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1. Materials Today Volume 20, Number 4 May 2017 RESEARCH
Exploring metal organic frameworks for
energy storage in batteries and
supercapacitors
Guiyin Xu, Ping Nie, Hui Dou*, Bing Ding, Laiyang Li and Xiaogang Zhang*
Jiangsu Key Laboratory of Material and Technology for Energy Conversion, College of Material Science and Engineering, Nanjing University of Aeronautics and
Astronautics, Nanjing, 210016, PR China
High energy density batteries and high power density supercapacitors have attracted much attention
because they are crucial to the power supply of future portable electronic devices, electric automobiles,
unmanned aerial vehicles, etc. The electrode materials are key components for batteries and
supercapacitors, which influence the practical energy and power density. Metal-organic frameworks
possessing unique morphology, high specific surface area, functional linkers, and metal sites are
excellent electrode materials for electrochemical energy storage devices. Herein, we review and comment
on recent progress in metal-organic framework-based lithium-ion batteries, sodium-ion batteries, lithium-
air batteries, lithium-sulfur/selenium batteries, and supercapacitors. Future perspectives and directions of
metal-organic framework-based electrochemical energy storage devices are put forward on the basis of
theoretical knowledge from the reported literature and our experimental experience.
Introduction
The energy crisis has gradually become a critical problem that
hinders the social development and ultimately threatens human
survival [1,2]. Electrochemical energy storage has attracted much
interest because of its high energy efficiency and clean power
systems [3–5]. Batteries and supercapacitors are the most impor-
tant electrochemical energy storage devices [6–9]. Lithium-ion
batteries (LIBs) with high energy density and low weight are widely
used in mobile phones, computers, portable electronic devices,
and environmentally friendly electric or hybrid electric vehicles
[10–14]. Sodium-ion batteries (SIBs) have similar chemistry to LIBs
[15]. More importantly, sodium sources are abundant in nature.
Therefore, SIBs have been regarded as a low-cost alternative battery
technology for promising electrical power systems [16–20]. More-
over, supercapacitors have been paid significant attention and are
widely applied in electric vehicles and aerospace systems due to their
high power density, long cycle life, and competitive price [21–23].
Electrode materials are critical and have become an
active research area to further develop the energy storage devices
mentioned above [24–26]. LIBs and SIBs absorb and release energy
by intercalation/deintercalation chemistry. Supercapacitors oper-
ate on the mechanism of adsorption and desorption of ions in an
electrolyte. Therefore, the suitable electrode materials for LIBs,
SIBs, and supercapacitors should have a high surface area, excel-
lent electrical conductivity, and tailored pore size. Recently, metal-
organic frameworks (MOFs) have been a research hotspot because
of their controllable morphology, abundant pores, high specific
surface area, and multifunctionalities [27–35]. MOFs are composed
of metal sites and organic linkers [36]. The metal sites can be ions
of transition metals, alkaline earth metals, or lanthanides. The
organic linkers are typically multidentate molecules with N- or O-
donor atoms (pyridyl, polyamines, carboxylates, and so on). MOFs
with high specific surface area and low density have been claimed
to be promising electrode materials for next-generation recharge-
able batteries and supercapacitors (Fig. 1) [37–40]. Batteries have a
high energy density and suffer from a low power density. Con-
versely, supercapacitors have a high power density and suffer from
a low energy density. MOFs can be customized in accordance with
their final application; for example, choosing specific metal sites
and tailoring their pore sizes. The metal sites and functional
linkers in MOFs have chemical interactions with polysulfides
RESEARCH:
Review
*Corresponding authors:. Dou, H. (dh_msc@nuaa.edu.cn),
Zhang, X. (azhangxg@nuaa.edu.cn)
1369-7021/ß 2016 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). http://dx.doi.org/10.1016/
j.mattod.2016.10.003 191

2. [41], thus improving the cycling performance for lithium–sulfur
(Li-S) batteries. Moreover, both organic linkers and metal centers
in MOFs demonstrate catalytic activity, signifying that MOFs are
superior electrode materials for high energy density lithium–air
(Li-O2) batteries [42].
To deeply understand the distinctive mechanism between the
morphology, specific surface area, functional linkers, and metal
sites in MOFs and their electrochemical performance, we review
the recent progresses of MOFs and their derivatives in the
development of LIBs, SIBs, Li-S/Se batteries, Li-O2 batteries,
and supercapacitors, and discuss possible strategies for improv-
ing the energy density, power density, and electrochemical
stability for these energy storage devices.
MOFs in advanced battery technologies
LIBs
MOFs for LIBs
LIBs have three essential components: a cathode electrode, an
anode electrode, and an electrolyte. Lithium ions are deinterca-
lated from the cathode electrode when LIBs are charging. Then,
RESEARCH Materials Today Volume 20, Number 4 May 2017
FIGURE 1
Overview of MOFs in electrochemical energy storage applications. Reproduced with permission from Refs. [37–41]. Copyright Royal Society of Chemistry
(2013, 2015), Wiley-VCH (2013, 2014) and American Chemical Society (2014).
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3. the deintercalated lithium ions move through the electrolyte and
intercalate to the anode electrode. During this process, LIBs obtain
and store energy. When LIBs are discharging, the lithium ions
move back to the cathode electrode; LIBs release the stored energy
during this process. Several studies have been focused on cathode
and anode materials. The conventional electrode materials suffer
from a complicated synthesis process, limited energy/power den-
sity, and short cycle life. MOFs are promising electrode materials
for LIBs because of their unique structure, high specific surface
area, well-developed porosity, and high lithium storage capacity
[43]. Metal cations act as active sites in MOFs for redox reactions,
and open crystal frameworks support effective and reversible
insertion/extraction for ions [44,45]. de Combarieu et al. [46]
reported FeIII
(OH)0.8F0.2[O2C-C6H4-CO2] (MIL-53(Fe)) as a cathode
material to reversibly insert Li+
. The experimental results showed
that up to 0.6 Li+
per Fe3+
could intercalate into MIL-53(Fe) at C/40
with no structural alteration, showing that MOFs are excellent
cathode materials for LIBs.
Except for cathode materials, MOFs can be used as anode
materials for LIBs. Maiti et al. [47] synthesized Mn-1,3,5-benzene-
tricarboxylate MOFs (Mn-BTC MOFs) by a simple solvothermal
strategy. The COO– groups in Mn-BTC MOFs play an important
role for the Li+
insertion/extraction (Fig. 2a). The obtained high
specific capacity of 694 mAh g1
and approximately 83% capacity
retention over 100 cycles at 103 mA g1
demonstrated an out-
standing electrochemical performance of Mn-BTC MOFs
(Fig. 2b,c). Lin et al. [48] presented a hydrophobic and polar
functionalized MOF (BMOF), which showed excellent thermal
and chemical stability. They demonstrated that lithium ions were
primarily stored by pores in BMOF. Nitrogen atoms in the amine
groups of BMOF also contributed to the high specific capacity.
Therefore, the specific capacity of BMOF could be further en-
hanced by increasing the specific surface area, pore volume, and
active nitrogen-rich functionality content. An et al. [49] obtained a
flexible layered nickel-based MOF (Ni-Me4bpz: C20H24Cl2N8Ni) by
a solvothermal method with 3,30
,5,50
-tetramethyl-4,40
-bipyrazole
(H2Me4bpz) and NiCl26H2O. The rotation of adjacent pyrazolate
rings is flexible for a framework with a lower energy state and
repeated quadrilateral units (Fig. 3a,b). Moreover, an sql topology
is gained due to the vertex and edge effect from the H2Me4bpz
ligands and the Ni(II) ion (Fig. 3c). Owing to its two-dimensional
characteristics, layered structure, flexibility, and high stability, Ni-
Me4bpz delivered an initial specific capacity of 320 mAh g1
at
50 mA g1
and maintained at 120 mAh g1
over 100 cycles. Han
et al. [50] reported the preparation of metal-1,4,5,8-naphthalene-
tetracarboxylates (Metal-NTC, Metal = Li and/or Ni) by a hydro-
thermal reaction and subsequently annealing process. Li/Ni-NTC
exhibited initial discharge and charge capacities of 1084 and
601 mAh g1
, which decreased to 482 and 475 mAh g1
over 80
cycles, respectively (Fig. 3d). The electrochemical performance
indicated that Li/Ni-NTC interfused the advantages of Li-NTC
and Ni-NTC, thereby exhibiting a good cyclical stability. Our
group demonstrated that M3
II
[CoIII
(CN)6]2nH2O (M = Co, Mn)
could be applied as anode materials [51]. The Co3[Co(CN)6]2
electrode material showed good electrochemical activities with
a reversible capacity of 299.1 mAh g1
. Surprisingly, the capacity
retention was up to 34% when the current density was increased
from 20 to 2000 mA g1
. The predominant rate capability was due
to the small particle size and fast lithium ion transportation
through large passages in the open skeleton.
