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Hierarchically porous carbon derived from
polymers and biomass: effect of interconnected
pores on energy applications
Saikat Dutta,*
a
Asim Bhaumik*
b
and Kevin C.-W. Wu*
a
Hierarchically porous carbons (HPCs) with 1D to 3D network are attracting vast interest due to their
potential technological application profile ranging from electrochemical capacitors, lithium ion batteries,
solar cells, hydrogen storage systems, photonic material, fuel cells, sorbent for toxic gas separation and
so on. Natural raw-materials such as biomass-biopolymer derived hierarchical nanostructured carbons
are especially attractive for their uniform pore dimensions which can be adjustable over a wide range of
length scales. Good electrical conductivity, high surface area, and excellent chemical stability are unique
physicochemical properties which are responsible for micro/nanostructured porous carbon to be highly
trusted candidate for emerging nanotechnologies. This review focuses on the ‘out-of-the-box’synthetic
techniques capable of deriving HPC with superior application profiles. The article presents the promising
scope of accessing HPCs from (1) hard-templating, soft-templating, and non-templating routes, (2)
biopolymers with a major focus on non-templating strategies. Subsequently, emerging strategies of
hetero-atom doping in porous carbon nanostructures are discussed. The review will highlight the
contribution of synergistic effect of macro–meso–micropores on a range of emerging applications such
as CO
2
capture, carbon photonic crystal sensors, Li–S batteries, and supercapacitor. Mechanism of ion
transport and buffering, electrical double layer enhancement have been discussed in the context of pore
structure and shapes. We will also show the differences of HPC and ordered mesoporous carbon (OMC)
in terms of their synthesis strategies and choices of template for self-assembly. How the remarkable
mechanical strength of the HPCs can be achieved by selecting self-assembling template, whereas
collapse of mesostructure via decomposition of framework occurs due to poor thermal stability or high
N-content of the carbon source will be discussed.
Broader context
The inter-connected pore network in hierarchically porous carbons (HPCs) consisting of macro, meso, and micropores is currently an attractive candidate for
advanced technological applications ranging from electrochemical supercapacitors, Li batteries, solar cells, and fuel cells. However, despite the ceaseless
development of porous carbons, limited success has been achieved in the synthesis of HPCs by conventional hard-templating methods. Besides, non-renewable
carbon sources have been utilized for the fabrication of HPCs. From a long-term perspective, looking for biorenewable carbon sources for HPCs is essential
along with developing unconventional non-templating routes that retain the framework of the pore-networks and also offer high specic surface areas and
desired pore sizes and shapes. We herein review how polymers, block-copolymers, and biopolymers have been utilized as carbon sources to offer HPCs with
versatile nanostructures. Heteroatom doped mesoporous carbon monoliths with hierarchical pores are accessible from N-containing polymeric resins. The
derived HPCs demonstrated the synergistic effect of macro-, meso- and microporosity in energy-related applications where ion-transport, buffering, electrical
double layer enhancement, and ion diffusion play signicant roles. The effect of interconnected pores in CO
2
adsorption and photonic crystal sensors are
described. The advantages of HPC over OMC are discussed. The current progress of HPCs derived from biopolymers, challenges, and future applications are
critically presented.
1. Introduction
Naturally occurring species are employed as resource for a wide
range of materials which suggests that a signicant fraction of
complex functionalities of living systems are based on their
hierarchical structures.
1
Compared to conventional porous
materials with uniform pore dimensions that can be adjusted
over a wide range of length scales, hierarchical porous materials
a
Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan
10617. E-mail: saikatdutta2008@gmail.com; kevinwu@ntu.edu.tw; Tel: +886
33669534
b
Department of Materials Science, Indian Association for the Cultivation of Science,
Jadavpur, Kolkata-700032, India. E-mail: msab@iacs.res.in; Fax: +91 33-24732805
Cite this: DOI: 10.1039/c4ee01075b
Received 4th April 2014
Accepted 29th May 2014
DOI: 10.1039/c4ee01075b
www.rsc.org/ees
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with well-dened pore dimensions and topologies offer mini-
mized diffusive resistance to mass transport by macropores,
and high surface area for active site dispersion over the micro-
and/or mesopores.
2
Such a novel type of interconnected porous
carbon materials with a 1D to 3D network are currently
attracting a great degree of interest due to their potential
technological application prole ranging from electrochemical
capacitors,
3
lithium ion batteries,
4
solar cells,
5
hydrogen storage
systems,
6
photonic materials,
7
fuel cells,
8
and sorbents for toxic
gas separation.
9
Good electrical conductivity, high surface area,
and excellent chemical stability are certain unique physico-
chemical properties which have caused micro/nanostructured
porous carbon to be a highly trusted candidate for emerging
nanotechnologies. Novel porous carbon materials with
controlled morphology, porosities, and architectures; especially
carbon frameworks with hierarchical porosity, namely, meso-
pores in combination with macropores or micropores; are
highly desirable due to their unique structural features
compared with carbon materials containing macropores con-
nected with mesopores. The design of hierarchical nano-
structured carbons (HNCs) with tailored macropores/
mesopores and doping of electron-donating element has
emerged as a promising eld of further investigation with an
extensive scope. The most commonly used synthetic technique
for fabrication of HNCs is “nanocasting”(hard templating) with
hierarchical nanostructured silica (HNS) as template to
impregnate with an appropriate carbon source, followed by
carbonization of the composite, and subsequent removal of the
template. Primarily, apart from obtaining ordered mesoporous
carbons (OMCs) (Fig. 1a),
10
the same with macro/mesoporous
arrays, disordered HNCs with macro/mesoporosity, and hollow
macroporous core/mesoporous shell were also obtained using a
nanocasting strategy.
11
Major challenges for the fabrication of ordered HNCs with
3D-interconnected macroporous and mesoporous structures
Fig. 1 Schematic representation of (a) 2D mesoporous carbon, (b)
three-dimensionally ordered macroporous monolithic carbon
(3DOM/m C) and a monolithic carbon–carbon nanocomposites
material (3DOM/m C/C); (c) 3D hierarchical porous texture of core,
walls, and pores (mesoporous walls, microporous, and macroporous
cores) of hierarchically porous graphitic carbon materials.
Professor Asim Bhaumik
received his PhD from NCL,
Pune in 1997. Aer postdoctoral
research as a JSPS fellow at the
University of Tokyo, (1997–
1999) and an associate
researcher at Toyota Central
R&D Labs. Inc., Japan (1999–
2001), in 2001 he had joined the
faculty in IACS, India. His
research focuses on several
aspects of energy, environment
and biomedical science,
including porous materials for adsorption, gas storage, catalysis,
sensing, photocatalysis, DSSC and drug delivery applications. He
is coauthor of 250 research publications and inventor of 13
patents. He is a board member of several journals and a Fellow of
the Royal Society of Chemistry.
Kevin C.-W. Wu is currently an
associate professor at the
Department of Chemical Engi-
neering, National Taiwan
University (NTU), Taiwan. He
received his PhD degree from the
University of Tokyo, Japan in
2005. He worked on the orien-
tational control of 2D hexagonal
mesoporous thin lms with
Professor Kazuyuki Kuroda
(Waseda University, Japan,
2005–2006) and with Professor
Victor S.-Y. Lin's group (Iowa State University, USA, 2006–2008) as
post-doc. He started his own research group in NTU in August
2008. His current interest is the synthesis of porous nanoparticles
and thin lms with desired structural orientation and function-
alities for biomedical and energy-related applications.
Saikat Dutta obtained his PhD
from IISc, Bangalore in 2008. He
worked as postdoctoral fellow in
Taiwan, India and in the USA. He
received a Fulbright Postdoctoral
Fellowship in 2012 while working
at the Department of Chemistry,
University of Florida, USA. Aer a
job as a scientist in India for a
short period (2013–2014), he
joined Professor Kevin C.-W.
Wu's laboratory at the Depart-
ment of Chemical Engineering,
National Taiwan University. Dr Dutta is co-author of more than 30
research publications in scientic journals. His research experience
includes chemical and material aspects of biopolymers, energy
applications of materials, and enzymatic biofuel production.
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arise from the complexity of interconnected meso/macroporous
HNSs with long-range order. Particularly interesting are those
nanostructured porous carbon materials
12
or carbon materials
exhibiting three-dimensional (3D) hierarchical porous textures
(containing pores at different scales, from micropores to mes-
opores, up to macropores) that combine high specic surface
areas with proper channels and allow efficient diffusion of any
substance (e.g., analytes, adsorbates, electrolytes etc.) to the
entire surface of the material.
13
Meso- and macro porosity exert
a signicant effect in introduction of more graphitic, nitrogen
doped carbon into the mesopores of a three-dimensionally
ordered macroporous monolithic carbon (3DOM/m C) by
chemical vapor deposition to produce a monolithic carbon–
carbon nanocomposites material (3DOM/m C/C) (Fig. 1b).
13a
Constructing different nanoscaled pores with interconnections
is very important and this greatly depends on the synthetic
strategies of the hierarchically porous carbons (HPCs) and the
level of the microstructure. To date, well-dened HPCs are
accessible via hard-/so-templating approaches and post-acti-
vation combined methods. For example, Cheng and co-workers
prepared a 3D periodic hierarchical porous graphitic carbon by
using alkaline system consisting of Ni(OH)
2
/NiO-phenolic resin
as a hard template (Fig. 1c),
14
and Lu and co-workers described
the HPCs obtained by post-activation of a Pluronic F127-tem-
plated phenolic resin.
15
However, most of the templates are expensive and the post-
synthetic removal of the template to produce a carbon replica
requires additional processing steps that are usually very much
time-consuming and harmful for environmental safety. These
limitations impart to HPCs an uncompetitive price-to-perfor-
mance ratio as compared with other materials and thus limit
their commercial viability. Obviously, the problem can be fully
eliminated with the incorporation of hierarchical porosity by
using any auxiliary template. Therefore, building a controllable
hierarchical porous structure from a biorenewable source
through a template-free method is a big challenge today.
2. Scope
In this review, so-templating and hard-templating, as well as
non-templating, strategies developed for the fabrication of
hierarchical porous carbon nanomaterials from various carbon-
rich precursors such as polymers, copolymers and biomass-
derived polymers as the carbon source are surveyed. The aim of
this article is to emphasize the newly explored carbon precur-
sors and naturally occurring biopolymers for their wondrous
future prospects in deriving nanostructured HPC materials.
Subsequently, a perspective on advanced applications of HPCs
for emerging areas of energy storage and generation including
reversible CO
2
capture for clean energy technology, carbon
photonic crystals, lithium–sulfur batteries, and supercapacitors
is provided. The review will mainly focus on the application
proles of the hierarchical porous carbons emphasizing the
effect of the interconnectivity of the pore-network on the effi-
ciency of the materials for a specic application. Thus this will
help in better understanding of the interconnected-pores–
property interrelationship. A comparative description of the
synthesis strategies of HPCs and ordered mesoporous carbons
(OMCs) is provided which emphasizes the factors responsible
for the growth of hierarchical porous nanostructures unlike the
nanostructure of OMCs. The article is completed with brief
discussion on the applications of HPCs derived from bio-
renewable sources and relevant conclusions.
3. Fabrication of HPCs from polymers
and biomass
3.1. Hard-templating
A large number of techniques have been explored to access
HPCs with combined macro- and mesoporosity mainly based
on the dual-templating strategy where two templates with
dimensions at different length scales are combined to originate
multimodal pores.
15,16
Primarily, these strategies involve nano-
casting (hard templating) or a combination of hard and so
templates.
