Enhancing electrochemical methods for producing and regenerating alane by using electrochemical catalytic additive

Abstract

A process of using an electrochemical cell to generate aluminum hydride (AlH.sub.3) and other high capacity hydrides is provided. The electrolytic cell uses an electro-catalytic-additive within a polar non-salt containing solvent to solubilize an ionic hydride such as NaAlH.sub.4 or LiAlH.sub.4. The resulting electrochemical process results in the formation of AlH.sub.3 adduct. AlH.sub.3 is obtained from the adduct by heating under vacuum. The AlH.sub.3 can be recovered and used as a source of hydrogen for the automotive industry. The resulting spent aluminum can be regenerated into NaAlH.sub.4 or LiAlH.sub.4 as part of a closed loop process of AlH.sub.3 generation.

Claims

1. An electrochemical process of producing AlH.sub.3 comprising: supplying an anode; supplying a cathode; placing said anode and said cathode in an electrolytic solution consisting of THF, an electro-catalytic-additive wherein said electrode-catalytic additive is a halide according to the formula MX where M is Li or Na and X is F, Cl, Br, or I, and an electrolyte selected from the group consisting of NaAlH.sub.4, LiAlH.sub.4, KAlH.sub.4, triethylenediamines, aluminum etherates, and combinations thereof, wherein the electro-catalytic-additive does not act as an electrolyte within the electrolytic solution by having no significant effect on resistance or conductivity of the electrolytic solution; and passing a current through the electrochemical cell to form an alane adduct, with the electro-catalytic-additive increasing the current passing through the electrochemical cell thereby increasing an efficiency in the formation in the alane adduct; and recovering AlH.sub.3 from the alane adduct.

2. The process according to claim 1 wherein said anode is an aluminum or palladium anode.

3. The process according to claim 1 wherein said cathode is a platinum or palladium hydride cathode.

4. The process according to claim 1 comprising the additional step of removing AlH.sub.3 from a surface of said anode.

5. The process according to claim 1 wherein said cathode is platinum and, atomization of hydrogen occurs at said cathode.

6. The process according to claim 1 wherein anode is an aluminum and said cathode is platinum and the electrolyte comprises NaAlH.sub.4 and a triethylenediamine to generate AlH.sub.3-triethylenediamine.

7. The electrochemical process of producing AlH.sub.3 comprising: supplying an anode selected from the materials of palladium, titanium, zirconium, aluminum, magnesium, calcium, or hydride forming metals; supplying a cathode selected from platinum or a metallic hydride; recovering aluminum from dehydrided AlH.sub.3; forming LiAlH.sub.4 from direct hydrogenation of the recovered aluminum; placing said anode and said cathode in an electrolytic solution consisting essentially of THF, an electro-catalytic-additive wherein said electrode-catalytic additive is a halide according to the formula MX where M is Li or Na and X is F, Cl, Br, or I, and the formed LiAlH.sub.4, wherein the electro-catalytic-additive does not act as an electrolyte within the electrolytic solution by having no significant effect on resistance or conductivity of the electrolytic solution; passing a current through the electrochemical cell to form AlH.sub.3 adduct with the electro-catalytic-additive increasing the current passing through the electrochemical cell thereby increasing an efficiency in the formation in the AlH.sub.3 adduct; and, heating the AlH.sub.3 adduct in a vacuum and thereby recovering AlH.sub.3.

8. An electrochemical process of producing an alane comprising: supplying an, anode selected from the materials of palladium, titanium, zirconium, aluminum, magnesium, calcium, and, combinations thereof; supplying a cathode selected from the materials of platinum, a metallic hydride, and combinations thereof; placing said anode and said cathode in an electrolytic solution consisting essentially of THF, an electro-catalytic-additive wherein said electrode-catalytic additive is a halide according to the formula MX where M is Li or Na and X is F, Cl, Br, or I, and an electrolyte selected from the group consisting of NaAlH.sub.4, LiAlH.sub.4, KAlH.sub.4, triethylenediamines, aluminum etherates, and combinations thereof, wherein the electro-catalytic-additive does not act as an electrolyte within the electrolytic solution by having no significant effect on resistance or conductivity of the electrolytic solution; and passing a current through the electrochemical cell to form an alane adduct with the electro-catalytic-additive increasing the current passing through the electrochemical cell thereby increasing an efficiency in the formation in the alane adduct.

