Metal–Organic Framework (MOF)‐Derived Nanoporous Carbon

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Title: Recent Advances of MOF-Derived Nanoporous Carbon Materials

Authors: Freddy Marpaung; Konstantin Konstantinov; Yusuke Yamauchi; Md.Shahriar A. Hossain; Jongbeom Na; Jeonghun Kim

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Recent Advances of MOF-Derived Nanoporous Carbon Materials

Freddy Marpaung,+1,2

Konstantin Konstantinov,1 Yusuke Yamauchi,*

2,3,4,5, Md. Shahriar A. Hossain

2,6

Jongbeom Na,*2,3

and Jeonghun Kim*

2

1 Australian Institute for Innovative Materials (AIIM), University of Wollongong, North Wollongong,

NSW 2500, Australia

2 Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland,

Brisbane, QLD 4072, Australia

3 International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials

Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

4 School of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia

5 Department of Plant & Environmental New Resources, Kyung Hee University, 1732 Deogyeong-

daero, Giheunggu, Yongin-si, Gyeonggi-do 446-701, South Korea

6 School of Mechanical & Mining Engineering, The University of Queensland, Brisbane, QLD 4072,

Australia

+ These authors equally contributed to this work.

E-mails: [email protected]; [email protected]; [email protected]

Abstract

Metal-organic frameworks (MOFs)-derived nanoporous carbon materials have received significant interest

due to their advantages of controllable porosity, good thermal/chemical stability, high electrical conductivity,

catalytic activity, easy modification with other elements and materials, etc. Thus, MOF-derived carbons

have been used in numerous applications such as environment, energy storage systems (i.e., batteries,

supercapacitors), catalysts, etc. Until now, many strategies have been developed in order to enhance the

properties and performances of MOF-derived carbons. In this review, we introduce and summarize recent

important approaches for advanced MOF-derived carbons focusing on precursor control, heteroatom doping,

shape/orientation control, hybridization with other functional materials.

Keywords: nanoporous carbon; metal-organic frameworks; functional carbon; chemical synthesis

Introduction

Carbon is one of the most important elements and can be existed with different forms such as graphite,

diamond, fullerene, and amorphous carbon. To improve their performances, nanoporous structure have been

introduced into carbon because nanopores can give a large surface area. Importantly, nanoporous carbon

materials (NPCs) have showed unique properties such as good electrical conductivity, catalytic activity, and

good thermal/chemical stability. These outstanding properties make them attractive to be used in various

applications such as purification of environments, energy storage systems (i.e., batteries, supercapacitor),

and catalysts. Until now various carbon materials with different pore sizes ranging from micropores to

macropores have been extensively studied. There are lots of carbon materials produced from series of


carbonization and activation process which possess controllable pore structures, which include activated

carbon, templated porous carbons, etc. Templated carbon materials have been prepared by various

approaches such as direct carbonization from carbon precursors, soft- and hard-templating methods. In order

to enhance the electrochemical performance of the electrode materials, heteroatoms, such as nitrogen (N),

sulfur (S), and boron (B), have been doped into porous carbons.

However, the large-scale synthesis is difficult in terms of cost-effectiveness because these methods

require numerous synthesis steps. To overcome the shortcomings, researchers have tried to solve those

issues by using metal-organic frameworks (MOFs) as sacrificial templates/precursors.[1-2]

MOFs have gained

a considerable attention in recent years because of their abundant diversity in structure and composition. In

addition, the porosity of MOFs can be tuned by organic linkers. More interestingly, their structures and

functions can be tailored for on demand applications through the transition metal ions and organic linkers.[3-

5]

In this review article, we summarize recent works for the synthesis, structural characteristics and

functionalization of MOF-derived NPCs. Much attention will be given on the new discoveries of MOF-

derived NPCs which are derived from MOFs precursors with different organic or inorganic functional

moieties such as N, Co, Sn, Ni, P and hybrid NPCs incorporated with metal oxides, metal nanoparticles. We

also elaborate how these added functionalities may affect the final performance of NPCs in electrochemical

applications.

History of MOFs-derived NPCs

Xu et al. reported the fabrication of MOF-derived NPC using MOF template with organic additives.[6]

The

furfurals alcohol (FA) having a molecular dimension of 8.4×6.4×4.3 Å3 has been used as a carbon source.

