Metal-organic framework nanocrystal-derived hollow porous materials: Synthetic strategies and emerging applications

Metal-organic frameworks (MOFs) have garnered multidisciplinary attention due to their structural tailorability, controlled pore size, and physicochemical functions, and their inherent properties can be exploited by applying them as precursors and/or templates for fabricating derived hollow porous nanomaterials. The fascinating, functional properties and applications of MOF-derived hollow porous materials primarily lie in their chemical composition, hollow character, and unique porous structure. Herein, a comprehensive overview of the synthetic strategies and emerging applications of hollow porous materials derived from MOF-based templates and/or precursors is given. Based on the role of MOFs in the preparation of hollow porous materials, the synthetic strategies are described in detail, including (1) MOFs as removable templates, (2) MOF nanocrystals as both self-sacrificing templates and precursors, (3) MOF@secondary-component core-shell composites as precursors, and (4) hollow MOF nanocrystals and their composites as precursors. Subsequently, the applications of these hollow porous materials for chemical catalysis, electrocatalysis, energy storage and conversion, and environmental management are presented. Finally, a perspective on the research challenges and future opportunities and prospects for MOF-derived hollow materials is provided.


INTRODUCTION
Metal-organic frameworks (MOFs) or porous coordination polymers (PCPs), which consist of inorganic metal nodes linked by organic linkers via coordination bonds, represent a new class of crystalline porous materials. 1,2 During the past few decades, MOF materials have allowed extraordinary achievements in both the synthesis of novel nanostructures and a wide variety of potential applications, such as energy storage and conversion, sensing, catalytic reaction, environmental management, and so on. MOFs have highly ordered crystalline framework structures, and these structures are durable enough to remove the contained guest species and yield permanent pores. In particular, through the rational design and/or control of organic ligands, metal nodes, and synthesis environment, the structure and related peculiarity of MOFs can be conveniently adjusted and modified to meet specific requirements. These attractive advantages are particular to MOFs and not easily accessible in most of the other traditional porous nanomaterials.
Due to their excellent intrinsic attributes, MOFs can be used as precursors or templates to fabricate MOF-derived secondary hollow porous nanomaterials (HPMs): metal carbides, metal oxides, metal sulfides, layered double hydroxides, porous carbon materials, and their hybrid composites. MOF-derived porous nanomaterials have also played a significant role in photocatalysis, energy storage and conversion, and particularly electrocatalysis, owing to their superior attributes, such as large specific surface area, micro-or mesoporosity for efficient electron and mass transport during the reaction, inherited structural versatility, high tolerance to acid/alkaline environments, and electrical conductivity, as well as diverse chemical components.
Among these MOF-derived hollow nanomaterials, discrete, hollow structures have garnered considerable attention. In most cases, the synthesis of MOFderived HPMs essentially relies on a decomposition of the MOF or its composites under a certain condition or chemical reaction with desirable reagents: (1) hollow SiO 2 derived from a secondary constituent of MOF@shell composite, (2) hollow porous carbon materials derived from the organic components of the MOF and its shell or guest molecules, and (3) hollow metal oxides, sulfides, carbides, supported MNPs, etc., derived from the metallic components of the MOF or MOF composite. Traditional synthetic methods for HPMs require a hard template, for example, SiO 2 , and polystyrene (PS), while a significant drawback is that harsh conditions are required for template removal. Compared with the traditional hard template method, the conditions of preparing hollow porous materials from MOFs as a template and precursor system are mild, and there is no need to remove the template. Furthermore, in contrast to the traditional molecular sieve and amorphous porous materials, hollow nanomaterials derived from MOFs demonstrate numerous advantages, including uniform hollow morphology and chemical constituents, tunable distribution of chemical and structural compositions, and tunable porosity. Benefiting from these attributes, MOF-derived hollow nanomaterials exhibit excellent performance in chemical catalysis, electrocatalysis, energy storage and conversion, and other applications.
Although several reviews have summarized the MOF-derived hollow nanomaterials and their potential applications, [3][4][5][6][7][8][9] most of these review articles are mainly focused on MOF-derived hollow materials and their applications in energy storage and conversion. Up to now, a comprehensive summary of the synthetic strategies and emerging applications of hollow porous materials derived from MOFbased templates and/or precursors is lacking in the literature. In this context, we aim to provide a systematic and detailed overview of the most recent achievements of MOFs as templates and/or precursors for fabricating hollow nanomaterials and their application in chemical catalysis, electrocatalysis, environmental management, energy storage, and energy conversions. Finally, this review concludes with some personal insights that we hope open a broad new avenue for future directions in this attractive research field.

DESIGN STRATEGIES FOR MOF-DERIVED HOLLOW POROUS MATERIALS
For this part, we will provide various detailed synthetic strategies for fabricating HPMs, such as hollow porous carbons and their composites, hollow layered double hydroxides, hollow metal-based compounds, and their composites (Table S1).

MOF nanocrystals as removable templates
Usually, the sacrificial template controls the size and morphology of the hollow inorganic nanostructures. He and co-workers proposed a synthetic route for SiO 2 -based hollow nanocrystals with tunable components. 10 As shown in Figure 1A, a layer of SiO 2 was uniformly coated on ZIF nanocrystals from a soluble TEOS precursor through the Stöber method. This produces a ZIF-8@SiO 2 coreshell material. Subsequently, hollow SiO 2 -based materials were obtained from the ZIF-8@SiO 2 core-shell composite by different ways of treatment: (1) a ZnO@SiO 2 yolk-shell composite was formed by calcination in the air or (2) a ZnO@SiO 2 yolk-shell composite and a hollow SiO 2 polyhedron were formed by thermal treatment in acidic solution at 100 C for 2 and 24 h, respectively. Inspired by the above research, we proposed a route to prepare hollow TiO 2 using ZIF nanocrystals as a removable template and gave rare examples of hollow cubic and polyhedral morphologies ( Figure 1B). It was particularly interesting to see that the surface of ZIF-8 can promote the growth of TiO 2 coatings. 11 The ZIF-8 core could be removed completely and quickly without affecting the integrity of the TiO 2 shells. The sacrificial ZIF-8 template controls the shape and size of the obtained hollow TiO 2 structure. These TiO 2 materials demonstrated exceptional textural properties. Upon using ZIF-8 nanocrystals injected with Pt nanoparticles (NPs) as templates, the Pt NPs were successfully encapsulated into the hollow TiO 2 cubes. 11 Brinker and colleagues also proposed a novel concept for the functionalization of the surface of MOFs based on the direct coordination of a phenolic-inspired lipid molecule, DPGG ( Figure 1C). Consequentially, ll The Innovation 3(5): 100281, September 13, 2022 MIL-88 nanocrystals and UiO-66 were also used as a removable template for the production of the hollow polymer. 12 Taken together, when an MOF is used as a template to fabricate hollow materials, the shape and size of the obtained hollow structures are controlled by the sacrificial MOF template. And the other advantage is that the MOF template can be completely and rapidly eliminated without affecting the integrity of the hollow shells.

