Thermoelectrochemical converters with high density metal electrodes.

Abstract

A thermoelectric converter is provided that includes a working fluid and a pair of membrane electrode assemblies (MEAs). Each MEA includes a pair of electrodes that are electronically conductive and permeable to the working fluid, and a thin-film electrolyte membrane sandwiched between the electrodes. The membrane is conductive to the ions of the working fluid and has a thickness of 0.03 μm to 10 μm. At least one electrode of each MEA includes a non-porous, dense metal. One electrode of each MEA contacts the working fluid at a first pressure on the high-pressure side, while the other electrode contacts the working fluid at a second pressure on the low-pressure side. The first MEA is configured to compress the working fluid from the second pressure to the first pressure, while the second MEA is configured to expand the working fluid from the first pressure to the second pressure.

Description

[Technical field]

[0001] (CROSS REFERENCE TO RELATED APPLICATIONS) This application claims priority to U.S. Provisional Patent Application No. 63 / 069380, filed August 24, 2020, the entire disclosure of which is incorporated herein by reference. [Background technology]

[0002] Membrane electrode assemblies (MEAs), which have an ion-conducting membrane sandwiched between two electrodes, are employed in many electrochemical applications. Some common applications are batteries, fuel cells, and gas separation processes such as hydrogen or oxygen separation. In general, an ionized working fluid is passed through the MEA by being oxidized on the inlet side, which causes electrons to be separated from the atoms. The resulting ions are conducted through the membrane to the opposite electrode, also called the outlet side. Meanwhile, the electrons are conducted through an external circuit to the opposite electrode. The ions are reduced as they recombine with electrons at the outlet electrode.

[0003] It is generally desirable to pass the ionized working fluid through the MEA as efficiently as possible. Resistance to passage of the ionized working fluid through the membrane is an issue in nearly all applications of interest. For example, in hydrogen or oxygen gas separation applications, the increased pressure drop across the membrane as the working fluid passes through it means that a greater amount of energy, and therefore higher costs, is required to supply the feed gas at the increased pressure.

[0004] A similar situation occurs in fuel cells. The most common type of fuel cell is a proton exchange membrane fuel cell that employs an MEA with a proton conducting membrane (PCM). This class of fuel cell supplies hydrogen to one of the electrodes and oxygen to the other. Hydrogen ions are conducted to the oxygen side of the fuel cell through the PCM under the chemical reaction potential of hydrogen and oxygen. Electrons associated with the chemical reaction are conducted from the hydrogen electrode to the oxygen electrode through an external load. The electrons and hydrogen ions reconstitute hydrogen and complete the reaction with oxygen on the oxygen side of the cell, resulting in the production of water that is exhausted from the fuel cell system. A continuous current is maintained by a continuous supply of hydrogen and oxygen to the fuel cell.

[0005] Alkali metal thermoelectrochemical conversion (AMTEC) cells have been designed as thermoelectrochemical heat engines. AMTEC heat engines use pressure to generate electrical potential and current by passing an ionized working fluid, such as sodium, through an electrochemical cell at high temperature. Electrodes couple the current to an external load. Electrical work is performed as a pressure difference across an electrolyte separator forces molten sodium atoms through the electrolyte. As the sodium flows into the electrolyte it becomes ionized, thereby releasing electrons to an external circuit. On the other side of the electrolyte, the sodium ions recombine with electrons as they leave the electrolyte to reform sodium, much like the process that occurs in battery and fuel cell type electrochemical cells. The reformed sodium, at low pressure and high temperature, leaves the electrochemical cell as an expanding gas.

[0006] The Johnson Thermoelectrochemical Converter (JTEC) system (disclosed in U.S. Patent Application Publication No. 2003 / 0133634 filed on April 28, 2003) is also a thermoelectrochemical heat engine that uses MEAs to convert heat to electrical energy. The JTEC uses a pair of MEA stacks, more specifically hydrogen concentration cells, connected in a back-to-back configuration, one at a relatively high temperature and the other at a relatively low temperature. Hydrogen circulates through the engine between the two MEA stacks via a countercurrent recuperative heat exchanger. The low temperature MEA stack coupled to a heat sink functions as the "electrochemical compressor" stage of the engine, while the high temperature MEA coupled to a heat source functions as the "electrochemical expansion" stage of the engine. As with any thermodynamic engine, the expansion process occurring at high temperature generates enough power to drive the compression process occurring at low temperature, providing a net output power to an external load.

[0007] However, this conventional engine design is often complicated by the need for a large membrane-to-electrode surface area ratio and the need for a large number of cells to be electrically connected in series to achieve a practical output voltage level. Specifically, unlike conventional fuel cells, whose open circuit voltage can be higher than 1 V, in thermoelectrochemical heat engines, the net output voltage generated by the difference in Nernst voltage between the hot and cold MEAs due to the hydrogen pressure ratio is only in the range of about 0.1 volts at moderate high and low pressure operating conditions. Thus, a large number of cells are generally required to be connected in series to achieve a useful output voltage level. Furthermore, the internal impedance of the MEA pair has a large effect on the output power capacity.

[0008] The main efficiency losses associated with MEAs are the gas pressure flow losses to and from the porous electrodes, the activation energy required to oxidize and reduce the working fluid at the electrode / membrane interface, and the impedance to ionic conduction through the membrane. Pressure losses associated with gas flow are addressed by optimizing the electrode thickness, pore size, and pore distribution. Activation losses are generally fixed material properties. Efforts to minimize activation energy losses usually focus on optimizing catalyst loading and distribution, as well as the type of catalyst used.

