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Recent Progress in Metal-Organic Frameworks for
Applications in Electrocatalytic and Photocatalytic
Water Splitting
Wei Wang, Xiaomin Xu, Wei Zhou, and Zongping Shao*
DOI: 10.1002/advs.201600371
1. Introduction
The global demand for energy has
increased rapidly and continuously
in recent decades due to the quickly
expanding human population and indus-
trialization; as a result, there has been a
significant increase in the utilization of
traditional fossil fuels, which has caused
severe environmental problems, such as
the greenhouse effect, air pollution and
water pollution. The quickly expanded
energy consumption has resulted in
major concerns about energy crises due
to the limited fossil fuels resources. For
a sustainable future, the development
of alternative energy material that is
clean and sustainable is highly desirable
but remains a major challenge. Among
the various energy carriers (materials),
hydrogen is one of the most ideal and
cleanest energy materials for the future
due to its high gravimetric energy den-
sity (120 vs. 44 MJ kg
−1
for gasoline),
high combustion efficiency, non-toxicity, clean exhaust prod-
ucts, and renewable and storable nature. During the past two
decades, tremendous attention has been given to the field of
hydrogen energy by researchers and governments around
the world. However, the success of the hydrogen economy
is strongly determined by the availability of useful routes for
the large-scale generation of hydrogen. Currently, the pro-
duction of hydrogen mainly relies on steam reforming and
partial oxidation of fossil fuels (natural gas or other hydro-
carbons), causing concerns about serious CO
2
emissions
and limited natural resources.
[1–3]
Water, one of the most
abundant resources on earth, is composed of hydrogen and
oxygen atoms. Water splitting is one of the most effective
ways to produce hydrogen. Among the various routes for
hydrogen generation from water at low temperature, direct
water splitting using solar/electrical energy over photo-
catalysts/electrocatalysts is highly promising because of its
sustainability.
[4–9]
Water splitting (H
2
O → H
2
+ 1/2O
2
) consists of two half
reactions, known as the oxygen evolution reaction (OER)
and the hydrogen evolution reaction (HER). However, these
reactions have sluggish kinetics and require catalysts. In the
electrochemical process, the OER and the HER are gener-
ally catalyzed by precious metal (Ir/Ru and Pt, respectively)
The development of clean and renewable energy materials as alternatives
to fossil fuels is foreseen as a potential solution to the crucial problems of
environmental pollution and energy shortages. Hydrogen is an ideal energy
material for the future, and water splitting using solar/electrical energy is one
way to generate hydrogen. Metal-organic frameworks (MOFs) are a class of
porous materials with unique properties that have received rapidly growing
attention in recent years for applications in water splitting due to their
remarkable design flexibility, ultra-large surface-to-volume ratios and tun-
able pore channels. This review focuses on recent progress in the application
of MOFs in electrocatalytic and photocatalytic water splitting for hydrogen
generation, including both oxygen and hydrogen evolution. It starts with the
fundamentals of electrocatalytic and photocatalytic water splitting and the
related factors to determine the catalytic activity. The recent progress in the
exploitation of MOFs for water splitting is then summarized, and strategies
for designing MOF-based catalysts for electrocatalytic and photocatalytic
water splitting are presented. Finally, major challenges in the field of water
splitting are highlighted, and some perspectives of MOF-based catalysts for
water splitting are proposed.
Dr. W. Wang, Prof. Z. Shao
Department of Chemical Engineering
Curtin University
Perth, WA 6845, Australia
E-mail: zongping.shao@curtin.edu.au,
shaozp@njtech.edu.cn
X. Xu, Prof. W. Zhou
Jiangsu National Synergetic Innovation Center for
Advanced Materials (SICAM)
State Key Laboratory of Materials-Oriented Chemical Engineering
College of Chemical Engineering
Nanjing Tech University (NanjingTech)
Nanjing 210009, P. R. China
Prof. Z. Shao
Jiangsu National Synergetic Innovation Center for
Advanced Materials (SICAM)
State Key Laboratory of Materials-Oriented Chemical Engineering
School of Energy Science and Engineering
Nanjing Tech University (NanjingTech)
Nanjing 210009, P. R. China
This is an open access article under the terms of the Creative Commons
Attribution License, which permits use, distribution and reproduction in
any medium, provided the original work is properly cited.