MOF nanocrystals as both self-sacrificing templates and precursors
In addition to the removable templates, MOF nanocrystals can be used as both self-sacrificing templates and precursors for fabricating hollow nanostructures. Many approaches have been developed to synthesize MOF derivatives, for instance, hollow metal hydroxides, metal sulfides, metal oxides, metal phosphides, hollow carbon matrixes, and so on. 13-16 ZIF-8 (Zn) and ZIF-67 (Co) were applied as self-sacrificial template and precursor to prepare M-Co-LDH (M = Mg, Co, Ni) nanocages with hollow structures ( Figure S1A). 14 Control of the simultaneous reactions, the precipitation of the shells, and the template etching is extremely crucial to the preparation of perfect nanocages. Subsequently, similar synthesis strategies were used for producing other LDH and metal hydroxide nanocages, such as Mg-Co LDH, 13,14 Ni-Co LDH, 14 Co 2+ -Co 3+ LDH, 17 Ni-Fe LDH, 18 and a series of metal hydroxides, including Co-OH, NiCo-OH, Ni 2 Co-Mn 1 -OH, Ni 1 Co-Mn 2 -OH, NiCoMn-OH-AD, CoMn-OH, and FeOOH@Ni(OH) 2 . 15,16 The process of MOF-derived hollow LDH could be observed by in situ technical characterization: etching of the NPs and growth of LDHs on the NP surfaces. 19 The conversion process of ZIF-8 nanocubes and ZIF-8 rhombic dodecahedrons into LDH nanocages is observed by in situ transmission electron microscopy (TEM). Conversion of ZIF-67@ZIF-8 coreshells into "shell-in-shell" LDH nanocages is demonstrated in Figures S1B and S1C.
The metal components of the MOF template are known to be converted into metal oxide by oxidation and pyrolysis in an air atmosphere. Thus, MOF nanomaterials are very suitable as sacrificial templates and precursors for producing various hollow metal oxides. Hollow Co 3 O 4 tetrahedrons were successfully synthesized through the thermolysis of [Co 3 L 2 (TPT) 2 $xG] n (G = guest molecules) at 773 K for 4 h in the air. 20 Guo   Schematic illustrations of the strategies for using ZIF nanocrystals as sacrificial templates for the synthesis of (A) hollow SiO 2 -based structures 10 and (B) hollow TiO 2 -based structures. 11 (C) Schematic illustration of the surface functionalization of MOF particles by a phase transfer reaction. 12  an octahedron and a yolk-shell octahedron by calcinating MIL-53 (Fe) in the air for 2 and 6 h, respectively ( Figure S1D). 21 Other hollow metal oxides, such as ZnO, 22 (Figures 2A and 2B). 33 To prepare hollow porous carbon, MOF precursors are usually pyrolyzed under an Ar or N 2 atmosphere, and metal species will experience evaporation and/or subsequent leaching in the pyrolysis process. For example, Zhang et al. utilized ZIF-8 as a sacrificial template and precursor for fabricating a hollow carbon matrix ( Figure 2C). 34 First, the inner parts of the ZIF-8 template were etched with a tannic acid solution, and then hollow ZIF-8 was treated at 873 K under an inert atmosphere, with subsequently etching of the obtained sample with acid solution to eliminate the remaining Zn species, and the hollow carbon matrix was acquired. In addition, Wang et al. proposed a hostguest strategy to fabricate a hollow N-doped porous carbon electrocatalyst with Fe-Co dual sites. 36 Reasonable introduction of other species into MOFs and then pyrolysis can provide an excellent opportunity to manufacture the targeted hollow material with the desired compositions. For example, Wu et al. proposed an MOF-assisted strategy for synthesizing MoC x NPs confined within hollow nano-octahedrons of porous carbon ( Figure 2D). 35 Figures 2E and 2F). Many research groups extended the fabrication method to other hollow nanostructures, such as hollow metal sulfides, metal selenides, and metal phosphides, that were synthesized by using MOF nanocrystals as both sacrificial templates and precursors.

MOF@secondary-component core-shell composites as precursors
The growth of MOF@secondary-component core-shell structures is very popular for fabricating MOF-derived hollow porous materials. Hu et al.
utilized phenolic acids as surface functionalization and etching reagents to prepare hollow MOFs ( Figure 3A). 37 Metal-phenolic networks (MPNs) were formed on the MOF surfaces by the coordination of phenolic acid and metal ions and finally functionalized the surface of the resulting hollow MOFs. As a seminal study, we proposed the synthetic routes toward the first N-HPC capsule, and then the N-HPC capsules encapsulated NPs with tunable components employing ZIF-8 nanocrystals as a template ( Figure 3B). 38 First, ZIF-8@K-TA was prepared by coating the metal-phenolic coordination (K-TA) as an auxiliary material on the surface of the ZIF-8 crystal. Afterward, ZIF-8@K-TA was pyrolyzed under an Ar atmosphere at 900 C. It is worth noting that this process converted the organic components of ZIF-8@K-TA into hollow microcapsules with the shape of the ZIF-8 templates. During the pyrolysis process, the Zn species vaporized and escaped from the material to form N-HPC capsules. Interestingly, post-synthetic ion exchange of the K + ions in the K-TA shell by Co(II) or Ni(II) could be realized by facilely immersing ZIF-8@K-TA in a methanolic solution of Co(NO 3 ) 2 or Ni(NO 3 ) 2 , respectively. Subsequently, monometallic Co or Ni NPs encapsulated in N-HPC capsules could be achieved by heating ZIF-8@Co-TA or ZIF-8@Ni-TA, respectively, under an Ar atmosphere. We further extended this strategy to synthesize supported multicomponent metal NPs. The heating of ZIF-8/ Pt@M-TA or ZIF-8/Pt@Mmix-TA results in N-HPC capsules with Pt-based alloyed NPs embedded in the capsule walls. This approach can be extended to prepare NPs composed of four different metals. 41 Inspired by the above successes, we subsequently fabricated a series of atomically dispersed metal catalyst-decorated hollow carbon capsules, including H- Similarly, the synthetic route toward an N, P, S-co-doped carbon shell (NPS-HCS) was reported. 39 They first prepared ZIF-67 nanocrystals and then deposited a cross-linked PZS layer on the surface of ZIF-67 to obtain a ZIF-67@PZS core-shell structure ( Figure 3C). After heat treatment in Ar and acid etching, NPS-HCSs were obtained ( Figure 3D). Inspired by the study, Fe single atoms supported on an N, P, S-co-doped hollow carbon polyhedron were fabricated. They revealed that the long-range interaction of the active metal center with sulfur and phosphorus facilitated the formation of a hollow structure and improved catalytic performance. 43 Moreover, Liu et al. fabricated NC@Co-NGC nanocages via pyrolysis of a ZIF-8@ZIF-67 core-shell composite. 40 The core-shell composites were originally obtained by coating the epitaxial ZIF-67 shell on the surface of the ZIF-8 core ( Figure 3E). After pyrolysis under N 2 atmosphere and subsequent etching treatment, double-shelled NC@Co-NGC nanocages with surface-anchored carbon nanotubes (CNTs) were obtained. It should be noted that the formation of hollow nanostructures can be attributed to surface-stabilized shrinkage of core-shell ZIF-8@ZIF-67 nanocrystals at high temperatures. Similarly, Pan and colleagues used core-shell ZIF-8@ZIF-67 composites to prepare hollow composites of CoP NPs embedded in N-doped CNTs through a continuous pyrolysis-oxidation-phosphorylation process. 44 In addition, various hollow carbon 34,39 and hollow carbon-supported metal-based materials, including metals, 45 metal oxides, 46 metal sulfides, metal phosphides, 47 and metal selenides, 48 can be easily produced through the reaction of MOF@secondary-component templates with the corresponding thermolysis.

