WO2008151060A1

WO2008151060A1 - Use of photoelectrochemical water splitting to generate materials for sequestering carbon dioxide

Info

Publication number: WO2008151060A1
Application number: PCT/US2008/065387
Authority: WO
Inventors: C. Deane Little; Joseph V. Kosmoski; Timothy C. Heffernan
Original assignee: New Sky Energy, Inc.
Priority date: 2007-05-30
Filing date: 2008-05-30
Publication date: 2008-12-11

Abstract

An integrated system for sequestering CO2 and producing renewable hydrogen, oxygen, acid and base includes a photoelectrochemical electrolysis unit, a hydrogen sequestration tank, an oxygen sequestration tank, an acid sequestration tank, a base sequestration tank and a gas contact area. The photoelectrochemical electrolysis unit is adapted to split water into oxygen and hydrogen using sunlight and includes a cathode adapted to produce hydrogen and base, an anode adapted to produce oxygen and acid, and an aqueous electrolyte solution. The hydrogen, base, oxygen and acid sequestration tanks collect and process hydrogen, base, oxygen and acid, respectively. The gas contact area is adapted to react CO2 with one of the base or a CO2 sequestering solution prepared using the acid. The integrated system provides a method of incorporating CO2 into valuable carbon-based products. Sales of these carbon negative products may subsidize renewable hydrogen production from water while reducing global atmospheric CO2.

Description

USE OF PHOTOELECTROCHEMICAL WATER SPLITTING TO GENERATE MATERIALS FOR SEQUESTERING CARBON DIOXIDE

BENEFIT CLAIM

[0001] This application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Patent Application No. 60/932,542, filed on May 30, 2007, entitled "Use of Photoelectrochemical Water Splitting to Generate Materials Suitable for Trapping Carbon Dioxide from the Atmosphere or Gas Streams", which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to the field of hydrogen production and carbon dioxide sequestration. More specifically, the present invention relates to an integrated system that uses solar energy in combination with photoelectrochemical water electrolysis to generate hydrogen and sequester carbon dioxide.

BACKGROUND

[0003] The electrochemical cleavage of water has traditionally been viewed as a method of producing hydrogen and oxygen gas. In traditional alkaline water electrolysis, two molecules of hydroxide base are produced and consumed for every molecule of hydrogen generated. A variant of water electrolysis is photoelectrochemical water splitting in which sunlight directly strikes photosensitive materials immersed in water, resulting in electrical excitation and splitting of the water molecule to form hydrogen gas and hydroxide base at the cathode and oxygen gas and acid at the anode.

[0004] Removing carbon dioxide from the atmosphere or from a gaseous source requires a very large energy input to overcome the entropic energies associated with isolating and concentrating diffuse gases. Current equipment and strategies for sequestering carbon dioxide from the atmosphere or for producing hydrogen are either inefficient or cost prohibitive. SUMMARY

[0005] In one embodiment, the present invention encompasses an integrated system for sequestering carbon dioxide from a gas stream and producing renewable hydrogen, oxygen, acid and base. The integrated system includes a photoelectrochemical electrolysis unit, a hydrogen sequestration tank, an oxygen sequestration tank, an acid sequestration tank, a base sequestration tank and a gas contact area. The photoelectrochemical electrolysis unit is adapted to split water into hydrogen and oxygen using sunlight and includes at least one cathode in a cathode region adapted to produce hydrogen and concentrated base in the form of hydroxide ions, at least one anode in an anode region adapted to produce oxygen and concentrated acid in the form of protons, and an aqueous electrolyte solution in contact with the cathode and the anode. The hydrogen and base sequestration tanks collect and process hydrogen and base, respectively, produced at the cathode region. The oxygen and acid sequestration tanks collect and process oxygen and acid, respectively, produced at the anode region. The gas contact area is adapted to react gaseous carbon dioxide with the base generated at the cathode or a carbon dioxide sequestering solution prepared using acid generated at the anode.

[0006] The integrated system set out above may include additional components. In one embodiment, the carbon dioxide reacts with the hydroxide base to form carbonate salt or bicarbonate salt. In another embodiment, the hydrogen, oxygen, acid, base, carbon dioxide, carbonate salts or bicarbonate salts are processed into value-added products. In another embodiment, the integrated system produces substantially no carbon dioxide, resulting in a net removal of carbon dioxide from the gas stream. [0007] According to other embodiments, the present invention encompasses a photoelectrochemical apparatus for generating renewable hydrogen and sequestering atmospheric carbon dioxide or carbon dioxide from a gas stream. The apparatus includes a photoelectrochemical electrolysis unit adapted to split water into hydrogen and oxygen using sunlight, a gas contact assembly, gas supply equipment and a separation chamber. The photoelectrochemical electrolysis unit is adapted to split water using sunlight and includes at least one cathode adapted to produce hydrogen and concentrated base in the form of hydroxide ions, at least one anode adapted to produce oxygen and concentrated acid in the form of protons, and an aqueous electrolyte solution. The gas contact assembly is adapted to receive carbon dioxide from an air or gas stream and hydroxide ions produced in the photoelectrochemical electrolysis unit so that the carbon dioxide contacts and reacts with the hydroxide ions to form bicarbonate or carbonate ions in solution. The separation chamber is connected to the gas contact assembly and is adapted to separate the bicarbonate or carbonate ions from the solution.

