The invention generally relates to the production of hydrogen, and more particularly to the use of a redox ion exchange membrane for hydrogen production.
As the price of fossil fuels increases, and as concern about climate change linked to fossil fuel combustion increases, the demand for alternative energy sources has grown. One promising energy source for a green economy is hydrogen. Hydrogen can be produced from water via an electrolysis process, and when burned hydrogen produces water once again, without creating carbon compounds or other undesirable byproducts. However, the electrolysis process is energy-intensive, and to the extent that fossil fuels are burned to generate electricity for the electrolysis process, that process is unhelpful to the environment.
As renewable energy sources reach higher grid penetration, water electrolysis to produce hydrogen is a promising solution for large-scale, time-shift energy storage, as well as a promising solution for the generation of hydrogen for use as fuel. Water electrolysis can be operated either in acidic or alkaline environments, respectively, with proton exchange membrane (PEM) electrolysis and alkaline or anion exchange membrane (AEM) being the existing commercialized technologies. Notably, the former has been developed to date for high efficiency, but suffers from the high cost and scarce abundance of noble metals, while the latter has been developed to date for robust reliability rather than high efficiency.
Membrane thickness is important for PEM performance. However, a tension exists between thicker membranes, which are more durable in chemical and mechanical stress, and thinner membranes, which are more suitable for proton exchange from the anode side to the cathode side. Further, a PEM degrades in use due to the chemical environment. The intermediate radicals of peroxide or hydroperoxide act to decomposes the PEM during operation. Iron and copper ions form in the PEM due to electron collector plate decay. These ions act as an accelerating agent for intermediate radical formation, which is damaging to the PEM. Additionally, over time, platinum ions from the anode and/or cathode form a layer over the membrane via a reduction process, which decreases performance and durability.
One example of a PEM is a NAFION™ brand perfluorosulfonic acid (PFSA) membrane, of The Chemours Company of Wilmington, Del. NAFION™ brand PEMs have a well-defined porous structure, and multiple functional groups, but are expensive, and susceptible to contamination and clogging, and are subject to the disadvantages described above with regard to PEMs in general. For NAFION™ brand PEMs, acid-based electrolyte is used, and hydronium ion (proton) exchange occurs across the membrane. The exact mechanism of action of a NAFION™ brand PEM is still the subject of research. However, referring to
As with PEMs in general, NAFION™ brand PEMs are used for water electrolysis in an acid electrolyte. Upon occurrence of electrochemical gas evolution reactions, protons are converted to H2 gas at the cathode and water molecules are oxidized to O2 gas and protons at the anode. Consequently, a gradient of proton concentration evolves between the two sides of the NAFION™ brand PEM. This concentration gradient can be translated to a biased chemical potential, which drives protons from the anode side through the NAFION™ brand PEM to the cathode side.
Anion exchange membrane (AEM) electrolysis resembles PEM in that it is used for water electrolysis, but in an alkaline electrolyte, and the membrane provides ionic pathways (either H− or OH−). Unlike PEM, AEM electrolysis eliminates the preferential usage of noble metal electrocatalysts and anti-corrosive building materials. Instead, period-4 transition metals (e.g., Fe, Co, Ni) can be used to formulate high efficient electrocatalysts (e.g., 1.0 A/cm2 at 1.5 V), especially when operating at elevated temperatures.
Bipolar AEMs (e.g., FUMASEP® brand AEMs of FUMATECH BWT GmbH, Bietigheim-Bissingen, Germany) have been developed in search of high ionic conductivity and robust mechanical stability. Referring to
Unfortunately, anion exchange membranes (AEMs) share most of the structural defects and performance degradation issues as PEMs. The degradation issue is even worse for AMEs, because their polymer backbones and functional groups can be easily attacked by OH− and radicals. In addition, AEMs are also known for their relatively poor ionic conductivity, because the diffusion coefficient of OH− is much lower than H+; a higher ion exchange capacity (IEC) is needed for this reason. However, higher IEC leads to the sacrifice of mechanical properties, due to excessive polymer swelling.
The Department of Energy has set a goal of reducing the cost of hydrogen production from $4/kg today to $1/kg by 2030. Existing technologies at or near the $4/kg cost range cannot meet that $1/kg goal. Thus, there is an unmet need for a technology that facilitates continuous high-volume production of hydrogen economically and easily.
In the present invention, a low-cost hydrogen production system is achieved through a new type of membrane, where protons or anions are transported through that membrane via simultaneous chemical reversible reduction and oxidation, instead of via conventional chemical ion gradient driven mass transport. Such membranes can be used with both alkaline or acid electrolytes, with high efficiency and high reliability.
