Method for supplying molten carbonate fuel cell with electrolyte and molten carbonate fuel cell using the same
10777834 ยท 2020-09-15
Assignee
Inventors
- Sung Pil Yoon (Seoul, KR)
- Jonghee Han (Seoul, KR)
- Hyung Chul Ham (Seoul, KR)
- Chang Won YOON (Seoul, KR)
- Sun-Hee Choi (Seoul, KR)
- Hyun Seo Park (Seoul, KR)
- Yeong Cheon Kim (Seoul, KR)
- Chang-Whan Lee (Seoul, KR)
- Hyoung-Juhn Kim (Seoul, KR)
- Tae Hoon LIM (Seoul, KR)
- Suk Woo Nam (Seoul, KR)
- Seong Cheol JANG (Seoul, KR)
Cpc classification
H01M8/04283
ELECTRICITY
Y02E60/50
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
Y02E60/36
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
C01D7/00
CHEMISTRY; METALLURGY
International classification
C01D7/00
CHEMISTRY; METALLURGY
H01M8/04276
ELECTRICITY
Abstract
Disclosed are a method for supplying molten carbonate fuel cell with electrolyte and a molten carbonate fuel cell using the same, wherein a molten carbonate electrolyte is generated from a molten carbonate electrolyte precursor compound in a molten carbonate fuel cell and is supplied to the molten carbonate fuel cell.
Claims
1. A method for supplying molten carbonate fuel cell with electrolyte, the method comprising: providing a molten carbonate electrolyte precursor vessel outside a molten carbonate fuel cell comprising a molten carbonate electrolyte; generating a gaseous molten carbonate electrolyte precursor compound from a solid or liquid molten carbonate electrolyte precursor compound in the molten carbonate electrolyte precursor vessel; and generating additional molten carbonate electrolyte from the gaseous molten carbonate electrolyte precursor compound generated in the molten carbonate electrolyte precursor vessel, thereby providing the additional molten carbonate electrolyte to the molten carbonate fuel cell, wherein the additional molten carbonate electrolyte is generated from at least one of carbonate ion, carbon dioxide and oxygen; and the gaseous molten carbonate electrolyte precursor compound.
2. A method for supplying molten carbonate fuel cell with electrolyte, the method comprising: providing a molten carbonate electrolyte precursor vessel outside a molten carbonate fuel cell comprising a molten carbonate electrolyte; generating a gaseous molten carbonate electrolyte precursor compound from a solid or liquid molten carbonate electrolyte precursor compound in the molten carbonate electrolyte precursor vessel; and generating additional molten carbonate electrolyte from the gaseous molten carbonate electrolyte precursor compound generated in the molten carbonate electrolyte precursor vessel, thereby providing the additional molten carbonate electrolyte to the molten carbonate fuel cell, wherein the gaseous molten carbonate electrolyte precursor compound reacts with at least one of carbonate ion, carbon dioxide and oxygen to form a molten carbonate electrolyte, and the gaseous molten carbonate electrolyte precursor compound is a material having a higher vapor pressure than lithium carbonate (Li.sub.2CO.sub.3), sodium carbonate (Na.sub.2CO.sub.3), or potassium carbonate (K.sub.2CO.sub.3) at an operating temperature of the molten carbonate fuel cell.
3. A method for supplying molten carbonate fuel cell with electrolyte, the method comprising: providing a molten carbonate electrolyte precursor vessel outside a molten carbonate fuel cell comprising a molten carbonate electrolyte; generating a gaseous molten carbonate electrolyte precursor compound from a solid or liquid molten carbonate electrolyte precursor compound in the molten carbonate electrolyte precursor vessel; and generating additional molten carbonate electrolyte from the gaseous molten carbonate electrolyte precursor compound generated in the molten carbonate electrolyte precursor vessel, thereby providing the additional molten carbonate electrolyte to the molten carbonate fuel cell, wherein the gaseous molten carbonate electrolyte precursor compound is at least one selected from the group consisting of metal lithium (Li), Li.sub.2O, LiOH and a hydrate of LiOH, LiF, LiCl, Li, Li.sub.3N, LiNH.sub.2, Li.sub.2C.sub.2, LiMoO.sub.4, LiAlH.sub.4, LiSn, LiPb, LiTi, LiHg, Li.sub.3Sb.sub.2, Li.sub.3Bi, Li.sub.2SO.sub.4, LiOCH.sub.3, LiOC.sub.2H.sub.5, metal sodium (Na), Na.sub.2O, NaOH and a hydrate of NaOH, NaF, NaCl, NaI, Na.sub.3N, NaNH.sub.2, Na.sub.2SO.sub.4, Na.sub.2C.sub.2O.sub.4, NaC.sub.2H.sub.3O.sub.2, NaOCH.sub.3, NaOC.sub.2H.sub.5, metal potassium (K), K.sub.2O, KOH and a hydrate of KOH, KF, KCl, KI, K.sub.3N, KNH.sub.2, and K.sub.2SO.sub.4.
4. A method for supplying molten carbonate fuel cell with electrolyte, the method comprising: providing a molten carbonate electrolyte precursor vessel outside a molten carbonate fuel cell comprising a molten carbonate electrolyte; generating a gaseous molten carbonate electrolyte precursor compound from a solid or liquid molten carbonate electrolyte precursor compound in the molten carbonate electrolyte precursor vessel; and generating additional molten carbonate electrolyte from the gaseous molten carbonate electrolyte precursor compound generated in the molten carbonate electrolyte precursor vessel, thereby providing the additional molten carbonate electrolyte to the molten carbonate fuel cell, wherein the gaseous molten carbonate electrolyte precursor compound comprises LiI and KI; LiI and NaI; LiI, KI, and NaI; LiI and KOH; or LiI, KOH, and NaI.
5. A method for supplying molten carbonate fuel cell with electrolyte, the method comprising: providing a molten carbonate electrolyte precursor vessel outside a molten carbonate fuel cell comprising a molten carbonate electrolyte; generating a gaseous molten carbonate electrolyte precursor compound from a solid or a liquid molten carbonate electrolyte precursor compound in the molten carbonate electrolyte precursor vessel; supplying the gaseous molten carbonate electrolyte precursor compound to a first electrode of the molten carbonate fuel cell; supplying a reaction gas containing carbon dioxide to a second electrode of the molten carbonate fuel cell, wherein the additional molten carbonate electrolyte is generated from at least one of a carbonate ion, carbon dioxide, or oxygen, and the gaseous molten carbonate electrolyte precursor compound, wherein the carbonate ion is generated from the reaction gas and transfers to an electrochemical reaction site in the first electrode, and carbon dioxide is not supplied to the first electrode, wherein oxygen generated from carbonate ion reacts with the molten carbonate electrolyte precursor compound to produce a molten carbonate electrolyte intermediate precursor compound, and the molten carbonate electrolyte intermediate precursor compound reacts with the carbon dioxide generated from carbonate ion to generate the molten carbonate electrolyte.
6. A method for supplying molten carbonate fuel cell with electrolyte, the method comprising: providing a molten carbonate electrolyte precursor vessel outside a molten carbonate comprising a molten carbonate electrolyte; generating a gaseous molten carbonate electrolyte precursor compound from a solid or liquid molten carbonate electrolyte precursor compound in the molten carbonate electrolyte precursor vessel; supplying the molten carbonate electrolyte precursor compound generated in the molten carbonate electrolyte precursor vessel to a first electrode in a gaseous state; supplying a reaction gas containing carbon dioxide to a second electrode; and generating a molten carbonate electrolyte from carbonate ion and the molten carbonate electrolyte precursor compound in the molten carbonate fuel cell, thereby providing the molten carbonate electrolyte to the molten carbonate fuel cell; wherein the carbonate ion is generated from the reaction gas and transfers to an electrochemical reaction site in the first electrode, and carbon dioxide is not supplied to the first electrode, and wherein a mixed gas of a gaseous molten carbonate electrolyte precursor compound and a carrier gas which does not react with the first electrode is provided to the first electrode.
7. The method for supplying molten carbonate fuel cell with electrolyte according to claim 6, wherein the generating the molten carbonate electrolyte precursor compound from the solid molten carbonate electrolyte precursor compound comprises melting a molten carbonate electrolyte precursor compound powder.
8. The method for supplying molten carbonate fuel cell with electrolyte according to claim 6, wherein the generating the gaseous molten carbonate electrolyte precursor compound from the solid molten carbonate electrolyte precursor compound comprises dissolving a molten carbonate electrolyte precursor compound powder in a solvent and heating the solvent.
9. A method for supplying molten carbonate fuel cell with electrolyte, the method comprising: providing a molten carbonate electrolyte precursor vessel outside a molten carbonate fuel cell comprising a molten carbonate electrolyte; generating a gaseous molten carbonate electrolyte precursor compound from a solid or liquid molten carbonate electrolyte precursor compound in the molten carbonate electrolyte precursor vessel; and generating additional molten carbonate electrolyte from the gaseous molten carbonate electrolyte precursor compound generated in the molten carbonate electrolyte precursor vessel, thereby providing the additional molten carbonate electrolyte to the molten carbonate fuel cell, wherein the gaseous molten carbonate electrolyte precursor compound is a compound comprising at least one selected from the group consisting of lithium (Li), sodium (Na) and potassium (K); or a compound comprising at least one selected from the group consisting of lithium (Li), sodium (Na) and potassium (K) and additionally comprising at least one selected from the group consisting of cesium (Cs), lanthanum (La), and rubidium.
10. The method for supplying molten carbonate fuel cell with electrolyte according to claim 9, wherein the additional molten carbonate electrolyte is a LiK-based molten carbonate electrolyte, a LiNa-based molten carbonate electrolyte, or a LiNaK-based molten carbonate electrolyte; or the additional molten carbonate electrolyte is a LiK-based molten carbonate electrolyte, a LiNa-based molten carbonate electrolyte, or a LiNaK-based molten carbonate electrolyte which additionally comprises at least one selected from the group consisting of (Cs)Cs.sub.2O, CsO.sub.2, CsH, CsF, CsCl, CsBr, CsI, CsOH and a hydrate of CsOH, Cs.sub.2CO.sub.3, metal rubidium (Rb), Rb.sub.2O, RbH, RbF, RbCl, RbI, RbOH and a hydrate of RbOH, Rb.sub.2CO.sub.3, metal lanthanum (La), La.sub.2O.sub.3, LaH, LaN, LaC, LaF, LaCl.sub.2, LaCl.sub.3, LaBr.sub.3, La.sub.3, LaC.sub.2, and La.sub.2CO.sub.3.
