Method for transporting nitride ions in an electrochemical cell

Abstract

A method for transporting nitride (N.sup.3) ions in an electrochemical cell includes providing nitrogen to a first side of a solid electrolyte membrane to form nitride ions and transporting the nitride ions across the solid electrolyte membrane. The solid electrolyte membrane includes a metal nitride. The method may be used for ionically-mediated separation and/or compression of nitrogen or to form ammonia.

Claims

1. A method for transporting nitride ions (N.sup.3) across a solid electrolyte comprising a metal nitride in an electrochemical cell, the electrochemical cell comprising: a first catalyst electrode layer; a second catalyst electrode layer; and the solid electrolyte between the first catalyst electrode layer and the second catalyst electrode layer; the method comprising: applying a voltage to the electrochemical cell; providing nitrogen (N.sub.2) to the first catalyst electrode layer, wherein the nitrogen reacts to form nitride ions at a surface of the first catalyst electrode layer; and transporting the nitride ions across the solid electrolyte; wherein the electrochemical cell is operated at a temperature in the range of from 150 C. to 250 C.; wherein the nitrogen (N.sub.2) is provided at a pressure in the range of from 1 bar to 10 bar; and wherein the solid electrolyte comprises lithium nitride.

2. The method of claim 1, wherein at least one of the first catalyst electrode layer and the second catalyst electrode layer comprises an alkali promoted ruthenium catalyst.

3. The method of claim 1, wherein the metal nitride electrolyte was formed in situ in an electrochemical cell.

4. The method of claim 1, wherein the electrochemical cell is located in an electrochemical stack comprising a plurality of cells.

5. The method of claim 1, wherein one of the first catalyst electrode layer and the second catalyst electrode layer comprises a ruthenium (Ru) catalyst.

6. The method of claim 5, wherein one of the first catalyst electrode layer and the second catalyst electrode layer does not comprise the ruthenium (Ru) catalyst.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The following is a brief description of the drawings, which are presented for the purposes of illustrating the exemplary embodiments disclosed herein and not for the purposes of limiting the same.

(2) FIG. 1 is a flow chart showing the process steps involved in ammonia synthesis using the Haber-Bosch process.

(3) FIG. 2 is a flow chart showing a method for producing an electrochemical stack in accordance with some embodiments of the present disclosure.

(4) FIG. 3 is a cross-sectional view of an electrochemical stack in accordance with some embodiments of the present disclosure.

(5) FIG. 4 is an exploded view of another electrochemical stack in accordance with some embodiments of the present disclosure.

(6) FIG. 5 schematically illustrates ammonia synthesis using a solid-state nitride conducting membrane in accordance with some embodiments of the present disclosure.

(7) FIG. 6 illustrates system integration of a modular electrochemical ammonia generator stack (MEGA) with a H.sub.2 production process (steam reforming, SR), an air separation process (cryogenic distillation) and an electricity generation process (gas turbine) in accordance with some embodiments of the present disclosure.

(8) FIG. 7 schematically illustrates nitrogen compression using a nitride-conducting solid membrane in accordance with some embodiments of the present disclosure.

(9) FIGS. 8A and 8B show a comparison of preliminary electrochemical impedance spectroscopy measurements of Li.sub.3N ionic conductivity with existing literature and the crystal structure of -Li.sub.3N, respectively.

(10) FIG. 9 includes photographs showing the synthesis of a 1.5 mm thick lithium nitride membrane by reacting Li in 1 bar N.sub.2 at 220 C. in which a prototype electrochemical stack (3 cm.sup.2 active area) was fabricated using a cathode catalyst (4 mg/cm.sup.2 Pt/Ru), an anode catalyst (4 mg/cm.sub.2 Pt) and Li metal. The Li metal was reacted under passage of N.sub.2 to form the lithium nitride membrane in situ.

(11) FIG. 10 illustrates cyclic voltammogram (10 mV/s) demonstrating NH.sub.3 synthesis at 180 C. in a preliminary cell operated with PtRu catalyst (N.sub.2) and Pt catalyst (H.sub.2). The NH.sub.3 produced was detected using a 100 ppm sensor, which saturated at higher values of applied potential.

(12) FIG. 11 is a graph showing some of the results of the proof-of-concept demonstration for nitrogen generation.

(13) FIG. 12 is a photograph of a portion of a disassembled, layered electrochemical cell formed in accordance with one of the examples of the present disclosure.

(14) FIG. 13 is a graph showing the change in pressure over time when an electric field was applied to the device of FIG. 12.

(15) FIG. 14 is a graph showing the change in pressure over time when the electric field applied in FIG. 13 was reversed.

(16) FIG. 15 illustrates an exemplary thermal profile in accordance with some embodiments of the present disclosure.