Many MOFs are unstable in atmosphere and moisture [52]. The
chemical stability of MOFs suffers from more severe challenges in
the complex electrochemical environment of LIBs. Therefore, the
thermal, chemical, and structural stabilities of MOFs are in high
demand, which are critical to the cycling performance of LIBs in
practical applications. In addition, the electrical conductivity of
MOFs is important to the practical specific capacity and rate
performance. Thus, MOFs with high electrical conductivity and
stability are desirable electrode materials for LIBs.
Materials Today Volume 20, Number 4 May 2017 RESEARCH
FIGURE 2
(a) Probable Li+
insertion/extraction sites for coordination with Li in the
organic fraction of Mn-BTC MOF. Electrochemical performance of Mn-BTC
MOF, (b) galvanostatic discharge/charge profiles, and (c) cycling
performance. Reproduced with permission from Ref. [47]. Copyright
American Chemical Society (2015).
FIGURE 3
Crystal structure of MOF (Ni-Me4bpz): (a) coordination mode, (b) planar
structure, (c) sql topological structure. Ni: red, Cl: green, N: blue, C: black.
(d) Cycling performance of Li-NTC, Ni-NTC, and Li/Ni-NTC, the inset is the
voltage profile of the samples in the initial cycle. Reproduced with
permission from Refs. [49,50]. Copyright Elsevier (2012, 2015).
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4. MOF-derived oxides for LIBs
Oxides have been considered as attractive electrode materials
because of their high theoretical capacity and intrinsic safety
[53,54]. MOF-derived oxides show high specific capacity and long
cycle life for LIBs [55]. Porous Co3O4 nanocages, which were
obtained by sintering Prussian blue analog (PBA, Co3[Co(CN)6]2),
exhibited a high and stable capacity of 1465 mAh g1
over
50 cycles at 300 mA g1
[56]. The superior electrochemical perfor-
mance was attributed to the small size, porous shell, and high
surface area of porous Co3O4 nanocages. Fang et al. [57] proposed
a facile liquid-phase deposition method to prepare porous metal
oxide nanosheets on 3D substrates using MOF precursors.
Benefiting from high specific surface area, porous nanosheet
structure and highly electro-conductive 3D substrates, the
Co3O4-3D nickel foam hybrid exhibited a high-rate capability
and long-term cyclic stability for LIBs. Fe2O3 is a prospective
anode material because of its high theoretical capacity
(1000 mAh g1
), nontoxicity, and low treatment cost. Zhang
et al. [58] reported Fe2O3 microboxes with hierarchically struc-
tured shells, which was synthesized by annealing Prussian blue
(PB) microcubes. The Fe2O3 microboxes exhibited a high specific
capacity of 950 mAh g1
at 200 mA g1
. Xu et al. [59] synthesized
spindle-like porous a-Fe2O3 from Fe3O(H2O)2Cl(BDC)3nH2O
(MIL-88-Fe). At 0.2 C, the specific capacity of a-Fe2O3 could
maintain up to 911 mAh g1
over 50 cycles. Even at 10 C, a-
Fe2O3 obtained a comparable capacity of 424 mAh g1
. There-
fore, MOFs are promising precursors to obtain high-performance
Fe2O3-based electrode materials. CuO has been widely applied in
various fields because it is abundant, inexpensive, and eco-friend-
ly. Nanostructured CuO with pyramidal shape was achieved by
pyrolysis of MOF-199 and exhibited a high initial capacity of
1208 mAh g1
[60]. Moreover, it showed a good cycle perfor-
mance for the tested 40 cycles and the Columbic efficiency
was up to 99%, clearly demonstrating that Cu-based MOF-derived
CuO had excellent electrochemical ability.
However, the inherent poor electrical conductivity and large
volume expansion of oxides led to the poor rate performance and
pulverization during repeated cycling. MOFs can serve as sacrificial
framework precursors to synthesize porous transition metal oxides
embedded in porous carbon matrices for enhancing the electro-
chemical performance of electrode materials [61,62]. Li et al. [63]
reported a simple and cost-effective method to synthesize micro-
cuboid-like carbon-decorated iron oxide (C-Fe3O4) from Fe-MOFs
precursors. C-Fe3O4 showed a high reversible capacity of
975 mAh g1
over 50 cycles at 100 mA g1
and an excellent rate
performance with high capacities of 1124, 1042, 886, and
695 mAh g1
at different current densities of 100, 200, 500, and
1000 mA g1
, respectively. Yang et al. [64] reported ZnO quantum
dots@hierarchically porous carbon, which was obtained through a
one-step controlled pyrolysis of isoreticular MOF-1. ZnO quantum
dots@hierarchically porous carbon showed a high specific capacity
of approximately 1200 mAh g1
, stability for 50 cycles with nearly
100% capacity retention at 75 mA g1
, and prominent rate perfor-
mance of approximately 400 mAh g1
at 3750 mA g1
. The out-
standing electrochemical performance and novel and facile
synthesis of ZnO quantum dots@hierarchically porous carbon
provided a prospective insight into its practical application in
future LIBs.
MOF-derived sulfides for LIBs
Sulfides have attracted widespread attention as promising elec-
trode materials for LIBs because of their high theoretical specific
capacity and abundance [65]. However, the low electronic con-
ductivity and the poor structure stability of sulfides during the
charge/discharge process restrict their practical specific capacity
and cycling performance. One effective solution is to design novel
porous sulfide composites such as porous sulfide/carbon materials.
The porous structure can accommodate the volume change during
the lithiation/delithiation process and improve the contact area
between the electrode material and electrolyte for rapid ion diffu-
sion. The carbon materials can improve the structural integrity
and increase the electronic conductivity of electrode materials.
Reduced graphene oxide (rGO)@CoSx and CoSx–rGO–CoSx compo-
sites were obtained by a thermal sulfurization of MOFs/graphene
oxide (GO) precursors [66]. These composites showed high specific
capacities, excellent rate capabilities, and long cycle life for LIBs,
demonstrating that MOFs are promising precursors for high-perfor-
mance sulfides.
In addition, porous carbon materials have been widely applied
to sustainable energy and alternative clean technologies. Porous
carbon materials with a particular morphology, large surface area,
unique pore size distribution, and prominent properties can be
obtained through a thermal carbonization of MOFs in inert atmo-
spheres and have provided prospective applications in LIBs [67–
73]. Therefore, the preparation of MOFs and MOF-derived materi-
als is a very attractive technology to synthesize electrode materials
for next-generation high-energy LIBs. Importantly, the reaction
mechanism, that is, the interaction effect between the novel
structure and specific property of these materials should be thor-
oughly explained for LIBs.
