ELECTRODE MATERIAL

Abstract

The present invention relates to an electrode material for oxygen evolution reaction. The electrode material comprises crystal structures of AlM.sub.2B.sub.2, and crystal structures of [M2B2] and oxidised M, wherein M is selected from Fe, Mn, and Cr. The present invention further relates to an electrode for oxygen evolution reaction and a system for water electrolysis.

Claims

1. An electrode material for oxygen evolution reaction, the electrode material comprising crystal structures of AlM.sub.2B.sub.2, and crystal structures of [M.sub.2B.sub.2] and oxidised M, wherein M is selected from Fe, Mn, and Cr.

2. The electrode material according to claim 1, wherein the electrode material at least partially is in form of particles, having an extension from 30 nm to 3 000 nm, preferably 100 nm to 500 nm.

3. The electrode material according to claim 1, wherein the oxidised M is selected from the group consisting of M-oxide, M-oxyhydroxide, and M-hydroxide, or combinations thereof, preferably the oxidised M is M.sub.3O.sub.4, M.sub.2O.sub.3, or MO.sub.2.

4. The electrode material according to claim 1, wherein the oxidised M is in form of particles having an extension from 2 nm to 20 nm, positioned on a surface of, within, or between crystals of [M.sub.2B.sub.2] or AlM.sub.2B.sub.2.

5. The electrode material according to claim 1, wherein the AlM.sub.2B.sub.2 and the [M.sub.2B.sub.2] each are characterised by being in form of a layered crystalline structure.

6. The electrode material according to claim 1, wherein M is Fe.

7. An electrode for oxygen evolution reaction formed by a support structure and the electrode material according to claim 1, wherein the electrode material is provided as a coating on the support structure.

8. The electrode for oxygen evolution reaction according to claim 7, wherein the support structure is composed of metal, preferably porous metal or a metal structure of grid-type.

9. The electrode according to claim 8, wherein the support structure is composed of porous Nickel or porous Cobalt.

10. The electrode according to claim 7, wherein the electrode material has a density on the support structure in the range of 0.1-5 mg cm.sup.−2, preferably 2 mg cm.sup.2.

11. A system for water electrolysis comprising an anode and a cathode, wherein the anode is an electrode according to claim 7.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1 is a schematic illustration of an electrode material according to an embodiment.

[0033] FIG. 2a-b shows HAADF-STEM images and the corresponding STEM-EDX elemental maps of AlFe.sub.2B.sub.2 particles before (a) and after (b) electrocatalytic activation in a 1 M KOH electrolyte solution according to an embodiment.

[0034] FIGS. 3 and 4 are schematic illustrations of an electrode for oxygen evolution reaction according to an embodiment.

[0035] FIG. 5 is a schematic illustration of the crystal structure of AlFe.sub.2B.sub.2 according to an embodiment.

[0036] FIGS. 6-10 shows the performance of different OER electrocatalysts in a 1 M KOH electrolyte solution according to an embodiment.

[0037] FIG. 11 illustrates current density vs. applied potential curves obtained with an electrode according to an embodiment.

[0038] FIGS. 12a and 12b shows Fe-L.sub.2,3 edge (12a) and O—K edge (12b) EELS spectra of AlFe.sub.2B.sub.2 particles according to an embodiment, before and after electrocatalytic activation. The reference spectra of α-Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, and FeO are shown for comparison.

[0039] FIG. 13 shows HRTEM image of an activated electrode material according to an embodiment, and the corresponding ED and FT ring patterns, which are indexed with the unit cell parameters of Fe.sub.3O.sub.4.

[0040] FIG. 14 shows the hypothesized mechanism for the in-situ formation of OER electrocatalysis from an AlFe.sub.2B.sub.2 scaffold according to an embodiment.

[0041] FIG. 15 shows a comparison of OER electrocatalytic activity and stability of previously reported boride- and phosphide-based systems, to the performance of the AlFe.sub.2B.sub.2-based electrode according to an embodiment.