(17) FIG. 16 illustrates an exemplary chemical (gas) profile in accordance with some embodiments of the present disclosure.

(18) FIG. 17 in a scanning electron microscope (SEM) image of a membrane produced according to one of the examples at 37 magnification and 1000 magnification (inset).

(19) FIG. 18 is an EDX linescan of nitrogen content across a cross-section of a membrane made according to one of the examples.

(20) FIG. 19 is a graph illustrating Faradaic efficiency and ammonia flux in an electrochemical cell made in accordance with one of the examples.

DETAILED DESCRIPTION

(21) A more complete understanding of the membranes, stacks, systems, and methods disclosed herein can be obtained by reference to the accompanying drawings. These figures are merely schematic representations based on convenience and the ease of demonstrating the existing art and/or the present development, and are, therefore, not intended to indicate relative size and dimensions of the assemblies or components thereof.

(22) Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. In case of conflict, the present document, including definitions, will control. Preferred methods and materials are described below, although methods and materials similar or equivalent can be used in practice or testing of the present disclosure. The materials, methods, and articles disclosed herein are illustrative only and not intended to be limiting. For example, lithium nitride solid electrolytes are discussed throughout this application, particularly in the Examples section. However, reactant metals other than lithium are also contemplated. Similarly, ammonia production is discussed throughout this application but other applications are also contemplated.

(23) The singular forms a, an, and the include plural referents unless the context clearly dictates otherwise.

(24) As used in the specification and in the claims, the term comprising may include the embodiments consisting of and consisting essentially of. The terms comprise(s), include(s), having, has, can, contain(s), and variants thereof, as used herein, are intended to be open-ended transitional phrases that require the presence of the named ingredients/steps and permit the presence of other ingredients/steps. However, such description should be construed as also describing compositions, mixtures, or processes as consisting of and consisting essentially of the enumerated ingredients/steps, which allows the presence of only the named ingredients/steps, along with any impurities that might result therefrom, and excludes other ingredients/steps.

(25) Unless indicated to the contrary, the numerical values in the specification should be understood to include numerical values which are the same when reduced to the same number of significant figures and numerical values which differ from the stated value by less than the experimental error of the conventional measurement technique of the type used to determine the particular value.

(26) All ranges disclosed herein are inclusive of the recited endpoint and independently combinable (for example, the range of from 2 to 10 is inclusive of the endpoints, 2 and 10, and all the intermediate values). The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value; they are sufficiently imprecise to include values approximating these ranges and/or values.

(27) As used herein, approximating language may be applied to modify any quantitative representation that may vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as about and substantially, may not be limited to the precise value specified, in some cases. The modifier about should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the expression from about 2 to about 4 also discloses the range from 2 to 4. The term about may refer to plus or minus 10% of the indicated number. For example, about 10% may indicate a range of 9% to 11%, and about 1 may mean from 0.9-1.1.

(28) For the recitation of numeric ranges herein, each intervening number there between with the same degree of precision is explicitly contemplated. For example, for the range of 6-9, the numbers 7 and 8 are contemplated in addition to 6 and 9, and for the range 6.0-7.0, the number 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, and 7.0 are explicitly contemplated.

(29) The present disclosure relates to electrochemical stacks including a solid electrolyte membrane. Methods for making and using the electrochemical stacks and solid electrolyte membranes are also disclosed. In some embodiments, the membrane comprises a metal nitride, a metal oxide or a mixed-metal oxide, a metal fluoride, a metal iodide, or a metal sulfide.

(30) The lithium nitride (e.g., Li.sub.3N) membranes of the present disclosure act as a solid nitride-ion (N.sup.3) conducting electrolyte to facilitate ion electromigration under an applied electric field. The membrane is a good ionic conductor at intermediate temperatures (.sub.453K=4.5 mS cm.sup.1), with a high nitride ion mobility (t.sub.N30.4).

(31) Metals are generally easier to machine than metal oxides due to their greater sectility and ductility. It is easier to assemble an electrochemical stack starting with a reactive base metal, and minimize the detrimental effects of mechanical stresses during assembly. The production of the ion transport membrane through an in situ reaction reduces the likelihood of membrane failure as a result of physical membrane cracking.

(32) Additionally, membranes made using the systems and methods of the present disclosure retain the properties of bulk films, and are more polycrystalline in nature. This results in lithium nitride membranes with ionic conductivities approaching single-crystal values, which are significantly higher than values for sintered lithium.

(33) The membrane can be combined with catalyst electrodes to form a membrane electrode assembly (MEA), which is the building block of the highly modular electrochemical stack. The membrane is synthesized by reacting Li metal sheet and N.sub.2 gas, which occurs in situ once the membrane electrode assembly (MEA) has been assembled, which will lend itself to reel-to-reel processing, simplifying stack assembly and dramatically reducing fabrication costs.