9. The process according to claim 8 further comprising forming LiAlH.sub.4 from a dehydrided AlH.sub.3, wherein the electrolyte in said electrolytic solution is LiAlH.sub.4.

10. The process according to claim 8 wherein said cathode is platinum and, atomization of hydrogen occurs at said cathode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A fully enabling disclosure of the present invention, including the best mode thereof to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying drawings.

(2) FIG. 1 is a schematic diagram describing the process of a reversible alane formation.

(3) FIG. 2 is a schematic diagram of an electrolytic apparatus which may be used with a non-aqueous electrolyte to form AlH.sub.3.

(4) FIG. 3 is an X-ray showing diffraction analysis graph of AlH.sub.3 produced by an electrochemical cell.

(5) FIG. 4 is an X-ray diffraction analysis graph of AlH.sub.3-TEDA produced by an electrolytic cell.

(6) FIG. 5 is a comparative graph showing formation of AlH.sub.3-TEDA with and without the ECA additive.

(7) FIGS. 6A-6D are photograph comparisons of the buk electrolysis of production of AlH.sub.3-TEDA using comparative electrochemical cells with (6D) and without (6B) the ECA.

(8) FIG. 7 is a comparison of the circuit voltage performed on the two electrochemical cells with and without the ECA.

(9) FIG. 8 is a comparison of the electrochemical and impedance stectroscopy on two electrochemical cells with and without the ECA.

(10) FIG. 9 is an X-ray diffraction analysis graph of alpha alane produced by an electrolysis cell.

(11) FIG. 10 is an IR spectra of AlH.sub.3-TEDA product from the electrochemical cell of LiAlH.sub.4-Et.sub.2O.

(12) FIG. 11 is an X-ray diffraction analysis graph of AlH.sub.3-TEDA product from the electrochemical cell of LiAlH.sub.4-Et.sub.2O.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(13) Reference will now be made in detail to the embodiments of the invention, one or more examples of which are set forth below. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations as come within the scope of the appended claims and their equivalents. Other objects, features, and aspects of the present invention are disclosed in the following detailed description. It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only and is not intended as limiting the broader aspects of the present invention, which broader aspects are embodied in the exemplary constructions.

(14) In describing the various figures herein, the same reference numbers are used throughout to describe the same material, apparatus, or process pathway. To avoid redundancy, detailed descriptions of much of the apparatus once described in relation to a figure is not repeated in the descriptions of subsequent figures, although such apparatus or process is labeled with the same reference numbers. The methodology described herein relates to an electrochemical process described in U.S. application Ser. No. 11/891,125 filed on Aug. 9, 2007 and to PCT Application No. PCT/US2008/009536 filed Aug. 8, 2008, both of which are incorporated herein by reference.

(15) In accordance with the present invention, it has been found that a complex hydride such as NaAlH.sub.4, LiAlH.sub.4, or KAlH.sub.4 may be dissolved in the polar solvent such as THF within an electrolytic cell. The use of an organic solvent prevents the oxidation of the product and allows for the dissolving of the product which would interfere with the desired reaction. The reaction products may be recovered later. Using a cathode of platinum and an anode of aluminum results in the electrolytic formation of AlH.sub.3 adduct. The adduct allows recovery of AlH.sub.3 by heating under a vacuum. While the AlH.sub.3 adduct will tend to accumulate on the anode, it has been found that mixing ether with the THF or adding more THF solvent will dissolve the AlH.sub.3 adduct from the anode and allow the reaction to continue. The aluminum hydride can be crystallized and separated later from adduct by evaporating the solvent under vacuum. Preferably, when solvent is heated to a temperature of between 70 and 80 degrees centigrade in order to form alpha alane. Although other alane phases can be formed by means of varying temperature used in separation from the adduct alpha alane formed using electrochemistry is most desired for its stability. Alternatively, it is envisioned that a mechanical scraper, ultrasonic vibration, or similar processes can be used to periodically or continuously remove the deposited AlH.sub.3 from the anode. However, it is believed that direct formation of AlH.sub.3 is not desired in that the crystallization from an adduct is needed in order to bring about stabilization of the resulting alane molecule.

