Recent advances in the type, synthesis and electrochemical application of defective metal-organic frameworks

Modulation approach

The modulator approach is the most common strategy used to synthesize defective MOFs^[106]. In this method, in addition to organic ligands, a large number of monocarboxylic acids are added during the synthesis to improve the crystallinity of MOFs^[107-109]. There are two usual mechanisms for inducing defect formation using modulators: (1) the conjugate acid of the modulator protonates the linker from the node; and (2) the conjugate base of the modulator competes with the linker for coordination to the node^[107], resulting in the defect of missing linker^[80] and/or missing cluster^[110]. The chemistry of the defects formed during the synthesis of MOFs is closely related to the type and number of modulators used. Up to now, several classical monocarboxylate modulators, such as formic acid (FA)^[111], acetic acid (AA)^[89,112,113], trifluoroacetic acid (TFA)^[114], dichloroacetic acid (DCA)^[115], difluoro acetic acid (DFA)^[95], and L-phenylalanine (L-Phe)^[116], have been frequently introduced into MOFs to induce defect formation.

In 2016, Shearer et al. proposed a close correlation between the acidity (pKa) and/or concentration of modulators and the defect concentration in the framework of MOFs^[95]. They have systematically explored the effect of various modulators (AA, DFA, and FA) on Uio-66 defect concentration. They found that the defect concentration was positively correlated with the modulator acidity/pKa [Figure 5A]. The pKa or concentration of modulators can be reasonably controlled to regulate the density of defects in MOFs. Furthermore, all the modulators selected in this study were completely miscible with the solvent, thus neglecting the possibility that solubility may affect the synthesis in the defective MOFs. Therefore, it is essential to consider the effect of modulator solubility or steric effects in the synthesis process in defective MOFs, besides the influence of the acidity and concentration of the modulator on the formation of defects in the framework of MOFs. Recently, Assaad et al. found that the distribution density of defects could be effectively regulated and controlled by changing the type and amount of the modulator, such as using different ratios of acetic acid and trifluoroacetic acid as the modulator of Uio-66 and HRSEM researched the morphology of Uio-66, and the SEM images are shown in Figure 5B-G^[117]. Among them, trifluoroacetic acid causes a larger volume of defects than acetic acid, and the higher the proportion of the modulator, the higher the distribution density of defects formed. In addition, Wu et al. proposed that the concentration of defective missing linkers could be effectively regulated by reasonably varying the modulator concentration and synthesis time, thus increasing the porosity and further improving the gas adsorption performance of the defective MOFs^[80].

Figure 5. (A) The number of defective linkers per Zr₆ formula unit against modulator molar equivalents in UiO-66. Reproduced with permission^[95]. Copyright 2016, American Chemical Society. (B) SEM images of the studied MOF nanoparticles UiO-66, (C) UiO-66-12AA, (D) UiO-66-36AA, (E) UiO-66-100AA, (F) UiO-66-12TFA, and (G) UiO-66-36TFA. Reproduced with permission^[117]. Copyright 2020, American Chemical Society. (H) TGA curves, and (I) catalytic performance over these UiO-66 samples. Reproduced with permission^[114]. Copyright 2020, The Royal Society of Chemistry.

MOF structures have promising applications in catalysis due to their high density of metal nodes. Unfortunately, the metal nodes in MOF structures are completely occupied by organic ligands, resulting in catalytic performance rarely superior to that of microporous solid catalysts (such as zeolites)^[118,119]. Therefore, tuning the active sites in the structure of MOFs to improve their catalytic activity remains a large challenge. In 2020, Wang et al. reported the preparation of defective MOF Uio-66 with trifluoroacetic acid (TFA) as a modulator and systematically investigated it as a catalyst for the liquid reaction of cyclohexanone to cyclohexanol via hydrogen transfer^[114]. Not all structural defects are catalytic for cyclohexanone. Only the open metal sites generated by the modulating agent are effective for the catalytic conversion of cyclohexanone [Figure 5H and I]. According to the findings, under identical reaction circumstances, missing linker defects and missing cluster defects created an increase in the quantity of functional open metal sites and enhanced their catalytic activity. In 2019, Kang et al. first proposed a new strategy of templated electrosynthesis of vacancy MOFs using ionic liquids as electrolytes and templates^[120]. The mesoporous Cu(II)-MOF MFM-100 was successfully prepared by the rapid electrosynthesis method, and the presence of defects in Cu(II)-MOF MFM-100 was confirmed by advanced characterization methods. Additionally, in recent years, many researchers have synthesized a series of hierarchically porous MOFs by controlling the reaction conditions of the modulation approach, such as the introduction of mesopores in microporous MOFs, thus making the application of MOFs more extensive^[121-123]. The modulation approach is widely used by researchers as a more mature method.

Mixed linker approach

The mixed linker approach is a common strategy for introducing defects in MOFs. As the name implies, it involves directly adding two or more structurally similar organic linkers as precursors to the reaction. According to the type of mixed linkers, they can be mainly divided into isostructural mixed linker (IML) and heterostructural mixed linker (HML)^[124]. Different linkers can be mixed to form defective MOFs with different sizes, shapes, and porosities^[74,125]. The location of defects in MOFs synthesized by mixed organic linkers depends on the relative rates of coordination bond formation and exchange. If the framework growth rate is faster than the exchange rate, the defects will be contained in the framework, and if the exchange rate is faster than the framework growth rate, the defects will reside on the crystal surface.

In 2014, Kozachuk et al. reported the synthesis of a series of single-phase defect variants Ru₃(btc)_2-x(pydc)_xX_y] (D1-D4) by using mixed linkers of 1,3,5-tricarboxylate (H₃btc) and pyridine-3,5-dicarboxylic acid (H₂pydc) [Figure 6A]^[126]. The structure and charge distribution of linkers btc^3- and pydc^2- are very similar, but pydc^2- has one less carboxylate ligator site compared to btc^3-. And pydc^2- acts as a second defective linker, leading to the formation of the defect in Ru-MOF. In addition, the incorporation of pydc^2- induced the reduction of some Ru metal sites. In 2016, Yuan et al. proposed a strategy using the thermodynamically guided synthesis of mixed linker Zr-MOFs with pillared-layer structure, namely PCN-133 and PCN-134 [Figure 6B]^[127]. Among them, PCN-134 not only shows high porosity and excellent chemical stability, but also maintains the integrity of its framework, while tolerating the absence of some auxiliary linkers, so that the defect density can be controlled by reasonably adjusting the ratio of auxiliary linkers to further improve the performance of PCN-134. The thermal stability of the modulator also contributes critically to the formation of defects. In 2017, Bueken et al. exploited the difference in decomposition temperatures of BDC (terephthalic acid) and CDC (trans-1,4-cyclohexanedicarboxylate) linkers to successfully introduce missing linker defects in the mixed linker UiO-66 [Figure 6C], thus also providing researchers with a comprehensive insight into the relationship between defect structure and properties of UiO-66 MOFs^[128].

Figure 6. (A) Mixed linker approach in Ru-HKUST-1 leads to modified node defects. Reproduced with permission^[126]. Copyright 2014, Wiley-VCH. (B) PCN-134 formed by 6-connected Zr 6 cluster, BTB, and TCPP. Reproduced with permission^[127]. Copyright 2016, American Chemical Society. (C) Schematic illustration of defect formation by thermal decomposition of labile linker. Reproduced with permission^[128]. Copyright 2017, American Chemical Society. SEM images of MTV Uio-66, (D) UiO-66(3A:1B), (E) UiO-66(1A:1B), (F) UiO-66(1A:3B), (G) UiO-66(3A:1C), (H) UiO-66(1A:1C) and (I) UiO-66(1A:3C). Reproduced with permission^[130]. Copyright 2021, Elsevier. (J) The XRD patterns of ZIF-8-SV-5 μm and ZIF-8-SP-5 μm (insert SEM images). Reproduced with permission^[131]. Copyright 2017, Elsevier. SEM images of UIO-66 with different synthesized conditions, (K) Zr-H-2 h, (L) Zr-H-10 h, (M) Zr-H-120 °C, (N) Zr H-140 °C. Reproduced with permission^[132]. Copyright 2021, Wiley.

