WATER SPLITTING

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U. S. Provisional Patent Application No. 62/509,857 filed May 23, 2017, which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

A. Field of the Invention

[0002] The invention generally concerns a surface-immobilized catalyst for electrochemical or photoelectrochemical total water-splitting. In particular, the surface- immobilized catalyst includes an acid resistant surface that includes electrically conductive metal and an organic ligand capable of bonding to the electrically conductive metal and complexing with a catalytic metal.

B. Description of Related Art

[0003] Solar assisted water-splitting systems can combine the harvesting of solar energy with water electrolysis to generate chemical energy in the form of gaseous hydrogen. In catalytic assisted electrolysis of water, the catalyst can mediate bond rearranging in the water- splitting reaction. In an electrochemical cell current flows between an anode and cathode. At the negatively charged cathode, a reduction reaction takes place, with electrons from the cathode transferred to the hydronium ion to form hydrogen. At the positively charged anode, an oxidation reaction occurs, generating oxygen gas and giving electrons to the anode to complete the circuit. The standard reduction potentials for the O2/H2O and H ⁺ /H2 half-reactions are given in Equations (1) and (2), respectively. Equation (1) is the oxygen evolution reaction (OER) and Equation (2) is the hydrogen evolution reaction (HER).

H ₂ 0 ^ ► 0 ₂ +4H ⁺ + 4 e ^" _n

^{2 2} E°= +1.23 V (1). 4H ⁺ + 4 e- ^ ► H _{2 E} o _{= +0 0Q} y ₍₂₎

The total standard potential of water electrolysis is the difference in potentials, (See, Equation (3)), which is -1.23 V at 25 °C, pH of 0, and a H ⁺ concentration of 1 molar. At 25 °C, pH of 0, and a H ⁺ concentration of 1 x 10 ^"7 molar, the potential is unchanged due to the Nernst equation.

E°cell = E°cathode - E°anode E°cell = - 1.23 V (3). For a catalyst to be efficient for this conversion, the catalyst should operate at voltages close to the thermodynamic value of each half reaction, i.e. Voltage in addition to E° that is required to attain a given catalytic activity is referred to as overpotential. The largest overpotential occurs at the anode, which limits the energy conversion efficiency. [0004] In general, noble metals and metal oxides have been the most effective catalyst for the F£ER and OER, respectively, due to their high current density at low over potentials. However, noble metal catalysts are limited by high cost and scarcity. To overcome this problem, numerous efforts have been made to produce catalysts that do not use noble metals or produce single-site surface catalysts (SSSC) with minimal noble metal loadings. By way of example, Joya et al. (Angewandte Chemie, 2012, Vol. 38:9601-9605 and U.S. Patent Application Publication No. 2013/0220825) describes mono-iridium complexes immobilized on metal oxide surfaces to produce O2 and H ⁺ from water with an overpotential of 1.75 V vs. normal hydrogen electrode (NHE). However, this catalyst can suffer from long-term stability under various pH conditions and complex synthetic methods. [0005] Despite various efforts directed at making metal organic complexes for use in water-splitting reactions, many of these systems suffer from pH instability as well as inefficient and/or economical disadvantages.

SUMMARY OF THE INVENTION

[0006] A solution to some of the problems associated with catalysts for water-splitting systems has been discovered. The discovery is premised on using a single-site surface catalyst attached to an acid resistant surface that includes an electrically conductive metal. The catalyst is attached (bound) to the acid resistant surface (e.g., a nickel (Ni°) metal or a titanium (Ti°) metal mesh) by linking groups of an organic ligand. Such a catalyst configuration provides a pH stable catalyst (e.g., stable at pH of 0 to 14) while using minimal amounts of catalytic metals for total water-splitting. As exemplified in a non-limiting manner in the Examples, the HER and OER of the catalysts of the present inventions achieved 10 mA/cm ² current densities at over potentials of less than 200 mV at neutral pH. Furthermore, the process to produce the catalysts of the present invention provides an elegant and efficient methodology. By way of example, an electrically conductive metal mesh can be contacted with the organic ligand under conditions sufficient to coat and bind the organic ligand to the electrically conductive metal in the mesh to form a coated mesh that includes the organic ligand bound to the electrically conductive material. The coated mesh can be contacted with a catalytic metal precursor solution under conditions to complex the catalytic metal to the ligand, thereby forming a catalyst of the present invention in which the ligand serves as a linker between the electrically conductive material and the catalytic metal. The process can take place under ambient temperatures and pressures. [0007] In some aspects of the present invention, single-site surface-immobilized total water-splitting catalysts are described. The catalysts can include an acid-resistant surface comprising an electrically conductive metal and an organic ligand attached to the electrically conductive metal and complexed with a catalytic metal. The acid-resistant surface can include an electrically conductive metal can be a Ni° or Ti° metal mesh. The organic ligand can be capable of passivating the surface such that formation of surface oxides are reduced, or even not formed, during use, thus, reducing chemical corrosion and promoting stability of the catalyst. The organic ligand can include two or more linking groups. At least one of two linking groups can be attached to the surface of the electrically conductive metal. Multiple ligands can be attached to the acid-resistant surface through the electrically conductive metal. The organic ligand can have general structure (I): R ₃

