Metal-organic framework membranes: From synthesis to electrocatalytic applications
Metal-organic framework membranes: From synthesis to electrocatalytic applications
Electrocatalysis, one of core technologies for electrochemical energy conversion, plays a key role in solving problems of energy shortage and environmental crisis. However, the electrocatalysis severely relies on the high-load precious metal-based catalysts with disadvantages of low durability, low abundance and high cost, which is a crucial factor of restricting its widespread applications. In the last two decades, metal-organic frameworks (MOFs), as an emerging family of nano-porous materials composed of metal ions or clusters and organic ligands with various topologies and physicochemical properties, have attracted tremendous attention in wide applications, including gas storage and separation, drug delivery, chemical sensing and catalysis [1-7]. MOFs and their derivatives can be designed with high porous channels, large surface areas, diversified and designable frameworks, and great chemical stability , which have been widely studied and applied for electrocatalysis of oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and CO2 reduction reaction (CO2RR) [9, 10] with following advantages: (a) Metal ions can directly serve as catalytic active centers species; (b) Ultra-large specific areas can expose abundant active sites; (c) Large porosity can provide more channels for diffusion of reactants and electrolyte; (d) Doping of heteroatoms into carbon frameworks or combination of metals-heteroatoms-carbon can enhance the electrocatalytic activity [11-13]. Therefore, MOFs-based materials are effective alternatives to replace the precious metals catalysts for the electrochemical catalysis.
To date, numerous MOFs and MOFs derivatives have been successfully applied in the electrochemical areas [7, 14-16]. MOFs as sacrificial templates have been converted to various nanostructural derivatives, which expose high densities of active sites and exhibit excellent electrochemical performance. However, MOFs-based electrocatalysts are usually obtained in the form of powdery crystals/particles , which need to be fabricated using polymer binders or other additives to prevent peeling off and achieve special form requirements [18-20]. These additives may reduce the proton or electron conductivity, block the pores, deteriorate the active sites, and weaken the electrocatalytic properties. Therefore, MOF membranes as a layer of intact and well-intergrown MOF crystals are promising and effective strategy in the field of electrocatalysis. MOF membranes and their derivatives with tunable thickness from microscale to nanoscale, high homogeneity for improved exposure of catalytic active sites and no binders are highly desirable to achieve good proton or electron conductivity and mechanical stability for qualified electrocatalytic properties .
Currently, synthesis and preparation of MOF membranes have been widely reported [22-27]. The techniques in literatures to synthesize MOF membranes involve in situ directed synthesis and secondary growth [4, 5, 28, 29]. The in situ directed synthesis includes only one step process, in which the substrates are directly immersed into the mother solutions to obtain the MOF membranes without any pre-decorated crystals on the surfaces [30, 31]. The seed-crystal secondary growth for preparing MOF membranes includes two processes of crystal seeds coating and membranes growth, which can get rid of the substrate limitation [32-34]. Further, several other techniques have also been reported for synthesis of MOF membranes, including self-assembled monolayers, layer-by-layer deposition, colloidal deposition, microwave, electro-spinning technique and electrochemical deposition [11, 16, 22]. Among these methods, electrochemical deposition is promising for fabricating high-quality MOF membranes owing to its advantages of facile preparation, short growth time, and ease to regulate and scale up . Additionally, ultrathin 2D MOF nanosheets those can be also defined as a kind of nanoscale MOF membranes, attract increasing research attention in electrocatalysis application owing to ultrathin nano-thickness, large surface areas and highly exposed active sites [36-38]. In this minireview, we summarize a brief introduction to the various methods for the fabrication of MOF membranes and focus on their promising performance in electrocatalysis applications.
2. Methods for fabrication of MOF membranes
Tremendous efforts have been devoted to the fabrication of MOF membranes and many novel methods have been developed. Various substrates (such as silica, alumina, titania, metal nets, graphite) have been used as the platforms for preparation of MOF membranes. In general, the challenge of preparing continuous and well-intergrown MOF membranes is to promote the heterogeneous nucleation of MOF crystals on the substrate surface. Several techniques have been described in literatures to synthesize MOF membranes, including directed synthesis, secondary growth and electrochemical deposition.
2.1 Directed synthesis
The directed synthesis is performed through directly immersing substrates into the grown mother solution without any crystals previously attached. The nucleation, growth and cross-linking of the MOF crystals are all completed in the same one step. Due to random crystals nucleation and orientation on the substrate surfaces, the substrates are generally placed downward or vertically to avoid crystals accumulation. The advantages of directed synthesis include facile operation, wide applicability and low investment cost. Lai et al. reported the continuous and well-intergrown MOF-5 membrane, which was successfully synthesized on porous α-alumina substrate without any surface modification via in situ solvothermal synthesis . The covalent bonds are formed between the carboxyl groups of 4-benzenedicarboxylic acid (BDC) and the hydroxyl groups on the alumina surface. The high-quality UiO-66 membrane was developed on a predesigned porous alumina hollow fiber using in situ solvothermal method (Fig. 1) . The BDC ligand also acted as the surface modifier during the synthesis via constructing coordination bonds between carboxylate oxygens and aluminum atoms on substrate, so that the nucleation and growth of UiO-66 were promoted to form a dense membrane. Cao et al. prepared an integrated Cu-BTC membrane on the potassium hexatitanate (K2Ti6O13) substrate via in situ solvothermal growth . The substrate is found to easily absorb Cu2+ in a weak acidic environment for growing Cu2+ -containing MOF membranes. And the continuous ZIF-69 membrane was also fabricated on porous α-alumina without any surface modification .
