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INTRODUCTION

The rapid economic growth has led to a significant increase in global energy requirements, while the overuse of fossil fuels has intensified severe environmental pollutions and resource shortages.¹ With this regard, the pursuit of renewable energy and sustainable storage technologies has been a global research goal to address those energy and environment crises.² To date, various rechargeable energy storage devices, such as supercapacitors, sodium‐ion batteries (SIBs), lithium‐sulfur batteries (LSBs), lithium‐ion batteries (LIBs), and so forth,^3‐6 have been developed to relieve the energy pressure, the success of which relies on the exploration of novel high‐performance electrode materials.^7,8

Carbon‐based materials have been widely used as energy storage materials because of their large specific surface area, high electrical conductivity, as well as excellent thermal and chemical stabilities.^9‐14 However, the traditional synthetic methods, such as pyrolysis of organic molecules or biomass materials, vapor phase decomposition methods, high‐temperature solvothermal and hydrothermal methods, and so forth, have suffered from certain limitations of morphology, specific surface area, and size controls, which hinders the exploitation of their electrochemical performance and also the exploration of their reaction mechanisms.^15‐18 To overcome the above limitations, metal‐organic framework (MOF)‐derived carbon materials have become a research hotspot. MOF compound, also known as porous coordination polymer (PCP), is a new type of porous organic‐inorganic hybrid material with metal ions (or metal clusters) and organic ligands.¹⁹ MOF owns the unique advantages of being able to control its composition and structure by selecting suitable organic ligands and metals. Therefore, MOF materials are characterized by rich composition and structure, diverse pore channels, and high porosity,²⁰ all of which make them excellent templates and precursors for the preparation of porous carbon materials. MOF‐derived carbon materials effectively retain the pore structure and large specific surface area of MOF precursors and have strong electrical conductivity and high stability.^21‐23 By selecting suitable MOF‐based precursors, the prepared carbon materials can even achieve in situ heteroatom doping (such as N, P, S, B) with regular and uniform atomic distribution matrix. Additionally, the morphology, specific surface area, and particle size of MOF‐derived carbon materials can also be tuned through designed synthetic control, making them as a competitive type of carbon materials especially for energy applications.^24‐27 Therefore, MOF‐derived porous carbon materials typically show a superior performance in many fields such as energy storage devices, oxygen reduction reactions (ORRs), and so on, which in turn leads to rapid promotion and development of MOF synthesis and applications.^28‐32 Compared with other types of carbon materials, MOF‐derived carbon materials generally have lower density, more exposed active sites, and easier to be fully contacted with the reaction medium. Additionally, the internal cavity of the hollow carbon materials can not only increase the load of the active material and unblock the diffusion pathway but also provide a buffer space for the volume expansion of the active material for sustainable uses,³³ thereby effectively improve the overall stability of the active material.^34‐37 In particular, hollow porous carbon nanoparticles prepared from MOFs can exhibit a variety of morphologies, like dodecahedron, cube, sphere, and tube structure, and their particle and cavity sizes range from tens to hundreds of nanometers.^38‐41 In all, the as‐prepared hollow carbon materials have rich structural diversity and controllable morphology, pore channels, and cavity size, which shows great advantages especially in the fields of energy storage applications.

In this review, we aim to give a brief introduction of the MOF‐derived carbon materials, including their synthetic methods, structures, and compositions, for the uses of energy storage devices (summarized in Scheme 1). Especially, the strategies to modify the morphology and composition for desirable electrochemistry are highlighted. Finally, a prospective in view of the development trends and the remaining challenges with future works of MOF‐derived energy materials are presented. The electrode materials described in this review are listed in Table 1.

View Image - 1 Scheme. Structures and applications for the MOF‐derived carbon energy materials. LIB, lithium‐ion battery; LSB, lithium‐sulfur battery; MOF, metal‐organic framework; SIB, sodium‐ion battery

1 Scheme. Structures and applications for the MOF‐derived carbon energy materials. LIB, lithium‐ion battery; LSB, lithium‐sulfur battery; MOF, metal‐organic framework; SIB, sodium‐ion battery

1 TableMOF‐derived carbon materials for battery and supercapacitor applications

MOFs	Sample	Current density	Capacitance	Cycle number	Capacitance retention	References
LIBs cathode
MOF‐74	C⊃NiS	60 mA g⁻¹	…	100	300 mA h g⁻¹	42
ZIF‐67	HCSP⊂GCC	544 mA g⁻¹	536 mA h g⁻¹	150	365 mA h g⁻¹	43
Ni‐MOFs	NiSb⊂CHSs	100 mA g⁻¹	667.4 mA h g⁻¹	100	497.3 mA h g⁻¹	44
MIL‐100(Fe)	LFP/N‐CNWs	1700 mA g⁻¹	93.60 mA h g⁻¹	…	…	45
V‐MOFs	LVP/P‐C	10 C	65 mA h g⁻¹	1100	58.5 mA h g⁻¹	46
MIL‐101	LVP@M‐101	0.5 C	143.5 mA h g⁻¹	1000	113.1 mA h g⁻¹	47
LIBs anode
ZIF‐8	Si@ZIF‐8‐700N	50 mA g⁻¹	1050 mA h g⁻¹	50	1020 mA h g⁻¹	48
Sn‐MOF	Sn@NPC	0.2 A g⁻¹	1099 mA h g⁻¹	1500	507 mA h g⁻¹	49
IRMOF‐1	C/ZnO quantum dot	75 mA g⁻¹	1200 mA h g⁻¹	50	1200 mA h g⁻¹	50
ZIF‐67	Graphene/Co₃O₄	5000 mA g⁻¹	877 mA h g⁻¹	200	714 mA h g⁻¹	51
ZIF‐67	CoS@PCP/CNTs	0.3 A g⁻¹	1270 mA h g⁻¹	…	…	52
ZIF‐8	ZnSNR@HCP	100 mA g⁻¹	1388 mA h g⁻¹	300	840 mA h g⁻¹	53
ZIF‐67	Co₃O₄/CNT	0.1 A g⁻¹	1281 mA h g⁻¹	200	…	54
MIL‐125(Ti)	TiO₂/C	5 A g⁻¹	190 mA h g⁻¹	10 000	140 mA h g⁻¹	55
Cu‐Co‐ZIF	CCP	100 mA g⁻¹	793 mA h g⁻¹	50	740 mA h g⁻¹	56
Ti‐MOF	Li₄Ti₅O₁₂/C	500 mA g⁻¹	277 mA h g⁻¹	1000	181 mA h g⁻¹	57
Co‐MOF	CoO@N, S‐C	1000 mA g⁻¹	812 mA h g⁻¹	500	809 mA h g⁻¹	58
MOF‐5	ZnO/C	100 mA g⁻¹	1928 mA h g⁻¹	100	750 mA h g⁻¹	59
LSBs
ZIF‐67	Co‐N‐GC	0.05 C	1670 mA h g⁻¹	500	…	60
ZIF‐67	ZIF‐67	0.05 C	1674 mA h g⁻¹	850	…	61
ZIF‐8	NC‐800	300 mA g⁻¹	1158 mA h g⁻¹	100	785 mA h g⁻¹	62
ZIF‐8	HNC	167.5 mA g⁻¹	1025.5 mA h g⁻¹	500	…	39
NH₂‐MIL‐101	CN@NSHPC	1 C	739 mA h g⁻¹	500	445 mA h g⁻¹	63
ZIF‐8	HNPC	2 C	781 mA h g⁻¹	800	562 mA h g⁻¹	40
Ce‐MOF‐2	Ce‐MOF‐2/CNT	1 C	1021.8 mA h g⁻¹	800	838.8 mA h g⁻¹	64
MOF‐5	HPCN‐S	837.50 mA g⁻¹	1177 mA h g⁻¹	50	730 mA h g⁻¹	65
Al‐MOF	FLHPC	167.2 mA g⁻¹	1206 mA h g⁻¹	100	856 mA g⁻¹	66
Zn‐TDPAT	FMNCN	0.1 C	1645 mA h g⁻¹	200	1220 mA h g⁻¹	67
SIB
ZIF‐8	P@N‐MPC	0.15 A g⁻¹	710 mA h g⁻¹	100	600 mA h g⁻¹	68
ZIF‐8	HNPC	10 000 mA g⁻¹	100 mA h g⁻¹	1000	99 mA h g⁻¹	69
Ni‐MOF	NiO/Ni/graphene	200 mA g⁻¹	483 mA h g⁻¹	200	290 mA h g⁻¹	70
MIL‐101	Fe₂O₃@C	500 mA g⁻¹	710 mA h g⁻¹	200	662 mA h g⁻¹	71
Ni‐Co‐MOF	Ni₃S₂/Co₉S₈/N‐C	100 mA g⁻¹	425 mA h g⁻¹	100	419.9 mA h g⁻¹	72
MOF‐5	CPC	100 mA g⁻¹	240 mA h g⁻¹	100	233.4 mA h g⁻¹	73
ZIF‐67	CoP@C‐RGO‐NF	100 mA g⁻¹	1043 mA h g⁻¹	120	630 mA h g⁻¹	74
Supercapacitor
ZIF‐8	HNC	253.6 F g⁻¹	1.0 A g⁻¹	20 000	233.6 F g⁻¹	38
ZIF‐8	NPCF	332 F g⁻¹	1.0 A g⁻¹	5000	328.4 F g⁻¹	75
ZIF‐8	NCF	264 F g⁻¹	5.0 A g⁻¹	10 000	259.2 F g⁻¹	76
MIL‐101‐NH₂	HCCN	272 F g⁻¹	0.1 A g⁻¹	2000	…	77
ZIF‐8	ZIF‐8@PZS‐C	333 F g⁻¹	1 mV s⁻¹	…	…	78
MOF‐5	NPC	222 F g⁻¹	0.05 A g⁻¹	…	…	79
Zn‐MOF	WO3/C	125.1 F g⁻¹	5 A g⁻¹	3000	110.5 F g⁻¹	80
Mn‐MOF	MCG	348 F g⁻¹	5 A g⁻¹	2000	341.4 F g⁻¹	81
Ni‐MOF‐74	Ni/C	585 F g⁻¹	5 A g⁻¹	10 000	468 F g⁻¹	82
Mn‐MOF	MnO_x‐CSs	156 F g⁻¹	5 A g⁻¹	2000	147.7 F g⁻¹	83

Abbreviations: LIB, lithium‐ion battery; LSB, lithium‐sulfur battery; MOF, metal‐organic framework; SIB, sodium‐ion battery.