(16) The electrolytic conditions can be varied to bring about a more efficient production of AlH.sub.3. For instance, operating the electrolytic process under high pressure will facilitate the reaction speed. Likewise, using the electrolytic process at high temperatures will also favor a more rapid and efficient reaction rate of AlH.sub.3 production. Since the electrolytic conditions are using non-volatile polar solvents, loss of solvents to higher temperatures is not a limitation. In addition, the cathode forms LiH or NaH along with the evolution of hydrogen gas at the anode.

Example 1

(17) An electrolytic cell schematically illustrated in FIG. 2 was used to produce AlH.sub.3 on a palladium anode and an aluminum cathode and an electrolyte of NaAlH.sub.4 dissolved in THF. The reaction occurred at ambient pressure at room temperature using 5 v and 4 mA over a 2 hour period producing 10 mg of AlH.sub.3. The formation of AlH.sub.3 was detected on the anode. The formation of AlH.sub.3 was confirmed using X-ray diffraction as set forth in FIG. 4.

Example 2

(18) A high pressure electrochemical cell was utilized to generate AlH.sub.3. The non-aqueous electrolyte NaAlH.sub.4, dissolved in THF, was used in conjunction with a palladium anode and a platinum cathode and an electrolyte of NaAlH.sub.4 dissolved in THF. The electrochemical cell was operated under an elevated hydrogen pressure of 500 psi H.sub.2 and at a temperature of 60° C. using a voltage of 10 volts over a 2 hour period. 3 mg of AlH.sub.3 was produced. The formation of AlH.sub.3 was detected on the palladium anode and was subsequently confirmed by X-ray analysis.

Example 3

(19) Under ambient conditions of temperatures and pressures, using aluminum and platinum as electrodes and LiAlH.sub.4 dissolved in THF, AlH.sub.3 adduct was produced and alpha alane was crystallized from the adduct by heating to 70° C. under vacuum and 2 grams of the alpha alane were obtained. 1.5-2 v and 30 mA over an 8 hour time period was applied to the electrochemical cell. The pure alpha alane was confirmed by X-ray as seen in FIG. 9.

Example 4

(20) As seen in reference to FIG. 3, an AlH.sub.3 Amine adduct was made using an electrochemical cell to generate AlH.sub.3-triethylenediamine (AlH.sub.3-TEDA). The electrolyte was made using NaAlH.sub.4 and THF which was mixed with TEDA dissolved in THF, the combination being used as the electrolyte with a platinum electrode as the cathode and an aluminum electrode as the anode. Using ambient pressure and room temperature and operating conditions of 1.5 v and 30 mA over an 8 hour time period, 10 gm of AlH.sub.3-TEDA were precipitated out of solution. The x-ray diffraction pattern set forth in FIG. 4 shows the recovered product produced by the electrochemical process in comparison to a standard obtained through conventional methodologies. The additional peaks of the competitive standard represent aluminum and LiAlH.sub.4 which are not present in the electrochemically produced AlH.sub.3-TEDA.

(21) The AlH.sub.3-TEDA made by conventional methodologies is known to be a desirable hydrogen storage material in that the material can store 2.7 times its weight at 88° C. as reported by J. Gretz et al in the J. Phys. Chem. 2000, Vol. 111, page 19148.

(22) As seen in reference to FIGS. 1, 3 and 9 and the Examples provided, set forth the ability to use an electrochemical cell having dissolved NaAlH.sub.4 as an electrolyte, to subsequently form AlH.sub.3. The process allows for the desirable production of a reliable source of AlH.sub.3 as part of a cyclic process loop. The AlH.sub.3 product can be used to generate hydrogen gas for automotive or other commercial purposes. The resulting aluminum metal (spent aluminum) can be combined with NaH and hydrogen in the presence of a titanium catalyst to regenerate NaAlH.sub.4 as is known in the art and as set forth and described in the following publications. B. Bogdanovic and M. Schwickardi. J. Alloys Comp. 253-254 (1997); C. M. Jensen, R. Zidan, N. Mariels, A. Hee and C. Hagen. Int. J. Hydrogen Energy 24 (1999), p. 461; R. A. Zidan, S. Takara, A. G. Hee and C. M. Jensen. J. Alloys Comp. 285 (1999), p. 119; C. M. Jensen, R. A. Zidan, U.S. Pat. No. 6,471,935 (2002); and B. Bogdanovic, R. A. Brand, A. Marjanovic, M. Schwickardi and J. Tölle. J. Alloys Comp. 302 (2000), p. 36;
all of the above publications are incorporated herein by reference for all purposes.