[0009] Depending on the application, the impedance to ionic conduction through the membrane is a material property. To achieve efficient energy conversion, it is desirable for the membrane to have high gas diffusion barrier properties, since diffusion of the working fluid (e.g., hydrogen gas) under pressure differential across the membrane results in a loss of electrical output and efficiency. The membranes utilized must also have good ionic conductivity. However, many of the known and available membrane materials with good ionic conductivity, such as Nafion manufactured by DuPont Corp., generally have very poor molecular diffusion barrier properties. Poor molecular diffusion barrier properties would require a thicker membrane to inhibit diffusion, which in turn would result in a higher conduction resistance and thus would be self-destructive. Conversely, known and available membrane materials with high molecular diffusion barrier properties generally have relatively low ionic conductivity, and the use of such materials would result in high system impedance and associated high polarization losses. Thus, thin membranes are needed to achieve practical power densities while minimizing internal resistive polarization losses, and large membrane areas are needed to achieve practical power levels.

[0010] Thus, there is a need for a practical manner to provide a thermoelectrochemical heat engine that can approximate the Carnot equivalent cycle using available high barrier, low conductivity membrane materials, can operate over a wide range of heat source temperatures, and avoids the reliability and inefficiency problems associated with mechanical engines. The solid-state heat engine of the present invention meets this need. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] U.S. Patent No. 7,160,639 Summary of the Invention

[0012] In one embodiment, the present invention relates to a thermal-electrical converter comprising an ionized working fluid and a first membrane electrode assembly and a second membrane electrode assembly electrically coupled to each other. Each membrane electrode assembly comprises a first electrode conducting electrons and permeable to the ionized working fluid, a second electrode conducting electrons and permeable to the ionized working fluid, and a thin-film electrolyte membrane sandwiched between the first and second electrodes. The thin-film electrolyte membrane is conductive to ions of the ionized working fluid and has a thickness of 0.03 μm to 10 μm. At least one of the first and second electrodes of each membrane electrode assembly comprises a non-porous, dense metal. Due to a working fluid pressure difference across each membrane electrode assembly, one of the first and second electrodes of each membrane electrode assembly is in contact with the working fluid at a first pressure, and the other of the first and second electrodes of each membrane electrode assembly is in contact with the working fluid at a second pressure lower than the first pressure. The first membrane electrode assembly is configured to expand the working fluid from a first pressure to a second pressure, and the second membrane electrode assembly is configured to compress the working fluid from the second pressure to the first pressure.

[0013] According to the aforementioned embodiment, the first and second membrane electrode assemblies operate at different temperatures from each other. Electrical power is applied to the second membrane electrode assembly to pump the working fluid through the second membrane electrode assembly to the high pressure side and maintain a pressure differential as heat is removed therefrom, resulting in the second membrane electrode assembly operating at a first voltage. The first membrane electrode assembly is also subjected to a pressure differential and heat is supplied thereto so that the working fluid expands through the first membrane electrode assembly to the low pressure side, thereby operating at a second voltage different from the first voltage.

[0014] In one embodiment, which may be combined with any of the previous embodiments, a non-porous, dense metal is deposited or mounted on a porous substrate.

[0015] In one embodiment that may be combined with any of the preceding embodiments, at least one of the first and second electrodes of at least one of the first and second MEAs includes a catalyst configured to promote oxidation and reduction of the working fluid as the working fluid passes through the respective thin film electrolyte membrane.

[0016] In one embodiment that may be combined with any of the preceding embodiments, at least one of the first and second electrodes of at least one of the first and second MEAs comprises palladium or an alloy thereof.

[0017] In one embodiment that may be combined with any of the preceding embodiments, the thermal-to-electrical converter further comprises a recuperative heat exchanger coupling the flow of working fluid at the first pressure and the second pressure between the first MEA and the second MEA.

[0018] In one embodiment, which may be combined with any of the preceding embodiments, the thermal to electrical converter has a tubular configuration.

[0019] In one embodiment that may be combined with any of the preceding embodiments, each of the first and second MEAs has a tubular configuration.

[0020] In one embodiment, which may be combined with any of the preceding embodiments, the interior of each of the first and second MEAs constitutes a first conduit for the flow of a working fluid at a first pressure.

[0021] In one embodiment that may be combined with any of the preceding embodiments, the thermal-to-electrical converter further comprises a housing at least partially enclosing the first and second MEAs, the space between the first and second MEAs and the housing constituting a second conduit for the flow of a working fluid at a second pressure.

[0022] In one embodiment, which may be combined with any of the preceding embodiments, one of the first and second electrodes is electrically common to both the first and second MEAs.

[0023] In one embodiment, which may be combined with any of the preceding embodiments, the thermal-to-electrical converter further comprises an external circuit for electrically coupling the first and second MEAs.

[0024] In one embodiment that may be combined with any of the preceding embodiments, the present invention relates to a thermal to electric converter system comprising a plurality of the above-mentioned converters, the plurality of converters being coupled between a first stream of working fluid at a first temperature and a second stream of working fluid at a second temperature lower than the first temperature.

