination polymers were sys-tematically studied. The above results illustrate the anisotropicorigin of the internal electrical properties of the 2D crystals.55

Based on these structural characterizations and the charge-transport modes of 2D conductive MOFs, Deng et al.assembled two 2D conductive MOFs into a completely new

type of 2D conductive MOF to increase the conductivity and thefinal electrochemical performances.116 The authors integratedtwo layered MOFs, namely Ni-MOF-24 and Cu3(HITP)2, into onehomogeneous and oriented 2D conductive MOF array using Co(OH)2 as a specific template (Fig. 4a). Ni-MOF-24 served as theelectrochemically active unit, and Cu3(HITP)2 (HITP =2,3,6,7,10,11-hexaiminotriphenylene) acted as the conductingcomponent to activate adjacent Ni-MOF-24. After Ni-MOF-24was separately assembled, Cu3(HITP)2 and the Ni//Cu MOF arrayon carbon fiber also served as electrodes. The electrochemicaltest based on these three electrodes showed that the Ni//CuMOF array achieved a specific capacitance as high as 1424 F g−1

at a current density of 2 A g−1 (Fig. 4b), which is much higherthan that of the precursor Co(OH)2 and Ni-MOF-24. The electro-chemical tests under a two-electrode system suggested that thishybrid capacitor delivered a maximum energy density of 57 W hkg−1 at a power density of 1500 W kg−1 and exceptional capaci-tance retention of 94.2% after 7000 cycles in an alkaline electro-lyte (KOH), as shown in Fig. 4c.116

To create a new type of ionic polymer metal composite(IPMC) actuator with high durability in an air environment,Zhang and coworkers started from the microstructure andthree-dimensional construction of electrode materials and pro-posed a high-performance electrochemical actuator based on a2D conductive Ni-CAT nanowire array (NWA) in 2021.117 A one-step in situ hydrothermal method was carried out to grow thecore–shell structure of the Ni-CAT NWA electrode material onthe surface of carbon nanofibers (CNFs) (Fig. 4d). The π–d con-jugated 2D conductive Ni-CAT not only has a periodic porousstructure similar to that of graphene, but it also has the advan-tages of high conductivity and large specific surface area(Fig. 4e). In addition, the ordered hierarchical pore structureand high conductivity of the Ni-CAT NWAs/CNF electrodes

Fig. 3 Structures of 2D MOFs 1 and 2. (A and E) The one-dimensional chain of (A) 1 and (E) 2. (B and F) Structures of ligands (B) m-H4TTFTB and (F)H8TTFOC. (C and G) The carboxylate groups in black bind to In3+ ions, whereas the red ones are unbound carboxylic acid groups; the 2D network of(C) 1 and (G) 2. (D and H) 3D stacking mode of (D) 1 and (H) 2. Color scheme: red, O; dark gray, C; yellow, S; green, H; cyan, In. Reproduced withpermission.114

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results in effective and rapid transmission of electrons andions. The four-probe method was used to measure the conduc-tivity of Ni-CAT NWAs/CNF and CNF, which was 4.63 × 10 Scm−1 and 7.04 S cm−1, respectively. After testing without anyadditional conductive additives or binders, the somewhat rec-tangular CV curve showed that the Ni-CAT NWAs/CNF electrodeexhibited EDLC behavior, and the area capacitance was ashigh as 40.5 mF cm−2 at a sweep rate of 5 mV s−1. The constantcurrent charge/discharge results indicated that the Ni-CATNWAs/CNF electrode underwent a lower iR drop at highcurrent densities, and when the current density was 0.1 mAcm−2, the maximum area capacitance was 48.5 mF cm−2. Inaddition, due to the high surface area and layered porousstructure, the Ni-CAT NWAs/CNF electrode exhibited satisfac-tory rate performance, with capacitances of 42.1, 33.9, 25.8,and 13.6 mF cm−2 at current densities of 0.2, 0.5, 1, and 2 mAcm−2, respectively. The Nyquist diagram illustrates that thecharge-transfer resistance (RCT) of the Ni-CAT NWAs/CNF elec-trode was as low as 7.4 Ω, and the equivalent series resistance(RS) was only 4.9 Ω, which was much lower than that of the Ni-CAT powder electrode (RS, 9.4 Ω; RCT, 14.9 Ω). The aboveresults show that the direct growth of Ni-CAT NWAs on theCNF current collector can provide a more optimal trans-mission path and promote the diffusion of ions in the one-dimensional pores, thereby reducing the transmission resis-tance of ions in the electrode.117

3. 2D layered MOFs as electrodematerials for SCs

Compared with 2D conductive MOFs, there are more diversemethodologies available to prepare 2D layered MOFs because

the fabrication of 2D layered MOFs can be realized not onlyfrom 2D pillar-layered MOFs but also from several three-dimen-sional MOFs (preparing 2D nanosheets or nanoflakes throughnanotechnological strategies). In 2011, the first use of a top–down method called Tyndall scattering for the synthesis of 2DMOF nanosheets, namely {Zn(TPA)(H2O)·DMF}n, occurred.118

Then, in 2013, Kondo et al. used a wet process to synthesizenanosheets through the same method based on a pillar-layeredMOF, namely [Cu(bpy)2(OTf)2]8 (bpy, 4,4′-bipyridine actingas the conducting unit and Otf, trifluoromethanesulfonateserving as the frame unit).119 In 2016, Xu et al.synthesized luminescent 2D MOF nanosheets, denoted asTi2(HDOBDC)2(H2DOBDC) (H2DOBDC = 2,5-dihydroxyter-ephthalic acid), for fast response and highly sensitive sensingof Fe3+ by way of top–down delamination.120 In these examples,the ‘top–down’ strategy was used, which usually includesmicromechanical, sonication-assisted liquid, and solvent-induced delamination exfoliation.84,121–133 The ‘bottom–up’method is the reverse strategy of ‘top–down’.134–141 Jiang et al.reported lanthanide-based MOF nanosheets with unique fluo-rescence-quenching properties (Fig. 5a).142 Additionally, theCliffe group reported a hafnium-based MOF nanosheet con-structed via defect-mediated transformation (Fig. 5b).143 Thesetwo examples are typical examples of the ‘bottom–up’ process,which can also involve hydro-/solvo-thermal methods, inter-face-mediated processes, and the Langmuir–Blodgett (LB)method.33 After these constructions, due to the excellent physi-cal and chemical properties and unique structure of layeredMOFs (LMOFs), there was increased interest in the applicationof LMOFs in the fields of catalysis, sensing, and biology, aswell as the SC field.44,45,144

Qu et al. synthesized a nickel-based, pillared DABCO-MOF(denoted as DMOF) with similar topologies, in the form of [Ni