However, this method is difficult for preparing continuous MOF membranes on bare substrates due to the poor heterogeneous nucleation of MOF crystals. Therefore, the metal substrates with same metal centers of MOFs were selected to improve the heterogeneous nucleation and enhance the interfacial bonding for the formation of various continuous MOF membranes. Guo et al. reported that HKUST-1 membrane was successfully synthesized on a copper net by a "twin copper source" technique . A "single nickel source" method was also used to fabricate a Ni2(Lasp)2(bipy) membrane on a nickel net , in which nickel net played an important role not only as the sole nickel source for the MOF crystals growth but also as a support for MOF membrane. Nevertheless, the metal substrates severely limit the kinds of metal centers of MOFs. Chemical modification of the substrates is another effective strategy to improve the heterogeneous nucleation and crystals intergrowth. Huang et al. developed a facile strategy of using polydopamine (PDA) as a covalent linker to modify porous Al2O3 substrate and stainless-steel-net (Fig. 2a), on which the well-intergrown ZIF-8 membranes were successfully anchored (Figs. 2b-d) [43, 44]. The metal-chelating capability of PDA layer on substrates promote the heterogeneous nucleation and growth of well-intergrown MOFs membranes. Based on this, they also fabricated various continuous MOF membranes on PDA-modified substrates, including ZIF-7 , ZIF-90 [45, 46] and ZIF-100 . The 3-aminopropyltriethoxysilane (APTES) was introduced as the covalent linker to prepare various MOF membranes, including ZIF-7, ZIF-8, ZIF-22, ZIF-90 and UiO-66-NH2 [48-52]. The ethoxy groups of APTES react with the hydroxyl groups of substrate surface, and the amino groups of APTES react with the organic ligands of MOFs via imines condensation. Then the heterogeneous nucleation and crystals growth of MOF membranes could be achieved onto various substrates.
Besides organic molecules modification, inorganic matters modified substrates have also attracted lots of research attention. The principle of inorganic modification is to introduce a metal precursor with same kinds of metal centers of MOFs onto the substrate. The introduced metal source has a certain attraction to the ligands in the solution, thereby promoting the crystallization of MOFs onto the substrates surfaces. Caro et al. introduced a urea hydrolysis method to prepare an asymmetric ZnAl-CO3 layered double hydroxide (LDH) buffer layer onto porous Al2O3 substrate, and then well-intergrown MOF (ZIF-7, ZIF-8, ZIF-90) membranes were directly fabricated on the ZnAl-CO3 LDH buffer layermodified substrates .
2.2 Secondary growth
The direct synthesis method is often limited by the substrate, and thus it is usually applied after a series of complex modification for the surface of substrate. Secondary growth has been widely studied owing to easy control of the crystal orientation and the resultant high-quality MOF membranes without cracks or intercrystalline gaps. In the method of secondary growth, how to form a uniformly continuous seed layer on the substrates is of vital important to successful preparation of MOF membranes . The crystal seeds on the substrate provide nucleation sites for secondary growth of MOF crystals, which makes MOF membranes more inclined to intergrow on the substrate, solving the low nucleation rates of bare substrate surface. Several seeding techniques are widely performed, including rubbing, dip coating [54, 55], spin coating , microwave assisted crystallization [57, 58] and chemical reaction coating .
Rubbing the substrates with dry MOF particles can embed the crystal seeds into the pores of substrates surfaces, which could provide rich nucleation sites for MOFs growth and improve the combination strength of MOF membranes to the substrates. The size of MOF seeds should be generally smaller than the pore size of the substrates. The rubbing process could modify a uniform seed layer onto various substrates such as α-Al2O3 [60, 61] and polyethersulfone (PES) . Carreon et al. modified the α-Al2O3 tube by manually rubbing with ZIF-8 seed crystals, and successively produced a continuous ZIF-8 membrane . Caro et al. modified a ceramic α-Al2O3 substrate with 300 nm octahedral UiO-66 crystal seeds and successfully prepared a thin UiO-66 membrane with highly crystallographic orientation . It has been found that some polymers like polyethyleneimine (PEI) can effectively enhance the interaction between the seeds and the substrates through hydrogen bonding to promote the formation of continuous seeds layer [54, 64, 65]. The ZIF-7 seeds modified alumina substrate was prepared by dip coating technique using ZIF-7 nanocrystals dispersed in PEI solution, and then a well-intergrown and ultra-microporous ZIF-7 membrane was successfully fabricated . Microwave-assisted solvothermal synthesis is also carried out to perform the secondary growth , in which pretreated substrate is placed in mother MOF liquid and MOF crystal seeds are rapidly formed by microwave heating. The microwave heating can efficiently and uniformly heat the substrates, accelerating nucleation rate of MOF crystals. The asprepared MOF seeds layer shows strong bonding force with the substrates. The MOF-5, ZIF-8, ZIF-7, ZIF-67 and SIM-1 membranes have been successfully prepared by microwave-assisted seeding crystallization [33, 57, 58].
