PRODUCTION

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U. S. Provisional Patent Application

No. 63/020,401 filed May 5, 2020, which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

A. Field of the Invention

[0002] The invention generally concerns an integrated photo-electro-chemical (PEC) reactor for solar hydrogen (¾) production from water using concentrated sunlight and alkaline media. The reactor can be resistant to alkaline media and can include an oxygen evolution reaction (OER) catalyst, a hydrogen evolution reaction (HER) catalyst, a permeable alkaline ion exchange membrane, and a solar cell. The solar cell can be isolated from the alkaline media.

B. Description of Related Art

[0003] Hydrogen produced by solar water splitting can be a clean and sustainable method. For example, photovoltaic (PV) electrolysis systems use PV panels/modules connected physically and electrically in series with an electrolyzer. Small scale PV electrolysis systems have demonstrated solar to hydrogen (STH) efficiencies of 10%. However, these systems suffer from high capital and balance of system costs.

[0004] Other devices include photo-electrochemical (PEC) devices that integrate III-V PV cells with electrocatalysts. Small-scale units can have a STH efficiency of more than 10% under 1-sun conditions. These devices suffer in that 1-sun conditions can have high associated costs. Furthermore, PEC devices suffer from stability (e.g., a 5 to 20% drop in performance) due to corrosion or heat damage to the components of the PV cell.

[0005] Various attempts to make more cost-effective systems have been described. For example, Kang et al. (Nature, Vol. 2, 2017, 17043) describes immersing a photocathode cell and photoanode cell co-integrated on a glass support in one chamber containing an acidic media. This PEC type device suffers in that the cells are exposed to acidic media. Further, this device has less than 15% STH. In another example, Modestino et al. (Energy & Environmental Science, 2014, Vol. 7, 297-301) describes a PEC device having two chambers with a channel joining the chambers to allow recirculation of the pH neutral media. This device suffers in that the channel was required for production of ¾ and had to be optimized to prevent gas crossover.

[0006] While various attempts to make PEC reactors have occurred, these devices typically suffer from inefficient STH and/or may not be cost-effective devices that produce hydrogen efficiently.

SUMMARY OF THE INVENTION

[0007] A solution has been discovered that addresses at least some of the problems associated with PEC reactors. In one aspect, the solution can include an integrated PEC reactor design. Design of the reactor provides for stable production of hydrogen and oxygen at high efficiencies. As shown in a non-limiting manner in the Examples, hydrogen can be produced with the systems and methods of the present invention at greater than 13% efficiency for over 100 hours. The design of the reactor can provide any one, any combination, or all of the following advantages. First, a solar cell can be directly integrated with the OER catalyst to avoid ohmic losses, thus increasing the efficiency of the system. In addition, while expensive current collector components can be used, they are not needed. Second, an alkaline electrolyte can be used, providing the advantage of using cost effective OER catalysts. A third advantage is that an alkaline electrolyte can be used to cool the solar cell, therefore reducing or even eliminating the need of external thermal management. A fourth advantage is that multijunction solar cell, which are more efficient than other types of solar cells ( e.g Si-based PV cells) can be used as the heat generated from the solar cell can be transferred to the electrolytic solution, which in turn helps in the conversion to chemical energy (hydrogen). A fifth advantage is that use of the integrated solar cell and OER catalyst can allow for a non-zero gap configuration as opposed to PEM based systems. A sixth advantage is that partially or fully isolating the solar cell from the electrolyte can reduce or even prevent corrosion of the solar cell. A seventh advantage is that the light concentrator can be capable of concentrating light, which can provide the ability to use at least 2 to 1500 sun (kW/m). As shown in a non-limiting manner in the Examples, maximum theoretical STH efficiency at ~ 18.3±0.7 % under light fluxes of 10 kW/m ² and 15 kW/m ² can be obtained with the systems and methods of the present invention. At higher light flux up to 207 kW/m ² , photocurrent densities up to 2.24 A/cm ² and associated H2 production rates of - 3.10 mL/min corresponding to 13%±0.6 % STH efficiency can be obtained.