Fig. 4 (a) Illustration showing MOF//MOF synthesis. (b) Galvanostatic profiles of Ni//Cu MOF, Ni-MOF-24, and Co(OH)2 at the current density of 2 Ag−1. (c) The stability test of the capacitor. Reproduced with permission.116 (d) Schematic illustration showing the fabrication of a Ni-CAT NWAs/CNFhybrid membrane by in situ hydrothermal growth of Ni-CAT NWAs on CNF. (e) Structural schematics of Ni-CAT along the c-axis. Color code: O, redspheres; C, gray spheres; Ni, blue spheres. Reproduced with permission.117

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(L)(DABCO)0.5] (where L denotes the functionalized BDC linkerand DABCO denotes 1,4-diazabicyclo[2.2.2]-octane) and directlyutilized this pillar-layered MOF as an SC electrode.145 A struc-ture simulation indicated that the as-prepared DMOF possesseda layered topology, which could be described as metal paddle-wheel clusters connected by functionalized BDC ligands to form2D layers that are further pillared together by DABCO ligands toform a 3D structure. After being directly used as electrodes in athree-electrode system, this MOF exhibited specific capacitancealteration from 552 to 438 F g−1 as the current densitiesincreased from 1 to 20 A g−1, indicating the excellent Coulomb

efficiency of 2D layered MOF-based materials. Moreover, theexceptional capacitance retention of 98% after 16 000 cycles atthe current density of 10 A g−1 and no structural change beforeand after the cycles proved that Ni-DMOF-ADC possesses long-lasting cycle stability.145 Due to the excellent performance of the2D layered MOF in the electrochemical test under the three-elec-trode environment, their performances under the two-electrodesystem were further explored, namely, assembling them intoenergy-related devices.

For instance, Feng and coworkers reported a facile layer-by-layer method (Fig. 6a) towards on-chip micro-supercapacitors

Fig. 5 (a) Schematic illustration of the overall process developed to produce 2D MOF nanosheets via an intercalation and chemical exfoliationapproach. Reproduced with permission.142 (b) Exfoliation and characterization of MOF-Ln nanosheets. Reproduced with permission.143

Fig. 6 (a) The synthesis of PiCBA and a schematic diagram of a PiCBA film through LBL fabrication. Reproduced with permission.146 (b) SEM andTEM images and the molecular structure of the 2D MOFs. Reproduced with permission.148 (c) Galvanostatic discharge curves of the porous Ni-MOFelectrode material at discharge current densities of 2, 5, 10, 20, and 25 A g−1, where the potential window ranged from 0 to 0.45 V. (d) Cycling per-formance at a current density of 5 A g−1. (e) Ragone plots of the device. Reproduced with permission.147

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(MSCs) based on a 2D coordination compound (marked asPiCBA). The PiCBA-based MSCs delivered a maximum volu-metric capacitance of 34.1 F cm3 at a sweep rate of 50 mV s−1

with a high energy density of 4.7 mW h cm3 due to the excel-lent carrier mobility of PiCBA, the permanent dipole momentof the azulene skeleton, and the ultralow bandgap of PiCBA.146

Additionally, in 2018, Zhang et al. synthesized another 2Dlayered Ni-MOF through a simple solvothermal method andused it as an electrode material for supercapacitors.147 The Ni-MOF-based electrode exhibited an excellent specific capaci-tance of 1,668.7 F g−1 at 2 A g−1 (Fig. 6c) and a capacitanceretention of 90.3% after 5000 cycles at 5 A g−1 (Fig. 6d). Thedouble electrode test showed that the energy density of thehybrid supercapacitor reached 57.29 W h kg−1 at a powerdensity of 160 W kg−1 (Fig. 6e).147

In 2019, Pang et al. synthesized ultrathin 2D cobalt-organicframework (Co-MOF) nanosheets by the ‘bottom–up’ strategyusing a facile surfactant-assisted one-pot hydrothermal syn-thesis (Fig. 6b).148 The three-electrode system revealed that theCo-MOF had a specific capacitance of 1159 F g−1 at a currentdensity of 0.5 A g−1, and the specific capacitance of the Co-MOF electrode was only attenuated by 3.3% after 6000cycles.148 In comparison with the 2D conductive MOFs,conductivity is an essential factor that hinders the practicalutilization of pillar-layered MOFs. The combination of thepillar-layered MOFs with a conductive matrix (includingcarbon-based materials) is a typical strategy to address thisproblem.

In order to verify the feasibility of a strategy to combineMOFs with a conductive matrix, Zhou reported the preparationof MOF nanomembranes on cellulose nanofibers (CNFs),where CNFs were used as substrates for the growth of MOFnanomembranes, and the performance was strongly depen-

dent on interfacial action.149 The obtained composite nano-fiber, namely CNF@c-MOF, was assembled into a self-support-ing nanopaper and possessed the characteristics of high con-ductivity (100 S cm−1), hierarchical micropores, and excellentmechanical properties. Furthermore, the as-synthesized nano-paper could be used as an electrode for flexible and foldablesupercapacitors. The authors proposed that the high conduc-tivity of the electrode promoted rapid charge transfer andefficient electrolyte transfer. As a result, the capacitance reten-tion rate was higher than 99% when the assembled deviceexperienced 10 000 unceasing charging and discharging cycles(Fig. 7a).149 In 2020, Ran designed a method for synthesizingultrathin 2D MOF nanosheets in situ interpenetrated by func-tional carbon nanotubes (CNTs, Fig. 7b).150 The thickness andeffective surface area of the novel 2D hybrid nanosheets couldbe precisely adjusted by controlling the addition of CNTs. Theas-prepared Ni-MOF/C-CNT nanosheets exhibited a superiorspecific capacity of 680 C g−1 at the current density of 1 A g−1.The assembled hybrid electrode exhibited a maximum energydensity of 44.4 W h kg−1 at a power density of 440 W kg−1.150

Cheng reported the fabrication of hybrid electrodes con-taining self-assembled 2D MOF/reduced graphene oxide (rGO)papers. Consequently, the flexible asymmetric SCs possessedan energy density of 1.87 mW h cm−3 at a power density of250 mW cm−3.14 In addition to carbon-based materials, metaloxides can also be combined with 3D MOFs to integrate intoone homogeneous MOF. Pang et al. reported an ultrathin Cu-MOF@δ-MnO2 nanosheet based on metal oxide and 3D Cu-MOF composites (also called HKUST-1). Gratifyingly, they grewthe 3D MOF on manganese dioxide nanosheets to realize a 2Dlayered structure. The as-obtained electrodes achieved a gravi-metric capacitance of 340 F g−1 (1 A g−1) within a 2.0 V poten-tial window.151 In an attempt to enhance the performance of

Fig. 7 (a) A schematic diagram of the synthesis procedure for the CNF@c-MOF and its electrochemical performances. Reproduced with per-mission.149 (b) A schematic illustration for the synthesis of ultrathin Ni-MOF/C-CNT nanosheets. Reproduced with permission.150

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the layered MOFs, compositing them with other materials isalso widely adapted to realize this purpose.