Moreover, as the research on MOFs is intensified, they are oriented to a wider range of applications. At present, many MOFs have been functionalized by introducing organic ligands with different functional groups. In 2010, Deng et al. conducted a series of systematic research^[129]. Based on MOF-5, they first introduced functional groups -NH₂, -Br, -(Cl)₂, -NO₂, -(CH₃)₂, -C₄H₄, -(OC₃H₅)₂ and -(OC₇H₇)₂ into BDC structure, and then selected two or more of these ligands to synthesize 18 types of MTV-MOF-5 (Multivariate MOFs), which are similar structure to MOF-5 and have various functionalization. Recently, Jrad et al. proposed the combination strategy of multivariate approach with modulated synthesis to develop two highly defective and functionalized MTV-UiO-66 series, namely MTV-UiO-66(COOH)₂ and MTV-UiO-66(OH)₂, and the SEM images are shown in Figure 6D-I^[130]. Furthermore, the comparative analysis of 33 different UiO-66 catalysts for the conversion of butyl butyrate showed that the density of active sites, as well as the number of defects or the number of functional groups on the linker, had a greater influence on the conversion of butyl butyrate relative to the surface area and catalyst loading level. These findings also provide a theoretical basis for expanding the application of Uio-66 in biofuel catalysts.

Fast crystallization approach

In addition, the fast crystallization approach is another effective strategy for creating defects in the MOF structure. The fast crystallization approach is mainly assisted by microwave-assisted synthesis and the addition of high concentrations of precursors to promote the fast crystallization of MOF crystals resulting in defects. Depending on the effect of modulators on MOFs in terms of reaction kinetics and crystallinity, the fast crystallization process leads to the formation of defects, such as ligand or metal center defects^[76]. In 2017, Chaemchuen et al. synthesized three different types of defective ZIF-8 [Figure 6J], MOF-5 and Zn-DABCO using a fast crystallization strategy and compared their catalytic properties on this basis^[131]. The defects formed in MOFs due to different crystal growth rates were found to affect the structure, morphology and other properties of the materials, such as particle shape and specific catalytic properties. Recently, Chu et al. used concentrated hydrochloric acid as a modulator to design and synthesize a series of defective Uio-66 by a rapid crystallization method [Figure 6K-N]^[132]. The prepared defective Uio-66 not only has the advantages of low preparation temperature, short preparation time and large-scale preparation but also removal of DBT from ODS exhibits excellent catalytic performance. More importantly, the fast crystallization approach provides a new strategy for introducing defects in the structure of MOFs, further expanding the application prospects of MOFs materials.

Etching treatment

Postsynthetic treatment refers to the MOFs activation and purification while maintaining the structural integrity of materials. Etching is an ancient fabrication method and one of the simplest and most efficient to generate defects among MOFs during the post-synthesis process^[139]. Therefore, replacing the linker in the MOF framework with an acid/base group may create defects in the post-synthetic parent MOF^[133,140]. For example, in 2017, Koo et al. reported a new strategy for introducing mesopores into water-stabilized microporous MIL-100(Fe) using a selective acid etching process [Figure 7A]^[101]. Due to the selective nature of the etching process, this method preserves the inherent crystallinity and external morphology of MIL-100(Fe) on the basis of allowing fine-tuning of the porosity [Figure 7B and C]. In addition, this strategy was also shown to be applicable to the synthesis of mesopores introduced in other water-stabilized microporous MOFs. In addition to acid etching, gas etching has also been applied to create defects in the framework of MOFs. On the contrary, in 2019, Doan et al., inspired by the fact that phosphoric acid can tune the porosity of MIL-100(Fe) by selective acid etching, successfully prepared defective hierarchical porous MOFs framework by phosphoric acid as an etching agent in water unstable HKUST-1 MOF^[141]. At the same time, it was also demonstrated that the extent of etching on MOFs was affected by the combined effect of pH and etching time. Recently, Albolkany et al. proposed an ammonia gas phase etching method to format mesopores within carboxylate-based microporous MOFs (HKUST-1, [Cu₃BTC₂], BTC = 1,3,5-benzenetricarboxylic acid) and thus produce hierarchically porous MOFs [Figure 7D]^[142]. Ammonia as gas phase etching agent is first uniformly adsorbed into the microporous MOFs, ensuring uniform etching within the MOFs crystal. The mesopores were prepared under heated conditions by cutting the carboxyl-metal coordination bonds using the strong coordination between ammonia and metals. In the HKUST-1 model, the size of the mesopores is controlled by the etching temperature and the volume of the mesopores can be modulated by changing the pressure of the etchant ammonia [Figure 7E and F]. This strategy can precisely control the mesopore size and volume without affecting the crystal morphology.

Figure 7. (A) Illustration of the etching process for MIL-100(Fe), (B and C) TEM images of MIL-100(Fe)-40. Reproduced with permission^[101]. Copyright 2017, The Royal Society of Chemistry. (D) Schematic illustration of the formation of hierarchically porous MOF by ammonia etching of HKUST-1, (E) SEM images of HP-HKUST-1 prepared using 20 mL NH₃ at 160 °C, (F) SEM images of HP-HKUST-1 prepared using 20 mL NH₃ at 200 °C. Reproduced with permission^[142]. Copyright 2021, Wiley-VCH. (G) Schematic illustration of the synthesis of mesoCu@Al-bpydc. (H) SEM images of mesoCu@Al-bpydc, (I) TEM images of mesoCu@Al-bpydc. Reproduced with permission^[143]. Copyright 2019, Wiley. (J) Schematic illustration of the synthesis pathway of Co-MOF and the generation of polycrystalline 2D materials at room temperature. (K and L) SEM images of Co-MOF nanoarray-derived. Reproduced with permission^[144]. Copyright 2018, American Chemical Society.

In 2019, Chang et al. proposed a facile and original in situ defect formation strategy to construct hierarchical MOFs using tetrafluoroborate [M(BF₄)_x] as both the functional site and etching agent^[143]. Firstly, microporous MOF Al-bpydc (bpydc = 2,2′-bipyridine-5,5′-dicarboxylic acid) and Cu(BF₄)₂ were selected as the parent MOFs and tetrafluoroborate, respectively, and then Cu(BF₄)₂ was doped into Al-bpydc by wet impregnation method. Thus mesoCu@Al-bpydc was constructed [Figure 7G-I]. This strategy not only maintains the structural stability of MOFs, but also rationally designs and controls the chemical environment of hierarchically porous MOFs. Furthermore, the nanoscale engineering of crystal boundary defects can significantly improve the catalytic activity of nanoelectrocatalysts in reactions such as OER and CO₂ reduction. Therefore, theoretically, the introduction of nanoscale crystal boundary defects within 2D materials can further improve the electrocatalytic performance of 2D materials. In 2018, Zhang et al. first synthesized Co-MOF nanosheet arrays as precursors at room temperature, and then obtained three-dimensional network structures of nanoscale polycrystalline two-dimensional nanosheets by etching/reconfiguration of MOF in an ethanol solution system at room temperature [Figure 7J-L]^[144]. The material exhibited excellent OER activity and stability under alkaline conditions.

