Iron-based catalysts

Abstract

Described herein is a catalyst comprising a nitrogen-doped carbon matrix, iron atoms, iron carbide particles, and carbon nanotubes, wherein the carbon nanotubes are dispersed and bonded to the nitrogen-doped carbon matrix, the iron atoms, and the iron carbide particles are dispersed and bonded to the nitrogen-doped carbon matrix and / or the carbon nanotubes. The catalyst may be used to coat an electrode for use in an electrochemical cell and electrolysis.

Description

IRON-BASED CATALYSTSREFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to Singapore patent application number 10202402523R with a filing date of 20 August 2024 and titled “Iron-based catalysts for electrochemical mineral recovery from waste streams and CO2 capture" and is incorporated herein by reference.FIELD OF THE INVENTION

[0002] The present disclosure relates to iron-based catalysts and their use in an electrochemical cell and electrolysis.BACKGROUND OF THE INVENTION

[0003] Electrochemical redox mediation technologies, which leverage oxygen reduction reactions (ORR) to produce crucial hydroxide (OH_) ions necessary for the formation of metal hydroxide or carbonate species, have been demonstrated in WO 2025 / 071496 as promising alternatives for CO2 capture and mineral extraction from waste streams such as seawater, desalination brine or mining wastewater. However, existing cathode catalysts rely on precious platinum (Pt). Despite its effectiveness, the scarcity and high cost of Pt hinder its widespread adoption, limiting the scalability and economic feasibility of the patented technology. Furthermore, Pt-based catalysts may encounter challenges associated with side reactions. For instance, under high current working conditions, the kinetically more favorable hydrogen evolution reaction (HER) competes simultaneously with the ORR, reducing efficiencies and lowering mineralization capacity. The generated hydrogen bubbles over the Pt catalyst may block the active sites and induce the detachment of the catalyst from the electrode, which brings interior stability under demanding reaction conditions. As a result of the side reactions, there is a waste of the energy supplied (i.e. high energy consumption) and low recovery efficiency. This implies that Pt-based catalysts may only operate effectively within a limited potential window where the ORR dominates.

[0004] In harsh environments like seawater, brine, or mining wastewater, electrocatalysts must not only exhibit strong intrinsic activity but also demonstrate high resistance to corrosion induced by chlorine. Moreover, their active sites should feature an optimal electronic and chemical bonding structure to enhance catalytic activity and stability. Therefore, the strategic design of single atomic electrocatalysts could confer superior resistance to chlorine poisoning, making them highly promising for applications in seawater electrolysis.

[0005] Zhan et al. (Chemical Engineering Journal, 2022, 443, 136456) describe the synthesis of a catalyst made of iron single atoms on N-doped activated carbon which exhibited notable ORR efficacy. However, the study solely assessed the anticorrosion properties of iron in a sodium chloride electrolyte.

[0006] One promising candidate garnering significant attention is a hybrid catalyst comprising iron carbide (FesC), Fe-Nx, and carbon nanotubes. On one hand, the abundance and cost-effectiveness of iron-based materials address concerns regarding scarcity and production costs. On the other hand, the combination of FesC and Fe-Nx demonstrates exceptional activity and selectivity for the oxygen reduction reaction (ORR), even in challenging environments such as seawater, making them well-suited for catalyzing CO2 mineralization. Moreover, the integration with carbon nanotubes further bolsters stability and efficiency by providing a conductive support structure. Despite the promising properties, current synthesis methods for these hybrid catalysts often entail complex procedures, multiple steps, or the use of toxic solvents. Consequently, there is also a need to develop a simplified and convenient protocol to streamline synthesis and render catalyst production more accessible.SUMMARY OF THE INVENTION

[0007] In a first aspect, there is provided a catalyst comprising a nitrogen-doped carbon matrix, iron atoms, iron carbide particles, and carbon nanotubes, wherein the carbon nanotubes are dispersed and bonded to the nitrogen-doped carbon matrix, the iron atoms, and the iron carbide particles are dispersed and bonded to the nitrogen-doped carbon matrix and / or the carbon nanotubes.

[0008] In an embodiment, the carbon nanotubes are present from 20 % v / v to 50 % v / v. Preferably, the carbon nanotubes are present from 40 % v / v to 50 % v / v.

[0009] In an embodiment, the iron atoms and the iron carbide particles and the carbon nanotubes are bonded to a plurality of nitrogen atoms in the nitrogen-doped carbon matrix.

[0010] In an embodiment, a X-ray photoelectron spectroscopy N 1 s spectra of the catalyst shows characteristics peaks of pyridinic-N, metal-N, pyrrolic-N, graphitic-N, and oxidized-N at 398.6 eV, 399.6 eV, 401.2 eV, 402.7 eV, and 405.2 eV respectively, wherein the peaks at 402.7 eV and 405.2 eV is lower in intensity than the peaks at 398.6 eV, 399.6 eV, and 401 .2 eV.

[0011] Preferably, the peak at 398.6 eV has the highest intensity followed by the peak at 401.2 eV and subsequently the peak at 399.6 eV,

[0012] In an embodiment, the peak at 398.6 eV has from 30 to 40% of the nitrogen species as measured in the X-ray photoelectron spectroscopy N 1s spectra of the catalyst. Preferably, the peak at 398.6 eV has from 33 to 37% of the nitrogen species.

[0013] In an embodiment, the peak at 401.2 eV has from 13 to 23% of the nitrogen species as measured in the X-ray photoelectron spectroscopy N 1s spectra of the catalyst. Preferably, the peak at 401 .2 eV has from 18 to 20% of the nitrogen species.

[0014] In an embodiment, the peak at 399.6 eV has from 15 to 25% of the nitrogen species as measured in the X-ray photoelectron spectroscopy N 1s spectra of the catalyst. Preferably, the peak at 399.6 eV has from 16 to 20% of the nitrogen species.

[0015] In an embodiment, the peak at 402.7 eV has from 10 to 20% of the nitrogen species as measured in the X-ray photoelectron spectroscopy N 1s spectra of the catalyst. Preferably, the peak at 402.7 eV has from 13 to 17% of the nitrogen species.

[0016] In an embodiment, the peak at 405.2 eV has from 7 to 17% of the nitrogen species as measured in the X-ray photoelectron spectroscopy N 1 s spectra of thecatalyst with the percentages of the five nitrogen species adding up to 100%.Preferably, the peak at 402.7 eV has from 10 to 1 % of the nitrogen species,

[0017] In an embodiment, the nitrogen-doped carbon matrix is or is formed from an iron-doped zeolitic imidazolate framework.

[0018] In an embodiment, a X-ray photoelectron spectroscopy Fe 2ps / 2 spectra of the catalyst shows characteristics peaks at 711.7 eV, 710.2 eV, and 708.4 eV corresponding to Fe3+, Fe2+, and metallic Fe or FesC respectively.