A redox ion exchange membrane may include an electrically-conductive material; and redox-active materials associated with that material, the redox-active materials having reversible oxidation and reduction properties.
A hydrogen production device may include a cathode, an anode, and a redox ion exchange membrane positioned between the cathode and the anode, the redox ion exchange membrane having a first surface and a second surface opposed to the first surface.
A fuel cell device may include a first electrode; a second electrode; and a redox ion exchange membrane positioned between the first and second electrodes, where the redox ion exchange membrane has a first surface and a second surface opposed to the first surface.
A method of producing hydrogen gas may include providing a cathode, an anode, and a redox ion exchange membrane positioned between the cathode and the anode, and applying electrical power to the cathode and the anode; where that applying causes simultaneous reciprocal reduction and oxidation reactions on opposite sides of the redox ion exchange membrane, such that H+ is released on one side of the redox ion exchange membrane
The use of the same reference symbols in different figures indicates similar or identical items.
Referring to
The cathode 6 and the anode 8 may be of conventional construction, and each may be coated with any suitable catalyst. According to some embodiments, at least one of the cathode 6 and the anode 8 is a platinum coil electrode. According to other embodiments, the cathode 6 has a platinum or platinum-based coating thereon. According to other embodiments, the anode 8 has an iridium or iridium oxide coating thereon. According to other embodiments, at least one of the cathode 6 and the anode 8 has an additional, or different, coating thereon. According to other embodiments, at least one of the cathode 6 and the anode 8 does not have a coating defined thereon.
The RIEM 4 will now be described in greater detail. As used in this document, a “redox ion exchange membrane” is defined to mean a membrane that, subjected to an electromagnetic field (such as from a cathode and an anode), performs reduction and oxidation reactions simultaneously to generate protons and anions, causing those ions to penetrate the membrane in opposite directions, regardless of a concentration gradient across the membrane. Simultaneous oxidation and reduction reactions that are dependent on one another are the hallmark of a redox reaction. The oxidation reaction by itself, and the reduction reaction by itself, each may be referred to as a half-reaction, because two half-reactions occur together to form a complete redox reaction.
According to some embodiments, referring to
The electrically-conductive material 22 may be continuous across the RIEM 4, or may be discontinuous. The electrically-conductive material 22 may be a plurality of independent islands across the RIEM 4. The electrically-conductive material 22 may be a single layer of material, multiple adjacent layers of material, a crystal structure, a nanostructure or nanostructure, or any other suitable structure onto which the inorganic microstructures 28 may be fabricated, grown, placed, or otherwise manufactured. Similarly, the inorganic nanostructures 28 may be continuous across the RIEM 4, or may be discontinuous. The inorganic nanostructures 28 may be a plurality of independent islands across the RIEM 4. The inorganic nanostructures 28 may be a single layer of material, multiple adjacent layers of material, a crystal structure, a nanostructure or nanostructure, and/or any other suitable structure fabricated, grown, placed, or otherwise manufactured on the electrically-conductive material 22. Referring to
Titanium is a preferred transition metal for use as the electrically-conductive material 22, and titanium oxide (TiO2) is a preferred transition metal oxide for use as the inorganic microstructures 28. However, any other transition metal or metals may be used instead of or in addition to titanium, including scandium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, cadmium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold, mercury, actinium, thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium, lawrencium, rutherfordium, dubnium, seaborgium, hassium, meitnerium, darmstadtium, roentgenium, and copernicium. Further, any other oxide, oxides, chalcogenide, or chalcogenides of those transition metals may be used instead of or in additional to titanium oxide, such as but not limited to: FeO, Fe2O3, FeS, CoS, CoO, NiO, MnO2, ZrO2, and/or Cr2O3. Further, any salt of those transition metals may be used instead of or in additional to titanium oxide, such as but not limited to: Fe4[Fe(CN)6]3, CoTiO3, and/or LiSeO4.
According to some embodiments, at least part of the electrically-conductive material 22 of the RIEM 4 is fabricated from a non-metallic material. Such non-metallic materials may at least one of: carbon materials (such as but not limited to carbon dots, graphite, graphene, carbon fibers, carbon nanotubes, and/or carbon black), sulfur, arsenic, selenium, boron, phosphorus, oxides of the foregoing materials (such as but not limited to A2SO3, SeO2, As2O5, B2O3 and/or P2O5), and salts of the foregoing materials (such as but not limited to NaAsO2, Na2SeO3, LiPO4, and/or NaBO3).