Pyrolysis treatment

Post-synthetic thermal treatment is a rapidly modifying method for changing the structure and properties of metals and polymers, and an efficient alternative method for creating defects in MOFs. The linker defects or cluster defects may be created through the control of the temperature and heating time of the decomposition of specific organic groups during the calcination process. In 2014, Gadipelli et al. proposed a facile and versatile method to promote the decomposition of carboxylate BDC (BDC = benzene dicarboxylate, -OOC-C₆H₄-COO-) by thermal annealing of the MOF post-synthesis below the framework decomposition temperature, resulting in the formation of defects in MOF-5 [Figure 8A]^[135]. The results showed that the annealed MOF-5 exhibited excellent CO₂ adsorption capacity compared to the untreated MOF-5 with a maximum uptake of 2 mmol·g^-1 at the same conditions of 25 °C and 1 bar. Nanoparticle@MOFs composites have attracted much attention from researchers for their excellent selective catalytic performance. However, there is an urgent need to investigate a simple method to solve the balance between catalytic efficiency and selectivity. For instance, Meng et al. proposed to successfully prepare Pt@UiO-66-NH₂, Pt@UiO-66, Pt@ZIF-8, and Au@ZIF-8 with hierarchical porosity by taking advantage of the thermal instability of the inherent defects of the encapsulated nanoparticles and selecting a suitable temperature annealing [Figure 8B]^[145]. where the position and dispersion of mesopores can be located and controlled, respectively. Unexpectedly, the resulting Pt@UiO-66-NH₂ exhibited not only enhanced catalytic efficiency but also improved catalytic rate and selectivity. In addition, this approach can also be extended to be applied to other MOFs and their derivatives for the production of hierarchical porous structures due to the prevalence of structural defects in MOFs. Recently, Jia et al. have proposed an air-heat treatment strategy to investigate with the assistance of high enthalpy pressurized air to remove residual molecules and incompletely coordinated linkers from MOFs in order to maximize the porosity of the obtained MOFs^[146]. [Figure 8C] The obtained MOFs with hierarchical micro/mesoporous structures exhibited excellent adsorption capacity and selectivity, such as CuBTC-A (BTC = 1,3,5-benzenetricarboxylic acid) obtained after thermal treatment of CuBTC improved CO₂ uptake and CO₂/N₂ selectivity by 22% and 86%, respectively.

Figure 8. (A) Schematic illustration of framework formation defect due to MOF-5 annealing. Reproduced with permission^[135]. Copyright 2014, American Chemical Society. (B) Schematic illustration of the synthesis of hierarchically porous Pt@MOFs using inherent defects. Reproduced with permission^[145]. Copyright 2018, Wiley-VCH. (C) Schematic of air-thermal processing strategy. Reproduced with permission^[146]. Copyright 2020, The Royal Society of Chemistry. (D) Schematic illustration for the structural evolution of MIL-101 under thermal treatment, (E) TEM image of Uio-66, (F) TEM image of Uio-67. Reproduced with permission^[136]. Copyright 2020, Wiley-VCH.

Moreover, post-synthesis thermal treatments are becoming increasingly popular among researchers as a common approach to optimize the structure and improve the performance of MOFs, and numerous MOF-derived materials were designed and synthesized. However, the wide selection of extreme thermo-treatment conditions (typically above 500 °C) not only does not improve the performance of MOFs, but also leads to the degradation of intrinsic properties. In 2020, Pan et al. calcined MIL-101(Cr) in the range of 200-280 °C and investigated the effect of the thermal treatment at different temperatures on MOFs and revealed that the thermal treatment could introduce unsaturated CrO_x clusters and generate partial deligandation [Figure 8D-F]^[136]. Calcinating MIL-101(Cr) at 280 °C created defects as active catalytic sites that improved the oxidation activity of styrene, which was ten folds more active than that of the original MIL-101.

Mechanical treatment

Mechanical treatment is a physical means to break the coordination bonds in the structure of MOFs, resulting in structural defects in the framework. In 2016, Bennett et al. ball-milled MOFs to break the metal-ligand bonds in parts of the framework of MOFs such as UiO-66, MIL-140B and MIL-140C^[147]. DFT calculations confirm the existence of defects in the MIL-140B framework. At the same time, the mechanical treatment strategy is also applicable to ZIF-8 and MIL-53^[137,148]. Recently, Steenhaut et al. treated HKUST-1 with ball milling and found that the number of defects was related to the duration of milling, and CO₂ adsorption experiments showed that the adsorption capacity of HKUST-1 was mainly dependent on the type and number of defects in MOFs rather than their surface area^[138]. Although mechanical treatment is much simpler compared with other post-synthetic modification processes and can control the number of defects formed, it is still difficult to control the type of defects introduced, so mechanical treatment can be combined with other post-synthetic modification methods to jointly regulate and thus synthesize MOFs with excellent properties.

Photocatalysis

Defects in MOFs and MOF derivatives can improve the photocatalytic ability of the materials mainly through two aspects: inhibition of electron-hole complexation and optimization of the energy band structure^[160,161]. Additionally, two parts of the energy barrier are involved in the photocatalytic process: the absorption of light by the ligand (E_abs) and the energy required to transfer photogenerated electrons to the unoccupied d orbitals of the metal (E_LMCT). The absence of ligands or clusters exposes varying degrees of metal sites, changing the reaction energy barrier and affecting photocatalytic performance^[162].

The generation of structural defects is often accompanied by a series of changes such as chemical bond breaks and reorganization, lattice distortion, and electron localization, which in turn have a very important impact on the physicochemical properties of the material^[163]. As the investigation progressed, researchers discovered that structural defects can in many cases enhance the properties of materials and even give them novel functions. Therefore, how to optimize the performance by creating structural defects in a rational way has received much attention from researchers in recent years. In 2019, Ma et al. used a zirconium-based MOF with excellent stability and easily tunable structure, UiO-66-NH₂, as a model for their study^[89]. The degree of linker defects in the structure was tuned by varying the amount of modulator acetic acid during the synthesis process to obtain UiO-66-NH₂-X with a progressively increasing degree of defects, and Pt@UiO-66-NH₂-X was further obtained by introducing co-catalyst Pt nanoparticles into the MOFs pores [Figure 9A]. More interestingly, when the introduction of the modulator in the synthesis reaches a certain level, further increase in its content leads to a further increase in the degree of defects and the photocatalytic hydrogen production activity does not continue to increase but appears to drop. The structural defects and photocatalytic performance showed a very interesting volcano-type variation trend overall [Figure 9B and C]. At the same time, the MOFs have good photocatalytic cycling stability. The results suggest that the presence of MOF structural defects can modulate the photocatalytic performance of the material, and that creating the right number of structural defects is essential for optimizing the performance. However, the investigation of how the defect type and the degree of ligand vacancies in UiO-66 tune the photocatalytic performance have not been found. Wang et al. prepared a series of UiO-66-NH₂ with different defect types and various vacancies degree by altering dicarboxylate ligands [Figure 9D], and used them to study their photocatalytic CO₂ reduction performance^[162]. The experimental results showed that the ligand-defective UiO-66-NH₂-ML-100 exhibited optimal photocatalytic activity with a maximum photocatalytic CO yield of 21.3 μmol·g^-1·h^-1, which was 2.2 times higher than that of the cluster defective UiO-66-NH₂-MC-150 [Figure 9E and F]. DFT calculations show that the introduction of ligand defects produces defective states dominated by ligand unsaturated Zr clusters, decreasing the E_LMCT, and electrons are easily excited from the ligands to the unoccupied d orbitals of the metal Zr atoms, improving the photocatalytic efficiency. The cluster defective MOFs, on the other hand, reduce the light absorption capacity (increases E_abs) and weaken the photocatalytic activity due to the lack of metal clusters and surrounding connected ligands.

