Water splitting system and method

Abstract

A system and method for photocatalytic water splitting with separated hydrogen and oxygen is provided. A water splitting system includes a hydrogen generation chamber including a hydrogen production port, an oxygen generation chamber including an oxygen collection port, an ion exchange membrane coupling the hydrogen generation chamber and the oxygen generation chamber, and a photocatalytic structure including a first catalyst portion disposed in the hydrogen generation chamber and a second catalyst portion disposed in the oxygen generation chamber. The first catalyst portion is configured to produce hydrogen via the hydrogen production port. The second catalyst portion is configured to produce oxygen via the oxygen production port.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 336,9526, filed April 29, 2022, entitled “Photocatalytic Water Splitting with Separate H2 and O2 Production,” the disclosure of which is incorporated herein by reference in its entirety. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with Government support under Contract No. W56HZV-21-C-0076 awarded by the United States Army. The Government has certain rights in this invention. [Technical field]

[0003] FIELD OF THE DISCLOSURE This disclosure relates generally to photocatalytic water splitting. [Background technology]

[0004] Solar hydrogen production from water and sunlight is a promising and economical strategy for the storage and conversion of renewable energy. To date, research on high-purity solar hydrogen production has mainly focused on the costly and complicated photovoltaic electrolysis (PV-E) and photoelectrochemical (PEC) routes.

[0005] Photocatalytic solar water splitting for green hydrogen production has been considered to be a cost-effective approach compared to the extensively studied PV-E and PEC water splitting. Photocatalytic water splitting does not involve electrical wires and biases, thus avoiding the significant ohmic losses that often limit the performance of PV-E and PEC water splitting systems. Importantly, conventional PEC water splitting is often performed in an electrolyzer, which involves the use of highly conductive electrolytes such as sulfuric acid, significantly limiting its practical application. In contrast, photocatalytic water splitting can utilize pure water, seawater, or tap water, significantly reducing system costs and mitigating issues related to photocatalytic corrosion, stability, and safety, making it much more suitable for practical application. For example, commonly reported high-efficiency PEC water splitting devices exhibit very low stability, and stable operation in a practical two-electrode configuration is often limited to a few hours. Photocatalytic water splitting is predicted to have a very low levelized cost of hydrogen production due to its long-term stability and simplicity.

[0006] However, a major challenge of photocatalytic water splitting is the co-production of a mixture of H2 / O2 gases in the same reaction chamber. The resulting hydrogen will inevitably be mixed with oxygen during the entire water splitting process, undesirably increasing the cost of hydrogen / oxygen separation. Moreover, the mixed hydrogen and oxygen also leads to safety issues. Summary of the Invention

[0007] According to one aspect of the disclosure, a water splitting system includes a hydrogen generation chamber including a hydrogen generation port, an oxygen generation chamber including an oxygen collection port, an ion exchange membrane coupling the hydrogen generation chamber and the oxygen generation chamber, and a photocatalytic structure. The photocatalytic structure includes a first catalytic portion disposed in the hydrogen generation chamber and a second catalytic portion disposed in the oxygen generation chamber. The first catalytic portion is configured to generate hydrogen via the hydrogen generation port. The second catalytic portion is configured to generate oxygen via the oxygen generation port.

[0008] According to another aspect of the present disclosure, a method for splitting water includes immersing one of an oxygen evolution reaction (OER) photocatalytic structure and a hydrogen evolution reaction (HER) photocatalytic structure in water contained in a chamber in which a species of a redox pair is present, exposing the chamber to light to illuminate the OER photocatalytic structure or the HER photocatalytic structure, converting the species of the redox pair by the illumination, recovering one of the oxygen and hydrogen produced by the illumination, regenerating the species of the redox pair in the water, and recovering the other of the oxygen and hydrogen produced during the regeneration of the species of the redox pair.

[0009] According to yet another aspect of the present disclosure, a water splitting system includes a hydrogen generation chamber including a hydrogen production port, an oxygen generation chamber including an oxygen collection port, a liquid flow path connecting the hydrogen generation chamber and the oxygen generation chamber for exchange of redox pairs, a hydrogen evolution reaction (HER) photocatalytic structure disposed within the hydrogen generation chamber, and an oxygen evolution reaction (OER) photocatalytic structure disposed within the oxygen generation chamber.