[0008] In one aspect of the present invention, integrated photo-electro-chemical (PEC) reactors for solar hydrogen (Eh) production from water using harvested sunlight and alkaline media are described. A PEC reactor can include: (a) a housing comprised of material resistant to alkaline media; (b) an O2 producing chamber (first chamber) in the housing; (c) a Eh producing chamber (second chamber) in the housing, (d) a permeable alkaline ion exchange membrane positioned between the first chamber and the second chamber, and (e) a solar cell positioned outside the interior volume of the first chamber.

[0009] The first chamber can include an oxygen evolution reaction (OER) catalyst, a first electrolyte composition inlet, and an O2 composition outlet. The second chamber can include a hydrogen evolution (HER) catalyst, a second electrolyte composition inlet, and a H2 composition outlet. At least a portion of each of the housing, the first chamber, and the second chamber can include an alkaline resistant material. The first and second chambers can include a first and a second electrolyte composition, respectively, having a pH of 8 or higher, preferably 10 to 14, or more preferably 11 to 13 or about 12. In some embodiments, the first and second chambers are flow through chambers.

[0010] The permeable alkaline ion exchange membrane can be positioned between the first chamber and the second chamber and proximate the OER catalyst such that a gap exists between the permeable alkaline ion exchange membrane and the OER catalyst. The OER catalyst can be positioned 0.1 mm to 5 mm, preferably, 0.5 to 2 mm from the permeable alkaline ion exchange membrane. Such a gap can provide an advantage of allowing electrolyte to flow between the OER catalyst and the permeable alkaline ion exchange membrane such that excess heat due to high light flux can be dissipated from the cell. The excess heat maybe transferred and/or absorbed by electrolyte media. For example, the heat generated and transferred to electrolyte from energy not used by the solar cell. Heat generated can be from light flux of 5 sun or more, 10 sun or more, 20 sun or more, 30 sun or more, 40 sun or more, 50 sun or more. This is in contrast to conventional OER/permeable proton membranes configuration, where the permeable proton membrane is positioned next to the OER catalyst such that no gap exists.

[0011] The OER catalyst in fluid communication with the first chamber and can be in the form a film or a foil. A portion of the surface of the OER catalyst can be attached to a portion of the housing or first chamber. In a preferred embodiment, the total geometric area of the OER catalyst is larger than the total geometric area of the solar cell positioned on the OER catalyst surface, preferably the ratio of the OER catalyst geometric area to the solar cell geometric area is 1 to 100, more preferably 1 to 50, 1 to 25, 1 to 15, 1 to 10, or 2 to 10. The OER catalyst can include nickel, preferably nickel foil, NixFei-x, NixFeyCo(i-x-y), C0P2 or Nix FeyZna-x-y), where x and y are any number between 0 and 1 and x+y <1. In a preferred aspect, OER catalyst inhibits corrosion of the solar cell and dissipates heat generated by the solar cell.

[0012] The HER catalyst can include a noble metal, preferably, platinum, or a non-noble metal, preferably NixFeyCo(i-x), where x and y are between 0 and 1 and x+y < 1. In some aspects, the HER catalyst can be supported on a porous conductive material ( e.g Ti mesh, stainless steel mesh, nickel mesh or carbon paper, or a combination thereof).

[0013] The solar cell is electrically coupled to the HER catalyst and is capable of accepting harvested light and/or concentrated harvested light. At least a portion of a surface of the solar cell includes the OER catalyst. The solar cell can be a solar cell that provides a voltage from 1.7 to 3.5 V, preferably about 1.7 V. In preferred embodiments, the solar cell can be a photovoltaic (PV) cell, more preferably a gallium (Ga) based cell, most preferably a GalnP/GalnAs/Ge PV cell, InGaP/GaAs PV cell, or a GalnAsP/GalnAs PV cell.