[0008] The apparatus described above may include additional components. In one embodiment, the apparatus includes gas supply equipment adapted to route carbon dioxide to the gas contact assembly. In another embodiment, the apparatus includes equipment for isolating and processing the hydrogen, oxygen, acid, base, carbonate or bicarbonate.

[0009] According to other embodiments, the present invention is a method of generating renewable hydrogen and sequestering carbon dioxide from a gas stream. The method first includes supplying sunlight to a photoelectrochemical electrolysis unit having an anode located in an anode region and a cathode located in a cathode region. Both the anode region and cathode region are in contact with an aqueous electrolyte. The method further includes producing oxygen gas and acid in the form of protons at the anode, producing hydrogen gas and base in the form of hydroxide ions, collecting the hydrogen gas, collecting the oxygen gas, removing acid from the anode region, removing base from the cathode region and contacting the hydroxide ions with a source of gaseous carbon dioxide to sequester carbon dioxide in solution as bicarbonate, carbonate or a mixture thereof.

[0010] The method set out above may include additional steps. In one embodiment, the method includes reacting the bicarbonate or carbonate with the acid produced in the photoelectrochemical electrolysis unit to generate concentrated carbon dioxide gas or super critical carbon dioxide. In another embodiment, the method includes isolating and processing at least one of the hydrogen, oxygen, acid, base, carbonate or bicarbonate. In another embodiment, the method includes utilizing at least one of the hydrogen, oxygen, acid, base, carbonate or bicarbonate as a reagent to produce a value-added product containing the sequestered carbonate, bicarbonate or stable form of carbon dioxide. In another embodiment, the method includes supplying direct current electricity from an energy source to the photoelectrochemical electrolysis unit. The energy source may be a renewable energy source, resulting in substantially no CO2 emissions.

[0011] In each of the embodiments set out above, the photoelectrochemical electrolysis unit may include additional features. In one embodiment, the photoelectrochemical electrolysis unit is one of a photovoltaic unit, a semiconductor- liquid junction unit or a photovoltaic/semiconductor-liquid junction unit. In another embodiment, the cathode is a photo-cathode. In another embodiment, the anode is a photo-anode. In another embodiment, at least one of the anode and cathode is a photo-electrode. In another embodiment, the photo-electrode is a p- or n- type semiconductor.

[0012] These and other aspects, processes and features of the invention will become more fully apparent when the following detailed description is read with the accompanying figures and examples. However, both the foregoing summary of the invention and the following detailed description of it represent one potential embodiment, and are not restrictive of the invention or other alternate embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a schematic diagram of a photoelectrochemical water splitting and carbon dioxide sequestration system, according to various embodiments of the present invention.

[0014] FIG. 2 is a schematic diagram of a photoelectrochemical electrolysis unit of the photoelectrochemical water splitting and carbon dioxide sequestration system of FIG. 1, according to various embodiments of the present invention. [0015] FIG. 3 is a schematic diagram of a photoelectrochemical electrolysis unit of the photoelectrochemical water splitting and carbon dioxide sequestration system of FIG. 1 connected to a solar source, according to various embodiments of the present invention.

[0016] FIG. 4 is a schematic diagram of value-added products that may be processed from the photoelectrochemical water splitting and carbon dioxide sequestration system of FIG. 1, according to various embodiments of the present invention. [0017] While the invention is amenable to various modifications and alternative forms, some embodiments have been illustrated by way of example in the drawings and are described in detail below. The intention, however, is not to limit the invention by those examples and the invention is intended to cover all modifications, equivalents, and alternatives to the embodiments described in this specification.

DETAILED DESCRIPTION

[0018] FIG. 1 shows a schematic diagram of an integrated phototelectrochemical (PEC) water splitting and carbon dioxide sequestration system 10 for generating renewable hydrogen and capturing carbon dioxide (CO₂), according to one embodiment To decrease the amount of carbon dioxide released into the atmosphere annually, it is critical to develop economically viable equipment and processes to provide renewable hydrogen as a fuel source and to remove vast quantities of carbon dioxide from the atmosphere by either sequestering it in a stable form or by converting it to valuable commodity products. The integrated PEC water splitting and carbon dioxide sequestration system 10 and its components produce hydrogen, oxygen, acid and base (hydroxide) through photoelectrochemical water splitting, followed by subsequent processing of one or more of these products to capture carbon dioxide as carbonate salt, bicarbonate salt or mineral carbonates. Photoelectrochemical water splitting is known in the art and is described, for example, in an article by Antonio Currao entitled "Photoelectrochemical Water Splitting", published in Vol. 61, No. 12 oϊChimia in 2007, pages 815-819, which is incorporated herein by reference. Using the base produced by the integrated PEC water splitting and carbon dioxide sequestration system 10 to capture carbon dioxide, renewable hydrogen is generated as a carbon negative rather than carbon neutral fuel and can be used as a large-scale application for reducing global carbon dioxide pollution. The integrated PEC water splitting and carbon dioxide sequestration system 10 creates carbon negative energy strategies for producing clean hydrogen fuel and reducing atmospheric carbon dioxide. The phrase "carbon dioxide negative" refers to the net overall reduction of carbon dioxide in the atmosphere or an air or gas stream. Thus, in stating that the integrated PEC water splitting and carbon dioxide sequestration system 10 is carbon dioxide negative, it is meant that the integrated PEC water splitting and carbon dioxide sequestration system 10 removes substantially more carbon dioxide than it produces. In addition, unlike traditional methods of manufacturing hydroxide base, no substantial carbon dioxide or chlorine gas is produced.