11. A method for supplying molten carbonate fuel cell with electrolyte, the method comprising: providing a molten carbonate electrolyte precursor vessel outside a molten carbonate fuel cell comprising a molten carbonate electrolyte; generating a gaseous molten carbonate electrolyte precursor compound from a solid or liquid molten carbonate electrolyte precursor compound in the molten carbonate electrolyte precursor vessel; generating additional molten carbonate electrolyte from the gaseous molten carbonate electrolyte precursor compound generated in the molten carbonate electrolyte precursor vessel, thereby providing the additional molten carbonate electrolyte to the molten carbonate fuel cell; supplying the gaseous molten carbonate electrolyte precursor compound to a first electrode of the molten carbonate fuel cell; and supplying a reaction gas containing carbon dioxide to a second electrode of the molten carbonate fuel cell, wherein the reaction gas transfers to an electrochemical reaction site in the first electrode by either of a concentration difference of the reaction gas between the first and second electrodes or a pressure difference between the first and second electrodes.
12. The method for supplying molten carbonate fuel cell with electrolyte according to claim 11, wherein: the reaction gas comprises carbon dioxide, water vapor and hydrogen; or the reaction gas comprises carbon dioxide and oxygen or oxygen-containing gas, and the additional molten carbonate electrolyte is generated by a reaction of at least one of carbon dioxide and oxygen with the gaseous molten carbonate electrolyte precursor compound.
13. A method for supplying molten carbonate fuel cell with electrolyte, the method comprising: providing a molten carbonate electrolyte precursor vessel outside a molten carbonate fuel cell comprising a molten carbonate electrolyte; generating a gaseous molten carbonate electrolyte precursor compound from a solid or liquid molten carbonate electrolyte precursor compound in the molten carbonate electrolyte precursor vessel; generating additional molten carbonate electrolyte from the gaseous molten carbonate electrolyte precursor compound generated in the molten carbonate electrolyte precursor vessel, thereby providing the additional molten carbonate electrolyte to the molten carbonate fuel cell; supplying the gaseous molten carbonate electrolyte precursor compound to a first electrode of the molten carbonate fuel cell; and supplying a reaction gas containing carbon dioxide to a second electrode of the molten carbonate fuel cell, wherein the additional molten carbonate electrolyte is generated from carbonate ion and the gaseous molten carbonate electrolyte precursor compound, and wherein the carbonate ion is generated from the reaction gas and transfers to an electrochemical reaction site in the first electrode, and carbon dioxide is not supplied to the first electrode.
14. The method for a supplying molten carbonate fuel cell with electrolyte according to claim 13, wherein the molten carbonate fuel cell is operated in a fuel cell mode in which a current is output, the first electrode is a fuel electrode and the second electrode is an air electrode, a reaction gas containing oxygen or oxygen-containing gas and carbon dioxide is supplied to the air electrode, and carbonate ion generated in the air electrode by generating a current transfers to the fuel electrode and a molten carbonate electrolyte is generated at the fuel electrode.
15. The method for supplying molten carbonate fuel cell with electrolyte according to claim 13, wherein the additional molten carbonate electrolyte is generated by a reaction of at least one of oxygen and carbon dioxide generated from carbonate ion with the gaseous molten carbonate electrolyte precursor compound.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The above and other aspects, features and advantages of the disclosed example embodiments will be more apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
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DESCRIPTION OF REFERENCE NUMERALS
(24) 100: Molten carbonate fuel cell (MCFC) 110: Matrix 120 and 130: First and second electrodes 140 and 150: First and second gas supply devices 145 and 155: First and second carrier gas supply devices 160: Molten carbonate electrolyte precursor vessel 161: Storage of molten carbonate electrolyte precursor vessel 162: Thermocouple 170: Power supply device 180: Current supply device 190: Heating device 200: Molten carbonate fuel cell (MCFC) stack
DETAILED DESCRIPTION
(25) Example embodiments are described more fully hereinafter. The invention may, however, be embodied in many different forms and should not be construed as limited to the example embodiments set forth herein. Rather, these example embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the description, details of features and techniques may be omitted to more clearly disclose example embodiments.
(26) It will be understood that when an element or layer is referred to as being on or connected to another element or layer, the element or layer can be directly on or connected to another element or layer or intervening elements or layers. In contrast, when an element is referred to as being directly on or directly connected to another element or layer, there are no intervening elements or layers present. As used herein, the term and/or includes any and all combinations of one or more of the associated listed items. It will be understood that, although the terms first, second, third, etc., may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. Spatially relative terms, such as below, lower, upper and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as below or lower relative to other elements or features would then be oriented above relative to the other elements or features. Thus, the exemplary term below can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms a, an and the are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. The terms first, second, and the like do not imply any particular order, but are included to identify individual elements. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguished one element from another. It will be further understood that the terms comprises and/or comprising, or includes and/or including when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.
(27) Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. All methods described herein can be performed in a suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., such as), is intended merely to better illustrate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention as used herein.
Term Definition
(28) In this disclosure, the term that a molten carbonate fuel cell is operated in a water electrolysis mode means that an electrolysis reaction takes place in a fuel electrode of the molten carbonate fuel cell so as to generate hydrogen from water as a current is applied to the molten carbonate fuel cell from the outside.
(29) In this disclosure, the term that a molten carbonate fuel cell is operated in a fuel cell mode means that a fuel cell reaction takes place and thus a current is generated (output) from the molten carbonate fuel cell and water is generated.
(30) In this disclosure, an air electrode means an air electrode that supplies air (oxygen) which is referred as such usually in a fuel cell, and a fuel electrode means a fuel electrode that supplies fuel (hydrogen) which is referred as such usually in a fuel cell.
(31) In this disclosure, the term that a cross-over phenomenon occurs means that hydrogen gas supplied to a fuel electrode of a molten carbonate fuel cell and oxygen gas supplied to an air electrode of the molten carbonate fuel cell are physically and chemically mixed.
(32) In this disclosure, the term that a N.sub.2 cross-over phenomenon occurs in a molten carbonate fuel cell (MCFC) means that a nitrogen concentration increases to a certain value or more at an outlet portion of a fuel electrode when air and carbon dioxide (for example, usually under a condition having an oxygen utilization rate of 40%) are put as the air electrode gas of the molten carbonate fuel cell and hydrogen, carbon dioxide, and water vapor (for example, usually under a condition having a hydrogen utilization rate of 40%) are put as the fuel electrode gas.
(33) Typically, it may be predicted that a performance of MCFC begins to deteriorate by the depletion of the molten carbonate electrolyte when the incidence rate of nitrogen cross-over is 1% or more. The incidence rate of nitrogen cross-over may be calculated by measuring a gas composition at an outlet of a fuel electrode of a molten carbonate fuel cell by gas chromatography.
(34) In this disclosure, the expression that performances of molten carbonate fuel cell (MCFC) begin to deteriorate means that the incidence rate of nitrogen cross-over increases to 1% or more at a fuel electrode of MCFC. A molten carbonate electrolyte may be additionally supplied at the time point at which the performance of the molten carbonate fuel cell begins to deteriorate. However, only the degree of recovery of cell performance is affected but the effect by an electrolyte addition method of example embodiments of the present invention is the same even if the electrolyte addition method of example embodiments of the present invention is used before or after the incidence rate of nitrogen cross-over reaches 1%.
(35) Hereinafter, example embodiments of the present invention will be described in detail with reference to the accompanying drawings.
(36) Method of Supplying Electrolyte of Molten Carbonate Fuel Cell
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(44) In a method for supplying a molten carbonate fuel cell (may be referred to as MCFC) with electrolyte according to example embodiments of the present invention, a molten carbonate electrolyte is provided to a MCFC by generating the molten carbonate electrolyte from a molten carbonate electrolyte precursor compound in the MCFC. In particular, the method may be performed through the following processes.
(45) In example embodiments, first, a molten carbonate electrolyte precursor compound in a gaseous or vapor state may be supplied to a first electrode of MCFC, and a reaction gas containing carbon dioxide may be supplied to a second electrode of the MCFC. Herein, carbon dioxide may not be supplied to the first electrode. In addition, the supply of the precursor compound and the like may be performed at an operating temperature and pressure of the MCFC. For example, a temperature of the MCFC may be maintained at the MCFC operating temperature, for example, in the range of 500 C. to 800 C., and a heating device 190 for melting or vaporizing the precursor compound may be maintained at a temperature in the range of 50 C. to 1000 C. The first and second electrodes disposed so as to face each other with the matrix interposed therebetween in the MCFC may function as the air electrode and the fuel electrode (in the water electrolysis mode) or the fuel electrode and the air electrode (in the fuel cell mode), respectively.
(46) In example embodiments, the molten carbonate electrolyte precursor compound is capable of reacting with one or more of carbonate ion, carbon dioxide, and/or oxygen to form a molten carbonate electrolyte. The molten carbonate electrolyte precursor compound may also be a compound having a higher vapor pressure than the molten carbonate electrolyte such as lithium carbonate (Li.sub.2CO.sub.3), sodium carbonate (Na.sub.2CO.sub.3), or potassium carbonate (K.sub.2CO.sub.3), etc., at the operating temperature of the MCFC.
(47) In example embodiments, the molten carbonate electrolyte precursor compound may comprise at least one selected from the group consisting of lithium (Li), sodium (Na) and potassium (K), or further comprise at least one selected from the group consisting of cesium (Cs), lanthanum and rubidium (Rb) in addition thereto.
(48) In a non-limited example, the molten carbonate electrolyte precursor compound may be, for example, metal lithium (Li), Li.sub.2O, LiOH and a hydrate thereof, LiF, LiCl, LiI, Li.sub.3N, LiNH.sub.2, Li.sub.2C.sub.2, LiMoO.sub.4, LiAlH.sub.4, LiSn, LiPb, LiTi, LiHg, Li.sub.3Sb.sub.2, Li.sub.3Bi, Li.sub.2SO.sub.4, LiOCH.sub.3, LiOC.sub.2H.sub.5, metal sodium (Na), Na.sub.2O, NaOH and a hydrate thereof, NaF, NaCl, NaI, Na.sub.3N, NaNH.sub.2, Na.sub.2SO.sub.4, Na.sub.2C.sub.2O.sub.4, NaC.sub.2H.sub.3O.sub.2, NaOCH.sub.3, NaOC.sub.2H.sub.5; metal potassium (K), K.sub.2O, KOH and a hydrate thereof, KF, KCl, KI, K.sub.3N, KNH.sub.2, K.sub.2SO.sub.4, and/or any combination thereof.
(49) In example embodiments, the molten carbonate electrolyte precursor compound may be supplied to the first electrode in a vapor or gaseous state by melting each or a mixture of the powders of the compounds described above or dissolving them in a solvent such as water, methanol, or ethanol, etc. at a room temperature and then heating the solution.