[0010] In relation to any one of the preceding aspects, the systems, apparatus and / or methods described herein may alternatively or additionally include or include any combination of one or more of the following aspects or features: The first catalytic portion includes a first side of the photocatalytic structure. The second catalytic portion includes a second side of the photocatalytic structure. The water splitting system further includes a separator disposed between the hydrogen production chamber and the oxygen production chamber. The photocatalytic structure is disposed along and integral with the separator. The photocatalytic structure includes a plurality of nanowires extending into the hydrogen production chamber. The first catalytic portion includes a plurality of nanowires extending outwardly from a substrate of the photocatalytic structure and a distribution of catalytic nanoparticles spanning the plurality of nanowires. Each nanowire of the plurality of nanowires is configured for photo-generation of charge carriers. The second portion includes a catalytic layer supported on a substrate of the photocatalytic structure. The second portion further includes a metal layer disposed between the catalytic layer and the substrate. The second portion includes a distribution of catalytic nanoparticles supported on a metal substrate of the photocatalytic structure. The regeneration of the species of the redox pair includes switching between immersing the OER photocatalytic structure and the HER photocatalytic structure in the water in the chamber, and illuminating the OER or HER photocatalytic structure immersed in the water after switching between the OER and HER photocatalytic structures. The method further includes switching again between the OER and HER photocatalytic structures after regeneration of the species of the redox pair, and repeating the exposing the chamber to light, collecting oxygen or hydrogen, and regenerating the species of the redox pair. The regeneration of the species of the redox pair includes applying a voltage to the water via a pair of electrodes immersed in the water. The application of the voltage is configured for electrolytic reduction of the other species of the redox pair in the water. The method further includes ceasing the application of the voltage to the water after regeneration of the species of the redox pair, and repeating the exposing the chamber to light, collecting oxygen or hydrogen, and regenerating the species of the redox pair. The liquid flow path includes a channel between the hydrogen generation chamber and the oxygen generation chamber. The water splitting system further includes a separator disposed along the hydrogen-producing chamber and the oxygen-producing chamber such that the liquid flow passage includes openings in the separator.The OER photocatalytic structure includes a first substrate, a first plurality of nanowires extending outwardly from the substrate, and a first distribution of nanoparticle catalysts present across the plurality of nanowires. The HER photocatalytic structure includes a second substrate, a second plurality of nanowires extending outwardly from the substrate, and a second distribution of catalyst nanoparticles present across the plurality of nanowires. Each nanowire of the first and second plurality of nanowires is configured for photogeneration of charge carriers.

