Chuangang Hu1, Jia Qu2, Ying Xiao3, Shenlong Zhao4, Hao Chen2, Liming Dai1,2,3,4. 1. Center of Advanced Science and Engineering for Carbon (Case4Carbon), Department of Macromolecular Science and Engineering, Case Western Reserve University (CWRU), 10900 Euclid Avenue, Cleveland, Ohio 44106, United States. 2. Institute of Advanced Materials for Nano-Bio Applications, School of Ophthalmology & Optometry, Wenzhou Medical University, 270 Xueyuan Xi Road, Wenzhou, Zhejiang 325027, China. 3. College of Energy, Beijing University of Chemical Technology, Beijing, China. 4. UNSW-BUCT-CWRU International Joint Laboratory, School of Chemical Engineering, The University of New South Wales, Sydney, New South Wales 2052, Australia.
Abstract
Along with the wide investigation activities in developing carbon-based, metal-free catalysts to replace precious metal (e.g., Pt) catalysts for various green energy devices, carbon nanomaterials have also shown great potential for biorelated applications. This article provides a focused, critical review on the recent advances in these emerging research areas. The structure-property relationship and mechanistic understanding of recently developed carbon-based, metal-free catalysts for chemical/biocatalytic reactions will be discussed along with the challenges and perspectives in this exciting field, providing a look forward for the rational design and fabrication of new carbon-based, metal-free catalysts with high activities, remarkable selectivity, and outstanding durability for various energy-related/biocatalytic processes.
<nclass="Chemical">spaclass="Chemical">n class="Chemical">Alclass="Chemical">n>ong with the wide investigation activities in developing <class="Chemical">spn>an class="Chemical">carbon-based, <class="Chemical">span class="Chemical">metal-free catalysts to replace precious metal (e.g., Pt) catalysts for various green energy devices, carbon nanomaterials have also shown great potential for biorelated applications. This article provides a focused, critical review on the recent advances in these emerging research areas. The structure-property relationship and mechanistic understanding of recently developed carbon-based, metal-free catalysts for chemical/biocatalytic reactions will be discussed along with the challenges and perspectives in this exciting field, providing a look forward for the rational design and fabrication of new carbon-based, metal-free catalysts with high activities, remarkable selectivity, and outstanding durability for various energy-related/biocatalytic processes.
Cat<nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n>y<class="Chemical">spn>an class="Chemical">sis is the core
for many sustainable energy conver<class="Chemical">span class="Chemical">sion devices and advanced biorelated
technologies.[1−3] Precious metals (e.g., Pt, Pd, Rh, Ru, Ir) and their
oxides are commonly used catalysts. However, their scarcity and high
price have limited certain commercial applications.[4] Most carbon materials are earth-abundant, ecofriendly,
and biocompatible, and, some of them are even catalytically active
and stable. Therefore, carbon-based, metal-free catalysts (C-MFCs)
have attracted worldwide interest as alternatives to precious metal-based
catalysts, particularly for energy/biorelated applications.[4−7] Compared with conventionalmetal-based catalysts, C-MFCs also display
a high and broad tunability because of rich surface chemistries and
lack of metal dissolution and poisoning. In this context, the introduction
of heteroatom(s) into the carbon skeleton (i.e., heteroatom-doping),
by either in situ doping during the nanocarbon synthesis or through
post-treatment (i.e., postdoping) of preformed carbon nanostructures,[4,5,8] has been demonstrated to cause
electron modulation of carbon atoms for facilitating catalytic reactions[4,6] and the surface property changes for biorelated applications.[7,9] Since the discovery of nitrogen-doped vertically aligned CNTs (VA-NCNTs)
for oxygen reduction reaction (ORR) in 2009,[10] worldwide efforts have been dedicated to the development of various
C-MFCs for the ORR,[4,11−13] oxygen evolution
reaction (OER),[14] hydrogen evolution reaction
(HER),[15,16] two-electron (2e–) transfer
ORR to produce H2O2 (an energy carrier and green
oxidizer),[17] I3– to I– reduction in dye-sensitized solar cells,[18] CO2 reduction reaction (CO2RR) for direct conversion of CO2 into fuels,[19,20] N2 reduction reaction (N2RR) for synthesis
of NH3 under ambient environment,[21] sustainable generation of green energy from sunlight and water,[22] biosensing, environmental monitoring,[23] and even commodity chemical production.[24,25]
By
creating a variety of class="Chemical">coexisticlass="Chemical">ng active <class="Chemical">n class="Chemical">span class="Chemical">sites, C-MFCs can possess
multiple cat<spclass="Chemical">n>an class="Chemical">alytic functionalities, which is otherwise difficult,
if not impossible, using metal-based catalysts. This significant advantage
allows for the design of new C-MFCs capable of catalyzing different
chemical reactions and/or bioprocesses simultaneously. Of particular
interest, certain C-MFCs have been demonstrated to be effective bifunctional
electrocatalysts for OER and ORR in rechargeable metal-air batteries
for efficient energy storage[26] as well
as OER and HER in photocatalytic/photoelectrochemicalwater splitting
systems to produce H2 and O2 gases from water
and sunlight.[27] In conjugation with photocatalysis,
these bifunctional electrocatalysts could be employed to harvest,
convert, and then store the solar energy, offering the possibility
for developing light-driven energy systems.
Apart from the fabrication
of C-MFCs for energy class="Chemical">coclass="Chemical">nver<class="Chemical">n class="Chemical">span class="Chemical">sion and storage, <spclass="Chemical">n>an class="Chemical">nanocarbons have also
been recently used for various biomedical applications.[28] Particularly, certain carbon nanomaterials were
demonstrated as stable and effective C-MFCs for detecting H2O2 released from living cells, while a novel solid-state
fluorescent sensor was fabricated by simply dipping a piece of filter
paper into carbondots with polyhedral oligomeric silsesquioxane (CDs@POSS)
solutions for efficient detection of ions (e.g., Fe3+)
of biological importance.[29] More recently,
certain rationally designed biocompatible carbon nanomaterials have
shown great potential in photodynamic therapy, sonodynamic therapy,
and catalytic nanomedicine.[30−33]
In this focused and critic<nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n> review, we summarize
recent advances in developing C-MFCs for energy and biorelated applications.
The ch<class="Chemical">spn>an class="Chemical">allenges and opportunities in this exciting field are presented
as well, <class="Chemical">span class="Chemical">along with elucidation of the structure–property relationship
and mechanistic understanding of recently developed C-MFCs in energy
and biorelated processes, providing a look forward for rational design
and fabrication of various C-MFCs with high activities, remarkable
selectivity, and outstanding durability for various energy/biocatalytic
processes.
Advanced Carbon Nanomaterials
Depending
on the arrangement of <nclass="Chemical">spaclass="Chemical">n class="Chemical">carboclass="Chemical">nclass="Chemical">n> atoms, <class="Chemical">spn>an class="Chemical">carbon has been tradition<class="Chemical">span class="Chemical">ally
divided into three categories: amorphous carbon, graphite, and diamond.[4] The recent discoveries of C60, CNTs,
and graphene (graphene nanosheets, graphene quantum dots, graphene
nanoribbon) opened up an important field in carbon material science
and technology (Figure ).[4] Using these individualcarbon nanomaterials
as building blocks, three-dimensional (3D) carbon architectures (Figure ) have been devised
as efficient porous C-MFCs, exhibiting a large specific surface area
(SSA) with numerous accessible active centers, high electrical conductivity
and ion diffusibility, and even good mechanical strength.[4,34−36]
Figure 1
Structure of carbon nanomaterials: diamond, graphite,
C60, CNTs, graphene, and 3D graphene-CNT hybrid materials.[34] Images reprinted with permission from ref (34). Copyright 2012, Elsevier.
St<nclass="Chemical">spaclass="Chemical">n class="Chemical">ruclass="Chemical">n>cture of <class="Chemical">spn>an class="Chemical">carbon nanomateri<class="Chemical">span class="Chemical">als: diamond, graphite,
C60, CNTs, graphene, and 3D graphene-CNT hybrid materials.[34] Images reprinted with permission from ref (34). Copyright 2012, Elsevier.
In addition to the <nclass="Chemical">spaclass="Chemical">n class="Chemical">graphiticclass="Chemical">n> <class="Chemical">spn>an class="Chemical">carbon
materi<class="Chemical">span class="Chemical">als mentioned above, nanodiamond (ND) derived materials consisting
of sp3carbon atoms have also been developed as superior
catalysts for special chemical reactions,[37−40] nanocarriers/probes for drug
delivery,[41,42] and biomedical imaging.[43,44] Because of the interplay between B- and N- doping,[40,45] B,N codoped nanodiamond (BND) exhibited a high efficiency and stability
toward selective CO2 reduction to ethanol as it passivated
the competitive HER reaction.[39] On the
other hand, as ND materials are both scalable and biocompatible, functionalized
ND particles have been used for targeted delivery of drugs,[41,42] and in vitro or in vivo imaging.[46−48] For instance, Ho et
al. demonstrated nanodiamonds ensured great enhancements toward high
efficacy and safe drug (doxorubicin) delivery in both murine liver
tumor and mammary carcinoma models.[49] Specifically,
fluorescent NDs (ND-phosphorylcholine composites) exhibited good dispersibility
in water, strong fluorescence, and low toxicity, making the ND-based
materials ideal for cell imaging.[44] In
particular, Zhang et al. have developed a versatile ND construct that
incorporates a targeting, imaging, and chemo-therapeutic agent attractive
for drug delivery with capabilities of targeting, imaging, and controlled
drug releasing, simultaneously.[50] Therefore,
the structure/charge modulation and surface functionalization of carbon-based
materials offer possibilities for advancing C-MFCs for energy and
biorelated applications.