STRUCTURE AND COMPOSITION OF MOF‐DERIVED CARBON MATERIALS

Composition

Normally, using MOFs as the precursor, the metal or metal compound‐carbon composite materials can be synthesized through the organic ligands in MOFs carbonized in a suitable calcination process under the protection of an inert gas. Some MOFs (ZIF‐8, ZIF‐67, etc) containing heteroatoms (like N) in ligands can obtain heteroatom‐doped porous carbon materials under certain calcination conditions.^25,84 After removing metal ion or metal compound components, carbon materials with multiscale pore structures can be obtained.^85‐88 For instance, Liu et al²¹ obtained MOF‐derived carbon materials using MOF‐5 as a template and furfuryl alcohol as an external carbon source by calcination at high temperature. Due to the porous structure of the MOF‐5 template and the volatilization of Zn elements during the calcination process, the derived carbon material contains abundant pore structure. They found that the MOF‐5‐derived porous carbon material has a hydrogen absorption rate of 2.6% of the carbon material mass at 1.01 × 10⁵ Pa, which is much higher than the hydrogen adsorption capacity (1.3%) of the pristine MOF‐5 under the same conditions. In addition, MOF‐5‐derived porous carbon materials can also be used as the electrode materials for supercapacitors with a specific capacity of up to 258 F g⁻¹ at the current density of 250 mA g⁻¹. Wang et al calcined ZIF‐8 in a nitrogen atmosphere, then treated with 35% hydrochloric acid to etch the Zn component (it should be noted here that concentrated hydrochloric acid or HF may be used in the etching process, and readers should take precautions when using such chemicals). Finally, through KOH activation, they obtained a graded porous carbon material with a specific surface area up to 3680.6 m² g⁻¹, which showed good performance as electrodes of supercapacitors.⁸⁹ In addition to the porous carbon materials, metal atom‐doped porous carbon materials can also be obtained by regulating the composition, structure, and calcination conditions of MOF precursors.^90‐93 For example, Xia et al⁹⁴ prepared nitrogen‐doped polyhedral carbon/Co composites with ZIF‐67 as the precursor by pyrolysis at high temperature,⁹⁴ whose performance in catalytic applications related to redox reactions (ORR) is comparable to the best‐performing carbon‐based ORR catalysts ever reported. Yin et al⁹⁵ took bimetallic MOF (ZnCo‐ZIF) as the precursor and obtained the monoatomic Co/N‐C composite catalyst by calcination at high temperature, with the loading capacity of monoatomic Co up to 4%. The composite catalyst has a half‐wave potential of 0.881 V in the electrocatalytic ORR, which is better than the commercial Pt/C of 0.811 V. In the preparation of metal‐carbon composites, in addition to using intrinsic metal elements in MOFs as doping sources, external metal elements can also be introduced during the reaction.⁹⁶ Liu et al calcined Zn/Ni‐MOF‐2 in Ar gas at high temperature and etched with hydrochloric acid. Then, the obtained porous carbon microcubes (CMCs) were mixed with selenium powder (mass ratio, 1:2) and wet ground in agate mortar. The Se/CMCs composites were finally prepared by melt‐diffusion method under Ar gas atmosphere at 260°C for 12 hours.⁹⁷ The initial capacity of the as‐prepared Se/CMCs electrode could reach 780.4 mA h g⁻¹ in Li‐Se battery. After 100 cycles, it still had a specific capacity of 452.2 mA h g⁻¹. In addition to porous carbon/metal composites, nonmetal heteroatom‐doped porous carbon materials can also be obtained by using MOFs as precursors.^98‐100 Li et al¹⁰¹ mixed MOF‐5 with N, P, S source dicyandiamide (DCDA), triphenylphosphine, dimethyl sulfoxide, and then calcined at 900°C for 5 hours in nitrogen condition. The metal component was then removed in hydrochloric acid and washed with deionized water to obtain NPS ternary heteroatom‐doped carbon material (NPS‐C‐MOF‐5). In the ORR catalytic application, NPS‐C‐MOF‐5 shows a high starting potential similar to commercial Pt‐C, and at the voltage of 0.6 V, the current density can reach 11.6 mA cm⁻², which was 1.2 times higher than that of commercial Pt‐C catalyst. The material also shows superior methanol tolerance and long‐term cycle stability in alkaline media than commercial Pt‐C catalysts.

In summary, carbon materials derived from MOF precursors can realize in situ doping without external elements during the preparation process, and the components can be controlled easily by adjusting the type of MOF precursors and synthesis routes, which is superior to other methods of synthesizing carbon materials.

Structure

In recent years, researchers have prepared a variety of MOF‐derived materials with different structures by regulating the precursor structure or reaction conditions. On the one hand, the partially prepared structures (such as core‐shell structure and hollow structure) can alleviate the impact of MOF‐derived materials during use, thereby showing excellent cycle performance. On the other hand, by modulating the structure of MOF‐derived materials, their active sites are fully exposed, thereby maximizing their performance. This section mainly introduces the three main structures: fibrous structure, hollow structure, and core‐shell structure.

Fibrous structure

Fibrous MOF‐derived carbon materials have a hierarchical porous structure and large specific surface area, which is beneficial for the material to make full contact with the electrolyte and enable the active sites to fully react. Fibrous MOF‐derived materials can be prepared through various methods, including electrospinning, template method, and so forth.^102‐105 As shown in Figure 1, Wang et al³⁸ once prepared ZIF‐8/PAN fiber by electrospinning mixed solution of ZIF‐8/PAN/DMF, followed by carbonizing it in nitrogen atmosphere to obtain nanoporous carbon fiber (NPCF) material. On the one hand, this one‐dimensional (1D) nitrogen‐doped carbon material has high conductivity, which facilitates the transfer of electrons and charges during the charge and discharge processes; on the other hand, porous carbon derived from ZIF‐8 is uniformly distributed in nitrogen‐doped carbon derived from polyacrylonitrile (PAN), making full use of the large specific surface area of MOF‐derived carbon materials. The specific capacitance of the prepared nitrogen‐doped porous carbon (NPC) reaches 332 F g⁻¹, which is higher than PAN‐derived carbon (PAN‐C) and ZIF‐8‐derived nanoporous carbon (ZIF‐8‐NPC). Moreover, the retention rate of the porous carbon specific capacitance was 98.9% after 5000 cycles at the current density of 1 A g⁻¹. On this basis, Li and Tang¹⁰⁶ employed electrostatic spinning to prepare the fibrous material with mixed ZIF‐8 with PAN, which was then calcined and etched at high temperature to obtain NPCFs.¹⁰⁶ The NPCFs were further mixed with Mn₃O₄ to prepare Mn₃O₄/NPCFs by sol‐gel method. Based on the cavity structure of the 1D composite and the strong interaction between Mn₃O₄ and NPCFs, the as‐prepared material had an ultrahigh specific capacity of 1058 mA h g⁻¹ at the current density of 50 mA g⁻¹, and showed excellent cycling performance and multiplier performance, making it an excellent electrode material for LIB. Zhang and his colleagues once adopted Te nanofiber as a template to synthesize Te@ZIF‐8 nanofibers, which was later calcined and phosphated to gain the Te/P codoped carbon nanofiber.¹⁰⁰ The high specific surface area and porous characteristics of this derivative material made it easy for electrolyte ions to access the active site. Thus when used as the ORR catalyst, the initial potential and half‐wave potential of the codoped carbon material is approximately −0.07 and −0.161 V, respectively. Meantime, the material shows showed better methanol tolerance than that of Pt/C electrode material.

View Image - 1 Figure. A, Preparation process of nanoporous carbon fibers (NPFCs). B, Scanning electron microscopy (SEM) image of ZIF‐8, inset in (B) shows transmission electron microscopy (TEM) image. C and D, SEM images. E and F, TEM images of ZIF‐8/PAN. G, SEM image of ZIF‐8‐NPC, inset in (G) is TEM image. H and I, SEM images of NPCF, inset of (I) is the corresponding sectional image of NPCF. Reproduced with permission from Reference: Copyright 2017, The Royal Society of Chemistry.38

1 Figure. A, Preparation process of nanoporous carbon fibers (NPFCs). B, Scanning electron microscopy (SEM) image of ZIF‐8, inset in (B) shows transmission electron microscopy (TEM) image. C and D, SEM images. E and F, TEM images of ZIF‐8/PAN. G, SEM image of ZIF‐8‐NPC, inset in (G) is TEM image. H and I, SEM images of NPCF, inset of (I) is the corresponding sectional image of NPCF. Reproduced with permission from Reference: Copyright 2017, The Royal Society of Chemistry.38

Hollow structure

MOF‐derived carbon materials with hollow structure have attracted lots of attention from relevant researchers owing to the special cavity structure, which enables carbon material to effectively store substances and shorten the transmission path.^107‐111 Currently, there are two main methods of synthesizing hollow MOF‐derived materials, including template method and thermionic exchange method.^112,113 As shown in Figure 2, Guan et al¹¹² assembled ZIF‐67 on the surface of the PS sphere to form a yolk‐shell structure and then carbonized it to obtain a single‐hole cobalt/nitrogen‐doped carbon hollow material (Co/N). The as‐prepared carbon material combined the advantages of hollow porous materials and Co‐CN composites. It has a hollow porous structure and excellent electrocatalytic activity in alkaline environments. Its half‐wave potential was 0.87 V, which is better than commercial Pt/C electrocatalysts. Bai et al obtained a layered double hydroxide (LDH) hollow nanocage (Co‐Co) in two steps, consisting of in situ codeposition and thermal ion exchange. ZIF‐67 was first deposited on the graphene sheet, which was then hydrolyzed by extra Co(NO₃)₂ to produce proton. Meanwhile, it was etched so that Co²⁺ on ZIF‐67 could be released,¹¹³ and NO³⁻ and O₂ partially oxidize Co²⁺ to Co³⁺, hence Co²⁺ and Co³⁺ were codeposited on the surface of ZIF‐67 to prepare the Co‐Co‐LDH and graphene composite material. This composite material has an ultrahigh specific capacitance of 1205 F g⁻¹, an excellent rate performance, and cyclic stability. When used as an electrode in the asymmetric supercapacitor, it exhibited a high‐energy density of 49.5 W h kg⁻¹ and a high‐power density of 7000 W kg⁻¹, showing an outstanding electrochemical performance. In addition, Tian et al¹¹⁴ also used a microwave‐assisted method to sulfide Ni‐BTC MOF to obtain NiS₂ hollow microspheres. When it was used as a hydrogen evolution reaction catalyst, only 219 mV is required to reach 10 mA cm⁻² current density in the alkaline medium.