SIBs
MOFs for SIBs
Because of the extensive distribution and low cost of sodium
resources, SIBs are competitive candidates to LIBs and one of
the most promising candidates for next-generation energy storage
systems [74,75]. The working principle and components of SIBs are
similar to LIBs. By analogy with LIBs, a variety of materials posses-
sing good lithium storage have also been explored for sodium
storage applications. Compared with lithium ion insertion mate-
rials, the exploration of sodium-ion insertion materials is limited,
owing to the poor kinetics of the sodium-ion insertion/extraction
caused by the larger radius (102 pm) and heavier atomic mass of
Na+
than those of Li+
[76–78]. Na+
diffusion needs a larger tunnel
size. Organic compounds possess a structural multiplicity and
flexibility compared to inorganic compounds [79]. Therefore,
organic compounds are deemed promising candidates for SIBs.
MOFs are assembled by linking metal ions and desirable organic
ligands; thus, they are ideal electrode materials for SIBs. Wessells
et al. [80] reported the bulk PBA nickel hexacyanoferrate nanopar-
ticle for the insertion/extraction of Na+
and K+
operated in safe and
low-cost aqueous electrolytes. The open-framework electrode pos-
sesses an excellent rate capability, high round trip energy efficien-
cy, and long cycle life. A proportion of 67% of its maximum low
rate capacity is retained at a high current density of 41.7 C and no
capacity fades over 5000 cycles at 8.3 C in aqueous sodium elec-
trolyte. Moreover, copper hexacyanoferrate exhibited ultralong
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5. cycle life and high power performance in aqueous batteries.
Around 83% of the original capacity could be delivered after
40,000 cycles at 17 C, with a high capacity retention at 83 C
(67% of capacity compared with that of 0.83 C) [81]. Wu et al.
[82] reported vacancy-free NaxCoFe(CN)6 (NaCoHCF) nanocrys-
tals by controlling crystallization strategies and studied the Na
storage properties in aqueous electrolyte. Benefiting from its ex-
cellent crystal and lattice integrity, the obtained material certified
a high capacity of 130 mAh g1
corresponding to invertible inser-
tion reactions of 1.7 Na+
per NaCoHCF. Furthermore, NaCoHCF
showed an outstanding capacity retention of 90% after 800 cycles,
indicating a feasible way to significantly strengthen the cyclability
and Na storage capacity of PBA. The superior performance, scalable
production, and low-cost aqueous electrolyte make PBA attractive
candidates for large-scale energy storage applications [83].
However, operating in aqueous electrolytes limits the stable
battery voltage to only 1.5 V due to the electrochemically stable
window of water. Various PB and its analogs (KMFe(CN)6, M = Fe,
Mn, Ni, Cu, Co and Zn), and the inexpensive NaxMnFe(CN)6
cathode have been demonstrated as new cathode materials for
SIBs with the saturated NaClO4 in 1:1 EC/DEC (vol:vol) electrolyte
[84,85]. Wang et al. [86] reported an iron-oxalato open 3D frame-
work (K4Na2[Fe(C2O4)2]32H2O) synthesized by a hydrothermal
process (Fig. 4a,b). K4Na2[Fe(C2O4)2]32H2O showed a reversible
Na+
intercalation/extraction, which had a one-dimensional un-
seal channel in the oxalato-bridged structure supplying ion
approachability up to two Na+
in each formula unit (Fig. 4c).
The discovery of a new species of ion intercalation material is
highly pleasing because of its significant prospects for applica-
tions in SIBs.
Materials Today Volume 20, Number 4 May 2017 RESEARCH
FIGURE 4
Crystal structure of K4Na2[Fe(C2O4)2]32H2O, with iron atoms in blue, carbon atoms in gray, oxygen atoms in red. (a) Coordination environment around Fe.
(b) 3D iron-oxalato framework. Non-bridging atoms, such as potassium and sodium, are omitted for clarity. (c) Crystal structure of K4Na2[Fe(C2O4)2]32H2O,
with iron atoms in blue, carbon atoms in gray, oxygen atoms in red, sodium atoms in yellow, and potassium atoms in purple. 1D open channel for K1 (channel 1)
and 1D closed channel for Na/K2 (channel 2) along the (c) direction are shown. Reproduced with permission from Ref. [86]. Copyright Wiley-VCH (2015).
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6. MOF-derived metal oxides for SIBs
Metal oxides as anodes for LIBs suffer from low electronic conduc-
tivity and poor structure stability in conversion reactions. The
radius of sodium ions (0.102 nm) is larger than that of lithium ions
(0.059 nm). Therefore, SIBs have much lower kinetics and more
severe volume change during the charge/discharge process. Tran-
sition metal oxides such as Fe2O3 [87], Co3O4 [88], and CuO [89]
have been used as anodes for SIBs. With similar strategies for
LIBs, porous nanostructure engineering and carbon modifica-
tions are a typical treatments to improve the conversion reac-
tion kinetics, increase the electronic conductivity, and
accommodate the volume change. Metal oxides have been
prepared using MOF precursors for SIBs. A hollow ball-in-ball
nanostructure NiO/Ni/graphene was synthesized from Ni-based
metal-organic frameworks (Ni-MOFs) [90]. SIBs using the
NiO/Ni/graphene anode showed a good rate performance
(207 mAh g1
at 2 A g1
) and stable cycle life (0.2% specific
capacity fading per cycle). The unique hierarchical hollow
ball-in-ball structure in NiO/Ni/graphene could accommodate
the volume change of active materials during repeated cycles.
Moreover, the highly conductive graphene not only facilitated
the fast electron transfer, but also promoted the formation of a
stable solid electrolyte interface (SEI) film.
MOF-derived carbon materials for SIBs
Mesoporous carbon obtained by a nanocasting strategy that
applies silica as template significantly enhances the kinetics and
performance for Na+
storage [91]. The direct pyrolysis of MOFs was
found to be an easy and effective method to synthesize porous
carbon with a narrow pore size distribution and high surface area
[67]. Qu et al. [92] synthesized microporous carbon (ZIF-C) with a
well-proportioned pore size of 0.5 nm by the pyrolysis of 2-methy-
limidazole zinc salt (ZIF-8). ZIF-C showed comparatively higher
capacity and more preferable reversibility than CMK-3. Further-
more, the microporous ZIF-C could dramatically reduce the irre-
versible capacity loss at the initial cycle and strengthen the
reversible storage of Na+
after subsequent cycles. The template-
free synthesis and excellent electrochemical performance of ZIF-C
show that MOF-derived carbon materials are promising anodes for
SIBs.
MOFs have been widely applied in new-generation SIBs. How-
ever, it is worth noting that the new electrochemical reaction
process, energy storage mechanism, and interaction effect be-
tween the novel structure and Na+
storage performance must be
clearly understood. With regard to MOF-derived carbon mate-
rials, the influence of porous structure, specific area, pore
volume, and heteroatom doping on the Na+
storage capability
should be thoroughly studied, serving as the basis for designing
excellent electrode materials and further promoting the devel-
opment for SIBs. The LiCoO2–graphite LIBs can deliver a spe-
cific energy of 491 Wh L1
and has been widely applied in
electric vehicles [93]. The specific energy can be further im-
proved by silicon hybridization into the graphite anode.
Although the above nanomaterials exhibit a good electrochem-
ical performance for LIBs and SIBs in laboratory tests, some
nanomaterials are difficult to produce on a large scale. There-
fore, the overall synthesis process should be simple and easy to
handle.
Li-O2 batteries
Li-O2 batteries are based on an oxygen cathode and a metal lithium
anode. The metal lithium releases lithium ions. Then, the lithium
ions move through the electrolyte to the cathode, react with
oxygen, and transform into Li2O and Li2O2 in the oxygen cathode
[94]. Li-O2 batteries are attractive because they can theoretically
store a high gravimetric energy density of 11,140 Wh kg1
, meet-
ing the requirements of future electric vehicle applications [95–
99]. However, Li-O2 batteries still face numerous challenges in
their practical applications, including (1) the extremely sluggish
kinetics of the oxygen reduction reaction (ORR) and oxygen
evolution reaction (OER) which not only increase the overpoten-
tial, but also influence the overall performance of Li-O2 batteries
[100–102] and (2) the insoluble discharge products (Li2O2) and the
unavoidable formation of side products (Li2CO3) deposited on the
cathode gradually block the catalytic sites as well as the diffusion
pathways of oxygen and electrolyte, inevitably leading to a tre-
mendous overpotential [103,104]. Therefore, the specific capacity,
rate capability, and cyclic stability of Li-O2 batteries are limited. In
response, it is highly desirable to rationally explore optimal cath-
ode catalysts involving porous structure, high activity and stability
to mitigate the sluggish ORR/OER kinetics and the cathode chan-
nel blockage, thereby increasing the electrochemical performance
of Li-O2 batteries [105–108].