(50) For example, the powders of LiOH, NaOH, KOH, and/or any hydrates thereof may be supplied to the first electrode in a vapor state by melting each or a mixture of them or dissolving them in water at a room temperature and then heating the solution.
(51) In addition, for example, the powders of LiF, LiCl, LiI, NaF, NaCl, NaI, KF, KCl, and/or KI may be supplied to the first electrode in a gaseous state by melting each or a mixture of them or dissolving them in water at a room temperature and then heating the solution.
(52) In a non-limited example, it is preferable that he molten carbonate electrolyte precursor compound comprises LiI and KI.
(53) In example embodiments, the molten carbonate electrolyte precursor compound in a vapor or gaseous state may be supplied to the first electrode as a mixed gas with a carrier gas which does not react with the first electrode. The carrier gas may contain, for example, an inert gas such as helium, nitrogen, or argon and/or water vapor, and preferably, it may contain an inert gas, such as helium, which is light and has a high diffusion coefficient so as that the diffusion coefficient of the mixed gas is increased and the reaction for generating the molten carbonate electrolyte can thus easily take place in the interior of the first electrode and at the electrochemical reaction sites. The mixed gas does not contain carbon dioxide.
(54) Meanwhile, in example embodiments, in the water electrolysis mode, the reaction gas may further contain hydrogen and water vapor in addition to carbon dioxide, and may be supplied to the second electrode. Alternatively, the reaction gas may further contain an oxygen or an oxygen-containing gas, such air and water vapor, in addition to carbon dioxide. In this case, the reaction gas may be supplied to the second electrode in an oxygen partial pressure range in which the second electrode is not oxidized.
(55) In example embodiments, in the fuel cell mode, the reaction gas may further contain oxygen or oxygen-containing gas such as air and water vapor in addition to carbon dioxide, and may be supplied to the second electrode.
(56) Meanwhile, in the water electrolysis mode or the fuel cell mode, the reaction gas may be supplied to the second electrode as a mixed gas with a carrier gas containing an inert gas.
(57) In an example embodiment, an electrochemical method in which a current is applied to the first and second electrodes may be performed. Referring to
(58) Alternatively, in another example embodiment, an electrochemical method in which a current is generated from the first and second electrodes may be performed. Referring to
(59) In the water electrolysis mode (see
(60) In a non-limiting example, more specifically, the minimum amount of current applied (A) at which all of the molten carbonate electrolyte precursor mixture supplied into the MCFC can react to generate a molten carbonate electrolyte may be calculated by Mathematical Equation (1).
Minimum amount of current applied (A)=FWAv (L/sec)/22.4 (mol/L)[Mathematical Equation 1]
(61) In [Mathematical Equation 1], F is the faraday constant (96,485 A.Math.sec/mol), W is the vapor pressure (mole fraction) of the molten carbonate electrolyte precursor compound at the MCFC operating temperature, and Av is the flow rate of the carrier gas. At this time, W can be referred to, for example, [Table 1].
(62) TABLE-US-00001 TABLE 1 LiOH vapor KOH vapor NaOH vapor Temperature pressure.sup.1,3 pressure.sup.2,3 pressure.sup.2,3 ( C.) (mole fraction) (mole fraction) (mole fraction) 600 2.34 10.sup.7 1.04 10.sup.4 1.81 10.sup.6 650 1.10 10.sup.6 3.31 10.sup.4 8.40 10.sup.6 700 4.65 10.sup.6 9.20 10.sup.4 2.26 10.sup.5 900 1.75 10.sup.4 4.00 10.sup.2 1.25 10.sup.3 (see 1. Ditmars, Walter E., and Herrick L. Johnston. Vapor Pressures of Inorganic Substances. X. Dissociation Pressures of Lithium Hydroxide between 650 and 800 K. 1. Journal of the American Chemical Society 75.8 (1953): 1830-1832; 2. Yaws, Carl L. Handbook of Vapor Pressure: Volume 4: Inorganic Compounds and Elements. Vol. 4. Gulf Professional Publishing, 1995; and 3. STANJAN chemical equilibrium solver (v3.93L IBM-PC Stanford University 1981, 1984, 1985, 1986, 1987)
(63) In addition, in a case where the molten carbonate electrolyte used in MCFC operation is, for example, Li.sub.2CO.sub.3 and/or K.sub.2CO.sub.3, the amount of the molten carbonate electrolyte consumed during the MCFC operation may be calculated by [Mathematical Equation 2] and/or [Mathematical Equation 3].
Amount of Li.sub.2CO.sub.3 consumed (mg/cm.sup.2)=(2.402t.sup.1/2+6.525)SM.sub.Li2CO310.sup.3[Mathematical Equation 2]
Amount of K.sub.2CO.sub.3 consumed (mg/cm.sup.2)=(0.158t.sup.1/2+9.391)SM.sub.K2CO310.sup.3[Mathematical Equation 3]
(64) In Mathematical Equations 2 and 3, t is the operating time (h), S is the reaction area (cm.sup.2) of the unit cell or cell stack, and M.sub.Li2CO3 and M.sub.K2CO3 are the molecular weight of Li.sub.2CO.sub.3 (73.89 g/mol) and the molecular weight of K.sub.2CO.sub.3 (138.20 g/mol), respectively. The above equations are empirical formulas, and the consumed amounts of each may vary depending on the experimental conditions and the MCFC constituents. Therefore, the consumed amount can be used as a reference on the tendency towards the consumption of MCFC electrolyte, namely, what kind of electrolyte is consumed first and in what percentage the electrolyte is consumed.
(65) The carbonate ion may be generated through Reaction Formula 1 when the reaction gas contains carbon dioxide, hydrogen, and water vapor. Alternatively, the carbonate ion may be generated through Reaction Formula 2 when the reaction gas contains carbon dioxide and oxygen or oxygen-containing gas.
CO.sub.2+H.sub.2O+2e.sup..fwdarw.CO.sub.3.sup.2+H.sub.2[Reaction Formula 1]
CO.sub.2+O.sub.2+2e.sup..fwdarw.CO.sub.3.sup.2[Reaction Formula 2]
(66) The carbonate ion generated from the reaction gas may transfer from the second electrode to the first electrode and generate carbon dioxide at least partially in the first electrode through Reaction Formula 3.
Co.sub.3.sup.2.fwdarw.CO.sub.2+O.sub.2+2e[Reaction Formula 3]
(67) Meanwhile, when generating a current in the MCFC, the MCFC is operated in the fuel cell mode (see
(68) Accordingly, carbonate ion, carbon dioxide, and oxygen may be present in the first electrode, and at least some of these carbonate ion, carbon dioxide and oxygen may react with the molten carbonate electrolyte precursor compound vapor at the electrochemical site in the first electrode to generate a molten carbonate electrolyte.
(69) The molten carbonate electrolyte may be, for example, a LiK-based molten carbonate electrolyte, a LiNa-based molten carbonate electrolyte, or a LiNaK-based molten carbonate electrolyte, and it may additionally contain Cs.sub.2CO.sub.3, Rb.sub.2CO.sub.3, and/or La.sub.2CO.sub.3.
(70) In example embodiments, when the molten carbonate electrolyte precursor compound is LiOH, NaOH, and/or KOH, a molten carbonate electrolyte can be generated through [Reaction Formula 4] to [Reaction Formula 6]. The molten carbonate electrolyte generated may be, for example, Li.sub.2CO.sub.3, Na.sub.2CO.sub.3, and/or K.sub.2CO.sub.3.
2LiOH+CO.sub.2.fwdarw.Li.sub.2CO.sub.3+H.sub.2O[Reaction Formula 4]
2NaOH+CO.sub.2.fwdarw.Na.sub.2CO.sub.3+H.sub.2O[Reaction Formula 5]
2KOH+CO.sub.2.fwdarw.K.sub.2CO.sub.3+H.sub.2O[Reaction Formula 6]
(71) In example embodiments, when the molten carbonate electrolyte precursor compound does not contain oxygen, that is, for example, the molten carbonate electrolyte precursor compound is metal lithium (Li), metal sodium (Na), metal potassium (K), LiH, NaH, KH, and/or any combination thereof, the molten carbonate electrolyte precursor compound vapor may first react with oxygen to form a molten carbonate electrolyte intermediate precursor compound in the form of an oxide such as Li.sub.2O, Na.sub.2O, K.sub.2O, etc. and this can react with carbon dioxide to finally generate molten carbonate electrolytes such as Li.sub.2CO.sub.3, Na.sub.2CO.sub.3, K.sub.2CO.sub.3 and/or any combination thereof.
(72) More specifically, when the molten carbonate electrolyte precursor compound contains, for example, metal lithium (Li) vapor, a molten carbonate electrolyte Li.sub.2CO.sub.3 may be generated through [Reaction Formula 7] and [Reaction Formula 8]. Alternatively, when the molten carbonate electrolyte precursor compound contains, for example, LiH, the molten carbonate electrolyte Li.sub.2CO.sub.3 may be generated through [Reaction Formula 9] and [Reaction Formula 10].
2Li(g)+O.sub.2(g).fwdarw.Li.sub.2O(c)[Reaction Formula 7]
Li.sub.2O(c)+CO.sub.2(g).fwdarw.Li.sub.2CO.sub.3(c)[Reaction Formula 8]
2LiH(g)+O.sub.2(g).fwdarw.Li.sub.2O(c)+H.sub.2O(g)[Reaction Formula 9]
Li.sub.2O(c)+CO.sub.2(g).fwdarw.Li.sub.2CO.sub.3(c)[Reaction Formula 10]
(73) Meanwhile, the processes described above may be performed when the MCFC performance begins to deteriorate. After the MCFC lifetime is substantially ran out, or when all the molten carbonate electrolyte inside the MCFC electrode is substantially depleted and to this end the molten carbonate electrolyte in the matrix begins to be consumed, it may be difficult to restore the performance of the MCFC to the initial level because of the deterioration in performance due to corrosion even if the molten carbonate electrolyte is additionally supplied through the processes described above. Furthermore, the processes described above may be repeatedly performed one or more times in order to maintain the MCFC performance. Hence, it is preferable to repeatedly perform the processes described above one or more times to additionally supply the molten carbonate electrolyte before the significant performance deterioration of the MCFC, for example, when an increase in internal resistance (IR) of the MCFC is less than 200% based on the internal resistance value at the initial stage of operation or the incidence rate of nitrogen cross-over is less than 1%.