View Image - 2 Figure. A, Schematic illustration of the controlled formation of PS@ZIF‐67 composite particle (I) and its conversion to single‐holed Co/NC hollow particle by a subsequent thermal treatment (II). B and C, Field emission scanning electron microscope (FESEM) and (D and E) transmission electron microscopy (TEM) images of PS@ZIF‐67 particles. F,H, and J, FESEM, (G,I, and K) TEM, (L and M) magnified TEM, (N) high‐resolution transmission electron microscopy images of single‐holed Co/NC hollow particles. Reproduced with permission from Reference: Copyright 2017, Wiley.112

2 Figure. A, Schematic illustration of the controlled formation of PS@ZIF‐67 composite particle (I) and its conversion to single‐holed Co/NC hollow particle by a subsequent thermal treatment (II). B and C, Field emission scanning electron microscope (FESEM) and (D and E) transmission electron microscopy (TEM) images of PS@ZIF‐67 particles. F,H, and J, FESEM, (G,I, and K) TEM, (L and M) magnified TEM, (N) high‐resolution transmission electron microscopy images of single‐holed Co/NC hollow particles. Reproduced with permission from Reference: Copyright 2017, Wiley.112

Core‐shell structure

Core‐shell structure is a dominant structure. Normally the material with core‐shell structure can combine the advantages of different components to build superb performance.^115‐118 There are two main strategies for preparing MOF‐derived carbon with core‐shell structures: (a) by regulating the structure of MOFs precursors; (b) by regulating the reaction conditions. Huang et al used (3‐aminopropyl)trimethoxysilane and sodium polystyrene sulfonate to conduct surface modifications of various substrates, which was coated with ZIF‐8 in the next step. After the heat treatment, they obtained a variety of MOF‐derived carbon composite materials with core‐shell structures. Then, through electrochemical tests on two core‐shell structure materials (CNTs@ZnCo₂O₄ and NiO@ZnCo₂O₄), it was found that the initial specific discharge capacity of CNTs@ZnCo₂O₄ is 682.7 mA h g⁻¹, and the value increased to 750 mA h g⁻¹ after 100 cycles.¹¹⁹ During the cycles, the specific discharge capacity of NiO@ZnCo₂O₄ was maintained at ~1000 mA h g⁻¹, which even increased to 1002 mA h g⁻¹ after 100 cycles. Huang et al tested the specific capacities of the two materials at different current densities and found that both of them had excellent rate performance, proving that the two core‐shell materials were excellent LIB electrode materials. As shown in Figure 3, based on the advantages of core‐shell structure, Ge et al¹²⁰ tried to grow ZIF‐67 on GO/nickel foam (NF), and then it was carbonized and phosphated to obtain the final CoP@C‐RGO‐NF composite with core‐shell structure.¹²⁰ The core‐shell structure can provide sufficient contact area between the electrode and the electrolyte, which gives the material with excellent sodium‐ion storage performance. At the current density of 100 mA g⁻¹, after 100 cycles, the specific capacity of the material was still maintained at 473.1 mA h g⁻¹. Sikdar et al¹²¹ calcined Co‐MOF‐1 under the condition of hydrogen to obtain Co/NCNTs (nitrogen‐doped carbon nanotubes), and then partially oxidized it in air to obtain Co₃O₄@Co/NCNT with core‐shell structure. This material exhibited excellent dual catalytic function. In the ORR catalytic reaction, when the current density was −1 mA cm⁻², the voltage of Co₃O₄@Co/NCNT was 0.88 V; in the OER catalytic reaction, when the current density was 10 mA cm⁻², the voltage of Co₃O₄@Co/NCNT was 1.61 V. Except for those conventional core‐shell structures, there are some novel structures based on core‐shell, such as yolk‐shell,^122,123 double‐shell,^124,125 and so forth. For example, Su et al¹²⁶ obtained the CdS microtubule with yolk‐shell structure through simple microwave‐assisted heat treatment of the Cd‐Fe Prussian blue analog. The material had excellent photocatalytic hydrogen evolution activity and stability. The catalytic hydrogen evolution rate was 3051.4 μmol h⁻¹ g⁻¹, which was about 2.43 times that of traditional CdS nanoparticles.

View Image - 3 Figure. A, Schematic illustration of the synthesis process of CoP@C‐RGO‐NF. B and C, Scanning electron microscopy (SEM) images of ZIF‐67‐GO‐NF. D‐F, SEM images of CoP@C‐RGO‐NF. G, Transmission electron microscopy image of CoP@C‐RGO. Reproduced with permission from Reference: Copyright 2017, Elsevier.120

3 Figure. A, Schematic illustration of the synthesis process of CoP@C‐RGO‐NF. B and C, Scanning electron microscopy (SEM) images of ZIF‐67‐GO‐NF. D‐F, SEM images of CoP@C‐RGO‐NF. G, Transmission electron microscopy image of CoP@C‐RGO. Reproduced with permission from Reference: Copyright 2017, Elsevier.120

Others

In addition to the three typical structures mentioned above, MOF‐driven carbon materials also have some other structures, such as normal porous, nanosheets, and so forth. Furthermore, as is known to all, LiFePO₄ is an ideal cathode material because of its high discharge capacity (170 mA h g⁻¹), suitable operating voltage (3.42 V), low cost, environmental friendliness, and high safety. However, the poor conductivity and low ion conductivity of LiFePO₄ seriously affect its electrochemical performance.^127‐129 Based on the above disscussion, Liu et al⁴⁵ prepared a LiFePO₄ combined with nitrogen‐doped porous carbon composite (LFP/N‐CNWs) by using an iron‐containing MOF material (MIL‐100(Fe)) as an iron and carbon source. As it turns out that NPC skeleton not only significantly improved the conductivity of the material but also facilitated electrolyte penetration and lithium‐ion transmission. Consequently, LFP/N‐CNWs exhibited a large current at 20 C and a discharge capacity of 93.60 mA h g⁻¹ when used as cathode materials of LIBs. Tan et al¹³⁰ chose MIL‐100(Fe) as the precursor and obtained an N‐doped graphene/Fe‐Fe₃C (NG/Fe‐Fe₃C) nanocomposite by a simple one‐pot method. The NG/Fe‐Fe₃C nanocomposite exhibits excellent electrochemical performance, including ultrahigh Coulomb efficiency of almost 100%, excellent rate capacity, 1098 mA h g⁻¹ discharge capacity after 48 cycles, and excellent cycle performance. The obtained material also has excellent rate performance, with a current density of 1000 mA g⁻¹ and a discharge capacity of 609 mA h g⁻¹.

PREPARATION STRATEGIES

By pyrolysis in an inert atmosphere, MOFs can be easily converted into carbon‐based nanoporous materials.^131,132 Based on the control of pyrolysis temperature and posttreatment, the pore structure characteristics of MOF precursors can be transferred to porous carbon materials, thereby providing ideal surface properties and microstructure. Meanwhile, the composition of MOF‐derived carbon materials can be achieved by replacing metal ions or bridging ligands, such as doping different heteroelements in the carbon framework and loading of metals. In addition, by using a secondary carbon source, other heteroatoms such as phosphorus (P) and sulfur (S) can be doped. This section summarizes the strategies for the morphology and composition of MOF‐derived carbon materials and provides guidance for the future design and application development of MOF‐derived carbon materials.

Strategies of morphology control

MOFs directly convert to NPCs

MOFs have become attractive precursors/templates for the preparation of NPCs due to their highly designable structure, high surface area, high adjustable porosity, and high organic component content. Previous strategies mainly obtained derived NPCs by carbonizing MOFs directly at high temperature, and the obtained NPCs inherited the framework structure of the precursor.²² For example, by carbonizing MOF‐5 at 600°C and then 900°C in nitrogen atmosphere, a three‐dimensional (3D) porous carbon with high specific surface area can be obtained, in which micropores and mesopores coexist.¹³³ This preparation strategy has many advantages, such as a wide range of precursors, a simple and fast reaction process.

In recent years, the preparation and application of hollow‐structured MOFs have attracted wide attention. Many strategies have been reported for the synthesis of MOF capsules/nanobubbles. One of the common methods is the self‐assembly of soft/hard templates. The hollow‐structured MOFs synthesized by this method are mostly polycrystalline, with larger particles and poor stability.^134,135 Zhang et al⁶⁹ reported a method for controlling the synthesis of single‐crystal hollow MOFs by corrosion, under the conditions of controlled concentration and immersion time, ZIF‐8 was etched with tannic acid, and hollow ZIF‐8 was obtained after drying. After high‐temperature pyrolysis treatment, the derived carbon well maintained the hollow structure of the precursor. In addition, through the combination of different morphology of MOFs, some special structures of derived porous carbon can also be obtained.¹³⁶

Matrix in situ growth of MOF‐derived carbon materials

To reduce particle agglomeration, increase specific surface area, and further improve electrical conductivity, the researchers covered MOF particles with different matrix materials by in situ growth to form composite carbon materials.

For example, on two‐dimensional graphene, graphene is functionalized with polyvinylpyrrolidone to uniformly modify the amino groups on the surface of graphene oxide (GO). Then, ZIF‐8 was grown in situ by adsorbing Zn²⁺ and adding ligands, which were uniformly dispersed on the graphene sheet. After high‐temperature carbonization, nitrogen‐doped flake porous carbon with a specific surface area of 911 m² g⁻¹ was obtained.¹³⁷ In addition to growing on carbon substrates, MOF particles can also be generated in situ on metal substrates. Li et al¹³⁸ immersed CoAl‐LDH in a mixed solution containing cobalt nitrate and 2‐methylimidazole to obtain a mixed material CoAl‐LDH@ZIF‐67 covered with ZIF‐67. After carbonization at 800°C, porous carbon with a honeycomb surface was obtained, which had a large specific surface of 220 m² g⁻¹ and a hierarchical micro/mesoporous structure.

Strategies of composition control

In addition to the morphology and degree of graphitization, the material's structural composition is also an important factor affecting performance. Therefore, controlling the material composition is also an effective means to optimize performance.

Heteroatom doping

Doping heteroatoms such as nitrogen (N), sulfur (S), boron (B) or phosphorus (P) in the carbon structure can effectively adjust its intrinsic properties, including electronic properties, surface and local chemical characteristics, and mechanical properties. Among them, the doping of the N atom is considered an ideal choice because it has an atomic size close to carbon and five valence electrons, which is conducive to forming strong valence bonds with carbon atoms, and thus has a wide range of applications. N‐doped NPCs can be obtained by direct carbonization of MOF precursors containing N ligands.¹³⁹ The process is simple and the active sites of metal‐nitrogen‐carbon (M‐N‐C) are evenly distributed in the framework. Therefore, N‐doped NPCs show excellent performance in the field of electrochemistry. In addition, to increase the nitrogen content of MOFs, nitrogen‐containing secondary carbon source molecules can be further added as precursors.

Similarly, other heteroatoms such as P or S can also be doped with a secondary carbon source by immersing MOFs in an organic solution containing these elements. Notably, P or S heteroatom doping can induce redistribution of carbon atom charges and weaken O–O bonds, thereby further enhancing the electrochemical performance of M‐N‐C nanocatalysts. Jiang et al synthesized a bimetallic ZIF precursor that combines the advantages of ZIF‐8 and ZIF‐67. When the Zn/Co molar ratio reaches 20, the electrocatalyst (CNCo‐20) has the best catalytic activity.¹⁴⁰ After doping with P, the ORR catalytic performance of P‐CNCo‐20 can be further improved. In addition, P‐CNCo‐20 also has excellent long‐term stability and methanol resistance.