[0019] In an embodiment, a X-ray diffraction pattern of the catalyst shows characteristic peaks at 37.7°, 43.4°, 44.5°, 45.7°, 48.7°, 54.4°, and 57.7°.

[0020] In an embodiment, the catalyst is free from platinum and / or palladium.

[0021] Preferably, the catalyst consists essentially of the nitrogen-doped carbon matrix, the iron atoms, the iron carbide particles, and the carbon nanotubes.

[0022] More preferably, the catalyst consists of the nitrogen-doped carbon matrix, the iron atoms, the iron carbide particles, and the carbon nanotubes.

[0023] In a second aspect, there is provided a method of preparing an iron-doped zeolitic imidazolate framework, the method comprising stirring a mixture under suitable conditions to provide the iron-doped zeolitic imidazolate framework, the mixture comprising water, an iron salt, a zinc salt, a compound of Formula I and a compound of Formula II, wherein Formulaeach of R1, R2, and R3is independently selected from a group consisting of hydrogen, a halogen, a nitro, and a C1 to C3 substituted or unsubstituted alkyl; andR4is selected from a group consisting of hydrogen, a C1 to C3 substituted or unsubstituted alkyl, a substituted aryl or an unsubstituted aryl, R5is selected from agroup consisting of hydrogen, a halogen, a hydroxyl, or a C1 to C3 substituted or unsubstituted alkyl; and isolating the iron-doped zeolitic imidazolate framework.

[0024] In an embodiment, R1is the C1 to C3 substituted or unsubstituted alkyl and preferably R2and R3are both hydrogens. For example, R1is selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, and cyclopropyl. In an example, the compound of Formula I is 2-methylimidazole.

[0025] In an embodiment, the iron salt is a Fe2+salt. In an example, the iron salt is an inorganic Fe2+anhydrous or hydrated salt. The inorganic Fe2+anhydrous or hydrated salt may be selected from the group consisting of FeSC , FeCh, and Fe(NO3)2.

[0026] In an embodiment, the zinc salt is an inorganic Zn2+anhydrous or hydrated salt. The inorganic Zn2+anhydrous or hydrated salt may be selected from the group consisting of Zn(NOs)2, ZnCl2, and ZnSC .

[0027] In an embodiment, a molar ratio of the iron salt to the zinc salt is from 1 :10 to 1 :1 , preferably the molar ratio is from more than 1 :10 to less than 1 :1 , more preferably the molar ratio is 1 :5.

[0028] In an embodiment, a second molar ratio of the compound of Formula I to a total amount of metal ions is at least 1 :1 , preferably the second molar ratio is at least 3:1. In an embodiment, the second molar ratio is at most 10: 1 , preferably the second molar ratio is at least 4: 1 to 10: 1 .

[0029] In an embodiment, a third molar ratio of the compound of Formula I to the compound of Formula II is from 10:1 to 1 :10, preferably the third molar ratio is from 5:1 to 1 :5, more preferably the third molar ratio is from 3:1 to 1 :3, even more preferably the third molar ratio is 1 :1 .

[0030] In an embodiment, R4is hydrogen, and preferably R5is hydrogen.

[0031] In an embodiment, stirring the mixture comprises adding a first aqueous solution of the iron salt and the zinc salt to a second aqueous solution of the compound of Formula I and the compound of Formula II; and stirring the mixture for at least 3 hours, preferably for at least 4 hours.

[0032] In an embodiment, isolating the iron-doped zeolitic imidazolate framework comprises separating the iron-doped zeolitic imidazolate framework from themixture, preferably by filtration or centrifugation; washing the separated iron-doped zeolitic imidazolate framework; and drying the separated iron-doped zeolitic imidazolate framework.

[0033] In a third aspect, there is provided a method of preparing a catalyst, comprising preparing the iron-doped zeolitic imidazolate framework according to the second aspect; and calcining the iron-doped zeolitic imidazolate framework at a temperature of at least 900 °C to form the catalyst.

[0034] In an embodiment, calcining the iron-doped zeolitic imidazolate framework is conducted under an inert atmosphere, preferably under a continuous flow of an inert gas like argon or nitrogen.

[0035] In a fourth aspect, there is provided a product obtained by the third aspect.

[0036] In a fifth aspect, there is provided an electrode coated with the catalyst according to the first aspect or fourth aspect, or prepared by the method according to the third aspect.

[0037] In a sixth aspect, there is provided a method of preparing a catalytically active electrode for an electrolytic cell, the method comprises providing the catalyst according to the first aspect or prepared according to the third aspect; coating an electrode with the catalyst; and drying the electrode.

[0038] In a seventh aspect, there is provided an electrolytic cell comprising a first electrode according to the fifth aspect or prepared by the sixth aspect; a second electrode; and an energy source electrically coupled to the first electrode and the second electrode, wherein the first electrode and the second electrode are each at least partly immersed in a first electrolyte and a second electrolyte respectively when the electrolytic cell is in use, wherein the first electrolyte and the second electrolyte is the same electrolyte or separated electrolytes, and if the first electrolyte and the second electrolyte are separated electrolytes, an ion exchange means to allow movement of ions between the first compartment and the second compartment.

[0039] Preferably, the electrolytic cell further comprises a first compartment to accommodate the first electrolyte and the first electrode; and a second compartment to accommodate the second electrolyte and the second electrode.

[0040] In an embodiment, the ion exchange means is an ion exchange membrane or a salt bridge. In an embodiment, the second electrode comprises iridium oxide on a second carbon electrode. In an embodiment, the electrochemical cell further comprises a gas outlet adapted to allow a gas produced at the second electrode to flow out, wherein the first electrolyte is a sodium chloride solution. Preferably, the second electrolyte is a sodium chloride solution, more preferably the sodium chloride solution is brine.

[0041] In an eight aspect, there is provided a method of performing electrolysis, the method comprising performing a reduction reaction at a first electrode according to the fifth aspect or prepared by the sixth aspect, the first electrode is at least partly immersed in a first electrolyte; and performing an oxidation reaction at a second electrode at least partly immersed in a second electrolyte, wherein the first electrode and the second electrode are electrically coupled to an energy source, wherein the first electrolyte and the second electrolyte is the same electrolyte or separated electrolytes, and if the first electrolyte and the second electrolyte are separated electrolytes, an ion exchange means allows movement of ions between the first compartment and the second compartment.

[0042] Preferably, the first electrode and the electrolyte are accommodated in a first compartment, and wherein the second electrode and the electrolyte are accommodated in a second compartment.

[0043] Preferably, the first electrolyte is a waste stream. The waste stream may be selected from the group consisting of seawater, desalination brine, industrial wastewater, and mining wastewater.

[0044] In an embodiment, the electrolyte comprises metal ions wherein the metal carbonate has a solubility in water of less than 30 mg / L at 25 °C, preferably the metal ions comprises magnesium cations and / or calcium cations.

