CATALYST FOR HYDROGEN PRODUCTION

Provided herein are catalysts for producing hydrogen via the hydrogen evolution reaction (HER) during water splitting, methods of producing hydrogen via photocatalytic water splitting using the catalysts, and compositions for use in photocatalytic water splitting that include the catalysts. In some embodiments, a catalyst hereof is a metal complex of Formula I,

[M(L1)(L2)][A]   Formula I

wherein M is a transition metal, L1 and L2 are both ligands independently forming one or more coordinate bonds with the metal M, and A is an anion, and
        wherein L1 is a tetrapyridyl-amine (Py4N) having four pyridyl groups and an amine group each forming a coordinate bond with the metal M.

BACKGROUND

Water splitting is a process used to produce hydrogen, H2, from water with oxygen, O2, as a byproduct. Water splitting is currently a focus in clean and sustainable energy efforts as it may provide the key to implementing widespread usage of hydrogen as an industrial energy source.

Water splitting generally involves decomposition of water into oxygen and hydrogen via two half-reactions: the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). The OER involves the oxidation of water to oxygen. The HER involves the reduction of protons to hydrogen. The OER is thermodynamically more favorable at basic conditions, while the HER is thermodynamically preferred at low pHs.

A significant challenge to leveraging water splitting on a large scale are the reaction kinetics of the OER and the HER and, more particularly, the overpotentials for the OER and the HER that create kinetic energy barriers for driving water splitting. Catalysis is generally required to minimize the overpotentials for the OER and the HER. For example, the water splitting reactions may be driven by electrolysis using electric energy, thermolysis using thermal energy, or photolysis using sunlight or solar energy. Electrolysis and/or photolysis may be particularly advantageous water splitting techniques for producing clean hydrogen energy using clean and/or energy sources from electricity and/or sunlight.

Metal complexes based on cobalt, nickel, and iron have been developed for electro- and photocatalytic hydrogen production. The efficiency of electro- and photocatalysts for the HER is strongly pH-dependent. Only a very limited number metal complexes are known for catalyzing HER under basic conditions. The advantage of performing water splitting under basic conditions is that the OER is energetically much more challenging than the HER where, compared to HER which involves a two electron/two proton process, the OER involves a four electron/four proton process. Therefore, it would be thermodynamically advantageous to provide more efficient and effective catalysis of HER at basic conditions that are preferred for the OER when the water splitting is occurring in the same media from the coupling of the OER to the HER.

A shortcoming of metal complexes that have been reported for catalyzing the HER under basic conditions is that the conditions also include a presence of organic solvents. The highly oxidizing conditions needed for the OER suggest that organic solvents should be avoided for photocatalysis.

A need exists in the industry for catalysts that can drive the HER in basic or alkaline environments that include purely aqueous compositions in the absence of any organic solvent.

SUMMARY

Embodiments of the present disclosure relate to catalysts for producing hydrogen via the hydrogen evolution reaction during water splitting. The catalysts advantageously have hydrogen evolution reaction activity in basic compositions that are thermodynamically favorable toward the oxygen evolution reaction via oxidation of water in the water splitting process. In this way, the catalysts hereof may facilitate the hydrogen evolution reaction and the oxygen evolution reaction occurring in mutually compatible compositions.

In one independent aspect, provided herein is a catalyst for producing hydrogen via the hydrogen evolution reaction during water splitting, wherein the catalyst is a metal complex of Formula I,

[M(L1)(L2)][A]   Formula I

In some aspects, the amine group of Py4N is a tertiary amine group.

In some aspects, the tetrapyridyl-amine (Py4N) has the Formula II and R1, R2, R3, and R4 are each independently selected from the group consisting of C1-C6 alkyl.

In some aspects, the tetrapyridyl-amine (Py4N) has the Formula II, R1, R2, R3, and R4 are each a C1 alkyl, and the ligand L1 is of Formula IIa,

In some aspect, the catalyst is a metal complex of Formula Ia,

In some aspects, the catalyst is the metal complex of Formula Ia and X and Y are the same integer between 1 to 3.

In some aspects, the catalyst is the metal complex of Formula Ia and the metal M is cobalt.

In some aspects, the catalyst is a cobalt complex of Formula Ia-i,

In some aspects, the catalyst is a cobalt complex of Formula Ia-ii,

In another independent aspect, provided herein is a method of producing hydrogen via photocatalytic water splitting, the method comprising contacting an aqueous composition with a catalyst as described herein.

In some aspects, the catalyst has photolytic hydrogen evolution reaction activity in the aqueous composition to drive the hydrogen evolution reaction in response to light exposure.

In some aspects, the catalyst is a metal complex of Formula I as described herein. In some aspects, the ligand L1 is a tetrapyridyl-amine (Py4N), such as the tetrapyridyl-amine of Formula II.

In some aspects, the catalyst is a cobalt complex of Formula I as described herein, wherein the ligand L1 is a tetrapyridyl-amine (Py4N), such as the tetrapyridyl-amine of Formula II, and the metal M is cobalt, such as Co(II) or Co(III).

In some aspects, the catalyst is a metal complex of Formula Ia as described herein.

In some aspects, the catalyst is a cobalt complex of Formula Ia as described herein, wherein the metal M is cobalt, such as Co(II) or Co(III).

In some aspects, the catalyst is a cobalt complex of Formula Ia-I as described herein, a cobalt complex of Formula Ia-ii as described herein, or a combination thereof.

In some aspects, the aqueous composition is configured such that the oxygen evolution reaction in the aqueous composition is thermodynamically favored in response to the light exposure over the hydrogen evolution reaction in the absence of the catalyst in the aqueous composition.

In some aspects, the aqueous composition is an aqueous alkaline composition having a pH of greater than 8.

