Modified mxene materials

Abstract

This disclosure describes freestanding or delaminated two-dimensional (2D) sheets of at least dual modified functionalised MXene materials, Mn+1XnTx, having electronegative surface groups and non-metal dopant and electrochemically active dopant atoms in the materials, suitable as OER catalysts.

Description

[0001] MODIFIED MXENE MATERIALS

[0002] Cross-Reference

[0003]

[0001] The present application claims priority to Australian provisional patent application AU 20249011 14 filed on 19 April 2024, the entire contents of which are incorporated herein by crossreference.

[0004] Technical Field

[0005]

[0002] The invention relates to modified MXene materials suitable as electrocatalysts for water splitting, such as for OER in alkaline aqueous solutions, including seawater.

[0006] Background

[0007]

[0003] Seawater has emerged as a preferred and unlimited raw material for sustainable / green hydrogen production via electrolysis. The two half-reactions of water splitting are the hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode. Both require highly efficient electrocatalysts to overcome the required overpotentials that make water splitting inefficient. Some excellent electrocatalysts for OER and HER have been developed, but catalysts that are stable and highly active in seawater are still lacking. Overall, H2production at the cathode is strongly dependent on the efficiency and stability of the OER at the anode. In real seawater, microorganisms and insoluble precipitate impurities inhibit performance by shielding catalytic active sites on the catalyst surface. Chloride products corrode the catalyst and deactivate active sites, thereby interfering with the driving OER and causing rapid decline in activity and stability. OER catalysts for selective seawater oxidation that inhibit chloride ion oxidation reactions are scant, due to anodic side reactions in seawater splitting.

[0008]

[0004] Seawater spitting is challenging, particularly for OER at the anode compared to more straightforward HER at the cathode side, due to competing anodic chloride ion oxidation which occurs due to the presence of approx. 0.5 M chloride anions in real seawater. The cathode chemistry is more straightforward than the anode chemistry, as the thermodynamic potentials of competing HER reactions at the cathode are not as close as the thermodynamic potentials for OER and chloride ion oxidation reactions which occur on the anode side. OER, as a four-electron oxidation process, involves a high energy barrier reaction pathway with at least two or three intermediates, and thus, is a much more sluggish reaction than the chlorine evolution reaction (CER) which is a kinetically faster reaction as it involves only a two-electron transfer process with a single catalytic intermediate. CER is pH independent and occurs at relatively high potentials. In contrast, the OER potential is pH dependent and much closer to the CER potential under acidic conditions than under alkaline conditions where the OER potential is much lower. Kinetically, CER is more favourable than OER, so in low pH media, despite its slightly higher redox potential, CER competes with OER under acidic conditions. Indeed, as CER competes with OER in seawater, the anodic reaction efficiencies are poor and suppression of OER at increased current densities becomes the bottleneck in seawater splitting.

[0009]

[0005] In alkaline media, as a result of the lower OER potential, there is an overall wider difference in the thermodynamic potentials for OER and chloride ion oxidation at the anode. Indeed, at pH > 7.5, OER is highly selective over CER within an overpotential window of 480 mV. However, at such high pHs in seawater, alongside OER, undesirable hypochlorite anion formation occurs as the dominant competitive anodic reaction to OER. Such competing reactions could be avoided by using an electrocatalyst active for OER below a 480 mV overpotential for alkaline seawater splitting. However, current densities at such low overpotential are typically low. Ideally, an industrial current density (1 A cmr2) below a 480 mV overpotential for alkaline seawater splitting would be desirable. Practical / commercial applications require catalysts to operate at high current densities above 500 mA cm2which require substantially higher overpotentials, but unfortunately, at such required higher overpotentials, hypochlorite formation is promoted even in alkaline seawater. Cl- corrosion means seawater splitting occurs with reduced efficiency and stability and catalyst deactivation occurs rapidly.

[0010]

[0006] Precious metal oxides, e.g. RuC>2 and lrC>2, are the current benchmark catalysts for OER in acidic media, but their scarcity, high cost, and compromised performance in seawater hamper their large-scale application. Engineered non-noble metal-based transition metal compounds are under study to improve OER kinetics to realise direct seawater splitting, but stability is challenging in the presence of the massive concentrations of Cl' in seawater.

[0011]

[0007] Therefore, developing improved highly conductive, long term stable anode materials that can operate as water / seawater splitting catalysts at large current densities under alkaline conditions for longer periods at low overpotentials that inhibit chlorine evolution or at higher overpotentials but involve a design that avoids hypochlorite formation is an important goal. Other desirable goals include better performance, stability, and greater economy compared to commercial RuO2 and I rO2 catalysts for OER. High stability would make electrolysis systems integrable with renewable power sources.

[0012]

[0008] MXene materials generally have formula Mn+iXnTx, where M is an early transition metal, X is carbon and / or nitrogen and Tx represents hydrophilic surface terminating groups are conventionally considered to have limited activity and stability as anode materials in seawater and exhibit poor OER performance. It is believed that the electrochemical neutrality of MXenes, caused by poor redox potential and propensity to undergo surface oxidation especially during the OER process, has traditionally limited their application in OER especially in complex and corrosive systems like seawater. As such, there is need for alternative materials that represent an improvement over traditional MXene performance for OER.

[0013]

[0009] A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that the document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

[0014]

[0010] Unless the context requires otherwise, where the terms "comprise”, “comprises” and "comprising” are used in the specification (including the claims), they are to be interpreted as specifying the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

[0015] Statements of the Invention

[0011] In a first aspect, the invention provides a freestanding or delaminated two-dimensional (2D) sheets of at least a dual modified functionalised MXene material, Mn+iXnTx, where n is 1 , 2 or 3, and M is an early transition metal (Group 3 to 6 metal), X is C and / or N, and Txis a hydrophilic surface functional group selected from one or more of O, F, OH, and Cl, wherein:

[0016] - the MXene material is functionalised such that a majority of the hydrophilic surface functional groups comprise an electronegative atom selected from one or more of S, N, and O, preferably O;

[0017] - the MXene material is modified by replacement of a portion of the early transition M atoms with one or more dopant non-metal atoms selected from B, N, C, O, P, Si, S, As and Se, preferably B; and

[0018] - the MXene material is further modified by replacement of at least one hydrophilic surface functional group and / or at least one early transition metal M atom with one or more electrochemically active metals selected from the group consisting of: Fe, Ni, Co, Cu, Cr, Mo and W, preferably Fe, Ni and Co, most preferably Fe. In one embodiment, the one or more electrochemically active metals are different to the early transition metal M atom.

[0019]

[0012] In some embodiments, the MXene material of the first aspect above is further modified by replacement of at least one hydrophilic surface functional group and at least one early transition metal M atom with one or more electrochemically active metals selected from the group consisting of: Fe, Ni, Co, Cu, Cr, Mo and W, preferably Fe, Ni and Co, most preferably Fe. In another embodiment, the MXene material of the first aspect above is further modified by replacement of at least one hydrophilic surface functional group with one or more electrochemically active metals selected from the group consisting of: Fe, Ni, Co, Cu, Cr, Mo and W, preferably Fe, Ni and Co, most preferably Fe. In yet another embodiment, the MXene material of the first aspect above is further modified by replacement of at least one early transition metal M atom with one or more electrochemically active metals selected from the group consisting of: Fe, Ni, Co, Cu, Cr, Mo and W, preferably Fe, Ni and Co, most preferably Fe.

[0020]

[0013] As used herein, the term “precursor MXene material” refers to an MXene of formula Mn+iXnTxwithout post-synthetic modification of the M or T groups and introduction of dopant atoms. It will be understood that some precursor MXene materials do contain a mixture of metals constituting M.

[0021]

[0014] As used herein, the term “functionalised MXene material” refers to an MXene of formula Mn+iXnTx with modification of the hydrophilic surface groups such that a majority comprise electronegative atoms selected from one or more of S, N and O. In some embodiments, a functionalised MXene material containing this surface functionalisation may be a naturally occurring precursor MXene material, and thus it will not be necessary to post-synthetically modify a precursor MXene material to achieve a majority electronegative atoms selected from one or more of S, N and O. In other embodiments, a separate functionalisation step will be required to form a functionalised MXene material. By way of non-limiting example only, in some embodiments, some as-synthesised MXenes have Tx as majority F, with some OH, and least O, and modification by treating the MXene with a strong base, such as NaOH, may eliminate a substantial portion of F and OH groups (e.g. , at least 50%, at least 60%, at least 70%, at least 80% or at least 90% F and OH groups) and replace them with O.

[0022]

[0015] As used herein, the term “dual modified functionalised MXene material” refers to an MXene of formula Mn+iXnTxwith functionalisation of the hydrophilic surface groups as per the definition of “functionalised MXene material” above, but with the additional dual modifications of (1) doping of the early transition M atoms with one or more dopant non-metal atoms and (2) replacement of some of the hydrophilic surface groups and / or early transition metal M atoms with one or more electrochemically active metals selected from the group consisting of: Fe, Ni, Co, Cu, Cr, Mo and W. The term “modified MXene material” may be used herein synonymously with “dual modified functionalised MXene material”.