Except for the compositing strategy, layered MOFs can alsobe used as a template to fabricate derived lamellar structures.For example, Liu and coworkers designed a negative electrodematerial (cobalt–manganese-layered double hydroxide (CoMn-LDH)) for SCs. They designed a scheme to synthesize CoMn-LDH using so-called Co-ZIF-L as the sacrificial MOF template/precursor and different manganese-containing salt solutionsas manganese sources. Moreover, they explored the potentialmechanism of the phase transformation through variouscharacterizations of the as-synthesized materials. As to thedetailed performance, CoMn-LDH-SO4 delivered a high arealcapacity of 582.07 mC cm−2 at 2 mA cm−2. Two-electrodemeasurement suggested that the energy density of CoMn-LDH-SO4 was 0.096 mW h cm−2 with a power density of1.5 mW cm−2, and its exceptional capacitance retentionreached 89% even after 18 000 cycles.152

Unlike other groups’ research on the performances of 2Dlayered MOFs, our group mainly focused on the mechanismused by active 2D layered MOFs in SC electrodes. In 2019, ourgroup synthesized two kinetically stable CuI-MOFs with 2Dlayered architectures by the introduction of β-[Mo8O26]

4− clus-ters.153 The electrochemical tests showed that both MOFs hada specific capacitance greater than 800 F g−1 when the currentdensity was settled at 1 A g−1, and over 100% of the originalspecific capacitance was maintained after 5000 continuouscycles. Due to the battery-type electrochemical behavior, thepower-law theory mechanism was documented to analyze thedistribution of the current, and the results suggested that thecapacitance-controlled part (i ∝ v) and diffusion-controlled (i ∝v1/2) section of the current contributed nearly equally during

the faradaic reaction process (Fig. 8a).153 Then, in order toexplore the alternated product of the 2D layered MOFs duringthe charge–discharge cycles, we reported two identical layeredMOFs (namely, CoFRS and NiFRS) based on 1,10-bis(1,2,4-triazol-1-yl) building blocks and 1,4-benzenedicarboxylic acidligands.154 Both electrodes exhibited excellent electrochemicalperformances. After assembling with the activated carbon (AC)negative electrode, the CoFRS//AC device delivered an energydensity value of 28.7 W h kg−1, which was correlated with apower density value of 400 W kg−1 (Fig. 8b and c).154 After thenew intermediates for both electrodes before and after long-lifespan cycling were tested, a substance being ascribed toneither the original MOF nor the metal hydroxide was found,and it was different from that of recent reports.145,155 Recently,we also reported a 2D Fe-based MOF (labeled as FeSC1) basedon the reaction among Fe salts, a tripodal ligand 4,4′,4″-stria-zine-2,4,6-triyl-tribenzoate (H3TATB), and an N-containingligand.156 Three-electrode measurement indicated that thenew intermediates in two electrodes before and after long life-span cycling could be assigned to FeOOH, originating frompartial phase transformation of Fe-MOFs. Moreover, the two-electrode device was also assembled. The correspondingdevice, namely sss-FeSC1//AC BSH, exhibited a maximumenergy density of 40 W h kg−1 at a power density of 799 W kg−1

with exceptional capacitance retention of 93.1% after 5000cycles (Fig. 8d and e). More importantly, the LED demo groupsassembled with six LED lights were illuminated for 5 min,indicating excellent practicability for the layered MOFs.156

Apparently, unlike 2D layered MOFs, 2D conductive MOF-based SC electrodes possess excellent conductivities andendurance stabilities even without the assistance of conductiveadditives. Even under the circumstance of supplementing the

Fig. 8 (a) Electrochemical characterization of NAU-1 and NAU-2.153 (b) Endurance test for NiFRS//AC BSH devices at a current density of 3 A g−1.Inset: CP curves at the first 10 cycling numbers and the LED display. Reproduced with permission. (c) Ragone plots of energy density and powerdensity for CoFRS//AC and NiFRS//AC BSH devices. Reproduced with permission.154 (d) CD profile of the FeSC2//AC sss-BSH device at variouscurrent densities. (e) 3D Ragone plots of energy-power density for the obtained sss-BSH devices. Reproduced with permission.156

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conducting matrixes, it is highly desirable to achieve moreoptimal practicability such as greater flexibility and moreextended lifespan cycling. As to the 2D layered MOFs, althoughcompositing MOFs with conductive supplements to increasethe conductivity and endurance stability is still a commonstrategy, several groups are now choosing ‘pillar’ and ‘layer’ligands with the greatest conductivity to co-construct the 2Dpillar-layered MOFs. It is believed that conductive 2D layeredMOFs will possess more accessible pores to favour fast ion/electron transport, and with the aid of the ‘pillar’ ligands, itwill therefore not be necessary to construct dense stacks of 2Dplanar MOFs.

4. Challenges and outlooks

It should be noted that although this review divides 2D MOFsinto two types based on the conductive aspect (2D conductiveMOFs and 2D layered MOFs (including pillar-layered MOFsand 2D nanosheets)), there is no absolute boundary, butrather, there is a complementary relationship between them.For the 2D conductive model, there have been several notableachievements such as obtaining single crystals and under-standing the conductive mechanisms. However, research on2D conductive MOFs for use as SC electrode materials is stillrequired in the following directions: (i) how to systematicallyadopt these existing research results to control the fabricationof a unique 2D conductive MOF to overcome dense stacks andthus further improve the transportability for ions or electrons;(ii) it is still difficult to prepare single crystals of 2D conductiveMOFs due to the complicated coordination mode for thecurrent planar π-conjugated ligand; (iii) the matching mecha-nism between 2D conductive MOFs and electrolyte (or electro-lyte ions) remains unclear, and it is still unknown whether thecontact between the electrolyte and the electrode determinesthe utilization of the electrode surface area and pores.Therefore, clarifying the interaction between conductive MOFsas electrodes in the SC system and the electrolyte is of greatsignificance to further improve and guide the development ofconductive MOFs; (iv) SCs with greater endurance can beapplied to a broader range of fields, and enhancement of theelectrochemical stability is vital to promote their industrializ-ation. Until now, there have been no reports emphasizing thechemical and thermal stability of 2D conductive MOF electro-des; and (v) it is still challenging to use either 2D conductiveMOFs as the templates or use precursors to fabricate thecorresponding derived electrode materials.

As to the 2D layered MOFs or MOF-based nanosheets,several issues need to be addressed: (i) how to increase theconductivity of the layered MOFs to thus further improve theirperformance; (ii) the compositing mechanism or the detailedsynergistic mechanism is unclear, although MOFs can be com-bined with inorganic materials or carbon-based materials toenhance their conductivities, which will thereby result in moreoptimal performances; (iii) the structural alteration before andafter the cycling process is still in divergence in current

reports, such as maintaining the original structures or chan-ging them into metal hydroxides under alkaline electrolyte.Although most researchers believe that conjugated planarstructures are generally more stable than 3D MOFs in electro-chemical cycling tests, additional experimental data are stillrequired to confirm this opinion because there currently is nodirect evidence to prove this opinion. Thus, defining thespecific structure for the intermediates is essential so that theworking mechanisms for the 2D pillar-layered MOFs or MOF-based nanosheets may be established.