The reactive seeding method is also used for MOF crystal seeds preparation, in which the substrate directly reacts as the metal source with the organic ligands to form a uniformly high-density seeds layer . Lee et al. synthesized a uniform and well-intergrown MIL-53 membrane on alumina porous substrate using reactive seeding method (Fig. 3a). The substrate was acted as aluminum precursor instead of Al(NO3)3, which reacted with 1, 4-benzenedicarboxylic acid (H2BDC) to generate a seed layer (Fig. 3b) . The seed layer morphology was similar to that of MIL-53 crystals (Fig. 3c). Then MIL-53 membrane was successfully fabricated by following secondary growth process (Figs. 3d and e). Based on this method, various MOF membranes were prepared, including MIL-96 membrane [66, 67] and ZIF-68 membrane . The layer-by-layer (LBL) growth method has also been risen to fabricate the MOF crystal seeds layer [34, 69]. Kitagawa et al. demonstrated the successful fabrication of crystalline NAFS-1 membrane using LBL growth technique . The step-by-step LBL method was applied to fabricate a uniform HKUST-1 seeds layer on a porous alumina substrate , in which the BTC3- and Cu2+ were alternately deposited by coordination interaction. Then HKUST-1 membrane was synthesized on the seeded substrate by secondary growth. Highly crystalline and continuous ZIF-8  and Ni-MOF-74  membranes have also been prepared via LBL method. The method has the advantages that it provides a good control of crystals orientation and numbers of layers in vertical direction. However, LBL method has also encountered a certain number of problems in scale-up production due to the relatively complicated synthesis process.
2.3 Electrochemical method
The directed synthesis and secondary growth are powerful means to fabricate MOF membranes on various substrates. However, preparation of MOF membranes using the two methods often bears cumbersome operation and long synthesis time. Electrochemical method for preparation of MOF membranes is pioneered by BASF [72, 73], which has advantages of facile preparation, short synthesis time, and ease to control membrane structure and thickness by altering the voltage . The anode dissolving replaces the metal salts as the metal sources, eliminating the impact of anions such as nitrate, sulfate, chlorate or chloride . De Vos et al. demonstrated that electrochemical method is an effective and versatile technique for preparing various MOF membranes [75, 76]. HKUST-1 membrane was electrochemically synthesized on a copper substrate in a short time at mild temperature. The crystal size could be perfectly controlled by adjusting the water content in the synthesis mother solution. Electrophoretic deposition (EPD) is also developed for preparation of MOF membranes . Hupp et al. demonstrated the EPD assembly of MOF membranes on various conductive surfaces (Fig. 4a). NU-1000, UiO-66, HKUST-1, Al-MIL-53 and MOF-525 membranes were successfully fabricated on the FTO substrates, respectively (Figs. 4b-f) [26, 78]. Different metal precursors can be electrodeposited as the metal sources required in the target MOF membranes. Wang et al. introduced the EPD strategy to modify the stainless-steel net substrate with metal oxide/hydroxide nanostructures as the metal sources, such as ZnO nanorods, Co(OH)2 nanosheets and Cu2O nanocubes . Then continuous MOF membranes of ZIF-8, ZIF-67 and HKUST-1 could be synthesized by further in situ growth.
2.4 Fabrication of 2D MOF nanosheets
Additionally, ultrathin 2D MOF nanosheets those can be defined as a category of nanoscale MOF membranes, possess abundant highly-accessible active sites and large specific surface areas, which are significantly advantageous to electrocatalytic applications [36, 37]. Two main methods of top-town and bottom-up strategies are achieved for synthesizing high-quality MOF nanosheets . Top-town strategy refers to peeling off MOF crystals to nanosheets by applying mechanical forces. Yang et al. reported a soft-physical technology to prepare highly crystalline 2D [Zn2(benzimidazole)4(OH)(H2O)]m (Zn2(bim)4) nanosheets with 1 nm thickness through breaking van der Waals force or hydrogen bonding, following by ultrasonic exfoliation in volatile solvent [80, 81]. Freeze-thaw approach was used to obtain 2D MAMS-1 membrane by a mild exfoliation strategy in suitable solvent systems . The top-town method is a facile and practical method for preparing 2D MOF nanosheets, but layered structure of MOF crystals are required, and the nanosheets may be damaged during the peeling off process. So top-town method is probably not feasible for high-yield preparation of ultrathin 2D MOF nanosheets.
Bottom-up strategy means straightforward fabrication of ultrathin MOF nanosheets in the grown solution of metal ions and organic ligands . Gascon et al. reported a bottom-up synthesis of three-layer approach for preparation of copper 1, 4-benzenedicarboxylate MOF nanosheets with lateral dimension of 0.5-4 μm and thickness of 5-25 nm . A series of 10 nmthickness MOF nanosheets, i.e., M-TCPP (M = Zn, Cu, Cd or Co, TCPP = tetrakis(4-carboxyphenyl)porphyrin) nanosheets, were fabricated by a facile surfactant-assisted synthesis , in which the anisotropic growth of nanosheets is achieved using the surfactant polyvinylpyrrolidone as the blocking agent to inhibit the bulk growth of layered molecules, thereby preparing the nanosheets with thickness of 3-10 nm and diameter of micron-level. Feng et al. prepared large-area and free-standing THTNi (THT = 1, 2, 5, 6, 9, 10-triphenylenehexathiol) single-layer MOF nanosheets consisting of triphenylene-fused nickel bis(dithiolene) complexes using Langmuir-Blodgett method . The nanosheets exhibited thickness of 0.7-0.9 nm and diameter of millimeters.