[0019] As shown in FIG. 1, the integrated PEC water splitting and carbon dioxide sequestration system 10 includes a solar energy source 12, an electrolysis unit 14 including a cathode 16, an anode 18 and an aqueous electrolyte source 20, a hydrogen sequestration tank 22, an oxygen sequestration tank 24, a base sequestration tank 26, an acid sequestration tank 28, a captured carbon dioxide apparatus 30 connected to the base sequestration tank 26, a carbon dioxide capture apparatus 32 connected to the acid sequestration tank 28, a carbon dioxide product system 34 and a fuel cell 36. Suitable electrolysis units are known in the art and several electrolysis units are compatible with the present PEC water splitting and carbon dioxide sequestration system 10. In one embodiment, the electrolysis unit 14 includes a standard electrolysis apparatus driven by a photovoltaic cell, such as a solar panel, for converting sunlight to direct current.

[0020] In an alternative embodiment shown in FIG. 2, the solar energy source 12 can accept direct sunlight, in which case the cathode 16 is replaced with a photo-cathode and the anode 18 is replaced with a photo-anode. Typically, the photo-cathode 16 and photo-anode 18 are made from p-type semiconductors and n-type semiconductors, respectively. When photo-electrodes are used, the redox reaction occurs directly at the semiconductor-liquid interface.

[0021] In yet another embodiment shown in FIG. 3, the solar energy source 12 accepts solar energy from a combination of direct sunlight and a photovoltaic cell 38. In this case, either the cathode 16 is a photo-cathode or the anode 18 is a photo-anode. All combinations of photovoltaic driven electrodes and semiconductor -liquid junction photo-electrodes are within the scope of this invention.

[0022] Referring back to FIG. 1, the cathode 16 and anode 18 should be stable in aqueous solutions with a wide pH rangeand should also have band gaps and band edges suitable for trapping sunlight and splitting water. To maximize efficiency of the electrolysis unit 14, the band gap of the cathode 16 or anode 18 should substantially overlap with the solar spectrum and the electron transfer energies should overlap with the dissociation energy of the water molecule. Optionally, a multi-step photon trapping device may be used to more comprehensively utilize the solar light spectrum. To ensure that enough energy is provided to drive the reaction in the electrolysis unit 14, direct current voltage generated by another source, such as windpower, hydroelectric, geothermal or a fuel cell, can also be used to supplement the energy provided from the sun. In addition, the supplemental energy provided by the direct current voltage reduces the energy required from the sun to a range that better aligns with the solar spectrum. In one embodiment, the supplemental electrical energy is generated in the fuel cell 36 using some of the hydrogen produced in the electrolysis unit 14. By using electrical energy generated by the fuel cell 36, more acid and base may also be generated. Examples of other sources of electrical energy include waste organic matter in a fuel cell or inexpensive fuels, such as methanol. [0023] Generally, the electrolysis unit 14 includes the cathode 16 located in a cathode region 40, the anode 18 located in an anode region 42 and the aqueous electrolyte solution 20. The cathode and anode regions 40, 42 may be separated by any suitable means, such as an ion selective membrane, an electric potential applied to at least one metal screen, or a porous glass frit. In addition, gravity feed, active pumping, or gas pressure displacement may be used to divert basic and acidic electrolytes produced in the cathode and anode regions 40, 42 into the base and acid sequestration tanks 26, 28, respectively. Alternatively, simple acid and base trapping resins may also be placed in contact with the aqueous electrolyte solution 20a to trap protons and hydroxide ions produced in the electrolytic unit 14 for later use.

[0024] The integrated PEC water splitting and carbon dioxide sequestration system 10 also includes a gas contact assembly 44, gas supply equipment 46 and a separation chamber 48. The gas contact assembly 44 is connected to a gas stream containing carbon dioxide and is adapted to receive hydroxide ions so that the carbon dioxide contacts and reacts with the hydroxide ions to form bicarbonate or carbonate ions in solution.. In one embodiment, the gas contact assembly 44 receives hydroxide ions from the base sequestration tank 26.

[0025] As shown in FIG. 1, the gas supply equipment 46 is adapted to route a gas stream containing carbon dioxide to the gas contact assembly 44. In one embodiment, the gas supply equipment 46 is connected to the captured carbon dioxide apparatus 30 and supplies the carbon dioxide to the gas contact assembly 44. In an alternative embodiment, the gas contact assembly 44 receives carbon dioxide directly from the captured carbon dioxide apparatus 30. In another embodiment, the captured carbon dioxide apparatus 30 and the gas contact assembly 44 are integrated into one device such that the gas contact assembly 44 receives carbon dioxide directly. In these two embodiments, the PEC water splitting and carbon dioxide sequestration system 10 can function without gas supply equipment.

[0026] The separation chamber 48 concentrates, processes or isolates the bicarbonate and carbonate formed from the reaction of hydroxide ions with carbon dioxide in the gas contact assembly 44.