In some aspects, the aqueous composition is an aqueous alkaline composition comprising the water as a primary solvent.

In some aspects, the aqueous alkaline composition is substantially free of organic solvents and/or non-aqueous solvents.

In some aspects, the method further comprises exposing the catalyst to light via a light source to drive the hydrogen evolution reaction. In some aspects, the light source comprises sunlight, one or more UV lamps, one or more Xenon lamps, or a combination thereof.

In another independent aspect, provided herein is a composition for use in photocatalytic water splitting. The composition comprises water and a catalyst as described herein.

In some aspects, the catalyst has photolytic hydrogen evolution reaction activity in the aqueous composition to drive the hydrogen evolution reaction in response to light exposure.

In some aspects, the catalyst is a metal complex of Formula I as described herein. In some aspects, the ligand L1 is a tetrapyridyl-amine (Py4N), such as the tetrapyridyl-amine of Formula II.

In some aspects, the catalyst is a cobalt complex of Formula I as described herein, wherein the ligand L1 is a tetrapyridyl-amine (Py4N), such as the tetrapyridyl-amine of Formula II, and the metal M is cobalt, such as Co(II) or Co(III).

In some aspects, the catalyst is a metal complex of Formula Ia as described herein.

In some aspects, the catalyst is a cobalt complex of Formula Ia as described herein, wherein the metal M is cobalt, such as Co(II) or Co(III).

In some aspects, the catalyst is a cobalt complex of Formula Ia-I as described herein, a cobalt complex of Formula Ia-ii as described herein, or a combination thereof.

In some aspects, the composition is configured such that the oxygen evolution reaction in the composition is thermodynamically favored in response to light exposure over the hydrogen evolution reaction in the absence of the catalyst in the composition.

In some aspects, the composition is an aqueous alkaline composition comprising the water as a primary solvent, wherein the aqueous alkaline composition is substantially free of organic solvents and/or non-aqueous solvents.

Objects, features, and advantages will become in part apparent and in part pointed out when reading the present disclosure in its entirety.

DETAILED DESCRIPTION

Provided herein are catalysts having photolytic hydrogen evolution reaction (HER) activity. The HER is a key reaction in water splitting for producing hydrogen as an alternative energy source. The ability to produce hydrogen using photocatalysis may provide an optimal solution to environmental and energy conservation as it allows the production of a clean energy source, hydrogen, from a renewable energy source, light. In this way, the catalysts hereof may address the shortcomings of current solar-driven water splitting processes and may provide economic, eco-friendly, and sustainable pathways toward widespread use of hydrogen energy on an industrial scale.

Advantageously, the catalysts provided herein can catalyze photolytic HER in an alkaline or basic environment. In this way, the catalysts can be used in environments that are thermodynamically favorable toward the OER via oxidation of water in the water splitting process. The catalysts hereof may thereby facilitate the HER and the OER occurring in mutually compatible compositions. Moreover, in embodiments, the catalysts may function in aqueous environments that may be substantially free of organic solvent and/or non-aqueous solvents. In embodiments, the catalysts may be functional in aqueous environments having a pH of greater than 8, such as a pH between 8 to 10, or a pH of about 9.

In some embodiments, the catalysts include molecular metal complexes such as cobalt complexes. Without being bound by a particular theory, density functional theory calculation suggests that the metal complexes may facilitate a modified electron transfer (E)—proton transfer (C)—electron transfer (E)—proton transfer (C) (mod-ECEC) pathway for hydrogen production from the protonation of Metal cation—H species. For example, cobalt complexes may provide the mod-ECEC pathway for hydrogen production from the protonation of Co(II)—H species.

Surprisingly, molecular metal complexes hereof have high photocatalytic HER activity in basic aqueous compositions and significantly outperform other catalysts in neutral and basic compositions. Certain molecular metal complexes are demonstrated herein to drive photocatalytic HERs in purely aqueous compositions with a turnover number (TON) of 218,000 and a turnover frequency (TOF) of 12500/hour over the course of several hours (e.g., five hours). The remarkable photocatalytic activity of the catalysts hereof may result from subtle structural change of the ligand scaffold in the metal complex. This may signal the importance of structure-function relationships in the molecular catalyst design and provides a pathway to further development of the photocatalytic HER in alkaline or basic environments. The present disclosure thereby significantly advances the development of molecular metal catalyst for solar- or light-driven HER in more challenging alkaline aqueous compositions that may facilitate significant advances in solar-driven water-splitting systems.

Terms and Definitions

Terms used in this description have the following meanings unless expressly stated otherwise or the context clearly indicates otherwise.

The “HER” means the hydrogen evolution reaction, or half reaction, that is involved in water splitting to generate hydrogen, H2, via reduction of protons.

The “OER” means the oxygen evolution reaction, or half reaction, that is involved in water splitting to generate oxygen, O2, via oxidation of water.

Co(I), CoI, and CoI+ refer to the +1 cation of Cobalt (Co). Co(I), CoI, and CoI+ may be used interchangeably.

Co(II), CoII, and Co2+ refer to the +2 cation of Cobalt (Co). Co(II), CoII, and Co2+ may be used interchangeably.

Co(III), CoIII, and Co3+ refer to the +3 cation of Cobalt (Co). Co(III), CoIII, and Co3+ may be used interchangeably.

“Py” used in the context of chemical compounds means pyridine or a pyridyl (or pyridinyl) group.

“Bpy” used in the context of chemical compounds means bipyridine or a bipyridyl or bipyridinyl group, for example, 2,2′-bipyridine or a 2,2′-bipyridyl (or 2,2′-bipyridinyl) group. Unless expressly stated otherwise, the presence of at two pyridyl or Py groups in a chemical compound does not imply that the chemical compound could include a bipyridyl or Bpy group.