Creation of Active Sites
in C-MFCs
In 2009,
the first heteroatom-<nclass="Chemical">spaclass="Chemical">n class="Chemical">dopedclass="Chemical">n> C-MFC was reported when a VA-NCNT array
was discovered to be an effective electrode toward cat<class="Chemical">spn>an class="Chemical">alyzing the
ORR in an <class="Chemical">span class="Chemical">alkaline medium.[10] The high catalytic
performance of the metal-free VA-NCNTs for ORR was ascribed to the
charge redistribution among C atoms and the adjacent N heteroatoms
induced by N-doping, which changed the oxygen molecule adsorption
mode to weaken the “O–O” bond, facilitating the
ORR (Figure a,b).[10] This groundbreaking work was followed by enormous
research worldwide.[4,6,11,51−54] Subsequently, various efficient
C-MFCs, including nonmetal atom (e.g., N, O, S, F, B, P, Br, Cl, or
As)-dopedgraphene, CNTs, carbon quantum dots, carbon nitride, and
graphite nanoplates, have been developed for advanced energy conversion/storage
involving ORR,[11−13] HER,[16,53] OER,[54,55] CO2RR,[56] and/or N2RR,[21] and biorelated applications (e.g.,
reactive oxygen species, ROS, generation/detection).[29,57]
Figure 2
(a)
Charge density distribution based on calculations for nitrogen-doped
CNTs (N-CNTs). (b) Possible adsorption modes of an O2 molecule
on CNTs (top) and N-CNTs (bottom).[10] (c)
Atomic-resolution transmission electron microscopy (AR-TEM) image
of the SeGnP edge. (d) IRR mimetic diagram on the SeGnP surface.[59] (e) Schematic representation of charge transfer
and of ORR process on PDDA-CNT.[60] (f) Free-energy
values for ORR of different kinds of defects.[67] (g) AR-TEM image of DG. Hexagons: orange, pentagons: green, heptagons:
blue, and octagons: red.[68] Images reprinted
with permission from refs (10, 59, 60, 67, and 68). Copyright 2009 AAAS, 2016 AAAS,
2011 American Chemical Society, 2015 American Chemical Society, and
2016 Wiley–VCH.
(a)
Charge den<nclass="Chemical">spaclass="Chemical">n class="Chemical">siclass="Chemical">n>ty distribution based on c<class="Chemical">spn>an class="Chemical">alculations for <class="Chemical">span class="Chemical">nitrogen-doped
CNTs (N-CNTs). (b) Possible adsorption modes of an O2 molecule
on CNTs (top) and N-CNTs (bottom).[10] (c)
Atomic-resolution transmission electron microscopy (AR-TEM) image
of the SeGnP edge. (d) IRR mimetic diagram on the SeGnP surface.[59] (e) Schematic representation of charge transfer
and of ORR process on PDDA-CNT.[60] (f) Free-energy
values for ORR of different kinds of defects.[67] (g) AR-TEM image of DG. Hexagons: orange, pentagons: green, heptagons:
blue, and octagons: red.[68] Images reprinted
with permission from refs (10, 59, 60, 67, and 68). Copyright 2009 AAAS, 2016 AAAS,
2011 American Chemical Society, 2015 American Chemical Society, and
2016 Wiley–VCH.
The heteroatom-<nclass="Chemical">spaclass="Chemical">n class="Chemical">doclass="Chemical">n>ping induced cat<class="Chemical">spn>an class="Chemical">alytic performance of C-MFCs
is strongly dependent on the <class="Chemical">span class="Chemical">dopant type, dopant location, and co-/multidoping
effect(s).[58] It was found that the ball-milling
generated edge-selenated graphene nanoplatelets (SeGnPs) were applied
as the counter electrode for dye-sensitized solar cells (DSSCs) with
I–/I3– or Co(II)/Co(III)
electrolytes to display an even higher photovoltaic performance than
those of DSSCs with the Pt counter electrode in SM315 or N719 sensitizers
(Figure c).[59] The calculations based on density functional
theory (DFT) and nonequilibrium Green’s function clarified
the I3– reduction mechanism by this special
edge doping (Figure d).
Furthermore, intermolecular charge transfer can be re<nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n>ized
by phy<class="Chemical">spn>an class="Chemical">sic<class="Chemical">span class="Chemical">ally doping nanocarbon materials with electron acceptor(s)
or donor(s) (Figure e), as exemplified by good ORR activities from the polyelectrolyte
(e.g., PDDA) adsorbed graphiticcarbon materials (e.g., CNTs, graphene).[60−62] In this particular case, amine groups in the PDDA possess a strong
electron acceptability, which can extract electrons from C atoms in
the carbon skeleton, forming partially positively charged carbon atoms
as active centers for efficient electrocatalysis of ORR,[60−62] OER,[63,64] and CO2RR.[65]
Recent studies indicate that certain pure <nclass="Chemical">spaclass="Chemical">n class="Chemical">carboclass="Chemical">nclass="Chemical">n>
nanocages and <class="Chemical">spn>an class="Chemical">graphene quantum <class="Chemical">span class="Chemical">dots/graphene nanoribbon hybrids without
any dopants and free from physically adsorbed polyelectrolyte can
also have high ORR activities.[66,67] DFT calculations disclose
the charge transfer between the defects of pentagon/zigzag edge and
the basal plane (Figure f), indicating ORR activities arising from the charge transfer induced
by defects.[67] Compared to catalytic activities
induced by the heteroatom-doping-induced intramolecular charge transfer and polyelectrolyte-adsorption-induced intermolecular charge transfer discussed above, the investigation of defective
induced catalytic actives in C-MFCs is still in its fancy. However,
many carbon nanomaterials with well-controlled defects (e.g., porous
graphene, Figure g)
have been reported to show good catalytic performance, even superior
to some C-MFCs from heteroatom-dopedgraphene, for various electrochemical
reactions (e.g., ORR, OER, HER).[68] This
is obviously a valuable field for future work.[69]
Recent Advances in the Development of C-MFCs
An exciting feature
of C-MFCs is that different <nclass="Chemical">spaclass="Chemical">n class="Chemical">doclass="Chemical">n>pant types and chemic<class="Chemical">spn>an class="Chemical">al complexities
of cat<class="Chemical">span class="Chemical">alytic active centers can introduce different catalytic functions
(e.g., ORR, OER, HER), and the catalytic performance could be enhanced
by improving the density and distribution of individual catalytic
active sites. Thus, the structural control and surface modification
of carbon-based materials can serve as a promising approach to further
enhance the catalytic activity of C-MFCs.
Of particular interest,
the 3D assembly st<nclass="Chemical">spaclass="Chemical">n class="Chemical">ruclass="Chemical">n>ctures (morphologies) of C-MFCs can be used to
increase the number/den<class="Chemical">spn>an class="Chemical">sity of acces<class="Chemical">span class="Chemical">sible active sites, improve the
number of active sites to be exposed, and facilitate electron transport,
mass transfer of reactants, and electrolyte diffusion.[4,6,16,58,70−77] Thus, the first reported N-doped CNTs with aligned structure exhibited
an ORR performance that greatly surpassed the nonaligned counterpart
due to the enhanced electrolyte/reactant diffusion and accessibility
to the active sites on the vertically aligned N-CNT surface.[10]
Unusu<nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n>
surface, mechanic<class="Chemical">spn>an class="Chemical">al, and electric<class="Chemical">span class="Chemical">al properties can be obtained by
tuning micro-/meso-/macroporous 3D carbon architectures. Such properties,
along with remarkable electrolyte transport capability for achieving
a large triphase (gas–liquid–solid) interface with abundant
accessible active sites, could significantly enhance the sluggish
kinetics of the catalytic reactions.[34] Functional
reduced graphene oxide (RGO) with a 3D porous structure can be obtained
by high-temperature thermal annealing GO with other precursor containing
heteroatom(s). In this context, Sheng et al., developed a facile approach
to N-doped graphene with a 3D porous structure by pyrolysis of a mixture
containing GO and melamine.[78] The N content
in the resulting N-doped graphene was up to 10.1 atom %, and the resultant
C-MFC catalyzed a high-efficient four electron (4e–) ORR process in 0.1 M alkaline medium.[78] Additionally, a powerful template-free approach, involving polymerization
followed by pyrolysis, has also been developed to produce various
C-MFCs with 3D porous structure.[26,78−81] The pyrolysis of a polyaniline aerogel derived from phytic acid
and aniline monomers generated a mesoporous C-MFCs with N,P codoping,
which exhibited an SSA over 1600 m2 g–1 and high electrocatalytic activities for both OER and ORR[26] attributable to the codoping in conjunction
with the smooth mass and charge transfer associated with the 3D mesoporouscarbon.
In addition, <nclass="Chemical">spaclass="Chemical">n class="Chemical">silicaclass="Chemical">n> colloids were employed as a hard
template for <class="Chemical">spn>an class="Chemical">polymerization of N-contained monomers, followed by pyroly<class="Chemical">span class="Chemical">sis,
to produce 3D N-dopedmesoporouscarbons.[82] Subsequent heat treatment under NH3 further modified
the mesoporouscarbons into meso-/microstructures with a high SSA,
leading to a high improvement in the ORR activity.[82] Also, 3D carbon nanocages with N doping were successfully
synthesized by annealing the pyridine in the presence of MgO or AAO
template,[83] which is attractive for metal-free
electrocatalysis in various applications. These results suggest the
importance of the hierarchical structures to the electrocatalytic
performance of C-MFCs.
Hierarchic<nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n> porous st<class="Chemical">spn>an class="Chemical">ructures were <class="Chemical">span class="Chemical">also
constructed to further enlarge the SSA and improve the electrolyte
diffusion. It was found that abundant edges in the pores could enhance
interfacial catalytic performances.[84] In
this context, a highly efficient HER electrocatalyst was developed
by in situ growing mesh-on-mesh networks of mesoporousg-C3N4 sheets on a mesoporousgraphene support.[85] The in-plane mesoporous structure in this system
affords abundant active sites at the exposed edges and defects for
hydrogen adsorption, leading to a high HER activity for this unique
C-MFC—even higher than that of some non-noble metallic catalysts.
Furthermore, N-doped holey graphene (N-HG) and B,N codoped HG (B,N-HG)
with hierarchically porous structures were successfully prepared as
highly efficient air electrodes in Li-O2 batteries and
Li-CO2 batteries, respectively.[8,86] The
efficient utilization of active sites in HG with abundant holes, coupled
with the enhanced density of active sites and electrolyte/gases/electron
transports, imparts superb cell performance. This provides new opportunities
for designing reversible and stable metal-air batteries with a high
energy efficiency.
