GB2570485A - Porous carbonaceous materials and methods for their manufacture - Google Patents

Abstract

A method for producing a porous carbonaceous material is disclosed. A metal-organic framework (MOF) material is provided. The MOF material comprises secondary building units (SBUs) comprising at least one SBU metal linked via organic ligands. The MOF material comprises pores within the crystal structure of the MOF interconnected via pore apertures. Guest species are introduced into the MOF material to form a guest-MOF material. The guest-MOF material is then subjected to carbonization heat treatment to form the porous carbonaceous material. The guest species are introduced into the MOF material using a metal salt solution, the guest species having a diameter not greater than a diameter of the pore apertures, the guest species thereby being introduced into the pores within the crystal structure of the MOF. The metal salt is selected from the group consisting of: metal acetates, metal arsenates, metal carbonates, metal hypochlorites, metal chlorites, metal chlorates, metal perchlorates, metal cyanides, metal nitrites, metal nitrates, metal oxalates, metal phosphates, metal sulphites, metal sulphates, metal thiocyanates, metal thiosulphates, metal halides, thiometallates, organometallic compounds (except carbonyls).

Description

POROUS CARBONACEOUS MATERIALS AND METHODS FOR THEIR MANUFACTURE

BACKGROUND TO THE INVENTION

Field of the invention

The present invention relates to porous carbonaceous materials and methods for their manufacture.

Related art

3D carbon-based structures can be formed from low-dimensional nanostructured building blocks (e.g. carbon dots, fullerenes, nanofibers/tubes and graphene). They have numerous applications, such as lightweight but strong structures and energy storage. Multi-step procedures and complex templates are typically used to fabricate these materials, but few regularly-ordered superstructures have been formed via facile bottomup approaches. Recent work has explored carbonizing metal-organic frameworks (MOFs), and in some cases with catalytically active metals this has resulted in carbonfiber assemblies.

Reference 17 discusses the formation of nano-structured transition metal carbides. A copper-based MOF host incorporates a molybdenum-based polyoxometalate. This material is formed by using the molybdenum-based polyoxometalate as a co-precursor in the synthesis of the copper-based MOF. The MOF material is then subjected to a heat treatment at 800°C for 6 hours under a N₂ atmosphere. The resultant material comprises mesoporous molybdenum carbide nanocrystallites.

Reference 18 reports the synthesis of hollow frameworks of nitrogen-doped carbon nanotubes (CNTs) from MOFs, specifically from purple ZIF-67 particles, which contain C, Co and N. Heat treatment was at 700°C under an Ar/H2 atmosphere followed by an acid wash to remove accessible Co particles.

Reference 10 is a review article based on the design of porous materials and nanocomposites from MOFs. It is acknowledged in Reference 10 that previous work has used MOFs as precursors for the formation of nanoporous carbon materials.

Reference 24 discloses a cage-confinement pyrolysis strategy for the formation of ultrasmall tungsten carbide nanoparticles. W(C0)6 is confined in the nanocages of MAF6 prior to pyrolysis.

Reference 21 discloses the formation of oriented CNTs from MOFs.

Reference 22 sets out a review of the formation of complex nanostructures derived from MOF-based precursors, these materials being of interest for electrochemical energy storage and conversion.

SUMMARY OF THE INVENTION

The inventors have realised that it would be advantageous to be able to manufacture porous carbonaceous materials via a facile approach, permitting a wide variety of structures to be formed. The present invention has been devised in order to address at least one of the above problems. Preferably, the present invention reduces, ameliorates, avoids or overcomes at least one of the above problems.

Accordingly, in a first preferred aspect, the present invention provides a method for producing a porous carbonaceous material comprising the steps:

providing a metal-organic framework (MOF) material comprising secondary building units (SBUs) comprising at least one SBU metal linked via organic ligands, the MOF comprising pores within the crystal structure of the MOF interconnected via pore apertures;

introducing guest species into the MOF material to form a guest-MOF material;

subjecting the guest-MOF material to carbonization heat treatment to form the porous carbonaceous material, wherein the guest species are introduced into the MOF material using a metal salt solution, the guest species having a diameter not substantially greater than a diameter of the pore apertures, the guest species thereby being introduced into the pores within the crystal structure of the MOF.

In this manner, it is found that the MOF material can be a widely available MOF material, which is cost effective to manufacture or purchase. The guest species can be chosen depending on the intended use of the carbonaceous material. Incorporation of the guest species into the MOF material creates the guest-MOF material. It is found that subjecting the guest-MOF material to carbonization heat treatment has the effect of forming a carbonaceous material having the SBU metal and the guest species (typically now in metallic form) present in the carbonaceous material.

As the skilled person readily understands, the SBU refers to the assembled basic unit comprising a metal component (typically a metal ion or cluster) and an organic linker (ligand). Thus, the SBU metal is intended to refer to the metal component of the SBU.

The guest species can be one or a combination of an anion, cation or neutral species.

The diameter of the guest species, where the guest species is an ion, may be determined with reference to the effective ionic radius considering both the ion centre and the solvation shell. This depends in part on the solvent. The effective ionic radius may be determined as disclosed in Michov (2013), where it is referred to as the electrokinetic radius.

The dimensions of the pore apertures (sometimes referred to as “windows”) of the MOF can be determined from the crystal structure, e.g. using single-crystal x-ray diffraction by a process well understood by those skilled in the art. Some examples are shown in Fig. 2 of Jiao et al (2016) and in Fig. 2 of Ma and Balbuena (2012).

For some MOFs, at least some of the pore apertures may have different diameters in different directions. In this case, the guest species preferably have a diameter not substantially greater than a diameter of the pore apertures in at least one direction.

Furthermore, it is noted here that the MOF material may have structural flexibility, and so guest species that are slightly larger than the aperture can still be received into the pore via the aperture.

In some embodiments, the guest species have a diameter smaller than a diameter of the pore apertures.

The carbonization heat treatment preferably takes place in an inert atmosphere. For example, an atmosphere of argon may be used.

The temperature in the carbonization heat treatment may be at least 600 °C, more preferably at least 700 °C and still more preferably at least 750 °C. For example, a carbonization heat treatment temperature may be about 800 °C. Preferably the carbonization heat treatment temperature is not more than 1000 °C.

After the carbonization heat treatment, the porous carbonaceous material may be subjected to a washing step to remove at least some of the SBU metal. Preferably, in the washing step, the porous carbonaceous material is contacted with a washing agent which selectively removes the SBU metal.

Preferably, the SBU metal and the guest species comprise different metals. In this way, the washing agent used in the washing step can be selected so as not to remove the guest species metal.

The guest species may be based on a metal selected from the group consisting of: Ni, Co, Mo, W, Fe, Pd, Rh, Ru, Os, Ir, Pt, Au, Nb, Ta, Mn, Tc, Re, Ag. More preferably, the guest speices is based on a metal selected from the group consisting of: Ni, Co, Mo, W, Fe.

The SBU metal is selected from the group consisting of: Cu, Zn. This is advantageous in the sense that MOFs comprising such SBU metals are well known and their synthesis and properties are clearly understood.

Alternatively, it is possible for the MOF to be formed using a different SBU metal. For example, the SBU metal may be selected from the group consisting of: Ni, Co, Mo, W, Fe, Pd, Rh, Ru, Os, Ir, Pt, Au, Nb, Ta, Mn, Tc, Re, Ag. In this case, the guest species may be based on a metal selected from the group consisting of: Cu, Zn. In this way, a similar combination of metals may be provided in the porous carbonaceous material, but from a different starting point.

Preferably, the metal salt for the guest species is not a simple oxide, carbonyl, or polyoxymetallate. The present inventors have found to date that such metal salts have relatively poor performance in forming suitable porous carbonaceous materials.