Figure 9. (A) Photocatalytic hydrogen production over Pt@UiO-66-NH₂-X with structural defects, (B) Trends in volcanic type activity of Pt@UiO-66-NH₂-X with different degrees of defects, (C) 10-round activity cycle study of Pt@UiO-66-NH₂-100. Reproduced with permission^[89]. Copyright 2019, Wiley-VCH. (D) Application of UiO-66 with different ligand vacancy degrees and defect types for photocatalytic reduction of CO₂, (E) Photoreduction of CO₂ to CO, (F) recycling performance. Reproduced with permission^[162]. Copyright 2022, The Royal Society of Chemistry. (G and H) TEM images of MIL-53, (I and J) TEM images of D-1, (K) TEM images of D-2. Reproduced with permission^[164]. Copyright 2019, Elsevier.

Moreover, introducing defects into MOFs can promote their electron-hole separation and thus improve the photocatalytic activity of the materials. Jiang et al. selected HCl as the modulator to synthesize the defective MIL-53 (TEM images are shown in Figure 9G-K) with increased specific surface area and easier electron-hole separation^[164]. More unexpectedly, when the tetracycline concentration was 10 mg·L^-1, the photocatalytic degradation efficiency of defective MIL-53 for tetracycline could reach more than 95%, and the photocatalytic activity was 1.5 times higher than that of the intact MIL-53. However, the design of catalysts and reaction interfaces for directly converting permanent gases (such as CO₂, O₂, and N₂) into liquid fuels is still extremely challenging. Recently, Hao et al. proposed that gas-permeable MOF membranes can facilitate the diffusion, activation, and reduction of gas molecules (e.g., CO₂, O₂) through modification of the electronic structure and catalytic properties of metal single atoms^[165]. Under visible light irradiation conditions, the MOF NH₂-UiO-66 containing specific defects with Ir single atom as the active center reduces CO₂ to HCOOH at the three-phase reaction interface at 420 nm with an apparent quantum efficiency of 2.51%.

Electrocatalysis

MOFs and their derivatives are regarded as one of the most promising electrocatalytic materials because of their various active sites and easy modification^[7]. However, the low electrocatalytic activity and poor electrochemical performance of most MOFs hinder the electrochemical reaction process. Therefore, enhancing active sites in MOFs and their derivatives is important for designing and developing novel and high-efficiency electrocatalysts^[166,167]. In recent years, researchers have found that defective engineering can effectively increase the active sites and accelerate the catalytic process, such as OER, HER, ORR, and CO₂RR.

OER: The OER has a vital role to play in many energy conversion technologies. However, the kinetic process of OER is slow, the efficiency of the reaction remains very low even with the help of noble metal catalysts, and the high cost and scarcity of noble metal catalysts hinder its large-scale application, and it is challenging to develop non-precious metal OER electrocatalysts with high catalytic activity and stability^[168,169].

The MOF materials with isolated active sites and large specific surface areas are candidates for high-efficiency catalysts^[170,171]. In 2018, Xu et al. cleverly prepared hollow structured CoS nanocages using metal organic frameworks (MOFs) as self-sacrificing templates, and then grew CeO_x nanoparticles in situ on the CoS surface to achieve precise tuning of the Co²⁺/Co³⁺ ratio and the generation of active defect sites [Figure 10A], thus significantly facilitating the water oxidation process^[172]. Meanwhile, the protective layer of CeO_x formed on the hollow structured CoS surface can effectively prevent the oxidation of CoS and the corrosion loss of the active component elements; thus, the catalyst also exhibits excellent stability [Figure 10B-G]. The method also provides a novel idea to explore for the synthesis of efficient and stable non-noble gold-based electrocatalyst materials. Up to now, the methods available to improve the performance of MOFs-based electrocatalysts are mainly divided into two types: direct synthesis and post-treatment. Among them, post-treatment mainly introduces defects in the structure of MOFs in a targeted manner, which leads to the appearance of electron localization, lattice distortion and chemical bond breakage, thus increasing the active sites and improving the local properties. Therefore, Zou et al. selected CoCu-MOF as the target of their study^[173]. Firstly, the CoCu-MOF was thermally treated to eliminate some of the functional groups to form defects while retaining the original framework structure of CoCu-MOF. Secondly, the defective CoCu-MOF was phosphorylated to form a highly active CoP/MOF, and the electrocatalyst shows a low overpotential of 295 mV at 10 mA·cm^-2 and long-term OER stability, which are mainly attributed to the enlarged electrochemical active surface area and accelerated charge-transfer rate in this unique structure.

Figure 10. (A) Design synthesis strategy of CeO_x/CoS, (B and C) SEM and TEM images of CoS, (D-G) SEM and TEM images of CeO_x/CoS. Reproduced with permission^[172]. Copyright 2018, Wiley-VCH. (H) Schematic diagram of Ni-MOF@Pt catalyst synthesis, (I) Performance of Ni-MOF@Pt catalyst HER. Reproduced with permission^[179]. Copyright 2019, American Chemical Society. (J) The process for the preparation of porous CoFeNi-LDH catalyst, (K) SEM images of p-Co_0.34Fe_0.33Ni_0.33-LDH, (L) TEM images of p-Co_0.34Fe_0.33Ni_0.33-LDH, (M) HR-TEM images of p-Co_0.34Fe_0.33Ni_0.33-LDH, (N) SEM images of h-Co_0.34Fe_0.33Ni_0.33-LDH, (O) TEM images of h-Co_0.34Fe_0.33Ni_0.33-LDH, (P) HR-TEM images of h-Co_0.34Fe_0.33Ni_0.33-LDH. Reproduced with permission^[181]. Copyright 2020, Wiley-VCH.

The activity of MOFs in electrocatalytic oxygen evolution reactions is currently receiving increasing attention. In addition to its high catalytic performance, the structure-activity relationship (especially the active site) is still poorly understood. Zhao et al. reported that the Ni_0.5Co_0.5-MOF-74 catalyst was characterized and analyzed during the OER reaction^[174]. The study found that the Ni_0.5Co_0.5-MOF-74 material creates higher defects and higher chemical valence in situ during the OER process, which is responsible for the higher OER performance of the catalyst. When the ratio of Ni and Co in the material is altered, the in situ generated active species in the catalyst changes in composition structure and electronic structure so that the catalytic activity is modulated. In addition, noble metals, which are expensive and less stable, have been used as catalysts for OER due to their high catalytic activity. To replace the noble metal catalyst, 3d transition metal-based materials, which are relatively inexpensive and have adequate catalytic activity, have been widely studied as promising candidates for OER catalysts. However, their moderate OER performance presents an obstacle to practical application. The introduction of oxygen vacancies is an effective way to tune the various properties of catalysts, which, in turn, can enhance the catalytic activity. Recently, Kang et al. reported a simple preparation strategy from MOFs to Co(OH)₂-Co₃O₄ hybrid catalysts with oxygen vacancies^[175]. They found that the Co-based catalysts exhibited different morphology, composition and oxygen vacancy ratio to achieve higher activation catalytic performance by controlling the dosage of hexamethylenetetramine (HMT). As the results show, the heterogeneous Co(OH)₂-Co₃O₄ catalyst has excellent activity, with significantly higher OER performance at a current density of 10 mA·cm^-2 and low overpotential of 280 mV. This strategy also expands new methods for preparing high oxygen vacancy ratio catalysts from MOFs.

