Patent Application
20060144701
The present invention relates to the electrochemical reduction of alkali metal salts with applications in alkali metal production.
Electrochemical processes are important in the chemical industry, but they are also large energy consumers. For example, the electrochemical production of inorganic chemicals and metals in the United States consumes about 5% of all the electricity generated annually, and about 16% of the electric power consumed by industry. Energy consumption contributes to the cost of production and, in many larger scale electrochemical manufacturing processes, it is the dominant cost. Therefore, it is desirable to significantly reduce this cost.
For example, sodium metal is typically generated from the electrolysis of sodium chloride using a plurality of cells arranged in parallel. The electrolysis of sodium chloride to make sodium metal and chlorine gas is shown in equations (1a) and (1b). Sodium chloride electrolysis generally employs carbon anodes, which degrade over time.
Cathode: Na++e−→Na (1a)
Anode: Cl−−e−→½Cl2 (1b)
In a parallel configuration, the cells operate at about 4-8 V. In an industrial manufacturing facility, large DC-DC converters are required to step down the voltage from its input level of hundreds of volts to the low voltage required for the electrolysis performed in parallel. Since large amounts of current are required to generate sodium at an appreciable rate, large current carrying bars are employed to minimize the risk of overheating and melting the current carriers.
The present invention is directed to electrochemical processes and apparatus for producing alkali metals from alkali metal salts. In accordance with one aspect of the invention, a stack of electrolysis cells electrically connected in series is provided and the voltage does not need to be stepped down, thus eliminating the need for large DC-DC converters and permitting the use of smaller current carriers than those typically used for the electrolysis of alkali metal salts in parallel.
In accordance with a further aspect of the invention, an aqueous sodium hydroxide solution is converted to sodium metal in a stack of electrolysis cells electrically connected in series.
In accordance with another aspect of the invention, molten sodium hydroxide is converted to sodium metal in a stack of electrolysis cells electrically connected in series.
In accordance with another aspect of the invention, an aqueous sodium hydroxide solution is converted to sodium metal and hydrogen is oxidized at the anode in a stack of electrolysis cells electrically connected in series.
In accordance with another aspect of the invention, molten sodium hydroxide is converted to sodium metal and hydrogen is oxidized at the anode in a stack of electrolysis cells electrically connected in series.
A complete understanding of the present invention may be obtained by reference to the accompanying drawings when considered in conjunction with the following detailed description, in which:
In accordance with the present invention, an alkali metal salt is reduced to an alkali metal through electrolysis in a series of electrolytic cells wherein each cell comprises a separator between the anode and cathode compartments. The series of electrolytic cells can operate at high voltage and thus eliminate the need for large DC-DC converters. Further, the tolerance for voltage within a current carrier is much greater than the tolerance for current, which permits the use of smaller current carriers.
The process of the invention can be used in the electrolytic generation of any metal from a corresponding metal salt, if a separator material is available that is both an ionic conductor of that metal ion and an electrical insulator. Preferable examples of metals in the process of the invention include alkali metals such as sodium, lithium and potassium, among others. Preferred ionic conductor/electrical insulators include lithium-β-aluminum oxide, lithium-β/β″-aluminum oxide, lithium-β″-aluminum oxide, lithium analogs of NaSICON ceramics, LiSICONs, lithium ion conductors with perovskite structure, sodium-β-aluminum oxide, sodium-β″-aluminum oxide, sodium-β/β″-aluminum oxide, NaSICON ceramics, potassium-β-aluminum oxide, potassium-β″-aluminum oxide, potassium-β/β″-aluminum oxide, and potassium analogs of NaSICON ceramics, KSICON.
In the present invention, to convert an alkali metal salt to an alkali metal in an electrolytic cell, the cathode compartment is preferably seeded with the alkali metal during or prior to operation. The term “seeded” as used herein means providing an amount of alkali metal to the cathode compartment sufficient to make electrical contact between the cathodic side of the ion conducting membrane and the current carrier leading to the anode of the next cell in the stack or to the current carrier leading to the power supply. The amount of alkali metal is an amount sufficient to be the current carrier between two or more cells, or between a cell and the power supply. One of ordinary skill in the art can readily select an appropriate seeding amount given the teachings herein and the size of the cell under consideration. The alkali metal is preferably provided to the cathode compartment before or between applications of an electric current. The cathode compartment is preferably maintained at a temperature at or above the melting point of the alkali metal, so that at least a portion of the metal is in a liquid state to facilitate electrical contact. The cathode and/or the anode compartment may be maintained under an inert atmosphere such as nitrogen or argon to protect alkali metal generated in the cathode compartment from air and moisture.