17,18
Among other techniques, template replication of
hierarchical inorganic materials
13a
and sol–gel
19,20
methods are
known. A major challenge to date has been the development of
HPCs with very high surface areas, pore volumes and porosities
at all three different length scales: macro-, meso-, and micro in a
simple material platform. Additional challenges with the
synthesis techniques include the requirement to synthesize
porous inorganic materials or special nanoparticles as hard
templates which involve time-consuming multiple steps and
high expense. Furthermore, most of the already explored so-
templates are based on rather expensive and non-renewable
surfactants and block-copolymers. Moreover, the size of the
mesopores may be difficult to tune due to aggregation of the
nanoparticle carbon precursor matrix. Ice templating has been
known for its potential for fabricating macroporous and hier-
archical nanostructures.
21,22
Based on the dual templating
strategy, combined ice templating beside a hard template
(colloidal silica) followed by physical activation generates
excellent interconnected macro-, meso- and microporosity,
respectively as shown by Estevez and co-workers.
23
It is observed
that combined silica particles (hard template) and glucose
molecules (as carbon source) were expelled away from growing
ice crystals (hard template) by plunging the mixture into liquid
nitrogen. The glucose–silica composite scaffold offers an inter-
connected macroporous carbon–silica structure which remains
intact during pyrolysis and silica etching (Fig. 2a). Most
importantly the resulting HPC scaffold contains macroporous
walls made of interconnected mesoporous carbons (Fig. 2b). A
distinct advantage is that this approach minimizes the aggre-
gation of hydrophilic silica particles by instantaneous locking
within the glucose matrix and subsequent carbonization. The
process offers the bimodal nature of mesoporosity observed in
the pore size distribution when using silica particles of different
size. Moreover, the pore size of the carbon material increases
due to the aggregation during freezing. Ice-template–silica-
particle derived HPCs have a macro- and mesopores (multi-
modal) dominated texture with tight control and tenability of
their porosity in terms of size and extent. This offers an HPC
monolith with desired shape and size along with high pore
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volumes and large pore sizes, making this as excellent candi-
dates for amine based CO
2
capture.
3.2. So-templating
So-templating can be a good alternative method to access
HPCs in which pore structure collapse can be minimized using
molecular species in the reaction media, which aer gelation,
stabilize the pores from collapsing during drying and carbon-
ization.
24
It is proposed that micelles formed during the process
act as templates for the construction of pores. HPCs of tunable
pore size through so-templating can be obtained when a
cationic polyelectrolyte poly(diallyldimethylammonium) chlo-
ride (PDADMAC) is utilized as sotemplate while using resor-
cinol–formaldehyde (RF) gel as carbon source.
25
There is an
stabilizing effect of the cationic polyelectrolyte on the sol–gel
nanostructure in which the porosity of the gel is maintained
during the drying process. The introduction of a scaffold for the
mass production of HPC monoliths while using triblock
copolymer F127 as sotemplate and phenol–formaldehyde (PF)
resin as carbon precursor offers HPCs decorated with micro-
and meso-porosity prepared by surface coating and solvent
evaporation-induced self-assembly (EISA).
26
In this so-tem-
plating method of transformation of the monolith into the HPC
with ordered mesopores through thermo-polymerization, the
hierarchical porous architecture is retained while the bulk
structure of the scaffold (sugarcane bagasse) was destroyed.
This so-templating method simplies greatly the production
of porous carbon by making it unnecessary to use a complex
drying procedure. A dual templating (hard–so) approach can
give rise to HPC with new nanostructure. A ow-enabled self-
assembly approach using hierarchically assembled amphiphilic
diblock copolymer micelles and inorganic nanoparticles which
were craed over large areas results one-step hierarchical self-
organization, i.e. parallel threads comprising amphiphilic
diblock copolymer micelles and inorganic nanoparticles on the
nanometer scale.
27
This type of hierarchical assembly obtained
from block copolymer micelles would open up ways to fabricate
novel HPCs.
So far, few cases of HPC fabrication using polymers and
biomass-derived molecules as the carbon source and hard- or
so-templating strategy have been investigated. Among these,
dual templating (hard–so) have been found more promising
for fabricating interconnected porosity. The major hurdle is the
efficient fabrication of a hierarchical nanostructured silica or
other template with a tailored hierarchical porosity of meso/
macropores. An additional difficulty associated with the hard
templating process is template removal under concentrated
basic conditions. Either decomposition or etching are the
strategies to remove the hard template. The removal of any
porous inorganic template requires time-consuming multiple
steps. At the same time, most of the sotemplates used for self-
assembly are based on surfactants and block-copolymers, which
rather expensive and non-renewable. In some cases, the size of
the mesopores can also be difficult to ne-tune because of the
aggregation of the nanoparticles in the polymerizing carbon
precursor matrix.
27
In addition, so-templating techniques like
sol–gel suffer from the critical drawbacks associated with the
long synthesis time required for gelation, solvent exchange and
supercritical drying. Further, major difficulties are associated
with template removal in the nanocasting and so-templating
methods
28
irrespective of the carbon source; an efficient non-
templating approach to access porous carbons with a
Fig. 2 Ice-templating route to access HPC dominated by macro and mesopores. SEM images: (a) glucose–silica composite; (b) carbonized HPC
materials (HRSEM); (c) macroporous walls of the glucose–silica material. ((a)–(c) are reproduced from the ESI of ref. 26 with permission,
Copyright, RSC 2013.)
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controllable pore structure and good mechanical strength is
highly desirable. Thus, a self-assembly process of the carbon
source without a surfactant in the reaction system is a chal-
lenging task.
3.3. Non-templating
The 3D nanoscaled architecture not only provides a continuous
electron pathway to ensure good electrical contact, but also
facilitates ion transport by shortening diffusion pathways.
29–32
The major challenge for the development of carbon based
electrode materials for high-performance energy-storage is how
to achieve desirable properties such as large surface area, high
conductivity, efficient porosities in the micro-, meso- or macro-
pores, 3D nanoarchitecture and high-level heteroatom-doping.
Among the non-template based strategies, a noteworthy
approach based on polypyrrole (PPy) microsheets as precursors
for carbon and KOH activation offers HPC with hierarchical
porous microstructures exhibiting macroporous frameworks,
mesoporous walls, and microporous textures. This pore
network is ideal for diffusion of the active ions.
33
In PPy-derived HPCs, mesopores with diameter 10–50 nm
were detected in the carbon walls, which are wider as compared
to that of the traditional HPCs and activated carbons (pores
smaller than 4 nm). This is due to the efficient phase separation
between the hydrophobic carbon and water during the KOH
activation process. This material also exhibits an annealing-
temperature-dependent stability of the pore network. For
example, a pore widening maximum of 500 nm at 700 C was
recorded. Among the advantages of HPCs derived from PPy
sheet, the most signicant is the high surface area that leads to
an electrode/electrolyte interface sufficient to form electric
double layers. Another advantage is that the hierarchical porous
network not only ensures fast ion diffusion by shortening the
diffusion pathways, it also utilizes macroporous frameworks as
ion-buffering reservoirs, mesoporous walls as ion-highways for
fast ion transmission, and microporous textures for charge
accommodation. Additionally, these features also provide
continuous electron pathways, important for achieving high-
rate performance. Non-templating chemical activation methods
have also offered porous carbons with micro- (1.2 nm)
34
and
mesoporosity, which are obtained from the hydrothermal
carbonization of polysaccharides (starch and cellulose). Bio-
resourced carbon can also be derived from yeast, which
contains an amorphous matrix and brillar network that burns
offto deliver carbon material.
35
Here, the fraction of narrower
micropore (<0.7 nm) formation depends on the degree of acti-
vation of the carbons.
Fabrication of polystyrene-derived HPC (PS-HPC) with a
unique hierarchical porous nanonetwork depends on con-
structing carbonyl cross-linking bridges between PS chains via a
template-free method.
2,36
Fabrication of spherical HPC by linear
polystyrene depends on the construction of carbonyl (–CO–)
crosslinking bridges between linear polystyrenes in which
carbonyl crosslinking bridges simultaneously provide the
resulting hierarchical porous polystyrene (HPP) with a high
crosslinking density and sufficient oxygen atoms. This forms a
hierarchical pore structure during carbonization.
2–4,37
Intro-
ducing an appropriate pre-cross-linking density into poly-
styrene nanospheres could minimize the chances of the
distortion of spherical shapes during the process of swelling
and crosslinking. Indeed, –C
6
H
4
–crosslinking bridges by
incorporating divinylbenzene (DVB) ensure the stability of
styrene–divinyl benzene (St–DVB) copolymer nanospheres with
spherical shape, and result in mesopores of dimension 2–50 nm
via the compact aggregation of St–DVB nanospheres.
37
This
non-templating strategy is quite different from PPy microsheet-
derived HPCs which greatly depend on the chemical activation
method for 3D hierarchical pore construction. In this process,
–C
6
H
4
–crosslinking bridges provide stability to the nano-
spheres during swelling and –CO–crosslinking bridges offer
good nanostructure inheritability in carbonization. In this
process, access to the small sized monodisperse nanoparticles
is essential for the fabrication of HPC with mesopores aer
aggregation of the particles. Dispersion polymerization and
delay in addition ensures the formation of smaller nano-
particles (55 nm) by using a low styrene nucleation concentra-
tion which further ensures the spherical shape of the St–DVB
nanospheres. Carbonyl (–CO–) crosslinking bridges in CCl
4
result in a hierarchical pore network by intra-/inter-sphere
crosslinking of the polystyrene chains of the nanospheres
(Fig. 3). As a result, the intra-sphere space is subdivided into
numerous micropores by the as-constructed intra-sphere
crosslinking bridges. The attack of carbocations (CCl
3+
) on the
surface of nanospheres, resulting in inter-sphere –CCl
2
–cross-
linking bridges and stacking of nanospheres in a certain
orientation, leads to a 3D network nanostructure containing
micropores inside the nanospheres. The formation and growth
of network nanoparticles from a template-free method involves
the crosslinking of linear polystyrene chains that are hard to
control in terms of the shape and size distribution of the
particles, which limits the fabrication of HPCs with tunable
structures.
A hierarchical porous network of 3D interconnected micro-,
meso-, and macropores can also be accessible from the
carbonization of an hierarchical porous polyaromatic precursor
obtained from aromatic hydrocarbons (AHC) (polynaphthalene,
polypyrrole).
38
When the origin of the hierarchical pore network
in aggregates of carbon spheres is based on the methylene
bridges between phenyl rings, a fast ion transport/diffusion
behavior and increased surface area usage in electric double-
layer was found. The special nature of these AHC-derived HPCs
depends on their micropores in a 3D interconnected network
inside the cross-linked AHC microspheres imparting excep-
tionally high electrochemically accessible surface area for
charge accumulation. The method of HPCs obtained through
methylene bridges between aromatic rings offers a maximum
surface area 455 m
2
g
1
(S
BET
), which is indeed less than that
can be obtained via –CO–bridged polystyrene copolymer
derived HPCs (S
BET
887 m
2
g
1
). The total pore volume (max
0.41 cm
3
g
1
) of AHC-derived HPC is much less than that of the
polystyrene copolymer derived HPCs which indicates the
advantages of the introduction of –CO–crosslinking bridges for
nanostructure inheritability during carbonization. The superior
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electrical conductivity offered by AHC-derived HPC is perhaps
due to the formation of aggregates of spherical carbon spheres
composed of turbostratic carbon with a weakly ordered
graphitic microstructure which is different from the origin of
micropores from direct cabonization or activation processes.
Turbostratic carbon
39
is generally a variant of h-graphite,
stacked up by graphene layers with regular spacing but different
degree of stacking ordering. This weakly ordered graphitic
microstructure can enhance the electrical conductivity.
HPCs can be obtained via hydrothermal carbonization (HTC)
of glucose and fructose as carbon sources, which generally
undergo dehydration to 5-hydroxymethylfurfural
40
under HTC
conditions, and the resulting polyfuran-type molecules aggre-
gate into secondary spherical particles of the nal size. In this
non-templated process, poly(ionic liquid)s (PILs) act as a
stabilizer of primary nanoparticles formed at the initial stage
and allow only growth by further addition of monomers.