Hollow MOF nanocrystals and their composites as precursors
Similar to the previously mentioned design strategy, direct thermolysis of hollow MOFs is a facile route to fabricate functional hollow porous materials. Zou and co-workers 49 fabricated NiO/Ni/graphene composites with hierarchical hollow structure through annealing of Ni-MOF with hierarchical hollow structure. Figure 4A shows that hierarchical hollow Ni-MOF nanocrystals were prepared with a uniform diameter. Then, the pyrolysis of the hollow MOF nanocrystals under a N 2 gas environment resulted in Ni/graphene core-shell composites. After further annealing treatment under air, the final sample NiO/Ni/graphene composites were acquired. 49 Moreover, hollow ZnO/C composites, 53 NiO x /Ni@C composites, 54 Z67-Co 3 O 4 /C-4, and Z9-Co 3 O 4 /C-4 55 were also synthesized by a similar process. Similarly, Yu et al. 50 fabricated hierarchical CoS 2 hollow prisms by using ZIF-67 as a precursor ( Figure 4B). Nanosized CoS 4 bubble-like subunits were prepared via sulfidation of ZIF-67 hollow prisms with thioacetamide in ethanol solution. During the sulfidation process, Co 2+ cations in the ZIF-67 were converted into hollow CoS 4 nanocrystals. After pyrolysis under the N 2 atmosphere, the hierarchical CoS 2 with multistage hollow interiors was acquired. 50 Moreover, Zhang et al. also employed Zn/Ni-MOF-5 and nanocubes as the precursor to prepare Zn/Ni-MOF-2 nanosheet with hierarchical hollow nanocubes. 56 External templates can also be used to construct hollow porous materials derived from MOF nanocrystals. Su and co-workers reported a multicavity hollow Au@ZIF-8 nanoreactor ( Figure 4C) produced via the exterior template method. 51 As depicted in Figure 4D, silica was employed as the template source coating on the surface of Au NPs to produce Au NP@silica core-shell composites. Subse-quently, the growth of ZIF-8 led to packaging of multiple Au NP@silica particles embedded in its surface to form "raisin bun"-like structures. Then, multicavity hollow Au NP@ZIF-8 nanoreactors were formed after etching the silica. The hollow products feature intrinsic monodispersed micropores and introduced macropores, and each microvoid unit structure has only one Au NP inside. NP@MOF 57 and yolk-shell nanocrystal@ZIF-8 58 were also synthesized with a similar process.  Moreover, hollow MOF nanocrystals and their composites have also been applied for synthesizing periodic hollow porous MOF structures. For instance, 52 3D hollow SOM-ZIF-8 with oriented and ordered macro-micropores were acquired by packaging ZIF-8 and PS spheres (PSs) as a well-assembled "precur-sor@PS" template ( Figure 4E). Specifically, PSs were assembled into a highly ordered 3D opal structure, and then ZIF-8 precursors filled the PS monolith interstices to form "precursor@PS." It was immersed in the CH 3 OH and NH 3 $H 2 O solution to reach a balanced process between the growth of the ZIF-8 crystal and the removal of the PSs. As a result, the highly oriented and ordered macropores SOM-ZIF-8 were obtained ( Figure 4F). Subsequently, Guo et al. synthesized Zn-N-HOPCPs with ordered pores through confined growth and pyrolysis of ZIF-8 crystalline template voids. 59 As shown in Figure S2A, in the preparation of ZIF-8, SiO 2 CCT was added for the preparation of SiO 2 -CCT@ZIF-8 precursors. The obtained SiO 2 -CCT@ZIF-8 was pyrolyzed under N 2 conditions and then SiO 2 -CCT was removed in 1 M NaOH to obtain the final products.
In addition, multishelled hollow MOFs can be fabricated by controlling crystal production and etching or self-assembly strategies. As shown in Figure S2B, Liu et al. developed a rational method to prepare single-, double-, and triple-shelled hollow MIL-101 with single-crystalline shells by step-by-step crystal growth and subsequent etching processes. 60 The cavity size and shell thickness of each layer can be tailored by rational regulation of MOF nucleation and crystallization. Choe and colleagues demonstrated the process of a solid MOP transformed to a hollow MOF with controlled layers of shells through self-assembly ( Figure S2C). 61 First, they immersed solid UMOM-1 (MOP) in DABCO solution to generate a core-shell structure. The external surface of the MOP crystal was coordinated with the linker; thus, the center of the MOP can be retained even if the reaction were stopped halfway. Due to the difference in solubility, the core was selectively dissolved in methanol solution and formed a single-crystal hollow MOF with a single shell. By repeating the above process, a single-crystal hollow MOF with multiple shell layers can be obtained. Similarly, Liu and co-workers 62 synthesized a multishelled ZIF-8 by selectively dissociating ZIF-67 from the multilayered ZIF-67@ZIF-8 ( Figure S2D). The shell number of hollow ZIF-8 can be tuned by controlling the epitaxial layer-by-layer overgrowth of ZIF-8 and ZIF-67. Furthermore, the interactions between different guests can also be tuned by precisely immobilizing them in MOF shells or by encapsulating them in cavities between MOF shells. 62

APPLICATIONS OF MOF NANOCRYSTAL-DERIVED HOLLOW POROUS MATERIALS Chemical catalysis
Chemical catalysis is a widespread phenomenon and a central topic for modern industry. To date, tremendous efforts have been committed to preparing and constructing excellent chemical catalysts. Recently, various novel MOF-derivative constructions have been reported and used in selective hydrogenation, goaldirected oxidation, CO 2 reduction reaction, and so on. 63 Selective hydrogenation. Hollow MOF composites with noble metals possess remarkable selectivity in hydrogenation reactions. Tsung and coworkers fabricated Pd on ZIF-8, yolk-shell Pd@ZIF-8, and core-shell Pd@ZIF-8 by utilizing Pd nanocrystals and hollow ZIF-8. 58 Through the gas-phase hydrogenation reaction of ethylene, cyclohexene, and cyclooctene, the molecular size selectivity of the prepared catalysts was studied. Experimental results revealed that all of the catalysts demonstrated superior activity for ethylene hydrogenation, while only the Pd on the ZIF-8 catalyst exhibited excellent catalytic performance for the cyclooctene. This is because ethylene molecules (2.5 Å) can diffuse through the pore size (3.4 Å) of the ZIF-8 shell, while cycloalkene molecules (5.5 Å) are much larger than the pore size of the ZIF-8 shell ( Figure 5A). Similarly, Yang et al. fabricated hollow Pd@ZIF-8 nanosphere catalysts with different thicknesses and studied their selectivity in liquid-phase hydrogenation. 64 As shown in Figure 5B, when Pd@ZIF-8 was applied to the hydrogenation reactions of 1-hexene, trans-stilbene, and tetrastyrene, experimental results showed that the smaller the size of the reactant, the higher the conversion. Moreover, Pd@ZIF-8 catalysts with thicker shells (Pd@ZIF-8(S)) possessed lower conversion efficiency. A similar conclusion was obtained when void@HKUST-1/Pd@ZIF-8 was used as a hydrogenation reaction catalyst. 67 Moreover, hollow PtAuDNP@HKUST-1 petalous heterostructures also exhibited excellent performance in the hydrogenation of olefin. 57 Noble-metal catalysts exhibited remarkable activity in selective hydrogenation, but the high cost limited their practical application in the hydrogenation reaction. One strategy is to combine noble metals with low-cost non-noble metals, which can reduce costs and maintain activity and selectivity. Analogously, yolk-shell PdCu@Fe III -MOF-5, 68 yolk-shell (PTA) @CdCu@MOF-5(FeIII), 69 and hollow Pd@Zn-Co ZIF 70 exhibited excellent selective hydrogenation performance.