DETAILED DESCRIPTION OF THE INVENTION

[0042] The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person. Although individual features may be included in different embodiments, these may possibly be combined in other ways, and the inclusion in different embodiments does not imply that a combination of features is not feasible. In addition, singular references do not exclude a plurality. In the context of the present invention, the terms “a”, “an” does not preclude a plurality.

[0043] It shall be realised that the electrode material as used herein may be defined as a pre-catalyst material in the sense that it is in a form where at least part of the AlM.sub.2B.sub.2 has been converted to [M.sub.2B.sub.2] and oxidised M that the electrode material catalyses OER. It has been unexpectedly realised that AlM.sub.2B.sub.2 may be converted to [M.sub.2B.sub.2] and oxidised M, for example, during use of the electrode material under electrolytic conditions, thus converting the AlM.sub.2B.sub.2 to a material active for OER. Material comprising crystal structures of AlM.sub.2B.sub.2, wherein M is selected from Fe, Mn, and Cr, is referred to as electrode material herein. Further, [M.sub.2B.sub.2] refers to M.sub.2B.sub.2 as part of structures composed of units of [M.sub.2B.sub.2], such as for example crystal structures of [M.sub.2B.sub.2] units.

[0044] In FIG. 1 an electrode material 1 for oxygen evolution reaction is schematically illustrated. The electrode material 1 comprises crystal structures of AlM.sub.2B.sub.2 2 wherein M is selected from Fe, Mn, and Cr.

[0045] The electrode material 1 for oxygen evolution reaction comprising crystal structures of AlM.sub.2B.sub.2, may be a pre-catalyst or a pre-electrode material for oxygen evolution reaction comprising crystal structures of AlM.sub.2B.sub.2, wherein M is selected from Fe, Mn, and Cr.

[0046] Although not illustrated in FIG. 1, the electrode material may at least partially be in form of particles, having a dimension or extension from 30 nm to 3 000 nm, preferably 100 nm to 500 nm. FIG. 2a illustrates a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of an example of such a particle consisting of AlFe.sub.2B.sub.2. FIG. 2b illustrates a HAADF-STEM images of an example of the electrode material further comprising crystal structures of [M.sub.2B.sub.2], and oxidised M, in the illustrated example [Fe.sub.2B.sub.2] and Fe.sub.3O.sub.4. In the illustrated example, electrode material of FIG. 2a comprising AlFe.sub.2B.sub.2 has at least partly been converted to [Fe.sub.2B.sub.2] and Fe.sub.3O.sub.4, i.e. an active form for OER, in this example by electrocatalytic activation in a 1 M KOH electrolyte solution. In such an activation, Al are leaching out from the electrode material. Thereby, the electrode material may be considered to be converted from a pre-catalytic form to a catalytic form. It will be appreciated that similar conversion may be realised with electrode materials where M being eg. Cr or Mn. Such electrode material comprising AlM.sub.2B.sub.2, wherein M is selected from Fe, Mn, and Cr, provides for an efficient electrode material made from abundant materials.

[0047] In FIG. 3 an electrode 4 for oxygen evolution reaction formed by a support structure 6 and the electrode material 1 according to the first aspect, wherein the electrode material 1 is provided as a coating 8 on the support structure 6 is schematically represented.

[0048] In FIG. 4 a portion of an electrode 4 for oxygen evolution reaction formed by a support structure 6 and the electrode material 1 according to the first aspect, wherein the electrode material 1 is provided as a coating 8 on the support structure 6 is schematically represented. FIG. 4 schematically illustrates Fe.sub.3O.sub.4 on and between crystal layers and electrocatalyst of H.sub.2O under formation of O.sub.2.

[0049] The support structure 6 may be composed of metal, such as Ni or Co, preferably porous metal or a metal structure of grid-type. With such a support structure 6, the coating 8 of electrode material 1 may be present within pores (not illustrated) of the support structure 6. For example, the electrode material 1 may have a density on the support structure 6 in the range of 0.1-5 mg cm.sup.−2, preferably 2 mg cm.sup.−2.