In addition, The high‐density uniformly distributed metal ions in the MOFs framework make this material show great potential in the preparation of monoatomic catalysts. Wu et al reported a Fe monoatomic catalyst, they added a certain amount of Fe(NO₃)₃ in the synthesis of ZIF to obtain iron‐doped MOF(Fe‐ZIF).¹⁴¹ It is worth noting that the size can be adjusted by changing the concentration of metal salts. After calcination at 907°C, Zn will evaporate, thereby obtaining Fe, N‐doped porous carbon. Monodispersed Fe active sites can greatly increase the active catalytic sites and enhance stability, resulting in excellent electrocatalytic performance.

Metal composite carbon material

Metal nanoparticles exhibit unique activity in the electrochemical field due to their small size, large specific surface area, and lattice characteristics. Surfactants are currently commonly used to coat the surface to stabilize nanoparticles, or to load nanoparticles onto porous materials with a large surface area (such as graphene, CNTs, etc) to prevent their agglomeration, increase effective surface area, and improve their dispersibility. However, the former causes the active sites of the nanoparticles to be masked, which affects the catalytic activity of the nanocrystals; the latter's matrix structure and loading process are usually complicated, and it is difficult to obtain an ideal dispersion. MOFs are composed of strong coordination of organic groups at metal nodes, which can be directly converted into metal‐modified carbon nanomaterials (M/C composites) by pyrolysis in an inert atmosphere, which has great advantages in preparing metal composite porous carbon.

Li et al¹⁴² used Fe‐MIL‐88A as the precursor to carbonize for 1 hour at 500°C under N₂ to obtain magnetic γ‐Fe₂O₃/C composites.¹⁴² The experimental results revealed that the calcination temperature and calcination time played an important role in the composition of the final product. When Fe‐MIL‐88A was annealed in N₂ atmosphere for 1 hour, and the temperature is 600°C, the products mainly include α‐Fe₂O₃, γ‐Fe₂O₃, and Fe₃C, and when the temperature was 700°C, the products mainly include α‐Fe₂O₃ and Fe₃C. When under the low temperatures (500°C), α‐Fe₂O₃ and Fe₃C can also be obtained by extending the baking time (over 1 hour).

APPLICATIONS

Lithium‐ion batteries

As a typical representative of green electrochemical energy sources, secondary batteries are playing an increasingly important role in daily lives. Among various secondary batteries, LIB is currently the most commonly used energy storage device and has many advantages such as high‐energy density, high operating voltage, low self‐discharge, and environmental friendliness. However, there are still some unresolved problems hindering its further development, such as unstable structures, unsatisfied energy density and cycling life, and so forth. Therefore, high‐energy density electrode materials have become a target pursued by many researchers today. Materials with higher theoretical specific capacities, such as silicon and germanium, have been tried to replace commercial graphite anode materials, but the huge volume change of such materials during the charge‐discharge reaction process can easily lead to pulverization and peeling of electrodes, which seriously hinders its commercial applications. In contrast, carbon‐based materials have a stable structure and good electrical conductivity. By properly constructing micro‐nanostructures, their electrochemical activity and specific capacity can be improved. MOF‐derived hollow carbon materials have a unique porous and internal hollow structure that effectively shortens the diffusion path of lithium ions and increases the effective contact area between the electrodes and electrolyte. Besides, its higher specific surface area and lower mass density also lay the foundation for specific capacity improvement.

LIBs cathode

When MOF‐derived carbon is utilized as electrode materials of LIBs, they are mainly used as carbon substrates to further compound with metal compounds, such as metal oxides (MO) or metal sulfides (MS), to prevent the growth or aggregation of internal nanoparticles. MOF is a highly ordered material, so carbon‐coated metal sulfide (C⊃MS) materials prepared by this method are expected to induce fairly uniformly dispersed metal nanoparticles in a porous carbon matrix. In addition, rich MOF materials with a variety of structures and adjustable pores can construct various functional C⊃Ms materials, which can not only achieve various porous carbon matrices with different porosities but also easily fix different metal nanoparticles.

Qian and Chen et al prepared nanosized Ni particles (5‐10 nm) in carbon matrix by calcining MOF‐74 (Ni) in a reducing environment and converting the Ni particles into NiS by in situ reaction. As a cathode of LIB, C⊃NiS with small particle size (~50 nm) and uniform porosity revealed better electrochemical performance compared with bare NiS. After 100 cycles, the obtained C⊃NiS electrode maintained a reversible capacity of about 300 mA h g⁻¹, while the bare NiS decayed to 100 mA h g⁻¹ after 20 cycles.⁴² As shown in Figure 4, Liu et al synthesized a novel electrode material consisting of hollow cobalt sulfide nanoparticles embedded in graphite carbon nanocage (HCSP⊂GCC). As the carbon nanocage‐encapsulated Co₉S₈ has good mechanical flexibility and obvious structural stability, the obtained HCSP⊂GCC showed excellent lithium‐ion storage capacity of 536, 489, 438, 393, 345, and 278 mA h g⁻¹ at 0.20, 0.50, 1, 2, 5, and 10 C (1 C = 544 mA g⁻¹), respectively, and it also owned a high rate capability and stable cycling performance.⁴³ As is known to all, Li₃V₄(PO₄)₃ is also an attractive cathode material for LIBs due to its good thermal stability, high operating voltage (3.0‐4.3 V), and theoretical capacity (133 mA h g⁻¹).¹⁴³ However, there is the main problem that the conductivity of Li₃V₄(PO₄)₃ is relatively low, which immensely hindered its commercial application.¹⁴⁴ Here, Wang et al⁴⁶ synthesized a series of P‐doped carbon envelope Li₃V₄(PO₄)₃ composites (Li₃V₄(PO₄)₃/P‐C) with different morphologies by using V‐containing MOFs as precursor through high‐temperature forging and firing treatment at N₂. The uniform coating of P‐doped carbon was proved to effectively accelerate the transportation of electrons and stabilize the structure of the material. Therefore, Li₃V₄(PO₄)₃/P‐C showed excellent rate performance and long cycle stability: it had a large discharge capacity of 138 mA h g⁻¹ at 0.1 C, which was still retained at 65 mA h g⁻¹ at 10 C, and 90% of the initial capacity could be maintained after 1100 cycles. Similarly, Liao et al reported a carbon‐coated Li₃V₄(PO₄)₃ nanocomposite synthesized by a selected V‐containing MOFs material (MIL‐101(V)) reacting with LiOH·H₂O and NH₄H₂PO₄ at high temperature. The as‐prepared composite material also exhibited excellent electrochemical properties when used as the cathode material of LIB.⁴⁷

View Image - 4 Figure. A, Schematic illustration of the fabrication of uniform HCSP⊂GCC cathode. B, Detailed formation mechanism of hollow particle subunits' evolution. C, Scanning electron microscopy and (D) Transmission electron microscopy images of uniform HCSP⊂GCC products. E, CV curves at a scanning rate of 0.1 mV s−1 in the voltage range of 1.0 to 3.0 V. F, Voltage‐capacity curves at 0.25 C rate. G, Cycling performances of HCSP⊂GCC cathode at 0.25 C rate. H, Voltage‐capacity curves at different rates. I, Rate capability at different rates. J, Long cycling performances of HCSP⊂GCC cathode at 1 C rate. Reproduced with permission from Reference: Copyright 2016, Wiley.43

4 Figure. A, Schematic illustration of the fabrication of uniform HCSP⊂GCC cathode. B, Detailed formation mechanism of hollow particle subunits' evolution. C, Scanning electron microscopy and (D) Transmission electron microscopy images of uniform HCSP⊂GCC products. E, CV curves at a scanning rate of 0.1 mV s−1 in the voltage range of 1.0 to 3.0 V. F, Voltage‐capacity curves at 0.25 C rate. G, Cycling performances of HCSP⊂GCC cathode at 0.25 C rate. H, Voltage‐capacity curves at different rates. I, Rate capability at different rates. J, Long cycling performances of HCSP⊂GCC cathode at 1 C rate. Reproduced with permission from Reference: Copyright 2016, Wiley.43

LIBs anode

As is well‐known, silicon‐based, tin‐based, and transition MO/MS are typical high‐energy density anode materials because their theoretical capacities are about 2 to 11 times that of graphite materials. Unfortunately, as anode materials for LIBs, these materials exhibited some serious problems such as poor conductivity and large volume changes during charging and discharging. To overcome these limitations, on the one hand, a variety of porous nanostructures can be prepared to release the stress caused by the volume change while leaving enough space for volume expansion; on the other hand, using carbon coating can improve the conductivity of the material properties, further stabilizing the structure of the material. However, to achieve the above requirements through traditional manufacturing processes requires a complex process, and the uniformity of the prepared materials is not ideal. Herein, by combining MOF‐derived carbon materials with other materials, the high‐energy density anode materials that meet the above requirements can be obtained through some simple high‐temperature processes.

Wang et al reported a simple mechanochemical method, they wrapped a layer of ZIF‐8 in situ on the surface of silicon nanoparticles, which was then heated to 700°C for 1 hour in an inert atmosphere to obtain NPC‐clad Si composites (Si@ZIF‐8‐700N).⁴⁸ The NPC (ZIF‐8‐700N) with the porous structure completely encapsulates the silicon nanoparticles, which buffers the volume change of Si during the charge and discharge process, so Si @ ZIF‐8‐700N has excellent long‐life cycle stability and rate performance as LIB anode material. This study demonstrates a simple and effective method for the preparation of silicon‐carbon composites. Yu et al synthesized carbon hollow spheres embedded with NiSb (NiSb⊂CHSs) using Ni‐MOFs and SbCl₃ through simple annealing and galvanic replacement reactions. Luckily, NiSb⊂CHS retained the advantages of Ni‐MOF such as ideal porosity, high specific surface area, and hollow structure. Thus this material exhibited good electrochemical performance when applied as an anode of LIBs. When paired with nanoscale LiMn₂O₄ cathode, NiSb⊂CHSs//LiMn₂O₄ full cell showed high rate capability of 210 mA h g⁻¹ at 2000 mA g⁻¹ and the excellent Coulomb efficiency of 99%.⁴⁴