(74) As described above, it is possible to generate and supply the molten carbonate electrolyte directly in the MCFC in which the electrochemical reaction take place by supplying a molten carbonate electrolyte precursor compound in a gaseous or vapor state to the first electrode and a reaction gas containing carbon dioxide to the second electrode, and then applying a current to the first and second electrodes or generating a current from the first and second electrodes.
(75) Particularly, when consumption of the electrolyte occurs in an MCFC to be operated by using a liquid electrolyte, the electrolyte of the MCFC can be additionally refilled to the required amount through the method described above without changing the operating temperature and/or pressure of the MCFC. Hence, a problem can be solved that the temperature of the MCFC may increase and the electrode structure may be changed by the consumption of the electrolyte and the accompanying cross-over phenomenon, resulting in performance deterioration and limited operation time of MCFC. Moreover, deterioration of the cell performance due to consumption of the electrolyte of the existing liquid electrolyte-using MCFC may be fundamentally solved, thus the lifetime of MCFC can be extended to the ultimate operating time that is determined by the MCFC constituents, and the long-term driving stability of MCFC can be effectively secured.
(76) In other example embodiments, according to the method for supplying an electrolyte to a molten carbonate fuel cell (MCFC) of the present invention, when there is a portion where defects such as matrix cracks or pinholes are caused, by using these defects, it is possible to diffuse the reaction gas containing carbon dioxide from the second electrode to the first electrode by a concentration difference. Accordingly, in the case of a MCFC having mechanical defects such as matrix cracks or pinholes, it is possible to easily refill the molten carbonate electrolyte directly in the interior thereof (see
(77) Accordingly, even in the case of MCFC which may exhibit a relatively low performance due to the above-described defects, the molten carbonate electrolyte can be easily supplied directly to the interior of the MCFC. Furthermore, when there is a defect, the molten carbonate electrolyte can be directly supplied to the interior of the MCFC by using only the pressure difference between the first and second electrodes and/or the concentration difference of the reaction gas without using an electrochemical method, which thereby provides an additional advantage.
(78) Molten Carbonate Fuel Cell
(79)
(80) In case of using an MCFC including a molten carbonate electrolyte supply device as illustrated in
(81) The MCFC as illustrated in
(82)
(83) Meanwhile, even in case of a MCFC having the structure illustrated in
(84) More specifically, the constituents and structures of molten carbonate fuel cells (MCFC) including the molten carbonate electrolyte supply device according to an example embodiment of the present invention illustrated in
(85) Referring to
(86) The matrix 110 may be disposed between the first and second electrodes 120 and 130 in the MCFC 100 and may contain a LiK-based molten carbonate electrolyte, a LiNa-based molten carbonate electrolyte, or a LiNaK-based molten carbonate electrolyte. The matrix 110 may contain, for example, -LiAlO.sub.2 or -LiAlO.sub.2.
(87) The first and second electrodes 120 and 130 may be disposed in the MCFC 100 so as to face each other with the matrix 110 interposed therebetween and provided as either of a fuel electrode or an air electrode of the MCFC 100 to be different from each other. More specifically, the first and second electrodes 120 and 130 may be respectively provided as an air electrode and a fuel electrode when the MCFC 100 is operated in the water electrolysis mode, and they may be respectively provided as a fuel electrode and an air electrode when the MCFC 100 is operated in the fuel cell mode. The first or second electrode 120 or 130 may contain lithiated NiO when provided as an air electrode and a Ni alloy, for example, NiAl, NiCr, NiAlCr, etc. when provided as a fuel electrode.
(88) The first gas supply device 140 may be connected to the first electrode 120 so as to supply the molten carbonate electrolyte precursor compound in a gaseous or vapor state to the first electrode 120 together with the carrier gas. The first carrier gas supply device 145 may be connected to the first gas supply device 140 as a supply device for supplying a separate carrier gas as described above. This makes it possible to supply the molten carbonate electrolyte precursor compound in a gaseous or vapor state to the first electrode 120 in the form of a mixed gas with the carrier gas.
(89) Meanwhile, only one first gas supply device 140 and one first carrier gas supply device 145 are illustrated in
(90) The second gas supply device 150 may be connected to the second electrode 130 so as to supply the reaction gas containing carbon dioxide and/or with the carrier gas to the second electrode 130. The second carrier gas supply device 155 may be a supply device for supplying a separate carrier gas, and may be connected to the second gas supply device 150. This makes it possible to supply the reaction gas to the second electrode 130 in the form of a mixed gas with the carrier gas.
(91) Meanwhile, only one second gas supply device 150 and one second carrier gas supply device 155 are illustrated in
(92) The molten carbonate electrolyte precursor vessel 160 may store a molten carbonate electrolyte precursor compound and heat and melt it to provide the molten carbonate electrolyte precursor compound in a gaseous or vapor state to the first electrode 120. Alternatively, the vessel 160 may dissolve the molten carbonate electrolyte precursor compound powder in a solvent and then heat the solution to provide the molten carbonate electrolyte precursor compound in a gaseous state. Herein, as illustrated in
(93) The molten carbonate electrolyte precursor vessel 160 may be mounted, for example, so as to be inserted into the first gas supply device 140 as illustrated in
(94) Referring to
(95) By mounting the thermocouple 162 in the molten carbonate electrolyte precursor vessel 160, it is possible to monitor the temperature of the molten carbonate electrolyte precursor vessel 160, and to easily control the temperature of the molten carbonate electrolyte precursor vessel 160 to a suitable temperature range for providing the molten carbonate electrolyte precursor compound in a gaseous or vapor state by using the heating device 190.
(96) Meanwhile, a unit MCFC has been described with reference to
(97) Hereinafter, specific Examples according to example embodiments of the present invention will be described in more detail. However, it should be understood that the present invention is not limited to the following Examples, various forms of Examples may be implemented within the scope of the appended claims, and the following Examples are only intended to facilitate the complete disclosure of the invention and to facilitate the practice of the invention to those skilled in the art.
(98) <Experiment 1: Case of Using LiOH/KOH>
(99)
(100)
(101)
(102)
(103)
(104)
Example 1: Water Electrolysis Mode (Current Application) Before Cell Performance Deterioration
(105) Typically, MCFC is operated at a temperature of 650 C. or lower, but in the present Example, a MCFC unit cell is operated at 720 C., which is higher than the normal operating temperature, in order to observe the performance deterioration due to the consumption of molten carbonate electrolyte in a short period of time. The operating conditions of the MCFC unit cell used are as presented in [Table 2].
(106) TABLE-US-00002 TABLE 2 Temperature 720 C. Pressure 1 atm Sealing pressure 0.2 MPa Gas utilization 0.1 at (anode: H.sub.2/cathode: O.sub.2 and CO.sub.2) 150 mA/cm.sup.2 Gas flow rate Anode 357 sccm Cathode 952 sccm Input gas Anode H.sub.2/CO.sub.2/H.sub.2O = 0.72:0.18:0.1 Composition Cathode Air/CO.sub.2 = 0.7:0.3 Anode NiNi5 wt % Al Cathode Lithiated NiO Matrix -LiAlO.sub.2 Electrolyte (Li/K).sub.2CO.sub.3 = (70:30)
(107) A MCFC unit cell of 55 cm.sup.2 is operated under the conditions presented in Table 2, the operation thereof is temporarily stopped while maintaining the operating temperature and pressure, and nitrogen of an inert gas is then sufficiently supplied into the first gas supply device for 30 minutes for purging.
(108) The reaction gas such as carbon dioxide and oxygen that may possibly remain in the air electrode (first electrode portion) of the MCFC is thus completely removed. Thereafter, the molten carbonate electrolyte is supplied by performing the processes to be described below before the sudden performance deterioration of the MCFC occurred, that is, when the incidence rate of nitrogen cross-over is less than 1%.
(109) In each of molten carbonate electrolyte precursor vessels, 100 mg of LiOH powder and 100 mg of KOH powder, which are solid at a room temperature, are respectively put, and the vessels are respectively inserted into the first gas supply devices connected to the air electrode (first electrode) of the MCFC.
(110) Subsequently, by using a heating device mounted outside the first gas supply device so as to enclose the vessel, the temperature of the vessels is controlled to a temperature of 462 C. or higher to be suitable for melting and vaporizing the electrolyte precursor compound (since the melting point of LiOH is 462 C. and the melting point of KOH is 360 C.), and finally, the temperature of the LiOH vessel is heated to 900 C. and the KOH vessel is heated to 700 C.
(111) Herein, monitoring the temperature of the vessel by using a thermocouple mounted inside the vessel make it possible to keep the temperature of the vessel constant so that the molten carbonate electrolyte precursor powder is maintained in a molten state and has a desired vapor pressure. As a result, LiOH vapor (about 1.7510.sup.4, see [Table 1]) is generated from the completely melted LiOH powder, and KOH vapor (about 9.2010.sup.4, see [Table 1]) is generated from the completely melted KOH powder. The vapors thus generated are supplied to the air electrode (first electrode) of the MCFC together with a carrier gas containing helium gas (97%) and water vapor (3%, 25 C. saturator) at a flow rate (Av) of 1.5910.sup.2 L/sec (952 ml/min).
(112) In addition, a reaction gas containing hydrogen (72%), carbon dioxide (18%), and water vapor (10%) is supplied to the fuel electrode (second electrode) of the MCFC through the second gas supply device at a flow rate of 6.6710.sup.3 L/sec (400 ml/min).
(113) Next, a current of 10 mA/cm.sup.2 [a current greater than the current required for the reaction (1.1 mA/cm.sup.2) is applied so as to generate more amounts of oxygen and carbon dioxide, see Mathematical Equations 4 and 7] is applied to the MCFC for 320 minutes (see Mathematical Equations 5 and 8) so that water electrolysis reaction takes place in the MCFC. Herein, the amount of current applied is calculated by Mathematical Equations 4 and 7, and this is the sum of the calculated values. The time for current application is calculated by Mathematical Equations 5 and 8, and this represents the longer time among the calculated values.
Amount of current applied (mA/cm.sup.2)=(W.sub.LiOH,900 C.1.5910.sup.2 (L/sec)/22.4 (L/mol))F(A.Math.sec/mol)1000(mA/A)/25 cm.sup.2=1.1 mA/cm.sup.2[Mathematical Equation 4]
Time required for LiOH evaporation (sec)=(amount of LiOH supplied (100 mg)/1000(mg/g)/M.sub.LiOH)/(W.sub.LiOH,900 C.1.5910.sup.2 (L/sec)/22.4 (L/mol))=320 min[Mathematical Equation 5]
(114) The amount of Li.sub.2CO.sub.3 generated can be calculated by Mathematical Equation 6 if the entire amount of LiOH added reacts with carbon dioxide supplied by the electrochemical reaction of water electrolysis (see Reaction Formula 4) to generate Li.sub.2CO.sub.3 in the MCFC.
