Metal coordination complexes

Abstract

A crystalline metal coordination complex comprising a ligand coordinated to a metal, wherein the metal is iron, cobalt, nickel, or a combination thereof, and the ligand is quinizarin, a quinizarin derivative, 6,11-dihydroxy-5,12-naphthacenedione, 5,8-dihydroxy-1,4-napthoquinone, 1,4-dichloroanthracene-9-10- dione, 1,8-dihydroxynaphthalene, dithranol or anthracene-14,9,10-tetraol, and method of making same.

Description

[0001] METAL COORDINATION COMPLEXES

[0002] Field of the invention

[0003] The invention relates to a solid metal coordination complex and a method of preparing the same. The metal coordination complex is a metal-ligand complex wherein: the ligand is quinizarin, a quinizarin derivative, 6,1 1 -di hydroxy-5,12-naphthacenedione, 5,8-dihydroxy-1 ,4-napthoquinone, 1 ,4- dichloroanthracene-9-10-dione, 1 ,8-dihydroxynaphthalene, dithranol or anthracene-14,9,10-tetraol; and the metal is iron, cobalt, nickel, or a combination thereof. In particular, the metal coordination complex is a metal-ligand complex wherein the ligand is quinizarin and the metal is iron, cobalt, nickel, or a combination thereof. Complexes of the invention have been found to be particularly effective as electrode catalysts for the oxygen evolution reaction which occurs during the electrocatalytic splitting of water.

[0004] Background to the invention

[0005] Development of renewable energy conversion technology is a major imperative in alleviating the energy crisis and building a low carbon society. Electrochemical water splitting is a promising option, not least because it produces hydrogen without the emission of CO2. Electrochemical water splitting involves the following two half cell reactions: an oxygen evolution reaction at the anode and a hydrogen evolution reaction at the cathode. The oxygen evolution reaction suffers from sluggish kinetics, is energy demanding, and requires a relatively high overpotential. The overpotential is the applied potential required to give oxygen production. Known catalysts for the oxygen evolution reaction include I rOz and RuO2, but as a result of high cost and scarcity of Ir and Ru, scale-up is challenging. In addition, the preparation of such transition metal oxide catalysts typically requires high reaction temperatures and includes calcination at temperatures of from 350 to 1000 °C, for example. These catalysts often require a binder to aid attachment of the catalyst to the current collector and / or the addition of conducting carbon black particles, in the assembly of an electrode.

[0006] Key technical challenges to be addressed in advancing this technology are performance, durability and cost. It is desirable to achieve a balance of all three. In particular it is desirable to provide an electrode catalyst for the oxygen evolution reaction which (i) has a low overpotential and thus can be operated efficiently and at a low cost, (ii) can be prepared by a simple process and / or a process that is not very energy intensive, (iii) can be formed into an electrode by way of a simple / efficient process, e.g. a process that does not involve the use of a binder or conducting carbon black particles, and / or (iv) has a long lifetime.

[0007] It is an object of the present invention to provide a compound that can act as an electrocatalyst and that solves one or more of the above-mentioned problems or meets one or more of the above-mentioned desires. It is an object of the present invention to provide a new compound that can act as an electrocatalyst and that solves one or more of the above-mentioned problems or meets one or more of the above-mentioned desires. Summary of the invention

[0008] This invention is based on the finding of a new metal coordination complex that is particularly effective as an electrocatalyst for the oxygen evolution reaction which occurs during the electrocatalytic splitting of water. The oxygen evolution reaction which occurs during the electrocatalytic splitting of water is also referred to as OER herein.

[0009] In a first aspect of the invention, there is provided a crystalline metal coordination complex comprising a ligand coordinated to a metal, wherein the metal is iron, cobalt, nickel, or a combination thereof, and the ligand is quinizarin, a quinizarin derivative, 6,1 1 -dihydroxy-5,12-naphthacenedione, 5,8-dihydroxy- 1 ,4-napthoquinone, 1 ,4-dichloroanthracene-9-10-dione, 1 ,8-dihydroxynaphthalene, dithranol or anthracene-14,9,10-tetraol. In particular, the ligand is quinizarin. In addition to being useful as electrocatalysts for the OER, complexes of the invention have been found to have excellent thermal stability.

[0010] In a second aspect of the invention, there is provided a method for preparing a crystalline metal coordination complex, the method comprising:

[0011] (i) heating a solution comprising: (a) a source of metal elected from a source of iron, a source of cobalt and / or a source of nickel; (b) a ligand chosen from quinizarin, a quinizarin derivative, 6,1 1 -di hydroxy-5,12-naphthacenedione, 5,8-dihydroxy-1 ,4-napthoquinone, 1 ,4- dichloroanthracene-9-10-dione, 1 ,8-dihydroxynaphthalene, dithranol and anthracene-14,9,10- tetraol; (c) a polar aprotic solvent; and (d) a base, to a temperature of from about 50 °C to about 200 °C;

[0012] (ii) maintaining the solution at the temperature for a period of about 3 hours to about 24 hours; and

[0013] (iii) cooling the solution to form crystals of the metal coordination complex.

[0014] In particular, the ligand is quinizarin. The solution of step (i) contains water, preferably up to about 20 vol %.

[0015] The method of the second aspect of the invention can be a method for preparing a crystalline metal coordination complex according to the first aspect of the invention. The invention extends to crystalline metal coordination complexes obtained / obtainable by the method of the second aspect of the invention. Thus the complexes of the invention can be prepared using a relatively simple method which employs moderate temperatures and which takes a moderate length of time.

[0016] In a third aspect of the present invention, there is provided a dispersion comprising the crystalline metal coordination complex of the first aspect of the invention or the crystalline metal coordination complex obtained / obtainable by the second aspect of the invention, and a solvent. Such a dispersion can be described as a catalyst ink or an electrocatalyst ink and is useful for the preparation of an electrode. The words catalyst, electrocatalyst and electrode catalyst are used interchangeably herein. In a fourth aspect of the invention, there is provided a method of preparing a supported electrocatalyst, comprising depositing a dispersion of the crystalline metal coordination complex in a solvent on a support material and removing at least a portion of the solvent. Notably, a robust supported electrocatalyst can be prepared without the use of binders to bind the electrocatalyst to the support.

[0017] In a fifth aspect of the invention there is provided a supported electrocatalyst comprising a crystalline metal coordination complex of the first aspect of the invention or a crystalline metal coordination complex obtained / obtainable by the second aspect of the invention, supported on a support material. Included in this aspect of the invention is a supported electrocatalyst obtained / obtainable by the fourth aspect of the invention.

[0018] In a sixth aspect of the invention, there is provided an electrode comprising a supported electrocatalyst according to the fifth aspect of the invention. Electrodes of the invention have been found to be efficient, e.g. they have a low overpotential for the OER and thus less applied potential will be required to drive the electrolysis of water using these electrodes. Electrodes of the invention have been found to retain their electrocatalytic activity after repeated use (based on cyclic voltammetry studies), indicating good retention of the electrocatalyst on the electrode and a long life. Notably, the electrochemical activity of the electrodes is achieved without using additives such as conductive carbon particles.

[0019] In a seventh aspect there is provided an electrochemical cell for generating hydrogen and oxygen from water comprising an anode and a cathode disposed in an aqueous electrolyte, wherein the anode is an electrode according to the sixth aspect of the invention.

[0020] In an eighth aspect of the invention, there is provided a method of generating hydrogen and oxygen from water using an electrochemical cell according to the seventh aspect of the invention, the method comprising applying an electrical potential between the anode and the cathode to effect the conversion of water to hydrogen and oxygen.

[0021] In a ninth aspect of the invention, there is provided a method of activating the crystalline metal coordination complex of the first aspect of the invention or a crystalline metal coordination complex obtained / obtainable by the second aspect of the invention comprising applying an electrical potential between the anode and the cathode of the electrochemical cell of the seventh aspect of the invention cyclically until stable performance is obtained.

[0022] Brief Description of the Drawings

[0023] Figures 1 a and 1 b are Scanning Electron Microscope (SEM) images of the iron-quinizarin complex prepared in Example 1 at magnifications showing scales of 1 pm and of 500 nm, respectively. Figure 1 c is an SEM image of the activated iron-quinizarin crystals with dendritic structures under low magnification. Figures 1 d and 1 e are SEM images of the activated iron-quinizarin dendritic crystals under high magnification showing the surface morphology with well -defined boundaries. Figures 1 f, 1 g and 1 h are representative areas of TEM images of the iron-quinizarin complex sample as prepared. Figures 1 i, 1 j and 1 k are representative areas of TEM images of iron-quinizarin complex sample after degradation by 2500 OER CV cycles.

[0024] Figure 2a shows an X-ray diffraction (XRD) pattern of the iron-quinizarin complex of Example 1 . Figure 2b shows an FTIR spectra of the iron-quinizarin complex of Example 1 (solid line) and quinizarin (dotted line). Figure 2c shows the thermogravimetric curves of the iron-quinizarin complex of Example 1 (solid line) and quinizarin (dotted line). Figure 2d is the N2 adsorption desorption isotherm of the iron-quinizarin complex of Example 1 (from the Brunauer-Emmett-Teller (BET) surface area analysis).

[0025] Figure 3a is an XPS spectrum (XPS survey scan) of the iron-quinizarin complex of Example 1 . Figure 3b is an XPS spectrum of the Fe2p peaks (Fe2p scan).

[0026] Figure 4 shows LSV polarization curves of OER activity for the iron-quinizarin complex of Example 1 on Fluorine-doped Tin Oxide (FTO) in 0.1 M KOH with a scan rate of 5 mVs-1at room temperature after 5 LSV activation cycles (solid line). Figure 4 includes a polarization curve for a bare FTO with no catalyst (dotted line) under equivalent conditions.

[0027] Figure 5a is an SEM image of the nickel-quinizarin complex synthesized at 60°C of Example 4. Figure 5b is an SEM image of the nickel-quinizarin complex synthesized at 80°C of Example 3. Apart from the temperature, all other reaction conditions including reaction time and concentrations were same in the syntheses of Examples 3 and 4. Figure 5c is an SEM image of the activated nickel-quinizarin complex synthesized at 80°C of Example 3.

[0028] Figure 6a shows images of the crystal structure obtained by single X-ray crystallography of the nickel- quinizarin complex of Example 3 with complex packing (the formula calculated from these results is (Ci4H7O4)2(H2O)2Nii(C3H7NO). This includes a molecule of the solvent DMF (C3H7NO), not coordinated to the nickel.

[0029] Figure 6b shows the packing of molecules in crystal.

[0030] Figure 6c illustrates the coordination around the Ni atom in the complex with 4 bonds to oxygen atoms of the quinizarin determined by synchrotron single crystal diffraction (the other four oxygens from quinizarin in this figure are not coordinated to the metal). The two other sites around the metal when approximately octahedrally complexed (labelled A and B), as is the case for the nickel-quinizarin complex of Example 3, are either occupied by other oxygen coordinated species such as water (as in the figure) or other solvent molecules. Sites A and B are not occupied by quinizarin. The Ni atom has a coordination number of 6. The nickel atom has 6 bonds. Two bonds to each of the quinizarin molecules (there are two quinizarin molecules so accounts for four bonds) and one bond to each axial ligand (there are two axial ligands, A and B, accounting for the other two bonds). A and B are suspected to be the sites on the metal that are used to facilitate OER reaction binding hydroxide and releasing oxygen. The figure does not imply a particular geometry of the orientations of the quinizarin aromatic groups.

[0031] Figure 6d shows the experimental PXRD (determined at room temperature) and calculated PXRD pattern deduced from the single crystal determined structure (determined at low temperature) of the complex. The radiation wavelength is 1 .54182 A (0.154 nm). The small differences in peak positions of the two patterns is attributed to the usual thermal expansion of the sample.

[0032] Figure 7a shows an SEM image of the nickel-cobalt-quinizarin complex of Example 2 as prepared. Figure 7b is an SEM image of the degraded / spent form of the nickel-cobalt-quinizarin complex of Example 2. Figures 7c and 7d are SEM images of the activated complex of Example 2 at different magnifications.

[0033] Figure 8a shows an SEM image of the nickel-iron-quinizarin complex of Example 6a as prepared. Figure 8b shows an SEM image of the activated nickel-iron-quinizarin complex of Example 6a. Figure 8a shows an SEM image of the degraded / spent form of the nickel-iron-quinizarin complex of Example 6a.

[0034] Figure 9 shows the polarisation curves obtained by Linear Sweep Voltammetry (LSV), carried out to check electrochemical activity of the nickel-quinizarin complex synthesized at 80 °C of Example 3 (dotted line), nickel-cobalt-quinizarin complex of Example 2 (solid line) and the cobalt-quinizarin complex of Example 4 (dashed line).

[0035] Figure 10a shows the polarisation curves obtained by Linear Sweep Voltammetry (LSV) carried out to check electrochemical activity of the electrocatalysts of Examples 6a, 6b, 6c and 6d. The stability over time (while undergoing CV cycles) of the electrocatalyst of Example 6a was also tested and the results are presented in the form of the polarisation curves of Figure 10b, which show a stability after over 4500 cycles.

[0036] Figure 1 1 shows magnetic susceptibility data for Fe(lll)Q: 1 / % vs absolute temperature. The gradient of this plot can be used to determine the peff and number of unpaired electrons (5).

[0037] Detailed Description

[0038] In a first aspect of the present invention, there is provided a crystalline metal coordination complex comprising a ligand coordinated to a metal, wherein the metal is iron, cobalt, nickel, or a combination thereof, and the ligand is quinizarin (1 ,4-dihydroxyanthraquinone), a quinizarin derivative, 6,1 1 - dihydroxy-5,12-naphthacenedione, 5,8-dihydroxy-1 ,4-napthoquinone, 1 ,4-dichloroanthracene-9-10- dione, 1 ,8-dihydroxynaphthalene, dithranol or anthracene-14,9,10-tetraol. When the metal of the complex is iron, cobalt or nickel, the complex is a single metal complex, i.e. it contains one type of metal only. The metal can be iron (III), cobalt (II) or nickel (II).

[0039] When the metal of the complex is a combination of iron, cobalt or nickel, the complex is a mixed metal complex, i.e. it contains more than one type of metal. When the metal is a combination of iron, cobalt or nickel, the metal will be a combination of two or more of iron, cobalt and nickel. The metal of the complex can be two of more of iron (II), iron (III), cobalt (II) and nickel (II).

[0040] The complex comprises a ligand coordinated to a metal wherein the ligand is quinizarin, a quinizarin derivative, 6,1 1 -di hydroxy-5,12-naphthacenedione, 5,8-dihydroxy-1 ,4-napthoquinone, 1 ,4- dichloroanthracene-9-10-dione, 1 ,8-dihydroxynahthalene, dithranol or anthracene-14,9,10-tetraol. Each of these ligands is a multidentate ligand, i.e. it is capable of co-ordinating to a metal centre through more than one bond. The complex comprises one type of multidentate ligand and this is chosen from quinizarin, a quinizarin derivative, 6,1 1 -dihydroxy-5,12-naphthacenedione, 5,8-dihydroxy- 1 ,4-napthoquinone, 1 ,4-dichloroanthracene-9-10-dione, 1 ,8-dihydroxynahthalene, dithranol or anthracene-14,9,10-tetraol. In other words, the complex comprises one type of multidentate ligand only. Thus the complex does not contain a mixture of multidentate ligands. For example, the complex does not comprise a combination of ligands chosen from multidentate ligands including quinizarin, a quinizarin derivative, 6,1 1 -dihydroxy-5,12-naphthacenedione, 5, 8-dihydroxy-1 ,4-napthoquinone, 1 ,4- dichloroanthracene-9-10-dione, 1 ,8-dihydroxynahthalene, dithranol and anthracene-14,9,10-tetraol. Further, the complex does not contain any multidentate ligands other than quinizarin, a quinizarin derivative, 6,1 1 -dihydroxy-5,12-naphthacenedione, 5, 8-dihydroxy-1 ,4-napthoquinone, 1 ,4- dichloroanthracene-9-10-dione, 1 ,8-dihydroxynahthalene, dithranol or anthracene-14,9,10-tetraol. So for example the complex does not comprise any ligands such as dithiooxamide, I, 8-hydroxyquinoline and 1 ,4,5,8-hydroxyanthraquinone. The complex can comprise other ligands such as water, OFT, NH3, triethylamine, Ci-Ce alkanol (e.g. ethanol) and dimethylformamide. These other ligands are not multidentate ligands, are smaller in size than the multidentate ligands and, for example, originate from solvents used in the preparation of the complexes. These other ligands are believed to occupy axial positions when the metal is octahedrally complexed. Unless otherwise indicated reference to ligand herein is reference to the multidentate ligand, i.e. to the ligand chosen from quinizarin, a quinizarin derivative, 6,1 1 -dihydroxy-5,12-naphthacenedione, 5, 8-dihydroxy-1 ,4-napthoquinone, 1 ,4- dichloroanthracene-9-10-dione, 1 ,8-dihydroxynahthalene, dithranol and anthracene-14,9,10-tetraol.