(34) The electrochemical stacks of the present disclosure may be used to produce a variety of nitrogenous chemicals when the membrane comprises a metal nitride. For example the electrochemical stack may produce hydrazine (N.sub.2H.sub.4), hydrazoic acid (HN.sub.3), or ammonia, which may be produced through the overall reactions below. The reaction products are determined by the choice of catalyst and operating conditions of voltage, gas composition and temperature. This enables the on-demand synthesis of these chemicals using much less hazardous intermediates.
3N.sub.2+H.sub.2.fwdarw.2NH.sub.3
2N.sub.2+H.sub.2.fwdarw.N.sub.2H.sub.4
N.sub.2+3H.sub.2.fwdarw.2NH.sub.3

(35) The intermediate temperature operation and the use of a N.sup.3 conducting solid electrolyte avoids the two primary problems in current ammonia generation technologies:

(36) 1. low ammonia conversion due to thermodynamic ammonia decomposition at high temperatures in the Haber-Bosch Process; and

(37) 2. low ammonia conversion due to parasitic hydrogen evolution in electrochemical processes.

(38) The modular stack design has no moving parts and a much lower capital cost than state-of the-art ammonia generation processes such as Haber-Bosch for small-scale NH.sub.3 generation. This small-scale ammonia generation enables the harnessing of energy that is currently lost during natural gas flaring or curtailed renewables.

(39) During operation, feed gases (N.sub.2/H.sub.2) enter the Modular Electrochemical Generation of Ammonia (MEGA) stack, maintained at intermediate temperature (e.g., from about 200 to about 250 C.). The thermodynamic decomposition of NH.sub.3 at this temperature is avoided by operating at a moderate hydrostatic pressure (e.g., less than about 5 bar). The separation of the feed gases using a solid membrane enables the use of N.sub.2-selective Ru catalysts without risk of H.sub.2 poisoning. At the cathode, N.sub.2 is reduced (N.sub.2+3e.sup..fwdarw.N.sup.3) and N.sup.3 migrates across the membrane and reacts with H.sub.2 (3/2H.sub.2+N.sup.3.fwdarw.NH.sub.3+3e.sup.) to produce NH.sub.3.

(40) MEGA's modularity is well-suited for use in conjunction with wind farms or gas flares, which vary considerably by location and intensity. The MEGA stack may be integrated with a small-scale steam reformer and air separation unit.

(41) A preliminary cost model to assess the production cost assumes a scaling exponent of 0.6 for small-scale steam reformer (SR) and air separation unit (ASU) plants and no cost for flared gas. The resulting NH.sub.3 cost using MEGA compares favorably to a much larger Haber-Bosch facility and allows reduction of NH.sub.3 transportation costs for manufacture of fertilizer. For example, the flared gas in North Dakota could be used to supply >40% of the NH.sub.3 demand in the Corn Belt. Process optimization during the integration of MEGA, SR, and ASU could result in even more favorable energetics and economics.

(42) FIG. 2 is a flow chart illustrating an exemplary method for producing a solid electrolyte membrane in situ in an electrochemical stack. Initially, a metal metal layer 105 is provided. In the depicted embodiment, the metal layer is provided in the form of a roll. The metal layer 105 is cut and laminated 160 with a first catalyst layer 130, a second catalyst layer 135, and sealing gaskets 132, 137 to form a membrane electrode assembly. The membrane electrode assembly is assembled 165 into a reactive metal stack by the provision of current collectors 150, 155 (e.g., bipolar plates). Finally, reactant gas (e.g., nitrogen gas, ammonia gas, or a nitrogen oxide gas) at relatively low temperature and pressure (compared to the Haber-Bosch process) is provided to the reactive metal stack and reacts 170 with the reactive metal layer to form a solid electrolyte membrane 120 in situ in the electrochemical stack.

(43) In some embodiments, the solid electrolyte membrane comprises a nitride, an oxide, an iodide, or a fluoride ion conductor.

(44) The solid electrolyte membrane is formed by reacting a reactant metal with a reactant gas. The reactant metal can include one or more of the following elements: lithium (Li), sodium (Na), magnesium (Mg), calcium (Ca), strontium (Sr), zirconium (Zr), yttrium (Y), scandium (Sc), cerium (Ce), and gadolinium (Gd).

(45) In some embodiments, the reactant gas comprises nitrogen (N.sub.2), ammonia (NH.sub.3), oxygen (O.sub.2), ozone (O.sub.3), hydrogen (H.sub.2), or fluorine (F.sub.2).

(46) When the membrane comprises a nitride, the reactant metal may contain lithium (Li), sodium (Na), magnesium (Mg), calcium (Ca), and/or strontium (Sr) and the reactant gas may be nitrogen (N.sub.2), ammonia (NH.sub.3), or hydrazine (N.sub.2H.sub.4).