In some embodiments, a redox-active coating or layer is deposited on the electrically-conductive material 22 and/or inorganic nanostructures 28. Such a redox-active coating or layer may be either the same material or two different materials. In rare cases, upon application of a bias, a redox active layer such as an under-potential-deposited hydrogen (denoted as Hupd) layer can form on one side of the substrate (e.g., metal thin films). The Hupd layer can diffuse through the substrate to the other side. In rare cases, the surface Hupd layer on a metal thin-film can be viewed as a redox-active material. Further, a redox-active organic or polymer material can be added as a co-catalyst to enhance the rate and overall performance of ion transport.
At least one organic species, such as a small molecule or polymer, may be applied to the electrically-conductive material 22 and/or inorganic nanostructures 28, and/or may be applied to and/or form part of another part of the RIEM 4 as a coating. The use of at least one organic species as a coating may be useful for bonding layers of the RIEM 4 (as described below), tuning affinity for water, such as local hydrophilic or hydrophobic properties of the RIEM 4, and/or creating mechanical flexibility of the RIEM 4. Redox-active polymers and macromolecules (such as (poly)quinone, polyimide, and polypyrrole), hydrophilic and/or hydrophobic additives (such as surfactants, PTFE nanoparticles, and zwitterionic compounds), and crosslinking agents (such as glutaraldehyde, silicone gel, and interfacial polymerization) may be usefully applied to the electrically-conductive material 22.
The RIEM 4 may be a single structure, or may be composed of two or more layers. Referring also to
According to some embodiments, one of the layers 24, 26 of the RIEM 4 is a solid layer. According to some embodiments, the solid layer is a piece of foil composed of transition metal or transition metal oxide. According to some embodiments, the solid layer extends across the entire RIEM 4. According to other embodiments, the solid layer extends across part of the RIEM 4. The use of one or more solid layers in the RIEM 4 improves the separation efficiency across the redox-active membrane, such as by minimizing the crossover effect in a fuel cell. According to other embodiments, the solid layer 24 is omitted from the RIEM 4.
According to some embodiments, liquid or gel is mixed with the RIEM 4 to create a pseudo-solid electrolyte. According to other embodiments, the RIEM 4 may be completely solid, in which case the RIEM 4 is a solid electrolyte.
Referring also to
Next, at block 104, electrical power is applied to the cathode 6 and anode 8. The amount of voltage and current applied depends on the properties of the first liquid 14 and the second liquid 16, and the properties of the RIEM 4. The proton-transporting capacity of the RIEM 4 depends on its relative potential, such that the voltage applied to the cathode 6 and anode 8 is greater than that potential to cause proton transport. Examples are provided below. The electrical power applied to the cathode 6 and anode 8 generates an electromagnetic field that is applied to the RIEM 4.
Next, at block 106, as a consequence of the application of electrical power to the cathode 6 and anode 8, and the application of an electromagnetic field to the RIEM 4, protons (hydronium ions) and anions (OH− ions) are simultaneously transported across the RIEM 4. Electrons are also transported across the RIEM 4 as a consequence of the application of an electromagnetic field to the RIEM 4. The ion motion direction across the RIEM 4 is not driven by the concentration gradient across the RIEM 4. Instead, the electrochemical driving force (that is, the relative voltage against the cathode and anode) determines the transport direction and causes ions to penetrate and cross the RIEM 4. Further, electron transfer across the RIEM 4 is driven by the potential bias across the RIEM 4, therefore enabling a minimal iR drop. For example, during water electrolysis using the RIEM 4 a lower OH− concentration catholyte (i.e., an acidic solution) and a high OH− concentration anolyte (i.e., an alkaline solution), OH− ions are transported from the cathode side to the anode side by the electrochemical driving force.
According to some embodiments, during operation of the RIEM 4, OH− is neither oxidized or reduced. Instead, redox-active species in the RIEM 4 are oxidized and/or reduced, absorbing or releasing OH−. Similarly, according to some embodiments, during operation of the RIEM 4, H+ is neither oxidized or reduced. In this way, the RIEM 4 can be operated for a substantial period of time without a need to replenish the electrolyte(s) of the first liquid 14 and/or the second liquid 16. Further, operation of the RIEM 4 does not utilize ion transport across the RIEM 4. Thus, the RIEM 4 may be solid, or may include solid components, without degrading operation of the RIEM 4.