[0041] The crystalline metal coordination complex comprises a ligand coordinated to a metal, wherein the metal is iron, cobalt, nickel, or a combination thereof, and the ligand is quinizarin (1 ,4- dihydroxyanthraquinone), a quinizarin derivative, 6,1 1 -dihydroxy-5,12-naphthacenedione, 5,8- dihydroxy-1 ,4-napthoquinone, 1 ,4-dichloroanthracene-9-10-dione, 1 ,8-dihydroxynaphthalene, dithranol or anthracene-14,9,10-tetraol, i.e., wherein the complex does not comprise a mixture of multidentate ligands. The complex can be a metal-quinizarin complex. The complex can contain a single metal and thus is an iron-quinizarin complex, a cobalt-quinizarin complex or a nickel-quinizarin complex. The complex can contain a combination of metals, i.e. the complex can be a mixed metal complex. The complex can be a mixed metal complex which contains two or three metals chosen from iron, cobalt, and nickel, and thus be a nickel-cobalt-quinizarin complex, a nickel-iron-quinizarin complex, a cobalt-iron- quinizarin complex or a nickel-cobalt-iron-quinizarin complex. The complex can be an iron-quinizarin complex, a cobalt-quinizarin complex, a nickel-quinizarin complex, a nickel-cobalt-quinizarin complex, a nickel-iron-quinizarin complex, a cobalt-iron-quinizarin complex, or an iron-cobalt-nickel-quinizarin complex. The complex can be an iron-quinizarin complex, a cobalt-quinizarin complex, a nickel- quinizarin complex, a nickel-cobalt-quinizarin complex, a nickel-iron-quinizarin complex, or a cobalt-iron-quinizarin complex. The complex can be an iron-quinizarin complex, a cobalt-quinizarin complex, a nickel-quinizarin complex, a nickel-cobalt-quinizarin complex, or a nickel-iron-quinizarin complex.

[0042] The complex can be a metal-quinizarin derivative complex, a metal-quinizarin derivative complex, a metal-6,1 1 -dihydroxy-5,12-naphthacenedione complex, a metal-5, 8-dihydroxy-1 ,4-napthoquinone complex, a metal-1 ,4-dichloroanthracene-9-10-dione complex, a metal-1 ,8-dihydroxynaphthalene complex, a metal-dithranol complex or a metal-anthracene-14,9,10-tetraol complex. In each case, the complex can be a single metal complex or a mixed metal complex as set out above for a metal- quinizarin complex.

[0043] It will be appreciated that the complex involves one or more ligands coordinated to one or more metal centres. As each of the ligands contains more than one co-ordinating functional group, a spectrum of structures is possible. As each quinizarin contains more than one co-ordinating functional group, a spectrum of structures is possible. For example, we refer to quinizarin, which has the structure (I) shown below:

[0044] Quinizarin can coordinate to a metal centre through the oxygen centres of its carbonyl and hydroxyl groups and thus can form a bridge between two metal centres. Thus, the complex may contain a range of structures depending on whether the ligands are bridging or not bridging metal centres. The complex may comprise ligands that are coordinated to one metal centre only and / or ligands that are coordinated to two metal centres, i.e. where the ligand forms a bridge between two metal centres. Further, it is believed that the metal centre of the metal coordination complex may be coordinatively unsaturated, i.e. it has at least one ‘empty’ site where no ligand is coordinated to the metal (see e.g. Figure 6c). Without wishing to be bound by any theory, it is believed that a coordinatively unsaturated metal centre may be more active for catalysis through an ‘empty’ site than a coordinatively saturated metal centre. It will be appreciated that reference to quinizarin as a ligand, is reference to quinizarin in the form in which it coordinates to a metal centre. For example, this can be a deprotonated form. Similarly, it will be appreciated that reference to any of the other multidentate ligands that can form the complex of the invention, is reference to that compound in the form in which it coordinates to a metal centre. The following table A shows the structures of 6,1 1 -dihydroxy-5,12-naphthacenedione, 5,8-dihydroxy- 1 ,4-napthoquinone, 1 ,4-dichloroanthracene-9-10-dione, 1 ,8-dihydroxynaphthalene, dithranol or anthracene-14,9,10-tetraol.

[0045] Table A

[0046] The molar ratio of the metal to the ligand in the complex may be from about 1 :0.5 to about 1 :2.5. The molar ratio of the metal to the ligand in the complex may be from about 1 :0.83 to about 1 :2 or 1 :0.7 to 1 : 1 .2. The molar ratio of metal to ligand is intended to refer to the average molar ratio of the complex, as it will be appreciated that the complex may contain a range of structures, e.g. depending on whether the ligands are bridging or not bridging metal atoms.

[0047] The molar ratio of the metal to the quinizarin in the complex may be from about 1 :0.5 to about 1 :2.5 or 1 :0.7 to 1 : 1 .2. The molar ratio of the metal to the quinizarin in the complex may be from about 1 :0.83 to about 1 :2. The molar ratio of metal to quinizarin is intended to refer to the average molar ratio of the complex, as it will be appreciated that the complex may contain a range of structures, e.g. depending on whether the ligands are bridging or not bridging metal atoms.

[0048] The metal complex may comprise ligands that are coordinated to one metal centre only. For example, for the nickel-quinizarin complex of Example 3, the metal is nickel and ligand is quinizarin and the metal complex comprises ligands that are coordinated to one nickel atom only, and the molar ratio of metal to ligand is 1 :2. In this embodiment, the metal complex is believed to have a distorted octahedral structure, with the metal having a coordination number of 6 and each quinizarin forming two bonds to the nickel atom giving a square planar coordination of bonds in an equatorial plane around the nickel atom. The two axial sites of the metal atom are occupied by “other ligands” as described above such as water, hydroxide or ethanol, for example. The metal coordination chemistry is distorted octahedral overall. The nickel is Ni(ll) and the resultant structure of the Ni(Quinizarin)z is shown in Figure 6c.

[0049] The complexes of the invention may have a BET surface area of greater than 35 m2 / g, greater than 60 m2 / g greater than 80 m2 / g, greater than 90 m2 / g or greater than 100 m2 / g (see, for example, Table 2).

[0050] Iron-ligand complexes

[0051] The complex may comprise quinizarin, a quinizarin derivative, 6,1 1 -dihydroxy-5,12-naphthacenedione, 5,8-dihydroxy-1 ,4-napthoquinone, 1 ,4-dichloroanthracene-9-10-dione, 1 ,8-dihydroxynaphthalene, dithranol or anthracene-14,9,10-tetraol, coordinated to iron. The iron may be iron (III), i.e. in a +3 oxidation state. The molar ratio of iron to ligand may be from about 1 :0.5 to about 1 :2.5 or from about 1 :0.7 to 1 :1 .2. The molar ratio of iron to ligand may be from about 1 :1 to about 1 :1 .5 (2:3). The molar ratio of iron to ligand may be about 1 :1 .

[0052] Iron-guinizarin complexes

[0053] The complex may comprise quinizarin coordinated to iron, i.e. the complex may be an iron-quinizarin complex. The iron may be iron (III), i.e. in a +3 oxidation state. The molar ratio of iron to ligand may be from about 1 :0.5 to about 1 :2.5 or from about 1 :0.7 to 1 :1 .2.. The molar ratio of iron to ligand may be from about 1 :1 to about 1 :1 .5 (2:3). The molar ratio of iron to ligand may be about 1 :1 . The complex may have an X-ray powder diffraction pattern comprising peaks at 9.09°,15.4° and 20.54° ±0.5°2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. Preferably, the iron is iron (III), the molar ratio of iron to quinizarin is about 1 :1 and the complex has an X-ray powder diffraction pattern comprising peaks at 9.09°,15.4°, and 20.54° ±0.5°2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0054] Cobalt-ligand complexes

[0055] The complex may comprise quinizarin, a quinizarin derivative, 6,1 1 -dihydroxy-5,12-naphthacenedione, 5,8-dihydroxy-1 ,4-napthoquinone, 1 ,4-dichloroanthracene-9-10-dione, 1 ,8-dihydroxynaphthalene, dithranol, antharuffin or anthracene-14,9,10-tetraol, coordinated to cobalt. The molar ratio of cobalt to ligand may be from about 1 :0.5 to about 1 :2.5 or from about 1 :0.7 to 1 :1 .2. The molar ratio of cobalt to ligand may be from about 1 :1 to about 1 :2. The molar ratio of cobalt to quinizarin may be from about 1 :1 to about 1 :1 .5. The molar ratio of cobalt to quinizarin may be about 1 :1 or about 1 :1 .5 (i.e. 2:3). The cobalt may be Co(ll).

[0056] Cobalt-ouinizarin complexes

[0057] The complex may comprise quinizarin coordinated to cobalt, i.e. the complex may be a cobalt- quinizarin complex. The molar ratio of cobalt to quinizarin may be from about 1 :0.5 to about 1 :2.5 or from about 1 :0.7 to 1 :1 .2. The molar ratio of cobalt to quinizarin may be from about 1 :1 to about 1 :2. The molar ratio of cobalt to quinizarin may be from about 1 :1 to about 1 :1 .5. The molar ratio of cobalt to quinizarin may be about 1 :1 or about 1 :1 .5 (i.e. 2:3). The complex may have an X-ray powder diffraction pattern comprising a peak at 9.34°±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. The peak can be described as a sharp peak. The ligand is quinizarin and the molar ratio of cobalt to quinizarin is about 1 :1 and the complex has an X-ray powder diffraction pattern comprising a peak at 9.34°±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. Preferably the cobalt is Co(ll).

[0058] Nickel-ligand complexes

[0059] The complex may comprise The complex may comprise quinizarin, a quinizarin derivative, 6,1 1 - dihydroxy-5,12-naphthacenedione, 5,8-dihydroxy-1 ,4-napthoquinone, 1 ,4-dichloroanthracene-9-10- dione, 1 ,8-dihydroxynaphthalene, dithranol or anthracene-14,9,10-tetraol, coordinated to nickel. The molar ratio of nickel to ligand may be from about 1 :0.5 to about 1 :2.5 or from about 1 :0.7 to 1 :1 .2. The molar ratio of nickel to ligand may be from about 1 :0.8 to less than 1 :2.3. The molar ratio of nickel to ligand may be from about 1 :1 to about 1 :2. The molar ratio of nickel to ligand may be from about 1 :2. The nickel may be Ni(ll).

[0060] Nickel-quinizarin complexes

[0061] The complex may comprise quinizarin coordinated to nickel, i.e.,the complex may be a nickel-quinizarin complex. The molar ratio of nickel to quinizarin may be from about 1 :0.5 to about 1 :2.5 or from about 1 :0.7 to 1 :1 .2. The molar ratio of nickel to quinizarin may be from about 1 :0.8 to less than 1 :2.3. The molar ratio of nickel to quinizarin may be from about 1 :1 to about 1 :2. The molar ratio of nickel to quinizarin may be about 1 :1 or about 1 :2. Preferably, the ligand is quinizarin. The complex may have a structure as shown in Figure 6c and described above. The complex may have an X-ray powder diffraction pattern comprising peaks at 7.12°, 1 1 .38°, 14.38°, 17.61 °, 24.77°±0.5° 2Q as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. Preferably, the molar ratio of nickel to quinizarin is about 1 :2 and the complex has an X-ray powder diffraction pattern comprising a sharp peak at 7.12°, 1 1 .38°, 14.38°, 17.61 °, 24.77°±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. Preferably the nickel is Ni(ll).

[0062] Nickel-lron-liqand complexes

[0063] The complex may comprise quinizarin, a quinizarin derivative, 6,1 1 -dihydroxy-5,12-naphthacenedione, 5,8-dihydroxy-1 ,4-napthoquinone, 1 ,4-dichloroanthracene-9-10-dione, 1 ,8-dihydroxynaphthalene, dithranol, antharuffin or anthracene-14,9,10-tetraol, coordinated to a combination of nickel and iron. The iron can be Fe(ll) or Fe(lll) and the nickel can be Ni(ll).

[0064] The molar ratio of metal (i.e. nickel and iron) to ligand may be from about 1 :0.5 to about 1 :2.5, or from about 1 :0.7 to 1 :1 .2, or from 1 :0.8 to 1 :2.3. The molar ratio of metal (i.e. nickel and iron) to ligand may be about 1 :1 .

[0065] Preferably, the molar ratio of iron to nickel is less than about 1 :1 . The molar ratio of iron to nickel may be less than about 0.2:1 , less than about 0.15:1 , less than about 0.05:1 or less than 0.04:1 . The molar ratio of iron to nickel may be from: (0.001 or 0.002 to 0.05):1 ; (0.001 or 0.002 to 0.15):1 ; or (0.001 or 0.002 to 0.2) :1 . Preferably, the molar ratio of nickel to iron is greater than about 1 :1 . The molar ratio of nickel to iron may be greater than about 1 :0.2, greater than about 1 :0.15, greater than about 1 :0.05, or greater than 1 :0.04. The molar ratio of nickel to iron may be from: 1 :0.001 or 1 :0.002 to 1 :0.05; 1 :0.001 or 1 :0.002 to 1 : 0.15; or 1 :0.001 or 1 :0.002 to 1 :0.2.

[0066] Nickel-iron-i

[0067] The complex may be a nickel-iron-quinizarin complex. The molar ratio of metal (i.e. nickel and iron) to quinizarin may be from about 1 :0.5 to about 1 :2.5, or from about 1 :0.7 to 1 :1 .2, or from 1 :0.8 to 1 :2.3. Preferably, the molar ratio of metal (i.e. nickel and iron) to ligand is about 1 :1 . The iron may be Fe(ll) or Fe(lll) and the nickel may be Ni(ll). The present inventors have found that, when the nickel-iron-quinizarin complex comprises a smaller amount of iron compared with the amount of nickel, a lower overpotential may be achieved (see Table 3). Thus, preferably, the molar ratio of iron to nickel is less than about 1 :1 . The molar ratio of iron to nickel may be less than about 0.2:1 , less than about 0.15:1 , less than about 0.05:1 or less than 0.04:1 . The molar ratio of iron to nickel may be from: (0.001 or 0.002 to 0.05):1 ; (0.001 or 0.002 to 0.15):1 ; or (0.001 or 0.002 to 0.2) :1 . Put another way, when the nickel and iron complex comprises a larger amount of nickel compared with the amount of iron, a lower overpotential may be achieved (see Table 3). Thus, preferably, the molar ratio of nickel to iron is greater than about 1 :1 . The molar ratio of nickel to iron may be greater than about 1 :0.2, greater than about 1 :0.15, greater than about 1 :0.05, or greater than 1 :0.04. The molar ratio of nickel to iron may be from: 1 :0.001 or 1 :0.002 to 1 :0.05; 1 :0.001 or 1 :0.002 to 1 : 0.15; or 1 :0.001 or 1 :0.002 to 1 :0.2.

[0068] The molar ratio of nickel to iron to quinizarin may be from about 1 :0.001 :1 to about 1 :1 :1 , i.e. 1 :(0.001 to 1 ):1 . The molar ratio of nickel to iron to quinizarin may be from about 1 :0.002:1 to about 1 :0.2:1 or 1 :0.15:1 . The molar ratio of nickel to iron to quinizarin can be about 1 :0.15:1 . The molar ratio of nickel to iron to quinizarin may be from about 1 :0.001 :1 to about 1 :0.10:1 . Preferably, the molar ratio of nickel to iron to quinizarin may be from about 1 :0.002:1 to about 1 :0.05:1 or 1 :0.04:1 . The molar ratio of nickel to iron to quinizarin can be about 1 :0.05:1 . The molar ratio of nickel to iron to quinizarin can be about 1 :0.01 :1 . The molar ratio of nickel to iron to quinizarin can be about 1 :0.005:1 . Preferably, the molar ratio of nickel to iron to quinizarin is about 1 :0.05:1 or about 1 :0.01 :1 or about 1 :0.005:1 .

[0069] The complex may have an X-ray powder diffraction pattern comprising a peak at 9.63° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0070] The complex may have a molar ratio of nickel to iron to quinizarin of about 1 :0.005:1 and have an X- ray powder diffraction pattern comprising a peak at 9.63° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0071] The complex may have an X-ray powder diffraction pattern comprising a peak at 9.57° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0072] The complex may have a molar ratio of nickel to iron to quinizarin of about 1 :0.01 :1 and have an X-ray powder diffraction pattern comprising a peak at 9.57° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0073] The complex may have an X-ray powder diffraction pattern comprising a peak at 9.7° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. The complex may have a molar ratio of nickel to iron to quinizarin of about 1 :0.05:1 and have an X-ray powder diffraction pattern comprising a peak at 9.7° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0074] The complex may have an X-ray powder diffraction pattern comprising a peak at 9.56° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0075] The complex may have a molar ratio of nickel to iron to quinizarin of about 1 :0.15:1 and have an X-ray powder diffraction pattern comprising a peak at 9.56° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0076] Nickel-cobalt-liqand complexes

[0077] The complex may comprise quinizarin, a quinizarin derivative, 6,1 1 -dihydroxy-5,12-naphthacenedione, 5,8-dihydroxy-1 ,4-napthoquinone, 1 ,4-dichloroanthracene-9-10-dione, 1 ,8-dihydroxynaphthalene, dithranol or anthracene-14,9,10-tetraol, coordinated to a combination of nickel and cobalt. The molar ratio of metal (i.e. nickel and cobalt) to ligand may be from about 1 :0.5 to about 1 :2.5, or from 1 :0.7 to 1 :1 .2, or from 1 :0.8 to 1 :2.3. The molar ratio of metal (i.e. nickel and cobalt) to ligand may be 1 :0.83 (i.e.3:2.5) or about 1 :1 . The molar ratio of nickel to cobalt to ligand may be about 1 :2:2.5. The nickel can be Ni(ll) and the cobalt can be Co(ll).