MOFs for Li-O2 batteries
Because of the high surface area, accessible metal sites, and flexible
structure, MOFs are good catalysts to enhance the cycling perfor-
mance of Li-O2 batteries [109–112]. Wu et al. [110] demonstrated
that MOFs could increase the O2 concentration in the micropores,
which was 18 times higher than the concentration of pure oxygen
at 273 K under ambient pressure (Fig. 5a). The Mn-MOF-74/Super P
cathode exhibited a capacity of 9420 mAh g1
under 1 atm of O2 at
room temperature, which was more than four times higher than
the capacity of the Super P cathode (Fig. 5b). The superior elec-
trochemical performance for Li-O2 batteries was attributed to the
high surface area and open metal sites of Mn-MOF-74, which
enhanced the concentration of O2 molecules in the micropores.
More importantly, hollow MOFs and heterometallic MOFs have
attracted extensive attention, especially in catalytic reactions in-
volving gases [111,113,114]. Zhang et al. [111] synthesized hierar-
chical Zn/Ni-MOF-2 nanosheet assembled hollow nanocubes
through a facile solvothermal process without any surfactant
(Fig. 6). The hollow MOFs have a high surface area, efficient
catalytic active sites, gas absorption/storage properties, and large
void space to accommodate the discharge products. Taking ad-
vantage of these outstanding inherent properties, hollow MOFs
can serve as a highly efficient electrocatalyst for Li-O2 batteries. In
particular, when active metal nanoparticles are incorporated in
the pores, these hollow MOFs supply numerous catalytically active
sites, improving the overall performance of Li-O2 batteries. The
introduced structural defects in MOFs have significant effects on
the catalytic activity [115]. Therefore, this also offers novel oppor-
tunities for improving the catalytic performance of MOFs in Li-O2
batteries.
Currently, most studies on Li-O2 batteries have been conducted
under a pure oxygen environment. There are enormous difficulties
that hinder the practical application of Li-O2 batteries in the
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7. ambient environment. CO2 and the moisture from the atmo-
sphere lead to serious parasitic reactions, thereby degrading the
overall performance of Li-O2 batteries [116,117]. A promising
method to deal with this problem is to incorporate an oxygen
selective membrane that allows only oxygen diffusion and pre-
vents CO2 and moisture penetration. Cao et al. [118] reported a
novel mixed matrix membrane synthesized by incorporating poly-
dopamine-coated MOF crystals of CAU-1-NH2 into a polymethyl-
methacrylate (PMMA) polymer. This membrane exhibited a good
performance under a real ambient atmosphere with 30% relative
humidity as an oxygen selective membrane for the cathode of Li-
O2 batteries. Therefore, MOFs can offer an unprecedented oppor-
tunity as oxygen selective membranes for the practical application
of Li-O2 batteries because of their tunable pore size and ligands
[119–121].
MOF-derived materials for Li-O2 batteries
MOF derivatives include metal oxides (or mixed metal oxides),
carbon materials, metal oxide/carbon composites, and sulfides.
Metal oxides/sulfides derived from MOFs exhibit a large surface
area, hierarchical porous structure, and efficient channels for mass
transport, providing an insight into optimizing the intrinsic catal-
ysis for Li-O2 batteries [112,122,123]. Zhang and his group syn-
thesized porous spinel-type Co–Mn–O nanocubes from MOFs and
employed them as cathode catalysts for Li-O2 batteries [112]. The
as-prepared oxygen electrode of Li-O2 batteries exhibited a high
catalytic activity, low overpotential, and improved cycle life with a
limited capacity of 500 mAh g1
up to 100 cycles. Recently, low-
cost sulfides have been used as catalysts for Li-O2 batteries. The
mesoporous two-dimensional cobalt sulfide nanosheets were ap-
plied as a bifunctional electrocatalyst for rechargeable Li-O2 bat-
teries and showed an initial discharge capacity of approximately
5917 mAh g1
with a high reversibility of 95.72% [123]. MOF-
derived sulfides exhibit a large surface area and optimally hierar-
chical porous structure, holding great potential for high-perfor-
mance Li-O2 batteries.
The low conductivity of metal oxides derived from MOFs limits
their electrochemical performance. Further research indicated
that MOFs with very high void volume filled by well-dispersed
nitrogen/carbon precursors may be beneficial for synthesizing
efficient ORR and OER catalysts, thereby significantly increasing
the amount of catalyst active site and electronic conductivity. Li
et al. [124] depicted a new approach to produce nitrogen-doped
graphene/graphene tube composites using MOFs as templates
(Fig. 7). The graphene/graphene tube-rich MOF template nitrogen
iron catalysts (N-Fe-MOF) for Li-O2 batteries exhibited superior
ORR and OER activity than traditional carbon black. Similarly, Yin
et al. [125] fabricated hierarchical porous ZnO/ZnFe2O4/C (ZZFC)
nanocages from MOF-based nanocage templates, delivering the
first discharge capacity exceeding 11,000 mAh g1
at 300 mA g1
and an improved cyclability for 15 cycles with a capacity of
5000 mAh g1
. The superior electrochemical performance is as-
cribed to the hollow MOF-based nanocage structure, mesoporous
nanocage walls, and high electronic conductivity.
In summary, MOFs and MOF-derived materials not only act as
highly efficient nonprecious metal cathode catalysts for Li-O2
batteries, but also work as oxygen selective membranes for the
practical application of Li-O2 batteries in the ambient environ-
ment. MOFs with optimized structures should be designed to
provide uniformly dispersed catalytic active sites, large surface
area, and high electronic and ionic conductivity. The combination
of these outstanding properties is beneficial to enhance the elec-
trocatalytic performance of Li-O2 batteries. Moreover, bimetallic
MOFs show a unique structure and catalytic performance, which
are potential catalysts for Li-O2 batteries.
Li-S batteries
Recently, Li-S batteries are one of the most promising candidates
for energy storage systems since a sulfur cathode has a much
higher theoretical capacity (1675 mAh g1
) than traditional cath-
ode materials used in LIBs [96]. Moreover, sulfur is naturally
abundant, cheap, and environmentally friendly. Li-S batteries
are based on the metal lithium anode and the sulfur cathode.
The working principle of Li-S batteries is [126]:
S8 þ 2e
! S8
2
(1)
S8
2
$ S6
2
þ
1
4
S8 (2)
2S6
2
þ 2e
$ 3S4
2
(3)
3S4
2
þ 2e
$ 4S3
2
(4)
2S3
2
þ 6Liþ
þ 2e
$ 3Li2S2 (5)
Li2S2 þ 2Liþ
þ 2e
$ 2Li2S (6)
Materials Today Volume 20, Number 4 May 2017 RESEARCH
FIGURE 5
(a) Schematic illustration of a Li-O2 cell using MOF-Super P composite as
the O2 electrode. Relative sizes of oxygen molecules are reduced for clarity.
(b) Discharge profiles of the Li-O2 cells using MOF-Super P composites or
Super P only under O2 atmosphere with 50 mA g1
at room temperature.
Reproduced with permission from Ref. [110]. Copyright Wiley-VCH (2014).
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8. Despite these advantages, the commercialization of Li-S batter-
ies has thus far been limited. First, the electrically insulating nature
of S and Li2S2/Li2S inevitably leads to a low active material utiliza-
tion. Second, long-chain lithium polysulfide intermediates Li2Sn
(3 n 8, Eqs. (1)–(4)) readily diffuse into organic liquid electro-
lyte, leading to rapid capacity fading and low Coulombic efficiency
for Li-S batteries [127]. There are techniques tackling those pro-
blems: embedding sulfur into porous carbon materials by the
physical absorption [128], strengthening sulfur species with metal
oxide additives through chemical adsorption [129–131], and coat-
ing sulfur with conductive polymers by physical blocking [132].