HER: HER catalysis is a significant step in the development of hydrogen energy, and worthwhile results have been achieved for strategies such as semiconductor-based photocatalysis, photocatalysis, and metal-based electrocatalysis^[176-178]. However, it is still important to explore high-efficiency HER catalysts with high abundance and low price.

In recent years, MOF derivatives have been widely concerned by researchers for electrochemical applications due to their abundant defects, many active sites and diverse structures. For example, in 2019, Rui et al. reported on an efficient strategy for the rapid construction of noble metal/2D MOF heterostructures^[179]. The method involves in situ growth of noble metal nanoparticles on the defect-rich surfaces of ultrathin MOF nanosheets [Figure 10H]. The 2D Ni-MOF@Pt composite catalysts with well-defined interfaces can be used for electrochemical HER. The Ni-MOF@Pt current densities of 241 mA·mg^-1 at -100 mV in 1 M KOH and 126 mA·mg^-1 at -35 mV in 0.5 M H₂SO₄ outperformed the pure Pt NPs (146 and 54 mA·mg^-1) [Figure 10I]. It indicates that Ni-MOF@Pt has excellent catalytic activity under acid-base conditions.

Layered double hydroxide (LDH) is an excellent electrocatalyst for OER. However, its application in electrocatalytic hydrolysis is limited by the sluggish kinetics of the Volmer step of the HER in an alkaline medium. Currently, some researchers have successfully reduced the hydrolysis barrier by introducing Ru/Ir into LDH, thus achieving better HER performance^[180]; however, the introduction of noble metals will undoubtedly increase the manufacturing cost of the catalyst. In 2020, Sun et al. used a simple strategy using MOF as a precursor for in situ topological structure transformation to prepare holey ternary CoFeNi-LDH catalysts on the surface of nickel foam [Figure 10J-P]^[181]. Such polygonal pores expose atomistic edge steps and lattice defects, forming a large number of unsaturated coordination metal centers, which, together with the modulation of the metal valence state by Fe, synergistically promote the hydrolysis kinetics of HER, ultimately obtaining excellent electrolytic water properties. The optimized catalysts exhibited overpotentials of 71 mV and 195 mV for HER and OER, respectively, which was superior to the Pt/C-RuO₂ combination. It also has excellent stability and can operate continuously for more than 25 h at a current density of 300 mA·cm^-2. In addition, the ratio of different doping metals has a tremendous effect on the performance of the catalyst. Gopi et al. designed and synthesized Ni/Fe bimetallic catalysts with various molar ratios. It was demonstrated that when the molar ratio of Ni and Fe was 0.06:0.06, the catalysts exhibited excellent catalytic performance for both OER and HER in acidic and alkaline mediums^[182]. In addition, vanadium doping effectively improved the OER and HER properties of Ni-Fe MOF/GO composites, while in vanadium doping, the defects on the MOF/GO surface contributed significantly to the corrosion potential of V-Ni_xFe_y-MOF/GO electrocatalysts. The electrocatalytic properties of typical defective MOFs and derivatives are summarized in Table 3.

ORR: The development of green and sustainable next-generation energy conversion technologies will have great significance in alleviating the energy crisis from environmental pollution. However, its core reaction ORR involves multi-step proton-electron coupling, and its reaction kinetics are very sluggish, which severely restricts the development of technologies such as fuel cells^[183-185]. Currently, noble metal Pt-based catalysts exhibit the optimal ORR catalytic activity, but their scarcity in abundance and high prices make it difficult to spread their popularity^[186,187]. Therefore, it is a significant challenge to develop novel ORR catalysts that are low-cost, environmentally friendly and high efficiency.

Single-atom catalysts have shown larger potential for application in many catalytic reaction systems due to their unsaturated coordination environment, highest atomic utilization, and unique electronic structure. In particular, the transition metal monoatomic catalysts loaded on carbon carriers have outstanding catalytic performance. In 2021, Yuan et al. proposed a strategy for constructing defect-rich MOF-derived single-atom catalysts by decarboxylation-induced effects for enhancing the electrocatalytic activity of transition metal Co-N₄ single-atom sites [Figure 11A]^[188]. The Zn-MOF (DMOF), which is rich in both carboxyl functional groups and nitrogen, was firstly constructed by the “mixed linker approach”, and then the single-atom derivatization catalyst (Co@DMOF-900) was obtained by the gas-phase migration method using DMOF as the precursor and CoCl₂ as the Co source, realizing the simultaneous in situ construction of Co-N₄ single-atom sites and carbon framework defects. It is further demonstrated by a series of characterization techniques and theoretical calculation methods that carbon defects induced by decarboxylation can effectively enhance the activity of metal single-atom sites in the electrocatalytic ORR process. This is mainly attributed to the increased activity of the Co-N₄ active site in Co@DMOF catalysts with defect-rich carbon structure, with conversion frequency TOF and mass current density j_m up to 2.015 e·s^-1·site^-1 and 3.3 A·mg^-1, respectively. In addition, defect-rich carbon materials are considered to be a class of highly active catalytic reaction sites with electrocatalytic properties comparable even to those of noble metal electrocatalysts. Recently, Wu et al. reported the use of Zn-MOF (ZIF-7) as a specific precursor to control the removal and reconstitution of carbon atoms through thermal redox reactions in the restricted ZIF-7 carbon cavity with ZnO quantum dots and the generation of CO₂ gas [Figure 11B]^[189]. The resulting porous carbon contains ultra-high density carbon defects (HDPC) with densities up to 2.46 × 10¹³ cm^-2 (The structure is characterized as shown in Figure 11C-E). When applied to oxygen reduction reactions, the porous carbon has excellent catalytic properties in both alkaline and acidic medium, with half-wave potentials of 0.90 or 0.75 V at 0.1 M KOH or HClO₄, respectively. Furthermore, the normalized specific activity and density functional theory calculation reveal a gradient “proximity effect” between carbon defects with different spatial distance, indicating that the quantitative control of carbon defect density is the key to enhancing electrocatalytic activity.

Figure 11. (A) Schematic illustration of MOFs decarboxylation-induced effect to construct defect-enhanced single-atom catalysts. Reproduced with permission^[188]. Copyright 2021, Wiley-VCH. (B) Schematic illustration of the preparation of porous carbon with high defect density, (C) TEM image of ZnO@ZIF-7, (D and E) HRTEM image of ZnO quantum dots encapsulated by ZIF-7. Reproduced with permission^[189]. Copyright 2022, Elsevier. (F) Schematic illustration of the preparation of CoFe/N CNFs, (G and H) SEM image (I) TEM image, (J) HAADF-STEM image, (K) SAED pattern, (L) EDS mapping of CoFe/N CNFs. Reproduced with permission^[190]. Copyright 2022, Springer. (M) Schematic illustration of the synthesis process of SA-Fe-NGM. Reproduced with permission^[191]. Copyright 2022, American Chemical Society.