[7]

FA was infiltrated into the micropores of the MOF-5 by CVD process and subsequently polymerized. Then,

the decomposition of MOFs and the formation of NPC frameworks occurred simultaneously during

carbonization in an inert atmosphere. The resulted NPC how a high surface area (2,872 m2

g-1

) after

carbonization at 1000 ºC. At low carbonization temperatures, such as 530 ºC (the decomposition point) or

800 ºC, the surface areas were 217 or 417 m

2 g

-1, respectively. FA has been successfully infiltrated into

MOF and MOF acted as both sacrificial template and secondary carbon source. Later, same group reported

the NPCs with surface area in range of 1,141 to 3,040 m2

g-1

through the wet impregnation method using FA

under a low carbonization temperature.[8]

After polymerization of FA and pyrolysis process, hierarchically

porous carbon materials were successfully obtained. From these approaches, it has been found that the pore

size was depended on the method of penetration of carbon source into the cavities of MOF-5 and the

carbonization condition. These methods opened the utilization of many alternative carbon sources such as

glycerol, carbon tetrachloride, ethylenediamine, and phenolic resin for preparation on MOF-derived NPCs.[9-

10]

Owing to large amount of carbon source from organic linker in MOFs, direct pyrolysis of MOFs

without any additional carbon sources can also lead to synthesize the NPCs.[11-12]

Cendrowski et al.


demonstrated of NPCs through direct carbonization of MOF-5 at different temperature under argon

atmosphere.[13]

After carbonization at temperature between 700 and 900 ºC, they showed a tendency for

agglomeration which significantly reduced the surface areas from 2,524 m2

g-1

to 1,153 m2

g-1

and 1,892 m2

g-1

, respectively. From these cases, a highly nanoporous carbons can be obtained through the carbonization

at a lower temperature.

NPCs derived from MOFs possess many advantages such as extremely high surface area, uniform

pore sizes, large pore volume, good thermal and chemical stability. We can expect a wide range of

applications including water purification for drinking water, energy harvesting, energy and gas storage,

absorbents, and catalysts. Recently, many efforts have been made to upgrade the performance of the MOF-

derived NPCs.

Various types of MOFs as precursors

Up till now, considerable efforts have been conducted to synthesis NPCs with uniform particle sizes and

shapes due to their potential applications in many aspects. So far, a wide variety of MOFs, such as MOF-

5,[14]

ZIF-8,[15]

ZIF-67,[16]

MOF-199,[17]

UiO-66,[18]

MIL-53,[19]

and Cu3(BTC2)[20]

have been considered as

promising carbon precursors for synthesizing NPCs. A wise selection of MOFs as precursors (inorganic

connectors and organic ligands) plays an important role for determination of the resulting structural

properties of NPCs. Following the above report, Chang et al. controlled the particle sizes of MOF-525 from

100 nm to 750 nm.[21]

After carbonization, they carefully investigated the effect of variation of the particle

sizes on the electrochemical performance have been investigated.

Kim et al. successfully investigated the direct pyrolysis of a series of well-known nonporous Zn-

containing MOFs and found that they can lead to the formation of NPCs with high nanoporosity.[22]

The

same group also proposed a linear relationship between the Zn:C atomic ratio from the starting MOF and the

corresponding SA of the obtained NPCs. This result showed an easy control of surface area by Zn/C ratio of

the precursors. Interestingly, the field of MOF structures has been extended towards the metal-organic gels

(MOGs).[23-24]

The coordination framework of the synthesized materials were made by hydrogen bonding

and Van der Waals force.[25]

Zou et al. reported a facile way to prepare NPCs from Al-based MOGs (Al-

MOGs).[23]

The resulted NPCs exhibited an impressive porous property of exceptionally high surface area

and large pore volume of 3,770 m2

g-1

and 2.62 cm3 g

-1, respectively because of a hierarchical porous

architecture. These NPCs showed a considerable hydrogen uptake and superior electrochemical.

Zhao et al. develop carbonized nanoparticles, which are derived from monodisperse nanoscale metal

organic frameworks (MIL-88B-NH3), as the high-performance ORR catalysts.[26]

The obtained onset

potential and the half-wave potential for the ORR represent the best ORR activity of all the non-noble metal

catalysts reported so far. Thus, we should explore new MOFs as sacrificial templates for realizing the best

catalytic performance.