Recent <nclass="Chemical">spaclass="Chemical">n class="Chemical">carboclass="Chemical">nclass="Chemical">n> architectures have been developed
as C-MFCs from zero-dimen<class="Chemical">spn>an class="Chemical">sion<class="Chemical">span class="Chemical">al (0D)-carbon quantum dots, one-dimensional
(1D)-CNTs, two-dimensional (2D)-graphene, and 3D-porous carbon materials.[70] Of particular interest, Pham et al. prepared
a 3D carbon architecture of CNT-bridged graphene via Coulombic interactions
between cationic surfactants-grafted CNTs and graphene oxide sheets
with negatively charge (Figure a,b).[87] Within the pillared 3D
structures, graphene layers were intercalated by CNTs to enhance the
accessible SSA and allow for fast ion diffusion, leading to a high-performance
electrochemical catalyst for metal-free catalysis or supercapacitor
for energy storage.
Figure 3
(a, b) Schematic and scanning electron microscope (SEM)
image of the 3D SWCNT-bridged graphene block on the cross-section.[87] (c) Schematic for the synthesis and microstructures
of a 3D graphene-RACNT fiber. (d) SEM image (top view) of the 3D graphene-RACNT
fiber. (e, f) TEM images of graphene sheet connecting to the open
tips of RACNTs under different magnifications. (g) SEM image of the
wire-shaped DSSC from the cross-section view.[36] Images reprinted with permission from refs (87 and 36). Copyright 2015 American Chemical
Society, 2015 AAAS.
(a, b) Schematic and scanning electron mic<nclass="Chemical">spaclass="Chemical">n class="Chemical">rosclass="Chemical">n>cope (SEM)
image of the 3D SWCNT-bridged <class="Chemical">spn>an class="Chemical">graphene block on the c<class="Chemical">span class="Chemical">ross-section.[87] (c) Schematic for the synthesis and microstructures
of a 3D graphene-RACNT fiber. (d) SEM image (top view) of the 3D graphene-RACNT
fiber. (e, f) TEM images of graphene sheet connecting to the open
tips of RACNTs under different magnifications. (g) SEM image of the
wire-shaped DSSC from the cross-section view.[36] Images reprinted with permission from refs (87 and 36). Copyright 2015 American Chemical
Society, 2015 AAAS.
<nclass="Chemical">spaclass="Chemical">n class="Chemical">Siclass="Chemical">n>nce the cov<class="Chemical">spn>an class="Chemical">alent bonding
between <class="Chemical">span class="Chemical">carbon atoms in the carbon skeleton of CNTs and graphene is
stronger than the van der Waals interaction in the transverse direction
between the carbon layers, thermal, electrical, and mechanical properties
of 1D CNTs and 2D layered graphene are strongly direction-dependent.
However, high through-thickness thermal and/or electrical conductivities
are usually required for many practical applications. Theoretical
studies indicated CNTs and graphene with a seamless nodal junction
could have both satisfied out-of-plane and in-plane properties.[88] In this context, Dai and co-workers have developed
a one-step approach (template-assisted chemical vapor deposition)
to a special 3D carbon architecture—graphene-CNT hollow fibers
with radially aligned CNTs (RACNTs) seamlessly sheathed by a cylindricalgraphene layer (Figure c–f).[36] The resultant fibers, with
meso-and micropores, a large SSA, and extraordinary electrical properties,
are superior electrodes, as exemplified by their use as counter electrodes
in all-solid-state wire-shaped DSSCs with high-performance (Figure g). The concept of
“seamless nodal junction” can be extended to other carbon
architectures, such as CNTs and C60, graphene and C60, even graphene and C3N4, for the fabrication
of new kinds of C-MFCs with unexpected properties. These are just
the first few examples for the special 3D carbon architectures, and
many more will be sure to follow.
Recent
Advances in Mechanistic Understanding of C-MFCs
A robust atomic-sc<nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n>e
understanding of the active <class="Chemical">spn>an class="Chemical">sites and related cat<class="Chemical">span class="Chemical">alytic activities
will enable the mechanistic understanding of catalytic reactions and
discovery of fundamentally new catalysis concepts and active site
structures,[89] which, ultimately, will lead
to great advances in catalysis applicable to critical reactions in
energy and biorelated systems. Therefore, it is important to identify
specific active centers for C-MFCs or their model catalysts.
Among many of the recent breakthroughs, the clarification of which
type of “class="Chemical">N” is reclass="Chemical">n class="Chemical">spon<span class="Chemical">sible for ORR from experiment and
theory is another milestone for the development of C-MFCs.[90] The intrin<span class="Chemical">sic active sites toward ORR in N-doped
C-MFCs were determined by using a suite of well-defined N-doping species
within various highly oriented pyrolytic graphites (HOPGs), including
pyridinic N-/graphitic N-/edges-/clean-HOPG (Figure a).[90] On the basis
of studies of these structurally well-controlled HOPG model catalysts,
Nakamura and co-workers have demonstrated that the positively charged
carbon atoms (C+) around pyridinic nitrogen in N-doped
C-MFCs are the active sites for ORR (Figure b).[90] An oxygen
molecule initially adsorbs on the C+ around the pyridinic
N, which is then followed by proton-coupled electron transfer to the
adsorbed oxygen via either a 2e– or 4e– pathway (Figure c). Therefore, the improved electrocatalytic performance of C-MFCs
results from the intrinsic activities of heteroatom-dopedcarbon coupled
with doping-induced charge transfer,[58] rather
than metal residuals.[91] This fundamental
understanding of the ORR mechanism in N-dopedcarbon materials is
critical as this principle can also provide the guideline to design
C-MFCs beyond ORR for fuel cells. More recently, Zhao et al. introduced
sp-hybridized N (sp-N) atoms into a few-layer graphdiyne host by replacing
the carbon atoms in acetylene groups (Figure d,e),[92] and found
that sp-N-dopedgraphdiyne exhibited overall enhanced ORR activities
in the presence of sp-N atoms (Figure f), which facilitated O2 adsorption and
electron transfer on the carbon surface. Thus, the introduction of
sp-N atoms in carbon skeleton provides a powerful approach for the
preparation of highly active C-MFCs.
Figure 4
(a) N 1s XPS spectra of HOPG model catalysts.
(b) ORR activities for these model catalysts in (a). The inset: nitrogen
contents of the catalyst models. (c) Schematic pathway for ORR on
N-doped carbon nanomaterials.[90] (d) Geometries
of N atoms (sp-N, pyri-N, amino-N, and grap-N atoms) in graphdiyne.
(e) N K-edge XANES of N doped graphdiyne (NFLGDY) prepared under temperatures
of 700, 800, and 900 °C. A broaden and red-shifted peak ca. 397.4
eV appeared when the temperature increased, indicating the presence
of the sp-N doping type. (f) ORR current densities @0.65 V for samples
with different amounts of sp-N atoms.[92] (g) Schemes of proposed active sites for the H2O2 generation on mild reduction of GO (F-mrGO) and F-mrGO(600)
(F-mrGO annealed at 600 °C).[98] Images
reprinted with permission from refs (90, 92, and 98). Copyright 2016 AAAS, 2018 Nature
Publishing Group, and 2018 Nature Publishing Group.
(a) N 1s XPS class="Chemical">spectra of HOPG model cat<class="Chemical">n class="Chemical">span class="Chemical">alysts.
(b) ORR activities for these model cat<spclass="Chemical">n>an class="Chemical">alysts in (a). The inset: nitrogen
contents of the catalyst models. (c) Schematic pathway for ORR on
N-dopedcarbon nanomaterials.[90] (d) Geometries
of N atoms (sp-N, pyri-N, amino-N, and grap-N atoms) in graphdiyne.
(e) N K-edge XANES of N dopedgraphdiyne (NFLGDY) prepared under temperatures
of 700, 800, and 900 °C. A broaden and red-shifted peak ca. 397.4
eV appeared when the temperature increased, indicating the presence
of the sp-Ndoping type. (f) ORR current densities @0.65 V for samples
with different amounts of sp-N atoms.[92] (g) Schemes of proposed active sites for the H2O2 generation on mild reduction of GO (F-mrGO) and F-mrGO(600)
(F-mrGO annealed at 600 °C).[98] Images
reprinted with permission from refs (90, 92, and 98). Copyright 2016 AAAS, 2018 Nature
Publishing Group, and 2018 Nature Publishing Group.
Recent Advances in Metal-Free
Electrocatalysis by C-MFCs
Electrochemical Production of H2O2
ORR can, in principle, go through either a 4e process to produce
electricity and <class="Chemical">spaclass="Chemical">n class="Chemical">class="Chemical">n class="Chemical">H2O in a fuel cell or a 2e– process with <spclass="Chemical">n>an class="Chemical">H2O2 as an intermediate.[4] H2O2 has been commonly
used not only as a green oxidizer for cleaning,[93,94] but also as fuel to power a rocket and a clean energy carrier to
run automobiles with water and O2 as byproducts (http://www.americanenergyindependence.com/peroxide.aspx). The technology currently used industrially to synthesize H2O2 is based on the anthraquinone process, which
is very energy intensive,[95] and cannot
compete financially with other reagents as it involves the costly
and energy-intensive separation and concentration steps. Direct synthesis
of H2O2 that combines H2 and O2, through 2e– ORR has been achieved using
noble metal catalysts through an electrochemical reaction.[95] Recently, it turns out to be possible that carbon catalysts with heteroatom-doping could be employed
as effective catalysts for electricity generation and H2O2 production in fuel cells.[17,96−99]
Of particular interest, <nclass="Chemical">spaclass="Chemical">n class="Chemical">siclass="Chemical">n>ngle-w<class="Chemical">spn>an class="Chemical">all <class="Chemical">span class="Chemical">graphitic carbon nanohorns
doped with N were obtained by annealing of polydopamine as 2e– transfer ORR C-MFCs for high-yield H2O2 production.[96] The as-prepared
C-MFCs showed a high performance over a wide pH range (1–13)
with an optimized Faradaic efficiency as high as 98%. Furthermore,
both the activity and selectivity for H2O2 generation
were correlated with the oxygen content of the surface oxidized carbon
nanomaterials,[97,98] and the carbon atoms adjacent
to oxygen functional groups (−COOH and C–O–C)
were confirmed as the active sites for ORR via the 2e– pathway (Figure g).[98] Thus, the O-CNTs exhibited greatly
improved activity and selectivity (∼90%) for H2O2 production when compared with pure CNTs.