Preferably, the metal salt for the guest species is selected from the group consisting of metal acetates, metal arsenates, metal carbonates, metal hypochlorites, metal chlorites, metal chlorates, metal perchlorates, metal cyanides, metal nitrites, metal nitrates, metal oxalates, metal phosphates, metal sulphites, metal sulphates, metal thiocyanates, metal thiosulphates, metal halides, thiometallates, organometallic compounds (except carbonyls). With respect to organometallic compounds, these include chemical compounds containing at least one chemical bond between a carbon atom and a metal, including alkaline, alkaline earth, and transition metals, and/or metalloids such as boron, silicon, and tin. Aside from bonds to organyl fragments or molecules, bonds to 'inorganic' carbon, such as carbon monoxide, cyanide, or carbide, are generally considered to be organometallic as well.

In a second preferred aspect, the present invention provides a porous carbonaceous material obtained by or obtainable by subjecting a guest-MOF material to carbonization heat treatment, the guest-MOF material comprising a metal-organic framework (MOF) material comprising secondary building units (SBUs) comprising at least one SBU metal linked via organic ligands, the MOF comprising pores within the crystal structure of the MOF interconnected via pore apertures with guest species introduced into the MOF material to form the guest-MOF material, wherein the SBU metal and the guest species comprise different metals.

Preferred or optional features set out with respect to the first aspect may also be applied to the second aspect.

In a third preferred aspect, the present invention provides a treated porous carbonaceous material obtained by or obtainable by subjecting the porous carbonaceous material of the second aspect to a washing step to remove at least some of the SBU metal.

The first, second and/or third aspect of the invention may have any one or, to the extent that they are compatible, any combination of the following optional features.

Further optional features of the invention are set out below.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 provides a summary of the microstructure and fabrication of materials according to embodiments of the invention and materials for comparison.

Figure 2 shows the results of characterization of material B2.

Figure 3 shows a summary for combined effects of substituting the MOF or guest on the morphology of nano-diatoms after carbonization.

Figure 4 illustrates that the hierarchically structured nano-diatom (B2) works as an anode material for Li-ion batteries.

Figure 5 shows N2-adsorption isotherms for A1 (adsorption in black and desorption in red) and B1 (adsorption in green and desorption in blue).

Figure 6 shows powder X-ray diffraction (XRD) patterns for as-synthesized HKUST1(Cu), A1, B1 and D1.

Figure 7 shows Fourier difference maps obtained from single crystal XRD for B1 (Figure 7a) and C1 (Figure 7b) revealing the electron density (red contours) distributed in the void of the HKUST-l(Cu) framework ([1 1 0] direction).

Figure 8 shows powder XRD patterns for as-synthesized HKUST-l(Zn), E1 and F1.

Figure 9 shows the temperature profile for the thermal treatment of the samples.

Figure 10 shows powder XRD patterns of carbonized B1, carbonized B1 washed with FeCh (aq) and water and B2 (carbonized B1 washed with FeCh (aq), HCI (aq) and water).

Figure 11 shows powder XRD patterns of carbonized F1 and F2 (i.e. carbonized F1 washed with HCI (aq) and water). The peaks in carbonized F1 are from ZnO, the crystalline by-product of HKUST-l(Zn) pyrolysis.

Figure 12 shows SEM images of thermochemically treated DMF@HKUST-1(Cu) (C2): a SE-SEM image at lower magnification (Figure 12a) showing the overall shape retention from its precursor (C1) and a SE-SEM image at higher magnification (Figure 12b) revealing the surface of C2.

Figure 13 shows 15 keV BSE-SEM images for B2: (Figure 13a) a low magnification image showing the overall nano-diatom morphology and (Figure 13b) image at higher magnification shows the fiber-like building blocks underneath the surface.

Figure 14 shows STEM images for B2: Figure 14a is a low magnification dark-field image showing ground B2 distributed on a carbon grid, and Figure 14b is bright-field image of a web-like surface.

Figure 15 shows enlarged STEM images for fibre-like part of B2: Figure 15 a is brightfield image and Figure 15b is a dark-field image. The round feature indicates a pore created removing the Cu particle.

Figure 16 shows an electron-diffraction pattern revealing the amorphous structure in the fibre-like part of B2 shown in Figure 15.

Figure 17 shows nitrogen physisorption measurements for A2 (0.0382 g) and B2 (0.0328 g). Figure 17a shows nitrogen-adsorption isotherms; Figure 17b shows BET surface area plots; and Figure 17c shows pore size distributions.

Figure 18 shows TGA-FTIR results for heating A2 and B2 in air (combustion): Figure 18a shows FTIR mapping of the emitted gas species upon burning A2; Figure 18b shows FTIR mapping of the emitted gas species upon burning B2; Figure 18c shows combustion TGA profiles for A2 and B2.

Figure 19a shows a schematic drawing for a 2-probe conductivity measurement. Figures 19b-19d shows current-voltage plots for A1, A2 and B2 respectively.

Figure 20 shows TGA-FTIR results for heating A1 and B1 in argon (thermal decomposition).

Figure 21 shows C 1s, Mo 3d, S 2s and S 2p XPS spectra for B1 treated up to various temperatures.

Figure 22 shows powder XRD results for B1 treated up to various temperatures.

Figure 23 shows SE-SEM images of B1 treated up to 400 °C, 600 °C and 800 °C (800 °C for 10 min and 2 hrs).

Figure 24 summarizes the processes occurring under different estimated temperature ranges and the associated characterization techniques used to identify them.

Figure 25 shows the results of cyclic voltammetry (CV) testing for B2 as LiB anode material (sample prepared the same way as those for LiB half-cell test) at a scan rate of 0.2 mV/s.

Figure 26 shows the results of the 1^st, 2^nd and 10^th discharge/charge tests at 1 A/g with a LiB half-cell using B2 as the anode material.

Figure 27 shows cycling performance during fast charge/discharge (1 A/g), observing less than 20% decrease (to about 500 mAh/g) after 200 cycles.

Figure 28 shows the hydrogen evolution reaction performance of the B2 catalysts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS, AND FURTHER OPTIONAL FEATURES OF THE INVENTION

Summarising the following disclosure of preferred embodiments of the invention: 3D carbon-based superstructures have proven remarkably useful for tailoring material properties in applications such as structural mechanics and energy storage. One approach to obtain them has been carbonization of selected metal-organic frameworks (MOFs) with catalytic metals, but this is not applicable to most common MOF structures. Here we present a strategy to transform common MOFs, via guest inclusions and hightemperature MOF-guest interactions, into complex carbon-based superstructures. As an example, we introduce metal salt guests in HKUST-1-like MOFs to generate a family of carbon-based “nano-diatoms” (named for morphological similarities with the naturallyexisting diatomaceous species) with two-to-four levels of structural hierarchy. We report control of morphology by facile changes in the chemistry of the MOF and guest, with implications for the formation mechanisms. We demonstrate that one of these superstructures has unique advantages as a fast-charging Li-ion battery (LiB) anode. The tunability of composition allow the growth of various novel carbon-based superstructures based on the wide variety of currently-available MOF candidates.

3D carbon-based superstructures!¹-⁷! from low-dimensional nanostructured building blocks (e.g. carbon dots, fullerenes, nanofibers/tubes and graphene) can greatly exceed the properties of bulk carbon materials in numerous applications, such as lightweight but strong structures¹⁴! and energy storage^15-8!. Multi-step procedures and complex templates are commonly used to fabricate these materials, but few regularly-ordered superstructures have been formed via facile bottom-up approaches.!³⁵1 Recent work has explored carbonizing metal-organic frameworks (MOFs),^[7-17i and in a few cases with catalytically active metals this has resulted in carbon-fiber assemblies.^[18-231 Here, we present an approach based on MOF-guest^[24-271 precursors and show a general strategy to transform common (non-catalytically active) MOFs into hierarchical carbon-based structures via a simple thermochemical treatment (pyrolysis followed by washing). By changing the type of guest and host, we produce a family of carbon-based “nanodiatoms” with ordered hierarchical structures and investigate the combined effects of substituting the MOF or guest with control over the morphology of the resulting carbonaceous material. One of the nano-diatoms is demonstrated to be superior for fastcharging Li-ion battery (LiB) anodes^128-30! _wjth high storage capacity and stability. These high-temperature guest-induced phenomena, applied to the broad range of available MOF-guest combinations,!³¹! allow bottom-up fabrication of a new generation of 3D carbon-based superstructures for practical use in energy storage, energy conversion, and sensing.