(I) where: Ri is an aliphatic group, an aromatic group, or a hetero-aromatic group; and R2, R3, R4 are linking groups and are each independently a hydrogen atom, a thiol group, a substituted thiol group, an amino group, a substituted amino group, an hydroxy group, a carbonyl group, a substituted carbonyl group, or a hetero-aromatic group, and at least one or more of R2, R3, or R ₄ is attached to the electrically conductive metal. In some embodiments, Ri can be an aliphatic group, R2 and R3 can be sulfur atoms that can be attached to the electrically conductive metal, and R ₄ can be a hydrogen atom. In a preferred embodiment, the organic ligand is -SCH2CH2S- (1,2-ethanedithiol) and at least one of the sulfur atoms is attached to the electrically conductive metal. Ri can be a hetero-aromatic group, and R2 and R4 can each be independently a hetero- aromatic group. R3 can be a sulfur atom and is attached to the electrically conductive metal. In a another instance, the organic ligand is 2,2':6',2"-terpyridine-4'-thiol and the sulfur atom of the thiol group is attached to the electrically conductive metal. At least one of the two linking groups can be complexed with a catalytic metal (e.g., a transition metal). The transition metal can be iridium (Ir), ruthenium (Ru), rhenium (Rh), cobalt (Co), cadmium (Cd), iron (Fe), pallidum (Pd), silver (Ag), platinum (Pt), or cerium (Ce), or any alloy or combination thereof. In a preferred instance, the transition metal can be Pd, Ru, or Fe. Use of a transition metal can lower the overpotential for water oxidation and, thus enhance hole (h ⁺ ) transport. Further, transition metals tend to be a more cost-effective material when compared with noble metals. In a preferred embodiment, the catalyst is a nickel-EDT-Pd catalyst, a Ni-EDT-Ru catalyst, or a Ni-EDT-Fe catalyst. The catalyst of the present invention can be formed as part of an electrode. In a water-splitting process, the catalyst can be used with or without external bias and/or in combination with other catalysts and/or electrodes (e.g., photocatalysts, Ru nanoparticles counter electrode, Pt spurted Ni-mesh counter electrode, and the like). [0008] In another aspect of the invention, processes for making the supported photocataly st of the present invention are described. A process can include contacting a support that includes an electrically conductive metal with a solution that includes the organic ligand of structure (I) under conditions suitable to bond the organic ligand to the electrically conductive metal mesh. The support bonded organic ligand can be contacted with a metal solution that includes a catalytic metal precursor under conditions suitable to complex the catalytic metal of the catalytic metal precursor to the organic ligand. In some embodiments, the electrically conductive support can be subjected to conditions suitable (e.g., acidic conditions) to reduce metal oxides of the electrically conductive metal prior to contact with the organic ligand.

[0009] In another aspect of the invention, methods for producing Fh are described. A method can include providing a current to cathode comprising the catalyst of claims 1 to 10 in an aqueous solution under conditions sufficient to generate hydrogen and hydroxide ions (OH ^" ) from the aqueous solution. The hydroxide ions can be contacted with an anode to generate O2 and electrons. The electrons can be provided to the cathode. The cathode can have a HER overpotential of 20 to 200 mV at a current density of 10 mA/cm ² . In some embodiments, the pH of the aqueous solution is at least 7, and the anode can include the catalyst of the present invention. Such an anode can have an OER overpotential of 50 to 500 mV at a current density of 10 mA/cm ² . In some embodiments, the anode and cathode can include the same catalyst of the present invention (e.g. Fe-SCifcCiFiS-Ni catalyst).

[0010] In the context of the present invention 20 embodiments are described. Embodiment 1 is a surface-immobilized total water-splitting catalyst comprising: (a) an acid-resistant surface comprising an electrically conductive metal; and (b) an organic ligand attached to the electrically conductive metal and complexed with a catalytic metal, wherein the organic ligand has a general structure (I):

R ₃

(I) where: Ri is an aliphatic group, an aromatic group, or a hetero-aromatic group; and R2, R3, R4 are linking groups and are each independently a hydrogen atom, a thiol group, a substituted thiol group, an amino group, a substituted amino group, an hydroxy group, a carbonyl group, a substituted carbonyl group, or a hetero-aromatic group, and at least one or more of R2, R3, or R ₄ is attached to the electrically conductive metal. Embodiment 2 is the catalyst of embodiment 1, wherein the acid-resistant surface is a nickel metal (Ni°) mesh or titanium metal (Ti°) mesh surface. Embodiment 3 is the catalyst of any one of embodiments 1 to 2, wherein Ri is an aliphatic group, R2 and R3 are sulfur atoms, and R ₄ is a hydrogen atom and R2 and/or R3 are attached to the electrically conductive metal. Embodiment 4 is the catalyst of any one of embodiments 1 to 3, wherein the organic ligand has a formula of -SCH2CH2S-, and at least one of the sulfur atoms is attached to the electrically conductive metal. Embodiment 5 is the catalyst of embodiment 1, wherein Ri is a hetero-aromatic group, and R3 is a sulfur atom, and R2 and R ₄ are each independently a hetero-aromatic group and R3 is attached to the electrically conductive surface. Embodiment 6 is the catalyst of embodiment 5, wherein the organic ligand has the structure of:

2,2':6' ,2"-Te rpyridin e-4'-thiol wherein the sulfur atom is attached to the surface of the electrically conductive metal. Embodiment 7 is the catalyst of any one of embodiments 1 to 6, wherein the catalytic metal is a transition metal and at least one of the two linking groups is complexed with the transition metal. Embodiment 8 is the catalyst of embodiment 7, wherein the transition metal is iridium (Ir), ruthenium (Ru), rhenium (Rh), cobalt (Co), cadmium (Cd), iron (Fe), pallidum (Pd), silver (Ag), platinum (Pt), or cerium (Ce). Embodiment 9 is the catalyst of embodiment 8, wherein the transition metal is Pd. Embodiment 10 is the catalyst of embodiment 8, wherein the transition metal is Ru. Embodiment 11 is the catalyst of any one of embodiments 1 tolO, wherein the catalyst is stable at a pH of 0 to 14.

[0011] Embodiment 12 is a process for making the catalyst of any one of embodiments 1 to 11, the process comprising: (a) contacting a support comprising an electrically conductive metal with a solution comprising the organic ligand of structure (I) under conditions suitable to bond the organic ligand to the electrically conductive metal of the support; and (b) contacting the support bonded organic ligand with a metal solution comprising a catalytic metal precursor under conditions suitable to complex the catalytic metal of the catalytic metal precursor to the organic ligand. Embodiment 13 is the process of embodiment 12, wherein the electrically conductive metal support is a nickel metal (Ni°) or titanium metal (Ti°) mesh and the ligand comprises a sulfur atom bonded to the Ni metal or Ti metal. Embodiment 14 is the process of any one of embodiments 11 to 13, wherein the organic ligand solution comprises 2 to 10 vol.% of the organic ligand. Embodiment 15 is the process of any one of embodiments 11 to 14, wherein the catalytic metal precursor comprises a transition metal, and the transition metal is iridium (Ir), ruthenium (Ru), rhenium (Rh), cobalt (Co), cadmium (Cd), iron (Fe), pallidum (Pd), silver (Ag), platinum (Pt), cerium (Ce), preferably Pd, Ru, or Fe. Embodiment 16 is the process of any one of embodiments 11 to 15, wherein, prior to contact with the organic ligand, the electrically conductive support is subjected to conditions suitable to reduce metal oxides of the electrically conductive metal. [0012] Embodiment 17 is an electrochemical process to produce hydrogen (Fh) and oxygen (O2) from water, the electrochemical process comprising: (a) providing a current to a cathode comprising the catalyst of any one of embodiments 1 to 10 in an aqueous solution under conditions sufficient to generate hydrogen and hydroxide ions (OH ^" ) from the aqueous solution; (b) contacting the OH ^" with an anode to generate O2 and electrons; and (c) providing the electrons to the cathode of step (a). Embodiment 18 is the electrochemical process of embodiment 17, wherein the cathode has a hydrogen evolution reaction (HER) overpotential of 20 to 200 mV at a current density of 10 mA/cm ² . Embodiment 19 is the electrochemical process of any one of embodiments 17 to 18, wherein the pH of the aqueous solution is at least 7, and wherein the anode comprises the catalyst of any one of claims 1 to 12 and has an oxygen evolution reaction (OER) overpotential of 50 to 500 mV at a current density of 10 mA/cm ² . Embodiment 20 is the electrochemical process of embodiment 19, wherein the anode and cathode comprise the same catalyst, preferably Fe-SCH2CH2S-Ni.

[0013] Other embodiments of the invention are discussed throughout this application. Any embodiment discussed with respect to one aspect of the invention applies to other aspects of the invention as well and vice versa. Each embodiment described herein is understood to be embodiments of the invention that are applicable to other aspects of the invention. It is contemplated that any embodiment discussed herein can be implemented with respect to any method or composition of the invention, and vice versa. In further embodiments, features from specific embodiments may be combined with features from other embodiments. For example, features from one embodiment may be combined with features from any of the other embodiments. In further embodiments, additional features may be added to the specific embodiments described herein. [0014] The following includes definitions of various terms and phrases used throughout this specification.

[0015] The phrase "single-site catalyst" refers to a catalyst that contains at least one single metal center (e.g. a transition metal). The metal atom can have an open-site or an easily replaceable group that can bind to substrates and a site where bonds are broken and formed. [0016] The terms "attached" and "bonded" are defined to include a chemical bond, which includes a covalent bond, a hydrogen bond, Van der Walls interaction, an ionic bond, a metal- metal bond, or a metal-element (e.g., MS, M-P, M-N) bond.

