metal–organic frameworks as platforms for clean energy

Energy &Environmental Science

REVIEW

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Metal–organic fram

National Institute of Advanced Industrial Sci

563-8577, Japan. E-mail: [email protected]; Fa

Cite this: DOI: 10.1039/c3ee40507a

Received 13th February 2013Accepted 18th March 2013

DOI: 10.1039/c3ee40507a

www.rsc.org/ees

This journal is ª The Royal Society of

eworks as platforms for cleanenergy

Shun-Li Li and Qiang Xu*

In order to void environmental pollution and an energy shortage, the application of clean and renewable

energy, such as solar, instead of fossil fuel is foreseen as a prospective issue. It is urgent and important to

develop and optimize various energy storage and conversion technologies and materials aimed at

utilization of different clean energy sources. Metal–organic frameworks (MOFs), a new class of porous

crystalline materials, act as an outstanding candidate in this field based on their high surface areas,

controllable structures and excellent electrochemical properties. Here, selected recent and significant

advances in the development of MOFs for clean energy applications are reviewed, and special emphases

are shown to the applications of MOFs as platforms for hydrogen production and storage, fuel cells,

Li-ion rechargeable batteries, supercapacitors and solar cells.

Broader context

With increasing environmental pollution and energy shortages, a major scientic and societal challenge is the conversion from a fossil fuel-based energyeconomy to a sustainable and clean one. Clean energy, such as solar, has a much lower environmental impact than conventional energies and will not run out,which is rapidly becoming a global issue. Different kinds of technologies have been used to convert and store energy from clean energy, such as hydrogen fuelcells, Li-ion rechargeable batteries, supercapacitors, solar cells and so on. At the same time, various materials have been optimized to increase energy storageand conversion efficiency. MOFs, with high yield and low cost, are promising candidates for application to clean energy based on their high surface area,adjusted pore sizes, controllable structures, low densities and so on. In this review, we present a survey on the research progresses for MOFs as platforms inhydrogen storage, fuel cells, Li-ion rechargeable batteries, supercapacitors and solar cells.

1 Introduction

Today, an important and urgent global problem for us is thewide use of fossil fuels and consequent lack of fuel and envi-ronmental pollution (green house effect etc.). This world is at anenergy crossroads. Thus, the development of clean energy hasbeen the subject of recent attention. Various energy storage andconversion systems are being developed aimed at utilization ofdifferent clean energy sources. Some reasonable and construc-tive plans have been considered. Free solar energy is a clean,safe, and renewable source of energy, which can be converted toelectric energy and stored in a rechargeable battery, and canproduce hydrogen from water by photocatalysis and so on.1–3

Hydrogen is one of the most promising candidates to replacenon-renewable fuel sources used nowadays because it is high inenergy, and can react with oxygen to generate electric energy infuel cells without producing pollution.4–10 Although differentkinds of materials have been employed in this eld,11–15 it is stilla huge challenge to effectively store and convert hydrogen foruse when needed with the highest efficiency and near zerocarbon emission.

ence and Technology (AIST), Ikeda, Osaka

x: +81 72 751 9629; Tel: +81 72 751 9562

Chemistry 2013

Emerging as a new class of porous crystalline materials,MOFs,16–20 assembled by two main components of inorganicvertices (metal ions or clusters) and organic linkers, havebecome a rapidly developing research area and have attractedtremendous attention in the last two decades. As a sub-branchof coordination polymers,21 one of the advantages of MOFs isthat their structures can be designed according to targetedproperties by careful selection of metal centers and differentfunctional linkers.22–33 A key structural feature in MOFs is thehigh porosity as well as high surface area, which plays a crucialrole in the functional properties, typically in gas storage andseparation,34–41 hosting guest molecules or nanoparticles (NPs)or as nanoreactors,42 thin lms,43–46 sensing or recognition,47–49

proton conduction50–52 and drug delivery.53,54 In addition, MOFsdisplay versatile excellent properties in magnetism,55 uores-cence,56–58 catalysis,59–62 electrochemistry63 and so on.64,65 As akind of multifunctional material, MOFs can be obtained in highyield by using cheap starting materials, combined with a lowframework density and high thermal stability, which makethem a candidate for applications related to clean energy.66

The topic of this review is on MOFs as platforms for cleanenergy. We aim to present the progress investigation, discus-sion and challenges related to the applications of MOFs in theelds of hydrogen energy (hydrogen sorption, nanoconnement

Energy Environ. Sci.

http://dx.doi.org/10.1039/c3ee40507a

http://pubs.rsc.org/en/journals/journal/EE

Scheme 1 The applications of MOFs in clean energy.

Energy & Environmental Science Review

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of chemical hydrides, hydrogen generation), fuel cells, Li-ionrechargeable batteries, supercapacitors and solar cells accord-ing to MOF’s roles (Scheme 1). We sincerely hope that this willinspire the interest of readers in clean energy investigation, whoare advised to further read the cited articles and reviews.

2 MOFs for hydrogen energy

Hydrogen is an attractive option because it has a high energycontent (120 MJ kg�1 compared to 44 MJ kg�1 for gasoline) andproduces a clean exhaust product (water vapor without CO2 orNOx), which is one of the most promising candidates as arenewable, environmentally friendly energy carrier. Muchattention has been paid to hydrogen energy by researchers andgovernments. In September 2011, the US Department of Energy(DOE) set the new targets for developing and verifying on-boardhydrogen storage systems: 5.5 wt% in gravimetric capacity and0.040 kg L�1 volumetric capacity at mild conditions (�40 to60 �C) by 2017,67 which are for a complete system, includingtank, material, valves, regulators, piping, mounting brackets,insulation, added cooling capacity, and/or other balance-of-plant components. With the merits of high crystallinity, purity,adjustable high porosity and controllable structural

Shun-Li Li was born in 1979 inJilin, P. R. China. She receivedBS (2002) and PhD degree(2008) at the Department ofChemistry, Northeast NormalUniversity (NENU) under thesupervision of Prof. Jian-FangMa. Then she carried out post-doctoral studies with Prof.Zhong-Min Su in EnvironmentalChemistry at NENU. In 2012,she joined Prof. Qiang Xu’sgroup at National Institute of

Advanced Industrial Science and Technology (AIST, Japan) as aJSPS (Japan Society for the Promotion of Science) invitation fellow.She is an associate professor of Inorganic Chemistry at NanjingNormal University (NJNU, China). Her current research interestlies in the development of the syntheses, structures and propertiesof MOFs.


characteristics, MOFs have been proven to be excellent candi-dates in the eld of hydrogen storage.

2.1 MOFs as absorbents for hydrogen

The most direct method of hydrogen storage is to use hydrogensorption materials. Since the rst investigation on MOF (4.5wt% at 77 K and 1 atm for MOF-5, Zn4O(BDC)3, H2BDC ¼ 1,4-benzenedicarboxylic acid) for hydrogen storage was reported byYaghi and co-workers in 2003,68 MOFs have attracted worldwideattention in the area of hydrogen storage. In just a few years,there have been numerous articles published on hydrogensorption in MOFs, which evaluate MOFs as physi-sorbents forhydrogen storage applications and show superior performancescompared to other porous materials. It is an important foun-dation for the development of hydrogen storage from experi-mental and theoretical research. Lots of important researchmethods and results have been reviewed by some independentgroups,69–77 and therefore we shall not describe here in detail.Some factors, which inuence hydrogen sorption in MOFs, havebeen summarized, such as exposed metal sites, surface areasand pore volumes, topological structures (interpenetration ornot), properties of linkers, chemical doping for MOFs, spillover,weak interaction between hydrogen and frameworks and so on.Constantly updated test and calculation methods, adsorptionmechanisms and so on, and the corresponding strategies havealso been suggested, which promote the fast development ofthis eld. Up to now, the highest excess H2 storage capacityreported is 99.5 mg g�1 at 56 bar and 77 K in NU-100 (NU ¼Northwestern University, Cu3(TCEPEB)(H2O)3, TCEPEB ¼ 1,3,5-tris[(1,3-carboxylic acid-5-(4-(ethynyl)phenyl))ethynyl]-benzene)with a high Brunauer–Emmett–Teller (BET) surface area of 6143m2 g�1, which has a total capacity of 164 mg g�1 at 77 K and 70bar.78 The highest total H2 storage capacity reported is 176 mgg�1 (excess 86 mg g�1) in MOF-210 (Zn4O(bte)4/3(bpdc), H3bte ¼4,40,40 0-[benzene-1,3,5-triyl-tris(ethyne-2,1-diyl)]tribenzoic acid

Qiang Xu received his PhDdegree in Physical Chemistry in1994 at Osaka University,Japan. Aer one year working asa postdoctoral fellow at OsakaUniversity, he started his careeras a Research Scientist in OsakaNational Research Institute in1995. Currently, he is a ChiefSenior Researcher at NationalInstitute of Advanced IndustrialScience and Technology (AIST,Japan) and adjunct professor at

Kobe University. He received the Thomson Reuters Research FrontAward in 2012. His research interests include porous and nano-structured materials and related functional applications, espe-cially for clean energy. He has published more than 250 papers inrefereed journals.

This journal is ª The Royal Society of Chemistry 2013


Review Energy & Environmental Science

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and H2bpdc ¼ biphenyldicarboxylic acid, and BET surface areaof 6240 m2 g�1) at 77 K and 80 bar.79 Based on the weak inter-molecular force (van der Waals), hydrogen storage in MOFs byphysisorption is necessary to apply low temperature to achieve asufficient adsorbed amount, and the release of hydrogenmolecules is fast. A huge challenge is how to reach the DOEtargets by use of MOFs at convenient temperatures, whichinspire researchers to nd effective strategies for hydrogenstorage. A possible solutionmight be to optimize various factorsthat affect hydrogen storage in MOFs (categories of metals,characteristics of linkers, large enough surface area and porevolume, modication of MOFs and so on) and combine them.

2.2 Nanoconnement of chemical hydrides in MOFs forchemical hydrogen storage

Chemical hydrogen storage, which involves the storing ofhydrogen in the form of chemical bonds, is one of the safe andefficient alternatives to physical hydrogen storage. Over the pastdecade, solid-state hydrogen storage materials have receivedconsiderable attention as promising chemical hydrogen storagematerials due to their attractive features, such as high hydrogendensity, relative stability, safe storability and so on.80–82 Smallinorganic molecules, such as ammonia borane (NH3BH3, AB)with a hydrogen capacity of 19.6 wt%, exceeding that of gaso-line,83–85 anhydrous hydrazine (H2NNH2) with that of 12.5wt%86,87 and so on, are attractive candidates for chemicalhydrogen-storage applications.88–95 Metal borohydrides M(BH4)nare promising hydrogen storage materials, with theoreticalhydrogen capacities of 18.3 wt%, and 121 kg m�3 for lithiumborohydride (LiBH4).96 Lightweight metal hydrides (MgH2, NaHand NaAlH4) are also considered as hydrogen storage media.97,98

Nanoconnement is of increasing interest and may lead tosignicantly enhanced kinetics, higher degree of stability and/or more favourable thermodynamic properties,92 which areemployed in different types of hydrogen storage materials.99–102

MOFs have a cavity size that varies from microporous (smallerthan 2 nm) to mesoporous (between 2 and 50 nm), which isregarded as bridging the gap between zeolites and mesoporoussilica, and are very suitable as host matrices to support guest

Table 1 Nanoconfinement of chemical hydrides by MOFs

MOF Conned hydridea Ref.

JUC-32-Y AB 108Mg-MOF-74 AB 115Ti@Mg-MOF-74 NaAlH4 123Zn-MOF-74 AB 117MIL-101 AB 118Ni@MIL-101 AB 118Pt@MIL-101 AB 119ZIF-8 DMAB 120

AB 140HKUST-1 NaAlH4 121

LiBH4 124

a AB: ammonia borane (NH3BH3); DMAB dimethylamine borane(H3B$NMe2H).


molecules.103–106 MOFs offer an attractive alternative to tradi-tional scaffolds because their nanoscale pores and orderedcrystalline lattice provides a highly controlled and inherentlyunderstandable environment. Chemical hydrides have beenincorporated into MOFs through liquid impregnation andvapor phase inltration methods to form chemical hydride@-MOF composites, in which the appropriate dimensional poresin MOFs not only accommodate the chemical hydrides but also,more importantly, nanoconne the chemical hydrides in them(Table 1).

JUC-32-Y,107 (Y(BTC)(H2O)$DMF (H3BTC ¼ 1,3,5-benzene-tricarbocylic acid, DMF ¼ N,N0-dimethylformamide)) is used forthe rst successful synthesis of a MOF-conned AB (ammoniaborane) system (denoted as AB@JUC-32-Y) with approximately8 wt% AB introduced by the infusion method in anhydrousmethanol solvent.108 In this case, MOF as the host material canimprove the thermal hydrogen release of AB. The coordinatelyunsaturated metal Y3+ sites of JUC-32-Y interacted with AB tocompletely prevent the formation of ammonia. Neat AB has atwo-step decomposition near its melting point (112 �C) and�150 �C.109 For AB@JUC-32-Y, as shown in Fig. 1, AB couldrelease 8.0 wt% hydrogen within 10 min at even a low temper-ature of 85 �C, where the decomposition temperature of ABdecreases �30 �C. More importantly, no volatile products (e.g.,borazine, ammonia, and diborane) were found during thedecomposition process. In this case, MOF acts as both the hostand catalyst, and hydrogen generation is promoted by a syner-gistic effect of nanoconnement of the MOF pores and catalysisby the metal centers.

Additionally, some classic MOFs (MIL-101,110 MIL: Mate-rials of Institute Lavoisier, M3F(H2O)2O(BDC)3$nH2O, M ¼ Cror Fe; Mg-MOF-74,111,112 Mg2(DOBDC), DOBDC ¼ 2,5-dioxido-1,4-benzenedicarboxylate; Zn-MOF-74,113 Zn2(DOBDC) andzeolitic-imidazolate-framework ZIF-8,114 [Zn(MeIm)2], MeIm ¼2-methylimidazolate) have been employed for nanoconne-ment of AB and its derivatives.115–119

Fig. 1 Time dependences of hydrogen release from AB@JUC-32-Y and neat ABat different temperatures. Reprinted with permission from ref. 108. Copyright2010, American Chemical Society.