Therefore, in order to overcome these problems, several rec-ommendations for 2D conductive MOFs and pillar-layeredMOFs (or MOF-based nanosheets) are presented as follows: for2D conductive MOFs, (i) it is suggested to employ long-chainplanar p-conjugated organic ligands to synthesize 2D conduc-tive MOFs with larger inner spaces, which could furtherincrease the conductivity and eliminate the need for a densestacking mode for 2D MOFs; (ii) except for simulating thecrystal structure with PXRD technology, novel growth strategiesto attain single crystals of 2D conductive MOFs are highly desir-able; (iii) architecting 2D conductive MOF films with multi-metal-centers is strongly recommended to improve the activeredox sites and therefore enhance the gravimetric capacitances;(iv) the electrochemical stability of 2D conductive MOFs couldbe tuned by changing the bonding strength of metal–ligandstructural units or combining 2D conductive MOFs with highlystable pseudocapacitance materials; (v) in future research,theoretical predictions based on computational modeling cansignificantly assist in designing and discovering the mostoptimal new MOFs for energy storage applications. For pillar-layered MOFs or MOF-based nanosheets, some suggestions are:(i) the structural alteration before and after the cycling processshould be thoroughly investigated by conducting more elaboratecharacterizations based on the current research. Additionally,theoretical predictions based on computational modelingwould be an important strategy to provide reasonable sugges-tions regarding the possible structural changes that could occurduring the electrochemical process; (ii) borrowing the experi-ence from preparing 2D conductive MOFs, suitableπ-conjugated ‘pillar’ and ‘layered’ ligands to co-build 2D con-ductive-layered MOFs should be carefully chosen in future SCresearch; (iii) the preparation of homogeneous composites isvery important to achieve more optimal synergistic effects; (iv)adjusting the morphologies/sizes and enhancing the wettabilityof MOF-based electrodes is recommended to enhance the trans-port rate for electrolyte ions or electrons. Although there arenumerous issues to be addressed, it is believed that 2D MOFswith higher conductivity, increased robust structural stability,and more optimal electrochemical performance, as well as theirderived MOF-based active components, will be promising candi-dates as electrodes in future SCs.

Conflicts of interest

There are no conflicts to declare.

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Acknowledgements

This work was supported by Fundamental Research Funds forthe Central Universities (KYGD202107) and the NaturalScience Foundation of Jiangsu Province (BK20180514). QZthanks the start-up funding support from City University ofHong Kong.

References

1 X. Jiang, S. Li and L. Shao, Energy Environ. Sci., 2017, 10,1339–1344.

2 N. Choudhary, C. Li, J. Moore, N. Nagaiah, L. Zhai, Y. Jungand J. Thomas, Adv. Mater., 2017, 29, 1605336.

3 D. Y. Lee, D. V. Shinde, E.-K. Kim, W. Lee, I.-W. Oh,N. K. Shrestha, J. K. Lee and S.-H. Han, MicroporousMesoporous Mater., 2013, 171, 53–57.

4 K. Wang, H. Wu, Y. Meng and Z. Wei, Small, 2014, 10, 14–31.5 Y. Chen, M. Han, Y. Tang, J. Bao, S. Li, Y. Lan and Z. Dai,

Chem. Commun., 2015, 51, 12377–12380.6 S. Zhang, Z. Yang, K. Gong, B. Xu, H. Mei, H. Zhang,

J. Zhang, Z. Kang, Y. Yan and D. Sun, Nanoscale, 2019, 11,9598–9607.

7 R. Díaz, M. G. Orcajo, J. A. Botas, G. Calleja and J. Palma,Mater. Lett., 2012, 68, 126–128.

8 L. Kang, S.-X. Sun, L.-B. Kong, J.-W. Lang and Y.-C. Luo,Chin. Chem. Lett., 2014, 25, 957–961.

9 D. Bradley, Mater. Today, 2016, 19, 554.10 W. Zuo, R. Li, C. Zhou, Y. Li, J. Xia and J. Liu, Adv. Sci.,

2017, 4, 1600539.11 M. Gao, K. Le, D. Xu, Z. Wang, F. Wang, W. Liu, H. Yu,

J. Liu and C. Chen, J. Alloys Compd., 2019, 804, 27–34.12 X. Cao, Y. Liu, Y. Zhong, L. Cui, A. Zhang, J. M. Razal,

W. Yang and J. Liu, J. Mater. Chem. A, 2020, 8, 1837–1848.13 A. Gopalakrishnan and S. Badhulika, J. Power Sources,

2020, 480, 228830.14 J. Cheng, S. Chen, D. Chen, L. Dong, J. Wang, T. Zhang,

T. Jiao, B. Liu, H. Wang, J.-J. Kai, D. Zhang, G. Zheng,L. Zhi, F. Kang and W. Zhang, J. Mater. Chem. A, 2018, 6,20254–20266.

15 P. Du, Y. Dong, C. Liu, W. Wei, D. Liu and P. Liu, J. ColloidInterface Sci., 2018, 518, 57–68.

16 Z. Tang, G. Zhang, H. Zhang, L. Wang, H. Shi, D. Wei andH. Duan, Energy Storage Mater., 2018, 10, 75–84.

17 J. Wen, B. Xu, J. Zhou and Y. Chen, J. Power Sources, 2018,402, 91–98.

18 C. Wang, H. Wang, B. Dang, Z. Wang, X. Shen, C. Li andQ. Sun, Renew. Energy, 2020, 156, 370–376.

19 K. Wang, R. Bi, M. Huang, B. Lv, H. Wang, C. Li, H. Wuand Q. Zhang, Inorg. Chem., 2020, 59, 6808–6814.

20 K. Wang, B. Lv, Z. Wang, H. Wu, J. Xu and Q. Zhang,Dalton Trans., 2020, 49, 411–417.

21 B. Xu, M. Zheng, H. Tang, Z. Chen, Y. Chi, L. Wang,L. Zhang, Y. Chen and H. Pang, Nat. Nanotechnol., 2019,30, 204002.

22 X. Wang, Y. Zhang, J. Zheng, X. Liu and C. Meng,J. Colloid Interface Sci., 2019, 554, 191–201.

23 N. Devillers, M.-C. Péra, D. Bienaimé and M.-L. Grojo,J. Power Sources, 2014, 270, 391–402.

24 P. Magne, B. Nahid-Mobarakeh and S. Pierfederici, IEEETrans. Ind. Appl., 2013, 49, 2352–2360.

25 N. A. Kyeremateng, T. Brousse and D. Pech, Nat.Nanotechnol., 2017, 12, 7–15.

26 L. Li, M. Zhang, X. Zhang and Z. Zhang, J. Power Sources,2017, 364, 234–241.

27 R. Li, C. He, L. Cheng, G. Lin, G. Wang, D. Shi, R. K.-Y. Liand Y. Yang, Composites, Part B, 2017, 121, 75–82.

28 J.-L. Zhuang, X.-Y. Liu, H.-L. Mao, C. Wang, H. Cheng,Y. Zhang, X. Du, S.-B. Zhu and B. Ren, J. Power Sources,2019, 429, 9–16.