42
This
occurs by electrostatic repulsion exerted by the PIL to minimize
agglomeration by lowering the particle size to <50 nm. From
this stage, nal hierarchical particle (Fig. 4) forms via ordered
mesoporous carbons (OMCs) through the use of block copoly-
mers as sotemplate, typically using resorcinol–formaldehyde
resins as the carbon source.
41
The multivalent binding power of
PILs has been explored over ILs for the formation of pores apart
from its catalytic effect on the HTC process.
42
This reveals that,
in the PIL-based HTC process, the macromolecular architecture
of PIL and the nature of anion control the formation of HPCs
with the desired pore network and functionality.
In order to optimize the structural features of hierarchical
porous carbon monoliths (HCMs), an approach of incorpo-
rating host foreign components in the macropores, particularly
those showing high CO
2
capture capability, would be a novel
strategy. The synthesis of such HCM-based composites allows
further improvement of their volumetric CO
2
capture ability. A
recent study of incorporating metal–organic frameworks into
the hierarchical pores of HCMs with a MOF (Cu
3
(BTC)
2
) (BTC ¼
1,3,5-benzenetricarboxylic acid), known for its promising CO
2
capture ability, was reported (Fig. 5).
43
In HCM–Cu
3
(BTC)
2
Fig. 3 Mechanism for the formation of hierarchical porous carbon (HPC) by intra-/inter-sphere crosslinking of polystyrene chains of nano-
spheres via a template-free method.
Fig. 4 Hierarchical porous N-doped carbon nanostructure using PILs as a multipurpose agent. (Figure is reproduced from ref. 42 with
permission, Copyright Wiley-VCH, 2013.)
Fig. 5 HPC monolith containing MOF crystallites inside the macro-
pores and an SEM micrograph of HPC–Cu
3
(BTC)
2
. (Figure is repro-
duced from ref. 35 with permission, Copyright, American Chemical
Society, 2012.)
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composites, macropores of the HCM matrix provide a micro-
environment for the growth of Cu
3
(BTC)
2
crystallites and are
dispersed within the HCM matrix involving a restriction effect
of the carbon skeleton of the HCM. The HCM's polar surface
and high bulk density with incorporation of MOF crystallites in
the macropores of HCM provides a novel strategy for practical
CO
2
adsorption.
Alkali (KOH or NaOH) treatment is an effective method for
activating porous carbon in order to enhance the porous
structure and pore widening, which exerts superior electro-
chemical performance for example with high capacitance and
excellent rate capability in both aqueous and non-aqueous
electrolytes.
44–46
This method can be more effective for the
preparation of hierarchical porous carbon using a non-tem-
plating route. A poly(vinyldene uoride) (PVDF) derived porous
carbon can be further improved by a two-step carbonization–
activation process in which alkali (NaOH) acts as an interceptor
of HF and an activation agent.
47
Such a trick improves the
percentage of meso- and macropores in the hierarchical inter-
connected network. With the increase of the concentration of
the NaOH activator, the percentage of meso- and micropores
increases and thus the pore volume with thinner pore walls
shows a higher activation degree. This results in a very high
pore volume of 2.28 cm
3
g
1
and BET surface area of 2711 m
2
g
1
. When comparing one-step activation with the two-step by
NaOH an improved electrochemical performance is recorded
with the one-step activated material, which reveals the efficiency
of the former method and is perhaps due to the molten NaOH
poured over the fresh surface of the micropores obtained by
PVDF pyrolysis making a higher degree of activation with an
increase in the abundance of mesopores with a wider distri-
bution in the hierarchical interconnected pore network.
Activation of such polystyrene-derived HPC with KOH can
give rise to a unique 3D interconnected large meso- and mac-
roporous structure (27.3 and 68.5 nm, respectively) in the
carbon network.
48
The high surface area of KOH-activated PS-
HPC (3023 m
2
g
1
) is even higher than that of the KOH activated
ordered mesoporous carbon (2060 m
2
g
1
) but, however, lower
than that of the KOH-activated polypyrrole-derived carbons
which possess irregularly shaped platelets or particles of larger
size (20 mm).
49
The special nature of these PS-HPC materials is
that a large number of micropores and small mesopores whose
pore size are large enough for ILs to access are formed within
the carbon framework during KOH activation. This provides a
large interface for the formation of the electric double layer. An
ideal non-templating strategy is therefore using the inexpensive
to manufacture and easily sourced carbon; however, limited
methods have been reported, which allows a huge scope for the
future.
4. Biopolymer derived porous carbon
The application of biomaterials as biological template is known
for the nanostructuration of various inorganic materials and
metal nanoparticles for which cellulose and polysaccharide
nanocrystals play a signicant role.
50–52
However, scope of
accessible porous materials containing ordered well-dened
pores from biomass has not been explored until recently. As a
promising renewable resource, biomass offers an attractive raw
material, and starch has already been explored for the prepa-
ration of hierarchical porous carbons.
53–55
Alginate, a naturally
occurring polysaccharide extracted from marine brown algae,
has attracted substantial attention for applications in the
immobilization of enzymes and proteins and as a template in
the fabrication of nanostructured semiconductor materials.
56,57
Alginate has abundant carboxyl and hydroxyl groups in its
polymeric carbon matrix and it is particularly convenient as
template in the aqueous phase due to the presence of negative
charges of the glucoronic and mannuronic units.
58,59
When all
these functional groups have been converted to carbon oxides
and water, the polymeric carbon matrix can be converted
naturally to carbonaceous materials upon carbonization,
making alginate a suitable precursor for the fabrication of
porous carbons.
The simplest approach to access porous materials from
biomass is pyrolysis of native biomaterial under closed condi-
tions or an inert atmosphere. For potential applications like
supercapacitors, the uniformity of porosity in biomass would be
most advantageous to obtain hierarchical porous carbon
nanostructures derived from natural renewable resources,
making the strategy highly economical. In such case, the
application of homogeneous biopolymer hydrogels or hydro-
thermal carbonization is essential. In addition to these,
understanding the process of formation of hierarchical pore
networks in the carbon monolith obtained via carbonization is
essential. Controlled carbonization of alginic acid bers,
prepared through a simple wet spinning method, offers varied
mesopores and micropores with different shapes and sizes
around the nanoparticles that construct a hierarchical porous
network.
60
The wet-spinning method offers preservation of the regular
and ber-like shape of the alginic bers, which can be retained
aer carbonizing with laments exhibiting uniform 1D
morphology with a diameter 10 mm. Under higher magni-
cation (Fig. 6a), however, it is clear that the lament is
composed of nanosized carbon particles (less than 10 nm) with
a porous texture and no homogeneity in pore arrangements.
Fig. 6b shows the morphology of a single lament, which
exhibits a smooth surface both on the exterior and cross section
of the lament under low magnication. Abundant interspaces
around the nanoparticles revealed from the TEM image (Fig. 6c)
that the carbon bers are composed of an interconnecting 3D
network of carbon particles and form a hierarchical porous
structure composed of large mesopores (19 nm pore diameter)
and abundant micropores (<2 nm). However, more essential is
to understand the pathways of formation of the hierarchical
porous network from a controlled carbonization process of an
alginate framework in which oxygen might play a signicant
role in formation of graphenic carbon atoms; as observed in the
pyrolysis of alginate in presence of phosphorous source, which
offers a P-doped graphene.
61
Developing an efficient and facile synthesis protocol using
the precursors accessible directly from natural sources for the
preparation of 1D nanoporous carbon with well-tailored
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architectures would be of great signicance. Toward this
direction, it is demonstrated that electrospun ber-like natural
cocoon microbers can be directly transformed into 1D carbon
microbers with an average diameter of 6 mm. The most rele-
vant fundamental question would be how a unique 3D porous
network, consisting of 1D carbon microbers built with
numerous carbon nanoparticles (10–40 nm), was obtained via
carbonization (Fig. 7).
62
The recipe of the network of hierar-
chical porous structure is that these carbon nanoparticles
contain micropores and their compact and loose aggregation
leads to the formation of mesopores and macropores, respec-
tively. Interlacing of the 1D carbon microbers themselves in
the carbonized cocoon leads to the formation of large macro-
pore. Due to their effective pore interconnectivity, high surface
area and large pore volume, as well as high nitrogen content,
the as-prepared 1D HPC microbers (HPCMF) exhibit superior
electrochemical performance as binder-free electrodes for
supercapacitors and show promising adsorption behavior for
organic vapors.
Biomass-derived porous carbons, such as from fungi,
63
corn
grain, lignocellulosic materials,
64
sh scales,
65
starch,
66
celtuce
leaves
67
have already been reported. These showed great
potential as electrode materials for supercapacitors or as solid
adsorbents for CO
2
capture. In such pyrolysis methods, by
optimizing the carbonization temperature, the porosity and
capacitance of the resulting porous carbon can be balanced. For
example, porous activated carbon derived from celtuce leaves
(CL) by air-drying and pyrolysis offers a continuous and dis-
torted layered microstructure which can facilitate KOH
impregnation and activation (Fig. 8). Indeed nanoscale pores
and local curvature were formed in as-prepared carbon due to
the KOH etching of CL and generated substantial micro/meso-
pores of extremely small size (0.5 to 5 nm) and large mesopores
and/or textural macropores (30 to 60 nm) that were
interconnected and distributed homogeneously throughout the
porous structure.
Alkali (KOH or NaOH) treatment can also be used as effective
strategy for constructing or improving the hierarchical pore
network in biomass-derived porous carbon, as demonstrated
for HPC microspheres derived from porous starch.
68
In this
case, the KOH activation of the carbon microspheres resulted in
a new porous structure on the surface of the ladder-like channel
(Fig. 9f). The pores created by KOH along with the macropores
inherited from the precursor offer a 3D hierarchical structure
with an incredibly high BET surface area of 3251 m
2
g
1
. Most of
the macropores were inherited from the precursor. There are
four main regions, including the ultrane 0.4–0.7 nm and 0.9–
1.3 nm micropore region. The 4–14 mm pores in the starch
precursor contribute more to the total volume of HPC micro-
spheres. The predominant pore size in HPC microspheres is in
the range of 0.7–4mm which is likely to be due to shrinkage
during carbonization.
Natural cellulose substances such as lter paper and cotton
possess a macro-to-nanoscopic random morphological hier-
archy consisted of b-D-glucose chains. Replication of this
sophisticated network at the nanometer level was realized
previously by coating ultrathin metal oxide gel lms on each
cellulose nanober surface via a surface sol–gel process.
69
Stable
suspensions of nanocrystalline cellulose (NCC) can be obtained
through hydrolysis of bulk cellulosic material with sulfuric acid.
In water, suspensions of NCC organize into a chiral nematic
phase that can be preserved upon slow evaporation, thereby
resulting in chiral nematic lms.
70,71
NCC–silica composite
lms may also be used to generate mesoporous carbon (MC)
with a high specic surface area and excellent chiral nematic
organization (Fig. 10).
72
Though so-templating is a more
promising strategy, however, while using NCC as structural
template, the use of mesoporous silica is essential for meso-
porosity and preservation of the long-range chiral organization.
This is due to the formation of linkages between the carbon
regions during pyrolysis, which may be prevented without using
silica or using higher silica loading and the formation of thicker
silica walls. At an optimum silica concentration, maximum BET
surface are (1465 m
2
g
1
) and micropore volume (1.22 cm
3
g
1
)
can be achieved. Chiral nematic mesoporous carbon (CMC) also
exhibits locally aligned pores originating from local nematic
organization. This CMC lm contains smooth surfaces with a
repeating layered structure perpendicular to the surface. Apart
Fig. 6 Typical SEM image of as-prepared HPCFs obtained from alginic acid fibers: (a) regular fiber-like shape of alginic fibers after cabonization;
(b) morphology of a single filament with a smooth surface both on the exterior and cross section of the filament; (c) under higher magnification
the filament is composed of nanosized carbon particles surrounded by fissures and hillocks with a inhomogeneous porous structure. (Published
from the ref. 17 with permission, Copyright Wiley-VCH, 2010.)