[0045] In an embodiment, the first electrolyte comprises oxygen and / or carbon dioxide, preferably the first electrolyte is saturated with oxygen and / or carbon dioxide. In an embodiment, the method comprises feeding oxygen and / or carbon dioxide into the first electrolyte.

[0046] Advantageously, the iron-based catalyst and electrode described herein provides a significantly cheaper alternative to platinum catalysts and electrodes. The iron-based catalyst and electrode has comparable or better operational parameters than the standard platinum catalysts including a simple and environmentally friendly synthetic process, a broad operational current window, lower cell voltage, higher recovery efficiency of metals, and greater stability and anti-corrosion properties.BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure (FIG.) 1 shows a schematic illustration of technology for preparing Fe / FesC@Fe-Nx@CNT for CO2-promoted mineral in disposal brine

[0048] FIG. 2 shows in the left panel (a) and the right panel (b) scanning electron microscope and transmission electron microscope images of Fe / FesC@Fe-Nx@CNT respectively.

[0049] FIG. 3 shows in panel (a) the XRD pattern of the Fe-based catalyst, in panel (b) the high-resolution XPS spectra for Fe 2p of the Fe-based catalyst, in panel (c) the high-resolution XPS spectra for N 1 s of the Fe-based catalyst, and in panel (d) the EXAFS spectra of the Fe-based spectra.

[0050] FIG. 4 in panel (a) shows the current-potential curves at a scan rate of 20 mV s’1, panel (b) shows the Tafel plot, panel (c) shows the electron transfer number and H2O2 efficiency of Fe / FesC@Fe-Nx@CNT and commercial Pt / C in O2 saturated brine in 02-saturated brine. Panels (c) and (d) show the stability of Fe / Fe3C@Fe-Nx@CNT and commercial Pt / C in brine and in 0.5 M KCI where I, Io is the current at the time t and initial current, respectively.

[0051] FIG. 5 panel (a) shows a schematic representation of a two-compartment featuring a two-electrode configuration in non-flowing brine electrolyte (pH 8.0). Panel (b) shows the current-potential (J-V) profile for Fe / Fe3C@Fe-Nx@CNT-loaded carbon paper in brine saturated with argon or oxygen using a three-electrode system. Panel (c) shows the multistep current profiles of Pt / C and Fe / Fe3C@Fe-Nx@CNT-loaded carbon paper in brine electrolyte with and without O2 supply in a full cell (two-electrode) system. The cellvoltages were reported without applying any iR correction. Panels (d) and (e) show the mineral recovery efficiency for Mg2+(panel (d)) and Ca2+(panel (e)) respectively after running the reaction at different currents for 2 hours (h) in brine. The electrolyte volume in each compartment was 100 mL of brine water (pH 8). The CO2 and O2 were simultaneously supplied under controlled flow rates of 5 seem and 20 seem, respectively.

[0052] FIG. 6 shows an image of prototype electrochemical cell.DETAILED DESCRIPTION OF THE INVENTION

[0053] In the following description, numerous specific details are set forth in order to provide a thorough understanding of various illustrative embodiments of the invention. It will be understood, however, to one skilled in the art, that embodiments of the invention may be practiced without some or all of these specific details. Embodiments described in the context of one of the methods or products are analogously valid for the other methods or products. Similarly, embodiments described in the context of a method are analogously valid for a product, and vice versa.

[0054] Where a combination is disclosed, each subcombination of the elements of that combination is also specifically disclosed and is within the scope of the invention. Conversely, where different elements or groups of elements are disclosed, combinations thereof are also disclosed. Where any element of an invention is disclosed as having a plurality of alternatives, examples of that invention in which each alternative is excluded singly or in any combination with the other alternatives are also hereby disclosed; more than one element of an invention can have such exclusions, and all combinations of elements having such exclusions are hereby disclosed.

[0055] Where a range of values is recited, it is to be understood that each intervening integer value, and each fraction thereof, between the recited upper and lower limits of that range is also specifically disclosed, along with each subrange between such values. The upper and lower limits of any range can independently be included in or excluded from the range, and each range where either, neither orboth limits are included is also encompassed within the invention. Where a value being discussed has inherent limits, for example where a component can be present at a concentration of from 0 to 100%, or where the pH of an aqueous solution can range from 1 to 14, those inherent limits are specifically disclosed. Where a value is explicitly recited, it is to be understood that values which are about the same quantity or amount as the recited value are also within the scope of the invention, as are ranges based thereon.

[0056] The term "alkyl" as used herein is a branched or unbranched saturated monovalent hydrocarbon radical of 1 to 24 carbon atoms (C1 -C24), such as methyl, ethyl, n-propyl, isopropyl, nbutyl, isobutyl, s-butyl, f-butyl, n-pentyl, isopentyl, s- pentyl, neopentyl, hexyl, heptyl, octyl, nonyl, decyl, dodecyl, tetradecyl, hexadecyl, eicosyl, tetracosyl, and the like. The alkyl group can be cyclic or acyclic. The alkyl group can be branched or unbranched. The alkyl group can also be substituted or unsubstituted. For example, the alkyl group can be substituted with one or more groups including, but not limited to, alkyl, cycloalkyl, alkoxy, amino, ether, halide, hydroxy, nitro, silyl, sulfo-oxo, or thiol, as described herein.

[0057] There is a need to develop alternative noble-metal-free catalysts to address the problems with existing catalysts. Bearing the above concerns in mind, the present disclosure herein is directed towards developing a straightforward and facile method for preparing a cost-effective catalyst with enhanced efficiency, as an alternative or replacement of Pt-based catalysts. The catalyst is anticipated to function effectively under high current densities, while also mitigating the occurrence of side reactions. This will enable high CO2 capture and enhance mineral productivity with minimal energy consumption.

[0058] Described herein is the development of iron-nitrogen-carbon based catalysts (Fe-N-C) with a unique structure consisting of atomically-disperse Fe-Nx, FesC, and carbon nanotubes (CNTs) and the preparation method thereof. The catalyst is synthesized through a process that prioritizes simplicity and environmental friendliness.

[0059] Focus was directed at the utilization of innovative iron-based materials as efficient and cost-effective cathodic catalysts to revolutionize the landscape of CO2-induced mineral recovery in disposal waste streams (desalination brine, wastewater from mining, or concrete industries). Iron-based (Fe) catalysts are proving to be superior to platinum (Pt) noble metal catalysts as cathode catalysts due to their ability to promote the oxygen reduction reaction (ORR) while avoiding the unfavorable hydrogen evolution reaction (HER). The described Fe-based catalyst, in particular, has a broad operational current (represents reaction rate) window, making it perfect for use at high current densities, thereby increasing productivity significantly. These unique features make Fe-based catalysts ideal for large-scale applications in CO2 mineralization and metal recovery processes, leading to a significant breakthrough in catalysis and environmental sustainability.