Electrochemical
Reduction of CO2 (CO2RR)
Following
the disclass="Chemical">covery of <class="Chemical">n class="Chemical">span class="Chemical">N-doped CNTs for ORR in 2009,[10] numerous C-MFCs have been developed for various reactions,
including graphene foams doped with N as counter electrodes for DSSCs
in 2012,[18] N-dopedgraphite nanomaterials
for OER,[14] and g-C3N4/N-doped graphene catalyst for HER in 2014,[16] N-dopedcarbon nanofibers (N-CNFs), and N-doped porous carbon for
CO2RR[19] in 2013 and N2RR[21] in 2018, respectively. Since the
research and development of C-MFCs for ORR, OER, HER, including their
multifunctional systems, and counter electrodes for DSSCs have been
reviewed recently,[4,57,69,100,101] a focus of
this article is given to the recent advances in the development of
C-MFCs for emerging multielectron transfer electrocatalysis, such
as CO2RR, N2RR, and photo/electro-/biorelated
reactions.
Electrochemic<nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n> reduction of <class="Chemical">spn>an class="Chemical">CO2 to the
v<class="Chemical">span class="Chemical">alue-added organic molecules (e.g., HCOOH, CH3OH, C2H4, CH4, CO) is a potential way for
the preparation of carbon compounds to store chemical energy.[58] In this regard, Kumar et al., developed N-CNFs
by electrospinning polyacrylonitrile, followed by pyrolysis, as the
first C-MFC for CO2RR in 2013.[19] Recently, a series of carbon materials doped with heteroatom (e.g.,
N, B, F, N/B, N/S) were developed as highly active, selective, and
stable C-MFCs for CO2RR.[19,36−39,102−106] In practical, it was demonstrated that pyridinic N, among those
doping N species in carbon skeletons, is responsible for CO2RR.[102] Electrons of graphitic N are limited
in the π antibonding orbital, which are less easily for binding
CO2, while pyrrolic N atoms are usually located in the
center of the carbon skeleton that often makes their lone pairs difficult
to access by CO2 molecule(s).
Recently, a bifunction<nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n>
C-MFC, porous <class="Chemical">spn>an class="Chemical">Si and N co<class="Chemical">span class="Chemical">doped carbon (SiNC), with catalytic activity
for both CO2RR and OER, was first realized (Figure a,b), and allowed for cost-effective
electrochemical splitting of CO2 into CO and O2 to supplement the naturalcarbon cycle (Figure c).[107] Partial
current densities of CO2-to-CO and OER on SiNC in neutral
medium were enhanced by 2 orders of magnitude in the Tafel regime
relative to single-doped comparisons (SiC and NC). Mechanism studies
suggested the elastic electron structure of -Si(O)–C-N- unit
in SiNC as the highly active site for CO2RR and OER simultaneously
by lowering the free energy of CO2RR and OER intermediates
adsorption.[107] Some detailed studies on
CO2RR by C-MFCs, including their synthesis, structure characterization,
and performance, are summarized in one of our previous review articles.[58]
Figure 5
(a) Schematic of bifunctional SiNC with catalytic activity
for both CO2RR and OER for the CO2 overall splitting.
(b) HAADF image of porous SiCN. (c) Current density, cathode, and
electrode potentials of CO2 overall splitting with two
SiNC electrodes.[107] (d) A generic mechanism
for N2RR to NH3 on heterogeneous catalysts.[111] (e) Schematic illustration of the preparation
of NPCs. (f) Production rates of NH3 and current efficiency
of NPCs sample during 10 cycles at a given potential of −0.9
V.[21] (g) The catalyst model with N-doped
highly disordered carbon for N2RR. (h) TEM image of the
corresponding catalyst of (g).[114] Images
reprinted with permission from refs (107, 111, 21, and 114). Copyright 2018 Wiley–VCH, 2016 Elsevier, 2018 American Chemical
Society, and 2018 Elsevier.
(a) Schematic of bifunction<nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n> <class="Chemical">spn>an class="Chemical">SiNC with cat<class="Chemical">span class="Chemical">alytic activity
for both CO2RR and OER for the CO2 overall splitting.
(b) HAADF image of porous SiCN. (c) Current density, cathode, and
electrode potentials of CO2 overall splitting with two
SiNC electrodes.[107] (d) A generic mechanism
for N2RR to NH3 on heterogeneous catalysts.[111] (e) Schematic illustration of the preparation
of NPCs. (f) Production rates of NH3 and current efficiency
of NPCs sample during 10 cycles at a given potential of −0.9
V.[21] (g) The catalyst model with N-doped
highly disorderedcarbon for N2RR. (h) TEM image of the
corresponding catalyst of (g).[114] Images
reprinted with permission from refs (107, 111, 21, and 114). Copyright 2018 Wiley–VCH, 2016 Elsevier, 2018 American Chemical
Society, and 2018 Elsevier.
Electrochemical Reduction of N2 (N2RR)
class="Chemical">Coclass="Chemical">nverticlass="Chemical">ng earth-abuclass="Chemical">ndaclass="Chemical">nt <class="Chemical">n class="Chemical">span class="Chemical">N2 to <span class="Chemical">NH3 is an important
approach for acquiring the widely used agricultural fertilizer[108] and hydrogen intermediate for energy storage.[109] As an alternative green fuel, NH3 has a volumetric energy density nearly double that of liquid hydrogen,
and is easier for storage, shipment, and distribution than hydrogen.[109] Industrial synthesis of NH3 involves
the energy- and capital-intensive Haber-Bosch process at high temperature
of 350–550 °C and high pressure between 150 and 350 atm.
Under ambient conditions, the photo-/electrochemicalN2RR to NH3 without additional reducing agent (H2) and greenhouse emission has attracted a great deal of attention.[110] As the strong N≡N bond with no dipole
moment in N2 molecules, it is a challenge to find an efficient
way for activating N2.[110] Generally
speaking, electrocatalytic N2RR on the surface of a heterogeneous
catalyst involves two reaction processes: associative and dissociative.[111] In an associative process, the two nitrogen
centers bind together as a N2 molecule is hydrogenated,
and the product (NH3) is released along with the cleavage
of the N–N bond. The alternating approach requires each of
the two nitrogen centers to be adsorbed on the active sites in the
catalyst, and the second NH3 molecule will be generated
just after the removal of the first NH3 molecule (Figure d). The dissociative
mechanism involves the cleavage of the N≡N bond before hydrogenation
occurs, forming adsorbed N atoms on the catalytic surface, followed
by the final step to produce NH3.
<nclass="Chemical">spaclass="Chemical">n class="Chemical">Ruclass="Chemical">n>, Au, and Fe/CNTs
have been used for <class="Chemical">spn>an class="Chemical">N2RR,[112,113] though their
large overpotenti<class="Chemical">span class="Chemical">al and poor N2 adsorption seriously impede
the efficiencies and yields of NH3. Also, various N-doped
porous carbon materials (NPCs) obtained by the pyrolysis of ZIFs (Figure e) have recently
been used for electrocatalytic N2RR.[21] The high SSA of the NPCs (896 cm3 g–1) can provide a large amount of active sites, and benefit the contact
and aggregation of N2 on the catalytic surface as well,
and hence the resultant high ammonia production rate of ∼1.4
mmol g–1 h–1 and energy conversion
efficiency of ∼1.42% at −0.9 V (Figure f).[21]
As
the maximum reaction free energies for <class="Chemical">spaclass="Chemical">n class="Chemical">class="Chemical">n class="Chemical">N2RR on pyridinic
N and pyrrolic N <spclass="Chemical">n>an class="Chemical">sites are 0.45 and 0.56 eV (form DFT calculations),
respectively. The values are quite similar; therefore, pyridinic and
pyrrolic N are considered the active sites for ammonia generation
on NPCs. The possible catalytic mechanism of N2RR on NPC,
followed the alternating approach, was also presented (*N≡N
→*NH≡NH → *NH2–NH2 → 2NH3), as we discussed above. To date, the investigation
of C-MFCs for electrochemicalN2RR is still a very early
development, and it is desirable to also study N2RR induced
by the defective and intermolecular charge transfer, and the mechanistic
understanding of relevant reactions. It was found that the performance
of N2RR on N-dopedcarbon can be enhanced by coupling with
defective effects (Figure g,h).[114] The moiety, consisting
of three adjacent pyridinic N atoms around one carbon vacancy in the
carbon skeleton, can effectively adsorb a N2 molecule to
dissociate “N≡N” for the following protonation
process. Thus, N-doped porous carbon showed a remarkable NH3 production rate of as high as 3.4 × 10–6 mol
cm–2 h–1 with a Faradaic efficiency
(FE) of 10.2% at a given potential of 0.3 V vs RHE in 0.1 M KOH medium.
Photoelectrochemical Catalysis
Heterogeneous photocat<nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n>y<class="Chemical">spn>an class="Chemical">sis
could be employed to harvest, convert, and store solar energy for
gene<class="Chemical">span class="Species">rating green fuels and environmental protections. In this context,
carbon nanomaterials with tunable energy bands are a class of promising
C-MFCs for photoelectrochemical catalysis.[115] Due to their distinctive physicochemical/optical/electrical properties,
graphitic carbon nitride (g-C3N4)-based materials
have been widely studied as polymeric semiconducting photocatalysts
to promote various photoelectrochemical reactions. Like most photocatalysts,
the photocatalytic activity of g-C3N4-based
materials is strongly determined by its band gap, which, along with
some theoretical potentials of various redox reactions at pH 7, are
shown in Figure a.[116] As can be seen, g-C3N4 owns a moderate bandgap of 2.7 eV, which corresponds to an optical
wavelength around 460 nm for visible light irradiation. More importantly,
the energy level of the top conductive band (CB) of g-C3N4 is much more negative when compared with those of conventional
inorganic semiconductors, the H2-evolution, and CO2/O2 theoretical reduction potentials (Figure a,c), which suggests
that photoinduced electrons in g-C3N4 have a
strong thermodynamic driving force to reduce different kinds of molecules.