Amount of Li.sub.2CO.sub.3 generated (mg/cm.sup.2)=(amount of LiOH supplied (100 mg)/1000(mg/g)/M.sub.LiOH)M.sub.Li2CO3/25 cm.sup.2=3.5 mg/cm.sup.2[Mathematical Equation 6]
(115) In the same manner, in case of KOH, the amount of current applied can be calculated by Mathematical Equation 7, and the time required for KOH evaporation can be calculated by Mathematical Equation 8. In addition, the amount of K.sub.2CO.sub.3 generated in the MCFC can be calculated by Mathematical Equation 9 if the entire amount of KOH added reacts with carbon dioxide supplied by the electrochemical reaction of water electrolysis (see Reaction Formula 4) to generate K.sub.2CO.sub.3 in the MCFC.
Amount of current applied (mA/cm.sup.2)=(W.sub.KOH,700 C.6.6710.sup.3 (L/sec)/22.4 (L/mol))F(A.Math.sec/mol)1000(mA/A)/25 cm.sup.2=1.1 mA/cm.sup.2[Mathematical Equation 7]
Time required for KOH evaporation (sec)=(amount of KOH supplied (100 mg)/1000(mg/g)/M.sub.KOH)/(W.sub.KOH,700 C.6.6710.sup.3 (L/sec)/22.4 (L/mol))=109 min[Mathematical Equation 8]
Amount of K.sub.2CO.sub.3 generated (mg/cm.sup.2)=(amount of KOH supplied (100 mg)/1000(mg/g)/M.sub.KOH)M.sub.K2CO3/25 cm.sup.2=3.7 mg/cm.sup.2[Mathematical Equation 9]
(116) In Mathematical Equations 4 to 9, F is the Faraday constant, W.sub.LiOH,900 C. is the LiOH vapor pressure 1.7510.sup.4 at the vessel temperature (900 C.), and W.sub.KOH,700 C. is the KOH vapor pressures 2.0410.sup.3 at the vessel temperature (700 C.). M.sub.LiOH and M.sub.KOH are respectively the molecular weight of LiOH (23.95 g/mol) and the molecular weight of KOH (74.11 g/mol), and M.sub.Li2CO3 and M.sub.K2CO3 are respectively the molecular weight of Li.sub.2CO.sub.3 (73.89 g/mol) and the molecular weight of K.sub.2CO.sub.3 (138.20 g/mol).)
(117) As a result of calculation by Mathematical Equations 6 and 9, it can be seen that the Li.sub.2CO.sub.3 molten carbonate electrolyte is generated by 3.5 mg/cm.sup.2 and the K.sub.2CO.sub.3 molten carbonate electrolyte is generated by 3.7 mg/cm.sup.2.
(118) Thereafter, 10 mg of LiOH of a molten carbonate electrolyte precursor compound is repeatedly supplied five times in total (see
(119) Referring to
(120) Referring to
(121) Referring to
(122) Meanwhile, in case of supplying a small amount of electrolyte precursor compound (for example, the seventh addition of molten carbonate electrolyte to supply 5 mg of LiOH and 5 mg of KOH, respectively, see
Example 2: Water Electrolysis Mode (Current Application) After Initial Cell Performance Deterioration
(123) Referring to
(124) First, 100 mg of LiOH.H.sub.2O electrolyte precursor compound is melted to produce a vapor while maintaining the temperature of the MCFC at 720 C., and the vapor (900 C.) thereof is supplied to the air electrode (first electrode portion) of the MCFC together with the carrier gas at a flow rate of 1.5910.sup.2 L/sec (952 ml/min) through the first gas supply device. Subsequently, a current of 10 mA/cm.sup.2 (0.25 A) is applied thereto for 320 minutes (calculated by Mathematical Equation 4 and Mathematical Equation 5) so that the water electrolysis reaction take place. As a result, a Li.sub.2CO.sub.3 molten carbonate electrolyte is generated and added in the interior of the MCFC (first additional supply of molten carbonate electrolyte).
(125) As a result, it is confirmed that the performance of the MCFC is slightly improved (see
(126) Thereafter, a Li.sub.2CO.sub.3 molten carbonate electrolyte is additionally supplied (second additional supply of molten carbonate electrolyte) by using 100 mg of LiOH.H.sub.2O electrolyte precursor compound in a water electrolysis mode, and a K.sub.2CO.sub.3 molten carbonate electrolyte is additionally supplied (third additional supply of molten carbonate electrolyte) by using 100 mg of KOH electrolyte precursor compound in a water electrolysis mode as third addition. Herein, the temperature of the MCFC is maintained at 720 C. and the carrier gas is supplied at a flow rate of 6.6710.sup.3 L/sec (400 ml/min).
(127) As a result, there is no significant change in the performance of the MCFC after the addition of Li.sub.2CO.sub.3 molten carbonate electrolyte (second additional supply of molten carbonate electrolyte) and the addition of K.sub.2CO.sub.3 molten carbonate electrolyte (third additional supply of molten carbonate electrolyte)(see
Example 3: Water Electrolysis Mode (Current Application) After Remarkable Cell Performance Deterioration
(128) Referring to
(129) First, mixed powders of 175 mg of LiOH.H.sub.2O powder and 144 mg of KOH powder are melted together in a precursor vessel at 900 C. to generate LiOH vapor and KOH vapor (vapor pressure: 1.7510.sup.4 and 4.0010.sup.2, respectively, see [Table 1]), and these vapors are supplied to the air electrode of the MCFC together with a helium (100%) carrier gas at a flow rate (Av) of 1.5910.sup.2 L/sec (952 ml/min). A reaction gas containing hydrogen (72%), carbon dioxide (18%), and water vapor (10%) is supplied to the fuel electrode of the MCFC through the second gas supply device at a flow rate of 6.6710.sup.3 L/sec (400 ml/min). Subsequently, a current of 10 mA/cm.sup.2 (0.25 A) is applied thereto for 320 minutes (calculated by Mathematical Equation 4 and Mathematical Equation 5) so that the water electrolysis reaction take place. As a result, a Li.sub.2CO.sub.3 molten carbonate electrolyte and a K.sub.2CO.sub.3 molten carbonate electrolyte are generated and added in the interior of the MCFC.
(130) As a result, both the internal resistance improvement (see
(131) Particularly, in case of adding Li.sub.2CO.sub.3 electrolyte and K.sub.2CO.sub.3 electrolyte at the same time by mixing LiOH.H.sub.2O and KOH when the cell performance is greatly deteriorated, it is confirmed that the performance of the MCFC is immediately restored as illustrated in
(132) However, the MCFC performance is not recovered to the initial level as in case of Example 1 in which the molten carbonate electrolyte is additionally supplied before the cell performance deterioration (when the incidence rate of nitrogen cross-over (%) is less than 1%) (see
Example 4: Fuel Cell Mode (Current Generation) After Remarkable Cell Performance Deterioration
(133) The MCFC unit cell of which the performance is recovered according to Example 3 is subjected to harsh operation so as to exhibit performance of 0.35 V at 160 mA/cm.sup.2 or less again (high current density operation at 400 mA/cm.sup.2), and a molten carbonate electrolyte is then additionally supplied in the period in which the cell lifetime almost runs out (fifth additional supply of molten carbonate electrolyte). The processes performed are as follows.
(134) The operation of the unit MCFC of 55 cm.sup.2 being operated at 720 C. is temporarily stopped, and then the same process as described in Example 1 is performed to purge the interior of the fuel electrode (first electrode) of the MCFC. The reaction gas such as carbon dioxide and oxygen that possibly remained in the MCFC is thus completely removed.
(135) Thereafter, 175 mg of LiOH.H.sub.2O powder and 144 mg of KOH powder which are solid at a room temperature are mixed so as to have a molten carbonate electrolyte eutectic composition (62 mol % Li.sub.2CO.sub.3/38 mol % K.sub.2CO.sub.3) and put in a molten carbonate electrolyte precursor vessel, and the vessel is inserted into the first gas supply device connected to the fuel electrode (first electrode) of the MCFC. Subsequently, the LiOH vapor and the KOH vapor are generated by melting the mixed powder of LiOH.H.sub.2O powder and KOH powder by controlling the vessel temperature to 700 C. Herein, the temperature of the vessel is maintained at 700 C. (melting point of LiOH: 462 C. and melting point of KOH: 360 C.) by monitoring the temperature of the vessel using a thermocouple mounted inside the vessel. The vapors thus generated are supplied to the fuel electrode (first electrode) of the MCFC together with a carrier gas containing nitrogen gas (32%), hydrogen (50%), and water vapor (18%, 70 C. saturator) at a flow rate (Av) of 6.6710.sup.3 L/sec (400 ml/min). In addition, a reaction gas containing air (70%) and carbon dioxide (30%) is supplied to the air electrode (second electrode) of the MCFC through the second gas supply device at a flow rate of 6.6710.sup.3 L/sec (400 ml/min).
(136) The MCFC is operated in the fuel cell mode by generating a current of 10 mA/cm.sup.2 (0.25 A) for 320 minutes (see Mathematical Equation 4 and Mathematical Equation 5) in a state of maintaining the cell temperature and the flow rate of carrier gas. As a result, a Li.sub.2CO.sub.3 electrolyte and a K.sub.2CO.sub.3 electrolyte are generated and added in the interior of the MCFC.
(137) Referring to
Example 5: Addition of Molten Carbonate Electrolyte Using Reaction Gas Diffusion by Concentration Difference
(138) A unit MCFC having an open circuit voltage to be 90% or less of the theoretical open circuit voltage and an incidence rate of nitrogen cross-over of 2% or more is subjected to the following processes.
(139) LiOH.H.sub.2O (lithium hydroxide monohydrate) powder and NaOH.H.sub.2O (sodium hydroxide monohydrate) powder which are solid at a room temperature are mixed at a ratio of 52 mol % Li.sub.2CO.sub.3:48 mol % Na.sub.2CO.sub.3 of the molten carbonate eutectic composition. This mixed powder is used to generate a mixed vapor of LiOH.H.sub.2O and NaOH.H.sub.2O, and the mixed vapor thus generated is supplied to the air electrode (first electrode portion) of the MCFC together with a carrier gas containing helium gas (97%) and water vapor (3%, 25 C. saturator) at a flow rate (Av) of 6.6710.sup.3 L/sec (400 ml/min). In addition, a reaction gas containing hydrogen (72%), carbon dioxide (18%), and water vapor (10%) is supplied to the fuel electrode (second electrode) of the MCFC through the second gas supply device at a flow rate of 6.6710.sup.3 L/sec (400 ml/min). Subsequently, by using the concentration difference of the reaction gas between the air electrode (first electrode portion) and the fuel electrode (second electrode portion) of the MCFC, the carbon dioxide-containing reaction gas is diffused from the fuel electrode (second electrode portion) to the air electrode (first electrode portion) through the matrix crack-generated and/or pinhole-generated portion. Accordingly, a two-component molten carbonate electrolyte containing Li.sub.2CO.sub.3 and Na.sub.2CO.sub.3 is generated in the air electrode (first electrode portion) and the matrix crack-generated portion of the MCFC.