[0014] In another aspect of the present invention, systems for producing H2 and O2 from water using solar energy are described. A system can include at least one of the PEC reactors of the present invention and a light concentrator positioned to provide harvested sunlight to the solar cell of the PEC reactor. The light concentrator is capable of directing concentrated light of at least 2 sun (kW/m), preferably 2 to 1500 sun (kW/m) to the solar cell. The light concentrator can be a Fresnel lens or a combination of a Fresnel lens and a secondary optic. The Fresnel lens can include polymethyl methacrylate (PMMA) or silicone on glass, and the secondary optic comprises PMMA, quartz or glass. In some embodiments, the system can include an array of PEC reactors. For example, the system can include at least two PEC reactors, where the O2 gas composition outlet of the first PEC reactor is in fluid communication with O2 composition outlet of the second PEC reactor and the H2 composition outlet of the first PEC reactor is in fluid communication with the H2 composition outlet of the second PEC reactor. [0015] In yet another aspect of the present invention, methods of producing oxygen (O2) and hydrogen (H2) using the reactors and/or systems of the present invention are described. A method can include: (a) providing harvested light to the solar cell; (b) providing a first aqueous electrolyte solution having a pH of 8 or more to the first chamber via the first electrolyte composition inlet and contacting the first electrolyte solution with the OER catalyst to produce O2 gas, and removing at least a portion of the electrolyte and O2 gas from the first chamber via the O2 composition outlet; and (c) providing a second aqueous electrolyte solution having a pH of 8 or more to the second chamber via the second electrolyte composition inlet and contacting the second electrolyte solution with the HER catalyst to produce a H2 gas, and removing at least a portion of the electrolyte and H2 gas from the second chamber via the H2 composition outlet. The method can also include separating O2 gas from the O2 composition and/or separating H2 gas from the H2 composition. The harvested light can be concentrated (e.g, by passing the light through a Fresnel lens and/or a secondary optic) and provided to the solar cell.

[0016] 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 or aspect discussed herein can be combined with other embodiments or aspects discussed herein and/or implemented with respect to any method or composition of the invention, and vice versa. Furthermore, compositions of the invention can be used to achieve methods of the invention.

[0017] The following includes definitions of various terms and phrases used throughout this specification.

[0018] The term “excess heat” refers to the energy from the sun that is not used by the solar cell. By way of example, a solar cell can use about 30% of the energy from the sun (e.g, transfers to current). The remaining energy (about 70%) becomes heat, which is transferred to the catalyst.

[0019] 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%. [0020] The terms “wt.%,” “vol.%,” or “mol.%” refers to a weight percentage of a component, a volume percentage of a component, 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.

[0021] The term “substantially” and its variations are defined to include ranges within 10%, within 5%, within 1%, or within 0.5%.

[0022] 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.

[0023] The term “effective,” as that term is used in the specification and/or claims, means adequate to accomplish a desired, expected, or intended result.

[0024] 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.”

[0025] 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.

[0026] The PEC reactors 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 phrase “consisting essentially of,” in one non limiting aspect, a basic and novel characteristic of the PECs of the present invention are their abilities to produce Eh and O2 from water using solar energy.

[0027] Other obj ects, 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. 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.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] 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.

[0029] FIGS. 1A and IB are schematics of the integrated PEC reactor of the present invention.

[0030] FIG. 2 is a schematic of cross-sectional view of the PEC reactor of FIG. 1A. [0031] FIG. 3 A is a schematic of the integrated PEC reactor of the present invention with a

Fresnel lens and secondary optic system for concentrating light.

[0032] FIG. 3B is a cross-sectional illustration of the PEC reactor of FIG. 3 A.

[0033] FIG. 4 is schematic of a PEC module, which consists of a cluster of individual PEC reactors of FIGS. 1A and IB connected together. The electrolyte flows into the module using two different lines for ¾ and O2.

[0034] FIG. 5 is schematic of the modules of FIG.4showing the gas collection system.

[0035] FIG. 6 is an illustration of the fabricated PEC reactor of the present invention.

[0036] FIG. 7 is a photograph of the PEC system of the present invention that includes the

PEC rector and Fresnel lens. [0037] FIG. 8 A is a graphical representation of H2 and O2 flow rates as a function of light concentration from the integrated PEC reactor. [0038] FIG. 8B is a graphical representation of H2/O2 ratio as a function of light concentration.