As such, g-C3N4 based materials are useful for
many photocatalytic applications, including photocatalytic H2 evolution and overall water splitting,[117,118] environmental remediation,[119,120] CO2RR,[121,122] N2RR,[123,124] and selective organic transformations
and disinfection[125,126] under light irradiation with
suitable wavelength(s). However, the bulk g-C3N4 materials often exhibit a low photocatalytic efficiency resulting
from multiple drawbacks: a high recombination rate of electron–hole,
insufficient visible light absorption, limited SSA (∼10 m2 g–1) and insufficient active centers for
interfacial photocatalytic reactions, slow kinetics for reaction on
the photocatalyst surface, and low charge mobility.[116,127] Various bandgap engineering strategies, such as heteroatom-doping,
defective site control, materials dimensionality tuning, pore texture
tailoring, heterojunction constructing, cocatalyst coupling, and nanocarbon
hybridization, have been introduced to address the above issues, and
thus improve the performance of g-C3N4 photocatalysts.[116,127]
Figure 6
(a)
The redox potentials of various reactions with respect to the theoretical
position of the g-C3N4 band edges at pH 7.[116] (b) TEM image of the CDots-C3N4 composite. The inset: A magnified TEM image from region marked
in red. (c) Band structure diagram for CDots-C3N4. VB, valence band; CB, conduction band. (d) H2 and O2 production from water splitting with CDots-C3N4 as photochemical catalyst.[145] (e)
AFM image of the mesoporous g-C3N4 nanomesh
with monolayer structure. The inset: enlarged view of the rectangle
area. (f) Height profiles along the lines in (e).[147] (g) UV–vis diffuse reflectance spectra of OCN-tubes.[153] (h) Schematic illustration of valence and conduction
bands of H-terminated diamond, respectively. (i) Comparison of NH3 yield from H-terminated and O-terminated diamond samples.[124] Images reprinted with permission from refs (116, 145, 147, 153, and 124). Copyright 2017 Elsevier, 2015 AAAS, 2016 American Chemistry Society,
2017 Wiley-VCH, and 2013 Nature Publishing Group.
(a)
The re<nclass="Chemical">spaclass="Chemical">n class="Chemical">doclass="Chemical">n>x potenti<class="Chemical">spn>an class="Chemical">als of various reactions with reclass="Chemical">spect to the theoretic<class="Chemical">span class="Chemical">al
position of the g-C3N4 band edges at pH 7.[116] (b) TEM image of the CDots-C3N4 composite. The inset: A magnified TEM image from region marked
in red. (c) Band structure diagram for CDots-C3N4. VB, valence band; CB, conduction band. (d) H2 and O2 production from water splitting with CDots-C3N4 as photochemical catalyst.[145] (e)
AFM image of the mesoporousg-C3N4 nanomesh
with monolayer structure. The inset: enlarged view of the rectangle
area. (f) Height profiles along the lines in (e).[147] (g) UV–vis diffuse reflectance spectra of OCN-tubes.[153] (h) Schematic illustration of valence and conduction
bands of H-terminated diamond, respectively. (i) Comparison of NH3 yield from H-terminated and O-terminated diamond samples.[124] Images reprinted with permission from refs (116, 145, 147, 153, and 124). Copyright 2017 Elsevier, 2015 AAAS, 2016 American Chemistry Society,
2017 Wiley-VCH, and 2013 Nature Publishing Group.
Of particular interest, heteroatom
(e.g., S,[128,129] O,[130] C,[131] I,[132,133] and P[134,135] ) <nclass="Chemical">spaclass="Chemical">n class="Chemical">doclass="Chemical">n>ping and C, P-co<class="Chemical">spn>an class="Chemical">doping[136] of <class="Chemical">span class="Chemical">g-C3N4 have been confirmed as effective approaches
to narrow its bandgap and improve the light-harvesting capability.
Specifically, doping with heteroatoms could enhance the π-conjugated
electron delocalization, which is significant for the enhancement
of the conductivity, charge mobility, and separation of photoinduced
electrons to improve the photocatalytic performance of g-C3N4. Copolymerization with appropriate aromatic derivatives
or organic small molecules could also enhance π-electron delocalization
to modulate the band gap, electronic structures, physical/chemical
properties for enhancing the photochemical activities of g-C3N4.[115,137]
Apart from the
heteroatom <nclass="Chemical">spaclass="Chemical">n class="Chemical">doclass="Chemical">n>ping, the enhancement in cryst<class="Chemical">spn>an class="Chemical">allinity of <class="Chemical">span class="Chemical">g-C3N4 could also lead to significantly improved photocatalytic
activity by improving the charge-carrier mobility and separation.[118,140] Because of the defect-induced catalytic activities,[66−69,139−142] however, the generation of nitrogen vacancies[143] or amorphous structures[144] in
g-C3N4 has been recently developed as an effective
strategy to enhance the visible-light performance. Such disorder structure
in amorphous g-C3N4 can narrow the bandgap to
1.9 from 2.7 eV, which corresponds to a red-shift of the absorption
wavelength edge from 460 to 682 nm.[144] The
above results provide a new approach to develop visible light-driven
photocatalysts from amorphous/defective g-C3N4 samples.
In 2015, Liu et <nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n>.[145] reported that a C-MFC of <class="Chemical">spn>an class="Chemical">carbon <class="Chemical">span class="Chemical">dots and C3N4 (C dots-C3N4, Figure b) can photocatalytic
water into H2 and O2 with a high quantum efficiency
(QE) of 16% and 6.3% at λ = 420 ± 20 and 580 ± 15
nm, respectively. In this particular case, C dots-C3N4 catalyzed water splitting into H2 and O2 molecules via a two-electron, two-step approach under visible light;
C3N4 and C dots are responsible for the first
step (photocatalysis) and second step (chemical catalysis), respectively.
C3N4 usually suffers from poisoning by H2O2 produced by a 2e– process,
which is usually difficult to be removed from the photocatalyst (C3N4) surface.[146] Nevertheless,
the synergism between C3N4 (H2O2 generation sites) and C dots (H2O2 decomposition
sites) in the composite offered a smooth approach for high efficient
O2 generation. Also, the incorporation of C dots into the
C3N4 results in an expanded ultraviolet visible
(UV–vis) absorption of the (photo)catalyst to the entire wavelength
range, and thus enhanced values of the QE and “solar-to-hydrogen”
conversion efficiency (Figure c). More importantly, C dots-C3N4 maintains
a high durability for the generation rate of H2 and O2 for 200 cycles (more than 200 days, Figure d). These results demonstrated that the C
dots-C3N4 can be used as a promising photocatalyst
for water splitting driven by visible light.
To further improve
the photocat<nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n>ytic activity of <class="Chemical">spn>an class="Chemical">g-C3N4, Qu and
co-workers produced atomic<class="Chemical">span class="Chemical">ally thin mesoporous nanomesh (Figure e,f)[147] by exfoliating the g-C3N4 bulk with mesoporous to tune the energy levels and enhance the electron
transfer, light harvesting, and accessible active sites. The resultant
g-C3N4 nanomesh exhibited a superb photocatalytic
HER rate (8510 μmol h–1 g–1) under a wavelength >420 nm and a quantum efficiency of 5.1%
at a wavelength of 420 nm, which outperformed all the metal-free g-C3N4 nanosheet photocatalysts. Delamination of layered
bulk into single atom sheets could induce extraordinary physical properties,
such as a large SSA, ultrafast carrier mobility, and fine-tuned energy
band structure, leading to highly efficient photocatalytic water splitting.
Production of H2 and O2 gases from water splitting
driven by visible light is a promising approach for green energy conversion.
Apart from the electrochemic<nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n> reduction of <class="Chemical">spn>an class="Chemical">CO2 and
<class="Chemical">span class="Chemical">N2 discussed above, the use of solar energy and CO2 for artificial photosynthesis of hydrocarbon fuels is also
considered as one of the ideal routes for solving the greenhouse effect.[148−152] In this context, Nam and co-workers coupled the N-doped graphene
quantum sheets (N-GQSs) with a silicon nanowire as the photocatalyst
for photocatalyzing CO2 into CO.[149] O-dopedg-C3N4 nanotubes (OCN-tube) with a
porous structure possess a CB potential of −0.88 V, which was
more positive than that of the bulk g-C3N4 (−1.02
V) and attractive for photocatalytic CO2 into liquid fuels
(Figure g).[153] OCN-tube was a promising photocatalyst for
CH3OH generation with a photocatalytic production rate
up to 0.88 μmol/(g·h) without obvious decay after three
cycles. The high performance of OCN-tube was assigned to a good CO2 uptake capacity, excellent light harvesting, and high separation
efficiency of photogenerated charge carriers resulting from its hierarchical
porous structure coupled with oxygendoping.
In recent years,
C-MFCs have <nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n>so been developed for photocat<class="Chemical">spn>an class="Chemical">alytic <class="Chemical">span class="Chemical">nitrogen fixation.[123] In particular, it was found that the boron-doped
diamond (BDD) yielded electron emissions into liquids to form solvated
electrons, which reacted quickly with protons to form neutral atomic
hydrogen to reduce N2 into NH3.[124] In this case, diamond is a semiconductor (bandgap:
5.5 eV) and lies ∼0.8–1.3 eV above the
vacuum level when its surface is terminated with hydrogen (Figure h). Thus, the photocatalytic
activity of BDD depends strongly on the surface property and correlates
well with the solvated electron generation (Figure i). The optimized activity of the C-MFC of
BDD is even superior to that of Ru/TiO2 under same conditions.