[0025] In one embodiment, which may be combined with any of the previous embodiments, the present invention relates to a method for generating electricity using the above-mentioned thermal to electrical converter as a heat pump, the method comprising the steps of: coupling a first membrane electrode assembly to a heat source at a first temperature and coupling a second membrane electrode assembly to a heat sink at a second temperature, the first temperature being lower than the second temperature; applying electrical power to the second membrane electrode assembly to pump a working fluid from a second pressure to a first pressure with heat being removed at the second temperature and a first voltage; and extracting electrical power from the first membrane electrode assembly as the working fluid expands from the first pressure to the second pressure with heat being supplied thereto at the first temperature and the second voltage, the first voltage being higher than the second voltage.

[0026] In one embodiment that may be combined with the preceding embodiments, the method further comprises the step of connecting an external power source in series with the first membrane electrode assembly.

[0027] In one embodiment, which may be combined with any of the previous embodiments, the present invention relates to a method for generating electricity using the above-mentioned thermal-to-electrical converter as a heat engine, the method comprising the steps of: coupling a first membrane electrode assembly to a heat source at a first temperature and coupling a second membrane electrode assembly to a heat sink at a second temperature, the first temperature being greater than the second temperature; applying electrical power to the second membrane electrode assembly to pump a working fluid from a second pressure to a first pressure with heat being removed at the second temperature and a first voltage; and extracting electrical power from the first membrane electrode assembly as the working fluid expands from the first pressure to the second pressure with heat being supplied thereto at the first temperature and a second voltage, the second voltage being greater than the first voltage. [Brief description of the drawings]

[0028] [Figure 1] FIG. 1 is a cross-sectional view of a thermoelectrochemical converter including two thin film membrane electrode assemblies according to one embodiment of the present invention. [Diagram 2] 1 is a graphical representation of hydrogen permeability of several metals suitable for use as electrodes in a thin film membrane electrode assembly of a converter, according to an embodiment of the invention. [Diagram 3] 1 is a table identifying several commercially available hydrogen-permeable metal alloys. [Figure 4] FIG. 2 is a cross-sectional view of a thermoelectrochemical converter including two thin film membrane electrode assemblies according to another embodiment of the present invention. [Figure 5A] FIG. 2 is a top view of the converter shown in FIG. [Figure 5B] FIG. 2 is a bottom view of the converter shown in FIG. [Figure 6] 1 illustrates a number of thermal to electrical converter tubes operating between a high temperature working fluid flow stream and a low temperature working fluid flow stream according to another embodiment of the present invention. [Figure 7] 1 is a graphical representation of some exemplary proton conducting materials suitable for use as electrolyte materials forming an ion conducting membrane in accordance with an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] The following detailed description of the preferred embodiments of the invention will be better understood when read in connection with the accompanying drawings, in which: For the purpose of illustrating the invention, there are shown in the drawings embodiments that are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.

[0030] In the following description, certain terminology is used for convenience of explanation only and not for limitation. The terms "proximal", "distal", "upward / upper", "downward / lower", "bottom" and "top" designate directions in the drawings to which reference is made. The terms "inward / inside" and "outward / outside" refer to directions toward and away from the geometric center of the device and designated parts thereof, respectively, according to the present invention. Unless otherwise specified herein, the terms "a", "an" and "the" should be read as meaning "at least one" and not limited to one element. The terminology includes the above terms, derivatives thereof and synonyms. It should also be understood that terms such as "first", "second", etc. are provided for clarity purposes only. Elements or components identified by these terms and their operations can be easily interchanged.

[0031] The present invention relates generally to an improved thin film membrane electrode assembly, particularly for use in thermoelectrochemical converters, and more particularly to a thermoelectrochemical converter having an ionizable working fluid and at least one electrochemical concentration cell including a thin film electrolyte layer and at least one electrode, more preferably an MEA comprising a pair of electrodes sandwiching the thin film electrolyte layer.

[0032] Referring to FIG. 1, a converter 100 according to an embodiment of the present invention is shown. The converter 100 includes a first electrochemical cell 9 and a second electrochemical cell 11. Each cell 9, 11 includes an ion-conducting MEA, which comprises an ion-conducting membrane sandwiched between two electrodes. More specifically, the first electrochemical cell 9 includes a first MEA 40, and the second electrochemical cell 11 includes a second MEA 42. Both the first and second MEAs 40, 42 have a generally tubular structure, meaning that the electrodes and their membranes have a tubular configuration, which will be described in more detail below. It should be understood by those skilled in the art that each cell may include more than one MEA, and that the MEA need not have a tubular configuration. In the converter 100 of FIG. 1, the first electrochemical cell 9 includes a heat sink Q L , and the second electrochemical cell 11 is coupled to a heat source Q H Heat sink Q L and heat source Q H are at different temperatures, as will be described in more detail below.

[0033] The first MEA 40 of the first electrochemical cell 9 comprises a membrane 6 located between a first electrode 5 and a second electrode 10. The second MEA 42 of the second electrochemical cell 11 comprises a membrane 8 located between the first electrode 5 and a second electrode 14. The membranes 6, 8 and electrodes 5, 10, 14 all have a tubular configuration (a cross-sectional view is shown in FIG. 1).

[0034] Each membrane 6, 8 is an ion-conducting membrane. More preferably, each membrane 6, 8 is in the form of a thin-film electrolyte layer, most preferably in the form of a dense thin-film electrolyte layer. Preferably, each thin-film electrolyte membrane 6, 8 has a thickness of less than 10 μm, more preferably between 0.03 μm and 10 μm. Preferably, each thin-film electrolyte membrane 6, 8 is conductive for the ions of the working fluid. More specifically, the dense thin-film electrolyte layers (membranes) 6, 8 are preferably non-porous and conductive for the ions of the working fluid, but are substantially impermeable to the non-ionized components of the working fluid. The ion-conducting membranes 6, 8 therefore limit the diffusion of non-ionized working fluid therethrough. The thin-film electrolyte structure of each MEA 40, 42 allows the thermoelectrochemical converter 100 to achieve a high power density, even when using electrolyte materials with relatively low ion conductivity.