[0039] FIG. 9 is a graphical representation of the system efficiency of the PEC system of the present invention as a function of light concentration from the integrated PEC reactor.

[0040] FIG. 10 shows stability tests of integrated PEC reactor under alternating 100 and 200 suns light flux using a Fresnel lens together with H2 and O2 production rates plotted as a function of time. The green shaded area shows the data when operating under 200 suns.

[0041] FIG. 11 shows a depth profile of nickel surface where the ⁹⁰ NiOx secondary ion signal was monitored after electrochemical oxidation reaction. Inset shows the oxide layer thickness measured from the Secondary Ion Mass Spectrometry (SIMS) experiment plotted against electrochemical current density.

[0042] 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

[0043] A solution to at least some problems associated with producing hydrogen gas from water using solar energy has been discovered. In one aspect, the solution can include a PEC reactor that uses concentrated solar irradiation and produces H2 and O2, preferably stoichiometric H2 and O2, under alkaline conditions. Other embodiments of the invention are discussed throughout this application. The PEC reactor of the present invention uses (i) an integrated device, which can reduce the balance of the system cost, (ii) concentrated sunlight, which can reduce the photo-absorber cost, and (iii) alkaline electrolyte, which can reduce catalyst cost and eliminate the need of external thermal management. Notably, the PEC reactor of the present invention can be stable over 100 hours with a constant STH efficiency.

[0044] 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. PEC Reactor

[0045] Referring to FIGS. 1 A, IB, and 2, a PEC reactor of the present invention is described in more detail. FIGS. 1 A and IB are schematics of one PEC reactor of the present invention. FIG. 2 is a cross-sectional view of the PEC reactor of 1A. PEC reactor 100 includes housing 102, first chamber 104, second chamber 106, permeable alkaline ion exchange membrane 108, solar cell 110, OER catalyst 112, and HER catalyst 114. FIG. IB provides InGaP/InGaAs/Ge as one non-limiting example of a solar cell 110 that can be used in the context of the present invention.

[0046] Housing 102 encompasses first chamber 104, second chamber 106 and can be chemically resistant to alkaline media ( e.g ., materials that when contacted with media having a pH greater than 8 for greater than 100 hours does not or slowly decomposes). First chamber 104 includes OER catalyst 112, first electrolyte composition inlet 116, and O2 gas composition outlet 118. Second chamber 106 includes HER catalyst 114, second electrolyte composition inlet 120, and H2 gas composition outlet 122. Inlets 116 and 120 allow electrolyte media to flow into chambers 104 and 106 respectively. Outlets 118 and 112 allow the electrolyte and gas compositions to exit chamber 104 and 106, respectively during production of ¾ and O2 from water using solar energy. For example, a composition that includes O2 can exit O2 outlet 118 and a composition that includes H2 can exit H2 outlet 122. The housing, first and second chambers, the first and second inlets and the first and second outlets can be made from materials resistant to alkaline media. For example, such resistant materials (e.g., do not or slowly corrode or decompose over time) can be exposed to an aqueous media having a pH from 8 to 14, or from 8, 9, 10, 11, 12, 13, 14 or any range or value there between. Non-limiting examples of alkaline resistant materials include poly(methyl)methacrylate (PMMA), polytetrafluoroethylene (PTFE), acrylonitrile butadiene styrene (ABS), acetal copolymer, polyethylene, polypropylene homopolymer, polyphenylsulfone, phenyl sulfone, polyvinylidene fluoride, and the like.