In a separate study, heterostructure junctions between g-C3N4 and rGO were constructed,[154] leading to a superior charge transfer to overwhelm the electron–hole
recombination to significantly improve the NH4+ generation rate and catalytic stability.[154] Therefore, the rational design and development of hierarchically
structured carbon hybrids provide a promising approach for high-performance
C-MFCs for efficient photocatalytic nitrogen fixation and beyond.
Recent Advances in Energy Harvesting on C-MFCs
<nclass="Chemical">spaclass="Chemical">n class="Chemical">Alclass="Chemical">n>though many C-MFCs
have shown comparable, or even better, performance to commerci<class="Chemical">spn>an class="Chemical">al Pt/C
cat<class="Chemical">span class="Chemical">alysts toward ORR in alkaline electrolytes,[10,13] their performance in acidic electrolytes needs to be further enhanced
to commercialize the polymer electrolyte membrane fuel cells (PEMFCs)
that have a more significant economic impact than alkaline fuel cells.[155,156] Therefore, it is important, but still challenging, to develop effective
C-MFCs for ORR in acidic electrolytes.[58,157,158]
By properly tuning the class="Chemical">species aclass="Chemical">nd types of
the <class="Chemical">n class="Chemical">span class="Chemical">dopants, together with the hierarchic<spclass="Chemical">n>an class="Chemical">al carbon structures, considerable
improvement has been realized in the development of C-MFCs toward
ORR in acidic electrolytes. In particular, VA-NCNTs as ORR catalysts
produced strong ORR signals in a PEMFC analogous acidic medium due
to its superior vertical array structure.[158] Subsequently, CNTs with N,S- codoping (NS-CNTs),[159] graphite nanofibers with N,F-codoping,[160] RGO with F,N,S-tridoping (F,N,S-rGO),[161] and zigzag-edged graphene nanoribbons[162] were also demonstrated to show superior ORR activities
in both acidic and alkaline medium with respect to their single doping
counters. A confined polymerization/carbonization within layered montmorillonite
approach was introduced to prepare pyridinic N-doping dominant graphene
with relatively good ORR performance under acidic conditions.[79] Graphene/CNT layer-by-layer composites (Figure a–c) also
displayed higher ORR catalytic activities in acidic medium than that
of their individual components of graphene or CNTs.[157]
Figure 7
(a) Schematic illustrations for the assembly of a MEA with the
VA-NCNTs. C.E., R.E., W.E., stand for counter electrode, reference
electrode, and working electrode, respectively. (b) SEM image of the
VA-NCNTs. (c) Digital photo and SEM images of the MEA after a durability
test. (d) Cross-section SEM image of the N-G-CNT/KB/Nafion
catalyst layer. The purple arrows indicate the parallelly separated
N-G-CNT sheets with interdispersed KB particles. (e, f) Schematic
illustrations of O2 diffusion through KB separated N-G-CNT
sheets and the densely packed N-G-CNT sheets, respectively. Images
reprinted with permission from ref (157). Copyright 2016, AAAS.
(a) Schematic illust<class="Chemical">spaclass="Chemical">n class="Species">ratclass="Chemical">n>ions for the assembly of a MEA with the
VA-NCNTs. C.E., R.E., W.E., stand for counter electrode, reference
electrode, and working electrode, reclass="Chemical">spectively. (b) SEM image of the
VA-NCNTs. (c) Digit<span class="Chemical">al photo and SEM images of the MEA after a durability
test. (d) C<span class="Chemical">ross-section SEM image of the N-G-CNT/KB/<class="Chemical">span class="Chemical">Nafion
catalyst layer. The purple arrows indicate the parallelly separated
N-G-CNT sheets with interdispersed KB particles. (e, f) Schematic
illustrations of O2 diffusion through KB separated N-G-CNT
sheets and the densely packed N-G-CNT sheets, respectively. Images
reprinted with permission from ref (157). Copyright 2016, AAAS.
Shui et <nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n>.[157] reported a well-defined
3D <class="Chemical">spn>an class="Chemical">N-doped <class="Chemical">span class="Chemical">carbon architecture constructed with graphene, CNTs, and
Ketjen black (KB) particles with N doping (N-G-CNT-KB). In this rationally
designed carbon system, N-doped graphene (N-G) ensures a large SSA
to provide enormous active sites for ORR reaction, CNTs contribute
conductivity, and KB particles separate the graphene sheets for efficient
electrolyte and O2 diffusion, which showed a catalytic
activity for ORR as high as the state-of-the-art nonprecious metal
catalyst (i.e., Fe–N–C) but a better long-term stability
in acidic PEMFCs.[157] Specifically, the
C-MFC of N-G-CNT-KB catalyst, delivered a current density of 30 A
g–1 at a given potential of 0.8 V and a peak power
density of 300 W g–1. Moreover, this N-G-CNT-KB
catalyst exhibited a much higher stability in acidic PEMFCs than that
of the Fe/N/C nonprecious metal catalyst running at 0.5 V,[157] whether at a high or low loading amount of
2.0 mg cm–2 or 0.5 mg cm–2, respectively.
This study opens avenues for green energy production from PEMFCs based
on well-defined C-MFCs.
On another front, class="Chemical">N,P-<class="Chemical">n class="Chemical">span class="Chemical">doped porous <spclass="Chemical">n>an class="Chemical">nanocarbon
(NPMC) was developed as the first bifunctional C-MFC for both ORR
and OER in 2015 (Figure a–f),[26] and then was employed as
air electrodes for highly efficient primary and secondary Zn-air batteries.
Since then, research achievements have further illustrated that many
well-defined carbon-based materials could be used as bifunctional
C-MFCs for OER and HER in water splitting devices to generate H2 and O2 gases,[163−165] for ORR/OER in metal-air
batteries,[8,166−169] and even trifunctional catalysis for ORR/OER/HER in integrated energy
systems (Figure j).[22,170]
Figure 8
(a)
SEM image of NPMC-1000. (b) Scheme of the structure of NPMC foam and
its building block. (b, c) Linear sweep voltammetry (LSV) curves of
NPMC-1000, NPMC-1100, the RuO2, and the Pt/C on an RDE
electrode in 0.1 M KOH medium. (d) ORR and (e) OER volcano plots for
N-doped graphene, P-doped graphene, and N,P-doped graphene, respectively.
(f) Schematic of a primary Zn-air battery. A carbon paper coated with
NPMC was employed as the air cathode (enlarge part), a Zn foil was
used as anode, and the separator a glass fiber membrane filled KOH
solution.[26] (g) Schematic of graphene with
N, P, and F tridoping as a multifunctional C-MFC for simultaneous
ORR, OER, and OER. (h) Polarization and power density curves of Zn-air
batteries using N, P, and F tridoped graphene as ORR/OER catalyst
in the air electrode. (i) O2 and H2 generation
volumes with respected to the water-splitting time, the N, P, and
F tridoped graphene was used as the HER/OER catalyst.[170] (j) Schematic of the integrated green energy
devices based on a superior multifunctional C-MFC (N,S-3DPG).[22] Images reprinted with permission from refs (26, 170, and 22). Copyright 2015 Nature Publishing Group, 2016 Wiley-VCH, and 2017
Elsevier.
(a)
SEM image of <class="Chemical">spaclass="Chemical">n class="Chemical">class="Chemical">n class="Chemical">NPMC-1000. (b) Scheme of the st<spclass="Chemical">n>an class="Chemical">ructure of NPMC foam and
its building block. (b, c) Linear sweep voltammetry (LSV) curves of
NPMC-1000, NPMC-1100, the RuO2, and the Pt/C on an RDE
electrode in 0.1 M KOH medium. (d) ORR and (e) OER volcano plots for
N-doped graphene, P-doped graphene, and N,P-doped graphene, respectively.
(f) Schematic of a primary Zn-air battery. A carbon paper coated with
NPMC was employed as the air cathode (enlarge part), a Zn foil was
used as anode, and the separator a glass fiber membrane filled KOH
solution.[26] (g) Schematic of graphene with
N, P, and F tridoping as a multifunctional C-MFC for simultaneous
ORR, OER, and OER. (h) Polarization and power density curves of Zn-air
batteries using N, P, and F tridopedgraphene as ORR/OER catalyst
in the air electrode. (i) O2 and H2 generation
volumes with respected to the water-splitting time, the N, P, and
F tridopedgraphene was used as the HER/OER catalyst.[170] (j) Schematic of the integrated green energy
devices based on a superior multifunctional C-MFC (N,S-3DPG).[22] Images reprinted with permission from refs (26, 170, and 22). Copyright 2015 Nature Publishing Group, 2016 Wiley-VCH, and 2017
Elsevier.
For high-performance <nclass="Chemical">spaclass="Chemical">n class="Chemical">Zclass="Chemical">nclass="Chemical">n>-air batteries,
one of the major ch<class="Chemical">spn>an class="Chemical">allenges is to increase ORR and OER efficiency
of the air electrodes. Thus, it is important to prepare inexpen<class="Chemical">span class="Chemical">sive
ORR/OER bifunctional electrocatalysts to facilitate the discharge
and charge processes and to reduce the cost for a Zn-air battery.
In this context, ORR and OER bifunctionalmetal-free 3D NPMC foams
(Figure a, b) were
prepared through polymerization of aniline in the presence of phytic
acid and subsequent pyrolysis.[26]
First-principle DFT c<nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n>culations (Figure d,e) reve<class="Chemical">spn>an class="Chemical">aled that the 3D <class="Chemical">span class="Chemical">NPMC foam required
lower minimum ORR and OER overpotentials than those on the N-doped
or P-doped counterparts, due to the synergistic effect between the
N,P codoping and abundant edges in the 3D carbon structure. When used
as the C-MFC for air electrodes in primary or rechargeable Zn-air
batteries (Figure f), the NPMC-based batteries displayed a cell performance comparable
to that of batteries with commercial Pt/RuO2 electrodes.