[0060] FIG. 1 shows a schematic illustration of a method for preparing the Fe / FesC@Fe-Nx@CNT catalyst for CO2-promoted miner recovery in disposal brine.

[0061] The method for preparing the catalyst of the invention includes the following steps:

[0062] Step I. Precipitating a catalyst precursor comprising combining aqueous solutions of salts of zinc (Zn) and iron (Fe) in the presence of an organic ligand;

[0063] Step II. Separating off the catalyst precursor precipitated in step I;

[0064] Step III. Calcining the catalyst precursor separated off in step II to yield the catalyst.

[0065] In step I of the method for preparing the catalyst, a catalyst precursor is precipitated by combining aqueous solutions of salts of Zn2+and Fe2+under appropriate conditions. In this step, the Zn2+will react with an organic ligand, 2- methylimidazole, to form a well-known zeolitic imidazolate framework (ZIF) that precipitates from their respective aqueous precursor salt solutions. Meanwhile, the presence of Fe2+acts as a dopant for forming Fe-doped ZIF8.

[0066] General Procedure for step I

[0067] 2-methylimidazole (2-Melm, 32 mmol) and benzylamine (32 mmol) were added to 80 mL H2O under ultrasonication (labeled as Solution A). A separate solution containing 1.33 mmol FeSO4'7H2O and 6.67 mmol Zn(NO3)2'6H2O was dissolved in 80 mL of water under mild stirring for 15 min (labeled as Solution B). Subsequently, solution B was slowly added to solution A. The slurry thus obtainedmay be stirred or otherwise agitated for some time, preferably 4 hours. The entire step I is preferably carried out at ambient temperature.

[0068] Benzylamine is used as an additive to accelerate the nucleation rate of ZIFs in water and allows to obtain more uniform and smaller size ZIFs particles. Other amine derivatives may potentially be used with careful investigation of their quantities. In particular, the benzylamine and other amine derivatives form the precursor and final catalyst with the specific characteristics described herein, especially the size and morphology homogeneity.

[0069] It is possible that other variations to 2-methylimidazole may be used for synthesizing ZIF-8, but with careful investigation. Different size and position of alkyl groups can influence the acid-base properties of the imidazole variants, consequently, the properties of the resulting ZIF-8. Similarly, different amounts or equivalence of the reactants may affect the properties of the resulting ZIF-8.

[0070] As water is used as the sole solvent in step I, Fe2+salts are preferred to Fe3+salts as they are less prone to hydrolysis in aqueous solution. However, Fe3+salts may still possibly be used. Iron and zinc salts of strong inorganic acids such as sulfate and chloride (for example FeSC , FeCl2, Zn(NOs)2, ZnCl2, and ZnSC ) are preferred due to their high solubility and stability in water, minimizing the risk of anion hydrolysis and interaction with organic ligands.

[0071] The molar ratio of Fe to Zn salts can be adjusted within a practical range, typically from 1 :10 to 1 :1 , or from more than 1 :10 to less than 1 :1 , or the molar ratio is 1 :5, to optimize the desired properties and performance of the method.

[0072] The ratio of 2-Melm (and other imidazole variants) to total metal ions should be at least 1 :1. For example, the ratio may be from 1 :1 to 10:1 , with an optimal ratio observed at 4:1. For the sake of simplifying the procedure, 2-Melm (and other imidazole variants) and benzylamine (and other benzylamine variants) are used in molar equivalence but may be used in other equivalence provided that the imidazole and benzylamine (including their variants) are used in excess relative to the total amount of metal ions.

[0073] General Procedure for step II

[0074] In the second step (Step II), the catalyst precursor, precipitated during Step I, is separated, preferably through filtration or centrifugation of the slurry obtained in Step I. The isolated solid catalyst precursor is then purified, preferably by undergoing three washes with methanol. Prior to the calcination process in the third step (Step III), it is advisable to eliminate excess water by subjecting it to drying at an elevated temperature, preferably around 50°C under vacuum conditions.

[0075] General Procedure for step III

[0076] The resultant catalyst precursor, thus prepared, undergoes calcination in Step III. During this calcination step, the temperature should be sufficiently high to convert the metal-organic framework into a metallic form and eliminate zinc from the precursor. The preferred calcination temperature is equal to or higher than 900°C. Preferably, the calcination process is conducted under dynamic conditions (for example continuous flow of an inert gas), ideally in an inert atmosphere like nitrogen or a noble gas. The catalyst precursor is preferably heated to the desired temperature at a ramping rate of 5°C / min and maintained at this temperature for 2 hours. During the annealing process, the evaporation of the Zn species occurs, leading to an augmentation in the specific surface area and the introduction of defects in the resultant carbon material. Simultaneously, Zn, acting as a self- sacrificial template, efficiently mitigates the agglomeration of iron nanoparticles. The majority of Fe atoms undergo a reaction with nitrogen atoms, forming Fe-Nxspecies. Excess Fe tends to crystallize into Fe nanocrystals, subsequently interacting with carbon atoms to induce carbide formation and catalytic graphitization. This process leads to the development of carbon layers encapsulating the iron carbide particles. Concurrently, a portion of the Fe-ZIF transforms into a one-dimensional hollow structure, facilitating the catalytic growth of carbon nanotubes (CNT), wherein the presence of Fe likely catalyzed their formation. Consequently, the Fe / FesC@Fe-Nx@CNT hybrid material is produced. Upon cooling down naturally in the argon atmosphere, the Fe / Fe3C@Fe-Nx@CNT was obtained and directly used for further characterization.

[0077] In the process where amorphous carbon layers encapsulate Fe / FesC particles, these layers are subjected to graphitization, resulting in the formation ofcarbon nanotubes (CNTs), with iron (Fe) serving as the catalytic agent. This method facilitates a more robust integration of FesC particles within the CNT structure than physically mixing FesC with CNTs. Fe single atoms are encapsulated in a carbon support structure that involves a nitrogen-doped carbon matrix, where nitrogen atoms provide coordination sites for single-atom iron, forming stable Fe-Nxstructures within the carbon lattice. Meanwhile, Fe / FesC particles are either loaded on the nitrogen-doped carbon matrix or carbon nanotubes (CNTs). The amount of CNTs present is in a range from 20 % v / v to 50 % v / v (percentage volume per volume or alternatively percentage volume versus volume). The amount of CNTs may vary based on the molar ratio of the iron salt to the zinc salt, with a 1 :5 molar ratio of the iron salt to zinc salt, the CNTs is present in 40% v / v to 50% v / v of the catalyst. The amount of CNTs may be derived from Scanning Electron Microscopic image-based estimation. There is currently no method available for the direct and accurate quantification of carbon nanotubes (CNTs) within the composite catalyst. Distinguishing CNTs from other carbon forms, such as graphite, also presents a challenge.