Fig. 7 Silk cocoon as source of hierarchical carbon.
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from the great possibility of CMC materials as hard-templates
capable of transferring chiral nematic structure to the other
material, this also displays semiconducting properties which
increase with rising temperature.
As a promising nanochemical resource, biomass-derived
materials have several advantages. They are naturally abundant
and many are extracted industrially on a large scale as low-value
waste products; for example, lignin from krapulping.
Currently, new approaches for utilizing biomass-derived cellu-
lose as a source of HPC and other new materials have been
reported, however, in the near future, we hope that both
extracted biopolymers and native biomass will be explored.
5. Hetero-atom doped porous
carbon monolith
The incorporation of heteroatoms, such as B, N, P and O, into
the carbon lattice can enhance signicantly the mechanical,
semiconducting, eld-emission, and electrical properties of
carbon materials.
73
These heteroatoms in the porous carbon
may enhance the electrical conductivity, inuence the wetta-
bility, or consequently maximize the electroactive surface
area.
74–77
Recent studies suggests that surface functional groups
or doped heteroatoms play important roles in improving the
performance of the carbon electrodes.
78–80
Doping carbon
materials with electron rich atoms has its own advantages for
advanced technological applications. Such a strategy becomes
ideal when the biomass-derived proteins serve as precursors for
synthesizing carbon materials with unique structure, high
specic surface area (805.7 m
2
g
1
), partial graphitization, and
very high bulk nitrogen content (10.1 wt%). However, the above
method depends on the templating strategy of egg white-
derived protein to template the structure of mesoporous cellular
foam.
77
Current carbon supercapacitors have extensively used
redox reactions to increase their charge-storage capacity. Redox
reactions associated with oxygen-containing phenol or quinone/
hydroquinones can contribute one electron; phosphorus atoms
which stabilize oxygen functional groups during electro-
chemical charging and gas-phase catalysis. This improves
reaction stability and selectivity.
81,82
Boron atoms in a carbon
lattice are able to promote the chemisorption of oxygen for a
more-reactive carbon surface.
83
The advantage of nitrogen
doping to the porous carbon surface causes a shiof the Fermi
level to the valence band in the carbon electrode that is essential
for supercapacitor applications. The combined effect of
nitrogen/oxygen-containing functional groups was also evident
in capacitance-enhancement.
74
Two methods are commonly used to prepare nitrogen-rich
carbon materials. One is the simple heat treatment of nitrogen-
containing precursors (such as melamine-based polymers,
Fig. 8 Carbonated celtuce leaves’surface morphology and cross-section morphology showing the pore network.
Fig. 9 The SEM images of (a) the porous starch precursor, (b) a porous
starch granule under high magnification, (c) the hierarchical porous
carbon microspheres, (d) the hierarchical porous carbon microspheres
under high magnification, (e) the internal ladder-like microstructure of
the hierarchical porous carbon microspheres and (f) the selected part
in (e) under high magnification.
Fig. 10 Chiral nematic porous carbon via NCC–silica composite films,
pyrolysis at 900 C, and silica removal from the carbon–silica
composite.
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polyacrylonitrile, vinylpyridine resin, and silk broin) under an
inert atmosphere.
84–86
The other method is the low-temperature
treatment (250–350 C) of carbon materials in a mixture of
ammonia and air with different volume ratios.
87
The location of
the N-centers of the carbon lattice of the porous carbon
monolith contributes to the efficiency of the materials for
supercapacitor and gas adsorption applications. Generally, HPC
obtained from the carbonization of nitrogen-containing
precursors offers more quaternary nitrogen centers (N-Q)
(located at the center and valley position of the carbon lattice)
which lack an ammonia-assisted carbonization method.
Ammonia-assisted carbonization of thermosetting-type
phenolic resin obtained via magnesium hydroxide templating
offers signicant nitrogen doping to carbon, which increases
the surface area with the involvement of different nitrogen
groups for ion transport and also depends on the relative redox
activities of the nitrogen groups.
88
In the case of magnesium
hydroxide templating, heat treatment in ammonia results in an
unexpected increase of the BET specic surface area and
micropore volume. This happens due to edge-nitrogen atoms
connected with two carbon atoms, a motif which prevents the
growth of the carbon lattice, leaving more space in the form of
micropores. The generation of micropores or their expansion
occurs due to the release of hydrogens which etch carbons,
causing removal of carbon atoms from lattice.
The CO
2
adsorption capacities of nitrogen-rich porous
carbons have been less studied as compared with amine-func-
tionalized porous silica. As compared to commercially available
activated carbons, polymer-based synthetic porous carbons
have high purity, good reproducibility, and well-dened pore
structures. Thus, such porous carbons have also been modied
to nitrogen-rich porous carbons by deliberate selection of N-
containing carbon precursors, such as melamine. For instance,
highly nitrogen-enriched porous carbons were prepared from
melamine–formaldehyde resins.
89
Recently, various nitrogen-
containing porous carbon monoliths were fabricated and used
for CO
2
capture and separation.
90
Using resorcinol–formalde-
hyde as carbon precursors and the amino acid L-lysine as the
catalyst, a type of nitrogen-doped porous carbon monolith was
synthesized by Lu and co-workers which possesses a maximum
CO
2
capture capacity of 3.13 mmol g
1
at 25 C and 1 atm under
aow of pure CO
2
.
82,91
Subsequently, a nitrogen-containing
porous carbon monolith with fully interconnected macro-
porosity and mesoporosity was fabricated, which could with-
stand a pressure of up to 15.6 MPa.
89,91
To avoid using harsh chemicals for activation and sacricial
templates, physical activation to obtain activated carbon
materials is highly desirable from both environmental and
economic points of view. The CO
2
uptake capacity can be
enhanced by incorporating basic N-functional groups and
narrow micropores (<1 nm) with high adsorption potential.
Porous activated carbon monoliths (ACMs) can be accessed via
activation–carbonization of mesoporous polyacrylonitrile (PAN)
monoliths in an oxidizing CO
2
environment (Fig. 11).
92
The
formation of carbon with a lamellar phase occurred by
carbonization of rigid polymer with an extended 2D-framework
obtained from the PAN monolith via cyclization inside the
polymer framework. Cyclization–aromatization imparts extra
rigidity to the polymer network which may be responsible for
the creation of narrow micropores. During carbonization,
removal of nitrogen from the framework causes fusion of the
molecular ladders which result in extended sheet-like lamellar
carbon frameworks. Micropores randomly distributed all over
the skeleton of the microstructures and N-rich porous surface
are held responsible for CO
2
uptake with maximum of
11.51 mmol g
1
at 273 K.
Phosphate-functionalized carbon materials are interesting
because of the widening of the operational voltage window with
aqueous electrolytes and the subsequent increase of the energy
that can be attained.
93
A resorcinol-based deep-eutectic solvent
(DES), composed of resorcinol, choline chloride and glycerol,
assisted the synthesis of phosphate-functionalized carbon
monoliths (PfCMs) via the polycondensation of formaldehyde
in the presence of phosphoric acid as catalyst, giving access to a
bicontinous porous network built with highly cross-linked
clusters that are aggregated and assembled into a stiff, inter-
connected structure.
94
Bi-continuous (with micropores and
mesopores) structures contain macropores of 0.56 mm resulting
from the spinodal decomposition process at the resorcinol
polycondensation stage in which a polymer-rich phase may
form with the segregation of the non-condensed matter. The
DES-assisted synthesis offers hierarchical porosity in the
phosphate functionalized carbon monolith with direct
assembly of carbon cylinders, which are excellent for super-
capacitor electrodes. In this process, DES plays multiple roles
such as tailoring the textural properties and composition of the
resulting carbon materials as a structure directing agent and
carbonaceous precursor. Moreover, the template effect of DESs
was reected in HPC monoliths with a maximum pore surface
area of 600 m
2
g
1
and narrow mesopore diameter
distributions.
N and B co-doped carbon monoliths with hierarchical pores
can be accessible via a self-assembly-carbonization process
starting from poly(benzoxazine-co-resol) and ionic liquid
[C
16
mim][BF
4
] as the carbon and boron sources, respectively.
95
A
high skeleton density and fully interconnected pore network
with ultramicropores (pore width < 1 nm) was obtained. This
pores are suitable for the diffusion of electrolyte ions by mini-
mizing the molecular diffusion limitation, thus potentially
advantageous for the enhancement of supercapacitor perfor-
mance. The nanostructure of the carbon–boron co-doped
carbon (CNB) contains a random combination of graphitic and
turbostratic stacking with short range ordering and displays a
graphite-like microstructure with good electrical conductivity
Fig. 11 Formation path of ACMs with narrow micropores in the carbon
framework.
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due to the multi-length-connected carbon framework. The
electron conductivity in the carbon materials is enhanced due
to the redistribution of pelectrons in the presence of
substituted boron and nitrogen that weakens C–C bonds and
strengthens C–O bonds when exposed to air. The ionic liquid
plays the role of introducing heteroatoms into the carbon
framework, through particle involvement in the condensation
process.
From the above discussion, it is revealed that for improved
conductivity of ions for a range of applications such as super-
capacitors, the introduction of heteroatoms to the local
graphitic carbon matrix is essential. Multiple heteroatom
incorporation into a carbon matrix is a comparatively new
strategy to enhance electron conductivity for electrochemical
energy storage applications. This strategy depends on the
molecular design of monomers and a choice of solvent/media
which is capable of supplying heteroatoms to the carbon matrix
during the condensation process. Simultaneous control of
nanoporosity and electrochemical accessibility of the nitrogen
atoms are essential for nitrogen-enriched nanoporous carbons
and a general synthetic pathway is desired. However, the most
frequently used strategy so far is the use of well-dened block
copolymers as precursor for nitrogen-enriched carbon, and the
sacricial block serves as a source of mesoporosity.
6. Synergistic effect of macro–
meso–micropores for applications
Compared to conventional porous materials consisting of
uniform pore dimensions that can be adjusted over a wide
range of length scales, hierarchical porous materials with well-
dened pore dimensions and topologies can exhibit minimized
diffusive resistance to mass transport from macropores and a
high surface area for active site dispersion from micro- and/or
mesopores. Depending on the specic requirement of many
emerging applications related to energy storage, environmental
cleaning, and sensing, a hierarchical nanoarchitecture of the
carbon materials is essential. A number of emerging applica-
tions of carbon materials is identied and the desired hierar-
chical interconnecting porous network is discussed.
6.1. CO
2
storage materials
The development of new porous materials as sorbents for CO
2
removal via selective adsorption nds potential application in
ue gas treatment and natural gas upgrading, which are
considered as clean energy technologies. This captured CO
2
can
further be used as feedstock to produce liquid fuels or can be
offered to microbes that consume CO
2
and produce fuels.
Carbon capture and sequestration in the form of CO
2
adsorp-
tion is a coherent extension of solar-to-fuel and biomass
conversions to biofuels. For the capture of CO
2
from ue gas
mixtures, the materials are usually operated under ambient
conditions. Thus, gas diffusion properties become the domi-
nant factor which indicates balance among the surface area,
pore size and interconnected pore structures. Generally, a
highly porous adsorbent when doped with nitrogen,
9
or in
monolithic form under relatively mild conditions, would facil-
itate large-scale application. The reversible absorption/desorp-
tion based on the humidity swing is very similar to a green leaf,
which displays net CO
2
uptake in sunlight and output in the
dark; a functional porous material would act as an “articial
leaf”that is able to capture and release CO
2
in ambient air
under dry and wet conditions, respectively.
96
The porous
support materials, consisting of immobilized quaternary
ammonium cations with hydroxide, bicarbonate, or carbonate
counter anions, would be desired materials under dry and
humid conditions.