(23) As seen in reference to FIG. 1, the entire process loop results in no unused byproducts, but provides for a closed system. The aluminum metal may be again converted into AlH.sub.3. Since no byproducts are produced, there is little waste associated with the process.

(24) The ability to generate AlH.sub.3 has been demonstrated using a non-aqueous solvent under both ambient conditions and elevated pressure and temperature conditions. While aluminum or palladium anodes and platinum or palladium hydride cathodes were utilized in the experiments, it is believed that other material choices for anodes and cathodes may be used.

(25) For instance, suitable anodes provided by palladium, titanium, zirconium, and other hydride forming metals are suitable for forming AlH.sub.3, borohydrides, and other alanates and complex hydrides. Likewise, suitable cathodes include materials such as platinum or a metallic hydride such as palladium hydride or titanium hydride. Where platinum is used as the cathode, it is noted that hydrogen gas is evolved from the surface of the cathode.

(26) In addition, it is believed that without undue experimentation, one having ordinary skill in the art can evaluate various process conditions for the electrolytic cell so as to optimize the production of AlH.sub.3 using various combinations of voltage, operating temperature, and operating pressure. It is also understood that the ability to regenerate aluminum into aluminum hydride holds enormous possibilities as a fuel source of hydrogen for transportation needs. Accordingly, it is recognized that within an overall energy budget, the most desirable operating conditions for generating AlH.sub.3 in the electrolytic system described above may be under conditions that may not achieve the highest yield, but does achieve a commercial product in the most cost effective manner.

(27) It is envisioned that the AlH.sub.3 can be provided to the automotive industry for use as a hydrogen source at various supply stations. The spent aluminum metal may be collected and subsequently treated at a commercial facility to regenerate the aluminum metal into an AlH.sub.3 using the polar, non-aqueous electrochemical cell. Depending upon the processing facility, the electrolytic cell may be operated under high pressure and/or high temperature conditions so as to generate a more favorable reaction rate.

(28) The methodology reported herein is not limited to the specific electrolyte and specific electrodes. For instance, a variety of aluminum etherates such as Al-TEA and boronates such as LiBH.sub.4-TEDA and other borohydride adducts may be employed. The electrochemical methodology described herein is a new method of making organo-metallic hydrides such as AlH.sub.3-TEDA or Al(BH.sub.4).sub.3-TEDA or other MH-Amine combinations where M is a metal that can have application in hydrogen storage for the automotive industry and portable energy systems such as batteries and fuel cells. The methodology lends itself to economical charging and re-charging systems as part of a renewable fuel cell.

(29) Heretofore, electrolytic processes involving the formation of alanes and other complex hydrides involve the use of salt containing electrolytic solutions, which are detrimental to the desired pathway described herein. In comparison, the present chemical formation process has a very high yield in that there are no competing side reactions that result in undesired end products.

(30) An additional variation of the methods reported herein include the use of an electo-catalytic-additive (ECA) which has been found useful in enhancing the yield of the electrochemical method for producing and generating alanes.

(31) The electrochemical method for producing and regenerating alane is highly enhanced by the use of an Electo-Catalytic-Additive (ECA). The ECA is an additive made of a halide (e.g. MX, M=Li, Na and X=F, Cl, Br and I) dissolved in a polar solvent.