“DPA” used in the context of chemical compounds means dipicolyl amine.

“Me” used in the context of chemical compounds means methane or a methyl group.

Chemical elements are identified using their ordinary element symbols.

“DPA-Bpy” used in the context of chemical compounds means N,N-bis(2-pyridinyl-methyl)-2,2′-bipyridine-6-methanamine.

“Py3Me-Bpy” used in the context of chemical compounds means 6-[6-(1,1-di-pyridin-2-yl-ethyl)-pyridin-2-ylmethyl]-[2,2′]bipyridinyl.

“Py4NMe” used in the context of chemical compounds means 1-(6-(1,1-di(pyridin-2-yl)ethyl)pyridin-2-yl)-N-methyl-N-(pyridin-2-ylmethyl)methanamine.

“Alkyl” used in the context of chemical compounds refers to a straight or branched chain moiety comprising up to 10 carbon atoms. Non-limiting examples of suitable alkyl groups include methyl, ethyl, propyl, butyl, pentyl, and hexyl. The alkyl group may be a straight-chain alkyl group or a branched alkyl group (e.g., isopropyl). In some embodiments, the alkyl group is optionally independently substituted with one or more substituents (e.g., methyl, ethyl, methoxyl, carboxyl, etc.).

“Aryl” used in the context of chemical compounds refers to an aromatic moiety comprising from 6 to 14 carbon atoms. The aryl group may be optionally independently substituted with one or more substituents (e.g., methyl, ethyl, methoxyl, carboxyl, etc.). Non-limiting examples of suitable aryl groups include phenyl, naphthyl, benzyl, tolyl, and xylyl.

“Alkoxyl” used in the context of chemical compounds refers to a group of the form —OR′, wherein R′ is alkyl as defined herein. For example, the group —OCH3 may be referred to herein as “methoxyl.” The group —OCH2CH3 may be referred to herein as “ethoxyl.”

“Aryloxyl” used in the context of chemical compounds refers to a group of the form —OR′, wherein R′ is aryl as defined herein. For example, the group —O(C6H6) may be referred to herein as “phenoxyl.”

“Carboxyl” used in the context of chemical compounds refers to a group of the form —C(O)OH.

“Hydrogen” used in the context of chemical compounds includes both stable isotopes of hydrogen, namely 1H (also known as protium) and 2H (also known as deuterium). “Hydrogen” used in the context of the product of the HER refers to H2.

“TON” used in the context of catalytic properties means the catalyst turnover number.

“TOF” used in the context of catalytic properties means the catalyst turnover frequency.

“Aqueous” used in the context of compositions, solvents or solutions refers to compositions, solvents or solutions that include water as the primary solvent. Aqueous compositions, solutions or solvents of the present disclosure may be substantially free of organic solvents and/or non-aqueous solvents.

An “aqueous composition” or “aqueous solution” can include water as the sole component or optionally can include one or more components in addition to water.

“Metal complex,” “complex” and like terms mean a coordination compound formed by the union of one or more electronically rich molecules or atoms (e.g., one or more ligands) with one or more electronically poor molecules or atoms such as a metal (e.g., a transition metal). For example, a metal complex of this disclosure may include a central metal (e.g., a central transition metal) and one or more ligands having one or more pairs of unshared electrons (e.g., via donor atoms) for forming one or more coordinate covalent bonds or “coordinate bonds” with the central metal. The ultimate composition of the complex catalyst may also contain an additional ligand, for example, an anion satisfying the coordination sites or nuclear charge of the metal in the complex.

EXAMPLE CATALYSTS

The embodiments of this disclosure relate to, inter alia, catalysts for the production of hydrogen via the HER of water splitting. Surprisingly, the catalysts hereof may have photolytic HER activity in a basic or high pH composition or solution, such as a composition or solution having a pH of greater than 8, a pH between 8 to 10, or a pH of about 9. In some embodiments, the catalysts hereof have photolytic HER activity in aqueous compositions or solutions, such as basic aqueous compositions or solutions. In some embodiments, the catalysts hereof have a TON of 218,000 in a basic composition or solution (e.g., a composition or solution having a pH of about 9), such as a basic aqueous composition or solution. In some embodiments, the catalysts hereof have a TOF of 12500/hour over the course of several hours (e.g., five hours) in a basic composition or solution (e.g., a composition or solution having a pH of about 9), such as a basic aqueous composition or solution.

In some embodiments, the catalyst comprises a metal catalyst. The metal catalyst may include a transition metal. In some embodiments, the metal catalyst comprises cobalt, nickel, or iron. In some embodiments, the metal catalyst comprises cobalt. The metal may be a metal ion, such as an ion of the metals described above. The metal catalyst may include a metal complex such as a transition metal complex. Preferably, the metal complex comprises a cobalt complex, a nickel complex, or an iron complex. More preferably, the metal catalyst comprises a cobalt complex.

In some embodiments, the metal catalyst comprises a metal complex of Formula I,

[M(L1)(L2)]X+[A]Y−   Formula I

The metal M can include a transition metal as described above, such as cobalt, nickel, or iron. In preferred embodiments, the metal M comprises cobalt and the metal complex can be referred to as a cobalt complex.

For simplicity, [M(L1)(L2)]X+ may be referred to as a cation and the additional ligand or anion A may be referred to as an anion. The anion A may satisfy remaining coordination sites of the metal M and/or a nuclear charge of the cation [M(L1)(L2)]X+. In some embodiments, the cation [M(L1)(L2)]X+ has the positive charge X+ that is satisfied by a negative charge Y− of the anion A. X and Y are each integers. X and Y may be the same integer. In some embodiments, each of X and Y is an integer between 1 to 3. For example, each of X and Y may be 1, or each of X and Y may be 2, or each of X and Y may be 3. Formula I can also be written as [M(L1)(L2)][A] when X and Y are the same integer.