(47) When the membrane comprises an oxide, the reactant metal may contain zirconium (Zr), yttrium (Y), scandium (Sc), cerium (Ce), and/or gadolinium (Gd) and the reactant gas may be oxygen (O.sub.2) or ozone (O.sub.3).

(48) When the membrane comprises a fluoride, the reactant gas may be fluorine (F.sub.2) and the reactant metal may be lanthanum (La) or lead (Pb).

(49) When the membrane comprises an iodide, the reactant gas may be iodine (I.sub.2), and the reactant metal may be silver (Ag).

(50) When the membrane comprises a sulfide, the reactant gas may be hydrogen sulfide (H.sub.2S) and the reactant metal may be silver (Ag).

(51) FIG. 3 illustrates a non-limiting embodiment of an electrochemical stack 100 in accordance with some embodiments of the present disclosure. The stack 100 includes a solid electrolyte membrane 120 (e.g., a solid metal nitride electrolyte membrane such as a lithium nitride membrane). Catalyst electrode layers 130, 135 are provided on each side of the electrolyte membrane 120. The first catalyst electrode layer 130 is located between the electrolyte membrane 120 and a first, microporous layer 140. The second catalyst electrode layer 135 is located between the electrolyte 120 and a second, microporous layer 145. The first microporous layer 140 is located between the first catalyst electrode layer 130 and a first current collector 150. The second microporous layer 145 is located between the second catalyst electrode layer 135 and a second current collector 155. The current collectors 150, 155 include inlets and outlets 152, 157 for introducing gases.

(52) The electrolyte 120 may have a thickness in the range of from about 50 m to about 2,000 m, including from about 50 m to about 250 m and from about 50 m to about 100 m.

(53) The reactant metal of the metal nitride electrolyte 120 may include at least one metal selected from lithium, magnesium, aluminum, scandium, gadolinium, calcium, cerium, and barium.

(54) In some embodiments, the nitride is a mixed metal nitride (i.e., contains more than one metal). The mixed nitride may contain lithium in combination with one or more metals selected from sodium, potassium, magnesium, aluminum, scandium, gadolinium, calcium, cerium, and barium.

(55) In particular embodiments, the nitride may be a lithium-aluminum nitride (Li.sub.3AlN.sub.2).

(56) The performance of the electrochemical cell relies on the electrolyte membrane to have a high ionic conductivity for the nitride ion (e.g., greater than 1 mS/cm), low area-specific resistance (e.g., less than 100 ohm-cm.sup.2), and poor electrical conductivity. Additionally, the membrane has very low permeability to gases in order to prevent mixing of the feed gases.

(57) The catalyst layers 130, 135 contain a conductive metal catalyst.

(58) The catalyst layers may have the same composition or different compositions. For example, in applications wherein only one side of the membrane is exposed to hydrogen, the catalyst layer of the unexposed side may contain ruthenium whereas the catalyst layer of the exposed side does not contain ruthenium.

(59) In some embodiments, the catalyst layers 130, 135 independently contain at least one metal selected from gold, iridium, palladium, platinum, and ruthenium.

(60) One or both catalyst layers 130, 135 may consist of ruthenium. In other embodiments, one or both catalyst layers contains ruthenium in combination with at least one metal selected from gold, iridium, palladium, and platinum.

(61) In some embodiments, the catalyst is an alkali promoted catalyst.

(62) The thicknesses of the catalyst layers 130, 135 may be the same or different. The catalyst layer may comprise a catalyst coated gas diffusion electrode (e.g., 250 m carbon paper or 400 m carbon cloth). The thickness depends upon the choice of electrode used.

(63) The catalyst layer may ensure a high specific surface area (e.g., from about 10 to about 60 m.sup.2/g).

(64) The catalyst-coated gas diffusion electrode enhances the reaction rate by increasing the three-phase (gas, catalyst, and electrolyte) contact area available for reaction while simultaneously providing a path for the easy collection of electrons produced during the reaction. For this reason, the gas diffusion electrode is made of a good electrical conductor such as carbon, covered with a layer of carbon felt to ensure a high surface area. The electrode surface may occasionally be post-treated with a binder (e.g., PTFE) to ensure mechanical stability.

(65) The microporous layers 140, 145 (also known as gas transport or gas diffusion layers) may be the same or different in terms of composition and thickness. These layers are porous to enable transport of gases.

(66) In some embodiments, each microporous layer independently contains a porous material selected from carbon cloth, carbon felt, carbon paper, metal mesh, and metal foam (e.g., nickel).