[0016] As used herein, the term “majority” refers to at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95%, or at least 99.5%, or of from 50-100%, or of from 70-100%, or of from 60-90%, or of from 70-80%, or of from 50-85%, or of from 65-95%, on a mass% or mol% basis.

[0023]

[0017] As used herein, the term “portion” refers to at least 0.5%, at least 1 %, or at least 5%, or at least 10%, or at least 20%, or at least 30%, or at least 40%, or up to 50%, or up to 40%, or up to 30%, or up to 20%, or up to 10%, or up to 5%, or of from 0.5-10%, or of from 1 -15%, or of from 5-20%, or of from 5-40%, or of from 10-45%, on a mass% or mol% basis.

[0024]

[0018] As used herein, the term “at least one” in the context of electrochemically active metal replacement of either hydrophilic surface functional groups or early transition metal M atoms refers to one or more hydrophilic surface functional groups, or one or more early transition metal M atoms, or one or more of both hydrophilic surface functional groups and early transition metal M atoms, being replaced by an electrochemically active metal atom / ion.

[0025]

[0019] As used herein, the terms “early transition metal M atom”, and “M atom” are used interchangeably to describe the metal (M) of the dual modified functionalised MXene material of general formula Mn+iXnTx.

[0026]

[0020] In one embodiment, there is disclosed freestanding or delaminated two-dimensional (2D) sheets of at least a dual modified functionalised MXene material, Mn+iXnTx, where n is 1 , 2 or 3, and M is an early transition metal (Group 3 to 6 metal), X is C and / or N, and Txis a hydrophilic surface functional group selected from one or more of O, F, OH, and Cl, wherein:

[0027] - the MXene material is functionalised such that a majority of the hydrophilic surface functional groups comprise an electronegative atom selected from one or more of S, N, and O, preferably O;

[0028] - the MXene material is modified by replacement of a portion of the early transition M atoms with one or more dopant non-metal atoms selected from B, N, C, O, P, Si, S, As and Se, preferably B; and

[0029] - the MXene material is further modified by replacement of at least one hydrophilic surface functional group and / or at least one early transition metal M atom with one or more electrochemically active metals selected from the group consisting of: Fe, Ni, Co, Cu, Cr, Mo and W, preferably Fe, Ni and Co, most preferably Fe, wherein the one or more electrochemically active metals are different to the early transition metal M atom.

[0030]

[0021] In another embodiment, there is disclosed freestanding or delaminated two-dimensional (2D) sheets of at least a dual modified functionalised MXene material, Mn+iXnTx, where n is 1 , 2 or 3, and M is an early transition metal (Group 3 to 6 metal), X is C and / or N, and Txis a hydrophilic surface functional group selected from one or more of O, F, OH, and Cl, wherein:

[0031] - the MXene material is functionalised such that a majority of the hydrophilic surface functional groups comprise an electronegative atom selected from one or more of S, N, and O, preferably O; - the MXene material is modified by replacement of a portion of the early transition M atoms with one or more dopant non-metal atoms selected from B, N, C, O, P, Si, S, As and Se, preferably B; and

[0032] - the MXene material is further modified by replacement of at least one hydrophilic surface functional group with one or more electrochemically active metals selected from the group consisting of: Fe, N I, Co, Cu, Cr, Mo and W, preferably Fe, Ni and Co, most preferably Fe, wherein the one or more electrochemically active metals are different to the early transition metal M atom.

[0033]

[0022] As described herein, the dopant non-metal atoms replace some of the early transition metal M atoms, where such substitution forms non-metal atom-electronegative atom bonds (e.g., non-metal atom-0 bonds) underneath outer layers of the precursor MXene material. The non-metal is preferably positioned underneath at least the top layers of precursor MXene material and provides vacancies, e.g., oxygen vacancies. In addition to substitution of some of the surface functionality for electrochemically active metals, these modifications activate the otherwise dormant OER potential of the precursor MXene. Further, the doping breaks van der Waals interactions between adjacent MXene nanosheets, generating freestanding sheets of the new at least dual modified MXene material.

[0034]

[0023] In an example of the first aspect, the invention provides freestanding or delaminated two- dimensional (2D) sheets of a dual modified functionalised MXene material, Mn+iXnTx, where n is 2, and M is Ti, X is C and / or N, and Txis hydrophilic surface functional group selected from one or more of O, F, OH, and Cl, wherein:

[0035] - the MXene material is functionalised such that a majority of the hydrophilic surface functional groups comprise an electronegative atom O;

[0036] - the MXene material is modified by replacement of a portion of the Ti atoms with dopant non-metal atom B; and

[0037] - the MXene material is further modified by replacement of at least one hydrophilic surface functional group and / or at least one early transition metal M atom with electrochemically active metal Fe.

[0038]

[0024] Again, in one embodiment, the one or more electrochemically active metals are different to the early transition metal M atom.

[0039]

[0025] In a second aspect, the invention provides use of freestanding or delaminated sheets according to the first aspect as an electrocatalyst, preferably as an OER electrocatalyst for water splitting, particularly where the water splitting involves of splitting of seawater, preferably alkaline seawater, alkaline DI, alkaline tap water, or alkaline wastewater. Desirably, the OER is selective OER by inhibiting OER. Typical operating conditions are at room temperature and atmospheric pressure, but temperatures up to 80 °C may be used.

[0040]

[0026] In a third aspect, the invention provides a method of forming a MXene based OER electrocatalyst, comprising the steps of forming freestanding or delaminated two-dimensional (2D) sheets of a functionalised and dual modified MXene, Mn+iXnTx, where n is 1 , 2 or 3, and M is an early transition metal (Group 3 to 6 metal), X is C and / or N, and Txare hydrophilic surface functional groups selected from one or more of O, F, OH, and Cl, by modifying a precursor MXene material, wherein the modifying steps involve:

[0041] - functionalising the precursor MXene material such that a majority the hydrophilic surface functional groups comprise an electronegative atom selected from one or more of S, N, or O, preferably O; - replacing at least a portion of the M atoms in the functionalised MXene with one or more dopant non- metal atoms selected from B, N, C, O, P, Si, S, As and Se, preferably B; and

[0042] - replacing at least one hydrophilic surface functional group with one or more electrochemically active metals selected from the group consisting of: Fe, Ni, Co, Cu, Cr, Mo and W, preferably Fe, Ni and Co, most preferably Fe. In one embodiment, the one or more electrochemically active metals are different to the early transition metal M atom.

[0043]

[0027] During formation, the electronegative atom terminations at the surface of the functionalised Mn+iXnTx aid non-metal dopant and electronegative metal atom adsorption to the surface during the initial deposition, with the electrochemical metal (preferably of lower oxidation state than that of M) interacting with the surface functional group terminations / electronegative atoms via electrostatic attractions due to its opposite charges with the surface. Thereafter, the non-metal and electrochemically active metal may be incorporated into the functionalised Mn+iXnTx upon thermal treatment, with the non- metal dopant replacing some of the M, while the electrochemically active metal replaces some of the surface termination and / or some of the M atoms. The doping also breaks van der Waals interactions between adjacent MXene nanosheets, generating freestanding sheets of the new material.

[0044]

[0028] In a fourth aspect, the invention provides a process involving inclusion of at least one non-metal atom dopant atom and at least one electrochemically active metal in an electronegative atom- functionalised MXene to increase the activity of a MXene based OER electrocatalyst, wherein the one or more dopant non-metal atoms are selected from B, N, C, O, P, Si, S, As and Se; and wherein the one or more electrochemically active metals are selected from the group consisting of: Fe, Ni, Co, Cu, Cr, Mo and W, preferably Fe, Ni and Co. Suitably the electronegative atom is selected from one or more of S, N, or O, preferably O.

[0045]

[0029] If a non-metal dopant cation of lesser oxidation state replaces M, e.g., Ti4+in its oxide, vacancies, e.g., oxygen vacancies, are created. For example, if B with a maximum oxidation state of +3 (e.g., B(OH)3) replaces some of the Ti4+in Ti3CzO2, oxygen vacancies are created.

[0046]

[0030] Desirably, the activity is OER which is selective OER by inhibiting CER.

[0047]

[0031] In a fifth aspect, the invention provides a product obtained by or obtainable by the process of the third or fourth aspects of the invention.