97
Materials with high specic surface areas
generally contain a large number of micro- and mesopores,
which may decrease the kinetics of absorption and desorption
due to diffusional limitations. Additionally, the capillary forces
in the micropores can also decrease the sorption rates. It would
be highly desirable to obtain porous carbon materials acces-
sible from macroporous polymers for reversible CO
2
capture
under ambient air by humidity swing with an improved
absorption/desorption kinetics controlled by the hierarchical
porous framework. Given that recent studies show that both
high porosity and interconnectivity between pores makes best-
performing materials, such materials can be obtained via
sacricial templating. Functional material of same porosity can
be accessible from a suitable carbon source, which retains the
structure of the template upon carbonization.
For CO
2
capture and storage, the inclusion of analogues of
water soluble amines into the walls of solid porous materials
and the construction of nitrogen-rich porous adsorbents
provide several advantages as compared with the costly regen-
eration step in amine-based solution absorption processes. A
nitrogen-rich porous carbon with a hierarchical micro/meso-
porous structure exhibiting fast adsorption–desorption kinetics
is highly desirable.
14
Nitrogen-rich porous carbons open the
door for the preparation of highly effective carbonaceous
adsorbents for CO
2
capture. The relatively low synthetic cost
and easy fabrication, combined with an excellent efficiency of
CO
2
adsorption and separation, make nitrogen rich porous
carbons highly promising for CO
2
-selective adsorption in prac-
tical applications. Porphyrin based nitrogen rich porous
organic polymers (POPs) are another interesting candidate for
CO
2
capture from ue gas.
98
A wide range of POPs can be
synthesized through extended aromatic substitution involving
the condensation of pyrrole with aromatic dialdehydes in the
presence of the Lewis acid FeCl
3
under solvothermal condi-
tions. The high surface area and N-rich polymeric network
together with porosity facilitates their great capacity for CO
2
capture. It has been observed that ordered 3D-hexagonal mes-
oporous silica HMS-4 functionalized with vinyl groups at the
surface of the mesopores could adsorb 5.5 mmol g
1
(24.3 wt%)
under 3 bar pressure at 273 K
99
due to its advantages of cage-like
pore openings, very high surface area and organic functionali-
zation over related mesoporous silica-based materials.
6.2. Carbon photonic crystals as sensors
It has been observed that glassy carbon nanostructures
obtained via porous silicon templating function as highly
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efficient adsorbents for volatile organics and also act as optical
vapor sensors.
7
This photonic properties of the carbon-inl-
trated composite pSi lm were found to be amended from the
Si-based sensor and the sensing strength depended on the
porous nature of the surface, containing two different types of
mesopores. Upon removal of the Si-template, the porous carbon
membrane can be retained when immersed in liquid (meth-
anol, ethanol, hexane, toluene) and this can act as a photonic
crystal. From highly porous carbon replicas with extremely high
structural delity in the silicon template, a carbon nanober
array can be obtained. This carbon nanober material in liquid
media would be a better alternative to photonic sensors. A
similar material can also be used for adsorption–desorption
phenomena. The enhancement of the refractive index of the
pore interior to red-shiof the position of the photonic stop
band is an essential criterion. Thus, a porous layer of carbon
acts as an organic toolkit for the silicon surface since carbon is a
superior adsorbent which covers uniformly the Si layer.
6.3. Li–S batteries
Li–S batteries are considered to hold great potential for the next
generation of large-scale and high energy density energy-storage
devices. In order to tackle the problem of high solubility of
intermediate soluble polysulde ions which results in a shuttle
phenomenon between the anode and cathode and a loss of
active mass, carbon materials with a hierarchical pore structure
can encapsulate and sequester elemental sulfur for high-
performance Li–S batteries. This improves electrical conduc-
tivity and prevents polysulde dissolution.
100
It was found that
HPCs with mesoporous walls and interconnected macropores
encapsulate elements that result in an improvement in the
performance of Li–S batteries. The contribution of mesopores
in encapsulation of sulfur suggests the presence of an hierar-
chically ordered structure.
101
This material’s walls are composed
of spherical mesopores, which indicates a hierarchically
ordered porous structure in which elemental sulfur could be
impregnated into the mesoporous walls of HPC in a highly
dispersed state, which inhibit aggregation of sulfur and initiate
essential electrical contact. The mesopores while serving as
container, traps elemental sulfur and subsequent lithium pol-
ysuldes during the charge–discharge process. In this process,
an appropriate ratio of mesopores/macropores facilitates the
transition of Li
+
during electrochemical cycling by reducing the
ionic and electronic conduction distance. Thus integration of
mesopores (sulfur lithiation) and macropores (ion transport) is
an essential feature for the next generation of hierarchically
porous carbon materials for Li–S battery applications (Fig. 12).
6.4. Supercapacitor
Effective utilization of intermittent renewable sources facilitates
energy storage, and electrochemical energy is stored in super-
capacitors (SCs) to transport high power within a short period in
portable electronic devices and hybrid electric vehicles. The
development of an electrode having both a high end specic
capacitance is closely related to the availability of inter-
connected meso-micropores and high rate capability. A
hierarchical structure with well-interconnected small and large
pores would provide the opportunity to optimize the specic
capacitance and rate capability of carbon materials as super-
capacitor electrodes. In general, since SCs store energy physi-
cally at the electrode/electrolyte interface based on an
electrochemical double-layer mechanism for which a high
surface area is the basic requirement in carbon-based electrode
materials. However, for commercial porous carbon, poor rate
performance is usually observed due to the low conductivity,
high ion-transport resistance and insufficient ionic diffusion
within the tortuous micropores, which limits their application
in high-power energy storage devices. What is missing in
commercially available carbon is an additional second-order
structure of meso/macropores which needs to be induced.
Three-dimensional (3D) porous nanostructures are desirable for
high-performance electrode materials. The 3D nanoscaled
architecture can not only provide a continuous electron
pathway to ensure good electrical contact, but also facilitate ion
transport by shortening diffusion pathways. The superior elec-
trochemical performance of the carbon is attributed to its
unique hierarchical porous structure, which provides the crit-
ical features required for advanced supercapacitors: abundant
micropores and mesopores provide the electrode with a high
accessible surface area, resulting in a large capacitance and
high energy density, while interconnected mesopores and
macropores facilitate ion transport, which ensure high rate
capability and high power density.
102
Hierarchical porous
carbon (HPC) materials have elicited porous structures and are
able to exhibit the advantages of each pore size with a syner-
gistic effect during the electrochemical charge–discharge
process. Macro/mesopores facilitate rapid ion transport by
serving as ion-buffering reservoirs and ion-transport path-
ways,
13d,14
and the micropores enhance the electrical double
layer.
103,104
Porous carbon materials, when used as electrodes in
electrochemical capacitors, suffer from electrode kinetic prob-
lems which are due to the inner-pore ion transport resulting in
Fig. 12 The electrochemical reaction process inside the pores of the
HOPC/S nanocomposite cathode.
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poor performance.
103,105
In spite of the fast ion transport ability
as well as the high utilization of surface area, HPCs still suffer
from a low energy density, a universal bottleneck for electric
double-layer capacitors (EDLCs).
6.4.1. Mechanism of ion transport and buffering. The exact
mechanism of ion transport within porous materials is highly
complex: various factors such as the shape of pores, connec-
tivity, and pore-size distribution should be considered. Apart
from the pore structure and shape, nature of electrolyte and
solid–liquid interface must all be considered. Among these
factors, the inner-pore ion transport resistance and diffusion
distance are the most important factors. Generally, micropores
(d< 2 nm) exhibit large specic surface areas but the pore walls
may interfere with ion transportation resulting in poorer power
density. When the pore diameters are smaller than solvated
ions, the formation of a double layer is supposed to be impos-
sible. However, in certain cases an electric double layer is
formed even when the solvated ions are larger than pore
diameters when using an organic electrolyte (LiClO
4
in
propylene carbonate and dimethoxy ethane).
106
In the hierar-
chical porous structure, electrolyte ions can be delivered
smoothly through meso/macropores to micropore surfaces with
large specic surface areas. From the fundamental viewpoint,
an analysis of the contribution of capacitance by meso/macro-
pores and micropores, separately is essential, however, in the
case of porous carbon, pore sizes are continuously distributed
from micropores to mesopores, thus making the analysis
complicated. In addition, electrolyte transport in their narrow
pores causes kinetic polarization and thus the capacitance may
be underestimated. It is believed that pores substantially larger
than the size of the electrolyte ion and its solvation shell are
required for high capacitance. The demonstration of charge
storage in pores smaller than the size of solvated electrolyte ions
will lead to an enhanced understanding of ionic transport in
porous media. These ndings should also permit the design of
specic supercapacitor applications for longer discharge times
where energy density is at a premium, such as in hybrid electric
vehicles, where extremely narrow pores should prove optimal,
but for pulse power applications, increasing the pore size might
be benecial.
6.4.2. Electrical double layer enhancement. In the case of
ordered mesoporous materials, interconnectivity of the chan-
nels is benecial in improving ion diffusion properties. An
efficient pore network and interconnectivity of the channels can
lead to much lower impedance to ion transport within both the
channels and the micropores in the carbon wall, and thus have
better electric double layer performance. Doping with an elec-
tron decient element (e.g. B) with valance electrons, can
introduce a hole charge carrier once it replaces a carbon atom in
the carbon lattice. This would increase the charge density and
further improve the double layer capacitance. This doping also
improves the polarity of a carbon matrix by improving the
wettability, and allows an easy diffusion of the electrolyte ions
into micropores. The local graphitic heteroatom-incorporated
carbon matrix presents a higher conductivity due to their
exceptional nanostructure and surface characteristics. Accord-
ing to Largeot et al. 2008, when the pore size of the electrode
materials is close to the size of an ion in the electrolyte, a
maximum capacitance can be obtained.
107
Taking into account
the fact that the solvated ion size of K
+
is between 0.36 nm and
0.42 nm, it can be predicted that the pores of these rst two
regions will contribute the most to the formation of the electric
double-layer, leading to a high capacitance as described by
Eliad et al.
108
The other two regions are the 1.3–3.4 nm micro/
mesopore region, which plays a role in electrolyte ion diffusion
paths, and the >50 nm macropore region which serves as an ion-
buffering reservoir and thus decreases the ion transport
distance during electrochemical processes for a starch-derived
hierarchical carbon. The contribution of the meso/macro and
micropores to C
DL
can be analyzed by the following equation.
C
DL
¼c
dl,meso
S
meso
+c
dl,micro
S
micro
where ¼c
dl,meso
and c
dl,micro
are the specic electric double layer
capacitance of meso/macropores and micropores, respectively.
Here the ion sieving effect of micropores comes into play in the
case of electrolytes with a positive ion radius slightly larger than
the micropore radius.
106
6.4.3. Ion diffusion. The unique hierarchical porous
structure of the HPCs favors the rapid diffusion of electrolyte
ions into the pores in supercapacitor electrodes. In a hierar-
chical porous architecture, large mesopores would provide a
fast diffusion channel for the electrolyte and the diffusion
distance would also be very short as the micropores are located
within the mesopore wall. Micropores are expected to be most
efficient in a double-layer formation. The energy and power
limitations normally observed at high rate are associated with
the complex resistance and the tortuous diffusion pathways
within the porous textures. At high discharge current, only some
parts of the pores (mainly the outer regions) can be accessed by
ions, whereas at low current, both the outer- and the inner-pore
surfaces are used for charge storage. The good energy and
power performances of HPCs conrm that most of micropores
within the mesoporous skeleton can be utilized effectively for
charge storage. A reduced capacitance of microporous carbons
at large discharging current densities was found; however, such
a reduced capacitance also exists for mesoporous carbon
materials, probably because of the solute diffusion process.
7. HPC versus ordered mesoporous
carbon (OMC)
The links between the hierarchically porous structure and their
role in performing energy conversion and storage can promote
the design of novel nanostructures with advanced properties.