Goal-directed oxidation. MOF-derived hollow porous materials have demonstrated remarkable performance in goal-directed oxidation of organic matters. Shen et al. reported a single-crystalline SOM-ZIF-8 with the 3D ordering of macro-micropores. 52 They investigated the catalytic performance of SOM-ZIF-8 in the Knoevenagel reaction of benzaldehydes and malononitrile. As shown in Figures 5C and 5D, SOM-ZIF-8 catalysts possessed significantly improved performance compared with the other catalysts and exhibited superior structural stability and enhanced recyclability. Doping heteroatoms can significantly improve the catalytic ability of hollow porous materials in various organic reactions. MOF-derived hollow Co 3 O 4 polyhedrons exhibited superior catalytic performance and excellent stability, and the complete conversion was up to 100% for toluene oxidation. 71 MOF-derived yolk-shell Co@C-N demonstrated prominently improved catalytic performance in the aqueous oxidation of alcohols while yielding >99% conversion. The enhanced catalytic performance is due to the unique yolk-shell structure, which can accelerate the transfer rate of reactants/ products and the synergistic effect between the Co NPs and the N-doped carbon nanosheet. 72 Similarly, both MOF-derived hollow Fe-Co nanocatalyst 73 and hollow yolk-shell Co@CN catalyst 74 also afforded high productivity and excellent selectivity for oxidation of HMF. In addition, MOF derivatives can be used as catalysts for CO 2 conversion. Dai and co-workers fabricated hierarchical hollow Ni@C spheres derived from Ni-MOFs, which exhibited superior catalytic performance and enhanced stability for CO 2 reduction reaction ( Figure 5E). 65 Novel HPC with ultrahigh concentrations of Zn single atoms ( Figure 5F) was synthesized for catalytic CO 2 cycloaddition with epoxides under light and showed excellent performance. The hollow structure of the porous carbon cavity can convert the absorbed light energy into heat energy, which significantly improves the conversion of endothermic CO 2 . Meanwhile, single Zn atoms act as a Lewis acid site, and a Lewis base site can cooperate to boost substrate activation. Moreover, the carbon shell with hierarchically porous character accelerated CO 2 enrichment and improved the transport rate of reactants/products. That was the first report on integrating the photothermic effect into endothermic CO 2 conversion. 66 Briefly, hollow porous materials derived from MOFs have the following advantages as catalysts in industrial catalytic processes: (1) the porous MOF shell can serve as a host matrix to prevent NP aggregation and (2) be applied to realize selective catalysis, and (3) the presence of porosity allows for promoting mass transport of chemical species. It is noteworthy that the thickness of the shell has a certain degree of influence on the catalytic performance.

Electrocatalysis
ORR. Fuel cells and metal-air batteries have garnered extensive attention owing to their being a possible solution to the fossil energy shortage and increasing environmental pollution. 75 ORR is important for fuel cells and metalair batteries. 5 The precious metal Pt is applied as an outstanding and effective four-electron transfer electrocatalyst for ORR. However, the low abundance and preciousness limit its widespread applications. Among all of the reported electrocatalysts, MOF-derived nanomaterials with high specific surface area, an abundance of accessible active sites, and improved mass/charge transfer rate have been viewed as highly beneficial for the ORR process (Table 1).
Among the studied non-noble metals, Fe and Co are the most active metal species for ORR. 76,90,91 A lot of excellent studies about Fe and Co as efficient cathode electrocatalysts have been reported. Tang and co-workers reported carbonized Fe NPs derived from MOFs (MIL-88B-NH 3 ), which showed excellent ORR property with E onset and E 1/2 reaching 1.03 and 0.92 V (versus RHE), respectively, in alkaline medium. In real ADMFC, carbonized Fe NPs possessed high output power density and were evenly 1.7 times higher than the 20% Pt/C. 77 Guo and colleagues fabricated Co@Co 3 O 4 @core@bishell NPs derived from MOFs into a highly ordered porous CM to prepare the catalyst Co@Co 3 O 4 @C-CM used for ORR. 78 This work created a solid interaction/contact between the metal oxide and the carbon shell linked to the porous CM, which is significant for facilitating the electron transfer rate between NPs and the porous CM and improving mass transport of O 2 and electrolytes, making the NPs hard to detach from the porous CM support. It was found that, in alkaline medium, Co@Co 3 O 4 @C-CM displayed almost identical catalytic performance but enhanced stability and better methanol tolerance for ORR relative to the 20% Pt/C.