[0017] The phrase "aliphatic group" refers to an acyclic or cyclic, saturated or unsaturated hydrocarbon group, excluding aromatic compounds. A linear aliphatic group does not include tertiary or quaternary carbons. A branched aliphatic group includes at least one tertiary and/or quaternary carbon. A cyclic aliphatic group is includes at least one ring in its structure. Polycyclic aliphatic groups may include fused, e.g., decalin, and/or spiro, e.g., spiro[5.5]undecane, polycyclic groups. Aliphatic group substituents can include a halogen, a hydroxyl, an alkyoxy, a haloalkyl, a haloalkoxy, a carbonyl, an amine, an amide, a nitrile, an acyl, a thiol, and a thioether group. An aliphatic group as used herein can be referred to as an alkyl group.

[0018] A "carbonyl" refers to a group having a carbon oxygen double bond (i.e, C=0). Non-limiting examples of carbonyl groups are ketones, aldehydes, esters, and carboxylic acids.

[0019] The phrase "aromatic group" refers to a substituted or unsubstituted, mono- or polycyclic hydrocarbon with alternating single and double bonds within each ring structure. Aromatic group substituents can include an alkyl, a halogen, a hydroxyl, an alkyoxy, a haloalkyl, a haloalkoxy, a carbonyl an amine, an amide, a nitrile, an acyl, a thiol, and a thioether group.

[0020] The phrase "hetero-aromatic group" refers to a mono-or polycyclic hydrocarbon with alternating single and double bonds within each ring structure, and at least one atom within at least one ring is not carbon. Hetero-aromatic group substituents can include an alkyl, a halogen, a hydroxyl, an alkyoxy, a haloalkyl, a haloalkoxy, a carbonyl, an amine, an amide, a nitrile, an acyl, a thiol, and a thioether group.

[0021] The terms "about" or "approximately" are defined as being close to as understood by one of ordinary skill in the art. In one non-limiting embodiment, the terms are defined to be within 10%, preferably within 5%, more preferably within 1%, and most preferably within 0.5%.

[0022] The terms "wt.%," "vol.%," or "mol.%" refers to a weight, volume, or molar percentage of a component, respectively, based on the total weight, the total volume of material, or total moles, that includes the component. In a non-limiting example, 10 grams of component in 100 grams of the material is 10 wt.% of component. [0023] The term "substantially" and its variations are defined to include ranges within 10%, within 5%, within 1%, or within 0.5%.

[0024] The terms "inhibiting" or "reducing" or "preventing" or "avoiding" or any variation of these terms, when used in the claims and/or the specification includes any measurable decrease or complete inhibition to achieve a desired result. [0025] The term "effective," as that term is used in the specification and/or claims, means adequate to accomplish a desired, expected, or intended result.

[0026] The use of the words "a" or "an" when used in conjunction with any of the terms "comprising," "including," "containing," or "having" in the claims, or the specification, may mean "one," but it is also consistent with the meaning of "one or more," "at least one," and "one or more than one."

[0027] The words "comprising" (and any form of comprising, such as "comprise" and "comprises"), "having" (and any form of having, such as "have" and "has"), "including" (and any form of including, such as "includes" and "include") or "containing" (and any form of containing, such as "contains" and "contain") are inclusive or open-ended and do not exclude additional, unrecited elements or method steps.

[0028] The catalysts of the present invention can "comprise," "consist essentially of," or "consist of particular ingredients, components, compositions, etc. disclosed throughout the specification. With respect to the transitional phase "consisting essentially of," in one non- limiting aspect, a basic and novel characteristic of the catalysts of the present invention are their abilities to catalyze electrochemical water-splitting to produce H2 and O2. [0029] Other objects, features and advantages of the present invention will become apparent from the following figures, detailed description, and examples. It should be understood, however, that the figures, detailed description, and examples, while indicating specific embodiments of the invention, are given by way of illustration only and are not meant to be limiting. Additionally, it is contemplated that changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Advantages of the present invention may become apparent to those skilled in the art with the benefit of the following detailed description and upon reference to the accompanying drawings.

[0031] FIGS. 1A and IB depict schematic of the surface-immobilized catalyst of the present invention.

[0032] FIG. 2 depicts a schematics of the process to produce the surface-immobilized catalysts of the present invention. [0033] FIG. 3 is a schematic of a two-electrode electrolysis system of the present invention for total water-splitting.

[0034] FIG. 4 shows acitivites of the catalysts of the present invention at 10 m A/cm ² under various pH conditions.

[0035] FIG. 5 shows chronopotentiometric curves of the Ir-EDT-Ni-mesh of the present invention and Co-EDT-Ni-mesh of the present invention at constant current density of 10 mA/cm ² .

[0036] FIGS. 6A and 6B show X-ray photoelectron spectroscopy (XPS) spectrum of Ir- EDT-Ni mesh: (6A) S 2p before, after HER and after OER; (6B) Ir 4f before, after HER and after OER. [0037] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings. The drawings may not be to scale.

DETAILED DESCRIPTION OF THE INVENTION

[0038] A discovery has been made that addresses at least some of the problems associated with electrochemical water-splitting to produce H ₂ (g) and 0 ₂ (g). The discovery is premised on a catalyst structured such that it has an acid-resistant surface having an electrically conductive metal and a catalytic metal. An organic ligand having general structure (I) can be used to serve as a linker between the electrically conductive metal and the catalytic metal. The catalysts of the present invention, as described and exemplified in a non-limiting manner the Examples section, are stable at various pH conditions (e.g., 0 to 14) and/or have enhanced OER and HER as compared to currently known catalysts. The catalysts of the present invention can be used with or without an electrical bias. By way of example, the catalysts can be used with a photocatalyst that generates sufficient electrons to continue the anodic reaction.