Dai et al⁴⁹ calcined Sn‐MOF under a protective atmosphere to obtain Sn/C composites, in which Sn quantum dots with tiny particle size were evenly distributed in the NPC framework (named Sn@NC). The tight combination of porous NC and Sn quantum dots could accommodate large volume changes to maintain the integrity of the structure. Therefore, Sn@N‐C had an outstanding and stable lithium storage performance. It can still maintain the capacity of 507 mA h g⁻¹ after 1500 cycles at the current density of 1 A g⁻¹. Yang et al⁵⁰ took IRMOF‐l as the precursor and obtained an amorphous porous carbon‐coated ZnO quantum dot composite simply by one step of high‐temperature forging. The size of ZnO in this composite was tiny, about 3.5 nm, and it was evenly distributed in the carbon skeleton, which could greatly shorten the transmission distance of lithium ions. Meanwhile, the carbon skeleton was hollow and porous so that it could accommodate the volume expansion in the process of ZnO‐embedded lithium. Hence, when applied as an LIB anode, this composite showed excellent cyclic stability, almost 100% of the initial capacity can be maintained at a current density of 75 mA g⁻¹ after 50 cycles, and excellent rate performance, of about 400 mA h g⁻¹ can be maintained at the current density of 3750 mA g⁻¹. This study provides a simple and potential method for large‐scale production of carbon‐coated MO quantum dot composites. In addition, Qu et al composited ZIF‐67 and GO in situ, then calcined the composite in inert atmosphere and oxidized at high temperature in air condition. Finally, they obtained the graphene/Co₃O₄ composite material. This graphene/Co₃O₄ composite material exhibited an initial discharge capacity of 1029 mA h g⁻¹, and can be maintained at 877 mA h g⁻¹ even under the large current of 5000 mA g⁻¹, showing superior rate capability.⁵¹ Wu et al⁵² employed ZIF‐67 as the precursor, which was heated to 600°C under the protection of an inert atmosphere, then introducing sulfur vapor to react with ZIF‐67. During this process, the Co‐ions within the ZIF‐67 framework were converted to uniform cobalt sulfide nanoparticles while the organic ligands were in situ carbonized to porous carbon polyhedra. Under the catalysis of CoS nanoparticles, some CNTs were grown on the carbon skeleton in the reduced position, after which a composite material named CoS@PCP/CNTs was finally obtained. CoS@PCP/CNTs showed excellent electrochemical performance as an anode material of LIBs, thanks to its unique structure and composition advantages: (a) nanoscale CoS can effectively shorten the transmission distance of lithium ions, which is beneficial to increase the rate performance; (b) hollow and porous carbon skeleton can not only buffer volume expansion but also facilitate the penetration of electrolyte; (c) partial graphitized carbon and CNTs can enhance the conductivity of the material and facilitate the rapid transport of electrons. This vulcanization method can also be extended to other MOF materials to prepare corresponding carbon‐encapsulated metal sulfide anode materials. For example, Chen et al⁵³ vulcanized ZIF‐8 with sulfur vapor at high temperature to obtain a porous carbon polyhedron‐coated ZnS nanorod composite (ZnSNR@HCP), which had excellent electrochemical performance when used for lithium anode materials. In addition, the MOF‐derived porous (PCNFs) and hollow carbon nanofibers (HCNFs) obtained by the electrospinning method in most cases have high surface area, good conductivity, and layered porous structure, which has made it a promising electrode material for rechargeable batteries. For instance, as shown in Figure 5, Lou et al synthesized Co₃O₄/CNT layered microtubules by in situ growth of ZIF‐67 on electrospun PAN nanofibers. The as‐prepared material showed a layered hollow structure, which could effectively reduce the volume change that occurred during the operation process. Therefore, the Co₃O₄/CNT microtubes derived from ZIF‐67/PAN showed good rate performance and cycle stability, and the capacity just decreased slightly after 200 cycles under the current of 1 A g⁻¹.⁵⁴

View Image - 5 Figure. A, Formation of the hierarchical CNT/Co3O4 microtubes. B‐D, Field emission scanning electron microscope of the CNT/Co3O4. E, Transmission electron microscopy of the CNT/Co3O4. F and G, High‐resolution transmission electron microscopy images of the synthesized hierarchical CNT/Co3O4 microtubes. H, Charge‐discharge voltage profiles at 0.1 A g−1. I, Rate performance. J, Cycling performance and Coulombic efficiency. Reproduced with permission from Reference: Copyright 2016, Wiley.54

5 Figure. A, Formation of the hierarchical CNT/Co3O4 microtubes. B‐D, Field emission scanning electron microscope of the CNT/Co3O4. E, Transmission electron microscopy of the CNT/Co3O4. F and G, High‐resolution transmission electron microscopy images of the synthesized hierarchical CNT/Co3O4 microtubes. H, Charge‐discharge voltage profiles at 0.1 A g−1. I, Rate performance. J, Cycling performance and Coulombic efficiency. Reproduced with permission from Reference: Copyright 2016, Wiley.54

Lithium‐sulfur batteries

LSB is a kind of secondary battery with metallic lithium as the negative electrode and sulfur element as the positive electrode and normally has a high theoretical specific capacity (1675 mA h g⁻¹) and high‐energy density (up to 2600 W h kg⁻¹). Simultaneously, sulfur element is environment‐friendly, abundant on earth, and relatively inexpensive, making LSBs one of the most promising next‐generation secondary batteries.¹⁴⁵ Nevertheless, LSBs can hardly reach their theoretical capacity in practical applications mainly due to the poor conductivity of the elemental sulfur (5 × 10⁻³⁰ S m⁻¹), and the long‐chain polysulfide (Li₂S_x, 4 ≤ x ≤ 8) generated by the sulfur positive electrode in electrochemical reactions is easily dissolved in the electrolyte and shuttles between the positive and negative electrodes of the battery, resulting in the deposition of the short insulating chain Li₂S₂ or Li₂S generated on the electrode surface and causing the loss of active material and poor cycling performance.¹⁴⁶ In addition, the sulfur element undergoes huge volume deformation during the lithium‐ion intercalation and delithiation reactions, which can easily lead to pulverization and shedding of the electrode material, further causing the electrode structure to collapse and shortening the battery life.¹⁴⁷

To date, the most effective way to solve these problems is to confine the active sulfur in the conductive carrier material so as to improve the conductivity of the electrode while suppressing the shuttle effect of polysulfides. In this regard, MOF‐derived hollow porous carbon materials have become an excellent host carrier for confined active sulfur due to their large specific surface area, high porosity, and uniform heteroatom doping.¹⁴⁸ Commonly, the MOFs used as precursors are ZIF‐8,^149‐152 ZIF‐67,^{60,61,153,154} and HKUST‐1.^155,156 Taking ZIF‐67 as an example, Li et al⁶⁰ employed ZIF‐67 as the precursor and directly carbonized it to obtain Co‐ and N‐doped porous carbon materials (Co‐N‐GC). The rich mesoporous carbon skeleton of Co‐N‐GC was proven to not only promote the transport of electrons and ions but also effectively inhibit the dissolution and loss of polysulfides. Besides, the doped Co and N heteroatoms could enhance the interaction between the carbon skeleton and polysulfides and had synergistic catalytic effect on the conversion of polysulfides at the same time. Therefore, after loading sulfur, S@Co‐N‐GC revealed excellent electrochemical performance as a lithium‐sulfur cathode. In detail, the discharge capacity reached 1670 mA h g⁻¹ at 0.05 C, which was almost close to the theoretical capacity (1675 mA h g⁻¹), and the Coulomb efficiency can still be maintained at 100% after 500 cycles at 1 C. Furthermore, ZIF‐67 can also be compounded with CNTs,¹⁵⁷ GO,¹⁵⁴ 3D carbon fibers,¹⁵³ and layered bimetal hydroxides⁶¹ so as to be converted into carbon‐based composites to achieve higher sulfur loading and better electrochemical performance. Dong's group designed and constructed a quasi‐2D Co@N–C composite with honeycomb architecture by in situ nucleation and directed epitaxial growth of ZIF‐67s onto the surfaces of the CoAl‐layered double hydroxide (CoAl‐LDHs) nanosheets, followed by a sintering and acidification process.⁶¹ This carbon material with special structure achieved high sulfur load, and combined with the good conductivity and the catalytic action of Co and N, it can also have a good rate and cycle performance under such a high loading, which is obviously better than the carbon material obtained by direct carbonization of ZIF‐67.

As shown in Figure 6, recently, Xiong's group prepared porous nitrogen‐doped carbon materials (NC) with different functions and structural characteristics, including different N contents, different graphitization degrees, and different specific surface areas, by adjusting the carbonization temperature (700°C‐900°C) of ZIF‐8.⁶² NC‐800 (10.45 wt% N, 1032.4 m² g⁻¹) showed excellent electrochemical performance, and the high specific surface area and high N content synergistically increased the capacity of the sulfur electrode. Moreover, graphitized and quantized N atoms are beneficial to improve the electrical conductivity of the electrode, thereby increasing the initial discharge capacity, while relying on the micropores of the porous carbon material to effectively fix and adsorb polysulfide molecules, alleviating the shuttle effect. Therefore, this study is an ideal example that successfully got cathode materials with high specific capacity and good cycle performance for Li‐S batteries.

6 Figure. A, Schematic illustration of the synthesis route to NC materials from ZIF‐8 crystals and its interaction with polysulfides during charge/discharge processes in a Li‐S cell. B, Scanning electron microscopy (SEM) images of ZIF‐8 precursor. C, SEM images of NC‐800 after calcination, (D) SEM images of NC‐800‐S after loading 60 wt% sulfur. B‐D, Scale bars = 200 nm. E, Cyclic performance. F, Rate capability electrodes at different current densities. G, Galvanostatic charge‐discharge voltage profiles of NC‐800‐S60 at a current density of 800 mA g−1 in the voltage window of 1.7 to 2.8 V. H, Initial galvanostatic charge‐discharge profile of NC‐800‐S60 at 300 mA g−1. I, Cycling performance of NC‐800‐S60 at a rate of 300 mA g−1 when the areal sulfur loading is up to 3.0 mg cm−2. Reproduced with permission from Reference: Copyright 2017, Wiley.62

Compared with the simple nitrogen doping, multiple heteroatom‐doped hollow carbon materials exhibit superior performance when used as the carrier of the positive electrode of sulfur. For example, Zhang et al⁶³ immersed varying mass of ammonium thiocyanate into the interior of NH₂‐MIL‐101(Al) by double‐solvent method, after the removal of aluminum metal by heat treatment and pickling, nitrogen‐sulfur codoped hollow carbon material (CN@NSHPC) containing g‐C₃N₄ nanodots were prepared. The pore structure of NH₂‐MIL‐101(Al) had an effect on the growth of g‐C₃N₄ nanodots, limiting the particle size to less than 5 nm. The synergy of the various components promoted CN@NSHPC to exhibit superior polysulfide adsorption performance than that of NPC or g‐C₃N₄ obtained by the same pyrolysis strategy. After loaded active sulfur, the prepared S/CN@NSHPC electrode has a specific capacity of 1447 mA h g⁻¹ at the current density of 0.2 C and a multiplier specific capacity of 387 mA h g⁻¹ at the current density of 5.0 C. Moreover, after 500 cycles at 1.0 C, the specific capacity loss of each cycle is only 0.048%. In addition to using MOFs alone to prepare hollow porous carbon materials, composites consisting of MOFs and other precursors can also be used to prepare hollow carbon materials with porous structures. Li et al⁴⁰ selected the ZIF‐8 complex coated with ionic imidazolinium organic polymer as the precursor, and they exchanged the bromide ions in the organic polymer for DCDA. Because of the low thermal stability of the organic polymer and the poor chemical and water stability of ZIF‐8, the organic polymer decomposed and released volatile molecules that were adsorbed by ZIF‐8, further etching the ZIF‐8 during the pyrolysis process. As the pyrolysis process continued, the hollow porous nitrogen‐doped carbon material (HNPC) was finally produced, and it showed excellent stability and multistage pore characteristics. The hollow structure and high doped nitrogen content were extremely beneficial for the adsorption and mass transfer of polysulfides, and the composite structure could provide a guarantee for the rapid transmission of electrons in the whole electrode, so the HNPC materials showed excellent overall performance in LSBs. After loading 65% (w/w) sulfur element, the HNPC‐900‐65S positive electrode had a high specific capacity of 562 mA h g⁻¹ after circulating 800 times at the current density of 2.0 C, the average capacity loss rate per cycle was as low as 0.035%.