[0035] Referring to FIG. 7, various exemplary proton conducting materials suitable for use as electrolyte materials forming ion conducting membranes in the present invention are shown. These materials shown in FIG. 7 are particularly suitable for use where hydrogen is utilized as the working fluid. Based on the data provided in FIG. 7, an optimal material can be selected for a given application based primarily on maximizing conductivity for the target operating temperature of the membrane. Some of the preferred membrane / electrolyte materials in FIG. 7 are oxides. The preferred membrane / electrolyte materials in FIG. 7 are suitable for application by sputtering or other physical deposition methods. Polybenzimidazole (PBI) is a preferred polymer and can therefore be applied by injection or other polymer coating techniques.

[0036] The first electrode 5 and the second electrode 10, 14 are preferably electronically conductive and permeable to the ionized working fluid. The permeability of the electrodes 5, 10, 14 may be due to atomic or molecular diffusion through the non-porous electrode material, or the electrodes 5, 10, 14 may be permeable due to porosity. In one embodiment, at least one of the electrodes 5, 10, 14 may include one or more additives that promote electronic conductivity and / or one or more catalysts that promote a desired electrochemical reaction.

[0037] Preferably, at least one of the first electrode 5 and second electrode 10 of the MEA 40 and at least one of the first electrode 5 and second electrode 14 of the MEA 42 comprise a non-porous metal, more preferably a non-porous dense metal. More preferably, both electrodes 5,10 and 5,14 of both MEAs 40,42 comprise a non-porous metal, more preferably a non-porous dense metal. In a preferred embodiment, at least one of the electrodes 5,10,14, more preferably all of the electrodes 5,10,14 comprise a solid non-porous metal or nano-porous material as a support for the thin-film electrolyte layers 6,8.

[0038] More specifically, in one embodiment, at least one of the electrodes 5, 10, 14 comprises a nanoporous material mounted or coated on a substrate that provides mechanical support for the nanoporous material, with the thin film electrolyte supported directly on the nanoporous material, thereby forming a nanoporous electrode. In one embodiment, the substrate is a porous substrate. The nanoporous electrode has sufficient surface smoothness to serve as a substrate for a thin film electrolyte coating, preferably having a thickness of 0.03 μm to 10 μm. Such electrodes may be composed of, for example, microporous carbon or nanoporous carbon. Examples of such microporous or nanoporous carbon electrodes and how they may be formed are disclosed in U.S. Patent No. 9,046,784 to Wang et al., U.S. Patent No. 6,632,849 to Yao et al., and in a publication by Kondyurin et al. entitled "Nanostructured Carbonized Thin Films Produced by Plasma Immersion Ion Implantation of Block-Copolymer Assemblies" (pp. 155-160), Version of Record online: October 29, 2007 | DOI: 10.1002 / ppap.200700111. The substrate can be, for example, a silicon sheet, a glass sheet, or a metal sheet.

[0039] In other embodiments, at least one of the electrodes 5, 10, 14 comprises a non-porous material mounted on a substrate that provides mechanical support to the non-porous material, thereby forming a non-porous electrode with a thin-film electrolyte supported on the non-porous material. In one embodiment, the substrate is a porous substrate. Preferably, at least one of the electrodes 5, 10, 14 comprises a non-porous metal material mounted on a porous electrode substrate, thereby forming a non-porous metal electrode with a thin-film electrolyte supported on the non-porous metal material. More specifically, at least one of the electrodes 5, 10, 14 may comprise a thin, non-porous metal membrane, permeable to the working fluid, supported on a porous substrate.

[0040] Alternatively, at least one of the electrodes 5, 10, 14 comprises a dense self-supporting metal sheet or foil that is permeable to the working fluid.

[0041] In one embodiment, each porous electrode 5, 10, 14 may optionally include a working fluid permeable dense metal coating on the surface of the metal substrate at the interface with the electrolyte membrane 6, 8. The dense metal coating may include a catalyst to promote oxidation and reduction as the working fluid flows into and out of the electrolyte membrane 6, 8. The metal coating may also be supported by a conductive or non-conductive porous substrate such as porous anodized aluminum (Whatman Anodisc® available from SigmaAldrich, USA) or a conductive material such as nickel foam.

[0042] Metal alloys and laminates that are particularly permeable to the working fluid are suitable for forming the electrodes 5, 10, 14. Referring to FIG. 2, several different metals are shown that may be suitable for use as the electrodes 5, 10, 14 of the present invention. More specifically, FIG. 2 shows the hydrogen permeability of several metals. Palladium and palladium alloys are particularly suitable when hydrogen is the working fluid. There are numerous metals that have higher permeability than palladium, but these other materials tend to fail due to embrittlement when placed in a hydrogen environment. On the other hand, palladium remains stable and has catalytic properties to promote hydrogen reactions. Palladium is also established as a suitable catalyst support substrate for thin electrolyte coatings. However, in addition to catalyzing the incoming hydrogen molecules with the cathode, palladium also catalyzes the reduction reaction of the electrolyte at the electrolyte interface. It has therefore been found to be beneficial to include a low-reactivity interfacial layer between the palladium and the electrolyte.