[0039] These and other non-limiting aspects of the present invention are discussed in further detail in the following sections with reference to the figures. A. Catalyst

[0040] The surface immobilized catalysts of the present invention can include an acid- resistant surface that includes an electrically conductive metal with an organic ligand attached to the conductive metal and complexed to a catalytic metal. The catalytic metal can be a single- site metal. In a preferred embodiment, the acid-resistant surface is the electrically conductive metal. By way of example, the acid-resistant surface can be a nickel, titanium, or nickel- titanium metal mesh. In other embodiments, the acid-resistant surface can be a non-oxide metal semiconductor. Non-limiting examples of non-oxide metal semi-conductors include cadmium (Cd), cadmium zinc sulfide (CdZnS), cadmium nickel sulfide (CdNiS), indium nitride (InN), gallium nitride (GaN), indium gallium nitride (InGaN), indium arsenide (InAs), gallium arsenide (GaAs), indium gallium arsenide (InGaAs), indium phosphide (InP), gallium phosphide (GaP), indium gallium phosphide (InGaP), aluminum gallium indium phosphide (AlGalnP), indium aluminum phosphide (InAlP), or combinations thereof. In these semiconductors, the organic ligand of the present invention attaches to the metal of the semiconductor. The acid-resistant surface contains little to no metal oxides prior to use in a water-splitting reaction. In some embodiments, the organic ligands are covalently bonded to the metal surface. The ligand can include at least two linker groups where one linker group can passivate the surface of non-oxide metal bonds (e.g., Ti or Ni dangling bonds) by filling them with-atoms that form covalent bonds with the metal (e.g., Ni-S and/or Ti-S) and simultaneously act as a ligand for attaching metal ion catalysts. Surface treatment of the acid- resistant surface can inhibit or reduce the formation of surface oxides that are the source of chemical corrosion and instability, and, thus, providing the advantages of catalyst longevity, and/or pH stability. For example, surface treating the electrically conductive metal surface with a short-chain sulfur-containing compound (e.g., 1,2-di thiol) can terminate the surface of non-oxide metal bonds (e.g., Ni or Ti dangling bonds) by filling the surface with S-atoms that form covalent bonds with the metal (e.g., Ni and/or Ti) which passivates the surface of substrate. Together with the ligand passivation, attachment of a metal catalyst (e.g., a catalytic transition metal) can effectively suppress charge/hole recombination on semiconductor substrates (which can alleviate the effects of Fermi level pinning), lower the reaction overpotential, and enhance heterogeneous reaction kinetics across an electrode interface.

[0041] Referring to FIGS. 1A and IB, catalyst 100 is depicted with an organic ligand having at least two linker groups attached to one side of support 102 having an acid-resistant surface that includes an electroconductive metal (FIG. 1A with R ₂ and R ₃ shown as linker groups), or to two sides of the acid-resistant surface (FIG. IB and R ₂ , R ₃ and R ₄ shown as linker groups). For ease in illustration, an ethylene carbon chain is shown, however, it should be understood that any of the organic ligands of the present invention can be attached to the linker group. Catalytic metal (M) can be coordinated with a linking group (e.g., R ₂ and R ₃ , R ₂ and R ₄ , R ₃ and R ₄ , or R ₂ , R ₃ and R ₄ ) of the ligand as described throughout this specification. As shown in FIG. 1 A, support 102 can be a metal mesh (e.g., material having a web-like, net-like, or mesh-like pattern such that a web-like, net-like, or mesh-like metal network is formed). It should be understood that organic ligands can be attached to electrically conductive metals on all sides of the support. The catalytic metal can be a transition metal. Non-limiting examples of transition metals include metals from Columns 8-12 of the Periodic Table. In some instances, the metal can be iridium (Ir), ruthenium (Ru), rhenium (Rh), cobalt (Co), cadmium (Cd), iron (Fe), pallidum (Pd), silver (Ag), platinum (Pt), or cerium (Ce), or combinations or alloys thereof. In a preferred instance, the metal is Pd, Ru, or Fe, or combinations thereof, or alloys thereof. [0042] The organic ligand can have at least two linker groups (e.g., R ₂ and R ₃ , R ₂ and R ₄ , R ₃ and R ₄ , or R ₂ , R ₃ and R ₄ ) and have the structure of: where: Ri is an aliphatic group, an aromatic group, or a hetero-aromatic group; and R2, R3, R4 are linking groups and are each independently a hydrogen atom, a thiol group, a substituted thiol group, an amino group, a substituted amino group, an hydroxy group, a carbonyl group, a substituted carbonyl group, or a hetero-aromatic group, and at least one or more of R2, R3, or R ₄ is attached to the electroconductive metal. Ri can have 1 to 15, 2 to 10, 3 to 5, or 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or any range there between of carbon atoms. When Ri is an aliphatic group, R ₄ can be hydrogen, and Ri can have the formula of (0¾)η where n is 1 to 15, preferably 2 to 5, more preferably 2 (e.g., disubstituted ethane). Ri, R2, R3, and/or R ₄ can be a substituted phenyl group (aromatic group), a nitrogen containing aromatic group, an oxygen- containing aromatic group, or a sulfur-containing aromatic group (e.g., thiophene group). In a preferred embodiment, Ri, R2, and R ₄ are nitrogen-containing hetero-aromatic compounds. Non-limiting examples of nitrogen-containing hetero-aromatic compounds include pyridines, pyrroles, or triazines, preferably 1, 3, 5-triazine. Representative structures of aromatic and hetero-aromatic groups. In these structures, R3 can be bonded to a carbon atom in the structure or be a heteroatom in the ring structure.