[0011] For a more complete understanding of the present disclosure, reference should be made to the following detailed description and the accompanying drawings, in which like reference numerals identify like elements throughout the drawings. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram of a water splitting system with separate production of hydrogen and oxygen, according to one example. [Diagram 2] FIG. 1 is a schematic diagram of a water splitting system with photocatalytic structure (eg, wafer) exchange and redox shuttle for the separate production of hydrogen and oxygen, according to one example. [Diagram 3] FIG. 1 is a schematic and photographic representation of a multiple chamber water splitting system for the separate production of hydrogen and oxygen, according to one example. [Figure 4] 1 shows scanning electron microscope (SEM) images of multiple InGaN nanowires for photocatalytic water splitting with H2 / O2 separation, along with graphical representations of the photoluminescence spectra, band edge potentials, and hydrogen production rates of the InGaN nanowires, according to one example. [Diagram 5] FIG. 1 is a schematic diagram of a water splitting system with integrated photocatalytic structures (eg, wafers) for the separate production of hydrogen and oxygen, according to one example. [Figure 6] According to another example, a schematic diagram of a water splitting system with an integrated photocatalytic structure (e.g., wafer) for the separate production of hydrogen and oxygen is shown along with an SEM image of multiple GaN nanowires on a Ni substrate for photocatalytic water splitting and a graphical illustration of hydrogen production rate using a Pt / GaN-Ni / Ir device. [Figure 7A] FIG. 1 is a schematic diagram of a water splitting system with a redox shuttle for the separate production of hydrogen and oxygen, according to one example. [Figure 7B] FIG. 1 is a schematic diagram of a water splitting system with a redox shuttle for the separate production of hydrogen and oxygen, according to one example. [Figure 7C] FIG. 1 is a schematic diagram of a water splitting system with a redox shuttle for the separate production of hydrogen and oxygen, according to one example. [Figure 7D] FIG. 1 is a schematic diagram of a water splitting system with a redox shuttle for the separate production of hydrogen and oxygen, according to one example. [Figure 8] (a) is a graph of selective hydrogen production via a photocatalytic wafer having a Rh / Cr2O3-InGaN catalyst array in a 0.1M K1 solution, and (b) is a graph of selective oxygen production via a photocatalytic wafer having a CoOx / InGaN catalyst array in a 0.1M K1O3 solution. [Figure 9] FIG. 1 is a schematic diagram of a water splitting system having interconnected chambers for redox shuttle exchange and separate production of hydrogen and oxygen, according to one embodiment. [Figure 10] FIG. 2 is a schematic diagram of a water splitting system having interconnected chambers for redox shuttle exchange and separate production of hydrogen and oxygen according to another embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The disclosed systems, devices and methods may take a variety of forms. Particular embodiments are shown in the drawings and will be described below with the understanding that the disclosure is intended to be exemplary. The disclosure is not intended to limit the invention to the specific embodiments described and illustrated herein.

[0014] Systems and methods for photocatalytic water splitting with separated hydrogen and oxygen are described. In some cases, the disclosed systems and devices include a photocatalytic structure integrated into a multiple chamber configuration. The photocatalytic structure can include a photocatalytic wafer or other device. In other cases, multiple chambers are used to generate hydrogen and oxygen separately and simultaneously via exchange and / or regeneration of redox pairs or shuttle species. In still other cases, separate production of hydrogen and oxygen is achieved via redox pairs in a single chamber with regeneration based on light and / or electroreduction.

[0015] The disclosed method and system can generate hydrogen and oxygen simultaneously or sequentially. In the case of simultaneous, hydrogen and oxygen can be collected separately using separate ports of each chamber. In the case of sequential, one or both of the hydrogen and oxygen generation can be performed during illumination (e.g., sun exposure) of the photocatalytic wafer. In some cases, hydrogen and oxygen generation can be performed over a daily cycle including illumination (e.g., daylight hours) and non-illumination (such as nighttime). For example, a bias voltage can be applied to the electrolytic reduction for nightly oxygen generation.

[0016] The disclosed methods and systems provide photocatalytic solar water splitting for scalable and cost-effective production of green hydrogen. The disclosed methods and systems address the challenge of co-generation of H2 / O2 mixed gases, possibly involving the same reaction chamber. The disclosed methods and systems utilize one of two strategies to overcome the challenges of photocatalytic water splitting. In the first approach, a photocatalytic wafer is integrated with a suitable proton exchange membrane and placed between two compartments of the reaction chamber. In some examples, the photogenerated charge carriers are spatially separated on the front and back sides of the photocatalytic wafer, thereby leading to the separate generation of H2 and O2 at the device level. In the second approach, the entire water splitting reaction is separated into two half-reactions, including a cathodic half-reaction for H2 production and an anodic reaction for O2 production. The two reactions are carried out either at separate locations or at different times mediated by a redox shuttle, thereby leading to the spatial separation of H2 and O2 at the device level. In some examples, the photocatalytic water splitting reactor or system includes two interconnected compartments for separate H2 and O2 production mediated by a suitable redox shuttle. In other examples, the photocatalytic water splitting reactor includes a single compartment for sequential (at different times) production of H2 and O2 for effective H2 and O2 separation. In each approach, the redox reactions (hydrogen evolution and oxygen evolution) can be separately driven by light, electricity, or a combination of light and electricity, thereby providing flexibility in the design, integration, and operation of the system.

