The present invention relates primarily to the production of hydrogen and oxygen from water by using nano engineered photoelectrochemical (PEC) devices while harnessing solar energy. Because of similarity in fundamental energy conversion theory and practice, this invention is also suitable for applications in photovoltaic, photo sensors and imaging technologies.
Hydrogen is the most promising fuel for future energy and economy. Growing demand for low cost clean hydrogen has drawn significant attention worldwide in recent years. However, in meeting upcoming challenges, the competitiveness of existing technologies is severely diminished as a result of problems related to their expensive cost, insecurity or environmentally harmful effects.
Efficient and cost effective renewable technologies for hydrogen production hold great promise. Among these renewable approaches, hydrogen production via PEC process is the most attractive one since it generates hydrogen from water by harvesting solar energy. Despite its remarkable potential, advances in this direction have been exceptionally underperformed. So far, it has been recognized that problems associated with low Solar To Hydrogen (STH) conversion efficiency, poor device operation durability and expensive construction materials are critical limiting factors which prevent the technologies from commercialization.
United States Patent Application Publication US 2006/0100100 titled “Tetrahedrally-Bonded Oxide Semiconductors For Photoelectrochemical Hydrogen Production”. The application relates to a photocatalyst that includes a tetrahedrally-bonded oxide semiconductor. In contrast to present invention, the application is strictly limited to the use of a tetrahedrally-bonded oxide semiconductor having an energy band gap in the range of 1.5 eV to 3.2 eV. The tetrahedrally-bonded semiconductor serves as a photocatalyst for the decomposition of water. The application lacks the photoactive lattice of multiple unit nano photo cells (hereinafter referred to as UNPCs) of the present invention and does not convert water to hydrogen gas at the same level of efficiency as the present invention.
United States Patent Application Publication US 2003/0121543 titled, “Photocatalytic Film Of Iron Oxide, Electrode With Such A Photolytic Film, Method Of Producing Such Films, Photoelectrochemical Cell With The Electrode And Photoelectrochemical System With The Cell, For The Cleavage Of Water Into Hydrogen And Oxygen.” This application teaches the use of iron oxide as a photocatalytic film that when illuminated by light oxidizes water to oxygen. The application is limited to the use of iron oxide for the direct cleavage of water with visible light and poor efficiency in hydrogen production. Thus the application exhibits shortcomings overcome by the present invention.
U.S. Pat. No. 6,409,893 titled, “Photoelectrochemical Cell” teaches an electrolyte composition comprising a polymer compound formed by polymerizing an ionic liquid crystal monomer containing at least one polymerizable group. Also disclosed are an electrochemical cell, a nonaqueous secondary cell and a photoelectrochemical cell, each comprising the electrolyte composition. The patent requires in its broadest claim at least one polymerizable group, and at least one substituted or unsubstituted alkyl or alkenyl group. In contrast to the present invention, the patent is directed at teaching a novel electrolyte for use in a photoelectrochemical cell, rather than a novel 3D UNPC lattice space structure incorporated in an anode, as taught by the present invention.
Generally speaking, the prior art devices and methods described and disclosed in these above mentioned patents and publications have at least one of the following shortcomings:
Inefficient photocatalysis
Poor material interface and corrosion
Rapid hole-electron recombination
Inadequate hole mobility
Poor operational durability of devices
Expensive construction materials
Therefore, cost effective hydrogen production remains as a major issue when adopting those existing approaches. Obviously, there is a compelling and crucial need in the art for highly efficient and durable PEC devices that produce hydrogen from water under sunlight illumination.
This invention provides a novel methodology that applies nano-scaled engineering to maximize STH conversion efficiency. The nano-engineered PEC anode invented in the current art improves photo current density by over ten times in magnitude when compared with technologies disclosed in the prior arts. Fundamentally, it overcomes all of aforementioned problems by nano scaled engineering.