Apart from providing large surface areas, HPCs can provide
interfacial transport, dispersion of active sites at different
length scales of pores (macro, meso, micro), and shorter a
diffusion path. Some theoretical calculations predict that the
hierarchically macro-, meso-, microporous structured catalysts
can reduce the diffusion limitations.
109,110
The low resistance
and short diffusion pathway facilitate fast electron and mass
transport to enhance the electrochemical energy storage
performance.
111
HPCs exhibit outstanding behavior in
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diffusion-assisted adsorption applications driven by the
different ranges of interconnected pores. This is very different
from the ordered mesoporous network which provides shorter
diffusion pathways along the carbon particles, and enhances
the diffusivity of molecules and/or ions. Generally, while the
construction process of HPCs, removal of hard-templates and
carbonization of the carbon source material results in micro-
porosity in the resulting carbon and mesoporous network,
generated as a result of incomplete pore lling of the hard
template.
Ordered mesoporous carbons (OMC) have received consid-
erable attention owing to their large surface area, tunable pore
structure, uniform and adjustable pore size, mechanical
stability and good conductivity. In spite of these outstanding
features, most of the mesoporous carbons derived so far have a
highly hydrophobic surface and a limited number of specic
active sites, which impedes their practical application. The
synthesis of HPC and OMCs depends on several factors such the
choice of C-containing precursor or N source for N-doped
porous carbons. For example, direct self-assembly of organic–
organic amphiphilic block copolymers has been a versatile
route to access OMCs, however these strategy suffers from
inferior pore structure with poor thermal stability or low N
content. The reason behind the collapse of the mesostructure is
the pluoronic surfactant with high oxygen content which
promotes decomposition of the N-containing part of the
precursor. In addition, pyrolysis at high temperature can also
accelerate the decomposition of the frameworks, resulting in a
collapsed mesostructure. When a direct synthesis of ordered
mesoporous carbons from organic–organic self-assembly with a
high level of doped element (e.g. N) suffers from high N content
and large surface area, this is a great challenge and in most
cases direct carbonization offers material with a wide pore size
and disordered mesostructure.
112
Hao et al. reported the direct self-assembly of poly-
(benzoxazine-co-resol) followed by a carbonization process,
obtaining N-doped porous carbons with well-dened hierar-
chical porosities
90
that contained dened multiple-length-scale
pore structures (macro-, meso-, and micropores) of fully inter-
connected macroporosity and mesoporosity with cubic Im3m
symmetry. This offers a remarkable mechanical strength to the
material. The generation of highly interconnected pores in the
nanostructures is dependent on the self-assembly of poly-
(benzoxazine-co-resol) and the carbonization process. In
this process, polybenzoxazine segments form hydrogen bonds
(Ar–O–H/O) with the EO segment of F127 to a signicant
extent, which guide the mesostructure assembly within polymer
species and the amphiphilic copolymer template (F127). During
the following curing step, the unreacted resorcinol and form-
aldehyde copolymerize with polybenzoxazine. In this case, both
the polybenzoxazine and poly(resorcinol-formaldehyde)
together form the hybrid skeletons which lead to the formation
of stable mesostructures.
In the case of OMCs the N source is external, while for
thermosetting polymerization the N source is incorporated.
However, in the case of using polybenzoxazine-co-resol self-
assembly in the presence of F127, the N source is attached to the
copolymer, undergoing self-assembly before carbonization. The
difference is that resorcinol–dicyandiamide obtains an ordered
mesoporous framework and a porous framework with an
interconnected multi-length-scale pore structure via a copoly-
mer self-assembly is obtained when using polybenzoxazine-co-
resol. Control experiments revealed that mesopores were
generated via the assembly of polybenzoxazine and the ethylene
oxide (EO) segment of F127 due to the oxygen affinity of the O
centers. This reveals that the presence of surfactant F127 is
essential for mesopore formation. Further, the condensation of
the resorcinol with polybenzoxazine and assembly of the
resorcinol segment of the poly(benzoxazine-co-resol) with the
EO segment offers hierarchical pore formation. When using a
resol-based copolymer such as poly(benzoxazine-co-resol) with
F127 as template, two segments (benzoxazine and resol)
interact separately with F127's EO segment via H-bonding and
electrostatic interactions, and this more complex self-assembly
forms in solution, which upon evaporation of solvent and
pyrolysis produces inter-connected pore networks in the nano-
structure (Fig. 13).
112
This comparative study on the synthesis of
OMC and HPCs reveals that the carbon source polymer played a
crucial role in determining the nanostructure of the porous
carbon when using the same polymer template with oxygen
containing blocks.
Ordered mesoporous structures are slightly degenerated, as
revealed from the small-angle X-ray direction (SAXS) patterns of
the P6m2D hexagonal mesostructure of H-NMC obtained with
various mass ratio of dicyandiamide (DCDA) to resol. In this
material, uniform mesopores are periodically aligned over a
large domain as revealed from cross-sectional eld-emission
scanning electron microscope (FESEM) analysis.
113
Unique nanoscale spherical OMCs with extremely high
bimodal porosities are an attractive material for applications
involving rapid charge–discharge capacity and good recycla-
bility. A two-step nanocasting process to obtain a spherical
mesoporous carbon nanoparticle with hierarchical pores
involves the application of a silica inverse opal which was used
as template for a triconstituent precursor solution containing
resol, tetraethylorthosilicate (TEOS) and Pluronic F127 as
structure directing agent.
114
This process involves carbonization
and etching of the silica inverse opal template, and results in
OMC with hierarchical porosity. In this case, silica inverse opal
templating creates OMC with a high inner pore volume
(2.32 cm
3
g
1
), a very high surface area (2445 m
2
g
1
) and a
bimodal pore size distribution with large and small mesopores
of 6 nm and 3.1 nm. This bimodal distribution arises from the
porous walls formed by etching the silica from the carbon–silica
nanocomposite walls.
8. Future potentials of HPCs in
emerging nanotechnologies
The hierarchical pore arrangement of HPCs has placed them far
ahead other conventional porous materials for tackling the
energy related problems based on their high specic surface
area, thermal stability, mass productivity, and fast adsorption–
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desorption kinetics. The possibility of utilizing HPCs derived
from a wide range of biomass sources has yet to be employed for
next generation technological applications relevant to energy
solutions; for example, applications of HPCs in unitized
regenerative fuel cells (URFCs), energy-storage systems for
uninterrupted power supplies, solar-powered aircra, and
satellites. The bioresourced highly-reproducible 3D morphology
with macro-to-mesoporous architecture can resemble the
intricate structure of the source. Chemical tailoring of such
hierarchically-porous, carbon converted, biogenic structures is
envisaged to result in a high degree of enzyme loading for rapid
enzymatic applications. Such carbon structure can also be
amplied with carboxylic groups by chemical modication and
metal deposition, opening the scope for new applications.
Several hydrolase enzymes can be attached to the carboxy-
functionalized HPCs via electrostatic attachment and depend-
ing on the loading of enzyme, several biocatalytic process can be
derived.
HPC coating on other hierarchically structured materials,
especially magnetic materials with a carbon-coated surface with
a nanostructured interior, can be suitable candidate for Li-ion
batteries due to their unique carbon shell. However such
materials open signicant scope for energy storage applications
in other devices as well. Apart from exploring HPCs for appli-
cation in storing and generation of energy, HPCs are also
promising candidates for the design of catalysts required for
selective biomass conversion processes. Highly uniform and
conformal coatings on both surfaces and to inltrate HPCs
derived from biomass will have immense signicance. Highly
dispersed organic and inorganic species on high surface area
materials with complex topology are another emerging area,
where atomic layer deposition can also be used as alternate
method for preparing composites with HPCs.
The enhancement of the energy density of supercapacitors
depends greatly on optimized porosity or hybrid devices by
employing pseudocapacitive elements. The effect of the low
charge carrier density of carbon on the total material capaci-
tance is not considered attentively. It considered that the
increase in density of states (DOS) of low density of charge
carriers in carbon materials leads to a substantial increase in
capacitance as the electrode potential increases.
115
A signicant
tool would be to improve the carbon capacitor performance,
doping with highly graphitic carbons for a stronger degree of
electrochemical doping and high skeleton density that may
result in enhancement of capacitance.
These apart, HPC would nd potential applications in the
area of enhancement of photocatalytic applications based on
hierarchically organized porous structures which might result
in a slow-photon effect and this photonic structural property of
HPCs is yet to be explored for solar thermal storage and articial
photosynthesis. A highly opened-up surface structure and
hierarchical order would offer a framework with properties of
accelerating charge collection and separation,
116
thus a so–
photocatalytic interface favoring mass transfer may be
accessible.
Accessing highly ordered DNA nanowires which are used in
waveguides, photodetection, nanophotonic switches, logic
devices, etc. is an attractive eld. Controlled evaporation-
induced self-assembly of a DNA aqueous solution on the HPC
surface may offer aligned DNA nanowires, however the process
would depend on the control of ow within the evaporating
solution.
117
Such controlled evaporation self-assembly would be
remarkable and introduce a new avenue for craing DNA-based
HPC nanostructures for these applications. Further, hierar-
chical micro/mesopores in an HPC can act as drug-loaded
nanocontainers to enhance the targeting capability to tumor
Fig. 13 Formation of HPC and OMC using F127 as soft-templates, where resol and resol-based block copolymer are used as the carbon sources.
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tissues in vitro and inhibit tumor growth with minimal side
effects in vivo.
118
9. Conclusions
Several energy and environmental solutions require the
advancement of highly stable, ordered porous nanostructured
materials to enhance the performance of core devices, for
purposes such as solar and chemical energy storage, separation
of gas molecules, and pollutant removal from air. Particularly,
there is a surge in demand for cutting down the emission-levels
of CO
2
from nonrenewable sources. The short-term goals of
carbon capture and sequestration require the absorptive
removal of CO
2
from air. Control and optimization of pore
geometry, size, volume, surface area and intrinsic basicity at the
surface are crucial to achieve efficient CO
2
capture at pre- and
post-combustion power stations. High surface area and ultra-
high total pore volume are signicantly desired for enhanced
applications of hierarchically porous materials. Given their rich
properties, HPCs are potential candidates for further pore
optimization/functionalization for energy storage, especially as
super-capacitor electrode materials, methane storage systems,
and Li-ion adsorption–desorption in batteries. Obviously, the
selection of a precursor plays a critical role in obtaining desir-
able porosities in the sp
2
-bonded graphenic carbons
119
and N-
doping can considerably enhance their CSS application through
favorable dipolar interaction with the CO
2
molecules.
In spite of recent advances on the control of the porosity of
carbon materials through mainly hard templating (nano-
casting) procedures, and the development of novel carbon
materials (graphene, carbon nanotubes), porous carbons with
an inter-connected pore hierarchy would be the choice for the
construction of new generation electrodes for commercial
supercapacitors and other energy related applications. To
comply with future energy demands, higher control over the
textural properties (pore size, volume, and aspect ratio) must be
maximized to improve power densities, making them suitable
for medium to long-term solutions. Furthermore, it is essential
to reduce the cost of accessing carbon materials by exploring
precursors such as biomass, which is low-cost, readily available
and renewable, thus will play a key role for various purposes.
Strategies for deriving hetero-atom doped carbon from
biopolymers and their further application as electrodes for use
in alkaline supercapacitors depends on the controlled ammonia
assisted carbonization when starting from a biopolymer
without N-content. Strategies of deriving doped-hierarchically
porous carbon from biopolymers can also be developed for
other elements such P, B etc. Ex situ electrochemical spectros-
copy can be used to investigate the evolution of N-functional
groups on the surface of the N-doped carbon electrodes in the
supercapacitor cell.
The major difficulty faced in constructing interconnected
pores in the macro–meso–microporous carbon system from a
non-templating route or from a biorenewable source is
controlling the pyrolysis step. A dual templating strategy
has been evolved for the construction of a hierarchical pore
network for crystalline TiO
2
lms
120
for application in
photoelectrochemical water-splitting due to structural homo-
geneity and integrity. Consequently, such strategies to access
HPC materials from biopolymers have not yet been developed.