As one example, referring to
It will be appreciated that in the above example, one reaction occurs on one side of the RIEM 4, while the opposite reaction occurs simultaneously on the opposite side of the RIEM 4. The Ti in both species TiO2 and TiOOH changes its chemical valence—that is, the Ti in the RIEM 4 takes part in a redox reaction. However, OH− is neither reduced nor oxidized, although it participates in that redox reaction.
As another example, referring to
It will be appreciated that in the above example, one reaction occurs on one side of the RIEM 4, while the opposite reaction occurs simultaneously on the opposite side of the RIEM 4. The Ti in both species TiO2 and TiOOH changes its chemical valence—that is, the Ti in the RIEM 4 takes part in a redox reaction. However, OH− is neither reduced nor oxidized, although it participates in that redox reaction.
As described above with regard to
Next, returning to
At the end of the operation 100, at block 110, the hydrogen gas formed in the first compartment 18 is collected and stored for later use.
It is known in the art that an electrolysis system can be run in reverse as a fuel cell. Consequently, the RIEM 4 is suitable for use in a fuel cell as well as in an electrolysis system for producing hydrogen. For example, an RIEM 4 that utilizes an electrically-conductive material 22 that is composed of Ti and inorganic nanostructures 28 that are composed of TiO2 may be utilized in an alkaline fuel cell system. That RIEM 4 may be utilized with a first liquid 14 and second liquid 16 that are both a solution of water with 1.0M KOH, with H2 and O2 feeding gases. Instead of the cathode 6 and anode 8, the fuel cell includes two electrodes on opposite sides of the RIEM 4. H2 is oxidized at the negative electrode while O2 is reduced at the positive electrode, outputting a voltage and a current via the electrodes. The RIEM 4 is biased versus the two electrodes. Consequently, TiO2/Ti will be reduced, while releasing OH− ions, close to the side of the RIEM 4 that faces the negative electrode. As such, the TiO2/Ti (reduced form) will be oxidized close to the other side of the RIEM 4, uptaking OH− ions from its environment.
Where the RIEM 4 is used in a fuel cell with two electrodes, according to some embodiments, at least one of the electrodes is fabricated at least in part from a noble metal. A “noble metal” is any metallic or semimetallic element that does not react with a weak acid and give off hydrogen gas in the process, which is a set that includes the six platinum group metals (platinum, gold, ruthenium, rhodium, palladium, osmium, and iridium), copper, mercury, technetium, rhenium, arsenic, antimony, bismuth, polonium, and silver.
To demonstrate the concept of a RIEM 4, experiments were performed with titanium and titanium oxide. In Example 1, a commercial titanium thin film that measured 12.5 μm in thickness, the surface of which is usually covered by an oxide layer (TiO2) on the scale of a few nanometers, was utilized as a material. To amplify the amount of the surface oxide, a titanium thin-film disc measuring 1.0 inch in diameter was subjected to a two-step procedure of treatment. That titanium thin-film disc was first electrochemically oxidized at 20.0 V for 4.0 hours in a mixture solution of 1.0M NH4Cl and 5.0M KOH, to roughen the surface and thusly enhance the efficient surface area for the subsequent formation of TiO2 nanostructures (
As another example of a demonstration concept of a redox-active membrane, a commercial titanium felt was cleaned by an ultrasonic bath, and subsequently further cleaned in a solution of acetone, ethanol, and deionized water. As in Example 1, the surfaces of individual titanium fibers (
The same procedure as in Example 2 was applied to convert a commercial titanium filter to a nanoporous TiO2/Ti-filter membrane. The titanium filter starting material was composed of sintered micro-powders (
In another example, a RIEM 4 including Ni(OH)2 was grown on a nickel mesh via an enhanced hydrolysis method at 90° C. in an aqueous solution containing 50M Ni(NO3)2 and 1.0M of urea. A starting material of a commercial nickel mesh included of interconnected nickel fibers of substantially 8 μm (
A TiO2/Ti RIEM 4 was prepared using the same methods as in Examples 1, 2 and 3 above. RuO2 was electrochemically deposited on the prepared TiO2/Ti in an electrolytic bath of 0.5M H2SO4 and 50 mM RuCl3. The electro-deposition was achieved in room temperature with a three-electrode system. A platinum foil and an Ag/AgCl electrode, respectively, were used as the counter electrode and the reference electrode. The electro-deposition adopted a chronoamperometric step-function, that is, −0.6 V for 5 minutes followed by −0.4 V for 0.5 minutes. All the potentials are cited against the Ag/AgCl reference electrode. The successful formation of RuO2 on the TiO2/Ti membrane was confirmed by SEM color mapping and EDS (