(140) As a result, it is confirmed that the measurement open circuit voltage of the MCFC is recovered to 98% or more of the theoretical open circuit voltage and the incidence rate of nitrogen cross-over is improved from 2% to 0.5% or less. Hence, it can be seen that it is possible to additionally supply the molten carbonate electrolyte to a MCFC having defects such as matrix cracks and/or pinholes by the above-described method and thus to solve the problem of deterioration in performance and lifetime of cell. Furthermore, it can be seen that it is possible to effectively improve the performance and lifetime of cell by the above-described method even in a case in which the performance and lifetime of cell are deteriorated by the depletion of electrolyte at the electrolyte sealing site.
Example 6: Addition of Molten Carbonate Electrolyte Using Reaction Gas Transfer by Pressure Difference
(141) A unit MCFC having an open circuit voltage to be 85% or less of the theoretical open circuit voltage and an incidence rate of nitrogen cross-over of 2.5% or more is subjected to the following processes.
(142) LiOH.H.sub.2O (lithium hydroxide monohydrate) powder and KOH.H.sub.2O (potassium hydroxide monohydrate) powder which are solid at a room temperature are mixed at a ratio of 62 mol % Li.sub.2CO.sub.3:38 mol % Na.sub.2CO.sub.3 of the molten carbonate eutectic composition. This mixed powder is used to generate a mixed vapor of LiOH.H.sub.2O and KOH.H.sub.2O, and the mixed vapor thus generated is supplied to the fuel electrode (first electrode portion) of the MCFC together with a carrier gas containing hydrogen (72%), nitrogen gas (18%), and water vapor (10%, 70 C. saturator) at a flow rate (Av) of 3.3310.sup.3 L/sec (200 ml/min). In addition, a reaction gas containing air (70%) and carbon dioxide (30%) is supplied to the air electrode (second electrode) of the MCFC through the second gas supply device at a flow rate of 110.sup.2 L/sec (600 ml/min). Subsequently, by using the pressure difference between the fuel electrode (first electrode portion) and the air electrode (second electrode portion) of the MCFC due to the flow rate difference, the carbon dioxide-containing reaction gas is diffused from the air electrode (second electrode portion) to the fuel electrode (first electrode portion) through the matrix crack-generated and/or pinhole-generated portion. Accordingly, a two-component molten carbonate electrolyte containing Li.sub.2CO.sub.3 and K.sub.2CO.sub.3 is generated in the fuel electrode (first electrode portion) and the matrix crack-generated portion of the MCFC.
(143) As a result, it is confirmed that the measurement open circuit voltage of the MCFC is recovered to 99% or more of the theoretical open circuit voltage and the incidence rate of nitrogen cross-over is improved from 2.5% to 0.45% or less. Hence, it can be seen that it is possible to additionally supply the molten carbonate electrolyte to a MCFC having defects such as matrix cracks and/or pinholes by the above-described method and thus to solve the problem of deterioration in performance and lifetime of cell. Furthermore, it can be seen that it is possible to effectively improve the performance and lifetime of cell by the above-described method even in a case in which the performance and lifetime of cell are deteriorated by the depletion of electrolyte at the electrolyte sealing site.
Example 7: Control of Generation Amount of Two or More Molten Carbonate Electrolyte Precursor Compound Vapors/Water Electrolysis Mode (Current Application)
(144) MCFC operated at 650 C. for 500 hours or longer so that the internal resistance value thereof is increased from 0.3 .Math.cm.sup.2 of the initial to 0.42 .Math.cm.sup.2 and the cell performance also deteriorated from 0.85 V at160 mA/cm.sup.2 to 0.77 V at160 mA/cm.sup.2 is subjected to the following processes under the same conditions presented in Table 2 except that the operating temperature is 650 C.
(145) First, two different first gas supply devices equipped with a heating device are connected to the air electrode (first electrode) of the MCFC and purged by supplying nitrogen gas to each of the first gas supply devices at 6.6710.sup.3 L/sec (400 ml/min).
(146) Thereafter, 175 mg of LiOH.H.sub.2O powder is prepared and filled in the first electrolyte precursor compound vessel, and this is inserted into the first gas supply device. The temperature of the first electrolyte precursor compound vessel is controlled to and maintained at 900 C. by using a heating device mounted to the first gas supply device. In addition, 144 mg of KOH powder is prepared and filled in a second electrolyte precursor compound vessel, and this is inserted into a separate first gas supply device different from the first gas supply device into which the first electrolyte precursor compound vessel is inserted. The temperature of the second electrolyte precursor compound vessel is controlled to and maintained at 600 C. by using a heating device mounted to the separate first gas supply device. Accordingly, LiOH vapor having a vapor pressure of 1.7510.sup.4 is generated from the first electrolyte precursor compound vessel maintained at 900 C., KOH vapor having a vapor pressure of 1.0510.sup.4 is generated from the second electrolyte precursor compound vessel maintained at 600 C. (see Table 1), and the mixed vapor having a composition of 62 mo % LiOH: 32 mol % KOH is supplied to the air electrode (first electrode) of the MCFC.
(147) Subsequently, the MCFC is operated in the water electrolysis mode by applying a current of 10 mA/cm.sup.2 (0.25 A) for 13 hours (see
Example 8: Addition of Molten Carbonate Electrolyte After Intentional Shortage of Initial Molten Carbonate Electrolyte Amount/Water Electrolysis Mode (Current Application) and Fuel Cell Mode (Current Generation)
(148) In the present Example, MCFC unit cell is subjected to the following processes under the same conditions presented in [Table 2] except that the operating temperature is 650 C.
(149) In general, the initial input amount of electrolyte is determined on the assumption that 100% of the matrix porosity, 40% of the air electrode porosity, and 20% of the fuel electrode porosity are filled with the molten carbonate electrolyte, but in the present Example, for the condition in which the electrolyte is in an initial amount smaller than such a general amount, the molten carbonate electrolyte is intentionally input in an amount capable of filling only 70% of the matrix porosity at the time of the unit cell mounting. The initial internal resistance and incidence rate of nitrogen cross-over of the unit cell after reaching the operating temperature of 650 C. are respectively 0.7 .Math.cm.sup.2 and 0.8% which are about two-fold or more the internal resistance (0.3 .Math.cm.sup.2) and the incidence rate of nitrogen cross-over (0.4%) of a general cell in which the molten carbonate electrolyte is sufficiently input. In addition, the initial cell performance is 0.62 V at 160 mA/cm.sup.2, which is much lower than 0.85 V at 160 mA/cm.sup.2 of the initial performance of the general cell. In order to perform a process of adding the molten carbonate electrolyte to the MCFC having such initial shortage of molten carbonate electrolyte, a large-capacity electrolyte adding vessel is used in the form of a saturator as illustrated in
(150) The specific processes performed are as follows. That is, LiOH powder is prepared and filled in the first electrolyte precursor compound saturator (a device for allowing the carrier gas to pass through the liquid precursor compound and to reach the equilibrium vapor pressure) of
(151) Herein, the carrier gas (helium gas used) supplied to the first gas supply device 140 is allowed to bubble in the large precursor compound vessel 160 serving as a saturator and supplied to the first electrode 120. In addition, KOH powder is prepared and filled in the second electrolyte precursor compound saturator of
(152) Accordingly, LiOH vapor having a vapor pressure of 1.7510.sup.4 is generated from the first electrolyte precursor compound saturator maintained at 900 C., KOH vapor having a vapor pressure of 1.0510.sup.4 is generated from the second electrolyte precursor compound saturator maintained at 600 C. (see [Table 1]), and the mixed vapor having a composition of 62 mo % LiOH: 32 mol % KOH is supplied to the air electrode (first electrode) of the MCFC. Herein, each carrier gas (helium) is input at a flow rate of 1 L/min.
(153) Subsequently, the MCFC is operated in the water electrolysis mode by applying a current of 10 mA/cm.sup.2 (0.25 A) for 20 hours (four-fold the amount of electrolyte input in the previous Examples, that is, 400 mg of LiOH and 576 mg of KOH) (see
(154) As a result, the internal resistance of the MCFC decreases from 0.7 .Math.cm.sup.2 to 0.4 .Math.cm.sup.2 to be close to the internal resistance of a general cell and the cell performance is improved from 0.62 V at 160 mA/cm.sup.2 (initial value) to 0.78 V at 160 mA/cm.sup.2 to be close to the performance of a general cell. In addition, after continuous measurement of the incidence rate of nitrogen cross-over at the outlet of the fuel electrode of the unit cell during the above-described processes, it is confirmed in real time that the occurrence of nitrogen cross-over is improved.
(155) Thereafter, the following processes are performed to additionally supply a molten carbonate electrolyte to the unit cell (second addition of molten carbonate electrolyte).
(156) The same processes as the processes in the first addition of molten carbonate electrolyte are performed to generate a mixed vapor having a composition of 62 mol % LiOH: 32 mol % KOH. The vapors thus generated are supplied to the fuel electrode (first electrode) of the MCFC together with a carrier gas containing nitrogen gas (32%), hydrogen (50%), and water vapor (18%, 70 C. saturator) at a flow rate (Av) of 1.6710.sup.2 L/sec (400 ml/min) through the first gas supply device after passing through the respective electrolyte precursor compound saturators of
(157) Subsequently, the MCFC is operated in the fuel cell mode by applying a current of 10 mA/cm.sup.2 (0.25 A) for 10 hours (two-fold the amount of electrolyte input in the previous Examples, that is, 200 mg of LiOH and 288 mg of KOH) (see
(158) As a result, the internal resistance of the MCFC slightly decreases from 0.4 .Math.cm.sup.2 to 0.35 .Math.cm.sup.2 and the cell performance is improved from 0.78 V at 160 mA/cm.sup.2 to 0.81 V at 160 mA/cm.sup.2 as compared to those before the second addition of molten carbonate electrolyte.
(159) <Experiment 2: Case of Using LiI/KI>
(160) In the following Comparative Examples and Examples, the impedance (10,000 Hz to 0.01 Hz) is measured by electrochemical impedance spectroscopy using an apparatus Solatron S1287 and 1255B in the OCV state.