[0048] Brief Description of the Figures

[0049]

[0032] Embodiments of the invention will herein be illustrated by way of example only with reference to the accompanying drawings in which:

[0050]

[0033] Figure 1 illustrates a) XRD pattern of various phases in B and Fe doped T CzOz (FBT) . b) TEM image of FBT. c) and d) SAED of FBT. e) HRTEM image of FBT with lattice fringes (insert);

[0051]

[0034] Figure 2 illustrates a) XPS survey spectra; and the fitted high-resolution XPS spectra of b) Ti 2p, c) C 1 s, d) O 1 s, e) B 1 s, f) Fe 2p of FBT. NEXAFS spectra for g) O K-edge, h) Ti L-edge, i) Fe L- edge, and j) B K-edge of FBT ; and

[0052]

[0035] Figure 3 illustrates a) FBT, RuOz and Ni Foam Polarization curves in 6 M alkaline seawater electrolyte with 100% iR correction, b) Comparison of the overpotentials achieved by FBT, RuOz and Ni Foam in 6 M alkaline seawater electrolyte at different current densities, c) Tafel slopes of the electrocatalysts from the polarization curves in a), d) Nyquist plots of FBT. e) Double layer capacitance (Cdl) curves of FBT. f) FBT, Fe-doped MXene (FT), B- doped MXene (BT) and TisC2O2 Polarization curves in 1 M KOH in D.L water electrolyte with 100% iR correction. G) FBT and RuO2 Polarization curves in 6 M KOH in D.L water electrolyte with 100% iR correction.

[0053]

[0036] Figure 4 illustrates TEM images of a) CBT and b) NBT. c) Polarisation curve of CBT in 6 M alkaline seawater electrolyte.

[0054] Detailed Description of the Invention

[0055]

[0037] MXene materials typically have limited activity and stability as an anode in seawater and thus exhibit poor OER performance in seawater splitting. However, the inventors have now developed a modified MXene material that addresses these deficiencies. In particular, by promoting surface termination and breaking the electrochemical neutrality of a traditional MXene surface, materials described herein not only surprisingly preserve the MXene 2D morphology, but also improve the material flexibility without negatively impacting the electrochemical activity, and do so in a way that prevents the surface termination from leaching.

[0056]

[0038] The material of the invention is provided in the form of freestanding or delaminated two- dimensional (2D) sheets of at least a dual modified functionalised MXene material, Mn+iXnTx, where n is 1 , 2 or 3, and M is an early transition metal (e.g., a Group 3 to 6 metal), where X is C and / or N, and Txare hydrophilic surface functional groups selected from one or more of O, F, OH, and Cl, wherein: the MXene material is functionalised such that a majority of the Txsurface functional groups comprise an electronegative atom selected from one or more of S, N, or O, preferably O; the MXene material is modified by replacement of a portion of M early transition metal atoms with one or more dopant non-metal atoms selected from B, N, C, O, P, Si, S, As and Se; and the MXene material is further modified by replacement of at least one Txsurface functional group and / or at least one additional M early transition metal atom with one or more electrochemically active metals selected from the group consisting of: Fe, Ni, Co, Cu, Cr, Mo and W, preferably Fe, Ni and Co.

[0039] In some embodiments, MXenes are inorganic materials comprising transition metal carbides, carbonitrides or nitrides, having a layered structure of general formula Mn+iXnTx, wherein M is an early transition metal, X is nitrogen and / or carbon and Txare surface functional groups such as -OH, -F, -Cl, or -O. As noted above, MXenes without modification of the M or T are referred to herein as “precursor MXenes”.

[0057]

[0040] In some embodiments, the invention is provided in the form of freestanding or delaminated 2D sheets of a dual modified functionalised MXene material, Mn+iXnTx, where n is 2, and M is Ti, X is C and / or N, and Txis hydrophilic surface functional group selected from one or more of O, F, OH, and Cl, wherein the MXene material is functionalised such that a majority of the hydrophilic surface functional groups comprise an electronegative atom O; the MXene material is modified by replacement of a portion of the Ti atoms with dopant non- metal atom B; and the MXene material is further modified by replacement of at least one hydrophilic surface functional group and / or at least one Ti atom with electrochemically active metal Fe.

[0058] In some examples, the free standing or delaminated 2D sheets of the above embodiment is in the form of B- and Fe-doped Ti3C2O.

[0059]

[0041] Desirably, the starting or precursor MXene material is a transition metal carbide; a transition metal nitride or a transition metal carbon nitride, such as TisCsTx. Exemplary precursor MXenes include 2-1 MXenes such as Ti2C, V2C, Nb2C, M02C, M02N, Ti2N, (Ti2-yNby)C, (V2-yNby)C, (Ti2-yVy)C, W1.33C, Nbi.33C, M01.33C, M01 33Y0.67C; 3-2 MXenes such as TisC2, TisCN, ZrsC2 and Hf3C2; 4-3 MXenes such as Ti4N3, Nb4C3, Ta4C3, V4C3, (Mo,V)4C3; 5-4 MXenes such as Mo4VC4; double transition metal MXenes such as 2-1 -2 MXenes: Mo2TiC2, Cr2TiC2, M02SCC2; and 2-2-3 MXenes such as Mo2Ti2C3.

[0060]

[0042] A preferred functionalised MXene is Ti3C2O2, which is an O-functionalised titanium carbide.

[0061]

[0043] Desirably, the material of the invention is a dual modified functionalised MXene, such as a dual modified functionalised transition metal carbide; a dual modified functionalised transition metal nitride or a dual modified functionalised transition metal carbon nitride. The typical modifications are described above. Desirably, the dual modified MXene comprises lattice defects as a result of at least the non- metal dopant which replaces some early transition metal M atoms in the precursor MXene. The electrochemical metal substitution in some embodiments replaces some of the precursor surface functionality and can also result in defects, particularly vacancies. Desirably, overall the modifications described herein introduce defects (e.g., unoccupied atom sites / vacancies, particularly oxygen vacancies, dislocations, misalignments, substitutional impurity / doped atoms, holes between atoms, etc.) into the precursor MXene material’s lattice structure, preferably at least under top layers of the precursor MXene. Dopant elements / compounds, that is, the electrochemically active non-noble transition metals, and non-metals such as one or more of B, S, C, P, etc., generate one or more such defects, and / or charged points, tuned surfaces and variable electronic properties that improve electrochemical performance. Additionally, such defects and features may serve as chloride ion repulsion / shielding regions and / or may positively attenuate or otherwise enhance catalytic active sites. It is believed that introduction of electrochemically active metals and / or dopant atoms into the inorganic material results in tuning I generation of defects (e.g. vacancies, dislocations, misalignments, holes, and / or new surface functionalisations) in the uppermost surface layers of the inorganic which can provide additional catalytic sites. The dopants also result in the freestanding sheet I delaminated sheet format of the new material.

[0062]

[0044] Desirably, the early transition metal M is selected from one or more of: Sc, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta and W. The electrochemically active metal sites provided are thought to be mainly responsible for the improved OER activities, while non-metal doping generates the much-needed unoccupied site vacancies (by substituting some of the early transition metal M atoms, e.g., Ti atoms) without destroying the 2D morphology, thus exposing more active sites for OER activities in the inorganic components.

[0063]

[0045] Preferably, the electrochemically active metal has an oxidation state that is less than that of the early transition metal M atom(s), e.g., Ti, which is replaced. For example, Ti has an oxidation state of +4 in Ti3Cz, and thus the electrochemically active metal preferably has an oxidation state of +3 or less. This ensures the formation of the required vacancies. For example, an Fe dopant into Ti3CzOz is confined to incorporation of the some of the O surface termination of the functionalised MXene. The material includes Fe in the +2 and +3 oxidation states. This leaves oxygen vacancies in the material.

[0064]

[0046] Preferably, the non-metal dopant has an oxidation state that is less than that of the early transition metal M atom(s), e.g., Ti, which is replaced. For example, Ti has an oxidation state of +4 in Ti3C2, and thus the non-metal dopant preferably has an oxidation state of +3 or less, such as B(lll). This forms vacancies where the non-metal replaces some of the M atoms in the precursor.

[0065]

[0047] Preferably, both the electrochemically active metal and the non-metal dopant, have an oxidation state that is lower than an oxidation state of the early transition metal M, in the MXene.

[0066]

[0048] In some examples, desirably, the hydrophilic surface functional groups are majority O. Desirably, the electrochemically active metal is Fe. Desirably, the non-metal is B.

[0067]

[0049] In other examples, the dual modified MXene is a modified titanium carbide, Ti3CzTx, wherein Txis a surface functional group selected from one or more of O, F, OH, and Cl.

[0068]

[0050] In one example, the dual modified MXene is a dual modified titanium carbide, Ti3CzO2. In one example, the dual modified MXene is a dual modified O-functionalised titanium carbide, Ti3CzOz.

[0069]

[0051] Suitably, the dual modifications may be (i) the non-metal dopant is B and (ii) the electrochemically active metal Fe.

[0070]

[0052] In some examples, the precursor MXene may include a mixture two or more phases.

[0071]

[0053] After the modifications of the invention, a preferred material produced is twin doped Fe- and B- doped TisCzTx, particularly B and Fe doped Ti3CzOz, which is specifically designated herein as ‘FBT’.

[0072]

[0054] Preferably, the twin doping strategy of a precursor MXene (Ti3CzTx, where Tx is surface functional groups such as -OH, -F, -Cl, or -O) produces the at least twin doped modified MXene material of the invention as a multiphase thin 2D material.