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materials to achieve favorable reaction kinetics.
[10–13]
Unfortu-
nately, noble metals suffer from low abundance and high cost,
hindering their large-scale use in water electrolysis. To ensure
sustainable hydrogen production, it is of great importance
to seek earth-abundant alternatives to precious metal-based
catalysts with excellent activity and robust stability.
[14–22]
For
example, several electrocatalysts composed of earth-abundant
elements (e.g., Fe, Co and Ni) were found to be promising
alternatives to precious catalysts, achieving high OER and
HER activity.
[14–18]
In addition, some carbon-based or heter-
oatom-doped carbon materials have been evaluated as inno-
vative options as electrocatalysts for the OER and HER.
[19–22]
The availability of different carbons (nanotubes, graphene,
etc.) with adjustable compositions has markedly increased
the number of candidates for OER/HER electrocatalysis.
[23–25]
Solar-driven H
2
generation from water using semiconductor-
based photocatalysts is another attractive route to solve the
energy and environmental problems.
[26–30]
To date, a number
of metal oxide-based photocatalysts have been demonstrated to
be effective for water splitting under UV light irradiation.
[31–33]
In particular, TiO
2
has been reported as a benchmark for the
UV-light-driven water splitting reactions due to its good photo-
stability, low toxicity, large abundance and low cost. Unfor-
tunately, TiO
2
has a large band gap of 3.2 eV, which can only
be used in the UV light range, which includes only 5% of all
solar energy (solar conversion efficiency in UV light is only
2% compared with 16% when visible light up to 600 nm can
be utilized). Thus, the development of new photocatalysts
with high photocatalytic activity under visible light irradiation
is one of the most attractive research topics in photocatalytic
water splitting.
[34–37]
In addition to the material composition,
the activity of catalysts for electrocatalytic/photocatalytic water
splitting relies heavily on the morphology of the catalyst.
[38–41]
Thus, optimizing the catalyst composition and morphological
structure is of critical importance to achieve highly efficient
hydrogen production from water splitting.
Metal-organic frameworks (MOFs) are a new class of
porous materials with unique electronic, optical and catalytic
properties.
[42,43]
In addition, they can be used as precursors
for the fabrication of various metal, metal oxide-carbon com-
posites or pure carbon materials with rich morphological
structures and versatile properties.
[43]
In applications as elec-
trocatalysts or photo catalysts or their precursors, MOFs offer
several advantages, such as high design flexibility, tunable
pore channels, large surface-to-volume ratios, flexibility to be
functionalized with various ligands and metal centers, and
rich compositions.
[43]
The metal centers separated by organic
linkers in MOFs can be considered as quantum dots; conse-
quently, short diffusion lengths of the charge carriers can be
achieved during the electrocatalytic and photocatalytic reac-
tions.
[44]
The specific surface areas and band gaps of MOFs
can be tailored by tuning the organic ligands and/or metal
centers, so their electrocatalytic and photocatalytic activities
can be tailored to maximize their performance. In recent
years, MOFs have been exploited directly as photocatalysts or
as their precursors for hydrogen generation from water split-
ting, the degradation of organic pollutants and the reduction
of CO
2
into useful fuels.
[45–49]
Recently, MOF-based materials
have also proved to be particularly suitable for electrocatalytic
water splitting.
[50–52]
In the last five years, tremendous efforts
have been made to apply MOFs as photocatalysts and elec-
trocatalysts for water splitting, and interest in this research
field is projected to continue increasing. Thus, a review of
the recent advances and challenges of MOF-based materials
in photocatalytic and electrocatalytic water splitting is highly
desirable.
Herein, the recent development of MOF-based materials
for electrocatalytic and photocatalytic water splitting reactions
is presented. Several critical factors that determine the activity
for water splitting reactions are summarized, and strategies
related to the design of catalysts are emphasized. Major chal-
lenges in the fields of photocatalytic and electrocatalytic water
splitting are highlighted, and some perspectives from the cur-
rent progress in the development of MOF-based catalysts are
given. Directions of the future research are also presented,
with emphasis on achieving the desired MOF functionality
and establishing structure-property relationships to identify
and rationalize the factors that determine the catalytic perfor-
mance. This paper aims to provide a comprehensive review
of the recent progress in this dynamic field, as well as some
guidelines for the further development of highly efficient
photocatalysts and electrocatalysts based on MOFs for water
splitting.