(161) The performance is measured by using an electric loader (ELTO DC Electronics Co., ESL 300Z), and the current density is measured at 50 mA/cm.sup.2, 100 mA/cm.sup.2, and 150 mA/cm.sup.2 at a constant current and an operating temperature of 650 C.
(162)
(163)
(164)
(165)
(166)
(167)
Comparative Example 1: Direct Input of (Li.SUB.0.62.K.SUB.0.38.).SUB.2.CO.SUB.3
(168) For comparison, an electrolyte [(Li.sub.0.62K.sub.0.38).sub.2CO.sub.3] is forcibly input into the interior of a specially fabricated unit cell. The experimental results are presented in
(169) TABLE-US-00003 TABLE 3 Input order 1 2 3 4 5 6 7 Li.sub.2CO.sub.3 0.0289 g 0.0207 g 0.0240 g 0.0251 g 0.0248 g 0.0297 g 0.0248 g K.sub.2CO.sub.3 0.0311 g 0.0223 g 0.0258 g 0.0269 g 0.0267 g 0.0318 g 0.0266 g Total 0.0600 g 0.0430 g 0.0498 g 0.0520 g 0.0515 g 0.0615 g 0.0514 g
(170) After the normal gas input, stabilization is performed for 90 hours, and then the first input of electrolyte (Li.sub.0.62K.sub.0.38).sub.2CO.sub.3 is performed (about 0.05 g each time).
(171) Referring to
(172) Air contains N.sub.2 as well as O.sub.2 and thus affects the reaction of O.sub.2 which is a main reaction material at the reaction site in the air electrode. Here, it is possible to determine flooding of cell through the O.sub.2 gain if the mass transfer resistance at the air electrode is increased by the excessive electrolyte.
(173) Usually, it can be judged as flooding when the difference between the air gain and the O.sub.2 gain is more than 90 mV at 150 mA/cm.sup.2. Here, the O.sub.2 gain in the 5th, 6th, and 7th inputs in which a decrease in performance is observed gradually increased to 82 mV, 87 mV, and 90 mV, respectively, at 150 mA/cm.sup.2.
(174) Meanwhile, the EIS measurement results in
(175) Finally, in case of IR change (see
(176) As a result, it can be judged that flooding has occurred. From these results, it can be seen that the addition of electrolyte to the interior of cell helps to improve the performance until the electrolyte is added in a certain amount at the initial stage, but the cell performance is deteriorated by an increase in the mass transfer resistance on the air electrode side when the electrolyte is added in more amount than this.
(177) In particular, as a characteristic of performance deterioration due to excess of electrolyte, the N.sub.2 cross-over at the outlet of the fuel electrode is almost similar to that at the initial stage (about 1% or less) while the cell performance deteriorates, resistance components of about 10 Hz or less increase in the impedance analysis, and the performance difference between pure oxygen and air is about 90 mV or more.
Example 1: (Li.SUB.0.62.K.SUB.0.38.)I Vapor Input
(178) The calculations are first performed by using STANJAN chemical equilibrium solver (V3.93L IBM-PC, Stanford University 1981, 1984, 1985, 1986, 1987). As a result, it can be predicted that 100% of reactants (LiI and KI) react to generate Li.sub.2CO.sub.3 and K.sub.2CO.sub.3 when they are put as a solid phase.
(179) As illustrated in
(180) Before inputting LiI and KI vapor, input of carbon dioxide gas and air (oxygen) of the air electrode gas is paused and the vessel is purged with nitrogen gas, and a carrier gas (inert gas such as helium or nitrogen, or the like) is supplied into each of the Li and KI vessels so as to supply Li vapor and KI vapor to the air electrode 120 in the MCFC unit cell 100.
2(Li/K/Na)I+CO.sub.2+O.sub.2.fwdarw.(Li/K/Na).sub.2CO.sub.3+I.sub.2 (electrochemical reaction site in air electrode)[Reaction Formula 11]
(181) The operating conditions for the electrolyte precursor input experiment are summarized in [Table 4].
(182) TABLE-US-00004 TABLE 4 Temperature 650 C. Pressure 1 atm Sealing pressure 0.2 MPa Gas utilization 0.1 at (anode: H2/cathode: O.sub.2 and CO.sub.2) 150 mA/cm.sup.2 Gas flow rate Anode 357 sccm Gas flow rate Cathode 952 sccm Input gas Anode H.sub.2/CO.sub.2/H.sub.2O = 0.72:0.18:0.1 composition Input gas Cathode Air/CO.sub.2 = 0.7:0.3 composition Anode NiNi5 wt % Al Cathode In-situ Lithiated NiO Matrix -LiAlO.sub.2 Electrolyte (Li/K).sub.2CO.sub.3 = (70:30)
(183) A MCFC unit cell of 55 cm.sup.2 is operated under the conditions presented in [Table 4], the operation thereof is temporarily stopped while maintaining the operating temperature and pressure, and nitrogen of an inert gas is then sufficiently supplied into the first gas supply device for 30 minutes for purging.
(184) A reaction gas such as carbon dioxide and oxygen that possibly remain in the air electrode (first electrode portion) of the MCFC is thus completely removed. Thereafter, the molten carbonate electrolyte is supplied by performing the processes to be described below before the sudden performance deterioration of the MCFC occurs, that is, when the incidence rate of nitrogen cross-over is less than 1%.
(185) In each of molten carbonate electrolyte precursor vessels 160, 300 mg of LiI powder and 228 mg of KI powder, which are solid at a room temperature, are respectively put, and the vessels are respectively inserted into the first gas supply device connected to the air electrode (first electrode) of the MCFC.
(186) Subsequently, by using a heating device mounted outside the first gas supply device so as to enclose the vessel, the temperature of the vessels is controlled to a temperature of 469 C. or higher and 681 C. or higher being suitable for melting and vaporizing the electrolyte precursor compounds (since the melting point of LiI is 469 C. and the melting point of KI is 681 C.), and finally, the temperature of the vessels is raised to 640 C. and 750 C.
(187) Herein, by monitoring the temperature of the vessel by using a thermocouple mounted inside the vessel, it is possible to keep the temperature of the vessel constant so that the molten carbonate electrolyte precursor powder is maintained in a molten state and has a desired vapor pressure.
(188) A carrier gas (100 ml/min [1.6710.sup.3 L/sec] of nitrogen) is input into the LiI powder completely melted in the precursor vessel 160 so that LiI vapor (about 9.262410.sup.4, see [Table 4]) is supplied to the air electrode (first electrode) of the MCFC.
(189) In addition, a carrier gas (100 ml/min [1.6710.sup.3 L/sec] of nitrogen) is input into the KI powder completely melted in the other precursor vessel 160 so that KI vapor (about 5.703910.sup.4, see [Table 4]) is generated and supplied to the air electrode (first electrode) of the MCFC.
(190) In addition, a reaction gas containing hydrogen (72%), carbon dioxide (18%), and water vapor (10%) is supplied to the fuel electrode (second electrode) of the MCFC through the second gas supply device at a flow rate of 357 ml/min.
(191) Next, a current of 0.4279 mA/cm.sup.2 (0.011 A, see Mathematical Equation 10 and Mathematical Equation 13) is applied for 542 minutes (see Mathematical Equation 11 and Mathematical Equation 14) so that the water electrolysis reaction takes place in the MCFC.
(192) Herein, the amount of current applied is calculated by Mathematical Equation 10 and Mathematical Equation 13, and this is the sum of the calculated values. The time for current application (time required for evaporation) is calculated by Mathematical Equation 11 and Mathematical Equation 14, and the longer time among the calculated values is considered.
Amount of current applied (mA/cm.sup.2)=(W.sub.LiI,640 C.6.6710.sup.3 (L/sec)/22.4 (L/mol))F(A.Math.sec/mol)1000(mA/A)/25 cm.sup.2[Mathematical Equation 10]
Time required for LiI evaporation (min)=(amount of LiI supplied (g)/(W.sub.LiI,640 C.0.1 (L/min)/22.4 (L/mol)(M.sub.LiI))[Mathematical Equation 11]
(193) The amount of Li.sub.2CO.sub.3 generated can be calculated by Mathematical Equation 12 if the entire amount of LiI added reacts with carbon dioxide supplied by the electrochemical reaction of water electrolysis (see Reaction Formula 5) to generate Li.sub.2CO.sub.3 in the MCFC.
Amount of Li.sub.2CO.sub.3 generated (mg/cm.sup.2)=(amount of LiI supplied (g)/(2(M.sub.LiI))M.sub.Li2CO3/25 cm.sup.2[Mathematical Equation 12]
(194) In the same manner, the amount of current applied can be calculated by Mathematical Equation 13 in the case of KI, and the time required for KI evaporation can be calculated by Mathematical Equation 14. In addition, the amount of K.sub.2CO.sub.3 generated in the MCFC can be calculated by Mathematical Equation 15 if the entire amount of KI added reacts with carbon dioxide supplied by the electrochemical reaction of water electrolysis (see Reaction Formula 5) to generate K.sub.2CO.sub.3 in the MCFC.
Amount of current applied (mA/cm.sup.2)=(W.sub.KI,750 C.6.6710.sup.3 (L/sec)/22.4 (L/mol))F(A.Math.sec/mol)1000(mA/A)/25 cm.sup.2[Mathematical Equation 13]
Time required for KI evaporation (min)=(amount of KI supplied (g))/(W.sub.KI,750 C.0.1 (L/min)/22.4 (L/mol)(M.sub.KI))[Mathematical Equation 14]
Amount of K.sub.2CO.sub.3 generated (mg/cm.sup.2)=(amount of KI supplied (g)/(2M.sub.KI)M.sub.K2CO3/25 cm.sup.2[Mathematical Equation 15]
(195) In Mathematical Equations 10 to 15, F is the Faraday constant, W.sub.LiI,640 C. is the LiI vapor pressure 9.262410.sup.4 at the vessel temperature (640 C.), and W.sub.KI,750 C. is the KI vapor pressures 5.703910.sup.4 at the vessel temperature (750 C.).
(196) M.sub.LiI and M.sub.KI are respectively the molecular weight of LiI (133.85 g/mol) and the molecular weight of KI (166.0028 g/mol), and M.sub.Li2CO3 and M.sub.K2CO3 are respectively the molecular weight of Li.sub.2CO.sub.3 (73.89 g/mol) and the molecular weight of K.sub.2CO.sub.3 (138.20 g/mol).