29 W. Jiang, J. Pan, J. Wang, J. Cai, X. Gang, X. Liu andY. Sun, Carbon, 2019, 154, 428–438.

30 S. Sanati, R. Abazari, A. Morsali, A. M. Kirillov, P. C. Junkand J. Wang, Inorg. Chem., 2019, 58, 16100–16111.

31 Y. Liu, Y. Wang, H. Wang, P. Zhao, H. Hou and L. Guo,Appl. Surf. Sci., 2019, 492, 455–463.

32 K.-B. Wang, Q. Xun and Q. Zhang, EnergyChem, 2020, 2,100025.

33 K.-B. Wang, R. Bi, Z.-K. Wang, Y. Chu and H. Wu, New J.Chem., 2020, 44, 3147–3167.

34 Y. Lei, C. Fournier, J.-L. Pascal and F. Favier, MicroporousMesoporous Mater., 2008, 110, 167–176.

35 L. L. Zhang and X. S. Zhao, Chem. Soc. Rev., 2009, 38,2520–2531.

36 H. Jiang, C. Li, T. Sun and J. Ma, Nanoscale, 2012, 4, 807–812.

37 G. Wang, L. Zhang and J. Zhang, Chem. Soc. Rev., 2012,41, 797–828.

38 K. Xie, X. Qin, X. Wang, Y. Wang, H. Tao, Q. Wu, L. Yangand Z. Hu, Adv. Mater., 2012, 24, 347–352.

39 J. Yan, Z. Fan, W. Sun, G. Ning, T. Wei, Q. Zhang,R. Zhang, L. Zhi and F. Wei, Adv. Funct. Mater., 2012, 22,2632–2641.

40 K. Wang, X. Ma, Z. Zhang, M. Zheng, Z. Geng andZ. Wang, Chemistry, 2013, 19, 7084–7089.

41 F. Zhang, T. Zhang, X. Yang, L. Zhang, K. Leng, Y. Huangand Y. Chen, Energy Environ. Sci., 2013, 6, 1623.

42 Y. Gao, J. Wu, W. Zhang, Y. Tan, J. Gao, B. Tang andJ. Zhao, RSC Adv., 2014, 4, 36366–36371.

43 J. A. Rajesh, Y.-H. Lee, Y.-H. Yun, V. H. V. Quy, S.-H. Kang,H. Kim and K.-S. Ahn, J. Electroanal. Chem., 2019, 850,113371.

44 (a) J. Dong, Z. Qiao, Y. Pan, S. B. Peh, Y. D. Yuan, Y. Wang,L. Zhai, H. Yuan, Y. Cheng, H. Liang, B. Liu and D. Zhao,Chem. Mater., 2019, 31, 4897–4912; (b) C. Li, K. Wang, J. Liand Q. Zhang, ACS Mater. Lett., 2020, 2, 779–797.

45 X. Liu, Z. Yan, Y. Zhang, Z. Liu, Y. Sun, J. Ren and X. Qu,ACS Nano, 2019, 13, 5222–5230.

46 (a) S. Jia, X. Xiao, Q. Li, Y. Li, Z. Duan, Y. Li, X. Li, Z. Lin,Y. Zhao and W. Huang, Inorg. Chem., 2019, 58, 12748–12755; (b) C.-J. Yao, Z. Wu, J. Xie, F. Yu, W. Guo, Z. J. Xu,

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. View Article Online

D.-S. Li, S. Zhang and Q. Zhang, ChemSusChem, 2020, 13,2457–2463; (c) Z. Wu, D. Adekoya, X. Huang, M. J. Kiefel,J. Xie, W. Xu, Q. Zhang, D. Zhu and S. Zhang, ACS Nano,2020, 14, 12016–12026; (d) F. Yu, W. Liu, S.-W. Ke,M. Kurmoo, J.-L. Zuo and Q. Zhang, Nat. Commun., 2020,11, 5534; (e) F. Yu, W. Liu, B. Li, D. Tian, J.-L. Zuo andQ. Zhang, Angew. Chem., Int. Ed., 2019, 58, 16101–16104;(f ) Z. Lin, J. Xie, B. Zhang, J. Li, J.-N. Weng, R.-B. Song,X. Huang, H. Zhang, H. Li, Y. Liu, Z. J. Xu, W. Huang andQ. Zhang, Nano Energy, 2017, 41, 117–127.

47 K. Wada, K. Sakaushi, S. Sasaki and H. Nishihara, Angew.Chem., Int. Ed., 2018, 57, 8886–8890.

48 R. Dong, Z. Zheng, D. C. Tranca, J. Zhang,N. Chandrasekhar, S. Liu, X. Zhuang, G. Seifert andX. Feng, Chemistry, 2017, 23, 2255–2260.

49 C. Feng, C.-P. Lv, Z.-Q. Li, H. Zhao and H.-H. Huang,J. Solid State Chem., 2018, 265, 244–247.

50 I. Stassen, N. Burtch, A. Talin, P. Falcaro, M. Allendorfand R. Ameloot, Chem. Soc. Rev., 2017, 46, 3185–3241.

51 J. Liu and C. Wöll, Chem. Soc. Rev., 2017, 46, 5730–5770.52 P. Ramaswamy, N. E. Wong and G. K. H. Shimizu, Chem.

Soc. Rev., 2014, 43, 5913–5932.53 L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R. P. Van

Duyne and J. T. Hupp, Chem. Rev., 2012, 112, 1105–1125.54 B. Li, H.-M. Wen, Y. Cui, W. Zhou, G. Qian and B. Chen,

Adv. Mater., 2016, 28, 8819–8860.55 J. H. Dou, M. Q. Arguilla, Y. Luo, J. Li, W. Zhang, L. Sun,

J. L. Mancuso, L. Yang, T. Chen, L. R. Parent,G. Skorupskii, N. J. Libretto, C. Sun, M. C. Yang, P. V. Dip,E. J. Brignole, J. T. Miller, J. Kong, C. H. Hendon, J. Sunand M. Dinca, Nat. Mater., 2021, 20, 222–228.