Figure 1 provides a summary of the microstructure and fabrication of materials according to embodiments of the invention and materials for comparison. Figure 1a shows a secondary electron SEM (SE-SEM) image of HKUST-l(Cu) particles (A1); the MOF particles retain their polyhedral particle-like morphology after pyrolysis (A2) at 800 °C under Ar followed by washing as observed in Figure 1b. In contrast, a guest-MOF system, (NH^MoSV dimethylformamide(DMF)@HKUST-1(Cu) (B1, pictured in Figure 1c), turns into carbon-based fibers/webs (Figure 1d) after the same treatment (B2). All the SE-SEM images in Figure 1 are accompanied by the maps produced by energydispersive X-ray spectroscopic signals from SEM (EDS-SEM) for Cu, Mo and S.

Pyrolysis (at 800 °C, under Ar) and washing of either only a MOF precursor (Figure 1a) or one impregnated with a metal-based guest (Figure 1c) yields two carbon-based structures that largely retain the polyhedral geometry of the original MOF crystals but have drastically different internal morphology. From the pure MOF we obtain an amorphous carbon structure (Figure 1b), whereas the MOF-guest transforms into a hierarchical carbon-based nano-diatom (Figure 1d). During thermal treatment the MOF serves as a carbon source, a reaction container and a catalyst for nanostructure formation. The washing step dissolves some metal-based by-products from the MOF host. We study in detail the Cu-based MOF (HKUST-1(Cu),^[32’³³¹ A1) impregnated with a Mo-based guest (ammonium tetrathiomolybdate,i³⁴i (NH^MoS^ ATM) in DMF solution to form our MOF-guest system ((NH₄)2MoS4/DMF@HKUST-1(Cu), B1) (detailed below). Here, the open nanoscale MOF porosity enables homogeneous host-guest mixing and inhibits the segregation induced by reactions, resulting in a unique catalyst composition which eventually transforms the precursor into the structure of B2 (Figure 2).

Figure 2 shows the results of characterization of material B2. For the web-like surface, (Figures 2a and 2b) SE-SEM images; (Figure 2c) dark-field scanning transmission electron microscope (DF-STEM) images; and (Figure 2d and Figure 2e) energydispersive X-ray spectroscopy (EDS) elemental maps of C and Mo/S. For fiber-like structure, (Figures 2f and 2g) SE-SEM images; (Figure 2h) dark-field scanning transmission electron microscope (DF-STEM) images; and (Figures 2i and 2j) EDS elemental maps of C and Mo/S. SE-SEM image, (Figure 2k) reveals the conjunction between the web-like surface and the fiber-like structure. EDS elemental maps reveal an abundance of carbon as well as homogeneous incorporation of Mo and/or S (the Mo La peak overlaps with that of S Ka). XPS spectra are also shown: (Figure 2I) C 1s; (Figure 2m) Mo 3d + S 2s; and (Figure 2n) S 2p. The XPS results also confirm the presence of C, Mo and S. The C 1s signal is strongly dominated by a C-C peak at around 284.8 eV. The Mo 3d peak at 228.7 eV (for Mo°)i³⁵i suggests Mo-C bond formation. The S 2p doublets at ca. 164 eV (S 2p₃/2) and ca. 165 eV (S 2pi/2) indicates the formation of S-C bonding.!²⁹) The presence of Mo and S support the unique guest-MOF interactions during pyrolysis.

As illustrated by Figure 2, detailed morphological characterization reveals a hierarchical carbon-based nano-diatom B2 with four levels of regularly-ordered structure at a wide range of scales. At the outermost level (i) the boundaries of the original MOF-guest particle are still visible (typically ca. 15 pm, shown in Figure 1 d, Figure S12). These particles are composed of (ii) webs (typically 0.5-1 pm in width, shown in Figure 2a-e and Figure S13b) at the surface and (iii) fibers (typically ca. 100 nm in diameter and ca. a few pm in length, shown in Figure 2f-g and Figures 15a and 15b) inside. Additionally, both the webs and fibers have (iv) mesopores (typically ca. 20 nm in diameter, shown in the high-resolution images of Figure 2c, Figure 2h, Figure 14b, and Figures 15a and 15b) created by Cu-particle removal.

To reveal the elementary processes contributing to the structure formation of B2 we performed systematic characterizations at various temperatures and stages of the reaction. We confirm that the carbonization process (i.e. pyrolysis) starts between 300 °C and 400 °C with the help of thermogravimetric analysis coupled with Fourier transform infrared spectroscopy (TGA-FTIR) (Figure 20), X-ray photoelectron spectroscopy (XPS) (Figure 21) and powder X-ray diffraction (PXRD) (Figure 22) on B1 treated at various intermediate temperatures. This process entails decomposition of the organic ligands of the MOF (1,3,5-benzenetricarboxylate) through dissociation of carboxylate groups and gasification as well as the production of metallic Cu particles. Comparing the products of A1 and B1 after identical carbonization conditions (Figure 1), only the host-guest precursor B1 yields a hierarchically structured nano-diatom (B2), whereas treatment of A1 gives rise to amorphous carbonized structures (A2) with the shape of the initial MOF crystals. We also prepared a control sample only impregnated with DMF (DMF@HKUST-1(Cu), C1). C1 behaves similarly to A1, yielding no significant morphological changes upon thermal treatment (Figure 12).

We investigated the potential mechanisms that lead to the structure of B2 and contrast them with the processes occurring in structures A1 and C1, which only produce amorphous morphologies under the same conditions. We start to observe carbon nanostructure formation after heating the guest-impregnated precursor B1 to around 600 °C (Figure 23). The fiber/web-like features become more apparent at 800 °C.

Unlike the transition from A1 to A2, which is governed by a non-catalytic pyrolytic process¹³⁶¹ (a common process involving decomposition, gasification, shrinkage and bond reformation of organic precursors), we hypothesize the unique hierarchical superstructure of our nano-diatom (B2) is produced through a hybrid reaction-diffusion process¹³⁷¹ combining both pyrolysis and metal-catalyzed carbon formation (known for Fe, Ni and Co-based catalysts¹³⁷’³⁸¹). These processes reconfigure the carbon in the original MOF material to create fiber and web-like carbon structures through reactions we later show depend strongly on the identities of the metals in the MOF and guest. Metallic Cu by itself is typically ineffective for catalysis of carbon-based nanostructure growth¹³⁹·⁴⁰¹, as shown for the A1->A2 transition. Mo alone (present in the guest) is also unable to catalyze the formation of carbon-based nanostructures, as its strong interaction with carbon yields inactive molybdenum carbides.¹¹⁷·⁴¹¹ Cu combined with Mo, however, can contribute synergistically to the development of low-dimensional carbon-based building blocks, as investigated previously by Holmes et al.¹⁴⁰’⁴²¹. Their calculations based on density functional theory (DFT) predict that some Cu-Mo metal complexes can have sufficient binding energy to stabilize carbon-based structures such as nanotubes. Thus, in our Cu-based MOF host and Mo-based guest system, metallic Cu (produced by thermal decomposition) becomes more active in the presence of well-dispersed Mo and leads to the development of carbonaceous superstructures from the pyrolyzed intermediates. We also observe XPS features indicating the formation of Mo-C and S-C bonds (Figure 2m, n and Figure 21) at 800°C, which are likely the result of side reactions. Meanwhile, we notice that the nanostructured carbonaceous building blocks are confined by the boundaries of the original MOF-guest crystal B1. Metal-carbon interactions and mobility of metal-based species may account for such confined growth. Mo prefers to stay in the carbonaceous matrix due to its strong affinity to C^[40_421. A significant amount of Cu, however, migrates out of the carbonized matrix due to weak Cu-C interactions!³⁹! and condenses on the surface. This accumulation of Cu on the surface disrupts the Cu/Mo ratio necessary for catalysis and inhibits outward growth of the fibers.^[4°i Consequently, the fibers are confined in a cage (i.e. the web-like structure observed in Figure 2k and Figure 13) with dimensions based on the original MOF host. Subsequent removal of Cu particles in the washing step thus yields significantly higher mesopore density at the web-like surface of B2 (Figure 2c) than on the fiber-like inner building blocks (Figure 2h). To verify these mechanistic hypotheses, we systematically investigated a series of MOF-guest combinations, where we replaced the reactive metals either with homologous metals, or with oxide-forming rather than carbide-forming species which would inhibit the metal-catalyzed carbon-forming reaction (Figure 3).