VO2 nanosheets were grown on a stainless steel (sst) mesh membrane to form a VO2/stainless steel (sst) membranes for potassium batteries by an electro-deposition method in room temperature with a three-electrode system. A platinum foil and an Ag/AgCl electrode were, respectively, used as the counter electrode and the reference electrode. The electrolytic bath consisted of 0.5M H2SO4 and 50 mM VO·SO4·H2O (vanadium (IV) oxide sulfate). The electrodeposition occurred at −0.75 V versus Ag/AgCl (3M KCl filled) with a variety of lengths of time. The VO2/sst membranes were characterized by SEM (
To further demonstrate the concept and the workability of the RIEM 4, nanoporous membranes were created, composed of nanoscaled metal oxides surrounding a metallic backbone. Referring to
A RIEM 4 configured as shown in
It is known that for water splitting at 25° C., the electrolyzing voltage is 1.48V based on thermodynamic calculations. Thus, if the system of the present invention were viewed incorrectly as two electrochemical cells connected in tandem and separated by a bipolar electrode, a voltage of 2.96V (i.e., 2 times 1.48V) would be the minimal requirement to anchor water-splitting reactions. However, referring to
A solid titanium foil measuring 0.5 mil (12.5 μm) in thickness was subjected to hydrothermal oxidation in a 10M NaOH solution at elevated temperatures. The formation of surface TiO2 was confirmed by SEM/EDS, and permeation experiments indicated that the foil remained solid and waterproof. As a result, a RIEM 4 was created.
The essential role of the redox-active species in transporting protons was studied by control experiments of water electrolysis in an acidic media wherein a Ti@TiO2 RIEM 4 with and without an additional deposit of RuO2 were studied in parallel. The Ti@TiO2 membrane measured 0.8 millimeters in thickness and featured a nanoporous structure.
Based on the experiments above, Ti@TiO2 and RuO2/Ti@TiO2 were chosen as an anion-conducting and cation-conducting membrane, respectively. A commercial titanium fiber felt disc (53%-56% porosity) measuring 25 mm in diameter and 0.25 mm in thickness was subjected to a hydrothermal reaction for 24 hours in a PTFE-lined autoclave reactor. The titanium surface was oxidized to titanium oxide nanowires that surround individual titanium fibers; the whole disc was converted to a nanoporous structure.
To evaluate the performance of that Ti@TiO2 membrane, platinum coil electrodes were placed in an H-shaped vessel that was separated by that Ti@TiO2 membrane. The chronoamperometric response in water electrolysis to a pre-determined voltage was recorded and compared to a device using a FUMASEP® brand AEM membrane of FUMATECH BWT GmbH, Bietigheim-Bissingen, Germany.
The use of the RIEM 4 in a process for generating hydrogen has been described above. It will be appreciated that the RIEM 4 is useful in other applications. As one application, the RIEM 4 may be used to separate electrodes and/or conduct ions in a fuel cell; a fuel cell is essentially a reverse process of the hydrogen-generating process described above. As another application, the RIEM 4 may be used in a metal air/oxygen battery, such as a zinc-air battery. As another application, the RIEM 4 may be used in a lithium-ion battery. As another application, the RIEM 4 may be used in an electrochemical supercapacitor or pseudo-supercapacitor.
As used in this document, and as customarily used in the art, terms of approximation, including the words “substantially” and “about,” are defined to mean normal variations in the dimensions, measurements and physical properties of items and processes in the physical world that may be associated with accuracy, precision, and/or tolerances.
While the invention has been described in detail, it will be apparent to one skilled in the art that various changes and modifications can be made and equivalents employed, without departing from the present invention. It is to be understood that the invention is not limited to the details of construction, the arrangements of components, and/or the method set forth in the above description or illustrated in the drawings. Statements in the abstract of this document, and any summary statements in this document, are merely exemplary; they are not, and cannot be interpreted as, limiting the scope of the claims. Further, the figures are merely exemplary and not limiting. Topical headings and subheadings are for the convenience of the reader only. They should not and cannot be construed to have any substantive significance, meaning or interpretation, and should not and cannot be deemed to indicate that all of the information relating to any particular topic is to be found under or limited to any particular heading or subheading. Therefore, the invention is not to be restricted or limited except in accordance with the following claims and their legal equivalents.
This application claims the benefit of priority to U.S. Provisional application No. 63/294,371, filed Dec. 28, 2021, which is hereby incorporated by reference in its entirety.
Number | Date | Country | |
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63294371 | Dec 2021 | US |