[0078] Nickel-cobalt-guinizarin complexes

[0079] The complex may comprise quinizarin coordinated to nickel and cobalt. Thus the complex may be a nickel-cobalt-quinizarin complex. The molar ratio of metal (i.e. nickel and cobalt) to ligand may be from about 1 :0.5 to about 1 :2.5, or from 1 :0.7 to 1 :1 .2, or from 1 :0.8 to 1 :2.3. The molar ratio of metal (i.e. nickel and cobalt) to ligand may be 1 :0.83 (i.e.3:2.5) or 1 :1 . The molar ratio of nickel to cobalt to ligand may be about 1 :2:2.5. The complex may have an X-ray powder diffraction pattern comprising peaks at 8.66 and 9.38°±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. The molar ratio of nickel to cobalt to quinizarin can be about 1 :2:2.5 and the complex has an X- ray powder diffraction pattern comprising peaks at 8.66 and 9.38°±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. Preferably, the molar ratio of metal (i.e. nickel and cobalt) to quinizarin is about 1 :1 and the complex has an X-ray powder diffraction pattern comprising peaks at 8.66 and 9.38°±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. Preferably the nickel is Ni(ll) and the cobalt is Co(ll).

[0080] Complexes of the invention: have been found to be particularly thermally stable (Example 1 ); can have a large surface area (Table 2); can have a relatively low overpotential for the OER when used as an electrocatalyst (Table 3) and thus present an efficient and low cost option for the OER; are stable / durable as an electrocatalyst (see, e.g., the stability studies in the examples). The second aspect of the invention provides a method for preparing a crystalline metal coordination complex, the method comprising:

[0081] (i) heating a solution comprising (a) a source of iron, a source of cobalt and / or a source of nickel; (b) a ligand chosen from quinizarin, a quinizarin derivative, 6,1 1 -dihydroxy-5,12- naphthacenedione, 5,8-dihydroxy-1 ,4-napthoquinone, 1 ,4-dichloroanthracene-9-10-dione, 1 ,8-dihydroxynaphthalene, dithranol or anthracene-14,9,10-tetraol; (c) a polar aprotic solvent; and (d) a base, to a temperature of from about 50 °C to about 200 °C;

[0082] (ii) maintaining the solution at the temperature for a period of about 3 hours to about 24 hours; and

[0083] (iii) cooling the solution to form crystals of the metal coordination complex.

[0084] The ligand can be quinizarin. The method may comprise a further step (iv) of isolating the crystals. This can be achieved by means known in the art such as filtering the product of step (iii). The method may further comprise a step of washing the crystals and / or a step of drying the crystals. The crystals may be dried by means known in the art, such as under vacuum. The method can be a method for preparing a crystalline metal coordination complex according to the first aspect of the invention.

[0085] Step (i) involves heating a solution comprising (a) a source of iron, a source of cobalt and / or a source of nickel; (b) ligand chosen from quinizarin, a quinizarin derivative, 6,1 1 -dihydroxy-5,12- naphthacenedione, 5, 8-dihydroxy-1 ,4-napthoquinone, 1 ,4-dichloroanthracene-9-10-dione, 1 ,8- dihydroxynaphthalene, dithranol or anthracene-14,9,10-tetrao (c) a polar aprotic solvent; and (d) a base, to a temperature of from about 50 or 60 °C to about 200 °C.

[0086] Prior to step (i), a solution comprising components (a) to (d) can be prepared by adding (a) a source of iron, a source of cobalt and / or a source of nickel to a solution of (b) ligand chosen from quinizarin, a quinizarin derivative, 6,1 1 -dihydroxy-5,12-naphthacenedione, 5, 8-dihydroxy-1 ,4-napthoquinone, 1 ,4-dichloroanthracene-9-10-dione, 1 ,8-dihydroxynaphthalene, dithranol or anthracene-14,9,10-tetrao (c) a polar aprotic solvent; and (d) a base. The source of iron, the source of cobalt and / or the source of nickel is in the form of a solution when it is added to the solution of (b) quinizarin (c) a polar aprotic solvent; and (d) a base. The solution contains water and can contain an alcohol such as a Ci-Ce alkanol, for example ethanol. The resultant solution can be stirred. The stirring can be for about 30 minutes at room temperature, for example.

[0087] Thus the solution of step (i) comprises water. Water may be present in the solution of step (i) in an amount of up to 20 vol %, from 1 vol % or 2 vol % to 20 vol %, from 5 to 15 vol % or from 8 vol % to 12 vol% or from 5 vol % to 10 vol %, based on the volume of the solution of step (i).

[0088] The solution of step (i) comprises (a) a source of iron, a source of cobalt and / or a source of nickel. The source of the metal may be a metal salt. For example, the source of iron may be an iron salt, the source of cobalt may be a cobalt salt, and the source of nickel may be a nickel salt. The iron salt, cobalt salt, and / or nickel salt may each independently be a chloride or an acetate. The solution may comprise (a) a source of iron(ll), a source of iron (III), a source of cobalt (II) and / or a source of nickel (II). When preparing a single metal complex, the solution may comprise a source of iron (III), a source of cobalt (II) or a source of nickel (II). When preparing a mixed metal complex, the solution may comprise two or more of: a source of iron (II); a source of iron (III); a source of cobalt (II); and a source of nickel (II). The source of iron (II) may be iron (II) acetate tetrahydrate, the source of iron (III) may be iron (III) chloride, the source of cobalt (II) may be cobalt (II) acetate tetrahydrate, and / or the source of nickel (II) may be nickel (II) acetate tetrahydrate.

[0089] The solution also comprises a ligand chosen from quinizarin, a quinizarin derivative, 6,1 1 -dihydroxy- 5,12-naphthacenedione, 5,8-dihydroxy-1 ,4-napthoquinone, 1 ,4-dichloroanthracene-9-10-dione, 1 ,8- dihydroxynaphthalene, dithranol and anthracene-14,9,10-tetraol. The molar ratios of the metal to the ligand in the solution disclosed herein may reflect the molar ratios of the metal to the ligand in the complex. For example, the molar ratio of the metal to the ligand in the solution may be from about 1 :0.5 to about 1 :2.5 so that the molar ratio of the metal to the ligand in the complex will be from about 1 :0.5 to about 1 :2.5. For example, the molar ratio of the metal to the ligand in the solution may be from about 1 :0.7 to about 1 :1 .2 so that the molar ratio of the metal to the ligand in the complex will be from about 1 :0.7 to about 1 :1 .2.

[0090] The solution may comprise quinizarin. In the method, quinizarin complexes with the metal. The molar ratios of the metal to the quinizarin in the solution disclosed herein may reflect the molar ratios of the metal to the quinizarin in the complex. For example, the molar ratio of the metal to the quinizarin in the solution may be from about 1 :0.5 to about 1 :2.5 so that the molar ratio of the metal to the quinizarin in the complex will be from about 1 :0.5 to about 1 :2.5. For example, the molar ratio of the metal to the quinizarin in the solution may be from about 1 :0.7 to about 1 :1 .2 so that the molar ratio of the metal to the quinizarin in the complex will be from about 1 :0.7 to about 1 :1 .2.

[0091] The solution comprises (c) a polar aprotic solvent. The polar aprotic solvent may be any polar aprotic solvent that the ligand is soluble in. The polar aprotic solvent may be selected from N, N-dimethylformamide, dimethyl sulfoxide, diethyl ether, acetone, acetonitrile, tetrahydrofuran, and ethyl acetate, or a combination thereof. The polar aprotic solvent may be N, N-dimethylformamide. The use of one or more polar aprotic solvent(s) facilitates the reaction of a nucleophile as part of the coordination reaction.

[0092] The solution comprises (d) a base. Any suitable base may be employed. For example, the base may be triethylamine. The base serves to deprotonate the ligand. . It is believed that the polar aprotic solvents acts to stabilise the intermediate formed after the deprotonation of the ligand. The base can be added so that the ligand is singly or doubly ionised. The extent of ionisation of the ligand is controlled by the pH of the solution comprising (b) the ligand chosen from quinizarin, a quinizarin derivative, 6,1 1 - dihydroxy-5,12-naphthacenedione, 5,8-dihydroxy-1 ,4-napthoquinone, 1 ,4-dichloroanthracene-9-10- dione, 1 ,8-dihydroxynaphthalene, dithranol or anthracene-14,9,10-tetrao (c) a polar aprotic solvent; and (d) a base. Consequently the extent of ionisation of the ligand can be controlled by the pH of the solution of step (i).

[0093] In step (i), the solution may be heated to a temperature of from about 50 °C to about 90 °C, from about 60 °C to about 80 °C. The solution may be heated to about 60 °C. The solution may be heated to about 80 °C.

[0094] Step (ii) involves maintaining the solution at a temperature (as defined hereinabove for step (i)) for a period of about 3 hours to about 24 hours. Preferably, the method comprises maintaining the solution at the temperature for about 3 to about 5 hours.

[0095] Steps (i) and (ii) may include heating at a controlled rate, heating to any temperature within the range specified and maintaining it at that temperature for the period of time specified, and also includes heating until a set predetermined temperature is reached and subsequently maintaining the reaction mixture at that predetermined temperature for a period of time.

[0096] Step (iii)

[0097] Step (iii) involves cooling the solution to crystallise the metal coordination complex. This step may comprise cooling the solution to room temperature over a period of 2 to 10 hours at a rate of about 5 to 10 °C per hour, preferably over a period of 3 to 6 hours. For instance, the solution may be slowly cooled in successive stages from 80 °C to 70, 60, 50, 40 and 30 °C, pausing and maintaining the temperature for one hour at each stage.

[0098] It will be appreciated that the selected temperature, heating rate, and cooling rate, will affect the obtained crystallite size. A desired crystallite size can therefore readily be obtained by selection of the temperature, and heating and cooling rates, using methods known within the art. For example, in the examples provided herein it can be seen that crystalline complexes obtained at 60 °C and 80 °C had different sizes and morphologies. Generally, it is believed that a slower cooling rate leads to larger crystals. The terms crystallite and crystal are used interchangeably herein.

[0099] Advantageously, the method of the second aspect of the invention involves a moderate temperature and, for example, does not comprise a calcining step. Thus the complexes disclosed herein can be prepared by a simple process that is not very energy intensive, thereby reducing cost, when compared to the preparation of transition metal-based OER catalysts of the prior art (which preparation requires a calcining step). Other advantages are that the complexes can be prepared using readily available solvents and in a reasonably short timescale.

[0100] The invention extends to a crystalline metal coordination complex obtained / obtainable by the method according to the second aspect of the invention. Exemplary methods of the second aspect of the invention are discussed below. Iron-ligand complexes

[0101] The method of the second aspect of the invention provides a method for preparing a crystalline metal coordination complex, wherein the solution in step (i) comprises a source of iron.

[0102] The molar ratio of iron to ligand in the solution may be the molar ratio of iron to ligand as set out for iron-ligand complexes disclosed herein. The molar ratio of iron to ligand in the solution may be from about 1 :0.5 to about 1 :2.5 or from 1 :0.7 to 1 :1 .2. The molar ratio of iron to ligand in the solution may be from about 1 :0.83 to about 1 :2. The molar ratio of iron to ligand in the solution may be from about 1 :1 to less than about 1 :1 .5 (2:3). The molar ratio of iron to ligand in the solution may be 1 :1 .

[0103] Preferably, the source of iron is a source of Fe3+, the molar ratio of iron to ligand in the solution is about 1 :1 , and the solution is heated to a temperature of about 80 °C.

[0104] Preferably, the source of iron is a source of Fe3+, the molar ratio of iron to ligand in the solution is about 1 :1 , the solution is heated to a temperature of about 80 °C. An exemplary method comprises:

[0105] (i) heating a solution comprising (a) a source of iron; (b) a ligand chosen from quinizarin, a quinizarin derivative, 6,1 1 -dihydroxy-5,12-naphthacenedione, 5,8-dihydroxy-1 ,4- napthoquinone, 1 ,4-dichloroanthracene-9-10-dione, 1 ,8-dihydroxynaphthalene, dithranol, antharuffin and anthracene-14,9,10-tetraol; (c) a polar aprotic solvent; and (d) a base, to a temperature of 80 °C;

[0106] (ii) maintaining the solution at the temperature for a period of about 4 hours;

[0107] (iii) cooling the solution to form crystals of the metal coordination complex; and

[0108] (iv) isolating the crystals; wherein the source of iron is a source of Fe3+and the molar ratio of iron to ligand in the solution is about 1 :1 . The solution of step (i) contains water, for example up to 20 vol % of water. The polar aprotic solvent may be N, N-dimethylformamide and the base may be triethylamine. The source of the iron may be iron(lll) chloride. The method may further comprise

[0109] (v) washing the crystals; and

[0110] (vi) drying the crystals.

[0111] Iron-guinizarin complexes

[0112] The method of the second aspect of the invention provides a method for preparing a crystalline metal coordination complex, wherein the solution in step (i) comprises a source of iron and the ligand is quinizarin.

[0113] The molar ratio of iron to quinizarin in the solution may be the molar ratio of iron to quinizarin as set out for iron-quinizarin complexes disclosed herein. The molar ratio of iron to quinizarin in the solution may be from about 1 :0.5 to about 1 :2.5 or from 1 :0.7 to 1 :1 .2. The molar ratio of iron to quinizarin in the solution may be from about 1 :0.83 to about 1 :2. The molar ratio of iron to quinizarin in the solution may be from about 1 :1 to less than about 1 :1 .5 (2:3). The molar ratio of iron to quinizarin in the solution may be 1 :1 .

[0114] Preferably, the source of iron is a source of Fe3+, the molar ratio of iron to quinizarin in the solution is about 1 :1 , and the solution is heated to a temperature of about 80 °C.

[0115] Preferably, the source of iron is a source of Fe3+, the molar ratio of iron to quinizarin in the solution is about 1 :1 , the solution is heated to a temperature of about 80 °C and the iron-quinizarin complex has an X-ray powder diffraction pattern comprising peaks at 9.09", 15.4" and20.54" ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. An exemplary method comprises:

[0116] (i) heating a solution comprising (a) a source of iron; (b) quinizarin; (c) a polar aprotic solvent; and (d) a base, to a temperature of 80 °C;

[0117] (ii) maintaining the solution at the temperature for a period of about 4 hours;

[0118] (iii) cooling the solution to form crystals of the metal coordination complex; and

[0119] (iv) isolating the crystals; wherein the source of iron is a source of Fe3+and the molar ratio of iron to quinizarin in the solution is about 1 :1 . The solution of step (i) comprises water, for example up to 20 vol % of water. The complex may have an X-ray powder diffraction pattern comprising peaks at 9.09°, 15.4°, and 20.54° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. The polar aprotic solvent may be N, N-dimethylformamide and the base may be triethylamine. The source of the iron may be iron(lll) chloride. The method may further comprise

[0120] (v) washing the crystals; and

[0121] (vi) drying the crystals.

[0122] Cobalt-ligand complexes

[0123] The method of the second aspect of the invention provides a method for preparing a crystalline metal coordination complex, wherein the solution in step (i) comprises a source of cobalt.

[0124] The molar ratio of cobalt to ligand in the solution may be the molar ratio of cobalt to ligand as set out for cobalt-ligand complexes disclosed herein. The molar ratio of cobalt to ligand in the solution may be from about 1 :0.5 to about 1 :2.5 or from 1 :0.7 to 1 :1 .2. The molar ratio of cobalt to ligand in the solution may be from about 1 :0.83 to about 1 :2. The molar ratio of cobalt to ligand in the solution may be from about 1 :1 to about 1 :2. The molar ratio of cobalt to ligand in the solution may be from about 1 :1 to about 1 :1 .5. The molar ratio of cobalt to ligand in the solution may be about 1 :1 or about 1 :1 .5 (i.e. 2:3).

[0125] Preferably, the source of cobalt is a source of cobalt (II), the molar ratio of cobalt to ligand in the solution is about 1 :1 or about 1 :1 .5, and the solution is heated to a temperature of about 80 °C.

[0126] An exemplary method comprises:

[0127] (i) heating a solution comprising (a) a source of cobalt; (b) a ligand chosen from quinizarin, a quinizarin derivative, 6,1 1 -dihydroxy-5,12-naphthacenedione, 5,8-dihydroxy-1 ,4- napthoquinone, 1 ,4-dichloroanthracene-9-10-dione, 1 ,8-dihydroxynaphthalene, dithranol and anthracene-14,9,10-tetraol; (c) a polar aprotic solvent; and (d) a base, to a temperature of 80 °C;

[0128] (ii) maintaining the solution at the temperature for a period of about 4 hours;

[0129] (iii) cooling the solution to form crystals of the metal coordination complex; and

[0130] (iv) isolating the crystals; wherein the source of cobalt is a source of cobalt (II) and the molar ratio of cobalt to quinizarin in the solution is about 1 :1 or 1 :1 .5. The solution of step (i) comprises water, for example up to 20 vol % of water. The polar aprotic solvent may be N, N-dimethylformamide and the base may be triethylamine. The source of the cobalt may be cobalt (II) acetate tetrahydrate. The method may further comprise

[0131] (v) washing the crystals; and

[0132] (vi) drying the crystals.