[0078] The presence of CNTs may enhance the conductivity of these samples and serve as highways for electron-conducting, and is thus beneficial to high electrocatalytic performance in ORR.

[0079] The scanning electron microscope (SEM) image presented in panel (a) of FIG. 2 reveals the presence of aggregated nanoparticles in the range of several tens of nanometers, accompanied by the observation of one-dimensional nanotubes. Examination of the transmission electron microscope (TEM) image indicates the emergence of irregular dark nanoparticles, also in the tens of nanometers, distributed randomly, implying the formation of Fe species particles distinct from their incorporation into the carbon framework as shown in panel (b) of FIG. 2.

[0080] The X-ray diffraction (XRD) patterns presented in FIG. 3 panel (a) show the crystalline structure of Fe-based catalysts. Conspicuous peaks corresponding to metallic Fe (PDF 06-0696) and FesC (PDF 35-0772) were observed. Additionally, notable broad peaks associated with graphite were also evident. FIG, 3 panel (a)shows a XRD pattern with a series of XRD peaks located at 37.7, 43.4, 44.5, 45.7, 48.7, 54.4 and 57.7°, which corresponds to (210), (102), (220), (112), (131 ), (230) (301 ) plane of FesC (JCPDS standard 35-0772). The powder X-ray diffraction (XRD) patterns were collected on a Rigaku Oxford powder diffraction meter with a Cu-Ka resource (Ka= 0.134 nm) within the 20 range of 10° to 70°.

[0081] The high-resolution X-ray photoelectron spectroscopy (XPS) spectra of Fe 2p3 / 2 in a Fe-based catalyst (FIG. 3 panel (b)) could be resolved into three peaks, including the Fe(lll) peak at 711.7 eV, the Fe(ll) peak at 710.2 eV, accompanied by a satellite at a higher binding energy region. The appearance of a peak at 708.4 eV indicates the presence of metallic Fe or FesC. Simultaneously, the intricate N 1s spectra can be dissected into five distinct peaks: pyridinic-N at 398.6 eV, metal-N at 399.6 eV, pyrrolic-N at 401 .2 eV, graphitic-N at 402.7 eV, and oxidized-N at 405.2 eV (FIG. 3 panel (c)).The intensity of the peaks of the different Nitrogen species is from the lowest the oxidized-N at 405.2 eV, the graphitic-N at 402.7 eV, the metal- N at 399.6 eV, the pyrrolic-N at 401.2 eV, and the highest intensity peak is the pyridinic-N at 398.6 eV. The existence of M-N signifies the coordination between metal and nitrogen. The XPS spectra were acquired from the X-ray spectrometer (Thermo Scientific, ESCALAB 250). C 1 s at 248.6 eV was used as a reference to calibrate the peaks.

[0082] A suitable amount of nitrogen dopant in carbon can maintain good electrical conductivity while improving its electrocatalytic properties. Among different nitrogen functionalities, pyridinic-N is beneficial for ORR due to the availability of a single lone pair of electrons, which could strengthen the Lewis basicity and promote the neighboring C atom as the active site. The pyrrolic-N may have a negative effect on the ORR.

[0083] Considering that the absolute intensity varies between different instruments, the proportion of different N species was calculated from their corresponding peak area. In the example herein, the percentage of pyridinic-N, pyrrolic-N, Fe-N (or metal-N), graphitic N and oxidized-N (N-O) were 35%, 20%, 18%, 15% and 12%, respectively.

[0084] Furthermore, the bonding environment of Fe atoms was explored through extended X-ray absorption fine structure (EXAFS) analysis. As shown in FIG. 3 panel (d), a well-resolved peak at 2.2 A and a minor peak at 1.5 A, corresponding to Fe-Fe distances and the Fe-N scattering path, were observed. This suggests the co-existence of Fe-Nxsites and zero-valence Fe in Fe / FesC nanoparticles within the Fe-based catalyst. Considering these results collectively, it is plausible that the nanoparticles in the Fe-based catalyst are indeed Fe / Fe3C@Fe-Nx@CNT nanoparticles.

[0085] The initial evaluation of the electrocatalytic activity of the Fe / FesC@Fe-Nx@CNT or oxygen reduction reaction (ORR) was conducted using a single-compartment glass cell using a CHI760E (CH Instrument, China) electrochemical workstation with a rotating disk electrode (RDE) system. Catalyst ink (16 mg mL-1) was prepared by dispersing the catalyst in the mixture of 2-propanol and water with 50 L of Nation solution (5 wt.%). The ink underwent a 30-minute sonication process and was subsequently drop-casted onto a glassy carbon electrode (5 mm in diameter, loading amount of 1600 pg cm2), followed by rapid drying under an infrared lamp. A catalyst coated-glassy carbon electrode was used as the working electrode, whereas a Pt foil was used as a counter electrode. The reference electrode is a Hydroflex reverse hydrogen electrode (RHE, Gaskatel). FIG. 4 panel (a) compares the RDE polarization profiles of the Fe-NC and the Pt / C catalyst at 2000 rpm in Os-saturated brine. Three primary indicators, namely onset potential, half-wave potential, and limiting current density (current in the diffusion-limited region), were considered for comparing their electrocatalytic performance. The Fe / Fe3C@Fe-Nx@CNT catalyst demonstrated promising electrocatalytic properties, with onset potential and half-wave potential of only 20 mV and 10 mV lower than that of Pt / C. Notably, its limiting current density at 0.2 V vs. RHE reached -7.7 mA cm-2, nearly 1 .6 times that of a commercial Pt / C catalyst, showcasing its potential as an alternative to Pt / C as an electrode. The Tafel slopes shown in panel (b) of FIG. 4, derived from the corresponding current-potential profile in kinetic regions, were determined to be 131 and 168 mV dec-1forFe / FesC@Fe-Nx@CNT and Pt / C respectively, indicating that the former exhibits faster kinetics for oxygen reduction in brine.

[0086] In addition to evaluating the catalytic activity, the durability of the catalyst also plays a pivotal role in determining its practical viability. To assess the durability, both catalysts were subjected to a constant potential (0.2 V vs. RHE), and the response current was continuously monitored. The recorded current was then utilized to calculate the relative l / lo, representing the remaining current in comparison to the initial current. FIG. 4 panel (c) indicates that Fe / FesC@Fe-Nx@CNT catalysts maintained roughly 50% of their initial current density in brine, exceeding the commercial Pt / C counterpart, which declined by 70% in the same period. To rule out the possibility that the formation of precipitates over the electrode surface could potentially decrease the contact between the electrode and electrolyte and cause the decrease of current, Fe / FesC@Fe NX@CNT and Pt / C stability were re-evaluated in a 0.5 mol dm-3NaCI electrolyte resembling brine conditions. As shown in FIG. 4 panel (d), Pt / C still experienced over 30% reduction, possibly due to the chloride (Cl ) poisoning effect in high chloride media as reported before. Remarkably, in contrast, the Fe / FesC@Fe-Nx@CNT catalyst maintains approximately 90% of its initial current over 2 hours. This unexpected resilience suggests a notable resistance of the iron catalyst to the degrading effects observed in the chlorine-rich surroundings, highlighting its potential for enhanced durability in practical applications.