[0043] For example, a Ni / Pd foil with a 600 nm thick Ni layer has a melting point of about 5×10 -9 mol s -1 m -1 Pa -0.5 This hydrogen permeability is half that of pure Pd foil. This value indicates that the hydrogen permeability is at least 2 A cm in a fuel cell. 2 It is sufficient to provide an external current of 1000 kcal / cm. See "Thin Film Fuel Cell Based on Nanometer-Thick Membrane of Amorphous Zirconium Phosphate Electrolyte" (Journal of The Electrochemical Society, 158(8)B866-B870(2011)0013-4651 / 2011 / 158(8) / B866 / 5, The Electrochemical Society).

[0044] Efforts towards the use of solid metal electrodes for fuel cell applications have been very limited compared to the extensive work being done towards the development of membranes for hydrogen separation / generation applications. Figure 3 lists some commercially available hydrogen-permeable metal alloys, adapted from Yin et al., "A Review on the Production and Purification of Biomass-Derived Hydrogen Using Emerging Membrane Technologies," Catalyst (October 6, 2017). The addition of silver and / or other alloying metals improves the mechanical strength and stability of palladium-based membranes. For example, by alloying silver to palladium, the permeability of the membrane can be increased by up to five times compared to a pure Pd membrane. One study conducted by Tanaka et al. "Preparation and Characterization of Ceramic Supported Ultra-Thin (approximately 1 μm) Pd-Ag Membranes" (DAPJMembr.Sci.2017, 528, 12-23) analyzed ultra-thin (approximately 1 μm) Pd-Ag membranes produced by simultaneous ELP method, and the membranes showed a thermal expansion coefficient of 9.0–9.4 × 10 at 400 °C. -6 mol.m -2 s -1 Pa -1 and H2 / N2 selectivity of 3300-2000.

[0045] Each MEA 40, 42 may be constructed by applying a thin film coating of electrolyte material onto an electrode permeable to the working fluid. For example, the thin film electrolyte layer or film 6, 8 may be coated onto the first electrode by sputter deposition, laser ablation, chemical vapor deposition or any other well-known thin film deposition technique. The thin film electrolyte layer or film 6, 8 may also be applied onto the first electrode by spin coating with a nanoparticle slurry of the electrolyte material, by sol-gel or by atomic layer deposition techniques. For polymer-based electrolyte films, inkjet printing, solvent casting or spin coating are some of the well-known application techniques that may be used to form the thin film electrolyte film 6, 8. After the formation of the thin film electrolyte film 6, 8 on one of the electrodes 5, 10, 14, the other electrode 5, 10, 14 is applied on top of the thin film electrolyte film 6, 8 to complete the MEA 40, 42. In one embodiment, one or both of the electrodes 5, 10, 14 of the MEA 40, 42 may contain catalytic materials to promote the oxidation and reduction of the working fluid at the electrode-electrolyte interface, or the material of the electrode itself may be catalytic to promote these reactions.

[0046] In one embodiment, after a second electrode is applied on top of the thin film electrolyte membranes 6,8, the bonded structure may be folded or rolled to form MEAs 40,42 in a tubular configuration.

[0047] In a preferred embodiment, the working fluid is in the form of a gas. In a preferred embodiment, the working fluid is hydrogen. Thus, each concentration cell 9, 11 includes an MEA 40, 42 consisting of a proton-conducting electrolyte material sandwiched between two electrodes that are conductive of electrons and permeable to hydrogen. For converters in which hydrogen is the working fluid, suitable highly permeable metals for the electrodes of each MEA include, but are not limited to, palladium, niobium, yttrium, tantalum and alloys thereof, including alloys with silver and / or copper. The proton-conducting electrolyte material of each MEA is in the form of a thin membrane having a thickness of 0.03 μm to 10 μm sandwiched between two hydrogen-permeable electrodes.

[0048] The disclosure provided herein primarily uses hydrogen as an example working fluid, although those skilled in the art will appreciate that the present invention is not limited to hydrogen as a working fluid, and indeed the principles described herein apply to other ionizable working fluids as well.

[0049] In the embodiment of Figure 1, converter 100 has a tubular structure such that the MEA of converter 100 also has a generally tubular structure. However, those skilled in the art will appreciate that the present invention does not require the MEA to be so configured. For example, the MEA may have a flat layered structure and / or be arranged in a stacked configuration. However, the description provided herein focuses on converters with a tubular configuration.

[0050] In the embodiment of FIG. 1, the first electrode 5 extends from the MEA 40 of the first electrochemical cell 9 to the MEA 42 of the second electrochemical cell 11. Thus, in the embodiment of FIG. 1, the first electrode 5 is common, and more specifically, electrically common, to the MEAs 40, 42 of both the first and second electrochemical cells 9, 11. As noted above, the first electrodes 5 of the MEAs 40, 42 have a tubular form such that a passageway, or conduit 50, is formed inside the tubular electrode 5 and extends between the first electrochemical cell 9 and the second electrochemical cell 11. The distal end of this conduit 50 is closed by the plug 7. More specifically, the first electrodes 5 of the MEAs 40, 42 and the plug 7 together define a closed conduit 50 that couples the working fluid flow between the first electrochemical cell 9 and the second electrochemical cell 11.