The positive charge X+ may be defined by a nuclear charge of the metal M in the cation [M(L1)(L2)]X+. In particular, X+ designates the number of electrons needed to satisfy a primary valency or oxidation state of the metal M in the cation [M(L1)(L2)]X+. The metal M is referred to as a metal for simplicity but generally comprises a metal ion, such as an ion of the metals described above (e.g., a cobalt cation, a nickel cation, or an iron cation). The positive charge X+ may depend on both the primary valency or oxidation state of the metal M and the number of ligands L1 and/or L2 that contribute to satisfying the primary valency or oxidation state of the metal M. In some embodiments, one of the ligands L1 or L2 may contribute to satisfying the primary valency or oxidation state of the metal M. In some embodiments, none of the ligands L1 or L2 contribute to satisfying the primary valency or oxidation state of the metal M.

The primary valency or oxidation state of the metal M in Formula I may be an integer greater than or equal to 1. In some embodiments, the primary valency or oxidation state of the metal M may be between 1 to 3. Preferably, the primary valency or oxidation state of the metal M is 2 or 3. In preferred embodiments, the metal M is Co(II) or Co(III). Co(II) has a primary valency or oxidation state of +2. Co(III) has a primary valency or oxidation state of +3.

In some embodiments, the metal M is Co(III) and none of the ligands L1 or L2 contribute to satisfying its primary valency or oxidation state. In such embodiments, X is 3. In some embodiments, the metal M is Co(III) and one of the ligands L1 or L2 contributes to satisfying its primary valency or oxidation state. In such embodiments, X is 2. In some embodiments, the metal M is Co(II) and none of the ligands L1 or L2 contribute to satisfying its primary valency or oxidation state. In such embodiments, X is 2. In some embodiments, the metal M is Co(II) and one of the ligands L1 or L2 contributes to satisfying its primary valency or oxidation state. In such embodiments, X is 1.

In some embodiments, the anion A may vary depending on the positive charge X+. Non-limiting examples of the anion A include a halide ion (e.g., Cl−, Br−, I−, F−), (PF6−)2, (PF6−)3, and the like. In some embodiments, when X is 1, the anion A comprises a halide, preferably Cl−. In some embodiments, when X is 2, the anion A preferably comprises (PF6−)2. In some embodiments, when X is 3, the anion A preferably comprises (PF6−)3.

The ligands L1 and L2 include electronically rich molecules or atoms having one or more pairs of unshared electrons for forming one or more coordinate bonds with the metal M. The ligands L1 and L2 preferably satisfy the secondary valency or coordination number of the metal M. In some embodiments, the ligands L1 and L2 may also contribute to satisfying the primary valency as described above. Preferably, the ligands L1 and L2 entirely satisfy the secondary valency of the metal M. The anion A may only contribute to satisfying the primary valency of the metal M. In this regard, in some embodiments, the ligands L1 and L2 cooperatively form coordinate bonds with the metal M that are equal in number to the coordination number of the metal M. For example, the metal M may have a coordination number greater than 1, such as 2, 4, or 6.

In some embodiments, the metal M may have a coordination number of 6. For example, cobalt, iron, or nickel may each have a coordination number of 6. In these embodiments, the ligands L1 and L2 preferably cooperatively form six (6) coordinate bonds with the metal M. In certain embodiments, the ligand L1 forms more coordinate bonds with the metal M than L2. For example, where the coordination number of the metal M is 6, the ligand L1 may form five (5) coordinate bonds with the metal M and the ligand L2 may form one (1) coordinate bond with the metal M.

Preferably, the ligand L1 may be a tetrapyridyl-amine (Py4N) with four pyridyl groups and an amine group each forming a coordinate bond with the metal M. In some embodiments, none of the pyridyl groups of the ligand L1 form a bipyridyl group. For example, in some embodiments, none of the pyridyl groups of the ligand L1 form a 2,2′-bipyridyl group. In some embodiments, the amine group of the ligand L1 may include a tertiary amine.

In preferred embodiments, the tetrapyridyl-amine has a Formula II

In some embodiments, R1, R2, R3, and R4 may each be independently selected from the group consisting of alkyl, aryl, alkoxyl, and aroxyl, each of which may be optionally independently substituted with one or more substituents selected from the group consisting of methyl, ethyl, methoxyl, and carboxyl. In some embodiments, R1, R2, R3, and R4 are each independently selected from the group consisting of C1-C6 alkyl.

In preferred embodiments, R1, R2, R3, and R4 are each a C1 alkyl and the ligand L1 is of Formula IIa,

The ligand L1 of Formula IIa may also be referred to as Py4NMe or 1-(6-(1,1-di(pyridin-2-yl)ethyl)pyridin-2-yl)-N-methyl-N-(pyridin-2-ylmethyl)methanamine. Each nitrogen of the tetrapyridyl-amine ligand L1 of Formula II may form a coordinate bond with the metal M of Formula I. For example, when the ligand L1 is of Formula IIa, the metal complex is of Formula Ia,

The metal complex of Formula Ia can also be referred to as [M(Py4NMe)(L2)]X+[A]Y−.

In some embodiments, the metal M (e.g., cobalt) has a coordination number of 6. In these embodiments, five of the six coordination sites of the metal M may form a coordinate bond with a nitrogen of the tetrapyridyl-amine ligand L1 of Formula II and the remaining coordination site of the metal M may form a coordinate bond with the ligand L2. The ligand L2 may include a molecule or atom with a pair of unshared electrons for forming the remaining coordinate bond with the metal M. Non-limiting examples of the ligand L2 include a halide ion (e.g., Cl−, Br−, I−, F−) or a water molecule (OH2).