Additionally, some 2D nanomaterials could be selected as seed templates to obtain MOF nanosheets [86, 87]. Wang et al. developed a defect-free ZIF-8 membrane with thickness of 100 nm using ZIF-8/GO hybrid nanosheets templates . The amounts of ZIF-8 nanocrystals was vital for preparing a uniform seeding layer to facilitate crystals intergrowth. The seeding layer is also as an effective barrier, eliminating the defects during the contradiffusion process. Recently, a facile and universal synthesis method for preparation of bimetallic MOF nanosheets based on confinement transformation of metal oxide nanoflakes has been introduced. Amorphous metal oxide nanosheets, such as Fe-Co, Ni-Fe or Co-Cu oxide nanosheets, are used as sacrificial templates to form a series of MOF-74 or BTC MOF nanosheets with a flexible combination by confined ligand coordination (Fig. 5a) . Particularly, FeCo-MOF-74 nanosheet is enriched with coordinative unsaturated metal sites, exhibiting high activity of oxygen evolution reaction (Figs. 5b-d) .
3. Electrocatalytic applications of MOF membranes and their derivatives
Comparing with other porous materials, MOFs can be rationally fabricated with maximum exposure of active sites for electrocatalysis by the tunable redox-active metal centers, ligands, along with the building blocks featuring electron-donating and electron-accepting ability at the molecular level . However, MOFs and their derivatives-based electrocatalysts in the form of powders often face the issues of poor conductivity, instability, aggregation and deterioration, which would largely affect their catalytic activities; while MOF membranes and their derivativesbased electrocatalysts show unique advantages of good proton or electron conductivity, controllable structure, strong adhesion to electrode surface, and abundantly effective catalytic centers for high-performance electrocatalysis . For example, Zou et al. reported a cobalt-based ZIF-67 membrane on the melamine sponge, which was pyrolyzed to the hierarchical architecture catalyst, exhibiting excellent trifunctional electrocatalytic activity for ORR, OER, and HER (Fig. 6). ZIF-67 formed by bridging 2-methylimidazolate anions and cobalt cations is easily converted to highly graphitic nitrogen-doped carbon nanotubes (NCNTs) with Co-Nx moieties, which exhibit excellent electrocatalytic activity. The synergetic coupling between cobalt ions and nitrogen-doped carbon could generate a satisfactory electronic surrounding to further improve the electrochemical performance . The cobaltincorporated NCNT on 3D macroporous structure was synthesized via pyrolysis at a certain temperature and exhibited excellent trifunctional electrocatalytic activity due to the unique structures with abundant active sites and effective mass transport . However, MOF membranes and their derivatives-based electrocatalysts still suffer from some drawbacks, including low mass permeability and obstruction of active metal centers by organic ligands. Ultrathin 2D MOF nanosheets and their derivatives have been aroused attention to be as novel high-effective electrocatalysts, which possess the following advantages of nanometer thicknesses for rapid mass transport and superior electron transfer, abundantly exposed catalytic active sites, and easily identifiable and tunable surface atomic structures and bonding arrangements . Herein, we summarize recent progress of MOF membranes/ nanosheets and their derivatives for electrocatalysis, including ORR, OER, HER and CO2RR.
3.1 ORR electrocatalysis
A number of MOF membranes/nanosheets and their derivatives-based ORR electrocatalysts are summarized in Table 1. Several studies have reported MOF membranes directly as the ORR electrocatalytic platforms [93, 94]. For example, Ni3(HITP)2 (HITP = 2, 3, 6, 7, 10, 11-hexaiminotriphenylene) membrane was synthesized on glassy carbon and ITO via solvothermal synthesis, which is a conductive 2D MOF with highly ordered crystallization and electrical conductivity of 40 S/cm . Ni3(HITP)2 membrane showed the ORR activity with onset potential (j = -50 μA/cm2) of 0.82 V and Tafel slope of -128 mV/dec in 0.10 mol/L KOH solution as well as strong stability and methanol tolerance. Azzaron et al. presented a facile and straightforward strategy to synthesize ZIF-8/ PABA (ABA = 3-aminobenzylamine) for ORR in neutral solutions , in which the ZIF-8 membrane was fabricated onto PABA conductive substrate by in situ growth. The MOF membranes derivatives, such as the N-C-metal membranes materials, were also reported for electrocatalytic performance [95-100]. Duan et al. reported CoAl LDHs as the platform to fabricate well-defined ZIF-67 arrays membrane by in situ directed growth . The derivative of well-distributed hierarchical structural CoAl-LDH@ZIF-67 sheets has been pyrolysis to carbon-based framework as a high-active ORR electrocatalyst (Figs. 7a and b). By tuning the Co content in the LDH layer, LDH@ZIF-67-800 exhibited excellent ORR activity owing to unique hierarchical micro-/mesoporous structure and abundant active sites, which performed onset potential of 0.94 V and half-wave potential of 0.83 V in 0.1 mol/L KOH, as well as onset potential of 0.875 V and half-wave potential of 0.675 V in 0.1 mol/L HClO4, comparing favorably with commercial Pt/C catalyst (Figs. 7c and d).