[0050] The combination of the electrode material 1 with a support structure 6 provides for a supported electrode or catalyst structure with accessible catalytic sites and having high accessible surface area. For reasons including the above, there is provided with embodiments of the present invention an electrode material which may be coated on a support structure forming a electrode that may function as an efficient OER electrocatalyst.

[0051] The electrode for oxygen evolution reaction may comprise an electrode material comprising AlM.sub.2B.sub.2 in form of particles having an extension or particle size, from 100 nm to 500 nm, wherein M is Fe, Cr or Mn, provided as a coating preferably having a density of 0.1-5 mg cm.sup.−2 on a support structure comprising porous metal, preferably Nickel or Cobalt. The electrode material of the electrode as provided may further comprise crystal structures of [M.sub.2B.sub.2] and M.sub.3O.sub.4; or crystal structures of [M.sub.2B.sub.2], and M.sub.3O.sub.4 may be formed before or during use of the electrode.

[0052] By way of examples, and not limitation, the following examples identify a variety of electrode materials pursuant to embodiments of the present invention. Although examples comprise M=Fe, Fe may be interchanged with Cr or Mn.

[0053] According to one example, the electrode material comprising AlFe.sub.2B.sub.2 has a layered crystal structure, which structure is illustrated in FIG. 5, wherein the Al atoms are sandwiched between the [Fe.sub.2B.sub.2] layers and may be etched or leached away to open up the catalytically active transition metal sites that subsequently catalyze OER. The underlying structure of AlFe.sub.2B.sub.2, thus, may act as a robust conductive structure for the catalytically active sites separated by the partially etched Al layers. Theoretical analysis shows that the bonding between the Al and [Fe.sub.2B.sub.2] layers in this structure is weaker than the Fe—B and B—B bonds within the [Fe.sub.2B.sub.2] layer.

[0054] According to embodiments, layers of [Fe.sub.2B.sub.2] function to serve as a precursor for Fe-based OER electrocatalyst. The electrode material comprising AlFe.sub.2B.sub.2 offers efficient OER with a low overpotential and remarkably high stability of the electrode material. The AlFe.sub.2B.sub.2 acts as a robust scaffold for in situ formation of catalytically active Fe.sub.3O.sub.4 nanoclusters on the surface of the [Fe.sub.2B.sub.2] layers. Catalytic performance, long-term stability and efficient synthesis suggest that this system may serve as an efficient and inexpensive OER catalyst, which may be made from abundant material.

[0055] Preparation of Electrode Material:

[0056] According to one example, an electrode material for oxygen evolution reaction according to embodiments, was manufactured according to the description below. It will be evident that electrodes from thus prepared electrode material coated on a porous support material provide for the desired OER.

[0057] Manufacturing of electrode material was performed in an argon-filled dry box (content of O.sub.2<0.5 ppm). Powders of aluminum (99.97%), iron (98%), crystalline boron (98%), and iron boride (FeB, 98%) were obtained from Alfa Aesar. The iron powder was additionally purified by heating in a flow of H.sub.2 gas at 500° C. for 5 h. The other materials were used as received.

[0058] Starting materials were mixed in a Al:Fe:B=3:2:2 ratio (a total weight of 0.35 g) and pressed into a pellet, which was arc-melted in an argon-filled glovebox. The pellet was remelted 4 times to achieve uniform melting. To maximize the sample's homogeneity, it was sealed in a silica tube under vacuum (˜10.sup.−5 torr) and annealed at 900° C. for 1 week.

[0059] Powder X-ray diffraction analysis (PXRD) revealed AlFe.sub.2B.sub.2 as the major phase with Al.sub.13Fe.sub.4 as minor byproduct. The byproduct was removed by HCl/water 1:1 vol/vol. It was observed that the electrode material also may be dissolved in dilute HCl, although slower than the impurity.