Wei Wang obtained his
Ph.D. degree in Chemical
Engineering at Nanjing Tech
University, China, in June,
2013. He is now a post-
doctoral fellow in Curtin
University, Australia, since
December, 2013. His research
interests include anode
catalytic materials and coke
formation mechanism for
solid oxide fuel cells (SOFCs)
operating on hydrocarbons, fuel selection and applica-
tion for SOFCs, photocatalysts for degradation of organic
substances, photoanodes and counter electrodes for dye-
sensitized solar cells.
Xiaomin Xu received his
Bachelor (2013) and Master
(2016) degrees in Chemical
Engineering from Nanjing
Tech University, China. He is
now pursuing a Ph.D. degree
in Chemical Engineering at
Curtin University, Australia.
His research interests
include the structure, syn-
thesis and characterization
of perovskite materials and
their applications in the electrocatalytic water splitting
reactions.
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2. Fundamentals of Water Splitting Reactions
2.1. Electrocatalytic Water Splitting
2.1.1. Basic Principles
Electrocatalytic water splitting involves two half reactions (OER
and HER), and the mechanistic schemes of the OER and HER
have been proposed in the literature.
[53–56]
The OER, which is
a four-electron process, is more complex than the HER and
involves several surface-adsorbed intermediates. In the fol-
lowing section, we mainly focus on the mechanistic study of
the OER while that of the HER is described only briefly.
In the HER, the chemical adsorption and desorption of H
atoms are competitive processes. A good HER catalyst should
have a bond with the adsorbed H* (the asterisk indicates a
bond to the catalyst surface) that is sufficiently strong to enable
the proton-electron-transfer process and also sufficiently weak
to ensure easy bond breaking and release of the produced H
2
gas.
[53]
The change in the Gibbs free energy for H* adsorption
on an electrocatalyst surface (∆
H
*
G
) can be applied to evaluate
both H* adsorption and H
2
desorption using the HER free
energy diagram.
[54]
The optimal ∆
H
*
G
should be zero, under
which condition the HER reaches the maximum rate.
[53]
More
importantly, a “volcano curve” correlation has been proposed
between the experimental HER activity (HER exchange current
density) and the quantum chemistry-derived ∆
H
*
G
for various
catalyst surfaces.
[54]
As a result, the relationship between the
nature of the electrocatalyst surface and the HER kinetics can
be established.
The OER pathways, in acidic or alkaline media, include ele-
mentary steps that differ according to different mechanisms,
yet all involve the adsorption/desorption of intermediates, such
as HO*, O* and HOO*.
[55–57]
The free adsorption energies of
the OER intermediates at selected potentials on Pt (111) and
Au (111) and some other metals were studied in acidic environ-
ment by Rossmeisl et al. using density functional theory (DFT)
calculations.
[58]
The most difficult step in the OER is the for-
mation of HOO* on the metal surface by splitting water on an
adsorbed oxygen atom (O*). This step is downhill in free energy
at high electrode potentials. At lower potentials, although water
can dissociate to O*, the OER is initiated only on the oxidized
surface, which makes this step slower than the O* formation
process. In other words, the formation of OOH* from O* is
uphill for the OER at the equilibrium potential of 1.23 V vs.
reversible hydrogen electrode (RHE). Applying a voltage to
move the potential positively away from 1.23 V (the difference
defined as the overpotential) is thus necessary for spontaneous
OER. The calculations show that the OER on Pt and Au sur-
faces should start at approximately 1.8 V. Simple linear rela-
tions between the stability of different intermediates and OER
activity were found when the analysis was extended to other
metals, which suggests that the oxygen adsorption energy is a
good descriptor of the capability of a metal-based electrocatalyst
for the OER.
[58]
In addition to metallic catalysts, the OER mechanism on
oxide catalysts has also been studied using computational
methods.