Figure 3 shows a summary for combined effects of substituting the MOF or guest on the morphology of nano-diatoms after carbonization. Schematic illustrations of precursors are shown in the left column and SE-SEM images of products (after the thermochemical treatment) are shown in the two columns on the right for (a) A1 (top) and D1 (bottom); and (b) E1 (top) and F1 (bottom).

We first replaced Mo in our original guest (ATM) with the homologous W, another group VI element, to yield ammonium tetrathiotungstate^[34i ((NH4)2WS4 or ATT) and form MOFguest (NH4)2WS4/DMF@HKUST-1(Cu) (D1). After thermal carbonization, the product, D2, shows more apparent metal-catalyzed fibers covering the entire surface (Figure 3a). This is consistent with the work by Holmes et a/.!⁴²!, where they suggest W has stronger interaction with graphene than Mo and enhances carbon nanostructure development. The similar morphology of the fibers compared to those from the Mo-based guest suggests a similar reaction mechanism in this homologous system. The higher activity of the tungsten is likely to result in easier nucleation and therefore more numerous but shorter fibers, as observed. Furthermore, this higher activity is likely the reason there is some fiber outgrowth on these superstructures, since even small amounts of W in the Cu-rich surface will still catalyze the growth.

We also used the original guest (i.e. ATM) but replaced the MOF host with a Zn-based MOF with a similar crystal structure, HKUST-l(Zn) (E1).^[33] We prepared the host-guest (NH₄)₂MoS₄/DMF@HKUST-1(Zn) (F1, Figure 3b), which unlike HKUST-l(Cu), produces oxides (ZnO) after thermal decomposition (Figure 11). ZnO formation should inhibit the production of metal-catalyzed carbon fibers. The carbonized product (F2), therefore, may be largely formed by pyrolytic collapse due to decomposition and bond reformation. Nonetheless, unlike the reference carbonized sample (E2), F2 exhibits some micronsized rod-like features. The actual mechanism is still unclear, but probably involves some directional collapse assisted by Mo-based guest derivatives. Both new MOF-guest transformations (D1->D2 and F1->F2) yielded drastically different morphologies from their precursors, as in the case for our original system (B1), whereas samples prepared from the corresponding guest-free MOFs (A1->A2 and E1->E2) primarily retained their original morphologies. This suggests that our MOF-guest treatment procedure provides a general means of creating a broad range of carbon-based superstructures. While shape transformations are observed in all of these MOF-guest systems (B, D and F), the resulting morphology can vary significantly depending on the catalytic and diffusion properties of the metal-based species in both the guest and host.

Figure 4 illustrates that the hierarchically structured nano-diatom (B2) works as an anode material for Li-ion batteries. Figure 4a shows a schematic drawing to illustrate good performance for fast charge/discharge due to the material’s hierarchical structure. Figure 4b shows charge-discharge curves at 0.2 A/g. Figure 4c shows the results of a rate capability test. Figure 4d shows the results of cyclic stability tests at 2 A/g.

In addition to exploring a range of carbonaceous superstructures derived from MOFguest precursors, we also demonstrated the potential usefulness of these new 3D carbon-based materials, particularly the B2, as lithium-ion battery anode (LiB) materials (Figure 4 and discussed in more detail below). We note that the performance of our nano-diatom-based LiB anode is among the best achieved for carbon-based materials!⁵·²⁸!. We obtain 830 mAh/g at 0.2 A/g (10^th cycle) and over 600 mAh/g (10^th cycle) at 1 A/g (Figure 4b, c). The anode also has robust cycling performance during fast charge/discharge (1 and 2 A/g, Figure 27 and Figure 4d), with less than 25% decrease (to about 300 mAh/g) after 500 cycles and consistently high coulombic efficiency at 2A/g. B2 shows greater cycling ability than A2, and both materials have superior performance compared with commercial graphite electrodes (Figure 27). The hierarchical structure of our nano-diatom (B2) may account for its exceptional performance as a LiB anode material (Figure 4a). The open architecture of our nano-diatom provides free space to facilitate electrolyte infiltration, enabling fast kinetics for lithium storage.¹⁵¹ The mechanically strong and well-spaced fiber/web-like structures may withstand volume change during charge/discharge cycling. Furthermore, mesoporosity contributes to relatively high electrochemical utilization by increasing the ion-accessible surface area, thereby enhancing storage capacity.¹⁵¹ In addition, the incorporation of Mo and S (Figure 2) (< 10 wt% each) may also contribute to this high specific capacity.^[29·³⁰¹

In summary, we have produced carbon-based superstructures with multiple levels of hierarchy via a simple, bottom-up thermochemical treatment of guest-MOF complexes of common MOF precursors. In this process, a small amount of guest (Figure 7) empowers the MOF host to undergo dramatic morphological transformations to form guest/hostdependent carbonaceous materials. The results present are for some Cu/Zn-MOF hosts with Mo/W-based guests for illustration, it is recognized that there is an abundance of MOF-guest systems (thousands of MOFs exist¹³¹¹) that will be compatible with the process. It is therefore clear that highly diversified carbon-based structures can be derived from these MOF-guest systems. The use of cheap, commercially available or easy-to-obtain, and easy-to-handle MOFs (e.g. HKUST-l(Cu) in this work) enhances the potential for industrial-scale manufacturing of these superstructures. We have demonstrated the applicability of one of the hierarchical carbonaceous structures as a high-performance LiB anode material with fast charging capability. Thus, on a more general level, the hierarchical architectures of our MOF-guest derived functional materials can be used for a variety of applications, such as energy storage, energy conversion, and sensing.

Detailed experimental methods are set out below. Briefly, HKUST-l(Cu) and HKUST1(Zn) were prepared based on previously reported methods.i³³·⁴³·⁴⁴! MOF-guest precursors were prepared by loading guest-free MOFs with guest-in-DMF (1 g guest chemical with 100 ml DMF) solution, rinsing 3 times with DMF, and drying under nitrogen flow overnight (detailed in Table 1). The precursors were placed in Ar atmosphere then heated to 800 °C, kept at 800 °C for 120 min and cooled down naturally (detailed in Figure 9). The carbonised products were then washed (detailed below) to remove byproducts. Characterization methods are given below.

Materials, Samples and Their Preparation Methods, and Characterisation Methods

1.1 Materials

The following items were used as received: 1,3,5-benzenetricarboxylic acid (H3BTC, 98%, Acros Organics), copper(ll) nitrate trihydrate (Cu(NO)2'3H₂O, 99%, Acros Organics), zinc(ll) nitrate hexahydrate (Zn(NO)2'6H2O, 98%, Acros Organics), Milli-Q water (17 ΜΩ), ethanol (99.8+%, Fisher Scientific), dimethylformamide (DMF, 99+%, Fisher Scientific), Whatman® polyamide membrane filters (0.2 pm), ammonium tetrathiotungstate ((NFU^WS-tor ATT, 99.9+%, Alfa Aesa), ammonium tetrathiomolybdate ((NH₄)2MoS4or ATM, 99.98%, Acros Organics).