[0060] The, thus purified, electrode material was ball-milled at 1725 rpm for 1 h in an 8000M High-Energy Mixer/Mill (SPEX), using a stainless-steel ball-milling set. The milling was carried out under Ar to minimize surface oxidation. The PXRD of the ball-milled sample revealed broadening of diffraction peaks, in accord with the decreased particle size. No traces of impurity phases were observed, except for a minor peak of Al.sub.2O.sub.3 which would be dissolved for example by the basic conditions of electrochemical conditions.

[0061] Transmission electron microscopy (TEM) analysis (using JEM-ARM200F microscope with cold field-emission gun, probe and image aberration corrected, equipped with CENTURIO EDX detector and GIF Quantum) was used for particle analysis. TEM samples were prepared by crushing a sample of electrode material in an agate mortar in ethanol and depositing the obtained suspension on a Cu carbon holey grid. The TEM analysis showed that the particle sizes obtained from the ball-milling were in the range of approximately 30 to 500 nm. Larger and smaller particles of electrode material may be obtained by different conditions for ball-milling.

[0062] Electrode materials of AlCr.sub.2B.sub.2 and AlMn.sub.2B.sub.2 were also prepared:

[0063] AlCr.sub.2B.sub.2 was prepared by mixing the elements in a Al:Cr:B=30:1:1 ratio (a total weight of 1.0 g) and placing them into an alumina crucible, which was sealed in an evacuated silica tube. The mixture was heated to 1000° C. at 300° C./h, held at that temperature for 3 days, and cooled in 50° C. increments to room temperature. Once cooled, the product was treated with dilute HCl (1:1 vol/vol) for 10 min to eliminate the excess of Al. Prism-shaped crystals of AlCr.sub.2B.sub.2 could be selected from the product.

[0064] AlMn.sub.2B.sub.2 was synthesized by mixing elements in a Al:Mn:B=1.5:2:2 ratio (a total weight of 0.35 g). The mixture was pressed into a pellet and arc-melted multiple times to ensure uniform melting. The sample was sealed in an evacuated silica tube and annealed at 800° C. for 2 weeks. After cooling down to room temperature, the sample was ground and subjected to PXRD, which revealed AlMn.sub.2B.sub.2 as the major phase and Al.sub.10Mn.sub.3 as a minor byproduct (<5%).sub..

[0065] Preparation of Electrode:

[0066] The following is one example of preparing an electrode, according to embodiments, of the ball-milled electrode material. The ball-milled electrode material consisting of AlFe.sub.2B.sub.2 was converted into an electrode ink by ultrasonically dispersing 5 mg of the electrode material in 1000 μL of ethanol containing 50 μL of a Nafion solution (Sigma-Aldrich, 5 wt. %). To prepare an electrode for catalytic tests, 200 μL of the ink was loaded on a Ni foam (Heze Jiaotong, 110 pores per inch, 0.3 mm thick, cleaned by ultrasonication in 6 M HCl) with an exposed area of 1.0 cm.sup.2, leading to a loading density of approximately 1.0 mg cm.sup.−2, followed by drying under ambient conditions. The result was an example of an electrode for OER formed by a support structure comprising porous Nickel and an electrode material comprising AlFe.sub.2B.sub.2 provided as a coating on the support structure. The electrode material of the coating comprises particles resulting from the ball-milling.

[0067] In alternative experiments, the support structure may be for example in form of a grid. According to other examples, alternative metals, M, were used for the electrode material preparation, for example Cr.

[0068] Electrochemical Measurements:

[0069] The electrode prepared according to the above example, was used and verified according to the following one example. Electrochemical measurements were conducted at 25° C., using a Biologic VMP-3 potentiostat/galvanostat. The OER performance of various electrodes, including the one prepared according to the above, were evaluated in a three-electrode system using 1.0 M KOH electrolyte, in which the catalytic electrode, the saturated calomel electrode (SCE), and a Pt wire served as the working, reference, and counter electrodes, respectively. Prior to each measurement, the SCE electrode was calibrated in Ar/H.sub.2-saturated 0.5 M H.sub.2SO.sub.4 solution, using a clean Pt wire as the working electrode. Unless stated otherwise, all potentials according to the present invention were converted to a reversible hydrogen electrode (RHE) reference scale according to the following equation: E.sub.RHE=E.sub.SCE+0.059 pH+0.241. An iR-correction of 85% was applied in the polarization experiments to compensate for the voltage drop between the reference and working electrodes, which was evaluated by a single-point high-frequency impedance measurement. OER anodic polarization curves were recorded with a scan rate of 5 mV s.sup.−1 in the range from 1.0 to 1.7 V vs. RHE. Impedance spectroscopy measurements were carried out at the overpotential of 0.26 V in the frequency range from 10.sup.5 to 10.sup.−2 Hz with a 10 mV sinusoidal perturbation. The catalytic stability of the electrodes was evaluated as a function of time at a constant current density of 10 mA cm.sup.−2.

[0070] According to the example, current densities of 10, 100, and 300 mA cm.sup.−2 were achieved at remarkably low overpotentials (η) of only 240, 290, and 320 mV, respectively. FIG. 6 shows the evaluation of bare porous Ni, or Ni foam, and four other control electrocatalysts, FeB, Fe.sub.3O.sub.4, RuO.sub.2, and IrO.sub.2 under the same conditions, i.e. in a 1 M KOH electrolyte solution, as compared to the AlFe.sub.2B.sub.2 electrode material, wherein the current density vs. applied potential curves, with the inset showing an enlarged low-current part of the plot. Evidently, AlFe.sub.2B.sub.2 exhibits substantially lower overpotentials at all current densities, outperforming all the above mentioned reference systems. The kinetic behavior of the OER electrocatalysts was compared by means of Tafel and Nyquist plots, which, revealed that the electrode material comprising AlFe.sub.2B.sub.2 exhibits not only the lowest overpotential but also the smallest Tafel slope (T.sub.S) in comparison to the reference electrocatalysts, as demonstrated in FIG. 7. The electrode material comprising AlFe.sub.2B.sub.2 shows a T.sub.S value of 42 mV dec.sup.−1, indicating the fastest OER rate in the 1 M KOH electrolyte. Further, FIG. 8 shows the Nyquist plot, obtained from the AC impedance measurements, demonstrating a significantly smaller charge-transfer resistance for AlFe.sub.2B.sub.2 as compared to the reference electrocatalysts. In one example the O.sub.2 TOFs for the electrocatalysts were estimated as TOF (s.sup.−1)=(jA)/(.sub.4Fn), where j is the current density (A cm.sup.−2) at a given overpotential, A is the surface area of the electrode (1.0 cm.sup.2), F is the Faraday constant (96485 C mol.sup.−1), and n is the amount of metal in the electrode (mol), determined as n=1.0 mg cm.sup.−2×1.0 cm.sup.2×10.sup.−3/metal molar mass. It was assumed that all of the metal ions were catalytically active and thereby their TOFs were calculated. Notably, some metal sites were indeed inaccessible during OER, and thus the calculated TOFs represent the minimum possible values. In FIG. 9, the O.sub.2 turnover frequencies (TOFs) at various overpotentials, such as 0.25, 0.30, and 0.35 V, are demonstrated. The electrode material comprising AlFe.sub.2B.sub.2 shows a TOF value of 0.12 5.sup.−1 at the overpotential of 350 mV, at which the OER benchmarks IrO.sub.2 and RuO.sub.2 achieved substantially lower TOFs values of 0.05 5.sup.−1 and 0.04 5.sup.−1, respectively. Further, the stability of AlFe.sub.2B.sub.2 under the harsh OER conditions was evaluated, demonstrating excellent stability of the electrode material and electrodes according to embodiments. FIG. 15 shows the chronopotentiometric plot of the performance of the electrode material comprising AlFe.sub.2B.sub.2 at the constant current density of 10 mA cm.sup.−2, in the 1 M KOH electrolyte solution. At this particular constant current density, the electrode material comprising AlFe.sub.2B.sub.2 maintained an essentially constant overpotential of 240 mV for over a 10-day period. Hence, the electrochemical studies reveal the electrode material comprising AlFe.sub.2B.sub.2 as a suitable component in a highly active and inexpensive OER electrode with remarkable long-term stability.