[0027] When photons from the solar energy source 12 strike the integrated PEC water splitting and carbon dioxide sequestration system 10, electrons excited by the sunlight cause photoelectrochemical cleavage of the water to produce hydrogen, oxygen, base and acid. The electrolysis unit 14 reduces water at the cathode 16 to produce renewable hydrogen and hydroxide base and oxidizes water at the anode 18 to produce oxygen and protons. The protons combine with anions present in the aqueous electrolyte solution 20a to form acid. Similarly, the hydroxide ions combine with cations present in the aqueous electrolyte solution 20a to form base. This sequence of reactions is similar for both standard electrodes and photo-electrodes. [0028] In some embodiments, the aqueous electrolyte solution 20a contacts the cathode 16 and the anode 18 and is responsible for transferring charges and moving ions within the electrolysis unit 14. The aqueous electrolyte source 20 may include water or any highly concentrated electrolyte solution, such as sodium, potassium, calcium, or magnesium sulfate, nitrate, phosphate, or carbonate. According to various embodiments, the aqueous electrolyte includes an alkali salt that is a salt of the l(IA) or 2(1IA) groups of the periodic table. Exemplary electrolytes suitable for use with the present invention include, but are not limited to: sodium sulfate, potassium sulfate, calcium sulfate, magnesium sulfate, sodium nitrate, sodium phosphate, potassium nitrate, sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium phosphate or potassium carbonate. Other suitable electrolyte solutions include sea water and aqueous sea salt solutions. In one embodiment, the aqueous electrolyte source 20 contains substantially no chloride such that the integrated PEC water splitting and carbon dioxide sequestration system 10 produces essentially no chlorine gas. In another embodiment, the integrated PEC water splitting and carbon dioxide sequestration system 10 produces less than about 100 parts per million (ppm) of chlorine, particularly less than about 10 ppm of chlorine, and more particularly less than about 1 ppm of chlorine.

[0029] In an exemplary embodiment, the aqueous electrolyte solution 20a is a saturated solution of sodium sulfate prepared by adding an excess of sodium sulfate to about 1000 liters of clean distilled water placed in a 1200 liter electrolyte processing and storage reservoir. The solution is maintained at about 30 degrees Celsius (⁰C) while being mechanically mixed overnight. The resultant solution is pumped into the electrolysis unit 14 using a pump or gravity feed. Upon photoelectrochemical water splitting, the aqueous electrolyte solution 20a yields hydrogen and sodium hydroxide in the cathode region 40 while producing oxygen and sulfuric acid in the anode region 42. [0030] The concentration of the aqueous electrolyte solution 20a can vary depending on the demands of the electrolysis unit 14 and the overall integrated PEC water splitting and carbon dioxide sequestration system 10. The aqueous electrolyte concentration may also vary with changes in the temperature, ρH_} and/or the selected electrolytic salt. According to one embodiment, the concentration of the aqueous electrolyte solution 20a is approximately IM. According to another embodiment, a saturated aqueous electrolyte solution 20a is maintained within the electrolysis unit 14.

[0031] As mentioned above, the solar energy supplied to the electrolytic unit 14 from the solar energy source 12 causes photoelectrochemical cleavage of the water in the electrolytic unit 14 to produce hydrogen and base at the cathode 16 and cathode region 40 and oxygen and acid at the anode 18 and anode region 42. The rising gases within the aqueous electrolyte solution 20a cause dynamic fluid convection, which is optimized by the electrolysis design. The convection flow of the aqueous electrolyte solution 20a within the cathode 16 and anode 18 minimizes the recombination of the newly generated base and acid typically experienced by traditional electrolysis units. This allows the concentration of base and acid within the cathode 16 and anode 18, respectively, to increase by greater than one hundred-fold relative to their initial concentrations. Thus, in an aqueous electrolyte solution 20a producing hydroxide base and hydronium acid, the concentration of hydroxide ions at the cathode 16 is increased by more than one hundred-fold and the concentration of hydronium ions at the anode 18 is increased by more than one hundred-fold. [0032] The base and acid may be further purified using ion selective membranes, semi-permeable membranes or hydrogel barriers. The membranes or barriers 50 (shown in FIGS. 2 and 3) are positioned between the cathode region 40 and the anode region 42, enclosing a central salt feed solution. The membranes or barriers 50 direct cations toward the cathode region 40 to combine with hydroxides to form base and anions toward the anode region 42 to combine with protons to form acid. Salt contamination of the acid and base is thus minimized while simultaneously reducing loss of acid and base due to recombination. [0033] The continuous production of base and acid during photoelectrochemical water splitting results in a pH difference between the cathode region 40 and the anode region 42 of the electrolysis unit 14. According to one embodiment, the difference in pH between the cathode region 40 and the anode region 42 is at least about 4 pH units. According to another embodiment, the difference in pH between the cathode region 40 and the anode region 42 is at least about 6 pH units. In yet another embodiment, the difference in pH between the cathode region 40 and the anode region 42 can reach about 10 pH units or more. The difference in pH between the cathode region 40 and the anode region 42 can be maintained by preventing the catholyte formed in the cathode region 40 and the anolyte formed in the anode region 42 from combining. Alternately, the pH difference between cathode region 40 and the anode region 42 can be minimized by adding fresh electrolyte to the system via a central feed line or reservoir, or by using suitable pH buffers that allow accumulation of acid and base ions without apparent pH changes. Such a strategy may minimize the energy input required to split water.

