Bio-inspired general synthesis of superplastic metal-organic framework aerogels and their applications

Many proposed utilizations for metal-organic frameworks (MOFs) demand their assembly into three-dimensional (3D) monolithic architectures. The capability of sustaining structural integrity during considerable deformations is important to allow a monolithic material that works reliably. Nevertheless, it remains a signicant challenge to realize high superplasticity in 3D macroscopic MOF networks. Here we report the ice-template-driven assembly of MOF nanobelts to form superelastic MOF-based cellular aerogels. Inspired by the hierarchical architecture of natural cork, the resulting materials can fully and rapidly recover its initial architecture after 50% strain compression and unloading for 2000 cycles. The characteristic hierarchical structure can be extended to single (Ni-, Mn-, and Co-), binary (NiMn-, NiCo-, and CoMn-), and ternary (NiCoMn-) MOF aerogels with exceptionally structural and chemical properties. Potential application has been further demonstrated for NiMn-MOF aerogels in exible energy conversion, which can effectively electrocatalysize hydrogen evolution in natural seawater even in the presence of considerable electrode deformations. The successful fabrication of such a class of fascinating architectures opens up enormous opportunities for exploring new application of MOFs in a free-standing, structurally adaptive, and macroscopic form photocatalysis, carbon dioxide electroreduction, metal-air batteries, biosensors, and adsorption.


Introduction
Metal-organic frameworks (MOFs) represent a class of materials with the combination of extraordinary porous, structural, and chemical properties, 1,2 which would become a new emerging nansocale building block for constructing macroscopic assemblies for a broad range of applications. [3][4][5][6] Some techniques, such as self-gelation, 3 have recently been developed to fabricate highly porous MOF aerogels. However, similar to most of the existing porous materials, the resulting MOF aerogels are brittle and have small recoverable deformation before failure. Superelasticity that has been observed in 3D architectures made of many other materials 7,8 has not been achieved in MOFs. Indeed, MOFs are especially fragile, in the common view, and they are far from the stage of superelasticity development. Previous analysis suggests that it would be signi cantly challenging to realize superelasticity in three-dimensional (3D) MOF assembles. [3][4][5][6] The capability of sustaining structural integrity during considerable deformations for 3D MOF assemblies is not only crucial for fabricating new categories of exible energy devices, such as electrocatalysis, 1 batteries, 9,10 and supercapacitors, 2,11 but also critical for future design of macroscopic MOF-based biological tissues resistant for mechanical damping. 12 It is therefore highly desirable to explore new strategies to address this challenging yet very important problem.
Generally, the macroscopic behaviors of materials are dominated by their intrinsic microscopic properties, for example, the underlying geometries. [13][14][15] In nature, corks have been recognized as one of the oldest materials exploited by human beings. The state-of-the-art structural analyses have been conducted to examine the microstructures of corks that feature exceptional mechanical robust. 14,15 In a cork structure, one-dimensional (1D) nano bers of cell walls are intimately arranged in a highly oriented manner to reach the reasonable strength. Further, individual cells of tens of micrometres in size are then closely connected to produce a 3D honeycomb-like architecture, which has proven to be very useful to maximize bulk superplastic modulus. Therefore, we surmise that e cient and ordered assembly at hierarchical structural levels is critical for cork to realize high mechanical elasticity.
Inspired by the striking hierarchical architecture of nature cork, we consider that MOF monoliths might deliver excellent mechanical properties if assembly in a reasonable manner similarly to cork.
Consequently, based on our most recent research on the controllable manipulation of low-dimensional MOF materials, 16 here we report a general procedure for building MOF architectures, starting by a onestep hydrothermal method to synthesize 1D MOF nanobelts followed by the ice-template-driven assembly of MOF nanobelt into a 3D hierarchical aerogels. The as-resulted NiMn-MOF nanobelt aerogel exhibits superelasticity, in addition to many other excellent structural properties such as highly exposed active centers, hierarchical pore structure, and good electrical conductivity. We further demonstrate the NiMn-MOF aerogels as a new class of exible electrodes for hydrogen evolution reaction (HER) in natural seawater.