1.2 Sample Overview

Table 1 - Description of samples mentioned in the work.

1 (precursors)	2 (after washing)
A ThermallyactivatedHKUST-l(Cu)	Pyrolyzed A1 at 800 °C under Ar, then washed with FeCF (aq), HCI (aq), and deionized water

1 (precursors)	2 (after washing)
B	Thermally activated HKUST-l(Cu) impregnated with a solution of ATM in DMF, ATM@HKUST1(Cu)	Pyrolyzed B1 at 800 °C under Ar, then washed with FeCh (aq), HCI (aq), and deionized water
C	Thermally activated HKUST-l(Cu) impregnated DMF	Pyrolyzed C1 at 800 °C under Ar, then washed with FeCh (aq), HCI (aq), and deionized water
	Thermally activated HKUST-l(Cu) impregnated	Pyrolyzed D1 at 800 °C under Ar, then
D	with a solution of ATT in DMF, ATT@HKUST-	washed with FeCh (aq), HCI (aq), and
	1(Cu)	deionized water
		Pyrolyzed E1 at 800 °C under Ar, then
E	Thermally activated HKUST-l(Zn)	washed with HCI (aq) and deionized
		water
	Thermally activated HKUST-l(Zn) impregnated	Pyrolyzed F1 at 800 °C under Ar, then
F	with a solution of ATM in DMF, ATM@HKUST-	washed with HCI (aq) and deionized
	1(Zn)	water

1.3 Sample Preparation Methods

1.3.1 Preparation of Metal-organic Frameworks (MOFs)

HKUST-l(Cu) was synthesized based on the methods described in Yang et a/.^[S1l Approximately 5 g Cu(NO)2'3H2O was dissolved in 60 ml Milli-Q water and approximately 1.36 g H3BTC was dissolved in 60 ml ethanol. Both solutions were ultrasonicated for several minutes. The two as-prepared solutions were mixed together in a borosilicate glass bottle (with screw cap) followed by the addition of DMF (4 ml). After shaking, the sealed bottle with mixed solution (blue and transparent) was immersed in an oil bath, which was heated to 80 °C and kept at this temperature for 20 hours. The assynthesized HKUST-l(Cu) crystal was collected by filtration and washed with ethanol three times. It was then activated at 130 °C under nitrogen flow overnight to remove the solvent molecules (water and ethanol), forming A1 (a dark blue or violet in color).

The method to produce HKUST-l(Zn) is adapted from the protocols provided by Feldblyum et a/.^[S21 and Bhunia et a/.^[S3l About 2.55 g Zn(NO)2'6H2O and 0.6 g H3BTC were dissolved in 150 ml DMF (colorless and transparent) in a sealed glass bottle, which was then heated to 88 °C and kept at this temperature for 16 hours in an oil bath. The as-synthesized HKUST-l(Zn) crystal was collected by filtration and washed with DMF three times. It was then activated at 130 °C under nitrogen flow overnight to form E1. The prepared MOFs were stored in a vacuum desiccator.

1.3.2 MOF-quest Systems Preparation

Figure 5 shows N2-adsorption isotherms for A1 (adsorption in black and desorption in red) and B1 (adsorption in green and desorption in blue).

Figure 6 shows powder X-ray diffraction (XRD) patterns for as-synthesized HKUST1(Cu), A1, B1 and D1. Since the major peaks at low angles match well, the HKUST1(Cu) framework structure is mostly retained throughout the thermal activation and guest impregnation processes. The disappearance of the peak at about 6° (2 theta) for A1 indicates that a guest-free MOF host was prepared. The reappearance of the peaks at the same 2 theta for B1 and D1 indicates that some guests have been loaded in the MOF host.

Figure 7 shows Fourier difference maps obtained from single crystal XRD for B1 (Figure 7a) and C1 (Figure 7b) revealing the electron density (red contours) distributed in the void of the HKUST-l(Cu) framework ([1 1 0] direction). The electron density is more localized in B1 than in C1, consistent with the heavier elements in ATM (e.g. Mo and S) compared to DMF. There are 2608 in-void electrons per unit cell in B1, whereas there are 2231 in-void electrons (ca. 56 DMF molecules) per unit cell in C1. We underestimate the number of ATM per unit cell in B1 to be 3 by assuming only the additional electrons (i.e. 2608 - 2231 = 337 electrons) come from ATM. The concentration of ATM is, therefore, 53.6 parts per thousand DMF molecules, which is about 18 times higher than the concentration of the as-prepared solution (2.98 parts per thousand DMF molecules for 1 g ATM per 100 ml DMF at room temperature).

Sample B1: To prepare B1, A1 was immersed in ATM/DMF solution (1 g ATM per 100 ml DMF, a dark red color) for 2 hours at room temperature (22 °C, Cambridge, UK) with gentle stirring. The quantity of ATM added was chosen to keep the molar ratio of Mo:Cu to approximately 1:2 during the impregnation (based on the assumed chemical formula of A1 as Cu3(BTC)2, or CU3C18H6O12). The resultant solids were collected by filtration and then washed with DMF until the liquid filtrate was almost colorless. The solid (i.e. B1) was dried at room temperature under nitrogen flow overnight. B1 is green in color.

Sample C1: As a control sample, A1 was immersed in DMF for 2 hours at room temperature with gentle stirring. The solid (i.e. C1) was collected by filtration and dried at room temperature under nitrogen flow overnight. The liquid filtrate is transparent and colorless. C1 is blue in color.

Sample D1: To prepare D1, A1 was immersed in ATT/DMF solution (1 g ATT per 100 ml DMF, a yellow color) for 2 hours at room temperature with gentle stirring. The quantity of ATT added was chosen to keep the molar ratio of Mo:Cu to approximately 1:2 during the impregnation (based on the assumed chemical formula of A1 as Cu3(BTC)2, or CU3C18H6O12). The resultant solids were collected by filtration and then washed with DMF until the liquid filtrate was almost colorless. The solid (i.e. D1) was dried at room temperature under nitrogen flow overnight.

Sample F1: To prepare F1, E1 was immersed in ATM/DMF solution (1 g ATM per 100 ml DMF) for 2 hours at room temperature with gentle stirring. The quantity of ATM added was chosen to keep the molar ratio of Mo:Zn to approximately 1:2 during the impregnation (based on the assumed chemical formula of E1 as Zn₃(BTC)2 (DMF)₃or Zn₃C27N3H27Oi5). The resultant solids were collected by filtration and then washed with DMF until the liquid filtrate was almost colorless. The solid (i.e. F1) was dried at room temperature under nitrogen flow overnight. Note that unlike HKUST-l(Cu), the presence of DMF is important for HKUST-l(Zn) to prevent the structural collapse.^[S2S31

Figure 8 shows powder XRD patterns for as-synthesized HKUST-l(Zn), E1 and F1. Since the major peaks at low angles match reasonably well, the HKUST-l(Zn) framework structure is mostly retained throughout the thermal activation and guest impregnation processes.

1.3.3 Thermal Treatment

Samples A1-F1 were all thermally processed using the same procedure: the samples were heated to 800 °C, kept at 800 °C for 120 min and cooled down naturally in Ar (99.99+%) in an alumina combustion boat in a Carbolite STF 15/450 tube furnace. During the heating stage, the temperature increased at an average rate of ca. 10 °C/min. This process produced carbonized A1-F1.