MOFs for Li-S batteries
Porous MOF materials with high specific surface area and pore
volume are suitable sulfur hosts [133]. In 2011, Tarascon et al.
[134] first proposed the use of MOFs as sulfur host materials for Li-S
batteries. A remarkable increase in capacity retention of Li-S
batteries was presented using MIL-100(Cr) with a high specific
surface area (1485 m2
g1
). X-ray photoelectron spectroscopy
(XPS) demonstrated a weak binding between the oxygenated
framework and lithium polysulfides. Subsequently, increasing
attention has been paid to using MOFs to encapsulate sulfur. Qian
et al. applied two types of MOFs into Li-S batteries: HKUST-1 [135]
and ZIF-8 [136]. The suitable pore space and open Cu2+
site ensured
a strong sulfur confinement in HKUST-1. The HKUST-1S com-
posite with 40 wt% sulfur content showed a stable cycle life with
the capacity of 500 mAh g1
after 170 cycles. The mechanism of
MOFs in sulfur storage was further investigated in detail. Xiao et al.
[41] showed that a novel Ni-MOF, Ni6(BTB)4(BP)3 (BTB = benzene-
1,3,5-tribenzoate and BP = 4,40
-bipyridyl), could be adopted as a
suitable sulfur host. The sulfur composite showed a high capacity
retention up to nearly 89% after 100 cycles at 0.1 C. It was found
that the synergistic effects of the mesopores (2.8 nm) and micro-
pores (1.4 nm) in Ni-MOF (Fig. 8a) and the strong interactions
between the polysulfide base and Lewis acidic Ni(II) center signifi-
cantly inhibited the migration of soluble polysulfides out of the
RESEARCH Materials Today Volume 20, Number 4 May 2017
FIGURE 6
(a) Schematic illustration of crystal structure transformation of MOF combined with its shape evolution. (b) SEM, (c) TEM, (d) energy-dispersive X-ray
spectroscopy (EDX) elemental mapping of hierarchical Zn/Ni-MOF-2 NAHNs. Reproduced with permission from Ref. [111]. Copyright Wiley-VCH (2014).
198
RESEARCH:
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9. pores (Fig. 8b). Theoretical calculations also evidenced that the
strong coordination force occurred on soluble Li2S8/Li2S6/Li2S4.
The Lewis acid–base interactions proposed in this study provided a
new insight to develop Li-S battery system. Further, Li et al. [137]
investigated the electrochemical performance of sulfur composites
with different MOFs. They found that the small size of the MOF
hosts tended to increase sulfur utilization, and small apertures
with functionalities in the open framework had affinity with the
polysulfide anions through Lewis acid–base interactions, leading
to a stable cycling ability (Fig. 8c). They also showed that a higher
capacity was achieved with the decrease in ZIF-8 particle size
(20 nm: 950 mAh g1
at 0.5 C), while the best cycling perfor-
mance was obtained when the particle size was approximately
200 nm (75% over 250 cycles at 0.5 C) [138].
In addition, some researchers took advantages of abundant
micropores and mesopores in MOFs and good electronic conduc-
tivity of carbon materials to synthesize hybrid nanocomposites as
sulfur hosts to enhance the electrochemical performance of Li-S
batteries. Zhang et al. [139] reported the preparation of an MIL-
101(Cr)@rGO composite and used it as a sulfur host. MIL-101(Cr)
could exhibit open channels or cavities from the microscale to
mesoscale. Graphene sheets deposited on the surface of the
MIL-101(Cr) by the in situ chemical oxidation polymerization
effectively improved the electronic conductivity of MIL-101(Cr).
Chen et al. [140] incorporated sulfur into MIL-101(Cr) and gra-
phene sheets (GNS) were then wrapped on the MIL-101(Cr)/S
composite in order to build a conductive bridge for electron
transfer as well as a physical barrier to slow down the dissolution
of polysulfides. The prepared GNS-MIL-101(Cr)/S composite cath-
ode presented a good capture ability of lithium polysulfides and
high electron conductivity.
In conclusion, the electron conductivity of MOFs that are used
as the sulfur host needs to be increased to improve the electro-
chemical performance of Li-S batteries. The interaction mecha-
nism between MOFs and lithium polysulfides also needs to be
further clarified. S–O and S–M bonds could exist between the MOFs
and lithium polysulfides (Fig. 9) [41,131]. These bonds are condu-
cive to stabilize lithium polysulfides and enhance the cycling
performance of Li-S batteries. Therefore, MOFs are potential sulfur
hosts for high-capacity and long-life Li-S batteries.
MOF-derived carbon materials for Li-S batteries
Because of the low electron conductivity of MOF materials, the
sulfur content in MOFs/S composite is usually very low (50%)
and large amount of conductive agents are needed for fabricating
sulfur cathodes. As mentioned above, MOFs can be pyrolyzed
Materials Today Volume 20, Number 4 May 2017 RESEARCH
FIGURE 7
(a)–(d) Representative TEM images of the N-Fe-MOF catalyst prepared at 1000 8C with dominant graphene (green arrows) and graphene tube (red arrows)
nanostructures. (e) A typical graphene tube. (f) The open mouth of a graphene tube is marked by a yellow arrow. Reproduced with permission from Ref.
[124]. Copyright Wiley-VCH (2013).
199
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10. RESEARCH Materials Today Volume 20, Number 4 May 2017
FIGURE 8
(a) Crystal structure of Ni-MOF containing two different types of pores represented by dark yellow sphere and blue sphere: mesopore (yellow sphere
indicates pore volume; gray: C; red: O; green: Ni; blue: N); micropore (blue sphere indicates the pore volume). (b) Cycling performance of Ni-MOF/S@155 at
different rates (inset: the interaction between Ni-MOF and sulfur species). Reproduced with permission from Ref. [41]. Copyright American Chemical Society
(2014). (c) SEM image of ZIF-8, the schematic illustration of the interaction between ZIF-8 and sulfur species, and cycling performance of S/MOFs.
Reproduced with permission from Ref. [137]. Copyright Royal Society of Chemistry (2014).
200
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11. under an inert atmosphere to synthesize hierarchical carbons. The
resulting carbonized products still exhibit micro-, meso-, and
macropores with high specific surface areas and large pore
volumes. The porous MOF-derived carbon materials, which are
directly applied as sulfur hosts, not only improve the conductivity
of the composite, but also suppress the dissolution of lithium
polysulfides through channel adsorption.
Our group first reported the MOF-derived carbon material as the
sulfur host for Li-S batteries [141]. The hierarchically porous
carbon nanoplates (HPCN) were synthesized through one-step
pyrolysis from MOF-5 with an average thickness of approximately
50 nm, high specific surface area of 1645 m2
g1
, and large pore
volume of 1.18 cm3
g1
. Sulfur embedded in HPCN exhibited a
high specific capacity and excellent cycling performance
(Fig. 10a). The HPCN/S composite delivered a discharge capacity
of 730 mAh g1
after 50 cycles at 0.5 C. The conductive carbon
framework in HPCN significantly enhanced the electron transport,
and the porous structure of HPCN effectively suppressed the
diffusion of lithium polysulfides. Moreover, the remaining meso-
pore channels in the HPCN/S composite allow the fast diffusion of
Li+
and sufficient infiltration of the electrolyte. Kumar et al. [142]
prepared different carbon materials with unique hierarchical pore
structures via pyrolysis of four types of zinc-containing MOFs and
found that the pore size distribution and volume of the MOF-
derived carbon materials were of great concern for the electro-
chemical performance of Li-S batteries. Cathode materials based
on carbonized MOFs with higher mesopore volumes (2–50 nm)
exhibited larger initial discharge capacities, whereas carbon hosts
with higher micropore (2 nm) volumes lead to better cycle life.