Results
Synthesis and formation process of superplastic MOF aerogels. As a proof-to-concept experiment, we describe the preparation of NiMn-MOF aerogel by a two-step procedure (Fig. 1). In the rst step of producing 1D NiMn-MOF nanobelts, nickel and manganese acetate salts were mixed with organic ligand of 2-thenoic acid in an ethanol solution, which was subjected to hydrothermal treatment at 150 o C. By tuning different hydrothermal durations, we examined the reaction intermediates. We found irregular nano-particles without apparent elemental segregation for the NiMn-MOF precursor before 30 min by scanning electronmicroscopy (SEM) and SEM elemental maps, indicating that the metal ions and ligands began to nucleate within this period ( According to the literature 17,18 and XRD patterns ( Fig. 2i and Supplementary Fig. 16), the as-formed NiMn-MOF is consisted of alternating organic units (2-thenoic acid group) and inorganic units (Ni or MnO 6 ), where the carboxyl group of each ligand bridges two metal atoms, and each metal ion coordinates to two opposite carboxyl groups and four equatorial ethanol molecules (Fig. 1). Other MOF nanobelts, including Ni-MOF, Mn-MOF, Co-MOF, NiCo-MOF, CoMn-MOF, NiCoMn-MOF, were synthesized by a similar method except of using different metal sources (Supplementary Figs. 17,19).
In the second step of hierarchical assembly of 1D MOF nanobelts into 3D aerogels, NiMn-MOF was dispersed in aqueous solutions with different mass concentrations, and subjected to freeze-dry treatment (Inset of Fig. 2a). Both FT-IR (Fig. 2l, Supplementary Fig. 18) and Zeta potential analyses ( Supplementary  Fig. 4) verify the presence of a large number of hydroxyl and carboxyl functional groups on the surface of NiMn-MOF, which promote their good dispersibility in water ( Supplementary Fig. 5). The MOF nanobelts in aqueous solution was then freezed in liquid nitrogen, where they are forced to align along the direction of ice solidi cation movement (Figure 1a), 19,20 resulting in a highly ordered and anisotropic 3D structure. During this process, the as-produced microstructures are affected by a number of factors, such as dispersion concentrations, dry method and freezing speed.
The aerogel architecture obtained at a low concentration (< 1.3 mg mL -1 ) is randomly oriented nanobelt cross-linked uffy network structure, and did not shows any superplastic property during compressionunloading test (Fig. 1a). When the concentration of NiMn-MOF reaches 3.6 mg mL -1 and above (such as 5.9 mg mL -1 ), these 1D nanobelts can assemble into ordered 2D array, and further to honeycomb networklike superplastic 3D aerogel (Figs. 1b, c). In addition, the dry method is also important to the formation of Morphology and structure characterizations of superplastic NiMn-MOF aerogel. The optical image shows NiMn-MOF that is present as a solid yellow-gray cylinders of a few centimeters size with a small mass density of 3.6 mg cm -3 (inset of Fig. 2a), which is similar to previously reported MOF aerogels. 4 The macroscopic aerogel is made of ordered 2D sheet array with the adjacent sheet spacing of approximately 15 μm (Figs. 2a,b), which are further composed of exible 1D nanobelts of 400 nm width, 1.37 nm thickness, and tens of micrometers lengths (Figs. 2c-f). The high-resolution TEM (HRTEM) image shows good crystallinity of NiMn-MOF with lattice fringe spacing of 0.25 nm (Fig. 2e). Further, TEM element mapping shows the homogeneously distribution of Ni, Mn, S, O and C elements throughout nanobelts (Fig. 2f).
Commonly, MOFs are considered as poor electrical conductors (usually 10 -10 S m -1 ) that limit their applications in the eld of electrical devices and electrochemistry. 24 NiCoMn-(ternary counterpart) metal sites were selected for mechanical property studies. The optical images in Figs. 3a,d,g illustrate a compression-resilience cycle test for Co-MOF, NiMn-MOF and NiCoMn-MOF starting by applying 50% strain of initial state. Interestingly, these MOFs aerogel can restore its original shape once the applied force was unloaded, and nally to the fully extended initial state within 1 s. The compression-resilience cycle was repeated for 2,000 times, and their shape degradations are almost negligible, i.e., 8.3% for Co-MOF, 2.9% for NiMn-MOF, and 3.8% for NiCoMn-MOF (Figs. 3c,f,i).