In the metal complex of Formula Ia, the tetrapyridyl-amine ligand L1 of Formula II does not contribute to satisfying the primary valency of the metal M. The positive charge X+ of the cation [M(Py4NMe)(L2)]X+ may thereby be defined by the primary valency or oxidation state of the metal M and any ionic charge of the ligand L2. In some embodiments, the ligand L2 is not ionically charged (e.g., in the case where the ligand L2 is a water molecule) and the positive charge X+ of the cation in Formula Ia is equal to the primary valency or oxidation state of the metal M. In some such embodiments, the metal M may include Co(II) and X is 2, or the metal M may include Co(III) and X is 3. In other embodiments, the ligand L2 is ionically charged (e.g., in the case where the ligand L2 is a halide such as Cl−) and the positive charge X+ of the cation in Formula Ia is equal to the primary valency or oxidation state of the metal M less the ionic charge of the ligand L2 (e.g., −1). In some such embodiments where the ligand L2 comprises a halide, the metal M may include Co(II) and X is 1, or the metal M may include Co(III) and X is 2.

As described above, the anion A may vary depending on the positive charge X+ of the cation in Formula Ia. For example, when X is 1, the anion A comprises a halide, preferably Cl−. In some embodiments, when X is 2, the anion A preferably comprises (PF6−)2. In some embodiments, when X is 3, the anion A preferably comprises (PF6−)3.

In one exemplary embodiment, the metal complex is a cobalt complex of Formula Ia-i,

The cobalt complex of Formula Ia-i may also be referred to as [Co(Py4NMe)Cl][Cl].

In one exemplary embodiment, the metal complex is a cobalt complex of Formula Ia-ii,

The cobalt complex of Formula Ia-ii may also be referred to as [Co(Py4NMe)(OH2)][(PF6)3].

Methods and Compositions for Producing Hydrogen Via Water Splitting

Also provided herein are methods of producing hydrogen via water splitting. An exemplary method comprises contacting a composition or solution comprising water with a catalyst hereof, wherein the catalyst has photolytic HER activity to drive the HER in response to light exposure. In some embodiments, contacting the composition or solution with the catalyst may include dispersing the catalyst in the composition or solution and the method may include exposing the composition or solution with the catalyst dispersed therein to light, whereby catalyst drives the HER in response to the light exposure. In some embodiments, the catalyst may be formed as a catalyst layer on a substrate of an electrode and contacting the composition or solution with the catalyst may include immersing the electrode having the catalyst layer formed thereon in the composition or solution. In these embodiments, the method may include exposing the electrode having the catalyst layer formed thereon to light. The photolytic water splitting reaction using the catalyst to drive the HER can be performed using any suitable photocatalytic water splitting technique known in the art.

In some embodiments of the method, the catalyst, and/or an electrode having the catalyst formed thereon, may be exposed to light to drive the HER in the composition or solution. Any suitable light source may be used, such as sunlight, UV lamps, Xenon lamps, or the like. As described herein, “exposing the catalyst to light” and like phrases encompasses direct exposure of the catalyst to light or photons as well as exposure of the catalyst to excited charge carriers (e.g., electrons) that are generated in response to light exposure. For example, the catalyst may be exposed to excited charge carriers (e.g., electrons) that are part of an electron-hole pair generated by a semiconducting material (e.g., titanium dioxide, TiO2) in response to light exposure. In some embodiments, the catalyst may be formed as a catalyst layer on a substrate of an electrode and a further photocatalyst layer may be included on the electrode.

The catalyst used in the methods hereof may include any of the example catalysts described herein. In some embodiments of the method, the catalyst is a metal complex of Formula I wherein the ligand L1 is a tetrapyridyl-amine (Py4N), such as the tetrapyridyl-amine of Formula II. In some embodiments, the catalyst is a cobalt complex of Formula I, wherein the ligand L1 is a tetrapyridyl-amine (Py4N) (e.g., the tetrapyridyl-amine of Formula II) and the metal M is cobalt, such as Co(II) or Co(III). In some embodiments, the catalyst is a metal complex of Formula Ia as described herein. In some embodiments, the catalyst is a cobalt complex of Formula Ia, wherein the metal M is cobalt, such as Co(II) or Co(III). In some embodiments of the method, the catalyst is a cobalt complex of Formula Ia-i, a cobalt complex of Formula Ia-ii, or a combination thereof.

In some embodiments of the method, the composition or solution is configured such that the OER is thermodynamically favored in response to the light exposure over the HER in the absence of the catalyst. In some embodiments, the composition or solution is an alkaline composition or solution wherein the OER is thermodynamically favorable at basic conditions. For example, the alkaline composition or solution may have a pH greater than 8, such as a pH between 8 to 10, or a pH of about 9. In embodiments, the alkaline composition or solution comprises water and a base or hydroxide producer in the water. Non-limiting examples of the base include sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)2), lithium hydroxide (LiOH), and combinations thereof. In some embodiments, the alkaline composition or solution comprises water, a base, and a pH buffer. Non-limiting examples of the pH buffer include boric acid (H3BO3), phosphoric acid (H3PO4), acetic acid (CH3COOH), and combinations thereof. In some embodiments, the pH buffer may include a universal buffer, such as the Britton-Robinson buffer. In some embodiments, the alkaline composition or solution is an aqueous alkaline composition or solution and comprises water as the primary solvent. The alkaline composition or solution may be substantially free of organic solvents and/or non-aqueous solvents. For example, the alkaline composition or solution may include less than 25%, less than 20%, less than 15%, less than 10%, less than 5%, or less than 1% organic solvent and/or non-aqueous solvent.