Ultrathin 2D MOF nanosheets and their derivatives have been gradually used in ORR electrocatalysis, owing to high ratio of surface active atoms, short pathway for mass transport and abundantly accessible active sites [37, 101-103]. Dong et al. synthesized high-quality ZIF-67 nanosheets by salt-template assisted bottom-up strategy . The as-prepared ZIF-67 nanosheets was carbonized to Co/N-doped nanoporous as ORR electrocatalyst, which exhibited onset potential of 0.938 V (0.945 V for Pt/C) and half-wave potential of 0.869 V (0.846 V for Pt/C), demonstrating more active ORR electrocatalysis. Graphene oxide (GO), a typically 2D aromatic monolayer of carbon with ultrathin structure, high surface area, and excellent electrical conductivity, has emerged as an ideal substrate for fabricating MOF nanosheets [104-112]. Jahan et al. synthesized hybrid MOF nanosheets by combining pyridinie functionalized r-GO sheets with Fe-porphyrin MOF , in which oxygenated functional groups on r-GO could promote electron transfer and link with porphyrin ligands to facilitate ORR electrocatalytic activity via 4e reaction. Prussian blue (PB) MOF/GO nanosheets were used as the precursor/template to develop the porous Fe, N-based carbon nanosheets catalysts , which showed excellent ORR activity with onset potential of 1 V and half-wave potential of 0.93 V.
3.2 OER electrocatalysis
Numerous MOF membranes/nanosheets and their derivatives were reported to accelerate the OER electrocatalysis, which are summarized in Table 2. A lot of MOF membranes/nanosheets have been directly used for OER electrocatalysis [113-116]. Hupp et al. reported a free-standing single-crystalline NU-1000 membrane grown on the fluorine-doped tin oxide (FTO) conducting glass via in situ directed growth . Then the Co catalytic sites were deposited on the NU-1000 membrane (Co-AIM NU-1000) via atomic layer deposition method. The obtained Co-AIM NU-1000 membrane was demonstrated as the excellent OER electrocatalyst at pH 11 solution with a quick 4e transfer conversion of hydroxide ions to dioxygen. Wang et al. have synthesized a series of MOF membranes, including Ni-BTC, Fe-BTC and Fe/Ni-BTC, which all exhibited good electrocatalysis activity . The Fe/Ni-BTC membranes were fabricated on the nickel foam via facile electrochemical deposition, which showed the OER activity with low overpotential of 270 mV at 10 mA/cm2 and Tafel slope of 47 mV/dec. Tang et al. introduced an ultrathin bimetallic NiCo-MOF nanosheets-based membrane deposited on copper foam , which exhibited an overpotential of 189 mV at 10 mA/cm2. Wang et al. prepared NiFe-bimetallic MOF nanosheets, which showed the OER catalytic activity with the overpotential of 260 mV at 10 mA/ cm2 and Tafel slope of 30 mV/dec . Dou et al. reported the 2D Ni-MOF nanosheets deposited with Fe-MOF nanoparticles (Figs. 8a and b) , which exhibited better OER activity with the lowest overpotential of 265 mV at a current density of 10 mA/cm2 than Ni-MOF (370 mV) and IrO2 (365 mV) (Figs. 8c and d). The high catalytic performance is ascribed to the synergistic effect of Ni active centers and Fe species.
Additionally, the derivatives of MOF membranes/nanosheets were also widely used for OER electrocatalysis. Jiang et al. proposed a versatile strategy to fabricate three-dimensional MOF arrays (ZIF-67, Co/Ni-MOF-74, HKUST-1) on semiconducting nanostructures (CoO, NiO, Cu(OH)2) as self-sacrificing templates onto various substrates (Ni foam, Cu mesh, Fe mesh, Cu foil) . The MOF arrays-based membranes were treated by pyrolysis process to prepare metal/metal oxide porous carbon membranes as qualified OER electrocatalyst with the advantages of the oriented arrangement, improved conductivity and surface actives areas, and fast electron transfer and electrolyte penetration. rGO was incorporated as the template to deposit sandwich-type ZIF-67 nanosheets , which were further derived to CoP/rGO layered sheets via pyrolysis and phosphating process. The ZIF-67 nanosheets-derived CoP/rGO exhibited excellent OER/HER electrocatalysis, which performed the overpotential of 340 mV at 10 mA/cm2 and Tafel slope of 66 mV/dec in alkaline conditions for OER, and onset potential of 13 mV and overpotential of 105 mV at 10 mA/cm2 as well as Tafel slope of 50 mV/dec in acid solution for HER, demonstrating a bi-functional electrocatalyst for water splitting.