56 S. Zhao, H. Wu, Y. Li, Q. Li, J. Zhou, X. Yu, H. Chen,K. Tao and L. Han, Inorg. Chem. Front., 2019, 6, 1824–1830.

57 A. Cadiau, L. S. Xie, N. Kolobov, A. Shkurenko,M. Qureshi, M. R. Tchalala, S. S. Park, A. Bavykina,M. Eddaoudi, M. Dincă, C. H. Hendon and J. Gascon,Chem. Mater., 2019, 32, 97–104.

58 A. A. Lobinsky and V. P. Tolstoy, RSC Adv., 2018, 8, 29607–29612.

59 K. Rui, G. Zhao, Y. Chen, Y. Lin, Q. Zhou, J. Chen, J. Zhu,W. Sun, W. Huang and S. X. Dou, Adv. Funct. Mater., 2018,28, 1801554.

60 Y. Huang, M. Zhao, S. Han, Z. Lai, J. Yang, C. Tan, Q. Ma,Q. Lu, J. Chen, X. Zhang, Z. Zhang, B. Li, B. Chen, Y. Zongand H. Zhang, Adv. Mater., 2017, 29, 1700102.

61 G. Zhan and H. C. Zeng, Adv. Funct. Mater., 2016, 26,3268–3281.

62 X. Huang, B. Zheng, Z. Liu, C. Tan, J. Liu, B. Chen, H. Li,J. Chen, X. Zhang, Z. Fan, W. Zhang, Z. Guo, F. Huo,Y. Yang, L.-H. Xie, W. Huang and H. Zhang, ACS Nano,2014, 8, 8695–8701.

63 H. Jia, Y. Yao, J. Zhao, Y. Gao, Z. Luo and P. Du, J. Mater.Chem. A, 2018, 6, 1188–1195.

64 Y. Zhu, J. Cui, S. An, Z. Li, Y. Zhang and W. He, J. AlloysCompd., 2019, 810, 151932.

65 M. Muschi and S. Christian, Coord. Chem. Rev., 2019, 387,262–272.

66 J. Suarez-Guevara, V. Ruiz and P. Gomez-Romero, Phys.Chem. Chem. Phys., 2014, 16, 20411–20414.

67 L. Zhang, D. Huang, N. Hu, C. Yang, M. Li, H. Wei,Z. Yang, Y. Su and Y. Zhang, J. Power Sources, 2017, 342, 1–8.

68 E. A. Flügel, A. Ranft, F. Haase and B. V. Lotsch, J. Mater.Chem. A, 2012, 22, 10119.

69 T. R. Cook, Y. R. Zheng and P. J. Stang, Chem. Rev., 2013,113, 734–777.

70 J. Yang, C. Zheng, P. Xiong, Y. Li and M. Wei, J. Mater.Chem. A, 2014, 2, 19005–19010.

71 (a) P. Srimuk, S. Luanwuthi, A. Krittayavathananon andM. Sawangphruk, Electrochim. Acta, 2015, 157, 69–77;(b) Y. Pan, R. Abazari, Y. Wu, J. Gao and Q. Zhang,Electrochem. Commun., 2021, 126, 107024; (c) Y. Wu, Y. Li,J. Gao and Q. Zhang, SusMat, 2021, 1, 66–87; (d) Y. Yan,C. Li, Y. Wu, J. Gao and Q. Zhang, J. Mater. Chem. A, 2020,8, 15245–15270; (e) J. Gao, M. He, Z. Y. Lee, W. Cao,W.-W. Xiong, Y. Li, R. Ganguly, T. Wu and Q. Zhang,Dalton Trans., 2013, 42, 11367–11370; (f ) Q. Zhang, X. Bu,Z. Lin, T. Wu and P. Feng, Inorg. Chem., 2008, 47, 9724–9726.

72 B.-X. Dong, F.-Y. Bu, Y.-C. Wu, J. Zhao, Y.-L. Teng,W.-L. Liu and Z.-W. Li, Cryst. Growth Des., 2017, 17, 5309–5317.

73 Y. Jiao, J. Pei, D. Chen, C. Yan, Y. Hu, Q. Zhang andG. Chen, J. Mater. Chem. A, 2017, 5, 1094–1102.

74 R. R. Salunkhe, Y. V. Kaneti and Y. Yamauchi, ACS Nano,2017, 11, 5293–5308.

75 R. Zhu, J. Ding, L. Jin and H. Pang, Coord. Chem. Rev.,2019, 389, 119–140.

76 M. D. Allendorf, R. Dong, X. Feng, S. Kaskel, D. Matogaand V. Stavila, Chem. Rev., 2020, 120, 8581–8640.

77 G.-B. Xiao, X.-Q. Yao, H. Xie, H.-C. Ma, P.-J. Yan andD.-D. Qin, Polyhedron, 2019, 162, 39–44.

78 Z. Lu, K. Wang, Y. Cao, Y. Li and D. Jia, J. Alloys Compd.,2021, 871, 159580.

79 Y. Zhao, Y. Liu, Z. Wang, Y. Ma, Y. Zhou, X. Shi, Q. Wu,X. Wang, M. Shao, H. Huang, Y. Liu and Z. Kang, Appl.Catal., B, 2021, 289, 120035.

80 H. Sun and L. Liu, Sens. Actuators, B, 2021, 338, 129825.81 V. Posligua, D. Pandya, A. Aziz, M. Rivera, R. Crespo-

Otero, S. Hamad and R. Grau-Crespo, J. Phys. Energy,2021, 3, 034005.

82 F. Wang, X. Wu, X. Yuan, Z. Liu, Y. Zhang, L. Fu, Y. Zhu,Q. Zhou, Y. Wu and W. Huang, Chem. Soc. Rev., 2017, 46,6816–6854.

83 C. Guan, X. Liu, W. Ren, X. Li, C. Cheng and J. Wang, Adv.Energy Mater., 2017, 7, 1602391.

84 M. Zhang, G. Feng, Z. Song, Y.-P. Zhou, H.-Y. Chao,D. Yuan, T. T. Y. Tan, Z. Guo, Z. Hu, B. Z. Tang, B. Liu andD. Zhao, J. Am. Chem. Soc., 2014, 136, 7241–7244.

85 M. G. Campbell, D. Sheberla, S. F. Liu, T. M. Swager andM. Dincă, Angew. Chem., Int. Ed., 2015, 54, 4349–4352.

Perspective Dalton Transactions

11344 | Dalton Trans., 2021, 50, 11331–11346 This journal is © The Royal Society of Chemistry 2021

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ishe

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:13

AM

. View Article Online

86 M. G. Campbell, S. F. Liu, T. M. Swager and M. Dincă,J. Am. Chem. Soc., 2015, 137, 13780–13783.

87 Y.-P. Wu, W. Zhou, J. Zhao, W.-W. Dong, Y.-Q. Lan,D.-S. Li, C. Sun and X. Bu, Angew. Chem., Int. Ed., 2017,56, 13001–13005.