In addition to MOF-derived composites, MOF-derived materials with an M-N-C structure (M represents a metal atom) have also demonstrated supernormal electrocatalytic performance for ORR and have attracted widespread attention. 92 For instance, Wan et al. prepared a concave-shaped Fe-N-C SAC (TPI@Z8(SiO 2 )-650-C) with enhanced surface area and dense Fe-N 4 sites ( Figure S3A). 79 Further investigation revealed that TPI@Z8(SiO 2 )-650-C has the special properties of additional mesoporosity, high surface area, and high exposure to Fe-N 4 activesite density (Figures S3B and S3C). The obtained catalysts showed excellent PEMFC performance, achieving current densities of 0.022 A cm À2 at 0.9 V iR-free and 0.047 A cm À2 at 0.88 V iR-free , which achieved the DOE 2018 target. TPI@Z8(SiO 2 )-650-C achieved a superior performance of 129 mA cm À2 at 0.8 V iR-free under 1 bar H 2 -air and a P max of 1.18 W cm À2 under 2.5 bar H 2 -O 2 . This was better than the performance reported in most of the literature ( Figure S3D). Li and colleagues synthesized N-coordinated Fe/Co dual-site catalysts derived from Zn/Co bimetallic MOF-encapsulated FeCl 3 molecules ( Figure S3E). Multiple means of characterization confirmed the presence of Fe/Co single-atom dual sites. 36 According to EXAFS results, the dual metal center was named N 3 Fe-CoN 3 in Figure S3F. The catalyst with Fe-Co dual active sites exhibited remarkable ORR performance under acidic conditions with E 1/2 (0.863 V versus RHE) and enhanced stability compared with 20% Pt/C, individual Fe SAs/NC, and Co SAs/NC (Figures S3G and S3H). Density functional theory (DFT) calculations proved the single-atom dual sites can decrease the cleavage energy of O-O bonds and realize excellent performance toward ORR and high selectivity for the fourelectron reduction path ( Figure S3I). Recently, we reported on CrN@H-Cr-Nx-C derived from ZIF-8@Cr-TA core-shell nanocrystals ( Figure S4A). 80 Due to the synergies between CrNNPs and single-atomic-site CrN x , the enriched CrN x sites, discrete CrNNPs, and affluent micro/mesopores, as an electrocatalyst, CrN@H-Cr-N x -C displayed superior electrocatalytic performance for ORR in an acidic medium with outstanding OCV, and excellent current power densities were also observed when used as a cathode electrocatalyst applied in PEMFC ( Figure S4B). The CrN@H-Cr-Nx-C also showed enhanced stability, proved by durability tests (Figure S4C). In addition, Ce-SAS/HPNC with a hierarchically macro/meso/microporous structure derived from MOFs can also be applied as an electrocatalyst for ORR ( Figure S4D). 81 XAS analysis results verified that the Ce sites were stabilized by four coordinated N atoms and six O atoms (Ce-N 4 /O 6 ) ( Figures S4E and S4F). Remarkably, the Ce-SAS/HPNC displayed an outstanding ORR performance with E onset of 1.04 V, E 1/2 of 0.862 V, and J K of 2.673 mA cm À2 at 0.9 V compared with the referenced catalysts ( Figures S4G  and S4H). In addition, the doping of heteroatoms (such as S, P, or O) will change the intrinsic performance of M-N-C catalysts. In this regard, P-doped P-CNCo-20 93 and N/P-doped Co-N 2 P 2 82 were also reported to have enhanced ORR performance. In addition, MOF-derived hollow heteroatom-doped carbon materials also displayed outstanding electrocatalytic ORR activity. 83 As an attractive study, a N-doped carbon electrocatalyst with a negligible amount (0-0.08 wt %) of Fe (N-HPC) was synthesized. Such N-HPCs feature a hollow and hierarchically porous architecture, which shows excellent ORR activity and durability. 84 When used as cathode catalysts, the N-HPCs demonstrated distinguished power densities of 486 and 158 mW cm À2 for PEMFCs and Znair batteries, respectively. Interestingly, Fe sites do not contribute to ORR activity ( Figures 6A-6C). Further, spin-polarized DFT calculations and CHE methodology were applied to investigate the source of the original catalytic activity of N-HPCs for ORR. The results indicated that the non-covalent-bonded N-deficient/N-rich heterostructure in the hierarchically porous architecture of N-HPCs could accelerate electron transfer between the layers and provide the active sites for oxygen adsorption and activation ( Figures 6D-6F).
OER. OER plays an essential role in energy conversion technologies. To reduce the expensive cost of OER, extensive efforts have been devoted to exploiting highperformance and inexpensive OER electrocatalysts, with MOF-derived hollow materials being one of them (Table S2). 94 According to numerous reports, MOF-derived bimetal or metal oxide NPs (especially Fe, Co, and Ni) are beneficial to OER catalysis. [95][96][97][98][99][100] For example, Lou and co-workers reported NiCoP/C ( Figure 7A) to have a remarkable OER catalytic performance, 47 which displayed a low overpotential of 330 mV at 10 mA cm À2 , and enhanced stability of 96.5% of the initial current is retained after 10 h (Figures 7B and 7C). In addition, Yao and colleagues prepared two types of hollow Co 3 O 4 /C (Z67-Co 3 O 4 /C-4 and Z9-Co 3 O 4 /C-4) via carbonized and then oxidized ZIF-67 and ZIF-9 precursors. 55 Figures 7D and 7E demonstrate that the catalysts of Z67-Co 3 O 4 /C-4 and Z9-Co 3 O 4 /C-4 exhibited excellent activity for OER. This is because the unique hollow porous carbon structure improves the electron transfer rate, and vast oxygen vacancies in hollow Co 3 O 4 /C can improve the adsorption of water molecules to promote OER activity. Apart from MOF-derived metal oxides, MOF-derived CoSe 2 with a hollow structure was also synthesized as an OER electrocatalyst with improved performance. 48 The optimized CoSe 2 -450 microspheres displayed 10 mA cm À2 at h = 330 mV with a small Tafel slope of 79 mV dec À1 , better than the reference catalysts. This is due to the hollow structure and uniformly distributed active sites, which facilitate a fast mass and electron transport rate. Integration of MOF-derived materials with metallic oxide proved to be a facile and effective strategy to design high-activity OER catalysts. 101 MOF-derived LDH materials also showed excellent OER properties. Zhang et al. fabricated Ni-Fe LDH nanocages with tunable shells that used MIL-88A particles as sacrificial templates ( Figure 7F). 18 By mixing different proportions of ethanol and water, Fe-Ni LDH single-shelled nanocages (SSNCs) and double-shelled nanocages (DSNCs) were easily acquired. Owing to more Fe elements in the outer shell, the obtained Ni-Fe LDH DSNCs with effective surface area exposure showed superior OER performance and a high C dl value compared with the single shelled ones ( Figure 7G).