An exemplary electrolytic stack 100 suitable for the process of the invention is illustrated in
Each individual cell 102 comprises a cathode compartment containing a cathode comprised of an electrode material in electrical communication with a catholyte which comprises a molten alkali metal. The molten alkali metal in electrical contact with the cathode may act as an auxiliary cathode and is referred to herein as a “liquid cathode.” Each cell also comprises an anode compartment 104 containing an electrode wherein the oxidation occurs, and an anolyte in electrical communication with the anodic electrode. The anode compartment is charged with an anolyte comprising either a molten alkali metal salt or an aqueous solution of alkali metal salt. The cathode compartment is preferably seeded with at least one alkali metal.
Alkali metals are the Group I metals, and preferably are lithium, sodium, and potassium. Suitable alkali metal salts include hydroxide, carbonate, and borate salts. As used herein, preferred borate salts are represented by the formula zM2O.xB2O3.yH2O, wherein z is ½ to 5; x is 0.1 to 5; y is 0 to 10; and M is an alkali metal ion such as sodium, potassium, or lithium.
The cell unit 102 comprising the cathode-membrane-anode combination can be repeated any number of times, with an ionic insulator 118 that is an electrical conductor separating the anode compartment of a first cell and the cathode compartment of the next cell. The ionic insulator may be comprised of a material such as, but not limited to, nickel, carbon, graphite, steel, platinum, and nickel alloys. One face of the ionic insulator may comprise the anode of a cell unit. The face of the ionic insulator opposite the anode compartment may comprise the cathode of a second cell unit and/or be in contact with a liquid cathode. The number of cell units in the stack is generally related to the voltage input and the expected single-cell operating voltage, wherein the number of cell units can be determined by dividing the input voltage by the expected cell voltage.
The ionic conductor 106 which separates the anode and cathode compartments is preferably a water-impermeable, ceramic cation conductor. Suitable ceramic ionic conductors include lithium-β-aluminum oxide, lithium-β/β″-aluminum oxide, lithium-β″-aluminum oxide, lithium analogs of NaSICON ceramics, LiSICONs, lithium ion conductors with perovskite structure, sodium-β-aluminum oxide, sodium-β″-aluminum oxide, sodium-β/β″-aluminum oxide, NaSICON ceramics, potassium-β-aluminum oxide, potassium-β″-aluminum oxide, potassium-β/β″-aluminum oxide, and potassium analogs of NaSICON ceramics, KSICON. Any ionic conductor material may be used, provided that it operates as both an ionic conductor for the corresponding metal ion, and otherwise as an electrical insulator in the methods and apparatus of the invention.
As an electric potential is applied to the cell and electrons are transferred into the cathode, alkali metal ions are transferred through the ionic conductor 106 from the anolyte of the anode compartment 104 to the cathode compartment 108. The alkali metal ions are then reduced at the cathode, and alkali metal accumulates in the cathode compartment.
Optionally, hydrogen gas can be passed into the anode compartment to enable hydrogen-assisted electrolysis as disclosed in U.S. patent application Ser. No. 10/388,197 entitled “Hydrogen-Assisted Electrolysis Process,” the disclosure of which is hereby incorporated by reference in its entirety, and which has been shown to reduce overall cell voltage and improve electrolytic efficiency through the preferential electrochemical oxidization of H2 at the anode. The anode compartment may include an optional gas inlet means for supplying a gas stream comprising hydrogen. Non-limiting examples of the gas inlet means include pipes, spargers, hoses, and hydrogen gas diffusion materials, among others. Whether to use hydrogen assisted electrolysis may depend on the relative availability and cost of hydrogen gas and electricity.
In a preferred embodiment of the invention using, for example, the electrolytic stack depicted in
4NaOH→4Na+O2+2H2O (2)
The ionic sodium conductor membrane 106 which separates the anode and cathode compartments is preferably a water-impermeable, ceramic cation conductor such as, but not limited to, sodium-β-aluminum oxide, sodium-β″-aluminum oxide, sodium-β/β″-aluminum oxide or NaSICON ceramics. The membrane may be modified to protect it from reaction with or degradation by water, sodium hydroxide, or sodium metal, such as by coating with a perfluorosulfonate polymer.
As an electric potential is applied to the cell and electrons are transferred into the sodium electrode, sodium ions are transferred through the membrane from the sodium hydroxide anolyte in the anode compartment to the cathode compartment. The sodium ions are then reduced at the cathode according to Equation (2a), and sodium metal accumulates in the cathode compartment. At the anode, oxygen gas and water are formed according to Equation (2b). The overall reaction for the cell is provided by Equation (2):
Cathode 4Na++4e−→4Na (2a)
Anode 4OH−→O2+2H2O+4e− (2b)
Overall 4NaOH→4Na+2H2O+O2 (2)
In another embodiment, the anode compartment 104 of each cell in the stack is charged with an anolyte comprising an aqueous sodium hydroxide solution, and the cathode compartment is seeded with sodium metal. The aqueous sodium hydroxide anolyte may be maintained at ambient temperature, or the aqueous anolyte solution may be maintained at the same elevated temperature as the cathode for ease and convenience.