88 S. Bi, H. Banda, M. Chen, L. Niu, M. Chen, T. Wu,J. Wang, R. Wang, J. Feng, T. Chen, M. Dinca,A. A. Kornyshev and G. Feng, Nat. Mater., 2020, 19, 552–558.

89 L. Chai, Z. Hu, X. Wang, Y. Xu, L. Zhang, T.-T. Li, Y. Hu,J. Qian and S. Huang, Adv. Sci., 2020, 7, 1903195.

90 S. Zhang, W. Xia, Q. Yang, Y. Valentino Kaneti, X. Xu,S. M. Alshehri, T. Ahamad, M. S. A. Hossain, J. Na, J. Tangand Y. Yamauchi, Chem. Eng. J., 2020, 396, 125154.

91 L. Yang, M.-L. Li, J.-N. Lin, M.-Y. Zou, S.-T. Gu, X.-J. Hong,L.-P. Si and Y.-P. Cai, Inorg. Chem., 2020, 59, 2406–2412.

92 J. Li, W. Xia, J. Tang, H. Tan, J. Wang, Y. V. Kaneti,Y. Bando, T. Wang, J. He and Y. Yamauchi, NanoscaleHoriz., 2019, 4, 1006–1013.

93 X. Li, X. Yang, H. Xue, H. Pang and Q. Xu, EnergyChem,2020, 2, 100027.

94 S. Takaishi, M. Hosoda, T. Kajiwara, H. Miyasaka,M. Yamashita, Y. Nakanishi, Y. Kitagawa, K. Yamaguchi,A. Kobayashi and H. Kitagawa, Inorg. Chem., 2009, 48,9048–9050.

95 M. Hmadeh, Z. Lu, Z. Liu, F. Gándara, H. Furukawa,S. Wan, V. Augustyn, R. Chang, L. Liao, F. Zhou, E. Perre,V. Ozolins, K. Suenaga, X. Duan, B. Dunn, Y. Yamamto,O. Terasaki and O. M. Yaghi, Chem. Mater., 2012, 24,3511–3513.

96 L. Sun, M. G. Campbell and M. Dinca, Angew. Chem., Int.Ed., 2016, 55, 3566–3579.

97 T. Kambe, R. Sakamoto, K. Hoshiko, K. Takada,M. Miyachi, J. H. Ryu, S. Sasaki, J. Kim, K. Nakazato,M. Takata and H. Nishihara, J. Am. Chem. Soc., 2013, 135,2462–2465.

98 J. Cui and Z. Xu, Chem. Commun., 2014, 50, 3986–3988.99 D. Sheberla, L. Sun, M. A. Blood-Forsythe, S. Er,

C. R. Wade, C. K. Brozek, A. Aspuru-Guzik and M. Dincă,J. Am. Chem. Soc., 2014, 136, 8859–8862.

100 T. Pal, T. Kambe, T. Kusamoto, M. L. Foo, R. Matsuoka,R. Sakamoto and H. Nishihara, ChemPlusChem, 2015, 80,1255–1258.

101 X. Huang, P. Sheng, Z. Tu, F. Zhang, J. Wang, H. Geng,Y. Zou, C. A. Di, Y. Yi, Y. Sun, W. Xu and D. Zhu, Nat.Commun., 2015, 6, 7408.

102 M. G. Campbell, S. F. Liu, T. M. Swager and M. Dinca,J. Am. Chem. Soc., 2015, 137, 13780–13783.

103 M. G. Campbell, D. Sheberla, S. F. Liu, T. M. Swager andM. Dinca, Angew. Chem., Int. Ed., 2015, 54, 4349–4352.

104 A. J. Clough, J. W. Yoo, M. H. Mecklenburg andS. C. Marinescu, J. Am. Chem. Soc., 2015, 137, 118–121.

105 E. M. Miner, S. Gul, N. D. Ricke, E. Pastor, J. Yano,V. K. Yachandra, T. Van Voorhis and M. Dincă, ACS Catal.,2017, 7, 7726–7731.

106 J. Park, M. Lee, D. Feng, Z. Huang, A. C. Hinckley,A. Yakovenko, X. Zou, Y. Cui and Z. Bao, J. Am. Chem. Soc.,2018, 140, 10315–10323.

107 R. Sakamoto, T. Kambe, S. Tsukada, K. Takada,K. Hoshiko, Y. Kitagawa, M. Okumura and H. Nishihara,Inorg. Chem., 2013, 52, 7411–7416.

108 M. S. Yao, X. J. Lv, Z. H. Fu, W. H. Li, W. H. Deng,G. D. Wu and G. Xu, Angew. Chem., Int. Ed., 2017, 56,16510–16514.

109 D. Sheberla, J. C. Bachman, J. S. Elias, C. J. Sun, Y. Shao-Horn and M. Dinca, Nat. Mater., 2017, 16, 220–224.

110 W.-H. Li, K. Ding, H.-R. Tian, M.-S. Yao, B. Nath,W.-H. Deng, Y. Wang and G. Xu, Adv. Funct. Mater., 2017,27, 1702067.

111 D. Feng, T. Lei, M. R. Lukatskaya, J. Park, Z. Huang,M. Lee, L. Shaw, S. Chen, A. A. Yakovenko, A. Kulkarni,J. Xiao, K. Fredrickson, J. B. Tok, X. Zou, Y. Cui andZ. Bao, Nat. Energy, 2018, 3, 30–36.

112 D. K. Nguyen, I. M. Schepisi and F. Z. Amir, Chem. Eng. J.,2019, 378, 122150.

113 J. Liu, Y. Zhou, Z. Xie, Y. Li, Y. Liu, J. Sun, Y. Ma,O. Terasaki and L. Chen, Angew. Chem., 2020, 59, 1081–1086.

114 J. Su, W. He, X.-M. Li, L. Sun, H.-Y. Wang, Y.-Q. Lan,M. Ding and J.-L. Zuo, Matter, 2020, 2, 711–722.

115 H. Meng, Y. Han, C. Zhou, Q. Jiang, X. Shi, C. Zhan andR. Zhang, Small Methods, 2020, 4, 2000396.

116 T. Deng, X. Shi, W. Zhang, Z. Wang and W. Zheng,iScience, 2020, 23, 101220.

117 Y. X. Shi, Y. Wu, S. Q. Wang, Y. Y. Zhao, T. Li, X. Q. Yangand T. Zhang, J. Am. Chem. Soc., 2021, 143, 4017–4023.

118 P.-Z. Li, Y. Maeda and Q. Xu, Chem. Commun., 2011, 47,8436–8438.

119 A. Kondo, C. C. Tiew, F. Moriguchi and K. Maeda, DaltonTrans., 2013, 42, 15267–15270.

120 H. Xu, J. Gao, X. Qian, J. Wang, H. He, Y. Cui, Y. Yang,Z. Wang and G. Qian, J. Mater. Chem. A, 2016, 4, 10900–10905.