[0034] Once the concentrations of base and acid reach a minimal increase of one hundred-fold relative to their initial electrolyte concentration, resulting in a pH difference between the cathode 16 and anode 18 of about 4 or more, fresh aqueous electrolyte solution 20a is pumped from the aqueous electrolyte source 20 to the cathode 16 and anode 18. To equilibrate the volume of liquid in the cathode 16 and anode 18, resultant base and acid is removed from the cathode 16 and anode 18, respectively, creating a continuous flow electrolysis system. [0035] After water in the aqueous electrolyte solution 20a has been split to produce hydrogen, oxygen, base and acid, the products are sequestered and collected. The gases are routed from the cathode 16 or anode 18 to storage or flow systems designed to collect such gases. The low density of the gases relative to the aqueous electrolyte solution 20a causes the gases to rise. The reaction regions are designed to direct this flow up and out of the cathode 16 and the anode 18 and into adjacent integrated areas. The hydrogen, base, oxygen and acid are physically diverted for collection in the hydrogen sequestration tank 22, the base sequestration tank 26, the oxygen sequestration tank 24 and the acid sequestration tank 28, respectively. [0036] The hydrogen and oxygen are collected in the hydrogen sequestration tank 22 and the oxygen sequestration tank 24, respectively, and may be used to generate electricity to power a fuel cell (such as fuel cell 36). The hydrogen and/or oxygen may also be used internally to react with other products of the integrated PEC water splitting and carbon dioxide sequestration system 10 to create value-added products. In addition, hydrogen and/or oxygen may be removed from the integrated PEC water splitting and carbon dioxide sequestration system 10 as products to be sold or used as fuels or chemical feedstocks.

[0037] The base generated by the electrolysis unit 14 is sent to the base sequestration tank 26 and is sold, used as a carbon neutral commodity or chemically reacted with carbon dioxide gas to form carbonate or bicarbonate. When used to capture carbon dioxide, the carbon dioxide is chemically transformed to carbonate or bicarbonate salts. The carbon dioxide may be captured by reacting, sequestering, removing, transforming or chemically modifying gaseous carbon dioxide in the atmosphere or gas stream. The gas stream may be flue gas, fermenter gas effluent, air, biogas, landfill methane, or any carbon dioxide-contaminated natural gas source. The carbonate salts may subsequently be processed to generate a variety of carbon-based products. For example, the carbonate salts may be concentrated, purified, enriched, chemically reacted, diverted, transformed, converted, distilled, evaporated, crystallized, precipitated, compressed, stored or isolated.

[0038] The reaction of the base with the carbon dioxide can be passive, without any air-water mixing. An example of a passive reaction includes an open-air reservoir filled with hydroxide base, or a solution containing the base. This reaction is spontaneous and can be driven by increased concentrations of base. The reaction can also proceed by active mechanisms involving the base or carbon dioxide. An example of an active reaction includes actively spraying, nebulizing, stirring, or dripping a basic solution in the presence of the carbon dioxide. In another example, carbon dioxide is actively reacted with the base by bubbling or forcing the gas stream through a column or reservoir of base generated by the electrolysis unit 14. Combinations of active and passive carbon dioxide trapping systems are also envisioned. In both cases, bicarbonate and carbonate salts are formed by the integrated PEC water splitting and carbon dioxide sequestration system 10. These reactions may take place within the integrated PEC water splitting and carbon dioxide sequestration system 10 or the hydroxide base may be removed from the system and transported to another site for capturing carbon dioxide from the atmosphere or a gas stream using the passive or active techniques previously described. [0039] The acid produced by the electrolysis unit 14 is routed to the acid sequestration tank 28 and can be processed and removed from the integrated PEC water splitting and carbon dioxide sequestration system 10 for sale as a carbon neutral commodity. Used internally, the acid may be used to prepare certain carbon dioxide sequestering compounds, which are then used to capture carbon dioxide from the atmosphere or gas stream. For example, the acid can be reacted with a material that when exposed to a strong acid is converted to a carbon dioxide sequestering material. Exemplary materials that can be converted to a carbon dioxide sequestering material by reaction with a strong acid include, but are not limited to, the following: the mineral clay sepiolite, serpentine, talc, asbestos, and various mining byproducts such as asbestos mining waste. According to one exemplary embodiment, the common mineral serpentine can be dissolved in sulfuric acid producing a solution of magnesium sulfate while precipitating silicon dioxide as sand. Addition of sodium hydroxide creates a mixture of magnesium sulfate and magnesium hydroxide. The process also converts toxic asbestos and asbestos waste into non-toxic carbon dioxide binding materials. Subsequent exposure of the magnesium solution to carbon dioxide from the atmosphere or gas stream results in the formation of either magnesium carbonate or magnesite, both of which form precipitates. These precipitates are well-suited for the production of construction blocks. According to further embodiments of the present invention, the carbon dioxide sequestering material may be further reacted with strong acid to release carbon dioxide gas under controlled conditions. The carbon dioxide released from the carbon dioxide sequestering materials may be captured and stored for further processing.