(32) The electrolysis is carried out in an electrochemically stable, aprotic, and polar solvent such as THF or ether. MAlH.sub.4 (M=Li, Na) is dissolved in a polar solvent, forming an ionic solution. For example, lithium alanates dissolved in THF is used as an electrolyte.
LiAlH.sub.4/THFLi.sup.+/AlH.sub.4.sup.−/THF  (1)
The reaction in the electrochemical cell without the ECA can be described as such
At the Anode:
AlH.sub.4.sup.−.fwdarw.AlH.sub.3.nTHF+½H.sub.2↑++e.sup.−  (2)
Or
3AlH.sub.4.sup.−+Al (Anode).fwdarw.4AlH.sub.3.nTHF+3e.sup.−□  (3)
At the cathode:
Li.sup.++PdH (Cathode)+e.sup.+.fwdarw.LiH+Pd  (4)
When LiCl is added in THF, for example, to the electrolyte in equation 1
In addition to the electrolyte described in equation 11 the ionic electrolyte solution will contain
LiCl/THFLi.sup.+/Cl.sup.−/THF  (5)
At the Anode:
3Cl+Al (Anode).fwdarw.AlCl.sub.3+3e.sup.−  (6)
At the cathode:
Li.sup.++PdH (Cathode)+e.sup.−.fwdarw.LiH+Pd  (7)

(33) However, the formation of AlCl.sub.3 at the aluminum electrode will lead to the following reaction producing more alane (AlH.sub.3)
3LiAlH.sub.4+AlCl.sub.3.fwdarw.4AlH.sub.3+3LiCl  (8)

(34) The LiCl dissolves back in THF as in equation (5) and the cycle continues till all the LiAlH.sub.4 is converted into AlH.sub.3.

(35) The same concept applies to other halides and other and different solvent such as etherates

(36) Experimental Verification of ECA

(37) Test 1:

(38) To test the effect of the ECA two cells were prepared using LiAlH.sub.4 and TEDA in THF and including the ECA (1 gm of LiCl) in one of the cells. TEDA was incorporated in the solution to visually detect the formation of alane as AlH.sub.3-TEDA (white precipitate) during the experimental test. AlH.sub.3-TEDA was easily separated by filtering and then weighed. FIG. 5 shows the initial state of each cell and after 10 min of electrolysis. This figure clearly shows that cell 2 (cell with ECA in FIG. 6D) contains a larger amount of AlH.sub.3-TEDA. Also, it should be notice that a minimal amount or no dendrites were produced in the counter electrode in either cell.

(39) FIG. 6 shows the production of AlH.sub.3-TEDA formation (FIG. 6B) in a cell without ECA and results of an increased yield with the ECA additive present as seen in FIG. 6D. Respective controls of 6A and 6C indicate the condition of the electrodes prior to 10 min of electrolysis at 2.1 V. The high potential used during the electrolysis was required to produce enough alane in those 10 min for visualization purpose and eventually yield comparison. This figure shows an increase of 80% in the current when the ECA was used. The total charge for cell 2 was twice the total charge obtained with cell 1. The amount of yield of AlH.sub.3-TEDA was doubled when the ECA was used on cell 2. CVs for both cells are presented in FIG. 7. It should be noted that the open circuit voltage (OCV) for cell 2 is shifted to −1.5 V from the original cell #1 (OCV=−1.9). This means that the overpotential required for cell 2 is less when performing the electrolysis at 2.1 V. Consequently, lower energy is required for cell 2 to produce AlH.sub.3-TEDA, which implies that cell 2 is more efficient because it has more current with less energy input.

(40) Electrochemical Impedance Spectroscopy (EIS) was performed on the cells with and without the ECA. FIG. 8 shows that of the impedance, which represents the resistance of the cells, is about 112 Wcm.sup.2 for both cells. This shows that the ECA does not have a significant effect in the resistance (or conductivity) of the solution. That is, the ECA is not acting as an electrolyte. Consequently, the increase in current and efficiency discussed above are an electro-catalytic effect of the ECA.

(41) Electrochemical Cell Preparation and Tests

(42) Test 2:

(43) The electrochemical cells were prepared in an argon environment. Solutions from Sigma Aldrich consisting of 100-200 mL of 1 M MAlH.sub.4 (M=Na, Li) in THF or Et.sub.2O (except NaAlH.sub.4 in Et.sub.2O due to poor solubility) were used. The working electrode (anode) was an aluminum sheet of 1.56 cm.sup.2 (Alfa Aesar) and the counter electrode (cathode) was a platinum foil of 6.25 cm.sup.2 (Alfa Aesar). Prior to the experiments, the aluminum electrode was sanded in an inert environment to remove as much of the oxide layer as possible. A “leak-free” 3M KCl Ag/AgCl (Warner Instruments) was used as reference electrode.