[0051] The MEAs 40, 42 are at least partially contained or surrounded by an outer housing, preferably a tubular outer housing 24. A sealing ring 12 may be optionally provided to ensure an airtight or near-airtight seal between the MEAs 40, 42 and the outer housing 24. A conduit 50 is located within or traverses the length of the outer enclosure (outer housing) 24. The outer housing 24 is thus divided into separate chambers: a first, or inner, chamber within the conduit 50, and a second, or outer, chamber surrounding the conduit 50. The outer chamber thus becomes, in effect, a second conduit 52. The first conduit 50 is in direct contact with the first electrode 5, while the second conduit 52 is in direct contact with the second electrode 10 of the MEA 40 of the first cell 9 and the second electrode 14 of the MEA 42 of the second cell 11.

[0052] In one embodiment, the first and second conduits 50, 52 have a concentric arrangement with the first conduit 50 contained within the second conduit 52. The second conduit 52 also couples the working fluid flow between the first electrochemical cell 9 and the second electrochemical cell 11. As shown in Figures 1 and 4, the first and second electrochemical cells 9, 11, and indeed the entire converter 100, may be configured as a concentric tubular structure, particularly with the concentric tubular conduits 50, 52. A concentric tubular structure is advantageous in that the confined pressure stresses are primarily tensile and compressive, thereby allowing the use of minimal wall thicknesses.

[0053] The converter 100 is configured such that one electrode of each MEA 40, 42 is maintained at a relatively low pressure and the other electrode of each MEA 40, 42 is maintained at a relatively high pressure. Those skilled in the art should understand that the terms "high pressure" and "high concentration" are used interchangeably herein with respect to the working fluid. Those skilled in the art should also understand that the terms "low pressure" and "low concentration" are used interchangeably herein with respect to the working fluid. Preferably, the high concentration (i.e., high pressure) sides of the two electrochemical concentration cells 9, 11 are interconnected by a high pressure conduit (i.e., a conduit maintained at a relatively high pressure) and the low concentration (i.e., low pressure) sides of the two electrochemical concentration cells 9, 11 are coupled to each other by a low pressure conduit (i.e., a conduit maintained at a relatively low pressure).

[0054] More specifically, in converter 100 of Figure 1, first electrodes 5 of MEAs 40, 42 are maintained at a relatively high pressure and therefore constitute the high pressure side of converter 100. First conduit 50 thus couples the working fluid flow at high pressure between first electrochemical cell 9 and second electrochemical cell 11. Second electrodes 10, 14 of MEAs 40, 42 are maintained at a relatively low pressure and therefore constitute the low pressure side of converter 100. Second conduit 52 thus couples the working fluid flow at low pressure between first electrochemical cell 9 and second electrochemical cell 11.

[0055] 1, the central portion 28 of the converter 100 functions as a recuperative heat exchanger connecting the working fluid flow between the two electrochemical cells 9, 11. In the central portion 28 of the converter 100 of the embodiment of FIG. 1, a barrier layer or coating 26 is provided over the outer length of the electrode 5 that spans the central portion 28 (i.e., the area between the first cell 9 and the second cell 11). The barrier coating 26 prevents hydrogen permeation through the hydrogen permeable material of the electrode 5 in the area of ​​the central portion 28.

[0056] Alternatively, as shown in FIG. 4, the cells 9, 11 may not have an electrically common electrode 5. The converter 100' of FIG. 4 and its operation are identical to that of the converter 100 of FIG. 1, except for the following aspects: Each MEA 40, 42 comprises separate electrodes. Thus, the MEA 40 of the first electrochemical cell 9 comprises a membrane 6 sandwiched between a first electrode 55 and a second electrode 10, and the MEA 42 of the second electrochemical cell 11 comprises a membrane 8 sandwiched between a first electrode 65 and a second electrode 14, the first electrodes 55 and 65 being separate from each other. In the central portion 28', a tubular member 27 is provided and extends between the first electrodes 55, 65. The tubular member 27 is preferably made of a material that is not permeable to the working fluid. More specifically, tubular member 27 preferably accommodates heat transfer while preventing passive diffusion of working fluid from a higher pressure state to a lower pressure state without undergoing an electrochemical reaction. Thus, in the embodiment of Figure 4, conduit 50 is formed entirely within tubular member 27, first electrode 55 of MEA 40 of first cell 9, and first electrode 65 of MEA 42 of second cell 11.

[0057] Referring to FIG. 1, one of the electrochemical concentration cells 9, 11 is connected to, for example, a heat source Q H , and operates to convert the heat into electricity by expanding an ionized working fluid from high concentration or pressure to low concentration or pressure, the expansion occurring under the Nernst potential of the cell. This cell is referred to herein as the "high temperature cell" or "high temperature MEA." The other electrochemical concentration cell 9, 11 is preferably connected to a heat sink Q L and operates in response to an electrical power input to pump and compress an ionized working fluid from low concentration or pressure back to high concentration or pressure, the compression being performed under an applied voltage above the Nernst potential of the cell. Power is dissipated by the compression process and the heat of compression is rejected. This cell is referred to herein as a "low temperature cell" or "low temperature MEA."

[0058] More specifically, in one embodiment, the MEAs 40, 42 operate as part of a heat engine, with the MEA 42 of the second cell 11 being coupled to a heat source Q HPreferably, the heat sink Q to which the MEA 40 of the first battery 9 is coupled L 1. The high temperature MEA 42 has a higher Nernst voltage than the low temperature MEA 40. The operation of the engine (converter) 100 is such that the working fluid is compressed in the low temperature concentration cell 9 from the low voltage electrode 10 to the high voltage electrode 5 of the MEA 40. The compressed working fluid is then delivered by high pressure conduit 50 to the high voltage electrode 5 of the MEA 42 of the high temperature concentration cell 11. The working fluid is expanded through the MEA 42 of the high temperature cell 11 as it passes through the thin film electrolyte membrane 8 from the high voltage electrode 5 to the low voltage electrode 14. The working fluid is then delivered by low pressure conduit 52 back to the low voltage electrode 10 of the low temperature concentration cell 9. The voltage generated by the high temperature MEA 42 is high enough to exceed the Nernst voltage of the low temperature MEA 40 and leave enough voltage to power the series connected external load 34.