[0071] Upon examination of the catalyst's stability plot, with reference to FIG. 10, it was noticed an obvious decrease in the overpotential value in the very beginning of the reaction. By carrying out several electrocatalytic cycles and monitoring the current-potential curves, this feature was explored further. FIG. 11 shows the current density vs. applied potential curves recorded over the AlFe.sub.2B.sub.2/porous-Ni electrode after OER catalytic cycles in a 1 M KOH electrolyte solution.

[0072] Elemental mapping of the ball-milled sample prior to catalysis showed presence of a thin oxide layer, which indicates minor oxidation and the presence Al.sub.2O.sub.3. Activated sample of the electrode material comprising AlFe.sub.2B.sub.2, obtained after 20 initial OER cycles, appeared much more heterogeneous. This sample clearly reveals the formation of core-shell particles. The development of the shell structure in the electrocatalytically activated sample is illustrated FIG. 2b, wherein the shell is indicated with white arrows. In FIG. 2b, energy-dispersive X-ray spectroscopy (EDX) elemental mapping similarly shows the presence of a thick layer of iron oxide on surface of AlFe.sub.2B.sub.2 particles. The EDX elemental mapping also shows that the Al:Fe ratio is drastically decreased, from 1:2 in the initial electrode material, or pre-catalyst material, to 1:6 in the activated one resulting from Al being partially leached out of layered AlFe.sub.2B.sub.2 structure under the basic conditions of electrocatalysis. This hypothesis is supported by theoretical analysis of the relative bond strengths presented in Table 1.

[0073] Electron energy loss spectroscopy (EELS) was used to probe the changes in the nature of the Fe sites during OER and to confirm the presence and localization of boron, which is difficult to detect by EDX spectroscopy. It may be observed that B is consistently present in the core-shell nanoparticles of the electrode material comprising AlFe.sub.2B.sub.2, along with Fe. Taking into account the EDX results and combining them with the EELS data, it may be concluded that these particles consist of AlFe.sub.2B.sub.2 core shelled with a layer of iron oxide. Analysis of the EELS data indicates that prior to catalytic testing the AlFe.sub.2B.sub.2 particles mainly contain Fe.sup.0 sites, with minor Fe.sup.3+ impurities. After activation, the thick oxide shell appears to be magnetite, Fe.sub.3O.sub.4. This phase may be distinguished from α-Fe.sub.2O.sub.3 and FeO by examining the iron L-edge and oxygen K-edge EELS fine structure observed in the energy regions around 705-725 eV and 530-570 eV, respectively, which is shown in FIGS. 12a and 12b, respectively. In particular, the Fe L.sub.3 peak of the sample is shifted to lower energies as compared to the peak of α-Fe.sub.2O.sub.3 while the Fe L.sub.2 peak of the sample is shifted to higher energies as compared to the peak of FeO (FIG. 12b). The formation of Fe.sub.3O.sub.4 nanoparticles was also successfully confirmed by selected area electron diffraction (ED) patterns and high resolution TEM (HRTEM) imaging as illustrated in FIG. 13. The ED patterns were perfectly indexed using the unit cell parameters of Fe.sub.3O.sub.4, while the Fourier transform (FT) of the HRTEM image produced an identical ring diffraction pattern with the pronounced (111) spots characteristic of Fe.sub.3O.sub.4, which is due to a preferential orientation of the nanoparticles.