Lou et al. [143] further explored the MOF-derived carbon materials
and synthesized microporous carbon polyhedrons from ZIF-8 as
conductive hosts to load sulfur for Li-S batteries. The study
revealed that the sulfur content and preparation procedure had
a high impact on Li-S batteries. The composite with sulfur
completely embedded in micropores presented a stable cycle life
in both 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME) and eth-
ylene carbonate/diethyl carbonate (EC/DEC) electrolytes, while
additional sulfur outside the micropores exhibited an inferior
performance as well as a different electrolyte compatibility.
Yin et al. [144] prepared nitrogen-doped carbon with hierar-
chically microporous (0.5 nm) and mesoporous (22 nm) structures
from ZIF-8. They found that small sulfur molecules (S24) were
effectively confined inside micropores (0.5 nm) (Fig. 10b), which
highly increased the electrochemical stability. In addition, nitro-
gen doping could enhance the interaction between carbon and
sulfur species as well as the interface charge transfer kinetics.
Carbonized aluminum metal organic framework (Al-MOF) has
been synthesized [145]. The novel one-dimensional structured
French fries-like porous carbon (FLHPC) could effectively avoid
the mass loss of active sulfur materials and facilitate the e
/Li+
transport (Fig. 10c). The S/FLHPC cathode showed a high initial
discharge capacity of 1206 mAh g1
and discharge capacity of
856 mAh g1
after 100 cycles at 0.1 C. Therefore, MOF-derived
carbon materials are promising candidates as sulfur hosts for Li-S
batteries. These carbon materials with high specific surface area
and hierarchical porous structure could facilitate electric transport
and the electrolyte penetration for sulfur cathodes and effectively
confine sulfur species. Further modifying MOF-derived carbon
materials and controlling the suitable sulfur content are critical
for sulfur cathodes.
In addition, metal oxides can trap lithium polysulfides by polar
surface interactions, or the Lewis acid–base interactions [146,147],
improving the electrochemical performance of Li-S batteries.
MOFs can be designed to various kinds of metal oxides with
tailored metal atom and pore size. One effective role of the
obtained MOF-derived metal oxides is as for conventional metal
oxides: to mitigate the lithium polysulfide dissolution by the
chemical adsorption. The other is the porous structure in MOF-
derived metal oxides, which is beneficial to further confine lithium
polysulfides by the physical adsorption and accommodate the
volume change of sulfur during the charge/discharge process.
Therefore, MOF-derived metal oxides are promising hosts for Li-
S batteries and need to be further studied and developed.
Li-Se batteries
Recently, selenium (Se) has been regarded as one of the most
promising electrode materials because of its similar chemical
properties to those of sulfur. The lithiation/delithiation mecha-
nism is similar to sulfur as follows (Fig. 11) [148]:
2Liþ
þ Se þ 2e
$ Li2Se (7)
Moreover, selenium has a theoretical gravimetric capacity of
678 mAh g1
and a high theoretical volumetric capacity of
3253 mAh cm3
[149,150]. As one type of metalloids, the elec-
tronic conductivity of selenium (1 103
S m1
) is higher than
that of sulfur (5 1030
S m1
); thus, Se can facilitate a fast
electrochemical reaction during the lithiation/delithiation pro-
cesses [151]. However, the Se cathodes also have big challenges
such as the poor cycle performance and low Coulombic efficiency,
which are attributed to the dissolution of high-order polyselenides
[152,153].
The porous carbon–selenium composites can enhance the elec-
tronic conductivity and relieve the dissolution of lithium poly-
selenides. MOFs have been recognized as promising precursors for
porous carbon materials. Zhang et al. [154] synthesized a meso-
porous carbon material (Meso-C) from MOF-5 precursor. Meso-C
had a high electrical conductivity, high specific surface area, and
large pore volume. The Meso-C@Se cathode displayed an initial
discharge capacity of 641 mAh g1
and a reversible capacity of
Materials Today Volume 20, Number 4 May 2017 RESEARCH
FIGURE 9
Illustration of the chemical effect between MOFs and Li2S4.
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12. 306.9 mAh g1
at 0.5 C after 100 cycles. Dai et al. [155] reported a
novel selenium–carbon composite by impregnating Se into ZIF-8
polyhedron-derived microporous carbon polyhedra (MICP). MICP
with interconnected microporous structures showed a high spe-
cific surface area of 890 m2
g1
and a large micropore volume of
0.474 cm3
g1
, which could effectively confine a relatively high
amount of Se, trap the diffusion of polyselenides, and buffer the
large volume expansion during the discharge process. This novel
Se-MICP composite exhibited ultralong cycle life and superior rate
performance.
Yin et al. [156] synthesized a nitrogen-doped layered carbon
sponge (NCS) with abundant mesopores and micropores by car-
bonization of an Al-MOFs precursor under Ar and NH3 flow. NCS
consisting of interconnected rich micropores in carbon layers and
mesopores existing between carbon layers exhibited a large specif-
ic surface area of 1274.5 m2
g1
, a micropore volume of
0.358 cm3
g1
, and a mesopore volume of 0.755 cm3
g1
. The
NCS/Se-50 composite showed a high initial discharge capacity
of 982.6 mAh g1
, excellent cycle performance, and high rate
capability. As shown in Fig. 12, bulk Se are mostly confined in
0.4–0.55 nm micropores of NCS when heated at 260 8C under
vacuum. The 0.4–0.55 nm micropores in the carbon layers could
efficiently immobilize linear selenium species to avoid their loss
and dissolution during cycling. The 5- to 10-nm mesopores be-
tween carbon layers could absorb the electrolyte to ensure a quick
transfer of lithium ions and buffer the large volume changes
during repeated cycles. The nitrogen-doped robust carbon sponge
matrix with a good electric conductivity further facilitated the
charge transfer, leading to a better electrochemical performance.
Therefore, NCS derived from Al-MOFs had a stable electrochemical
performance for Li-Se batteries.
In summary, MOF-derived porous carbon with a high electrical
conductivity, high specific surface area, and large pore volume can
be used to confine selenium. In particular, microporous carbon
materials doped with heteroatoms (N, P, B) are excellent immobi-
lizers as selenium hosts for Li-Se batteries, because micropores can
effectively suppress the dissolution of lithium polyselenides, and
the heteroatoms can further enhance the electrical conductivity
RESEARCH Materials Today Volume 20, Number 4 May 2017
FIGURE 10
(a) Illustration of the HPCN/S composite cathode. Reproduced with permission from Ref. [141]. Copyright Royal Society of Chemistry (2013). (b) Discharge
and charge mechanism of the C–S hybrid cathode. Reproduced with permission from Ref. [144]. Copyright American Chemical Society (2015). (c) Illustration
of the synthesis route for the S/FLHPC composite. Reproduced with permission from Ref. [145]. Copyright Royal Society of Chemistry (2015).
202
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13. and the interaction between carbon and selenium species. Fur-
thermore, MOFs with a high specific surface area, large pore
volume, and metal sites can be used as frameworks to confine
Se, which exhibit not only physical absorption but also chemical
absorption, as shown in Fig. 9, to enhance the electrochemical
performance of Li-Se batteries. Similar to the effects for Li-S bat-
teries, MOF-derived oxides/sulfides show chemical adsorption
from the polar surface interactions and physical adsorption from
the porous structure, trapping lithium polyselenides and accom-
modating the volume change of selenium during the charge/
discharge process. The clear mechanism of the MOF-derived
oxides/sulfides for Li-Se batteries should be further developed and
thoroughly understood. The above studies on Li-O2 batteries, Li-S
batteries, and Li-Se batteries show a high specific energy in laboratory
tests. However, the lithium anode suffers from the lithium dendrite
issue, leading to the serious security problem and hindering their
practical applications. Therefore, metal lithium modification is also
quite important for their practical applications.