[0073]

[0055] In some examples, the FBT material of the invention may comprise, or consist essentially of, or consist of: (a) hexagonal shaped titanium carbide (Ti6C3.75) and (b) iron titanium oxide (FezTi3Og; pseudorutile), and (c) tetragonal shaped titanium boron oxide (TiB0.024Oz; B-doped rutile).

[0074]

[0056] Preferably, the at least dual modified MXene material of the invention is in thin 2D sheet form which is dramatically different from the precursor MXene material. Suitably, clusters of dense particles with different shapes (rectangular & hexagonal) may be present.

[0075]

[0057] Further doped examples include twin or tri doped derivatives of the MXene material, comprising two, three or more modifications of the nature described above, where at least one modification is a dopant non-metal, and at least one other modification is an electrochemically active metal. The dopants may be any combination of the above-mentioned dopant metal elements, or metalloid metals, metalloids, metal compounds or metalloid compounds, preferably those that form Lewis acid species, and especially strong Lewis acid species, within the inorganic material. Desirably the dopants are a combination of B and Fe; B and Co; or B and Ni.

[0076]

[0058] Suitably, the dual modified MXene comprises Fe3+surface terminations.

[0059] In an example, a preferred dual modified MXene is a dual modified titanium carbide, TisC2O2, and wherein the non-metal dopant is B and the electrochemically active metal is Fe. An example of a preferred dual or twin doped functionalised MXene is B-Fe-Ti3C2O2.

[0077]

[0060] Desirably, the dual modified MXene may comprise two or more phases of material. For example, the dual modified MXene comprises two or more phases of materials selected from: (a) hexagonal shaped titanium carbide (e.g., TieCsjs) and (b) iron titanium oxide (e.g., Fe2Ti30g); and (c) tetragonal shaped titanium boron oxide (e.g., TiBo.02402).

[0078]

[0061] Desirably, the dual modified MXene comprises lattice defects in the form of unoccupied vacancies as a result of at least the non-metal dopant B. Vacancies are generated when the non-metal, in this case B, replaces some of the Ti in the precursor, in this case Ti3C2O2-MXene, during material synthesis. Desirably, the material comprises M-0 (where M = Ti, B and Fe) bonds. In the preferred FBT material, C-Ti-Tx and Ti(ll)-Tx groups (where Tx is surface termination and can be Fe, B or both) are present, e.g., as confirmed by XPS analysis identifying presence of Ti in different oxidation states including 0, +2, and +4. Suitably, the material may comprise one or more of: TiC>2 (metal-O), Ti-O-Tx, C-Ti-(OH)Xand C-0 groups. Suitably, the material may comprise one or more of: Ti, O, C-C, C-O, and O-C=O groups. In FBT, the Fe active sites are thought to be mainly responsible for the improved OER activities, while B doping generates the much-needed unoccupied oxygen site vacancies (by substituting some of the Ti) without destroying the 2D morphology, thus exposing more active sites for OER activities in the inorganic components.

[0079]

[0062] Suitably, the electrochemical metal dopant, e.g., Fe dopant, may be confined to incorporation into, or replacement with, the some of the surface termination groups of the modified MXene material. The material may include Fe in the +2 and +3 oxidation states.

[0080]

[0063] The doping strategy of the invention as described herein dramatically improves MXene performance for OER, particularly in seawater, e.g., by promoting surface termination and breaking the inherent electrochemical neutrality of the precursor MXene. The 2D freestanding or delaminated sheet morphology favourably impacts electrochemical activity in a way in which prevents the newly included surface terminations I dopants from leaching. The doping and promotion of surface termination provides abundant accessible electroactive sites on the at least dual modified MXene inorganic material that are further enhanced by presence of the 2D morphology which is not present in the precursor MXene which has a connected multilayer or accordion like morphology. It is believed that the doping process causes delamination of the typical MXene multilayer (accordion-like) morphology to result in the desirable freestanding sheets I delaminated sheets of the material of the invention which greatly improves performance over the starting precursor material. Further, incorporation of electrochemically active metals, such as Fe, and non-metal dopants, such as B, into the precursor MXene significantly boosts electrochemical performance and results in much enhanced OER activity, particularly in seawater. The non-metal dopants can also create surface passivation by generating negatively charged polyanions on the inorganic material surface which form a Cl' repulsion layer on the inorganic material surfaces to prevent or inhibit chloride oxidation reactions. Polyanions such as borate, sulfate, phosphate, carbonate, and combinations thereof, may be formed by doping with one or more of non-metal dopants B, S, P, N, O, C atoms, or combinations thereof into the functionalised MXene material. These polyanions may provide anti -corrosive properties to the material as the polyanions modulate hydroxyl anions at the interface, e.g., via hydrolysis which repels the chloride anions present in seawater, thus preventing corrosive chlorine chemistry at the anode. Preferred combinations include S, P and B. Borate species are particularly preferred and may form from B doping. Borate species are a weak Lewis acid that acts as a local OH- modulator which preferentially binds OH- to Cl’, while the B-OH sites formed result in a layer of Lewis acid protection on the inorganic material. The surface borate and B-OH species which result from B doping act as a weak Lewis acid at the interface serving as a local OH’ modulator. Thus, in general terms, the B-doping modulates the electronic properties of the inorganic material and creates borate species that regulate hydroxyl ion adsorption. As a result of at least these attributes, introducing the boron, e.g., in a form of a B dopant into the material has a positive effect on the OER catalysis. Overall, the modifications described herein result in a new material that exhibits vastly improved OER performance in seawater splitting. The doping strategy described herein breaks the otherwise electrochemical inactivity of previously known MXene materials to drive ampere-level selective OER in seawater.

[0081]

[0064] Overall, the materials exhibit exceptional performance for selective anodic reactions, particularly OER, even in natural / fresh seawater, a most challenging substrate. Advantageously, when used as an electrocatalyst in seawater, preferred materials avoid chlorine gas evolution or hypochlorite formation by supporting the desirable excellent performance for OER not only at lower overpotentials that inhibit chlorine oxidation reactions at the anode, but at higher overpotentials resulting from OER at higher current densities due to shielding effect of many of preferred materials of the invention. The materials are particularly more active during operation than conventionally used electrocatalysts and in preferred materials, are resistant to Cl- based corrosion and deactivation of active sites.

[0082]

[0065] The modified MXenes materials can be made in accordance with methods known to the person skilled in the art. Methods for synthesising precursor MXene materials, such as those described above, are known in the art. Alternatively, some precursor MXenes are commercially available from sources such as Merck® / Sigma Aldrich® and Carbon-Ukraine Ltd., including Ti2CTx, Ti3C2Txand V2CTX.

[0083]

[0066] Methods of functionalising precursor MXene materials to form a majority of surface groups as S, N or O, include treatment with a strong reducing agent. As an example, a MXene in the form of a MXene precursor material as noted herein, such as titanium carbide (e.g., Ti3C2Tx) can be first treated with a strong reducing agent, such as NaOH, to form an O functionalised MXene materials, such as in this example Ti3C2O2(O functionalised titanium carbide MXene). In one example, a precursor carbide, nitride or carbonitride MXene material may be treated with aqueous NaOH, such as 0.1 M, for a period of time, such as 24 h, and then filtered, washed, and dried to yield a majority O-functionalised MXene. Similar processes using a mixture of CsBr / KBr / LiBr and Li2S to treat a precursor MXene with heating, e.g., to 550 °C, or a mixture of CsBr / KBr / LiBr and NaNH2to treat a precursor MXene heating, e.g., to 300 °C, can achieve majority S or N functionalisation of the MXene, respectively.

[0084]

[0067] These materials can then be modified by electronically tuning / engineering to promote surface termination and to break inherent electrochemical neutrality in the MXene by substituting at least some of the surface functionality with one or more electrochemical active metals such as Fe, and by replacing at least some of the early transition metal M atoms with a preselected non-metal, for example, B for Ti in the case of ‘FBT’.

[0085]

[0068] By way of example only, non-metal dopant doping may be achieved by mixing a compound comprising the non-metal, such as B(OH)3(B source), CO(NH2)2(urea) (N source), SC(NH2)2(thiourea) or pure sulfur (S source), or sodium hypophosphite (P source), in an appropriate amount in an aqueous mixture also comprising the functionalised MXene. An appropriate amount may comprise an amount sufficient to result in from 1 -20 wt% of the non-metal dopant being present in the MXene (relative to the total weight of the MXene), such as of from 2-7 wt%, 5-10 wt%, or 1 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%. Electrochemically active metal doping may be achieved by mixing a compound comprising the electrochemically active metal, such as a nitrate salt of Fe, Co, Ni (e.g. Fe(NO3)3.9H2O) or if a nitrate salt is not available, another water soluble salt of the active metal, in an appropriate amount also into the aqueous mixture comprising the functionalised MXene and non-metal dopant precursor. An appropriate amount may comprise an amount sufficient to result in from 1 -20 wt% of the electrochemically active dopant being present in the MXene (relative to the total weight of the MXene), such as of from 5-15 wt%, 5-15 wt%, 7-15 wt%, or 10-20 wt%, or 1 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%. After mixing, the solid can be dried and annealed, such as at a high temperature of between 400 °C - 800 °C, such as at 600 °C, for a period of time, such as 4-6 h, or 5 h, at a heating rate of about 5 °C / min under an inert atmosphere. The cooled solid can then be collected, washed and dried for use.