Also provided herein is a composition or solution for use in photocatalytic water splitting. The composition or solution comprises water and a catalyst having photolytic HER activity in the composition or solution. The catalyst may include any of the example catalysts described herein. In some embodiments, the catalyst is a metal complex of Formula I wherein the ligand L1 is a tetrapyridyl-amine (Py4N), such as the tetrapyridyl-amine of Formula II. In some embodiments, the catalyst is a cobalt complex of Formula I, wherein the ligand L1 is a tetrapyridyl-amine (Py4N) (e.g., the tetrapyridyl-amine of Formula II) and the metal M is cobalt, such as Co(II) or Co(III). In some embodiments, the catalyst is a metal complex of Formula Ia as described herein. In some embodiments, the catalyst is a cobalt complex of Formula Ia, wherein the metal M is cobalt, such as Co(II) or Co(III). In some embodiments, the catalyst is a cobalt complex of Formula Ia-i, a cobalt complex of Formula Ia-ii, or a combination thereof. In some embodiments, the composition or solution is configured such that the OER is thermodynamically favored in response to the light exposure over the HER in the absence of the catalyst. In some embodiments, the composition or solution is an alkaline composition or solution wherein the OER is thermodynamically favorable at basic conditions. For example, the alkaline composition or solution may have a pH greater than 8, such as a pH between 8 to 10, or a pH of about 9. In embodiments, the alkaline composition or solution comprises water and a base or hydroxide producer in the water. Non-limiting examples of the base include sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)2), lithium hydroxide (LiOH), and combinations thereof. In some embodiments, the alkaline composition or solution comprises water, a base, and a pH buffer. Non-limiting examples of the pH buffer include boric acid (H3BO3), phosphoric acid (H3PO4), acetic acid (CH3COOH), and combinations thereof. In some embodiments, the pH buffer may include a universal buffer, such as the Britton-Robinson buffer. In some embodiments, the alkaline composition or solution is an aqueous alkaline composition or solution and comprises water as the primary solvent. The alkaline composition or solution may be substantially free of organic solvents and/or non-aqueous solvents. For example, the alkaline composition or solution may include less than 25%, less than 20%, less than 15%, less than 10%, less than 5%, or less than 1% organic solvent and/or non-aqueous solvent.

EXAMPLES

The following non-limiting examples are provided to further illustrate the present disclosure. Reference is also made to the experimental procedures, results, analysis, and discussion provided in Ping Wang et al., “Photocatalytic Hydrogen Production with A Molecular Cobalt Complex in Alkaline Aqueous Solutions,” J. Am. Chem. Soc. 2024, 146, 14, 9493-9498 (Mar. 26, 2024), https://doi.org/10.1021/jacs.3c12928, and Supporting Information available at https://pubs.acs.org/doi/10.1021/jacs.3c12928?goto=supporting-info, the entire disclosure of which is hereby incorporated herein by reference.

Example 1: Experimental Equipment and Materials

The compounds provided herein can be synthesized using techniques described herein and, to the extent not described herein, techniques that are known to those skilled in the art.

Representative examples of methods that may be used to synthesize particular compounds within the scope of the present disclosure are set forth below. Unless otherwise indicated, the equipment and materials described below were used in the following examples. All commercial chemicals were used without any purification. The UV/Vis absorption spectra of Co complexes were obtained from a GENESYS™ M 10S UV-Vis Spectrophotometer. 1H NMR and 2D correlation spectroscopy (COSY) spectra were collected on a JEOL 400 MHz NMR spectrometer. JA values listed for NMR are the actual frequency differences and not the coupling constants. Electrospray ionization mass spectra (ESI-MS) were obtained from a Thermo Electron LCQ Advantage liquid chromatography-mass spectrometer. Elemental analyses were conducted by Atlantic Microlab, Inc., Atlanta, Georgia. The produced H2 was quantitively measured by gas chromatography using an HP 5890 series II Gas Chromatograph with a TCD detector (Molecular sieves 5 Å column). Cyclic voltammetry (CV) experiments were done by using CH Instruments potentiostat (model 660A). Photocatalytic hydrogen production was conducted using techniques known in the art. Britton-Robinson buffers were prepared with 0.5 M of boric acid (H3BO3), phosphoric acid (H3PO4), acetic acid (CH3COOH) and adjusted with NaOH to various pHs. Turnover numbers (TONs) for catalytic H2 production by catalysts were calculated by subtracting the amount of H2 produced in control experiments in the absence of catalyst under otherwise the same conditions. All experiments were conducted under an argon (Ar) atmosphere unless stated otherwise.

Example 2: Synthesis of Compounds of Formula Ia-i and Formula Ia-ii

Scheme 1 below provides the synthetic scheme for the Py4NMe ligand (Formula IIa) and the related Co complexes (Formula Ia-i and Formula Ia-ii). Description of each step of Scheme 1 to produce the related compounds is provided below.

The syntheses of 2,2′-[1-(6-methylpyridin-2-yl)ethane-1,1-diyl]dipyridine (Py3Me) was carried out according to techniques known in the art.

{[Co(Py4NMe)(H2O)](PF6)3}·H2O (Formula Ia-i·H2O). A solution of AgPF6 (700 mg, 2.8 mmol) in 10 mL H2O was added to an aqueous solution (20 mL) of Formula Ia-i (440 mg, 0.84 mmol) and the resulting reaction mixture was refluxed overnight. Water was removed after filtration of the reaction solution through Celite. The resulting residue was washed with methanol/diethyl ether 3 times. The obtained solid was then redissolved in H2O for overnight refluxing. A dark red solid (Yield, 471 mg, 62%) was obtained after filtration and water removal. ESI-MS: m/z 618.2 (calcd m/z 618.4 for {[Co(Py4NMe)(F)](PF6)}+. Anal. Calcd for C25H27N5CoP3F18O·H2O: C, 32.45; H, 3.16; N, 7.57. Found C, 32.31; H, 2.91; N, 7.53.