This work has initiated worldwide studies on the development of highly
efficient bifunctional/multifunctional C-MFCs for metal (Zn/Li/Mg/Al)-air
batteries and beyond.[166−170]
Be<nclass="Chemical">spaclass="Chemical">n class="Chemical">siclass="Chemical">n>des, a porous <class="Chemical">spn>an class="Chemical">carbon co<class="Chemical">span class="Chemical">doped with N and O, fabricated
by the pyrolysis of a zeolitic imidazolate framework-8 (ZIF-8) and
subsequent H2SO4 treatment, was demonstrated
as a high-performance OER/HER bifunctional catalyst for overall water
splitting. The catalyst delivered a current density of 10 mA cm–2 for more than 8 h at a cell voltage of ca. 1.82 V
under alkaline medium.[164] Similarly, porous
carbon with N, P, and O tridopants has also been developed for water
splitting with a current density of 10 mA cm–2 at
a potential of 1.66 V and a long-term electrochemical durability in
alkaline.[165] This catalyst also showed
the high catalytic performance and good long-term durability in acidic
or neutral environments. Furthermore, N, F, and P tridopedgraphene
materials were used as a trifunctional C-MFCs (Figure g) for electrochemicalwater splitting (OER
and HER) for the generation of O2 and H2 gases,
driven by a Zn-air battery with the same electrocatalyst as an air
electrode (ORR and OER).[170] The achieved
high gas generation rates for H2 (0.496 μL s–1) and O2 (0.254 μL s–1) showed great potential for practical applications (Figure h,i). Subsequently, trifunctional
C-MFCs for ORR, OER, and HER simultaneously have also been realized
by the fabrication of N, S codopedgraphitic sheets with stereoscopic
holes.[171] More recently, Yao and co-workers
reported highly efficient defect-abundant C-MFCs for simultaneous
ORR, OER, and HER by removal of N from a N-doped graphene.[68]
Furthermore, trifunction<nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n> C-MFCs with
a low-cost have great potenti<class="Chemical">spn>an class="Chemical">al for widely applications, eclass="Chemical">speci<class="Chemical">span class="Chemical">ally
in light-derived integrated clean energy systems.[22] By rationally designing the cross-linked 3D graphitic networks
with N, S codoping (N,S-3DPG), Dai and co-workers reported a highly
efficient trifunctional catalyst toward HER, OER, and ORR in alkaline
medium.[22] The extraordinary trifunctional
electrocatalytic activity of N,S-3DPG was assigned to the combined
effects induced from the N,S-codoping and the unique 3D porous architecture.
On the basis of the emerging trifunctional C-MFC toward ORR/OER/HER,
an integrated energy system was further developed, in which photoelectrochemicalwater splitting was driven by perovskite solar cells to generate H2 and O2 gases for a PEMFC for the renewable generation
of green electricity, representing a fresh concept for generation
of clean electricity just from sunlight and water without any pollutant
emission (Figure j).[22]
Recent Advances in Biorelated
Applications of Carbon Nanomaterials
Owing to their unique
chemic<nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n>, phy<class="Chemical">spn>an class="Chemical">sic<class="Chemical">span class="Chemical">al, optical, and even magnetic properties, carbon
nanomaterials have been widely studied for various biorelated applications.
Carbon nanomaterials have successfully attracted tremendous interest
for the development of novel biomedical tools, including biosensors,
imaging agents, and drug/gene carriers for medical diagnostics and
clinical treatments. Great progress has been made, and many excellent
reviews and books have appeared.[28] Therefore,
this section will focus on the recent advances in biorelated applications
of carbon nanomaterials, particularly C-MFCs.
Among various
biorelated applications of <nclass="Chemical">spaclass="Chemical">n class="Chemical">carboclass="Chemical">nclass="Chemical">n> nanomateri<class="Chemical">spn>an class="Chemical">als, one of the fascinating
recent developments is related to their photoluminescence (PL) features.
In this regard, researchers have paid much attention to <class="Chemical">span class="Chemical">graphene quantum
dots (GQDs) for biological imaging.[172] For
formaldehyde detection and bioimaging, optical and fluorescent properties
of GQDs were found to be tunable by heteroatom doping and/or surface
functionalization.[173] The quantum yield
of N-doped GQDs (NGQDs) can be reached as high as 22.9%, which is
around 23 times the pristine GQDs (1%). Of particular interest, the
fluorescence emission of the functionalized NGQDs can be reversibly
switched “on–off” via redox reactions.
Furthermore, the PL emis<nclass="Chemical">spaclass="Chemical">n class="Chemical">siclass="Chemical">n>on can be switched from excitation-independent
to excitation-dependent by modifying the surface states of NGQDs.
For instance, Dai and co-workers have successfully de<class="Chemical">spn>an class="Chemical">signed <class="Chemical">span class="Chemical">carbon
dots (CDs) and amine functionalized polyhedral oligomeric silsesquioxane
(CDs@POSS) nanocomposites for multifunctional applications (Figure a,b).[29] The emission intensity and fluorescence quantum
yield of the CDs@POSS nanocomposites were superior to those of the
pure CDs as the surface passivation of CDs in CDs@POSS. Furthermore,
the fabricated CDs@POSS-based liquid marbles showed obvious fluorescence
under UV excitation, demonstrating its potential for sensing and drug
delivery. More interestingly, the formation of a complex between Fe3+ ions and CDs@POSS can effectively quench the fluorescence
of CDs@POSS. On this basis, a novel solid-state fluorescent sensor
was fabricated by simply dipping a piece of filter paper into a CDs@POSS
solution, and the resultant solid-state fluorescent sensor exhibited
a good sensitivity for the detection of Fe3+.[29]
Figure 9
(a) Schematic drawing for the fabrication route of CDs@POSS.
(b) TEM image of CD@POSS. Inset is the size distributions of the CDs.[29] (c) Illustration of colorimetric detection of
glucose by using glucose oxidase (GOx) and g-C3N4 peroxidase-like catalytic reactions. (d) Schematic of H2O2 reduction with TMB catalyzed by g-C3N4.[177] Images reprinted with permission
from refs (29 and 177). Copyright
2016 Wiley-VCH and 2014 Elsevier.
(a) Schematic drawing for the fabrication route of <nclass="Chemical">spaclass="Chemical">n class="Chemical">CDsclass="Chemical">n>@POSS.
(b) TEM image of <class="Chemical">spn>an class="Chemical">CD@POSS. Inset is the <class="Chemical">span class="Chemical">size distributions of the CDs.[29] (c) Illustration of colorimetric detection of
glucose by using glucose oxidase (GOx) and g-C3N4 peroxidase-like catalytic reactions. (d) Schematic of H2O2 reduction with TMB catalyzed by g-C3N4.[177] Images reprinted with permission
from refs (29 and 177). Copyright
2016 Wiley-VCH and 2014 Elsevier.
Having good cat<nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n>ytic properties and biocompatibility, C-MFCs
have been widely used in many biorelated applications.[2,174,175] Certain <class="Chemical">spn>an class="Chemical">carbon nanomateri<class="Chemical">span class="Chemical">als
have been demonstrated as stable and effective C-MFCs for detecting
H2O2 released from living cells.[176] It was demonstrated that the g-C3N4 owned an intrinsic catalytic activity for the oxidation
of 3,3′,5,5′-tetramethylbenzidine (TMB) to generate
a colored solution (easily to be detected by the naked eye) in the
presence of H2O2 (Figure c,d).[177] Using
g-C3N4 peroxidase-like C-MFC and glucose oxidase
(GOx), therefore, a colorimetric approach for detecting glucose in
serum samples was developed, exhibiting a high sensitivity and selectivity
with a minimum detection density of 1.0 μM. In a separated study,
aminated 3,4,9,10-perylenetetracarboxylic acid (PTCA) coupled with
GQDs (GQDs/PTCA-NH2) realized solid-state GQD sensing.[178] Furthermore, the fabricated electrochemiluminescence
signal tag was decorated on the surface of the electrode, displaying
good film-forming property, excellent electronic conductivity, and
high stability. Carbon fibers sheathed with VA-CNTs have also been
developed for monitoring of ascorbate in rat brains,[179] which displayed a good selectivity, reproducibility, and
durability.
<nclass="Chemical">spaclass="Chemical">n class="Chemical">Superoxide aclass="Chemical">nioclass="Chemical">nclass="Chemical">n> (<class="Chemical">spn>an class="Chemical">O2•–) is one of the important <class="Chemical">span class="Chemical">ROS species, and closely related to many
diseases, including atherosclerosis, diabetes, neurodegeneration,
and cancer.[180] Just as carbon nanomaterials
could be employed as superior C-MFCs for 4e– and
2e– process ORR, hollow mesoporouscarbon spheres
with N-doping (N-HMCS, Figure a) were used as a sensitive enzyme-free C-MFC sensor
for electrochemical quantification of O2•– directly (Figure b). As unique features, for example, good conductivity, effective
N doping, apposite pore size/volume, and large SSA, N-HMCS showed
a higher sensitivity for detection of O2•– than solid carbon spheres and hollow mesoporouscarbon spheres without
N doping, even some of metal-/enzyme-based sensors.[181] More recently, microporous graphitic nanosheets with N,P-codoping
(N,P-CMP-1000) were fabricated by the pyrolysis of as-prepared conjugated
polymers and phytic acid, which was then employed as highly active
C-MFC toward overall ORR in a broad pH range. Specifically, having
a high electrocatalytic activity for ORR in neutral solution (Figure c), N,P-CMP-1000
was successfully used for DO electrochemical quantification with a
low detection limit of 1.89 μAmg–1 L and wide
detection range from 2.56 to 16.65 mg L–1.[182] In vitro cytotoxicity tests (CCK-8 and live/dead
cell double staining assay) further demonstrated a high biocompatibility
of N,P-CMP-1000 to human corneal epithelial cells (Figure d, e), indicating a good promise
as an eye-wearable biosensor for detecting DO in tears to monitor
eye health (Figure f).[182]
Figure 10
(a) SEM image of N-HMCS, the inset is
the corresponding TEM image. (b) Current–time response on N-HMCS
electrode with successive injection of O2•– into 0.1 M deoxidized PBS with pH 7.4 at a given potential of −0.15
V.[181] (c) LSV curves of the N,P-CMP-1000
catalyst in common PBS solution (top) and artificial tear (bottom),
respectively. Inset: dissolved oxygen (DO) concentrations detected
by a commercial DO sensor. In vitro cytotoxicity of N,P-CMP-1000 extracts
against (d) human corneal epithelial cells determined by CCK-8 assay
and (e) live/dead cell double staining assay. (f) Schematic of the
potential using N,P-CMP-1000 for sensing DO in the wearable glasses
to monitor eye health.[182] Images reprinted
with permission from refs[181 and 182]. Copyright 2018 Elsevier, 2018 Wiley-VCH.