HER. Like the OER, HER is an essential reaction for water splitting. Although the overpotential of HER is much lower than OER, electrocatalysts are still needed to improve the HER reaction. 102 Recently, earth-abundant transition metal carbides, especially MoC, have been extensively studied as high-efficiency HER catalysts under acidic and basic conditions. Mesoporous molybdenum carbide nano-octahedrons (MoC x nano-octahedrons) were prepared through confined carburization in an MOF. 35 The unique nanostructure MoC x nano-octahedrons showed outstanding catalytic performance for HER in acidic medium and alkaline conditions. Moreover, on repeated potential sweeps, the MoC x nano-octahedrons exhibited promising durability in acidic and alkaline media ( Figure 8A). Stability tests are demonstrated in Figure 8B. The current density of MoC x nano-octahedrons is generally stable for more than 10 h in 0.5 M H 2 SO 4 , with slight degradation observed during long-term operation in 1 M KOH. A TEM image conveyed that the nanostructure and crystallinity after degradation measurement are well retained, further confirming its good stability in an acidic environment ( Figure 8B). The HER activity under basic conditions is also superiorly favorable for many HER catalysts, such as graphene and CNTs, and even state-of-the-art b-Mo 2 C-based electrocatalysts. Subsequently, hollow Ni-decorated molybdenum-carbide was also synthesized through a similar strategy. 104 Benefiting from the advantages of combined composition (MoC and Ni NPs) and unique structure, the obtained samples showed a small overpotential of 123 mV at 10 mA cm À2 , a small Tafel slope of 83 mV dec À1 in alkaline medium, and superior stability. Zhang et al. reported ultrafine Pt-Co alloy NPs confined on surfaces of CNTs (Pt 3 Co@NCNT catalyst) and, benefiting from the synergistic effects of the bimetallic alloy components, the Pt 3 Co@NCNT catalyst showed remarkably enhanced HER performance in both acidic and alkaline media. The exquisite porous carbon shell structure host prevents the ultrasmall Pt 3 Co NPs from aggregating, thus ensuring long-term durability. 105 In a fascinating study, Huang et al. 106 synthesized a series of hollow Co-based bimetallic sulfide (M x Co 3Àx S 4 , M = Zn, Ni, and C) polyhedrals by using MOFs as self-templates. Zn 0.30 Co 2.70 S 4 exhibited outstanding electrocatalytic HER activity over a wide pH range, with overpotentials of 80, 90, and 85 mV at 10 mA cm À2 and 129, 144, and 136 mV at 100 mA cm À2 in 0.5 M H 2 SO 4 , 0.1 M phosphate buffer, and 1 M KOH, respectively. Zn 0.30 Co 2.70 S 4 further exhibited photocatalytic HER activity when working with an organic photosensitizer (Eosin Y) or semiconductors (TiO 2 and C 3 N 4 ). This study provides a reference for the synthesis of transition metal sulfides used for HER.
Bifunctional electrocatalysts. Electrocatalysis, as an important process in energy conversion, usually involves more than one process. Specifically, water splitting includes HER and OER, and metal-air batteries involve ORR and OER. The ORR is also the fundamental electrochemical reaction for fuel cells. The design of bifunctional electrocatalysts is significant for the electrochemical process and puts forward higher requirements on the componential tuning and morphological control of the catalyst. MOF-derived hollow structure materials are considered high-efficiency electrocatalysts due to their unique advantages, such as tailorable composition and multilevel porosity.
Transition metals doped with heteroatoms are considered to be highly efficient electrocatalysts. Pan et al. proposed the preparation of CoP/NCNHP by a successive pyrolysis-oxidation-phosphatization process applied to core-shell-structured ZIF-8@ZIF-67 as the precursor. 44 The excellent catalytic performance of both HER and OER of CoP/NCNHP can be attributed to the synergistic effects between highly active CoP NPs and NCNHP. And the overpotentials are 140 and 115 mV for the HER under acidic and alkaline conditions to achieve the current density of 10 mA cm À2 , and under alkaline conditions, the overpotential is 310 mV for OER. Hollow heterojunctions derived from MOFs can also be used as outstanding bifunctional electrocatalysts. For instance, hollow Co 3 S 4 @MoS 2 heterostructures 107 and hollow CoS x @MoS 2 microcubes 108 exhibited outstanding catalytic performance for HER and OER under acidic and alkaline media. The highly efficient electrocatalytic activities benefited from the distinct heterostructures and synergistic effects between Co 3 S 4 /CoS x and MoS 2 .
Heteroatom-doped carbon nanomaterials are considered to be promising electrocatalysts for the electrocatalytic reaction process. Direct formation of NCNTFs ( Figures 8C and 8D) derived from MOFs could be achieved through control of the pyrolysis atmosphere. 103 The as-prepared NCNTFs demonstrated higher E 1/2 , with 0.87 V, for ORR than the 20% Pt/C electrocatalyst with 0.84 V ( Figure 8E) and possessed superior stability after 5,000 cycles. In addition, the NCNTFs demonstrated superior electrocatalytic properties for the OER. As shown in Figure 8F, they gave a current density of 10 mA cm À2 at 1.60 V (versus RHE), which compared favorably with reported nanocarbon-based catalysts and was lower by $180 mV than the 20% Pt/C (10 mA cm À2 , about 1.78 V). The advantageous traits of NCNTFs, such as hierarchical shells of interconnected crystalline NCNT, optimum graphitic degree, and N-doping level, are the source of the excellent electrocatalytic activity of both ORR and OER.
In summary, MOF-derived hollow nanostructures are promising candidates for applications in electrocatalytic reactions owing to their hierarchical porous structures, accessible active sites, high chemical stability, and good electron conductivity. Electrocatalytic reactions such as ORR, HER, and OER are crucial reaction steps in energy conversion and require catalysts to accelerate their slow kinetics. The performance of the catalyst plays an important role in energy conversion efficiency. Although MOF-derived catalysts have made great achievements in electrocatalytic applications, several problems and challenges still exist, such as device durability, stability, cost issues, catalytic performance, structure-activity relationships, etc. Therefore, the above issues should be considered when designing electrocatalysts.

Energy storage and conversion
MOF-derived hollow nanostructures with extremely porous structures, low density, robust architecture, large surface area, and rich redox reactions of metal ions have verified their unique capabilities and exhibited improved energy storage and conversion performance. 109 Recently, MOF derivatives have been extensively applied in energy storage, especially for LIBs, SIBs, and supercapacitors, and showed high reversible capacity, superior rate, and cycling performance. 109,110 LIBs and SIBs. Various novel MOF-derived hollow structures, such as CuO, 26 hollow structures were prepared and applied for LIBs. 122 The experiment revealed that Co 3 O 4 @Co 3 V 2 O 8 shows superior rate capability, cycling stability, and reversible capacity. Moreover, after a long charge and discharge cycle, the complex hollow structure of Co 3 O 4 @Co 3 V 2 O 8 is almost unchanged, indicating the stability of the hollow structure in LIBs. In addition to LIBs, MOF-derived hollow structures, i.e., VN-NBs, 123 MHPCS/Se, 124 and Se x S y , 125 were also used as electrode materials in Li-S and Li-Se batteries.
The sluggishness of the LiPSs redox reactions is a great challenge in the largescale practical application of Li-S cells. 126 Aiming at the above problems, Li and co-workers constructed a hollow N-doped porous carbon (Ni-N 5 /HNPC) with an optimal Ni-N 5 active moiety, which acted as an ideal host for a sulfur cathode under the guidance of theoretical simulations. 127 First, the Ni-N x /C structure (x = 3-5) with the highest cathode performance was selected by first-principles calculation. The energy calculation results of the reaction step indicated that the Ni-N 5 /C structure is the best cathode candidate. Therefore, they fabricated Ni-N 5 /HNPC with Ni-N 5 active sites using MOFs as self-sacrificing templates. As cathode of Li-S batteries, Ni-N 5 /HNPC exhibited outstanding rate performance and longterm cycling stability.