Morphological controls


Peng and co-workers synthesized porous carbon polyhedral (PCP) materials that were derived from Zn-

PBAI MOF (Figure 1a).[27]

The Zn-PBAI was constructed through combining T-shaped ligand, 5(4-

pyridine-4-yl-benzoylamino) isophthalic acid ligands with pyridine and isophthalate. The formation of

polyhedron shape was initiated by the interaction between six Zn atoms and six PBAI ligands to generate an

octahedral supramolecular building block. A 3-D porous metal-organic polyhedral framework was created

through connecting each of octahedral cage. They carbonized the Zn-PBAI at different temperature (800,

900 and 1000 ºC) under a nitrogen atmosphere, the surface areas increased significantly from 1,037 m

2 g

-1

(PCP-800), 1,160 (PCP-900), and 1,254 m2

g-1

(PCP-1000), respectively. Furthermore, the porous carbons

showed high performance in electrochemical analysis. Especially, the PCP-1000 showed the high capacity

which was attributed to the high surface area which provide abundant of active venues for the storage and

transmission of Li ions. Besides, both of nitrogen and pyridinic can improve the functionalization of the PCP,

as well as strengthen the bonding of lithium species and receive more charge from Li ions.

The hollow carbon nanobubbles were successfully synthesized through chemical etching techniques

by Zhang et al. (Figure 2a).[28]

The hollow structures on the ZIF-8 were prepared by chemical etching using

tannic acid, in which the tannic acid was used either as a protecting agent or an etching agent. In order to

further expand the mesopores, they carbonized it at 600 oC under N2 atmosphere followed by removal of

remaining Zn with HCl solution. As a result, a wide range pore size distribution was shown for the hollow

carbon nanobubbles, including micropores (< 2 nm) and mesopores in the range of 6 to 40 nm. However, the

surface area of hollow carbon nanobubbles was only 700 m2

g-1

, which was lower than that of the non-

hollow carbon nanoparticles (900 m2

g-1

).

Recently, ordered-porous metal-free carbon (o-PMFC) with uniform rod-like morphology was

successfully synthesized (Figure 1d) by using Zn-MOF nanorods.[29]

Interestingly, the formation of rod-

shaped morphology (Figure 1e and f) occurs without surfactants and modulators. After the sample was

carbonized in N2 atmosphere, the o-PMFC was successfully obtained. This material demonstrates good

water dispersibility and strong interaction with glycans. When used to capture N-glycans, the o-PMFC

showed a higher absorption capacity compared with activated carbon. Recently, Xu et al. reported a self-

templated, catalyst-free strategy for the synthesis of one-dimensional carbon nanorods by morphology-

preserved thermal transformation of rod-shaped MOFs. The as-synthesized non-hollow (solid) carbon

nanorods can be transformed into graphene nanoribbons through sonochemical treatment followed by

chemical activation. This approach without any complicated steps is readily scalable and could be used to

produce carbon nanorods and graphene nanoribbons on industrial levels.[30]

Other group has successfully synthesized nano-diatom porous carbon by introduced of guest species,

as shown in Figure 3d.[31]

They obtained porous carbon with different shapes of nanoparticles such as

polyhedral, fiber and web-like (Figure 3b-e) after pyrolysis at 800 ºC under argon atmosphere. When applied

to Li-B batteries, the nano-diatom electrode can deliver a capacity of up to 830 mA h g-1

in the primary

discharge/charge after 10 cycles at a current density of 0.2 A g-1

and over 600 mA h g-1

after 10 cycles at a

current density of 1 A g-1

. Interestingly, this electrode material showed capacity retention of over 75% after


500 cycles. When compared with current commercial graphite electrodes, the nano-diatom electrode showed

a better stability and performance. Such excellent performance was attributed to their unique structures

which were important for improving electrolyte access to the active sites.

For practical applications of supercapacitors, direct preparation of electrode on the conductive

substrates is crucial in order to diminish the resistance between the electrode material and the current

collector. Therefore, recently our group developed a new method for fabricating ZIF-8 and ZIF-67

nanowires through a simple and facile chemical vapor deposition on a conductive carbon cloth (Figure 4).[32]

This approach opens up a new way for solvent-free synthetic route of MOFs. The Co3O4-loaded NPC

nanowire arrays were fabricated by further thermal treatment. Co3O4 is known as a faradic metal oxide with

a high theoretical capacitance. Due to the presence of Co catalyst, ZIF-67 was successfully converted to

highly conductive sp2 type carbon. The obtained hybrid electrodes exhibited a high areal capacitance with

stable cycling. A simple synthesis of MOFs via the chemical vapor method offers a promising new platform

to design conductive, ultra-high surface area carbon electrodes.