UNPC is the key novel nano engineering concept of the present invention in maximizing STH. Each UNPC is comprised of a first component consisting of conductive or semi conductive material including photo active and inactive compositions, a second component consisting of a photo active semi conductive material or materials, and a third component consisting of a carrier mobility promoter. The second component acts as a photo active center utilizing the energy from photons to separate electrons from holes when the first component is conductive. The second component and the first component cooperate with each other to increase electron-hole separations when the first component is photo sensitive. The first component forms a backbone spreading out in a 3D structural fashion, forming a framework for UNPCs to attach on and conducts electrons when it is conductive only. The first component both conducts electrons and separates electrons from holes when it is photo sensitive. Sites having combined photo activities of component 1 and component 2 are referred to as photo active sites.
Multiple UNPCs are joined together through the first component or through the first component jointly with the second component to form a backbone, which directly contacts a conductive common bottom plate. On the opposite end, the UNPCs are linked by continuous carrier mobility promoters which lead to a common top plate. Thus, all UNPCs are bounded by a common top plate and a common bottom plate, forming a UNPC photo active mass in a Bravais lattice structural order or a hybrid structure. A nano engineered anode is formed when these multiple UNPCs are bounded by the top plate and the bottom plate in this manner.
A PEC cell basically consists of an anode, a cathode, water or an electrolyte, and a zone separator which prevents hydrogen and oxygen from mixing. When applying an adequate bias voltage, internal or external, electrons and holes from the UNPCs move into two separate flow directions, minimizing electron-hole recombination. The common top plate discharges the energy of the holes from the photo active sites into water and generates oxygen. Electrons from the photo active sites flow through the common bottom plate and feed into the cathode where water is reduced to form hydrogen gas.
Another embodiment of this invention provides advantages to fabricate flexible photo anodes such as bending photo plate anodes and fiber photo tube anodes in addition to rigid flat plates. By using the UNPC nano engineering design concept, a variety of flexible conductive substrates can be used as long as adequate nano materials can be fitted in. Fine metal woven, fiber glass and fiber glass cloth (proper coating with conductive materials) are partially transparent and flexible. It is conceivable that construction of UNPCs on these substrates achieves unique photo sensitivity or photon energy conversion efficiency suitable for powerful applications such as sensors in photonics and photovoltaic due to their potential optimum energy conversion efficiencies.
a is conceptual drawing of a UNPC 11 in accordance with a preferred embodiment of the present invention;
b is an illustration of the UNPC 11 structure of
a displays a typical mapping image of carbon (C) which is incorporated in a backbone of a nano structured anode as illustrated in
b displays a typical mapping image of tungsten (W) which is incorporated in a backbone of a nano structured anode as illustrated in
c exhibits a mapping image of sulfur (S) in component 3 which binds to photo active sites as illustrated in
d illustrates a mapping image of fluorine (F) in component 3 in connecting with photo active sites as illustrated in
The first embodiment of the invention is the novel design of unit nano photo cell (UNPC) 11 as shown in
This invention facilitates a variety of PEC functionalities by modifying 3D structural integration of these UNPCs. The structural integration includes crystal system manipulation (triclinic, monoclinic, orthorhombic tetragonal, hexagonal, cubic), junction adjustment between: 1) the component 1 and the component 2 and 2) the component 1 and carrier mobility promoter 3 and 3) the component 2 and carrier mobility promoter 3 and junction engineering within component 2. A higher order of the backbone lattice structure may be subject to more defect issues and imposes a more stringent demand on fabrication processes. On the other hand, a lower structural order or a hybrid structure tends to provide more flexible solutions as long as they are statistically viable. Conventional characterization methodology such as porosity measurement and surface morphology may provide some sensible information regarding how the anode film surface is presented. A deeper understanding of junctions, doping and spacing in a specific UNPC, however, provides the most profound solution to the optimum efficiency of a PEC cell.
As shown in
2H2O+2e−=H2+2OH− (reduction at cathode)
4OH−═O2+2H2O+4e− (oxidation at anode)
Or
2H++2e−=H2 (reduction at cathode)
2H2O=4H++O2+4e− (oxidation at anode)
Combined reaction: 2H2O=2H2+O2
As can be seen, applying this top plate 6 also prevents liquid electrolytes between anode 7 and separator 9 from direct contact with photo active material and thus improves photo anode operation durability. Clearly, this invention allows broad application of materials in anode fabrication and fundamentally eliminates the corrosion issue which is one of the major road blocks in prior arts.