[0040] The acid may also be used by the integrated PEC water splitting and carbon dioxide sequestration system 10 as a chemical reagent to create other value-added products. In one case, the carbonate and bicarbonate salts are isolated after reacting the base with carbon dioxide. The acid can then be combined with carbonate and bicarbonate salts, for example from the separation chamber 48, to release the carbon dioxide from the carbonate or bicarbonate salts in a controlled manner to further process the released carbon dioxide into value-added products. These products may include, but are not limited to: carbon monoxide, super-critical carbon dioxide, pressurized carbon dioxide, liquid carbon dioxide or solid carbon dioxide. The acid may also be used as a chemical commodity for any process requiring acid. [0041] Alternatively, base and/or acid may be removed from the PEC water splitting and carbon dioxide sequestration system 10 and transported to another site to capture carbon dioxide from the atmosphere or a gas stream using the passive or active techniques previously described. When powered by solar energy, the system produces the base and/or acid that may be used to capture carbon dioxide from the atmosphere or a gas stream. In this mode, the overall integrated PEC water splitting and carbon dioxide sequestration system 10 sequesters substantially more carbon dioxide than it creates, resulting in a net negative carbon dioxide footprint. Any significant carbon dioxide trapping makes all of the products produced by the system carbon dioxide negative, particularly those carbon products synthesized or produced from atmospheric carbon dioxide. The process creates numerous strategies to produce carbon dioxide negative products that are normally manufactured as CO2 polluting petrochemicals, building materials or in other greenhouse gas emitting industries. [0042] FIG.4 illustrates value-added products that may be processed from the carbon dioxide captured using the base and/or acid produced by the integrated PEC water splitting and carbon dioxide sequestration system 10. Many carbon-based products can be manufactured from carbon dioxide trapped by the integrated PEC water splitting and carbon dioxide sequestration system 10. Commercial products manufactured from carbon dioxide trapped by the integrated PEC water splitting and carbon dioxide sequestration system 10 are carbon dioxide negative, resulting in an overall net decrease in atmospheric carbon dioxide as gaseous carbon dioxide is converted to value-added carbon products. Sale of these products may dramatically subsidize renewable hydrogen production, making clean hydrogen an inexpensive byproduct of an industrial process focused on converting atmospheric carbon dioxide into valuable carbon-based products. Exemplary value-added products manufactured using the hydrogen, oxygen, acid and base produced by the integrated PEC water splitting and carbon dioxide sequestration system 10 include those disclosed in U.S. Patent Application Serial No. 12/062,269 entitled "Electrochemical Methods to Generate Hydrogen and Sequester Carbon Dioxide"; U.S. Patent Application Serial No. 12/062,322 entitled "Electrochemical Apparatus to Generate Hydrogen and Sequester Carbon Dioxide"; U.S. Patent Application Serial No. 12/062,374 entitled "Renewable Energy System for Hydrogen Production and Carbon Dioxide Capture"; and PCT Application Serial No. PCT/2008/059310 entitled "Electrochemical System, Apparatus and Method to Generate Renewable Hydrogen and Sequester Carbon Dioxide", all filed on April 3, 2008.

[0043] The integrated PEC water splitting and carbon dioxide sequestration system 10 processes the value-added products from the center of the diagram outward. As previously mentioned, base generated is reacted with carbon dioxide to produce carbonate and bicarbonate salts. The carbon dioxide, carbonate/bicarbonate salts can in turn be converted to carbon monoxide by chemical reduction or reaction with hydrogen. The combination of carbon monoxide and hydrogen is Syngas, a critical cornerstone of synthetic organic chemistry. Through additional processing of these central products, a number of chemical building blocks, such as methane, urea, ethylene glycol, acetaldehyde, formaldehyde, limestone, acetic acid, methanol, formic acid, acetone and foπnamide can be formed. The value-added chemical building blocks can be removed from the integrated PEC water splitting and carbon dioxide sequestration system 10 for sale as products or remain in the integrated system for further processing to a second class of value-added products. These value-added end products are then removed from the integrated PEC water splitting and carbon dioxide sequestration system 10 and sold, resulting in profitable conversion of carbon dioxide into carbon negative products. Simultaneous production of renewable hydrogen is subsidized by sale of these carbon products, creating a carbon negative energy strategy with potentially dramatic impacts on global warming. [0044] The center circle of FIG.4 depicts exemplary products that can be produced from the reaction of hydroxide base with carbon dioxide, or (in the case of carbon monoxide) by reaction of captured carbon dioxide with hydrogen. These chemical compounds include carbon dioxide, carbon monoxide, carbonate and bicarbonate, all of which can be easily inter-converted. They can be further processed to create standard chemical building blocks. In many cases, the hydrogen, oxygen, acid and base generated by the electrolysis unit 14 can be used for this secondary processing. The building blocks can also be further processed within the integrated PEC water splitting and carbon dioxide sequestration system 10 to make many valuable carbon based products, exemplary embodiments of which are shown in FIG. 4. [0045] The commercial products manufactured from carbon dioxide trapped by the integrated PEC water splitting and carbon dioxide sequestration system 10 represent carbon negative commodities, with the integrated PEC water splitting and carbon dioxide sequestration system 10 producing an overall net decrease in gaseous carbon dioxide while creating value-added carbon products. Sale of these products may dramatically subsidize renewable hydrogen production, making clean hydrogen an inexpensive by-product of an industrial process focused on converting atmospheric carbon dioxide into valuable carbon-based products.

Embodiments

[0046] Embodiment 1 is an integrated system for sequestering carbon dioxide from a gas stream and producing renewable hydrogen, oxygen, acid and base, the integrated system comprising: a) a photoelectrochemical electrolysis unit adapted to split water into hydrogen and oxygen using sunlight; i) at least one cathode in a cathode region adapted to produce hydrogen and concentrated base in the form of hydroxide ions; and ii) at least one anode in an anode region adapted to produce oxygen and concentrated acid in the form of protons; and iii) an aqueous electrolyte solution in contact with the cathode and the anode; b) a hydrogen sequestration tank for collecting and processing the hydrogen produced at the cathode; c) an oxygen sequestration tank for collecting and processing the oxygen produced at the anode; d) an acid sequestration tank for collecting and processing the acid produced at the anode; e) a base sequestration tank for collecting and processing the base produced at the cathode; and f) a gas contact area adapted to react gaseous carbon dioxide with the base generated at the cathode region or a carbon dioxide sequestering solution prepared using the acid generated at the anode region. [0047] The integrated system of embodiment 1, wherein the photoelectrochemical electrolysis unit is one of a photovoltaic unit, a semiconductor-liquid junction unit or a photovoltaic/semiconductor-liquid junction unit.