To minimize the chance of oxidation at elevated temperatures, the tube was vacuumed and refilled with Ar before heating. This process was repeated three times. To prevent overstress on the tube from thermal expansion upon heating, the heating power was initially set to be 30% of its maximum output power. After 60 min, when the furnace reached above 600 °C, the power was increased to 50%. The temperature profile of the crucible containing the samples was measured for the first 90 minutes of heating with a thermocouple (plotted in Figure 9). Error bars give the standard error from the triplicates.

1.3.4 Washing Procedures

Carbonized A1-D1 were washed with excess amounts of 0.5 M FeCb (aq) followed by excess amounts of 10% (v/v) HCI (aq) and plenty of Milli-Q water to remove most of the Cu-containing by-products.

Figure 10 shows powder XRD patterns of carbonized B1, carbonized B1 washed with FeCb (aq) and water and B2 (carbonized B1 washed with FeCb (aq), HCI (aq) and water). B2 is also described as the nano-diatom in the article. FeCh can effectively remove Cu particles (the by-product during HKUST-1(Cu)’s pyrolysis)^[S41, but it produced a small amount of CuCI, which was removed by HCI (aq, 10% v/v) treatment.

Carbonized E1 and F1 were washed with excess amounts of 1 M HCI (aq) followed by plenty of MilliQ water to remove most of the Zn-containing by-products.

Figure 11 shows powder XRD patterns of carbonized F1 and F2 (i.e. carbonized F1 washed with HCI (aq) and water). The peaks in carbonized F1 are from ZnO, the crystalline by-product of HKUST-l(Zn) pyrolysis.

1.4 Characterization Methods

SEM: Secondary electron SEM (SE-SEM) images and mappings with energy-dispersive spectroscopy (SEM-EDS) were acquired using a FEI Nova NanoSEM™ with a secondary electron detector (electron acceleration voltage: 5 kV) and EDS detector (electron acceleration voltage: 15 kV). Backscattered electron SEM (BSE-SEM) images were obtained from a Phenom ProX Desktop microscope (electron acceleration voltage: 15 kV).

Powder X-ray diffraction (XRD): Powder XRD patterns were collected on a Bruker D8 ADVANCE with a Ni 0.012 filter between the X-ray source and the sample (20 from 3.5° to 80° and a step size of 0.04°). Samples were uniformly distributed on a silicon disc supported by a round holder. The holder and the disc were rotated (30 rpm) during the measurement. The illumination area is fixed so that the exposure area forms a circle (16 mm in diameter) with the rotation.

Single Crystal (SC) XRD: Crystal structure determination was carried out using an Oxford Gemini E Ultra diffractometer, Mo Ka radiation (λ = 0.71073 A), equipped with an Eos CCD detector. Data collection and reduction were conducted using CrysAliPro (Agilent Technologies). An empirical absorption correction was applied with the 0lex2 platform.^[SS| The structure was solved using ShelXT^[S6] and refined by ShelXL^[S?1.

Table 2 - Information for SC XRD.

Sample	B1	C1
Empirical formula	CU3C18H6O15	CU3C18H6O15
Temperature/K	299.0(9)	298.3(4)
Crystal system	cubic	cubic
Space group	Fm3m	Fm3m
a/A	26.3531(8)	26.3032(6)
a/°	90	90
Volume/A3	18301.8(16)	18198.2(11)
pcaicg/cm3	0.948	0.949
p/mnr1	1.420	1.423
F(000)	5136.0	5184.0
Crystal size/mm3	0.18 x 0.12 x 0.1	0.23 x 0.144 x 0.09
Radiation	ΜοΚα (λ = 0.71073)	ΜοΚα (λ = 0.71073)
2Θ range for data collection/0	3.09 to 56.574	4.38 to 56.49
Reflections collected	4078	4026
Independent reflections	11 06 [Rint = 0.0931, Rsigma = 0.0731]	1091 [Rint = 0.0926, Rsigma = 0.0582]
Data/restraints/parameters	1106/0/36	1091/0/36
Goodness-of-fit on F2	1.084	1.174
Final R indexes [Ι>=2σ (I)]	R1 = 0.0932, wR2 = 0.2702	R1 = 0.0878, wR2 = 0.2529
Final R indexes [all data]	R1 = 0.1404, wR2 = 0.3063	R1 = 0.1062, wR2 = 0.2795

X-ray photoelectron spectroscopy (XPS): The XPS system was equipped with a

SPECS PHOIBOS 150 electron energy analyzer with a total energy resolution of 500 meV. The measurements were done using a monochromatic Al Ka x-ray source (1486.6 eV). A low-energy electron flood gun was used to compensate the positive charge during the measurements. All spectra were aligned to the C 1s at 284.8 eV. For analysis of the C 1s, Mo 3d, and S 2p spectra, a Shirley background was subtracted.

Scanning TEM (STEM)/STEM-EDS: STEM images were acquired on an FEI Osiris operating at 200 keV fitted with bright field (BF) and annular dark field (ADF) detectors. Energy dispersive spectra were simultaneously recorded on four Bruker silicon drift detectors. Diffraction patterns were recorded on a Gatan UltraScan 1000XP CCD camera. STEM samples were prepared by drop-casting 100 pl of sample suspension (ground sample powder dispersed in ethanol) on carbon grids.

Thermogravimetric analysis with Fourier transform infrared spectroscopy (TGAFTIR): TGA was performed with a TA Instruments Q500 thermogravimetric analyser. Samples were heated from room temperature up to 1000 °C at a rate of 10 °C/min. Thermal decomposition was carried out in Ar, whereas combustion was performed in air. Gaseous species emitted from the thermogravimetric analyzer were passed to a Thermo Scientific™ Nicolet™ iS™ 10 FTIR spectrometer for analysis. A spectrum was acquired every 40 sec. The FTIR sampling chamber was kept at 400 °C. The FTIR optical path length is ca. 100 mm.

Nitrogen physisorption measurements: Nitrogen physisorption measurements were undertaken at 77 K (-196 °C) using a MicroMeritics TriStar 3000 Porosimeter. Prior to the N2 adsorption test, samples were quickly taken out from the vacuum desiccator and evacuated for 1 hour at 120 °C under nitrogen flow. The surface area of the carbonized samples was quantified using the Brunauer-Emmett-Teller (BET) method.^lS8i

Two-probe electric current-voltage (l-V) measurement: Electrical measurements were conducted using a combination of a Keithley 2182 nanovoltmeter to apply voltage and a Keithley 2440 5A source meter to read current. The voltage was varied from +5 V to -5 V and then back to +5 V, with a step size of 0.5 V. The detailed sampling configuration and measurement procedures are shown below.

Electrochemical properties characterization and lithium-ion battery (LiB) half-cell test: The electrochemical properties of the B2 electrode were investigated using 2032type coin cells with a lithium foil counter electrode and 1 M LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DMC) (1:1 v/v) as the electrolyte. The working electrode was fabricated using a mixture of 90 wt.% B2 and 10 wt.% (polyvinylidene fluoride, PVDF), used as a binder; the mixture was applied on a Cu foil. Each working electrode had a surface area of 1.13 cm², and the density of active material in the electrode was approximately 1 mg/cm². For comparison, electrodes were also prepared using A2 or commercial graphite mixed with 10 wt.% binder. The specific capacities of all the electrodes were calculated from the total masses of active materials, and their electrochemical characteristics using were examined by cyclic voltammetry (CV) and charge-discharge curves using a potentiostat/galvanostat (IVIUM/ LAND) within a 0.01-3 V range against Li⁺/Li.

SEM Images of Reference Sample C2

Figure 12 shows SEM images of thermochemically treated DMF@HKUST-1(Cu) (C2): a SE-SEM image at lower magnification (Figure 12a) showing the overall shape retention from its precursor (C1) and a SE-SEM image at higher magnification (Figure 12b) revealing the surface of C2.