Moreover, the microstructures of Co-, NiMn-, and NiCoMn-MOFs during compression-resilience cycle test were examined by SEM images. Figs. 3b,e,h clearly displays the anisotropic layered structure of Co-MOF, NiMn-MOF and NiCoMn-MOF aerogels, where the adjacent layer spacing change from 22 to 6 μm, 95 to 20 μm and 25 to 6 μm after applying 50% strain to the MOF architecture, respectively. The layer spacing restore to 21 μm for Co-MOF, 90 μm for NiMn-MOF and 23 μm for NiCoMn-MOF after the external force release within 1 s. The above phenomenon is similar to many other superplastic materials like graphene and polymers, 8,27 indicating excellent superplasticity of our MOF aerogels.
Next, we demonstrated the potential applications of our superplastic MOF aerogels as a new class of exible electrodes for hydrogen evolution reaction (HER) in seawater. As we know, exible energy systems have attracted considerable interest because of their remarkable properties such as small-size unit, lightweight, and shape conformability, which are promising components for versatile portable, foldable, and wearable devices. 16,[28][29][30] However, the currently reported hydrogen evolution systems are generally fabricated in the form of bulky and heavy architectures, indicating that they are far behind the requirement of exibility. Conventional electrodes are synthesized by depositing of powder or thin-lm form electrocatalysts, such as MOFs and others, 31-33 onto rigid substrates (like glassy carbon 31,32 and FTO glass 33 ), only resulting in bulk and fragile electrolytic devices. Very recently, several soft current collectors (such as carbon-ber paper 30 and metal foams 16,28 ) have been employed to make HER catalyst electrodes by taking advantages of their excellent properties of high electrical conductivity, interconnected porous networks, and mechanical robust. Integrating these soft substrates with catalytic active species has been achieved by the synthetic strategies of solution-casting or direct growth. However, owning to the intrinsically fragile nature of powders or thin-lms, the exible behaviors of asresultant electrodes are still compromised, leading to considerable activity decay in the process of electrode deformations.
Here we rstly demonstrated the electrochemical hydrogen evolution by using a superplastic MOF aerogels with favorable activity, exibility, kinetics, and stability. Technically, NiMn-MOF was interwoven with nickel foam to produce a catalytic electrode. The as-resultant electrode can promote hydrogen  Table 7).
Notably, the activities of catalyst electrodes show little change after structural deformations. The overpotentials after one-, two-, and three-foldings have slightly increased to 270, 264 and 258 mV at the current density of 10 mA cm -2 , which are 354, 343, and 361 mV at 50 mA cm -2 , and 362, 362, and 386 at 100 mA cm -2 (Figs. 4a,b, Supplementary Table 6). Even under the high current density of 200 mA cm -2 , the HER overpotential only increases to 364, 372, and 394 mV after signi cant deformations of one-, two-, and three-folds (Figs. 4a,b, Supplementary Table 6). To our best knowledge, such mechanically robust electrodes with excellent activities have never been reported before.
Importantly, the catalyst electrodes also show excellent reaction kinetics after deformations, as veri ed by the similar Tafel slopes and charge-transfer resistance (R ct ) from EIS analysis in Fig. 4c and Supplementary Remarkably, all the electrodes with different folds exhibit excellent electrochemical durability, as revealed by the chronopotentiometry with negligible overpotential increase for 12 hrs (Fig. 4e) and identical LSV and EIS before and after testing (Fig. 4f). This excellent electrochemical durability is further con rmed by the SEM, elemental mappings, XRD and FT-IR before and after testing (Supplementary Figs. 27-29). All the above results show that the NiMn-MOF electrode has high physical exibility, stability and mechanical integrity, and can be used as a exible catalytic electrode for natural seawater splitting.