[0047] The interior volume of first chamber 104 is separated from the interior volume of second chamber 106 by permeable alkaline ion exchange membrane 108. The outer surfaces of membrane 108 are coupled to interior surfaces of housing 102 using known methodology (e.g, friction, adhesive or the like). Having the arrangement of first chamber 104/membrane 108/second chamber 106 prevents gas (e.g, H2 and O2 gases) crossover between the chambers. Membrane 108 can allow hydroxide ions (OH ) generated in second chamber 102 to permeate into first chamber 104. Membrane 108 can be any alkaline ion exchange membrane. Non limiting examples of an alkaline ion exchange membrane include Sustainion® X37-50 (Dioxide Materials, Inc. USA), Fumasep FAPQ-375 (FuMA-Tech GmBH, Germany), polybenzimidazole fiber (PBI), AMI-7001 (Membranes International, USA), and the like. Membrane 108 is positioned proximate OER catalyst 112 to form gap 124 (See, FIG. 2). Gap 124 allows electrolyte to flow between membrane 108 and OER catalyst 112 such that excess heat from the cell, due to high light flux, is dissipated from the OER catalyst. A distance between membrane 108 and OER catalyst 112, or a size of gap 124, can be 0.1 to 5 mm, preferably 0.5 to 2 mm, or 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5,

1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6,

3.7, 3.8, 3.9, 4.0, 4.1, 4.2, 4.3, 4.4., 4.5, 4.6, 4.7, 4.8, 4.9, or 5.0, or any value or range there between.

[0048] Referring to FIG. 2, OER catalyst 112 can be positioned in opening 126 of housing 102. As shown, OER catalyst 112 is integrated with surface 128 of housing 102 and surface 130 of first chamber 104. In some embodiments, surface 130 of first chamber 104 is also a housing surface. Integration of the OER catalyst with the housing and/or first chamber can be done using known adhesive methodology. For example, an epoxy resin can be used to adhere the OER catalyst to the housing and/or first chamber surfaces. At least a portion of surface 132 of OER catalyst 114 is in fluid contact with the interior volume of first chamber 104. OER catalyst can be any known OER catalyst and can be in the form of a film or metal foil. In preferred instances, the OER catalyst includes nickel (Ni), cobalt (Co), iron (Fe), zinc (Zn) or combinations thereof. For example, the OER catalyst can be nickel foil, NixFei-x, NixFe _y Co(i- x-y), C0P2 or Nix FeyZno-x-y), where x and y are any number between 0 and 1 and x+y <1.

[0049] A portion of OER catalyst surface 134, which is opposite of surface 132 and outside of the interior volume of first chamber 104, can be integrated with solar cell 110. Integration of solar cell 110 with surface 134 of OER catalyst can be performed using known methods. For example, a metal composition ( e.g a silver paste or gallium eutectic paste) can be applied to surface 134 and solar cell 110 positioned on the metal composition. In a preferred embodiment, a silver paste can be used due to its good electrical and thermal conductivity. The composition can be heated to attach the solar cell to the OER catalyst. Because the OER catalyst is between the interior volume of chamber 104 and solar cell 110, the OER catalyst can inhibit or limit corrosion of the solar cell as the electrolyte solution does not contact the solar cell. The OER catalyst can also dissipate or transfer heat generated by the solar cell during use to the electrolyte media in gap 124. The heat may be absorbed by the electrolyte media. The OER catalyst can have a total geometric area larger than the total geometric area of the solar cell. For example, the ratio of the OER catalyst geometric area to the geometric solar cell area can be from 1 to 100, more preferably between 2 and 10, or from 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 100, or any value or range there between.

[0050] Solar cell 110 can be any solar cell that provides a voltage of 1.7 to 3.5, or 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, or any value or range there between to the system. In one embodiment, the voltage is about 1.7 V. In some embodiments, the solar cell can be a double junction (2J), a triple junction (3 J) photovoltaic cell, or both integrated with the OER catalyst. Non-limiting examples of photovoltaic cells include gallium (Ga)-based cells. Non-limiting examples of Ga-based cells GalnP/GalnAs/Ge PV cell, InGaP/GaAs/Ge PV cell, or a GalnAsP/GalnAs PV cell. As shown in FIG. IB, the solar cell is, for example, a GalnP/GalnAs/Ge PV cell with the Ge portion of the cell attached to the OER catalyst. The solar cell accepts light from light concentrator, which is discussed in more detail with reference to FIGS. 3A and 3B and transforms the light to energy. Solar cell 110 is electrically coupled through connector 136 ( e.g a wire or a foil) to the HER catalyst.