Ma et al. also reported hybrid porous nanowire arrays composed of strongly interacting Co3O4 and

carbon were prepared by a facile carbonization of the MOF grown on Cu foil. The resulting material,

possessing a high surface area and a large carbon content, can be directly used as the working electrode for

oxygen evolution reaction without employing extra substrates or binders. The resulting hybrid Co3O4-carbon

porous nanowire arrays can also efficiently catalyze oxygen reduction reaction, featuring a desirable four-

electron pathway for reversible oxygen evolution and reduction, which is potentially useful for rechargeable

metal–air batteries, regenerative fuel cells, and other important clean energy devices.[33]

Meng et al. develop a general method for the oriented synthesis of precise carbon-confined

nanostructures by low-pressure vapor superassembly of a thin MOF shell and subsequent controlled

pyrolysis.[34]

The selected nanostructured metal oxide precursors not only act as metal ion sources but also

orient the superassembly of gaseous organic ligands through the coordination reactions under the low-

pressure condition, resulting in the formation of a tunable MOF shell on their surfaces. This synthetic

approach and proposed mechanism open new avenues for the development of functional nanostructured

materials in many frontier fields.

Introduction of heteroatoms into MOF-derived carbons

Nowadays, introduction of heteroatoms such as sulphur, boron, nitrogen into the carbon structure can

enhance the physical, chemical and electrical properties of NPCs.[35-37]

For example, nitrogen-doping can

enhance electrochemical performance, which has promising potentials as catalysts, H2 storage, CO2

adsorbents, and electrode materials. Wang et al. reported the functionalization of NPCs via nitrogen doping

that could enhance the catalytic performance. The porous carbon was prepared by direct carbonization of

ZIF-8 precursors at different temperature (700, 800, and 900 ºC) in N2 followed with removal of zinc oxide

nanoparticles using acid treatment (Figure 5).[38]

The resulting materials were applied as a electrocatalyst in

the electrochemical reduction of carbon dioxide. Interestingly, the Faradaic efficiencies to carbon monoxide


(∼78%) reached as high as about 78%. The carbonization temperature can control the amount and the

accessibility of N species in the NPCs, in which pyridinic-N and quaternary-N species play key roles in the

selective formation of carbon monoxide.

Wu and co-workers synthesized dual-heteroatom doped-hollow carbon via synergistic etching and

surface functionalization by using tannic acid.[39]

ZIF-8 were served as both a precursor and a nitrogen-

source, while 1,4-benzene diboronic acid (DBBA) was used as a boron-source. After carbonization with the

assistance of tannic acid, N/B co-doped hollow carbon materials were obtained. The excellent ORR

performances can be assigned to the large surface area and the existence of co-doping B and N which can act

as active sites to facilitate oxygen absorption and reduction.

Very recently, Ma et al. investigated the tri-doped (N, P, and S) hollow carbon shells (NPSC).[40]

The

NPSC materials were prepared by mixing ZIF-67 with poly(cyclotriphosphazeneeco-4,40-sulfonyldiphenol)

followed with pyrolysis at different temperature in N2 and acid etching (Figure 6a). The porous hollow shells

were successfully doped with N, S, and P, as shown in Figure 6b-f. Such heteroatom-doped carbon materials

are attractive catalysts for peroxymonosulfate (PMS) activation and environment remediation. The

synergistic effects of N, P, and S doping greatly break the electroneutrality of pristine carbon structures,

promote the O-O breakage of PMS, and facilitate the generation of radicals.

Oxygen co-doped NPC is useful for enhancement of absorption capacity for methylene blue (MB).

This NPCs were prepared by direct carbonization process of MOFs under an inert gas streamed with water

steam.[35]

The water steam acts as the oxidizing and activating agent. The steam-water treatment increased

the concentration of O in the sample. When evaluated the absorption capacity for methylene blue (MB)

showed that sample without streamed of water steam has a higher absorption capacity.

Functionalization with nanoparticles (metals, metal oxides, metal sulfides, etc.)

It is interesting to extend the synthetic method to further enhancement the properties of NPCs through the

deposition of functional metal, metal oxide, metal sulphide nanoparticles into NPCs. Magnetic Co/NPC

composites were prepared by direct one-step carbonization of ZIF-67, a well-known Co-containing MOF.[41]

Unlike the previously mentioned methods, this approach can maintain a rich content of highly crystalline

magnetic Co nanoparticles that are uniformly distributed over the entire NPCs. Therefore, the obtained

Co/NPC composite shows a strong magnetic response. Cheng et al. reported a successful deposition of Cu

nanoparticles on MOF-5-C which was fabricated by direct carbonization of MOF-5 without any additional

carbon sources.[42]