121 Z. Li, D. Wang, Y. Wu and Y. Li, Natl. Sci. Rev., 2018, 5,673–689.

122 C. Liu, L. Wang, J. Qi and K. Liu, Adv. Mater., 2020, 32,2000046.

123 D. Rodríguez-San-Miguel, C. Montoro and F. Zamora,Chem. Soc. Rev., 2020, 49, 2291–2302.

124 K. Wang, X.-L. Lv, D. Feng, J. Li, S. Chen, J. Sun, L. Song,Y. Xie, J.-R. Li and H.-C. Zhou, J. Am. Chem. Soc., 2016,138, 914–919.

125 M. Hu, Y. Ju, K. Liang, T. Suma, J. Cui and F. Caruso, Adv.Funct. Mater., 2016, 26, 5827–5834.

126 L. Vilà-Nadal and L. Cronin, Nat. Rev. Mater., 2017, 2,17054.

127 E.-X. Chen, M. Qiu, Y.-F. Zhang, Y.-S. Zhu, L.-Y. Liu,Y.-Y. Sun, X. Bu, J. Zhang and Q. Lin, Adv. Mater., 2018,30, 1704388.

128 P. Chandrasekhar, A. Mukhopadhyay, G. Savitha andJ. N. Moorthy, J. Mater. Chem. A, 2017, 5, 5402–5412.

Dalton Transactions Perspective

This journal is © The Royal Society of Chemistry 2021 Dalton Trans., 2021, 50, 11331–11346 | 11345

Publ

ishe

d on

14

July

202

1. D

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d on

11/

24/2

021

9:01

:13

AM

. View Article Online

129 L. Zhuang, L. Ge, H. Liu, Z. Jiang, Y. Jia, Z. Li, D. Yang,R. K. Hocking, M. Li, L. Zhang, X. Wang, X. Yao andZ. Zhu, Angew. Chem., 2019, 58, 13565–13572.

130 A. Mukhopadhyay, V. K. Maka, G. Savitha andJ. N. Moorthy, Chem, 2018, 4, 1059–1079.

131 L. Cao, T. Wang and C. Wang, Chin. J. Chem., 2018, 36,754–764.

132 B. Garai, A. Mallick, A. Das, R. Mukherjee andR. Banerjee, Chem. – Eur. J., 2017, 23, 7361–7366.

133 K. Zhao, W. Zhu, S. Liu, X. Wei, G. Ye, Y. Su and Z. He,Nanoscale Adv., 2020, 2, 536–562.

134 Y. Lin, H. Wan, D. Wu, G. Chen, N. Zhang, X. Liu, J. Li,Y. Cao, G. Qiu and R. Ma, J. Am. Chem. Soc., 2020, 142,7317–7321.

135 F. Mercuri, M. Baldoni and A. Sgamellotti, Nanoscale,2012, 4, 369–379.

136 L. Ma, F. Svec, Y. Lv and T. Tan, Chem. – Asian J., 2019, 14,3502–3514.

137 T. Rodenas, I. Luz, G. Prieto, B. Seoane, H. Miro,A. Corma, F. Kapteijn, F. X. Llabrés i Xamena andJ. Gascon, Nat. Mater., 2015, 14, 48–55.

138 R. Makiura, S. Motoyama, Y. Umemura, H. Yamanaka,O. Sakata and H. Kitagawa, Nat. Mater., 2010, 9, 565–571.

139 F. Nouar, J. F. Eubank, T. Bousquet, L. Wojtas,M. J. Zaworotko and M. Eddaoudi, J. Am. Chem. Soc.,2008, 130, 1833–1835.

140 J. Park, Q. Jiang, D. Feng, L. Mao and H.-C. Zhou, J. Am.Chem. Soc., 2016, 138, 3518–3525.

141 X. Xiao, L. Zou, H. Pang and Q. Xu, Chem. Soc. Rev., 2020,49, 301–331.

142 Y. Ding, Y. P. Chen, X. Zhang, L. Chen, Z. Dong,H. L. Jiang, H. Xu and H. C. Zhou, J. Am. Chem. Soc., 2017,139, 9136–9139.

143 H.-S. Wang, J. Li, J.-Y. Li, K. Wang, Y. Ding and X.-H. Xia,NPG Asia Mater., 2017, 9, e354–e354.

144 R. Dong, Z. Zhang, D. C. Tranca, S. Zhou, M. Wang,P. Adler, Z. Liao, F. Liu, Y. Sun, W. Shi, Z. Zhang,E. Zschech, S. C. B. Mannsfeld, C. Felser and X. Feng, Nat.Commun., 2018, 9, 2637.

145 C. Qu, Y. Jiao, B. Zhao, D. Chen, R. Zou, K. S. Walton andM. Liu, Nano Energy, 2016, 26, 66–73.

146 C. Yang, K. S. Schellhammer, F. Ortmann, S. Sun,R. Dong, M. Karakus, Z. Mics, M. Loffler, F. Zhang,X. Zhuang, E. Canovas, G. Cuniberti, M. Bonn andX. Feng, Angew. Chem., Int. Ed., 2017, 56, 3920–3924.

147 C. Zhang, Q. Zhang, K. Zhang, Z. Xiao, Y. Yang andL. Wang, RSC Adv., 2018, 8, 17747–17753.

148 Y. Zheng, S. Zheng, Y. Xu, H. Xue, C. Liu and H. Pang,Chem. Eng. J., 2019, 373, 1319–1328.

149 X. K. S. Zhou, B. Zheng, F. Huo, M. Stromme and C. Xu,ACS Nano, 2019, 13, 9578–9586.

150 F. Ran, X. Xu, D. Pan, Y. Liu, Y. Bai and L. Shao, Nano-Micro Lett., 2020, 12, 46.

151 J. Xu, Y. Wang, S. Cao, J. Zhang, G. Zhang, H. Xue, Q. Xuand H. Pang, J. Mater. Chem. A, 2018, 6, 17329–17336.

152 X. Liu, L. Zhang, X. Gao, C. Guan, Y. Hu and J. Wang, ACSAppl. Mater. Interfaces, 2019, 11, 23236–23243.

153 K. Wang, Z. Wang, S. Wang, Y. Chu, R. Xi, X. Zhang andH. Wu, Chem. Eng. J., 2019, 367, 239–248.

154 K. Wang, Q. Li, Z. Ren, C. Li, Y. Chu, Z. Wang, M. Zhang,H. Wu and Q. Zhang, Small, 2020, 16, e2001987.

155 X. Liu, C. Shi, C. Zhai, M. Cheng, Q. Liu and G. Wang,ACS Appl. Mater. Interfaces, 2016, 8, 4585–4591.

156 K. Wang, S. Wang, J. Liu, Y. Guo, F. Mao, H. Wu andQ. Zhang, ACS Appl. Mater. Interfaces, 2021, 13, 15315–15323.

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