Fig. 2 (a) Amount and (b) rate of H2 released from NaAlH4@HKUST-1 comparedwith neat NaAlH4. Reprinted with permission from ref. 121. Copyright 2009,American Chemical Society.


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With the specic one-dimensional hexagonal pore structure,Mg-MOF-74 facilitates AB loading in methanol with a large massfraction up to approximately 26 wt%, which corresponds to anAB/Mg molar ratio of 1 : 1.115 Different amounts of AB can beconned in the pores of MOFs. The H2 release temperature andkinetics are dependent on the amount of AB encapsulated.AB@Mg-MOF-74 can release 10–12 wt% of clean H2 in 20 minslightly above 100 �C, and 9 wt% within 25, 90 and 420 min at 95,85 and 75 �C, respectively. In addition, the authors also demon-strated that Ni-MOF-74 and HKUST-1116 (Cu3(TMA)2(H2O)3,TMA ¼ benzene-1,3,5-tricarboxylate) are not suitable for thenanoconnement of AB molecules, showing that metal centersplay a signicant role in this case. Recently, Srinivas et al.employed Zn-MOF-74 to conne AB molecules with AB/Zn ratiosof 1 : 1 and 2 : 1 as 1AB@Zn-MOF-74 and 2AB@Zn-MOF-74,respectively.117 During the hydrogen generation by pyrolysis, noammonia, borazine and diborane were observed for 1AB@Zn-MOF-74, while 2AB@Zn-MOF-74 shows trace amounts ofbyproducts. Thus, the metal center is important for nano-connement of AB molecules based on the above two cases.

In 2011, Si et al. synthesized a series of nanocomposites ofMIL-101 and Ni@MIL-101 (prepared by immersing MIL-101 in aNiCl2 solution) with different amounts of AB.118 For pyrolysis ofAB, MIL-101 and Ni@MIL-101 doped with AB samples start toevolve H2 at about 50 �C and give broad desorption peakscentered at 85 and 75 �C, respectively, which are lower thanother AB nanocomposite systems. More importantly, it (75 �C) islower than the proton exchange membrane (PEM) fuel cellworking temperature (80 �C). Thus, MOFs are effective hostmaterials to improve the thermal decomposition of AB and theintroduction of a catalyst into the nanocomposites is a feasiblemethod for improving both thermodynamics and kinetics.

When conning both a chemical hydride and metal nano-particle catalyst in the MOF pores, metal nanoparticles mayprovide an additional catalytic effect to the nanoconnement.Recently, we reported the pyrolysis of AB from the AB/Pt@MIL-101 composite, which showed the rst and second H2 evolutionpeaks at about 88 and 130 �C, respectively, and �30 �C lowerthan those for the pristine AB. The effective depression ofvolatile byproducts and the absence of material foamingobserved for AB/Pt@MIL-101 clearly demonstrate the synergeticeffect of the Pt NP catalysis and nanoconnement of the MIL-101 framework during the pyrolysis process.119

Fischer and co-workers employed a vapor phase inltrationmethod to conne dimethylamine borane (DMAB ¼H3B$NMe2H) in the pores of ZIF-8.120 Interestingly, DMAB@ZIF-8 can change to (H2B$NMe2)2@ZIF-8 through dehydrocouplingreaction at room temperature.

In 2009, Allendorf and co-workers loaded NaAlH4 to HKUST-1 in THF, and on average there are eight THF molecules andeight formula units of NaAlH4 per large pore.121 For the pyrolysisof nanoscale hydrides in MOF, it can start desorbing H2 at 70 �Cand desorb�80% of the total H2 at 155 �C, during whichMOF isnot decomposed (Fig. 2). However, bulk NaAlH4 shows the onsetdehydrogen temperature of 150 �C and 70% of the total H2 isdesorbed at 250 �C. Then, they prepared the NaAlH4@HKUST-1with NaAlH4 cluster size of 1 nm, and quantitatively


investigated the thermodynamics and kinetics of H2 desorp-tion. The results show that the size of NaAlH4 clusters exerts agreater inuence on the thermodynamics and reaction ratesthan other factors, such as interactions between NaAlH4 andpore walls and so on.122 When conning both hydrogen storagematerials and catalysts in pores of MOFs, the sample of Ti-doped nano-NaAlH4@MOF-74 exhibits an onset temperaturefor hydrogen desorption of�50 �C and releases 3.6 wt% in 2.5 hat 150 �C, which is similar to that of undoped nano-NaAlH4@MOF-74 (4.5 wt%) at 200 �C. Although the presence of titaniumis not necessary for this increase in desorption kinetics, itenables rehydriding to be fully reversible, where it displaysminimal capacity loss (from 4.1 to 3.6 wt% for hydrogengeneration) in four dehydrogenation/rehydrogenation cyclesunder H2 pressure.123

In another report, Yu, Guo and co-workers also used dehy-drated HKUST-1 as host for loading LiBH4 in ether solution.124

The dehydrogenation of LiBH4@HKUST-1 started from around60 �C, which is lower than that for the pristine LiBH4 (380 �C).Aer heating up to 200 �C, a total gas release of 4.8 mmol g�1

was observed for the LiBH4@HKUST-1 sample, indicating apartial decomposition of loaded LiBH4 below this temperature(7 mmol g�1 for a complete decomposition of the connedLiBH4 to H2). However, boric acid, the oxidative product, isproduced during the process of LiBH4 decomposition. Inter-estingly, when loading LiBH4 into hydrated HKUST-1, thecoordinated water molecules in HKUST-1 can react with LiBH4

to release H2 during the loading process at room temperature.Nanoconnement of chemical hydrides and their interaction

with the active metal centers in MOFs are important. Comparedwith neat chemical hydrides, nanoconned hydride@MOF



Scheme 2 Representation of preparation of Pt@MIL-101 by double solventsmethod.


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materials show advantages for the improvement of hydrogenrelease thermodynamics and kinetics, lower operationaltemperature, and high purity of released hydrogen duringpyrolysis. Up to now, the corresponding reports about hydri-de@MOFs are still scarce, which may be due to requirementsfor MOFs. For solid-state chemical hydride@MOF systems, theMOFs should have appropriate pore sizes, active metal sites,and especially thermal and chemical stability, which is the mostimportant for application of MOFs in this eld.

Fig. 3 (a and b) HAADF-STEM, (c) TEM images and (d) reconstructed slice bytomography of 2 wt% Pt@MIL-101. (e) Hydrogen generation from aqueousNH3BH3 in the presence of Pt@MIL-101 catalysts at room temperature. Reprintedwith permission from ref. 119. Copyright 2012, American Chemical Society.

2.3 MOFs as catalysts, catalyst supports or precursors ofcatalysts for hydrogen generation from chemical hydrides

MOFs may prove to be very useful in catalysis, which can bedivided into three aspects: (i) MOFs act as host materials forsupporting catalysts;125–127 (ii) the framework itself acts as acatalyst, where its activity may be frommetal centers, organic orpseudo-organic linkers and other functional groups from post-synthetic modication; and (iii) MOFs act as precursors ofcatalysts by framework decomposition.128–130

Chemical hydrides can release H2 rapidly in the presence ofmetal nanoparticle (NP) catalysts immobilized to MOFs.131–133 In2011, we reported bimetallic Au–Pd NPs immobilized in meso-porous MIL-101 as efficient catalysts for the decomposition offormic acid for hydrogen generation.134MIL-101 was chosen as asupport because of its large pore sizes (2.9–3.4 nm), windowsizes (1.2–1.4 nm) and its hybrid pore surface, which facilitatesthe encapsulation of metal NPs and the adsorption of thesubstrate formic acid inside the pores. We graed the electron-rich functional group ethylenediamine (ED) into MIL-101 (ED-MIL-101) for improving the interactions between the metalprecursors and the MIL-101 support. By using a simple liquidimpregnation method, the resulting bimetallic Au–Pd NPsimmobilized in the MIL-101 and ED-MIL-101 (Au–Pd/MIL-101and Au–Pd/ED-MIL-101) represent the rst highly active MOF-immobilized metal catalysts for the complete conversion offormic acid to hydrogen at a convenient temperature.

Then, our group reported highly dispersed Ni NPs (�2.7 nm)immobilized by the framework of ZIF-8 by chemical vapordeposition (CVD) and chemical liquid deposition (CLD)approaches, followed by hydrogen reduction, to obtain CVD-Ni@ZIF-8 and CLD-Ni@ZIF-8, respectively.135 For CLD-Ni@ZIF-8, the hydrolysis of the AB reaction can be completed with therelease of hydrogen of H2/AB ¼ 3.0 in 19 min (Ni/AB ¼ 0.019),corresponding to a TOF value of 8.4 min�1. The sampleprepared by using CVD approach showed a higher activity, forwhich the reaction can be completed (H2/AB ¼ 3.0) in 13 min(Ni/AB ¼ 0.016), giving a TOF value of 14.2 min�1.

In order to avoid NP aggregation on the external surface ofthe MOF, we recently reported the use of a “double solvents”method, which is based on a hydrophilic solvent (water) and ahydrophobic solvent (hexane), the former containing the metalprecursor with a volume set equal to or less than the porevolume of the adsorbent (MIL-101), which can be absorbedwithin the hydrophilic adsorbent pores, and the latter, in a largeamount, playing an important role to suspend the adsorbentand facilitate the impregnation process (Scheme 2).119 Based on


this efficient method, the Pt precursor was introduced into theMIL-101 pores, followed by hydrogen reduction. TEM andelectron tomographic measurements clearly demonstrated theuniform three dimensional distribution of the ultrane Pt NPsthroughout the interior cavities of MIL-101 (Fig. 3). For the H2

generation by hydrolysis of aqueous AB (ammonia borane) atroom temperature in the presence of 2 wt% Pt@MIL-101 cata-lysts (Pt/AB ¼ 0.0029 in molar ratio), the reaction is completedwithin 2.5 min, corresponding to a catalytic activity 2 timeshigher than that of 2 wt% Pt/g-Al2O3, the most active Pt catalystfor this reaction reported so far. Pt@MIL-101 retains highcatalytic activity aer ve runs.

The wide range of adjustable metal centers and theirsurrounding environments as well as a variety of organic orpseudo-organic linkers provide multiple opportunities to createvarious MOFs with different catalytic properties. A considerablenumber of catalytic reactions over MOFs have been repor-ted,136,137 and here we only give some selected recent progress onthe framework itself as a catalyst in hydrogen generation.

Li and Kim used ZIF-9 (ref. 138) (Co(PhIm)2$(DMF)$(H2O),PhIm ¼ benzimidazolate) as a catalyst in NaBH4 hydrolysis forhydrogen generation for the rst time.139 The initial hydrogengeneration rate for the ZIF-9 catalyst is relatively slow due to thegradual formation of CoB active centers, and then it increasesrapidly aer the formation of CoB. The hydrogen generationrate at 40 �C can reach 3641.69 mL min�1 g�1 (Co). Aer vecycles, ZIF-9 maintains its basic structure and crystallinity, butthe long range order of ZIF-9 was destroyed to a certain extentduring the operation. In this case, metal centers (Co) form CoBactive centers rstly, which can promote the NaBH4 hydrolysis.Thus, ZIF-9 plays two roles: a catalyst and a precursor forpreparing the catalyst.



Scheme 3 The reaction scheme of photochemical hydrogen production fromwater using Ru-MOFs as HPC in the presence of Ru(bpy)3

2+, MV2+, and EDTA-2Na.


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Very recently, ZIF-8 was employed as a catalyst for thepyrolysis of AB through solid-state mixing instead of conne-ment.140 AB/ZIF-8 shows a lower dehydrogenation temperaturethan that of neat AB, even if the content of AB increases to 90wt%. Compared with the results of a nanoconned sample ofAB@ZIF-8-MeOH (immersing the AB in ZIF-8), it demonstratesthat ZIF-8 acts as a catalyst. PXRD patterns of AB/ZIF-8 aerdehydrogenation show no major changes in the ZIF-8 frame-work, indicating that ZIF-8 is stable during dehydrogenationand potentially reusable as a catalyst.

By efficient and mild in situ reduction using AB as a reducingagent141 in methanol, Li and co-workers prepared Ni NPs using aNi-based MOF ([Ni(4,40-bipy)(HBTC)], 4,40-bipy ¼ 4,40-bipyr-idine) as a precursor.128 With the high reaction rate and lowactivation energy, the catalyst showed a high catalytic activity,with which the H2 release was completed within 5 min fromaqueous AB solution (0.32 M) at room temperature. Further-more, the catalyst can be recycled by centrifugal separation andreused for up to 20 cycles without obvious loss of activity. Then,by the in situ reduction with AB in methanol (MOF/AB ¼ 0.02),two catalysts were obtained from MOF precursors [Ni(pyz)]-[Ni(CN)4] (pyz ¼ pyrazine) and Ni3[Fe(CN)6]2.129 With three Ni0-based catalysts (1.0 mol%) from the precursors of the above-mentioned MOFs, the thermal decomposition of AB shows asignicantly lower reaction onset temperature, a decreasedactivation energy and remarkably accelerated kinetics. At 80 �C,AB with MOF-based catalyst (prepared from [Ni(4,40-bipy)(HBTC)]) released 7.5 wt% H2 in 2 h without any inductionperiod. At 90 �C, 6.0 wt% H2 can be released from AB with thecatalyst prepared from [Ni(pyz)][Ni(CN)4] within only 20 min.Recently, this group also prepared a highly active Co(0) catalystby the same in situ reduction reaction using reducing agentNaBH4 by introducing the aqueous solution of AB from MOFprecursor Co2(BDC)2(dabco) (dabco ¼ 1,4-diazabicyclo[2.2.2]-octane).130 It is interesting that the active Co(0) sites, generatedby reduction in the micropores and channels, are surroundedby the organic linkers and stabilized in the residue of theframework. The hydrogen generation from aqueous AB solution(0.32 M) was completed within 1.4 min (MOF/NaBH4/AB ¼0.057 : 0.08 : 1) at room temperature.