The surface area of MOF-5-C (2,524 m2

g-1

) was higher than that of Cu@MOF-5-C

(2,039 m2

g-1

). In the catalytic activity for the A3-coupling reactions, the Cu@MOF-5-C had a higher

catalytic activity, even with smaller surface area. This occurs due to synergetic interaction between Cu

nanoparticles and the matrix of porous carbon. Guo et al. synthesized Sn@NPC in two steps; (1)

carbonization of ZIF-67 at temperature of 800 ºC under argon atmosphere followed with removal Co

nanoparticles using hydrochloric acid, and (2) chemical reduction of Sn species in the NPCs. The Sn

nanoparticles were uniformly dispersed into the carbon matrix.[43]

The surface area of the NPC before Sn


deposition was 547 m2

g-1

, the Sn deposition largely decreased to 134 m2

g-1

. When this material was used as

anode materials for Lithium-ion battery, it showed good electrochemical properties which achieved high

initial discharge and charge capacities (1,440 mA h g-1

and 818 mA h g-1

at a current density of 200 mA g-1

).

In addition, this material had 51% capacity retention (741 mA h g-1

) even after 200 cycles at a current

density 200 mA g-1

. Such high Li storage capacity and excellent rate of capability can be associated to the

unique carbon frameworks, which provide abundant active sites and to buffer the strain due to volume

change during charging and discharging process upon cycling.

Jiao et al. has successfully synthesized magnetic NPCs (Fe3O4/NPC) which was derived from MOF-5

and Fe salt that were used as a carbon and a magnetic precursor, respectively (Figure 7a).[44]

The synthesized

Fe3O4/NPC was obtained through in situ chemical co-precipitation of Fe2+

and Fe3+

on the NPCs. Then it is

followed by carbonization at 900 ºC in nitrogen atmosphere. The TEM and SEM images (Figure 7b-e)

clearly show that nano-sized Fe3O4 particles were dispersed homogeneously on the matrix of NPC. The

adsorption performance also showed high adsorption capacity (292.4 mg g-1

at 298 K) and rapid adsorption

rate for organic dye methylene blue (MB). This material can be used as an absorbent for various organic

dyes. Haldorai et al. synthesized NPC-Co3O4 composite from ZIF-67 at temperature 900 ºC under nitrogen

atmosphere with removal of Co nanoparticles with 10% HF solution (Figure 8a).[45]

At evaluation of

electrodes for pseudocapacitors, the synthesized materials that is NPC-Co3O4 composite showed a high

specific capacitance of 885 F g-1

at 2.5A g-1

. An excellent stability was gained with only 6.0% capacitance

loss after 10,000 cycles. Besides that, the NPC-Co3O4 could be used as an electrode for the detection of

glucose. The improvement is from the synergistic effects between components in the nanostructured NPC-

Co3O4 composite, as well as a high surface area and higher ionic conductivity than that of the NPC.

Chen et al. also prepared porous carbon composites (Ni1Co4S@C) using Ni substituted ZIF-67 as

precursor via a one-step simultaneous carbonization and sulfurization process.[46]

The metal sulphide

nanoparticles were homogeneously embedded into the matrix of porous carbon that remarkably strengthens

the interaction between metal sulphides and carbon matrix. The large surface area is provide enough sites for

diffusion of ions through the highly porous substructures, which are crucial for the electrochemical activity

the electrodes. These findings pave a way to develop effective and promising alternative electrocatalysts

towards OER, ORR and HER in the next generation of energy storage and conversion technologies. Another

group also reported a porous Cu@graphitic octahedron carbon cages from a Cu-based MOF. The resulting

material exhibited good enrichment ability for four fluoroquinolones (FQs) due to their superior chemical

affinities to the target analytes.[20]

Unique catalytic effect of Co nanoparticles to grow carbon nanotubes

Meng et al. synthesized ZIF-67, followed with thermal treatment at a low temperature of 435 ºC under argon

(Figure 9a).[47]

Interestingly, the reducing gasses such as H2 and NH3 were released during pyrolysis reaction,

which it could alter metal ions into metal nanocatalysts that beneficial to catalyze the organic ligands and

generate a highly oriented CNTs. The growth direction of CNTs is from inside to outside, which create a


hollow interior voids after the carbonization process. The obtained hollow dodecahedra (N-CNTs-650)

showed increasing catalytic activities toward ORR (0.85 V), compared to Pt/C (0.83 V) in 0.1 mol L-1

KOH

solution saturated by oxygen. In addition to the outstanding ORR mentioned above, the composite displayed

a discharge capacity of 1,030 mA h g-1

after 250 cycles at a current rate of 0.1 A g-1

. The hollow interior

space and multilevel pores also allow fast diffusion of charged species onto the surface of the active sites.