(197) As a result of calculation by Mathematical Equations 12 and 15, it is found that the Li.sub.2CO.sub.3 molten carbonate electrolyte is generated by 3.5 mg/cm.sup.2 and the K.sub.2CO.sub.3 molten carbonate electrolyte is generated by 3.7 mg/cm.sup.2.
(198) The vapor pressure of LiI at the respective temperatures is presented in [Table 5] and the vapor pressure of KI at the respective temperatures is presented in [Table 6].
(199) TABLE-US-00005 TABLE 5 N.sub.2 mole LiI mole fraction fraction Number Number Temp. (vapor (vapor of moles of moles ( C.) pressure) pressure) of N.sub.2 of LiI LiI 550 0.99992 0.000084203 0.0040873 3.4419E07 0.3 g + 598 0.99968 0.00032336 0.0040873 1.3221E06 N.sub.2 600 0.99966 0.00034083 0.0040873 1.3935E06 100 ccm 610 0.99956 0.00044166 0.0040873 1.806E06 640 0.99907 0.00092624 0.0040873 3.7893E06 650 0.99883 0.0011719 0.0040873 4.7955E06
(200) TABLE-US-00006 TABLE 6 N.sub.2 mole KI mole fraction fraction Number Number Temp. (vapor (vapor of moles of moles ( C.) pressure) pressure) of N.sub.2 of KI KI 700 0.9998 0.00019935 0.0040873 8.1496E07 0.228 g + N.sub.2 100 ccm 750 0.99943 0.00057039 0.00408727 2.3327E06
(201) Meanwhile, Table 7 shows a change in the composition of the electrolyte generated by a change in the temperature of the electrolyte precursor vessel.
(202) TABLE-US-00007 TABLE 7 LiI precursor KI precursor vessel Number of mol mol Number of vessel temperature moles of Gas % % moles of gas temperature 550 3.44E07 30% 70% 8.15E07 700 598 1.32E06 62% 38% 8.15E07 700 600 1.39E06 63% 37% 8.15E07 700 610 1.81E06 69% 31% 8.15E07 700 650 4.8E06 85% 15% 8.15E07 700 640 3.79E06 62% 38% 2.33E06 750
(203) It is possible to match the mole fraction of the precursor vapor phase not only at the temperatures presented in the above table but also at various other temperatures.
(204) By using an experimental device as illustrated in
(205) Particularly, in order to have a composition of electrolyte of (Li.sub.0.62K.sub.0.38).sub.2CO.sub.3, 100 ccm of carrier gas (inert gas such as N.sub.2, He, etc.) is allowed to flow into each precursor vessel while maintaining the temperature of the precursor vessel 160 containing 0.3 g of LiI at 640 C. and the temperature of the precursor vessel 160 containing 0.228 g KI at 750 C. by using the temperature controller 190 of the precursor input device so that the vapor pressure and molar ratio of LiI/KI vapor phase discharged from each precursor vessel 160 is 62:38.
(206) In order to control the space time (retention time in the cell frame) and vaporization time of the precursor, 100 ccm of carrier gas is allowed to flow.
(207) The time (min) required for vaporization of 0.3 g of LiI at 640 C. is calculated as 542.2 minutes by Mathematical Equation 11.
(208) The time (min) required for vaporization of 0.228 g of KI at 750 C. is calculated as 539.4 minutes by Mathematical Equation 6.
(209) The amount of current required to be applied at the time of EC mode operation is also calculated by the respective Mathematical Equations 10 and 13. The amount of current applied is 0.2659 (mA/cm.sup.2) for LiI at 640 C. and 0.1638 (mA/cm.sup.2) for KI at 750 C.
(210) Meanwhile, [Table 8] shows the input schedule of (Li.sub.0.62K.sub.0.38)I electrolyte precursor vapor.
(211) TABLE-US-00008 TABLE 8 Input order 1 2 3 4 5 6 7 8 Date Jun. 13, Jun. 14, Jun. 16, Jun. 17, Jun. 18, Jun. 20, Jun. 21, Jun. 22, 2017 2017 2017 2017 2017 2017 2017 2017 Lil 0.0500 g 0.0864 g 0.0773 g 0.1079 g 0.0914 g 0.1183 g 0.1140 g 0.5172 g Li.sub.2CO.sub.3 0.0147 g 0.0254 g 0.0228 g 0.0318 g 0.0269 g 0.0349 g 0.0336 g 0.1524 g KI 0.0380 g 0.0656 g 0.0587 g 0.0821 g 0.0694 g 0.0899 g 0.0866 g 0.3931 g K.sub.2CO.sub.3 0.0158 g 0.0273 g 0.0244 g 0.0342 g 0.0289 g 0.0374 g 0.0361 g 0.1636 g Lil + KI 0.0880 g 0.1520 g 0.1360 g 0.1900 g 0.1608 g 0.2082 g 0.2006 g 0.9103 g molten salt
(212) The amount of LiI and KI supplied to the cell is controlled by controlling the input time when inputting the LiI and KI electrolyte precursor vapor.
(213) In other words, in case of inputting the entire amount of 0.3 g of LiI, it takes 9 hours according to Mathematical Equation 7 under the above experimental conditions, but the desired amount of LiI is input by inputting the LiI vapor for 1 hour and 30 minutes corresponding to in order to actually supply 0.05 g of LiI in the first input. The amount of electrolyte precursor vapor input for each order is determined in this manner.
(214) Gas phase input of LiI and KI is started after the normal gas is input, a stabilization time of 100 hours elapsed, and then the performance begins to decrease. Although deterioration in performance is slightly observed after initial input, this is regarded in the range of error.
(215) It can be seen that the performance is improved through repeated gas phase input of the electrolyte precursor thereafter, and ultimately, the initial performance is maintained for a long time.
(216) In the final input, in order to confirm the addition of molten carbonate electrolyte through the electrolyte precursor, the performance and impedance behavior of the cell are observed after excessive electrolyte precursor is intentionally input.
(217) In the eighth input of
(218)
(219) A phenomenon that the mass transfer resistance at 10 Hz or less greatly increased is observed when (Li.sub.0.62K.sub.0.38)I electrolyte precursor vapor is excessively input in the same manner as in the case of direct input of (Li.sub.0.62K.sub.0.38).sub.2CO.sub.3 electrolyte in
(220) This is in accordance with the phenomenon that the air electrode polarization resistance, particularly the air electrode mass transfer resistance, increases by the presence of an excessive amount of the molten carbonate electrolyte.
(221) That is, as in the EIS results for direct input of (Li.sub.0.62K.sub.0.38).sub.2CO.sub.3, the R.sub.CT at 10 Hz band does not change much but IR slightly increases and R.sub.MT rapidly increases in the region of 10 Hz or less.
(222) In both cases in which (Li.sub.0.62K.sub.0.38).sub.2CO.sub.3 electrolyte is directly input and (Li.sub.0.62K.sub.0.38)I electrolyte precursor vapor is input, the mole fraction of N.sub.2 measured at the outlet of the fuel electrode is all 1% or less. It is found that the electrolyte is reliably supplied through the electrolyte precursor since the oxygen gain greatly increases to about 300 mV particularly after the (Li.sub.0.62K.sub.0.38)I electrolyte precursor vapor is excessively input.
(223) Accordingly, it can be seen that the flooding phenomenon also occurs when the (Li.sub.0.62K.sub.3.38)I electrolyte precursor vapor.
(224) From this, it has been confirmed that the purpose of electrolyte addition can be effectively achieved by inputting (Li.sub.0.62K.sub.0.38)I electrolyte precursor vapor.
(225) In this regard, a clear performance improvement may be shown when LiOH et al. are used as the precursor compound for the electrolyte supplying method according to the example embodiments but it may be difficult to supply a desired composition of electrolyte due to their low vapor pressure in the long-term operation. LI and KI may be used since their vapor pressure is higher than that of LiOH et al. and they may form Li.sub.2CO.sub.3 and K.sub.2CO.sub.3 by a reaction of O.sub.2 and CO.sub.2, and further they are not hazardous to human health. Thus, LiI and KI may replace LiOH and KOH.
(226) According to example embodiments of the present invention, it is possible to directly supply a molten carbonate electrolyte in the interior of a molten carbonate fuel cell by supplying a molten carbonate electrolyte precursor compound to a first electrode in a gaseous state and a reaction gas containing carbon dioxide to a second electrode and applying a current to the first and second electrodes or generating a current in the first and second electrodes.
(227) In particular, in case in which the consumption of electrolyte occurs in a molten carbonate fuel cell to be operated by using a liquid electrolyte, it is possible to additionally replenish the electrolyte of the molten carbonate fuel cell by a required amount without changing the operating temperature and/or pressure of the molten carbonate fuel cell. Consequently, it is possible to solve the ultimate problem of a MCFC that the temperature of the molten carbonate fuel cell increases and the electrode structure changes by the consumptuion of the electrolyte and the accompanying cross-over phenomenon, resulting in deteriorated performance and limited operating time.
(228) Moreover, in case in which the cell performance is much deteriorated by mechanical defects such as cracks or pinholes in the matrix and/or defects due to shortage of the molten carbonate electrolyte in the matrix caused by a long-term operation, it is possible to respectively supply a molten carbonate electrolyte precursor compound in a gaseous state or a vapor state and a reaction gas containing carbon dioxide to the first and second electrodes and then to diffuse the reaction gas from the second electrode to the first electrode by using the concentration difference of the reaction gas between the first and second electrodes and/or the pressure difference between the first and second electrodes.
(229) Accordingly, even in case of MCFC which may exhibit relatively low performance by the above-described defects, it is possible to easily supply the molten carbonate electrolyte directly into the interior thereof (electrodes, matrix, wet seal areas, and the like). Particularly, in case in which the cell performance is low or greatly deteriorated by the above-described defects, it is possible to directly supply the molten carbonate electrolyte into the interior of the MCFC by using only the pressure difference and/or the concentration difference of the reaction gas, which thus provides an additional advantage.
(230) In addition, according to example embodiments of the present invention, when it is required to add an electrolyte, a separate electrolyte inlet is not required for each unit cell (bipolar plate) of a large-capacity MCFC in the form of a stack but only the existing gas inlet is sufficient. As described above, it is possible to add the electrolyte without opening the stack during the operation and thus to minimize thermal damage and to cut down the cost required for manufacture and replacement of the constituents. Consequently, it is expected to be able to operate the MCFC stack up to 80,000 hours as well as to exceed the 40,000 hours of operation that breaks the breakeven point of the MCFC stack.
(231) The embodiments or Examples of the present invention disclosed above are for illustrative purposes only and may be embodied in various forms and should not be construed as limited to the embodiments or Examples described herein.
(232) Further, while the present invention has been described with respect to the specific embodiments or examples, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.