[0051] HER catalyst 104 is positioned in chamber 106 and proximate alkaline ionic membrane 108. HER catalyst 104 can include catalytic metal deposited on a support. The catalytic metal can be a noble metal or a non-noble metal. Noble metals include palladium (Pd), platinum (Pt), gold (Au), rhodium (Rh), ruthenium (Ru), rhenium Noble (Re), osmium (Os), iridium (Ir) or any combinations or alloys thereof. Non-noble metals include nickel (Ni), molybdenum (Mo), or combinations thereof. The support can be a porous conductive material such as, for example, a conductive metal mesh or carbon paper. Non-limiting examples of supports include titanium (Ti) mesh, stainless steel mesh, nickel mesh or carbon paper, or combinations thereof. The catalytic metal can be deposited onto the support by any technique such as sputtering, electrodeposition, drop casting, impregnation, deposition precipitation, and the like.

B. PEC Reactor System

[0052] Referring to FIGS. 3A and 3B, PEC system 200, which includes light concentrator 138 in combination with PEC reactor 100 is illustrated. FIG. 3 A is a schematic of light concentrator and FIG. 3B is a cross-sectional representation of the system shown in FIG. 3A. Light concentrator 138 can include a solar concentrator 140. Solar concentrator 140 can be any known solar concentrator. Solar concentrator 140 harvests sunlight, concentrates the sunlight, and provides the concentrated sunlight to the solar cell. Solar concentrator 140 can be a Fresnel lens. The Fresnel lens can be made of PMMA or silicone on glass (SOG) and be any size or shape. The size and/or shape can be determined based on the amount of solar concentration required by the system. For example, the solar concentrator can provide least 2 sun (kW/m), 2 to 1500 sun (kW/m), 10 to 1000 sun (kW/m), 50 to 500 sun (kW/m) or any range or value there between to the solar cell. In some embodiments, optional secondary optic 142 can be adhered ( e.g ., glued with optical glue) to at least a portion of surface 144 of solar cell 110 that is opposite of the surface coupled to the OER catalyst. Optional secondary optic 142 can be used for uniform light irradiation of solar cell 110. Non-limiting examples of optional secondary optic 142 materials include PMMA, quartz or glass. In some embodiments, solar concentrator, optional secondary lens and PEC reactor are all one unit. For example, the solar concentrator and/or the optional secondary optic 142 can be attached to the PEC reactor and sold or used as a PEC reactor.

[0053] Referring to FIG. 4, modular form 400 of the PEC reactors is illustrated. Module 400 can include at least two reactors (100, 100’) joined together. As shown, the array or module has 15 PEC reactors connected together. As shown, electrolyte enters the module via inlets 402 and 404. O2 gas composition exits through outlet 406 and Eh gas composition exits through outlet 408. In such a module, the first PEC reactor is in fluid communication with PEC electrolyte outlets of the second PEC reactor (See, FIG. 5). Electrolyte and produced O2 flow from the first reactor, second and h ^L reactor and exit through the O2 outlet 406. Similarly, Eh composition outlet of the first PEC reactor is in fluid communication with the Eh PEC electrolyte composition outlet of the second PEC reactor (See, FIG. 5). Electrolyte and produced O2 flow from the first reactor, second reactor, and n ^th and exit through the Eh outlet 408. Referring to FIG. 5, a schematic of the gas collection system is illustrated. In module 400, the electrolyte flows into each module via electrolyte inlets 402 and 404. Using two different conduits for collection of the gas compositions. For example, Fh is collected with Fh outlet 408 and O2 composition is collected using outlet 406.