Preferably, the cathode compartment is maintained at temperatures so that the metal in the cathode compartment is in a molten state. For sodium, this temperature is from above about 95° C., and preferably above about 97.8° C. The molten sodium is in contact with an ionic sodium conductor 106 which separates the cathode and anode compartments.
When a potential greater than 2.2 V, preferably greater than 2.8 V, and most preferably greater than 3.5 V, is applied across the anode and cathode, sodium ions are transported across the ionic membrane to form sodium metal according to Equation (2a), and the sodium metal builds up in the cathode. At the anode, oxygen gas and water are formed according to Equation (2b).
In another preferred embodiment, the anode compartment 106 of each cell in the stack is charged with an anolyte comprising molten sodium hydroxide, and the cathode compartment is seeded with sodium metal. Cells with a molten hydroxide anolyte may generally be operated at a reduced cell voltage compared with the cells with an aqueous anolyte.
The sodium hydroxide anolyte is preferably maintained at temperatures from above about 318° C. to maintain the molten state. Preferably, the cathode compartment is maintained at temperatures from above about 95° C., and more preferably above about 97.8° C., so that the sodium metal in the cathode compartment is in a molten state, and in contact with the ionic sodium conductor 106 separating the cathode and anode compartments. Both anolyte and catholyte may be maintained at the same temperature, or at different temperatures, provided that at least a portion of both the anolyte and the catholyte are in the liquid or molten liquid state.
When a potential greater than 2.0 V, preferably greater than 2.2 V, and most preferably about 2.8 V, is applied across the anode and cathode, sodium ions are transported across the ionic membrane to form sodium metal according to Equation (2a), and the sodium metal builds up in the cathode. At the anode, oxygen gas and water are formed according to Equation (2b). The overall reaction for the cell is shown by Equation (2).
In another preferred embodiment, hydrogen is optionally supplied to the anode compartment 106 of at least one cell in the stack to lower the overall cell voltage. In this case, instead of generating oxygen at the anode as in Equation (2), hydrogen is oxidized at the anode as in equation (3), and the preferred voltage of the cell is lowered. When a potential is applied across the anode and cathode, sodium ions are transported across the ionic membrane to form sodium metal according to Equation (3a), and the sodium metal builds up in the cathode. At the anode, hydrogen is oxidized and water is formed according to Equations (3b) and (3c). The overall reaction for the cell is provided by Equation (3):
Cathode 4Na++4e−→4Na (3a)
Anode 2H2→4H++4e− (3b)
Anolyte 4H++4OH−→4H2O (3c)
Overall 4NaOH+2 H2→4Na+4H2O (3)
The method of operating the electrolytic stack, the operating conditions such as temperature, and the cell stack designs for hydrogen assisted electrolysis are as described as above. However, operating voltage requirements are reduced.
In one aspect of the hydrogen assisted electrolysis, an aqueous solution of sodium hydroxide is charged to the anode compartment 106. When a potential greater than 1.4 V, preferably greater than 1.8 V, is applied across the anode and cathode, sodium ions are transported across the ionic membrane to form sodium metal according to Equation (3a), and the sodium metal builds up in the cathode. Hydrogen is oxidized and water is formed at the anode according to Equations (3b) and (3c).
In another aspect of the hydrogen assisted electrolysis, molten sodium hydroxide is charged to the anode compartment 106. When a potential greater than 0.9 V, preferably greater than 1.2 V, is applied across the anode and cathode, sodium ions are transported across the ionic membrane to form sodium metal according to Equation (3a), and the sodium metal builds up in the cathode. Hydrogen is oxidized and water is formed at the anode according to Equations (3b) and (3c).
The number of cell units in the stack is related to the voltage input, and the expected single-cell operating voltage. The number of cell units may typically be determined by dividing the input voltage by the expected cell voltage. As an example, for a 120 V power supply with hydrogen gas supplied to the anodes, approximately 100 cells will be in the stack with each cell operating at about 1.2 volts. Stack resistance may lower the number of cells. Without hydrogen at the anode, about 42 cells may be expected to be present in the stack, each operating at about 2.8 volts. The number of cells in the stack is flexible and can be chosen to match a convenient input voltage.
The following examples further describe and demonstrate features of the present invention. The examples are given solely for illustration purposes and are not to be construed as a limitation of the present invention.