Recently, SIBs have been proposed as a potential alternative to LIBs due to the advantage of the high abundance and low cost of the sodium source, high system safety, and wide distribution, as well as storage mechanisms and components similar to those of lithium. 3 Similar to LIBs, MOF-derived hollow metal materials can also be applied as electrodes with remarkable rate capability, cycling stability, and reversible capacity to SIBs. 34,49,[128][129][130][131] Liu et al. reported on Ni 3 S 2 / Co 9 S 8 /N-doped carbon composites via carbonization and sulfurization of binary Ni/Co MOFs (Ni-Co-MOF). 132 Due to the integrated merits of ultrafine Ni 3 S 2 and Co 9 S 8 NPs ($7 nm), hollow porous structure, and an ultrathin N-doped carbon coating, the final composite material exhibited excellent performance when used as an anode in SIBs. Experiments showed that a reversible specific capacity of 419.9 mAh g À1 was achieved after 100 cycles at 0.1 A g À1 , and a superior capacity retention rate of 98.6% was achieved. In addition, excellent rate performance was observed: at a current density of 2 A g À1 , an average capacity of 323.2 mAh g À1 can be reached. The formation of metal sulfide and N-doped carbon layers provides larger capacity and enhanced conductive surface coating. With these virtues, Ni 3 S 2 /Co 9 S 8 /N-doped carbon composite materials possess fast sodium storage kinetics and high conductivity, thereby achieving high rate properties.
The construction of composite nanomaterials can effectively alleviate the huge volume expansion/contraction of antimony-based materials when used in sodium-ion batteries, resulting in low cycle life. For instance, Huang and co-workers proposed a NiN-Sb 2 Se 3 @C composite with a hierarchical nanodot-in-nanofiber structure in which antimony selenide (Sb 2 Se 3 ) nanocrystallites are confined by both 0D and 1D carbon layers. 133 As expected, the NiN-Sb 2 Se 3 @C composite anode achieved enhanced capacity and exceptional cycle lifespan for over 10,000 cycles at 2.0 A g À1 ( Figure S5A). Figure S5B demonstrates the process of the structure evolution of micro-Sb 2 Se 3 , Sb 2 Se 3 @C, and NiN-Sb 2 Se 3 @C during electrochemical processes. It is seen that the NiN-Sb 2 Se 3 @C composite can bear the volume-change-induced strain and avoid aggregation of Sb 2 Se 3 NPs. Similarly, the (CoS NP@NHC)@MXene composite manifests distinguished electrochemistry performance when used as electrode material for all of the LIBs, SIBs, and PIBs, which benefit from the synergistic effect of the components ( Figure S5C). 134 Supercapacitors. Supercapacitors have become an ideal choice of energy storage device for their various advantages such as low internal resistance, high power density, and excellent cycle life. 135,136 The surface area and chemical composition of the electrode have a great influence on the properties of the supercapacitor. In this regard, MOF-derived porous hollow structures meet requirements such as affording rich redox chemistry with abundant active sites, inducing the hollow structures for larger capacitance, forming carbon-based nanomaterials for improving surface areas, and enhancing conductivity (Table S3). 110 144 H-NiS 1-X / C-50, 145 Co 9 S 8 @NiO, 146 NiCoMn-S, 147 CoSNP/CoS-NS DSNBs 148 ) have seen widespread use as anode materials for supercapacitors. For instance, Ni x Co 3Àx O 4 with a hollow structure was used as anode material in supercapacitors and demonstrated outstanding specific capacitance of 2,870.8 F g À1 at 1 A g À1 and excellent cyclic stability with 81% capacitance retention after 5,000 cycles. 139 In addition, Ho and colleagues fabricated hybrid NiCo-LDH/Co 9 S 8 (C/ LDH/S) with outstanding performance in supercapacitors. 141 As Figure S5D shows, galvanostatic CD indicated excellent capacitive behavior with highly reversible and rapid reaction kinetics of C/LDH/S. In addition, it exhibited excellent cycle stability and 95.4% retention capacitance after 3,000 cycles. They found that the uniform combination of multiple metal species, the production of heterosulfide-hydroxide, and the arrangement of the hollow structure optimize the catalytic site and enhance conductivity and hydrogen adsorption of NiCo-LDH/Co 9 S 8 .
Pang and co-workers proposed a general approach for preparing hollow 3D Mxene/MOF composites (Ti 3 C 2 T X /ZIF-67/CoV 2 O 6 ) by in situ growth of MOFs and subsequent ion exchange. 149 Ti 3 C 2 T X /ZIF-67/CoV 2 O 6 overcomes the disadvantage of poor conductivity of traditional MOFs. 150,151 Notably, the Ti 3 C 2 TX/ZIF-67/CoV 2 O 6 electrode demonstrated excellent performance with a high specific capacitance of 253.8 F g À1 at 5 A g À1 and a high coulombic efficiency of 94.4% after 4,000 GCD cycles at 3 A g À1 .
Composite materials can overcome the defects of a single material and integrate the advantages of different nanomaterials, thus facilitating the performance of the target application. Therefore, a composite could obtain neoteric physical and chemical properties that cannot be achieved by a single component.

Environmental applications
Environmental pollution has become a critical issue in human health and environmental protection. 152,153 Benefiting from their tunable configuration, controllable composition, permanent porosity, and larger specific surface area, MOFderived hollow structures have displayed fascinating physicochemical properties and amassed extensive attention in catalytic degradation of pollutants in our environment. Li and colleagues fabricated ZnO@C-N-Co core-shell nanostructures that used a hollow Zn/Co-ZIF matrix as precursor toward efficient degradation of MO and displayed excellently improved performance and remarkable recyclability. 154 The CeO 2 /Au@SiO 2 hollow nanotubes obtained demonstrated high catalytic performance for 4-nitrophenol reduction. The kinetic reaction rate constant (k) of Ce-MOF/Au@SiO 2 (0.71 min À1 ) is higher than that of Au@SiO 2 (typically 0.1 min À1 ). Moreover, the catalytic activity was decreased by only $17.6% after five successive cycles, indicating that the CeO 2 /Au@SiO 2 catalyst has superior stability and reusability. The outstanding catalytic performance was attributable to the unique and small size of the Au NPs, as well as the strong synergistic effect between the CeO 2 and the Au NPs. 155 In addition, hollow materials have abundant metal active sites and can also realize the elimination of organic pollutants by exciting free radicals with strong oxidizing properties. For example, the homogeneous bimetallic hollow C-CoM-HNC derived from a MOF achieves the effective removal of RhB by activating persulfate. 156 According to reports, similar studies have used MOF-derived hollow Co 3 O 4 /carbon as an effective activator of PMPS and achieved the degradation of BPA. 157 Adsorption technology is widely used to purify environmental pollutants due to its advantages of simple operation, simple regeneration, and large-scale applications. [158][159][160] The inherent specialties of MOF nanomaterials, including higher surface area, abundant pore structures, and adjustable chemical composition, have been allowing them to serve as prospective candidates for supernormal adsorbents. Yang and co-workers 161 reported MOF-derived porous Ni 1-x Co x Fe 2 O 4 microcubes as adsorbents for efficient removal of nitrophenol. The equilibrium quantity of Ni 1-x Co x Fe 2 O 4 for nitrophenol was 47 mg g À1 of ferrite accomplished in 7 min. High specific surface area and mesoporous nature are the keys to the excellent adsorption performance of Ni 1-x Co x Fe 2 O 4 on nitrophenol. 161 Analogously, carbon aerogels bearing a hierarchical structure (Biomass-C@MIL-53-C) were prepared via direct carbonization of kapok fibers assembled with MIL-53 on its surface. 162 As an adsorbent, it demonstrated superior adsorption performance, excellent hydrophobic property, and outstanding cycling stability. Specifically, the adsorption capacities of Biomass-C@MIL-53-C were 35-119.5 times their own weight toward various kinds of oils and organic solvents. Moreover, after eight adsorption-squeezing cycles, it could retain 77.2%-96.7% of its initial adsorption capacity, implying that Biomass-C@MIL-53-C was an outstanding adsorbent for organic pollutant purification.