Phenyl Pyridine Pyrrole 1 ,3 ,5-Triazine Thiophe ne

R2, R3, and/or R ₄ can be a hydrogen (H) atom, a thiol group (— SH), an amino group (— NH2), a hydroxyl (— OH), a carboxylic acid (— COOH), an ester (— CO2R5, where R5 is aliphatic group, an aromatic group, or a hetero-aromatic group), or an amide (— CONH2) group. In a preferred embodiment, R2 and R3 are thiol groups. Non-limiting examples of organic ligands with a sulfur linking group are 1,2-ethanedithiol (HSCH2CH2SH (R3 and R2 = S, R ₄ =H and Ri is CH2) and 2,2':6',2"-terpyridine-4'-thiol shown below (R3=S, and Ri, R ₄ , and R2 are triazole), where the sulfur atom is attached (bonded) to the electrically conductive metal.

[0043] The amount of organic ligand and/or the catalytic metal to be used can depend, inter alia, on the catalytic activity of the catalyst. In some embodiments, the amount of organic ligand in the photocatalyst can be up to 2 wt.%, or from 0.0001 to 2 wt.% or 0.1 to 1.5 wt.%, or 0.5 to 1 wt.%) or any value or range there between, based on the total weight of the photocatalyst. The catalytic metal present in the catalyst be up to 3 wt.%, or from 0.0001 to 3 wt.%) or 0.1 to 2.5 wt.%, 0.5 to 2 wt.%, or 1 wt.% to 1.5 wt.% or any value or range there between, based on the total weight of the photocatalyst.

B. Preparation of the Catalyst

[0044] The surface-immobilized catalyst of the present invention can be made as exemplified in the Examples. Referring to FIG. 2, a schematic of the process 200 to prepare the surface-immobilized catalyst is depicted. In step 1, substrate 202 can be obtained from a commercial supplier or manufactured. By way of example, a nickel mesh can be cut with a laser to a desired sized {e.g., 0.1 x 2 cm, or 0.5 x 1 cm, or the like). Substrate 202 can include electrically conductive metal 204 (shown by hatches) and metal oxide layer 206. Substrate 202 can be contacted with a reducing solution that removes metal oxide layer 206 to produce substrate 102 having an acid-resistant surface 208 that includes electrically conductive metal 204. A non-limiting example of a reducing solution can include an alcoholic solution of an acid {e.g., acetic acid, citric acid, hydrochloric acid, or the like in methanol or ethanol). The solution can include greater than, equal to or any range between any two of: 25, 30, 35, 40, 45, 50, 55, 60 vol.% acid. In some embodiments, metal oxide layer 206 is not present so step 1 is not necessary. In step 2, Substrate 208 can be contacted {e.g., immersed, sprayed, dipped, rolled) with ligand solution 210 for a time sufficient to bond Lg to electrically conductive metal 204 of acid-resistant surface 208 to form ligand immobilized material 212 and to functionalize {e.g., passivate) acid-resistant surface 208. Substrate 208 can be contacted with organic ligand solution 210 at 20 to 35 °C, or about 20 °C to 40 °C, or 25 °C to 35 °C, or at ambient temperature for a desired period of time {e.g., 0.1 hour to 24 hour, or 0.5 to 10 hour, or 5 to 6 hours). Organic ligand solution 210 can be a neat solution (e.g., 100 vol.% organic solution) or a mixture of organic ligand and solvent (e.g., methanol, ethanol, propanol, acetonitrile etc.). The volume of organic ligand in the solvent can range from 0.5 vol.% to 99 vol.%, from 1 vol.%) to 50 vol.%), 10 vol.%) to 40 vol.%> or about 5 vol.%>, or any value or range there between. In instances, where only portions of surface 208 are functionalized, only that portion is contacted with the organic ligand solution 210. In one embodiment, substrate 102 is dipped in organic ligand solution 212. In step 3, immobilized ligand material 212 can be contacted with catalytic metal (M) precursor solution 214 to complex the catalytic metal (M) with Lg (e.g., sulfur atom) of the organic ligand at 20 to 35 °C, or about 25 °C to 30 °C for a desired period of time (e.g., 0.5 hour to 24 hour, or 0.5 to 10 hour, or 0.5 to 1 hour) to form surface immobilized catalyst 100. In this step, the catalytic metal precursor ligands (e.g., halides or nitrites) can be a metal halide, a metal nitrate, a metal chlorite, a metal hydroxide dissolved in a solvent such as aqueous alcohol or aqueous acetonitrile solution (e.g., 1 : 1 to 10: 1, 2: 1, to 8: 1, or 3 : 1 to 6: 1, or about 5: 1 organic solvent to water). Non-limiting examples of metal precursors include metal chlorides and metal nitrates. The solvent can include at least 1 x 10 ^"6 to 10 x 10 ^{" 6} , 1 x 10 ^"6 to 5 x 10 ^"6 x or about 3 x 10 ^"6 moles of catalytic metal precursor. The volume of the catalytic metal precursor in the solvent can be 0.1 to 10 vol.%>, 1 to 8 vol.%>, or about 5 vol.%>, or any value or range there between. By way of example, Pd catalysts can be attached to the sulfur linking groups of the organic ligand by immersing the ligand immobilized material 212 for 30 min in 10 mg PdCh dissolved in 1 mL acetonitrile to form surface-immobilized catalyst 100 having Pd complexed by at least on linker group (e.g. R ₂ in the Fig.) of the organic ligand. Surface immobilized catalyst 100 can be dried under at flow of nitrogen at 20 to 40 °C or about ambient temperature. Such a method of making creates a small mono-layer of catalytic metal on the acid-resistant surface. Thus, providing a cost effective method of making the catalytic system when employing noble metals.