In contrast, except for electrodes, the separator is also quite an important part for LSBs, which can not only separate the positive and negative poles of lithium batteries to avoid short circuit, but also provide smooth microchannel for lithium‐ion transmission. However, a traditional commercial separator can rarely prevent the “shuttle effect” of polysulfide in LSBs. In response to this problem, researchers began trying to modify traditional commercial separators with some functional materials, such as GO,¹⁵⁸ CNTs,¹⁵⁹ metal composites,¹⁶⁰ and so forth. These functional materials can prevent polysulfides from passing through the separator and serve as the upper current collector that reuses the polysulfide adsorbed on it. Inspired by the above work, MOF‐derived carbon‐based composites have gradually begun to be applied as the membrane modification layer. For instance, Hong et al⁶⁴ applied two Ce‐based MOF materials (Ce‐MOF‐1 and Ce‐MOF‐2) compounded with CNTs as a modification layer to optimize the separator of LSBs. As the coordination unsaturation of the Ce(IV) metal center in Ce‐MOF‐2 is higher than that in Ce‐MOF‐1, the interaction between Ce‐MOF‐2 and polysulfide is stronger, so after combining the high conductivity of CNTs, Ce‐MOF‐2/CNT composites could efficiently adsorb and convert polysulfides, which effectively suppressed the shuttle effect of polysulfides. Compared with the original separator, Ce‐MOF‐2/CNT modified separator effectively improved the performance of LSBs. The initial capacity of the battery is 1021.8 mA h g⁻¹ at 1 C, which could be maintained at 838.8 mA h g⁻¹ after 800 cycles, and the Coulomb efficiency showed almost no attenuation. Even under the high sulfur load of 6 mg cm⁻², the LSB with Ce‐MOF‐2/CNT modified separator could also have an initial capacity of 993.5 mA h g⁻¹ at 0.1 C, which was even maintained at 89.2% of the initial capacity after 200 cycles.

Sodium‐ion batteries

Due to the small reserves and uneven distribution of lithium resources on the earth, the cost of lithium batteries remains relatively high, which limits its wider application. Sodium, which is similar to lithium, is abundant and widely distributed on earth, and its extraction cost is relatively low. Meanwhile, sodium element has similar physical and chemical properties with lithium, so SIB is expected to be the most promising alternative to LIBs.¹⁶¹ Although the working principles of the two batteries are similar, the radius and atomic weight of sodium ions are much larger than those of lithium ions, which seriously affects the migration of sodium ions in electrochemical reactions. As a result, sodium ions cannot be reversibly de‐intercalated in the graphite material, so that graphite cannot be used as a negative electrode material of a sodium‐ion battery.¹⁶² Therefore, the development of high‐performance sodium storable cathode materials has become one of the critical factors for the development and application of SIBs, and researchers have made great efforts to develop novel cathode materials. As shown in Figure 7, Li et al⁶⁸ first calcined and etched ZIF‐8 to synthesize NPC material (N‐MPC) and further phosphated it to obtain an amorphous red phosphorus/nitrogen‐doped microporous carbon composite material (P@N‐MPC). Red phosphorus has a theoretical specific capacity as high as 2595 mA h g⁻¹, and the pore size of P@N‐MPC is about 1 nm, which can shorten the electrolyte ion diffusion path. And the existence of carbon materials is able to make up for the shortcomings of insufficient red phosphorus conductivity and play a limiting role for red phosphorus, which enhances the structural stability during cycling, thereby making P@N‐MPC an excellent electrode material for SIB. In SIB applications, the specific capacity of P@N‐MPC was about 600 mA h g⁻¹ at the current density of 0.15 A g⁻¹, and the capacitance retention rate was 99.8%, reaching 450 mA h g⁻¹ after 1000 cycles at the current density of 1 A g⁻¹.

View Image - 7 Figure. Schematic illustration of the preparation process for A, P@N‐MPC and (B) sodiation process of P@N‐MPC. C, Scanning electron microscopy, (D) transmission electron microscopy, and (E) high‐resolution transmission electron microscopy images of P@N‐MPC. F, Cyclic voltammograms of P@N‐MPC at a scan rate of 0.2 mV s−1. G, Capacity and Coulombic efficiency‐cycle number curves of P@N‐MPC electrode at a cycling rate of 0.15 A g−1. H, Capacity of P@N‐MPC composite as a function of cycling rate (0.3‐8 A g−1). I, Excellent cycle performance of P@N‐MPC electrodes at 1 A g−1 with activation first at low current density. Reproduced with permission from Reference: Copyright 2017, Wiley.68

7 Figure. Schematic illustration of the preparation process for A, P@N‐MPC and (B) sodiation process of P@N‐MPC. C, Scanning electron microscopy, (D) transmission electron microscopy, and (E) high‐resolution transmission electron microscopy images of P@N‐MPC. F, Cyclic voltammograms of P@N‐MPC at a scan rate of 0.2 mV s−1. G, Capacity and Coulombic efficiency‐cycle number curves of P@N‐MPC electrode at a cycling rate of 0.15 A g−1. H, Capacity of P@N‐MPC composite as a function of cycling rate (0.3‐8 A g−1). I, Excellent cycle performance of P@N‐MPC electrodes at 1 A g−1 with activation first at low current density. Reproduced with permission from Reference: Copyright 2017, Wiley.68

Through chemical etching, Zhang et al first synthesized bubble‐shaped ZIF‐8 precursors with a shell thickness of 10 nm,⁶⁹ which retained the nanocrystalline polyhedron morphology with the particle size of about 100 nm and also kept the characteristics of the single crystal. After zinc or zinc oxide is removed from the material by carbonization and pickled in an inert atmosphere, HNPC material with a high specific surface area (700 m² g⁻¹) can be successfully developed. The as‐prepared carbon material exhibited a large number of micropores, mesopores, and cavities (~40 nm). As a cathode material for SIBs, hollow carbon materials exhibit better multiplier and cycling performance than that of unetched solid carbon. After 1000 cycles at the current density of 10 A g⁻¹, the specific capacity of the hollow carbon electrode showed almost no attenuation and maintained 99% Coulomb efficiency.

Chen et al took electrostatic spinning technology to produce a composite fiber paper of zinc acetate, cobalt acetate, and PAN, then the composite fiber paper was soaked in an ethanol solution containing 2‐methylimidazole, and through the action of the ligand metal and ligand, a bimetallic imidazole framework polymer layer was grown on the PAN fibers to obtain a fibrous composite PAN/Zn (Ac)₂/Co(Ac)₂@BMZIF with a core‐shell structure.¹⁶³ The fibrous composite was further carbonized at 700°C, during which period the ZIF‐8 in the BMZIF layer was transformed into porous carbon with high nitrogen content, and ZIF‐67 was formed with a highly graphitized carbon material after carbonization. Besides, ZnO formed by the degradation of Zn(Ac)₂ has an etching effect on the internal PAN fibers, and the hollow graphitized carbon nanofibers (HGCNFs) are successfully produced after the Zn metal is removed by acid washing. It was proved that the carbon fiber still had good toughness and flexibility and could be used as a flexible electrode material. Without the addition of the conductive agent and the adhesive, the as‐prepared HGCNFs could be directly used as the cathodes of SIBs and exhibited high discharge specific capacity and outstanding rate performance. After 10 000 cycles at a high current density of 4.5 A g⁻¹, the specific capacity (~140 mA h g⁻¹) did not decrease significantly, showing excellent cycle stability. Most importantly, the HGCNFs electrode material still maintains the initial hollow structure after 10 000 long cycles, and its morphology and structure remained intact without significant changes as well. These excellent properties were mainly attributed to the specially designed hollow porous fiber structure. Specifically, the considerable interlayer distance and abundant defects of carbon materials ensured sufficient sodium storage sites, while the high specific surface area and the porous structure helps electrolyte to make full contact with the active material and shorten the sodium‐ion diffusion path, ensuring fast charge‐transfer reaction, and the high length‐to‐diameter ratio of tubular structure achieved fast 1D electron transmission.

Supercapacitor

Supercapacitor, also called electrochemical capacitor, is a unique energy storage device between traditional capacitors and secondary batteries. Supercapacitors combine the high‐power density of traditional capacitors and the high‐energy density characteristics of the secondary batteries, thus they possess many advantages such as excellent rate performance, high safety performance, long cycle life, and environmental friendliness. At present, supercapacitor is widely used in many fields, including uninterruptible power supplies and automotive industry, and has attracted considerable attention from the academic and industrial communities. There are two main ways of energy storage for supercapacitors: one is using electric double layer to reach capacitance storage, that is, to store energy through the separation of charges formed at the interface between electrolyte ions and electrodes, and the other is Faraday's capacitor energy storage, which uses a reversible redox reaction occuring on the electrode surface or bulk phase to achieve capacitance storage. Presently, the performance of supercapacitors is mainly determined by the electrode materials. Among various electrode materials, carbon is the most widely used one in the supercapacitor the market currently, mainly because of its numerous advantages of extensive source, environmental friendliness, good electrical conductivity, and large specific surface area.¹⁶⁴

Generally, carbon materials store energy by forming an electric double layer through the separated charges of electrolyte ions on the surface, so the structure with a large specific surface area and appropriate pore size is the key method to achieve high capacitance of carbon materials.¹⁶⁵ Although some organic precursors can be transformed into carbon materials with high specific surface through some physical or chemical activations, the pore structure of such carbons often has sorts of defects, which easily lead to kinetic problems of ions transmission, thereby resulting in the poor rate performance of the material.¹⁶⁶ Studies have shown that high porous and partially graphitized carbon materials can effectively improve the utilization of the specific surface area, reduce high rate polarization, and achieve a high‐performance electrode material for supercapacitors. MOF‐derived carbon materials meet well with the above requirements, for the porous carbon materials prepared with MOFs as precursors generally possess adjustable large specific surface areas and regular nanopore structures, laying a foundation for the construction of highly active electrode materials.^167,168 However, MOF‐derived carbon materials also have some drawbacks. For example, because most MOF precursors own the microporous structures, the corresponding carbon materials also basically maintain the microporous structure. Hence the pore structure hinders the rapid diffusion and penetration of electrolyte ions, resulting in a decrease of effective contact area and the poor‐rate performance.¹⁶⁹ Recently, hollow porous carbon materials based on MOF precursors have received more and more attention, this is mainly because hollow porous carbon materials have a higher contact area compared with solid microporous carbon materials, which facilitates the rapid ionization transmission, which can adapt to the volume change during the operation period and promote the reaction kinetics of the electrode.^170‐172