Another type of NPCs was prepared from a bimetallic ZIF that is composed of Co2+

and Zn2+

with 2-

MIM as an organic linker (Figure 10a).[48]

After carbonization, the samples were washed extensively with

acid HF solution to remove the residual Zn and Co. Interestingly, a large portion of multiwalled CNTs

(MWCNTs) on the matrix of NPCs was observed. When evaluated as electrode for capacitor, the NPC

showed the potential window up to 0.8V and specific capacitance at about 10 Fg-1

at all current densities,

even at high current density (10 A g-1

). Such outstanding electrochemical behaviour can be ascribed to the

presence of CNTs, which provide abundant active sites for improving the electrical conductivity and

enhancing ion diffusivity. Kim and co-workers successfully synthesized another hybrid NPC derived from

ZIF-8 (Zn2+

) and ZIF-67 (Co2+

) (Figure 11a), and subsequently carbonized the obtained material at

temperature of 800 ºC under argon condition.[49]

The presence of hairy-like CNT on the surface of NPC

derived from Co/Zn-ZIF, as shown in Figure 11c and d. The electrochemical performance of the materials

showed the high capacitance values of 286 F g-1

at 2.5 A g-1

and 103.7 F g-1

at 10 A g-1

by investigation in

the 3 electrode configuration using 0.5 M H2SO4 electrolyte.

The highly uniformed hybrid carbon material has been prepared by in-situ growth carbon nanotubes

(CNTs) from MOFs, with the cobalt species as the catalyst for CNTs growth and dicyandiamide (DICY) as a

sacrificial agent for the formation of graphitic carbon.[50]

The polygonal structures and CNTs could be

observed on the edge of polygon carbon from the SEM images (Figure 12b-d). The cobalt nanoparticles and

heteroatom nitrogen can help in immobilizing sulfur species, resulting in the formation of a hybrid

composite (CNT@Co-N-C/S). This electrode material is capable of attaining up to 970 mA h g-1

of energy

density at 0.2C. Furthermore, it showed a good stability with nearly 79.8% of the capacity retention after

500 cycles. This excellent performance is supported by the formation of Co nanoparticles which are

distributed on the carbon matrix (Figure 12e). In addition, the CNT@Co-N-C/S showed high electrical

conductivity because of CNT on the carbon-sulphur composites that can not only shorten the penetration of

ions into the porous structure but also facilitate access of electrolyte. Similar phenomenon was reported by

Huang et al. New nanocomposites consisting of nanostructured Co3O4 and multiwalled carbon nanotubes

(MWCNTs) is highly desirable for the improvement of electrochemical performance of lithium-ion

batteries.[51]

Hybridization of MOF-derived NPCs with carbon nanotubes, graphene nanosheets, and related

nanomaterials

Hybridization has been known as an effective way to upgrade the functionalities and stabilities of NPCs.

Tan et al. reported the hybridization of ZIF-8 with CNTs, as shown in Figure 13a.[52]

The ZIF-8 was used as


precursors and CNT as a skeleton for assembly of ZIF-8 nanocrystals. The obtained hybrid materials after

carbonization at 1,000 ºC in a N2 atmosphere were clearly confirmed by the SEM and TEM images (Figure

13b and c). The relatively high specific capacitance (up to 75.1 F g-1

at 1 A g-1

in 1M H2SO4 solution) was

realized, which was attributed to the strong interaction between carbons and CNTs. Another group also

developed the following similar approach.[53]

The ZIF-CNT hybrid material was carbonized at 1000 ºC

under Ar atmosphere. The additional injection of NH3 (10 vol%) increased the N content in the obtained

material. The optimized sample showed a half potential (E1/2) of 0.899 V in 0.1 M KOH, which was higher

than that of Pt/C (E1/2 = 0.881 V). Furthermore, this catalyst showed a higher ORR onset potential of 1.05 V,

compared to Pt/C (1.02 V). The specific activity at 0.80 V was over 700 mA cm-2

, which was also much

higher than that of Pt/C catalyst (114.4 mA cm-2

). The superior catalytic performance was mainly associated

to the synergistic effects between NPC and CNT along with abundant N content. Bao et al. reported a

hierarchical architecture of multi-walled carbon nanotubes @ mesoporous carbon (MWCNT@Meso-C)

using unique multi-walled carbon nanotubes @ metal-organic framework (MWCNT@MOF-5) as both

template and precursor. Active sulfur is encapsulated into the MWCNT@Meso-C matrix for high

performance lithium sulfur battery. The resulting MWCNT@Meso-C is a promising host material for the

sulfur cathode in the lithium sulfur battery applications.[54]