[0059] More specifically, the converter 100, 100' of Figures 1 and 4 operates as a heat engine as follows: Starting at a low pressure, low temperature state 1, electrical energy is supplied to the low temperature MEA 40 to pump the working fluid flow 16 across the MEA 40 from the low pressure, low temperature state 1 to the high pressure, low temperature state 2. The temperature of the working fluid increases during the compression process as a heat Q LThe temperature of the working fluid is kept nearly constant by removing protons from the proton conducting membrane 6. The thin membrane 6, preferably less than 10 μm thick, does not support large temperature gradients, so the nearly isothermal assumption for the process is valid, provided sufficient heat is transferred to the membrane 6 and its substrate. From the high pressure, low temperature state 2, the working fluid passes as flow 18 through a recuperative countercurrent heat exchanger (i.e., central section 28) where it is heated under nearly constant pressure to a high pressure, high temperature state 3 and flows to the high temperature MEA 42. The heat 30 required to raise the temperature of the working fluid from the high pressure, low temperature state 2 to the high pressure, high temperature state 3 is transferred from the working fluid flow 20 flowing in the opposite direction in the heat exchanger. More specifically, central section 28 acts to couple heat 30 from the working fluid flow 20 leaving the high temperature MEA 42 and flowing in conduit 52 to the working fluid flow 18 moving in conduit 50 to the high temperature MEA 42. In the high temperature MEA 42, power is generated as the working fluid flow 22 moves across the MEA 42 and expands from a high pressure, high temperature state 3 to a low pressure, high temperature state 4. As the working fluid expands, heat Q is applied to the MEA 42 to maintain a nearly constant temperature. H is provided. To switch from the low pressure, high temperature state 4 to the low pressure, low temperature state 1, the working fluid flow 20 flows through a recuperative heat exchanger (i.e., central section 28) where its temperature is reduced in a constant pressure process by heat transfer from the high pressure, low temperature state 2 to the working fluid flow 18 through the high pressure, high temperature state 3. The cycle continues as the working fluid is pumped by the low temperature MEA 40 from low pressure to high pressure.

[0060] During operation of the converter 100, 100', the working fluid is compressed in the low-temperature electrochemical cell 9 by supplying an electric current at a voltage sufficient to exceed its Nernst potential, thereby moving the working fluid from the low-pressure side to the high-pressure side of the membrane 6. Meanwhile, the working fluid expands in the high-temperature electrochemical cell 11 as an electric current (power) is extracted under its Nernst potential. In the high-temperature electrochemical cell 11, a flow of electric current is generated as the working fluid expands from the high-pressure side to the low-pressure side of the membrane 8. As with any thermodynamic engine employing a working fluid that follows the nature of compressible gases, in the converter 100, more work (electricity) is extracted during the high-temperature expansion than the work (electricity) input required for the low-temperature compression. The difference between the heat energy input to maintain a constant temperature during the high-temperature expansion of the converter 100 and the heat energy removed to maintain a constant temperature during the low-temperature compression is given as the difference between the electric energy output by the high-temperature expansion process and the electric energy consumed by the low-temperature compression process.

[0061] According to the Nernst equation, the high temperature electrochemical cell 11 will have a higher voltage than the low temperature electrochemical cell 9. Since the current (I) is the same through both cells, the voltage difference means that the power generated through the expansion of the working fluid in the high temperature electrochemical cell 11 is higher than that of the low temperature electrochemical cell 9. The power (VHT*I) output by the high temperature electrochemical cell 11 is sufficient to drive the compression process (VLT*I) in the low temperature electrochemical cell 9 and provide a net power output ((VHT*I)-(VLT*I)) to the external load 34. This voltage difference provides the basis for the operation of the converter 100.

[0062] In other embodiments in which the MEAs 40, 42 operate as part of a heat pump application, the heat source Q to which the MEA 42 of the second cell 11 is coupled H The heat sink Q to which the MEA 40 of the first battery 9 is bonded L Therefore, the working fluid has a low temperature relative to the heat source Q H As the heat is drawn from the heat source Q HThe working fluid is expanded at low temperature in the MEA 42 coupled to the heat sink Q L The cold expanding MEA 42 is compressed at high temperature in the MEA 40 coupled to the cold MEA 42, and the heat of compression is rejected upon heating. Due to its low operating temperature, the cold expanding MEA 42 generates a Nernst voltage that is less than the Nernst voltage of the hot compressing MEA 40. Therefore, an external power source is connected in series with the cold MEA 42 to provide a resultant voltage high enough to exceed the Nernst potential of the hot MEA 40, thereby causing the compression process to proceed therethrough.

[0063] 5A-5B show end views of converter 100 configured as a tubular structure. The tubular structure of converter 100 is intended to function as an active heat pipe for heat transport, either as a generator or as a heat pump. As such, converter 100 may be configured with a relatively small diameter tubular structure, whereby multiple tubes may be arranged to form a structure similar to a bank of "heat pipes" used to transfer heat between a heat source and a heat sink, as shown in FIG.