[0017] The disclosed methods and systems avoid the use or reliance on downstream H2 / O2 separators. Such downstream separators significantly increase the overall system cost and footprint, thereby reducing the overall solar-to-hydrogen conversion efficiency, and further impose limitations on gas flow rates, pressures, and reaction chamber design. Moreover, such H2 / O2 separators have failed to provide a solution with relatively low cost, low power consumption, and ultra-high purity H2 separation.

[0018] Although described in the context of nanowire-based photocatalytic arrays, the disclosed methods and systems can use a wide variety of nanostructures and / or other catalytic arrays. For example, photocatalytic wafers can include various types and shapes of scaffolds or frameworks to support the distribution of catalytic nanoparticles and / or other catalysts.

[0019] The disclosed methods and systems may alternatively or additionally use still other photocatalytic structures or configurations. For example, the disclosed systems and methods are therefore not limited to wafer-based photocatalytic structures or devices. In some cases, the disclosed methods and systems may alternatively or additionally include or use suspended photocatalytic particles or other structures.

[0020] Although described in the context of InGaN-based and GaN-based nanostructures for photogeneration of charge carriers, the disclosed methods and systems can use a wide variety of semiconductor materials. Thus, the disclosed methods and systems are not limited to the use of III-nitride semiconductors, semiconductor alloys, or semiconductors. For example, various oxides (e.g., TiO2, SrTiO3), oxynitrides, and other photocatalytic materials can be used. The composition and other properties of the nanoparticles distributed throughout the nanowire can also differ from the examples described herein. For example, the nanoparticles can be Pt and Pd for hydrogen generation, and NiO for oxygen generation. x and IrO2. The composition and other properties of the substrate of the device can also differ from the examples described herein. For example, the substrate can be composed of or include sapphire, Mo, Ti, etc.

[0021] FIG. 1 shows an example of a water splitting system 100 in which a photocatalytic wafer 102 is integrated with two chambers 104, 106. One chamber is for hydrogen production and the other chamber is for oxygen production. In this example, the photocatalytic wafer 102 is integrated with a suitable proton exchange membrane and placed between the two chambers 104, 106 (or two compartments of a reaction chamber). The photogenerated charge carriers are spatially separated. For example, in the example shown, the electrons migrate to the nanowire surface, while the holes migrate towards the backside of the substrate. By deposition of a suitable co-catalyst, the hydrogen evolution reaction occurs in chamber 104 (or compartment) and the oxygen evolution reaction occurs in chamber 106 (or compartment), thereby leading to the separate production of H2 and O2 at the device level.

[0022] The water splitting system 100 further includes an ion (or proton) exchange membrane connecting the hydrogen production chamber and the oxygen production chamber. For example, a Naflon membrane can be used. In the example of FIG. 1, the membrane is disposed along a wall or other separator between the hydrogen production chamber 104 and the oxygen production chamber 106.

[0023] The arrangement of the wafers in the water splitting system 100 provides spatial separation of the photogenerated electrons and holes. The example of Figure 1 provides a fully integrated solar hydrogen device with efficient charge carrier separation and extraction. Other techniques for addressing the challenges of charge carrier separation and extraction are described below.

[0024] 2 illustrates an example of a water splitting system 200 and method in which the water splitting reaction is separated into two half-reactions, including a cathodic half-reaction for H2 production and an anodic half-reaction for O2 production. In this example, the two reactions occur sequentially, i.e., at different times, mediated by a redox shuttle, thereby resulting in spatial separation of H2 and O2 at the device level. Alternatively or additionally, the two reactions occur in separate locations (e.g., separate chambers).

[0025] The approach shown in Figure 2 provides flexibility in designing reaction chambers for overall water splitting and separate H2 / O2 generation. For example, iodate / iodide (IO3 / I - ) redox couples can be employed to produce high purity hydrogen and oxygen at different times or different locations. System 200 can operate at near-neutral pH conditions using a proton-coupled electron transfer redox shuttle. Utilizing this approach, a solar-to-hydrogen efficiency of approximately 2.5% was achieved, the highest reported value for solar water splitting with simultaneous H2 / O2 separation capability.