Another work was performed by Park et al., in which NPCs were functionalized with crumpled

graphene balls (CGBs) (Figure 14a).[55]

The CGBs act as a conductive matrix for the formation of

polyhedral MOF which utilized vander Waals forces between the CGBs and organic precursor. The

polyhedral MOF was carbonized at 700 ºC in an argon atmosphere, followed with spray pyrolysis of CGBs

and diffusion of Se into the micropores of carbon matrix. The discharge capacity of the NPC/CGB-Se

electrode in lithium-selenium battery was up to 998 mA h g-1

at current density of 0.5 C in a non-aqueous

electrolyte. In addition, the NPC/CGB-Se had a remarkable cycle stability with discharge capacity 462 mA h

g-1

over 1000 cycles. Such excellent of electrochemical properties can be associated to the unique pore

structure of the composite, which can not only increase Se-ion diffusion but also enhance electrolyte

penetration. Ge and co-workers reported a synthesis of MOF-derived core/shell structured CoP@C

polyhedrons anchored on 3D reduced grapheme oxide (RGO) on nickel foam (NF) as binder-free anode for

high performance sodium-ion battery (Figure 15).[56]

The ZIF-67-GO-NF was first prepared at room

temperature and then carbonized and phosphatized at 400 oC. Further carbonization of ZIF-67-RGO-NF was

performed at temperature 350 ºC in argon atmosphere to obtained ZIF-67-derived core/shell Co@C

polyhedral structure. After that, an in-situ low-temperature phosphidation process was carried out to obtain

CoP@C polyhedrons. This hybrid material showed a higher initial discharge/charge capacity

(2,455.6/1,163.5 mA h g−1

) compared to CoP@C (1,908.5/746.2 mA h g−1

) in sodium-ion battery test. In

addition, the electrode was able to retain 47.3% of its original capacity after 100 charge-discharge cycles.

Such high initial charge/discharge capacity and stability can be ascribed to the unique core/shell structured

CoP@C and RGO networks, which can accommodate diffusion of species through the highly porous


substructures and buffer of volume changes during charge and discharge process, which are essential for the

electrochemical activity of electrodes.

Jin and co-workers reported that natural halloysite nanotubes (HNTs) were hybridized with MOFs to

prepare novel composites.[57]

Pre-treatment using sulfuric acid and nitric acid mixed solution was key obtain

halloysite with carboxylic group. Then, the halloysite was mixed with triethylamine and DMF to produce

MOF/ halloysite composite, followed by calcination in N2. The resulting carbon material showed good

hydrogen absorption of 0.23 wt% at 2.65 MPa. Further decoration with transition metals such as Al, Pd and

Ni enhanced the hydrogen absorption performance. The Pd-loaded carbon material had higher hydrogen

absorption of 0.32 wt% at 25 ºC and 2.65 MPa.

Conclusion and Perspectives

In our review, we demonstrate that the NPCs synthesized from MOFs through appropriate functionalization

methods can be applied to numerous applications. By precisely tuning the pore structures, shapes,

framework compositions, many benefits can be realized such as improving magnetic property, catalytic

activity, absorption capacity, and capacitance. By controlling the shapes of the starting MOF crystals, we

can prepare many kinds of MOF-derived NPCs such as hexagonal, cubes, hollow spheres, flakes, tubes, and

foams. Heteroatom doping and encapsulation of nanoparticles dispersed on the NPC matrixes are very

effective to optimize the physical and chemical properties. However, there are still many obstacles that need

to be overcome in order to synthesis highly ordered NPCs derived from MOFs. A better approach to finely

control the pore sizes of carbon nanoparticles is required. Future works should figure out how to determine

the shapes and sizes of carbon nanopores and find the best way to transfer guest species onto the nanopores

carbon surface. We are expecting to discover new structures of NPCs derived from MOFs, and we believe

that the resulting new NPCs will be extended into other undiscovered research sectors to diverse

applications with more investigation in the further.


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Figure 1. (a) Schematic synthesis of porous carbon polyhedral (PCP), (b, c) SEM images of PCP.