(67) The gas transport layer has adequate porosity to ensure high gas permeability (e.g., less than 10 s), low electrical area-specific resistance (less than 0.013 ohm-cm.sup.2, and high tensile strength (greater than 5 N/cm).

(68) In some embodiments, the gas transport layer is covered with a microporous layer (e.g., less than 50 m thick) made from carbon black. The purpose of this layer is to ensure good areal contact between the gas transport layer and, in PEM fuel cell, encourage the wick-off of produced water. In this embodiment, the microporous layer reduces the electrode's porosity in the vicinity of the solid electrolyte, creating a high capillary pressure to ensure that the lithium nitride does not completely submerge the catalyst.

(69) In some embodiments, the catalyst electrode layers and/or gas transport layers are optional.

(70) In some embodiments, the current collectors are in physical contact with the electrolyte membrane.

(71) In general, the current collectors 150, 155 exhibit good electrical conductivity, high thermal conductivity, high chemical resistance, high corrosion resistance, mechanical stability against compressive forces, and low permeability to the gases used in a particular application (e.g., hydrogen).

(72) The current collectors may contain one or more conductive materials selected from graphite, metals, and metal alloys. In some embodiments, the metal is stainless steel (e.g., Type 304 and/or Type 316), copper, or titanium.

(73) The current collectors generally include one or more flow fields for enhanced gas distribution. In some embodiments, the flow field(s) is/are serpentine.

(74) The current collectors generally have a high electrical conductivity (e.g., greater than 10.sup.4 S/m), low gas permeability and chemical stability towards the feed and product gases, high tensile strength to allow operation at intermediate pressure (e.g., greater than 5 bar), and good machinability to enable the machining of flow fields.

(75) In some embodiments, each current collector has a thickness within the range of from about 1 to about 5 mm.

(76) The electrochemical stack 100 of FIG. 3 includes a single cell. However, it should be understood that the electrochemical stacks of the present disclosure may also include a plurality of cells. FIG. 4 is an exploded view illustrating an electrochemical stack 200 with two cells. Each membrane electrode assembly 210 includes a solid electrolyte membrane and catalyst electrodes and is sandwiched between two current collectors 250. In some embodiments, a stack module contains from about 50 to about 100 cells. For example, a 50-cell stack (20 cm20 cm) operating at a current of 250 mA/cm.sup.2 may be able to produce 1 kg of ammonia per hour.

(77) In some embodiments, the electrochemical stack is used to produce ammonia. A non-limiting example of an exemplary ammonia production method is schematically shown in FIG. 5. The system includes a solid, metal nitride electrolyte 320, first and second catalyst electrode layers 330, 335, and first and second current collectors 350, 355. In the depicted embodiment, the electrolyte 320 is a lithium nitride electrolyte, the first catalyst electrode layer 330 is a ruthenium catalyst layer, and the second catalyst electrode layer 335 is a platinum catalyst layer. Nitrogen gas is provided through the first current collector 350 via inlet 374 and hydrogen gas is provided through the second current collector 355 via inlet 372. Unreacted nitrogen may be removed via outlet 376 and is optionally recycled. Nitride ions (N.sup.3) are generated at the first catalyst layer 330. The electrolyte 320 is selectively permeable to the nitride ions, which pass therethrough. The nitride ions reaching the second catalyst layer 335 react with hydrogen to form ammonia. Ammonia generated in the electrochemical stack may be removed from the system via outlet 378. The solid electrolyte 320 is not permeable to hydrogen. Therefore, the hydrogen provided via inlet 372 does not reach the ruthenium catalyst 330. Thus, the solid electrolyte 320 prevents the deleterious poisoning of the catalyst 330.

(78) The systems and methods of the present disclosure enable distributed generation of ammonia from an underutilized energy source (e.g., natural gas flares, curtailed or stranded renewables such as wind farms and solar farms). During operation, the stack is maintained at an intermediate temperature and the reactant gases (N.sub.2 and H.sub.2) enter the stack at moderate pressure. In some embodiments, the intermediate temperature is less than about 250 C. The temperature may be in the range of from about 150 C. to about 250 C. In some embodiments, the moderate pressure is less than about 10 bar. The pressure may be in the range of from about 1 bar to about 10 bar. The nitrogen is typically separated from air by cryogenic distillation, membrane separation or pressure swing adsorption. The hydrogen is typically obtained by steam reforming of natural gas at flared gas site or through water electrolysis at a renewable energy farm. When an active voltage is applied, N.sub.2 is reduced at the cathode (i.e., first catalyst layer 330 in FIG. 5) at the catalyst surface to form nitride ions (N.sup.3):
N.sub.2+6e.sup..fwdarw.2N.sup.3

(79) The resulting N.sup.3 migrates to the anode (i.e., second catalyst layer 335 in FIG. 5) where it reacts with H.sub.2 to generate ammonia:
3H.sub.2+2N.sup.3.fwdarw.2NH.sub.3+6e.sup..