C. Systems and Method of Use

[0045] The surface immobilized catalyst of the present invention can be used in a system to produce H ₂ and 0 ₂ from water. FIG. 3 depicts system 300 for electrochemical water- splitting. System 300 includes anode 302, cathode 304, and aqueous solution 306, membrane 308, in container 310. Anode 302 and cathode 304 are connected to energy storage device 312. Aqueous solution 306 can include water at a desired pH. The pH of the solution can be greater than, equal to, or between any two of: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14. The pH of the aqueous solution can be controlled by the known techniques. For example, addition of a buffer compound, an acid compound or a base compound (e.g., phosphate buffer, sulfuric acid, or sodium hydroxide) to water to obtain the desired pH. Anode 302 can include surface- immobilized catalyst 100 attached to inert electrode material 314. By way of example, a nickel mesh containing an EDT-Pd material can be attached to an inert electrode. Cathode 304 can include surface-immobilized catalyst 100' attached to inert electrode material 314. Surface- immobilized catalyst 100' can be the same or different than surface-immobilized catalyst 100. Energy storage device 312 (e.g., a battery) can provide current to cathode 304 to catalyze the splitting of water to generate H ₂ and OH ^" . H ₂ stream 3 16 can exit container 310 and be collected, stored, transported, or a combination thereof. The OH ^" can permeate membrane 308 contact with anode 302 to generate 0 ₂ , H ⁺ ions, and electrons (e ^~ ). Generated electrons (e ^~ ) travel can be provided to energy storage device 312 and/or to cathode 304. 0 ₂ stream 318 can exit container 310 and be collected, stored, transported, or a combination thereof. The streams can also be sold as products. H ⁺ ions can permeate membrane 308 and travel to cathode 304.

[0046] Depending on the pH and voltage applied, various surface-immobilized catalysts can be used. The surface-immobilized catalysts can generate low HER overpotentials at 10 mA/cm ² . As exemplified in the Examples, at a pH of 0 the surface-immobilized catalyst on the cathode can include EDT-Pd (HER = 40 mV), EDT-Ce (160 mV), or EDT-Co (HER = 180 mV). At a pH of 7, the surface-immobilized catalyst can include EDT-Ag (HER = 210 mV) or EDT-Ce (HER = 240 mV). At a pH of 14, the surface-immobilized catalyst can include EDT-Fe (HER = 130 mV) or EDT-Pd (HER= 20 mV). In another example, at a pH 7, the surface-immobilized catalyst on the anode can include EDT-Co (OER = 500 mV) or EDT-Rh (OER = 500 mV). At a pH of 14, the surface-immobilized catalyst on the anode can include EDT-Ru (OER = 90 mV) or EDT-Fe (OER = 160 mV).

[0047] In some embodiments, the surface-immobilized catalyst is used in combination with a photocatalyst for photoelectrolysis of water. By way of example, the cathode can be the catalyst of the present invention (e.g., Ni-EDT-Pt catalyst) and the anode can be a photoanode that includes an InGaN nanowire based and GaAs/InGaAs multi -junction based photocatalyst.

EXAMPLES

[0048] The present invention will be described in greater detail by way of specific examples. The following examples are offered for illustrative purposes only, and are not intended to limit the invention in any manner. Those of skill in the art will readily recognize a variety of noncritical parameters which can be changed or modified to yield essentially the same results.