In particular, ZIF‐8 is often selected as a precursor for preparing porous carbon materials for supercapacitors due to its simple synthesis, controllable morphology, and high nitrogen content. For example, Li et al took electrolyte‐modified silicon dioxide as a template, and it was deposited and grown ZIF‐8 on the surface.³⁸ After the carbonization process and the removal of the template, a hollow nitrogen‐doped carbon shell framework was prepared. When the as‐prepared carbon material is used as an electrode material for supercapacitors, the specific capacitance of the electrode reached 253.6 F g⁻¹ at the current density of 1.0 A g⁻¹, which could be kept at 79% at the improved current density of 50.0 A g⁻¹, and the specific capacitance retention rate was 92.1% after 20 000 cycles. Besides, a symmetrical supercapacitor assembled by this ZIF‐8‐derived carbon material displayed a high voltage of 1.6 V in 1 mol L⁻¹ Na₂SO₄ electrolyte, and its energy density could reach 13.3 W h kg⁻¹. As shown in Figure 8, also using ZIF‐8 as a precursor, Wang et al tried to disperse ZIF‐8 nanoparticles (ZIF‐8 NPs) into PAN solution in N,N‐dimethylformamide (DMF), and then they obtained ZIF‐8 and PAN composite fiber (ZIF‐8/PAN) by electrostatic spinning.⁷⁵ Finally, the 1D NPCF was obtained after the carbonization process at high temperature. During the carbonization process, ZIF‐8 NPs were transformed into a hollow cubic shell and were firmly bound to the 1D carbon fibers obtained by the pyrolysis of PAN. Due to the unique 1D hollow fiber structure, NPCF exhibited better electrochemical performance than that of the pure ZIF‐8‐derived carbon material. Under different sweep speeds, the cyclic voltammetry curves were regular rectangles, indicating that the specific capacitance of NPCF electrode mainly came from electric double‐layer capacitors. The smaller voltage drop of the constant current charge‐discharge curve indicated that NPCF had better conductivity compared with the ZIF‐8‐NPC. Therefore, when NPCF is used as an electrode to assemble a supercapacitor, it showed excellent electrochemical performance. The specific capacitance was 332 F g⁻¹ at the current density of 1.0 A g⁻¹, and the capacitance retention rate was 98.9% after 5000 cycles under the same current density.

View Image - 8 Figure. A, Scheme depicting the fabrication of NHCSF‐3 from ZIF‐8/PP‐SiO2‐3 precursor. Scanning electron microscopy (SEM) images of (B) ZIF‐8 and (C) NHCSF‐0. D and E, Low‐ and high‐resolution transmission electron microscopy (TEM) images of NHCSF‐0. SEM images of (F) SiO2, (G) PP‐SiO2‐3, (H) ZIF‐8/PP‐SiO2‐3, (I) ZIF‐8/SiO2‐3, and (J) NHCSF‐3 (inset showing the enlarged image). K, Low‐resolution SEM image of NHCSF‐3 and the corresponding elemental mapping images of C, O, and N, respectively. L and M, Low‐ and high‐resolution TEM images of NHCSF‐3. N, current‐voltage (CV) profiles of NHCSFs at a scan rate of 10 mV s−1. O, CV profiles of NHCSF‐3 at various scan rates. P, Galvanostatic charge‐discharge (GCD) profiles of NHCSFs at a current density of 1 A g−1. Q, GCD profiles of NHCSF‐3 at various current densities. R, Nyquist plots and equivalent circuit of NHCSFs (the inset showing the magnified plots). S, Cycling stability of NHCSF‐0 and NHCSF‐3. Reproduced with permission from Reference: Copyright 2018, Wiley.75

8 Figure. A, Scheme depicting the fabrication of NHCSF‐3 from ZIF‐8/PP‐SiO2‐3 precursor. Scanning electron microscopy (SEM) images of (B) ZIF‐8 and (C) NHCSF‐0. D and E, Low‐ and high‐resolution transmission electron microscopy (TEM) images of NHCSF‐0. SEM images of (F) SiO2, (G) PP‐SiO2‐3, (H) ZIF‐8/PP‐SiO2‐3, (I) ZIF‐8/SiO2‐3, and (J) NHCSF‐3 (inset showing the enlarged image). K, Low‐resolution SEM image of NHCSF‐3 and the corresponding elemental mapping images of C, O, and N, respectively. L and M, Low‐ and high‐resolution TEM images of NHCSF‐3. N, current‐voltage (CV) profiles of NHCSFs at a scan rate of 10 mV s−1. O, CV profiles of NHCSF‐3 at various scan rates. P, Galvanostatic charge‐discharge (GCD) profiles of NHCSFs at a current density of 1 A g−1. Q, GCD profiles of NHCSF‐3 at various current densities. R, Nyquist plots and equivalent circuit of NHCSFs (the inset showing the magnified plots). S, Cycling stability of NHCSF‐0 and NHCSF‐3. Reproduced with permission from Reference: Copyright 2018, Wiley.75

Subsequently, Chen et al⁷⁶ used ZIF‐8 and PAN as precursors, similarly, they employed the electrospinning method to prepare nitrogen‐doped carbon fibers containing hollow nanoparticles. As an electrode material of supercapacitor, the specific capacitance of the fiber electrode at the current density of 1.0 and 50.0 A g⁻¹ are 307.2 and 193.4 F g⁻¹, respectively. The maximum energy density at the power density of 25 000 W kg⁻¹ is 10.96 W h kg⁻¹. The fiber electrode also exhibited excellent cycling stability, with a loss rate of only 1.8% after 10 000 cycles at the current density of 5.0 A g⁻¹. In addition to common nitrogen atom doping, sulfur, phosphorus, and other heteroatom doping may further increase the hydrophilicity of carbon materials, improving the electrolyte wettability and providing more surface Faraday reaction sites, thereby effectively promoting their electrochemical performance. For instance, Zhu et al⁷⁷ introduced thiourea into MIL‐101‐NH₂ through the dual‐solvent method and pyrolyzed under an inert atmosphere to prepare hollow nanocapsule‐like nitrogen and sulfur codoped porous carbon materials. The material owned a multistage pore (macropore, mesopore, and micropore) structure, which effectively increased the contact area and facilitated the rapid transfer of charges. By the synergistic effect of the multistage pore structure and the heteroatom doping, the carbon material exhibited excellent electrochemical performance. When used as an electrode for a supercapacitor, the specific capacitance of the carbon electrode was 240 F g⁻¹ when the current density is 10.0 A g⁻¹. While using ionic liquid as the electrolyte, the energy density of the electrode could reach 46.7 W h kg⁻¹ at the power density of 8250 W kg⁻¹.

Zhang et al⁷⁸ once grew organic polymers on the surface of ZIF‐8 nanoparticles and then successfully synthesized hollow polyhedral nitrogen, phosphorus, and sulfur codoped porous carbon materials (ZIF‐8@PZS‐C) after carbonization and acid washing.⁷⁸ Compared with the pure ZIF‐8‐derived carbon material (ZIF‐8‐C), the ZIF‐8@PZS‐C had a larger specific surface area, higher conductivity, and excellent hydrophilic surface. Regardless of the sweep speed, ZIF‐8@PZS‐C material showed higher specific capacitance than that of ZIF‐8‐C, indicating that ZIF‐8@PZS‐C possessed better double‐layer capacitor performance. Even after 10 000 cycles at the current density of 10.0 A g⁻¹, the constant current charge‐discharge curve of ZIF‐8@PZS‐C was basically the same as that of the initial cycle, indicating the outstanding cyclic stability of the material. These excellent properties are mainly due to the synergistic effect of the special structure and multiple heteroatom doping.

Others

In addition to the above applications, MOF‐derived carbon materials can also be used in some other electrochemical energy storage devices, including lithium‐oxygen (Li‐O₂) batteries, lithium‐selenium (Li‐Se) batteries, and fuel cell.

As is well‐known, selenium and sulfur elements belong to the same main group, so their chemical properties are quite similar, as well as the electrochemical lithium insertion and delithiation reaction mechanisms. Specifically, its theoretical mass specific capacity of 678 mA h g⁻¹, and its theoretical volume specific capacity can reach 3253 mA h cm⁻³.¹⁷³ While compared with sulfur, selenium has higher conductivity (1 × 10⁻³ S m⁻¹), which can realize a fast electrochemical reaction process.¹⁷⁴ However, similar to LSBs, there are also some tough problems in the electrochemical reaction of selenium, such as large volume expansion and the dissolution of reaction intermediates in the electrolyte during the electrochemical reactions.¹⁷⁵ These problems could lead to low utilization of active materials, poor cycle stability, and low Coulomb efficiency. To alleviate the above limitations, Park et al¹⁷⁶ dispersed ZIF‐8 nanoparticles into the solution of PAN and obtained ZIF‐8/PAN nanofibers (ZIF‐8/PAN NF) by electrospinning. After the metal zinc was removed by carbonization and pickling, ZIF‐8 was transformed into hollow polyhedral carbon, while PAN was transformed into carbon fiber with mesoporous structure. The composite product M‐CNF consisting of the above two was activated by KOH at 800°C to prepare the dual‐porous material BP‐CNF with both microporous and mesoporous structures. The BP‐CNF material was then compounded with the selenium elemental substance to finally obtain the BP‐CNF/Se cathode material. In this cathode electrode material, the mesoporous structure enabled the material to have better electrolyte wettability, and a large number of micropores could improve the utilization rate of short‐chain selenide. Therefore, when the BP‐CNF/Se material was used as a cathode of a lithium‐selenium battery, it showed a high specific discharge capacity and superior rate performance. At the current density of 0.5 C, the specific capacities of the second and 300th cycle of the BP‐CNF/Se cathode electrode were 742 and 588 mA h g⁻¹, which was equivalent to the capacity retention rate of 79.2%, and at the high rate of 10.0 C, the electrode could still maintain the specific capacity of 568 mA h g⁻¹. Liu et al¹⁷⁷ adopted ZIF‐8 as the precursor, and it underwent the heat treatment to obtain cubic carbon materials (HMCNCs) with a hollow core, mesoporous inner shell layer, and a microporous outer shell layer. They first coated mesoporous silica (m‐SiO₂) with different thicknesses of the ZIF‐8 cubic nanoparticles, and then they carbonized this composite material to get hollow HMCNCs with the mesoporous shell. Because carbonization was preferentially performed at the interface between the m‐SiO₂ layer and the ZIF‐8 particles, and the higher thickness of m‐SiO₂ layer could offset the inward shrinkage of the core‐shell material during the carbonization process, the internal ZIF‐8 shrinked outward during the pyrolysis process, which induced the composite material to generate hollow HMCNCs with mesoporous shell after removing m‐SiO₂. The as‐prepared material had a pore size of up to 25 nm, a large specific surface area, and pore volume of 1086 m² g⁻¹ and 3.77 cm³ g⁻¹. After loading SeS₂, the SeS₂/HMCNCs cathode material could maintain the specific capacity of 812.6 mA h g⁻¹ after 100 cycles at the current density of 0.2 A g⁻¹. At the same time, it possessed high rate capability that could maintain the specific capacity of 455.1 mA h g⁻¹ even at the high current density of 5.0 A g⁻¹.