Alternatively, Formula Ia-ii can be prepared from the reaction of the Py4NMe ligand with [Co(CH3CN)6](PF6)2 in acetone/H2O to afford complex [Co(Py4NMe)(OH2)](PF6)2·H2O, which can be converted to Formula Ia-ii by refluxing with AgPF6 in water.

[Co(Py4NMe)(OH2)](PF6)2·2H2O. [Co(CH3CN)6](PF6)2 (0.797 g, 1.5 mmol) in 60 mL acetone was slowly added to a refluxing solution of Py4NMe (0.541 g, 1.4 mmol) in 30 mL acetone and the resulting solution was continued to reflux overnight. Acetone solvent was removed after filtration over celite and the solid was washed with acetone/ether 4 times. Degassed Millipore water (˜100 mL) was then added to the dried solid and refluxed overnight. Another filtration was performed, and water was removed to afford a dark red solid (Yield, 0.549 g, 52.7%). Anal. Calcd for C25H31CoF12N5O3P2: C, 37.61; H, 3.91; N, 8.77. Found C, 37.81; H, 3.76; N, 8.37.

[Co(Py4NMe)(OH2)](PF6)3 (Formula Ia-ii). A solution of AgPF6 (134.4 mg, 0.53 mmol) in 10 mL Millipore H2O was added to an aqueous solution (50 mL) of [Co(Py4NMe)(OH2)](PF6)2·2H2O (400.2 mg, 0.53 mmol) and the resulting reaction mixture was refluxed overnight. Water was removed after the reaction solution was filtered through Celite. The resulting residue was washed with methanol/diethyl ether 3 times. The obtained solid was then redissolved in Millipore H2O and refluxed overnight at 103° C. A red solid (Yield, 279 mg, 58.5%) was obtained after cold filtration and water removal. Anal. Calcd for C25H27CoF18N5OP3: C, 33.09; H, 3.00; N, 7.72. Found C, 33.32; H, 3.18; N, 7.53.

Example 3: Analysis and Discussion

The efficiency of electro- and photocatalytic HER is well-known to be strongly pH-dependent. While electro- and/or photocatalytic hydrogen production under basic solutions have been reported for heterogeneous catalysts, only a limited number of molecular metal complexes are known for HER under basic conditions. The reported photocatalytic HER catalyzed by molecular catalysts under basic conditions are generally conducted in a mixture of organic solvents and aqueous solutions. However, the highly oxidizing conditions needed for water oxidation suggest that organic solvents should be avoided in solar-driven water splitting devices. As best understood, visible-light driven HER catalyzed by molecular metal complexes in purely aqueous solutions at basic pHs has not been reported. Therefore, it remains a significant challenge in developing molecular transition metal complexes that can catalyze photolytic HER in basic aqueous solutions in the absence of any organic solvent.

Molecular cobalt complexes with polypyridyl ligands for photocatalytic/electrocatalytic hydrogen production in aqueous solutions have been reported. Studies have shown that the optimal pHs for photocatalytic HER by cobalt complexes depend on the electronic and structural properties of ligand scaffolds. Analyzed herein with the complexes of Formula Ia-i and Formula Ia-ii are two comparative complexes, Complex C1 and Complex C2, shown below. Complex C1 displays maximum HER activity at pH 4 using ascorbic acid as electron donor and [Ru(Bpy)3]2+ as photosensitizer. Complex C2 displays the highest activity for photocatalytic HER at pH 7 aqueous solutions with much improved activity and stability compared to Complex C1. The complexes of Formula Ia-i and Formula Ia-ii display remarkable activity for photocatalytic HER with exceptional stability and TON of 218, 000 in a pH 9 aqueous solution, which further elucidate the electronic and structural factors that govern the pH-dependent photocatalytic HER.

The crystal structure of the cation of Formula Ia-i was observed as a distorted octahedral geometry with an axial chloride ligand. Vapor diffusion of benzene into a methanol solution of Formula Ia-ii yields crystals whose X-ray structure shows the binding of a fluoride ion to the CoIII center due to the decomposition of PF6− anion, consistent with the ESI/MS data obtained for Formula Ia-ii. Furthermore, the axial chloride ligand observed for the cation of Formula Ia-i and the fluoride ligand of the cation of Formula Ia-ii following the vapor diffusion lie trans and cis, respectively, to the pyridyl linked to the —CH2N(Me)CH2Py moiety. The UV-vis spectrum of Formula Ia-ii in water shows a broad peak at 488 nm from the d-d transition of the CoIII—OH2 form, similar to that of Complex C1, while no significant absorption was observed for the complex of Formula Ia-i in acetonitrile from 300 nm to 800 nm.

Cyclic voltammograms (CV) of the complex of Formula Ia-i, and Formula Ia-i in the presence of ferrocene, in 0.1 M Bu4NPF6/CH3CN solution were conducted using a scan rate of 100 mV/s, a glassy carbon working electrode, an Ag/AgCl reference electrode, a Pt wire counter electrode, and ferrocene as internal reference. The CV of Formula Ia-i in CH3CN displays three quasi-reversible redox events at −0.10, −1.72, and −2.29 V (vs Fc+/Fc), corresponding to CoIII/CoII, CoII/CoI, and CoI/Co0 (or ligand-based) redox couples, respectively. The CoII/CoI couple at −1.72 V (vs Fc+/Fc) for the complex of Formula Ia-i is more negative than those of the CoII—Cl forms of Complex C1 (−1.58 V vs Fc+/Fc) and Complex C2 (−1.54 V vs Fc+/Fc) due to the substitution of the Bpy group in Complex C1 and Complex C2 with more basic amine moiety.