[0086]

[0069] By way of example only, the solid for use can be fabricated into an electrode for electrochemical splitting of water by mixing the modified MXene with a high surface area carbon black and a binder and drop casting the mixture on a conductive substrate, such as dried nickel foam.

[0087]

[0070] For FBT, the Fe and B doping can be readily achieved by thermal treatment of Ti3C2O2, whereby B doping for some of the Ti atoms and Fe replacement of some of the surface termination, results in Fe3+in surface termination via electrostatic attractions due to its opposite charges with the surface. The Fe interaction is believed to break van der Waals interactions between adjacent MXene nanosheets, to produce freestanding 2D nanosheet, which serve as the doped inorganic template required for solid / liquid assisted synthetic process described below.

[0088]

[0071] Following an analogous process, any electrochemically active metal element as described herein can be introduced into the functionalised MXene material.

[0089]

[0072] Desirably, the twin doping strategy of the precursor MXene (e.g., with dopant Fe and B) produces new material “FBT” which is provided as a multiphase thin 2D material including a mixture of three phases of (a) hexagonal shaped titanium carbide (Ti6C3.75) and (b) iron titanium oxide (Fe2Ti3O3; pseudorutile), and (c) tetragonal shaped titanium boron oxide (TiBo.02402; B-doped rutile). In some examples, the freestanding thin 2D nanosheets may be associated with clusters of smaller hexagonal nanosheets at some of the edges. It is believed that the non-metal B substitutes for some of the Ti in the MXene matrix to create defects, particularly in the form of unoccupied vacancies, that may act as conductivity enhancers and / or more active catalyst sites.

[0090]

[0073] The invention extends to a use of freestanding or delaminated sheets of the at least dual modified MXene material described above as an electrocatalyst, preferably as an OER electrocatalyst for water splitting, particularly where the water splitting involves seawater, preferably alkaline seawater, alkaline DI, alkaline tap water, and alkaline wastewater. Desirably, the OER is selective OER by inhibiting CER. Generally, the new materials are useful electrocatalysts operable in deionised (DI) water and / or fresh seawater at high current densities of over 500 mA.crrr2e.g., as determined by linear scanning voltammetry at room temperature (25°C). Advantageously, when used as an electrocatalyst in fresh seawater, the materials readily avoid chloride ion oxidation (such as chloride gas or hypochlorite formation) by supporting the necessary excellent current densities for OER at not only lower overpotentials of <480 mV, but also at much higher current density of >1 A. cm-2which are typically associated with higher overpotentials of >480 mV where completing chloride ion oxidation reactions are problematic. Due to the excellent Ch repulsion ability of preferred catalysts of the invention, chlorine oxidation reactions and associated by chloride product formation is inhibited even at high current densities and overpotentials >480 mV. Thus, advantageously, preferred materials are selective towards OER and inhibition of chloride oxidation reaction such as hypochlorite formation in high pH seawater or chloride gas evolution in acidic seawater.

[0091]

[0074] In some embodiments, selective OER in seawater at very high current densities of up to 1 .75 A erm2are achievable. The materials described herein overcome problems experienced in direct electrolytic seawater splitting which arises from impurities present and unwanted side reactions which hinger long term performance of anode in OER reactions in seawater, particularly natural (untreated) seawater. The materials can also be used for OER in deionised water or wastewater. The materials of the invention represent a significant advance in the art.

[0092]

[0075] The invention further extends to providing a method of forming a MXene based OER electrocatalyst, comprising the steps of forming freestanding or delaminated two-dimensional (2D) sheets of a functionalised and dual modified MXene, Mn+iXnTx, where n is 1 , 2 or 3, and M is an early transition metal (Group 3 to 6 metal), X is C and / or N, and Txis a hydrophilic surface functional group selected from one or more of O, F, OH, and Cl, by modifying a precursor MXene material, wherein the modifying steps involve: functionalising the precursor MXene material such that a majority of the hydrophilic surface functional groups are oxygen; replacing at least a portion of the additional early transition metal M atoms in the functionalised MXene with one or more dopant non-metal atoms selected from B, N, C, O, P, Si, S, As and Se; and replacing at least one hydrophilic surface functional group and / or at least one additional early transition metal M atom, with one or more electrochemically active metals selected from the group consisting of: Fe, Ni, Co, Cu, Cr, Mo and W, preferably Fe, Ni and Co.

[0093]

[0076] In some embodiments, the method involves the inclusion of at least one non-metal atom dopant atom and at least one electrochemically active metal in an O-surface functionalised MXene to increase the activity of a MXene-based OER electrocatalyst, wherein the one or more dopant non-metal atoms are selected from B, N, C, O, P, Si, S, As and Se; and wherein the one or more electrochemically active metals are selected from the group consisting of: Fe, Ni, Co, Cu, Cr, Mo and W.

[0094]

[0077] In preferred embodiments, the one or more electrochemically active metals are different to the early transition metal M atom. Most preferred materials are selective towards OER in alkaline natural seawater, while avoiding chloride oxidation reactions, such as CER in acidic pH or hypochlorite formation in high pH media (pH>7). To mitigate by-product formation during seawater splitting, preferred electrocatalysts must be active enough to generate industrially high current density (typically >500 mA.crrr2), most preferably at an overpotential of 2 480 mV in an alkaline medium.

[0095]

[0078] Generally, preferred materials of the invention favour hydroxyl ion adsorption over chloride adsorption thereby promoting OER while inhibiting chlorine reactions to prevent corrosion and / or fouling of the electrocatalyst when used in seawater splitting.

[0096]

[0079] Advantageously, the materials of the invention obtained by or obtainable by the above process when used in water splitting applications result in excellent performance and stability of the materials of the invention when used as electrocatalysts, for example, in water splitting applications involving water including deionised water, wastewater, or natural (untreated, filtered) seawater. Preferred materials are sufficiently active during electrocatalysis to enable commercial levels of direct electrolytic seawater splitting, noting natural seawater is a particularly onerous medium for water splitting due to chlorine ion concentration and other contaminants present. Typically, materials of the kind described herein require a low overpotential of -521 mV to achieve a current density of 1 A cm2, which is 225% higher than the current density achieved by the typical commercial catalyst, RuC>2, at the same overpotential in alkaline seawater.

[0097]

[0080] Preferred electrocatalysts obtained by or obtainable by the process of the invention result in OER in seawater with a remarkably high current density of at least 0.5 A crrr2at low overpotentials of <480mV at ambient conditions (room temperature, 25 °C, and 1 atm pressure). Particularly preferred catalysts of the invention result in OER at higher current densities of over 0.5 A crrr2at overpotentials of <480 mV while retaining OER selectivity due to the Ch shielding ability. Such performance is typically significantly higher than that of commercially used lrO2 in seawater splitting.

[0098]

[0081] In terms of OER performance, the specific FBT material described herein generates high current densities at overpotential <480 mV in alkaline 6M KOH DI and 6M KOH seawater medium, demonstrating exclusive selectivity towards OER while avoiding chlorine oxidation byproduct formation at room temperature, 25 °C, and 1 atm pressure. This exclusive OER selectivity is attributed to the material’s impressively high OER activities in seawater, hence making FBT an excellent catalyst for industrial seawater splitting.

[0099] Applications

[0100]

[0082] The materials of the invention have utility in many applications, including hydrogen electrochemistry, water electrolysis, fuel cells, and their H related reactions. In a preferred application, the invention extends to a use of material of the invention as an electrocatalyst, particularly as an OER catalyst for water splitting, preferably alkaline water, alkaline seawater, deionised water, tap water or waste water.

[0101] Embodiments

[0102]

[0083] Embodiment 1. Freestanding or delaminated two-dimensional (2D) sheets of at least a dual modified functionalised MXene material, Mn+iXnTx, where n is 1 , 2 or 3, and M is an early transition metal (Group 3 to 6 metal), X is C and / or N, and Txis a hydrophilic surface functional group selected from one or more of O, F, OH, and Cl, wherein: the MXene material is functionalised such that a majority of the hydrophilic surface functional groups comprise an electronegative atom selected from one or more of S, N, or O, preferably O; the MXene material is modified by replacement of a portion of the early transition M atoms with one or more dopant non-metal atoms selected from B, N, C, O, P, Si, S, As and Se, preferably B; and the MXene material is further modified by replacement of at least one hydrophilic surface functional group and / or at least one early transition M atom with one or more electrochemically active metals selected from the group consisting of: Fe, Ni, Co, Cu, Cr, Mo and W, preferably Fe, Ni and Co, most preferably Fe.