[0133] Cobalt-guinizarin complexes

[0134] The method of the second aspect of the invention provides a method for preparing a crystalline metal coordination complex, wherein the solution in step (i) comprises a source of cobalt and the ligand is quinizarin.

[0135] The molar ratio of cobalt to quinizarin in the solution may be the molar ratio of cobalt to quinizarin as set out for cobalt-quinizarin complexes disclosed herein. The molar ratio of cobalt to quinizarinin the solution may be from about 1 :0.5 to about 1 :2.5 or from 1 :0.7 to 1 :1 .2. The molar ratio of cobalt to quinizarinin the solution may be from about 1 :0.83 to about 1 :2. The molar ratio of cobalt to quinizarin in the solution may be from about 1 :1 to about 1 :2. The molar ratio of cobalt to quinizarin in the solution may be from about 1 :1 to about 1 :1 .5. The molar ratio of cobalt to quinizarin in the solution may be about 1 :1 or about 1 :1 .5 (i.e. 2:3).

[0136] Preferably, the source of cobalt is a source of cobalt (II), the molar ratio of cobalt to quinizarin in the solution is about 1 :1 , and the solution is heated to a temperature of about 80 °C.

[0137] Preferably, the source of cobalt is a source of cobalt (II), the molar ratio of cobalt to quinizarin in the solution is about 1 :1 , the solution is heated to a temperature of about 80 °C, and the complex has an X-ray powder diffraction pattern comprising a peak at 9.34°±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0138] An exemplary method comprises:

[0139] (i) heating a solution comprising (a) a source of cobalt; (b) quinizarin; (c) a polar aprotic solvent; and (d) a base, to a temperature of 80 °C;

[0140] (ii) maintaining the solution at the temperature for a period of about 4 hours;

[0141] (iii) cooling the solution to form crystals of the metal coordination complex; and

[0142] (iv) isolating the crystals; wherein the source of cobalt is a source of cobalt (II) and the molar ratio of cobalt to quinizarin in the solution is about 1 :1 . The solution of step (i) comprises water, e.g. up to about 20 vol % of water. The complex may have an X-ray powder diffraction pattern comprising a peak at 9.34°±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. The polar aprotic solvent may be N, N-dimethylformamide and the base may be triethylamine. The source of the cobalt may be cobalt (II) acetate tetrahydrate. The method may further comprise

[0143] (v) washing the crystals; and

[0144] (vi) drying the crystals.

[0145] Nickel-liqand complexes

[0146] The method of the second aspect of the invention provides a method for preparing a crystalline metal coordination complex, wherein the solution in step (i) comprises a source of nickel.

[0147] The molar ratio of nickel to ligand in the solution may be the molar ratio of nickel to ligand as set out for nickel-ligand complexes disclosed herein. The molar ratio of nickel to ligand in the solution may be from about 1 :0.5 to about 1 :2.5 or from 1 :0.7 to 1 :1 .2. The molar ratio of nickel to ligand in the solution may be from about 1 :0.8 to less than 1 :2.3. The molar ratio of nickel to ligand in the solution may be from about 1 :1 to 1 :2. The molar ratio of nickel to ligand in the solution may be about 1 :1 or about 1 :2.

[0148] Preferably, the source of nickel is a source of nickel (II), the molar ratio of nickel to ligand in the solution is about 1 :2, and the solution is heated to a temperature of about 80 °C.

[0149] An exemplary method comprises:

[0150] (i) heating a solution comprising (a) a source of nickel; (b) a ligand chosen from quinizarin, a quinizarin derivative, 6,1 1 -dihydroxy-5,12-naphthacenedione, 5,8-dihydroxy-1 ,4- napthoquinone, 1 ,4-dichloroanthracene-9-10-dione, 1 ,8-dihydroxynaphthalene, dithranol and anthracene-14,9,10-tetraol; (c) a polar aprotic solvent; and (d) a base, to a temperature of 80 °C;

[0151] (ii) maintaining the solution at the temperature for a period of about 4 hours;

[0152] (iii) cooling the solution to form crystals of the metal coordination complex; and

[0153] (iv) isolating the crystals; wherein the source of nickel is a source of nickel (II) and the molar ratio of cobalt to quinizarin in the solution is about 1 :2. The solution of step (i) comprises water, e.g. up to about 20 vol % of water. The complex may have an X-ray powder diffraction pattern comprising a sharp peak at 7.12°, 1 1 .38°, 14.38°, 17.61 °, 24.77°±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. The polar aprotic solvent may be N, N-dimethylformamide and the base may be triethylamine. The source of the nickel may be nickel (II) acetate tetrahydrate. The method may further comprise

[0154] (v) washing the crystals; and

[0155] (vi) drying the crystals. Nickel-guinizarin complexes

[0156] The method of the second aspect of the invention provides a method for preparing a crystalline metal coordination complex, wherein the solution in step (i) comprises a source of nickel and the ligand is quinizarin.

[0157] The molar ratio of nickel to quinizarin in the solution may be the molar ratio of nickel to quinizarin as set out for nickel-quinizarin complexes disclosed herein. The molar ratio of nickel to quinizarin in the solution may be from about 1 :0.5 to about 1 :2.5 or from 1 :0.7 to 1 :1 .2. The molar ratio of nickel to quinizarin in the solution may be from about 1 :0.8 to less than 1 :2.3. The molar ratio of nickel to quinizarin in the solution may be from about 1 :1 to 1 :2. The molar ratio of nickel to quinizarin in the solution may be about 1 :1 or about 1 :2.

[0158] Preferably, the source of nickel is a source of nickel (II), the molar ratio of nickel to quinizarin in the solution is about 1 :1 , and the solution is heated to a temperature of about 80 °C.

[0159] Preferably, the source of nickel is a source of nickel (II), the molar ratio of nickel to quinizarin in the solution is about 1 :1 , the solution is heated to a temperature of about 80 °C and the complex has an X-ray powder diffraction pattern comprising peaks at 7.12°, 1 1 .38°, 14.38°, 17.61 °, 24.77°±0.5° 2Q as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0160] An exemplary method comprises:

[0161] (i) heating a solution comprising (a) a source of nickel; (b) quinizarin; (c) a polar aprotic solvent; and (d) a base, to a temperature of 80 °C;

[0162] (ii) maintaining the solution at the temperature for a period of about 4 hours;

[0163] (iii) cooling the solution to form crystals of the metal coordination complex; and

[0164] (iv) isolating the crystals; wherein the source of nickel is a source of nickel (II) and the molar ratio of cobalt to quinizarin in the solution is about 1 :1 . The solution of step (i) comprises water, e.g. up to about 20 vol % of water. The complex may have an X-ray powder diffraction pattern comprising a sharp peak at 7.12°, 1 1 .38°, 14.38°, 17.61 °, 24.77°±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. The polar aprotic solvent may be N, N-dimethylformamide and the base may be triethylamine. The source of the nickel may be nickel (II) acetate tetrahydrate. The base may be present so that the pH of the solution of step (i) is from about 10.5 to 13. This can be measured using titration data (UV-VIS spectra) taken from a similar system, It is believed that at this pH, the quinizarin ligands are singly ionised. The method may further comprise

[0165] (v) washing the crystals; and

[0166] (vi) drying the crystals.

[0167] Nickel-iron-liqand complexes The method of the second aspect of the invention provides a method for preparing a crystalline metal coordination complex, wherein the solution in step (i) comprises a source of nickel and a source of iron.

[0168] The molar ratio of nickel to iron to ligand in the solution may be the molar ratio of nickel to iron to ligand as set out for nickel-iron-ligand complexes disclosed herein.

[0169] The molar ratio of metal (i.e. nickel and iron) to ligand in the solution may be from about 1 :0.5 to about 1 :2.5. or from 1 :0.8 to 1 :2.3.

[0170] Preferably, the molar ratio of nickel to iron in the solution is greater than about 1 :1 . The molar ratio of nickel to iron in the solution may be greater than about 1 :0.2, greater than about 1 :0.15, greater than about 1 :0.05, or greater than 1 :0.04. The molar ratio of nickel to iron in the solution may be from: 1 :0.001 or 1 :0.002 to 1 :0.05; 1 :0.001 or 1 :0.002 to 1 : 0.15; or 1 :0.001 or 1 :0.002 to 1 :0.2.

[0171] The molar ratio of nickel to iron to ligand in the solution may be from about 1 :0.001 :1 to about 1 :1 :1 . The molar ratio of nickel to iron to ligand in the solution may be from about 1 :0.002:1 to about 1 :0.2:1 or 1 :0.15:1 . The molar ratio of nickel to iron to ligand in the solution may be from about 1 :0.002:1 to about 1 :0.10:1 . The molar ratio of nickel to iron to ligand in the solution may be from about 1 :0.002:1 to about 1 :0.05:1 or 1 :0.04:1 . The molar ratio of nickel to iron to ligand in the solution can be about 1 :0.05:1 . The molar ratio of nickel to iron to ligand in the solution can be about 1 :0.01 :1 . The molar ratio of nickel to iron to ligand in the solution can be about 1 :0.005:1 .

[0172] Preferably, the source of nickel is a source of nickel (II), the source of iron is a source of iron (II) and the molar ratio of nickel to iron to ligand in the solution is about 1 :0.005:1 , 1 :0.01 :1 , 1 :0.05:1 or 1 :0.15:1 , and the solution is heated to a temperature of about 80 °C.

[0173] An exemplary method comprises:

[0174] (i) heating a solution comprising (a) a source of nickel and a source of iron; (b) a ligand chosen from quinizarin, a quinizarin derivative, 6,1 1 -dihydroxy-5,12-naphthacenedione, 5,8- dihydroxy-1 ,4-napthoquinone, 1 ,4-dichloroanthracene-9-10-dione, 1 ,8- dihydroxynaphthalene, dithranol and anthracene-14,9,10-tetraol; (c) a polar aprotic solvent; and (d) a base, to a temperature of 80 °C;

[0175] (ii) maintaining the solution at the temperature for a period of about 4 hours;

[0176] (iii) cooling the solution to form crystals of the metal coordination complex; and

[0177] (iv) isolating the crystals; wherein the source of nickel is a source of nickel (II), the source of iron is a source of iron (II) and the molar ratio of nickel to iron to ligand in the solution is about 1 :0.005:1 , about 1 :0.01 :1 , about 1 :0.05:1 or about 1 :0.15:1 . The solution of step (i) comprises water, e.g. up to 20 vol % of water. The polar aprotic solvent may be N, N-dimethylformamide and the base may be triethylamine. The source of the nickel may be nickel (II) acetate tetrahydrate. The source of the iron (II) may be iron (II) acetate tetrahydrate The method may further comprise

[0178] (v) washing the crystals; and

[0179] (vi) drying the crystals.

[0180] Nickel-iron-i

[0181] The method of the second aspect of the invention provides a method for preparing a crystalline metal coordination complex, wherein the solution in step (i) comprises a source of nickel and a source of iron, and the ligand is quinizarin.

[0182] The molar ratio of nickel to iron to quinizarin in the solution may be the molar ratio of nickel to iron to quinizarin as set out for nickel-iron- quinizarin complexes disclosed herein.

[0183] The molar ratio of metal (i.e. nickel and iron) to quinizarin in the solution may be from about 1 :0.5 to about 1 :2.5, or from 1 :0.7 to 1 :1 .2, or from 1 :0.8 to 1 :2.3.

[0184] Preferably, the molar ratio of nickel to iron in the solution is greater than about 1 :1 . The molar ratio of nickel to iron in the solution may be greater than about 1 :0.2, greater than about 1 :0.15, greater than about 1 :0.05, or greater than 1 :0.04. The molar ratio of nickel to iron in the solution may be from: 1 :0.001 or 1 :0.002 to 1 :0.05; 1 :0.001 or 1 :0.002 to 1 : 0.15; or 1 :0.001 or 1 :0.002 to 1 :0.2.

[0185] The molar ratio of nickel to iron to quinizarin in the solution may be from about 1 :0.001 :1 to about 1 :1 :1 . The molar ratio of nickel to iron to quinizarin in the solution may be from about 1 :0.002:1 to about 1 :0.2:1 or 1 :0.15:1 . The molar ratio of nickel to iron to quinizarin in the solution may be from about 1 :0.002:1 to about 1 :0.10:1 . The molar ratio of nickel to iron to quinizarin in the solution may be from about 1 :0.002:1 to about 1 :0.05:1 or 1 :0.04:1 . The molar ratio of nickel to iron to quinizarin in the solution can be about 1 :0.05:1 . The molar ratio of nickel to iron to quinizarin in the solution can be about 1 :0.01 :1 . The molar ratio of nickel to iron to quinizarin in the solution can be about 1 :0.005:1 .

[0186] Preferably, the source of nickel is a source of nickel (II), the source of iron is a source of iron (II) and the molar ratio of nickel to iron to quinizarin in the solution is about 1 :0.005:1 , 1 :0.01 :1 , 1 :0.05:1 or 1 :0.15:1 , and the solution is heated to a temperature of about 80 °C.

[0187] Preferably, the source of nickel is a source of nickel (II), the source of iron is a source of iron (II) , the molar ratio of nickel to iron to quinizarin in the solution is about 1 :0.005:1 and the complex has an X- ray powder diffraction pattern comprising a peak at 9.63° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0188] Preferably, the source of nickel is a source of nickel (II), the source of iron is a source of iron (II) , the molar ratio of nickel to iron to quinizarin in the solution is about 1 :0.01 :1 and the complex has an X-ray powder diffraction pattern comprising a peak at 9.57° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0189] Preferably, the source of nickel is a source of nickel (II), the source of iron is a source of iron (II) , the molar ratio of nickel to iron to quinizarin in the solution is about 1 :0.05:1 and the complex has an X-ray powder diffraction pattern comprising a peak at 9.7° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0190] Preferably, the source of nickel is a source of nickel (II), the source of iron is a source of iron (II) , the molar ratio of nickel to iron to quinizarin in the solution is about 1 :0.15:1 and the complex has an X-ray powder diffraction pattern comprising a peak at 9.56° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0191] An exemplary method comprises:

[0192] (i) heating a solution comprising (a) a source of nickel and a source of iron; (b) quinizarin; (c) a polar aprotic solvent; and (d) a base, to a temperature of 80 °C;

[0193] (ii) maintaining the solution at the temperature for a period of about 4 hours;

[0194] (iii) cooling the solution to form crystals of the metal coordination complex; and

[0195] (iv) isolating the crystals; wherein the source of nickel is a source of nickel (II), the source of iron is a source of iron (II) and the molar ratio of nickel to iron to quinizarin in the solution is about 1 :0.005:1 , about 1 :0.01 :1 , about 1 :0.05:1 or about 1 :0.15:1 . The solution of step (i) comprises water, e.g. up to 20 vol % of water. The complex may have an X-ray powder diffraction pattern comprising a peak at 9.63° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. The complex may have an X-ray powder diffraction pattern comprising a peak at 9.57° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. The complex may have an X-ray powder diffraction pattern comprising a peak at 9.7° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. The complex may have an X-ray powder diffraction pattern comprising a peak at 9.56° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. The polar aprotic solvent may be N, N-dimethylformamide and the base may be triethylamine. The source of the nickel may be nickel (II) acetate tetrahydrate. The source of the iron (II) may be iron (II) acetate tetrahydrate The method may further comprise

[0196] (v) washing the crystals; and

[0197] (vi) drying the crystals.

[0198] Nickel-cobalt-liqand complexes

[0199] The method of the second aspect of the invention provides a method for preparing a crystalline metal coordination complex, wherein the solution of step (I) comprises a source of nickel and a source of cobalt. The molar ratio of nickel to cobalt to ligand in the solution may be the molar ratio of nickel to cobalt to ligand as set out for nickel-cobalt-ligand complexes disclosed herein. The molar ratio of metal (i.e. nickel and cobalt) to ligand in the solution may be from about 1 :0.5 to about 1 :2.5. or from 1 :0.8 to 1 :2.3. The molar ratio of metal (i.e. nickel and cobalt) to ligand in the solution may be 1 :0.83 (i.e.3:2.5). The molar ratio of metal (i.e. nickel and cobalt) to ligand in the solution may be 1 :1 . The molar ratio of nickel to cobalt to ligand in the solution may be about 1 :2:2.5.

[0200] Preferably, the source of nickel is a source of nickel (II), the source of cobalt is a source of cobalt (II), the molar ratio of nickel to cobalt to ligand in the solution is about 1 :2:2.5, and the solution is heated to a temperature of about 80 °C.

[0201] An exemplary method comprises:

[0202] (i) heating a solution comprising (a) a source of nickel and a source of cobalt; (b) a ligand chosen from quinizarin, a quinizarin derivative, 6,1 1 -dihydroxy-5,12-naphthacenedione, 5,8- dihydroxy-1 ,4-napthoquinone, 1 ,4-dichloroanthracene-9-10-dione, 1 ,8- dihydroxynaphthalene, dithranol and anthracene-14,9,10-tetrao; (c) a polar aprotic solvent; and (d) a base, to a temperature of 80 °C;

[0203] (ii) maintaining the solution at the temperature for a period of about 4 hours;

[0204] (iii) cooling the solution to form crystals of the metal coordination complex; and

[0205] (iv) isolating the crystals; wherein the source of nickel is a source of nickel (II), the source of cobalt is a source of cobalt (II) and the molar ratio of nickel to cobalt to ligand in the solution is about 1 :2:2.5. The solution of step (i) comprises water, e.g up to 20 vol % of water. The polar aprotic solvent may be N, N-dimethylformamide and the base may be triethylamine. The source of the nickel may be nickel (II) acetate tetrahydrate. The method may further comprise

[0206] (v) washing the crystals; and

[0207] (vi) drying the crystals.