[59]
Rossmeisl and co-workers investigated the trends
in the electrocatalytic properties of the most stable (110)
surfaces of RuO
2
, IrO
2
and TiO
2
. Similar to the findings on
metal surfaces, the binding energies of O*, HO* and HOO*
on the (110) surfaces of these rutile oxides showed universal
linear relations. Based on this, a volcano plot was constructed
to describe the trends in OER activity according to a simplified
descriptor, the O* binding energy. It was found that RuO
2
binds
oxygen slightly too weakly, while IrO
2
binds oxygen too strongly,
leading to a higher overpotential, which was also observed in
experiments.
[60]
However, TiO
2
binds O* too weakly, and it dis-
plays a low OER activity. These results suggest that a material
that binds oxygen slightly more strongly than RuO
2
is expected
to exhibit even better OER activity.
The origin of the overpotential for OER catalysis was also
studied using DFT calculations on various oxides.
[61]
A universal
scaling relation between the binding energies of the HOO* and
HO* intermediates was identified, which defined the lowest
theoretical overpotential for the OER on oxide surfaces. This led
to a general description of OER activity with the introduction of
a single descriptor (∆G
O*
−∆G
HO*
). For the oxides considered,
the OER activity could not be greatly enhanced beyond RuO
2
by tailoring the binding between the intermediates and the
oxide surface. To avoid the limitations defined by the universal
scaling relationship, relative stabilization of HOO* compared to
HO* must be achieved. In this regard, three-dimensional (3D)
structures are likely to stabilize HOO*.
2.1.2. Factors to Determine the Electrocatalytic Activity
Generally, the catalytic activity of an electrocatalyst for water
splitting is determined by the intrinsic activity and the number
of active sites. For oxide-based electrocatalysts, the intrinsic
activity is often related to the material composition, mixed
valence states of the compositional cations (redox couples),
crystal structure, metal-oxygen bond energy, oxygen vacancy
concentration, electronic conductivity and charge transfer capa-
bility.
[56,62–65]
The number of active sites can be increased by
building high-surface-area structures, tuning the morphology
and creating nanostructured catalytic systems. Compositing
with other catalytic materials or conductive supports can result
in hybrids with enhanced activity and more active sites, which
is sometimes known as the synergistic effect. The most effec-
tive methods to maximize the HER/OER activity include tai-
loring the surface and/or bulk properties (by the selection of
cations and anions), optimizing the morphology (by the use of
advanced synthetic procedures), enhancing the charge transfer
process (by the functional modification of the surface electronic
structure) and forming composite or hybrid catalysts. These
methods may produce more active sites for HER/OER and ideal
pathways for the transportation of reactants and gaseous prod-
ucts (i.e., hydrogen and oxygen). The strategies for enhancing
electrocatalytic activity are not limited to oxides and can be, in
principle, applied to other types of electrocatalysts. Additionally,
researchers often take advantage of several combined strategies
to improve the efficiency of electrocatalysts in the HER/OER.
The morphology and microstructure are crucial charac-
teristics for electrocatalysts because they have a direct cor-
relation with the number of active sites and, therefore, the
catalytic activity.
[66,67]
For example, a simple self-template
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strategy was developed to fabricate hollow Co-based bime-
tallic sulfide (M
x
Co
3−x
S
4
, M = Zn, Ni and Cu) polyhedra from
homogenous bimetallic MOFs.
[66]
The combination of polyhe-
dral morphology, hollow structure, homo-incorporation of a
second metal element and high Brunauer-Emmett-Teller (BET)
surface area significantly enhanced the HER activity of Co
3
S
4
.
The hollow Zn
0.30
Co
2.70
S
4
exhibited the highest catalytic activity
among the four electrocatalysts, indicating that cation selection
is very important to achieve high electrocatalytic activity for
water splitting.
In addition, nanostructured electrocatalysts generally ben-
efit from increased specific surface area and, therefore, have
more active sites for the electrocatalysis, which can be tailored
by the preparation methods and annealing conditions.
[68,69]
For
example, Shi et al. utilized an in situ carburization method
to prepare MoC encapsulated by a graphitized carbon shell
(nanoMoC@GS) electrocatalyst from a Mo-based MOF.