Moreover, Li et al¹⁷⁸ employed the MOF with a cage structure as the precursor to prepare a hybrid catalyst composed of graphene sheets and graphene tubes, which was used as the cathode of the Li‐O₂ battery.¹⁷⁸ The special MOF precursor was prepared by using 4,6‐tris(4‐pyridyl)‐1,3,5‐triazine as the ligand, and its size was about 1.8 nm. This unique structural feature of cage structure promoted the subsequent combination of DCDA and iron acetate in the pores, thereby forming MOF‐based hybrids, which then were used as precursors for the preparation of various carbon‐based catalysts. After calcining the obtained MOF‐based mixtures at different temperatures, three carbon‐based nanostructures with different morphologies were formed, namely, onion‐like carbon/Fe₃C hybrids, N‐doped carbon tubes, and N‐doped graphite polyene sheet/graphene tube hybrid. When used as the ORR catalyst, the performance of N‐doped graphene sheets/graphene tube hybrids was close to the commercial Pt/C catalyst and higher than that of the other two carbon‐based materials (ie, onion‐like carbon/Fe₃C hybrids and N‐doped carbon tubes). It is worth noting that this mixed catalyst also showed high half‐wave potential in alkaline electrolyte. The N‐doped graphene sheet/graphene tube mixed catalyst was further used as the cathode material for a Li‐O₂ battery, wherein it could provide a high initial discharge capacity of up to 5300 mA h g⁻¹, excellent cycle stability (73% capacity retention after 50 cycles at a current density of 400 mA g⁻¹), indicating the potential application of MOF‐derived catalysts in Li‐O₂ batteries.

MOF‐derived nanostructured carbon materials have been extensively studied by many researchers as electrocatalysts in fuel cells.^179,180 For example, Hong et al proposed highly graphitized nitrogen‐doped porous carbon (NGPC) derived from ZIF‐8 as an effective ORR electrocatalyst. In the carbonization process, ZIF‐8 acted as both carbon source and nitrogen source, so that NGPC products not only retained the morphology of the precursor MOF, but also had rich nitrogen, high surface area, and good conductive network. The excellent electrocatalytic activity of this metal‐free catalyst is attributed to the synergistic effect of abundant graphite N active sites, high surface area, and a high degree of graphitization. To obtain in situ NPC with large surface areas and narrow pore sizes distribution, it was recommended to introduce the second carbon source into the MOFs.¹⁸¹ Glucose was used as a secondary carbon precursor, which penetrated the pores of the MOF precursor zinc‐benzimidazole (ZIF‐7). After carbonization at 950°C for 5 hours, the proposed NPC was synthesized. Importantly, the addition of carbon source of glucose not only improved the graphitization of the product but also facilitated the removal of Zn metal and Zn compound impurities from ZIF‐7, which enabled the fuel cell to achieve true metal‐free electrocatalyst. Compared with ZIF‐7‐derived carbon and glucose‐derived carbon, ZIF‐7/glucose‐derived carbon showed the best ORR activity. Its initial potential was 0.8 V (vs reversible hydrogen electrode) and it owned nearly four‐electron selectivity (the electron transfer number is 3.68 at 0.3 V) in O₂ saturated 0.1 M KOH, which is very close to commercial Pt/C. In addition to ORR activity, this nitrogen‐doped carbon had better stability and methanol tolerance than that of commercially available Pt/C catalysts. The above results show that the nitrogen‐containing MOFs/carbon source composites can be used as ideal precursors for the preparation of NPCs with the highly efficient electrocatalysis due to their graphene‐like morphology, high specific surface area, and high porosity.

SUMMARY AND OUTLOOK

Porous carbon materials are widely used in energy storage devices due to their large specific surface area, wide source of raw materials, high stability, and no pollution to the environment. Compared with common porous carbon materials made from organic molecules or biomass materials, MOF‐derived carbon material generally does not require subsequent complicated physical or chemical activation to obtain carbon materials with large specific surface area and high porosity. Moreover, such materials usually retain the regular network pore structure and unique morphology of MOFs precursors. At the same time, the heteroatoms in the ligands of MOFs can provide rich and uniform heteroatom doping for such carbon materials. In addition, because MOFs have a variety of structures and abundant compositions, different types of MOFs precursors can be selected to prepare porous carbon materials with diverse pore structures, multiple heteroatom doping, and different specific surface areas, so as to control the structure and properties of the obtained porous carbon materials. On this basis, MOF‐derived carbon materials show more structural advantages. Compared with solid porous carbon, MOF‐derived hollow porous carbon has better electrolyte wettability, higher effective electrochemical contact area, shorter ion diffusion, and electron transmission distance, and can accommodate larger volume expansion during charge and discharge process. Therefore, porous carbon with a hollow structure can achieve high mass transfer rate and has outstanding large rate performance and further exhibits excellent electrochemical activity. Based on this, this paper introduces recent research progress of hollow porous carbon materials derived from MOFs, including the structure of such carbon materials and their applications in secondary batteries, supercapacitors, and other fields. Although the MOF‐derived carbon materials have become research hotspots in recent years due to their unique structural advantages, and some research progress has been made, their future development still faces many problems and challenges.

(1)
Although there are thousands of MOFs, most of the MOFs currently used as precursors are those with classical structures, such as ZIF or MIL series. Due to limited precursors selection, carbon materials often show similar morphologies and pore structures, which is not conducive to enriching the structural types of such materials and further improving and enhancing the performance. Therefore, in the future, it is still necessary to continuously explore and develop other structures and compositions of MOFs as precursors of such carbon materials, to obtain hollow porous carbon materials with better performance.
(2)
MOFs contain two components, metal ions and organic ligands. After carbonization at high temperature, the organic ligands are transformed into carbon materials, while the metals are also retained and form the composite with the carbon materials. Although MOFs containing metal Zn can be removed by high‐temperature volatilization during pyrolysis, most of the MOFs still need to be treated with acid or alkali to remove metal after carbonization to obtain carbon materials, which makes the preparation process more complicated; and the use of high concentrations of acid and alkali also has certain risks. In addition, the heteroatoms in the organic ligands are largely lost during the high‐temperature carbonization process, resulting in a lower doping content. It is an urgent problem to search for the preparation conditions with mild process and the maximum retention of heteroatoms.
(3)
Compared with the porous carbon materials prepared by traditional methods, the porous carbon materials prepared with MOFs as precursors show more structural and performance advantages, but also have the fatal disadvantage namely, its high cost. Although some classic MOFs can use cheap metal ions and ligands, their low yields often lead to rising costs. Therefore, the further development of MOF‐derived carbon materials needs to overcome the problem of low yields and develop cheap organic ligands to achieve large‐scale production.
(4)
Hollow porous carbon materials can be prepared by template method or pre‐etched MOFs precursor method. In addition, MOFs can also obtain hollow porous carbon materials through pyrolysis carbonization and subsequent metal removal. However, the structure of the material obtained by the above methods depends on the template or has a great degree of structural uncertainty, which makes the composition and structure of the final material difficult to control, which causes a great obstacle to the study of its structure‐activity relationship. Therefore, it is still necessary to further explore the structure of the MOFs precursor and its structural evolution during the carbonization process, so as to realize the structural control of the final carbon material to meet the actual demand.

In conclusion, hollow carbon materials prepared with MOFs as the precursor system have more structural advantages than carbon materials prepared with traditional methods, and show better electrochemical performance in secondary batteries, supercapacitors, and other fields. It is believed that with the deepening of research, some existing problems will gradually be improved and solved, and further development of this study field will be promoted, which will open up new directions and paths for the application of MOFs and the development and practicality of carbon materials.

ACKNOWLEDGMENTS

This study acknowledges the supports by the Shenzhen Science and Technology Innovation Commission under Grant JCYJ20180507181806316, the City University of Hong Kong under project Fundamental Investigation of Phase Transformative Materials for Energy Application (Project No. 9610399), and the Shenzhen Research Institute, City University of Hong Kong.

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Metal‐organic frameworks (MOFs) are of quite a significance in the field of inorganic‐organic hybrid crystals. Especially, MOFs have attracted increasing attention in recent years due to their large specific surface area, desirable electrical conductivity, controllable porosity, tunable geometric structure, and excellent thermal/chemical stability. Some recent studies have shown that carbon materials prepared by MOFs as precursors can retain the privileged structure of MOFs, such as large specific surface area and porous structure and, in contrast, realize in situ doping with heteroatoms (eg, N, S, P, and B). Moreover, by selecting appropriate MOF precursors, the composition and morphology of the carbon products can be easily adjusted. These remarkable structural advantages enable the great potential of MOF‐derived carbon as high‐performance energy materials, which to date have been applied in the fields of energy storage and conversion systems. In this review, we summarize the latest advances in MOF‐derived carbon materials for energy storage applications. We first introduce the compositions, structures, and synthesis methods of MOF‐derived carbon materials, and then discuss their applications and potentials in energy storage systems, including rechargeable lithium/sodium‐ion batteries, lithium‐sulfur batteries, supercapacitors, and so forth, in detail. Finally, we put forward our own perspectives on the future development of MOF‐derived carbon materials.

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Title

Recent progress on MOF‐derived carbon materials for energy storage

Author

Ren, Jincan¹; Huang, Yalan¹; Zhu, He¹; Zhang, Binghao²; Zhu, Hekang¹; Shen, Shenghui³; Tan, Guoqiang⁴; Wu, Feng⁴; He, Hao⁵; Si Lan⁶; Xia, Xinhui³; Liu, Qi²

¹ Department of Physics, City University of Hong Kong, Hong Kong, China
² Department of Physics, City University of Hong Kong, Hong Kong, China; Shenzhen Research Institute, City University of Hong Kong, Shenzhen, China
³ State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, and Department of Materials Science and Engineering, Zhejiang University, Hangzhou, China
⁴ School of Materials Science & Engineering, Beijing Institute of Technology, Beijing, China
⁵ Shenzhen Research Institute, City University of Hong Kong, Shenzhen, China; School of Materials Science & Engineering, Changsha University of Science & Technology, Changsha, Hunan, China
⁶ Shenzhen Research Institute, City University of Hong Kong, Shenzhen, China; School of Materials Science and Engineering, Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and Technology, Nanjing, China

Pages

176-202

Section

REVIEWS

Publication year

2020

Publication date

Jun 2020

Publisher

John Wiley & Sons, Inc.

e-ISSN

26379368

Source type

Scholarly Journal

Language of publication

English

DOI

https://doi.org/10.1002/cey2.44

ProQuest document ID

2544911401

Recent progress on MOF‐derived carbon materials for energy storage

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