[27] (d)

Schematic preparation procedure of porous metal free carbon (PMFC), (e, f) SEM images of PMFC.[29]


Figure 2. (a) Schematic illustration of the synthesis of NPC nanobubbles (b) The SAED pattern of NPC

nanobubbles, (c) The particle size distribution of NPC nanobubbles.[28]


Figure 3. (a) Preparation precedures for fabrication of nano-diatom derived from guest impregnation of

polyhedral MOF. The SEM images of nano-diatom for (b, c) web-like surface and (d, e) fiber-like

structure.[31]


Figure 4. Scheme showing the synthetic process to generate (a) ZnO@ZIF-8-x-y and

Co(CO3)0.5(OH)·0.11H2O@ZIF-67-x-y using 2-Melm vapor where x and y are the synthesis temperature

and synthesis time, respectively. (b) Advantages of Co3O4/NC hybrid materials. (c, g) SEM images of (c) the

original ZnO-arrays and (g) Co(CO3)0.5(OH)·0.11H2O-arrays grown on conductive carbon cloth. (d) Low

magnification and (e) high magnification SEM images, and (f) TEM image of the ZnO@ZIF-8 sample

grown at 100 °C for 15 h (i.e., ZnO@ZIF-8-100-15). (h) Low magnification and (i) high magnification SEM

images, and (j) TEM image of the Co(CO3)0.5(OH)·0.11@ZIF-67 sample grown at 90 °C for 15 h (i.e.,

Co(CO3)0.5(OH)·0.11H2O@ZIF-67-90-15).[32]


Figure 5. (a) Preparation procedures for synthesis of N-doped NPC. i) Room temperature synthesis of ZIP-8

crystals. ii) Pyrolysis of the ZIF-8 crystals in N2. iii) Acid leaching to generate the N-doped NPC catalyst.

The TEM images of (b) ZIF-8 and (c) N-doped NPC.[38]


Figure 6. (a) Preparation of NPSC at different temperature (600, 700, 800 °C) from ZIP-67 and PZS. The

SEM images of (b) ZIF-67, (c) NPSC-600, (d) NPSC-700, and (e) NPSC-800; TEM images of (f) ZIF-67,

(g) NPSC-600, (h) NPSC-700, and (i) NPSC-800.[40]


Figure 7. (a) Preparation procedures for synthesis of Fe3O4/NPC. The SEM images of (b) NPC, and (c)

Fe3O4/NPC. The TEM images of (d) NPC and (e) Fe3O4/NPC.[44]


Figure 8. (a) Schematic images of the fabrication of NPC-Co3O4 composite for glucose sensor and

supercapacitor applications. (b) The SEM image of the as-synthesized ZIF-67 and (c) TEM image of NPC-

CO3O4.[45]


Figure 9. (a) Preparation procedures for the synthesis process of N-CNTs from ZIF-67. The corresponding

SEM images of (b) fresh ZIF-67 and (c) samples prepared after low-temperature pyrolysis at 435 °C in argon

for 0.5 h and (d) 8 h, respectively. (e, f) SEM, (g, h) TEM and (i) HRTEM images of N-CNT-assembled

hollow dodecahedra.[47]


Figure 10. (a) Schematic procedure for synthesis of bimetallic Co/Zn ZIF. The SEM images of (b)

bimetallic ZIF and (c) carbonized hybrid ZIF. The TEM images of (d) residual Co particles and (e) graphitic

carbon nanotudes (CNTs) on the surface of the as-prepared nanoporous carbon.[48]


Figure 11. (a) Schematic procedure for synthesis of hybrid Co/Zn (2:1)-ZIF from metal precursors and 2-

methylimidazole ligand, and carbonization to form amorphous and graphitic nanoporous carbon. The SEM

images of (b) hybrid Co/Zn (2:1)-ZIF, (c) carbonized nanoporous carbon (NPC), and (d) CNT on the surface

of NPC. (e) High resolution (HR)-TEM image of CNT on the surface of NPC. (f, g) TEM images, (h) high-

angle annular dark field – scanning TEM (HAADF-STEM) image, and (i) EDS mapping of hybrid Co/Zn-

ZIF.[49]


Figure 12. (a) Illustration of the synthesis of CNT@Co-N-C, SEM images of (b) ZIF-67, (c) Co-N-C and

(d) CNT@Co-N-C carbonization at 700 oC with 30% DICY, (e) TEM images of CNT@ Co-N-C.

[50]


Figure 13. (a) Schematic illustration of the synthesis of N-doped NPC from ZIF-8/CNT, NPC composite

images (b) SEM, (c) TEM.[52]


Figure 14. (a) The schematic diagram of NPC/CGB-Se Composite generation, NPC/CGB-Se images (b)

SEM, (c) TEM.[55]


Figure 15. (a) Schematic illustration of the synthesis of CoP@C-RGO-NF, CoP@C-RGO-NF. (b) SEM

images of CoP@C-RGO-NF and (c) TEM image of CoP@C-RGO-NF.[56]

Documents

Metal–Organic Framework (MOF)‐Derived Nanoporous Carbon