[0026] Further examples of the system design approaches shown in Figures 1 and 2 are described below. These examples provide alternative parameters, transport and kinetic processes for efficient solar-to-hydrogen conversion. In some examples, the photocatalytic water splitting reactor or system includes two interconnected compartments or chambers for separate H2 and O2 generation mediated by appropriate redox shuttles. In other examples, the photocatalytic water splitting reactor or system includes a single compartment for sequential (at different times) production of H2 and O2 for effective H2 and O2 separation. In each approach, the redox reactions (hydrogen generation and oxygen generation) can be separately driven by light, electricity, or any combination of light and electricity, thereby allowing great flexibility in the design, integration and operation of the system. Additionally, the two compartments or chambers can be further integrated in a tandem configuration for optimal solar light absorption and utilization.

[0027] In each example, the systems and methods include or use wafer-scale InGaN nanowire photocatalytic arrays on silicon or other substrates. Further details regarding nanostructure-based photocatalytic arrays are described in International Application No. PCT / US2021 / 056804 ("Water Splitting Device Protection") and U.S. Patent No. 9,112,085 ("Highly Efficient Broadband Semiconductor Nanowire Devices"), the entire disclosures of which are incorporated herein by reference. Other nanostructure shapes and arrangements can be used. The disclosed systems and methods can alternatively or additionally use or include still other photocatalytic structures, including, for example, planar structures.

[0028] 3 shows an example of a water splitting system 300 in which two chambers are connected to a quartz holder. Two holes in the quartz holder are used to install the photocatalytic wafer and membrane, respectively.

[0029] As shown in Figure 4(a), a photocatalyst wafer is fabricated by growing p-type InGaN nanowires on a p-type silicon wafer. As shown in Figure 4(b) and (c), the InGaN nanowires have a band gap of 2.45 eV and a band edge potential suitable for the water redox reaction. In this example, platinum (Pt) nanoparticles for hydrogen generation are deposited on the surface of the InGaN nanowires, and Ni, Au, and Ir layers for trapping holes and generating oxygen are sequentially deposited on the backside of the silicon wafer.

[0030] As shown in the schematic diagram of Figure 5, upon photoexcitation, the InGaN nanowires generate photogenerated electrons and holes. The photogenerated electrons are transferred to the Pt nanoparticles for hydrogen generation, and the photogenerated holes are transferred to the Ir layer for oxygen generation. As a result, 8.5 μmol -2 h -1 Hydrogen is selectively generated in the left chamber (e.g., at a wafer) at a production rate of 0.1 to 1.0 nm, allowing separation of H2 and O2 at the device level.

[0031] FIG. 6 shows an example of a water splitting system 600 with a photocatalytic wafer configured to accelerate charge transfer between GaN nanowires and an Ir layer. In this example, a metallic Ni wafer is directly used instead of semiconducting silicon as the substrate for GaN. The Ni wafer forms good ohmic contact with the p-doped Ga(In)N nanowire photocatalytic light absorber, thereby enabling more efficient collection of photogenerated holes. Also, in this example, Ir nanoparticles are used as co-catalysts for oxygen generation. The Ir nanoparticles exhibit a larger specific surface area than the Ir layer. Similarly, GaN nanowires grow well on the Ni wafer, as shown in FIG. 6(c). During operation, hydrogen and oxygen are generated on the GaN-supported Pt nanoparticles and Ni-supported Ir nanoparticles, respectively. As a result, hydrogen is generated at 0.07 mmol cm in the left chamber, as shown in FIG. 6(d). -2 h -1 It is selectively generated at a high rate.

[0032] In the above example, the charge transfer rate between the two sides of the photocatalytic wafer may limit further improvement of the hydrogen production efficiency in photocatalytic water splitting.

[0033] To improve efficiency, many examples based on redox couples provide an economical and highly efficient H2 / O2 source separation approach. In this approach, iodate / iodide (IO3 - / I - ) redox couples are employed to produce separated high purity hydrogen and oxygen at different times or spaces. In this way, H2 / O2 separation at the device level can be achieved. Figures 7A-7D show some examples of methods and systems implementing this approach.