[0048] The integrated system of embodiment 1, wherein the carbon dioxide reacts with the hydroxide base to form carbonate salt or bicarbonate salt. [0049] The integrated system of embodiment 1, wherein the carbon dioxide reacts with the hydroxide base to form carbonate salt or bicarbonate salt and wherein the hydrogen, oxygen, acid, base, carbon dioxide, carbonate salts or bicarbonate salts are processed into value-added products.

[0050] The integrated system of embodiment 1, wherein the integrated system produces substantially no carbon dioxide, resulting in a net removal of carbon dioxide from the gas stream. [0051] The integrated system of embodiment 1, wherein the anode is a photo-anode. [0052] The integrated system of embodiment I, wherein the cathode is a photo- cathode.

[0053] The integrated system of embodiment 1, wherein at least one of the anode and cathode is a photo-electrode.

[0054] The integrated system of embodiment 1, wherein at least one of the anode and cathode is a p- or n- type semiconductor.

[0055] Embodiment 2 is a photoelectrochemical apparatus for generating renewable hydrogen and sequestering atmospheric carbon dioxide or carbon dioxide from a gas stream, the apparatus comprising: a) a photoelectrochemical electrolysis unit adapted to split water into hydrogen and oxygen using sunlight, wherein the electrolysis unit comprises: i) at least one cathode adapted to produce hydrogen and concentrated base in the form of hydroxide ions; ii) at least one anode adapted to produce oxygen and concentrated acid in the form of protons; and iii) an aqueous electrolyte solution; b) a gas contact assembly adapted to receive carbon dioxide and hydroxide ions produced in the photoelectrochemical electrolysis unit so that the carbon dioxide contacts and reacts with the hydroxide ions to form bicarbonate or carbonate ions in solution; and c) a separation chamber connected to the gas contact assembly and adapted to separate the bicarbonate or carbonate ions from the solution.

[0056] The apparatus of embodiment 2, wherein the photoelectrochemical electrolysis unit is one of a photovoltaic unit, a semiconductor-liquid junction unit or a photovoltaic/semiconductor-liquid junction unit.

[0057] The apparatus of embodiment 2, further comprising gas supply equipment adapted to route carbon dioxide to the gas contact assembly. [0058] The apparatus of embodiment 2, further comprising equipment for isolating and processing the hydrogen, oxygen, acid, base, carbonate ions or bicarbonate ions. [0059] The apparatus of embodiment 2, wherein the anode is a photo-anode.

[0060] The apparatus of embodiment 2, wherein the cathode is a photo-cathode.

[0061] The apparatus of embodiment 2, wherein at least one of the anode and cathode is a photo-electrode.

[0062] The apparatus of embodiment 2, wherein at least one of the anode and cathode is a p- or n- type semiconductor.

[0063] Embodiment 3 is a method of generating renewable hydrogen and sequestering carbon dioxide from an air or gas stream comprising: a) supplying sunlight to a photoelectrochemical electrolysis unit including an anode located in an anode region and a cathode located in a cathode region, wherein the anode and the cathode are in contact with an aqueous electrolyte; b) producing oxygen gas and acid at the anode, wherein the acid is in the form of protons; c) producing hydrogen gas and base at the cathode, wherein the base is in the form of hydroxide ions; d) collecting the hydrogen gas; e) collecting the oxygen gas; f) removing acid from the anode region; g) removing base from the cathode region; and h) contacting the hydroxide ions in the base with a source of gaseous carbon dioxide to sequester carbon dioxide in solution as bicarbonate, carbonate or a mixture thereof.

[0064] The method of embodiment 3, wherein the photoelectrochemical electrolysis unit is one of a photovoltaic unit, a semiconductor-liquid junction unit or a photovoltaic/semiconductor-liquid junction unit.

[0065] The method of embodiment 3, further comprising reacting the bicarbonate or carbonate with the acid produced in the photoelectrochemical electrolysis unit to generate concentrated carbon dioxide gas or super critical carbon dioxide. [0066] The method of embodiment 3, further comprising isolating and processing at least one of the hydrogen, oxygen, acid, base, carbonate or bicarbonate.

[0067] The method of embodiment 3, further comprising utilizing at least one of the hydrogen, oxygen, acid, base, carbonate or bicarbonate as a reagent to produce a value-added product.

[0068] The method of embodiment 3, further comprising supplying direct current voltage from an energy source to the photoelectrochemical electrolysis unit.

[0069] The method of embodiment 3, further comprising supplying direct current voltage from an energy source to the photoelectrochemical electrolysis unit, wherein the energy source is renewable energy source.

[0070] The method of embodiment 3, wherein the anode is a photo-anode.

[0071] The method of embodiment 3, wherein the cathode is a photo-cathode.

[0072] The method of embodiment 3, wherein at least one of the anode and cathode is a photo-electrode.

[0073] The method of embodiment 3, wherein at least one of the anode and cathode is a p- or n- type semiconductor.