2.4 MOFs for photocatalytic hydrogen production

The photochemical reduction of water into hydrogen moleculesusing a photocatalyst is not dependent on fossil fuels, and istherefore an ideal method for producing clean energy. Besidebeing active photocatalysts for the degradation of organicmolecules and dye molecules,142 MOFs are also useful photo-catalysts for the reduction of water into hydrogen molecules.143

In 2009, Mori and co-workers employed Ru-MOFs ([Ru2(p-BDC)2]n, Ru2(CH3COO)4BF4 and [Ru2

II,III(p-BDC)2BF4]n) ashydrogen production catalysts (HPCs), which showed catalyticactivity for the photochemical reduction of water into hydrogenmolecules in the presence of light harvesting systems (LHSs)with photosensitizer (PS) Ru(bpy)3

2+ (bpy ¼ 2,20-bipyridine),electron relay (ER)MV2+ (methyl viologen) and sacricial donorsEDTA-2Na (EDTA ¼ ethylenediaminetetraacetic acid) under


visible light irradiation (Scheme 3).144 The initial slopes of H2

evolution for the three photocatalysts (0.01 mmol) weremeasured to be 16.1 mmol h�1 for [Ru2(p-BDC)2]n, 1.58 mmol h�1

for Ru2(CH3COO)4BF4, and 7.33 mmol h�1 for [Ru2II,III(p-

BDC)2BF4]n. Aer 4 h irradiation, 41 mmol H2 was produced byusing [Ru2(p-BDC)2]n with the turnover number (TON) of 8.16,and the apparent quantum yield is 4.82% at 450 nm.Comparing the three catalysts, the evolution rate of H2

increased as follows: non-porous Ru2(CH3COO)4BF4 < porous[Ru2

II,III(p-BDC)2BF4]n < porous [Ru2(p-BDC)2]n, indicating that aporous MOF is favorable for improving the activity of thephotochemical water reduction.

Following this research result, another investigation of Ru-MOFs ([Ru2(p-BDC)2X]n, X ¼ Cl, Br and BF4) by modicationwith different counter-ions as catalysts showed that the order ofobserved catalytic activity was as follows: Ru–Br > Ru–BF4 > Ru–Cl.145 The initial slopes of H2 evolution for Ru–Cl, Ru–Br andRu–BF4 (0.005 mmol) were measured to be 5.55, 15.2 and7.33 mmol, respectively. Prolonged irradiation led to an increasein H2 production. The most active catalyst [Ru2(p-BDC)2Br]n,showed the highest catalytic activity with 46.7 mmol H2 evolu-tion and a TON of 18.7 aer 4 h of irradiation. Aer the catalyticreaction, the Cl� and BF4

� counter-ions disappeared and weresubstituted with EDTA, and only the Ru–Br catalyst maintainedthe coordination geometry between Ru and Br� counter-ions,implying that the catalytic activity of MOF with counter-ionsaltered noticeably when the counter-ions were manipulated bymolecular catalysts, and the design of MOF catalysts requiredan adequate examination of counter-ions for the purpose ofreaction.

In 2010, two Rh-MOFs ([Rh2(p-BDC)2]n and [Rh2(C6H5COO)4])were used as hydrogen production catalysts for photochemicalreduction of water under visible light irradiation in the presenceof the above-mentioned multicomponent system containingRu(bpy)3

2+, MV2+, and EDTA-2Na.146 Aer 10 h irradiation, pro-longed irradiation led to increased H2 production; the totalamount of hydrogen production reached 62.2 and 22.1 mmolbased on [Rh2(p-BDC)2]n and [Rh2(C6H5COO)4] (0.005 mmol),respectively, which indicated that MOFs involving Rh2 active sitesobviously have an advantage.

An interesting three-dimensional interpenetrated bimetallicMOF [Zn{Pd(INA)4}]n (INA ¼ isonicotinate) was also used as aHPC in the same LHS. The initial slope of the H2 evolution forthe HPC (0.005 mmol) was determined to be 37.3 mmol h�1 with



Fig. 4 The photocatalytic reactions involving Al/Zn-PMOF, Pt and EDTA (i) withand (ii) without MV. Reprinted with permission from ref. 150. Copyright 2012,Wiley-VCH.

Fig. 5 (a) Scheme showing the synergistic photocatalytic hydrogen evolutionprocess via photoinjection of electrons from the light harvesting MOF frameworksinto the Pt NPs. (b) Diffuse reflectance spectra of four samples, and a photographof suspensions of these samples (in the inset). (c) Relationship between the ratioof K2PtCl4/Ir added in the reaction solution and the ratio of Pt/Ir deposited insidethe MOF. (d) Time-dependent hydrogen evolution curves of four samples.(1: Zr6O4(OH)4(bpdc)5.94(L1)0.06 and 2: Zr6O4(OH)4(L2)6$64DMF). Reprinted withpermission from ref. 151. Copyright 2012, American Chemical Society.


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a TOF of 14.9 h�1, and 70.6 mmol H2 gas was observed aer 4 hirradiation. It exhibits a higher catalytic activity than that of thereference sample [PdII(H-INA)2(INA)2].147 Recently, Miyazakiet al. used another bimetallic MOF [Zn2(H2O)3{PdCl2(pydc)2}]n(H2pydc ¼ 3,5-pyridine-dicarboxylic acid) based on the buildingunit of trans-[PdCl2(H2pydc)2], and 50.6 mmol H2 gas wasobserved aer 4 h irradiation based on this catalyst (0.005mmol). Its catalytic activity is higher than that of the corre-sponding building unit.148

Llabres i Xamena and co-workers applied two notably water-stable Zr-containing MOFs (UiO-66: [Zr6O4(OH)4(BDC)12] andUiO-66(NH2): [Zr6O4(OH)4(ATA)12]; ATA ¼ 2-amino-terephthalate) as photocatalysts for H2 generation in methanolor water/methanol upon irradiation.149 The apparent quantumyield for H2 generation using monochromatic light at 370 nm inwater–methanol (3 : 1) was 3.5% for UiO-66(NH2). When usingthe mixtures of platinum nanoparticles and Zr-containingMOFs as catalysts, maximum amounts of 2.4 and 2.8 mL of H2

generation were obtained aer 3 h of irradiation over UiO-66and UiO-66 (NH2) (45 mg), respectively. Although the apparentquantum yield is still low in this case, the nding triggers a newresearch line aimed at exploiting MOFs as semiconductors,particularly for water splitting.

In 2012, two non-interpenetrated water-stable porphyrin-based MOFs, Al-PMOF (H2TCPP[AlOH]2(DMF3(H2O)2,H2TCPP ¼ meso-tetra(4-carboxyl-phenyl) porphyrin)) and Al/Zn-PMOF (Zn0.986(12)TCPP[AlOH]2), from post-synthetic chemicalmodication by porphyrin metalation of Al-PMOF were used forvisible-light driven hydrogen generation from water.150 In twodifferent hydrogen generation systems, MOF/MV2+/EDTA/Ptand MOF/EDTA/Pt (Fig. 4), H2 production at rates of 100 and200 mmol g�1 h�1 for Al/Zn-PMOF and Al-PMOF (3.5 mg) wasobserved aer an induction period of about 3 h, respectively.Repeated reactions with the same catalysts showed goodreproducibility.

Another important investigation was made by Lin’sgroup. Two stable, porous and phosphorescent MOFs(Zr6O4(OH)4(bpdc)5.94(L1)0.06 and Zr6O4(OH)4(L2)6$64DMF)(H2bpdc ¼ biphenyldicarboxylic acid, L1 ¼ IrIII(ppy)2(dcbpy),L2 ¼ IrIII(ppy)2(dbzpy), ppy ¼ 2-phenylpyridine, dcbpy ¼ (2,20-


bipyridine)-5,50-dicarboxylate, dbzpy ¼ (2,20-bipyridine)-5,50-dibenzoate) were built from two [Ir(ppy)2(bpy)]

+-derived (bpy ¼2,20-bipyridine) dicarboxylate ligands with short dcbpy and longdbzpy, and Zr6(m3-O)4(m3-OH)4(carboxylate)12 secondarybuilding units.151 Then, Pt nanoparticles of 2–3 and 5–6 nmdiameter, respectively, were loaded into the two MOFs viaMOF-mediated photoreduction of K2PtCl4 in a mixed solvent oftetrahydrofuran/triethylamine/H2O (20 : 1 : 1 v/v/v) by ultravi-olet light. The resulting Pt@MOF assemblies serve as effectivephotocatalysts for hydrogen evolution by synergistic photoex-citation of the frameworks and electron injection into the Ptnanoparticles (Fig. 5). Comparing the different ratios ofK2PtCl4/Ir added in the reaction solution, the optimized Pt/Irratios in the MOF sample were 18.6 and 17.8 for the twoPt@MOFs, respectively (Fig. 5c). The two Pt@MOFs (based on0.04–0.05 mmol of the Ir-MOF) gave the Ir-TONs (Ir-TON ¼n(1/2H2)/n(Ir)) of 730 and 1621 for the rst cycle aer 6 h using a450 W Xe-lamp with a 420 nm cutoff lter. Pt@MOF by thedbzpy ligand gave an Ir-TON of 7000 (Fig. 5d), approximatelyve times the value afforded by the homogeneous control[Ir(ppy)2(bpy)]Cl/K2PtCl4 under their respective conditions, andcould be readily recycled and reused. MOFs thus provide aversatile and tunable platform to hierarchically integratedifferent functional components for solar energy utilization.

Matsuoka and co-workers prepared a Ti-MOF-NH2 based on2-aminobenzenedicarboxylic acid,152 which showed the iso-structure with MIL-125.153 Then, Pt nanoparticles as co-catalystswere deposited onto Ti-MOF-NH2 via a photodeposition process(Pt/Ti-MOF-NH2). When using 0.01 M triethanolamine as asacricial electron donor, the total evolution of hydrogen aer



Scheme 4 MOF-templated synthesis of Fe2O3@TiO2 by coating MIL-101(Fe)with TiO2 followed by calcination, and its use for photocatalytic hydrogenproduction after depositing Pt particles. Reprinted with permission from ref. 154.Copyright 2012, Wiley-VCH.

Fig. 6 Representation of a hydrogen fuel cell. Reprinted with permission fromref. 160. Copyright 2011, American Chemical Society.


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9 h visible-light irradiation (l > 420 nm) reached 33 mmol basedon the catalyst of Pt/Ti-MOF-NH2 (10 mg) at room temperature.

Recently, Lin and co-workers developed a novel MOF-tem-plated approach to synthesize a mixed metal oxide nano-composite material by rst creating core–shell particles andcalcining to decompose the core.154 As shown in Scheme 4, thisfacile method produces crystalline octahedral nanoshellscomposed of hematite Fe2O3 nanoparticles embedded inanatase TiO2 with some Fe doping. With triethylamine (TEA) asa sacricial reducing agent, the photocatalytic activity of theFe2O3@TiO2 material (0.5 mg: 5.0 mmol Fe and 1.25 mmol Ti) incombination with K2PtCl4 (3.70 mmol) for photoinducedhydrogen production from water show that the amount of H2

produced increases linearly during this entire time period, witha total of 30.0 mmol H2 per mg of material produced aer 48 h.As a heterogeneous catalyst, Fe2O3@TiO2 can be recovered froma reaction mixture and reused with a fresh solution. Thiscomposite material has interesting photophysical properties asit enables photocatalytic hydrogen production from water usingvisible light, which showsmore excellent properties than that ineach component. In this case, MOF plays two roles. One is atemplate and the another is a precursor for Fe2O3. Combiningmultifunctional properties of MOFs in one reaction is thus anefficient way to kill two birds with one stone.

Polyoxometalates (POMs),155,156 a large family of solubleanionic metal oxide clusters of d-block transitionmetals in highoxidation states, constitute ideal building blocks as alternaivesto metal ions for targeting new multifunctional POM-basedMOF (POMOF) materials due to their redox and catalytic prop-erties. One example is to use POMs instead of simple metalcenters to obtain a POM-based MOF [H[{Ce(H2O)5}2-{Ce(pdc)2(H2O)4}{Ce(pdc)3}(PW12O40)]$2H2O (pdc ¼ pyridine-2,6-dicarboxylate)], which is constructed from 2D cationic[{Ce(H2O)5}2{Ce(pdc)2(H2O)4}{Ce(pdc)3}]

2+ layers pillared bya-Keggin [PW12O40]

3� anions. The POM-based MOF, with a


loading of 1.2 wt% Pt by in situ photoreduction using H2PtCl6solution, shows photocatalytic activity for hydrogen evolutionwith an average rate of 3.4 mmol h�1 from 100 mL of 20%methanol solution with 0.1 g catalyst.157 Recently, Wang and co-workers employed an ionothermal method in the synthesisof POM-based porous material to obtain a 3D (TBA)2-[CuII(BBTZ)2(a-Mo8O26) (TBA ¼ tetrabutylammonium cation,BBTZ ¼ 1,4-bis(1,2,4-triazol-1-ylmethyl)-benzene),158 whichshows photocatalytic activity for hydrogen evolution with anaverage rate of 0.78 mmol h�1 under UV radiations in 100 mL20% methanol solution by using 0.1 g POM-MOF as a catalystand Pt NPs as co-catalyst.

2.5 Conclusions

MOFs are emerging as one of the most promising platforms forhydrogen storage and generation due to their adjustable poresize, and wide variety of selective metal centers and changeablelinkers. However, there are still challenges in these elds: (i)how to improve the hydrogen adsorption at convenienttemperatures; (ii) how to synthesize more MOFs with highthermal stability to nanoconne chemical hydrides for pyrolysisto generate hydrogen; (iii) how to increase catalytic activity ofMOFs for hydrogen production; and (iv) how to develop othermethods for hydrogen production, such as an interesting elec-trocatalytic hydrogen evolution reaction reported recently withelectrodes fabricated using 3D POMOFs159 ((TBA)3[PMo8

VMo4-VIO36(OH)4Zn4][C6H3(COO)3]4/3$6H2O ¼ 3(trim4/3)), whichshowed an activity higher than platinum. While there are somehurdles to overcome, more and more investigations show theadvantages of MOFs in this eld.