3.3 HER electrocatalysis
The HER electrocatalytic performance of various MOF membranes/nanosheets and their derivatives are summarized inTable 3. Hupp et al. deposited the hexagonal rod-shaped NU-1000 on FTO substrate as membrane electrode to further electrodeposit Ni-S (Fig. 9a) , which showed excellent HER electrocatalytic activity with the overpotential of 238 mV and Tafel slope of near 120 mV/ dec (Figs. 9d and e). Strikingly, the original purpose was to deposit Ni-S onto NU-1000 membrane, but actually Ni-S was electrodeposited to form a layer of film on the bottom of NU-1000 rods membrane (Figs. 9b and c). The enhanced HER electrocatalysis could be ascribed that NU-1000 rods modified the environment of electrocatalyst and synergy effect of Zr6 cluster and Ni-S improved the proton transfer activity . Marinescu et al. integrated cobalt dithiolene catalysts into 2D BHT-based MOF (BHT = benzenehexathiol) nanosheets through a liquid-liquid interfacial synthesis , which exhibited high HER catalytic activity and remarkable acidic stability, reaching the overpotential of 0.34 V at 10 mA/cm2 and Tafel slope of 149 mV/dec. Feng et al. prepared the freestanding 2D Ni-THT-MOF nanosheets with large active area and thickness of 0.7-0.9 nm by Langmuir-Blodgett method at air/water interface . The Ni-THT nanosheet-based membrane immobilized on glassy carbon electrode exhibited excellent HER electrocatalysis with Tafel slope of 80.5 mV/dec and overpotential of 333 mV at 10 mA/cm2, superior to most CNT-supported molecular catalysts and heteroatom-doped graphene catalysts. Cobalt or nickel dithiolene species have been considered as the most efficient electrocatalysts for the HER [120, 121]. Based on this, a series of 2D MOF nanosheets from different metal centers (Co, Ni) and ligands by bottom-up interfacial synthesis . Catalytic molecular MSxNy (M = Co and Ni, x/y = 2:2, 0:4 and 4:0) was introduced into the obtained 2D MOF nanosheets by the Langmuir-Blodgett (LB) method. The HER electrocatalytic performance of THTA-Co and THTA-Ni MOF nanosheets membranes valuated by RDE technique in 0.5 mol/L H2SO4 showed the overpotentials of 283 mV and 315 mV at 10 mA/cm2, and the Tafel slope of 71 mV/dec and 76 mV/dec. Additionally, THTA-Co/GO was also prepared and showed a high-active HER electrocatalysis with the overpotential of 230 mV at 10 mA/cm2 and Tafel slope of 70 mV/dec, superior than THTA-Co. Additional, some copper or hafnium species also have been studied as the efficient HER catalysts [122-124]. Recently, Roy et al. reported a novel MOF, UU-100(Co), composed of a tetranucleating cobaloxime linker with carboxylate anchors ([Co(dcpgH)(dcpgH2)]Cl2), in which the cobaloximes act as metalloid-linkers between hexanuclear zirconium clusters . The UU-100(Co) membrane was deposited onto the glassy carbon (GC) electrode pretreated by carboxylic acids. UU-100(Co) membrane showed the HER electrocatalysis with the onset potential of -0.15 V vs. RHE and Tafel slope of 250 mV/dec in pH 4 acetate buffer, superior than most of other comparable cobaloxime catalysts. The high porosity and large internal channels in UU-100(Co) membrane promote the easy access of electrolyte and the high electron hopping rates. The cobaloxime catalytic sites in UU-100(Co) membrane could also exhibit excellent stability in electrocatalytic HER performance.
3.4 CO2RR electrocatalysisz
Global warming caused by CO2 has become a severe environmental issue of worldwide concern [125, 126]. Conversion of CO2 to some valuable chemicals and fuels has become an important task [127, 128]. Electrocatalytic CO2 reduction reaction (CO2RR) has attracted increasing attention owing to the significant advantage of conversion at normal temperature and pressure, which products involve formaldehyde, methanol, carbon monoxide, methane, oxalic acid, ethylene and ethanol . MOFs and MOF membranes have been widely used for CO2 adsorption, separation and storage in the past decades [130, 131]. Recently, the selective CO2RR using MOF membranes as the efficient electrocatalysts has aroused widespread attraction [14, 16, 131, 132]. The CO2RR electrocatalytic performance of various MOF membranes are summarized in Table 4.
Cu catalyst could reduce carbon dioxide to carbon monoxide, formic acid, methanol, ethanol and other alkane; therefore, Cu-based MOFs have been particularly studied [133-135]. A uniform Cu3(BTC)2 membrane coated on glassy carbon was used as an efficient electrocatalyst for the selective reduction of carbon dioxide . The CV results revealed that the electrochemically generated Cu(Ⅰ) formed the adduct with carbon dioxide and oxalate anion through formation of carbon dioxide radical anion, which could form the oxalic acid via a 2-electron reduction and dimerization mechanism. Metalloporphyrins are widely studied as catalysts for CO2RR under assistant of light or electricity . Recently, metalloporphyrins-based MOFs have attracted the attention in CO2RR catalytic application, in which the metalloporphyrin enhances the absorption of light to improve the electron-hole separation rate, and MOFs structure offers the large specific surface areas and high porosity [136-138]. Hupp et al. prepared a Fe-porphyrin-based MOF-525 membrane deposited on FTO surface via electrophoretic deposition (EPD) method . The obtained membrane was used as an effective electrocatalyst for CO2RR owing to great molecular-scale porosity, excellent chemical stability and high conductivity. The mainly products of CO2RR were mixtures of CO and H2 with ~100% Faradaic efficiency (FE), which was higher than most of heterogeneous molecular catalyst for CO2RR previously reported. A porphyrin-based MOF of PCN-222 (Fe) membrane on carbon paper was fabricated through the simple dip-coating method , which exhibited excellent CO2RR catalytic property of converting CO2 to CO with overpotential of 494 mV, FECO of 91%, and TOF of 0.336 site-1 s-1. The high CO2RR performance of PCN-222(Fe) is ascribed to the intrinsic activity of porphyrin molecule, great adsorption to CO2, and high conductivity of carbon paper. Yang et al. introduced a novel MOF membrane of Al2(OH)2TCPP-Co onto (TCPP = 4, 4', 4'', 4'''-(porphyrin-5, 10, 15, 20-tetrayl)tetrabenzoate) a conductive substrate via a dissolution-recrystallization method as an effective electrocatalyst for CO2RR (Figs. 10a and b) , which showed the selection of CO production was reached 76 %, and Tafel slope was 165 mV/decade in low-overpotential region. The stability test showed that the current density was no obviously decrease in CO2-saturated bicarbonate buffer over to 7 h with a TON of 1400 (Figs. 10c and d) . The majority of catalytic centers in cobalt-metalated porphyrin units in this MOF are redox-accessible where Co(Ⅱ) is reduced to Co(I) during catalysis. Immobilization of porphyrin molecular into MOF membranes has been proved to be the great potential for conversion of CO2 to sustainable fuels.