[0034] In the example system and method shown in FIG. 7A, iodide (I - A chamber containing a Rh / Cr2O3-InGaN catalyst array (e.g., Rh / Cr2O3 nanoparticles on InGaN nanowires) reacts with hydrogen and iodate (IO3 -The hydrogen produced is stored in a hydrogen tank via a compressor. Next, the wafer with the Rh / Cr2O3-InGaN catalyst array is placed on a CoO x - Replace the wafer with one that has an InGaN catalyst array (e.g., CoO on InGaN nanowires). x nanoparticles). CoO x -InGaN wafers are the IO3 produced in the previous step. - Again I - Meanwhile, oxygen is also produced and stored in the oxygen tank. Next, CoO x The system is returned to its initial state by replacing the Rh / Cr2O3-InGaN wafer with the Rh / Cr2O3-InGaN wafer. This procedure can therefore be repeated to generate hydrogen and oxygen at different times, thereby providing separation of hydrogen and oxygen. With this approach, a maximum STH of 4.95 mmol / h is achieved, which corresponds to a maximum STH of 4.5%, as shown in Figure 8. -1 / cm -2 High production rates of have been experimentally achieved for hydrogen production.

[0035] Referring to FIG. 7B, another example water splitting system and method is configured to increase or maximize daytime solar energy utilization efficiency via electrolytic reduction-assisted photocatalytic water splitting. In this case, I - The chamber containing the Rh / Cr2O3-InGaN catalyst array was heated with hydrogen and IO3 during the day. - After the hydrogen is stored, the generated IO3 - For example, at night I - The hydrogen is directly electroreduced to the hydrogen gas. To do this, a bias voltage is applied through a pair of electrodes. Meanwhile, oxygen is produced separately. As a result, the system is regenerated by electrolytic reduction. Therefore, the procedure can then be repeated to produce separated hydrogen and oxygen. This method also reduces the complexity of the operation and design of the system.

[0036] The electrolytic reduction can be carried out to any desired extent in conjunction with one or more of the other embodiments described herein.

[0037] Alternatively or additionally, a bias voltage is applied during hydrogen production. The bias voltage can be applied with or without simultaneous photocatalysis.

[0038] FIG. 7C shows another example water splitting system and method involving the exchange of photocatalytic wafers or other devices. In this case, the reaction system uses two separated chambers. - and Rh / Cr2O3-InGaN are placed in the left chamber, and IO3 - and CoO x In the first stage, hydrogen and oxygen are generated in the left and right chambers, respectively. Meanwhile, the I - and right ventricular IO3 - are IO3 - and I - After the first stage, the photocatalytic wafers in the two chambers are exchanged with each other. As a result, the reaction system is regenerated. Therefore, this procedure can be repeated to continuously produce high-purity hydrogen with source separation capability.

[0039] FIG. 7D shows yet another example water splitting system and method that replaces the photocatalytic wafer or other device. Again, economical and highly efficient source separation was achieved based on a redox couple. In this case, iodate / iodide (IO3 - / I - ) redox couples are used to produce high-purity hydrogen and oxygen separated in different time or space. As shown in Figure 7D, hydrogen is first reacted with Rh / Cr2O3 / CoO x -I on InGaN wafers - IO3 - In the second step, unloaded InGaN nanowires on a separate wafer are used to fabricate IO3 - I - During this process, oxygen is generated and the solution is regenerated. As a result, hydrogen and oxygen are generated at different times, and H2 / O2 source separation is achieved. First, I - and IO3 -We demonstrated that two half-reactions using I as the electron donor and acceptor selectively produced hydrogen and oxygen on the corresponding photocatalysts, respectively. This indicates that I is a promising candidate for photocatalytic water splitting with hydrogen / oxygen source separation. - and IO3 - This study examines the feasibility of using as a redox couple.