Example 1

(Synthesis Surface-Immobilized Catalysts of the Present Invention)

[0049] General Information: All the reagents were purchased from commercial sources and in analytical or reagent grade when possible. All of the chemicals either nitrate or acetonitrile soluble chlorite except ethylene dithiol (EDT). The laser cut Ni-mesh with the size of (0.5 cm x 1.0 cm) was used as the substrate. Stock solutions of catalytic metal sources-(C): 10 mg/mL in acetonitrile (CH3CN) solutions of various metals nitrites or chlorites were prepared. A stock solution of EDT in methanol (B): 5 v/v(%) EDT in methanol solution was prepared. A stock solution for cleaning Ni-mesh (A): 50 v/v(%) acetic acid in methanol was prepared.

[0050] General Method: At ambient temperature (about 25 to 30 °C) and pressure, the following procedure was used to produce the surface-immobilized catalysts of the present invention. The laser cut nickel mesh was dipped into solution A for 10 sec to remove native metal oxides from the surface of the metal and produce a cleaned nickel mesh. The cleaned nickel mesh was dipped into stock solution B for 6 hours to from an EDT-Ni mesh. The EDT- Ni mesh was dipped into stock solution C for 0.5 hours to form catalytic metal -EDT-Ni mesh material of the present invention.

Example 2

(HER and OER Performance using the Catalyst of the Present Invention)

[0051] Analytical equipment: The liner scan voltammagram (LSV) measurements were conducted with Bio-Logic VMP3 multichannel potentiostat/galvanostat with a built in EIS analyzer (Bio-Logic Science Instruments, France). The working electrodes were catalysts prepared in Example 1, with a surface area of 0.5 cm ² , the counter electrode for HER was a Pt spurted Ni-mesh and for OER was a Ru nanoparticles attached Ni-mesh. Ag/AgCl was used as the reference electrode. Electrode solutions were pH=0 (1M H2SO4), pH=7 (phosphate buffer) and pH=14 (lM NaOH).

[0052] The catalytic performances for HER and OER of the catalysts of the present invention at three different pH were measured by using LSV. The results are shown in FIG. 4 and listed in Table 1. FIG. 4 is a graphical depiction of activities of the catalysts of the present invention at 10 mA/cm ² under various pH conditions. The initial oxidation state is that of the metal cations as prepared. Table 1

[0053] Among the HER molecule catalysts, Ir-EDT, Pt-EDT, and Pd-EDT operated at 10 mA/cm ² with over potentials less than 30 mV in acidic solution. Among the OER catalysts in basic condition, non-noble metal based EDT-metal based complexes such as Fe-EDT and Ni- EDT, noble metal based catalysts such as Rh-EDT and Ru-EDT at 10 mA/cm ² current densities had over potentials of less than 200 mV, which are the better than those reported in the literature (See, McCrory, C. C. L. et al., "Benchmarking Hydrogen Evolving Reaction and Oxygen Evolving Reaction Electrocatalysts for Solar Water Splitting Devices," Journal of the American Chemical Society 2015, Vol. 137, pp. 4347-4357 (the best non-noble OER = 0.33 for 10 mAcm ^"2 in 1M NaOH) and Xu et al., "A nickel iron diselenide-derived efficient oxygen- evolution catalyst", Nature Communications 2016, Vol. 7, pp. 12324 (OER = 195 mV for 10 mAcm ^"2 using a nickel iron diselenide in 1M KOH). At neutral conditions (pH = 7), Ce-EDT and Rh-EDT catalysts of the present invention showed the lowest over potentials.

Example 3

(pH Stability of the Catalysts of the Present Invention)

[0054] The stabilities of the Ir-EDT-Ni-mesh and Co-EDT-Ni-mesh were evaluated by time-dependent potential change in a pH = 7 buffer solution. In these tests, current drops in both HER and OER measurements were not observed within 10 hours. FIG. 5 depicts chronopotentiometric curves of the Ir-EDT-Ni-mesh and Co-EDT-Ni-mesh at constant current density of 10 mA/cm ² .

Example 4

(Characterization of the EDT-Ir on Nickel Mesh Catalyst Before and After testing) [0055] XPS spectra of the Ir-EDT-Ni mesh were obtained before and after HER and OER testing. FIG. 6 shows the Ir 4f ₇ /2 and Ir 4f ₅ / ₂ XPS data for the freshly deposited EDT-Ir on Ni- mesh, giving peak binding energies of 62.3 and 65.3 eV respectively, same as the value expected for Ir ³⁺ (62.3 and 65.2 eV). After the over potential measurement for HER and OER, the Ir peaks were shifted to 61.8 and 64.8 eV. Without wishing to be bound by theory, it is believed this was from structural changes of the Ir complex on the Ni mesh surface. In addition, the Ir contents decreased after both HER and OER reactions owning to release of physically absorbed Ir-EDT complex, which were not connected to the Ni surface through Ni-S bond. Since no decease in activities in both OER and HER were observed within 10 hour of stability tests (FIG. 5), it is believed that the active site was chemically attached to the surface. The peaks at about 68 eV were assigned for Ni 3p. There are were no obvious changes on the position of S 2p, however, the peak intensities decreased, which confirmed dissolution of physically attached EDT on the surface. Furthermore, no S 2p peaks between 166-169 eV were detected, which indicated that S atoms were bound to the Ni in the thiol form and that no sulfate species were formed through oxidation of the sulfur during the catalytic cycle.