The Re-based MOF (ReL(CO)3Cl (L = 2, 2'-bipyridine-5, 5'-dicarboxylic acid) membrane was deposited on FTO substrate by a liquid phase epitaxy method, which exhibited excellent electrocatalysis for conversion of CO2 to CO with a high faradaic efficiency of 93 ± 5%, an order of magnitude larger than previous reported MOF membrane-based catalyst . Eddaoudi et al. systematically investigated the electrocatalytic CO2RR activity of a variety of MOF membranes (ZIF-8, Cu-BDC, Re-fcu-MOF, Al2(OH)2TCPP) grown on Au nanostructured microelectrodes (AuNMEs) via layer-by-layer method and solvothermal deposition. These MOF membranes on AuNMEs promoted the reduction products of CH4 and C2H4, but completely suppressed CO production during the CO2RR process . The morphology and crystallization of MOF membranes also significantly affect CO2RR electrocatalysis. The ultrathin 2D Ni-MOF nanosheets were exfoliated and employed as CO2RR electrocatalyst for the conversion of CO2 to CO [141, 142]. By controlling the thickness of nanosheets, the 5 nm-thickness Ni-MOF showed high CO2RR catalytic selectivity, fast reaction kinetics and high TOF, exhibiting high faradaic efficiency of 78.8% and surface active site density of 1.68 × 10-7 mol/cm2 .
4. Conclusions and perspectives
In this mini-review, we summarize the controllable synthesis of MOF membranes and their electrocatalytic applications. The strategies for fabrication of MOF membranes are introduced, including in situ directed synthesis, secondary growth, and electrochemical method. Additionally, construction of 2D MOF nanosheets are also mentioned . MOF membranes/nanosheets and their derivatives possess several intrinsic advantages for electrocatalysis, including: (a) the form of membrane onto the electrode could avoid the aggregation and peeling off of MOFs crystals; (b) controllable orientation and arrangement of MOF crystals to form MOFs membranes could improve the proton or electron conductivity, increase active surface areas, expose more catalytic sites, and accelerate the release of gas bubbles; (c) abundantly unsaturated metal sites, porous structure and diverse surface functionalization of MOF membranes could provide more active sites for electrocatalysis; (d) calcination, pyrolysis or chemical reaction for MOF membranes could obtain M-N-C (M = Fe, Co, etc.) structures with highly active moieties, structural stability and fast mass diffusion; (e) ultrathin 2D MOFs nanosheets with nanometer thickness possess rapid electron and electrolyte transport, abundant catalytic active sites, easy identification and tunability of surface atomic structures and bonding arrangements.
There is still a long way to achieve practical applications of MOF membranes-based electrocatalysts. Further understanding the mechanism of action for the MOF membranes-based electrocatalysts requires an in-depth investigation. The future challenges to researchers may involve the following aspects:
(a) Selective preparation and precise control of MOF membranes according to actual requirements of electrocatalytic applications. The microstructures of crystal size and orientation, thickness, crystal boundary structure, and localization activity point in MOF membranes would affect their electrocatalytic properties and service lifetime. Hierarchical pores structure can be fabricated by introducing multi-metals or mixed ligands to increase active centers species for electrochemical catalysis [21, 22, 144].
(b) Further development of high conductive MOF membranes. Most MOF membranes are still severely restricted by the poor conductivity in electrocatalysis. Although some conductive MOFs have been synthesized in the laboratory, they are still limited in number and difficult to design molecular structures [145-147]. Ultrathin MOF nanosheets or the MOF nanosheets on GO templates with high-plane orientation possess the nano-thickness and unsaturated metal sites, which are beneficial to improve electron transfer. How to both enhance the electrical properties of MOF membranes and preserve the crystal structure characters is of great significance for electrocatalytic performance.
(c) Detailed mechanism analysis of structure-activity relationship between the atomic composition, electronic configuration, microstructure and surface morphology of MOF membranesbased electrocatalysts and electrocatalytic reactions at molecular and electronic levels. MOF membranes possess abundant, highly-ordered and complicated hierarchical M-N, M-C, N-C, M-N-C (M = metal center ions) structure, which are the active center species for electrocatalytic reactions. Combining the structure-activity relationship and the theoretical calculation to develop advanced MOF membranes-based electrocatalysts with high density of catalytic sites and excellent electrochemical activity is the main direction of future development.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
This work is financially supported by National Natural Science Foundation of China (Nos. 21805103, 21805104, 21802048), Fundamental Research Funds for the Central Universities (Nos. 2018KFYYXJJ121, 2019KFYXJJS073) and National 1000 Young Talents Program of China.