In 1 M potassium phosphate solution at pH 7.0, the CV of Formula Ia-ii displays a quasi-reversible redox event at 0.20 V (vs SHE) which is assignable to the CoIII/CoII couple. The Pourbaix diagram of the CoIII/CoII couple of Formula Ia-ii in universal buffer shows a pH dependent redox potential change from pH 3.5 to pH 11.5 with a slope of 58.5 mV/pH, suggesting a PCET process. Based on the Pourbaix diagram, the pKa's of CoIII—OH2 and CoII—OH2 for Formula Ia-ii were derived as 3.5 and 11.5, respectively. At more negative potentials, the CV of Formula Ia-ii in 1 M phosphate buffer at pH 7.0 shows one irreversible reduction event at −1.12 V (vs SHE) before catalysis (CV conducted using 1 mM of Formula Ia-ii in 1 M pH 7.0 sodium phosphate buffer, scan rate, 100 mV s−1; Hg pool working electrode, Ag/AgCl reference electrode, Pt mesh counter electrode). Compared to Complexes C1 and C2, the redox potential of −1.12 V for the CoII/CoI couple of 3b is much more negative than those of Complex C1 (−0.84 V) and Complex C2 (−0.70 V vs SHE).

Due to the higher stability of the complex Formula Ia-ii in air and its one-electron reduction leads to the formation of the complex of Formula Ia-i, only Formula Ia-ii was used for the HER catalysis study. The photocatalytic HER activity of Formula Ia-ii was investigated in a similar way as reported for Complexes C1 and C2 using 50 nM of Formula Ia-i, 0.1 M ascorbic acid as electron donor and 0.5 mM [Ru(Bpy)3]2+ as photosensitizer in 0.5 M Britton-Robinson buffer. At pH 7, Formula Ia-ii (50 nM) catalyzed HER with a TON of 22,400, significantly higher than those reported for Complexes C1 and C2 at pH 7 solutions. Surprisingly, a pH screening of the photocatalytic HER by Formula Ia-ii demonstrated that Formula Ia-ii is more active at basic aqueous solutions. At pH 9, Formula Ia-ii displays the highest activity with a TON of 218,000 after ˜40 hour photolysis with a TOF of 12500/hour during the first 5-h photolysis. Under the same conditions, the Complexes C1, C2 and CoSO4 produced negligible amounts of H2 close to that of control experiment in the absence of Formula Ia-ii. Therefore, Formula Ia-ii is the most active catalyst for photocatalytic HER in alkaline solutions among the Complexes C1, C2 and the complex of Formula Ia-ii. Mercury poison test also confirmed the molecular nature of the complex of Formula Ia-ii during photocatalytic HER at pH 9.0. At 1.0 and 10 μM of Formula Ia-ii, the amount of H2 produced was 0.628 mmol with TON of 62800 and 0.757 mmol with TON of 7570, respectively, demonstrating Formula Ia-ii is truly effective for photocatalytic HER in basic solutions.

As shown in Table 1, photocatalytic HER in alkaline solutions have been reported for metal complexes in systems containing different photosensitizers and sacrificial electron donors at various pHs with TONs ranging from ˜10 to 11333 in mixed organic aqueous solutions. The complex of Formula Ia-ii is demonstrated as a rare example for photocatalytic HER in alkaline aqueous solutions without using any organic solvents with a remarkable TON of 218000 at pH 9.0. While a direct comparison of the complex of Formula Ia-ii to the reported catalysts is not practical due to the different photocatalysis conditions reported in literature, the TON and TOF of the complex of Formula Ia-ii are certainly among the highest reported for homogeneous photocatalytic HER in alkaline aqueous solutions.

Photocatalytic hydrogen production in alkaline solutions by transition metal complexes.

Sacrificial
Light

Catalyst
Photo-sensitizer
agents
source
pH
Solvents
TON
TOF

buffer

complexes

complexes

In conclusion, the results demonstrate visible light-driven hydrogen production catalyzed by a molecular Co complex in alkaline aqueous solutions that may be important for coupling to the oxidation of water for overall water splitting in alkaline solutions. Also demonstrated herein is the extraordinary performance of the complexes of Formula Ia-i and Formula Ia-ii, in terms of both TON and TOF, for photocatalytic HER especially at basic pHs.

The phrases, unless otherwise specified, “consists essentially of” and “consisting essentially of” do not exclude the presence of other steps, elements, or materials, whether or not, specifically mentioned in this specification, so long as such steps, elements, or materials, do not affect the basic and novel characteristics of the present disclosure, additionally, they do not exclude impurities and variances normally associated with the elements and materials used.

Numerical ranges used herein include the numbers recited in the range. For example, the numerical range “from 1 wt % to 10 wt %” comprises 1 wt % and 10 wt % within the recited range.

All numerical values within the detailed description herein are modified by “about” the indicated value, and take into account experimental error and variations that would be expected by a person having ordinary skill in the art.

All documents described herein are incorporated by reference herein, including any priority documents and or testing procedures to the extent they are not inconsistent with this text. As is apparent from the foregoing general description and the specific embodiments, while forms of the present disclosure have been described, various modifications can be made without departing from the spirit and scope of the present disclosure. Accordingly, it is not intended that the present disclosure be limited thereby.

As used herein, the term “comprising” is considered synonymous with the term “including” for purposes of United States law. Whenever a composition, an element or a group of elements is preceded with the transitional phrase “comprising,” it is understood that the transitional phrases “consisting essentially of,” “consisting of,” “selected from the group of consisting of,” or “is” can additionally or alternatively precede the recitation of the composition, element, or elements and vice versa.