(a) SEM image of N-<class="Chemical">spaclass="Chemical">n class="Geclass="Chemical">ne">HMCSclass="Chemical">n>, the inset is
the correclass="Chemical">sponding TEM image. (b) Current–time response on N-<span class="Gene">HMCS
electrode with successive injection of <span class="Chemical">O2•– into 0.1 M deoxidized <class="Chemical">span class="Chemical">PBS with pH 7.4 at a given potential of −0.15
V.[181] (c) LSV curves of the N,P-CMP-1000
catalyst in common PBS solution (top) and artificial tear (bottom),
respectively. Inset: dissolved oxygen (DO) concentrations detected
by a commercialDO sensor. In vitro cytotoxicity of N,P-CMP-1000 extracts
against (d) human corneal epithelial cells determined by CCK-8 assay
and (e) live/dead cell double staining assay. (f) Schematic of the
potential using N,P-CMP-1000 for sensing DO in the wearable glasses
to monitor eye health.[182] Images reprinted
with permission from refs[181 and 182]. Copyright 2018 Elsevier, 2018 Wiley-VCH.
Addition<nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n>ly, because of their unique st<class="Chemical">spn>an class="Chemical">ructure and property
advantages, including large <class="Chemical">span class="Chemical">SSA, rich surface chemical functionalities,
nanoscale size, and biocompatibility, carbon nanomaterials have shown
great potential as drug carriers and mediators for target cancer therapy
to minimize the side effects to surrounding normal tissues caused
by conventionalcancer therapies, such as radiation,[183] ultrasound,[30] and microwave
therapies.[184] Through well-controlled functionalization,
for example, CNTs could be employed as nanocarriers for the targeted
delivery of anticancer drugs, genes, and immunotherapy components
for chemotherapy.[31] They have been used
as mediators to damage cancer cells without severe damage to normal
tissue in photothermal therapy (PTT) and photodynamic therapy (PDT)
(Figure a).
Figure 11
(a) Processes
of PTT and PDT using CNTs.[31] (b) Schematic
drawing of the sonosensitization process of PMCS for cancer therapy.[32] (c) Synthetic procedures for GODFe3O4@DMSNs nanocatalysts. (d) The schematic diagram of the
catalytic-therapeutic mechanism of GFD NCs for generating hydroxyl
radicals toward cancer therapy.[33] Images
reprinted with permission from refs (31−33). Copyright 2016 Dove Medical Press, 2018 Wiley-VCH, and 2017 Nature
Publishing Group.
(a) Processes
of class="Chemical">PTT aclass="Chemical">nd PDT u<class="Chemical">n class="Chemical">span class="Chemical">sing CNTs.[31] (b) Schematic
drawing of the sonosen<spclass="Chemical">n>an class="Chemical">sitization process of PMCS for cancer therapy.[32] (c) Synthetic procedures for GODFe3O4@DMSNs nanocatalysts. (d) The schematic diagram of the
catalytic-therapeutic mechanism of GFD NCs for generating hydroxyl
radicals toward cancer therapy.[33] Images
reprinted with permission from refs (31−33). Copyright 2016 Dove Medical Press, 2018 Wiley-VCH, and 2017 Nature
Publishing Group.
Because of the therapy
safety, relatively long tissue penet<nclass="Chemical">spaclass="Chemical">n class="Species">ratclass="Chemical">n>ion depth, and cost-effectiveness,
sonody<class="Chemical">spn>an class="Gene">namic therapy (SDT) has <class="Chemical">span class="Chemical">also led to widespread interest recently.[185] However, the discovery of sonosensitizers with
high sonosensitization efficacy and good stability is still a great
challenge. Very recently, a metal–organic framework (MOF)-derived
carbon nanostructure (PMCS) was developed as a superior sonosensitizer
(Figure b).[32] Compared to amorphous carbon nanospheres, the
sonosensitization effect of PMCS was shown to be closely linked to
the porphyrin-like macrocycle in MOF derived carbon nanomaterials.
Their intrinsic large bandgap allowed for a high yield ROS production
(Figure b), as validated
by electron spin resonance and dye measurement, for a high tumor inhibition
efficiency (85%). This study offers a good perspective for understanding
of structure-dependent SDT enhancement, which should lead to new carbon-based
materials with advanced structures for high-performance SDT.
To achieve a high <class="Chemical">spaclass="Chemical">n class="Disease">tumorclass="Chemical">n> class="Chemical">specificity, specific <span class="Disease">tumor microenvironment
(TME) has been deliberately developed,[186] in which the metabolism, biosynthetic process, and phy<span class="Chemical">sic<class="Chemical">span class="Chemical">al–chemical
environment are different from those in normal tissues.[187,188] Nevertheless, the used toxic anticancer medicine could cause some
destruction of normal tissues.[189,190] If the substances
in the TME are toxicity-free and biocompatible, and could be converted
into effective chemicals just against tumors, the stimuli of intratumor-delivered
nontoxic agent/agents would trigger the therapeutic process in tumorous
tissues without any damage to normal tissue. Very recently, Hou et
al. developed a smart biocompatible catalyst for catalytic nanomedicine
via TME-responsive reactions with a highly efficient, tumor-specific
therapy.[33] During the therapy process,
glucose oxidase (GOD) as the starting enzyme catalyst was introduced
to catalyze the glucose into a certain amount of H2O2 in the tumor region (as the intracellular H2O2 level is low). The generated H2O2 was
then reacted with Fe3O4 NPs supported by dendritic
mesoporoussilica nanoparticles via Fenton-like reactions for hydroxyl
radicals generation,[191] further leading
to apoptosis of the tumor (Figure c,d), resulting in a tumor-specific therapy without
damage to normal organisms and tissues. As can be seen, mesoporoussilica-based versatile nanomaterials were usually chosen as catalyst
supports for Fe3O4 loading (Figure c).[33,192] In this respect, nontoxic carbon-based nanomaterials (biodegradable
mesoporouscarbon)[70,193] with a large specific area and
good biocompatibility could be alternatives to silica-based versatile
nanosystems.[33] Indeed, various simple and
feasible methods have been designed for developing multidimensional
and multifunctionalcarbon materials with controlled architectures
and pore sizes[4,26,18,58,70,171,194] to support various
C-MFC active sites and/or other catalyst NPs for biomedical sensing,
imaging, diagnosis, and therapy. Having the unique advantages of the
tunable intrinsic structure and surface structure of the carbon nanomaterials,
the well-defined carbon could further act as a C-MFC to reduce/replace
Fe3O4 NPs. In this context, C-MFCs show great
potential as advanced materials for cancer therapy for various biomedical
applications.
Looking Forward: Future Directions
and Perspectives
The introduction of heteroatoms into the
<nclass="Chemical">spaclass="Chemical">n class="Chemical">carboclass="Chemical">nclass="Chemical">n> skeleton causes electron modulation that could gene<class="Chemical">spn>an class="Species">rate de<class="Chemical">span class="Chemical">sirable
electronic structures of C-MFCs for various catalysis. Although significant
progress has been achieved in developing C-MFCs for many important
reactions associated with energy conversion/storage and biomedical
applications, fundamental understanding of the reaction mechanisms
associated with C-MFCs is still largely lacking with respect to metal-based
catalysts. Further research work on the active sites engineering (e.g.,
dopant type, location, content) and characterization are needed to
reveal the structure–performance relationship for the development
of efficient heteroatom-doped C-MFCs.
class="Chemical">Compared with the st<class="Chemical">n class="Chemical">span class="Species">rategy
of heteroatom <spclass="Chemical">n>an class="Chemical">doping, the investigation on the defective effect is
an emerging direction. Some research work has indicated that the catalytic
performance induced by defects for ORR, OER, and HER could be even
superior to that of N-doping.[68] This finding
prompted more and more studies on the effects of defect types (e.g.,
topological defects, edge defects) on the catalytic performance, along
with the defect control and identification.
The atomic level
in<nclass="Chemical">spaclass="Chemical">n class="Chemical">siclass="Chemical">n>ght into the relationship between active <class="Chemical">spn>an class="Chemical">sites and cat<class="Chemical">span class="Chemical">alytic activities
is still largely lacking. In-depth theoretical and experimental investigations
should be devoted to explaining the nature of the active sites and
the electrochemical catalytic mechanisms, providing more guidelines
and opportunities for discovery of C-MFCs with breakthrough activities
for specific reactions. At the current stage, the theoretical calculations
have been employed to explain the experimental phenomena in most of
the cases. However, more attention should be paid to predict structures
of promising catalytic active sites and then provide guidance for
designing highly efficient C-MFCs for various reactions. On one hand,
the in situ real-time characterization techniques (e.g., X-ray absorption
spectra, scanning tunneling microscope, Raman, and/or atomic force
microscopy) should be developed and employed to detect chemical reactions
and disclose the electrochemical active sites. On the other hand,
considerations on the optimization of synthetic methods should be
taken to enhance the density and exposure of the corresponding catalytic
active sites.
Multifunction<nclass="Chemical">spaclass="Chemical">n class="Chemical">alclass="Chemical">n> C-MFCs are currently highly de<class="Chemical">spn>an class="Chemical">sirable
for advanced energy-related technologies, including <class="Chemical">span class="Chemical">Zn-air batteries,
water-splitting systems self-powered by Zn-air batteries, and integrated
devices of water-splitting with fuel cells for the continuous generation
of clean electricity. In another frontier, intracellular catalysis
is very promising for biomedical applications (e.g., cancer therapy)
while carbon nanomaterials have been used for biosensing, bioimaging,
and drug/gene delivery. Because of the large difference between in
vivo and in vitro environments, however, much more work is required
before clinic implications to be within insight. Continued work in
these emerging fields is of great value.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11