However, HPCs have not yet been exploited for asymmetric
supercapacitors (ASCs) with a faradaic electrode as the energy
source and a capacitive electrode as a power source, in which an
effective approach to increase the cell voltage has plenty of
scope. Additionally, potential raw materials like bacterial
cellulose have yet to be explored for the design of HPCs with a
network of pores, either as a template or precursor for carbon.
Acknowledgements
SD and KCW Wu thank the Ministry of Science and Technology
(MOST), Taiwan (101-2628-E-002-015-MY3, 101-2623-E-002-005-
ET, 101-2923-E-002-012-MY3, 103-2811-E-002-018 and 103-2218-
E-002-101), National Taiwan University (101R7842 and
102R7740), and the Center of Strategic Materials Alliance for
Research and Technology (SMART Center) of National Taiwan
University (102R104100) for funding supports. AB wishes to
thank GITA, New Delhi for funding through India-Taiwan S&T
Cooperation Program.
References
1 J. Aizenberg, J. C. Weaver, M. S. Thanawala, V. C. Sundar,
D. E. Morse and P. Fatzl, Science, 2005, 309, 275.
2 C. Zou, D. C. Wu, M. Z. Li, Q. C. Zeng, F. Xu, Z. Y. Huang and
R. W. Fu, J. Mater. Chem., 2010, 20, 731.
3 B.-Z. Fang, J.-H. Kim, M.-S. Kim, A. Bonakdarpour, A. Lam,
D. Wilkinson and J.-S. Yu, J. Mater. Chem., 2012, 22, 19031.
4 H. L. Wang, Y. Yang, Y. Y. Liang, J. T. Robinson, Y. G. Li,
A. Jackson, Y. Cui and H. J. Dai, Nano Lett., 2011, 11, 2644.
5 B.-Z. Fang, M.-W. Kim, S.-Q. Fan, J.-H. Kim, D. Wilkinson,
J.-J. Ko and J.-S. Yu, J. Mater. Chem., 2011, 21, 8742.
6 B.-Z. Fang, J.-H. Kim, M.-S. Kim and J.-S. Yu, Langmuir,
2008, 24, 12068.
7 T. L. Kelly, T. Gao and M. J. Sailor, Adv. Mater., 2011, 23,
1776.
8 B.-Z. Fang, J.-H. Kim, M.-S. Kim and J.-S. Yu, Chem. Mater.,
2009, 21, 789.
9 P.-Z. Li and Y. Zhao, Chem.–Asian J., 2013, 8, 1680.
10 J. Liu, T. Yang, D.-W. Wang, G. Q. Lu, D. Zhao and
S. Z. Qiao, Nat. Commun., 2013, 4, 2798.
11 B. Fang, J. H. Kim, M.-S. Kim and J.-S. Yu, Acc. Chem. Res.,
2013, 46, 1397.
12 (a) S.-W. Woo, K. Dokko, H. Nakano and K. Kanamura, J.
Mater. Chem., 2008, 18, 1674; (b) C. Liang, K. Hong,
G. A. Guiochon, J. W. Mays and S. Dai, Angew. Chem., Int.
Ed., 2004, 43, 5785; (c) Y. Meng, D. Gu, F. Zhang, Y. Shi,
H. Yang, Z. Li, C. Yu, B. Tu and D. Zhao, Angew. Chem.,
Int. Ed., 2005, 44, 7053.
13 (a) Z.-Y. Wang, F. Li, N. S. Ergang and A. Stein, Chem.
Mater., 2006, 18, 5543; (b) L.-Z. Fan, Y.-S. Hu, J. Maier,
P. Adelhelm, B. Smarsly and M. Antonietti, Adv. Funct.
Mater., 2007, 17, 3083; (c) M. C. Gutierrez, F. Pic,
F. Rubio, J. M. Amarilla, F. J. Palomares, M. L. Ferrer,
Energy Environ. Sci. This journal is © The Royal Society of Chemistry 2014
Energy & Environmental Science Minireview
Published on 29 May 2014. Downloaded by University of Florida on 23/07/2014 16:59:02.
View Article Online
F. Del Monte and J. M. Rojo, J. Mater. Chem., 2009, 19, 1236;
(d) D. Carriazo, F. Pic, M. C. Gutierrez, F. Rubio, J. M. Rojo
and F. Del Monte, J. Mater. Chem., 2010, 20, 773; (e)
H.-J. Liu, X.-M. Wang, W.-J. Cui, Y.-Q. Dou, D.-Y. Zhao
and Y.-Y. Xia, J. Mater. Chem., 2010, 20, 4223.
14 D. W. Wang, F. Li, M. Liu, G. Q. Lu and H. M. Cheng, Angew.
Chem., Int. Ed., 2008, 47, 373.
15 A. H. Lu, G. P. Hao, Q. Sun, X. Q. Zhang and W. C. Li,
Macromol. Chem. Phys., 2012, 213, 1107.
16 H. Nishihara and T. Kyotani, Adv. Mater., 2012, 24, 4473.
17 Y. Deng, C. Liu, T. Yu, F. Liu, F. Zhang, Y. Wan, L. Zhang,
C. Wang, B. Tu, P. A. Webley, H. Wang and D. Zhao,
Chem. Mater., 2007, 19, 3271.
18 Z. Wang and A. Stein, Chem. Mater., 2008, 20, 1029.
19 J. Biener, M. Stadermann, M. Suss, M. A. Worsley,
M. M. Biener, K. A. Rose and T. F. Baumann, Energy
Environ. Sci., 2011, 4, 656.
20 S. L. Candelaria, R. Chen, Y. H. Jeong and G. Cao, Energy
Environ. Sci., 2012, 5, 5619.
21 L. Estevez, A. Kelarakis, Q. Gong, E. H. Da'as and
E. P. Giannelis, J. Am. Chem. Soc., 2011, 133, 6122.
22 M. C. Guti´
errez, M. Jobb´
agy, N. Rap´
un, M. L. Ferrer and
F. Del Monte, Adv. Mater., 2006, 18, 1137.
23 L. Estevez, R. Dua, N. Bhandari, A. Ramanujapuram,
P. Wang and E. P. Giannelis, Energy Environ. Sci., 2013, 6,
1785.
24 Y. Xia, Z. Yang and R. Mokaya, Nanoscale, 2010, 2, 639.
25 J. Balach, L. Tamnorini, K. Sapag, D. F. Acevedo and
C. A. Barbero, Colloids Surf., A, 2012, 415, 343.
26 C.-H. Huang and R.-A. Doong, Microporous Mesoporous
Mater., 2012, 147,47
–52.
27 S. Han and T. Hyeon, Chem. Commun., 1999, 1955.
28 N. Pal and A. Bhaumik, Adv. Colloid Interface Sci., 2013, 189–
190,21–41.
29 Z. Li, L. Zhang, B. S. Amirkhiz, X. Tan, Z. Xu, H. Wang,
B. C. Olsen, C. M. B. Holt and D. Mitlin, Adv. Energy
Mater., 2012, 2, 431.
30 B. G. Choi, M. Yang, W. H. Hong, J. W. Choi and Y. S. Huh,
ACS Nano, 2012, 6, 4020.
31 H. Jiang, P. S. Lee and C. Li, Energy Environ. Sci., 2013, 6, 41.
32 L. Qie, W. Chen, H. Xu, X. Xiong, Y. Jiang, F. Zou, X. Hu,
Y. Xin, Z. Zhang and Y. Huang, Energy Environ. Sci., 2013,
6, 2497.
33 B. Li, W. Han, B. Jiang and Z. Lin, ACS Nano, 2014, 8, 2936.
34 M. Sevilla and A. B. Fuertes, Energy Environ. Sci., 2011, 4,
1765.
35 W. Shen, Y. He, S. Zhang, J. Li and W. Fan, ChemSusChem,
2012, 5, 1274.
36 D. Wu, C. M. Hui, H. Dong, J. Pietrasik, H. J. Ryu, Z. Li,
M. Zhong, H. He, E. K. Kim and M. Jaroniec,
Macromolecules, 2011, 44, 5846.
37 Q. Zeng, D. Wu, C. Zou, F. Xu, R. Fu, Z. Li, Y. Liang and
D. Su, Chem. Commun., 2010, 46, 5927.
38 X. Huang, S. Kim, M. S. Heo, J. E. Kim, H. Suh and I. Kim,
Langmuir, 2013, 29, 12266.
39 Z. Q. Li, C. J. Lu, Z. P. Xia, Y. Zhou and Z. Luo, Carbon, 2007,
45, 1686.
40 (a) S. Dutta, RSC Adv., 2012, 2, 12575; (b) S. Dutta, S. De and
B. Saha, ChemPlusChem, 2012, 77, 259.
41 (a) Y. Huang, H. Cai, D. Feng, D. Gu, Y. Deng, B. Tu,
H. Wang, P. A. Webley and D. Zhao, Chem. Commun.,
2008, 2641; (b) M. C. Gutierrez, F. Pico, F. Rubio,
M. Amarilla, F. J. Palomares, M. L. Ferrer, F. Del Monte
and J. M. Rojo, J. Mater. Chem., 2009, 19, 1236.
42 P. Zhang, J. Yuan, T.-P. Fellinger, M. Antonietti, H. Li and
Y. Wang, Angew. Chem., Int. Ed., 2013, 52, 6028.
43 D. Qian, C. Lei, G.-P. Hao, W.-C. Li and A.-H. Lu, ACS Appl.
Mater. Interfaces, 2012, 4, 6125.
44 J. Wang and S. Kaskel, J. Mater. Chem., 2012, 22, 23710.
45 X. He, R. Li, J. Qiu, K. Xie, P. Ling, M. Yu, X. Zhang and
M. Zheng, Carbon, 2012, 50, 4911.
46 B. Xu, S. Hou, G. Cao, F. Wu and Y. Yang, J. Mater. Chem.,
2012, 22, 19088.
47 B. Xu, S. Hou, G. Cao, M. Chu and Y. Yang, RSC Adv., 2013,
3, 17500.
48 H. Zhong, F. Xu, Z. Li, R. Fu and D. Wu, Nanoscale, 2013, 5,
4678.
49 L. Wei, M. Sevilla, A. B. Fuertes, R. Mokaya and G. Yushin,
Adv. Funct. Mater., 2012, 22, 827.
50 N. Lin, J. Huang and A. Dufrense, Nanoscale, 2012, 4, 3274.
51 S. J. Eichhorn, SoMatter, 2011, 7, 303.
52 K. E. Shopsowitz, H. Qi, W. Y. Hamad and
M. J. MacLachlan, Nature, 2010, 468, 422.
53 (a) E. Raymundo-Pinero, F. Lerous and F. Beguin, Adv.
Mater., 1877, 2006, 18; (b) S. H. Guo, J. H. Peng, W. Li,
K. B. Yang, L. B. Zhang, S. M. Zhang and H. Y. Xia, Appl.
Surf. Sci., 2009, 255, 8443.
54 F. Zhang, K. X. Wang, G. D. Li and J. S. Chen, Electrochem.
Commun., 2009, 11, 130.
55 W. Z. Shen, Z. F. Qin, H. G. Wang, Y. H. Liu, Q. J. Guo and
Y. L. Zhang, Colloids Surf., A, 2008, 316, 313.
56 S. Dutta, A. K. Patra, S. De, A. Bhaumik and B. Saha, ACS
Appl. Mater. Interfaces, 2012, 4, 1560.
57 M. Buaki-Sogo, M. Serra, A. Primo, M. Alvaro and H. Garcia,
ChemCatChem, 2013, 5, 513.
58 M. Chtchigrovsky, Y. Lin, K. Ouchaou, M. Chaumontet,
M. Robitzer, F. Quignard and F. Taran, Chem. Mater.,
2012, 24, 1505.