3 MOFs for fuel cells

A fuel cell is an electrochemical conversion device that has acontinuous supply of fuel such as hydrogen, natural gas, ormethanol and an oxidant such as oxygen, air, or hydrogenperoxide.160 Each fuel cell consists of an anode (negative side), acathode (positive side) and an electrolyte that allows charges tomove between the two sides of the fuel cell (Fig. 6),161,162 whichoffers a highly efficient way to use diverse energy sources and, asa result, has demonstrated lower energy use and emissionswhen compared with conventional technologies. The key




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technical objectives of the DOE are to develop a fuel cell systemfor portable power (<250 W) with an energy density of 900 W hL�1 by 2015, and to achieve a direct hydrogen fuel cell powersystem for transportation with a peak-efficiency of 60%, 5000 hdurability and mass-produced cost of $30 kW�1 by 2017.163

There are remaining challenges aimed at achieving high effi-ciency and durability along with low material andmanufacturing costs, which encourage improving the capacitiesof hydrogen storage and generation, electrode catalysis, protonconductivity of polymer electrolyte membrane, and so on. Withuncoordinated metal sites, larger surface areas and higher porevolumes, MOFs exhibit various properties of redox, protonconduction, catalysis and so on. More importantly, MOFs canbe prepared with high yields and from relatively cheap

Table 2 Proton conductivities of MOFs and MOF-based composites

MOF-based sample

(HOC2H4)2dtoaCu (dtoa ¼ dithiooxamide)H2dtoaCu(HOC3H6)2dtoaCu(H5C2)2dtoaCu(NH4)2(adp)[Zn2(ox)3]$3H2O{NH(prol)3}[MnIICrIII(ox)3](NH4)4[MnCr2(ox)6]3$4H2O{NMe3(CH2COOH)}[FeCr(ox)3]$nH2O{NEt3(CH2COOH)}[MaMb(ox)3] (MaMb ¼ MnCr, FeCr orFeFe){NBu3(CH2COOH)}[MaMb(ox)3] (MaMb ¼ MnCr, FeCr orFeFe)[Zn(H2PO4)2(Tz)2]n (Tz ¼ triazole)[Ca(D-Hpmpc)(H2O)2]$2HO0.5}n composite membrane (D-H3pmpc ¼ D-1-(phosphonomethyl)piperidine-3-carboxylicacid)1,2,4-Triazole@PCMOF2Zn3(L)(H2O)2$2H2O (L ¼ [1,3,5-benzenetriphosphonate]6�)PCMOF21/2[La(H5L)(H2O)4] (L ¼ 1,2,4,5-tetrakisphosphonomethylbenzene)La(H5DTMP)$7H2O (H8DTMP ¼ hexamethylenediamine-N,N,N0,N0-tetrakis(methylenephosphonic acid)MgH6ODTMP$6H2O (H6ODTMP ¼octamethylenediamine-N,N,N0,N0-tetrakis(methylenephosphonic acid)Zr(O3PCH2)2N–C6H10–N(O3CH2P)2H4$4.5H2OGd3(H0.75O3PCHOHCOO)4$xH2OIm@[Al(OH)(1,4-NDC)]n (Im ¼ imidazole, 1,4-NDC ¼ 1,4-naphthalenedicarboxylate)Histamine@[Al(m2-OH)(1,4-NDC)]nM(OH)(BDC-R)(H2O) (M ¼ Al or Fe, R ¼ –NH2, –H, –OH,and –COOH)H2O-HKUST-1CsHSO4@Cr-MIL-101H2SO4@MIL-101H3PO4@MIL-101Histamine@Zn-MOF-74Sulphated MIL-53(Al)[Zn(l-LCl)(Cl)](H2O)2[Zn(d-LCl)(Cl)](H2O)2 (L ¼ 3-methyl-2-(pyridin-4-ylmethylamino)butanoic acid)[{(Zn0.25)8(O)}Zn6(L)12(H2O)29(DMF)69(NO3)2]n (H2L ¼ 1,3-bis(4-carboxyphenyl)imidazolium)


precursors, which make it possible to use MOFs as electrolytesand electrode catalysts for fuel cells.

3.1 MOFs as electrolytes for fuel cells

The PEM fuel cell is believed to be the most promising fortransportation applications. Its operating temperature is near100 �C with ultra low or zero emissions of environmentalpollutants. The high proton conductivity is one of the importantfactors for evaluating a PEM. MOFs exhibit proton conductivityby the framework itself or by including protonic charge carriers(water, acids, heterocycles and so on) in their pores164 (Table 2).

Kitagawa, Nagao and co-workers, for the rst time, employeddithiooxamide derivatives with Cu ions to synthesize a series of

Protonconductivity(S cm�1) T (�C) RH (%) Ref.

1.2 � 10�5 27 83 16510�6 27 75 1662.0 � 10�6 27 100 1674.2 � 10�6 23 100 1698 � 10�3 25 98 1701 � 10�4 25 75 1711.1 � 10�3 22 96 1720.8 � 10�4 25 65 173�1 � 10�7 25 65 174

�1 � 10�7 25 85 174

1.2 � 10�4 150 0 1752.8 � 10�5 25 53 176

5 � 10�4 150 0 513.5 � 10�5 25 98 1772.1 � 10�2 85 90 1784 � 10�3 62 98 1798 � 10�3 25 98 180

1.6 � 10�3 19 100 181

1.0 � 10�4 80 95 1823.2 � 10�4 21 98 1832.2 � 10�5 120 0 52

1.7 � 10�3 150 0 18510�9 to 10�6 25 95 187

1.5 � 10�5 25 Methanol vapor 18810�2 200 0 1891 � 10�2 150 0.13 1903 � 10�3 150 0.13 1904.3 � 10�9 146 0 18630 20 100 1914.45 � 10�5 30 98 1924.42 � 10�5 30 98 192

2.3 � 10�3 25 95 193



Fig. 7 Comparison of the Arrhenius plots of Tz and b-PCMOF2 without and withdifferent amounts of Tz measured in anhydrous H2 atmospheres. Reprinted withpermission from ref. 51. Copyright 2009, Nature Publishing Group.


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2D MOFs, which can encapsulate different concentrations ofguest water molecules and exhibit proton conductions from10�6 to 1.2 � 10�5 S cm�1 at 27 �C with high relative humidity(RH > 75%).165–169 Then, Kitagawa and co-workers selectedanother 2D oxalate-bridged anionic layer framework[Zn2(ox)3]N

2� (ox ¼ oxalate) as a host and introduced NH4+ into

the pores as counter cations and adipic acid (adp) molecules toprovide additional protons.170 This is the rst example of a MOF((NH4)2(adp)[Zn2(ox)3]$3H2O) to exhibit a super protonconductivity of 8 � 10�3 S cm�1 at 25 �C and 98% RH.Encouraged by the results, they expanded to a series of oxalate-bridged bimetallic complexes {NH(prol)3}[M

IICrIII(ox)3] (MII ¼

MnII, FeII and CoII) with hydrophilic tri(3-hydroxypropyl)ammonium (NH(prol)3+).171 The frameworks show protonconduction from 1.2 � 10�10 to 4.4 � 10�10 S cm�1 under 40%RH and �1 � 10�4 S cm�1 under 75% RH and 25 �C. In2011, another anionic oxalate-bridged bimetallic MOF [MnII-CrIII(ox)3]

4+ was reported by Train, Verdaguer and co-workers,which hosts ammonium cations and water molecules in func-tionalized channels and exhibits a proton conductivity of 1.1 �10�3 S cm�1 at room temperature due to the guest molecules.172

A continuity of the work by Okawa, Kitagawa and co-workerswas to investigate the inuence of the hydrophilicity of thecationic ions with different substitutes on the proton conduc-tivity of the 2D oxalate-bridged bimetallic complexes{NR3(CH2COOH)}[MCr(ox)3]$nH2O (R ¼ Me (methyl), Et (ethyl),or Bu (n-butyl), and M¼Mn or Fe; Me-FeCr, Et-MnCr, Bu-MnCr,and Bu-FeCr).173 The proton conductivity of the MOFs increasedwith increasing hydrophilicity of the cationic ions {NMe3(CH2-COOH)}+ > {NEt3(CH2COOH)}+ > {NBu3(CH2COOH)}+. So themost hydrophilic complex (Me-FeCr) shows a high protonconductivity of �10�4 S cm�1 even at a low humidity of 65% RHand ambient temperature. Then, the isostructural Et-MOFs (Et-MnCr$2H2O, Et-FeCr$2H2O, Et-FeFe$2H2O) and Bu-MOFs (Bu-MnCr, Bu-FeCr, and Bu-FeFe) have been investigated by thesame group.174 At 25 �C, the Et-MOFs and Bu-MOFs have similarconductivities of 3 � 10�11 and 1 � 10�11 S cm�1, respectively,at 45% RH. With increasing RH, the conductivities of the Et-MOFs increase sharply to �1 � 10�7 S cm�1 at 65% RH, andthat of the Bu-MOFs increases less sharply to �1 � 10�7 S cm�1

near 85% RH, which also may be due to the inuence of thehydrophilicity of the cations. Very recently, Horike and Kita-gawa synthesized a 2D layer MOF ([Zn(H2PO4)2(Tz)2]n, Tz ¼triazole), in which partial deprotonated (H2PO4)

� anions asterminal ligands coordinate to the Zn ion from both sides of the(Zn(Tz)2)n

2+ layer and extend hydrogen bonds in the layers.175

This anhydrous MOF shows a proton conductivity of >10�4 Scm�1 at 150 �C without any guest supports and the protontransport occurs mainly in the ab plane, which demonstratesintrinsic proton conduction by the MOF itself. This importantresult offers promising prospects for creating proton conduc-tors by framework itself of MOF as well as investigating theproton-hopping mechanism.

Very recently, Zhu and co-workers synthesized a chiral two-dimensional MOF, {[Ca(D-Hpmpc)(H2O)2]$2HO0.5}n (D-H3pmpc ¼D-1-(phosphonomethyl)piperidine-3-carboxylic acid), whichpossesses protonated tertiary amines as proton carriers and


hydrogen-bonding chains serving as proton-conducting path-ways.176 When fabricating a MOF–polymer composite (MOF–PVP-x, x¼ the percentage of MOF) by assembling polyvinylpyrrolidone(PVP) with different contents of MOFs, MOF-PVP-50 (50%MOF inthe sample) shows a proton conductivity at 25 �C ranging from2.8� 10�5 S cm�1 at�53%RH to 5.7� 10�5 S cm�1 at�65%RH.

Some 3D porous MOFs based on sulfonate and phosphonateligands are used as the preferred hosts to encapsulate guestmolecules. In 2009, two important investigations were made bysome independent groups. Shimizu and co-workers reported aproton-conducting MOF (PCMOF2, Na3(2,4,6-trihydroxy-1,3,5-benzenetrisulfonate), a-PCMOF2: a lower temperature phase,and b-PCMOF2: a higher temperature phase) with triazole, whichresulted in a signicant enhancement of both the protonconductivity and the maximum operating temperature.50,51 Theywere also able to control the quantity of guest molecules withinthe channels, which enabled modulation of the conductivity andled to an optimal conductivity value of approximately 5 � 10�4 Scm�1 at 150 �C in anhydrous H2 for the rst time—higher thanthe conductivity of either pure PCMOF2 or pure triazole (Fig. 7).In addition, the partially loaded MOF (b-PCMOF2(Tz)0.45) wasalso incorporated into a H2/air membrane electrode assembly.The resultingmembrane proved to be gas tight, and gave an opencircuit voltage of 1.18 V at 100 �C. Then they investigated aphosphonate PCMOF-3 (Zn3(L)(H2O)2$2H2O, L ¼ [1,3,5-benzenetriphosphonate]6�), in which free water moleculesarranged ordered chains.177 The proton conductivity is 3.5� 10�5

S cm�1 at 25 �C and 98%RH, and the activation energy for protontransfer is as low as 0.17 eV. Very recently, they partially replacedNa3(2,4,6-trihydroxy-1,3,5-benzenetrisulfonate) in the b-PCMOF2with isomorphous 1,3,5-benzenetriphosphonate by making apellet of a-PCMOF2 and pressing onto it a pellet of Na3H3(1,3,5-benzenetriphosphonate) at a proper temperature andhumidity.178 When replacing one-third Na3(2,4,6-trihydroxy-1,3,5-benzenetrisulfonate), PCMOF21/2 (Na3(2,4,6-trihydroxy-1,3,5-benzenetrisulfonate)2/3(1,3,5-benzenetriphosphonate)1/3) was



Fig. 9 Views of the structure of MIL-53(M) (left) and Arrhenius plots of theproton conductivities of four functionalized MOFs (right). Reprinted withpermission from ref. 187. Copyright 2011, American Chemical Society.