Additional Characterisations of B2

Figure 13 shows 15 keV BSE-SEM images for B2: (Figure 13a) a low magnification image showing the overall nano-diatom morphology and (Figure 13b) image at higher magnification shows the fiber-like building blocks underneath the surface.

Figure 14 shows STEM images for B2: Figure 14a is a low magnification dark-field image showing ground B2 distributed on a carbon grid, and Figure 14b is bright-field image of a web-like surface.

Figure 15 shows enlarged STEM images for fibre-like part of B2: Figure 15 a is brightfield image and Figure 15b is a dark-field image. The round feature indicates a pore created removing the Cu particle.

Figure 16 shows an electron-diffraction pattern revealing the amorphous structure in the fibre-like part of B2 shown in Figure 15.

Figure 17 shows nitrogen physisorption measurements for A2 (0.0382 g) and B2 (0.0328 g). Figure 17a shows nitrogen-adsorption isotherms; Figure 17b shows BET surface area plots; and Figure 17c shows pore size distributions.

BET theoryi^S8i dealing with multi-layer physisorption was used to determine the specific surface area of A2 and B2. By obtaining the specific quantity, Q (cm³/g), of N2 adsorbed in a sample as a function of pressure, P, a linear plot can be obtained with equation (1):

— = — (-) + — (1)

Q(——1) Qm^c Qm^c where, Q_m (cm³/g) is the specific quantity of N2 adsorbed to form a monolayer on the

C —1 sample and c is the BET constant. Q_m can be determined from the slope,---, and the yQm^c intercept, — of the linear plot:

Qm^c xm ~ / c-i _| 1 λ wm^c Qm^c'

The specific surface area, S (m²/g), can then be determined:

$ _ Qm^A^s (2) (3) where, N_A is Avogadro's number (mol·¹), s is the adsorption cross-sectional area of N₂ (0.162 nm²)i⁹i and V is the molar volume of N₂, which is about 22400 cm³/mol under standard temperature and pressure (STP).

For 0.0382 g A2, the slope is 0.009402 ± 0.000127 g/cm³ and the y-intercept is 0.000035 ± 0.000016 g/cm³. The specific surface area is 464.7184 ± 6.3607 m²/g.

For 0.0328 g B2, the slope is 0.202991 ± 0.000639 g/cm³ and the y-intercept is 0.001070 ± 0.000079 g/cm³. The specific surface area is 21.3328 ± 0.0673 m²/g.

Figure 18 shows TGA-FTIR results for heating A2 and B2 in air (combustion): Figure 18a shows FTIR mapping of the emitted gas species upon burning A2; Figure 18b shows FTIR mapping of the emitted gas species upon burning B2; Figure 18c shows combustion TGA profiles for A2 and B2. SO2 is produced in the combustion experiment of B2, confirming S incorporation. Compared with A2, B2 has ca. 11 wt.% solid product (likely to be ΜοΟδ) remaining after combustion at ca. 500 °C. We thus estimate an about

7.4 wt.% Mo incorporation in B2. The maximum S incorporation is ca. 9.8 wt.% provided all the S comes from M0S4²· and is retained in B2.

The method used to obtain the conductivities and current-voltage plots of A2 and B2 was adapted from those reported previously for carbon black powder^[S101 and metal powder^[S11]. The setup is shown in Figure 19a. Briefly, sample powder was confined in a mold and shaped under pressure to be a cylinder with defined radius, r, and height, h. The top and bottom of the sample cylinder were in contact with the aluminum electrodes separately whereas the rest of the cylinder was surrounded by an electrically insulating Teflon mold. A voltage (V) was applied across the sample and the current (I) travelling through the cylinder was measured. For each sample, the voltage starts from 5 V and then goes down stepwise (0.5 V per step) to -5 V. It was then increased stepwise back to 5 V.

The electrical conductivities, o, of A2 and B2 were determined by fitting a line on the data from -2 V to 2 V. The slope (l/V) is 1/R, where R is resistance. The slope is about 0.05 Ω'¹ for A2 and ca. 0.01 Ω'¹ for B2. The value of h is 2.43 mm for A2 and 2.84 mm for B2. The value of r for A2 is the same as that for B2, which is 1 mm. o is determined by:

_ 1 _ h. _ h.

p ^RA (t)^{777 2} (4) where, p is resistivity and A is the area of the cylindrical top or bottom.

The conductivities of A2 and B2 are, therefore, ca. 0.39 S/m and ca. 0. 1 S/m respectively.

Figure 19a shows a schematic drawing for a 2-probe conductivity measurement. Figures 19b-19d shows current-voltage plots for A1, A2 and B2 respectively.

Characterizing the Transformation during the Thermal Treatment of B1

Figure 20 shows TGA-FTIR results for heating A1 and B1 in argon (thermal decomposition). Figure 20a shows FTIR mapping of the emitted gas species upon heating A1; Figure 20b shows FTIR mapping of the emitted gas species upon heating B1; Figure 20c shows TGA profiles for A1 and B1. In Figure 20b, the presence of NH3 indicates the thermal decomposition of ATM; DMF comes from physical desorption of the solvent (DMF). At around 365 °C, CO2 comes from the dissociation of carboxylate group from BTC ligands, which indicates the occurrence of pyrolysis.

Figure 21 shows C 1s, Mo 3d, S 2s and S 2p XPS spectra for B1 treated up to various temperatures. The disappearance of O-C=O bonding in the C 1s spectra between 300 °C and 400 °C is consistent with the occurrence of pyrolysis. The Mo 3d + S 2s spectrum for B1 has a doublet at 227.7 eV and 230.8 eV, matching amorphous MoS3^[S12] with a slight shift to lower binding energy overlapping with the S 2s peak. The peak shift towards higher binding energy from room temperature to 200 °C verifies the decomposition of ATM.I^S13S141 Additionally, a peak for Mo° emerges at ca. 228.7 eV at around 800 °C. Neither Mo-S nor Mo-0 can give rise to the Mo° peak. This Mo° peak as well as the overall Mo 3d spectrum for the sample treated at 800 °C for 2 hours are consistent with the MoC_x study carried out by Wan et al.^[S15]. The result supports the formation of Mo-C bonding. The S 2p peak shift towards higher binding energy at around 800 °C indicates the formation of S-C bondingi^s16i. Samples were treated up to targeted elevated temperatures using the tube furnace under the temperature profile shown in

Figure 9. When a sample reached the target temperature, it was kept at this temperature for 10 min to reach equilibrium, unless mentioned specifically.

Figure 22 shows powder XRD results for B1 treated up to various temperatures. The significant pattern mismatch between 200 °C and 300 °C indicates the collapse of the host organic framework. Above 400 °C, XRD patterns only show peaks for metallic Cu, which verifies the occurrence of pyrolysis between 300 °C and 400 °C. Samples were treated up to targeted elevated temperatures using the tube furnace under the temperature profile shown in Figure 9. When a sample reached the target temperature, it was kept at this temperature for 10 min to reach equilibrium, unless mentioned specifically.

Figure 23 shows SE-SEM images of B1 treated up to 400 °C, 600 °C and 800 °C (800 °C for 10 min and 2 hrs). At around 600 °C, the development of carbonaceous nanostructures initiates. These nanostructures will eventually grow to fibers/webs at 800 °C (after 2 hours of treatment). Samples were treated up to targeted elevated temperatures using the tube furnace under the temperature profile shown in Figure 9. When a sample reached the target temperature, it was kept at this temperature for 10 min to reach equilibrium, unless mentioned specifically.

Figure 24 summarizes the processes occurring under different estimated temperature ranges and the associated characterization techniques used to identify them. This summary only covers the transformation of B1, which is treated according to the temperature profile mentioned discussed above. The processes are also summarized below:

ATM decomposition:

-» Afo5_x + NIP + H - S species (incop orated in MOF) or H_ZS

HKUST-l(Ou) decomposition:

-+ — WjWWii iL — MSerf ^: 4- Crt: F

Note: €^Η₃(€00)/~ (i.e. 8TC) Is the organic ligand for HKUST-l(Cu).