[0208] Nickel-cobalt-guinizarin complexes

[0209] The method of the second aspect of the invention provides a method for preparing a crystalline metal coordination complex, wherein the solution of step (I) comprises a source of nickel and a source of cobalt and the ligand is quinizarin.

[0210] The molar ratio of nickel to cobalt to quinizarin in the solution may be the molar ratio of nickel to cobalt to quinizarin as set out for nickel-cobalt-quinizarin complexes disclosed herein. The molar ratio of metal (i.e. nickel and cobalt) to quinizarin in the solution may be from about 1 :0.5 to about 1 :2.5or from 1 :0.7 to 1 :1 .2 or from 1 :0.8 to 1 :2.3. The molar ratio of metal (i.e. nickel and cobalt) to quinizarin in the solution may be 1 :0.83 (i.e.3:2.5). The molar ratio of metal (i.e. nickel and cobalt) to ligand in the solution may be 1 :1 . The molar ratio of nickel to cobalt to quinizarin in the solution may be about 1 :2:2.5. Preferably, the source of nickel is a source of nickel (II), the source of cobalt is a source of cobalt (II) the molar ratio of nickel to cobalt to quinizarin in the solution is about 1 :2:2.5, and the solution is heated to a temperature of about 80 °C.

[0211] Preferably, the source of nickel is a source of nickel (II), the molar ratio of nickel to cobalt to quinizarin in the solution is about 1 :2:2.5, the solution is heated to a temperature of about 80 °C and the complex has an X-ray powder diffraction pattern comprising peaks at 8.66 and 9.38° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0212] An exemplary method comprises:

[0213] (i) heating a solution comprising (a) a source of nickel and a source of cobalt; (b) quinizarin,; (c) a polar aprotic solvent; and (d) a base, to a temperature of 80 °C;

[0214] (ii) maintaining the solution at the temperature for a period of about 4 hours;

[0215] (iii) cooling the solution to form crystals of the metal coordination complex; and

[0216] (iv) isolating the crystals; wherein the source of nickel is a source of nickel (II), the source of cobalt is a source of cobalt (II) and the molar ratio of nickel to cobalt to quinizarin in the solution is about 1 :2:2.5. The solution of step (i) comprises water, e.g. up to 20 vol % of water. The complex may have an X-ray powder diffraction pattern comprising a sharp peak at 8.66 and 9.38° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. The polar aprotic solvent may be N, N-dimethylformamide and the base may be triethylamine. The source of the nickel may be nickel (II) acetate tetrahydrate. The method may further comprise

[0217] (v) washing the crystals; and

[0218] (vi) drying the crystals.

[0219] In a third aspect, the invention provides a dispersion comprising the crystalline metal coordination complex of the first aspect of the invention or the crystalline metal coordination complex obtained / obtainable by the second aspect of the invention, and a solvent. A suitable solvent is a solvent in which the crystalline metal coordination complex is not soluble in. The solvent may be a polar protic solvent such as ethanol. The dispersion can be prepared using methods known in the art. For example the dispersion may be prepared by mixing the complex with the solvent and sonicating the mixture. Typically, the dispersion contains from 0.5 to 1 .5 mg of crystalline metal coordination complex per 10 mL of solvent. The dispersion is useful for the preparation of electrodes.

[0220] In a fourth aspect of the invention, there is provided a method of preparing a supported electrocatalyst, comprising depositing a dispersion of the crystalline metal coordination complex in a solvent on a support material. At least a portion of the solvent is then removed, e.g. by allowing the deposited dispersion to dry, for example, in air. Alternatively the deposited dispersion may be dried The support material may, for example, be fluorine-doped tin oxide (FTO), indium tin oxide (ITO), a metal foam, or a metal mesh. The metal foam may be a copper foam. The metal foam may be nickel foam. The metal mesh may be a copper mesh. The metal mesh may be a nickel mesh. Preferably, the support material is selected from fluorine-doped tin oxide (FTO), nickel foam or nickel mesh. Prior to depositing the dispersion on the support material, the support material can be treated with acetone, then washed with ethanol and dried in air.

[0221] In a fifth aspect of the invention there is provided a supported electrocatalyst comprising a crystalline metal coordination complex of the first aspect of the invention or a crystalline metal coordination complex obtained / obtainable by the second aspect of the invention, supported on a support material. Included in this aspect of the invention is a supported electrocatalyst obtained / obtainable by the fourth aspect of the invention.

[0222] In a sixth aspect of the invention, there is provided an electrode comprising a supported electrocatalyst according to the fifth aspect of the invention. The electrode can be the supported electrocatalyst according to the fifth aspect of the invention. An advantage of using the metal coordination complexes as catalysts on an electrode is that the electrode has a long lifetime. For example, complexes of the invention have been found to be thermally stable to a high temperature (e.g. up to -570 °C, see Figure 2(c)), and to withstand many repeated CV cycles of the OER (e.g. up to 4500 cycles, see Figure 10(b)). Notably, this maintenance of good OER activity over extended use is achieved without the use of binders to bind the electrocatalyst to the support material or the presence of additional conducting particles.

[0223] The seventh aspect of the invention provides an electrochemical cell for generating hydrogen and oxygen from water comprising an anode and a cathode disposed in an aqueous electrolyte, wherein the anode is an electrode according to the sixth aspect of the invention.

[0224] In water electrolysis, different materials can be used for the anode and the cathode. Upon application of a potential across the electrodes, the negatively charged hydroxide ions (OHj migrate to the positively charged anode where water / OH- oxidation, i.e. the oxygen evolution reaction takes place and oxygen (O2) evolves. Simultaneously, the positively charged protons (H+) diffuse to the negatively charged cathode where water / proton reduction, i.e. the hydrogen evolution reaction (HER) takes place and hydrogen gas (H2) evolves.

[0225] The cathode (not comprising the complex) may include platinum, ruthenium oxide, iridium oxide, etc. (See Alia, S. M., & Anderson, G. C. (2019) Iridium oxygen evolution activity and durability baselines in rotating disk electrode half-cells, Journal of The Electrochemical Society, 166(4), F282-F294.). The cathode may be Pt, but may also be a graphite rod, or a gold mesh.

[0226] The electrolyte may have a neutral pH, or contain an acid or an alkali. Preferably the electrolyte contains an alkali, e.g. KOH. The electrolyte may have pH of about 9.7.

[0227] A reference electrode may also be present and may be any standard reference electrode known in the art, such as Ag / AgCI or a saturated calomel electrode (SCE). A reference electrode is not required for the reaction to occur. The purpose of a reference electrode is to provide a stable and known electrode potential for measurement purposes.

[0228] In an eighth aspect of the invention, there is provided a method of generating hydrogen and oxygen from water using an electrochemical cell according to the seventh aspect of the invention, the method comprising applying an electrical potential between the anode and the cathode to effect the conversion of water to hydrogen and oxygen. Conditions for operating water electrolysis cells are known in the art. Typically the method comprises applying an electrical potential of from about 0 V to about +1 .6 V versus the Ag / AgCI (3M KCI) at the anode and then converting to that vs. reversible hydrogen electrode (RHE) using equation

[0229] ERHE = 0.212 + 0.0592 X pH + EAg / Agci

[0230] Here ERHE represents the transformed potential against RHE, EAg / Agci is the observed potential recorded against Ag / AgCI (3M KCI) electrode, and 0.212 is the standard potential of Ag / AgCI (3M KCI) at standard conditions. Typically the pH of the electrolyte ranges from 9.0 to 14 in alkaline media Typically the method may be carried out at room temperature (about 20°C) and at atmospheric (ambient) pressure (about 100kPa).

[0231] In a ninth aspect of the invention, there is provided a method of activating the crystalline metal coordination complex of the first aspect of the invention or a crystalline metal coordination complex obtained / obtainable by the second aspect of the invention comprising applying an electrical potential between the anode and the cathode of the electrochemical cell of the seventh aspect of the invention cyclically until stable performance is obtained. Stable performance is also referred to herein as steady performance. This method can involve LSV. Stable performance is obtained when a stable LSV curve is obtained, i.e. when the LSV curve does not change substantially between cycles. The as-prepared complex is not catalytically active and requires activation in order to become catalytically active. Activation is also known as conditioning or running-in. The electrocatalyst undergoes morphological changes, observable using SEM, when it is activated (see Figures 7 and 8). Accordingly, the method of activating the crystalline metal coordination complex may involve applying the potential until morphological changes to the electrocatalyst are observed by scanning electron microscopy (SEM), for example until dendritic structures are formed. The electric potential can be from 0 V to 1 .6 V (vs Ag / AgCI (3M KCI)). Typically the method involves cyclic voltammetry in which the potential of the anode is cycled between a potential of 0 V to 1 .6 V (vs. Ag / AgCI (3M KCI)). Typically, activation should occur between 5 to 25 CV cycles at a scan rate of 50 mV s-1. The method of activating the electrode may be carried out at room temperature (about 20 °C) and at atmospheric (ambient) pressure (about 10OkPa).

[0232] Metal coordination complexes according to the invention have been shown to be effective as electrocatalysts for use in the oxygen evolution reaction (OER) in the electrolysis of water. In particular, when used as an electrode catalyst, these complexes advantageously provide one or more of the following advantages: (i) an electrode having a relatively low overpotential and thus can be operated more efficiently and at a lower cost, (ii) a catalyst that can be prepared by a not very energy intensive / simple process, (iii) a complex can be formed into an electrode by way of a simple / efficient process, e.g. a process that does not involve the use of a binder or conducting carbon black particles, and / or (iv) a catalyst with a long lifetime.

[0233] As used herein, singular forms "a," "an" and "the" also include plural forms unless the context clearly dictates otherwise. Use of the singular includes the plural unless specifically stated otherwise. The terms "comprising", "containing" and "including" as well as other forms (e.g., "include," "comprise" and "contain") are not limiting. As used herein, the term “comprising” is intended to encompass as alternative embodiments, the phrases “consisting essentially of’ and “consisting of’ where “consisting of’ excludes any element or step not specified and “consisting essentially of’ permits the inclusion of additional unrecited elements or steps that do not materially affect the essential or basic and novel characteristics of the composition or method under consideration.

[0234] The disclosure provides aspects and embodiments as set out in the following clauses:

[0235] 1 . A crystalline metal coordination complex comprising a ligand coordinated to a metal, wherein the metal is iron, cobalt, nickel, or a combination thereof, and the ligand is quinizarin.

[0236] 2. A crystalline metal coordination complex according to clause 1 , wherein the molar ratio of the metal to quinizarin is from about 1 :0.5 to about 1 :2.5 or about 1 :0.7 to about 1 :1 .2.

[0237] 3. A crystalline metal coordination complex according to clause 2, wherein the molar ratio of the metal to quinizarin is from about 1 :0.83 to about 1 :2.

[0238] 4. A crystalline metal coordination complex according any one of the preceding clauses, wherein the metal is iron, cobalt, nickel, a combination of nickel and cobalt, a combination of nickel and iron, or a combination of cobalt and iron.

[0239] 5. A crystalline metal coordination complex according to clause 4, wherein the metal is iron, cobalt, nickel, a combination of nickel and cobalt, or a combination of nickel and iron.

[0240] 6. A crystalline metal coordination complex according to any one of the preceding clauses, wherein the metal is iron.

[0241] 7. A crystalline metal coordination complex according to clause 6, wherein the iron is in a +2 or +3 oxidation state.

[0242] 8. A crystalline metal coordination complex according to any one of clauses 5 to 7, wherein the molar ratio of iron to quinizarin is from about 1 :1 to about 1 :1 .5.

[0243] 9. A crystalline metal coordination complex according to clause 8, wherein the molar ratio of iron to quinizarin is about 1 :1 . 10. A crystalline metal coordination complex according to any one of clauses 6 to 9, wherein the complex has an X-ray powder diffraction pattern comprising peaks at 9.09°, 15.4° and 20.54° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0244] 11. A crystalline metal coordination complex according to clause 6, wherein the metal is iron in a +2 oxidation state, the molar ratio of iron to quinizarin is 1 :1 and the complex has an X-ray powder diffraction pattern comprising peaks at 9.09°, 15.4° and 20.54° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0245] 12. A crystalline metal coordination complex according to any one of clauses 1 to 5, wherein the metal is cobalt.

[0246] 13. A crystalline metal coordination complex according to clause 12, wherein the molar ratio of cobalt to quinizarin is from about 1 :1 to about 1 :2 or to about 1 :1 .5.

[0247] 14. A crystalline metal coordination complex according to clause 13 wherein the molar ratio of cobalt to quinizarin is about 1 :1 or about 1 :1 .5.

[0248] 15. A crystalline metal coordination complex according to any one of clauses 12 to 14, wherein the complex has an X-ray powder diffraction pattern comprising a peak at 9.36° ±0.5°2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0249] 16. A crystalline metal coordination complex according to clause 12, wherein the molar ratio of cobalt to quinizarin is about 1 :1 .5 and the complex has an X-ray powder diffraction pattern comprising a peak at 9.36° ±0.5°2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0250] 17. A crystalline metal coordination complex according to any one of clauses 1 to 5, wherein the metal is nickel.

[0251] 18. A crystalline metal coordination complex according to clause 17, wherein the molar ratio of nickel to quinizarin is from about 1 :08 to about 1 :2.3.

[0252] 19. A crystalline metal coordination complex according to clause 18, wherein the molar ratio of nickel to quinizarin is from about 1 :1 to about 1 :2.2.

[0253] 20. A crystalline metal coordination complex according to clause 19, wherein the molar ratio of nickel to quinizarin is about 1 :1 or about 1 :2. 21 . A crystalline metal coordination complex according to any one of clauses 17 to 20, wherein the complex has an X-ray powder diffraction pattern comprising peaks at 7.12°, 1 1 .38°, and 14.38°, 17.61 and 24.77 ±0.5°2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0254] 22. A crystalline metal coordination complex according to clause 17, wherein the molar ratio of nickel to ligand is about 1 :1 and the complex has an X-ray powder diffraction pattern comprising peaks at 7.12°, 1 1 .38°, and 14.38°, 17.61 and 24.77 ±0.5°2© as measured by X- ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0255] 23. A crystalline metal coordination complex according to any one of clauses 1 to 5, wherein the metal is nickel and iron.

[0256] 24. A crystalline metal coordination complex according to clause 23, wherein the molar ratio of (nickel and iron) to quinizarin is from about 1 :0.8 to about 1 :2.3.

[0257] 25. A crystalline metal coordination complex according to clause 23 or clause 24, wherein the molar ratio of nickel to iron is greater than about 1 :1 , greater than about 1 :0.2.

[0258] 26. A crystalline metal coordination complex according to clause 25, wherein the molar ratio of nickel to iron is greater than about 1 :0.15, greater than about 1 :0.05, or greater than 1 :0.04.

[0259] 27. A crystalline metal coordination complex according to clause 25, wherein the molar ratio of nickel to iron is greater than about 1 :0.10.

[0260] 28. A crystalline metal coordination complex according to clause 27, wherein the molar ratio of nickel to iron is from 1 :0.001 or 1 :0.002 to 1 :0.05; from 1 :0.001 or 1 :0.002 to 1 : 0.15; or from 1 :0.001 or 1 :0.002 to 1 :0.2.

[0261] 29. A crystalline metal coordination complex according to clause 23, wherein the molar ratio of nickel to iron to quinizarin is from about 1 :0.001 :1 to about 1 :1 :1 .

[0262] 30. A crystalline metal coordination complex according to clause 23, wherein the molar ratio of nickel to iron to quinizarin is from about 1 :0.001 :1 to about 1 :0.10:1 .

[0263] 31 . A crystalline metal coordination complex according to clause 23, wherein the molar ratio of nickel to iron to ligand is from about 1 :0.002:1 to about 1 :0.2:1 or 1 :0.15.

[0264] 32. A crystalline metal coordination complex according to clause 23, wherein the molar ratio of nickel to iron to ligand is from about 1 :0.002:1 to about 1 :0.05:1 or 1 :0.04:1 . 33. A crystalline metal coordination complex according to clause 23, wherein the molar ratio of nickel to iron to ligand is 1 :0.005:1 , 1 :0.01 :1 , 1 :0.05:1 or 1 :0.15:1 , and is preferably 1 :0.01 :1 or 1 :0.005:1 .