59 A. Primo, M. Liebel and F. Quignard, Chem. Mater., 2009,
21, 621.
60 X. L. Wu, L.-L. Chen, S. Xin, Y.-X. Yin, Y.-G. Guo, Q.-S. Kong
and Y.-Z. Xia, ChemSusChem, 2010, 3, 703.
61 M. Latorre-Sanchez, A. Primo and H. Garcia, Angew. Chem.,
Int. Ed., 2013, 52, 11813.
62 Y. Liang, D. Wu and R. Fu, Sci. Rep., 2013, 3, 1119.
63 H. Zhu, X. L. Wang, F. Yang and X. R. Yang, Adv. Mater.,
2011, 23, 2745.
64 F. C. Wu, R. L. Tseng, C. C. Hu and C. C. Wang, J. Power
Sources, 2004, 138, 351.
65 W. X. Chen, H. Zhang, Y. Q. Huang and W. K. Wang, J.
Mater. Chem., 2010, 20, 4773.
66 L. Wei, M. Sevilla, A. B. Fuertesm, R. Mokaya and G. Yushin,
Adv. Energy Mater., 2011, 1, 356.
This journal is © The Royal Society of Chemistry 2014 Energy Environ. Sci.
Minireview Energy & Environmental Science
Published on 29 May 2014. Downloaded by University of Florida on 23/07/2014 16:59:02.
View Article Online
67 R. Wang, P. Wang, X. Yan, J. Land, C. Peng and Q. Xue, ACS
Appl. Mater. Interfaces, 2012, 4, 5800.
68 S.-H. Du, L.-Q. Wang, X.-T. Fu, M.-M. Chen and C.-Y. Wang,
Bioresour. Technol., 2013, 139, 406.
69 J. Huang and T. J. Kunitake, J. Am. Chem. Soc., 2003, 125,
11834.
70 J. Pan, W. Y. Hamad and S. K. Strauss, Macromolecules,
2010, 43, 3851.
71 J. F. Revol, L. Godbout and D. G. Gray, J. Pulp Pap. Sci., 1998,
24, 146.
72 K. E. Shopsowitz, W. Y. Hamad and M. J. MacLachlan,
Angew. Chem., Int. Ed., 2011, 50, 10991.
73 (a) A. Y. Liu and M. L. Cohen, Science, 1989, 245, 841; (b)
Y. D. Xia and R. Mokaya, Adv. Mater., 2004, 16, 1553.
74 D. Hulicova-Jurcakova, M. Seredych, G. Q. Lu and
T. J. Bandosz, Adv. Funct. Mater., 2009, 19, 438.
75 L. F. Chen, X. D. Zhang, H. W. Liang, M. Kong, Q. F. Guan,
P. Chen, Z. Y. Wu and S. H. Yu, ACS Nano, 2012, 6, 7092.
76 M. Zhong, E. K. Kim, J. P. McGann, S.-E. Chun,
J. F. Whitacre, M. Jaroniec, K. Matyjaszewski and
T. Kowalewski, J. Am. Chem. Soc., 2012, 134, 14846.
77 Z. Li, Z. Xu, X. Tan, H. Wang, C. M. B. Holt, T. Stephenson,
B. C. Olsen and D. Mitlin, Energy Environ. Sci., 2013, 6, 871.
78 Z. Wen, X. Wang, S. Mao, Z. Bo, H. Kim, S. Cui, G. Lu,
X. Feng and J. Chen, Adv. Mater., 2012, 24, 5610.
79 H. R. Byon, B. M. Gallant, S. W. Lee and Y. Shao-Horn, Adv.
Funct. Mater., 2013, 23, 1037.
80 Y. Mao, H. Duan, B. Xu, L. Zhang, Y. Hu, C. Zhao, Z. Wang,
L. Chen and Y. Yang, Energy Environ. Sci., 2012, 5, 7950.
81 D. Hulicova-Jurcakova, A. M. Puziy, O. I. Poddubnaya,
F. Surez-Garcia, J. M. D. Tascn and G. Q. Lu, J. Am. Chem.
Soc., 2009, 131, 5026.
82 J. Zhang, X. Liu, R. Blume, A. H. Zhang, R. Schlogl and
D. S. Su, Science, 2008, 322, 73.
83 D. W. Wang, F. Li, Z. G. Chen, G. Q. Lu and H. M. Cheng,
Chem. Mater., 2008, 20, 7195.
84 D. Hulicova, J. Yamashita, Y. Soneda, H. Hatori and
M. Kodama, Chem. Mater., 2005, 17, 1241.
85 C. O. Ania, V. Khomenko, E. Raymundo-Pinero, J. B. Parra
and F. Beguin, Adv. Funct. Mater., 2007, 17, 1828.
86 Y. J. Kim, Y. Abe, T. Yanaglura, K. C. Park, M. Shimizu,
T. Iwazaki, S. Nakagawa, M. Endo and M. S. Dresselhaus,
Carbon, 2007, 45, 2116.
87 K. Jurewicz, K. Babel, R. Pietrzak, S. Delpeux and
H. Wachowska, Carbon, 2006, 44, 2368.
88 D.-W. Wang, F. Li, L.-C. Yin, X. Lu, Z.-G. Chen, I. R. Gentle,
G.-Q. Lu and H.-M. Cheng, Chem.–Eur. J., 2012, 18, 5345.
89 (a) L. Meng and X. Junmin, J. Phys. Chem. C, 2014, 118,
2507; (b) C. Ma, J. Shi, Y. Song, D. Zhang, X. Zhai,
M. Zhong, Q. Guo and L. Liu, Int. J. Hydrogen Energy,
2012, 7, 7587.
90 G.-P. Hao, W.-C. Li, D. Qian, G.-H. Wang, W.-P. Zhang,
T. Zhang, A.-Q. Wang, F. Schuth, H.-J. Bongard and
A.-H. Lu, J. Am. Chem. Soc., 2011, 133, 11378.
91 G.-P. Hao, W.-C. Li, D. Qian and A.-H. Lu, Adv. Mater., 2010,
22, 853.
92 M. Nandi, K. Okada, A. Dutta, A. Bhaumik, J. Maruyama,
D. Derks and H. Uyama, Chem. Commun., 2012, 48, 10283.
93 D. Carriazo, M. C. Gutierrez, F. Pico, J. M. Rojo,
J. L. G. Fierro, M. L. Ferrer and F. Del Monte,
ChemSusChem, 2012, 5, 1405.
94 D. Carriazo, M. C. Gutierrez, M. L. Ferrer and F. Del Monte,
Chem. Mater., 2010, 22, 6146.
95 D.-C. Guo, J. Mi, G.-P. Hao, W. Dong, G. Xiong, W.-C. Li and
A.-H. Lu, Energy Environ. Sci., 2013, 6, 652.
96 T. Wang, K. S. Lackner and A. Wright, Environ. Sci. Technol.,
2011, 45, 6670.
97 H. He, W. Li, M. Zhong, D. Konkolewicz, D. Wu, K. Yaccato,
T. Rappold, G. Sugar, N. E. David and K. Matyjaszewski,
Energy Environ. Sci., 2013, 6, 488.
98 A. Modak, M. Nandi, J. Mondal and A. Bhaumik, Chem.
Commun., 2012, 48, 248.
99 A. Dutta, M. Nandi, M. Sasidharan and A. Bhaumik,
ChemPhysChem, 2012, 13, 3218–3222.
100 X. L. Ji, K. T. Lee and L. F. Nazar, Nat. Mater., 2009, 8, 500.
101 B. Ding, C. Yuan, L. Shen, G. Xu, P. Ni and X. Zhang, Chem.–
Eur. J., 2013, 19, 1013.
102 Z. Chen, J. Wen, C. Yan, L. Rice, H. Sohn, M. Shen, M. Cai,
B. Dunn and Y. Lu, Adv. Energy Mater., 2011, 1, 551.
103 J. Chmiola, G. Yushin, Y. Gogotsi, C. Portet, P. Simon and
P. L. Taberna, Science, 2006, 313, 1760.
104 W. Xing, C. Huang, S. Zhuo, X. Yuan, G. Wang, D. Hulicova-
Jurcakova, Z. Yan and G. Lu, Carbon, 2009, 47, 1715.
105 W. Xing, C. C. Huang, S. P. Zhuo and X. Yuan, Langmuir,
2009, 25, 7783.
106 H. Yamada, I. Moriguchi and T. Kudo, J. Power Sources,
2008, 175, 651.
107 C. Largeot, C. Portet, J. Chmiola, P. L. Taberna, Y. Gogotsi
and P. Simon, J. Am. Chem. Soc., 2008, 130, 2730.
108 L. Eliad, G. Salitra, A. Soffer and D. Aurbach, J. Phys. Chem.
B, 2001, 105, 6880.
109 G. Wang, E. Johannessen, C. R. Kleijn, S. W. de Leeuw and
M.-O. Coppens, Chem. Eng. Sci., 2007, 62, 5110.
110 G. Wang and M.-O. Coppens, Chem. Eng. Sci., 2010, 65,
2344.
111 Y. H. Deng, C. Liu, T. Yu, F. Liu, F. Q. Zhang, Y. Wan,
L. Zhang, C. Wang, B. Tu, P. A. Webley, H. Wang and
D. Zhao, Chem. Mater., 2007, 19, 3271.
112 (a) J. S. Lee, X. Q. Wang, H. M. Luo, G. A. Baker and S. Dai, J.
Am. Chem. Soc., 2009, 131, 4596; (b) X. Q. Wang and S. Dai,
Angew. Chem., Int. Ed., 2010, 49, 6664; (c) W. Yang,
T. P. Fellinger and M. Antonietti, J. Am. Chem. Soc., 2011,
133, 206; (d) J. S. Lee, X. Q. Wang, H. M. Luo and S. Dai,
Adv. Mater., 2010, 22, 1004.
113 J. Wei, D. Zhou, Z. Sun, Y. Deng, Y. Xia and D. Zhao, Adv.
Funct. Mater., 2013, 23, 2322.
114 J. Schuster, G. He, B. Mandlmeier, T. Yim, K. T. Lee, T. Bein
and L. F. Nazar, Angew. Chem., Int. Ed., 2012, 51, 3591.
115 P. Ganesh, P. R. C. Kent and V. Mochalin, J. Appl. Phys.,
2011, 110, 073506.
116 J. Zhang, J. Sun, K. Maeda, K. Domen, P. Liu, M. Antonietti,
X. Fu and X. Wang, Energy Environ. Sci., 2011, 4, 675.
Energy Environ. Sci. This journal is © The Royal Society of Chemistry 2014
Energy & Environmental Science Minireview
Published on 29 May 2014. Downloaded by University of Florida on 23/07/2014 16:59:02.
View Article Online
117 B. Li, W. Han, M. Byun, L. Ahu, Q. Zou and Z. Lin, ACS
Nano, 2013, 7, 4326.
118 Z. Luo, X. W. Ding, Y. Hu, S. J. Wu, Y. Xiang, Y. F. Zeng,
B. L. Zhang, H. Yan, H. C. Zhang, L. L. Zhu, J. J. Liu,
J. H. Li, K. Y. Cai and Y. L. Zhao, ACS Nano, 2013, 7,
10271–10284.
119 G. Srinivas, V. Krungleviciute, Z.-X. Guo and T. Yildirim,
Energy Environ. Sci., 2014, 7, 335.
120 R. Zhang, D. Shen, M. Xu, D. Feng, W. Li, G. Zheng, R. Che,
A. A. Elzatahry and D. Zhao, Adv. Energy Mater., 2014, 4,
DOI: 10.1002/aenm.201301725.
This journal is © The Royal Society of Chemistry 2014 Energy Environ. Sci.
Minireview Energy & Environmental Science
Published on 29 May 2014. Downloaded by University of Florida on 23/07/2014 16:59:02.
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