[0103]

[0084] Embodiment 2. The freestanding or delaminated 2D sheets of Embodiment 1 , wherein the dual modified functionalised MXene is a dual modified functionalised transition metal carbide (TMC); a dual modified functionalised transition metal nitride (TMN) or a dual modified functionalised transition metal carbon nitride (TMN).

[0104]

[0085] Embodiment 3. The freestanding or delaminated 2D sheets of Embodiment 1 or Embodiment 2, wherein the dual modified functionalised MXene comprises lattice defects as a result of at least the non-metal dopant.

[0105]

[0086] Embodiment 4. The freestanding or delaminated 2D sheets of any one of the preceding Embodiments, wherein the early transition metal M is selected from one or more of: Sc, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta and W.

[0106]

[0087] Embodiment 5. The freestanding or delaminated 2D sheets of any one of the preceding Embodiments, wherein the electrochemically active metal, the non-metal dopant, or both the electrochemically active metal and the non-metal dopant, have an oxidation state that is lower than an oxidation state of the early transition metal, M, in the MXene.

[0107]

[0088] Embodiment 6. The freestanding or delaminated 2D sheets of any one of the preceding Embodiments, wherein the electrochemically active metal is Fe.

[0108]

[0089] Embodiment 7. The freestanding or delaminated 2D sheets of any one of the preceding Embodiments, wherein the MXene material is a dual modified functionalised titanium carbide (TMC), Ti3C2Tx, wherein Txis a surface functional group selected from one or more of O, F, OH, and Cl.

[0109]

[0090] Embodiment 8. The freestanding or delaminated 2D sheets of any one of the preceding Embodiments, wherein the MXene material is a dual modified O-functionalised titanium carbide (TMC), TisC2O2.

[0110]

[0091] Embodiment 9. The freestanding or delaminated 2D sheets of any one of the preceding Embodiments, wherein dual modifications are (i) non-metal dopant is B and (ii) electrochemically active metal is Fe.

[0111]

[0092] Embodiment 10. The freestanding or delaminated 2D sheets of any one of the preceding Embodiments, wherein the dual modified functionalised MXene comprises two or more phases of material.

[0112]

[0093] Embodiment 11. The freestanding or delaminated 2D sheets of any one of the preceding Embodiments, wherein the dual modified functionalised MXene is a dual modified O-functionalised titanium carbide (TMC), Ti3C2O2, wherein the non-metal dopant is B and the electrochemically active metal is Fe.

[0094] Embodiment 12. The freestanding or delaminated 2D sheets of any one of the preceding Embodiments, wherein the electrochemically active metal is Fe, and the dual modified functionalised MXene comprises Fe3+surface terminations.

[0113]

[0095] Embodiment 13. The freestanding or delaminated 2D sheets of any one of the preceding Embodiments, wherein M is Ti, the non-metal dopant is B, and the electrochemically active metal is Fe, and the dual modified functionalised MXene comprises two or more phases of materials selected from: (a) hexagonal shaped titanium carbide (e.g., Ti6C3.75) and (b) iron titanium oxide (e.g., Fe2Ti30g); and (c) tetragonal shaped titanium boron oxide (e.g., TiBo.02402).

[0114]

[0096] Embodiment 14. The freestanding or delaminated 2D sheets of any one of the preceding Embodiments, wherein the dual modified functionalised MXene comprises lattice defects in the form of unoccupied vacancies as a result of at least the non-metal dopant.

[0115]

[0097] Embodiment 15. Use of freestanding or delaminated sheets according to any one of Embodiments 1 to 14 as an electrocatalyst.

[0116]

[0098] Embodiment 16. Use of freestanding or delaminated sheets according to Embodiment 15, as an OER electrocatalyst for water splitting.

[0117]

[0099] Embodiment 17. Use of freestanding or delaminated sheets according to Embodiment 15, as an OER electrocatalyst for water splitting of seawater, preferably alkaline seawater.

[0118]

[0100] Embodiment 18. Use of freestanding or delaminated sheets according to Embodiment 16 or 17 wherein the OER is selective OER by inhibiting GER.

[0119]

[0101] Embodiment 19. A method of forming a MXene-based OER electrocatalyst, comprising the steps of forming freestanding or delaminated two-dimensional (2D) sheets of a functionalised and dual modified MXene, Mn+iXnTx, where n is 1 , 2 or 3, and M is an early transition metal (Group 3 to 6 metal), X is C and / or N, and Txis a hydrophilic surface functional group selected from one or more of O, F, OH, and Cl, by modifying a precursor MXene material, wherein the modifying steps involve: functionalising the precursor MXene material such that a majority of the hydrophilic surface functional groups comprise an electronegative atom selected from one or more of S, N, or O, preferably O; replacing at least a portion of the M atoms in the functionalised MXene with one or more dopant non-metal atoms selected from B, N, C, O, P, Si, S, As and Se, preferably B; and replacing at least one hydrophilic surface functional group and / or at least one early transition M atom with one or more electrochemically active metals selected from the group consisting of: Fe, Ni, Co, Cu, Cr, Mo and W, preferably Fe, Ni and Co, most preferably Fe.

[0120]

[0102] Embodiment 20. A process involving inclusion of at least one non-metal atom dopant atom and at least one electrochemically active metal in an O-surface functionalised MXene to increase the activity of a MXene-based OER electrocatalyst, wherein the one or more dopant non-metal atoms are selected from B, N, C, O, P, Si, S, As and Se, preferably B; and wherein the one or more electrochemically active metals are selected from the group consisting of: Fe, Ni, Co, Cu, Cr, Mo and W, preferably Fe, Ni and Co, most preferably Fe.

[0121]

[0103] Embodiment 21 . Product obtained by or obtainable by the process of Embodiments 19 or 20.

[0104] Embodiment 22. Freestanding or delaminated two-dimensional (2D) sheets of a dual modified functionalised MXene material, Mn+iXnTx, where n is 2, and M is Ti, X is C and / or N, and Txis hydrophilic surface functional group selected from one or more of O, F, OH, and Cl, wherein the MXene material is functionalised such that a majority of the hydrophilic surface functional groups comprise an electronegative atom O; the MXene material is modified by replacement of a portion of the Ti atoms with dopant non- metal atom B; and the MXene material is further modified by replacement of at least one hydrophilic surface functional group and / or at least one Ti atom with electrochemically active metal Fe.

[0122] Embodiment 23. The freestanding or delaminated 2D sheets of Embodiment 22, in the form of B- and Fe- doped Ti2C2O2.

[0123] Examples

[0124]

[0105] The present invention is described with reference to the following examples. It is to be understood that the examples are illustrative of and not limiting to the invention described herein.

[0125] Example 1: B, Fe-dooed TiaCzOs (‘FBT’)

[0126]

[0106] A modified Ti3C2Tx-MXene was engineered by substituting regular Ti3C2Tx-MXene surface functionality with electrochemically active Fe and partially replacing Ti with the non-metal B. Providing B underneath the top layer generates vacancies. The presence of the resultant B-0 bond improves reaction kinetics by modulating the adsorption of OH' ions at the interface. The presence of vacancies and surface termination resulting from the doping in the modified MXene, provide for enhanced OER activity in application described herein. OER evaluation shows that FBT required low overpotentials of 521 to achieve 1 A cm-2in seawater.

[0127] MXene (Ti3CsTx) synthesis

[0128]

[0107] 2 g of Ti3AIC2(Sigma) powder was gradually added to 40 mL of 10% HF solution (Sigma) and the mixture was stirred at 500 rpm and 35 °C for 24 h. The resultant mixture divided into three 50 mL centrifuge tubes that already contains about 25 mL of milli-Q water. The tubes were then sealed, placed inside a centrifuge and centrifuged at 5000 rpm for 5 minutes. The supernatant was discarded and milli- Q water was added to the sediment for more washing and the process was repeated until a pH of ~7 was obtained. To ensure all the F was removed, the neutral sediment (wet Ti3C2Tx) was dispersed in 1 L of milli-Q water and the solid recovered via vacuum filtration and using 0.2-micron PES filter membrane. After filtration, the product was vacuum dried at 50 °C overnight.

[0129] TiaCzTx surface modification using 0.1 M NaOH

[0130]

[0108] 2g of dried Ti3C2Txwas treated with 20 mL of 0.1 M NaOH (Sigma) solution with stirring at room temperature for 24 hours. The solid product was recovered by centrifuging and followed with multiple washing with DI water until a pH of ~7 was obtained. The neutral solid was subsequently redispersed in DI water and sonicated for 15 minutes. The solid product was recovered by vacuum filtration, dried overnight under vacuum at 50 °C and the final Ti3C2O2was labelled T.