[0074] Based on results above, it may be concluded that the OER performance by AlFe.sub.2B.sub.2 is, at least in part, due to partial etching of Al from the structure, followed by the surface oxidation of the exposed [Fe.sub.2B.sub.2] layers, as reflected by the following idealized reaction sequence:

2AlFe.sub.2B.sub.2+2KOH+6H.sub.2O=2K[Al(OH).sub.4]+4“FeB”+3H.sub.2,

12“FeB”+6KOH+17O.sub.2=3K.sub.2B.sub.4O.sub.7+4Fe.sub.3O.sub.4+3H.sub.2O,

wherein “FeB” stands for the modified AlFe.sub.2B.sub.2 with partially etched Al layers. Thus, AlFe.sub.2B.sub.2 acts as a pre-catalyst, with the [Fe.sub.2B.sub.2] layers providing a robust support for in situ generated Fe.sub.3O.sub.4 nanoclusters. Hence, FIG. 14 illustrates that the partial etching of Al atoms in alkaline electrolyte exposes the [Fe.sub.2B.sub.2] layers, which are subsequently surface-oxidised to afford the electrocatalytically active ultra-small Fe.sub.3O.sub.4 nanoclusters. FIG. 15 shows a comparison with other non-oxide OER catalysts (mainly borides and phosphides), which reveals a stable and efficient performance of the catalytic system based on the AlFe.sub.2B.sub.2 comprising electrode of the present invention.

[0075] In summary, the electrode material according to embodiments serves as an excellent OER pre-catalyst, maintaining high electrocatalytic activity for more than 10 days under alkali conditions. The present invention is not limited to exemplified electrode material comprising AlFe.sub.2B.sub.2, in fact other AlM.sub.2B.sub.2, which are isostructural to AlFe.sub.2B.sub.2, wherein M may be Cr or Mn, are also possible.

TABLE-US-00001 TABLE 1 Distances and integral crystal orbital Hamilton population (—ICOHP) values calculated for the five shortest interatomic contacts in the structure of AlFe.sub.2B.sub.2. Bond Distance (Å) —ICOHP (eV/bond) B—B 1.605 5.13 Al—Fe 2.622 1.02 Al—B 2.430 1.06 2.048 2.75 Al—B 2.199 2.15

[0076] The person skilled in the art realizes that the present inventive concept by no means is limited to the preferred variants described above. On the contrary, various modifications, variations and equivalents are possible within the scope of the appended claims.

Itemized List of Embodiments

[0077] 1. An electrode material for oxygen evolution reaction, the electrode material comprising crystal structures of AlM.sub.2B.sub.2, wherein M is selected from Fe, Mn, and Cr.
2. The electrode material according to item 1, wherein the electrode material at least partially is in form of particles, having a dimension from 30 nm to 3 000 nm, preferably 100 nm to 500 nm.
3. The electrode material according to item 2, wherein the electrode material further comprising crystal structures of [M.sub.2B.sub.2], and oxidised M.
4. The electrode material according to item 3, wherein the oxidised M is selected from the group consisting of M-oxide, M-oxyhydroxide, and M-hydroxide, or combinations thereof, preferably the oxidised M is M.sub.3O.sub.4, M.sub.2O.sub.3, or MO.sub.2.
5. The electrode material according to item 3 or 4, wherein the oxidised M is in form of particles having a dimension from 2 nm to 20 nm, positioned on a surface of, within, or between crystals of [M.sub.2B.sub.2] or AlM.sub.2B.sub.2.
6. The electrode material according to any one of items 3 to 5, wherein the AlM.sub.2B.sub.2 and the [M.sub.2B.sub.2] each are characterised by being in form of a layered crystalline structure.
7. The electrode material according to anyone of items 1 to 6, wherein M is Fe.
8. An electrode for oxygen evolution reaction formed by a support structure and the electrode material according to anyone of items 1-7, wherein the electrode material is provided as a coating on the support structure.
9. The electrode for oxygen evolution reaction according to item 8, wherein the support structure is composed of metal, preferably porous metal or a metal structure of grid-type.
10. The electrode according to item 9, wherein the support structure is composed of porous Nickel or porous Cobalt.
11. The electrode according to any one of items 8 to 10, wherein the electrode material has a density on the support structure in the range of 0.1-5 mg cm.sup.−2, preferably 2 mg cm.sup.−2.
12. A system for water electrolysis comprising an anode and a cathode, wherein the anode is an electrode according to items 8-11.