Moreover, Chen et al. developed a novel fabrication method to construct 3D unique defect-rich electrocatalysts in which irregular hollow CoFe nanospheres are embedded in N-doped carbon nanofibers (CoFe/N CNF) [Figure 11F], the morphology, elemental and structural information of CoFe/N CNFs are shown in Figure 11G-L^[190]. The diameter of the optimized CoFe-ZIF is about 25 nm, which provides enriched active sites. The defect-rich carbon and irregular hollow CoFe nanospheres exhibited superior catalytic activity under 10 mA·cm^-2 in alkaline media with a potential difference of only 0.67 V between OER potential and ORR half-wave potential, which is comparable to commercial catalysts with 20% Pt/C and RuO₂ blends. In addition, MOFs are regarded as ideal precursors for the synthesis of Fe single-atom catalysts. However, most MOF-derived catalysts have 3D morphology, resulting in numerous Fe-N_x catalytic active sites buried inside the catalyst particles, which cannot directly participate in the reaction. Therefore, it is necessary to construct catalyst morphologies with high exposure of Fe-N_x active sites and combine them with mesoporous channels to promote the utilization of active sites. Li et al. have adopted an exfoliation strategy to firstly exfoliate Zn ZIF-containing nanosheets into ultrathin two-dimensional graphene nanonetworks (NGM) employing KCl and LiCl as exfoliating as well as etching agents, and secondly obtain hierarchical porous structures with desired pore sizes by regulating the ratio of LiCl to KCl, and eventually selected as a precursor for the formation of Fe-N_x active sites by high-temperature pyrolysis of FePc to obtain the Fe-single-atom catalysts (SA-Fe-NGM) [Figure 11M]^[191]. Furthermore, The SA-Fe-NGM is found to deliver a superior oxygen reduction reaction (ORR) activity in acidic media (half-wave potential = 0.83 V vs. RHE) and a high-power density of 634 mW·cm^-2 in the H₂/O₂ fuel cell test.

CO₂RR: The appropriate number of defects can modify the local properties of a solid material and enhance its performance. Therefore, regulating the electronic and geometric structure of MOF catalysts by defect engineering to improve electrocatalytic CO₂ reduction activity is a very effective strategy.

Heteroatom-doped materials are widely used in electrocatalytic CO₂ reduction by modulating the absorption/desorption of reaction intermediates through the electronic properties and chemical reactivity of carbon materials. For example, recently, Ling et al. developed a K⁺-assisted synthesis strategy that utilizes freely movable dimethylammonium cations (MeNH²⁺) within the one-dimensional pores of the electronegative framework material bio-MOF-1 to replace MeNH²⁺ within the pores with K⁺via an ion-exchange strategy to obtain K⁺@bio-MOF-1 materials^[192]. The final defective carbon material K-defect-C-1100 with a single carbon defect center-vacancy type V₁₂ (missing 12 near-neighboring carbon atoms at the defect) was obtained by high-temperature pyrolysis of K⁺@bio-MOF-1 material [Figure 12A]. Unexpectedly, K-defect-C generated CO by CO₂ electroreduction at -0.45 V with a Faraday conversion efficiency of 99% [Figure 12B and C], far exceeding that of MOF-derived carbon without K⁺ etching. In addition, single-atom sites are also considered promising catalytic modulators. However, in electrochemical CO₂RR reaction systems, the introduction of single atoms triggers a competitive HER, which leads to a decrease in CO₂RR product selectivity. Hu et al. proposed a bimetallic single-atom catalyst strategy consisting of transition metal and main group metal sites. The mesoporous N-doped carbon-anchored Cu/In bimetallic site catalysts (Cu-In-NC) were prepared by pyrolysis of Cu and In co-doped ZIF-8 (Cu-In-ZIF-8) under argon atmosphere [Figure 12D]^[193]. During thermal conversion, a large number of defective sites on Cu-In-ZIF-8 further expand to form mesopores [Figure 12E and F]. The formation of mesopores can improve the catalyst mass transfer efficiency and thus enhance the accessibility of the active center. In CO₂RR, the HER inert in site substantially enhances the performance of the Cu site for CO₂ reduction. The CO faradic efficiency FE CO of Cu-In-NC reaches 96% at -0.7 V, which is significantly better than that of Cu-NC (55% for FE CO) and In-NC (54% for FE CO).

Figure 12. (A) Schematic illustration of K⁺-assisted construction of intrinsically defective carbon material, (B) LSV curves, (C) FEs of CO at different applied potentials. Reproduced with permission^[192]. Copyright 2022, Wiley-VCH. (D) Cu-In-NC preparation schematic, (E) SEM image of Cu-In-ZIF-8, (F) TEM image of Cu-In-ZIF-8. Reproduced with permission^[193]. Copyright 2022, Wiley-VCH. (G) SEM image of Cu-foam electrode, (H and I) SEM images of Cu₂(L)-e/Cu. Reproduced with permission^[194]. Copyright 2020, Springer Nature.

Moreover, MOF compounds with infinite metal sites and pore structures have promising potential in the field of electrochemical CO₂ reduction. The metal centers can be used as active catalytic sites, and the porosity facilitates the adsorption of CO₂. In 2020, Kang et al. invented the method of modifying Cu-MOF with high defective sites on Cu foam [Figure 12G]^[194]. By selecting ligands with a high spatial site resistance, the Cu-MOF with more defects on the surface of Cu foam was rapidly modified using electrochemical growth methods. The EPR analysis shows that the Cu-MOF obtained by electrochemical growth has 15.3% Cu²⁺ defect sites, while the Cu-MOF obtained by the conventional thermal synthesis method contains only 1.5% Cu²⁺ defect sites [Figure 12H and I]. Meanwhile, the Cu-MOF/Cu electrode in acetonitrile electrolyte produced a current density of 65.8 mA·cm^-2 at -1.8 V vs. Ag/Ag⁺ with a formic acid Faraday efficiency of 90.5%. Compared with the single metal catalyst, the performance improvement of bimetallic catalysts is mainly attributed to the multiple factors among structure, interface, and various synergistic effects resulting from the coupling of two metals. For the rational design of Cu-based bimetallic catalysts and understand the promotion mechanism between Cu-based bimetallic catalysts on CO₂ reduction. In 2019, Albo et al. reported the performance of Cu(II) and Bi(III)-based metal-organic framework (HKUST-1 and CAU-17, respectively) blends into the electroreduction of CO₂ to alcohols in a filter-press electrochemical cell^[195]. The results revealed a synergic effect of Cu and Bi-based MOFs, associated with a favored interplay between the actives sites and reaction intermediates, prompting methanol formation and C-C coupling reaction to ethanol. The optimum values for a more selective CO₂ reduction to alcohols can be set at a Bi content of 12% and a acurrent density of j = 20 mA·cm^-2. Similarly, Albo et al. synthesized four MOPMs (HKUST-1, CuAdeAce, CuDTA and CuZnDTA) containing Cu as gas diffusion electrodes^[196]. The electrodes show relatively high surface areas, accessibilities, and exposure of the Cu catalytic centers as well as favorable electrocatalytic CO₂ reduction performance. The maximum cumulative Faradaic efficiencies for CO₂ conversion at HKUST-1, CuAdeAce, CuDTA, and CuZnDTA based electrodes are 15.9, 1.2, 6, and 9.9%, respectively, at a current density of 10 mA·cm^-2, an electrolyte-flow/area ratio of 3 mL·min·cm^-2, and a gas-flow/area ratio of 20 mL·min·cm^-2.

Others

It has long been important that defects in crystalline materials are controlled as they are related to the properties of the material. Ye et al. have synthesized defective UiO-66, which has shown excellent catalytic activity in the oxidative desulfurization reactions of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT)^[197]. They synthesized UiO-66-free and UiO-66-solvent with 2.16 and 1.53 linkers missing per unit cell, respectively, where the higher degree of defective UiO-66-free could improve the removal of DBT up to 99.6%, while the corresponding value of UiO-66-solvent was 80.5%. More surprisingly, the difference between UiO-66-free and UiO-66-solvent for 4,6-DMDBT removal was even greater, 98.1% and 52.5%, respectively. Recently, Xu et al. used defect design strategies to introduce two types of defects into the MIL-100-Cr framework^[198]. Meanwhile, Ru NPs with defective sites anchoring the restricted domains were used to achieve the Ru-loaded DEMOF catalysts (Ru@DEMOF), and the obtained Ru@DEMOF catalysts simultaneously enhanced the activity and cycling stability of the reaction for the selective hydrogenation of glucose to prepare sorbitol.