[68]
The
nanoMoC@GS showed favorable activity in acidic media as an
electrocatalyst for HER, which stemmed from the synergistic
effects of the ultrafine MoC, ultrathin and conductive GS, high
porosity and high surface area.
[68]
Other methods to improve the surface area, such as syn-
thesizing nanoparticles (NPs) and combining NPs with high-
surface-area supports, have been used to enhance the activity
of electrocatalysts for water splitting.
[70,71]
For example, Li
et al. synthesized a nitrogen-doped Fe/Fe
3
C@graphitic layer/
carbon nanotube hybrid (Fe/Fe
3
C@NGL-NCNT) using MIL-
101 (Fe) MOF as the precursor.
[70]
This Fe/Fe
3
C@NGL-NCNT
hybrid showed superior OER activity and stability compared
with the commercial Pt/C, which may originate from the
abundant active sites and the synergistic effect of the unique
architecture.
The charge transfer capability is also essential for achieving
high electrocatalytic activity for water splitting, and the coupling
of some functional materials, such as reduced graphene oxide
(RGO), to MOF-based electrocatalysts can improve the charge
transfer capability (conductivity).
[72,73]
For example, Tang et al.
used a simple pyrolyzing method to synthesize a porous Mo-
based hybrid from a polyoxometalate-based MOF and graphene
oxide (POMOFs/GO), which showed improved performance for
the HER.
[73]
2.2. Photocatalytic Water Splitting
2.2.1. Mechanism and Reaction Steps
Studies on splitting water into hydrogen and oxygen using
light (photons) originated from the discovery of the Honda-
Fujishima effect in 1967. Water splitting using photocatalysts
has since been widely investigated.
[74–78]
Previous reviews of
water splitting using semiconductors as photocatalysts have
demonstrated the basic principles of the water splitting pro-
cess.
[76–78]
The electrons in the valence band (VB) of the photo-
catalyst are transferred to the conduction band (CB), and
holes are left in the VB after absorbing UV and/or visible light,
creating electron-hole pairs. The photogenerated electron-hole
pairs can induce redox reactions similar to water electrolysis.
Specifically, water molecules are reduced by the electrons to
generate H
2
and are oxidized by the holes to produce O
2
, com-
pleting the water splitting reactions.
Water splitting into H
2
and O
2
is an energetically uphill
reaction with a standard Gibbs free energy change (∆G) of
+237 kJ mol
−1
(corresponding to 1.23 eV). Therefore, the
band gap of the photocatalysts and the edges of the CB and
VB must be suitable for water splitting. The bottom level of
the CB should be more negative than the redox potential of
H
+
/H
2
(0.0 V vs. normal hydrogen electrode (NHE)), while the
top level of the VB should be more positive than the redox
potential of O
2
/H
2
O (1.23 V vs. NHE).
[76]
The theoretical
minimum band gap for water splitting is therefore 1.23 eV,
which is equivalent to a light wavelength of approximately
1100 nm. However, not all semiconductors meet the require-
ments for water splitting. For metal oxide-based photocatalysts,
the VB mainly consists of O 2p orbitals, and the top level of
the VB is much higher than 1.23 V vs. NHE. Therefore, the
oxidation-reduction potentials (ORPs) of O
2
/H
2
O and H
+
/H
2
are
positioned between the top level of the VB and the bottom level
of the CB. Higher photon energy than the band gap of the photo-
catalyst is needed due to an activation barrier in the charge
transfer process in the water splitting reactions. At the same
time, the much wider band gaps of these materials make them
only photoactive in UV light. As approximately 50% of the solar
spectrum consists of visible photons (400 <
λ
< 800 nm), it is
critical to develop active photocatalysts with high activity under
visible light for photocatalytic water splitting.
The development of photocatalysts is very important to
enable water splitting with visible light. The main steps in the
photocatalytic water splitting reactions should be tailored to
meet the requirements for photocatalysts capable of water split-
ting. There are three steps in the photocatalytic water splitting
reactions, which has been demonstrated in some informative
reviews.