[0064] More specifically, referring to Figure 6, there is shown a bank of thermal-to-electrical converter tubes coupled between a hot working fluid flow stream 32 (i.e., heat source) and a cold working fluid flow stream 30 (heat, i.e., heat sink). A hot MEA 42 for expansion of the working fluid is exposed to and heated thereby, and a cold MEA 40 for compression of the working fluid is exposed to and cooled thereby, the cold working fluid flow stream 30. The hot and cold working fluid flow streams 32, 30 are isolated from each other by recuperative heat exchanger sections 38, as the working fluid flows in its respective heat exchanger (central section 28) of each converter tube coupling flow in its concentric tubes between the respective cold MEA 40 and the respective hot MEA 42.

[0065] It will be appreciated by those skilled in the art that changes may be made to the above-described embodiments without departing from the broad inventive concept thereof. It is understood, therefore, that the invention is not limited to the particular embodiments disclosed, but that it is intended to cover modifications within the spirit and scope of the invention as defined by the following claims.

Claims

1. an ionized working fluid; a first membrane electrode assembly and a second membrane electrode assembly electrically coupled to each other; Each membrane electrode assembly is provided with: a first electrode that conducts electrons and is permeable to the ionized working fluid; a second electrode that conducts electrons and is permeable to the ionized working fluid; a thin-film electrolyte membrane sandwiched between the first electrode and the second electrode, the thin-film electrolyte membrane having conductivity for ions of the ionized working fluid and a thickness of 0.03 μm to 10 μm; At least one of the first and second electrodes of each membrane electrode assembly comprises a non-porous, dense metal; Due to a working fluid pressure difference applied to each membrane electrode assembly, one of the first and second electrodes of each membrane electrode assembly is in contact with the ionized working fluid at a first pressure, and the other of the first and second electrodes of each membrane electrode assembly is in contact with the ionized working fluid at a second pressure lower than the first pressure; the first membrane electrode assembly is configured to compress the ionized working fluid from the second pressure to the first pressure; The second membrane electrode assembly is configured to expand the ionized working fluid from the first pressure to the second pressure.

2. 2. The thermal to electric converter of claim 1, wherein said non-porous dense metal is laminated or mounted on a porous substrate.

3. 3. The thermal to electric converter of claim 1 or 2, wherein at least one of the first and second electrodes of at least one of the first and second membrane electrode assemblies includes a catalyst configured to promote oxidation and reduction of the ionized working fluid as the ionized working fluid passes through the respective thin film electrolyte membrane.

4. 4. The thermal-to-electric converter of claim 1, wherein at least one of the first and second electrodes of at least one of the first and second membrane electrode assemblies comprises palladium or an alloy thereof.

5. 5. The thermal to electric converter of claim 1, further comprising a recuperative heat exchanger coupling a flow of the ionized working fluid at the first pressure and the second pressure between the first membrane electrode assembly and the second membrane electrode assembly.

6. A thermal to electrical converter as claimed in any preceding claim, wherein the thermal to electrical converter has a tubular configuration.

7. The thermal to electric converter of claim 1 , wherein each of the first and second membrane electrode assemblies has a tubular configuration.

8. 8. A thermal to electric converter as claimed in any one of claims 1 to 7, wherein an interior of each of the first and second membrane electrode assemblies constitutes a first conduit for the flow of the ionized working fluid at the first pressure.

9. 9. The thermal to electric converter of claim 1, further comprising a housing at least partially surrounding the first and second membrane electrode assemblies, a space between the first and second membrane electrode assemblies and the housing constituting a second conduit for the flow of the ionized working fluid at the second pressure.

10. A thermal to electric converter as claimed in any one of claims 1 to 9, wherein one of the first and second electrodes is electrically common to both the first and second membrane electrode assemblies.

11. 11. The thermal to electric converter of claim 1, further comprising an external circuit for electrically coupling the first and second membrane electrode assemblies.

12. 10. A thermal to electric converter system comprising a plurality of the thermal to electric converters of claim 6 coupled between a first stream of the ionized working fluid at a first temperature and a second stream of the ionized working fluid at a second temperature lower than the first temperature.

13. 13. A method for generating electricity using the thermal-to-electrical converter of claim 1 as a heat pump, comprising: coupling the first membrane electrode assembly to a heat sink at a first temperature and coupling the second membrane electrode assembly to a heat source at a second temperature, the first temperature being greater than the second temperature; applying power to the first membrane electrode assembly to pump the ionized working fluid from the second pressure to the first pressure while heat is being removed at the first temperature and first voltage; extracting power from the second membrane electrode assembly while heat is being supplied thereto at the second temperature and second voltage as the ionized working fluid expands from the first pressure to the second pressure, the first voltage being greater than the second voltage; A method for providing the above.

14. The method of claim 13 , further comprising the step of connecting an external power source in series with the second membrane electrode assembly.

15. 13. A method for generating electricity using the thermal-to-electrical converter of claim 1 as a heat engine, comprising the steps of: coupling the first membrane electrode assembly to a heat sink at a first temperature and coupling the second membrane electrode assembly to a heat source at a second temperature, the first temperature being lower than the second temperature; applying power to the first membrane electrode assembly to pump the ionized working fluid from the second pressure to the first pressure while heat is being removed at the first temperature and first voltage; extracting power from the second membrane electrode assembly while heat is being supplied thereto at the second temperature and second voltage as the ionized working fluid expands from the first pressure to the second pressure, the second voltage being greater than the first voltage; A method for providing the above.

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