Advanced oxidation processes (AOPs) are a robust system to degrade refractor organic pollutants in environmental pollution management. MOFderived SACs have been widely used due to the maximum utilization of metal atoms and the unique electronic properties of metal sites and ultralow metal loads. 163 Mi et al. reported that atomically dispersed Co-SA catalyst derived from MOFs applied to peroxymonosulfate (PMS) activation. 164 DFT calculations revealed that CoN 2+2 was the definite active site ( Figure 9A). 1 O 2 was the predominant reactive oxygen species, and the proportion was 98.89% ( Figures 9B and  9C). The generated 1 O 2 showed excellent degradation activity for organic pollutants in a wide pH range.

Other applications
In addition to the above-mentioned emerging applications, the unique properties of MOF-derived hollow structures enable their applications in gas storage/ separation, 166 gas sensors, 22,31,[167][168][169][170][171][172][173] extraction of uranium from seawater, 165,174 etc. Xu and co-workers synthesized ZnO/ZnFe 2 O 4 hollow nanocages and used them as a sensing material for gas sensors. It demonstrated an improved response to acetone (25.8) with a detection limit of 1 ppm. 167 Moreover, MOF-derived Ag/Au HPNS@FO were used for electrochemical As(III) determination and exhibited high sensitivity (922.5 mA ppb À1 ) and sustained stability. 175 Recently, our group applied the MOF-derived adsorption-electrocatalyst Fe-N x -C-R in uranium extraction from seawater and demonstrated considerable results. 165 Real seawater experiments showed that Fe-N x -C-R has a superior uranium extraction capacity of $1.2 mg g À1 in 24 h. Interestingly, we found that the isolated Fe-N x site first reduced UO 2 2+ to UO 2 + , and then oxidized U(V) to U(VI) in the presence of Na + , finally obtaining the product Na 2 O(UO 3 $H 2 O) x ( Figures 9D and 9E). To our knowledge, the developed system is the first to yield a U(VI) solid product (i.e., Na 2 O(UO 3 $H 2 O) x ) in uranium extraction from seawater by an adsorption-electrocatalysis system. We further demonstrated that an amidoxime-functionalized indium-nitrogen-carbon catalyst (In-N x -C-R) offers a 5-fold higher uranium extraction capacity in seawater compared with our aforementioned Fe-N x -C-R system. X-ray absorption spectroscopy and in situ Raman spectroscopy allowed the relationship between the In-N x -C-R structure and the adsorption-electrocatalytic mechanism for uranium extraction from seawater to be understood. 174 This provides meaningful clues for the study of uranium extraction from seawater. The adsorption-reduction of U(VI) to U(IV) is an efficient technique for the extraction or preconcentration of U(VI) from aqueous solutions.
In addition to the above applications, MOF-derived porous hollow materials also exhibited excellent performance in photocatalytic and electrocatalytic CO 2 RR. For example, Ma et al. 176 synthesized crystalline MoS 2 @TiO 2 nanohybrids with MOFs as precursors through a simple hydrothermal method. The optimal material showed prominent catalytic activity for HER with a high hydrogen production rate of 10,046 mmol h À1 g À1 under visible light. The catalyst obtained also displayed excellent electrocatalytic activity. The extensive and close contact interface and synergistic effect of different components are regarded as the source of enhanced catalytic activity. Moreover, a novel ZnO@C-N-Co core-shell nanocomposite was also reported as a highly efficient photocatalyst for pollutant photodegradation. 154 The most significant advantage of this material is the recycling performance, which benefits from the assistance of magnetic Co NPs inside the material. Reducing CO 2 toward the generation of valuable chemicals is one of the important strategies to achieve the dual carbon goal. 177,178 Wang et al. presented a carbon-confined indium oxide electrocatalyst for efficient CO 2 RR toward the direct production of formic acid. 179 Experimental results showed that the formate selectivity exceeds 90% in a wide potential window from À0.8 to À1.3 V versus RHE in a liquid-phase flow cell. The high selectivity and activity for CO 2 RR benefited from the carbon protective layer preventing the reductive corrosion of indium oxide and the carbon layer, optimizing the adsorption of reaction intermediates.

CONCLUSION AND OUTLOOK
We have systematically summarized the design strategies of MOFs as precursors and/or soft templates in the fabrication of hollow porous materials, including hollow carbon, metals, metal oxides, metal carbides, metal sulfides, metal hydroxides, and their hybrid composites. Due to various favorable structural features, MOF-derived hollow nanomaterials are considered promising candidates for multifield applications: an enhanced surface area, low density, mesoporous structure,

Review ll
The Innovation 3(5): 100281, September 13, 2022 higher loading capacity, and shortened transport distance for mass and charge. Here, the promising applications of MOF-derived hollow structures for chemical catalysis, electrocatalysis, energy storage and conversion, environmental applications, and so on have been summarized.
Despite the intriguing progress and great achievements that have occurred in the synthesis and extensive application of various MOF-derived HPMs, the study is at a burgeoning stage, and more effort is needed to deal with the issues that will be encountered in the future development process. Here, we propose several challenges and research directions that MOF-derived hollow materials may face in future development: (1) more effort should be focused on controllably synthesizing complex structures and regulating components of hollow MOFs and derivatives according to the needs of specific applications; (2) combination with other functional nanomaterials, for example, NPs, clusters, CNTs, and GO, etc., is an economical and facile strategy to regulate the properties of hollow MOFs and their derivatives; (3) the design of hollow MOFs or their derivatives with different sizes and geometrical structures will greatly enrich the storage of HPMs and ultimately affect their internal properties; (4) realizing the large-scale yield of hollow MOFs or derivatives is significant in practical applications; (5) strengthening the deep understanding of structure-activity relationships of hollow material structures is important; particularly, the mechanisms of various MOF-derived hollow materials in electrochemistry have not been thoroughly investigated; and (6) future work should combine experimental and computational methods to explore the interior reaction mechanisms, revealing the influence of structure-property-function and their synergistic interactions.