[0040] As an example, photocatalytic water splitting with hydrogen / oxygen source separation was performed using Rh / Cr2O3 / CoO x The experiments were carried out in 0.050MKI and 0.050MKI03 for -InGaN and pristine InGaN. Each cycle included 1 h HER and 2 h OER, contributing to the production of stoichiometric H2 and O2 ratio of 2:1. The hydrogen purity during HER reached more than 95%. Meanwhile, the STH efficiency reached more than 2.5% for H2 production in four stable cycles. The STH and hydrogen purity depended on the concentration of redox couple. When the concentration of redox couple was increased to 0.05M, the hydrogen purity was more than 95% and the STH slightly decreased.

[0041] 9 and 10 show further examples of redox shuttle-based water splitting systems and methods. In these cases, the separate production of hydrogen and oxygen can be achieved without the assistance of electrolytic reduction.

[0042] In Figure 9, two chambers are connected to each other through a channel. The connection is made by a redox couple, e.g. IO3 - / I - In this example, two photocatalytic wafers are installed in different chambers without exchange during operation. The species of redox pairs generated during operation are spontaneously diffused to the corresponding regions to generate hydrogen and oxygen, respectively, through the channel between the two chambers. Separation of hydrogen and oxygen generated in photocatalytic water splitting is achieved despite the coupling, as the gases rise toward the exit port.

[0043] As shown in Figure 9, the system may use two light sources: one light source to illuminate the hydrogen generation chamber; and one light source to illuminate the oxygen generation chamber. In other cases, a single light source may be used.

[0044] FIG. 10 shows an example where a single light source, e.g., natural sunlight, is used to illuminate both chambers. For that purpose, two photocatalytic wafers may be placed in close proximity. In this example, the wafers are placed parallel to each other in the two chambers and / or aligned in other ways. The two photocatalytic wafers are spatially separated by a separator, e.g., a quartz clapboard. In this case, natural sunlight illuminates the two photocatalytic wafers simultaneously, which is useful in conjunction with concentrated sunlight. As in the above example, the chambers are illuminated with a chemical species of a redox pair (e.g., IO3 - / I - ) are coupled to each other (e.g., at the bottom of the chamber) to allow for the movement and exchange of hydrogen and oxygen, which allows for continuous hydrogen and oxygen generation upon splitting of the photocatalytic wafer. In this example, the coupling is provided through an opening in the separator. Again, hydrogen is selectively and separately generated in the chamber with the Rh / Cr2O3-InGaN photocatalytic wafer, despite the presence of the opening.

[0045] Either or both of the redox reactions (hydrogen and oxygen evolution) in the above examples can be driven by light, electricity, or any combination of light and electricity. Thus, the disclosed methods and systems offer flexibility in design, integration, and operation.

[0046] Iodide / iodate (IO3 - / I - Although the present invention has been described in connection with a redox shuttle comprising a bromine / bromide and Fe redox couple, additional or alternative redox couples can be used. For example, the disclosed methods and systems can be used with a bromine / bromide and Fe redox couple. 3+ / Fe 2+ may be used or included.

[0047] The above-described systems and methods overcome the challenges of photocatalytic water splitting to produce high-purity solar hydrogen from water and sunlight. The above examples establish experimental demonstration of a fully integrated photocatalytic solar water splitting system with separate H2 and O2 production. For example, with the use of InGaN photocatalytic nanostructures, the disclosed methods and systems achieved solar-to-hydrogen efficiencies of approximately 5%.

[0048] The present disclosure has been described with reference to specific examples which are intended to be illustrative only and not limiting of the disclosure. Modifications, additions and / or deletions can be made to the examples without departing from the spirit and scope of the disclosure.

[0049] The above description has been given for clarity of understanding only, and no unnecessary limitations should be understood therefrom.

Claims

1. a hydrogen generation chamber including a hydrogen generation port; an oxygen generating chamber including an oxygen collection port; an ion exchange membrane connecting the hydrogen generation chamber and the oxygen generation chamber; A photocatalytic structure, The photocatalyst structure is a first catalyst portion disposed within the hydrogen generation chamber; a second catalytic portion disposed within the oxygen generating chamber; the first catalyst portion is configured to produce hydrogen through the hydrogen production port; The second catalytic portion is configured to produce oxygen via the oxygen collection port.