MOFs in supercapacitor technologies
MOFs for supercapacitors
Supercapacitors can be divided to two distinct families [157]. One
is electrochemical double-layer capacitors. Electrode materials
adsorb and release ions from the electrolyte during the charging
and discharging processes. The other is pseudocapacitors. Redox
reactions mainly occur on the electrode material surface. MOFs,
which have a high surface area and tailored pore size, are suitable
electrode materials for supercapacitors. Inspired by the pioneering
work of Dı́az et al., the investigation of MOFs as electrode materials
for supercapacitors has been expanded and raised broad interest
[158–160]. Shortly, Han’s group [161,162] also observed a good
pseudocapacitor behavior and charge retention in Co-based MOFs
Materials Today Volume 20, Number 4 May 2017 RESEARCH
FIGURE 11
Illustration of the charge and discharge process of Li-Se batteries.
203
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14. in aqueous electrolytes. A series of different MOF nanocrystals
with multiple organic functionalities, metal ions, and a variety of
structure types have been prepared and examined for the possibil-
ity of use in supercapacitor energy storage application. These
nanocrystals of MOFs showed obvious differences in electrochem-
ical properties [163]. Unfortunately, the conductivity issue causes
an obstacle for their practical application, causing a poor rate
performance and capacity fading over cycles, because of the fact
that MOFs are mostly electrically insulating.
To this end, the tailored design of MOFs with a unique structure
[164–168] and the introduction of additives have thus been made
[169]. Wang et al. [170] developed an effective electrochemical
deposition strategy to overcome the insulating problem of MOFs
by interweaving flexible ZIF-67 crystals/carbon cloth with conduc-
tive polyaniline (PANI) polymer, which could act as bridges for
electron transportation between the external circuit and the in-
ternal surface of MOFs (Fig. 13). The results showed that the
composites exhibited an exceptional areal capacitance of
2146 mF cm2
at 10 mV s1
. Moreover, a remarkable areal capaci-
tance of 35 mF cm2
and a power density of 0.833 W cm3
at
0.05 mA cm2
were achieved with the assembled symmetric flexi-
ble solid-state supercapacitor, which are the highest values among
all MOF-based supercapacitors reported to date.
Because of the high electrical conductivity, surface area, and
outstanding electrochemical stability, rGO has attracted extensive
attention as an electrical double-layer supercapacitor material.
However, the application of MOFs/rGO composites in supercapa-
citors has been scarcely investigated [163,171,172]. Ni-doped
MOF-5 with rGO has recently been synthesized in gram-scale
quantities. Nickel in Ni-doped MOFs was found to engage in a
two-electron, reversible redox reaction shuttling between Ni and
Ni(OH)2 in an alkaline electrolyte, and the addition of a conduc-
tive rGO significantly reduces the charge transfer resistance, lead-
ing to three times larger capacitance than the algebraic sum of
contributions from two components [173]. The direct growth of
Ni-MOFs on the carbon nanotube (CNT) surface has been realized
RESEARCH Materials Today Volume 20, Number 4 May 2017
FIGURE 12
Schematic discharge/charge mechanism of the NCS/Se-50 composite
cathode. Reproduced with permission from Ref. [156]. Copyright Royal
Society of Chemistry (2015).
FIGURE 13
Illustration of the synthesis route for polyaniline-ZIF-67-carbon cloth. Reproduced with permission from Ref. [170]. Copyright American Chemical Society
(2015).
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15. by a solvothermal method for asymmetric supercapacitors, which
presented a high energy density of 36.6 W h kg1
and excellent
cycle life with only 5% loss of the initial capacitance after 5000
deep cycles [174].
PBAs have been explored for grid-scale energy storage applica-
tions because of their easy synthesis, large octahedral interstitial
sites, and open channels for ionic diffusion, which enables fast
and highly reversible alkali-ion intercalation in both aqueous and
organic electrolytes. PBAs were initially studied as potential elec-
trode materials for supercapacitors early in 2008 by Huang’s group
[175], where nanoscale Ni3(Fe(CN)6)2H2O prepared by a co-pre-
cipitation method was investigated in three different electrolytes
(1 M KNO3, 1 M LiNO3, and 1 M NaNO3). A specific capacitance of
574.7 F g1
was obtained at 0.2 A g1
in 1 M KNO3 solution with
an excellent capacitance retention after 1000 cycles. Li et al. [176]
reported the facile preparation of cobalt hexacyanoferrate
(CoHCFe) nanoparticles as the electrode material for high power
capacitors. In neutral 0.5 M Na2SO4, CoHCFe exhibited a high
specific capacitance of 250 F g1
, excellent rate capability, and
ultrahigh cycling stability with 93.5% capacitance retention after
5000 cycles. By pairing with a carbon black-modified graphene
negative electrode to construct an asymmetric supercapacitor, an
incredible working voltage of approximately 2.4 V, high energy
density of 34.4 Wh kg1
, and power density up to 25 kW kg1
could be delivered. Subsequently, Yue et al. [177] reported the
pseudocapacitive behavior of various mesoporous metal hexa-
cyanoferrates, M = NiII
, CoII
, and CuII
. The capacitive perfor-
mances of these materials were comparable to that of
conventional hybrid graphene/MnO2 nanostructured textiles.
Moreover, a reversible multivalent ion intercalation in PBAs from
aqueous electrolyte was reported, which demonstrated a decent
specific capacity and cycling stability [178,179]. High-power alu-
minum-ion asymmetric capacitor was constructed using
Al0.2CuFe–PBA as the cathode and activated carbon as the anode
[180]. When the current density was set as high as 100 C
(34 mA cm2
), the discharged capacity remained more than
65% at 1 C. The device also maintained approximately 90% of
its initial capacity after 1000 cycles at 5 C, indicating a great promise
for future energy storage applications.
Very recently, the composites consisting of inorganic pseudo-
capacitance materials and MOFs have been investigated as poten-
tial electrode materials for supercapacitors [181–183]. MOFs can be
used in another important supercapacitor application, the separa-
tor membrane because of their unique ion transport behavior
[184].
MOF-derived metallic oxides/sulfides for supercapacitors
The global demand for nanostructured metal oxides/sulfides with
a unique structure in pseudocapacitive applications has strongly
promoted the need for scientific study in this area. Finding an
efficient way to prepare high-performance inorganic functional
materials is very important. To date, the thermal treatment of
MOFs represents one of the most promising methods for the
preparation of nanoporous metal oxides/sulfides. Since MOFs
are constructed by joining metal ions or clusters with organic
ligands, the large fraction of metal-containing units can be utilized
as the metal source. The organic components could provide
abundant nanopores when they are removed through simple
thermal treatment.
Yamauchi and co-workers demonstrated the application of
nanoporous NiO in supercapacitors. Nanoporous NiO, which
possessed tunable porosity and crystallinity, was synthesized by
thermal decomposition of two-dimensional metal–cyanide hybrid
coordination polymers [185,186]. Salunkhe et al. [187] reported a
‘two-for-one’ idea to design an asymmetric supercapacitor, where
nanoporous carbon and nanoporous cobalt oxide (Co3O4) materi-
als were selectively prepared from a single precursor zeolitic imi-
dazolate framework (ZIF-67), by optimizing the annealing
conditions (Fig. 14). The fabricated asymmetric supercapacitors
(Co3O4//carbon) with an optimal mass loading could be operated
in a wide potential window of 0.0–1.6 V, giving a high specific
energy of 36 Wh kg1
and specific power of 8000 W kg1
at
2 A g1
. The simple thermal decomposition of MOFs at a proper
temperature has been widely applied to prepare other metal oxides
or carbon-based composites for supercapacitors, including CeO2
[188], nitrogen-doped carbon/Mn3O4 composites [189], hollow
Co3O4 [190], ZnCo2O4 nanoparticles [191], NixCo3xO4 nanopar-
ticles [192], Fe3O4/carbon composite [193], and Cu–Cu2O–CuO/C
composites [194].