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obtained, which showed a high conductivity of 2.1� 10�2 S cm�1

at 85 �C and 90% RH. When changing 1,2,4,5-tetraki-sphosphonomethylbenzene as a ligand, a new water-stable 3DLa-based PCMOF-5 exhibited a proton conductivity of 4 � 10�3 Scm�1 at 98% RH and 62 �C.179

Recently, Cabeza and co-workers employed exible tetra-phosphonates (hexamethylenediamine-N,N,N0,N0-tetrakis(me-thylenephosphonic acid) and octamethylenediamine-N,N,N0,N0-tetrakis(methylenephosphonic acid)) to construct La-based andMg-based MOFs, which show a proton conductivity of 10�3 Scm�1 at room temperature and high RH,180,181 higher thanthat of zirconium tetraphosphonate.182 When they used a car-boxyphosphonate instead of tetraphosphonate, a family oflanthanide MOFs were obtained, among which Gd-carbox-yphosphonate material has a proton conductivity of 3.2 �10�4 S cm�1 at 98% RH and 21 �C.183

Another impressive example of hybridized proton conduc-tors under anhydrous conditions is encapsulation of a proton-carrier molecule—imidazole (Im)184 by sublimation based onhost–guest interaction using aluminium compounds [Al(m2-OH)(1,4-NDC)]n (1,4-NDC ¼ 1,4-naphthalenedicarboxylate) andMIL-53(Al) (Al(OH)(BDC)(H2O)).52 The results show that thedifferent values of conductivity of imidazole in two MOFs (Im@[Al(m2-OH)(1,4-NDC)]n: 2.2 � 10�5 S cm�1, and Im@MIL-53(Al):1.0 � 10�7 S cm�1 at 120 �C) are consistent with the dynamicproperties of imidazole adsorbed in the pores, which is due tothe properties (hydrophobic or hydrophilic) of pores and theinteractions of pores and guest molecules (Fig. 8). The samegroup reported a further study, in which they selected hista-mine instead of imidazole as a proton conductor by immersioninto hosts of [Al(m2-OH)(1,4-NDC)]n.185 The proportion betweenguest molecules and metal ions of hosts in histamine@[Al(m2-OH)(1,4-NDC)]n is twice as large as the proton carrier in Im@[Al(m2-OH)(1,4-NDC)]n (mol/mol), which results in a remarkableimprovement of conductivity of over 10�3 S cm�1 at 150 �Cunder anhydrous conditions. This material is regarded as asuperionic conductor and the porous MOF supports are prom-ising for the creation of hybrid conducting materials.

Recently, histamine molecules were introduced in the poresof Zn-MOF-74 by immersing and reuxing the mixture ofdehydrated Zn-MOF-74 and histamine.186 The low protonconductivity of 4.3 � 10�9 S cm�1 at 146 �C for histamine@Zn-MOF-74 may be caused by the low mobility of histamine in theframework, limiting the proton hopping between imidazolerings.

Fig. 8 (a) Imidazole molecules are densely packed in the bulk solid. Imidazoleaccommodated in a nanochannel containing the active sites with a high affinity toimidazole (b) with and (c) without the strong host–guest interaction. Reprintedwith permission from ref. 52. Copyright 2009, Nature Publishing Group.


The proton conductivities of a series of isostructuralMIL-53(M) (M ¼ Al or Fe; Al(OH)(BDC)(H2O), Al(OH)(BDC–NH2)(H2O), Al(OH)(BDC–OH)(H2O)1.5, and Fe(OH)(BDC–(COOH)2)(H2O)) with different substitutions of amino, hydroxyl,and carboxylic groups in organic linkers were discussed byKitagawa and co-workers.187 As shown in Fig. 9, the order ofproton conductivities observed for the substituted MIL-53derivatives (–NH2 < –H < –OH < –COOH) and those of the acti-vation energies (–NH2, –H > –OH > –COOH) correlate well withthe order of the pKa values of meta-substituted benzoic acids(R ¼ –NH2, –H, –OH, and –COOH: 4.74, 4.19, 4.08, and 3.62),respectively. This is the rst example in which proton conduc-tivity has been widely controlled by the substitution of ligandfunctional groups in an isostructural series of MOFs.

In 2012, Farha, Hupp and co-workers discussed the inu-ences of node-coordinated molecules and pore-lling solventsin MOFs on proton conductivities.188 The post-synthesis modi-cation of HKUST-1 via coordination of H2O at open CuII (node)sites led to a large (�75-fold) enhancement in proton conduc-tivity relative to a version containing nodes modied by aceto-nitrile once the material’s channels were infused with MeOH.Then, the conductivity became almost immeasurably smallwhen methanol was replaced by hexane as the pore-llingsolvent (Fig. 10).

For MIL-101, the (1 � x)CsHSO4�x@Cr-MIL-101 nano-composite system, assembling proton conductors of CsHSO4 in

Fig. 10 Comparison of proton conductivities of H2O-HK, pristine-HK, EtOH-HK,MeCN-HK, MeOH-HK, and bulk MeOH under a MeOH or n-hexane atmosphere.Reprinted with permission from ref. 188. Copyright 2012, American ChemicalSociety.



Fig. 11 Cyclic voltammograms of a glassy carbon electrode coated with MOFs in0.5 M H2SO4 containing different ethanol concentrations (0.0, 0.5, 1.0 and 2.0 M,sweep rate ¼ 100 mV s�1). Reprinted with permission from ref. 207. Copyright2010, Wiley-VCH.


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pores of MOF, has a proton conductivity of 10�7 to 10�2 S cm�1

over the temperature range 50–200 �C and a chemical stabilityup to 190 �C.189 The unusual behavior of CsHSO4 in the nano-space of Cr-MIL-101 deals with the appreciable interface inter-action and disordered state of the salt in the nanospace of themesoporous Cr-MIL-101, which causes the conductivityincrease. Very recently, Ponomareva et al. reported the protonconductivities of H2SO4@MIL-101 and H3PO4@MIL-101 by theimpregnation of MIL-101 in non-volatile acids H2SO4 andH3PO4.190 The conductivities at T ¼ 150 �C are 1 � 10�2 S cm�1

for H2SO4@MIL-101 and 3 � 10�3 S cm�1 for H3PO4@MIL-101with RH of 0.13%, respectively, which are greater than those ofany other MOF-based compounds and could be compared withthe best proton conductors, such as Naon. By a post-syntheticfunctionalization method, the framework of MIL-53(Al) wassulfated up to 50% by reaction with triuoromethanesulfonicanhydride and H2SO4 in nitromethane as solvent at roomtemperature.191 At 20 �C and 100%RH, the sulphatedMIL-53(Al)material shows a proton conductivity of ca. 0.3 S m�1.

Besides these classic MOFs, some new porous MOFs havebeen reported as hosts for proton conductivities. Banerjee andco-workers investigated the inuence of hydrogen bondsbetween halogen atoms and hydrogen atoms from watermolecules in the 1D channels on proton conductivity of fourhomochiral MOF isomers, [Zn(l-LCl)(Cl)](H2O)2, [Zn(l-LBr)(Br)](H2O)2, [Zn(d-LCl)(Cl)](H2O)2, and [Zn(d-LBr)(Br)](H2O)2(L ¼ 3-methyl-2-(pyridin-4-ylmethylamino)butanoic acid).192

MOFs [Zn(l-LCl)(Cl)](H2O)2 and [Zn(d-LCl)(Cl)](H2O)2 showproton conductivities of 4.45 � 10�5 and 4.42 � 10�5 S cm�1,respectively, higher than the other two MOFs, which is attrib-uted to the fact that [Zn(l-LCl)(Cl)](H2O)2 has a higher waterholding capacity than [Zn(l-LBr)(Br)](H2O)2. Another porousMOF [{(Zn0.25)8(O)}Zn6(L)12(H2O)29(DMF)69(NO3)2]n (H2L ¼ 1,3-bis(4-carboxyphenyl)imidazolium) based on Zn8O clusters andimidazolium-based ligands has been synthesized by Kitagawa,Bharadwaj and co-workers.193 Its proton conductivity increaseswith an increase of humidity and reaches 2.3 � 10�3 S cm�1 atambient temperature and 95% relative humidity.

In addition, it has been found that some 1D coordinationpolymers can also exhibit proton conductivity.194–198

3.2 MOFs as electrode catalysts for fuel cells

Today, a large challenge in the fuel cell eld is to nd highefficient non-platinum groupmetal (NPGM) catalysts for oxygenreduction reactions (ORR) at the cathode instead of platinum-based catalysts with high costs and limited reserves, and cor-responding catalysts used at the anode.199–202

The water-stable Cu-based MOF (copper(II)-2,20-bipyr-idinebenzene-1,3,5-tricarboxylate) as a NPGM catalyst for ORRshows a couple of well-dened redox peaks at ca. �0.15 V in aphosphate buffer (pH 6.0).203 The presence of O2 in the bufferclearly increases the reduction peak current, while it decreasesthe reversed oxidation peak current of the redox wave. With thepositive shi of the potential for ORR at the MOF-modiedelectrode compared with that at the bare graphene carbonelectrode, this demonstrates the electrocatalytic activity of the


Cu-bipy-BTC MOF towards ORR through an almost four-elec-tron reduction pathway.

A graphene–metalloporphyrin MOF ((G-dye–FeP)n) has beensynthesized by the use of pyridine-functionalized graphene(G-dye) as a building block with iron–porphyrin (5,10,15,20-tetrakis(4-carboxyl)-Fe–porphyrin)204,205 in the assembly of aMOF, because iron porphyrins play a vital role in oxygentransport and reduction reactions.206 The addition of G-dyechanges the crystallization process of iron–porphyrin in theMOF, increases its porosity, and enhances the electrochemicalcharge transfer rate of iron–porphyrin, and (G-dye–FeP)nexhibits interesting catalytic activity towards the ORR in alka-line medium with a facile four-electron ORR pathway.

The rst and important example of the utilization of anoble-metal-free MOF material, N,N0-bis(2-hydroxyethyl)dithiooxamidatocopper(II), as an electrocatalyst for ethanolelectrooxidation reactions was demonstrated by Kitagawa’sgroup.207 This MOF was coated on a glassy-carbon electrode, andcyclic voltammetry showed one redox peak centered at 0.35 V vs.Ag/AgCl and ascribed to CuI/CuII in 0.5 M H2SO4 solution(Fig. 11). With an increasing ethanol concentration, the currentdensity increased continuously. Electrolysis with controlledpotential (0.39 V) combined with gas chromatography analysisshows the oxidation of acetaldehyde (2 electron oxidation).

3.3 MOFs as precursors of catalysts for fuel cells

In 2011, Liu and co-workers reported the experimentaldemonstration of porous MOF as a new class of precursors forpreparing NPGM electrode ORR catalysts.208 The reported 3DMOF of [Co(Im)2$0.5DMA]N (ref. 209) (DMA ¼ N,N-dimethyla-niline) was employed as a precursor with the potential toproduce a uniformly distributed catalytic center Co–N4 (M–Nx

was suggested to be the active ORR sites210,211) and high active-site density. The MOF was active for the preparation of NPGMcatalysts through pyrolysis under an inert atmosphere at 750 �C,with an onset potential of 0.83 V vs. a reversible hydrogenelectrode and the electron transfer mechanism characterized by




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3.2–3.5 electrons per O2 molecule, which is comparable to thebest cobalt-based NPGM catalysts. Lefevre, Dodelet and co-workers reported that the ZIF serves as a microporous host forphenanthroline and ferrous acetate to form a Fe-based catalystprecursor, which was ball milled, pyrolyzed twice, rst in argonat 1050 �C, then in ammonia at 950 �C, to form a nitrogenatedmicroporous carbon structure hosting FeNx active sites.212 Acathode made with the optimized Fe/N/C electrocatalyst, testedin H2–O2, has a power density of 0.75 W cm�2 at 0.6 V. Theunprecedented catalytic activity and power density of these newFe-based catalysts bolster the prospect of a viable alternative toPt-based cathode catalysts in PEMFCs. Another work is the useof 3D [Fe3(Im)6(ImH)2]x (FeIM)213 as a precursor to prepareelectrocatalysts by pyrolysis, acid-washing, pyrolysis againunder a owing NH3 atmosphere to give the nal productsFeIM700, FeIM800 and FeIM900 based on the pyrolysistemperature (700, 800 and 900 �C), respectively.214 To study theeffect of ZIF-8, the catalyst named FeIM/ZIF-8 was prepared byball-milling the FeIM and ZIF-8 mixture with different ratiosbetween them followed by using the above-mentioned pyrolysisconditions.212 The catalytic property of FeIM/ZIF-8 as thecathode is better than that of the FeIM system, which shows anonset potential of 0.977 V and a measured volumetric currentdensity of 12 A cm�3 at 0.8 V in a single cell test.

3.4 Conclusions

Fuel cells, working without polluting the environment, are cleanand green cells, which are recognized as a clean energy tech-nology. However, there are still some signicant challenges,such as increasing efficiency, energy and power densities,improving reliability, and reducing cost. MOFs, with low costand various properties, can act as electrolytes, electrode cata-lysts and catalyst precursors by adjusting the structures toobtain the optimized materials for fuel cells.

4 MOFs for lithium-ion rechargeablebatteries

A lithium-ion battery (Li-ion battery or LIB)215–217 is a recharge-able battery in which lithium ions move from the negativeelectrode to the positive electrode during discharge, and backwhen charging, where it uses an intercalated lithium compoundas the positive electrode material. The three primary functionalcomponents of a lithium-ion battery are the negative electrode(anode)218,219 positive electrode (cathode)220 and the electro-lyte221,222 that conducts lithium ions. LIBs are used in all sorts ofmobile devices (mobile phones, notebook PCs, electric vehiclesand so on) because a small, lightweight battery can deliver lotsof power. As of 2011, LIBs account for 66% of all portablesecondary battery sales in Japan.223 Based on their excellentproperties, MOFs have been investigated as electrode andelectrolyte materials in LIBs.

4.1 MOFs as materials for lithium-ion rechargeable batteries

Based on their porous structure, uncoordinated sites andfunctional organic linkers, MOFs have been employed as LIB


electrode materials based on reversible Li-ion insertion/extrac-tion reaction.224,225 The MOFs with different metals investigatedin LIBs so far can be classied into two types: non-Li-MOF andLi-MOF.