Lithium-ion batteries

With the benefits of superior energy density and lower environmental pollution, LIBs stand out among many energy storage devices and have received more and more attention from researchers^[207-210]. Further improvement of electrochemical energy storage in LIBs is urgent due to the limited conductivity and theoretical capacity of conventional electrode materials^[211,212]. However, the simplest way to solve this problem is to synthesize new electrode materials. In recent years, the MOFs aroused a lot of attention due to their unique structures. In particular, the introduction of the defect in the MOF structure will maximize the exposure of the metal active site, which has many advantages as an electrode material^[213].

Recently, there has been an increasing interest in plasma technology for tuning the properties of electrode materials. Many beneficial effects can be created through strong interactions among charged particles on the electrode materials. For example, in 2021, Sun et al. proposed the strategy of combining MOF-derived nano/micro structure and plasma fabrication to prepare TiO₂/C nanocomposites with high defect concentrations [Figure 13A-D]^[214]. The composite exhibits excellent electrochemical properties with a reversible capacity of 316.9 mAh·g^-1 at 0.5 A·g^-1 and possesses a high-rate capacity of 186.1 mAh·g^-1 at 10 A·g^-1. The reason can be attributed to the fact that the energetic ion bombardment not only increases the specific surface area and active sites of the material, but also introduces a high concentration of defects. More importantly, the introduction of oxygen vacancies, Ti³⁺ ion and lattice distortion can improve the intrinsic conductivities, which is also verified by density functional theory calculations as well as the resistivity and GITT measurements. In addition to composite materials, MOF-derived materials are also used in lithium-ion battery electrode materials. Recently, Lin et al. used different Zn-Mn based MOFs as precursors to construct anode materials of oxygen vacancy-rich ZnMn₂O₄ and obtained excellent specific capacity and rate performance, effectively improving the electrochemical performance of ZnMn₂O₄ from the atomic level^[215]. The constant current charge/discharge test at 0.1 A·g^-1 confirmed the excellent specific capacities of BDC-O_V-ZMO [Figure 13E and F], BTC- O_V-ZMO and IN- O_V-ZMO (initial charge/discharge capacities of 637.5/993 mAh·g^-1, 836.9/1313.6 mAh·g^-1 and 479.9/750.8 mAh·g^-1, respectively). In addition, DFT calculations can further explain the effect of abundant oxygen vacancies on the lithium storage capacity of ZMO anode materials [Figure 13G and H] mechanistically.

Figure 13. (A) Schematic illustration of the preparation of plasma TiO₂/C, SEM images of TiO₂/C with different plasma power; (B) TM-0; (C) TM-90; (D) TM-150. Reproduced with permission^[214]. Copyright 2021, Elsevier. (E and F) SEM images of BDC-O_V-ZMO, TDOS and PDOS profiles of (G) ZMO and (H) O_V-ZMO. Reproduced with permission^[215]. Copyright 2022, Elsevier.

Zinc-ion batteries

In recent years, the aqueous ZIBs have received a lot of attention from researchers because of their environmental friendliness, safety and economic advantages^[216,217]. However, ZIBs still face some problems, such as the dissolution and collapse of electrode materials leading to poor full cell cycling performance; the strong electrostatic attraction between divalent Zn ions and host atoms leading to poor rate performance of the full cell^[218]. Therefore, it is significant to find suitable electrode materials for developing high-performance ZIBs.

Recently, Li et al. proposed the smart artificial SEI by calcination of MOF precursors to derive porous CeO₂ with abundant defects (MOF-CeO₂@Zn) as zinc anodes to enhance the cycle life of batteries [Figure 14A]^[219]. The crystal defects in MOF-CeO₂ can provide abundant active sites for the transfer of Zn ions, which can accelerate the transport of Zn ions [Figure 14B-E]. At the same time, this porous structure balances the ion flux and combines with an inhibition of the solvation effect, leading to a uniform deposition of Zn and suppressing the formation of Zn dendrites. As a result, the MOF-CeO₂@Zn anode exhibits an ultra-stable plating/stripping behavior (over 3,200 h) and high coulometric efficiency (99.5% coulometric efficiency at 2 mA·cm^-2). In addition, this multifunctional protective layer improves significantly the overall performance of the Zn||MnVO full cell, which still has a specific capacity of 163 mAh·g^-1 after 10,000 cycles at 5 A·g^-1. Leng et al. were the first to design and synthesize oxygen-rich defective MOF-derived MnV₂O₄/C materials to be used in aqueous zinc ion batteries^[220]. They selected two MOF precursors, [Mn(phen)H₂O][V₂O₆] and [Mn(bpy) V₂O₆]-1.16H₂O, to investigate the effects of the type of Mn-V MOF precursor and calcination temperature on the electrochemical behavior of the resulting MnV₂O₄/C composites [Figure 14F and G]. The optimal sample MnV₂O₄(p)/C-700 exhibited a capacity retention rate of 94.3% after 1000 charges/discharges at 20 A·g^-1 [Figure 14H]. Ex situ X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectra, as well as elemental mappings and X-ray photoelectron spectroscopy of MnV₂O₄(p)/C-700 discern the partial phase transformation mechanism of MnV₂O₄→Zn₃(OH)₂V₂O₇(H₂O)₂ during discharge/charge process. Sun et al. adopted a similar method to prepare a MnO/C hybrid material derived from Mn-BTC and rich in oxygen vacancies [Figure 14I and J]^[221]. The effect of air and argon on the formation of oxygen vacancies was investigated. The incorporation of oxygen vacancies can tune the electronic rearrangement and crystal microstructure via the different calcination atmospheres of air (Mn₂O₃) and Ar (MnO/C). Furthermore, the oxygen vacancy can effectively promote the reaction kinetics and electrochemical performance of manganese-based oxide hybrids. Meanwhile, the oxygen vacancy-enriched MnO/C, with a specific capacity of 336.8 mAh·g^-1 at 0.1 A·g^-1, exhibits excellent stability in long-cycle tests with 73.8% capacity retention over up to 10,000 cycles [Figure 14K].

Figure 14. (A) Schematic illustration of the synthesis of MOF-CeO₂ and the foil coating process in Zn, (B) SEM images of MOF-CeO₂/C, (C) SEM images of MOF-CeO₂ (D) SEAD analysis of MOF-CeO₂/C, (E) SEAD analysis of MOF-CeO₂. Reproduced with permission^[219]. Copyright 2023, Elsevier. (F) SEM images of MnV₂O₄(p)-700, (G) SEM images of MnV₂O₄(p)/C-700, (H) long-term cycle life of MnV₂O₄(p)/C-700. Reproduced with permission^[220]. Copyright 2021, Wiley-VCH. (I) SEM images of Mn-BTC, (J) SEM images of 700-Air, (K) long-term cycling performance of 700-Ar. Reproduced with permission^[221]. Copyright 2022, Elsevier.