[76–78]
The first step is the formation of electron-hole
pairs by incident photons. When the energy of incident light is
greater than the band gap energy, the electrons in the VBs can
be excited and transferred into the CB. Meanwhile, holes are
generated in the VB. However, the band structure is only a ther-
modynamic requirement. Other factors, such as charge separa-
tion, mobility and the lifetime of photogenerated electrons and
holes can also affect the photocatalytic activity for water split-
ting. The second step is charge separation and diffusion to the
catalyst surface without recombination of the photogenerated
carriers, which is drastically affected by the crystal structure,
particle size and crystallinity of the photocatalyst.
[79,80]
A higher
crystallinity can lead to superior charge migration efficiency
because the defects in a photocatalyst with lower crystallinity
act as recombination centers for the photogenerated electron-
hole pairs, which decreases the photocatalytic activity. A smaller
particle size of the photocatalyst also suppresses the possibility
of electron-hole pair recombination. The final step is the reduc-
tion and oxidation of surface-adsorbed species by the photogen-
erated electrons and holes to generate H
2
and O
2
, respectively.
In this step, the surface active sites of the photocatalyst play
vital roles in efficient water splitting. Co-catalysts, such as Pt,
are usually loaded onto the photocatalyst surface as active sites
to reduce the activation energy for the HER.
[81]
These processes
affect the overall efficiency of water splitting based on a semi-
conductor-based photocatalyst.
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2.2.2. Factors to Determine the Photocatalytic Activity
The main factors that determine the photocatalytic activity of the
photocatalysts for water splitting include the band gap energy/
visible light absorption capability, active sites/co-catalysts and
charge transfer/separation efficiency. Covalent modification is
a method to reduce the band gap energy of photocatalysts. For
example, the band gap of MOFs can be reduced by a diazo cou-
pling with amino-substituted ligands and other molecules.
[82]
The photocatalytic activity of these modified MOFs corresponds
to a red shift of the absorption edge, suggesting that the band
gap energy/visible light absorption capability plays a vital role
in the photocatalytic activity.
The incorporation of active co-catalysts is an effective way to
increase the number of active sites for water splitting.
[83–87]
For
example, the well-defined cages of MIL-101(Cr) MOF were used
to engage the molecules of a high-valent di-µ-oxo dimanga-
nese catalyst with high activity for photo-electrochemical (PEC)
water oxidation and the incorporation of MnTD ([(terpy)
Mn(µ-O)
2
Mn](terpy)]
3+
; terpy: 2,2′:6′,2′′-terpyridine) improved
the turnover number of MIL-101(Cr) more than 20-fold while
maintaining the initial high rate in the PEC water oxidation
reaction.
[83]
In another study, Hansen and Das found that
MnTD⊂MIL-101(Cr) showed superior activity to MIL-101(Cr)
and MnO
2
catalysts for the OER.
[84]
These studies suggested that
the incorporation of active co-catalysts can greatly enhance the
photocatalytic activity for water splitting, and the selection and
incorporation method of the co-catalysts should be optimized.
The charge transfer/separation efficiency of the photocatalyst
plays a critical role in photocatalytic water splitting. The con-
struction of heterojunctions is an effective way to enhance the
charge transfer/separation capability of electron-hole pairs.
[88,89]
For example, a MOF-derived Co
3
O
4
/TiO
2
composite photocata-
lyst with 2 wt.% Co loading and a p-n heterojunction, exhibited
a much higher hydrogen evolution rate than the conventional
Co
3
O
4
/TiO
2
nanocomposite (≈ 7-fold enhancement).
[88]
The photocatalytic activity of MOF-based photocatalysts is
determined by several crucial factors, such as the band gap
energy, active sites/co-catalyst and charge transfer capability.
These factors are often closely related. For example, an azo-
carboxylic acid can be used as an organic linker to construct a
Gd-based MOF with a reduced band gap.
[90]
Gd-MOF has high
photocatalytic activity for the HER due to its high visible light
absorption capability. The addition of Ag co-catalyst improved
the HER activity of Gd-MOF by providing more active sites and
improving the charge transfer capability.