Non-Li MOFs can be used as electrode and electrolytematerials according to their structural properties. In 2007, Fereyet al. rst reported FeIII(OH)0.8F0.2(BDC)$H2O (MIL-53(Fe)),226 aMOF with higher oxidation state metal, as a rechargeableintercalation electrode successfully. The MOF mixed with15 wt% carbon served as the positive electrode, and Li-foil wasused as the negative electrode.227 A maximum capacity ofLi insertion in the cathode reached x ¼ 0.6 forLixFe

III(OH)0.8F0.2(BDC)$H2O during discharge, which gives agravimetric capacity of 75 mA h g�1. The results of Mossbauerspectroscopic analysis showed that only the FeII and FeIII ratioin the material varied with the Li uptake. The composition wasidentied as mixed-valence Li0.5Fe0.45

IIFe0.55III at x¼ 0.5 and the

process is reversible as demonstrated by in situ X-ray absorptionne structure (XAFS).228 However, the cyclability was limited athigh energy density due to the poor electronic conductivity ofthe MOF, which was rationalized in terms of interactionsbetween Fe3+ and Fe2+ sites without any electronic delocaliza-tion.229 Density functional theory (DFT) study of the detailedmechanism of Li-insertion in the same material showed fourdifferent sites for Li insertion in the framework based on theamount of Li uptake.230,231 In order to enhance the electro-chemical capacity of MIL-53(Fe), Tarascon and co-workersintroduced a guest organic electroactive molecule such as 1,4-benzoquinone within its framework, but the extra capacity,unfortunately, faded rapidly upon cycling.232 In addition, redoxactive organic linkers were also selected to construct MOFs forpositive electrodes of LIBs, but the desired result was notachieved.233

In 2011, Tarascon employedmesoporous chromium trimesateMIL-100(Cr) (Cr3O(H2O)F(BTC)2)234 as a host of cathode materialin Li–S batteries.235 The 48% S was impregnated within the poresof MIL-100 by melt-diffusion at 155 �C to obtain MIL-100(Cr)/S@155, which was ball-milled with different amounts of carbonto form cathode materials. Among them, the MIL-100(Cr)/S@155with 50% carbon additive (MIL-100(Cr)/S@155 + 50% C) shows arelatively high initial capacity and capacity retention aer 60cycles. Comparing with different host materials with various poresizes and shapes, mesoporous carbon/S@155 composite andSBA-15/S�1@155 + 50% C (SBA-15 mesoporous silica) show ahigher initial capacity than MIL-100(Cr)/S@155 + 50% C.However, mesoporous carbon/S@155 exhibits the lowest capacityaer 32 cycles and the capacity decay of MIL-100(Cr)/S@155 +50% C is relatively slower than the other two, as shown in Fig. 12.This is caused by the small size of the pore windows in MIL-100,which can conne more polysulphides in the pores of MIL-100than the other hosts. It is worth noting that the connementeffect caused by the use of a nanoporous self-supporting S matrixis very important in Li–S batteries. Therefore, the structuralexibility and adjustable pore size of MOFs provides more spacefor battery performance improvement.

Non-Li-MOFs were also tested as anode materials. MOF-177(Zn4O(BTB)2, BTB

3� ¼ 1,3,5-benzenetribenzoate) was applied in



Fig. 13 (a) The scheme for the modification of MOF-74 to form the solid elec-trolyte. (b) Nyquist plots of the ac impedance data obtained for Mg2(DOBDC)$0.35LiOiPr$0.25LiBF4$EC$DEC pellets. Reprinted with permission from ref. 243.Copyright 2011, American Chemical Society.

Fig. 12 Cycling performance of MIL-100(Cr)/S@155 + 50% C, mesoporouscarbon/S@155 and SBA-15/S@155 + 50% C in 1 M of Li organic electrolyte at aC/10 discharge rate. Reprinted with permission from ref. 235. Copyright 2011,American Chemical Society.


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reversible lithium storage, but the result exhibited a relativelyhigh irreversible capacity in the rst discharge process and amuch lower reversible discharge–charge capacity in thefollowing electrochemical cycles, which proved the decompo-sition of MOF-177.236 In 2010, a series of diamondoid MOFs(Zn3(HCOO)6, Co3(HCOO)6 and Zn1.5Co1.5(HCOO)6) wereemployed for the Li storage using conversion reaction at lowpotential. Among them, the anode material of Zn3(HCOO)6shows an invariable capacity of 560 mA h g�1 (9.6 moles of Li)for up to 60 cycles with a 60 mA g�1 charging rate within thevoltage range 0.005–3.0 V.237 With ex situ PXRD, FTIR and TEMstudies, the electrochemical cycling suggests that metal formateframeworks react reversibly with Li through conversion reac-tion, according to reactions (1) and (2):

Zn3(HCOO)6 + 6Li+ + 6e� 4 3Zn + 6HCOOLi (1)

3Zn + 3Li+ + 3e� 4 3LiZn (2)

During the charge, MOFs react with Li+ to form Zn nano-particles, and then Li–Zn alloy, which plays a vital role inattaining the superior Li storage performance. During thedischarge, MOFs are regenerated by alloy slipping back to metalnanoparticles which in turn form zinc formate MOFs. Thematerials based on Co3(HCOO)6 and Zn1.5Co1.5(HCOO)6 showcapacities of 410 and 510 mA h g�1 aer 60 cycles, respectively,without the formation of alloy. This work demonstrates thatrobust, thermally stable MOFs are a prospective class of elec-trode materials for LIBs. Another anode material ZTO/ZIF-8,formed by ZIF-8 coating on the surface of Zn2SnO4 (ZTO)nanoparticles, was prepared by Wei’s group. They added as-prepared ZTO nanoparticles in the methanol solution of zincnitrate hexahydrate with 2-methyl imidazole to obtain ZTO/ZIF8-10, ZTO/ZIF8-20 and ZTO/ZIF8-30 by controlling thegrowth time.238 The capacities of ZTO, ZIF8-10, ZIF8-20 andZIF8-30 are 109.6, 349.2, 265.7 and 177.8 mA h g�1 aer 20cycles, respectively, which indicates the conductivity of elec-trode material can be greatly increased aer the surface of ZTOnanoparticles was coated by ZIF-8. Copper hexacyanoferrate(Cu3(Fe(CN)6)2) shows the excellent electrode performance,which can be cycled for over 40 000 charge/discharge cycles at


17C rate and retain 83% of the initial capacity (52.2 mA h g�1).Even at a very high cycling rate of 83C, two thirds of itsmaximum discharge capacity is observed.239,240

For non-Li-MOFs as electrolyte materials, it is found thatMOFs can display a high conductivity. In 2011, Long and co-workers evaluated the conductivity of MOF-177,241 Cu–BTTri(H3BTTri ¼ 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene),242 and Mg-MOF-74 soaked in a 1 M solution of LiBF4 in a 1 : 1 volumemixture of ethylene carbonate (EC) and diethyl carbonate (DEC),and the impregnated frameworks showed conductivities withan order of 10�9 to 10�6 S cm�1.243 Themost promising materialis Mg-MOF-74 functionalized with lithium alkoxide (LiOPr: Liisopropoxide, LiOMe: Li methoxide or LiOEt: Li ethoxide),where coordinatively unsaturated Mg2+ cations bonded byalkoxide anions make Li ions move freely. The uptake of LiOiPrin Mg-MOF-74 followed by soaking in a typical electrolytesolution (in a 1 M solution of LiBF4 in a 1 : 1 mixture of EC andDEC) leads to the optimized solid lithium electrolyteMg2(DOBDC)$0.35LiO

iPr$0.25LiBF4$EC$DEC (Fig. 13), whichshows a high conductivity of 3.1 � 10�4 S cm�1 at ambienttemperatures. The foregoing results demonstrate a promisingnew approach for creating solid lithium electrolyte materials244

by using MOFs.Li-based MOFs are considered as a good candidate for

electrode materials. Tetrahydroxybenzoquinone, includingelectrochemically active C]O functionalities, shows equilib-rium between carbonyls and enolates. Dolhem, Poizot and



Fig. 14 Growth illustration of agglomerated Co3O4 nanoparticles. Reprintedwith permission from ref. 270. Copyright 2010, Elsevier.


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co-workers used the Li salt of tetrahydroxybenzoquinone(Li2+xC6O6) as a cathode material, showing a high capacity of580 mA h g�1 for x ¼ 0, which demonstrated the feasibility ofsustainable LIBs with MOFs as electrode materials.245 Then theyalso investigated Li4C6O6, which can be both reduced to Li2C6O6

and oxidized to Li6C6O6.246 The results show the electrochemicalperformances vs. Li with a sustained reversibility of 200 mA hg�1 at an average potential of 1.8 V, which can construct a LIBwith the cycles between Li2C6O6 and Li6C6O6. Because of nocrystal structures of Li2+xC6O6, Goddard, Kang and co-workerspredicted the Li ion sites in Li4C6O6 as an organic cathodematerial through rst-principles multi-level computationalmethods, which is benecial to the design of the electrodematerials with high capacities.247 Encouraged by these results,systematic lithiated electrode materials based on benzoqui-none, 2,5-dihydroxycyclohexa-2,5-diene-1,4-dione, 1,5-dihy-droxy anthraquinone and their derivatives were investigated indetail.248–251 However, the reported crystal structures of Li-basedMOFs as electrode materials are still scarce.252–254 In 2008, Sunand co-workers conrmed the crystal structure of [Li2(C6H2O4)$2H2O] based on the linker of 2,5-dihydroxy-1,4-benzoqui-none.255 The anhydrous Li2(C6H2O4) was used as an electrodematerial and showed an initial discharge capacity of 176 mA hg�1 with a columbic efficiency of 93.18% in the rst cycle, and137 mA h g�1 aer 10 cycles. In 2009, Tarascon and co-workersprepared Li-based MOFs by the reaction of conjugated dicar-boxylic acid and LiOH or Li2CO3 in ethanol–water medium(terephthalic acid: Li2C8H4O4 or Li2TPA, and muconic acid:Li2C6H4O4).256 For Li2TPA,257 each Li ion is coordinated by fouroxygen atoms from different ligands to extend to a 3D MOF.258

By ball-milling powders of Li2C8O4H4 and Li2C6O4H4 with 30wt% carbon black as electrodes, they have overall uptakecapacities of 2.3 and 1.2 Li per formula unit at potentials of 0.8and 1.4 V, and show reversible capacities of about 300 and 170mA h g�1, which slowly decay to values of about 234 and 125 mAh g�1, respectively. The rst-principles calculations for Li2TPAshow that the storage of Li in the organic salts is different fromtraditional battery materials by utilizing multiple valences ofthe organic groups, which involves breaking the aromatic p-bond in the benzenoid ring and regrouping the electrons into aset of new p-bonds with a low Li diffusion barrier. (Li2(OBA)(H2OBA ¼ 4,40-oxybisbenzoic acid)),259 [Li6(PDA)3]$2EtOH(H2PDA ¼ 2,6-pyridinedicarboxylate),260 (Li2TDC) (H2TDC ¼4,40-tolane-dicarboxylate),261 LiO2C(CH ¼ CH)nCO2Li (n ¼ 1, 2, 3and 4),262 pyromellitic diimide dilithium salt263 and so on, basedon conjugated carboxylate acid ligands and their derivatives(different substituents, the length of conjugated chains,conformations etc.), have shown capacities of 100–200 mA hg�1. The corresponding investigations have also been extendedto sodium ion batteries.264–267

Fig. 15 Illustration of the formation of spindle-like porous a-Fe2O3 (upper left).TEM images of the spindle-like a-Fe2O3 (lower left). Rate performance withincreasing charge rate from 0.1 to 10 C, as compared with previous works. Thedischarge rate is fixed at 0.1 C (1 C ¼ 1000 mA g�1) (right). Reprinted withpermission from ref. 273. Copyright 2012, American Chemical Society.

4.2 MOFs as precursors of materials for lithium-ionrechargeable batteries

Transition metal oxides, which can react with Li reversibly viathe conversion reaction (eqn (3)), exhibit high reversiblecapacities at a relatively low potential as anode materials.268,269


MOFs can act as precursors for preparing metal oxides, micro-porous carbon materials and metal-oxide carbon composites.

MxOy + 2yLi+ + 2ye� 4 xM + yLi2O (3)

In 2010, we, for the rst time, developed a MOF route forsynthesizing agglomerated Co3O4 nanoparticles, with a diam-eter of around 250 nm, consisting of smaller nanoparticles witha size of about 25 nm, by converting cobalt oxide subunits in acobalt-MOF (Co3(2,6-NDC)3(DMF)4) via pyrolysis method in air(600 �C) (Fig. 14).270 An electrode was formed by mixing 85 wt%Co3O4 nanoparticles, 5 wt% carbon black and 10 wt% poly-vinylidene uoride (PVDF), which exhibited a reversiblecapacity of 965 mA h g�1 aer 50 cycles. This agglomeratedCo3O4 favors the enhanced capacity, improved rate capabilityand prolonged cycle life as an electrode material for LIB. Bycontrolling the agglomerated structures with varied primaryand secondary particle sizes, the Co3O4 electrode performancefor LIB can be further improved. Li et al. reported porous Co3O4

nanowires by pyrolysis (450 �C) of Co(NA)H (NA ¼ nitrilotri-acetic acid), which exhibits a reversible capacity of 810 mA h g�1

and stable cyclic retention until the 30th cycle.271 The porousnanostructure of Co3O4 provides pathways for easy accessibilityof electrolytes and fast transportation of lithium ions.

Recently, spindle-like porous a-Fe2O3 was prepared from theprecursor of MIL-88-Fe (Fe3O(H2O)2Cl(BDC)3$nH2O)272 by Cho’sgroup, where MIL-88-Fe was rst heated at 500 �C in N2 gas toobtain black FeOx–C composite, then calcinated at 380 �C in air




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to get the nal dark red a-Fe2O3 (removal of carbon residue).273 Asshown in Fig. 15, the spindle-like porous a-Fe2O3, consisting ofclustered Fe2O3 nanoparticles with sizes of <20 nm, shows thedimensions of length and width of 0.8 and 0.4 mm, respectively.This material was mixed with 10 wt% Ketjen Black and 10 wt%PVDF to form an electrode, which showed a capacity of 911 mA hg�1 aer 50 cycles at a rate of 0.2 C. Even at a rate of 10 C, acomparable capacity of 424 mA h g�1 could be reached. Then ahigher capacity of 950 mA h g�1 at 200 mA g�1 was reported byLou’s group.274 By utilizing the simultaneous oxidative decom-position of Prussian blue microcubes (Fe4[Fe(CN)6]3) and crystalgrowth of iron oxide shells, they prepared Fe2O3 microboxanisotropic hollow structures with various shell architectures. Asan anode material for LIBs, it shows an excellent cycling perfor-mance besides the high specic capacity.