To gain mechanism insights into the HER at NiMn-MOF catalyst, we have conducted a series of density functional theory (DFT) calculations to investigate the catalytic reactions by calculating the Gibbs free energy of H* adsorption, which has been considered as a key descriptor to characterize the HER activity of the electrocatalysts. [34][35][36][37] Firstly, the Gibbs free energy of H* adsorption on NiMn-MOF surface was calculated at different active sites (Ni, Mn, and O sites) with the most energy stable con gurations shown in Fig. 4h and supplementary Fig. 30. Generally, an electrocatalyst with a positive value represents low kinetics of hydrogen adsorption, while a negative value means low kinetics of hydrogen desorption. The optimum value of |ΔG| should be zero. For NiMn-MOF, the ΔG H* of Mn and O active sites is quite positive (0.991 eV) or negative (-1.51 eV), which indicates a strong interaction between H* and these active sites, manifesting in poor HER reaction kinetics. While at the Ni active sites, ΔG H* shows the optimal value of -0.709 eV, demonstrated signi cantly improved activity for HER (Fig. 4h). Therefore, Ni should be the main active sites for HER proceeding on NiMn-MOF surfaces. Further, the ΔG H* of NiMn-MOF is compared with those of bare Ni-MOF and Mn-MOF surfaces calculated by a similar method (Fig. 4i, Supplementary Fig.  31). NiMn-MOF shows a much smaller Gibbs energy of -0.709 eV than bare Ni-MOF (1.10 eV) and Mn-MOF (2.65 eV), thus indicating the strong synergistic effect between Ni and Mn atoms inside the hybrid MOF architecture (Fig. 4i). This result is also consistent to the con guration change inside NiMn-MOF with the Ni-H bonding length reduced by 0.01 angstrom as comparison to bare Ni-MOF without Mn doping.
The signi cantly enhanced HER performances of superplastic NiMn-MOF aerogel is originated from its excellent structural properties. Firstly, the large surface area of NiMn-MOF has been examined by N 2 isotherms, which shows the Brunauer-Emmett-Teller (BET) speci c surface area 2.6 times larger as comparison to its powder counterpart (11.8 vs. 4.6 m 2 g -1 , Supplementary Fig. 32, Table 3). 38 This result is consistent to electrochemical double-layer capacitance (C dl ) test in Supplementary Figs. 33,34 and Table 4, where the C dl of NiMn-MOF is 11.25 mF cm -2 and roughly 3 times larger than that of powder counterpart (3.92 mF cm -2 ). In addition, the competition co-ordination between metals (Ni or Mn) with different electronegativity and oxygen (O) forms an asymmetric M-O covalent bonds, which determine the exposure of cationic active sites. This result is consistent to Zeta potential, XRD and FT-IR change in Figs. 2i,l and 4g, which proves that the bimetallic synergy can produce higher exposure metal active sites. Secondly, we nd the NiMn-MOF with excellent electrical conductivity, which is different from traditional MOF materials. 1 The electrical conductivity have been examined by experiments, as well as ELF and DOS theoretically (Figs. 2j,k, Supplementary Fig. 20, Table 1), which can be explained by the strong coordination between metal ions and organic ligands 2 . The conductive MOF nature can ensure fast charge transport within electrodes that afford excellent catalytic activity.
Thirdly, from the perspective of microstructure, NiMn-MOF aerogel is a multi-level porous structure made of 1D nanobelts assembled into 3D macroscopic architecture. During applying strains, the mutual repulsion between organic groups (-OH and -COOH from 2-thenoic acid) on the surface of the nanobelts can reduce the effect of external forces which is easy to recover after loading release. 13,14,39 As a consequence, the NiMn-MOF electrode has worked stably even after three-folding deformation. The hierarchically porous structure of NiMn-MOF can also provide enormous voids between adjacent nanobelts that can buffer the strains/stress during vigorous gas evolution within electrodes, thereby allow for excellent durability in catalytic reactions.

Conclusion
In conclusion, this work has reported that superelastic MOF-based 3D monoliths could be fabricated by mimicking the hierarchical architecture of nature cork via a low-cost, solution-based procedure. Characteristic of the attractive properties of superelasticity with a good recovery rate, excellent electrical conductivity, high activity and stability of hydrogen evolution from seawater electrolysis all combined together, we expect that such exceptional functional materials will pave the way for a broad range of technological applications. Particularly, our work would allow for exploring the properties and utilizations of MOFs in a self-supporting, structurally exible, and 3D macroscopic form. Furthermore, a wide range of functional materials may be readily introduced into the open micro-, meso-, and macropores inside MOF aerogels, offering plenty of space to develop many new MOF-based hybrid nanomaterials for applications such as photocatalysis, carbon dioxide electroreduction, metal-air batteries, biosensors, and adsorption. Figure 1 Schematic synthesis of superplastic nickel, manganese (NiMn)-metal-organic framework aerogels.    Electrocatalytic performances and mechanism studies of nickel, manganese-metal-organic framework aerogels for hydrogen evolution in natural seawater. The exible catalyst electrode was prepared by intervolving MOF aerogels with nickel foam substrate, which was achieved by introducing nickel foam in the synthetic step of MOF arogels.