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Figure 1 SEM images (a and c, cross section; b, top view) and (d) EDXS mapping (corresponding to c) of the UiO-66 membranes. Zr signal, red; Al signal, light blue. Reproduced with permission . Copyright 2015, American Chemical Society.
Figure 2 (a) Reaction mechanism for dopamine polymerization. (b) Scheme of preparation of ZIF-8 membranes by using PDA as covalent linker between ZIF-8 layer and Al2O3 support. (c) Top view and (d) cross-section SEM images of the ZIF-8 membrane. Reproduced with permission . Copyright 2015, American Chemical Society.
Figure 3 (a) Schematic diagram of preparation of the MIL-53 membrane on alumina support via the reactive seeding method. SEM images of the (b) MIL-53 seed layer, (c) MIL-53 powders, (d) MIL-53 membrane surface and (e) cross-section. Reproduced with permission . Copyright 2015, Royal Society of Chemistry.
Figure 4 (a) A scheme illustrating the principal of MOFs EPD film growth, showing the attraction of charged MOF particles toward an oppositely charged electrode using an applied electric field. (b-e) Top view SEM images of NU-1000, UiO-66, HKUST-1, Al-MIL-53 and MOF-525, respectively. (a-e) Reproduced with permission . Copyright 2014, Wiley Publishing Group. (f) Reproduced with permission . Copyright 2015, American Chemical Society.
Figure 5 (a) Schematic illustration of the 2D oxide sacrifice approach conversion of M-ONS with H4dobdc ligand to form M-MNS. (b) OER curves of FeCoMNS-1.0, FeCo-ONS and RuO2 loaded on Ni foam with the loading amount of 2.0 mg/cm2 in 0.1 m KOH. (c) The overall water splitting activity of various catalysts. (d) Continuous amperometric i-t measurement at the cell voltage of 1.80 V in 1.0 mol/L KOH. Reproduced with permission . Copyright 2019, Wiley Publishing Group.
Figure 6 (a) Illustration of the synthesis process for the MSZIF-T electrocatalysts. (b) ORR polarization curves for the various electrocatalysts. (c) RRDE voltammograms of the hydrogen peroxide yields and electron-transfer numbers for MSZIF-900 and Pt/C. Electrochemical performance of the MSZIF-900 catalyst for (d) HER and (e) OER with i-R compensation. Reproduced with permission . Copyright 2017, Wiley Publishing Group.
Figure 7 (a) Schematic illustration for the synthesis of porous honeycomb-like carbon-based framework. (b) STEM images of LDH@ZIF-67-800. (c) Linear sweep voltammetry (LSV) curves of CoAl-LDH-800, ZIF-67-800, LDH@ZIF-67-800 and Pt/C catalyst in O2-saturated 0.1 mol/L KOH solution at a sweep rate of 10 mV/s and electrode rotation speed of 1600 rpm. (d) Peroxide yield and electron transfer number of LDH@ZIF-67-800 and Pt/C catalyst at various potentials based on the RRDE data. Reproduced with permission . Copyright 2016, Wiley Publishing Group.
Figure 8 (a) Schematic illustration of synthesis of Ni-MOF@Fe-MOF hybrid nanosheets. (b) HAADF-STEM image and corresponding EDS elemental mapping images of Ni-MOF@Fe-MOF hybrid. (c) Corresponding overpotential and current density of different catalysts at 10 mA/cm2 and 1.50 V vs. RHE, respectively. (d) Corresponding Tafel plots derived from the LSV curves. Reproduced with permission  Copyright 2018, Wiley Publishing Group.
Figure 9 (a) Illustration of Ni-S electrodeposition to create the NU-1000_Ni-S hybrid system. (b) SEM image of an NU-1000_Ni-S film showing the typical hexagonal rod-shaped crystals of NU-1000 on top of the FTO substrate. (c) Cross-sectional SEM image of NU-1000_Ni-S film. (d) J-V curves and (e) Tafel plots of variouis catalysts. Reproduced with permission . Copyright 2015, Macmillan Publishers Limited.
Figure 10 (a) The organic building units, in the form of cobalt-metalated TCPP, are assembled into a 3D MOF, Al2(OH)2TCPP-Co with variable inorganic building blocks. (b) Illustration of The MOFs deposited on a conductive substrate to achieve a functional CO2 electrochemical reduction system. (c) The selectivity for each product is tested over a potential range of -0.5 to -0.9 vs. RHE. (d) In the low-overpotential region, the Tafel slope of 165 mV/decade is closest to that of a one-electron reduction from CO2 to the CO2·- rate-limiting step. Reproduced with permission . Copyright 2015, American Chemical Society.
Table 1. The ORR electrocatalysis of MOF membranes/nanosheets and their derivatives.
Table 2. The OER electrocatalysis of MOF membranes/nanosheets and their derivatives.
Table 3. The HER electrocatalysis of MOF membranes/nanosheets and their derivatives.
Table 4. The CO2RR electrocatalysis of various MOF membranes.
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