[0265] 34. A crystalline metal coordination complex according to clause 23, wherein the complex has an X-ray powder diffraction pattern comprising a peak at 9.63° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0266] 35. A crystalline metal coordination complex according to clause 23, wherein the molar ratio of nickel to iron to quinizarin is about 1 :0.005:1 and the complex has an X-ray powder diffraction pattern comprising a peak at 9.63° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0267] 36. A crystalline metal coordination complex according to clause 23, wherein the complex has an X-ray powder diffraction pattern comprising a peak at 9.57° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0268] 37. A crystalline metal coordination complex according to clause 23, wherein the molar ratio of nickel to iron to quinizarin is about 1 :0.01 :1 and the complex has an X-ray powder diffraction pattern comprising a peak at 9.57° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0269] 38. A crystalline metal coordination complex according to clause 23, wherein the complex has an X-ray powder diffraction pattern comprising a peak at 9.7° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0270] 39. A crystalline metal coordination complex according to clause 23, wherein the molar ratio of nickel to iron to quinizarin is about 1 :0.05:1 and the complex has an X-ray powder diffraction pattern comprising a peak at 9.7° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0271] 40. A crystalline metal coordination complex according to clause 23, wherein the complex has an X-ray powder diffraction pattern comprising a peak at 9.56° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

[0272] 41 . A crystalline metal coordination complex according to clause 23, wherein the molar ratio of nickel to iron to quinizarin is about 1 :0.15:1 and the complex has an X-ray powder diffraction pattern comprising a peak at 9.56° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. A crystalline metal coordination complex according to any one of clauses 1 to 5, wherein the metal is nickel and cobalt. A crystalline metal coordination complex according to clause 42, wherein the molar ratio of nickel to cobalt to quinizarin is about 1 :2:2.5. A crystalline metal coordination complex according to clause 42 or clause 43, wherein the complex has an X-ray powder diffraction pattern comprising peaks at 8.66 and 9.38° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. A crystalline metal coordination complex according to clause 42, wherein the molar ratio of nickel to cobalt to quinizarin is about 1 :2:2.5 and the complex has an X-ray powder diffraction pattern comprising peaks at 8.66 and 9.38° ±0.5° 2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm. A method for preparing a crystalline metal coordination complex, the method comprising:

[0273] (i) heating a solution comprising (a) a source of metal chosen from a source of iron, a source of cobalt, a source of nickel and combination thereof; (b) quinizarin; (c) a polar aprotic solvent; and (d) a base, to a temperature of from about 50 °C to about 200 °C; and

[0274] (ii) maintaining the solution at the temperature for a period of about 3 hours to about 24 hours; and

[0275] (iii) cooling the solution to form crystals of the metal coordination complex; and preferably, wherein the solution of step (i) contains up to 20 vol % of water. A method according to clause 46, further comprising:

[0276] (iv) isolating the crystals. A method according to clause 47, further comprising:

[0277] (v) washing the crystals; and

[0278] (vi) drying the crystals. A method according to any one of clauses 46 to 48, comprising heating the solution to a temperature of from about 55 °C to about 200 °C. A method according to clause 49, comprising heating the solution to a temperature of from about 55 °C to about 90 °C. A method according to any one of clauses 46 to 50, comprising heating the solution to a temperature of about 60 °C. 52. A method according to any one of clauses 46 to 50, comprising heating the solution to a temperature of about 80 °C.

[0279] 53. A method according to any one of clauses 46 to 52, comprising maintaining the solution at the temperature for about 3 to about 5 hours, preferably 4 hours.

[0280] 54. A method according to any one of clauses 46 to 53, wherein the source of iron is a source of iron (II) or a source of iron (III), the source of cobalt is a source of cobalt (II) and / or the source of nickel is a source of nickel (II).

[0281] 55. A method according to any one of clauses 46 to 54, wherein the source of iron is an iron salt, the source of nickel is a nickel salt and the source of cobalt is a cobalt salt.

[0282] 56. A method according to clause 55, wherein each of the iron salt, nickel salt and / or cobalt salt is independently a chloride or an acetate.

[0283] 56. A method according to any one of clauses 46 to 55, wherein the iron salt is iron (III) chloride or iron (II) acetate tetrahydrate.

[0284] 57. A method according to any one of clauses 46 to 55, wherein the cobalt salt is cobalt (II) acetate tetrahydrate.

[0285] 58. A method according to any one of clauses 46 to 55, wherein the nickel salt is nickel (II) acetate tetrahydrate.

[0286] 59. A method according to any one of clauses 46 to 58, wherein the polar aprotic solvent is selected from N, N-dimethylformamide, dimethyl sulfoxide, diethyl ether, acetone, acetonitrile, tetrahydrofuran, and ethyl acetate, or a combination.

[0287] 60. A method according to clause 50, wherein the polar aprotic solvent is N, N-dimethylformamide.

[0288] 61 . A method according to any one of the clauses 46 to 60 wherein the base is triethylamine.

[0289] 62. A method according to any one of clauses 46 to 61 , comprising cooling the solution to room temperature over a period of 3 to 6 hours at a rate of about 5 to 10 °C per hour.

[0290] 63. A method according to any one of clauses 46 to 62, wherein the method is a method for preparing a crystalline metal coordination complex according to any one of clauses 1 to 45. 64. A method according to any one of clauses 46 to 62 wherein the source of metal is a source of iron (III) and the molar ratio of iron (III) to quinizarin is from about 1 :1 to about 1 :1 .5, or is about 1 :1 .

[0291] 65. A method according to clause 64, wherein the method comprises heating the solution to a temperature of about 80 °C and maintaining the solution at this temperature for about 4 hours.

[0292] 66. A method according to clause 64 or clause 65, wherein the method is a method for preparing a crystalline metal coordination complex according to any one of clauses 6 to 1 1 .

[0293] 67. A method according to any one of clauses 46 to 62, wherein the source of metal is a source of cobalt (II), the ligand is quinizarin, and the molar ratio of cobalt (II) to quinizarin is from about 1 :1 to about 1 :2, or is about 1 :1 or is about 1 .1 .5.

[0294] 68. A method according to clause 67, wherein the method comprises heating the solution to a temperature of about 80 °C and maintaining the solution at this temperature for about 4 hours.

[0295] 69. A method according to clause 67 or clause 68, wherein the method is a method for preparing a crystalline metal coordination complex according to any one of clauses 12 to 16.

[0296] 70. A method according to any one of clauses 46 to 62, wherein the source of metal is a source of nickel (II), the ligand is quinizarin, and the molar ratio of nickel (II) to quinizarin is from about 1 :08 to about 1 :2.3, or is about 1 :1 or is about 1 .2.

[0297] 71 . A method according to clause 70, wherein the method comprises heating the solution to a temperature of about 80 °C and maintaining the solution at this temperature for about 4 hours.

[0298] 72. A method according to clause 70 or clause 71 , wherein the method is a method for preparing a crystalline metal coordination complex according to any one of clauses 17 to 22.

[0299] 73. A method according to any one of clauses 46 to 62, wherein the source of metal is a source of nickel (II) and a source of iron (II) and the molar ratio of nickel (II) and iron (II) to quinizarin is from about 1 :08 to about 1 :2.3.

[0300] 74. A method according to clause 73, wherein the molar ratio of nickel to iron is greater than about 1 :0.15, greater than about 1 :0.05, or greater than 1 :0.04.

[0301] 75. A method according to clause 73 or clause 74, wherein the molar ratio of nickel to iron is greater than about 1 :0.10. 76. A method according to any one of clauses 73 to 75, wherein the molar ratio of nickel to iron to ligand is 1 :0.005:1 , 1 :0.01 :1 , 1 :0.05:1 or 1 :0.15:1 .

[0302] 77. A method according to any one of clauses 73 to 76, wherein the method comprises heating the solution to a temperature of about 80 °C and maintaining the solution at this temperature for about 4 hours.

[0303] 78. A method according to any one of clauses 73 to 77, wherein the method is a method for preparing a crystalline metal coordination complex according to any one of clauses 23 to 41 .

[0304] 79. A method according to any one of clauses 46 to 62, wherein the source of metal is a source of nickel (II) and a source of cobalt (II) and the molar ratio of nickel (II) and cobalt (II) to quinizarin is from about 1 :1 to about 1 :2.3, or the molar ratio of nickel (II) and cobalt (II) to quinizarin is about 1 :2:2.5.

[0305] 80. A method according to clause 79, wherein the method comprises heating the solution to a temperature of about 80 °C and maintaining the solution at this temperature for about 4 hours.

[0306] 81 . A method according to clause 79 or clause 80, wherein the method is a method for preparing a crystalline metal coordination complex according to any one of clauses 42 to 45.

[0307] 82. A crystalline metal coordination complex obtainable by a method according to any one of clauses 46 to 83.

[0308] 83. A dispersion comprising a crystalline metal coordination complex according to any one of clauses 1 to 45 or clause 82, and a solvent.

[0309] 84. A dispersion according to clause 83, wherein the solvent is a polar protic solvent.

[0310] 85. A dispersion according to clause 84, wherein the solvent is ethanol.

[0311] 86. A supported electrocatalyst comprising a crystalline metal coordination complex according to any of clauses 1 to 45 or clause 82, supported on a support material.

[0312] 87. A supported electrocatalyst according to clause 86, wherein the support material is selected from fluorine-doped tin oxide (FTO), indium tin oxide (ITO), a metal foam, or a metal mesh.

[0313] 88. A supported electrocatalyst according to clause 87, wherein the metal foam or metal mesh comprises copper or nickel. 89. A supported electrocatalyst according to clause 87, wherein the support material is selected from fluorine-doped tin oxide (FTO), nickel foam or nickel mesh.

[0314] 90. A method of preparing a supported electrocatalyst comprising depositing a dispersion according to any of clauses 83 to 85 on a support material and removing at least a portion of the solvent.

[0315] 91 . A method according to clause 90, wherein the method is a method of preparing a supported electrocatalyst according to any one of clauses 86 to 87.

[0316] 92. A method according to clause 90 or clause 91 , wherein a binder is not used to bind the electrocatalyst to the support material.

[0317] 93. A method according to any one of clauses 90 to 92, wherein conducting particles are not deposited on the support material.

[0318] 94. A supported electrocatalyst obtainable by the method according to any one of clauses 90 to 93.

[0319] 95. An electrode comprising the supported electrocatalyst according to any one of clauses 86 to 89 or clause 94.

[0320] 96. An electrochemical cell for generating hydrogen and oxygen from water comprising an anode, and a cathode disposed in an aqueous electrolyte, wherein the anode is an electrode according to clause 95.

[0321] 97. An electrochemical cell according to clause 96, wherein the aqueous electrolyte is alkaline.

[0322] 98. A method of generating hydrogen and oxygen from water using the electrochemical cell according to clause 96 or clause 97, the method comprising applying an electrical potential between the anode and the cathode to effect the conversion of water to hydrogen and oxygen.

[0323] 99. A method of generating hydrogen and oxygen from water according to clause 99, wherein the method is carried out at a temperature of about 20 °C and at ambient pressure (about 10OkPa).

[0324] 100. A method of activating the metal coordination complex according to any one of clauses 1 to 45 and clause 82, comprising applying an electrical potential between the anode and the cathode of the electrochemical cell of clause 96 or clause 97 cyclically until steady performance is obtained. 101 . A method according to clause 100, wherein the potential is applied until morphological changes to the electrocatalyst are observed by scanning electron microscopy (SEM).

[0325] The present invention will now be described by way of reference to the following examples. These examples are not to be construed as being limiting on the invention.

[0326] MCC stands for Metal Coordination Complex.

[0327] MOF stands for Metal-Organic Framework.

[0328] FT-IR stands for Fourier-Transform Infrared Spectroscopy.

[0329] TGA stands for Thermogravimetric Analysis.

[0330] BET (Brunauer-Emmett-Teller) adsorption isotherms, used in the measurement of the specific surface area of a material.

[0331] XPS stands for X-ray Photoelectron Spectroscopy.

[0332] FTO stands for Fluorine-doped Tin Oxide.

[0333] RHE stands for Reversible Hydrogen Electrode.

[0334] LSV stands for Linear Sweep Voltammetry.

[0335] PXRD stands for Powder X-Ray Diffraction.

[0336] SEM stands for Scanning Electron Microscopy.

[0337] FESEM stands for Field Emission Scanning Electron Microscopy.

[0338] OER stands for Oxygen Evolution Reaction.

[0339] ECSA stands for Electrochemical surface area

[0340] TOF stands for Turnover Frequency.

[0341] Tafel slope is a parameter that relates the rate of an electrochemical reaction to the overpotential.

[0342] EIS stands for Electrochemical Impedance Spectroscopy.

[0343] RCR stands for Randles Circuit.

[0344] CPE stands for Constant Phase Element.

[0345] CV stands for Cyclic Voltammetry.

[0346] SXRD stands for Single Crystal X-Ray Diffraction.

[0347] Materials characterization

[0348] A high-performance field emission scanning electron microscope (FEG-SEM system MIRA3) was used to characterize the morphology and structure of the products.

[0349] Average particle size was measured using XRD data and the X'pert HighScore program, and determined according to the Scherrer equation to estimate crystallite size.

[0350] Fourier Transform Infrared Spectrometry (FT-IR, NICOLET 380) was used to analyse the functional groups of the samples. A FT-IR spectrum of the pure quinizarin was also collected for comparison with the metal coordination complexes. X-ray diffraction (Panalytical X’Pert Pro -Cu radiation) was used to obtain powder XRD patterns of the samples. Cu-Ka radiation source (A = 1.5418 A) with accelerating voltage and tube current of 30 kV and 10 mA, respectively. Measurements were performed from 5° to 80° 2-theta (only data up to 40" 2-theta values are included as no peaks were evident in any complex after 2-theta 40").

[0351] Single crystal X-ray diffraction was carried out on VMXm. Variable beam size from 0.4 x 1 .3 to 9 x 13 pm (V x H) with fully tuneable energy (10 to 22 keV currently available) was applied. Data collection was performed using Pilatus3 6M. Samples were prepared on cryo-TEM grids and mounted on special holders to collect data.

[0352] Thermogravimetric analysis (TGA) of the synthesized material was carried out on a TGA Q 500 at a heating rate of 10 °C min-1 under an inert nitrogen atmosphere in the temperature range 25°C to 800°C.

[0353] Compositional analysis was carried out using CHN quantitative analysis and inductively coupled plasma optical emission spectrometry. CHN quantitative analysis was performed by combustion of the samples and metal analysis was carried out by inductively coupled plasma optical emission spectrometry measurements.

[0354] To deduce the specific surface area, BET adsorption isotherms were obtained using TriStar 3000 with nitrogen gas as the adsorptive and the analysis temperature of -195.8°C (immersed in liquid nitrogen). Calc. Model: N2 at 77 K on carbon (slit pore, NLDFT equilibrium model). The method used for measuring the specific surface area was according to Stephen Brunauer, P. H. Emmett and Edward Teller, J. Am. Chem. Soc. 1938, 60, 2, 309-319.

[0355] A Quorum Technologies Q150T ES Turbo-Pumped Sputter Coater / Carbon Coater was used for pre-treatment of the samples for sputtering with Pt targets (10 mm thickness) before taking SEM images and EDS measurements.

[0356] X-ray photoelectron spectroscopy (XPS) analysis was carried out using an Escalab 250XI spectrometer from Thermo Fisher Scientific (West Sussex, UK). The instrument was operating in constant analyser energy mode. A monochromatic Al-Kasource (1486.74 eV) was used. Survey scans were acquired using pass energy of 100 eV, with 5 scans being recorded using 0.5 eV steps and a dwell time of 50 ms. For narrow scans, the number of scans was 20, and a pass energy of 30 eV, step size of 0.05 eV, and dwell time of 50 ms were used.

[0357] Elemental analysis was carried out using Energy-Dispersive X-ray Spectroscopy (EDS) integrated with a FEG-SEM (MIRA3), operating at an accelerating voltage of 15 keV.

[0358] Electrochemical measurements A conventional three-electrode system was used to carry out the electrochemical measurements described herein. The system is an electrochemical cell which had a Pt wire as counter electrode, Ag / AgCI (3M KCI) as reference electrode (this electrode is an Ag wire in contact with AgCI in a 3M KCI solution) and a working electrode (at which the reaction of interest, OER, is occurring). In the examples according to the invention, the working electrode is an electrode according to the invention. 0.5 mM KOH solution (pH = 9.7) was used as an electrolyte. The Nernst equation was applied to convert potentials (recorded against Ag / AgCI (3M KCI) reference electrode) to the equivalent against the reversible hydrogen electrode.

[0359] ERHE = 0.212 + 0.0592 X pH + EAg / AgCi

[0360] ERHE in the above equation represents the transformed potential against RHE, EAg / Agci is the observed potential recorded against Ag / AgCI (3M KCI) electrode, and 0.212V is the standard potential of Ag / AgCI with 3M KCI solution.

[0361] The electrolytic solution was purged with N2 gas before commencement of each measurement. The measurements were taken at room temperature and under ambient pressure unless otherwise indicated.

[0362] Cyclic voltammetry (CV) measurements were performed at a scan rate of 50 mV s-1. This technique involves cycling the potential of the working electrode and measuring the resulting current. The potential is swept linearly with time starting from an initial potential of 0 V to a final potential of +1 .6 V vs. Ag / AgCI (3M KCI) and then back to the initial potential. The number of cycles (referred to herein as CV cycles) was varied to above 6000.

[0363] Linear sweep voltammetry (LSV) was employed to record polarization curves (current density versus working electrode potential) for the working electrode. These polarization curves are also referred to as LSV curves herein. In this technique, the potential is swept linearly with time, starting from an initial potential of 0 V to a final potential of +1 .6 V vs. Ag / AgCI (3M KCI). Linear sweep voltammetry is carried out by running the positive potential at a scan rate of 5 mV s-1in 0.5 mM KOH solution.

[0364] Stability measurements were made using an Autolab potentiostat in a three electrode configuration and involved LSV data (polarisation curve) collection, followed by 500 CV cycles and then repeating, typically until 2500 to 4500 CV cycles have been completed in total.