3. Recent Advances in MOF-Based Catalysts for
Water Splitting
Because of the many outstanding features of MOFs, such as
tunable pore channels, high specific surface area, easy tailoring
of the material composition, rich morphological structure, and
capability to act as precursors for the preparation of various
metal/metal oxide/carbon composites and carbon materials
of various properties, during the past five years, the applica-
tions of MOFs as catalysts or the precursors of catalysts for
electrocatalytic and photocatalytic water splitting reactions for
hydrogen generation have been extensively exploited. Both the
direct application of MOFs for water splitting and application as
a precursor for metal/metal oxide/carbon composites or porous
carbon materials (by leaching of the metal/metal oxide from
the composites), which were then applied as electrocatalysts or
photocatalysts, have been reported. Additionally, MOFs were
studied as catalysts for both the OER and HER, and the reac-
tions were conducted in acidic and alkaline electrolytes.
3.1. Electrocatalytic Water Splitting
For electrocatalytic water splitting, the direct application of MOFs
as electrocatalysts was first reported in 2011 by Nohra et al., who
pioneered the use of polyoxometalate-based MOFs (POMOFs) for
the HER.
[91]
The structural properties were investigated but their
electrocatalytic activity was only briefly studied and the efficiency
of POMOFs to replace Pt catalyst for the HER was not clearly
demonstrated. In 2015, Qin et al. reported a type of POMOFs
called [TBA]
3
[
ε
-PMo
V
8
Mo
VI
4
O
36
(OH)
4
Zn
4
][BTB]
4/3
·xGuest
(NENU-500, BTB = benzene tribenzoate, TBA
+
= tetrabutylam-
monium ion) as an ultrastable electrocatalyst for the HER.
[92]
It displayed a Tafel slope of 96 mV dec
−1
and an overpotential
of 237 mV at a current density of 10 mA cm
−2
(a metric associ-
ated with solar fuel synthesis), which was inferior to Pt/C (Tafel
slope of 30 mV dec
−1
and overpotential of 52 mV at 10 mA cm
−2
).
Very recently, Dai et al. demonstrated MoS
x
anchored on
Zr-MOF (UiO-66-NH
2
) prepared by a solvothermal method for
the HER.
[93]
The introduction of MoS
x
nanosheets to the MOFs
dramatically enhanced the HER activity due to the improved
electron transport, the increased number of active sites and the
favorable delivery of local protons in the Zr-MOF structure. By
optimizing the MoS
x
amount, the MoS
x
-MOF composite with
a Mo/Zr ratio of 0.5 displayed remarkable HER activity, with a
Tafel slope of 59 mV dec
−1
, which was only slightly higher than
that of Pt/C (32 mV dec
−1
).
[93]
In the study of MOFs as precursors for the preparation of
electrocatalysts for water splitting reactions, Chaikittisilp and
co-workers were the first to use a Co-based MOF (zeolitic imi-
dazolate framework-9, ZIF-9) as a precursor for the prepara-
tion of a nanoporous Co
x
O
y
-C hybrid as an electrocatalyst for
the OER.
[94]
The conversion of ZIF-9 to the Co
x
O
y
-C hybrid is
shown in Figure 1a. As depicted in Figure 1b, for the OER,
the Z9-700-250 and Z9-800-250 electrocatalysts exhibited more
negative onset potentials and higher current densities than
Z9-900-250 and Pt/carbon black. These results indicated that
the Z9-800-250 hybrid is a promising electrocatalyst for the
OER. Very recently, Aijaz et al. reported a highly active elec-
trocatalyst for the OER comprising core-shell Co@Co
3
O
4
NPs
embedded in CNT-grafted N-doped carbon-polyhedra, which
was obtained by the pyrolysis of a Co-based MOF in H
2
atmos-
phere and a subsequent controlled oxidative calcination.
[95]
This electrocatalyst displayed an overpotential of 410 mV at
10 mA cm
−2
, comparable to RuO
2
, which has been demonstrated
as the benchmark electrocatalyst for the OER.
Although nanoporous carbon was successfully synthesized
from MOFs in 2008
[96]
and was widely used in the oxygen reduc-
tion reaction (ORR),
[97–99]
the direct use of nanoporous carbon
derived from MOFs in water splitting was demonstrated only
Adv. Sci. 2017, 1600371
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