[0131] Twin Doping of H3C2O2 (T) with Iron and Boron

[0132]

[0109] Calculated amount of T, Fe(NO3)3.9H2O (Sigma) and B(OH)3(Sigma) (mass ratio: T: Fe(NO3)3.9H2O = 1 :1.5 and Fe(NO3)3.9H2O: B(OH)3= 3:10) were dispersed in DI water (for T = 50 mg, DI water = 20 ml) and stirred for 90 minutes at RT. After stirring, the water was evaporated until dryness, and the resultant power collected and annealed. The annealing was done at 600 °C for 5 hours, at a heating rate of 5 °C / min in a tubular furnace under Ar flow. Upon cooling to room temperature, the product was collected, washed with warm water (60 - 70 °C) followed by absolute ethanol and dried at overnight under vacuum at 50 °C. The resultant ‘FBT’ product was collected.

[0133]

[0110] An analogous process was followed to produce the Co and Ni analogues of FBT, using Ni(NO3)2.6H2O and Co(N03)2.6H20 in place of Fe(NO3)3.9H2O.

[0134] Electrochemical Testing

[0135]

[0111] The electrochemical testing was conducted on an electrochemical workstation (CHI760D) in a standard 3-electrode system at RT using 6 M KOH in DI water and seawater as electrolytes. Graphite rod and Ag / AgCI (1 M KCI) were employed as the counter and reference electrodes respectively. The working electrodes were prepared by drop-casting of prepared electrode ink on washed nickel foam (washed with 1 M HCI, DI water and ethanol). To prepare the electrodes ink, firstly, carbon black (CB) solution was prepared by dispersing 20 mg of carbon powder in 20 ml solvent (Isopropyl alcohol: water = 1 :4) and sonicated for 1 hour. Next, 4 mg of FBT, 1 ml of CB solution and 8 pL of PTFE (Sigma) binder was mixed and sonicated for about 30 - 40 minutes for homogeneous ink. The Wes were then prepared by drop-casting 200 L of the prepared ink on an area of 0.25 cm2on the washed and dried nickel foam, then dried in a vacuum over at 50 °C for 24 hours. All the LSV were performed at a scan rate of 0.1 V s'1with iR compensation. Also, the potentials were converted to RHE using E(V + RHE) = E(Ag / AgCI) + (0.059*pH) + 0.235. All tests were conducted at room temperature, 25 °C, and 1 atm of pressure.

[0136] Results & Discussion

[0137]

[0112] Tuning I engineering Ti3C2O2MXene such that B replaces some of the Ti, while Fe replaces some of the surface oxygen can effectively activate dormant OER potential of the precursor MXene Ti3C2Tx. The O functionalised Ti3C2O2 MXene was first synthesised by treatment of Ti3C2Txwith NaOH solution to generate O-terminations which aided simultaneously with Fe and B introduction / doping into the matrix of Ti3C2O2MXene to result in the new FBT material of the invention in 2D form. The XRD of FBT is show in Figure 1(a). TEM image revealed that the precursor Ti3C2O2MXene possesses the characteristic multilayered, accordion like nanosheets morphology associated with MXenes, while XRD confirmed the successful synthesis of MXene through presence of characteristic MXene XRD peaks (Figure 1(b) - bottom is TiC2O2, middle is FBT).

[0138]

[0113] During formation of FBT, the O-terminations at the surface of Ti3C2O2(after NaOH treatment) aided B and Fe adsorption to Ti3C2O2during the initial deposition, with Fe3+interacting with the surface termination via electrostatic attractions due to its opposite charges with the surface. Thereafter, Fe and B are incorporated into the Ti3C2O2upon thermal treatment, with B doping replacing some of Ti, while Fe replaced some of the surface termination. The doping breaks van der Waals interactions between adjacent MXene nanosheets, generating freestanding sheets of the new FBT material. The freestanding modified MXene nanosheets was confirmed by the disappearance of (002) peak in the XRD of FBT (Figure 1(b)). The retention of (006), (200) and (1 10) peaks located at 2© = 36.2°, 41.7° and 61 ° respectively for FBT confirms the preservation of most of original Ti3C2O2crystal structure upon Fe and B incorporation. However, the disappearance of (002) peak indicates the disruption of ordered stacking of multilayered MXene induced by the presence of Fe at the surface. Further, whilst XRD could not independently confirm the presence of Fe and B, the evolution of new peaks at 2© = -32°, -54.6° and -56.8° could be attributed to their influence upon incorporation into the Ti3C2O2-MXene structure.

[0139]

[0114] EDX mapping confirms the uniform distribution of Fe, B, Ti, C and O demonstrating successful incorporation of Fe and B, into the MXene matrix. TEM analysis revealed that FBT possess freestanding thin 2D nanosheets with clusters of smaller hexagonal nanosheets at some of the edges, and these shapes were confirmed through microstructural analysis of selected area electron diffraction (SAED) imaging. HR-TEM imaging of FBT reveals the 2D nanosheets are arranged in definite order. The magnified image (insert in 1€) reveals a section of the atomic structure of FBT nanosheet consisting of Ti and C atoms and sectional distance of -0.92 nm. Furthermore, a defect is apparent in the area marked with golden colour (most right square in image) which was attributed to unoccupied vacancies left behind when B substituted some of the Ti in Ti3C2O2 matrix.

[0140]

[0115] A XPS survey scan also confirmed successful synthesis of Ti3C2C>2-MXene via identification of Ti, C, and O in the X-ray photoelectron spectroscopy (XPS) survey scan and the presence of Ti-C bond in the high-resolution C 1s spectrum. Upon doping with only B, there was decrease in the intensity of Ti 2p peak confirming the successful replacement of some of the Ti in Ti3C2O2-MXene, which is supported by the presence B-0 bond in the high-resolution B 1s spectrum. Upon incorporation of only Fe, there was no significant observable decrease in the intensity of Ti 2p, confirming that Fe only interacted with the surface terminations of the Ti3C2O2.

[0141]

[0116] The presence of Ti, C, O, B and Fe in the survey scan of FBT confirm the successful incorporation of Fe and B into the Ti3C2O2-MXene matrix. High-resolution 0 1 s spectrum of FBT (Figure 2(a)), reveals four binding energy peaks located at 529.75, 531 .30, 532.15 and 533.42 eV corresponding to TiC>2 (lattice oxygen), (Ti or B)-O-Fe (defective oxygen vacancy site due to low oxygen coordination), C-Ti-(OH)Xand C-0 respectively were identified. Compared to Ti3C2O2, there was a decrease in the intensity of TiC>2 peak in FBT, indicating its conversion to (Ti or B)-O-Fe bonds. According to the principle of metal oxide defect chemistry, the presence of dopants often results in different lattice defects, causing oxygen vacancies and this depends on oxidation state. For example, if a cation of lesser oxidation state replaces Ti4+in its oxide, oxygen vacancies are created. Therefore, when B with a maximum oxidation state of +3 replaces some of the Ti4+(from TiOa peak), oxygen vacancies are created, explaining the decrease in TiC>2 peak intensity and the increase of (Ti or B)-O- Fe peak intensity. Furthermore, the peak area ratio of defective oxygen to lattice oxygen for Ti3C2O2, and FBT was calculated to be 0.33 & 1 .78 respectively, thus confirming more oxygen vacancies in FBT.

[0142]

[0117] The material’s OER activity in alkaline DI water and seawater was compared to a commercial RU©2 catalyst. FBT delivered ampere level current density performances at lower overpotentials, and outperformed the commercial RuO3catalyst (Figure 3(a) - (c)). Remarkably, FBT achieved the current density of 1 A cm-2at low overpotentials of 521 .2 mV in alkaline seawater and 453.1 mV in DI water. This performance was attributed to the redox potential of FBT that was triggered by the presence of Fe and B, and the 2D morphology that could enhance electrochemical active surface area (ECSA).

[0143]

[0118] The influence of Fe and B doping was also investigated by measuring the OER activities of Ti3C2O2-MXene, BT (no Fe doping) and FT (no B doping) in alkaline DI water electrolyte. The comparison makes it is apparent that Fe active sites are mainly responsible for improved OER activities. Conversely, B generated the much-needed vacancies thus exposing more active sites for OER activities in FBT.

[0144]

[0119] Also, the improved performance of FBT over FT could be attributed to its impressive large 2D morphology (Figure 1(b)) as compared to that of FT. Further, from the Nyquist plots (Figure 3(d)), FBT exhibited the smallest semicircle diameter than TigCsOa, BT and FT, indicating the least charge transfer resistance at the electrocatalyst-electrolyte interface, thus improved charge transfer capability and OER activities. Moreover, it was observed from Figure 3 that FBT possess higher ECSA than FT, BT and Ti2C3O2, indicating the presence of more active sites for OER activity, hence improved performances. In seawater electrolysis, exclusive selectivity towards OER while avoiding CER is highly desirable for electrocatalysts for industrial application. To mitigate CER during seawater splitting, the electrocatalyst must be active enough to generate high current density at overpotential <480 mV (equilibrium potential of CER) in alkaline medium. As shown in Figure 3, FBT could potentially mitigate CER due to its ability to generate high current densities at 480 mV. Moreover, the selectivity of FBT towards OER in seawater was investigated, by passing the gas generated during electrochemical testing through acetone to probe the evolution of chlorine gas though analysis for chloroacetone generation. Gas generated during 1 h of continuous electrochemical testing was passed through acetone, and only acetone could be detected on GC analysis, indicating the absence of chlorine gas. This exclusive OER selectivity was attributed to the materials’ impressively high OER activities in seawater, hence making them a catalyst of choice for seawater splitting in the industry.