(44) Preparation of the electrochemical cells to test the ECA was similar as described above, except that larger electrodes were employed and aluminum was also used as the counter electrode. In addition, about 1 g of LiCl was used in the cell for the test of the ECA and TEDA was added for visualization of the alane as AlH.sub.3-TEDA.

(45) Experiments were performed by attaching the electrochemical cell to a Schlenk line and were carried out at room temperature and atmospheric pressure. Electrochemical impedance spectroscopy (EIS), cyclic voltammograms (CV) and bulk electrolysis were performed using a Bio Logic VMP3 potentiostat. The impedance spectra were recorded using an amplitude of 20 mV around open circuit. The CVs were measured for 3 cycles at a scan rate of 10 mV/s or 50 mV/s.

(46) Characterization

(47) To confirm that alane was produced in the electrochemical cell of LiAlH.sub.4-Et.sub.2O, TEDA was used to precipitate the alane as AlH.sub.3-TEDA. For this, 70 mg of TEDA was added to 10 mL of THF and stirred until all solid was dissolved. Then this solution was added to 20 mL of the AlH.sub.3/Et.sub.2O solution collected from the electrochemical cell. The solid (formed immediately) was collected by filtration followed by a wash with two 20 mL aliquots of THF to remove any residual TEDA. The remaining solid was dried under vacuum for 24 hours. IR spectra and XRD of this sample are shown in FIGS. 10 and 11 respectively.

(48) The CVs for the electrochemical cells with MAlH.sub.4 (M=Na, Li) in THF and EtO.sub.2 show different oxidation and reduction reactions occurring in the system. The possible half-reactions with their respective equilibrium potentials for the cells are shown in Table 1. Although these values represent the potentials in aqueous solutions they give an approximation of the overpotentials at which these reactions can occurs. In general, the difference between the Na-related reactions and the analogous Li-related reactions is a potential shift of −0.28 to −0.33 V, except for the reactions (4) and (9) which has a shift of 0.04 V. However, the CVs show that the potentials between the NaAlH.sub.4-THF and LiAlH.sub.4-EtO.sub.2 cells are not shifted producing similar CVs with different current magnitude due to the difference in cell resistances.

(49) TABLE-US-00001 TABLE 1 Reduction potentials for the electrochemical cell reactions. Reactions E.sup.0 (V) vs SHE Eq. No. 4AlH.sub.3 + 3Na.sup.+ + 3e.sup.−     3NaAlH.sub.4 + Al −1.57 (1) AlH.sub.3 + ½H.sub.2 + Na.sup.+ + e.sup.−     NaAlH.sub.4 −1.73 (2) Al + 2H.sub.2 + Na.sup.+ + e.sup.−     NaAlH.sub.4 −2.28 (3) ½H.sub.2 + Na.sup.+ + e.sup.−     NaH −2.37 (4) Na.sup.+ + e.sup.−     Na −2.71 (5) 4AlH.sub.3 + 3Li.sup.+ + 3e.sup.−     3LiAlH.sub.4 + Al −1.89 (6) AlH.sub.3 + ½H.sub.2 + Li.sup.+ + e.sup.−     LiAlH.sub.4 −2.05 (7) Al + 2H.sub.2 + Li.sup.+ + e.sup.−     LiAlH.sub.4 −2.56 (8) ½H.sub.2 + Li.sup.+ + e.sup.−     LiH −2.33 (9) Li.sup.+ + e.sup.−     Li −3.04 (10) 

(50) Although preferred embodiments of the invention have been described using specific terms, devices, and methods, such description is for illustrative purposes only. The words used are words of description rather than of limitation. It is to be understood that changes and variations may be made by those of ordinary skill in the art without departing from the spirit or the scope of the present invention which is set forth in the following claims. In addition, it should be understood that aspects of the various embodiments may be interchanged, both in whole, or in part. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained therein.