[0365] An Autolab potentiometer was used in a three electrode configuration and with Linear Sweep Voltammetry (LSV) to measure the Tafel slopes.

[0366] Linear Sweep Voltammetry was used to check electrochemical activity of the electrodes and, in particular, to measure the overpotential. The overpotential is the extra amount of bias required, in addition to the thermodynamic potential of water oxidation reaction (taken to be 1 .229 V vs RHE), for electrochemical activity to be observed, i.e. for the OER to take place at the working electrode and a current to be produced. The overpotential is represented with Greek letter “q”. It is usually reported as the overpotential at which the current density is 10 mA cm-2. The potential (ERHE) at which an electrochemical cell produces current at current density of 10 mA cm2can be deduced from the LSV polarisation curve, “ ” is calculated for OER by using the following equation:

[0367] *7=^RHE ~ 1- 229 where the standard thermodynamic potential of the OER is E0H2O / O2 = 1 .229 V. (Anantharaj, S., & Kundu, S. (2018). Energy & Environmental Science, 11(4), 744-771). The experimentally determined potential at 10 mA cm-2(Ej=10) is used for OER performance evaluation.

[0368] Electrochemical impedance spectroscopy (EIS) was performed with the three electrode set-up described above, over the frequency range of 100 kHz -100 mHz by the application of a sinusoidal oscillating (AC) potential with an amplitude of 10 mV and measurement of the current response. The Nyquist plots were obtained and the data was fitted to an equivalent electrochemical circuit to determine the solution resistance (Rs) and charge transfer resistance (Ret) of the reaction.

[0369] Example 1 : Iron-quinizarin complex (FeQn)

[0370] Synthesis of complex

[0371] Iron (III) chloride (0.032 g, 0.02 mol) in doubly distilled water (10 ml) was mixed with quinizarin (0.72 g, 0.03 mol) in N, N-Dimethylformamide (70 ml) and triethyl amine (20 ml) and stirred for 30 minutes. The resulting mixture was heated at 80°C for 4 hours in 15 mL ‘Ace pressure tubes’ [capacity ~15 mL, L x O.D. 10.2 cm x 25.4 mm from Ace glass] and cooled slowly to room temperature. The sample was slowly cooled after for 4 hours at 80 °C. The set-temperature was then reduced to 70 °C and air cooled. On reaching 70 °C, the mixture was maintained at 70 °C for 1 hour. The mixture was then progressively cooled in a sequence of steps, by cooling and maintaining at 60, 50, 40 and 30°C for ~1 hour at each temperature. The mixture was then cooled overnight to room temperature. The product was filtered and washed with N, N-Dimethylformamide three times and dried under vacuum overnight to obtain a black powder.

[0372] Electrode fabrication

[0373] An electrode comprising the crystals of the iron-quinizarin complex as an electrocatalyst was prepared. Conductive FTO glass of dimensions 1 cm x 1 cm x 2 mm and a sheet resistance of 16 Q cm-2(Omniscience, South Korea) was employed as a substrate for the electrode. A dispersion of the crystals of the iron-quinizarin complex in a solvent was prepared by (i) adding 1 mg of the crystals to 10 ml analytical grade ethanol, and (ii) sonicating the resultant mixture for approximately 30 minutes. The dispersion is also referred herein as a catalyst ink. Prior to depositing the dispersion on the FTO substrate, the FTO substrate was washed with acetone, then washed with ethanol and dried in air. The electrode was prepared by depositing 5 pL of the catalyst ink on the active side of the FTO substrate. The active side of the FTO substrate is the side coated with FTO (fluorinated tin oxide). After placing the drop of catalyst ink on the surface of electrode, the electrode was allowed to dry in air.

[0374] Compositional analysis The results for the complex of this example and for the complexes of other examples are presented in Tables 1 a and 1 b as weight percentages of the elements, relative molar amounts of the elements and the empirical formulae derived from this information. The data in Table 1 b is derived from TEM / EDX measurements. Transmission Electron Microscopy (TEM) data was obtained on a Thermo Fisher Scientific Talos F200X G2 S / TEM while the sample was mounted on lacey mesh copper grid. Energy Dispersive Spectroscopy (EDS) spectra and analysis was performed on an Oxford Instruments X-maxN 80 EDS system. The data indicates that most of the complexes formed have a metal to ligand ratio of 1 :1 . The exception is the nickel-quinizarin complex of Example 3. The data indicates that it has a metal to ligand ratio of 1 :2 and indeed this is confirmed by the single crystal data reported herein.

[0375] Table 1a

[0376] * T = temperature at which the complex is synthesised at

[0377] ** The PXRD peak positions are peaks that are not present in PXRD spectra for the metal (M) or quinizarin (Q).

[0378] Table 1 b

[0379] * T = temperature at which the complex is synthesised at

[0380] ** The PXRD peak positions are peaks that are not present in PXRD spectra for the metal (M) or quinizarin.

[0381] +Carbon content from EDX can be elevated due to impurities

[0382] SEM and TEM images

[0383] The crystals of the complex as-prepared were imaged by SEM and the resultant images are shown in Figures 1 a and 1 b. The images show that the complex has a very rough morphology and comprises aggregated cuboidal particles of approximately the same size, with an average size of approximately 41 nm. The crystals of the complex as-prepared were also imaged by TEM and the resultant images are shown in Figures 1 f, 1 g and 1 h. In the TEM images, which were taken at a much higher magnification than the SEM images, the complex exhibits a well-defined morphology with distinct particle sizes, shapes, and crystal structures, including a square flake-like form of particle.

[0384] Figures 1 c, 1 d and 1 e are SEM images of the activated complex at different magnifications, from which it can be seem that the morphology of the complex has changed from that of the as prepared complex. After activation the complex forms dendritic flower-like / butterfly-like patterns. Figure 1 c shows that the crystals of complex have formed dendritic structures. Figures 1 d and 1 e show that these dendritic crystals have well defined boundaries.

[0385] Figures 1 i, 1j and 1 k are TEM images of the electrocatalyst after 2500 CV cycles of OER activity. The degradation of electrocatalyst can be observed in the change in morphology shown in these images. The TEM images show that the electrocatalyst is now in the form of agglomerates.

[0386] Powder X-ray diffraction (PXRD)

[0387] Figure 2a shows the PXRD pattern for the complex. Well-defined PXRD peaks were obtained which indicate a crystalline nature. Specifically, the complex has a powder X-ray diffraction pattern comprising peaks at 9.09°, 15.4°, and 20.54° ±0.5° 2© as measured by powder X-ray diffraction using an X-ray wavelength of 0.154 nm. Using the PXRD results (Scherrer equation), the crystallite size of these materials was calculated which was found to be approximately 41 nm.

[0388] FTIR spectroscopy Figure 2b shows the FTIR spectra of the complex (solid line) and of quinizarin (dotted line). For quinizarin, a band is observed at around 1638 cm-1. This peak is the carbonyl stretch and is a typical peak for uncoordinated quinizarin. A corresponding peak at 1638 cm-1not present in the spectrum for the complex. As the carbonyl stretch is not observed for the complex, coordination between quinizarin and the metal ion is indicated. Moreover, the appearance of sharp peaks at 713 cm-1in the spectrum for the complex indicate the formation of a metal-0 bond. These results indicate that the iron has coordinated with quinizarin in the complex.

[0389] XPS

[0390] The XPS survey spectrum, Figure 3, was obtained for the complex and shows an intense oxygen peak at 532 eV (O 1 s), a carbon peak at 285 eV (C 1 s), and intermediate iron signals at 71 1 eV (Fe 2ps / 2), weak iron signals at 94 eV (Fe 3s), and 56 eV (Fe 3p). The Fe 2p core level spectrum shown in Figure 3 indicates the presence of Fe2+or Fe3+,but not iron metal (Fe(0)).

[0391] Magnetic susceptibility

[0392] Figure 1 1 shows magnetic susceptibility data for Fe(lll)Q: 1 / % vs absolute temperature. The gradient of this plot can be used to determine the pieff and number of unpaired electrons (5 in this case) and hence confirm that the oxidation state of the metal is Fe3+. Magnetic data was obtained using Quantum Design cryogen-free Magnetic Property Measurement System (MPMS) at Maxwell institute, University of Cambridge, UK. It has an operational temperature range of 1 .8-400 K and a 7 Tesla magnet. The system has a magnetic moment sensitivity of better than 108emu. SquidLab software was used for performing flexible and robust background subtraction and dipole fitting on magnetization data.

[0393] Surface area and porosity

[0394] A nitrogen adsorption-desorption study of the complex was conducted to investigate the surface area and porosity of the material. As depicted in Figure 2d, the complex displays a typical type-IV isotherm with an evident hysteresis loop, indicating the presence of micropores and distinct mesopores in the structure of the material. The specific surface area measured by BET of the complex is 1 16.46 m2g-1and the total pore volume is 0.67 cm3g-1. The total pore volume was determined from the nitrogen adsorption data at a relative pressure close to 0.99, using the same BET isotherms obtained with the TriStar 3000 system, and calculated based on the assumption that pores are filled with liquid nitrogen at -195.8°C. This and the surface area and pore volume of the complexes of other examples described herein are set out in Table 2. Without wishing to be bound by any theory, it is believed this high specific surface area offers abundant open active sites for charge storage and thus boosts electrode-electrolyte interactions, ensuring increased electrical conductivities and more accessible channels for reducing the mass transport resistance, in turn leading to increased densities and reactivities of the exposed electrocatalytic active centres for OER. Table 2:

[0395] Thermal stability - thermoqravimetric analysis

[0396] Thermogravimetric analysis of the complex was performed and compared with that of quinizarin to confirm the coordination between metal ion and ligand. Figure 2c shows the thermal decomposition temperatures of the complex (solid line) and quinizarin (dotted line) in N2 atmosphere. The thermogravimetric curve for quinizarin shows that it decomposes completely by 310 °C while for the complex, three different phases where observed in the thermogravimetric curve. The first phase is the initial mass loss (7%) which occurs between 152 °C and 229 °C. This is succeeded by a subsequent phase, spanning 229 °C to 346 °C and characterized by a gradual mass reduction of 5.6%. After this, the majority of the material survives to approximately 600 °C, and the final residue obtained at 800 °C represents 47.4% of the original mass of the complex. These results indicate that the iron-quinizarin complex is far more thermally stable than quinizarin.

[0397] Comparison of the iron-quinizarin complex with an iron-quinizarin metal organic framework

[0398] The iron-quinizarin complex prepared in this example was confirmed to be a metal coordination complex rather than a metal organic framework (MOF) composed of iron and quinizarin. This was confirmed by PXRD data for the iron-quinizarin complex of Figure 2a showing distinct diffraction peaks at around 20 = 9.09°, 15.4° and 20.54°, which peaks are absent in the PXRD pattern of an iron-quinizarin metal organic framework disclosed in S. Agrawal et al, “Novel semiconducting iron-quinizarin metal-organic framework for application in supercapacitors”, Molecular Physics, 2019, Vol. 1 17, No. 22, 3424-3433. PXRD pattern of an iron-quinizarin metal organic framework shows weak peaks at 20 = 19°, 24°, 25.5°, 27° and 35.5°. SEM images of the iron-quinizarin metal organic framework indicate crystals of as size of about 10 pm, while SEM images of the iron-quinizarin complex prepared in this example indicate agglomerated crystals with an average crystal size of about 41 nm. The thermal degradation profile of the iron-quinizarin complex prepared in this example (Figure 2c, solid line) shows that the final residue obtained at 800 °C represents 47.4% of the original weight of the complex. In contrast, the iron-quinizarin metal organic framework, which starts to lose mass at approximately 270°C, leaves a residue at 700 °C which is approximately 23% of the original weight of the metal organic framework (S. Agrawal et al, “Novel semiconducting iron-quinizarin metal-organic framework for application in supercapacitors”, Molecular Physics, 2019, Vol. 117, No. 22, 3430-3431 ). Therefore, the iron-quinizarin complex of this example is far more thermally stable than the iron-quinizarin metal organic framework. Activation of the complex

[0399] The complex was activated by subjecting the working electrode to 15 CV cycles, i.e. 15 LSV cycles in the range of 0-+1.6 V. The catalyst is subjected to repeated CV cycles until the LSV curve does not change substantially between cycles.

[0400] Electrochemical activitv / stabilitv

[0401] The stability of the electrocatalyst was evaluated by the relative analysis of polarisation curves obtained by LSV before and after subjecting the activated electrocatalyst to consecutive CV cycles at a 50 mV / s scan rate. Figure 4a shows the polarization curve of bare FTO (dashed line) and the OER activity for the activated electrocatalyst of this example (solid line).

[0402] OER performance

[0403] The OER performance of the electrocatalyst was estimated in terms of the Tafel slope, as the Tafel slope provides an insight of the reaction kinetics and surface adsorbed species produced during the reaction. The Tafel slope was found by OER fitted LSV curves. The overpotential plotted against log (j) gives the Tafel slope, where j is the current density. The Tafel slope is determined from the Overpotential vs. Current Density (OER) curves by fitting the linear region of the log(current density) versus overpotential plot. The Tafel slope is calculated from the slope of this linear fit. The curve used was the same as used for overpotential calculation i.e. of activated catalyst. The Tafel slope obtained in this way for the electrode of this example gives a value of 38.8 mVdec-1. This value is small when compared to Tafel slope values of other electrocatalysts indicating that swift charge transport is offered by the new electrocatalyst (see the Tafel slope values given in Table 3 below, for example). OER performance is also indicated by the overpotential. Table 3 below shows the OER performance parameters of the electrocatalysts prepared in this example and other examples described herein compared to that reported for known Fe-based catalysts.

[0404] Table 3

[0405]

[0406] 1 . So.os-Fe-BTB / NF is S-doped Fe-based organic framework grown on nickel foam (NF).

[0407] 2. Coo.75Feo.25@COF-TB is Fe-doped Co-tannic acid coordination complex nanoflowers (denoted as FexCoi-x- TA nanoflowers).

[0408] 3. Fe_DES; 3-D iron alkoxides of glycerol synthesized in choline chloride (ChCI) / glycerol deep eutectic solvent (DES).

[0409] 4. FF is Fe Foam. a. Ling, C., Leng, X., Lu, X. J., Li, J. FL, Yang, Z., & Xu, A. W. (2022). A self-supported S-doped Fe-based organic framework platform enhances electrocatalysis toward highly efficient oxygen evolution in alkaline media. Journal of Materials Chemistry A, 10(33), 17246-17253. b. Xu, Y., Xie, M., Li, X., Shao, F., Li, S., Li, S., ... & Jiao, Y. (2022). Regulating the electronic structure of Fe- based metal organic frameworks by electrodeposition of Au nanoparticles for electrochemical overall water splitting. Journal of Colloid and Interface Science, 626, 426-434. c. Zhou, G., Ma, Y., Gu, C., Yang, J., Pang, H., Li, J., ... & Tang, Y. (2022). Fe incorporation -induced electronic modification of Co-tannic acid complex nanoflowers for high-performance water oxidation. Inorganic Chemistry Frontiers, 9(6), 1091 -1099. d. Shah, S. S., Albadrani, A., Fettouhi, M., Aziz, M. A., & Helal, A. (2024). Synthesis and Oxygen Evolution Reaction Application of a Co-Cd Based Bimetallic Metal-Organic Framework. Chemistry-An Asian Journal, e202301039. e. Liu, X., Feng, L., Li, Y., Xia, T., Sui, Z., & Chen, Q. (2022). Covalent organic frameworks composites containing bipyridine metal complex for oxygen evolution and methane conversion. Molecules, 27(16), 5193. f. Liu, S., Chen, T., Ying, H., Li, Z., & Hao, J. (2020). Deep Eutectic Solvent-Mediated Construction of Oxygen Vacancy-Rich Fe-Based Electrocatalysts for Efficient Oxygen Evolution Reaction. Advanced Sustainable Systems, 4(6), 2000038. g. Zhang, C., Zhang, B., Li, Z., & Hao, J. (2019). Deep eutectic solvent-mediated hierarchically structured Fe- based organic-inorganic hybrid catalyst for oxygen evolution reaction. ACS Applied Energy Materials, 2(5), 3343-3351.

[0410] Example 2: Nickel-cobalt-quinizarin complex (NiCoQH)

[0411] Synthesis of complex

[0412] Nickel (II) acetate tetrahydrate (0.248 g, 0.01 mol) and cobalt (II) acetate tetrahydrate (0.18 g, 0.01 mol) in ethanol (8 ml) and distilled water (2 ml) is mixed into a solution of quinizarin (0.48 g, 0.02 mol) in N, N-Dimethylformamide (70 ml) and triethyl amine (20 ml). The obtained mixture is then heated at 80°C for 4 hours in five 15 mL ace pressure tubes’ [capacity ~15 mL, L x O.D. 10.2 cm x 25.4 mm from Ace glass] and cooled slowly to room temperature. The product is filtered and washed with N, N- Dimethylformamide three times and dried in a vacuum hood overnight to obtain a dark purple precipitate.

[0413] Electrode fabrication

[0414] An electrode comprising the crystals of the nickel-cobalt-quinizarin complex as an electrocatalyst was prepared in the same manner as the electrode of Example 1 .

[0415] Compositional analysis

[0416] The results are presented in Table 1 .

[0417] Surface area and porosity

[0418] The results are set out in Table 2.