Very recently, hierarchical CuO buttery sheet-like nano-structures and hollow CuO nanotubes were obtained by thermaltreatment of the precursors, which basically preserved themorphology of the precursors: buttery-like Cu(C6H4NO2)2(H2O)4nanosheets and [Cu(C6H4NO2)(OH)]H2O nanorods (C6H5NO2 ¼isonicotinic acid).275 Compared with commercial CuO, the initialdischarge capacities of CuO nanosheets and 1D nanostructureswere calculated to be 813 and 933mA h g�1, respectively (337 mAh g�1 for commercial CuO), and the discharge capacities retain555 and 688mA h g�1 at 100mA g�1 aer 30 cycles (182mA h g�1

for commercial CuO). The results show that the dischargecapacity of the CuO samples would be affected by themorphology of the samples. Thus, preparing metal oxide mate-rials from MOFs is an effective method for LIBs application.

On the other hand, a combination of mixed-metal oxides,showing a good capacity, is regarded as anode materials in Li-recyclability.276 Courtel et al. prepared a series of mixed-metaloxide AMn2O4 (A ¼ Co, Ni, and Zn) by calcining the precursorsgenerated from oxalic acid, transition metal acetate and man-ganese(II) acetate.277 Among them, ZnMn2O4, showing betterperformance compared to its single metal oxides, Mn2O3 or ZnO,is regarded as a potential high-performance anode materialbecause of its low cost, environmental benignancy, low operatingvoltages and high energy density. Recently, Yang, Li, and co-workers developed a two-step process to prepare ower-likeZnMn2O4 by thermal transformation of nanoscale mixed metal–organic frameworks (MMOFs) as precursors.278 MMOFs wererstly synthesized by hydrothermal reaction with two kinds ofmetal acetates, NaOH and ptcda (perylene-3,4,9,10-tetracarboxylicdianhydride), and then spinel ZnMn2O4 was obtained by treatingMMOFs in air at 450 �C. By mixing 75 wt% active ZnMn2O4, 15wt% acetylene carbon black and 10 wt% PVDF as electrode, theoverall capacities for the rst discharge and charge are as high as1277 and 730 mA h g�1, respectively, an irreversible capacity of547 mA h g�1, and a retained capacity of 502 mA h g�1 aer 30cycles, which might be caused by the unique architecture ofassemblies of nanoplates consisting of well-crystallized agglom-erated nanoparticles. Based on the above results, mixed-metaloxides, obtained by calcination, show a porous structure whichcan enhance the electrolyte/ZnMn2O4 contact area, shorten the Li+

ion diffusion length, and accommodate the strain induced by thevolume change during the charge and discharge cycles.


4.3 Conclusions

Now, LIBs are commonplace. Although rapid developmentshave been made for LIBs, there are still some challenges, suchas electrode materials with a fairly good structural stability andminimal volume change over the entire operational Li inser-tion/extraction voltage range, electrode integrity over manydischarge–recharge cycles, improved capacity, good cell cyclelife, electrolyte with fully compatible with lithium metal, moreenvironmentally sustainable materials and so on. MOFs haveshown feasibility and great potential in LIB applications eitheras electrode materials or electrolytes. Integrating the advan-tages of MOF materials into traditional battery materials wouldbe benecial for battery research and development. The corre-sponding investigation is still ongoing.

5 MOFs as platforms for supercapacitors

Different from fuel cells and batteries, an electrical double-layercapacitor (EDLC), also called a supercapacitor, is a device thatstores electrical energy in the electrical double layer that formsat the interface between an electrolytic solution and an elec-tronic conductor.279 It consists of electrodes (mostly activatedcarbon with a very high surface area), an electrolyte (aqueous,organic solution or ionic liquid), and the separator, whichprevents facing electrodes from contacting each other. Theproperties of the electrode materials280 are important factors forthe supercapacitors, where the excellent electrode materialsshow high conductivity, high surface-area range, good corro-sion resistance, high temperature stability, controlled porestructure, processability and compatibility in composite mate-rials, and relatively low cost.281–284 Inheriting some of the above-mentioned properties from MOFs or their derivatives, by usingMOFs as precursors, suggests that MOFs are excellent candi-dates for the electrode materials for supercapacitors.

5.1 MOFs as electrode materials

Recently, two research groups employed as-synthesized MOFsas electrode materials. Co8-MOF-5 (Zn3.68Co0.32O(BDC)3-(DEF)0.75) has been used as an electrode material (Co8-MOF-5/carbon black/polytetrauoroethylene with 75 : 15 : 10 wt%) inan electrolyte composed of 0.1 M tetrabutylammonium hexa-uorophosphate in acetonitrile. However, the capacitancevalues are low (0.49 F g�1 at 25 mV s�1 from cyclic voltamme-tries, and 0.30 F g�1 at 10 mA g�1 from charge/discharge curveswhen referred to total mass of the electrode).285 Almost at thesame time, another Co-MOF composed of Co2+ and terephtalicligands was ground with polyethylene glycol (PEG), ethanol anddeionized water, and the blended semi-liquid paste was doctor-bladed on an indium tin oxide conducting glass substrate.286

The doctor-bladed lm was annealed at 250 �C to decomposethe PEG used as an organic binder in the lm, and the resultantmaterial showed a specic capacitance of 206.76 F g�1 at 0.6 Ag�1 in 1 M LiOH electrolyte. These two results show that the as-synthesized MOFs are feasible to apply as electrode materialsfor supercapacitors, and metal ions, ligands, structures and



Fig. 16 A diagrammatic view of preparation of nanoporous carbon using MOFas a template. Reprinted with permission from ref. 287. Copyright 2008, AmericanChemical Society.


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other factors may work together to inuence the capacitance ofthe devices.

5.2 MOFs as templates and/or precursors for electrodematerials

MOFs can be employed as templates and/or precursors forpreparing porous carbons and metal oxides, which have beenused as the electrode materials of supercapacitors with highperformances.

Table 3 Capacitance of carbon materials prepared using MOFs as templates and/

SampleCurrent density(mA g�1)

Sweep rate(mV s�1)

Capacitance(F g�1)

NPC — 5 204NPC530 50 — 158NPC650 50 — 222NPC800 50 — 151NPC900 50 — 148NPC1000 50 — 149MC 250 — 149/113MPC 250 — 170/111MAC 250 — 72/4MC-A 250 — 222/126MPC-A 250 — 187/110MAC-A 250 — 274/1683D HPC 600 — 175HPC 100 — 166HPCs-0 100 — 185HPCs-0.1 100 — 215HPCs-0.2 50 — 344HPCs-0.4 100 — 241C800 — 5 188C1000 — 5 161Z-700 — 50 23Z-800 — 50 130Z-900 — 50 128Z-1000 — 50 112MgC 500 — 180/173BaC 500 — 171/169C-S700 — 2 182C-S900 — 2 156C-C1700 — 2 117C-C1900 — 2 70


In 2008, we, for the rst time, employed a MOF as atemplate to prepare nanoporous carbon (NPC), where a MOF-5framework was used as template and furfuryl alcohol (FA) ascarbon precursor.287 Firstly, as shown in Fig. 16, FA moleculeswere introduced in a three-dimensional intersecting channelsystem of MOF-5 through vapour deposition, where FA mole-cules were polymerized. Then we obtained different carbonmaterials by pyrolysis at different temperatures in argon. Thecarbon material obtained at 1000 �C showed BET surface areaas high as 2872 m2 g�1. During the carbonization process, theMOF-5 framework decomposed, behaving as a self-sacricedtemplate and precursor and resulting in the formation of thenanoporous carbon with a large pore volume and high specicsurface area. When using the as-synthesized nanoporouscarbon as an electrode material (electrolyte, 1.0 M H2SO4) forEDLC, it showed a capacitance of 204 F g�1 at a sweep rate of5 mV s�1, which is higher than that of carbon materialssynthesized from SBA-15.288 The specic capacitance wasmaintained as high as 258 F g�1 even at a current density of250 mA g�1 (Table 3). Thus, the nanoporous carbon materialsfrom MOFs have excellent electrochemical properties as elec-trode materials for EDLC.289 We also introduced the carbonprecursor FA into the pores of MOF-5 by employing theincipient wetness technique instead of vapour deposition.290

The BET surface areas for samples obtained by carbonizing attemperatures from 530 to 1000 �C (NPC530, NPC650, NPC800,

or precursors

Surface area(m2 g�1) Electrolyte Ref.

2872 1.0 M H2SO4 2873040 1.0 M H2SO4 2901521 1.0 M H2SO4 2901141 1.0 M H2SO4 2901647 1.0 M H2SO4 2902524 1.0 M H2SO4 2901812 6 M KOH/1.5 M NEt4BF4 acetonitrile 2921543 292384 2921673 2921271 2922222 2921224 1 M NEt4BF4 2931391 6 M KOH 2941796 6 M KOH 2942137 6 M KOH 2942587 6 M KOH 2952857 6 M KOH 2942169 1.0 M H2SO4 2963405 1.0 M H2SO4 296520 0.5 M H2SO4 299720 0.5 M H2SO4 2991075 0.5 M H2SO4 2991110 0.5 M H2SO4 2992322 EMImBF4/30% KOH 304955 304817 6 M KOH 305704 6 M KOH 305311 6 M KOH 305199 6 M KOH 305



Fig. 17 (a and b) CVs at different scan rates and (c and d) galvanostatic charge/discharge profiles at different current densities for (a and c) C800 and (b and d)C1000 samples. Reprinted with permission from ref. 296. Copyright 2011,American Chemical Society.


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NPC900 and NPC1000) fall into the range from 1141 to 3040 m2

g�1 and the dependence of BET surface areas on carbonizationtemperatures shows a “V” shape. All the samples have a poresize distribution centered at about 3.9 nm. The highestspecic capacitances are observed for the carbon obtained at650 �C with a value as high as 167 and 222 F g�1 at a sweep rateof 5 mV s�1 and a current density of 50 mA g�1 (electrolyte,1.0 M H2SO4), respectively. The corresponding decompositionmechanism of MOF-5 has been demonstrated by Zhang andHu.291 Hu et al. prepared porous carbon materials (MC, MPCand MAC) through the direct thermolysis of MOF-5 without orwith phenolic resin or carbon tetrachloride and ethylenedi-amine as the additional carbon sources, and then these carbonmaterials were activated by KOH to further tune the porestructures and textures (MC-A, MPC-A and MAC-A).292 Theresults show that only porous carbon materials from addi-tional carbon sources (carbon tetrachloride and ethylenedi-amine) have an increasing BET surface area aer KOHactivation (before: 384 m2 g�1; and aer: 2222 m2 g�1), whichalso shows the largest capacitances of 274 and 168 F g�1 at acurrent density of 250 mA g�1 in aqueous and organic elec-trolytes, respectively. Recently, Zhan and co-workers synthe-sized 3D hierarchical porous carbons with different surfaceareas from pyrolysis of MOF-5 using glucose as the additionalcarbon sources by changing the reaction time.293 When thereaction time was 5 h, the pore size of the carbon was mainlycentered at 0.8 and 1.5 nm. However, the pore size of theresultant carbon increased, and mesopores with sizes of 2–10 nm and macropores exceeding 50 nm could also beobserved with the increase in reaction time (7, 15 and 20 h).The BET surface areas also changed from 823 to 1224 m2 g�1,and the highest initial specic capacity was 175 F g�1 at 0.6 Ag�1 in 1 M NEt4BF4/propylene carbonate electrolyte. Anotherexample is that a series of hierarchical porous carbons (HPCs)are synthesized from MOFs as a precursor and glycerol as acarbon source, and the pore size distribution and BET surfacearea can be modulated by adding different amounts ofBi(NO3)3$5H2O.294,295 These HPC materials show electro-chemical properties with the specic capacitance from 185 to241 F g�1 at 0.1 A g�1 in aqueous solution of 6 mol L�1 KOH.

In 2011, we prepared nanoporous carbon material (C800 andC1000) from highly porous ZIF-8 as both a precursor and atemplate and furfuryl alcohol as the second carbon source.296 Bychanging the calcination temperature (800 and 1000 �C), thesurface areas (2169 and 3405 m2 g�1) of the resultant carbonmaterials can be tuned simply. As shown in Fig. 17, the speciccapacitance is 200 F g�1 at a current density of 250 mA g�1

(electrolyte, 1.0 M H2SO4). It was also revealed that directcalcination of ZIF-8 at 1000 �C resulted in the formation ofcarbon with a surface area as high as 3184 m2 g�1.296 Aerchemical activation using KOH with a carbon/KOH weight ratioof 1 : 4 at 700 �C for 1 h, ZIF-templated carbons undergoenhancement of their porosity, and a sample carbonized at900 �C has the highest increase with BET surface area from 933to 3188 m2 g�1.297 Recently, Banerjee and co-workers alsodemonstrated that carbon materials from pyrolysis of iso-reticular ZIFs (ZIF-68, ZIF-69 and ZIF-70) with furfuryl alcohol


carbon sources showed the increasing BET surface areascompared with the corresponding ZIF precursors.298 Yamauchi,Ariga and co-workers reported a series of nanoporous carbons(Z-700, Z-800, Z-900 and Z1000) with different BET surface areas(520–1110 m2 g�1) through direct carbonization of a commer-cially available ZIF-8 without additional carbon sources byadjusted calcination temperature (700–1000 �C).299 Three ofthem show the capacitances from 112 to 130 F g�1 at a scan rateof 50 mV s�1 (electrolyte, 0.5 M H2SO4).