[0087] Fe / FesC@Fe-NC@CNT has demonstrated promising performance in the ORR with high selectivity for generating the hydroxide ion (OH-). It was then employed as a cathodic catalyst to leverage its potential for ORR-induced metal recovery and CO2 capture in brine. FIG. 5 panel (a) illustrates a schematic representation of a two-compartment system where Fe / Fe3C@Fe-NC@CNT-loaded carbon paper (6 cm2, loading amount of 1 mg cm-2) was used as the cathode, and lrO2 / C was used as the anode. FIG. 5 panel (b) shows the current-potential curves of Fe / Fe3C@Fe-Nx@CNT in oxygen (O2)- and argon (Ar)-saturated solutions. In an argon saturated electrolyte, noobservable current is noted until reaching a highly negative potential of -0.7 V, suggesting the initiation of the hydrogen evolution reaction (HER). Conversely, in an oxygen saturated electrolyte, the current begins at a more positive potential of 0.5 V, indicating that the ORR is more thermodynamically favorable. The gap between the potentials in Ar- and Ch-saturated electrolytes serves as an indicator of the working potential for ORR. A larger gap is considered more favorable, suggesting increased thermodynamic favorability for ORR.

[0088] The multistep-current in O2- and Ar-saturated solutions was further employed to compare the ORR favorability over Fe / Fe3C@Fe-Nx@CNT and commercial Pt / C FIG. 5 panel (C). In panel (c) of FIG. 5, the electric current density in the steps from left to right is 5 mA cm-2, 10 mA cm-2, 15 mA cm-2, 20 mA cm-2, and 25 mA cm-2. At first glance, it is observed that the utilization of ORR markedly reduces the required voltages to achieve similar currents compared to traditional HER. Secondly, the cell voltage required to obtain a specific current on Fe / Fe3C@Fe-Nx@CNT is comparable to that of Pt / C. The cell voltage gap between ORR and HER is narrowing with the increase of applied current. This is attributed to the fact that, at higher current, the favorability of thermodynamically favorable ORR becomes less significant due to the competitive nature of the kinetically favorable HER. In the case of Pt / C, this gap nearly disappeared at 20 mAcrrr2, indicating the possibility of both ORR and HER occurring simultaneously, thus reducing energy efficiency. What sets the Fe / Fe3C@Fe-Nx@CNT catalyst apart is that even at higher current levels of 25 mA cm'2, a substantial gap exists between ORR and HER and suggests that ORR remains the more favorable reaction. This discrepancy suggests that while Pt / C is a benchmark catalyst for HER, Fe / Fe3C@Fe-Nx@CNT, being different, offers a wider working potential window.

[0089] To comprehensively investigate the performance of Fe / FesC@Fe- NX@CNT and commercial Pt / C (used as a benchmark), two distinct current densities were selected based on a multistep current profile in panel (c) of FIG. 5. The initial value was set at a low current density, 10 mA cm-2, where a significant gap between ORR and HER exists for both Fe / FesC@Fe-Nx@CNT and commercial Pt / C. The second current density was chosen at a higher region, specifically 25mAcrrr2, assuming that HER occurs on Pt / C while ORR continues on Fe / FesC@Fe- Nx@CNT. Panel (d) of FIG. 5 illustrates that, for Mg2+, higher currents lead to increased removal of Mg2+for both Fe3C / Fe-Nx@CNT and commercial Pt / C. At low current density, the Mg2+removal is comparable for both catalysts. However, at a high current density of 25 mA cm-2, Fe / Fe3C@Fe-Nx@CNT exhibits a slightly better capacity of 38.6±9.4% for Mg2+removal compared to 25.2±3.2% for the commercial Pt / C catalyst. A notable difference arises in removing Ca2+(Panel (e) of FIG. 5), which reflects the CO2 capture ability. Pt / C shows a decrease in the efficiency of Ca2+removal at high current density, with only 27.0±5.8% removed at 25 mA cm-2, even lower than at 10 mA cm-2which had 36.6±5.2% removed under identical conditions. Conversely, for Fe / Fe3C@Fe-Nx@CNT, Ca2+removal continues to decrease with the current increase to 25 mA cm-2and achieves a 62.2±12.9% recovery which is significantly higher than of the commercial Pt / C. These results demonstrate that Fe / Fe3C@Fe-Nx@CNT, being less effective in HER even at a high current density, favors the ORR and facilitates the removal of both Mg and Ca ions without the interference of hydrogen bubble formation.

[0090] In an example, the electrochemical cell utilizes a two-compartment cell 100. An example of a two-compartment cell 100 is shown in FIG. 6. One compartment houses the cathode 105, and the anode 110 is in a second compartment. The two compartments are separated by an ion exchange membrane 115 (Nafion 117). This membrane 115 facilitates ion transport between the compartments and may be used in place of or replaced by a salt bridge. A gas inlet tube 120 supplies a gas to the cathode 105. The gas may contain carbon dioxide and / or oxygen, or air. A gas outlet tube 125 allows the evolved chlorine gas from the anode 110 to be removed from the cell 100. An electrolyte inlet tube 130 provides brine or other electrolyte to the cathode and an electrolyte outlet tube 135 removes the brine or electrolyte from the anode 110. These features are similar to those in a commercially available Proton exchange membrane (PEM) electrolyzer.

[0091] FIG. 7 shows an example of a system 200 with an electrochemical reactor 205. An example of the electrochemical reactor 205 is the two -compartment cell 100 shown in FIG. 6. Gas cylinders 210, 215 provide a supply of oxygen and carbondioxide to the electrochemical reactor 205. A gas mixer 220 may be used to mix the oxygen and carbon dioxide before feeding it to the electrochemical reactor 205. The evolved chlorine gas and excess carbon dioxide and oxygen gas may be passed out via a gas outlet 230. A brine tank 235 may be used to store bring and a peristaltic pump 240 is used to pump the brine into the electrochemical reactor 205 as the electrolyte in particular to the cathode. The treated brine is collected in a treated brine tank 245 with another peristaltic pump 240. A mixture containing the precipitate is collected in a precipitate settling tank 250 which may be separated into a supernatant and a precipitate. The supernatant may be collected in a supernatant tank 255 and pumped back into the electrochemical reactor with a pump 240. The precipitate may be collected in a precipitate storage tank 260.