This invention is applicable to variety of PEC cell structures. As shown in
Referring again to
It is conceivable that a variety of materials can be deployed to fabricate UNPC based PEC anodes, photovoltaic film stacks and sensor electrodes. This invention includes but is not limited to the following material systems: Fe2O3 with SiO2 doping; WO3 with carbon doping; TiO2/Ti; CdS with doped Si; CdTe/WO3; GaAs with doped Si; GaAs/WO3; TiO2/Fe2O3; InP; CuInSe2; copper indium gallium diselenide (CIGS) or variations and combinations thereof.
Solid Polymer Electrolyte such as Surlyn and Nafion etc. are good materials to serve as component 3 in the UNPCs as illustrated in
Other ionomers including polymeric electrolytes such as lithium poly(2-sulphoethyl methacrylate, sodium poly(phosphazene sulphonate), poly-diallydimethylammonium chloride and sodium polystyrene sulphonate are good to serve as component 3 if a proper treatment is received prior to applying for the UNPC application.
It is conceivable that proper carrier promoters can be deployed to fit photovoltaic and sensor electrode applications based on UNPC design concept Materials with conductivity dominantly attributed to n-doping, such as silicon doped with P or As, may serve as a promoter after applying adequate surface treatment for PV and sensor electrode applications.
Anode 7 construction includes but not limited by the following steps:
1. Bottom Plate 4 Preparation
2. Crystalline Material Growth
3. Photo Activity Enhancement
4. Top Down Capsulation
A PEC cell set-up depends on whether an external bias voltage is required. As shown in
An example of the nano-engineered PEC anode of stack 7 in
glass//F—SnO/C—WO3-Nafion/thin porous film/Pt gauze.
The thicknesses of the respective layers are approximately: 2 mm/2μ/4μ/10 μm/200 μm, respectively, for optimum sunlight or solar simulator illumination. More specifically, the fine structure of the UNPC in stack 7 shows nano crystalline photo active sites whose sizes are in the range of 20000-125000 nm3 and dimensions on each side are in the range of 25-50 nm. They are directly in contact with a layer of 2-6 nm carrier mobility promoters as demonstrated in
In certain specific embodiments, the film thickness, density and crystalline size of WO3 may be adjusted such that an optimum balance of backbone and promoter can be reached. For maximum photon capture, for example, the thickness of WO3 layer needs to be thick enough to allow a zero transmission for photons with wave length shorter 490 nm.
In certain embodiments, the PEC cell deploys metal substrates such as Ni, Ti and Al or metal grids, such as Ag and Au grids, supported conductive glass. Woven mesh, cloth and sheets are desirable to achieve special conductivity and structure and thus minimize resistive loss of conversion efficiency.
There are different PEC materials which can be deployed in a similar nano engineered fashion as aforementioned. One of the direct extensions of this UNPC is to apply Fe2O3, TiO2, WO3, CdS, CdTe, GaAs, etc for water splitting PEC processes. In addition to the methods described above, another way to optimize cell performance is to apply basic electrolytes such as aqueous solutions of NaOH, KOH or acidic electrolytes such as aqueous solutions of H2SO4 in addition to applying a SPE.
In such embodiments, flow dynamic processes can be used in a way that water or aqueous electrolytes can flow directly into the cell by applying mechanical or thermal transport means instead of bubbling in a static PEC cell.
Application of the nano engineered PEC anode on photovoltaic electrode is another embodiment of the current invention as shown in
The above detailed description of the present invention is given for explanatory purposes. It should be understood that all references cited herein are expressly incorporated herein by reference. It will be apparent to those skilled in the art that numerous changes and modifications can be made without departing from the scope of the invention. Hence, the whole of the foregoing description is to be construed in an illustrative and not a limitative sense, the scope of the invention being defined solely by the appended claims.