Al-based MOF is an excellent candidate as a template and/orprecursor for preparation of porous carbon materials.300–302

Yamauchi and co-workers introduced FA into pores of Al-basedMOF (Al(OH)(1,4-NDC)$2H2O)303 as precursors, which werecarbonized under an inert atmosphere to prepare microporouscarbon bers with an increase in the BET surface area (178–513 m2 g�1) upon increasing the loading amount of FA.301

The same group applied the direct carbonization ofAl(OH)(1,4-NDC)$2H2O under protection of an inert gas from500 to 800 �C. The samples were washed with a HF solution toremove the Al component, and showed a highest surface area of5500 m2 g�1.302

Beside these well-known MOFs, other MOFs have also beenused as templates and/or precursors for porous carbon mate-rials.304–306 In 2010, Zhou and co-workers prepared two meso-porous carbons (MgC and BaC) by simple pyrolysis ofcommercial magnesium or barium citrate and tested as elec-trode materials with specic capacitances of 180 and 171 F g�1

in an ionic liquid (EMImBF4 ¼ 1-ethyl-3-methylimidazoliumtetrauoroborate) electrolyte, and 173 and 169 F g�1 in aqueouselectrolyte at 500 mA g�1.304 Recently, mesoporous graphiticcarbon nanodisks (C-S700, C-S900, C-C1700 and C-C1900) werefabricated via catalytic carbonization (700 and 900 �C) of Fe-based MOFs constructed by 1,4,5,8-naphthalenetetracarboxylicdianhydride and different iron salts.305 Then the carbonizationproducts were washed in an acid solution and showed thespecial capacitance of 70–182 F g�1 at a scan rate of 2 mV s�1 ina 6 M KOH electrolyte.



Fig. 18 Representation of a dye-sensitized solar cell. Reprinted with permissionfrom ref. 309. Copyright 2001, Nature Publishing Group.


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As mentioned above, many factors, such as structures, poresizes, stability of MOFs, additional carbon sources, calcinationtemperature and so on play fundamental roles in the formationof the nal carbon materials. These factors work together andhave a signicant effect on the morphology, particle sizes, poretypes and properties as electrode materials for supercapacitors.It is a feasible method to preparing pore carbon materials aselectrode materials based on MOFs.

Apart from porous carbons, porous metal oxides can also beprepared fromMOFs. Nano/micro Co3O4, with the particle sizebetween 30 and 50 nm, were prepared from solid-stateannealing of MOF precursor (Co(BDC)2H2)n at 450 �C in air,which shows a BET surface area of 21.5 cm2 g�1, and anaverage pore size of 28.8 nm.307 The porosity of Co3O4 nano/micro superstructures is benecial for the rapid contact ofelectrolyte ions to the large surfaces of the electroactive Co3O4

materials. By using as an electrode material of super-capacitors, it exhibited a specic capacitance of 208 F g�1 at acurrent density of 1 A g�1, and a specic capacitance retentionof 97% aer 1000 continuous charge–discharge cycles in 6.0 Maqueous KOH solution.

5.3 Conclusions

With improving the properties of electrode materials, super-capacitors have been widely used in various elds as a renew-able energy storage device. As the shortcomings, such as poweronly being available for a short duration, low capacity, lowenergy density and so on, limit the promotion of super-capacitors, it is still a challenge to develop the electrode mate-rials with high specic surface areas, suitable pores, lowinternal electrical resistances, good cycling performances andlow costs, for which MOFs, as demonstrated above, are prom-ising platforms.

6 MOFs as platforms for solar cells

Sunlight is free, and solar energy conversion represents apromising route to green and renewable energy generation.Solar cells are a photoelectric conversion device.3,308 Amongthem, the dye-sensitized solar cell (DSSC),309,310 a photo-electrochemical system based on a semiconductor formedbetween a photo-sensitized anode and an electrolyte (Fig. 18),has attracted great interest as a potential candidate of “low-costsolar cells” toward the solution of global energy demand with anew record power-conversion efficiency of 12.3%.311 Semicon-ducting materials are important compositions in solar cells.Lots of results on the use of MOFs as semiconductors have beenreported from theoretical calculations and experiments, such asZn4O(2,6-NDC)3-(DMF)1.5(H2O)0.5$4DMF$7.5H2O (UTSA-38, 2,6-NDC ¼ 2,6-naphthalenedicarboxylate)142 and so on.312–314 Theseresults have demonstrated that the band gaps of MOF itself as asemiconductor can be adjusted by changing metal types, thesizes of metal oxide clusters, coordinating atoms from linkersand so on.315–323 Combining with the photoresponse of MOFs,324

the advantages of MOFs make it possible to use them in solarcells.


In 2007, a device using as-synthesized MOF-5 as the activematerial was prepared by Garcia and co-workers, which wasdeposited onto a transparent indium tin oxide (ITO) electrodewith a layer area and thickness of 1 � 1 cm and 50 mm,respectively, and its performance was tested in a rudimentaryphotovoltaic cell without sensitizing dyes and electrolyte. Theresults show voltage at open circuit VOC ¼ 0.33 V, current ISC ¼0.7 mA, and ll factor FF ¼ 44%, which demonstrates MOF-5 asan active component in photovoltaic cell.325 Then they also usedAl2(BDC)3 as a semiconductor to build photovoltaic cells anddemonstrated that Al2(BDC)3 including 1,4-dimethoxybenzenewith the lowest thickness (2.7 mm) showed the better efficiency(VOC ¼ 0.36 V, current density JSC ¼ 36 mA cm�2, and FF ¼ 40%)than that without guest molecules.326

In 2011, Li et al., for the rst time, used ZIF-8 to coat TiO2

with different thicknesses for use as electrodematerials in DSSCby immersing TiO2 in a fresh methanol solution containing1 mM Zn(NO3)2 and 2 mM 2-methyl imidazole from 0 to 40min.327 Among them, the thickness of thin ZIF-8 lm coated onTiO2 is about 2 nm aer a growth time of 30 min. Then TiO2/ZIF-8 was immersed in 0.3 mM ethanol solution of various dyesfor 3 h to sensitize the electrode material. The open circuitvoltage of DSSC was enhanced with increasing the thickness ofthe ZIF-8 coating layer. As shown in Fig. 19, the value of VOCincreased from 29 to 66 mV for the D205 (2-((E)-5-(1,2,3,3a,4,8b-hexahydro-4-(4-(2,2-diphenylyingyl)phenyl) cyclopenta[b]indole-7-yl)methyl)-3-octyl-5-(3-carboxymethyl-4-oxothiazolidin-2-yli-dene)rhodanine), black dye ([RuL(NCS)3]$3TBA, L ¼ 2,20:60,20 0-terpyridyl-4,40,40 0-tri-carboxylic acid and TBA ¼ tetra-n-buty-lammonium), C101 (2-ethylhexanoic acid cobalt(II) salt), N719(di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,20-bipyr-idyl-4,40-dicarboxylato) ruthenium(II)) and D131 ((E)-2-cyano-3-(4-(4-(2,2-diphenylvinyl)phenyl)-1,2,3,3a,4,8b-hexahydrocyclo-penta[b]indol-7-yl)acrylic acid), respectively, which was notcaused by the change in the conduction band edge of TiO2 dueto its possible coordination with 2-methyl imidazole, asdemonstrated by Mott–Schottky measurements. However, theshort circuit current decreased with the introduction of ZIF-8shell material; with a signicant increase in the adsorbed dyes



Fig. 19 (a) The relationship between ZIF-8 growth time and VOC. (b) The VOC ofthe TiO2 and TiO2/ZIF-8 electrodes sensitized by various dyes (with a growth timeof 40 min). Reprinted with permission from ref. 327. Copyright 2011, RoyalSociety of Chemistry.


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on the TiO2/ZIF-8 electrode, the core/shell structure inhibitedthe injection of electrons from dyes into the conduction bandedge of TiO2. Recently, Sahoo, Banerjee and co-workers calcinedtwo Zn-based homochiral MOFs (based on valine) with differenthalogen anions at 800 �C in air or N2 to obtain rod-shaped,hexagonal column-shaped, and elliptical aggregation of ZnOmorphologies, which were used to prepare ZnO lms with athickness of 12 mm by the doctor-blade method, annealing at450 �C and coating with sensitizer (N719 dye).328 The DSSCactivity measurements showed power conversion efficiencies aslow as 0.15 and 0.14% for ZnO derived from the two MOFs inair, and no DSSC activity was detected for that prepared in N2

due to poor conductivity and crystallinity of the carbon presentin as-synthesized ZnO samples.

The quantum dot-sensitized solar cell (QDSSC)329–331 isconsidered to be a simple analogue of a DSSC. The difference isthe replacement of the organometallic or organic dyes with QDsensitizers. A MOF is treated as a light-sensitive semiconductor,and the second building unit of the metal oxide cluster can beconsidered as a discrete QD analogue, which is stabilized andinterconnected by the conjugated organic linkers acting as thephoton antenna.

In 2004, Bordiga et al. testied that the Zn4O13 cluster inMOF-5 behaves as a ZnO quantum dot (QD) by UV-Vis DRS andphotoluminescence spectroscopy, combined with excitationselective Raman spectroscopy.332 The organic part acts as aphoton antenna able to efficiently transfer the energy to theinorganic ZnO-like QD part, where an intense emission at525 nm occurs. Inspired by this result, studies about MOFs asQDs and as hosts for encapsulating different QDs in the poreshave been carried out.333–337

In 2011, Falcaro et al. used highly luminescent CdSe–CdS–ZnS QDs to modify nanostructured poly-hydrate zinc phosphatemicroparticles, which acted as nucleation seeds for growingMOF-5.334 Recently, another luminescent multishell CdSe/CdS/ZnS QDs withinMOF-5 was achieved through a one-pot method.The QDs were dispersed into the DMF and DEF solvents by two-step solvent exchanges, and then were added in the motherbatch of MOF-5 precursors for growing QD@MOF-5, in which itretained the framework’s highly crystalline morphologyalthough the QD size is noticeably bigger than the MOF-5


cavities.325 By Fisher’s group, ZIF-8 was employed as a host toconne quantum dots gallium nitride and zinc oxide in itspores by annealing ZIF-8 including metal precursors, whichwere introduced by chemical vapor deposition.336,337

Very recently, Wiederrecht, Hupp and co-workers reportedtwo porphyrin-based MOFs: F-MOF ([5,15-dipyridyl-10,20-bis-(pentauorophenyl)porphinato]zinc(II)) and DA-MOF ([5,15-bis-[4-(pyridyl)ethynyl]-10,20-diphenylporphinato]zinc(II)).338 Fluo-rescence quenching experiments and theoretical calculationsindicate the photogenerated exciton migrates over a netdistance of up to �45 porphyrin struts within its lifetime in DA-MOF (but only �3 in F-MOF), with a high anisotropy along aspecic direction. The long distance and directional energymigration in DA-MOF suggests many promising applications ofthis compound or related compounds in solar energy conver-sion schemes as an efficient light-harvesting and energy-trans-port component. Then they modied the exterior surfaces oftwo porphyrin-based MOFs with CdSe/ZnS core/shell QDs bychemical bonds for the enhancement of light harvesting viaenergy transfer from the QDs to the MOFs.339 The photoexcita-tion of the QDs is followed by energy transfer to the MOFs withefficiencies of 84% by tuning the size of the QDs. This sensiti-zation approach can result in a >50% increase in the number ofphotons harvested by a single monolayer MOF structure with amonolayer of QDs on the surface of the MOF.

As one of the most viable clean energies, the research of solarcells develops rapidly, although it started relatively latecompared with other energy applications. MOFs, with thephotoresponse, semiconductor, and quantum dot properties,have been tried for use in solar cells. These results show thatMOFs, as multi-functional materials, are benecial for appli-cations in solar cells. Because the application of MOFs to solarcells is in its infancy, there is a long way to go for us to optimizethe excellent properties of MOFs to enhance the efficiency andperformance of solar cells.

7 Conclusions and perspective

Today, there is a growing concern related to clean energy, whichwill become our most important energy source in the future,because of the serious issues arising from the use of largeamounts of fossil fuels and consequent non-renewable energydepletion and environmental pollution. MOFs can show variousproperties by adjusting their structural characteristics throughcarefully selecting metal ions or metal clusters and multi-func-tional linkers. MOFs show their own merits in each of the above-mentioned elds, such as low costs, simple synthesis methods,controllable structures and versatile properties and functions.Here, we reviewed MOFs as platforms for clean and renewableenergy. For hydrogen energy, MOFs show high hydrogen sorptioncapacities, whereas it is still quite difficult to store hydrogen byphysisorption in porous materials at convenient temperatures.While chemical hydrogen storage with MOFs is another efficientway, the stability of MOFs needs to be considered duringhydrogen generation (pyrolysis and hydrolysis). For fuel cell, Li-ion renewable battery and supercapacitor applications, MOFs ashosts, catalysts and catalyst precursors have been widely reported




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with the corresponding proton conductivity, catalysis, redoxproperties and so on, however, there is still a long way to gotowards their industrialization in these three elds. The investi-gation into MOF applications for solar cells started lately and thecorresponding reports are still scarce.

A large number of MOF-based materials with high chemicaland thermal stabilities, and excellent properties have beenreported. How best to apply the existing MOF-basedmaterials toclean energy is one of our major aims, while another target ishow to synthesize new MOFs by optimizing metal ions orclusters, organic linkers, synthesis conditions and so on toprovide more excellent MOF candidates for the clean energyapplications. Presently, the investigation of MOFs as platformsfor clean energy is still in its infancy. Further efforts in this eldwill certainly contribute to the development and practicalapplications of future clean and renewable energy. Especially,the applications of MOFs in solar cells will be one of theresearch focuses in the future. It will be exciting to watch therapid development of such new materials in the years to come.

We have tried to present an up-to-date overview of such arapidly growing eld, while the subject is very active and manypapers are contributed each year (even during the writing of thisarticle) from chemists, energy, environmental and materialsscientists, etc. Therefore, it is hard to take into account allpublications in this eld in limited pages. We apologize here ifsome signicant contributions were le out.

Acknowledgements

The authors gratefully acknowledge the reviewers for valuablecomments and constructive suggestions and the editor for kindinvitation. The authors are pleased to acknowledge the nework of the talented and dedicated graduate students, post-doctoral fellows, and colleagues who have worked with us inthis area and whose names can be found in the references. Theauthors thank AIST and Q.X. thanks METI for nancial support.S.L.L. thanks JSPS for invitation fellowship.

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