Nori-vaporised C-based species are the feedstock for the C-based nanostructure development.

S-C and Mo-C bond formation:

MoS* in C — based matrix -+ Mo — C & S — C in C — based matrix

Exploration of Applications

5.1 Supporting Information for B2 as Anode Material for Li-ion Batteries (LiBs)

Figure 25 shows the results of cyclic voltammetry (CV) testing for B2 as LiB anode material (sample prepared the same way as those for LiB half-cell test) at a scan rate of 0.2 mV/s.

Figure 26 shows the results of the 1^st, 2^nd and 10^th discharge/charge tests at 1 A/g with a LiB half-cell using B2 as the anode material.

Figure 27 shows cycling performance during fast charge/discharge (1 A/g), observing less than 20% decrease (to about 500 mAh/g) after 200 cycles. B2 shows greater cycling ability than A2; and both materials have superior performance over commercial graphite electrodes.

5.2 B2 as a Hydrogen Evolution Reaction (HER) electrocatalyst

A catalyst suspension was prepared by dispersing 5 mg of the catalyst (B2) in 1 mL of solution containing 0.95 mL of ethanol and 50 pL of 0.5 wt% Nafion solution followed by ultrasonication for 1h.

Electrodes were prepared on two different substrates: fluorine-doped tin oxide (FTO) and carbon fiber paper. FTO electrodes were prepared by contacting FTO slides (Sigma31

Aldrich; surface resistivity: 7 Ohm/sq), cut to 5x7 mm², with copper wire using silver paste. The contact was sealed with epoxy (3M Scotch-Weld DP410), leaving an exposed electrode area of approximately 5x5 mm². Carbon fiber electrodes were prepared by contacting carbon fiber paper (Toray carbon paper 120, wet proofed), cut to 10x10 mm², with copper wire using silver paste. The contact side was sealed with epoxy (3M Scotch-Weld DP410). For all electrodes, 0.2 mg cm’² of the catalyst suspension was drop-cast with a micropipette on the electrodes and dried overnight at room temperature. Electrocatalysis measurements were performed in a home-made closed glass reactor. Tests were performed at a scan rate of 50 mV sec^-1. A three electrode configuration was used, with an Ag/AgCI electrode saturated with KCI as the reference electrode and a glassy carbon rod as the counter electrode. All potentials measured were converted to the reversible hydrogen electrode (RHE) scale according to the Nernst equation: E_RHe = EAg/Agci + 0.197 + 0.059 pH. The electrolyte was purged with H2 prior to every measurement through a porous glass frit. Electrochemical measurements were recorded and analyzed using an Ivium Compact Stat potentiostat and the Ivium Soft software. All data were corrected for internal resistance and the current densities were normalized to the geometric area of the electrodes.

Figure 28 shows the hydrogen evolution reaction performance of the B2 catalysts. LSV polarisation curve of B2 on FTO with 0.2mg cm'² loading (top) and B2 on carbon fiber paper (bottom), with insets showing Tafel plots.

The electrocatalytic activity of the B2 sample for the hydrogen evolution reaction (HER) was examined in ^-saturated 0.5 M H2SO4. Figure 28 shows the linear sweep voltammograms (LSVs) for B2 on FTO (top) and carbon fiber paper (bottom). The corresponding Tafel plots insets) are constructed based on this data. After internal resistance (IR) correction, the Tafel slope (b) is determined by fitting the polarization data to the Tafel equation (η = a + b log \j\, where η is the overpotential, b is the Tafel slope and j is the current density in mA/cm²). B2 on FTO electrodes exhibits an average Tafel slope = 125 mV dec¹ and η (/ = 10 mA cm^-2) = 430 mV, which suggests the Volmer (discharge) reaction to be the rate limiting step.i^s17i Here, the active material detaches rapidly from the FTO surface, so that only the first scan is reported. On carbon fiber paper electrodes, however, we observe increases in both catalytic activity (η (at j = 10 mA cm'²) = 197 mV) and stability. These improvements are most likely due to better dispersion and adhesion of the catalyst to the electrode surface.

B2 deposited on carbon fiber paper is more active for HER than carbon fiber paper by itself (bottom part of Figure 28). Mo and S incorporation (< 10 wt% each) (Figure 2 and Figure 18c & 22) are likely to contribute to this higher electrocatalytic activity.^{[S12 S181} The overpotential, η, of 197 mV aty = 10 mA cm ², is quite competitive compared with other carbonized MOF-based electrocatalysts.[^S4S19_S211

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

All references referred to above and/or listed below are hereby incorporated by reference.

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Claims (12)

1. A method for producing a porous carbonaceous material comprising the steps:

providing a metal-organic framework (MOF) material comprising secondary building units (SBUs) comprising at least one SBU metal linked via organic ligands, the MOF comprising pores within the crystal structure of the MOF interconnected via pore apertures;

introducing guest species into the MOF material to form a guest-MOF material;

subjecting the guest-MOF material to carbonization heat treatment to form the porous carbonaceous material, wherein the guest species are introduced into the MOF material using a metal salt solution, the guest species having a diameter not substantially greater than a diameter of the pore apertures, the guest species thereby being introduced into the pores within the crystal structure of the MOF.

2. A method according to claim 1 wherein, where the pore apertures have different diameters in different directions, the guest species have a diameter not substantially greater than a diameter of the pore apertures in at least one direction.

3. A method according to claim 1 or claim 2 wherein, after carbonization heat treatment, the porous carbonaceous material is subjected to a washing step to remove at least some of the SBU metal.

4. A method according to any one of claims 1 to 3 wherein the SBU metal and the guest species comprise different metals.

5. A method according to any one of claims 1 to 4 wherein the guest species is based on a metal selected from the group consisting of: Ni, Co, Mo, W, Fe, Pd, Rh, Ru, Os, Ir, Pt, Au, Nb, Ta, Mn, Tc, Re, Ag.

6. A method according to any one of claims 1 to 5 wherein the guest species is based on a metal selected from the group consisting of: Ni, Co, Mo, W, Fe.

7. A method according to any one of claims 1 to 6 wherein the SBU metal is selected from the group consisting of: Cu, Zn.

8. A method according to any one of claims 1 to 4 wherein the SBU metal is selected from the group consisting of: Ni, Co, Mo, W, Fe, Pd, Rh, Ru, Os, Ir, Pt, Au, Nb, Ta, Mn, Tc, Re, Ag, and the guest species is based on a metal selected from the group consisting of: Cu, Zn.

9. A method according to any one of claims 1 to 8 wherein the metal salt is not a simple oxide or carbonyl, or poly-oxymetallate.

10. A method according to any one of claims 1 to 9 wherein the metal salt is selected from the group consisting of: metal acetates, metal arsenates, metal carbonates, metal hypochlorites, metal chlorites, metal chlorates, metal perchlorates, metal cyanides, metal nitrites, metal nitrates, metal oxalates, metal phosphates, metal sulphites, metal sulphates, metal thiocyanates, metal thiosulphates, metal halides, thiometallates, organometallic compounds (except carbonyls).

11. A porous carbonaceous material obtained by or obtainable by subjecting a guestMOF material to carbonization heat treatment, the guest-MOF material comprising a metal-organic framework (MOF) material comprising secondary building units (SBUs) comprising at least one SBU metal linked via organic ligands, the MOF comprising pores within the crystal structure of the MOF interconnected via pore apertures with guest species introduced into the MOF material to form the guest-MOF material, wherein the SBU metal and the guest species comprise different metals.

12. A treated porous carbonaceous material obtained by or obtainable by subjecting the porous carbonaceous material of claim 11 to a washing step to remove at least some of the SBU metal.