C. Method of Producing Hydrogen (H2) and Oxygen (O2)

[0054] Using PEC reactor 100, system 200, and/or module 400, H2 and O2 can be produced from water using solar energy. Alkaline electrolyte (e.g., having a pH of 8 or more) can enter chamber 104 via electrolyte inlet 116 and chamber 106 via electrolyte inlet 120. The electrolyte can be moved into the chamber using standard fluid moving apparatus such as a pump. As electrolyte fills chambers 104 and 106, it can flow out of the chambers through outlets 118 and 122, respectively in a continuous manner or a non-continuous manner. Sun energy can be concentrated by light concentrator 138 and provided to reactor 100. In a preferred embodiment, sun energy is concentrated with solar concentrator 140 to increase the amount of sun energy (e.g, at least 2 sun (kW/m) provided to solar cell 110. In other embodiments, the sun energy is concentrated using solar concentrator 140 and then directed to solar cell 100 using optional secondary optic 142. Solar cell 110 energizes HER catalyst 108 and helps drives the water splitting reaction in the alkaline media to produce ¾ and OH ions. Produced ¾ composition exits the PEC reactor through outlet 122. The OH ions created in chamber 106 can permeate the alkaline ion exchange membrane 108 and enter chamber 104. In chamber 104, the contact of the alkaline media in the presence OER catalyst 112, produces O2. Produced O2 composition exits chamber 104 via O2 outlet 118. The produced compositions can be stored or further processed to purify the H2 and O2. In some embodiments, only H2 and/or O2 gas is produced and removed from PEC reactor 100.

EXAMPLES

[0055] 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

(PEC Reactor Manufacture)

[0056] The integrated PEC reactor was fabricated using PMMA as the housing ( e.g ., housing 102). A 3J III-V PV solar cell (e.g., solar cell 100, Azure Space) with GalnP/GalnAs/Ge structure was used in the reactor. The size of each cell was 0.3025 cm ² . FIG. 6 is an illustration of the PEC reactor (e.g, reactor 100). Nickel foil (e.g, 112 OER catalyst) with an area of 3 cm ² and thickness 0.25 mm was used to protect the PV cell and act as an OER catalyst. Sputtered platinum with (50 - 70) nm thickness supported onto Ti mesh was used as a HER catalyst (e.g, HER catalyst 114). The area of the Fresnel lens (PMMA, Edmund Optics, #43-025) used was 19.6 cm ² and the area of the light spot was 0.196 cm ² (See, FIG. 7). Example 2

(Generation of Eh and O2 using the PEC Reactor of the Current Invention)

[0057] General Procedure. An Asahi Spectra 320W solar simulator (350-1800 nm) was used as the light source. A Fresnel lens concentrated the light onto the 3J PV cell PMMA (Edmund Optics, #43-025) based Fresnel lens with area approximately of 19.6 cm ² was used to concentrate light onto 3 J PV cell kept at the focal distance from the lens (5 cm). The light flux was measured using a pre-calibrated monocrystalline silicon reference cell (Newport, 91150-KG5) and a pre-calibrated high concentration triple junction (3J) GalnP/GalnAs/Ge reference cell (Azure space, 3C42A). The light flux measurements were further verified using flux measurements by a spectroradiometer (Spectral Evolution SR- 500), in range of 350-1100 nm).

[0058] A potassium hydroxide/water composition (5M KOH) was pumped to the integrated PEC Reactor of the present invention where H2/O2 gases were generated from water. The generated H2/O2 gases were collected using two separate inverted eudiometers. The total volume of the KOH in the collection system was approximately 0.5 L. At 5M concentration, the solubility of oxygen was about 0.1 x 10 ³ moles/L/bar. Thus, maximum dissolved oxygen at 1 atm was determined to be about 0.2 x 10 ³ moles or 0.00892 mL O2; which wss negligible when compared to the amount of O2 detected. To monitor H2, a gas chromatograph (Agilent Technologies, 7890A) equipped with a thermal conductivity detector (TCD) connected to Porapak Q packed column (2 m long, 1/8 in. external diameter) at 45 °C was used with N2 as carrier gas (flow rate: 20 mL/min; pressure: 8 psi). Oxygen was monitored using another GC (Thermo Scientific, Trace 1300) equipped with a TCD connected to a packed molecular sieve (5 A) column (2 m long, 1/8 in. external diameter) with He as the carrier gas (flow rate: 1.5 mL/min; pressure: 22 psi).