[0092] The Fe / Fe3C@Fe-Nx@CNT catalyst described here possesses high ORR performance in near-neutral media (e.g. chlorine-containing brine water). The Fe- based catalyst exhibits comparable or even better performance to commercial Pt / C catalysts under identical conditions. The Fe / FesC@Fe-Nx@CNT catalyst possesses an extended operational potential window. This is a crucial advancement as the catalysts effectively suppress competitive hydrogen evolution reactions even at high currents and extend the operational potential window of the process, allowing for greater flexibility and control. The developed catalyst has increased mineral recovery efficiency and CO2 capture compared to commercial Pt / C catalyst. The developed catalysts demonstrate improved durability in brine, leading to a longer catalyst lifespan and sustained performance in chlorine-containing media.

[0093] The Fe / FesC@Fe-Nx@CNT catalyst may be used in is synthesized through a novel facile one-pot process that prioritizes simplicity and environmental friendliness, employing water as the primary solvent. When used as a cathodic catalyst, the Fe / Fe3C@Fe-Nx@CNT catalyst has been demonstrated to be superior to Pt noble metal catalysts as cathode catalysts due to their ability to promote the oxygen reduction reaction (ORR) while reducing the hydrogen evolution reaction (HER), making them perfect for use at high current densities, thereby increasing productivity significantly.

[0094] Table 1 : Comparison of Fe / Fe3C@Fe-Nx@CNT and commercial Pt / C catalyst

[0095] Table 1 shows a comparison of the properties of the developed Fe / FesC@Fe-Nx@CNT catalyst and commercial Pt / C catalyst. It may be seen that the developed iron based catalyst has a substantially larger working window and provides operation flexibility. The lower cell voltage for the iron based catalyst has 7% lower energy requirements than the Pt / C catalyst. The recovery efficiency for the developed iron based catalyst is higher for both magnesium and calcium, in particular it is 2.3 times higher efficiency for calcium and translates to a higher capture of carbon dioxide. The developed iron based catalyst is substantially more stable with better anti-corrosion properties compared to Pt / C and is 20 times cheaper in Singapore dollars. Taken together, it may be seen that the developed Fe / Fe3C@Fe-Nx@CNT catalyst has better operational properties, with lower energy requirements and costs but higher recovery efficiency. The synergy between these properties significantly make the Fe / Fe3C@Fe-Nx@CNT catalyst better than the commercial platinum catalyst in many aspects.

[0096] The described catalyst herein may be used to coat an electrode, like a cathode in a electrochemical cell and used in carbon capture and storage. This technology can be implemented in chemical manufacturing processes with substantial carbon emissions, including but not limited to coal-fired or natural gaspower plants, desalination plants, or steel manufacturing. Its application contributes to overall emission reduction goals, aligning with environmental targets. The electrochemical cell may also enable the recovery of valuable metals (such as magnesium and calcium) from disposal brine generated in various industrial processes, including desalination plants. The reclaimed minerals can serve as an alternative feedstock for cement applications, promoting circular economy practices and reducing dependence on traditional raw materials. It is worth noting that the technology is not limited to brine but can also be applied to other metal-containing sources, such as wastewater from mining or industrial processes. Other metals that can precipitate in hydroxide or carbonate forms are also recoverable using this technology, further expanding its scope and potential applications. The economically viable iron-based catalysts can replace platinum catalysts in oxygen reduction reactions (ORR) within various electrochemical applications, including fuel cells. This offers a cost-effective solution to replace platinum catalysts and electrodes.

Claims

Claims1 . A catalyst comprising a nitrogen-doped carbon matrix, iron atoms, iron carbide particles, and carbon nanotubes, wherein the carbon nanotubes are dispersed and bonded to the nitrogen-doped carbon matrix, the iron atoms, and the iron carbide particles are dispersed and bonded to the nitrogen-doped carbon matrix and / or the carbon nanotubes.

2. The catalyst according to claim 1 , wherein the carbon nanotubes are present from 20 % v / v to 50 % v / v, preferably the carbon nanotubes are present from 40 % v / v to 50 % v / v.

3. The catalyst according to claim 1 or claim 2, wherein the iron atoms and the iron carbide particles and the carbon nanotubes are bonded to a plurality of nitrogen atoms in the nitrogen-doped carbon matrix.

4. The catalyst according to claim 3, wherein a X-ray photoelectron spectroscopy N 1s spectra of the catalyst shows characteristics peaks of pyridinic-N, metal- N, pyrrolic-N, graphitic-N, and oxidized-N at 398.6 eV, 399.6 eV, 401.2 eV, 402.7 eV, and 405.2 eV respectively, wherein the peaks at 402.7 eV and 405.2 eV is lower in intensity than the peaks at 398.6 eV, 399.6 eV, and 401 .2 eV.

5. The catalyst according to claim 4, wherein the peak at 398.6 eV has the highest intensity followed by the peak at 401.2 eV and subsequently the peak at 399.6 eV,6. The catalyst according to claim 4 or claim 5, wherein the peak at 398.6 eV has from 30 to 40% of the nitrogen species as measured in the X-ray photoelectron spectroscopy N 1s spectra of the catalyst, preferably the peak at 398.6 eV has from 33 to 37% of the nitrogen species.

7. The catalyst according to any one of claims 4 to 6, wherein the peak at 401 .2 eV has from 13 to 23% of the nitrogen species as measured in the X-ray photoelectron spectroscopy N 1 s spectra of the catalyst, preferably the peak at 401 .2 eV has from 18 to 22% of the nitrogen species.

8. The catalyst according to any one of claims 4 to 7, wherein the peak at 399.6 eV has from 15 to 25% of the nitrogen species as measured in the X-ray photoelectron spectroscopy N 1 s spectra of the catalyst, preferably the peak at 399.6 eV has from 16 to 20% of the nitrogen species.

9. The catalyst according to any one of claims 4 to 8, wherein the peak at 402.7 eV has from 10 to 20% of the nitrogen species as measured in the X-ray photoelectron spectroscopy N 1 s spectra of the catalyst, preferably the peak at 402.7 eV has from 13 to 17% of the nitrogen species.

10. The catalyst according to any one of 6 to 9, wherein the peak at 405.2 eV has from 7 to 17% of the nitrogen species as measured in the X-ray photoelectron spectroscopy N 1 s spectra of the catalyst with the percentages of the five nitrogen species adding up to 100%, preferably the peak at 402.7 eV has from 10 to 14% of the nitrogen species,11. The catalyst according to any one of claims 1 to 10, wherein the nitrogen- doped carbon matrix is or is formed from an iron-doped zeolitic imidazolate framework.

12. The catalyst according to any one of claims 1 to 11 , wherein an X-ray photoelectron spectroscopy Fe 2ps / 2 spectra of the catalyst show characteristics peaks at 711.7 eV, 710.2 eV, and 708.4 eV corresponding to Fe3+, Fe2+, and metallic Fe or FesC respectively.