As a demonstration of the process in a single electrolytic cell, 200 mL of 50 wt-% NaOH aqueous solution was placed in an electrolytic cell, as illustrated in
Before operation of the electrolytic cell, the tube was seeded with about 1 gram of sodium metal. A nickel wire 208 was passed through the septum to make electrical contact with the sodium 210. Collectively, the nickel wire and the sodium formed the cathode.
A nickel wire 212 in the NaOH solution was the anode. This wire and the NaOH solution inside the reactor and outside the Na β-alumina tube comprised the anode compartment. The nitrogen outlet from the Na β″-alumina extended into the anode compartment above the aqueous NaOH solution 214, so the anode compartment also was protected from air. The atmosphere above the anode compartment did not enter the cathode compartment because of the positive pressure behind the outlet needle.
The apparatus (anode and cathode compartments) was heated to about 120° C. The cell potential was held at a constant 4 V (anode versus the cathode). The current increased as the number of milliamp-hours passing through the cell increased because the cathode grew in size as sodium ions were drawn through the membrane and reduced to the metal on the cathode.
The sodium generated in the cathode was hydrolyzed in a 95% ethanol/5% water solution, and 1440 mL of H2 was collected (20° C., 1 atm pressure). The sodium metal seed initially charged to the cathode compartment would yield 523 mL of H2 on hydrolysis. The charge passing through the cell totaled 2013 mAh, which would yield an additional 1.73 grams of sodium that would hydrolyze to yield 905 mL of H2. The total anticipated yield from hydrolysis of sodium is 1428 mL of H2. Within experimental error, the yield of sodium was quantitative.
The sodium generating process as described in Example 1 was repeated by connecting two cells as shown in
A two-cell stack was constructed as described in Example 2 and sodium metal was produced under the hydrogen-assisted sodium generation in molten NaOH conditions shown in Equation (2). Two heating mantles were placed next to each other, and an α-alumina crucible containing 400 g of NaOH was placed inside each mantle. The mantles were both held above about 350° C., with reactor 1 operated at about 351° C., and reactor 2 operated at about 368° C. This melt comprised the anolyte and the anode compartment. The anode in each reactor was a nickel tube terminating in a nickel sparger. Hydrogen gas from a common source passed through each sparger into the anode compartment. The gas outlets of the two reactors were joined together to feed a common bubbler, to equalize the hydrogen pressure in the two reactors.
A sodium β″-alumina tube charged with 0.5 g NaOH was inserted into each melt. A nickel wire was in contact with the molten NaOH inside each sodium β″-alumina tube. These wires were the cathodes and the interior of the tube was the cathode compartment. The tube walls were the membrane/separator. Sodium metal was generated at the cathode, eventually filling the compartment so that the sodium was in contact with the membrane, and sodium became the cathode.
A working electrode from a constant voltage source was attached to the cathode in reactor 1. The anode in reactor 1 was connected to the cathode in reactor 2 by a wire. The anode in reactor 2 was attached to the counter electrode of the constant voltage source.
A potential of 2.4 volts was applied across the stack. An average current of about 285 mA flowed through the cells, for about 4.2 hours so that 1200 mAh of current passed through each cell of the stack. After the 4.2 hour time period, the potential was removed from the stack, and the sodium production measured. The measured output from the stack was 1.73 grams of sodium metal. In theory, each cell each passing 1200 mAh should produce 1.03 grams of sodium. As there were two cells in the stack, the theoretical yield was 2.06 grams of sodium. The 1.73 grams actually measured represents a coulombic yield of 84%.
While the present invention has been described with respect to particular disclosed embodiments, it should be understood that numerous other embodiments are within the scope of the present invention. For example, while the above examples were initiated with NaOH in the cathode compartment, sodium metal could be used instead of sodium hydroxide. In large scale implementation an all metal cathode compartment would be preferred. The processes and apparatus of this invention also can be conducted in a potentiostatic (constant voltage) mode or a galvanostatic (constant current) mode, and the invention is not limited to either potentiostatic or galvanostatic operation. In addition, the alkali-metal containing salt is not limited to hydroxide and any alkali metal salt may be used.
The above description and drawings are only to be considered illustrative of exemplary embodiments, which achieve the features and advantages of the invention. Modification and substitutions to specific process conditions and structures can be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be considered as being limited by the foregoing description and drawings, but is only limited by the scope of the appended claims.
This application claims the benefit of U.S. Provisional Application Ser. No. 60/626,484, filed Nov. 10, 2004, the entire disclosure of which is incorporated herein by reference.
The invention was made with Government support under Cooperative Agreement No. DE-FC36-04G014008 awarded by the Department of Energy. The United States Government has certain rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 60626484 | Nov 2004 | US |