[0059] The system efficiency reported was defined as: System efficiency = Energy from H2 produced / Energy of total light incident onto the Fresnel lens. FIGS. 8A and 8B show that the rates of H2 and O2 increased linearly with sun concentration with H2/O2 ratio of 1 98±0.13. The calculated system efficiency was 12.5±0.6 as shown in FIG. 9. An example of how the system efficient was calculated for 100 suns is shown below:

H2 flow rate = (2.16 mL/min) / (22,400 mL/mol) / (60 s/min) = 1.08* 1 O ⁶ mol/s. Energy from H2 produced = H2 flow rate * Chemical energy stored in H2 = 1.08*1 O ⁶ mol/s * 237,000 J/mol = 0.256 W. Energy of total light incident (1 sun) onto the Fresnel lens = 0.1 W/cm ² * Fresnel lens area = 0.1 W/cm ² * 19.6 cm ² = 1.96 W/.

System efficiency = 0.256 W / 1.96 W * 100 = 13.1 %.

Example 3

(Long Term Stability Testing)

[0060] The PEC Reactor was the same as that of Example 1, with the exception that the nickel foil had an area of 4 cm ² . The procedure of Example 2 was followed to generate he H2/O2 gases from water. FIG. 10 shows the long-term stability test data of the integrated PEC reactor of the present invention under alternating 100 and 200 suns light fluxes. The PEC reactor of the present invention showed stable performance with constant H2/O2 production (ratio of 2: 1) at a STH efficiency of- 13% for more than 100 hours without degradation, which is the highest reported stability for PEC Eh production under concentrated light. The reactor demonstrated good thermal management at high operating photocurrents.

Example 4

(Depth profiling of OER Catalyst - Nickel Foil)

[0061] While stoichiometric H2/O2 production in Examples 2 and 3 indicated true water splitting, the role of the nickel catalyst in protecting the 3J PV cell from the electrolyte was analyzed using dynamic secondary ion mass spectrometry (SIMS). The nickel foil was subjected to electrochemical oxidation in conditions similar to those of the PEC tests under different current densities. The nickel samples for SIMS were prepared in a two-electrode setup with Pt as counter electrode. The electrolyte used was 5M KOH and was continuously purged with high purity (99.999%) N2 during the reaction. The nickel was oxidized using chrono-potentiometry measurements at different currents. After measurements, the nickel samples were washed with deionized water and dried with N2 gas before loading into the UHV chamber for SIMS depth profiling experiments. Depth profiling experiments on the nickel foil samples were performed using dynamic SIMS instrument from Hiden analytical company (Warrington-UK) operated under ultra-high vacuum conditions, typically 10 ⁹ torr. A continuous Ar ⁺ beam of 4 keV energy was employed to sputter the surface while the selected ions were sequentially collected using a MAXIM spectrometer equipped with a quadrupole analyzer. The sputtered area was determined to be 750 ^c 750 pm ² . In order to avoid the edge effect during depth profiling experiments, data was acquired from a small area located in the middle of the eroded region. Using an adequate electronic gating, the acquisition area was scaled down to about 75 x 75 mih ² . The conversion of the sputtering time to sputtering depth scale was carried out by measuring the depth of the crater generated at the end of the depth profiling experiment using a stylus profiler from Veeco Company, with a calculated average sputtering rate of about 5.5 nm/min.

[0062] FIG. 11 shows the depth profile of the nickel surface where the ⁹⁰ NiOx secondary ion signal was monitored. The point at which the signal from the ⁹⁰ MOx secondary ion fragment became constant represents the oxide/bulk nickel interface. The SIMS results indicated that the oxide layer thickness increased linearly with time at constant current and with the latter at constant time; see for example the difference between 25 and 50 hour samples at 180 mA/cm ² . From these data, the rate of Ni oxidation was extracted and found to be 0.088 nm per mA cm ² per hour. This oxidation rate is negligible and does not disturb the stoichiometry of H2/O2. To confirm nickel oxidation is an electrochemical process, SIMS depth profiling of nickel dipped in alkaline electrolyte under dark conditions was performed and no oxidation with time was seen.

[0063] Although embodiments of the present application and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the above disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein can be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.