[0074] Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

Claims

CLAIMS The following is claimed:

1. An integrated system for sequestering carbon dioxide from a gas stream and producing renewable hydrogen, oxygen, acid and base, the integrated system comprising:

a) a photoelectrochemical electrolysis unit adapted to split water into hydrogen and oxygen using sunlight comprising:

i) at least one cathode in a cathode region adapted to produce hydrogen and concentrated base in the form of hydroxide ions;

ii) at least one anode in an anode region adapted to produce oxygen and concentrated acid in the form of protons; and

iii) an aqueous electrolyte in contact with the cathode and the anode;

b) a hydrogen sequestration tank for collecting and processing the hydrogen produced at the cathode;

c) an oxygen sequestration tank for collecting and processing the oxygen produced at the anode;

d) an acid sequestration tank for collecting and processing the acid produced at the anode;

e) a base sequestration tank for collecting and processing the base produced at the cathode; and f) a gas contact area adapted to react gaseous carbon dioxide with the base generated at the cathode region or a carbon dioxide sequestering solution prepared using the acid generated at the anode region.

2. The integrated system of claim 1 , wherein the photoelectrochemical electrolysis unit is one of a photovoltaic unit, a semiconductor-liquid junction unit or a photovoltaic/semiconductor-liquid junction unit.

3. The integrated system of claim 1, wherein the carbon dioxide reacts with the hydroxide base to form carbonate salts or bicarbonate salts.

4. The integrated system of claim 3, wherein the hydrogen, oxygen, acid, base, carbon dioxide, carbonate salts or bicarbonate salts are processed into value-added products.

5. The integrated system of claim 1 , wherein the integrated system produces substantially no carbon dioxide, resulting in a net removal of carbon dioxide from the gas stream.

6. The integrated system of claim 1, wherein the anode is a photo-anode.

7. The integrated system of claim 1 , wherein the cathode is a photo-cathode.

8. The integrated system of claim 1, wherein at least one of the anode and cathode is a photo-electrode.

9. The integrated system of claim 8, wherein the photo-electrode is a p- or n- type semiconductor.

10. A photoelectrochemical apparatus for generating renewable hydrogen and sequestering atmospheric carbon dioxide or carbon dioxide from a gas stream, the apparatus comprising:

a) a photoelectrochemical electrolysis unit adapted to split water into hydrogen and oxygen using solar energy, wherein the electrolysis unit comprises:

i) at least one cathode adapted to produce hydrogen and concentrated base in the form of hydroxide ions;

ii) at least one anode adapted to produce oxygen and concentrated acid in the form of protons; and

iii) an aqueous electrolyte solution;

b) a gas contact assembly adapted to receive carbon dioxide and hydroxide ions produced in the photoelectrochemical electrolysis unit so that the carbon dioxide contacts and reacts with the hydroxide ions to form bicarbonate or carbonate ions in solution; and

c) a separation chamber connected to the gas contact assembly and adapted to separate the bicarbonate or carbonate ions from the solution.

11. The apparatus of claim 10, wherein the photoelectrochemical electrolysis unit is one of a photovoltaic unit, a semiconductor-liquid junction unit or a photovoltaic/semiconductor-liquid junction unit.

12. The apparatus of claim 10, further comprising gas supply equipment adapted to route carbon dioxide to the gas contact assembly.

13. The apparatus of claim 10, further comprising equipment for isolating and processing the hydrogen, oxygen, acid, base, carbonate ions or bicarbonate ions.

14. The apparatus of claim 10, wherein the anode is a photo-anode.

15. The apparatus of claim 10, wherein the cathode is a photo-cathode.

16. The apparatus of claim 10, wherein at least one of the anode and cathode is a photo-electrode.

17. The apparatus of claim 16, wherein the photo-electrode is a p- or n- type semiconductor.

18. A method of generating renewable hydrogen and sequestering carbon dioxide from an air or gas stream comprising:

a) supplying sunlight to a photoelectrochemical electrolysis unit including an anode in an anode region and a cathode in a cathode region, wherein the anode region and the cathode region are in contact with an aqueous electrolyte;

b) producing oxygen gas and acid at the anode, wherein the acid is in the form of protons;

c) producing hydrogen gas and base at the cathode, wherein the base is in the form of hydroxide ions;

d) collecting the hydrogen gas;

e) collecting the oxygen gas;

f) removing acid from the anode region; g) removing base from the cathode region; and

h) contacting the hydroxide ions in the base with a source of gaseous carbon dioxide to sequester carbon dioxide in solution as bicarbonate, carbonate or a mixture thereof.

19. The method of claim 18, wherein the photoelectrochemical electrolysis unit is one of a photovoltaic unit, a semiconductor-liquid junction unit or a photovoltaic/semiconductor-liquid junction unit.

20. The method of claim 18, further comprising reacting the bicarbonate or carbonate with the acid produced in the photoelectrochemical unit to generate concentrated carbon dioxide gas or super critical carbon dioxide.

21. The method of claim 18, further comprising isolating and processing at least one of the hydrogen, oxygen, acid, base, carbonate or bicarbonate.

22. The method of claim 18, further comprising utilizing at least one of the hydrogen, oxygen, acid, base, carbonate or bicarbonate as a reagent to produce a value-added product.

23. The method of claim 18, further comprising supplying direct current voltage from an energy source to the photoelectrochemical electrolysis unit.

24. The method of claim 23, wherein the energy source is renewable energy source.

25. The method of claim 18, wherein the anode is a photo-anode.

26. The method of claim 18, wherein the cathode is a photo-cathode.

27. The method of claim 18, wherein at least one of the anode and cathode is a photo-electrode.

28. The method of claim 27, wherein the photo-electrode is a p- or n- type semiconductor.