[0419] SEM images

[0420] The crystals of the complex as-prepared were imaged by SEM and the resultant image is shown in Figure 7a. The image shows that the crystals have plate-like structures. Figures 7c and 7d are SEM images of the activated complex at different magnifications. The activated forms have pillar-like structures which stand normal to the surface. Figure 7b is an SEM image of the degraded electrocatalyst. Figure 7b shows that the degraded electrocatalyst is more in the form of a lump, with only a few needle-like pillars remaining.

[0421] Powder X-ray diffraction (PXRD)

[0422] The complex has a powder X-ray diffraction pattern comprising peaks as indicated in Table 1 as measured by powder X-ray diffraction using an X-ray wavelength of 0.154 nm.

[0423] Activation of the complex The complex of this example was activated by subjecting the working electrode to 15 CV cycles.

[0424] Electrochemical activitv / stabilitv

[0425] A polarisation curve obtained by LSV was taken of the activated electrocatalyst before it was subjected to 15 CV cycles. The activated electrocatalyst was then subjected to 15 CV cycles. After 4500 CV cycles, there was a significant variation in polarisation curves of the freshly activated and the post- catalytic reaction electrode. This change of response indicates a degradation of the electrode / electrocatalyst.

[0426] PER performance

[0427] The PER performance of the electrocatalyst was evaluated in terms of overpotential. The results are shown in Table 3.

[0428] Example 3: Nickel-quinizarin complex (80 °C) (NiPH)

[0429] Synthesis of complex

[0430] Nickel (II) acetate tetrahydrate (0.496 g, 0.02 mol) in ethanol (8 ml) and distilled water (2 ml) is mixed into a solution of quinizarin (0.48 g, 0.02 mol) in N, N-Dimethylformamide (70 ml) and triethyl amine (20 ml). The obtained mixture is then heated at 80°C for 4 hours in five 15 mL ace pressure tubes’ [capacity ~15 mL, L x P.D. 10.2cm x 25.4mm from Ace glass] and cooled slowly to room temperature. The product is filtered and washed with N, N-Dimethylformamide and kept for drying in a vacuum hood overnight to obtain a dark purple precipitate.

[0431] Electrode fabrication

[0432] An electrode comprising the crystals of the nickel-quinizarin complex as an electrocatalyst was prepared in the same manner as the electrode of Example 1 .

[0433] Compositional analysis

[0434] The results are presented in Table 1 .

[0435] Surface area and porosity

[0436] The results are set out in Table 2.

[0437] SEM images

[0438] The crystals of the complex as-prepared were imaged by SEM and the resultant image is shown in Figure 5b. The image shows that the crystals have well-defined rod-like structures. The crystals had a size as large as 10 pm, allowing an image of the crystal structure to be obtained by Single crystal X-ray diffraction with complex packing. The formula calculated from nickel complex packing and single crystal XRD analysis is CuHgOsNh (which complements the results obtained from microanalysis in Table 1 ). The structure is shown in Figure 6. It is evident from these results that the complexes obtained are metal coordination complexes, rather than metal organic frameworks.

[0439] Powder X-ray diffraction (PXRD)

[0440] The complex has a powder X-ray diffraction pattern comprising peaks as indicated in Table 1 as measured by powder X-ray diffraction using an X-ray wavelength of 0.154 nm.

[0441] Magnetic susceptibility

[0442] Magnetic susceptibility data for this complex confirmed that the oxidation state of the metal is Ni2+.

[0443] Activation of the complex

[0444] The complex of this example was activated by subjecting the working electrode to about 20 cycles of cyclic voltammetry.

[0445] Electrochemical activity

[0446] A polarisation curve obtained by LSV was taken of the activated electrocatalyst. The polarisation curve for the activated catalyst is shown in Figure 9 (solid line) and this demonstrates that the electrocatalyst is electrochemically active.

[0447] PER performance

[0448] The PER performance of the electrocatalyst was evaluated in terms of overpotential. The results are shown in Table 3.

[0449] Example 4: Nickel-quinizarin complex (60 °C)

[0450] Synthesis of complex

[0451] Nickel (II) acetate tetrahydrate (0.496 g, 0.02 mol) in ethanol (8 ml) and distilled water (2 ml) is mixed into a solution of quinizarin (0.48 g, 0.02 mol) in N, N-Dimethylformamide (70 ml) and triethyl amine (20 ml). The obtained mixture is then heated at 60°C for 4 hours in five 15 mL ace pressure tubes’ [capacity ~15 mL, L x P.D. 10.2cm x 25.4mm from Ace glass] and cooled slowly to room temperature. The product is filtered and washed with N, N-Dimethylformamide and kept for drying in a vacuum hood overnight.

[0452] Electrode fabrication

[0453] An electrode comprising the crystals of the nickel-quinizarin complex as an electrocatalyst was prepared in the same manner as the electrode of Example 1 .

[0454] SEM images

[0455] The crystals of the complex as-prepared were imaged by SEM and the resultant image is shown in Figure 5a. The image shows that the crystals have flower-like morphology. From comparison of the SEM images of Figures 5a and 5b, it is evident that the temperature at which the synthesis reaction is carried out for nickel-quinizarin complexes affects the morphology of the product. It is evident from these results that the complexes obtained are metal coordination complexes, rather than metal organic frameworks.

[0456] Example 5: Cobalt-quinizarin complex (CoQH)

[0457] Synthesis of complex

[0458] Cobalt (II) acetate tetrahydrate (0.36 g, 0.02 mol) in ethanol (8 ml) and distilled water (2 ml) is mixed into a solution of quinizarin (0.48 g, 0.02 mol) in N, N-Dimethylformamide (70 ml) and triethyl amine (20 ml). The obtained mixture is then heated at 80°C for 4 hours in five 15 mL ace pressure tubes’ [capacity ~15 mL, L x O.D. 10.2 cm x 25.4 mm from Ace glass] and cooled slowly to room temperature. The product is filtered and washed with N, N-Dimethylformamide and kept for drying in a vacuum hood overnight to obtain a dark brown precipitate.

[0459] Electrode fabrication

[0460] An electrode comprising the crystals of the cobalt-quinizarin complex as an electrocatalyst was prepared in the same manner as the electrode of Example 1 .

[0461] Compositional analysis

[0462] The results are presented in Table 1 .

[0463] Surface area and porosity

[0464] The results are set out in Table 2.

[0465] Powder X-ray diffraction (PXRD)

[0466] The complex has a powder X-ray diffraction pattern comprising peaks as indicated in Table 1 as measured by powder X-ray diffraction using an X-ray wavelength of 0.154 nm.

[0467] Electrochemical activity

[0468] A polarisation curve obtained by LSV was taken of the activated electrocatalyst. The polarisation curve for the activated catalyst is shown in Figure 9 (dashed line) and this demonstrates that the electrocatalyst is electrochemically active.

[0469] PER performance

[0470] The PER performance of the electrocatalyst was evaluated in terms of overpotential. The results are shown in Table 3.

[0471] Examples 6a to 6d: Nickel-iron-quinizarin complexes with varying metal ratios

[0472] Synthesis of complexes These complexes were prepared using a method similar to the methods of Examples 2 and 3 adapted by varying the amounts of nickel acetate tetrahydrate and iron acetate tetrahydrate salts to provide complexes with molar ratios of nickel to iron from 6.67:1 to 200:1 . Specifically complexes having the following nickekiron molar ratios: 1 :0.005, e.g. NiFeo.oos (Example 6a); 1 :0.01 , e.g. NiFeo.01 (Example 6b); 1 :0.05, e.g. NiFeo.os (Example 6c); and 1 :0.15, e.g. NiFeo.15 (Example 6d) were prepared.

[0473] Electrode fabrication

[0474] An electrode comprising the crystals of the nickel-iron-quinizarin complex of each of Examples 6a to 6d as an electrocatalyst was prepared in the same manner as the electrode of Example 1 .

[0475] Compositional analysis

[0476] The results are presented in Table 1 .

[0477] SEM and TEM / EDS images

[0478] The crystals of the complex of Example 6b as prepared were imaged by SEM and the resultant image is shown in Figure 8a. Figure 8b is an SEM image of the activated complex, from which it can be seen that the very distinctive activated product forms a branched structure. It is evident from these images that significant morphological changes to the complex as prepared take place upon activation. Figure 8c is an SEM image of the degraded electrocatalyst, the degradation is evident by a change in morphology from the flower-like dendrites shown in Figure 8a to a more blob-like form shown in Figure 8c.

[0479] Powder X-ray diffraction (PXRD)

[0480] The complexes have powder X-ray diffraction pattern comprising peaks as indicated in Table 1 as measured by powder X-ray diffraction using an X-ray wavelength of 0.154 nm.

[0481] Activation of the complex

[0482] The complex of Example 6b was activated by subjecting the working electrode to 10 cycles of cyclic voltammetry.

[0483] Electrochemical activitv / stabilitv

[0484] Polarisation curves were taken of the electrocatalyst of Example 6b as prepared (solid line of Figure 10b) and of the activated catalyst (after 15 CV cycles, dashed line of Figure 10b). Further polarisation curves are taken after the electrocatalyst is subjected to 1000, 2500, 4500 and 6000 CV cycles. These are also shown in Figure 10b. After 10 CV cycles, there was a significant variation in polarisation curves of the freshly activated and post-catalytic electrode. This response change indicates a degradation of the electrode / electrocatalyst. The electrocatalysts prepared showed improved properties in terms of their electrochemical activity and stability over time (see e.g. Figure 10). As can be seen in Figures 9 and 10, there appears a peak around +1 .2 V vs. RHE when the catalysts are subjected to 15 CV cycles at a scan rate of 50 mVs-1. Without wishing to be bound by any theory, it is believed this may be due to formation of some species as a result of electrooxidation. As can be seen from Figure 10 b, the formed species are quite stable and it can be seen that the peak exists for at least up to 4500 cycles.

[0485] PER performance

[0486] LSV polarisation curves were taken of each of the electrocatalysts (Figure 10a). The PER performance of the each of the electrocatalysts was evaluated in terms of overpotential. The results are shown in Table 3. It can be seen from Table 3, that the complexes of this example have suitably low overpotential values, making them suitable for use as electrocatalysts. In particular, it can be seen that introducing small amounts of iron in the complexes provided especially low overpotential values, with nickel :iron molar ratios of 1 :0.01 (Example 6b) having a low overpotential of 180 mV (q) at 10 mAcrn-2.

[0487] Nomenclature for the complexes of each of the Examples used in figures

[0488] Example 1 FePn

[0489] Example 2 NiCoPH

[0490] Example 3 NiPH

[0491] Example 5 CoPH

[0492] Example 6a NiFeP4

[0493] Example 6b NiFePi

[0494] Example 6c NiFePz

[0495] Example 6d NiFePs

Claims

Claims1 . A crystalline metal coordination complex comprising a ligand coordinated to a metal, wherein the metal is iron, cobalt, nickel, or a combination thereof, and the ligand is quinizarin, a quinizarin derivative, 6,11 -dihydroxy-5,12-naphthacenedione, 5,8-dihydroxy-1 ,4- napthoquinone, 1 ,4-dichloroanthracene-9-10-dione, 1 ,8-dihydroxynaphthalene, dithranol or anthracene-14,9,10-tetraol.

2. A crystalline metal coordination complex according to claim 1 , wherein the molar ratio of the metal to ligand from about 1 :0.5 to about 1 :2.5 or from about 1 :0.7 to 1 :1.2 or from about 1 : 0.83 to 1 :2.

3. A crystalline metal coordination complex according to claim 1 or claim 2, wherein the ligand is quinizarin.

4. A crystalline metal coordination complex according to claim 3, wherein the metal is iron, and, optionally, wherein the molar ratio of iron to quinizarin is from about 1 :1 to about 1 :1 .5.

5. A crystalline metal coordination complex according to claim 4, wherein the complex has an X- ray powder diffraction pattern comprising peaks at 9.09°, 15.4° and 20.54° ±0.5°2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

6. A crystalline metal coordination complex according to claim 4 or claim 5, wherein the iron is in a +3 oxidation state.

7. A crystalline metal coordination complex according to claim 3, wherein the metal is cobalt, and, optionally, wherein the molar ratio of cobalt to quinizarin is about 1 :1 or about 1 :1 .5.

8. A crystalline metal coordination complex according to claim 7, wherein the complex has an X- ray powder diffraction pattern comprising a peak at 9.36° ±0.5°2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

9. A crystalline metal coordination complex according to claim 3, wherein the metal is nickel, and, optionally, wherein the molar ratio of nickel to quinizarin is about 1 :1 or about 1 :2.

10. A crystalline metal coordination complex according to claim 9, wherein the complex has an X- ray powder diffraction pattern comprising peaks at 7.12°, 11 .38°, and 14.38°, 17.61 and 24.77 ±0.5°2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.1 1 . A crystalline metal coordination complex according to claim 9 or claim 10, wherein the molar ratio of nickel to quinizarin is 1 :2 and each ligand is bound to one nickel atom only.

12. A crystalline metal coordination complex according to claim 3, wherein the metal is nickel and iron, and, optionally, wherein the molar ratio of nickel to iron is greater than about 1 :1 .

13. A crystalline metal coordination complex according to claim 12, wherein the molar ratio of nickel to iron to quinizarin is from about 1 :0.001 :1 to about 1 :1 :1 .

14. A crystalline metal coordination complex according to claim 12, wherein the molar ratio of nickel to iron to quinizarin is from about 1 :0.001 :1 to about 1 :0.10:1 .

15. A crystalline metal coordination complex according to any one of claims 12 to 14, wherein the complex has an X-ray powder diffraction pattern comprising a peak at 9.63 ±0.5o2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

16. A crystalline metal coordination complex according to any one of claims 12 to 14, wherein the complex has an X-ray powder diffraction pattern comprising a peak at 9.57 ±0.5o2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

17. A crystalline metal coordination complex according any one of claims 12 to 14, wherein the complex has an X-ray powder diffraction pattern comprising a peak at 9.7 ±0.5o2© as measured by X-ray powder diffraction using an X-ray wavelength of 0.154 nm.

18. A crystalline metal coordination complex according to claim 12 or claim 13, wherein the complex has an X-ray powder diffraction pattern comprising a peak at 9.56 ±0.5o2© as measured by X- ray powder diffraction using an X-ray wavelength of 0.154 nm.

19. A crystalline metal coordination complex according to claim 3, wherein the metal is nickel and cobalt, and, optionally, wherein the molar ratio of nickel to cobalt to quinizarin is about 1 :2:2.5.

20. A crystalline metal coordination complex according to claim 19, wherein the complex has an X- ray powder diffraction pattern comprising peaks at 8.66 and 9.38 ±0.5o2© as measured by X- ray powder diffraction using an X-ray wavelength of 0.154 nm.21 . A method for preparing a crystalline metal coordination complex according to any one of claims 1 to 20, the method comprising:(i) heating a solution comprising (a) a source of metal chosen from a source of iron, a source of cobalt, a source of nickel and combination thereof; (b) a ligand chosen form quinizarin, a quinizarin derivative, 6,1 1 -dihydroxy-5,12-naphthacenedione, 5,8-57 dihydroxy-1 ,4-napthoquinone, 1 ,4-dichloroanthracene-9-10-dione, 1 ,8- dihydroxynaphthalene, dithranol and anthracene-14,9,10-tetraol; (c) a polar aprotic solvent; and (d) a base, to a temperature of from about 50 °C to about 200 or from about to about 55 °C to about 90 °C; and(ii) maintaining the solution at the temperature for a period of about 3 hours to about 24 hours; and(iii) cooling the solution to form crystals of the metal coordination complex.

22. A method according to claim 21 , wherein the ligand is quinizarin.

23. A method according to claim 21 or claim 22 wherein the solution of step (i) has a water content of up to 20 vol %.

24. A method according to any one of claims 21 to 23, comprising:(iv) isolating the crystals.

25. A method according to any one of claims 21 to 24, comprising heating the solution to a temperature of from about 55 °C to about 90 °C, and maintaining the solution at the temperature for about 3 to about 5 hours.

26. A method according to any one of claims 21 to 25, wherein the source of metal is chosen from a source of iron (II), a source of iron (III), a source of cobalt (II), a source of nickel (II) and a combination thereof.

27. A crystalline metal coordination complex obtainable by a method according to any one of claims 21 to 26.

28. A dispersion comprising a crystalline metal coordination complex according to any one of claims 1 to 20 or claim 27, and a solvent.

29. A supported electrocatalyst comprising a crystalline metal coordination complex according to any of claims 1 to 20 or claim 27, supported on a support material.30 A method of preparing a supported electrocatalyst comprising depositing a dispersion according to claim 29 on a support material and removing at least a portion of the solvent.31 . An electrode comprising the supported electrocatalyst according to claim 29.

32. An electrochemical cell for generating hydrogen and oxygen from water comprising an anode, and a cathode disposed in an aqueous electrolyte, wherein the anode is an electrode according to claim 31 .

33. A method of generating hydrogen and oxygen from water using the electrochemical cell according to claim 32, the method comprising applying an electrical potential between the anode and the cathode to effect the conversion of water to hydrogen and oxygen.

34. A method of activating the metal coordination complex according to any one of claims 1 to 20 or claim 27, comprising applying an electrical potential between the anode and the cathode of the electrochemical cell of claim 32 cyclically until a stable performance is obtained.

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