(80) FIG. 6 is a flow chart illustrating an exemplary system which includes a modular electrochemical ammonia generator stack MEGA. Steam reformer SR may be similar to or different from the steam reformer of FIG. 1 and provides hydrogen to the MEGA stage. Air separation unit ASU may be similar to or different from the air separation unit of FIG. 1 and provides nitrogen to the MEGA stage. A TURBINE stage provides energy to the MEGA stage. In addition or as an alternative to the turbine, energy may be provided by wind or solar power. The MEGA stage produces ammonia at a considerably lower temperature and pressure in comparison to the HB stage in FIG. 1.

(81) In other embodiments, the electrochemical stack is used to separate nitrogen from an inert gas and/or compress nitrogen. A non-limiting example of an exemplary nitrogen compressing method is schematically shown in FIG. 7. The system includes a solid, metal nitride electrolyte 420, first and second catalyst electrode layers 430, 435, and first and second current collectors 450, 455. In the depicted embodiment, the electrolyte 420 is a lithium nitride electrolyte and the first and second catalyst electrode layers 430, 435 are ruthenium catalyst layers. In this case, ruthenium can be used in both catalyst layers 430, 435 because catalyst poisoning from hydrogen is not an issue since both feeds 472, 474 provide nitrogen to the electrochemical stack. The first inlet 472 provides low pressure nitrogen and the second inlet 474 provides high pressure nitrogen. Low pressure nitrogen is also recycled via recycle line 476. High pressure nitrogen leaves the system via outlet 478. Nitrogen reacts at the first catalyst layer 430 (i.e., cathode) to form nitride ions (N.sup.3) which the electrolyte 420 is permeable to. The nitride ions react in a reversible reaction at the second catalyst layer 435 (i.e., anode) to produce more nitrogen, thereby increasing the pressure of nitrogen in the second side of the cell. The reversible reaction at the second catalyst layer is shown below:
N.sub.2+6e.sup.N.sup.3.

(82) As a result, the gas pressure downstream from the N.sub.2 inlet increases. When multiple stages are stacked, the pressure of the product N.sub.2 gas can be increased substantially (e.g., to greater than about 100 bar at 250 C.). These conditions are adequate for nitrogen to exist as a supercritical fluid, and the resulting product can be a N.sub.2 feed for applications such as supercritical drying, or purification of porous materials.

(83) The systems and methods of the present disclosure may be used to separate N.sub.2|inert gas mixtures. Solid electrolyte membranes afford a modular structure that has better cost-scaling for small-scale gas processing applications. One example is the purification of noble gases (Ar, Kr, Ne, Xe). This is currently achieved using adsorption columns or cryogenic distillation, both of which require high installation and operation costs. Polymeric membranes for separation of N.sub.2|He suffer from low He selectivity or low flux. The Knudsen selectivity for these gases is low, and polymeric membranes to separate N2|He mixtures require multiple stages and a substantial membrane area. Ion-selective membranes offers much higher selectivity, and are desirable to improve the flux in separation of inert gases.

(84) The compressed N.sub.2 may also be used in the production of ammonia.

(85) The separation/compression systems and methods of the present disclosure overcome the deficiencies of the prior art for small scale applications. They may be useful for on-site chemical production, for remote chemical production based on the availability of inexpensive reagents at small scale, for enabling continued production during large plant maintenance, for modular expansion of existing plants, or for low-capital expenditure plants that can scale production based on real-time costs of reagents.

(86) The portable electrochemical nitrogen separators/compressors may be used for delivering portable compressed nitrogen for filling tires in automobiles. Oxide ion transport membranes need a high operating temperature to achieve this task.

(87) The portable electrochemical nitrogen separators/compressors may also be used for delivering high purity N.sub.2 to maintain an inert atmosphere (e.g., for storage of combustible or reactant products such as petroleum).

(88) The systems can also function as portable add-ons to ovens and glove boxes to maintain inert conditions without requiring industrial air.

(89) In other embodiments, the electrochemical stacks of the present disclosure may be used to detect ammonia.

(90) FIG. 8A is a graph showing a comparison of preliminary electrochemical impedance spectroscopy measurements of Li.sub.3N ionic conductivity with existing literature. FIG. 8B illustrates the crystal structure of -Li.sub.3N.