2. The first catalytic portion includes a first side surface of the photocatalytic structure, The water splitting system of claim 1 , wherein the second catalytic portion comprises a second side of the photocatalytic structure.

3. 2. The water splitting system of claim 1, further comprising a separator disposed between the hydrogen production chamber and the oxygen production chamber, the photocatalytic structure being disposed along and integral with the separator.

4. The water splitting system of claim 1 , wherein the photocatalytic structure comprises a plurality of nanowires extending into the hydrogen production chamber.

5. The first catalyst portion is A plurality of nanowires extending outward from a substrate of the photocatalytic structure; and a distribution of catalytic nanoparticles across the plurality of nanowires.

6. The water splitting system of claim 5 , wherein each nanowire of the plurality of nanowires is configured for photo-generation of charge carriers.

7. The water splitting system of claim 1 , wherein the second portion comprises a catalyst layer supported by a substrate of the photocatalytic structure.

8. The water splitting system of claim 7 , wherein the second portion further comprises a metal layer disposed between the catalyst layer and the substrate.

9. The water splitting system of claim 1 , wherein the second portion comprises a distribution of catalytic nanoparticles supported by a metal substrate of the photocatalytic structure.

10. 1. A method for water splitting comprising the steps of: Immersing one of the oxygen evolution reaction (OER) photocatalytic structure and the hydrogen evolution reaction (HER) photocatalytic structure in water contained in a chamber and in which a species of a redox pair is present; exposing the chamber to light to illuminate the OER or HER photocatalytic structure and converting species of the redox pair by said illumination; collecting one of oxygen and hydrogen produced by the illumination; regenerating the species of the redox couple in the water; and and collecting the other of the oxygen and hydrogen produced during regeneration of the species of the redox pair.

11. Regenerating the species of the redox pair comprises: Switching between the OER photocatalytic structure and the HER photocatalytic structure to be immersed in the water in the chamber; The method of claim 10, comprising illuminating the OER or HER photocatalytic structure immersed in water after switching between the OER and HER photocatalytic structures.

12. The method further comprises: after the species of the redox pair are regenerated, switching again between the OER photocatalytic structure and the HER photocatalytic structure; 12. The method of claim 11, comprising repeatedly collecting oxygen or hydrogen and regenerating the species of the redox pair.

13. 11. The method of claim 10, wherein regenerating the species of the redox pair comprises applying a voltage to the water via a pair of electrodes immersed in the water.

14. 14. The method of claim 13, wherein applying the voltage is configured for electrolytic reduction of the other species of the redox pair in water.

15. The method further comprises: after the redox couple species have been regenerated, stopping the application of the voltage to the water; 14. The method of claim 13, comprising repeating the exposing of the chamber, collecting oxygen or hydrogen, and regenerating the species of the redox pair.

16. a hydrogen generation chamber including a hydrogen generation port; an oxygen generating chamber including an oxygen collection port; a fluid flow path connecting the hydrogen generation chamber and the oxygen generation chamber for exchange of redox pairs; a hydrogen evolution reaction (HER) photocatalytic structure disposed within the hydrogen generation chamber; and an oxygen evolution reaction (OER) photocatalytic structure disposed within the oxygen generating chamber.

17. The water splitting system of claim 16 , wherein the liquid flow path includes a channel between the hydrogen-production chamber and the oxygen-production chamber.

18. 17. The water splitting system of claim 16, further comprising a separator disposed along the hydrogen-producing chamber and the oxygen-producing chamber, the liquid flow path comprising an opening in the separator.

19. the OER photocatalytic structure includes a first substrate, a first plurality of nanowires extending outwardly from the substrate, and a first distribution of catalytic nanoparticles throughout the plurality of nanowires; 17. The water splitting system of claim 16, wherein the HER photocatalytic structure includes a second substrate, a second plurality of nanowires extending outwardly from the substrate, and a second distribution of catalytic nanoparticles throughout the plurality of nanowires.

20. The water splitting system of claim 16 , wherein each nanowire of the first and second plurality of nanowires is configured for photo-generation of charge carriers.

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