[0145] Example 2: B, Co-doped T13C2O2 CCBT’)

[0146]

[0120] Another modified Ti3C2Tx-MXene was engineered by replacing the electrochemically active Fe (in example 1 ) with Co to develop cobalt and boron doped Ti3C2O2-MXene (‘CBT’).

[0147]

[0121] The TEM images of CBT (Figure 4 (a)) show that the ultrathin 2D sheet like morphology of the MXene is well preserved even after replacing Fe with Co. Figure 4(c) shows the OER linear sweep voltammetry polarisation curve of CBT. The result shows that CBT requires relatively low overpotentials of 480 and 630 mV to achieve the current densities of 0.5 and 1 A cm-2, respectively.

[0148] Example 3: B. Ni-doped T13C2O2 (‘NBT’)

[0149]

[0122] Another modified Ti3C2Tx-MXene was engineered by replacing the electrochemically active Fe (in example 1 ) with Ni to develop nickel and boron doped Ti3C2O2-MXene (‘NBT’).

[0150]

[0123] The TEM images of NBT (Figure 4 (b)) show that the ultrathin 2D sheet like morphology of the MXene is well preserved even after replacing Fe with Ni.

[0151] Conclusion

[0152]

[0124] OER activity of bare Ti3C2O2-MXene both in alkaline seawater and DI water can be enhanced using the doping method of the invention. The post synthesis analysis of the dopant modified MXene, FBT, reveals that B substituted some of the Ti in Ti3C2O2 forming B-0 bond underneath the outer layer, while Fe species substituted the surface termination. B underneath the top layer generated the much- needed oxygen vacancies.

[0125] Remarkably, FBT required very low overpotentials (521 .2 mV respectively) to deliver 1 A cm2. Similarly, CBT requires relatively low overpotentials of 480 and 630 mV to achieve current densities of 0.5 and 1 A cm-2, respectively.

[0153]

[0126] The major observation here is that, unlike other MXene-based HS OER catalysts where the MXene only acted as support that supplies electrical conductivity, the new FBT, NBT and CBT materials contribute to both OER activity and conductivity.

[0154]

[0127] Hence, this work demonstrates a new approach through which MXenes can be modified for enhanced applications, such as OER activities, both in alkaline seawater and DI water.

[0155]

[0128] It will be apparent to the person skilled in the art that while the invention has been described in some detail for the purposes of clarity and understanding, various modifications and alterations to the embodiments and methods described herein may be made without departing from the scope of the inventive concept disclosed in this specification.

Claims

Claims1. Freestanding or delaminated two-dimensional (2D) sheets of at least a dual modified functionalised MXene material, Mn+iXnTx, where n is 1 , 2 or 3, and M is an early transition metal (Group 3 to 6 metal, ‘M atom / s’), X is C and / or N, and Txis a hydrophilic surface functional group selected from one or more of O, F, OH, and Cl, wherein: the MXene material is functionalised such that a majority of the hydrophilic surface functional groups comprise an electronegative atom selected from one or more of S, N, or O; the MXene material is modified by replacement of a portion of the M atoms with one or more dopant non-metal atoms selected from B, N, C, O, P, Si, S, As and Se; and the MXene material is further modified by replacement of at least one hydrophilic surface functional group with one or more electrochemically active metals and / or replacement of at least one M atom with one or more electrochemically active metals that is different to the M atom, wherein the electrochemically active metal is selected from the group consisting of: Fe, Ni, Co, Cu, Cr, Mo and W.

2. The freestanding or delaminated 2D sheets of claim 1 , wherein the dual modified functionalised MXene material is a dual modified functionalised transition metal carbide, a dual modified functionalised transition metal nitride, or a dual modified functionalised transition metal carbon nitride.

3. The freestanding or delaminated 2D sheets of claim 1 or claim 2, wherein the dual modified functionalised MXene material comprises lattice defects as a result of at least the non-metal dopant.

4. The freestanding or delaminated 2D sheets of any one of the preceding claims, wherein the M atom is selected from one or more of: Sc, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta and W.

5. The freestanding or delaminated 2D sheets of any one of the preceding claims, wherein the electrochemically active metal, the non-metal dopant, or both the electrochemically active metal and the non-metal dopant, have an oxidation state that is lower than an oxidation state of M atom in the MXene.

6. The freestanding or delaminated 2D sheets of any one of the preceding claims, wherein the one or more electrochemically active metals are selected from Fe, Ni, and Co, optionally wherein the electrochemically active metal is Fe.

7. The freestanding or delaminated 2D sheets of any one of the preceding claims, wherein the MXene material is a dual modified functionalised titanium carbide, Ti3C2Tx, wherein Txis a surface functional group selected from one or more of O, F, OH, and Cl.

8. The freestanding or delaminated 2D sheets of any one of the preceding claims, wherein the hydrophilic surface functional group comprise electronegative atom O.

9. The freestanding or delaminated 2D sheets of any one of the preceding claims, wherein the MXene material is modified by replacement of a portion of the M atoms with B.

10. The freestanding or delaminated 2D sheets of any one of the preceding claims, wherein the MXene material is a dual modified O-functionalised titanium carbide, TigCsOa.

11. The freestanding or delaminated 2D sheets of any one of the preceding claims, wherein dual modifications are (i) non-metal dopant is B and (ii) electrochemically active metal is Fe.

12. The freestanding or delaminated 2D sheets of any one of the preceding claims, wherein the dual modified functionalised MXene comprises two or more phases of material.

13. The freestanding or delaminated 2D sheets of any one of the preceding claims, wherein the dual modified functionalised MXene material is a dual modified O-functionalised titanium carbide, Ti3C2O2, wherein the non-metal dopant is B and the electrochemically active metal is Fe.

14. The freestanding or delaminated 2D sheets of any one of the preceding claims, wherein the electrochemically active metal is Fe, and the dual modified functionalised MXene comprises Fe3+surface terminations.

15. The freestanding or delaminated 2D sheets of any one of the preceding claims, wherein M is Ti, the non-metal dopant is B, and the electrochemically active metal is Fe, and the dual modified functionalised MXene comprises two or more phases of materials selected from: a hexagonal shaped titanium carbide, an iron titanium oxide and a tetragonal shaped titanium boron oxide.

16. The freestanding or delaminated 2D sheets of claim 15, wherein the hexagonal shaped titanium carbide is TieCs.yo, the iron titanium oxide is FegTisOg, and / or the tetragonal shaped titanium boron oxide is TiB0024O2.

17. The freestanding or delaminated 2D sheets of any one of the preceding claims, wherein the dual modified functionalised MXene comprises lattice defects in the form of unoccupied vacancies as a result of at least the non-metal dopant.

18. Use of freestanding or delaminated 2D sheets according to any one of the preceding claims as an electrocatalyst, preferably, as an oxygen evolution reaction (OER) electrocatalyst for water splitting.

19. A method of forming a MXene-based oxygen evolution reaction (OER) electrocatalyst, comprising the steps of forming freestanding or delaminated two-dimensional (2D) sheets of a functionalised and dual modified MXene, Mn+iXnTx, where n is 1 , 2 or 3, and M is an early transitionmetal (Group 3 to 6 metal), X is 0 and / or N, and Txis a hydrophilic surface functional group selected from one or more of O, F, OH, and Cl, by modifying a precursor MXene material, wherein the modifying steps involve: functionalising the precursor MXene material such that a majority of the hydrophilic surface functional groups comprise an electronegative atom selected from one or more of S, N, or O, preferably O; replacing at least a portion of the M atoms in the functionalised MXene with one or more dopant non-metal atoms selected from B, N, C, O, P, Si, S, As and Se, preferably B; and replacing at least one hydrophilic surface functional group with one or more electrochemically active metals and / or replacement of at least one M atom with one or more electrochemically active metals that is different to the M atom, wherein the electrochemically active metal is selected from the group consisting of: Fe, Ni, Co, Cu, Cr, Mo and W, preferably Fe, Ni and Co, most preferably Fe.

20. Freestanding or delaminated two-dimensional (2D) sheets of a dual modified functionalised MXene material, Mn+iXnTx, where n is 2, and M is Ti, X is C and / or N, and Txis hydrophilic surface functional group selected from one or more of O, F, OH, and Cl, wherein the MXene material is functionalised such that a majority of the hydrophilic surface functional groups comprise an electronegative atom O; the MXene material is modified by replacement of a portion of the Ti atoms with dopant non- metal atom B; and the MXene material is further modified by replacement of at least one hydrophilic surface functional group and / or at least one Ti atom with electrochemically active metal Fe.

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