Lithium-sulfur batteries

Lithium-sulfur batteries (Li-S) have been considered as one of the most promising devices for energy storage attributed to their ultra-high energy density (2,567 Wh·kg^-1) and theoretical capacity (1,675 mAh·kg^-1)^[222-224]. However, the sluggish reaction kinetics and shuttle effect of the reaction intermediate lithium polysulfide have hindered the relevance of Li-S for practical applications^[225,226]. The preparation of anode carrier materials that can anchor lithium polysulfide is the simplest and most effective main method to solve the shuttle problem. Meanwhile, defect engineering is widely used to synthesis functional materials with electron-rich active sites^[227,228]. It is reported that many defect-engineered materials are superior to defect-free materials in accelerating polysulfide conversion. Therefore, it is significant to apply defect engineering to the conversion of lithium polysulfides to enhance the performance of Li-S.

In order to demonstrate that defects in the cathode host material of Li-S can be effective in their performance. In 2020, He et al. reported a study of the oxygen-deficient sulfur-containing material TiO₂(D-TiO₂) using DFT calculations^[229]. Based on the results of DFT calculations, the composite structure of 3D graphene aerogel containing oxygen defective TiO₂ (D-TiO₂@Gr) as an anode carrier was reconstructed. Electrochemical performance tests showed that the 3D graphene aerogel (D-TiO₂@Gr) composite of D-TiO₂ was proved to be an excellent sulfur host and the specific discharge capacity was 1,049.3 mAh·g^-1 with a sulfur loading of 3.2 mg·cm^-2 after 100 cycles at 1C. Even with the sulfur mass loading increasing to 13.7 mg·cm^-2, an impressive stable cycling is obtained with an initial areal capacity of 14.6 mAh·cm^-2, confirming the effective enhancement of electrochemical performance by the oxygen defects. Li et al. proposed the construction of defective UiO-66-NH₂-4/graphene electrocatalytic membrane (D-UiO-66-NH₂-4/G EM) [Figure 15A] by defect engineering strategy to achieve the excellent electrochemical performance of Li-S under the harsh conditions of high sulfur loading and low electrolyte/sulfur ratio^[230]. Their research revealed that the D-UiO-66-NH₂-4 framework effectively suppresses the shuttle effect through Lewis acid-base interaction between UiO(Zr) and polysulfides, provides sufficient sites for capturing polysulfides through strong chemical affinity interactions, and provides active sites for efficiently converting polysulfides with effective electrocatalytic activity. Most importantly, a Li-S battery with such an electrocatalytic membrane delivers a high capacity of 12.3 mAh·cm^-2 (1,013 mAh·g^-1) at a sulfur loading up to 12.2 mg·S·cm^-2 under a lean electrolyte condition (E/S = 5 μL·mg^-1-sulfur) at 2.1 mA·cm^-2 (0.1 C). In 2021, Zhang et al. synthesized amorphous MIL-88B (aMIL-88B) by a simple ligand competition method^[231] [Figure 15B]. SEM images revealed that the surface of aMIL-88B was rougher compared to that of cMIL-88B and was also accompanied by the appearance of some cracks, which could be confirmed by TEM images. Meanwhile, the direct immersion of crystalline MIL-88B (denoted as cMIL-88B) in dimethylimidazole solution triggers the partial substitution of amino terephthalate by 2-Methylimidazole (2-MeIM), followed by the hydroxylation that leaves the deficiency of organic ligand at these sites. Such amorphization endows the aMIL-88B [Figure 15C-E] with higher electrochemical performance, more active sites exposure, and higher adsorptive/catalytic activity to polysulfides compared with those of the parent MIL-88B counterpart. Therefore, Li-S batteries with the as-developed amorphous MOF (aMIL-88B)-modified separator realize efficient and reversible sulfur electrochemistry, exhibiting superb cyclability with high-capacity retention of 740 mAh·g^-1 after 500 cycles at 1 C, rate performance up to 5 C, and also decent areal capacity under a high sulfur loading of 4.3 mg·cm^-2. Recently, Wang et al. used thermal treatment to prepare porous 2D defective ZIF-7 with abundant active edges for application as sulfur carriers in Li-S^[232]. The 2D porous defective MOF (ZIF-7) was obtained by etching and creating pores through a simple thermal treatment. [Figure 15F] It was found that the ZIF-7 600 material obtained at a calcination temperature of 600 had the largest specific surface area as well as the largest sulfur loading, demonstrating the enormous potential of ZIF-7 600 [Figure 15G and H] as a sulfur carrier. As a result, the synthesized ZIF-7 600 for Li-S host material significantly improved the performance of Li-S, that is, exhibited high initial capacities of 1,206.3 and 972.8 mAh·g^-1 after 100 cycles at 0.2 C.

Figure 15. (A) Schematic illustration of D-UiO-66-NH₂ and TEM images. Reproduced with permission^[230]. Copyright 2021, American Chemical Society. (B) Schematic illustration of the synthesis of amorphous aMIL-88B and its applications, (C and D) SEM images of aMIL-88B, (E) TEM images of aMIL-88B. Reproduced with permission^[231]. Copyright 2021, Elsevier. (F) Schematic illustration of S/ZIF-7 600 synthesis, (G) SEM images of ZIF-7 600, (H) TEM images of ZIF-7 600. Reproduced with permission^[232]. Copyright 2022, Wiley-VCH.

Other batteries

The defective MOFs are also commonly used in other batteries besides those mentioned above, and the continuous efforts of researchers have reported numerous batteries with excellent performance.

Zinc-air batteries (ZABs) are considered one of the most promising alternatives to LIBs due to their high energy density (1,086 Wh·kg^-1), low cost, safety and environmental protection. Jiang et al. proposed the first innovative idea of using organic-inorganic hybrid materials by simultaneously introducing ligand vacancies and hierarchical pores into a heterometal imidazole framework to improve the performance of rechargeable zinc-air batteries^[233]. Synchrotron radiation absorption spectroscopy and DFT modeling were used to investigate the origin of the active site in depth. Regulation of the d-orbital electronic structure of the active site metal Co by proximity interactions of ligand vacancies and long-range interactions of the cobalt-zinc alliance. Among the prepared series of materials, BHZ-48 has the optimal adsorption/desorption energy state of the active intermediate, and therefore the energy barrier of its potential decisive step is significantly reduced. Among the prepared series of materials, BHZ-48 has the adsorption/desorption energy state of the optimal active intermediate so that the energy barrier of its potential decision step is significantly reduced, which can lead to higher energy density and cycle life of ZABs. Liu et al. proposed the introduction of Fe-dopants and S vacancies into MOFs-derived bimetallic nickel-cobalt sulfide composites using a two-step solvothermal method to prepare Fe-NiCo-S samples with excellent charge storage kinetics and activity, and the obtained Fe-NiCo-S nanosheets have ultra-high specific capacitance (2,779.6 F·g^-1 at 1 A·g^-1) and excellent rate performance (1,627.2 F·g^-1 at 10 A·g^-1)^[234]. Additionally, based on the density functional theory (DFT) calculation, defects cause more electrons to appear near the Fermi level, which is conducive to electron transfer in electrochemical processes. Wang et al. developed a method for partial removal of coordination functional groups to introduce isophthalic acid during the coordination of Cu ions with 1,3,5-bezenetricarboxylic acid, which resulted in the construction of numerous mesoporous defects as well as Cu metal centers with unsaturated coordination in microporous Cu-MOF^[235]. Experimental and theoretical calculations have shown that a large number of mesopores and unsaturated coordination of Cu metal centers can effectively reduce the spatial site resistance during the electrocatalytic process, thus enhancing mass transfer, lowering the electron conduction resistance, improving the affinity with the reactants, and at the same time providing more storage space for Li₂O₂, the discharge product of insoluble Li-air batteries. In addition, after loading Co on mesoporous Cu-MOF, the most optimized catalyst can produce a discharge capacity of nearly 7,000 mAh·g^-1 in Li-air batteries.