13. The catalyst according to any one of claims 1 to 12, wherein a X-ray diffraction pattern of the catalyst shows characteristic peaks at 37.7°, 43.4°, 44.5°, 45.7°, 48.7°, 54.4°, and 57.7°.

14. The catalyst according to any one of claims 1 to 13, wherein the catalyst is free from platinum and / or palladium.

15. The catalyst according to any one of claims 1 to 14 consisting essentially or consisting of the nitrogen-doped carbon matrix, the iron atoms, the iron carbide particles, and the carbon nanotubes.

16. A method of preparing an iron-doped zeolitic imidazolate framework, the method comprising stirring a mixture under suitable conditions to provide the iron-doped zeolitic imidazolate framework, the mixture comprising water, an iron salt, a zinc salt, a compound of Formula I and a compound of Formula II, wherein Formula I iseach of R1, R2, and R3is independently selected from a group consisting of hydrogen, a halogen, a nitro, and a C1 to C3 substituted or unsubstituted alkyl;R4is selected from a group consisting of hydrogen, a C1 to C3 substituted or unsubstituted alkyl, a substituted aryl or an unsubstituted aryl,R5is selected from a group consisting of hydrogen, a halogen, a hydroxyl, or a C1 to C3 substituted or unsubstituted alkyl; and isolating the iron-doped zeolitic imidazolate framework.

17. The method according to claim 16, wherein R1is the C1 to C3 substituted or unsubstituted alkyl and preferably R2and R3are both hydrogens.

18. The method according to claim 17, wherein R1is selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, and cyclopropyl, preferably the compound of Formula I is 2-methylimidazole.

19. The method according to any one of claims 16 to 18, wherein the iron salt is a Fe2+salt.

20. The method according to claim 19, wherein the iron salt is an inorganic Fe2+anhydrous or hydrated salt, preferably the inorganic Fe2+anhydrous or hydrated salt is selected from the group consisting of FeSCh, FeCh, and Fe(NO3)2.21 . The method according to any one of claims 16 to 20, wherein the zinc salt is an inorganic Zn2+anhydrous or hydrated salt, preferably the inorganic Zn2+anhydrous or hydrated salt is selected from the group consisting of Zn(NO3)2, ZnCl2, and ZnSC .

22. The method according to any one of claims 16 to 21 , wherein a molar ratio of the iron salt to the zinc salt is from 1 :10 to 1 :1 , preferably the molar ratio is from more than 1 : 10 to less than 1 :1 , more preferably the molar ratio is 1 :5.

23. The method according to any one of claims 16 to 22, wherein a second molar ratio of the compound of Formula I to a total amount of metal ions is at least 1 :1 , preferably the second molar ratio is at least 3:1 .

24. The method according to claim 23, wherein the second molar ratio is at most 10:1 , preferably the second molar ratio is at least 4:1 to 10: 1 .

25. The method according to any one of claims 16 to 24, wherein a third molar ratio of the compound of Formula I to the compound of Formula II is from 10:1 to 1 :10, preferably the third molar ratio is from 5:1 to 1 :5, more preferably the third molar ratio is from 3:1 to 1 :3, even more preferably the third molar ratio is 1 :1.

26. The method according to any one of claims 16 to 25, wherein R4is hydrogen, and preferably R5is hydrogen.

27. The method according to any one of claims 16 to 26, wherein stirring the mixture comprises adding a first aqueous solution of the iron salt and the zinc salt to a second aqueous solution of the compound of Formula I and the compound of Formula II; and stirring the mixture for at least 3 hours, preferably for at least 4 hours.

28. The method according to any one of claims 16 to 27, wherein isolating the iron-doped zeolitic imidazolate framework comprises separating the iron- doped zeolitic imidazolate framework from the mixture, preferably by filtration or centrifugation; washing the separated iron-doped zeolitic imidazolate framework; and drying the separated iron-doped zeolitic imidazolate framework.

29. A method of preparing a catalyst, comprising preparing the iron-doped zeolitic imidazolate framework according to any one of claims 16 to 28; and calcining the iron-doped zeolitic imidazolate framework at a temperature of at least 900 °C to form the catalyst.

30. The method according to claim 29, wherein calcining the iron-doped zeolitic imidazolate framework is conducted under an inert atmosphere, preferably under a continuous flow of an inert gas like argon or nitrogen.

31. An electrode coated with the catalyst according to any one of claims 1 to 15, or prepared by the method according to claim 29 or claim 30.

32. An electrolytic cell comprising a first electrode according to claim 31 ; a second electrode; and an energy source electrically coupled to the first electrode and the second electrode, wherein the first electrode and the second electrode are each at least partly immersed in a first electrolyte and a second electrolyte respectively when the electrolytic cell is in use, wherein the first electrolyte and the second electrolyte is the same electrolyte or separated electrolytes, and if the first electrolyte and the second electrolyte are separated electrolytes, an ion exchange means to allow movement of ions between the first compartment and the second compartment.

33. The electrolytic cell according to claim 32, comprising at least one of the following:(a) a first compartment to accommodate the first electrolyte and the first electrode; and a second compartment to accommodate the second electrolyte and the second electrode;(b) a gas outlet adapted to allow a gas produced at the second electrode to flow out, wherein the first electrolyte is a sodium chloride solution, and preferably the second electrolyte is a sodium chloride solution, more preferably the sodium chloride solution is brine.

34. The electrolytic cell according to claim 32 or claim 33, wherein the ion exchange means is an ion exchange membrane or a salt bridge and / or the second electrode comprises iridium oxide on a second carbon electrode.

35. A method of performing electrolysis, the method comprising performing a reduction reaction at a first electrode according to claim 31 or prepared by claim 33, the first electrode is at least partly immersed in a first electrolyte; andperforming an oxidation reaction at a second electrode at least partly immersed in a second electrolyte, wherein the first electrode and the second electrode are electrically coupled to an energy source, wherein the first electrolyte and the second electrolyte is the same electrolyte or separated electrolytes, and if the first electrolyte and the second electrolyte are separated electrolytes, an ion exchange means allows movement of ions between the first compartment and the second compartment.

36. The method according to claim 35, with at least one of the following:(a) the first electrode and the electrolyte are accommodated in a first compartment, and wherein the second electrode and the electrolyte are accommodated in a second compartment;(b) the first electrolyte is a waste stream;(c) the waste stream is selected from the group consisting of seawater, desalination brine, industrial wastewater, or mining wastewater;(d) the electrolyte comprises metal ions wherein the metal carbonate has a solubility in water of less than 30 mg / L at 25 °C, preferably the metal ions comprise magnesium cations and / or calcium cations;(e) the first electrolyte comprises oxygen and / or carbon dioxide, preferably the first electrolyte is saturated with oxygen and / or carbon dioxide;(f) the method further comprises feeding oxygen and / or carbon dioxide into the first electrolyte.

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