(91) One feature of the systems and methods of the present disclosure is the rapid nitride ion (N.sup.3) transport in a solid electrolyte at moderate temperatures, which enables efficient NH.sub.3 generation while avoiding parasitic H.sub.2 evolution or thermal decomposition of NH.sub.3. This is due to the unique crystal structure of -Li.sub.3N (FIG. 8B), which constitutes sheets of (Li.sub.2N).sup. and mobile Li.sup.+ channels. As a result, the material was previously thought of as a single-ion conductor. Lithium nitride has been known to be an effective Li.sup.+ conductor, and solid-state N.sup.3 transport has been demonstrated in proof-of-concept experiments discussed in the Examples section of this application. The disruptive manufacturing approach may involve roll-to-roll assembly of Li metal and the other layers into a complete membrane electrode assembly (MEA), followed by in situ membrane formation by Li nitridation:
3Li+N.sub.2.fwdarw.Li.sub.3N.

(92) This will dramatically reduce fabrication costs by avoiding more expensive membrane fabrication methods such as tape-casting and vapor deposition, enabling the fabrication of a solid ceramic ion conductor as a fully dense membrane with ionic conductivity and reduced gas permeability.

(93) This also enables the fabrication of membranes at thickness and processing conditions that allow for the characteristic length of a crystalline grain to be similar to or greater than the thickness of the film, such that conductivity approaches single-crystalline conductivity.

(94) The ability to fabricate the membrane inside the electrochemical cell, such that this fragile material does not need further handling and is mechanically protected by the cell, enables the use of much thinner membranes than would otherwise be possible, which in turn decreases the resistance of the cell, increases the current density and efficiency, and reduces the both the capital cost and electricity cost of the chemical that the stack produces.

(95) The systems and methods of the present application have numerous advantages over the prior art. One advantage is that the modular electrochemical stack has no moving parts (low maintenance costs) and scales very well with size (high efficiency at any size). This makes it particularly well suited for small scale ammonia generation (e.g., at flared gas sites and wind farms).

(96) Another advantage is that nitride ion transport prevents parasitic H.sub.2 evolution along with N.sub.2 reduction.

(97) An additional advantage is that current density may be comparable to Haber-Bosch by membrane optimization (using thinner membrane, reduced polycrystallinity) and improvements in N.sub.2-reduction catalysts.

(98) A further advantage is that the metal nitride provides a solid barrier which prevents H.sub.2 crossover and poisoning of the N.sub.2-selective Ru catalyst.

(99) Still another advantage is the process operates at intermediate temperatures and can potentially avoid thermodynamic decomposition of ammonia.

(100) Yet another advantage is that the simple manufacturing method avoids the need for expensive tape casting or chemical vapor deposition fabrication processes.

(101) The following examples are provided to illustrate the devices and methods of the present disclosure. The examples are merely illustrative and are not intended to limit the disclosure to the materials, conditions, or process parameters set forth therein.

Examples

(102) Formation of Lithium Nitride Layer

(103) An experiment was conducted to demonstrate the feasibility of membrane fabrication by reacting lithium (Li) metal in a nitrogen (N.sub.2) atmosphere (1 bar) at 220 C. At this temperature, the lithium rapidly reacted with nitrogen (Li+3/2 N.sub.2.fwdarw.Li.sub.3N) to yield a dark colored product. The results are shown in FIG. 9 which shows a lithium metal layer prior to exposure to the nitrogen environment (0 minutes; left photograph); after 10 minutes of exposure, before the lithium nitride layer was fully formed (middle photograph); and after the formation of the lithium nitride layer (90 minutes; right photograph). The lithium nitride membrane had a thickness of about 1.5 mm.

(104) Fabrication of Prototype Electrochemical Stack

(105) A prototype electrochemical stack was fabricated using a cathode catalyst (4 mg/cm.sup.2 Pt/Ru), an anode catalyst (4 mg/cm.sup.2 Pt), and lithium metal. The lithium metal was reacted under the passage of nitrogen gas to form the lithium nitride membrane in situ. Forming the membrane in situ is advantageous because the lithium metal layer is more flexible and simplifies assembly, whereas the lithium nitride layer is less flexible, more difficult to bend, and more likely to be damaged if assembly is performed after the membrane has formed.

(106) Ammonia Generation and Nitrogen Evolution Using Electrochemical Stack

(107) Experiments were performed to assess the feasibility of the proposed electrochemical stack for ammonia generation. The temperature dependence of the ionic conductivity is compared to existing literature data in FIG. 8A. It is known that single-crystal Li.sub.3N has a highly anisotropic conductivity. The preliminary membrane has an overall conductivity that has an intermediate value between that of the parallel (cross-plane) and perpendicular (in plane) ionic conductivities. It is also noted that the electronic conductivity of the material is at least 10.sup.4 higher, indicating this to be a pure ionic conductor.