Because of their exceptional thermal properties and chemical
reactivity, nanostructured MOF materials have also been exten-
sively used as ideal precursors/templates to develop advanced
functional materials with hollow or porous structures. Lou et al.
[195] reported a novel structure-induced anisotropic chemical
etching/anion exchange method to transform Ni-Co PBA nano-
cubes into well-defined cubic NiS nanoframes. Benefitting from its
structural merits including 3D hollow, porous structure, small
nanoparticle size, and good structural robustness, the as-prepared
NiS nanoframes exhibited enhanced electrochemical properties
for electrochemical capacitors and hydrogen evolution reaction in
an alkaline electrolyte. Similarly, the synthesis of amorphous cobalt
sulfide polyhedral nanocages from ZIF-67 nanocrystals and 2-methy-
limidazole in methanol under ambient conditions [196], the porous
Cu1.96S/C polyhedra from HKUST-1 and sulfur powders [197], and
Cr2O3 nanoribbons/carbon composite were also obtained [198].
Furthermore, MOFs are promising precursors and templates to
synthesize binary oxides and graphene-based composites. Utiliz-
ing Co3[Co(CN)6]2nH2O and the partial Fe(III)-substituted MOF-5
(FeIII
-MOF-5) as precursors and self-sacrificial templates, starfish-
shaped porous Co3O4/ZnFe2O4 hollow nanocomposites can be
prepared by a stepwise strategy [199]. The synergistic effect be-
tween Co3O4 and ZnFe2O4 enables the nanocomposites to deliver
a specific capacitance as high as 326.7 F g1
, and a high energy
density of 82.5 Wh kg1
at a power density of 675 W kg1
. rGO-
wrapped MoO3 composites were first reported by simply mixing
molybdenum-based MOFs with graphene oxide (GO) sheets fol-
lowed by an annealing process in a different atmosphere [200]. The
obtained rGO/MoO3 composite showed excellent electrochemical
properties for high-performance supercapacitor applications, in
particular in all-solid-state flexible supercapacitor devices. The
flexible supercapacitor showed a high energy density and superior
capacitance retention of ca. 80% over 5000 cycles at 2 A g1
,
suggesting the potential composite for the practical application
in flexible energy storage devices. Another class of promising
electrode materials, metal nanoparticles embedded in porous
Materials Today Volume 20, Number 4 May 2017 RESEARCH
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16. carbon, was also fabricated by the direct carbonization of MOFs for
high-performance supercapacitor applications [201–203].
MOF-derived carbon materials for supercapacitors
Carbon materials derived from MOFs have a large specific surface
area and pore volume [67,70,204], allowing a fast ion diffusion and
enhancing the interaction between the electrolyte and electrode.
Therefore, these carbon materials are promising electrode materi-
als for supercapacitors. Nanoporous carbons through the direct
carbonization of ZIF-8 exhibit an excellent electrochemical per-
formance and cycling stability [69]. The synthetic process is simple
and easy to handle. To further increase the specific surface area and
pore volume of carbon materials that are directly carbonized from
MOFs, these carbon materials were activated by KOH [205]. The as-
synthesized porous carbon materials exhibited a high specific
capacitance of 168 F g1
at a scan rate of 5 mV s1
.
Moreover, the nitrogen-doped porous carbons can be directly
obtained from MOFs. The nitrogen doping can increase the wet-
tability and electrical conductivity of carbon materials and the
specific capacitance by a pseudocapacitive effect [206,207]. Nune
et al. [208] reported the synthesis of hierarchical nitrogen-doped
porous carbon (HNPC) from a nitrogen-containing isoreticular
MOF (IRMOF-3). The carbonization temperature could control
the nitrogen content and specific surface area. When IRMOF-3
was carbonized at 950 8C, the obtained HNPC had a high capaci-
tance of 239 F g1
because of the large specific surface area, high
pore volume, and additional pseudocapacitance from nitrogen
doping.
The carbons’ high nitrogen content prepared from MOFs suffer
from low stability and amorphous carbon [209]. Yamauchi et al.
[210] reported the integration of nitrogen-doped carbon and
highly graphitic carbon originated from core–shell structured
ZIF-8@ZIF-67 crystals (Fig. 15). This new composite had a high
nitrogen content, large surface area, and graphitic structure, show-
ing a distinguished specific capacitance of 270 F g1
at 2 A g1
. The
unique structure and excellent performance indicates that the
composite is potentially useful for energy storage applications.
The specific energy of commercial electrochemical double-layer
capacitors can reach approximately 6–7 Wh kg1
[157]. These
electrochemical double-layer capacitors are now used in different
types of electronic devices and public transport systems. The next
challenge is to increase the specific energy to more than
10 Wh kg1
, thus satisfying the ever-increasing demands in
high-energy power electronics such as electric automobile and
unmanned aerial vehicle. Recent studies indicate that pseudoca-
pacitors have the potential for practical applications. Although
metallic oxides and sulfides as electrode materials for pseudoca-
pacitors show a high capacitance, the long life cycling perfor-
mance still needs further improvement. Moreover, the research
and technology gaps of supercapacitors and batteries are limited
by not only the electrode side, but also the electrolyte side.
Challenges, prospects, and opportunities
In summary, this study reviews and comments on the recent
development of MOFs and MOF-derived materials used in LIBs,
SIBs, Li-O2 batteries, Li-S/Se batteries, and supercapacitors based
RESEARCH Materials Today Volume 20, Number 4 May 2017
FIGURE 14
Schematic illustration of the preparation process of nanoporous carbon and nanoporous Co3O4 from a ZIF-67 polyhedron as the single precursor by
optimized thermal treatment. Reproduced with permission from Ref. [187]. Copyright American Chemical Society (2015).
206
RESEARCH:
Review

17. on theoretical knowledge and experiment experience, demon-
strating that MOFs and MOF-derived materials are potential elec-
trode materials for these energy storage applications because of
their controllable morphology, flexible structure, large pore vol-
ume, high specific surface area, and accessible metal sites.
However, there are still challenges for the practical application
of MOFs and MOF-derived materials: (1) the stability of MOFs in
moist air should be further improved, which would allow the
application of MOFs regardless of the environment. Moreover,
MOFs could be simply and largely prepared in air, resulting in low-
cost and feasible market availability. (2) The electrical conductivity
of MOFs and MOF-derived metallic oxides/sulfides should be
enhanced, which is very important for the increase in capacity
and rate performance of batteries and supercapacitors. The incor-
poration of carbon materials into MOFs and MOF-derived materi-
als is a promising strategy to increase the electrical conductivity of
the composite electrodes. (3) Dual or multi-metal sites in MOFs
should be further developed to improve the catalytic performance
of Li-O2 batteries. (4) The specific chemical interaction between
MOFs and polysulfides/polyselenides should be clarified, which is
significant to the ultralong cycling performance of Li-S and Li-Se
batteries. The Lewis acidic metal centers have high affinity to the
polysulfide/polyselenide anions. However, the presence of Lewis
basic amino groups in linkers is a disadvantage to polysulfides/
polyselenides. Thoroughly understanding the mechanism of
MOFs and MOF-derived materials in energy storage applications
is critical for their practical application. Thus, MOFs and MOF-
derived materials can meet the demand for the rapid development
of electric vehicle, unmanned aerial vehicle, and portable elec-
tronic devices.
Acknowledgements
We are deeply grateful to Lei Sun (Massachusetts Institute of
Technology), Decai Qin (NUAA), Aixiu Wang (NUAA), and Jiaren
Yuan (NUAA) for helpful discussions and suggestions. We are also
grateful to the National Key Basic Research Program 973 (No.
2014CB239701), the National Natural Science Foundation of
China (Nos. 51372116 and 51672128), and the Natural Science
Foundation of Jiangsu Province (Nos. BK2011030 and 20151468).
G.Y. Xu would like to thank Funding of Jiangsu Innovation
Program for Graduate Education (KYLX15_0300) and Outstanding
Doctoral Dissertation in NUAA (BCXJ15-07).
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