(108) The proof of concept experiments to demonstrate ammonia generation and nitrogen evolution were performed using a 3 cm.sup.2 electrochemical stack. Ammonia was generated using a feed of H.sub.2 (5.07% in Ar, 80 mL min.sup.1) and N.sub.2 (100 mL min.sup.1) at 180 C. The ammonia produced was detected using an electrochemical ammonia sensor (100 ppm detection limit). The experimental results conclusively demonstrate the synthesis of ammonia at rates sufficient enough to saturate the ammonia sensor. The maximum current obtained was 8.64 mA/cm.sup.2 at 1.5 V, which corresponds to an ammonia synthesis flux of 30 nmol cm.sup.2 s.sup.1 (90% faradaic efficiency). The results of the nitrogen generation experiments (performed using a different cell) demonstrate a maximum current density of 2 mA/cm.sup.2 is attained at a potential of 2.1 V. FIG. 10 is a cyclic voltammogram (10 mV/s) demonstrating the results. FIG. 11 is a graph showing some of the results of the proof-of-concept demonstration for nitrogen generation.

(109) In-Situ Reaction and Membrane Electrode Assembly

(110) A Swagelok cell was fabricated using a ID compression fitting, stainless steel current collector with gas inlet and outlet. FIG. 12 is a photograph of a portion of a disassembled, layered electrochemical cell formed by sandwiching a Li metal sheet (250 m thick) between an appropriate catalyst (4 mg/cm.sup.2 PtRu catalyst) and gasket (380 m Teflon). When heated at an appropriate temperature (240 C. on hot plate), the nitridation reaction rapidly converted the Li metal into lithium nitride, which was obtained as a 270 m thick purple-red disc. The lithium nitride with the catalyst and gasket represents a repeating unit in a multilayer electrochemical stack.

(111) Nitrogen Compression Experiment

(112) The Swagelok cell was set up as a symmetric cell (N.sub.2|Li.sub.3N|N.sub.2), operated at a temperature of 125-150 C. with an ambient pressure of 0.77 psi being applied on the downstream side. When an electric field was applied, the pressure of nitrogen gas increased in the downstream gas chamber, indicating that N.sub.2 was being reduced, transported across the lithium nitride membrane and re-oxidized back to N.sub.2. These results are shown in FIG. 13. When the electric field was reversed, the pressure decreased as nitrogen was removed from the chamber of interest. These results are shown in FIG. 14. This satisfactorily indicates that the system can be used to transport and compress nitrogen gas.

(113) Effect of Thermal and Chemical Profiles on Membrane Formation

(114) During membrane formation, the thermal and chemical (gas) profiles can be adjusted/optimized to ensure proper formation. Changing the temperature and/or gas supplied to the stack during the formation process may be beneficial.

(115) For example, the temperature profile of FIG. 15 includes a short ramp to a temperature modestly higher (<10 C.) than the melting point of the metal layer. This ensures that partial melting or deformation of the metal surface and ensure improved contact between the catalyst and membrane without deforming the bulk membrane structure. The membrane is then rapidly cooled to a temperature just below the metal melting point. Simultaneously, the reactant gas pressure is increased to ensure complete conversion of the membrane. Optionally, an inert gas, such as argon, may be used when the membrane is not actively undergoing nitridation as depicted in FIG. 16.

(116) Microstructure of Lithium Nitride Membranes Produced In Situ

(117) Particular examples of in situ nitrided lithium nitride membranes are illustrated in FIGS. 17 and 18. FIG. 17 includes SEM micrographs of the larger membrane (37 magnification) and (inset) a closer view (1000 magnification) of its cross-section. Energy dispersive X-ray (EDX) analysis (FIG. 18) of the nitrogen content in the membrane demonstrates consistent nitridation was achieved across the membrane thickness. This particular membrane was synthesized at a fixed reaction temperature of 185 C. and 50 mm N.sub.2 gauge pressure. The reaction temperature was maintained at slightly higher than the melting point of Li metal (180.5 C.), and the presence of N.sub.2 led to the formation of a lithium nitride solid film which helped ensure that the shape of the membrane was not deformed.

(118) Faradaic Efficiency and Ammonia Flux

(119) The Faradaic efficiency for ammonia synthesis was determined in a H.sub.2|Li.sub.3N|N.sub.2 electrochemical cell operated at 87 C. The potential was increased in steps of +0.25 V, and the current density recorded for at least 10 minutes per step. Upon completion of the experiments, H.sub.2 flow was replaced with N.sub.2, and the same voltage stepping experiment repeated in a symmetric N.sub.2|Li.sub.3N|N.sub.2 electrochemical cell. The baseline current in the symmetric cell was subtracted from the current when operated using H.sub.2, and used to estimate the Faradaic efficiency for ammonia production. The results are shown in FIG. 19. The Faradaic efficiency was nearly 100% when operated at about 0.5 V, and levels off at a value of 83% at 2V, demonstrating ammonia synthesis.

(120) It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.